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The effects of pressure on single crystals of Cr-doped gallium oxide (β-Ga2O3:Cr3+) and aluminum gallium oxide [(Al0.1Ga0.9)2O3] were examined by measuring the wavelength shift in the spectral R lines. Photoluminescence (PL) spectra of these materials were collected from samples in diamond anvil cells at pressures up to 9 GPa. The β-Ga2O3:Cr3+R lines were found to shift linearly under hydrostatic pressure. The (Al0.1Ga0.9)2O3R lines also show a linear shift but the R1 line shifted less than for β-Ga2O3:Cr3+. The ratio of R2 to R1 peak areas vs pressure is dominated by nonradiative recombination. X-ray diffraction measurements of (Al0.1Ga0.9)2O3 indicate that its equation of state is similar to that of β-Ga2O3. β-Ga2O3:Cr3+ was examined under non-hydrostatic conditions by using mineral oil as a pressure transmitting medium. Similar to the case in ruby, the R1 line is much more sensitive to non-hydrostatic stress than R2. Spatially resolved PL of a sample at 8 GPa in mineral oil showed significant variations in the R1 emission wavelength. These results suggest that the R1 line can serve as a sensitive probe of alloy composition and non-hydrostatic stress, while the R2 line is insensitive to these perturbations.

We demonstrate a method to achieve selectively patterned Mg-doped GaN layers using hydrogen drive-in through plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) films. Activated Mg-doped GaN layers were selectively deactivated by patterned PECVD SiNx films with low-temperature annealing and showed high-resistive behavior. Spatially resolved photoluminescence measurements were used to optically verify the deactivation of Mg acceptors and showed distinct features corresponding to activated and deactivated Mg in GaN. The method suggested here provides a simple and effective method to achieve patterned Mg-doped GaN regions without thermal and plasma damage, which could cause degradation of device performance. The proposed method could provide a way to achieve future high-performance GaN lateral and vertical devices that rely on laterally patterned doping.

Photoluminescence spectroscopy is a nondestructive optical method that is widely used to characterize semiconductors. In the photoluminescence process, a substance absorbs photons and emits light with longer wavelengths via electronic transitions. This paper discusses a method for identifying substances from their photoluminescence spectra using machine learning, a technique that is efficient in making classifications. Neural networks were constructed by taking simulated photoluminescence spectra as the input and the identity of the substance as the output. In this paper, six different semiconductors were chosen as categories: gallium oxide (Ga2O3), zinc oxide (ZnO), gallium nitride (GaN), cadmium sulfide (CdS), tungsten disulfide (WS2), and cesium lead bromide (CsPbBr3). The developed algorithm has a high accuracy (>90%) for assigning a substance to one of these six categories from its photoluminescence spectrum and correctly identified a mixed Ga2O3/ZnO sample.

Monoclinic gallium oxide (β-Ga2O3) is attracting intense focus as a material for power electronics, thanks to its ultra-wide bandgap (4.5–4.8 eV) and ability to be easily doped n-type. Because the holes self-trap, the band-edge luminescence is weak; hence, β-Ga2O3 has not been regarded as a promising material for light emission. In this work, optical and structural imaging methods revealed the presence of localized surface defects that emit in the near-UV (3.27 eV, 380 nm) when excited by sub-bandgap light. The PL emission of these centers is extremely bright—50 times brighter than that of single-crystal ZnO, a direct-gap semiconductor that has been touted as an active material for UV devices.

Photoluminescence spectroscopy is an important technique for characterizing optical properties of semiconductors. Using a confocal laser microscope system, 2D PL maps of technologically important materials were obtained with excellent signal-to-noise and resolution, from near-IR to UV.

