HVPE-grown AlN-GaN based
structures for UV spectral region
Alexander Usikov
Technologies and Devices International, Inc.
(www.tdii.com)
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
1
Collaborator teams
TDI
A. Usikov, A. Pechnikov, V. Soukhoveev, V. Sizov,
O. Kovalenkov, and V. Dmitriev;
Fox Group
B. O'Meara amd H. Helava;
Soft-Impact
S. Karpov;
Ioffe Institute
E. Arakcheeva, A. Zakhgeim, N. Shmidt,
N. Kuznetsov, A. Lavrent’ev, I. Kotousova,
A. Zubrilov, W. Lundin and S. Gurevich;
AFRL, Wright-Patterson AFB
G. Smith, T. Dang, J. Brown and T.R. Nelson
Naval Research Laboratory
Wright State University
J.A. Freitas, Jr.
D.C. Look
Virginia Commonwealth University
M. Reshikov
University of Florida
B. Luo, F. Ren, K.H. Baik, and S.J. Pearton
Texas Technical University
S. Nikishin
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
2
Outline
Introduction
•HVPE technology for thick GaN layers growth
•HVPE technology for thick AlGaN layers growth
•GaN and AlGaN single layers: Summary
Fabrication of AlGaN-based LED wafers
•HVPE growth
•Modeling of LED operation
Violet LEDs properties
UV LEDs development
•UV LED properties overview
•SIMS analysis
•Electrical properties
Summary
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
3
Hydride Vapor Phase Epitaxy (HVPE)
Is one of the first techniques employed for GaN growth
Ga or Al metals
substrate
Growth conditions
T ~ 1000oC
P atmospheric
Vg < 0.1 mm/hr
HCl
NH3
GaN or AlN
Sapphire or SiC
Common features
•Mature
•Simple
•Reliable
•Fast growth rate
•High material quality
Advanced features developed at TDI
•capability to combine deposition of thick
and thin layers in the same growth run.
•easy to grow high-quality AlGaN layers
in the whole composition range.
•P-type doping for GaN and AlGaN
•low-background impurity concentrations
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
4
GaN-on-sapphire layers: surface
Regular
Improved
AFM
Photo
Conclusions:
usingusing
improved
HVPEHVPE
processprocess
roughness
of GaN layers
was
Conclusions:
improved
roughness
of GaN
in about
times10 times
layers reduced
was reduced
in 10
about
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
5
GaN-on-sapphire Layers: SIMS Depth
Analysis
Undoped
Undoped
1E+09
Ga->
1E+08
1E+17
1E+07
1E+16
1E+06
Mg Co Fe Ni
1E+15
Cr
1E+05
1E+14
1E+04
Mn
1E+13
0
2
1E+05
1E+18
1E+03
1E+02
C
1E+15
0
1
4
5
CONCENTRATION
(atoms/cc)
C
DEPTH (microns)
2
3
4
1E+21
SECONDARY ION
INTENSITY (cts/sec)
CONCENTRATION
(atoms/cc)
1E+08
1E+07
1E+06
1E+05
1E+04
1E+03
1E+02
1E+01
1E+00
Al->
3
1E+00
5
6
Mg doped (p-type)
O
2
1E+01
Si
DEPTH (microns)
Si
1
1E+04
1E+16
6
LS-26
0
Al->
O
1E+17
Si doped (n-type)
Ga->
1E+06
1E+19
DEPTH (microns)
1E+21
1E+20
1E+19
1E+18
1E+17
1E+16
1E+15
1E+14
1E+07
Ga->
1E+14
1E+03
4
1E+20
1E+09
1E+08
1E+07
1E+06
1E+05
1E+04
1E+03
1E+02
1E+01
S772
1E+20
Ga->
Mg
1E+19
1E+18
1E+17
Al->
1E+16
1E+15
1E+14
0
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
2
4
DEPTH (microns)
6
8
6
SECONDARY ION
COUNTS
1E+18
1E+08
H20-19-1
SECONDARY ION INTENSITY
(cts/sec)
H20-19-1
1E+19
SECONDARY ION
COUNTS
CONCENTRATION
(atoms/cc)
1E+10
CONCENTRATION (atoms/cc)
1E+21
1E+20
GaN-on-sapphire layers: electrical properties
n-type GaN layers
Si-doped GaN
undoped GaN
1000
uncorrected
corrected for
interface layer
300
200
100
200
uncorrected
100
0
0
corrected for
interface layer
2
2
µ (cm /V s)
2000
µ~360 cm2/V sec,
