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Real structure of heteroepitaxial GaN and AlGaN layers is studied by diffuse x-ray scattering. A new developed method based on Monte Carlo simulation enabling to determine densities of threading dislocations in c-plane GaN and stacking faults in a-plane GaN is presented. The results of Monte Carlo simulations are compared with ones obtained by use of other conventional techniques. The advantages and limitations of the new method are discussed in detail. The methods accuracy is estimated as about 15%. We have shown that our method is a reliable tool for threading dislocations and stacking faults densities determination.

III-nitridy běžně krystalizují v hexagonální (wurtzitové) struktuře, zatímco kubická (sfaleritová) struktura je metastabilní a má pouze mírně vyšší energii. Jejich fyzikální vlastnosti jsou silně ovlivněny přítomností rozsáhlých defektů, které jsou v těchto dvou strukturách od sebe odlišné. U wurtzitových nitridů se jedná primárně o vláknové dislokace. Některé vláknové dislokace tvoří hluboké energetické stavy v zakázaném pásu, kterými ovlivňují elektrické a optoelektronické vlastnosti těchto materiálů. Oproti tomu, kubické nitridy obsahují množství vrstevných chyb, které představují lokální transformace do stabilnější wurtzitové struktury. Cílem této práce je charakterizovat rozsáhlé defekty v obou krystalových strukturách pomocí elektronové mikroskopie, mikroskopie atomárních sil a rentgenové difrakce. Prokázali jsme, že vzorky GaN/AlN a AlN s orientací (0001) rostlé na substrátu Si (111) pomocí epitaxe z organokovových sloučenin obsahují velkou hustotu vláknových dislokací. Nejčastější jsou dislokace s Burgersovým vektorem s komponentou ve směru a wurtzitové struktury, následované dislokacemi s Burgersovým vektorem s komponentou ve směru a+c, zatímco dislokace s Burgersovým vektorem s c komponentou jsou relativně vzácné. Pravděpodobný původ vláknových dislokací je diskutován v souvislosti s různými mechanismy růstu těchto vrstev. Prizmatické vrstevné chyby byly nalezeny v tenkých nukleačních vrstvách AlN, ale v tlustších vrstvách již nebyly přítomny. Na rozhraní AlN / Si byla nalezena amorfní vrstva složená ze SiNx a částečně taky z AlN. Navrhujeme, že by tato amorfní vrstva mohla hrát významnou roli při relaxaci misfitového napětí. Analýza elektrické aktivity rozsáhlých defektů v AlN byla provedena pomocí měření proudu indukovaného elektronovým svazkem. Zjistili jsme, že vláknové dislokace způsobují slabý pokles indukovaného proudu. Díky jejich vysoké hustotě a rovnoměrnému rozložení však mají větší vliv na elektrické vlastnosti, než mají V-defekty a jejich shluky. Topografické a krystalografické defekty byly studovány na nežíhaných a žíhaných nukleačních vrstvách kubického GaN deponovaných na 3C-SiC (001) / Si (001) substrátu. Velikost ostrůvků na nežíhaných vzorcích se zvyšuje s tloušťkou nukleační vrstvy a po žíhání se dále zvětšuje. Po žíhání se snižuje pokrytí substrátu u nejtenčích nukleačních vrstev v důsledku difúze a desorpce (nebo leptání atmosférou reaktoru). Vrstevné chyby nalezené ve vrstvách GaN, poblíž rozhraní se SiC, byly většinou identifikovány jako intrinsické a byly ohraničené Shockleyho parciálními dislokacemi. Jejich původ byl diskutován, jako i vliv parciálních dislokací na relaxaci misfitového napětí. Díky velkému množství vrstevných chyb byly podrobněji studovány jejich interakce. Na základě našich zjištění jsme vyvinuli teoretický model popisující anihilaci vrstevných chyb v kubických vrstvách GaN. Tento model dokáže předpovědět pokles hustoty vrstevných chyb se zvyšující se tloušťkou vrstvy.

In this thesis I present and discuss the results of optical spectroscopy performed on InGaN/GaN single and multiple quantum well (QW) structures. I report on the optical properties of InGaN/GaN single and multiple QW structures, measured at high excitation power densities. I show a correlation exists between the reduction in PL efficiency at high excitation power densities, the phenomenon so-called ``efficiency-droop'', and a broadening of the PL spectra. I also show a distinct change in recombination dynamics, measured by time-resolved photoluminescence (PL), which occurs at the excitation power densities for which efficiency droop is measured. The broadening of the PL spectra at high excitation power densities is shown to occur due to a rapidly redshifting, short-lived high energy emission band. The high energy emission band is proposed to be due to the recombination of weakly localised/delocalised carriers occurring as a consequence of the progressive saturation of the local potential fluctuations responsible for carrier localisation, at high excitation power densities. I report on the effects of varying threading dislocation (TD) density on the optical properties of InGaN/GaN multiple QW structures. No systematic relationship exists between the room temperature internal quantum efficiency (IQE) and the TD density, in a series of nominally identical InGaN/GaN multiple QWs deposited on GaN templates of varying TD density. I also show the excitation power density dependence of the PL efficiency, at room temperatures, is unaffected for variation in the TD density between 2 x107 and 5 x109 cm-2. The independence of the optical properties to TD density is proposed to be a consequence of the strong carrier localisation, and hence short carrier diffusion lengths. I report on the effects of including an InGaN underlayer on the optical and microstructural properties of InGaN/GaN multiple QW structures. I show an increase in the room temperature IQE occurs for the structure containing the InGaN underlayer, compared to the reference. I show using PL excitation spectroscopy that an additional carrier transfer and recombination process occurs on the high energy side of the PL spectrum associated with the InGaN underlayer. Using PL decay time measurements I show the additional recombination process for carriers excited in the underlayer occurs on a faster timescale than the recombination at the peak of the PL spectrum. The additional contribution to the spectrum from the faster recombination process is proposed as responsible for the increase in room temperature IQE.

