`
`November, 1973
`
`Herbert Paul Maruska
`
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`GALLIUM NITRIDE LIGHT-EMITTING DIODES
`
`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
`
`By
`
`Herbert Paul Maruska
`
`November 1973
`
`"7y...
`
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`
`
`I certify that I have read this thesis and that in my
`opinion it is fully adequate,
`in scope and quality, as
`a dissertation for the degree of Doctor of Philosophy.
`
`
`
`
`
`(Principal Adviser)
`
`I certify that I have read this thesis and that in my
`opinion it is fully adequate,
`in scope and quality, as
`a dissertation for the degree of Doctor of Philosophy.
`~
`
`~
`
`3%
`
`I certify that I have read this thesis and that in my
`opinion it is fully adequate,
`in scope and quality, as
`a dissertation for the degree of Doctor of Philosophy.
`
`6,erth «La-
`
`94 gaim
`
`'(Electrical Engineering)
`
`I certify that I have read this thesis and that in my
`opinion it is fully adequate,
`in scope and quality, as
`a dissertation for the degree of Doctor of Philosophy.
`
`(kw/a;flé‘éfw
`
`Electrical Engineering)
`
`Approved for the University Committee
`on Graduate Studies:
`
`Rafi“ f M5903
`
`Dean of Graduate Studies
`
`ii
`
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`E B
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`h—..
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`
`
`To Jacques I. Pankove, whose dedicated
`
`interest in science has been a source of
`
`inspiration to me.
`
`’WW
`
`iii
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`ACKNOWLEDGMENTS
`
`T would like to thank the following individuals for their contri-
`
`butions to this dissertation:
`
`Professor David A. Stevenson, principal advisor, for guidance,
`
`helpful discussions, and continuing interest.
`
`Professor Richard H. Bube, Professor James F. Gibbons, and Professor
`
`Gerald L. Pearson. members of the thesis reading committee, for helpful
`
`suggestions and discussions.
`
`Dr. Jacques I. Pankove of RCA Laboratories for starting me on a
`
`career in scientific research and for his lasting friendship.
`
`Dr. Robert A. Burmeister, Jr. and Dr. Egon E. Loebner of Hewlett
`
`Packard for a critical review of the manuscript.
`
`Dr. Wally Rhines of Texas Instruments for experimental assistance
`
`and many creative and stimulating discussions while he was at Stanford,
`
`and continuing friendship.
`
`Dr. Troy W. Barbee and Professor Robert H. Huggins for valuable
`
`suggestions and discussions.
`
`Mr. William L. Larson for much appreciated experimental assistance
`
`and good friendship.
`
`Mr. Steve Chiang, Dr. Alan L. Fahrenbruch, Mr. Bruce E. Liebert,
`
`Mr. John McKenzie, and Mr. Yves Verhelle for helpful discussions and
`
`friendship.
`
`Mr. Perfecto Mary for preparing the drawings and illustrations.
`
`Mrs. Valerie Faulkenburg for typing the original and final
`
`manuscripts.
`
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`The assistance of Mr. Loren Anderson, Miss Rosemary Koch, Mrs.
`
`Marta de Rojas and the entire technical and office staffs of the Center
`
`for Materials Research is gratefully acknowledged.
`
`Two years of fellowship support from RCA Laboratories is also
`
`gratefully acknowledged.
`
`And I would like to thank my wife, Claire, for all the good times
`
`we had in California.
`
`This research was supported by the Advanced Research Projects
`
`.
`
`Agency under Grant No. DAHClS 71—6-6.
`
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`
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`Chapter
`
`TABLE OF CONTENTS
`
`LIGHT—EMITTING DIODES .
`
`1.1
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`Introduction .
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`1.2 Classification and Basis of Operation
`of LED's .
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`1.3 LED Efficiencies .
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`1.4 Examples of LED Materials
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`1.5 Purpose of This Work .
`
`.
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`SYNTHESIS TECHNIQUES AND PROPERTIES OF
`GALLIUM NITRIDE .
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`2.1 Synthesis and Growth Techniques
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`2.2 Doping .
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`32
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`2.3 Optical Properties .
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`2.4 Electrical Properties
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`2.5 Electroluminescence
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`EXPERIMENTAL TECHNIQUES .
`3.1 Open—Flow Vapor Growth System
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`3.2 Doping Procedure .
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`3.3 Point=Contact LED‘s
`3.b
`Improved Large—Area Metal Contacts -
`
`.
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`3.5 Measurement of Wavelength and Intensity
`of Luminescence
`.
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`3.6 Electrical Characteristics
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`3.7 Scanning Electron Microscope .
