A. Chandolu

Controlled growth of GaN nanowires by pulsed metalorganic chemical vapor deposition

Controlled and reproducible growth of GaN nanowires is demonstrated by pulsed low-pressure metalorganic chemical vapor deposition. Using self-assembled Ni nanodots as nucleation sites on (0001) sapphire substrates we obtain nanowires of wurtzite-phase GaN with hexagonal cross sections, diameters of about 100nm, and well-controlled length. The nanowires are highly oriented and perpendicular to the growth surface. The wires have excellent structural and optical properties, as determined by x-ray diffraction, cathodoluminescence, and Raman scattering. The x-ray measurements show that the nanowires are under a complex strain state consistent with a superposition of hydrostatic and biaxial components.

HfO2 gate dielectric with 0.5 nm equivalent oxide thickness

Hafnium dioxide films have been deposited using reactive electron beam evaporation in oxygen on hydrogenated Si(100) surfaces. The capacitance–voltage curves of as-deposited metal(Ti)–insulator–semiconductor structures exhibited large hysteresis and frequency dispersion. With post-deposition annealing in hydrogen at 300 °C, the frequency dispersion decreased to less than 1%/decade, while the hysteresis was reduced to 20 mV at flatband. An equivalent oxide thickness of 0.5 nm was achieved for HfO2 thickness of 3.0 nm. We attribute this result to a combination of pristine hydrogen saturated silicon surfaces, room temperature dielectric deposition, and low temperature hydrogen annealing.

Green light emission from InGaN multiple quantum wells grown on GaN pyramidal stripes using selective area epitaxy

Selective area epitaxy has been used to grow pyramidal GaN stripes, followed by InGaN multiple quantum well (MQW) structures, in order to produce long-wavelength green light emission. Stripes oriented along ⟨112¯0⟩ produce smooth {11¯01} sidewall facets. The room-temperature optical properties are investigated by cathodoluminescence spectroscopy using a scanning electron microscope. MQWs grown in unmasked reference regions exhibit emission at 450 nm. The stripe sidewalls emit light with peak wavelength of 500 nm with consistent linewidth and intensity. The stripe ridge emits light with peak intensity at wavelength of ∼550 nm. Based on the spatial extent of the 550 nm emission, the ridge is estimated to be ∼250 nm wide. The large redshift is produced by the enhanced presence of indium species due to lateral vapor diffusion and surface migration in selective area epitaxy.

Solar-blind ultraviolet photodetectors based on superlattices of AlN/AlGa(In)N

We describe solar-blind photodetectors based on superlattices of AlN/AlGa(In)N. The superlattices have a period of 1.4 nm, determined by x-ray diffraction, and an effective band gap of 260 nm measured by optical reflectivity. Using simple mesa diodes, without surface passivation, we obtain low dark leakage currents of 0.2–0.3 pA, corresponding to the leakage current density of ∼0.3 nA/cm2, and high zero-bias resistance of ∼1×1011 Ω. Excellent visible cutoff is obtained for these devices, with six orders of magnitude decrease in responsivity from 260 to 380 nm. These results demonstrate the potential of junctions formed by short-period superlattices in large-band-gap devices.

Effect of stress and free-carrier concentration on photoluminescence in InN

We report photoluminescence (PL) studies of InN epilayers grown by plasma-assisted molecular beam epitaxy with free-electron concentration ranging from 5.9×1017to4.2×1018cm−3. X-ray diffraction measurements are used to determine strains, which are best described as a combination of hydrostatic and biaxial. The PL energy is affected by both strains along with free-carrier concentration through band filling. PL spectra are used to estimate the dependence of the Fermi level on free-carrier concentration, taking strain into account. The fundamental energy gap is found to be ∼0.70eV. PL broadening is well described based on band filling.

Deep Ultraviolet Light Emitting Diodes Based on Short Period Superlattices of AlN/AlGa(In)N

We report a systematic study of the optical properties of superlattices of AlN/Al0.08Ga0.92(In)N with periods in the range of 1.25–2.25 nm. The superlattices were grown on sapphire substrates using gas source molecular beam epitaxy with ammonia. Effective bandgaps between 4.5 eV (276 nm) and 5.3 eV (234 nm), as determined by optical reflectivity measurements, were obtained by adjusting the barrier and well thickness. These superlattices can be doped n- and p-type. We demonstrate double heterostructure light emitting diodes operating at wavelengths as short as 262±2 nm.

Short-period superlattices of AlN/Al0.08Ga0.92N grown on AlN substrates

High-quality short-period superlattices of AlN∕Al0.08Ga0.92N have been grown by gas-source molecular-beam epitaxy with ammonia on Al face of AlN (0001) substrates. A significant reduction was achieved in the dislocation density, down to 3×108cm−2. Complete removal of residual Al2O3 surface oxide is needed in order to obtain low dislocation density in homoepitaxy on AlN. We show that the presence of Al2O3 islands with the surface coverage as low as 0.2% results in increased dislocation density.

Optical phonon decay in bulk aluminum nitride

We report Raman studies of the E22 and A1(LO) symmetry phonons of bulk AlN from 13to375K. Based on observed dependences of the phonon energies and linewidths, and accounting for the temperature-dependent thermal expansion, we determine that both phonon lifetimes are limited by two-phonon decay. The E22 decays symmetrically. The A1(LO) asymmetrically decays into vibrations with energies 586 and 309cm−1 which we attribute to A1(TO) and LA phonons from the M point of the Brillouin zone.

Electrical properties of p–n junctions based on superlattices of AlN/AlGa(In)N

Measurements of acceptor activation energy in p–n junctions based on superlattices of AlN (1.25 nm thick) and Al0.08Ga0.92(In)N (0.5 nm thick), with the average AlN content greater than 0.6, are reported. Structural characteristics of superlattices were determined using transmission electron microscopy and x-ray diffraction. p–n junctions in mesa-etched diodes exhibit low leakage current densities of 3×10−10 A/cm2 at near zero bias. Acceptor activation energy of 207±10 meV, obtained from the temperature dependence of the forward current, is very similar to that of uniform alloy of Al0.08Ga0.92N that constitutes the well material. The acceptor activation energy thus appears controlled by the well material and remains low despite high average AlN content and large band gap.

Effect of impurity incorporation on emission wavelength in cathodoluminescence spectrum image study of GaN pyramids grown by selective area epitaxy

Metal-organic chemical vapor deposition has been used for selective epitaxy of GaN pyramids ranging in size from over 1μm to 500 nm in length at the base. Pyramids are terminated by {11¯01} crystal facets. The optical properties of the pyramids are investigated by cathodoluminescence (CL) in a scanning electron microscope. CL spectrum imaging reveals the pyramid apices to emit light at ∼363.4 nm corresponding to the emission wavelength of relaxed GaN. As the CL excitation is moved away from the apex a systematic redshift is observed. The redshift is ∼4 nm for pyramids with 3 μm base dimension and ∼2 nm for the 500 nm pyramids. The shift is attributed to diffusion of silicon and oxygen into the GaN pyramids due to SiO2 mask decomposition with negligible contribution from stress. The observations are backed by finite element simulations of diffusion and stress.