S. N. G. Chu

Ga2O3 films for electronic and optoelectronic applications

Properties of Ga2O3 thin films deposited by electron‐beam evaporation from a high‐purity single‐crystal Gd3Ga5O12 source are reported. As‐deposited Ga2O3 films are amorphous, stoichiometric, and homogeneous. Excellent uniformity in thickness and refractive index was obtained over a 2 in. wafer. The films maintain their integrity during annealing up to 800 and 1200 °C on GaAs and Si substrates, respectively. Optical properties including refractive index (n=1.84–1.88 at 980 nm wavelength) and band gap (4.4 eV) are close or identical, respectively, to Ga2O3 bulk properties. Reflectivities as low as 10−5 for Ga2O3/GaAs structures and a small absorption coefficient (≊100 cm−1 at 980 nm) were measured. Dielectric properties include a static dielectric constant between 9.9 and 10.2, which is identical to bulk Ga2O3, and electric breakdown fields up to 3.6 MV/cm. The Ga2O3/GaAs interface demonstrated a significantly higher photoluminescence intensity and thus a lower surface recombination velocity as compared to ...

Low‐temperature photoluminescence from InGaAs/InP quantum wires and boxes

InGaAs/InP quantum well layers grown by gas source molecular beam epitaxy have been used to fabricate quantum wires and boxes with transverse dimensions as small as ∼300 A. These artificial structures exhibit intense low‐temperature photoluminescence and show exciton shifts of 8–14 meV expected of low dimensional confinement. Low surface recombination velocity characteristic of InP and its alloys should allow luminescence studies of features as small as ∼30 A under moderate excitation intensities.

High quality GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia

We describe the growth of hexagonal GaN on Si(111) by gas source molecular beam epitaxy with ammonia. The initial deposition of Al, at 1130–1190 K, resulted in a very rapid transition to a two-dimensional growth mode of AlN. The rapid transition is essential for the subsequent growth of high quality GaN and AlGaN. This procedure also resulted in complete elimination of cracking in thick (>2 μm) GaN layers. For layers thicker than 1.5 μm, the full width at half maximum of the (0002) GaN diffraction peak was less than 14 arc sec. We show that a short period superlattice of AlGaN/GaN grown on the AlN buffer can be used to block defects propagating through GaN, resulting in good crystal and luminescence quality. At room temperature, the linewidth of the GaN exciton recombination peak was less than 40 meV, typical of the best samples grown on sapphire.

Low interface state density oxide‐GaAs structures fabricated by in situ molecular beam epitaxy

Several oxide‐GaAs heterostructures were fabricated using in situ multiple‐chamber molecular beam epitaxy. The oxides include SiO2, MgO, and Ga2O3(Gd2O3), all evaporated by an electron beam method. The SiO2 and Ga2O3(Gd2O3) films are amorphous while the MgO films are crystalline and part of the films are epitaxially grown on GaAs(100). Among these heterostructures, the Ga2O3(Gd2O3)–GaAs shows a photoluminescence intensity comparable to that of Al0.45Ga0.55As–GaAs, and forms accumulation and inversion layers as measured from capacitance voltage measurement in quasistatic and high frequency modes.

Misfit stress in InGaAs/InP heteroepitaxial structures grown by vapor‐phase epitaxy

Misfit stress in InGaAs/InP heteroepitaxial structures grown by hydride‐transport vapor phase epitaxy has been studied using an x‐ray technique to measure the wafer curvature and using an x‐ray double crystal diffractometer to measure mismatches. The stress calculated from the measured radius of curvature was, in all cases, found to be smaller than the value predicted from the lattice mismatch using the simple beam theory and assuming a coherent interface. A simple argument is used to show that the parallel mismatch is directly related to the density of misfit dislocations. By taking into account both the presence of misfit dislocations and tetragonal distortion, relationships between the curvature, density of interfacial misfit dislocations, relaxed lattice mismatch, and measured tetragonally distorted vertical and parallel mismatches are derived. Predicted values for the radius of curvature agree very well with measured values. The results also indicate that in the presence of interfacial misfit disloca...

