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GaN multiple-quantum-well diode for transferrable optoelectronics
Optical Materials 72 (2017) 20e24
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Simultaneous dual-functioning InGaN/GaN multiple-quantum-well diode for transferrable optoelectronics Zheng Shi a, Jialei Yuan a, Shuai Zhang a, Yuhuai Liu b, Yongjin Wang a, * a b
Peter Grünberg Research Centre, Nanjing University of Posts and Telecommunications, Nanjing, 210003, China Department of Electronics Engineering, Zhengzhou University, Science Road 100, Zhengzhou, 450001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2017 Received in revised form 9 May 2017 Accepted 21 May 2017
We propose a wafer-level procedure for the fabrication of 1.5-mm-diameter dual functioning InGaN/GaN multiple-quantum-well (MQW) diodes on a GaN-on-silicon platform for transferrable optoelectronics. Nitride semiconductor materials are grown on (111) silicon substrates with intermediate Al-composition step-graded buffer layers, and membrane-type MQW-diode architectures are obtained by a combination of silicon removal and III-nitride film backside thinning. Suspended MQW-diodes are directly transferred from silicon to foreign substrates such as metal, glass and polyethylene terephthalate by mechanically breaking the support beams. The transferred MQW-diodes display strong electroluminescence under current injection and photodetection under light irradiation. Interestingly, they demonstrate a simultaneous light-emitting light-detecting function, endowing the 1.5-mm-diameter MQW-diode with the capability of producing transferrable optoelectronics for adjustable displays, wearable optical sensors, multifunctional energy harvesting, flexible light communication and monolithic photonic circuit. © 2017 Published by Elsevier B.V.
Keywords: InGaN/GaN multiple-quantum-well diode Transferrable optoelectronics Simultaneous light-emitting light-detecting function Mechanical transfer
GaN-based multiple-quantum-well diodes (MQW-diodes) inherently serve multiple functions [1,2]. They can emit light under current injection and create a photocurrent when they are illuminated. In particular, the most intriguing phenomenon is the lightemitting light-detecting duality of the MQW-diodes, whereby the MQW-diodes simultaneously possess light-emitting light-detecting function [3]. It's of great interest to produce large-area and dual functioning MQW-diodes for transferrable optoelectronics, such as adjustable displays, wearable optical sensors and flexible light communications [4e7]. The excellent mechanical property afforded by GaN makes it feasible to transfer membrane-type GaN optoelectronic devices to foreign substrates. However, it is difficult to separate MQW-diodes grown on sapphire or SiC substrates using a wafer-level procedure [8,9]. To address this issue, Chung et al. demonstrated a transferrable thin-film light-emitting diode (LED) that uses graphene as a substrate for GaN growth [10]. Kobayashi et al. introduced BN layer as a release layer for mechanical transfer of GaN-based devices onto foreign substrates [11]. On the other hand, GaN grown on silicon substrate can offer a preferred choice to obtain suspended GaN optoelectronic devices by using the mature
* Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.optmat.2017.05.039 0925-3467/© 2017 Published by Elsevier B.V.
manufacturing technique of silicon [12,13]. Pivoted GaN microdisk and suspended GaN photonic crystal slab have been demonstrated by a combination of III-nitride semiconductor etching and silicon substrate undercut [14,15]. Comb-drive GaN micro-mirror and membrane gratings have been reported by using a double-sided process [16,17]. Since III-nitride epitaxial film is thick, an improved light efficiency could be achieved for membrane-type GaN-based devices with a controllable film thickness, which is produced by a combination of silicon removal and III-nitride film backside thinning. Here, we propose, fabricate and characterize the 1.5-mmdiameter dual functioning InGaN/GaN MQW-diodes on a GaN-onsilicon platform for transferrable optoelectronics. Suspended MQW-diodes that are obtained by a combination of silicon removal and III-nitride film backside thinning are mechanically transferred from silicon to foreign substrates such as metal, glass and polyethylene terephthalate (PET). The transferred MQW-diodes display strong electroluminescence under current injection and photodetection under illumination. In particular, they can demonstrate a simultaneous light-emitting light-detecting function. The transferrable MQW-diodes are implemented on a GaN-onsilicon platform, in which III-nitride epitaxial films are grown on (111) silicon substrate using a metal-organic chemical vapour phase deposition [18]. Firstly, the intermediate Al-composition step-
Z. Shi et al. / Optical Materials 72 (2017) 20e24
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Fig. 1. (A) Cross-sectional TEM image of an MQW-diode grown on silicon substrate; (B) High resolution TEM image of InGaN/GaN MQW active region.
