FENG Xiao-xiao，HAN Ming-yu，CHEN Mei-peng，F(xiàn)ANG Qian，WANG Yong-jin，LI Xin,2*

（1.GaN Optoelectronic Integration International Cooperation Joint Laboratory of Jiangsu Province, College of Telecommunications and Information Engineering, Nanjing University of Posts and Telecommunications,Nanjing 210003, China；2.Key Laboratory of Broadband Wireless Communication and Sensor Network Technology, Ministry of Education, Nanjing University of Posts and Telecommunications, Nanjing 210023, China）

* Corresponding author，E-mail: lixin1984@njupt.edu.cn

Abstract: The movement of objects is everywhere in nature.With the rapid development of smart vehicle and 6G mobile communications, the demand for highly Integrated Sensing and Communication (ISAC)devices with communication and motion sensing is increasing.Based on the coexistence of luminescence and detection characteristics of GaN multiple quantum wells, an integrated optoelectronic chip based on the epitaxial GaN multiple quantum wells material on sapphire substrate with sensitive motion detection and visible light communication.The transmitter of the optoelectronic chip transmits a visible light signal in blue band to the moving target object.The visible light signal modulated by the motion of the target object is reflected back to the receiver of the chip to stimulate the changing photocurrent.By analyzing the changing photocurrent, the motion of the target object rotating at different speeds can be detected.The change period of the photocurrent curve is consistent with the rotation period of the target object.We also study the optoelectronic characteristics and the visible light communication performance of the optoelectronic chip.This chip can be used as transceiver terminal of visible light communication system and can also process and transmit the motion detection signals collected by the chip.The optoelectronic chip based on GaN multiple quantum wells materials is a highly integrated ISAC terminal device with application value.

Key words: motion detection; multiple quantum wells; III-nitride; optoelectronic chips; visible light communication

1 Introduction

Integrated Sensing and Communication (ISAC)refers to the integration of communication technology and sensing technology, which realizes the perception of the environment while communicating,and uses a single terminal to provide complex functions.Environmental perception technology is a kind of essential technology for ISAC applications such as autopilot driving, Internet of Things (IoT),smart city and other adaptive intelligent systems[1-2].For the perception technology, the non-contact environmental perception ability is of critical importance.This technology has the characteristics of high sensitivity, various application scenarios, and nondestructive and non-interference detection[3-4].Noncontact motion detection technology has great application potential in security systems, IoT healthcare, autopilot, and other fields[5-7].

Traditional motion detection technology, including radar[8]and (Light Detection and Ranging,LiDAR)[9-10], is mainly used for motion tracking of large moving objects in a large field of view.Radar and LiDAR use the echo principle to analyze the flight time of the pulse by transmitting microwave or laser pulses and receiving echoes reflected from the surface of the target object, thereby obtaining the spatial position and motion of the object.It is more difficult to achieve the real-time object tracking because radar takes longer time to track the target object due to the need for full field of view detection scanning.LiDAR locates objects by laser pulses and therefore has a faster response time.However, both radar and LiDAR have high cost.In contrast, moving object tracking based on imaging technology has lower cost.Images of moving objects can be obtained by using imaging systems.The spatial location and motion information of target objects can be extracted from the images by using image processing and analysis algorithms.However,due to the factors such as relative displacement and motion blur, it is more difficult for imaging systems to simultaneously achieve high spatial resolution,high temporal resolution, and high signal-to-noise ratio[10].Each of the above motion detection techniques has its own advantages and limitations, so it is of application value and scientific significance to study new methods of accurate and reliable noncontact environmental motion perception, such as new techniques of environmental perception using visible light signals.

Motion detection techniques using visible light signals and LiDAR are based on similar physical principles, both of which realize motion detection by detecting the modulation of light signals by the target object.The difference being that the medium of the visible light signal is ordinary Gaussian light rather than a laser, resulting in lower power consumption, better environmental adaptability, smaller size, and lower cost.Abuella H and Ekin S of Oklahoma State University, USA, studied visible light motion detection systems for vehicle speed measurement and indoor localization by using Light Emitting Diode (LED) headlights of automobiles and interior lighting as light sources[11-13].The motion detection systems in the above studies require a separate light source and a network of photodetectors, so there are still some challenges, such as system size, high cost, and limited application scenarios.

As a third-generation semiconductor, nitride materials have excellent optical, electrical, mechanical and piezoelectric properties.By varying the content of In, Ga, and Al in the material system and modulating the band gap of nitride materials, optical signals covering the UV, visible, and near-infrared wavelength can be transmitted or detected[14-15].Nitride multiple quantum wells materials have superior optoelectronic properties compared to bulk nitride materials and also have an integrated function of light-emitting and detection.Lin Chen of Huazhong University of Science and Technology integrated light emitting diodes and photodetectors on a single photonic chip based on multiple quantum well materials, and developed a single integrated LED and photodiode (PD) chip, which can be used as both a light transmitter and a detector, and can detect changes in the signal of cardiac pulses without relying on external photodetectors[16].Aihua Zhong of Shenzhen University realized a capacitive hydrogen gas sensor using GaN (gallium nitride)material with honeycomb nanonetwork structure epitaxially grown on silicon substrate, which has the advantages of high safety and low energy consumption and it is an ideal device for hydrogen gas detection[17].Shuai Zhang of Nanjing University of Posts and Telecommunications implemented sensors by the piezoelectric effect of thin-film light-emitting diodes, these sensors were used for non-contact air flow detection, including flow sensing, pressure sensing, and environmental detection[18].The above study broadens the application scenario of III-nitride photonic integrated chip for non-contact visible light sensing and detection.Qingxi Yin in our group prepared multiple quantum wells diode devices with the same structure on a single optoelectronic chip as light-emitting and detecting devices,respectively.They built a free-space reverse optical communication system and explored the integrated chip for visible light communication and sensing[19].All these studies present that GaN optoelectronic chips have superior performance and promising development in ISAC and environmental detection.