n~ 3x1018 cm-3
(300 K)
400
µ (cm /V s)
µ~760 cm2/V sec;
n~ 4x1016 cm-3
(300 K)
0
300
0
100
200
300
T (K)
T (K)
p-type Mg-doped GaN
Au-Schottky barrier
F=10 kHz
p-type
i-type
1E+12
S773-2
1E+20
-2
1E+11
µ~10 cm2/V sec;
p~ 1x1018 cm-3
(300 K)
1E+19
-5
ρ > 1010 Ohm cm,
(300 K)
1E+10
ρ (Ω cm)
Concentration (cm -3)
Zn-doped GaN
1E+9
1E+8
4-
1E+7
1-3
1E+6
1E+18
0.012
0.014
0.016
Distance (µm)
0.018
0.020
1E+5
1
2
3
4
1000/T (K-1)
(for more information please visit http://ncsr.csci-va.com)
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
7
GaN-on-sapphire layers:optical properties
10
10
GaN:Si # TH1721
PL Intensity (rel. units)
Excitation Density - 0.1 W/cm
10
2
9
Temperature
10
10
10
10
15 K
50 K
100 K
200 K
300 K
345 K
8
7
6
5
1.5
2
2.5
3
3.5
Photon Energy (eV)
Photoluminescence spectra for undoped and Si-doped GaN layers grown on sapphire.
For undoped layer the FWHM of XADo peak is 1.7 meV (6 K).
(CL and PL measurements were done at NRL and VCU, respectively)
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
8
GaN-on-sapphire layers: relationship with
thickness
700
FWHM (arcsec)
600
500
400
300
200
0
10
20
Layer thickness (µm)
30
Variation of the FWHM of the x-ray (00.2) reflection with GaN layer thickness
¿ - without Si-doping;
› - 350 sccm of silane flow rate;
É - 150 sccm of silane flow rate.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
9
GaN-on-sapphire layers: lattice constant
variation with Si doping
c (Å)
5.192
5.19
Relation between a and c lattice constants.
5.188
Silane flow rate: ? - 0 sccm, ? - 50 sccm, +75 sccm, ¦ – 100 sccm, ? – 160 sccm, ∆ 300 sccm.
5.186
5.184
3.182
3.184
3.186
a (Å)
3.188
3.19
Variation of c lattice constant with Si doping
5.192
c (Å)
5.19
Proper
selection
of
Si-doping
concentration and growth conditions
is useful for strain engineering and
minimization crack formation in GaN
and AlGaN layers
5.188
5.186
5.184
0
40
80
120
160
Silane flow rate (sccm/min)
200
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
10
GaN-on-sapphire templates:
defect reduction
GaN-on-sapphire
layers: defects
reduction
New: low defect density GaN-on-sapphire
templates are fabricated without ELOG or
substrate puttering techniques.
GaN thickness
X-ray ω-scan rocking curves
(002)
(102)
6 – 12 microns
< 200 arc sec
< 400 arc sec
Substantial improvement of quality of GaN-on-sapphire templates make
them attractive low cost substrates for power blue, UV, and white LEDs
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
11
AlGaN-on-sapphire layers: compositional control
AlGaN-on-sapphire templates: composition control
6.5
6.0
AlxGa(1-x)N
30% 48% 52% 56% 70%
x10
x20
x100
Band Gap Energy (eV)
GaN
CL Intensity (arb. units)
CL
96K
5.5
5.0
4.5
Eg(x)=Eg(0) + ax +bx
b [eV]: 0
0.5
1.0
1.5
2
4.0
AlxGa1-xN on SiC
AlxGa1-xN on sapphire
3.5
3.0
3.0
3.5
4.0
4.5
5.0
Photon Energy ( eV )
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
GaN
AlN
x
Edge CL spectra for AlGaN
alloys with different composition
Band gap energy of HVPE grown
AlGaN alloys vs alloy composition x
70
70
60
AES relative intensity,%
80
AES relative intensity, %
80
60
O
N
50
40
30
Ga
30
Al
20
10
0
Al
40
Ga
20
10
C
0
200
O
N
50
16 mol. % AlN, l std. dev. 0.65%
400
Depht, nm
600
800
0
C
0
100
200
300
400
D e p ht, nm
500
600
AES depth profiles indicating uniform NATO ARW, June 2003, Vilnuis
material composition for AlGaN layers
Copyright TDI, Inc.
