[Show abstract][Hide abstract] ABSTRACT: n this work, we report results of a study of electronic transport in nominally undoped p-type GaSb wafers typically employed as substrate material for the epitaxial growth of InAs/GaInSb type-II superlattices. Magnetic field dependent Hall-effect measurements and high-resolution mobility spectrum analysis clearly indicate p-type conductivity due to carriers in both the heavy and light hole bands. The extracted hole concentrations indicate a thermal activation energy of 17.8 meV for the dominant native acceptor-like defects. A temperature-independent effective mass ratio of 9.0 ± 0.8 was determined from the ratio of measured heavy and light hole concentrations. Over the 56 K–300 K temperature range, the light hole mobility was found to be 4.7 ± 0.7 times higher than the heavy hole mobility. The measured room temperature mobilities for the light and heavy holes were 2550 cm2/Vs and 520 cm2/Vs, respectively.
[Show abstract][Hide abstract] ABSTRACT: Monitoring the nanomechanical movement of suspended cantilever structures has found use in applications ranging from biological/chemical sensing to atomic force microscopy. Interrogating these sensors relies on the ability to accurately determine the sub-nanometre movements of the cantilever. Here we investigate a technique based on the combination of integrated silicon photonics and microelectromechanical systems (MEMS) to create an optically resonant microcavity and demonstrate its use for monitoring of the position of cantilevers on the picometer scale under ambient conditions with dynamic range extending over several microns. The technique is interferometric, and we show it to be sufficiently sensitive to measure both the first and second modes of cantilever Brownian motion. We anticipate that application of this technique will provide a physically robust, picometer precision, integrated cantilever movement read-out technology which can take cantilever sensors from laboratory controlled environments into real world conditions, allowing everyday applications.
[Show abstract][Hide abstract] ABSTRACT: We report on the deposition, crystallization, and magnetic properties of cerium-substituted europium iron garnet having the general form of (CeEu) 3 (FeGa) 5 O 12 . The films were deposited on gallium gadolinium garnet and fused quartz substrates using biased target ion beam deposition at a rate of 2.7 nm/min. The Ce:EIG thin film has a composition of Ce 1.3 Eu 1.7 Fe 3 Ga 1.6 O 12 , with 30% of the Ce in the 4+ oxidation state and the remainder as Ce 3+ . The film exhibits the primary peaks of the garnet phase in X-ray diffraction patterns. In the visible part of the electromagnetic spectrum, the film on GGG exhibits a Faraday rotation of 3.3°/µm with coercivity of 0.58 kOe, whereas the film on fused quartz exhibits 1.1°/µm with a coercivity of 0.8 kOe. The film on the fused quartz substrate has a saturation magnetization of 17 emu/cm 3 at room temperature. Index Terms— Biased target ion beam deposition, cerium substituted europium iron garnets, crystallization, grazing angle X-ray diffraction (XRD), magnetic and magneto-optical (MO) properties, X-ray photoelectron spectroscopy (XPS).
IEEE Transactions on Magnetics 12/2014; 50(12). · 1.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Design of practically realizable unipolar HgCdTe nBn photodetectors has been studied in detail by numerical analysis. The simulations reported herein reveal that, by optimization of barrier doping, dark current levels can be reduced and collection efficiency substantially improved. It is shown that p-type doping of the barrier layer can significantly reduce the effective potential barrier arising from the valence band offset between the absorber and barrier regions, thus enabling HgCdTe nBn detector operation under near zero-bias conditions. However, relatively high electric fields in the space charge regions near the barrier/absorber interface result in enhanced trap-assisted Shockley–Read–Hall thermal generation. Our calculations indicate that nBn HgCdTe detectors with barriers engineered by use of HgTe/Hg0.05Cd0.95Te superlattices have, potentially, substantially better valence band alignment without the need for p-type doping.
