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InGaN micro-LEDs (μ LEDs) with their potential high-volume applications have attracted substantial research interest in the past years. In comparison to other III–V semiconductors, InGaN exhibits a reduced susceptibility toward non-radiative surface recombination. However, efficiency degradation becomes more prominent as dimensions shrink to a few μ m or less. Due to the high surface-to-volume ratio of the miniaturized devices, the non-radiative recombination increases and reduces the internal quantum efficiency. While many groups focus on surface passivation to mitigate surface defects, the influence of crystallographic orientation of the μ LED sidewall on the efficiency remains unexplored. This study addresses this gap by investigating the impact of crystallographic orientation of the sidewalls on the emission properties of the μ LEDs. Hexagonal and elongated μ LEDs with dimensions as small as 3.5 μ m and sidewalls with crystallographically well-defined m- and a-planes were fabricated. Electrical and optical properties were investigated using photo- and electroluminescence. External quantum efficiency (EQE) is assessed based on well-known carrier recombination models. It can be shown that μ LED performance intrinsically depends on the crystallographic orientation of the sidewalls. Comparing hexagonal μ LED structures with a-plane and m-plane sidewalls, an increase in the EQE by 33 % was observed for structures with a-plane sidewalls, accompanied by reduction in the current density of the peak EQE by a nearly two orders of magnitude compared to structures with m-plane sidewalls. By analyzing the EQE characteristics at the μ LED center and near the sidewalls, the improvements can be directly attributed to the increased radiative recombination from sidewalls with a-plane orientation. [ABSTRACT FROM AUTHOR]

Laser lift-off (LLO) is an important step in the processing chain of nitride-based light-emitting diodes (LEDs), as it enables the transfer of LEDs from the growth substrate to a more suitable carrier. A distinctive feature of LLO with ultrashort pulses is the ability to use either above- or below-bandgap radiation, since nonlinear absorption becomes relevant for ultrashort pulses. This study addresses the differences in the absorption scheme for below- and above-bandgap radiation and investigates the electrical and optical properties of InGaN/GaN LEDs before and after LLO with 347 and 520 nm laser light via current–voltage and power- as well as temperature-dependent photoluminescence measurements. LLO could be successfully realized with both wavelengths. The threshold fluence required for LLO is about a factor of two larger for 520 nm compared to that for 347 nm. Furthermore, an increase in leakage current by several orders of magnitude and a significant decrease in efficiency with laser fluence are observed for below-bandgap radiation. In contrast, leakage current hardly increases and efficiency is less dependent on the laser fluence for samples lifted with 347 nm. This degradation is ascribed to the absorption of laser light in the active region, which facilitates a modification of the local defect landscape. The effect is more severe for below-bandgap radiation, as more laser light penetrates deep into the structure and reaches the active region. Ultimately, we show that LEDs lifted with ultrashort laser pulses can exhibit good quality, making ultrashort pulse LLO a viable alternative to conventional LLO with nanosecond pulses. [ABSTRACT FROM AUTHOR]

High-temperature annealing (HTA) is one of the most promising techniques to produce high-quality, cost-efficient AlN templates for further epitaxial growth of AlGaN devices. Unfortunately, the yield of this process seems to be limited due to the restricting face-to-face configuration that is typically used, in which contaminations of the template surface can occur easily. A high yield is crucial for process transfer into industry. Indeed, templates that are annealed in open-face configuration suffer from surface degradation due to excessive AlN evaporation during the course of the annealing process. To highlight the physics that are restricting the open-face approach of the process, sublimation behavior of AlN at temperatures and atmospheres typically used in HTA processes has to be examined. In this study, we use the Knudsen effusion mass spectrometry technique to confirm the previously published results on equilibrium partial pressures of species above AlN. Based on the experimentally determined data and further AlN sublimation experiments, the apparent sublimation coefficient of AlN in N2 and Ar atmospheres at HTA process conditions can be derived. Despite N2 having a stabilizing effect on AlN during HTA, the still high decomposition rates of several hundred nanometers per hour can explain the excessive damage that is typically observed if AlN/sapphire templates are annealed in an open-face configuration. Finally, based on theoretical considerations, a strategy to reduce the sublimation of AlN during HTA in open-face configuration is suggested. [ABSTRACT FROM AUTHOR]

