Thick AlN Layers Research Articles

Patterned sapphire substrates cause a wavelength shift of green InGaN light-emitting diodes

InGaN/GaN multiple quantum wells (MQWs) were fabricated under the same growth conditions on the planar and patterned sapphire substrates (PSS) with 10 nm and 20 nm sputtering AlN layers, respectively. Photoluminescence and electroluminescence results both showed that MQWs samples have significant differences in emission wavelengths. The wavelength of the samples on planar substrate is about 20 nm longer than that on the PSS. For samples with the same substrate, but different AlN layer thickness, also exhibit a small wavelength shift. High-resolution X-ray diffraction also revealed that the periodic thickness of MQWs on a planar substrate is thicker than that on PSS. Thermodynamic simulation was carried out to verify the effect of PSS on the heat conduction of GaN film. The PSS embedded in GaN film will affect its heat dissipation ability, and thus influence the wavelength of the MQWs samples by affecting the growth temperature.

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

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

Experimental observation of high intrinsic thermal conductivity of AlN

AlN is an ultra-wide bandgap semiconductor which has been developed for applications including power electronics and optoelectronics. Thermal management of these applications is the key for stable device performance and allowing for long lifetimes. AlN, with its potentially high thermal conductivity, can play an important role serving as a dielectric layer, growth substrate, and heat spreader to improve device performance. However, the intrinsic high thermal conductivity of bulk AlN predicted by theoretical calculations has not been experimentally observed because of the difficulty in producing materials with low vacancy and impurity levels, and other associated defect complexes in AlN which can decrease the thermal conductivity. This work reports the growth of thick AlN layers by MOCVD with an air-pocketed AlN layer and the first experimental observation of intrinsic thermal conductivity from 130 K to 480 K that matches density-function-theory calculations for single crystal AlN, producing some of the highest values ever measured. Detailed material characterizations confirm the high quality of these AlN samples with one or two orders of magnitude lower impurity concentrations than seen in commercially available bulk AlN. Measurements of these commercially available bulk AlN substrates from 80 K to 480 K demonstrated a lower thermal conductivity, as expected. A theoretical thermal model is built to interpret the measured temperature dependent thermal conductivity. Our work demonstrates that it is possible to obtain theoretically high values of thermal conductivity in AlN and such films may impact the thermal management and reliability of future electronic and optoelectronics devices.

Homoepitaxial growth of AlN on a 2-in.-diameter AlN single crystal substrate by hydride vapor phase epitaxy

Homoepitaxial growth of a thick AlN layer on a 2-in.-diameter single crystal AlN(0 0 0 1) substrate prepared by physical vapor transport (PVT) was investigated by hydride vapor phase epitaxy (HVPE). A water-clear crack-free homoepitaxial layer with a thickness of 500 μm or more could be grown on the entire surface of the PVT-AlN substrate. No degradation of crystallinity was observed during homoepitaxial growth. X-ray rocking curve measurements of the homoepitaxial layer showed small full widths at half maximum of 11–16 arcsec, the same as those for the PVT substrates. A freestanding AlN substrate prepared from the homoepitaxial layer showed high optical transmittance in the deep ultraviolet region owing to low impurity concentrations.

Effect of excitation mode on the magnetic field detection limit of magnetoelectric composite cantilevers

Magnetic field excitation of strain-coupled magnetoelectric composite cantilevers in different bending modes is investigated for magnetic field sensing, yielding the sensitivity, noise, and magnetic field detection limit. An analytic theory covering the resonant magnetoelectric response and thermal vibration noise of arbitrary bending modes and the Johnson–Nyquist noise from the composite and electronics is presented, and detection limit results of thin film FeCoBSi–Si–AlN composite cantilevers are calculated for the first three bound–free and free–free bending modes over a wide range of dimensions. We use size-scaling to yield the same 1 kHz resonance frequency for all modes and dimensions, constant quality factors Qf = 1000, and thickness-independent experimental material parameters. Magnetic field detection limits in the 1 pT/Hz1/2 to 100 fT/Hz1/2 range are predicted for practical cantilever dimensions, whereby higher modes are found to yield lower detection limits at similar functional layer thicknesses but a greater cantilever size. All detection limits are found to be thermal vibration noise limited and for different modes to display the same 1/size2 scaling behavior but require different FeCoBSi–Si–AlN layer thickness ratios.

