InGa(Al)N quantum well heterostructures

Progress in III-Nitride-based optoelectronics is critical for a wide range of technologies including solid state lighting and illumination, ultra-high density optical storage, bio-chemical sensing and medical applications.

In particular, solid state lighting will be one of the major technological revolutions of the 21st century. By replacing conventional incandescent and fluorescent lamps with long-life, reliable, efficient and compact light emitting diodes (LEDs) it will be possible to drastically reduce energy consumption and with it the carbon footprint of domestic and commercial lighting. LED-based white appearance lighting can be obtained either by using phosphors to down-convert blue/UV light or by combining different wavelength LEDs (RGB mixing). State-of-the-art blue emitting laser diodes (LD) made from standard polar GaN, the device of choice for ultra-high density DVD-based optical data storage, are limited to a narrow range of wavelengths around 405nm because of technological constraints. This limits their range of application. Efficient blue to green emitting LDs and LEDs will enable new techniques in biological science, medical therapy, environmental sensing in addition to improvements in large area lighting technology.

Activities

LEDs, Ridge Laser and Multisection devices structures are fabricated within the cleanroom at Cardiff.

To determine the characteristics of the InGaN-GaN devices that we have made we use many different techniques. We measure the light-current-voltage characteristics which give us information about the recombination pathways. The optical gain is studied through the use of a multi-section device architecture which allows a full characterisation of the InGaN QWs and of the surrounding layers to be made. Using this detailed experimental data and by comparison with simulations of device behaviour we are able to gain a further understanding of the intrinsic properties of the InGaN-GaN structures.

History

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 focussed 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. Some of these issues have been researched here in Cardiff.