Temperature programmed desorption investigations of hydrogen and ammonia reactions on GaN

surface science ELSEVIER

Surface Science 381 (1997) L581 L588

Surface Science Letters

Temperature programmed desorption investigations of hydrogen and ammonia reactions on GaN Ratna Shekhar, Klavs F. Jensen * Department o["Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received 11 December 1996; accepted for publication 27 January 1997

Abstract

We report data for chemisorption and reaction of deuterium and isotopically labeled ammonia on single-crystalline GaN films grown on sapphire substrates. Temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) studies, following exposure of the clean GaN film at room temperature to the probe reactant species, were conducted under UHV conditions. Deuterium desorption took place over a wide temperature range, 525-800 K, with molecular deuterium as the only product. At low exposures, two distinct deuterium desorption peaks at ~ 660 and 770 K were observed. The deuterium desorption peak at 660 K shifted to lower temperatures with increasing D adatom coverages. TPD experiments after ammonia adsorption on GaN revealed small amounts of hydrogen desorbed at ~ 600 K and over a range 660 770 K, suggesting partial decomposition of ammonia. Molecular ammonia desorption was observed at ~ 560 and 600 K, with the low temperature desorption state growing with increasing ammonia exposures. Further studies on deuterium-precovered GaN films indicated that ammonia production resulted from recombination of NHx species and hydrogen adatoms on the surface. © 1997 Elsevier Science B.V. Keywords. Ammonia; III-V compound semiconductors; Gallium nitride; Hydrogen; Thermal programmed desorption

Organometallic chemical vapor deposition ( O M C V D ) with a m m o n i a a n d a trialkyl gallium (e.g., t r i m e t h y l gallium) is a m a j o r t e c h n i q u e for g r o w t h o f G a N films for light-emitting devices [ 13]. T h e difficulties in achieving n i t r o g e n i n c o r p o r a tion a n d the p o t e n t i a l effects o f h y d r o g e n i n c o r p o r a t i o n on d o p i n g p e r f o r m a n c e p r o v i d e incentives for u n d e r s t a n d i n g the r e a c t i o n s o f a m m o n i a a n d h y d r o g e n species on G a N . H y d r o g e n c h e m i s o r p tion a n d a m m o n i a r e a c t i o n s on o t h e r c o m p o u n d s e m i c o n d u c t o r surfaces, n o t a b l y G a A s , have been r e p o r t e d in the literature. H y d r o g e n was f o u n d to d e s o r b m o l e c u l a r l y f r o m single-crystal G a A s sur* Corresponding author. Fax: + 1 617 258 8224; e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0039-6028(97)00085-X

faces at 5 2 0 K , after s e c o n d - o r d e r h y d r o g e n a d a t o m r e c o m b i n a t i o n [4,5]. A n a d d i t i o n a l p e a k was o b s e r v e d at ~ 670 K on an A s - r i c h surface. M i n o r a m o u n t s o f arsine (ASH3) d e s o r p t i o n have also been o b s e r v e d following h y d r o g e n a d a t o m a d s o r p t i o n on clean G a A s surfaces [4,5]. T h e f o r m a t i o n o f N H x a n d H adspecies d u r i n g a m m o nia d e c o m p o s i t i o n on the clean G a A s ( 1 0 0 ) surfaces has been r e p o r t e d [6,7]. Singh et al. [7] also r e p o r t e d t h a t N H x a n d H adspecies u n d e r g o r e c o m b i n a t i v e d e s o r p t i o n to p r o d u c e a m m o n i a at ~ 4 5 0 K, with s o m e residual n i t r o g e n r e m a i n i n g on the surface after c o m p l e t e d e h y d r o g e n a t i o n o f the N H x adspecies. I n a d d i t i o n to t h e r m a l d e c o m p o s i t i o n [6,7], a d s o r b e d a m m o n i a on the clean G a A s ( 1 0 0 ) surface has been r e p o r t e d to u n d e r g o

