Clifton G. Fonstad

Defect structure and electronic donor levels in stannic oxide crystals

By measuring the conductivity of stannic oxide crystals as a function of oxygen partial pressure at elevated temperatures, it is shown that the dominant native defect in SnO2 is a doubly ionizable oxygen vacancy. Both donor levels of this defect, the first 30 meV deep and the second 150 meV deep, are identified and a model is presented that explains previous results. The behavior in hydrogen is contrasted to that in oxygen, and preliminary results are presented indicating that hydrogen introduces a donor 50 meV deep.

Electrical Properties of High‐Quality Stannic Oxide Crystals

Single crystals of the wide bandgap semiconductor stannic oxide, SnO2, have been grown and studied electrically. A chemical vapor deposition technique using chlorine transport, no inert carrier gases, and low pressure has been used to grow stannic oxide crystals of higher purity and with almost an order‐of‐magnitude higher low‐temperature Hall mobility, 8800 cm2/V sec at 80°K, than have previously been available. Measurements of Hall mobility, carrier concentration, and resistivity have been made between 20 and 625°K on crystals with room‐temperature carrier concentrations between 8×1015 and 2×1018 cm−3. The effects of the crystal anisotropy on these measurements have been investigated and found to be small (all results reported are for the a direction). A donor level ∼35‐meV deep due to antimony and another level ascribed to oxygen vacancies at ∼140 meV have been observed. Polar optical mode scattering with a dominant characteristic temperature of 1080°C is the main carrier scattering mechanism above 250...

Pseudomorphic In0.53Ga0.47As/AlAs/InAs resonant tunneling diodes with peak‐to‐valley current ratios of 30 at room temperature

Pseudomorphic In0.53 Ga0.47 As/AlAs/InAs resonant tunneling diodes have been grown on InP substrates by molecular beam epitaxy. Peak‐to‐valley current ratios as high as 30 at 300 K and 63 at 77 K are obtained on a structure with barriers of ten atomic layers AlAs, and a well consisting of three atomic layers of In0.53 Ga0.47 As, six atomic layers of InAs, and three atomic layers of In0.53 Ga0.47 As. For comparison pseudomorphic In0.53 Ga0.47 As/AlAs with In0.53 Ga0.47 As well structures have also been fabricated. For the In0.53 Ga0.47 As well structures, peak‐to‐valley current ratios as high as 23 have been obtained at 300 K, and, in other devices with lower current densities, two resonances are observed at room temperature.

Intrawell and interwell intersubband transitions in multiple quantum wells for far‐infrared sources

A theoretical study of electrically pumped unipolar lasers exploiting intrawell or interwell intersubband radiative transitions in multiple quantum‐well heterostructures for the generation of IR and FIR radiation is presented. The feasibility of these coherent sources critically depends on the non‐radiative intersubband transition rates. Numerical simulations of acoustical phonon, optical‐phonon, and electron‐electron scattering were implemented, including their temperature dependence. For far‐infrared coherent sources, electron‐electron scattering emerges as the dominating non‐radiative relaxation mechanism. Interwell schemes offer distinctive advantages such as simplicity in design, greater tolerance in design and fabrication errors, field tunability of the emission frequency, improved internal quantum efficiency and aid in establishing population inversion. Design rules are put forward for such long wavelength sources.

Phase-locked semiconductor laser arrays

A method and apparatus for phase-locking semiconductor laser arrays is described wherein individual light waves propagating in adjacent waveguides are phase-locked by intercoupling the waveguides in a specific manner. The waveguides are intercoupled in an unguided mode-mixing region wherein various modes of propagation mix by diffraction. The length of this region is fixed at a distance z which produces a phase difference an integral multiple of 2π between adjacent waveguides.

Three-dimensional multiwaveguide probe array for light delivery to distributed brain circuits

To deliver light to the brain for neuroscientific and neuroengineering applications like optogenetics, in which light is used to activate or silence neurons expressing specific photosensitive proteins, optical fibers are commonly used. However, an optical fiber is limited to delivering light to a single target within the 3D structure of the brain. Here, we describe the design and fabrication of an array of thin microwaveguides, which terminates at a three-dimensionally distributed set of points, appropriate for delivering light to targets distributed in a 3D pattern throughout the brain.

