Properties of thin film III-V/IV semiconductor alloys and nanostructures

121 Pages·2017·7.92 MB·English

Properties of thin film III-V/IV semiconductor alloys and nanostructures

By

Roger Jia

B.S.E. Materials Science and Engineering
University of Michigan, Ann Arbor, 2011

Submitted to the Department of Material Science and Engineering in Partial Fulfillment of the
Requirements for the Degree of

Doctor of Philosophy in Materials Science and Engineering
at the
Massachusetts Institute of Technology

September 2017

© 2017 Massachusetts Institute of Technology. All rights reserved.

Signature of Author:_____________________________________________________________
Department of Materials Science and Engineering
July 31, 2017

Certified by:___________________________________________________________________
Eugene A. Fitzgerald
Merton C. Flemings-SMA Professor of Materials Science and Engineering
Thesis Supervisor

Accepted by:___________________________________________________________________
Donald R. Sadoway
John F. Elliott Professor of Materials Chemistry
Chair, Departmental Committee on Graduate Students
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Properties of thin film III-V/IV semiconductor alloys and
nanostructures
By
Roger Jia
Submitted to the Department of Material Science and Engineering on July 31, 2017 in Partial
Fulfilment of the Requirements for the Degree of Doctor of Philosophy in Materials Science and
Engineering

ABSTRACT

A large amount of research and development has been devoted to engineering materials for the
next generation of semiconductor devices with high performance, energy efficiency, and
economic viability. To this end, significant efforts have been made to grow semiconductor thin
films with the desired properties onto lattice constants with viable, cost effective substrates.
Comparatively less effort has been made to explore III-V/IV heterovalent nanostructures and
alloys, which may exhibit properties not available in existing materials. The investigation of
these structures, grown using MOCVD, is the goal of this thesis and is motivated by two factors:
one, that III-V/IV nanostructures should be good thermoelectrics based on the “phonon glass
electron crystal” concept, and two, that (GaAs) (Ge ) alloys were observed to exhibit near-
1-x 2 x
infrared room temperature luminescence, a result that can have significant implications for low
bandgap optical devices. A survey of various growth conditions was conducted for the growth of
the model GaAs/Ge system using MOCVD to gain insight in the epitaxy involving heterovalent
materials and to identify structures suitable for investigation for their thermoelectric and optical
properties. A significant decrease in the thermal conductivities of GaAs/Ge nanostructures and
alloys relative to bulk GaAs and bulk Ge was observed. This reduction can be attributed to the
presence of the heterovalent interfaces. The electron mobilities of the structures were
determined to be comparable to bulk Ge, indicating minimal disruption to electron transport by
the interfaces. A further reduction in thermal conductivity was observed in an
(In Ga As) (Si Ge ) alloy; the alloy had a thermal conductivity of 4.3 W/m-K,
0.1 0.9 0.84 0.1 0.9 0.16
comparable to some state-of-the-art thermoelectric materials. Room temperature
photoluminescence measurements of various compositions of (GaAs) (Ge ) alloys revealed a
1-x 2 x
maximum energy transition of 0.8 eV. This bandgap narrowing is the result of composition
fluctuations; the fluctuations create regions of lower bandgap, resulting in a weak dependence on
luminescence as a function of Ge composition as well as lower bandgap than the homogeneous
alloy with the same composition. As silicon was added to the (GaAs) (Ge ) alloy, the bandgap
1-x 2 x
increased despite the composition fluctuations. Based on the results from this work III-V/IV
nanostructures show promise for thermoelectric and optical applications.

