DJM dissertation the absolute last version for upload-augmented

131 Pages·2011·6.31 MB·English

GALLIUM NITRIDE EPITAXY BY A NOVEL HYBRID VPE TECHNIQUE

A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND
ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF
STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

David J. Miller
June 2011
© 2011 by David J. Miller. All Rights Reserved.

Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons Attribution-

Noncommercial 3.0 United States License.
http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/hz462yv9251

ii
I certify that I have read this dissertation and that, in my opinion, it is fully adequate
in scope and quality as a dissertation for the degree of Doctor of Philosophy.
James Harris, Primary Adviser
I certify that I have read this dissertation and that, in my opinion, it is fully adequate
in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Michael McGehee
I certify that I have read this dissertation and that, in my opinion, it is fully adequate
in scope and quality as a dissertation for the degree of Doctor of Philosophy.
GLENN SOLOMON
Approved for the Stanford University Committee on Graduate Studies.
Patricia J. Gumport, Vice Provost Graduate Education
This signature page was generated electronically upon submission of this dissertation in
electronic format. An original signed hard copy of the signature page is on file in
University Archives.
iii
GALLIUM NITRIDE EPITAXY BY A NOVEL HYBRID VPE TECHNIQUE
David J. Miller, Ph.D.
Stanford University, 2011

