J. David Zook

Defeating Compensation in Wide Gap Semiconductors by Growing in H that is Removed by Low Temperature De-Ionizing Radiation

We propose a general method to obtain high conductivity of either type in wide gap semiconductors where compensation normally limits conductivity of one or both types. We suggest that the successes of Amano et al. and of Nakamura et al. in obtaining more than 1018 cm-3 holes in GaN are particular examples of the general process that we propose.

Ultraviolet and violet light‐emitting GaN diodes grown by low‐pressure metalorganic chemical vapor deposition

Both metal‐insulator‐semiconductor and p‐n junction electroluminescence have been observed in thin‐film, metalorganic chemical vapor deposition‐grown GaN diodes thermally annealed in N2. UV radiation, peaking near 380 nm, is emitted when electrons are injected from the undoped, n‐type material into the Mg‐doped, p‐type GaN. Violet light, peaking near 430 nm, is obtained by injecting electrons into p‐type material from either n‐type material or non‐ohmic metal contacts. The present results support and extend earlier interpretations of the nature of the recombination centers in GaN.

Electrical Properties of Heavily Doped Silicon

Measurements have been made of the temperature dependences of the electrical resistivity and Hall coefficient in samples of n‐ and p‐type silicon having impurity concentrations in the 1018 to 1020 cm−3 range. The resistivity data extend from 4° to 900°K, and the Hall data from 4° to 300°K. The results exhibit two noteworthy features: viz., (1) a hump or maximum in the resistivity vs temperature curves at or slightly below the degeneracy temperature in each sample, which is most pronounced in the least heavily doped samples and gradually fades out as the impurity concentration increases, and (2) an extension of the positive dependence of resistivity on temperature below the hump or degeneracy temperature to surprisingly low temperatures in each sample.

Capacitive microbeam resonator design

Resonant clamped-clamped microbeams sealed in a hard vacuum cavity are classified as transducers, which can measure physical variables by converting them into axial strain using an appropriate silicon microstructure. These devices can be constructed, using surface micromachining technology, on a single-crystal silicon substrate. They have fundamental resonant frequencies with high sensitivity to strain. Such devices use resonant frequency changes by variables such as pressure, temperature, force, and acceleration to measure these quantities. Electrostatically driven and sensed microbeam resonators may be used for sensor applications. In order to design such microbeam resonators it is useful to use electrical network theory. This requires that the mechanical parameters for the resonator are converted to electrical equivalents. For electrostatically driven and sensed microbeam resonators the drive voltage must contain a dc bias and a small amplitude sinusoid in order to drive the resonators at the resonant frequency. The effects of these dc biases and parasitics on the resonant frequency and the quality factor are clarified here with theoretical calculations using the electrical equivalents of electrostatic microbeam resonators and experimental results. As a result the dc bias and parasitics are dominant factors in determining the performance of capacitive microbeam resonators, especially effecting the resonant frequency and quality factor. The maximum vibration amplitude requirements for pure sinusoidal operation with low power dissipation in the capacitive microbeam resonator have also been identified.

Fiber-optic vibration sensor based on frequency modulation of light-excited oscillators

A sensor device for detecting vibration, including a light source for providing a laser light, a first optical fiber connected to the source for transmitting the laser light, an oscillator positioned to receive the transmitted laser light and adapted to reflect the light as a frequency modulated light; a second optical fiber positioned to capture the frequency modulated light to transmit the frequency modulated light; and a frequency modulated discriminator for receiving the frequency modulated light from the second optical fiber and producing a signal responsive of vibration of the oscillator. In a preferred embodiment, the frequency modulated discriminator further includes a frequency meter for determining the average number of cycles per unit time to provide a second signal responsive of the temperature of the oscillator. The optical fibers may be a pair of different fibers positioned for transmitting the laser light and the frequency modulated light respectively, or the same fiber positioned for transmitting both the laser light and the frequency modulated light. In this latter embodiment, the device further includes a beam splitter to direct the frequency modulated light to the discriminator. The preferred oscillator includes a microchip having a microbeam mounted on a thin silicon cantilever such that deflection of the beam perpendicular to the plane of the microchip changes the tension in the microbeam to change its resonant frequency. Also preferred is a microbeam including a thin metal deposit to create a bimorph structure.

