A. Y. Cheng

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.

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.

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.

Introduction of extrinsic defects into mercuric iodide during processing

Low temperature (4.2 K) photoluminescence spectroscopy (PL) measurements were performed on mercuric iodide (HgI2) crystals which were intentionally doped with copper or silver during KI etching. PL spectra obtained after these doping experiments show specific Cu and Ag features similar to those previously observed after deposition of Cu or Ag contacts on mercuric iodide crystals. The in‐diffusion of Cu or Ag into bulk HgI2 has also been confirmed a few days after doping. This diffusion introduces new recombination centers in the material. This work suggests that the processing steps used to fabricate mercuric iodide nuclear detectors can lead to the introduction of new defects which are detrimental to detector performance.

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.

Study of stoichiometry in mercuric iodide by low temperature photoluminescence spectroscopy

Low temperature (4.2 K) photoluminescence spectroscopy (PL) measurements were performed on mercuric iodide (HgI2) crystals that were surface-doped with either iodine or mercury. Two methods of treatment were used to achieve the surface doping. The first is the direct immersion of HgI2 samples into potassium iodide (KI) aqueous solution saturated with iodine or immersion into elemental mercury liquid. The second is the storage of HgI2 crystals under either iodine or mercury vapor. Certain features in the PL spectra were correlated with the stoichiometry of the HgI2/ crystals modified by the surface doping. It was also found that if HgI(subscript 2 was exposed to air, an iodine deficient surface layer would form within a one-day period due to the preferential loss of iodine. Finally, the behavior of a broad emission band in the PL spectra and its implication in the fabrication of high quality HgI2 nuclear detector is discussed.