Thick AlN Layers Research Articles

Introduction of dislocation filtering with different ammonia flows in low-temperature grown AlN (< 1200 °C)

High crystalline quality of AlN layers are commonly grown at temperatures above 1200 °C. However, this work revealed that introducing a rough surface of AlN layer, defined as dislocation filtering (DF) herein, can improve the crystalline quality of 1.5 μm thick AlN layers grown at 1175 °C via metal organic vapor phase epitaxy technique. With 7 slm of ammonia flow during the DF growth, the lowest threading dislocation density (TDD) of ∼2.9 × 109 cm−2 was obtained. The result was associated to the formation of 3D growths which become reasonably smaller, denser, and more uniformly distributed with increasing ammonia flow up to 7 slm. The 3D growths were suggested to promote lateral growth that can increase the possibility for TD inclination and hence, mutual annihilation. The 3D growths characteristic disappeared when the ammonia flow was 10 slm. The optimized AlN layer that was obtained with 7 slm of ammonia flow exhibited an atomically smooth surface with a few deep trenches and improved transparency. Meanwhile, the calculated in-plane tensile strain for the AlN layers grown on DF with 1–7 slm of ammonia was about 3 times higher than the AlN layer without DF. However, no evidence of microcracks on the surface was observed.

Enhancement of gallium nitride on silicon (111) using pulse atomic-layer epitaxy (PALE) AlN with composition-graded AlGaN buffer

The ability to configure the optimal buffer layer for GaN growth depends on the knowledge of relaxation processes that occurs during the cooling step while countering the tensile stresses due to the contrast of thermal expansion coefficient between GaN and Si(111) substrate. Here, we inaugurate the pulse atomic-layer epitaxy (PALE) AlN layer to reinforce the buffer layer to achieve a thick GaN epilayer which is crucial for high performance power devices. The characteristics of grown GaN on Si substrate based on PALE AlN thickness of 0 ~ 100 nm are investigated along with microstructural evolution between AlN NL and composition-graded AlGaN buffer layer. PALE AlN layer deposited with an optimum thickness of 50 nm and above was observed to exhibit a highly uniform coalesced GaN epilayer surface with root-mean square (RMS) roughness of 0.512 nm. The thickness of the PALE AlN layer substantially affected the crystallinity of the top GaN epilayer where the lowest value for symmetric (0 0 0 2) and asymmetric (1 0 -1 2) x-ray rocking curve analysis were achieved, indicating the reduction of threading dislocation density in the growth structure. Transition of the E2 (high) peak from the Raman spectrum shows that the strain compression in GaN epilayer is directly proportional to the thickness of the PALE AlN layer.

Impact of AlN interlayer on the electronic and I-V characteristics of In0.17Al0.83N/GaN HEMTs devices

Here, we study a simulation model of In0.17Al0.83N/GaN passivated high electron mobility transistors (HEMTs) on SiC substrate. The research focused systematically on the effet of AlN interlayer on the electronic and electric characteristics using the Nextnano simulation software. The 2D–electron gas density of In0.17Al0.83N/AlN/GaN HEMTs is investigated through the dependence on various AlN layer thickness, we report calculations of I-V characteristics, with 1.5 nm AlN thickness, we find the highest maximum output current of 1.81 A/mm at Vgs 1 V, and more than 450 mS/mm as a transconductance peak. The Results are in agreement with experimental data.

Impedance-matched high-overtone bulk acoustic resonator

We demonstrated a nearly impedance-matched high-overtone bulk acoustic resonator (HBAR) operating at super high frequency ranges using an epitaxial AlN piezoelectric layer directly grown on a conductive SiC cavity substrate with no metal layer insertion. The small impedance mismatch was verified from the variation in the free spectral range (FSR); the experimentally obtained FSR spectra were greatly reproduced using the Mason model. Broadband phonon cavity modes up to the K-band (26.5 GHz) were achieved at an AlN layer thickness of 200 nm. The high figure of merit of f×Q=1.3×1013 Hz at 10 GHz was also obtained. Our nearly impedance-matched high-quality HBAR will enable the development of microwave signal processing devices for 5G and future 6G communication systems, such as low-phase noise oscillators and acoustic filters, as well as research on high-frequency acoustic systems hybridized with electric, optical, and magnetic systems.

