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2005
Luminescence properties of defects in GaN
Michael A. Reshchikov
Virginia Commonwealth University, mreshchi@vcu.edu
Hadis Morkoç
Virginia Commonwealth University
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Reshchikov, M. A., Morkoc, H. Luminescence properties of defects in GaN. Journal of Applied Physics 97, 061301
(2005). Copyright © 2005 AIP Publishing LLC.
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APPLIED PHYSICS REVIEWS
Luminescence properties of defects in GaN
Michael A. Reshchikov
a兲
and Hadis Morkoç
Department of Electrical Engineering and Physics Department, Virginia Commonwealth University,
Richmond, Virginia 23284
共Received 13 July 2004; accepted 18 January 2005; published online 15 March 2005兲
Gallium nitride 共GaN兲 and its allied binaries InN and AIN as well as their ternary compounds have
gained an unprecedented attention due to their wide-ranging applications encompassing green, blue,
violet, and ultraviolet 共UV兲 emitters and detectors 共in photon ranges inaccessible by other
semiconductors兲 and high-power amplifiers. However, even the best of the three binaries, GaN,
contains many structural and point defects caused to a large extent by lattice and stacking mismatch
with substrates. These defects notably affect the electrical and optical properties of the host material
and can seriously degrade the performance and reliability of devices made based on these nitride
semiconductors. Even though GaN broke the long-standing paradigm that high density of
dislocations precludes acceptable device performance, point defects have taken the center stage as
they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and
lead to anomalies in electronic devices. The point defects include native isolated defects 共vacancies,
interstitial, and antisites兲, intentional or unintentional impurities, as well as complexes involving
different combinations of the isolated defects. Further improvements in device performance and
longevity hinge on an in-depth understanding of point defects and their reduction. In this review a
comprehensive and critical analysis of point defects in GaN, particularly their manifestation in
luminescence, is presented. In addition to a comprehensive analysis of native point defects, the
signatures of intentionally and unintentionally introduced impurities are addressed. The review
discusses in detail the characteristics and the origin of the major luminescence bands including the
ultraviolet, blue, green, yellow, and red bands in undoped GaN. The effects of important group-II
impurities, such as Zn and Mg on the photoluminescence of GaN, are treated in detail. Similarly, but
to a lesser extent, the effects of other impurities, such as C, Si, H, O, Be, Mn, Cd, etc., on the
luminescence properties of GaN are also reviewed. Further, atypical luminescence lines which are
tentatively attributed to the surface and structural defects are discussed. The effect of surfaces and
surface preparation, particularly wet and dry etching, exposure to UV light in vacuum or controlled
gas ambient, annealing, and ion implantation on the characteristics of the defect-related emissions
is described. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1868059兴
TABLE OF CONTENTS
I. INTRODUCTION............................ 3
II. FORMATION AND ENERGY LEVELS OF
POINT DEFECTS IN GaN.................... 5
A. Theoretical approach.................... 5
B. Native point defects..................... 6
1. Vacancies........................... 6
a. Gallium vacancy.................... 7
b. Nitrogen vacancy................... 7
c. Divacancy......................... 7
2. Interstitials and antisite defects.......... 7
a. Gallium interstitial.................. 7
b. Nitrogen interstitial.................. 7
c. Gallium antisite.................... 8
d. Nitrogen antisite.................... 8
C. Impurities............................. 8
1. Shallow donors...................... 8
2. Substitutional acceptors................ 8
3. Isoelectronic impurities................ 9
4. Hydrogen........................... 9
D. Complexes............................. 9
1. Shallow donor—gallium vacancy
complexes........................... 10
2. Shallow acceptor—nitrogen vacancy
complexes........................... 10
3. Hydrogen-related complexes............ 10
4. Other complexes..................... 11
E. Role of dislocations in the point defect
formation.............................. 11
III. LUMINESCENCE METHODS................ 12
A. Steady-state photoluminescence............ 12
1. Recombination statistics............... 12
2. Effect of temperature on PL intensity..... 13
3. Estimates of quantum efficiency......... 14
4. Effect of excitation intensity on PL
intensity............................ 14
JOURNAL OF APPLIED PHYSICS 97, 061301 共2005兲
0021-8979/2005/97共6兲/061301/95/$22.50 © 2005 American Institute of Physics97, 061301-1
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5. Estimates of acceptor concentration in
n-type GaN.......................... 15
B. Time-resolved luminescence.............. 15
C. Vibrational properties of deep-level defects.. 16
D. Photoluminescence excitation spectra....... 17
E. Spatially and depth-resolved
cathodoluminescence.................... 18
F. Optically detected magnetic resonance...... 18
IV. LUMINESCENCE RELATED TO POINT
DEFECTS IN UNDOPED GaN................ 18
A. Yellow luminescence band................ 19