Рост объемных эпитаксиальных пленок AlN полуполярной ориентации на подложках Si (001) и гибридных подложках SiC/Si (001)

Abstract The possibility of growing bulk (more than 7 μm thick) epitaxial semipolar AlN films on Si(001) and hybrid SiC/Si(001) substrates without cracks has been investigated. It is found that an AlN layer grown on the Si substrate is extended, whereas an AlN layer grown on the hybrid SiC/Si substrate is compressed. The limiting (critical) thickness of the semipolar AlN layer on the Si(001) substrate is determined to be ~7.5 μm. When the film thickness exceeds this value, an ensemble of cracks is formed in the film, leading to its total cracking and exfoliation from the substrate. The semipolar epitaxial AlN films with a thickness of more than 40 μm are grown on hybrid SiC/Si substrates without cracking and exfoliation from the substrate.

Особенности пироэффекта в эпитаксиальных слоях нитрида алюминия, полученных на Si-подложках

The pyroeffect was studied in quasi-bulk AlN layers with a thickness of 10-170 μm obtained by the technology of chloride-hydride vapor phase epitaxy on standard Si sub-strates. The pyroelectric current was measured by the method of thermal exposure to non-stationary (me-ander type) laser radiation, that together with the data of independent non-contact measure-ment of the active layer surface temperature dynamics made it possible to determine the value of the pyroelectric coefficient AlN in the composition of the bimorph AlN / Si structure for different thicknesses of AlN layers. It was found that the mean values are less than those that correspond to the material obtained by the same technology, but on SiC substrates; so, in or-der to achieve the pyrocoefficients of comparable magnitude with those in the case of “AlN on Si”, an increase in the thickness of AlN layers by 60 -70% is re-quired. At the same time, when the thickness of the AlN layer is large (110 μm, 170 μm) after the removal of the silicon substrate, the pyroelectric coefficient increased and reached ~8.6-9.0μC/(m2·K).

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.

A New Method of Growing AlN, GaN, and AlGaN Bulk Crystals Using Hybrid SiC/Si Substrates

The main principles of a new method of growing bulk single-crystal AlN, AlGaN, and GaN films with thickness from 100 μm and more on silicon substrates with a buffer silicon carbide layer with its subsequent detachment from Si substrates are presented. The main substance of this method is a combination of the method of chloride-hydride epitaxy that determines high growth rates of III nitride layers and the use a Si substrate with a buffer layer of nanoscale SiC film grown by the atomic substitution method as the growth substrate. The Si substrate with a SiC layer grown by the atomic substitution method has a number of structural, physical, and chemical features as compared to SiC layers grown on Si by the standard methods. It is shown that it is precisely this feature that enables the growth on their surfaces of thick crack-free AlN, AlGaN, and GaN layers with subsequent and quite simple their detachment from the substrate. The single-crystal crack-free AlN layers with thickness to 300 μm, AlGaN layers with thickness to 400 μm, GaN layers with thickness to 200 μm, and GaN films of the semipolar ($$11\bar {2}4$$) orientations with thickness to 30 μm have been grown.

