J. Phys. Chem. C 2008, 112, 5911-5921
5911
The Role of Band Bending in Affecting the Surface Recombination Velocities for Si(111) in Contact with Aqueous Acidic Electrolytes David J. Michalak, Florian Gstrein, and Nathan S. Lewis* Beckman Institute and KaVli Nanoscience Institute, DiVision of Chemistry and Chemical Engineering, 210 Noyes Laboratory, 127-72, California Institute of Technology, Pasadena, California 91125 ReceiVed: July 9, 2007; In Final Form: October 11, 2007
The role of band bending in affecting surface recombination velocity measurements has been evaluated by combining barrier height data with charge-carrier lifetime measurements for Si(111) surfaces in contact with a variety of acidic aqueous electrolytes. Charge-carrier lifetimes and thus surface recombination velocities have been measured by contactless radio frequency photoconductivity decay techniques for long bulk lifetime n-Si(111) samples in contact with 11 M (40% by weight) NH4F(aq), buffered (pH ) 5) HF(aq), 27 M (48% by weight) HF(aq), or concentrated 18 M H2SO4. Regardless of the sample history or surface condition, long charge-carrier lifetimes were observed for n-Si(111) surfaces in contact with 11 M NH4F(aq) or buffered HF(aq). On the basis of previous barrier height measurements, this behavior is consistent with the formation of an electrolyte-induced surface accumulation layer that reduces the rate of steady-state surface recombination even in the presence of a significant density of surface trap sites. A straightforward evaluation of the surface trap state density from the measured surface recombination velocities, S, is thus precluded for such Si/liquid contacts. In contrast, a wide range of S values, depending on the history of the sample and the state of the surface, were observed for n-Si(111) surfaces in contact with 27 M HF(aq). These results in conjunction with previously measured barrier height data indicate that the charge-carrier lifetimes measured for n-Si(111) in contact with 27 M HF(aq) can be directly correlated with the surface condition and the effective surface-state trap density. These conclusions were confirmed by measurements of the apparent S values of n-Si(111) surfaces in contact with various solutions in the presence of the known deep trap, Cu. For Si(111)/HF(aq) contacts, very high (g920 ( 270 cm s-1) surface recombination velocities were observed when 0.16 mM (10 ppm) Cu2+ was in the solution and/or adsorbed onto the Si(111) surface as Cu0 deposits, whereas low (100 ( 75 or 225 ( 20 cm s-1) apparent surface recombination velocities were measured for Cu-contaminated Si(111) samples in contact with 0.16 mM (10 ppm) Cu2+-containing 11 M NH4F(aq) or BHF(aq) solutions, respectively.
I. Introduction Si(111) surfaces in contact with strongly acidic aqueous solutions have among the lowest surface recombination velocities, S, reported to date for any semiconductor surface in contact with any ambient. Assuming a geometric capture cross section, the apparent surface recombination velocity of S ) 0.25 cm s-1 for Si(111) in 27 M (48% by weight) HF(aq) corresponds to less than 1 electrically active trap site in every 40 000 000 surface atoms.1 Similarly, low S values have been observed for HF-etched Si(111) in contact with other highly acidic (pH e 0) solutions (e.g., H2SO4 and HCl), but larger S values (near 10 cm s-1) were observed under less acidic (pH 1-3) conditions of the same solutions.1 One explanation for this pH dependence is the ability of the acid to passivate weakly basic electronically active defect sites, but the observed low S values seemed to correlate with acid molarity rather than pH.1 Curiously, etching of Si(111) in 27 M HF(aq) results in a significantly rougher surface,2 contaminated with more reactive3,4 dihydride (SiH2) and trihydride (SiH3) species, relative to the atomically smooth surfaces that can be prepared at higher pH values.2,5 These results suggest that several factors in combination may play a role in producing the extraordinarily low S values observed for Si in strongly acidic aqueous media. Another possible explanation for the pH-dependent S values is that different surface Fermi level positions could be produced by contacting the Si with electrolytes having different acidities. These different Fermi level positions would produce different surface recombination velocities even for surfaces having similar trap densities. Electrically defective silicon surfaces can revers* To whom correspondence should be addressed.
