Novel 4H-SiC MOS Devices for Self-Biased High-Efficiency Radiovoltaic Batteries

Stand-alone power supplies, such as lithium batteries, are compact power sources for most electronic devices, however, they are unsuitable for harsh environments, needs frequent recharging, and needs regular replacement as the capacity of lithium batteries reduces 70-90% within few years of usage. Hence, there is a need for reliable stand-alone power supplies with stable power output for prolonged periods for applications in harsh-environment electronics, remote sensing, and medical implants.

To address this, we propose to develop an innovative radiovoltaic battery based on metal/oxide/silicon carbide semiconductor (MOS) device that converts radiation energy from a radioactive source into electrical power without requiring an external biasing voltage (i.e. self-biased). Such batteries can power devices for decades without maintenance, making them ideal for space missions, remote sensing in isolated or harsh environments, deep-ocean equipment, and covert defense operation where conventional batteries and solar photovoltaics do not work.

With biofriendly radioactive sources like tritium, radiovoltaic batteries can also power life-saving medical implants such as pacemakers. The proposed silicon carbide (SiC) semiconductor device fabrication can be retrofitted with existing silicon device facilities, reducing production cost. Radiovoltaic batteries inherently have minimal carbon footprints, they are environment friendly due to their longevity, moreover, nuclear waste can be recycled to manufacture radioisotope batteries.

The project offers excellent research opportunities for graduate and undergraduate students in multidisciplinary fields.

The project aims to develop a novel metal oxide/4H-SiC vertical MOS betavoltaic device capable of reaching the theoretically predicted conversion efficiency of 25% in 4H-SiC. The MOS devices will be fabricated using Y2O3 and other high-k dielectrics deposited epitaxially on high-quality 4H SiC epilayers through pulsed laser deposition. Performance of the present day radiovoltaic devices are limited by bulk and interfacial defects causing charge trapping and short minority carrier diffusion length.

Therefore, the overall technical goals of the project are: i) investigate Y2O3/4H-SiC junction and interface properties to understand their role in enhancing the minority carrier diffusion length in the Ni/Y2O3/4H-SiC MOS structure; ii) explore the properties of other wide bandgap high-k dielectric (SiO2, Al2O3, HfO2) to study their passivation properties and in relative effectiveness of enhancing the betavoltaic cell properties; iii) study 4H-SiC converters with different thicknesses of epilayer to examine the effects of series resistance on the betavoltaic cell performance; iv) demonstrate betavoltaic cells with a conversion efficiency that is close to the theoretical limit. Electrical characterizations such as current-voltage and capacitance-voltage measurements will be carried out to study the junction properties, and alpha spectroscopic methods will be employed to determine the minority carrier diffusion lengths.

Deep level transient spectroscopy (DLTS) will be carried out to study the bulk and interfacial defects. To understand the surface chemistry and to optimize the band offsets between the dielectrics and 4H-SiC epilayers, x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and cross-sectional high-resolution scanning electron microscopy will be carried out.

These studies will be correlated with the conversion efficiency of the betavoltaic cells measured using a 63Ni beta particle source. Because of the high-quality of the 4H-SiC epilayers and efficient charge collection at zero bias, the proposed 4H-SiC betavoltaic devices are anticipated to deliver power and conversion efficiency close to that theoretically predicted for 4H-SiC.

Additionally, since the 4H-SiC growth and fabrication techniques are matured, the proposed devices will reduce the cost of device production substantially than that incurred for presently available diamond radiovoltaic devices.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.