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«by GOPI KRISHNA SAMUDRALA YOGESH K. VOHRA, CHAIR SHANE A. CATLEDGE JOSEPH G. HARRISON RAYMOND G. THOMPSON UDAY K. VAIDYA A DISSERTATION Submitted to ...»

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MULTIVARIABLE STUDY ON HOMOEPITAXIAL GROWTH OF DIAMOND ON

PLANAR AND NON-PLANAR SUBSTRATES

by

GOPI KRISHNA SAMUDRALA

YOGESH K. VOHRA, CHAIR

SHANE A. CATLEDGE

JOSEPH G. HARRISON

RAYMOND G. THOMPSON

UDAY K. VAIDYA

A DISSERTATION

Submitted to the graduate faculty of The University of Alabama at Birmingham, in partial fulfillment of the requirements for the degree of Doctor of Philosophy

BIRMINGHAM, ALABAMA

2009 Copyright by Gopi Krishna Samudrala 2009

MULTIVARIABLE STUDY ON HOMOEPITAXIAL GROWTH OF DIAMOND ON

PLANAR AND NON-PLANAR SUBSTRATES

GOPI KRISHNA SAMUDRALA

PHYSICS

ABSTRACT

In this dissertation research, the roles of three basic parameters in homoepitaxial diamond growth by microwave plasma chemical vapor deposition on planar (type Ib yellow diamond plates) and non-planar (type Ia brilliant cut natural diamonds) substrates have been investigated. Isotopically enriched carbon-13 methane gas has been utilized in the experiments as the source of carbon to clearly distinguish the grown diamond layer from the underlying substrate using Raman spectroscopy.

Nitrogen showed a catalytic effect in the growth of diamond on diamond anvil substrates. In the standard H2/O2/N2/13CH4 feed gas mixture, when nitrogen was varied between 0 to 3500 parts per million (ppm), an optimum value of 1250 ppm nitrogen resulted in the highest growth rate and smoothest surface morphology. This particular chemistry resulted in 100% success rate during the fabrication of designer diamond anvils. No such optimum value in nitrogen concentration was found for planar substrates indicating a strong dependence of diamond growth rate on the substrate geometry.

On planar substrates, the effects of nitrogen concentration, methane concentration and substrate temperatures were studied independently by varying each parameter carefully over a broad range. Dramatic changes in surface morphologies and growth rates were observed by optical and atomic force microscopy. The nitrogen incorporation in carbon-13 diamond layers was monitored through photoluminescence spectroscopy of iii nitrogen–vacancy complexes. A twentyfold increase in growth rate has been reported as a result of this research. An optimum substrate temperature of 1050 C resulted in the highest growth rate when 2% CH4/H2 was used in feed gas mixture. Use of high methane concentration in the feed gas mixture resulted in diamond films where twinning on the surface was completely absent.

Aggressive incorporation of nitrogen in CVD diamond has been observed at substrate temperatures above 1050 C indicating that species such as HCN, CN and NHX play an active role on the surface at those temperatures. The optimum temperature has been observed to shift to higher values as the C/N ratio in the feed gas mixture increased.

The roles of various growth parameters in high growth rate and high quality homoepitaxial diamond growth are discussed in this thesis.

–  –  –

First, I would like to thank my advisor Dr. Yogesh K. Vohra. I am very fortunate to have a research mentor like him. His enthusiasm towards scientific research has a great influence on me. Working in his lab beside many of his successful students has been inspiring and I consider it a privilege. His suggestions, patient discussions and explanations were of immense help. Without his mentoring and strong support, this thesis would not have been possible.

I must thank the members of my research committee Dr. Shane A. Catledge, Dr.

Joseph G. Harrison, Dr. Raymond G. Thompson and Dr. Uday K. Vaidya for their valuable time and suggestions. I would also like to thank the Carnegie – DOE alliance center (CDAC) and Lawrence Livermore National Laboratory (LLNL) for supporting my research efforts.

My time in graduate school has been fun because of the company of good friends.