Thesis Supervisor: Eugene A. Fitzgerald

Title: Merton C. Flemings-SMA Professor of Materials Science and Engineering
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Acknowledgements
This thesis is the culmination of six years of work, and it could not have been done without the
advice, guidance, and aid of the many people that I have met during my time here at MIT. I
would like to thank my advisor, Professor Eugene Fitzgerald, for guiding me not only through
the actual research project but also through the ups and downs of the PhD process. Particularly
with the downs; Gene always knew what to say whenever I came to him with big misgivings on
whether I was actually going through the “usual PhD valley of death” and not some struggle due
to an inability to grasp how to do research.
I would like to thank my thesis committee, Professor Carl Thompson and Professor Jeff
Grossman, for providing feedback and evaluating my work. Their advice was very helpful in
ultimately creating a more coherent work and final presentation.
I would like to thank my collaborators, Lingping Zeng from Professor Gang Chen’s group in the
department of mechanical engineering, Dongwook Lee from Professor Yang Shao-Horn’s group
in the department of materials science and engineering, and Tony Zhu from Professor Vladimir
Bulović’s group in the department of electrical engineering and computer science, for providing
me with the thermal conductivity, Seebeck coefficient, and low temperature photoluminescence
results in this work. In addition, I would like to thank several other members and alumni of
Professor Chen’s group, Kimberly Collins, Maria Luckyanova, and Lee Weinstein, for their help
in various early stage experiments.
I would like to make a special note of the Fitzgerald research group. The vast majority of the
research in the group relies on the operation of the MOCVD reactor. Not even six months of my
PhD passed when the reactor began to exhibit significant problems hindering normal operation,
and malfunctions continued to occur often enough for the following 3 years to never allow us to
feel comfortable. Maintaining reactor operation throughout the years would have been many
times more stressful if not for the great camaraderie of the group. I would also like to thank the
members for all their help over the years from making TEM samples to research advice and
suggestions: Adam Jandl, who was my first point of contact when I started out, Prithu Sharma,
Tim Milakovich, Kunal Mukherjee, Ryan Iutzi, Chris Heidelberger, Rushabh Shah, and Mayank
Bulsara.
I would like to thank Dr. Yong Zhang and Dr. Shiahn Chen of the MIT Center of Materials
Science and Engineering for their instruction and assistance with operation of the TEMs. I
would like to acknowledge funding support from the Solid-State Solar Thermal Energy
Conversion Center, an Energy Frontier Research Center of the U.S. Department of Energy, and
the Singapore-MIT Alliance for Research and Technology.
Lastly, this work could not be done without the huge support of my family. I hope this work
brings joy to them, especially my father, who had to abandon his own PhD ambitions 27 years
ago in order to provide for a certain newborn baby.

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Table of Contents

List of Figures ................................................................................................................................ 9
List of Tables ............................................................................................................................... 15
Chapter 1: Introduction and Background ................................................................................ 16
1.1. Motivation .......................................................................................................................... 17
1.2. Thesis Objectives and Organization ................................................................................... 19
1.3. III-V/IV epitaxy.................................................................................................................. 20
1.3.1. Polar on non-polar epitaxy .......................................................................................... 22
1.3.1.1. Antiphase Domains .............................................................................................. 23
1.3.2. Non-polar on polar epitaxy .......................................................................................... 24
1.3.3. Metastable (GaAs)1-x(Ge2)x alloys ............................................................................... 26
1.4. Overview of Applications .................................................................................................. 27
1.4.1. Thermoelectrics ........................................................................................................... 27
1.4.1.1. Thermoelectric figure of merit ............................................................................. 28
1.4.1.2. Nanostructuring.................................................................................................... 29
1.4.2. Low bandgap materials lattice matched to GaAs ........................................................ 30
Chapter 2: Materials Growth and Characterization ............................................................... 32
2.1. Film Epitaxy ....................................................................................................................... 33
2.2. Structural Characterization ................................................................................................. 34
2.2.1. Transmission Electron Microscopy (TEM) ................................................................. 34
2.2.2. Electron-Dispersive X-ray Spectroscopy (EDS) ......................................................... 36
2.3. Optical Characterization ..................................................................................................... 37
