Taofei Zhou

Dislocation cross-slip in GaN single crystals under nanoindentation

The dislocation multiplication and movement mechanism in GaN single crystals has been studied using nanoindentation and cathodoluminescence. Dislocation loops can multiply and move from plane to plane by cross-slip, thus producing a wide plastic deformation in GaN during indentation. This mechanism is further supported by the remarkable movement of indentation induced dislocations during annealing. Furthermore, the so-called pop-in events, in which the indenter suddenly enters deeper into the material without the application of any additional force, can be better understood by considering the cross-slip mechanism.

The bound states of Fe impurity in wurtzite GaN

A study on the bound states of Fe impurities in GaN by ultraviolet photoluminescence (PL) emissions is presented. Two elusive PL lines were observed at 3.463 eV (L1) and 3.447 eV (L2), respectively. The intensities of the two lines are proportional to the Fe concentration. The temperature dependence of L1 and L2 revealed acceptor-like and strong localized characteristic, respectively. Furthermore, Raman analysis indicated that L2 is correlated to an exciton bound to a nitride-vacancy (VN) related complex, i.e., [Fe2+-VN]. By co-doping with Si, the [Fe2+-VN]-related bound state will enable the spin-coupling between isolated iron ions.

A practical route towards fabricating GaN nanowire arrays

GaN nanowire (NW) arrays have been fabricated by the electrodeless photoelectrochemical (PEC) etching method for the first time. Under appropriate conditions, the etching process is just a dislocation-hunted process, in which the etching solution “digs down” along the threading dislocations, resulting in the formation of GaN NWs by preferentially etching away the defective parts of GaN with dislocations and retaining the flawless parts. The NWs have a density of 1∼2 × 107 cm−2, diameters ranging from 150 nm to 500 nm, and corresponding lengths ranging from 10 μm to 20 μm. Transmission electron microscopy (TEM) indicates that these GaN NWs possess few dislocations. High resolution X-ray diffraction (HRXRD) and micro-Raman measurements show that these GaN NWs are stress-free. Room temperature cathodoluminescence (CL) measurements show a single near-band-edge emission at 367 nm with a full width at half maximum (FWHM) of 8 nm from the NWs, indicating a high optical quality. Additionally, negative piezoelectric current pluses are generated from the GaN NWs when the conductive atomic force microscope is scanned cross the arrays in contact mode. Such GaN NW arrays are promising building blocks for exploring nanodevices with excellent performance.

Structure, Stress State and Piezoelectric Property of GaN Nanopyramid Arrays

GaN nanopyramid (NP) arrays have been fabricated by a convenient electrodeless photoelectrochemical etching method. Transmission electron microscopy measurement indicates that these NPs are composed of crystalline GaN surrounding a dislocation. High-resolution X-ray diffraction and the micro-Raman spectrum reveal a highly compressive stress relaxation in the NPs compared with compressed GaN subfilm. Additionally, negative piezoelectric current pluses are generated from the GaN NPs when the conductive atomic force microscope scans cross the arrays in the contact mode. The result demonstrates that the GaN NP arrays are a promising candidate for nanogenerators.

Redshift of A 1(longitudinal optical) mode for GaN crystals under strong electric field

We investigated the property of GaN crystals under a strong electric field. The Raman spectra of GaN were measured using an ultraviolet laser, and a remarkable redshift of the A 1(LO) mode was observed. The role of the surface depletion layer was discussed, and the interrelation between the electric field and phonons was revealed. First-principles calculations indicated that, in particular, the phonons that vibrate along the [0001] direction are strongly influenced by the electric field. This effect was confirmed by a surface photovoltage experiment. The results revealed the origin of the redshift and presented the phonon property of GaN under a strong electric field.