graded buffer layers are grown to manage the misfit of the coefficient of thermal expansion and the lattice mismatch between GaN and silicon. The 3400-nm-thick n-type GaN is then grown with a Si doping of 8 1018 cm 3. After growing a 9-pair InGaN/GaN MQW structure, the 115-nm-thick p-type GaN layer with an Mg doping of 2.0 1020 cm 3 is finally grown, leading to a formation of MQWbased p-n junction diode structure. Fig. 1A shows the crosssectional transmission electron microscopy (TEM) image of IIInitride epitaxial films. Thick III-nitride epitaxial films, whose total thickness is around 5.8 mm, can support many optical modes to decrease the light efficiency. Therefore, suspended III-nitride film backside thinning is required to obtain ultrathin membrane architecture, which leads to an improved light efficiency [19,20]. Fig. 1B illustrates the high resolution TEM image of InGaN/GaN MQW active region. The InGaN layer is 3 nm thick and the GaN layer is 10 nm thick. Fig. 2A schematically shows the wafer-level transfer procedure
of the MQW-diode-on-silicon to foreign substrates. Firstly, the 1.5mm-diameter MQW-diodes are fabricated on a GaN-on-silicon platform using GaN-based diode process, in which the support beams are defined and etched by inductively coupled plasma reactive ion etching (ICP-RIE) with Cl2 and BCl3 hybrid gases at the flow rates of 10sccm and 25sccm, respectively. Subsequently, suspended MQW-diodes are formed by removing silicon substrate, and backside ICP-RIE is then conducted to thin suspended IIInitride film to a thickness of ~2 mm. Finally, the cleaned samples are put onto foreign substrate, and the support beams are mechanically broken using probes, leading to a direct transfer of suspended MQW-diode from silicon to foreign substrate. The transferred MQW-diodes can be further fixed. Fig. 2B illustrates the transferred MQW-diode on a PET substrate. The MQW-diode-onPET is flexible, endowing it with the possibility to develop adjustable displays and wearable optical sensors. Fig. 2C demonstrates the transferred MQW-diode on glass substrate. The MQW-diodes
Fig. 2. (A) Schematic illustrations of the fabrication and transfer procedures for the MQW-diode-on-silicon. (B) Transferred MQW-diode on a PET substrate. (C) Transferred MQWdiode on a glass substrate. (D) Light emission image of the MQW-diode-on-glass under current injection.
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can emit light under current injection. Fig. 2D shows the light emission image of the MQW-diode-on-glass. The 20 nm Ni/180 nm Au bilayers are simultaneously used as the p-electrode and a metal reflector, and the emitted light mainly escapes from the bottom surface. The lateral emission is scattered into air from the in-plane escape cones parallel to the wafer surface. Such large-area MQWdiodes, acting as either light-emitting device or light-detecting device, can be transferred to foreign substrates using a simple procedure. Regarding the transferred MQW-diodes, silicon absorption of the emitted light is eliminated after silicon substrate is etched away. Ultrathin device architecture that is generated by backside thinning of III-nitride epitaxial films can reduce the number of confined optical mode as well as the defect absorption of the emitted light. The MQW-diodes are mechanically transferred by breaking the support beams, which will not negatively influence the device performance. Taking all these into account, the light efficiency of the transferred MQW-diodes can be improved. As a result, after being transferred onto foreign substrate, the MQWdiodes have stronger EL intensity compared to the normal MQWdiodes before the substrate transferring. Moreover, the light can be emitted from the top and bottom surfaces of the transferred MQW-diodes simultaneously. The custom-designed fiber probe is controlled by a three dimensional stage to directly capture the emitted light from the MQW-diode under current injection. When the MQW-diode operates as a light emitter, the collected light is coupled to a 300-mm-diameter multimode fiber and sent to the
spectrometer. Fig. 3A shows the electroluminescence (EL) spectra of the suspended MQW-diode on GaN-on-Si substrate under various injection currents at room temperature. In this case, silicon substrate is removed and the MQW-diode is suspended by support beams. The emission peak wavelength exhibits a slight red-shift from 453.7 to 454.9 nm as the injection current is increased from 5 to 50 mA. Regarding the MQW-diode-on-glass, the membrane is free and the optically induced charge carriers can give rise to the internal electric field, which affects the light emission. In addition to increase the light emission intensity, the dominant emission peak exhibits a distinct red-shift from 455.8 to 461.9 nm as the injection current is gradually increased from 5 to 50 mA, as shown in Fig. 3B. After silicon