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A novel technique for RTP annealing of compound semiconductors
PII:
Solid-State Electronics Vol. 42, No. 12, pp. 2335±2340, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-1101/98 $ - see front matter S0038-1101(98)00236-6
A NOVEL TECHNIQUE FOR RTP ANNEALING OF COMPOUND SEMICONDUCTORS M. FU1, V. SARVEPALLI1, R. K. SINGH2, C. R. ABERNATHY2, X. CAO2, S. J. PEARTON2 and J. A. SEKHAR1 Micropyretics Heaters International (MHI) Inc., 1776 Mentor Avenue, Cincinnati, OH 45212, U.S.A. 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, U.S.A.
1
(Received 6 January 1998; in revised form 17 March 1998; accepted 21 March 1998) AbstractÐWe introduce for the ®rst time a novel rapid thermal processing (RTP) unit called Zapper2, which has recently been developed by MHI Inc. and the University of Florida, for high temperature thermal processing of semiconductors. This Zapper2 unit is capable of reaching much higher temperatures (>15008C) than conventional tungsten±halogen lamp RTP equipment and achieving high ramp-up and ramp-down rates. We have conducted implant activation annealing studies of Si+-implanted GaN thin ®lms (with and without an AlN encapsulation layer) using the Zapper2 unit at temperatures up to 15008C. The electrical property measurements of such annealed samples have led to the conclusion that high annealing temperatures and AlN encapsulation are needed for the optimum activation eciency of Si+ implants in GaN. It has clearly been demonstrated that the Zapper2 unit has tremendous potential for RTP annealing of semiconductor materials, especially for wide band-gap (WBG) compound semiconductors that require very high processing temperatures. # 1998 Published by Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Recent interest in developing advanced electronic devices that operate at high temperature and/or high power has brought into focus many new challenges for semiconductor materials and the related processing technology. Compound semiconductors such as SiC and GaN have signi®cant advantages for such device applications because of their wider bandgaps (higher operating temperature), larger breakdown ®elds (higher operating voltage), higher electron saturated drift velocity (higher operating current), and better thermal conductivity (higher power density)[1,2]. Some of the examples of SiC and GaN-based electronic devices are SiC power MOSFET[3] and GaN junction ®eld eect transistor (JFET)[4]. In the development of advanced electronic devices, the technology of rapid thermal processing (RTP) plays a critical role at numerous points such as implant activation of dopant species, implantation-induced damage removal, alloying of ohmic contacts, maximization of sheet resistance in implant isolated regions, etc. High annealing temperature and short processing time have been identi®ed as two key requirements in RTP annealing of compound semiconductors such as GaN and SiC, especially for implant activation and damage removal[5]. It has been found in Si-implanted GaN that, although Si donors can be eciently activated at 11008C, complete removal of ion implantationinduced damage requires even higher annealing temperature[6]. A more recent study has shown that
implantation-induced damage in GaN can only be signi®cantly reduced by using annealing temperatures up to 14008C[7]. The existing commercial RTP equipment typically relies on a series of tungsten±halogen lamps as heat sources to rapidly heat up the semiconductor wafers[8]. However, this type of lamp-based RTP system suers from many problems such as point heat source, ¯uctuating lamp temperature during processing, and only modest temperature capacity (<11008C). Recent interest in developing wide band-gap (WBG) compound semiconductors has pushed the processing temperature requirements to much higher values (up to 15008C). Presently, there are no speci®c RTP systems that can operate at such high temperatures. In the study of annealing of GaN up to 14008C[7], a custom system (based on MOCVD system) that employed RF-heating was built and utilized. Thus, there is an urgent need in compound semiconductor processing particularly in GaN and SiC technology to develop new RTP systems which can provide uniform heating to very high temperatures (>15008C). To meet such an urgent need, Micropyretics Heaters International (MHI) Inc., in collaboration with the University of Florida, has recently developed a unique high temperature RTP unit called Zapper2. This novel RTP unit has been speci®cally designed to achieve very high uniform temperatures (>15008C) and is thus especially useful for high temperature thermal processing of semiconductor
