Epitaxy of group-III nitrides
Vanya Darakchieva
vanya@ifm.liu.se
Tel 5707
Room M323
Group-III nitrides
• binary compounds: GaN, AlN, InN;
• ternary: GaInN, AlInN, AlGaN and quaternary alloys AlInGaN
Group-III nitrides: unique properties
and applications
ƒ crystal structure
ƒ physical properties
ƒ band-gap energies
ƒ applications
Group-III nitrides: crystal structure
A
A
• stable wurtzite crystal structure
metastable – zinc blende structure
wurtzite structure:
2 lattice parameters:
a and c
Group-III nitrides: physical properties
• different atomic sizes and electronegativities of Me
cations and N anions Æ strongly ionic bonds
AlNÆGaNÆInN
• high bond strengths:
- high melting points Æ suitability for high-T devices
AlN: Td = 1040 °C<< Tm = 3500 °C (200 atm)
GaN: Td = 850 °C << Tm = 2800 °C (45 000 atm)
InN: Td = 630 °C << Tm = 2200 °C (>60 000 atm)
- high break-down fields Æ suitability for high-power
devices
6
α
β
AlN
AlN
Si
GaAs
• Large and direct band
gaps
AlN – 6.0 eV; GaN – 3.4 eV
InN – 0.7 – 1.9 eV?
5
4
UV
AlP
GaN
GaN
AlAs
3
AlSb
GaP
2
? polycrystalline
InN
InN
1
InP
GaAs
GaSb
? "high-quality"
InAs
0
3.0
3.5
4.0
4.5
5.0
IR
Eg (eV)
band gap energy
7
Al2 O3
6H-SiC
Group-III nitrides: band gap energies
5.5
6.0
InSb
6.5
equilibrium lattice constant
a0 (Å)
•Alloying
- enormous
technological
potential for
optoelectronic
devices from IR to UV
Group-III nitrides: applications
Group-III nitrides: applications
• visible light and UV LEDs : traffic lights, lights at home (white
LEDs), full-color displays, automotive lighting
• LDs in the blue, violet and UV: data storage applicationsDVD capacity of 28Gbytes, significant improvement of
printing, xerography etc.
• microwave and high power (> 1MW) electronics: military
(radars, satellites) and communication applications such as
third generation wireless cellular networks
• biological and chemical detection systems on UV optical
sources down to 280 nm
• spin-transport electronics (spintronics) in which the spin of
charge carrier is exploited: magnetic sensors and actuators, high
density ultra-low power memory and logic, spin-polarized light
emitters for optical encoding, optical switches and modulators
Epitaxial growth techniques for
group-III nitrides
ƒ metalorganic vapor phase epitaxy
ƒ molecular beam epitaxy
ƒ hydride vapor phase epitaxy
ƒ other techniques
MOVPE of group-III nitrides
Pyrolysis of organometalic
precursors and
hydrides on a heated
substrate involving gas
phase and surface
reactions at high V/III
ratio
Organometalic precursors: trimethyl-In,-Ga or –Al
Hydrides: NH3; V/III ratios > 2000:1
Growth T: 550°C for InN, > 900 °C for GaN and AlN
MOVPE of group-III nitrides
• the growth process is controlled by diffusion in the
crystallizing phase surrounding the substrate
(growth reaction at the interface)
• diffusion across the boundary layer is determined by
size of molecules, T, p, flow velocity and viscosity
Growth process
thermodynamics
kinetics
hydrodynamics
mass transport
MOVPE of group-III nitrides
1. Low T the growth is limited by
kinetics of the reaction: growth rate
increases with T
2. Intermediate T the growth is limited
by diffusion: growth rate constant with T
3. Elevated T desorption dominates the
growth: growth rate drops with T
MOVPE of group-III nitrides
• the metalorganics have relatively high
vapor pressures Æ allows transport to the
substrate using carrier gas
• P= 10-1000 hPa
• Doping: Bis-Mg and SiH4
Advantages: large-area growth capability, precise control of epitaxial deposition
and easy service
Disadvantages: toxic chemicals, relatively low grow rate, high-purity chemicals
and gases
MOVPE of group-III nitrides
Problems and difficulties:
• high growth T (high thermal stability of NH3) - alternative N precursors
(toxic, instable, high C contamination) or use of single source precursors
(low grow rate)
• carrier gases: H2 influences growth rate and film structure
• growth of InN – low decomposition T - alternative single source
precursors, plasma activated N2, high partial NH3 pressure
MOVPE of group-III nitrides
Problems and difficulties:
• growth of InGaN and InAlN alloys: In composition > 20% - tradeoff between
quality and amount of In incorporation
MBE of group-III nitrides
• film crystallization via reactions between
thermal molecular or atomic beams of the