`
`3.8 Transmission Electron Microscope .
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`3.9 Proton Bombardment and ProtoneAssisted Diffusion .
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`36
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`39
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`41
`41
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`47
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`53
`54
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`57
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`63
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`A
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`vi
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`TABLE OF CONTENTS (Contd)
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`Chapter
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`4.
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`EXPERIMENTAL RESULTS
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`4.1 Growth and Fabrication of GaN Light-
`Emitting Diodes
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`4.2 Emission Spectra of CaN:Mg Light—
`Emitting Diodes
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`4.3 Electrical Characteristics .
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`4.4 Temperature Dependence of Luminescence
`and Electrical Characteristics .
`.
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`4.5 Growth and Surface Morphology of GaN .
`
`4.6 Pattern of Light Emission
`
`.
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`4.7 Characteristics of the Insulating
`(Mg-doped) Region
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`4.8 Electrical Potential Distribution .
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`4.9 Diode Capacitance
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`4.10 Proton Bombardment and Proton—Assisted
`Diffusion
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`5.
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`DISCUSSION
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`5.1 Basic Mechanisms for Diode Operation .
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`5.2 Mechanism for CaN nei-n Diode Electrical
`Characteristics
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`143
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`143
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`154
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`160
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`5.3 Mechanisms for Electroluminescence .
`
`5.4
`
`Impact Ionization in GaN m—i—n Diodes
`
`5.5 Nature of the Potential Barrier
`
`.
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`5.6 Model for GaN Light—Emitting Diodes
`
`6.
`
`CONCLUSIONS .
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`vii
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`LIST OF FIGURES
`
`Number
`
`Page
`
`Figure 1.1
`
`Spectral sensitivity of
`
`the human eye.
`
`Figure 1.2
`
`Best reported values of luminous power con—
`version for light—emitting diodes.
`
`Figure 1.3
`
`Chromaticity diagram.
`
`Figure l.é
`
`The color matching functions V(x), V(y),
`and V(z),
`(red, green, and blue}.
`
`Figure 2.1
`
`Equilibrium vapor pressure of N2 over
`GaN(s)~Ga(£).
`
`Figure 3.1
`
`Open—Flow Vapor Growth System for pre—
`paring gallium nitride.
`
`Figure 3.2
`
`Quartz sample holder.
`
`Figure 3.3
`
`System for positioning dopant crucible
`in sidearm of vapor growth apparatus.
`
`Figure 3.4
`
`Graphite dopant crucible.
`
`Figure 3.5
`
`Gallium nitride light-emitting diode
`attached to a glass slide.
`
`Figure 3.6
`
`Schematic diagram of a gallium nitride light-
`emitting diode on a TO—S transistor header.
`
`Figure 3.7a Experimental arrangement for measuring the
`wavelength and intensity of electrolumi—
`nescence (schematic).
`
`Figure 3.7b Experimental arrangement for measuring the
`wavelength and intensity of photolumines—
`cence (schematic).
`
`Figure 3.8
`
`Spectral response of
`
`the 8—20 photocathode.
`
`Figure 3.9
`
`System for measuring l—V characteristics
`(schematic).
`
`Figure 3.10
`
`System for simultaneous measurement of power
`efficiency and I—V characteristics of a light—
`emitting diode (schematic).
`
`11
`
`15
`
`17
`
`18
`
`30
`
`43
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`46
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`49
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`52
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`56
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`58
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`59
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`60
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`62
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`64
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`66
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`V1”
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`LIST OF FIGURES (Contd)
`
`Number
`
`Page
`
`Figure 3.11 Schematic diagram of the scanning
`electron microscope.
`
`Figure 4.1
`
`Electroluminescence spectrum of point-contact
`GaN:Mg light-emitting diode (sample #6‘8.72).
`
`Figure 4.2
`
`Electroluminescence spectrum of GaNzMg m—i—n
`diode, sample #6-28-72 (forward bias).
`
`Figure 4.3
`
`Electroluminescence spectrum of typical
`GaN:Mg mri—n diode (forward bias).
`
`Figure 4.4
`
`Dependence of the peak wavelength of the emission
`on applied voltage (forward bias).
`
`Figure 4.5
`
`Typical reverse—bias electroluminescence
`spectrum for GaN:Mg m—i-n diodes.
`
`Figure 4.6
`
`Dependence of the peak wavelength of the emission
`on applied voltage (reverse bias).
`
`Figure 4.7
`
`Photoluminescence spectrum of Mg-doped GaN.
`
`Figure 4.8
`
`Dependence of light power output on electrical
`powar input for GaN:Mg lightwemitting diodes.
