To read the full-text of this research, you can request a copy directly from the authors.
Here, we report on the application of an electron source with high accelerating voltage (62 kV–200 kV) to simulate betavoltaic power generation capabilities of a planar GaN PIN (p-GaN/i-GaN/n-GaN) device. The in situ electrical characterization reported here enables detailed performance comparison of new device designs to conventional device configurations. In operando investigation of a GaN PIN device under irradiation by a modified transmission electron microscope is being reported here. A large-area planar GaN PIN (0.04 cm², 17.8 nA/cm² at 5 V reverse bias) device was irradiated with an electron beam of approximately equivalent spot size. At an approximate input current density of 5 nA/cm², the maximum power produced (MPP) decreases from 2.45 µW/cm² to 0.45 µW/cm² with an increase in the beam voltage from 62 kV to 200 kV. This reduction in power corresponds to reduced electron–hole pair generation and capture within the active region of the device. The inverse relation of MPP to beam voltage is modeled by CASINO2 Monte Carlo simulations of energy absorption and is found to be in good agreement with the experimental measurement. At a constant 62 kV beam voltage, MPP is shown to increase with beam current density up to 48.2 µW/cm² at 177 nA/cm². Repeated device dark current measurements following the irradiation indicate no degradation of the device. An irradiation dose of ∼10¹⁶ cm⁻², equivalent to exposure from a 10 mCi radioisotope source for 1 yr, was performed at an energy of 200 kV, with no appreciable deterioration in device performance.
Betavoltaic devices are suitable for delivering low-power over periods of years. Typically, their power density is on the order of nano to micro-Watts per cubic centimeter. In this work we evaluate the potential for using high-aspect ratio three-dimensional semiconductor structures to enhance the power and efficiency of these devices. The Monte Carlo transport code MCNP6 is used to provide realistic estimates of the theoretical levels of charge generation, which is in turn used to make predictions about the power output from three-dimensional betavoltaics. The focus of this work is on silicon and promethium-147, but other semiconductors and radioisotopes are considered as well. In the case of silicon diodes with three-dimensional features that are comparable to what is commercially available we estimate that power densities in the range of 20-25 mW/cm³ can be achieved at efficiencies of 2.9-5.8% when coupled with promethium-147 oxide.
Gallium Nitride based high electron mobility transistors (HEMTs) are attractive for use in high power and high frequency applications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20-30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN-based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitridematerial grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 1014 cm-2 or electrons at fluences near 1016 cm-2, both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances.
This paper presents a theoretical calculation model of GaN betavoltaic battery with the N63i radiation source. The output parameters of the GaN p-n junction battery and the GaN Schottky barrier battery are calculated based on Monte Carlo simulations. The calculation results show that when the thickness of N63i source is 2 μm, the GaN-N63i batteries nearly achieve optimized performance. For the GaN-based p-n junction battery, the maximum output power density, filling factor, energy conversion efficiency and total energy conversion efficiency are 0.359 μW/cm2, 94.6%, 23.2% and 6.96%, respectively. For the GaN-based Schottky barrier battery, the output performance is related to the selection of the Schottky metal. Among the metals of Ag, Ti, Au, Pd, Ni and Pt for the GaN Schottky barrier diodes, the Au-GaN Schottky barrier device is typically fabricated. The corresponding output parameters of the Au-GaN Schottky barrier battery are 0.223 μW/cm2, 92.2%, 14.4% and 4.33%, respectively.
A combined GaN 3D core-shell and planar pin structure is being developed and demonstrated to achieve the highest potential to increase energy transfer efficiency from the source (ηsrc) and power generated per cm² (PGaN/cm²) in a betavoltaic (BV) device configuration. Physics-based Sentaurus TCAD and Monte Carlo N-Particle extended (MCNPX) software are employed to obtain the maximum ηsrc and PGaN/cm² by a parametric study of device dimensions coupled with a ⁶³NiCl2 source. Idealized structure dimensions are determined to be 2 µm wide, 4 µm tall GaN pin core-shell mesas, with ⁶³Ni source conformally surrounding the structure with a 2 µm gap for maximum efficiency of energy transfer. For maximizing power deposited (10 µm mesa separation) a 3.75x increase in PGaN/cm² at approximately half the activity density compared to a planar device is achieved for 4 µm mesa height, with 5.82x improvement in ηsrc.
