Technical Insight
Nitrides leap ahead at ICNS-5
The III-nitrides are a growth area for compound semiconductors, and recently there has been rapid progress in performance and the variety of devices available. Richard Balmer reports from Nara.
The 5th International Conference on Nitride Semiconductors (ICNS-5) was held in the ancient Japanese city of Nara on May 26-30, 2003. The majority of the 649 delegates were from Asia, including 405 from Japan and 63 from South Korea. However, the attendance from elsewhere was also strong, with 78 US and 63 European delegates. A total of 406 papers were presented, although sadly 23 presentations were withdrawn by researchers from China, Taiwan and Singapore due to the SARS outbreak.
The number of presentations demonstrated not only the continued growth of activity in this field but also the tremendously rapid progress that has been made since ICNS-4 in 2001. There was a much more commercial feel at this meeting, which stems from the emergence of spin-off and start-up companies that are taking processes and devices to market, as well as the ever-present large corporations which are continually adding new products and improving existing ones. Some of the areas of significant progress included UV-LEDs with increased quantum efficiency, white LEDs with better color rendering, and HEMTs with significant improvements in breakdown voltage, current collapse and gate leakage.
The conference was opened with a series of plenary lectures, including one by Fernando Ponce from Arizona State University, who summarized the current status and identified the important issues for the growth of InGaN alloys, and one by Hiroshi Ogawa from Sony Corporation, who discussed the impact of nitride laser diodes on the evolution of optical data storage.
Bulk nitride substrates
The development of large-area III-nitride substrates for lattice-matched growth of GaN and related alloys is gathering momentum. As a result there was much discussion of recent progress, with most of the commercial developers represented at the meeting. R Vaudo from ATMI in Danbury, CT, described an investigation of free-standing semi-insulating Fe-doped GaN substrates grown by HVPE. Fe concentrations up to 2 x 1017/cm3 were achieved, but the highest resistivity observed was 2 x 109 Ωcm at room temperature for an Fe concentration of 4 x 1016/cm3.
Leo Schowalter of Crystal IS in Latham, NY, described the company s AlN bulk growth technology from which it produces substrates with a variety of orientations. Vladimir Dmitriev from TDI in Silver Spring, MD, described the development of freestanding AlGaN substrates fabricated using HVPE growth onto SiC with subsequent removal of the SiC. Dmitriev also presented work by TDI and co-workers on AlN substrates grown by vapor phase techniques, which have so far resulted in 35 mm diameter substrates, with 50 mm material expected by the end of 2003. Mike Leszczynski from Polish company TopGaN described the progress made with the company s high-pressure growth technique for bulk GaN substrates. Although the current samples are only about 12 mm in diameter, they show excellent low dislocation densities of about 100/cm2 and have been used to fabricate blue lasers.
The bandgap of indium nitride
In the two sessions devoted to InN, the fundamental bandgap of this material was a hot topic, with many researchers supporting the recent reports of a narrower bandgap than the previously accepted 1.9 eV. Deposition of high-quality (as determined by XRD) InN films on sapphire and Si(111) by RF-MBE was reported by Y Nanishi representing a group of researchers from Ritsumeikan University and Sony Corporation, Japan. A carrier concentration of 4.9 x 1018/cm3 and a room temperature mobility of 1130 cm2/Vs were measured. Absorption and PL measurements observed sharp luminescence at 0.8 eV and no peak at 1.9 eV.
A Yamamoto from Fukui University, Japan, and co-workers at the Ioffe Institute in Russia reported MOVPE growth of InN films with absorption between 0.7 and 2 eV. XPS studies indicate that oxygen contamination of the films is the mechanism for the absorption at 2 eV.
New processes yield better LEDs
The topic of LEDs was initiated with a well attended and stimulating discussion session on general lighting using white LEDs. Many of the critical issues were covered including lowering costs, raising the luminous efficiency and total flux per lamp, improving color rendering, and phosphor conversion versus multichip RGB LEDs. One of the highlights was a presentation by H Shimada, a surgeon at Kyoto University, and Takashi Mukai from Nichia on shadowless lighting during surgery using goggles mounted with high-power white LEDs (figure 1). The minimum luminance required for surgery is 20,000 lm/m2, and the local area of illumination is 15 x 15 cm2, so a total luminous flux of 450 lm is necessary.
