AlN Layer Research Articles

Ni/AlN Composite Coating for Corrosion and Elements Interdiffusion Resistance in Molten Fluoride Salts System

The Ni/AlN composite coating was prepared for increasing corrosion and elements interdiffusion resistance of GH3535 alloy in molten fluoride salts, and the effect of mechanical interlocking on adhesion strength between AlN layer and nickel coating was also studied. Results indicated that the adhesion strength between AlN layer and nickel coating could be significantly enhanced through mechanical interlocking effect, which effectively prevented the nickel coating from flaking off at elevated temperature. Through an etching pre-treatment of AlN layer, the corrosion resistance of the Ni/AlN coated GH3535 alloy in molten FLiNaK salt was further improved, and elements interdiffusion between the substrate and nickel coating was completely suppressed. AlN layer as a diffusion barrier remained compact and continuous in Ni/GH3535 system after high-temperature molten salt corrosion. Moreover, a Ni–P layer consisting of Ni3P and Ni phases formed in the Ni coating after corrosion.

Void containing AlN layer grown on AlN nanorods fabricated by polarity selective epitaxy and etching method

Creating voids between thin films is a very effective method to improve thin film crystal quality. However, for AlN material systems, the AlN layer growth, including voids, is challenging because of the very high Al atom sticking coefficient. In this study, we demonstrated an AlN template with many voids grown on AlN nanorods made by polarity selective epitaxy and etching methods. We introduced a low V/III ratio and NH3 pulsed growth method to demonstrate high-quality coalesced AlN templates grown on AlN nanorods in a metal organic chemical vapor deposition reactor. The crystal quality and residual strain of AlN were enhanced by the void formations. It is expected that this growth method can contribute to the demonstration of high-performance deep UV LEDs and transistors.

High thermoelectric power factor in ambient-stable semiconducting rare-earth ErN thin films

Erbium nitride (ErN) is an emerging semiconducting rare-earth pnictide with unique electronic and magnetic properties. ErN has attracted significant interest for spin superlattices and spintronic devices and as a second-stage regenerator for Gifford–McMahon cryo-coolers. Solid-solution alloys of ErN with III-nitride semiconductors such as GaN have been studied extensively for use in solid-state lasers, amplifiers, and light-emitting devices operating in the retina-safe and fiber-optic communication wavelength window of 1.54 μm. However, due to the high affinity of Er toward oxygen, ErN is prone to oxidation in ambient conditions. To date, no reports on the deposition of the high-quality ErN thin film and its thermoelectric properties have been published. In this Letter, semiconducting ErN thin films are deposited inside an ultrahigh-vacuum chamber and capped with thin (3 nm) AlN layers to stabilize it in ambient conditions. Structural, optical, and electronic characterization reveals that ErN thin films (a) grow with (111) and (002) orientations on (0001) Al2O3 and (001) MgO substrates with sharp and abrupt ErN–substrate interfaces, (b) demonstrate a direct bandgap of 1.9 eV, and (c) exhibit a high carrier concentration in the range of 4.3 × 1020 to 1.4 × 1021 cm−3. Thermoelectric measurements show a moderately high Seebeck coefficient of –72.6 μV/K at 640 K and a maximum power factor of 0.44 × 10−3 W/m K2 at 486 K. Demonstration of an ambient-stable semiconducting ErN thin film and its high thermoelectric power factor marks significant progress in rare-earth pnictide research and will help develop ErN-based spintronic and thermoelectric devices.

A first-principles research of two-dimensional AlN/C2N van der Waals heterostructure as photocatalyst

Van der Waals (vdW) heterostructures are ordinarily employed to furnish more possibilities for two-dimensional (2D) semiconductor materials. In this work, we explore thoroughly the electronic and optical properties of AlN/C 2 N heterostructure based on first-principles. The results reveal that the heterostructure with an indirect bandgap is a semiconductor material that is stable in energy and made by vdW interaction. The AlN/C 2 N heterostructure has type-II band alignment that can availably facilitate the segregation of photogenerated carriers and prolong the longevity of the carriers, and the built-in electric field ( E int ) formed inside the heterostructure can further encourage them. The band edge position crossing the water-splitting redox potential states that the heterostructure can serve to water-splitting catalytic process, while water oxidation and reduction reactions befell in the AlN layer and the C 2 N layer, respectively. Moreover, the heterostructure has higher carrier mobility and stronger light absorption capacity in the near-ultraviolet and visible light regions compared to two monolayers. It is worth noting that the light absorption of the heterostructure has redshift under compressive biaxial strain. The work demonstrates that the 2D AlN/C 2 N heterostructure has the compelling potential to be used as an efficient water-splitting photocatalyst and used for other optical devices. • The photogenerated carriers in the type-Ⅱ AlN/C 2 N vdW heterostructure can be effectively separated under the light. • The energy band of AlN/C 2 N spans the redox potential of water, so it can be used for photocatalytic water splitting. • The heterostructure possesses higher light absorption coefficient in the near-ultraviolet and visible light regions. • AlN/C 2 N heterostructure can be used in photocatalysis and other optical fields.