In this paper, an optoelectronic integrated chip with integrated transceiver function for visible light signals in the blue band is developed by using the co-existence of luminescence and detection characteristics of III-nitride multiple quantum wells structure.The nondestructive motion detection and stable visible light communication are realized using this chip.The visible light signal from the transmitter of this optoelectronic chip is modulated by the motion of the target object and reflected back to the receiver of the optoelectronic chip.By detecting the change of photocurrent of the receiver, the motion of the target object rotating at different speeds can be detected.In addition, the optoelectronic characteristics and visible light communication performance of the chip are also investigated.The study provides a valuable method for the application of optoelectronic integrated chips in the field of optical motion detection and communication with low cost, low power consumption and high integration.Finally, an integrated ISAC terminal device with application value was developed.

2 Design and fabrication of optoelectronic chips

2.1 Working principle and fabrication process of optoelectronic chip

As shown in Figure 1(a) (color online), under the forward bias, the nitride multiple quantum well diode device excites photons and transmits visible light signals, as a transmitter.When the diode device of the same structure is applied with reverse bias, the nitride multiple quantum wells transforms into photoelectric detection mode and absorbs the incident photons to excite electron-hole pairs and works as a receiver.Thus the diode device realizes the electro-optic/optical-electro bi-directional energy and information conversion in the homogeneous integrated system.The layered structure of the III-nitride multiple quantum wells material used in this study is shown in Figure 1(b) (color online).The nitride epitaxial layer, including buffer layer, n-GaN layer, Multiple Quantum Wells (MQWs) layer and p-GaN layer.The bottom substrate is a patterned sapphire substrate.The patterned substrate can compensate the material defects caused by the different lattice constants and thermal expansion coefficients of sapphire substrate and nitride epitaxial layer, and improve the performance of optoelectronic chips.As a high-reflectance mirror for a specific wavelength, the Distributed Bragg Reflection(DBR) is formed by periodic alternating layers of silicon dioxide (SiO2) and titanium dioxide (TiO2).The thickness and period of the DBR are determined by the target wavelength, and the optical thickness of each layer is 1/4 of the target wavelength,which can reduce the leakage of optical signals and improve the optical performance of optoelectronic chips.

As shown in Figure 2(a), we designed a III-nitride optoelectronic chip with a diode device in the center being used as a visible light signal transmitter.The surrounding diode devices in the chip were used as visible light signal receivers to facilitate the use of a larger number of receivers to collect the modulated light signals in a composite manner, for a larger photocurrent detection signal.However, when the actual motion detection system was used, we found that using four receivers at the same time made it difficult to adjust the reflected light path and thus result the uniform coverage of the modulated light signal to the four receivers.In addition, the photo currents between the four receivers connected in the peripheral circuit would cause crosstalk.Finally, after considering the feasibility of the optical path adjustment of the test system and the stability of the test results, one transmitter and one receiver were used for one-to-one testing in the experiment.As shown in Figure 2(b), a positive bias is applied to the transmitter by the signal generator, and the transmitter achieves electro-optical conversion and transmits a visible light signal in blue band to the surface of the target object, and the visible light signal modulated by the motion of the target object is reflected back to a receiver at the corners of the chip.When the receiver is applied negative bias, the modulated visible light signal will be output as a photocurrent signal by photoelectric conversion, and the motion detection of the target object can be achieved through the monitoring of photocurrent by the optoelectronic chip.

The fabrication process of III-nitride optoelectronic chips using standard semiconductor processes is shown as Fig.3 (color online).(a) The patterns of the transmitter's light-emitting region and the receiver's detection region are defined on the photoresist layer by photolithography, and the pattern is transferred to the nitride epitaxial layer by using Inductively Coupled Plasma (ICP) etching.(b) The patterns of the transmitter and receiver is defined on the photoresist layer using photolithography, and the pattern is transferred to the nitride epitaxial layer by using ICP etching.The overall etch penetrates the nitride epitaxial layer to achieve the electrical isolation of the transmitter and receiver, thus preventing the crosstalk current.(c) Magnetron sputter coating and etching of the Indium Tin Oxide (ITO) layer is completed to form a current

spreading layer on the transmitter and receiver surfaces.(d) is the process for the preparation of Ni/Al/Ti/Pt/Au positive and negative metal electrodes of the transmitter and receiver by using electron-beam evaporation and lift-off techniques.(e) is the process for the preparation of SiO2films to protect the transmitter and receiver surfaces using electron-beam evaporation.(f) is the process for the preparation of Ni/Al/Ti/Pt/Au lead electrodes for the transmitter and receiver using electron beam evaporation and lift-off techniques.

2.2 Morphological characterization of optoelectronic chips

Figure 4(a) shows the overall optical microscope image of the optoelectronic chip.The transmitter and receiver are connected to the circuit board with leads for the electrical connection of the motion detection experiment, respectively.Figure 4(b) shows the optical microscope image of the transmitting/receiving region, which is a square structure with a side length of 240 μm.Figure 4(c)shows the SEM image of the DBR layer with a microstructure of a hemispherical structure with a period of 2 μm hexagonal arrangement.The DBR layer at the bottom of the sapphire substrate can reduce the visible light signal leakage at the bottom of the chip, enhance the absorption of photons inside the chip, and improve the photoelectric conversion efficiency of the optoelectronic chip.Figure 4(d) shows the enlarged optical microscope image of a single transmitter/receiver, which can clearly show the good fabrication quality of the device.

