High density, uniform GaN nanodot arrays with controllable size have been synthesized by using template-assisted selective growth. The GaN nanodots with average diameter 40nm, 80nm and 120nm were selectively grown by metalorganic chemical vapor deposition (MOCVD) on a nano-patterned SiO2/GaN template. The nanoporous SiO2 on GaN surface was created by inductively coupled plasma etching (ICP) using anodic aluminum oxide (AAO) template as a mask. This selective regrowth results in highly crystalline GaN nanodots confirmed by high resolution transmission electron microscopy. The narrow size distribution and uniform spatial position of the nanoscale dots offer potential advantages over self-assembled dots grown by the Stranski–Krastanow mode. / Singapore-MIT Alliance (SMA)

Das Spurengas Lachgas (N₂O) trägt zur Klimaerwärmung und Zerstörung der Ozonschicht in der Atmosphäre bei. Mit einem Anteil von ca. 70% sind landwirtschaftliche Böden weltweit Hauptverursacher der hohen anthropogenenN₂O Emissionen. N₂O entsteht in Böden durch verschiedene mikrobiologische Prozesse, bei denen N₂O unter anderem aus düngerbürtigem N gebildet wird. Die Entwicklung effektiver Minderungsmaßnahmen wird erst möglich, wenn ein Verständnis der N₂O Quellprozesse und ihrer Dynamik in Böden vorhanden ist. In dieser Studie wurde die Denitrifikation als ein Quellprozess untersucht, der zusammen mit Nitrifikation und Nitrifizierer-Denitrifikation hauptsächlich für die N₂O Emissionen aus Böden verantwortlich ist. Die Denitrifikation beschreibt die Reduktion von Nitrat (NO₃^-) zu N2, wobei Nitrit (NO₂^-), Stickstoffmonoxid (NO) und N₂O Zwischenprodukte dieses Reaktionsweges sind. Lange Zeit galten heterotrophe Bakterien als alleinige Verursacher von N₂O Emissionen aus der Denitrifikation. Im Jahr 1972 wurde allerdings in Versuchen mit Pilzreinkulturen nachgewiesen, dass auch Pilze in der Lage sind, N₂O über die Denitrifikation zu bilden. Zwei Jahrzehnte später wurde gezeigt, dass den meisten Pilzen das Enzym N₂O-Reduktase fehlt. Somit ist nicht N₂, sondern N₂O das hauptsächliche Endprodukt der pilzlichen Denitrifikation. Dies lässt vermuten, dass die Bildung von N₂O durch pilzliche Denitrifikation noch unterschätzt wird, vorausgesetzt Pilze und Bakterien haben ähnliche Prozessraten. Bisher wurde jedoch nicht ausgiebig erforscht, welchen Anteil die einzelnen mikrobiellen Gemeinschaften an der N₂O Bildung tatsächlich haben. Zur Unterscheidung der N₂O Bildungsprozesse in Bezug auf die beteiligten Mikroorganismen stellt die Isotopenanalyse von N₂O eine vielversprechende Anwendung dar. Vor allem die ¹⁵N-Positionspräferenz im N₂O (SP = site preference, d.h. die Differenz zwischen den δ¹⁵N-Werten der außenständigen und zentralen N-Atome im linearen N₂O-Molekül) aus der Denitrifikation zeigte starke Unterschiede zwischen Reinkulturen einiger Bakterien (SP = -11 bis 0 ‰) und zwei untersuchten Pilzen (SP ~ 37 ‰). Jedoch wurden Bakterienreinkulturen bisher ausgiebiger untersucht als Pilzreinkulturen, auch wenn bekannt ist, dass sich die beteiligten Enzyme bei der Denitrifikation, bis auf die NO-Reduktase, zwischen Bakterien und Pilzen nicht unterscheiden. Die verschiedenen NO-Reduktasen sind vermutlich die Ursache für die unterschiedlichen SP-Werte des von Pilzen und Bakterien produzierten N₂O. Des Weiteren wurde bei Bakterien ein Austausch der Sauerstoffatome von Zwischenprodukten der Denitrifikation und dem umgebenden Wasser gefunden, der zwischen 4 und 100% beträgt. Ob es einen solchen Sauerstoffaustausch auch bei Pilzen gibt, ist bisher jedoch unerforscht. Würde der Sauerstoffaustausch bei pilzlicher Denitrifikation