Pyrene is a polycyclic aromatic hydrocarbon with fluorescence in the 370-400 nm range. The optical properties of pyrene deposited on different substrates were investigated from experimental and theoretical points of view. In contrast to prior studies of surface-enhanced fluorescence, in this work, all substrates were semiconductors or insulators. Experimentally, the intensities of photoluminescence (PL) and PL excitation (PLE) spectra were investigated. The PLE spectra of pyrene's monomer emission show that the intensities of the two main absorption peaks vary according to the chosen substrate. Calculations for pyrene on zinc sulfide (ZnS) and surface-oxidized diamond suggest that this connection arises from the alignment of the pyrene electronic levels relative to the band edges of the substrate. The absorption peak at 272 nm (4.56 eV) is shown to depend on the HOMO-1->LUMO orbital contribution, whereas the 335 nm (3.70 eV) peak depends upon the HOMO->LUMO transition. When the HOMO-1 level falls inside the substrate valence band, as in the case of ZnS, there is significant hybridization of the molecular state with surface states that causes an enhancement of the 272 nm peak. For pyrene on diamond, however, the HOMO-1 level falls in the substrate bandgap, leading to negligible hybridization and thus no enhancement.

β-Ga2O3 has demonstrated insulating properties with Mg, Fe, and Zn acceptor doping. Here we investigate Cu doping (0.25 at.%) in bulk Czochralski (CZ) and vertical gradient freeze (VGF) β-Ga2O3, with significant Cu incorporation, even with the expected Cu evaporation. Representative crystals were assessed for orientation, purity, optical, and electrical properties. The solubility and electronic behavior of Cu dopants are consistent with measured concentrations of 1×1018 - 1×1019 atoms/cm3 and electrical measurements that show high resistivities of 109-1010 Ω∙cm. Segregation and precipitation of Cu species in part of the VGF material was determined to be Cu2O by analysis with Raman spectroscopy, photoluminescence microscopy, energy-dispersive X-ray spectroscopy, and laser ablation inductively coupled plasma mass spectroscopy. With sufficient Cu concentration, β-Ga2O3 crystals excited with deep ultraviolet light photodarken rapidly and exhibit decreased resistivity. This darkened state remains at room temperature for several days before decaying.

MgxZn1−xO thin films were grown as metastable alloys via a sputtering technique in order to achieve single-phase wurtzite alloys with deep-UV optical bandgaps. As-grown alloys with Mg composition range 0–72% resulted in optical bandgaps spanning the UV-range of 3.3–4.4 eV. The thermal stability of the alloys was studied via post-growth controlled annealing experiments up to 900 °C. Alloys with low Mg up to 34% were found to be highly stable and retained their optical and material properties; however, alloys with higher Mg, up to 72%, were found to be unstable and were phase separated into wurtzite and cubic structural phases with respective optical bandgaps at ~ 3.5 and 6.0 eV. Both the as-grown and annealed alloys were studied using X-ray diffraction for structural identification, transmission spectroscopy for bandgap analysis, and Raman scattering for mapping the phonon mode-behavior. The experimental value for the solubility limit was found to be ~ 30%. A straightforward model calculation based on the Raman-mode saturation behavior yielded a similar value for the solubility limit of the alloys. The results are discussed in terms of available phase-diagrams for stable-state ceramics alloys that were grown under thermodynamics equilibrium conditions.

Research is described here that is directed toward obtaining p-type ZnO crystals either by doping or by creating native defects. Theoretically, zinc vacancies are shallow acceptors that should allow for p-type conduction. Bulk ZnO crystals can be grown by chemical vapor transport (CVT), melt solidification, and hydrothermal methods. Here we have explored annealing processes with the goal of creating zinc vacancies in bulk ZnO crystals. Positron annihilation spectra reveal the reproducible formation of high concentrations (>1020 cm−3) of isolated zinc vacancies (VZn) in oxygen-annealed, CVT-grown ZnO crystals in the ∼100–150 nm near-surface. Melt- and hydrothermally grown samples, in contrast, show insignificant levels of zinc vacancy creation. Photoluminescence (PL) emission spectra indicate a VZn(0/-1) acceptor level at ∼ 155–165 meV; red PL (∼1.7 eV) emission, related to the VZn(-1/-2) level, was also observed. Infrared absorption spectroscopy reveals the presence of a zinc vacancy complex with a hole binding energy range of 420–450 meV – and with a continuum suggesting a p-type region. XPS measurements support the deficiency of Zn after oxygen annealing the CVT-grown ZnO single crystal.

