Search

In recent years, electrochemical biosensors based on semiconductor and metal nanostructures have attracted a great deal of attention as new instruments in the healthcare arsenal that could substantially enhance early diagnostics capabilities and thus enable active health management. Among numerous materials studied, nanostructured ZnO has been recognized as a promising platform for biomedical applications owing to its low cost, relative ease of preparation leading to a rich variety of nanostructures with high aspect ratios (nanowires, nanobelts, nanoflakes), proven biocompatibility in the bulk form, electronic properties supporting various device types, and catalytic surface activity. In this contribution, we review the recent progress in development of enzymatic and non-enzymatic biosensors based on ZnO nanostructures. After a critical discussion of biocompatibility of nanostructured ZnO, we segue into the discussion of ZnO-based devices for detection of physiologically important analytes, including glucose, cholesterol, L-lactic acid, uric acid, metal ions, and pH. Special attention is given to ZnO nanorod based sensors for intracellular measurements. (C) 2017 The Electrochemical Society. All rights reserved.

The luminescent properties of Mg-doped GaN have recently received particular attention, e. g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional Mg-Ga acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (less than10(18) cm(-3)), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 +/- 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the Mg-Ga acceptor. (C) 2014 AIP Publishing LLC.

The luminescent properties of Mg-doped GaN have recently received particular attention, e. g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional Mg-Ga acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (less than10(18) cm(-3)), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 +/- 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the Mg-Ga acceptor. (C) 2014 AIP Publishing LLC.

The luminescent properties of Mg-doped GaN have recently received particular attention, e. g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional Mg-Ga acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (less than10(18) cm(-3)), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 +/- 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the Mg-Ga acceptor. (C) 2014 AIP Publishing LLC.

The luminescent properties of Mg-doped GaN have recently received particular attention, e. g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional Mg-Ga acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (less than10(18) cm(-3)), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 +/- 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the Mg-Ga acceptor. (C) 2014 AIP Publishing LLC.

The luminescent properties of Mg-doped GaN have recently received particular attention, e. g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional Mg-Ga acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (less than10(18) cm(-3)), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 +/- 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the Mg-Ga acceptor. (C) 2014 AIP Publishing LLC.

Si-doped GaN layers grown by metal organic vapor phase epitaxy on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). We have observed stacking fault (SF) related emission in the range of 3.29–3.42 eV for samples with moderate doping, while for the layers with high concentration of dopants, no CL lines related to SFs have been noted. Perturbation of the SF potential profile by neighboring impurity atoms can explain localization of excitons at SFs, while this effect would vanish at high doping levels due to screening.

Si-doped GaN layers grown by metal organic vapor phase epitaxy on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). We have observed stacking fault (SF) related emission in the range of 3.29–3.42 eV for samples with moderate doping, while for the layers with high concentration of dopants, no CL lines related to SFs have been noted. Perturbation of the SF potential profile by neighboring impurity atoms can explain localization of excitons at SFs, while this effect would vanish at high doping levels due to screening.

Si-doped GaN layers grown by metal organic vapor phase epitaxy on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). We have observed stacking fault (SF) related emission in the range of 3.29–3.42 eV for samples with moderate doping, while for the layers with high concentration of dopants, no CL lines related to SFs have been noted. Perturbation of the SF potential profile by neighboring impurity atoms can explain localization of excitons at SFs, while this effect would vanish at high doping levels due to screening.

Si-doped GaN layers grown by metal organic vapor phase epitaxy on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). We have observed stacking fault (SF) related emission in the range of 3.29–3.42 eV for samples with moderate doping, while for the layers with high concentration of dopants, no CL lines related to SFs have been noted. Perturbation of the SF potential profile by neighboring impurity atoms can explain localization of excitons at SFs, while this effect would vanish at high doping levels due to screening.

Si-doped GaN layers grown by metal organic vapor phase epitaxy on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). We have observed stacking fault (SF) related emission in the range of 3.29–3.42 eV for samples with moderate doping, while for the layers with high concentration of dopants, no CL lines related to SFs have been noted. Perturbation of the SF potential profile by neighboring impurity atoms can explain localization of excitons at SFs, while this effect would vanish at high doping levels due to screening.

