Growth Of AlN Layers Research Articles

Thermal-induced decomposition and mechanical properties of AlN/CrN superlattice coating

Here, the thermal-induced decomposition and mechanical properties of AlN/CrN superlattice coating with modulation period of ∼8 nm are investigated in detail. Introduction of CrN template layers stabilizes the cubic growth of AlN layers, where the coherent AlN/CrN coating exhibits the higher hardness value of ∼24.1 GPa than CrN (∼16.9 GPa) and AlN (∼18.5 GPa) monolithic coatings. After thermal exposure, the AlN/CrN coating decomposes into stable body-centered cubic Cr and wurtzite AlN via intermediate phases of hexagonal Cr2N and cubic AlN. Especially, the formation of metastable cubic AlN is observed due to the destruction of coherent interfaces. Meanwhile, the thermal load leads to the grain deformation, which causes the change of AlN and CrN sublayers from the crossover to pinch-off to finally dissociation. The driving force of grain deformation mainly arises from the N-loss of CrN sublayers and the high-density stacking faults formation of AlN sublayers. Furthermore, the occurrence of stacking faults and h-Cr2N phase offset the softening effect caused by the thermal decomposition. Correspondingly, the hardness almost remains unchanged between 900 and 1100 °C.

Seeded Growth of AlN Layers on Al‐ and N‐Polar AlN Seeds by the Cu–Al–Ca Solution Growth Method

Resistive Switching In article number 2300060, and colleagues have successfully grown AlN layers on both N-polar and Al-polar seed crystals using a Cu–Al–Ca solution system. The authors compared the surface morphologies by stereoscopic microscopy and optical microscope, growth rates by Cross-sectional SEM images, growth qualities by HRXRD, Raman and PL for the Al-polar and N-polar surfaces.

Polarity-controlled AlN/Si templates by in situ oxide desorption for variably arrayed MOVPE-GaN nanowires

In this paper, we present a comprehensive study of position-defined Al-polar AlN nucleation on lithographically patterned Si(111) substrates as a method to obtain ordered Ga-polar GaN nanowire arrays, with possible application in future nanowire-based devices such as LEDs and photoelectrochemical water-splitting cells. In a hydrogen processing step, ex situ prepared oxide on pre-structured Si-pillars could be selectively removed. This enabled Al-polar AlN nucleation on the Si-pillar’s sidewalls during the following metal–organic vapor phase epitaxy, while dominant N-polar AlN layer growth was observed on the still oxidized Si(111) horizontal substrate surface neighboring the pillars. 100% of the Ga-polar GaN wires are emerging on the Al-polar AlN growth sites, thus selective area epitaxy without any mask material could be realized. To gain a precise understanding of the growth mechanisms, the attainable Ga-adatom collection area per NW was varied by changing the Si-pillars’ placement pattern. The wire length and diameter increase with extended pitch. At a constant pitch, the size of the wires is adjustable by variation of the Si-pillars’ diameter, therefore growth of GaN wires of controllable dimension and pitch could be attained. Additionally, parasitic NW growth was completely suppressed for any pitch < 3.5 µm, while an increased pitch resulted in additional parasitic growth. Based on these results a model was derived, which includes the site-controlled removal of the oxide, the thus achieved local polarity control of AlN growth, and the influence of the collection area of each NW with respect to their size, whereas the collection area could be set in the experiment by adjustment of the lithographically controlled pitch and growth-parameter dependent Ga-adatom diffusion length. The mask-less polarity- and site-controlled growth of NWs with a height of 5.3 µm ± 0,33 µm and a diameter of 800 nm ± 160 nm at a pitch of 2.5 µm could be attained. Hence, a deep understanding of the growth mechanisms and the geometrical control of polarity- and site-controlled GaN NWs could be achieved, forming the base for development of NW-based devices on a conductive AlN/Si-template.

Void containing AlN layer grown on AlN nanorods fabricated by polarity selective epitaxy and etching method

Creating voids between thin films is a very effective method to improve thin film crystal quality. However, for AlN material systems, the AlN layer growth, including voids, is challenging because of the very high Al atom sticking coefficient. In this study, we demonstrated an AlN template with many voids grown on AlN nanorods made by polarity selective epitaxy and etching methods. We introduced a low V/III ratio and NH3 pulsed growth method to demonstrate high-quality coalesced AlN templates grown on AlN nanorods in a metal organic chemical vapor deposition reactor. The crystal quality and residual strain of AlN were enhanced by the void formations. It is expected that this growth method can contribute to the demonstration of high-performance deep UV LEDs and transistors.

