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The spontaneous polarization (SP) and piezoelectric (PZ) constants of BxAl1-xN and BxGa1-xN (0 ≤ x ≤ 1) ternary alloys were calculated with the hexagonal structure as reference. The SP constants show moderate nonlinearity due to the volume deformation and the dipole moment difference between the hexagonal and wurtzite structures. The PZ constants exhibit significant bowing because of the large lattice difference between binary alloys. Furthermore, the PZ constants of BxAl1-xN and BxGa1-xN become zero at boron compositions of ∼87% and ∼74%, respectively, indicating non-piezoelectricity. The large range of SP and PZ constants of BxAl1-xN (BAlN) and BxGa1-xN (BGaN) can be beneficial for the compound semiconductor device development. For instance, zero heterointerface polarization ΔP can be formed for BAlN and BGaN based heterojunctions with proper B compositions, potentially eliminating the quantum-confined Stark effect for c-plane optical devices and thus removing the need of non-polar layers and substrates. Besides, large heterointerface polarization ΔP is available that is desirable for electronic devices.

The epitaxial growth of technically-important β-Ga2O3 semiconductor thin films have not been realized on flexible substrates due to limitations by the high-temperature crystallization conditions and the lattice-matching requirements. In this report, for the first time single crystal β-Ga2O3 (-201) thin films is epitaxially grown on the flexible CeO2 (001)-buffered hastelloy tape. The results indicate that CeO2 (001) has a small bi-axial lattice mismatch with β-Ga2O3 (-201), thus inducing a simultaneous double-domain epitaxial growth. Flexible photodetectors are fabricated based on the epitaxial β-Ga2O3 coated tapes. Measurements show that the obtained photodetectors have a responsivity of 40 mA/W, with an on/off ratio reaching 1000 under 250 nm incident light and 5 V bias voltage. Such photoelectrical performance is already within the mainstream level of the β-Ga2O3 based photodetectors by using the conventional rigid single crystal substrates; and more importantly remained robust against more than 1000 cycles of bending tests. In addition, the epitaxy technique described in the report also paves the way for the fabrication of a wide range of flexible epitaxial film devices that utilize the materials with lattice parameters similar to β-Ga2O3, including GaN, AlN and SiC.

Wurtzite BAlN alloys are emerging ultrawide bandgap III-nitride semiconductors promising for optical and electronic devices. Yet the boron compositions of the grown alloys have been limited. In this Letter, we report on the demonstration of a thick single-phase wurtzite BAlN film with a boron composition over 20%. The growth was conducted at 1010 C and 150 Torr with continuous flows of group-III precursors and ammonia with a growth rate of 2.2 lm/h by metalorganic chemical vapor deposition. The boron composition was studied by x-ray diffraction (XRD), secondary neutral mass spectrometry (SNMS), and Rutherford backscattering spectrometry (RBS). The XRD 2h scan exhibited the clear wurtzite BAlN peak 1.82 larger than the AlN peak, indicating the boron composition of 30.9% based on the lattice constants of wurtzite AlN and BN. The SNMS and RBS experiments, independent of strain and defects, revealed that the boron content was 22%. The microstructures of the wurtzite BAlN film were further studied by transmission electron microscopy, showing an initial 5 nm thick layer free of crystal twinning followed by widespread crystal twinning with lattice rotations of 60 clockwise and anticlockwise. The optical transmission experiment manifested that the bandgap of the smaller-lattice BAlN film was 5.1 eV, smaller than that of larger-lattice AlN. This trend was the opposite of the conventional InGaAlN but consistent with theoretical predictions. This study would greatly facilitate the research of material, physics, and devices incorporating the wurtzite BAlN alloys.

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Extreme environments are often faced in energy, transportation, aerospace, and defense applications and pose a technical challenge in sensing. Piezoelectric sensor based on single-crystalline AlN transducers is developed to address this challenge. The pressure sensor shows high sensitivities of 0.4-0.5 mV per psi up to 900 °C and output voltages from 73.3 to 143.2 mV for input gas pressure range of 50 to 200 psi at 800 °C. The sensitivity and output voltage also exhibit the dependence on temperature due to two origins. A decrease in elastic modulus (Young's modulus) of the diaphragm slightly enhances the sensitivity and the generation of free carriers degrades the voltage output beyond 800 °C, which also matches with theoretical estimation. The performance characteristics of the sensor are also compared with polycrystalline AlN and single-crystalline GaN thin films to investigate the importance of single crystallinity on the piezoelectric effect and bandgap energy-related free carrier generation in piezoelectric devices for high-temperature operation. The operation of the sensor at 900 °C is amongst the highest for pressure sensors and the inherent properties of AlN including chemical and thermal stability and radiation resistance indicate this approach offers a new solution for sensing in extreme environments.

GaN electronics have hinged on invasive isolation such as mesa etching and ion implantation to define device geometry, which, however, suffer from damages, hence potential leakage paths. In this study, we propose a new paradigm of polarization isolation utilizing intrinsic electronic properties, realizing in situ isolation during device epitaxy without the need of post-growth processing. Specifically, adjacent III- and N-polar AlGaN/GaN heterojunctions were grown simultaneously on the patterned AlN nucleation layer on c-plane sapphire substrates. The two-dimensional electron gas (2DEG) was formed at III-polar regions but completely depleted in N-polar regions, thereby isolating the 2DEG channels with a large 3.5 eV barrier. Structures of polarization-isolated high electron mobility transistors (PI-HEMTs) exhibit significantly reduced isolation leakage currents by up to nearly two orders of magnitude at 50 V voltage bias compared to the state-of-the-art results. Aside from that, a high isolation breakdown voltage of 2628 V is demonstrated for the PI-HEMT structure with 3  μm isolation spacing, which is two-times higher than a conventional mesa-isolation HEMT. Moreover, the PI-HEMT device shows a low off-state leakage current of 2 × 10−8 mA/mm with a high Ion/Ioff ratio of 109 and a nearly ideal subthreshold slope of 61 mV/dec. This work demonstrates that polarization isolation is a promising alternative toward the plasma-damage-free isolation for GaN electronics.

Polarization matched c-plane III-nitride quantum wells structure," ABSTRACT Polarization-matched quantum wells (QWs) can lead to maximized electron-hole wave functions overlap and low efficiency droop at high current density. By using the modern theory of polarization with hexagonal reference, c-plane InAlN/InGaN QWs were explored and designed for polarization matching. The simulation results show that, even on cplane, polarization-matched structures can be achieved by adjusting strain and material composition. The In composition of larger than 35% of InAlN was required to match the total polarization of InGaN at any given composition. Considering the bandgap's bowing factors of III-nitride ternary alloys, In0~0.1Ga1.0-0.9N as quantum barrier (QB) provided enough potential barriers for In0.35~0.45Al0.65-0.55N to form a multiple QW (MQW) structure. The results indicated that improper resistance of MQW and the existing fixed charge between the interfaces of p-type region/MQW and n-type region/MQW could result in nonuniform carrier distributions and current leakage, respectively. Furthermore, we found that In0.41Al0.59N/In0.1Ga0.9N polarization-matched MQW had proper resistance; however, such structure produced a huge polarization fixed-charge between the junction interface. By studying the strain level of InAlN QW and GaN QB, which can be grown on AlN/GaN superlattice templates, the In0.33Al0.67N/GaN polarization-matched MQW structure has been specifically designed with small resistance and without inducing improper polarization fixed charge. By optimizing the number and thickness of QWs, the 425nm LED has relative IQE of 56% and efficiency droop of only 7% at high current density of 333 A/cm 2 . This study provides guidance for development of In-rich InAlN materials.