nicht erfolgen, ermöglichte dies neben der unterschiedlichen SP eine weitere Unterscheidung der Herkunft des N₂O. Der Sauerstoffaustausch würde signifikante Unterschiede in der O Isotopensignatur im N₂O pilzlicher bzw. bakterieller Herkunft verursachen. In der vorliegenden Studie, die Aufschluss über die pilzliche N₂O Produktion aus der Denitrifikation geben soll, wurden drei Hauptthemen behandelt. In einem Isotopen-Tracerexperiment mit <up>18</sup>O-angereichertem Wasser wurde untersucht, ob bei sechs Pilzreinkulturen ein Sauerstoffaustausch zwischen Wasser und Zwischenprodukten der Denitrifikation stattfindet. Die Pilzreinkulturen zeigten tatsächlich durch Inkorporation von ¹⁸O aus Wasser in N₂O einen Sauerstoffaustausch. Auch Pilze können bis zu 100% des O während der Denitrifikation austauschen. Eine Unterscheidung zwischen der Denitrifikation durch Bakterien und Pilze anhand der Sauerstoffsignatur ist somit nicht möglich. Das zweite Thema sollte Auskunft darüber geben, ob hohe SP-Werte des N₂O aus der Denitrifikation bei Pilzreinkulturen allgemeingültig sind. Neben den zwei bisher untersuchten wurden vier weitere Pilzreinkulturen inkubiert. Diese Studie zeigte für die getesteten Pilzarten ebenfalls höhere SP-Werte (SP = 19.7 bis 32.6 ‰) im Vergleich zum Wertebereich von Bakterienreinkulturen. Basierend auf den Ergebnissen zum Sauerstoffaustausch aus dem Isotopen-Tracerexperiment wurde für die jeweiligen sechs Pilze, anhand der im Rahmen dieses Versuchs ermittelten natürlichen Sauerstoffisotopensignaturen, Mechanismen zur O Isotopenfraktionierung untersucht. Dafür wurden, neben den Werten des Sauerstoffaustausches und der natürlichen O Isotopensignatur der Pilzreinkulturen, Werte für Fraktionierungseffekte aus der Literatur in einem Isotopenfraktionierungsmodell angewendet, um die Beteiligung der verschiedenen Enzyme, die während der Denitrifikation an dem Sauerstoffaustausch beteiligt sind, abzuschätzen. Im Vergleich zu den NO₃^-- und NO-Reduktasen wies die N₂O^--Reduktase einen maßgeblich höheren Sauerstoffaustausch auf. Die Erkenntnisse aus den Experimenten mit den Pilzereinkulturen sollten im Rahmen des dritten Themas auf Ihre Übertragbarkeit auf die mikrobiellen Gemeinschaften in Böden untersucht werden, indem Bodeninkubationsversuche mit selektiver Hemmung der Organismengruppen (Pilze und Bakterien) durchgeführt wurden. Bei dieser Modifizierung der Methode zur Substrat-induzierten Respiration mit selektiver Hemmung (SIRIN) sollte untersucht werden, ob sich die spezifischen SP-Werte für Bakterien und Pilze nach selektiver Wachstumshemmung von Bodengemeinschaften durch spezifische Antibiotika nachweisen lassen. Die Ausprägung des Hemmungseffekts auf SP-Werte in den drei getesteten Böden entsprach nicht den Erwartungswerten, die sich aus den SP-Werten der Pilz- und Bakterienreinkulturen ergaben. Die ermittelten SP-Werte lagen in den meisten Fällen im Bereich jener bakterieller Reinkulturen und eine Hemmung der Bakterien führte in keinem Fall zu der erwarteten Veränderungen der SP-Werte. Folglich konnten die SP-Werte dieser Versuche nicht dazu dienen, die N₂O Bildung in den gehemmten Varianten den verschiedenen Organismengruppen zu zuordnen. Ungeklärt blieb, ob dies durch fehlende Eignung der modifizierten SIRIN-Methode zu erklären ist, oder ob die an Reinkulturen beobachteten SP-Unterschiede zwischen Pilzen und Bakterien nicht auf mikrobielle Gemeinschaften der Versuchsböden übertragbar sind. Im Hinblick auf nach wie vor bestehende methodische Defizite bei der Untersuchung der Pilzdenitrifikation im Boden sollte dies in weitergehenden Studien geklärt werden.