Advisors: Glenn S. Solomon & James S. Harris

Gallium nitride is an important material for the production of next-generation
visible and near-UV optical devices, as well as for high temperature electronic
amplifiers and circuits; however there has been no bulk method for the production of
GaN substrates for device layer growth. Instead, thick GaN layers are
heteroepitaxially deposited onto non-native substrates (usually sapphire) by one of two
vapor phase epitaxy (VPE) techniques: MOVPE (metalorganic VPE) or HVPE
(hydride VPE). Each method has its strengths and weaknesses: MOVPE has precise
growth rate and layer thickness control but it is slow and expensive; HVPE is a low-
cost method for high rate deposition of thick GaN, but it lacks the precise control and
heterojunction layer growth required for device structures. Because of the large (14%)
lattice mismatch, GaN grown on sapphire requires the prior deposition of a low
temperature MOVPE nucleation layer using a second growth process in a separate
deposition system. Here we present a novel hybrid VPE system incorporating
elements of both techniques, allowing MOVPE and HVPE in a single growth run. In
this way, a thick GaN layer can be produced directly on sapphire. GaN growth
commences as small (50-100 nm diameter) coherent strained 3-dimensional islands
which coalesce into a continuous film, after which 2-dimensional layer growth
commences. The coalescence of islands imparts significant stress into the growing
film, which increases with the film thickness until catastrophic breakage occurs, in-
situ. Additionally, the mismatch in thermal expansion rates induces compressive
stress upon cooling from the growth temperature of 1025ºC. We demonstrate a
growth technique that mitigates these stresses, by using a 2-step growth sequence: an
initial high growth rate step resulting in a pitted but relaxed film, followed by a low
growth rate smoothing layer. As a result, thick (>50 µm) and freestanding films have
been grown successfully. X-ray rocking curve linewidth of 105 arcseconds and 10K
PL indicating no “yellow” emission indicate that the material quality is higher than
that produced by conventional MOVPE. By further modifying the hybrid system to
include a metallic Mn source, it is possible to grow a doped semi-insulating GaN
template for use in high frequency electronics devices.
iv
Acknowledgements:
First and foremost, I would like to thank my advisors Glenn Solomon and
James Harris for their constant support and encouragement to finish. My parents
Barry and Barbara Miller were instrumental in helping me finally attain this goal, and
I am grateful for their love and maintaining their faith in me over the years.
Additional thanks go to Julie Tell who helped me prepare for my thesis defense and
Jody Seltzer who effectively motivated me to put forth the effort to write this
dissertation.
The research for this dissertation was done at CBL Technologies, Inc. in
Redwood City, CA. I would like to thank Glenn again for his role as CEO and
director of the company in helping me define and shape this project. Our company’s
technicians Randy Carston and Rodney Worth greatly assisted me in performing
numerous crystal growth runs and constant system maintenance. The Matsushita
Electric Company of Japan gave generous financial support in the form of a joint
development agreement with CBL and provided Tetsuzo Ueda and Tadao Hashimoto,
two outstanding engineers who served in numerous roles at our facility. Finally, I
would like to thank Manfred Ramsteiner, Oliver Brandt, Achim Trampert, and Klaus
Ploog at the Paul Drude Institute for Solid State Electronics in Berlin for their
microstructural and optical characterization of our GaN material.
v
Table of Contents
Chapter 1: The Need for Gallium Nitride Substrates ...................................... 1
1.1 Introduction ........................................................................................... 1
1.2 Properties of GaN, AlN and InN ............................................................. 6
Chapter 2: A New Hybrid VPE Method for GaN ............................................ 9
2.1 Towards a gallium nitride substrate ......................................................... 9
2.2 Substrates for GaN heteroepitaxial growth ............................................ 14
2.2.1 Silicon ............................................................................................. 14
2.2.2 Silicon carbide ................................................................................ 16
2.2.3 Sapphire .......................................................................................... 16
2.2.4 Lithium gallate ................................................................................ 17
2.3 Summary of heteroepitaxial substrate choices ...................................... 17
2.4 Vapor-phase epitaxy of GaN ................................................................. 19
2.4.1 MOVPE .......................................................................................... 19
2.4.2 HVPE .............................................................................................. 22
2.4.3 Hot and Cold Walled Reactors ....................................................... 26
2.4.4 Near-equilibrium vs. far from equilibrium processes ..................... 28
2.4.5 Low Temperature Nucleation layer ................................................ 30
2.4.6 The Hybrid VPE system ................................................................. 33
Chapter 3: Hybrid MOVPE/HVPE GaN process optimization ..................... 35
3.1 Stress in heteroepitaxial GaN ................................................................ 35
3.1.1 Lattice mismatch stress ................................................................... 36
3.1.2 Coalescence stress in GaN on sapphire .......................................... 38
3.1.3 Thermal mismatch stress ................................................................ 40
3.2 Effects of stress ...................................................................................... 42
3.2.1 Cracking ......................................................................................... 43
3.2.2 Peeling and delamination ............................................................... 47
3.3 The surface morphology of HVPE GaN films ....................................... 48
3.3.1 Hillocks ........................................................................................... 49
3.3.2 Pits .................................................................................................. 52
3.3.3 Hexagonal pits ................................................................................ 53
3.3.4 Irregular pits .................................................................................... 59
3.3.5 Quantifiable roughness measurement .............................................. 61
3.4 Effects of substrate temperature, growth rate and V/III ratio ............. 64
3.5 smoothing layer growth .......................................................................... 67
3.6 The 2-step growth process ...................................................................... 70
vi
3.7 Summary of GaN deposition process optimization techniques .............. 72
Chapter 4: Microstructural Characterization of VPE-grown GaN ................ 74
4.1 Structure of thin GaN layers ............................................................... 74
4.2 A 3-zone layered growth model ............................................................. 77
4.3 Structural improvement with increasing thickness ................................. 79
4.4 X-ray methods for determining approximate dislocation density .......... 80
4.41 Accounting for the effect of bowing on XRD linewidth .................. 81
4.4.2 The relationship between FWHM and dislocation density ............. 84
4.43 Reduction in the dislocation density with increased thickness ........ 85
4.5 Summary of microstructural characterization ....................................... 88
Chapter 5: Photoluminescence characterization ............................................ 90
5.1 Photoluminescence characterization of GaN .......................................... 90
5.1 10K Photoluminescence ..................................................................... 90
5.1.2 Photoluminescence setup ................................................................. 92
5.2 Photoluminescence characterization of a 12 µm sample ....................... 93
5.3 Photoluminescence of 28 micron layer .................................................. 95
5.4 Emission from Freestanding 60 µm GaN .............................................. 96
5.5 Two-electron transitions ......................................................................... 97
5.6 Summary of photoluminescence results ................................................. 98
Chapter 6: A semi-insulating GaN Alloy: GaMnN ..................................... 100
6.1 Semi-insulating GaN for use in microwave amplifiers ........................ 100
6.2 Semi-insulating GaN through the incorporation of Mn using HVPE .. 101
6.3 Initial Characterization of GaMnN ....................................................... 104
6.4 TEM analysis: a second phase and a new crystal structure ................ 107
6.4.1 Mn-rich second phase in GaMnN ................................................. 108
6.4.2 Sublattice ordering – a new crystal structure ................................ 110
6.5 Summary: semi-insulating GaMnN ...................................................... 112
Chapter 7: Conclusions and future directions .............................................. 114
7.1 Conclusions ......................................................................................... 114
7.2 Future directions and research ............................................................. 116
Chapter 8: List of References ...................................................................... 118

