AlN Buffer Layer Research Articles

Structural and morphological characteristics of GaN-based hybrid heterostructures grown on por-Si

GaN/Si(111) heterostructures were grown by plasma-assisted molecular beam epitaxy on usual Si(111) substrates and compliant por-Si/Si(111) substrates without using AlN buffer layer. The positive influence of the high-temperature nitridation step, as well as the usage of a compliant substrate on crystalline quality, was confirmed. We got a crack-free 850-nm-thick GaN layer at room temperature by using the low-temperature GaN buffer layer with nanocolumnar morphology.

Electronic structure and optical characteristics of the hybrid GaN/por-Si heterostructures

GaN/Si(111) heterostructures grown by plasma-assisted molecular beam epitaxy on routine Si(111) substrates and compliant por-Si/Si(111) substrates without using AlN buffer layer was studied by using various structural and spectroscopy methods of analysis. XPS study revealed that the layer is grown on the compliant substrate of por-Si being closer to the stoichiometric composition. The shift of the A1(LO) mode in the Raman spectrum confirms the lattice-matched growth type on the compliant substrate in comparison with the routine c-Si substrate. The experimentally determined value of the optical bandgap width for the epitaxial GaN layer grown on por-Si substrate exceeds that of the layer grown on single-crystalline silicon c-Si by 0.1 eV

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.

Emission Properties of Heavily Doped Epitaxial Indium-Nitride Layers

The results of studies of the spontaneous photoluminescence and stimulated emission spectra of epitaxial n-InN layers with a concentration of free electrons of ~1019 cm–3 are reported. The layers are grown by molecular-beam epitaxy with the plasma activation of nitrogen on sapphire substrates with AlN and GaN buffer layers. It is found that, as the InN layers are grown under conditions with enrichment with nitrogen at a growth temperature elevated to 470°C close to the beginning of the decomposition of InN, the crystal quality of the layers is improved and the stimulated-emission threshold is lowered. As the conditions of growth change to conditions with enrichment with the metal, two emission bands separated by an energy of 100 meV are observed in the spontaneous-photoluminescence spectra of InN. For such layers, a substantial increase in the stimulated-emission threshold and, in some cases, the lack of a transition to stimulated emission are observed. In the study, an interpretation of the observed emission bands is given and some inferences as to their nature are made.

Effect of high‐temperature AlN buffer on anisotropy of semi‐polar (11‐22) GaN with two pressure growth stages

The low-temperature GaN (LT-GaN) and high-temperature AlN (HT-AlN) buffer layers were grown on m -plane sapphire by metal–organic chemical vapour deposition. The following semi-polar (11-22) GaN thin films were deposited under high- and low-pressure growth stages. Anisotropy of (11-22) GaN grown on HT-AlN buffer along two in-planar directions ([11-2-3] and [-1100]) were clearly suppressed, the maximum variation of the X-ray rocking curve full width at half maximum of (11-22) GaN on LT-GaN buffer was 0.2498°, that of (11-22) GaN on HT-AlN buffer was 0.0488°. The X-ray diffraction results of on-axis and off-axis both indicated that the crystal quality of the epitaxial GaN layer with HT-AlN buffer was obviously improved, and surface morphology was much smoother.

Epitaxial growth and characterization of GaN thin films on graphene/sapphire substrate by embedding a hybrid-AlN buffer layer

This study investigates that high-quality GaN thin films can be grown on a few-layer graphene (FLG)/sapphire substrate by embedding a hybrid AlN buffer layer (BL). The hybrid AlN BL is constructed by low-temperature AlN nucleation layer (LT-AlN NL) and high-temperature AlN BL grown respectively by sputtering and metal organic chemical vapor deposition (MOCVD). The high density of edge-type threading dislocation (TD) in the GaN sample without hybrid AlN BL provide current leakage paths, resulting in a symmetric and temperature-independent I-V characteristic curve for a Ni-based Schottky contact. The excellent adhesion and uniform coverage of LT-AlN NL on the FLG layer by sputtering can overcome the nucleation issue and prevent the thermal etching effect of graphene during MOCVD epitaxial process. The edge-type TD density and carbon concentration of the GaN thin films grown on the hybrid AlN BL/FLG/sapphire substrate can be reduced significantly, resulting in a lower intensity of blue, green, and orange luminescences on a 17-K photoluminescence spectrum. The Ni-based Schottky contact with a barrier height of 0.69 eV and leakage current density of 4.38 × 10−6 A/cm2 is obtained, which demonstrates that a high-quality GaN thin films can be grown onto an FLG substrate by embedding a hybrid AlN BL.

