Group-III Nitride Field Emitters

Field-emission devices (cold cathodes) having low electron affinities can be fabricated through lattice-mismatched epitaxial growth of nitrides of elements from group III of the periodic table. Field emission of electrons from solid surfaces is typically utilized in vacuum microelectronic devices, including some display devices. The present field-emission devices and the method of fabricating them were developed to satisfy needs to reduce the cost of fabricating field emitters, make them compatible with established techniques for deposition of and on silicon, and enable monolithic integration of field emitters with silicon-based driving circuitry. In fabricating a device of this type, one deposits a nitride of one or more group-III elements on a substrate of (111) silicon or other suitable material. One example of a suitable deposition process is chemical vapor deposition in a reactor that contains plasma generated by use of electron cyclotron resonance. Under properly chosen growth conditions, the large mismatch between the crystal lattices of the substrate and the nitride causes strains to accumulate in the growing nitride film, such that the associated stresses cause the film to crack. The cracks lie in planes parallel to the direction of growth, so that the growing nitride film becomes divided into microscopic growing single-crystal columns. The outer ends of the fully-grown columns can serve as field-emission tips. By virtue of their chemical compositions and crystalline structures, the columns have low work functions and high electrical conductivities, both of which are desirable for field emission of electrons. From examination of transmission electron micrographs of a prototype device, the average column width was determined to be about 100 nm and the sharpness of the tips was determined to be characterized by a dimension somewhat less than 100 nm. The areal density of the columns was found to about 5 x 10(exp 9)/sq cm . about 4 to 5 orders of magnitude greater than the areal density of tips in prior field-emission devices. The electric field necessary to turn on the emission current and the current per tip in this device are both lower than in prior field-emission devices, such that it becomes possible to achieve longer operational lifetime. Moreover, notwithstanding the lower current per tip, because of the greater areal density of tips, it becomes possible to achieve greater current density averaged over the cathode area. The thickness of the grown nitride film (equivalently, the length of the columns) could lie between about 0.5 microns and a few microns; in any event, a thickness of about 1 micron is sufficient and costs less than do greater thicknesses. It may be possible to grow nitride emitter columns on glass or other substrate materials that cost less than silicon does. What is important in the choice of substrate material is the difference between the substrate and nitride crystalline structures. Inasmuch as the deposition process is nondestructive, an ability to grow emitter columns on a variety of materials would be advantageous in that it would facilitate the integration of field-emitter structures onto previously processed integrated circuits

Radiation and Temperature Hard Multi-Pixel Avalanche Photodiodes

The structure and method of fabricating a radiation and temperature hard avalanche photodiode with integrated radiation and temperature hard readout circuit, comprising a substrate, an avalanche region, an absorption region, and a plurality of Ohmic contacts are presented. The present disclosure provides for tuning of spectral sensitivity and high device efficiency, resulting in photon counting capability with decreased crosstalk and reduced dark current

Solid-State Neutron Detector Device

The structure and methods of fabricating a high efficiency compact solid state neutron detector based on III-Nitride semiconductor structures deposited on a substrate. The operation of the device is based on absorption of neutrons, which results in generation of free carriers

Strained quantum well photovoltaic energy converter

An indium phosphide photovoltaic cell is provided where one or more quantum wells are introduced between the conventional p-conductivity and n-conductivity indium phosphide layer. The approach allows the cell to convert the light over a wider range of wavelengths than a conventional single junction cell and in particular convert efficiently transparency losses of the indium phosphide conventional cell. The approach hence may be used to increase the cell current output. A method of fabrication of photovoltaic devices is provided where ternary InAsP and InGaAs alloys are used as well material in the quantum well region and results in an increase of the cell current output