3 Characterization of optoelectronic performance of optoelectronic chips

3.1 Electrical characteristics test

Electrical characterization of optoelectronic chips is tested by an Agilent B1500A semiconductor device analyzer.The transmitter operates at positive bias and the receiver at negative bias.Therefore,the voltage is ranged from -5 V to 5 V.Due to equipment safety restrictions, the measured saturation current is 100 mA.Figure 5(a) shows the current-voltage characteristics of the transmitter/receiver, where the transmitter turns on at 2.5 V and reaches a saturation current of 100 mA at 4.0 V.The differential resistance of the transmitter is calculated from the linear region of the current-voltage (IV) curve to be about 15 Ω.The negatively biased region generates almost no current, indicating that the receiver has no significant leakage current phenomenon and good performance.The carrier compounding process occurs inside the multiple quantum wells and the transmitter emits visible light signal that is linearly modulated by the bias voltage.The receiver absorbs the visible light signal, then releases electron-hole pairs and generates photocurrent.The good electrical properties guarantee the operational stability of the optoelectronic chip.The capacitance-voltage curve in Figure 5(b) (color online) shows that the transmitter has a negative capacitance behavior in the positive bias voltage interval under AC signals with different frequencies.As the positive bias voltage increases, the capacitance decreases and drops to negative values after the transmitter reaches the turn-on voltage.Reducing the Resistance-Capacitance (RC) time constant (a constant indicating the transition response time, which in the resistor-capacitance circuit of the transmitter is the product of the resistance and the capacitance)can improve the response speed of the transmitter,and the smaller the absolute value of the negative junction capacitance, the smaller the RC time constant.The lower the frequency of the AC signal, the higher the positive bias voltage, and the larger the absolute value of the negative junction capacitance.The absolute value of the negative capacitance of the capacitance-voltage curve is in the pF magnitude, and the absolute value is very small, indicating that the transmitter has a good response speed and is suitable as a fast-response integrated sensing and communication device.

3.2 Optical characteristic test

Figure 6 (color online) shows the electroluminescence spectra and spectral responsivity of the optoelectronic chip measured at different injection currents.The visible light signal was collected at 5 mm from the upper surface of the transmitter using a 200 μm diameter multimode fiber in dark and coupled to an Ocean Optics USB4000 spectrometer for characterization.The five Gaussian distribution curves with different colors in Figure 6 are the electroluminescence spectra of the transmitter with different drive currents, and the peak wavelength is at 461.61 nm, which is the visible signal in the blue band.The luminescence intensity of the transmitter gradually increases for currents from 20 μA to 100 μA.The pink curve of the connecting point corresponding to the right axis in Figure 6 is the spectral responsivity of the receiver, which is measured by the IQE-200B quantum efficiency measurement system.The light blue region indicated by the arrow is the spectral overlap region between the electroluminescence spectrum and the spectral responsivity.The visible light signal transmitted from the photoelectric chip transmitter can be detected by the receiver on the same chip in the blue light band from 431 nm to 461 nm, which realizes the single integrated electro-optical/optical-electro conversion of the visible light signal and verifies the feasibility of using a single photoelectric chip to realize the sensing and communication function in principle.

Figure 7 shows the luminescence photographs for transmitter under currents of 10 μA, 50 μA, and 100 μA.Different currents were applied through the probe stage by using an Agilent B1500A semiconductor device analyzer in dark.It can be clearly observed that the intensity of the visible light signal in the blue band transmitted by the transmitter intensifies with increasing current.The experimental results show that when the optoelectronic chip is applied to motion detection, increasing the injected current can enhance the intensity of the receiver photocurrent converted by the visible light signal modulated by the object motion to be measured.It indicates that the non-destructive photodetection signal intensity of the optoelectronic chip can be freely and flexibly modulated by the driving current.

4 Motion detection of optoelectronic chips

The motion detection system of this optoelectronic chip is shown in Figure 8, and the main part of the system is the reflective optical path.The reflector is used as the measurement object to perform rotational motion driven by a rotation table.The reflector is a GMH-11-K9 reinforced aluminum standard precision planar reflector with 25.4 mm diameter and 4 mm thickness produced by United Optical Technology, and the base material is finely annealed H-K9L optical glass with a protective UVreflective aluminum film coating.The reflector has a reflectivity of approximately 90% in visible range.A DC power supply is used to drive the transmitter in positive bias mode to transmit visible light signals in the blue band.A semiconductor parameter analyzer is used to drive the receiver with zero-bias voltage for detecting the visible light signal modulated by the rotational motion of the reflector and converting it into photocurrent for motion detection.A convex lens is set in the middle of reflective optical path to converge the visible light signal.The distance from the optoelectronic chip to the lens is 10 cm, and the distance from the lens to the reflector is 5 cm.The visible light signal transmitted by the calibrated transmitter is focused on the reflector,and the visible light signal modulated by the reflector is focused by the convex lens again after passing through the reflected optical path, and is returned to the receiver.

4.1 Motion detection with variable speed

In this experiment, we apply the same positive bias voltage on the transmitter, change the rotation speed of the mirror, and study the effect of the variable rotation speed on the test results.As shown in Figure 9, the transmitter positive bias voltage was set to 2.9 V, and the reflector rotational speed was set to 100 rpm, 200 rpm and 319 rpm, as shown in Figure 9 for a test duration of 10 seconds.The visible light signal is modulated by a rotating mirror, so that the receiver photocurrent changes.The change period of the photocurrent curve is consistent with the rotation period of the mirror.

The effect of the transmitter bias voltage on the magnitude of the receiver photocurrent change is also investigated for a constant rotation speed.The same test duration of 10 seconds was set at the reflector speed of 200 rpm.The transmitter bias voltage is 2.7 V～2.9 V (Figure 10).The change period of the photocurrent curves at different bias voltages all correspond to the rotation speed of the reflector, and the change of the receiver photocurrent increases from 6 nA to 10 nA.It indicates that within the linear bias dynamic operating range of the transmitter, the larger the bias voltage, i.e., the greater the intensity of the transmitted visible light signal, the greater change in the excited receiver photocurrent pulse, and the better the corresponding motion detection effect.The above experiments show that the optoelectronic integrated chip can achieve non-contact and highly sensitive motion detection of the target object.

