Blue LED and LD
III-Nitride semiconductors: a new material for optoelectronics
Since the first demonstration of InGaN/GaN blue-violet laser diodes in the mid-90’s, research and development of this semiconductor material has rapidly increased. This has been driven by the use of InGaN as an efficient blue light emitter, a spectral region where previously, efficient semiconductor light sources were unavailable. The group III-Nitride 'triangle' of InN-GaN-AlN, of which InGaN is a member, forms a direct band-gap semiconductor system with band-gaps spanning the infra-red, visible and ultraviolet spectral regions (Fig.1). There are therefore also significant applications in ultraviolet light emission/detection, as well as in high-power and high-temperature electronics.
Figure 1
Research and development of III-nitride semiconductors continues to be an exciting and growing field, both from a fundamental as well as from a commercial perspective. Understanding the fundamental properties of this still somewhat mysterious material system as well as developing it for new devices and applications remains a considerable challenge. At the same time, the market for InGaN-based blue-violet laser diodes and InGaN-based light-emitting diodes is expected to grow to several billion dollars within the next few years. Blue-violet laser diodes are about to burst into the consumer electronics market with the next generation of DVD players. These Blu-ray players exploit the short wavelength of blue light to record up to 13 hours of standard video on a single DVD. Light-emitting diodes made out of the InGaN-material are about to grow into a multi-billion dollar market of their own, fuelled by their use in solid-state white light sources.
III-Nitrides fabricated by molecular beam epitaxy: the Sharp approach
Currently, commercial III-Nitride devices are grown by a growth technique known as metal organic chemical vapour deposition (MOCVD). In this vapour method, precursor gases flow over a substrate and then react at the surface to create the desired layers of the device. Molecular beam epitaxy (MBE) is an alternative growth technique, in which atoms/molecules are evaporated in an ultra-high vacuum and then allowed to settle on a substrate to build up the device layer by layer.
Despite numerous attempts by groups all over the world to make blue-laser diodes by MBE, none had succeeded until late 2003. Then, researchers at SLE achieved a world first by demonstrating a blue-violet laser diode grown by MBE. This opened the way for the MBE-growth of efficient nitride optoelectronic materials, and for exploiting the many advantages MBE offers, such as more accurate doping profiles, the possibility of in-situ monitoring of growth and lower consumption of source materials. For further information on the MBE technique, please click here.
SLE continues to be at the leading edge of nitrides research and growth by MBE, continuing the development of the MBE growth technique, and developing III-nitride materials and devices.
InGaN blue-violet laser diodes by MBE: a world first
Until recently, MBE had not been successful in producing high-quality InGaN based LEDs and lasers. Despite considerable efforts worldwide, the output power of MBE-grown LEDs had been limited to less than 1mW, and lasing had not been achieved. Significant progress in the MBE growth of nitrides for optoelectronic devices has been demonstrated at SLE in the last two years, with the first InGaN laser diodes grown by MBE and high-power InGaN LEDs grown by MBE with 3.75mW optical output power at 20mA current reported by SLE.
For further information on laser diode growth and fabrication, please click here.
The first InGaN laser diodes grown by MBE operated under pulsed current conditions at 0.01% duty cycle, with a room-temperature threshold current density of 30 kA/cm2 and a threshold voltage of 33 V (Fig.2).
Figure 2
These results were comparable with the first MOCVD-grown InGaN laser diodes and demonstrated the potential of MBE growth for nitride optoelectronic devices. The devices were grown on sapphire templates and subsequent improvements in growth and fabrication enabled a reduction in threshold current density to 7.7 kA/cm2.
Figure 3 shows a SEM micrograph of one facet of the fully processed laser diode. L-I characteristics of the improved SLE laser diode grown by MBE on a template substrate, compared to the first SLE laser diode on a template substrate are shown in Fig. 4.
Figure 3
Figure 4
These encouraging results opened up the question whether MBE will be able to produce material of high enough quality for room-temperature continuous-wave (cw) laser diodes. Achieving cw operation of MBE-grown laser diodes would be of fundamental interest as well as of commercial significance.
SLE is well on the way towards this milestone, having recently demonstrated 50% duty-cycle operation of InGaN laser diodes fabricated by MBE (Fig. 5). A far-field of such a laser grown on so-called free-standing GaN is shown in Fig. 6. Further work at SLE is now concentrating on improving the characteristics of the laser diode devices with the aim of demonstrating long lifetime, continuous wave operation, and evaluating the devices for commercial applications such as Blu-ray.
Figure 5
Figure 6
Simulation of blue laser diodes and LEDs
In order to simulate the electrical and optical device performance of nitride lasers and LEDs, SLE uses state-of-the-art simulation tools. These enable the optimisation of existing structures with fewer fabrication runs, as well as the investigation of novel device structures. A close feed-back to experimental work assists in validation of the modelled results and in further development of the simulators themselves.