Mg implantation in AlN layers on sapphire substrates

Mg ions were implanted in 1 μm thick AlN layers grown on sapphire substrates. The Mg implantation with a total dose of 5 × 1014 cm−2 introduced Al-vacancy related defects, which were decreased by annealing at temperatures over 1400 °C in an N2 ambient. We found that annealing temperatures over 1400 °C were necessary for an electrically conductive Mg-implanted AlN layer. The Mg-implanted AlN layer annealed at 1500 °C showed 1.1 nA at a bias of 100 V at room temperature and 7 nA at a bias of 10 V at 300 °C.

Influence of AlGaN n-type doping and AlN thickness on the two-dimensional electron gas density (ns) and resistance (R2DEG)

In this paper, electrical characterizations by C(VG) and ID(VG) and comparison with 1D Schrödinger-Poisson simulations are carried out to investigate the effects of AlN layer thickness and n-type AlGaN barrier doping on the two-dimensional electron gas resistance (R2DEG). Specifically, these effects are related to either the electron sheet density (ns) or the mobility (µ). We show that varying the AlN thickness from 0.7 nm (ns = 7.8 × 1012 cm−2) to 1.5 nm (ns = 9.0 × 1012 cm−2) leads to a linear increase of ns. However, simultaneous degradation of the transport properties probably due to intensifying roughness mechanism at high ns tends to limit the improvement of the device properties. Furthermore, we show that the heavily doped AlGaN barrier slightly increases ns and reduces the 2DEG mobility to ∼ 1770 cm2.V−1.s−1 leading to a degraded 2DEG resistance. However, the overall resistance is improved (R = 323 Ω/□) compared to the undoped case (R = 380 Ω/□) due to the simultaneous contribution of two conducting channels at VG = 0 V. To go further, the polarization charges at the AlGaN/AlN and AlN/GaN interfaces are computed in the 1D Schrödinger-Poisson simulations to account for the presence of the AlN layer. A reduced polarization for very thin AlN layers is considered to account for the experimental results. Finally, a very simple empirical model is proposed that predicts the enhancement of polarization charges (σ) as a function of AlN thickness.

Effect of the AlN strain compensation layer on InGaN quantum well red-light-emitting diodes beyond epitaxy.

An atomically thick AlN layer is typically used as the strain compensation layer (SCL) for InGaN-based-red light-emitting diodes (LEDs). However, its impacts beyond strain control have not been reported, despite its drastically different electronic properties. In this Letter, we describe the fabrication and characterization of InGaN-based red LEDs with a wavelength of 628 nm. A 1-nm AlN layer was inserted between the InGaN quantum well (QW) and the GaN quantum barrier (QB) as the SCL. The output power of the fabricated red LED is greater than 1 mW at 100 mA current, and its peak on-wafer wall plug efficiency (WPE) is approximately 0.3%. Based on the fabricated device, we then used numerical simulation to systematically study the effect of the AlN SCL on the LED emission wavelength and operating voltage. The results show that the AlN SCL enhances the quantum confinement and modulates the polarization charges, modifying the device band bending and the subband energy level in the InGaN QW. Thus, the insertion of the SCL considerably affects the emission wavelength, and the effect on the emission wavelength varies with the SCL thickness and the Ga content introduced into the SCL. In addition, the AlN SCL in this work reduces the LED operating voltage by modulating the polarization electric field and energy band, facilitating carrier transport. This implies that heterojunction polarization and band engineering is an approach that can be extended to optimize the LED operating voltage. We believe our study better identifies the role of the AlN SCL in InGaN-based red LEDs, promoting their development and commercialization.

