Dopability limits in Al-rich AlGaN alloys for far-UVC LEDs

Abstract

Transitioning to solid-state ultraviolet (UV) lighting is critical for reducing global energy utilization to meet net-zero targets. AlGaN-based far-UVC LEDs offer a mercury-free, energy-efficient alternative to conventional mercury lamps, yet their performance is severely bottlenecked by poor carrier injection at Al compositions exceeding 80\%. Point defects are known to significantly affect carrier concentrations and radiative recombination efficiency, however, systematic studies of point defects in AlGaN alloys remain scarce. In this work, we investigate intrinsic and extrinsic defects in high-Al-content Al$_{1-x}$Ga$_x$N alloys ($x$ = 1/6, 1/4, and 1/3). We reveal that explicit alloy modeling and proper treatment of the temperature dependence of the band gap are essential to bring calculated carrier concentrations in line with experimental observations. We uncover that Si dopants preferentially substitute minority Ga atoms, forming compensating negative-\textit{U} \textit{DX} centers in Al-rich environments that severely limit n-type conductivity. We identify carbon as the most detrimental unintentional impurity, while the impact of oxygen and hydrogen is negligible in Si-doped samples typically used for devices. These findings highlight the significance of explicit alloy modeling and provide valuable insights into the design of AlN-based alloys.

Figures (4)

• Figure 1: (a) Crystal structure and (b) Electronic band structure alongside the density of states with 0.2 eV Gaussian broadening of wurtzite AlN. (c) The band gap of Al$_{1-x}$Ga$_x$N as a function of $x$, compared with experimental data from ref.Shan1999. (d) Al-Ga-N ternary chemical potential diagram showing the stable region of Al$_5$GaN$_6$ due to the limits imposed by competing phases.
• Figure 2: Calculated defect formation energy as a function of the Fermi energy in Al$_{1-x}$Ga$_x$N alloys under N-rich and most N-poor growth conditions for (a,b) $x$ =1/6, (c,d) $x$ =1/4, (e,f) $x$ =1/3. The grey dash lines are the equilibrium Fermi levels, which are determined self-consistently to ensure charge neutrality at T = 1400 K, the typical AlGaN alloy growth temperature. (g) Charge transition levels of nitrogen vacancy in Al$_{1-x}$Ga$_x$N alloys at $x$ = 1/6, 1/4 and 1/3. (h) Total concentration of deep defects in Al$_{1-x}$Ga$_x$N for $x$ = 1/6, 1/4 and 1/3 under N-rich and most N-poor growth conditions with and without the dependence of temperature on band gap. (i) Calculated net carrier concentrations with temperature-dependent band gap renormalization as a function of annealing temperature and quenching at room temperature. The net carrier concentration is $\lvert n_e - n_h \rvert$, where $n_e$ and $n_h$ are electron and hole concentrations, respectively.
• Figure 3: Calculated defect formation energy as a function of the Fermi energy in Si-doped Al$_5$GaN$_6$ alloys under (a) N-rich and (b) most N-poor growth conditions. The equilibrium Fermi energy is calculated at an annealing temperature of 1400 K. (c) Charge transition levels and local structures of SiGa in Al$_{1-x}$Ga$_x$N alloys at $x$ = 1/6, 1/4 and 1/3. The light blue, dark blue, green and grey spheres represent Al, Si, Ga and O atoms, respectively. (d) Total concentration of deep defects (VAl, VGa, VN, VAl+SiAl, VAl+SiGa, VGa+SiAl and VGa+SiGa) in Al$_{1-x}$Ga$_x$N for $x$ = 1/6, 1/4 and 1/3 under N-rich and most N-poor growth conditions with and without the influence of temperature on band gap. (e) Calculated net carrier concentrations on the band gap as a function of annealing temperature and quenching at room temperature. The temperature-dependent band gap renormalization is included. The net carrier concentration is $\lvert n_e - n_h \rvert$, where $n_e$ and $n_h$ are electron and hole concentrations, respectively.
• Figure 4: Formation energies of unintentional extrinsic point defects (a) O, (b) C and (c) H in Al$_5$GaN$_6$ alloys under most N-poor growth conditions. The equilibrium Fermi energy is calculated at an annealing temperature of 1400 K. (d) The equilibrium Fermi levels of O, H, C doped Al$_{1-x}$Ga$_x$N alloys at $x$ = 1/6, 1/4 and 1/3.