Semipolar AlN Research Articles

Effects of Dual Moderate-Temperature-Grown AIN Interlayers on Structural and Optical Properties of Semipolar (1122) AIN Film

High quality semipolar (1122) AlN films have been grown on (1010) m-plane sapphire substrates with the help of dual moderate-temperature-grown (MTG) AlN interlayers by using metal-organic chemical vapor deposition technology. The layer thickness of the semipolar (1122) AlN film was determined by employing relative optical transmittance spectrum measured with ultraviolet-visible spectrophotometer. The effect of the insertion of 80 nm-thick MTG AlN interlayer on structural and optical properties was investigated in detail based on the characterization results of the atomic force microscopy, high-resolution X-ray diffraction, and Raman spectroscopy. Comparing with the semipolar (1122) AlN film grown without the MTG AlN interlayer, both the surface morphology and crystalline quality of the semipolar (1122) AlN film grown with the insertion of dual 80 nm-thick MTG AlN interlayers have been improved significantly. In fact, the root mean square value of the surface roughness decreased from 3.5 to 1.4 nm, and the full width at half maximum value of X-ray rocking curve decreased from 1667 to 1174 arcsec, respectively. These facts reveal that the insertion of the dual MTG AlN interlayers is a powerful method to improve the surface morphology and crystalline quality of the semipolar (1122) AlN films owing to the formation of nanoscale patterned substrate-like structure and its blocking effect on the propagation of the dislocations.

Evaluations of the microstructures at the interface between the semipolar (101̄3) AlN epilayer and the m-plane (101̄0) sapphire substrate

We investigate the microstructures at the interface between the semipolar (101̄3) AlN epilayer and the m-plane (101̄0) sapphire substrate using electron microscopy techniques.

Полуполярные широкозонные III-N-слои на кремниевой подложке: эпитаксия с контролем свойств структур (обзор)

The experimental results of the recent years on the synthesis of semipolar wide-band III-N layers on a nanostructured silicon substrate are summarized. The idea of synthesis involves the formation of Si(111) side walls on the silicon surface, then the epitaxial nucleation of the layer in the “c” direction of the crystal, followed by the fusion of blocks in the semipolar direction of the surface. Examples of orientation controlling epitaxy of semipolar AlN(10-11), GaN(10-11), GaN(11-22) layers synthesized on nanostructured Si(100), Si(113) substrates by methods of metalorganic vapor phase epitaxy and hydride vapor phase epitaxy are shown. The review presents a summary and the prospects for further developments in the field of optoelectronics based on the platform - semipolar GaN on Si.

Influence of growth temperature and miscut angle of m-plane sapphire substrate on the semi-polar (11–22) AlN film grown by HVPE

The high-quality semi-polar (11-22) AlN thin films were grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE). The surface morphology and crystalline quality of the AlN film were greatly influenced by the growth temperature and the substrate miscut angle. As the temperature increased, the grain size on the surface increased and the grain density decreased. In addition, the higher growth temperature also resulted in smaller values of the full width at half maximum (FWHM) of X-ray rocking curves (XRC) when temperature was more than 1,460 °C. At high temperature of 1,530 °C, the introduction of 1° -off miscut angle to the substrate resulted in smooth surface, low density of stacking faults and low FWHM of XRC. The misfit dislocation density was calculated from the tilt angle of epilayer measured by X-ray reciprocal space mappings along [−1−123] AlN. The misfit dislocation density of the sample grown on 1° -off substrates was 6.7 × 105 cm−2. The improvement of crystal quality is believed to be due to the enhancement of adatom mobility at higher temperatures and also the appropriate miscut variation.

Multi-step in situ interface modification method for emission enhancement in semipolar deep-ultraviolet light emitting diodes

Semipolar III-nitrides have attracted increasing attention in applications of optoelectronic devices due to the much reduced polarization field. A high-quality semipolar AlN template is the building block of semipolar AlGaN-based deep-ultraviolet light emitting diodes (DUV LEDs), and thus deserves special attention. In this work, a multi-step in situ interface modification technique is developed for the first time, to our knowledge, to achieve high-quality semipolar AlN templates. The stacking faults were efficiently blocked due to the modification of atomic configurations at the related interfaces. Coherently regrown AlGaN layers were obtained on the in situ treated AlN template, and stacking faults were eliminated in the post-grown AlGaN layers. The strains between AlGaN layers were relaxed through a dislocation glide in the basal plane and misfit dislocations at the heterointerfaces. In contrast, high-temperature ex situ annealing shows great improvement in defect annihilation, yet suffers from severe lattice distortion with strong compressive strain in the AlN template, which is unfavorable to the post-grown AlGaN layers. The strong enhancement of luminous intensity is achieved in in situ treated AlGaN DUV LEDs. The in situ interface modification technique proposed in this work is proven to be an efficient method for the preparation of high-quality semipolar AlN, showing great potential towards the realization of high-efficiency optoelectronic devices.

