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Group III-nitride materials, gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN) have direct band gaps with band gap energies ranging from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN) wave-lengths, covering the entire spectral range from 0.7 eV to 6.2 eV upon alloying. The invention of the GaN-based blue LEDs, for which the Nobel prize in Physics was awarded in 2014, has opened up avenues for exploration of III-Nitride mate-rial and device technologies, and has inspired generations of researchers in the semiconductor field. Group III-nitrides have also been demonstrated to be among the most promising semiconductors for next generation of efficient high-power, high-temperature and high-frequency electronic devices. The need to build a sustainable and efficient energy system motivates the development of vertical GaN transistors and diodes for applications with power ratings of 50-150 kW, e.g., in electric vehicles and industrial inverters. The key is to grow GaN layers with low concentration of defects (impurities and dislocations), which enables an expansion in both voltage and current ratings and reduction of cost. Despite intense investigations and impressive advances in the field, defects are still a major problem which hinders exploiting the full potential of GaN in power electronics. The aim of this thesis is to perform an in-depth investigation of the growth of GaN and AlGaN under several nucleation mechanisms provided by different underlying substrates. In that regard, four different epitaxial approaches based on different nucleation schemes have been studied: (i) growth of planar GaN layers trough NWs reformation. We investigated GaN layers with different thicknesses on reformed GaN NW templates and highlight this approach as an alternative to the expensive HVPE GaN substrates. The sapphire used as a substrate limits to some extent the reduction of threading dislocations, however, the resulting GaN material pr, Grupp III-nitrider är halvledare med direkta bandgap där bandgapsenergierna spänner från det infraröda till djupt ultravioletta banden. Tillräknade i den gruppen är galliumnitrid (GaN), aluminiumnitrid (AlN) samt indiumnitrid (InN) som tillsammans kan realisera alla bandgapsenergier från 0.7 eV (InN) till 6.2 eV (AlN) genom legering. Utvecklingen av GaN-baserade blå LED:er, som tilldelades 2014 års Nobelpris i fysik, har öppnat många nya dörrar inom III-nitridforskning och skapat många nya tillämpningar av halvledarmaterial. Till exempel har grupp III-nitrider påvisats mycket lovande som nästa generations högeffekts- och högfrekvenskomponenter inom elektroniken. Efterfrågan på hållbara och effektiva energisystem har drivit utvecklingen av vertikala GaN-transistorer och dioder för tillämpning inom 50-150 kW omfånget, så som elektriska fordon och industriella växelriktare. Nyckeln ligger i att växa lager av GaN med låg konsentration av defekter (orenheter och dislokations), som både kan öka spänningsfönstret och strömstyrkan och samtidigt reducera kostnaden. Defekter har däremot varit svåra att kontrollera och trots mänger av framsteg är det fortfarande den stora utmaningen för att fullt kunna utnyttja potentialen av GaN inom elektronik. Målet i denna avhandling är att utföra fördjupade undersökningar av GaN- och AlGaN-tillväxt vid olika betingade tillväxtmekanismer som funktion av tillväxtsubstrat. Fyra olika epitaxiella tillvägagångsätt har studerats med tillhörande nukleationsmekanismer. (i) Tillväxt av plana GaN-lager genom nanotråd-reformation. Vi har undersökt GaN med olika tjocklekar på omformade GaN nanotråd-mallar och påvisar att metoden är ett alternativ till dyra HVPE GaN-substrat. Safiren som används som substrat begränsar till viss del en reducering av slingrande dislokationer men den resulterande GaN-ytan är jämn och har en termisk ledningsförmåga nära GaN i bulk. (ii) Homoepitaxiell GaN-tillväxt. Vi utvecklade en hetväggs MOCVD-epitaxi som möjliggör en l, Funding agencies: Swedish Research Council (VR) under Grant No. 2016 − 00889, (ii) the Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program, Grant No. 2016 − 05190, (iii) the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No. 2009 − 00971, and (iv) the Swedish Foundationfor Strategic Research (SSF), under Grant No. EM16 − 0024.

In this work, we present the fabrication and analysis of fully-vertical GaN FinFETs with a gate length of 550 nm. The devices with fin widths of around 100 nm reveal normally-OFF behavior and subthreshold swings (SSs) very close to the 60-mV/dec limit. Low hysteresis values indicate low defect densities at the oxide/GaN interface. The devices exhibit low specific ON-resistances at a maximum of around 90 V breakdown voltage, which is reasonable for the drift layer thickness of 1 mu m. The capacitances in the devices were modeled and identified with capacitance voltage measurements, which could also be used to approximate the effective and field effect mobility in the channel and reveal to around 164 and 54 cm(2)/(Vs) at higher gate voltages, which is a slight improvement to reported values for similar devices., Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA)through the Competence Center Program [2022-03139]; Swedish Research Council (VR) [2016-00889, 2022-04812]; Swedish Foundation for Strategic Research [EM16-0024]

