The use of ultra-wide bandgap transparent conducting beta gallium oxide (b-Ga2O3) thin films as electrodes in ferroelectric solar cells is reported. In a new material structure for energy applications, we report a solar cell structure (a light absorber sandwiched in between two electrodes – one of them -transparent) which is not constrained by the ShockleyeQueisser limit for open-circuit voltage (Voc) under typical indoor light. The solar blindness of the electrode enables a record-breaking bulk photovoltaic effect (BPE) with white light illumination (general use indoor light). This work opens up the perspective of ferroelectric photovoltaics which are not subject to the Shockley-Queisser limit by bringing into scene solar-blind conducting oxides.

https://doi.org/10.1016/j.mtener.2019.100350

Which the actual critical electrical field of the ultra-wide bandgap semiconductor b-Ga2O3 is? Even that it is usual to find in the literature a given value for the critical field of wide and ultra-wide semi-conductors such as SiC (3 MV/cm), GaN (3.3 MV/cm), b-Ga2O3 (~8 MV/cm) and diamond (10 MV/cm), this value actually depends on intrinsic and extrinsic factors such as the bandgap energy, material residual impurities or introduced dopants. Indeed, it is well known from 1950’s that reducing the residual doping (NB) of the semiconductor layer increases the breakdown voltage capability of a semiconductor media(e.g. as N􀀁3=4 by using the Fulop’s approximation for an abrupt junction). A key limitation is, therefore,he residual donor/acceptor concentration generally found in these materials. We report that doping with amphoteric Zinc a p-type b-Ga2O3 thin films shortens free carrier mean free path (0.37 nm), resulting in the ultra-high critical electrical field of 13.2 MV/cm. Therefore, the critical breakdown field can be, at least, four times larger for the emerging Ga2O3 power semiconductor as compared to SiC and GaN. We further explain these wide-reaching experimental facts by using theoretical approaches based on the impact ionization microscopic theory and thermodynamic calculations.

Materials Today Physics 15 (2020) 100263

Congratulations to  Dr. Zeyu Chi!!

11/07/2023, Versailles

Ultra-wide band gap semiconductors β-Ga2O3 and ZnGa2O4 electronic properties

Abstract:
In context of necessity of accelerating climate-saving actions by improving energy efficiency and reducing power consumption, ultra-wide band gap (Eg > 4.5 eV) β-Ga2O3 and ZnGa2O4 oxides are considered as promising materials for efficient power electronics devices. The objective of my PhD theses work was to investigate electronic properties of β-Ga2O3 and ZnGa2O4 thin films in order to disclose their real potentiality for applications in optoelectronics. By using multiple experimental characterization and analysis techniques for studding point defects the two problems were addressed: the realization/control of n-type and p-type conductivity and estimation/engineering critical electrical fields.

In this work we have demonstrated the first time that Zn doping of β-Ga₂O₃/r-sapphire thin film grown by MOCVD technique can exhibit a long-time stable room-temperature hole conductivity with a conductivity activation energy of around 86 meV. The origin of this level might be attributed then the donor-acceptor complex . (Figure) We believe that this study will add new evidences and will help to break a “taboo” related to the feasibility of room temperature hole conductivity in Ga₂O₃ via traditional growth technique and doping. We hope that we can inspire researchers to further study experimentally the point defects in β-Ga₂O₃.

The spinel group is a growing family of materials with general formulation AB₂X₄ (the X anion typically being a chalcogen like O and S) with many advanced applications for energy. At the time being, the spinel zinc gallate (ZnGa₂O₄) arguably is the ternary ultra-wide bandgap bipolar oxide semiconductor with the largest bandgap (~5eV), making this material very promising for implementations in deep UV optoelectronics and ultra-high power electronics. In this work, we further demonstrate that, exploiting the rich cation coordination possibilities of the spinel chemistry, the ZnGa₂O₄ intrinsic conductivity (and its polarity) can be controlled well over 10 orders of magnitude. p-type and n-type ZnGa₂O₄ epilayers can be grown by tuning the pressure, oxygen flow and cation precursors ratio during metal-organic chemical vapor deposition. A relatively deep acceptor level can be achieved by promoting antisites (Zn_Ga) defects, while up to a (n > 10¹⁹ cm^-3) donor concentration is obtained due to the hybridization of the Zn-O orbitals in the samples grown in Zn-rich conditions. Electrical transport, atomic and optical spectroscopy reveal a free hole valence band conduction (at high temperature) for p-ZnGa₂O₄ while for n-ZnGa₂O₄ a (Mott) variable range hopping (VRH) and negative magnetoresistance phenomena take place, originated from “self-impurity” band located at Ev+ ~3.4 eV. Among arising ultra-wide bandgap semiconductors, spinel ZnGa₂O₄ exhibit unique self-doping capability thus extending its application at the very frontier of current energy optoelectronics.

While there are several n-type transparent semiconductor oxides (TSO) for optoelectronic applications (e.g. LEDs, solar cells or display TFTs), their required p-type counterpart oxides are known to be more challenging. At this time, the n-type TSO with the largest bandgap (Eg=5 eV) is Ga2O3 that holds the promise of extending the light transparency further into the deep ultraviolet. In this work, it is demonstrated that strongly compensated Ga2O3 is also an intrinsic (or native) p-type TSO with the largest bandgap for any reported p-type TSO.

J. Mater. Chem. C, 2019, 7, 10231–10239 | 10235