Nibir K. Dhar

A Perspective on Nanowire Photodetectors: Current Status, Future Challenges, and Opportunities

One-dimensional semiconductor nanostructures (nanowires (NWs), nanotubes, nanopillars, nanorods, etc.) based photodetectors (PDs) have been gaining traction in the research community due to their ease of synthesis and unique optical, mechanical, electrical, and thermal properties. Specifically, the physics and technology of NW PDs offer numerous insights and opportunities for nanoscale optoelectronics, photovoltaics, plasmonics, and emerging negative index metamaterials devices. The successful integration of these NW PDs on CMOS-compatible substrates and various low-cost substrates via direct growth and transfer-printing techniques would further enhance and facilitate the adaptation of this technology module in the semiconductor foundries. In this paper, we review the unique advantages of NW-based PDs, current device integration schemes and practical strategies, recent device demonstrations in lateral and vertical process integration with methods to incorporate NWs in PDs via direct growth (nanoepitaxy) methods and transfer-printing methods, and discuss the numerous technical design challenges. In particular, we present an ultrafast surface-illuminated PD with 11.4-ps full-width at half-maximum (FWHM), edge-illuminated novel waveguide PDs, and some novel concepts of light trapping to provide a full-length discussion on the topics of: 1) low-resistance contact and interfaces for NW integration; 2) high-speed design and impedance matching; and 3) CMOS-compatible mass-manufacturable device fabrication. Finally, we offer a brief outlook into the future opportunities of NW PDs for consumer and military application.

Planar p-on-n HgCdTe heterostructure infrared photodiodes on Si substrates by molecular beam epitaxy

We have developed a low temperature procedure for molecular beam epitaxy of CdTe buffer layers on {211} Si wafers and have used Si/ZnTe/CdTe composite substrates for molecular beam epitaxy of double layer Hg1−xCdxTe heterostructures. Planar p-on-n double layer heterostructures were formed by an implantation technique and test diodes were fabricated and characterized. At 77 K, devices with 30×30 μm2 junction area had R0A values in the range 1.5×106–1×107Ω cm2 with a uniform cut-off wavelength of 4.65 μm.

Heteroepitaxy of CdTe on {211} Si using crystallized amorphous ZnTe templates

CdTe films were grown by molecular‐beam epitaxy on As‐passivated nominal {211} Si substrates using thin interfacial ZnTe buffer layers. ZnTe layers were grown by the following two growth methods: (1) migration enhanced epitaxy (MEE) directly on Si and (2) MEE on 80 A templates of crystallized amorphous ZnTe buffers deposited directly on Si. CdTe films thicker than 8 μm had threading dislocation densities in the range of 2.5 to 4×106 cm−2 and 1 to 2×106 cm−2 for methods (1) and (2), respectively. Double crystal x‐ray rocking curve peak widths decreased from 550 to 120 arcsec as CdTe film thickness increased from 3 μm to 9 μm for samples grown by method (1), and for samples grown by method (2), peak widths decreased from 300 to 84 arcsec in the same thickness range. CdTe film grown on vicinal {211} Si substrates misoriented by 5° toward [111] with ZnTe buffer layer prepared by method (2) had a dislocation density of 8×105 cm−2 and an x‐ray peak width of 72 arcsec.

The effect of As passivation on the molecular beam epitaxial growth of high-quality single-domain CdTe(111)B on Si(111) substrates

The effect of As passivation of Si(111) substrates on the subsequent molecular beam epitaxial growth of CdTe(111) is investigated through a detailed comparison of the microstructures of two types of films. The film grown on a substrate treated with a Te flux is found to exhibit a rough film-substrate interface and has very poor crystalline quality with a (111)A orientation. In contrast, a CdTe film grown under identical conditions except for the Si substrate treated with an As flux is observed to have an atomically abrupt film-substrate interface and a single-domain structure in the technologically more relevant (111)B orientation. A growth mechanism for the formation of these high-quality single-domain CdTe(111)B films is proposed.