4.2 Variable speed motion detection

Considering that most of the objects in nature move at variable speeds, a speed-adjustable rotating motor with a speed range of 0 to 100 rpm as shown in Figure 11(a) is used to control the target object with variable speed motion to test the detection performance of objects with variable speed by the optoelectronic chip.As shown in Figure 11(b), the test was performed with the bias voltage of the optoelectronic chip transmitter at 3 V and the bias voltage of the receiver at 0 V.The initial rotation speed of the target object is 30 rpm, and the rotation speed increases by 30 rpm every 10 seconds, and finally the rotation speed reaches 90 rpm, and the current change period of each time period is consistent with the rotation period of the target object, then good and stable motion detection of the variable speed object is achieved.It shows that the optoelectronic chip can not only detect objects with stable motion in real time, but its effect of detecting objects with variable speed motion is still very good.

5 Visible light communication testing of optoelectronic chips

5.1 Single-emission visible light communication test

A free-space visible light communication test was performed to investigate the communication performance of the optoelectronic chip.As shown in Figure 12(a), digital signals were loaded by a signal generator (Keysight, 81160A) to a transmitter.A commercial avalanche photodiode (Hamamatsu,C12702-12) is used as the receiver to convert the received optical signal into an electrical signal.The transceiver signal is characterized by an Agilent DSO9254A digital storage oscilloscope.As shown in Figure 12(b), the transceiver signal waveforms are well maintained during transmission at a transmission rate of 25 Mbps.Figure 12(c) shows the eye diagram at the corresponding transmission rate with acceptable overshoot amplitude, clear open eye,small signal amplitude distortion, and clear rising and falling edges, indicating that the signal noise is small and the visible light communication transmission performance of the optoelectronic chip as a transmitter is good.Figure 12(d) (color online)shows the 3 dB bandwidth of the optoelectronic chip as a transmitter tested with the Keysight E5080A vector network analyzer, which characterizes the visible light communication response characteristics of the optoelectronic chip.At 2 V, 2.2 V, and 2.4 V bias voltages, the 3 dB band width are 10.1 MHz, 16.7 MHz, and 23.1 MHz, respectively.It can be seen that as the bias voltage increases, the 3 dB bandwidth becomes progressively larger and the communication response speed of the optoelectronic chip increases.The above experiments show that the optoelectronic chip as a transmitter can realize high-speed visible light communication transmission.

5.2 Transceiver-integrated visible light communication test

In the transceiver-integrated visible light communication test, the optoelectronic chip works as both transmitter and receiver, as shown in Figure 13(a).The visible light signal loaded with digital signals transmitted by the transmitter is reflected back to the same optoelectronic chip through the reflected light path of the reflector and then received by the receiver.Figure 13(b) shows the visible light communication transceiver signal at 5 Kbps speed with the optoelectronic chip acting as transmitter/receiver at the same time.The random digital signal transmitted by the optoelectronic chip as a transmitter is well retained at the receiver end, realizing the visible light communication with integrated transceiver.The up and down overshoot amplitude of the eye diagram in Figure 13(c) is small, the open eye is clear, the signal amplitude distortion is not large,and the rising and falling edges are clear although there is a certain degree of zero-crossing distortion.It indicates that the signal noise is small and the optoelectronic chip can be used as the transceiver terminal of the visible light communication system with good transmission performance.

6 Conclusion

Based on the co-existence of luminescence and detection characteristics of Group III nitride multiquantum wells structure, an integrated optoelectronic chip based on epitaxially grown multi-quantum wells GaN material on sapphire substrate is proposed.As an ISAC terminal device, this optoelectronic chip has motion detection and visible light communication functions.The diode on the optoelectronic chip works in transmitter mode when a forward bias voltage is applied, transmitting a visible light signal in the blue band with a central wavelength of 462 nm.The diode on the optoelectronic chip works in receiver mode when a negative bias voltage is applied, detecting visible light signals in the same band.The electroluminescence spectrum of the transmitter has a spectral overlap of about 30 nm with the spectral responsivity of the re-

——中文對照版——

1 引 言

通感一體化是將通信技術與感知技術相融合，在通信的同時實現(xiàn)對周圍環(huán)境的感知，利用單一終端提供復合功能。環(huán)境感知技術是自動駕駛、物聯(lián)網和智慧城市等自適應智慧運行系統(tǒng)等通感一體應用場景的重要支撐技術[1-2]。對于感知技術, 非接觸式環(huán)境感知能力極其重要，該技術具有靈敏度高、適用場景多樣和無損無干擾檢測的特點[3-4]。非接觸式運動探測技術在安防系統(tǒng)、物聯(lián)網、醫(yī)療保健、自動駕駛等領域都有巨大的應用潛力[5-7]。

傳統(tǒng)運動探測方式，包括雷達[8]和激光雷ceiver, enabling integrated communication and sensing applications for transceiver in this band.

The transmitter of this optoelectronic chip sends visible light signals to the target object, and enables motion detection of the target object at a maximum speed of 319 rpm by monitoring the photocurrent converted from the modulated light signals received by the receiver in a non-contact detection mode.The bias voltage of the transmitter increases from 2.7 V to 2.9 V when the target object moves at 200 rpm, and the corresponding photocurrent variation of the receiver increases from 6 nA to 10 nA.This paper also investigates the visible light communication performance of the optoelectronic chip, which can achieve 25 Mbps digital signal communication as a transmitter and 5 Kbps digital signal communication as a transceiver terminal under reflected light path.The optoelectronic chip as an ISAC terminal device can process and transmit the signals collected by motion detection, and is suitable for application scenarios such as Internet of Things, autopilot, and smart cities.This study provides a promising way to implement nitride optoelectronic chips in low-cost, low-power, and highly integrated ISAC terminal devices.達[9-10]等，主要用于針對大視場、大型運動物體的運動追蹤。雷達和激光雷達利用回波原理，通過發(fā)送微波或激光脈沖并接收經目標物體表面反射的回波，計算出脈沖的飛行時間，從而獲得物體的空間位置和運動情況。由于需要進行全視場探測掃描，雷達要花費較長的時間追蹤目標物體，因此較難實現(xiàn)實時物體追蹤。激光雷達利用激光脈沖進行物體定位，響應速度更快。但雷達和激光雷達成本均較高。與之相比，基于成像技術的運動物體追蹤技術成本較低。其利用成像系統(tǒng)獲得運動物體圖像，利用圖像處理和分析算法從圖像中提取目標物體的空間位置和運動信息。然而由于相對位移和運動模糊等因素，成像系統(tǒng)較難同時實現(xiàn)高空間分辨率、高時間分辨率和高信噪比[10]。上述運動探測技術各有優(yōu)點和局限性，需繼續(xù)研究準確可靠的非接觸式環(huán)境運動感知新手段，如利用可見光信號的環(huán)境感知新技術，具有重要的實用價值和科學意義。

