Aluminum nitride

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Rare earth-doped aluminum nitride thin films for optical applications

Rare earth-doped aluminum nitride thin films for optical applications

This chapter is dedicated to investigate the optical behavior of cerium-doped AlN (Ce-AlN) thin films prepared by RF reactive magnetron sputtering. The crystal structure of the prepared samples was investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM) at low and high resolution (HRTEM). The chemical composition was analyzed by Rutherford backscattering spectrometry (RBS) and Energy-dispersive X-ray (EDX) techniques. The oxidation states of cerium (Ce 3+ and Ce 4+ ) have been probed using Electron Energy Loss Spectroscopy (EELS). The optical response has been examined by photoluminescence (PL) under different optical excitation wavelengths. Photoluminescence excitations measurements (PLE) have been performed in order to explore the excitation mechanisms. In addition, the PL intensity evolution with low temperature variation was used to gain more information about the PL thermal quenching mechanism. It is found that oxygen plays an important role in the PL response by changing the oxidation state of Ce ions from optically inactive (Ce 4+ ) to the optically active one (Ce 3+ ). In addition, the importance of the oxidation is further confirmed by the excitation mechanisms responsible for blue emission and determined by PLE measurements. This role has been discussed to the light of the correlation between PL, PLE and EELS results. Besides that, the post-deposition annealing for our samples is found to be essential for activating the photoluminescence. A comprehensive study is presented to understand the optical mechanisms of Ce-doped aluminum nitride and aluminum (oxy) nitride materials. Based on a proposed approach, PL manipulation has been achieved to offer different emission colors (blue, green and white). Our findings can be used to understand and manipulate the behavior of RE-doped Al(O)N for lightening applications.
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Defect Analysis of Aluminum Nitride

Defect Analysis of Aluminum Nitride

In our case however, the temperature dependence of the defect related luminescence cannot be explained by the presence of a simple, deep trap level, since in this case it is impossible t[r]

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Unprecedented thermal stability of inherently metastable titanium aluminum nitride by point defect engineering

Unprecedented thermal stability of inherently metastable titanium aluminum nitride by point defect engineering

Figure 1 (a) shows a schematic of conventionally employed reactive magnetron sputtering: the reactive gas, in this case N2, is introduced in the deposition cham- ber homogeneously in order to obtain constant product quality over the whole deposited area. The high flow of nitrogen leads to poisoning of the metal target, mean- ing that a nitride covers the target surface and has to be sputtered. The vapor that interacts with the growing film therefore consists of metals and atomic and molecular nitrogen. Part of the vapor is ionized and there is a high partial pressure of inert sputtering gas. While the reactive sputtering process is on a global level rather well under- stood [ 19 , 20 ], there is no theory available that describes (off-) stoichiometry, that is, the nitrogen-to-metal ratio N/M of the thin film.
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Unprecedented thermal stability of inherently metastable titanium aluminum nitride by point defect engineering

Unprecedented thermal stability of inherently metastable titanium aluminum nitride by point defect engineering

For comparison, (Ti,Al)N x was also deposited by sput- tering a poisoned TiAl target [ 42 ]. A poisoned target surface is obtained by sputtering in a N 2 abundant atmo- sphere resulting in larger nitride formation rate than the nitride sputtering rate at the target, meaning that a nitride is sputtered rather than metals. Here we intro- duce a flow of N 2 of 50 sccm homogeneously into the deposition system, resulting in a N 2 partial pressure of 95 mPa. HPPMS was used with a frequency of 800 Hz, duty cycle of 4% and a time-average power of 3000 W resulting in peak power density of 0.4 kW/cm 2 . The deposition time on amorphous carbon substrates was 5 min and on sapphire 30 min, where the first samples were used to enable optimum Rutherford back-scattering spectrometry (RBS) measurements conditions while the second set was used for nanoindentation, thermal sta- bility and atom probe investigations. In order to avoid
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Electrical transport properties of doped gallium nitride nanowires