Cost-Effective Formation of Ti/Ni/Ti/Au Ohmic Contacts to n-type SiC Using SiO2 Encapsulation Layer during Phosphorous Implant Activation

The purpose of this research is to develop a cost-effective formation of ohmic contacts to n-type SiC through ion implantation and activation. Although Nitrogen is commonly used as an n-type dopant, especially for in-situ doping during epitaxial growth, Phosphorus was chosen as dopant for this work due to the following reasons: Phosphorus has higher solubility than Nitrogen. Thus, it is easier to achieve higher concentration locally through ion implantation, which is needed for quality ohmic contact.Phosphorous has a higher activation rate at lower activation temperature, which could be beneficial if the availability of furnace equipment and cap layer material is limited.Phosphorus-implanted doping profile can be accurately detected through SIMS analysis against a constant in-situ doped nitrogen background doping of the n-type epi layer. Phosphorus was implanted using the implantation series shown in Table 1. As indicated, the three multiple energy implants, with the highest energy of 200keV and the lowest energy of 50keV, will form a 200-nm-deep box profile against a n- epitaxial layer (0.4μm, 2×1017cm-3) used as a RESURF layer for device fabrication (see Figure 1 and 2). Obtaining a good box profile with high doping concentration is necessary in achieving quality ohmic contact with low contact resistance.After ion implantation, the dopants must be thermally activated, and crystal damage must be recovered through annealing at high temperature between 1400~1700°C. The process of ion implantation and activation is shown in Figure 1. Traditionally, cap layers, such as pyrolyzed negative photoresist and AlN, have been used during implant activation to prevent surface deterioration. But using pyrolyzed negative photoresist as a cap layer requires a complex furnace setup, where inert gasses, such as Ar or N2, are needed to prevent the cap layer from oxidizing and evaporating. The deposition of a thick AlN layer requires a DC sputtering system where a 3-inch AlN sputtering target could cost over $500~. The AlN cap layer also requires KOH etch for removal after activation, which can also etch the SiC layer underneath. Because of the above reasons, we propose the use of 1µm thick SiO2 to encapsulate the front and back of implanted sample using PECVD deposition. The SiO2 encapsulation layer withstands temperature up to ~1500°C in air ambient, and the activation temperature and the ambient are set to these values for evaluation and analysis. As shown in Figure 3, if there is sufficient thickness (>1µm) of SiO2, the surface is protected from the deterioration caused by high temperature, and surface roughness is suppressed to the minimum as summarized in Table 2.Ti/Ni/Ti/Au (20nm/90nm/5nm/120nm) were deposited using e-beam evaporation system and annealed at 1050℃ for 2min using Heatpulse 610 RTA (Rapid Thermal Annealing) system in N2 ambient. This unique metal contact stacks are effective in forming quality ohmic contacts even when oxidation during annealing is inevitable. It has been widely understood that only Ni layer is necessary to form nickel silicide during thermal formation of ohmic contacts, but when oxygen is present from incomplete inert gas ambient or poor vacuum conditions, contact layer surface starts to oxides, forming a green NiO (nickel oxide) layer. This unwanted layer significantly deteriorates current conduction. The Ti layer (5nm) over the Ni layer in the proposed metal stacks reacts with oxygen easily, preventing further oxidation inward and achieving quality ohmic contact through effective nickel silicide formation. Even when state-of-the-art semiconductor fabrication equipment is unavailable, the proposed ohmic metal contact stacks will still be effective.Two-contact two-terminal patterns using the above ohmic metal stacks were initially fabricated for the confirmation of activation process. The plots in Figure 5 show the current conduction capability of evaluation pattern with an activation temperature of 1300°C and 1500°C, compared with a non-implanted epitaxial layer. As shown in the figure, with 1300°C annealing, the activation ratio is only ~20% and negative effects from the epi layer degradation dominates over carrier activation. Whereas, with 1500°C annealing, the activation ratio is roughly 70% and current conduction increases by a factor of 4 through the dopant activation process.The specific contact resistance ρC is significantly improved through N+ implant with an obtained value of 2.461×10-5 Ω·cm2 from the TLM evaluation plot in Figure 6. While the N+ implant is effective in reducing contact resistance, the high temperature activation annealing still slightly deteriorates the conductivity of the drift layer, where increase in sheet resistance RS is observed.The proposed ohmic metal stacks and SiO2 encapsulation enable cost-effective formation of ohmic contacts to n-type SiC where complex fabrication equipment setup is unavailable. Figure 1

