J . Phys. Chem. 1990, 94. 6842-6847
6842
a substantial coverage of phosphorus on the surface, as determined by the Auger spectrum. The experiments with a 1:2 DMMP O2mixture at 945 and 1075 K left phosphorus on the P d ( l l 1 ) surface after 3600-s exposure. These results therefore suggest that Pd( 1 1 1) is an effective catalyst for the continuous decomposition of DMMP in excess O2at 1075 K. However, catalytic activity is not sustained at 945 K on Pd( 1 1 1) at a DMMP:02 incident flux ratio of 0.051. Comparison of Mo, Pd, and Ni for the Catalytic Decomposition of DMMP under Oxidizing Conditions. We have previously reported that a sustained catalytic decomposition of DMMP may be achieved on Mo(l I O ) at 898 K using a flux ratio of 0.5 DMMP:02.2 The present work indicates that Pd( 1 1 1) exhibits sustained catalytic decomposition capability at a flux ratio of 0.051 DMMP:02 at 1075 K and that surface phosphorus does not deposit at this temperature. Under the same incident gas ratio conditions on Pd( 1 1 1) at 945 K substantial phosphorus deposition occurs. Thus, somewhere between 945 and 1075 K surface phosphorus oxidation occurs at a rate sufficient to maintain the Pd( 1 1 1) surface clean for DMMP decomposition. Ni( 1 1 1) and preoxidized Ni( 1 1 I ) surfaces deposit phosphorus which cannot be removed by oxygen treatment at 1075 K. We therefore have established the order of catalytic reaction activity among three transition metals for sustained catalytic decomposition of DMMP under oxidizing conditions, namely, Mo > Pd > Ni, based on the temperature needed to prevent phosphorus build up under continuous reaction conditions. In both cases where sustained catalytic decomposition is observed (Mo, Pd), it has been found that the removal of surface phosphorus by oxygen is the key to maintenance of a stable DMMP catalytic decomposition process over long periods of time. Directionsfor Future Research. On the basis of the comparison between Mo (early transition metal), Pd, and Ni, it may be speculated that the early transition metals will be more useful than the late transition metals for the catalytic decomposition of organophosphorus compounds. Metals in group IVA (4) and VA
+
(5) should be studied to determine whether lower temperatures for the catalytic decomposition of organophosphonates are possible compared to Mo. The key issue is probably the reduction of the activation energy for surface phosphorus oxidation by proper choice of the transition metal, and it is possible that this objective may be achieved by moving to the very early transition metals. In addition, specific promoters for phosphorus oxidation on transition metals should be sought.
Summary The decomposition of DMMP has been studied on Ni( 11 1) and Pd( 1 1 I ) . The results are summarized as follows: 1. DMMP decomposes on Pd( 11 1) into H2(g), CO(g), and H,O(g), leaving P(a) and a small amount of C(a) residues on the surface. Both residues can be removed by oxidation using 02(g) at 1075 K. 2 . On Ni( 1 1 I ) , DMMP decomposes into H2(g), and CO(g), leaving P(a) on the surface which cannot be removed by oxidation at 1075 K. P(a) is removed only by Ar+ sputtering. 3. A sustained condition for continuous DMMP catalytic decomposition can be reached on Pd(l11) with a 0.051 D M M P 0 2 flux ratio at a surface temperature of 1075 K. The removal of surface phosphorus by oxidation is the key process, and the DMMP oxidation reaction appears to occur on a Pd( 11 1) surface that contains only small coverages of P(a), O(a), and C(a) under the conditions of this experiment. 4. These results, compared to earlier studies on Mo(1 IO), suggest that the early transition metals are more active for catalytic decomposition of organophosphonate compounds under oxidizing conditions. Acknowledgment. We gratefully acknowledge the support of this work by the Army Research Office under Contract No. DAAL 03-89-K-0005. Registry No. DMMP, 756-79-6; Ni, 7440-02-0; Pd, 7440-05-3; P, 7723-14-0.
Studies of Surface Recomblnation Velocity at Cu/CdS( 1120) Interfaces Y . Rosenwaks,+ L. Burstein,+ Yoram Shapka,' and D. Huppert*,t Department of Electron Devices and Materials, Faculty of Engineering, and Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviu University, Ramat Aviv 69978, Israel (Receiued: January 2, 1990: In Final Form: April 13, 1990)
Direct measurement of the surface recombination velocity (SRV) on etched CdS( 1 120) and at its interface with Cu (deposited by aqueous CuS04solutions and in situ thermal evaporation) was achieved by using ultrafast time-resolved photoluminescence. Correlation with interfacial composition and chemistry was based on Auger electron spectroscopy (AES) and atomic absorption spectroscopy. The surface electronic structure was studied by using surface photovoltage spectroscopy. The results show that the original CdS SRV increases sharply as a function of Cu coverage. The SRV stabilizes around 1 X IO6 cm/s (3 orders of magnitude above the clean surface value) at Cu coverage of about 1 monolayer, as estimated from AES results on in situ deposited Cu/CdS. The results are explained in terms of Cu-S compound-derived recombination centers at the Cu/CdS interface. The results are compared with thermally evaporated Cu/CdS interfaces, where a similar Cu coverage dependence and SRV mechanism are apparent.
