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We propose, fabricate and demonstrate on-chip photonic integration of suspended InGaN/GaN multiple quantum wells (MQWs) devices on the GaN-on-silicon platform. Both silicon removal and back wafer etching are conducted to obtain membrane-type devices, and suspended waveguides are used for the connection between p-n junction InGaN/GaN MQWs devices. As an in-plane data transmission system, the middle p-n junction InGaN/GaN MQWs device is used as a light emitting diode (LED) to deliver signals by modulating the intensity of the emitted light, and the other two devices act as photodetectors (PDs) to sense the light guided by the suspended waveguide and convert the photons into electrons, achieving 1 × 2 in-plane information transmission via visible light. Correspondingly, the three devices can function as independent PDs to realize multiple receivers for free space visible light communication. Further, the on-chip photonic platform can be used as an active electro-optical sensing system when the middle device acts as a PD and the other two devices serve as LEDs. The experimental results show that the auxiliary LED sources can enhance the amplitude of the induced photocurrent.
© 2016 Optical Society of America
1. Introduction
Light generation, propagation, detection, amplification and modulation of semiconductor materials have been investigated for more than a decade, and the integration of various photonic devices, including photodetectors (PDs), microcavity lasers, high-electron-mobility transistor (HEMT), metal-oxide-semiconductor fieldeffect transistor (MOSFET), waveguides, and light emitting diodes (LEDs), is of great interest for monolithically integrating onto compact photonic platforms with on-chip technologies [1–5 ]. Monolithic integration of AlGaN/GaN HEMTs and blue LEDs on a GaN-on-sapphire platform has been achieved by metal organic chemical vapor deposition selective growth technique [6]. LED and PD that are optically coupled by waveguides or free space optical transmission have been integrated on-chip to achieve an intrachip optical interconnect based on nanowire photonics and sophisticated growth techniques [7,8 ]. To realize the monolithic integration of LED, planar waveguide and PD, a material system with multiple optoelectronic functions and a similar fabrication process of various optical components are required [9,10 ]. The selectable functionalities of light emission, transmission and detection can be provided by GaN materials, which makes it possible to produce complicated monolithic photonic integrated systems. High-quality GaN-based epitaxial films on silicon substrates have been produced by managing the thermal and lattice mismatches between GaN and silicon [11,12 ]. The obvious advantages, such as use of the cheaper, widely available silicon wafer and the ability to use automated back-end manufacturing tools in silicon fabs [13,14 ], make mass production of monolithic photonic integration possible. The p-n junction, GaN-based InGaN/GaN multiple quantum wells (MQWs) are excellent light emitters. However, from a material growth point of view, thick epitaxial films are used to compensate for the large thermal and lattice mismatches between GaN and silicon. Therefore, some emitted light is absorbed by the silicon substrate and several optical modes of emitted light are confined inside the thick epitaxial films [15]. On the other hand, planar waveguide devices can be developed due to the trapped light in the visible wavelength range, and the light is effectively confined and propagated within the waveguide [16,17 ]. The GaN-based LED can be switched on and off at very high rates [18,19 ] and can act as a photodetector under zero or reverse voltages. As a result, the monolithic integration of multifunctional photonic devices is promising to provide novel functions in visible light communication.
Here, we demonstrate the double-side process for the fabrication of suspended membrane monolithic photonic integration of the LEDs, waveguides and PDs on the GaN-on-silicon platform by removing the silicon substrate and back wafer thinning of the epitaxial film. The suspended p-n junction InGaN/GaN MQWs are used for the fabrication of the devices. After the removal of the silicon substrate and back wafer thinning of the epitaxial film, the carrier concentration is increased, the spreading resistance is significantly decreased, and the residual stress between the silicon substrate and the epitaxial films is released, leading to improvements in the electronic performances of the suspended membrane LED [20,21 ]. The high-index contrast between GaN materials and the air of the suspended membrane device results in high optical confinement of the illumination light in the PD to enhance its performance. The proposed suspended membrane monolithic photonic integration plays an important role in various fields, such as planar optical communication and active electro-optical (EO) sensing.