removal, the suspension of MQW-diode is obtained by support beams, which can sustain the stress caused by heating effect at high injection current. On the other hand, the transferred MQW-diodes can be deformed freely in space when they are put on the glass substrate and are probed by two probes. The membrane is deformed due to heating effect at high injection current. Therefore, the red-shift shown in Fig. 3A is much smaller than that presented in Fig. 3B. When the MQW-diodes are transferred on metal substrates, the heating effect can be further managed because of their good thermal conductivity of metal substrates. Fig. 3C shows the light-current-voltage (L-I-V) characteristics of the MQW-diode-on-glass measured at room temperature. The plot of the light output power varies as a function of the injection current, which is in good agreement with the I-V plot of the MQW-diode-on-glass. Compared with organic LED devices
Fig. 3. (A) Room-temperature EL spectra of suspended MQW-diode under current injection. (B) EL spectra of the MQW-diode-on-glass under current injection. (C) L-I-V characteristics of the MQW-diode-on-glass measured at room temperature. (D) The modulation characteristics of the MQW-diode-on-glass at 3 Mbps.
Z. Shi et al. / Optical Materials 72 (2017) 20e24
[21,22], GaN-based MQW-diodes acting as light emitters may offer many advantages such as high efficiency, reliability and long-term stability. These large-area transferrable GaN-based MQW-diodes are promising for wearable displays as well as organic LED devices. When the MQW-diode-on-glass is directly driven by an arbitrary waveform generator to emit the modulated light, the out-of-plane light emission is collected by the fiber probe and sent to a photodiode module, leading to a free-space data transmission using visible light. Fig. 3D shows the transmitted and received pseudorandom binary sequence data at a transmission rate of 3 Mbps, which is limited by the used direct current probe. The modulation speed can be further improved by reducing the p-electrode size of the MQW-diode [23]. These results experimentally indicate that the 1.5-mm-diameter MQW-diode under light-emitting operation mode can be used as transferrable displays as well as light communications. The characteristics of the MQW-diode under illumination are shown in Fig. 4. When the 410 nm laser beam is coupled to the multimode fiber, the fiber probe can directly illuminate the MQWdiode, which operates as a photodiode to sense light. The photogenerated electron-hole pairs lead to an induced voltage that is directly characterized by a digital storage oscilloscope without amplification. The MQW-diode serves as a single light-detecting device at zero current injection. Fig. 4A shows that the induced voltages of the MQW-diode-on-glass without current injection. When the MQW-diodes are irradiated with a pulse illumination of
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500 ms, the induced voltage amplitude increases and finally reaches a maximum value. When the illumination is stopped, the voltage amplitude gradually comes back to the baseline, exhibiting a decay behavior. Since most of the illuminated light is reflected back by the large-area p-electrode, the light absorption efficiency can be further enhanced by directly irradiating the bottom surface of the MQW-diodes. Therefore, the 1.5-mm-diameter MQW-diodes can be used as flexible solar cells to harvest energy from ambient light sources by integrating a capacitor circuit [24]. The InGaN/GaN MQW active layer emits light under current injection, and the MQW-based p-n junction diode generates an induced photocurrent under light irradiation. In reality, both of them can coexist to generate the most intriguing phenomenon. The MQW-diodes simultaneously possess the light-emitting lightdetecting duality when the current injection and the light irradiation happen at the same time. Fig. 4B shows the light-emission and light-detection behavior of the MQW-diode-on-glass with the current injection of 40 mA. When the MQW-diodes are irradiated with a pulse illumination of 500 ms, the light-detecting plots exhibit the sharp decay processes because the photogenerated electronhole pairs are quickly cancelled by the injected electrons and holes. It means a faster light-detecting mechanism under this dual operation mode. The light-emitting light-detecting duality of the MQW-diode makes it possible to produce full-duplex data communication using visible light. Moreover, 1.5-mm-diameter membrane is large enough to integrate transmitter, waveguide,
Fig. 4. (A) Single light-detecting behavior of the MQW-diode-on-glass. (B) Simultaneous light-detecting light-emitting behavior of the MQW-diode-on-glass. (C) Continuously lightdetecting response of the MQW-diode-on-glass. (D) The induced voltage as a function of the incident light power for the MQW-diode-on-glass.
