1. Introduction

Group-III-nitride compounds, represented by (Al, In, Ga)N, have been widely used in the fabrication of photodetectors over the last decade. The intrinsic properties of III-nitrides including high drift velocity, large carrier mobility, strong optical absorption near the band edge and high resistance to radiation compared to other materials, make them highly suitable for the development of photodetectors that cover both visible and ultraviolet ranges [1–4]. However, some issues have been reported by different research groups, such as indistinct absorption edges, poor detection contrasts, high dark currents, or bad rectifying contacts in bulk-based InGaN devices due to alloy segregation, clustering, high carrier concentration near the surface and nanoscale inhomogeneities [5–9]. In order to circumvent the aforementioned problems, photodetectors based on InGaN/GaN multiple quantum wells (MQWs) are considered as an alternative to bulk-based devices. MQW-based photodetectors have been primarily studied in the ranges of UV and infrared [10–13]. Remarkable advantages are offered by the use of MQWs in the active region of standard structures, giving low dark currents, an easier integration of emitter and detector functions, and extra flexibility to tune the detection-edge by adjusting well widths and the layer composition, and the possibility of the fabrication of multicolor detection pixels [14–17]. Out-of-plane light can couple into the in-plane waveguide with selectable functionalities operating either in polarization beamsplitter mode [18] or in wavelength-division multiplexer mode [19,20]. Moreover, the suspended InGaN/GaN MQWs diode can operate either as a light-emitting diode (LED) or photodiode [21]. In our previous work [22], we demonstrated a passive photonic integration of the optically-pumped region and six waveguides with different propagation directions. Active photonic integration of InGaN/GaN MQWs diodes with waveguide have been presented in Refs [23–25]. It is of great interest to integrate waveguide and photodetector on a single chip for integrated optics. Furthermore, GaN-on-silicon is an attractive platform for the monolithic large-scale integration of photonic and electronic circuits. It can potentially allow monolithic integration of photodetector, waveguide and transceiver circuits, which may greatly improve the device performance and reliability.

Here, we develop a wafer-level technique to make the on-chip photonic integration of photodetector and waveguide on the GaN-on-Si platform. Suspended device architecture, which is obtained by a combination of silicon removal and back wafer etching of suspended membrane, leads to a highly confined structure to manipulate light [26–29]. Scanning electron microscope (SEM) and atomic force microscope (AFM) are used to analyze the physical characteristics of fabricated devices. Light illumination measurements are conducted to evaluate the suspended waveguide photodetector through a micro-reflectance setup. The rough surface naturally formed in the process of backside etching is beneficial to light insertion with an improved performance of photodetectors. Furthermore, the metal contacts could be used as mirrors to reflect part of incident light, resulting in an enhanced photovoltaic effect of the fabricated device. The incident light coupling into the suspended waveguide is guided and detected by the photodetector. Both the responsivity spectra and the on/off switching performance are experimentally characterized.

2. Experimental results and discussion

The proposed integrated devices in this study are fabricated by a double-sided process on the GaN-on-silicon platform. The epitaxial films consisting of p-GaN layer, InGaN/GaN MQWs, n-GaN layer, undoped GaN layer and Al(Ga)N buffer layer are grown on a silicon substrate. The thickness of the p-GaN layer, InGaN/GaN MQWs, n-GaN layer, undoped GaN layer, Al(Ga)N buffer layer and silicon substrate are ~220 nm, ~250 nm, ~3.2 µm, ~400 nm, ~900 nm and 200 µm, respectively. For the p-n junction InGaN/GaN MQW device, the top layer is firstly defined by photolithography and etched down to n-GaN to form the isolation mesa. The 20 nm Ni/ 180 nm Au metallization stacks were evaporated onto both p- and n-electrode regions using the lift-off technique, serving as the metal contacts. After the lift-off process, the sample was annealed at 500°C in a N₂ atmosphere for 5 min. The waveguide was subsequently patterned and etched by reactive ion etching (RIE) with Cl₂ and BCl₃ hybrid plasma at flow rates of 10 and 25 sccm, respectively. Silicon removal was conducted by deep RIE of silicon to obtain the membrane-type device structure, and back wafer thinning was then performed. After removing the residual photoresist, the suspended waveguide photodetector is finally obtained.