Ratna Shekhar, Klavs F Jensen / SurJace Science 381 (1997) L581-L588

[7-9] and photon-induced [ 10] decomposition. Recently, hydrogen desorption and ammonia adsorption on a polycrystalline GaN surface were studied by Chiang et al. [11] using time-of-flight scattering and recoil spectroscopy (TOF-SARS) technique. In this study, two different states for deuterium desorption were identified; one at ~ 525 K for D2 desorption from Ga sites and the other at ~ 775 K for desorption from N sites. No formation of NH3 was observed during hydrogen desorption experiments on the polycrystalline GaN surface [11], in contrast to GaAs studies where arsine (ASH3) desorption occurred [4,5]. Ammonia was noted to chemisorb dissociatively on the GaN surface at room temperature [ll]. In the present investigation, temperature programmed desorption and isotope-labeling methods are used to delineate the coverage-dependent pathways of hydrogen interaction and ammonia decomposition on clean and deuterium pre-/postexposed GaN(0001) surfaces. All experiments were carried out in a stainless steel ultra-high-vacuum chamber equipped with a differentially pumped quadrupole mass spectrometer (VG) multiplexed with a computer for TPD experiments, as described in detail elsewhere [12]. All TPD data collections were done by placing the sample within 2 mm of the opening (3 ram) of the QMS shielding. During experiments, as many as 16 different masses (m/e values) were monitored simultaneously. The chamber was also equipped with LEED/AES (Fison) facilities and an ion sputtering gun. The chamber base pressure was ~7 × 10-~° Torr. GaN(0001 ) film ( ~ 1 ~tm) was grown on a sapphire substrate [13]. The crystallinity of the film was verified again by X-ray diffraction (XRD) where prominent peaks (20) only at 34.4 and 72.8 degrees were observed, indicating a strong (0001) orientation [ 14]. Nonetheless, it is to be noted that GaN films typically have lattice defects and stacking faults, leading to localized electronic effects [15-17]. The 1 5 m m × 7 m m GaN sample was clamped to a button heater with the help of tantalum (0.25mm) strips spot-welded to the button heater. A 3000 A thick tantalum film was also sputter-deposited on the backside of the sap-

phire to facilitate uniform heating of the GaN film. Prior to use, the GaN film was degreased with methanol, washed with distilled water and finally, blow dried with nitrogen. In situ cleaning of the film was accomplished with cycles of mild Ar + sputtering (1 kV, 15 ~tA) and annealing at 1000 K. GaN surface cleanliness was monitored by collecting the Auger spectra at different locations on the film, before and after in situ cleaning. A recent detailed study of GaN surface cleaning [18] indicates, however, that some residual impurities, carbon and oxygen, could still be present in small quantity. The linear heating rate used in this study was ~3 K/s. A k-type thermocouple, glued at the back edge of the sample with inorganic ceramic, was used to monitor the sample temperature. The temperature values recorded in this study are internally consistent as all studies were performed on the same sample. A simple heat transfer analysis for the thin sample used (< 1 mm) in this study indicates that the temperature difference between the front and the back of the sample is _<5K. Research-grade deuterium (Matheson, 99.5%) and ammonia (CIL ND3 with 99% D content and Matheson N15H3 with 99% N 15 content) were used. The GaN surface was exposed to probe molecules at room temperature via dynamic backfilling of the UHV chamber. Deuterium atoms were generated by dissociating D 2 on a "whitehot" (~ 2000 K) tungsten filament placed ~ 5 cm away from the surface [19,20]. During this process, the surface temperature went up by ~ 50 K. It was difficult to estimate the atomic deuterium exposures onto the GaN surface and therefore, only relative exposures based on background molecular deuterium pressure are indicated below. Exposures are expressed in Langmuirs (L) where 1 L represents 10- 6 Torr. s. In order to avoid confounding our observations by hydrogen in the chamber background, deuterium (De) was used to model hydrogen interaction with the GaN surface. When GaN was exposed to large amounts of molecular deuterium (dose as large as 5 × 10 -s Torr for 600 s), no deuterium desorption was observed in the temperature programmed desorption experiments. However, dideuterium (D2) was observed as the major