Effect of high intensity short-pulsed ion irradiation on optical properties of multilayer coatings of aluminum and silicon nitrides

The effect of high-intensity short-pulsed ion irradiation on the optical properties of multilayer coatings, consisting from thin layers of aluminum and silicon nitrides, deposited by reactive magnetron sputtering on glass, steel and silicon substrates, has been investigated. The larger the thickness of AlN layers, the stronger the effect of growth and radiation defects on multilayer coating properties. When the concentration of growth defects increases and its interaction intensifies, the radiation resistance of coatings grows. In general, the radiation resistance of coatings was due to the wide band gap of their constituent nitrides and the high concentration of strongly interacting growth defects distributed along the inter-crystallite and interlayer boundaries.

Effect of Gas Annealing on the Electrical Properties of Ni/AlN/SiC.

It is shown in this work that annealing of Schottky barrier diodes (SBDs) in the form of Ni/AlN/SiC heterojunction devices in an atmosphere of nitrogen and oxygen leads to a significant improvement in the electrical properties of the structures. Compared to the non-annealed device, the on/off ratio of the annealed SBD devices increased by approximately 100 times. The ideality factor, derived from the current-voltage (IV) characterization, decreased by a factor of ~5.1 after annealing, whereas the barrier height increased from ~0.52 to 0.71 eV. The bonding structure of the AlN layer was characterized by X-ray photoelectron spectroscopy. Examination of the N 1 s and O 1 s peaks provided direct indication of the most prevalent chemical bonding states of the elements.

Suppression of current dispersion in AlGaN/GaN MISHFETs with in-situ AlN passivation layer

We demonstrate effective reduction of interface states and current collapse in AlGaN/GaN metal insulator semiconductor heterostructure field effect transistor (MISHFETs) with Al2O3 and in-situ AlN passivation layer. Here, first, a 3 nm-thick in-situ AlN layer is grown on AlGaN/GaN structure by using MOCVD at 1070 °C, and second a 7 nm-thick atomic layer deposition (ALD) Al2O3 layer is immediately deposited on the AlN layer. The X-ray photoelectron spectroscopy (XPS) analyses for the Ga 3d core level and multi-frequency capacitance–voltage (C–V) measurement show that the density of interface states originated from the Ga-O bonds, which is dramatically reduced with the in-situ AlN passivation layer. The AlGaN/GaN-based MISHFET with the Al2O3/in-situ AlN layers exhibits the low gate-lag effect (8%), remarkably less than that of the device with the typical GaN capping layer (21.5%). These results indicate that the Al2O3/in-situ AlN double passivation layer is very effective in suppressing the current collapse, and hence, it is found to be the promising method for the fabrication of AlGaN/GaN MISHFETs with enhanced performance.

Structural and optical properties of self-assembled AlN nanowires grown on SiO2/Si substrates by molecular beam epitaxy

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO2/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO2 interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20–30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1–2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy.

Electrical Spin Injection into the 2D Electron Gas in AlN/GaN Heterostructures with Ultrathin AlN Tunnel Barrier

AbstractThe spin injection into 2D electron gas (2DEG) in AlN/GaN heterostructures is studied by magneto‐transport measurements. An ultrathin AlN layer at the hetero‐interface acts as a barrier to form high‐quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection to overcome the conductance mismatch issue. In this study, Hanle signals and inversed Hanle signals are observed, proving that the spin injection is achieved in the 2DEG in the AlN/GaN heterostructure rather than in the interfacial states. The spin‐relaxation time in 2DEG at 8 K is found to be as long as 860 ps, which almost keeps constant with bias and decreases with increasing temperature. The spin‐relaxation process is illustrated as Rashba spin‐orbit coupling dominated D'yakonov Perel’ mechanisms above 8 K. These results show the promising potential of 2DEG in AlN/GaN heterostructures for spin field‐effect transistor applications.

