Improvement of Crystal Quality of AlN Films with Different Polarities by Annealing at High Temperature

High-quality AlN film is a key factor affecting the performance of deep-ultraviolet optoelectronic devices. In this work, high-temperature annealing technology in a nitrogen atmosphere was used to improve the quality of AlN films with different polarities grown by magnetron sputtering. After annealing at 1400–1650 °C, the crystal quality of the AlN films was improved. However, there was a gap between the quality of non-polar and polar films. In addition, compared with the semi-polar film, the quality of the non-polar film was more easily improved by annealing. The anisotropy of both the semi-polar and non-polar films decreased with increasing annealing temperature. The results of Raman spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy revealed that the annihilation of impurities and grain boundaries during the annealing process were responsible for the improvement of crystal quality and the differences between the films with different polarities.

Effect of reactive gas condition on nonpolar AlN film growth on MnS/Si (100) by reactive DC sputtering

The effects of reactive gas flow conditions on nonpolar AlN film growth on MnS/Si (100) substrates using reactive DC magnetron sputtering were investigated. During AlN deposition at a substrate temperature of 750 °C, the MnS surface can be unintentionally nitrided, resulting in a decrease in the crystallinity of the AlN. Low temperature growth of the AlN layer at 300 °C prevents this nitridation and results in the crystallization of nonpolar AlN. A N2 flow equal to 30% of the Ar sputtering gas flow was found to improve the crystallinity of the nonpolar AlN and to reduce nitrogen defects, which play an important role in interfacial reactions. Nitrogen defects promote the formation of alloys such as AlMn and MnSi that degrade the interface and can significantly decompose the MnS. A higher proportion of N2 improves the nonpolar AlN crystallinity, reduces the concentration of defects and suppresses reactions at the AlN/MnS interface.

Piezoelectric MEMS Acoustic Transducer with Electrically-Tunable Resonant Frequency

The paper presents a technique to obtain an electrically-tunable matching between the series and parallel resonant frequencies of a piezoelectric MEMS acoustic transducer to increase the effectiveness of acoustic emission/detection in voltage-mode driving and sensing. The piezoelectric MEMS transducer has been fabricated using the PiezoMUMPs technology, and it operates in a plate flexural mode exploiting a 6 × 6 mm doped silicon diaphragm with an aluminum nitride (AlN) piezoelectric layer deposited on top. The piezoelectric layer can be actuated by means of electrodes placed at the edges of the diaphragm above the AlN film. By applying an adjustable bias voltage Vb between two properly-connected electrodes and the doped silicon, the d31 mode in the AlN film has been exploited to electrically induce a planar static compressive or tensile stress in the diaphragm, depending on the sign of Vb, thus shifting its resonant frequency. The working principle has been first validated through an eigenfrequency analysis with an electrically induced prestress by means of 3D finite element modelling in COMSOL Multiphysics®. The first flexural mode of the unstressed diaphragm results at around 5.1 kHz. Then, the piezoelectric MEMS transducer has been experimentally tested in both receiver and transmitter modes. Experimental results have shown that the resonance can be electrically tuned in the range Vb = ±8 V with estimated tuning sensitivities of 8.7 ± 0.5 Hz/V and 7.8 ± 0.9 Hz/V in transmitter and receiver modes, respectively. A matching of the series and parallel resonant frequencies has been experimentally demonstrated in voltage-mode driving and sensing by applying Vb = 0 in transmission and Vb = −1.9 V in receiving, respectively, thereby obtaining the optimal acoustic emission and detection effectiveness at the same operating frequency.

An Experimental and Theoretical Study of Impact of Device Parameters on Performance of AlN/Sapphire-Based SAW Temperature Sensors

The impact of device parameters, including AlN film thickness (hAlN), number of interdigital transducers (NIDT), and acoustic propagation direction, on the performance of c-plane AlN/sapphire-based SAW temperature sensors with an acoustic wavelength (λ) of 8 μm, was investigated. The results showed that resonant frequency (fr) decreased linearly, the quality factor (Q) decreased and the electromechanical coupling coefficient (Kt2) increased for all the sensors with temperature increasing from −50 to 250 °C. The temperature coefficients of frequency (TCFs) of sensors on AlN films with thicknesses of 0.8 and 1.2 μm were −65.57 and −62.49 ppm/°C, respectively, indicating that a reduction in hAlN/λ favored the improvement of TCF. The acoustic propagation direction and NIDT did not obviously impact the TCF of sensors, but they significantly influenced the Q and Kt2 of the sensors. At all temperatures measured, sensors along the a-direction exhibited higher fr, Q and Kt2 than those along the m-direction, and sensors with NIDT of 300 showed higher Q and Kt2 values than those with NIDT of 100 and 180. Moreover, the elastic stiffness of AlN was extracted by fitting coupling of modes (COM) model simulation to the experimental results of sensors along different directions considering Euler transformation of material parameter-tensors. The higher fr of the sensor along the a-direction than that along the m-direction can be attributed to its larger elastic stiffness c11, c22, c44, and c55 values.

