Im Mittelpunkt der vorliegenden Arbeit steht die Herstellung ultradünner epitaktischer Galliumnitrid-Schichten auf einem Siliziumkarbid-Substrat mit dem Verfahren der ionenstrahlgestützten Molekularstrahlepitaxie. Für die Analyse der Oberflächentopographie der Galliumnitrid-Schichten direkt nach der Abscheidung – ohne Unterbrechung der Ultrahochvakuum-Bedingungen – wurde ein Rastersondenmikroskop in die Anlage integriert. Als weitere Hauptanalysenmethode wurde die Reflexionsbeugung hochenergetischer Elektronen zur Bestimmung der Oberflächenstrukturen in situ während der Schichtabscheidung eingesetzt. Weiterhin wurden die Galliumnitrid-Schichten hinsichtlich ihrer strukturellen Eigenschaften mittels Röntgenstrahl-Diffraktometrie, Röntgen-Photoelektronenspektroskopie und Transmissionselektronenmikroskopie ex situ charakterisiert. Wesentliches Ziel dieser Arbeit war die Herausstellung des Einflusses maßgeblicher Abscheidungsparameter (vor allem Substrattemperatur und Gallium-Depositionsrate) auf die Schichteigenschaften sowie die Optimierung dieser Wachstumsparameter. Besonderes Augenmerk lag auf der Untersuchung der Auswirkungen des Stickstoffion-zu-Galliumatom-Verhältnisses und des Einflusses der niederenergetischen Ionenbestrahlung auf das Galliumnitrid-Schichtwachstum im Frühstadium. Dies betrifft hauptsächlich den Wachstumsmodus (zwei- oder dreidimensional) und die Bildung der hexagonalen oder der kubischen Phase.:Kapitel 1 1 Einführung 1 Kapitel 2 5 Grundlagen 5 2.1 Kristallstruktur und Eigenschaften von Galliumnitrid 5 2.2 Wechselwirkung niederenergetischer Ionen mit der Oberfläche 8 2.2.1 Energiefenster für die optimale ionenstrahlgestützte Epitaxie 9 2.3 Einfluss der Teilchenenergie auf die Oberflächenmobilität 11 2.3.1 Strukturzonenmodelle 11 2.3.2 Thermisch induzierte Oberflächenmobilität 13 2.3.3 Ballistisch induzierte Oberflächenmobilität 14 2.4 Herstellungsverfahren 15 2.4.1 Metallorganische Gasphasenepitaxie 15 2.4.2 Molekularstrahlepitaxie 16 2.4.3 Ionenstrahlgestützte Molekularstrahlepitaxie 17 2.5. Schichtwachstum 18 2.5.1 Frühstadium des Wachstums 18 2.5.2 Wachstumsmodi 20 2.5.3 Wachstum an Stufen 21 2.5.4 Ionenstrahlgestütztes GaN-Wachstum 23 2.6 Modellsystem GaN auf 6H-SiC 25 2.6.1 Epitaxiebeziehungen von GaN-Schicht und 6H-SiC(0001)-Substrat 25 2.6.2 Schichtspannungen 28 Kapitel 3 32 Experimentelle Bedingungen 32 3.1 Experimenteller Aufbau 32 3.1.1 UHV-Anlage zur ionenstrahlgestützten Abscheidung 32 3.1.2 Gallium-Effusionszelle 33 3.1.3 Stickstoff-Hohlanoden-Ionenquelle 34 3.1.4 RHEED-System 35 3.1.5 UHV-STM 37 3.2 Probenherstellung 40 3.2.1 Vorbehandlung 40 3.2.2 Abscheide- und Ionenstrahlparameter 41 3.3 Charakterisierung 43 3.3.1 Kristallographische Struktur 44 Reflexionsbeugung hochenergetischer Elektronen 44 Röntgenstrahl-Diffraktometrie 49 Röntgenstrahl-Reflektometrie 52 Transmissionselektronenmikroskopie 53 3.3.2 Oberflächentopographie 54 Rastertunnelmikroskopie 54 Rasterelektronenmikroskopie 56 3.3.3 Chemische Zusammensetzung 57 Röntgen-Photoelektronenspektroskopie 57 Kapitel 4 59 Ergebnisse und Diskussion 59 4.1 GaN-Wachstum auf 6H-SiC(0001) bei Variation des Ion/Atom-Verhältnisses 59 4.1.1 Oberflächenstruktur und Oberflächentopographie 59 4.1.2 Kristallographische Struktur und chemische Zusammensetzung 63 4.1.3 Diskussion: Einfluss des I/A-Verhältnisses 70 4.2 Inselwachstum: Oberflächenstruktur und Oberflächentopographie 74 4.2.1 Einfluss des I/A-Verhältnisses 75 4.2.2 Einfluss der Substrattemperatur 75 4.2.3 Einfluss der Depositionsdauer 78 4.2.4 Diskussion: Einfluss von I/A-Verhältnis, Depositionsdauer und Substrattemperatur auf die Oberflächentopographie der 3D-GaN-Schichten 82 4.3 Inselwachstum: Kristallographische Struktur und Morphologie 84 4.3.1 Morphologie der inselförmigen GaN-Schichten 84 4.3.2 Gitteranpassung und mechanische Spannungen 86 4.3.3 Diskussion: Kubisches GaN und Spannungsaufbau 88 4.4 Zweidimensionales Schichtwachstum: Oberflächenstruktur und Oberflächentopographie 90 4.4.1 Einfluss des I/A-Verhältnisses 90 4.4.2 Oberflächentopographie ultradünner 2D-Schichten 93 4.4.3 Einfluss der Depositionsdauer 96 4.4.4 Einfluss der Substrattemperatur 99 4.4.5 Diskussion: Einfluss von I/A-Verhältnis, Depositionsdauer und Substrattemperatur auf die Oberflächentopographie der 2D-GaN-Schichten 102 4.5 Zweidimensionales Schichtwachstum: Kristallographie und Morphologie 106 4.5.1 Einfluss des I/A-Verhältnisses 106 4.5.2 Einfluss der Depositionsdauer 107 4.5.3 Einfluss der Substrattemperatur 109 4.5.4 Morphologie und Gitterfehlanpassung 112 4.5.5 Koaleszenz von Inseln im Anfangswachstum 116 4.5.6 Diskussion: Einfluss von I/A-Verhältnis, Depositionsdauer und Substrattemperatur auf Struktur und Morphologie der 2D-GaN-Schichten 119 4.6 Moduswechsel von 3D- zu 2D-Wachstum bei sequenzieller Abscheidung unter Verringerung des I/A-Verhältnisses 124 Kapitel 5 128 Zusammenfassung und Ausblick 129 Anhang A 134 Anhang B 136 Literaturverzeichnis 137 Danksagung 145 Veröffentlichungen 146 Lebenslauf des Autors 147 Selbständigkeitserklärung 148

