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An investigation of the passivating effects of hydrogen sulphide on the GaAs(100) surface
Materials Science and Engineering, B9 (1991 ) 37-41
37
An investigation of the passivating effects of hydrogen sulphide on the GaAs(100) surface G. J. Hughes, L. Roberts, M. O. Henry and K. McGuigan School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9 (Ireland)
G. M. O'Connor, F. G. Anderson, G. P. Morgan and T. Glynn Department of Experimental Physics, University College, Galway (Ireland)
Abstract In this paper we investigate the passivating effects of exposing a freshly etched GaAs(100) surface to hydrogen sulphide gas. The effectiveness of this passivation procedure is assessed in a comparison between the characteristics of the treated and untreated surface by a range of techniques. Spectra of the treated surface obtained by deep-level transient spectroscopy reveal a significant reduction in the intensity of a peak attributed to interface states, which is clearly detected on the untreated surface. The idealities of the diodes fabricated on the treated surface are comparable with those obtained for diodes fabricated on the freshly etched GaAs(100) surface. Both photoluminescence and Raman spectroscopy measurements also indicate a reduction in the interface state densities of the treated surface compared with the untreated surface. Variations in the effectiveness of the passivation were observed, which appear to depend on the precise procedure followed during the preparation of the surfaces prior to hydrogen sulphide exposure.
1. Introduction A variety of chemical surface treatments have been developed with the aim of reducing the density of defect states found at the surface or interfacc of semiconductor devices. A high density of defect states at the interface, which can be caused by surface oxidation, has generally a detrimental effect on the electronic characteristics of the device. Efforts have been concentrated on the use of inorganic sulphides such as sodium sulphide and ammonium sulphide in wet chemical preparation of the passivating surface layer [1-6]. A photoemission study of these surfaces [7] has indicated that they are predominately oxidized, and the fraction of the surface atoms bonded to sulphur is small. A much higher fraction of sulphur bonding to surface atoms was found from photoemission studies of the passivation of the GaAs surface by exposure to hydrogen sulphide (HzS) gas in ultrahigh vacuum [8]. The present study reports the passivation of the GaAs(100) surface and metal-semiconductor interface by exposure of the chemically etched surface to HzS at room temperature. The passiva0921-5107/91/$3.50
tion was determined by the comparison of spectra from a surface which was exposed to H2S gas immediately after etching with a sample which was left in air following etching. The non-contact techniques of photoluminescence (PL) and Raman spectroscopy were used to determine the effects of the passivation on the GaAs surface. The semiconductor-metal interface passivation was determined by the measurement of the current-voltage ( l - V ) characteristics and from deep-level transient spectroscopy (DLTS) of diodes fabricated on the treated surface. Both of these techniques have been shown to be capable of detecting interface state densities [9-14]. Our studies indicate that H2S treatment prolongs the as-etched surface characteristics which quickly degrade in air in the absence of this treatment.
2. Experimental details The crystals used in the studies consisted of n-type GaAs(100) of two doping ranges. Highly doped crystals, of the order of 1018 cm -3, were required for the Raman experiments, while 1016 © ElsevierSequoia/Printedin The Netherlands
38
cm- 3 silicon doped crystals were used for the I- V and DLTS analysis. PL measurements were made on both doping types and this was used to ensure that both semiconductors had been subjected to the same passivating process. The GaAs(100) samples which had one side polished were initially washed in acetone and methanol before being etched in a H2SO4-H202-H20 solution (5 : 1 : 1) for 1 min and thoroughly rinsed in deionized water. Ohmic contacts were made to the back of the samples for electrical measurements by applying an In-Ga paint which was annealed into the samples at 400 °C in a nitrogen atmosphere. The samples were then directly inserted in a dosing chamber where they were exposed to an atmosphere of H2S for 30 min. Schottky diodes were then fabricated on these samples by depositing gold contacts. In order to determine the effectiveness of the passivating procedure, treated surfaces were exposed in air for periods up to 18 h prior to metal deposition. The electrical measurements were made using a commercial Polaron DLTS system which has been interfaced to a PC and a computer-controlled I - V system. On average, 10 individual diodes were fabricated on a sample; therefore any variations in the effective passivation across the surface could be detected. PL spectra of the passivated and unpassivated samples were taken at liquid-helium temperatures in an Oxford Instruments CF1204D flow cryostat. Luminescence was excited with a continuous-wave argon ion laser operating on all lines with a power density of approximately 110 mW cm -2. The luminescence was analysed using a Spex 1704 spectrometer of 1 m focal length and a North Coast EO-817 germanium detector cooled to 77 K. The Raman experiments were performed using the 514.5 nm line from an argon ion laser with the power density at the surface less than 2 W cm -2. The scattered light from the sample was collected and analysed using a Spex 0.85 m double monochromator and an EMI 9863B photomultiplier.