ibly yield a variety of S values for the same surface trap-state density, depending on the position of the Fermi level relative to the band edges of silicon.6-9 Different Fermi level positions can result from differences in charge equilibration between the silicon and the various liquids. For example, an electrically defective native oxide Si(111) surface reversibly exhibits S values >1000 cm s-1 when measured under nearly flat band conditions (e.g., when under a nitrogen ambient or when in contact with a tetrahydrofuran-decamethylferrocene+/0 solution that maintains the Fermi level near midgap) but reversibly displays significantly lower S values of 37 ( 13 cm s-1 when in contact with a CH3OH-0.05 M ferrocene+/0 solution that creates an inversion layer at the surface and stabilizes the Fermi level near the valence band maximum.6,9 The reversible surface recombination velocity behavior can be explained through the Shockley-Read-Hall (SRH) expression for surface recombination, eq 1.10-12
Un,s ) Up,s ) Us ) NT
(
kn,skp,s(nsps - ni2)
kn,s(ns + n1) + kp,s(ps + p1)
ns ) ns,0 + δn,
ps ) ps,0 + δp
)
(1) (2a)
ni ) NCe[-(EC-Ei)/kBT] ) NVe[-(Ei-EV)/kBT] ) xNCNV e[-Eg/2kBT] n1 ) NCe[-(EC-ET)/kBT],
(2b)
p1 ) NVe[-(ET-EV)/kBT] (2c)
In the SRH equation, Us (cm-2 s-1) is the surface recombination rate, which is assumed to be equal to the individual rates
10.1021/jp075354s CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
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of recombination for electrons, Un,s, and holes, Up,s; NT (cm-2) is the surface trap-state density; kn,s and kp,s (cm3 s-1) are the rate constants for electron and hole capture by the surface trap states, respectively; ni (cm-3) is the intrinsic charge-carrier concentration, which can be expressed either as function of the effective density of states in the conduction band, NC (cm-3), the energy of the conduction-band minimum, EC, and the Fermi level position for intrinsic (undoped) silicon, Ei, or as a function of the effective density of states in the valence band, NV (cm-3), the energy of the valence band maximum, EV and Ei, or in terms of NC, NV, and the energy of the band gap, Eg, where kB is Boltzmann’s constant and T is the temperature in Kelvin (eq 2b). The variables ns and ps (cm-3) represent the surface electron and hole concentrations, respectively, under the presence of illumination, each of which is the sum of an equilibrium (dark) electron or hole surface charge-carrier concentration, ns,0 or ps,0 respectively, and an injected charge-carrier electron or hole concentration due to illumination, δn and δp, respectively. The constants n1 and p1 (cm-3) depend on the energy of the trap state, ET. Assuming that the electron quasi-Fermi level, Fn, and the hole quasi-Fermi level, Fp, remain flat across the space-charge region, that low-level injection conditions apply (defined as δn ) δp , nb + pb), and that bulk trapping is negligible such that the photoinjected charge-carrier concentrations are equal (δn ) δp), the SRH equation can be rearranged13 and re-expressed in terms of the surface recombination velocity, S (cm s-1), as given by eq(3):
S)
Us ) δp (kn,skp,s)1/2NT(nb + pb) 2{ni cosh[(ET - Ei)/kBT - u0] + n/i cosh[u/s - u0]}
u0 ) ln
(3)
(x )
kp,s 1 , u/s ) (E/F - Ei)/kBT, E/F ) (Fn + Fp) kn,s 2 (4a) n/i ) xnbpb ) nie[(Fn-Fp)/2kBT] ns ) n/i eus , *
ps ) n/i e-us
*
(4b) (4c)
The surface recombination velocity, S (cm s-1), can be thought of as the ratio between the recombination rate per unit surface area, Us, and the injection level just beneath the spacecharge region, δp. Alternatively, S can be envisioned as the speed at which carriers recombine at the surface, with larger S values corresponding to faster electron-hole recombination rates. As demonstrated in eq 3, S is proportional to the trapstate density, NT, so a lower defect density will result in a lower recombination rate. The terms in the denominator, however, indicate that S is also dependent on the band structure and the energy of the trap state. The first term in the denominator represents the dependence of S on the energy of the trap state, ET. This term is smallest (leading to the largest S values and fastest recombination rates) when the energy of the trap is offset from midgap (Ei) by an amount proportional to the ratio of the carrier-capture rate constants, u0.14 The second term in the denominator represents the dependence of S on the surface voltage under illumination, u/s . Because of the functional form of the hyperbolic cosine in the denominator of eq 3, nonzero u/s values will lead to a larger denominator and smaller S values. Nonzero u/s values result