700
9 mol. % AlN, l std. dev. 1.29%
12
AlGaN-on-sapphire layers: surface
rms = 0.251nm, box rms = 0.226nm
AFM image for AlGaN/GaN/sapphire sample [1]
RHEED images for AlGaN layers
grown on sapphire
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
13
AlGaN-on-sapphire layers: electrical properties
AlGaN-on-sapphire templates: electrical properties
Hg-probe
F=10 kHz
p-type
Concentration (cm -3)
1E+019
Nd-Na , cm-3
S795s1 (AlGaN:Mg/GaN:Mg)
1E+19
1E+018
1E+017
-1
-4
1E+18
-2
-5
-3
4
5
2
3
1
1E+17
10
20
30
40
50
% Al
0.03
0.04
0.05
0.06
Distance (µm)
Dependence of concentration
Nd-Na on AlN concentration for
undoped AlGaN layers
Depth
profiles
of
concentration
Na-Nd measured in several locations for
p-Al15Ga85N layer grown on p-GaN/sapphire
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
14
GaN and AlGaN templates: Summary
TDI has developed advanced HVPE technology to produce low defect GaN, AlGaN, and
AlN layers on sapphire substrates. Developed technology allows us to control n-type and ptype doping for GaN and AlGaN materials in a wide concentration range.
X-ray diffraction studies revealed that c- and a- lattice constants in GaN layers vary nonmonotonically with Si-doping. In heavily Si doped GaN layers (ND-NA >1.8x1018 cm-3),
biaxial stress can reverse sign.
Optimal thickness of a crack-free GaN-on-sapphire template in terms of reliability and
reproducibility in structural properties lies in the range from 5 to 10 µm. By proper selection
of the initial growth conditions, this range can be extended toward larger thickness.
HVPE homoepitaxial growth of undoped GaN layers on GaN-on-sapphire templates
improves the structural crystal quality and surface morphology. To grow heavily Si-doped
GaN layers on sapphire substrates with improved structural properties, a thick undoped
GaN layer can be combined with the thin GaN cap layer doped with Si.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
15
HVPE of AlGaN for device structures
Recently, we have demonstrated novel HVPE technology
capable to produce multi layer submicron structures for
device applications.
For the first time, AlGaN/GaN-based HEMTs [1] and
violet LEDs [2] grown by HVPE technology were
demonstrated
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
16
LED Fabrication [2]
LED Structures Growth by HVPE
Al metal
source
Sapphire
Ga metal
substrate
source
HCl
NH3
Ar
HCl
Cross Section SEM View
GaN:Mg contact layer
Reactor
chamber
Exhaust
Sapphire subs trate
Dies processing
Growth conditions:
Growth temperature ~ 1050 C
Reactor pressure
1 atm
Growth rate
0.2 - 3.0 µm/min
Carrier gas
argon
Doping
Mg, SiH4
Ni/Au transparent
contact
p-contact pad
n-contact pad
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
17
EL of a violet LED structure [2]
In contact
droplet
Ni/Au p-type
contact pads
Sapphire
Electroluminescence from a test contact pad ( ~250 µm in diameter) at
40 mA dc. Emission wavelength is about 420 nm.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
18
Modeling: Carrier concentration in LED
structure
Design of the
~ 5 - 6 µm
GaN:Mg contact layer
Sapphire substrate
Carrier concentration ( cm-3 )
violet LED structure
24
10
21
10
Ub= 3.0 V
active
region
1018
1015
1012
109
106
electrons
holes
3
10
100
400
600
800
1000
1200
Distance (nm)
The model used to optimize the LED
structure accounts for polarization
charges at every interface and nonradiative carrier recombination on
threading dislocation cores.
Modeling shows the hole injection to be
the factor controlling the LED efficiency.
To enhance the hole injection, both
increase in hole concentration and pAlGaN emitter composition is found to
be beneficial.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
19
Modeling: Band diagrams of LED upon
bias
Ub= 2.0 V
2
active
region
Electron bands (eV)
Electron bands (eV)
3
1
0
-1
Fn
Fp
-2
-3
-4
400
3
Ub= 3.0 V
2
Luminescence intensity (arb.)