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modulator, and receiver into a single chip, opening feasible ways to fabricate transferrable monolithic photonic circuit [25,26]. Fig. 4C shows the continuous photoresponse of the MQW-diode-on-glass, in which the pulse function arbitrary generator modulates the repetitive illumination by square waves with 50% duty cycle at 1 kHz. Because the decay processes of the photogenerated electron-hole pairs are distinctly reduced by the injected carriers, the MQWdiode-on-glass exhibits a faster continuous photoresponse of dual-functioning operation than that of single-functioning operation. Fig. 4D shows the induced voltage as a function of the illumination power for the MQW-diode-on-glass. The induced voltage amplitude increases as the illumination power is gradually increased, demonstrating responsivity of ~100 V/W of singlefunctioning operation and ~10 V/W of dual-functioning operation at the 410 nm illuminations, respectively. GaN-on-silicon platform provides a feasible way to mechanically transfer large-area MQW-diodes from silicon to foreign substrates such as metal, glass and PET. During the fabrication procedure, thick epitaxial films could be thinned to a desired thickness, which leads to an improved light efficiency. Aiming at wearable displays and flexible light communications, the transferred 1.5-mm-diameter MQW-diodes operating as emitter exhibit strong electroluminescence as well as modulation speed. The photoresponse of the MQW-diodes under light irradiation offers a promising solution to develop GaN-based transferrable optical sensors and energy harvesting devices. In particular, the intriguing light-emitting light detecting duality of the MQW-diodes would create a variety of new possibilities for transferrable optoelectronics, especially for monolithic photonic circuit. Acknowledgements We thank the Peter Grünberg Research Centre for the use of their equipment. The work is jointly supported by the National Natural Science Foundation of China (61322112, 61531166004), research Projects (2016YFE0118400), and the Natural Science Foundation of Jiangsu Province (BE2016186). References [1] S. Nakamura, The roles of structural imperfections in InGaN-based blue lightemitting diodes and laser diodes, Science 281 (1998) 956e961.
[2] Y. Nanishi, Nobel prize in physics: the birth of the blue LED, Nat. Phot. 8 (2014) 884e886. [3] Y. Yang, et al., Multi-dimensional spatial light communication made with onchip InGaN photonic integration, Opt. Mater. 66 (2017) 659e663. [4] H. Matsubara, et al., GaN photonic-crystal surface-emitting laser at blue-violet wavelengths, Science 319 (2008) 445e447. [5] H. Nakazato, et al., Micro fluorescent analysis system integrating GaN-lightemitting-diode on a silicon platform, Lab. Chip 12 (2012) 3419e3425. [6] S.Y. Lee, et al., Water-resistant flexible GaN LED on a liquid crystal polymer substrate for implantable biomedical applications, Nano Energy 1 (2012) 145e151. [7] X. Dai, et al., Flexible light-emitting diodes based on vertical nitride nanowires, Nano Lett. 15 (2015) 6958e6964. [8] L. Guoqiang, et al., GaN-based light-emitting diodes on various substrates: a critical review, Rep. Prog. Phys. 79 (2016) 056501. [9] C.-H. Cheng, et al., Transferring the bendable substrateless GaN LED grown on a thin C-rich SiC buffer layer to flexible dielectric and metallic plates, J. Mater. Chem. C 5 (2017) 607e617. [10] K. Chung, et al., Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices, Science 330 (2010) 655e657. [11] Y. Kobayashi, et al., Layered boron nitride as a release layer for mechanical transfer of GaN-based devices, Nature 484 (2012) 223e227. [12] A. Dadgar, et al., Growth of blue GaN LED structures on 150-mm Si(111), J. Cryst. Growth 297 (2006) 279e282. [13] K. Maki, et al., Optically pumped lasing properties of (1101) InGaN/GaN stripe multiquantum wells with ridge cavity structure on patterned (001) Si substrates, Appl. Phys. Express 8 (2015) 022702. [14] H.W. Choi, et al., Lasing in GaN microdisks pivoted on Si, Appl. Phys. Lett. 89 (2006) 211101. ~ o, et al., Continuous wave blue lasing in III-nitride nanobeam cavity [15] N.V. Trivin on silicon, Nano Lett. 15 (2015) 1259e1263. [16] Y. Wang, et al., Comb-drive GaN micro-mirror on a GaN-on-silicon platform, J. Micromech. Microeng. 