Device-Level Impact of Highly Anisotropic Thermal Conductivity of AlN/GaN Digital Alloys

AlN/GaN digital alloys (i.e., short period superlattices) have the advantage of being an ultra-wide bandgap due to the quantum confinement effect, preservation of high mobility (no strong alloy scattering), and higher thermal conductivity than AlGaN random alloys.Herein, we investigate the influence of AlN/GaN thickness combinations on digital alloy (DA) thermal conductivity. Using time-domain thermoreflectance and steady-state thermoreflectance, we measure the anisotropic thermal conductivity of AlN/GaN DA’s, with thickness combinations: (2 nm/1 nm), (4 nm/1 nm), (6 nm/1 nm), (11 nm/1 nm). DA's with thick AlN layers exhibit highly anisotropic thermal conductivity (i.e., 4x difference between cross-plane and in-plane).Using finite-element modeling, we thermally simulate AlGaN and AlN/GaN DA devices. Improved lateral heat spreading in DA devices results in lower hot-spot temperatures compared to AlGaN devices. As the first thermal measurements of AlN/GaN DA’s, this study provides guidance for future device design.

An A0 mode Lamb-wave AlN resonator on SOI substrate with vertically arranged double-electrodes

In this paper, a novel vertically arranged double-electrodes A0 mode Lamb-wave AlN resonator on SOI substrate with a high electromechanical coupling coefficient and high figure of merit (FOM) is reported. The AlN resonator has a sandwich structure with aluminum and N-type doped silicon as electrode layers and a 500 nm thick AlN film as piezoelectric layer. The resonator has only two electrodes vertically arranged rather than horizontal interdigitated (IDT) electrodes which is common in conventional Lamb-wave resonators. The electrode gaps for the vertically arranged double-electrodes resonators are defined by AlN layer thickness rather than by photolithography for lateral field excitation resonators, which results in higher electric field strength and higher electromechanical coupling efficient (). Compared with conventional thickness field excitation (TFE) resonators with floating bottom electrodes, the vertically arranged double-electrodes resonators have higher electric field strength as the potential difference is larger between the top electrode and bottom electrode than that between the IDT electrodes and floating electrode. As a result, a higher electromechanical coupling coefficient is achieved. Furthermore, the resonant frequency of the vertically arranged double-electrodes resonator presented in this work can be defined by photolithography by controlling the width of the silicon layer. The of the vertically arranged double-electrodes resonator calculated from the measurement results of admittance versus frequency by numerically fitting with the Butterworth Van Dyke model shows an increase by 3.85 times, from 0.073% to 0.281% compared with conventional TFE resonators, and the FOM also increases by three times, from 2.66 to 7.99. This work provides a new structure to design future AlN Lamb-wave resonators on SOI substrate.

Investigation of AlN/Si based heterogeneous Junction using inter-digitated electrodes for enhanced UV light detection

A 200 nm thick AlN layer is fabricated by Hydride Vapor Phase Epitaxial (HVPE) technique onto the n-type Silicon < 111 > wafer. After epitaxial growth the AlN is masked and 100 nm thick interdigitated electrodes of Silver are deposited by the atomistic scaled Physical Vapor Deposition (PVD) system. These interdigitated electrodes are further examined with five possible circuit configurations of AlN/Silicon under multiple biases and temperatures. Different electrical and electro-optical characterization such as Current-Voltage, Charge Deep Level Transient Spectroscopy and Kinetics of Dark and UV current; have been performed for each configuration. For all such cases the Ultra-Violet (UV) light has been sensed and found that the reversed biased silicon coupled with interdigitated AlN schemes may provide largest responsivity under both photovoltaic and photoconductive modes. Also, the photo-conversion gain (ratio of UV Current / Dark Current) of each case under different physical conditions have also been emulated and found that the same reverse biased configuration leads higher extent of photo-conversion gain. The charge based transient analysis using (Q-DLTS) have been performed for all said configurations and it is found that the same reverse biased scheme offers largest magnitude of capture cross section i.e. ~ 10−18 cm2, which may capture larger extent of free charge carriers in UV mode and could be opted for enhanced UV light detection.

