Transition between amorphous and crystalline phases of SiC deposited on Si substrate using H3SiCH3

This paper presents a study of the transition between amorphous and crystalline phases of SiC films deposited on Si(1 0 0) substrate using H₃SiCH₃ as a single precursor by a conventional low-pressure chemical vapor deposition method in a hot-wall reactor. The microstructure of SiC, characterized by X-ray diffraction and high-resolution transmission electron microscopy, is found to vary with substrate temperature and H₃SiCH₃ pressure. The grain size decreases with increasing MS pressure at a given temperature and also decreases with reducing temperature at a given MS pressure. The deposition rates are exponentially dependent on the substrate temperature with the activation energy of around 2.6 eV. The hydrogen compositional concentration in the deposited SiC films, determined by secondary ion mass spectrometry depth profiling, is only 2.9% in the nanocrystalline SiC but more than 10% in the amorphous SiC, decreasing greatly with increasing deposition temperature. No hydride bonds are detected by Fourier transform infrared spectroscopy measurements. The chemical order of the deposited SiC films improves with increasing deposition temperature.     

Epitaxial films of metals from the platinum group (Ir,Rh, Pt and Ru) on YSZ-buffered Si(1 1 1)

In the present work we have grown twin-free single crystal metal films of iridium (Ir), rhodium (Rh), platinum (Pt) and ruthenium (Ru) on silicon (1 1 1) substrates via an yttria-stabilized zirconia (YSZ) buffer layer. A prerequisite for the realisation of heteroepitaxial metal films without additional texture components was the twin-free deposition of the YSZ films by pulsed laser deposition (PLD). For the metal films on top, a novel two-step growth process was applied with an extremely low deposition rate for the first 20 nm. For all metals, a drastic texture improvement by up to a factor of 9 could be observed compared to the oxide buffer layer. Minimum values were 0.18° (Ir) and 0.12° (Rh) for tilt and twist, respectively. For all four metals investigated, twin-free epitaxial films could be grown on YSZ/Si(1 1 1) whereas the twinning problem for platinum films was solved by decoupling the Pt-YSZ interface via an additional iridium interlayer. The grown metal/YSZ/Si(1 1 1) multilayer samples offer the possibility to integrate a variety of interesting nanostructures and functional materials on silicon. They are now available in 4 in wafer size.     

Selective area growth of GaN nanorods on patterned W/SiO2/Si substrates by RF-MBE

We report on the selective area growth (SAG) of GaN nanorods on Si substrates masked with W or SiO₂ and also on bare Si substrates by RF plasma-assisted molecular beam epitaxy (RF-MBE). The growth of GaN (i.e. irradiation of Ga and RF plasma-activated N₂) on the W mask layer results in the appearance of a ring reflection high-energy electron diffraction (RHEED) pattern coming from α-W. In contrast, broken ring RHEED patterns from GaN nanorods are clearly observed on SiO₂ and Si surfaces. Ex-situ scanning Auger microscopy analysis confirms that no growth of GaN takes place on W. Utilizing this phenomenon, we have demonstrated the SAG of GaN nanorods on Si substrates partly masked with W. We will discuss this phenomenon in terms of the difference in the desorption energy of Ga on W, SiO₂, and Si.     

Low-temperature deposition of crystalline silicon nitride nanoparticles by hot-wire chemical vapor deposition

The nanocrystalline alpha silicon nitride (α-Si₃N₄) was deposited on a silicon substrate by hot-wire chemical vapor deposition at the substrate temperature of 700 °C under 4 and 40 Torr at the wire temperatures of 1430 and 1730 °C, with a gas mixture of SiH₄ and NH₃. The size and density of crystalline nanoparticles on the substrate increased with increasing wire temperature. With increasing reactor pressure, the crystallinity of α-Si₃N₄ nanoparticles increased, but the deposition rate decreased.     

Low-temperature RPCVD of Si,SiGe alloy,and Si1−yCy films on Si substrates using trisilane (Silcore)

Si homo-epitaxial growth by low-temperature reduced pressure chemical vapor deposition (RPCVD) using trisilane (Si₃H₈) has been investigated. The CVD growth of Si films from trisilane and silane on Si substrates are compared at temperatures between 500 and 950 °C. It is demonstrated that trisilane efficiency increases versus silane's one as the surface temperature decreases. Si epilayers from trisilane, with low surface roughness, are achieved at 600 and 550 °C with a growth rate equal to 12.4 and 4.3 nm min⁻¹, respectively. It is also shown that Si_1−xGe_x layers can be deposited using trisilane chemistry.     