MBE growth and device processing of MWIR HgCdTe on large area Si substrates

The traditional substrate of choice for HgCdTe material growth has been lattice matched bulk CdZnTe material. However, as larger array sizes are required for future devices, it is evident that current size limitations of bulk substrates will become an issue and therefore large area Si substrates will become a requirement for HgCdTe growth in order to maintain the cost-efficiency of future systems. As a result, traditional substrate mounting methods that use chemical compounds to adhere the substrate to the substrate holder may pose significant technical challenges to the growth and fabrication of HgCdTe on large area Si substrates. For these reasons, non-contact (indium-free) substrate mounting was used to grow mid-wave infrared (MWIR) HgCdTe material on 3″ CdTe/Si substrates. In order to maintain a constant tepilayer temperature during HgCdTe nucleation, reflection high-energy electron diffraction (RHEED) was implemented to develop a substrate temperature ramping profile for HgCdTe nucleation. The layers were characterized ex-situ using Fourier transform infrared (FTIR) and etch pit density measurements to determine structural characteristics. Dislocation densities typically measured in the 9 106 cm−2 to 1 107 cm−2 range and showed a strong correlation between ramping profile and Cd composition, indicating the uniqueness of the ramping profiles. Hall and photoconductive decay measurements were used to characterize the electrical properties of the layers. Additionally, both single element and 32 32 photovoltaic devices were fabricated from these layers. A RA value of 1.8 106-cm2 measured at −40 mV was obtained for MWIR material, which is comparable to HgCdTe grown on bulk CdZnTe substrates.

Atomically thin layers of B-N-C-O with tunable composition

Atomically thin quaternary alloy of boron, nitrogen, carbon and oxygen, 2D-BNCO with tunable composition. In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp2-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm2 V−1 s−1, and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < Eg < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform.

Advances in Infrared Detector Array Technology

This Chapter covers recent advances in Short Wavelength Infrared (SWIR), Medium Wave‐ length Infrared (MWIR) and Long Wavelength Infrared (LWIR) materials and device technol‐ ogies for a variety of defense and commercial applications. Infrared technology is critical for military and security applications, as well as increasingly being used in many commercial products such as medical diagnostics, drivers’ enhanced vision, machine vision and a multi‐ tude of other applications, including consumer products. The key enablers of such infrared products are the detector materials and designs used to fabricate focal plane arrays (FPAs).

Electrooptical Characterization of MWIR InAsSb Detectors

InAs1−xSbx material with an alloy composition of the absorber layer adjusted to achieve 200-K cutoff wavelengths in the 5-μm range has been grown. Compound-barrier (CB) detectors were fabricated and tested for optical response, and Jdark–Vd measurements were taken as a function of temperature. Based on absorption coefficient information in the literature and spectral response measurements of the midwave infrared (MWIR) nCBn detectors, an absorption coefficient formula α(Ε, x, T) is proposed. Since the presently suggested absorption coefficient is based on limited data, additional measurements of material and detectors with different x values and as a function of temperature should refine the absorption coefficient, providing more accurate parametrization. Material electronic structures were computed using a k·p formalism. From the band structure, dark-current density (Jdark) as a function of bias (Vd) and temperature (T) was calculated and matched to Jdark–Vd curves at fixed T and Jdark–T curves at constant Vd. There is a good match between simulation and data over a wide range of bias, but discrepancies that are not presently understood exist near zero bias.

Device physics and focal plane array applications of QWIP and MCT

Infrared sensor technology is critical to many commercial and military defense applications. Traditionally, cooled infrared material systems such as indium antimonide, platinum silicide, mercury cadmium telluride, and arsenic doped silicon (Si:As) have dominated infrared detection. Improvement in surveillance sensors and interceptor seekers requires large size, highly uniform, and multicolor IR focal plane arrays involving medium wave, long wave, and very long wave IR regions. Among the competing technologies are the quantum well infrared photodetectors based on lattice matched or strained III-V material systems. This paper discusses cooled IR technology with emphasis on QWIP and MCT. Details will be given concerning device physics, material growth, device fabrication, device performance, and cost effectiveness for LWIR, VLWIR, and multicolor focal plane array applications.

Multispectral EO/IR Sensor Model for Evaluating UV, Visible, SWIR, MWIR and LWIR System Performance

Next Generation EO/IR Sensors using Nanostructures are being developed for a variety of Defense Applications. In addition, large area IRFPAs are being developed on low cost substrates. In this paper, we will discuss the capabilities of a EO/IR Sensor Model to provide a robust means for comparing performance of infrared FPAs and Sensors that can operate in the visible and infrared spectral bands that coincide with the atmospheric windows - UV, Visible-NIR (0.4-1.8μ), SWIR (2.0-2.5μ), MWIR (3-5μ), and LWIR (8-14μ). The model will be able to predict sensor performance and also functions as an assessment tool for single-color and for multi-color imaging. The detector model can also characterize ZnO, Si, SiGe, InGaAs, InSb, HgCdTe and Nanostructure based Sensors. The model can predict performance by also placing the specific FPA into an optical system, evaluates system performance (NEI, NETD, MRTD, and SNR). This model has been used as a tool for predicting performance of state-of-the-art detector arrays and nanostructure arrays under development. Results of the analysis can be presented for various targets for each of the focal plane technologies for a variety of missions.