GaN Light-Emitting Diodes
`
`November, 1973
`
`Herbert Paul Maruska
`
`TCL 1028, Page 1
`LOWES 1028, Page 1
`
`LOWES 1028, Page 1
`
`
`
`November, 1973
`
`Herbert Paul Maruska
`
`TCL 1028, Page 1
`LOWES 1028, Page 1
`
`LOWES 1028, Page 1
`
`
`
`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
`
`BRerbert Paul Maruska
`
`November 1973
`
`;
`
`TCL 1028, Page 2
`LOWES1028, Page 2
`
`LOWES 1028, Page 2
`
`
`
`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.
`
`
`beableLt Mesanam
`
`(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.
`
`~
`
`Pde.
`
`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.
`
`p
`Gos
`ALee KG L SONIA
`
`“(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.
`
`OE,Liblrns
`/XBlectrical Engineering)
`
`Approved for the University Committee
`on Graduate Studies:
`
`Lovcbe € Mepoee
`
`Dean of Graduate Studies
`
`ii
`
`TCL 1028, Page 3
`LOWES 1028, Page 3
`
`-2
`
`LOWES 1028, Page 3
`
`
`
`To Jacques I, Pankove, whose dedicated
`
`interest in science has been a source of
`
`inspiration to me.
`
`Se
`
`iii
`
`TCL 1028, Page 4
`LOWES 1028, Page 4
`
`LOWES 1028, Page 4
`
`
`
`ACKNOWLEDGMENTS
`
`I 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 FE, 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 Prefessor 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,
`
`TCL 1028, Page 5
`LOWES 1028, Page 5
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`LOWES 1028, Page 5
`
`
`
`a ee
`
`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
`
`pratefully acknowledged,
`
`And I would like to thank my wife, Claire, for all the good times
`
`we had in Californta.
`
`This research was supported by the Advanced Research Projects
`
`Agency under Grant No, DAHC15 71-G-6,
`
`.
`
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`
`
`TABLE OF CONTENTS
`
`Chapter
`
`1.
`
`LIGHT-EMITTING DIODES .
`
`.
`
`.
`
`.
`
`1. ww eee
`
`1.1
`
`Introduction .
`
`1.2 Classification and Basis of Operation
`of LED's @e ea ak © ae Swe wR ES
`
`1.3 LED Efficiencies .
`
`1.
`
`1.
`
`6
`
`1 we we ee ht
`
`1,4 Examples of LED Materials ....
`
`1.5 Purpose of This Work .
`
`-
`
`.
`
`-
`
`6 «2 es ee se ee
`
`SYNTHESIS TECHNIQUES AND PROPERTIES OF
`GALLIUM NITRIDE .
`.
`1
`6 ee ee ee ee ee ee
`
`2.1 Synthesis and Growth Techniques
`
`.
`
`.
`
`« ss se wee
`
`2.2 Dopine «
`
`sw Ww «we we eo
`
`2,3 Optical Properties .
`
`6
`
`»
`
`2.
`
`2
`
`1
`
`© ew ee te
`
`2.4 Electrical Properties ....
`
`2.5 Electroluminescence
`
`. +.
`
`6 se we
`
`© eh ee ew
`
`EXPERIMENTAL TECHNIQUES .
`
`1.
`
`6
`
`«©
`
`©
`
`©
`
`8
`
`8 ee eh ee ee
`
`3.1 Open-Flow Vapor Growth System
`
`3.2 Doping Protéduré »
`
`«
`
`«8 ee 8
`
`em ee ee we a
`
`3.3 Point-Contact LED's
`
`.
`
`,
`
`3.4
`
`Improved Large-Area Metal Contacts .
`
`3.5 Measurement of Wavelength and Intensity
`of Luminescence
`.«
`.
`.
`1.
`6
`+ oes he ee ee
`
`3.6 Electrical Characteristics
`
`3,7 Scanning Electron Microscope
`
`3.8 Transmission Electron Microscope . .. «1
`
`«1
`
`ew «4
`
`3.9 Proton Bombardment and Proton-Assisted Diffusion .
`
`10
`
`23
`
`25
`
`25
`
`ils
`
`32
`
`36
`
`39
`
`41
`41
`
`47
`
`54
`
`oh
`
`63
`
`67
`
`70
`
`ue
`
`:
`3
`
`|
`
`vi
`
`TCL 1028, Page 7
`LOWES 1028, Page 7
`if
`
`LOWES 1028, Page 7
`
`
`
`TABLE OF CONTENTS (Contd)
`
`Chapter
`
`4,
`
`EXPERIMENTAL RESULTS
`
`.
`
`.
