3240
J. Phys. Chem. 1995,99, 3240-3250
Observation of the Potential-Dependent Second Harmonic Response from the Si(11l)/Electrolyte and Si(11l)/SiOfllectrolyte Interfacial Regions John L. Daschbach,’ P. R. Fischer, D. E. Gragson, D. Demarest? and G. L. Richmond* Department of Chemistry, University of Oregon, Eugene, Oregon 97403 Received: June 30, 1994; In Final Form: October 11, 1994@ The optical second harmonic (SH) response from the n-Si( 111) surface immersed in aqueous electrolyte solutions has been examined and is observed to be potential dependent. This potential dependence is observed for both the monohydrogen-terminated surface immersed in NH4F as well as for surfaces which are photoanodically oxidized in H2SO4. The potential dependence from the latter is screened in part by the presence of the insulating overlayer. Furthermore, a minimum is observed in the potential-dependent response which is shifted well anodic of the flatband potential. The SH observations are attributed to field effects within the space charge region of the semiconductor which are manifested in the higher order bulk SH response. Photocurrent transient analysis is utilized as a means of determining the flatband potential of these surfaces immersed in electrolyte solution. Experiments performed in UHV are discussed and compared to that observed in solution.
I. Introduction The reactivity of semiconductor surfaces can influence the performance of the semiconductor as an electrical component, usually in an adverse manner. In a reactive environment, such as immersed in an electrolytic solution, surfaces such as Si, Ge, and GaAs readily react to create surface defects and form oxides and other surface species. For silicon surfaces, an increased understanding of surface reactivity and stability in electrolytic solutions in recent years has been gained through infrared reflectance,’%* ultra-high vacuum (UHV)transfer experiment^,^-^ scanning tunneling microscope (STM)6-’5 and atomic force microscope (AFM)I6imaging, and second harmonic generation (SHG).’7-24 This progress can be attributed both to improved experimental methodologies as well as to advances in surface preparation procedures, e.g., hydrogen termination of the silicon surface.* However, these studies have only begun to describe the complex behavior of Si electrodes and the properties of the depletion layer. The use of surface second harmonic generation as a probe of interfacial electronic properties of semiconductors extends back to some of the earliest work of SHG on interface^.^^,^^ Recently a number of workers have returned to examining in more detail the SH response from silicon immersed in aqueous electrolyte systems in an attempt to exploit the inherent sensitivity and temporal response of the technique as an in-situ probe. Aktsipetrov et ~ l . have * ~ developed a detailed theory of the potentialdependent response from semiconductor surfaces in general, taking into account the free carrier density in the material, flatband potential, and surface state density. They have observed good agreement between experiments and their model for both Si and Ge electrodes in aqueous electrolytes. Sorg and co-workers20.21 have examined Si(111) electrodes in aqueous electrolytes at a single potential and examined the sensitivity to laser-induced adsorption of metals on the Si electrode. Characterization of the silicon surface in a reactive environment and the Si/SiO2 interfacial region has proven to be extremely complex. Recent advances in the chemical wet
’ Present address: Molecular Science Research Center, Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352. Present address: Department of Physics, Grinnell College, Grinnell, IA 501 12. Abstract published in Advance ACS Abstracts, February 1, 1995. @
etching of the Si( 111) surface predominantly characterized by multiple internal reflection infrared spectroscopy has led to an improved understanding of the “clean” silicon surface and has provided a relatively simple preparation procedure for obtaining such a surface.2 The clean surface, which is obtained at room temperature by etching the native surface oxide in an N W / HF solution buffered to pH 8 (by addition of NI&OH) followed by a thorough water rinsing, has been shown to be atomically smooth with large ideally mono-H-terminated terraces and two atom step edges. Furthermore, these surfaces are relatively stable in air and immersed in solution (until driven to oxidation under anodic bias in the presence of suprabandgap light). In contrast, surfaces etched only in HF result in mono-, di-, and tri-Hi-termination and exhibit microscopically rough surfaces. Higashi et al.I1 observed that these surfaces exhibit reversible surface roughening or smoothing if immersed in HF then in the buffered solution, respectively, or vice versa. Thus, the roughening observed with an HF etch does not reflect the morphology of the initial Si/SiOz interfacial region but is instead a result of the etching process. The Si/SiO2 morphology is complex and in spite of the numerous experiments employed to examine this interface there does not yet exist an unequivocal description of the buried interfacial r e g i ~ n . ~ ~ The - ~ Ocomplexity of this interfacial region arises from a transition layer which is composed of a nonstoichiometric suboxide and is dependent upon the experimental conditions under which the oxide layer is formed. The width, composition, and structure of this transition layer continue to be topics of discussion in the literature. An earlier study from this l a b ~ r a t o r y ’demonstrated ~ the sensitivity of both the relative phase and magnitude of the SH response from both the Si( 11l)/electrolyte and Si( 11l)/SiOZ/ electrolyte interfaces. In that paper a highly potential dependent response for H-terminated n-Si( 111) surfaces biased in N h F and oxide covered surfaces biased in H2S04 was observed. This paper describes the more detailed studies of these earlier results and expands the work to the Si( 11])/vacuum and Si( 111)/SiO2/ vacuum interfaces. This has been done in an attempt to address more explicitly the correlation between the phase in the SH response and potential induced band bending, as well as the relative contributions from the surface, bulk, and space charge
0022-3654/95/2099-3240$09.00/0 0 1995 American Chemical Society
Interfacial Electronic Properties of Semiconductors region. For both the oxidized and H-terminated surface it is found that the relative phase between the in-plane and out-ofplane response behaves consistently as long as the semiconductor is maintained under flatband conditions. Important to our understanding of the SH response is the ability to determine the flatband potential for these systems. By taking photocurrent transient data concurrently with the SHG measurements, a reasonably accurate determination of the flatband potential can be achieved. As the bands are increasingly bent (depletion condition), by the application of an external potential, the relative phase is seen to progressively increase. The degree of increase of the relative phase with applied potential is less for the oxidized sample relative to the native surface due to the insulating nature of the overlayer. In addition, comparative experiments have been performed in UHV. For a H-terminated Si(ll1) surface transferred to UHV, the relative phase is significantly higher than that observed in the electrochemical environment at flatband. Furthermore, experiments were performed at the Si( 11l)/SiOz/vacuum interface with samples of varying thicknesses of thermally grown Si02. The relative phase is seen to decrease with increasing oxide thickness. These experiments have been performed to help describe the nature of the response from these semiconductor surfaces as they exist in the solution environment.
11. Experimental Considerations
J. Phys. Chem., Vol. 99, No. IO, 1995 3241 The samples were degreased by ultrasonification in separate baths of methylene chloride, acetone, and methanol and were then dried with nitrogen. The back of the wafer was etched for 1 min in 48% hydrofluoric acid to remove the native oxide and was then mounted on Ga-In eutectic that had been placed on an embedded copper contact in a Kel-F shaft. A mask containing an embedded acid-resistant fluorocarbon O-ring was used to seal the surface from the electrolyte. The details of the electrochemical cell have been published elsewhere.34 The counter electrode was a platinum wire and all potentials are referenced to the saturated calomel electrode (SCE). Anisotropy measurements were performed by rotating the sample about the surface normal with a computer-controlled stepper motor. The front of the silicon wafer was etched for 3 min using a buffered W F / H F (7:l ratio) solution at pH 8.0 (by the addition of N b O H ) . This has been shown to produce smooth wellordered hydrogen-terminated surface^.^*^^ The sample was rinsed with ultrapure water and then immersed in the aqueous electrolyte. Experiments were performed in 0.1 M NI&F, 0.1 M H2S04, 0.1 M KC1, and 0.1 M KOH. All solutions were prepared with ultrapure water (Barnstead, Sybron Corp.) and were deaerated with Nz. For some experiments the cell was connected to a peristaltic pump to flow fresh electrolyte through the cell. This also allowed electrolyte solutions to be changed while using a single electrode. A BioAnalytical Systems Model CV-27 potentiostat was used for potential control. Photocurrent transient data was recorded on a LeCroy 9410 digital oscilloscope. The electrochemical setup was not modified to the standard design utilized for recording photocurrent transient^.^^ Therefore, the transient data presented below is affected by the response of the cell/ potentiostat combination and as such is considerably more complex than that expected for a photoelectrode alone.
