InGaN Light-Emitting Diodes with Quantum-Well Structures
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AIInGaP LEDs is an emission wavelength of 570 nm (yellowish green) and maximum external quantum efficiency of 1% [6]. When the emission wavelength is reduced to the green region, the external quantum efficiency drops sharply because the band structure of AIInGaP approaches an indirect transition band structure. Therefore, high-brightness pure green LEDs, which have high efficiency of above 1 % at the peak wavelength between 510-530 nm with a narrow FWHM, have not been commercialized yet. Among II-VI materials, ZnSSe- and ZnCdSe-based materials have been intensively studied for use in green light-emitting devices, and much progress has been made recently. The recent performance of Il-VI green LEDs is an output power of 1.3 mW, external quantum efficiency of 5.3 % at 10 mA and peak wavelength of 512 nm [7]. However, the lifetime of Il-VI-based devices is still short, which prevents their commercialization at present. Recently, high-brightness single-quantum-well-structure (SQW) blue, green and yellow InGaN LEDs with luminous intensity above 10 cd for green LEDs have been achieved and now commercialized [8,9]. Here, recent performance of 11I-V nitride based SQW LEDs is described. EXPERIMENT III-V nitride films were grown by the two-flow metalorganic chemical vapor deposition (MOCVD) method. Details of the two-flow MOCVD are described elsewhere [10]. The growth was conducted at atmospheric pressure. Sapphire with (0001) orientation (C face) and of twoinch diameter was used as a substrate. The growth conditions of each layer are described elsewhere [11]. p-electrode p-GaN
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p-Alo.2Gao.sN Ino.2Gao.SN•" n-GaN --
n-electrode
GaN buffer layer-
Sapphire substrate
I
Fig. 1. Structure of a blue SQW LED.
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A blue LED device structure (Fig. 1) consists of a 300 A GaN buffer layer grown at a low temperature (550 *C), a 4-ltm-thick layer of n-type GaN:Si, a 20-A-thick active layer of undoped In 0 .2Ga 0 .8N, a 1000-A-thick layer of p-type Al0 .2 Ga0 .8 N:Mg, and a 0.5-4m-thick layer of p-type GaN:Mg. The active region forms a SQW structure consisting of a 20 A In 0 .2 Ga 0 .8N well layer sandwiched by 4 gtm n-type GaN and 1000 A p-type Al0. 2 Ga 0 .8 N barrier layers. The indium mole fraction of the InGaN active layer was varied between 0.05 and 0.8 in order to change the peak wavelength of the InGaN SQW LEDs from blue to orange. Fabrication of LED chips was accomplished as follows. The surface of the p-type GaN layer was partially etched until the n-type GaN layer was exposed. Next, Ni/Au contact was evaporated onto the p-type GaN layer and a Ti/Al contact onto the n-type GaN layer. Then the wafer was cut into rectangles (350 gtm x 350 gtm). These chips were set on a lead frame, and were then molded. The characteristics of LEDs were measured under direct current (DC)-biased conditions at room temperature. RESULTS AND DISCUSSION
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Fig.2. Electroluminescence of (a) blue, (b) green and (c) orange SQW LEDs at a forward current of 20 mA.
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Figure 2 show
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