Mechanical properties and epitaxial growth of TiN/AlN superlattices

TiN/AlN superlattice coatings combine the superlattice effect and the outstanding properties of AlN in its non-equilibrium face-centred cubic (NaCl-type, rs) structure. In comparison with their non-layered counterparts, the superlattice films exhibit superior and strongly bilayer-period-dependent mechanical properties. Here we analyse the structure and mechanical properties of superlattices with AlN layer thicknesses of 0.9, 1.2, and 1.5 nm and different TiN layer thicknesses. We found that the maximum AlN layer thickness, up to which AlN can be fully stabilized in its metastable rs structure, depends on the substrate material and is at least 1.5 nm for coatings on MgO (100) and 0.9 nm for the coatings grown on Si (100) or Al2O311¯02. Our data also show that with increasing thickness of the template layer, the crystallization of AlN in the stable wurtzite structure is favoured. TiN/AlN superlattice coatings on MgO (100) exhibit the highest hardness values, peaking at 37.0 ± 0.5 GPa for AlN layer thicknesses of 0.9 nm and a bilayer period of 2.5 nm. Films with ultra-small bilayer periods exhibit high hardness combined with low values of elastic modulus suggesting a high elastic-strain-to-failure for these coatings. The highest toughness was found for superlattice films grown on MgO (100), scaling inversely with the bilayer period.

Impact of intermediate high temperature annealing on the properties of AlN/sapphire templates grown by metalorganic vapor phase epitaxy

In this work, we investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses. To avoid cracking of the MOVPE layers during HTA at 1700 °C these layers have to be grown under low incorporation of tensile strain. For each sample the growth is stopped at a certain thickness between 230 nm and 1.2 μm followed by HTA. X-ray rocking curve FWHM of 0002 and 10–12 reflections lead to an estimation of threading dislocation densities (TDD) as low as cm−2 for the thickest AlN layers of 880 nm and 1.2 μm after annealing. For all layer thicknesses MOVPE growth is subsequently continued to a total AlN layer thickness of 1.5 μm to reach for a smooth surface. The change of the AlN strain state after HTA leads to an increased wafer curvature at room temperature and compressive strain in the subsequently grown AlN layers increasing the wafer bow at room temperature. Hence, to serve as a base layer, e.g. for ultraviolet light emitting diodes, a trade-off between low TDD, surface smoothness and wafer bow has to be found.

Ultra-Energy-Efficient Photoelectrode Using Microstriped GaN on Si

A prototype photoelectrode with a unique design has been fabricated using GaN microstripes grown on a patterned Si substrate. The photoelectrode has demonstrated a record-high photocurrent density of 11 mA/cm2 upon one sun illumination and a H2 generation rate of up to 2.67 mL·cm–2·h–1. This performance with a step-change has been achieved due to the contribution from both the GaN and the silicon substrate, as such a combination covers a wide spectral region (from the ultraviolet region due to the GaN bandgap to the infrared region due to the silicon bandgap). Unlike conventional GaN grown on a silicon substrate, where a thick AlN layer is required to separate GaN from the silicon in order to avoid the well-known Ga melt-back issue, a GaN/silicon heterojunction in our photoelectrode can be formed as a result of a weak Ga melt-back reaction, which is due to the specially designed configuration of our photoelectrode grown using the microstripes. Two reference photoelectrodes have been fabricated for compari...

Overgrowth and characterization of (11-22) semi-polar GaN on (113) silicon with a two-step method

A two-step approach has been developed for the growth of semi-polar (11–22) GaN on patterned (113) silicon substrates, which effectively eliminates Ga melt-back etching at a high temperature, one of the most challenging issues. A (113) Si substrate is patterned into groove trenches by means of using a standard photolithography technique and then anisotropic chemical etching, forming (111) facets with an inclination angle of 58˚ with respect to c-axis in addition to the un-etched (113) facets. A thick AlN layer is subsequently epitaxially grown on the patterned silicon to cover all the facets ensuring to eliminate the melt-back, followed by selectively depositing SiO2 masks on the (113) facets only. Further GaN overgrowth is performed only on the exposed (111) facets, forming (11–22) semi-polar GaN with high crystal quality along the vertical direction. Stimulated emission at room temperature has been observed with a low threshold. Low-temperature photoluminescence measurements confirm a significant reduction in basal stacking faults density. This method provides a promising approach to effectively suppress the Ga melt-back etching issue, which is particularly important for Al(Ga)N growth on semi-polar GaN that requires a high growth temperature. The presented results are crucially important for developing monolithic on-chip integration of electronics and photonics on silicon.

