Removal of elemental arsenic by water from the GaAs surface

Vacuum/volume 41/numbers 4-6/pages 821 to 823/1990

0042-207X/9053.00 + .00 © 1990 Pergamon Press plc

Printed in Great Britain

Removal of elemental arsenic by w a t e r from the GaAs surface V I Belyi, N P Sysoeva

and

B A K o l e s o v , Institute of Inorganic Chemistry, Siberian Branch of the USSR

Academy of Sciences, Novosibirsk, USSR

In this work we investigated directly the behaviour of elemental arsenic on the GaAs surface by Raman spectroscopy and ellipsometry after de-ionized water treatment. The water is saturated with oxygen and nitrogen and treatment was done with and without uv illumination. The slices of GaAs were oxidized at the temperature 4 0 0 - 5 5 0 ° C in air. Under these conditions the native oxide film consists of two layers. The top layer is composed of Ga20 3 alone. The layer adjacent to the GaAs surface is a mixture of Ga203 and polycrystalline As. As a result we find that the water saturated by oxygen removes As, but it does not dissolve Ga20 3. The process accelerates under uv illumination and is completely suppressed for water saturated by nitrogen. The mechanism of the arsenic dissolution is disscussed in detail.

1. Introduction The phenomenon which is known as Fermi level pinning on the surface of GaAs prevents us solving many technological problems on this material such as stability of Schottky barriers, p - n junctions, laser and transistor structures and so on. The nature of surface states which are responsible for the Fermi level pinning is the controversial question up to date. The most correct hypothesis, in our opinion, is that the elemental arsenic which is the constituent part of the native oxide on GaAs plays an important role in this phenomenonL According to this hypothesis different procedures were developed in the last decade, all of them based on the removal of the elemental arsenic with treatments in the solutions of Ru3+, 2 sulfides3'4 or de-ionized water 5. In this work we investigated directly the behaviour of elemental arsenic on the GaAs surface by g a m a n scattering (RS) spectroscopy and multiple incidence angle (MAI) ellipsometry after water treatment. The water was saturated with oxygen or nitrogen, in the dark or under uv lamp illumination.

2. Preparation of samples and experimental arrangement The conditions of the GaAs processing were chosen to enable detection of the elemental As by RS. The GaAs n-type slices Ne = 2 × 10 ~8cm -3 have been etched in the mixture H2SO 4 : HaO 2 : H20 (3 : 1 : 1) and then oxidized in air at 450, 500 and 550°C for 15 min. After RS and MAI measurements they were placed for 30-40 s in a 10% NH4OH solution and the measurements were repeated. Then the slices have been placed in the quartz glass, with the de-ionized water saturated by the flow of air or nitrogen. During processing half of the experiments were performed in the dark and the other half under uv lamp illumination. After each processing, the RS spectra investigations and MAI ellipsometric measurements were repeated.

The measurements are carried out with a LEF-3M-type ellipsometer in a polarizer-compensator-sample-analyser arrangement. The H e - N e laser (2 = 632.8 nm) was taken as the light source. The ellipsometric parameters A and ~ are measured in the two zones at 7 - 9 different angles of incidence (o0 in the range 50-80 ° . As distinct from ellipsometry of anodic oxides, for which the one-layer model is quite acceptable, the thickness and optical parameters of the thermal oxides have been obtained using a two-layer model. The choice of this model is based on the fact that the part of the native oxide adjacent to the GaAs surface is highly enriched by elemental arsenic and absorbs the light. However, the upper part of the native oxide is free of elemental As and consists of GaaO 3 only which is transparent to the light (Figure 1). The optical parameters of two layers (Figure 1) were determined from ellipsometric measurements. A common approach for doing this is the method, which consists of minimizing the

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n2 ~ k2, d2

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\\~1

/_/.//////////A Figure I. Two-layer model of the thermal oxide on the GaAs surface: d~ and d2 are the thicknesses: n~ and n2 are refractive indices; kj and k2 are absorpiton indices of the GaAs-adjacent and top layers of the oxide films accordingly. 821

V I Belyi et al: Removal of arsenic by water from GaAs function which accounts for differences between the measured A and ~ and those calculated for a certain model. In our case this function was the weighted sum of squares: S= N

,= \

-~__

/

(O)

1" - O m i n 3

1" " 3 0 r a i n

-~ZL,

Here, ¢im, A i m a r e measured and $i¢, Aic are calculated ellipsometric parameters for the angle of incidence (Poi; (5~,and 6a, are standard deviations of ~'im and Aim; N is the number of angle ~o0. Its minimum was found by the method developed in the work 6. In the calculations we used the following optical constants of GaAs: refractive index nGaAs= 3.83 and absorption index kGaAs=0.187. The optical constants of the elemental arsenic were obtained by ellipsometry on the fresh surface of the arsenic monocrystal after cleavage and found equal to has = 3.5 and kAs = 3.2. The RS spectra were taken by DFS-24 spectrometer with use of the argon laser (2 = 488.0 nm) as a light source. The polycrystalline arsenic was detected as intense TO (201 cm -1) and LO (258 c m - ~) bands in the Raman spectra together with TO (268cm - l ) and LO (292cm -1) modes for crystalline GaAs (Figure 2).

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-Omin 3

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3 - LO(Ascr.) 4 -- TO(A$cr.)