from an imbalance in the surface electron and hole concentrations under illumination (ns * ps * n/i , eq 4c), and such an imbalance results from residual band bending that exists under illumination. At equilibrium (u/s f us ) (EF - Ei)/kBT), large us values are obtained for EF values that are significantly displaced from midgap (Ei). Under these conditions, a large amount of band bending exists for relatively intrinsic silicon samples. Severe cases of equilibrium band bending, such as the presence of inversion or accumulation layers, will have sufficiently large us values at equilibrium that even under illumination nonzero u/s values can persist. In such a situation, S values will be decreased independently of the surface trap-state density. This passivation mechanism has been predicted for metalinsulator-semiconductor devices designed to have an inversion layer, but passivation due to the presence of an accumulation layer had not been explicitly discussed.15 This passivation mechanism can be understood physically because recombination requires participation from both electrons and holes. An n-type silicon wafer under accumulation at the surface will have a large concentration of electrons at the surface but very few holes. Under such conditions, the recombination rate will be slow and rate-limited by transport of holes to the surface. For an n-Si(111) surface under inversion, the hole concentration at the surface will be large, but recombination will be slow and limited by the rate at which electrons can reach the surface. If kp,s ) kn,s, the maximum recombination rate occurs when the surface electron and hole concentrations are equal (u/s ) 0) because ns ) ps ) n/i under such conditions (eq 4c). Observed S values are thus a function of both the surface trap concentration and the amount of band bending. The question of interest is whether the unusually low surface recombination velocity values for silicon in contact with aqueous acidic or fluoride-containing solutions are due to a direct reduction of the density of electrically active trap states, NT, or are due to the formation of either an inversion layer or an accumulation layer at the surface (nonzero u/s values). The barrier heights (which are indicative of us values) for various Si/liquid contacts have recently been measured by electrochemical impedance spectroscopy on Si electrodes and by use of opencircuit channel impedance data on n+-p-Si(111)-n+ and p+-nSi(100)-p+ solution-gated devices. The results indicate that n-type silicon surfaces are under accumulation (|us| . 0) when in contact with 11 M NH4F(aq) or with buffered HF(aq) solutions and are under depletion conditions (us ≈ 0) when in contact with 27 M HF(aq). For contact with 18 M H2SO4, the n-Si and p-Si surfaces are both under inversion conditions (|us| . 0). This difference in the surface Fermi level position when the Si is in contact with the different electrolytes implies that among the electrolytes considered only the charge-carrier lifetimes, or corresponding S values, measured for silicon in contact with 27 M HF(aq) can be used to directly infer the density of surface trap states, because only this contact results in roughly equal surface concentrations of electrons and holes / (u s ≈ 0) under the measurement conditions. To demonstrate the validity of this model, in this work we report measurement of the charge-carrier lifetimes and corresponding S values of silicon in contact with these various electrolyte solutions. We also evaluate the relationship of the measured surface recombination velocity to the actual trap density at the surface under the measurement conditions of interest. Long charge-carrier lifetimes and low S values measured for electrically defective silicon samples in contact with either 11 M NH4F(aq) or buffered HF(aq) would further demonstrate that the apparent passivation in such electrolytes is due to the