Excellent uniformity and very low (<50 A/cm2) threshold current density strained InGaAs quantum well diode lasers on GaAs substrate

We report the growth and fabrication of InGaAs/GaAs strained quantum well (QW) lasers with a very low threshold current density, Jth, of <50 A cm−2 emitting at 0.98 μm. The lasers, 1350 μm long, had two InGaAs 80 A quantum wells in the active region and Al0.6Ga0.4As in cladding layers, and were grown on 3° off (100) towards 〈111〉A GaAs substrate. Misorienting the substrate towards 〈111〉A improves the material quality and device performance substantially for x≊0.6 in the cladding layer, but degrades it somewhat for x≊0.35. The Jth increased about 25% with decreasing x from 0.6 to 0.35 due to decreased optical confinement. Single QW stripe lasers with x=0.35 tested on a 3.0 cm×1016 μm size bar, representative of the whole 5‐cm‐diam substrate, exhibited a yield of ≳90% and an excellent spatial uniformity of Jth and emission wavelengths which were 212±4 A cm−2 and 989±1 nm, respectively.

Observation of large second order susceptibility via intersubband transitions at λ∼10 μm in asymmetric coupled AlInAs/GaInAs quantum wells

We report the first observation of second harmonic generation associated with intersubband transitions in AlInAs/GaInAs quantum wells. The measured second order susceptibility in our new asymmetric coupled well structure lattice matched to InP is 4.8×10−8 m/V, about 100 times larger than bulk values in InP, GaAs, and InAs.

Surface layer spinodal decomposition in In1−xGaxAsyP1−y and In1−xGaxAs grown by hydride transport vapor‐phase epitaxy

Composition modulation due to spinodal decomposition in In1−xGaxAsyP1−y quaternary and In1−xGaxAs ternary alloys is observed by transmission electron microscopy in epitaxial layers grown by hydride transport vapor phase epitaxy at 700 °C on (001) InP substrate. A quasi‐periodic fine contrast oriented along [100] and [010] directions is found in all samples over a composition range of 0.20≤x≤0.53 and 0.37≤y≤1. Contrary to the result reported on materials grown by liquid‐phase epitaxy, complete mixing at composition outside the miscibility gap predicted by thermodynamics for bulk crystals is not observed. The evidence indicates a rapid spinodal decomposition by surface diffusion during vapor phase deposition. Furthermore, transmission electron microscopy diffraction contrast experiments on the (110) cross‐sectional view of planar and nonplanar epitaxial layers reveal a columnar structure oriented along the growth direction which is consistent with the surface layer spinodal formation. Additional coarse cont...

Critical layer thickness in strained Ga1−xInxAs/InP quantum wells

We use a combination of electrical, optical, and structural characterization techniques to determine the critical layer thickness of strained Ga1−xInxAs/InP quantum wells. Well compositions covering the entire range of strain available, from −3.8% (GaAs) to +3.2% (InAs), were investigated. We find that the critical layer thickness in this material system is unambiguously described by the classical Matthews and Blakeslee force balance model [J. Cryst. Growth 27, 118 (1974)]. Reverse leakage current of strained‐well samples grown in a p‐i‐n configuration is shown to be the most direct and reliable measure of the pseudomorphic limit.

Quantum cascade laser: Temperature dependence of the performance characteristics and high T0 operation

The design and temperature dependence of the performance characteristics of a quantum cascade intersubband laser operating pulsed in the midinfrared (λ≂4.3 μm) are reported. The threshold current density varies exponentially with temperature [exp(T/T0)] from ≊6.0 kA/cm2 at 50 K to ≊9.3 kA/cm2 up to the maximum operating temperature (125 K) with a T0∼112 K. This weak temperature dependence, compared to interband lasers operating at similar wavelengths, is due to the intersubband nature of the laser transition, to the physics of optical phonons scattering, and to the negligible intersubband Auger transition rates. The measured peak optical power varies from 32 mW at 10 K to 18 mW at 80 K for a 1.2‐mm cavity length. The measured slope efficiency is 52 mW/A at 80 K which corresponds to an estimated differential quantum efficiency of ≂3.4×10−2 per facet per stage.