Design and growth of GaN-based blue and green laser diodes

REVIEWS . . . . . . . . . . . . . . . . . . . . . . . . . . SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 18 March 2020 | https://doi.org/10.1007/s40843-020-1275-4 Sci China Mater 2020, 63(8): 1348–1363 SPECIAL ISSUE: Optical Gain Materials towards Enhanced Light-Matter Interactions Design and growth of GaN-based blue and green laser diodes 1† 1,2† Aiqin Tian , Lei Hu 1 , Liqun Zhang , Jianping Liu ABSTRACT GaN-based laser diodes (LDs) extend the wavelength of semiconductor LDs into the visible and ultraviolet spectrum ranges, and are therefore expected to be widely used in quantum technology, bio & medical instruments, laser displays, lighting and materials processing. The development of blue and green LDs is still challenging, even though they are based on the same III-nitride materials as GaN-based lightemitting diodes. The challenges and progress of GaN-based blue and green LDs are reviewed from the aspects of epitaxial growth and layer structure design. Due to large differences in lattice constants and growth conditions for InN, GaN, and AlN, considerable effort is required to improve the quality of InGaN multiple quantum well (MQW) gain medium for blue and especially green LDs. p-type doping profiles, conditions and layer structures are critical to reduce the internal losses and to mitigate the degradation of InGaN MQWs. Hole injection is also a key issue for GaN-based LDs. Keywords: GaN-based LDs, InGaN, thermal degradation, In segregation, optical loss, carrier injection INTRODUCTION The invention and development of GaN-based lightemitting diodes (LEDs) have extended the wavelengths of semiconductor optoelectronic devices into the visible and ultraviolet spectrum ranges. GaN-based blue LEDs have been widely used in lighting and displays, and the Nobel Prize in Physics of 2014 honored the inventors of efficient blue LEDs—Professors Akasaki I, Amano H and Nakamura S [1–5]. GaN-based laser diodes (LDs), however, are different kinds of light sources with high brightness (4 orders of magnitude brighter than that of LEDs) and high spectral purity. Semiconductor LDs are now the 1,2* 1,2 and Hui Yang most widely used lasers because of low costs, small sizes, high efficiencies and long lifetimes. Like the LEDs, GaNbased LDs have extended the visible and ultraviolet spectrum ranges. GaN-based violet LD with wavelength of 405 nm for the application of high-density optical storage was the first research focus of GaN-based LDs. GaN-based blue and green LDs have been studied intensely due to great potential for wide applications in displays, lighting, quantum technology, optical clock, medical instruments, materials processing and underwater communications and detection (Fig. 1a). Laser displays using red, green and blue LDs are emerging technologies with larger color gamuts, higher color saturation, and capabilities for both pico-projectors and larger displays [6–11]. Fig. 1b compares the color gamuts of laser displays vs. other technologies [11]. GaNbased direct blue and green emission semiconductor LDs are thus desirable for laser displays. However, their fabrication is more challenging, compared with that of LEDs [12,13]. There are four main difficulties to fabricate high-performance GaN-based blue and green LDs. First, as shown in Fig. 2, the LD epitaxial structures are more complex and more strained to form the optical cavity, which increases the complexity of epitaxial growth and causes more crystalline defects. Second, LDs are more sensitive to defects, including non-radiative recombination centers and inhomogeneity, both of which reduce the peak gain of LDs. Meanwhile, InGaN-based LEDs are not sensitive to defects because of the localization effect. Spectral broadening due to indium (In) composition fluctuations and interface roughness is a particular issue for green LDs whose gain medium is high-In-content InGaN multiple 1 Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, Hefei 230026, China † These two authors contributed equally to this work. * Corresponding author (email: ) 2 1348 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 August 2020 | Vol. 63 No. 8 SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REVIEWS Figure 1 (a) Applications of GaN-based blue and green LDs. (b) Color gamut in the Commission Internationale de l´Eclairage (CIE) chromaticity diagram of laser, LED, liquid crystal display (LCD) and cathode ray tube (CRT) display [11]. Figure 2 Schematic of a conventional GaN-based LD with a palladium (Pd)/platinum (Pt)/gold (Au) electrode or a hybrid GaN-based LD with indium tin oxide (ITO) cladding layer. Reprinted with permission from Ref. [29]. Copyright 2020, Chinese Laser Press. quantum wells (MQWs) [14]. Third, LDs are more sensitive to impurities because light will be strongly absorbed when propagating forward and backward many times within the cavity. Fourth, hole injection is usually inhomogeneous among MQWs because of the large effective masses of holes and the large potential barrier. Lack of holes in n-side quantum wells (QWs) will lead to high light absorption and, therefore, high threshold currents and reduced slope efficiencies for the LDs. In LEDs, either holes are injected into bottom QWs via V-pit sidewalls, or without hole injection, and light emission in the bottom QWs is not an issue. The first GaN-based blue LDs were invented in 1999 by Nichia [15], while the first high-brightness LEDs was reported in 1993. The performance was improved in 2001 by using a free-standing GaN substrate [16]. The output power of GaN-based blue LDs was improved to 200 mW in 2005 [17], 500 mW in 2006 [18], 1 W in 2008 [19], and 5.25 W most recently [20]. However, the epitaxial growth August 2020 | Vol. 63 No. 8 and layer structures of GaN-based LDs have not been reported in significant detail. We have also obtained blue LDs with output powers more than 2 W [21], and re−1 cently have improved slope efficiencies to 1.65 W A [21,22]. The development of GaN-based green LDs (λ >500 nm) has been even more challenging. They were reported in 2009 by Osram Corp. [6]. Since then, green InGaN LDs grown on c-plane [6,23–29], (1122) plane [30], and (2021) plane [31–34] have been realized. Challenges and recent progresses on GaN-based blue and green LDs will be discussed below. CHALLENGES Crystalline defects The first challenge is the preparation of high-quality InGaN/GaN MQWs, which is the gain medium of LDs, especially for high-In-content InGaN/GaN MQW green LDs. The quantum efficiency of GaN-based LEDs drops significantly as emission wavelength extends into the “green gap” [35,36], as shown in Fig. 3a. The efficiency of GaN-based LDs drops even more severely due to m (...truncated)