I value the friendship and support of Steve Fox, Jonathan Williams, Andrew Stemshorn, Walter Uhoya, Parimal Bapat, Alisa Holley, Nicole Hadiyah and Bhagvanth Reddy Sangala. I thank Dr. Wei Qiu for his help during the early stages of my research career.

The previous works of graduate students who worked in Dr. Yogesh Vohra’s lab proved

–  –  –

taking care of the documentation process and helping me have a trouble free time. My thanks to Mr. Jerry Sewell of physics machine shop for his help many times over the course of the last four years.

Without the support and guidance of my cousin Mr. Durga Prasad Chaturvedula; I would not have been able to realize my goal of coming to USA to pursue a career in scientific research. The support of my parents, sister is what makes life livable. Some call it luck and I call it the grace of the God. I am fortunate to have the blessings of the God.





My dearest Ekta, I cannot thank you enough. None of this would have been possible without your love and support through tough times.

–  –  –

ABSTRACT

DEDICATION

ACKNOWLEDGMENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS

CHAPTER I. INTRODUCTION

1.1 Diamond growth from vapor phase

1.2 MPCVD system description

1.2.1 Microwave generator and guiding system

1.2.2 Gas flow control system

1.2.3 Deposition chamber and vacuum control system

1.3 Substrates used in diamond growth experiments

1.4 Dependence of diamond growth on various parameters

2. DIAMOND GROWTH ON NON-PLANAR SUBSTRATES

2.1 Introduction

2.2 Role of nitrogen

2.3 Experimental details and characterization techniques

2.4 Results and discussion

viii3. DIAMOND GROWTH ON PLANAR SUBSTRATES

3.1 Introduction

3.2 Experimental details

3.3 Results and discussion

4. DIAMOND GROWTH MECHANISM

4.1 Introduction

4.2 Qualitative study on diamond growth

4.3 Discussion on diamond growth mechanism

5. CONCLUSIONS

6. FUTURE WORK

LIST OF REFERENCES

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1 Surface roughness values of non-planar samples

2 Summary of growth conditions for planar substrates

3 Summary of nitrogen incorporation trends on planar substrates

–  –  –

1.1 Schematic of generic CVD system

1.2 Schematic of MPCVD system

1.3 Photograph of actual MPCVD system

1.4 Photograph of substrates used in the experiments

2.1 Arrangement of diamond anvils in a cell

2.2 Stages in preparation of designer diamond anvils

2.3 Raman spectrum showing C-12 and C-13 peaks

2.4 Growth rate as a function of N2 concentration for non-planar substrates.............22

2.5 AFM images of samples prepared with different N2 concentrations

2.6 Thermodynamic calculations showing relative concentrations of species............25

2.7 PL comparison of samples prepared with high and low N2 in plasma..................26

3.1 Raman spectrum of sample 7 from table 3.1

3.2 Growth rate as a function of N2 concentration for planar substrates

3.3 PL spectra of samples prepared with different N2 concentrations

3.4 Optical micrographs of samples prepared with different N2 concentrations.........37

3.5 AFM images of samples prepared with different N2 concentrations

3.6 AFM image showing distinct change in morphology

–  –  –

3.9 Optical micrographs of samples prepared at different substrate temperatures......43

3.10 AFM images of samples prepared at different substrate temperatures..................44

3.11 Growth rate as a function of CH4 concentration for planar substrates..................46

3.12 PL spectra of samples prepared with different CH4 concentrations

3.13 AFM images of samples prepared with different CH4 concentrations.................48

3.14 Optical micrographs of samples prepared with different CH4 concentrations......49

3.15 Extended omega scan of sample 11 from Table 3.1

3.16 Comparison of omega scans for different samples

4.1 Illustration of hydrogen abstraction reaction

4.2 Effect of oxygen on CVD diamond growth

4.3 Effect of temperature on CVD diamond growth with 6% CH4/H2 in plasma.......63

4.4 Substrate holder designs

4.5 Optical micrographs of samples prepared with high CH4 concentration..............66

6.1 Mass spectroscopy studies done in real time

–  –  –

Diamond has long been a topic of intense research interest owing to its excellent

material properties. Some properties of diamond which have practical use in industry are:

