History of Gallium-Nitride based blue light diode
December 10, 2014
The creation of long lived blue semiconductor laser was thought extremely unlikely until the mid-1990s when Shuji Nakamura at Nichia Chemical Industries, Japan, announced the room temperature operation of a GaN based laser diode. Until then research on ZnSe based II-VI semiconductors had yielded room temperature laser operation but device degradation was a major problem. Since then Nichia have demonstrated lifetimes of 10000h for their GaN based laser diodes.
One of the first challenges encountered for growing GaN was to find a suitable substrate. This was due to the GaN having a small lattice constant, 3.2Å, compared to the lattice constant of GaAs at 5.6Å used for laser diodes in the red-infrared regimes. Sapphire and SiC emerged as the two leading contenders though they still have a lattice mismatch and SiC was found to be very expensive. The lattice mismatch created a high defect density but light emission and lasing was still possible in these structures. Another major problem has been obtaining good quality p-type GaN with a high enough hole density to reduce the operating voltage to the near to the band gap of the active region of the device, which is the case with other III-V materials. Typically a GaN LED or laser would be expected to operate at 3-4 V.
The long lifetime achieved by Nichia was due to a feature called epitaxial layer over growth (ELOG), which leads to a reduced defect density, and a new method of annealing p-type material to give a higher hole density. More recently research has focused on the substrate and initial growth to limit the defect density.
The majority of GaN LEDs and Laser structres have been grown on sapphire substrates due to cost issues over SiC. However, Sapphire is an insulator and so more advanced processing techniques have had to be applied so that the n-type material can be exposed to create a metal contact. In addition it is difficult to obtain good ohmic contacts to the p-type material and this has required the creation of novel p metal contacts. —