Mn DOPING OF GaN LAYERS GROWN BY MOVPE

In this contribution we present a growth of Ga1-xMnxN layers by MOVPE. Mn doped GaN layers were grown with and without undoped GaN templates on (0001) sapphire substrates in a quartz horizontal reactor. For the deposition of Ga1-xMnxN layers (MCp)2Mn was used as a Mn – precursor. The flow of the Mn precursor was 0.2-3.2 μmol.min-1. The deposition of Ga1-xMnxN layers was carried out under the pressure of 200 mbar, the temperature 1050 °C and the V/III ratio of 1360. For the growth of high quality GaN:Mn layers it was necessary to grow these layers on a minimally partially coalesced layer of pure GaN. The direct deposition of GaN:Mn layer on the low temperature GaN buffer layer led to a three-dimensional growth during the whole deposition process. Another investigated parameter was the influence of nitrogen on the layer’s properties. A nearly constant ferromagnetic moment persisting up to room temperature was observed on the synthesized thin films

Fabrication and properties of gallium phosphide variable colour displays

The unique properties of single-junction gallium phosphide devices incorporating both red and green radiative recombination centers were investigated in application to the fabrication of monolithic 5 x 7 displays capable of displaying symbolic and alphanumeric information in a multicolor format. A number of potentially suitable material preparation techniques were evaluated in terms of both material properties and device performance. Optimum results were obtained for double liquid-phase-epitaxial process in which an open-tube dipping technique was used for n-layer growth and a sealed tipping procedure for subsequent p-layer growth. It was demonstrated that to prepare devices exhibiting a satisfactory range of dominant wavelengths which can be perceived as distinct emission colors extending from the red through green region of the visible spectrum involves a compromise between the material properties necessary for efficient red emission and those considered optimum for efficient green emission

Characterization of low conductivity wide band gap semiconductors

This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures.The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples.The investigated AlₓGa₁₋ₓN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlₓGa₁₋ₓN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlₓGa₁₋ₓN:Si samples showed the incorporation properties of Si in AlₓGa₁₋ₓN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects.The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InₓAl₁₋ₓN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated.The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InₓAl₁₋ₓN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs.This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures.The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples.The investigated AlₓGa₁₋ₓN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlₓGa₁₋ₓN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlₓGa₁₋ₓN:Si samples showed the incorporation properties of Si in AlₓGa₁₋ₓN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects.The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InₓAl₁₋ₓN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated.The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InₓAl₁₋ₓN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs

Gallium nitride-based microwave high-power heterostructure field-effect transistors

The research described in this thesis has been carried out within a joint project between the Radboud Universiteit Nijmegen (RU) and the Technische Universiteit Eindhoven (TU/e) with the title: "Performance enhancement of GaN-based microwave power amplifiers: material, device and design issues". This project has been granted by the Dutch Technology Foundation STW under project number NAF 5040. The aims of this project have been to develop the technology required to grow state-of-the-art AlGaN/GaN epilayers on sapphire and semi-insulating (s.i.) SiC substrates using metal organic chemical vapor deposition (MOCVD) and to fabricate microwave (f > 1 GHz) high-power (Pout > 10 W) heterostructure field-effect transistors (HFETs) on these epitaxial films. MOCVD growth of AlGaN/GaN epilayers and material characterization has been done within the group Applied Materials Science (AMS) of RU. Research at the Opto-Electronic Devices group (OED) of TU/e has focused on both electrical characterization of AlGaN/GaN epilayers and design, process technology development, and characterization of GaN-based HFETs and CPW passive components. Although a considerable amount of work has been done during this research with respect to processing of CPW passive components on s.i. SiC substrates, this thesis focused on active AlGaN/GaN devices only. GaN is an excellent option for high-power/high-temperature microwave applications because of its high electric breakdown field (3 MV/cm) and high electron saturation velocity (1.5 x 107 cm/s). The former is a result of the wide bandgap (3.44 eV at RT) and enables the application of high supply voltages (> 50 V), which is one of the two requirements for highpower device performance. In addition, the wide bandgap allows the material to withstand much higher operating temperatures (300oC - 500oC) than can the conventional semiconductor materials such as Si, GaAs, and InP. A big advantage of GaN over SiC is the possibility to grow heterostructures, e.g. AlGaN/GaN. The resulting two-dimensional electron gas (2DEG) at the AlGaN/GaN heterojunction serves as the conductive channel. Large drain currents (> 1 A/mm), which are the second requirement for a power device, can be achieved because of the high electron sheet densities (> 1 x 1013 cm-2) and high electron saturation velocity. These material properties clearly indicate why GaN is a very suitable candidate for next-generation microwave high-power/high-temperature applications such as high-power amplifiers (HPAs) for GSM base stations, and microwave monolithic integrated circuits (MMICs) for radar systems. In this thesis we have presented the design, technology, and measurement results of n.i.d. AlGaN/GaN:Fe HFETs grown on s.i. 4H-SiC substrates by MOCVD. These devices have submicrometer T- or FP-gates with a gate length (Lg) of 0.7 µm and total gate widths (Wg) of 0.25 mm, 0.5 mm, and 1.0 mm, respectively. The 1.0 mm devices are capable of producing a maximum microwave output power (Pout) of 11.9 W at S-band (2 GHz - 4 GHz) using class AB bias conditions of VDS = 50 V and VGS = -4.65 V. It has to be noted that excellent scaling of Pout with Wg has been demonstrated. In addition, the associated power gain (Gp) ranges between 15 dB and 20 dB, and for the power added efficiency (PAE) values from 54 % up to 70 % have been obtained. These results clearly illustrate both the successful development of the MOCVD growth process, and the successful development and integration of process modules such as ohmic and Schottky contact technology, device isolation, electron beam lithography, surface passivation, and air bridge technology, into a process flow that enables the fabrication of state-of-the-art large periphery n.i.d. AlGaN/GaN:Fe HFETs on s.i. SiC substrates, which are perfectly suitable for application in e.g. HPAs at S-band