2.3.1. Photoluminescence (PL) .............................................................................................. 37
2.3.2. Absorption Spectroscopy ............................................................................................. 38
2.4. Time-Domain Thermoreflectance (TDTR) ........................................................................ 38
2.5. Electrical Characterization ................................................................................................. 39
2.5.1. Van der Pauw – Hall method ....................................................................................... 39
2.5.2. Seebeck Measurements................................................................................................ 41
Chapter 3: Epitaxy of the GaAs/Ge System ............................................................................. 42
3.1. Introduction ........................................................................................................................ 43
3.2. Growth of GaAs/Ge superlattices ...................................................................................... 44
6
3.2.1. 650°C and 100 Torr growth (samples 1 and 2) ........................................................... 46
3.2.2. 500°C and 100 Torr growth (samples 3 and 4) ........................................................... 47
3.2.3. 650°C and 250 Torr growth (samples 5 – 12) ............................................................. 50
3.2.3.1. Reduction in period thicknesses (samples 9 – 12) ............................................... 54
3.2.4. 500°C and 250 Torr growth (samples 13 and 14) ....................................................... 55
3.3. Growth of (GaAs)1-x(Ge2)x alloys ...................................................................................... 56
3.4. Summary ............................................................................................................................ 63
Chapter 4: Thermoelectric Properties of III-V/IV Nanostructures ....................................... 65
4.1. Introduction ........................................................................................................................ 66
4.1.1. Structural quality of materials ..................................................................................... 66
4.2. Design of experiment ......................................................................................................... 67
4.2.1. GaAs/Ge superlattices ................................................................................................. 67
4.2.1.1. Determination of interface density....................................................................... 69
4.2.2. III-V/IV alloy structures .............................................................................................. 71
4.3. GaAs/Ge structures ............................................................................................................ 72
4.3.1. Thermal conductivity ................................................................................................... 72
4.3.2. Electrical mobility ....................................................................................................... 75
4.3.3. Seebeck coefficient ...................................................................................................... 79
4.4. Estimation of thermoelectric potential for the GaAs/Ge system........................................ 79
4.5. Thermoelectric properties of (In0.1Ga0.9As)0.84(Si0.1Ge0.9)0.16 ............................................. 82
4.5.1. Optimization of (In1-xGaxAs)1-z(Si1-yGey)z ................................................................... 84
4.6. Summary ............................................................................................................................ 87
Chapter 5: Optical Properties of III-V/IV Alloy Structures ................................................... 89
5.1. Introduction ........................................................................................................................ 90
5.2. Optical measurements of the (GaAs)1-x(Ge2)x alloys ......................................................... 91
5.3. Optical measurements of (GaAs)1-y(Si1-xGex)y and (In1-xGaxAs)1-z(Si1-yGey)z ................... 95
5.4. Possible explanations for the origin of photoluminescence ............................................... 99
5.4.1. Bandgap narrowing due to composition fluctuation.................................................... 99
5.4.2. Radiative and non-radiative defects .......................................................................... 100
5.4.3. Recombination at interfaces due to spatial separation of photoexcited carriers ........ 101
5.5. Summary .......................................................................................................................... 102
Chapter 6: Conclusions and Future Work ............................................................................. 104
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6.1. Summary of results........................................................................................................... 105
6.1.1. MOCVD of GaAs/Ge superlattices and (GaAs)1-x(Ge2)x alloys ............................... 105
6.1.2. Thermoelectric properties of III-V/IV structures ...................................................... 106
6.1.3. Optical properties of III-V/IV alloys ......................................................................... 107
6.2. Future directions ............................................................................................................... 108
Appendix A ................................................................................................................................ 110
References .................................................................................................................................. 116