Optical properties of AlN layers obtained by magnetron sputtering

Abstract The growth of AlN layers on glass substrates using magnetron sputtering method was performed and the grown layers were subjected to optical measurements. Transmission spectra of the layers grown at different content of N2 in the atmosphere were obtained. The transmission spectra as well as energy gap depended on N2 content. The annealing of the layers in air led to transmission changes and influenced energy gap and refractive index values.

Crystal defect analysis in AlN layers grown by MOVPE on bulk AlN

MOVPE growth of AlN layers on bulk AlN substrates with low threading dislocation density (<105 cm−2) can result in enhanced defect formation. Chemo-mechanical polishing (CMP) of bulk AlN using colloidal silica sol can produce locally disturbed surface with SiOx residuals. These surface disturbances lead to generation of threading dislocations (∼108 cm−2) in homoepitaxially grown AlN layers. These dislocations show a tendency to form nano- and even micropipes, which may be associated with oxygen accumulation along the dislocation lines. As result, the subsequently grown AlGaN-based layer structures exhibit a high number of v-pits and micropipes. Inductively coupled plasma etching of AlN substrate surface prior to MOVPE growth results in clean AlN surfaces and improves the AlGaN layer quality.

MOVPE growth and high-temperature annealing of (10[formula omitted]0) AlN layers on (10[formula omitted]0) sapphire

Heteroepitaxial growth of AlN layers on (101¯0) m-plane sapphire substrates was investigated by metalorganic vapour phase epitaxy. Different nucleation temperatures were used to achieve single phase (101¯0) AlN layers. The crystallinity of the layers further increased with increasing annealing temperature and annealing time. The full-width at half maximum of the symmetric (101¯0) X-ray rocking curves decreased from about 2900/1940 arcsec to 1030/800 arcsec along [0001]/[112¯0]AlN. The density of basal stacking faults of the annealed layers was found to decrease from ∼2.8 × 105 to ∼1.5 × 104 cm−1. The annealed layers had a larger effective optical bandgap energy of ∼6.15 eV than ∼6.04 eV of the as-grown layers due to their better crystallinity and structural order.

Epitaxial growth of single-crystalline AlN layer on Si(111) by DC magnetron sputtering at room temperature

The epitaxial growth of an AlN layer on a Si(111) substrate at room temperature by DC magnetron sputtering was investigated. The predeposition of a 5-nm-thick Al layer on the Si substrate before the AlN deposition was found to be crucial for the epitaxial growth of the AlN layer. The orientation relationships of AlN/Al/Si were observed to be AlN ∥ Al ∥ Si and AlN ∥ Al[011] ∥ Si, indicating the epitaxial growth of the AlN layer on the Si(111) substrate. This epitaxial growth of the AlN layer was attributed to the smaller lattice mismatches between AlN and Al and AlN and Al[011] than that between AlN and Si.

Semipolar AlN on Si(100): Technology and properties

Initial stages of the semipolar AlN layer formation by chloride-hydride vapor phase epitaxy on a misoriented Si(100) substrate with a thin intermediate nano-SiC layer grown by atomic substitution method are investigated. It is found that once the growth rate of AlN layer on a nano-SiC/Si(100) substrate is about 0.02μm/h, the layer is formed in a polar direction. On the other hand, at the growth rate of 1μm/h the layer is formed in a semipolar direction. We propose the model of the semipolar AlN layer growth based on the formation of the special structure on the SiC layer synthesized by atomic substitution and on the possibility of the matching of the quasi-lattices of Al(20−23) and 3CSiC(100) planes assuming bonding of four atoms from the layer to four atoms of the 3CSiC quasi-lattice consisting of three lattice unit cells in one of the directions.

High quality Al0.99Ga0.01N layers on sapphire substrates grown at 1150 °C by metalorganic vapor phase epitaxy

We systematically investigated metalorganic vapor phase epitaxy (MOVPE) growths of AlN layers with trimethylgallium (TMGa) supply on sapphire substrates at 1100–1250 °C. We found that Ga incorporations into the AlN layers contributed to smooth surfaces covered with step terraces at the early stage of the Al(Ga)N growth. In addition, a GaN mole fraction leading to the smooth surfaces was found to be around 2–3% at the beginning of growth. The Ga supply during the AlN layer growth at 1150 °C provided very smooth Al0.99Ga0.01N layers on sapphire substrates.