The optical measurements were performed by utilizing the fundamental 1.064 pm output from a 10 Hz Nd3+:YAG laser (Quanta Ray) producing 10 nsec pulses. The angle of incidence on the cell was 45", therefore 32" on the sample in solution. The incident fluence of the pulses was %0.25 J/cm2, which is below the damage threshold for silicon. Standard polarization and filtering was used on the input and output beams and 111. Theoretical Considerations for Data Interpretation detection was via a monochromator with photomultiplier tube and gated electronics. The cell and detection optics were housed Second harmonic generation is forbidden in the bulk of a in a light tight box with the input beam passing through a pair centrosymmetric medium under the electric dipole approximaof long pass RG-1000 filters (Schott Glass Technologies Inc.). tion due to the presence of inversion symmetry. However, at A port was provided normal to the electrode for illumination the surface where truncation of the centrosymmetric lattice with a HeNe laser when it was necessary to photoanodically occurs, SHG is allowed. The expression for the second-order grow a silicon dioxide overlayer. induced polarization in the medium, P$i(2w), as a function of The ultra high vacuum (UHV) experiments were performed the fundamental driving field, E(w),can be seen in terms of in a chamber with a base pressure of 3.5 x lop9Torr which both surface and bulk contribution^:^^^^^ utilized a manipulator capable of 360" azimuthal rotation. The surface cleanliness and crystallinity were monitored by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). In one study, the silicon sample was sputtered (Ne+, C 1 kV) and annealed (1000 K) to clean the surface and induce -V 120 x [zM:E(w)E(o)](1) the Si( 111)-7x 7 reconstruction. Relative phase measurements are based on an interference The material susceptibility parameters, 2, are second-rank t e c h n i q ~ e ~ and l-~~ were performed by insertion of a harmonic tensors which describe the material's response to the driving waveplate (crystalline quartz, CVI Optics) in the beam path. field for each term in this multipolar e x p a n s i ~ n . ~ The ~ , first ~~ This leaves the polarization of the fundamental beam unaltered two terms of eq 1 are electric dipole in nature, the third and and creates a reference SH wave. The sum of the intensity of fourth describe the electric quadrupole contribution, and the last the reference SH and generated SH signals are then detected. term is the magnetic dipole contribution. The first and third The waveplate is then translated toward the sample resulting in terms are generated at the surface whereas the other terms an interference which originates from the dispersion of the originate from the bulk. While the magnetic dipole contribution fundamental and SH frequencies in the ambient air. The change has been observed:' it is negligible in these experiments. The in relative phase induced by the application of the applied dc surface quadrupolar term reflects structural discontinuity created field can be determined from the difference between the by the interface. The second and fourth terms represent field interference patterns observed at different applied potentials. discontinuities across the surface; it is through these terms that the coupling of the applied perpendicular dc field occurs The silicon wafers (International Wafer Service) were n-doped (predominated by &. with phosphorus having a resistivity of 3.0-6.5 Qcm, representing a defect doping density of approximately lOI5 ~ m - ~ . SHG derives its surface sensitivity from not only the dipoleThe samples obtained were 0.75 in. in diameter, 660-71 1 pm allowed surface terms but also higher order (quadrupole) terms which are inseparable from the dipole terms. The induced thick, with one side flattened to represent the [Oli] direction.
3242 J. Phys. Chem., Vol. 99,No. IO, 1995
Daschbach et al.
nonlinear surface polarization can be written in terms of an effective susceptibility as42,43
~ 3 2 0= ) ~~$;:E(u)E(u)
(2)
where 2:; is the second-order susceptibility tensor reflecting the optical as well as the symmetry properties of the surface layer and is composed of contributions from the bulk and the surface. If the SH intensity is recorded as a function of azimuthal angle, the variation in intensity reflects the overall symmetry of the surface and allows determination of the tensorial properties of A portion of these studies were done by rotating the surface azimuthally and monitoring the modulation in the second harmonic intensity as a function of the rotation angle.44 The azimuthal angle q5 is defined as the angle between the [2ii] direction and the projection of the incident wave vector parallel to the surface. This angular variation (rotational anisotropy) is directly related to the overall symmetry of the surface atomic structure. The intensity of this azimuthal dependence for a (1 11) surface can be described using phenomenological equations as a function of polarization condition^:^^^^^,^^
zfi.
I;;+$>
= la(-)
+ c(3)cos(3l$)l2
(3)
esu'(4> = lb(3)sin(34>12
(4)
The subscripts p and s denote the beam polarizations for the fundamental and second harmonic, respectively. The quantity d-1 is the isotropic coefficient and is related to the out-of-plane response (those with a z component) of the polarized electrons; this corresponds to the nonzero, independent surface tensor elements xzzz,xzii, and xizi (i = x, y) as well as the isotropic bulk element. The anisotropic coefficients b(3)and d3)describe the in-plane response of the electrons which includes the nonzero, independent surface tensor element and bulk element. Both the Fresnel coefficients as well as the material susceptibility tensor elements manifest themselves in the isotropic and anisotropic coefficients. Depending upon the chosen polarization, the fit to these equations allows information to be extrapolated about the nature of the SH response from the coefficients d-1, b(3),and d3). Much of the data presented here is obtained using p-polarized fundamental light and monitoring the p-polarized second harmonic, therefore, the rotational anisotropy must arise from the interference between d 3 )and d-).The fit to eq 3 above results in the ratio d3)/u("") which contains both a magnitude and relative phase, each of which are indicative of the extent of interference for the given experimental conditions. When a dc field is applied across the interface, as has been done in the studies reported here, additional factors in the polarizability can occur and can be described by24
xu
P::i(2w) = ji$;:E(w)E(w)
oriented in the z-direction (normal to the electrode surface) with the surface and bulk optical polarization. This additional polarization can be written as the following:46
+ ji$i:E(w)E(w)E(dc)
(5)
The third-order term refers to the potential dependent part where E(&) is the static electric field oriented normal to the surface. For these studies this perpendicular static field is due to an applied potential on the order of 0-5 V which drops predominantly across the space charge region of the semiconductor. The strength of this dc field can be on the order of lo6- lo7 V/m depending on the depth of the space charge region which is governed largely by the doping density of the semiconductor for a given applied potential. This can be viewed as a mixing of a static field that induces a polarization strictly
The overall symmetry of the response remains unchanged since both x(2)and ~ ' ( ~have 1 the same symmetry constraints imposed by the electrode surface. The net effect is that most tensor elements (including the bulk terms because of the extension of the static field into the bulk semiconductor) will be enhanced by the interfacial ~ h a r g i n g . ~ ~ . ~ ~ For the general discussion of the observed potential dependence, eq 5 can be expressed in terms of the SH intensity and is written in a simpler form as
(7) where
corresponds to the linear Fresnel factors,
x","
and is the effective cubic nonlinearity arising from the static field which includes all terms that vary linearly with field strength.49 A@ is the potential drop across the semiconductor space charge region and is proportional to the difference between the applied (Eapp)and the flatband (Efa) potential. When the SH response is dominated by the cubic nonlinearity term, the observed potential dependence should be parabolic with a minimum near the flatband potential. Such behavior has been observed in numerous studies of metaYelectrolyte Under conditions where the surface andor bulk quadratic nonlinearities dominate, parabolic potential dependence with a minimum shifted from flatband would be observed.