Surface Recombination Velocities for Si(111) formation of accumulation layers (|us| . 0). Si surfaces terminated with electrically defective silicon oxides cannot be used for such a study, because silicon oxide layers are actively etched by fluoride-containing aqueous electrolytes. Transition metal elements, such as Cu, Fe, and Au, are known to produce very high recombination rates at Si surfaces.16-20 For example, the presence of Cu at the Si-SiO2 interface has been shown to deleteriously affect the performance of metal-oxide-semiconductor devices.19 In situ effective lifetime measurements of silicon samples immersed in 27 M HF(aq) have demonstrated that addition of trace amounts of copper(II) compounds produces a significant decrease in the effective charge-carrier recombination lifetime of the silicon surfaces.21 Hence, introduction of Cu provides an approach to probe whether 11 M NH4F(aq) and buffered HF(aq) solutions, which produce a significant accumulation of electrons at the Si surface, are capable of producing low S values, despite the high level of electrically active charge-carrier trapping sites. II. Experimental Section A. Materials. Samples were double-side polished, float-zone, 3 ms lifetime, 4-6 kΩ cm resistivity, n-type silicon (111) wafers with a thickness of either 500 ( 20 or 200 ( 5 µm and a miscut angle of < ( 0.5°. The samples were cut into ∼1.5 cm × 1.5 cm pieces for use in surface recombination velocity measurements. An 11 M (40% by weight) NH4F(aq) solution and a proprietary mixture of NH4F and HF (pH ) 5.0), “buffer HF improved,” (BHF) were used as received from Transene Inc. Concentrated 18 M H2SO4 and 27 M (48% by weight) HF(aq) were used as received from EM Science. CAUTION: concentrated sulfuric acid is highly toxic and corrosive and can cause serious burns. CAUTION: fluoride-containing solutions such as 11 M (40% by weight) NH4F, buffered HF, and 27 M (48% by weight) HF pose a serious contact hazard. Hydrofluoric acid is highly toxic and corrosive and may cause serious burns which may not be immediately painful or visible. Fluoride ions readily penetrate the skin and can cause destruction of deep tissue and bone. All H2O was freshly obtained from a Barnsted Millipore purification system and had a resistivity of >18 MΩ cm. The Cu2+ solutions were prepared by dissolving 21 mg of CuCl2 (formula weight ) 134.45 g mol-1) into 100 mL of the desired aqueous fluoride solution. This 1.6 mM (100 ppm) stock solution was then diluted by 10 with the same aqueous fluoride solution to form the 0.16 mM (10 ppm) Cu2+ solution of interest. B. Surface Recombination Velocity Measurements. A contactless 450 MHz radio frequency (rf) conductivity apparatus was used to measure photoconductivity decays.6,9 A SpectraPhysics INDI-30 Nd:yttrium-aluminum-garnet laser (1064 nm) operating at a repetition rate of 10 Hz was used to illuminate samples with 10 ns pulses. Neutral density filters were used to adjust the power density of the expanded incident beam to ∼1 × 10-3 mJ cm-2 pulse-1, as determined using a power meter (Coherent Fieldmaster GS) equipped with a pyroeletric sensor (Coherent LM-P10i). The rf output from a Wavetek or Rohde & Schwarz signal generator was split into sample and reference paths. The reference path passed through a constant impedance phase shifter, as well as an amplifier and an attenuator, before being sent into the local oscillator port of a double-balanced frequency mixer. After amplification, the sample path was sent into the coupled port of a 20 dB directional coupler. The input port of the directional coupler was connected to a shielded aluminum box that contained a ground-terminated 1-12 pF matching capacitor in parallel with a series network of a λ/2 length of coaxial cable (where λ is the wavelength), a 1-12 pF
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5913
Figure 1. Representative photoconductivity transients for a 200 micron thick Si(111) wafer in contact with 27 M HF(aq) and subsequently in contact with air after a H2O rinse. The amplitude of the decay is recorded in volts as the dc output from the double-balanced frequency mixer. The laser pulse occurred at time t ) 0, and the solid lines represent single-exponential fits to the data with the reported lifetime values. The responses from both decays have been normalized to their maximum values, for clarity in display. The magnitude of the signal for the measurement in 27 M HF(aq) was much lower due to the large dark conductivity of the solution relative to that of the air contact. This difference this is responsible for the lower signal-to-noise for those data.
