Roger J. Reeves

Buried p-type layers in Mg-doped InN

Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage (CV) analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. Early indications suggest the Mg acceptor level in InN may lie near 110meV above the valence band maximum. The development of p-type doping techniques offers great promise for future InN based devices.

Influence of spontaneous polarization on the electrical and optical properties of bulk, single crystal ZnO

Hall effect, photoluminescence, and Schottky diode measurements were made on the Zn-polar and O-polar faces of undoped, bulk, single crystal, c-axis ZnO wafers. Significant polarity related differences were observed in the PL and Schottky diode characteristics of low carrier concentration, hydrothermally grown wafers. Increased emission from free exciton recombinations and from recombinations between 3.3725 and 3.3750eV was observed on the Zn-polar face. Conversely, emission between 3.3640 and 3.3680eV was more intense on the O-polar face. The barrier heights of silver oxide Schottky diodes were approximately 130meV larger on the Zn-polar face compared to the O-polar face.

Stimulated Raman scattering of picosecond pulses in barium nitrate crystals

Abstract Stimulated Raman Scattering (SRS) of picosecond pulses was investigated in Ba(NO3)2 crystals. The results are in good agreement with transient SRS theory. First and second Stokes conversion efficiencies were measured to be 25% and 5%, respectively.

Oxidized noble metal Schottky contacts to n-type ZnO

Deliberately oxidized iridium, platinum, and palladium Schottky contacts were fabricated on the Zn-polar and O-polar faces of hydrothermal bulk ZnO by eclipse pulsed laser deposition in an oxygen ambient. The barrier heights of these oxidized contacts were significantly higher than their plain metal counterparts, with ideality factors approaching the image-force-controlled limit for laterally homogeneous interfaces. The key aspects of this technique are a low deposition energy and the use of an oxidizing environment which reduces interfacial defects, particularly oxygen vacancies. In each case, the barriers on the Zn-polar face were 210–260 meV higher than those on the O-polar face.

Structural and photoluminescence properties of Gd implanted ZnO single crystals

We present the structural and photoluminescence properties of 30 keV gadolinium implanted and subsequently annealed zinc oxide (ZnO) single crystals. Rutherford backscattering and channeling results reveal a low surface region defect density which was reduced further upon annealing. For low implantation fluence, around 85% of the Gd atoms are estimated to be in sites aligned with the ZnO lattice, while for higher fluences the Gd is largely disordered and likely forms precipitates. The Raman spectra of the implanted samples show defect-induced modes, which match the one-phonon density of states for the most heavily implanted samples. Annealing eliminates these features implying the removal of Gd-associated lattice disorder. Low temperature photoluminescence spectra revealed a red-shift in the defect emission, from green to orange/yellow, indicating the suppression of a deep level, which is thought to be due to oxygen vacancies. It is suggested that the orange/yellow emission is unmasked when the green emis...

Defects in hydrothermally grown bulk ZnO

Hydrothermally grown bulk ZnO (Tokyo Denpa) was investigated using junction-capacitance spectroscopy on silver oxide Schottky contacts (barrier height of 1.20eV, ideality factor of 1.04). Two main shallow defects, T1 and T2, with thermal activation energies of 13 and 52meV, respectively, were identified. Two closely lying, deep defect levels E3∕E3′ at approximately 320meV below the conduction band were found in higher concentrations (mid-1014cm−3) than the shallow donors. 4K photoluminescence showed dominant emission from excitons bound to three neutral donors, aluminum, hydrogen, and an unassigned impurity, with donor binding energies close to the thermal activation energy of T2.

Studies of the spectroscopic properties of Pr3+ doped LaF3 nanocrystals/glass

Abstract Emission and excitation spectra are presented for praseodymium (Pr 3+ , 0.01 at%) doped oxyfluoride glass host containing LaF 3 nanocrystals. Two types of Pr 3+ ions, those in LaF 3 nanocrystals and those in the glass host, are characterized by spectroscopic and dynamical studies. Pr 3+ ions in the glass are selectively excited by pumping the 4f5d band and UV emission is observed. For Pr 3+ ions in the glass, the relaxation of 3 P 0 is dominated by multi-phonon processes to 1 D 2 .

Properties of nitrogen implanted and electron beam annealed bulk ZnO

The optical properties of bulk ZnO ion implanted with nitrogen ions, at an energy of 23 keV have been studied as a function of implantation fluence and electron beam (EB) annealing conditions. Nuclear reaction analysis and Raman results have revealed the implanted N concentration and its structural changes with respect to various nitrogen ion fluences. The optical properties of nitrogen implanted bulk ZnO were investigated by low temperature photoluminescence measurements. An enhanced peak at 3.235 eV has been attributed to donor-accepter pair (DAP) emission involving the implanted N acceptor in ZnO. The emission near 3.3085 eV is attributed to a free electron to acceptor transition. We also report a broad band emission feature at ∼3.09 eV in the nitrogen implanted with 1–2×1015 ions cm−2 and EB annealed at 800–900 °C. This is assigned to a thermally activated nitrogen acceptor transition as it is unique only to nitrogen implanted samples. An ionization energy of 377 meV indicates that this line may corre...

Reactive ion etched gallium nitride : metastable defects and yellow luminescence

Gallium nitride has been reactive-ion etched with SF6 and argon plasmas. The Ar-etched samples show a striking transition from a dominant blue luminescence band to a dominant yellow luminescence band after less than 5 min of low power illumination. The observation of metastable defects which are associated with both the yellow and blue bands has important consequences for our understanding of defect-related luminescence in gallium nitride.

Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in CaF2:Yb2+

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF(2):Yb(2+). The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.