Introduction It is widely accepted that surface recombination of carriers at semiconductor surfaces severely limits the performance of minority-carrier devices such as solar cells, detectors, and bipolar transistors. Surface recombination velocity (SRV) is a good means of parametrizing such a surface property. For example, it was recently demonstrated',2 that an improvement in the SRV value Faculty of Engineering $School of Chemistry.
of GaAs by 2 orders of magnitudes had led to a 60-fold improvement in the gain of a heterojunction bipolar transistor. To date, little has been done regarding research, investigation, or characterization of the factors affecting the SRV. Most of the works are limited to evaluation of SRV on thin film surfaces as ( 1 ) Nottenburg, R. N.; Sandroff, C. J. S.; Humphrey, D. A,; Hollenbeck, T.H.; Bhat, R. Appl. Phys. Lett. 1988, 52, 218. (2) Sandroff, C. J.; Nottenburg, R. N.; Bischoff, J . C.; Bhat, R. Appl.
Phv.7. Lerr. 1987, 5 1 , 3 3 .
0022-3654/90/2094-6842$02.50/0
0 1990 American Chemical Society
Surface Recombination at Cu/CdS( 1 120) Interfaces this greatly simplifies the measurement and the mathematical a n a l y ~ i s . ~In, ~previous studies5v6we reported on a technique for direct measurements of SRV by time-resolved photoluminescence and demonstrated it for CdS crystals immersed in different aqueous electrolyte solutions. We noticed that immersion of CdS M), in CuSO, solution, even at very low concentrations ( has a very strong effect on the SRV value. We then predicted that the changes in SRV are due to chemisorbed Cu2+ ions. In this work we combined four independent experimental techniques, time-resolved photoluminescence (PL), Auger electron spectroscopy (AES), atomic absorption spectroscopy (AAS), and surface photovoltage spectroscopy (SPS) in order to correlate the SRV and the chemical and electronic structure of the crystal surface. It is found that the changes in SRV are due to submonolayer coverage of Cu-S compound (formed as a result of Cu2+ ions deposited from the CuSO, solutions) which lead to formation of midgap surface states. Experimental Section
The CdS crystals were 1-2 mm thick “ultrahigh purity” (Cleveland Crystals) n-type with donor concentrations of -2 X 10l6 The (1 120) surfaces were mechanically polished down to 0.015 pm, etched for 1-3 min in 2:3 H20:HCI, and rinsed in distilled water. The PL experiments were conducted while the crystal was immersed in the C u S 0 4 solutions before and after strong stirring for 15 min. After such measurements the crystal was rinsed in distilled water and kept there until its insertion into the ultra-high-vacuum (UHV) chamber. Ohmic back contacts were made by evaporating high-purity AI in a high-vacuum system. Details of the time-resolved picosecond PL apparatus were described earlier.’ The crystals were excited by a 25-ps fwhm laser pulse at 352 nm which was amplified and frequency tripled from a pulse generated by a passively mode-locked Nd:YAG oscillator. The photoluminescence was collected from the front surface of the crystal and was focused through colored glass filters onto the entrance slit of a streak camera (Hamamatsu C939). The output of the streak camera was recorded and digitized on a PAR 1205D optical MCA for computerized data processing. Auger electron spectra were obtained by using an electron gun with a beam diameter of - 5 pm and a CMA with resolution better than 0.3% (VG Scientific). All spectra were obtained with the CdS surface positioned at a constant orientation to the incident beam and the CMA. The same electron beam energy (3 keV), primary beam current (2.0 PA), peak-to-peak (p-p) modulation voltage (3 V), and electron multiplier bias were used for all spectra, in order to ensure a precise comparison between the Auger intensity ratios on the different samples. The measurements were preformed at a number of points on each sample and averaged digitally. Cu was evaporated in the same UHV chamber (base pressure 5 X IO-” Torr) and its thickness was monitored by a quartz oscillator positioned next to the semiconductor surface. Cd ion concentration analysis of the C u S 0 4 solutions in which the crystals were immersed was performed by AAS (Perkin-Elmer 5000). The SPS technique measures the contact potential difference (CPD) between a vibrating Au reference probe and the semiconductor surface. The CPD is a measure of the band bending via the relationship = 6 A u - x - Vf - VB (1) where c $is ~the~gold work function, x is the CdS electron affinity, Vr is the energy difference between El and the bulk E,, and VB is the surface band bending [=E,(surface) - E,(bulk)], where Ef and E, are the Fermi level and conduction-band edge, respectively. Photovoltage measurements were carried out using monochromatic VCPD
(3) Nelson, R. J.; Williams, J . S.; Leamy, H. J.; Miller, B.; Casey, Jr., H.
c.;Parkinson, B. A.; Heller, A. Appl. Phys. Lett. 1980, 36, 76.
