III-V족 화합물반도체의 산화막 형성 기구에 관한 연구

정찬화(a), W. J. Mitchell*, S. I. Yi, Evelyn L. Hu*, W. Henry Weinberg

Department of Chemical Engineering, University of California at Santa Barbara

Center for Quantized Electronic Structures (QUEST), UCSB*

The Oxidation Mechanism of III-V Compound Semiconductor Films

Chan-Hwa Chung(a), W. J. Mitchell*, S. I. Yi, Evelyn L. Hu*, W. Henry Weinberg

Department of Chemical Engineering, University of California at Santa Barbara

Center for Quantized Electronic Structures (QUEST), UCSB*

INTRODUCTION

The preparation of stable and insulating oxides on III-V compound semiconductor is a subject of considerable importance due to applications in device manufacturing. The unique properties of GaAs and AlAs oxide films formed by wet oxidation have recently attracted much attention. It is well known that AlAs oxidizes much faster than all other group III-As materials (e.g. GaAs or even AlxGa1-xAs with x<1), resulting in a "selective" oxidation which effectively terminates at the AlAs/III-As interface. Such oxides may provide a viable GaAs-based MOS or GaAs-on-insulator technology. Moreover, vertical cavity surface emitting lasers which utilize incompletely oxidized AlAs epilayers for both injected current and lasing optical-mode confinement exhibit significantly lower threshold currents at which laser action begins.

Despite the many technological applications of AlAs and GaAs oxides, little is known at a fundamental level about the evolution of the oxide, an objective which is best achieved under a well-controlled UHV environment in which the many steps of the oxidant-surface interaction may be unambiguously isolated and understood. In order to address this concern, we have investigated, under ultrahigh vacuum conditions, the mechanism of water adsorption and dissociation and subsequent growth of thin oxide films on the GaAs(001) and AlAs(001) surfaces using AES, TPD, LEED, and HREELS (High-Resolution Electron Energy Loss Spectroscopy). These methods allow us to determine a more complete mechanism of the initial stages of oxidation and provide insight into the key parameters which affect the "quality" of the final oxide.

EXPERIMENTAL DETAILS

All experiments were conducted in a turbopumped stainless steel UHV system with a base pressure of 1×10-10 Torr. Briefly, the chamber contains a commercial HREEL spectrometer (LK Technologies, LK-2000-14-R), a single-pass cylindrical mirror analyzer (Perkin-Elmer, PHI 3017) for AES, four-grid rear-view LEED optics, and a differentially pumped quadrupole mass spectrometer (UTI-100C) equipped with a cryo-shroud for TPD.

The experiments used n-type GaAs(001) and 50□ thick (undoped) AlAs(001) substrates grown by molecular beam epitaxy (MBE), which were vacuum-transferred or As capped to prevent oxidation by air upon exposure to the atmosphere.

(a) Current address: Dept. of Chemical Engineering, Sung Kyun Kwan University, Suwon, Korea

RESULTS AND DISCUSSION

A. Initial Stages of Oxidation [1,2]

After 0.2 L H2O exposure at 100 K [see Fig. 1(a)], we observed the H2O scissoring mode at 1630 cm-1 and the O―H stretching mode at 3670 cm-1. The absence of Ga―H or As―H stretching modes (at approximately 1800 cm-1 and 2100 cm-1, respectively) and the presence of the molecular H2O mode at 1630 cm-1 suggest that water adsorbs molecularly on this GaAs surface at 100 K.

Annealing the H2O exposed surface results in the dissociation of water. This is evident from HREEL spectrum shown in Figs. 1(b) and 1(c). At 200 K, the partial dissociation of water has been monitored; the As―H stretching mode at 2110 cm-1 has appeared and there is a buildup of adsorbed hydroxyls (an increase in the 3670 cm-1 peak), but the molecular H2O mode at 1630 cm-1 still exists. Upon further annealing to 310 K, the H2O mode at 1630 cm-1 has disappeared while As―H stretching mode at 2110 cm-1 and the O―H stretching mode at 3670 cm-1 has been retained. Furthermore, no evidence of Ga―H bond formation was found, implying that the hydrogen produced by the dissociation of water upon annealing preferentially adsorbs at As sites.

Fig. 1.  Sequence of HREEL spectra of adsorbed H2O on GaAs(001)-(4x2): (a) after 0.2L exposure at 100K, (b) after annealing to 200K, (c) 310K, (d) 550K.

To investigate the details of this behavior, thermal desorption spectra were measured after various exposures of the GaAs(001)-(4x2) surface to D2O at 100 K. In Fig.2, clear evidence for D2O dissociation on this GaAs surface is provide by the D2 desorption peaks at temperatures between 600 and 700 K, which are associated with the thermal decomposition (dehydrogenation) of hydroxyls (―OD). Further evidence of D2O dissociation is provided by the thermal desorption spectra of D2O which was obtained after various exposures of D2O at 100 K. Five desorption features are evident; these are labeled □1, □2, □, □1, and □2 in Fig. 3. The thermal desorption peaks labeled □1 and □2, correlating with a depletion of the vibrational mode (1630 cm-1) of molecularly adsorbed water in the HREEL spectrum above 310 K [see Fig. 1(c)] are assigned to the rehydrogenation of hydroxyls which has been produced by dissociation of water below 200 K. Since As―H mode has been observed in HREEL spectra only at temperatures between 200 and 500 K, the □1 peak (at 300-500 K) is assigned to recombinative desorption of hydroxyl and hydrogen [Ga―OD+As―D→D2O(g)+Ga+As], where as the □1 peak (at 600-700 K) is thought to result from the disproportionation of hydroxyls [2 Ga―OD → D2O(g) + Ga]. Although these TPD data provide clear evidence of D2O dissociation, some fraction of molecularly adsorbed D2O remains intact and desorbs at 180 K, as judged by the □1 peak (Fig. 3).