`
`1 wp ew
`
`ew
`
`ew
`
`we te
`
`4.1 Growth and Fabrication of GaN Light-
`Emitting Diodes ... 4.
`6
`e+ ee ew eae
`
`4.2 Emission Spectra of GaN:Mg Light-
`Emitting Diodes
`.. 6.
`4 ee ee ee nae
`
`4.3 Electrical Characteristics .
`
`4.4 Temperature Dependence of Luminescence
`and Electrical Characteristics ,....,
`
`4.5 Growth and Surface Morphology of GaN .
`
`.
`
`4.6 Pattern of Light Emission
`
`....4.4-.
`
`4.7 Characteristics of the Insulating
`(Mg-doped) Region
`.
`.
`. 1. ee an
`
`4.8 Electrical Potential Distribution
`
`4.9 Diode Capacitance
`
`4.10 Proton Bombardment and Froton-Assisted
`DALESHUOTY
`ay
`seo
`ann
`Per
`see dee Rs
`wes
`owt
`my Oe ae
`
`DISGUSSTON
`
`¢§
`
`& was aw we
`
`&
`
`@
`
`8
`
`5.1 Basic Mechanisms for Diode Operation, ..
`
`5.2 Mechanism for GaN mi-n Diode Electrical
`Characteristics
`
`103
`
`li?
`
`~.
`
`134
`
`.
`
`.
`
`.
`
`141
`
`143
`
`5.3 Mechanisms for Electroluminescence .
`
`.
`
`5.4
`
`Impact Ionization in GaN m-i-n Diodes
`
`..
`
`5.5 Nature of the Potential Barrier ...
`
`160
`
`180
`
`5.6 Model for GaN Light-Emitting Diodes
`
`»
`
`»
`
`L185
`
`CONCLUSIONS ,
`
`vil
`
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`LOWES 1028, Page 8
`
`
`
`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 1.4
`
`The color matching functions V(x), V(y),
`and V(z),
`(red, green, and blue).
`
`Figure 2.1]
`
`Equilibrium vapor pressure of No over
`GaN(s)-Ga(g).
`
`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-5 transistor header.
`
`Figure 3./a Experimental arrangement for measuring the
`wavelength and intensity of electroiumi-
`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 I-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
`
`1?
`
`18
`
`30
`
`43
`
`AG
`
`49
`
`52
`
`56
`
`58
`
`59
`
`60
`
`62
`
`64
`
`66
`
`pent
`
`TCL 1028, Page 9
`LOWES1028, Page 9
`
`LOWES 1028, Page 9
`
`
`
`LIST OF FIGURES (Contd)
`
`Number
`
`Page
`
`Figure 3.11 Schematic diagram of the scanning
`electron microscope.
`
`Figure 4.1
`
`Electreluminescence spectrum of point-contact
`GaN:Mg light-emitting diode (sample #6-8-72).
`
`Figure 4,2
`
`Electroluminescence spectrum of GaN:Mg m-i-n
`diode, sample #6-28-72 (forward bias).
`
`Figure 4,3
`
`Electroluminescence spectrum of typical
`GaN:Mg m-i-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
`power input for GaN:Mg light-emitting diodes,
`
`Figure 4.9
`
`External quantum efficiency of GaN:Mg light-
`emitting diodes.
`
`Figure 4,10 Typical discde 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
`
`76
`
`78
`
`79
`
`81
`
`82
`
`83
`
`85
`
`86
`
`87
`
`88
`
`90
`
`o1
`
`93
`
`94
`
`95
`
`
`
`ix
`
`TCL 1028, Page 10
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`(a
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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 facetted surface developed in GaN
`grown on (1102)-sapvhire,
`
`Figure
`
`4.194
`
`Gallium nitride films on quartz substrates,
`showing crystallite size at various distances
`downstream from the ammonia inlet:
`(a) 2";
`(b) 3";
`(ce) 4", Magnification 200X.
`
`Figure
`
`4.20
`
`Laue pattern of (1013) -GaN (grown in (1102)-
`sapphire substrate).
`
`Figure
`
`4,21
`
`Gallium nitride light-emittingdiode 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 200X magnification.
`
`Figure
`
`§.24
`
`Same diodes as in Figure 4.23 with external
`illumination and light spot pattern super-
`imposed (200X).
`(1013)-orientation,
`
`Figure
`
`4,25
`
`Gallium nitride light-emitting diode
`#12°7°72 with external illumination.
`(t013)-orientation.
`
`Figure
`
`4.26
`
`Gallium nitride light-emitting diode
`#12¢7°72 (cf. Figure 4.25) with forward
`bias,
`illustrating light spot pattern.
