A Diamond Gammavoltaic Cell

Lead Research Organisation: University of Bristol
Department Name: Physics

Abstract

Using in-house fabrication facilities, coupled with internal and external testing facilities, this project aims to construct a diamond, transverse Schottky diode-based photovoltaic, optimised for high-energy radiation such that it can supply power in harsh gamma environments - a gammavoltaic. The first stages of the project focus on finding an optimal pair of contacts, one ohmic and one rectifying, that strike a balance between high in-built voltage and low resistance. For this, several literature-standard contact types and a number of more novel types have been/ are being compared. Device simulation in softwares packages such as GEANT4, COMSOL and in-house programs will link these experimental results back to theory, and influence the thicknesses chosen for the growth of the diamond layers. Growth is undertaken by microwave plasma-enchanced chemical vapour deposition (MWCVD) and will also potentially be by hot-filament CVD (HFCVD).

For the ohmic contacts, the literature standard is an Au/Ti multilayer contact, deposited onto a highly boron-doped section of substrate, and annealed in vacuum such that a TiC interlayer develops. Many other metal combinations have also been published, generally consisting of carbide-forming metals such as Mo and Ti, but also a significant number focussing on oxygen-adherent metals such as Cr, and some focussing on diamond-soluble metals such as Pd and Co. There is also the less-investigated sp2-rich/ graphic contact, made by deliberately non-ideal diamond growth or by laser-writing, which shows promise as an ohmic contact technology but, as it is less developed, is being investigated in parallel to lessen risk to the project. Schottky contacts have historically been made with metals such as Al, Cu and Zr, which readily oxidise and adhere to an O-terminated diamond surface, and these metals will both be investigated. Additional novelty will be introduced in the Schottky contact investigations by using transferrable graphene sheets, which are known to for a Schottky barrier when adhered to diamond. This, coupled with the sp2-type contact, reveals the possibility for an all-carbon device with high radiation hardness. For the Schottky interface, a boron concentration of four to five magnitudes lower than for the ohmic contact is desirable. Secondary Ion Mass Spectrometry can, to a certain extent, confirm such concentrations, although Cathodoluminescence Spectroscopy is a non-destructive, complimentary and preferred technique should a suitable facility be found.

The test facilities for the devices comprise of MRC Harwell, a 4TBq Co-60 source chamber in Oxfordshire provided for free use, a small in-house Am-241 source used for preliminary testing, and finally several commercial sites such as Sellafield in Cumbria, which will be the first full deployment site and provide test runs for the final prototypes.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R51245X/1 01/10/2017 30/09/2021
1955046 Studentship EP/R51245X/1 01/10/2017 30/09/2021 Gordon MacKenzie
 
Description A gammavoltaic cell design has been successfully found. Stacking two cells in series, it was found to be possible to deliver a "burst" of power that flashed an LED. Stacking three cells in series was able to power a bluetooth transmitter for long enough (~10s) that an ambient environmental measurement could be transmitted to a laptop. This brings us close to the ultimate aim of the project, which is to deploy a gammavoltaic-powered sensor package in a waste store, for greatly increased safety and information resolution.

Work on a single cell, using a synchrotron x-ray source, has shown a fundamental link between the type of photon scattering in the device, and the ability of the device to generate power. This is a novel result for a voltaic device, as solar photovoltaics operate in a regime where only one photoelectric mechanism is present.
Exploitation Route Whilst some research has been done towards gammavoltaics by previous researchers, this is the first demonstration of a gammavoltaic actually powering something. This has been possible through a fundamentally different design approach. Once the results are published, other researchers can use and improve this design methodology to advance the development of gammavoltaics. Installation of such devices in nuclear waste stores, as previously mentioned, will greatly increase the safety of such stores and help to address the legacy waste issue. Other applications of the technology have been noted, such as powering for small satellites and for gamma harvesting inside reactor cores, but these are still highly speculative.
Sectors Energy,Environment

 
Description Synchrotron analysis of diamond gammavoltaics 
Organisation Diamond Light Source
Country United Kingdom 
Sector Private 
PI Contribution Conception of experiment, fabrication of samples, experimental design and running the experiment
Collaborator Contribution Use of equipment, design of experiment, consultation and running the experiment
Impact A much greater understanding of the behaviour of diamond gammavoltaics under irradiation from different photon energies, and the resultant dependency of output power on photon scattering mechanism.
Start Year 2019