Investigation of Interfacial Microstructures of TiN/AlN Multilayers by Neutron Reflectometry

Three alternate TiN/AlN nanostructured multilayers with the TiN layer thicknesses of 60, 70, and 120 Å and the AlN layer thickness of 10 Å were fabricated using dc reactive magnetron sputtering. Microstructural characterizations of the three nano-scale films were performed using nonpolarized specular neutron reflectometry. The results showed that the three TiN/AlN multilayer thin films were typical superlattice films and the thicknesses of the TiN layer and AlN layer in the multilayers were consistent with the design thickness nearly. The interface roughness was asymmetric in all the samples. The interface of AlN growing on TiN was much sharper than that of TiN growing on AlN and the latter was the diffusion interface in the TiN/AlN multilayer films.

Improvement of 650-nm red-emitting GaIn0.17N/GaIn0.38N multiple quantum wells on ScAlMgO4 (0001) substrate by suppressing impurity diffusion/penetration

This study aims to improve the crystalline quality of 650-nm GaIn0.17N/GaIn0.38N red-emitting multiple quantum wells (MQWs) fabricated on a ScAlMgO4 (SCAM) substrate. When using the SCAM substrate, the diffusion and/or penetration of impurities, including Mg, Sc, O, and Al, from the SCAM substrate poses as a challenge. To address this issue, we introduced an Al0.74In0.26N layer between the SCAM substrate and MQWs, which was lattice-matched to the substrate. The Al0.74In0.26N layer effectively blocked the diffusion of impurities from the SCAM substrate into the adjacent layers during the metal-organic vapor epitaxy (MOVPE) growth. For further suppression, a thick AlN layer was deposited on the back of the SCAM substrate before the MOVPE growth, which effectively suppressed impurity penetration from the growth surface. The structure proposed in this study improved the crystallinity and the surface roughness of MQWs, resulting in the improvement of internal quantum efficiency by approximately three times compared to that of the conventional sample.

Separation of AlN layers from silicon substrates by KOH etching

In this work, the AlN/Si(111) epitaxial structures grown consistently by plasma assisted molecular beam epitaxy (PA MBE) and hydride vapour phase epitaxy (HVPE) methods were studied. The PA MBE AlN buffer layers were synthesized via coalescence overgrowth of self-catalyzed AlN nanocolumns on Si(111) substrates and were used as templates for further HVPE growth of thick AlN layer. It was shown that described approaches can be used to obtain AlN layers with sufficiently smooth morphology. It was found that HVPE AlN inherited crystallographic polarity of the AlN layer grown by PA MBE. It was demonstrated that the etching of such AlN/Si(111) epitaxial structures results in partial separation of the AlN epilayers from the Si(111) substrate and allows to form suspended structures. Moreover, the avoidance of surface damage and backside overetching was achieved by use thin Cr film as surface protective coating and by increasing the layer thickness accordingly.

