GeSn films were simultaneously deposited on Si (100), Si (111), c-plane sapphire (Al2O3), and fused silica substrates to investigate the impact of the substrate on the resulting GeSn film. The electronic, structural, and optical properties of these films were characterized by temperature-dependent Hall-effect measurements, x-ray diffractometry, secondary ion mass spectrometry, and variable angle spectroscopic ellipsometry. All films were polycrystalline with varying degrees of texturing. The film on Si (100) contained only GeSn (100) grains, 40.4 nm in diameter. The film deposited on Si (111) contained primarily GeSn (111) grains, 36.4 nm in diameter. Both films deposited on silicon substrates were fully relaxed. The layer deposited on Al2O3 contained primarily GeSn (111) grains, 41.3 nm in diameter. The film deposited on fused silica was not textured, and the average grain size was 35.0 nm. All films contained ∼5.6 at. % Sn throughout the layer, except for the film deposited on Al2O3, which contained 7.5% Sn. The films deposited on Si (111), Al2O3, and fused silica exhibit p-type conduction over the entire temperature range, 10–325 K, while the layer deposited on the Si (100) substrate shows a mixed conduction transition from p-type at low temperature to n-type above 220 K. From ∼175 to 260 K, both holes and electrons contribute to conduction. Texturing of the GeSn film on Si (100) was the only characteristic that set this film apart from the other three films, suggesting that something related to GeSn (100) crystal orientation causes this transition from p- to n-type conduction.

The transport properties of W-doped thermochromic V1-xWxO2 (x=0 and 0.0074) thin films prepared by pulsed laser deposition were studied to understand the effect of doping on the electrical properties of these films. Temperature dependent magneto-transport measurements (Hall effect) in magnetic fields up to 9 Tesla were performed on thin film vanadium dioxide (VO2) across the Mott metal-insulator transition (MIT). The Hall carrier density increases by 4 orders of magnitude at MIT. The Hall mobility varies little across the MIT and remains low at ~ 0.05 cm2 /V sec. The majority carriers are electrons. Magneto-resistance is small and positive. Comparison of the three Hall parameters including carrier concentration, conductivity and mobility between various doping levels on both metallic and insulating state are reported and a model has been proposed. A correlation between carrier concentration and conductivity of VO2 films is observed but doesn’t exist between carrier concentration and mobility.

Disorder arising from random locations of charged donors and acceptors introduces localization and diffusive motion that can lead to constructive electron interference and positive magnetoconductivity. At very low temperatures, 3D theory predicts that the magnetoconductivity is independent of temperature or material properties, as verified for many combinations of thin-films and substrates. Here, we find that this prediction is apparently violated if the film thickness d is less than about 300 nm. To investigate the origin of this apparent violation, the magnetoconductivity was measured at temperatures T = 15 – 150 K in ten, Sn-doped In 2 O 3 films with d = 13 – 292 nm, grown by pulsed laser deposition on fused silica. We observe a very strong thickness dependence which we explain by introducing a theory that postulates a second source of disorder, namely, non-uniform interface-induced defects whose number decreases exponentially with the interface distance. This theory obeys the 3D limit for the thickest samples and yields a natural figure of merit for interface disorder. It can be applied to any degenerate semiconductor film on any semi-insulating substrate.

ZnGa2O4 is a ultra-wide band bandgap transparent oxide with electron transport properties similar to those of the popular b-Ga2O3 but with a higher lattice symmetry. Recent experimental work has been producing high-quality ZnGa2O4 crystals. Here we present our ongoing first-principles modeling work on the structural, thermal, and electronic properties of ZnGa2O4. Elastic stiffness tensor modeling based on symmetry-allowed deformations provides an insight into the effect of symmetry on lattice dynamical properties. We obtain thermodynamical properties such as thermal expansion in the quasiharmonic approximation based on phonons from finite-displacement supercell approach. The phonon dispersions and density of states are compared to those of b-Ga2O3. We find a large number of optical phonons at low energies <15 meV but with higher symmetry by comparison to b-Ga2O3. The large number of optical modes has a signature both in quantum magnetoconductance measurements and in the breakdown field, the latter being a metric relevant for applications in power electronics.

