AlN Buffer Layer Research Articles

Comparative study on the molecular beam epitaxial growth and characterization of AlGaN nanowire structures on AlN buffer layer and on Si

AlGaN in the form of nanowires is an important platform for semiconductor ultraviolet light sources on Si. In the past, significant efforts have been devoted to improving the quality of AlGaN nanowires. In this context, we present a comparative study on the molecular beam epitaxial growth and characterization of AlGaN nanowire structures on the AlN buffer layer on Si and on Si directly. It is found that AlGaN nanowires grown on the AlN buffer layer shows an improved internal quantum efficiency, compared with the nanowires grown on Si directly. This improvement is attributed to the reduced nanowire coalescence due to the improved vertical alignment of the nanowires grown on the AlN buffer layer.

Low-temperaturevan der waals epitaxy of GaN films on graphene through AlN buffer by plasma-assisted molecular beam epitaxy

Low-temperature growth of III-nitride semiconductor materials on non-single-crystalline substrates is necessary to realize inexpensive and large-area transferable GaN-films for flexible devices. In this work, the growth conditions and material properties of III-nitride films have been explored using plasma-assisted molecular beam epitaxy and AlN as a buffer layer. Migration enhanced epitaxy mode is the most suitable for the realization of low-temperature van der Waals epitaxy (vdWE). An AlN buffer layer with thickness of 10 nm grown by the MEE mode results in the optimizedcrystalline quality and surface flatness, which is extremely beneficial for the subsequentGaN film epitaxy. A single-crystalline GaN film with a flat surface has been obtained on AlN/graphene/quartz substrate at 530 °C, possessinga dislocation density about 1.10 × 1011 cm−2 and a RMS surface roughness of 2.04 nm for 10 × 10 μm2 area. The results obtained in this work can be applied to vdWE of III-nitrides on other 2D material/non-single-crystalline substrates for their further applications in flexible devices or circuits.

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

Comparison study on light trapping of GaN UV photocathode

To obtain optimal light absorption of GaN photocathode, the absorption spectra of GaN photocathode have been studied under different emission layer thicknesses, buffer layer thicknesses and buffer layer materials in detail based on finite difference time domain (FDTD) method. The results show that, whether photocathode is reflection-mode or transmission-mode, its absorption spectra has same characteristics and trends. The appropriate thickness of GaN emission layer is in range of 100–150 nm, and at this time, maximum absorption efficiency can be achieved. For GaN photocathode with ultra-thin emission layer, AlN buffer layer becomes an important factor affecting light trapping. Compared with thickness of buffer layer, different Al composition in buffer layer cause a more significant change in absorption spectrum. In addition, absorption spectrum of AlxGa1−xN with variable Al composition as emission layer has also been studied. It is found that although emission layer with variable Al compositions can generate a built-in electric field to help electrons escape, it would reduce trapping of incident light, narrow absorption sideband, and shift cutoff wavelength below 300 nm, which is suitable for “sun blind” detection applications. Our research can provide theoretical guidance for designing GaN photocathodes with optimal light absorption properties.

Direct van der Waals epitaxy of stress-free GaN films on PECVD grown graphene

Residual stress is generated in GaN epitaxial layers due to the mismatch during GaN epitaxy on sapphire using the traditional method. Therefore, the use of graphene to reduce residual stress and dislocation densities in GaN epitaxy has become an important research direction. However, growing a stress-free GaN film on graphene substrate remains challenge. In this work, we directly grew graphene on sapphire via plasma enhanced chemical vapor deposition (PECVD) to obtain an epitaxial graphene with characteristic orientation, and ultra-low stress GaN films can then be obtained through metal organic chemical vapor deposition (MOCVD) assisted with the sputtering AlN buffer layer. Through this method, we successfully obtained continuous and flat GaN films with ultra-low biaxial compressive stress (0.023 GPa) without the complicated stress engineering during epitaxial growth. First principle calculation was employed to confirm that the characteristic orientation of epitaxial graphene is crucial to release the stress in GaN. The obtained GaN films can also be easily transferred because of small van der Waals force on graphene. The transferred GaN heterojunction was directly fabricated into a metal-insulator-semiconductor (MIS) device from which typical electrical properties can be obtained. Our work reveals the stress-releasing mechanism and excellent stress-releasing effect of graphene and provides a new epitaxial strategy to guide crystallographic epitaxy.