MOVPE growth of single-crystal hexagonal AlN on cubic diamond

We have obtained single-crystal aluminum nitride (AlN) layers on diamond (1 1 1) substrates by metalorganic vapor-phase epitaxy (MOVPE). When the thermal cleaning temperature of the substrate and growth temperature of the AlN layer were below 1100 °C, the AlN layer had multi-domain structures mainly consisting of rotated domains. An interface layer, consisting of amorphous carbon and poly-crystal AlN, was formed between the AlN layer and the diamond substrate. On the other hand, when the thermal cleaning temperature and growth temperature were above 1200 °C, a single-crystal AlN layer was grown and no interface layer was formed. Therefore, we attribute the multi-domain structures to the interface layer. Even at the growth temperature of 1100 °C, by performing the thermal cleaning at 1200 °C, the single-crystal AlN layer was obtained, indicating that the thermal cleaning temperature of the substrate is a critical factor for the formation of the interface layer. The epitaxial relationship between the single-crystal AlN layer and the diamond (1 1 1) substrate was determined to be [0 0 0 1]_AlN∥[1 1 1]_diamond and [1 0 1¯ 0]_AlN∥[1 1¯ 0]_diamond. The AlN surface had Al polarity and no inversion domains were observed in the AlN layer.     

The effect of temperature and ammonia flux on the surface morphology and composition of InxAl1−xN epitaxial layers

An interesting recent development in the Group III nitrides is the growth of InAlN lattice matched to GaN, with applications in distributed Bragg reflectors (DBRs), high electron mobility transistors (HEMTs) and as etch-layers. This work presents a systematic study of the effects of changing the key growth conditions of ammonia flux and growth temperature in InAlN growth by metal-organic vapour phase epitaxy (MOPVE) and describes our current optimised parameter set. We also particularly concentrate on the details of surface morphology assessed by atomic force microscopy (AFM). The nanoscale surfaces are characterised by low hillocks and dislocation pits, while at a larger scale microscopic indium droplets are also present. However, these droplets are eliminated when the layers are capped with GaN. Other trends observed are that increasing the growth temperature will lower the indium incorporation approximately linearly at a rate of approximately 0.25% per °C, and that increasing the ammonia flux from 44.6 to 178.6 mmol min⁻¹ increased the indium incorporation, but further increases to 446 mmol min⁻¹ did not result in any further increase.     

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality AlxGa1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al_xGa_1−x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al_xGa_1−xN was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al_0.1Ga_0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al_0.65Ga_0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al_xGa_1−x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.     

HRTEM investigation of high-reflectance AlN/GaN distributed Bragg-reflectors by inserting AlN/GaN superlattice

A high-quality AlN/GaN distributed Bragg-reflectors (DBR) was successfully grown on sapphire substrate by low-pressure metal-organic chemical vapor deposition using ultra-thin AlN/GaN superlattice insertion layers (SLILs). The reflectivity of AlN/GaN DBR with ultra-thin AlN/GaN SLIL was measured and achieved blue peak reflectivity of 99.4% at 462 nm. The effect of ultra-thin AlN/GaN superlattice insertion layer was examined in detail by transmission electron microscopy, and indicated that the crack of AlN/GaN DBR can be suppress by inserting AlN/GaN SLIL. For electronic properties, the turn on voltage is about 4.1 V and CW laser action of vertical-cavity surface-emitting laser (VCSEL) was achieved at a threshold injection current of 1.4 mA at 77 K, with an emission wavelength of 462 nm.     

Growth of undoped and Zn-doped GaN nanowires

Undoped and Zn-doped GaN nanowires were synthesized by chemical vapor deposition (CVD), and the effects of substrates, catalysts and precursors were studied. A high density of GaN nanowires was obtained. The diameter of GaN nanowires ranged from 20 nm to several hundreds of nm, and their length was about several tens of μm. The growth mechanism of GaN nanowires was discussed using a vapor–liquid–solid (VLS) model. Furthermore, room-temperature cathodoluminescence spectra of undoped and Zn-doped GaN nanowires showed emission peaks at 364 and 420 nm, respectively.     