`
`99
`
`100
`
`101
`
`102
`
`104
`
`105
`
`107
`
`108
`
`109
`
`lil
`
`112
`
`TCL 1028, Page 11
`LOWES1028, Page 11
`a
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`LOWES 1028, Page 11
`
`
`
`LIST OF FIGURES (Contd)
`
`Number
`
`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
`
`Light spots observed 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 100X 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 electron micrograph of diode
`#12°13+72 at 100X magnification with
`reverse bias (-22 volts).
`
`Figure 4.34
`
`Scanning electron micrograph of diode
`#12°13°72 at 500X magnification with
`reverse bias.
`The insulating (Mg-doped)
`GaN layer can be clearly distinguished.
`(1102)-oriented sapphire substrate,
`
`Figure 4.35
`
`Scanning electron micrograph of diode
`#12°13°72 at 1LOOOX magnification with
`no bias.
`
`113
`
`115
`
`116
`
`120
`
`121
`
`122
`
`124
`
`126
`
`xi
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`LOWES 1028, Page 12
`
`
`
`LIST OF FIGURES (Contd)
`
`Number
`
`Figure
`
`4.36
`
`Figure
`
`4,37
`
`Figure
`
`4.38
`
`Scanning electron micrograph of diode
`#12°13+72 at LOOOX magnification with
`forward bias.
`The bias was removed
`during part of the scan, as indicated.
`
`Scanning electron micrograph of diode
`#12°13*72 ac 1000X 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 240,000.
`
`Figure
`
`4,39
`
`Superimposed zero bias and forward bias
`line scan traces and SEM photograph of a
`gallium nitride light-emitting diode.
`Magnification 1000xX,
`
`Figure
`
`4.40
`
`Superimposed zero bias and forward bias
`line scan traces and SEM photograph of a
`gallium nitride light-emitting diode.
`Magnification 2500X,
`
`Figure
`
`4.4)
`
`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 2500X.
`
`Figure
`
`4.42
`
`Electric potential distribution across a
`gallium nitride light-emitting diode when
`forward bias is applied.
`
`Figure
`
`4.43
`
`Figure
`
`4.44
`
`Electric potential distribution across a
`gallium nitride light-emitting diode when
`reverse bias is applied,
`
`The capacitance and conductance as functions
`of applied voltage for a typical GaN diode.
`Also included is the quantity, 1/C*, as a
`function of voltage.
`
`Figure
`
`Current~-voltage characteristic of a GaN
`m-i-n diode prepared by proton bombardment.
`
`|
`
`128
`
`129
`
`131
`
`132
`
`133
`
`135
`
`136
`
`138
`
`140
`
`xii
`
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`TCL 1028, Page 13
`
`LOWES 1028, Page 13
`
`
`
`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 GaN:Mg light-
`emitting diode current-voltage character-
`istics at various temperatures.
`cf. Figure 4,14,
`
`Figure
`
`5.3
`
`Plot of log I versus log V for GaN:2n
`light-emitting diode (after PANKOVE,
`1972, Figure 8).
`
`Figure
`
`5.4
`
`The I-V characteristic of the GaN:2n
`light-emitting diode shown in Figure 5.3,
`replotted to the Fowler-Nordheim Equation,
`
`Figure
`
`5.5
`
`Log of electroluminescence intensity versus
`(voltage)1/2, Cf, Figure 4,12,
`
`Figure
`
`5.6
`
`Log of forward and reverse bias electro-
`luminescence intensity versus (voltage 2!”
`
`Figure
`
`he 7
`
`Log of electroluminescence intensity versus
`(voltage) 1/2 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
`
`3.9
`
`Carrier multiplication factor M as a
`function of applied voltage for a GaN
`light-emitting dicde,
`
`Figure 5.10
`
`Figure
`
`5.11
`
`Page
`
`155
`
`156
`
`157
`
`158
`
`163
`
`164
`
`165
`
`166
`
`176
`
`179
`
`Plot of (external quantum efficiency) x
`“1/2
`w1/2
`
`versus (voltage)
`
`(voltage)
`
`Charge concentrations, electric fields, and
`electric potentials at a p=n junction.
