Effect of thermal treatment on ZnO substrate for epitaxial growth

ZnO is a highly efficient photon emitter, and has optical and piezoelectric properties that are attractive for a variety of applications in sensors and potentially optoelectronic devices such as emitters. Due to its identical stacking order and close lattice match to GaN, it is also being developed as a substrate material for GaN epitaxy. However, the surface finish of the ZnO is such that much of the damage induced by sawing and follow up mechanical polishing remains. We developed a thermal treatment method to eliminate surface damage on the 0 face of ZnO (0 0 0 1) to prepare it for epitaxial growth. Atomic force microscopy images of ZnO (0 0 0 1) annealed at 1050 °C for 3 h etc. show that residual scratches from mechanical polishing are removed and atomically flat, terrace-like surfaces are attained. In addition, low-temperature photoluminescence and high-resolution X-ray diffraction measurements have been employed to investigate the effect of annealing on ZnO substrates.     

Effects of a Rotating Magnetic Field on Gas Transport during Detached Crystal Growth in Space

During the detached Bridgman growth of semiconductor crystals, the melt has a short free surface, which is detached from the ampul wall near the crystal–melt interface, thus eliminating the crystal defects caused by contact with the ampul wall. Recent modeling has indicated that initiation and continuation of detached growth depends on the rate of transport of dissolved gas from the crystal–melt interface, where gas is rejected into the melt, to the detached free surface, where evaporating gas maintains the pressure on the free surface. Here we use numerical modeling to investigate whether the application of a rotating magnetic field increases or decreases the transport of rejected gas to the detached free surface. Unfortunately, the results show that a rotating magnetic field almost always decreases the evaporation rate at the detached free surface. The exception is an insignificant increase for a short period at the beginning of crystal growth for a few circumstances. The evaporation rate decreases as the strength of the rotating magnetic field is increased.     

Observation of surface charging at the edge of a Schottky contact

Scanning Kelvin probe microscopy was used to detect reverse-bias-induced surface potential changes near the Schottky contact of a GaN Schottky diode. After application of a reverse bias, the surface potential near the Schottky contact gradually decreased with time, indicating an increase of band bending. Surface potential traces recorded after turning off the reverse bias indeed revealed increased band bending near the Schottky contact. A higher reverse bias caused a larger increase of band bending. The authors suggest that a reverse bias facilitates electron tunneling at the edge of the Schottky contact by decreasing the potential barrier width. Capture of these tunneled electrons by surface states causes the observed increase of band bending.