Ratna Shekhar, Klavs F. Jensen / Surface Science 381 (1997) L581-L588

desorption product during T P D experiments when G a N was exposed to different amounts of atomic deuterium. Similar results of molecular versus atomic deuterium exposures onto GaAs(}00), GaAs( 111 ) and polycrystalline GaN surfaces have been reported in previous studies [4,5,11 ]. Small amounts of H D were also observed in the desorption spectra as a result of cracking of background H 2 and subsequent surface recombination with D adatoms. No other products, such as ammonia (ND3), was observed. Deuterium T P D spectra, following increasing exposures of atomic deuterium onto the clean GaN surface, are shown in Fig. 1. For very small exposures, a deuterium desorption peak was first observed at ~770 K. Another desorption state was noted at ~660 K which began to progress toward lower temperature and ultimately, a large peak at 625 K. The lowering

625 K

U} ¢..

m

e" e" m

E o

I-

0

1500 L

(/) U~

100 L

30 L 5L

400

500 600 700 Temperature (K)

800

Fig. 1. D2 (m/e = 4 ) T P D spectra following increasing exposures of D atoms onto GaN(0001 ) film.

of desorption peaks with increasing cover~ ge is indicative of second-order recombinative de~orption at ~660 K. As compared to GaAs sl adies [4,5], the deuterium desorption from GaN occurred over a wide temperature range. The onset of molecular deuterium desorption occurr,:d at 450 K and the high temperature shoulder continued until 800 K. However, the relative amotmt of deuterium desorption above 700 K was negligible. Fig. 2a depicts the TPD spectra after the clean G a N surface was exposed to 30 L of ammonia (ND3). The only peaks observed during ammonia T P D were m/e=20, 14 and 4. Our observation of gaseous molecular deuterium and nitrogen following ammonia TPD suggests that ammonia partly decomposed on the GaN surface. The parent molecule (m/e=20) desorbed at 545 and 600 K, with a high-temperature shoulder. The molecular deuterium desorbed over a temperature range of 480-780 K. The onset of deuterium desorption at 480 K suggested that partial ammonia decomposition occurred below 480 K. To avoid interfi~rence with the background CO (m/e = 28), nitrogen was monitored by collecting the m/e = 14 signal. A very small nitrogen peak was observed at 830 K. No m/e=69 or 83 signals were observed, sug~:esting no detectable thermal desorption of Ga ~ or GaN * species. On the basis of the decomposition product yields, the relative yield of ammonia decomposition versus desorption seemed to be very small (5%). Auger electron spectroscopy, tbllowing completion of the ammonia TPD, was unable to notice any significant change in the N KLL Auger peak. In order to assess whether any nitrogen exchange between ammonia and the GaN surface took place during the ammonia T P D discussed above, ammonia decomposition studies on GaN were performed with N~SH3 . No nitrogen exchange between the substrate and the reactant was observed. The cracking pattern of ammonia desorbing from the G a N surface remained identical to that obtained for the probe reactant ammonia. For example, the m/e= 15 signal over the 480-650 K temperature range resulted solely from the cracking of molecular (N ~5H3) ammonia. Furthermore, no high-temperature peak for m/e= 14 was observed when isotopically labeled (N~SH 3) ammonia was used,

Ratna Shekhar, Klavs F. Jensen / Surface Science 381 (1997) L581-L588

~I(a)

NE

~[(b)

600K

zl ~

N15H3] .0K

J

[(C) 560K

NlSH3

I'

545K

t

%~' m/e=14(x8

i = W

mJe=-15(x4)

:!(0

1 350 450 550 650 750 850 Temperature

(K)

m/e=14(xl0) I t I 500 70O Temperature

J

(K)

9OO

I

I

I

I

400 600 Temperature

800

(K)

Fig. 2. TPD product spectra following (a) 30 L ND3 exposure to the clean GaN(0001 ) film; (b) 30 L N 15H3 exposure to the clean GaN(0001 ) film; (c) increasing exposure of N15H3 onto the clean GaN(0001 ) surface.

