J. E. Van Nostrand

Observation of 430 nm Electroluminescence from ZnO/GaN Heterojunction Light-Emitting Diodes

In this work, we report on the growth, fabrication, and device characterization of wide-band-gap heterojunction light-emitting diodes based on the n-ZnO/p-GaN material system. The layer structure is achieved by first growing a Mg-doped GaN film of thickness 1 μm on Al2O3(0001) by molecular-beam epitaxy, then by growing Ga-doped ZnO film of thickness 1 μm by chemical vapor deposition on the p-GaN layer. Room-temperature electroluminescence in the blue-violet region with peak wavelength 430 nm is observed from this structure under forward bias. Light–current characteristics of these light-emitting diodes are reported, and a superlinear behavior in the low current range with a slope 1.9 and a sublinear behavior with a slope 0.85 in the high current range are observed.

Arsenic for antimony exchange on GaSb, its impacts on surface morphology, and interface structure

We quantify the rates and total amounts of the arsenic for antimony exchange on both the Sb-terminated and Ga (or In)-terminated GaSb (001) surfaces using in situ real time line-of-sight mass spectrometry (LOS-MS) during molecular beam epitaxy. On the Sb-terminated GaSb (001) surface, an As for Sb exchange is observed to occur at all values of incident As2 flux considered. At high substrate temperature, three-dimensional (3D) nanometer-sized clusters from as a consequence of As/Sb exchange and lattice mismatch strain between GaAs and GaSb. The 3D clusters are found to have lateral dimensions of ∼10–30 nm and heights of 1–3 nm by atomic force microscopy (AFM). By contrast, at lower substrate temperatures a two-dimensional surface morphology is maintained, and AFM reveals an array of atomically flat terraces. On the surface terminated by one monolayer (ML) of Ga or In, there exists a critical As2 flux below which the As/Sb exchange is greatly diminished. The net amounts of Sb leaving the surface during one ...

Deformation potentials of the E2(high) phonon mode of AlN

AlN layers grown on (111)-oriented silicon substrates were studied by Raman spectroscopy. The deformation potentials of the nonpolar E2(high) phonon mode of hexagonal AlN were derived from phonon frequency shifts under biaxial stress applied to the layer. Stress was applied by mechanical bending of the wafer with resulting in-plane biaxial stress in AlN. The technique allows one to avoid the uncertainty of x-ray diffraction strain determination inherent to experimental methods commonly used for deformation potentials determination in III–V nitrides. The obtained values for the phonon deformation potentials are in reasonably good agreement with previous theoretical calculations. For pure biaxial stress, we determine a phonon frequency shift of 3 cm−1/GPa.

Growth of vertically self-organized InGaAs quantum dots with narrow inhomogeneous broadening

We have fabricated vertically self-organized multiple sets of In0.6Ga0.4As quantum dots (QDs) on GaAs (001) that combines the concepts of variable amount deposition and shape stabilization and size equalization of QDs. The inhomogeneous broadening of optical emission from these dots reached a record low value of 18.4 meV at a wavelength of ∼1185 nm (4 K). The seed layer and the second dot layer have essentially the same dot density of ∼250 μm−2 due to the high degree of dot vertical alignment. The deposition amount for the second dot layer was selected to be 9 monolayers, which resulted in dots with convergent lateral size (∼62 nm) and stabilized facets, close to {011}. The third layer, with the same amount of InGaAs as the second layer, had a dot density of ∼350 μm−2, an average lateral dot size of ∼71 nm, an average dot height of ∼11 nm, and shallower side facets close to {023}.

The effect of Al on Ga desorption during gas source-molecular beam epitaxial growth of AlGaN

In this letter, we report on the impact aluminum has on gallium desorption kinetics in AlGaN alloys grown by gas source-molecular beam epitaxy. Aluminum is found to preferentially incorporate into the AlGaN films over the range of fluxes and temperatures investigated [0.05⩽Ji(Ga)⩽0.5 ML/s; 0.1⩽Ji(Al)⩽0.2 ML/s; 700 °C⩽Ts⩽775 °C]. As a result, Ga is not observed to incorporate into the film until the NH3 flux exceeds that required to grow stoichiometric AlN. This preferential incorporation stems from two facts: (a) Al has an ammonia cracking efficiency ∼2.5 times greater than that of Ga, and (b) Al participates in a Al-for-Ga exchange. As a result of these factors and under NH3 limited growth conditions, the aluminum mole fraction in a layer can be controlled by changing the incident NH3 flux.