Pulsed sputter deposition has been demonstrated to be a viable process for the growth of high quality GaN and InGaN/GaN LEDs. It enables the fabrication of nitride LEDs with a red emission wavelength at large areas. In this study, we explore details on the epitaxial sputter deposition of GaN with a particular emphasis on ion damage. By changing the argon to nitrogen ratio, we adjust the growth mode from island to layer growth. TEM revealed speckles in the epitaxial GaN, which could be identified as isolated basal stacking faults, acting as non-radiative recombination centers. Using Monte Carlo methods, we modeled the energies of backscattered and sputtered atoms in order to get information on the ion damage mechanisms. Considering the collisions on the way from the target to the substrate, we found energetic nitrogen to induce the speckles. A shielding mechanism based on metallic gallium has been identified, leading to the strongly increased luminescence quality in comparison to the non-shielded material. [ABSTRACT FROM AUTHOR]

We studied the near-band-edge emission (NBE) and yellow defect luminescence (YL) of GaN under below bandgap excitation with 40-fs laser pulses at a wavelength of 775 nm. Even though in this case three-photon absorption processes are required for the excitation of band-to-band transitions, fourth-order (INBE ∝ Iexc4) and sub-linear (IYL ∝ Iexc0.5) dependencies were observed for the intensity of the NBE and YL as a function of the excitation intensity, respectively. Modelling the carrier dynamics with a few-level rate-equation model revealed that, for high excitation irradiances, the electron-hole population generated by three-photon absorption is such that the NBE recombination rate is intensified (exponent > 3) and, at the same time, the competition between the electron capturing and the defect-level emission rate suppresses the YL (exponent < 1). [ABSTRACT FROM AUTHOR]

Three dimensional GaN structures with different crystal facets and doping types have been investigated employing the surface photo-voltage (SPV) method to monitor illumination-induced surface charge behavior using Kelvin probe force microscopy. Various photon energies near and below the GaN bandgap were used to modify the generation of electron-hole pairs and their motion under the influence of the electric field near the GaN surface. Fast and slow processes for Ga-polar c-planes on both Si-doped n-type as well as Mg-doped p-type GaN truncated pyramid microstructures were found and their origin is discussed. The immediate positive (for n-type) and negative (for p-type) SPV response dominates at band-to-band and near-bandgap excitation, while only the slow process is present at sub-bandgap excitation. The SPV behavior for the semi-polar facets of the p-type GaN truncated pyramids has a similar characteristic to that on its c-plane, which indicates that it has a comparable band bending and no strong influence of the polarity-induced charges is detectable. The SPV behavior of the non-polar m-facets of the Si-doped n-type part of a transferred GaN column is similar to that of a clean c-plane GaN surface during illumination. However, the SPV is smaller in magnitude, which is attributed to intrinsic surface states of m-plane surfaces and their influence on the band bending. The SPV behavior of the non-polar m-facet of the slightly Mgdoped part of this GaN column is found to behave differently. Compared to c- and r-facets of p-type surfaces of GaN-light-emitting diode micro-structures, the m-plane is more chemically stable. [ABSTRACT FROM AUTHOR]

The effect of the V/III flux ratio and substrate temperature on magnetotransport properties and lattice parameters of Ga0.96Mn0.04As grown by molecular-beam epitaxy (MBE) are investigated. The results suggest that the lattice parameter of Ga0.96Mn0.04As increased with decreasing V/III ratio and increasing growth temperature.