Growth kinetics of SiGe/Si superlattices on bulk and silicon-on-insulator substrates for multi-channel devices

We have studied in reduced pressure chemical vapor deposition the growth kinetics of Si and Si_0.8Ge_0.2 on bulk Si(0 0 1) and on silicon-on-insulator (145 nm buried oxide/20 nm Si over-layer) substrates. For this, we have grown at 650 °C, 20 Torr 19 periods (Si_0.8Ge_0.2 19 nm/Si 32 nm) superlattices on both types of substrates that we have studied in secondary ion mass spectrometry, X-ray diffraction and cross-sectional transmission electron microscopy. The Si and SiGe growth rates together with the Ge content are steady on bulk Si(0 0 1), with mean values around 9.5 nm min⁻¹ and 20.2%, respectively. In contrast, growth rates decrease from ∼9.5 nm min⁻¹ down to values around 7.0 nm min⁻¹ (SiGe) and 6.3 nm min⁻¹ (Si), when the deposited thickness on SOI increases from 0 up to slightly more than 100 nm. They then go back up to values around 8.8–9.0 nm min⁻¹ as the thickness increases from 100 up to 400 nm. They then slowly decrease to values around 8.4–8.6 nm min⁻¹ as the thickness increases from 400 up to 800 nm. The Ge concentration follows on SOI exactly the opposite trend: an increase from 19.9% (0 nm) up to 20.6% (∼100 nm) followed by a decrease to values around 20.1% (400 nm) then a slow re-increase up to 20.4% (800 nm). These fluctuations are most likely due to the following SOI surface temperature variations: from 650 °C down to 638 °C (100 nm), back up to 648 °C (400 nm) followed by a slow decrease to 646 °C (800 nm). These data curves will be most useful to grow on conventional SOI substrates large number of periods, regular Si/Si_0.8Ge_0.2 superlattices that will serve as the core of multi-channel or three-dimensional nano-wires field effect transistors.     

The epitaxial growth of sputtered silicon layers on differently oriented substrates at low temperature - An electron microscopic study

We investigated epitaxial silicon films deposited on differently oriented substrates by pulsed magnetron sputtering at temperatures of 500-550 °C. Our scanning and transmission electron microscopic as well as electron backscattering investigations show that epitaxial films grow not only on (1 0 0)-oriented substrates, but also on (2 1 0)-, (4 1 1)- and (3 1 1)-oriented ones. A change to the (1 0 0) orientation is found for the growth on (1 1 1)- , (3 2 1)- and close to (1 1 0)-oriented substrates. For these orientations transmission electron microscopic investigations show stacking faults, microtwins and small amorphous inclusions in a region starting at the substrate-film interface up to thicknesses of 150-200 nm. With increasing film thickness above 200 nm the crystalline perfection of the epitaxial layers improves.     

Effects of initial stages on the crystal quality of nonpolar a-plane AlN on r-plane sapphire by low-pressure HVPE

A 4–6 μm thick a-plane (1 1 2¯ 0) AlN was grown on r-plane sapphire substrate by low-pressure hydride vapor phase epitaxy (LP-HVPE), using a direct growth without any nitridation and buffer layer, a single-step nitridation growth, a two-step nitridation growth and a two-step buffer growth method. For the two-step buffer growth procedure, smoother surface is observed with the lower full widths at half maximum (FWHM) of X-ray rocking curves (XRC) compared with the other two kinds of nitridation procedures. A smaller FWHM of in-plane XRC peak anisotropy features are reversed, which is consistent with the smaller in-plane stress anisotropic distribution in a-plane AlN, when the two-step nitridation or buffer growth method is used. In four kinds of initial growth procedures, the two-step buffer method is the suitable method for the growth of a-plane AlN by HVPE with the high crystal quality and more isotropic distribution.     

Nature of luminescence and strain in gallium nitride nanowires

Photo- and cathodo-luminescence measurements of a variable-diameter ensemble of GaN nanowires revealed a diameter-dependent, spectral emission distribution between 350 nm and 850 nm. Spectral analysis indicated that wires with a diameter less than 400 nm were dominated by a yellow luminescence with a weaker near UV/violet emission also present. Examination of this ensemble showed that there was a general trend in the ratio of near-UV-to-yellow emission intensities with increasing nanowire diameter. Additionally, a broad green emission appears in the nanowires with a diameter above approximately 200 nm. A calculation based on the nanoheteroepitaxy model indicates that this diameter represents a transitional thickness where strain is relieved by defect formation mechanisms with a characteristic green emission.     