2335
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materials. In this paper, we ®rst give a brief introduction about the Zapper2 unit. We then report the implant activation annealing studies of Si+implanted GaN thin ®lms (with and without an AlN encapsulation layer) using the Zapper2 unit at temperatures up to 15008C. The electrical analysis and characterization results of such annealed samples are presented and discussed. Based on these experiments, the signi®cant potential of the MHI Zapper2 unit in the development of advanced electronics is summarized. 2. A NOVEL RTP EQUIPMENT ± THE ZAPPER2 UNIT
Most existing RTP equipment utilizes either an array of ten or more tungsten±halogen lamps, or a single arc lamp as heat sources[8]. This type of lamp-based RTP equipment can achieve only modest processing temperatures (<11008C), primarily because of the point-like sources and large thermal mass of the systems. To realize higher temperature capacity, new types of heat sources have to be employed. In the past few years, MHI Inc. has developed and patented a series of novel molybdenum intermetallic composite heaters that may be used in air at temperatures up to 19008C[9,10]. These heaters have high emissivity (up to 0.9) and allow heat up time of the order of seconds and heat ¯uxes up to 100 W cmÿ2. Using proprietary fabrication technology the heaters can be made into various sizes and shapes. After extensive research on
the existing RTP systems and based on the availability of such high temperature heaters, we have designed and constructed a novel furnace-based RTP unit called Zapper2. A picture of a prototype Zapper2 unit, which includes furnace and its supporting assembly, control electronics, sample susceptor and motor-driven actuator, is shown in Fig. 1. This novel RTP unit is capable of achieving much higher temperatures than the lamp-based RTP equipment. Figure 2 shows some typical time± temperature pro®les at 15008C. Unlike the lampbased RTP equipment which rely on heat source temperature ¯uctuation to rapidly heat up and cool down the wafer, the Zapper2 unit relies on wafer movement (in/out of furnace horizontally) to achieve rapid ramp-up and ramp-down rates. The motor-driven actuator, which controls the sample moving-in/out speed and dwell time in the furnace, features up to 10 programs in memory. Each program can do an unlimited number of steps, time delays and repetitions. The maximum moving speed that can be attained is 10 in. sÿ1. The wafer susceptor is made of quartz that is primarily transparent to the incident radiation and thus has low thermal mass. The wafer can be processed either in a vacuum or sealed ambient environment. A unique electronics system that enables the Zapper2 to perform from simple to complicated annealing treatments has also been developed. The process controller is capable of up to 20 programs with 16 segments per program. Each program recipe can
Fig. 1. A photograph showing the working model of the novel RTP unit (Zapper2).
RTP annealing of compound semiconductors
2337
3. EXPERIMENTAL PROCEDURE
Fig. 2. Time±temperature pro®les for RTP annealing of GaN at 15008C.
have designated ramp up and ramp down rates. Communications option is also available for connecting to PLC, PC software. The prototype Zapper2 RTP unit shown in Fig. 1 are mostly based on the Phase I eort of a BMDO SBIR project (Contract No. F19628-97-C0092) for developing a novel RTP system. The proposed temperature uniformity for this prototype unit in the Phase I of the project was targeted to be 288C (at 15008C) over a 1 in.2 area. By utilizing MHI's large area high emissivity heating elements and carefully designing the heating chamber, the temperature uniformity of the newly developed Zapper2 system is better than expected. In fact, we have achieved a uniformity of R48C over a 8.5x6' area at 15008C. The unit is ready for commercial use in the batch mode.