The representative TPD data for a 30 L (N15H3) ammonia exposure onto the GaN surface is depicted in Fig. 2b where molecular nitrogen desorbed at ~850K. The molecular nitrogen desorbed after recombination of N 15 adatoms produced after complete decomposition of N15H3 ammonia. This observation indicated that no nitrogen species from the GaN film lattice were released during ammonia reaction studies, unlike previous observation of GaAs etching during ammonia reaction where AsH3 and As2 desorbed at high temperatures [4,5]. Fig. 2c depicts desorption states after a series of ammonia exposures. At low exposure, ammonia desorbed at 550 and 600 K, similar to the previous observation in Fig. 2a. However, the higher temperature desorption state in Fig. 2c saturated soon afterwards and the peak at 560 K grew larger. In order to gain insight into the ammonia decomposition mechanism on the GaN surface, ammonia TPD experiments on D-precovered GaN(0001) surfaces were conducted. At first, the clean GaN surface was exposed to 150 L of D z via the "white-hot" filament method to produce deuterium adatoms. The prepared GaN surface was then exposed to 100L of ammonia. The TPD results, depicting deuterated ammonia production, are presented in Fig. 3. The observation of Do-, D1- and Dz-ammonia at ~ 560 K indicated that

~

450

NH3

550 650 Temperature (K)

750

l l l l r l l l

450

550 650 750 Temperature

(K)

850

Fig. 3. Ammonia TPD product spectra following 100L N 15H3 exposure onto a D-precovered GaN surface.

the ammonia decomposed on GaN film to produce NHx adspecies which later recombined with D adatoms to produce gaseous ammonia with differing amounts of deuteration. The ratio of deuterated versus non-deuterated ammonia production at ~ 565 K in the TPD experiment shown in Fig. 3a was approximately 0.25. Furthermore, the desorption of HD at ~ 600 K with a high-temperature tail-end also suggested that a substantial amount of ammonia underwent thermal decomposition on the GaN surface. However, dehydrogenation of

Ratna Shekhar, Klavs F. Jensen / Surflace Science 381 (1997) L581-L588

NHx adspecies on GaN was completed by 750 K. N adatoms thus released later recombined to desorb as molecular nitrogen at ~ 850 K. In another set of experiments, the clean GaN surface was first exposed to 100 L of NtSH3 at room temperature and then annealed at ca. 550-560 K for 2 min, in order to synthesize small amounts of NHx fragments on the surface. The prepared surface was post-dosed with 150 L D 2 exposure via the "white-hot" filament method. The results of this experiment are presented in Fig. 4 where D~ (i=0, 1, 2) ammonia desorption peaks were observed at 630 and 675 K. As depicted earlier in Fig. 3a, these high-temperature ammonia desorption states were convoluted with the larger desorption peak at 560 K and hence could not be resolved clearly. The most important observation is the enhancement in the relative yield of deuterated versus non-deuterated ammonia products. In comparison to the yield of deuterated ammonia products at 560 K, the relative yield of deuterated ammonia production shown in Fig. 4 was nearly equal to that of the non-deuterated ammonia. The relative product yield distribution following TPD experiments is shown as an inset in Fig. 4. The relatively smaller yields of NHD2 production in TPD experiments depicted in Fig. 3 and 4 indicates that NH 2 adspecies were mostly present after ammonia decomposition on the GaN surface. No molecular deuterium adsorption on GaN film at room temperature was observed in this study. The absence of interaction of molecular deuterium with III-V semiconductors, like GaAs(100), GaAs(110) and polycrystalline GaN surfaces [4,5,18], has been noted previously. At low exposures (< 10 L), deuterium desorption took place at a peak at ~770 K and a second peak at 660 K appeared on increasing coverages. The second peak at 660 K grew larger, shifting to lower temperature with increasing doses of atomic deuterium. Eventually, there appeared to be only one strong deuterium desorption peak at 625 K after an exposure as large as 3000 L. Shifting of deuterium desorption peak temperature with increasing coverage indicates a second-order recombinative desorption, as suggested in previous studies [4, 5, 11 ]. The high-temperature deuterium desorption state above 700 K was completely convoluted

