The effect of external excitation on the sulphur passivation of GaAs surfaces

Thin Solid Films, 215

( 1992) I79 183

179

The effect of external excitation on the sulphur passivation surfaces A. S. Weling, Semiconductor (Received

K. K. Kamath

Devices Research

August

Lahorrrtory,

I, 1991; accepted

February

of GaAs

and P. R. Vaya Centre fbr Systems

and Devices, Indian Institute

qf Technology,

Madras

600036 (India)

21, 1992)

Abstract A simple chemical treatment using alkaline sulphides as an effective and robust means of surface passivation of GaAs has been widely studied. A new method of external excitation in the form of light and heat during sulphorization is investigated in the present study. This method of passivation is found to be more effective. The stoichiometry and the electronic properties of the passivated GaAs surfaces have been characterized by X-ray photoelectron spectroscopy, photoluminescence and I- V measurements on Schottky contacts. The recently proposed advanced unified defect model is found to explain the apparently contradictory results of decrease in surface state density accompanied by an increase in the surface band bending.

1. Introduction

2. Experimental

The use of alkaline sulphide solutions as a convenient and robust means of chemical passivation of GaAs surfaces has gained wide attention [ 11. The coating of Na,S .9H,O and (NH,),S has been reported to yield a remarkable decrease in surface recombination velocities [ 11, an increase in photoluminescence (PL) efficiency [2], an increase in the current gain of heterojunction bipolar transistors (HBTs) at low emitter currents [3, 41, an improvement in the performance of solar cells [5] and p-n junctions [6], and greater sensitivity of Schottky barriers to metal work functions [7]. However, the study of the exact mechanism of sulphur incorporation needs more attention in the light of some of the more recent results which reveal the limitation of this method in obtaining a completely unpinned surface [8, 93. We have carried out an extensive study of the effectiveness of external excitation on the chemical treatment using alkaline sulphides, primarily aqueous solutions of sodium sulphide nanohydrate. X-ray photoelectron spectroscopy (XPS) was used to study the change in stoichiometry of the surface and the sulphur incorporation mechanism. The effect of this change on the surface band bending and surface recombination velocity has been observed by PL spectroscopy. A rough estimate of the change in surface band bending has been made using J- V measurements on aluminium Schottky contacts. The recently developed advanced unified defect model (AUDM) [8, 91 is found to give a good correlation between all these results.

2.1. Surface passivation The n-GaAs wafers were degreased and treated with a strong oxide stripping etch of a 3: 1: 1 solution of H,S0,:H,0,:H20, followed by rinsing in hot HCl. The sulphide treatments were carried out on (100) surfaces that had been freshly etched in a 1:8:500 solution of H,S0,:H,0,:H20, this mild etching treatment being used as a reference to estimate the effect of chemical passivation. The wafers were soaked in solutions of Na,S of varied concentration (0.5-l M) for around 10 min. Experiments were carried out to study the effect of external excitation in the form of heat and illumination on the reactant species during the course of passivation. The sulphide solution was kept in a water bath at various temperatures and also with and without illumination from a white light source. Evidence of the greater effectiveness of such excitation was best reflected in PL studies. The usual method of sulphorization produced surfaces which exhibited poor adherence to metal contacts and hence were highly unsuitable for forming metal contacts. On the contrary, a deionized (DI) water rinse following the sulphorization reduces the effectiveness of the passivation. However, sulphorization in the presence of external excitation is found to be very effective. The PL intensity remains almost the same even after a DI water rinse (see Fig. 4). This gave Schottky contacts with aluminium which did not exhibit any degradation in their characteristics even after six weeks.