22)are the surface and bulk susceptibilities, and %;A@
IV. Results A. H-terminated Si(ll1) Surfaces. Figure 1 shows the SH response for a n-Si(ll1) sample as a function of azimuthal rotation for p-in, p-out polarization (Figure la) and p-in, s-out polarization (Figure lb). The sample is biased in 0.1 M NJ&F solution near the flatband potential (-0.65 V vs SCE) as determined from photocurrent transient measurements. The surface was etched for 3 min in buffered N&F/HF at pH 8.0 to produce a H-terminated surface prior to immersion. Once prepared, this surface should remain relatively free of any photogenerated surface oxides due to the competing rates of dissolution and photooxidation in fluoride containing electrol y t e ~ . ~The ' response for both measurements exhibits the threefold symmetry of the surface region. For the former case, the fit of the data to eq 3 gives a value for d3)/dW) = 2.2ei2I0. The s-polarized output data is fit by eq 4 and gives a value of b(3)= 0.45. As the voltage is varied within the region of stable photocurrent, the response from the H-terminated surface is found to be highly potential dependent. This is evident in the p-in, p-out experiment where there is a large increase in the amplitude and relative phase angle of the ratio d3)/&)as the potential is driven anodically into the depletion region. Figure 2 illustrates the pronounced shift in both the relative magnitude and relative phase of the quantity d3)/u('")as manifested in the rotational anisotropy as the potential is increased positive of flatband. Within the potential range depicted in the figure, the relative phase angle changes from 21' at flatband (Figure la) to 126" at 1.0 V (1.65 V positive of the flatband potential). The uncertainty in the phase angle measurements is f5". The magnitude of the ratio of d3)/u(")increases by a factor of 2 over this potential range. Although measurements conducted at
J. Phys. Chem., Vol. 99, No. 10, 1995 3243
Interfacial Electronic Properties of Semiconductors
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Figure 1. SH rotational anisotropy from n-Si( 11 1) immersed in 0.1 M NH@ biased at the flatband potential (-0.65 V vs SCE). Open circles are the data, solid lines are the fits to eq 3 or eq 4. (a) p-in, p-out polarization; d3)/u('")= 2.2ei2'",(b) p-in, s-out polarization; b(3i= 0.45. 1.5
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Figure 2. Plot of the fit parameters to the potential-dependent rotational anisotropy from n-Si( 111) immersed in 0.1 M N H P ; all figures are for the p-in, p-out polarization condition. The relative magnitude of d3)/u(-)is given by the open circles, 0,on the left ordinate. The relative is given by the closed circles, 0,on the right ordinate. phase of d3)/a('") In order to provide an alternative means of visualizing how the anisotropy pattern, and implicitly the fit parameters, changes at different potentials, the rotational anisotropies observed at three different potentials displayed in the Figure have been inset above. Each anisotropy displays the SH Intensity as a function of rotation angle.
potentials cathodic of the flatband are more difficult (due to hydrogen evolution in some electrolytes), a reduction in the phase angle with potential is the general observation. For the
p-in, s-out experiment a potential dependence is also observed; the intensity being highest near the flatband and decreasing as the sample is biased anodically. Figure 2 displays an interesting trend in both the relative magnitude and relative phase with the applied potential. While both the magnitude and relative phase of the ratio appear to be parabolic at low overpotentials with minima occumng near flatband, this does not correlate with any simple physical models for this system. Whether this is indeed a strong effect, indicating sensitivity to the depth of the SCR, remains to be determined by doping density studies. It is important to identify exactly what has been depicted in Figure 2. The figure displays both the magnitude ratio and relative phase obtained from the fit to eq 3 from each rotational anisotropy pattern as a function of applied potential. This figure displays how d3)and a(") interfere at each potential shown. It is important to distinguish this situation from that discussed below for Figure 3 where the magnitude of each the isotropic and anisotropic coefficients are isolated at one particular surface orientation. Experiments were performed which isolate the potential dependence of the amplitude and phase of the anisotropic, inplane (d3)or b(3))and isotropic, out-of-plane (a'")) response. For the amplitude measurements, both terms display a parabolic potential dependence with a higher overall signal level from the anisotropic contribution (Figure 3). The isotropic response has a minimum near +0.26 V, nearly 900 mV from the flatband. This out-of-plane response would be expected to couple to the perpendicularly applied static dc field, which for a sample at +0.3 V has a space charge region (SCR) with a depth on the order of 1200 nm. The minimum of the anisotropic in-plane response is estimated by extrapolation to be 1.6 V, considerably further from the flatband potential. For the phase measurements, a significant interference was observed only in the anisotropic component, ld3),and a relative phase difference of 84 f 3" was observed between -0.65 and f 0 . 3 V, consistent with the anisotropy results. The isotropic component displayed a nonzero, yet minimal response, but a relative phase shift was not observed within the signal to noise limits. It is worth noting that there is a slight hysteresis in the SH response between the time at which the sample is immersed in the solution and after the sample has been held under a potential
+
3244 J. Phys. Chem., Vol. 99, No. 10, 1995
2n ”‘1 --d‘------Daschbach et al.
6
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Figure 4. SH rotational anisotropies from the n-Si( 11l)/vacuum interface at the p-in, p-out polarization condition. The solid lines are the fit to eq 3. (a) Initially H-terminated sample which was sputtered = and annealed resulting in the (7 x 7) reconstructed surface; d3)/dm) 1.8elZ8’.(b) Hydrogen-terminated surface transferred to vacuum, no further surface cleaning was performed; d3)/a(‘) = 2.8ei6’”.