Applying CLIPS to control of molecular beam epitaxy processing

A key element of U.S. industrial competitiveness in the 1990's will be the exploitation of advanced technologies which involve low-volume, high-profit manufacturing. The demands of such manufacture limit participation to a few major entities in the U.S. and elsewhere, and offset the lower manufacturing costs of other countries which have, for example, captured much of the consumer electronics market. One such technology is thin-film epitaxy, a technology which encompasses several techniques such as Molecular Beam Epitaxy (MBE), Chemical Beam Epitaxy (CBE), and Vapor-Phase Epitaxy (VPE). Molecular Beam Epitaxy (MBE) is a technology for creating a variety of electronic and electro-optical materials. Compared to standard microelectronic production techniques (including gaseous diffusion, ion implantation, and chemical vapor deposition), MBE is much more exact, though much slower. Although newer than the standard technologies, MBE is the technology of choice for fabrication of ultraprecise materials for cutting-edge microelectronic devices and for research into the properties of new materials

Influence of HiPIMS pulse widths on the deposition behaviour and properties of CuAgZr compositionally graded films

In this work, the influence of different pulse widths (25, 50 and 100 μs) during high power impulse magnetron sputtering (HiPIMS) of copper, silver and zirconium was investigated in terms of plasma properties and properties of combinatorial composition gradient CuAgZr film libraries. In situ plasma diagnostics via optical emission spectroscopy (OES), time-of-flight mass spectrometry (TOFMS), and modified quartz crystal microbalance (m-QCM), followed by film ex situ X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations allowed to determine the effect of deposition parameters on the thin films' microstructural changes. Changing the pulse width, while keeping the duty cycle constant, modified the discharge composition in the target region and the ionised fraction of the sputtered species in the substrate region. The maximum Cu ionised fraction (19 %) was found for 50 μs, resulting in compact and smooth morphology for Cu-rich films, whereas short 25 μs pulses provided porous columnar films with rough surfaces, as the result from Ar+ bombardment. For Ag-rich films, Ag segregation allowed the deposition of dense layers, regardless of the used pulse width. Furthermore, low Ag (<10 at.%) CuAgZr films produced via HiPIMS and direct-current magnetron sputtering (DCMS) were compared in terms of structural and mechanical property changes as a function of Zr contents. For the studied chemical composition range, a linear relationship between Zr content, XRD phase shift and mechanical properties was observed for HiPIMS films, in contrast to DCMS's more abrupt transitions. An increase in hardness and elastic modulus (up to 44 % and 22 %, respectively) was found for the HiPIMS films compared to DCMS ones. The obtained results highlight HiPIMS's flexibility in providing a wide range of tailoring possibilities to meet specific application requirements, such as crystalline microstructure, density and associated mechanical properties

Improved Nanoreinforced Composite Material Bonds with Potential Sensing Capabilities

Dramatic increase in the bond strength of composite/adhesive interfaces of nanoreinforced composite material joints and structures has been achieved using laser-assisted fabrication of Micro-Column Arrays (MCA) on the surface of the two materials prior to bonding. Several advantages of the MCA technology resulting in drastic improvement of virtually any bond include: interlocking of the adhesive material between micro columns, about 10-fold increase of the specific surface area, inherent elasticity of the micro columns, enhanced resistance to hygrothermal failures, substantially improved wettability, and control over the surface chemistry. In order to take advantage of the unique electrical properties of both composite materials and the Carbon Nanotube (CNT) reinforced adhesive, experiments were performed to evaluate the potential of using electrical response of the bonded material to applied stress

Long mean free paths of room-temperature THz acoustic phonons in a high thermal conductivity material

We report measurements of room-temperature mean free paths of long-lived THz acoustic phonons in wurtzite GaN. Longitudinal phonon wave packets are excited and probed by femtosecond laser pulses in two InGaN-GaN multiple quantum well structures separated by a GaN layer. By measuring the temperature dependence of the phonon attenuation in the range 80–300 K we isolate the intrinsic phonon mean free path at 300 K which is found to be 5.3 and 3.5 μm at 1.06 and 1.43 THz, respectively. The measurements are found to be in good agreement with ab initio calculations which show that the main channel of the acoustic phonon decay is a three-phonon scattering process involving the acoustic phonon and two high-frequency optical phonons. Our results indicate that the contribution of low-THz acoustic phonons to thermal transport in GaN is relatively smaller than in Si; thus finite size effects are expected to be less important in GaN