AlN Templates Research Articles

Detailed analysis of Ga-rich current pathways created in an n-Al0.7Ga0.3N layer grown on an AlN template with dense macrosteps

To clarify the behavior of the AlGaN in 20 nm wide Ga-rich current pathways in an n-AlGaN layer, which assists carrier localization in AlGaN-based light-emitting diodes, we performed a detailed analysis using an n-Al0.7Ga0.3N layer on AlN with dense macrosteps on a 1.0° miscut sapphire substrate. Energy-dispersive X-ray spectra, obtained using cross-sectional scanning transmission electron microscopy calibrated by Rutherford backscattering and cross-sectional cathodoluminescence spectra, indicated that AlN mole fraction in the Ga-rich current pathways was nearly ~2/3. This result is consistent with those of other research groups, suggesting that metastable Al2/3Ga1/3N is created in Ga-rich current pathways.

UVB LEDs Grown by Molecular Beam Epitaxy Using AlGaN Quantum Dots

AlGaN based light emitting diodes (LEDs) will play a key role for the development of applications in the ultra-violet (UV). In the UVB region (280–320 nm), phototherapy and plant lighting are among the targeted uses. However, UVB LED performances still need to be improved to reach commercial markets. In particular, the design and the fabrication process of the active region are central elements that affect the LED internal quantum efficiency (IQE). We propose the use of nanometer-sized epitaxial islands (i.e., so called quantum dots (QDs)) to enhance the carrier localization and improve the IQE of molecular beam epitaxy (MBE) grown UVB LEDs using sapphire substrates with thin sub-µm AlN templates. Taking advantage of the epitaxial stress, AlGaN QDs with nanometer-sized (≤10 nm) lateral and vertical dimensions have been grown by MBE. The IQE of the QDs has been deduced from temperature dependent and time resolved photoluminescence measurements. Room temperature IQE values around 5 to 10% have been found in the 290–320 nm range. QD-based UVB LEDs were then fabricated and characterized by electrical and electroluminescence measurements. On-wafer measurements showed optical powers up to 0.25 mW with external quantum efficiency (EQE) values around 0.1% in the 305–320 nm range.

Sec-Eliminating the SARS-CoV-2 by AlGaN Based High Power Deep Ultraviolet Light Source.

The world‐wide spreading of coronavirus disease (COVID‐19) has greatly shaken human society, thus effective and fast‐speed methods of non‐daily‐life‐disturbance sterilization have become extremely significant. In this work, by fully benefitting from high‐quality AlN template (with threading dislocation density as low as ≈6×108 cm−2) as well as outstanding deep ultraviolet (UVC‐less than 280 nm) light‐emitting diodes (LEDs) structure design and epitaxy optimization, high power UVC LEDs and ultra‐high‐power sterilization irradiation source are achieved. Moreover, for the first time, a result in which a fast and complete elimination of SARS‐CoV‐2 (the virus causes COVID‐19) within only 1 s is achieved by the nearly whole industry‐chain‐covered product. These results advance the promising potential in UVC‐LED disinfection particularly in the shadow of COVID‐19.

High Temperature Annealing SP-AlN Ameliorates the Crystal Quality ofAl<sub>0.5</sub>Ga<sub>0.5</sub>N Regrowth

In this study, the effect of a high-temperature annealing process on AlN is investigated. The high-temperature annealing process reduces the screw dislocation density of the AlN film to 2.1x107 cm-2. The AlN surface is highly flat. Through HRXRD and Raman spectroscopy, the stress mode changes in the sputtered AlN film before and after high-temperature annealing were studied in depth. Based on the HTA-AlN template, a high-quality, high-Al composition AlGaN epitaxial wafer, with a (0002) plane rocking curve FWHM of 246 arcsec , was prepared at 1080°C The growth mode of AlGaN grown directly on the AlN template at low temperature is summarized.