`WN,,
`Np are respectively the concentrations of
`acceptors and donors; Fas Fy: and F are
`respectively the electric field due to the
`
`xidti
`
`TCL 1028, Page 14
`LOWES1028, Page 14
`
`LOWES 1028, Page 14
`
`
`
`LIST OF FIGURES (Contd)
`
`Number
`
`ionized acceptors, electric field due to the
`ionized donors, and total electric field;
`Pas th and ¢ are respectively the potential
`due to the jonized acceptors, potential due
`to the ionized donors, and total potential;
`and oe 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 tslands on a (1102)
`sapphire substrate;
`(b)} Growth of GaN
`islands, until they meet;
`(c) Growth of
`continuous GaN film after the islands
`the
`have met,
`indicating the posttion of
`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
`LOWES1028, Page 15
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`LOWES 1028, Page 15
`
`
`
`LIST OF SYMBOLS
`
`constant; lattice parameter (A); activity
`area (em@)3 constant
`
`constant
`
`barrier height
`(eV)
`constant; lattice parameter
`
`(A)
`
`capacitance (pF)
`
`thickness of insulator (cm)
`
`energy (eV)
`
`activation energy (eV)
`
`band gap energy (eV)
`
`minimum energy for impact fonization (eV)
`
`electric field (volts/cm)
`
`luminous power
`
`(lumens)
`
`Gibbs free energy (Kcal)
`
`standard Gibbs free energy (Keal)
`~15 eV-sec)
`
`Planck's constant (4.14 x 10
`
`insulating
`
`current
`(amps)
`current density (amp fem”)
`Boltzmann constant
`(0.864 x 107¢ eV/°K)
`
`equilibrium constant
`
`length (cm); free path (cm); dimensionless parameter
`characterizing trap distribution
`
`|
`
`mean free path (cm)
`
`metal
`
`XV
`
`TCL 1028, Page 16
`LOWES1028, Page 16
`
`LOWES 1028, Page 16
`
`
`
`LIST OF SYMBOLS (Contd)
`
`mass of electron (9.1 x 10°
`
`31 Kg)
`
`m*
`
`effective mass of electron (Kg)
`
`electron carrier density tem) : numerical constant
`
`effective charge density in depletion region fom 3
`acceptor density fen}
`donor density (em 9)
`luminescent center density Com”)
`2
`
`hole carrier density (cm_
`
`light power output (watts);
`
`electronic charge (1.60 x 10.
`
`tonization probability; pressure
`19 coul)
`
`resistance (%); gas constant
`
`(1.987 cal/°K-mole)
`
`temperature (°C,
`
`°K)
`
`effective electron temperature (°K)
`
`bias voltage (volts)
`
`eye sensitivity function
`
`concentration of positively charged vacancies on the
`nitrogen lattice
`
`width of depletion region (cm)
`
`distance (cm)
`absorption coefficient ten*)
`
`permittivity (farads/m)
`
`permittivity of free space (8.85 x 10°
`
`la farads/m)
`
`efficiency
`
`external quantum efficiency
`
`"bad
`
`radiative efficiency of recombination
`
`xvi
`
`TCL 1028, Page 17
`LOWES1028, Page 17
`
`LOWES 1028, Page 17
`
`
`
`
`
`LIST OF SYMBOLS (Contd)
`
`wavelength (A)
`mobility (cm2/V-sec)
`frequency (sec?)
`conductivity (a-em-); cross-section (cm*)
`
`lifetime
`
`electric potential (volts)
`
`barrier potential (volts)
`
`xvil
`
`TCL 1028, Page 18
`LOWES1028, Page 18
`
`LOWES 1028, Page 18
`
`
`
`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 grown 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 i018 em>,
`Insulating GaN was produced through
`
`compensation by doping with the acceptor impurities 2n and Mg. Light-
`
`emitting diodes were prepared by growing onto a sapphire substrate a
`
`film of highly conducting undoped GaN (n=-layer), followed 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-doped 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 smail
`
`(<10 u
`
`xviii
`
`TCL 1028, Page 19
`LOWES1028, Page 19
`1
`
`LOWES 1028, Page 19
`
`
`
`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 sitghtly
`
`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
`10° volts/cm are developed at the mi and i~n junctions, but not
`
`throughout the bulk of the i-layer. 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 subsequent 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
`LOWES1028, Page 20
`
`xix
`
`LOWES 1028, Page 20
`
`
`
`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
`LOWES1028, Page 21
`wg
`
`LOWES 1028, Page 21
`
`
`
`
`
`'Let there be light,'
`"God said,
`and there was light.
`God saw that the
`light was good, and God divided light
`from darkness."
`
`Gn 1: 3, 4
`
`xxi
`
`TCL 1028, Page 22
`LOWES1028, Page 22
`
`LOWES 1028, Page 22
`
`
`
`
`
`PLATE I.
`
`Forward biased GaN:Mg light-emitting diode,
`
`xxii
`
`TCL 1028, Page 23
`LOWES1028, Page 23
`
`LOWES 1028, Page 23
`
`
`
`
`
`PLATE II. Reverse biased GaN:Mg light-emitting dicde.