(4) Yablonovitch, E.; Allara, D. L.; Chang, C. C.; Gimitter, T.;Bright, T. B. Phys. Rev. Lett. 1986, 57, 249. ( 5 ) Evenor, M.; Gottesfeld, S.; Harzion, Z.; Huppert, D.;Feldberg, S.J .
Phys. Chem. 1984,88, 6213. (6) Benjamin, D.;Huppert, D. J . Phys. Chem. 1988, 92, 4676. (7) Huppert, D.; Koldney, E. Chem. Phys. 1981. 63, 401.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6843
Cu+*/CdS( I l i O )
0 TIME ( p s )
Figure 1. Experimental (dotted lines) and simulated_(solidlines) photoluminescence decay curves for an etched CdS( 1 120) single crystal immersed in (a) distilled water, SRV 51 X IO’ cm/s; (b) 4 X IOd M CuSO, solution, SRV = 5 X IO3 cm/s; ( c ) 1 X IO” M CuS04solution, SRV = 1.2 X IO4 cm/s; (d) 2.5 X M CuSO,, SRV = 6 X IO4 cm/s; M CuS04, SRV 23 X IO5 cm/s. (e) 5 X
light from a Jarrell-Ash grating monochromator (0.6 eV < hv < 2.8 eV) which was focused onto the CdS surface positioned to within a fraction of a millimeter of the vibrating kelvin probe (supplied by Delta-Phi-Elektronik, Julich, FRG). Spectra were acquired within the range (0.68 eV < hv < 2.75 eV) with a monochromator resolution of Ahv = 0.02 eV and AVcpD N 0.02 v. Positive changes in slope AVCpD/Ahvoccurring at an energy Eo correspond to transitions from a surface state situated at an energy Eo below the conduction band edge E, (at E, - Eo). The population of a surface state situated at an energy Ei above E, (at E, + Ei) is observed by a negative AvcpD/Ahvchange at a photon energy hv = Ei. Additional details of the SPS technique are given elsewhere.*s9 Results A . Time-Resolved PL Measurements. In previous works5-6
we demonstrated that by fitting simulated and experimental PL decay curves (measured for bulk semiconductors) we can quantitatively evaluate SRV. The computer simulation of the timedependent PL intensity I ( t ) is given by I ( t ) = K r l A n 2 ( x , t )exp(-ax) dx
(2)
based on the solution of the ambipolar diffusion equation.I0 An is the local concentration of excess carriers, cy is the edge absorption coefficient, and K, is the second-order radiative rate constant. The edge PL decay curves of I ( t ) yield values of SRV through the boundary condition (3) where D* is the ambipolar diffusion coefficient. A detailed discussion of the simulation process is given in ref 5 . In short, the model takes into account all factors contributing to the PL decay, such as first-order surface and bulk recombination, ambipolar diffusion of photogenerated carriers, self-absorption of the edge photoluminescence, and the finite width of the laser pulse and the detection system time resolution. However, it should be reemphasized here that the luminescence decay is not affected by band bendingdue to the high power of the exciting laser ( I O l 9 photons Figure 1 shows experimental and simulated PL decay curves for the CdS single crystal immersed in CuSO, solutions of different concentrations. The following parameters were used for the fitting process: D* = 1.0 cm2/s, 1/a(532 nm) = 7 X cm, and (8) Gatos, H. C.; Lagowsky, J . J . Vac. Sci. Technol. 1973, I O , 130. (9) Brillson, L. J. Surf. Sci. 1975, 51, 45. Shapira, Y.; Brillson, L. J.; Heller, A. J . Vac. Sri. Technol. 1983, BI, 618. (IO) Vaitkus, J. Phys. Status Solidi 1976, 34A, 769.
6844
Rosenwaks et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, I990
l
o
~
.c
s
1
i
u
,0.2 2,
0
Y
lo3!