Distribution of dislocations in AlN crystals grown on evaporating SiC substrates

By the use of high-resolution X-ray diffractometry and synchrotron radiation topography, the distribution of dislocations in AlN crystals grown on evaporating SiC substrates is studied. The growth of AlN layer in one process with the evaporation of SiC substrate gave the possibility to prevent the relaxation of thermal stresses through cracking of AlN during cooling the structure. The continuous 0.2-1.5 mm thick plates were used as the model objects for the study of dislocation structure near the AlN/SiC interface. Analysis of the broadening of the Bragg reflection peaks, the shape of scattering maps in reciprocal space and topographs showed that dislocations formed the mosaic structure, different from the distribution of threading dislocations in GaN epilayers. A theoretical model for misfit strain relaxation in the growing AlN layer is suggested. The results enable to clarify the dislocation processes during sublimation growth of industrial quality AlN crystals.

Electron microscopy characterization of AlGaN/GaN heterostructures grown on Si (111) substrates

AlGaN/GaN buffer heterostructures were grown on “on axis” and 4 deg off Si (111) substrates by MOVPE. The electron microscopy study reveals the very good epitaxial growth of the layers. Almost c-plane orientated nucleation grains are achieved after full AlN layer growth. Step-graded AlGaN layers were introduced, in order to prevent the stress relaxation and to work as a dislocation filter. Thus, a crack-free smooth surface of the final GaN epitaxial layer is achieved in both cases, making the buffer structure ideal for the forthcoming growth of the heterostructure (used for HEMT device applications). Finally, the growth of the AlGaN/GaN heterostructure on top presents characteristic and periodic undulations (V-pits) on the surface, due to strain relaxation reasons. The AlN interlayer grown in between the heterostructure demonstrates an almost homogeneous thickness, probably reinforcing the 2DEG electrical characteristics.

XPS study of impurities in Si‐doped AlN film

This paper reports an XPS study of impurities in a 100‐nm‐thick AlN film grown by metalorganic chemical vapor deposition (MOCVD) under low pressure on the n‐type 6H‐SiC substrate. The Si‐doped AlN film was characterized by the X‐ray photoelectron spectroscopy (XPS) in a high vacuum system, which reveals the content distribution and chemical states of impurities along depth. The XPS analysis of AlN film before and after argon‐ion etching indicates that there always exist Ga, O and C contaminations in AlN film. Especially, O contamination on the AlN film surface is mostly introduced during the growth of AlN layer by MOCVD. Meanwhile, most of O atoms bind with Al or Ga in Al―O and Ga―O chemical states. In particular, the Ga atoms in AlN film are always in two chemical states, i.e. Ga―Ga bond and Ga―O bond, which demonstrates that the aggregation of Ga is accompanying with AlN growth. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Demonstration of isotype GaN/AlN/GaN heterobarrier diodes by NH3-molecular beam epitaxy

The results of vertical transport through nitride heterobarrier structures grown by ammonia molecular beam epitaxy are presented. Structures are designed with binary layers to avoid the effects of random alloy fluctuations in ternary nitride barriers. The unintentional incorporation of Ga in the AlN growth is investigated by atom probe tomography and is shown to be strongly dependent on both the NH3 flowrate and substrate temperature growth parameters. Once nominally pure AlN layer growth conditions are achieved, structures consisting of unintentionally doped (UID) GaN spacer layers adjacent to a nominally pure AlN are grown between two layers of n+ GaN, from which isotype diodes are fabricated. Varying the design parameters of AlN layer thickness, UID spacer layer thickness, and threading dislocation density show marked effects on the vertical transport characteristics of these structures. The lack of significant temperature dependence, coupled with Fowler-Nordheim and/or Milliken-Lauritsen analysis, point to a prevalently tunneling field emission mechanism through the AlN barrier. Once flatband conditions in the UID layer are achieved, electrons leave the barrier with significant energy. This transport mechanism is of great interest for applications in hot electron structures.

Dependence of the AlN-Layer growth rate on source-substrate clearance for the sublimation growth method

The proposed study is devoted to investigating the growth conditions of AlN thick layers and bulk crystals in the sublimation sandwich method. For the first time, the dependences of the layer-growth rate on source-substrate clearance are experimentally obtained for this material at two nitrogen-pressure values in the reactor: 0.3 and 0.6 bar. The obtained results point to the fact that the basic mechanism of the transport of components of the source to the substrate is diffusion. The contribution of kinetic effects becomes appreciable at the smallest clearance of 0.3 mm, especially, at a lower nitrogen pressure of 0.3 bar.

