Interface state band between GaAs and its anodic native oxide

Thin Solid Films, 56 (1979) 183-200 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

183

I N T E R F A C E STATE BAND B E T W E E N GaAs A N D ITS A N O D I C NATIVE OXIDE T. SAWADA AND H. HASEGAWA

Department of Electrical Engineering, Facultyof Engineering, Hokkaido University, Sapporo060 (Japan) (Received March 20, 1978: accepted July 21, 1978)

The basic interface properties of GaAs metal-oxide-semiconductor (MOS) structures formed using an anodic oxidation process in a mixed solution of glycol and water (the AGW process) were investigated. Detailed measurements of the capacitance-voltage and conductance-voltage characteristics and of the transient behaviour of the thermal and optical MOS capacitanc e revealed various anomalies which are not encountered in the silicon MOS system. On the basis of an analysis of such anomalies an interface state band (ISB) model is presented for the GaAs-anodic oxide MOS system, and the origin and properties of the ISB are discussed. The present ISB does not pin the surface Fermi potential but severely limits the range of its movement at steady state to the lower half of the energy gap.

I. INTRODUCTION

In recent years the formation of high quality insulating thin solid films on GaAs and related compound semiconductor materials has received considerable attention in connection with the fabrication of high speed metal-oxide-semiconductor (M OS) devices, with the optical matching and surface passivation of optoelectronic devices and with various device processing applications. In contrast with silicon technology, a high temperature process seems to be unacceptable here owing to the high volatility of Group V elements such as arsenic, and low temperature processes such as anodic oxidation and low temperature plasma oxidation are to be preferred. In particular the stable and easily controlled room temperature oxidation process using anodization in glycol and water (the AGW process) 1 has already produced some promising results for MOS field-effect transistor (FET) applications 2' 3. In spite of such recent achievements, however, the basic interface properties of compound semiconductor MOS structures are not well understood 4~ and no selfconsistent model for their behaviour which fully accounts for the anomalies observed in the capacitance-voltage (C-V) data has so far been established. In this paper we present a novel interface state band (ISB) model for the G a A s anodic oxide MOS system 7 on the basis of detailed quasistatic and dynamic MOS C-V and conductance-voltage (G-V) measurements carried out over wide ranges of various parameters on samples prepared by the AGW process. The ISB model can explain the observed anomalies in a consistent way and seems to clarify the

184

T. SAWADA, H. HASEGAWA

characteristic features and difficulties associated with the "compound" nature of the material. 2. EXPERIMENTAL As the starting materials, p- and n-type GaAs prepared by horizontal Bridgman vapour phase epitaxy or liquid phase epitaxy with carrier concentrations of l014l0 is c m - a and orientations of (100), (111) A and B and (110) were used. Sn-Ag or Z n - I n - A g back contacts, depending on the conduction type, were provided by alloying. Anodic oxidation was carried out using a mixture of propylene glycol and a 3~o aqueous solution of tartaric acid (buffered by N H 4 O H to pH 6.3) in the volume ratio 2:1 1. The standard oxidation procedure was to anodize in the constant-current mode first at a current density of 0.5 mA c m - 2 to a predetermined formation voltage and then to change to the constant-voltage mode until the current decayed to about 0.01 mA c m - 2. For n-type material the anode was illuminated by a collimated beam of light from a tungsten lamp in order to create holes for the anodic reaction s. For cleaning purposes the as-grown oxide was dissolved in a concentrated HCI solution and the oxide was grown again to make the MOS sample. The oxide thickness was typically 1500-2000/~. The MOS capacitors were formed by the vacuum deposition of aluminium or gold through a metal mask. The field plate area was typically 1 x 1 0 - 3 c m 2.

As a standard post-growth annealing process, samples were kept at 300 °C for 3 h in a flow of high purity hydrogen. Annealing in other gases such as oxygen or nitrogen yielded nearly the same results. The ramped and pulsed MOS C - V and G - V measurements were performed in a light-tight electrically shielded box using an automatic plotter with phase-sensitive detection for the frequency range 1 Hz-10 MHz and for the temperature range from liquid nitrogen temperature to 200 °C. Samples were kept in a flow of dry nitrogen during the experiment. The ramped field plate bias was supplied from a voltage ramp generator with a variable ramp speed from 1 mV s-1 to l0 V s-1. The a.c. signal amplitude for the admittance measurement was always kept small compared with the mean thermal energy kT. The impurity concentration was determined bY combined use of the van der Pauw technique, Schottky C - V measurements and MOS deep depletion C - V measurements at various temperatures. 3. RESULTS AND DISCUSSION

3.1. Capacitance-voltage anomalies As has been reported previously 9, apparently improved and well-defined C - V behaviour was obtained after the post-growth annealing. Annealing at temperatures higher than 350 °C resulted in a reduction of the electrical breakdown field strength of the oxide and an increase in the pre-breakdown leakage current. The C - V curves obtained for such leaky samples were very different from those of the normal samples discussed below and were similar to those described by Chang and Sinha I o for plasma oxides, showing no flat response corresponding to inversion at high frequencies.

