Veit Hoffmann

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0  0) and on semipolar (1 0  2), (1 1  2), (1 0  1) as well as (2 0  1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0  1) < (1 1  2) = (0 0 0 1) < (2 0  1) < (1 0  0) = (1 0  2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0  0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0  0) and the semipolar (1 0  2) and (2 0  1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1  2) and (1 0  1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

Temperature and excitation power dependent photoluminescence intensity of GaInN quantum wells with varying charge carrier wave function overlap

For the realization and the improvement of GaN-based optoelectronic devices (light emitting diodes and laser diodes) emitting from the ultraviolet to the red wavelength range GaInN quantum well structures with high internal quantum efficiency are of great importance. To determine parameters which affect the internal quantum efficiency, we have analyzed the emission intensity of GaInN quantum well structures with varied electron and hole wave function overlap by temperature and excitation power dependent and by time-resolved photoluminescence. The quantum confined Stark effect reduces the temperature dependent photoluminescence emission intensity for thick polar quantum wells at low temperature. But near room temperature, these thick polar GaInN quantum wells feature less relative intensity loss than thinner polar quantum wells. This behavior can partially be assigned to increased screening effects and higher quantum well barriers for thicker quantum wells. Additionally, excitation power dependent photolum...

InGaN–GaN Disk Laser for Blue-Violet Emission Wavelengths

An optically pumped InGaN-GaN surface-emitting laser with external cavity emitting at 393 nm has been realized. The peak output power is 300 W, the slope efficiency 3.5%, and the threshold pump power density 700 kW/cm2. Critical parameters in the design of the laser will be discussed and ways to improve the performance will be suggested.

Laser Scribing for Facet Fabrication of InGaN MQW Diode Lasers on Sapphire Substrates

In this letter, a novel method for the fabrication of high-quality facets for III-nitride lasers grown on c -plane sapphire substrates as well as on GaN substrates is presented. Based on a laser scribing process gain-guided laser diodes with smooth facets were fabricated, which showed threshold current densities of 6.5 kA/cm2 at an emission wavelength of 405 nm under pulsed operation.

Influence of the barrier composition on the light output of InGaN multiple-quantum-well ultraviolet light emitting diodes

MOVPE grown InGaN multiple-quantum-well (MQW) light emitting diodes (LEDs) on c-plane (0001) sapphire emitting at 375 nm with GaN, Al0.16Ga0.84N and InxAl0.16GaN-barrier layers were investigated in order to study the influence of the barrier composition on the light output characteristics of near UV devices. By substituting the GaN barrier layers with Al0.16Ga0.84N the output power increased 30-fold due to the increased band-offset between the In0.03Ga0.97N QWs and the barriers. The addition of 3.3% indium to the AlGaN barriers resulted in a reduction of the FWHM, and a 50-fold increase in light output power compared to LEDs with GaN barriers. Even though the band-offset and hence the carrier confinement for the InAlGaN barriers is smaller than in the case of AlGaN barriers, strain compensated In0.03Al0.16Ga0.79N barrier layers seem to be greatly beneficial for the external quantum efficiency of the near UV LEDs. The effect of an n-type Al0.23Ga0.77N hole-blocking-layer, which was inserted below the MQW stack to prevent hole carrier leakage from UV LED active region, on the light output was also investigated. By incorporating strain compensated In0.03Al0.16Ga0.79N barriers and an Al0.23Ga0.77N hole blocking layer we were able to realize 375 nm LEDs with an output of 1 mW (measured on-wafer) at 100 mA. Finally, the wavelength dependence of the light output from UV LEDs with InGaN MQWs emitting between 375 nm and 381 nm with peak output power of 4 mW at 200 mA for the longer wavelength devices is shown.

Optically Pumped DFB Lasers Based on GaN Using 10th-Order Laterally Coupled Surface Gratings

An optically pumped GaN-based laser structure with 10th-order laterally coupled surface grating is demonstrated. The fabrication involved i-line photolithography and dry etching, avoiding more complex technologies such as multiple epitaxy steps. The lasing threshold of the laterally coupled distributed-feedback (LC-DFB) laser was similar to that of a ridge waveguide Fabry-Perot (RW-FP) laser. Single-peak emission with a full width at half maximum of 0.06 nm at 404.2 nm was achieved for LC-DFB lasers. In contrast to the RW-FP lasers, the LC-DFB laser is shown to exhibit a smaller shift of the emission wavelength with temperature.

