Nichia warms the pace for CW blue laser

Nichia W a r m s the Pace for C W Blue Laser by Roy Szweda A p p l i e d Physics Letters j o u r n a l Dr Shuji In t h e

Nakamura and his colleagues have r e c o r d e d their success in "the first continuous-wave o p e r a t i o n of lnGaN multi-quantumwell-structure laser diodes at 233K". Pulsed o p e r a t i o n at RT has previously b e e n d e m o n s t r a t e d (see IlI-Vs Review Vol. 9 No. 3, p22) but the target is for CW o p e r a t i o n in order to rcalise practical manufaeturable blue emitting lasers for high density data storage for DVD etc. This r e c e n t a c h i e v e m e n t is a notable step in the path to this goal. As b e f o r e , t h e l a s e r s w e r e p r e p a r e d using the Nichia-developed two-flow MOCVD m e t h o d at atmospheric pressure using (001) C-face sapphire substrate. The s t r u c t u r e of the laser diode is c o m p l e x and b e s t s e e n as a p i c t o r i a l r e p r e s e n t a t i o n as s h o w n here. Key to the g r o w t h and integrity of the layers are the n-type InGaN buffer and thick A1GaN w h i c h prevents film cracking. As Dr N a k a m u r a says, t h e s t r u c t u r e of the ridge-geom e t r y InGaN MQW was almost the same ms that used before. This was formed by etching of the surface of the p-type GaN layer. The mirror facet was formed by dry etching also. Since the objective of the w o r k was to achieve RT operation these diodes w e r e actually tested under t h e s e c o n d i t i o n s at this t e m p e r a t u r e but "were easily b r o k e n within one second due to heat generation". However, CW operation was subsequently confirmed when the t e m p e r a t u r e was red u c e d to 233K (i.e. -40"C)

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by immersion in ethanol c o o l e d by d w ice. It was under these conditions that DC cleetrical characterization was performed. tip to a threshold c u r r e n t of 210 mA no stimulated emission took place. However, a differential quantum efficiency of 8% p e r facet and 9.5 mW p e r facet o u t p u t w e r e g a i n e d at 250 mA. It is w o r t h pointing out that the p a p e r d e s c r i b e s i m p r o v e m e n t in operating voltage - these measurements w e r e made at 11 V contrast this to that used before i.e. 30V. This was achieved by "adjusting growth, ohmic contact and d o p i n g profile conditions". Pulsed o p e r a t i o n of these diodes was also successful with lasing o c c u r r i n g bet w e e n 283K a n d 343K. T e m p e r a t u r e range was det e r m i n e d by the limits of t h e m e a s u r e m e n t equipment. Dr Nakamura estimates the temperature d e p e n d e n c e of the threshold c u r r e n t to be 162K.

Temperature control was also a p r o b l e m for m e a s u r e m e n t of emission spectra. In fact it is unknown as yet h o w much l o n g e r than the 30 rain used here that these diodes would continue to o p e r a t e at 233K but "it is e x p e c t e d to bc longer". The optical characteristics of the diode w e r e also m e a s u r e d and s p o n t a n e o u s emission noted at a peak wavelength of 409 nm with a broad spectral width of 20 nm. It is n o t e d that observed peaks had separations w h i c h differ from those seen with pulsed o p e r a t i o n . Dr N a k a m u r a infers that the previously seen multiple sharp peaks w e r e characteristic of the pulsed c u r r e n t flow. However, it may also derive from i n h o m o g e n c i t i e s in film thickness and c o m p o sition of layers and so o n . The p a p e r also discusses e x c i t a t i o n b e h a v i o u r and considers their relation to e m i s s i o n m e c h a n i s m in this type of diode. It is d e e m e d unlikely that ex-

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citons are related to emission at RT, u n l i k e c i r c u m s t a n c e s in o t h e r devices such as S- and M-QW LEDs.

N e x t Issue In the next issue we will return to the LED developments at Nichia and also look at their growing application in outdoor displays.

Reference The paper is: S. Nakamura, M. Senoh, S. Nagahama, N. lwasa, T. Yamada, T. Matsushita, Y, Sugimoto and H. Kiyoku, "The first continuous-wave operation of InGaN m u l t i - q u a n t u m - w e l l s t r u c t u r e laser diodes at 233K", App!. Phys. Le~, N6~

vember 11 (1996) in press. • Contact: Dr S N a k a mura, Dept. o f Research a n d Development, Nichia Chemical Industries, Ltd., 491 Oka, Kaminaka, A n a n , T o k u s h i m a 774, Japan. Tel/fax: [81] 884 22 2 3 1 1 / 23 1802. Email: shuji(~i nichia.co.jp URL :http :\ \ w w w l a. meshnet. o r j p / nichia