In-situ X-ray diffraction analysis of GaN growth on graphene-covered amorphous substrates

In-situ X-ray diffraction was used to investigate the initial stages of GaN growth on graphene-covered SiO2 substrates with or without AlN buffer layers by plasma-assisted molecular beam epitaxy. The in-plane lattice parameter of GaN remained almost constant during growth without AlN buffer layers, confirming weak interactions between GaN and graphene. On the substrate with AlN buffer layers, the GaN lattice parameter continuously changed from an AlN value to GaN value, indicating that the AlN islands act as seeds for GaN growth. Besides, GaN has preferential orientations epitaxial to graphene. Graphene can be used as a template of GaN growth on amorphous substrates.

An Increase of Threading Dislocations Filtering Efficiency in Al2O3 Templates with Faceted Surface Morphology During a Growth by Molecular Beam Epitaxy

We present the results of a transmission electron microscopy and X-ray diffractometry investigation of AlN/c-Al2O3 templates with GaN ultrathin insertions grown by plasma-assisted molecular beam epitaxy. It has been shown that AlN buffer layers with faceted surface morphology provide a greater threading dislocation density reduction than smooth layers. The filtering effect of GaN ultrathin insertions has been confirmed.

Performance analysis of HfO2/InAlN/AlN/GaN HEMT with AlN buffer layer for high power microwave applications

We present a performance enhancement evaluation of n + doped graded InGaN drain/source region-based HfO 2 /InAlN/AlN/GaN/AlN on SiC metal-oxide-semiconductor high electron mobility transistor (MOS-HEMTs) with a T-shaped gate. Impact on the device characteristics with the inclusion of a HfO 2 surface passivation layer and an AlN buffer layer in the MOS-HEMT structure as a performance booster has been analyzed for the HEMT device with 30 nm gate length using Silvaco ATLAS TCAD. The proposed MOS-HEMT exhibits an outstanding performance, with an enhanced power gain cut-off frequency (f max ) of 366 GHz, a current gain cut-off frequency (f t ) of 426 GHz, and a off-state breakdown voltage (V br ) of 81 V. The high -k (high permittivity) HfO 2 based metal oxide semiconductor HEMT device experiences a low off-state gate leakage current (I g ~ 10 −11 A/mm) and a high I on /I off ratio of 10 9 . The InAlN/GaN/AlN heterostructures demonstrate improved two-dimensional electron gas (2DEG ~ 5.3 × 10 13 cm −2 ), carrier mobility (μ) of 1256 Cm 2 /V-s and drain current density of (I ds ) 2.7 A/mm. A large signal analysis performed at 30 GHz yielded a maximum of 28% power-added efficiency. The high JFoM of 34.506 THz V (Johnson Figure of Merit = f t × V br ) and (f t .f max ) 1/2 of 394.86 GHz indicate the potential applicability of the HfO 2 /InAlN/GaN MOS-HEMTs in high-frequency and high-power applications.

2 in. Free-standing GaN grown by HVPE with sputtered AlN buffer on GaAs substrate

A magnetron sputtered AlN buffer layer is applied for preparing 2 in. free-standing (FS) GaN on the GaAs substrate by hydride vapor phase epitaxy (HVPE). The AlN buffer layer with excellent compactness can prevent As atoms from diffusing to the GaN growth surface. Thus it serves as an effective protective layer for GaAs substrate and also a seed layer for the deposition of GaN in the early stage of the high-temperature GaN growth. Benefiting from the decomposition property of GaAs at temperature higher than 900 °C, thick GaN film self-separated completely from GaAs substrate during growth. By this way, a 2 in. FS-GaN layer with the thickness of 1.3 mm is finally obtained after the long growth time about 20 h.

