Optical and Electrical Control of Conductivity and DX-Like Deep Donors in Ge-Doped AlN

Optically-triggered AlGaN power handling devices are of interest to mitigate the electromagnetic interference and reduce the complexity of gate drivers. In these devices, optically-activated carriers can modulate the gate charge and threshold voltage; these are called photo-gating effects. However, due to the wide band gap of these semiconductors, photo-generated carriers based on band-to-band transitions require ultra-violet illumination sources, which are challenging to integrate. It would be far easier to use visible light to excite sub-band gap, defect-to-band transitions, if sufficient device functionality can be achieved.<br/>In this work, we demonstrate optical and electrical control of conductivity in highly Ge-doped epitaxial AlN. Group IV dopants, such as Si and Ge, form deep donor levels in Al-rich AlGaN, that behave like DX defects. They are typically undesirable because they limit the equilibrium conductivity, but they also produce giant and persistent photoconductivity. We made Schottky contacts on epitaxial AlN thin films, with Ge concentration of 2x10¹⁸ cm³ and higher, and measured the spectral photoconductivity. We observe a photoconductive response threshold energy of 3 eV, far below the 6.1 eV band gap of AlN. Sub-band gap excitation can increase the conductivity by 10,000 times, compared to the dark current, and the photoconductivity persists for hours after light exposure. The same films are extremely resistive in the dark at equilibrium. These are all signatures of defect-assisted photoconductivity and DX-like defects.<br/>Illumination can switch on conductivity quickly, but the persistent (<i>e.g.</i> hours-long) decay times limit the device concept. We previously demonstrated that conductivity in materials with DX-like defects can be rapidly switched by electrical charge injection at suitably-designed heterojunctions, achieving orders-of-magnitude faster response than using light, of interest for hysteretic electronic devices for memory and computing [1, 2]. Here, we demonstrate the usefulness of this concept for conductivity switching in AlN: we show that voltage pulses after illumination can suppress the persistent photoconductivity in seconds instead of hours. Also, the persistent current levels can be reconfigured and programmed with voltage pulses of variable amplitude and duration. We explain our results with a model of charge state transitions of DX-like defects in the contact space charge region.<br/>Our study enhances fundamental understanding of deep, DX-like donors in AlN. Persistent photoconductivity due to defect-to-band transitions excited by visible light, combined with rapid electrical reconfigurability, makes doped AlN promising for optically-triggered power handling devices. The programmability of the persistent photocurrents may also make these devices of interest for memory and neuromorphic computing.<br/>[1] H. Yin, A. Kumar, J. M. LeBeau, and R. Jaramillo, <i>Defect-Level Switching for Highly Nonlinear and Hysteretic Electronic Devices</i>, Phys. Rev. Applied <b>15</b>, 014014 (2021).<br/>[2] J. Dong and R. Jaramillo, <i>Modeling Defect-Level Switching for Nonlinear and Hysteretic Electronic Devices</i>, J. Appl. Phys. <b>135</b>, 224501 (2024).