Conductive homoepitaxial Si-doped β-Ga2O3 films were fabricated by pulsed laser deposition with an as-deposited 2323 S cm−1 conductivity (resistivity = 4.3 × 10−4 Ω-cm, carrier concentration = 2.24 × 1020 cm−3, mobility = 64.5 cm2 V−1 s−1, and electrical activation efficiency = 77%). High quality homoepitaxial films deposited on commercial (010) Fe-compensated β-Ga2O substrates were determined by high-resolution transmission electron microscopy and x-ray diffraction. The β-Ga2O3 films have ∼70% transparency from 3.7 eV (335 nm) to 0.56 eV (2214 nm). The combination of high conductivity and transparency offers promise for numerous ultrawide bandgap electronics and optoelectronic applications.

The gallium vacancy is one of the dominant native point defects in β-Ga2O3, one that, together with its complexes, can have a major effect on free carrier densities and transport in this wide bandgap semiconductor. We used a combination of depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to identify the optical and energy-level properties of these defects as well as how their defect densities and spatial distributions vary with neutron irradiation and temperature-dependent-forming gas anneals. These studies reveal optical signatures that align closely with theoretical energy-level predictions. Likewise, our optical techniques reveal variations in these defect densities that are consistent with hydrogen passivation of gallium vacancies as a function of temperature and depth from the free Ga2O3 surface. These techniques can help guide the understanding and control of dominant native point defects in Ga2O3.

Quantum magnetoconductance, delta-sigma(B)=sigma(B)-sigma(0), offers a new, unique way to study phonons in heavily-doped, complex semiconductors, including beta-Ga2O3 and Sn-doped In2O3 (ITO). At low temperature, theory predicts delta-sigma(B)=2.908B^(1/2) S/cm, shown to be true for thick, but not thin, samples. We grew ten ITO films by PLD on fused silica, d=13–292 nm. The thickness dependence was explained by a new delta-sigma(d)-vs-d theory based on a second source of disorder, interface-generated defects that decrease exponentially with distance from the ITO/FS interface. A fit of delta-sigma(d)-vs-d gives three parameters, including d*, the thickness above which the surface is not affected by interface damage.

In this study, the growth of scandium nitride (100) single crystals with high electron mobility and high thermal conductivity was demonstrated by physical vapor transport (PVT). Single crystals were grown in the temperature range of 1900 °C–2140 °C under a nitrogen pressure between 15 and 20 Torr. Single crystal tungsten (100) was used as a nearly lattice constant matched seed crystal. Growth for 20 days resulted in a 2 mm thick crystal. Hall-effect measurements revealed that the layers were n-type with a 300 K electron concentration and a mobility of 2.17 × 1021 cm−3 and 73 cm2/V s, respectively. Consequently, this ScN crystal had a low electrical resistivity, 3.94 × 10−5 Ω cm. The thermal conductivity was in the range of 51–56 W/m K, three times higher than those in previous reports for ScN thin films. This study demonstrates the viability of the PVT crystal growth method for producing high quality bulk scandium nitride single crystals.

Recent developments in growing highly n-doped wide bandgap oxides such as beta-gallium oxide (beta - Ga2O3) and more recently zinc gallate (ZnGa2O4) have opened avenues toward important applications, such as transparent electrodes and ohmic contacts. Magnetoconductivity measurements provide a unique method to assess the contribution of phonons to mobility over a wide range of temperatures. For beta - Ga2O3 and ZnGa2O4, initial attempts to interpret the measured magnetoconductivity raised fundamental questions about the interplay between the large number of phonon modes in these lattices, electron-phonon scattering, and lattice disorder. Here, we use density functional theory modeling of electron-phonon scattering to help rationalize magnetoconductivity measurements for a wide range of electron concentrations n and temperatures in beta - Ga2O3 and ZnGa2O4. The results provide a first-principles understanding of dominant low-field mobility features suggested by phenomenological models used traditionally for semiconductors with high lattice symmetry. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

