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Abstract
InGaN/GaN multiple quantum well (MQW) diodes perform multiple functions, such as optical emission, modulation and reception. In particular, the partially overlapping spectral region between the electroluminescence (EL) and responsivity spectra of each diode results in each diode being able to sense light from another diode of the same MQW structure. Here, we present a noncontact, optical proximity sensing system by integrating an MQW-based light transmitter and detector into a tiny GaN-on-sapphire chip. Changes in the external environment modulate the light emitted from the transmitter. Reflected light is received by the on-chip MQW detector, wherein the carried external modulation information is converted into electrical signals that can be extracted. The maximum detection proximity is approximately 17 mm, and the displacement detection accuracy is within 1 mm. Based on the detection of distance, we extend the application of the sensor to vibration and pressure detection. This monolithic integration design can replace external discrete light transmitter and detector systems to miniaturize reflective sensor architectures, enabling the development of novel optical sensors.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Proximity sensors can perceive the proximity of a target object and acquire essential information regarding its position and velocity. This technology enables various functionalities, such as precise position recognition, efficient obstacle avoidance, automated control mechanisms, and seamless human–machine interactions [1]. The application of this technology is prevalent in various domains, including automatic control, aerospace engineering, and electronic skin (e-skins) [2,3]. There are many different sensing methods for proximity, including capacitive sensors [4–6], ultrasonic sensors [7–9], magnetic induction sensors [10,11], and triboelectric sensors [12,13]. These approaches all have merits and drawbacks. Capacitive sensors are susceptible to interference and exhibit limited linearity. Although ultrasonic sensors are effective for long-range detection, they present challenges in accurately measuring distances within a narrow range. Magnetic induction sensors demonstrate sensitivity exclusively toward conductive materials, but their response time is slow, and they are unreliable. Triboelectric sensors have limitations in detecting static forces and displaying vulnerability to environmental disturbances [14].
In comparison, optoelectronic sensors offer many advantages, such as wide detection of objects, simple structure, fast response speed, high resolution and long lifespan [15,16]. With ongoing miniaturization and multifunctionalization of electronic devices, there is a growing opportunity for optical sensor integration [17]. In the domain of silicon-based optoelectronic devices, extensive research efforts are underway to address the challenges associated with the indirect bandgap of silicon, which poses a significant obstacle to the development of high-quality light-emitting sources. This endeavor aims to facilitate a more seamless integration of diverse functionalities within monolithic systems. [18,19]. By leveraging the principle of coexistence between light emission and detection, it is possible to revolutionize the conventional architecture of various types of optoelectronic sensors [20–22], including fiber optic sensors [23–25], that rely on separate components for these functionalities. This principle has been demonstrated in our previous studies of on-chip optical communication [26–28]. In the same way, we can use this principle to achieve off-chip communication and sensing. The emission of light signals from the transmitter, their modulation by the surrounding environment, and subsequent reception by the homogeneous detector enable the sensing of external environmental information. This technology has been proven to be applicable for monitoring various environmental and human physiological indicators, such as human pulses [29], solution concentrations [30], and air humidity [31]. Thus, optoelectronic proximity sensors can be realized through the use of light emission, object reflection, and light reception modes. Optoelectronic proximity sensors have faster response rates, higher sensitivity, and a wider range of applications. Additionally, utilizing the coexistence of light emission and detection can enhance the compactness of the sensor structure. In this study, we fabricated a homogeneously integrated GaN MQW proximity sensor to measure displacement, and innovatively adopt a simple structure to realize the monitoring of multiple environmental information of displacement, vibration and pressure with one device.