Figure 1(a) illustrates the optical microscope image of the fabricated devices obtained from the silicon side. The metallization stacks can be clearly observed because GaN is transparent in the visible wavelength region. The incident light can illuminate the sample through the hole, and the metallization stacks are then used as the bottom metal mirror to reflect the incoming light back for re-absorption. Figure 1(b) shows a scanning electron microscope (SEM) image of the fabricated device structure. The gap between mesa and n-electrode is 10 μm. These suspended electrodes are connected to the contact pads without silicon removal. Figure 1(c) shows the SEM image of suspended waveguide with a waveguide width of 10 μm. When the out-of-plane light illuminates the waveguide, the light can couple into the waveguide and propagate as a confined optical mode, leading to the realization of the in-plane light coupling between waveguide and InGaN/GaN MQWs diode [28,29]. Figure 1(d) demonstrates the three dimensional AFM image of electrode regions, where the measured heights are 600 nm and 200 nm for the isolation mesa and the metallization stacks, respectively. And the surface of the suspended membrane has a root-mean-square roughness of 1.64 nm, which means the surface is smooth.

 

Fig. 1 (a) Optical micrograph of the suspended waveguide photodetector obtained from silicon side; (b) SEM image of the fabricated photodetector and waveguide; (c) SEM image of the suspended waveguide; (d) AFM image of the electrode region.

Download Full Size | PDF

Figure 2(a) shows a micro-reflectance setup that is built up to characterize the suspended device. When a Bentham WLS100 white light source is used, a fiber-coupled light beam is focused onto the sample through an objective with a numerical aperture of 0.75, and the reflected light can be sent to a CCD camera for imaging the sample or a fiber-coupled Ocean Optics USB4000 spectrometer (resolution: 0.1 nm) for measuring the reflectance spectra. In association with a Horiba iHR320 monochromator (resolution: 0.06 nm) and Agilent B1500A semiconductor device analyzer, the induced photocurrent of the suspended device can be measured by the setup. Figure 2(b) and 2(c) illustrate the schematic illumination areas, which are represented by the blue circles. The incident light is focused onto the bottom surface through the hole and the waveguide end facet, respectively. There are black regions beside the waveguide because the silicon substrate and epitaxial films are completely removed. And the size in diameter of the illumination spot is ~20 μm.

 

Fig. 2 (a) The micro-reflectance setup; Schematic of illumination spot focused onto the suspended device (b) bottom surface; (c) waveguide end facet.

Download Full Size | PDF

The chip is wire-bonded for illumination measurements. Figure 3(a) shows measured reflectance spectra of the suspended device. The oscillations observed in the reflectance spectra are due to the well-known interference phenomena. The inset is the optical image of suspended photodetector obtained by the CCD camera. It can be clearly seen that the etched surface is rough and the p-electrode region is brighter than the neighborhood region because the metal contacts can reflect part of the incident light back. As a result, the measured reflectance of suspended membrane with the p-contact is higher than those of suspended membrane without p-contact. Therefore, a fraction of the reflected light by the metal contacts is absorbed by the p-n junction, leading to an improved photocurrent effect of the fabricated device. When the device operates in detection mode, a fiber-coupled 435 nm wavelength light beam is focused onto the sample, and the current-voltage (I-V) characteristics are tested under conditions of dark and light illumination. All measurements are carried out at room temperature. Figure 3(b) depicts the measured photocurrents of the suspended device at bias voltages ranging from −2 V to 4 V. We use measured current values without illumination to subtract other measured current values with illumination to obtain the induced photocurrents. Two operation modes can be observed. Below the turn-on voltage, the photocurrent values decrease with increasing bias voltage. Above the turn-on voltage, the InGaN/GaN MQWs diode can emit and detect light at the same time. Under this operation mode, the photocurrent values increases with increasing bias voltage. Moreover, the measured photocurrent values increase with increasing illumination power from 11.6 μW to 22.7 μW. Figure 3(c) shows the measured spectral response of the fabricated device at 0 V bias voltage, where the illumination power is 22.7 μW at the wavelength of 435 nm. The highest optical power is measured at the wavelength of ~500 nm. The maximum photocurrent is induced at ~435 nm. The capacitance-voltage measurements are performed in the bias range of −2 V to 2 V at different frequencies. The capacitance of the suspended p-n junction waveguide photodetector is mainly determined by the depletion width. As shown in Fig. 3(d), the capacitance decreases as signal frequency increases. Moreover, the calculated depletion region has a width range from ~41.8 nm to ~105.1 nm and becomes narrower as the bias voltage increases.

