Mohammad A. Mannan

Effect of Z1/2, EH5, and Ci1 deep defects on the performance of n-type 4H-SiC epitaxial layers Schottky detectors: Alpha spectroscopy and deep level transient spectroscopy studies

Spectroscopic performance of Schottky barrier alpha particle detectors fabricated on 50 μm thick n-type 4H-SiC epitaxial layers containing Z1/2, EH5, and Ci1 deep levels were investigated. The device performance was evaluated on the basis of junction current/capacitance characterization and alpha pulse-height spectroscopy. Capacitance mode deep level transient spectroscopy revealed the presence of the above-mentioned deep levels along with two shallow level defects related to titanium impurities (Ti(h) and Ti(c)) and an unidentified deep electron trap located at 2.4 eV below the conduction band minimum, which is being reported for the first time. The concentration of the lifetime killer Z1/2 defects was found to be 1.7 × 1013 cm−3. The charge transport and collection efficiency results obtained from the alpha particle pulse-height spectroscopy were interpreted using a drift-diffusion charge transport model. Based on these investigations, the physics behind the correlation of the detector properties viz., ...

Correlation of Deep Levels With Detector Performance in 4H-SiC Epitaxial Schottky Barrier Alpha Detectors

High resolution Schottky barrier detectors for alpha particles were fabricated using 20 μm thick detector grade n-type 4H-SiC epitaxial layer. The Schottky barrier detectors were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Deep level transient spectroscopic (DLTS) measurements were carried out to identify and characterize the electrically active defect levels present in the epitaxial layers. The detection properties of the Schottky detectors were characterized in terms of alpha particle peak widths in pulse height spectra obtained using a standard alpha emitting radioisotope source. The differences in the performance of different detectors were correlated on the basis of the barrier properties and the deep level defect type, concentration, and capture cross-section. Varying degree of the presence of deep level defects was found to be the reason behind the leakage current variation and the difference in the ultimate detector performance observed among the detectors. From the DLTS data it was found that at least two defect centers located at Ec-0.6 eV (Z1/2) and at Ec -1.6 eV (EH6/7),both related to carbon vacancies, affected the detector performance the most.

Deep Levels in n-Type 4H-Silicon Carbide Epitaxial Layers Investigated by Deep-Level Transient Spectroscopy and Isochronal Annealing Studies

Deep levels were investigated by the capacitance mode deep-level transient spectroscopy (C-DLTS) on 4H-SiC Schottky barrier diodes fabricated on 50 μm-thick n-type 4HSiC epitaxial layers. C-DLTS scans from 80 K to 800 K revealed the presence of Ti(c), Z1/2, EH5, and EH6/7 defect levels in the energy range from 0.17 to 1.6 eV below the conduction band edge. The annealing out of primary defects and generation of secondary defects were investigated by systematic and thorough C-DLTS studies from prior and subsequent isochronal annealing in the temperature range from 100 °C to 800 °C. The capture cross-section of Ti(c) was observed to decrease up to 400 °C and remained unchanged at higher annealing temperatures. Defect densities were shown to decrease up to 200 °C and gradually increase at higher temperatures. The Z1/2 and EH6/7 defect parameters showed similar variation for the temperature range studied. The thermal evolutions of these deep levels in n-type 4H-SiC epitaxial layers are analyzed and discussed for the first time.

Surface passivation and isochronal annealing studies on n-type 4H-SiC epitaxial layer

Schottky barrier radiation detectors were fabricated on the Si-face of 50 μm thick detector grade n-type 4H-SiC epitaxial layers. The junction properties of the fabricated detectors were investigated by current-voltage (I-V) and capacitancevoltage (C-V) measurements. The radiation detector performances were evaluated by alpha pulse height spectroscopy using a 0.1 μCi 241Am radiation source. Deep level transient spectroscopy (DLTS) measurements were carried out to identify and characterize the electrically active defect levels present in the epitaxial layers. The performance of the detector was found to be limited by the presence of electrically active defect centers in the epilayer. Deep level defects were reduced significantly by isochronal annealing. Surface passivation studies were conducted on n-type 4H-SiC epilayers for use on radiation detectors for the first time. Energy resolution of the detector was found to have improved after passivation and the life time killing defects that were responsible for preventing full charge collection were reduced significantly. Systematic and thorough C-DLTS studies were conducted prior and subsequent to isochronal annealing to observe evolution of the deep level defects.

Characterization of high-resistivity CdTe and Cd0.9Zn0.1Te crystals grown by Bridgman method for radiation detector applications

CdTe and Cd0.9Zn0.1Te (CZT) crystals have been studied extensively for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from zone refined ultra-pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled, and optimized by a computer simulation and modeling program developed in our laboratory. The grown crystals were thoroughly characterized after cutting wafers from the ingots and processed by chemo-mechanical polishing (CMP). The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ~10 μm for CdTe and ~8 μm for CZT crystal. The etch pit density was ≤ 5×104 cm-2 for CdTe and ≤ 3×104 cm-2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ~ 1.5×1010 Ω-cm for CdTe and 2-5×1011 Ω-cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτe and μτh) showed μτe = 2×10-3 cm2/V (μτh = 8×10-5 cm2/V) and 3-6×10-3 cm2/V (μτh = 4- 6×10-5 cm2/V) for CdTe and CZT, respectively. Detectors in single pixel, Frisch collar, and coplanar grid geometries were fabricated. Detectors in Frisch grid and guard-ring configuration were found to exhibit energy resolution of 1.4% and 2.6 %, respectively, for 662 keV gamma rays. Assessments of the detector performance have been carried out also using 241Am (60 keV) showing energy resolution of 4.2% FWHM.

