Laser doping of GaN for advanced optoelectronic applications

GaN is an important compound semiconductor for optoelectronic applications including light-emitting diodes, ultraviolet lasers and photodectectors. p-type and n-type semiconductors and ultimately p-n junctions are necessary to fabricate these types devices. GaN can be doped with n-type dopant atoms, such as silicon, relatively easily; however, doping GaN with a p-type dopant such as Mg is challenging. For example, ion implantation can be used to increase the concentration of electrically active impurities in the source and drain regions of the device, but this process requires high temperatures for electrical activation, along with capping layers to prevent GaN decomposition. Additionally, ion implantation creates lattice damage that is difficult to remove via annealing and acts to compensate the dopants. Two laser doping techniques, based on 1) gas immersion and 2) molten precursor laser doping are modified to improve p-type doping in GaN. Bis-magnesium dihydrate [Mg(TMHD)2] is the precursor used in both cases to supply Mg atoms. In the gas immersion method, the precursor powder is heated to 300°C in a bubbler and then a carrier gas, nitrogen, is introduced to the bubbler to transport the Mg-containing vapor to a laser doping chamber that was originally under a vacuum of 0.1 mTorr. In the molten precursor technique, on the other hand, a bed of [Mg(TMHD)2] powder is sandwiched between the GaN wafer and a soda lime glass slide at the wafer surface. The soda lime glass slide allows the incident laser (532 nm wavelength) irradiation to reach the GaN wafer surface, restrains the [Mg(TMHD)2] powder at the GaN surface and confines the molten precursors to the surface. Various laser processing parameters and the dopant concentration are discussed in this paper.