1. Effect of temperature.................. 20
2. Effect of excitation intensity............ 22
3. Effect of hydrostatic pressure........... 22
4. Effect of electron irradiation............ 23
5. Time-resolved PL..................... 23
6. Resonant excitation................... 25
7. Vibrational model of the YL............ 26
8. Comparison with the positron
annihilation results.................... 26
9. ODMR on the YL.................... 27
10. Effect of doping on the YL............. 27
B. Yellow and green luminescence in
high-purity GaN........................ 28
1. Effect of excitation intensity............ 29
2. Resonant excitation................... 30
3. Time-resolved PL..................... 31
4. Effect of temperature.................. 33
C. Ultraviolet 共shallow DAP兲 band........... 34
1. Steady-state PL...................... 34
2. Time-resolved PL..................... 36
3. ODMR and identification of the shallow
acceptor............................ 37
D. Blue luminescence band.................. 38
1. Steady-state PL...................... 38
2. Time-resolved PL..................... 40
3. Spatially and depth-resolved
cathodoluminescence.................. 40
4. Origin of the BL band in undoped GaN... 40
E. Red luminescence band.................. 41
F. Red and green luminescence bands in
Ga-rich GaN grown by MBE.............. 42
1. Effect of excitation intensity............ 42
2. Effect of temperature.................. 43
3. Time-resolved PL..................... 44
4. Resonant excitation of the GL2 and RL2
bands.............................. 45
5. Origin and model of the GL2 and RL2
bands.............................. 45
G. Other broad bands in undoped GaN........ 46
H. Characteristics and identification of
radiative defects in undoped GaN.......... 47
V. INTENTIONALLY INTRODUCED IMPURITIES
AND NATIVE DEFECTS. . ................... 48
A. Luminescence in Zn-doped GaN........... 48
1. Blue luminescence band............... 49
a. Effect of temperature................ 49
b. Effect of excitation intensity.......... 50
c. Time-resolved PL................... 51
d. Resonant excitation and vibrational
properties......................... 51
e. ODMR and defect identification....... 52
2. Green, yellow, and red luminescence
bands.............................. 52
B. Luminescence in Mg-doped GaN.......... 52
1. Ultraviolet luminescence band in lightly
Mg-doped GaN...................... 53
2. Effect of potential fluctuations on PL..... 54
3. UVL and BL bands in compensated and
heavily Mg-doped GaN................ 56
a. Effects of growth conditions and
annealing.......................... 56
b. Effect of excitation intensity.......... 57
c. Effect of temperature................ 58
d. Time-resolved PL................... 60
e. Effect of hydrostatic pressure......... 60
f. Effect of electron irradiation.......... 60
g. Optically detected magnetic resonance... 61
h. DLTS, positron annihilation, and the
infrared spectra..................... 61
4. Yellow and red luminescence bands...... 62
5. Luminescence in GaN:Mg codoped with
shallow donors....................... 62
6. Identification of defects in Mg-doped
GaN............................... 62
C. Luminescence in GaN doped with other
impurities............................. 62
1. Doping with shallow donors............ 62
a. Silicon doping...................... 62
b. Oxygen doping..................... 63
c. Selenium doping.................... 63
d. Germanium doping.................. 63
2. Doping with acceptors................. 63
a. Carbon doping..................... 63
b. Beryllium doping................... 64
c. Calcium doping.................... 64
d. Cadmium doping................... 65
e. Manganese doping.................. 65
f. Other acceptors in GaN.............. 65
3. Doping with isoelectronic impurities..... 65
a. Arsenic doping..................... 65
b. Phosphorus doping.................. 66
4. Radiative defects introduced by
irradiation........................... 66
5. Transition and rare-earth elements....... 67
a. Transition metals................... 67
b. Rare-earth elements................. 67
VI. DEFECT-RELATED LUMINESCENCE IN
CUBIC GaN. . ............................. 67
A. Undoped material....................... 67
1. Exciton emission..................... 67
2. Shallow DAP band................... 67
3. Deep defects......................... 68
B. Doped material......................... 68
1. Carbon doping....................... 68
2. Magnesium doping................... 69
3. Silicon doping....................... 69
VII. EXCITONS BOUND TO POINT DEFECTS. . . . 69
061301-2 M. A. Reshchikov and H. Morkoç J. Appl. Phys. 97, 061301 共2005兲
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A. Free excitons........................... 69
B. Bound excitons......................... 71
1. Excitons bound to shallow donors....... 71
2. Excitons bound to acceptors............ 73
3. Haynes rule in GaN................... 74
VIII. UNUSUAL LUMINESCENCE LINES IN
GaN.................................... 75
A. Y
i
lines............................... 75
1. Effects of sample treatments and
experimental conditions on the Y
i
lines... 76
a. Effect of hot wet chemical etching..... 76
b. Effect of photoelectrochemical
etching............................ 77
c. Evolution of PL and memory effect.... 77
d. Effect of excitation intensity.......... 77
e. Effect of temperature................ 77
2. Characteristics of the Y
i
lines........... 78
a. The 3.45-eV line 共Y
1
兲.