The Si(100) surface and Ga surface phases up to 1 ML on their oxidation have been studied by XPS and LEED. The selective growth of Ga on the SiO2/Si structures fabricated by EBL has been analyzed using SEM and AFM methods. It was proved that Ga clusters grow in structures beside the oxide. The structure of alumina on Ni3Al(111) and NiAl(110) substrates was fully determined by combining the results of STM measurements and DFT simulations. It was determined the alumina/NiAl(110) does not form a suitable template for ordered Fe and Co clusters growth. However, the next research confirmed the alumina/Ni3Al(111) forms template appropriate to clusters growth purpose.

Ce mémoire traite de l’étude de la croissance de nanofils (In,Ga)N par épitaxie en phase vapeur par la méthode aux hydrures (HVPE). L’objectif est de contrôler l’homogénéité, la composition et les dimensions des fils. La première partie de ce travail est consacrée à l’étude de la croissance auto-organisée de nanofils InGaN par HVPE. Les résultats montrent qu’il est possible de maitriser la composition des fils en indium, de 7 % à 90 %, en ajustant la composition de la phase vapeur. Des caractérisations structurales confirment une très bonne qualité cristalline des nanofils InGaN obtenus ainsi qu’une parfaite homogénéité sur leurs longueurs. Ce travail expérimental a été couplé à un travail de modélisation théorique basée sur des calculs thermodynamiques. Un deuxième travail portant sur l’étude de l’épitaxie sélective de réseaux de fils (In,Ga)N sur des templates GaN/c-Al2O3, masqués par un diélectrique, est réalisée. La croissance, parfaitement sélective et reproductible, de nano- et microrods d’InN de très bonne qualité cristalline est démontrée pour la première fois par HVPE. Une étude systématique sur l’influence des paramètres de croissance a permis de déterminer la hiérarchie des vitesses de croissance des différentes facettes des rods et d’identifier les phénomènes physiques mis en jeu. Un modèle de croissance basé sur les énergies de surface et d’interface est proposé afin d’expliquer la présence d’un creux dans les rods d’InN. Des études en photoluminescence sur des ensembles de nanorods d’InN ont mis en évidence un fort dopage de type n et indiquent également la présence d’une surface d’accumulation des porteurs de charge sur les parois des rods. Enfin, l’étude de la faisabilité de la croissance sélective d’InGaN par HVPE est initiée. / This thesis focuses on a comprehensive study of (In, Ga)N nanowires grown by hydride vapor phase epitaxy (HVPE), combining the growth technology, complementary chemical and structural analyses and theoretical modeling. The first part of this work is devoted to the study of the self-induced growth of InGaN nanowires by HVPE. The end result shows that growth of vertically aligned InGaN nanowires with a high crystalline quality can be synthesized by this cost-effective technique. The indium content can be varied from 7 % to 90 % with a high degree of homogeneity along the nanowire length with a good crystal quality. This is achieved by understanding the kinetics of interconnected chemical reactions in the vapor phase, and coupling them with the kinetically controlled composition of solid nanowires. The second focus section of this work looks at the selective area growth of (In, Ga)N nanorods. Well-ordered and vertically aligned InN nano and microrods with high aspect ratio and high crystalline quality are synthetized by HVPE using the SAG approach. The growth occurs through the apertures of a SiNx masked Ga-polar GaN/c-Al2O3 template for adjusted growth temperature and V/III ratio. A systematic study of the evolution of InN nanorods shape under various growth conditions: growth temperature, growth time and the input NH3 partial pressure, is investigated. A growth model based on surface and interface energies is proposed to explain the presence of a void in these InN nanorods. Photoluminescence measurements on InN nanorods reveal strong n-type doping and indicate the presence of a carrier accumulation on the nanorods surfaces. Finally, the selective growth feasibility of InGaN nanorods by HVPE is initiated.

