The band structure of germanium changes significantly when alloyed with a few percent concentrations of tin, and while much work has been done to characterize and exploit these changes, the corresponding deep-level defect characteristics are largely unknown. In this paper, we investigate the dominant deep-level defects created by 2 MeV proton irradiation in Ge1 − xSnx (x = 0.0, 0.020, 0.053, 0.069, and 0.094) diodes and determine how the ionization energies of these defects change with tin concentrations. Deep-level transient spectroscopy measurements approximate the ionization energies associated with electron transitions to/from the valence band (hole traps) and conduction band (electron traps) in the intrinsic regions of p-i-n diode test structures. The prominent deep-level hole traps may be associated with divacancies, vacancy–tin complexes, and vacancy–phosphorous complexes (V2, V–Sn, and V–P, respectively), with the presumed V–P hole trap dominating after room temperature annealing. The ionization energy level of this trap (approximated by the apparent activation energy for hole emission) is close to the intrinsic Fermi level in the 0% and 2% Sn devices and decreases as the tin concentration is increased, maintaining an approximately fixed energy spacing below the indirect conduction band edge. The other hole traps follow this same trend, and the dominant electron trap ionization energies remain roughly constant with changes in tin concentrations, indicating they are likewise pinned to the conduction band edge. These results suggest a pattern that may, in many cases, apply more generally to deep-level defects in these alloys, including those present in the “as-grown” materials.

Direct-bandgap germanium-tin (Ge-Sn) alloys are highly sought-after materials for applications in silicon photonic integrated circuits. Other than crystal quality, two main factors determine the transition from the indirect to direct bandgap: the high Sn concentration and the strain relaxation in the materials. Using ion implantation and pulsed laser melting, we demonstrate a fully-relaxed Ge-Sn alloy with a Sn concentration of 6at. %. This concentration is at least 10 times higher than the equilibrium solubility of Sn in Ge. Cross-sectional transmission electron microscopy shows unconventional threading-like defects in the film as the mechanism for the strain relaxation. Due to the high degree of strain relaxation and the good crystal quality, photoluminescence could be obtained from the samples to examine the indirect-direct bandgap transition in the alloys.

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.

Ge-Sn alloys with a sufficiently high concentration of Sn is a direct bandgap group IV material. Recently, ion implantation followed by pulsed laser melting has been shown to be a promising method to realize this material due to its high reproducibility and precursor-free process. A Ge-Sn alloy with ~9 at.% Sn was shown to be feasible by this technique. However, the compressive strain, inherently occurring in heterogeneous epitaxy of the film, evidently delays the material from the direct bandgap transition. In this report, an attempt to synthesize a highly-relaxed Ge-Sn alloy will be presented. The idea is to produce a significantly thicker film with a higher implant energy and doses. X-ray reciprocal space mapping confirms that the material is largely-relaxed. The peak Sn concentration of the highest dose sample is 6 at.% as determined by Rutherford backscattering spectrometry. Cross-sectional transmission electron microscopy shows unconventional defects in the film as the mechanism for the strain relaxation. Finally, a photoluminescence (PL) study of the strain-relaxed alloys shows photon emission at a wavelength of 2045 nm, suggesting an active incorporation of Sn concentration of ~6 at.%. The results of this study pave way to produce high quality relaxed GeSn alloy using an industrially scalable method.

The structural, electrical and optical properties of a unintentionally doped Ge1-xSnx (x = 6.2%) alloy semiconductor both as-grown and after a hydrogen plasma treatment or 'hydrogen passivation' are investigated by high resolution x-ray diffraction, atomic force microscopy, and temperature-dependent Hall-effect and photoluminescence (PL) measurements. The samples were grown via reactions of SnD4 and Ge3H8 on Ge buffered Si wafers at 305 degrees C and subsequently subjected to a hydrogen plasma environment generated using an inductively coupled H-2 approach (ICP). The plasma treatments showed no appreciable change in the structural, compositional and morphological properties of the samples indicating no measurable degradation of the materials quality has occurred. However, the H passivation significantly alters the electrical activity of as-grown defects and impurities in the epilayer, resulting in electrical conductivity of passivated sample that is more than 2 times higher at 300 K and 30 times higher at 10 K. The as-grown samples showed conduction-type changes with temperature (manifested by singularities in the apparent carrier concentrations around 14 and 255 K), while the background n-type carrier concentration of the hydrogen treated analogs varied smoothly with temperature. In particular at 300 K, the carrier concentration was reduced from the background 4.61 x 10(17) cm(-3) in the as-grown material to 5.68 x 10(16) cm(-3) in the H treated counterpart due to potential passivation of deleterious point defects, a highly desirable outcome for effective device performance. The room temperature PL intensity increased similar to 5 times (more than 3 times at 5 K) upon hydrogen treatment due to electrical passivation of deep acceptor states. Hydrogen plasma treatments are thus found to enhance the electrical and optical responses of the samples, suggesting that conventional ICP treatments could be used as a processing step to improve the properties of newly developed Sn-based group IV semiconductors using straightforward and relatively low temperature protocols.