.............. 78
b. The 3.42-eV line 共Y
2
兲.
.............. 79
c. The 3.38-eV line 共Y
3
兲.
.............. 79
d. The 3.35-eV line 共Y
4
兲.
.............. 79
e. The 3.34-eV line 共Y
5
兲.
.............. 79
f. The 3.32-eV line 共Y
6
兲.
.............. 79
g. The 3.21-eV line 共Y
7
兲.
.............. 80
h. The 3.08-, 2.85-, 2.80- and 2.66-eV
lines 共Y
8
–Y
11
兲.
..................... 80
3. Y
i
lines and structural defects........... 80
a. Atomic force microscopy............. 80
b. X-ray diffraction.................... 80
c. Transmission electron microscopy...... 81
B. Oil-related 3.31- and 3.36-eV lines......... 81
C. Identification of the Y
i
lines............... 82
IX. UNSTABLE LUMINESCENCE FROM
DEFECTS................................. 82
A. Unstable luminescence bands.............. 82
1. Blue band from the etched GaN surface.. 83
2. Blue and yellow unstable bands......... 83
B. Manifestation of surface states in
photoluminescence...................... 85
1. Band bending at the surface of GaN..... 85
2. Effect of UV illumination on PL........ 86
3. Effect of ambient on intensity and shape
of PL bands......................... 86
4. Effect of passivation on PL............. 87
X. SUMMARY................................ 88
I. INTRODUCTION
Gallium nitride and its alloys with InN and AIN have
emerged as important semiconductor materials with applica-
tions to green, blue, and ultraviolet portions of the spectrum
as emitters and detectors and as high-power/temperature ra-
dio frequency electronic devices. However, further improve-
ments in device performance hinge on understanding and
reduction of extended and point defects. The lack of native
substrates makes the fabrication of efficient and reliable de-
vices particularly difficult, which is typified by dislocation
densities in the range of 10
9
–10
10
cm
−2
on sapphire sub-
strates unless special precautions are taken. Isolated point
defects and defects related to dislocations are responsible for
a variety of ailments in devices. In detectors, they manifest
themselves as excess dark current, noise, and reduced re-
sponsivity. In light-emitting devices, they reduce radiative
efficiency and operation lifetime. Furthermore, the point de-
fects and complexes are generally the culprits for parasitic
current paths. Moreover, they decrease the gain and increase
the noise—particularly the low-frequency noise—in elec-
tronic devices, increase the threshold current, the slope effi-
ciency and operation lifetime of lasers, and are source of
instability particularly in devices relying on charge control
and high electric fields such as field-effect transistors.
It is customary to bring a variety of techniques to probe
the optical and electrical signatures associated with point de-
fects. Luminescence is a very strong tool for detection and
identification of point defects in semiconductors, especially
in wide-band-gap varieties where application of electrical
characterization is limited because of large activation ener-
gies that are beyond the reach of thermal means. In spite of
considerable progress made in the last decade in light-
emitting and electronic devices based on GaN, understanding
and identification of point defects remain surprisingly enig-
matic. One of the reasons is a vast number of controversial
results in the literature. Therefore, a critical review of the
state of understanding of point defects, particularly the issues
dealing with their manifestation in luminescence experi-
ments, is very timely. Even though the optical properties of
GaN have been reviewed by Monemar,
1–4
only a small frac-
tion of those reviews concerned themselves with defect-
associated luminescence in GaN. It should also be noted that
a brief review of point defects and their optical properties in
GaN can be found in earlier reviews and books prepared as
part of the general properties of GaN.
5–13
In many original
works and reviews, analysis of the luminescence is limited to
excitonic emission, a field which is well understood, leaving
out an earnest discussion of point defects in GaN which still
remain unidentified. Traditionally by point defects one means
native defects, impurities, and complexes with the size com-
parable to the nearest atomic distance. Besides point defects,
the crystal lattice may contain extended defects, such as dis-
locations, clusters, domains, voids, etc. The latter commonly
do not contribute to the luminescence, although may signifi-
cantly affect the optical and electronic properties of the ma-
terial by trapping carriers or gettering the point defects.