The intensity and the polarization anisotropy of spatially indirect photoluminescence of type-II ZnSe/BeTe structures with four possible combinations of normal and inverted interfaces in a wide range of photoexcited carrier densities are studied. The photoluminescence (PL) polarization degree at interfaces with Be-Se and Zn-Te bonds amounts to about 50% and approximately 70%, respectively.

Photoluminescence spectroscopy, x-ray techniques and transmission electron microscopy were used to examined Be-enhanced formation of CdSe quantum dots in CdSe/ZnSe heterostructures.

The number of extended defects in ZnSe based II-VI heterostructures grown by molecular beam epitaxy on (001) GaAs were reduced through the application of BeTe buffer layers. Three kinds of etch pits could be discerned by optical microscopy after defect selective chemical etching. The use of a thin BeTe buffer layer allows the reduction of the density of paired Frank type staking faults to below 1000/sq.cm. The function of Se in the background pressure of the defect nucleation at the II-IV/GaAs interface was found to be significant.

A study was conducted to examine the band alignment at the interface of pseudomorphic BeTe/ZnSe(100) heterojunctions for different interface terminations using photoelectron spectroscopy. The heterostructures were prepared by molecular beam epitaxy and supported high structural quality. Results indicated the importance of the large reduction in the valence band for applications in optoelectronic devices.

The optical characteristics of ZnTe epilayers, Zn(sub 1-x)Mg(sub x)Se(sub y)Te(sub 1-y) epilayers and ZnTe/Zn(sub 1-x)Mg(sub x)Se(sub y)Te(sub 1-y) quantum wells grown on (100)-InAs substrates by molecular beam epitaxy were studied. There were several sharp photoluminescence lines close to the excitonic position and no noticeable luminescence from deep levels in ZnTe epilayers. Bright luminescence was observed from Zn(sub 1-x)Mg(sub x)Se(sub y)Te(sub 1-y) epilayers which were lattice matched with InAs.

Resonant tunneling through ZnSe/BeTe double-barrier, single-quantum-well structures has been observed. Negative differential resistance was seen in the current-voltage characteristics up to room temperature. Because of a conduction-band offset of more than 2 eV, four resonances with negative differential resistance could be detected for this semiconductor material combination at liquid-helium temperature. The structures showed a peak-to-valley ratio up to 6:1 at 4.2 K.

The valence band offset in the semimagnetic CdTe/Cd(1-x)Mn(x)Te quantum wells is determined by comparing spectroscopic techniques. Energy difference between heavy and light-hole excitons are estimated to calculate a valence band offset. Zeeman splitting analyses reveal a decrease in valence band offset with an increase in mu content. Change in magnetic properties have no influence on the (HH-LH) energy difference.

High resolution transmission electron microscopy (HRTEM) facilitated the study of the structural properties of CdMgTe/CdTe superlattices, revealing two distinct mechanisms associated with misfit dislocation generation. Molecular beam epitaxy facilitated the growth of these superlattices on CdTe. HRTEM delineated superlattice structural quality on an atomic scale.

A limit in the n-type doping of CdMgTe is an intrinsic dopant property and is not a result of self-compensation due to nonideal growth conditions. The origin of the limitation in CdMgTe doping, which was studied using bromine and chlorine during molecular beam epitaxy, exhibited DX-like behavior in donors.

The investigation of oxygen on CdTe substrates by means of X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED), reveals that a Tellurium oxide layer 15 angstroms thick is found on the surface of as-delivered CdTe substrates that are mechanically polished. The investigation also shows that the oxide does not evaporate at temperatures below 350 degrees centigrade.

Reflection high energy electron diffraction was used to study the growth mechanism of CdTe durign molecular beam epitaxy. Cd and Te sticking coefficients were derived from comparisons of kinetic models of molecular beam epitaxial growth. The sticking coefficients were found to be dependent on the surface concentration of free Cd and Te sites. It was also shown that kinetic model growth rates coincide with those derived from experimental models.