0.6 ~ .'•m

_~o..tl::~!!it o., tli iii]

NH3 NDH 2 ND2H

"~

NH 3

t._

2

I 550

I

I

600 650 700 Temperature (K)

750

Fig. 4. TPD product spectra depicting NDiH 3 i (i=0, 1, 2) after post-dosing D atoms on the GaN surface covered with NHx fragments; the inset indicates ND~H 3 ~(i=0, 1, 2) product distribution following experiments.

with the more prominent low-temperature desorption state. Using the TOF-SARS technique, Chiang et al. [11] have proposed the presence of two distinct adsorption sites; deuterium adsorption on Ga sites and on N sites. While there is evidence of two states of molecular deuterium desorption

Ratna Shekhar, Klavs F. Jensen / Surface Science 381 (1997) L581 L588

tN surface in our study also, the hightemperature desorption state could be resolved only at very low exposures. Similar to the analysis of Auger electron spectra of clean GaN surface reported by Khan et al. [21], our Auger results (the ratio of nitrogen and gallium Auger intensities) for the clean GaN film indicated that the surface was gallium rich; a similar conclusion was reported recently by Ponce et al. [17]. The deuterium desorption peak in this study was more than 100 K higher than those reported in the corresponding studies on GaAs surfaces [4,5]. This difference, in contrast to GaAs studies, could be due to the different electronic environment around the Ga sites on the GaN surface, e.g., larger ionicity of GaN. Furthermore, the deuterium desorption from GaN took place over a wide temperature range which suggests more than one binding state arising from localized electrostatic effects [22]. The small presence of high-temperature desorption states in our study ( > 7 0 0 K ) could also be due to defect sites on the GaN film. No deuterium desorption was observed when the sample alone, after numerous deuterium TPD studies, was annealed at higher temperatures (> 1000 K). However, subsurface diffusion of deuterium atoms into the GaN sample cannot be ruled out completely. A recent study by Pearton et al. [23] has suggested that hydrogen readily diffuses into binary and tertiary nitrides at temperatures in the range 85-250°C and that hydrogen removal from subsurface lattice sites occurs above 800°C. Overall, as suggested previously by Chiang et al. [ 11 ], hydrogen adatoms are not likely to be present on the growing GaN film surface and hence, will not block the adsorption sites in the operating temperature range for GaN growth processes

[1-31. Unlike hydrogen chemisorption on GaAs surfaces [4,5] where arsine (ASH3) was reportedly observed to desorb from the surface and cause surface etching, hydrogen chemisorption on GaN in this study suggested lack of such activity. Similar observation during hydrogen chemisorption on polycrystalline GaN surface has been noted previously [ 18]. The different reactivities of hydrogen on GaAs and GaN surfaces are possibly due to the thermodynamic energetics of the respective

