Thick AlN Layers Research Articles

Bistability in a multiferroic composite resonator

Bistable characteristics of a nonlinear multiferroic composite resonator containing ferromagnetic and piezoelectric layers are investigated. The resonator was a borosilicate glass substrate of 25 mm × 2 mm dimensions and 150 μm thickness with a 2 μm thick amorphous ferromagnetic FeCoSiB layer and a 2 μm thick piezoelectric AlN layer deposited on its sides by magnetron sputtering. The resonator was excited by ac voltage at a frequency of 156 kHz, matching its longitudinal acoustic resonance frequency. The bistability loops were observed with increasing and decreasing frequency at constant excitation voltage and with increasing and decreasing voltage at constant frequency. With increasing excitation voltage, the resonator frequency first decreases by ∼0.7 kHz and then increases again to the initial value. A bistability model is suggested that uses Lorentzian shape resonance line and measured dependences of the resonance frequency and transmission coefficient on the output signal, which quantitatively describes experimental data. It is shown that bistability in a multiferroic resonator arises due to the nonlinearity of the ferromagnetic layer.

Surface hole gas enabled transparent deep ultraviolet light-emitting diode

The inherent deep-level nature of acceptors in wide-band-gap semiconductors makes p-ohmic contact formation and hole supply difficult, impeding progress for short-wavelength optoelectronics and high-power high-temperature bipolar electronics. We provide a general solution by demonstrating an ultrathin rather than a bulk wide-band-gap semiconductor to be a successful hole supplier and ohmic contact layer. Free holes in this ultrathin semiconductor are assisted to activate from deep acceptors and swept to surface to form hole gases by a large electric field, which can be provided by engineered spontaneous and piezoelectric polarizations. Experimentally, a 6 nm thick AlN layer with surface hole gas had formed p-ohmic contact to metals and provided sufficient hole injection to a 280 nm light-emitting diode, demonstrating a record electrical-optical conversion efficiency exceeding 8.5% at 20 mA (55 A cm−2). Our approach of forming p-type wide-band-gap semiconductor ohmic contact is critical to realizing high-efficiency ultraviolet optoelectronic devices.

Novel design and performance of the solidly mounted resonator with an AlN-buffered ZnO piezoelectric film

In this report, an AlN-buffered ZnO double-layer piezoelectric layer based SMR (solidly mounted resonator) was designed and fabricated, in which the nano AlN film acted as a buffer layer. For optimization of the piezoelectric layer, a series of SMRs were fabricated with different AlN layer thicknesses from 15 nm to 100 nm by radio-frequency magnetron sputtering (RFMS). Optimization of the AlN-buffered ZnO thin films for the SMR devices performance were investigated through XRD (X-ray diffraction), SEM (Scanning electron microscopy), AFM (atomic force microscope) and NWA (network analyzer) with the different AlN layer thickness. From the investigations, we concluded that existence of nano AlN layer obviously affected the resonant performance of the ZnO based SMR device and it was found that the best ZnO thin film was exhibited when AlN thickness was 55 nm which shows better return losses and higher Q factor than devices exhibiting an effective method to optimize ZnO SMR.

Growth of High‐Quality AlN and AlGaN Films on Sputtered AlN/Sapphire Templates via High‐Temperature Annealing

We investigated the fabrication and characterization of AlN and AlGaN epilayers grown on sputtered and annealed AlN/sapphire templates by metal organic vapor phase epitaxy. The surfaces of the AlN homoepilayers exhibited clear step‐terrace structures with minimal root mean square roughness values. X‐ray rocking curve (XRC) measurements showed that the AlN homoepilayers had smaller (102)‐plane full width at half maximum (FWHM) values (192 arcsec) than the AlN/sapphire templates did (214 arcsec), and that they had larger (0002)‐plane FWHM values (97 arcsec) than the AlN/sapphire templates (22 arcsec). The latter results may be due to the wafer curvature increasing as the AlN layer thickness increases. We also present results for the growth of AlGaN heteroepilayers with different AlN compositions on AlN/sapphire templates that either did or did not contain AlN regrowth layers. High‐AlN mole fraction AlGaN heteroepilayers had larger (0002)‐plane FWHM values and smaller (102)‐plane FWHM values than low‐AlN mole fraction AlGaN. Furthermore, the presence of an AlN regrowth layer tended to have a negative effect on the high‐AlN mole fraction AlGaN layers. The above results are discussed using lattice mismatch, wafer curvature, surface condition, and growth temperature parameters.