Stress control in thick AlN/c-Al2O3 templates grown by plasma-assisted molecular beam epitaxy

Stress evolution was studied in up to ~3 µm thick AlN templates, comprising ∼65 nm thick AlN nucleation layers (NLs) and thick buffer layers (BLs), grown using different growth modes and conditions by plasma-assisted molecular beam epitaxy (PA MBE) on c-Al2O3. Growth of both the NL and BL in a standard PA MBE mode at N-rich conditions (at flux ratio Al/N2* ∼ 0.5) led to instant generation of a relatively high tensile stress (∼1.5 GPa) which is maintained throughout the entire growth. On the contrary, NLs, grown using a migration-enhanced epitaxy (MEE), demonstrated a transition from the initial compressive stress to stress-free growth, which is usually observed in the Volmer–Weber films. Further growth of thick AlN BLs on the MEE–NLs at various Al/N2* ratios revealed a wide variety of stress evolution processes. The BL growth by using metal-modulated epitaxy (MME) at Me-rich conditions with Al/N2* ∼ 1.33 led to a gradual decrease in the initial compressive stress in the 2D AlN layers, whereas standard PA MBE growth of 3D BL at N-rich conditions (Al/N2* ∼ 0.92) exhibited a fast transition from the initial compressive stress to tensile stress. Moreover, we succeeded in achieving the quasi-stress-free growth of a 3.1 µm thick AlN BL using the MME growth mode at the optimum flux ratio Al/N2* = 1.05. These results were compared with the results of other authors and explained using a kinetic approach to description of stress evolution during PA MBE of AlN/c-Al2O3 templates, taking into account several simultaneously acting competitive mechanisms of continuous generation of tensile and compressive stresses.

Adsorption behaviors and crystal structure of AlN films on TiN (001) surfaces by first-principles study

Abstract The initial adsorption behaviors of Al and N atoms on TiN (001) surfaces and the structural stability of rocksalt AlN (rs-AlN) and wurtzite AlN (wz-AlN) films on TiN (001) substrates were calculated by first-principles. The results show that Al atom is adsorbed above N atom of TiN (001) surface initially, and then atomic bondings of Al to N and N to Ti are energetically favorable to form a continuous adsorption. The crystal structure formed by AlN adatoms is investigated by calculating the bonding energy of AlN/TiN interface and strain energy in AlN film. rs-AlN film is inclined to grow on TiN (001) substrate, and it will not transform into the stable wz-AlN structure when the thickness of AlN layer exceeds 8.9 nm or more. This calculation result provides guidance for the preparation of a thick rs-AlN film in the experiments.

Effect of nitrogen capture ability of quantum dots on resistive switching characteristics of AlN-based RRAM

This Letter studies the effect of the nitrogen capture ability of quantum dots on resistive switching characteristics of AlN-based resistive random access memory. We prepared a single layer AlN device and four types of AlN/PbS quantum dot stacked structure devices with different concentrations. Compared with the single layer AlN device, the AlN/PbS quantum dot stacked structure devices exhibit excellent resistive switching characteristics, such as forming-free, low power consumption, and excellent stability. We propose that the resistive switching process is determined by the migration of nitrogen ions and the lead sulfide (PbS) quantum dot layer as a natural nitrogen ion reservoir, which can improve the resistive switching characteristics. Moreover, the size of the natural nitrogen ion reservoir can be modulated by changing the concentration of quantum dots.

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

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

Высокочастотный резонансный магнитоэлектрический эффект в структуре FeCoSiB-AlN на диэлектрической подложке

A high-frequency magnetoelectric (ME) effect has been discovered in a planar structure containing a ferromagnetic FeCoSiB layer and a piezoelectric AlN layer produced by magnetron sputtering on a borosilicate glass substrate. The structure was excited by a magnetic field at a frequency of the thickness acoustic vibration mode of 32.4 MHz, and the electric voltage generated by the piezoelectric layer was measured. The coefficient of ME conversion at the frequency of the second thickness mode is αE ≈ 19 V / A at a bias magnetic field of 120 A / m, corresponding to the maximum of the piezomagnetic coefficient of the ferromagnetic layer.

Crystallographic orientation and strain distribution in AlN seeds grown on 6H–SiC substrates by the PVT method

AlN crystal is prepared on 6H–SiC substrate by PVT method, the growth pattern is [0001]AlN‖[0001]6H–SiC and [101̄0]AlN‖[101̄0]6H–SiC and the stress in AlN layer decreases with the increase of thickness.

The stability of graphene and boron nitride for III-nitride epitaxy and post-growth exfoliation.

