High temperature annealing of GaN, InN, AlN and related alloys

Solid-Slave Hecrronics Vol.41, No. 5. pp. 681-694. 1997 8 1997 Elsevier Science Ltd. All rights reserved

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TEMPERATURE ANNEALING OF GaN, AND RELATED ALLOYS

InN, AlN

J. HONG’, J. W. LEE’, C. B. VARTULI’, J. D. MACKENZIE’, S. M. DONOVAN’, C. R. ABERNATHY’, R. V. CROCKETT’, S. J. PEARTON’, J. C. ZOLPER* and F. REN1 ‘Department

of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, U.S.A. %andia National Laboratories, Albuquerque, NM 87185, U.S.A. 3Bell Laboratories, Lucent Technologies, Murray Hill. NJ 07974, U.S.A. (Received 26 July 1996; in revised form 17 September 1996)

Abstract-Transient thermal processing is employed for implant activation, contact alloying, implant isolation and dehydrogenation during III-nitride device fabrication. We have compared use of InN and AIN powder as methods for providing a N? partial pressure within a graphite susceptor for high temperature annealing of GaN, InN AIN, InAlN and InGaN. The AlN powder provides adequate surface protection to temperatures of _ 1100°C for AIN, 2 1050°Cfor GaN, -600°C for InN and - 800°C for the ternary alloys. While the InN powder provides a higher NZ partial pressure than AIN powder, at temperatures above -750°C the evaporation of In is sufficiently high to produce condensation of In droplets on the surfaces of the annealed samples. 0 1997 Elsevier Science Ltd

so it is difficult to maintain pristine surfaces over the whole wafer areas. In principle furnace annealing in a hydride or Hz/group V ambient is an ideal solution. In practice, however, it may have a number of shortcomings, the furnace must be large to accommodate all the plumbing required and so has long heating times and a slow turnaround because of the need to completely flush the system after the anneal and before removing the wafer. In the particular case of GaN and related materials, the use of NH, as an annealing ambient raises the possibility of unintentional hydrogen passivation of p-type dopants, such as occurs during cool-down after metalorganic chemical vapor deposition growth. In this paper we compare results for high temperature annealing of III-nitrides using either AlN or InN powder as a source of N2 overpressure within a Sic-coated graphite susceptor. These susceptors are widely used in GaAs device technology because they eliminate slip formation that can occur due to temperature differentials of as little as 7°C between the center and edge of a wafer.

INTRODUCTION

The vapor pressure above III-V materials at elevated temperatures is dominated by that of the more volatile group V element, either P, As, N or Sb. The partial pressures of these species are generally many orders of magnitude higher than those of the group III elements In, Ga or Al[l]. The group V element is typically in the form of molecules of either two or four atoms (e.g. Pz or P,)[2]. The partial pressures of these species dominate those of monomer group V atoms. At the elevated temperatures needed for implant activation (typically 5&60% of the material’s melting temperature) the group V vapor pressures are appreciable fractions of an atmosphere. Accordingly, stringent efforts must be made to protect the semiconductor surface during annealing[3-191. Existing approaches seek either to provide a group V overpressure or to encapsulate the surface. The group V overpressure may be provided by introducing PH3, ASHY or NH, into the heating chamber (or equivalently an organometallic compound such as tertiarybutylphosphine or tertiarybutylarsine) or by transporting group V vapor in HZ or another gas[19-221. The capless proximity method of placing the wafer face-to-face with another uncapped wafer of the same type is one of the most commonly used for research studies. However, slices can move slightly during insertion or removal from the furnace, creating microscratches. Any dust or contamination on one wafer will also tend to be baked on to the other wafer,

EXPERIMENTAL The GaN, AlN, InN and Ino.sAlosN layers were grown on GaAs or A120, substrates by metal organic molecular beam epitaxy as described previously [23,24]. The samples were annealed between 600 and 1150°C for 10 s at the peak temperatures within an AG Associates 410T rapid thermal anneal furnace. 681

J. Hong er

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al.

1100 9OOOC, 10 set -

1000 900 800 700 600 500 400 300 200 100

01 ““’ 0 20

40

““’

60

80

100 TIME

Fig.

1. Time-temperature

““““’ 120 140

160

180

bed

responses for GaN wafers annealed on a Si wafer, or in two different types of susceptors.

