Characteristics of thermal oxides grown on GaAs1 − xPx

Thin Solid Films, 56 (1978) 129-142 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

129

CHARACTERISTICS O F T H E R M A L OXIDES G R O W N O N GaAsl _xPx G. J. KUHLMANN, R. K. PANCHOLY AND D. H. PHILLIPS* Electronics Research Center, Rockwell International Corporation, Anaheim, Calif. 92803 (U.S.A.) (Received April 14, 1978; accepted May 18, 1978)

Oxides were thermally grown on GaAsl _ xPx of various mole fractions x using dry oxygen or steam at various temperatures. The elemental composition of the oxide films as a function of depth was examined using ion microprobe mass analysis. Our results indicated that the grown layers were arsenic deficient through the bulk of the oxide and arsenic rich near the oxide-semiconductor interface region. Capacitance-voltage (C-V) characteristics of metal-insulator-semiconductor (MIS) capacitor structures displayed the deep depletion behavior typically observed with wide band gap semiconductor MIS devices. The characteristics also exhibited a hysteretic nature indicative of electron trapping at the oxide-semiconductor interface. The interface trap densities estimated from analysis of the C-V characteristics were in the range (5-9) x 101 ~ cm-2 eV- 1. Post-oxidation annealing of the films in argon or nitrogen generally resulted in increased dielectric leakage current and in reduced C-V hysteresis effects. The correlation of electrical and ion microprobe data demonstrates the important role of arsenic in determining the electrical properties of the MIS devices.

1. INTRODUCTION Several material properties of GaAs and the alloy GaAs 1 _xPx, including a high electron mobility, a large band gap and a low minority carrier lifetime, make these IlI-V compound semiconductors attractive for high speed, high temperature device applications. Many of these properties can be optimized for particular applications by proper selection of the phosphorus mole fraction x. However, the development of many useful III-V semiconductor devices such as metal-oxide-semiconductor (MOS) field-effect and charge-coupled structures has been hampered by the lack of a practical method for forming a good quality native dielectric suitable for gate insulator and surface passivation applications. Several methods have been investigated for forming native dielectrics on GaAs. These include thermal oxidation 1-3, anodic oxidation 4-7 and plasma oxidation s-10. In general, all of the oxides formed have exhibited inferior electrical properties owing to dielectric leakage, large interface state densities and charge trapping in the oxide. Oxides grown using the anodic and plasma techniques require post-oxidation *Present address: LockheedMicroelectronicsCenter,Sunnyvale,Calif.94086, U.S.A.

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G . J . KUHLMANN, R. K. PANCHOLY, D. H. PHILLIPS

annealing treatments at high temperature (300-650°C) for stabilization. In addition, the anodic and plasma oxidation methods are not'well suited to large-scale device production because of the oxide growth procedure. The major difficulties associated with the thermal oxidation of GaAs are related to the incongruent formation of gallium and arsenic oxides and to the high equilibrium vapor pressure of arsenic oxides at elevated temperatures. The result is a lack of arsenic in the oxide, non-stoichiometry at the oxide-semiconductor interface and a film composed primarily of crystalline I~-Ga20 3 which does not have suitable dielectric and surface passivation characteristics. Uniform oxide films have previously been grown on GaAso.sPo.5 without severe decomposition of the semiconductor surface 11, and results on these films have indicated the possible importance of phosphorus, through the presence of stable GaPO4, in forming a higher quality dielectric than is attainable by simply oxidizing GaAs. In this paper the characteristics of oxides thermally grown on GaAsl _xPx of various mole fractions x are reported. Ion microprobe mass analysis (IMMA) was used to examine the elemental composition of the insulators with depth. The electrical properties of the oxide and of the interface region were investigated using current-voltage (I-V) and capacitance-voltage (C-V) measurements. Correlation of these results with the ion microprobe data suggests the importance of arsenic in the dielectric in determining the electrical characteristics of metal-insulatorsemiconductor (MIS) devices. 2. EXPERIMENTAL

