Impurities and conductivity control in Al-rich AlGaN alloys

Summary form only given. AlGaN alloys are recognized as promising materials for applications in optoelectronic devices in deep ultraviolet (DUV) spectral range. Highly conductive (p- and n-type) AlGaN alloys are essential for device applications. Although n-type AlN has been reported by Si doping, further improvements in the material quality and conductivity are still needed for many device applications. One of the major difficulties in obtaining highly conductive n-type Al-rich AlGaN alloys is the generation of deep level defects that compensate free electrons. By employing DUV photoluminescence spectroscopy, we have identified three electron compensating centers - isolated cation-vacancy with three negative charges (V_m)^3- , cation-vacancy-complexes with two negative charges (V_m- complex)^2- , and cation-vacancy-complexes with one-negative charge, (V_m- complex) ^-1. The energy levels of these three deep level centers in Al_xGa_{1- x}N (0 ≤ x ≤ 1) alloys are pinned to a common energy level in vacuum. It was observed that that suppressing the concentrations of these defects could significantly improve the conductivity and material quality of Al-rich AlGaN alloys and hence the performance of DUV LEDs. However, a far more serious issue for the realization of high performance DUV LEDs is the poor p type conductivity of Al-rich Al_xGa_{1- x}N, which is inherent to AlGaN. This issue is caused by the large acceptor activation energies (E_A) of Al_xGa_{1- x}N (as large as ~500 meV in AlN). Since the free hole concentration (p) depends exponentially on acceptor energy level,p~exp(-E_A/kT), an E_A value around 500 meV translates to only 1 free hole for roughly every 2 billion (2 ×10⁹ ) incorporated Mg impurities (at 300 K). This causes an extremely low free hole injection efficiency- - into the quantum well (QW) active region and is a major obstacle for the realization of high performance AlGaN-based DUV LEDs. We report on the growth, doping and optical and transport properties of p-type layers optimized for DUV emitter structure incorporation. Benefits of using AlN epi-templates in DUV emitter structures will also be elaborated.