bias in the solution for a few minutes. For samples examined in 0.1 M NH4F aqueous electrolyte, the signal is observed to increase slightly and the signal to noise to improve during one to three rotational scans following the initial immersion. During this approximately 12 min period the potential is held at flatband. Nevertheless, fits to the data obtained from these initial scans yield similar values for the d3)/&) ratio. The data also show good reproducibility over time for the same applied potential provided no excursions are made far into the anodic photocurrent regime (> 1.O V). Similar SH measurements were also conducted on related samples in the UHV environment. All surfaces were initially prepared in a H-terminated state prior to mounting in the chamber. In the first case, the surface was sputtered and annealed in order to obtain an oxide-free surface. The rotational anisotropy in the SH response was measured and the p-in, p-out results are shown in Figure 4a. For the annealed surface the rotational anisotropy is consistent with previous studies of this (7 x 7) reconstructed ~ u r f a c e . ~Figure ~ , ~ ~ 4b shows the corresponding results for the H-terminated sample which was placed in the chamber and evacuated to the base pressure. No additional surface cleaning was performed. For the p-in, p-out data of Figure 4b, the results correspond to the unreconstructed surface and give a relative phase angle of 61” when fit to eq 3. The results are similar qualitatively to that obtained in solution. However, the relative phase angle is considerably higher than the phase angle normally observed for this surface immersed in solution at the flatband potential. Assuming that the relative phase angle is indeed indicative of the band bending at the surface of the semiconductor, one would conclude that the surface in vacuum is not at flatband but that there is actually a potential drop of several hundred millivolts across the space charge region (depletion condition). This is consistent with the
predicted effect of surface states inducing a depletion condition by capturing some of the bulk carriers.54 In comparing the annealed surface to the H-terminated surface in UHV, it is clear that the response from the reconstructed surface is considerably higher than the response from the nonreconstructed surface. This is consistent with the increase in long-range order observed for the reconstructed s u r f a ~ e . ~ ~ , ~ ~ B. Transient Photocurrent Measurements. Transient photocurrent measurements were made in conjunction with the SH measurements to determine the flatband potential for each experiment and to understand the effect of photogeneration of carriers resulting from the above band gap energy (Ebg = 1.12 eV) of the probe laser (&dent = 1.17 eV). A typical photocurrent transient from a sample immersed in 0.1 M N b F at a potential of -0.60 V is shown in Figure 5a. It is apparent from this figure that the transients are distinctly bipolar, consisting of two discernible “lobes” each of which are clearly evident and are labeled as “peak 1” and “peak 2”. As mentioned above, the response of the potentiostatkell combination is not optimum for transient measurements and thus the photocurrent transients display this bipolar character. Specifically peak 1 is considered to be a “ring” which we attribute to an inductive response in the circuit. The lobe of opposite sign (peak 2 ) contains most of the charge. Figure 5b displays the same sample now held at a potential of -0.5 V. It is apparent that even a small change in potential has a significant effect on the character of the lobes in the photocurrent transient. It is for this reason that extensive analysis of the photocurrent transients was undertaken as a means of determining the flatband potential. Figure 6 shows the photocurrent analysis for n-Si( 111)
Interfacial Electronic Properties of Semiconductors 1.2e-5
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J. Phys. Chem., Vol. 99, No. 10, 1995 3245 I
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Figure 6. Photocurrent transient analysis as a function of potential. (a) 0.1 M KOH, pH = 13; (b) 0.1 M N H P , pH = 6.0; (c) 0.1 M H2S04, pH = 1.0. For all figures, the curves are delineated as follows: (curve A) the average integrated photocurrent for all combined lobes of the transient; (curve B) the maximum peak current observed in either lobe after the light pulse minus the precurrent; (curve C) the minimum peak current observed in either lobe after the light pulse minus the precurrent; (curve D) the precurrent recorded prior to the light pulse. This is sensitive to the residual redox events taking place in the absence of light.
immersed in three different electrolytes, (a) 0.1 M KOH, (b) 0.1 M "4F,and (c) 0.1 M H2S04, as a function of potential. For all solutions an inflection point in the photocurrent is observed. In particular the KOH data is more clearly resolved (compared to what is observed in N W or in H2S04) due in large part to the basicity of the electrolyte which extends the potential region before the onset of hydrogen evolution. Each of the plots in Figure 6 shows the average integrated photocurrent for all combined lobes of the transient (A), the maximum peak current minus the precurrent (B), the minimum peak current minus the precurrent (C), and the precurrent recorded prior to the laser pulse (D). The precurrent is sensitive to the residual redox events taking place at the surface in the absence of light. The precurrent is subtracted from each to eliminate the current due to any surface redox reactions (e.g., hydrogen
evolution). We have found it useful to show both the peak photocurrents as well as the integrated photocurrent because the peak photocurrent appears smoother than the integrated photocurrent for each electrolyte. Although we have not found a perfect model for our instrumental response (which leads to the appearance of the ring), it is clear from inspection of Figure 6 that this provides an alternative means of assigning the flatband potential for these systems. These data provide the best representation of the band bending conditions in the silicon sample. By comparing curve A to curves B and C in Figure 6a, it is apparent that the peak current for the positive lobe is at a minimum approximately where the integrated photocurrent starts to rise. Similarly, the peak current for the negative lobe is at a maximum approximately where the integrated photocurrent starts to rise. For KOH in Figure 6a this occurs at a potential of -1.0 V vs SCE (indicated by the arrow). The integrated photocurrent provides another estimate of the flatband potential, though not as dramatic as the analysis from the transient data. The integrated photocurrent shows the same trends as expected from CW illumination, with an increase in photocurrent with increasing anodic bias near flatband and reaching a plateau at large anodic biases relative to flatband. Thus we have demonstrated that analysis of the photocurrent transient provides a clear determination of the flatband potential under our experimental conditions. Figure 6b is the photocurrent analysis for n-Si( 111) immersed in 0.1 M N h F (pH = 6.0). The analysis from a sample in N&F is not as obvious as in the case of KOH. This is because hydrogen evolution is occurring in the region of the flatband potential. This is demonstrated by the large negative precurrent (curve D) which subsequently influences both the maximum and minimum peak currents. The flatband potential is determined from the behavior of curves B and C (which have eliminated this redox behavior by subtraction) and the value of the flatband potential is determined to be -0.65 V vs SCE. Figure 6c shows the analysis of transient data taken as a function of potential in 0.1 M H2S04. In all cases the data obtained for N&F is quite reproducible in both the magnitude of the observed photocurrent as a function of applied potential and the flatband potential. In contrast, data obtained in HzS04 and other nonetching electrolytes is dependent on the history of the sample, particularly the amount of anodic oxide present on the electrode as well as expected dependence on solution pH. However, reasonably reproducible data is obtained on freshly prepared samples or samples which have been etched in situ. The H2S04 data is also limited by the onset of hydrogen evolution but it is not as dramatic as that observed in N&F. Furthermore, assignment of the flatband potential using the integrated photocurrent trace alone would lead to an erroneous value for the flatband potential. It is clear that analysis of the photocurrent transients provides an alternate means of assessing the flatband potential. To summarize, the flatband potentials determined from transient measurements are -0.65, -0.4, - 1.O.
and-0.5Vin0.lMN&F(pH6.0),0.lMKCl(pH6.5),0.1 M KOH (pH 13), and 0.1 M H2SO4 (pH l.O), respectively. The data provides additional evidence for the conclusion that the minimum observed in the amplitude of either the isotropic and anisotropic response (Figure 3) is not at the flatband potential. The magnitude of the transients reach a steady state at any fixed potential in N&F, and to a lesser extent in H2S04. Upon stepping to more anodic potentials, the transients are seen to dramatically increase in magnitude and to then decay over a period of 10-100 laser shots to a steady state. Near flatband the integrated charge in each photocurrent transient is approximately lo-' C in 0.1 M "8. As a limiting case we
3246 J. Phys. Chem., Vol. 99, No. 10, 1995
Daschbach et al.
