Tuning carrier lifetime in InGaN/GaN LEDs via strain compensation for high-speed visible light communication

www.nature.com/scientificreports OPEN received: 20 June 2016 accepted: 25 October 2016 Published: 14 November 2016 Tuning carrier lifetime in InGaN/ GaN LEDs via strain compensation for high-speed visible light communication Chunhua Du1,*, Xin Huang1,*, Chunyan Jiang1, Xiong Pu1, Zhenfu Zhao1, Liang Jing1, Weiguo Hu1 & Zhong Lin Wang1,2 In recent years, visible light communication (VLC) technology has attracted intensive attention due to its huge potential in superior processing ability and fast data transmission. The transmission rate relies on the modulation bandwidth, which is predominantly determined by the minority-carrier lifetime in III-group nitride semiconductors. In this paper, the carrier dynamic process under a stress field was studied for the first time, and the carrier recombination lifetime was calculated within the framework of quantum perturbation theory. Owing to the intrinsic strain due to the lattice mismatch between InGaN and GaN, the wave functions for the holes and electrons are misaligned in an InGaN/GaN device. By applying an external strain that “cancels” the internal strain, the overlap between the wave functions can be maximized so that the lifetime of the carrier is greatly reduced. As a result, the maximum speed of a single chip was increased from 54 MHz up to 117 MHz in a blue LED chip under 0.14% compressive strain. Finally, a bandwidth contour plot depending on the stress and operating wavelength was calculated to guide VLC chip design and stress optimization. In addition to Si and Ge semiconductors, compound semiconductor materials (like ZnO, CdS, and GaN) have been intensively studied and successfully applied in many novel devices such as piezoelectric nanogenerators (NGs)1–4, sensors5–7, photodetectors8–10, high-electron-mobility transistors11,12, photovoltaic cells13,14, and logic devices15,16. Due to their lack of crystal lattice symmetry, most compound semiconductors also have a strong piezoelectric property. The piezoelectric effect, together with semiconducting properties and photoexcitation properties, known as the piezo-phototronic effect17,18, plays a pivotal role in enhancing the performance and expanding the applications of novel electronic/optoelectronic devices such as optical memories19, personalized handwriting20, visible light communication (VLC)21 and biomedical imaging22. The basic mechanism lies in using the piezopotential at the interface as a gate to tune/control the carrier generation, transport, separation and/ or recombination via external strain, thus tuning the device performance17,18. This field experienced very rapid development and exhibited great potential in beyond-Moore devices. However, until now, all theoretical and experimental works on the piezo-phototronic effect have focused on the quasi-equilibrium state without considering the carrier dynamic process. The carrier dynamic process dominates the light absorption/emission and carrier transport and therefore has important physical meaning and potential applications. Compound semiconductors can operate in the entire visible light region via bandgap engineering by linearly altering the alloy composition23,24, which has been widely used to fabricate many optoelectronic devices25,26, greatly improving modern life. Among these technologies, VLC based on GaN light emitting diodes (LEDs) has attracted much attention, and a huge potential market exists. The rapid response and easy modulation and integration suggest a great potential in ultra-high speed wireless communication. In today’s information-rich era, the ability to process large volumes of data is an urgent and endless demand. In VLC, the modulation bandwidth of the LED is the most significant bottleneck. JJD Mckendry et al. used an LED chip with a 60 MHz 3 dB modulation 1 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National Center for Nanoscience and Technology (NCNST), Beijing, 100083, P. R. China. 2School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States. *These authors contributed equally to this work. Correspondence and requests for materials should be addressed to W.H. (email: ) or Z.L.W. (email: ) Scientific Reports | 6:37132 | DOI: 10.1038/srep37132 1 www.nature.com/scientificreports/ bandwidth, significantly higher than that of commercially available LEDs, to achieve a 3 GB/s communication speed through orthogonal frequency division multiplexing (OFDM)27. Some companies, such as Apple Inc., have argued that a LiFi network based on VLC is a hundred times quicker than a WiFi network. In addition, the natural conjunction of the existing lighting network and VLC technology and the immunity from electromagnetic interference makes the ubiquitous coverage of wireless communication possible, providing a new type of broadband access with a great capacity of information, flexible deployment, convenient maintenance, security and confidentiality, and economy28,29. Because of this, the application fields of VLC have already been extended from the military and the aerospace industry to civil engineering. The compound semiconductor devices mentioned above operate based on minority-carrier transport and recombination, which are characterized by the minority-carrier lifetime30. The carrier lifetime in GaAs nanowires31 has been intensively investigated, and its applications from photovoltaics to single-photon emitters have been considered. It has been demonstrated that instead of the RC (resistance-capacitance) time delay and doping concentration, the carrier recombination lifetime in InGaN/GaN QW LEDs has the most significant influence on the modulation bandwidth of the VLC system32. In InGaN/GaN QW LEDs, the radiative recombination is definitely contributed to by the band-to-band transition of carriers33 whose lifetime is dominated by the transition rate and thus can be modulated by the built-in electric field. It is reported that a built-in electric field as high as 2.45 MV/ cm is generated in an In0.2Ga0.8N/GaN quantum well due to the internal strain along the c-axis caused by the large lattice mismatch of GaN and InGaN34. Controlling the atomic ratio of InGaN alloys can not only tune the operation wavelength of the LED from blue to green but also change the piezoelectric coefficients and mismatch strain. In this paper, we experimentally and theoretically demonstrate how the piezo-phototronic effect can be utilized to tune the carrier lifetime of an InGaN/GaN LED operating at the blue or green wavelength via strain compensation. The highest bandwidth of a single-chip blue LED was up to 117 MHz under 0.14% compressive strain. Finally, a bandwidth contour plot depending on the stress and operating wavelength was calculated to guide VLC chip design and strain optimization. Results and Discussion InGaN/GaN LEDs commonly operate in the wide wavelength band from blue to green. Two blue and green InGaN/Ga (...truncated)