A variety of undoped GaN layers 03 mm thick were grown at 010508C by metal organic chemical vapor deposition using trimethylgallium and ammonia. Growth was preceded by deposition of thin (0200 AÊ) GaN or AlN buers on the Al2O3 substrates. Capacitance±voltage measurements on the GaN showed typical n-type background carrier concentrations of R3 10ÿ6 cmÿ3. Si+ was implanted to a dose of 5 1015 cmÿ2, 100 keV, producing a maximum Si concentration of 06 1020 cmÿ3 at a depth of 0800 AÊ. Some of the samples were encapsulated with 1000±1500 AÊ of AlN deposited in one of two ways. In the ®rst, AlN was deposited by reactive sputtering of pure AlN targets in 300 mTorr of 20% N2 in Ar. The deposition temperature was 4008C. In the second method, AlN was grown by metal organic molecular beam epitaxy (MOMBE) at 7508C using dimethylamine alane and plasma dissociated nitrogen[11]. The samples were sealed in quartz ampoules under N2 gas. To ensure good purity of this annealing ambient the quartz tube (with sample inside and one end pre-closed) was subjected to an evacuation/ N2 purge cycle for 3 repetitions before the other end of the tube was closed, producing a ®nal N2 pressure of 015 psi. This negative pressure was necessary to prevent blowout of the ampoule at elevated annealing temperatures. The samples were then annealed at 1100±15008C, for a dwell time of 010 s (Fig. 2). The time dierence for reaching the annealing temperature between inside and outside the ampoule was found to be 4±6 s. To compensate for this heating time lag inside the ampoule, the dwell time was purposely extended to 015 s. Ramp rates were 0808C sÿ1 from 25±10008C and 308C sÿ1 from 1000±15008C, producing an average ramp-up rate over the entire cycle of 0508C sÿ1. The typical ramp-down rate was 0258C sÿ1. Measurements of temperature uniformity over a typical wafer size were 288C at both 1400 and 15008C. After removal of the samples from the ampoules they were examined by scanning electron microscopy (S.E.M.), atomic force microscopy (A.F.M.) and van der Pauw geometry Hall measurements obtained with alloyed HgIn eutectic. 4. RESULTS AND DISCUSSION
Figure 3 shows S.E.M. micrographs of unencapsulated GaN surfaces annealed at 12008C (top left), 13008C (top right), 14008C (bottom right) or 15008C (bottom right). The 12008C annealing does not degrade the surface, and the samples retain the same appearance as the as-grown material. After 13008C annealing, there is a high density (0108 cmÿ2) of small hexagonal pits due to incongruent evaporation from the surface. The 1400 8C annealing produces complete dissociation of the
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M. Fu et al.
Fig. 3. S.E.M. micrographs of GaN surfaces after unencapsulated annealing at 12008C (top left), 13008C (top right), 14008C (bottom left), and 15008C (bottom right).
GaN, and only the underlying AlN buer survives. Annealing at 15008C also causes loss of this buer layer, and a smooth exposed Al2O3 surface is evident. Corresponding AFM scans for this set of samples are shown in Fig. 4. By sharp contrast to the results for GaN, both the sputtered and MOMBE AlN were found to survive annealing above 13008C. For the sputtered material we often observed localized failure of the ®lm, perhaps due to residual gas agglomeration. For the MOMBE ®lms this phenomenon was absent. Also in the sputtered material the root-mean-square (RMS) surface roughness tended to go through a maximum, due to some initial localized bubbling, followed by the ®lm densi®cation (Fig. 5). The clear result from all of this data is that the implanted GaN needs to be encapsulated with AlN in order to preserve the surface quality. We have previously shown that AlN is selectively removable
from GaN using KOH-based solution[12,13]. Figure 6 shows the sheet carrier concentration and electron mobility in the Si+-implanted GaN, for both unencapsulated and AlN-encapsulated material, as a function of annealing temperature. For unencapsulated annealing we see an initial increase in electron concentration, but above 13008C the GaN layer blows-o. By contrast, for AlN-encapsulated samples the Si+ implant activation percentage is higher (090%) and peaks around 14008C. This corresponds to a peak carrier concentration of r5 1020 cmÿ3. For 15008C annealing both carrier concentration and mobility decrease, and this is consistent with Si-site switching as observed in Si+implanted GaAs at much lower temperatures[14]. The results in Fig. 6 are convincing evidence of the need for high annealing temperatures and the concurrent requirement for eective surface protection of the GaN.
