Applications for HFETs

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1 Applications for HFETs Ga-face Quantum well is formed at the interface AlGaN GaN Buffer P SP P SP P PE -σ s +σ int 2DEG + ve φ b d σ comp AlGaN σ int E 0 GaN E c E F c-plane sapphire σ 2DEG σ surf Higher Al composition Higher sheet charge but lower critical thickness Lower thickness of AlGaN lower breakdown field for HFET Band engineering based on quaternary nitride growth can help in reducing strain (for higher Al %) and increase critical thickness However growth of quaternaries are difficult (In segregation)!

2 Applications in Optoelectronics Independent control of the strain (via lattice constant) and band offset leading to better quantum efficiency Provides better thermal match to the GaN substrate, so reduces stress during cooldown phase (from growth temperature to room temperature) after growth AlInGaN/ InGaN MQW However growth of quaternaries are difficult! AlInGaN/InGaN MQW LED on sapphire

3 Summary of III-nitrides until now Comparison of III-nitrides with other semiconductors Applications overview of nitrides (based on the unique properties) High power microwave devices Power electronic devices UV lasers and LEDs Crystal and band structure overview Polarization: Physics and consequences Formation and use of quaternary alloys of nitrides

4 Thermo Dyn. of crystal growth I: Nucleation In order for the crystal to grow, the crystalline phase must have lower chemical potential than the phase from which the crystal is forming The surface energy required to create a nucleus with radius r is 4πr 2 σ. The energy released due to the formation of a nucleus of radius r is - 4πr 3 µ/(3ρ), where ρ is the molar density πr µ The free energy is then give by G = 4πr σ 3ρ The stability of the nucleolus is achieved at G σρ = 0 r = 2 r µ Then the free energy barrier is given by G b 3 16πσ ρ = 2 3 µ 2

5 Thermodynamics of crystal growth II If the nucleus is below the critical size it will dissolve. G b is the potential barrier for crystallization. Growth is always performed in this region where no new nucleolus may appear. The only process that occur in this region is the growth of the existing nuclei.

6 Basic epitaxial growth modes Frank-van der Merwe (FM) Volmer-Weber (VW) Stranski-Krastanov (SK) Epilayers like to adhere to substrate, more than to each other Example: Growth of GaAs layers by Liquid phase epitaxy (LPE) Epilayers does not like to adhere to substrate Example: Initial nucleation of GaN buffer layer after the nucleation layer growth Epilayers first wet subsrate and then balls up Example: selforganized quantum dots of InAs on GaAs substrate

7 Surface and interface energy Frank-van der Merwe (FM) Volmer-Weber (VW) Stranski-Krastanov (SK) γ n < 0 γ n > 0 First, γ n < 0 Then, γ n > 0 γ n = γ FV + γ FS - γ SV, where n refers to n th layer growth γ FV = surface energy of film-vapor interface γ FS = interfacial energy between film and substrate γ SV = substrate surface energy Note: Difference between FM and SK arises primarily due to elastic strain associated with the film

8 The concept of Supersaturation Supersaturation: If a phase is brought outside its stability region, then the difference between the chemical potential of this phase and the chemical potential of a thermodynamically stable phase is called supersaturation. Supersaturation is the driving force for all phase transformations. Supersaturation µ = µ g - µ s, µ g = chemical potential of the gas phase, µ s = chemical potential of the solid phase Example: Supersaturation for a gas-liquid or gas-solid equilibrium is given by = kt ln P P µ 0 P is the actual pressure, P 0 is the equilibrium pressure

9 Other epitaxial growth modes, summary Other epitaxial growth modes Supersaturation vs. misfit percentage

10 Summary Total energy minimization occurs in FM mode by lateral (2D) growth and in SK by 3D growth. VW changes from one to the other. SK and VW growth modes are usually observed for vapor phase epitaxy, and give rise to islands that coalesce later and proceeds by columnar growth or step flow growth, giving rise to a high density of dislocations 2D growth is possible for lower supersaturation in presence of steps or defects such as screw dislocations Step bunching (common for SiC growth) can occur in presence of large step density and large supersaturation Dislocation mediated growth morphology is extremely common for GaN where growth is initially columnar, followed by 2D growth. Screw-Island growth is also common in MBE grown GaN on MOCVD grown GaN templates and SiC growth Growth by LPE is an example of near-equilibrium growth, others usually non-equilibrium growth

11 Common growth techniques Bulk crystal growth Czochralski: Liquid to solid (Si, GaAs) Bridgman: Liquid to solid (CdTe, other II-VI compounds) Sublimation: Vapor to solid (SiC, GaN?) Epitaxial crystal growth Liquid phase epitaxy: substrate moves from one melt to another (Ex: GaAs) Adv: Simple, high quality defect free layers, equilibrium growth Disadv: Multiple layers difficult, QW structure difficult, poor layer interface Chemical vapor deposition (Ex: all compound semiconductors, Si) Adv: Very high quality, high throughput Disadv: Toxic gases, non-uniform, not as abrupt as MBE Molecular beam epitaxy (Ex: all compound semiconductors) Adv: Very high quality, no toxic gases, very abrupt Disadv: Low throughput, can be unstable from run to run

12 Growth of Nitrides Epitaxial growth of nitrides play a much more important role than in other technologies because. Major Problem: Non-availability of native (or lattice and thermally matched) substrate Therefore. Heteroepitaxy is done: by MBE or MOCVD Result: Huge density of dislocations to release strain So strategies to reduce dislocations are very important for III-nitride technology.