Gruppe III-Nitrid basierte UVC LEDs und Laser mit transparenten AlGaN:Mg Schichten und Tunneldioden, hergestellt mittels MOVPE

In this work, AlGaN-based light-emitting diodes (LEDs) and lasers with emission wavelengths in the deep ultraviolet (UVC) spectral range are produced, analyzed, and optimized. Here, the focus is on the UV transparency of the structures, enabling high light extraction efficiency for UVC LEDs and being a necessary condition for UVC laser diodes, however at the same time challenging due to low electrical conductivity. AlN and AlGaN layers as well as heterostructures for devices are grown by metalorganic vapor phase epitaxy. A systematic analysis of the influence of individual layer properties on the emission properties of LEDs and lasers is provided. Defect reduced (ELO) AlN layers on sapphire and AlN substrates serve as basis for the epitaxial growth of AlN and AlGaN layers. By analyzing the influence of substrate offcut on surface morphology, atomically smooth AlN layers are reproducibly obtained on both types of substrates for offcut angles < 0.17°. For the realization of n-type AlGaN:Si cladding layers, the influence of growth parameters such as temperature, gas phase composition and growth rate was separately analyzed. Highly conductive, uniform and smooth AlGaN:Si layers were obtained by the implementation of a superlattice concept with 10 s growth interruptions to increase the diffusion length of metal adatoms. Despite high compressive strain, pseudomorphic laser structures with three-fold quantum wells were obtained with emission wavelength at 270 nm by the choice of Al0.7Ga0.3N waveguide composition, whereas lower aluminum contents lead to partial strain relaxation. In addition, the formation of V-pits acting as scattering centers in the waveguide was successfully reduced by increasing the growth temperature from 900 ℃ to 1080 ℃. Finally, the influence of these individual optimization steps on laser properties was analyzed. Optically pumped UVC lasers with laser threshold, spectral linewidth reduction, and TE polarized emission above threshold were shown near 270 nm. By reducing the surface roughness, the laser thresholds were reduced by a factor of seven. Electrical injection mechanisms were experimentally analyzed by electroluminescence measurements on transparent UVC LEDs with waveguide system, and combined with simulations of optical modes and the corresponding losses. By the variation of composition and layer thickness of waveguide and cladding layers an optimized heterostructure design for UVC laser diodes with 200 nm thick Al0.76Ga0.24N:Mg cladding layers was found. This design simultaneously enables efficient carrier injection and sufficient mode confinement with low optical losses of 40 cm-1. As an unconventional alternative to resistive AlGaN:Mg layers, tunnel junctions (TJ) in reverse bias configuration were implemented into the UVC LED heterostructure for efficient injection of holes. By the initial optimization of individual TJ components, such as doping concentrations at the TJ interface or the composition of an interlayer, the first demonstration of functional TJ-LEDs with AlGaN tunnel homojunction was achieved, as well as the first demonstration of AlGaN-based TJ-LEDs grown by metalorganic vapor phase epitaxy. Based on these devices, the interlayer thickness was varied to exploit polarization charges at the interface in order to reduce the space charge region width and enhance tunneling probabilities. Using 8 nm thick GaN interlayers, a reduction of the operation voltage by 20 V was achieved, as well as TJ-LEDs with external