The Structural Evolution of Semipolar (11-22) Plane AlN Tem-Plate on m-Plane Sapphire Prepared by Sputtering and High Temperature Annealing.

In this work, the epitaxial semipolar (11–22) AlN was prepared on nonpolar m-sapphire substrate by combining sputtering and high-temperature annealing. According to our systematic measurements and analysis from XRD, Raman spectra, and AFM, the evolution of crystalline structure and morphology was investigated upon increasing AlN thickness and annealing duration. The annealing operation intensively resets the lattice and improves the crystalline quality. By varying the film thickness, the contribution from the AlN-sapphire interface on crystalline quality and lattice parameters during the annealing process was investigated, and its contribution was found to be not so obvious when the thickness increased from 300 nm to 1000 nm. When the annealing was performed under durations from 1 to 5 h, the crystalline quality was found unchanged; meanwhile, the evolution of morphology was pronounced, and it means the crystalline reorganization happens prior to morphology reset. Finally, the annealing treatment enabled a zig-zag morphology on the AlN template along the sapphire [0001] direction in the plane, which potentially affects the subsequent device epitaxy process. Therefore, our results act as important experience for the semipolar nitride semiconductor laser device preparation, particularly for the epitaxy of microcavity structure through providing the crystalline evolution.

Improved surface morphology and crystalline quality of semi-polar ([formula omitted]) AlN epilayer with dual moderate-temperature-grown AlN interlayers

Semi-polar (112‾2) AlN epilayers have been grown on (101‾0) m-plane sapphire substrates with the help of dual moderate-temperature-grown (MTG) AlN interlayers by metal-organic chemical vapor deposition. Significant improvements in both surface morphology and crystalline quality of the semi-polar (112‾2) AlN epilayers have been achieved with the insertion of the MTG AlN interlayers due to the formation of nano-scale patterned substrate-like structure and the reduction in the basal-plane stacking faults with their associated partial dislocations. The effect of the variation in the thickness of the MTG AlN interlayer ranged from 20 to 100 nm was investigated in detail based on the characterization results of the atomic force microscopy (AFM) and X-ray diffraction. It was revealed that all the semi-polar AlN epilayer samples grown with the dual AlN interlayers on (101‾0) m-plane sapphire substrates were uniquely [112‾2]-oriented regardless of the variation in the thickness of the MTG AlN interlayer. Moreover, it was found that the most remarkable reduction in the surface roughness and the most effective improvement of structural quality could be obtained when the thickness for the inserted dual MTG AlN interlayers was approximately 80 nm. In fact, the smallest root mean square roughness of the semi-polar (112‾2) AlN epilayer surface was determined to be 1.4 nm by AFM measurement.

Initial Stages of Growth of Semipolar AlN on a Nanopatterned Si(100) Substrate

Initial Stages of Growth of Semipolar AlN on a Nanopatterned Si(100) Substrate

Semi-polar (11–22) AlN epitaxial films on m-plane sapphire substrates with greatly improved crystalline quality obtained by high-temperature annealing

This study addresses the difficulty of obtaining relatively low-cost semi-polar (11–22) AlN films epitaxially grown on m-plane sapphire substrates with high crystalline quality by applying a cost-effective high-temperature annealing process conducted at 1700 °C to films grown via standard-temperature metal organic chemical vapor deposition. The X-ray diffraction rocking curves of the annealed films along the [11–23] and [10–10] directions obtain full width at half maximum values of 0.152° and 0.193°, respectively. Atomic force microscopy results demonstrate the occurrence of a significant recrystallization process in the columnar formations of AlN films during annealing, resulting in increased AlN crystal grain size. Raman spectroscopy measurements reveal that the semi-polar (11–22) AlN films are subject to increased compressive stress after the annealing process. Transmission electron microscopy analyses confirm that our semi-polar AlN films provide appropriately low dislocation and basal stacking fault densities of 5.6 × 109 cm−2 and 1.05 × 105 cm−1, respectively. The proposed approach therefore obtains low-cost AlN films that are suitable for the mass manufacture of efficient optoelectronic devices.