A Monte Carlo source model using PENELOPE was developed to investigate
different tritiated metals in order to design a better radioisotope source for
betavoltaic batteries. The source model takes into account the self‐absorption
of beta particles in the source which is a major factor for an efficient source
design. The average beta energy, beta flux, source power output, and source
efficiency were estimated for various source thicknesses. The simulated results
for titanium tritide with 0° and 90° angular distributions of beta particles were
validated with experimental results. The importance of the backscattering effect
due to isotropic particle emission was analyzed. The results showed that the
normalized average beta energy increases with the source thickness, and it
reaches peak energy depending on the density and the specific activity of the
source. The beta flux and power output also increase with increasing source
thickness. However, the incremental increase in beta flux and power output
becomes minimal for higher thicknesses, as the source efficiency decreases
significantly at higher thicknesses due to the self‐absorption effect. Thus, a
saturation threshold is reached. A low‐density source material such as beryllium
tritide provided a higher power output with higher efficiency. A maximum
power output of approximately 4 mW/cm3 was obtained for beryllium tritide
with SiC. A form factor approach was used to estimate the optimum source
thickness. The optimum source thickness was found near the thickness where
the peak beta particle average energy occurs.
We report here for the first time a fabrication of betavoltaic battery prototype consisting of 200 single conversion cells based on Schottky barrier diamond diodes which have been vertically stacked with ~24% ⁶³Ni radioactive isotope. The maximum electrical output power of about 0.93 μW was obtained in total volume of 5 × 5 × 3.5 mm³. We used the ion-beam assisted lift-off technique to obtain conversion cells of minimal thickness comparable with the characteristic penetration length of beta-particles emitted by ⁶³Ni isotope. The obtained value of 15 μm was limited by the mechanical strength of produced structures and process reliability. To check the performance of thin diamond based conversion cells we carried out IV-curves measurements at electron beam irradiation in SEM. We found that the sacrificial layer for the splitting of such thin conversion cell from HPHT diamond substrate did not cause a considerable degradation of device charge collection efficiency. As a result, the fabricated prototype provided the output power density of about 10 μW/cm³, that is the best known value for nuclear batteries based on ⁶³Ni radioisotope. Moreover, the long half-life of ⁶³Ni isotope gives the battery specific energy of about 3300 mWh/g that is an order of magnitude higher than the typical value of commercial chemical cells.
The method of Monte Carlo and numerical model co-simulation is adopted to research the radiation-voltaic effect in semiconductor device and used in the optimal design of ¹⁴⁷Pm SiC-based cell in this paper. According to the energy spectrum of ¹⁴⁷Pm, the ionization energy deposition in the cell is calculated by Monte Carlo method. The result is converted into the non-equilibrium carrier information and mapped into the device grid generated by the numerical software, so as to simulate output characteristics of the cell. The simulation results based on the SiC PIN betavoltaic cell show the conversion efficiency firstly goes up and then decreases as the I layer thickness increases, and the conversion efficiency decreases when the doping concentration of I layer increases. The conversion efficiency will be 3.74% when the doping concentration and thickness of I layer are 5 × 10¹⁴cm⁻³ and 20 μm respectively. According to the analysis, the recombination loss of radiation-induced carriers in I layer is the main factor influencing the improvement of conversion efficiency. To improve the conversion efficiency, the “graded N layer” SiC PN cell is proposed in this paper to replace conventional I layer with two N layers with different doping concentrations; the electric field is introduced to reduce the recombination loss of the radiation-induced carriers. The conversion efficiency will be 4.58% when the thickness of two layers are 10 μm and the doping concentrations are respectively 5 × 10¹⁴cm⁻³ and 1 × 10¹⁶cm⁻³.