Microsensor technologies based on nitride semiconductor materials were developed as options for improved exhaust gas sensors in diesel exhaust systems. The main goals were to develop new sensors that can meet the requirements given by Peugeot PSA to meet upcoming EU emissions regulations for NO, NO2, and NH3 detection. Two different sensor technologies were developed based on Schottky junction and high electron mobility transistor (HEMT) devices. Novel materials such as BGaN and BGaN/GaN superlattice structures are explored. For each device, a comprehensive analytical model is developed and simulations are carried out to optimize and design the sensor devices. Materials growth is then conducted for the different semiconductor layers, followed by materials characterizations to ensure high quality materials. Device prototypes are fabricated using various materials and functional layer designs. For device testing, an experimental setup is developed. Our experimental results show excellent sensitivity; we also report selectivity between NO and NO2 for the first time for these types of devices. Finally, we modify our devices for other sensing applications such as the detection of other harmful gases and pollutants in liquid environments.

In this dissertation, GaN growth on porous templates by metalorganic chemical vapor deposition (MOCVD) was studied. The motivation of this research is pursuing an effective reduction of defects in GaN by its submicron-scale and nano-scale epitaxial lateral overgrowth (ELO) on these porous templates, which included porous TiN/GaN (P-TiN), imprint lithography patterned Ti/GaN (IL-Ti), carbon-face nano-porous SiC (C-PSC), and silicon-face nano-porous SiC (Si-PSC). The porous TiN/GaN was formed in situ in MOCVD reactor by annealing a Ti-covered GaN seed layer. This simplicity makes the GaN ELO on the P-TiN more cost-efficient than the conventional ELO which requires ex situ photolithography and/or etching. Both the GaN nano-ELO and the GaN micron-ELO could be realized on P-TiN by controlling the GaN nucleation scheme. The reduction efficacy of edge threading dislocation (TD) was ~15 times. The optical characterization indicated that the non-radiative point-defects in GaN grown were reduced significantly on the P-TiN.The imprint lithography patterned Ti/GaN had uniformly distributed submicron Ti pads on GaN seed layer. These Ti pads acted as GaN ELO masks. The TD reduction efficacy of the IL-Ti was only ~2 due to the low coverage of Ti (~25%) on the GaN seed layer and the low pressure (30 Torr) employed during GaN ELO. Even with a small reduction of TDs, the point-defects in GaN were effectively lowered by the IL-Ti. Hydrogen polishing, sacrificial oxidation, and chemical mechanical polishing were employed to remove surface damage on the PSC substrates. Nitrogen-polarity GaN grown on the C-PSC was highly dislocated because the rough surface of C-PSC induced strong misorientation between GaN nucleation islands. The efficacy of Si-PSC on defect reduction primarily depended on the GaN nucleation schemes. A high density of GaN nano-nucleation-islands was required to realize the GaN nano-ELO extensively. With such a nucleation scheme, the GaN grown on Si-PSC had a ~20 times reduction on the density of the mixed and screw TDs compared with control sample. This growth method is promising for effective defect reduction within a small GaN thickness. Reducing the GaN nucleation density further lowered the TD density but also diminished the efficacy of Si-PSC. These results were explained by a growth model based on the mosaic structure of GaN.

In this thesis, we first present a brief overview of various theoretical approaches used to examine the electronic structure of defects in GaN. Using the recently developed hybrid density functional theory (HSE06) along with the experimental measurements, we propose a new explanation of the nature of the yellow luminescence band in carbon-doped GaN. We conduct a systematic study of electronic and optical properties of defects (Carbon, Oxygen, Silicon related) that are candidates for the origin of yellow luminescence. We show that the CN-ON complex is significantly more likely to form compared to isolated carbon configurations. In contrast to the properties of the isolated carbon acceptor, calculated defect levels and optical transitions involving deep level of the CN-ON complex agree quite well with our thermal luminescence quenching data as well as with the experimentally measured C-doped GaN luminescence spectra. Hence, the CN-ON complex, rather than isolated C impurity, is more likely to resolve a long-standing problem of the yellow luminescence in GaN.