I
IO-^
I
10-5
I
1u4
I
I
la2
I
IO-'
J
o
CuSO4 CONCENTRATION (M)
Figure 2. S R V of CdS( I 120) surfaces as a function of the CuSO, solution concentration. The SRV values were derived from the curves such as shown in Figure I .
l/a(352 nm) = 8 X IO4 cm. The value of the bulk recombination time rb = 5 X s was obtained from the fitting process, its effect becoming dominant only at the tail of the decay curves. The simulation process yielded the following SRV values: (a) In distilled water SRV I1 X IO3 cm/s. (b) In 4 X IOd M solution M solution SRV = 1.2 SRV = 5 X IO3 cm/s. (c) In 1 X X IO4 cm/s. (d) I n 2.5 X M solution SRV = 6 X lo4 cm/s. M solution SRV 1 3 X lo5 cm/s. (e) In 5 X Figure 2 shows the values of SRV (obtained from the simulation process shown in Figure 1 ) as a function of the CuSO, solution concentration. Clearly the solution concentration has a strong effect on the SRV (in our sensitivity range of lo3 5 SRV 5 IO6) even at very dilute solutions. I n > 5 X M solutions SRV saturates, a point which will be discussed later. Rinsing the crystals in deionized water did not change the original luminescence profile. B. A E S Measurements. AES spectra of the crystals showed 0 and C contaminations which did not change after immersion in the different CuSO, solutions. In order to minimize these contaminations the crystals were kept in deionized water until insertion to the UHV chamber. The relative surface concentration by C was estimated as -20% while the 0 concentration was 10%. The presence of 0 and C on the surface caused a decrease of the S (149 eV)/Cd(376 eV) intensity ratio, indicating that the S atoms are "shadowed" preferentially on the (1 120) surface as was also reported by B r i l l ~ o n . Presence ~ of Cu was evident on the crystals whose PL characteristics changed after the immersion in the CuSO, solutions. The Cu peak-to-peak (p-p) intensity increased with the solution concentration while the Cd p-p intensity decreased. The S p p intensity changed due to the different 0 and C contaminations but with no correlation to the C p-p intensity changes. Figure 3 shows the ratio of the Cu p p (91 5 eV) to the Cd p-p (376 eV) intensities as a function of the CuS04 solution concentration. The Cu LMM features could not be detected on the surfaces of the crystals which had been immersed in solutions M. At 1 X M concentration the diluted below I X features were sometimes observable only by obtaining d2N/dE2 spectra. Clearly the Cu/Cd ratio reaches a constant value at and IO-" M. At these solution concentrations between 5 X solution concentrations, the Cu relative surface concentration is
-
-0.2.
I n order to evaluate the Cu coverage on the solution-treated crystals we measured the AES Cu/Cd signal ratio on CdS crystals on which Cu was evaporated in the UHV chamber. The crystals had been etched in the same manner as the solution-treated crystals. The results are shown in Figure 4. As will be discussed later, the Cu coverage was estimated by comparing the Cu/Cd
CuSO4
C0NCENTRP;TK)N ( M I
Figure 3. Cu/Cd A E S peak-to-peak signal ratio obtained from CdS(1 120) surfaces as a function of the CuS04 solution concentration.
I
I
I
I
]
o
a1 g 0.2
0
E
o!
I
I 2
I 4
I 6
Cu COVERAGE ( I 1
Figure 4. Cu/Cd A E S peak-to-peak signal ratio of CdS( 1120) surfaces as a function of in situ deposited Cu coverage.
AES signals ratio of the vacuum-deposited to the solution-treated crystals. We also measured the time-resolved PL of Cu-covered CdS crystals. The deposition was carried out in a high-vacuum system. The results are presented in Figure 5. It shows three experimental PL decay curves (dotted lines) obtained from the CdS surface in air (a) and covered with 0.5 A (b) and 1.5 A (c) of Cu. Evaporation of thicker Cu layers did not change the PL time profile. The solid lines represent simulated curves which yield SRV values of 3 X IO4 and 3 X IOs cm/s for the 0.5- and 1.5-A Cu coverage, respectively. We have measured the Cd2+ion concentration in the different CuSO, solutions by AAS. We have found that the Cd2+ ion concentration in the CuS04 solutions increases with the CuS04 solution concentration. It has been known that immersion of CdS in hot and concentrated (-1 M) CuSO, solutions produces Cu,-,S/CdS (0 < x < 0.2) heterojunctions through a series of reactions." The first one is a chemical ion exchange: CdS + Cu2+ --e CuS Cd2+. Such a reaction increases the Cd2+ ion concentration in the CuSO, solutions after the immersion process.
+
( I 1) Saksena, S . ; Pandya. 94. 223.