We study the electrical and optical properties of degenerate ZnGa2O4 films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire and semi-insulating films grown by pulsed laser deposition (PLD) on fused silica. After a forming-gas anneal at 700 °C, the MOCVD film is highly conducting, with a room-temperature carrier concentration of 2 × 1020 cm−3, a mobility of 20 cm2/V s, and direct bandgap absorptions at 3.65 eV and 4.60 eV. Under the same annealing conditions, the PLD film is semi-insulating, with a direct bandgap absorption at 5.25 eV. The phonon structure, important for electrical and thermal conduction as well as superconductivity and other quantum phenomena, is very complicated due to the large number of atoms (and, thus, phonon branches) in the unit cell. However, we show that the phonon contributions to electron mobility (μph) can be directly measured by quantum-based magnetoconductivity over the temperature span T = 10–200 K. From an approximate analytical formula, μph = function (Tph, T), we calculate an effective phonon energy kTph(T) that takes account of all phonon contributions at temperature T. For T = 10–200 K, the value of kTph ranges from about 10 to 90 meV, consistent with the energy range of the ZnGa2O4 phonon density of states (at 0 K) calculated by density functional theory. The total measured mobility can then be modeled by μtot−1 = μii−1 + μph−1, where μii is the mobility due to ionized-impurity scattering. With a high bandgap, controllable conductivity, high breakdown voltage, and bulk-growth capability, ZnGa2O4 offers opportunities for high-power electronics and UV detectors.

We used depth-resolved cathodoluminescence spectroscopy (DRCLS), absorption spectroscopy, and temperature-dependent Hall effect (TDH) measurements to study the effects of fluence dependent neutron irradiations on deep level defects and the associated changes of electrical properties of β-Ga2O3 grown by low pressure chemical vapor deposition and pulsed laser deposition. DRCLS enabled us to monitor systematic increases of three deep level defects after neutron irradiation which correlated with TDH measurements of significant free carrier removal and mobility decrease. The correlations between defect profiles and electrical property changes vs. irradiation dose link these dominant electrically active native point defects in Ga2O3 with their contributions to free carrier mobility, carrier density, and donor/acceptor depth profiles, further revealing their donor/acceptor electrical behavior and physical nature, consistent with the formation of compensating defects. After irradiation, temperature-dependent forming gas (FG) anneals were performed to reverse the radiation-induced damage and carrier removal. The evolution of defect concentrations with increasing neutron dose and their depth-resolved distributions with FG anneal temperature reveal an interplay between specific defects to control electronic properties.

InGaZnO (IGZO) is an excellent semiconductor material for thin-film transistors (TFTs) used in direct-current and radio-frequency (RF) switching applications, especially since it can be grown at low temperatures on a wide variety of substrates. IGZO thin films with a composition of InGaZnO4 have been deposited and used as channel layers in TFTs for many applications to date; however, IGZO compositions can also be easily changed to vary their properties. These different compositions of IGZO may have different defect properties. In this study, we report the growth of IGZO with composition of In2O3:Ga2O3:5ZnO (In2Ga2Zn5O11) by pulsed laser deposition (PLD) and its electronic defects studied by thermally stimulated current (TSC) spectroscopy. It was found that the as-grown sample has a DC activation energy of 0.62 eV, and four major traps with activation energies between similar to 0.16-0.50 eV and one at similar to 0.90 eV. Electrical properties and the deep traps of PLD-grown IGZO thin film were found to be varied by post-processing conditions such as thermal history and measurement conditions such as the bias and light exposure applied to the samples. The instabilities induced by these conditions can be explained by structural and stoichiometric features-the ZnO4 tetrahedra and GaO6 octahedra in the structure may become distorted, caused by oxidation or reduction, so that the different defect states could be changed and/or lattice energy variations from the distortion can be observed. This work demonstrates that current-based trap emission, such as that associated with TSC, can effectively reveal electronic defects in highly-resistive semiconductor materials, especially those that are not amenable to capacitance-based techniques, such as deep-level transient spectroscopy, and provide an effective manner to study the trap instabilities in IGZO.

