III-V족 화합물 반도체의 표면보호막 생성에 대한 표면화학적 연구

정 찬 화,  S. I. Yi*, W. Henry Weinberg*

성균관대학교 화학공학과, 경기도 수원시 장안구 천천동 300

*Dept. of Chemical Engineering, Univ. of California, Santa Barbara, CA 93106, U.S.A.

Long-Term Surface Passivation of III-V Compound Semiconductor

Chan-Hwa Chung, S. I. Yi*, and W. Henry Weinberg*

Dept. of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea

*Dept. of Chemical Engineering, Univ. of California, Santa Barbara, CA 93106, U.S.A.

INTRODUCTION

Sulfidization of GaAs(001) surfaces has been observed to passivate the surface electronically by removing the surface pinning states that detrimentally affect the performance of GaAs-based devices[1]. Many studies have reported sulfidization of GaAs using Na2S9H2O and (NH4)2S□ in wet chemical treatments, but they also reported poor reproducibility, degradation in ambient air, and contamination[2,3]. Dry passivation of GaAs in vacuum, using H2S as the sulfur source, was an alternative that improved purity and reproducibility, but again the passivated GaAs suffered from long-term degradation by oxidation in ambient air[4]. Long-term surface passivation was, hence, provided by growth of an insulating GaS film achieved by the deposition of [t-butyl(GaS)]4 using metallorganic chemical vapor deposition (MOCVD)[5].

We report here the surface chemistry of GaS film evolution in the dry passivation processes using H2S and [t-butyl(GaS)]4 molecule as the sulfur source.

EXPERIMENTAL METHODS

All experiments were performed in a turbopumped stainless-steel ultrahigh-vacuum (UHV) system with a base pressure of 〈1×10-10Torr. The chamber contains a  HREELS (LK-2000-14-R), an AES (Perkin-Elmer PHI 3017), a four-grid and rear-view LEED optics, and a differentially pumped quadrupole mass spectrometer (UTI-100C) equipped with a cryoshroud for thermal desorption measurements. The HREEL spectrometer was aligned for a dipolar scattering geometry where an electron beam of impact energy 7eV strikes the sample surface at an incident angle of 60°with respect to the surface normal, while keeping the analyzer in the specular direction. For a typical TPD measurement, the sample was positioned less than 10mm from the opening in the quadrupole mass spectrometer (QMS) shroud. A thermal desorption spectrum was collected by ramping the temperature at 1 K/sec.

The experiments used n-type GaAs(001) grown by molecular beam epitaxy (MBE), which were doped with 1017-1018 cm-3 of silicon. These sample were As capped in MBE system, and transferred and mounted to a molybdenum sample holder by indium bonding. Once returned to vacuum, the samples were cleaned in situ by Ar ion sputtering (500eV, ∼1.5□A/cm2, 15min), removing the surface contaminants and oxides while maintaining most of the As capping. The As-rich (1×1) surface is obtained by annealing to 650K, followed by the (2×4) surface after annealing to 700K. Clean Ga-rich (4×2) surfaces were prepared repeatedly on the sample by Ar ion sputtering followed by annealing to 800K. Cleanliness of surfaces prepared by this method was verified by HREELS and AES before H2S and [t-butyl(GaS)]4 exposures.

Hydrogen sulfide was dosed through a directional 5□m pinhole gas doser. The exposures were measured in Torr·sec, which is the product of the backing pressure in the gas reservoir and the exposure time. It was found in a previous calibration with acetylene that 80 Torr·s of acetylene with this doser is roughly equivalent 1 L (1 L = 1 Langmuir ≡ 1×10-6 Torr·s) of background pressure dosing. Using the relationship that the flux is proportional to (mass)-0.5, 70 Torr·s of H2S is equal to 1 L. For [t-butyl(GaS)]4 exposures, we used an UHV pinhole doser cell containing the [t-butyl(GaS)]4, which was docked onto a magnetically coupled linear feedthrough and pumped down to 2×10-8 Torr in the load-lock chamber. Then the doser cell, heated to 100℃ for sublimation of the [t-butyl(GaS)]4, was introduced into the UHV chamber and positioned approximately 3cm from the GaAs crystal. The chamber pressure remained below 5×10-10 Torr, resulting in a deposition rate of 1 ML/min.

RESULTS AND DISCUSSION

The HREEL spectra were recorded after varying exposures of H2S to GaAs(001)-(4×2) surface at 100K, and the results are presented in Fig. 1. At lower exposures, the H2S scissoring mode at 1170cm-1 due to molecular adsorption and Ga―H mode at 1870cm-1 are not observed, whereas the S―H stretching mode at 2550cm-1 and the As―H stretching mode at 2110cm-1 are evident. These results suggest that initial adsorption of H2S is dissociative even at 100K, producing HS― and H― species, the latter of which are bonded only to As atoms, and the molecular adsorption of H2S occurs only after saturation of the dissociative adsorption.

Upon annealing to 700K, the thermal desorption of H2S, AsH3, and H2 is observed, and the HREELS peaks due to hydrogenic modes on the surface also disappeared. In thermal desorption spectra shown in Fig. 2, three desorption features are evident: these are labeled the □1, □2 (lower coverage states), and □ (a higher coverage state). The □ state populates in the high-exposure regime as the additional loss peak appears at 1170cm-1 in HREELS, suggesting this state is due to weakly bound molecular H2S. The origins of the high-temperature H2S desorption peaks (□1, □2) are related to the recombination reaction of the products of H2S dissociation. In our TPD experiments, we observed the desorption of neither AsS□, As2, nor S2 upon annealing to 700K. When the sample was heated to temperatures above 750K, only the desorption of Ga2S (m/e=170) was detected. It is generally understood that Ga―S bonds are dominant after annealing[6,7].