The semi-insulating single crystal β-Ga2O3 is becoming increasingly useful as a substrate for device fabrication. Fe doping is a method for producing such substrates. Along with Fe dopants, β-Ga2O3:Fe also contains Cr3+. Photoluminescence (PL) emission peaks at 690 nm (1.80 eV) and 696 nm (1.78 eV), as well as a broad feature around 709 nm (1.75 eV), are observed in β-Ga2O3:Fe. PL mapping of the 690 nm emission showed high and low intensity bands due to impurity striations introduced during crystal growth. PL mapping also revealed surface defects showing broad emissions around 983 nm (1.26 eV) and 886 nm (1.40 eV) that were spatially localized, occurring at discrete spots on the sample surface. Raman mapping of an 886 nm emission center revealed peaks at 2878 and 2930 cm−1, consistent with an organometallic or hydrocarbon compound. Raman mapping of the 983 nm center showed a peak at 2892 cm−1. Bright UV emission centers showed Raman peaks at 2910 and 2968 cm−1, which are attributed to Si–CH3 groups that may originate from silica polishing compounds or annealing in a silica ampoule.

Zinc-doped monoclinic gallium oxide (β-Ga2O3:Zn) has semi-insulating properties that could make it a preferred material as a substrate for power devices. Infrared and UV/Visible spectroscopy were used to investigate the defect properties of bulk β-Ga2O3:Zn crystals. As-grown crystals contain a single O-H stretching mode at 3486.7 cm−1 due to a neutral ZnH complex. A deuterium-annealed sample displays the corresponding O-D stretching mode at 2582.9 cm−1, confirming the O-H assignment. A strong Ir4+ electronic transition at 5147.6 cm−1 is also observed, along with sidebands attributed to ZnIr pairs. These sidebands show distinct differences compared with Mg-doped samples; most importantly, several peaks are attributed to Ir4+ paired with a Zn on the tetrahedral Ga(I) site. Annealing under an oxygen atmosphere produced an insulating material with a resistance above 1 TΩ.

The ability to define the crystal phase of oxide semiconductors could benefit transparent electronics and catalysis. In this paper, laser-induced localized phase transitions of titanium dioxide (TiO2) thin films are reported. Irradiation was performed with a 532 nm continuous wave laser. Raman spectroscopy and micro-Raman mapping were used to identify the phase transformations. A Raman map of the anatase Eg mode (144 cm−1) and rutile Ag mode (608 cm−1) revealed the formation of crystalline microstructures due to the laser treatment. Laser irradiation under vacuum results in an anatase-to-rutile phase transition. Irradiating the rutile region in air changes the crystal structure back to anatase, despite the thermodynamic stability of rutile. The results suggest that irradiated photons are absorbed by defects, resulting in localized electronic excitation that leads to a mixture of amorphous and crystalline regions. The phase of the crystalline regions depends strongly on the ambient conditions (vacuum versus air).

In this study, the formation and character of gallium vacancies (VGa) and their complexes in near surface and bulk regions of single crystal β-Ga2O3 were explored using unintentionally doped single crystals grown by the Czochralski method. As-grown and O2 annealed (up to 1550 °C) samples were investigated using positron annihilation spectroscopy (PAS) to study the top 0.05–6 μm, and also current–voltage measurements and infrared (IR) spectroscopy, with hydrogenated samples to probe VGa, to study the bulk. After annealing in O2 > 1000 °C, the β-Ga2O3 resistivity begins increasing, up to ∼109 Ω cm for 1550 °C treatment, with the top 0.5 mm being many orders of magnitude more resistive. PAS measurements of the top 6 μm (S values) and very near surface 200 nm (diffusion length, L) indicate differential behavior as a function of peak annealing temperature. At least four temperature regimes of behavior are described. VGa are present in the bulk after growth, but considerable changes occur upon annealing at a temperature ≈1000 °C, where L and S decrease simultaneously, suggesting an increasing defect concentration (L) but a decreasing defect volume (S). Annealing at a temperature ≈1400 °C increases S again, showing an increasing volume concentration of VGa, with IR absorption showing a large signature of VGa-2H, indicative of increased VGa formation that was not present when annealing at a temperature ≈1000 °C. These results suggest that defect changes from annealing in oxygen are depth dependent, and that VGa configuration may not be the same near the oxygen-exposed surface of the sample and in the bulk.

