Radiative and non-radiative recombination in GaInN/GaN quantum wells

Die Gruppe-III-Nitride Galliumnitrid (GaN), Indiumnitrid (InN) und Aluminiumnitrid (AlN) bilden ein Halbleitermaterialsystem, dessen ternäre und quaternäre Mischverbindungen Al(x)In(y)Ga(1-x-y)N direkte Bandlücken besitzen, die sich über einen Energiebereich von 0.65 bis 6.2 eV erstrecken. Das Materialsystem deckt damit einen Spektralbereich ab, der von infrarot, über das gesamte sichtbare, bis nach ultraviolet reicht, wobei die direkte Bandlücke eine effiziente Lichterzeugung in diesen Halbleitern ermöglicht. So werden Quantenfilmstrukturen aus dem Materialsystem in optoelektronischen Bauteilen wie LEDs und Halbleiterlasern verwendet. Die Untersuchungen in dieser Arbeit beschäftigen sich mit dem Auftreten von invers pyramidalen Aussparungen mit hexagonalem Grundriss (V-Defekte) in der Kristallstruktur von mittels Metallorganischer Gasphasenepitaxie gewachsenen GaInN/GaN Quantenfilmstrukturen und den Auswirkungen, die die V-Defekte bzw. die Quantenfilmstrukturen auf den Kristallfacetten der V-Defekte auf die Emissions- und Rekombinationseigenschaften der gesamten GaInN/GaN Quantenfilmstrukturen haben. Dabei wird insbesondere auf die Wirksamkeit der V-Defekte zur Unterdrückung nichtstrahlender Verlustprozesse von optisch erzeugten Überschussladungsträgern im Quantenfilmsystem und auf die Dynamik der Rekombinationsprozesse eingegangen. Ausserdem werden die nichtstrahlenden Verluste in Halbleiterlaserstrukturen mit GaInN/GaN Quantenfilmen als verstärkendes Medium, die Bedeutung von V-Defekten für Laserstrukturen und die Auswirkungen der Laseralterung auf die Quantenfilmstruktur untersucht. Die Quantenfilmstrukturen wurden mittels temperaturabhängiger zeitaufgelöster Photolumineszenz, temperatur- und leistungsabhängiger Photolumineszenz im Dauerstrichbetrieb und der optischen Verstärkungsspektroskopie mit der variablen Strichlängenmethode studiert.The group-III-nitrides, gallium nitride (GaN), indium nitride (InN), and aluminium nitride (AlN), represent a system of semiconductors which ternary and quaternary compositions Al(x)In(y)Ga(1-x-y)N have direct bandgaps with energies from 0.65eV to 6.2eV. So, the group-III-nitrides cover a spectral range from infrared, via the whole visible, to the ultaraviolet spectral region. The direct bandgap allows for an efficient light generation in these semiconductors. Therefore, group-III-nitride quantum well structures are used in optoelectronic devices like LEDs or semiconductor lasers. These studies deal with the formation of inverse pyramidal pits with hexagonal base (V-shaped pits) in the crystalline structure of MOVPE-grown (metal organic vapour phase epitaxy) GaInN/GaN quantum well structures and with the effects of the V-Shaped pits or the quantum wells on the V-shaped pit's facets on the emission and recombination characteristics of the whole GaInN/GaN quantum well structure. Especially the effectiveness of the V-shaped pits, suppressing the nonradiative recombination of optically generated excess carriers in the quantum well structure, and the recombination dynamics of excess carriers in the quantum wells are analyzed. Furthermore, the nonradiative recombination in semiconductor lasers with GaInN/GaN quantum wells in the active region, the relevance of V-shaped pits for laser structures, and the effects of aging on the GaInN/GaN quantum wells of laser structures are studied. The quantum well structures are examined with temperature dependent time-resolved photoluminescence, temperature and excitation power dependent contiuous-wave photoluminescence, and optical gain measurements with the variable stripe length method