Journal of Electronic Materials 12/2014; · 1.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Crystallization of germanium (Ge) thin films was investigated by depositing Ge on glass and silicon substrates using electron beam (e-Beam) evaporation. The hole carrier concentration in Ge thin films deposited on glass substrates was found to decrease slightly with increasing annealing temperature, whereas its Hall mobility was found to increase monotonically. For all crystallized Ge thin films herein reported, the conductivity was found to be dominated by holes (p-type). This characteristic was then exploited to fabricate p-Ge/n-Si heterojunction diodes on n-type silicon substrates, which were then investigated by analysis of their current–voltage and capacitance–voltage characteristics. It was found that increasing annealing temperatures lead to significant improvements in on/off ratio and ideality factor, as well as increased built-in voltage. After crystallization of the top p-Ge layer through annealing at 600 °C, the devices indicated an on/off ratio of 106, an ideality factor of 1.25 and a built-in potential of 0.58 eV. The improvement in device performance is correlated with the crystallization of the Ge thin films, as confirmed by Hall effect and X-ray diffraction measurement, which indicated increases in hole mobility and improved solid-phase crystallization with increases in annealing temperature. The photoresponse of the devices was characterized employing a two-dimensional mapping approach, which provides insight into the optical generation and recombination processes at the Ge/Si heterojunction. The results herein presented indicate that thermally crystallized Ge thin films may be of significant promise for electronic and optoelectronic device applications.
[Show abstract][Hide abstract] ABSTRACT: Modern devices consisting of multiple semiconductor layers often result in the population of numerous distinct carrier species. Conventional Hall measurements at a single-magnetic-field strength provide only a weighted average of the electron mobility and carrier concentration of a semiconductor structure and, therefore, are of limited use for the extraction of carrier transport information. In recent years, mobility spectrum analysis techniques, which have been developed to extract a mobility spectrum from magnetic field-dependent conductivity-tensor measurements, have been applied in the analysis of carrier conductivity mechanisms of numerous semiconductor structures and devices. Currently there is a severe lack of reported studies on theoretical calculations of the mobility distribution of semiconductor structures or devices. In addition, the majority of reports on experimental mobility spectrum analysis are of complex, multi layered structures such as type-II superlattices, and the interpretation of the mobility spectra has been difficult. Therefore, a good understanding of the mobility spectrum has yet to be developed. For example, it is often assumed that distinct peaks of a mobility spectrum result from fundamentally different conduction mechanisms such as the bulk and surface conduction of narrow-band-gap semiconductors. In this article, we present calculations of the electron mobility distribution of bulk GaAs, which predict the existence of multiple mobility spectrum peaks that result from electron conductivity in the Γ conduction band. This report serves as an important and simple test case upon which experimentally measured mobility spectra can be compared. It also presents insight into the general nature of electron mobility distributions.
New Journal of Physics 11/2014; 16(11):113033. · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Modulation of the sub-band electron population in the inversion channel of 10-nm planar fully depleted silicon-on-insulator MOSFETs is evidenced by the bias dependence of inversion layer transport parameters. Two distinct inversion-layer electron species were detected by magnetic-field-dependent magnetoresistance measurements and high-resolution mobility spectrum analysis. According to self-consistent Poisson–Schrödinger calculations, these species correspond to carriers in distinct sub-bands within the Si channel region. The mobility peak of the carrier with the highest sheet density occurs under gate bias conditions that result in a minimum perpendicular effective electric field.
IEEE Electron Device Letters 09/2014; 35(11). · 3.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A method is described where the valence band discontinuity in HgCdTe-based nBn detectors will be eliminated. The method relies on doping modulation technique, where grading the material composition and doping concentration of the barrier layer at the same time will lead to elimination of the valence band discontinuity in HgCdTe-based nBn detectors. The method is not limited to the nBn structure and can be applied to any barrier detector structure with xBx (with x = n, p) to eliminate the energy band discontinuity in the valence band or conduction band.