Demonstration of lateral epitaxial growth of AlN on Si (1 1 1) at low temperatures by pulsed reactive sputter epitaxy

We present a pulsed reactive magnetron sputter process for high quality AlN on Si (111) beneficially avoiding any high-temperature growth. Initially, metallic aluminium with a nominal thickness of about one monolayer is deposited at a substrate temperature around 850 °C in an Ar plasma followed by sputtering in an Ar/N plasma. For 250 nm thick AlN layers a surface roughness below 0.2 nm rms is obtained as determined by atomic force microscopy (AFM). Using an Al nucleation step prior to AlN growth substantially improves the crystalline properties of AlN. The FWHM values of the AlN (0 0 0 2) and the AlN (1 0 1 0) diffraction peaks with 0.45° and 0.86°, respectively, are comparable to state-of-the-art AlN on Si layers grown by metalorganic vapour phase epitaxy (MOVPE). Two different N-precursor gases, namely N2 and NH3, lead to distinct layer qualities as revealed by atomic force microscopy and transmission electron microscopy. Only with NH3 substantial lateral growth can be achieved at T = 850 °C which is mandatory to obtain smooth surface morphologies. In MOVPE such lateral AlN growth is typically only achieved at high growth temperatures (T > 1000 °C).

Thick AlN layers grown on micro-scale patterned sapphire substrates with sputter-deposited annealed AlN films by hydride vapor-phase epitaxy

Misaligned crystallite growth is found on the sidewalls of dome-shaped patterns which hinders the coalescence of the c-plane AlN layer growing on the c-axis-oriented sapphire plane. In this study, thick AlN layers without cracks were grown by hydride vapor-phase epitaxy on micro-scale patterned sapphire substrates using annealed sputter-deposited AlN with a thickness of 200 nm. It was observed that undesired misaligned AlN growth can be prevented by selecting an appropriate growth temperature. A two-step approach was used to promote the lateral growth and coalescence of AlN. A relatively smooth surface without discontinuities and holes was obtained. The influence of the multi-oriented sapphire facets provided by micro-scale patterned sapphire substrates was investigated. According to the geometry of the misaligned crystallites, the grown AlN deviating from the c-axis on the near sapphire r-plane sidewalls has less influence on the coalescence of AlN, compared to the near n-plane sidewalls.

Evaluation of residual stress of c oriented AlN/Si (111) and its impact on mushroom-shaped piezoelectric resonator

Aluminium nitride-based MEMS resonators are one of the interesting recent research topics for its tremendous potential in a wide variety of applications. This paper focuses on the detrimental effect of residual stress on the AlN-based MEMS resonator design for acoustic applications. The residual stress in the sputtered c-axis (< 001 >) preferred oriented AlN layers on Si (111) substrates are studied as a function of layer thickness. The films exhibited compressive residual stresses at different thickness values: − 1050 MPa (700 nm), − 500 MPa (900 nm), and − 230 MPa (1200 nm) with ± 25 MPa accuracy. A mushroom-shaped AlN-based piezoelectric MEMS resonator structure has been designed for the different AlN layer thicknesses. The effect of the residual stresses on the mode shapes, resonant frequencies, and quality factor (Q) of the resonator structures are studied. The resonant frequency of the structures are altered from 235 kHz, 280 kHz, and 344 kHz to 65 kHz, 75 kHz and 371 kHz due to the residual stress of − 1050 MPa (thickness: 700 nm), − 500 MPa (thickness: 900 nm) and − 230 MPa (thickness: 1200 nm), respectively. At no residual stress, the quality factors of the resonator structures are 248, 227, 241 corresponding to the 700 nm, 900 nm, and 1200-nm-thick AlN layers, respectively. The presence of the residual stress reduced the Q values from 248 (thickness: 700 nm), 227 (thickness: 900 nm), 241 (thickness: 1200 nm) to 28, 53, and 261, respectively.

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.

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.

Simulation of a Normally-off HEMT Transistor Based on a GaN/AlGaN with a p-Gate

We perform the numerical simulation of the characteristics of normally-off GaN/AlGaN HEMT transistors with a p-gate. The dependences of the threshold voltage on the thickness of the main AlGaN barrier and additional spacer and stop AlN layers are determined. The effect of the incomplete compensation of the spontaneous charge during passivation on the output characteristics of the transistor is analyzed. The effect of the ohmic resistance of the source on the saturation current of the transistor is studied. It is shown that the saturation current with the optimal design parameters can reach 1 A/mm at a gate voltage of Vg ~ 3 V.