2. Experimental results and discussion
The integration of the emitter, waveguide and photodetector is implemented on a commercial GaN-on-silicon wafer from Lattice Power Corporation, Jiangxi, China and is based on a p-n junction In GaN/GaN multiple quantum wells (MQWs) structure. The layer structures comprise a ~220 nm-thick p-GaN layer, ~250 nm InGaN/GaN MQWs, ~3.2 μm-thick n-GaN, a ~400 nm undoped GaN layer, a ~900 nm-thick Al(Ga)N buffer layer and a 200 μm silicon substrate. The total thickness of the top device layer is ~4.97 μm. The suspended devices are fabricated on a GaN-on-silicon platform by a double-sided process. Silicon removal is performed to achieve suspended p-n junction devices and highly confined GaN-based waveguides. Back wafer thinning of the suspended membrane is then conducted to obtain ultrathin devices with controllable membrane thickness, leading to improved optical performance. Figure 1(a) shows an optical micrograph of the fabricated devices. Three suspended p-n junction InGaN/GaN MQWs devices are achieved on a suspended membrane, and the devices are connected by three 80μm long, 10μm wide and 3μm thick suspended waveguides. Both silicon substrate and epitaxial films are completely removed among the waveguides and thus, no light can reflect back, leading to the black regions. Three devices have the similar structures, the gap between mesa and n-electrode is 10μm, and the p-electrode with 80μm in diameter is fabricated on a 115μm × 115μm mesa in the suspended membrane region. Figure 1(b) illustrate a schematic of the side view of the fabricated integrated devices. silicon substrate underneath the device region is totally removed to form suspended device architecture.
Fig. 1 (a) Optical micrograph of the fabricated integrated devices obtained from the top. (b) Schematic of the side view of the fabricated integrated devices.
The electrical characteristics of the middle fabricated device operated under LED mode were investigated. An Agilent B1500A semiconductor device analyzer was used to measure the current-voltage (I-V) characteristic curve of the middle suspended membrane LED. Figure 2(a) shows a typical rectifying behavior of the middle suspended membrane LED with a turn-on voltage of ~2.8 V. A leakage current of 53.3 pA was measured at −4 V, and currents of 7.54 nA, 3.24 mA and 20.1 mA were measured at the forward voltages of 2.0 V, 4.5 V and 6.5 V, respectively. Above the turn-on voltage, the current increased rapidly with increasing applied bias voltage, and the current increased to 50.8 mA under the forward bias voltage of 8 V, which indicates excellent I-V performance. The existence of high dislocation density is related to the large lattice mismatch between GaN and silicon, which results in a reduction of the carrier mobility in epitaxial film, and the negative influence of the spreading resistance [22]. The removal of the silicon substrate and back wafer thinning of the epitaxial film release the compressive stress between the III–V epitaxial layers and the silicon substrate. Then, the carrier concentration and the conductivity of the GaN epitaxial layer are both increased [23], and the I-V performance of the suspended membrane LED is effectively enhanced.
Fig. 2 (a) Measured I-V characteristics of the middle LED. (b) Measured EL spectra of the middle LED.
The optical characteristics of the middle suspended membrane device operated under LED mode, as a function of forward bias voltage in the visible range, were investigated from the topside with an Ocean Optics HR4000 spectrometer. Figure 2(b) illustrates that the EL spectra of the middle suspended membrane LED at forward bias voltage levels of 3 V to 6 V. The two insets show the light emission images of the middle LED at 4 V and 5 V, respectively. With increasing forward bias voltage, the emission intensity is enhanced. The dominant EL peak, which is attributed to the excitation of the InGaN/GaN MQWs active layers, was measured at 461 nm and is stable. After removing the silicon substrate and back wafer thinning of the epitaxial film, the silicon absorption of the emitted light is eliminated, and the emitted light is able to escape to free space from the backside. Some of the emitted light is reflected from the bottom membrane interface to the topside [21]. Moreover, the confined modes inside the epitaxial film are reduced by the back wafer thinning of the epitaxial film [24,25 ]. Hence, as a suspended membrane LED, the light efficiency is significantly improved, which is beneficial for the application of visible light communication. Data transmission in visible light communication is achieved by modulating the intensity of the emitted light to transform an electronic signal to an optical signal [14,26 ]. The ratio of the dominant EL peak between 6 V and 3 V is approximately 3.7, indicating that the output intensity can be strongly modulated by the forward voltage. These features of the suspended membrane LED enable its application for on-chip planar and free-space visible light communication. The three fabricated devices have similar electrical characteristics and optical characteristics when they are operated under LED mode.
As a single emitter, dual receiver system, the coupling responses among the LED, six waveguides and two PDs were characterized by an Agilent B1500A semiconductor device analyzer. The power-dependent photoresponses of the left device operated under PD mode, with the middle device operated under LED mode at different applied voltage levels of 2.8 V to 3.6 V, are illustrated in Fig. 3(a) . With increasing applied voltage of the middle LED, the current-voltage (I-V) curves of the left PD shift to higher bias voltage levels. Negative currents are observed. The maximum negative currents were measured at −103.0 nA with a bias voltage of 1.68 V and −163.8 nA with a bias voltage of 1.72 V when the middle LED was at the applied voltage levels of 3.4 V and 3.6 V, respectively. The experimental results of the right PD are similar to the left PD, indicating that the light signals of the middle LED can be received by the PDs located on both sides. Although the LED has a strong out-of-plane emission, the in-plane light propagation through the waveguide plays the dominant role in the induced photocurrent because both the LED and the PD are fabricated on the same membrane.
Fig. 3 (a) The induced photocurrent of the left PD versus the forward voltage levels of the middle LED. (b) Waveform of the left PD. (c) Eye diagrams of the left PD. (d) Schematic of the real time data transmission.
Moreover, the communication functions of the monolithic photonic integration were further investigated. The middle LED was directly driven by an Agilent 33522A arbitrary waveform generator. A random binary sequence with bit rate 1 Mbps, offset 3.5 V and amplitude 1 V was outputted to the middle LED. With the middle LED delivering light signals into the six waveguides, the left and right PDs converted the photons back into electrons to complete the information process. The signals received by the left PDs with a bias voltage of 0 V are illustrated in Fig. 3(b). Peaks and troughs are clear despite some attenuation, indicating on-chip optical coupling among the LED and the PD connected by several waveguides. Figure 3(c) shows the eye diagram of the left PD. Open eyes are clearly shown at 1 Mbps. The same experiments were conducted on the right PD, and similar results were obtained. Subsequently, real time 1 × 2 data transmission based on the monolithic photonic integration system was realized. A schematic of the real time data transmission is shown in Fig. 3(d). These results show that the monolithic integrated devices are capable of realizing on-chip multiple-input-multiple-output (MIMO) optical interconnects in the field of planar visible light communication. The light coupling efficiency of the LED with the waveguides and PDs can be significantly improved by proper design of the devices, such as optimizing the waveguide structures and adjusting the electrode sizes. The sensitivity of the PD can be enhanced by proper auxiliary illumination or bias voltage.
Photoresponse measurements are conducted to characterize the three fabricated devices operated under PD mode by a combination of an Agilent B1500A semiconductor device analyzer and an iHR320 monochromator. As p-n junction diodes, rectifying behavior is exhibited for the three photodetectors without illumination. Distinct currents of the three PDs are observed for bias voltages higher than 1.84 V, 1.80 V and 1.76 V, respectively. In the measured range, the currents of the three PDs reach 7.64 nA, 7.06 nA and 8.74 nA under the forward voltage of 2 V, whereas the currents are −10.3 nA, −18.2 nA and −9.95 nA for the three LEDs, which are illuminated with a 450 nm, 70 μW light beam generated by an iHR320 monochromator from the topside at room temperature, as shown in Figs. 4(a)-4(c) . These results indicate that because they are independent, the PDs can effectively sense the transmitted light that interacts with an external object, and the information about the object can be derived. Subsequently, a DaHeng GCI-73 mechanical shutter and an iHR320 monochromator were used to measure the photoresponse characteristics of the three independent PDs. Three PDs at 0 V bias voltages are illuminated by a continuous light beam with 450 nm wavelength and 70 μW illumination power. A rectangular illumination pulse is generated by a mechanical shutter with a 0.2 s on and 0.2 s off switching cycle. The photocurrents are approximated as a rectangular pulse, and the duration is the same as the illumination pulse, as shown in Figs. 4(d)-4(f). The light currents are approximately −24 nA, −36 nA and −27 nA with the mechanical switch open, whereas the dark currents are nearly 0 A with the mechanical switch close. Hence, the on/off ratios between the light currents and dark currents are up to three orders of magnitude when the illumination power is only 70 μW. Compared to silicon bulk devices, the suspended membrane design has a relatively large on/off ratio due to the characteristics of the large band gap InGaN/GaN MQWs, and the MQWs PD is promising for improved signal to noise ratios [9,27 ].Furthermore, some of the illumination light is confined into the suspended membrane due to the large refractive index contrast between GaN and air and is not absorbed by silicon substrate under the epitaxial films, in contrast to a PD with a silicon substrate [28]. The suspended membrane PD’s detection efficiency is significantly enhanced.
Fig. 4 (a)(b)(c) I-V characteristics of the three PDs versus the illumination power. (d)(e)(f) Induced photocurrent temporal traces of the three PDs.
Figure 5(a) shows the induced current of the middle device operated under PD mode with illumination by a 450 nm, 70 μW light source generated by an iHR320 monochromator. The left device operated under LED mode was used as an auxiliary light source at different applied voltage levels of 2 V to 5 V. The induced photocurrent of the middle PD with auxiliary illumination is remarkably enhanced in comparison with the PD without auxiliary illumination. The left LED is not lit because the forward voltage of 2 V is lower than the turn-on voltage. With increasing applied voltage of the left LED, the negative currents gradually increase. Negative currents were measured when the applied voltage was higher than 4 V. The maximum negative currents were observed at −0.432 μA with a forward bias voltage of 1.8 V and at −1.417 μA with a forward bias voltage of 2.0 V when the left LED had an applied voltage level of 4 V and 5 V, respectively. To further investigate the auxiliary illumination effects on the photoresponse of the PDs, the left and right devices under LED mode are simultaneously subjected to forward voltage levels of 2 V to 5 V. The measured negative currents reached −0.742 μA with a forward bias voltage of 1.82 V and −2.67 μA with a forward bias voltage of 2.0 V, as illustrated in Fig. 5(b). The negative current of the middle PD with the two LEDs under a forward voltage of 2 V is 1.88 times higher than that with a single LED under a forward voltage of 2 V, which demonstrates the positive influence of auxiliary illumination. These results indicate that the induced photocurrent of the photodetector for novel active EO sensors can be amplified by the integrated devices. Moreover, the auxiliary source can operate at the range where the emitted light intensity and the voltage level have an approximate linear relation when a suitable bias point is used [29].
Fig. 5 (a) I-V characteristics of the middle PD versus the illumination power and one auxiliary light source. (b) I-V characteristics of the middle PD versus the illumination power and two auxiliary light sources.
3. Conclusions
In conclusion, we describe the monolithic photonic integration of the LED, waveguide and PD on a GaN-on-silicon platform fabricated by the double-side process. The optical coupling among the middle LED and the PDs located on both sides by planar waveguides has been achieved, which demonstrates the realization of two channels for in-plane photon information transmission in visible light communication. With three suspended p-n junction InGaN/GaN MQWs PDs, spatial light signals can be independently perceived, and three channels for optical signal detection and reception are accomplished. Moreover, the emission of the LEDs is useful to amplify the sensitivity of the middle PD, and the detection of spatial optical signals is achieved more easily.
Acknowledgments
This work is jointly supported by the National Natural Science Foundation of China (NSFC) (61322112, 61531166004) and research project (2014CB360507, RLD201204, BJ211026).
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