(1) The gem quality diamond is one of the most in-demand precious stones; (2) As the hardest known material (hardness of 100 GPa), it has wide ranging applications in the cutting tool industry; (3) Diamond’s unsurpassed thermal conductivity (2000 W/mK at room temperature) merits it as good heat sink; (4) Undoped diamond is an excellent electrical insulator; (5) Also, diamond can be doped to form p-type and n-type semiconductors, with room-temperature drift mobilities of 4500 cm2/V.s for electrons and 3800 cm2/V.s for holes [1]; (6) Diamond has resistance towards damage from heat, acid corrosion and radiation.

There are other fields in which diamond has applications such as high power electronic devices and solid state electronics [2]. For many applications, a key requirement is a large area single crystal diamond sample. Although single crystal diamond samples have been prepared by high pressure – high temperature (HPHT) method, the cost involved and the issues with metal impurities incorporation in diamond prepared by such method have limited its applicability. HPHT method has limitations on the size of the size of the diamond crystal that can be grown. The limitation of the

–  –  –

which High pressure – High temperature conditions can be created using the beltapparatus and other multi-anvil devices. Chemical vapor deposition (CVD) is rapidly becoming an excellent alternative to HPHT synthesis. It is not until very recently [3] that high growth rates have been reported in diamond growth by chemical vapor deposition.

However, the added advantage the CVD readily offered over HPHT synthesis has been greater control over the inclusion of impurities into the grown diamond film. The following section offers a brief description of a generic CVD system and the basic mechanism of growth of diamond from vapor phase.

–  –  –

At the typical growth conditions of low pressure (50 to 200 torr) – high temperature (850 to 1300 C) synthesis of diamond in CVD; thermodynamically stable state of carbon is graphite, not diamond. Thus the diamond synthesis by CVD is carried out under conditions where diamond is in a metastable state. However, the presence of hydrogen and moderate amounts of oxygen enables the growth of diamond instead of graphite. The detailed mechanism will be discussed in chapter 5. The basic gas precursors needed are hydrogen and a source of carbon, usually CH4 and sometimes CO2 and C2H2.

The schematic of a generic CVD system is shown in Figure 1.1.

A source of energy is needed to dissociate the gas phase precursors. The source of energy can be hot-filament or microwaves or dc arcjet or laser radiation. Depending on

–  –  –

conducted as part of this thesis, a 1.2 KW microwave generator has been used as the source of energy needed to dissociate gas phase precursors. The energy supplied by the source readily dissociates the components of the feed gas mixture and produces the carbon radicals and atomic hydrogen necessary for diamond growth. Since pressures inside the CVD chamber are of the order of 50 to 200 torr, the mobility and diffusion of the growth radicals is well supported near the substrate surface. Another added benefit of relying on microwave plasma CVD for diamond growth is that at the aforementioned pressure ranges, it is possible to obtain a very intensive localized discharge that has little tendency to spread.

Atomic hydrogen completely covers the substrate surface and its abstraction creates the radical sites where carbon growth radicals would come and adsorb. It also ensures that the carbon being incorporated into the lattice is sp3 bonded instead of sp2.

The complete understanding of roles of each individual growth radical has not yet been achieved and that is why much of the research on homoepitaxial diamond growth is qualitative.

1.2: Microwave Plasma Chemical Vapor Deposition (MPCVD) system description An understanding of the individual subsystems constitutes to the description of the MPCVD system. The major components of the MPCVD system used to study the homoepitaxial diamond growth can be listed as (a) Microwave generator and guiding

–  –  –

The schematic of the MPCVD system used in the experiments is shown in Figure 1.2 and part of the actual system is shown in Figure 1.3. A short description of each of the subsystems will follow next. For a detailed description of the system, its design issues please refer to [4].

–  –  –



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