Materials with persistent photoconductivity (PPC) experience an increase in conductivity upon exposure to light that persists after the light is turned off. Although researchers have shown that this phenomenon could be exploited for novel memory storage devices, low temperatures (below 180 K) were required. In the present work, two-point resistance measurements were performed on annealed strontium titanate (SrTiO3, or STO) single crystals at room temperature. After illumination with sub-gap light, the resistance decreased by three orders of magnitude. This markedly enhanced conductivity persisted for several days in the dark. Results from IR spectroscopy, electrical measurements, and exposure to a 405 nm laser suggest that contact resistance plays an important role. The laser was then used as an “optical pen” to write a low-resistance path between two contacts, demonstrating the feasibility of optically defined, transparent electronics.

Calcium and magnesium doped beta-phase gallium oxide (β-Ga2O3) single crystals were Czochralski grown in an iridium crucible and subsequently annealed in hydrogen at ~800° C. IR spectroscopy was used to investigate the roles of Mg and Ca dopants as well as H and Ir impurities. An IR peak at 3492 cm-1 was previously assigned to an O-H bondstretching mode of a MgH complex in hydrogen-annealed Ga2O3:Mg and an IR peak at 3541 cm-1 is tentatively assigned to a similar mode in a CaH complex for H annealed Ga2O3:Ca. An IR peak at 5148 cm-1 is attributed to the presence of Ir4+ when magnesium is present. Excitation with several different wavelengths was used to confirm the position of the Ir level within the band gap. Our results suggest that isolated substitutional Ca dopants are less effective as deep acceptors than Mg. Polarization dependence of the iridium IR spectra is also discussed.

Electron paramagnetic resonance (EPR) is used to identify and characterize neutral zinc acceptors in Zn-doped β-Ga2O3 crystals. Two EPR spectra are observed at low temperatures, one from Zn ions at tetrahedral Ga(1) sites (the Z n Ga 1 0 acceptor) and one from Zn ions at octahedral Ga(2) sites (the Z n Ga 2 0 acceptor). These Zn acceptors are small polarons, with the unpaired spin localized in each case on a threefold coordinated oxygen O(I) ion adjacent to the Zn ion. Resolved hyperfine interactions with neighboring 69Ga and 71Ga nuclei allow the EPR spectra from the two acceptors to be easily distinguished: Z n Ga 1 0 acceptors interact equally with two Ga(2) ions and Z n Ga 2 0 acceptors interact unequally with a Ga(1) ion and a Ga(2) ion. The as-grown crystals are compensated, with the Zn ions initially present as singly ionized acceptors ( Z n Ga 1 − and Z n Ga 2 −). Exposing a crystal to 325 nm laser light, while being held at 140 K, primarily produces neutral Z n Ga 2 0 acceptors when photoinduced holes are trapped at Z n Ga 2 − acceptors. This suggests that there may be significantly more Zn ions at Ga(2) sites than at Ga(1) sites. Warming the crystal briefly to room temperature, after removing the light, destroys the EPR spectrum from the shallower Z n Ga 2 0 acceptors and produces the EPR spectrum from the more stable Z n Ga 1 0 acceptors. The Z n Ga 2 0 acceptors decay in the 240–260 K region with a thermal activation energy near 0.65 eV, similar to Mg Ga 2 0 acceptors, whereas the slightly deeper Z n Ga 1 0 acceptors decay close to room temperature with an approximate thermal activation energy of 0.78 eV.

Pressure-dependent phase transitions of In2Se3 bulk powders and nanowire samples were studied at room temperature using synchrotron X-ray diffraction and a diamond anvil cell. γ-In2Se3, metastable under ambient conditions, transforms into to the stable high pressure β phase between 2.8 and 3.2 GPa in bulk powder samples and at slightly higher pressures, between 3.2 and 3.7 GPa, in nanowire samples. While the γ phase bulk modulus is similar to that of the β phase, the decrease due to pressure in the unit cell parameter ratio, c/a, is less than half the decrease seen in the β phase. First-principles calculations show that γ-In2Se3 has a higher energy and unit-cell volume than β-In2Se3, consistent with the experimental observations.