2.1. Starting material The semiconductor material used in this study was n-type (tellurium-doped) epitaxial GaAs~ _xPx with (100) surface orientation. Phosphorus mole fractions of 0.3-0.7 were investigated in order to determine any major interface effects due to differences in the relative arsenic:phosphorus composition of the starting semiconductor. The epitaxial layers were of various donor concentrations (1 x 10161 × 1017 cm-3) and were grown by chemical vapor deposition on heavily doped ntype (p ~ 0.003 ~ cm) GaAs substrates (GaP for x = 0.7). A region of linearly graded phosphorus composition was grown between the substrate and the GaAs l_xPx epitaxial layer in order to minimize lattice mismatch. 2.2. Dielectric growth Prior to oxidation the samples were cleaned and degreased in trichloroethane, acetone and deionized water. After this operation a chemical etch in 5H2504: H202:H20 at room temperature for 5 min was carrie0 out to remove surface damage. The etch removed an estimated 2-4 lam of the surface region of the semiconductor. The oxides were grown in an open tube using either dry oxygen or steam as the oxidizing ambient. Various post-oxidation anneal ambients and temperatures were investigated in order to determine any significant effects on interface properties. All the steam oxides were annealed in dry oxygen for 2 h at the growth temperature. Some of the steam oxides and dry oxides were then annealed further in either nitrogen or argon at various temperatures.

CHARACTERISTICSOF THERMALOXIDESGROWNON GaAs~ _ xPx

131

2.3. M I S capacitor fabrication MIS capacitors were formed with aluminum vacuum evaporated through a metal shadow mask. The thickness of the aluminum gates was generally 1 I,tm. Some MIS capacitors which were analyzed using the ion microprobe had a thinner (8002000 A) aluminum layer to facilitate more rapid profiling. Residual oxide formed on the GaAs substrate during oxidation was removed by lapping with 5 lam alumina. Indium was then soldered onto the substrate and was pulse alloyed to form ohmic contacts.

2.4. Ion microprobe analysis of the film composition IMMA, which is fundamentally a secondary ion mass spectrometry analysis of micro-area regions, was used to examine the elemental depth composition of the insulator. The depth profiles were obtained on an ARL ion microprobe mass analyzer using a primary beam of ~802+ ions with a beam energy of 20 keV, the samples being maintained at + 1.5 kV; these conditions result in an effective ion energy of 9.25 keV for tsO+ ions. The use of an oxygen primary beam results in higher secondary ion yields than when an inert beam is used. The 18O isotope allows detection of 160 in the oxide without primary beam interference effects with the secondary ions.

2.5. Electrical characterization Measurements of the d.c. leakage current were made using a shielded probe and a Keithley model 610C electrometer. A.c. C - V measurements of the MIS capacitors were recorded for several different frequencies using a PAR model 124 lock-in amplifier. Additional C - V measurements at 1 MHz were made using a Boonton model 71A capacitance meter. 3. RESULTSAND DISCUSSION

3.1. Dielectric growth The growth behavior of oxides grown on GaAs~ _xPx in dry oxygen has been presented elsewhere ~t, ~2. Table I is a comparison of the oxidation time, temperature and thickness for various films grown during this study. The insulator thicknesses were determined by ellipsometry at 5460 ,~ or by step profiling after chemical TABLE I COMPARISON OF OXIDE G R O W T H ON G a A s I

Substrate GaAso.sPo.5 GaAso.sPo.5 GaAso.sP0.s GaAs0.sPo.5 GaAs0.6P04 GaAs0.6P0.* GaAso.6Po.4 GaAso.6Po.4 GaAso.3 Po.7

Ambient Steam Steam Dry 02 Dry 02 Steam Steam Steam Dry 02 Dry 02

_xPx IN DRY OXYGEN A N D STEAM

Temperature

Oxidation time

(°C)

(min)

Approximateoxide thickness (A)