INTERFACE STATE BAND BETWEEN

GaAs

185

A N D ITS A N O D I C OXIDE

Examples of the C - V curves of the normal samples subjected to the standard post-growth annealing process are shown in Fig. 1 for a p-type and an n-type sample. To avoid complexity complete C - V traces are given only for 100 Hz; for the rest, only the traces from the inversion side towards the accumulation side are drawn. In spite of the large improvements apparent in the C - V curves as a result of the annealing mentioned above, we see in Fig. 1 that several distinct anomalies that are not encountered in the silicon M O S system are still present in the annealed samples as summarized in the following. (1) There is an a n o m a l o u s frequency dispersion of the a c c u m u l a t i o n capacitance. As is clearly seen in Fig. 1, the n-type sample exhibits an a n o m a l o u s capacitance behaviour on the accumulation side, where the capacitance decreases drastically roughly within the range from 10 k H z to 1 MHz. A similar but much less p r o n o u n c e d decrease of the accumulation capacitance was observed in the p-type sample within the measured frequency range up to 10 MHz. (2) Large values of the inversion capacitance are found. While in the case of the n-type sample in Fig. 1 the capacitance value of the flat portion of the C - V curves on the inversion side agrees well with the calculated high frequency inversion capacitance Cinv shown by the broken line, a large deviation is seen in the case of the p-type sample. Such a deviation was always found in p-type samples. N-type samples also showed some deviation when the carrier concentration was high but the deviation was always found to be much smaller than for p-type samples. (3) A hysteresis effect was found. As shown by the arrows in Fig. 1, both p-type and n-type samples showed so-called injection-type hysteresis effects even at r o o m temperature. Figure 2 shows an example of the measured temperature variation of the C - V curves for a p-type sample, and here we see the fourth a n o m a l y : (4) a n o m a l o u s temperature behaviour. 130 °C

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Fig. 1. The MOS C-V curves for (a) a p-type (100)sample (NA = 1.2 x 1016era- a) and (b) an n-type (100) sample (ND = 1.2 x 1015cm-3) for various frequencies at room temperature: sweep rate 300 mV s- 1. Fig. 2. The temperature dependence of the 1 MHz MOS C-V curves for an annealed p-type GaAs (100) sample: p ~ 1 x 1016cm- 3, tox ~ 2000 A, sweep rate 300 mV s- i.

186

T. SAWADA, H. HASEGAWA

Additional anomalies will be pointed out later in connection with MOS transient capacitance studies under dark and illuminated conditions.

3.2. Anomalous frequency dispersion of the accumulation capacitance The anomalous frequency dispersion of the accumulation capacitance was reported first by the present authors 4 for the case of anodic native oxides on n-type GaAs but similar effects were reported on n-type GaAs metal-insulatorsemiconductor capacitors with various insulators such as deposited insulators containing oxygen (e.g. A1203, SiO 2 etc.) 11-14, a thermally grown oxide or an O +implanted GaAs layer 14, and recently on plasma-grown M OS capacitors I s, 16; so far they have been explained by various mechanisms other than the present ISB model described later. The mechanisms put forward to explain the present anomaly in the case of n-type GaAs have included the following: (a) a frequency dispersion of the permittivity of the bulk oxide; (b) the formation of a highly resistive GaAs surface layer due to oxygenS1 ; (c) the formation of a "metamorphic layer" near the interface 12 which is a lossy dielectric whose gross electrical behaviour can be represented by parallel connection of a capacitance and a conductance in series with the bulk oxide capacitance; (d) Quast's trap model ~a, 14 in which traps introduced into the GaAs surface during oxide formation produce a potential maximum in the GaAs conduction band at the accumulation bias and the capacitance and conductance show frequency dispersions corresponding to the diffusion of majority carriers over this maximum. In addition to these, we add (e) our model, the ISB model, in which we assert that an anomalous band-like high density distribution.of acceptor interface states appears at a certain position of the GaAs energy gap connected with the basic oxidation mechanism of the compound semiconductor material. The models (a)-(c) assume that the semiconductor surface potential is in the accumulation region, i.e. that it is pushed against the bottom of the conduction band. The model (d) uses the same assumption except for the presence of the potential maximum underneath. In contrast, the present model (e) assumes that the surface potential is pushed against the bottom of the acceptor interface state band (the a-ISB). The small signal a.c. equivalent circuit representations of the models (a)(e) are given in Fig. 3. For model (e) we assume the simplest circuit with a single time constant for simplicity, and the time constant dispersion effect will be discussed later. Now we shall show that only model (e) can explain our experimental results in a consistent way. Firstly, Fig. 4 shows the variations of the measured MOS capacitance with frequency at fixed accumulation and inversion biases for n-type MOS samples having oxides of different thicknesses on the same GaAs substrate. For convenience each capacitance value is normalized by its corresponding lowest frequency value CLV. The result in Fig. 4 clearly excludes model (a) by the following simple argument. When an oxide layer with a thickness d and a frequency-dependent permittivity cox(co)is connected in series with the semiconductor capacitance C o (per unit area), the normalized capacitance C/CLF is given by C CLF

