epitaxy of group-iii nitrides
TRANSCRIPT
Group-III nitrides
• binary compounds: GaN, AlN, InN;• ternary: GaInN, AlInN, AlGaN and quaternary alloys AlInGaN
Group-III nitrides: unique properties and applications
crystal structurephysical propertiesband-gap energiesapplications
Group-III nitrides: crystal structure
• stable wurtzite crystal structuremetastable – zinc blende structure
A
A
wurtzite structure: 2 lattice parameters:
a and c
Group-III nitrides: physical properties
• different atomic sizes and electronegativities of Me cations and N anions strongly ionic bonds AlN GaN InN
• high bond strengths:- high melting points 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 suitability for high-powerdevices
Group-III nitrides: band gap energies
• Large and direct band gaps
•Alloying- enormous
technologicalpotential for
optoelectronicdevices from IR to UV
AlN – 6.0 eV;GaN – 3.4 eV
InN – 0.7 – 1.9 eV?
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.50
1
2
3
4
5
6
7
? "high-quality"
? polycrystalline
InN
GaN
α βAlN
GaA
s
IRU
V
InAs
AlAs
GaAsInP
GaP
AlP
GaSb
InSb
AlSb
GaN
InN
AlN
6H-S
iC
Al 2O
3
Si
band
gap
ene
rgy E g
(eV
)
equilibrium lattice constant a0 (Å)
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 applications-
DVD 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 opticalsources 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
metalorganic vapor phase epitaxymolecular beam epitaxyhydride vapor phase epitaxyother techniques
MOVPE of group-III nitrides
Pyrolysis of organometalicprecursors and hydrides on a heatedsubstrate involving gas phase and surface reactions at high V/III ratio
Organometalic precursors: trimethyl-In,-Ga or –AlHydrides: NH3; V/III ratios > 2000:1Growth 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 limitedby diffusion: growth rate constant with T
3. Elevated T desorption dominates thegrowth: growth rate drops with T
MOVPE of group-III nitrides
Advantages: large-area growth capability, precise control of epitaxial depositionand easy service
Disadvantages: toxic chemicals, relatively low grow rate, high-purity chemicalsand gases
• the metalorganics have relatively high vapor pressures allows transport to the substrate using carrier gas
• Doping: Bis-Mg and SiH4
• P= 10-1000 hPa
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
• growth of InGaN and InAlN alloys: In composition > 20% - tradeoff between quality and amount of In incorporation
Problems and difficulties:
MBE of group-III nitrides
• film crystallization via reactions betweenthermal molecular or atomic beams of theconstituents and a substrate surface at elevaed T in UHV
• the growth process is controled by kinetics of the surface processes: adsorption, migrationand dissociation, incorporation of atoms intothe crystal lattice, thermal desorption
• application of rf plasma or cyclotronresonance source to produce N radicals
MBE of group-III nitrides
• 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
MOVPE – step-flow mode
MBE – Ga-rich MBE – N-rich
MBE of group-III nitridesAdvantages:• low growth T (InGaN, InN, InAlN)• excellent control of epitaxial deposition – compositionally sharpinterfaces• 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 defectdensity
HVPE of group-III nitrides
PumpingSystemGas Exhaust Thermocouple
Quartz tubes
Closing hatch
Ga-boat
HCl/N2
Substrate
NH /N3 2
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
300 400 500 600 700 800 900 100010-12
10-10
10-8
10-6
10-4
10-2
1
Par
tial p
ress
ure
(atm
)
HCl
GaCl3
GaCl
H2
ΣPi: 1.0 atm, PoHCl: 6.0x10-3 atm, Fo: 0.0
(GaCl3)2
GaCl2
IG
(a) Ga source zone
300 400 500 600 700 800 900 100010-12
10-10
10-8
10-6
10-4
10-2
1
AlCl3
AlCl2
AlCl
(AlCl3)2HCl
H2
IG
Source zone temperature (°C)
(b) Al source zone
300 400 500 600 700 800 900 100010-12
10-10
10-8
10-6
10-4
10-2
1
InCl3
InCl2
InCl
(InCl3)2
HCl
H2
IG
(c) In source zone
(g)1/2H Cl(g)III HCl(g) l)or III(s 2+⋅=+ (g)H (g)ClIII 2HCl(g) l)or III(s 22 +⋅=+
(g)3/2H (g)ClIII 3HCl(g) l)or III(s 23 +⋅=+ (g))Cl(III (g)Cl2III 233 ⋅=⋅
. (4)
1. 2. 3. 4.
Source zone
HVPE of group-III nitrides
5.
6.
Growth zone
(g)H HCl(g) GaN(s) (g)NH GaCl(g) 23
7.
8.
++=+
(g)1/2H (g)GaCl HCl(g) GaCl(g) 22 +=+
(g)H (g)GaCl 2HCl(g) GaCl(g) 23 +=+
(g))(GaCl (g)2GaCl 233 =
0.7 0.8 0.9 1.0 1.1 1.2 1.3-6
-4
-2
0
2
4
6
8
10
12
14
16
1860070080090010001100
1000/T (K-1)
Log
K
Temperature (°C)
InCl(g) + NH3(g) = InN(s) + HCl(g) + H2(g)
InCl3(g) + NH3(g) = InN(s) + 3HCl(g)
GaCl3(g) + NH3(g) = GaN(s) + 3HCl(g)
GaCl(g) + NH3(g) = GaN(s) + HCl(g) + H2(g)
AlCl(g) + NH3(g) = AlN(s) + HCl(g) + H2(g)
AlCl3(g) + NH3(g) = AlN(s) + 3HCl(g)
500
HVPE of group-III nitrides
Advantages:• high growth rate (up to 900 μm/h)• low cost quasi-substrates• high quality
hot platethick HVPE-GaN
sapphire
pulsed
decompositionregion
laser beam
scanning
HVPE of group-III nitrides
Disadvantages: • harsh environment (HCl)• Si and O impurities from the quartz tubes high e- concentration• long runsProblems 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 thesputtered metal atoms at the substrate surface; magnetic field is appliedto 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 andinterdiffusion at interfaces
• Disadvantages: Me targets are easily oxidized; oxides – reduction of sputteringyield and need of long-term pre-sputtering
Critical issues in the epitaxy of group-III nitrides
substratesstrain 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
m-plane
a-plane
c-plane
a2
a3
a1
c • 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 dislocationsat the interface
• 14% (very large) lattice mismatch for GaN/sapphire – growth of individualand 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 strainenergy
Group-III Nitrides: mosaic crystal model
• mosaic blocks (single crystallites) with vertical and lateral coherence lengths
• mosaic tilt: out-of-plane rotation of the blocks perpendicular to the surface normal
tilt
twist
• mosaic twist: in-plane rotation of the blocks around the surface normal
Improving-quality concepts
buffer layers, nucleation modificationsepitaxial lateral overgrowthpendeoepitaxy
Group-III nitrides: buffer layers
• Buffer layers: to provide nucleation centers with the same orientation as the substrate, to promote lateral growth and to accommodate partly the strain
with BL without BL
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 thicknessfor 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 flowrate 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 extendslaterally 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 defectsin 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: pendeoepitaxy
• Pendeo epitaxy: growth selectively begins on the side walls of a tailoredmicrostructure previously etched into the seed layer, applied in MOVPEon SiC and Si
• advantages: maskless, reduced contamination, dislocation reduction - 105 cm-2
• disadvantages: wing tilting, generation of defects in the coalescence regions
R.F. Davis et al.