`
`Figure 4.9
`
`External quantum efficiency of GaN:Mg light-
`emitting diodes.
`
`Figure 4.10 Typical diode I~V characteristic.
`
`Figure 4.11 Log~log plot of diode I—V characteristic
`(Sample #7'7'72).
`
`Figure 4.12 Luminous intensity as a function of
`applied voltage.
`
`Figure 4.13 Temperature dependencies of diode current and
`luminous intensity at constant voltage.
`
`Figure 4.14 Temperature dependencies of diode current and
`luminous intensity at various constant voltages.
`
`Figure 4.15 Temperature dependence of photoluminescence
`intensity.
`
`68
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`76
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`78
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`81
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`83
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`LIST OF FIGURES (Contd)
`
`Number
`
`Figure 4.16 Hexagonal islands of GaN nucleated on (0001)—
`sapphire.
`
`Figure 4.17 Hexagonal islands of GaN, which have coalesced
`to form a continuous film on the left side of
`
`the picture.
`
`(0001)-orientation.
`
`Figure 4.18 Surface of gallium nitride sample #5-19-72.
`grown on (1102)-sapphire.
`
`Figure 4.19 Typical facgtted surface developed in GaN
`grown on (1102)-sapohire.
`
`Figure 4.19A Gallium nitride films on quartz substrates,
`showing crystallite size at various distances
`downstream from the ammonia inlet:
`(a) 2";
`(b) 3";
`(c) 4". Magnification 200x.
`
`Figure 4.20 Laue pattern of (1013)—GaN (grown in (1102)—
`sapphire substrate).
`
`Figure 4.21 Gallium nitride light—emitting_diode with
`external illumination.
`The (1102) sapphire
`substrate is removed, exposing the n~GaN
`layer. Magnification 28X.
`
`Figure 4.22 Light spot pattern observed during forward
`biasing of the diode shown in Figure 4.21.
`Magnification 28X.
`
`Figure 4.23 Light spot pattern of forward—biased diode
`at 200K magnification.
`
`Figure 4.24
`
`Same diodes as in Figure 4.23 with external
`illumination and light spot pattern super-
`imposed (ZOOX).
`(1013)—orientation.
`
`Figure 4.25 Gallium nitride lightwemitting diode
`#1212'72 with external illumination.
`(1013)“orientation.
`
`Figure 4.26 Gallium nitride light—emitting diode
`#12‘7‘72 (cf. Figure 4.25) with forward
`bias,
`illustrating light spot pattern.
`
`Page
`
`97
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`99
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`100
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`101
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`102
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`104
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`105
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`107
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`108
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`109
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`112
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`x
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`LIST OF FIGURES (Contd)
`
`Number
`
`Page
`
`Figure 4.27 Diode #12°7-72 (cf. Figure 4.25) with
`reverse bias,
`illustrating different set of
`light spots appearing with reverse bias than
`forward bias.
`
`Figure 4.28 Diode mounted perpendicular to a glass slide
`showing sapphire substrate, GaN, and metal
`contact.
`
`Figure 4.29 Lightspotsobserved when diode shown in
`Figure 4.28 was forward biased.
`The light
`spots occur within the GaN region.
`Magnification 500x.
`
`Figure 4.30 Perpendicular View of diode #12-13-72, as
`seen in the scanning electron microscope.
`The entire device (including the (1102)~
`sapphire substrate) and the two metal
`contacts (to the n— and i-layers) are
`visible. Magnification 25X.
`
`Figure 4.31 Scanning electron micrograph of diode
`#12'13'72 at lOOX magnification——no bias.
`(1102)~oriented sapphire substrate.
`
`Figure 4.32 Scanning electron micrograph of diode
`#12-13'72 at lOOX magnification with
`forward bias (+22 volts).
`
`Figure 4.33 Scanning electr0n micrograph of diode
`#12-13‘72 at lOOX magnification with
`reverse bias (F22 volts).
`
`Figure 4.34 Scanning electron micrograph of diode
`#12'13‘72 at 500x magnification with
`reverse bias.
`The insulating (Mg—doped)
`Gafl_layer can be clearly distinguished.
`(1102)~oriented sapphire substrate.
`
`Figure 4.35 Scanning electron micrograph of diode
`#12-13'72 at lOOOX magnification with
`no bias.
`
`113
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`115
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`116
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`120
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`121
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`122
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`123
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`124
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`126
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`i
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`Number
`
`Figure
`
`Figure
`
`é.37
`
`Figure
`
`5.38
`
`Figure
`
`4.39
`
`Figure
`
`4.40
`
`Figure
`
`4.41
`
`Figure
`
`G.42
`
`Figure
`
`4.43
`
`Figure
`
`4.&4
`
`LIST OF FIGURES (Contd)
`
`Scanning electron micrograph of diode
`#12-13-72 at lOOOX magnification with
`forward bias.