21 (035012) (2011). [17] Z. Shi, et al., Guided-mode resonances in GaN membrane grating, IEEE Photonics J. 6 (2014) 1e7. [18] Y. Sun, et al., Room-temperature continuous-wave electrically injected InGaNbased laser directly grown on Si, Nat. Phot. 10 (2016) 595e599. [19] J.J. Wierer, et al., III-nitride photonic-crystal light-emitting diodes with high extraction efficiency, Nat. Phot. 3 (2009) 163e169. [20] S. Noda, et al., Light-emitting diodes: photonic crystal efficiency boost, Nat. Phot. 3 (2009) 129e130. [21] T.-H. Han, et al., Extremely efficient flexible organic light-emitting diodes with modified graphene anode, Nat. Photonics 6 (2012) 105e110. [22] M.S. White, et al., Ultrathin, highly flexible and stretchable PLEDs, Nat. Photonics 7 (2013) 811e816. [23] R. Koester, et al., High-speed GaN/GaInN nanowire array light-emitting diode on Silicon(111), Nano Lett. 15 (2015) 2318e2323. [24] N. Oh, et al., Double-heterojunction nanorod light-responsive LEDs for display applications, Science 355 (2017) 616. [25] M.D. Brubaker, et al., On-chip optical interconnects made with gallium nitride nanowires, Nano Lett. 13 (2013) 374e377. [26] X. Lu, et al., Monolithic integration of enhancement-mode vertical driving transistors on a standard InGaN/GaN light emitting diode structure, Appl. Phys. Lett. 109 (2016) 053504.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Simultaneous dual-functioning InGaN/GaN multiple-quantum-well diode for transferrable optoelectronics Zheng Shi a, Jialei Yuan a, Shuai Zhang a, Yuhuai Liu b, Yongjin Wang a, * a b
Peter Grünberg Research Centre, Nanjing University of Posts and Telecommunications, Nanjing, 210003, China Department of Electronics Engineering, Zhengzhou University, Science Road 100, Zhengzhou, 450001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2017 Received in revised form 9 May 2017 Accepted 21 May 2017
We propose a wafer-level procedure for the fabrication of 1.5-mm-diameter dual functioning InGaN/GaN multiple-quantum-well (MQW) diodes on a GaN-on-silicon platform for transferrable optoelectronics. Nitride semiconductor materials are grown on (111) silicon substrates with intermediate Al-composition step-graded buffer layers, and membrane-type MQW-diode architectures are obtained by a combination of silicon removal and III-nitride film backside thinning. Suspended MQW-diodes are directly transferred from silicon to foreign substrates such as metal, glass and polyethylene terephthalate by mechanically breaking the support beams. The transferred MQW-diodes display strong electroluminescence under current injection and photodetection under light irradiation. Interestingly, they demonstrate a simultaneous light-emitting light-detecting function, endowing the 1.5-mm-diameter MQW-diode with the capability of producing transferrable optoelectronics for adjustable displays, wearable optical sensors, multifunctional energy harvesting, flexible light communication and monolithic photonic circuit. © 2017 Published by Elsevier B.V.
Keywords: InGaN/GaN multiple-quantum-well diode Transferrable optoelectronics Simultaneous light-emitting light-detecting function Mechanical transfer
GaN-based multiple-quantum-well diodes (MQW-diodes) inherently serve multiple functions [1,2]. They can emit light under current injection and create a photocurrent when they are illuminated. In particular, the most intriguing phenomenon is the lightemitting light-detecting duality of the MQW-diodes, whereby the MQW-diodes simultaneously possess light-emitting light-detecting function [3]. It's of great interest to produce large-area and dual functioning MQW-diodes for transferrable optoelectronics, such as adjustable displays, wearable optical sensors and flexible light communications [4e7]. The excellent mechanical property afforded by GaN makes it feasible to transfer membrane-type GaN optoelectronic devices to foreign substrates. However, it is difficult to separate MQW-diodes grown on sapphire or SiC substrates using a wafer-level procedure [8,9]. To address this issue, Chung et al. demonstrated a transferrable thin-film light-emitting diode (LED) that uses graphene as a substrate for GaN growth [10]. Kobayashi et al. introduced BN layer as a release layer for mechanical transfer of GaN-based devices onto foreign substrates [11]. On the other hand, GaN grown on silicon substrate can offer a preferred choice to obtain suspended GaN optoelectronic devices by using the mature
* Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.optmat.2017.05.039 0925-3467/© 2017 Published by Elsevier B.V.
