【International Papers】κ-Ga₂O₃/(B)GaAs/GaAs Heterostructures: Study of Optically Active Defects, Design, and Modeling of Solar Cells Based on These Heterostructures

      Researchers from the ACS Omega have published a dissertation titled "κ-Ga₂O₃/(B)GaAs/GaAs Heterostructures: Study of Optically Active Defects, Design, and Modeling of Solar Cells Based on These Heterostructures" in ACS Omega.

Background

      The global shift toward renewable energy has driven intense research into advanced photovoltaic (PV) technologies capable of outperforming traditional single-junction solar cells (SCs). Among the most promising approaches, there is the intermediate band solar cell (IBSC), a concept proposed to overcome the Shockley–Queisser limit by enabling the absorption of low-energy (sub-bandgap) photons via an intermediate energy level forming an intermediate band (IB) positioned within the host semiconductor bandgap. This IB allows for two-step photon absorption: from the valence band (VB) to the IB, and from the IB to the conduction band (CB), thereby enhancing photocurrent generation while preserving high output voltage. Hence, the realization of efficient IBSCs requires advanced material systems with engineered band structures and tailored optical and electronic properties. A particularly attractive platform to achieve IBs involves wide/narrow bandgap heterostructures. The IB formation might be achieved at the interface via quantum confinement, defect states, or impurity-induced band anticrossing effects. In this study, we present a detailed investigation of the emission characteristics of such heterostructures through temperature-dependent PL and e-beam-energy-dependent CL to highlight the defect-assisted and radiative recombination mechanisms. Furthermore, the device performance of Ga₂O₃/GaAs and Ga₂O₃/BGaAs/GaAs heterostructures is predicted, specifically looking at their application in IBSCs. Band alignment and energy level formation at the interfaces, quantum efficiency (QE), and J–V characteristics under standard AM1.5 illumination are simulated using SCAPS-1D. This work aims at establishing the experimental and theoretical bases for the fabrication of Ga₂O₃-based IBSC architectures and offers novel insights into the role of oxide/III–V interfaces and boron incorporation in enabling high-performance SCs.

Abstract

      This work focuses on the steady-state luminescence of pure-phase κ-Ga₂O₃/GaAs and κ-Ga₂O₃/BGaAs/GaAs heterostructures grown by metal–organic chemical vapor deposition. The films were characterized by electron beam (e-beam) energy-dependent cathodoluminescence (CL) and steady-state photoluminescence (PL), supported by one-dimensional Solar Cell Capacitance Simulator (SCAPS-1D) simulation, to assess their possible application as intermediate energy band layers in solar cells. The power- and temperature-dependent PL and CL show that the luminescence in κ-Ga₂O₃/GaAs and κ-Ga₂O₃/a-BGaO/BGaAs/GaAs heterostructures, where a-BGaO represents a thin amorphous BGaO interlayer between the κ-Ga₂O₃ film and BGaAs template, is dominated by donor–acceptor transitions. These transitions originated from point defect ensembles tuned by the disorder at the interface, which give rise to the formation of minibands. The boron-to-gallium substitution, boron segregation, gallium diffusion, and point defects clustering at the interface are supposed to play a crucial role in the luminescence mechanisms. The heterojunction features are promising in view of the development of a new class of active layers in photodetectors working in the visible and infrared regions. We propose a novel solar cell structure in which the insertion of a BGaAs/a-BGaO interlayer between GaAs and κ-Ga₂O₃ may substantially enhance the solar cell parameters. The SCAPS-1D simulations suggest that the κ-Ga₂O₃/a-BGaO/BGaAs/GaAs heterostructure may reach a power conversion efficiency (PCE) of 23.76% with an open-circuit voltage (V_oc) of 0.92 V, a short-circuit current density (J_sc) of 32.61 mA/cm², and a fill factor (FF) of 78.66%.

Conclusion

      κ-Ga₂O₃/GaAs and κ-Ga₂O₃/BGaAs/GaAs heterostructures, prepared by MOCVD, were characterized by PL and CL. Low-temperature PL measurements of κ-Ga₂O₃/GaAs structure evidenced a point defect-related luminescence, a broadening, and a red shift of the buried GaAs layer with respect to pristine GaAs. This is believed to be related to the diffusion of Ga atoms from the top κ-Ga₂O₃ layer into the GaAs substrate, which reduces the vacancy density and generates Ga_As acceptors.

      PL and CL measurements show that luminescence in buried GaAs is not of an excitonic nature and more likely dominated by donor-to-acceptor transitions related to V_Ga that form PDEs distributed in the bandgap. The observations made on κ-Ga₂O₃/GaAs are useful to understand the luminescence mechanisms in the κ-Ga₂O₃/BGaAs/GaAs heterostructure. In the latter case, the CL intensity is double in comparison to that of κ-Ga₂O₃/GaAs, with the emission tuned by PDEs forming band-tail states.

      Based on the properties of the κ-Ga₂O₃/BGaAs/GaAs heterostructure, we proposed a SC prototype that includes this material stack. The simulation of the TCO/Ga₂O₃/BGaAs/GaAs/Au prototype suggests a total QE of ∼90%, six times higher than TCO/Ga₂O₃/GaAs/Au and a broadened response ranging from the deep UV to the infrared spectrum (200–1000 nm). The insertion of a BGaAs interlayer in the SC suggests a PCE of 23.76%, V_oc of 0.92 V, J_sc of 32.61 mA/cm², and an FF of 78.66%. This preliminary demonstration of the κ-Ga₂O₃/(B)GaAs/GaAs heterostructure is expected to pave the way to functional and physical integration of Ga₂O₃ with other materials. The fabrication of fully functional devices and their testing, including accelerated aging, thermal cycling, electrical characterization after prolonged illumination, and bias stress tests, combined with structural and compositional analyses, are necessary in order to quantify degradation mechanisms and confirm the long-term stability of the heterostructures.