X. J. Bao

Incorporation of defects during processing of mercuric iodide detectors

The effects of chemical etching in KI solution, heating, and vacuum exposures of HgI2 were individually studied by low‐temperature photoluminescence (PL) spectroscopy. Each of these processing steps is important in the manufacturing of mercuric iodide detectors and may be responsible for the incorporation of carrier traps both in the near‐surface region and in the bulk. The results of etching experiments showed that the near‐surface region has a different defect structure than the bulk, which appears to result from iodine deficiency. Bulk heating at 100 °C also modifies the defect structure of the crystal. Vacuum exposure has an effect similar to chemical etching, but it does not cause significant degradation of the stoichiometry for recently KI‐etched specimens. These studies suggest that some features in the PL spectra of HgI2 are associated with stoichiometry of the specimens.

Hydrogen treatment effect on shallow and deep centers in GaSb

It is shown by spreading resistance and capacitance–voltage measurements that atomic hydrogen passivates shallow acceptors and donors in GaSb. Deep level passivation by hydrogen also occurs, as revealed by deep level transient spectroscopy measurements on Schottky diode structures. Effective diffusion coefficients for hydrogen were determined for both n+ and p+ GaSb; in the former case the diffusion is thermally activated with the relationship DH=3.4×10−5e−0.55 eV/kT, whereas in p+ material DH=1.5×10−6e−0.45 eV/kT over the temperature range 100–250 °C. Reactivation of passivated shallow and deep levels occurs for temperatures of 250–300 °C.

Investigation of copper electrodes for mercuric iodide detector applications

Copper diffusion in mercuric iodide was studied by low‐temperature photoluminescence (PL) spectroscopy and Auger electron spectroscopy. A broad radiative emission band at a wavelength of about 6720 A in the PL spectra was found to be related to Cu incorporation in the crystal. PL spectra obtained from surface doping experiments indicate that Cu is a rapid diffuser in HgI2 bulk material. Auger electron spectroscopy performed as a function of depth from the crystal surface confirms the rapid bulk diffusion process of Cu in HgI2. Fabrication of HgI2 nuclear detectors with Cu electrodes indicates that Cu is not acceptable as an electrode material, which is consistent with the fact that it diffuses easily into the bulk crystal and introduces new radiative recombination centers.

Effects of indium and tin overlayers on the photoluminescence spectrum of mercuric iodide

Mercuric iodide (HgI2 ) crystals with semitransparent metal overlayers of indium and tin were characterized using low‐temperature photoluminescence (PL) spectroscopy. The PL spectra were found to differ for points beneath the thin metal overlayers and points that were masked off during each deposition. The photoluminescence data were compared with PL measurements taken on HgI2 photodetectors with indium‐tin‐oxide (ITO) entrance electrodes. The similarities of the spectra for the HgI2 samples with In, Sn, and ITO conducting overlayers indicate that the regions in the ITO‐contacted photodetectors with relatively poor photoresponses are associated with the interaction of indium or tin with the mercuric iodide substrate.

Low-temperature photoluminescence studies of mercuric-iodide photodetectors

Mercuric‐iodide (HgI2 ) photodetectors with sputtered indium‐tin‐oxide (ITO) entrance electrodes were studied using low‐temperature photoluminescence spectroscopy. The photoluminescence spectrum obtained on each photodetector was found to differ for points beneath the ITO contact and points adjacent to it, indicating that the contact fabrication process introduces new carrier traps and radiative recombination centers within the ITO‐HgI2 interfacial region. In particular, a new broad band was observed in the spectra taken from points beneath the ITO electrode. Photocurrent‐versus‐position measurements showed that the intensity of this broad band was enhanced in regions having relatively poor photoresponse.

High resolution 4.2 K near band‐gap photoluminescence spectrum of mercuric iodide

We have investigated in detail (resolution up to 0.35 A) the near‐band‐gap 4.2 K photoluminescence spectrum of undoped Hgl2 in its red tetragonal form. At least 26 emission lines are resolved in the wavelength region between 5290 and 5400 A. Many of these are reported for the first time. We have also tabulated the steplike emission lines between 5220 and 5290 A.

STUDY OF SEMITRANSPARENT PALLADIUM CONTACTS ON MERCURIC IODIDE BY PHOTOLUMINESCENCE SPECTROSCOPY AND THERMALLY STIMULATED CURRENT MEASUREMENTS

Semitransparent palladium contacts on mercuric iodide were studied by low temperature photoluminescence spectroscopy and thermally stimulated conductivity. These contacts were deposited either by thermal evaporation or by plasma sputtering. Changes due to palladium deposition were found in the photoluminescence spectra and were attributed to modifications in the stoichiometry within the palladium/mercuric iodide interfacial region. Thermally stimulated conductivity measurements revealed two dominant traps with activation energies of 0.010 and 0.54 eV. The importance of these traps in the application of nuclear detection is discussed.

Correlations between mercuric iodide photoluminescence spectra and nuclear detector performance

Abstract Low temperature photoluminescence spectroscopy was performed on a variety of HgI 2 samples and also on graded HgI 2 nuclear detectors. Correlations were found between features in the photoluminescence spectra and a crystals ability to produce high-quality detectors. The intensity of a broad emission band centered at 6200 A (designated as band 3) is weaker in crystals that yield high-quality detectors. Therefore, the defects responsible for this emission band are undesirable in the fabrication of HgI 2 nuclear detectors. The measurements also revealed that stronger emission in the exciton region (designated as band 1) is associated with crystals which produce high-quality detectors, indicating that a high degree of structural perfection is important for HgI 2 detector applications. These correlations, together with earlier results from studies of processing-induced defects, lead to suggestions regarding improvement of the manufacturing yield of high-quality HgI 2 detectors.

Carrier traps and transport in mercuric iodide

Abstract Thermally stimulated current spectroscopy (TSC) was performed on a variety of mercuric iodide samples and detectors to determine the nature and origin of deep traps in this material. It is shown that the trap type and concentration is a function of the metal overlayer employed as a contact material. The energy barrier height as well as the type (electron or hole) of barrier at the metal/semiconductor interface has also been determined by internal photoemission measurements. When polarization effects are not present, as is the case in most Pd contacted samples, the barrier height can be accurately determined by this technique. A value of 1.05 eV was measured for a hole barrier at the Pd/Hgl 2 interface.

Photoluminescence investigations of defects introduced during processing of mercuric iodide nuclear detectors

Abstract Low-temperature photoluminescence (PL) spectroscopy was performed on a variety of HgI 2 samples to determine the effects of chemical etching with Kl and HNO 3 solutions and the modifications in the PL spectra due to the presence of carbon, chromium and parylene films. These investigations reveal that the processing steps used to manufacture HgI 2 nuclear detectors can lead to the incorporation of new defects into the near-surface region of the crystals. Moreover, correlations between the photoluminescence spectra and detector performance show that some of these defects are undesirable for producing high-quality devices.