Group-III nitride materials, gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN) have direct band gaps with band gap energies ranging from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN) wavelengths and covering the entire spectral range from 0.7 eV to 6.2 eV upon alloying. The invention of the GaN-based blue LEDs, for which the Nobel prize in Physics was awarded in 2014, has opened up avenues for exploration of IIINitride material and device technologies and has inspired generations of researchers in the semiconductor field. Group-III nitrides have also been demonstrated to be among the most promising semiconductors for next generation of efficient high-power, high-temperature and high-frequency electronic devices. The need to build a sustainable and efficient energy system motivates the development of vertical GaN transistors and diodes for applications with power ratings of 50-150 kW, e.g., in electric vehicles and industrial inverters. The key is to grow GaN layers with low concentration of defects (impurities and dislocations), which enables an expansion in both voltage and current ratings and reduction of cost. Despite intense investigations and impressive advances in the field, defects are still a major problem hindering exploiting the full potential of GaN in power electronics. This Licentiate thesis focuses on the development of two different epitaxial approaches in MOCVD for reducing dislocation densities in GaN with controlled doping for power device applications: i) growth of planar GaN layers trough NWs reformation, which can be further exploited as templates for a subsequent growth of thick drift layers and ii) homoepitaxial GaN growth. Special attention is put on understanding homoepitaxial growth under different nucleation schemes and thermal stability of GaN. We have established conditions in homoepitaxy to deliver state-of-the-art GaN material with low impurity levels combined with a reasonable growt, Funding agencies: The Swedish Research Council (VR) under Grant No. 2016-00889, The Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program, Grant No. 2016-05190, The Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No.2009-00971, The Swedish Foundation for Strategic Research (SSF), under Grant No. EM16-0024

Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 x 1015 cm-3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices., Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA) [2016-05190]; Linkoping University; Chalmers University of technology; Ericsson; Epiluvac; FMV; Gotmic; Hexagem; Hitachi Energy Research; On Semiconductor; Saab; SweGaN; aUMS; Volvo Cars; Swedish Research Council VR [2016-00889]; Swedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant [2009-00971]; NanoLund

AlxGa1-xN ternary alloys are emerging ultrawide band gap semiconductor materials for high-power electronics applications. The heat dissipation, which mainly depends on the thermal conductivity of the constituent material in the device structures, is the key for device performance and reliability. However, the reports on the thermal conductivity of AlxGa1-xN alloys are very limited. Here, we present a comprehensive study of the thermal conductivity of AlxGa1-xN in the entire Al composition range. Thick AlxGa1-xN layers grown by metal-organic chemical vapor deposition on GaN/sapphire and GaN/SiC templates are examined. The thermal conductivity measurements are done by the transient thermoreflectance method at room temperature. The effects of the Al composition, dislocation density, Si doping, and layer thickness on the thermal conductivity of AlxGa1-xN layers are thoroughly investigated. All experimental data are fitted by the modified Callaway model within the virtual crystal approximation, and the interplay between the different phonon scattering mechanisms is analyzed and discussed., Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA) [2016-05190]; Swedish Research Council [2016-00889, 2017-03714]; Swedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant SFO Mat LiU [CBET-1336464, DMR-1506159]

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Thermal conductivity of AlxGa1-xN layers with 0 <= x <= 0.96 and variable thicknesses is systematically studied by combined thermoreflectance measurements and a modified Callaway model. We find a reduction in the thermal conductivity of AlxGa1-xN by more than one order of magnitude compared to that of GaN, which indicates a strong effect of phonon-alloy scattering. It is shown that the short-mean free path phonons are strongly scattered, which leads to a major contribution of the long-mean free path phonons to the thermal conductivity. In thin layers, the long-mean free path phonons become efficiently scattered by the boundaries, resulting in a further decrease in the thermal conductivity. Also, an asymmetry of thermal conductivity as a function of Al content is experimentally observed and attributed to the mass difference between Ga and Al host atoms., Funding Agencies|Swedish Governmental Agency for innovation systems (VINOVA) under Competence Center Program [2016-05190]; Swedish Research Council VRSwedish Research Council [2016-00889, 2017-03714]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant SFO Mat LiU [2009-00971]; NSFNational Science Foundation (NSF) [CBET-1336464, DMR-1506159]

Herein, the potential of reformed GaN nanowires (NWs) fabricated by metalorganic chemical vapor deposition (MOCVD) for device-quality low-defect density templates and low-cost alternative to bulk GaN substrates is demonstrated. The effects of epilayer thickness and NW reformation conditions on the crystalline quality and thermal conductivity of the subsequent GaN epilayers are investigated. Smooth surfaces with atomically step-like morphologies with no spirals are achieved for GaN epilayers on the reformed NW templates, indicating step-flow growth mode. It is further found that annealing of the NWs at a temperature of 1030 degrees C in the presence of NH3 and H-2, followed by a coalescence done at the same temperature under planar growth conditions, leads to the most efficient screw dislocation density reduction by nearly an order of magnitude. At these optimized conditions, the growth takes place in a layer-by-layer fashion, producing a smooth surface with a root mean square (RMS) roughness of 0.12 nm. The highest thermal conductivity of k = 206 W m(-1) K-1, approaching the respective value of bulk GaN, is obtained for the optimized 2 mu m-thick GaN layer. The thermal conductivity results are further discussed in terms of the phonon-dislocation and the phonon-boundary scattering., Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program [2016-05190]; Linkoping University; Chalmers University of Technology; ABB; Ericsson; Epiluvac; FMV; Gotmic; On Semiconductor; Saab; SweGaN; UMS; Swedish Research Council VRSwedish Research Council [2016-00889]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [FL12-0181, RIF14-055, EM16-0024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkping University [2009-00971]