Rajendra Dahal

Self-powered micro-structured solid state neutron detector with very low leakage current and high efficiency

We report on the design, fabrication, and performance of solid-state neutron detector based on three-dimensional honeycomb-like silicon micro-structures. The fabricated detectors use boron filled deep holes with aspect ratio of over 12 and showed a very low leakage current density of ∼7 × 10−7 A/cm2 at −1 V for device sizes varying from 2 × 2 to 5 × 5 mm2. A thermal neutron detection efficiency of 4.5% ± 0.5% with discrimination setting of 500 keV and gamma to neutron sensitivity of (1.1 ± 0.1) × 10−5 for single layer was measured without external bias for these devices. Monte-Carlo simulation predicts a maximum efficiency of 45% for such devices filled with 95% enriched 10boron.

Boron filling of high aspect ratio holes by chemical vapor deposition for solid-state neutron detector applications

A multiple deposition and etching process has been developed to enable high fill factor boron deposition in high aspect ratio holes fabricated in a (100) silicon substrate. The boron deposition was carried out using low-pressure chemical vapor deposition and the etching was done by inductively coupled plasma reactive ion etching technique. The boron deposition processes were carried out under different conditions in order to find a baseline process condition. The boron etching processes done under different conditions with the photoresist as the mask are also discussed. Finally, the fabricated neutron detector with the highest fill factor was characterized for the thermal neutron detection efficiency.

High detection efficiency micro-structured solid-state neutron detector with extremely low leakage current fabricated with continuous p-n junction

We report the continuous p-n junction formation in honeycomb structured Si diode by in situ boron deposition and diffusion process using low pressure chemical vapor deposition for solid-state thermal neutron detection applications. Optimized diffusion temperature of 800 °C was obtained by current density-voltage characteristics for fabricated p+-n diodes. A very low leakage current density of ∼2 × 10−8 A/cm2 at −1 V was measured for enriched boron filled honeycomb structured neutron detector with a continuous p+-n junction. The neutron detection efficiency for a Maxwellian spectrum incident on the face of the detector was measured under zero bias voltage to be ∼26%. These results are very encouraging for fabrication of large area solid-state neutron detector that could be a viable alternative to 3He tube based technology.

4H-SiC n-Channel Insulated Gate Bipolar Transistors on (0001) and (000-1) Oriented Free-Standing n − Substrates

We experimentally demonstrate 4H-SiC n-channel, planar gate insulated gate bipolar transistors (IGBTs) on 180-μm thick lightly doped free-standing n- substrates with ion-implanted collector regions, and metal-oxide-semiconductor gates on (0001) and (000-1) surfaces. The IGBTs show an ON-state current density of 20 A/cm2 at a power dissipation of 300 W/cm2. The threshold voltages are measured to be 7.5 V and 10.5 V on Si-face and C-face, respectively. Both IGBTs show a small positive temperature coefficient of the forward voltage drop, which is useful for easy parallelization of devices.

Growth of hexagonal boron nitride on (111) Si for deep UV photonics and thermal neutron detection

Hexagonal boron nitride (hBN) growth was carried out on (111) Si substrates at a temperature of 1350 °C using a cold wall chemical vapor deposition system. The hBN phase of the deposited films was identified by the characteristic Raman peak at 1370 cm−1 with a full width at half maximum of 25 cm−1, corresponding to the in-plane stretch of B and N atoms. Chemical bonding states and composition of the hBN films were analyzed by X-ray photoelectron spectroscopy; the extracted B/N ratio was 1.03:1, which is 1:1 within the experimental error. The fabricated metal-hBN-metal devices demonstrate a strong deep UV (DUV) response. Further, the hBN growth on the vertical (111) surfaces of parallel trenches fabricated in (110) Si was explored to achieve a thermal neutron detector. These results demonstrate that hBN-based detectors represent a promising approach towards the development of DUV photodetectors and efficient solid-state thermal neutron detectors.

