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We investigate the impact of Mg-doping on the performance and degradation kinetics of AlGaN-based UV-C light-emitting diodes (LEDs). By comparing LEDs from three wafers with different nominal doping levels [Mg/(Al+Ga) ratio: 0.15%, 0.5%, and 1% in the gas phase during epitaxy] in the AlGaN:Mg electron-blocking layer (EBL), we demonstrate the following results: (i) A higher Mg-doping in the EBL results in a higher optical power at low current levels, which is ascribed to an increased hole injection efficiency. (ii) The reduction of the optical power follows a non-exponential trend, which can be reproduced by using the Hill's formula and is ascribed to the generation/activation of defects within the quantum wells. (iii) A higher Mg-doping in the EBL mitigates the degradation rate. An interpretation of the experimental data is proposed, assuming that hydrogen, which is present in and moving from the EBL, can reduce the rate of de-hydrogenation of point defects in the active region, which is responsible for degradation. [ABSTRACT FROM AUTHOR]

The threshold current density of narrow (1.5 μm) ridge waveguide (In,Al)GaN based laser diodes is found to strongly depend on the ridge etch depth. By solving the complex-value two-dimensional waveguide equation, it is shown that, for shallow-ridge devices with a small built-in index step, the dependence of the modal gain on the material gain is strongly influenced by antiguiding effects. Taking into account the lateral extension of the material gain beyond the ridge as well as the optical mode absorption in the unpumped regions of the quantum wells, the observed differences in the threshold current density can be reproduced by simulations. [ABSTRACT FROM AUTHOR]

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 photoluminescence points to a transition from a radiative recombination based on excitons at 10 K to a bimolecular recombination at room temperature for thick c-plane quantum wells. This transition may also affect the intensity decrease by a changed carrier diffusivity. [ABSTRACT FROM AUTHOR]

Laser gain is investigated for AlGaN wurtzite quantum-well structures emitting in the wavelength range from ∼270 to 340 nm. The calculations show that gain properties vary notably with aluminum concentration in the quantum well. The TE gain dominates over the entire spectral range, although an enhancement of TM gain is observed for AlGaN quantum wells with the high aluminum mole fraction. The calculations also predict an increase in threshold current density for the shorter-wavelength lasers. [ABSTRACT FROM AUTHOR]

Abstractauthoren Optical polarization is a fundamental property of light emission from m-plane InGaN quantum wells. It is a result of band structure, anisotropic strain, the degree of polarization depending on thermal occupation, and band-filling of valence subbands. We analyze the optical-polarized light of nonpolar samples at low and at room temperature over a wide range of excitation density. We observe that the measured energy separation and linewidth of the orthogonally polarized emission depend on carrier density, too. A consistent explanation of these effects is possible in the context of Fermi-Dirac statistics for a degenerate carrier density [ABSTRACT FROM AUTHOR]

The effects of the aluminum content x and the magnesium doping concentration in the Al [ABSTRACT FROM AUTHOR]

The deterioration of the InGaN active region of laser diode structures emitting around 440 nm is observed in-situ during epitaxial growth and analyzed ex-situ by cathodoluminescence spectroscopy and transmission electron microscopy. The growth conditions of the p-layers on top of the InGaN active region are found to affect the homogeneity of the InGaN material properties which can be monitored by the in-situ reflectance signal at 950 nm. The deposition of the p-layers at 950 °C results in the formation of metallic indium platelets as well as voids changing the refractive index of the active region and thus the reflectance. A reduction of the p-layer deposition temperature by 30 °C prevents this undesirable decomposition. [ABSTRACT FROM AUTHOR]

The impact of dislocations on surface topology as well as on quantum well emission in c-plane, semipolar, and nonpolar InGaN/GaN heterostructures is being analyzed by micro-photoluminescence and white-light-interferometry. V-pits with (10 [ABSTRACT FROM AUTHOR]