The temperature (T)-dependent photoluminescence (PL) from Ge 1− y Sn y ( y = 4.3%–9.0%) alloys grown on Ge-buffered Si substrates was studied as a function of the Sn content. The PL from Ge 1− y Sn y alloys with high Sn contents (≥7.0%) exhibited the typical characteristics of direct bandgap semiconductors, such as an increase in the PL intensity with decreasing T and a single PL peak corresponding to a transition from the direct bandgap (Γ-valley) to the valence band at all temperatures from 10 to 300 K. For the Ge 1− y Sn y alloys with low Sn contents (≤6.2%), the PL emission peaks corresponding to both the direct bandgap ( E D ) and the indirect bandgap ( E ID ) PL appeared at most temperatures and as T was increased, the integrated PL intensities of E D initially increased, then decreased, and finally increased again. The unstrained E D and E ID energies estimated from the PL spectra at 75 and 125 K were plotted as functions of the Sn concentration, and the cross-over point for unstrained Ge 1− y Sn y was found to be about 6.4%–6.7% Sn by using linear fits to the data in the range of Sn contents from 0% to 9.0%. Based on the results at 75 and 125 K, the cross-over Sn concentration of unstrained Ge 1− y Sn y should be about 6.4%–6.7% Sn content at room temperature. The E D energies of the Ge 0.925 Sn 0.075 alloys were estimated from the T-dependent photoreflectance spectra, and the E D values was consistent with those obtained from PL spectra.

Room temperature photoluminescence (PL) was observed from GeSn layers fabricated by ion implantation of Sn into bulk Ge followed by pulsed laser melting using an Nd:YAG laser at 355 nm. PL measurements indicate regions of high-crystalline quality with Sn concentrations of up to 9%.

Thermally stimulated techniques—thermally stimulated current (TSC) spectroscopy and thermally stimulated depolarization current (TSDC) spectroscopy—were used to comparatively study the electrical properties and deep level defects in β-Ga2O3 pulsed laser deposited thin films and Czochralski-grown bulk crystals. It was found that the samples are highly resistive and each sample may have different dark current activation energy. Deep level defects revealed by the thermally stimulated techniques vary from sample to sample. In addition to the common traps E1 (∼0.56 eV), E2 (∼0.84 eV), and E3 (∼0.99 eV), reported in the literature and revealed by DLTS studies of Ga2O3 bulk crystals, that were also found in our samples by the thermally stimulated techniques, a trap at ∼110 meV and several other traps are revealed specifically by TSDC between 105 and 225 K.

Temperature-dependent photoluminescence (PL) of two sets of ternary samples with fixed tin concentrations of ~5.2% (Ge0.924Si0.024Sn0.052, and Ge0.911Si0.036Sn0.053) and ~7.3% (Ge0.900Si0.027Sn0.073, and Ge0.888Si0.04Sn0.072) were measured along with their binary counterparts (Ge0.948Sn0.052 and Ge0.925Sn0.075). The variations of direct bandgap emission (ED) and indirect bandgap emission (EID) with temperature were studied for both ternary and binary alloys by means of Gaussian curve fitting, and the results are compared. The bandgap widths of ternaries clearly increase after Si incorporation into the GeSn with similar Sn concentrations. It is found that for the ternaries both ED and EID peak energies are blue shifted, and the energy separation of ED and EID peaks becomes larger than that of binaries for similar Sn concentrations. Moreover, both ED and EID peaks appear at room temperature (RT) in the GeSiSn spectra, but the ED peak position is greater than EID, indicating these ternaries are indirect bandgap materials. Low temperature PL validates the existence of indirect PL emission in Ge0.90Si0.027Sn0.073 and direct gap behavior in Ge0.925Sn0.075, indicating GeSn becomes a direct bandgap material at lower Sn concentration than GeSiSn. The PL intensities of these ternaries are generally weaker and the spectra become more complicated than those of binaries, probably due to increased strain and defects in the ternaries. Finally, it is found that the effect of large differences in strain of ternary samples on PL peak positions can be greater than that of small Si composition differences in ternaries. A large compressive strain in ternaries can also make splitting of the ED into ED,HH (conduction band minimum-Γ valley to heavy hole maximum) and ED,LH (conduction band minimum-Γ valley to light hole maximum) transitions more observable in the PL spectra.