Solid-state neutron detectors based on thickness scalable hexagonal boron nitride

This paper reports on the device processing and characterization of hexagonal boron nitride (hBN) based solid-state thermal neutron detectors, where hBN thickness varied from 2.5 to 15 μm. These natural hBN epilayers (with 19.9% 10B) were grown by a low pressure chemical vapor deposition process. Complete dry processing was adopted for the fabrication of these metal-semiconductor-metal (MSM) configuration detectors. These detectors showed intrinsic thermal neutron detection efficiency values of 0.86%, 2.4%, 3.15%, and 4.71% for natural hBN thickness values of 2.5, 7.5, 10, and 15 μm, respectively. Measured efficiencies are very close (≥92%) to the theoretical maximum efficiencies for corresponding hBN thickness values for these detectors. This clearly shows the hBN thickness scalability of these detectors. A 15 μm thick hBN based MSM detector is expected to yield an efficiency of 21.4% if enriched hBN (with ∼100% 10B) is used instead of natural hBN. These results demonstrate that the fabrication of hBN th...

Towards high efficiency solid-state thermal and fast neutron detectors

Variety of applications of fast neutron detection utilize thermal neutron detectors and moderators. Examples include homeland security applications such as portal monitors and nuclear safeguards which employ passive systems for detection of fissile materials. These applications mostly rely on gas filled detectors such as 3 He, BF3 or plastic scintillators and require high voltage for operation. Recently there was considerable progress in the development of solid-state neutron detectors. These operate by detection of charged particles emitted from neutron interactions with a converter material. In order to increase neutron detection efficiency to a usable level, the thickness of the converter material must exceed the range of the charged particles in the converter, which limits the efficiency of planar detectors to several percent. To overcome this limitation three di- mensional structured solid-state devices are considered where the converter can be thicker but still allow the charged particles to escape into the semiconductor. In the research described here this was accomplished by a semiconductor device that resembles a honeycomb with hexagonal holes and thin silicon walls filled with the converter material. Such design can theoretically achieve about 45% thermal neutron detection efficiency, experimentally about 21% was observed with a partially filled detector. Such detectors can be fabricated in variety of sizes enabling designs of directional fast neutron detectors. Other converter materials that allow direct detection of fast neutrons were also considered by both simulation and experiments. Because the semiconductor thickness is less than a few hundred microns, the efficiency of these detectors to g-ray(s) is very low. With further developments these new solid-state neutron detectors can replace gas ionization based detectors in most applications.

Experimental Demonstration of High-Voltage 4H-SiC Bi-Directional IGBTs

We experimentally demonstrate, for the first time, bi-directional 4H-SiC planar gate, insulated gate bipolar transistors fabricated on 250-μm thick, lightly doped free-standing substrates. On Si face, forward voltage drop (at 50 A/cm2) of 9.7 V was obtained at room temperature, with a differential ON-resistance of 140 mQ · cm2, indicating good conductivity modulation. We have also demonstrated control over minority carrier injection in static characteristics of the BD-IGBTs by application of a back-gate bias.

Characteristics of 4H-SiC P-i-N diodes on lightly doped free-standing substrates

This paper presents static and dynamic electrical characteristics of implanted 4H-SiC PiN diodes fabricated on Si-face and C-face of lightly doped free-standing substrates. The device performance is found to be comparable to conventional diodes. Carrier lifetime of about 2.5 μs was measured for the drift region.

Single-Crystal CdTe Homojunction Structures for Solar Cell Applications

We report two different CdTe homojunction solar cell structures. Single-crystal CdTe homojunction solar cells were grown on GaAs single-crystal substrates by metalorganic chemical vapor deposition. Arsenic and iodine were used as dopants for p-type and n-type CdTe, respectively. Another homojunction solar cell structure was fabricated by growing n-type CdTe directly on bulk p-type CdTe single-crystal substrates. The electrical properties of the different layers were characterized by Hall measurements. When arsine was used as arsenic source, the highest hole concentration was ~6 × 1016 cm–3 and the activation efficiency was ~3%. Very abrupt arsenic doping profiles were observed by secondary ion mass spectrometry. For n-type CdTe with a growth temperature of 250°C and a high Cd/Te ratio the electron concentration was ~4.5 × 1016 cm–3. Because of the 300 nm thick n-type CdTe layer, the short circuit current of the solar cell grown on the bulk CdTe substrate was less than 10 mA/cm2. The open circuit voltage of the device was 0.86 V. According to a prediction based on measurement of short circuit current density (Jsc) as a function of open circuit voltage (Voc), an open circuit voltage of 0.92 V could be achieved by growing CdTe solar cells on bulk CdTe substrates.