AlN Templates Research Articles

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.

High Crystallinity and Highly Relaxed Al0.60Ga0.40N Films Using Growth Mode Control Fabricated on a Sputtered AlN Template with High‐Temperature Annealing

Herein, a method for growing AlGaN using an intermediate AlN molar fraction on a sputtered AlN template with high‐temperature annealing via an AlN homoepitaxial layer is investigated. The growth mode dependence of AlGaN on the amount of Ga doping in the AlN homoepitaxial layer is examined in detail. As a result, homoepitaxial growth of AlN on sputtered AlN improve the surface flatness. In addition, it is found that the macroscopic flatness is improved by increasing the amount of Ga doping, but this decreases the microscopic flatness. Moreover, when Al0.60Ga0.40N is grown on homoepitaxial AlN layers with different amounts of Ga doping, a difference in the growth mode is observed, and this reveal a remarkable difference in the dislocation density of Al0.60Ga0.40N. The dislocation density dependence of the lasing threshold power density in a UV–B optical pumped laser is also investigated. As a result, the lasing threshold power density is reduced from ≈220 to 50 kW cm−2 by reducing the dislocation density from 1.6 × 109 to 8.0 × 108 cm−2. It is confirmed that it is, therefore, essential to reduce the dislocation density to reduce the threshold power density of the UV–B laser.

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 .

AlN nanostructures and flat, void-less AlN templates formed by hydride vapor phase epitaxy on patterned sapphire substrates

Severe growth irregularity usually encountered in AlN growth on patterned sapphire substrates (PSSs) was completely suppressed in hydride vapor phase epitaxial (HVPE) growth on a PSS having small cone-pattern with a height of 220 nm. At low temperature, the AlN growth on the PSS led to a formation of regular array of hexagonal AlN nano-rods on the cone-tops. At high temperatures, the growth mode turned into a layered fashion and a void-less AlN template with an atomically smooth surface and high crystal quality were realized with an HVPE growth thickness less than 1 μm.

Enhanced Strain Relaxation in AlGaN Layers Grown on Sputter‐Based AlN Templates

Strain plays a crucial role in the performance of ultraviolet (UV) optoelectronic devices. The strain states of AlGaN layers grown on different AlN templates are investigated. Two types of AlN templates are prepared: one is grown solely by metal‐organic chemical vapor deposition (MOCVD), and the other is fabricated with the combination of direct current (DC) sputtering, high‐temperature annealing, and MOCVD growth. The qualities of 4 μm‐thick MOCVD AlN and 1.2 μm‐thick sputter‐based AlN are almost equivalent in terms of dislocation density and surface morphology. However, the strain relaxation ratio of the AlGaN layer is higher for the 1.2 μm sputter‐based AlN than the 4 μm MOCVD AlN, indicating that the strain state depends on the total thickness of the epilayers on the sapphire substrate. In addition, it is found that the curvature of the sample at room temperature is governed mainly by the thickness of the AlGaN layer. As a result, sputter‐based AlN templates allow for the enhanced strain relaxation of the AlGaN layer with a small sample bowing.

An Initial Study of Ultraviolet C Optical Losses for Monolithically Integrated AlGaN Heterojunction Optoelectronic Devices

An initial study of losses in n‐AlxGa1−xN planar waveguides at λemission ≈ 280 nm using monolithically integrated AlxGa1−xN multiple quantum wells (MQWs)‐based light‐emitting diodes and detectors is presented. The epilayer structure for the integrated devices is grown on an AlN (3.5 μm thick) template over sapphire substrates. Emitter–detector optical coupling and the directional independence of radiation within the epistructure are experimentally established. A model for estimating the attenuation coefficient under these conditions is developed. The attenuation coefficient for a planar n‐Al0.65Ga0.35N waveguide is measured to be 5–6 cm−1, and it primarily arises from the free‐carrier absorption rather than surface roughness‐dependent Rayleigh scattering.

Atomic simulation of AlGaN film deposition on AlN template

In this article, we study the deposition of AlGaN film on AlN template by molecular dynamics (MD) simulations. The effects of growth temperature and film thickness on the dislocation of deposited AlGaN film are simulated and studied. The atomic structure of deposited AlGaN film is also investigated. We find that the dislocations usually occur at the interface between AlN template and AlGaN film and then extend towards the growth direction. The dislocation density decreases with the increase of AlGaN film thickness, which indicates that increasing the thickness of deposited AlGaN film to a certain extent is beneficial to reducing dislocation. In addition, increasing the growth temperature can also effectively reduce the dislocation in deposited AlGaN film. Furthermore, the crystallinity of deposited AlGaN film could be improved by increasing the growth temperature. This is consistent with the dislocation discussion. The mobility of adatoms increases as the growth temperature increases. So it is easier for adatoms to find their ideal lattice points at higher temperature. Thus the dislocation and other defects can be effectively reduced and the crystal quality of deposited AlGaN film could be improved.

