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We present an innovative approach to integrate arrays of isolated, strain-free GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donor-bound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that a reduction of threading dislocations is achieved in their top part.

A transmission electron microscopy of pyramidal inversion domains induced by Mg doping in MOVPE and bulk GaN is presented. Based on high resolution observations and EDX analysis, two atomic models are proposed for the Mg-rich (0001) inversion domain boundaries. These structural defects appearing for Mg concentrations in the 10(19) cm(-3) range are shown to be possible origins for the auto-compensation and the blue luminescence.

A transmission electron microscopy study of a wide range of p-type GaN samples reveals that high Mg doping has a strong influence on the polarity of GaN. The main characteristic of Mg-doped metal organic vapour phase epitaxy (MOVPE) and bulk GaN is the presence of pyramidal inversion domains (PIDs). It is shown that the appearance of PIDs is correlated with a decrease of the free hole concentration and with the appearance of the blue photoluminescence band which is characteristic of MOVPE-grown Mg-doped GaN. A tentative model based on electrostatic considerations is proposed for this blue luminescence band. (C) 2002 Elsevier Science B.V. All rights reserved.

International audience; We study the influence of defect states (shallow donors and deep acceptors) on the carrier relaxation dynamics of gallium nitride in the picosecond regime for different excitation intensities and different lattice temperatures. Time-resolved luminescence, degenerate, and nondegenerate four-wave mixing experiments show a saturation threshold in the blue and yellow spectral region, which is found to disappear for lattice temperatures below 200 K. When analyzing all these results in the frame of a rate-equation model, we give a relaxation scenario for the carriers, the lifetimes of the population of the different states, and identify radiative and nonradiative transitions. After filling defect states by an optical excitation, the ambipolar diffusion coefficient of GaN is measured through degenerate four-wave mixing experiments. A law value of 0.16 cm(2)/s at room temperature is determined, indicating that defect states still influence the diffusion. Nondegenerate four-wave mixing experiments exhibit a competition between an electronical and a thermal contribution to the nonlinear susceptibility in GaN. [S0163-1829(99)00340-9].

Gallium nitride (GaN) has gained a lot of attention due to its high range of applications as solid state light emitters and detectors. However, the nitride samples often evidence deep level optically active defects affecting their performance on the high energy spectral region. This is the case for the commonly observed yellow luminescence (YL) band detected in bulk samples and thin films. Despite the large amount of literature reports on the processes that rule this emission in GaN films, there is still some controversy regarding the origin of the defects from where the YL originates. Furthermore, decreasing the dimensionality by analysing GaN nanowires provides new information on the luminescence behaviour when compared with epilayers. By using room temperature optical analysis, we aim to contribute to a deeper insight into the understanding of the YL detected in GaN NW, where YL was induced/enhanced by ion implantation and annealing treatments. YL was seen to be strongly dependent on different surface modifications, the analysis environment and illumination/irradiation excitation time. (C) 2013 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim

An AlN buffer layer and a thick-GaN layer for high-electron-mobility transistors (HEMTs) were grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The structural and morphological properties of the layers were investigated by high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) techniques. The optical quality of the thick-GaN layer was also evaluated in detail by a photoluminescence (PL) measurement. It was found that the AlN buffer layer possesses high crystal quality and an atomically flat surface with a root-mean-square (rms) roughness of 0.16 nm. The screw-and edge-type dislocation densities of the thick-GaN layer were determined as 5.4 x 10(7) and 5.0 x 10(9) cm(-2) by means of the mosaic crystal model, respectively. It was observed that the GaN layer has a smooth surface with an rms of 0.84 nm. Furthermore, the dark spot density of the GaN surface was estimated as 6.5 x 10(8) cm(-2) over a scan area of 4 mu m(2). Turkish State Planning Organization [2001K120590]; TUBITAKTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [105E066, 105A005, 106E198, 106A017]; Turkish Academy of SciencesTurkish Academy of Sciences This work was supported by the Turkish State Planning Organization (Project No. 2001K120590). This work is also supported by TUBITAK under Project Nos. 105E066, 105A005, 106E198, and 106A017. One of the authors (E. O.) also acknowledges partial support from the Turkish Academy of Sciences.

The epitaxial growth of GaN by employing a technique previously reported of vapor phase reaction between GaCl and NH_3 in an Ar carrier gas ambient is described. Single crystal layers 10--100 μm thick and -- 0. 5 cm^2 in area, were obtained on (0001) oriented sapphire substrate at deposition temperature about 1045 ℃The best undoped layers obtained had carner concentration of 2--7 X 10^ cm^ and electron mobility about 90 cm^2/V sec., III-V化合物GaN（窒化ガリウム）は，直接選移形のエネルギーバンド構造を有し，その結晶構造はウルツ鉱形で六方晶系に属する。その禁制帯幅は室温で約3.4eVであり，光学的基礎吸収端が3650Åの近紫外部にある。この大きな禁制帯幅のため，種々の応用が考えられている。既に今日までに，不純物（主としてII-B族元素Zn，Mg等）を添加し，絶縁体としたGaNと，undopedのn形GaNによるi-n形ダイオードで，青色・緑色・黄色・赤色の発光が報告されている。特にZnを添加したものからは最高パワー効率0.1%が得られている。しかし，現在までに得られたundoped GaNは常にn形を示し，その自由電子濃度も10^17cm^{-3}以上と高く，又p形結晶はまだ確認されていない。ドナーの原因としては，窒素の空格子点が考えられている。我々は，高効率青色発光素予の材料としてGaNを取り上げ，その結晶成長を試みている。本論文には，我々の行ったundoped GaNの作成とその電気的性質について報告する。

GaN grown by three different methods (MOVPE, GSMBE and HVPE) have been studied using temperature (T) dependent reflectivity and photoluminescence (PL). Both non intentionally doped (MOVPE, GSMBE and HVPE), n- and p-doped samples (MOVPE and GSMBE) have been investigated. Reflectivity is used to obtain intrinsic transition energies. These energies vary with the amount of strain in the crystal. Growth parameters influencing this strain state are discussed. Using MOVPE (T-g approximate to 1050 degrees C) and GSMBE (T-g approximate to 800 degrees C), it is possible to grow samples whose low temperature PL spectra are dominated by free and bound excitons and their phonon replica. Intentional n-type doping up to 10(20) cm(-3) is easily achieved with Si. For n much greater than 10(18) cm(-3), the spectra broaden and exhibit a blue shift, attributed to band filling. p-Type doping has been attempted using Mg, C and Ca. Ca doping led to compensated samples. C doping using CCl4 resulted in n-type samples, due to simultaneous oxygen incorporation in the layers; a strong enhancement of the 3.27 eV donor acceptor pair PL is also observed in this case. p-Type doping up to 10(18) cm(-3) has been achieved with Mg. With increasing densities, a deepening of the donor acceptor pair PL energy is observed. For high Mg doping, the spectra are dominated by a blue band in the 2.8 eV range, involving deep electron states. (C) 1997 Elsevier Science S.A.