Blow-off N1 from a LN2 tank was used to flush air from the system. The wafers were placed within the commercially available susceptor, in which the reservoirs were filled with either AlN or InN powder (5 IO pm particle size) obtained from AESAR. The annealed surfaces were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) or energy dispersive X-ray spectrometry (EDAX).

reconstruction and annealing of defects in the heteroepitaxial material. Zolper et a1.[25] reported significant improvements in luminescence intensity and surface morphology from GaN annealed up to

API powder

RESULTS AND DISCUSSION

A typical time-temperature response for the susceptor is shown in Fig. 1. The heating and cooling times are significantly longer than for a simple proximity geometry (wafers sitting on a Si support wafer) and for a susceptor that does not contain in the figure) reservoirs (labeled “conventional” because of its much greater thermal mass. The root-mean-square (RMS) surface roughness measured by AFM are shown as a function of annealing temperature in Fig. 2. In the case of AlN powder, the In-based materials show clear thresholds for surface roughening (>6OO”C for InN, > 700°C for InAlN), whereas the GaN and AlN show little change even at 1150°C. In the case of InN powder all of the materials show higher surface roughness, although the sharp threshold for InN is no longer evident. The AFM scans for GaN annealed with either AIN powder or InN powder are shown in Figs 3 and 4, respectively. There are clear surface morphology changes, with little overall change in roughness for the AlN powder case. This is likely due to surface

600

000

Anneal Tempmtm

rooo &I

1240 %4

Fig. 2. Normalized RMS surface roughness as a function of annealing temperature for nitrides annealed in a susceptor with either AIN powder (top) or InN powder (bottom) in the reservoirs.

High temperature

annealing

of GaN,

683

InN and AIN

GaN (AIN powder)

control

looo”c

800°C

X 1.000 &div 2 500.000 nln/div

900°C Fig. 3. AFM

scans

as a function

lloo”c of anneal

temperature reservoirs.

for GaN

annealed

with AlN powder

in the

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GaN (InN powder)

control

X 1.000 pm/div 2 500.000 nm / div

900°C Fig. 4. AFM

scans as a function

1050°C of anneal

temperature reservoirs.

for GaN

annealed

with InN powder

in the

High temperature

annealing

of GaN.

InN and AIN

685

Fig. 5. SEM micrographs of GaN untreated surface (top left), after annealing at 1100°C with AIN powder (bottom left), or at 800°C (top right) or 1000°C (bottom right) with InN powder.

1100°C. The samples annealed with InN powder between 800 and 1050°C show the presence of features with micron-size dimensions. The SEM micrographs of GaN samples are shown in Fig. 5. With AlN powder, the surface of GaN still appears smooth after 1100°C annealing (bottom left), whereas there are metallic droplets present on

10.24 X-ray Energy (keV) Fig. 6. EDAX spectrum of GaN after annealing with InN powder in the reservoirs.

at 1000°C

surfaces annealed with InN powder (top and bottom right). The identity of these droplets was probed by EDAX. A typical spectrum from a GaN sample annealed with InN powder in the reservoir at 1000°C is shown in Fig. 6. Clearly the metallic droplets are due to InN. Thus, it appears that for temperatures preferential loss of Nz from the of - 500-700°C InN powder occurs, producing an additional Nz partial pressure within the susceptor, but above -750°C there is sufficient vapor transport of In that it condenses on the GaN surface upon the completion of the anneal. This was borne out by the presence of silvery deposits throughout the susceptor after annealing above 750’C. These deposits were not present when AIN powder was used in the reservoirs. AFM scans for AlN annealed with AlN powder or InN powder are shown in Figs 7 and 8, respectively. Once again there was little change in overall surface roughness with the AlN powder anneals, but the In droplets were detected for annealing temperatures up the AFM to - 1050°C. At still higher temperatures data showed an apparent smoothing. The reason for the smoothing is obvious from the SEM micrographs of Fig. 9. The In droplets are present on the 1000°C annealed samples done with

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al.

AIN (AIN powder)

?-

control

X 1.000 pm/div 2 500.000 nm/div

X

1.000 pm/div 2 500.000 nm/div

1ooo”c

1150°C

Fig. 7. AFM scans as a function of anneal temperature for AIN annealed with AIN powder in the reservoirs.

.

High temperature

annealing

of GaN,

687

InN and AIN

AlN (InN powder)

control

X 1.000 pm/div

Z 500.000 nm/div

X 1.000 pm/div 2 500.000 nm/div

1150°C

looo”c Fig. 8. AFM

scans

as a function

of anneal

temperature reservoirs.

for AIN annealed

with InN powder

in the

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Fig. 9. SEM micrographs of AIN untreated surface (top left), after annealing at 1000°C with AlN powder (bottom left), or at 1000°C (top right) or 1100°C (bottom right) with InN powder.