`
`xxiit
`
`TCL 1028, Page 24
`LOWES1028, Page 24
`
`LOWES 1028, Page 24
`
`
`
`
`
`Chapter lL
`
`LIGHT-EMITTING DICDES
`
`1.1
`
`Introduction
`
`Light-emitting diodes (LED's) are devices 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 electroluminescence 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. Commerciaily 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 applieation,
`
`then, it is necessary that the emitted
`
`light fall in the visible portion of the electromagnetic spectrum
`(4006 A to 7000 A), 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 sources:
`
`reliability, long life, low
`
`cost, small size, and low power requirements and operating voltages.
`
`The last three items are particularly important for compact battery
`
`operated equipment.
`
`The operating characteristics of semiconductor
`
`LED's are compatible with silicon integrated circuits, which makes them
`
`especially suitable for visuai displays in computer systems.
`
`TCL 1028, Page 25
`LOWES1028, Page 25
`
`LOWES 1028, Page 25
`
`
`
`1.2 Classification and Basis of Operation of LED's
`
`It is possible to distinguish five classes of semiconductor light-
`
`emitting diodes. These are,
`
`(1)
`
`p-n homojunction diodes
`
`(2)
`
`p-n heterojunction diodes
`
`(3) Schottky barrier and m-i-s 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 involves 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
`
`—?-
`
`TCL 1028, Page 26
`LOWES1028, Page 26
`
`LOWES 1028, Page 26
`
`
`
`carriers,
`
`The excess electrons are situated in the semiconducter'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
`
`curve,
`
`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,
`
`tneluding the purposely intro-
`
`duced dopant atoms.
`
`In such a case the photon energy will be the
`
`—3—
`
`TCL 1028, Page 27
`LOWES1028, Page 27
`
`LOWES 1028, Page 27
`
`
`
`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 pessible 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,
`
`=h=
`
`TCL 1028, Page 28
`LOWES1028, Page 28
`
`LOWES 1028, Page 28
`
`
`
`only one conductivity type, which, depending on the compound,
`
`is either
`
`nor p, but not beth, 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 4.
`
`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 inte 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
`
`aréa.
`
`It is also sometimes possible to obtain electroluminescence from
`
`materials in which one cannot form p=-n junctions by the use of Schottky
`
`barriers or m-i-s structures (type 3 didoes).
`
`A Schottky barrier is
`
`formed at a non-ohmic (1.¢e., rectifying) metal contact to a semi-
`
`conductor, and Light can be generated in the high-fleld region occurring
`
`at this contact.
`
`In the m-i-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 insulating layer is inadvertantly formed when fabricating
`
`a Schottky barrier if the metal used for the contact diffuses into the
`
`-5-
`
`TCL 1028, Page 29
`LOWES1028, Page 29
`
`LOWES 1028, Page 29
`
`
`
`semiconductor, rendering it insulating (by compensating the dopant
`
`already in it).
`
`In some cases it is possible to inject minority carriers from the
`
`metal
`
`(which contains large quantities of beth electrons and holes near
`
`its Fermi level) into the high-field barrier or insulating region in the
`
`semiconductor, whereupon recombination can occur with majority carriers,
`
`But some materials resist injection of minority carriers even by this
`
`process, and then one must resort to the creation of the minority
`
`carriers within the material. There are two possible mechanisms for
`
`this:
`
`impact fonization and internal field emission.
`
`Impact tonization
`
`occurs when majority carriers are accelerated by the applied electric
`
`field to such high energies they can remove other carriers from a filled
`
`band by colliding with them.
`
`In this way minority carriers appear in
`
`the formerly filled band. Alternatively, it 1s pessible to remove
`
`majority carriers from trap levels within the forbidden energy gap by
`
`this collision process, so that empty states are formed for recombi-
`
`nation. With the internal field emission process, carriers are directly
`
`removed from a filled band or from filled localized levels by the action
`
`of the electric field (quantum mechanical tunneling).
`
`For materials
`
`where the band separation (or separation between a level and a band)
`
`is
`
`of sufficiently large energy so that visible light may be emitted
`
`during recombination,
`
`the calculated field strengths required for
`
`internal field emission appear to be prohibitively large.
`
`A device which also operates without minority carrier injection is
`
`the electroluminescent cell. Here a fine powder of a luminescent
`
`material is dispersed in an insulating plastic binder and placed between
`
`=e
`
`TCL 1028, Page 30
`LOWES1028, Page 30
`
`LOWES 1028, Page 30
`
`
`
`two flat,
`
`transparent electrodes,
`
`forming a capacitor. This assemblage
`
`is not conducting to a dir
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