The influence of the dislocation density and the carrier-confining potentials on the temperature dependent behavior of the external quantum efficiency (EQE) of near-ultraviolet light-emitting diodes (LEDs) at different current densities has been investigated. The LED efficiencies were found to be very temperature sensitive with characteristic temperatures ranging from 48 to 207 K. In the low current density region the temperature dependence of the EQE is mainly dominated by the Shockley–Read–Hall recombination and hence the dislocation density of the device. In the high current density region, carrier leakage from the quantum wells is the main factor influencing the decrease in the EQE with ambient temperature, and in this region the LEDs become less temperature sensitive with increasing quantum well barrier height. [ABSTRACT FROM AUTHOR]

For AlGaN-based multi-quantum-well light emitters grown on c-plane substrates there is a tendency for the polarization of the emitted light to switch from transverse electric (TE) polarization to transverse magnetic (TM) polarization as the wavelength decreases. This transition depends on various factors that include the strain in the quantum well. Experimental results are presented that illustrate the phenomenon for nitride light emitting diodes (LEDs) grown on sapphire and on bulk AlN. Model calculations are presented which quantify the dependence of the TE/TM switch on the quantum well strain and the Al composition in the barriers surrounding the well. [ABSTRACT FROM AUTHOR]

The optical and structural properties of In0.08Ga0.92N/In0.02Ga0.98N multiple quantum wells (MQWs) grown at different temperatures and with different supplies of indium were analyzed by atomic force microscopy and spectrally resolved cathodoluminescence (CL). By comparing the contrasts of monochromatic CL images with high-resolution secondary-electron images of the sample surface, it is shown that almost all contrasts of the CL images can be explained by lateral inhomogeneities of both the thickness and the InN mole fraction of the InGaN layers. Dark contrasts in the CL images solely related to dislocations were not observed, indicating very weak nonradiative recombination correlated with threading dislocations in the InGaN quantum wells. The lateral inhomogeneities of layer thickness and indium incorporation depend strongly on the growth conditions. [ABSTRACT FROM AUTHOR]

Polarized photoluminescence of strained quantum wells grown on c-plane, semipolar (1012), (1122), (1011), (2021) planes, and nonpolar GaN substrates was studied experimentally and in theory. The observed optical polarization switching between the substrate orientations (1012) and (1122) is in accordance with our general model of polarization switching, based on a kvector ·pvector model of arbitrary substrate orientation. Spectrally resolved measurements of the polarization degree stemming from (1012) samples show that the maximum of the polarization degree is red-shifted with respect to the maximum of the photoluminescence intensity. We ascribe this effect to an increased polarization of the transitions for higher indium content. [ABSTRACT FROM AUTHOR]

The electroluminescence from near ultraviolet (UV) light emitting diodes containing InGaN multiple quantum wells (MQWs) with GaN, AlGaN, and InAlGaN barriers was investigated. Based on band-structure calculations the observed wavelength shift in the peak emission with increasing injection current is attributed to the screening of the polarization fields and to band gap renormalization. InGaN MQWs with almost zero net polarization have been realized. No blueshift in the emission spectra of these devices was observed over the entire current range. [ABSTRACT FROM AUTHOR]

In an AlGaN quantum-well laser, the presence of Al affects the optical properties of the gain-generating active region partly because of the distinct difference in the band structure between AlN and GaN or InN. The intricate connection between band structure and internal-electric-field effects leads to a noticeably stronger influence of barrier composition on optical gain, lasing polarization, and threshold current than in conventional near-infrared III-V lasers. [ABSTRACT FROM AUTHOR]