The performance of gallium nitride (GaN) p-i-n diodes were investigated for use as a betavoltaic device. The GaN p-i-n structure was grown by metalorganic chemical vapor deposition (MOCVD) and consisted of 80 nm p = 4x10¹⁷ cm⁻³ GaN/ 1 µm unintentionally doped (UID) n = 10¹⁶ cm⁻³ GaN/ 2 µm 10¹⁸ cm⁻³ n⁺-GaN layers. Dark IV measurements showed a turn on-voltage of approximately 3.2 V, specific-on-resistance of 15.1 mΩ-cm² and a reverse leakage current of -0.14 mA/cm² at -10 V. A clear photo-response was observed when IV curves were measured under a light source at a wavelength of 310 nm (4.0 eV). In addition, GaN p-i-n diodes were tested under an electron-beam in order to simulate common beta radiation sources ranging from that of ³H (5.6 keV average) to ⁶³Ni (17 keV average). From this data, we estimated output powers of 53 nW and 750 nW with overall efficiencies of 0.96% and 4.4% for our device at average incident electron energies of 5.6 keV and 17 keV corresponding to ³H and ⁶³Ni beta sources respectively.
A single crystal diamond large area thin membrane was assembled as a p-doped/Intrinsic/Metal (PIM) structure and used in a betavoltaic configuration. When tested with a 20 keV electron beam from a high resolution scanning electron microscope, we measured an open circuit voltage (Voc) of 1.85 V, a charge collection efficiency (CCE) of 98%, a fill-factor of 80%, and a total conversion efficiency of 9.4%. These parameters are inherently linked to the diamond membrane PIM structure that allows full device depletion even at 0 V and are among the highest reported up to now for any other material tested for betavoltaic devices. It enables to drive a high short-circuit current Isc up to 7.12 μA, to reach a maximum power Pmax of 10.48 μW, a remarkable value demonstrating the high-benefit of diamond for the realization of long-life radioisotope based micro-batteries.
In order to improve the performance of betavoltaic converters based on synthetic IIb diamond Schottky structure, we performed comparative studies of converters with Al, Hf, Pt, and Au metals forming the Schottky barrier by means of the electron beam-induced current (EBIC) method. Nearly full collection of generated electron-hole pairs was found for all fabricated structures. The aluminum contact showed the lowest energy loses due to negligible electrons absorption and backscattering. But the conversion efficiency of the Al-contact Schottky diode was just about 2% because of its low open-circuit voltage. We attributed the reduction of the Schottky barrier height to local defects observed by EBIC. The diamond cells with platinum contact showed the best performance of about 6% with a relatively high open-circuit voltage >1V and a good incident beam multiplication factor. Thereby platinum forms the most stable and effective Schottky barrier contact for diamond betavoltaic cells.
Realization of an 18.6% efficient 4H-silicon carbide (4H-SiC) large area betavoltaic power source using the radioisotope tritium is reported. A 200 nm 4H-SiC P+N junction is used to collect high-energy electrons. The electron source is a titanium tritide (TiH3x) foil, or an integrated titanium tritide region formed by the diffusion of tritium into titanium. The specific activity of the source is directly measured. Dark current measured under short circuit conditions was less than 6.1 pA/cm2. Samples measured with an external tritium foil produced an open circuit voltage of 2.09 V, short circuit current of 75.47 nA/cm2, fill factor of 0.86, and power efficiency of 18.6%. Samples measured with an integrated source produced power efficiencies of 12%. Simulations were done to determine the beta spectrum (modified by self absorption) exiting the source and the electron hole pair generation function in the 4H-SiC. The electron-hole pair generation function in 4H-SiC was modeled as a Gaussian distribution, and a closed form solution of the continuity equation was used to analyze the cell performance. The effective surface recombination velocity in our samples was found to be 105–106 cm/s. Our analysis demonstrated that the surface recombination dominates the performance of a tritium betavoltaic device but that using a thin P+N junction structure can mitigate some of the negative effects.