AlGaN multiple quantum wells by PA-MBE for deep UV emission: Effect of growth interruptions

Growth of AlGaN multiple quantum wells by Plasma Assisted-Molecular Beam Epitaxy has been optimized for use in deep ultraviolet light emitting diodes. A series of samples were grown on c-plane sapphire using an AlN buffer layer, an ~1 μm AlGaN layer, followed by 40-pairs of wells and barriers. The III/V flux ratio was varied from near stoichiometric to excess group III and a plasma exposure step was introduced for some samples after the deposition of wells and barriers. The quality of the well-barrier interface was estimated by the number and intensity of X-Ray Diffraction (XRD) superlattice peaks. Our results indicate that interface abruptness is greatly enhanced by the addition of this process step. While the room temperature photoluminescence (PL) intensity increases significantly upon introduction of the interruption after the barrier layer, this effect is reversed with additional interruption after the well layers. Temperature dependent PL measurements show an anomalous variation with increasing temperature with a maxima around 110°-150°C. This is due to photogeneration of e-h pairs in the underlying AlGaN layer and its subsequent recombination in the quantum wells by diffusion through percolative barrier layers with in-plane potential fluctuations. This is arrested at lowest temperatures causing a reduction in PL intensity. The PL intensity ratio I300K/I4K is highest for samples grown under excess group III, but reduces with the addition of the growth interruption step after the barrier layer. Growth under stoichiometric conditions together with dual growth interruptions was found to be optimal, with improved PL properties while maintaining high interface quality.

Effects of the surface preparation and buffer layer on the morphology, electronic and optical properties of the GaN nanowires on Si

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaOx buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm−2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaOx semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

Epitaxy of GaN(0001) and GaN(10$$\bar {1}$$1) Layers on Si(100) Substrate

Two different approaches to epitaxy of 4-μm-thick layers of polar GaN(0001) and semipolar GaN(10 $$\bar {1}$$ 1) on a V-shaped nanostructured Si(100) substrate with nanometer-thick SiC and AlN buffer layers have been experimentally demonstrated. The GaN(0001) layers were synthesized by hydride vapor-phase epitaxy, and GaN(10 $$\bar {1}$$ 1) layers, by metal-organic vapor-phase epitaxy, with the growth completed by hydride vapor-phase epitaxy. It was shown that layers of the polar GaN(0002) have a longitudinal elastic stress of –0.45 GPa and the minimum full width at half-maximum of the X-ray diffraction rocking curve ωθ ~ 45 arcmin, whereas for the semipolar GaN(10 $$\bar {1}$$ 1), these values are –0.29 GPa and ωθ ~ 22 arcmin, respectively. A conclusion is drawn that the combined technology of semipolar gallium nitride on a silicon (100) substrate is promising.

Stimulated emission of AlGaN layers grown on sapphire substrates using ammonia molecular beam epitaxy

By optimising the growth temperature of the AlGaN layers and using high-temperature AlN buffer layers, high-quality AlxGa1 – xN layers (x = 0.15, 0.21, 0.26, and 0.3) were obtained, in which stimulated emission in the UV spectral range 330 – 297 nm was implemented with the threshold intensity of excitation Ith ≈ 0.7 – 1.4 MW cm−2, respectively. It is found that the threshold value of the stimulated emission of AlGaN layers grown by molecular beam epitaxy is largely determined by the intensity of the process of thermal decomposition of GaN, which affects the surface morphology and, consequently, the amount of optical scattering loss. It is shown that no pronounced localisation of nonequilibrium charge carriers occurs in the AlGaN layers, which is manifested in the absence of a large Stokes shift and in the realisation of optical amplification at transitions in an electron-hole plasma, and also indicates a relatively homogeneous material composition.

