Nanowire (NW) photodetectors are an important building block for photonic circuits. The one dimensional nature of NW architecture leads to unique and novel material properties and concomitantly enables adaptation of fabrication processes from thin film technology. The relaxation of any lattice mismatch constraint, a small foot print, high surface to volume ratio, superior optical trapping and feasibility of implementing in different NW configurations can be strategically used to improve detector performance and heterogeneous integration of compound semiconductor based optical devices with traditional Si technology. In this proposed work SAM avalanche photodetector (APD) concepts from thin film form will be adapted toward bandgap engineering of GaAsSb/GaAs heterostructure in a unique manner exclusive to the NW architecture. The GaAsSb material system has been chosen as it encompasses bandgaps that are tunable in the telecommunication wavelength region. Two NW architectures, namely axial and radial also commonly known as core-shell configuration will be the focus of investigation to implement a SAM APD design. This proposed work is built on the recent success of our work on high quality self-catalyzed growth of GaAsSb/GaAs NWs in both of these configurations and realization of Schottky barrier based photodetectors on Si using Ga- assisted molecular beam epitaxy. Our principal focus in this proposal is growth and design optimization of the heterostructure in these two configurations using a variety of material and device characterization techniques. The experimental work will be complimented by modeling using different software packages. The proposed work will focus towards fundamental aspect of engineering enhancement of the electric field in 3D and achieving deeper insight into the avalanche mechanism in these two NW configurations. The performance in these two configurations will be evaluated to engineer an optimized design in the final phase for high gain and robust nano-APD������������������s in the near infrared region.

The proposed work is focused on comprehensive investigation of the dilute nitride GaAsSbN nanowire (NW) ensemble and single NW based near infrared photodetectors. Vertically aligned NW ensembles of GaAsSbN in both radial and axial configurations will be grown on (111) Si substrates. The vapor-liquid-solid growth mode will be used via a self-catalyzed process. The enabling growth technology will be molecular beam epitaxy. Growth study will focus on developing fundamental knowledge that will allow for better understanding on the effect of the growth parameters and NW configuration on compositional uniformity, structural and optical quality. Also these are interesting materials issues from the viewpoint of evaluation of the defects, both in the NWs as well as at interfaces, because the presence of N in 2D structures not only are responsible for large band gap bowing but also induces defects and recombination centers. Annealing of these films is found to be essential in 2D structures to suppress the defect states as attested by our extensive previous work 14,15 on thin films. Our very recent work7,8 on dilute nitride NWs reveals that the temperature dependence of photoluminescence efficiency is considerably enhanced in these NWs and room temperature PL emission has been routinely obtained even without any annealing. Raman spectral line shape and shifting of the Raman modes were found to be strongly influenced by the nature of the defects.8 The point defects lead to symmetric line shape while redshifting the Raman TO mode. Further, the I-V characteristics of the dilute nitride NWs exhibit rectifying characteristics with Schottky barrier height being dependent on the point defects. We used both Raman and I-V characteristics successfully to differentiate between the contribution of the planar and point defects and have shown annihilation of point defects is very efficient in the nanowires.8 This has significant potential impact on providing a pathway to eliminate defects and to achieve excitonic PL emission enabling novel devices that are inconceivable using higher order 2D and 3D layers. We propose to build upon these promising results to do an extensive characterization of GaAsSbN NWs using various advanced nanoscale architectures offered by the one dimensional configuration, namely both axial and radial heterostructures, as a function of N incorporation and determine the effect of the surfactants on the incorporation of N and red shifting the wavelength as well as enhancing the PL intensity. Focus will also be on annihilation of defects and the nature of the defects being annihilated in 1D structure and its impact on the PL emission and absorption, with annealing as well as the NW architecture and other growth parameters, namely , growth temperature, growth interruption. Finally these investigations will be used to demonstrate a p-i-n photodetector in the 1.3 ���������m region. The material characterization would include low temperature ���������-PL (intensity and temperature dependencies), Raman spectroscopy, x-ray diffraction, scanning electron microscopy, bright field transmission electron microscopy (TEM), high resolution TEM (HRTEM) and electron energy loss spectroscopy (EELS), electrical characteristics of Schottky barrier and pulsed I-V measurements for the estimation of carrier lifetime. Device characterization would include I-V measurements, spectral response, quantum efficiency and estimation of the carrier lifetime using pulsed laser measurements. Our experimental work will be guided by finite element modeling of our nanostructure and related devices using combination of several available softwares such as Comsol Multiphysics, MATLAB, Lumerical Suite������������������s Tools etc. Successful completion of the work will accelerate the development of these NWs for the next generation single photon detection applications in this important NIR regime. Further, we also propose to explore the ���������integration of these nanowires with graphene in collaboration with Dave Snyder������������������s group from Penn State University. Two techniques will be explored for the growth using the deposition of a single layer of graphene on SiOx layer on Si substrate and with atomic layer deposition of ~1 nm thick SiOx layer grown on graphene deposited on (111) oriented Si substrate.

