Gallium oxide (Ga2O3) continues to rapidly develop as promising material platform for next-generation
power electronics adopting wider (ultrawide) bandgap semiconductors beyond SiC and GaN. This is in
large part due to the availability of high quality single crystal substrates and epilayers that have allowed
for exploration of important material properties, processing and growth recipes, and device designs.
While p-type doping remains out of reach, controllable n-type doping has now been demonstrated via a
variety of approaches and dopants, yet has remained challenging in wider-band gap alloys such as those
incorporating Al. One primary concern is the role of cation vacancies, which have been shown to be
favorable in Ga2O3 and can act as compensating centers, as well as form complexes with a variety of
defects. Here we survey the current understanding of these and other native point defects and their
interactions with other common dopants and impurities in Ga2O3, focusing on their potential optical and
electrical consequences from insights gained through first-principles-based calculations employing hybrid
functionals. We discuss how vacancies can influence defect kinetics and how their incorporation may be
influenced by growth and processing steps. These results provide guidance for controlling the
conductivity in Ga2O3 for facilitating next-generation power electronics based on this ultra-wide bandgap
semiconductor.
This work was partially performed under the auspices of the U.S. Department of Energy by Lawrence
Livermore National Laboratory under Contract DE-AC52-07NA27344 and partially supported by LLNL
LDRD funding under Project No. 22-SI-003 and by the Critical Materials Institute, an Energy Innovation
Hub funded by the U.S. DOE, Office of Energy Efficiency and Renewable Energy, Advanced
Manufacturing Office.
The effect of oxygen defects on the gradual degradation rates of power and nonradiative carrier recombination in ~800 nm laser diodes was studied experimentally. While intentional introduction of oxygen at low levels (<5×10^15 cm^-3) was observed to degrade lasing performance prior to aging, no variation in gradual degradation rate of lasing power was observed. This suggests that degradation in these devices is not due to nonradiative recombination at low levels of point defects. Simulation of our data indicates that the power degradation may arise from increased intracavity absorption.
Ga2O3 is an ultrawide bandgap semiconductor material platform for next-generation power electronics, largely owing to commercially-available single-crystal substrates that can be grown via industrially-scalable processes. Transition metals such as Ir, Cr, and Fe incorporated during growth can strongly influence the resulting optical and electrical properties of single crystals, with Fe now the de facto dopant for achieving semi-insulating substrates. Several other transition metals have been shown to exhibit diverse electronic behavior, acting as deep acceptors, deep donors, or even as efficient shallow donors, depending on how they incorporate into the lattice. Here we survey the current understanding of transition metal point defects in Ga2O3, focusing on their potential optical and electrical consequences from insights gained through first-principles-based calculations employing hybrid functionals.
Properties of monoclinic β-Ga2O3 are strongly dependent upon orientation. In the case of optical properties, polarization can reveal differences in optical bandgap and absorptions related to transition metal ions. This phenomenon is known as pleochroism and has been extensively studied in minerology. β-Ga2O3 bulk single crystals doped with Zn, Mn, Cr, or Cu were grown by the Czochralski and vertical gradient freeze methods. Ultraviolet-visible-near infrared spectroscopy and photoluminescence (PL) revealed polarization- and orientation-dependent optical absorptions in β-Ga2O3. Crystals were annealed in reducing and oxidizing environments in an attempt to alter the intensities of absorptions characteristic to the different transition metal oxidation states in a given ligand field. Visible pleochroism was strongest in (001) oriented Mndoped samples as shown by polarized optical microscopy. All samples were electrically insulating, indicative of acceptor doping, aside from Cr-doped samples where Cr acts as a deep donor.
Gallium oxide has emerged as a promising ultrawide-bandgap semiconductor for electronic applications. Part of the attraction of Ga2O3 is its ability to be alloyed with other materials (such as Al2O3) for band-gap engineering or doped with other elements (such as silicon) for modifying its electrical conductivity. But what is still unknown is how these alloying capabilities extend into the orthorhombic phases, or how well the ultrawide-bandgap AlGO alloys can be n-type doped. Here hybrid density functional theory calculations are used to determine the electronic structure of AlGO alloys. Conduction-band offsets of AlGO alloys in the orthorhombic phase are calculated, as are donor ionization energies as a function of Al content. In light of these results, we discuss band engineering and doping strategies in AlGO alloys for electronic device applications.
Point defects are at the heart of the important properties of wide band-gap and oxide semiconductors for power electronics applications, and therefore understanding the details of point defects and their role in determining the properties becomes imperative. Beta-Ga2O3 has received significant attention recently due to its unique advantages, including high breakdown voltage and availability as bulk substrates, which make it a viable candidate for next-generation power device applications. Here we present the first direct microscopic observation of the formation of interstitial-divacancy complexes within beta-Ga2O3 lattice using atomic resolution scanning transmission electron microscopy. We directly observed that cation atoms are present in multiple interstitial sites, and each interstitial atom is paired with two adjacent vacancies. The observed structure of the complexes is consistent with the calculation using density functional theory (DFT), which predicts them to be compensating acceptors. The number of the observed complexes increase as a function of Sn doping concentration, which matches with the increase in the concentration of the trap state at Ec - 2.1 eV measured using deep level optical spectroscopy, which strongly suggests that the defects corresponds to that trap level. Our finding provides new crucial information on the exact origin of the properties of beta-Ga2O3 that has been unobtainable using other methods. The results also provide new important insight on the material’s unique response to the impurity incorporation that can impact their properties, which can ultimately guide the development of growth and doping of new-generation materials for power electronics.