High-Temperature Annealing of AlGaN

In the past few years, high-temperature annealing of AlN has become a proven method for providing AlN layers with low dislocation densities. Herein, the example of Al0.77Ga0.23N is used to investigate whether annealing can also improve the material quality of the ternary alloy. A detailed analysis of the influence of annealing temperature on structural and optical material properties is presented. It is found that with increasing annealing temperature, the threading dislocation density can be lowered from an initial value of 6.0 × 109 down to 2.6 × 109 cm−2. Ga depletion at the AlGaN surface and Ga diffusion into the AlN buffer layer are observed. After annealing, the defect luminescence between 3 and 4 eV is increased, accompanied by an increase in the oxygen concentration by about two orders of magnitude. Furthermore, due to annealing optical absorption at 325 nm (3.8 eV) occurs, which increases with increasing annealing temperature. It is assumed that the reason for this decrease in ultraviolet (UV) transmittance is the increasing number of vacancies caused by the removal of group-III and N atoms from the AlGaN lattice during annealing

Temperature-Dependent Charge Carrier Diffusion in [0001¯] Direction of GaN Determined by Luminescence Evaluation of Buried InGaN Quantum Wells

Temperature-dependent transport of photoexcited charge carriers through a nominally undoped, c-plane GaN layer toward buried InGaN quantum wells is investigated by continuous-wave and time-resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in [0001⎯⎯] direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low-temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN-based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers

Status and Prospects of AlN Templates on Sapphire for Ultraviolet Light-Emitting Diodes

Herein, the scope is to provide an overview on the current status of AlN/sapphire templates for ultraviolet B (UVB) and ultraviolet C (UVC) light-emitting diodes (LEDs) with focus on the work done previously. Furthermore, approaches to improve the properties of such AlN/sapphire templates by the combination of high-temperature annealing (HTA) and patterned AlN/sapphire interfaces are discussed. While the beneficial effect of HTA is demonstrated for UVC LEDs, the growth of relaxed AlGaN buffer layers on HTA AlN is a challenge. To achieve relaxed AlGaN with a low dislocation density, the applicability of HTA for AlGaN is investigated. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Selective Growth of GaP Crystals on CMOS-Compatible Si Nanotip Wafers by Gas Source Molecular Beam Epitaxy

Gallium phosphide (GaP) is a III–V semiconductor with remarkable optoelectronic properties, and it has almost the same lattice constant as silicon (Si). However, to date, the monolithic and large-scale integration of GaP devices with silicon remains challenging. In this study, we present a nanoheteroepitaxy approach using gas-source molecular-beam epitaxy for selective growth of GaP islands on Si nanotips, which were fabricated using complementary metal–oxide semiconductor (CMOS) technology on a 200 mm n-type Si(001) wafer. Our results show that GaP islands with sizes on the order of hundreds of nanometers can be successfully grown on CMOS-compatible wafers. These islands exhibit a zinc-blende phase and possess optoelectronic properties similar to those of a high-quality epitaxial GaP layer. This result marks a notable advancement in the seamless integration of GaP-based devices with high scalability into Si nanotechnology and integrated optoelectronics.Deutsche Forschungsgemeinschaft 10.13039/501100001659European Commission 10.13039/501100008530Peer Reviewe

Improving AlN Crystal Quality and Strain Management on Nanopatterned Sapphire Substrates by High-Temperature Annealing for UVC Light-Emitting Diodes

Herein, AlN growth by metalorganic vapor-phase epitaxy on hole-type nanopatterned sapphire substrates is investigated. Cracking occurs for an unexpectedly thin-layer thickness, which is associated to altered nucleation conditions caused by the sapphire pattern. To overcome the obstacle of cracking and at the same time to decrease the threading dislocation density by an order of magnitude, high-temperature annealing (HTA) of a 300 nm-thick AlN starting layer is successfully introduced. By this method, 800 nm-thick, fully coalesced and crack-free AlN is grown on 2 in. nanopatterned sapphire wafers. The usability of such templates as basis for UVC light-emitting diodes (LEDs) is furthermore proved by subsequent growth of an UVC-LED heterostructure with single peak emission at 265 nm. Prerequisites for the enhancement of the light extraction efficiency by hole-type nanopatterned sapphire substrates are discussed. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Status and Prospects of AlN Templates on Sapphire for Ultraviolet Light‐Emitting Diodes