D. K.; Chopra, K . L. Thin Solid Films 1980,
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6845
Surface Recombination at Cu/CdS( 1 120) Interfaces
0.7 TIME ( p s )
TABLE I: Correlation of the Number of Cd2+ Ions Released into the Different CuSO, Solutions (Measured by AAS), with the SRV and the AES Results Cu/Cd AES SRV, Cd2+ ions CUSO, solution concn, M lo5 cm/s released, X10’5 c n r 2 p p signal ratio 0.03 0.12 0.1 1 I x 10-5 2.5 x 10-5 0.6 0.5 0.1 5 x 10-5 23 1.3 0.15 I x 10-4 23 2.2 0.3 I x 10-3 23 2.2 0.3
-
0.25
23 23 23
2.2 2.2 2.2
0.3 0.3 0.3
The results are given in Table I. The SRV values obtained from the simulation process for different solution concentrations are given in column 2. Column 3 comprises the corresponding number of CdZ+ions (calculated from the AAS results). The Cu/Cd AES signal ratio is given in column 4. The SRV values, the number of Cd2+ions released into the CuSO, solutions, and the Cu/Cd AES signal ratio seem to be directly related. The increase in the SRV values is followed both by increase in the CdZ+ion concentration and by the Cu/Cd AES signal ratio. C. SPS Measurements. The surface photovoltage spectra of four CdS samples are shown in Figure 6. Figure 6A is a spectrum taken from the etched crystal, (B) from the crystal that had been M M CuSO, solution, (C) in 2.5 X immersed in a 1 X CuSO,, solution, and (D) in the 5 X M CuSO, solution. The marked increase in CPD at around 2.4 eV (=E8) is due to the decrease in band bending caused by band-to-band transitions. The spectrum of the etched sample shows distinct slope changes at hv 0.83 and 1.95 eV, which correspond to surface states at E, 0.83 eV and E , - 1.95 eV, respectively. These levels are probably associated with ambient-derived contaminations as was also reported by B r i l l ~ o n . ~ The solution-treated crystals exhibited a much greater band bending which increased with the solution concentration. (Accurate calculations of band bending were prevented by power limitations of our lamp.) In addition, two distinct slope changes appeared in the solution-treated crystals (especially above the concentration of 1 X M CuSO,) at around hu 1.2 eV and at hu 2.3 eV while the original surface states at E, = +0.83 eV and E, = -1.95 eV disappeared. The former is a negative AVCpD/Ahvchange and corresponds to a surface state at E, = - I .2 eV and the latter is a positive change corresponding to a surface state at E, = +2.3 eV.
+
-
-
I
1.5
1
1.9
I 1
2.3
Y 2.7
Energy [eV]
Figure 5. Experimental (dotted lines) photoluminescence decay curves for CdS(I 120) surfaces (a) in air, (b) covered with 0.5 A of Cu, and (c) covered with 1.5 A of Cu. Computer simulation curves (solid lines) are given for SRV values (a) 51 X lo3 cm/s, (b) 2 X IO4 cm/s, and (c) 23 x IO5 cm/s.
1 x 10-2 1 x Io-’
I
1.1
-
Discussion The AES and the AAS results both indicate the formation of Cu-S compound at the crystal surface. The Cu Auger spectra
Figure 6. Surface photovoltage spectra of etched CdS(1120) surface (A) and after treatments (B) in 1 X M CuS04 solution; (C) in 2.5 X IO” M CuSO, solution and (D) in 5 X 10” M C u S 0 4 solution. Arrows indicate transitions to and from sub-bandgap surface and interface states.
show a 4-5-eV shift to higher binding energies compared to bulk Cu, indicating compound formation. Furthermore, as the CuS04 solution concentration increases, the relative Cd/S surface concentration ratio decreases, again indicating the exchange reaction mentioned before. This is supported by our AAS measurements which showed that the concentration of Cd2+ ions (released by the reaction into the solution) also increased with the CuSO, solution concentration. A simple plausible explanation to the ion exchange reaction is based on ionic equilibria in aqueous solutions. For CdS the while solubility product in water Ksp= [CdZ+][S2-] = 1.4 X CuS solubility in water has a much lower product (Ksp = 4 X 10-38).’2Therefore, it is expected that Cd2+ is released into the CuSO, solution while Cu2+ ions react with the surface S2-ions to form a relatively insoluble surface layer of Cu-S. A series of experiments that we performed with solutions containing other metal cations support this assumption. Other sulfide-forming cations that have lower solubilities than CdS increase the SRV value, while ions whose solubility product with Sz- is greater than that of CdS do not alter the SRV even at high solution concentration (-0.5 M). This will be reported in detail elsewhere. One of the limiting steps of the ion-exchange reaction forming the Cu-S layer is the transport of Cu2+ions to the crystal surface, which is diffusion-limited. At low Cu2+concentration (about 1 X 10” M) the time needed to form a monolayer (neglecting the time required for the exchange reaction itself) is estimated to be 20 min.’3J4 The exchange reaction rate can be increased by strong stirring of the CuSO, solutions. We repeated the PL experiments after immersing the crystal in a vigorously stirred solution for about 15 min. In this case the previous SRV values (measured in the unstirred solutions) were obtained in solutions diluted by 1 order of magnitude. However, the Cu/Cd AES signal ratio and the CdZ+ion concentration in these solutions were the same as the ones obtained for the same SRV values. This supports our conclusions that the reaction rate is diffusion-limited and that the SRV is governed by the Cu coverage, regardless of its production method. We now estimate the Cu-S coverage associated with the different SRV values. The estimation is based on the relative AES p-p ratios obtained from the solution treated and the in situ deposited surfaces. Figure 2 shows that the highest SRV value of 3 X lo5 cm/s (limited by our simulation procedure as explained before5) is achieved at a CuSO, solution concentration of 5 X M. At this solution concentration the Cu/Cd AES signal ratio (12) Mahan, B. H . Uniuersity Chemistry, 3rd ed.; Addison Wesley: Reading, MA, 1975; p 204. (13) Delahay, D.; Trachtenberg, I . J . Am. Chem. SOC.1957, 79, 2355. (14) Gileadi, E.; Rabin, B. T.; Bockris, J. O.’M. J . Phys. Chem. 1965,69, 3335.
