Timothy C. Hayes

Layered GaTe Crystals for Radiation Detectors

In this work we investigated a new method of growing detector grade large GaTe layered chalcogenide single crystals. GaTe ingots (2″ diameter) were grown by a novel method using graphite crucible by slow crystallization from a melt of high purity (7N) Ga and Te precursors in an argon atmosphere. GaTe samples from the monocrystalline area of the ingot have been cleaved mechanically and characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis by x-rays (EDAX), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), transmission line matrix method (TLM), resistivity measurements using van der Pauw technique, Hall Effect, and Capacitance-Voltage measurements. Our investigations reveal high potential for developing superior quality GaTe crystals using this growth technique for growing large volume inexpensive GaTe single crystals for nuclear radiation detectors.

Fabrication and characterization of Cd0.9Zn0.1Te Schottky diodes for nuclear radiation detectors

We have fabricated and characterized cadmium zinc telluride (CZT) Schottky diodes with low reverse leakage current for high resolution radiation detector applications. The diodes were made using Cd0.9Zn0.1Te detector grade crystals grown by the low temperature tellurium solvent method. The diodes were characterized using electron beam induced current (EBIC) technique to investigate crystallographic defects. The EBIC images were correlated with transmission infrared (TIR) images of CZT crystals and the EBIC contrast was attributed to the nonuniformities in spatial distribution of Te. Further characterization by the thermally stimulated current (TSC) spectroscopy revealed shallow and deep level centers with activation energies 0.25- 0.4 eV and 0.65 - 0.8 eV respectively, which we attribute to intrinsic defects associated with excess of Te. Pulse height spectra (PHS) measurements were carried out using a 241Am (59.6 keV) radiation source on the Frisch collar radiation detectors made from the suitable portions of the CZT ingot used for Schottky diode fabrication, and an energy resolution of ~4.2% FWHM was obtained.

Defect correlation studies on 4H-SiC crystals and epitaxial layers for radiation detector applications

Nuclear radiation detectors in the energy range of soft x-rays have been fabricated using bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates. The devices have been characterized by optical microscopy, current-voltage (I-V) measurements, thermally stimulated current (TSC) spectroscopy (94K – 650 K), Hall effect, van der Pauw measurements, and electron beam induced current (EBIC) technique. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D- centers. Deep level centers in the n-type epitaxial layer peaked at ∼ 400 K (Ea ∼ 1.1 eV) and ∼ 470 K were correlated with IL2 defect and 1.1 eV center in high purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ∼ 290 K (Ea = 0.82 – 0.87 eV) that was attributed to IL1 and HK2 centers, and at ∼ 525 K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band gap. Results of EBIC and optical microscopy characterization showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layers. The I-V characteristics of the devices on SI epitaxial layers obtained in wide temperature range (94K – 650 K) exhibited steps at ∼ 1 V and ∼ 70 V corresponding to the ultimate trap filling of deep centers peaked at > 500 K and at ∼ 250 K (Ea ∼ 0.57 eV), & ∼ 300 K (Ea ∼ 0.85 eV) respectively. The high temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep level electron trap centers. The responsivity of the n-type epitaxial SiC sensors to low energy x-rays is reported for the first time.

Characterization of Cd 0.9 Zn 0.1 Te schottky diodes for high resolution nuclear radiation detectors

High barrier cadmium zinc telluride (CZT) Schottky diodes with very low reverse leakage current have been fabricated and characterized for high resolution gamma ray detectors. The diodes were made using Cd0.9Zn0.1Te detector grade crystals grown from zone refined Cd, Zn, and Te (7N) precursor materials using low temperature tellurium solvent method. Various crystallographic defects due to Te-inclusions/precipitates were identified and characterized using electron beam induced current (EBIC) measurement technique for the first time. The EBIC images were correlated very well with transmission infrared (TIR) images of CZT crystals and the EBIC contrast was attributed to the nonuniformities in spatial distribution of Te inclusions. Further characterization by the thermally stimulated current (TSC) spectroscopy revealed shallow and deep level centers with activation energies 0.25– 0.4 eV and 0.65 – 0.8 eV respectively, which was attributed to intrinsic defects associated with Te inclusions. Pulse height spectra (PHS) measurements were carried out using 137Cs (662 keV) radiation source and energy resolution of ∼1.51% FWHM was obtained from the as-grown boule.

Surface and defect correlation studies on high resistivity 4H SiC bulk crystals and epitaxial layers for radiation detectors

Radiation detectors have been fabricated using bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates. The devices have been characterized by optical microscopy, current-voltage (I-V) measurements, thermally stimulated current (TSC) spectroscopy (94K - 650 K), Hall effect, van der Pauw measurements, and electron beam induced current (EBIC) technique. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D- centers. Deep level centers in the n-type epitaxial layer peaked at ~ 400 K (Ea ~ 1.1 eV) and ~ 470 K were correlated with IL2 defect and 1.1 eV center in high purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ~ 290 K (Ea = 0.82 - 0.87 eV) that was attributed to IL1 center and 3C inclusions, and at ~ 525 K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band gap. The TSC spectra of the SI epitaxial layer exhibited peaks with different current polarity which we attributed to the built-in electric field reversal. Results of EBIC and optical microscopy characterization showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layers and presence of stacking faults and 3C-SiC inclusions in both epitaxial layers. The I-V characteristics of the devices on SI epi obtained in wide temperature range (94K - 650 K) exhibited steps at ~ 1 V and ~ 70 V corresponding to the ultimate trap filling of deep centers peaked at > 500 K and at ~ 250 K (Ea ~ 0.57 eV), & ~ 300 K (Ea ~ 0.85 eV) respectively. Slow processes of the injected carrier capture on traps resulted in the I-V characteristic with negative differential resistance (NDR). The high temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep level electron trap centers.