M. Kuball

Recombination dynamics in InGaN quantum wells

Transient photoluminescence measurements are reported on a thin InGaN single quantum well, encompassing the high injection regime. The radiative processes that dominate the recombination dynamics, especially at low temperatures, show the impact of localized electronic states that are distributed over a large energy range (∼100 meV). We suggest that these states originate from microstructural disorder in the InGaN/GaN system.

Time‐resolved pump‐probe experiments with subwavelength lateral resolution

We demonstrate picosecond time‐resolved pump‐probe experiments with subwavelength resolution using a reflection near‐field scanning optical microscope (NSOM). The sample was a laterally patterned gold nanostructure. Strain‐induced reflectivity changes as low as ΔR/R≊10−4 were detected in the near‐field.

GAIN SPECTROSCOPY ON INGAN/GAN QUANTUM WELL DIODES

We have investigated spectroscopically the emergence of gain in InGaN/GaN quantum well diodes under high current injection (>kA/cm2). The spectral characteristics suggest that the electronic states responsible for blue laser action in this material are strongly influenced by the presence of microscopic crystalline disorder.

Near-field optical study of InGaN/GaN epitaxial layers and quantum wells

We have employed near-field scanning optical microscopy to investigate the influence of specific microstructural defects on the optical properties of thin InGaN/GaN epilayers and quantum wells. These defects are empty “pinholes” with a hexahedron cone morphology that are nucleated by threading dislocations from the GaN buffer layer. By correlating atomic force microscopy with spatially and spectrally resolved photoluminescence (PL) on a 100 nm spatial scale, we find that the pinholes have no clearly observable effect on the PL efficiency, at least partly due to the strong carrier localization in the InGaN nonrandom alloy.

Terahertz oscillations in an In0.53Ga0.47As submicron planar Gunn diode

The length of the transit region of a Gunn diode determines the natural frequency at which it operates in fundamental mode—the shorter the device, the higher the frequency of operation. The long-held view on Gunn diode design is that for a functioning device the minimum length of the transit region is about 1.5 μm, limiting the devices to fundamental mode operation at frequencies of roughly 60 GHz. Study of these devices by more advanced Monte Carlo techniques that simulate the ballistic transport and electron-phonon interactions that govern device behaviour, offers a new lower bound of 0.5u2009μm, which is already being approached by the experimental evidence that has shown planar and vertical devices exhibiting Gunn operation at 600u2009nm and 700u2009nm, respectively. The paper presents results of the first ever THz submicron planar Gunn diode fabricated in In0.53Ga0.47As on an InP substrate, operating at a fundamental frequency above 300u2009GHz. Experimentally measured rf power of 28u2009μW was obtained from a 600u2009nm lo...

Reliability of AlGaN/GaN high electron mobility transistors on low dislocation density bulk GaN substrate: Implications of surface step edges

To enable gaining insight into degradation mechanisms of AlGaN/GaN high electron mobility transistors, devices grown on a low-dislocation-density bulk-GaN substrate were studied. Gate leakage current and electroluminescence (EL) monitoring revealed a progressive appearance of EL spots during off-state stress which signify the generation of gate current leakage paths. Atomic force microscopy evidenced the formation of semiconductor surface pits at the failure location, which corresponds to the interaction region of the gate contact edge and the edges of surface steps.

Nitride lasers on SiC substrates

Laser diode (LD) structures were fabricated by metal-organic chemical vapor deposition (MOCVD) from the AlN-InN-GaN system on single crystal 6H-SiC substrates. A conducting buffer layer was developed for these devices which uses an AlGaN buffer layer and provides a conduction path between SiC substrate and the active device region. Violet and blue multiple quantum well (MQW) separate confinement heterojunction (SCH) LDs were fabricated having InGaN wells and GaN barriers. The lowest pulsed operation threshold current density obtained for lasing was 7.1 kA/cm/sup 2/ in a 4-well structure.

Improvements in thermionic cooling through engineering of the heterostructure interface using Monte Carlo simulations

A self-consistent Ensemble Monte Carlo (EMC) model was developed to simulate the thermionic effect in heterostructure barrier coolers. The model was validated on an InGaAs-InGaAsP heterostructure device of variable barrier height and width, producing good quantitative agreement with previous literature results. The operation of the cooler was found to be a complex and intricate process depending on the field, conduction band and details of barrier structure. When applied to a GaAs-AlGaAs micro-cooler there was good agreement with the experimental results. Importantly, very small alterations in the barrier structure were found to lead to considerable changes in device performance.

InGaN/GaN lasers grown on SiC

Single crystal thin films with compositions from the AlN-InN- GaN system were grown via metal-organic chemical vapor deposition (MOCVD) on single crystal 6H-SiC substrates. Blue light emitting (LED) and laser diode (LD) structures were fabricated. A conducting buffer layer was developed which uses an AlGaN buffer layer which provides a conduction path between SiC and the active device region. This conducting buffer layer was utilized in both the LEDs and the LDs. The external quantum efficiency of the LEDs was 3% at 20 mA (3.6V) with a peak emission wavelength of 430 nm. Violet and blue LDs were fabricated which consisted of an 8-well InGaN/GaN multiple quantum well (MQW) active region in a separate confinement heterostructure (SCH) design. The devices lased at room temperature under pulsed and continuous wave operation with an emission wavelength of 404-435 nm. The lowest pulsed operation threshold current density obtained for lasing under was 10.4 kA/cm2.

Micro-cooler enhancements by barrier interface analysis

A novel gallium arsenide (GaAs) based micro-cooler design, previously analysed both experimentally and by an analytical Heat Transfer (HT) model, has been simulated using a self-consistent Ensemble Monte Carlo (EMC) model for a more in depth analysis of the thermionic cooling in the device. The best fit to the experimental data was found and was used in conjunction with the HT model to estimate the cooler-contact resistance. The cooling results from EMC indicated that the cooling power of the device is highly dependent on the charge distribution across the leading interface. Alteration of this charge distribution via interface extensions on the nanometre scale has shown to produce significant changes in cooler performance.