A novel process for low-temperature (LT) epitaxial growth of silicon carbide (SiC) by replacing the growth precursor propane with chloro-methane was recently developed at Mississippi State University. However, only limited information was available about the defects and impurity incorporation in the various types of epitaxial layers produced by this new method like blanket epitaxial layers, selectively grown epitaxial mesas, and highly doped epitaxial layers, prior to their comprehensive characterization in this work. Molten potassium hydroxide (KOH) etching, mechanical polishing and a variety of other characterizing techniques were used to delineate and identify the defects both in the epilayer and substrates. Under optimum growth conditions, the concentration of defects in the epitaxial layers was found to be less than that in the substrate, which established the good quality of the LT growth process. Defect concentrations, on selectively grown epitaxial layers, strongly depended on the crystallographic orientation of the mesa sidewall. The addition of HCl to the growth process, aimed at increasing the growth rate, caused a significant concentration of triangular defects (TDs) to be formed in the epitaxial layers. The TDs were traced down to the substrate by a combination of repeated polishing and molten KOH etching steps. The TDs were found not to originate from any substrate defects. Their origin was traced to polycrystalline silicon islands which form on the surface during growth and subsequently get evaporated away, which had made it impossible to detect them and suspect their influence on the TD generation prior to this work. The TDs were found to include single or multiple stacking faults bound by partial dislocations and, in some cases, inclusions of other SiC polytypes. Gradual degradation of the epitaxial morphology was found in heavily aluminum doped p+ layers, with an increase in the level of doping, followed by much steeper degradation when approaching the solubility limit of Al in 4H-SiC. Precipitates were the dominating defect at the highest levels of doping and were observed beyond a doping of 3.5x1020 cm-3. A dislocation generation model for heavily doped epitaxial layers was developed accounting for the stress in the lattice caused by Al doping.

OBJETIVOS: Descrever os resultados obstétricos e perinatais em três grupos de gestações gemelares monocoriônicas diamnióticas (MCDA) conduzidas de forma expectante: com diástole zero (DZ) e/ou reversa (DR) fixas na Dopplervelocimetria de artérias umbilicais (AU) e restrição de crescimento fetal seletiva (RCFs) (tipo II de Gratacós); com DZ e/ou DR intermitente e RCFs (tipo III de Gratacós) e com DZ e/ou DR intermitente sem RCFs (Sem RCFs). MÉTODOS: Estudo retrospectivo, realizado no período de abril de 2007 a abril de 2017. Foram incluídas todas as gestações MCDA no período com DZ e/ou DR no Doppler de AU com IG < 26 semanas e ausência de Síndrome de Transfusão feto-fetal (STFF) ou malformações fetais e das quais foi possível obtenção dos dados obstétricos, fetais e dos recém-nascidos (RNs) até a alta hospitalar. Todas as variáveis foram analisadas descritivamente. Além disso, para as variáveis qualitativas foram calculadas as frequências absolutas e relativas. RESULTADOS: 33 pacientes preencheram os critérios de inclusão; 6 no tipo II, 22 no tipo III e 5 no grupo Sem RCFs. A mediana da IG do diagnóstico foi semelhante em todos os grupos. As discordâncias entre os pesos dos fetos e RNs foram maiores no tipo II. Quando a DZ e/ou DR eram intermitentes (tipo III e Sem RCFs), a latência entre o diagnóstico e o óbito fetal (OF) foi superior. Quanto aos resultados obstétricos, a IG mediana do parto foi de 29; 32,71 e 30,78 semanas nos tipos II, III e Sem RCFs, respectivamente, e as indicações foram, em sua maioria, por deterioração dos parâmetros ultrassonográficos ou biofísicos fetais. A taxa de OF foi maior no tipo II (33,3%), ocorreu em 20% no Sem RCFs e 11,36% no tipo III. Em relação aos dados perinatais gerais, o pior resultado ocorreu no tipo II, em que nenhuma paciente levou 2 RNs vivos para casa, sendo que isso sucedeu em 72,7% no tipo III e 60% no Sem RCFs. No que concerne à morbidade neonatal, o peso de nascimento foi menor no tipo II. Já, a HIV mostrou-se mais comum no tipo III (30,3%), porém, no tipo II, a ocorrência foi similar (28,57%), não acometeu nenhum RN no grupo Sem RCFs. CONCLUSÃO: A prematuridade foi preponderante. O acompanhamento da vitalidade dos fetos é de suma importância, uma vez que a piora de seus parâmetros indica a resolução da