Three-dimensional GaN columns recently have attracted a lot of attention as the potential basis for core-shell light emitting diodes for future solid state lighting. In this study, the fundamental insights into growth kinetics and mass transport mechanisms of N-polar GaN columns during selective area metal organic vapor phase epitaxy on patterned SiOx/sapphire templates are systematically investigated using various pitch of apertures, growth time, and silane flow. Species impingement fluxes on the top surface of columns Jtop and on their sidewall Jsw, as well as, the diffusion flux from the substrate Jsub contribute to the growth of the GaN columns. The vertical and lateral growth rates devoted by Jtop, Jsw and Jsub are estimated quantitatively. The diffusion length of species on the SiOx mask surface λsub as well as on the sidewall surfaces of the 3D columns λsw are determined. The influences of silane on the growth kinetics are discussed. A growth model is developed for this selective area metal organic vapor phase epitaxy processing. [ABSTRACT FROM AUTHOR]

The electron spin polarization on n-type modulation doped Cd(sub 1-x)Mn(sub x)Te/Cd(sub 1-x-y)Mn(sub x)Mg(sub y)Te single quantum wells is investigated. Results show that in magnetic fields, the s-d and p-d exchange interactions with localized magnetic moments cause the electron states to undergo an initial giant splitting. This splitting yields an initial spin polarization which will be enlarged by the many-body effects of the electron self-energy corrections.

We investigate the photoluminescence properties of ZnO nanorods grown by vapor phase epitaxy. The room temperature photoluminescence spectra exhibit an energy shift of about 80 meV to lower energy in comparison with that of bulk ZnO as well as ZnO epilayers. The emission band observed at 3.31 eV at low temperature dominates the photoluminescence at room temperature. The high internal quantum efficiency of about 33% is explained in terms of this band, which seems to result from excitons bound to surface defect states. [ABSTRACT FROM AUTHOR]

The influence of ZnO seed crystals and postgrowth annealing on low-temperature aqueous chemically grown ZnO nanorods is analyzed. At the seed crystal/nanorod interface a high density of structural defects leads to emission at 3.332 eV, attributed to excitons bound to structural defects. This peak is absent for seed crystals, very pronounced for rods of shorter lengths grown on seed crystals, and reduced for longer nanorods. After annealing in oxygen and nitrogen atmosphere, the near-band-edge excitonic transitions sharpen and deep-level emission is strongly reduced. Time-resolved photoluminescence measurements show a striking similarity between donor-bound excitons and excitons bound to structural defects. [ABSTRACT FROM AUTHOR]

The optical properties of ZnO nanorods realized by an advanced low-temperature aqueous chemical growth on both silicon and plastic substrates are presented. Systematic photoluminescence investigations in the temperature range of 4–293 K reveal strong and well-resolved near-band-edge emission even for rods on plastic substrate, and a weak deep-level emission. At intermediate temperatures phonon replicas of excitonic lines are observable. The optimum molar concentration range of the solution for obtaining nanorods of good optical quality is shown to lie between 0.025M and 0.075M. The large linewidth of the near-band-edge emission (∼10 meV), its temperature dependence, and the absence of sharp excitonic transitions indicate that this emission is a result of transitions from a band of donor states. [ABSTRACT FROM AUTHOR]

The structural characteristics of the ZnO film grown on sapphire substrate using a thin MgO buffer layer were studied using transmission electron microscopy and high-resolution x-ray diffraction. The growth was carried out in a modified plasma-molecular beam epitaxy system. The observed misfit dislocations were well confined at the sapphire overgrown interface exhibiting domain matching epitaxy, where the integral multiples of lattice constants match across the interface. The main extended defects in the ZnO film were the threading dislocations having a mean density of 4×109 cm-2. The formation of the MgO buffer layer as well as the ZnO growth were monitored in situ by reflection high-energy electron diffraction. The very thin ∼1 nm, MgO buffer layer can partially interdiffuse with the ZnO as well as react with the Al2O3 substrate forming an intermediate epitaxial layer having the spinel (MgO/Al2O3) structure. [ABSTRACT FROM AUTHOR]