tuning capacitor, and a 2-turn 4 mm inner diameter flat coil connected to ground. A small hole in the Al box was made above the 2-turn coil and tuning capacitor so that silicon samples could be situated on top of the coil. The output port of the directional coupler was fed into the rf input of the doublebalanced frequency mixer. The direct current (dc) component of the output from the mixer was observed on a Tektronix TDS210 oscilloscope. After placement of the sample near the detection coil, the matching capacitor was tuned in the dark so that all power was delivered to ground and no power was reflected back to the mixer. Photogenerated conductivity in the silicon created an impedance mismatch in the front end, and a transient dc photoconductivity decay signal was obtained from the output of the mixer. Transient photoconductivity decays were measured for silicon samples placed in cells that were formed from the base and lid of 35 mm × 10 mm polystyrene petri dishes (Falcon Inc.). Prior to measurement, the silicon samples were sandwiched between two drops of solution and the polystyrene plates of the lid and base of the Petri dish, with the Petri dish base on the top and the inverted Petri dish lid on the bottom of the Si. This arrangement limited the total dark conductivity of the cell by minimizing the amount of electrolyte solution and hence increased the signal-to-noise in the rf decay measurement. Because the volume of liquid varied slightly from cell to cell, the matching capacitor and rf frequency were tuned separately for each cell to achieve reproducibility between nominally identically prepared Si/liquid junctions. The average of 128 photoconductivity decay curves was fitted to a single exponential of the form y ) y0 + A exp(t/τ), where τ is the time constant for the decay, corresponding to the lifetime of free carriers in the sample. Some representative photoconductivity decay transients are presented in Figure 1. The observed lifetime of the transient photoconductivity decay, τ, can be related to the bulk lifetime, τb, and to the surface recombination velocity, S, using eq 5
(
)
1 1 d d2 ) + + 2 τ τb 2S π D
-1
(5)
5914 J. Phys. Chem. C, Vol. 112, No. 15, 2008 where d (cm) is the sample thickness and D (cm2 s-1) is the ambipolar diffusion coefficient for carriers in Si.20,22 The S (cm s-1) values reported in this work were calculated assuming that the experimentally observed lifetime is dominated by surface recombination with no contribution from charge-carrier recombination in the bulk (i.e., assuming τb ) ∞). The S values are thus maximum values, because as specified by the Si wafer manufacturer, τb ) 3 ms for the bulk samples used in this study. For the most rapid observed decays, even for infinite S values, a finite lifetime, τ, will be observed due to the finite rate of diffusion of carriers to the surface (accounted for by the second term in the parentheses of eq 5). Assuming infinite S and τb values and assuming an ambipolar diffusion constant of D ) 9 cm2 s-1, a diffusion-limited lifetime, τ, of 25 or 5 µs would be observed for the 500 or 200 µm thick Si samples, respectively. This diffusion-limited lifetime sets a minimum bound of ∼1000 and ∼2200 cm s-1 on the S values extracted from the shortest measurable lifetimes for the 500 and 200 µm thick Si samples, respectively. C. Channel Conductance Devices. Channel devices in the n+-p-Si(111)-n+ configuration were fabricated as described elsewhere.7,8,23,24 Briefly, standard lithographic techniques were used to diffuse two n+ source and drain contacts into a 1015 cm-3 boron-doped p-Si(111) substrate such that a p-Si(111) channel, 1 mm wide by 8 mm long, remained between the source and drain contacts. Two separate Al contact pads were made to each source or drain region to verify that an ohmic contact had been formed for each junction. The entire device, except the central p-Si(111) channel and a small portion of the adjacent source and drain contacts, was encapsulated with insulating epoxy or paraffin wax. The surface of the p-Si(111) channel region was made atomically smooth by immersion in 11 M NH4F(aq), followed by a quick rinse in H2O, and was dried under N2(g). Devices were then exposed to the ambient of choice and impedance spectra, employing a 10 mV alternating current amplitude of various frequencies ranging from 10-3-106 Hz with a data spacing of 10 points per decade, were collected across the source and drain contacts. For a given device/ambient contact, the magnitude of the impedance (Ω) and the phase angle (degrees) between the current and voltage sinusoids were recorded at each frequency. All measurements were made in the dark inside a metal mesh Faraday cage with unstirred electrolyte solutions. When unperturbed, the low-frequency source-to-drain impedance, Zlf, was very high, because either the n+-p or the p-n+ junction was reversed biased. When the surface of p-Si(111) channel was driven into inversion by contact to certain liquids, however, Zlf became very low, because the potential barrier to carriers moving to an inversion layer from an oppositely doped source or drain contact was low. Under these conditions, the total source-to-drain impedance, Zlf, is well approximated by only the resistance of inversion layer of the p-Si(111) channel, Rch. Rch is of interest because this resistance can be directly related to the barrier height for the p-Si(111) channel/liquid interface.8,23,24 This method thus provided an estimate of the barrier height of the p-Si(111)/liquid interface without inducing possible surface chemistry changes that could result from the application of a probing voltage across the solid/liquid interface.24 A more detailed description of the methods used to analyze transconductance data is described elsewhere.24 III. Results A. Surface Recombination Velocities of Si(111) Surfaces in Contact with Aqueous Electrolytes. Figure 2 shows the
Michalak et al.