Influence of off-cut angle of r-plane sapphire on the crystal quality of nonpolar a-plane AlN by LP-HVPE

The effect of the off-cut angle of an r-plane sapphire substrate has been investigated on the growth of a-plane AlN thick layer by low-pressure hydride vapor phase epitaxy (LP-HVPE). The off-cut angle (θ) was changed from +5.0° (close to c-axis) to −5.0° (close to m-axis). Results show that the crystalline quality and surface morphology are very sensitive to the sign of θ off-angle. The plus θ off-angle is found to be dramatically reduce the full-widths at half-maximum (FWHM) of X-ray rocking curves (XRC), compared with the minus θ off-angle. In-plane FWHM anisotropic feature marked as M- or W-shape dependence on azimuth angle was observed for a-plane AlN. The shape and degree of anisotropy depend on the sign of θ off-angle, while the plus of θ off-angle will leads to the W-shape and the decreased anisotropy. The minimum crystal tilts and twists of the films are observed for the vicinal sapphires with the plus off-angles of +0.2° to +1.0°.     

Electron microscopy analysis of dislocation behavior in HVPE-AlGaN layer grown on a stripe-patterned (0 0 0 1) sapphire substrate

Microstructures were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in order to clarify the dislocation behavior in AlGaN layers HVPE-grown on a stripe-patterned sapphire (0 0 0 1) substrate. SEM observation revealed very clearly the growth process: if AlGaN starting to grow from the side-wall of patterned substrate develops, a poly-crystalline region is formed up to the top surface of thin film. When the growth from the upper side (terrace) of patterned substrate is predominant, AlGaN becomes a single-crystalline layer with a flat surface. Threading dislocations (TDs) generated from the interface to the terrace propagate upwards, inclining to the wing regions. They are scarcely merged with one another. The AlGaN layer on the patterned substrate with a wider groove has a smaller density of dislocation to be about 1×10⁹ cm⁻². There are four types of dislocations: (1) TDs inclining toward 〈1 1¯ 0 0〉 normal to their Burgers vector B; (2) TDs inclining toward 〈2 1¯ 1¯ 0〉 on their slip-plane; (3) TDs inclining largely or horizontal dislocations (HDs) along 〈2 1¯ 1¯ 0〉 and (4) roundly curved HDs lying on (0 0 0 1) plane. Some TDs change the direction of inclination, suggesting that internal stress changed intricately during the growth.     

Photoluminescence study of Si-doped a-plane GaN grown by MOVPE

Si-doped a-plane GaN films with different doping concentrations were grown by metal-organic vapor phase epitaxy. A mirrorlike surface without pits or anisotropic stripes was observed by optical microscopy. Detailed optical properties of the samples were characterized by temperature- and excitation-intensity-dependent PL measurements. A series of emission peaks at 3.487, 3.440, 3.375–3.350, 3.290 and 3.197 eV were observed in the low-temperature PL spectra of all samples. The origin of these emissions is discussed in detail.     

Growth of InN films and nanostructures by MOVPE

Since a few years, a lot of research efforts have been devoted to InN, the least known of the semiconducting group-III nitrides. Most of the samples available today have been grown using the molecular beam epitaxy technique, and fewer using the metal organic vapor phase epitaxy (MOVPE) method. Whatever the method, the growth of InN is extremely challenging, in particular due to the fact that no lattice matched substrate is available.     

Fabrication of a GaN lateral polarity junction by metalorganic chemical vapor deposition

A fabrication process for growth of GaN lateral polarity junctions consisting of Ga-polar and N-polar domains grown simultaneously side-by-side on c-plane sapphire was developed using the polarity control scheme. An ammonia-annealing step following deposition and patterning of a thin low-temperature AlN nucleation layer played a crucial role in avoiding mixed-polarity growth of the remaining AlN nucleation layer, as well as in nitriding the bare sapphire surface to facilitate growth of N-polar GaN. The achievement of both polar domains, free from inversion domains within a contiguous domain, led to Ga-polar domain exhibiting featureless morphology with highly resistive characteristics, while N-polar domains exhibited hexagonally faceted morphology and were highly conductive.     

Characterizations of GaN films and AlGaN/GaN heterostructures on vicinal sapphire (0 0 0 1) substrates grown by MOCVD

GaN films and AlGaN/GaN heterostructures grown on vicinal sapphire (0 0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that surface morphologies of GaN films depend on the vicinal angle, however, they are not sensitive to the inclination directions of the substrate. The optimized vicinal angle for obtaining excellent surface morphology is around 0.5°. This conclusion is also confirmed by characterizing the electrical property of two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure.