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Photoreflectance (PR) and microwave-reflectance photoconductivity-decay (μ-PCD) have been applied to study the surface band bending in Mg-doped InN layers with various Mg concentrations (∼1 × 1017 to ∼7 × 1019 cm−3) and thicknesses (∼400 to ∼3600 nm). A PR resonance related to the band-to-band transition at the Γ point below the surface electron accumulation is observed for moderately doped samples ([Mg]: 1.3 × 1017 cm−3 to 8.7 × 1017 cm−3). Samples with the Mg doping concentration in the range of ∼5 × 1018 ≤ [Mg] ≤ 7 × 1019 cm−3 are p-type and show very weak or no PR resonance. It is shown that the results correlate with the surface layer depletion thickness. The strongest PR signal is observed for thick depletion layers. The results are confirmed by μ-PCD measurements that show the longest carrier lifetimes for moderately doped samples with the thickest surface depletion layer. [ABSTRACT FROM AUTHOR]

The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) are addressed by using its solid suboxide, SiO, as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux ( Φ SiO ) from the source at different temperatures (TSiO) confirmed SiO molecules to sublime with an activation energy of 3.3 eV. The TSiO-dependent Φ SiO was measured in vacuum before and after subjecting the source material to an O2-background of 10 − 5 mbar (typical oxide MBE regime). The absence of a significant Φ SiO difference indicates negligible source oxidation in molecular O2. Mounted in an oxygen plasma-assisted MBE, Si-doped β-Ga2O3 layers were grown using this source. The Φ SiO at the substrate was evaluated [from 2.9 × 109 cm−2 s−1 (TSiO = 700 °C) to 5.5 × 1013 cm−2 s−1 (TSiO = 1000 °C)] and Si-concentration in the β-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., N Si from 4 × 1017 cm−3 (TSiO = 700 °C) up to 1.7 × 1020 cm−3 (TSiO = 900 °C). For a homoepitaxial β-Ga2O3 layer, a Hall charge carrier concentration of 3 × 1019 cm−3 in line with the provided Φ SiO (TSiO = 800 °C) is demonstrated. No SiO-incorporation difference was found between β-Ga2O3(010) layers homoepitaxially grown at 750 °C and β-Ga2O3(−201) heteroepitaxial layers grown at 550 °C on c-plane sapphire. However, the presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease in doping concentration (particularly at TSiO < 800 °C), which has been tentatively explained with a simple model. Degassing the source at 1100 °C reverted this oxidation. Concepts to reduce source oxidation during MBE-growth are referenced. [ABSTRACT FROM AUTHOR]

The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) are addressed by using its solid suboxide, SiO, as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux ( Φ SiO ) from the source at different temperatures (TSiO) confirmed SiO molecules to sublime with an activation energy of 3.3 eV. The TSiO-dependent Φ SiO was measured in vacuum before and after subjecting the source material to an O2-background of 10 − 5 mbar (typical oxide MBE regime). The absence of a significant Φ SiO difference indicates negligible source oxidation in molecular O2. Mounted in an oxygen plasma-assisted MBE, Si-doped β-Ga2O3 layers were grown using this source. The Φ SiO at the substrate was evaluated [from 2.9 × 109 cm−2 s−1 (TSiO = 700 °C) to 5.5 × 1013 cm−2 s−1 (TSiO = 1000 °C)] and Si-concentration in the β-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., N Si from 4 × 1017 cm−3 (TSiO = 700 °C) up to 1.7 × 1020 cm−3 (TSiO = 900 °C). For a homoepitaxial β-Ga2O3 layer, a Hall charge carrier concentration of 3 × 1019 cm−3 in line with the provided Φ SiO (TSiO = 800 °C) is demonstrated. No SiO-incorporation difference was found between β-Ga2O3(010) layers homoepitaxially grown at 750 °C and β-Ga2O3(−201) heteroepitaxial layers grown at 550 °C on c-plane sapphire. However, the presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease in doping concentration (particularly at TSiO < 800 °C), which has been tentatively explained with a simple model. Degassing the source at 1100 °C reverted this oxidation. Concepts to reduce source oxidation during MBE-growth are referenced. [ABSTRACT FROM AUTHOR]

SnO/ ε -Ga2O3 vertical p–n diodes with planar geometry have been fabricated on c-plane Al2O3 and investigated by current–voltage measurements. The effects of the in-plane conduction through the Si-doped ε -Ga2O3 layer on the diode performance and their relevance have been evaluated. A significant series resistance is observed, which shows typical features of the variable range hopping transport observed in Si-doped ε -Ga2O3; this in-plane transport mechanism is probably induced by the columnar domain structure of this polymorph. The dependence of the series resistance on the geometry of the diode supports the interpretation. A simple equivalent model is presented to describe the experimental behavior of the diode, supported by preliminary impedance spectroscopy investigation. [ABSTRACT FROM AUTHOR]