constituents and a substrate surface at
elevaed T in UHV
• the growth process is controled by kinetics of
the surface processes: adsorption, migration
and dissociation, incorporation of atoms into
the crystal lattice, thermal desorption
• application of rf plasma or cyclotron
resonance source to produce N radicals
MBE of group-III nitrides
MBE – Ga-rich
MBE – N-rich
MOVPE – step-flow mode
• N-stable growth (low III/V flux) – faceted surface morphology and
tilted columnar structure
• Ga-rich conditions (high III/V flux) – reduction of structural defects,
step flow growth
MBE of group-III nitrides
Advantages:
• low growth T (InGaN, InN, InAlN)
• excellent control of epitaxial deposition – compositionally sharp
interfaces
• compatibility with surface sensitive diagnostic methods (RHEED,
AES)
Disadvantages:
• low growth rate – ML/s (0.5 – 1 μm/h)
• high cost (UHV)
• complex maintanance (UHV)
Problems and difficulties:
• no possibility for advanced nucleation schemes - high defect
density
HVPE of group-III nitrides
NH3/N2
Pumping
System
Gas Exhaust
HCl/N2
Substrate
Thermocouple
Closing hatch
Quartz tubes
Ga-boat
the growth process is controlled by:
Forming of group-III Me chloride gas –source zone
(typically 850 °C for GaN)
Reaction of group-III-Me-chloride with NH3
(typically 1060-1100 °C for GaN, 1300 °C for AlN)
HVPE of group-III nitrides
Source zone
1. III(s or l) + HCl(g) = III ⋅ Cl(g) + 1/2H2 (g)
2. III(s or l) + 2HCl(g) = III ⋅ Cl2 (g) + H 2 (g)
3. III(s or l) + 3HCl(g) = III ⋅ Cl3 (g) + 3/2H 2 (g) 4. 2III ⋅ Cl3 (g) = (III ⋅ Cl3 ) 2 (g)
ΣPi: 1.0 atm, P
o
HCl:
-3
o
6.0x10 atm, F : 0.0
(b) Al source zone
(a) Ga source zone
1
Partial pressure (atm)
10-2
.
1
IG
GaCl
10
GaCl2
HCl
10-6
GaCl3
1
IG
10-2
AlCl
H2
H2
-4
(c) In source zone
IG
10-2
InCl
H2
AlCl3
10
-4
AlCl2
(4)
10
-4
HCl
10-6
10-6
-8
-8
InCl2
10
-8
10
-10
10-10
10-10
10
-12
-12
-12
10
HCl
(GaCl3)2
(AlCl3)2
10
InCl3
(InCl3)2
300 400 500 600 700 800 900 1000
10
10
300 400 500 600 700 800 900 1000
Source zone temperature (°C)
300 400 500 600 700 800 900 1000
HVPE of group-III nitrides
Growth zone
Temperature (°C)
18
1100 1000 900
800
700
600
16
5. GaCl(g) + NH3 (g) = GaN(s) + HCl(g) + H 2 (g)
14
12
6. GaCl(g) + HCl(g) = GaCl2 (g) + 1/2H 2 (g)
+
l(g)
AlC
10
N
+
(s)
AlN
=
g)
H 3(
H
)+
Cl(g
500
g)
H 2(
7. GaCl(g) + 2HCl(g) = GaCl3 (g) + H 2 (g)
8.
2GaCl3 (g) = (GaCl3 ) 2 (g)
Log K
8
GaCl(g) + NH3(g) = GaN(s) + HCl(g) + H2(g)
6
4
AlCl3(g) + NH (g) = AlN
3
(s)
2
+ 3HCl(g)
0
-2
InCl3 (g
)+
-4
-6
0.7
GaCl (g)
3
+
NH3(g) =
GaN(s) +
InN(s)
3HCl(g)
+ 3HCl(
g)
InCl(g) + NH3(g) = InN(s) + HCl(g)
+ H2(g)
NH3(g)
=
0.8
0.9
1.0
1000/T (K-1)
1.1
1.2
1.3
HVPE of group-III nitrides
Advantages:
• high growth rate (up to 900 μm/h)
• low cost
• high quality
quasi-substrates
pulsed
laser beam
decomposition
region
scanning
sapphire
thick HVPE-GaN
hot plate
HVPE of group-III nitrides
Disadvantages:
• harsh environment (HCl)
• Si and O impurities from the quartz tubes
• long runs
high e- concentration
Problems and difficulties:
• reproducibility problems – parasitic deposition
long-time
cleaning
• difficulties to obtain p-type doping
• difficulties to grow on Si – melt-back etching
special
buffer layers
• growth of InN and InGaN – need of large NH3 overpressure
• growth of AlN – violent reaction between AlCl3 and quartz
special coatings of the quartz tubes, alternative precursors
Other techniques for growth of
group-III nitrides
• Magnetron sputter epitaxy: similarity with MBE (UHV, low growth T,
compatibility with surface diagnostic methods)
principle: Nitrogen gas (typically dilluted with noble gas) reacts with the
sputtered metal atoms at the substrate surface; magnetic field is applied
to increase the ionization efficiency of the sputtering process
• Advantages: low growth T - In containing alloys, use of Si and GaAs as
substrate material, reduction of thermally activated diffusion of dopants and
interdiffusion at interfaces
• Disadvantages: Me targets are easily oxidized; oxides – reduction of sputtering
yield and need of long-term pre-sputtering
Critical issues in the epitaxy of
group-III nitrides
substrates
„ strain phenomenon
„ defects
„
Group-III nitrides: substrate
issues
• Lack of native substrates
- growth from solution, sodium melt and in
supercritical ammonia small size ≤ 1 cm2