High‐Quality AlN Template Prepared by Face‐to‐Face Annealing of Sputtered AlN on Sapphire

This study comprehensively investigates the properties of metalorganic vapor phase epitaxy (MOVPE)‐grown AlN films on high‐quality face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) templates on sapphire substrates, which are highly important to control the surface morphology for various applications, such as UV light‐emitting diodes and laser diodes. The conditions of thermal cleaning and AlN growth by MOVPE are investigated to remove numerous small islands on as‐annealed FFA Sp‐AlN. Subsequent to thermal cleaning in H2 + NH3 at 1300 °C, MOVPE growth is performed with varying NH3 flow rate and growth temperature (Tg) under a constant pressure and group‐III flow rate. An atomically flat surface with an atomic step‐and‐terrace structure is obtained at a growth rate of ≈1.0 μm h−1 and a Tg of 1300 °C. Transmission electron microscopy images and secondary‐ion mass spectrometry reveal low dislocation densities and impurity concentrations. Finally, the effects of compressive strain in FFA Sp‐AlN on the lattice constant and curvature of the MOVPE‐grown AlN film on FFA Sp‐AlN are investigated. The compressive strain of the AlN film, which is carried over from FFA Sp‐AlN, can prevent crack formation but leads to a large wafer curvature after cooling down from the Tg.

Compressive strain induced enhancement of transverse-electric polarized ultraviolet light emission for AlGaN quantum wells

The impact of strain on optical polarization of c-plane Al0.37Ga0.63N/Al0.5Ga0.5N multiple quantum wells (QWs) has been studied by the polarized photoluminescence spectroscopy at room temperature. By adopting multiple alternation cycles of low- and high-temperature growth, the strain state of AlN template can be modulated. With increasing in-plane compressive strain of QWs layers, the polarization degree can be enhanced from 41.5% to 61.9%. Furthermore, the strain dependence of the polarization degree has been confirmed by the numerical simulation based on k·p method, revealing the dominant role of energy level shift and wavefunction overlap. It is found that the relative position of the three valence subbands is responsible for the steep increase of the polarization at smaller strain, while the wavefunction overlap induced by piezoelectric effect modulates the optical polarization at larger strain. These results are of great importance in the design and analysis of AlGaN based ultraviolet light emitting devices.

Mask-free three-dimensional epitaxial growth of III-nitrides

A novel catalyst-free and maskless growth approach is presented to form an ordered geometrical array of three-dimensional (3D) AlGaN/AlN microrods. The growth method is composed of a single growth step using metalorganic vapor phase epitaxy, achieving microstructures with homogeneous diameters, shapes and sizes over relatively large scale (on 2-in. wafer). The 3D AlGaN/AlN heterostructures are grown in a form of micro-sized columns elongated in one direction perpendicular to the substrate surface and with a hexagonal cross section. A careful examination of growth steps revealed that this technology allows to suppress coalescence and lateral overgrowth, promoting vertical 3D growth. Interestingly, two distinct morphologies can be obtained: honeycomb-like hexagonal arrangement perfectly packed and with twisted microrods layout, by controlling strain state in AlN buffer layers. Consequently, 3D AlGaN microrods on tensile-strained AlN templates show a 0° twisted morphology, while on compressive-strained templated a 30° twisted arrangement. Moreover, the optical and crystalline quality studies revealed that the top AlGaN layers of the examined 3D semiconductor structures are characterized by a low native point-defect concentration. These 3D AlGaN platforms can be applied for light emitting devices or sensing applications.Graphic abstract

Improvement of AlN Material Quality by High-Temperature Annealing Toward Power Diodes

AlN devices have the potential to outperform current GaN devices, especially in the high-voltage region, thanks to AlN’s larger critical electric field and thermal conductivity. In order for the AlN-based power diodes to be realized and performance to be demonstrated, high-quality AlN crystals would be the prerequisite. In this article, significantly improved AlN quality has been demonstrated through high-temperature treatment of AlN thin layers at high temperatures in the range of 1670–1730 °C, followed by regrowth of AlN layers on top. The produced AlN layer shows much improved crystalline quality when compared to AlN layer without thermal treatment. The thermal treatment of AlN nucleation layers recrystallizes the AlN materials and results in a significant reduction of the full-width at half-maximum (FWHM) values of X-ray rocking curves of AlN (002) and (102) planes. The annealing of AlN nucleation also produced much pronounced atomic step features and improves the transmittance for the nucleation layers. Regrowth of AlN templates on such annealed nucleation improves AlN crystalline quality further, producing high-quality templates for power diode applications.

