speed, 1 to 50 mg of PFA could be applied in each coating step. 13. The coated tubes were placed inside a quartz pipe 57 mm in diameter, fitted with end caps designed to hold the coated tubes in the center while they were being rotated. The quartz pipe was fitted into a temperature- controlled furnace and purged with scientific-grade He (total impurities 1 part per million) at a flow rate of 100 cc/min for 15 min. The temperature was raised at a rate of 5.0°C/min to the final temperature (473 to 873 K) and held there for 120 min. To ensure uniformity during pyrolysis, we rotated the tubes at 30 rpm. 14. The gas was introduced on the core (feed) side of the NPCM at a pressure of 300 kPa, and the shell side pressure response was measured continuously. The membrane module was evacuated and returned to atmospheric pressure on both the core and shell sides before the introduction of the next probe gas. All experiments were conducted at 295 K. 15. J. W. Hutchinson and Z. Suo, Adv. Appl. Mech. 29, 63 (1991). 16. R. C. Reid, J. M. Prausnitz, B. E. Poling, The Properties of Gases and Liquids (McGraw-Hill, New York, ed. 4, 1987). 17. P ss and P cs (Pa) are the pressures on the shell side and core side of the tubular membrane, respectively; t (s) is time; A (m 2 ) is the membrane area; R (m 3 Pa mol -1 K -1 ) is the gas constant; T (K) is the temperature; V ss (m 3 ) is the shell side volume; ' is the gas perme- ability (mol m –1 sec –1 Pa –1 ); and  (m) is the mem- brane thickness. 18. L. M. Robeson, J. Mem. Sci. 62, 165 (1991). 19. Supported by Department of Energy Office of Basic Energy Science, State of Delaware Research Partner- ship, and DuPont. 30 March 1999; accepted 9 August 1999 Room Temperature Lasing at Blue Wavelengths in Gallium Nitride Microcavities Takao Someya, 1 * Ralph Werner, 2 Alfred Forchel, 2 Massimo Catalano, 3 Roberto Cingolani, 4 Yasuhiko Arakawa 1 Lasing action has been demonstrated at blue wavelengths in vertical cavity surface-emitting lasers at room temperature. The microcavity was formed by sandwiching indium gallium nitride multiple quantum wells between nitride- based and oxide-based quarter-wave reflectors. Lasing action was observed at a wavelength of 399 nanometers under optical excitation and confirmed by a narrowing of the linewidth in the emission spectra from 0.8 nanometer below threshold to less than 0.1 nanometer (resolution limit) above threshold. The result suggests that practical blue vertical cavity surface-emitting lasers can be realized in gallium-nitride– based material systems. Blue nitride-based semiconductor laser diodes (LDs) and light-emitting diodes (LEDs) have been developed over the past few years (1–13) and are having a large impact on industry as well as on fundamental research. Blue vertical cavity surface-emitting lasers (VCSELs) have attracted increasing attention (14, 15 ) because they are expected to surpass conventional blue nitride LDs in many applications. In particular, the use of two-dimensional arrays of blue VCSELs would drastically reduce the read-out time in high-density optical storage (compact disc and digital video disc) and increase the scan speed in high-resolution laser printing technol- ogy (14 ). The short vertical cavity configuration is also especially suitable for reducing the threshold current in blue lasers, because wide band gap materials have large optical gain due to the giant joint density of states (1, 2). In addition, field patterns from VCSELs are natu- rally completely circular, whereas the aspect ratio between vertical and horizontal modes is still 4 in conventional edge-emitting nitride la- sers (10). The main obstacle to the room-temperature operation of blue VCSELs is the crystal growth of highly reflective nitride mirrors consisting of GaN and AlN or GaN and Al x Ga 1-x N with high aluminum content x on which the high- quality InGaN active regions are subsequently grown (16 –19). The problems that arise in epi- taxial growth of high-quality films are the large difference in thermal expansion coefficients be- tween GaN (5.6  10 -6 /K) and AlN (4.2  10 -6 /K) and the large difference in their lattice constants (2.7%) (20). However, progress in crystal growth technology is now opening the door to the synthesis of highly lattice-mis- matched nitride semiconductor systems. We report here the fabrication of blue nitride VCSELs and the observation of lasing action at room temperature. Using a micro- cavity with a high Q factor of 500, we ob- served lasing action at a wavelength of 399 nm under optical excitation. Disk-shaped VCSEL structures 18 m in diameter are formed from a planar multilayer by reactive ion etching and arrayed in a two- 1 Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan. 2 Lehrstuhl fu ¨r Technische Physik, Universita ¨t Wu ¨rz- burg, Am Hubland, 97074 Wu ¨rzburg, Germany. 3 Con- siglio Nazionale delle Ricerche, Istituto Studio Mate- riali per l’Elettronica, Universita’ di Lecce, via Arne- sano 73100 Lecce, Italy. 4 Istituto Nazionale Fiscica Materia, Universita’ di Lecce, Dipartimento Ingegneria Innovazione, via Arnesano 73100 Lecce, Italy. *To whom correspondence should be addressed. E- mail: someya@iis.u-tokyo.ac.jp Fig. 1. (A) Scanning electron microscope image of a two-dimensional array of GaN-based VC- SELs. (B) Cross-sectional image of the VCSEL structure observed by transmission electron microscopy. The structure of the GaN-based multilayer, grown on a (0001)-oriented sap- phire substrate, is as follows: a 30-nm GaN nucleation layer, 400-nm GaN, a nitride distrib- uted Bragg reflector (DBR) consisting of 43 pairs of 38-nm GaN (dark layers) and 40-nm Al 0.34 Ga 0.66 N (bright layers), 195-nm GaN, multiple quantum wells (MQWs) consisting of 26 periods of 5-nm In 0.01 Ga 0.99 N barrier and 3-nm In 0.1 Ga 0.9 N quantum well, and 18-nm GaN. The aluminum content x = 0.34 of the Al x Ga 1-x N was determined by x-ray diffraction measurements. The oxide DBR consisting of 15 pairs of 48-nm ZrO 2 (dark layers) and 68-nm SiO 2 (bright layers) was evaporated on the top of the GaN-based multilayer to form a vertical cavity. R EPORTS www.sciencemag.org SCIENCE VOL 285 17 SEPTEMBER 1999 1905 on March 29, 2018 http://science.sciencemag.org/ Downloaded from