3. Results
Figure 1 illustrates the DLTS spectra obtained for the passivated and unpassivated GaAs samples. These spectra clearly show the well-characterized EL2 and EL6 bulk electron trap features [15]. The comparison is made between two
g'3 r(a) U m 0
)assivated ]
(b) H2S P a s s i v a t e d
N
EL6 E L
< Z
2 p (v)
09i 1.0
O0 [:nO
100 200 300 400 500
100 2 0 0 3 0 0 4 0 0 5 0 0
TEMPERATURE (N) Fig. 1. DLTS spectra of (a) unpassivated (n = 1.6) and (b) H2S-passivated ( n = l . 1 ) GaAs, for a reverse voltage of - 1 V, and pulse voltages Vp as shown. T h e rate window is 100 s - J. I.S. denotes the interface state feature.
samples: one which was chemically etched and left in air for 1 h prior to gold deposition, and the other sample which was exposed to H2S immediately after etching and then exposed to air for 1 h before a metal contact was deposited. The additional spectral features observed on the unpassivated GaAs surface are only detected in the forward-bias pulse sequence of the DLTS; therefore the electron traps corresponding to these features are located within the intrinsic depletion region of the diode. Two distinct overlapping peaks are clearly detected for high forward-bias voltages. Further investigations are under way at present to determine the origin of these features. The shift in the peak maxima of these features to lower temperatures as a function of forward bias is indicative of broad bands of interface state energies, rather than discrete bulk-like levels [9]. The ideality parameters determined from the I- V characteristics for the unpassivated sample are significantly higher (1.6 < n < 2) than for the H2Sexposed surface (n < 1.1). These latter idealities are similar to those routinely obtained for GaAs diodes where the gold overlayer is deposited immediately following etching. The DLTS spectra taken on these diodes exhibit characteristics similar to the passivated sample, i.e. no interface states were detected during the forward-bias pulse sequence of a DLTS scan. The idealities of the diodes fabricated on the passivated surface which was exposed to air for 18 h prior to metal deposition were in the range n = 1.1-1.2, and there was some evidence of an increase in the interface state density from the DTLS spectra, The PL spectrum of the unpassivated GaAs sample in Fig. 2 shows strong near-band-edge
39
emission in the 1.45-1.5 eV region which can be attributed to unresolved donor-to-acceptor and conduction-band-to-acceptor transitions [16]. The broad bands observed at 0.9-1.4 eV have been attributed to donor-vacancy complexes [17, 18]. The PL spectrum of the H2S-passivated sample shows a dramatic reduction in the inten-
,D
sity of the donor-vacancy bands relative to the near-band-edge emissions. Both 1 0 TM c m - 3 and 1016 c m - 3 doped samples used in the study exhibited the same post-passivated PL spectra. The Raman spectra shown in Fig. 3 are also of the passivated and unpassivated samples. Selection rules predict that the peaks observed in Raman spectra can be attributed to the longitudinal optical (LO) phonon mode (292 cm -1) arising from the carrier-depleted region at the surface, and a coupled LO phonon-plasmon mode originating from an undepleted carder region further from the surface [4]. This phonon-plasmon mode gives rise to two peaks: a sharp L - peak (267 cm -~) and a broader L+ peak not shown in the figure. Passivation by H2S results in a change in the relative intensity of the photon-plasmon peak relative to the uncoupled peak.
(a) n-GaAs:Si
Unpassivated
Z r-~ r.~
A
(b) n-G~:Si
HzS Passlvated
II
I
z 750
950
1150
1350
4. Discussion
1550
ENERGY (meV)
There is a great deal of controversy in the literature as to the nature of the bonding of sulphurcontaining compounds to the GaAs surface. All the studies using X-ray photoelectron spectroscopy agree that the sulphur layer formed is very
Fig. 2. PL spectra at 4.2 K of (a) the unpassivated and (b) the H.2S-.passivated GaAs(100) surface. Near-band-edge emission is observed in the 1.45-1.5 eV region, while the broad bands from 0.9 to 1.4 eV are attributed to donor-vacancy complexes.