`The bias was removed
`
`during part of the Seen, as indicated.
`
`Scanning electron micrograph of diode
`#12-13-72 at lOOOX magnification with
`reverse bias.
`The bias was removed
`
`during part of the scan, as indicated.
`
`Transmission electron micrograph of
`insulating (Mg—doped) region of a GaN
`LED. Notice no evidence of precipitates
`or dislocations. Magnification 240,000X.
`
`Superimposed zero bias and forward bias
`line scan traces and SEM photograph of a
`gallium nitride light—emitting diode.
`Magnification 1000K.
`
`Superimposed zero bias and forward bias
`line scan traces and SEM photograph of a
`gallium nitride light—emitting diode.
`Magnification 2500K.
`
`Superimposed zero bias and reverse bias
`(~6, —12, and ‘18 volts) line scan traces
`and SEM photograph of a gallium nitride
`light—emitting diode. Magnification 2500K.
`
`Electric potential distribution across a
`gallium nitride light—emitting diode when
`forward bias is applied.
`
`Electric potential distribution across a
`gallium nitride lightflemitting diode when
`reverse bias is applied.
`
`The capacitance and conductance as functions
`of applied voltage for a typical CaN diode.
`Also included is the quantity,
`l/cz, as a
`function of voltage.
`
`Figure
`
`Current—voltage characteristic of a GaN
`m—i—n diode prepared by proton bombardment.
`
`xii
`
`127
`
`128
`
`129
`
`131
`
`132
`
`133
`
`135
`
`136
`
`138
`
`1&0
`
`TCL 1028, Page 13
`LOWES 1028, Page 13
`
`VIZIO EX. 1028 Page 0013
`
`LOWES 1028, Page 13
`
`VIZIO Ex. 1028 Page 0013
`
`
`
`LIST OF FIGURES (Contd)
`
`Number
`
`Figure 5.1
`
`Figure 5.2
`
`Fowler-Nordheim plot of GaN:Mg light-
`emitting diode current—voltage charaCter—
`istic. Cf. Figure 4.11.
`
`Fowler—Nordheim plot of GaNrfig light-
`emitting diode current-voltage character—
`istics at various temperatures.
`CE. Figure 4.14.
`
`Figure 5.3
`
`Plot of log I versus log V for GaN:Zn
`light—emitting diode (after PANKOVE,
`1972, Figure 8).
`
`Figure 5.4
`
`The I—V characteristic of the GaN:Zn
`light-emitting diode shown in Figure 5.3,
`replotted to the Fowler~Nordheim Equation.
`
`Figure 5.5
`
`Log of electroluminescence intensity versus
`(voltage)—l/2. Cf. Figure 4.12.
`
`Figure 5.6
`
`Log of forward and reverse bias electro-
`
`lumineseence intensity Versus (voltage)F
`
`1/2
`
`Figure 5.7
`
`Logof'electroluminescence intensity‘versus
`(voltage)_lf2 at various temperatures.
`Cf. Figure 4.14.
`
`Figure 5.8
`
`Log of luminous intensity versus log of
`diode current at various temperatures.
`Cf. Figure 4.14.
`
`Figure 5.9
`
`Carrier multiplication factor M as a
`function of applied Voltage for a CaN
`light—emitting diode.
`
`Figure 5.10 Plot of (external quantum efficiency) x
`{2
`-1/2‘
`(voltage)-1
`versus (voltage)
`
`Figure 5.11 Charge concentrations, electric fields, and
`
`NA,
`electric potentials at a p=n junction.
`ND are respectively the concentrations of
`
`acceptors and donors; FA’ FD, and F are
`respectively the electric field due to the
`
`Page
`
`155
`
`156
`
`157
`
`153
`
`163
`
`164
`
`165
`
`166
`
`176
`
`179
`
`xiii
`
`TCL 1028, Page 14
`LOWE's 1028, Page 14
`
`VIZIO EX. 1028 Page 0014
`
`LOWES 1028, Page 14
`
`VIZIO Ex. 1028 Page 0014
`
`
`
`LIST OF FIGURES (Contd)
`
`Nil—mtg
`
`ionized acceptors, electric field due to the
`ionized donors, and total electric field;
`
`¢A, ¢D’ and ¢ are respectively the potential
`due to the ionized acceptors, potential due
`to the ionized donorsI and total potential;
`
`and ¢B is the barrier potential.
`
`Figure 5.12
`
`Schematic of processes responsible for
`electroluminescence in GaN m—i—n diodes.
