Oxygen vacancies and donor impurities in β-Ga2O3

Abstract

Using hybrid functionals we have investigated the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3. We find that oxygen vacancies are deep donors, and thus cannot explain the unintentional n-type conductivity. Instead, we attribute the conductivity to common background impurities such as silicon and hydrogen. Monatomic hydrogen has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors.

Deep level defects in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy

“…To date, there have been some theoretical calculations that predict states near these energy values based on the various oxygen and gallium vacancy related defects of several charge states and configurations but more work must be carried out in conjunction with careful experiments to elucidate further. 16,42,[45][46][47] The remaining states at E C À 1.27 eV (detected in trace amounts by DLOS), and the set of levels at E C À 0.18 eV À 0.21 eV found by DLTS, were not seen in the earlier EFG studied material. The role of vastly different growth methods, or the use of a different dopant could be responsible for these differences, and this would not be surprising.…”

“…Theoretical calculations have predicted the presence of vacancy-related energy levels near E C À 0.8 eV, though there is some controversy regarding the specific vacancy type (oxygen or gallium) in those works. 16,45,46 If indeed this is a vacancy-related state, it would not be surprising given that the different thermodynamic conditions of PAMBE vs. melt-based bulk crystal growth would generate different types and concentrations of Ga and O vacancies. However, a recent study has theoretically predicted that Fe Ga substitutional impurities can also manifest as a state near E C À 0.8 eV, and indeed an experimental correlation between the Fe concentration and the concentration of this trap within bulk substrate materials was observed along with an insensitivity to high energy particle irradiation that corroborates an extrinsic source.…”

“…17 In addition, Ge 4þ is predicted to preferentially incorporate on the tetrahedral Ga(I) site, making it a potentially more effective dopant than Sn 4þ which favors the octahedrally coordinated Ga(II) site. 3,16 Moreover, the much higher vapor pressure of Ge oxides compared with Si oxides avoids source oxidation, 17,18 and a wide range of controllable electron concentrations (10 16 -10 20 cm À3 ) for Ge-doped b-Ga 2 O 3 has been demonstrated. 19 Coupling these observations with the successful demonstrations of Ge-doped PAMBE-grown b-Ga 2 O 3 MOSFETs 3 and (Al 1-x Ga x ) 2 O 3 /Ga 2 O 3 MODFETs 14 implies that Ge might become the dopant of choice for bGa 2 O 3 devices.…”

“…Promising n-type dopants for b-Ga 2 O 3 are Si, Ge, and Sn, since each is predicted to substitute on Ga sites. 16 For Si-doping, growth of uniformly doped thick layers by PAMBE is challenged by Si source oxidation in the oxygen plasma environment, creating a self-limiting process. 13 The other two candidates, Ge 4þ and Sn 4þ , are identified as a better match with Ga 3þ cationic size.…”

Ultrawide‐Bandgap Semiconductors: Research Opportunities and Challenges

“…It can be anticipated that the combination of a very heavy valence band effective mass and a strong Fröhlich interaction will lead to formation of polarons with very low conductivity. [162] In β-Ga 2 O 3 -based alloys and heterostructures, transport is completely unexplored. 2D electron gases in heterostructures are yet to be created in this material system, though this is expected to happen as the material matures.…”