700
500
500
Z 400
300
200
1 O0
! 25D
i 270
i
v 290
i
i 310
!
! 330
RAMAN SHIRT ( c m - I )
Fig. 3. Raman scattering spectra of both the unpassivated (+) and the passivated ( x ) samples. The uncoupled LO phonon and the L-coupled phonon-plasmon peaks appear at 292 cm- ~and 269 cm ~respectively.
40
thin, probably only a monolayer [5]. The precise mechanism by which sulphur passivates the surface is also the subject of much debate. This is primarily due to the wide variations reported in the reactivity of these sulphur compounds with the surface. Shin et al. [5] have suggested that there is a kinetic barrier to sulphur, strongly bonding to the GaAs surface; therefore the degree of sulphur uptake on a particular surface depends on whether there is sufficient energy to surmount this kinetic barrier. They have also reported that the uptake of sulphur on the GaAs(100) etched surface varies depending on the extent to which the surface is oxidized prior to H2S exposure. Kuhr and coworkers [19, 20] found in their investigations of the interaction of H2S with the GaAs surface that the amount adsorbed critically depended on the order and composition of the surface. The results of our investigations indicate that the H:S passivation of the etched GaAs surface has the effect of maintaining the surface with the post-etched characteristics. This has been established from the similarities between the electrical characteristics of the passivated sample and those of the etched surface which was immediately metallized. The etching process removes the surface oxide and the subsequent passivation or the immediate metallization of the etched surface limits the regrowth of a surface oxide, thereby giving rise to the near-ideal electrical characteristics observed. Even for the passivated sample which was subjected to prolonged exposure to atmospheric conditions, the electrical measurements indicated that the surface had remained predominantly unoxidized. This contrasts markedly with the samples exposed to air where a significant deterioration in the electrical characteristics of the surface was detected, even over a time scale of 1 h. Quantitative comparisons of different PL spectra are difficult in the present experimental arrangement; therefore we only compare the relative intensities of features within the same spectrum. The reduction in the intensity of the donor-vacancy bands with respect to the band edge emissions on H2S exposure may be caused by a modification of the donor-vacancy defect and/or an increase in the number of donor species contributing to the near-band-edge luminescence, both of which are consistent with a reduction in interface state density. The Raman spectra clearly indicate the effect of H2S passiva-
tion on the GaAs surface. By knowing the penetration depth of the laser light into the sample, approximately 0.1/~m, the ratio of the intensities of the L- to the uncoupled LO peak can be used to determine the depth of the depletion region and hence the magnitude of the band bending at the surface [4]. The post-passivating increase in the relative intensity of the phonon-plasmon peak at 267 cm-l, relative to the uncoupled phonon peak at 292 cm-1, signifies a reduction in the band bending and this can be attributed to a reduction in the surface state density. Therefore both the PL and the Raman spectra can be interpreted in terms of H2S surface passivation of the etched surface when compared with the unpassivated surface. We have obtained significantly varying results depending on the precise procedure followed immediately after etching and prior to H2S exposure. The longer that an etched surface is exposed to air prior to HzS treatment, the less effective is the passivating treatment. Our findings would support the proposal by Spindt and Spicer [21] who suggested that the sulphurization is effective in passivating the surface owing to its ability to inhibit the growth of surface oxides which lead to the extensive disruption of the GaAs surface, increasing the density of surface states. The slight deterioration in the electrical characteristics of the passivated surface which underwent prolonged air exposure is also consistent with this proposal. It would be difficult to envisage a process involving the exposure of a surface to a gas which would totally passivate a surface to the extent that no subsequent oxidation would occur, bearing in mind the larger heats of formation of oxides compared with sulphides [21]. Although we cannot comment on the chemistry of the passivated surface, it is obvious from the fact that surface oxidation is almost totally inhibited by this treatment that the surface coverage of sulphur must be close to one monolayer as has previously been reported [5]. In conclusion, we have found that the exposure of freshly etched GaAs(100) surfaces to H2S leads to the chemical stabilization of the surface to the extent that it retains the as-etched electrical characteristics in air. This can be contrasted with the rapid deterioration of the electrical characteristic of air-exposed untreated surfaces. The effectiveness of the passivation has been found to depend on the degree to which the surface has been exposed to air prior to HzS treatment.