TABLE 1: Fits to Rotational Anisotropy Data: oxide thickness vapp- vn, %o 8, (H2S04) 15 A 25 A
3
a)
0 V (FBI +0.5 V +l.OV +1.5 V f2.0v +2.5 V +3.0 V $3.5 v
g
3 ,
1
d
C
$
2
1/1
1
0 0
60
120
180
240
300
360
Rotation Angle Figure 7. SH rotational anisotropy from the n-Si(11 1) immersed in 0.1M HzS04, all scans taken at the p-in, p-out polarization condition. (a) Initial H-terminated sample immersed in H2S04 and biased at the flatband potential, E = -0.5 V; d3)/a("")= 1.8ei50".(b) Same sample that has had 15 8, oxide photoanodically grown on tbe surface; d3)/a("") = 2.4eI3'". (c) Same sample that has a total of 25 A oxide grown on the surface; d3)/a'")= 2.3e'48".The signal enhancement in the presence of the oxide is apparent. assume that all the photocurrent is generated within the beam profile of 0.05 cm2 giving about C cm-2 per laser shot. From numerous of the photoelectrochemistry of n-silicon, it is known that illumination of an anodically biased surface results in formation of an oxide layer which passivates the surface. In fluoride containing electrolytes the dissolution of this oxide competes with the photooxidation process producing a steady-state photocurrent dependent on solution composition and anodic polarization. On the basis of the previous studies, we estimate that it takes approximately 300 laser shots to oxidize a monolayer. This estimate is an upper limit since it is based on an unlikely high quantum yield of one for the oxidation of silicon, a four-electron process. The growth of the stable oxide in acidic solutions has been estimated to be about 10% e f f i ~ i e n t . ~ ~ It is difficult to make a comparison of these photocurrent results with previous studies considering that the earlier has been performed under CW illumination. While similarities do exist, any qualitative results are purely speculative due to the difference of the form of excitation of the semiconductor. However, the flatband potential as determined from the photocurrent analysis is in good agreement with previous studies of silicon surfaces immersed in fluoride containing electrolytes.10,20,62.63 C. Buried Si(lll)/SiOflnterface. The sensitivity of SHG to trace oxides on the surface and the effect of thicker surface oxides on the interfacial response has been examined in several studies. In the first set of experiments the initially prepared H-terminated surface was immersed in 0.1 M H2SO4. Unlike an N W solution, the surface Si02 generated either by the probe laser light or simply by the presence of the solution will remain on the surface due to the insolubility of the oxide in H2S04. Figure 7a is the scan of the response from this surface at flatband
1.8
2.4 ei3"
2.9 ei55'
4.2 ei75" 4.6 ei106° 4.0 ei1269
2.3 2.8 ei61" 3.4 ell2' 4.0 e1899 5.0 e1113" 4.7
c(%(")
40 8, 2.5 2.8 e>42c 3.2 els1' 3.5 ei58" 3.7 ei70" 4.5 e'92c 5.1 e'"o' 4.8 ei120"
(-0.5 V) in 0.1 M H2S04 for p-in and p-out polarization. From photocurrent measurements we estimate the oxide thickness to be less than 3 ML. Parts b and c of Figure 7 were performed on a sample for which respectively an estimated 15 and 25 8, of oxide had been photoanodically grown on the surface in 0.1 M H2S04, These traces were taken at flatband, -0.2 and 0.0 V, respectively. These oxide overlayers were grown on the surface by a controlled stepwise increase in potential while the surface was illuminated with the diffused beam from a HeNe laser (632 nm), all the while keeping the anodic current approximately 20 p A cm-2. For all samples in Figure 7, the response arising from the C3" symmetry of the Si( 111) surface region is clearly evident in the presence of the oxide as has been reported in previous ambient studies. In contrast to the H-terminated surface in Figure la, the oxidized samples show a significantly enhanced signal intensity. However, there is no evidence of an isotropic contribution from the amorphous oxide layer. It is known that the nonlinear response from this insulating overlayer is small. For the sample directly immersed in 0.1 M H2S04 (Figure 7a), the response is similar to what is obtained for the H-terminated surface in the 0.1 M N K F solution near the flatband, Le., d3)/ a(-) is 1.7ei4', although a distinct difference is observed. In NH4F the SH response is highly reproducible, however, in H2S04 the phase can vary between 30" and 60" from one experiment to the next. This is consistent with the observation that the flatband potential for these samples also varies from one experiment to the next and is indicative of the variability of the properties of the initial stages of oxide growth on the surface and the possible storage of charge at the interface. At flatband the phase angle is near 40" for the samples shown and for samples of greater thicknesses. A summary of the potential dependence in d3)/dm) for surfaces with varying amounts of oxides photoanodically grown on the surface is shown in Table l . I 9 The data were taken with p-in, p-out polarization for the samples biased at several potentials. As the oxide thickness increases, the potential region accessible for study also increases due to the screening nature of the oxide. For example, with the 15 8, sample, the stable potential range can be extended to +2.0 V whereas for the 40 8, sample the range is f5.0 V. For all oxidized samples, there is a positive shift in flatband potential as determined by the current transient measurements. The oxide layer thickness has been determined by ellipsometry measurements and etch back times. Because the electrochemically grown oxides have substantial water content in the first monolayers of oxide, with this ratio decreasing as the thickness increases,59these ellipsometrically determined thicknesses are viewed as an upper limit. The increased screening introduced by the oxide is evident in the reduced phase shift for the same applied potential as the oxide layers are made thicker. In-situ etching experiments have been performed to understand if the above observations are consistent with the removal of surface oxides. Photocurrent transients are monitored in
Interfacial Electronic Properties of Semiconductors 0.8
-
3 .*
,
I
J. Phys. Chem., Vol. 99, No. 10, 1995 3241 1.0e-4
8.0e-5
0.6
Y
3
0.5
6.0e-5
0.4
3
i z -0
4.0e-5
5 B 00 a a B
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O3
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2
a
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0.7
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81
0.1
0
500
1000
1500
2000
Time (seconds) Figure 8. In-situ etching experiments performed on n-Si( 111) samples with an estimated 40 A photoanodically grown oxide on the surface. Isolation of the isotropic SH response via 4 = 30' and p-in, p-out polarization. The open circles represent the SH data (units given by the left ordinate) and the solid line represents the photocurrent obtained simultaneously (units given by the right ordinate). The samples are initially immersed in 0.1 M &Sod; at the point indicated by the arrow 0.1 M N H P is started flowing into the cell at a rate of 30 mL/min.
conjunction with the SH. Typical results for the anisotropic response (p-in, s-out polarization, fixed azimuthal angle of 4 = 30") are presented in Figure 8. A similar result is obtained for the isotropic response. In these experiments an anodic oxide is grown under constant current conditions (20 p A cm-*) in 0.1 M H2S04 under diffuse HeNe illumination while the potential is driven to 5.0 V. The HeNe light is then blocked and the potential is set to 0.5 V, the potential at which the etching is performed. The SH response is then collected as the electrolyte solution is changed to 0.1 M NH4F through the peristaltic pump at a flow rate of approximately 30 mL/min, in the cell which has a capacity of 5 mL. The onset of photocurrent was used as an indicator of the removal of oxide from the surface since minimal photocurrent was observed in the presence of the oxide. As the etching begins and proceeds within the first minute, the photocurrent is nonexistent and the small change in the SH response is attributed to the changing field in the SCR as the potential dropped across the oxide diminishes as the oxide thins. When the oxide layer becomes thinner there is a substantial anodic current observed, which is also present in the absence of illumination. This current is assumed to be due to the complete oxidation of incompletely oxidized silicon in the interfacial region as the last few monolayers of oxide are etched.59 Roughly concurrent with the large increase in anodic current there is an increase in the photocurrent transients and a dramatic decrease in the SHG, consistent with previous Two factors are responsible for the decrease in SH intensity which are not separable from each other in these experiments. As noted above, the presence of the anodic oxide enhances the signal, and the removal of this oxide will therefore decrease the signal. The second effect is the change in the field of the SCR when the insulating oxide is removed. The potential used for etching is much closer to the potential at which the minimum in the SH from a sample in N b F occurs than for an oxide covered sample which has been shifted anodic by the screening of the oxide layer. Comparative experiments for the Si( 11l)/SiOz/vacuum interface have been performed. It is important to distinguish that the Si(ll1)ISiOz samples utilized in this experiment have been
8 ,
0
I
60
120
180
240
300
360
Rotation Angle Figure 9. SH rotational anisotropy from n-Si( 11 l)/SiOz/vacuurn = interfaces for various thermal oxide thicknessex (a) 57 A, c(~)/u("") 3.5e'80".(b) 73 A, ci3)/u("")= 3.0e176".(c) 108 A, d3)/d6') = 3.1e1660, (d) 180 A. c%(-) = 3.ie154".