Investigation of AlGaN/GaN Heterostructures Grown on Sputtered AlN Templates with Different Nucleation Layers.

In this work, a sputtered AlN template is employed to grow high-quality AlGaN/GaN heterostructures, and the effects of AlN nucleation layer growth conditions on the structural and electrical properties of heterostructures are investigated in detail. The optimal growth condition is obtained with composited AlN nucleation layers grown on a sputtered AlN template, resulting in the smooth surface morphology and superior transport properties of the heterostructures. Moreover, high crystal quality GaN material with low dislocation density has been achieved under the optimal condition. The dislocation propagation mechanism, stress relief effect in the GaN grown on sputtered AlN, and metal organic chemical vapor deposition AlN nucleation layers are revealed based on the test results. The results in this work demonstrate the great potential of AlGaN/GaN heterostructures grown on sputtered AlN and composited AlN nucleation layers for microelectronic applications.

Two-dimensional analysis of the nonuniform quantum yields of multiple quantum wells for AlGaN-based deep-ultraviolet LEDs grown on AlN templates with dense macrosteps using cathodoluminescence spectroscopy

AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) incorporating uneven multiple quantum wells (MQWs) with inclined and terrace zones, which were fabricated on an AlN template with dense macrosteps, have exhibited a high internal quantum efficiency (IQE). To investigate the microscopic structure of uneven MQWs, cathodoluminescence (CL) mapping characterization was carried out, and the maps of the CL intensity at 300 K relative to that at 38 K were obtained for uneven MQWs that targeted 265 and 285 nm LEDs. At an electron beam current of less than 1.0 nA, the signals from inclined and terrace zones of the uneven MQWs were confirmed to satisfy the nonsaturated excitation condition at 300 K. Nonradiative recombination (NR) was insufficiently frozen even at 38 K, specifically on the terraces in the 265 nm MQW, suggesting high concentrations of NR centers due to point defects (PDs). In contrast, NR in the 285 nm MQW at 38 K was closer to freeze-out. The concentration of PDs in the 285 nm MQW was likely to be lower than that in the 265 nm MQW. Finally, the ratios of the CL intensity at 300 K to those at 38 K were mapped, demonstrating an approach to creating an approximate map of IQE. The values in the CL intensity ratio maps are discussed by considering the analytical error factors. The results support the model of localized current injection through Ga-rich stripe zones in the n-AlGaN cladding layer.

Broadband Ultraviolet Emission from 2D Arrays of AlGaN Microstructures Grown on the Patterned AlN Templates

Broadband ultraviolet (UV) emission is achieved using AlGaN microstructure 2D arrays. 2D arrays of trenches are initially formed on AlN templates on sapphire (0001) substrates. AlGaN‐based quantum wells (QWs) are subsequently regrown on top of the patterned templates. Bunched steps are formed within the trench, inducing variations in the Al composition and AlGaN thickness in the QWs. Regions with and without bunched steps coexist and cause emission wavelength variations. The formation mechanism of bunched steps is attributed to the position‐dependent AlN growth rate due to variations of the source–precursor flow within narrow trenches.

Influence of thin MOCVD-grown GaN layer on underlying AlN template

Abstract We have studied the direct growth of a GaN film on an AlN template/sapphire substrate by metalorganic chemical vapor deposition. It was found that the GaN layer causes marked deformation of the underlying AlN template at the initial growth. The intensity of x-ray diffraction from AlN drops by a factor of 5 and the full widths at half maximum of the rocking curves of the on- and off-axis planes are increased from 50 to 300 arcsec. With increasing GaN growth time, these values gradually recover, and the crystalline quality of the GaN film is improved. No alloy formation is observed at the interface between AlN and GaN. An AlN template on a sapphire substrate seems to act as a buffer layer, adjusting the lattice constant to improve the crystallinity of the direct grown GaN. Compared with a GaN film grown on a sapphire substrate, GaN grown directly on an AlN template forms a smoother surface with better crystalline quality in a shorter growth time, and at a lower temperature with fewer nonradiative defects in the band gap.