Ultraviolet Stimulated Emission in AlGaN Layers Grown on Sapphire Substrates Using Ammonia and Plasma‐Assisted Molecular Beam Epitaxy

Ammonia and plasma‐assisted (PA) molecular beam epitaxy modes are used to grow AlN and AlGaN epitaxial layers on sapphire substrates. It is determined that the increase in the thickness of the AlN buffer layer grown by ammonia molecular beam epitaxy (MBE) from 0.32 to 1.25 μm results in the narrowing of 101 X‐Ray rocking curves (XRCs), whereas no clear effect on the 002 XRC width is observed. It is shown that strong GaN decomposition during growth by ammonia MBE causes AlGaN surface roughening and compositional inhomogeneity, which leads to deterioration of its lasing properties. AlGaN layers grown by ammonia MBE at an optimized temperature demonstrate stimulated emission (SE) which peaks at λ = 330, 323, 303, and 297 nm, with SE threshold values of 0.7, 1.1, 1.4, and 1.4 MW cm−2, respectively. In comparison with these, AlGaN layers grown using PA MBE pulsed modes (migration‐enhanced epitaxy, metal‐modulated epitaxy, and droplet elimination by thermal annealing) show SE with a relatively low threshold (0.8 MW cm−2) at a considerably shorter wavelength of λ = 267 nm.

Material-to-device performance correlation for AlGaN-based solar-blind p–i–n photodiodes

We report on crystalline quality-to-device performance correlation for self-powered Al0.40Ga0.60N, p–i–n ultraviolet (UV) photodetectors on c-plane sapphire. The active p–i–n detector stack was grown over an AlN buffer. Careful optimization of the nucleation density on growth surface helped achieve a two-orders and one-order of magnitude reduction in the screw and edge dislocation density in the buffer layer, respectively. This resulted in a nine-orders of magnitude reduction in the reverse leakage current from 4.3 mA to 4.2 pA (at 10 V). Correspondingly, a thirteen-fold enhancement in the zero-bias external quantum efficiency (EQE) from 3.4% to 45.5%, when measured under 289 nm front-illumination was also observed. The detector epi-stack grown over the optimal AlN buffer layers led to the realization of high-performance p–i–n detectors with a dark current density below 4 nA cm−2 at 10 V and a zero-bias EQE of 74.7% under back-illumination. This is one of the highest zero-bias EQE reported for solar-blind detectors realized on template-free and mask-free III-nitrides grown using metal organic chemical vapor deposition on any substrate. The deep-UV-to-visible rejection ratio exceeded 106 while the deep-UV-to-near UV rejection exceeded 103. The thermal-noise limited detectivity was estimated to be 4 × 1014 cm Hz1/2 W−1. Hopping conduction along screw dislocation-mediated localized trap states was found to be the dominant carrier transport mechanism in the samples exhibiting high reverse leakage. For these samples, the responsivity (photocurrent) exhibited an exponential variation with reverse bias and a nonlinear variation with input optical power. This is explained using a hole-trapping associated gain mechanism and its impact on the transient characteristics of the detectors is investigated. A 6 × 1 linear array of the highest EQE detectors was realized and detector performance parameters were found to be comparable before and after wire bonding. This study is expected to enhance the understanding of III-nitrides-based vertical, self-powered detectors and benefit the development of high-performance, focal plane arrays using less complicated growth techniques.

Defect Structure Determination of GaN Films in GaN/AlN/Si Heterostructures by HR-TEM, XRD, and Slow Positrons Experiments

The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e., thickness of layers, crystal structures, growth orientation, elemental diffusion depths, edge, and screw dislocation densities), within two GaN/AlN/Si heterostructures, that alter their efficiencies as positron moderators. The structure of the GaN film, AlN buffer layer, substrate, and their growth relationships were determined through high-resolution transmission electron microscopy (HR-TEM). Data resulting from high-resolution X-ray diffraction (HR-XRD) was mathematically modeled to extract dislocation densities and correlation lengths in the GaN film. Positron depth profiling was evaluated through an experimental Doppler broadening spectroscopy (DBS) study, in order to quantify the effective positron diffusion length. The differences in values for both edge and screw dislocation densities, and correlation lengths (Le, Ls) found in the 690 nm GaN film, were associated with the better effective positron diffusion length (Leff) of = 43 ± 6 nm.

Research on adhesion strength and optical properties of SiC films obtained via RF magnetron sputtering

SiC is widely used in various mechanical applications as a protective film because of its strength, thermal stability and good mechanical hardness. Here, amorphous SiC thin films with AlN as a buffer layer were deposited on glass and Si substrates through RF magnetron sputtering at different RF powers. The influence of the AlN buffer layer thickness on the morphological and the mechanical properties of the composite films was investigated. Results demonstrate that the AlN buffer layer can effectively improve the adhesion strength of SiC thin films, which has increased gradually from 26.78 N to 37.66 N. The transmittance of SiC thin films was measured using a UV–Vis–NIR spectrophotometer over a spectral range of 300–1200 nm. The average transmittance of SiC films decreases with increasing RF power, and their optical band gap values have varied from 3.31 eV to 3.50 eV.