RTP annealing of compound semiconductors
2339
Fig. 4. AFM scans of GaN surfaces after unencapsulated annealing at dierent temperatures.
5. SUMMARY AND CONCLUSIONS
There is clear evidence from both ion channeling and TEM measurements that temperatures above 13008C are required to completely remove implantation damage in GaN. Since the residual damage tends to produce n-type conductivity, it is even more imperative in acceptor-implanted material to completely remove its in¯uence. However, a premium is placed on prevention of surface dissociation, because loss of nitrogen also leads to residual n-type conductivity in GaN. The combination of RTP annealing in the Zapper2 unit at
1400±15008C and high quality AlN encapsulants produce metallic doping levels (05 1020 cmÿ3) in Si+-implanted GaN. The Zapper2 unit is clearly capable of achieving much higher temperatures than conventional tungsten±halogen lamp-based RTP equipment, and displays excellent time±temperature responses. Presently the Zapper2 unit is still in its prototype development stage. Further work is currently under progress to perfect its design and further extend its temperature capacity. Nevertheless, as demonstrated by the implant activation annealing experiments in
2340
M. Fu et al.
Fig. 5. RMS surface roughness of GaN and AlN samples after unencapsulated annealing at dierent temperatures.
the present work, the Zapper2 unit has shown signi®cant potential for high temperature annealing of compound semiconductors and should play a major role in the development of advanced electronic devices. AcknowledgementsÐThis work is partially supported by the Ballistic Missile Defense Organization (BMDO) of the U.S. Department of Defense through the SBIR program under Contract No. F19628-97-C-0092. Dr. Joseph Lorenzo is the contract monitor. MHI funding of the University of Florida eort is gratefully acknowledged. REFERENCES
1. See for examples in MRS Bulletin, 1997, 22, February Issue: GaN and Related Materials for Device Applications, ed. S. J. Pearton and C. Kuo. MRS, Pittsburgh, PA, 1997. 2. See for examples in MRS Bulletin, 1997, 22, March Issue: SiC Electronic Materials and Devices, ed. M. A. Capano and R. Trew. MRS, Pittsburgh, PA, 1997. 3. Shenoy, J .N., Cooper, J. A.Jr. and Melloch, M. R., IEEE Elec. Dev. Lett., 1997, 18, 93. 4. Zolper, J. C., Shul, R. J., Baca, A. G., Pearton, S. J., Wilson, R. G. and Stall, R. A., Appl. Phys. Lett., 1996, 68, 273.