Recent Advances in Fabricating Wurtzite AlN Film on (0001)-Plane Sapphire Substrate

Ultrawide bandgap (UWBG) semiconductor materials, with bandgaps far wider than the 3.4 eV of GaN, have attracted great attention recently. As a typical representative, wurtzite aluminum nitride (AlN) material has many advantages including high electron mobility, high breakdown voltage, high piezoelectric coefficient, high thermal conductivity, high hardness, high corrosion resistance, high chemical and thermal stability, high bulk acoustic wave velocity, prominent second-order optical nonlinearity, as well as excellent UV transparency. Therefore, it has wide application prospects in next-generation power electronic devices, energy-harvesting devices, acoustic devices, optical frequency comb, light-emitting diodes, photodetectors, and laser diodes. Due to the lack of low-cost, large-size, and high-ultraviolet-transparency native AlN substrate, however, heteroepitaxial AlN film grown on sapphire substrate is usually adopted to fabricate various devices. To realize high-performance AlN-based devices, we must first know how to obtain high-crystalline-quality and controllable AlN/sapphire templates. This review systematically summarizes the recent advances in fabricating wurtzite AlN film on (0001)-plane sapphire substrate. First, we discuss the control principles of AlN polarity, which greatly affects the surface morphology and crystalline quality of AlN, as well as the electronic and optoelectronic properties of AlN-based devices. Then, we introduce how to control threading dislocations and strain. The physical thoughts of some inspirational growth techniques are discussed in detail, and the threading dislocation density (TDD) values of AlN/sapphire grown by various growth techniques are compiled. We also introduce how to achieve high thermal conductivities in AlN films, which are comparable with those in bulk AlN. Finally, we summarize the future challenge of AlN films acting as templates and semiconductors. Due to the fast development of growth techniques and equipment, as well as the superior material properties, AlN will have wider industrial applications in the future.

Highly Homogeneous Current Transport in Ultra-Thin Aluminum Nitride (AlN) Epitaxial Films on Gallium Nitride (GaN) Deposited by Plasma Enhanced Atomic Layer Deposition

This paper reports an investigation of the structural, chemical and electrical properties of ultra-thin (5 nm) aluminum nitride (AlN) films grown by plasma enhanced atomic layer deposition (PE-ALD) on gallium nitride (GaN). A uniform and conformal coverage of the GaN substrate was demonstrated by morphological analyses of as-deposited AlN films. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analyses showed a sharp epitaxial interface with GaN for the first AlN atomic layers, while a deviation from the perfect wurtzite stacking and oxygen contamination were detected in the upper part of the film. This epitaxial interface resulted in the formation of a two-dimensional electron gas (2DEG) with a sheet charge density ns ≈ 1.45 × 1012 cm−2, revealed by Hg-probe capacitance–voltage (C–V) analyses. Nanoscale resolution current mapping and current–voltage (I–V) measurements by conductive atomic force microscopy (C-AFM) showed a highly homogeneous current transport through the 5 nm AlN barrier, while a uniform flat-band voltage (VFB ≈ 0.3 V) for the AlN/GaN heterostructure was demonstrated by scanning capacitance microscopy (SCM). Electron transport through the AlN film was shown to follow the Fowler–Nordheim (FN) tunneling mechanism with an average barrier height of <ΦB> = 2.08 eV, in good agreement with the expected AlN/GaN conduction band offset.

Effect of High-Temperature Nitridation and Buffer Layer on Semi-Polar (10–13) AlN Grown on Sapphire by HVPE

We have investigated the effect of high-temperature nitridation and buffer layer on the semi-polar aluminum nitride (AlN) films grown on sapphire by hydride vapor phase epitaxy (HVPE). It is found the high-temperature nitridation and buffer layer at 1300 °C are favorable for the formation of single (10–13) AlN film. Furthermore, the compressive stress of the (10–13) single-oriented AlN film is smaller than polycrystalline samples which have the low-temperature nitridation layer and buffer layer. On the one hand, the improvement of (10–13) AlN crystalline quality is possibly due to the high-temperature nitridation that promotes the coalescence of crystal grains. On the other hand, as the temperature of nitridation and buffer layer increases, the contents of N-Al-O and Al-O bonds in the AlN film are significantly reduced, resulting in an increase in the proportion of Al-N bonds.

Fabrication of a 3.5-GHz Solidly Mounted Resonator by Using an AlScN Piezoelectric Thin Film

In this study, a 3.5-GHz solidly mounted resonator (SMR) was developed by doping scandium in aluminum nitride to form AlScN as the piezoelectric thin film. Molybdenum (Mo) of 449 nm thickness and silicon dioxide (SiO2) of 371 nm thickness were used as the high and low acoustic impedance films, respectively, which were alternately stacked on a silicon substrate to form a Bragg reflector. Then, an alloy target with atomic ratio of 15% Sc was adopted to deposit the piezoelectric AlScN thin film on the Bragg reflector, using a radio frequency magnetron sputtering system. The characteristics of the c-axis orientation of the AlScN thin films were optimized by adjusting sputtering parameters as sputtering power of 250 W, sputtering pressure of 20 mTorr, nitrogen gas ratio of 20%, and substrate temperature of 300 °C. Finally, a metal top electrode was coated to form a resonator. The X-ray diffraction (XRD) analysis showed that the diffraction peak angles of the AlScN film shifted towards lower angles in each crystal phase, compared to those of AlN film. The energy dispersive X-ray spectrometer (EDX) analysis showed that the percentage of scandium atom in the film is about 4.5%, regardless of the sputtering conditions. The fabricated resonator exhibited a resonance frequency of 3.46 GHz, which was a small deviation from the preset resonance frequency of 3.5 GHz. The insertion loss of −10.92 dB and the electromechanical coupling coefficient of 2.24% were obtained. As compared to the AlN-based device, the AlScN-based resonator exhibited an improved electromechanical coupling coefficient by about two times.