Epitaxy of group-iii nitrides. Vanya Darakchieva Tel 5707 Room M323

Transcription

1 Epitaxy of group-iii nitrides Vanya Darakchieva Tel 5707 Room M323

2 Group-III nitrides binary compounds: GaN, AlN, InN; ternary: GaInN, AlInN, AlGaN and quaternary alloys AlInGaN

3 Group-III nitrides: unique properties and applications crystal structure physical properties band-gap energies applications

4 Group-III nitrides: crystal structure A A stable wurtzite crystal structure metastable zinc blende structure wurtzite structure: 2 lattice parameters: a and c

5 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 ( atm) InN: Td = 630 C << Tm = 2200 C (> atm) - high break-down fields suitability for high-power devices

6 Group-III nitrides: band gap energies band gap energy E g (ev) H-SiC α AlN GaN β AlN GaN Al2 O 3 Si GaAs AlP AlAs GaP AlSb 2? polycrystalline InP InN InN 1 GaAs? "high-quality" GaSb InAs InSb equilibrium lattice constant a 0 (Å) IR UV Large and direct band gaps AlN 6.0 ev;gan 3.4 ev InN ev? Alloying - enormous technological potential for optoelectronic devices from IR to UV

7 Group-III nitrides: applications

8 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 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

9 Epitaxial growth techniques for group-iii nitrides metalorganic vapor phase epitaxy molecular beam epitaxy hydride vapor phase epitaxy other techniques

10 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: NH 3 ; V/III ratios > 2000:1 Growth T: 550 C for InN, > 900 C for GaN and AlN

11 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

12 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

13 MOVPE of group-iii nitrides the metalorganics have relatively high vapor pressures allows transport to the substrate using carrier gas P= hpa Doping: Bis-Mg and SiH 4 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

14 MOVPE of group-iii nitrides Problems and difficulties: high growth T (high thermal stability of NH 3 ) - alternative N precursors (toxic, instable, high C contamination) or use of single source precursors (low grow rate) carrier gases: H 2 influences growth rate and film structure growth of InN low decomposition T - alternative single source precursors, plasma activated N 2, high partial NH 3 pressure

15 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

16 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

17 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 MBE Ga-rich MBE N-rich MOVPE step-flow mode

18 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

19 HVPE of group-iii nitrides NH /N 3 2 HCl/N 2 Pumping System Gas Exhaust 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 NH 3 (typically C for GaN, 1300 C for AlN)

20 HVPE of group-iii nitrides 1. III(sor l) + HCl(g) = III Cl(g) + 1/2H2(g) 2. III(sor l) + 2HCl(g) = III Cl2 (g) + H2(g) 3. III(sor l) + 3HCl(g) = III Cl3 (g) + 3/2H2(g) 4. 2III Cl3(g) = (III Cl3) 2(g) ΣP i : 1.0 atm, P o HCl: 6.0x10-3 atm, F o : 0.0 Source zone 1 (a) Ga source zone IG 1 (b) Al source zone IG 1 (c) In source zone IG 10-2 GaCl 10-2 H 2 AlCl 10-2 InCl Partial pressure (atm) H 2. GaCl 2 AlCl 2 (4) HCl GaCl 3 (GaCl 3 ) AlCl 3 HCl (AlCl 3 ) H 2 InCl 3 HCl InCl Source zone temperature ( C) (InCl 3 )

21 HVPE of group-iii nitrides Growth zone Temperature ( C) AlCl(g) + NH3(g) = AlN(s) + HCl(g) + H2(g) GaCl(g) + NH 3 (g) = GaN(s) + HCl(g) + H 2 (g) GaCl(g) + HCl(g) = GaCl 2 (g) + 1/2H 2 (g) GaCl(g) + 2HCl(g) = GaCl 3 (g) + H 2 (g) 2GaCl 3 (g) = (GaCl 3 ) 2 (g) GaCl(g) + NH 3 (g) = GaN(s) + HCl(g) + H 2 (g) Log K AlCl 3 (g) + NH 3 (g) = AlN(s) + 3HCl(g) GaCl 3 (g) + NH 3 (g) = GaN(s) + 3HCl(g) InCl 3 (g) + NH 3 (g) = InN(s) + 3HCl(g) -2 InCl(g) + NH 3 (g) = InN(s) + HCl(g) + H 2 (g) /T (K -1 )

22 HVPE of group-iii nitrides Advantages: high growth rate (up to 900 μm/h) lowcost high quality quasi-substrates decomposition region pulsed laser beam scanning sapphire thick HVPE-GaN hot plate

23 HVPE of group-iii nitrides Disadvantages: harsh environment (HCl) Si and O impurities from the quartz tubes high e - concentration long runs 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 NH 3 overpressure growth of AlN violent reaction between AlCl 3 and quartz special coatings of the quartz tubes, alternative precursors

24 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

25 Critical issues in the epitaxy of group-iii nitrides substrates strain phenomenon defects

26 Group-III nitrides: substrate issues Lack of native substrates - growth from solution, sodium melt and in supercritical ammonia small size 1 cm 2 high impurity concentration cm -3 - HVPE free-standing quasi-substrates a 2 c-plane a-plane a 3 r-plane c Foreign substrates: sapphire, SiC, Si - different lattice parameters - different thermal expansion coefficients m-plane a 1

27 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

28 Strain phenomenon in nitrides: origin and types

29 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

30 Defects in nitride epilayesrs: dislocations dislocations of edge, screw and mixed type high density (typically cm -2 ) for epilayers grown directly on the substrates

31 Defects in nitride epilayesrs: large scale defects columnar highly conductive region with free-carriers of cm -3 crack formation critical thickness for appearance to release the strain energy

32 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

33 Improving-quality concepts buffer layers, nucleation modifications epitaxial lateral overgrowth pendeoepitaxy

34 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

35 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)

36 Group-III nitrides: buffer layers Buffer layers: improvement of surface morphology, structural and optical properties, reduction of dislocations down to x10 7 cm -2, elimination of the columnar interfacial region, higher critical thickness for crack appearance

37 Group-III nitrides: nucleation modifications Si x N y : 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

38 Group-III nitrides: epitaxial lateral overgrowth ELOG: growth selectively begins from homoepitaxial windows and extends laterally over mask wings (mask material: SiO 2 - Usui et al., W Hiramatsu et al.) advantages: reduction of dislocations in the ELOG material down to 10 5 cm -2 disadvantages: complicated growth process, wing tilting, generation of defects in the coalescence regions, enhanced impurity incorporation

39 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

40 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

41 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 cm -2 disadvantages: wing tilting, generation of defects in the coalescence regions