Figure 2. Transient photoconductivity decay lifetimes and the corresponding S values for 500 µm thick Si(111) samples immersed in a given etching solution after sequential rinsing steps. Exposure number 1 corresponds to solvent-rinsed surfaces covered with a native oxide measured under air. Exposure number 2 was measured while the sample was immersed in solution immediately after placing the native-oxideterminated sample directly into the etchant solution of interest with no pre-etch. Exposure numbers 3-10 represent the measured lifetimes and corresponding S values obtained sequentially for silicon samples immersed in the solution of interest after a brief water-rinsing and N2(g)-drying procedure. Finally, exposure number 11 represents the measured lifetimes and corresponding S values for the water-rinsed and N2(g)-dried samples measured in contact with air. The error bars represent the standard deviations obtained as a result of repeating the same procedure for at least three nominally identical wafer pieces.
observed charge-carrier lifetimes and the corresponding S values of 500 µm thick n-Si(111) samples during sequential exposures to either aqueous 27 M HF(aq), buffered HF(aq), or 11 M NH4F(aq). Exposure number 1 corresponds to solvent-rinsed, nativeoxide-terminated Si(111) samples that were measured in contact with air. Diffusion-limited lifetimes of ∼25 µs (S g 1000 cm s-1) were obtained under such conditions. In contrast, when these same samples were immersed into either 27 M HF(aq) (gray bars), BHF (white bars), or 11 M NH4F(aq) (striped bars) (Figure 2, exposure 2), the corresponding photoconductivity decay curves were all characterized by relatively long lifetimes (2000 < τ < 3000 µs) and therefore had very low surface recombination velocities (8.3 < S < 12.5 cm s-1). Except for repeated contact with 27 M HF(aq), which exhibited a continual increase in S with increased sample rinsing and reimmersion, the S values remained relatively stable as a function of repeated electrolyte exposures and water rinses (Figure 2, exposures 3-10). The final measurement was performed under an air ambient after a final H2O rinse and N2(g) dry (Figure 2, exposure 11). For the data presented in Figure 2, a given wafer piece was subjected to repeated exposures of only one and the same liquid, either 27 M HF(aq), BHF, or 11 M NH4F(aq). Figure 3 shows the photoconductivity decay lifetimes and corresponding S values observed for a single 200 µm thick Si(111) sample during sequential exposure to various different electrolyte solutions. While the results from only a single wafer are shown in this figure, the results from this sample are representative of the trends observed from other wafers subjected to a similar procedure. In short, diffusion-limited lifetimes (∼4 µs) were observed for the as-received, native-oxide terminated Si(111) sample in contact with air (Figure 3, exposure 1). Similar lifetimes were also observed after a subsequent solvent-rinsing procedure (Figure 3, exposure 2) and after exposure to an 80100 °C 3:1 mixture of concentrated H2SO4 and 30% H2O2(aq) (hereafter referred to piranha solution) (Figure 3, exposure 3). The piranha solution is used to chemically remove metallic and hydrocarbon contaminants from the surface; this process also grows an electrically defective oxide layer into the surface of
Surface Recombination Velocities for Si(111)
Figure 3. Transient photoconductivity decay lifetimes and the corresponding S values for a 200 µm thick n-Si(111) wafer demonstrating the immersion history effect. The lifetime of an n-Si(111) wafer was measured as received with a native oxide (exposure number 1), after solvent rinsing (exposure 2), and after a 30 s buffered HF(aq) pre-etch and subsequent 10 min exposure to a 3:1 mixture of concentrated H2SO4 and H2O2(aq) (Piranha) solution (exposure 3). The wafer was then measured while immersed in a 27 M HF(aq) solution (exposure 4), in 11 M NH4F(aq) (exposure 6), in 27 M HF(aq) solution again (exposure 8), and finally in 11 M NH4F(aq) again (exposure 10). The sample was then again exposed to the piranha solution for 10 min, as indicated by the vertical broken line. The sample was subsequently measured in 27 M HF(aq) solution (exposure 12), in 11 M NH4F(aq) (exposure 14), and finally again in 27 M HF(aq) (exposure 16). Exposure numbers 5, 7, 9, 11, 13, and 15 correspond to measurements made in air after H2Orinsing and N2(g)-drying steps. It should be noted that the S values for Si immersed in 27 M HF(aq) are considerably smaller (exposure 4) prior to contact with other electrolytes than afterward (exposure 8). Low S values can be recovered (exposure 12) by an oxidative clean of the surface, but re-exposure to a variety of electrolytes causes the S value to increase again (exposure 16). While this figure describes the performance of only one sample, the results from this sample are representative of the trends observed with other samples. Error bars are not included graphically because the standard error in the lifetime, obtained from fitting the transient photoconductivity decay data to a single exponential, was less than 1%.