650 600 700 680 550 550 550 680 680

90 45 370 1200 60 180 360 1200 1200

7000 1250 1180 2000 100 410 2000 2300 1500

132

G . J . KUHLMANN, R. K. PANCHOLY, D. H. PHILLIPS

etching. The region of the insulator near the oxide surface is etched in room temperature N H + O H at a rate of about 1000 A m i n - 1. This etching process reaches its limit before complete removal of the oxide. A residual layer a few hundred fingstr6ms thick remains which can be removed by a dip etch in 50H20:H2SO+: H202 at room temperature. This etch step appears to lift off the residual layer by attacking the underlying semiconductor surface. The amount of G a A s l _ x P x removed during the dip etch is less than 35 ,~. The nature of this interface and its possible influence on electrical characteristics will be discussed later. The growth rate of the oxides increases for decreasing phosphorus content in the semiconductor. The relationship between the growth kinetics and the mole fraction x of phosphorus available for incorporation into the oxide is not completely understood at present and requires further study.

3.2. Dielectric composition Previous studies of the oxidation of GaAs have indicated that the primary oxidation products at low temperatures ( < 300 °C) are G a 2 0 3 and As20313-15 and that the As203 becomes volatile or decomposes at higher temperatures. In contrast, the thermal oxidation of G a P has been found to result in either a mixture of G a 2 0 3 and G a P O 4 1 or simply GaPO+ 16 To examine the effect of the phosphorus content of the alloy GaAs I _xPx on the oxide composition, ion microprobe depth profiles were obtained for oxides grown on material of various mole fractions x. Figures l(a)-l(c) show profiles for oxides grown on GaASo.TP0.3, GaAs0.sPo. 5 and GaAso.3Po. 7. These films were grown for 370 min in dry oxygen at 700 °C. The profiles are uncorrected for differences in sputter rate and ion yield between the elements. A semiquantitative analysis of the

108 Ga

107

100

105

lO4 8 103

lO2

lO 1 3100 A - -

lo(

0

800

1200

1~0 TIME(SEC)

Ia }

CHARACTERISTICSOF THERMAL OXIDES GROWN ON GaAs1 -xPx

133

108

Ga 107

~

105 ~

-

-

0

104

103

io2 101 ~

600

1200

1800

TIME (SEe)

(b)

lO8 Ga 107

106

P

105

~

104

103

102

101

--

~I~OA 100

(c)

I

I 840O

I

I 1200

I

=] I

1800

TIME (SEC)

Fig. 1. Ion microprobe depth profiles for oxides grown in dry oxygen on (a) GaAso.vPo.3, (b) GaAso.sPo.s and (c) GaAso.3Po.7. I M M A data is in progress and will be presented elsewhere. However, some general observations regarding the behavior of the p h o s p h o r u s and arsenic during the oxidation can be made. The similarity between the p h o s p h o r u s and oxygen profiles

134

G . J . KUHLMANN, R. K. PANCHOLY, D. H. PHILLIPS

in the bulk of the oxide indicates the probability of a chemically bonded compound such as GaPO4. Other possible phosphorus compounds, e.g. P205, are not expected to be present in the film because of their volatility at the growth temperature. The behavior of the arsenic is similar to that observed in oxides grown on GaAs. The rise in arsenic counts near the surface of the oxide is similar to that noted with plasma-grown oxides on GaAs 14. The major portion of the oxide is arsenic deficient and there is apparently an arsenic-rich region near the oxidesemiconductor interface which is similar for all the phosphorus concentrations. Although it cannot be conclusively stated that there is an arsenic-rich interface region until more quamitative results have been obtained, such a result would not be unexpected in view of previous Auger profile studies on anodic 17, plasma 14 and thermaP 8 oxides grown on GaAs. In addition, as discussed earlier, the differences between the etching behaviors of the interfacial layer and of the remainder of the oxide suggest a varying composition with depth. Figure 2 shows an IMMA profile of an oxide grown on GaAso.sPo. 5 in steam at 600 °C for 225 min. After growth the sample was annealed in dry oxygen for 2 h at 600 °C to minimize possible instabilities due to O H - ions. Very little additional growth occurs during this step. The same general features exist in the profiles of both the steam oxides and the dry oxides; however, the phosphorus and the oxygen appear to be more uniformly distributed through the bulk of the oxide grown in steam. The effect of post-oxidation argon annealing on the oxide composition is illustrated in the IMMA profiles in Figs. 3(a) and 3(b). Figures 4(a) and 4(b) are similar profiles for MIS capacitor structures with unannealed and (premetallization) argon-annealed oxides respectively. In both cases the effect of the annealing is to decrease the amount of arsenic near the surface of the oxide. No