gestação em considerável parcela desses casos. O tipo II foi o que apresentou os desfechos perinatais mais desfavoráveis, além da maior discordância de peso entre os fetos e os RNsAbsent and/or reversed end-diastolic flow in the umbilical arteries / OBJECTIVES: To describe obstetric and perinatal outcomes in three groups of monochorionic diamniotic (MCDA) twin pregnancies under expectant management: with absent and/or reversed end-diastolic Doppler flow (AREDF) in the umbilical arteries (UA) and selective intrauterine growth restriction (sIUGR) (Gratacós\' type II); with intermittent AREDF (iAREDF) and sIUGR (Gratacós\' type III) and with iAREDF without sIUGR (Without sIUGR). METHODS: This was a retrospective study, from April 2007 to April 2017. All the MCDA twin pregnancies presenting AREDF or iAREDF in the UA at less than 26 weeks of gestational age (GA), without signs of Twin to Twin Transfusion Syndrome (TTTS) or fetal anomalies and of whom it was possible to obtain obstetrical data and information of the fetuses and neonates until hospital discharge were included. All the variables were descriptively analysed. Furthermore, absolute and relative frequencies were calculated for the qualitative variables. RESULTS: 33 patients were included; 6 in type II, 22 in type III and 5 in the group Without sIUGR. Median GA at diagnosis was similar among the groups. Fetal and neonate\'s weight discordances were greater in type II. The groups with iAREDF (type III and Without sIUGR) had the longest latency between diagnosis and intrauterine fetal demise (IUFD). Regarding obstetric outcome, median GA at delivery were 29, 32.71 and 30.78 weeks for type II, III and Without sIUGR respectively and sonographic and biophysical parameters deterioration was the main reason to indicate deliveries. IUFD was also more frequent in type II (33,3%), occurred in 20% of type III and 11,36% in the group Without sIUGR. As far as general perinatal outcome is concerned, type II had the worst result, in which no patient took 2 neonates home; it happened in 72.7% in type III and 60% in the group Without sIUGR. In relation to perinatal morbidity, type II neonates presented the lowest weight at birth. Type III and II had the highest, but similar proportions of intraventricular hemorrhage (30.3% and 28.57% respectively), it didn\'t happen in the group Without sIUGR, though. CONCLUSION: Prematurity was preponderant. It is of the highest importance to monitor fetal wellbeing, since the great majority of deliveries were indicated by the deterioration of its parameters. Type II sIUGR showed the most unfavorable perinatal outcome and additionally, the greatest fetal and neonatal weight discordances

OBJETIVOS: Descrever os resultados obstétricos e perinatais em três grupos de gestações gemelares monocoriônicas diamnióticas (MCDA) conduzidas de forma expectante: com diástole zero (DZ) e/ou reversa (DR) fixas na Dopplervelocimetria de artérias umbilicais (AU) e restrição de crescimento fetal seletiva (RCFs) (tipo II de Gratacós); com DZ e/ou DR intermitente e RCFs (tipo III de Gratacós) e com DZ e/ou DR intermitente sem RCFs (Sem RCFs). MÉTODOS: Estudo retrospectivo, realizado no período de abril de 2007 a abril de 2017. Foram incluídas todas as gestações MCDA no período com DZ e/ou DR no Doppler de AU com IG < 26 semanas e ausência de Síndrome de Transfusão feto-fetal (STFF) ou malformações fetais e das quais foi possível obtenção dos dados obstétricos, fetais e dos recém-nascidos (RNs) até a alta hospitalar. Todas as variáveis foram analisadas descritivamente. Além disso, para as variáveis qualitativas foram calculadas as frequências absolutas e relativas. RESULTADOS: 33 pacientes preencheram os critérios de inclusão; 6 no tipo II, 22 no tipo III e 5 no grupo Sem RCFs. A mediana da IG do diagnóstico foi semelhante em todos os grupos. As discordâncias entre os pesos dos fetos e RNs foram maiores no tipo II. Quando a DZ e/ou DR eram intermitentes (tipo III e Sem RCFs), a latência entre o diagnóstico e o óbito fetal (OF) foi superior. Quanto aos resultados obstétricos, a IG mediana do parto foi de 29; 32,71 e 30,78 semanas nos tipos II, III e Sem RCFs, respectivamente, e as indicações foram, em sua maioria, por deterioração dos parâmetros ultrassonográficos ou biofísicos fetais. A taxa de OF foi maior no tipo II (33,3%), ocorreu em 20% no Sem RCFs e 11,36% no tipo III. Em relação aos dados perinatais gerais, o pior resultado ocorreu no tipo II, em que nenhuma paciente levou 2 RNs vivos para casa, sendo que isso sucedeu em 72,7% no tipo III e 60% no Sem RCFs. No