the silicon. Consistent with the results of Figure 2, long chargecarrier lifetimes were observed for the sample subsequently immersed in 27 M HF(aq) (Figure 3, exposure 4). In this case, the HF(aq) removed the strained surface oxide and resulted in an electrically passive H-terminated surface. After a waterrinsing step, the lifetime in contact with air was quite short (Figure 3, exposure 5), but subsequent exposure of the sample to 11 M NH4F(aq) restored the long lifetime (Figure 3, exposure 6), indicating either the elimination of or a relative lack of sensitivity to the presence of most of the defects that had been introduced by the exposure of the sample to air. Subsequent lifetimes measured in contact with air (Figure 3, exposure 7) or then in contact with 27 M HF(aq) (Figure 3, exposure 8) were significantly shorter. The short lifetime and large S value obtained in 27 M HF(aq) at this point (Figure 3, exposure 8) presumably reflected an increase in surface defect levels that had been introduced as a result of the repeated reimmersion, rinsing, and handling of the sample. This behavior is consistent with the decline in the lifetime observed for repeated exposures only to 27 M HF(aq), as shown in Figure 2, exposures 3-10; however, it is more severe in magnitude. Although subsequent measurement in 11 M NH4F(aq) displayed lifetimes that were significantly shorter than those previously observed for that sample in NH4F(aq) originally (Figure 3, exposure 6), the lifetimes in NH4F(aq) were significantly higher than those in 27 M HF(aq). This behavior is also consistent with that observed in Figure 2 for repeated exposures to NH4F(aq).
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5915
Figure 4. Charge-carrier lifetimes for a 200 µm thick n-Si(111) wafer after sequential exposure to various etching solutions after initial immersion and after 5 and 10 min of equilibration in a given solution. Data for exposures 1, 3, 5, 7, 9, 11, 13, and 15 were obtained in air after a H2O rinse and N2(g) dry; data for all other exposures were measured in contact with the labeled solution. During the first several solution measurements, the sample lifetimes and corresponding S values showed no dependence on immersion time, but the last measurements in 11 M NH4F(aq) (exposures 14, 16) displayed initially low lifetimes that rose with increasing immersion time. While the presented results were only obtained from one sample, they adequately represent the trends observed for many other samples with similar sequential exposure studies. Error bars are not included graphically because the standard error in the lifetime, obtained from fitting the transient photoconductivity decay data to a single exponential, was no more than 1%.