41
References 1 E. Yablonovitch, C. J. Sandroff, R. Bhat and T. Gmitter,
A ppl. Phys. Lett., 51 ( 1987 ) 439. 2 C. J. Sandroff, R. N. Nottenburg, J. C. Bischoff and R. Bhat, Appl. Phys. Letr, 51 (1987) 33. 3 C. J. Sandroff, M. S. Hedge, L. A. Farrow, C. C. Chang and J. P. Harbison, Appl. Phys. Lett., 54 (1989) 362. 4 L. A. Farrow, C. J. Sandroff and M. C. Tamargo, Appl. Phys. Lett., 51 (1987) 1931. 5 J. Shin, K. M. Geib, C. W. Wilmsen and Z. LillentalWeber, J. Vac. Sci. Technol. A, 8 (1990) 1894. 6 K. M. Geib, J. Shin and C. W. Wilmsen, J. Vac. Sci. Technol. B, 8(1990) 838. 7 T. Tiedje, K. M. Colbow, D. Rogers, Z. Fu and W. Eberhardt, J. Vac. Sci. Technol. B, 7(1989) 837. 8 T. Tiedje, P. C. Wong, K. A. R. Mitchell, W. Eberhardt, Z. Fu and D. Sondericker, Solid State Commun., 70 (1989) 355. 9 K. Yamasakai, M. Yoshida and T. Sugano, Jpn. J. Appl. Phys., 18(1979) 113.
10 H. Zhang, Y. Aoyagi, S. Iawi and S. Namba, Appl. Phys. Lett., 50(1987) 341. 11 W. Platen, D. Kohl, K. A. Brauchle and K. Wolter, Mater. Sci. Forum, 10-12 (1986) 223. 12 L. Roberts and G. J. Hughes, Proc. Conf. EMRS, November, 1990, Elsevier, 199 l, in the press. 13 A. B. McC16an, I. M. Dharmadasa and R. H. Williams, Semicond. Sci. Technol., 1 (1986) 137. 14 W. Platen, H. J. Schmutzler, D. Kohl, K. A. Brauchle and K. Wolter, J. Appl. Phys., 64 (1988) 218. 15 F. D. Auret, A. W. R. Leitch and J. S. Vermaak, J. Appl. Phys., 59(1986) 158. 16 J. Van den Ven, W. J. A. M. Hartmann and L. J. Giling, J. Appl. Phys., 60(1986) 3735. 17 E.W. Williams, Phys. Rev., 168(1968) 922. 18 T. Instone and L. Eaves, J. Phys. C, 11 (1978) L771. 19 H.J. Kuhr, W. Ranke and J. Finster, Surf. Sci., 192 (1986) 171. 20 W. Ranke, H. J. Kuhr and J. Finster, Surf. Sci., 192 (1987) 81. 21 C.J. Spindt and W. E. Spicer, AppL Phys. Letr, 55 (1989) 1653.
37
An investigation of the passivating effects of hydrogen sulphide on the GaAs(100) surface G. J. Hughes, L. Roberts, M. O. Henry and K. McGuigan School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9 (Ireland)
G. M. O'Connor, F. G. Anderson, G. P. Morgan and T. Glynn Department of Experimental Physics, University College, Galway (Ireland)
Abstract In this paper we investigate the passivating effects of exposing a freshly etched GaAs(100) surface to hydrogen sulphide gas. The effectiveness of this passivation procedure is assessed in a comparison between the characteristics of the treated and untreated surface by a range of techniques. Spectra of the treated surface obtained by deep-level transient spectroscopy reveal a significant reduction in the intensity of a peak attributed to interface states, which is clearly detected on the untreated surface. The idealities of the diodes fabricated on the treated surface are comparable with those obtained for diodes fabricated on the freshly etched GaAs(100) surface. Both photoluminescence and Raman spectroscopy measurements also indicate a reduction in the interface state densities of the treated surface compared with the untreated surface. Variations in the effectiveness of the passivation were observed, which appear to depend on the precise procedure followed during the preparation of the surfaces prior to hydrogen sulphide exposure.
