BACKGROUND

Nichia Chemical Industries Ltd. is the world's leading manufacturer of GaN-based blue, green, and white LEDs and phosphors for lighting and CRTs. The company is very entrepreneurial and was founded in 1956 by Nobuo Ogawa to develop fluorescent lamp phosphors. The company has gained worldwide recognition for Dr. Shuji Nakamura's achievements of bright blue, green, and white LEDs and, more recently, long life CW blue laser diodes. Dr. Nakamura started the GaN effort in 1989 at a time when all the other optoelectronic companies were pursuing II-VI technology. This foresight has allowed the company to develop a large technology lead in nitride semiconductor technology.

Due in large part to its technical accomplishments in GaN materials and devices, Nichia has grown very rapidly recently: In 1997 it had sales of approximately $330 million, compared to 1995 sales of approximately $200 million (Nichia 1997). The company is privately held and proud to be a major employer for the people of Shikoku Island.

TECHNOLOGY DISCUSSION

GaN Light Emitting Diodes

Nichia controls the market for wide bandgap GaN LED devices and is selling approximately 20 million LEDs every month. At the time of the TTEC panel's visit, the best external efficiency Dr. Nakamura has achieved for the blue and green LEDs is 10% and 12% external quantum efficiencies, respectively. The white LEDs are approximately 5% efficient and have a luminous efficacy of 10 lumens/watt, which is becoming competitive with existing incandescent sources. Nichia is only interested in the optoelectronic applications of GaN and is not pursuing high temperature electronic applications.

When asked about the high temperature performance of GaN LEDs, Dr. Nakamura mentioned that the newly developed amber LEDs are far superior to conventional AlGaInP amber LEDs. The wavelength shift as a function of temperature is much smaller in GaN than in GaAs-based LEDs. He showed a comparison of InGaN to AlGaInP-based LEDs at an elevated temperature of 80°C in which the InGaN LED light output is decreased by only 20%, whereas the AlGaInP LED light output is down by 70%. This excellent temperature performance for GaN-based LEDs in comparison to conventional GaAs-based LEDs is also seen when GaN-based LEDs are compared to GaP green and AlGaAs red LEDs (Nakamura and Fasol 1997). The InGaN yellow LEDs are not as bright as transparent substrate AlGaInP LEDs, but their performance rivals absorbing substrate AlGaInP LEDs. At 20 mA, the new yellow LED is 4 candela and has an external efficiency of 3.3% at 594 nm (Mukai et al. 1998). Dr. Nakamura has successfully operated the GaN LEDs at maximum temperatures of 120°C for a packaged lamp and 300°C for a bare chip. Recently, Nichia developed a 2% efficient UV (372 nm) LED. UV LEDs are expected to find new applications in UV plastic curing, lighting, sterilization, medical, and counterfeit currency detection. Nichia's LED products have many new markets.

Dr. Nakamura emphasized that GaN is the most "environmentally friendly" LED material available. In comparison to toxic GaAs LEDs or even mercury-containing fluorescent lamps, GaN offers a truly safe lighting solution. Dr. Nakamura is very optimistic that the GaN LED will improve the energy efficiency of lighting and be beneficial to the environment.

GaN Blue Laser Diodes

Nichia was the first company in the world to successfully achieve a blue laser diode in a GaN materials system. Initially the GaN lasers lasted only a few seconds, but reliability has improved dramatically to a point where Dr. Nakamura has achieved CW operation for up to 4000 hours at room temperature. Elevated temperature testing at 50°C has projected that the actual lifetime is in excess of 20,000 hours at 2 mW. The highest CW output power previously achieved was 400 mw. A key development in obtaining reliable CW laser performance has been defect reduction by using an epitaxial lateral overgrowth GaN (ELOG) substrate. In this technique a silicon dioxide mask is propagated by block dislocation, and a "defect free" film is achieved in the laterally overgrown region. After 100 microns of growth, a fully coalesced GaN thin film is achieved, and a proprietary process removes the sapphire substrate. The ELO process will be described in more detail in the section on GaN materials. Laser diodes with InGaN/GaN multiple quantum well (MQW) active regions are then grown on top of this virtual bulk GaN substrate. The active regions are then defect free and can survive under high current operation (3 kA/cm²). The panel was shown a CW blue laser diode housed in a small held package being run off a watch battery. Dr. Nakamura believes that a blue laser diode product will be available soon from Nichia. Nichia is developing the laser for use in the largest market first, that being the next generation of high density digital versatile disc (DVD) optical storage systems. Current estimates are that the reduction in spot size that can be achieved with blue GaN lasers will yield a storage capacity of 15 GB per conventional size compact discs, enough for recording a full-length motion picture in high resolution mode.

When asked what the most important technology for high quality GaN is, Dr. Nakamura mentioned crystal growth. In particular, Nichia's proprietary 2-flow metalorganic chemical vapor deposition (MOCVD) is one of the key technologies for achieving high quality GaN materials and devices. The defect reduction technique of ELOG defect free substrate also plays a major role in achieving long life reliable laser diodes.

GaN LED Traffic Signals

One of the most impressive demonstrations of GaN technology the panel saw during its Japan trip was the LED traffic signal demonstration. Nichia had constructed a large outdoor display comparing LED-based signals to incandescent-based signal heads. The brightness, color, and viewing angle of the LED traffic signals was as good or better than the conventional 70 watt (Japan) incandescent light-bulb plus filter. The savings in energy is tremendous (See Table Nichia.1), and the payback period based on energy savings alone is 1 to 3 years. Given that the solid-state reliability and lifetime of the LED signal head is at least 5 years, maintenance costs and safety are much improved for LED traffic signals. The impact of this energy savings alone is enormous, and if all the traffic signals in Japan were switched to LEDs, the move would save the construction of at least one nuclear power plant. This clearly is an area where GaN technology will have an impact on the public.

Table Nichia.1

Traffic Signals Comparison to 70 W Japanese Standard

COMPANY TOUR

The panel was allowed to visit the impressive product showroom of Nichia and to witness some recently developed products. Along with Nichia's wide range of GaN LED colors, a full-color large screen LED TV was on display. Nichia LEDs have been used to build even larger screen (building size) LED TVs. These can be seen at major Shinkansen (bullet train) stations like Nagoya (See Fig. Nichia.1). The LED TV is particularly impressive because of the wide range of colors afforded by the color purity in the green (520 nm) and blue (470 nm) LEDs, in comparison to less color-pure phosphors. Nichia has also developed a white foot-lamp and overhead multi-color reflector lamp. Other emerging products using GaN LEDs are a compact color scanner, a LCD backlight, an automobile dashboard backlight, and UV LEDs for counterfeit currency detection. During the product discussion, Dr. Nakamura also displayed Nichia's most recent product successes, including a CW blue laser pointer, amber LEDs, and newly developed UV LEDs.

Fig.Nichia.1. LED TV

REFERENCES