Electrical characteristics and deep-level transient spectroscopy of a Ge0.873Si0.104Sn0.023 photodiode grown by ultra-high vacuum chemical vapor deposition on a p++ Ge platform are investigated. The photodiode shows good rectifying I-V characteristics, and the dark current exhibits an activation energy of E-dc = 0.43 eV at high temperature while the reverse bias leakage current in the film is low but increases with temperature. Capacitance-voltage measurements show the diode has a built-in potential of 0.37 V at 300 K; the depth profile obtained from capacitance-voltage measurements is in agreement with secondary ion mass spectrometry analysis reported previously. Deep level transient spectroscopy shows two electron traps at similar to 100 K and at similar to 165 K with energy levels at similar to 0.09 eV and similar to 0.36 eV from the conduction band, respectively; and at least one hole trap at similar to 275 K with energy level at similar to 0.61 eV from the valence band (similar to 0.33 eV from the conduction band) existing in the device.

Indium-doped ZnO bulk crystals grown by the hydrothermal method are highly-conductive, with resistivity at 0.01 Omega cm at room temperature as revealed by Hall-effect measurement. In this paper we report on structural and optical properties of these crystals. The grown In:ZnO crystals have been studied by high resolution X-ray diffraction, micro-Raman scattering and low-temperature photoluminescence and cathodoluminescence. It was found that the c lattice parameter of the grown In:ZnO crystal expanded 0.06% with respect to the lithium-doped ZnO crystal seed, and the In-doped ZnO overgrew the seed crystal pseudomorphically but with high quality crystallinity; the X-ray rocking curves show the FWHM of the Zn face and O faces are only 0.05 degrees and 0.1 degrees; and the indium concentration in the crystal reaches the solubility limit. Raman spectra show strain relaxation gradually from the regrowth interface as well as a weak spectral feature at 723 cm(-1). The peak at 312 cm(-1) noticed in hydrothermally grown In:ZnO nanostructures does not appear in our In-doped crystals, indicating that this peak may be associated with specific defects (e.g. surface related) of the nanostructures. Photoluminescence measurements show that an indium donor bound exciton peak I-9 ((InX)-X-0) is the dominant peak in the PL spectrum, located at 3.3586 eV on the zinc face and 3.3577 eV on the oxygen face. Both of them deviated from the consensus literature value of 3.3567 eV, probably due to strain in the crystal induced by impurities.

Simultaneous epitaxial growth of film and nanowire array on a substrate is of both scientific significance and practical importance for nanoscale optoelectronics. Nevertheless, in situ building conducting connection between individually isolated nanowires grown on insulating substrates is still challenging. Herein, we demonstrate a novel and facile strategy for the simultaneous epitaxial growth of nonpolar a-plane ZnO film and obliquely aligned nanowire array on Au-coated r-plane sapphire substrate. The morphology, structure, components, and optical properties of the as-synthesized ZnO nanostructures were investigated using field-emission scanning electron microscopy, X-ray diffraction, field-emission transmission electron microscopy, energy-dispersive spectroscopy, X-ray photo-electron spectroscopy, and photoluminescence spectroscopy. A cooperative growth mechanism is proposed: Au-catalyzed vapor transport initiates the co-occurrence of nonpolar a-plane and polar c-plane ZnO nuclei, and subsequently, the non-upward directed Au catalyst helps the nonpolar a-plane ZnO nuclei develop into a ZnO conductive film at the bottom and zinc self-catalyzed vapor–liquid–solid growth helps the polar c-plane ZnO nuclei develop simultaneously into obliquely aligned nanowire arrays. The proposed strategy realized in situ synthesis of nanowires with conductive connection and it can benefit the application of ZnO nanowires in optoelectronics.