[Show abstract][Hide abstract] ABSTRACT: Three-dimensional (3D) topological insulators (TI) are a new state of quantum matter in which surface states reside in the bulk insulating energy bandgap and are protected by time-reversal symmetry. It is possible to create an energy bandgap as a consequence of the interaction between the conduction band and valence band surface states from the opposite surfaces of a TI thin film, and the width of the bandgap can be controlled by the thin film thickness. The formation of an energy bandgap raises the possibility of thin-film TI-based metal-oxide-semiconductor field-effect-transistors (MOSFETs). In this paper, we explore the performance of MOSFETs based on thin film 3D-TI structures by employing quantum ballistic transport simulations using the effective continuous Hamiltonian with fitting parameters extracted from ab-initio calculations. We demonstrate that thin film transistors based on a 3D-TI structure provide similar electrical characteristics compared to a Si-MOSFET for gate lengths down to 10 nm. Thus, such a device can be a potential candidate to replace Si-based MOSFETs in the sub-10 nm regime.
Journal of Applied Physics 08/2014; 116(8):084508-084508-8. · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an experimental demonstration of a novel, integrated readout approach for measuring the suspended height of micro-electro-mechanical systems (MEMS) structures. The approach is based on creating a resonant optical cavity between the suspended MEMS structure and the substrate that the MEMS structure is anchored to. The resulting interferometric effect causes modulation of an optical laser signal which is strongly dependent on the position of the MEMS device.
[Show abstract][Hide abstract] ABSTRACT: Photostriction-based all-optical actuation of silicon micro-cantilevers has been investigated through experimental characterization of structures fabricated on silicon-on-insulator substrates, and through numerical modeling and analysis of their semiconductor device and micromechanical characteristics. Since the pressure coefficient of the bandgap is negative in Si, photostriction-induced photoactuation in Si-based cantilevers was evident as upward mechanical defections (away from the substrate) in response to pulsed laser illumination on the cantilevers’ top surface; which is in contrast to the downward deflections typical of photothermal effects. For the numerical modeling of photostriction induced effects, carrier lifetime and excess-carrier concentrations were determined from transient photoconductance measurements. The experimentally determined parameters were then employed to simulate carrier-density profiles across the modeled structure. The modeled cantilever deflections were found to be in excellent agreement with experimentally determined deflections. It is also shown that 100-um-long Si cantilevers were deflected by up to 10 nm, and generated a force of 0.14 nN, when optically actuated by a 405-nm laser power density of 400 W/cm2.
Journal of Microelectromechanical Systems 06/2014; · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work, GaSb is proposed as a new alternative substrate for the growth of HgCdTe via molecular beam epitaxy (MBE). Due to the smaller mismatch in both lattice constant and coefficient of thermal expansion between GaSb and HgCdTe, GaSb presents a better alternative substrate for the epitaxial growth of HgCdTe, in comparison to alternative substrates such as Si, Ge, and GaAs. In our recent efforts, a CdTe buffer layer technology has been developed on GaSb substrates via MBE. By optimizing the growth conditions (mainly growth temperature and VI/II flux ratio), CdTe buffer layers have been grown on GaSb substrates with material quality comparable to, and slightly better than, CdTe buffer layers grown on GaAs substrates, which is one of the state-of-the-art alternative substrates used in growing HgCdTe for the fabrication of mid-wave infrared detectors. The results presented in this paper indicate the great potential of GaSb to become the next generation alternative substrate for HgCdTe infrared detectors, demonstrating MBE-grown CdTe buffer layers with rocking curve (double crystal x-ray diffraction) full width at half maximum of ̃60 arcsec and etch pit density of ̃106 cm-2.
Journal of Electronic Materials 01/2014; 43(8). · 1.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we present the results of a numerical study on the influence of discrete dopant atom distribution and crystal orientation on the electrical characteristics of $p$-channel silicon nanowire-based transistors using 3-D quantum simulations. The valence band was modeled employing a three-band $k.p$ Hamiltonian with optimized Lüttinger parameters, while the device characteristics were obtained by modeling hole carrier transport through a self-consistent solution of Poisson's equation and the nonequilibrium Green's function formalism. Simulation of various discrete impurity configurations show that impurities located near the center of the channel region have the greatest impact on threshold voltage. It is shown that the effect of donor-like impurities on device threshold voltage is virtually independent of the nanowire's crystallographic orientation, whereas discrete acceptor-like impurities induce smaller threshold voltage shifts in nanowire transistors oriented along the  and  directions; thus suggesting that the devices oriented along these directions would be relatively less sensitive to the effects of unintentional impurities on threshold voltage.
