Influence of strain relaxation of the AlxGa1−xN barrier on transport properties of the two-dimensional electron gas in modulation-doped AlxGa1−xN/GaN heterostructures

Influences of the thickness of the Si-doped n-type ${\mathrm{Al}}_{\mathrm{0.22}}{\mathrm{Ga}}_{\mathrm{0.78}}\mathrm{N}$ barrier and the thickness of the ${\mathrm{Al}}_{\mathrm{0.22}}{\mathrm{Ga}}_{\mathrm{0.78}}\mathrm{N}$ spacer on mobility and density of the two dimensional electron gas (2DEG) in modulation-doped ${\mathrm{Al}}_{\mathrm{0.22}}{\mathrm{Ga}}_{\mathrm{0.78}}\mathrm{N/GaN}$ heterostructures were investigated. 2DEG mobilities of 1274 cm²/V s at 300 K and 4495 cm²/V s at 77 K were reached. Both 2DEG mobility and density decrease dramatically when the ${\mathrm{Al}}_{\mathrm{0.22}}{\mathrm{Ga}}_{\mathrm{0.78}}\mathrm{N}$ barrier becomes partially relaxed, indicating that transport properties of the 2DEG are influenced significantly by the piezoelectric polarization of the ${\mathrm{Al}}_{\mathrm{0.22}}{\mathrm{Ga}}_{\mathrm{0.78}}\mathrm{N}$ layer. From our results, the critical thickness of an ${\mathrm{Al}}_{\mathrm{0.22}}{\mathrm{Ga}}_{\mathrm{0.78}}\mathrm{N}$ layer on GaN is estimated to be between 65 and 75 nm, which is much higher than that predicted by theoretical calculation. This may be attributed to the interaction of misfit dislocations and the presence of a high density of extended defects in the ${\mathrm{Al}}_{\mathrm{0.22}}{\mathrm{Ga}}_{\mathrm{0.78}}\mathrm{N}$ layer.