Formation of conductive AlN buffer layer using spontaneous via-holes and realization of vertical AlGaN Schottky diode on a Si substrate

A conductive AlN epitaxial layer is successfully realized by spontaneously forming via-holes filled with n-AlGaN inside an AlN buffer layer on a Si substrate. The via-holes are found to originate from the formation of an Al–Si alloy, produced from a small amount of Al supplied to the Si substrate at the initial stage of the crystal growth of the n-AlN buffer layer using metal organic chemical vapor deposition and successive selective growth of n-AlN on the Si surface. The via-holes are filled with conductive n-AlGaN by successive epitaxial growth of n-AlGaN, making the insulating n-AlN buffer layer conductive. The vertical conductivity through this n-AlN buffer layer is enhanced more than 540 times compared with an n-AlN buffer layer without via-holes. Using this conductive n-AlN buffer layer on the Si substrate, we successfully fabricated a vertical n-AlGaN Schottky diode on the Si substrate for the first time.

Improvement of emission efficiency with a sputtered AlN buffer layer in GaInN-based green light-emitting diodes

We examined a sputtered AlN (sp-AlN) buffer layer to improve the emission efficiency of GaInN-based green LEDs. Both the electroluminescence and photoluminescence emission efficiencies are improved in a LED with a sp-AlN buffer layer compared to that with a conventional low-temperature GaN buffer layer. To identify the origin of the improvements, we carried out the analysis including X-ray diffraction, surface morphology, initial nucleation growth, and temperature-dependent PL. Through analysis and consideration, we showed that the improvements are mainly caused by less strain and a lower degree of carrier localization in quantum wells (QWs), that is highly influenced by the grain diameter of the GaN island at initial growth. These results indicate that a sp-AlN buffer layer induces less compositional pulling that results in less strain and alloy fluctuation in QWs, simultaneously.

Influence of surface nitridation and an AlN buffer layer on the growth of GaN nanostructures on a flexible Ti metal foil using laser molecular beam epitaxy

GaN nanorods (NRs) and hollow nanocolumns (HNCs) were grown on flexible Ti foils using laser-assisted molecular beam epitaxy at a growth temperature of 700 °C. The shape, size and density of the GaN nanostructures were tuned by surface nitridation and AlN buffer layer growth temperature on a Ti foil. Sparse (∼ 5.5 × 108 cm−2) GaN NRs were obtained on the bare surface whereas dense (∼3.47 × 109 cm−2) GaN NRs were grown on the nitridated Ti foil. The shape of the GaN changed from NRs to HNCs by introducing an AlN buffer layer on nitridated Ti foil. Raman spectroscopy showed the grown GaN nanostructures have a wurtzite crystal structure. Room-temperature photoluminescence spectroscopy measurements show that the GaN nanostructures possess an intensive near band edge emission at ∼3.42 eV with a negligible defect-related peak. The growth of tunable GaN nanostructures on flexible metal foils is attractive for flexible optoelectronics and sensor devices.

Temperature effect on Al predose and AlN nucleation affecting the buffer layer performance for the GaN-on-Si based high-voltage devices

An AlN buffer layer allows epitaxial growth of GaN on silicon substrates. We have studied the early AlN nucleation stage performed at high and low process temperatures. We show that the temperature has a crucial effect on the chemical reactions on the Si substrate during the initial growth stage. We have observed that large clustered defects are formed at 1000 °C. These defects are responsible for degradation of the vertical leakage current (VLC) blocking capability of the buffer layer. Formation of the defects is prevented if the temperature is lowered to 800 °C, which is explained by a carbonization of the Si surface. Formation of the SiC interlayer leads to the stable AlN/Si(111) interface during subsequent high-temperature growth of the buffer structure. We demonstrate that very low VLCs in superlattice-based buffer are achieved using the low-temperature nucleation process, which makes it suitable for fabrication of high voltage AlGaN/GaN high electron mobility transistor devices.

Correlation of donor-acceptor pair emission on the performance of GaN-based UV photodetector

High responsivity UV photodetector has been realized by fabricating single crystalline epitaxial GaN-based devices grown on silicon substrate by using molecular beam epitaxy system. The influence of trap states existing within the forbidden gap on GaN-based optoelectronic devices has been investigated. The quality and performance of the fabricated GaN based UV photodetector with distinct AlN buffer layer grown at low and high substrate temperature are substantiated. A detailed analysis reveals that high temperature buffer can yield improved crystalline quality of GaN with similar morphological properties as compared to buffer layer grown at low temperature. However, the distinct buffer layer influence the optical properties as the photoluminescence analysis explains that both the films possess superior band-to-band edge emission where GaN grown with high temperature AlN buffer consists of dominant acceptor defect states in comparison to GaN with low temperature AlN buffer layer. The existence of acceptor states has been accredited by the presence of Ga vacancy related states in the band gap. The fabricated devices yield high photoresponsivity of 2.1 and 1.5 A/W at 1 V from GaN films bearing low and high accepter defect states which explain a clear correlation of device performance with trap states within the band-gap region.

