Electrical Characteristics and Deep Traps Spectra of Undoped GaN Films Grown on Si Using Different Strain-Relieving Buffer Types

Electrical properties of GaN films grown on Si by molecular beam epitaxy using various types of strain-relieving layers have been studied by means of Hall/van der Pauw measurements, capacitance-voltage profiling, admittance spectroscopy, and deep levels transient spectroscopy with electrical and optical injection. The electrical properties of all grown films were determined by relatively deep electron traps N1, N2, and N3 with aggregate concentration of ~10¹⁷ cm^-3. Freezing out of these traps led to the films freezing out down to the depth corresponding to the nearest underlying heterointerface where a strong band bending caused a sharp nonuniformity of charge carriers concentration. For AlN or Al-rich AlGaN underlying films, this band bending could cause formation of hole sheet charge leading to apparent conductivity to appear p-type in Hall. Other deep traps detected in the grown films were N4 and N5 acceptors with levels near E_c - 0.6 eV, and hole traps H1 and H2 with levels near E_v + 0.9 eV. Possible consequences of the observed phenomena for designing the thick GaN stand-off films in power transistors are briefly discussed.