Walter Henderson

Molecular beam epitaxy of complex metal-oxides: Where have we come, where are we going, and how are we going to get there?

A renewal of interest in molecular beam epitaxy (MBE) of complex metal oxides has occurred in recent years. This revival of interest is due in part to advances in the technology of MBE oxide epitaxy and in part to the potential of such materials for insertion into unique applications. Some of the key advances in oxide epitaxy are reviewed, including dramatic quality improvement in complex oxides, particularly the perovskite oxide class. Some of the latter advances include high quality integration of perovskites on silicon pioneered by McKee et al. [Phys. Rev. Lett. 81, 3014 (1998)] and demonstration of interface charge control and oxide–oxide heterointerfaces with mobility exceeding 10000cm2∕Vs by Huang et al. [Physica E 22 712 (2004)]. A new demonstration of LiNbO3 epitaxy on SiC using a novel chemistry will also be detailed. Whether it is applications in mainstream silicon such as alternative gate dielectric replacements or scarcely examined multifunctional oxides, MBE holds promise for a variety of app...

III-nitride integration on ferroelectric materials of lithium niobate by molecular beam epitaxy

Integration of III-nitride electrical devices on the ferroelectric material lithium niobate (LiNbO3) has been demonstrated. As a ferroelectric material, lithium niobate has a polarization which may provide excellent control of the polarity of III-nitrides. However, while high temperature, 1000°C, thermal treatments produce atomically smooth surfaces, improving adhesion of GaN epitaxial layers on lithium niobate, repolarization of the substrate in local domains occurs. These effects result in multi domains of mixed polarization in LiNbO3, producing inversion domains in subsequent GaN epilayers. However, it is found that AlN buffer layers suppress inversion domains of III-nitrides. Therefore, two-dimensional electron gases in AlGaN∕GaN heterojunction structures are obtained. Herein, the demonstration of the monolithic integration of high power devices with ferroelectric materials presents possibilities to control LiNbO3 modulators on compact optoelectronic/electronic chips.

Transient atomic behavior and surface kinetics of GaN

An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination.

Complementary oxide memristor technology facilitating both inhibitory and excitatory synapses for potential neuromorphic computing applications

A new family of crystalline oxides is identified that provide a method of producing complementary memristance (both n and p-type having been demonstrated) with unusually large demonstrated memristance behavior. To the best of our knowledge, these are the only devices having a large enough memristance to have measureable memristance at the macroscopic (10s to 100s of um device size) scale. Additionally, the oxides are highly conducting (low loss) with resistivities for both n and p-type variants in the ~5E-4 ohm-cm range. Complementary Oxide Memristors (both n-type and p-type) have been demonstrated in the same material contrasting all other known memristor technologies which are unipolar. Such behavior could be useful in future neuromorphic computing since n-type material exhibits inhibitory synaptic response (increasing resistance with time/voltage) while p-type material exhibits excitatory synaptic response (decreasing resistance with time/voltage). In principle (not yet demonstrated) this core complementary technology can fully implement neuron/synapse brain function without the need for traditional CMOS.

III-Nitride Growth on Lithium Niobate: A New Substrate Material for Polarity Engineering in III-Nitride Heteroepitaxy

Herein, we discuss the use of a novel new substrate for III-Nitride epitaxy, Lithium Niobate. It is shown that Lithium Niobate (LN) has a smaller lattice mismatch to III-Nitrides than sapphire and can be used to control the polarity of III-Nitride films grown by plasma assisted molecular beam epitaxy. Results from initial growth studies are reported including using various nitridation/buffer conditions along with structural and optical characterization. Comparisons of data obtained from GaN and AlN buffer layers are offered and details of the film adhesion dependence on buffer layer conditions is presented. Lateral polarization heterostructures grown on periodically poled LN are also demonstrated. While work is still required to establish the limits of the methods proposed herein, these initial studies offer the promise for mixing III-Nitride semiconductor materials with lithium niobate allowing wide bandgap semiconductors to utilize the acoustic, pyroelectric/ferroelectric, electro-optic, and nonlinear optical properties of this new substrate material as well as the ability to engineer various polarization structures for future devices.

Mg Doped GaN Using a Valved, Thermally Energetic Source: Enhanced Incorporation, Control and Quantitative Optimization

In this study, a thermally-energetic Mg source with an independent, valved-flux control was used to study the behavior of Mg incorporation into GaN. To observe effects of the thermal energy of the Mg flux on Mg incorporation, two Mg flux temperatures were investigated: one (900°C) well above the melting point of Mg and one (625°C) slightly below the melting point of Mg. Alternating Mg-doped and undoped GaN layers were grown at steps of increasing Mg flux, retaining a constant thermal energy, from below the saturation limit, to above the saturation limit. Results were analyzed and compared using secondary ion mass spectroscopy (SIMS). For a constant measured Mg flux, the incorporated Mg increased by more than an order of magnitude when the Mg thermal source temperature was raised from 625°C to 900°C. During SIMS analysis, the energy spectra of sputtered Ga atoms were fairly constant for a Mg flux above the saturation limit, and shifts for a Mg flux slightly below the critical flux for saturation, indicating a conductivity change, and possibly providing a quantitative means of optimizing p-type conduction. Furthermore, Mg incorporation into GaN strongly depends on the III-V flux ratio. During this study it was also observed that Mg incorporation into GaN was enhanced on a rough growth-layer surface under N-rich conditions, while a smoother growth-layer surface resulted in lower Mg incorporation, even under N-rich conditions1.