3. Results and discussions 2

The samples of the native oxide layers were analysed by means of an electroactive graphite paste electrode as it was done previously for the native oxides on InSb 8 and InAs 9. It was found that the thermal oxide on GaAs was consisted in our case of the polycrystalline G a 2 0 3 and As only. The consideration of the ternary phase diagram for G a - A s - O system ~° together with the results of the analysis support the model given in Figure 1. The optical parameters of the thermal native oxides are given in Table 1. As one can see, the refractive indices of the oxide layer adjacent to the GaAs are much higher then for the top layer (n I >>n2). In our calculations k 2 was accepted as equal to zero. The adsorption indices follow the sequence kAs > kl >> kc~As. It means that the layer which is adjacent to the GaAs is enriched by the elemental arsenic. The RS spectra are shown in Figures 2(a-c). Figure 2(a) shows that the processing of the native oxides in the de-ionized water during one hour with uv illumination (and without) does not change the amount of elemental arsenic in the film. It means that the upper part of the native oxide film protects the arsenic from dissolution. It is known ~ that gallium hydroxides easily form the soluble methagallates MeGaO2 or their hydrates MeGa(OH)4, where Me = K, Na, N H 4. By analogy it is possible to represent the formation of the gallates from gallium sesquioxide by the

I 300

I 250

I 300

1 200

I I ZSO 200 v ( c m "I)

I 300

I 250

t 200

Figure 2. The Raman spectra of the GaAs oxidized for 15 min at 500°C processed for different times z (min), in the air-saturated de-ionized water: (a) without the etching in 10% NH4OH; (b, c), (~ = 0') after etching in 10% NH4OH for 40 s; (a, b) under uv illumination; (c) without uv illumination.

equations: Ga:O3 + 2NH4OH = 2NH4GaO2 + H20, or

Ga203 + 2NH4OH = 2NH4Ga(OH)4. So to etch off partly the top layer of the native oxide on GaAs the samples were immersed in the 10% NH4OH solution for 30-40 s and then processed in the water. The etching time of the native oxides in the ammonia hydroxide solution was chosen such that the optical parameters did not change at all but the thickness d 2 of the top oxide layer became less. For instance the thickness of the oxide d2 = 6 nm obtained at 450°C after etching became equal to 1.5 nm, its protecting properties were lost, and access of oxygen and water to the arsenic was opened. The RS spectra as well as the ellipsometric measure-

Table 1. Parameters of GaAs native oxide films as obtained by multiple angle of incidence ellipsometry Temperature of oxidation, T (°C)

Time of oxidation, ~ (min)

Surface adjacent Thickness dt (nm)

layer of the oxide film Refractive Absoption index index nI kL

Top layer of the oxide film Thickness Refractive d2 index (nm) /'12

Absorption index k2

450 500 550

15 15 15

5 10 12

2.5 2.6 2.8

6 6 13

0 0 0

*Such low indices of refraction are due to porosity of the films. 822

1.8 2.0 2.3

1.5" 1.6" 1.8

V I Belyi et al: Removal of arsenic by water from GaAs

ments support the conclusion that the arsenic was not removed during the etching (Figures 2(a-c); z = 0). The following processings of the etched samples by the de-ionized water with the air flow passing through it and with uv illumination led to the complete removal of the arsenic from the film. This conclusion is certainly right only up to the level of sensitivity of the RS ( ~ l016 atoms of As cm-2,12 Figure 2(b). The same processings without the illumination lead also to the removal of the arsenic but at a much slower rate. As one can see in Figure 2(c), the time to practically complete removal of the As can last up to one hour, and when the flow of nitrogen is bubbled through the water, the amount of arsenic in the film does not change at all. The next explanations can be given for these experiments. Under the uv illumination, the hydrogen peroxide arises in the water. The origin of the HEO 2 is considered to be the insertion of singlet oxygen atoms into watert3:

in Table 1 for the same temperature we can see that nl ~ nz "~, has and kAs,> k~. This means that the elemental arsenic is removed from native oxide, oxidizing into mszO 3 with subsequent dissolution of it in the water. 4. Conclusions By means of the RS spectroscopy and MAI ellipsometry it was shown that the water dissolved oxygen oxidizes the elemental arsenic and that this process accelerates itself under uv illumination. However, the thickness of the film does not change during the processing in the water. It means that only the elemental arsenic dissolves, the gallium sesquioxide does not. According to ref 5, this processing lowers the surface state density. In this case, our results support the hypothesis that the elemental arsenic is responsible for the surface states and the Fermi level pinning.

O('D) ÷ H20 ---+H20 2. The hydrogen peroxide oxydizes the elemental arsenic to H3AsO3: 2As + 3H20 2 = 2H3AsO 3. The dissolved O2 also oxidizes an arsenic in the film: 4As + 302 + 6H20 = 4H3AsO3, but much more slowly in comparison with the process under uv illumination. When the flow of the inert nitrogen bubbles through the water it diminishes the oxygen content in it to the minimum, and oxidation of the elemental arsenic stops completely. The ellipsometric data also supports the conclusion that after the processing of the native oxides in 10% NH4OH and H 2 0 the elemental arsenic removes itself from the film. For example, for the samples oxidized at 450°C after 60 min of the processing in 10% NH4OH, and then in the water saturated by oxygen and under uv illumination, these parameters for the native oxide layers have been obtained: d~ --- 5 nm, n~ = 1.6, k = 0.8; d2 = 1.5 nm, nz = 1.5, k 2 = 0. Comparing these data with given

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