The collaborative efforts of North Carolina A&T State University, North Carolina State University and N5 Sensors are focused on comprehensive investigation of GaAsSb nanowire (NW) arrays based near infrared photodetectors. Vertically aligned NW arrays of GaAsSb in both radial and axial configurations will be grown on (111) Si substrates. The vapor-liquid-solid growth mode will be used via a self-catalyzed process. The enabling technology to be used is molecular beam epitaxy (MBE) which allows stringent control of the growth parameters. Patterned NW arrays will be created using electron beam lithography. Optoelectronic and structural properties of the NWs will be investigated using a variety of characterization techniques. Performance of GaAsSb NW photoconductors (PD) and p-i-n diodes will be the subject of detailed investigation using various advanced nanoscale architectures offered by the one dimensional configuration. Low frequency noise (LFN) measurements will be used to probe traps in the NW, their impact on the device performance and will be correlated to the optoelectronic and structural properties of the nanowires. Effects of different design considerations on NW configuration, growth and process parameter optimization and tunnel contacts on LFN will also be the subject of the detailed investigation. Experimental work will be supported by theoretical modeling. Thus this comprehensive study will provide a fundamental understanding of the microscopic mechanism of carrier dynamics and trap states in the NW, better understanding of the underlying physics and growth mechanisms and their impact on the photodetector performance. Successful completion of the work will accelerate the development of these NWs for the next generation PD applications. The proposed research work will be suitably integrated to the educational programs at NCA&TSU to create a strong diverse workforce. Thus this highly experienced multi-institutional scientific team with synergistic activities and complementary individual expertise will provide an innovative solution to the Army������������������s challenges in opteoelectronic devices.

For the past forty years, device processing relied on electric charge as state variables. Spin states (spin-up and spin-down) and controlled magnetic properties can offer an alternative that will lead to memory and logic devices that scale beyond CMOS in both density and power/operation. Dilute Magnetic Semiconductor (DMS) can be part of several functional electric device structures. This allows controlled magnetic and electronic properties in a single device structure. While a voltage signal can be used to program and interrogate the device, internally it uses magnetism as the storage element. We propose to develop a non volatile memory device structure, based on controlling the magnetic state of a dilute magnetic semiconductor (DMS) via an interfacial induced magnetization in heterostructures at room temperature. We propose to build 1) a memory device structures that manipulate the spin states to show that manipulation of spin states is feasible in the proposed GaN memory devices. Specifically we will build memory devices based on GaMnN and GaCrN DMS heterostructures. Also we will utilize our finding in control magnetism at interfaces to build a magnetic tunnel junction (MTJ) utilizing manganese or chromium doped GaN. We will outline results to show that the presence of free carriers (holes) in GaMnN and GaCrN is required to mediate ferromagnetism (FM) at room temperature in the ferromagnetic GaN/p-GaN heterostructures. Applying an electric field to GaMnN or GaCrN to deplete the holes results in the DMS changing its properties from FM to paramagnetic (erase). Upon reversing the direction of the electric field to enhance holes concentration, the magnetic properties change from paramagnetic to FM (write). Several device structures, such as (FM)/p-n, FM/p layers separated by a very thin barrier layer, and (ferromagnetic/insulating/ferromagnetic) tunnel junction devices will be investigated. Special attention will be given to (Ga,TM)N/AlGaN/p-GaN (TM= Mn or Cr) multilayer structures. In such structures, the GaTMN layer will offer the magnetic dipole, whereas the p-GaN layers offer holes in the GaN valence band that are needed to mediate the FM properties and AlGaN is the barrier layers that will be used to control the magnetic states in the devices.