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180

A. S. D2"lhT,g et al. / S passil:ation o/' GaAs smjaces

2.2. X-ray photoelectron spectroscopy analysis of the surfaces 2.2.1. Observations Two sets of magnesium XPS analyses have been performed, using a VG Scientific mark 2 system, on four differently treated n-GaAs (100) samples. Eight regions of the photoelectron spectrum were scanned, each with a range of 25 eV binding energy and step size of 0.05 eV. The C ls core level recorded at 284.8 eV in the spectra of all the samples was used as a reference for calibration of the other spectral regions. The results obtained can be summarized as follows. (i) As 3d core level spectra (Fig. 1) gave a wealth of information regarding the chemical composition of the surface. The sample treated with only the mild etch had a strong As203 peak at 44.1 eV which dominated the arsenic metal peak at 41 eV (Fig. l(a)). A strong new peak at 42.5 eV in the sluphide-treated samples was attributed to an As~ Sy sulphide phase (Fig. l(b)). The sulphide-to-oxide ratio (calculated from areas under the corresponding peaks) varied from about 2.5 for the room temperature treatment without illumination to about 3.2 for the treatment at 80 C and white light illumination. (ii) Ga 3d core level spectra revealed similar results with a decrease in the Ga203 phase after sulphide treatment (Fig. 2). The spectra were resolved into two peaks: gallium metal at 19.0 eV and oxide at 20.0 eV. The increase in Ga,,~t~:Gao×~ ratio compared with the untreated sample varied from 2.5- to 3.5-fold. (iii) The 2p spectra of sulphur were more helpful in characterizing the sulphur incorporation mechanism. The samples passivated under external excitation exhibited stronger peaks at 160.5 eV (Fig. 3(b)) than those passivated without external excitation (Fig. 3(a)), which indicates the presence of more metal sulphide [10] on

•

2X

,?,! .....

15.00

..... , .........

17.00

19.00

21.00

Energy

23,00

,

25.00

(eV)

Fig. 2. XPS spectra of Ga 3d core level (a) without passivation and (b) with passivation.

.~ (b)

x

155.00

1 60.00

165.00

170.00

Energy leVI

Fig. 3. XPS spectra of S 2p core level, passivated (a) without and (b) with heat and illumination.

surfaces passivated with external excitation. The peak at 165.0 eV corresponds to elemental sulphur. (iv) O ls core level spectra at around 531.5 eV were used to estimate the relative oxide content in the surface layers of each of the above samples. Oxide concentrations were observed to fall to about 40% in samples passivated without heat and illumination and to about 30% in samples passivated with heat and illumination. (v) The spectra of arsenic and gallium were used to estimate the relative concentrations of gallium and arsenic species at the surface. The approximate formula used for calculation of the fractional coverages of arsenic and gallium based on the uniform layer model

(b)

35.00

(b)

"~

&O.O0 Energy

45.00 (eY)

50.00

Fig. 1. XPS spectra of As 3d core level (a) without passivation and (b) with passivation.

[1o] is IG~/IA ~=

S(;~,rca,nojo,,(KE(~,) I/2/SA~rA~nAJA~( KEAs )1/-, (1)

A. S. Weling et al. / S passivation of GaAs sur~wes

where S is the spectrometer transmission function, r is the photoelectron cross-section, n is the atomic density, l is the photoelectron mean free path, and KE is the kinetic energy of the emitted photoelectron which is the difference between the X-ray photon energy (1253.6 eV in this case) and the binding energy of the core level under consideration. Data for S, r and l are available with the instrument reference guide. The atomic density ratio n(Asvot,i):n(Gaxot,i) as calculated using the above formula was found to be in the neighbourhood of 1.25 for a freshly etched untreated sample. The maximum decrease in this ratio is seen in the sample passivated with heat and illumination. In general, however, there is slight gallium enrichment of the surface with passivation which decreases with exposure to air. 2.2.2. Discussion The mechanism of sulphur passivation of the GaAs surface is more or less clear from the examination of the stoichiometry of the surface. The reduction in oxide content of the surface from 3d spectra (Figs. 1 and 2) confirms the hypothesis that the alkaline nature of the sulphide prevents initial oxidation of the surface. The appearance of an As2S3 phase together with the presence of an elemental sulphur phase indicates that the surface is terminated with A s - S bonds as well as S - S network [11]. The most important result is the marked reduction in the ratio n(Asvot~0:n(Gavota0, which is a definite indication of reduction in the arsenic pile-up at the surface. 2.3. Electronic properties of the passivated surface The effect of the reduction in arsenic pile-up at the surface due to passivation is to change the density of surface states, which consequently affects the surface band bending and surface recombination velocity. These effects were studied using PL and J - V techniques. 2.3. I. Effect on photoluminescence spectra PL has been extensively used to characterize the non-radiative surface recombination in GaAs both at room temperature and at low temperatures. Sandroff and coworkers have reported an enormous increase in the PL yield with the application of Na2S • 9 H 2 0 films by spin coating [2]. We have carried out the room temperature PL study not explicitly to measure the surface recombination velocity but simply to verify qualitatively the change in the PL yield with passivation. The 488 nm line of an argon ion laser was used as the excitation source for the PL and the emission spectra were scanned using a SPEX spectrometer. PL spectra obtained are shown in Fig. 4. Samples analysed were of three major types: those pre-treated with the mild etch, those passivated with sodium sulphide in the

