Thermodynamic of GaN growth by HVPE method

Prepared by: Kawan Anil

Thermodynamic:

• Chemical thermodynamics is the study of the interrelation of energy with chemical reactions and chemical transport and with physical changes of state within the confines of the laws of thermodynamics.

• Zeroth law• First• Second• Third

GaN

• Gallium Nitride is a wide-bandgap, compound semiconduc-tor. Due to its unique material properties, GaN is a disruptive technology across a wide range of electronic applications.

• Deriving from its inherent material properties, devices based on gallium nitride can deliver vastly superior performance compared to currently available silicon and III-V solutions, the most important of which are the ability to operate with:

• High Power (V*I) • High Voltage • High Temperature • High Speed • High tolerance to Radiation • Low Noise

Properties: Property GaN SiC Si Ga GaAs

Bandgap (eV)

Direct3.42

Indirect3.2

Indirect1.1

Indirect0.66

Direct1.43

Thermal conductivity(W/cm k)

1.8-2.4 3.6-4.9 1.3 0.58 0.46

Melting point (°C )

2500 3100 1412 937 1240

HVPE:

• 1st: 1966; Tietjen & Amick

Thermodynamic of GaN

• Partial pressures of gaseous species in equilibrium with GaN are calculated for temperatures, input GaCl partial pressures, input V/III ratios and mole fractions of hydrogen relative to the inert gas atoms.

• GaCl, GaCl3, NH3, HCl, H2 and inert gas(IG)

• Ga precursors are obtained by following reaction:

• Galiq+HClg ⇔ GaClg +½H2g

• GaClg+2HClg ⇔ GaCl3g+H2g

Two thermodynamic reaction pathway for deposition of GaN:

• GaClg+NH3g ⇔ GaN +HClg+H2g

• 3GaClg+2NH3g ⇔ 2GaN + GaClg+3H2g

• K1= (PHClPH2)/(PGaClPNH3)

• K2=(PGaCl3PH2)/(PGaClPHCl)

• ΣPi= PGaCl + PGaCl3 + PNH3 + PHCl+ PH2 + PIG

• P°GaCl - PGaCl = P°

NH3 - PNH3

• A=½(PGaCl + 3PGaCl3 + PHCl) / (3/2PNH3 + ½PHCl+ PH2 + PIG)

• F= ½(3PNH3 + PHCl+ 2PH2) / (3/2PNH3 + ½PHCl+ PH2 + PIG)

• NH3(g)→(1-α)NH3(g)+ α/2N2(g)+3 α /2H2(g)

Equilibrium partial pres-sures over GaN as a func-tion of growth tempera-

ture.

Equilibrium partial pressure over GaN as a function of in-put partial pressure of GaCl.

Equilibrium partial pres-sures over GaN as a func-tion of hydrogen partial

pressure in the carrier gas, parameter F.

Driving force for the GaN deposition as a

function of growth tem-perature with various

parameters F.

Comparison between calculated growth rates and experimental data.

Values of gas flows used for the HVPE growth of GaN

Gas Flow in SCCM

HCl source 20-30a

N2 source 83-73a

Additional HCl 0-200b

N2 Carrier gas 1480-2000b

NH3 300

Major problems:

• Dislocation• the substrate of choice has been sapphire (Al2O3), which has a

14% lattice-size mismatch and a 34% mismatch in thermal ex-pansion coefficient. As a result of growth along (0001) GaN on Al2O3, high concentrations of misfit and threading dislocations are formed. The main concerns are threading dislocations be-cause they will propagate to the active parts of devices grown on top of the underlying GaN layers, due to the fact that dislo-cations cannot terminate inside the material unless they form half-loops. One of the growth techniques that give smaller dis-location density is hydride vapor-phase epitaxy (HVPE). The lower density is due to the fact that large thickness of GaN can be grown, allowing more interactions between dislocations and lowering their density