R. M. Kolbas

Growth of high optical and electrical quality GaN layers using low‐pressure metalorganic chemical vapor deposition

We report on the low‐pressure metalorganic chemical vapor deposition of high quality single‐crystal GaN layers over basal plane sapphire substrates. Optimization of growth conditions resulted in material with carrier densities of 1017 /cm3 at room temperature and corresponding mobilities around 350 cm2 /V s. The photoluminescence linewidths improved from 160 meV [full width at half maximum (FWHM)] to 25 meV (FWHM). With improved material quality we were able to observe the polar optical mode and the ionized impurity scattering regimes in the mobility versus temperature data. Good quality Schottky barriers were formed on the as‐grown material using a tungsten probe and an alloyed indium contact. Our observations indicate a direct correlation between electrical and optical characteristics of good material and strongly question nitrogen vacancies as the sole explanation for the high carrier densities observed in poor quality GaN growths.

Improved breakdown voltage in GaAs MESFETs utilizing surface layers of GaAs grown at a low temperature by MBE

A metal-semiconductor field-effect transistor (MESFET) utilizing surface layers of GaAs grown at a low temperature by MBE (LT GaAs) under the gate electrode has been fabricated. The high trap density of LT GaAs reduces the surface fields of the FET, suppresses gate leakage, and increases the gate-drain breakdown voltage without sacrificing current drive capability. An undoped AlAs layer is incorporated between the LT GaAs layer and the channel as a barrier to the diffusion of excess As from the LT GaAs layer to the channel. A 74- mu m-gate-width device demonstrated an improved breakdown voltage of 34.85 V with a g/sub m/ of 144 mS/mm and an I/sub dss/ of 248 mA/mm.<<ETX>>

ZnSe light‐emitting diodes

We report the successful fabrication of ZnSe p‐n junction light‐emitting diodes in which Li and Cl are used as p‐type and n‐type dopants, respectively.

Photoluminescence characteristics of AlGaN‐GaN‐AlGaN quantum wells

AlxGa1−xN‐GaN quantum wells were grown on basal plane sapphire by low‐pressure metalorganic vapor deposition. The photoluminescence spectra of samples of different well thicknesses and x values were measured. The experimental data were compared with the calculated solutions of the finite square quantum well and the bound states involved in the optical transition were identified.

Light emission from thermally oxidized silicon nanoparticles

Light emission characteristics from silicon nanoparticles consisting of a crystalline core encased in an amorphous oxide shell are presented. The particles were thermally oxidized in the open atmosphere at 800 °C for times from 5 to 160 min in order to decrease the Si core dimensions. Photoluminescence spectra, at low excitation levels, reveal that the light emission shifts to shorter wavelengths as the oxidation time is increased. At high excitation levels, photoluminescence spectra show little or no shift. These results indicate that there are at least two mechanisms involved with light emission from Si nanoparticles, one associated with quantum size effects and another which is independent of size distribution.

Light emission from crystalline silicon and amorphous silicon oxide (SiO x ) nanoparticles

Bright orange-red light emission was observed from single crystal silicon nanoparticles and silicon oxide (SiOx) nanoparticles. The emission peak was recorded at about 1.5 eV both at room temperature and 77K. Varying the mean silicon particle size, we observed no effect of particle diameter on the emission wavelength. Amorphous silicon oxide (SiOx) nanoparticles also showed essentially the same emission spectrum as the crystalline particles. The absence of change in the photoluminescence (PL) spectrum with variations in particle size and crystallinity indicates that quantum confinement is not the controlling PL mechanism. An examination of the hydrogen content with relation to the PL intensity showed no direct correlation; however, all samples did contain some hydrogen, so its effect on PL cannot be ruled out. To test for the presence of photoluminescent siloxene on the surface of the particles, nitric acid was applied; a violent reaction occurred with the silicon particles, while the SiOx particles showed no reaction. Taken in conjunction with the emission data, these experiments demonstrate that the PL of the SiOx is also not dependant on siloxene. Evidence points to an amorphous coating as the source of photoluminescence.

Absorption Coefficient and Refractive Index of GaN, AIN and AlGaN Alloys

The design of optoelectronic devices fabricated from III-nitride materials is aided by knowledge of the refractive index and absorption coefficient of these materials. The optical properties of GaN, AIN and A1GaN grown by MOVPE on sapphire substrates were investigated by means of transmittance and reflectance measurements. Thin (less than 0.5 Vm) single crystal films were employed to insure that transmission measurements could be obtained well above the optical band gap. The influence of alloy broadening on the absorption edge was investigated by using a series of AIGaN alloy samples with a range of Al compositions. The optical absorption coefficient above the band gap was obtained for AIGaN having up to 38% A] composition. The refractive index below the band gap was determined for the same series of samples. These properties provide information critical to the optimal design of solar blind detectors or other optoelectronic devices.

Quantum confinement of E1 and E2 transitions in Ge quantum dots embedded in an Al2O3 or an AlN matrix

Alternating layers of Ge quantum dots embedded in either Al2O3 or AlN matrices were deposited on sapphire substrates by pulsed-laser deposition. The characteristics of the dots are shown to be independent of the surrounding matrix. The dots size (73, 130, 160, and 260 A±5%) was controlled by the laser energy density and deposition time, and was characterized by high-resolution transmission electron microscopy. The dots were single crystalline with no apparent GeOx interfacial layers. Transmission spectroscopy at room temperature and 77 K was used to probe the above-band-edge absorption of the Ge nanodots. The spectral positions of both E1/E1+Δ1 and E2 transitions were found to shift to higher energy in the absorption spectra with decreasing nanodot sizes. This indicates that strong quantum-confinement effect permits the optical properties of Ge dots to be modified in a controlled manner.

Photoluminescence characterization of AlGaN-GaN pseudomorphic quantum wells and calculation of strain induced bandgap shifts

The low temperature (77 K) photoluminescence characteristics of AlxGa1-xN-GaN strained layer quantum wells with differentx values grown by metalorganic chemical vapor deposition (MOCVD) were investigated. The photoluminescence spectra were useful in analyzing both quantum confinement effects and strain induced energy shifts. The strain induced shifts were found to be a strong function of aluminum compositionx. A model was developed to calculate the strain induced bandgap shifts atk = 0. The values predicted by this model which took into account the wurtzite crystal structure of the material system, were in good agreement with (i.e. within 2 meV of) the experimentally measured shifts.

Strain‐induced phase separation in annealed low‐temperature grown Al0.3Ga0.7As

Data are presented to show that the hydrostatic strain in a metastable Al0.3Ga0.7As film grown at low temperatures induces a one‐dimensional compositional modulation upon annealing at 600 °C, a direct evidence of the existence of a miscibility gap in strained AlxGa1−xAs. A strain‐driven vacancy‐assisted mechanism is proposed to account for the compositional modulation and segregation of As clusters.