OPTICAL CHARACTERIZATION OF INGAN HETEROSTRUCTURES FOR BLUE LIGHT EMITTERS AND VERTICAL CAVITY LASERS: EFFICIENCY AND RECOMBINATION DYNAMICS By Serdal Okur, Ph.D. A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2014. Major Director: Ümit Özgür, Associate Professor, Electrical and Computer Engineering This thesis explores radiative efficiencies and recombination dynamics in InGaN-based heterostructures and their applications as active regions in blue light emitters and particularly vertical cavities. The investigations focus on understanding the mechanism of efficiency loss at high injection as well as developing designs to mitigate it, exploring nonpolar and semipolar crystal orientations to improve radiative efficiency, integration of optimized active regions with high reflectivity dielectric mirrors in vertical cavity structures, and achieving strong exciton-photon coupling regime in these microcavities for potential polariton lasing. In regard to active regions, multiple double heterostructure (DH) designs with sufficiently thick staircase electron injection (SEI) layers, which act as electron coolers to reduce the overflow of hot electrons injected into the active region, were found to be more viable to achieve high efficiencies and to mitigate the efficiency loss at high injection. Such active regions were embedded in novel vertical cavity structure designs with full dielectric distributed Bragg reflectors (DBRs) through epitaxial lateral overgrowth (ELO), eliminating the problems associated with semiconductor bottom DBRs having narrow stopbands and the cumbersome substrate removal process. Moreover, the ELO technique allowed the injection of carriers only through the high quality regions with substantially reduced threading dislocation densities compared to regular GaN templates grown on sapphire. Reduced electron-hole wavefunction overlap in polar heterostructures was shown to hamper the efficiency of particularly thick active regions (thicker than 3 nm) possessing three-dimensional density of states needed for higher optical output. In addition, excitation density-dependent photoluminescence (PL) measurements showed superior optical quality of double heterostructure (3 nm InGaN wells) active regions compared to quantum wells (2 nm InGaN wells) suggesting a minimum limit for the active region thickness. Therefore, multiple relatively thin but still three dimensional InGaN active regions separated by thin and low barriers were found to be more efficient for InGaN light emitters. Investigations of electroluminescence from light emitting diodes (LEDs) incorporating multi DH InGaN active regions (e.g. quad 3 nm DH) and thick SEIs (two 20 nm-thick InGaN layers with step increase in In content) revealed higher emission intensities compared to LEDs with thinner or no SEI. This indicated that injected electrons were cooled sufficiently with thicker SEI layers and their overflow was greatly reduced resulting in efficient recombination in the active region. Among the structures considered to enhance the quantum efficiency, the multi-DH design with a sufficiently thick SEI layer constitutes a viable approach to achieve high efficiency also in blue lasers. Owing to its high exciton binding energy, GaN is one of the ideal candidates for microcavities exploiting the strong exciton-photon coupling to realize the mixed quasiparticles called polaritons and achieve ideally thresholdless polariton lasing at room temperature. Angle-resolved PL and cathodoluminescence measurements revealed large Rabi splitting values up to 75 meV indicative of the strong exciton-photon coupling regime in InGaN-based microcavities with bottom semiconductor AlN/GaN and a top dielectric SiO2/SiNxDBRs, which exhibited quality factors as high as 1300. Vertical cavity structures with all dielectric DBRs were also achieved by employing a novel ELO method that allowed integration of a high quality InGaN cavity active region with a dielectric bottom DBR without removal of the substrate while forming a current aperture through the ideally defect-free active region. The full-cavity structures formed as such were shown to exhibit clear cavity modes near 400 and 412 nm in the reflectivity spectrum and quality factors of 500. Although the polar c-plane orientation has been the main platform for the development of nitride optoelectronics, significant improvement of the electron and hole wavefunction overlap in nonpolar and semipolar InGaN heterostructures makes them highly promising candidates for light emitting devices provided that they can be produced with good crystal quality. To evaluate their true potential and shed light on the limitations put forth by the structural defects, optical processes in several nonpolar and semipolar orientations of GaN and InGaN heterostructures were investigated. Particularly, stacking faults were found to affect significantly the optical properties, substantially influencing the carrier dynamics in nonpolar (1-100), and semipolar (1-101) and (11-22)GaN layers. Carrier trapping/detrapping by stacking faults and carrier transfer between stacking faults and donors were revealed by monitoring the carrier recombination dynamics at different temperatures, while nonradiative recombination was the dominant process at room temperature. Although it is evident that nonpolar (1-100)GaN and semipolar (11-22)GaN require further improvement of material quality, steady-state and time-resolved PL measurements support that (1-101)-oriented GaN templates and InGaN active regions exhibit optical performance comparable to their highly optimized polar c-plane counterparts, and therefore, are promising for vertical cavities and light emitting device applications.