manufacturing technique of silicon [12,13]. Pivoted GaN microdisk and suspended GaN photonic crystal slab have been demonstrated by a combination of III-nitride semiconductor etching and silicon substrate undercut [14,15]. Comb-drive GaN micro-mirror and membrane gratings have been reported by using a double-sided process [16,17]. Since III-nitride epitaxial film is thick, an improved light efficiency could be achieved for membrane-type GaN-based devices with a controllable film thickness, which is produced by a combination of silicon removal and III-nitride film backside thinning. Here, we propose, fabricate and characterize the 1.5-mmdiameter dual functioning InGaN/GaN MQW-diodes on a GaN-onsilicon platform for transferrable optoelectronics. Suspended MQW-diodes that are obtained by a combination of silicon removal and III-nitride film backside thinning are mechanically transferred from silicon to foreign substrates such as metal, glass and polyethylene terephthalate (PET). The transferred MQW-diodes display strong electroluminescence under current injection and photodetection under illumination. In particular, they can demonstrate a simultaneous light-emitting light-detecting function. The transferrable MQW-diodes are implemented on a GaN-onsilicon platform, in which III-nitride epitaxial films are grown on (111) silicon substrate using a metal-organic chemical vapour phase deposition [18]. Firstly, the intermediate Al-composition step-
Z. Shi et al. / Optical Materials 72 (2017) 20e24
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Fig. 1. (A) Cross-sectional TEM image of an MQW-diode grown on silicon substrate; (B) High resolution TEM image of InGaN/GaN MQW active region.
graded buffer layers are grown to manage the misfit of the coefficient of thermal expansion and the lattice mismatch between GaN and silicon. The 3400-nm-thick n-type GaN is then grown with a Si doping of 8 1018 cm 3. After growing a 9-pair InGaN/GaN MQW structure, the 115-nm-thick p-type GaN layer with an Mg doping of 2.0 1020 cm 3 is finally grown, leading to a formation of MQWbased p-n junction diode structure. Fig. 1A shows the crosssectional transmission electron microscopy (TEM) image of IIInitride epitaxial films. Thick III-nitride epitaxial films, whose total thickness is around 5.8 mm, can support many optical modes to decrease the light efficiency. Therefore, suspended III-nitride film backside thinning is required to obtain ultrathin membrane architecture, which leads to an improved light efficiency [19,20]. Fig. 1B illustrates the high resolution TEM image of InGaN/GaN MQW active region. The InGaN layer is 3 nm thick and the GaN layer is 10 nm thick. Fig. 2A schematically shows the wafer-level transfer procedure
of the MQW-diode-on-silicon to foreign substrates. Firstly, the 1.5mm-diameter MQW-diodes are fabricated on a GaN-on-silicon platform using GaN-based diode process, in which the support beams are defined and etched by inductively coupled plasma reactive ion etching (ICP-RIE) with Cl2 and BCl3 hybrid gases at the flow rates of 10sccm and 25sccm, respectively. Subsequently, suspended MQW-diodes are formed by removing silicon substrate, and backside ICP-RIE is then conducted to thin suspended IIInitride film to a thickness of ~2 mm. Finally, the cleaned samples are put onto foreign substrate, and the support beams are mechanically broken using probes, leading to a direct transfer of suspended MQW-diode from silicon to foreign substrate. The transferred MQW-diodes can be further fixed. Fig. 2B illustrates the transferred MQW-diode on a PET substrate. The MQW-diode-onPET is flexible, endowing it with the possibility to develop adjustable displays and wearable optical sensors. Fig. 2C demonstrates the transferred MQW-diode on glass substrate. The MQW-diodes
Fig. 2. (A) Schematic illustrations of the fabrication and transfer procedures for the MQW-diode-on-silicon. (B) Transferred MQW-diode on a PET substrate. (C) Transferred MQWdiode on a glass substrate. (D) Light emission image of the MQW-diode-on-glass under current injection.