6846
The Journal of Physical Chemistry, Vol. 94, No. 17. 1990
is -0.15 as can be seen from Figure 3. The AES signal ratio rises to its highest value of -0.3 at a solution concentration of around 1 X IO4 M. This ratio does not increase further even at very concentrated solutions. Our results indicate that the Cd2+ ions from the CdS lattice are replaced chemically by Cu2+ according to the exchange reaction described earlier. This is a kinetically controlled reaction that stops as smn as a fully reacted Cu-S layer forms, as seen from the stabilization of the Cu surface concentration. The constant Cu/Cd AES signal ratio may indicate a formation of approximately one monolayer of Cu-S compound. Hence we approximate that the highest SRV value (corresponding to a Cu/Cd AES signal ratio of 0.15 which is a half of the highest value) occurs at around monolayer coverage. The AES signal ratios obtained after evaporation of Cu on the same CdS samples seem to support these assumptions. It is seen from Figure 4 that the Cu/Cd signal ratio of 0.1 is obtained after evaporation of 1.5-2 A of Cu. However, it must be emphasized that the CdS surface was subjected to HCI etching, a process which roughens the crystal surface9 and increases its surface area. Furthermore, we have no clear evidence that the evaporated Cu reacts with CdS (in contrast to the behavior of the Cu2+ions) and hence it may diffuse into the crystal causing a reduction of the Cu/Cd AES signal ratio. Therefore, we can approximate that the AES Cu/Cd signal ratio of 0.15 which corresponds to the highest SRV value and was obtained after evaporation of 2 A indeed corresponds to monolayer coverage. The AAS results support these assumptions. The number of Cd2+ ions released into the C u S 0 4 solution does not increase with solution concentration after 1 X 1 0-4 M, the same concentration at which the AES Cu/Cd signal ratio gets its highest value (-0.3) and which we had attributed to I monolayer coverage. The number of Cd2+ions released into the solution is 2.2 X IOl5 (the total crystal surface area is 0.9 cm2) which is not a bad approximation for 1 monolayer ( - 7 X lOI4 coverage taking into account the rough crystal surface. At around monolayer coverage, according to our Cd2+ ions are released. previous assumptions, 5 X l O I 4 Following similar arguments it is approximated that the SRV value of 1.2 X IO4 cm/s which is associated with an AES Cu/Cd signal ratio of 0.03 and with 1 X 1OI4 cm-2 Cd2+released ions (see Table I ) is measured after 1/10 monolayer coverage. In short, SRV values are found to strongly increase as a function of Cu coverage, saturating at monolayer. In the past, there were some attempts to correlate the surface chemical composition and the measured value of SRV. Nelson et aL3 showed that chemisorbed Ru ions on the surface of GaAs reduced the SRV. By using Rutherford backscattering they found surface Ru (with a depth resolution limit of 35 A). They did not report on any changes in the SRV following changes in the relative surface chemical composition. We now discuss the reasons for the increase in the SRV value caused by the Cu-S formation. It has been theoretically pred i ~ t e d ' ~ that - ~ ' the Cu2-,S/CdS interface has characteristic interface states. BoerI8 has proposed that these states are the result of Cd interstitials in very close proximity to S dangling bonds, causing a large fraction of these donor-acceptor pairs to become effective recombination centers. The origin of the S dangling bonds is considered by him to be the lattice mismatch between the CdS and the Cu2S lattices, while the Cd interstitials compensate the negative interface charges produced. These recombination centers give rise to an interface recombination velocity on the order of - 5 X lo6 cm/s. This is calculated according to
-
-
-
SRV = u,~u,V,
(4)
where uth is the thermal velocity of free carriers, u is the recombination cross section ( = 3 X 1 0-15cm2),I8and N , ( N 1.7 X l O I 4 cm-*) is the density of interface states. ( I 5 ) Martinuui, S.; Mallem, D.; Cabot, T. Phys. Status Solidi 1976,36A,
227.
(16) Lindmayer. J.; Revesz, A. G. Solid State Electron 1971. / I ,647. (17) Balkanski, M.; Chone, B. Rec. Phys. Appl. 1966, I , 179. (18) B k r . K . W . J . Appl. Phys. 1979, SO. 5356.