Epitaxial growth of polarization-graded AlGaN-based solar-blind ultraviolet photodetectors on pre-grown AlN templates

In this letter, based on the method of introducing one mesothermal AlN (MT-AlN) interlayer, a high-quality AlN template is firstly obtained by adjusting the growth rate of the high-temperature AlN (HT-AlN) epilayers on both sides of the MT-AlN interlayer. Then, the epitaxial growth of polarization-graded AlGaN-based solar-blind ultraviolet photodetector (SUV-PD) structural material is implemented on the pre-grown AlN template by introducing an n-AlxGa1−xN gradient layer. It leads to the elimination of band discontinuity and forms a polarization-graded induced field resulting in a high-performance back-illuminated AlGaN-based SUV-PD. This research provides an important reference for the development of high-performance back-illuminated AlGaN-based SUV-PDs.

Deposition of carbon-containing hole injection layers on p-type Al0.8Ga0.2N grown by metalorganic vapor phase epitaxy

A carbon-containing amorphous film is deposited on metalorganic-vapor-phase-epitaxy-grown AlN or AlGaN templates by flowing propane, aluminum, and nitrogen gases at 1010 °C. The deposited ∼1-nm-thick layers show p-type conductivity with a sheet carrier density of ∼1 × 1013 cm−2. When the film is deposited on Mg-doped Al0.8Ga0.2N prior to metallization, it acts as a hole injection layer into p-type AlGaN and shows nearly a double increase in hole current. The transmittance of the deposited layer is ∼90% in the 200–400-nm wavelength range. Hence, the deposited layer is a promising hole injection layer into high-Al-content p-type AlGaN.

Impact of High‐Temperature Annealing on Boron Containing AlN Layers Grown by Metal Organic Vapor Phase Epitaxy

Herein, the impact of high‐temperature (HT) annealing on the crystalline structure of metal organic vapor phase epitaxy (MOVPE)‐grown boron‐containing AlN layers is investigated. High‐resolution X‐ray diffraction studies reveal AlBN in the wurtzite configuration for nonannealed 300 nm‐thick layers containing several percent of boron. After 3 h of annealing at 1700 °C, the AlBN‐related reflex is weakened, showing a strong impact of the HT treatment on the crystalline structure of this material. After annealing, high‐resolution transmission electron microscopy micrographs reveal grain formation with moiré patterns, giving strong evidence of different crystal phases or orientations, alongside well‐oriented wurtzite regions. High‐angle annular dark‐field (HAADF) imaging and electron energy loss spectroscopy indicate stronger compositional inhomogeneities for the annealed sample in comparison with the as‐grown layer, most likely related to phase separation between AlN and BN. In addition, a significant diffusion of B out of the surface region is observed. AlBN with about ten times a lower boron content, for which defect propagation from the AlN template into the AlBN layer is visible, shows a much more homogeneous contrast in HAADF investigations after annealing, although the formation of granular structures is still observed.

Overcoming the excessive compressive strain in AlGaN epitaxy by introducing high Si-doping in AlN templates

The influence of compressive strain in high-quality AlN templates on the subsequent growth of AlGaN-based device layers was investigated. The AlN templates showed compressive strain of ∼−0.29% and threading dislocation densities (TDDs) below 6.5 × 108 cm−2. By introducing high Si-doping in MOVPE-grown AlN, the compressive strain was relaxed while preserving the low TDD. By this method, the low TDD was transferred from the AlN template to the micron-thick n-Al0.63Ga0.37N. A 275 nm LED was demonstrated with a ∼2.5 times power enhancement than the same LED on conventional MOVPE-grown AlN template under low current injection. The maximum external quantum efficiency (EQE) was enhanced from 1.6% to 2.2% with an improved n-AlGaN.

The Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB‐LED Structures

AbstractStrain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB‐LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 109 and 3 × 109 cm−2 and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 109 cm−2 as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB‐LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology.

Coherent epitaxial growth of superconducting NbN ultrathin films on AlN by sputtering

We investigated the structural and electrical properties of superconducting NbN films epitaxially grown on AlN single-crystalline films using a sputtering technique. The NbN(111) films grown on AlN under optimized temperatures exhibited clear peaks with Pendellösung fringes attributed to the growth of the atomically flat surfaces in 2θ/ω X-ray diffraction patterns. Scanning transmission electron microscopy also confirmed the formation of sharp NbN/AlN interfaces. Reciprocal space mapping revealed that the NbN films were coherently grown on the AlN templates, which indicates that the NbN films have the same in-plane lattice constants as AlN. It was also determined that the shape of the unit cell of NbN depends strongly on the epitaxial growth temperature. The NbN films coherently grown on AlN exhibited superconducting transition temperatures (Tc) ranging from 12 to 16 K, which also depends on the epitaxial growth temperature. These results indicate that the epitaxial strain (or change in crystal structure) in NbN modifies Tc.

Crystalline quality improvement of face-to-face annealed MOVPE-grown AlN on vicinal sapphire substrate with sputtered nucleation layer

Face-to-face annealed metalorganic vapor phase epitaxy (MOVPE)- grown AlN templates with sputtered AlN nucleation layers (FFA MOSp-AlN templates) were fabricated on vicinal sapphire substrates. The sputtered thin AlN layer supplies AlN seeds that are well aligned to the c-axis. The MOVPE growth with a relatively thick AlN enlarges crystal grains with a low concentration of impurities. Face-to-face annealing at high temperatures reduces the twist component of AlN films due to recrystallization, and AlN films with low threading dislocation densities are formed. The full width at half maximum (FWHM) of the (10−12) X-ray rocking curves (XRCs) of AlN templates decreased as the thickness of the MOVPE-grown AlN layer increased. The FWHMs for the (0002) and (10−12) XRCs of the 300-nm-thick AlN templates with an off-cut angle of 0.6° were 9 and 337 arcsec, respectively. The concentrations of O, Si, and C impurities in the FFA MOSp-AlN templates were lower than those in the face-to-face annealed sputtered AlN templates. Si doped Al0.7Ga0.3N films, which are typically used as n-type AlGaN layers for UVC LEDs, were grown on the FFA MOSp-AlN templates with various off-cut angles. The AlGaN films with smooth flat surfaces were obtained using the FFA MOSp-AlN template with larger off-cut angles due to the suppression of the large hillock formation.

Molecular beam epitaxy and characterization of wurtzite ScxAl1−xN

We demonstrate the growth of pure wurtzite phase ScxAl1−xN with a Sc composition as high as x = 0.34 on GaN and AlN templates using plasma-assisted molecular beam epitaxy. The wurtzite structure is well maintained even at high growth temperatures up to 900 °C for Sc0.2Al0.8N. Smooth surface morphology (root mean square roughness less than 1 nm) and excellent crystal quality [(002) plane rocking curve full-width at half maximum below 450 arc sec] are achieved over the range of x ≤ 0.34. Optical absorption studies indicate a decreasing bandgap with increasing Sc with a linear relationship of Eg(x) = 6.1 − 3.39x, which is in good agreement with the theoretical prediction. A monotonically tunable refractive index between AlN and GaN is further measured for ScxAl1−xN with various Sc compositions. This work provides a viable path for the epitaxy of wurtzite ScxAl1−xN with high Sc compositions. The distinct effect of substitutional Sc on bandgap and refractive index could be used in designing high-performance optoelectronic, electronic, and piezoelectric devices, and III-nitride integrated photonics and optical cavities.