2. Experimental section
The proposed GaN MQW-based optoelectronic proximity sensor is prepared using standard semiconductor processing and are easy to mass produce. The fabrication process is depicted in Fig. 1. The sensor is implemented on a 4 inch III-nitride-on-sapphire platform, which includes several layers: a sapphire substrate, unintentionally doped GaN(u-GaN), Si-doped n-GaN, InGaN/GaN MQWs, and Mg-doped p-GaN. The steps involved in the fabrication process are as follows. Step a: Photolithography is used to define mesa regions measuring 1 × 1 mm. Inductively coupled plasma (ICP) was applied through p-GaN and the MQWs to expose the n-GaN. Then, deep ICP etching is conducted to completely remove the epitaxial III-nitride films, ensuring electrical isolation between the transmitter and detector adjacent to each other. Step b: A transparent indium tin oxide (ITO) current spreading layer is deposited using sputtering. Rapid thermal annealing is performed after the deposition. The ITO layer is defined and etched away, exposing the n-GaN surface. Step c: Ni/Al/Ti/Pt/Ti/Pt/Au/Ti/Pt/Ti metal stacks are deposited as ohmic contact electrodes on both the n-GaN and ITO surfaces. Rapid thermal annealing is performed after the metal lift-off process. Step d: A SiO2 layer is deposited on the wafer using plasma-enhanced chemical vapor deposition. The contact windows between the electrodes and bonding pads are defined by photolithography. Dry ICP etching was performed to remove the SiO2 layer, exposing the windows. Step e: The Ni/Al/Ti/Pt/Ti/Pt/Au metal stack is deposited on the open windows using e-beam evaporation. The final steps were as follows: The sapphire substrate was lapped and polished to a thickness of 200 µm. The chips are diced using ultraviolet nanosecond laser micromachining. This fabrication process allows for the creation of the proposed GaN MQW-based optoelectronic proximity sensor on a III-nitride-on-sapphire platform. After the sensor was manufactured, it was fixed on a 2 cm * 2 cm circuit board for easy testing and application.
3. Results and discussion
In this section, we initially characterize the optical and electrical properties of the sensor’s transmitter and detector. Subsequently, we perform comprehensive evaluations of the sensor's performance as a proximity detector. This includes ascertaining the DC bias conditions for the transmitter and quantifying the sensor's measurement scope and precision. Building upon the established displacement detection capabilities, we extend the investigation to explore the sensor's potential applications in vibration and force sensing, corroborating these through rigorous experimental validation.
3.1 Basic parameters
Figure 2(a) shows the I-V curve of the transmitter obtained using a Keysight B1500A semiconductor parameter analyzer. The data show that the transmitter turns on at approximately 2.2 V, and the forward bias voltage at a current injection of 30 mA is approximately 2.31 V. The inset in Fig. 2(a) demonstrates a strict proportional relationship between the excitation light intensity of the transmitter and the driving current. This allows us to control the LED more precisely to obtain the desired light output power, resulting in a light source with excellent performance when performing optical sensing. Figure 2(b) shows a superimposed plot of the electroluminescence spectrum of the transmitter and the detection spectrum of the detector. The peak of the emission spectrum is located at 525.88 nm under a current of 30 mA. According to band theory, GaN MQW devices can absorb light with a shorter wavelength than their own emission wavelength. Figure 2(b) shows that there is an overlap region of approximately 45 nm between the detection spectrum and the emission spectrum. This overlap region enables the realization of an integrated proximity sensor with both transition and detection functions on a single chip [32].
Fig. 2. (a) I-V characteristic of the transmitter. The inset shows the light output intensity as a function of the injection current; (b) EL spectrum of the MQW-LED as well as the spectral responsibility of the MQW-PD; (c) Plot of the photocurrent of the detector versus the driving current of the transmitter; (d) I-V characteristics in logscale of the detector when the transmitter operates at different currents.
Figures 2(c) and (d) depict the relationship between the photocurrent of the detector and the driving current of the transmitter under dark-field conditions, further confirming the detection capability of the detector for the excitation light of the emitter. Figure 2(c) shows the photocurrent of the detector measured at a 0 V bias voltage under different driving currents of the emitter, demonstrating a highly linear response region. This can provide accurate light intensity measurement, which is very beneficial for applications requiring high-precision light intensity detection, such as spectral analysis and environmental monitoring. Figure 2(d) indicates that the dark current of the detection end is less than 10−7 A, and there is low background noise. When the transmitter operates at a current above 20 mA, the photocurrent of the detector exceeds 10−5 A. Based on the above data, this proximity sensor has a low opening voltage at the transmitting side, which is conducive to reducing the energy consumption of the sensor. Both transmitter and detector have good linearity, which is conducive to the measurement of environmental parameters and reduces the difficulty of signal processing. The lower dark current helps to reduce background noise.