Ga

P

4

oo

1

0

i 0

i 1300

I

I 2600

I

I 3~00 ~

~

,r~llO0 A

TIME (SEC)

Fig. 2. An I M M A profile o f a GaAso.sPo. 5 steam oxide (see text): SOA, SOA-1, tox ~ 3400 A. The sample was annealed in argon at 600 °C for 30 min.

CHARACTERISTICS

OF THERMAL

OXIDES

GROWN

GaAs 1-~Px

ON

135

Ga

7

6 P 5

4

3

2

1

I

0

I

I

I

1100

0

I

I

2200

/

3300

3700 A

TIME (SEC)

(a)

7 ~

Ga

,,,

.,.,,-,-

6

~2

s

p

...i

'I 0

I 0

(b)

,4" I 800

I

I 1600

I

I

~ 2800A

2400

TIME (SEC)

Fig. 3. IMMA profiles of GaAs0.6P0. 4 dry oxides prepared in oxygen at 680°C: (a) unannealed, tox ~ 2100 A; (b) annealed in argon for 2 h at 680 °C, to, ~ 1800 A,

significant changes appear to occur in the arsenic peak at the oxide-semiconductor interface. Results similar to those shown in Figs. 3 and 4 were obtained for oxides grown on GaAs0.sP0. 5. The arsenic loss near the surface is probably due to the volatilization o f A s 2 0 a (the As20 a vapor pressure is 1 atm at 457 °C) which diffuses out during the growth process. Although the exact magnitude and location of the interface arsenic peak are

136

G.J.

KUHLMANN,

R. K. PANCHOLY,

D. H. PHILLIPS

A~

J"

P

I

I

I

I

900

1800

I

I'~] I ~75eeA 2700

TIME (SEC)

(a)

7

~

-v

6

s

A o

2 0 1

0

(b)

O

~ 6900 A

I 900

1800

I 2700

TIME (SEC)

Fig. 4. IMMA profiles of GaAso.6Po.4 MIS structures heated in oxygen at 680°C for 20 h: (a) unannealed, tox ~ 2300 A; (b) annealed in argon for 2 h at 680 °C, tox ,~ 2300 ,~. n o t k n o w n at present, it is possible that arsenic m a y be piling up on the s u b s t r a t e side of the interface. In this case n o changes in the p e a k are expected with annealing. However, the p o s t - a n n e a l i n g C-l/results described later suggest that s o m e changes m a y be o c c u r r i n g at this interface which result in changes in electrical properties.

3.3. Dielectric current-voltage and breakdown characteristics F i g u r e 5 shows a plot of the I - V characteristics of GaAso.sPo.5 M I S c a p a c i t o r s