Herein, the scope is to provide an overview on the current status of AlN/sapphire templates for ultraviolet B (UVB) and ultraviolet C (UVC) light‐emitting diodes (LEDs) with focus on the work done previously. Furthermore, approaches to improve the properties of such AlN/sapphire templates by the combination of high‐temperature annealing (HTA) and patterned AlN/sapphire interfaces are discussed. While the beneficial effect of HTA is demonstrated for UVC LEDs, the growth of relaxed AlGaN buffer layers on HTA AlN is a challenge. To achieve relaxed AlGaN with a low dislocation density, the applicability of HTA for AlGaN is investigated.BMBF, 03ZZ0112A&B, Zwanzig20 - Advanced UV for Life - Verbundvorhaben: AlN-SubstrateBMBF, 03ZZ0134B&C, Zwanzig20 - Advanced UV for Life - Verbundvorhaben: UV PowerBMBF, 03ZZ0138A&B, Zwanzig20 - Advanced UV for Life - Verbundvorhaben: UV-LEDs für ultrakurze Wellenlängen um 230 nm auf Basis von AIN-Substraten (AIN-230nm)DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Role of substrate quality on the performance of semipolar (11 2 - 2) InGaN light-emitting diodes

We compare the optical properties and device performance of unpackaged InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) emitting at ∼430 nm grown simultaneously on a high-cost small-size bulk semipolar (11 2 - 2) GaN substrate (Bulk-GaN) and a low-cost large-size (11 2 - 2) GaN template created on patterned (10 1 - 2) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ∼ and basal-plane stacking fault (BSF) density of 0 cm-1, while the PSS-GaN substrate has the TDD of ∼2 × 108cm-2 and BSF density of ∼1 × 103cm-1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density

Improving AlN Crystal Quality and Strain Management on Nanopatterned Sapphire Substrates by High‐Temperature Annealing for UVC Light‐Emitting Diodes

Herein, AlN growth by metalorganic vapor‐phase epitaxy on hole‐type nanopatterned sapphire substrates is investigated. Cracking occurs for an unexpectedly thin‐layer thickness, which is associated to altered nucleation conditions caused by the sapphire pattern. To overcome the obstacle of cracking and at the same time to decrease the threading dislocation density by an order of magnitude, high‐temperature annealing (HTA) of a 300 nm‐thick AlN starting layer is successfully introduced. By this method, 800 nm‐thick, fully coalesced and crack‐free AlN is grown on 2 in. nanopatterned sapphire wafers. The usability of such templates as basis for UVC light‐emitting diodes (LEDs) is furthermore proved by subsequent growth of an UVC‐LED heterostructure with single peak emission at 265 nm. Prerequisites for the enhancement of the light extraction efficiency by hole‐type nanopatterned sapphire substrates are discussed.BMBF, 03ZZ0134B, Zwanzig20 - Advanced UV for Life - Verbundvorhaben: UV Power; TP2: Entwicklung von high-power UVB-LEDs um 300 nmDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Temperature Dependence of Dark Spot Diameters in GaN and AlGaN

Threading dislocations in c-plane (Al,Ga)N layers are surrounded by areas with reduced light generation efficiency, called “dark spots.” These areas are observable in luminescence measurements with spatial resolution in the submicrometer range. Dark spots reduce the internal quantum efficiency in single layers and light-emitting devices. In cathodoluminescence measurements, the diameter of dark spots (full width at half maximum [FWHM]) is observed to be 200–250 nm for GaN. It decreases by 30–60% for AlxGa1−xN with x ≈ 0.5. Furthermore, the dark spot diameter increases with increasing temperature from 83 to 300 K in AlGaN, whereas it decreases in GaN. Emission energy mappings around dark spots become less smooth and show sharper features on submicrometer scales at low temperature for AlGaN and, on the contrary, at high temperature for GaN. It is concluded that charge carrier localization dominates the temperature dependence of dark spot diameters and of the emission energy distribution around threading dislocations in AlGaN, whereas the temperature-dependent excitation volume in cathodoluminescence and charge carrier diffusion limited by phonon scattering are the dominant effects in GaN. Consequently, with increasing temperature, nonradiative recombination related to threading dislocations extends to wider regions in AlGaN, whereas it becomes spatially limited in GaN