Fig. 6. Sheet carrier density and electron mobility in capped and uncapped Si-implanted GaN, as a function of annealing temperature. 5. Zolper, J. C. and Pearton, S. J., in Proc. Wide Bandgap Semiconductor Symp., ed. J. Dismukes et al., pp. 268±275. The Electrochemistry Soc., Pennington, NJ, 1997. 6. Zolper, J. C., Tan, H. H., Williams, J. S., Zou, J., Cockayne, D. J. H., Pearton, S. J., Crawford, M. H. and Karlicek, R. F.Jr., Appl. Phys. Lett., 1997, 70, 2729. 7. Zolper, J. C., Han, J., Biefeld, R. M., Van Deusen, S. B., Wampler, W. R., Reiger, D. J., Pearton, S. J., Williams, J. S., Tan, H. H., Karlicek, R. J., Jr. and Stall, R. A., J. Appl. Phys., 1997, submitted. 8. Roozeboom, F., in Rapid Thermal Processing: Science and Technology, ed. R. B. Fair, pp. 349±423. Academic Press, New York, 1993. 9. Sekhar, J. A., Penumella, S. and Fu, M., in Transient Thermal Processing Techniques in Electronic Materials, ed. N. M. Ravindra and R. K. Singh, pp. 171±175. TMS, Warrendale, PA, 1996. 10. MHI Heating Element Handbook, Version 8. Cincinnati, OH, 1997. 11. Abernathy, C. R., Mat. Sci. Eng. Rep., 1995, 14, 203. 12. Mileham, J. R., Pearton, S. J., Abernathy, C. R., MacKenzie, J. D., Shul, R. J. and Kilcogne, S. P., Appl. Phys. Lett., 1995, 67, 1119. 13. Vartuli, C. B., Pearton, S. J., Lee, J. W., Abernathy, C. R., MacKenzie, J. D., Zolper, J. C., Shul, R. J. and Ren, F., J. Electrochem. Soc., 1996, 143, 3681. 14. Pearton, S. J., Int. J. Mod. Phys. B, 1993, 7, 4687.
Solid-State Electronics Vol. 42, No. 12, pp. 2335±2340, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-1101/98 $ - see front matter S0038-1101(98)00236-6
A NOVEL TECHNIQUE FOR RTP ANNEALING OF COMPOUND SEMICONDUCTORS M. FU1, V. SARVEPALLI1, R. K. SINGH2, C. R. ABERNATHY2, X. CAO2, S. J. PEARTON2 and J. A. SEKHAR1 Micropyretics Heaters International (MHI) Inc., 1776 Mentor Avenue, Cincinnati, OH 45212, U.S.A. 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, U.S.A.
1
(Received 6 January 1998; in revised form 17 March 1998; accepted 21 March 1998) AbstractÐWe introduce for the ®rst time a novel rapid thermal processing (RTP) unit called Zapper2, which has recently been developed by MHI Inc. and the University of Florida, for high temperature thermal processing of semiconductors. This Zapper2 unit is capable of reaching much higher temperatures (>15008C) than conventional tungsten±halogen lamp RTP equipment and achieving high ramp-up and ramp-down rates. We have conducted implant activation annealing studies of Si+-implanted GaN thin ®lms (with and without an AlN encapsulation layer) using the Zapper2 unit at temperatures up to 15008C. The electrical property measurements of such annealed samples have led to the conclusion that high annealing temperatures and AlN encapsulation are needed for the optimum activation eciency of Si+ implants in GaN. It has clearly been demonstrated that the Zapper2 unit has tremendous potential for RTP annealing of semiconductor materials, especially for wide band-gap (WBG) compound semiconductors that require very high processing temperatures. # 1998 Published by Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Recent interest in developing advanced electronic devices that operate at high temperature and/or high power has brought into focus many new challenges for semiconductor materials and the related processing technology. Compound semiconductors such as SiC and GaN have signi®cant advantages for such device applications because of their wider bandgaps (higher operating temperature), larger breakdown ®elds (higher operating voltage), higher electron saturated drift velocity (higher operating current), and better thermal conductivity (higher power density)[1,2]. Some of the examples of SiC and GaN-based electronic devices are SiC power MOSFET[3] and GaN junction ®eld eect transistor (JFET)[4]. In the development of advanced electronic devices, the technology of rapid thermal processing (RTP) plays a critical role at numerous points such as implant activation of dopant species, implantation-induced damage removal, alloying of ohmic contacts, maximization of sheet resistance in implant isolated regions, etc. High annealing temperature and short processing time have been identi®ed as two key requirements in RTP annealing of compound semiconductors such as GaN and SiC, especially for implant activation and damage removal[5]. It has been found in Si-implanted GaN that, although Si donors can be eciently activated at 11008C, complete removal of ion implantationinduced damage requires even higher annealing temperature[6]. A more recent study has shown that