high impurity concentration ≥ 1019 cm-3
- HVPE free-standing quasi-substrates
r-plane
a-plane
a2
c
a3
c-plane
m-plane
a1
• Foreign substrates: sapphire, SiC, Si
- different lattice parameters
- different thermal expansion coefficients
Strain phenomenon in nitrides:
origin and types
• different lattice parameters of layer and substrates: growth strain
• different thermal expansion coefficients of layer and substrates:
thermal strain
• incorporation of dopants and impurities: hydrostatic strain
Strain phenomenon in nitrides:
origin and types
Defects in nitride epilayers:
dislocations
• formation mechanism: lattice mismatch between substrate and film
strain
elastic strain energy increases with film thickness
• critical thickness: energetically favorable to introduce misfit dislocations
at the interface
• 14% (very large) lattice mismatch for GaN/sapphire – growth of individual
and isolated islands rather than as a continuous film
Defects in nitride epilayesrs:
dislocations
• dislocations of edge, screw and mixed type
– high density (typically 109-1010 cm-2) for epilayers grown directly
on the substrates
Defects in nitride epilayesrs:
large scale defects
• columnar highly conductive region with free-carriers of 1020 cm-3
• crack formation – critical thickness for appearance to release the strain
energy
Group-III Nitrides: mosaic crystal
model
• mosaic blocks (single crystallites) with vertical and lateral coherence
lengths
tilt
twist
• mosaic tilt: out-of-plane rotation of the blocks perpendicular to the surface normal
• mosaic twist: in-plane rotation of the blocks around the surface normal
Improving-quality concepts
buffer layers, nucleation modifications
„ epitaxial lateral overgrowth
„ pendeoepitaxy
„
Group-III nitrides: buffer layers
with BL
without BL
• Buffer layers: to provide nucleation centers with the same orientation as the
substrate, to promote lateral growth and to accommodate partly the strain
Group-III nitrides: buffer layers
• MOVPE: low-T GaN (S. Nakamura) and AlN (H. Amano) buffer layers
(similar for MBE)
• HVPE: ZnO (R. Molnar) and high-T AlN (T. Paskova) buffer layers and
MOVPE-GaN templates (T. Paskova)
Group-III nitrides: buffer layers
• Buffer layers: improvement of surface morphology, structural and
optical properties, reduction of dislocations down to 108 – 5x107 cm-2,
elimination of the columnar interfacial region, higher critical thickness
for crack appearance
Group-III nitrides: nucleation
modifications
• SixNy: introduced in MOVPE just before the growth of LT buffer layer or
alternatively at HT as intermediate layer
- formation of small nucleation islands that can enhance the lateral
growth of GaN leading to reduction of threading dislocation density
• modulation epitaxy: growth interruptions (time modulation) or flow
rate modulation in MOVPE and HVPE
- defect reduction and increase of critical thickness for crack appearance
due to enhanced lateral Ga diffusion and self-limiting growth mechanism
Group-III nitrides: epitaxial
lateral overgrowth
• ELOG: growth selectively begins from homoepitaxial windows and extends
laterally over mask wings (mask material: SiO2 - Usui et al., W – Hiramatsu et al.)
• advantages: reduction of dislocations in the ELOG material down to 105 cm-2
• disadvantages: complicated growth process, wing tilting, generation of defects
in the coalescence regions, enhanced impurity incorporation
Group-III nitrides: epitaxial
lateral overgrowth
• ELOG: nucleation at the mask edges, further GaN islands are generated in the
window and coalesce forming a rough surface with many pits
• ELOG: high growth rate in [0001]
and slow growth of the {1-101}
facets (stable surfaces) until the
island is composed of two [1-101]
facets, further lateral overgrowth
over the mask
Group-III nitrides: epitaxial
lateral overgrowth
• ELOG: successfully applied in HVPE on sapphire and MOVPE on
sapphire, SiC and Si; does not work in MBE
Group-III nitrides: pendeo
epitaxy
• Pendeo epitaxy: growth selectively begins on the side walls of a tailored
microstructure previously etched into the seed layer, applied in MOVPE
on SiC and Si R.F. Davis et al.
• advantages: maskless, reduced contamination, dislocation reduction - 105 cm-2
• disadvantages: wing tilting, generation of defects in the coalescence regions