A challenging approach, but one providing a key solution to material growth, remote epitaxy (RE)—a novel concept related to van der Waals epitaxy (vdWE)—requires the stability of a two-dimensional (2-D) material. However, when graphene, a representative 2-D material, is present on substrates that have a nitrogen atom, graphene loss occurs. Although this phenomenon has remained a hurdle for over a decade, restricting the advantages of applying graphene in the growth of III-nitride materials, few previous studies have been conducted. Here, we report the stability of graphene on substrates containing oxygen or nitrogen atoms. Graphene has been observed on highly decomposed Al2O3; however, graphene loss occurred on decomposed AlN at temperatures over 1300 °C. To overcome graphene loss, we investigated 2-D hexagonal boron nitride (h-BN) as an alternative. Unlike graphene on AlN, it was confirmed that h-BN on AlN was intact after the same high-temperature process. Moreover, the overgrown AlN layers on both h-BN/AlN and h-BN/Al2O3 could be successfully exfoliated, which indicates that 2-D h-BN survived after AlN growth and underlines its availability for the vdWE/RE of III-nitrides with further mechanical transfer. By enhancing the stability of the 2-D material on the substrate, our study provides insights into the realization of a novel epitaxy concept.

Tunneling Enhanced Structure for Improving the Performance of Ultraviolet Light-emitting Diodes

The novel AlGaN-based ultraviolet light-emitting diode (UV-LED) with a tunneling enhancement structure is proposed. The tunneling enhanced structure consists of an ITO layer, an extremely thin AlN layer, and a p+-GaN layer. Under forward bias, compared to traditional devices, tunneling enhanced UV-LEDs can shorten the width of the tunneling region. In addition, the dielectric constant of the AlN layer in the tunnel area is small, which can effectively increase the transport of holes from the ITO layer to the MQWs active region. Due to the increased hole concentration and drift speed, for a 300 ㎛ × 254 ㎛ UV-LED chip, the optical output power increased by 19.6% at 120 ㎃. Therefore, the proposed tunneling enhanced structure provides a simple and effective way to increase the light output power of UV-LEDs.

Tuning electrical and thermal conductivities of the two-dimensional electron gas in AlN/GaN heterostructures by piezoelectricity

We investigate the electrical and thermal conductivities of the two-dimensional electron gas (2DEG) confined in the quantum well formed at the heterojunction between a thin GaN layer and an AlN layer strained by an Al x Ga1−x N capping layer in the temperature range from 10 to 360 K. The experimental protocol developed to deduce from calorimetric and Hall-effect measurements at a variable temperature the critical characteristics and transport properties of the confined 2DEG is presented. It is found that, in the measured temperature range (10–360 K), the electrical conductivity of the 2DEG is temperature-independent, due to the predominance of scattering processes by interface defects. However, the thermal conductivity shows a linear temperature dependence, mirroring the specific heat of free electrons. The temperature-independent relaxation time associated with the overall electron scattering means that the values obtained for electrical and thermal conductivities are in excellent agreement with those stipulated by the Weidemann–Franz law. It is also found that for weak strain fields in the AlN layer, both the electrical and thermal conductivities of the two-dimensional interfacial electrons increase exponentially with strain. The importance of 2DEG in AlN/GaN quantum wells lies in the fact that the strong piezoelectricity of AlN allows the transport properties of the 2DEG to be tuned or modulated by a weak electric field even with the high density of lattice mismatch induced defects at the AlN-GaN interface .

Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers: Experimental and ab initio analysis

We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow determination of the bandgaps of the investigated AlN samples and their temperature dependencies. Next, they are compared with the literature data obtained by other experimental techniques for bulk AlN crystals and layers grown on different substrates. The obtained results revealed that the AlN bandgap depends on the substrate. The theoretical analysis using density functional theory calculations showed that the effect is induced by the tetragonal strain related to the lattice mismatch between the substrate and the AlN layer, which has a strong influence on the spectral positions of the intrinsic excitons, and consequently on the bandgap of AlN layers.

Structural and dynamic properties of short-period GaN/AlN superlattices grown by submonolayer digital epitaxy

Structural and dynamic properties of short-period GaN/AlN superlattices with the thicknesses of the constituent layers varying from two to several monolayers, grown using the method of submonolayer digital molecular beam epitaxy, are experimentally and theoretically studied. It is established that in the grown samples there are two types of periodicity. One type is formed by periodic sequence of GaN and AlN layers in the superlattice, and the second one is related with periodic interruptions in the growth of superlattice for the evaporation of excess Ga metal. The dependences of the positions and intensities of the lines in the Raman spectra on the period of the superlattice are determined, and microscopic nature of optical phonon modes is established. The doublet structure of the E(TO) lines localized in the GaN and AlN layers of superlattice, genetically related to the E 2(high) and E1 phonon branches of the bulk crystal, is first discovered and explained. A strong dependence of the polar modes localized in the AlN layer on the thickness of layer forming the superlattice is revealed. The results of complex studies will improve the accuracy of quantitative estimation of important parameters of superlattice structures and can be used to optimize growth parameters for the fabrication of structurally perfect short-period GaN/AlN superlattices.