A 340-nm-band ultraviolet laser diode composed of GaN well layers

We have demonstrated the laser operation of a short-wavelength ultraviolet laser diode with multiple-quantum-wells composed of GaN well layers. The laser action has been achieved in 340-nm-band far from the wavelength corresponding to GaN band gap under the pulsed current mode at room temperature. The device has been realized on the Al(0.2)Ga(0.8)N underlying layer. The AlN mole fraction of the underlying layer is 0.1 lower than that of the underlying layer which was used for the previously reported 342 nm laser diode. These results provide a chance to the next step for a shorter-wavelength ultraviolet laser diode.

Unintentional doping in GaN

The optimisation of GaN-based electronic and optoelectronic devices requires control over the doping of the material. However, device performance, particular for lateral transport electronic devices, is degraded by the presence of unintentional doping, which for heteroepitaxial GaN layers grown in the polar (0001) orientation is mainly confined to a layer adjacent to the GaN/substrate interface. The use of scanning capacitance microscopy (SCM) has demonstrated that this layer forms due to the high rate of incorporation of gas phase impurities, primarily oxygen, during the early stages of growth, when N-rich semi-polar facets are often present. The presence of such facets leads to additional unintentional doping when defect density reduction strategies involving a three-dimensional growth phase (such as epitaxial lateral overgrowth) are employed. Many semi-polar epitaxial layers, on the other hand, exhibit significant unintentional doping throughout their thickness, except when a three-dimensional growth phase is introduced to aid in defect density reduction resulting in the presence of (0001) and non-polar facets which incorporate less dopant. Non-polar epitaxial samples exhibit behaviour more similar to (0001)-oriented material, but oxygen diffusion from the sapphire substrate along prismatic stacking faults also locally affects the extent of the unintentional doping in this case.

Imaging dislocations in gallium nitride across broad areas using atomic force microscopy

We have employed an atomic force microscope with a high sampling rate to image GaN samples grown using an epitaxial layer overgrowth technique and treated with silane and ammonia to enlarge the surface pits associated with threading dislocations (TDs). This allows TDs to be identified in high pixel density images tens of microns in size providing detailed information about the spatial distribution of the TDs. An automated software tool has been developed, which identifies the coordinates of the TDs in the image. Additionally, we have imaged the same sample using Kelvin probe force microscopy, again at high pixel density, providing data about the local changes in surface potential associated with hundreds of dislocations.