Persistent Room-Temperature Photodarkening in Cu-Doped ß-Ga_{2}O_{3}.

ß-Ga_{2}O_{3} is an ultrawide band gap semiconductor with emerging applications in power electronics. The introduction of acceptor dopants yields semi-insulating substrates necessary for thin-film devices. In the present work, exposure of Cu-doped ß-Ga_{2}O_{3} to UV light >4 eV is shown to cause large, persistent photo-induced darkening at room temperature. Electron paramagnetic resonance spectroscopy indicates that light exposure converts Cu^{2+} to Cu^{3+}, a rare oxidation state that is responsible for the optical absorption. The photodarkening is accompanied by the appearance of O─H vibrational modes in the infrared spectrum. Hybrid function calculations show that Cu acceptors can favorably complex with hydrogen donors incorporated as interstitial (H_{i}) or substitutional (H_{O}) defects. When Cu_{Ga}-H_{O} complexes absorb light, hydrogen is released, contributing to the observed Cu^{3+} species and O─H modes.

Acceptor and surface states of ZnO nanocrystals: a unified model.

Semiconductor nanocrystals have the potential for a range of applications in optoelectronics and nonlinear optics. As the surface-to-volume ratio increases, surface emission processes become more important. Using infrared (IR) and photoluminescence (PL) spectroscopy, we have developed a unified model for the acceptor and intragap surface states of ZnO nanocrystals. A PL peak was observed at 2.97 eV, in agreement with an acceptor level previously observed in the IR (Teklemichael et al 2011 Appl. Phys. Lett. 98 232112). The temperature dependence of the IR absorption peaks, which correspond to a hole binding energy of 0.46 eV, showed an ionization activation energy of only 0.08 eV. This activation energy is attributed to thermal excitation of the hole to surface states 0.38 eV above the valence band maximum. Therefore, while the acceptor is deep with respect to the bulk valence band, it is shallow with respect to surface states. A strong red PL emission centered at 1.84 eV, with an excitation onset of 3.0 eV, is attributed to surface recombination.

Resonant interaction between hydrogen vibrational modes in AlSb:Se.

Vibrational modes and their interactions affect numerous physical processes in condensed-matter systems. In the present work, hydrogen vibrations in Se-doped AlSb were investigated with first-principles calculations. Vibrational frequencies were calculated for the longitudinal, transverse, wag (bending), and stretch modes of the Al-H complex. The Al-H stretch mode interacts with a combination mode involving a wag overtone and transverse fundamental. This resonant interaction yields vibrational states that are linear superpositions of the stretch mode and the combination mode. The spatial extent of such excitations is significantly larger than that of a local vibrational mode.

Measuring the volume of a fluid in a diamond anvil cell using a confocal microscope.

Confocal microscopy is a potentially powerful technique for obtaining equation-of-state (EOS) data for fluids in a diamond anvil cell. Unlike conventional microscopy, a confocal microscope scans the cell in three dimensions. From the intensity profile of the reflected laser light, we calculated the index of refraction and optical thickness of the sample contained in the cell. These measurements, combined with the cross-sectional area of the sample, enabled us to calculate the volume. As a test of the experimental technique and analysis, we produced a pressure-volume curve for liquid water at 300 K. The results agree with published EOS data within experimental error.

Ferromagnetism in Ga(1-x)Mn(x)P: evidence for inter-Mn exchange mediated by localized holes within a detached impurity band.

We report an energy gap for hole photoexcitation in ferromagnetic Ga(1-x)Mn(x)P that is tunable by Mn concentration (x < or = 0.06) and by compensation with Te donors. For x approximately 0.06, electrical transport is dominated by excitation across this gap above the Curie temperature (TC) of 60 K and by thermally activated hopping below TC. Magnetization measurements reveal a moment of 3.9 +/- 0.4 muB per substitutional Mn while the large anomalous Hall signal demonstrates that the ferromagnetism is carrier mediated. In aggregate these data indicate that ferromagnetic exchange is mediated by holes localized in a Mn-derived band that is detached from the valence band.

Conformation of p-terphenyl under hydrostatic pressure.

The conformation of p-terphenyl (C18H14) and deuterated p-terphenyl (C18D14) has been investigated, using high-pressure infrared spectroscopy at liquid-helium temperatures. First-principles calculations, together with the experimental results, were performed to determine the structure of p-terphenyl in the twisted conformation. At low temperatures and pressures, p-terphenyl belongs to the C2 point group of symmetry. In this configuration, the central ring is twisted with respect to the plane of the outer rings. The symmetry of the molecule is nearly C2h, consistent with previous x-ray diffraction measurements.

Resonant interaction between localized and extended vibrational modes in Si: 18O under pressure.

The interaction between localized and extended vibrational modes in solids is of central importance in understanding how local vibrational modes decay into phonons. Interstitial oxygen (O(i)) in silicon is a model system for studying such interactions. Using hydrostatic pressure, we have brought the antisymmetric stretch mode of (18)O(i) in silicon into resonance with the second harmonic of the (18)O(i) resonant mode. Infrared spectroscopy was used to observe an anticrossing between these two vibrational modes at pressures near 4 GPa. A model of the interaction between these modes produced excellent agreement with the experimentally observed frequencies and linewidths.