使用可見光信號的運動探測技術和激光雷達的物理原理類似：都是通過探測目標物體對光信號的調制來實現(xiàn)運動探測。區(qū)別是可見光信號的媒介是普通的高斯光而不是激光，功耗更低，環(huán)境適應性更好，體積更小，成本更低。美國俄克拉何馬州立大學的Abuella H 和Ekin S 以汽車的LED（發(fā)光二極管）大燈和室內照明為光源，研究了用于車速測量和室內定位的可見光運動探測系統(tǒng)[11-13]。上述研究中的運動探測系統(tǒng)需設置單獨光源和光電探測器網絡，仍存在一些挑戰(zhàn)，如系統(tǒng)體積大、成本高、使用場景有限等。

作為第三代半導體，氮化物材料具有優(yōu)異的光學、電學、機械和壓電性能。通過改變 In、Ga、Al 等元素在材料體系中的含量，調控氮化物材料的禁帶寬度可以發(fā)射或探測覆蓋紫外、可見光及近紅外波段的光信號[14-15]。氮化物多量子阱材料的光電性能相較于體形態(tài)的氮化物材料更為優(yōu)異，并且具有發(fā)光探測一體效應。華中科技大學的陳林將發(fā)光二極管和光電探測器集成在基于多量子阱材料的單片光子芯片上，研發(fā)了單片集成LED 和PD（光電二極管）芯片。該芯片同時作為光發(fā)射器和探測器，可在不依賴外部光電探測器的情況下探測心臟脈沖的信號變化[16]。深圳大學的鐘愛華利用在硅襯底上外延生長的具有蜂窩狀納米網絡結構的GaN（氮化鎵）材料實現(xiàn)了電容式氫氣氣體傳感器。其具有高安全性和低能耗，是檢測氫氣的理想設備[17]。南京郵電大學的張帥利用薄膜發(fā)光二極管的壓電效應實現(xiàn)的傳感器可以進行非接觸式空氣流量檢測，可用于流量傳感、壓力傳感和環(huán)境探測[18]。上述研究拓寬了用于非接觸式可見光傳感探測的III 族氮化物光子集成芯片的應用場景。本課題組的尹清溪將具有相同結構的量子阱二極管器件制備在單一光電子芯片上，分別作為發(fā)光和接收器件，構建自由空間逆向光通信系統(tǒng)，探索了可見光通信感知一體化芯片及關鍵技術[19]。這些研究都表明氮化鎵光電子芯片在通感一體和環(huán)境探測方面有著優(yōu)越的性能和有前景的發(fā)展空間。

本文利用III 族氮化物多量子阱結構的發(fā)光探測并存的特點，研發(fā)了具有藍光波段可見光信號收發(fā)一體功能的光電子集成芯片，并利用該芯片實現(xiàn)了無損式運動探測和穩(wěn)定的可見光通信。該光電子芯片發(fā)射器發(fā)出的可見光信號受目標物體運動情況調制，反射回光電子芯片的接收器。檢測接收器的光電流變化就可以探測以不同速度旋轉的目標物體的運動情況。同時還研究了該芯片的光電特性和可見光通信性能。本文研發(fā)了具有實用價值的高集成度通感一體終端器件。本研究為光電子集成芯片在低成本、低功耗、高集成度的光學運動探測和通信領域的應用提供了有價值的途徑。

2 光電子芯片的設計與制備

2.1 光電子芯片的工作原理與制備流程

如圖1(a)（彩圖見期刊電子版）所示，氮化物多量子阱二極管器件在正向偏壓作用下激發(fā)光子，發(fā)射可見光信號，作為發(fā)射器工作。同樣結構的二極管器件施加反向偏壓時，氮化物多量子阱轉變?yōu)楣怆娞綔y模式，吸收入射光子激發(fā)電子空穴對，作為接收器工作，完成同質集成體系下電光/光電雙向能量及信息轉換。本研究使用的III 族氮化物多量子阱材料的分層結構如圖1(b)（彩圖見期刊電子版）所示。氮化物外延層包括緩沖層、n-GaN 層、多量子阱層(Multiple Quantum Well, MQW)及p-GaN 層。底部為圖形化的藍寶石襯底，圖形化襯底可以彌補藍寶石襯底和氮化物外延層的晶格常數(shù)、熱膨脹系數(shù)不同造成的材料缺陷，提升光電子芯片性能。分布式布拉格反射鏡（Distributed Bragg Reflection, DBR）由二氧化硅(SiO2)和二氧化鈦(TiO2)周期性交替層疊而成，厚度和周期由目標波長決定，每層的光學厚度為目標波長的1/4，作為針對特定波長的高反射率反射鏡，可減少光信號泄漏，提升光電子芯片的光學性能。

Fig.1 (a) GaN materials with multiple quantum wells can realize the bi-directional conversion of optoelectronic/electro-optic signals; (b) layered structure of III-nitride materials with multiple quantum wells圖1 (a)可實現(xiàn)光電/電光信號雙向轉換帶有多量子阱結構的GaN 材料；(b) III 族氮化物多量子阱材料的分層結構

