Samuel T. Teklemichael

Acceptors in ZnO nanocrystals

While zinc oxide (ZnO) has potential for optoelectronic applications, the lack of reliable p-type doping remains a major challenge. We provide evidence that ZnO nanocrystals contain uncompensated acceptors. IR absorption peaks at liquid-helium temperatures suggest a hydrogenic acceptor with a hole binding energy of 0.4–0.5 eV. Electron paramagnetic resonance (EPR) measurements in the dark showed a resonance at g=2.003, characteristic of acceptors that involve a zinc vacancy. An EPR resonance due to vacancy hydrogen complexes was observed after exposure to light. Given the lack of alternatives, vacancy complexes may provide a feasible route toward p-type conductivity.

Acceptors in ZnO

Zinc oxide (ZnO) has potential for a range of applications in the area of optoelectronics. The quest for p-type ZnO has focused much attention on acceptors. In this paper, Cu, N, and Li acceptor impurities are discussed. Experimental evidence indicates these point defects have acceptor levels 3.2, 1.4, and 0.8 eV above the valence-band maximum, respectively. The levels are deep because the ZnO valence band is quite low compared to conventional, non-oxide semiconductors. Using MoO2 contacts, the electrical resistivity of ZnO:Li was measured and showed behavior consistent with bulk hole conduction for temperatures above 400 K. A photoluminescence peak in ZnO nanocrystals is attributed to an acceptor, which may involve a Zn vacancy. High field (W-band) electron paramagnetic resonance measurements on the nanocrystals revealed an axial center with g⊥ = 2.0015 and g// = 2.0056, along with an isotropic center at g = 2.0035.

Pressure-induced phase transformation of In2Se3

In2Se3 has potential as a phase-change material for memory applications. Understanding its phase diagram is important to achieve controlled switching between phases. Using x-ray diffraction and a diamond-anvil cell, the pressure-dependent structural properties of In2Se3 powder were studied at room temperature. α-In2Se3 transforms into the β phase at 0.7 GPa, an order of magnitude lower than phase-transition critical pressures in typical semiconductors. The β phase persists upon decompression to ambient pressure. Raman spectroscopy experiments confirm this result. The bulk moduli are reported and the c/a ratio for the β phase is shown to have a highly nonlinear dependence on pressure.

Equations of state for ZnO and MgZnO by high pressure x-ray diffraction

MgZnO alloys are potentially important semiconductors for UV optoelectronics and other applications. Since device heterostructures are under significant strain, it is important to determine the elastic properties of these materials, in the bulk and nanoscale. In this work, the equations of state were obtained for ZnO and MgxZn1-xO alloys under hydrostatic pressure, using x-ray diffraction. The equations of state were found to be insensitive to Mg content up to x = 0.2. The pressure dependence of the c/a ratio also did not depend on Mg content, within experimental uncertainty. Combining these results with previous ultrasonic measurements, the following elastic constants were determined to be valid for wurtzite MgxZn1-xO (x≤ 0.2): C11 = 207 GPa, C12 = 118 GPa, C33 = 210 GPa, and C13 = 106 GPa. These elastic constants yield a bulk modulus of 143 GPa and c/a slope of -3×10-4GPa-1. Nanocrystals showed the same bulk moduli as their bulk counterparts. However, nanocrystals showed consistently steeper c/a slopes,...

Dopants in nanoscale ZnO

Zinc oxide (ZnO) is a metal-oxide semiconductor that has attracted resurgent interest as an electronic material for a range of device applications. In our work, we have focused on how defect properties change as one goes from the bulk to the nanoscale. Infrared (IR) reflectance spectra of as-grown and hydrogen-annealed ZnO nanoparticles were measured at near-normal incidence. The as-grown particles were electrically semi-insulating, and show reflectance spectra characteristic of insulating ionic crystals. Samples annealed in hydrogen showed a significant increase in electrical conductivity and free-carrier absorption. A difference was observed in the reststrahlen line shape of the conductive sample compared to that of the as-grown sample. In addition to hydrogen doping, we successfully doped ZnO nanoparticles with Cu. To probe the electronic transitions of Cu 2+ impurities in ZnO nanoparticles, IR transmission spectra were taken at liquid-helium temperatures. Two absorption peaks were observed at energies of 5781 and 5821 cm -1 . Finally, we tentatively assign a series of IR spectral lines to Na acceptors.