Despite enormous efforts and investments, the efficiency of InGaN-based green and yellow-green light emitters remains relatively low, and that limits progress in developing full color display, laser diodes, and bright light sources for general lighting. The low efficiency of light emitting devices in the green-to-yellow spectral range, also known as the “Green Gap”, is considered a global concern in the LED industry. The polar c-plane orientation of GaN, which is the mainstay in the LED industry, suffers from polarization-induced separation of electrons and hole wavefunctions (also known as the “quantum confined Stark effect”) and low indium incorporation efficiency that are the two main factors that contribute to the Green Gap phenomenon. One possible approach that holds promise for a new generation of green and yellow light emitting devices with higher efficiency is the deployment of nonpolar and semi-polar crystallographic orientations of GaN to eliminate or mitigate polarization fields. In theory, the use of other GaN planes for light emitters could also enhance the efficiency of indium incorporation compared to c-plane. In this thesis, I present a systematic exploration of the suitable GaN orientation for future lighting technologies. First, in order to lay the groundwork for further studies, it is important to discuss the analysis of processes limiting LED efficiency and some novel designs of active regions to overcome these limitations. Afterwards, the choice of nonpolar orientations as an alternative is discussed. For nonpolar orientation, the (1-100)-oriented (m-plane) structures on patterned Si (112) and freestanding m-GaN are studied. The semi-polar orientations having substantially reduced polarization field are found to be more promising for light-emitting diodes (LEDs) owing to high indium incorporation efficiency predicted by theoretical studies. Thus, the semi-polar orientations are given close attention as alternatives for future LED technology. One of the obstacles impeding the development of this technology is the lack of suitable substrates for high quality materials having semi-polar and nonpolar orientations. Even though the growth of free-standing GaN substrates (homoepitaxy) could produce material of reasonable quality, the native nonpolar and semi-polar substrates are very expensive and small in size. On the other hand, GaN growth of semi-polar and nonpolar orientations on inexpensive, large-size foreign substrates (heteroepitaxy), including silicon (Si) and sapphire (Al2O3), usually leads to high density of extended defects (dislocations and stacking faults). Therefore, it is imperative to explore approaches that allow the reduction of defect density in the semi-polar GaN layers grown on foreign substrates. In the presented work, I develop a cost-effective preparation technique of high performance light emitting structures (GaN-on-Si, and GaN-on-Sapphire technologies). Based on theoretical calculations predicting the maximum indium incorporation efficiency at θ ~ 62º (θ being the tilt angle of the orientation with respect to c-plane), I investigate (11-22) and (1-101) semi-polar orientations featured by θ = 58º and θ = 62º, respectively, as promising candidates for green emitters. The (11-22)-oriented GaN layers are grown on planar m-plane sapphire, while the semi-polar (1-101) GaN are grown on patterned Si (001). The in-situ epitaxial lateral overgrowth techniques using SiNx nanoporous interlayers are utilized to improve the crystal quality of the layers. The data indicates the improvement of photoluminescence intensity by a factor of 5, as well as the improvement carrier lifetime by up to 85% by employing the in-situ ELO technique. The electronic and optoelectronic properties of these nonpolar and semi-polar planes include excitonic recombination dynamics, optical anisotropy, exciton localization, indium incorporation efficiency, defect-related optical activities, and some challenges associated with these new technologies are discussed. A polarized emission from GaN quantum wells (with a degree of polarization close to 58%) with low non-radiative components is demonstrated for semi-polar (1-101) structure grown on patterned Si (001). We also demonstrated that indium incorporation efficiency is around 20% higher for the semi-polar (11-22) InGaN quantum wells compared to its c-plane counterpart. The spatially resolved cathodoluminescence spectroscopy demonstrates the uniform distribution of indium in the growth plane. The uniformity of indium is also supported by the relatively low exciton localization energy of Eloc = 7meV at 15 K for these semi-polar (11-22) InGaN quantum wells compared to several other literature reports on c-plane. The excitons are observed to undergo radiative recombination in the quantum wells in basal-plane stacking faults at room temperature. The wurtzite/zincblende electronic band-alignment of BSFs is proven to be of type II using the time-resolved differential transmission (TRDT) method. The knowledge of band alignment and degree of carrier localization in BSFs are extremely important for evaluating their effects on device properties. Future research for better understanding and potential developments of the semi-polar LEDs is pointed out at the end.