Electrical transport properties of doped gallium nitride nanowires

Chapter 2 Introduction Group III-nitrides have been considered as a promising material base for semiconductor device applications since 1970. The group III-nitrides, with the binary compounds aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) are excellent candidates for optoelectronic applications such as blue-, UV- and IR-light emitting diodes, because they form a continuous alloy system (InGaN, InAlN, and AlGaN) whose direct optical bandgap for the hexagonal wurtzite phase ranges from 0.7 eV for InN over 3.4 eV for GaN to 6.2 eV for AlN [4, 5] as depicted in Figure 2.1. The wide band gap with large breakdown electric fields makes the material suitable for high power applications as well. Heterostructures with a discontinuity in total polarization are used to build high electron mobility transistors (HEMT) based on a 2-dimensional electron gas [4, 6].
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Point defects in oxide and nitride semiconductors

Point defects in oxide and nitride semiconductors

Semiconducting oxides and nitrides are, due to their large band gap energies, the two most interesting groups to employ in optoelectronic devices operating in the visible and UV spectral range. There are to mention Zinc Oxide (ZnO) and Gallium Oxide (Ga2O3) on the oxide side and Gallium Nitride (GaN) together with Indium Nitride (InN) and Aluminum Nitride (AlN) for the nitrides. These semiconducting materials combine unique properties on their crystallography and growth mechanisms, as well as on their optical, electrical and magnetic properties. Hence it is not surprising that with these materials it was possible to build novel displays, light emitters, data storages, bio- and environmental-sensors and energy generating- or saving-devices. For any device application one has to solve problems related to the growth mechanisms of the materials. Defect characterization of the materials is a necessity, since relevant physical properties are affected by intrinsic and extrinsic defects. There are various characterization tools ranging from the electrical- or optical- and magnetic methods to microscopy’s such as electron- or atomic force microscopy which give information on the structural- or surface-properties. The choice which one suits best to achieve the given purpose depends on the specific information one needs.
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Raman Spectroscopy on Graphene Encapsulated in Hexagonal Boron Nitride

Raman Spectroscopy on Graphene Encapsulated in Hexagonal Boron Nitride

As a consequence, Raman spectroscopy is a highly suitable tool for sample characterization in graphene research, since such short-ranged strain variations have been shown to be a limitin[r]

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High temperature stability of nitride-based power HEMTs

High temperature stability of nitride-based power HEMTs

The ohmic contacts to GaN and its ternary alloys like InAlN are commonly based on the formation of an alloyed metal nitride interface providing a path for electron tunneling. The common layer stack developed during the last decade has been Ti/Al with a Ni/Au overlay. According to their phase diagrams [4], many phases can be formed above the melting point of Al, essentially generating chemical activity of Ti for an interfacial reaction well below its melting temperature.

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Group III-Nitride Nanowires as Multifunctional Optical Biosensors

Group III-Nitride Nanowires as Multifunctional Optical Biosensors

Group-III nitride materials exhibit a high electrochemical stability [13–15]. In addition, AlGaN surfaces reveal non-toxic properties in contact with living cells [15]. This qual- ifies the electrodes for biological applications and in vivo studies. Other nanomaterials that are commonly used for optical biomolecule detection are nanoparticles (NPs), such as gold NPs or surface modified NPs like surface modified quantum dots (QDs). The detection mechanism here is based on the quenching of the nanoparticles´ PL intensity due to hole transfer from the QDs to the molecules [89]. In contrast to the InGaN/GaN NWH electrodes, the working point of such NPs cannot be defined. Thus, the applicabil- ity for the detection of one individual biomolecule is limited. Besides, quantum dots have been reported to possess electrochemical stability only in a small pH range in the neutral range of 6 < pH < 9 [90] or 6 < pH < 8 [91]. For application in different environments, an irreversible decrease of the photoluminescence intensity was observed.
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III-nitride-based optochemical transducers for gas detection

III-nitride-based optochemical transducers for gas detection

The first promising research successes date back to some 50 years ago where first works on GaN based transistors were published [13]. As GaN materials have to be produced via epitaxial growth processes, appropriate substrates are mandatory. As bulk GaN crystals were not available then, the progress of research on GaN was very slow in the beginning. It was only in the 1980s that the nitride material science rapidly progressed as these materials could then be grown on sapphire substrates using GaN and AlN as nucleation layers [14,15]. In the 1990-ties p-type GaN was discovered us- ing magnesium as a dopant [16]. Some years later, after high quality GaN films could be manufactured, the successful commercialization of GaN devices set in. GaN based light emitting (LED) and laser diodes found their way into consumer products such as Blue Ray discs and lighting bulbs. The inventors of high efficient GaN based LEDs were rewarded with the Nobel prize in 2014 [17]. Other successful products were high- electron-mobility transistors (HEMT) for high frequency applications [18].
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Solvothermal and Ionothermal Approaches to Carbon Nitride Chemistry

