Highly conductive modulation doped composition graded $p$-AlGaN/(AlN)/GaN multiheterostructures grown by metalorganic vapor phase epitaxy

In this study, we present theoretical and experimental results regarding highly conductive modulation doped composition graded $p$-AlGaN/(AlN)/GaN multiheterostructures. Based on simulation results, several multiheterostructures were grown by metalorganic vapor phase epitaxy. Using high resolution x-ray diffraction and x-ray reflectometry, the abruptness of the AlGaN/AlN/GaN interfaces could be determined. Using electron holography, the energetic profile of the valence band could be measured, yielding important information about the vertical carrier transport in such multiheterostructures. The electrical properties of the samples were investigated by measuring the lateral $\left({\sigma }_L\right)$ and vertical $\left({\sigma }_V\right)$ conductivity, respectively. The free hole concentration of a sample optimized in terms of lateral conductivity was measured to be $1.2\times {10}^{19}{\text{cm}}^{-3}$ (295 K) with a mobility of $7{\text{cm}}^2/\text{V}\text{s}$, yielding a record ${\sigma }_L$ of $13.7{\left(\Omega \text{cm}\right)}^{-1}$. Low temperature Hall measurements (77 K) proved the existence of a two-dimensional hole gas at the AlN/GaN interface, as the lateral conductivity could be increased to $30{\left(\Omega \text{cm}\right)}^{-1}$ and no carrier freeze out was observable. By substituting the $p$-GaN layer in a light emitting diode (LED) with an AlGaN/GaN multiheterostructure, the overall voltage drop could be reduced by more than 100 mV $\left(j=65\text{A}/{\text{cm}}^2\right)$. Furthermore improved current spreading on the $p$-side of LEDs with integrated AlGaN/AlN/GaN multiheterostructures could be proved by $\mu$-electroluminescence, respectively.