Stabilization of sputtered AlN/sapphire templates during high temperature annealing

In this work we investigated annealing of 350 nm and 450 nm thick sputtered (SP) AlN on sapphire in the temperature range from 1650 °C to 1730 °C. The most distinct decrease in the threading dislocation density (TDD), from initially 3 × 1010 cm−2 to 4 × 108 cm−2, was achieved for 450 nm AlN at an annealing temperature of 1680 °C. Unfortunately, for temperatures ≥1680 °C the reaction between AlN and sapphire partially disturbs the AlN layer by formation of polycrystalline AlON at the AlN/sapphire interface. In addition, we found epitaxially oriented (1 1 1) AlON on top of the AlN layers that hinders further epitaxial growth. This problem can be prevented by lower annealing temperatures (≤1680 °C) and thick SP AlN layers. The importance of an appropriate AlN sputtered layer thickness and of a precise temperature control during high temperature annealing are shown in this paper to ultimately achieve a balance between low TDD and suppression of excessive AlON formation.

Crystallographic orientation dependent maximum layer thickness of cubic AlN in CrN/AlN multilayers

Metastable rock-salt (face centered cubic, c-) AlN can be grown in CrN/AlN multilayers when the AlN layer is thin enough. Exceeding a certain critical thickness, the thermodynamically stable wurtzite (w) structure grows. In this work, a bilayer-period-gradient (21 repeated blocks, each consisting of 10 bilayers with AlN layer-thicknesses ranging from 1.0 nm to 10.0 nm), ∼2.0 μm-thick, reactively magnetron sputtered multilayer was characterized in detail with a spherical aberration-corrected transmission electron microscope (TEM). The studies are complemented by DFT (density functional theory) calculations.The high resolution TEM (HRTEM) studies reveal that the <111> growth-orientation is not as effective as the <110> and <100> growth-orientations in stabilizing the metastable c-AlN. The critical thickness for the c-AlN layers (before the thermodynamically stable w-AlN forms) is around ∼2.0 nm for the <111> growth-orientation but reaches as high as 4.1 nm for both <110> and <100> growth-orientations. Contrary to the <111> orientation, in both <110> and <100> orientations several unusually highly mismatched c-CrN/w-AlN interface structures form as soon as w-AlN is present. DFT studies suggest that the larger critical thickness of the AlN layers in <100> and <110> orientation is allowed by the lower surface energy and higher cubic/wurtzite interfacial energy. The combination of HRTEM and DFT studies allows answering open questions on the impact of crystallographic orientations and interface structures, and also provides a better understanding on the growth mechanisms of c-AlN, necessary for the outstanding mechanical properties of AlN-containing multilayers.

Microstructure and properties of TiAlN/AlN multilayers with different modulation periods

The effects of thicknesses of AlN layer on microstructures and properties of TiAlN/AlN multilayers were studied. TiAlN/AlN multilayers had been deposited on 304 stainless steels by magnetron sputtering system. The microstructure and properties of the multilayers were characterized by X-ray diffraction, vicker hardness tester, atomic force microscopy, and friction abrasion tester. The results show that under “template effect” of TiAlN layer, AlN layer is changed into face-centered cubic and grow epitaxially with TiAlN layer. The superhardness effect could be generated in the region with sharp interface. With the thickness of AlN layer in TiAlN/AlN multilayers, the properties of multilayer are firstly increase and then decrease. When the AlN layer thickness is 3.2 nm, the multilayer has excellent properties. The hardness of multilayer is 30.98 GPa, the roughness of multilayer is 20.2 nm, and the coefficient of friction of multilayer is 0.61.