CHARACTERISTICS OF THERMAL OXIDES GROWN ON GaAs~ _ xPx

137

10-5 BREAKDOWN

10-7

10.0

10"9

10"10

10-11

10-12

10"13

10-14

.--~

I 2

I

I 3

I

I 4

I

I 5

(VOLTS) 1/2

Fig+5. l-VcharacteristicsofGaAso.~Po.5 MIScapacitors fabricatedwith various annealing procedures: tox ~ 2000 ,~, A ~ 3.2 x 10- 3cm2; O, positive VG; D, negative VC. The sampleswereoxidizedfor 18 h in oxygen at 700 °C. with oxides that had been grown during the same oxidation run but that had undergone nitrogen anneals for different times. These point-by-point measurements were taken rapidly after the sample had been kept at the maximum bias for 15 rain. The results show an increase in leakage current with increasing nitrogen annealing time at 700 °C. Similar results were obtained for argon annealing. The linear relation between log I and V 1/2 and the polarity dependence of the current shown in Fig. 5 are indicative of Poole-Frenkel emission of electrons from coulombic-type traps. This mechanism is further demonstrated by the fact that all the devices had I - V characteristics which exhibited a transient behavior similar to that reported for anodic oxides on GaAs ~9, 20. Figure 6 shows the decay behavior of the dielectric leakage current for both positive and negative fields applied to the gate electrode. The general nature of this transient leakage current behavior is similar to the decay behavior of the hysteresis in the C - V characteristics and is apparently due to charging and discharging of the electron traps in the oxides. This phenomenon will be discussed further in relation to the C - V characteristics. Catastrophic dielectric breakdown occurred when the leakage current reached about 1 mA cm-2. Breakdown voltage measurements were made by "walking out" the I - V characteristic to avoid large transient currents due to trap filling or emptying. For the device in Fig. 5 breakdown voltages ranged from 18 V for 2000 A oxides annealed for long times (4.75 h in nitrogen) to 35 V for unannealed oxides. These values correspond to dielectric strengths of 9 x 105 and 1.75 x 106 V c m respectively. The increase in dielectric leakage current and the associated decrease in

138

G . J . KUHLMANN, R. K. PANCHOLY, D. H. PHILLIPS

1 .Of

0.8

o .J

0.6

0.4

0.2

0

I 10

I 100

1000

t (SEC)

Fig. 6. The decay behavior of the dielectric leakagecurrent; tox ,~ 2000 A. breakdown voltage with annealing are probably related to the loss of arsenic or earlier.

As203 from the film, as discussed

3.4. Capacitance-voltage characteristics Figure 7 shows the type of high frequency C - V characteristic typically observed for the GaAs~_xP x MIS structures. These devices exhibit depletion-type characteristics at room temperature as a consequence of the wide band gap and the low minority carrier generation rate of GaAs I -~Px. The C - V characteristics were identical over the frequency range from 100 Hz to 1 MHz, in contrast with the behavior reported for GaAs oxides 5' 9 but similar to that reported for G a P oxides 21. The hysteresis in the C - V characteristics is like that observed in films grown on both GaAs 5 and G a P 21. The magnitude of this hysteresis depends on the maximum bias applied during the sweep. In all cases the device under test was biased at the m a x i m u m sweep bias (both positive and negative) for 2 min before sweeping or resweeping. Figure 8 illustrates the effect of the bias sweep rate on the C - V hysteresis. The hysteretic behavior of these devices is apparently due to electron injection and trapping in the oxide near the oxide-semiconductor interface. The large amount of hysteresis in MIS devices fabricated on wide gap semiconductors such as GaAs~ _xPx (1.4 ~< Eg ~< 2.25 eV for 0 ~< x ~< 1) can be attributed to the fact that the emission rate of the electron traps is too low for them to follow the d.c. voltage variation during the sweep 22. Illumination of the devices with intense microscope light removes most of the hysteresis by increasing the trap emission rate, as shown in Fig. 7. No significant change in hysteresis was noted for the oxides grown on material of different phosphorus mole fractions. However, post-oxidation annealing resulted in a reduction in hysteresis, as shown in Fig. 9. Figure 9(a) illustrates the C - V

CHARACTERISTICS OF THERMAL OXIDES G R O W N ON G a A s I

_xPx

139

180

80

60

40

L --12

--8

_ 1 _ _

~

m

--4

+4

+8

V G (VOLTS)

Fig. 7. A typical C - V characteristic of a GaAso.sPo. s M I S structure (device 10-1-1): f = A = 3.2 x 10 -3 cm 2, C i = 84.5 pF, tox ~ 2200 A,, N d ~ 1 x 1017 cm -3, sweep rate 130 m V s - 1.

1 MHz,

100

80

60

40

--12

-8

-.4

1

. I . m

4

8

12

VG (VOLTS)

Fig. 8. A GaAso.sPo. s M I S C - V characteristic s h o w i n g the effect of bias sweep rate o n the hysteresis: curve 1, sweep rate 130 m V s - l ; curve 2, sweep rate 20 m V s - l ; f = 100 kHz, A = 3.5 × 10 -3 c m 2, t o x = 2000 A.