IEEE Transactions on Electron Devices 01/2014; 61(2):386-393. · 2.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an integrated readout technique for interrogating the suspension height of micro-electro-mechanical systems (MEMS) structures. This readout technique is envisaged to be useful in applications such as MEMS-based biological and chemical sensing, where it is necessary to obtain the accurate position of a MEMS beam. The approach is based on the suspended MEMS structure modulating light transmission in an underlying optical waveguide via Fabry-Perrot phenomena. The performance of the technique is predicted via finite difference time domain (FDTD) simulations the results of which are confirmed by experimental measurements.
[Show abstract][Hide abstract] ABSTRACT: A miniature Fabry-Perot etalon was designed and fabricated to provide spectral filtering capability at the resonance wavelength of 10 μm. A high transmission peak of 85% and a relatively broad bandwidth of 500 nm are expected based on optical modeling. Optimal deposition conditions for process durable thin film materials were developed and optical constants of these materials were characterized. Fabrication of devices was accomplished using standard surface micromachining technique. Released mirrors exhibited a deflection of 400 nm over a length of 150 μm.
[Show abstract][Hide abstract] ABSTRACT: An electron transfer quantum well infrared photodetector (QWIP) consisting of repeating units of two coupled quantum wells (QWs) is capable of exhibiting a two color voltage dependent spectral response. However, significant electron transfer between the coupled QWs is required for spectral tuning, which may require the application of relatively high electric fields. Also, the band structure of coupled quantum wells is more complicated in comparison to a regular quantum well and, therefore, it is not always obvious if an electron transfer QWIP can be designed such that it meets specific performance characteristics. This paper presents a feasibility study of the electron transfer QWIP and its suitability for spectral tuning. Self consistent calculations have been performed of the bandstructure and the electric field that results from electron population within the quantum wells, from which the optical characteristics have been obtained. The band structure, spectral response, and the resonant final state energy locations have been compared with standard QWIPs. It is shown that spectral tuning in the long-wave infrared band can be achieved over a wide wavelength range of several microns while maintaining a relatively narrow spectral response FWHM. However, the total absorption strength is more limited in comparison to a standard QWIP, since the higher QW doping densities require much higher electric fields for electron transfer.
Journal of Applied Physics 11/2013; 114:194501. · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Spin-on dopants offer an expedient, straight forward, and low cost method for doping semiconductors. These sol-gel dopant films contain sufficient impurity content to obtain surface concentrations well above the solubility limit of most semiconductors (e.g., Si, Ge). While much has been published about spin-on dopants for degenerate doping of Si, there is to the authors' knowledge no report in the literature of such films being used to achieve degenerate p-type doping of Ge, as there are a number of technical challenges associated with this technique. In this work, we present a novel technique for degenerate p-type doping of Ge using Ga spin-on dopant films in a regular horizontal tube furnace. This technique gives both excellent uniformity, higher doping concentrations and better potential for ultra-shallow junctions than diffusion from solid sources, is much preferred to rapid melt/alloyed junctions (especially for optoelectronic applications), and is readily applicable to rapid thermal processing. In this preliminary investigation, we report doping concentrations exceeding 1020cm-3 which are shown to be fully electrically activated. We report on the use of a traditional "pre-dep"/"drive-in" approach, which could be used to give shallow dopant profiles, or tailored dopant concentrations, for a wide variety of electronic and optoelectronic applications, and to form back surface fields in photovoltaic and thermo-photovoltaic devices. Values of 3.4 ± 0.2 eV and 3.2 ± 0.4 eV are extracted for the activation enthalpies of the diffusion processes into p-type 1 0 0 and n-type 2 1 1 Ge substrates, respectively, which are shown to be in good agreement with previously published data [1,2]. We conclude that Ga is a more suitable dopant for reliable, low-cost, degenerate p-type doping of Ge, rather than B (also investigated in this work), which is known to be problematic.