2.20E 03

i

181 ,

i

i

° ~

O.OOf O0 8000.00

8500,00 WavelemJth

9000.00

(~)

Fig. 4. PL spectra from an unpassivated surface (spectrum a), a surface passivated without heat and illumination (spectrum b) and a surface passivated with heat and illumination (spectrum c).

dark and at room temperature, and those undergoing varying degrees of heat and illumination. The best results were obtained with a temperature of 70-80 °C and white light illumination (200 W source kept at a distance of 30 cm). The increase in PL yield has been attributed to two different mechanisms. First, the passivation, if it reduces the surface state density at all, gives rise to reduced non-radiative surface recombination. Secondly, the increase in surface band bending with sulphide passivation gives rise to an increased field which repels the electrons from the surface [12]. Since both of these mechanisms work simultaneously it is difficult to isolate the effect of reduction in surface recombination velocity on the PL yield. 2.3.2. J - V measurements Although PL results suggest an increase in surface band bending together with a decrease in non-radiative surface recombination, it is essential to quantify these changes. J - V measurements on A1-GaAs Schottky contacts were used to obtain rough estimates of the change in barrier height and the reduction in density of surface states. The J - V characteristics obtained for the three cases, unpassivated, passivated without heat and illumination and passivated with heat and illumination, are given in Fig. 5. This also shows the straight line fit to the part of the curve with the best ideality (maximum slope). Since the overall ideality factors for these contacts were quite large (1.5-1.95) and the region of unit ideality factor was quite obscured, we used a numerical method of isolating the thermionic emission and the recombination current components. The effects of series resistance R s and shunt leakage resistance RsH also

182

A. S. Weling et al. / S passivation ~] GaAs sur/iwes

Q

-S

-10 z J

-15

-20

0

"

0-2

0.8

0.4

1.0

1.2

V (votts)

Fig. 5. J V characteristics of Schottky contacts on three samples: spectrum a, unpassivated: spectrum b, passivated without external excitation: spectrum c, passivated with external excitation.

were taken care of. The overall equation used was [13] I = Is{exp[q(V

-

1}

- IRs)/2kT]

-

- IRs)/kT]

+ IR{exp[q(V

+ (V - IRs)/Rsn

1}

Here, an interesting result is encountered. The sulphide treatment of the surface results in a decrease in the thermionic component of the current, indicating an increase in the barrier height, together with a decrease in the recombination component, indicating reduction in surface recombination velocity. As explained earlier, the PL results also support these results. This directly contradicts the Bardeen model, wherein a decrease in density of surface states should result in the surface Fermi level moving towards the Mort limit [13], giving a reduction in surface band bending. The recently developed A U D M [8, 9] explains satisfactorily the pair of apparently contradictory results obtained. However, the improvement in the overall ideality factor with the sulphide treatment indicates a larger reduction in the surface recombination velocity compared with the reduction in density of surface states. This gives rise to the idea of dissociating the density of surface states from the surface recombination velocity. This will be explained by an argument based on the midgap position of the surface states and the charge state of the deep states in the following section.

(2)

where the first part is the thermionic current component, the second part is the so-called 2 k T recombination component and the third part is the leakage component. The barrier heights obtained are given in Table 1 together with those obtained by the extrapolation of the best ideality factor part of the ln(J) vs. V plot (Fig. 5). It is observed that the barrier height obtained from the conventional method approaches that of our component separation technique as the overall ideality factor decreases. This indicates the flexibility of this approach compared with the conventional method. The barrier heights of 0.74 eV for unpassivated and 0.84 eV for passivated samples agree very well with those obtained by Besser and Helms [8] who have used a surface conductivity technique, wherein they compare the channel resistances of gated and ungated field effect transistors in n-GaAs to measure the surface Fermi level position.