Smooth interfaces and surfaces are beneficial for most (opto)electronic devices that are based on thin films and their heterostructures. For example, smoother interfaces in (010) β-Ga2O3/(AlxGa1-x)2O3 heterostructures, whose roughness is ruled by that of the β-Ga2O3 layer, can enable higher mobility 2-dimensional electron gases by reducing interface roughness scattering. To this end we experimentally prove that a substrate offcut along the [001] direction allows to obtain smooth β-Ga2O3 layers in (010)-homoepitaxy under metal-rich deposition conditions. Applying In-mediated metal-exchange catalysis (MEXCAT) in molecular beam epitaxy at high substrate temperatures (Tg = 900 °C) we compare the morphology of layers grown on (010)-oriented substrates having different unintentional offcuts. The layer roughness is generally ruled by (i) the presence of (110)- and -facets visible as elongated features along the [001] direction (rms < 0.5 nm), and (ii) the presence of trenches (5–10 nm deep) orthogonal to [001]. We show that an unintentional substrate offcut of only ≈ 0.1° almost oriented along the [001] direction suppresses these trenches resulting in a smooth morphology with a roughness exclusively determined by the facets, i.e. rms ≈ 0.2 nm. Since we found the facet-and-trench morphology in layer grown by MBE with and without MEXCAT, we propose that the general growth mechanism for (010)-homoepitaxy is ruled by island growth whose coalescence results in the formation of the trenches. The presence of a substrate offcut in the [001] direction can allow for step-flow growth or island nucleation at the step edges, which prevents the formation of trenches. Moreover, we give experimental evidence for a decreasing surface diffusion length or increasing nucleation density on the substrate surface with decreasing metal-to-oxygen flux ratio. Based on our experimental results we can rule-out step bunching as cause of the trench formation as well as a surfactant-effect of indium during MEXCAT. [ABSTRACT FROM AUTHOR]

By antimony doping tin oxide, SnO2:Sb (ATO), below 1.0% Sb concentration, controllable n-type doping was realized. Plasma-assisted molecular beam epitaxy has been used to grow high quality single crystalline epitaxial thin films of unintentionally doped (UID) and Sb-doped SnO2 on r-plane sapphire substrates. A UID thickness series showed an electron concentration of 7.9×1018 cm-3 for a 26 nm film, which decreased to 2.7×1017 cm-3 for a 1570 nm film, whereas the mobility increased from 15 to 103 cm2/V s, respectively. This series illustrated the importance of a buffer layer to separate unintentional heterointerface effects from the effect of low Sb doping. Unambiguous bulk electron doping was established by keeping the Sb concentration constant but changing the Sb-doped layer thickness. A separate doping series correlated Sb concentration and bulk electron doping. Films containing between 9.8×1017 and 2.8×1020 Sb atoms/cm3 generated an electron concentration of 1.1×1018–2.6×1020 cm-3. As the atomic Sb concentration increased, the mobility and resistivity decreased from 110 to 36 cm2/V s and 5.1×10-2 to 6.7×10-4 Ω cm, respectively. The Sb concentration was determined by secondary ion mass spectrometry. X-ray diffraction and atomic force microscopy measurements showed no detrimental effects arising from the highest levels of Sb incorporation. Temperature dependent Hall measurements established a lower limit for the Sb electron activation energy of 13.2 meV and found that films with greater than 4.9×1019 electrons/cm3 were degenerately doped. Within experimental uncertainties, 100% donor efficiency was determined for Sb-doped SnO2 in the range studied. [ABSTRACT FROM AUTHOR]

Optical transitions in InAs/InP quantum dot structures are studied and interpreted in terms of heavy- (hh) and light-hole (lh) excitons originating in InAs valence band. Appearance of a step-like absorption and two activation energies in photoluminescence spectra of quantum dots under temperature elevation is described as a transition from the excitonic states to the two-dimensional-like states realized in the InAs/InP quantum dots. A mixture of hh and lh states determines the anisotropy of the transmission spectra and may result in rotation of the polarization plane for light transmitted through the InAs/InP quantum dot system. [ABSTRACT FROM AUTHOR]

Using a doped-channel Al0.3Ga0.7As-GaAs-In0.2Ga0.8As Greek-cross micro-Hall device as model structure, the size, and geometry effects on the absolute magnetic sensitivity and noise were investigated. The data show that although the absolute magnetic sensitivity is higher for devices with larger active areas, the self-heating effects at moderate and high electric fields are more detrimental for larger devices than for smaller ones. For this reason, the scaled absolute sensitivity is greater for smaller devices. The geometry is further optimized using finite element method simulations. Rounding the corners within the Greek-cross geometry results in lower peak electric fields and therefore a higher signal-to-noise sensitivity. [ABSTRACT FROM AUTHOR]

Antimony doping tin oxide is used for realizing Sn[O.sub.2]:Sb (ATO) and plasma-assisted molecular beam epitaxy is used for growing high quality single crystalline epitaxial thin films of unintentionally doped (UID) and Sb-doped Sn[O.sub.2] on r-plane sapphire substrates. X-ray diffraction and atomic force microscopy measurements have not shown any detrimental effects arising from the highest levels of Sb incorporation.

We here present an experimental study on (010)-oriented β-Ga2O3 thin films homoepitaxially grown by plasma assisted molecular beam epitaxy. We study the effect of substrate treatments (i.e., O-plasma and Ga-etching) and several deposition parameters (i.e., growth temperature and metal-to-oxygen flux ratio) on the resulting Ga2O3 surface morphology and growth rate. In situ and ex-situ characterizations identified the formation of (110) and ( 1 ¯ 10) -facets on the nominally oriented (010) surface induced by the Ga-etching of the substrate and by several growth conditions, suggesting (110) to be a stable (yet unexplored) substrate orientation. Moreover, we demonstrate how metal-exchange catalysis enabled by an additional In-flux significantly increases the growth rate (>threefold increment) of monoclinic Ga2O3 at high growth temperatures, while maintaining a low surface roughness (rms < 0.5 nm) and preventing the incorporation of In into the deposited layer. This study gives important indications for obtaining device-quality thin films and opens up the possibility to enhance the growth rate in β-Ga2O3 homoepitaxy on different surfaces [e.g., (100) and (001)] via molecular beam epitaxy. [ABSTRACT FROM AUTHOR]