I. Melngailis

GaN avalanche photodiodes grown by hydride vapor-phase epitaxy

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.

The Role of Impurities in Hydride Vapor Phase Epitaxially Grown Gallium Nitride

Gallium nitride (GaN) films grown by hydride vapor phase epitaxy on a variety of substrates have been investigated to study what role silicon and oxygen impurities play in determining the residual donor levels found in these films. Secondary ion mass spectroscopy analysis has been performed on these films and impurity levels have been normalized to ion implanted calibration standards. While oxygen appears to be a predominate impurity in all of the films, in many of them the sum of silicon and oxygen levels is insufficient to account for the donor concentration determined by Hall measurements. This suggests that either another impurity or a native defect is at least partly responsible for the autodoping of GaN. Additionally, the variation of impurity and carrier concentration with surface orientation and/or nucleation density suggests either a crystallographic or defect-related incorporation mechanism.

TEMPERATURE AND COMPOSITIONAL DEPENDENCE OF LASER EMISSION IN Pb1−xSnxSe

Laser emission has been obtained in Pb1−xSnxSe diodes with x up to 0.28, and the temperature dependence of the emission has been studied in the range from 1.5 to 100°K. The results strongly support a band model in which the conduction and valence band edge states cross as the Sn content is increased from 0 to 0.28. For x ≤ 0.10, the temperature coefficient of the energy gap is positive whereas for x ≥ 0.19, the temperature coefficient is negative as predicted by the band model. Also, the results provide evidence that the energy gap is direct on both sides of the crossover point which at 4.2°K occurs for x ≈ 0.15.

FAR‐INFRARED PHOTOCONDUCTIVITY IN HIGH‐PURITY EPITAXIAL GaAs

Extrinsic far‐infrared photoconductivity has been observed at 4.2°K in high‐purity n‐type epitaxial layers of GaAs grown on Cr‐doped semi‐insulating GaAs substrates. Measurements of the responsivity and noise in the detection system at 300 Hz in a 1‐Hz bandwidth yield an NEP of 1.2 × 10−11 W at 195 μ, 1.4 × 10−12 W at 337 μ and 6 × 10−11 W at 902 μ. The time constant of the detector has been determined to be shorter than 1 μsec using a Ge avalanche modulator to chop the incident radiation. A time constant of about 5 nsec has been measured using impact impurity ionization in the GaAs.

Optically pumped GaN/Al0.1Ga0.9N double‐heterostructure ultraviolet laser

Molecular‐beam epitaxy was used to grow a 100 nm Al0.1Ga0.9N/100 nm GaN/500 nm Al0.1Ga0.9N double heterostructure on a 10‐μm‐thick GaN buffer layer grown with hydride vapor phase epitaxy on (0001) sapphire. Lasing from the 100 nm GaN active layer has been obtained at ∼359 nm at liquid‐nitrogen temperature (77 K) and at ∼365 nm at room temperature (295 K), using transverse optical pumping at 337.1 nm with a 600 ps transversely excited atmospheric pressure pulsed nitrogen laser. Threshold pump fluences were measured to be 0.3 and 0.5 mJ/cm2 at 77 and 295 K, respectively, for a laser with 65 μm cavity length. In a laser of 23 μm cavity length, longitudinal cavity modes were observed with 0.56 nm spacing, corresponding to a group refractive index of 5.0 at the lasing wavelength.

Monolithic integrated InxGa1−xAs Schottky‐barrier waveguide photodetector

InxGa1−xAs Schottky‐barrier diodes for detection in the 0.9–1.06‐μm wavelength range have been incorporated in high‐purity GaAs planar waveguides using a selective epitaxial growth process. A quantum efficiency of 60% at 1.06 μm has been obtained for these detectors, and current gain has been observed.

PHOTOVOLTAIC EFFECT IN PbxSn1−xTe DIODES

Photovoltaic response has been observed in p‐n junctions of PbxSn1−xTe at wavelengths up to 9.5 μ at 77°K and up to 12 μ at 12°K. These results together with previous photoluminescence data and the proposed model for the band structure of PbxSn1−xTe (ref. 1) indicate that these alloys have considerable potential for efficient infrared detection throughout the 8 to 14 μ atmospheric window and well beyond.

The Use of Lasers in Pollution Monitoring

Optical techniques have opened up new possibilities in air pollution monitoring because of their remote-sensing capability, very high specificity, and short observation time. Techniques involving the use of lasers include Raman scattering, emission either from resonantly excited or from hot gases, and resonant absorption. Unique advantages in these applications are provided by the recently developed tunable lasers, including organic dye lasers, parametric oscillators, spin-flip Raman lasers, and semiconductor lasers. The absorption technique which promises to have the widest range of application has been tested in the laboratory by using tunable diode lasers. High-resolution absorption spectra have been measured for SF6, NH3, and C2H4 at 10.6 ?m, SO2 at 8,6 ?m, CO at 4.7 ?m, and NO at 5.2 ?m, and the C2H4 (ethylene) content of automobile exhausts has been measured by means of a derivative technique. A sensitivity sufficient to detect one ppm of NH3 in air has been achieved in a 10-cm-long gas cell. The 50 to 100-kHz resolution achievable with diode lasers is about four orders of magnitude better than the resolution of conventional spectrometers and more than adequate to resolve Doppler broadened gas absorption lines which are on the order of 100 MHz wide.

Gallium Nitride Thick Films Grown by Hydride Vapor Phase Epitaxy

Gallium nitride (GaN) thick films (to 150 μm) have been deposited by hydride vapor phase epitaxy (HVPE). These films are unintentionally doped n-type (n = 1–2 × 10 17 cm −3 at 300 K) and exhibit structural and electronic properties which are comparable with the best reported for GaN films grown by organometallic vapor phase epitaxy. Additionally, these properties are found to be uniform over 2-in diameter films grown on sapphire substrates. The use of either a GaCl or ZnO surface pretreatment has been found to substantially enhance the nucleation density, resulting in improved surface morphology and film properties, even though it appears that the ZnO film is thermochemically desorbed early on in the growth. Dislocation densities as low as ˜5×10 7 cm −2 have been attained for films 40 μtm thick. Homoepitaxial overgrowths both by electron-cyclotron-resonance plasma enhanced molecular beam epitaxy and OMVPE proceed in a straightforward manner, essentially replicating the defect structure of the HVPE GaN film.

Light scattering in high‐dislocation‐density GaN

Light scattering by edge dislocations and the resulting loss coefficient have been modeled for GaN layers. Phase‐front deformation caused by the refractive‐index variation in the dislocation’s strain field has been considered and the resulting scattering loss calculated. We show that the high dislocation densities observed in recent GaN layers can result in significant large loss coefficients. The present work also offers some insights for improved lasers.