 

Fig. 3 (a) Measured reflectance spectra of suspended membrane with/without metal contacts, and the inset is the optical image of suspended photodetector obtained by the CCD camera; (b) Measured photocurrent at an illumination wavelength of 435 nm; (c) Spectral response of the suspended photodetector; (d) The capacitance-voltage curves for suspended photodetector at different frequencies (1-5 MHz).

Download Full Size | PDF

Figure 4(a) shows the induced photocurrent at bias voltages ranging from −2 V to 4 V when the light beam is incident onto the waveguide output facet. The induced photocurrent under illumination indicates that the incident light can couple into the waveguide and propagate along the suspended waveguide, and the guided light can be detected by the photodetector and converted into current. Figure 4(b) and 4(c), respectively, illustrate the responsivity spectra of the suspended device with bias 0 V and 3 V, where on both figures the measured responsivity were observed to be different at p-electrode and the waveguide region. As shown in Fig. 4(b), the peak value of the responsivity spectra is measured as ~7.58 mAW⁻¹ for p-electrode, while that for the waveguide region is measured as ~2.67 mAW⁻¹, where the peak values are located around 435 nm at 0 V bias. Figure 4(c) also shows the peak values of the responsivity spectra, but the bias is increased to 3 V. For p-electrode, the peak value is measured as ~140 mAW⁻¹; for the waveguide region, the peak value is measured as ~130 mAW⁻¹. The peak values are located around 401 nm at 3 V bias. Figure 4(d) shows an experiment of the photocurrent temporal trace for the waveguide region of the suspended device which is performed by using a mechanical shutter to generate a rectangular illumination pulse with 0.1 s on and 0.1 s off switching cycles. It is clearly seen that a negative photocurrent of ~-66 nA is acquired when the sample is under an illumination power of 22.7 μW. In contrast, the dark photocurrent is significantly lower, approximately −13 pA. Hence, the suspended device has a larger on/off ratio around 5.07 × 10³ compared with silicon bulk devices due to the large band gap nature of InGaN/GaN MQWs, indicating that the suspended device has a remarkable characteristic of temporal response, leading to a distinct on/off switching performance. In addition, the proposed photodetector can achieve higher response speed in a photonic integrated circuit [28], in which an open eye diagram at 1 Mbps is obtained at the photodetector with the bias voltage of 0 V.

 

Fig. 4 (a) Measured photocurrent at an illumination wavelength of 435 nm, and the inset is the optical image of suspended waveguide; (b) Responsivity spectra of the suspended waveguide photodetector at 0 V bias; (c) Responsivity spectra of the suspended waveguide photodetector at 3 V bias; (d) The induced photocurrent temporal trace of the suspended waveguide photodetector.

Download Full Size | PDF

The light coupling properties of the suspended waveguide are evaluated using the finite difference time domain method (FDTD). For simplicity, the used refractive index of GaN is 2.45, and a point-like source with the wavelength of 435 nm is adopted for simulation. Figure 5(a) illustrates the FDTD simulation of the suspended waveguide, which is 30 μm long and 10 μm wide. The contour legend from 0.0 to 0.6 presents the relative intensity of the incident light that propagates along the z-direction of the suspended waveguide. It can be seen that most of the energy of the incident light is concentrated in the waveguide, which means the incident light can effectively couple into the suspended waveguide and propagate inside the highly confined suspended waveguide structure [29]. Figure 5(b) shows TM field profiles of guided modes at the device output facet for suspended waveguide with a waveguide height of 3 μm. The contour legend from 0.0 to 0.004 presents the energy of every single point of the whole cross-section. It illustrates that the energy distribution at the device output facet is highly scattered. Compared to the waveguide dimension, the light at the 435 nm wavelength is highly confined inside the suspended waveguide due to the intrinsic characteristic of the high index of the GaN material in comparison with air.

 

Fig. 5 (a) Plan view FDTD simulation of 10 μm wide suspended waveguide; (b) TM field profile of guided modes.

Download Full Size | PDF

3. Conclusion

In conclusion, we have fabricated a suspended waveguide photodetector featuring p-n junction InGaN/GaN MQWs using a wafer-level technique and demonstrated the feasibility of integrating the two elements on a chip. The light can illuminate the suspended integrated device from the backside, and the metallization stacks serve as the metal mirror to reflect the incoming light back, leading to an improved photocurrent response. The out-of-plane light coupling into the suspended waveguide is guided and detected by the photodetector. The peaks of the responsivity spectra for the suspended waveguide photodetector are located around 401 nm at 3 V bias and 435 nm at 0 V bias, respectively. These results pave a promising way to develop the suspended waveguide photodetector for diverse applications in the visible wavelength region.