quantum efficiencies of 2.3% and emission powers of 6.6 mW at 268 nm and 0.26 mW at 232 nm.In dieser Arbeit werden AlGaN-basierte Leuchtdioden (LEDs) und Laser mit Emissionswellenlängen im tiefen ultravioletten (UVC) Spektralbereich hergestellt, charakterisiert und optimiert. Dabei liegt die UV-Transparenz der Strukturen im Fokus, die hohe Lichtextraktionseffizienz für UVC LEDs ermöglicht und eine notwendige Bedingung für UVC Laserdioden darstellt, gleichzeitig aber aufgrund geringer elektrischer Leitfähigkeit herausfordernd ist. AlN und AlGaN Schichten sowie Heterostrukturen für Bauelemente werden mittels metallorganischer Gasphasenepitaxie hergestellt und der Einfluss einzelner Schichteigenschaften auf die Emissionseigenschaften von LEDs und Lasern systematisch analysiert. Defektreduzierte (ELO) AlN Schichten auf Saphirsubstraten sowie AlN Substrate dienen als Basis für das epitaktische Wachstum von AlN und AlGaN Schichten. Durch die Analyse des Einflusses des Substratfehlschnittes auf die Oberflächenmorphologie konnten atomar glatte AlN Schichten auf beiden Substrattypen für Fehlschnittwinkel < 0.17° reproduzierbar hergestellt werden. Die AlGaN:Si Wachstumsparameter Temperatur, Gasphasenzusammensetzung und Wachstumsrate wurden separat variiert. Leitfähige, homogene und glatte AlGaN:Si Schichten konnten durch die Umsetzung eines Übergitterkonzeptes mit je 10 s Wachstumsunterbrechung zur Erhöhung der Diffusionslänge von Metalladatomen realisiert werden. Pseudomorphe Laserstrukturen mit Dreifach-Quantenfilmen und Emissionswellenlängen von 270 nm wurden trotz stark kompressiver Verspannung mittels Al0.7Ga0.3N Wellenleitern realisiert, wogegen geringere Aluminiumgehalte zu Teilrelaxation der Verspannung führen. Zudem konnte die Ausbildung von V-Pits als Streuzentren im Wellenleiter durch Erhöhung der Wachstumstemperatur von 900 ℃ auf 1080 ℃ erfolgreich reduziert werden. Schließlich wurde der Einfluss dieser einzelnen Optimierungsschritte auf die Lasereigenschaften analysiert. Optisch gepumpte UVC Laser mit spektraler Einschnürung, Laserschwelle sowie TE polarisierter Emission nahe 270 nm wurden gezeigt. Durch Reduktion der Oberflächenrauheit konnte die Laserschwelle schrittweise um den Faktor sieben reduziert werden. Elektrische Injektion wurde mittels Elektrolumineszenz an transparenten UVC LEDs mit Wellenleitersystem experimentell analysiert und mit Simulationen optischer Moden und deren Verluste kombiniert. Durch die Variation von Zusammensetzung und Schichtdicke von Wellenleiter- bzw. Mantelschichten konnte ein optimiertes Heterostrukturdesign für UVC Laserdioden mit 200 nm dicken Al0.76Ga0.24N:Mg Mantelschichten gefunden werden, welches gleichzeitig effiziente Ladungsträgerinjektion und ausreichenden Modeneinschluss mit geringen optischen Verlusten von 40 cm-1 ermöglicht. Als unkonventionelle Alternative zu resistiven AlGaN:Mg Schichten wurden Tunneldioden (TJ) zur Löcherinjektion implementiert. Durch die anfängliche Optimierung individueller Komponenten wie der Zusammensetzung einer Zwischenschicht oder der Dotierlevel an der Grenzfläche, wurde die erste Demonstration AlGaN-basierter TJLEDs ermöglicht, die mit metallorganischer Gasphasenepitaxie gewachsen wurden. Auf dieser Basis wurde die Zwischenschichtdicke gezielt variiert, um Polarisationsladungen an der Grenzfläche zur Reduktion der Raumladungszonenbreite auszunutzen und die Tunnelwahrscheinlichkeit zu erhöhen. Mit 8 nm GaN Zwischenschichten wurde eine Spannungsreduktion um 20 V erreicht, sowie TJ-LEDs mit externer Quanteneffizienz von 2,3% und Emissionsleistung von 6,6 mW bei 268 nm und 0,26 mW bei 232 nm