Образование полуполярных III-нитридных слоев на поверхности Si(100), структурированной с помощью самоформирующейся наномаски

The epitaxial growth of AlN and GaN layers was studied by Metalorganic Vapor Phase Epitaxy, on a Si(100) substrate, on the surface of which a V-shaped nanostructure with sub-100 nm element size (NP-Si(100)) was formed. It is shown that a corrugated surface is formed from semipolar AlN(10-11) planes with opposite "c"axes during the formation of a semipolar AlN layer at the initial stage of epitaxy. Then, during the growth of the GaN layer, the transition from the symmetric state of two semipolar AlN planes to an asymmetric state with a single orientation of the "c"-axis of the semipolar GaN(10-11) layer occurs, and the "c" direction in the growing semipolar layer coincides with the direction of the flow of N2+ ions to the silicon surface during the formation of a nanomask.

Plastic Relaxation of Stressed Semipolar AlN($$10\bar {1}1$$) Layer Synthesized on a Nanopatterned Si(100) Substrate

Plastic relaxation of a stressed semipolar AlN( $$10\bar {1}1$$ ) layer synthesized on a nanopatterned Si(100) substrate has been investigated using scanning electron microscopy. It is shown that the application of a nanorelief consisting of triangular nanogrooves with inclined faces close to the Si(111) plane in a semipolar AlN layer can lead to the formation of cracks only in the direction perpendicular to a groove. Model concepts of plastic relaxation of the stressed semipolar layer are based on comparison of the threshold stress, above which cracks appear, with the thermomechanical stresses emerging because of the difference between the thermal expansion coefficients of the AlN/Si structure.

Single-phase high-quality semipolar (10–13) AlN epilayers on m-plane (10–10) sapphire substrates

Single-phase flat semipolar (10–13) AlN epilayers on m-plane (10–10) sapphire substrates grown by ammonia-free high-temperature metalorganic vapor phase epitaxy are firstly demonstrated. It is found that twins and basal-plane stacking faults are undetectable by X-ray diffraction and cross-sectional characterization. An X-ray rocking curve (XRC) shows the full widths at half maximum (FWHM) of the (10–13) and the (0002) diffraction peaks from a ∼2.3 μm thick AlN film as narrow as 322 and 373 arcsec, respectively, indicating a high structural quality. Semipolar AlN epilayers hold great promise for high performance deep-ultraviolet (DUV) optoelectronic device applications.

Indium incorporation and optical properties of polar, semipolar and nonpolar InAlN

Indium incorporation and the optical properties of InxAl1−xN layers (0 ≤ x ≤ 0.45) grown by metal-organic vapour phase epitaxy have been investigated simultaneously on polar (0001), untwinned semipolar (101̅3) and nonpolar (101̅0) AlN templates, which were prepared on planar sapphire substrates. The InN mole fraction () of the layers was tuned by changing growth temperature from 660°C to 860°C. determined by x-ray diffraction was found to be comparable for the polar, semipolar and nonpolar surface orientations. This is consistent with comparable effective bandgap energy of the layers obtained from optical transmission measurements at room temperature. The bandgap bowing parameter was found to be strongly composition-dependent. Room-temperature photoluminescence measurements showed impurity transitions for the layers with ≤ 0.2, while InAlN near-band-edge luminescence was observed for the layers with higher .

Рост объемных эпитаксиальных пленок AlN полуполярной ориентации на подложках Si (001) и гибридных подложках SiC/Si (001)

Abstract The possibility of growing bulk (more than 7 μm thick) epitaxial semipolar AlN films on Si(001) and hybrid SiC/Si(001) substrates without cracks has been investigated. It is found that an AlN layer grown on the Si substrate is extended, whereas an AlN layer grown on the hybrid SiC/Si substrate is compressed. The limiting (critical) thickness of the semipolar AlN layer on the Si(001) substrate is determined to be ~7.5 μm. When the film thickness exceeds this value, an ensemble of cracks is formed in the film, leading to its total cracking and exfoliation from the substrate. The semipolar epitaxial AlN films with a thickness of more than 40 μm are grown on hybrid SiC/Si substrates without cracking and exfoliation from the substrate.

Hydride Vapor-Phase Epitaxy of a Semipolar AlN(10$$\bar {1}$$2) Layer on a Nanostructured Si(100) Substrate

We propose a method for the synthesis of hexagonal AlN layer on Si(100) substrate with a V-groove nanostructured surface where the angle between the sloped nanoridge surface and Si(100) plane amounts to 47°. It is established that metalorganic hydride vapor-phase epitaxy (HVPE) on this substrate leads to the formation of semipolar AlN(10$$\bar {1}$$2) layers having an X-ray rocking curve with a minimum FWHM value of ωθ ~ 60 arcmin. Raman spectra of this epilayer display additional peaks related to A1(TO) and E1(TO) phonons in contrast to the spectrum of a polar AlN(0001) layer containing an additional A1(LO) peak.

