D. Wong

Sputter deposition of YBa2Cu3O6+x on alumina and the influence of ZrO2 buffer layers

The deposition of films of YBa2Cu3O6+x by rf diode sputtering on alumina has been investigated. Although a stoichiometric (123) target was employed, the film composition differs from that of the target and varies as a function of position on the substrate. Films displaying a broad resistive transition have been produced in a reproducible manner. It is demonstrated that the use of a ZrO2 buffer layer decreases the transition width significantly and consistently yields films which have a more metallic‐like behavior above Tc.

The effect of annealing treatments on defect structure and diffusion lengths in bulk n‐type GaAs

Specimens from n‐type GaAs wafers have been annealed at high temperature (900–1050 °C) and then characterized using deep‐level transient spectroscopy (DLTS) and electron‐beam‐induced current techniques. Relatively short anneals result in substantial changes to the electron trap structure in this material. Diffusion lengths are, at best, marginally increased by short (16 min and below) anneals but have been found to be significantly increased by longer anneals of between 40 and 80 min. DLTS measurements of hole traps in both unannealed and annealed n‐type material suggest that a hole trap we term HCX may be an important recombination center in this class of material. The results obtained demonstrate the effectiveness of wafer annealing as a technique for creating a (10 μm deep) near‐surface zone in n‐type bulk material in which defects are suppressed and diffusion lengths improved.

Optical detection of impurities and defects in detector-grade mercuric iodide

Abstract We report the results of two different optical techniques, Fourier transform infrared (FTIR) spectroscopy and low-temperature photoluminescence, used to study impurities and native defects in mercuric iodide crystals and nuclear detectors. Several absorption bands associated with the presence of impurities are observed in transmission infrared spectra. Our measurements indicate that significant amounts of water are likely contained in the HgI 2 crystals, and that the concentration of the water can be reduced by appropriate storage of the material. The FTIR spectra also show the existence of hydrocarbons in the bulk material. Several other weak absorption bands due to contaminants are detected in the transmission measurements. Low-temperature photoluminescence studies were also conducted on fully processed nuclear detectors to elucidate the relation between the measured luminescence and the detector response. At 78 K we observe a spectral line that appears to be correlated with the detector quality. In general, the photoluminescence results suggest that detector fabrication steps cause significant modifications to the stoichiometry in the near-surface region of the HgI 2 crystals.

HgI2 near‐band‐gap photoluminescence structure and its relationship to nuclear detector quality

The low‐temperature photoluminescence spectra of several mercuric iodide detectors and off‐stoichiometric bulk material have been characterized. Phonon energies have been determined with Raman spectroscopy over a range of temperatures. In earlier work some of the near‐band‐gap photoluminescence features were identified as phonon replicas. After careful examination of Raman and photoluminescence data, we find that one or perhaps more of these features is probably due to shallow electronic levels related to native defects. Suggestions as to the relationship between photoluminescence peaks and detector quality are made.

Hysteresis model for polycrystalline high‐Tc superconductors

The hysteresis exhibited by ceramic superconductors can be modeled using a phenomenological critical current density of the form Jc=α/(B0+B) with suitable extensions. At low temperatures (near 4 K), satisfactory results can be obtained using this form and neglecting the equilibrium magnetization. However, we show that at higher temperatures the incorporation of the equilibrium magnetization becomes more important. At 77 K, the commonly observed shape of the curves is dominated by the equilibrium magnetization. An approximate model including the equilibrium magnetization is described and compared to measurements made at 77 K.

Trap suppression by isoelectronic In or Sb doping in Si‐doped n‐GaAs grown by molecular‐beam epitaxy

The effects of isoelectronic doping of GaAs by In or Sb on the electron deep levels in n‐GaAs grown by molecular‐beam epitaxy have been investigated in the growth temperature range 500–600 °C for Si doping levels of 4–7×1016 cm−3 and As‐stabilized conditions. The two dominant traps M3 and M6 are drastically reduced in concentration by up to three orders of magnitude for M3 (from 1015 cm−3 down to <1012 cm−3) and two and a half orders of magnitude for M6 by introducing 0.2–1 at.% In or Sb and increasing growth temperatures from 500 to 550 °C. The trap concentrations of M3 and M6 were also significantly reduced by increasing the growth temperature to 600 °C without In or Sb doping and by decreasing the growth rate from 1.0 to 0.3 μm/h. The incorporation coefficients of In and Sb have been measured and are found to decrease with increasing growth temperature. The growths with high M3 and M6 trap densities are shown to have short minority‐carrier diffusion lengths. Indium isoelectronic doping, which is presum...

Low‐field structure in the magnetization of polycrystalline YBa2Cu3O7−x and ErBa2Cu3O7−x

We have studied the structure present at small fields (<50 Oe) in the magnetic hysteresis of polycrystalline samples of YBa2Cu3O7−x and ErBa2Cu3O7−x. This structure is absent from measurements on a powder of YBa2Cu3O7−x. We conclude that the structure results from interactions among the grains and is not an intrinsic property of the compounds. Flux creep associated with this structure is found to be proportional to ln(t) over three decades of observation time.

Suppression of the dominant recombination center in n‐type GaAs by proximity annealing of wafers

n‐type GaAs specimens have been annealed in sealed quartz ampoules and characterized with electron‐beam‐induced current and photoresponse measurements, deep‐level transient spectroscopy, and photoluminescence spectroscopy. By correlating changes in the concentrations of defects with minority‐carrier diffusion length (Lp) it is shown that the dominant recombination center in this material is a hole trap termed HCX (Ev+0.29 eV). Increases in Lp of up to a factor of 3, which can be achieved by proximity annealing at 950 °C for 16 h, are related to the loss of As from the specimen surfaces during the early stages of annealing. The beneficial effect of the annealing is associated with a limited source diffusion process since the total amount of As loss is determined by the ratio of the ampoule volume to the GaAs surface area. Proximity protection of the surfaces is necessary to prevent the generation of a Ga vacancy‐related hole trap HCZ (Ev+0.57 eV).

Reduction of bulk and surface recombination in GaAs for improved solar cell performance

A technique for suppressing recombination centers and increasing minority-carrier diffusion lengths in bulk n-type GaAs is described. It is shown that such pretreatment of the material leads to substantially improved long-wavelength photocurrent collection in Zn-diffused solar cells fabricated directly in the bulk substrates. Passivation of the front surface of the cells with ammonium sulfide improves their short-wavelength photoresponse. These results suggest the possibility of realizing high-efficiency GaAs solar cells fabricated without the use of costly epitaxial technology.<<ETX>>

Photoluminescence variations associated with the deposition of palladium electrical contacts on detector-grade mercuric iodide

Specimens of mercuric iodide with evaporated semitransparent palladium contacts have been studied using low‐temperature photoluminescence spectroscopy. Distinct differences were found between spectra taken from beneath the Pd contacts and those taken from regions on the HgI2 sample that were masked during the Pd deposition, indicating that contact fabrication can change the defect structure near the contact/substrate interface. Comparison of the spectra from spots beneath the contacts with spectra from bulk material specimens and HgI2 detectors graded in terms of their nuclear detection performance suggests that the processing steps used to deposit electrical contacts and the choice of contact material may have a significant influence on detector performance.