The desorption of H2S and arsine (AsH3) upon thermal annealing to 700K frees sites on the surface for further sulfur chemisorption. These vacant sites were monitored by atomic hydrogen postexposure. As shown in Fig. 3(a), only a Ga―H stretching mode at 1870cm-1 appeared. This occurred without any apparent hydrogenation of As or sulfur, suggesting that only the Ga sites are available for hydrogenation chemisorption after the first exposure of H2S at 100K and annealing to 700K. To monitor the details of the surface states, TPD spectra and AES were recorded during the cycling treatment: one cycle consists of a dose of 3 L H2S at 100K and heating to 700K. In TPD spectra, both the □1 and □2 states disappear during the repeated cycles, whereas the □ state due to molecularly adsorbed H2S increases, and the depletion of As is observed in the AES measurements (cf. Fig. 4). It was also found by atomic hydrogen postexposure that the repeated cycling completely passivated the surface by sulfur atoms., i.e., after four cycles, atomic hydrogen exposure results in sulfur hydrogenation, is shown in Fig. 3(b) by the appearance of S―H stretching mode at 2550cm-1 and H2S scissoring mode at 1170cm-1 without the appearance of Ga―H or As―H modes. This result strongly suggests that complete passivation is accomplished after the cycling treatment, and this leads to a well-defined (2×1) LEED pattern. The thickness of sulfur layer, however, is still less than 2 ML even after 7 cycles, which is not sufficiently thick enough to passivate the surface during the long period of time.

The thicker films of GaS were grown by the exposure of [t-butyl(GaS)]4 molecule at 650K on the fully passivated surface after cycling treatment of H2S. After the 480 ML exposure of [t-butyl(GaS)]4 molecule, the resulting surface has a (1×1) structure that may be interpreted as an unreconstructed surface of a cubic phase GaS film. The lattice constant deduced from this (1×1) LEED pattern is 4%±2% smaller than that observed for the (2×1), (4×2), and (2×4) reconstructions, in accordance with the 0.236nm Ga―S bond length in the gas-phase [t-butyl(GaS)]4 molecule[8]. The uncertainty with this measurement is based on the distribution of the (4×2), (2×4), and (2×1) lattice constants measured from the LEED patterns. The AES spectra showed a carbon-free surface with a S to Ga peak-intensity ratio of ∼8 which is identical to that observed for MOCVD-grown 5000Å thick GaS film on GaAs(001) at TriQuint, Inc.

CONCLUSIONS

At a surface temperature of 100K, dissociative adsorption of H2S occurs to form As―H bonds on the both GaAs(001)-(2×4) and (4×2) surfaces via a molecularly adsorbed precursor. In the thermal desorption of H2S, the □1 and □2 states are resolved and assigned to the recombinative desorption of mercaptan (HS―) species which are bound to submonolayer surface. After several repeated cycles of H2S exposure at 100K followed by annealing to 700K, saturation sulfide coverage is achieved. Only sulfur atoms were available for post-hydrogenation on this sulfide layer. Formation of a clean and thick GaS film from deposition of [t-butyl(GaS)]4 at a surface temperature of 650K was verified with AES. □-hydride elimination of the t-butyl ligands occurred at a surface temperature of 650K that released both iso-butylene and hydrogen. As a multilayer of GaS was deposited, the LEED pattern changed to (1×1) of which the lattice constant is 4%±2% smaller than that observed for the (2×1), (4×2), and (2×4) reconstructions.

REFERENCES

[1] C.R. Zeisse, J.J. Messick, and D.L. Lile, J. Vac. Sci. Technol. 14, 957 (1977).

[2] M.S. Carpenter, M.R. Melloch, and T.E. Dungan, Appl. Phys. Lett. 53, 66 (1988).

[3] C.J. Sandroff, M.S. Hedge, and C.C. Chang, J. Vac. Sci. Technol. B 7, 841 (1989).

[4] H. Kawanishi, Y. Sugimoto, and K. Akita, J. Appl. Phys. 70, 805 (1991).

[5] P.P. Jenkins, A.N. MacInnes, M.B. Power, and A.R. Barron, Science 263, 1751 (1994).

[6] C.-H. Chung, S.I. Yi, and W.H. Weinberg, Appl. Phys. Lett. 69, 3369 (1996).

[7] C.-H. Chung, S.I. Yi, and W.H. Weinberg, J. Vac. Sci. Technol. A 15, 1163 (1997).

[8] S.I. Yi, C.-H. Chung, and W.H. Weinberg, J. Vac. Sci. Technol. 16, 1650 (1998).

Fig. 1. HREEL spectra of adsorbed H2S on GaAs(001)-(4×2) at 100K.	Fig. 2. Thermal desorption spectra of H2S and H2 (inset) monitored during the cycling treatment. The each cycle ia an exposure of 3L H2S at 100K followed by heating to 700K.

Fig. 3. HREEL spectra of postexposure of 5L atomic hydrogen to the GaAs(001) surface (a) that was annealed at 700K after 40L exposure of H2S at 100K, and (b) that was treated by four cycles of 40L H2S at 100K and heated to 700K.	Fig. 4. The dependence of the S/Ga and As/Ga Auger peak ratios on the number of cycles. Each cycle is an exposure of 3L H2S at 100K followed by heating to 700K.