140

G. J. KUHLMANN, R. K. PANCHOLY, D. H. PHILLIPS

6O

55

50

2 IVIIN AT -15 V

40

35

30

.~L -15

-10

-5

+5

VG (VOLTS)

(a) SO

45

4O

35

F= 3O

25

2O <

o~

-15

(b)

-10

-5

.03

vc (VOLTS]

Fig. 9. GaAso.6Po. 4 MIS C - V characteristics: (a) unannealed sample (4N14), dry oxide, A ~ 2.15 x 10-3 cm 2, tox ~ 2350 A, N d ~ 7 x 10 ~6 era-3 j . = I M Hz, sweep rate 90 mV s-~; (b) argon-annealed sample (4A13), dry oxide, A ~ 1.58x 10 -3 cm 2, tox ~ 2200 A, N d ~ 7x 1016 c m - a , f = 1 MHz, sweep rate 90 mVs -~ characteristic of an unannealed GaAso.6P0: MIS device which exhibited a s i g n i f i c a n t a m o u n t o f h y s t e r e s i s . T h e c h a r a c t e r i s t i c for a d e v i c e o x i d i z e d a t t h e s a m e t i m e b u t w h i c h h a d u n d e r g o n e a 2 h p o s t - o x i d a t i o n a r g o n a n n e a l a t 6 8 0 °C is s h o w n in Fig. 9(b). T h e r e d u c t i o n in h y s t e r e s i s is a c c o m p a n i e d b y a n i n c r e a s e in t h e

CHARACTERISTICS OF THERMAL OXIDES G R O W N ON

GaAs~ _ xPx

141

dielectric leakage current, as discussed earlier. The ion microprobe profiles do not exhibit any significant compositional changes near the oxide-semiconductor interface. It is in this region that any changes in composition are expected to result in measurable changes in trapping behavior and hysteresis. It is possible that with other analytical techniques, e.g. X-ray photoelectron spectroscopy, it may be possible to detect more subtle changes in bonding structure which may be causing changes in trapping characteristics. Annealing in argon or nitrogen at 450 °C after oxidation at 680 °C did not result in a reduction in the C - V hysteresis. At this temperature a significant loss of arsenic from the interface region is not expected; some loss may still occur at the oxide surface however. It is possible that annealing at a lower temperature ( ~ 300 °C) in a reactive gas such as hydrogen may be effective in reducing trapping states without causing a loss of arsenic and an increase in oxide conductivity. Such anneals have had some success when applied to GaAs anodic oxides 4. An estimate of the interface state density was made by applying Terman's differential C - V method 23 to the positive-going curve (Fig. 8), since this portion of the curve was not sensitive to the sweep rate. Values obtained using this approach were in the range (5-9)x 1011 cm -2 eV -1. It is not clear whether this and other conventional interface analysis techniques, such as the a.c. conductance and quasistatic methods, remain valid for III-V compound semiconductor interfaces where thermal equilibrium conditions may not exist owing to a low thermal generation rate and a high dielectric leakage current. 4.

SUMMARIZING REMARKS

The chemical and electrical properties of oxides thermally grown on GaAsl_xP x were investigated over the range of phosphorus mole fraction 0.3 ~< x ~< 0.7. Results of ion microprobe measurements indicate that the oxides are composed primarily of gallium, phosphorus and oxygen, with arsenic being depleted from the major portion of the film. Arsenic-rich regions appear both at the oxide surface and at the interface with the semiconductor. Post-oxidation annealing at the growth temperature (680-700 °C) results in arsenic loss near the oxide surface. The hysteresis observed in the C - V characteristics of MIS devices is attributed to electron trapping in the oxide. Annealing reduces this hysteresis but also results in an increase in dielectric leakage current, apparently due to the loss of arsenic from the oxide. The results of this work indicate that the presence of phosphorus in the semiconductor does affect the thermal oxidation kinetics and inhibits severe decomposition of the surface. The incorporation of phosphorus into the grown oxide in a uniform manner is apparently responsible for the somewhat improved electrical behavior of these oxides compared with the behavior of many thermal oxides grown on GaAs. However, the importance of controlling the arsenic in the oxide during the oxidation and annealing processes is clearly demonstrated for this alloy, as it has been previously for GaAs. It is possible that steam oxidation processes at lower temperature in conjunction with a low temperature reactive gas anneal could provide such control of the arsenic loss.