High temperature

annealing

of GaN.

InN and AIN

689

InN (AlN powder)

X 1.000 pm/div Z 500.000 nm/div

control

X 1.000 pm/div Z 500.000 nm/div

600°C

X 1.000 pm/div Z 500.000 nm/div

650°C

700°C Fig.

10. AFM

scans

looo”c as a function

of anneal

temperature reservoirs.

for InN annealed

with AIN powder

in the

J. Hong et al.

690

InN (InN powder)

Control

600°C

1.000 pmldiv 2 500.000 nmldiv

X 1.000 pm/div 2 500.000 nm/div

Fig. 1I. AFM scans as a function of anneal temperature for InN annealed with InN powder in the reservoirs.

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High temperature annealing of GaN, InN and AIN

Fig. 12. SEM micrographs of InN untreated surface (top left), after annealing at 700°C with InN powder (bottom left), or at 700°C (top right) or 800°C (bottom right) with AlN powder.

InN powder (top right), but have evaporated by 1100°C (bottom right), leaving behind small (l-3 pm) spherical features. AFM scans for InN samples as a function of annealing temperatures are shown for the AlN powder case (Fig. 10) and the InN powder case (Fig. 11). At 600°C the surface roughness remains the same for the material annealed with AlN powder as for the control samples, but already at this temperature the In droplet formation has begun in the case of the InN powder annealing. Neither type of powder is particularly effective in preventing InN surface dissociation above N 650°C. Figure 12 shows SEM micrographs from samples annealed at 700°C with either InN powder (lower left), on which many small In droplets are evident, or with AlN powder (upper right), where the surface is also degraded. At 800°C with AlN powder the In droplets are blown-off, leaving a pitted surface (lower right). InAlN would be expected to have better thermal stability than pure InN and the AFM scans of Fig. 13 show that 700°C annealing with AlN powder does not degrade the surface, but by 800°C In droplets are already evident. By contrast, with InN powder there

is severe roughening at 700°C because of the In vapor condensation (Fig. 14). The difference in droplet size between those caused solely by N2 loss from the InAlN surface (i.e. the AlN powder case) and those caused by vapor condensation (i.e. the InN powder case) is evident in the SEM micrographs of Fig. 15.

SUMMARY

AND CONCLUSION

We have examined two different susceptor reservoir materials for rapid thermal annealing of GaN and related alloys. AlN powder is effective in maintaining the surfaces of AlN and GaN epitaxial layers to 2 105O”C, while InN is severely degraded above -600°C. An attempt to increase the N2 partial pressure within the susceptor by using InN powder in the reservoirs was unsuccessful because the vapor transport of In is sufficiently high at 2750°C that droplets condense on the surface of the nitride samples being annealed. A superior alternative to capless annealing may be the use of sputtered AlN capping layers that can easily withstand temperatures up to

J. Hong rl a/.

692

InAlN (AIN powder)

control

X 1.000 pmm/div

2 500.000 nm/div

950°C

700°C

X 1.000 pm/div 2 500.000 nm/div

800°C Fig. 13.AFM

scans as a function

1ooo”c of anneal

temperature reservoirs.

for InAlN

annealed

with AIN powder

in the

High temperature

annealing

of GaN.

InN and AIN

693

InAlN (InN powder)

control

X 1.000 pm/div Z 500.000 nm/div

X 1.000 pm/div Z 500.000 nm/div

1ooo”c

800°C Fig. 14. AFM

scans as a function

of anneal

temperature reservoirs.

for InAlN

annealed

with InN powder

in the

694

J. Hong et al tered by AFOSR (G. L. Witt). The work at Sandia supported by DOE contract DE-AC04-94AL85000.

is

REFERENCES

Fig. 15. SEM micrographs of InAlN untreated surface (top), or after 800°C annealing with AlN powder (center) or InN powder (bottom) in the reservoirs.

115O”C[26] and which can be selectively removed KOH solutions[27]. Acknowledgements-The work Microfabritech facility and is AASERT grant through AR0 (NOOO14-92-J-1895) administered (ECS-9522887) and a DARPA

in

at UF is performed in the partially supported by an (Dr J. M. Zavada), a URI by ONR, an NSF grant grant (A. Husain) adminis-

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