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(1)

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INTERFACE STATE BAND BETWEEN

GaAs AND ITS ANODIC OXIDE

187

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C/CLF in strong accumulation (Co, C D L F ~ 0 0 ) should be dependent on frequency but not on oxide thickness, whereas C/CLF in strong inversion (C o finite) should be dependent on both frequency and thickness irrespective of whether COLF is finite or infinitely large at the lowest frequency used. Thus if model (a) is valid the curves A and B in Fig. 4 should coincide and the curves C and D should split and should show different and frequency-dependent behaviours. The result is totally contrary to this and clearly indicates that the present anomaly is related not to the oxide bulk but to the interface region. Additionally, the observed carrier concentration dependence and orientation dependence of the anomalous frequency dispersion (Fig. 5) contradict model (a) because it is difficult to understand how the oxide permittivity dispersion can be so dependent on the electron concentration and orientation of the semiconductor when the oxidation process parameters and the refractive index of the oxide (the square root of the optical value of the oxide permittivity) are both

188

T. SAWADA, H. HASEGAWA

independent of conduction type, carrier concentration and orientation 1. Furthermore, the Auger in-depth profile analysis of our oxides performed by Watanabe e t al. 17 at the authors' University showed no appreciable difference between p-type and n-type samples either in the oxide bulk or in the interface region. We would therefore expect the same oxide permittivity with the same relaxation frequency for n-type and p-type samples, whereas the observed dispersion of the accumulation capacitance is very different for n-type and p-type samples, as is seen in Fig. 1. 1,0

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Secondly, the observed frequency dispersion of an n-type sample is compared in Fig. 6 with the theoretical curves for models (b)-(e). The curves were computed using the equivalent circuits in Fig. 3, and the parameters were chosen such that the curves approached the observed capacitance values in the low frequency and high frequency limits and that they fitted as closely as possible to the observed variation in the intermediate frequency range. As can be seen in Fig. 6, a reasonably good fit can only be obtained with models (c) and (e). Some of the data and arguments given above for excluding model (a) also provide additional grounds for excluding models (b) and (d). Furthermore, the introduction of oxygen into the GaAs during anodic oxidation to provide the traps assumed in models (b) and (d) appears to be somewhat improbable because a recent analysis 18 has shown that the oxygen ions are not the principal moving species. Finally, we exclude model (c) for the following reasons. 1

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I N T E R F A C E STATE B A N D B E T W E E N

GaAs

A N D ITS A N O D I C O X I D E

189

(1) The absence of a frequency dispersion on the inversion side within the measured frequency range (see Figs. 1 and 4) is not consistent with model (c). (2) It is difficult to explain the concentration dependence in Fig. 5 with model (c) but it can be explained in a straightforward way on model (e) by the difference in the semiconductor depletion layer capacitance. (3) The thickness of the metamorphic layer deduced by the curve fitting using the equivalent circuit in Fig. 3(c) is unrealistically large (over 1000 A) while the interfacial transition layer thickness revealed by Auger analysis was typically 100/~ or below. (4) Judging from the similarity of the oxidation process with a sufficient supply of holes and from the results of the Auger analysis, the thickness and electrical properties of such a layer, if it is formed, are expected to be more or less the same for n- and p-type samples, whereas the observed dispersions of the accumulation capacitance are quite different. We may thus conclude that model (e), which assumes the presence of an a-ISB, is the most probable mechanism for the anomalous frequency dispersion of the accumulation capacitance in n-type samples. We shall show in the following section that this a-ISB causes a different anomaly in p-type samples. However, the less pronounced frequency dispersion of the accumulation capacitance in the p-type sampl e with a higher relaxation frequency (see Fig. 1 and also Fig. 7) can be explained by the presence of a donor interface state band (d-ISB) near the valence band edge, as will be mentioned later. 3.3. Inversion capacitances Except for the anomaly (2) in Section 3.1 concerning the theoretical and experimental inversion capacitance values, the behaviour of the C-V curves of the ptype sample on the inversion side (Fig. 1) looks quite normal, apparently indicating the deep-depletion-inversion type transient. For this reason anomaly (2) was not given much attention in previous preliminary assessments of p-type samples4' 9 in which the capacitance deviation was attributed to possible doping fluctuations in the bulk materials used and the fiat response of the capacitance was interpreted as caused by normal inversion. However, our more extensive study revealed that this disagreement occurs consistently in all p-type samples even when the doping was carefully determined by various means. This experimental fact suggested a new interpretation that the observed behaviour on the inversion side of p-type samples corresponds to a "pseudoinversion" where instead of the conduction band the a-ISB is filled with electrons. Obviously this interpretation is more consistent with the ISB model for n-type samples. If the a-ISB were formed only in n-type samples, it would be difficult to understand its mechanism. Disagreements between experimental and theoretical inversion capacitances were observed also in highly doped n-type samples during the early stages of the present study and led to difficulty in achieving a unified understanding. However, we found that these were only virtual and were the result of two factors. Firstly, in highly doped n-type samples the net ionized donor density in the depletion region is not in general equal to the carrier concentration owing to the small effective density of states in the GaAs conduction band which causes degeneracy and impurity

