Recent Developments in InGaN-Based Blue LEDs and LDs

Shuji Nakamura

Department of Research and Development, Nichia Chemical Industries, Ltd.,
 491 Oka, Kaminaka, Anan, Tokushima 774, Japan
Tel: +81-884-23-7787, Fax: +81-884-23-1802, e-mail: shuji@nichia.co jp

Gallium nitride and other III-V nitride-based semiconductors have a direct band gap that is suitable for blue light-emitting devices. The band gap energy of aluminium gallium indium nitride (AlInGaN) varies between 6.2 and 2.0 eV, depending on its composition at room temperature. Thus, using these semiconductors, red to UV emitting devices are fabricated. High efficient UV, blue and green InGaN single-quantum-well (SQW) structure light-emitting diodes (LEDs) have been fabricated with the external quantum efficiencies of 7.5 % at 371 nm (UV), 11.2 % at 468 nm (blue) and 11.6 % at 520 nm (green), respectively, which were the highest values ever reported for the LEDs with those emission wavelengths [1]. The luminous efficiencies of blue and green LEDs were 5 lm/W and 30 lm/W, respectively, which values are almost identical to that of the white conventional incandescent bulb lamp (20 lm/W). By combining the blue, green and red LEDs, we could fabricate white LEDs with a luminous efficiency of 20-30 lm/W which is almost comparable to that of the incandescent bulb lamp. These blue and green InGaN SQW LEDs are already used for many applications, such as LED full color displays, LED traffic lights, lighting, and etc. The shorter wavelength means that the light can be focused more sharply, which would increase the storage capacity of magnetic and optical disks. Digital versatile disks (DVDs), which came onto the market in 1996, rely on red AIInGaP semiconductor lasers and have a data capacity of about 4.7 Gbytes, compared to 0.65 Gbytes for compact disks (CDs). By moving to violet wavelengths using III-V nitride-based semiconductors, the capacity could be increased to 15 Gbytes. At the end of 1995, first current injection III-V nitride-based LD was reported by the group of present author [1]. First RT-CW operation was achieved in December, 1996 after the success of the low temperature CW operation at 233K in November, 1996 [1]. Then, the latest paper showed that the lifetime became as long as 10,000 hours [2] under RT CW operation using epitaxially laterally over grown GaN (ELOG) substrate and the AlGaN/GaN modulation doped strained-supperlattice (MD-SLSs). Thus, great progress has been achieved on III-V nitnde-based LEDs and LDs recently. Here, the present status of InGaN/GaN/AlGaN-based LDs are described.

The III-V nitride films were grown using the two-flow metal-organic chemical vapor deposition (MOCVD) method [1]. First, selective growth of GaN was performed on a 2-mm-thick GaN layer grown on a (0001) C-face sapphire substrate under a low pressure of 100 Torr. The 2-mm-thick silicon dioxide (SiO2) mask was patterned to form 3-mm-wide stripe windows with a periodicity of 13 mm in the GaN <1-100> direction. Following the 20-mm-thick GaN growth on the SiO2 mask pattern, the coalescence of the selectively grown GaN enabled the achievement of a flat GaN surface over the entire substrate. This coalesced GaN was designated the ELOG. Cross-sectional transmission electron microscopy (TEM) images of the ELOG substrate were obtained. Threading dislocations, originating from the GaN/sapphire interface, propagate to the regrown GaN layer within the window regions of the mask. In contrast, there were no observable threading dislocations in the overgrown layer. However, very few short edge-on dislocation segments parallel to the interface plane were observed in the GaN layer on the SiO2 mask area. These dislocations were parallel to the (0001) plane via the extension of the vertical threading dislocations after a 90o bend in the regrown region. These dislocations did not subsequently propagate to the surface of the overgrown GaN layers. We examined the defect density by plan-view TEM observation of the surface of the ELOG substrates. The number of dislocations on the SiO2 mask area was almost zero within the area of 10 mm x 10 mm, and that on the window area was approximately 1 x 107/cm2.

The InGaN MQW-structure LD was grown on the above-mentioned ELOG substrate. The InGaN MQW-structure LD consisted of a 3-mm-thick layer of n-type GaN:Si, a 0.1-mm-thick n-type In0.1Ga0.9N:Si, a Al0.14Ga0.86N/GaN MD-SLS cladding layer consisting of 120 2.5-nm-thick Si-doped GaN separated by 2.5-nm-thick undoped Al0.14Ga0.86N layers, a 0.1-mm-thick layer of Si-doped GaN, an In0.15Ga0.85N/In0.02Ga0.98N MQW structure consisting of four 3.5-nm-thick Si-doped Inl0.15Ga0.85N well layers foming a gain medium separated by 10.5-nm-thick Si-doped In0.02Ga0.98N barrier layers, a 20.0-nm-thick layer of p-type Al0.2Ga0.8N:Mg, a 0.1- mm-thick layer of Mg-doped GaN, a Al0.14Ga0.86N/GaN MD-SLS cladding layer consisting of 120 2.5-nm-thick Mg-doped GaN separated by 2.5-nm-thick undoped Al0.14Ga0.86N layers and a 0.05-mm-thick layer of p-type GaN:Mg. The ridge-geometry LDs were fabricated with an area of 4 x 450 mm. A mirror facet was also formed by dry etchlng. High-reflection facet coatings (50 %) consisting of 2 pairs of quarter-wave TiO2/SiO2 dielectric multilayers were used to reduce the threshold current.

Figure 1 shows typical voltage-current (V-I) characteristics and the light output power per coated facet of the LD as a function of the forward DC current (L-I) at RT. No stimulated emission was observed up to a threshold current of 53 mA, which corresponded to a threshold current density of 3 kA/cm2, as shown in Fig.l. Figure 2 shows the results of a lifetime test for CW-operated LDs carried out at 20oC, in which the operating culrent is shown as a function of time under a constant output power of 2 mW per facet controlled using an autopower controller (APC). After 2,200 hours of operation, the operating current is still increasing gradually with increasing operating time. The LDs is still survive after 2,200 hours of operation. The lifetime of some of the LDs was estimated to be longer than 10,000 hours from the degradation speed. The degradation speed was defmed to be the derivatives of dI/dt (mA/100 hours), where I is the operating current of the LDs and t is the time. Using this degradation speed, the estimated lifetime was obtained to be the time when the operating current became twice of the initial operating current of the LDs. Emission spectra of the LDs were measured under RT-CW operation. Usually, the LDs showed a single-mode stimulated emission at a wavelength of around 400 nm. However, some LDs showed a self pulsation with increasing the DC forward currents at RT. In summary, InGaN MQW LDs with the MD-SLS cladding layers grown on the ELOG substrate were demonstrated to have an estimated lifetime of more than than 10,000 hours under RT CW operation. These results suggest an imminent commercialization of the III-V nitride-based short-wavelength LDs.
 

Fig.l. Typical L-I and V-I characteristics of InGaN MQW LDs measured under CW operation at RT.

Fig.2. Operating current as a function of time under a constant output power of 2 mW per facet controlled using an autopower controller. The InGaN MQW LDs with Md-SLS cladding layers grown on the ELOG substrate were operated under DC at 20oC.

References
[1] For a review, see S. Nakamura and G. Fasol, The blue laser diode (Springer-Verlag, Heidelberg, Germany,1997) 1 st ed.
[2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, Jpn. J. Appl. Phys. 36, L15768 (1997).