Fractal Electrode Formation in Metal–Insulator Composites Near the Percolation Threshold

Over the past 20 years, there have been a variety of experiments that have revealed a large increase in the low-frequency capacitance of devices constructed from metal-insulator nanocomposite materials. These capacitive increases are typically reported as dramatic increases in the effective relative dielectric constants of the nanocomposite materials. A class of these materials that operate at room temperature and have metal particle concentrations near the percolation threshold have been shown to have dramatic increases in the effective dielectric constant on the order of 104-1010. The simulations in this paper reveal that electrical contact to large metal clusters inside the composite material near the percolation threshold form fractal-like electrodes that deeply penetrate into the host dielectric material, which result in an effective dielectric constant that is 104-105 times greater than the host dielectric. Furthermore, insulating the planar electrodes so that no electrical contact is made to the metallic clusters inside a near-percolation-threshold composite material reveals a much smaller increase (40×) in the effective dielectric constant. Finally, a new physical scaling model and a simple geometric model for capacitance estimation in metal-insulator composites are developed and used to enhance understanding of the physical effects behind the results of these numerical simulations.

Monolithic integration of AlGaN/GaN-LiNbO/sub 3/ optical-electronic-structures

We present an innovative technology directed at monolithic integration of GaN electronic devices with lithium niobate (LiNbO/sub 3/) waveguides. GaN FETs have been fabricated for use in systems occupying complete phase and amplitude control and measurement capabilities.

Molecular Beam Epitaxial Growth of AlN/GaN Multiple Quantum Wells

AlN/GaN multiple quantum wells (MQWs) were grown on sapphire substrates by plasmaassisted molecular beam epitaxy. Growth temperature, III/V ratio, growth rate, and other growth parameters were optimized for the buffer layer and the MQWs, separately. The growth of AlN buffer was kept as Al-rich as possible while the formation of Al droplets was avoided. A GaN buffer layer was also tried but proved to be inferior to AlN buffer probably due to its larger surface roughness, higher dislocation density, and larger lattice mismatch with the AlN barrier layers in the MQWs. Very flat surfaces with a RMS roughness of 0.7nm were observed by atomic force microscopy (AFM) on the samples with both AlN buffer layer and 20 MQWs deposited under the optimized growth conditions. Abrupt interfaces and excellent periodicities of the MQWs were confirmed by X-ray diffraction (XRD) and reflectivity measurements with MQWs’ satellite peaks clearly visible up to the 10th order. Room-temperature intense ultraviolet (UV) photoluminescence (PL) emission with wavelength in the range of 320-350nm was also observed from the MQWs with well width ranging from 1.0 to 1.5nm. These MQW structures can potentially be used for UV light emitters and quantum cascade lasers.

A mechanistic study of the interaction of water-soluble borate glass with apatite-bound heterocyclic nitrogen-containing bisphosphonates

UNLABELLED Long-term oral and intravenous use of nitrogen-containing bisphosphonates (N-BPs) is associated with osteonecrosis of the jaw. Although N-BPs bind strongly to bone surfaces via non-covalent bonds, it is possible for extrinsic ions to dissociate bound N-BPs from mineralized bone by competitive desorption. Here, we investigate the effects and mechanism of using an ionic cocktail derived from borate bioactive glass for sequestration of heterocyclic N-BPs bound to apatite. By employing solid-state and solution-state analytical techniques, we confirmed that sequestration of N-BPs from bisphosphonate-bound apatite occurs in the presence of the borate-containing ionic cocktail. Simulations by density functional theory computations indicate that magnesium cation and borate anion are well within the extent of the risedronate or zoledronate anion to form precipitate complexes. The sequestration mechanism is due to the borate anion competing with bisphosphonates for similar electron-deficient sites on the apatite surface for binding. Thus, application of the borate-containing ionic cocktail represents a new topical lavage approach for removing apatite-bound heterocyclic N-BPs from exposed necrotic bone in bisphosphonate-related osteonecrosis of the jaw. STATEMENT OF SIGNIFICANCE Long-term oral consumption and injections of nitrogen-containing bisphosphonates (N-BPs) may result in death of the jaw bone when there is traumatic injury to the bone tissues. To date, there is no effective treatment for such a condition. This work reported the use of an ionic cocktail derived from water-soluble borate glass microfibers to displace the most potent type of N-BPs that are bound strongly to the mineral component on bone surfaces. The mechanism responsible for such an effect has been identified to be cation-mediated complexation of borate anions with negatively-charged N-BPs, allowing them to be released from the mineral surface. This borate-containing cocktail may be developed into a novel topical rinse for removing mineral-bound N-BPs from exposed dead bone.