Хлорид-гидридная газофазная эпитаксия полуполярного слоя AlN(1012) на наноструктурированной подложке Si(100)

A new approach is proposed to the synthesis of a semipolar AlN on a Si(100) substrate at the surface for which the angle between the inclined plane of the nanogrooves and Si(100) is 47°. It is shown that the hydride vapor-phase epitaxy on a such nano-patterned substrate enables formation of a semipolar layer AlN (1012) characterized by the full width at half maximum value as low as ωθ ~60 arcmin for the x-ray diffraction rocking curve. It is found that the Raman spectra of the semipolar AlN(10-12) layer contain additional peaks on the Raman curves associated with phonons A1(TO) and E1(TO), in contrast to the polar AlN(0001) layer, where the peak A1(LO) is additionally manifested.

Temperature-dependence of Cl2/Ar ICP-RIE of polar, semipolar, and nonpolar GaN and AlN following BCl3/Ar breakthrough plasma

The authors report a comprehensive investigation of temperature-dependence of inductively coupled plasma reactive ion etching (ICP-RIE) of polar (0001), semipolar (11−22), and nonpolar (11−20) GaN and AlN, in the temperature range of 22–205 °C. The main objective is to study the effect of ICP etching near and beyond the boiling point of the volatile etch end-products: GaCl3 (201 °C) for GaN and AlCl3 (180 °C) for AlN. High-temperature ICP-RIE is beneficial in quicker removal of surface oxides and may permit the use of a single-step Cl2/Ar ICP-RIE for etching all orientations of GaN and AlN. However, the best results are still obtained with a combination of BCl3-plasma based surface oxide removal pretreatment and Cl2/Ar ICP-RIE etching, which provides a constant etch rate with a smooth surface morphology irrespective of the etching temperature.

Metalorganic vapor phase epitaxy of pit-free AlN homoepitaxial films on various semipolar substrates

Semipolar AlN homoepitaxial films, which are expected to act as underlying layers of highly efficient light emitters, are fabricated on 15°-off (0001), (11¯02), and (112¯2) AlN substrates using the metalorganic vapor phase epitaxy method. In conventional (0001) AlN growth, low reactor pressures are preferred to enhance the migration of Al adatoms and to suppress parasitic reactions between trimethylaluminum and ammonia. In contrast, low-pressure growth generates numerous pits on the surface of semipolar AlN grown homoepitaxially, which are derived from defects formed in the initial growth stage. Herein we experimentally demonstrate that higher-pressure growth can drastically decrease the pit density. A higher-pressure growth realizes atomically smooth surfaces, strong near-band-edge emissions with narrow line widths (∼1–2 meV), and well-suppressed deep level emissions. The optimal reactor pressure to eliminate pits is 500 Torr in terms of the growth rate and nucleation density.

New Semipolar Aluminum Nitride Thin Films: Growth Mechanisms, Structure, Dielectric and Pyroelectric Properties

Surface topography, dielectric and pyroelectric properties of semipolar AlN thin films are studied. The growth of AlN was carried out by hydride-chloride vapor phase epitaxy on hybrid SiC layers specially prepared for the growth of semipolar AlN layers. SiC layers were grown on Si substrates of p-type (KDB) with (100)-orientation. The dynamic pyroelectric response of the AlN layer was observed. The pyroelectric current kinetics was analyzed in the frequency range of 10–1000 Hz, and the calculated pyroelectric coefficient was nearly equal to 9·10−10 C/(cm2K).

Controlled crystal orientations of semipolar AlN grown on an m-plane sapphire by MOCVD

The way to control AlN orientations on an m-plane sapphire in metal-organic chemical vapor deposition has not been clarified. We investigated the effect of the nitridation process on the crystal orientation of semipolar AlN on an m-plane sapphire. In terms of substrate heating, (10-1-3) AlN was obtained under H2 without NH3, whereas (11–22) AlN was obtained by heating with H2 and NH3. Temperature-dependent nitridation demonstrated that the AlN orientation changed from (10-1-3) to (11–22) at the nitridation temperature of 500 °C, which can be explained by the formation of r-plane nanofacets on m-plane sapphire under significant nitridation at a high temperature. At a nitridation temperature of 1100 °C, a small amount of NH3 led to the formation of (11–22) AlN, indicating that care must be taken to keep the reactor conditions for producing desired AlN orientations with reproducibility.