Performance prediction of AlGaAs/GaAs betavoltaic cells irradiated by nickel-63 radioisotope

Highlights

• Performance of Al0.1Ga0.9 A s/GaAs-based betavoltaic heterostructure was evaluated using a comprehensive analytical model.

• The adoption of a radioactive source with an activity density of 10 mCi/cm2 results in a good cell performance.

• The studied heterostructure performs a conversion efficiency (η) in excess of 33%.

Abstract

The performance of a n/p betavoltaic heterostructure, i.e. aluminum gallium arsenide (Al_xGa_1-xAs) on gallium arsenide (GaAs) substrate has been evaluated by nickel-63 (Ni⁶³) beta-particles irradiation with anaverage kinetic energy of 17.1 keV. The thickness of Al_xGa_1-xAs emitter layer was set to 1.2 μm with an aluminum molar fraction of 0.1. The thickness of GaAs base region was fixed to be 3 μm. During the calculations, the reflection from the front surface, the metallurgical interface, and the limits of the depletion region were carefully taken into account. Moreover, the equivalent circuit accounts for the ohmic losses. The simulation results reveal that by using a radioactivity density of 10 mCi/cm², the conversion efficiency (ɳ) of an optimized device structure increases up to 33%. The other cell electrical parameters, such as the short-circuit current density (J_sc), the open-circuit voltage (V_oc), and the maximum electrical power density (P_max) are observed to be 438.09 nA/cm², 0.97 V, and 337.35 nW/cm², respectively.

Introduction

In the recent years, a tremendous development of the information technology has significantly increased the interest for long-life power batteries in improving the human lifestyle. In particular, after numerous theoretical and experimental investigations, significance of the potential of radioisotope batteries has been highlighted for their use in highly sensitive applications, for instance in implantable prosthetic devices, deep-sea explorations, automatic weather stations, space flights, and in all situations where it is inconvenient or impossible to replace batteries.

Radioisotope batteries consist of semiconductor-based devices that uses energy from the decay of a radioactive isotope to generate electricity according to the working principle of photovoltaic cells. The selection criteria for radioactive and absorbing semiconductor materials for radioisotope batteries depend on several factors, specifically half-life of radioisotope limited to the task duration, type of decay, and energy content. Simultaneously, the semiconductor being used must be efficient and resistant to radiation. In particular, beta-particles exhibit few advantages over the alpha-particles such as high penetration ability and less radiation damage. Owing to these characteristics of beta-particles, most of the researchers focused their efforts to develop betavoltaic cells. The idea to develop radioisotope batteries was first suggested by Mosely in 1913 [1], and since 1950s the advances in betavoltaic batteries have seen a rapid progress and significantly drawn the focus of researchers to investigate materials like gallium arsenide (GaAs) [[2], [3], [4], [5], [6], [7]], aluminium gallium arsenide (AlGaAs) [[8], [9], [10]], silicon carbide (SiC) [[11], [12], [13], [14], [15], [16], [17]], gallium nitride (GaN) [[18], [19], [20], [21], [22], [23]], and indium gallium phosphide (InGaP) [24,25]. However, until now, a very limited number of materials have been used to produce nuclear batteries. In fact, different technological issues, such as radioisotopes, converters, shielding, costs, and restrictions on trade and transportation need to be still addressed, and therefore this field remains fertile for researchers.

The design of a betavoltaic cell needs careful selection of radioactive source so that the maximum kinetic energy of β-particles should not exceed the semiconductor radiation damage threshold [26]. Most recently, the interest to investigate GaAs and its related compound has been increased constantly [[2], [3], [4], [5], [6], [7], [8], [9], [10]]. The performance of several GaAs-based betavoltaic cells presented in literature is summarized in Table 1. The advantages of Al_xGa_1-xAs over other compounds are its good lattice and bandgap matching, radiation damage resistance, and high theoretical conversion efficiency.

Note that the theoretical efficiency limit values for the Al_0.1Ga_0.9As and GaAs-based structures are 32.02% and 31.66%, respectively. These results were obtained using the following expression describing the conversion efficiency of a semi-infinite semiconductor material [[27], [28], [29], [30]]:

$$\eta \approx \frac{E_g}{2.8E_g+0.5}\times 100\%$$

where E_g is the band gap energy.

It is clear, therefore, that the driving potential efficiency of these technologies is practically in its infancy and far from the theoretical limits. In this context, future developments and improvements need great efforts in modelling activities (analytical and numerical analysis) that contribute significantly towards cost-effective experimental tests related to hazardous and expensive materials.

The present article studies the optimized use of Al_xGa_1-xAs/GaAs n/p heterostructures for radioisotope batteries regarding understanding of physical and geometrical device parameters that can affect the cell performance. Moreover, based on the comprehensive analytical model presented in the following, we have compiled a simulation software that investigates the role of various design parameters to determine the performance of an Al_0.1Ga_0.9As/GaAs betavoltaic cell irradiated by Ni⁶³ beta-particles. These parameters are the doping concentration in the n and p regions, the junction depth, the surface recombination velocity, and the radioactivity density of Ni⁶³. The simulation of the current density-voltage J(V) and power density-voltage P(V) characteristics would help to extract the key figures of merit (FOMs) of the cell, i.e. conversion efficiency (ɳ), maximum output power density (P_max), open-circuit voltage (V_oc), and short-circuit current density (J_sc) as a function of fundamental physical parameters. Al_xGa_1-xAs/GaAs heterostructures with x ranging from 0.1 to 0.35 have been recently fabricated by the Ioffe Russian Institute and evaluated under a flow of beta-particles emitted by the radioactive isotope H³ [[8], [9], [10]].

This work is supported by authors’ preliminary investigations focused on the exploitation of radiation resistant materials with good reliability [[31], [32], [33], [34], [35], [36], [37]].