Advancement of Gallium Nitride PIN Diode and Development of Novel 3D+planar Core-shell Microstructures for Betavoltaic Device Technology

Abstract

A betavoltaic (BV) device converts the kinetic energy from β-electrons emitted by radioactive decay into electricity, with output power in the <1 mW regime. AlxGa1-xN is promising for use as the converter material in a BV device due to its wide bandgap, superior radiation tolerance, chemical inertness, and physical hardness. With the emergence of micro- and nanotechnology, many low power systems require self-sustained power sources where replacement is difficult or impossible in harsh environments, for which BV batteries are a prime candidate. BV batteries have been investigated for several decades; however, the technology still lacks in efficient energy conversion and limited power output. The work reported here has addressed some of the challenges associated with, and improved upon, the GaN-based beta-energy converter, along with engineering new methods for device qualification under high energy irradiation and radioisotope sources. Uniquely, a converter structure has been designed, simulated, and implemented with the growth of novel GaN 3D+planar core-shell microstructures. High performing GaN PIN planar diodes were fabricated by tailoring the fabrication process for BV specific needs such as a “beta-transparent” p-contact, low forward leakage by a KOH wetetch passivation treatment and implementing a large area mesa for greater electron absorption. Electron energy irradiation was performed to mimic radioisotope exposure using an electron flood gun (4-16 keV) and a custom in-operando TEM setup (62 – 200 keV). Beam voltage and beam current dependence is reported for several devices. For the electron flood gun irradiation, the highest efficiency of energy conversion at 7% is reported for GaN PIN exposed to 16 keV electron irradiation. Direct measurement of these GaN PIN diodes under exposure to solid metal radioisotope and liquid radioisotope solution is also reported, for both 63Ni and 147Pm isotopes. Moreover, methodology for device testing in custom enclosures was developed and executed. A combined 3D (planar+core-shell) PIN device has been proposed as the optimal design for GaN application as a BV battery. This optimized structure layout leads to enhanced conversion (depletion) region volume compared to that in a planar device under equilibrium conditions. Monte Carlo simulation indicates there is a 3.75x increase in the amount of power absorbed in the GaN layers (PGaN/cm2) at approximately half the activity density for a 3D structure with 4 μm mesa height compared to planar designs with 10 μm 63Ni thickness with a 5.8x improvement in energy transfer efficiency (ηsrc). Metalorganic chemical vapor deposition (MOCVD) growth of the proposed combined 3D+planar GaN PIN was achieved in both a fin and pillar geometry. High aspect ratio n-GaN seeds with controlled facet stabilization were obtained by optimization of the MOCVD growth conditions. An optimized growth condition is achieved where GaN semipolar sidewalls are replaced by m-plane nonpolar sidewalls characterized by their 90° angle with respect to the c-plane (substrate), which was selected as the seed condition for the combined 3D+planar core-shell structure. The fin combined 3D+planar PIN showed bipolar diode behavior with a threshold voltage of ~3 V.