`The situation for forward bias is
`illustrated.
`
`Figure 5.13
`
`Growth and device operation of a gallium
`nitride light—emitting diode:
`(a;
`Nucleation of GaN islands on a (1102)
`sapphire Substrate;
`(b) Growth of GaN
`islands, until they meet;
`(c) Growth of
`continuous GaN film after the islands
`
`the
`indicating the position of
`have met,
`sub-grain boundaries;
`(d) Complete GaN
`m—i~n diode, showing the structural origin
`of the electroluminescence with forward
`bias.
`
`Page
`
`182
`
`187
`
`189
`
`xiv
`
`TCL 1028, Page 15
`LOWE's 1028, Page 15
`
`VIZIO EX. 1028 Page 0015
`
`LOWES 1028, Page 15
`
`VIZIO Ex. 1028 Page 0015
`
`
`
`LIST OF SYMBOLS
`
`constant; lattice parameter (E); activity
`
`area (cmz); constant
`
`constant
`
`barrier height
`
`(eV)
`
`constant; lattice parameter
`
`(E)
`
`capacitance (pF)
`
`thickness of insulator (cm)
`
`energy (eV)
`
`activation energy (eV)
`
`band gap energy (eV)
`
`minimum energy for impact ionization (eV)
`
`electric field (volts/cm)
`
`luminous power
`
`(lumens)
`
`Gibbs free energy (Kcal)
`
`standard Gibbs free energy (Keel)
`
`Planck's constant(4.14 x 10‘15 eV—sec)
`
`insulating
`
`current
`
`(amps)
`
`current density (amp/cmz)
`
`Boltzmann constant (0.864 x 10‘
`
`4 eV/°K)
`
`equilibrium constant
`
`length (cm); free path (cm); dimensionless parameter
`characterizing trap distribution
`
`mean free path (cm)
`
`metal
`
`XV
`
`TCL 1028, Page 16
`LOWES 1028, Page 16
`
`VIZIO EX. 1028 Page 0016
`
`LOWES 1028, Page 16
`
`VIZIO Ex. 1028 Page 0016
`
`
`
`LIST OF SYMBOLS (Contd)
`
`mass of electron (9.1 x 10,
`
`31 Kg)
`
`effective mass of electron (Kg)
`
`electron carrier density (cm—3); numerical constant
`
`effective charge density in depletion region (cm—3)
`
`acceptor density (cm—3)
`
`donor density (cm—3)
`
`luminescent center density (cm—3)
`
`hole carrier density (cm—3)
`
`light power output (watts);
`
`ionization probability; pressure
`
`electronic charge (1.60 x 10—
`
`19 coul)
`
`resistance (0); gas constant
`
`(1.987 cal/“K—mole)
`
`i
`
`temperature (°C,
`
`°K)
`
`effective electron temperature (°K)
`
`bias voltage (volts)
`
`eye sensitivity function
`
`concentration of positively charged vacancies on the
`nitrogen lattice
`
`V V
`
`(A)
`
`[Vii]
`
`width of depletion region (cm)
`
`distance (cm)
`
`absorption coefficient
`
`(cm—
`
`1)
`
`permittivity (farads/m)
`
`permittivity of free space (8.85 x 10_12 farads/m)
`
`efficiency
`
`external quantum efficiency
`
`radiative efficiency of recombination
`
`xvi
`
`TCL 1028, Page 17
`LOWE's 1028, Page 17
`
`...__‘
`
`VIZIO EX. 1028 Page 0017
`
`LOWES 1028, Page 17
`
`VIZIO Ex. 1028 Page 0017
`
`
`
`
`
`LIST OF SYMBOLS (Contd)
`
`wavelength (A)
`
`mobility (cm2/V-sec)
`
`frequency (sec_ )
`
`canductivity (Q—cm_1); cross-section (cmz)
`
`lifetime
`
`electric potential (volts)
`
`barrier potential (volts)
`
`xvii
`
`TCL 1028, Page 18
`LOWES 1028, Page 18
`
`VIZIO EX. 1028 Page 0018
`
`LOWES 1028, Page 18
`
`VIZIO Ex. 1028 Page 0018
`
`
`
`ABSTRACT
`
`‘
`
`The synthesis and characterization of hetero-epitaxial gallium
`
`nitride (GaN) films were undertaken with particular reference to the
`
`phenomenon of light emission. Gallium nitride was gIDWn by the chemical
`
`vapor deposition technique using sapphire substrates. Techniques for
`
`characterization included optical, scanning electron, and transmission
`
`electron microscopy and measurements of various electrical and optical
`
`properties of the films.