Electrical and optical investigation of MBE grown Si-doped AlxGa1−xN as a function of Al mole fraction up to 0.5

Si-doped Al x Ga 1-x N grown by gas source MBE was investigated as a function of Al mole fraction up to 0.5 using temperature dependent Hall effect (TDH) and cathodoluminescence measurements. The band gap energies for the Al x Ga 1-x N layers estimated from the bound exciton peaks agree well with the linearly extrapolated band gaps only for x ≤ 0.3. TDH measurements reveal the presence of a highly degenerate n-type region at the Al x Ga 1-x N/sapphire interface. The donor concentrations of Al x Ga 1 x N layers are estimated to be 4.5, 5.0, 15, 19, and 8 x 10 18 cm -3 for x = 0.1, 0.2, 0.3, 0.4, and 0.5, respectively, compared with the nominal doping value of 10 18 Si cm - 3. The activation energies of the Si donors are 6, 11, 40, 60, and 68 meV for x = 0.1 0.2, 0.3, 0.4, and 0.5, respectively. Both the electrical and optical measurements indicate that good quality Al x Ga 1-x N films can be grown for x up to 0.3 by gas source MBE using a sapphire substrate and an AlN buffer layer, but lesser quality films are obtained for x > 0.3.

Phonon deformation potentials of the E2(high) phonon mode of AlxGa1−xN

Micro-Raman spectroscopy was applied to study the E2(high) phonon deformation potentials in AlxGa1−xN material, which are required to convert phonon frequency shifts into stress values. AlxGa1−xN layers were grown by molecular beam epitaxy directly on (111)-oriented Si substrates. Mechanical bending was applied to introduce biaxial stress in the AlxGa1−xN layers and Raman shifts were measured as a function of the applied deformation. The Si phonon mode provided a reference for the applied stress and allowed determination of phonon deformation potential values for AlxGa1−xN. Deformation potentials of the GaN- and AlN-like E2(high) phonon mode of AlGaN alloys were found to be similar to the corresponding values of binary compounds.

Cooperative nucleation and evolution in InGaAs quantum dots in multiply stacked structures

We report a type of vertical self-organization of In0.6Ga0.4As quantum dots (QDs) wherein the dot density for the top layer (NT) doubles that of the seed layer (NS). The experimental observation fits well with a scheme of QD vertical self-organization accompanied by additional cooperative dot nucleation at the face centers of an approximate two-dimensional array defined by the embedded seed dots. The dots in the top layer undergo a shape transition from {023} to {011} and a simultaneous shape stabilization and size equalization.

Cathodoluminescence, microstructure, and morphology of tensile-strained AlxGa(1−x)N epitaxial films grown by gas source molecular beam epitaxy

Tensile-strained AlxGa(1−x)N (0⩽x⩽0.4) alloys are deposited on 2-μm-thick GaN on Al2O3(0001) by gas source molecular beam epitaxy using ammonia as the nitrogen source. The evolution of the surface morphology of AlxGa(1−x)N epitaxial films as a function of mole fraction is studied using atomic force microscopy. Surface roughness initially increases with Al mole fraction, but then decreases with a corresponding onset of defects. Extended ridge-like defects are observed along the 〈0010〉 of the AlxGa(1−x)N for high Al mole fractions. Band-edge and subband-edge emission at 6 K is investigated using cathodoluminescence. Strong emission, ascribed to donor-bound excitons, shallow donor to shallow-acceptor pair emission, and deep emission associated with the “yellow” band of GaN are observed. The energy level of emission from donor-bound excitons is found to exhibit a linear dependence on Al more fraction, suggesting a lack of band bending in this material system. Finally, microstructure is investigated using a tr...

Optical investigation of MBE grown Si-doped AlxGa1-xN as a function of nominal Al mole fraction up to 0.5

Al x Ga 1 - x N epitaxial films were grown by gas source molecular beam epitaxy and investigated as a function of Al mole fraction. Cathodoluminescence, photoluminescence, and optical absorption measurements were used to characterize 1 μm thick layers of Al x Ga 1 - x N with nominal x values of 0.1, 0.2, 0.3, 0.4, and 0.5 as well as GaN and the AIN buffer layer. The GaN and Al x Ga 1-x N layers were doped with 1 x 10 18 cm of Si from a Knudsen source. Typical spectra contain a donor bound exciton peak with its phonon replicas and donor acceptor pair (DAP) peaks. From the observed bound exciton peak positions and the absorption data, the band gap energies for the Al x Ga 1 - x N were estimated, and these values were compared with the linearly extrapolated band gaps. This study indicates that the MBE growth of 1 μm thick Al x Ga 1-x N layers provides good quality films for x values up to 0.3, and lesser quality films for x values above this mole fraction, requiring further improvement and/or modifications to the MBE growth technique to obtain quality Al x Ga 1 - x N alloy material for semiconductor devices.