First principles study of hBN-AlN short-period superlattice heterostructures

We report a theoretical study of the structural, electronic, and optical properties of hBN-AlN superlattice (SL) heterostructures using a first-principles approach based on standard and hybrid density functional theory. We consider a short-period (L &amp;lt; 10 nm) SL and find that its properties depend strongly on the AlN layer thickness LAlN. For LAlN≲1 nm, AlN stabilizes into the hexagonal phase and the SL displays insulating behavior with type II interface band alignment and optical gaps as small as 5.2 eV. The wurtzite phase is formed for thicker AlN layers. In these cases, built-in electric fields lead to the formation of polarization compensating charges as well as two-dimensional conductive behavior for electronic transport along interfaces. We also find defect-like states localized at interfaces which are optically active in the visible range.

Effect of Nitride Layer Thickness on Reciprocating Sliding Wear Behavior of AlN Layers on Spray-Formed Al Alloys

Dry sliding friction and wear behavior of nitrided and non-nitrided spray-formed Al alloys against AISI52100 steel were investigated using a linear-reciprocating ball on flat configuration under different normal loads. In case of nitrided spray-formed Al alloys, an increase in the applied normal load leads to a steady reduction of the stationary stage of the coefficient of friction depending on the sustainability of the AlN/Al compound. With regard to the nitride layer, thickness-specific wear regimes are distinguished for both spray-formed Al alloys. Nitrided spray-formed AlSi alloys show mild abrasive wear independent of normal load in case of AlN layers with up to 3 µm thickness. The layers exhibit an increased wear resistance compared to the non-nitrided condition for all applied test loads. Thick AlN layers > 4 µm show a steep increase in wear rate at loads exceeding 10 N, indicating a severe adhesive wear. These layers fail due to nitride layer delamination. Nitrided spray-formed AlMg alloys show mild abrasive wear for thin AlN layers < 4 µm up to normal loads of 12 N. Thick AlN layers almost immediately fail upon the initiation of the sliding contact due to crack formation within the nitride layer.

Improved crystal quality of semipolar AlN by employing a thermal annealing technique with MOVPE

AlGaN light-emitting diodes (LEDs) have been studied for their use as highly efficient light sources in the deep ultraviolet (DUV) region. The spontaneous and piezoelectric polarization in c-plane AlGaN, produces a strong electric field, which degrades the performance of LEDs. To overcome this polarization problem, growth on less widely used substrates, such as those with semipolar/nonpolar orientations has been carried out. However, the expected improvements have not been forthcoming for AlGaN alloys in the DUV region because of the poor crystallinity obtained. Here, we report on the improved crystallinity of semipolar AlN grown on m-plane sapphire using a high-temperature annealing process. For (10-1-3) and (11-22) AlN layers grown on m-plane sapphire, the crystal quality improved with increasing annealing temperature up to 1700 °C. The crystallinity of (11-22) AlN layer was further improved by further growth of AlN. A best FWHM of 280arcsec was obtained for a 3-μm thick (11-22) AlN layer grown after high-temperature annealing.

High brightness ultraviolet light-emitting diodes grown on patterned silicon substrate

We report on the fabrication of high brightness AlGaN-based ultraviolet light emitting diodes (UV-LED) on patterned silicon. Using the lateral epitaxial overgrowth approach, we demonstrate the growth of a 6 µm thick AlN layer of high crystalline quality. X-ray diffraction characterization showed a rocking curve with a full width at half maximum of 553 and 768″ for the (00.2) and (10.2) planes, respectively. The low dislocation density of the AlN template enabled the growth of bright AlGaN/GaN quantum wells emitting at 336 nm. By appropriate flip-chip bonding and silicon substrate removal processing steps, the patterned AlN surface was exposed and efficient bottom-emission UV-LEDs were realized. Improvement of the AlN quality and the structure design allowed the optical output power to reach the milliwatt range under pulsed current, exceeding the previously reported maximum efficiency. Further investigations of the optical power at different pulsed currents and duty cycles show that thermal management in this device structure is still challenging, especially in continuous wave mode operation. The strategy presented here is of interest, since AlN crystalline quality improvement and optimization of the light extraction are the main issues inhibiting efficient UV emitter on silicon fabrication.