`
`The thin films of GaN were doped during
`
`growth with zinc and magnesium to form n—i junctions.
`
`Such material
`
`provided the basis for the fabrication of m—i—n light—emitting diodes,
`
`which emitted light in the high—energy violet region of the visible
`
`spectrum with Mg doping and green light with Zn doping.
`
`Undoped GaN layers were ordinarily n—type with carrier concentra-
`
`tions in excess of 1018 cm_3.
`
`Insulating GaN was produced through
`
`compensation by doping with the acceptor impurities Zn and Mg.
`
`Light—
`
`emitting diodes were prepared by growing onto a sapphire substrate a
`
`film of highly conducting undoped GaN (n-layer),
`
`folIOWed by a film of
`
`insulating GaN (i—layer), and then placing a metal contact over the
`
`Surface (m—layer).
`
`Such a structure forms an m—i—n diode, which con—
`
`tains two electrical junctions, m-i and i—n. Most of the diodes studied
`
`had a Mg~d0ped i—layer, and such diodes exhibited violet electro—
`
`luminescence which peaked at 2.9 eV with a 400 meV halfwidth. External
`
`quantum efficiencies of about 0.005% were obtained in these diodes
`
`with a bias of 20 volts.
`
`The light is emitted from the diodes in the form of small
`
`(<10 u
`
`xviii
`
`TCL 1028, Page 19
`LOWES 1028, Page 19
`I
`
`VIZIO EX. 1028 Page 0019
`
`LOWES 1028, Page 19
`
`VIZIO Ex. 1028 Page 0019
`
`
`
`diameter), discrete spots. These light spots are generated at the
`
`junction in the diode which forms the cathode. Although Laue patterns
`
`indicate that the GaN films are single-crystalline.
`
`the surfaces of the
`
`layers show a strongly facetted structure, and the positions of the
`
`light spots correspond to these facets. Gallium nitride is nucleated
`
`in the form of a large number of individual islands which subsequently
`
`coalesce to form a continuous film.
`
`The coalesced islands are slightly
`
`misaligned, resulting in the formation of subgrain boundaries.
`
`No
`
`evidence for precipitates or dislocations within the individual cells
`
`was found.
`
`The electric potential distribution across the m-i—n diodes was
`
`obtained through the voltage—contrast capability of the scanning
`
`electron microscope. Potential gradients and electric fields of about
`
`105 volts/cm are developed at the mri and i-n junctions, but not
`
`throughout the bulk of the ivlayer. These observations, coupled with
`
`a study of the diode current—voltage characteristics,
`
`including their
`
`temperature dependence,
`
`lead to a model of current control in GaN
`
`diodes by tunnelling through a potential barrier.
`
`The I—V character—
`
`istics follow the Fowler-Nordheim field-emission equation.
`
`It is proposed that impact ionization, occurring at the subgrain
`
`boundaries in the GaN films,
`
`is responsible for the electroluminescence.
`
`High—energy electrons empty the luminescent centers by collisions, and
`
`the subsaquent recombination of electrons in thermal equilibrium in the
`
`conduction band with these centers results in the generation of light.
`
`This model is consistent with photomultiplication measurements and with
`
`the voltage dependence of the external quantum efficiency.
`
`The models
`TCL 1028, Page 20
`LOWES 1028, Page 20
`
`xix
`
`VIZIO EX. 1028 Page 0020
`
`LOWES 1028, Page 20
`
`VIZIO Ex. 1028 Page 0020
`
`
`
`developed for the diode electrical properties and for the electro—
`
`luminescence are useful in understanding the characteristics of m—i-n
`junctions.
`
`
`
`TCL 1028, Page 21
`LOWE's 1028, Page 21
`I
`
`VIZIO EX. 1028 Page 0021
`
`LOWES 1028, Page 21
`
`VIZIO Ex. 1028 Page 0021
`
`
`
`
`
`'Let there be light,‘
`”God said,
`and there was light.
`God saw that the
`light was good, and God divided light
`from darkness."
`
`Gn l: 3,
`
`1:
`
`xxi
`
`TCL 1028, Page 22
`LOWE'S 1028, Page 22
`
`VIZIO EX. 1028 Page 0022
`
`LOWES 1028, Page 22
`
`VIZIO Ex. 1028 Page 0022
`
`
`
`
`
`PLATE I.
`
`Forward biased GaN:Mg light-emitting diode.
`
`xxii
`
`TCL 1028, Page 23
`LOWES 1028, Page 23
`
`VIZIO EX. 1028 Page 0023
`
`LOWES 1028, Page 23
`
`VIZIO Ex. 1028 Page 0023
`
`
`
`
`
`PLATE II. Reverse biased GaNzMg light—emitting diode.
