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  1. To date, it has remained challenging to achieve N-polar AlN, which is of great importance for high power, high frequency, and high temperature electronics, acoustic resonators and filters, ultraviolet (UV) optoelectronics, and integrated photonics. Here, we performed a detailed study of the molecular beam epitaxy and characterization of N-polar AlN on C-face 4H-SiC substrates. The N-polar AlN films grown under optimized conditions exhibit an atomically smooth surface and strong excitonic emission in the deep UV with luminescence efficiency exceeding 50% at room temperature. Detailed scanning transmission electron microscopy (STEM) studies suggest that most dislocations are terminated/annihilated within ∼200 nm AlN grown directly on the SiC substrate due to the relatively small (1%) lattice mismatch between AlN and SiC. The strain distribution of AlN is further analyzed by STEM and micro-Raman spectroscopy, and its impact on the temperature-dependent deep UV emission is elucidated.

     
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    Free, publicly-accessible full text available December 1, 2024
  2. N-polar AlGaN is an emerging wide-bandgap semiconductor for next-generation high electron mobility transistors and ultraviolet light emitting diodes and lasers. Here, we demonstrate the growth and characterization of high-quality N-polar AlGaN films on C-face 4H-silicon carbide (SiC) substrates by molecular beam epitaxy. On optimization of the growth conditions, N-polar AlGaN films exhibit a crack free, atomically smooth surface (rms roughness ∼ 0.9 nm), and high crystal quality with low density of defects and dislocations. The N-polar crystallographic orientation of the epitaxially grown AlGaN film is unambiguously confirmed by wet chemical etching. We demonstrate precise compositional tunability of the N-polar AlGaN films over a wide range of Al content and a high internal quantum efficiency ∼74% for the 65% Al content AlGaN film at room temperature. Furthermore, controllable silicon (Si) doping in high Al content (65%) N-polar AlGaN films has been demonstrated with the highest mobility value ∼65 cm2/V-s observed corresponding to an electron concentration of 1.1 × 1017 cm−3, whereas a relatively high mobility value of 18 cm2/V-s is sustained for an electron concentration of 3.2 × 1019 cm−3, with an exceptionally low resistivity value of 0.009 Ω·cm. The polarity-controlled epitaxy of AlGaN on SiC presents a viable approach for achieving high-quality N-polar III-nitride semiconductors that can be harnessed for a wide range of emerging electronic and optoelectronic device applications.

     
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    Free, publicly-accessible full text available October 30, 2024
  3. Surface-emitting semiconductor lasers have been widely used in data communications, sensing, and recently in Face ID and augmented reality glasses. Here, we report the first achievement of an all-epitaxial, distributed Bragg reflector (DBR)–free electrically injected surface-emitting green laser by exploiting the photonic band edge modes formed in dislocation-free gallium nitride nanocrystal arrays, instead of using conventional DBRs. The device operates at ~523 nm and exhibits a threshold current of ~400 A/cm 2 , which is over one order of magnitude lower compared to previously reported blue laser diodes. Our studies open a new paradigm for developing low-threshold surface-emitting laser diodes from the ultraviolet to the deep visible (~200 to 600 nm), wherein the device performance is no longer limited by the lack of high-quality DBRs, large lattice mismatch, and substrate availability. 
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  4. Photocatalytic water splitting is a wireless method for solar-to-hydrogen conversion. To date, however, the efficiency of photocatalytic water splitting is still very low. Here, we have investigated the design, synthesis, and characterization of quadruple-band InGaN nanowire arrays, which consist of In 0.35 Ga 0.65 N, In 0.27 Ga 0.73 N, In 0.20 Ga 0.80 N, and GaN segments, with energy bandgaps of ∼2.1 eV, 2.4 eV, 2.6 eV, and 3.4 eV, respectively. Such multi-band InGaN nanowire arrays are integrated directly on a nonplanar wafer for enhanced light absorption. Moreover, a doping gradient is introduced along the lateral dimension of the nanowires, which forms a built-in electric field and promotes efficient charge carrier separation and extraction for water redox reactions. We have demonstrated that the quadruple-band InGaN nanowire photocatalyst can exhibit a solar-to-hydrogen efficiency of ∼5.2% with relatively stable operation. This work demonstrates a novel strategy using multi-band semiconductor nanostructures for artificial photosynthesis and solar fuel conversion with significantly improved performance. 
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  5. Ternary III-nitride-based nanowires with highly efficient light-emitting properties are essential for a broad range of applications. By using the selective area molecular-beam epitaxy method, InGaN/AlGaN quantum disks (QDs) embedded in hexagonal GaN nanowires were successfully grown. With the help of atomic-scale-resolved transmission electron microscopy and atom probe tomography, atomic ordering and other related structural information, such as crystallography and local chemistry, have been unambiguously revealed to provide unique insights into the exceptionally strong photoluminescence enhancements. A boomerang-shaped InGaN/AlGaN QD was identified, and atomic-level 1 : 1 periodic chemical ordering within the boomerang shaped AlGaN layers along the c -direction was revealed, confirming the preferential site occupation of Al-atoms. This type of growth provides a strong suppression of the quantum-confined Stark effect and is thus likely a very strong contributor to the exceptional properties. This work therefore enables us to directly establish the key structural elements necessary to understand the exceptionally strong emission exhibited by these materials. Optimization of the configurations of QDs could be an alternative design tool for developing various advanced LED device applications with well-designed structure and desirable optical properties. 
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