The influence of AlN buffer layer on the growth of self-assembled GaN nanocolumns on graphene

GaN nanocolumns were synthesized on single-layer graphene via radio-frequency plasma-assisted molecular beam epitaxy, using a thin migration-enhanced epitaxy (MEE) AlN buffer layer as nucleation sites. Due to the weak nucleation on graphene, instead of an AlN thin-film we observe two distinguished AlN formations which affect the subsequent GaN nanocolumn growth: (i) AlN islands and (ii) AlN nanostructures grown along line defects (grain boundaries or wrinkles) of graphene. Structure (i) leads to the formation of vertical GaN nanocolumns regardless of the number of AlN MEE cycles, whereas (ii) can result in random orientation of the nanocolumns depending on the AlN morphology. Additionally, there is a limited amount of direct GaN nucleation on graphene, which induces non-vertical GaN nanocolumn growth. The GaN nanocolumn samples were characterized by means of scanning electron microscopy, transmission electron microscopy, high-resolution X-ray diffraction, room temperature micro-photoluminescence, and micro-Raman measurements. Surprisingly, the graphene with AlN buffer layer formed using less MEE cycles, thus resulting in lower AlN coverage, has a lower level of nitrogen plasma damage. The AlN buffer layer with lowest AlN coverage also provides the best result with respect to high-quality and vertically-aligned GaN nanocolumns.

Effect of annealing atmosphere (CO and N2 gas flow) on surface morphology and crystal quality of AlN buffer layer

In this study, the effect of the annealing atmosphere (N2–CO and pure N2 atmosphere) on the crystal quality, surface morphology, and surface O content of the annealed AlN buffer layers was studied. The FWHM of the (0002) plane of an XRD rocking curve showed that the screw dislocation density of the as-grown AlN film can be reduced by annealing in both N2–CO and pure N2 atmosphere. The AlN film showed an improvement in crystal quality under pure N2 annealing atmosphere. As annealing the AlN buffer layer improves the crystal quality, surface roughness, and surface O content of the AlN film, it should be well controlled for the subsequent growth of III-nitride layers. The surface oxidation that occurred on the AlN films during annealing in N2–CO atmosphere resulted in an increase in surface O content and surface roughness of the AlN films, which is undesirable for the subsequent growth of III-nitride layers. We observed that the surface O content (in the depth of 2-nm) of all annealed AlN films was reduced in the N2 annealing atmosphere, which could be due to the O outgassing from the AlN surface to the pure N2 annealing atmosphere. The O outgassing of the surface O content in the AlN surface layer was related to the volume of space in the annealing setups. However, a large volume of annealing space under pure N2 gas flow resulted in higher thermal decomposition on the annealing AlN films, leading to pits forming on the annealed AlN surface and a smearing of the original step-terrace structure. Therefore, we found that there is an optimal annealing atmosphere space in maintaining the original surface morphology and achieving low surface O content in the annealed AlN film.

Повышение эффективности фильтрации прорастающих дислокаций в темплейтах AlN/c-Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- с фасетированной морфологией поверхности во время их роста методом молекулярно-пучковой эпитаксии

The results of transmission electron microscopy study and X-ray diffraction analysis of AlN/c Al2O3 templates with GaN ultrathin insertions grown by plasma-assisted molecular beam epitaxy are presented. It is shown that AlN buffer layers with faceted surface morphology provide a much higher threading dislocations density reduction then with smooth layers. The filtering action of ultrathin GaN insertions is confirmed.