1. Introduction A variety of chemical surface treatments have been developed with the aim of reducing the density of defect states found at the surface or interfacc of semiconductor devices. A high density of defect states at the interface, which can be caused by surface oxidation, has generally a detrimental effect on the electronic characteristics of the device. Efforts have been concentrated on the use of inorganic sulphides such as sodium sulphide and ammonium sulphide in wet chemical preparation of the passivating surface layer [1-6]. A photoemission study of these surfaces [7] has indicated that they are predominately oxidized, and the fraction of the surface atoms bonded to sulphur is small. A much higher fraction of sulphur bonding to surface atoms was found from photoemission studies of the passivation of the GaAs surface by exposure to hydrogen sulphide (HzS) gas in ultrahigh vacuum [8]. The present study reports the passivation of the GaAs(100) surface and metal-semiconductor interface by exposure of the chemically etched surface to HzS at room temperature. The passiva0921-5107/91/$3.50
tion was determined by the comparison of spectra from a surface which was exposed to H2S gas immediately after etching with a sample which was left in air following etching. The non-contact techniques of photoluminescence (PL) and Raman spectroscopy were used to determine the effects of the passivation on the GaAs surface. The semiconductor-metal interface passivation was determined by the measurement of the current-voltage ( l - V ) characteristics and from deep-level transient spectroscopy (DLTS) of diodes fabricated on the treated surface. Both of these techniques have been shown to be capable of detecting interface state densities [9-14]. Our studies indicate that H2S treatment prolongs the as-etched surface characteristics which quickly degrade in air in the absence of this treatment.
2. Experimental details The crystals used in the studies consisted of n-type GaAs(100) of two doping ranges. Highly doped crystals, of the order of 1018 cm -3, were required for the Raman experiments, while 1016 © ElsevierSequoia/Printedin The Netherlands
38
cm- 3 silicon doped crystals were used for the I- V and DLTS analysis. PL measurements were made on both doping types and this was used to ensure that both semiconductors had been subjected to the same passivating process. The GaAs(100) samples which had one side polished were initially washed in acetone and methanol before being etched in a H2SO4-H202-H20 solution (5 : 1 : 1) for 1 min and thoroughly rinsed in deionized water. Ohmic contacts were made to the back of the samples for electrical measurements by applying an In-Ga paint which was annealed into the samples at 400 °C in a nitrogen atmosphere. The samples were then directly inserted in a dosing chamber where they were exposed to an atmosphere of H2S for 30 min. Schottky diodes were then fabricated on these samples by depositing gold contacts. In order to determine the effectiveness of the passivating procedure, treated surfaces were exposed in air for periods up to 18 h prior to metal deposition. The electrical measurements were made using a commercial Polaron DLTS system which has been interfaced to a PC and a computer-controlled I - V system. On average, 10 individual diodes were fabricated on a sample; therefore any variations in the effective passivation across the surface could be detected. PL spectra of the passivated and unpassivated samples were taken at liquid-helium temperatures in an Oxford Instruments CF1204D flow cryostat. Luminescence was excited with a continuous-wave argon ion laser operating on all lines with a power density of approximately 110 mW cm -2. The luminescence was analysed using a Spex 1704 spectrometer of 1 m focal length and a North Coast EO-817 germanium detector cooled to 77 K. The Raman experiments were performed using the 514.5 nm line from an argon ion laser with the power density at the surface less than 2 W cm -2. The scattered light from the sample was collected and analysed using a Spex 0.85 m double monochromator and an EMI 9863B photomultiplier.
3. Results
Figure 1 illustrates the DLTS spectra obtained for the passivated and unpassivated GaAs samples. These spectra clearly show the well-characterized EL2 and EL6 bulk electron trap features [15]. The comparison is made between two
g'3 r(a) U m 0
)assivated ]
(b) H2S P a s s i v a t e d
N
EL6 E L
< Z
2 p (v)
09i 1.0
O0 [:nO
100 200 300 400 500
100 2 0 0 3 0 0 4 0 0 5 0 0
TEMPERATURE (N) Fig. 1. DLTS spectra of (a) unpassivated (n = 1.6) and (b) H2S-passivated ( n = l . 1 ) GaAs, for a reverse voltage of - 1 V, and pulse voltages Vp as shown. T h e rate window is 100 s - J. I.S. denotes the interface state feature.
