Building better blue and green lasers
Full-colour laser projectors and laser TVs are set to benefit from blue
and green laser diodes with very impressive wall-plug efficiencies.
BY YOSHITAKA NAKATSU, TSUYOSHI HIRAO, TOMONORI MORIZUMI, YOJI NAGAO, SHINGO MASUI, TOMOYA YANAMOTO AND SHIN-ICHI NAGAHAMA FROM NICHIA CORPORATION
Founded in 1956 and headquartered in Anan, Japan, our company, Nichia, is a well-known manufacturer of chemical materials, LEDs and laser diodes. For more than 50 years we have been playing an active role in the business related to displays. Back in 1970, we started to manufacture phosphors for colour TVs, and a year on we began to produce phosphors that are used in our customer’s high-pressure mercury lamps, a key element in projection displays. We shot to worldwide fame in the 1990s with the invention of the high-brightness blue LED, which we started to manufacture in 1993. Building on this success, in 1995 we produced the world’s first electron-injection InGaN-based laser diodes. That particular success has played a pivotal role in the development of laser-based displays, such as laser projectors and laser TVs. The market for these displays, which employ lasers emitting the three primary colours of red, green, and blue, continues to grow.
A direct emission portfolio
As well as being the pioneer of the blue laser diode, we are the first company to commercialise watt-class green laser diodes. Both feature in our laser diode portfolio, which has sources spanning 375 nm in the UV to 532 nm in the green. Makers of displays can use our lasers alongside red laser diodes, which are commercially produced by a number of other companies.
The combination of red, green and blue lasers has much promise for projectors, thanks to its prowess on the colour diagram. When united, these three sources produce an expanded colour gamut, ensuring that a wider variety of colours are produced, ultimately making it possible to create images that stimulate the senses. There is also an appeal from a business standpoint: laser diodes can offer a low power consumption and a long life, helping to trim the running costs of a cinema.
Here we detail the latest developments in our blue and green laser diodes. When these sources, plus variants in the red, emit narrow wavelengths, this enables the creation of displays that express extremely rich colours. In terms of maturity, red and blue lasers are out in front, with mass production well established. The most common green source is actually one based on the second-harmonic-generation of a 1064 nm laser – the result is a complex contraption with a power conversion efficiency for a commercial device of typically only about 6.5 percent. Due to this shortfall in performance, high-efficiency green laser diodes are eagerly anticipated for driving the deployment of laser projectors.
Figure 1. Nichia’s multi-mode blue and green laser diodes.
Recording-breaking blues and greens
Why is it so difficult to make a green laser diode? If your starting point is its blue cousin, you have to shift emission to the green by increasing the indium content in the quantum well, because this narrows the bandgap. Unfortunately, there is a downside to this move, with strain introduced into the active region due to InN having a much bigger lattice constant than GaN. Strain leads to degradation, hampering the radiative recombination between electrons and holes in the quantum wells. What’s more, piezoelectric polarisation is introduced, as the indium content increases due to the distortion of the crystal structure, and this alters the profile of the energy band structure. Impacts include a shift in the emission wavelength, and a decrease in luminescence efficiency that stems from a reduction in the carrier recombination probability.
Figure 2. (a) The emission spectrum under 3 A CW and (b) current-light (I-L) and current-voltage (I-V) characteristics of a Nichia blue laser diode under CW operation at 25 ºC.
To suppress the piezoelectric field, some groups turn to non-polar GaN substrates, which are limited in size and availability. We prefer an alternative approach with greater commercial viability, involving a thinning of the quantum wells to realise better uniformity and a greater overlap of electron and hole wavefunctions. Through changes to the epitaxial structure and the device design, we have produced laser diodes with a high wall-plug efficiency.
We grow the epitaxial layer structures of our lasers by MOCVD. Their production involves loading 2-inch c-plane free-standing GaN substrates into an MOCVD reactor and depositing n-type, active and p-type layers. We process these epiwafers into devices by using etching to form a ridge structure on the p-side, before adding n-type and p-type electrodes to the top and bottom of the chip (see Figure (1)). The mirrors at the ends of these edge-emitting structures are created by cleaving the wafer and then coating the bare facets with dielectric mirrors. To reduce optical absorption losses, we gave much care to the design of the optical confinement structures of the epitaxial stack. The fabrication of our lasers finishes by mounting the chip, with a junction down method, in a TO-can package that suppresses thermal resistance.
Figure 3. Current-WPE (I-WPE) characteristics for a Nichia blue laser diode under CW operation at 25 ºC
Blues beyond 50 percent
Back in 2001, we reported a key milestone for the blue laser diode: the first milliwatt output power for this device. Rapid progress came over the next few years, leading us to unveil the first watt-class laser diode in 2007. Recently, we have broken another noteworthy barrier, realising a WPE of 50 percent. For our blue lasers emitting at 455 nm, spectral width is typically 2 nm, implying that all the light energy exists in only about one-tenth of the spectral width of an LED. It is quite astonishing that half of the electrical input power is converted into laser light spanning just a 2 nm range. Note, though, that this level of efficiency is below that of the best GaAs-based and InP-based laser diodes, so we can expect further improvements in efficiency in the future.
Earlier this year we revealed an optical output of 5.90 W and a WPE of 51.6 percent from a 455 nm blue laser driven at 3 A (see Figures 2 and 3). These laser diodes have raised the bar for output power and efficiency through advances in chip design. Specifically, improved performance has come from suppressing absorption losses inside the laser diode element, increasing the carrier injection efficiency and trimming the operating voltage via resistance reduction.
An important characteristic of our blue lasers is their exceptional reliability. They have demonstrated this merit in a 30,000 hour lifetime test at the rated 3 A (see Figure 4). Thanks to this attribute, we can expect our blue laser diodes to serve in projector displays for several years or more – that’s much longer than conventional lamp light sources, which need to be replaced after several thousand hours.
To reach any desired output power, we can mount a number of our blue laser diodes together in a package. Using this approach, we have produced sources with optical output powers from 20 W to 100 W.
In recent years, there has been a global shift from lamps to solid-state light sources. This trend will continue as products are replaced. At the centre of this transformation is the laser diode. There’s no doubt that laser diodes will become mainstream, with the shift from lamps to lasers spreading to all markets.
Figure 4. Lifetime test results of Nichia’s blue laser diodes under an automatic current control of 3.0 A CW operation at a case temperature of 65 °C. Operating current is normalised by its initial value.
Another recent triumph for us is increasing the WPE of our 525 nm green laser diode to nearly 24 percent. This laser is capable of completely covering the standard for digital cinema in the green region. To realise such success, we have combatted the challenges associated with piezoelectric polarisation, which reduces the overlap of electrons and holes and thus the radiative efficiency. Our solution is to reduce the active layer thickness. While piezoelectric polarisation still occurs, by turning to a very thin active layer thickness, we suppress the decrease of luminescence