surfaces. The cohesive energies for GaAs and GaN are approximately 155 and 218 kcal/mol, respectively [24,25]. Also, due to the partial double bond character of the N - G a bond, the surface bond in GaN is shorter and stronger than that in GaAs [22]. Therefore, hydrogen adatoms on GaN encounter a higher barrier for the etching reaction, i.e., volatile ammonia evolution from GaN following hydrogen adatom adsorption. However, under high temperature (~900°C) and hydrogen ambient (1 atm), GaN(0001) surfaces grown on sapphire have been reported to react with H 2 to form Ga droplets and NH 3 [26,27]. Following ammonia doses onto clean GaN, molecular ammonia desorption in large amounts at ~ 5 5 0 K was observed. Small amounts of hydrogen and nitrogen as TPD products during ammonia reaction on GaN were also noted, suggesting minor ammonia decomposition pathways. However, observation of multi-deuterated ammonia production at ca. 560 K indicated the presence of NHx adspecies below 480 K which underwent hydrogenation to produce volatile ammonia as a major TPD product. The distribution of deuteration in volatile ammonia produced during isotope labeling TPD experiments is suggestive of the presence of significant amounts of NH2 species on the surface which either desorbed as gaseous ammonia or decomposed non-selectively to produce hydrogen and nitrogen adatoms. This recombination reaction of NH2 and hydrogen adspecies to produce molecular ammonia has been reported to occur on other semiconductor surfaces as well [6-10,28]. In similar isotope-labeling ammonia TPD studies on the GaAs(100) surface, Singh et al. [7] have reported the production of gaseous NHD 2, NHzD and NH3 at ca. 460 K. In a study involving laser-induced interaction of ammonia with GaAs(100), using TPD and HREELS results, Zhu et al. [10] confirmed the presence of NH2 adspecies as the most dominant surface species which reacted to produce NH 3 and H 2 between 300 and 700 K. In a related study on the surface chemistry of 1,1-dimethylhydrazine on GaAs(100) [28], the formation of ammonia via NH2(ad) and hydrogen adatom recombination in the temperature range 580-600 K was recently noted. In the above study [28], it was further suggested that

Ratna Shekhar. Klavs F Jensen / Surjace Science 381 (1997) L581-L588

NH2 adspecies underwent simultaneous decomposition and hydrogenation reactions. The evolution of small amounts of N H D 2 as well as dinitrogen at high temperature in the present study indicates that NH2(ad) decomposed via sequential dehydrogenation. However, it is also plausible that some H - D exchange reactions between N H 2 and D adspecies to produce volatile NHD2 took place. Unfortunately, the method of depositing deuterium atoms on the surface via the "white-hot" filament method leads to uncertainties in estimating the surface coverage and hence, no modeling to verify isotopic scrambling in ammonia products could be undertaken. In the light of the conflicting reports on ammonia decomposition on GaAs surfaces [7,9,10], it is important to note that no leakage currents due to stray electrons from the quadrupole mass spectrometer (QMS) were observed in the present UHV study. It is also unlikely that electroninduced dissociation would lead to ammonia decomposition to an extent observed in this study. Before isotope labeling studies, the product yields after ammonia decomposition on GaN were negligible in comparison to the amounts of molecular ammonia desorbed at --~550 K. The extent of thermal dissociation of ammonia could be ascertained in the present UHV study only after the observation of multi-deuterated ammonia products at ~550 K. In the recent UHV study by Chiang et al. [11], ammonia was observed to be reactive on both clean and hydrogen-terminated amorphous G a N surfaces at an adsorption temperature as low as 2 5 C where ammonia dissociatively adsorbed to produce both G a l l and N H species in the time-of-flight mass analyses. These observations of ammonia decomposition on GaN surfaces under UHV conditions are in contrast to an earlier high-temperature study [29] where GaN itself was shown to have little influence on ammonia decomposition. However, the coexistence of Ga and G a N was observed to induce significant catalytic decomposition of NH3 to N2 and extensive ammonia decomposition occurred (up to ~60%) above a threshold temperature of 950-1000°C [29]. Molecular hydrogen does not react with GaN at room temperature. No film etching due to hydrogen adatom interaction with GaN was

observed and negligible amounts ot . remained on the surface above 750 K. The ammonia chemisorbed dissociatively on GaN to produce NHx and hydrogen adspeeies. The majority of NHx adspecies on G a N desorbed as molecular ammonia after recombination reaction with H adatoms and a minor reaction pathway for NH~ adspecies was further dehydrogenation to nitrogen and hydrogen. The ammonia reaction on GaN did not lead to any etching or surface decomposition reaction.

Acknowledgements This work was supported in part by Mitsubishi Chemicals and by the National Science Foundation.