設計了III 族氮化物光電子芯片，如圖2(a)所示。其中心的二極管器件作為可見光信號發(fā)射器，四周的二極管器件作為可見光信號接收器，這樣有利于使用較多的接收器復合收集調制光信號，獲得更大的光電流探測信號。然而在實際運動探測系統(tǒng)的調試中，我們發(fā)現(xiàn)，同時使用4 個接收器會使反射光路調整困難，難以實現(xiàn)調制光信號對4 個接收器的均勻覆蓋。此外，在外圍電路中連接的4 個接收器之間的光電流也會出現(xiàn)串擾。最終通過權衡測試系統(tǒng)光路調整可行性和測試結果穩(wěn)定性后，在本文實驗中使用一個發(fā)射器和一個接收器進行一對一測試。如圖2(b)所示，由信號發(fā)生器施加正偏壓，發(fā)射器實現(xiàn)電光轉換后發(fā)射藍光波段的可見光信號到目標物體表面，經目標物體運動情況調制的可見光信號反射回芯片四角的一個接收器。對接收器施加負偏壓，將調制可見光信號進行光電轉換輸出為光電流信號，監(jiān)測光電流可以實現(xiàn)光電子芯片對目標物體的運動探測。

Fig.2 (a) Schematic diagram of III-nitride optoelectronic chip and its (b) schematic diagram of motion detection system圖2 (a) III 族氮化物光電子芯片示意圖及其(b)運動探測系統(tǒng)示意圖

利用標準半導體工藝制備III 族氮化物光電子芯片的加工流程如圖3（彩圖見期刊電子版）所示：(a)利用光刻技術在光刻膠層確定發(fā)射器的發(fā)光區(qū)和接收器的探測區(qū)的圖形結構，并利用感應耦合等離子體刻蝕法(Inductively Coupled Plasma，ICP) 將圖形結構轉移至氮化物外延層。(b)利用光刻技術在光刻膠層確定發(fā)射器和接收器的整體圖形結構，并利用ICP 刻蝕將圖形結構轉移至氮化物外延層，整體刻蝕穿透氮化物外延層，實現(xiàn)發(fā)射器和接收器的電隔離，防止串擾電流的出現(xiàn)。(c)磁控濺射鍍膜并刻蝕氧化銦錫(ITO)層，形成發(fā)射器和接收器表面的電流拓展層。(d)利用電子束蒸鍍和剝離技術制備發(fā)射器和接收器的Ni/Al/Ti/Pt /Au 正負金屬電極。(e)利用電子束蒸鍍制備二氧化硅薄膜保護發(fā)射器和接收器表面。(f)利用電子束蒸鍍和剝離技術制備發(fā)射器和接收器的Ni/Al/Ti/Pt /Au 引線電極。

Fig.3 Fabrication process of III-nitride optoelectronic chip圖3 III 族氮化物光電子芯片的加工流程圖

2.2 光電子芯片的形貌表征

圖4(a)為光電子芯片整體光鏡圖，發(fā)射器和接收器分別用引線連接在電路板上，用于進行運動探測實驗的電連接。圖4(b)為發(fā)射/探測區(qū)域的放大光鏡圖，發(fā)射/探測區(qū)域為邊長240 μm 的正方形結構。圖4(c)為DBR 層的電子顯微鏡圖，微觀結構為周期2 μm 六邊形排布的半球狀結構。藍寶石襯底底部的分布式DBR 層能夠減少芯片底部的可見光信號泄露，增強光子在芯片內部的吸收，提升光電子芯片的光電轉換能力。圖4(d)為單個發(fā)射器/接收器的局部放大光鏡圖，可以清晰看出器件加工質量良好。

Fig.4 Morphological images of optoelectronic chip.(a)Overall optical microscope image of optoelectronic chip; (b) optical microscope image of transmitting/receiving region; (c) SEM image of DBR layer;(d) enlarged optical microscope image of a single transmitter/receiver圖4 光電子芯片形貌圖。(a)光電子芯片的整體光鏡圖；(b)發(fā)射/接收區(qū)域光鏡圖；(c) DBR 層電子顯微鏡圖；(d)單個發(fā)射器/接收器的局部放大光鏡圖

3 光電子芯片的光電性能表征

3.1 電學特性測試

使用安捷倫B1500A 半導體器件分析儀對光電子芯片進行電學特性測試。發(fā)射器在正向偏壓下工作，接收器在負向偏壓下工作。因此電壓取值范圍為-5 V～5 V，由于設備安全限制，測量飽和電流為100 mA。圖5(a)為發(fā)射器/接收器的電流-電壓曲線，發(fā)射器開啟電壓為2.5 V，到4.0 V時達到飽和電流100 mA，從電流-電壓（I-V）曲線的線性區(qū)域計算發(fā)射器的微分電阻約為15 Ω。負偏壓區(qū)域幾乎無電流產生，說明接收器無明顯漏電流現(xiàn)象，性能良好。在多量子阱內部發(fā)生載流子復合過程，發(fā)射器發(fā)射可見光信號，并且受偏置電壓線性調制。接收器吸收可見光信號釋放電子-空穴對，產生光電流。良好的電學性能保障了光電子芯片的工作穩(wěn)定性。圖5(b)（彩圖見期刊電子版）的電容-電壓曲線表明在不同頻率交流信號下發(fā)射器在正偏壓區(qū)間具有負電容行為。隨著正偏置電壓增加，發(fā)射器達到開啟電壓后電容減小并下降到負值。降低RC 時間常數(shù)（表示過渡反應時間的常數(shù)，在發(fā)射器的電阻電容電路中，是電阻和電容的乘積）可以提高發(fā)射器的響應速度，負結電容絕對值越小，RC 時間常數(shù)越小。交流信號頻率越低，正偏置電壓越高，負結電容絕對值越大。電容-電壓曲線的負電容絕對值在pF 量級，絕對數(shù)值很小，說明發(fā)射器具有良好的響應速度，適合作為快速響應的通感一體器件。

Fig.5 Electrical characteristics test results.(a) Currentvoltage curve of transmitter/receiver; (b) capacitance-voltage curve of transmitter/receiver圖5 光電芯片電性能測試結果。(a)發(fā)射器/接收器的電流-電壓(I-V)曲線；(b)電容-電壓(C-V)曲線