Design and development of an AlGaN/GaN heterostructure nano‐ATT oscillator: experimental feasibility studies in THz domain

Numerical analysis followed by experimental investigations are done with heterostructure GaN/AlGaN single drift MIxed Tunnelling and Avalanche Transit Time (MITATT) devices grown on Si〈111〉 substrate, at around 1 THz. A generalised self-consistent, nonlinear Mixed Quantum Drift-Diffusion (MQDD) simulator is developed and used for designing p++-n−-n-n++ type room temperature solid-state source within 0.75–1.1 THz regime. The epitaxial heterojunction layers are grown by III–V nitride molecular beam epitaxy which has enabled the realisation of GaN/AlGaN periodic structure on Si〈111〉 substrate with AlN buffer layer. For the first time, the realisation of heterostructure III–V Nitride MITATT oscillator at sub-mm wave/terahertz (THz) frequencies is done. The device is observed to oscillate at 1 THz with ∼4%. A comparison of MQDD model with experimental (On wafer DC testing) results for heterojunction GaN/AlGaN MITATT diodes shows generalised agreement in terms of breakdown voltage (∼27 V) and efficiency (∼4%). First experimental results of GaN/AlGaN heterostructure MITATT diode are reported and compared with the theoretical predictions obtained through MQDD model.

Enhancement of 2D Electron Gas Mobility in an AlN/GaN/AlN Double‐Heterojunction High‐Electron‐Mobility Transistor by Epilayer Stress Engineering

Herein, 2D electron gas (2DEG) enhancement in an AlN/GaN/AlN double‐heterojunction high‐electron‐mobility transistor (DH‐HEMT) is achieved by epilayer stress engineering. The epistructures are grown on a SiC substrate using plasma‐assisted molecular beam epitaxy (PA‐MBE). The stress in the AlN buffer is systematically studied as a function of the III/V ratio. An optimized AlN buffer layer with the relaxation of 66% and a smooth surface morphology with a root mean square (RMS) roughness of 0.5 nm is grown using the two‐step growth method with recovery via metal consumption. It is observed that the stress in the GaN channel is influenced by the stress in the AlN buffer nonlinearly. Furthermore, the carrier mobility increases as the stress in the GaN channel is increased. However, the 2DEG density remains unchanged. An optimized AlN/GaN/AlN DH‐HEMT epistructure with very high 2DEG sheet carrier density of 4.1 × 1013 cm−2 and carrier mobility of 613 cm2 V−1 s−1 is achieved, which leads to higher device output power density.

Improved Reverse Leakage Current in GaInN-Based LEDs With a Sputtered AlN Buffer Layer

In this study, the improvement of reverse leakage current characteristics with a sputtered (SP) -AlN buffer layer in GaInN-based green light-emitting diodes (LEDs) has been presented for the first time. To understand the origin of the improvement, a detailed review and careful analysis of reverse leakage current characteristics were performed. The review and analysis identified that the improvement was primarily caused by the suppression of variable-range-hopping process obtained by replacing conventional low-temperature GaN buffer. Verification that threading dislocations and V- defects can enhance the variable-range-hopping process has been received. We believe that this study will contribute to the realization of green LEDs with advantages of high reliability, a long lifetime, and electrical robustness.

AMMONIA MOLECULAR BEAM EPITAXY OF AlGaN HETEROSTRUCTURES ON SAPPHIRE SUBSTRATES

In order to develop a technology for the growth of Al(Ga)N-based heterostructures, the effect of different molecular beam epitaxy growth conditions on the properties of AlN and AlGaN layers was studied. The optimal conditions for the growth of AlN buffer layers were established, which made it possible to achieve a root mean square roughness as small as 0.7 nm. It was shown that an increase of AlN layer thickness leads to a decrease of density of edge dislocations, while no explicit dependence of the screw dislocation density on the layer thickness was observed. The minimal obtained dislocations density values for 1.25μm-thick AlN layer were n edges = 5.9×10 9 cm -2 and n screw = 2.2×10 7 cm -2 for edge and screw dislocations respectively. As a result of optimization of the AlGaN growth temperature, a series of 0.15μm-thick layers was grown, which showed stimulated emission at wavelengths λ = 330 nm, 323 nm, 303 nm, and 297 nm with threshold power densities of 0.7 MW/cm 2 , 1.1 MW/cm 2 , 1.4 MW/cm 2 and 1.4 MW/cm 2 , respectively. The determined optimal epitaxy conditions for AlN and AlGaN layers were used to grow the AlGaN/GaN high electron mobility transistor structure on a sapphire substrate with two-dimensional electron gas, which had a mobility of 1950 cm 2 /(Vs) at a concentration of 1.15×10 13 cm -2 . The obtained results are important for creating of nitride-based UV-emitting optoelectronic semiconductor devices, as well as high-power and high-frequency electronic devices.