que concerne à morbidade neonatal, o peso de nascimento foi menor no tipo II. Já, a HIV mostrou-se mais comum no tipo III (30,3%), porém, no tipo II, a ocorrência foi similar (28,57%), não acometeu nenhum RN no grupo Sem RCFs. CONCLUSÃO: A prematuridade foi preponderante. O acompanhamento da vitalidade dos fetos é de suma importância, uma vez que a piora de seus parâmetros indica a resolução da gestação em considerável parcela desses casos. O tipo II foi o que apresentou os desfechos perinatais mais desfavoráveis, além da maior discordância de peso entre os fetos e os RNsAbsent and/or reversed end-diastolic flow in the umbilical arteries / OBJECTIVES: To describe obstetric and perinatal outcomes in three groups of monochorionic diamniotic (MCDA) twin pregnancies under expectant management: with absent and/or reversed end-diastolic Doppler flow (AREDF) in the umbilical arteries (UA) and selective intrauterine growth restriction (sIUGR) (Gratacós\' type II); with intermittent AREDF (iAREDF) and sIUGR (Gratacós\' type III) and with iAREDF without sIUGR (Without sIUGR). METHODS: This was a retrospective study, from April 2007 to April 2017. All the MCDA twin pregnancies presenting AREDF or iAREDF in the UA at less than 26 weeks of gestational age (GA), without signs of Twin to Twin Transfusion Syndrome (TTTS) or fetal anomalies and of whom it was possible to obtain obstetrical data and information of the fetuses and neonates until hospital discharge were included. All the variables were descriptively analysed. Furthermore, absolute and relative frequencies were calculated for the qualitative variables. RESULTS: 33 patients were included; 6 in type II, 22 in type III and 5 in the group Without sIUGR. Median GA at diagnosis was similar among the groups. Fetal and neonate\'s weight discordances were greater in type II. The groups with iAREDF (type III and Without sIUGR) had the longest latency between diagnosis and intrauterine fetal demise (IUFD). Regarding obstetric outcome, median GA at delivery were 29, 32.71 and 30.78 weeks for type II, III and Without sIUGR respectively and sonographic and biophysical parameters deterioration was the main reason to indicate deliveries. IUFD was also more frequent in type II (33,3%), occurred in 20% of type III and 11,36% in the group Without sIUGR. As far as general perinatal outcome is concerned, type II had the worst result, in which no patient took 2 neonates home; it happened in 72.7% in type III and 60% in the group Without sIUGR. In relation to perinatal morbidity, type II neonates presented the lowest weight at birth. Type III and II had the highest, but similar proportions of intraventricular hemorrhage (30.3% and 28.57% respectively), it didn\'t happen in the group Without sIUGR, though. CONCLUSION: Prematurity was preponderant. It is of the highest importance to monitor fetal wellbeing, since the great majority of deliveries were indicated by the deterioration of its parameters. Type II sIUGR showed the most unfavorable perinatal outcome and additionally, the greatest fetal and neonatal weight discordances

The diploma thesis deals with morphology of cobalt thin film on clean Si(111) and on silicon dioxide thin film on Si(111) studied by AFM and XPS. It is also study of selective growth of cobalt on lattice made by focused ion beam and electron lithography. In the last part, the growth of metals (Fe, Co) on surface oxide on Ni3Al(111) was studied.

This thesis deals with deposition of gallium nitride thin films on silicon substrates covered by negative HSQ rezist. Rezist was patterned via electron beam lithography to create masks, where the selective growth of crystals was achieved. Growth of GaN layers was carried out by MBE method. For achievement of desired selective growth, the various deposition conditions were studied.

This thesis deals with deposition of GaN thin films and GaN selective growth utilizing pyrolyzed resist masks. Carbon masks were prepared on silicon substrates by electron-beam litography and resist pyrolysis. As a further step, Ga and GaN were deposited on the masked substrates by Moleculer Beam Epitaxy (MBE) method. A selective growth of Ga droplets was achieved. These results were used for preparation of GaN crystallites by pulse deposition. It is also shown that direct MBE deposition of GaN on the masked substrates leads to a selective growth of GaN thin films with GaN film growing only on the areas which are not covered by the carbon mask. The results are explained by enhanced surface diffusion of gallium atoms on the surface of the carbon mask.