samples: one which was chemically etched and left in air for 1 h prior to gold deposition, and the other sample which was exposed to H2S immediately after etching and then exposed to air for 1 h before a metal contact was deposited. The additional spectral features observed on the unpassivated GaAs surface are only detected in the forward-bias pulse sequence of the DLTS; therefore the electron traps corresponding to these features are located within the intrinsic depletion region of the diode. Two distinct overlapping peaks are clearly detected for high forward-bias voltages. Further investigations are under way at present to determine the origin of these features. The shift in the peak maxima of these features to lower temperatures as a function of forward bias is indicative of broad bands of interface state energies, rather than discrete bulk-like levels [9]. The ideality parameters determined from the I- V characteristics for the unpassivated sample are significantly higher (1.6 < n < 2) than for the H2Sexposed surface (n < 1.1). These latter idealities are similar to those routinely obtained for GaAs diodes where the gold overlayer is deposited immediately following etching. The DLTS spectra taken on these diodes exhibit characteristics similar to the passivated sample, i.e. no interface states were detected during the forward-bias pulse sequence of a DLTS scan. The idealities of the diodes fabricated on the passivated surface which was exposed to air for 18 h prior to metal deposition were in the range n = 1.1-1.2, and there was some evidence of an increase in the interface state density from the DTLS spectra, The PL spectrum of the unpassivated GaAs sample in Fig. 2 shows strong near-band-edge
39
emission in the 1.45-1.5 eV region which can be attributed to unresolved donor-to-acceptor and conduction-band-to-acceptor transitions [16]. The broad bands observed at 0.9-1.4 eV have been attributed to donor-vacancy complexes [17, 18]. The PL spectrum of the H2S-passivated sample shows a dramatic reduction in the inten-
,D
sity of the donor-vacancy bands relative to the near-band-edge emissions. Both 1 0 TM c m - 3 and 1016 c m - 3 doped samples used in the study exhibited the same post-passivated PL spectra. The Raman spectra shown in Fig. 3 are also of the passivated and unpassivated samples. Selection rules predict that the peaks observed in Raman spectra can be attributed to the longitudinal optical (LO) phonon mode (292 cm -1) arising from the carrier-depleted region at the surface, and a coupled LO phonon-plasmon mode originating from an undepleted carder region further from the surface [4]. This phonon-plasmon mode gives rise to two peaks: a sharp L - peak (267 cm -~) and a broader L+ peak not shown in the figure. Passivation by H2S results in a change in the relative intensity of the photon-plasmon peak relative to the uncoupled peak.
(a) n-GaAs:Si
Unpassivated
Z r-~ r.~
A
(b) n-G~:Si
HzS Passlvated
II
I
z 750
950
1150
1350
4. Discussion
1550
ENERGY (meV)
There is a great deal of controversy in the literature as to the nature of the bonding of sulphurcontaining compounds to the GaAs surface. All the studies using X-ray photoelectron spectroscopy agree that the sulphur layer formed is very
Fig. 2. PL spectra at 4.2 K of (a) the unpassivated and (b) the H.2S-.passivated GaAs(100) surface. Near-band-edge emission is observed in the 1.45-1.5 eV region, while the broad bands from 0.9 to 1.4 eV are attributed to donor-vacancy complexes.
700
500
500
Z 400
300
200
1 O0
! 25D
i 270
i
v 290
i
i 310
!
! 330
RAMAN SHIRT ( c m - I )
Fig. 3. Raman scattering spectra of both the unpassivated (+) and the passivated ( x ) samples. The uncoupled LO phonon and the L-coupled phonon-plasmon peaks appear at 292 cm- ~and 269 cm ~respectively.