190

T. S A W A D A , H. H A S E G A W A

deionization. Acceptors and deep donors may also complicate the situation. The ionized impurity concentration of these highly conducting n-type samples was therefore carefully determined by analysis of the deep depletion behaviours of M O S C - V curves taken at various sweep speeds and at various temperatures. Secondly, for highly doped n-type samples it becomes difficult to determine the oxide capacitance to the required accuracy in view of the small overall capacitance variation with bias owing to the slow saturation with bias and to the slow frequency dispersion of the capacitance on the accumulation side. We found that a reasonably accurate value can be obtained by biasing the n-type samples sufficiently positive and by measuring the capacitance at sufficiently low frequency. To show this, the frequency dispersion of an n-type sample at a high positive bias ( + 50 V) is compared in Fig. 7 with that of a p-type sample having the same thickness but biased at a lower accumulation bias ( - 1 6 V). We see that both capacitances C a at accumulation biases become close to each other on the low frequency side, giving the same oxide permittivity value. By taking account of the above-mentioned factors, we came to the conclusion that n-type samples always show band bending corresponding to normal inversion, or at least very close to it within the experimental accuracy of 0.1 eV or so, at high inversion biases. ×

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Fig. 7. Plots of the accumulation capacitanceC, and the inversioncapacitance Ci vs. frequencyfor an ntype GaAs (100) sample (ND÷ = 7 x 1017 cm -3) and for a p-type GaAs (il-i) sample (NA = 6 x 1017 cm- 3) with nearly the same impurity concentrations. In Fig. 7 the variations of capacitance CI with frequency on the inversion side are shown. These two samples have nearly the same value of the ionized impurity concentration but C i for the n-type sample is smaller than that for the p-type sample. The interpretation is that the former corresponds to normal inversion and the latter to pseudo-inversion at the a-ISB. It should be noted that C a for the n-type sample approaches the Ci of the p-type sample in the high frequency limit, indicating that the position of the bottom of the a-ISB is the same for both samples. Most well-prepared n- and p-type samples showed only the so-called high frequency curves at room temperature within the measured frequency range down to l Hz. This seems to be consistent with the slow carrier generation times for normal inversion and pseudo-inversion as measured by the M O S capacitance

INTERFACE STATE BAND BETWEEN

GaAs AND ITS ANODIC OXIDE

191

transient experiments. At elevated temperatures, however, low frequency curves were observed even at 1 MHz, as seen in Fig. 2 for a p-type sample. 3.4. Interface state band model

On the basis of the interpretation and discussion given in preceding sections, the range of semiconductor band bending obtained by biasing is plotted in Fig. 8 as a function of the ionized impurity concentration, in terms of the corresponding semiconductor depletion layer capacitance. The depletion layer capacitance was determined by quasi-steady-state high frequency MOS capacitance measurements. Theoretical values of the depletion layer capacitance corresponding to strong inversion and that corresponding to the surface potential at mid-gap are shown by the solid line and the broken line respectively. The striking conclusion is that the movement of the surface potential is limited to the lower half of the energy gap irrespective of conduction type, impurity concentration and crystal orientation. If we interpret this by the present ISB model it is found that the bottom of the aISB is located at the same place in all the samples studied--at mid-gap. The basic idea of the model is schematically shown in Fig. 9. The a-ISB is an acceptor-like empty interface state band whose partial filling causes pseudo-accumulation with an anomalous frequency dispersion of the capacitance in n-type samples and pseudoinversion in p-type samples. o ~ -

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Before discussing the details of the ISB model we present in the following section the results of the MOS capacitance transient studies because they provide further support for the basic idea of the present model from the dynamic point of view. 3.5. M O S capacitance transients

MOS capacitance transients at 1 MHz were measured at room temperature both in the dark and with illumination by collimated monochromatic light of

192

T. SAWADA, H. HASEGAWA

various wavelengths and intensities using the simple configuration shown in Fig. 10. Figures l0 and 11 show the typical measured capacitance variations as a function of time for a p-type sample and an n-type sample respectively under dark and illuminated conditions. The samples were kept in the dark at the accumulation bias of - 1 5 V or + 15 V for a sufficiently long time to achieve quasi-steady-state capacitance values and then at t = 0 the bias was switched to the deep depletion bias of + 15 V or - 15 V and, in the case of illuminated conditions, the light was turned on at the same instant.