Success of the photovoltaic industry depends upon fabrication of solar cells that exhibit high efficiency at a reasonable cost, and silicon based cells are currently considered to be the most promising candidate to achieve this goal. Although our previous SiSoC research has clearly shown that impurities and defects can limit cell performance and fabrication yield, there is still a need to understand the fundamental relationships between carrier lifetime and processing steps, defects and/impurities on samples of CCZ grown Si provided by our member companies. In this collaboration between NCSU and the University of Manchester, we shall use our array of spectroscopic and electrical techniques to identify and quantify the recombination centers in CCZ Si that can limit lifetime and hence cell performance. Techniques to be used are microwave photoconductive decay, deep level transient spectroscopy, quasi-steady-state photoconductance, deep level transient spectroscopy and minority carrier transient spectroscopy. The results of this investigation should enable us to obtain novel data on recombination centers and thus suggest methods for process optimization. We shall provide a baseline of electrical and spectroscopic data, for example, lifetime, trap energies, concentrations, etc., with a goal toward recombination center identification and an assessment of how the cell fabrication process modulates the physical characteristics measured.

This proposal is a request to extend the Silicon Solar Consortium (SiSoC), an existing multi-university National Science Foundation Industry/University Cooperative Research Center (I/UCRC) beyond its current Phase I 1998-2013 NSF funding period to a 2013-2018 Phase II funding. SiSoC at NCSU is the lead "Parent" site while Georgia Tech is a "Partner" site. SiSoC has also brought together six additional silicon materials driven solar cell academic institutions at Lehigh U, Texas Tech U, the U of Washington, Rochester Institute of Technology, Hanyang U (Korea), and the U of Manchester(England) who collectively add a broad range of photovoltaics related research activities of value to our industrial members. This proposal outlines how the SiSoC university consortium has advanced our underlying materials science and engineering issues that solar cell industrial companies face in developing high efficiency, cost competitive, solar cell photovoltaic devices.

The mission of the Center is to create a unique multi-university, multi-company culture which addresses exploratory silicon-based PV research issues, identified by the international PV industry, to meet the future needs of electricity production from solar energy. The education of graduate and post-graduate students in silicon engineering and PV science is a critical component of the Center's activities.

Develop optimized molecular beam epitaxial growth processes for catalyst-free GaAsSb axial and core shell heterostructure nanowires with wavelengths in the IR (0.9-1.3 ìm) range for potential solar cell, light emitting diode, and photodetector applications. The choice of this material system has been dictated by the following factors: (a) The Sb-based materials are ideally suited for infrared optoelectronics, due to their low optical band gap and high optical gain achieved in 2D structures. The bandgap of GaAsSb can be tailored from 0.87 ìm (GaAs) to 1.7 ìm (GaSb) by adjusting the alloy composition. This near-infrared range is particularly attractive for infrared countermeasures and also it encompasses the 1.3 and 1.55 ìm wavelengths, of great interest for optical fiber communication applications. (b) Sb-based NWs are relatively unexplored, though significant work has been carried out in other, related semiconductor NWs, namely GaAs and GaN. These will serve as benchmarks for the synthesis and fabrication of our proposed devices.