While β-Ga2O3 has recently attracted a lot of attention for applications in UV detection and high-power electronics as an ultrawide-band gap semiconductor with a direct band gap Eg ≈ 4.8 eV, it has been also found that crystalline β-Ga2O3 doped with trivalence titanium (Ti3+:β-Ga2O3) is a potential material for ultrafast and tunable laser applications. As a semiconductor and as a laser medium, the optical and electrical properties are critical to its broad applications. In this paper we report the optical and electrical properties of intentionally, Ti-doped β‐Ga2O3 bulk crystals (TiGaO) grown by the float-zone method in comparison with their counterpart: the unintentionally doped ones. The optical and electrical properties of TiGaO crystals, both the as-grown and those annealed in 600 °C and 1000 °C, were investigated by transmittance and reflectance spectroscopy and by thermally stimulated current (TSC) spectroscopy. The optical constants were determined by our approximation-free approach, and the photocurrents (PCs) and electronic defects of the crystals due to titanium doping were analyzed and compared. We measured reflectance and transmittance on the double-side polished samples, and then derived closed-form expressions for the absorption and reflection coefficients, α and R, respectively, in terms of measured reflectance and transmittance, Rm and Tm. The real (η) and imaginary (κ) parts of the index of refraction (n = η + iκ) can be also obtained from α and R. Then, we used TSC spectroscopy to study the electrical properties and deep level defects in TiGaO crystals. The Ti-doped TiGaO crystals are resistive but annealing further increases the resistivity and shrinks slightly the bandgap. The as-grown samples have little photoresponse but after annealing they show negative PCs upon UV illumination, indicating annealing generates multiple recombination centers in the crystals. Nevertheless, they all have a dark current activation energy of 1.01 eV, which might be attributed to titanium doping that is different to the undoped ones which usually have an activation energy of 0.84 eV attributed to a trace amount of iron as is reported in literature.

The conductivity σ, quantum-based magnetoconductivity Δσ = σ(B) − σ(0), and Hall coefficient RH (= µH/σ) of degenerate, homoepitaxial, (010) Si-doped β-Ga2O3, have been measured over a temperature range T = 9–320 K and magnetic field range B = 0–10 kG. With ten atoms in the unit cell, the normal-mode phonon structure of β-Ga2O3 is very complex, with optical-phonon energies ranging from kTpo ~ 20–100 meV. For heavily doped samples, the phonon spectrum is further modified by doping disorder. We explore the possibility of developing a single function Tpo(T) that can be incorporated into both quantum and classical scattering theory such that Δσ vs B, Δσ vs T, and µH vs T are all well fitted. Surprisingly, a relatively simple function, Tpo(T) = 1.6 × 103{1 − exp[−(T + 1)/170]} K, works well for β-Ga2O3 without any additional fitting parameters. In contrast, Δσ vs T in degenerate ScN, which has only one optical phonon branch, is well fitted with a constant Tpo = 550 K. These results indicate that quantum conductivity enables an understanding of classical conductivity in disordered, multi-phonon semiconductors.

With a high bandgap of 4.7 eV, β-Ga2O3 can be made semi-insulating by doping with Fe or Mg and thereby possesses a very high breakdown field necessary for high-powered switches. Somewhat surprisingly, β-Ga2O3 can also be made highly conductive by doping with Si, which leads to great potential for n+ohmic contacts and transparent current spreading layers. In the latter application, the goal is to achieve both high conductivity (high concentration n and mobility μ) and high transparency in the visible and UV regions. Recently we have achieved n = 2 x 1020 cm-3 in β-Ga2O3, using pulsed laser deposition (PLD) with a Ga2O3 target containing 1-wt%-SiO2. Although n is temperature-independent, µ is not, and by fitting µ vs T, we can determine donor ND and acceptor NA concentrations. However, at higher temperatures, µ is strongly affected by longitudinal optical (LO) phonon scattering, which is much more complicated to model in Ga2O3 (9 LO phonons) than in ZnO, GaN, and other binary semiconductors (1 LO phonon). Highly-doped samples have another complication, disorder in the dopant placement. Fortunately, this disorder leads to small quantum corrections delta sigma in the conductivity which are also affected by LO phonons. Indeed, the study of delta sigma vs T and vs magnetic field B at low temperatures is crucial in understanding mobility at 300 K. We demonstrate calculations of ND and NA in PLD-grown β-Ga2O3 under the assumption that the dominant acceptor is the Ga vacancy in various charge states.

This review presents recent research advances in measuring native point defects in ZnO nanostructures, establishing how these defects affect nanoscale electronic properties, and developing new techniques to manipulate these defects to control nano- and micro- wire electronic properties. From spatially-resolved cathodoluminescence spectroscopy, we now know that electrically-active native point defects are present inside, as well as at the surfaces of, ZnO and other semiconductor nanostructures. These defects within nanowires and at their metal interfaces can dominate electrical contact properties, yet they are sensitive to manipulation by chemical interactions, energy beams, as well as applied electrical fields. Non-uniform defect distributions are common among semiconductors, and their effects are magnified in semiconductor nanostructures so that their electronic effects are significant. The ability to measure native point defects directly on a nanoscale and manipulate their spatial distributions by multiple techniques presents exciting possibilities for future ZnO nanoscale electronics.