40
thin, probably only a monolayer [5]. The precise mechanism by which sulphur passivates the surface is also the subject of much debate. This is primarily due to the wide variations reported in the reactivity of these sulphur compounds with the surface. Shin et al. [5] have suggested that there is a kinetic barrier to sulphur, strongly bonding to the GaAs surface; therefore the degree of sulphur uptake on a particular surface depends on whether there is sufficient energy to surmount this kinetic barrier. They have also reported that the uptake of sulphur on the GaAs(100) etched surface varies depending on the extent to which the surface is oxidized prior to H2S exposure. Kuhr and coworkers [19, 20] found in their investigations of the interaction of H2S with the GaAs surface that the amount adsorbed critically depended on the order and composition of the surface. The results of our investigations indicate that the H:S passivation of the etched GaAs surface has the effect of maintaining the surface with the post-etched characteristics. This has been established from the similarities between the electrical characteristics of the passivated sample and those of the etched surface which was immediately metallized. The etching process removes the surface oxide and the subsequent passivation or the immediate metallization of the etched surface limits the regrowth of a surface oxide, thereby giving rise to the near-ideal electrical characteristics observed. Even for the passivated sample which was subjected to prolonged exposure to atmospheric conditions, the electrical measurements indicated that the surface had remained predominantly unoxidized. This contrasts markedly with the samples exposed to air where a significant deterioration in the electrical characteristics of the surface was detected, even over a time scale of 1 h. Quantitative comparisons of different PL spectra are difficult in the present experimental arrangement; therefore we only compare the relative intensities of features within the same spectrum. The reduction in the intensity of the donor-vacancy bands with respect to the band edge emissions on H2S exposure may be caused by a modification of the donor-vacancy defect and/or an increase in the number of donor species contributing to the near-band-edge luminescence, both of which are consistent with a reduction in interface state density. The Raman spectra clearly indicate the effect of H2S passiva-
tion on the GaAs surface. By knowing the penetration depth of the laser light into the sample, approximately 0.1/~m, the ratio of the intensities of the L- to the uncoupled LO peak can be used to determine the depth of the depletion region and hence the magnitude of the band bending at the surface [4]. The post-passivating increase in the relative intensity of the phonon-plasmon peak at 267 cm-l, relative to the uncoupled phonon peak at 292 cm-1, signifies a reduction in the band bending and this can be attributed to a reduction in the surface state density. Therefore both the PL and the Raman spectra can be interpreted in terms of H2S surface passivation of the etched surface when compared with the unpassivated surface. We have obtained significantly varying results depending on the precise procedure followed immediately after etching and prior to H2S exposure. The longer that an etched surface is exposed to air prior to HzS treatment, the less effective is the passivating treatment. Our findings would support the proposal by Spindt and Spicer [21] who suggested that the sulphurization is effective in passivating the surface owing to its ability to inhibit the growth of surface oxides which lead to the extensive disruption of the GaAs surface, increasing the density of surface states. The slight deterioration in the electrical characteristics of the passivated surface which underwent prolonged air exposure is also consistent with this proposal. It would be difficult to envisage a process involving the exposure of a surface to a gas which would totally passivate a surface to the extent that no subsequent oxidation would occur, bearing in mind the larger heats of formation of oxides compared with sulphides [21]. Although we cannot comment on the chemistry of the passivated surface, it is obvious from the fact that surface oxidation is almost totally inhibited by this treatment that the surface coverage of sulphur must be close to one monolayer as has previously been reported [5]. In conclusion, we have found that the exposure of freshly etched GaAs(100) surfaces to H2S leads to the chemical stabilization of the surface to the extent that it retains the as-etched electrical characteristics in air. This can be contrasted with the rapid deterioration of the electrical characteristic of air-exposed untreated surfaces. The effectiveness of the passivation has been found to depend on the degree to which the surface has been exposed to air prior to HzS treatment.
41
References 1 E. Yablonovitch, C. J. Sandroff, R. Bhat and T. Gmitter,
A ppl. Phys. Lett., 51 ( 1987 ) 439. 2 C. J. Sandroff, R. N. Nottenburg, J. C. Bischoff and R. Bhat, Appl. Phys. Letr, 51 (1987) 33. 3 C. J. Sandroff, M. S. Hedge, L. A. Farrow, C. C. Chang and J. P. Harbison, Appl. Phys. Lett., 54 (1989) 362. 4 L. A. Farrow, C. J. Sandroff and M. C. Tamargo, Appl. Phys. Lett., 51 (1987) 1931. 5 J. Shin, K. M. Geib, C. W. Wilmsen and Z. LillentalWeber, J. Vac. Sci. Technol. A, 8 (1990) 1894. 6 K. M. Geib, J. Shin and C. W. Wilmsen, J. Vac. Sci. Technol. B, 8(1990) 838. 7 T. Tiedje, K. M. Colbow, D. Rogers, Z. Fu and W. Eberhardt, J. Vac. Sci. Technol. B, 7(1989) 837. 8 T. Tiedje, P. C. Wong, K. A. R. Mitchell, W. Eberhardt, Z. Fu and D. Sondericker, Solid State Commun., 70 (1989) 355. 9 K. Yamasakai, M. Yoshida and T. Sugano, Jpn. J. Appl. Phys., 18(1979) 113.
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