Abstract:

Nanoscale epitaxial lateral overgrowth (NELO) of GaN is performed using PECVD-grown SiO₂ with nanoscale windows as the growth mask.  It is found that the overgrown GaN epilayers exhibit significant reduction in the threading dislocations (TDs) as characterized by atomic force microscopy.  Low and uniform TDs density were observed over the whole 2-inch wafer, which is different from ELO GaN layers grown on conventional SiO₂ stripes in which the coherent GaN layers grown in the window regions still suffer from high dislocation density.  LED structures with active regions consisting of five-period multiple quantum wells (MQWs) were fabricated.  Detailed characterizations of the optoelectronic properties of the devices were performed by I-V and electroluminescence (EL) measurements.  Significant improvements in the EL intensity are observed among the devices grown on the NELO GaN layers (type N) compared to the control devices which have a conventional structure (type C).  It is interesting to note that a 15 nm blue shift in the EL peak is observed among the type N devices.  Detailed high resolution X-ray diffraction and reciprocal space mapping characterizations were performed.  The experimental data demonstrate substantial stress relaxation in the type N MQWs resulting in significant lowering in the quantum confinement Stark effect in the MQWs which is believed to underlie the observed blue shift in the EL peak.   The reduction in the quantum confinement Stark effect is also partly responsible for the observed improvement in the quantum efficiency of the device.  Detailed investigations in the degradation of the devices due to hot-electron injection indicate that stress relaxation in the MQWs utilizing NELO growth technique results in improvements in the hot-electron hardness of the LEDs.