The current-generation goggles contain 16 power LED modules mounted on an AlN plate, supplied by packaging specialist Kyocera, and a heat sink, which is cooled with microfans. Each module gives 23 lm at 350 mA from a 1 mm2 die, allowing a head-to-patient distance of 52 cm. The drawbacks are that the heat generated from non-radiative processes prevents the use of the goggles for more than 15 minutes before they are too hot to wear, and poor color rendering in the red band makes muscle tissue appear blue, making it difficult to distinguish between arteries and veins. The next-generation goggles will incorporate the latest white LED modules, which achieve superior color rendering by utilizing improved phosphor technology.
S Kamiyama from Meijo University, in collaboration with Kyocera Corporation and the Advanced Materials Laboratory in Tsukuba, Japan, reported violet LEDs grown on a zirconium diboride (ZrB2) substrate. This material is reported to have excellent electrical and thermal conductivities (130 W/mK) as well as a perfect lattice match for Al0.26Ga0.74N. On-wafer measurements yielded an operating voltage of 3.5 V and an L-I curve with excellent linearity compared with equivalent devices on sapphire, attributed to reduced output power saturation due to self heating.
Nichia s Mukai also gave an invited talk covering the status of LED and laser technology at Nichia and the efforts to improve the devices. In particular he described the advances in LED efficiency through the use of a patterned sapphire substrate, which scatters some of the downward-directed emission back into the structure that would otherwise be lost through the substrate. He also discussed the further enhancement to extraction efficiency through the use of a Rh metal mesh top electrode (figure 2).
Mukai s colleague Daisuke Morita reported a GaN-free 365 nm UV LED, which incorporates the mesh electrode and exhibits a peak output power of more than 200 mW. Removing the GaN reduces the losses due to UV photon absorpuion. The LEDs are initially grown on a sapphire substrate with a GaN buffer layer. Absorption in the GaN layer is avoided by transferring the device to a CuW substrate bonded to the top surface using AuSn solder. The sapphire and the GaN buffer layer are then removed, leaving a GaN-free device. A 50% increase in the extraction efficiency was obtained by replacing the NiAu with a Rh mesh electrode, taking the extraction efficiency above 10% for both pulsed and DC operation of a 365 nm LED for the first time.
Other groups also reported impressive efficiency gains using patterned sapphire substrates, including Mitsubishi Cable and Stanley Electric, which together have developed 405 nm LEDs grown on grooved sapphire. Mitsubishi s Hiromitsu Kudo described a systematic study to investigate how the groove depth influenced the photoluminescence of the LED structure. Increasing the depths of the grooves increased the PL intensity and removed unwanted interference fringes from the spectrum. The 405 nm devices had an output power of 26.1 mW and an impressive external quantum efficiency of 43%, the highest to date for violet LEDs.
The number of presentations demonstrated not only the continued growth of activity in this field but also the tremendously rapid progress that has been made since ICNS-4 in 2001. There was a much more commercial feel at this meeting, which stems from the emergence of spin-off and start-up companies that are taking processes and devices to market, as well as the ever-present large corporations which are continually adding new products and improving existing ones. Some of the areas of significant progress included UV-LEDs with increased quantum efficiency, white LEDs with better color rendering, and HEMTs with significant improvements in breakdown voltage, current collapse and gate leakage.
The conference was opened with a series of plenary lectures, including one by Fernando Ponce from Arizona State University, who summarized the current status and identified the important issues for the growth of InGaN alloys, and one by Hiroshi Ogawa from Sony Corporation, who discussed the impact of nitride laser diodes on the evolution of optical data storage.
Bulk nitride substrates
The development of large-area III-nitride substrates for lattice-matched growth of GaN and related alloys is gathering momentum. As a result there was much discussion of recent progress, with most of the commercial developers represented at the meeting. R Vaudo from ATMI in Danbury, CT, described an investigation of free-standing semi-insulating Fe-doped GaN substrates grown by HVPE. Fe concentrations up to 2 x 1017/cm3 were achieved, but the highest resistivity observed was 2 x 109 Ωcm at room temperature for an Fe concentration of 4 x 1016/cm3.
Leo Schowalter of Crystal IS in Latham, NY, described the company s AlN bulk growth technology from which it produces substrates with a variety of orientations. Vladimir Dmitriev from TDI in Silver Spring, MD, described the development of freestanding AlGaN substrates fabricated using HVPE growth onto SiC with subsequent removal of the SiC. Dmitriev also presented work by TDI and co-workers on AlN substrates grown by vapor phase techniques, which have so far resulted in 35 mm diameter substrates, with 50 mm material expected by the end of 2003. Mike Leszczynski from Polish company TopGaN described the progress made with the company s high-pressure growth technique for bulk GaN substrates. Although the current samples are only about 12 mm in diameter, they show excellent low dislocation densities of about 100/cm2 and have been used to fabricate blue lasers.