Optical spectroscopy is a powerful approach for detecting defects and impurities in ZnO, an important electronic material. However, knowledge of how common optical signals are linked with defects and impurities is still limited. The Cu-related green luminescence is among the best understood luminescence signals, but theoretical descriptions of Cu-related optical processes have not agreed with experiment. Regarding native defects, assigning observed lines to specific defects has proven very difficult. Using first-principles calculations, we calculate the properties of native defects and impurities in ZnO and their associated optical signals. Oxygen vacancies are predicted to give luminescence peaks lower than 1 eV; while related zinc dangling bonds can lead to luminescence near 2.4 eV. Zinc vacancies lead to luminescence peaks below 2 eV, as do the related oxygen dangling bonds. However, when complexed with hydrogen impurities, zinc vacancies can cause higher-energy transitions, up to 2.3 eV. We also find that the Cu-related green luminescence is related to a (+/0) deep donor transition level.
Alloying of CdZnTe (CZT) with selenium has been found to be very promising and effective in reducing the overall concentration of secondary phases (Te precipitates/inclusions) and sub-grain boundary networks in the crystals. These two types of defects are the main causes for incomplete charge collection, and hence they affect the yield of high-quality CZT, resulting in a very high cost for large-volume, high-quality detector-grade CZT detectors. The addition of selenium was also found to very effective in increasing the compositional homogeneity along the growth direction of the CdZnTeSe (CZTS) ingots grown by the traveling heater method (THM) technique. The compositional homogeneity along the growth direction can enhance the overall yield of detector-grade CZTS, which should therefore be possible to produce at a lower cost compared to CZT. The electrical properties and detector performance of the CZTS crystals will be presented and discussed.
KEYWORDS: Interfaces, Copper indium gallium selenide, Crystals, Thin films, Process control, Thin film solar cells, Manufacturing, Thin film manufacturing, Thin film devices, Control systems
Optimizing the buffer layer in manufactured thin-film PV is essential to maximize device efficiency. Here, we describe a combined synthesis, characterization, and theory effort to design optimal buffers based on the (Cd,Zn)(O,S) alloy system for CIGS devices. Optimization of buffer composition and absorber/buffer interface properties in light of several competing requirements for maximum device efficiency were performed, along with process variations to control the film and interface quality. The most relevant buffer properties controlling performance include band gap, conduction band offset with absorber, dopability, interface quality, and film crystallinity. Control of an all-PVD deposition process enabled variation of buffer composition, crystallinity, doping, and quality of the absorber/buffer interface. Analytical electron microscopy was used to characterize the film composition and morphology, while hybrid density functional theory was used to predict optimal compositions and growth parameters based on computed material properties.
Process variations were developed to produce layers with controlled crystallinity, varying from amorphous to fully epitaxial, depending primarily on oxygen content. Elemental intermixing between buffer and absorber, particularly involving Cd and Cu, also is controlled and significantly affects device performance. Secondary phase formation at the interface is observed for some conditions and may be detrimental depending on the morphology. Theoretical calculations suggest optimal composition ranges for the buffer based on a suite of computed properties and drive process optimizations connected with observed film properties.
Prepared by LLNL under Contract DE-AC52-07NA27344.
Advances in thin-film photovoltaics have largely focused on modifying the absorber layer(s), while the choices for other layers in the solar cell stack have remained somewhat limited. In particular, cadmium sulfide (CdS) is widely used as the buffer layer in typical record devices utilizing absorbers like Cu(In,Ga)Se2 (CIGSe) or Cu2ZnSnS4 (CZTS) despite leading to a loss of solar photocurrent due to its band gap of 2.4 eV. While different buffers such as Zn(S,O,OH) are beginning to become competitive with CdS, the identification of additional wider-band gap alternatives with electrical properties comparable to or better than CdS is highly desirable.
Here we use hybrid density functional calculations to characterize CdxZn1-xOyS1-y candidate buffer layers in the quaternary phase space composed by Cd, Zn, O, and S. We focus on the band gaps and band offsets of the alloys to assess strategies for improving absorption losses from conventional CdS buffers while maintaining similar conduction band offsets known to facilitate good device performance. We also consider additional criteria such as lattice matching to identify regions in the composition space that may provide improved epitaxy to CIGSe and CZTS absorbers. Lastly, we incorporate our calculated alloy properties into device model simulations of typical CIGSe devices to identify the CdxZn1-xOyS1-y buffer compositions that lead to the best performance.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Department of Energy office of Energy Efficiency and Renewable Energy (EERE) through the SunShot Bridging Research Interactions through collaborative Development Grants in Energy (BRIDGE) program.
Titanium dioxide is a versatile material with ubiquitous applications, many of which are critically linked to either light absorption or transparency in the visible spectral range in addition to electrical conductivity. Doping is a well-known way to influence those properties in order to bring them into a desired range. Working towards a comprehensive understanding of the electronic and optical properties of TiO2 (as well as of the link between them) we review and summarize electronicstructure results that we obtained using cutting-edge theoretical spectroscopy techniques. We focus on the formation of electron and hole polarons and we elucidate the influence of doping on the optical properties of TiO2. In addition, we present new results for the reflectivity of pure TiO2.
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