ZnO is an efficient luminescent material in the UV-range ∼3.4 eV with a wide range of applications in optical technologies. Sputtering is a cost-effective and relatively straightforward growth technique for ZnO films; however, most as-grown films are observed to contain intrinsic defects which can significantly diminish the desirable UV-emission. In this research the defect dynamics and optical properties of ZnO sputtered films were studied via post-growth annealing in Ar or O2 ambient, with X-ray diffraction (XRD), imaging, transmission and Urbach analysis, Raman scattering, and photoluminescence (PL). The imaging, XRD, Raman and Urbach analyses indicate significant improvement in crystal morphology and band-edge characteristics upon annealing, which is nearly independent of the annealing environment. The native defects specific to the as-grown films, which were analyzed via PL, are assigned to Zni related centers that luminesce at 2.8 eV. Their presence is attributed to the nature of the sputtering growth technique, which supports Zn-rich growth conditions. After annealing, in either environment the 2.8 eV center diminished accompanied by morphology improvement, and the desirable UV-PL significantly increased. The O2 ambient was found to introduce nominal Oi centers while the Ar ambient was found to be the ideal environment for the enhancement of the UV-light emission: an enhancement of ∼40 times was achieved. The increase in the UV-PL is attributed to the reduction of Zni-related defects, the presence of which in ZnO provides a competing route to the UV emission. Also, the effect of the annealing was to decrease the compressive stress in the films. Finally, the dominant UV-PL at the cold temperature regime is attributed to luminescent centers not associated with the usual excitons of ZnO, but rather to structural defects.

To realize the many potential applications of ZnO films, it is vital to produce films with high optical quality that exhibit strong UV luminescence. By combining annealing at an optimal temperature followed by the deposition of a coating, one can achieve a significant enhancement of photoluminescence (PL). The effectiveness of the coating over time is a crucial point to be considered. Three types of coating materials were investigated: MgO, SiO2, and Al2O3. Due to its strong bond energy, MgO was found to be the most effective coating material for passivation of the surfaces of the ZnO films; SiO2 was the second best. The UV-PL intensity of MgO coated ZnO was found to increase by a factor of 52 relative to an uncoated film. The effectiveness of the coatings exhibited a linear correlation with their bond energies and is discussed in terms of competing mechanisms to surface passivation such as the adsorption of OH-groups; these can act as surface traps and diminish the UV-PL intensity. Annealing at 900 °C prior to the deposition of the coating was found to be an important step in realizing the optimal performance of the coating due to the reduction of Zn interstitials accompanied by improved crystallinity. Exposure to the environment, up to 294 days, results in the degradation of the UV-PL of the MgO coated film; this effect was not observed for the film coated with SiO2. This effect is discussed in terms of the strong reactivity of MgO with environmental contaminants from the OH-groups.

Titanium dioxide (TiO2) has a range of applications including catalysis, hydrogen production, and water purification. In this work, anatase TiO2 was annealed in vacuum at 800 °C, resulting in a conductive thin film. Exposure to subgap laser light (532 nm wavelength) caused a seven order-of-magnitude increase in resistance. Laser-irradiated regions showed an increase in optical transmission, consistent with a reduction in oxygen vacancy concentration. Scanning electron microscopy and Raman spectroscopy indicate that laser irradiation did not change the morphology, composition, or phase of the material.

Confocal laser scanning microscopy (CLSM) is a preferred method for obtaining optical images with submicrometer resolution. Replacing the pinhole and detector of a CLSM with a digital camera [charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS)] has the potential to simplify the design and reduce cost. However, the relatively slow speed of a typical camera results in long scans. To address this issue, in the present investigation a microlens array was used to split the laser beam into 48 beamlets that are focused onto the sample. In essence, 48 pinhole-detector measurements were performed in parallel. Images obtained from the 48 laser spots were stitched together into a final image.