Joseph Van Nostrand

Selective area deposited blue GaN–InGaN multiple-quantum well light emitting diodes over silicon substrates

We report on fabrication and characterization of blue GaN–InGaN multi-quantum well (MQW) light-emitting diodes (LEDs) over (111) silicon substrates. Device epilayers were fabricated using unique combination of molecular beam epitaxy and low-pressure metalorganic chemical vapor deposition growth procedure in selective areas defined by openings in a SiO2 mask over the substrates. This selective area deposition procedure in principle can produce multicolor devices using a very simple fabrication procedure. The LEDs had a peak emission wavelength of 465 nm with a full width at half maximum of 40 nm. We also present the spectral emission data with the diodes operating up to 250 °C. The peak emission wavelengths are measured as a function of both dc and pulse bias current and plate temperature to estimate the thermal impedance.

Local transport properties, morphology and microstructure of ZnO decorated SiO2 nanoparticles

We report on a novel, surfactant free method for achieving nanocrystalline ZnO decoration of an SiO(2) nanoparticle at ambient temperature. The size distributions of the naked and decorated SiO(2) nanoparticles are measured by means of dynamic light scattering, and a monodisperse distribution is observed for each. The morphology and microstructure of the nanoparticles are explored using atomic force microscopy and high resolution transmission electron microscopy. Investigation of the optical properties of the ZnO decorated SiO(2) nanoparticles shows absorption at 350 nm. This blue shift in absorption as compared to bulk ZnO is shown to be consistent with quantum confinement effects due to the small size of the ZnO nanocrystals. Finally, the local electronic transport properties of the nanoparticles are explored by scanning conductance atomic force microscopy. A memristive hysteresis in the transport properties of the individual ZnO decorated SiO(2) nanoparticles is observed. Optical absorption measurements suggest the presence of oxygen vacancies, whose migration and annihilation appear to contribute to the dynamic conduction properties of the ZnO decorated nanoparticles. We believe this to be the first demonstration of a ZnO decorated SiO(2) nanoparticle, and this represents a simple yet powerful way of achieving the optical and electrical properties of ZnO in combination with the simplicity of SiO(2) synthesis.

UV Schottky-barrier detector development for possible Air Force applications

This paper discusses the collaborative optical and electrical characterization of the first photovoltaic (PV) III-nitride based detectors grown and fabricated by the Air Force Research Laboratory (AFRL). These 2.6 micrometer thick, n-type GaN Schottky detector structures doped with Si were grown by molecular beam epitaxy (MBE) on (0001)-oriented sapphire substrates, and incorporated palladium (Pd) as the Schottky metal contact. Working Schottky-barrier detector sizes ranged from 50 micrometer to 1600 micrometer in diameter. Flood- illuminated spectral responsivities of these Schottky detectors were as high as 0.12 A/W (for a 1600 micrometer diameter device biased at -1.5 V) at a peak wavelength of 273 nm. The typical measured frequency response of these detectors was flat from dc to the chopper limit of 700 Hz, and the 1/e response time of a 1600 micrometer diameter Schottky- barrier GaN detector was found to be as low as 50 microsecond(s) at zero bias. Noise characterization of these detectors was also performed, and noise equivalent powers (NEPs) of sample GaN Schottky-barrier detectors are reported.

Type-II InGaAs/GaAsSb superlattice for photodetection in the near infrared

The optical properties of an (formula available in paper) type-II superlattice lattice matched to InP(001) was characterized by photo luminescence and near infrared photoresponse. The samples were designed for optical emission near 1.8micrometers and were grown by molecular beam epitaxy. At 4K, a strong type-II luminescence at 1.8micrometers (689meV0 with a full width at half maximum (FWHM) of 18 meV was observed. Similarly, the onset of the band edge photoresponse occurred at 1.8micrometers (693 meV) at 10K. We believe this to be the first observation of both luminescence and photoresponse from the InGaAs/GaAsSb/InP materials system grown by any technique.

Reliability of fully-integrated nanoscale ReRAM/CMOS combinations as a function of on-wafer current control

Resistive random access memory (ReRAM) is a novel form of non-volatile memory expected to replace FLASH memory in the near future. As a further step toward use of ReRAM in computer architectures, we have developed a CMOS/ReRAM hybrid process, integrating ReRAM into a CMOS process flow. In this study, we investigate yield, reliability, endurance, threshold voltages (forming, set and reset voltages) of ReRAM devices implemented in the back end of the line (BEOL). The focus of this study is on single ReRAM devices operated with an external transistor (1T1R configuration). Yield and endurance of these 1T1R configured devices show a strong dependence on device position on the wafer, the device size, and the current compliance used during the initial forming and subsequent set sweeps. Elevated temperatures resulted in a shift of the optimum operation conditions and could be used to improve device performance. Variance in performance of the ReRAM devices was directly correlated with manufacturing issues and material properties to gain maximum information for future improvements.

Comparison of random telegraph noise, endurance and reliability in amorphous and crystalline hafnia-based ReRAM

Resistive random access memory (ReRAM) is a novel form of non-volatile memory expected to replace FLASH memory in the near future. To optimize the switching parameters of ReRAM we investigated fab-friendly HfOx based devices with an either amorphous or crystalline active layers. Our devices are fabricated with a copper bottom electrode, a 50 nm sub-stoichiometric hafnia layer, and a platinum top electrode. These devices operate according to the electrochemical metallization model. We compared endurance, reliability and random telegraph noise (RTN) with pulse-based cycling/readout. Initial endurance measurements show 4 million and 70 million consecutive cycles for the amorphous and crystalline hafnia, respectively. The transmission rate was shown to be slightly higher for the amorphous active layer with a confidence of 85%. Furthermore, it is shown that the relative difference in resistance during RTN is not dependent on the crystallinity, but increases with an increase in high resistive state. A high variety of noise patterns were observed, including transition rates from 1 s-1 up to 12000 s-1 and multi-state traps.

Tunneling atomic force microscopy characterization of cuprous oxide thin films

In this work we characterized thermally grown cuprous oxide thin films using tunneling atomic force microscopy (TUNA) and optical reflection measurements. Significant hysteresis was observed in the I-V curves measured at the nanometer contact under various bias voltages. Histogram analysis of the barrier voltage distribution indicated the barrier height is related to electrochemical potentials for oxidation/ reduction of copper atoms. Changes in chemical state of copper atoms were identified by optical reflectance measurements in the UV-VIS-NIR wavelength region. The peak shift observed in the optical reflection measurements from the short to the long wavelength is attributed to the quantum size confinement effects of the nanometer-scale cuprous particles formed in the films. The grain size, including surface roughness, was measured by topographic AFM imaging. The fluctuations in the I–V measurements are likely due to changes of electrochemical properties of cuprous ions in the film, including the grain size distribution. The asymmetric distribution in the barrier height may indicate that a different probability for injecting an electron in and withdrawing an electron from the films.

Electrically-Tunable Group Delays Using Quantum Wells in a Distributed Bragg Reflector

We present an optical delay line structure incorporating InxGa1-xAs quantum wells in the GaAs quarter- wave layers of a GaAs/AlAs distributed Bragg reflector. Applying an electric field across the quantum wells shifts and broadens the e1-hh1 exciton peak via the quantum- confined Stark effect. Resultant changes in the index of refraction thereby provide a means for altering the group delay of an incident laser pulse. Theoretical results predict tunable delays on the order of 50 fs for a 30-period structure incorporating 3 quantum wells per GaAs layer. Structure design, growth and fabrication are detailed. Preliminary group delay measurements on large-area samples with no applied bias are presented.