A novel fluctuation-based approach is applied to consider the unsolved problems in the realm of nitride heterostructure field effect transistors (HFETs). Fluctuations originate at a microscopic level and provide information on the processes responsible for device operation and degradation. The novel fluctuation-based approach is impelled by recent demonstration of strong correlation of microwave hot-electron fluctuations with frequency performance and degradation of nitride HFETs. The correlation has its genesis in the dissipation of the LO-mode heat accumulated by the non-equilibrium longitudinal optical phonons (hot phonons) confined in the channel that hosts the high-density hot-electron gas subjected to a high electric field. The LO-mode heat causes additional scattering of hot electrons and facilitates defect formation in a different manner than conventional heat accumulated by acoustic phonons. The heat accumulation depends on the supplied electric power and the rate of heat dissipation. We treat the problem in terms of the hot-phonon lifetime responsible for conversion of non-migrant optical phonons into migrant modes. The lifetime is measured over wide ranges of electron densities and supplied electric power. The optimal conditions for heat dissipation are determined and associated with plasmon-assisted disintegration of LO phonons. Signatures of plasmons are experimentally resolved in fluctuations, dissipation, hot-electron transport, transistor frequency performance, and device reliability. In particular, a slower degradation and a faster operation of GaN-based HFETs is demonstrated when the plasmon-assisted ultrafast dissipation of the LO-mode heat comes into play., The original document contains color images.

Rediscovered in the last decade, zinc oxide (ZnO) has attracted much attention as a promising material for many optoelectronics and to some extent microelectronics applications. However, a clear majority of effort expended in this fast developing field has been limited to heteroepitaxial structures grown on foreign substrates with lattice-parameter and thermal-expansion mismatch with ZnO which is detrimental. Recognizing the importance, the effort has shifted to include developing technologies capable of producing freestanding ZnO wafers in large-scale for ZnO based device applications, which is the subject matter of this manuscript. Three competing approaches -- hydrothermal method, melt growth (modifications of the well known Bridgman technique), and seeded vapor transport growth (or sublimation) -- have now reached the level which can be construed as commercialization. In this article, we discuss the progress, outstanding problems, and prospects of these growth methods employed for commercial manufacturing of ZnO wafers., U.S. Government or Federal Rights License. The original document contains color images.

Studies on the optically active rare earth (RE) ions doped III-nitrides QWs and SLs are very scarce, however available data indicates significant improvement in the PL intensities of Eu and Er doped AlGaN/GaN superlattices. Despite encouraging initial results, the mechanism for transfer of excitation from electrons and holes produced by electron or photon pumping in such structures is still not entirely clear. As demonstrated by GaN laser diodes, light emitting diodes (LED), and GaN based electronic devices, all GaN based devices must take advantage of quantum well (QW) structures such as GaN/AlGaN, InGaN/GaN and GaN/AlN. In order to optimize the device design using RE-doped III-nitrides, it is necessary to study and understand the physical properties of RE doped nitride QWs and SLs as well as the QW structural effects on the device performance.

We report on the magnetic field and power density dependences of resonant Raman scattering by interface phonons in GaAs---AlxGa1-xAs superlattices. Strong photoexcitation leads to quenching of the nominally forbidden (and sample-dependent) scattering while a dramatic enhancement of the intensity is observed in the presence of a magnetic field. Alternative mechanisms that partially account for the experimental findings are discussed.

We report resonant Raman scattering from Be acceptors in GaAs-AlxGa1-xAs quantum well structures, grown by molecular beam epitaxy. Center- and edge-doped samples with well widths in the range 70-165 A were investigated as a function of temperature and uniaxial stress. The data show confinement-induced shifts and splittings of the lowest-lying acceptor levels in good agreement with recent calculations, and also excitations that may involve impurity states derived from higher subbands. The stress dependence of the spectra reveals coupling of the lowest acceptor transition to transverse acoustic phonons. Confinement-split lines exhibit a not fully understood intensity exchange with increasing temperature.

Studies on the optically active rare earth (RE) ions doped III-nitrides QWs and SLs are very scarce, however available data indicates significant improvement in the PL intensities of Eu and Er doped AlGaN/GaN superlattices. Despite encouraging initial results, the mechanism for transfer of excitation from electrons and holes produced by electron or photon pumping in such structures is still not entirely clear. As demonstrated by GaN laser diodes, light emitting diodes (LED), and GaN based electronic devices, all GaN based devices must take advantage of quantum well (QW) structures such as GaN/AlGaN, InGaN/GaN and GaN/AlN. In order to optimize the device design using RE-doped III-nitrides, it is necessary to study and understand the physical properties of RE doped nitride QWs and SLs as well as the QW structural effects on the device performance.