This thesis explores the improvement of quantum efficiencies for InGaN/GaN heterostructures and their applications in light-emitting diodes (LEDs) and vertical cavity surface-emitting lasers (VCSELs). Different growth approaches and structural designs were investigated to identify and address the major factors limiting the efficiency. (1) Hot electron overflow and asymmetrical electron/hole injection were found to be the dominant reasons for efficiency degradation in nitride LEDs at high injection; (2) delta p-doped InGaN quantum barriers were employed to improve hole concentration inside the active region and therefore improve hole injection without sacrificing the layer quality; (3) InGaN active regions based on InGaN multiple double-heterostructures (DHs) were developed to understand the electron and hole recombination mechanisms and achieve high quantum efficiency and minimal efficiency droop at high injection; (4) the effect of stair-case electron injectors (SEIs) has been investigated with different active region designs and SEIs with optimized thickness greatly mitigated electron overflow without sacrificing material quality of the active regions. The active regions showing promising performance in LEDs were incorporated into VCSEL designs. Hybrid VCSEL structures with bottom semiconductor AlN/GaN and a top dielectric SiO2/SiNx DBRs have been investigated, and quality factors as high as 1300 have been demonstrated. Finally, VCSEL structures with all dielectric DBRs have been realized by employing a novel ELO-GaN growth method that allowed integration of a high quality InGaN cavity active region with a dielectric bottom DBR without removal of the substrate while forming a current aperture through the ideally dislocation-free region. The full-cavity structures formed as such exhibited quality factors 500 across the wafer.

This thesis uses the techniques of atomic force microscope (AFM) and conductive AFM (C-AFM) to study the conduction properties of n-type GaN films. A total of 16 samples were examined and grouped according to their surface morphologies and conduction behaviors. The most common type of surface morpliology was that of Ga-rich samples having undulating "hillocks" with interspersed holes. Although most of the samples had this common morphology, their local conduction behaviors were not all similar. Local I-V spectra of the tip-sample Schottky contact could be grouped according to three major types: low leakage, high leakage, and "p-type". The highest quality samples with low leakage were usually grown at moderate temperatures (~650°C). For such samples, localized leakage only occurred at screw dislocations located at small pits terminating surface hillocks. I-V spectra taken on and off such hillocks were fit in forward bias to determine whether field emission or Frenkel-Poole conduction were dominant. Although field emission is a good fit compared to Frenkel-Poole, yielding reasonable values for the barrier height, the results are not yet conclusive without variable temperature studies.