Solvothermal and Ionothermal Approaches to Carbon Nitride Chemistry

With the focus based on properties and possible applications of graphite-type networks, carbon nitride materials experience a third period of prosperity. Nearly quarter of a century ago, the pioneering work of Liu and Cohen evoked the “harder than diamond” fever, predicting compressibility comparable to diamond for still hypothetical binary sp 3 -hybridized carbon nitride C 3 N 4 . [31] Adopting β-Si 3 N 4 structure, β-C 3 N 4 was calculated to have a bulk modulus (which describes a substance’s resistance to uniform compression and is inversely proportional to both interatomic distances and ionicity) higher than diamond. [32] Nevertheless, not only compression but also shearing strains have to be taken into account for the hardness of solids, thus suggesting a slightly lower hardness for β-C 3 N 4 in comparison with diamond. [33] A multitude of further modifications for sp 3 - hybridized C 3 N 4 was predicted to be stable, among these α- C 3 N 4 (hexagonal), pseudocubic C 3 N 4 (defect ZnS-type) and cubic C 3 N 4 (Willemite-II- type). [33] However, not only theoretical work but especially numerous attempts to synthesize sp 3 -C 3 N 4 were published, including mostly physicochemical methods like CVD- and PVD processes, laser techniques and sputter processes. [33-35] Another approach is derived from DFT-calculations, predicting the conversion of theoretically more stable layered graphitic C 3 N 4 into sp 3 -C 3 N 4 to
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Investigations into s-Heptazine-Based Carbon Nitride Precursors

Investigations into s-Heptazine-Based Carbon Nitride Precursors

Cyameluric acid and cyamelurates (cf. Chapter 7) also have a vast potential for prepar- ing salts and adducts. The structural investigation of cyameluric acid trihydrate proved the preference of a keto-like tautomer. Structural investigations of other compounds containing cyameluric acid or cyamelurates fully support the observations made for the trihydrate. Study- ing solutions of cyameluric acid in aqueous ammonia three salts were yielded. The structure of these compounds was solved using single-crystal XRD. The results shed light on the struc- tural properties possible for cyamelurate ions. Especially the copper salt shows new coordina- tion types involving this ion, since copper cyamelurate bonds are the first metal cyamelurate bonds notably affecting bond lengths within the cyamelurate itself. This is probably due to the rather soft Cu 2+ ions’ tendency to establish bonds with an increasingly covalent character also explaining the increasing affinity towards Cu-N bonds. Magnetic properties have been studied for this compound showing antiferromagnetic ordering at low temperatures (9 K). Hopefully these results will be helpful in order to establish an improved access to metal cyamelurate salts. This would allow for a better study of their properties as well as their potential use as precursors for carbon nitride-based networks
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Insights into the reversibility of aluminum graphite batteries

Insights into the reversibility of aluminum graphite batteries

The electrolyte anodic and cathodic stability was evaluated by linear sweep voltammetry (scan rate of 0.1 mV s 1 ), using a three electrode conguration with glassy carbon rods as the working and counter electrodes, and aluminum wire as the reference electrode. 2 The cycling stability of the aluminum metal (Al 99.99% Alfa Aesar) in the ionic liquid-based electrolyte was evaluated by continuous stripping/deposition tests on symmetrical Al/Al cells. The electrochemical characterization of the complete (Al//PG) cells was performed using aluminum metal as the counter electrode, a GF/A Whatman® glass ber soaked by the electrolyte as separator and a pyrolytic graphite disk (Panasonic PG, 100 mm thickness, 8.66 mg cm 2 loading, herein named PG for brevity) as the working electrode. The specic currents (mA g 1 ) and the specic capacities (mA h g 1 ) in the whole manuscript are referred to the cathode (PG) weight. The cycling tests of Al/EMIMCl:AlCl 3 /PG cells were carried out by applying increasing specic currents (from 25 to 100 mA g 1 ) in the voltage range of 0.4–2.4 V, and for the long term cycling test a current rate of 25 mA g 1 was applied for the rst ve cycles and a current rate of 75 mA g 1 for the following cycles. All galvanostatic cycling tests were carried out at 25  C in a thermostatic climatic chamber (with a possible deviation of 1  C), using a Maccor 4000 Battery Test System.
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Investigations into carbon nitrides and carbon nitride derivatives