Growing AlN crystals on SiC seeds: Effects of growth temperature and seed orientation

In this study, the detailed correlation between growth conditions and AlN features was studied specifically regarding the effects of growth temperature ranging from 1770 to 1980 °C and orientation of SiC seeds in 4° off-axis C-face of SiC and on-axis 4° and 8° off-axis Si-face of SiC seeds on the characteristics of the as-grown AlN layers. An optimal growth condition was derived at approximately 1920 °C on off-axis Si-face of SiC seeds. The other growth conditions caused various deficiencies compared with the optimal conditions. Increasing each 4° off-axis angle of the Si-face was equivalent to raising the growth temperature approximately 70 ± 10 °C, lowering the growth temperature on large off-axis SiC seeds to suppress SiC decomposition. An AlN growth active energy of 400 ± 35 kJ/mol is determined as Si and C impurities are present, which is approximately one-third smaller than that predicted with no impurities. These results provide a reference model for hetero-growth of AlN crystals on SiC seeds to obtain large AlN template seeds that are necessary to achieve more than 2-in. AlN crystals.

Status and Prospects of AlN Templates on Sapphire for Ultraviolet Light‐Emitting Diodes

Herein, the scope is to provide an overview on the current status of AlN/sapphire templates for ultraviolet B (UVB) and ultraviolet C (UVC) light‐emitting diodes (LEDs) with focus on the work done previously. Furthermore, approaches to improve the properties of such AlN/sapphire templates by the combination of high‐temperature annealing (HTA) and patterned AlN/sapphire interfaces are discussed. While the beneficial effect of HTA is demonstrated for UVC LEDs, the growth of relaxed AlGaN buffer layers on HTA AlN is a challenge. To achieve relaxed AlGaN with a low dislocation density, the applicability of HTA for AlGaN is investigated.

Suppression of dislocation-induced spiral hillocks in MOVPE-grown AlGaN on face-to-face annealed sputter-deposited AlN template

AlGaN films were grown on face-to-face annealed sputter-deposited AlN/sapphire (FFA Sp-AlN) templates via metalorganic vapor phase epitaxy (MOVPE), and the growth behavior of the AlGaN films was investigated. The sapphire substrates with small off-cut exhibited poor surface flatness of AlGaN grown on the FFA Sp-AlN templates owing to the formation of large hillock structures. To understand the origin of these hillock structures, the crystallinity and surface morphology of conventional fully MOVPE-grown AlN/sapphire (MOVPE-AlN) templates and the FFA Sp-AlN template were comprehensively studied. The screw- and mixed-type threading dislocation density of the FFA Sp-AlN template was estimated to be approximately 1.8 × 106 cm−2, which was two orders of magnitude lower than that of the MOVPE-AlN template. Consequently, the uniquely observed growth of the hillock structures in the FFA Sp-AlN templates was attributed to their low density of screw- and mixed-type threading dislocations. The large surface off-cut sapphire substrates suppressed the hillock structures on the FFA Sp-AlN templates. The improvement in surface flatness resulted in better optical properties of multiple quantum wells grown on the AlGaN layer. These results demonstrate a promising method for achieving highly efficient and cost effective AlGaN based deep ultraviolet light-emitting diodes.

Heteroepitaxial Integration of ZnGeN2 on GaN Buffers Using Molecular Beam Epitaxy

Recently theorized hybrid II-IV-N2/III-N heterostructures, based on current commercialized (In,Ga)N devices, are predicted to significantly advance the design space of highly efficient optoelectronics in the visible spectrum, yet there are few epitaxial studies of II-IV-N2 materials. In this work, we present heteroepitaxial ZnGeN2 grown on GaN buffers and AlN templates. We demonstrate that a GaN nucleating surface is crucial for increasing the ZnGeN2 crystallization rate to combat Zn desorption, extending the stoichiometric growth window from 215 °C on AlN to 500 °C on GaN buffers. Structural characterization reveals well-crystallized films with threading dislocations extending from the GaN buffer. These films have a critical thickness for relaxation of 20–25 nm as determined by reflection high energy electron diffraction (RHEED) and cross-sectional scanning electron microscopy (SEM). The films exhibit a cation-disordered wurtzite structure, with lattice constants a = 3.216 ± 0.004 Å and c = 5.215 ± 0.005 Å determined by RHEED and X-ray diffraction (XRD). This work demonstrates a significant step toward the development of hybrid ZnGeN2-GaN integrated devices.