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List of Figures

Figure 1.1. Bandgap vs. lattice parameter diagram of II-VI, III-V, and group IV semiconductors.
Lines denote ternary compositions (binary in case of SiGe) of the constituent endpoint
semiconductors. Line colors represent the conduction band valley lowest in energy; materials
with a Γ-valley have direct bandgaps, while materials with X- or L- valleys have indirect
bandgaps. Dashed lines indicate estimated compositions based on a linear trend (no
experimental data available). Image adapted from Woodall Research Group.21 ......................... 19
Figure 1.2. GaAs-Ge phase diagram. Mutual immiscibility is observed in the material system.
Phase separation would be expected at all compositions absent kinetic limitations. Adapted from
Ansara22......................................................................................................................................... 21
Figure 1.3. Primary modes of thin film growth a) Volmer-Weber (island growth) b) Frank-van
der Merwe (layer-by-layer) c) Stranski-Krastanov (layer-by-layer up to a critical thickness,
followed by island growth).23 The growth mode of a material depends on the interaction
energies of the adatom and surface. .............................................................................................. 22
Figure 1.4. Diagram of a GaAs lattice on a (001) group IV substrate with a single atomic step,
resulting in the formation of an antiphase boundary (vertical dashed line).25 .............................. 24
Figure 1.5. Diagram of a GaAs lattice on a (001) group IV substrate with a double atomic step.
No antiphase boundary forms since the lattice on either side of the step has the same
orientation.25.................................................................................................................................. 24
Figure 1.6. Model of the GaAs 𝒄(𝟒×𝟒) surface reconstruction.29 ............................................. 25
Figure 1.7. Model of the GaAs 𝒄(𝟖×𝟐) surface reconstruction.29 ............................................. 26
Figure 1.8. (002) dark-field cross-section transmission electron microscopy (XTEM) image of
(GaAs) (Ge ) layers grown by MOCVD, showing pronounced phase separation. Dark regions
1-x 2 x
are Ge-rich.9 .................................................................................................................................. 27
Figure 1.9. Conflicting nature of the thermoelectric properties. Axis scale: Thermal conductivity,
κ, 0 to 10 W m-1 K-1. Seebeck coefficient, α, 0 to 500 µV K-1. Electrical conductivity, σ, 0 to
5000 Ω-1 cm-1. Based on Bi2Te3 data.33 ....................................................................................... 29
Figure 1.10. Dependence of the thermoelectric figure of merit, zT, of various materials as a
function of temperature.33 ............................................................................................................. 29
Figure 1.11. Direct bandgap of (GaAs) (Ge ) with composition. Image adapted from Koiller et
1-x 2 x
al.8 ................................................................................................................................................. 31
Figure 2.1. Schematic of an MOCVD system.50 ........................................................................... 33
Figure 2.2. Diffraction pattern of a GaAs crystal in the [110] zone axis. ..................................... 35
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Figure 2.3. Diagram of a) brightfield imaging and b) off-axis darkfield imaging in TEM.51 ...... 35
Figure 3.1. (002) Darkfield XTEM image of GaAs/Ge superlattices grown at 650°C and 100 Torr
with substrate oriented in (a) the exact (001) direction (sample 1) and (b) (001) 6° offcut towards
the <111>A direction (sample 2). Antiphase boundaries (APBs) in the GaAs layers were
confirmed through observing contrast reversal in adjacent domains in (002) darkfield and (00-2)
darkfield images (not shown)........................................................................................................ 46
Figure 3.2. XTEM images of GaAs/Ge superlattices grown at 500°C and 100 Torr. (a) (002)
darkfield and (c) (00-2) darkfield images of sample 3, grown using a substrate oriented in the
exact (001) direction. (b) (002) darkfield and (d) (00-2) darkfield images of sample 4, grown
using a substrate oriented (001) 6° offcut towards the <111>A direction. APBs were determined
from observing contrast reversal in adjacent domains in the (002) darkfield and (00-2) darkfield
images. Stacking faults (SF) are observed in sample 3, and are also present in sample 4 (not
shown). .......................................................................................................................................... 49
Figure 3.3. Dependence of Ge growth rate on total system pressure. Growth was done at 750°C
on GaAs substrates with orientations (001) 3° offcut towards <110>, (110), and As terminated
(111).60 .......................................................................................................................................... 51
Figure 3.4. (002) Darkfield XTEM images of GaAs/Ge superlattices grown at 650°C and 250
Torr using the N anneal surface preparation method. (a) Sample 5, grown on a substrate oriented
2
in the exact (001) direction. (b) Sample 6, grown on a substrate oriented (001) 6° off towards the
<111>A direction. APBs in the GaAs layers were confirmed through observing contrast reversal
in adjacent domains in (002) darkfield and (00-2) darkfield images (not shown)........................ 52
Figure 3.5. (002) Darkfield XTEM images of GaAs/Ge superlattices grown at 650°C and 250
Torr using the TMGa pulse surface preparation method. (a) Sample 7, grown on a substrate
oriented in the exact (001) direction. (b) Sample 8, grown on a substrate oriented (001) 6° off
towards the <111>A direction. APBs in the GaAs layers were confirmed through observing
contrast reversal in adjacent domains in (002) darkfield and (00-2) darkfield images (not shown).
....................................................................................................................................................... 53
Figure 3.6. (002) Darkfield XTEM images of GaAs/Ge superlattices grown at 650°C and 250
Torr. Sample 9 (a) and sample 11 (b) were grown on substrates oriented in the exact (001)
direction, while sample 10 (c) and sample 12 (d) were grown on substrates oriented (001) 6°
offcut towards the <111>A direction. N anneal was used in (a) and (c) while TMGa pulsing
2
was used in (b) and (d). Precursor flows and flow times during deposition of each GaAs and Ge
layer were the same for all samples. APBs in the GaAs layers were confirmed through observing
contrast reversal in adjacent domains in (002) darkfield and (00-2) darkfield images (not shown).
....................................................................................................................................................... 55
Figure 3.7. (002) Darkfield XTEM images of GaAs/Ge superlattices grown at 500°C and 250
Torr. (a) substrate oriented in the exact (001) direction (sample 13). (b) substrate oriented (001) 6°
offcut towards the <111>A direction (sample 14). Stacking faults are observed in both samples.
....................................................................................................................................................... 56
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