Thermally stimulated current (TSC), thermally stimulated depolarization current (TSDC), and thermally stimulated luminescence (TSL) spectroscopies were combined to study trapping phenomena in undoped bulk SrTiO 3 crystals. Electrical measurements were also performed and showed that the crystals are highly resistive in the dark but exhibited an unusually high photocurrent upon 400-nm illumination. Several traps were revealed in both TSC and TSDC spectra between 83 K and 450 K in such a broad temperature range and their activation energies were extrapolated from the trap positions (peaks). TSL spectra demonstrate similar characteristics comparable to TSC and TSDC spectra, though there are some differences because of different excitation and recombination mechanisms. This work reveals the presence of large number of traps in SrTiO 3 single crystals, which are most likely the source of many of the interesting phenomena in SrTiO 3 such as transient and persistent photoconductivity.

InGaZnO (IGZO) is a promising semiconductor material for thin-film transistors (TFTs) used in DC and RF switching applications, especially since it can be grown at low temperatures on a wide variety of substrates. Enhancement-mode TFTs based on IGZO thin films grown by pulsed laser deposition (PLD) have been recently fabricated and these transistors show excellent performance; however, compositional variations and defects can adversely affect film quality, especially in regard to electrical properties. In this study, we use thermally stimulated current (TSC) spectroscopy to characterize the electrical properties and the deep traps in PLD-grown IGZO thin films. It was found that the as-grown sample has a DC activation energy of 0.62 eV, and two major traps with activation energies at similar to 0.16-0.26 eV and at similar to 0.90 eV. However, a strong persistent photocurrent (PPC) sometimes exists in the as-grown sample, so we carry out post-growth annealing in an attempt to mitigate the effect. It was found that annealing in argon increases the conduction, produces more PPC and also makes more traps observable. Annealing in air makes the film more resistive, and removes PPC and all traps but one. This work demonstrates that current-based trap emission, such as that associated with the TSC, can effectively reveal electronic defects in highly-resistive semiconductor materials, especially those are not amenable to capacitance-based techniques, such as deeplevel transient spectroscopy (DLTS).

For single slabs of uniform material, such as bulk semiconductors, we derive closed-form expressions for absorption and reflection coefficients, ∝ and R, respectively, in terms of measured reflectance and transmittance, Rm and Tm. The formula for α can replace the several commonly used approximations for ∝ as a function of Tm, and in particular does not require ∝d >> 1, where d is the thickness. Thus, it can be applied to weak impurity absorptions, such as Fe absorption in Fe-doped GaN. Finally, the real (η) and imaginary (κ) parts of the index of refraction (n = η + iκ) can be obtained from ∝ and R and agree well with η and κ results obtained from other experiments. For multi-layer structures, “effective” values of ∝, R, η, and κ are obtained, but they can often be assigned to a particular layer. This new technique has been successfully applied to many bulk and layered structures.

We used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure the nature and spatial distribution of native point defects at Zn- and O-polar ZnO interfaces with iridium oxide (IrOx) and their impact on Schottky barrier formation. IrOx and other metal oxides exhibit higher Schottky barriers than their pure metal counterparts, consistent with wider depletion regions and potentially useful for ohmic contacts to p-type semiconductors. DRCLS with I-V and 1/C2-V barrier height and carrier profile measurements showed high zinc vacancy VZn and CuZn defect densities that compensate free carrier densities, increase depletion widths, and form higher effective barriers than Ir/ZnO contacts. Zn-polar versus O-polar ZnO interfaces with IrOx exhibit 40% higher VZn + CuZn interface segregation and lower carrier densities within a wider depletion region, accounting for the significantly higher (0.89 vs. 0.67 eV) barrier heights. Both the depth of VZn density segregation and the Zn-deficient layer thickness measured microscopically match the depletion width and applied electric fields comparable to spontaneous polarization fields across similar layers displaying analogous defect segregation. These results account for the difference in polarity-dependent segregation due to the electric field-driven diffusion of native defects near ZnO interfaces.