ReviewIII-nitride semiconductor lasers grown on Si
Introduction
Silicon photonics compatible with large-scale and large-wafer-size manufacturing foundries are deemed as a promising way to revolute communication and computation technologies [[1], [2], [3]]. However, Si with an indirect band structure, usually used for complementary metal oxide semiconductor (CMOS), cannot emit light efficiently. Recently, Ge and SiGe alloys with a hexagonal crystal structure were reported to emit light efficiently [4]. Nevertheless, they are not fully compatible with CMOS-used Si with a cubic lattice structure. Therefore, the lack of an efficient on-chip laser source remains as the major roadblock of Si photonics.
III-V compound semiconductor lasers on Si may be a potential option for on-chip light source [[5], [6], [7], [8]]. Given the low throughput of the off-chip integration via chip bonding process, it is highly desirable to achieve lasers directly grown on large-size Si [5,8]. Liu’s group grew high-quality III-V semiconductor epilayer with a recorded threading dislocation density (TDD) on the order of 105 cm−2, by combining a nucleation layer and dislocation filter layers on Si, and then realized high performance III-V quantum dot lasers grown on Si [5]. However, the defect density in the laser material is still a little high for III-V semiconductor lasers. In contrast, III-nitride semiconductors are less sensitive to structural defects [9], and thus III-nitride lasers on Si may be a potential on-chip light source for Si photonics.
III-nitride semiconductor materials with a direct bandgap have a wide emission ranging from ultraviolet (UV) to near infrared (IR) region, and have been widely used for highly efficient light-emitting diodes (LEDs) and laser diodes (LDs) with a huge commercial success [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]]. The 2014 Nobel Prize in physics was awarded to the inventors of GaN-based blue LEDs. The state-of-art threshold current density (Jth) of III-nitride LDs is less than 0.6 kA/cm2, and the highest wall-plug efficiency is over 50% from Nichia [14]. And III-nitride LDs have been widely used in displays [14,15], lighting and optical storage [22], and also showed a great potential for wide applications in monolithic integration [23], visible light communications [24], optical clock [19], material processing [15], quantum technology [19], and medical instruments [17].
Today most of the GaN-based LDs are produced on 2-inch free-standing (FS) GaN substrates (~$4000/pc), which makes the LD chip cost 2-3 orders of magnitude higher than that of LEDs grown on Si or sapphire substrates. Additionally, FS GaN substrates often have issues of non-uniformity in offcut angle, defect density, and residual stress [25], which affect the device yield, further increasing the LD chip cost. GaN-based LDs grown on large-size low-cost Si substrates may revolutionize the LD industry by slashing the cost and further boost their applications [[26], [27], [28], [29]]. Therefore, III-nitride LDs grown on Si have received wide attention and been pursued for decades.
Similar with GaN-on-GaN LDs, GaN-on-Si LDs can be processed by standard micro/nanofabrication technologies, including lithography, dry etching, deposition, facet cleavage, etc. However, distinctly different from the homoepitaxy of GaN-on-GaN LDs, the heteroepitaxy of GaN-on-Si LDs encounters the issues of huge mismatch in both lattice constant and thermal expansion coefficient between GaN and Si. These issues usually not only induce a large tensile stress, wafer bowing, and even microcrack networks in the epilayer, but also cause a high TDD (108-109 cm−2), 2-3 orders of magnitude higher than that of GaN-on-GaN LDs. The large tensile stress poses a daunting challenge for the growth of GaN-on-Si LDs including about 2-μm-thick AlGaN cladding layers, while the high TDD affects the internal quantum efficiency and the optical gain by causing severe non-radiative recombination and formation of V-shape pit defects along with deteriorated interfaces of multiple quantum wells (MQWs).
This review article began with a discussion on the key challenges of the direct growth of III-nitride semiconductor lasers on Si, followed by the recent progress of GaN-on-Si lasers and their monolithic integration. Additionally, various methods of further improving the material quality and several novel device structures are suggested for boosting the device performance and reliability.
Section snippets
Key challenges in the direct growth of III-nitride lasers on Si
The first lasing action of III-nitride semiconductor materials grown on Si under optical pumping was reported by S. Bidnyk et al. from Oklahoma State University in 1998 [30]. The lasing action was observed in the GaN pyramids, which were laterally overgrown on a patterned GaN/AlN seeding layer on Si(111) substrate.
Afterwards, Yablonskii’s group from National Academy of Sciences (NAS) of Belarus reported the lasing action of GaN film grown on Si substrate in 2002 [31], and then realized a lasing
Recent progress of III-nitride semiconductor LDs grown on Si
In this section, the recent progress of III-nitride semiconductor LDs grown on Si and their monolithic integration will be reviewed, including the Fabry-Pérot (F–P) cavity lasers, the microdisk lasers, and the lasers with nanostructures.
Outlook
Unlike III-nitride lasers grown on FS GaN substrates, III-nitride lasers grown on Si often have a large residual strain and a high density of TDs due to the CTE and lattice mismatch, affecting the device performance and reliability. Recently, great progress has been made in III-nitride LDs grown on Si. For III-nitride F–P cavity LDs grown on Si, RT electrically pumped lasing was realized with a lasing wavelength ranging from near UV to blue [[26], [27], [28]]. And the Jth and threshold voltage
Summary
III-nitride LDs grown on Si have attracted much attention and been widely studied due to their important applications as cost-effective laser sources for laser display and lighting, and efficient on-chip light source for Si photonics. The heteroepitaxial growth of III-nitride LDs on large-size Si substrates encounters large mismatch in both CTE and lattice constant, usually resulting in a large tensile strain and high TDD, which hindered the realization of III-nitride LDs grown on Si for about
Declaration of competing interestCOI
The authors declare no competing financial interest.
Acknowledgements
The authors are grateful for the financial support from Key-Area R&D Program of Guangdong Province (Grant Nos. 2019B010130001, 2019B090917005, 2019B090904002, 2019B090909004, and 2020B010174004), Strategic Priority Research Program of CAS (Grant Nos. XDB43000000, and XDB43020200), Key Research Program of Frontier Sciences, CAS (Grant Nos. QYZDB-SSW-JSC014, and ZDBS-LY-JSC040), CAS Interdisciplinary Innovation Team, National Natural Science Foundation of China (Grant Nos. 61534007, 61775230,
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These authors contributed equally to this work.