3.2 光學特性測試

圖6（彩圖見期刊電子版）為不同注入電流下測得的光電子芯片的電致發(fā)光光譜和光譜響應度。在黑暗無光環(huán)境下，利用200 μm 直徑的多模光纖在發(fā)射器上表面5 mm 處收集可見光信號，并耦合到Ocean Optics USB4000 光譜儀中進行表征。圖中5 條不同顏色的高斯分布曲線為不同驅動電流下發(fā)射器的電致發(fā)光光譜，主光譜峰為461.61 nm，為藍色波段可見光信號。電流從20 μA增大到100 μA，發(fā)射器的發(fā)光強度逐漸增加。圖中右軸對應的連點粉紅色曲線為接收器的探測譜,其采用IQE-200B 量子效率測量系統(tǒng)測得。箭頭所指淺藍色區(qū)域為電致發(fā)光光譜和響應度光譜間的光譜重疊區(qū)域。即在431 nm～461 nm 的藍光波段范圍內，光電子芯片發(fā)射器發(fā)射的可見光信號可以利用同一芯片上的接收器進行探測，實現(xiàn)可見光信號的單片集成式電光/光電轉換，從原理上驗證了利用單個光電子芯片實現(xiàn)通感一體功能的可行性。

Fig.6 Electroluminescence (EL) spectrum of transmitter and spectral responsivity of receiver圖6 光電子芯片發(fā)射器的電致發(fā)光(EL)譜和接收器的探測譜

圖7 為發(fā)射器注入電流為10 μA、50 μA、100 μA時的發(fā)光圖片。在黑暗環(huán)境下，采用安捷倫B1500A半導體器件分析儀通過探針臺施加不同的電流?？擅黠@觀察到發(fā)射器發(fā)射的藍光波段可見光信號強度隨著電流增加而加強。實驗結果表明，光電子芯片應用于運動探測時，提升注入電流可以提升經待測物體運動情況調制的可見光信號轉換的接收器光電流強度。表明光電子芯片的無損光探測信號強度可經由驅動電流進行自由靈活調控。

Fig.7 Luminescence photographs of transmitter with different injected currents圖7 注入不同電流的發(fā)射器發(fā)光圖片

4 光電子芯片的運動探測

III 族氮化物光電子芯片的運動探測系統(tǒng)如圖8 所示，系統(tǒng)主體為反射光路。反射鏡作為待測物體在旋轉工作臺的帶動下進行旋轉運動。反射鏡為聯(lián)合光科公司生產的GMH-11-K9 加強鋁標準精度平面反射鏡，直徑為25.4 mm，厚度為4 mm，基材為精退火H-K9L 光學玻璃，表面鍍層為保護性紫外反射鋁膜。在光子芯片工作的可見光波段，該反射鏡的反射率約為90%。采用直流電源以正偏壓模式驅動發(fā)射器發(fā)射藍光波段可見光信號。采用半導體參數(shù)儀，以零偏壓驅動接收器，用于探測經反射鏡旋轉運動調制的可見光信號，并將其轉化為光電流，實現(xiàn)運動探測。反射光路中間設置凸透鏡以匯聚可見光信號，光電子芯片到透鏡的距離為10 cm，透鏡到反射鏡的距離為5 cm。校準發(fā)射器發(fā)射出的可見光信號聚焦在反射鏡上，反射鏡調制的可見光信號經過反射光路，再次經過凸透鏡聚焦，回射入接收器。

Fig.8 Schematic diagram of motion detection system of optoelectronic chip圖8 光電子芯片的運動探測系統(tǒng)示意圖

4.1 勻速運動探測

首先對發(fā)射器施加相同的正偏壓，改變反射鏡轉速，研究轉速對測試結果的影響。如圖9 所示，發(fā)射器正偏壓設置為2.9 V，反射鏡轉速設置為100 rpm、200 rpm 和319 rpm，分別對應圖9(a)、9(b)、9(c)，測試時長為10 s。旋轉反射鏡調制可見光信號，使得接收器光電流發(fā)生變化。光電流曲線的變化周期與反射鏡的旋轉周期相符。

Fig.9 Detected photocurrent of the receiver from the reflector moving at different rotating speeds when the transmitter is applied 2.9 V bias voltage圖9 發(fā)射器偏壓為2.9 V 時，不同轉速反射鏡運動下接受器的探測光電流

同時研究反射鏡轉速不變的情況下，發(fā)射器偏置電壓對接收器光電流變化幅度的影響。反射鏡轉速為200 rpm 時，同樣設置10 s 測試時長。發(fā)射器偏置電壓為2.7 V、2.8 V、2.9 V，對應圖10(a)、10(b)、10(c)。可見，不同偏置電壓下光電流曲線的變化周期均與反射鏡的旋轉轉速相符，接收器的光電流變化幅度從6 nA 增加至10 nA。說明在發(fā)射器的線性偏壓動態(tài)工作范圍內，偏置電壓越大，即發(fā)射的可見光信號強度越大，激發(fā)的接收器光電流脈沖變化越明顯，相應的運動探測效果越好。上述實驗說明光電子集成芯片可以實現(xiàn)對目標物體運動情況的非接觸高靈敏探測。

Fig.10 Detected photocurrent of the receiver under 200 rpm, rotating speed of mirror when the bias voltages applied to the transmitter are set as 2.7 V,2.8 V, 2.9 V圖10 反射鏡的運動轉速為200 rpm，發(fā)射器偏壓分別設置為2.7 V、2.8 V、2.9 V 時接收器的探測光電流