Light Beam Deflector Performance: a Comparative Analysis

The performance of various types of analog light beam deflectors is summarized, and their relative positions in a deflector hierarchy are defined. The three types of deflectors considered are (1) mechanical (galvanometer) mirror deflectors, (2) acoustooptic deflectors, and (3) analog electrooptic deflectors. Material figures of merit are defined and compared, and the theoretical trade-off between speed and resolution is given for each type of deflector.

Optically excited self-resonant microbeams

Abstract Optically excited self-resonance of polysilicon microbeams sealed in a cofabricated vacuum enclosure has been achieved. Unmodulated low-power laser diodes from 650 to 840 nm have been used to excite resonances ranging from 65 to 750 kHz on microbeams ranging from 0.79 to 2.38 μm in thickness. The photovoltaic excitation mechanism uses a p-n junction photodiode underneath the microbeam. The structure forms an effective optomechanical modulator at the microbeam resonance frequency, and the resonance can be readily detected with the reflected laser light, which is modulated at levels that can approach 100%. Analysis of the conditions for self-resonance gives predictions of minimum Q -values for self-resonance. Observed Q -values (20 000 to 130 000) are well in excess of the required values. Thicknesses of the microbeam and vacuum gaps above and below it are critical for achieving low oscillation thresholds, which may be as low as 1 μW of optical power. The clamped—clamped microbeams are sensitive strain transducers with high gage factors, low temperature sensitivity, and wide dynamic range. These are the first optically powered active devices to achieve gain by interchanging optical, electrical, and mechanical energy in a merged structure. They uniquely combine silicon microfabrication technology with optoelectronic technology and can form the basis for a new class of fiber-optic sensors for pressure, temperature, acceleration, and other variables that can be converted to a strain using an appropriate silicon microstructure.

Theory of beam‐induced currents in semiconductors

An analytical model is presented for the current flowing in a Schottky diode or shallow p‐n junction when the current is generated by an electron beam or light beam. The model represents an exact solution to the three‐dimensional diffusion equation when the current‐collecting junction is at right angles to a grain boundary or to the semiconductor surface. The finite size of the electron‐hole generation region is readily included in the analysis. The results are given in terms of one‐dimensional integrals which can simply and accurately be evaluated using a programmable hand calculator. The theory provides a basis for using electron‐beam‐induced currents or light‐beam‐induced currents to measure the minority‐carrier diffusion length and surface recombination velocity at a grain boundary or surface.

Polysilicon sealed vacuum cavities for microelectromechanical systems

Sealed vacuum cavities are highly useful in silicon-based microelectromechanical systems. Sealed polysilicon cavities serve as reference chambers for commercially available pressure sensors and provide enclosures for high-Q mechanical resonators. They also provide isolation of resonant strain gauges from ambient gases and liquids for use in precision pressure transmitters. The residual pressure in such cavities is less than 10−5 Torr, as evidenced by the polysilicon resonator Q values which can be as high as 300 000. Measured values of Q can be dominated by parasitic electrical resistance losses in the electrical drive circuit. After these losses are accounted for, the dominant loss is due to mechanical radiation of acoustic waves from the supports to the rest of the die and package. The vibration of the beams can be excited and sensed in several different ways for testing purposes. Fiberoptically driven and sensed microbeams operating at up to 510 °C for several hours resulted in no loss of vacuum as evi...

OPTICAL PERFORMANCE OF HIGH-ASPECT LIGA GRATINGS II

The analysis of an IR tunable filter that uses a transmission grating with variable spacing is extended to include incident spherical waves instead of plane waves and to illustrate specifically the effects of finite conductivity in the metal. The model is based on a rigorous vector solution of Maxwell’s equations implemented in a finite elements solver. The modeling results based on tabulated optical properties of nickel are compared with experimental measurements made with filters fabricated in permalloy by the lithographie, galvanoformung, abformung (LIGA) process. The theoretical and experimental results compare quite well and suggest that the resolution of the filter is severely reduced at the shorter wavelengths (less than 4 ?m). At longer wavelengths, the resolution is limited by the aspect ratio of the LIGA structures. A spectral resolution of 0.1 ?m appears to be practical.