Rosenwaks et al. E(eV)
E (eV)
f
4
p 1.18
3
E.162
~
E = I 95
245
$245
~
a ) etched
b) C;+/CdS
Figure 7. Schematic energy band diagram of etched (a) and CuSO,
solution treated (b) CdS surface. The sub-bandgap surface and interface states shown correspond to the SPS features of Figure 6.
Although Boer's model1*is not directly applicable in our case of very low metal coverage, it indicates one of the mechanisms of interface defect formation. In our case this may be attributed to Cu-S bonding and resulting Cu-induced lattice disruption. Examination of our results (presented in Table I) shows that there is a direct correlation between the number of Cd2+ ions leaving the CdS surface and the SRV values. In the low solution concentrations of 1 X IO-s and 2.5 X 10-s M, SRV is even linear with the Cd2+ions concentration which means that the concentration of surface states is proportional to the number of the released Cd2+ ions or to the Cu-S bonds. After immersion in the 5 X M solution the number of Cd2+ions increases by a factor of -2.5 while the SRV value by a factor of 5 . There seem to be two possibilities. One is that at this solution concentration the number of surface states ceases to be linear with the number of Cu-S bonds, Le., each bond formation results in more than one surface state. Another possibility is that our simulated SRV value is not very sensitive in the high velocities range, Le., SRV values between 3 X I O 5 and 1 X IO6 cm/s can equally fit the lowest experimental decay curve in Figure 1. The insensitivity occurs in this range of velocities because the surface recombination process becomes effectively controlled by the rate of carriers diffusion toward the surface. In order that a chemisorbed species will affect the crystal SRV value it has to form a surface state whose energy lies in the semiconductor band gap. The closer to midgap the surface state is, the greater is its capture cross section for electron-hole pairs.I9 Our SPS results support this thesis. The surface-state levels derived from the surface photovoltage spectra shown in Figure 6 are represented schematically in the energy band diagrams in Figure 7. Also shown in this figure is the increase in band bending with the Cu solution concentration. This is due to the increase in surface-state density and the resulting decrease in the electron surface concentration. As was already mentioned, it was impossible to accurately determine the band bending at the surface, but we estimated it to be in the range of 300-500 mV. The surface-state density required to produce such bendings was calculated according to Poisson equation20 to be between 2.6 X 101jand 1.2 X 10ls As can be seen from Table I, these values are in quite a good agreement with the number of Cd2+ ions replaced by the Cu2+ ions on the crystal surface. Another evidence for the increase in SRV (caused by the chemisorbed Cu2+ions) is the lowering of the photovoltage response with increasing CuSO, solution concentration. The Cu2+ions produce two discrete surface-state levels at E, = -1.2 eV and at E, = +2.3 eV. The former is very close to midgap and accounts for the large increase in SRV. Heller et al.21.22 have proposed a model which qualitiatively estimates the position of the surface state (formed as a result of chemi(19) Henry, c . H.; Lang, D. V. Phys. Rev. B 1977, 15, 989. (20) Many, A.; Goldstein, Y . ;Grover, A. Semiconduclor Surfaces; North Holland: Amsterdam, 1977. ( 2 1 ) Heller. A.; Miller, B. Adc. Chem. Ser. 1980, 184, 215. ( 2 2 ) Heller. A . Arc. Chem. Res. 1981, 14, 154
J . Phys. Chem. 1990, 94, 6847-6852 sorption or another crystalsolution interaction) according to the metal-anion bond strength. They state that the weaker the bond is, the closer will its position be to midgap. Therefore, we believe that these midgap interface states are formed due to the relatively weak Cu-S bonds (as compared to the Cd-S bonds). Thus the observed increase in SRV is due to the increase in u resulting from the redistribution of the interface states closer to midgap. It is instructive to compare the effect on SRV of the solution-deposited Cu and the evaporated Cu. Based on Figure 4, and the above discussion, submonolayer coverages of evaporated and solution deposited Cu resulted in similar values of SRV. We have no clear evidence for the formation of Cu-S compound in the case of evaporation. Brucker and Brillsonz3 have also shown that evaporation of Cu on cleaved CdS under UHV conditions does not change the Cd/S surface concentration ratio up to about IO-A Cu coverage. They suggested that the unchanged ratio indicates that no Cu-S chemical reaction takes place similarly to Au/CdS interfaces and unlike reactive metals ( e g , AI or Ti) interfaces with CdS. Our results show that the SRV values are essentially the same for both deposition methods. The difference in bonding does not in our opinion rule out a common origin for the SRV increase. In the solution case, the increase is due to surface states formed as a result of Cu-S bonds formation, while the evaporated Cu leads to surface states formation due to weaker bonding and lattice disruption. Although the effect on the SRV values is essentially the same, the energy position of these states may be different. These energy positions may be identified by surface photovoltage measurements on Cu/CdS interfaces. In the past24we reported on the measurement of the SRV value on CdS surfaces covered with Au. We suggested that the measured increase in the SRV (23) Brucker, C. F.; Brillson, L. J. J . VUC.Sci. Technol. 1981, 19, 617. (24) Huppert. D.:Evenor, M.; Shapira, Y. J . Vac. Sci. Technol. 1984, A2, 532.