Growth of AlGaN/InGaN/GaN Heterostructure on AlN Template/Sapphire

GaN films are grown directly on AlN templates/sapphire substrates without using low‐temperature (LT) buffer layers by metalorganic chemical vapor deposition. AlN templates are complimentarily deformed at the initial growth of GaN, adjusting the a‐lattice constant and tilting crystal orientation slightly. Compared with the film on sapphire substrates using an LT buffer layer, the GaN on the AlN template forms a smoother surface and has better crystalline quality after a shorter growth time at a lower temperature. Higher‐quality InGaN films on the GaN/AlN template are subsequently grown to optimize the thickness exhibiting the minimum Urbach energy which is evaluated by photothermal deflection spectroscopy. A high‐quality AlGaN/InGaN heterostructure is fabricated in which the Shubnikov–de Haas oscillation can be clearly observed from the electron gas of the InGaN channel at the interface.

Untwinned semipolar (101̅3) AlxGa1-xN layers grown on m-plane sapphire

Heteroepitaxial growth of untwinned mono-crystalline semipolar (101̅3) Alx layers on (101̅3) AlN template was investigated by metalorganic vapour phase epitaxy. The templates were initially deposited on (101̅0) m-plane sapphire substrates by directional sputtering. Different Al/Ga ratios in gas phase were used to adjust the AlN mole fraction over the entire range of composition. All the layers show a triclinic distortion in the wurtzite unit cell due to anisotropic in-plane strain. The AlN mole fraction of the (101̅3) layers and c-plane co-loaded layers estimated by x-ray diffraction is comparable. This is consistent both with their comparable effective bandgap energy estimated from optical transmission measurements and their near band-edge emission energy obtained from room-temperature photoluminescence. The dependence of the bandgap and near band-edge emission energies on the AlN mole fraction indicates a bowing parameter of 0.9 eV.

Analysis of growth rate and crystal quality of AlN epilayers by flow-modulated metal organic chemical vapor deposition

Abstract AlN templates have been grown on sapphire by flow-modulated metal organic chemical vapor deposition (MOCVD). By analyzing the TMAl duty ratio R, TMAl utilization ratio β and parasitic reaction ratio α, we obtained the quantitative relationship between flow-modulated modes and growth rate, and then proposed two kinds of flow-modulated modes to increase the growth rate of AlN. Besides, we found the continuous growth in flow-modulated mode can transform the AlN growth mode from the step-bunching growth into the two-dimension growth and accordingly improve the crystal quality of AlN. Finally, the AlN template with a relatively fast growth rate (0.98 μm/h) and flat surface morphology (RMS = 0.5 nm) was obtained by NH3 pulse-flow mode with a small duty ratio of NH3.

Aluminium incorporation in polar, semi- and non-polar AlGaN layers: a comparative study of x-ray diffraction and optical properties

Growth of AlxGa1−xN layers (0 ≤ x ≤ 1) simultaneously on polar (0001), semipolar (10bar{{rm{1}}}3) and (11bar{{rm{2}}}2), as well as nonpolar (10bar{{rm{1}}}0) and (11bar{{rm{2}}}0) AlN templates, which were grown on planar sapphire substrates, has been investigated by metal-organic vapour phase epitaxy. By taking into account anisotropic in-plane strain of semi- and non-polar layers, their aluminium incorporation has been determined by x-ray diffraction analysis. Optical emission energy of the layers was obtained from room-temperature photoluminescence spectra, and their effective bandgap energy was estimated from room-temperature pseudo-dielectric functions. Both x-ray diffraction and optical data consistently show that aluminium incorporation is comparable on the polar, semi- and non-polar planes.

Stress evolution during growth of AlN templates on c-Al2O3 substrates by plasma-assisted molecular beam epitaxy

We describe stress evolution during plasma-assisted molecular beam epitaxy (PAMBE) of AlN nucleation and buffer layers on c-Al2O3 substrates at varying growth temperatures of 780 and 850°C. Moreover, different mechanisms of stress generation in the growing AlN films, related to the processes of grain coalescence and impact of Me-excess during PA MBE by migration enhanced epitaxy and metal-modulated epitaxy, are considered.

Scalable synthesis of multilayer h-BN on AlN by metalorganic vapor phase epitaxy: nucleation and growth mechanism

We studied the nucleation and growth of hexagonal BN (h-BN) on AlN template on c-plane sapphire by metalorganic vapor phase epitaxy as functions of growth temperature, deposition time, and triethylboron (TEB) partial pressure. A lateral growth rate of about 25 nm min−1 for h-BN nuclei was obtained by atomic force microscopy and a nucleation activation energy of 2.1 eV was extracted from the temperature dependence of the nucleation density. A large TEB flow rate strongly enhances the formation of h-BN nuclei. At a reduced TEB flow rate, we observed a significantly decreased nuclei density and a delay in nucleation due to TEB desorption. By fine tuning the growth parameters, single-crystalline multilayer h-BN was successfully formed on AlN surface, as confirmed by x-ray diffraction and transmission electron microscopy (TEM). The epitaxial relationship between h-BN and AlN was [0 0 0 1]h-BN || [0 0 0 1]AlN and [1 0 −1 0]h-BN || [1 1 −2 0]AlN from TEM and electron backscatter diffraction measurements. In addition, TEM showed that the initial h-BN layers are not parallel and tend to form half-domes. On those half-domes (cap-shaped-like) a 2D lateral growth sets on, resulting in a well-oriented 2D multilayer observed in TEM. Thus, the surface topography further develops to form a relatively flat surface without wrinkles and finally a typical hexagon-like wrinkled surface at thicker h-BN layers. Particularly, the small h-BN nuclei have dangling bonds at their periphery that can interact with the substrate, forming actual bonds with AlN. Hence the choice on the substrate is important, despite the basal planes of multilayer h-BN are bonded by a weak van der Waals force.

