Researches On ZnO Based Betavoltaic Battery And Relative Time Properties

With the rapid development of MEMS?microelectromechanical systems?,the micro power supply eventually become a current research hot-point.However,conventional power supplies need replenishment and have relatively short lifetimes,while betavoltaic batteries with high energy density,long lifetime,stable operation are suitable for operating in harsh inaccessible environments.Betavoltaic battery with wide bandgap semiconductor is preferred due to higher radiation resistance and conversion efficiency.However,lots of wide bandgap semiconductors suffer from the doping asymmetry problem,in that one kind of doping can be obtained rather easily,but the contrary conductivity is difficult to get,so it is important to take the technical maturity into account.Additionally,because the betavoltaic battery is designed for long term usage,it is necessary to acquire the time effect on the performance of the battery.In this work,a novel wide bandgap semiconductor ZnO is utilized as energy converting material,which is easier to grow,more economical and has higher radiation resistance than SiC and GaN.The suitable doping concentrations for ZnO homojunction and Schottky devices are given.By using Monte Carlo simulations,the time-related self-absorption and electrical properties are obtained.For TiT₂ source,the processes of tritium decaying,leaking,source swelling and ³He releasing are considered,and the component and density changes are calculated.Finally,the time-related electrical properties are evaluated.The main results are listed as follows:1.The bandgap of ZnO is similar to that of GaN's and higher than that of SiC's,and its theoretical conversion efficiency is relatively high.The density and electron density of ZnO are moderate,so it is convenient to fabricate micro-devices.For the growth method,it is easier to grow high quality ZnO film at lower temperature.Additionally,the radiation resistance of ZnO is much higher than that of Si,GaAs,GaN,etc.Alongwith its low cost,environmentally friendly characteristics,ZnO is definitely a potential energy converting material for betavoltaic battery.2.For solid sources,namely ⁶³Ni,¹⁴⁷Pm and TiT₂,at initial time,the increasement between surface power and thickness is slowing down because of the existence of self-absorption effect.The source with proper thickness should have relatively high surface power and low self-absorption rate.3.For ZnO homojunction,the scale of whole device should be larger than 17?m and 118?m when using ⁶³Ni and ¹⁴⁷Pm,respectively.With the doping concentrations increasing,the build-in potential increases slightly,while the depletion width decreases drastically.In order to match the energy deposition range and the depletion width,the doping concentrations should be lower to increase the depletion width.4.Au,Pd,Ni and Pt are suitable to fabricate Schottky contact with n-type ZnO,while Al,Ag and Ti are adequate to fabricating Ohmic contact.For ⁶³Ni source,the self-absorption rate increases slightly,and the surface power decreases exponentially in 200years.Among all devices?i.e.,ZnO homojunction and all Schottky devices?,ZnO homojunction has the best electrical properties and longest lifetime,while among all Schottky devices,Pt/ZnO and Ni/ZnO have relatively long lifetimes.Pt/ZnO device has the highest open circuit voltage,fill factor and maximum output power,while Ni/ZnO device has the largest short circuit current among all Schottky devices.For Schottky devices,although their electrical properties are inferior to pn junction's,they have their own advantages including fabricating easiness,independence of p-type ZnO and radiation hardness,that is to say,they can be alternatives to the energy converting structures of betavoltaic battery beside pn junction.5.TiT₂ source is affected by tritium decaying,swelling,³He storing and releasing along with tritium leaking.As time goes by,the source changes its component,density,volume and radioactivity,and ³He will be released.In this process,the adsorbed tritium content decreases approximately linear,the self-absorption rate and surface radioactivity also decrease.For pn junction based betavoltaic battery,open circuit voltage,fill factor and conversion efficiency drop suddenly,while short circuit current and maximum output power decreases approximately linear.At 3.5 a,the source begins to release ³He gas,while at 10.5 a,the short circuit current and maximum power remaining only about 1%of their initial quantities.At 10.73 a,the source is run out of tritium,and the betavoltaic battery loses its effectiveness.Considering all these factors,the battery has its best performance before 3.5 years because no ³He release and relatively high electrical properties.