The bandgap of indium nitride
In the two sessions devoted to InN, the fundamental bandgap of this material was a hot topic, with many researchers supporting the recent reports of a narrower bandgap than the previously accepted 1.9 eV. Deposition of high-quality (as determined by XRD) InN films on sapphire and Si(111) by RF-MBE was reported by Y Nanishi representing a group of researchers from Ritsumeikan University and Sony Corporation, Japan. A carrier concentration of 4.9 x 1018/cm3 and a room temperature mobility of 1130 cm2/Vs were measured. Absorption and PL measurements observed sharp luminescence at 0.8 eV and no peak at 1.9 eV.
A Yamamoto from Fukui University, Japan, and co-workers at the Ioffe Institute in Russia reported MOVPE growth of InN films with absorption between 0.7 and 2 eV. XPS studies indicate that oxygen contamination of the films is the mechanism for the absorption at 2 eV.
New processes yield better LEDs
The topic of LEDs was initiated with a well attended and stimulating discussion session on general lighting using white LEDs. Many of the critical issues were covered including lowering costs, raising the luminous efficiency and total flux per lamp, improving color rendering, and phosphor conversion versus multichip RGB LEDs. One of the highlights was a presentation by H Shimada, a surgeon at Kyoto University, and Takashi Mukai from Nichia on shadowless lighting during surgery using goggles mounted with high-power white LEDs (figure 1). The minimum luminance required for surgery is 20,000 lm/m2, and the local area of illumination is 15 x 15 cm2, so a total luminous flux of 450 lm is necessary.
The current-generation goggles contain 16 power LED modules mounted on an AlN plate, supplied by packaging specialist Kyocera, and a heat sink, which is cooled with microfans. Each module gives 23 lm at 350 mA from a 1 mm2 die, allowing a head-to-patient distance of 52 cm. The drawbacks are that the heat generated from non-radiative processes prevents the use of the goggles for more than 15 minutes before they are too hot to wear, and poor color rendering in the red band makes muscle tissue appear blue, making it difficult to distinguish between arteries and veins. The next-generation goggles will incorporate the latest white LED modules, which achieve superior color rendering by utilizing improved phosphor technology.
S Kamiyama from Meijo University, in collaboration with Kyocera Corporation and the Advanced Materials Laboratory in Tsukuba, Japan, reported violet LEDs grown on a zirconium diboride (ZrB2) substrate. This material is reported to have excellent electrical and thermal conductivities (130 W/mK) as well as a perfect lattice match for Al0.26Ga0.74N. On-wafer measurements yielded an operating voltage of 3.5 V and an L-I curve with excellent linearity compared with equivalent devices on sapphire, attributed to reduced output power saturation due to self heating.
Nichia s Mukai also gave an invited talk covering the status of LED and laser technology at Nichia and the efforts to improve the devices. In particular he described the advances in LED efficiency through the use of a patterned sapphire substrate, which scatters some of the downward-directed emission back into the structure that would otherwise be lost through the substrate. He also discussed the further enhancement to extraction efficiency through the use of a Rh metal mesh top electrode (figure 2).
Mukai s colleague Daisuke Morita reported a GaN-free 365 nm UV LED, which incorporates the mesh electrode and exhibits a peak output power of more than 200 mW. Removing the GaN reduces the losses due to UV photon absorpuion. The LEDs are initially grown on a sapphire substrate with a GaN buffer layer. Absorption in the GaN layer is avoided by transferring the device to a CuW substrate bonded to the top surface using AuSn solder. The sapphire and the GaN buffer layer are then removed, leaving a GaN-free device. A 50% increase in the extraction efficiency was obtained by replacing the NiAu with a Rh mesh electrode, taking the extraction efficiency above 10% for both pulsed and DC operation of a 365 nm LED for the first time.
Other groups also reported impressive efficiency gains using patterned sapphire substrates, including Mitsubishi Cable and Stanley Electric, which together have developed 405 nm LEDs grown on grooved sapphire. Mitsubishi s Hiromitsu Kudo described a systematic study to investigate how the groove depth influenced the photoluminescence of the LED structure. Increasing the depths of the grooves increased the PL intensity and removed unwanted interference fringes from the spectrum. The 405 nm devices had an output power of 26.1 mW and an impressive external quantum efficiency of 43%, the highest to date for violet LEDs.
