Highly Conductive Modulation-Doped Graded p-AlGaN/(AlN)/GaN Multi-Heterostructures

In this study we present theoretical and experimental results regarding modulation doped p-AlGaN/(AlN)/GaN multi-heterostructures. As the heterostructures should yield both, higher lateral and better vertical conductivity than p-doped GaN, band structure simulations have been performed prior to growth experiments. Based on the simulation results several samples were grown by metalorganic vapor phase epitaxy (MOVPE). High resolution X-ray diffraction was used to determine the effective Al-concentration as well as the period length of the multi-heterostructures. The electrical properties of the samples were investigated by measuring the lateral (σL) and vertical (σV ) conductivity. Moreover electron holography measurements were performed to determine the profile of the valence band. The free hole concentration of a sample optimized in terms of lateral conductivity was measured to be 5.5× 10 18 cm (295K) with a mobility of 12 cm/V s, yielding a σL of 10 (Ωcm) −1 . Low temperature Hall measurements (77K) proved the existence of a 2DHG at the AlN/GaN interface, as the lateral conductivity could be increased to 25 (Ωcm) . By substituting the p-GaN layer in a light emitting diode (LED) with an AlGaN/GaN multi-heterostructure optimized in terms of vertical conductivity, the overall voltage drop could be reduced by more than 100mV (j = 65A/cm), clearly demonstrating that p-type heterostructures are a candidate to increase the efficiency of high brightness LEDs.