Surface morphology and electronic structure of bulk single crystal β-Ga2O3(100)

Lovejoy, Tracy C.; Yitamben, E. N.; Shamir, N.; Morales, J.; Vı́llora, Encarnación G.; Shimamura, Kiyoshi; Zheng, Shuzhan; Ohuchi, F. S.; Olmstead, Marjorie A.

doi:10.1063/1.3086392

Cited by 60 publications

(36 citation statements)

References 18 publications

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“…This behaviour was explained in terms of an electron accumulation layer model of the Ga 2 O 3 surface [144]. The presence of an electron accumulation is also suggested by recent photoemission data from Ga 2 O 3 , where the surface Fermi level can be seen to be approximately 4.5 eV above valence band maximum, close to the conduction band minimum [145].…”

Section: Surface Conductivitymentioning

confidence: 70%

Conductivity in transparent oxide semiconductors

King

Veal

2011

J. Phys.: Condens. Matter

225

180

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Abstract. Despite an extensive research effort for over 60 years, an understanding of the origins of conductivity in wide band-gap transparent conducting oxide (TCO) semiconductors remains elusive. While TCOs have already found widespread use in device applications requiring a transparent contact, there are currently enormous efforts to (i) increase the conductivity of existing materials, (ii) identify suitable alternatives, and (iii) attempt to gain semiconductor-engineering levels of control over their carrier density, essential for the incorporation of TCOs into a new generation of multifunctional transparent electronic devices. These efforts, however, are dependent on a microscopic identification of the defects and impurities leading to the high unintentional carrier densities present in these materials. Here, we review recent developments towards such an understanding. While oxygen vacancies are commonly assumed to be the source of the conductivity, there is increasing evidence that this is not a sufficient mechanism to explain the total measured carrier concentrations. In fact, many studies suggest that oxygen vacancies are deep, rather than shallow, donors, and their abundance in as-grown material is also debated. We discuss other potential contributions to the conductivity in TCOs, including other native defects, their complexes, and in particular hydrogen impurities. Convincing theoretical and experimental evidence is presented for the donor nature of hydrogen across a range of TCO materials, and while its stability and the presence of interstitial and substitutional species are still somewhat open questions, it is one of the leading contenders for unintentional conductivity in TCOs. We also review recent work indicating that the surfaces of TCOs can support very high carrier densities, opposite to the case of conventional semiconductors. In thin-film materials/devices and, in particular, nanostructures, the surface can have a large impact on the total conductivity in TCOs. We discuss models that attempt to explain both the bulk and surface conductivity based on features of the bulk band structure common across the TCOs, and compare these materials to other semiconductors. Finally, we briefly consider transparency in these materials, and its interplay with conductivity. Understanding this interplay, as well as the microscopic contenders for the conductivity of these materials, will prove essential to the future design and control of TCO semiconductors, and their implementation into novel multifunctional devices.

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Section: Surface Conductivitymentioning

confidence: 70%

Conductivity in transparent oxide semiconductors

King

Veal

2011

J. Phys.: Condens. Matter

225

180

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“…Due to the very large band gap, it has an estimated breakdown field of ≈6–8 MV cm −1 and high transparency in the deep ultraviolet (UV) and visible wavelength region. Another advantage lies in the feasibility to mass produce high quality bulk Ga 2 O 3 at low cost using melting growth techniques such as edge defined film‐fed growth (EFG), floating zone (FZ), and Czochralski methods. Ga 2 O 3 can also be used as a conductive and transparent substrate for GaN based light emitting diodes (LEDs).…”

Section: Introductionmentioning

confidence: 99%

Towards High‐Mobility Heteroepitaxial β‐Ga₂O₃ on Sapphire − Dependence on The Substrate Off‐Axis Angle

Rafique

Han

Neal

et al. 2017

Physica Status Solidi (a)

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This paper presents the heteroepitaxial growth of β‐Ga2O3 thin films on off‐axis (0001) c‐sapphire substrates by low pressure chemical vapor deposition (LPCVD). (−201) oriented β‐Ga2O3 thin films are grown using high purity metallic gallium (Ga) and oxygen (O2) as the precursors. N‐type conductivity in silicon doped β‐Ga2O3 thin films is demonstrated. It is found that the film crystalline quality, surface morphology, and electrical conductivity are remarkably sensitive to the off‐axis angles. X‐ray phi‐scan measurements of the β‐Ga2O3 film grown on on‐axis c‐sapphire indicate the presence of six in‐plane rotational domains due to the substrate symmetry. With the increase of off‐axis angle toward <11–20> of sapphire, one of the in‐plane orientations is strongly favored. The use of off‐axis substrate also reduced the X‐ray rocking curve full width at half maximum and increased the intensities of the Raman peaks. The best electrical properties of the β‐Ga2O3 film are exhibited by the film grown on 6° off‐axis c‐sapphire. The room temperature electron Hall mobility was 106.6 cm2 V−1 s−1 with an n‐type carrier concentration of 4.83 × 1017 cm−3. The results from this study demonstrate high electrical quality β‐Ga2O3 thin films grown on off‐axis c‐sapphire substrates, which are promising for high power electronic and short wavelength optoelectronic device applications.

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“…Figure 3(a) shows several defects (dark in this empty state image) on an atomically flat terrace with "B" termination; the regularly spaced diagonal rows correspond to the atomic corrugation in the c direction. 8 Figure 3(b) shows an apparent-height profile through two defects along the line between the arrows in (a). The defects cause $ 0:4 Å deep, 1.3 nm wide depressions centered on the flat, regular atomic rows.…”

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confidence: 96%

Band bending and surface defects in β-Ga2O3

Lovejoy

Chen

Zheng

et al. 2012

Applied Physics Letters

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Surface band bending and surface defects on the UV-transparent conducting oxide β-Ga2O3 (100) are studied with hard x-ray photoemission spectroscopy and scanning tunneling microscopy. Highly doped β-Ga2O3 shows flat bands near the surface, while the bands on nominally undoped (but still n-type), air-cleaved β-Ga2O3 are bent upwards by ≳0.5 eV. Negatively charged surface defects are observed on vacuum annealed β-Ga2O3, which also shows upward band bending. Density functional calculations show oxygen vacancies are not likely to be ionized in the bulk, but could be activated by surface band bending. The large band bending may also hinder formation of ohmic contacts.

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Surface morphology and electronic structure of bulk single crystal β-Ga2O3(100)

Cited by 60 publications

References 18 publications

Conductivity in transparent oxide semiconductors

Conductivity in transparent oxide semiconductors

Towards High‐Mobility Heteroepitaxial β‐Ga₂O₃ on Sapphire − Dependence on The Substrate Off‐Axis Angle

Band bending and surface defects in β-Ga2O3

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Surface morphology and electronic structure of bulk single crystal β-Ga2O3(100)

Cited by 60 publications

References 18 publications

Conductivity in transparent oxide semiconductors

Conductivity in transparent oxide semiconductors

Towards High‐Mobility Heteroepitaxial β‐Ga2O3 on Sapphire − Dependence on The Substrate Off‐Axis Angle

Band bending and surface defects in β-Ga2O3

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Towards High‐Mobility Heteroepitaxial β‐Ga₂O₃ on Sapphire − Dependence on The Substrate Off‐Axis Angle