6847
was caused by the CdS surface disruption by the unreactive Au, which resulted in formation of midgap interface states. On the basis of similar SRV dependence we suggest that in the Au case the formation of midgap interface states is predominantly a result of the CdS lattice disruption and defect formation. It seems that, in the Cu case, the interface recombination centers are a result of a combined effect of weak bonding and lattice disruption.
Summary and Conclusions We have studied the effect of Cu coverage on the surface recombination velocity of CdS. It is found that very dilute CuSO., solutions produce controllable submonolayers of Cu-S compound and increase the value of SRV dramatically. SRV increases by almost 1 order of magnitude at an estimated Cu coverage of 1/ 10 monolayer. It continues to be a strong function of coverage up to monolayer where it saturates at values above 3 X lo5 cm/s. The Cu-S compound formation in such dilute solutions is due to its extremely low solubility in water relative to CdS. These conclusions were supported both by AES and AAS measurements. The SPS measurements have supplied the evidence that compound evolution led to midgap surface-state formation. We propose that weak bonding and lattice defects formed in the Cu-S/CdS interface are responsible for these states. Similar SRV values were obtained by UHV-deposited Cu. It is suggested that the increased SRV in this case is due to similar reasons. Cuinduced weak bonding and lattice defects create midgap interface states, which contribute to increase of SRV. This points to a direction in which SRV could be systematically controlled.
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Acknowledgment. Y.R. gratefully acknowledges the financial support of a grant from the Gordon Center for Energy Research at Tel Aviv University. We are grateful for the support of the Krantzberg Institute, The Israeli National Academy of Sciences, and the U S . AID-CDR Program. Registry No. Cu, 7440-50-8; CdS, 1306-23-6.
Chemisorption and Thermal Decomposition of Ethylene on a Pd( 1lo)( 1X2)-Cs Surface: Electron Energy Loss Spectroscopy and Thermal Desorption Studies T. Sekitani, J. Yoshinobu,+ M. Onchi,$ and M. Nishijima* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received: February 2, 1990; In Final Form: April 24, 1990)
The adsorbed state of ethylene on the Pd( 1 lo)( IX2)-Cs surface and its thermal decomposition have been studied by the use of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. For a small exposure (0.1 langmuir) at YO K, ethylene is a-bonded to the Pd( 1 lo)( 1 X2)-Cs surface. For a large exposure (2 langmuirs), a small amount of physisorbed ethylene exists in addition to T-bonded ethylene. The saturation coverage corresponds to 0.23 C2H4 molecules per Pd atom of the unreconstructed Pd( 1 IO) surface. The physisorbed ethylene is desorbed by heating to 1 IO K. A part of the *-bonded ethylene is desorbed intact at 200 K, and additionally at 270 K by a recombinative process. By heating to 300 K, the residual C2H4admolecules are decomposed, and methylidyne (CH) species are formed. Only carbon adatoms remain on the Pd( 1 IO)( 1X2)-Cs surface by heating to 500 K. These results are compared with those for the Pd( 1 IO) clean surface, and the effects of Cs adatoms on the surface reactions are discussed.
I. Introduction sorption and thermal decomposition of ethylene on the Pd(l IO) clean surface using electron energy loss spectroscopy (EELS), The adsorption and thermal decomposition of ethylene on well-defined surfaces (Fe,l.2 Ni,3-5Cu,6 Ru?v8 Rh?JO Pd,11-16*2w22 Ag," lr,'* Pt,I9 etc.) have been an object of many studies as a (1) Erley, W.; Bar6, A . M.; Ibach, H. Surf. Sci. 1982, 120, 273. prototype for the interaction of olefin hydrocarbons with catalysts. (2) Seip, U.; Tsai, M.-C.; Kiippers, J.; Ertl, G. Surf. Sci. 1984, 147, 65. (3) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. The ethylene-Pd( 1 IO) surface interaction has been studied by a (4) Stroscio, J . A.; Bare, S. R.; Ho, W. Surf. Sci. 1984, 148, 499. few research groups.13,2w22Recently, we have studied the ad( 5 ) Zaera, F.; Hall, R. B. Surf. Sci. 1987, 180, 1 . Present address: Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260. 'Professor Emeritus. +
0022-3654/90/2094-6847$02.50/0
(6) Nyberg, C.; Tengstil, C . G.; Andersson, S . ; Holmes, M. W. Chem. Phys. Lett. 1982, 87. 87. (7) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W . H. J . Am. Chem. Soc. 1986, 108, 3554.
0 1990 American Chemical Society