We have investigated the structural, optical, and vibrational properties of strained heteroepitaxial ZnO layers by high resolution x-ray diffraction, reflectivity, and Raman measurements. The ZnO layers were grown by metalorganic vapor phase epitaxy on sapphire substrates under varying growth conditions. A Poisson number of μ=0.303 and phonon deformation-potential parameters of a=-690 cm-1, b=-940 cm-1 for the high-energy E2 optical phonon mode have been determined. The shift of the excitonic resonances due to the strain in the layers agrees well with the experimentally determined Poisson ratio using the deformation-potentials D1–D4 determined by Wrzesinski and Fröhlich [Phys. Rev. B 56, 13087 (1997)]. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Gallium and nitrogen ions have been implanted into ZnO crystals and metal organic vapor phase epitaxy grown ZnO layers. Postimplantation annealing behavior in the temperature range between 200 and 900 °C has been studied by means of Raman scattering and low-temperature photoluminescence. The temperature for healing of the implantation-induced defects was found to be 800 °C. Implanted gallium acts as donor with a donor binding energy E[sub D] of 53 meV, thus allowing the control of n-type doping in ZnO. From photoluminescence measurements of the donor-acceptor pair transition of a series of nitrogen-implanted ZnO samples we estimate the binding energy E[sub A] of the nitrogen acceptor between 163 and 196 meV. Electrical characterization of nitrogen-implanted samples shows a behavior ranging from low n-type to highly compensated. But no unambiguous and reproducible type conversion could be achieved. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Structures with four possible configurations of normal and inverted interfaces have been investigated. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. The comparison of interfaces formed by the growth on anion (Se, Te) and cation (Zn, Be) terminated layers has shown that the latter demonstrate a significantly higher nonradiative recombination rate. In agreement with the quantum well (QW) symmetry, the photoluminescence (PL) of the structures with nonequivalent normal and inverted interfaces is highly linearly polarized both at low and high excitation densities. Unexpectedly, a similarly strong PL polarization has been found for structures with equivalent interfaces up to carrier densities of 10[sup 12] cm[sup -2] per QW. The polarization is explained by a built-in electric field, it decreases with increasing carrier concentration due to screening of the electric field. Finally, we have found that the PL polarization degree at interfaces with Be-Se and Zn-Te bonds amounts to about 50% and ≈70%, respectively. of Physics. [ABSTRACT FROM AUTHOR]

The fabrication of (CdMg)Te confinement quantum well laser structure is performed by molecular beam epitaxy using an elemental Mg and a CdTe compound source. Emission in the whole visible range is achieved by tuning the band gap. This tuning can be effected by adjusting Mg concentration.

Time-resolved photoluminescence of perpendicular transport and consequent capture of photoexcited carriers in superlattices of II-VI semiconductors is investigated using large sized quantum wells. Superlattice periods in the range of 30 to 80 angstroms are used as standards of trapping dynamics. A collection of electrons and holes in an enlarged cdTe quantum well exhibit the limitation by a short-period CdTe/Cd(1 to x)Mn(x)Te superlattice.

Deals with a study which examined the x-ray photoelectron spectroscopy of cadmium telleride and gallium arsenite interfaces. Information on the thin films; Methodology of the study; Results and discussion.

Investigates the optical properties of quantum wells structures of zinc telluride. Observed sharp photoluminescence lines close to the excitonic position in zinc telluride epilayers; Effect of the reduction of the band gap energy of the zinc telluride layer; Estimation of the valence band for zinc telluride layers.

Presents the results of an investigation on the tellurium-stabilized and the cadmium-stabilized surfaces of (100) cadmium telluride substrates. Effect of laser illumination on the cadmium telluride growing surface; Discussion of results.