implantation-induced damage in GaN can only be signi®cantly reduced by using annealing temperatures up to 14008C[7]. The existing commercial RTP equipment typically relies on a series of tungsten±halogen lamps as heat sources to rapidly heat up the semiconductor wafers[8]. However, this type of lamp-based RTP system suers from many problems such as point heat source, ¯uctuating lamp temperature during processing, and only modest temperature capacity (<11008C). Recent interest in developing wide band-gap (WBG) compound semiconductors has pushed the processing temperature requirements to much higher values (up to 15008C). Presently, there are no speci®c RTP systems that can operate at such high temperatures. In the study of annealing of GaN up to 14008C[7], a custom system (based on MOCVD system) that employed RF-heating was built and utilized. Thus, there is an urgent need in compound semiconductor processing particularly in GaN and SiC technology to develop new RTP systems which can provide uniform heating to very high temperatures (>15008C). To meet such an urgent need, Micropyretics Heaters International (MHI) Inc., in collaboration with the University of Florida, has recently developed a unique high temperature RTP unit called Zapper2. This novel RTP unit has been speci®cally designed to achieve very high uniform temperatures (>15008C) and is thus especially useful for high temperature thermal processing of semiconductor
2335
2336
M. Fu et al.
materials. In this paper, we ®rst give a brief introduction about the Zapper2 unit. We then report the implant activation annealing studies of Si+implanted GaN thin ®lms (with and without an AlN encapsulation layer) using the Zapper2 unit at temperatures up to 15008C. The electrical analysis and characterization results of such annealed samples are presented and discussed. Based on these experiments, the signi®cant potential of the MHI Zapper2 unit in the development of advanced electronics is summarized. 2. A NOVEL RTP EQUIPMENT ± THE ZAPPER2 UNIT
Most existing RTP equipment utilizes either an array of ten or more tungsten±halogen lamps, or a single arc lamp as heat sources[8]. This type of lamp-based RTP equipment can achieve only modest processing temperatures (<11008C), primarily because of the point-like sources and large thermal mass of the systems. To realize higher temperature capacity, new types of heat sources have to be employed. In the past few years, MHI Inc. has developed and patented a series of novel molybdenum intermetallic composite heaters that may be used in air at temperatures up to 19008C[9,10]. These heaters have high emissivity (up to 0.9) and allow heat up time of the order of seconds and heat ¯uxes up to 100 W cmÿ2. Using proprietary fabrication technology the heaters can be made into various sizes and shapes. After extensive research on
the existing RTP systems and based on the availability of such high temperature heaters, we have designed and constructed a novel furnace-based RTP unit called Zapper2. A picture of a prototype Zapper2 unit, which includes furnace and its supporting assembly, control electronics, sample susceptor and motor-driven actuator, is shown in Fig. 1. This novel RTP unit is capable of achieving much higher temperatures than the lamp-based RTP equipment. Figure 2 shows some typical time± temperature pro®les at 15008C. Unlike the lampbased RTP equipment which rely on heat source temperature ¯uctuation to rapidly heat up and cool down the wafer, the Zapper2 unit relies on wafer movement (in/out of furnace horizontally) to achieve rapid ramp-up and ramp-down rates. The motor-driven actuator, which controls the sample moving-in/out speed and dwell time in the furnace, features up to 10 programs in memory. Each program can do an unlimited number of steps, time delays and repetitions. The maximum moving speed that can be attained is 10 in. sÿ1. The wafer susceptor is made of quartz that is primarily transparent to the incident radiation and thus has low thermal mass. The wafer can be processed either in a vacuum or sealed ambient environment. A unique electronics system that enables the Zapper2 to perform from simple to complicated annealing treatments has also been developed. The process controller is capable of up to 20 programs with 16 segments per program. Each program recipe can
Fig. 1. A photograph showing the working model of the novel RTP unit (Zapper2).
