Michael J. Suscavage

New iodide method for growth of GaN

A new method, Iodine Vapor Phase Growth (IVPG), of growing GaN films and small single crystals by using elemental iodine instead of HCI has been developed. Elemental iodine was used to transport gallium metal into the reaction zone where the gallium iodide reacts with ammonia to produce GaN. GaN films were grown on sapphire substrates having a MOCVD template layer. Films with mobility as high as 660 cm 2 /Vs with carrier concentrations in the range of n = 5 x 10 16 to 3 x 10 17 cm -3 have been grown in a temperature range of 1000 to 1110 °C.

Electrical conductivity of zinc-based fluoride glasses

The dc electrical conductivity of R‐Zn‐Ba‐Yb‐Th fluoride glasses, where R is Li or Na, was measured as a function of RF content. The replacement of BaF2 by RF resulted in a decrease in the electrical conductivity. This finding is the reverse of that expected for oxide glasses, but is in agreement with reports of similar behavior in zirconium‐based fluoride glasses. The electrical conductivity of a glass containing LiF was always less than that of the corresponding glass containing NaF. It is suggested that these results are due to changes in the strain energy needed to move a fluorine ion from one site to another.

High Quality Hydrothermal ZnO Crystals

Zinc Oxide crystals have historically been grown in hydrothermal autoclaves with a basic mineralizer; however, doubts have been raised about the quality of such crystals because they have often exhibited large x-ray rocking curve widths and low photoluminescence (PL) yield with large linewidths. Several ZnO crystals were grown hydrothermally and sliced parallel to the c-plane. This resulted in opposite surfaces (the C + and C - ) exhibiting pronounced chemical and mechanical differences. Different surface treatments were investigated and compared by PL both at room temperature and liquid helium temperatures, and by double axis X-ray rocking curve measurements. The high quality of hydrothermally-grown ZnO is substantiated by the narrow rocking curve widths and sharp PL peaks obtained. A critical factor in obtaining these results was found to be surface preparation.

A Study of the Surface Morphological Features of the Polar Faces of ZnO by Atomic Force Microscopy (AFM) Methods and AlN Thin Films Deposited on ZnO Polar Faces by PLD

The effects of both temperature and atmosphere on the resulting morphological features of the polar faces of single crystal ZnO were investigated and characterized by atomic force microscopy (AFM). In studies where ZnO was thermally processed in flowing oxygen at atmospheric conditions within the temperature range of 500°C to 900°C for 30 minutes, the Znsurface (i.e., (000 1)) showed a tendency to reconstruct with increasing temperature until terraces became evident at 900°C. Terrace heights were as small as 0.9 nm. In contrast, the O-surface (i.e., (000 ī)) was observe to change very little during the O 2 -atmoshere, thermal treatment and remained comparatively rougher than the Zn-surface. ZnO samples which were thermally processed under high vacuum (i.e., 5 × 10 -7 Torr) conditions exhibited a more dramatic contrast. The vacuum annealed Zn-surface was observed to develop very smooth surface features (Roughness = 0.09 nm) at annealing temperatures within the 700 – 800°C range. In contrast, and as expected, the O-surface roughness increased due to surface reduction reactions. In addition to these findings, it is noted that AFM measurements may be utilized as a convenient method to distinguish between the two polar surfaces of ZnO. Aluminum nitride was deposited on the Zn- and O- surfaces from 700 to 850°C by pulsed laser evaporation. X-ray diffraction indicated that the AIN was c-axis oriented with no interface reaction products detected between the ZnO substrate and AIN film.

Cathodoluminescence Study of Ammonothermal GaN Crystals

GaN crystals grown in supercritical ammonia by the ammonothermal method were studied by cathodoluminescence (CL), both in image and spectrally resolved modes. The main extended defects and the incorporation of point defects and impurities in different growth sectors were revealed. The influence of the seeds, the role of the growth planes and the changes in the crystal quality during the growth run are discussed.

Gamma-induced coloring of doped heavy-metal fluoride glasses

Fluorozirconate glass compositions were irradiated with 1 . 1 73 and 1. 333 MeV gamma-rays from a cobalt-60 source to study the influence which fluorine network concentrations sodium doping indium doping cobalt doping cerium doping have on radiation damage. Lower concentrations of nonbridging fluorine ions were found to reduce color-center absorption and indium doping was found to eliminate visible and infrared radiation-induced colorization. 1.

Annealing of Induced Damage in Fluoride Glass Components

Heavy metal fluoride (HMF) glasses are of considerable interest for use as multispectral optical components. HMF glass samples of different compositions were prepared and subjected to different doses of gamma radiation atroom temperature. Optical absorption measurements showed that strong damage occurred in the UV and visible regions and no significant losses were detected in the infrared region.