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can emit light under current injection. Fig. 2D shows the light emission image of the MQW-diode-on-glass. The 20 nm Ni/180 nm Au bilayers are simultaneously used as the p-electrode and a metal reflector, and the emitted light mainly escapes from the bottom surface. The lateral emission is scattered into air from the in-plane escape cones parallel to the wafer surface. Such large-area MQWdiodes, acting as either light-emitting device or light-detecting device, can be transferred to foreign substrates using a simple procedure. Regarding the transferred MQW-diodes, silicon absorption of the emitted light is eliminated after silicon substrate is etched away. Ultrathin device architecture that is generated by backside thinning of III-nitride epitaxial films can reduce the number of confined optical mode as well as the defect absorption of the emitted light. The MQW-diodes are mechanically transferred by breaking the support beams, which will not negatively influence the device performance. Taking all these into account, the light efficiency of the transferred MQW-diodes can be improved. As a result, after being transferred onto foreign substrate, the MQWdiodes have stronger EL intensity compared to the normal MQWdiodes before the substrate transferring. Moreover, the light can be emitted from the top and bottom surfaces of the transferred MQW-diodes simultaneously. The custom-designed fiber probe is controlled by a three dimensional stage to directly capture the emitted light from the MQW-diode under current injection. When the MQW-diode operates as a light emitter, the collected light is coupled to a 300-mm-diameter multimode fiber and sent to the
spectrometer. Fig. 3A shows the electroluminescence (EL) spectra of the suspended MQW-diode on GaN-on-Si substrate under various injection currents at room temperature. In this case, silicon substrate is removed and the MQW-diode is suspended by support beams. The emission peak wavelength exhibits a slight red-shift from 453.7 to 454.9 nm as the injection current is increased from 5 to 50 mA. Regarding the MQW-diode-on-glass, the membrane is free and the optically induced charge carriers can give rise to the internal electric field, which affects the light emission. In addition to increase the light emission intensity, the dominant emission peak exhibits a distinct red-shift from 455.8 to 461.9 nm as the injection current is gradually increased from 5 to 50 mA, as shown in Fig. 3B. After silicon removal, the suspension of MQW-diode is obtained by support beams, which can sustain the stress caused by heating effect at high injection current. On the other hand, the transferred MQW-diodes can be deformed freely in space when they are put on the glass substrate and are probed by two probes. The membrane is deformed due to heating effect at high injection current. Therefore, the red-shift shown in Fig. 3A is much smaller than that presented in Fig. 3B. When the MQW-diodes are transferred on metal substrates, the heating effect can be further managed because of their good thermal conductivity of metal substrates. Fig. 3C shows the light-current-voltage (L-I-V) characteristics of the MQW-diode-on-glass measured at room temperature. The plot of the light output power varies as a function of the injection current, which is in good agreement with the I-V plot of the MQW-diode-on-glass. Compared with organic LED devices
Fig. 3. (A) Room-temperature EL spectra of suspended MQW-diode under current injection. (B) EL spectra of the MQW-diode-on-glass under current injection. (C) L-I-V characteristics of the MQW-diode-on-glass measured at room temperature. (D) The modulation characteristics of the MQW-diode-on-glass at 3 Mbps.