142

G.J. KUHLMANN, R. K. PANCHOLY, D. H. PHILLIPS

ACKNOWLEDGMENTS

The authors wish to thank R. Drouet and E. H. Romero for assistance with device fabrication and Dr. W. Stuckey and Mr. N. Marquez of the Aerospace Corporation for the IMMA work performed as part of this research. The encouragement and helpful suggestions of G. Kinoshita are also sincerely appreciated. This research was supported by the U.S. Army Night Vision Laboratory under Contract DAAK70-77-C-0122, by NASA under Contract NASI-15101 and by Rockwell International Corporation. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

14 15 16 17 18 19 20 21 22 23

M. Rubenstein, ,1. Electrochem. Soc., 113 (1966) 540. S.P. Murarka, Appl. Phys. Lett., 26 (1975) 180. C.W. Wilmsen and S. Szpak, Thin Solid Films, 46 (1977) 17. H. Hasegawa, K. E. Forward and H. L. Hartnagel, Appl. Phys. Lett., 26 (1975) 567. T. lkoma, H. Tokuda, H. Yokomizo and Y. Adachi, Jpn. J. Appl. Phys., 16 (1977) 475. C.R. Zeisse, L. J. Messick and D. L. Lile, J. Vac. Sci. Technol., 14 (1977) 957. S.M. Spitzer, B. Schwartz and G. D. Weigle, J. Electrochem. Soc., 122 (1975) 397. R.P.H. Chang and A. K. Sinha, Appl. Phys. Lett., 29 (1976) 56. N. Yokoyama, T. Mimura, K. Odani and M. Fukuta, Appl. Phys. Lett., 32 (1978) 58. L.A. Chesler and G. Y. Robinson, Appl. Phys. Lett., 32 (1978) 60. D.H. Phillips, W. W. Grannemann, L. E. Coerver and G. J. Kuhlmann, J. Electrochem. Soc., 120 (1973) 1087. R.K. Pancholy and D. H. Phillips, Thermal oxidation of GaAs 1 xPx, Paper presented at 19th Electronic Materials Conf., Ithaca, New York, June 1977. C.W. Wilmsen and R. W. Kee, Analysis of the oxide-semiconductorinterface using Auger electron spectroscopy and ESCA as applied to InP and GaAs, Paper presented at 5th Conf. on the Physics of Compound Semieonductor Interfaces, Los Angeles, California, January 1978, in J. Vac. Sci. Technol., 15(1978) 1513. C.C. Chang, R.P.H. ChangandS. P. Murarka, J. Electrochem. Soc.,125(1978)481. R.A. Logan, B. Schwartz and W. J. Sundburg, J. Electrochem. Soc., 120 (19.73) 1385. R. Nishitani, H. lwasaki, Y. Mizokawa and S. Nakamura, Jpn. J. Appl. Phys.,17 (1978) 321. C.C. Chang, B. Schwartz and S. P. Murarka, J. Electrochem. Soc., 124 (1977) 922. !. Shiota, N. Miyamoto and J. Nishizawa, J. Electroehem. Soc., 124 (1977) 1405. B. Weiss, E. Kohn, B. Bayraktaroglu and H. L. Hartnagel, Inst. Phys. Conf. Ser., 33a (1977) 168. G. Weimann, Thin Solid Films, 47(1977) 127. T. lkoma and H. Yokomizo, IEEE Trans. Electron Devices, 23 (1976) 521. A. Goetzberger and J. C. Irvin, 1EEE Trans. Electron Devices, 15 (1968) 1009. L.M. Terman, Solid-State Electron., 5 (1962) 285.