Session: MNS-01 MEMS/NEMS Power Sources, Sensors and Actuators, and Computing

Paper Number: 74406

Start Time: August 17, 11:10 AM

74406 - Betavolataic Power Sources for Low Power Electronics 

Betavoltaic Power Source for MEMS Sensors and Actuators

Betavoltaic devices are long life power sources, which convert nuclear energy released as beta particles directly into electrical energy. The power generated by these devices is low (nano-Watts to micro-Watts) but they provide a constant power source that is compact and very long lasting (i.e. has high energy density). The working principle of a betavoltaic device is similar to solar cells (photovoltaics), except power is generated by beta particles instead of photons. Since beta isotopes emit beta particles as they naturally decay over time, they can provide a continuous power for long periods of time unlike photovoltaics, which are dependent on sunlight. Tritium (3 H), Nickel-63 (Ni⁶³) and Promethium (Pm), with half lifetimes of 12.5 years, 100.1 years and 2.6 years respectively and are often used as beta sources because of their availability and their respective beta energies are below the semiconductor damage threshold for most wide bandgap semiconductors.

 

In this study, we used the Shockley and Queisser detailed balanced theory to approximate the efficiency limits for different energy band-gaps with respect to ³H, ⁶³Ni and ¹⁴⁷Pm beta flux distribution. Gallium Nitride (GaN) with theoretical coupling efficiency of 26% is an attractive choice for fabricating a betavoltaic device. In our previous studies, we measured an output power of 70 nW/5mm² with an estimated overall efficiency of 1.2% at the average energy emission of ³H (5.6 keV) using Electron Beam Induced Current (EBIC) measurements in a GaN pin diode grown on sapphire substrate. It is well known that the lattice mismatch between GaN films and sapphire substrate result in threading dislocation densities on the order of 10⁹ – 10¹⁰ cm^‑2 , which are known to provide leakage paths in vertical diodes and result in higher point defect concentrations – both known to degrade device performance. In the last decade, large advances in GaN substrate technology has resulted in true bulk GaN substrates grown via the ammonothermal method. Though expensive, and not readily available, these crystals have threading dislocations on the order of 10³-10⁴ cm^-2_. In this study, for the first time, we fabricate betavoltaic PIN diodes on high quality GaN material with epitaxial layers grown on bulk GaN substrates with low threading dislocation density and evaluate their device performance using EBIC using electron beam energies equivalent to the beta decay average energies of the previously mentioned radioisotopes.

 

Wide bandgap semiconductors have less advanced growth and fabrication techniques. Theoretically, they promise greater output power and higher efficiency compared to narrow bandgap semiconductors, but when actual devices are fabricated, their output power and efficiencies are less than half of what’s promised theoretically. On the other hand, theoretical efficiencies are far less for narrow bandgap semiconductors but when actual devices are fabricated, their efficiencies are closer to promised values due to advancements made in growing and fabricating narrow bandgap semiconductors. Therefore, we are investigating a wide bandgap (GaN) and a narrow bandgap (Indium gallium phosphide - InGaP) semiconductor for a betavoltaic device.

The semiconductor choice is also dependent on the beta-source used. Beta sources with high energy beta particles can be used to generate more power, but tend to damage the semiconductor junction, degrading its performance over time. For example, if InGaP is used with ¹⁴⁷Pm, the power source may stop generating power within days because high energy beta particles are likely to cause mechanical damage to InGaP due to its lower radiation resistance than GaN. When higher output power is desired, a wider bandgap semiconductor such as GaN can be used with promethium-147 to generate more power without the mechanical damage in the semiconductor junction. However, when low continuous output power is desired, low bandgap semiconductor such as InGaP can be used with low energy beta sources such as tritium. 

Presenting Author: Muhammad Khan NSWC, IHD

Authors:

Muhammad Khan NSWC, IHD