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Fig. 10. MOS capacitance transients for a p-type(100) sample under dark and illuminated conditions: NA~ 2 x 1017cm-3, frequency 1 MHz, VG-15 V---* + 15 V, tox = 2000,~. Fig. 11. MOS capacitance transients for an n-type(100) sample under dark and illuminated conditions: N o = 2. l x 1016cm- 3 frequency 1 MHz, VG + 15 V -~ - 15 V, tox= 2000A, constant incident quanta. Let us first consider the dark responses. It can be seen in Figs. 10 and 11 that there were considerable differences between n- and p-type samples: the response of p-type samples was quicker and roughly exponential whereas that of n-type samples was much slower and looked more normal, being indicative of carrier generation in the depletion layer. Figure 12 gives typical Zerbst plots 19 of the responses for both types. Except for the first 6-10 s linear Zerbst plots were obtained for most of the ntype samples, and from their slopes reasonable values of the carrier lifetime from several nanoseconds 3o several tens of nanoseconds were obtained. Our interpretation of the n-type results is that the response is characterized by the bulk generation of holes with a superposition of initial rapid electron emission from the partially occupied a-ISB to the conduction band. Indeed, when the bias was switched in the dark from the accumulation bias to 0 V, instead of driving into the deep depletion bias the hole generation response disappeared (Fig. 13) and only the electron emission response from the a-ISB remained; its amplitude increased with increase in the magnitude of the accumulation bias as shown. In contrast, the Zerbst plots of p-type samples were always highly non-linear and extrapolation of the tangent at any point intersected the negative ordinate axis, as shown by the example in Fig. 12. Further analysis on the basis of the equation 19 d Cox2 - - - 2 Cox dNs dt C 2 esN a dt

(2)

INTERFACE

STATE BAND BETWEEN

GaAs

AND ITS ANODIC OXIDE

193

where C is the M O S capacitance, Cox is the oxide capacitance, t the time, es the semiconductor permittivity, N B the semiconductor doping concentration and Ns the surface charge density, revealed that a strong exponential generation of surface charge is present whose time constant is of the order of several seconds. This result can be understood in terms of the thermal generation of electrons from the valence band into the a-ISB at the surface, leading to pseudo-inversion. 0.8

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frequency 1 M H z , VG + 15 V ~ - 15 V ; O , p - t y p e (100) s a m p l e , N A = 1.0 x 10 *~ c m - 3, f r e q u e n c y 1 M H z , VG - 15 V ~ + 15 V. The numbers at the data points are the time in seconds. initial capacitance 2&

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Fig. 13. Dark MOS transientsof an n-type (100) sample for switching from accumulation bias to zero bias: N o = 2.1 x 1016 cm -3, frequency 1 MHz. Secondly, the responses under illuminated conditions were also very different for p- and n-type samples, as can be seen in Figs. 10 and 11. As expected, both samples showed more or less similar enhanced generation behaviour when the photon energy was larger than the gap energy Eg. However, when the photon energy was below Eg, n-type samples still showed enhanced generation but p-type samples were not affected by the illumination. Photo-capacitance experiments on Schottky contacts on the same p- and n-type substrates indicated that the present drastic

194

T. SAWADA, H. HASEGAWA

effect is peculiar to MOS structures; this rules out the possibility of large contributions from bulk traps. The present phenomenon can be understood by assuming that the optical transition for electrons from the valence band to the a-ISB is inhibited whereas that from the a-ISB to the conduction band is allowed. Light with photon energy below E~ can reach the interface under the gate by diffuse scattering at the irregular back surface of the GaAs substrates. If such an interpretation is valid, the optically enhanced generation rate in the n-type samples should eventually be limited by the thermal transition of electrons from the valence band to the a-ISB. This was indeed the case, as is shown in Fig. 14 where the relative increase in the charge generation rate on illumination, as determined from capacitance transients, is plotted as a function of light intensity. The maximum speed of the transient was again several seconds. However, no such saturation was observed in either n- or p-type samples when the photon energy was larger than E s. Figure 15 shows the spectral response of the excess charge generation rate due to illumination for an n-type sample under the condition of constant incident quanta. Since the reflectance of the oxide-GaAs system obviously behaves in a complicated way 2°, the data cannot be analysed quantitatively, but they at least show the anomalous nature of the GaAs MOS structure under illumination compared with the silicon MOS structure. The sharp dip in response near the band gap wavelength is most probably due to the increased light absorption which drastically reduces the amount of light arriving at the interface underneath the gate by transmission and diffuse scattering. When the photon energy is above Eg, minority carriers generated by light within the diffusion length from the gate will contribute to the speeding-up of the transients in both p- and n-type samples. 10z ao

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Fig. 14. The excess charge g e n e r a t i o n rate caused by i l l u m i n a t i o n vs. light intensity for an n-type (100) s a m p l e u n d e r i l l u m i n a t i o n by m o n o c h r o m a t i c light (2 = 1200 nm) whose p h o t o n energy is below the b a n d g a p energy: ND = 2.1 x 10 ~6 c m - 3 frequency 1 M Hz, VG + 15 V --* - 15 V. Fig. 15. The spectral response of the excess charge g e n e r a t i o n rate to i l l u m i n a t i o n by c o n s t a n t incident q u a n t a for a n n-type (100) sample: N D ~ 3.2 x 10 t 7 c m - 3, frequency 1 M H z, Vo + 15 V --* - 15 V.