thermally grown on the substrate (Integrated Circuits Laboratory, Stanford University, resistivity of 1-4 Q-cm). While the thermal and anodic oxides are different in interfacial composition and to a small extent m ~ r p h o l o g ythe , ~ ~general trends observed do provide a basis for some comparison. The p-in, p-out SH response for the rotational anisotropy of four different thermal oxide thicknesses, 57, 73, 108, and 180 A, are seen in Figure 9, a-d, respectively. In all cases the symmetry of the surface region is maintained in the anisotropy, without a large isotropic response contribution from the oxide. Furthermore, the ampliremains essentially constant; however, the tude of d3)/dm) relative phase decreases with increasing oxide thickness. In comparison to the work in solution (Figure la), the response in vacuum is clearly not at the flatband condition. In fact, the response in vacuum is similar to a sample in solution biased a few hundred millivolts positive of flatband. Equivalently, a negative surface charge in UHV would result in the same effect. In vacuum, surface potentials in excess of 350 mV can build up on a surface, especially in the presence of the insulating Si02 layer. Assuming relatively constant surface charge from sample to sample, the thickness of the oxide should affect the relative phase in a similar manner to that in the electrochemical cell. A thicker oxide layer more effectively screens the surface charge from the underlying bulk and thus the band bending would be decreased. This is what is observed in UHV; the phase gets smaller, indicating less band bending (approaching flatband) for the thicker oxides. This is similar to what is observed in solution because the phase angle progressively gets smaller as the surface remains oxide free and approaches the flatband condition.
3248 J. Phys. Chem., Vol. 99, No. 10, 1995 Experiments which compare thermal and anodic oxides immersed in solution are currently underway in this laboratory.
V. Discussion SHG has been used to monitor a variety of silicon interfaces in both solution and vacuum. A strong potential dependence of the second harmonic signal from n-Si(ll1) in aqueous solution has been observed and shows a minimum in the potential-dependent response that is shifted well anodic of the flatband potential. Similar results are observed for Si( 111)/ SiOz/electrolyte interfaces where the potential dependence still manifests itself in this interfacial region but is screened in part by the presence of the oxide. Comparative experiments in vacuum have been performed and are consistent with that observed in solution if an allowance for surface charging in the vacuum is made. The source of our SH response arises from a combination of both surface and bulk contributions from the semiconductor. An additional source to the SH response also arises from the ordering of the solvent molecules at the interface as a function of applied p ~ t e n t i a l however, ,~~ the magnitude of this response is considerably smaller than the contributions which arise solely from the silicon surface and bulk SCR. It is important to realize that the electric field strength at a semiconductor/electrolyte interface is much smaller than that for metaYelectrolyte interfaces; this will have a direct effect on the ordering of the solvent molecules. Furthermore, if the silicon surface is even slightly oxidized, the surface roughness will impair the ordering of the solvent. Therefore, the magnitude of the SH response from the solvent for these systems is essentially negligible. If an appreciable combined surface and bulk response from the silicon is present, then the minimum in the effective potential-dependent response can be shifted from the flatband potential. This shift arises from interference between the f 3 ) response and the potential-independent surface and bulk second-order responses. While we observe a strong potential dependence of the SHG from n-Si( 111) in solution, it does not appear that the dominant contribution is a dc field dependent x(3)term (from eq 7) since both the isotropic and anisotropic components decrease as the electrode is biased positive of flatband. This stands in contrast to studies of metaYelectrolyte systems under nonresonant conditions, in which the surface response dominates the SHG and the bulk response is essentially negligible. Under these conditions, parabolic potential dependence is observed; the minimum in SH intensity occurring at the potential of zero charge and increasing as the potential increase^.^,^^^^^ Further evidence for this is the similar potential-dependent response observed for the p-in, s-out polarization combination. Since the electric field in the SCR is static and normal to the surface, the tensor elements of the effective second-order susceptibility will transform as the surface-allowed elements.66 However for the effective C3,,symmetry of the Si( 111) surface only the element will generate signal for the p-in, s-out polarization combination. Potential dependence for this polarization combination has been observed for the Ag( 111) surface and was attributed to a perturbation in the surface xyu electronic response47since the bulk quadrupole response is negligible for Ag. However in contrast to a metal, the bulk quadrupole response is of comparable magnitude to the surface response for oxidized Si away from resonance.39 Because the interference between potential-dependent and -independent terms would be expected to be different for different polarization conditions it is not unexpected that the isotropic and anisotropic potentialdependent response show minima at different potentials. However, it would be expected that the isotropic response (those
Daschbach et al. elements with a z-component) would show a much stronger coupling to the perpendicularly applied dc field and would therefore have a minima closer to the flatband potential than the response in the anisotropic case. In fact the opposite is observed. It would appear then that the potential-dependent coupling observed here is in part manifested through the bulk quadrupole response. True discrimination of the surface to bulk contribution cannot be achieved for a (1 11) surface even for experiments chosen at specific polarization conditions, but the fact that the potential-dependent response is similar in form for samples in both and H2SO4 indicates that the potential dependence arises predominantly from the bulk. A change in signal related to the relative free carrier density in the SCR, as has been observed in the metalization of Si( samples in vacuum, would be expected to show a trend similar to that observed here as the depth of the SCR increases with increasing applied potential positive of flatband. Since the penetration depth of the pump light (%l cm) is considerably greater than the depth of the SCR, the fundamental evenly samples the SCR. However the escape depth for SH light is on the order of 1250 nm and hence changes in the bulk quadrupolar susceptibility in the SCR are manifested in the observed response where the signal decreases as the depth of the SCR is increased, up to a point. Since the x(3)response must be increasing positive of flatband it starts to dominate at some point, giving rise to the expected increase of SH signal with increased band bending. The slight enhancement in signal and signal to noise observed in our experiments after one scan indicates, however, that the surface response is not negligible. There are a number of possible reasons for this observation. One possible contribution is from the Si/SiO2 boundary. The nature of this interface is the subject of many contradicting reports in the literature, with both a crystalline interphase region28@ and a completely amorphous i n t e r p h a ~ e ~being ’ . ~ ~ supported. However, it seems likely that a strained region of the Si lattice exists just below the interface for some surface preparation conditions. This subsurface has been shown to lead to a strong resonant enhancement in the nonlinear response near 3.3 eV68but was not present for annealed or H-terminated samples. A similar mechanism has been postulated to support the enhanced signal seen from the (7 x 7) reconstructed Si( 111) surface in vacuum. While in vacuum, the oxide-covered surface exhibits diminished signals relative to the reconstructed one, the nature of the anodically grown oxide is quite different from the native and thermally grown oxides. The experiments here, being far from the resonance at 3.3 eV, make direct comparison difficult. However, the same general feature is observed; the H-terminated sample has a lower signal relative to the oxidized samples. Further, the data reported here from H-terminated surfaces in vacuum indicates a lower signal from these surfaces relative to the oxide-covered surface. It is also possible that the enhanced signal is due in part to the increased density of surface states present at the Si/SiO2 boundary. This is supported by the rough correlation of stabilization of the increase in signal and improvement in signal to noise with the oxidation of one monolayer. An increase in signal due to local field effects on a microscopically roughened Si/Si02 interface are also possible but unlikely since the signal does not improve after one monolayer has been oxidized even though the surface would be increasingly roughened well beyond this point. The rotational anisotropy persists in the presence of the oxide and the signal from the oxide-covered surfaces is enhanced relative to the clean surface. This suggests that in the presence of the oxide, the signal from the Si(111) continues to dominate the response over the more isotropic and disordered oxide