RTP annealing of compound semiconductors
2337
3. EXPERIMENTAL PROCEDURE
Fig. 2. Time±temperature pro®les for RTP annealing of GaN at 15008C.
have designated ramp up and ramp down rates. Communications option is also available for connecting to PLC, PC software. The prototype Zapper2 RTP unit shown in Fig. 1 are mostly based on the Phase I eort of a BMDO SBIR project (Contract No. F19628-97-C0092) for developing a novel RTP system. The proposed temperature uniformity for this prototype unit in the Phase I of the project was targeted to be 288C (at 15008C) over a 1 in.2 area. By utilizing MHI's large area high emissivity heating elements and carefully designing the heating chamber, the temperature uniformity of the newly developed Zapper2 system is better than expected. In fact, we have achieved a uniformity of R48C over a 8.5x6' area at 15008C. The unit is ready for commercial use in the batch mode.
A variety of undoped GaN layers 03 mm thick were grown at 010508C by metal organic chemical vapor deposition using trimethylgallium and ammonia. Growth was preceded by deposition of thin (0200 AÊ) GaN or AlN buers on the Al2O3 substrates. Capacitance±voltage measurements on the GaN showed typical n-type background carrier concentrations of R3 10ÿ6 cmÿ3. Si+ was implanted to a dose of 5 1015 cmÿ2, 100 keV, producing a maximum Si concentration of 06 1020 cmÿ3 at a depth of 0800 AÊ. Some of the samples were encapsulated with 1000±1500 AÊ of AlN deposited in one of two ways. In the ®rst, AlN was deposited by reactive sputtering of pure AlN targets in 300 mTorr of 20% N2 in Ar. The deposition temperature was 4008C. In the second method, AlN was grown by metal organic molecular beam epitaxy (MOMBE) at 7508C using dimethylamine alane and plasma dissociated nitrogen[11]. The samples were sealed in quartz ampoules under N2 gas. To ensure good purity of this annealing ambient the quartz tube (with sample inside and one end pre-closed) was subjected to an evacuation/ N2 purge cycle for 3 repetitions before the other end of the tube was closed, producing a ®nal N2 pressure of 015 psi. This negative pressure was necessary to prevent blowout of the ampoule at elevated annealing temperatures. The samples were then annealed at 1100±15008C, for a dwell time of 010 s (Fig. 2). The time dierence for reaching the annealing temperature between inside and outside the ampoule was found to be 4±6 s. To compensate for this heating time lag inside the ampoule, the dwell time was purposely extended to 015 s. Ramp rates were 0808C sÿ1 from 25±10008C and 308C sÿ1 from 1000±15008C, producing an average ramp-up rate over the entire cycle of 0508C sÿ1. The typical ramp-down rate was 0258C sÿ1. Measurements of temperature uniformity over a typical wafer size were 288C at both 1400 and 15008C. After removal of the samples from the ampoules they were examined by scanning electron microscopy (S.E.M.), atomic force microscopy (A.F.M.) and van der Pauw geometry Hall measurements obtained with alloyed HgIn eutectic. 4. RESULTS AND DISCUSSION
Figure 3 shows S.E.M. micrographs of unencapsulated GaN surfaces annealed at 12008C (top left), 13008C (top right), 14008C (bottom right) or 15008C (bottom right). The 12008C annealing does not degrade the surface, and the samples retain the same appearance as the as-grown material. After 13008C annealing, there is a high density (0108 cmÿ2) of small hexagonal pits due to incongruent evaporation from the surface. The 1400 8C annealing produces complete dissociation of the
2338
M. Fu et al.
Fig. 3. S.E.M. micrographs of GaN surfaces after unencapsulated annealing at 12008C (top left), 13008C (top right), 14008C (bottom left), and 15008C (bottom right).