vii
List of Tables
Table 1.1. Room temperature band gap and lattice constants for GaN, InN and
AlN ................................................................................................................................. 7
Table 2.1. A comparison of lattice and thermal mismatch, chemical
compatibility issues, and cost for GaN heteroepitaxial substrates. .............................. 18
Table 6.1. Characterization summary of 5 µm thick GaMnN layers grown with
various HCl flow ratios. ............................................................................................. 105

viii
List of Figures
Figure 2.1. Schematic diagram showing the bonding arrangement and unit cell
for GaN. ........................................................................................................................ 10
Figure 2.2. Bandgap vs. lattice constant for group III-nitrides ....................... 11
Figure 2.3. The Ga - N phase diagram. ........................................................... 13
2
Figure 2.4. Temperature variation of the thermodynamic driving force for
MOVPE and HVPE. ................................................................................................... 26
Figure 2.5. Schematic diagram showing the different heating schemes used for
“hot-” and “cold-” walled reactors.. ............................................................................. 27
Figure 2.6. Atomic Force Microscope images of MOVPE nucleation layer
before and after recrystallization. ................................................................................. 32
Figure 2.7. Model of a novel hybrid MOVPE/HVPE deposition system
featuring hot-walled and cold-walled heating systems. ............................................... 34
Figure 3.1. A schematic drawing of a GaN (0001) unit cell overlaid onto the
(0001) sapphire unit cell. .............................................................................................. 37
Figure 3.2. A schematic representation of the evolution of coalescence stress in
heteroepitaxial GaN on sapphire. ................................................................................. 40
Figure 3.3. Illustration of the effects of the 33% mismatch in thermal
expansion coefficient between sapphire and GaN ........................................................ 41
Figure 3.4. A comparison of tensile and compressive stress cracking
mechanisms. ................................................................................................................. 45
ix
Figure 3.5. Plan-view micrograph of a 10 µm thick GaN film that has cracked
and buckled during the cool down process ................................................................... 46
Figure 3.6. 100x optical micrograph of a 2 µm film grown at 1050ºC, showing
presence of numerous hexagonal hillock-shaped prominences. ................................... 50
Figure 3.7. Cross-section optical micrograph of a hexagonal-shaped pit in a 24
µm HVPE GaN film. .................................................................................................... 53
Figure 3.8. The basic tetrahedral bonding arrangement between Ga and N
atoms in GaN.. .............................................................................................................. 54
Figure 3.9. The GaN unit cell as viewed along the [1010] azimuth. ............... 55
Figure 3.10. Schematic diagram of the distortion of hexagonal pits as a
mechanism for strain relief.. ......................................................................................... 58
Figure 3.11. The effect of surface thermal pretreatment prior to deposition of
the low temperature MOVPE layer. ............................................................................. 61
Figure 3.12. Plan-view and cross-sectional micrographs of pitted and smooth
HVPE GaN films. ......................................................................................................... 63
Figure 3.13. The effect of substrate temperature on resulting surface
morphology of 2 µm HVPE films. ............................................................................... 65
Figure 3.14. The effect of the growth rate on 2 µm thick HVPE GaN films
grown at 1025ºC.. ......................................................................................................... 65
Figure 3.15. The effect of the V/III ratio on the surface morphology of 2 µm
HVPE GaN films grown at 1025ºC. ............................................................................. 66
Figure 3.16. The effect of smoothing layer regrowth on a pitted film. ........... 68
x