Fig. 2.  TPD spectra of D2 (m/e=4) from GaAs(001)-(4x2) as a function of D2O exposure.

We have also observed the similar behaviors of water on AlAs surface as shown in the thermal desorption spectra (Fig. 4) recorded after exposure of the AlAs(001) surface to 3 L D2O at 100 K. The D2 desorption peak at 650 K is assigned to the thermal decomposition of aluminium hydroxide (Al―OD) produced by the partial dissociation of D2O below 200 K. The D2O desorption peak at 550 K is assigned to the recombinative desorption of D2O from aluminium hydroxide via either disproportionation or rehydrogenation. The desorption of arsine (AsD3) depletes the surface arsenic. This suggests that Al―O (and Ga―O in case of GaAs surface) bonds predominantly exist after annealing. The oxidation mechanism of AlAs deduced from the data is summarized in Table I. Note that the overlap of dissociation and desorption from the molecularly adsorbed state upon annealing implies that high concentrations of water remain on the surface as dissociation proceeds.

Fig. 4.  D2O, D2, and AsD3 thermal desorption spectra that result following a 3 L exposure of D2O to AlAs(001) surface at 100 K.

Fig. 3.  D2O thermal desorption spectra following the indicated exposures of D2O to the GaAs(001)-(4x2) surface at 100 K.

B. Evolution of Oxide

The rehydrogenation of hydroxyls, the thermal desorption of arsine, and the intact water desorption, which occur upon annealing to 750 K, free sites on the surface for further oxidation. Hence, we applied a cycling treatment, adsorption of water at 100 K followed by annealing to 750 K, to effectively oxidize the surface. The AES data showed  that Al―O bonds (or Ga―O in case of GaAs)  are  predominantly exist  after several cycles of adsorption/annealing treatments. The AES depth profiles  recorded from the oxidized AlAs also show  that metallic Al and As can diffuse  through  the oxide layer to the surface  (where they can react with D2O)

Table I. Summary of the wet oxidation mechanism of AlAs(001) surface.

□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□

100K

D2O(g) + AlAs	―→    AlAs---D2O(a)	Molecular adsorption

150-180K

―→       D2O(g)	Molecular desorption

∼150K

AlAs---D2O(a)	―→  Al―OD + As―D	Partial dissociation

∼150K

―→  Al―O + 2 As―D	Total dissociation

300-650K

As―D	―→       AsD3(g)	Reaction-limited desorption

160-300K

As―Dx	―→     AsD3(g) (x>1)	Desorption-limited desorption

350-700K

2 Al―OD	―→  Al―O + Al + D2O(g) OD disproportionation

350-700K

Al―OD + As―D	―→    AlAs + D2O(g)	OD rehydrogenation

500-700K

2 Al―OD	―→   2 Al―O + D2(g)	OD dehydrogenation

□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□

which explicitly reminds us that cationic diffusion also plays a part in the growth mechanism of multilayer oxide.

C. Electron Beam Enhanced Oxidation [3]

We also considered the effect of electron beam exposure on the AlAs wet oxidation reaction. The AlAs surface was first exposed to 16 L of H2O at 100 K and annealed to 300 K to produce a hydroxyl-rich surface. This partially oxidized surface was then exposed to a 3 keV electron beam (∼3□A current with a beam diameter on the order of 100□m) for different times, whereupon the peak-to-peak intensity ratios of the reduced Al (64eV) to the oxidized Al (54eV) Auger transitions were recorded [see Fig. 5]. It is clear from Fig. 6 that the surface Al undergoes further oxidation after a beam exposure. However, if the surface was annealed to temperatures at which the hydroxide (Al―OH) phase is depleted, electron beam enhanced oxidation was not observed, indicating that the presence of surface hydroxides is critical to the effect. This result may have implications for an in situ electron beam lithography application.

Fig. 5.  Peak-to-Peak Al(64eV)/Al3+(54eV) Auger intensity ratio as a function of exposure time to a 3 keV electron beam at a surface temperature of 300 K.

REFERENCES

1. C.-H. Chung, S.I. Yi, and W.H. Weinberg, J. Vac. Sci. Technol. A, 16(3) 1785 (1998).

2. W.J. Mitchell, C.-H. Chung, E. Hu, W.H. Weinberg, Surface Science, 384, 81 (1997).

3. S.I. Yi, W.J. Mitchell, C.-H. Chung, E.L. Hu, and W.H. Weinberg, J. Vac. Sci. Technol. B, 16(4), 2199 (1998).