The fairly quick non-radiative transition between the valence band and the aISB, as revealed here, may be related to the recently proposed degradation mechanism of GaAs photoluminescence intensity 21 in connection with the oxidation-induced degradation of GaAIAs double-heterojunction lasers.

INTERFACE STATE BAND BETWEEN

GaAs

AND ITS ANoDIC OXIDE

195

3.6. Distribution of interface states On the basis of the present interpretation of the surface potential positions in relation to the C - V behaviour, Terman's method 22 was applied to evaluate the interface state density distribution; the result is summarized in Fig. 16. The C - V curves at 10 M H z were used. Since the capacitance data of the p-type sample had still not reached their high frequency limits on the accumulation side even at this frequency, the corresponding inaccurate portion of the distribution is indicated by the dotted curve in Fig. 16. For comparison the recent result of Shimano et al. 23 using the saturation surface photovoltage technique and that of Zeisse et al. 24 using Berglund's technique 25 are roughly sketched as broken curves. All the results show U-shaped distributions but there is an obvious disagreement concerning the location of the distributions, i.e. concerning whether the U-shaped distribution is in the lower or upper half of the energy gap. Let us first comment on the surface photovoltage technique. According to Johnson 26 the basic assumption for direct measurement of the surface potential by the saturation photovoltage is that the presence of interface states does not disturb the charge redistribution in the semiconductor space charge layer, which takes place under photo-induced charge injection so as to preserve the overall charge neutrality. In silicon M O S structures this condition seems well satisfied but in the case of GaAs MOS structures we feel that the achievement of a photo-induced flat-band condition would be difficult in n-type samples owing to the presence of the anomalously high density a-ISB. The saturation value of the surface photovoltage may be obtained but it will most probably give the surface potential with respect to the bottom of the aISB rather than that measured from the flat-band condition. F r o m this viewpoint we can understand the discrepancy of about Eg/2 with regard to the location of the Ushaped distribution. In contrast, in carrying out Berglund's procedure Zeisse et al. seem to have assumed rather arbitrarily that a + 10 V bias corresponds to a potential of - 3kT/q, probably to correlate the data with the surface photovoltage result. Thus a proper determination of the additive constant 25 in Berglund's procedure may alter the result. In fact we tried the same technique on a few n-type samples with 1016-1017 carriers per cubic centimetre and we determined the additive constant by the method of Nicollian and Goetzberger 27; the result supported our distribution in the lower half of the energy gap. As can be seen in the U-shaped distributions in the lower half of the energy gap, the a-ISB actually shows a roughly exponential tailing into the lower half of the gap region. High density interface states also exist towards the valence band. F r o m these a more detailed and realistic ISB model may be pictured (Fig. 17). We assume the presence of an a-ISB and a d-ISB, both having tails into the lower half of the gap region. The point of minimum density of states is located near the halfway point of the lower half, i.e. ¼Eg above the valence band edge. According to experiment, the zero bias surface potential position also lies near this halfway point, showing a weak dependence on conduction type, carrier concentration and crystal orientation. Since we could only move the surface potential within the lower half of the gap region in both n- and p-type samples, the state density distribution in the upper half could not be obtained directly by any electrical measurement. However, we assume in our model shown in Fig. 17 that the extent of the a-ISB is large; i.e. we assume that

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We also assume the presence of a d-ISB near the valence band from the measurements on n-type samples. Since the accumulation capacitance of the p-type samples did not reach the high frequency limit within the measured frequency range up to 10 MHz, as mentioned previously, it appears highly probable that the portion of the distribution for the p-type sample shown by the dotted curve in Fig. 16 eventually relaxes at higher frequencies to become closer to the distribution shown for the n-type sample, accompanied by a fairly large "anomalous frequency dispersion" of the accumulation capacitance at higher frequencies. The salient features of the anomalous low temperature behaviour shown in Fig. 2 can be explained at least qualitatively by the model shown in Fig. 17. The main mechanisms involved are the same as those for the silicon MOS structure at low temperatures 2s but they take part more drastically. They include the curve shift caused by the movement of the bulk Fermi potential, non-equilibrium charging and discharging of interface states, the freezing-out of interface states and nonequilibrium band bending associated with the gate bias sweep. 3.7. Time constant dispersion and the origin o f the interface state bands

Although the simple single time constant model can explain the gross behaviour of the frequency dispersion of the capacitance caused by the a-ISB seemingly better than other models, as we have seen in Fig. 6 deviations from the simple theory are already clear, suggesting a time constant dispersion. Such deviations become apparently more pronounced in samples with higher carrier concentrations (Fig. 7).