Investigations into carbon nitrides and carbon nitride derivatives

The optical properties of the samples were investigated by UV-Vis diffuse reflectance spectroscopy. Fig. 4.4 shows the UV-Vis absorption spectra of CTO, melamine calcined for 16 h and CN-doped CTO materials. The absorption edge of the CTO sample occurs at around 370 nm, and the band gap energy is estimated to be about 3.5 eV as obtained by the Kubelka-Munk function (Table 4.1). After carbon nitride doping of CTO, the absorption edge shifts to the lower energy region. We can also see that the absorption edges of the doped samples shift remarkably to longer wavelengths with increasing calcination times. However, the calcination time should not be too long, as the absorption edge of the hybrid significantly shifts back to lower wavelengths below 420 nm when the calcination time is as long as 36 hours. Table 4.1 shows the band gap of CTO, melamine calcined for 16 h and CN doped CTO materials. The table suggests that the doped materials exhibit lower band gaps than CTO, even lower than that of calcined melamine alone. Therefore, the hybrid materials can absorb more visible light and this property will be advantageous for photocatalysis because of the extra availability of photons.
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Shape Factor Effect on Inclusion Sedimentation in Aluminum Melts

Shape Factor Effect on Inclusion Sedimentation in Aluminum Melts

Figure 1 presents the critical diameter of particles as a function of the density difference from 500 to 5000 (kg/ m 3 ) with molten pure aluminum having the density of 2360 kg/m 3 and the dynamic viscosity of 0.00125 kg/m s at 720 C. The shown ranges include the most common inclusions present in aluminum melts, i.e., Al 2 O 3, MgOÆAl 2 O 3 , MgO, TiB 2 , TiC, and SiC. Al 4 C 3 is also commonly detected in aluminum melts but has almost the same density as the liquid aluminum, which results in a very high critical diameter (approximately 290 lm). This limit covers even the clusters since Al 4 C 3 inclusions are typically found in diameter sizes of a few microns. [ 21 ] Over the shown critical diameters, Stokes law will not be valid and can be misleading. Therefore, the modeling part of this study will focus on particles up to 50 lm. The reason for this limitation is the narrow Re range of Stokes flow regime. On the other hand, Schiller and Naumann [ 20 ] approach is valid in the range 0.1 < Re < 800 and valid in the range 0.15 < Re < 1500 after the improvement by Tran-Cong. [ 10 ] Due to the validity in moderate Re, the critical diameter for Schiller and Naumann [ 20 ] drag will be higher than the critical diameter for Stokes.
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Atomic-Scale Insights into the Oxidation of Aluminum

Atomic-Scale Insights into the Oxidation of Aluminum

steadily amorphise completely. The full growth sequences for Al(100) and (111) surface facets are available in Videos SV1 and SV2 , respectively. Figure 4 shows the average thickness of the aluminum oxide formed on {100} and {111} surfaces as a function of oxygen pressure (3 × 10 −5 , 3 × 10 −6 , and 3 × 10 −7 Torr). More details of measuring growth rate are shown in Figure S3 . The Cabrera − Mott theory models low temperature oxidation as driven by the electric field produced by the charge separation due to the presence of metal ions at the oxide-gas interface and oxygen ions at the metal-oxide interface. 21 It predicts that oxide growth is rapid until an oxide completely covers the surface. After this the oxide film thickens at a slower rate, as a result of the increased charge separation across the growing oxide layer. All growth curves we observed follow the same general kinetic trends as predicted by the Cabrera −Mott theory ( Figure 4 ). The di fferences we observe for the early stages of nucleation and growth are likely due to the unavoidable pressure gradient as gas is introduced to the column as well as local surface variations (kinks, steps, etc.) that are not included in the general theory.
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Analysis of the green gap problem in III-nitride LEDs