References [l] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 31 (1992) [95. [2] M.A. Khan, D.T. Olson, J.N. Kuznia, Appl. Phys. Lett. 65 (1994) 64. [3] H. Morkoc, S.N. Mohammad, Science 267 (1995) 51. [4] W. Mokwa, D. Kohl, G. Heiland, Phys. Rev. B 29 (1984) 6709. [5] J.R. Creighton, J. Vac. Sci. Technol. A 8 (1990) 3984. [6] A. Tripathi, D. Mazzarese, W.C. Conner, K,A. Jones, J. Electron. Mater. 18 (1989) 45. [7] N.K. Singh, A.J. Murrell, J.S. Foord, Surf. Sci. 274 (1992) 341. [8] Y.-M. Sun, J.G. Ekerdt, J. Vac. Sci. Technol. B I I (1993) 610. [9] Y.~M. Sun, D.W. Sloan, T. Huett, J.M. White, J.G. Ekerdt, Surf. Sci. 295 (1993) L982. [10] X.-Y. Zhu, M. Wolf, T. Huett, J.M. White, J. Chem. Phys. 97 (1992) 5856. [11] C.-M. Chiang, S.M. Gates, A. Bensaoula. J.A. Schultz, Chem. Phys. Lett. 246 (1995) 51. [12] M. Xi, K.F. Jensen, Surf. Sci. 339 (1995) 310. [I 3] The GaN sample, grown by MOCVD, was obtained from Hewlett Packard Co., Palo Alto. CA 94303. [14] J.S. Hwang, A.V. Kuznetsov, S.S. Lee, H.S. Kim, J.G. Choi, P.J. Chong, J. Cryst. Growth 142 (1994) 5. [15] S.D. Lester, F.A. Ponce, M.G. Craford, D.A. Sleigerwald, Appl. Phys, Lett. 66 (1995) 1249. [16] N,-E. Lee, R.C. Powell, Y.-W. Kim, J.E. Greene~ J. Vac. Sci. Technol. A 13 (1995) 1249. [17] F.A. Ponce, D.P. Bour, W.T. Young, M. Saunders, J.W. Steeds, Appl. Phys. Lett. 69 (1996) 337.

Ratna Shekhar, Klavs F Jensen / Surface Science 381 (1997) L581 L588

[19] [20] [21] [22] [23]

th, S.W. King, R.J. Nemanich, R.F. Davis, J. Electr. Mater. 25 (1996) 805. D. Berennan, P.C. Fletcher, Proc. Roy. Soc. Lond. Ser. A 250 (1959) 389. J.W. Smith, W.L. Fite, J. Chem. Phys. 37 (1962) 898. M.A. Khan, J.N. Kuznia, D.T. Olson, R. Kaplan, J. Appl. Phys. 73 (1993) 3108. J.E. Jaffe, R. Pandey, P. Zapol, Phys. Rev. B 53 (1996) R4209. S.J. Pearton, R.J. Shul, R.G. Wilson, F. Ren, J.M. Zavada, C.R. Abernathy, C.B. Vartuli, J.W. Lee, J.R. Mileham, J.D. Mackanzie, J. Electron. Mater. 25 (1996) 845.

[24] [25] [26] [27]

J.R. Arthur, J. Phys. Chem. Solids 28 (1967) 2257. Z.A. Munir, A.W. Searcy, J. Chem. Phys. 42 (1965) 4223. Y. Morimoto, J. Electrochem. Soc. 121 (1974) 1383. C.J. Sun, P. Kung, A. Saxler, H. Ohsato, E. Bigan, M. Razeghi, D.K. Gaskill, J. Appl. Phys. 76 (1994) 236. [28] Y.-M. Sun, D.W. Sloan, M. McEllistrem, A.L. Schwaner, J.M. White, J. Vac. Sci. Technol. A 13 (1995) 1455. [29] S.S. Liu, D.A. Stevenson, J. Electrochem. Soc. 125 (1978) 1161.