Interfacial Electronic Properties of Semiconductors overlayer. The signal enhancement in the presence of the oxide indicates that the SHG is sensitive to the Si( 111) surface adjacent to the oxide and that it is not merely a bulk response, as was discussed above. It is conceivable that the signal enhancement is due to a change in the Fresnel factors which occurs as the oxide grows on the surface, thus altering the angle of incidence of the incoming optical field and therefore increasing the SH response. Previous studies of photoanodically grown oxides on silicon have shown that the composition of the oxide varies with oxide thickness; specifically, water is incorporated into the first few monolayers. As a result, a change in refractive index has been observed as the oxide grows.59 Therefore, the incoming field effectively samples a gradient in the dielectric properties of the oxide. This would be manifested in the nonlinear response through the Fresnel factors. However, since the complex part of the dielectric for Si02 varies to a negligible extent for the wavelength region employed here, there is very little to no absorption by the oxide. Therefore, the changes in the Fresnel factors would be expected to have only a minor effect. Angle of incidence experiments are underway in this laboratory to determine if this does indeed contribute to the SH response. Earlier SH work24had examined the Si/electrolyte interface; however, no information was given concerning the reference electrode employed, and the flatband potential versus the reference electrode was not reported. It appears that the electrode polarization was assumed to be absolute with respect to zero applied potential and a model of quadratic dependence of the SHG with applied potential was found to fit the data. However, the CV data presented would indicate that the flatband potential was probably negative of - 1.O V. With this correction the data clearly do not fit a quadratic dependence in applied field strength but they do resemble the data reported here. As noted above, the onset of the photocurrent and the behavior of the photocurrent transients yield a flatband consistent with the literature and well negative of the minimum in the SH signal. In the model put forth by Aktsipetrov et al., the minimum in the SH response occurs at the flatband potential, and the data presented in that work supports the model. It is clear, however, that for the results obtained in this work the minimum in the SH response occurs well positive of flatband and thus another explanation must be sought. Additional SH experiments have been performed utilizing different semiconductors and observe results which differ from those presented here. Lantz et al.66,69,70 have examined singlecrystal Ti02(001) electrodes and found the response to be dominated by the cubic susceptibility term, but only positive of flatband. Negative of flatband negligible SH response is observed, which is attributed to an optical screening of the field in the space charge region by the excess electronic density at the surface. The simple model for electric field strength at the surface used in their analysis (that is E works reasonably well for depletion conditions when the applied potential vs flatband is less than the bandgap of the semiconductor. We have performed a complete calculation using Seiwatz and Green’s formulation (specifically eq 25)71for the Ti02 case and it indicates the square root dependence on applied potential extends to about 2.5 V vs flatband with a very sharp transition to an exponential dependence on applied potential for the next few hundred millivolts beyond this point. The transition point is a function of doping density and moves further from flatband as doping density increases and occurs around the point at which the Fermi level reaches the valence band edge at the surface. The data reported by Lantz and c o - w o r k e r ~ do ~ ~not , ~ extend ~,~~ into this potential region for the doping densities reported. For
=&v
J. Phys. Chem., Vol. 99, No. IO, 1995 3249 Ti02 with Nd = 1.0 x 10l6 this transition point is about 2.75 V positive of flatband. For Si samples with Nd = 1.0 x lOI5 the transition point is about 0.75 V positive of flatband. This is close to the point at which the minimum in the isotropic response is observed for the Si samples in this study. Since the point at which this transition occurs is a function of doping density, a continuing investigation using samples of different doping densities is underway to quantitatively examine this effect, the results of which should be valuable for determination of the sensitivity of SHG to the depletion layer width as a function of applied potential, as well as distinguishing the source of the nonlinear optical response. It appears that for the Si samples investigated here the ~ ( ~ contribution (EFISH response) only becomes the dominant mechanism positive of the point at which the Fermi level reaches the valence band edge at the surface. The electric field at the surface increases more rapidly with applied potential than the square root dependence which occurs in the depletion regime. The observed response is therefore a superposition of a bulk quadrupole response with the EHSH mechanism. A significant difference between the work reported here and that in the Ti02 s t ~ d i e s ~is ~the , ~absorption ~ , ~ ~ of SH light by the sample; the large absorption coefficient limits the response to the top ~ 2 0 nm of the Ti02 sample. This short distance relative to the width of the SCR allows the authors to simplify the interpretation to be proportional to the magnitude of the electric field at the surface. In our work, a significant portion of the SCR is being sampled by the SH light, thus complicating the interpretation. In order to understand the interplay between the absorption of the SH light and the width of the SCR, experiments are underway at additional wavelengths in an attempt to further understand the contributions to the SH response.
VI. Conclusions The nature of the potential dependence in the optical second harmonic response has been investigated for a variety of Si/ electrolyte and Si/SiO2/electrolyte interfacial regions. The relative phase determined from rotational anisotropy measurements is a strong indicator of the potential-dependent SH response. Photocurrent transient analysis has been performed and provides a reasonably accurate indication of the flatband potential. The photocurrent data combined with the relative phase from the SH rotational anisotropy provide a means of assessing the degree of band bending in the space charge region of the semiconductor. This potential dependence arises predominantly from the coupling of the dc electric field to the second-order material susceptibility tensor elements for the silicon surface and bulk space charge region. For the case of Si/SiOz/electrolyte interfaces, the presence of the oxide screens a portion of the applied dc field and this is also manifested in the relative phase angle. The Si/vacuum comparative experiment agrees well with that observed in solution. In the case of the Si/SiO2/vacuumexperiments, the surfaces appear to be under a depletion condition and this is comparable to that observed in solution. Further studies are being performed in this laboratory in order to formulate a comprehensive understanding of the potential dependent SH response from the interfacial regions investigated here. Acknowledgment. The authors gratefully acknowledge funding from the Department of Energy, Basic Energy Sciences (DE-FG06-86ER45273). References and Notes (1) Jakob, P.; Chabal, Y. J.; Raghavachari, K.; Becker, R. S.; Becker, A. J. Surf: Sci. 1992, 275, 407.