INTERFACE STATE BAND BETWEEN

GaAs

AND ITS ANODIC OXIDE

197

More detailed and accurate measurements of the MOS capacitance and conductance as a function of frequency at each fixed bias revealed that a broad time constant dispersion does exist and cannot be fitted into the model of Nicollian and Goetzberger 27 by curve fitting. A similar observation has also been made recently by Kohn and Hartnagel 6 using Cole-Cole diagrams. By further careful investigations we finally came to the conclusion that an excellent fit of the experimental data can be obtained if we employ for the time constant dispersion the tunnelling model proposed by Preier 29 in which electrons are exchanged by tunnelling between the semiconductor and traps in the oxide. The mathematical and experimental details of such an analysis are beyond the scope of the present paper and will be presented elsewhere. Only the main conclusions are sumarized. (1) The best fit of the data is obtained for an exponentially decaying spatial distribution of traps into the oxide. (2) The density of traps, determined by curve fitting, shows a U-shaped distribution as a function of surface potential similar to those in Fig. 16, giving very high densities towards mid-gap and towards the valence band edge. (3) The electron capture cross section of the a-ISB is of the order of 1 0 - s - 1 0 - 9 c m 2 and is thus anomalously large compared with that usually found in silicon MOS structures (which is of the order of 10-i s_10-16 c m 2 ) . In contrast, the hole capture cross section of the d-ISB is of the order of 10-14-10 - 15 c m 2 and is much more normal. As an example, the measured frequency dispersions of the MOS capacitance C m and conductance G m a r e compared in Fig. 18 with the theoretical curves based on the tunnelling model with an exponential distribution of traps. The present understanding of the time constant dispersion is obviously consistent with the observed hysteresis effects (anomaly (3) in Section 3.1). Figure 19 shows an example of the observed logarithmic charging effect for the C - V shift as a function of biasing time. This again indicates the effect of oxide traps. The behaviour of the non-steady-state C - V curves obtained at fast sweep speeds or at low temperatures showed characteristic hysteresis effects that can be well explained by the theory developed by Simmons and Wei 3°' 31 on the dynamic behaviour of MIS systems containing surface traps. Thus we see that all the anomalies present in the GaAs-anodic oxide MOS system can be explained by a U-shaped ISB distribution resulting from a high density of interfacial traps. Depending on the distance from the interface, some of them exchange carriers very rapidly with the semiconductor while others do it very slowly, giving rise to a very large range of time constant values. If the C - V and G - V measurements are performed very rapidly before the establishment of the d.c. surface potential, which involves the filling-up or emptying of states with slow time constants within the ISBs, more "ideal" behaviour involving only "fast" states will be observed, and this is indeed what has been found by Fritzsche et al. 5 The a-ISB near mid-gap shows a remarkable correlation to the Fermi level pinning in vacuum-cleaved and oxygen-adsorbed GaAs (110) surfaces 32. Because of this the origin of the ISBs was first correlated 7 with the so-called GSCH model 32 which involves the formation of an intrinsic surface state band at mid-gap by gallium dangling bonds. However, recent developments 33 appear to contradict such a

198

T. SAWADA, H. HASEGAWA

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v~ = - 25 v. model. The results presented here indicate that the origin of the ISBs is related to the presence of a high density of defects in the interface region. We can visualize the existence of several interfacial glassy monolayers having a high density of defects of non-stoichiometric nature. If the defects are sufficiently dense, they will themselves form an amorphous-like modified surface band structure with tails, as shown in Fig. 17. The anomalously large value of the observed effective electron capture cross section may be understood on the basis of such a model. A large number of defects loosely connected by the overlapping of wavefunctions will effectively result in a very large cross section when the states in the tail exchange carriers with the GaAs bulk. The state distribution in Fig. 17 may be explained by a thin semiconductor layer with a reduced band gap at the interface. The a-ISB, which is the "conduction band" of this hypothetical semiconductor, may even allow surface electrical conduction. In fact the partial success of the reported "n-channel inversion" F E T a4 suggests such a surface conduction. Although nothing definite can be said from the electrical data presented here, the most probable source of the defects in the above argument is arsenic atoms in view of the difference in oxygen affinity between gallium and arsenic atoms. The selective oxidation of gallium near the interface will leave a high density of arsenic atoms. The existence of such arsenic atoms near the interface has been observed recently on GaAs by Chang et al. 35 using Auger analysis, although no such large amount of arsenic atoms was found in the Auger profile of our oxide ~7 probably because of the difference in the anodic processes used. 4. CONCLUSIONS Detailed quasistatic and dynamic electrical measurements were made on the GaAs-anodic oxide MOS system prepared by the AGW process. On the basis of such measurements the ISB model has been presented and the origin and properties of the ISBs have been discussed.