395 nm GaN-based near-ultraviolet light-emitting diodes on Si substrates with a high wall-plug efficiency of 52.0%@350 mA.

The high-performance 395 nm GaN-based near-ultraviolet (UV) light emitting diodes (LEDs) on Si substrates have been obtained by designing an AlN buffer layer to decrease the dislocations density of the GaN layer. By adopting a multi-layer structure with a high- and low-V/III ratio alternation, a high-quality AlN buffer layer has been obtained with a small full-width at half-maximum (FWHM) for AlN(0002) X-ray rocking curve (XRC) of 648 arcsec and a small root-mean-square roughness of 0.11 nm. By applying the optimized AlN buffer layer, the high-quality GaN layer with GaN(0002) and GaN(10-12) XRC FWHM of 260 and 270 arcsec have been obtained, and the high-performance GaN-based near-UV LED wafers and chips have been fabricated accordingly. The as-fabricated near-UV LED chips exhibit a light output power of 550 mW with a forward voltage of 3.02 V at 350 mA, corresponding to a wall-plug efficiency of 52.0%. These chips with outstanding performance are of paramount importance in the application of curing, sterilization, efficient white lighting, etc.

Enhanced Device Performance of GaInN-Based Green Light-Emitting Diode with Sputtered AlN Buffer Layer

In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly improved by just replacing the buffer layer with a sputtered-AlN layer. To understand the origin of the improvement in performance, the electrical and optical properties were compared by means of electro-reflectance spectroscopy, I–V curves, electroluminescence spectra, L–I curves, and internal quantum efficiencies. From the analysis of the results, we concluded that the improvement is mainly due to the mitigation of strain and reduction of the piezoelectric field in the multiple quantum wells active region.

GaN Heteroepitaxy on Strain-Engineered (111) Si/Si1−xGex

The metalorganic chemical vapor deposition growth of GaN on strained Si/Si1−xGex epilayers on (111) Si substrates was investigated. A multi-beam optical stress sensor (MOSS) was used for in situ stress measurements during growth of the entire heterostructure. MOSS was initially used to measure the extent of stress relaxation during the growth of constant-composition Si1−xGex layers on Si. The results compared favorably to that obtained by post-growth high-resolution x-ray diffraction. MOSS was also used to monitor stress during the growth of thin, tensile-strained Si on relaxed Si0.95Ge0.05/compositionally graded epilayers. The tensile-strained Si/Si1−xGex epilayers were then used as virtual substrates for the growth of GaN epilayers using a thin (90 nm) AlN buffer layer. GaN grown on tensile-strained Si exhibited a higher initial compressive stress and reduced final tensile stress compared to films grown directly on (111) Si, resulting in a lower crack density in the GaN along with a reduced density of threading dislocations. These results suggest that strain engineering of the Si surface prior to growth may provide an alternative method to improve the quality of GaN grown on (111) Si.

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ∼46 μm hr–1 at a high RF bias (400 W), and ∼7 μm hr–1 at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ∼3× after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency and power conversion efficiency UV-C LEDs.

Hetero-epitaxial growth of a GaN film by the combination of magnetron sputtering with Ar/Cl2 gas mixtures and a separate supply of nitrogen precursors from a high density radical source

Hetero-epitaxial growth of a gallium nitride (GaN) film on an AlN(0 0 0 1) buffer layer on a sapphire(0 0 0 1) substrate was demonstrated by supplying gallium precursors and nitrogen radicals separately from two individually operated plasma sources to control the V/III supplying ratio precisely. The sources were a reactive Ar-Cl2-mixture plasma sputtering of a gallium target and a remote low inductance antenna (LIA) for N2-H2 inductively coupled-plasma. Lateral growth of the GaN film was observed in 0.5%-Cl2-added Ar sputtering at a low growth temperature of 670 °C, whilst the growth mode coalesced at temperatures lower than 600 °C. With more than 2.0% of Cl2, no film was deposited due to etching by the reactive chlorine whenever the temperature was at 500 °C. At the growth temperature of 670 °C, crystallinity with narrow a X-ray rocking curve GaN (0 0 0 2) was obtained at the condition of 0.5% Cl2 and 27.4% N2, even though the background pressure was 10−4 Pa.