1
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(2) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656. (3) Bitzer, T.; Lewerenz, H. J. J. Electroanal. Chem. 1993, 359, 287. f4) Graf. D.: Bauer-Maver. S.: Schneeg. A. J. Vac. Sci. Technol. A 1993, 11, 940. (5) Thornton, J. M. C.: Williams, R. H. Phys. Scr. 1990, 41. 1047. (6) Hsiao, G. S.; Virtanen, J. A.; Penner, R. M.Appl. Phys. Lett. 1993, 63, 1119. (7) Houbertz, R.; Memmert, U.; Behm, R. J. Appl. Phys. Lett. 1993, 62, 2516. (8) Itaya, K.; Sugawara, R.; Morita, Y.; Tokumoto, H. Appl. Phys. Lett. 1992, 60, 2534. (9) Allongue, P.; Brune, H.; Gerischer, H. Surf: Sci. 1992, 275, 414. (10) Yau, S.-L.; Fan, F.-R. F.; Bard, A. J. J. Electrochem. SOC. 1992, 139, 2825. (11) Higashi, G. S.; Becker, R. S.; Chabal, Y. J.; Becker, A. J. Appl. Phys. Lett. 1991, 58, 1656. (12) Houbertz, R.; Memmert, U.; Behm, R. J. Appl. Phys. Lett. 1991, 58, 1027. (13) Memmert, U.; Behm, R. J. In Festkoerperprobleme (Advances in Solid State Physics); Oxford: New York, 1991; Vol. 31; pp 189. (14) Tomita, E.; Matsuda, N.; Itaya, K. J. Vac. Sci. Technol. A 1990, 8, 534. (15) Kaiser. W. J.: Bell. L. D.: Hecht. M. H.: Grunthaner. F. J. J. Vac. Sci. Technol. A 1988, 6, 519. (16) Kim, Y.; Lieber, C . M. J. Am. Chem. SOC. 1991, 113, 2333 (17) Fischer, P. R.; Daschbach, J. L.; Gragson, D. E.; Richmond, G. L. J. Vac. Sci. Technol. A 1994, 12 (5), 2617. (18) Kautek, W.; Sorg, N.; Kruger, J. Electrochim. Acta 1994,39, 1245. (19) Fischer, P. R.; Daschbach, J. L.; Richmond, G. L. Chem. Phys. Lett. 1994, 218, 200. (20) Sorg, N.; Kruger, J.; Kautek, W.; Reif, J. Ber. Bunsenges. Phys. Chem. 1993, 97, 402. (21) Krueger, J.; Sorg, N.; Reif, J.; Kautek, W. Appl. Surf: Sci. 1993, 69. ~. , 388. (22) Kravetsky, I. V.; Kulyuk, L. L.; Micu, A. V.; Shutov, D. A,; Strumban, E. E.; Cobianu, C.; Dascalu, D. Appl. Surf: Sci. 1993, 63, 269. (23) Aktsipetrov, 0. A.; Baranova, I. M.; Grigor'eva, L. V.; Evtyukhov, K. N.; Mishina, E. D.; Murzina, T. V.; Chernyi, I. V. Sov. J. Quantum Electron. 1991, 21, 854. (24) Lee, C.; Chang, R. K.; Bloembergen, N. Phys. Rev. Lett. 1967,18, 167. (25) Bloembergen, N.; Chang, R. K.; Jha, S. S.; Lee, C. H. Phys. Rev. 1968, 174, 813. (26) Grunthaner, F. J.; Grunthaner, P. J. Mater. Sci. Rep. 1986, I , 65. (27) Fukuda, H.; Yasuda, M.; Iwabuchi, T. Appl. Phys. Lett. 1992, 61, 693. (28) Ourmazd, A.; Taylor, D. W.; Rentschler, J. A,; Bevk, J. Phys. Rev. Lett. 1987, 59, 213. (29) Hollinger, G.; Himpsel, F. J. Appl. Phys. Lett. 1984, 44, 93. (30) Ishizaka, A.; Iwata, S.; Kamigaki, Y. Surf: Sci. 1979, 84, 355. (31) Chang, R. K.; Ducuing, J.; Bloembergen, N. Phys. Rev. Lett. 1965, 15, 6. (32) Koos, D. A. Ph.D. Dissertation Thesis, University of Oregon, 1991. (33) Georgiadis, R.; Richmond, G. L. J. Phys. Chem. 1991, 95, 2895. (34) Richmond, G. L. In Advances in Electrochemical Science and Enaineenna: Tobias. C. W.. Gerisher. H.. Eds.: VCH Publishers: New York. 19G1; Vol-2, pp 142. (35) Jakob, P.; Chabal, Y . J. J. Chem. Phys. 1991, 95, 2897. (36) Kenyon, C. N.; Ryba, G. N.; Lewis,.N. S. J. Phys. Chem. 1993, 97, 12928. V"
Daschbach et al. (37) Pershan, P. S. Phys. Rev. 1963, 130, 919. (38) Guyot-Sionnest, P.; Shen, Y. R. Phys. Rev. B 1988, 38, 7985. (39) Tom, H. W. K.; Heinz, T. F.; Shen, Y. R. Phys. Rev. Lett. 1983, 51, 1983. (40) Tom, H. W. K. Ph.D. Dissertation Thesis, University of California, Berkeley, 1984. (41) Reif, J.; Zink, J. C.; Schneider, C.-M.; Kirschner, J. Phys. Rev. Lett. 1991, 67, 2878. (42) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (43) Shen, Y. R. Nature 1989, 337, 519. (44) Richmond, G. L.; Robinson, J. M.; Shannon, V. L. frog. Surf: Sci. 1988, 28, 1. (45) Sipe, J. E.; Moss, D. J.: van Driel, H. M. Phys. Rev. B 1987, 35, 1129. (46) Koos, D. A.; Shannon, V. L.; Richmond, G. L. J. Phys. Chem. 1990, 94, 2091. (47) Bradley, R. A.; Georgiadis, R.; Kevan, S. D.; Richmond, G. L. J. Chem. Phys. 1993, 99, 5535. (48) Qi. J.; Yeganeh, M. S.; Koltover, I.; Yodh, A. G.; Theis, W. M. Phys. Rev. Lett. 1993, 71, 633. (49) Guyot-Sionnest, P.; Tadjeddine, A. J. Chem. Phys. 1990,92, 734. (50) Corn, R. M.; Romagnoli, M.; Levenson, M. D.; Philpott, M. R. Chem. Phys. Lett. 1984, 106, 30. (51) Gerischer, H.; Luebke, M. Ber. Bunsenges. Phys. Chem. 1988, 92, 573. (52) Heinz, T. F.; Loy, M. M. T.; Thompson, W. A. Phys. Rev. Lett. 1985, 54, 63. (53) Heinz, T. F.; Loy, M. M. T.; Thompson, W. A. J. Vac. Sci. Technol. B 1985, 3, 1467. (54) Momson, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (55) Gerischer, H.; Allongue, P.; Costa Kieling, V. Ber. Bunsenges. Phys. Chem. 1993, 97, 753. (56) Chazalviel, J.-N. Electrochimica Acta 1992, 37, 865. (57) Parkhutik, V. P. Electrochim. Acta 1991, 36, 1611. (58) Matsumura, M.; Morrison, S. R. J. Electroanal. Chem. 1983, 144, 113. (59) Lewerenz, H. J. Electrochim. Acta 1992, 37, 847. (60) Paolucci, F.; Peter, L. M.; Stumper, J. J. Electroanal. Chem. 1992, 341, 165. (61) Peter, L. M.; Blackwood, D. J.; Pons, S. J. ElectroanaL Chem. 1990, 294, 111. (62) Searson, P. C.; Zhang, X. G. Electrochim. Acta 1991, 36, 499. (63) Madou, M. J.; Loo, B. H.; Frese, K. W.; Morrison, S. R. Surf: Sci. 1981, 108, 135. (64) Kulyuk, L. L.; Shutov, D. A.; Strumban, E. E.; Aktsipetrov, 0. A. J. Opt. SOC. Am. B: Opt. Phys. 1991, 8, 1766. (65) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, 0. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Nature 1994, 368, 444. (66) Lantz, J. M.; Baba, R.; Corn, R. M. J. Phys. Chem. 1993,97,7392. (67) Arekat, S.; Kevan, S. D.; Richmond, G. L. Europhys. Lett. 1993, 22. 377. (68) Daum, W.; Krause, H.-J.: Reichel, U.; Ibach, H. Phys. Rev. Lett. 1993, 71, 1234. (69) Lantz, J. M.; Corn, R. M. J. Phys. Chem. 1994, 98, 4899. (70) Lantz, J. M.; Corn, R. M. J. Phys. Chem. 1994, 98, 9387. (71) Seiwatz, R.; Green, M. J. Appl. Phys. 1958, 29, 1034. JP94 1655N