III-nitride semiconductors are of great interest owing to their commercial and military applications due to their optoelectronic and mechanical properties. They have been synthesized successfully by many growth methods. Among them, molecular beam epitaxy (MBE) is a promising epitaxial growth method owing to precise control of growth parameters, which significantly affect the film properties, composition, and thickness. However, the understanding of growth mechanism of III-nitride materials grown in this growth regime is far from being complete.In this dissertation, GaN and AIGaN growth mechanism under metal-rich conditions were investigated. The Ga surface desorption behavior during the growth was investigated systematically using reflection high-energy electron diffraction (RHEED). It was found that desorption of Ga atoms from the (0001) GaN surfaces under different III-V ratios deviates from the zeroth-order kinetics in that the desorption rate is independent of the coverage of adsorbed atoms. The desorption energies of Ga are determined to be 2.76 eV with the Ga coverage closing to 100%, 1.89 eV for a ~45% coverage, and 0.82 eV for a 10% coverage, as monitored by the change of the RHEED specular beam intensity during growth. In addition, the GaN surface morphology under different III-V ratios on porous templates matches the dependence of Ga desorption energy on the metal coverage, and III/V ratio dominates the growth mode. In a related AIGaN growth mechanism study, a competition between A1 and Ga atoms to incorporate into the film was found under metal-rich conditions. Employing this mechanism, A1xGa1-xN layers with precisely controlled A1 mole fraction, x in the range 0xxGa1-xN films was determined to be about 1 eV. The A1xGa1-xN layers grown under metal-rich conditions, as compared to that under N-rich conditions, have a better structural and optical quality. Employing A1xGa1-xN layers grown under metal-rich conditions, a lateral geometry GaN/A1GaN MQW-based photodetector was fabricated. It exhibited a flat and narrow spectral response in the range of 297~352 nm in the backillumination configuration.

Different methods were developed for the preparation of bulk ZnO substrates. Remarkable improvement on the surface, optical and crystalline quality of the bulk ZnO substrate was achieved. ZnO substrates with an atomically flat surface exhibiting terrace-like features were used as a substrate for GaN grown by MBE. High-resolution x-ray diffraction and low temperature PL results show that similar high quality GaN layers can be achieved on both annealed O-face and Zn-face ZnO substrates. The prospect of the device applications of GaN epitaxy on ZnO, including AlGaN/GaN MODFET structure on ZnO and GAN/ZnO based p-n junction were discussed.

Time-resolved photoluminescence (TRPL) measurements paired with steady-state photoluminescence (SSPL) measurements can help to determine the PL lifetime, shape and position of unresolved bands, capture coefficients, and concentrations of free electrons and defects.PL bands that are obscured in the SSPL spectra can be accurately revealed by TRPL measurements. TRPL measurements are able to show if the PL band originates from an internal transition between different states of the same defect. The main defect-related PL bands in high-purity GaN grown by hydride vapor phase epitaxy (HVPE) which have been investigated are the ultraviolet, blue, green, yellow and red luminescence bands (UVL, BL, GL, YL and RL, respectively). The concentration of free electrons can be calculated from these measurements providing a contactless alternative to the Hall effect method. The lifetime of most defect-related PL bands decreases with increasing temperature. However, the lifetime of the GL band, with a maximum at 2.4 eV observed in the SSPL spectra only at high excitation intensity, increases as a function of temperature. By analyzing the PL intensity decay, the origin of the GL can be attributed to an internal transition from an excited state of the CN defect, which behaves as an optically generated giant trap, to the 0/+ level of the same defect. This first observation of an optically generated giant trap was detected by analyzing the cubic temperature dependence of the electron capture coefficient. Excitation intensity and temperature dependent studies on Mg-doped GaN grown by HVPE were performed. The position of the UVL (3.2 eV) peak blue-shifts with increasing excitation intensity, which can be explained by the presence of potential fluctuations. The BL peak (2.8 eV) also blue-shifts with increasing excitation intensity, and red-shifts as a function of temperature. These shifts can be explained by the transitions originating from a deep-donor to the MgGa acceptor, and the corresponding donor-acceptor pair nature.