thermodynamic of gan growth by hvpe method
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Thermodynamic of GaN growth by HVPE method
Prepared by: Kawan Anil
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Thermodynamic:
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• 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
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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
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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
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HVPE:
• 1st: 1966; Tietjen & Amick
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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
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• K1= (PHClPH2)/(PGaClPNH3)
• K2=(PGaCl3PH2)/(PGaClPHCl)
• ΣPi= PGaCl + PGaCl3 + PNH3 + PHCl+ PH2 + PIG
• P°GaCl - PGaCl = P°
NH3 - PNH3
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• 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)
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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.
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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.
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Comparison between calculated growth rates and experimental data.
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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
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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