`
`xxiii
`
`TCL 1028, Page 24
`LOWE's 1028, Page 24
`
`VIZIO EX. 1028 Page 0024
`
`LOWES 1028, Page 24
`
`VIZIO Ex. 1028 Page 0024
`
`
`
`
`
`Chapter 1
`
`LIGHT—EMITTING DIODES
`
`1.1 au_du_cti_oa
`
`Light—emitting diodes (LED's) are devicas that emit electromagnetic
`
`radiation in response to an applied electric field. Light emission from
`
`a material due to an electric field is termed electroluminescence.
`
`The
`
`light involved with the process of electroluminesconce is not determined
`
`simply by the temperature of the material, as is the case with incan—
`
`descence (black body radiation). Applications for LED'S are found in
`
`the areas of indicator lights, alpha-numeric displays, picture displays
`
`(solid-state color television) and IR sources. Commercially available
`
`red LED's are now being used as numerals in calculators and wrist-
`
`watches, and in place of panel meters in various types of electronic
`
`equipment.
`
`For most application,
`
`then, it is necessary that the emitted
`
`light fall in the visible portion of the electromagnetic spectrum
`
`(4000 A to 7000 R), since the purpose of the devices is to transmit
`
`information from instruments to people.
`
`Solid—state semiconductor materials are used for the manufacture of
`
`various light-emitting diodes. They feature many attractive qualities
`
`in contrast to incandescent light sourcas:
`
`reliability, long life, low
`
`cost, small size, and low power requirements and operating voltages.
`
`The last three items are particularly important for compact battery
`
`CPerated equipment.
`
`The operating characteristics of semicouductor
`
`LED's are compatible with silicon integrated circuits, which makes them
`
`especially suitable for visual displays in computer systems.
`
`TCL 1028, Page 25
`LOWE's 1028, Page 25
`
`VIZIO EX. 1028 Page 0025
`
`LOWES 1028, Page 25
`
`VIZIO Ex. 1028 Page 0025
`
`
`
`
`1.2 Classification and Basis of Operation of LEDLE
`
`It is possible to distinguish five classes of semiconductor light—
`
`emitting diodes. These are.
`
`(l)
`
`p~n homojunction diodes
`
`(2)
`
`p-n heterojunction diodes
`
`(3) Schottky barrier and m—iws diodes
`
`(4)
`
`electroluminescent cells (powders dispersed in organic
`
`binders)
`
`(5)
`
`infrared emitting diodes coated with phosphors that emit
`
`visible light
`
`(”up—converters").
`
`The first class of diodes inVOIVes a single material, part of which is
`
`doped p—type and the other part of which is doped n—type to form a p-n
`
`junction.
`
`The n—type side of the diode is prepared so as to contain an
`
`excess of electrons, while the p—type side has a deficit of electrons
`
`(that is, an excess of holes). Excess electrons are introduced into a
`
`semiconductor by substituting in place of a normal lattice constituent
`
`an element
`
`(dopant) which contains one more than the proper number of
`
`bonding electrons. Holes occur when the substituted dopant lacks one
`
`from the proper number of bonding electrons. When a potential differ-
`
`ence is applied to such a diode, electrons are injected from the n—type
`
`region across the junction into the p-type region, where they can
`
`recombine with holes. Conversely, holes can be injected into the n-side
`
`with subsequent recombination with electrons; usually one process is
`
`dominant in a given semiconductor.
`
`The carriers which have been
`
`injected into a region in which they were originally scarce are called
`
`minority carriers.
`
`The carriers present in excess are the majority
`
`_.2_
`
`TCL 1028, Page 26
`LOWES 1028, Page 26
`
`VIZIO EX. 1028 Page 0026
`
`LOWES 1028, Page 26
`
`VIZIO Ex. 1028 Page 0026
`
`
`
`carriers.
`
`The excess electrons are situated in the semiconductor's conduction
`
`band in the n—type region, while the holes reside in the valence band in
`
`the p—type region.
`
`The valence and conduction bands are separated by a
`
`forbidden energy region, called the "energy gap" or "band gap" of
`
`the
`
`material. This is just the energy required to move an electron from
`
`the valence band (where a hole is left behind)
`
`to the conduction band.
`
`Thus, for example, if electrons are injected from the conduction band
`
`on the n—side to the conduction band on the p—side, under the influence
`
`of an applied potential difference, and they recombine with holes in
`
`the valence band,
`
`the total energy difference involved is just the
`
`"band gap" energy. This energy of recombination can be released from
`
`the diode in the form of photons. All of the photons emitted by a
`
`light—emitting diode will not possess exactly the same energy because
`
`the electrons and holes which recombine are distributed in energy in
`
`their respective bands.