GaN, and only the underlying AlN buer survives. Annealing at 15008C also causes loss of this buer layer, and a smooth exposed Al2O3 surface is evident. Corresponding AFM scans for this set of samples are shown in Fig. 4. By sharp contrast to the results for GaN, both the sputtered and MOMBE AlN were found to survive annealing above 13008C. For the sputtered material we often observed localized failure of the ®lm, perhaps due to residual gas agglomeration. For the MOMBE ®lms this phenomenon was absent. Also in the sputtered material the root-mean-square (RMS) surface roughness tended to go through a maximum, due to some initial localized bubbling, followed by the ®lm densi®cation (Fig. 5). The clear result from all of this data is that the implanted GaN needs to be encapsulated with AlN in order to preserve the surface quality. We have previously shown that AlN is selectively removable
from GaN using KOH-based solution[12,13]. Figure 6 shows the sheet carrier concentration and electron mobility in the Si+-implanted GaN, for both unencapsulated and AlN-encapsulated material, as a function of annealing temperature. For unencapsulated annealing we see an initial increase in electron concentration, but above 13008C the GaN layer blows-o. By contrast, for AlN-encapsulated samples the Si+ implant activation percentage is higher (090%) and peaks around 14008C. This corresponds to a peak carrier concentration of r5 1020 cmÿ3. For 15008C annealing both carrier concentration and mobility decrease, and this is consistent with Si-site switching as observed in Si+implanted GaAs at much lower temperatures[14]. The results in Fig. 6 are convincing evidence of the need for high annealing temperatures and the concurrent requirement for eective surface protection of the GaN.
RTP annealing of compound semiconductors
2339
Fig. 4. AFM scans of GaN surfaces after unencapsulated annealing at dierent temperatures.
5. SUMMARY AND CONCLUSIONS
There is clear evidence from both ion channeling and TEM measurements that temperatures above 13008C are required to completely remove implantation damage in GaN. Since the residual damage tends to produce n-type conductivity, it is even more imperative in acceptor-implanted material to completely remove its in¯uence. However, a premium is placed on prevention of surface dissociation, because loss of nitrogen also leads to residual n-type conductivity in GaN. The combination of RTP annealing in the Zapper2 unit at
1400±15008C and high quality AlN encapsulants produce metallic doping levels (05 1020 cmÿ3) in Si+-implanted GaN. The Zapper2 unit is clearly capable of achieving much higher temperatures than conventional tungsten±halogen lamp-based RTP equipment, and displays excellent time±temperature responses. Presently the Zapper2 unit is still in its prototype development stage. Further work is currently under progress to perfect its design and further extend its temperature capacity. Nevertheless, as demonstrated by the implant activation annealing experiments in
2340
M. Fu et al.
Fig. 5. RMS surface roughness of GaN and AlN samples after unencapsulated annealing at dierent temperatures.
the present work, the Zapper2 unit has shown signi®cant potential for high temperature annealing of compound semiconductors and should play a major role in the development of advanced electronic devices. AcknowledgementsÐThis work is partially supported by the Ballistic Missile Defense Organization (BMDO) of the U.S. Department of Defense through the SBIR program under Contract No. F19628-97-C-0092. Dr. Joseph Lorenzo is the contract monitor. MHI funding of the University of Florida eort is gratefully acknowledged. REFERENCES
1. See for examples in MRS Bulletin, 1997, 22, February Issue: GaN and Related Materials for Device Applications, ed. S. J. Pearton and C. Kuo. MRS, Pittsburgh, PA, 1997. 2. See for examples in MRS Bulletin, 1997, 22, March Issue: SiC Electronic Materials and Devices, ed. M. A. Capano and R. Trew. MRS, Pittsburgh, PA, 1997. 3. Shenoy, J .N., Cooper, J. A.Jr. and Melloch, M. R., IEEE Elec. Dev. Lett., 1997, 18, 93. 4. Zolper, J. C., Shul, R. J., Baca, A. G., Pearton, S. J., Wilson, R. G. and Stall, R. A., Appl. Phys. Lett., 1996, 68, 273.
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