Long-lived and high-energy-density betavoltaics have a great potential as power supplies for remote and hostile environmental conditions, where volume power density and/or power lifetime are very important considerations. In this paper, we provide new results to aid in the design and optimization of betavoltaics made with Si space solar cells and beta sources. The new results were obtained by using a customized low-energy electron accelerator to characterize the radiation-hardened high-efficiency Si space solar cells while varying the electron beam energy and electron beam current density, i.e., electron beam flux. The betavoltaic conversion efficiency of Si space solar cells increases until 60 keV and then decreases with the increasing electron beam energy. The maximum efficiency (6%) obtained at the electron beam energy of 60 keV suggests that Pm-147 would be a good beta source to make high-efficiency nuclear batteries. The radiation ionization energy is ~ 3.90 eV per electron-hole pair for Si space solar cells. Some radiation damage-induced performance degradation was also observed when the Si space solar cells were exposed to the bombardment of 62-keV electrons with fluence up to 4.92 × 1018betas/cm2, which is equivalent to the radiation from a semi-infinite Pm-147 layer for ~ 2.26 years. The results in this paper suggest that beta-particle entrance window, betavoltaic cells' configuration structure, and device properties such as charge carriers' diffusion length are very important factors to be engineered to improve the conversion efficiency for practical betavoltaics.
This article reviews the effects of radiation damage on GaN materials and devices such as light-emitting diodes and high electron mobility transistors. Protons, electrons and gamma rays typically produce point defects in GaN, in contrast to neutron damage which is dominated by more extended disordered regions. Regardless of the type of radiation, the electrical conductivity of the GaN is reduced through the introduction of trap states with thermal ionization energies deep in the forbidden bandgap. An important practical parameter is the carrier removal rate for each type of radiation since this determines the dose at which device degradation will occur. Many studies have shown that GaN is several orders of magnitude more resistant to radiation damage than GaAs, i.e. it can withstand radiation doses of at least two orders of magnitude higher than those degrading GaAs with a similar doping level. Many issues still have to be addressed. Among them are the strong asymmetry in carrier removal rates in n- and p-type GaN and interaction of radiation defects with Mg acceptors and the poor understanding of interaction of radiation defects in doped nitrides with the dislocations always present.
Large-scale ab initio molecular dynamics method has been used to determine the threshold displacement energies E<sub>d</sub> along five specific directions and to determine the defect configurations created during low energy events. The E<sub>d</sub> shows a significant dependence on direction. The minimum E<sub>d</sub> is determined to be 39 eV along the < 1 010> direction for a gallium atom and 17.0 eV along the < 1 010> direction for a nitrogen atom, which are in reasonable agreement with the experimental measurements. The average E<sub>d</sub> values determined are 73.2 and 32.4 eV for gallium and nitrogen atoms, respectively. The N defects created at low energy events along different crystallographic directions have a similar configuration (a N–N dumbbell configuration), but various configurations for Ga defects are formed in GaN.
Gallium nitride (GaN) light emitting diodes (LEDs) were irradiated at room temperature with electrons in the range 300-1400 keV. A threshold energy of 440 keV was observed, corresponding to a gallium atom displacement energy of 19±2 eV. This value of the displacement energy compares with that of silicon carbide but is smaller than that of diamond and larger than that of gallium arsenide (GaAs). No threshold energy for the nitrogen atom was observed. It is concluded that the nitrogen sublattice repairs itself through annealing. The measured displacement energy is used to determine the Rutherford cross section, which permits a theoretical comparison of electron and proton irradiation damage in GaN. The effects of 2.5 MeV electrons on gallium nitride films have been studied by photoluminescence (PL), and according to the literature, they introduce transitions in the near infrared part of the spectrum. Experiments on gallium nitride films using 2 MeV protons are reported in this work. The same transitions in the near infrared part of the spectrum are observed by PL. It is deduced that 2 MeV protons are about 1000 times more damaging than 2.5 MeV electrons. The Rutherford cross section predicts a value of 214. The difference is attributed to the defect recombination rate which depends on the particle type. The nature of the transitions in the near infrared part of the spectrum is reviewed. The GaN films were annealed at 400°C for 30 min. As a result of annealing, another transition appears in the green part of the spectrum. Transitions involving the gallium vacancy in irradiated GaN are discussed.