Nano-optical analysis of GaN-based diode lasers

Near-field scanning optical microscopy (NSOM) is applied for analyzing GaN-based diode lasers. The measurements are carried out at the front facets of standard devices without any additional preparation. Four different schemes for luminescence and photocurrent detection are applied. The results allow for a detailed analysis of the epitaxial layer sequence, the waveguide mode, the impact of defect absorption, and efficiencies of carrier transfer into the quantum well. Moreover, the effective potential profile as formed by both layer structure and doping profile is imaged. Features being spatially separated by only 30 nm are safely resolved. Our results pave the way towards non-destructive nanoscopic analysis of wide-bandgap optoelectronic devices.

Spacer and well pumping of InGaN vertical cavity semiconductor lasers with varying number of quantum wells

We have investigated the dependence of the threshold pump power and slope efficiency of 415 nm (In)GaN vertical cavity surface emitting lasers on the wavelength of the pump source and the number of quantum wells. InGaN double quantum well resonant-periodic-gain structures with 6, 8, and 10 periods have been compared. By barrier and well pumping of the samples with a 375 nm dye laser, a nearly 10 times reduction of the laser threshold was observed compared to pumping with a 337 nm nitrogen laser source. The laser threshold was found to be independent of the number of quantum wells. The slope efficiency seems to be not affected by the pump wavelength and resonant-periodic-gain periods. The results are discussed with a rate equation model that takes into account the inhomogeneous pumping of the quantum wells and optical thickness variations in the resonant-periodic-gain structure.

Determination of polarization fields in group III-nitride heterostructures by capacitance-voltage-measurements

The polarization fields in wurtzite group III-nitrides strongly influence the optical properties of InAlGaN-based light emitters, e.g., the electron and hole wave function overlap in quantum wells. In this paper, we propose a new approach to determine these fields by capacitance-voltage measurements (CVM). Sheet charges generated by a change of the microscopic polarization at heterointerfaces influence the charge distribution in PIN junctions and therefore the depletion width and the capacitance. We show that it is possible to determine the strength and direction of the internal fields by comparing the depletion widths of two PIN junctions, one influenced by internal polarization fields and one without as a reference. For comparison, we conducted coupled Poisson/carrier transport simulations on the CVM of the polarization-influenced sample. We also demonstrate the feasibility and limits of the method by determining the fields in GaN/InGaN and GaN/AlGaN double heterostructures on (0001) c-plane grown by me...

Spatial inhomogeneities in Al x Ga1−x N quantum wells induced by the surface morphology of AlN/sapphire templates

The effects of the template on the optical and structural properties of Al0.75Ga0.25N/Al0.8Ga0.2N multiple quantum well (MQWs) laser active regions have been investigated. The laser structures for optical pumping were grown on planar c-plane AlN/sapphire as well as on thick epitaxially laterally overgrown (ELO) AlN layers on patterned AlN/sapphire. Two ELO AlN/sapphire templates were investigated, one with a miscut of the sapphire surface to the m-direction with an angle of 0.25°, the other with a miscut angle of 0.25° to the sapphire a-direction. The MQWs are studied by atomic force microscopy, plan-view cathodoluminescence (CL) at room temperature and 83 K as well as transmission electron microscopy using high-angle annular dark-field imaging and energy-dispersive x-ray spectroscopy. The results are compared to optical pumping measurements. It was found that the surface morphology of the templates determines the lateral wavelength distribution in the MQWs observed by spectral CL mappings. The lateral wavelength spread is largest for the laser structures grown on ELO AlN with miscut to sapphire a-direction caused by the local variation of the MQW thicknesses and the Ga incorporation at macrosteps on the ELO-AlN. A CL peak wavelength spread of up to 7 nm has been found. The MQWs grown on planar AlN/sapphire templates show a homogeneous wavelength distribution. However, due to the high threading dislocation density and the resulting strong nonradiative recombination, laser operation could not be achieved. The laser structures grown on ELO AlN/sapphire show optically pumped lasing with a record short wavelength of 237 nm.