Sandwich method to grow high quality AlN by MOCVD

We report pulsed atomic layer epitaxy growth of a very high crystalline quality, thick (~2 µm) and crack-free AlN material on c-plane sapphire substrates via a sandwich method using metal organic chemical vapor deposition. This sandwich method involves the introduction of a relatively low temperature (1050 °C) 1500 nm thick AlN layer between two 250 nm thick AlN layers which are grown at higher temperature (1170 °C). The surface morphology and crystalline quality remarkably improve using this sandwich method. A 2 µm thick AlN layer was realized with 33 arcsec and 136 arcsec full width at half maximum values for symmetric and asymmetric reflections of ω-scan, respectively, and it has an atomic force microscopy root-mean-square surface roughness of ~0.71 nm for a 5 × 5 µm2 surface area.

Interface control for pure ultraviolet electroluminescence from nano-ZnO-based heterojunction devices

Realization of pure and stable ultraviolet electroluminescence (UV EL) of ZnO light-emitting diode (LED) is still a challenging issue, due to complicated defects of intrinsic ZnO and the corresponding device interfaces. In this paper, we demonstrated a simple & feasible method to fabricate n-ZnO/AlN/p-GaN heterojunctions light-emitting devices. First, the vertically aligned ZnO nanorods (NRs) have been prepared as high quality active layer, and the nanostructured heterojunction LED arrays were constructed by directly bonding ZnO NRs onto AlN-coated p-GaN wafer. By optimizing the AlN layer thickness to be 20 nm, a strong and pure ultraviolet emission located at 387 nm can be observed. The energy band alignment of n-ZnO/AlN (20 nm)/p-GaN heterojunction LED has been studied by using X-ray photoelectron spectroscopy (XPS), the valence band offset between AlN and GaN was calculated to be 0.34 eV. On the other side, the conduction band offset (as large as 3.28 eV) between AlN and ZnO can block the flow of electrons from ZnO to p-GaN. Thus, electron-hole recombination takes place in the ZnO layer, and a pure UV EL could be observed. Our results provide a significant approach toward future of pure ultraviolet optoelectronic LEDs.

The structure of InAlGaN layers grown by metal organic vapour phase epitaxy: effects of threading dislocations and inversion domains from the GaN template.

Defects in quaternary InAlGaN barriers and their effects on crystalline quality and surface morphology have been studied. In addition to growth conditions, the quality of the GaN template may play an important role in the formation of defects in the barrier. Therefore, this work is focused on effects caused by threading dislocations (TDs) and inversion domains (IDs) originating from the underlying GaN. The effects are observed on the crystalline quality of the barrier and characteristic surface morphologies. Each type of TDs is shown to affect the surface morphology in a different way. Depending on the size of the corresponding hillock for a given pinhole, it was possible to determine the dislocation type. It is pointed out that the smallest pinholes are not connected to TDs whereas the large ones terminate either mixed type or edge type TDs. At sufficiently large layer thickness, the IDs originating from the GaN template lead to the formation of concentric trenches at the layer surface, and this is related to the change in growth kinetics on top and at the immediate surroundings of the ID.

Direct growth of thick AlN layers on nanopatterned Si substrates by cantilever epitaxy

Here, Demir et al. (Article No. 1600363) demonstrate the pulsed atomic layer epitaxy AlN layers that have been grown on a 200 nm period of nano-patterned Si (111) substrates by a cantilever epitaxy via MOCVD, and compared with AlN layers grown by a maskless lateral epitaxial overgrowth (LEO) on micro-patterned Si (111) substrates. The material quality of 5-10 μm thick AlN grown by LEO is comparable to that of the much thinner layers (2 μm) grown by cantilever epitaxy on the nano-patterned substrates. Back emission ultra violet light emitting diodes (UV LEDs) were fabricated by flip chip bonding to patterned AlN heat sinks, followed by a complete Si (111) substrate removal, demonstrating a peak pulsed power of ~0.7 mW at 344 nm peak emission wave-length. Additionally, this study features the first demonstration of UV LEDs on nano-patterned Si substrates.

