Aluminum nitride is a material of great potential for high power electronic devices, UV photonic devices as well as
acoustic devices. However, the lack of a good crystal growth technology for bulk material and substrate hinders the
development of these AlN-based devices. While AlN has been successfully grown on sapphire substrate for some time,
the presence of a large number of dislocations in the material is still a major barrier to overcome . In this work, we
demonstrate a low-dislocation-density AlN template on sapphire by inserting an AlN interlayer by metal-organic
chemical vapor deposition. The main idea of our approach is to change the growth mode in the course of the epitaxial
growth by decreasing growth temperature and changing V/III ratio. As the growth mode changes, dislocations tend to be
redirected and/or form dipole half loops via annihilation processes . The etch-pit-density of the AlN templates is
reduced from 3.6×109 cm-2 to 1.7×109 cm-2. Accordingly, the full width at half maximum of the (0002) x-ray rocking
curve is reduced from 37 arcsec to 12 arcsec. The result indicates that the AlN template has low screw and mixed type
dislocations. AlGaN/GaN Schottky diodes fabricated on this high quality AlN template exhibit very high breakdown
voltage (> 2000 V), which sets a record-high figure of merit of 1.15 GW/cm2.
The authors report on the growth of GaN on AlGaN/(111)Si micropillar array by metal-organic chemical vapor
deposition. Using the substrates with micropillar array, 2 μm-thick GaN films without cracks can be achieved.
Transmission electron microscopy, atomic force microscopy, and micro-Raman studies indicate that the dislocation
density and residual stress of the GaN grown on micropillar array are also reduced. The results reveal the potential of this
type of substrates for growing GaN-based devices as well as preparing GaN freestanding substrates.
Multiple quantum well (MQW) structure piezoelectric semiconductor can be treated as a piezoelectric transducer to generate nanometer wavelength and THz frequency acoustic waves. The generation mechanism of nano acoustic wave (NAW) in quantum wells induced by femtosecond optical pulses can be modeled by a macroscopic elastic continuum theory. The absorption of the MQW's modulated by NAW's through quantum confined Franz-Keldysh (QCFK) effect allows another femtosecond optical probe pulses to monitor the propagating NAW. Many applications in typical ultrasonics can be achieved by NAWs. The simultaneous waveform synthesis is demonstrated by an optical coherent control technique. The phase of the totally reflected NAW is studied. Acoustic coherent control can be achieved by designing the thickness of the cap layer on the MQW. We also demonstrated the feasibility to apply THz NAWs to acoustically control an electronic device with higher operation speed and spatial accuracy. Seismology, which is the first step toward ultrasonic imaging, was also demonstrated. The arrival time of the echo is obtained by processing the transient transmission changes of the probe. Ultrafast technique and nano technology are ready for nano ultrasonics.