`
`The energy distributions of the electrons and
`
`holes are governed by Fermi—Dirac statistics, but if the Semiconductor
`
`is not degenerately doped,
`
`the distributions in the valence and con—
`
`duction bands may be approximated by Maxwell—Boltzmann statistics.
`
`Since the carriers are distributed in energy in this way, a plot of
`
`number of photons emitted vs. energy has the form of a bell-shaped
`
`CUI'VB‘
`
`In many cases the electrons and holes recombine at a localized
`
`energy state within the forbidden energy gap.
`
`Such recombination sites
`
`are due to imperfections in the crystal,
`
`including the purposely intro—
`
`duced dopant atoms.
`
`In such a case the photon energy will be the
`
`w3.
`
`TCL 1028, Page 27
`LOWE's 1028, Page 27
`
`VIZIO EX. 1028 Page 0027
`
`LOWES 1028, Page 27
`
`VIZIO Ex. 1028 Page 0027
`
`
`
`energy separation between this level and one of the bands. Notice that
`
`two photons of different energies adding up to the band gap energy could
`
`be emitted, but in many situations of interest the localized state is
`
`quite close (a few tenths of an electron-volt) from either the valence
`
`or the conduction band, so that the energy released is approximately
`
`the band gap energy of the semiconductor.
`
`In any case the band gap
`
`determines the highest frequency range of light which may be emitted.
`
`In order for the light to be visible to the human eye, a minimum band
`
`gap of 1.8 eV is required.
`
`The largest band gap that will produce
`
`visible light is about 3.1 eV, although materials with still larger
`
`band gaps may contain appropriate recombination centers to provide
`
`visible light.
`
`In this first class of light-emitting diodes,
`
`then,
`
`the crystal is
`
`homogeneous throughout except for the change in dopant.
`
`The problems
`
`associated with joining dissimilar materials, such as interface states
`
`acting as electronic traps, dislocations, and lattice strains, are
`
`avoided. Various disruptions of the crystalline perfection usually
`
`decrease the number of photons emitted rather than just providing
`
`recombination centers, and the recombination energy is dissipated non-
`
`radiatively at the imperfections.
`
`However, it is not possible to form p—n junctions in all materials.
`
`There are substances which exhibit excellent luminescence when stimu-
`
`lated with external light sources (photoluminescence) or when bombarded
`
`with electron beams
`
`(cathodoluminescence), and therefore appear to be
`
`attractive candidates for electroluminescence. This is especially true
`
`if their band gaps fall in the visible region of the spectrum. However,
`
`-4-
`
`TCL 1028, Page 28
`LOWES 1028, Page 28
`
`VIZIO EX. 1028 Page 0028
`
`LOWES 1028, Page 28
`
`VIZIO Ex. 1028 Page 0028
`
`
`
`only one conductivity type, which, depending on the compound,
`
`is either
`
`n or p, but not both, can be achieved by doping.
`
`In the absence of a
`
`p—n homojunction, other schemes must be devised for creating the
`
`minority carriers necessary for recombination.
`
`Such approaches are
`
`involved in diodes of classes 2, 3, and A.
`
`In class 2 diodes, different
`
`materials are used on the two sides of a p~n junction,
`
`forming a hetero—
`
`junction.
`
`The semiconductors involved in such an arrangement are one
`
`which can only be doped p-type joined to another which can only be
`
`doped n—type.
`
`In most cases.
`
`the energy gaps are not
`
`the same in the
`
`two substances,
`
`leading to a further complication beyOnd the lattice
`
`strains involved:
`
`the potential barriers that must be surmounted at
`
`the junction will be different for electrons and holes.
`
`Few sets of
`
`materials fit into this heterojunction scheme, because the two compounds
`
`selected should be completely mutually soluble.
`
`If they are not, a two
`
`phase region with poor electrical properties can result in the junction
`
`area.
`
`It is also sometimes possible to obtain electroluminescence from
`
`materials in which one cannot form pen junctions by the use of Schottky
`
`barriers or m—i—s structures (type 3 didoes).
`
`A Schottky barrier is
`
`formed at a non—ohmic (i.e., rectifying) metal contact to a semi~
`
`conductor, and light can be generated in the high—field region Occurring
`
`at this contact.
`
`In the mHi—s
`
`(metal—insulator—semiconductor) structure,
`
`an insulating layer, either of the semiconductor itself or some other
`
`substance,
`
`is interposed between the metal contact and the semiconductor.
`
`Often such an insu