4.2 變速運動探測

考慮到自然界中大部分物體是變速運動，利用如圖11(a)所示的調速范圍為0 至100 rpm 的可調速旋轉電機，設置變速運動的目標物體，用來測試光電子芯片對變速運動物體的探測。如圖11(b)所示，在光電子芯片發(fā)射器的偏置電壓為3 V，接收器偏置電壓為0 V 時進行測試，目標物體起始轉速為30 rpm，每隔10 s 轉速增加30 rpm，最后轉速達到90 rpm?？梢?，每個時間段的電流變化周期均與目標物體的旋轉周期一致，說明所設計的光電子芯片實現(xiàn)了對變速物體良好穩(wěn)定的運動探測。

Fig.11 (a) Variable speed motor and (b) photocurrent curve of variable speed motion detection圖11 (a)變速電機和(b)變速運動探測的光電流曲線

表明該光電子芯片不僅可以對運動情況穩(wěn)定的物體進行實時探測，對于變速運動的物體探測效果依然良好。

5 光電子芯片的可見光通信測試

5.1 單發(fā)射可見光通信測試

進行自由空間可見光通信測試，研究光電子芯片的通信性能。如圖12(a)所示，數(shù)字信號由信號發(fā)生器（Keysight，81160A）加載到發(fā)射器上。以商用雪崩光電二極管（Hamamatsu，C12702-12）作為接收器，將接收到的光信號轉換為電信號。收發(fā)信號由安捷倫DSO9254A 數(shù)字存儲示波器表征。如圖12(b)所示，在25 Mbps 的傳輸速率下，收發(fā)信號波形在傳輸過程中保持良好。圖12(c)為對應傳輸速率下的眼圖，其過沖幅度可以接受，開眼清晰，信號幅度畸變小，上升、下降沿清晰，表明信號噪聲較小，作為發(fā)射器的光電子芯片的可見光通信傳輸性能整體良好。圖12(d)為使用矢量網絡分析儀Keysight E5080A 測試的光電子芯片作為發(fā)射器的3 dB 帶寬，用以表征光電子芯片的可見光通信響應特性。在2 V、2.2 V、2.4 V 偏置電壓下，3 dB 帶寬分別為10.1 MHz、16.7 MHz、23.1 MHz?？梢钥闯觯弘S著偏置電壓的增大，3 dB 帶寬值也逐漸變大，光電子芯片的通信響應速度不斷上升。上述實驗表明，該光電子芯片作為發(fā)射器能夠實現(xiàn)可見光通信高速傳輸。

Fig.12 (a) Visible light communication test system of optoelectronic chip as a transmitter.Signal waveforms (b) and eye diagram (c) of optoelectronic chip as a transmitter at 25 Mbps.(d) 3 dB bandwidth of optoelectronic chip as a transmitter at different bias voltages圖12 (a)作為發(fā)射器的光電子芯片的可見光通信測試系統(tǒng)。作為發(fā)射器的光電子芯片在25 Mbps 傳輸速率下的收發(fā)信號波形(b)和眼圖(c)，以及(d)不同電壓下作為發(fā)射器的光電子芯片3 dB 帶寬

5.2 收發(fā)一體的可見光通信測試

在收發(fā)一體的可見光通信測試中，光電子芯片同時作為發(fā)射器和接收器，如圖13(a)所示。發(fā)射器發(fā)射的加載數(shù)字信號的可見光信號，通過反射鏡的反射光路回到同一光電子芯片上，由接收器接收。圖13(b)為光電子芯片同時作為發(fā)射器/接收器在5 Kbps 速度下的可見光通信收發(fā)信號。光電子芯片作為發(fā)射器發(fā)射的隨機數(shù)字信號在接收器一端得到良好保留，實現(xiàn)了收發(fā)一體的可見光通信。圖13(c)中眼圖的上下過沖幅度較小，開眼清晰，信號幅度畸變不大，雖然存在一定程度的過零點失真，但上升、下降沿清晰。說明信號噪聲較小，光電子芯片可以作為可見光通信系統(tǒng)的收發(fā)一體終端，具有良好的傳輸性能。

Fig.13 (a) Visible light communication test system of the optoelectronic chip as a transceiver.(b) Signal waveform and (c) eye diagram at 5 Kbps transmission rate圖13 (a) 同時作為發(fā)射器和接收器的光電子芯片的可見光通信測試系統(tǒng)。作為收發(fā)一體終端的光電子芯片在5 Kbps 傳輸速率下的收發(fā)信號波形(b)和眼圖(c)

6 結 論

鑒于III 族氮化物多量子阱結構具有發(fā)光和探測并存的特點，提出了一種基于藍寶石襯底外延生長多量子阱氮化鎵材料的集成式光電子芯片。作為通感一體終端器件，該光電子芯片具有運動探測和可見光通信功能。光電子芯片上的二極管在施加正向偏置電壓時以發(fā)射器模式工作，發(fā)射中心波長為462 nm 的藍光波段的可見光信號。光電子芯片上的二極管在施加負向偏置電壓時以接收器模式工作，可以探測同一波段的可見光信號。發(fā)射器的電致發(fā)光光譜與接收器的響應光譜有30 nm 左右的光譜重疊，可以在該波段實現(xiàn)可見光信號的收發(fā)一體通信和傳感應用。

該光電子芯片發(fā)射器向目標物體發(fā)射可見光信號，以非接觸探測方式，通過監(jiān)控接收器接收的由調制光信號轉化的光電流，實現(xiàn)對最高轉速為319 rpm 的目標物體的運動探測。目標物體運動轉速為200 rpm 時，發(fā)射器偏置電壓從2.7 V 增加到2.9 V，相應接收器的光電流變化幅度從6 nA增加到10 nA。本文還研究了該光電子芯片的可見光通信性能，其作為發(fā)射器可實現(xiàn)33 Mbps 的數(shù)字信號通信，作為收發(fā)一體終端在反射光路下可實現(xiàn)5 Kbps 的數(shù)字信號通信。該光電子芯片作為通感一體終端器件，能夠對運動探測采集的信號進行處理和傳輸，適用于物聯(lián)網、自動駕駛、智慧城市等應用場景。本研究為氮化物光電子芯片在低成本、低功耗、高集成度的通感一體終端器件的實現(xiàn)提供了很有希望的途徑。