Our research goals are to design and implement novel schemes in order to utilize the optical and electrical properties of III-V semiconductors devices grown by MBE and to enhance their performance. Intimately tied to this is the characterization of optical devices such as modulators, lasers, and detectors as well as an understanding of the underlying optical processes involved. GaAs based materials as well as InGaAs and InP material systems are investigated for possible optoelectronic integration. Specifically, by using Gas Source MBE (GSMBE), we intend to obtain high quality InP based materials as well as GaAs electronic devices grown of InP substrates. The latter being an initial step towards the optoelectronic integration of GaAs with InP. GSMBE provides excellent control over the AS/P ration which cannot be achieved by conventional solid source MBE. Utilization of this technique will lead to the realization of low threshold, low current density CW laser in InGaAsP material system.

The research funded the following projects - Metal-Insulator- Semiconductor Field-Effect-Transistor(MISFET), Ge/GaAs Heterojunction Bipolar Transistor (HBT), power HBT, and a theoretical investigation on Direct and Resonant Tunneling Diode (RTD). This project investigated the properties of MISFET type structures based on compound semiconductors. MISFET's have advantages over the aforementioned transistors in areas where high speed, simplicity of fabrication, uniformity of threshold voltages, low standby power, and high gate operational voltages are required. A major research goal is to investigate Ge/GaAs heterojunctions towards incorporating Ge in well established GaAs Heterojunction Bipolar Transistors (HBT's). This novel material combination will result in enhanced device performance. For HBT's, critical parameters affecting the device performance are base doping, base contact resistance, and band discontinuities at the emitter base junction. The semiconductor Ge has excellent potential to improve the performance of GaAs/A1GaAs HBT's. The third investigates the current handling capability of A1GaAs/GaAs HBT's. The idea is to use the HBT as a high- power and high-speed device. Multi-finger emitter and emitter ballasting resistor are used in the design of the HBT. The direct and resonant tunneling diode project used a k dot p band model to calculate the complex wave-vectors of the tunneling electrons in the calculation of the I-V characteristics of these types of diodes. Another important point in the calculation is the inclusion of semiconductor band-bending. (jhd)

Traditionally, there have been two approaches to achieving high switchings speeds in semiconductor devices. One is to improve processing and lithographic techniques thereby reducing device dimensions and reducing parasitic losses. The other is to fabricate devices from material with improved transport properties such as GaAs or InP as opposed to Si. With the advent of epitaxial techniques which can grow thin reproducible heterolayer structures on substrates large enough for LSI and VLSI application, devices which employ 'conduction band engineering' to improve device performance are becoming strong contenders in the race for faster devices. Examples of devices which have been improved by conduction band engineering are heterojunction lasers, heterojunction bipolar transistors junction field effect transistors and metal semiconductor field effect transistor (MESFETS). In the aforementioned structures conduction and valence band discontinuities at the heterojunction are used to confine carriers to the active region of the device and/or reduce parasitic currents. A modulation doped heterostructure (MDH), on the other hand, is employed to actually improve or enhance the transport properties of the majority carriers in the channel of a MESFET. In a modulation doped (Al, Ga)As/GaAs heterostructure only the larger bandgap (Al, Ga)As is doped (n-type).

This project is aimed at developing the technology to fabricate A1.5Ga.5As gate GaAs FETs with submicron gate dimensions and to compare the dc, large-signal switching and small-signal microwave performance of both normally-on and normally-off devices with similar Schottky-barrier devices. (Author)

The research on modulation doped AlGaAs/GaAs FETs (MODFETs) and on AlGaAs buffer GaAs MESFETs has continued with; substantial progress. Current-voltage collapse observed in the drain characteristics of MODFETs in dark at 77K have for the first time been eliminated. The responsible mechanisms were found to relate to the defect concentration in AiGaAs and also to the gate recess width relative to the gate. Microwave S-parameter measurements were also made to deduce the device equivalent circuit parameters and compared to conventional GaAs MESFETs. The results are compared qualitatively to predictions. In GaAs FETs with high resistivity AlGaAs buffer layers it was found that no improvement (relative to GaAs buffer FETs) but degradation can result if the AiGaAs buffer GaAs active layer interface is not of high quality. The only way high quality can be assured has been found to be via the use of a thin superlattice at the heterointerface. Both optical and electrical properties of the GaAs layer grown on this superlattice is of sufficiently high quality to lead the expected improvements in the device performance. With the super lattice AlGaAs/GaAs MESFETs better rf performance has been obtained.