Kelvin J. Zavalla

Experimental Determination of Electron-Hole Pair Creation Energy in 4H-SiC Epitaxial Layer: An Absolute Calibration Approach

Electron-hole pair creation energy (e) has been determined from alpha spectroscopy using 4H-SiC epitaxial layer Schottky detectors and a pulser calibration technique. We report an experimentally obtained e value of 7.28 eV in 4H-SiC. The obtained e value and theoretical models were used to calculate a Fano factor of 0.128 for 5.48 MeV alpha particles. The contributions of different factors to the ultimate alpha peak broadening in pulse-height spectra were determined using the calculated e value and Monte-Carlo simulations. The determined e value was verified using a drift-diffusion model of variation of charge collection efficiency with applied bias.

Cd

Detector grade Cd0.9Zn0.1Te single crystals were grown using a tellurium solvent method. Single crystal blocks of volume ~1 cm3 were prepared for detector fabrication and characterization. The grown crystals were characterized using infra-red transmission imaging and Pockels effect measurements. Two detectors in single-polarity charge sensing configurations viz., small pixel, and virtual Frisch grid were fabricated on two crystals obtained from the same section of the ingot. Current-voltage measurements performed in planar configuration exhibited a very low leakage current of ~5 nA at 1000 V and resistivities of the order of 8.5×1010 Ω·cm. Electron drift mobilities of the order of 840 cm2/V.s and electron mobility-lifetime products of the order of 2.7×10-3 cm2/V were calculated from alpha spectroscopy using detectors in planar configuration. The small pixel and the virtual Frisch grid detector showed similar energy resolution of 3.7% for 662 keV gamma rays however, the virtual Frisch grid configuration revealed a better overall performance with a peak-to-Compton ratio of 2.8. A digital spectrometer and related software has been developed using a digitizer card and used to employ offline correction schemes to compensate for the charge loss effects, resulting in significant improvement of the 662 keV peak resolution (1.8% as compared to 3.7% without correction) obtained in the case of small pixel detector.

_{0.9}

Detector grade Cd0.9Zn0.1Te (CZT) single crystals were grown from zone refined Cd, Zn, and Te (∼7 N) precursor materials, using a tellurium solvent method. Detectors with virtual Frisch grid configuration were fabricated using these crystals. I-V measurements revealed low leakage currents at room-temperature, ∼11 nA for one such detector D1 and ∼8 nA for another detector D2 at 1100 V. The spectroscopic performances of the two CZT virtual Frisch grid detectors have been evaluated and compared for high energy gamma ray detection. Detector D1 showed a well-resolved pulse-height spectrum with an energy resolution of ∼1.6% for the 662 keV gamma rays. Detector D2 also showed a distinct 662 keV peak but with a broader pulse-height distribution. A digital biparametric correlation study of the depth of interaction and energy deposited by the 662 keV gamma rays was carried out. A different kind of correlation pattern from that observed normally for hole trapping was noticed in the case of detector D2. Correlation o...

Zn

In the present work high-resolution alpha particle detectors have been fabricated on high quality 20 μm thick n-type 4H-SiC epitaxial layers. Schottky barrier detectors have been fabricated by depositing 10 nm thick nickel contacts on the Si face of the epilayers. The detectors were characterized using current-voltage (I-V), capacitance-voltage (CV), alpha spectroscopic measurements, and deep level transient spectroscopy (DLTS). I-V measurements revealed a barrier height of ~1.6 eV, diode ideality factor of 1.09, and leakage current of the order of 14 pA at an operating bias of 110 V. C-V measurements revealed low effective doping concentrations of 3.1 × 1014 cm-3 in the epilayers. A micropipe density lower than 1 cm-2 was evaluated in the epilayers. Pulse-height spectroscopy exhibited energy resolution as high as 0.37 % for 5.48 MeV alpha particles with a detector active area of 11 mm2. A diffusion length of ~13.2 μm for holes has been determined in these detectors following a calculation based on a drift-diffusion model. Detailed electronic noise analysis in terms of equivalent noise charge (ENC) was carried out to study the effect of various noise components that contribute to the total electronic noise in the detection system. The noise analysis revealed that the white series noise due to the detector capacitance has substantial effect on the detector’s overall performance. DLTS measurements have revealed the presence of at least four majority (electron) carrier trap levels that can act as recombination/generation or trapping centers.

_{0.1}

Detector grade Cd0.9Zn0.1Te single crystals were grown using a tellurium solvent method. Crystal blocks of dimensions ~11×11×10 mm3 were prepared for characterization and detector fabrication. The crystals were characterized using optical techniques. Infra-red transmission images revealed a low Te inclusion/precipitation concentration. Pockels effect measurements showed fairly uniform depth-wise distribution of the electric field. Detectors in various configurations viz., small pixel, virtual Frisch grid, and coplanar grid were fabricated on the same crystal in turns, and characterized by electrical measurements, alpha spectroscopy, and high energy gamma ray spectroscopy. Current-voltage measurements exhibited very low leakage current of ~5 nA at 1000 V and resistivities of the order of 8.5×1010 Ω.cm. Electron drift mobilities of the order of 840 cm2/V.s and electron mobility-lifetime products of the order of 2.7×10-3 cm2/V were calculated using alpha spectroscopy. All the detectors exhibited a distinct peak for the 662 keV gamma rays from a 137Cs source. Among all other configurations, the virtual Frisch grid configuration revealed the best resolution of 4.3% (FWHM) for the 662 keV gamma rays.

Te Crystal Growth and Fabrication of Large Volume Single-Polarity Charge Sensing Gamma Detectors

The spectroscopic performance of two Frisch collar CdZnTe (CZT) detectors with different dimensions has been tested and evaluated after exposing to high energy gamma rays. The dimensions of one of the detectors, detector A, were 4.2×6.2×6.5 mm3 and that of detector B were 3.0×4.0×9.0 mm3. Detector grade CZT crystals were grown from zone refined Cd, Zn, and Te (~7N) precursor materials, using the tellurium solvent method. Detectors with virtual Frisch grid configuration were fabricated from these crystals. I-V measurements revealed low leakage currents at room-temperature, ~ 11 nA at 1100V for detector A and 14.5 nA at 1800V for detector B. For the spectroscopic measurements, the detectors were irradiated using a 137Cs source. Detector A showed an impressive energy resolution of 1.4% and detector B showed an energy resolution of ~5.9 % for 662 keV from Cs-137 radioisotope. A digital technique based biparametric correlation study showed that the performance of detector B was limited by the operating bias voltage. Further, a digital correction scheme has been described and was applied to recover the effect of charge (electron) – loss.