Robert D. Horning

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.

Temperature-dependent absorption measurements of excitons in GaN epilayers

Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the 1s A and B exciton transitions. Using polarization dependent absorption, the C exciton transition was identified. A broad absorption feature was observed at ∼3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of E(T)=E(T=0)−11.8×10−4T2(1414+T) eV for the A and B excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling.

Sealed-cavity resonant microbeam pressure sensor

A quasi-digital pressure sensor based on polysilicon resonant microbeams has been demonstrated. Pressure sensitivities of nearly 4000 counts per second per psi have been attained on a 10 psi device with a base frequency of 233 000 Hz. Short-term stability as low as 0.01 ppm of the base frequency is typical. The microbeams are fabricated with their own integral vacuum cavities, allowing high-Q operation in the differential pressure mode or in contact with liquids such as silicone oil. Design considerations include the effects of internal strain and lead to a push-pull layout configuration independent of microbeam strain or diaphragm thickness. Fabrication technology incorporates fine-grained polysilicon, surface micromachining, bulk micromachining, and reactive sealing. Packaging into precision avionics headers is being used for preliminary testing. Testing results indicate suitability for precision avionics, industrial, and commercial applications. Optical methods have been used to test resonant microbeam pressure sensors and verify the push-pull design methodology. Testing methods developed under this effort include electrostatic drive/piezoresistive sensing, optical drive/optical sensing, substrate piezoelectric drive/optical sensing, and electrostatic drive/laser vibrometer sensing. Wafer-level testing of 200 μm×46 μm×1.9 μm microbeams shows an average fundamental frequency of 553 150 and first overtone of 1 332 550 Hz. The standard deviations across the wafer are 0.15 and 0.10%, respectively. The internal strain and effective thickness can be determined with high resolution. Laser vibrometer measurements through the microbeam shell verify the fundamental frequency and reveal at least ten overtones up to 25 MHz.

Sealed-cavity resonant microbeam accelerometer

Abstract Resonant microbeam strain-sensing elements have been combined with a highly symmetric multiwafer silicon microstructure to form the first micromachined accelerometer with a polysilicon resonant microbeam pickoff. The MARIMBA (micromachined resonant integrated microbeam accelerometer) uniquely combines bulk-micromachining with advanced surface-micromachining techniques and a multiple-wafer structure to produce an all-silicon acceleration sensor capsule with a direct acceleration-to-digital frequency output. The microbeams are fabricated from thin films of fine-grained polysilicon with integral polysilicon vacuum encapsulation, electrostatic drive and piezoresistive sense. The silicon microstructure features a dual open-web suspension system formed monolithically with the silicon proof mass and frame. Silicon caps provide squeeze-film gas damping, overrange protection, and environmental protection. Initial devices have been fabricated with scale factors greater than 700 Hz g −1 on ±20 g devices with base microbeam frequencies of 500–700 kHz. Root Allan variances below 0.1 Hz and seven-day stability measurements less than 2 ppm of the base frequency on test microbeams indicate that milli- g stability and micro- g sensitivities are attainable.

Intrinsic exciton transitions in GaN

Intrinsic excitonic transitions in GaN have been studied using a variety of spectroscopic measurements. Sharp spectral structures associated with intrinsic free excitons could be observed in photoluminescence, reflection, and absorption spectra. The energy positions of excitonic transitions in GaN epitaxial layers were found to be influenced by the residual strain resulting from lattice-parameter and thermal-expansion mismatches between the epilayers and the substrates. The values of the four principal deformation potentials of wurtzite GaN were derived by using the strain tensor components determined by x-ray measurements. The observation of spectral features involving the emission of LO phonons in absorption and photoluminescence excitation spectra at energies above exciton resonances indicate that a phonon-assisted indirect excitation process, which simultaneously generates a free exciton and a LO phonon, is a very significant and efficient process in GaN. The lifetime of the free excitons is found to ...

LARGE OPTICAL NONLINEARITIES NEAR THE BAND GAP OF GAN THIN FILMS

The interband optical transitions in single-crystal GaN films grown by metal organic chemical vapor deposition have been studied at 10 K and room temperature using nondegenerate nanosecond optical pump-probe techniques. At low temperatures, strong, well-resolved features are seen in the absorption and reflection spectra corresponding to the 1s A and B exciton transitions. These features broaden and decrease in intensity due to the presence of a high density of photoexcited free carriers and are completely absent in the absorption and reflection spectra as the excitation density, Iexc, approaches 3 MW/cm2, resulting in induced transparency in transmission measurements. The absorption spectra also show induced absorption below the band gap as Iexc is increased. Both the observed induced transparency and induced absorption were found to be extremely large, exceeding 4×104 cm−1 as the pump density approaches 3 MW/cm2 at 10 K.

Optical studies of epitaxial GaN-based materials

A variety of spectroscopic techniques has been used to study the optical properties of epitaxial GaN based materials grown by metalorganic chemical vapor deposition and molecular beam epitaxy. The emphasis was on the issues vital to device applications such as stimulated emission and laser action, as well as carrier relaxation dynamics. Sharp exciton structures were observed by optical absorption measurements above 300 K, providing direct evidence of the formation of excitons in GaN at temperatures higher than room temperature. Using a picosecond streak camera, the time decay of free and bound exciton emissions was studied. By optical pumping, stimulated emission and lasing were investigated over a wide temperature range up to 420 K. In addition, the optical nonlinearity of GaN was studied using wave mixing techniques.

High performance infrared detector arrays using thin film microstructures

Honeywell has developed a unique uncooled thermal detector technology based on fabricating thin film structures with temperature sensitive detector materials. High TCR resistive materials such as VOx and YBaCuO, and pyroelectric PbTiO/sub 3/ have been used. Two dimensional imaging arrays of sizes up to 240/spl times/336 have been integrated with Si substrate electronics to achieve temperature sensitivities of less than .04 C operating at room temperature. The thin film detector materials are deposited on microstructure thin film pixels of 2 mil sizes which are subsequently thermally isolated from the substrate by etching away the underlying substrate. The thermal isolation of the microstructure pixel provides the temperature rise and the detector material provides the conversion to an electrical signal.

AlxGa1−xN polarity determination by x‐ray diffraction

The crystallographic polarities of (0001) oriented AlxGa1−xN epitaxial layers grown on basal plane sapphire were determined using the anomalous dispersion effect of x‐ray diffraction. Integrated intensities of three (00l) reflections were measured using the Lγ1 and Lβ2 excitation lines from a tungsten x‐ray source. Calculations that included the effects of anomalous dispersion were compared with the measurements to identify the polarity. Each epitaxial layer had a single polarity rather than a mixture of oppositely polarized grains. However, both A‐ and B‐oriented layers were observed. The method also provides a nondestructive estimate of the layer thickness.

GaN solid state electron emitter

We report the first experimental demonstration of a low voltage, room temperature electron emitter that is based on the GaN UV opto-electronic cathode. It consists of an ultra-violet light emitting diode (UV-LED) in direct contact with a photoemitter. The photons generated by the LED have energies 3.4 eV (the bandgap of GaN). The photons cause photoemission of electrons from the surface layer of low workfunction material such as LaB6 or CeB6 (0 = 2.5 eV). The photoemitted electrons are collected by an anode or can be focused. Modulated electron emission is obtained by modulating the LED.