Controllable N-Type Doping in Ultra-Wide Bandgap AlN By Chemical Potential Control

AlN, as an ultra-wide bandgap semiconductor (UWBG), is an alternative for high power devices and deep-UV optoelectronics. However, significant challenges in efficient n-type doping and point defect control in AlN epitaxy has precluded the development of AlN based devices. Si is typically employed as n-type dopant in AlN with activation energy of about 300 meV originally attributed to the DX^-1 formation. Recently, Bagheri et al. provided a direct proof of single electron occupancy for donors in Al rich AlGaN and AlN instead of DX^-1 formation.¹ This makes controllable n-type doping via epitaxy in AlN feasible but only if proper point defect management is attained. Si doped AlN exhibits a “knee behavior” resulting in a conductivity and carrier concentration maxima at a specific Si concentration. Hence a high doping limit exists for Si in AlN that lowers the maximum achievable carrier concentration that are necessary for AlN based optoelectronics. Similarly, a low doping limit (a minimum achievable carrier concentration with a corresponding maximum mobility) exists in AlN, similar to that in AlGaN and GaN, which makes unfeasible the implementation of AlN power electronics that require low doped drift regions. These two challenges translate into a major “point defect problem” that exists in AlN which should be solved for implementation of this technology.<br/>In this work, we demonstrate a systematic chemical potential control (CPC) based point defect control scheme where we relate the growth environment variables to the defect formation energy by determining and controlling the impurity chemical potentials and systematically engineer the growth environment accordingly for minimal point defect incorporation or generation.<br/>Accordingly, threading dislocations, C_N, and V_III-nSi_III complexes were identified as the primary defects responsible for the doping limits in AlN grown by metalorganic chemical vapor deposition (MOCVD). We demonstrate control over the knee point (improving the maximum carrier concentration by inhibiting the formation of complexes) and low doping limit (achieving minimum carrier concentrations along with the maximum mobility by reducing the carbon incorporation) via controlling the chemical potentials of III/N, growth temperature and reduction of threading dislocation density. Doping below 10¹⁸ cm^-3 Si concentration was severely restricted by dislocations on sapphire substrate (TDD~10⁹ cm^-2) independent of growth condition. On AlN single crystal, efficient doping up to 5×10¹⁷ cm^-3 Si was achieved by reduction of dislocation density and carbon incorporation under a N-rich growth environment. An increase in the N chemical potential and a 200°C increase in the growth temperature led to a mobility increase from 40 to 200 cm²/Vs at a carrier concentration of 10¹⁵ cm^-3 in Si doped AlN.<br/>This work opens pathways for controllable n-type doping in AlN providing one of the main building blocks for the realization of UWBG-based high power devices and deep-UV optoelectronics.<br/>1. Bagheri, P. <i>et al.</i> On the Ge shallow-to-deep level transition in Al-rich AlGaN. <i>J. Appl. Phys.</i> <b>130</b>, 055702 (2021).