Heteroepitaxy mechanisms of AlN on nitridated c- and a-plane sapphire substrates

We investigate the metalorganic vapor phase epitaxy of c-oriented AlN on c- and a-plane sapphire substrates, focusing on the effect of sapphire nitridation on the AlN structure. Prior to AlN growth, the sapphire surface is subjected to nitridation via an in-situ NH3 treatment. We demonstrate that nitridation without H2 thermal etching treatment realizes high quality AlN on both c- and a-plane sapphires, indicating that a reaction between NH3 and oxygen on the sapphire surface is a critical factor in the material growth. It is proposed that nitridation initially creates nanometer-scale inversion domains in the AlN epilayer, but as growth proceeds, the N-polar domains are annihilated, leaving voids. Such growth behaviors can be regarded as spontaneous selective area growth with strain-adsorbing void formation, and lead to crack-free, ∼5 μm thick AlN layers, which produce x-ray line widths as narrow as 180 and 483 arc sec for the (0002) and (101¯2) reflections, respectively, on c-plane sapphire, and 237 and 433 arc sec for these reflections on a-plane sapphire.

Influence of AlN buffer layer on growth of AlGaN by HVPE

To the end of improvement of layer morphology and crystalline perfection of thick AlGaN layers grown by hydride vapor phase epitaxy (HVPE) as well as for the suppression of crystallite formation during growth, the impact of AlN buffer layer properties on AlGaN growth was investigated. While the surface morphology of 500 nm thick AlN layer improves with higher V/III ratio toward 50 and lower growth temperatures of 1020 °C, its use as a buffer layer for thick AlGaN layers leads to strong degradation of AlGaN surface and even to formation of different crystallites during growth. Best surface morphology and crystalline perfection of thick AlGaN layers was observed for the growth on an AlN buffer layer with a higher crystal quality disregarding its 3D morphology and high surface roughness. Best results were achieved at medium V/III ratio of about 10–20 and AlN buffer growth temperature of 1060 °C.

Metamorphic Al0.5Ga0.5N:Si on AlN/sapphire for the growth of UVB LEDs

In this paper we investigate the growth of metamorphic Al0.5Ga0.5N:Si on c-plane AlN/sapphire. The structural properties of the AlGaN:Si pseudo substrates and the electro-optical characteristics of subsequently grown UVB LEDs are being examined. We demonstrate, that superlattices allow the controlled strain relaxation of Al0.5Ga0.5N by rearrangement of threading dislocations, thus preventing the formation of cracks. This study investigates AlN/GaN superlattices with a nominal GaN layer thickness between 1.0nm and 2.5nm at a fixed AlN layer thickness of 2.5nm. The number of superlattice-periods was also varied between 20 and 120. It was found that beyond a GaN layer thickness of 1.5nm three-dimensional structures are formed. Additionally, these three-dimensional structures reduce the local defect density of the subsequently grown Al0.5Ga0.5N layer. Although the Al0.5Ga0.5N layer appears to be almost fully relaxed, the relaxation state of this pseudo substrate, was found to be dependent on the GaN layer thickness in the superlattice. After optimizing the superlattice structure we were able to grow crack free 4μm thick Si-doped Al0.5Ga0.5N layers and on top UVB LEDs with a fully strained active region emitting at 310nm with output powers of more than 18mW at 500mA.

Passivation of InGaAs interface states by thin AlN interface layers for metal-insulator-semiconductor applications

The passivation of InGaAs by thin AlN layers allows a significant reduction of the interface state density compared to that of the widely used Al2O3/InGaAs structure. The influence of the AlN layer thickness on the interface electrical properties, as well as the role of the post-deposition annealing, was carefully examined. Ultrathin AlN layers (∼1 nm) provide high quality interfacial electrical properties after a mild anneal (400 °C). Thick AlN passivation layers require annealing at higher temperature (500 °C) to achieve low interface states density. Possible explanations of the observed trend are suggested.