4
4
active
region
1
0
Fn
-1
-2
Fp
-3
600
800
1000
1200
-4
400
Distance (nm)
600
800
1000
1200
Distance (nm)
If doping of n-AlGaN emitter
is
insufficient, a barrier to electrons
appears at the active region /emitter
interface. To avoid this un-desirable
effect, increase of n-doping is quite
helpful
The radiative recombination is
controlled by hole injection into
active region. This results in
luminescence intensity decaying
from the p-n junction position
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
20
EL properties of violet LED structures [2]
Energy (eV)
0.20
0.15
0.10
0.05
3.2
3
2.8
2.6
2.4
Intensity (arb.units)
0.25
3.4
p-AlGaN composition
EL Intensity (arb.units)
0.30
0.00
8
10
10
9
10
10
350
375
400
-2
425
450
475
500
525
Wavelength (nm)
Edge dislocation density (cm )
The p-AlGaN layer composition is not the major factor driving threading dislocation
generation. Improvement of p-type doping efficiency in p-AlGaN layers can further
increase EL intensity.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
21
3.0
2.1
2.5
1.8
1.5
2.0
1.2
1.5
0.9
1.0
0.6
0.5
0.0
Output power (mW)
Quantum efficiency (%)
Violet LED dies and lamps [2]
0.3
0.0
0
5
10
15
20
25
Electric current (mA)
The external light emission efficiency of ~2.5% is achieved.
The brightness of the violet LED lamp (420 nm) is as high as 400 - 500 mcd at
20 mA.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
22
Violet LEDs [2]: Summary
The capability of HVPE as an alternative cost-effective technique suitable for
fabrication of AlGaN-based violet LEDs has demonstrated.
Numerical modeling shows that higher composition and higher doping of pAlGaN injection layer are key parameters for bright AlGaN-based LED
fabrication.
X-ray diffractometry can be successfully used for express control of
threading dislocation density that is parameter critical to LED brightness.
The HVPE-grown AlGaN/GaN/AlGaN-based LED structures emit light at the
peak wavelength of 415- 420 nm. The peak position does not practically shift
with the current. The external light emission efficiency of ~2.5% is achieved.
The brightness of the violet LED lamp was as high as 500 mcd at 20 mA.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
23
UV LED development based on HVPE
growth technology
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
24
UV LED properties overview
AlGaN-based
active region
AlInGaN-based
active region
AlGaN-based
active region
AlInGaN-based
active region
5
Output power, mW
10
50 mA
20 mA
150 mA
100 mA
1
External efficiency (%)
1A (pulsed)
4
20 mA
3
20 mA
2
1
1 A (pulsed)
0
0.1
260
280
300
320
340
360
380
260
280
300
320
340
360
380
Wavelength, nm
Wavelength, nm
Optical output power and efficiency of UV LEDs in relation to emission peak wavelength.
? ,? - Univ. of South Carolina, USA; MOCVD grown structure;
˜ - NTT Corporation, Japan. MOCVD grown structure;
¿,¿ - Univ. of Tokushima, Japan; MOCVD grown structure
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
Source: Technical Digest of
ICNS-5, May 25-30, 2003,
Nara, Japan
25
UV LED design
Design of the UV
LED structure
Top view of the UV LED die
with attached wire bonds
X = 0.21-0.27
p-AlxGa1-xN:Mg
Y = 0.10-0.21
n-AlyGa1-yN
Z = 0.21-0.27
n-AlzGa1-zN:Si
mol. % AlN
~ 5 µm
p-GaN:Mg
n-GaN:Si base layer
Sapphire substrate
A number of device structures were grown on 2-inch sapphire substrates.
AlN concentration in AlGaN layers was varied for different samples. Different
compositions of the active regions were used to produce LEDs with different
peak emission wavelengths in the 300-350 nm spectral region.