In order to illustrate the myriad of optical transitions that
could be and have been observed in the luminescence spectra
of GaN associated with defects, we present a table summa-
rizing them 共Table I兲 as well as a figure 共Fig. 1兲 showing a
schematic description of the related transitions and energy
positions within the gap of the defect levels we are about to
discuss in detail throughout the review. In addition to the
luminescence energy-band positions, Table I tabulates their
nomenclature and provides brief comments and references to
the sections of this review where these lines and bands are
discussed in detail. The energy position of the luminescence
lines and bands may depend on strain in thin GaN layers,
temperature, and excitation intensity. Therefore, in Table I
a兲
Electronic mail: mreshchi@vcu.edu
061301-3 M. A. Reshchikov and H. Morkoç J. Appl. Phys. 97, 061301 共2005兲
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TABLE I. List of main luminescence lines and bands in GaN.
Maximum
position
共eV兲 Nomenclature Doping Comments
Reference to the text
共pages兲
3.478 FE, X
A
Undoped 69–71
3.471 DBE, D
0
X
A
Undoped, Si A few close lines 71–75
3.466 ABE, A
0
X
A
Undoped, Mg Best FWHM ⬍0.1 meV 73–75
3.44–3.46 TES Undoped Plethora of lines 71–72
3.455 ABE Zn A weaker peak at 3.39 eV 49, 71–72
3.45–3.46 Y
1
Undoped Correlates with inversion domains 75–82
3.41–3.42 Y
2
Undoped 75–82
3.397 Be e-A type 64
3.387 FE-LO Undoped 69–71
3.38 DBE-LO Undoped 71–73
3.38 Be DAP type 64
3.37–3.38 Y
3
Undoped 75–82
3.375 ABE–LO Undoped 73–74
3.364 ABE-LO Zn 49, 71–72
3.35–3.36 Y
4
Undoped 75–82
3.34 Y
5
Undoped 75–82
3.30–3.32 Y
6
Undoped 75–82
3.295 FE-2LO Undoped 69–71
3.288 DBE-2LO Undoped 71–75
3.283 ABE-2LO Undoped 71–75
3.28 UVL Undoped e-A type 34–37
3.272 ABE-2LO Zn 49, 71–72
3.27 DBE DBE in cubic GaN 67–68
3.26 UVL Undoped, Si DAP type 19, 34–37, 47–48, 63
3.1–3.26 UVL Mg e-A and DAP 53, 54, 56–62
3.21–3.23 Y
7
Undoped 75–82
3.16 Shallow DAP in cubic GaN 67–68
3.08 Y
8
Undoped 80
3.08 C In cubic GaN 68–69
3.0–3.05 BL C Broad 63–64
2.9–3.0 BL Undoped, Fe Broad, unstable intensity 83–84
2.9 BL P Broad, with fine structure 66
2.88 BL Undoped Broad, with fine structure 19, 38–41, 47–48
2.88 BL Zn Broad, with fine structure 48–52
2.86 Y
9
Undoped 80
2.8 Y
10
Undoped 80
2.8 BL Cd Broad, with fine structure 64–65
2.7–2.8 BL Mg Broad, large shifts 56–62
2.6–2.8 BL Undoped Broad, surface related 83
2.68 Y
11
Undoped 80
2.6 GL As Broad, with fine structure 65–66
2.6 GL Zn Broad 48, 52
2.56 AL Undoped Broad 47
2.51 GL3 Undoped Broad 47
2.5 Ca Broad 64
2.4–2.5 Mg–O Broad 62
2.48 GL Undoped Broad 29–34
2.43 Hg Broad 65
2.36 GL2 Undoped Broad 19, 42–48
2.2–2.3 YL Undoped, C Broad 19–34, 47–48, 63
1.9–2.1 C Broad, in cubic GaN 68–69
1.8–2.0 RL Undoped Broad 19, 41, 47–48
1.85 RL2 Undoped Broad 19, 42–48
1.8 Zn Broad 48, 52
1.7–1.8 Mg Broad 62
1.66 Undoped Broad 42
1.64 C Broad 63–64
1.3 共Fe兲 Sharp 67
1.27 Mn Broad 65
1.193 共Ti,Cr兲? Sharp 67
0.95 Undoped Sharp, irradiation induced 66
0.85–0.88 Undoped Sharp, irradiation induced 66
061301-4 M. A. Reshchikov and H. Morkoç J. Appl. Phys. 97, 061301 共2005兲
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