Direct growth of thick AlN layers on nanopatterned Si substrates by cantilever epitaxy

AlN layers have been grown on 200 nm period of nanopatterned Si (111) substrates by cantilever epitaxy and compared with AlN layers grown by maskless lateral epitaxial overgrowth (LEO) on micropatterned Si (111) substrates. The material quality of 5–10 µm thick AlN grown by LEO is comparable to that of much thinner layers (2 µm) grown by cantilever epitaxy on the nanopatterned substrates. Indeed, the latter exhibited root mean square (RMS) roughness of 0.65 nm and X-ray diffraction full width at half-maximum (FWHM) of 710 arcsec along the (0002) reflection and 930 arcsec along the (10–15) reflection. The corresponding room temperature photoluminescence spectra was dominated by a sharp band edge peak. Back emission ultra violet light emitting diodes (UV LEDs) were fabricated by flip chip bonding to patterned AlN heat sinks followed by complete Si (111) substrate removal demonstrating a peak pulsed power of ∼0.7 mW at 344 nm peak emission wavelength. The demonstrated UV LEDs were fabricated on a cost effective epitaxial structure grown on the nanopatterned Si substrate with a total thickness of 3.3 µm.

Signal-to-Noise Ratio in Cantilever Magnetoelectric Sensors

The signal-to-noise ratio (SNR) is investigated for compound magnetoelectric (ME) sensors on cantilever substrates (SUBs) for the detection of low-level magnetic fields. Operated at the mechanical resonance, the magnetic field deforming the magnetostrictive (MS) layer causes a resonant bending-mode response in the ME cantilever. The deformation of the piezoelectric (PE) layer allows for the extraction of a voltage or charge signal. Here, the influence of the PE layer thickness and electrode length on the SNR is evaluated in a theoretical study. The signal levels are calculated using the finite-element method. Noise voltages are calculated including the intrinsic electric noise of the ME sensor and amplifier noise for the case of a voltage amplifier and a charge amplifier. AlN and PZT are considered as PE materials. For a cantilever geometry with 10 mm-length, 10 mm-width, and 300 $\mu \text{m}$ -thick silicon SUB and a Metglas MS layer of 2 $\mu \text{m}$ thickness, a limit of detection (LOD) in the pT-range is predicted for $2~\mu \text{m}$ -thick AlN layers, while the LOD of PZT ME sensors is approximately one order of magnitude worse. A doubling of the SNR is obtained for choosing an upper electrode covering only the fixed side of the cantilever. Operation with a charge amplifier shows at least $\sim 50$ % better SNR values compared with PE voltage amplification.

Influence of AlN thickness on AlGaN epilayer grown by MOCVD

AlGaN/AlN layers were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The AlN buffer thickness was varied from 400 nm to 800 nm. The AlGaN layer thickness was 1000 nm. The crystalline quality, thickness and composition of AlGaN were determined using high resolution X-ray diffraction (HRXRD). The threading dislocation density (TDD) was found to decrease with increase of AlN layer thickness. Reciprocal space mapping (RSM) was used to estimate the strain and relaxation between AlGaN and AlN. The optical properties of AlGaN layers were investigated by temperature dependent photoluminescence (PL). PL intensities of AlGaN layers increases with increasing the AlN thickness. The surface morphology of AlGaN was studied by atomic force microscopy (AFM). Root mean square (RMS) roughness values were found to be decreased while increase of AlN thickness.

On a reduction in cracking upon the growth of AlN on Si substrates by hydride vapor-phase epitaxy

The main problem of the epitaxial growth of thick AlN layers on a Si substrate consists in the formation of cracks, which complicates the application of structures of this kind in the fabrication of semiconductor devices. The possibility of obtaining crack-free AlN layers with a thickness exceeding 1 μm and a mirror- smooth surface by hydride vapor-phase epitaxy is demonstrated. The properties of the layers are studied by X-diffraction analysis, optical and scanning electron microscopy, and Raman spectroscopy.

Infrared Heat Transfer Analysis in Aluminum Nitride Based Multilayer Pyroelectric IR Detector

A MEMS pyroelectric detector can be used as a pixel element in focal plane arrays of night vision cameras provided a significant rate of temperature change is available with the pyroelectric layer. The infrared heat transfer analysis intended for maximum thermal influence in pyroelectric layer is reported. The 3-D finite element modeling (FEM) is done by using COMSOL Multiphysics software. The device is simulated for different geometries to reduce the heat loss with substrate. The thickness of pyroelectric AlN and interconnected thin metal layer are being modulated for enhanced rate of temperature change. The simulation results designate that the IR detector mounted on diaphragm structure attains better temperature field as well rate of temperature change. In contrast of a bulk substrate mounted device.

