Epitaxy of group-III nitrides

Epitaxy of group-III nitrides

Vanya [email protected]

Tel 5707 Room M323

mailto:[email protected]

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


• 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


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


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


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


• 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)


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


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


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


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


• 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)


• 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


• 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


• 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.

Epitaxy of group-III nitrides

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