In many studies, the value of the experimentally determined internal piezoelectric field has been reported to be significantly smaller than theoretical values. We believe this is due to an inappropriate approximation for the electric field within the depletion region, which is used in the analysis of experimental data, and we propose an alternative method. Using this alternative, we have measured the strength of the internal field of InGaN p-i-n structures, using reverse bias photocurrent absorption spectroscopy and by fitting the bias dependent peak energy using microscopic theory based on the screened Hartree-Fock approximation. The results agree with those using material constants interpolated from binary values. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Electroabsorption measurements are reported for wurtzite InGaN/GaN quantum wells. The electroabsorption technique allows exact quantitative analysis of absorption and absorption changes in InGaN quantum wells and barrier layers, with recorded field-induced absorption changes as large as 7000 cm[sup -1] below and almost 20000 cm[sup -1] above the band edge. The technique thus allows precise determination of the strong internal fields that originate from strain-induced polarization and differences in spontaneous polarization. The fields measured on functioning diodes vary between 1.1 and 1.4 MV/cm for indium concentrations in InGaN quantum wells ranging from 7% to 9%. [ABSTRACT FROM AUTHOR]

Studies the phase separation that occurs in In[sub 0.33]Ga[sub 0.67]N/GaN multiple-quantum-well structures after annealing at high nitrogen pressures. Superlattice peaks showed by the x-ray diffraction (XRD) spectra of the as-grown samples; Transmission electron microscopy (TEM) images of the annealed samples; TEM and XRD measurements.

Indium-gallium nitride (In[sub x]Ga[sub 1-x]N) single-quantum-well (SQW) light emitting diodes (LEDs), grown by metalorganic chemical vapor deposition on sapphire, were transferred onto Si substrates. The thin-film In[sub x]Ga[sub 1-x]N SQW LED structures were first bonded onto a n[sup +]-Si substrate using a transient-liquid-phase Pd-In wafer-bonding process followed by a laser lift-off technique to remove the sapphire growth substrate. Individual, 250x250 μm[sup 2], LEDs with a backside contact through the n[sup +]-Si substrate were then fabricated. The LEDs had a typical turn-on voltage of 2.5 V and a forward current of 100 mA at 5.4 V. The room-temperature emission peak for the In[sub x]Ga[sub 1-x]N SQW LEDs was centered at 455 nm with a full width at half maximum of 19 nm. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Wafer bowing has a strong impact on the wavelength homogeneity of InGaN based light emitters due to the strong temperature dependence of the In incorporation. Using in-situ curvature measurements and pyrometry we have studied the impact of growth conditions on the lateral and vertical temperature distribution across the wafer and on wafer bow on sapphire and GaN substrates. From the pyrometry signal at 950nm the temperature of the substrate holder can be deduced. Using 400nm pyrometry the surface temperature was determined for sufficiently thick GaN layers. While total flow and satellite rotation were found to have no impact, the total pressure affects the vertical temperature profile and thus the wafer bow. However, the effect is small and can only be used for fine tuning. To compensate for the strain of the AlGaN cladding layer in laser structures precurved GaN substrates are shown to be advantageous. Photoluminescence confirmed a spatially uniform surface temperature distribution during the deposition of the InGaN quantum well active region with a peak wavelength around 500nm and a deviation as low as 3.5nm on flat sapphire substrates and precurved GaN substrates. [Copyright &y& Elsevier]

Abstract: InGaN/GaN multiple quantum well structures emitting in the blue/green wavelength region were grown by metal organic vapor phase epitaxy. By reducing the quantum well growth time the influence of the quantum well thicknesses between 3.8 and 1.1nm on the indium incorporation and the distribution of indium in the quantum wells in growth direction were investigated. X-ray diffraction measurements show that the average indium mole fraction in the quantum wells decreases with reducing quantum well width due to a delay in the indium incorporation at the barrier/well interface. Quantitative analysis reveals a segregation length of about 2nm as a measure of the graded region in growth direction. Cathodoluminescence imaging reveals that the spatial variation of the wavelength is reduced with decreasing quantum well thickness down to 1.7nm. Reducing the width of the quantum well further results in an increase of the spatial wavelength variation. [ABSTRACT FROM AUTHOR]