Jennifer I. Brand

A class of boron-rich solid-state neutron detectors

Real-time solid-state neutron detectors have been fabricated from semiconducting boron–carbon alloys, deposited by plasma-enhanced chemical vapor deposition. Single neutrons were detected and signals induced by gamma rays were determined to be insignificant. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron–carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron–carbon alloy layers enriched in 10B could lead to increased detection efficiency and active diodes could use the inherent micron scale spatial resolution, increasing the range of possible applications.

Chemical vapor deposition of boron carbide

Boron carbide is an important non-metallic material with outstanding hardness, excellent mechanical, thermal and electrical properties. Its low density, high chemical inertness and neutron capture section make boron carbide an attractive material for micro-electronic, nuclear, military, space and medical applications. Boron carbide based materials are widely deposited by chemical vapor deposition methods (CVD). This paper provides a comprehensive review on the recent literature of boron carbide CVD. Structure, properties and potential application areas of this material are also reviewed. The status of the theoretical modeling of boron carbide deposition and the developments on the experimental processes are reported. It was evident from this review that extensive research still remains to be done on the modeling of CVD boron carbides. Majority of the reviewed published research papers deals with characterization, and growth rate of deposited boron carbide phases. Some thermodynamic modeling based on Gibbs free energy minimization were attempted in classical CVD systems. However, these models were often not able to represent the actual growth mechanism. No significant modeling work has been reported in other CVD systems such as plasma enhanced chemical vapor deposition (PECVD), hot filament chemical vapor deposition (HFCVD), synchrotron radiation chemical vapor deposition (SRCVD). Reliable thermo-chemical data for boron carbides with various stoichiometries are also needed to study and model actual deposition reaction mechanisms of such complex systems.

A Handheld Neutron-Detection Sensor System Utilizing a New Class of Boron Carbide Diode

A handheld neutron-detection sensor application is described in this paper. The sensor system utilizes a new class of boron carbide diode that interacts with incoming neutrons. To interface with the boron carbide diode, an integrated front end is designed in a 1.5-mum standard CMOS technology. With the diode and front-end microchip, a handheld neutron-detection system was realized with an embedded microcontroller for real-time processing. The handheld detector operation was then tested with a plutonium-beryllium neutron source. Test and measurement results confirm the validity of the approach and the functionality of the design

The heteroisomeric diode

We have fabricated a new class of diode from two different polytypes of boron carbide. Diodes were fabricated by chemical vapour deposition from two different isomers of closo-dicarbadodecaborane: closo-1,2-dicarbadodecaborane (orthocarborane, C2B10H12) and closo-1,7-dicarbadodecaborane (metacarborane, C2B10H12), differing only by the carbon placement within the icosahedral cage. We find that the electronic structure (molecular orbitals) of these two isomer molecules and the resulting decomposition reflect the tendency of metacarborane to form an n-type semiconductor while orthocarborane is an effective source compound for a slightly p-type semiconducting boron carbide. The diodes of this novel class are effective solid state neutron detectors, and have a number of unique applications.

Boron carbide/n-silicon carbide heterojunction diodes

The fabrication, initial structural characterization, and diode measurements are reported for a boron carbide/silicon carbide heterojunction diode. Current–voltage curves are obtained for operation at temperatures from 24 to 351 °C. Plasma-enhanced chemical-vapor deposition (PECVD) -deposited undoped boron carbide material is highly crystalline and consists of a variety of polytypes of boron carbide (BC) with crystal sizes as large as 110 nm. Crystal phases are similar to those for PECVD BC on Si but only partially match known boron and boron-rich BC phases.

Evidence for multiple polytypes of semiconducting boron carbide (C2B10) from electronic structure

Boron carbides fabricated via plasma enhanced chemical vapour deposition from different isomeric source compounds with the same C2B10H12 closo-icosahedral structure result in materials with very different direct (optical) band gaps. This provides compelling evidence for the existence of multiple polytypes of C2B10 boron carbide and is consistent with electron diffraction results.

Semiconducting boron-rich neutron detectors

Semiconducting boron-rich boron-carbon alloys have been deposited by plasma-enhanced chemical vapor deposition. Heterojunction diodes made with 276nm thick nanocrystalline layers of these alloys have been used as real-time solid-state neutron detectors. Individual neutrons were detected and signals induced by gamma rays were determined to be insignificant. Linearity of detection was demonstrated over more than two orders of magnitude in flux. The neutron detection performance was unaffected by > 1 x 1015 neutrons / cm2. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron carbon alloy layers enriched in 10B could lead to increased detection efficiency.

Surface photovoltage effects on the isomeric semiconductors of boron-carbide

During exposure to synchrotron radiation, closo 1,7-dicarbadodecaborane (metacarborane) and closo 1,2-dicarbadodecaborane (orthocarborane) decompose, and are accompanied by increasingly evident photoemission surface photovoltage effects. We show that metacarborane and orthocarborane form self-doped n-type and p-type boron-carbides, respectively. Surface photovoltage effects dominate the photoemission final state, not the changes in electronic structure due to decomposition.

The local structure of transition metal doped semiconducting boron carbides

Transition metal doped boron carbides produced by plasma enhanced chemical vapour deposition of orthocarborane (closo-1,2-C2B10H12) and 3d metal metallocenes were investigated by performing K-edge extended x-ray absorption fine structure and x-ray absorption near edge structure measurements. The 3d transition metal atom occupies one of the icosahedral boron or carbon atomic sites within the icosahedral cage. Good agreement was obtained between experiment and models for Mn, Fe and Co doping, based on the model structures of two adjoined vertex sharing carborane cages, each containing a transition metal. The local spin configurations of all the 3d transition metal doped boron carbides, Ti through Cu, are compared using cluster and/or icosahedral chain calculations, where the latter have periodic boundary conditions.

Pairwise cobalt doping of boron carbides with cobaltocene

We have performed Co K-edge x-ray absorption fine structure and x-ray absorption near edge structure measurements of Co-doped plasma enhanced chemical vapor phase deposition (PECVD) grown “C2B10Hx” semiconducting boron carbides, using cobaltocene. Cobalt does not dope PECVD grown boron carbides as a random fragment of the cobaltocene source gas. The Co atoms are fivefold boron coordinated (R=2.10±0.02A) and are chemically bonded to the icosahedral cages of B10CHx or B9C2Hy. Pairwise Co doping occurs, with the cobalt atoms favoring sites some 5.28±0.02A apart.