Device structures emitting at a peak wavelength of 340 nm were selected for
device processing and packaging.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
26
SIMS depth analysis of a UV LED structure
1E+20
1E+04
Ga->
1E+19
1E+03
1E+18
Mg
1E+17
1E+02
1E+16
1E+01
Al->
1E+15
1E+14
1E+00
0
0.5
1
1.5
2
2.5
3
3.5
4
1E+21
1E+08
S751
1E+20
1E+07
Ga->
1E+06
1E+19
Si
1E+05
1E+18
1E+04
O
1E+17
1E+03
C
1E+16
1E+02
Al->
1E+15
1E+01
1E+14
1E+00
0
0.5
1
1.5
2
2.5
3
3.5
DEPTH (µm)
DEPTH (µm)
The background impurities concentration
distribution is similar to one for violet LED
structures:
•
Low background oxygen and carbon
concentration in thick GaN:Si layers;
•
Higher oxygen concentration in the AlGaN
cladding layers ;
•
The carbon concentration increases slightly
in the AlGaN layers.
Cross sectional SEM view of UV LED structure
near the surface
P-GaN cap layer thickness is about 100 nm
AlGaN active region thickness is less than 50 nm
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
27
SECONDARY ION INTENSITY
1E+05
S751
SECONDARY ION INTENSITY
CONCENTRATION (atoms/cc)
CONCENTRATION (atoms/cc)
1E+21
EL spectra of UV LED
300K electroluminescence spectra of UV LED
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
28
UV LED lamp view
(a)
(b)
UV LED lamps (λ~ 341 nm). One chip encapsulated without plastic glass (a);
three chips un-encapsulated without plastic glass (b).
UV emitting structures were grown by proprietary HVPE technology at TDI.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
29
Optical output for UV LED grown by HVPE
Optical Power (µ W)
2500
2000
Dmitriev
1500
1000
500
Shatalov
0
0
20
40
60
80
100
120
Current (mA)
Optical power vs. forward current measure at the Air Force Research Laboratory, Wright-Patterson AFB
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
30
Peak Emission
Peak Emission Wavelength (eV)
3.7
3.68
3.66
3.64
3.62
3.6
0
20
40
60
80
100
120
Current (mA)
The peak photon emission energy as a function of the pulsed input current.
Measurements were done at the Air Force Research Laboratory, Wright-Patterson AFB
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
31
UV LED Quantum efficiency
Quantum Efficiency (%)
0.63
0.62
0.61
0.6
0.59
0.58
0.57
0
20
40
60
80
100
120
Current (mA)
The external quantum efficiency of the UV LED as a function of input current
Measurements were done at the Air Force Research Laboratory, Wright-Patterson AFB
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
32
Place of TDI’s UV LEDs under the sun
AlGaN-based
active region
AlInGaN-based
active region
AlGaN-based
active region
5
AlInGaN-based
active region
Output power, mW
10
50 mA
20 mA
150 mA
100 mA
1
100 mA
External efficiency (%)
1A (pulsed)
4
20 mA
3
20 mA
2
1
1 A (pulsed)
20 mA
0
0.1
240
280
320
360
Wavelength, nm
400
240
280
320
360
400
Wavelength, nm
Optical output power and efficiency of UV LEDs in relation to emission peak wavelength.
? ,? - Univ. of South Carolina, USA; MOCVD grown structure;
˜ - NTT Corporation, Japan. MOCVD grown structure;
¿,¿ - Univ. of Tokushima, Japan; MOCVD grown structure ;
Ó-
resent result of TDI, HVPE grown structure
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
33
Summary
UV LEDs emitting in the wavelength range from 305 to 340 nm
based on HVPE-grown AlGaN/AlGaN heterostructures are
demonstrated.
At a peak wavelength of 341 nm, 0.5 mW and 2 mW of optical
output power was achieved in a packaged device at pulsed
injection currents of 20 mA and 110 mA, respectively.
The obtained results proved that the HVPE technique
developed at TDI has significant potential for growing of
AlGaN-based device structures for UV spectral range.
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
34
References
1. M. Mastro, D. Tsvetkov, V. Soukhoveev, A.Usikov,
V.Dmitriev, B.Lio, F. Ren, K.H. Baik, S.J. Pearton,
Solid State Electronics 47 (2003) 1075 - 1079
2. A. Usikov, D.V. Tsvetkov , M.A. Mastro1, et al.,
Technical Digest of the 5th Int. Conf on Nitride
Semiconductors, May 25-30, 2003, Napa, Japan, p.509
NATO ARW, June 2003, Vilnuis
Copyright TDI, Inc.
35