INTERFACE STATE BAND BETWEEN

GaAs

AND ITS ANODIC OXIDE

199

The origin of the ISBs appears to be associated with the "compound" nature of the material. Recently ISBs were also found to exist in the GaP-anodic oxide MOS system 36 and thus interfaces having ISBs may be a common feature of compound semiconductor MOS systems. If the hypothesis presented here concerning the origin of ISBs is correct, it appears to be possible to remove them by some catalytic reaction which oxidizes the arsenic atoms fully. In addition, a positive use of the ISB may be possible in, for instance, memory, IR detection and IR imaging applications. ACKNOWLEDGMENTS

The authors would like to thank Professors H. Tagashira, T. Yamashina, K. Watanabe, T. Sugano, Y. Shibata, T. Hariu, M. Hirose, R. Singh and H. Hartnagel for their helpful discussions during the course of this work. The present work was financially supported by a Grant-in-Aid for Special Research on "Surface Electronics" from the Japanese Ministry of Education. REFERENCES 1 2 3 4 5

H. Hasegawa and H. L. Hartnagel, J. Electrochem. Soc., 123 (1976) 713. D.L. Lile, A. R. Clawson and D. A. Collins, Appl. Phys. Lett., 29 (1976) 207. H. Tokuda, A. Adachi and T. Ikoma, Electron. Lett., 13 (1977) 761. T. Sawada and H. Hasegawa, Electron. Lett., 12 (1976) 471. D. Fritzsche, G. Weimann, W. Schlapp and H. W. Dinges, Paper presented at Fruhjahrstagung der

6 7

E. Kohn and H. L. Hartnagel, Solid-State Electron., 21 (1978) 409. H. Hasegawa and T. Sawada, Proc. 7th Int. Congr. and 3rd Conf. on Solid Surfaces, Vienna, September 1977, Vol. 1, Berger and S6hne, Vienna, 1978, p. 549. H. Hasegawa and T. Suzuki, Jpn. J. Appl. Phys., 15 (1976) 2489. H. Hasegawa, K. E. Forward and H. L. Hartnagel, Appl. Phys. Lett., 26 (1975) 567.

Deutschen Physikalischen Gessellschaft Fachgruppe Halbleiterphysik, Freudestadt, April 5-9, 1976.

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

R.P.H. ChangandA. K. Sinha, AppI. Phys. Lett.,29(1976)56. Y. Nakai, S. Yamagishi, T. Matsuo and T. Utagawa, Toshiba Rev., 24 (1969) 721. T. Itoh and Y. Sakai, Solid-State Electron., l 7 (1974) 751. W. Quast, Electron. Lett., 8 (1972) 419.

T.Miyazaki•N.Nakamura•A.D•iandT.T•kuyama•Jpn.J.App•.Phys.••4•Supp•.2•part2(•975) 441. K. Yamasaki and T. Sugano, Jpn. J. Appl. Phys., 17, Suppl. 17-1 (1978) 321.

T. Miura, N. Yokoyama, Y. NakayamaandM. Fukuta, Jpn. J. Appl. Phys.,SuppL17-1(1978) 153. K. Watanabe, M. Hashiba and T. Yamashina, Jpn. J. Appl. Phys., Suppl. 17-1 (1978) 335. D.J. Coleman, Jr., D. W. Shaw and R. D. Dobrott, J. Electrochem. Soc., 124 (1977) 239. M. Zerbst, Z. Angew. Phys., 22 (1966) 30. T. Ishii and B. Jeppson, J. Electrochem. Soc., 124 (1977) 1785. T. Suzuki and M. Ogawa, Appl. Phys. Lett., 31 (1977) 473. L.M. Terman, Solid-State Electron., 5 (1962) 285. A. Shimano, A. Moritani and J. Nakai, Jpn. J. Appl. Phys., 15 (1976) 939. C.R. Zeisse, L. J. Messick and D. L. Lile, J. Vac. Sci. Technol., 14 (1977) 957. C.N. Berglund, IEEE Trans. Electron Devices, 13 (1966) 701. E.O. Johnson, Phys. Rev., 111 (1958) 153. E.H. Nicollian and A. Goetzberger, Bell. Syst. Tech. J., 46 (1967) 1055. D . M . Brown and P. V. Gray, J. Electrochem. Soc., 115 (1968) 760. H. Preier, Appl. Phys. Lett., 12 (1967) 361. J.G. Simmons and L. S. Wei, Solid-State Electron., 16 (1973) 43.

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31 J.G. Simmons and L. S. Wei, Solid-State Electron., 16 (1973) 53. 32 P.E. Gregory, W. E. Spicer, S. Ciraci and W. A. Harrison, Appl. Phys. Lett., 25 (1974) 511. 33 W. Spicer, P. Pianetta, I. Lindau and P. W. Chye, J. Vac. Sci. Technol., 14 (1977) 917. 34 B. Bayraktaroglu, E. Kohn and H. L. Hartnagel, Electron. Lett., 12 (1976) 53. 35 C.C. Chang, B. Schwartz and S. P. Murarka, J. Electrochem. Soc., 124 (1977) 922. 36 H. Hasegawa and T. Sakai, J. Appl. Phys., 49 (1978) 4459.