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[21,22], GaN-based MQW-diodes acting as light emitters may offer many advantages such as high efficiency, reliability and long-term stability. These large-area transferrable GaN-based MQW-diodes are promising for wearable displays as well as organic LED devices. When the MQW-diode-on-glass is directly driven by an arbitrary waveform generator to emit the modulated light, the out-of-plane light emission is collected by the fiber probe and sent to a photodiode module, leading to a free-space data transmission using visible light. Fig. 3D shows the transmitted and received pseudorandom binary sequence data at a transmission rate of 3 Mbps, which is limited by the used direct current probe. The modulation speed can be further improved by reducing the p-electrode size of the MQW-diode [23]. These results experimentally indicate that the 1.5-mm-diameter MQW-diode under light-emitting operation mode can be used as transferrable displays as well as light communications. The characteristics of the MQW-diode under illumination are shown in Fig. 4. When the 410 nm laser beam is coupled to the multimode fiber, the fiber probe can directly illuminate the MQWdiode, which operates as a photodiode to sense light. The photogenerated electron-hole pairs lead to an induced voltage that is directly characterized by a digital storage oscilloscope without amplification. The MQW-diode serves as a single light-detecting device at zero current injection. Fig. 4A shows that the induced voltages of the MQW-diode-on-glass without current injection. When the MQW-diodes are irradiated with a pulse illumination of
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500 ms, the induced voltage amplitude increases and finally reaches a maximum value. When the illumination is stopped, the voltage amplitude gradually comes back to the baseline, exhibiting a decay behavior. Since most of the illuminated light is reflected back by the large-area p-electrode, the light absorption efficiency can be further enhanced by directly irradiating the bottom surface of the MQW-diodes. Therefore, the 1.5-mm-diameter MQW-diodes can be used as flexible solar cells to harvest energy from ambient light sources by integrating a capacitor circuit [24]. The InGaN/GaN MQW active layer emits light under current injection, and the MQW-based p-n junction diode generates an induced photocurrent under light irradiation. In reality, both of them can coexist to generate the most intriguing phenomenon. The MQW-diodes simultaneously possess the light-emitting lightdetecting duality when the current injection and the light irradiation happen at the same time. Fig. 4B shows the light-emission and light-detection behavior of the MQW-diode-on-glass with the current injection of 40 mA. When the MQW-diodes are irradiated with a pulse illumination of 500 ms, the light-detecting plots exhibit the sharp decay processes because the photogenerated electronhole pairs are quickly cancelled by the injected electrons and holes. It means a faster light-detecting mechanism under this dual operation mode. The light-emitting light-detecting duality of the MQW-diode makes it possible to produce full-duplex data communication using visible light. Moreover, 1.5-mm-diameter membrane is large enough to integrate transmitter, waveguide,
Fig. 4. (A) Single light-detecting behavior of the MQW-diode-on-glass. (B) Simultaneous light-detecting light-emitting behavior of the MQW-diode-on-glass. (C) Continuously lightdetecting response of the MQW-diode-on-glass. (D) The induced voltage as a function of the incident light power for the MQW-diode-on-glass.
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modulator, and receiver into a single chip, opening feasible ways to fabricate transferrable monolithic photonic circuit [25,26]. Fig. 4C shows the continuous photoresponse of the MQW-diode-on-glass, in which the pulse function arbitrary generator modulates the repetitive illumination by square waves with 50% duty cycle at 1 kHz. Because the decay processes of the photogenerated electron-hole pairs are distinctly reduced by the injected carriers, the MQWdiode-on-glass exhibits a faster continuous photoresponse of dual-functioning operation than that of single-functioning operation. Fig. 4D shows the induced voltage as a function of the illumination power for the MQW-diode-on-glass. The induced voltage amplitude increases as the illumination power is gradually increased, demonstrating responsivity of ~100 V/W of singlefunctioning operation and ~10 V/W of dual-functioning operation at the 410 nm illuminations, respectively. GaN-on-silicon platform provides a feasible way to mechanically transfer large-area MQW-diodes from silicon to foreign substrates such as metal, glass and PET. During the fabrication procedure, thick epitaxial films could be thinned to a desired thickness, which leads to an improved light efficiency. Aiming at wearable displays and flexible light communications, the transferred 1.5-mm-diameter MQW-diodes operating as emitter exhibit strong electroluminescence as well as modulation speed. The photoresponse of the MQW-diodes under light irradiation offers a promising solution to develop GaN-based transferrable optical sensors and energy harvesting devices. In particular, the intriguing light-emitting light detecting duality of the MQW-diodes would create a variety of new possibilities for transferrable optoelectronics, especially for monolithic photonic circuit. Acknowledgements We thank the Peter Grünberg Research Centre for the use of their equipment. The work is jointly supported by the National Natural Science Foundation of China (61322112, 61531166004), research Projects (2016YFE0118400), and the Natural Science Foundation of Jiangsu Province (BE2016186). References [1] S. Nakamura, The roles of structural imperfections in InGaN-based blue lightemitting diodes and laser diodes, Science 281 (1998) 956e961.
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