Defect characterization of Cd0.9Zn0.1Te crystals using electron beam induced current (EBIC) imaging and thermally stimulated current (TSC) measurements

Semi-insulating Cd0.9Zn0.1Te nuclear detector grade crystals were grown by a low temperature solution method from in-house zone refined (~7N) precursor materials. The processed crystals from the grown ingot were thoroughly characterized by using a non-destructive electron beam induced current (EBIC) contrast imaging method. The EBIC results were correlated with the infrared (IR) transmittance mapping, which confirms the variation of contrasts in EBIC is due to non-uniform distribution of tellurium inclusions in the grown CZT crystal. Electrical characteristics of defect regions in the fabricated detectors were further investigated by I-V measurements, and thermally stimulated current (TSC) measurements. Finally, to demonstrate the high quality of the grown CZT crystals, pulse height spectra (PHS) measurements were carried out using gamma radiation sources of 241Am (59.6 keV) and 137Cs (662 keV).

High-barrier Schottky contact on n-type 4H-SiC epitaxial layer and studies of defect levels by deep level transient spectroscopy (DLTS)

High barrier Schottky contact has been fabricated on 50 μm n-type 4H-SiC epitaxial layers grown on 350 μm thick substrate 8° off-cut towards the [11̅20] direction. The 4H-SiC epitaxial wafer was diced into 10 x 10 mm2 samples. The metal-semiconductor junctions were fabricated by photolithography and dc sputtering with ruthenium (Ru). The junction properties were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Detectors were characterized by alpha spectroscopy measurements in terms of energy resolution and charge collection efficiency using a 0.1 μCi 241Am radiation source. It was found that detectors fabricated from high work function rare transition metal Ru demonstrated very low leakage current and significant improvement of detector performance. Defect characterization of the epitaxial layers was conducted by deep level transient spectroscopy (DLTS) to thoroughly investigate the defect levels in the active region. The presence of a new defect level induced by this rare transition metal-semiconductor interface has been identified and characterized.

Investigation of thermally evaporated high resistive B-doped amorphous selenium alloy films and metal contact studies

Amorphous selenium (a-Se) alloy materials with arsenic, chlorine, boron, and lithium doping were synthesized for room temperature nuclear radiation detector applications using an optimized alloy composition for enhanced charge transport properties. A multi-step synthetic process has been implemented to first synthesize Se-As and Se-Cl master alloys from zone-refined Se (~ 7N), and then synthesized the final alloys for thermally evaporated large-area thin-film deposition on oxidized aluminum (Al/Al2O3) and indium tin oxide (ITO) coated glass substrates. Material purity, morphology, and compositional characteristics of the alloy materials and films were examined using glow discharge mass spectroscopy (GDMS), inductively coupled plasma mass spectroscopy (ICP-MS), differential scanning calorimetry (DSC), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive analysis by x-rays (EDAX). Current-Voltage (I-V) measurements were carried out to confirm very high resistivity of the alloy thin-films. We have further investigated the junction properties of the alloy films with a wide variety of metals with different work functions (Au, Ni, W, Pd, Cu, Mo, In, and Sn). The aim was to investigate whether the choice of metal can improve the performance of fabricated detectors by minimizing the dark leakage current. For various metal contacts, we have found significant dependencies of metal work functions on current transients by applying voltages from -800 V to +1000 V.

Deep levels in n-type 4H-SiC epitaxial Schottky detectors by deep level transient spectroscopy and effects of edge termination on energy resolution

High resolution Schottky barrier alpha detectors have been fabricated on 20 μm n-type 4H-SiC epitaxial layers. The junction properties of the detectors were studied by current-voltage (I-V) and capacitance-voltage (C-V) measurements. A thermionic emission model applied to the forward I-V characteristic revealed an effective surface barrier height of 1.72 eV and a diode ideality factor of 1.18 suggesting uniform spatial distribution of surface barrier height. The C-V measurements revealed a doping concentration of 1.8×1014 cm-3 which ensured a fully depleted (~20 μm) detector at bias voltages as low as ~70 V. Alpha spectroscopy measurements revealed an energy resolution of 0.5% for 5.48 MeV alpha particles. The performance of these detectors is limited by the presence of microscopic and macroscopic defects. Deep level transient spectroscopy (DLTS) studies revealed the presence of Ti(c), Z½, and possible Ci1 defect centers. An improvement in detector resolution to ~0.3% was observed after edge termination while evaluating effects of surface defects on device performance.