Homoepitaxial growth of AlN layers on freestanding AlN substrate by metalorganic vapor phase epitaxy

We investigated the optimum conditions for the homoepitaxial growth of AlN on freestanding AlN (0001) substrates (off angle: 0.6–0.9°) by metalorganic vapor phase epitaxy. The crystallinity, surface morphology, and full width at half maximum as obtained from X-ray rocking curves of the homoepitaxially grown AlN layer were strongly dependent on the growth conditions. When AlN layers are grown on an AlN substrate using a relatively low V/III ratio of 80, which is commonly used for AlN layer growth on a sapphire substrate, a high density of hillocks appeared at the AlN surface. In addition, the crystallinity of these hillocks was significantly worse than that of the substrate. By optimization of the growth temperature and V/III ratio, homoepitaxial AlN layers with good surface flatness and high crystallinity were realized.

An early in-situ stress signature of the AlN-Si pre-growth interface for successful integration of nitrides with (111) Si

The integration of Metal Organic Chemical Vapor Deposition (MOCVD) grown group III-A nitride device stacks on Si (111) substrates is critically dependent on the quality of the first AlN buffer layer grown. A Si surface that is both oxide-free and smooth is a primary requirement for nucleating such layers. A single parameter, the AlN layer growth stress, is shown to be an early (within 50 nm), clear (&amp;lt;0.5 GPa versus &amp;gt;1 GPa), and fail-safe indicator of the pre-growth surface, and the AlN quality required for successful epitaxy. Grain coalescence model for stress generation is used to correlate growth stress, the AlN-Si interface, and crystal quality.

Influence of source gas supply sequence on hydride vapor phase epitaxy of AlN on (0001) sapphire substrates

AlN layers were grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) at 1100°C with a source gas supply sequence of (1) NH3 preflow or (2) AlCl3 preflow. An Al-polarity AlN layer without inclusion of a N-polarity region was grown when AlCl3 was preflown to the sapphire surface prior to AlN growth, while N- and Al-polarity regions were both present in the same AlN layer when NH3 was preflown, since growth was performed on a nitrided sapphire surface. Compared with the AlN layers grown with NH3 preflow, the Al-polarity AlN layers grown with AlCl3 preflow had improved crystalline structural quality, a low concentration of oxygen impurity, and a photoabsorption edge energy of 6.08eV, which is close to an ideal value. Therefore, the source gas supply sequence has a significant influence on the growth of AlN layers on (0001) sapphire substrates. Thus, preflow of AlCl3 gas to a sapphire surface prior to AlN growth is a key process for high crystalline quality AlN layer growth with uniform Al-polarity on (0001) sapphire substrates by HVPE.

Carrier Gas Dependence at Initial Processes for a-Plane AlN Growth on r-Plane Sapphire Substrates by Hydride Vapor Phase Epitaxy

The influence of the carrier gas used during the thermal cleaning of r-plane sapphire substrates and the subsequent first AlN layer growth at 1050 °C on two-step growth of a-plane AlN layers by hydride vapor phase epitaxy (HVPE) was investigated. When hydrogen (H2) was used as the carrier gas, the decomposition of r-plane sapphire occurred during the thermal cleaning, and unintentional nitridation of the sapphire surface occurred at the beginning of the growth of the first AlN layer, which resulted in the occurrence of misoriented AlN grains in the second AlN layer grown at 1450 °C. When a mixture of H2 and nitrogen (N2) was used as the carrier gas, nitridation of the sapphire surface occurred during the thermal cleaning, which also resulted in the occurrence of misoriented AlN grains. A single-crystalline a-plane AlN layer free of misoriented grains could be obtained by using only N2 as the carrier gas during the thermal cleaning and the growth of the first AlN layer to prevent nitridation of the sapphire surface.

Carrier Gas Dependence at Initial Processes fora-Plane AlN Growth onr-Plane Sapphire Substrates by Hydride Vapor Phase Epitaxy

The influence of the carrier gas used during the thermal cleaning of r-plane sapphire substrates and the subsequent first AlN layer growth at 1050 °C on two-step growth of a-plane AlN layers by hydride vapor phase epitaxy (HVPE) was investigated. When hydrogen (H2) was used as the carrier gas, the decomposition of r-plane sapphire occurred during the thermal cleaning, and unintentional nitridation of the sapphire surface occurred at the beginning of the growth of the first AlN layer, which resulted in the occurrence of misoriented AlN grains in the second AlN layer grown at 1450 °C. When a mixture of H2 and nitrogen (N2) was used as the carrier gas, nitridation of the sapphire surface occurred during the thermal cleaning, which also resulted in the occurrence of misoriented AlN grains. A single-crystalline a-plane AlN layer free of misoriented grains could be obtained by using only N2 as the carrier gas during the thermal cleaning and the growth of the first AlN layer to prevent nitridation of the sapphire surface.