Direct Evidence of Ge Becoming a Deep Donor Instead of a DX- in Al Rich AlGaN

Abstract

Abstract Body: Controllable n-type doping in AlGaN and AlN is crucial for applications in electronics, optoelectronics, and power electronics. Si and recently Ge have been utilized as n-type dopants in AlxGa1-xN with low ionization energy for x0.8 and x>0.5, presenting challenges for novel applications employing Al rich AlGaN and AlN. While such a transition was previously observed in the AlGaAs system, it was confirmed to be an acceptor type DX-1. However, no such direct proof of the Si and Ge charge states in Al rich AlGaN and AlN exists. In this work we present direct evidence of single electron occupancy in the Ge deep state in AlxGa1-xN (x>0.5) i.e., Ge transitions to a deep neutral donor. Co-doping with the shallow donor Si (at x 0.5 showing that Ge transitions to a deep state.1 Si remained a shallow donor with low ionization energy up to x~0.8. Therefore, both dopants are expected to be shallow donors in Al0.4Ga0.6N forming a d+ state. However, possible electron compensation in Si and Ge codoped Al0.65Ga0.35N where Ge transitions into a deep state, would directly show the charge sate of the deep state Ge, either D0 or DX-1. Carrier concentration measurements versus Al content under constant Si and Ge concentrations of 1×1019 cm-3 and 8×1018 cm-3, respectively, were realized at RT and 600°C. Negligible compensation and a temperature independent carrier concentration are observed in Al0.4Ga0.6N were both Ge and Si are expected to be in their shallow states. Interestingly, a reduction in carrier concentration at x~0.65 corresponding only to the Ge concentration was observed, indicating that Ge is not providing free electrons nor compensating the free electrons provided by Si. This is the signature of a deep donor having a higher ionization energy and not of an acceptor state. At higher temperatures, the ionization of the deep donor Ge provides higher carrier concentrations as expected for such a donor. Contrary to the formation of a stable DX-1, which is expected to pin the Fermi level at the (-/+) thermodynamic transition3, transition to a deep donor allows for the possibility of highly conducting AlN and Al rich AlGaN by degenerately doping the system. Hence, this work cast light on the true nature of donor related deep states in nitrides, further opening pathways for doping engineering and achievement of high conductivity in Al rich AlGaN and AlN. 1. Bagheri, P. et al. The nature of the DX state in Ge-doped AlGaN. Appl. Phys. Lett. 116, 222102 (2020). 2. Collazo, R. et al. Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications. Phys. Status Solidi C 8, 2031–2033 (2011). 3. Gordon, L., Lyons, J. L., Janotti, A. & Van de Walle, C. G. Hybrid functional calculations of D X centers in AlN and GaN. Phys. Rev. B 89, 085204 (2014).