The sample was then oxidized for 10 min in piranha solution to remove any surface contamination. Because the piranha solution grows an electrically defective oxide layer into the surface of the silicon, lifetime values immediately following the piranha cleaning step were low (Figure 3, exposure 11). Subsequent contact of this sample with 27 M HF(aq) (Figure 3, exposure 12), which removes this oxide layer, and then with 11 M NH4F(aq) (Figure 3, exposure 14) produced long lifetimes that are consistent with those measured after the initial piranha clean (Figure 3, exposures 4 and 6). Upon additional rinsing and handling steps, the lifetime in contact with 27 M HF(aq) (Figure 3, exposure 16) again decreased, consistent with the results from exposure number 8. Hence, the electrically active defect sites, which arise either from rinsing and handling or contamination from solution, can be successfully removed through a 10 min exposure to piranha solution. In addition, the lifetimes for the Si(111) samples in 27 M HF(aq) (Figure 3, exposures 4, 8, 12, and 16) are much more sensitive to the state of the surface than are the lifetimes of Si(111) in contact with 11 M NH4F(aq) (Figure 3, exposures 6, 10, and 14). Figure 4 depicts charge-carrier lifetimes observed in a separate series of experiments that involved the sequential exposure of a single 200 µm thick n-Si(111) wafer to the various electrolytes of interest at three selected exposure times in each electrolyte solution. As in Figure 3, the results from only a single sample are displayed, but the data and trends therein are consistent with results measured from other samples that were subjected to similar procedures. The first several exposures (Figure 4, exposures 1-12) of the sample were in accord with the results displayed in Figures 2 and 3. The longer lifetimes observed in 27 M HF(aq) after exposure to various electrolytes and handling steps (cf., Figure 4, exposures 4 and 10, relative to Figure 3, exposures 8 and 16) indicate that the rinsing and handling steps in this specific experiment introduced less contamination than those used in Figure 3. While most silicon/liquid junctions produced S values that were independent of immersion time, the time dependence of the S values observed for the silicon sample exposed to 11 M NH4F(aq) (Figure 4, exposures 14 and 16) were, however, distinctly different in behavior. The charge-
5916 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Michalak et al. TABLE 1: Calculated Barrier Height Valuesa for the Low-Frequency Channel Impedance Data Presented in Figure 4
Figure 5. Low-frequency channel impedance data for an n+-p-Si(111)n+ device immersed sequentially in different etching solutions, demonstrating the history and time-dependent effects. The initial lowfrequency channel impedance value of ∼5 × 107 Ω for the air or N2(g) contacts (exposure 1) is so large that the barrier height for p-Si(111) is not under inversion. Sequential contact of the device to 11 M NH4F(aq) (exposure 2) led to a significant decrease in the low-frequency channel impedance value, and the barrier height for the p-Si(111)/11 M NH4F(aq) contact is calculated to be 1.075 V (Table 1). Similarly, low channel impedance and high barrier height values were obtained for subsequent contact of the device with buffered HF (exposure 3) and 11 M NH4F(aq) (exposure 4). A larger low-frequency channel impedance value was obtained for the device subsequently placed in contact with 27 M HF(aq) (exposure 5); this value led to a lower barrier height of 0.897 V (Table 1), consistent with previous measurements.24 Upon subsequent contact of the devices with 11 M NH4F(aq) (exposure 6) or buffer HF (exposure 7), the low-frequency channel impedance values were not as low, meaning that the barrier heights were no longer as large (1.002 and 1.000 V, respectively, Table 1). While longer exposure times (up to 20 min) to buffered HF (exposure 8) produced no significant change in the barrier height, longer exposure times in 11 M NH4F(aq) (exposures 9 and 10) led to an increase in the barrier height by ∼50 mV; this is indicative of an increase in the surface electron concentration by ∼ a factor of 10. Because the Fermi level is relatively close to the conduction-band edge under these conditions, the error in estimating the barrier height is about 5%, based on the error introduced by using equations derived from Maxwell-Boltzmann statistics rather than Fermi-Dirac statistics.24,47 Barrier height values based on a more correct quantum mechanical approach24,48 are presented in Table 1.
carrier lifetimes of exposures 14 and 16 (Figure 4) initially started out low (large S), gradually increased with exposure time, and after 10 min approached the values observed upon the initial exposure to 11 M NH4F(aq) (Figure 4, exposure 6). Hence, a slow change in the Si(111)/11 M NH4F(aq) interface produces an increase in the charge-carrier lifetime even for surfaces that initially have a relatively high apparent density of electrical trap sites. During the extended immersion times of 5 and 10 min (shaded and white bars, respectively), the samples were not removed from solution or from the containment cells, but they were removed from the exposure to the Nd:YAG laser to minimize saturation of trap states that can occur from the presence of excess photogenerated carriers.25 B. Channel Impedance Data of Si(111) Surfaces in Contact with Aqueous Electrolytes. Channel impedance data on solution-gated field-effect p+-n-p+ Si(111) devices were obtained to evaluate any variation in band bending due to time-dependent immersion in the various solutions of interest for comparison with the surface recombination velocity measurements. Figure 5 presents the magnitude of the in-phase (phase angle