Multiwaveguide implantable probe for light delivery to sets of distributed brain targets

Optical fibers are commonly inserted into living tissues such as the brain in order to deliver light to deep targets for neuroscientific and neuroengineering applications such as optogenetics, in which light is used to activate or silence neurons expressing specific photosensitive proteins. However, an optical fiber is limited to delivering light to a single target within the three-dimensional structure of the brain. We here demonstrate a multiwaveguide probe capable of independently delivering light to multiple targets along the probe axis, thus enabling versatile optical control of sets of distributed brain targets. The 1.45-cm-long probe is microfabricated in the form of a 360-μm-wide array of 12 parallel silicon oxynitride (SiON) multimode waveguides clad with SiO(2) and coated with aluminum; probes of custom dimensions are easily created as well. The waveguide array accepts light from a set of sources at the input end and guides the light down each waveguide to an aluminum corner mirror that efficiently deflects light away from the probe axis. Light losses at each stage are small (input coupling loss, 0.4 ± 0.3 dB; bend loss, negligible; propagation loss, 3.1 ± 1 dB/cm using the outscattering method and 3.2 ± 0.4 dB/cm using the cutback method; corner mirror loss, 1.5 ± 0.4 dB); a waveguide coupled, for example, to a 5 mW source will deliver over 1.5 mW to a target at a depth of 1 cm.

Enhanced photogeneration of carriers in a semiconductor via coupling across a nonisothermal nanoscale vacuum gap

Enhanced generation of carriers when a thermophotovoltaic cell is placed in submicron proximity to a heated surface is demonstrated using custom-designed InAs photodiodes and special silicon-based heater chips produced using microelectromechanical system techniques. The short-circuit current of the photocells is shown to increase sharply (up to fivefold) when the spacing between the heater and photodiode surfaces is reduced, while at the same time, the heater temperature decreases, consistent with increased radiative transfer between the two surfaces. By varying the spacing sinusoidally (at up to 1 kHz), it is demonstrated that the increase in the short-circuit current occurs in phase with the decrease in separation, thereby ruling out thermal effects. It is argued that the increase in short-circuit current is due to increased evanescent coupling of blackbody radiation from the hot surface to the cold photocell, consistent with recent theoretical predictions. The demonstration of this effect is the initia...

Theoretical gain of strained-layer semiconductor lasers in the large strain regime

The theoretical gain of strained-layer semiconductor lasers is analyzed in the large strain regime based on the density-matrix method, taking into account the modification of both the valence bands and the transition dipole moments. The wave functions for the valence-band states for an arbitrary wave vector at the Gamma point in the presence of stain are derived from diagonalization of the strain Hamiltonian using the original wave functions obtained from the k-p method. These wave functions are then used to obtain the dipole moment matrix elements at the band edges, which are found to be independent of the wave vector. >

Very large radiative transfer over small distances from a black body for thermophotovoltaic applications

The maximum amount of radiated heat intensity which can be transferred from a black body of refractive index D/sub BB/ to an object of refractive index D/sub OBJ/ located a short distance away is shown to be n/sub smaller//sup 2/ times the free space Planck distribution where n/sub smaller/ is the smaller of n/sub BB/ and n/sub OBJ/, and where n/sub BB/ and n/sub OBJ/ are assumed greater than unity. The implication is that the radiative power spectral density within a thermophotovoltaic cell could be designed to be much greater than the free space Planck distribution. The maximum radiative intensity transferred occurs when the index of the black body matches that of the object at wavelengths where the Planck distribution is sizeable. A simple expression is found for the transferred radiative intensity as a function of the refractive indices of and the distance separating, the black body and the object. This expression is interpreted in terms of the specific black body modes which are evanescent in the space between the black body and the object and which make the largest contribution to the transmission of radiation. The black body, the object, and the region are all modeled as lossless dielectrics.