3. Analysis of the results

The sulphide passivation of GaAs surfaces under external excitation gives rise to the following: (i) reduced arsenic pile-up, (decrease in n(Asxot~):n(Gavo~])), reduced oxide (decrease in ASoxide:ASMct,I, Gao×~dc:GaMet~) and the formation of a new arsenic sulphide phase (from XPS); (ii) reduced surface recombination (from PL and J V measurements); (iii) increased surface band bending (J V measurements); (iv) a larger decrease in surface recombination compared with the decrease in surface states ( J - V measurements). All these results can be explained satisfactorily using the A U D M developed by Spicer and coworkers [8, 9]. To account for the apparently contradictory pair of results of a large reduction in surface recombination velocity accompanied by a movement of the surface Fermi level away from the flat band condition, the A U D M proposes that surface states are made up of TABLE 1. Ideality factors and barrier heights deep donors compensated by acceptors. In this model the Fermi level position is determined by Nd, the numldeality factor q Barrierheight (~b (eV) Sample ber of donor states, and N,, the number of compensatdetermined by regression o v e r Conventional Numerical ing acceptors. For N d much greater than N, the Fermi level Er will be pinned at the donor state. For the appropriate ranges method method reversed case Ef will be near the acceptor state, and, for 0.80 0.74 Nd comparable with N~, Ef will lie at an energy between Unpassivated 1.95 0.86 0.84 Passivated without 1.9 these two levels. If now the passivation reduces N d then heat and light this will not only reduce the surface recombination 0.87 0.86 Passivated with 1.5 velocity but also move E r towards the valence band heat and light maximum (VBM), thus increasing the band bending.

A. S. Weling et al. / S passivation of GaAs suiJaces

The reduction in arsenic pile-up at the surface due to passivation as observed by XPS supports the argument that arsenic antisite defects give rise to the deep donor and gallium antisite deep acceptors. The results of the deep level transient spectroscopy experiments of Liu et al. [15] reveal two donor levels and possibly two acceptor levels. However, from SRH theory the acceptors are not as efficient recombination centres as donors owing to their position away from the midgap. Therefore, the reduction in deep donor concentration is of more significance. In the double-donor model the movement of the Fermi level towards the VBM due to the reduction in donor states gives rise to an increased fraction of donor states being in the ionized state. Therefore, reduction in the surface recombination is not only due to the reduction in surface states but also due to the repulsion of holes (needed for recombination) by the ionized donors. This clearly explains the result obtained by J V measurements, i.e. a larger decrease in recombination current compared with the reduction in surface state density and hence the improvement in the overall ideality factor.

4. Conclusions External excitation in the form of heat and illumination during the sulphide passivation of GaAs surfaces is observed to retain the effectiveness of passivation even after a DI water rinse following the passivation. It not only passivates the surface more effectively but also increases the stability of the surface. An increase in the GaMetal:Gaoxide and AsMet,j :ASoxide ratios shows that the sulphide passivation prevents the oxidation of the GaAs surface. The presence of an arsenic sulphide phase together with the presence of sulphur indicates that the surface is terminated with A s - S bonds as well as an S - S network. Moreover, the marked reduction in Asvo,,,:Ga~ota~ is a definite indication of a reduction in arsenic pile-up at the surface. A marked improvement in PL yield is corroborated by a reduction in recombination component of the forwardbiased current in the Schottky contact. A model of deep donors compensated by acceptors explains satisfactorily the various results obtained. A

183

reduction in arsenic pile-up at the surface with passivation combined with the increased band bending supports the argument that arsenic antisite defects are responsible for the deep donor states. A larger reduction in surface recombination compared with the reduction in surface states has been explained as due to the possibility of double-donor states as has been observed by Liu et al. [ 15].

Acknowledgments We gratefully acknowledge the help rendered by RSIC, Indian Institute of Technology, Madras, regarding the XPS experiments. The useful suggestion by Dr. J. Majhi during the course of this work and the help from Professor B. Vishwanathan in analysing XPS results are also acknowledged.

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