Prevention of AlN crystal from cracking on SiC substrates by evaporation of the substrates

The problem of prevention of AlN crystal layers from cracking under action of thermoelastic stresses during growth of these layers on SiC substrates has been studied. Calculation of residual thermoelastic stresses in AlN/SiC double-layer system has shown that cracking of the AlN layer during cooling is inevitable until this layer becomes at least 15 times thicker than a substrate. The required ratio of the thicknesses of the layer and the substrate can be reached by growing an AlN layer with simultaneous evaporation of the SiC substrate. Experimentally performed evaporation of SiC substrates in one process with growing AlN single layers on them using the sublimation sandwich method has made it possible to prevent these layers from cracking. Continuous (non-cracked) plates with 0.2–0.8 mm thickness without substrates have been obtained as a result of these experiments. According to X-ray images obtained in synchrotron radiation, they consist of single crystalline AlN of 2H polytype, contain dislocations, but do not contain cracks. The degree of crystallinity of these thin plates, which was estimated by the full widths at half-maximum of rocking curves of X-ray diffraction reflections, corresponds to the degree of crystallinity of thick (3–5 mm) AlN layers grown on nonevaporated SiC substrates.

V-pit to truncated pyramid transition in AlGaN-based heterostructures

The formation of three-dimensional truncated pyramids after the deposition of AlN/GaN superlattices onto (0001) AlN/sapphire templates has been analysed by atomic force microscopy as well as transmission electron microscopy. V-pits in AlN layers and the formation of nano-mounds around the v-pit edges are suggested to be responsible for the pyramid formation. Keeping the individual AlN layer thickness at 2.5 nm in the 80xAlN/GaN superlattice, the transformation to the three-dimensional pyramids is observed when the individual GaN layer thickness exceeds 1.5 nm. A subsequent overgrowth of the pyramidal structures by AlGaN results in inhomogeneous Ga distribution in the layers and laterally inhomogeneous strain states. Nevertheless, compared to the growth on planar layers, the overgrowth of the truncated pyramids leads to a slight reduction in dislocation density from 1 · 1010 cm−2 (for GaN thickness of 1 nm in SL) to 7 · 109 cm−2 (for GaN thickness of 2 nm in SL). The non-planar growth front and thus the compositional inhomogeneity in AlGaN vanish gradually with increasing AlGaN thickness. As a result, homogeneous 4 μm thick Al0.5Ga0.5N buffer layers suitable for the fabrication of UV-B LED structures can be obtained.

Superconductor–insulator transition in two-dimensional NbN/MgO and NbN/AlN/MgO films

In this study, we investigate the dependence of superconducting transition temperature (Tc) on sheet resistance (Rsq) for two series of NbN films with different homogeneities which were prepared on either MgO substrates (NbN/MgO) or MgO substrate covered with a 1nm thick AlN layer (NbN/AlN/MgO). The Rsq-dependent Tc (Tc(Rsq)) behavior of the homogeneous NbN/MgO films could be explained with Finkel’stein’s formula, which is based on localization theory using reasonable fitting parameters. On the other hand, the Tc(Rsq) values of the NbN/AlN/MgO films were larger than that of the NbN/MgO films in the region To understand the origin of this deviation from the theory for homogeneous systems, we assumed that the NbN/AlN/MgO films have the potential to form an inhomogeneous percolation structure. To confirm this assumption, the temperature (T) dependence of the upper critical magnetic field ( near Tc was analyzed using the form which is based on classical percolation theory. From the dependence of Rsq ((Rsq)) of both series, it was found that the (Rsq) of the NbN/AlN/MgO films deviates in the region from in a homogeneous NbN/MgO films. By introducing an effective sheet resistance to account for the inhomogeneous structure of the NbN/AlN/MgO films, we were able to qualitatively explain the differences between the Tc(Rsq) behaviors of the two series.