Analysis of the green gap problem in III-nitride LEDs

It is not clear whether these piezoelectric fields are desirable. It is agreed that the reduction of wavefunction overlap is deleterious to the IQE. However, without the QCSE, more indium would be needed to reach the same wavelength, and QWs with a higher indium content may be less efficient for reasons other than the piezoelectric fields (see § 2.6). Therefore, there still is the concrete possibility that the simultaneous removal of the piezoelectric fields and increase of the indium content will actually reduce the overall efficiency. The consensus in the scientific community seems to be that the performance of the III–nitride LEDs would improve if the piezoelectric fields were removed [Wal00], and this has driven a lot of research on the growth of GaN in non-polar and semipolar directions [Rom06, Spe09, Mas10, Sch12b].
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Ammonothermal synthesis of functional nitride oxides and ternary nitrides

Ammonothermal synthesis of functional nitride oxides and ternary nitrides

exploration of new materials. The obtained crystals of nitride oxide perovskites were frequently found at the bottom of the liner in druse-like agglomerations. [28] This melt seems to be essential for the growth of µm-range crystals. First attempts of different mineralizers for the synthesis of PrTaON 2 with alkaline azides (Na, K, Rb, Cs) showed that NaN 3 provides the most appropriate mineralizer for crystals growth in form of cube- like crystals for the same reaction conditions concerning temperature and pressure. Other mineralizers did not lead to the desired nitride oxide perovskite neither in form of microcrystalline powder nor in form of crystals. Further attempts in different synthetic conditions (temperature, pressure, heating and cooling rates) should be examined in order to elucidate suitable crystals growth conditions. In situ measurements will additionally help to determine the dissolution temperature of the product. One problem is that the obtained crystals of nitrides and nitride oxides are often fused and twinned. Furthermore the employed metal liner provides a surface with many seeds. The final cooling leads to leave the Ostwald-Miers area were a supersaturation of the solution takes places. Thus, a lot of seeds are formed and many small crystals are grown instead of few large ones. [35]
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Exploring multinary (Oxide) Nitride materials by the Ammonothermal approach

Exploring multinary (Oxide) Nitride materials by the Ammonothermal approach

on the basis of ammonothermally synthesized bulk samples. Within the second part, Chapters 4–6 report on the development of an ammonothermal access to (oxo)nitridophosphates, which significantly expands the structural diversity of ammonothermally accessible nitrides and emphasizes the great potential of the ammonothermal method itself for (oxide) nitride synthesis. In Chapter 7, finally, the lessons learned were used for further methodical development in terms of the ammonothermal synthesis of an oxonitridosilicate, representing a case study for future preparation of other (oxide) nitride materials. Within the following sections the presented results are briefly discussed in their scientific context and prospects for future investigations are provided that concern the field of ammonothermal synthesis.
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Indium gallium nitride nanostructures for optoelectronic applications

Indium gallium nitride nanostructures for optoelectronic applications

Kurzfassung Diese Arbeit befasst sich mit der Herstellung und Charakterisierung von Nanodr¨ahten des Indiumgalliumnitrid (InGaN) Materialsystems. Die Anwendung der Gruppe III- Nitride in optoelektronischen Bauelementen hat sich in den letzten Jahren fest etabliert. Sie sind daher von großem Interesse f¨ ur Forschung und Industrie. Insbesondere ist die Verwendung von Galliumnitrid (GaN) - InGaN Heterostrukturschichten als Basis der Licht-emittierenden Dioden (LEDs) zu erw¨ahnen. Diese werden nicht nur als energieef- fiziente Lichtquellen verwendet, sondern z.B. auch als blaue Laser in Blu-Ray-Playern oder als Hintergrundbeleuchtung in Displays. Durch Verwendung dieses Materia- lystems in Form von Nanostrukturen, insbesondere Nanodr¨ahten, soll eine gesteigerte Effizienz der Bauelemente erzielt werden. Durch ihre einzigartige Morphologie und das hohe Oberfl¨achen-zu-Volumen Verh¨altnis kann beispielsweise bei Anwendung in der Photovoltaik die Reflektion des einfallenden Lichts verringert und der Anteil an absorbiertem Licht erh¨oht werden. Zus¨atzlich bieten Nanodr¨ahte, im Gegensatz zu planaren Schichten, die M¨oglichkeit, den Einbau von Gitterfehlern beim Kristallwach- stum auf Fremdsubstraten, wie Silizium oder Saphir, zu reduzieren.
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