  Blue light-emitting diodes (LEDs) consisting of a buried n+-p+ GaN tunnel
junction, (In,Ga)N multiple quantum wells (MQWs) and a n+-GaN top layer are
grown on single-crystal Ga-polar n+-GaN bulk wafers by plasma-assisted
molecular beam epitaxy. The (In,Ga)N MQW active regions overgrown on the p+-GaN
show chemically abrupt and sharp interfaces in a wide range of compositions and
are seen to have high structural and optical properties as verified by X-ray
diffraction and spatially resolved cathodoluminescence measurements. The
processed LEDs reveal clear rectifying behavior with a low contact and buried
tunnel junction resistivity. By virtue of the top n+-GaN layer with a low
resistance, excellent current spreading in the LEDs is observed at low currents
in this device structure. A few of new device possibilities based on this
unique design are discussed.