(Invited) Recent Advances in III-Nitride Devices Using Ultrawide Bandgap AlxGa1-Xn Active Layers

III-Nitride devices with high-Al AlxGa1-xN (x>0.4) active layers have the potential of addressing several key application areas. For these high Al-compositions the direct bandgap leads to a cut-off wavelength<290 nm which makes the material ideal for solar blind detectors. Similarly, AlxGa1-xN (x>0.4) multiple quantum wells (MQWs) are used in the active region of the deep ultraviolet (DUV) light emitting diodes. More recently, to take advantage of the higher breakdown field for high voltage devices, several researchers including us have explored Heterojunction Field-Effect Transistors (HFETs) and metal-oxide-semiconductor (MOS) HFETs with high-Al AlxGa1-xN (x>0.4) channel layers. Currently nearly all the (UWBG) AlGaN devices are fabricated on 3-5 µm thick MOCVD grown high quality AlN/sapphire templates. This choice is dictated by the deep UV optical transparency, the easy availability and the low-cost of sapphire. Increasing the AlN template thickness to larger than 5 µm leads to stress related cracking. The low thermal conductivity of sapphire and this AlN growth thickness limitation leads to device performance limitations especially for high power operation. To avoid this, recently using a pulsed MOCVD approach, we have for the first time succeeded in growing 16 µm thick, crack-free, low-defect AlN template layers on basal plane sapphire substrates. These were then used to deposit epitaxial structures for the AlGaN channel HFET/MOSHFETs. In this presentation we will present studies to establish the role that the high conductivity thick AlN buffers play in improving the devices’ electrical and thermal performance. Recently we have also for the first time conducted optical waveguiding studies at UVC wavelengths. For these studies we fabricated monolithically integrated UVC LEDs, detectors and waveguides on an AlN/sapphire template platform. Optical attenuation coefficient as low as 4.8 cm-1 was measured for Al0.5Ga0.5N waveguide with an AlN clad layer. Details of our integrated device fabrication and optical losses measurements scheme will also be discussed. Acknowledgement: This work was partially supported by Defense Advanced Research Projects Agency, Army Research Office and Office of Naval Research (Monitors: Dr. M. Gerhold. Dr. Paul Maki and Dr. Lynn Petersen).

Performance Modulation for Back-Illuminated AlGaN Ultraviolet Avalanche Photodiodes Based on Multiplication Scaling

Back-illuminated Al0.1Ga0.9N ultraviolet avalanche photodiodes (APDs) of various multiplication widths were fabricated on AlN templates with a separate absorption and multiplication structure. The impacts of an increased multiplication scale on a device performance were investigated. The avalanche breakdown voltage was found to increase as the multiplication layer thickness (MLT) increases. The APD with 230-nm-MLT achieved a superior maximum multiplication gain of 5.4 × 10 4 , higher than that obtained in devices with 150-nm- and 300-nm-MLT. Theoretical simulations demonstrated that the critical electric field intensity in an avalanche region would decrease as the rising of MLT, indicating the modulating ability of multiplication scaling on the AlGaN APD performance. In addition, APDs fabricated on different AlN templates were employed to study the effects of crystalline quality on device properties.

Comparison of AlxGa1−xN multiple quantum wells designed for 265 and 285 nm deep-ultraviolet LEDs grown on AlN templates having macrosteps

AlGaN multiple quantum wells (MQWs) targeting 265 and 285 nm deep-ultraviolet LEDs grown on AlN templates with macrosteps were compared. In the cathodoluminescence images of the 285 nm MQW, high-intensity zones were observed along the edgelines of the macrosteps with wavelengths of 290–296 nm and on the terraces with wavelengths of 286–288 nm. Dark spots related to threading dislocations were seen in the entire 285 nm MQW. For 265 nm, high-intensity zones were limited along the edgelines and dark spots showed a low-contrast, which is likely to be caused by nonradiative recombination centers due to point defects.