3.2 Proximity sensor
The operating principle of the reflective integrated optoelectronic proximity sensor is shown in Fig. 3(a). The excitation light is modulated by an external moving silver mirror, resulting in changes in the intensity of the light reflected to the detector, which is then converted into an electrical signal. We fabricated a prototype test system, as shown in Fig. 3(b), using a slide rail kit (including a slide table, slide rail, motor, motor controller, and displacement limiter), a force gauge, a three-axis adjustable platform, and a semiconductor parameter analyzer. A force gauge was used only to fix the silver mirror during distance testing. During the test, the motion range of the slide table is constrained by the position limiter, and the speed of movement is controlled by the motor. A 2 cm*2 cm silver mirror was fixed at the top of the force gauge. The sensor is fixed on a three-axis adjustable platform during proximity detection, and the distance between the sensor and the silver mirror is manually controlled by the three-axis adjustable platform.
Fig. 3. (a) Schematic of the experimental measurements; (b) Schematic of the test system; (c) Signals generated by the detector for different driving currents of the transmitter; (d) and (e) Magnified images of the data shown in (c); (f) Measured photocurrent distribution of the sensor over 4000 s.
Primarily, we need to determine the driving current of the transmitter during operation. In the reflective optoelectronic detection mode, the emission intensity of the emitter affects the maximum value of distance detection. We let the slide table oscillate within a fixed range while maintaining the same distance from the detector each time and applying different driving currents to the emitter. This process yields the three graphs shown in Fig. 3(c), (d), and (e). Figure 3(d) and (e) are magnified portions of Fig. 3(c) for analysis. Figure 3(d) shows an analysis of the detection signal at low driving currents. A small driving current results in a weak excitation light intensity from the transmitter. After reflection, the light signal reaching the detector also weakens, resulting in a smaller amplitude of the generated electrical signal, which is submerged in background noise (approximately 2 nA amplitude). This effect leads to a low signal-to-noise ratio and requires high signal filtering and amplification capabilities. As the driving current gradually increases, the amplitude of the background noise of the signal increases from 2 nA (for driving currents below 20 mA) to more than 20 nA after the driving current exceeds 65 mA. The signal quality, which is depicted in Fig. 3(e), deteriorates significantly, and the signal is nearly hidden in the noise. This noise at the detector includes thermal noise, shot noise, generation-recombination (G-R) noise, and 1/f noise [33]. There are two reasons for the larger noise amplitude in Fig. 3(d). The primary factors contributing to this phenomenon are likely the amplified thermal effects of the transmitter at higher driving currents, leading to elevated temperatures of the adjacent detector and heightened thermal noise. In addition, when the driving current increases, the intensity of the reflected and direct light also increases, resulting in an increase in the photogenerated carrier concentration, which will randomly recombine after generation, resulting in a more clear change in the carrier concentration. The G-R noise caused by the change in carrier concentration also increases [33]. Considering the signal quality at various driving currents, encompassing the overall signal stability depicted in Fig. 3(c), we ultimately established the driving current of the transmitter to be 50 mA, which was subsequently employed. As shown in Fig. 3(f), the signal remains highly stable over a period of 4000 seconds, and the photocurrent curves inserted in the figure for three different time segments are almost identical. This confirms the durability and stability of the sensor.
During testing, when the transmitter is driven at 50 mA, the maximum detectable distance of the detector is approximately 17 mm. Beyond this distance, the detector produces a very small signal amplitude in response to displacement changes below 1 mm. Based on the fitting of the data from Fig. 4(a), we established an exponential relationship between the photocurrent and the back-and-forth motion distance, as illustrated in Fig. 4(b). With this fitted curve, we can accurately determine distance based on the measured photocurrent.
Fig. 4. (a) Dynamic response measured from a silver mirror moving back and forth repeatedly at different distances from the surface of the sensor; (b) Fitting diagram of photocurrents measured at different distances.
3.3 Vibration sensor
Then, we used the distance detection performance of the sensor to test the vibration. The related test equipment is shown in Fig. 5(a). Vibration signals are generated by a signal generator and, after being amplified by a power amplifier, drive the vibration table to vibrate. As shown in Fig. 5(b), when the detector is far from the vibrating object, the amplitude of the response signal generated by the detector to vibrations of the same frequency and amplitude will be significantly weakened. Due to the characteristics of mechanical vibration itself, the higher the frequency of vibration is, the smaller the vibration amplitude. This result signifies that the further away the detector is from the vibrating object, the weaker its detection performance for high-frequency vibrations. Figure 5(c) shows the response signal to vibrations of different frequencies at the same distance. According to the signal, which is limited by the performance of the vibrating machine, when the vibration frequency increases, the vibration amplitude decreases greatly. This trend results in a decrease in the amplitude of the response signal. Because the vibration table is not firmly secured to the anti-vibration platform, when measuring vibrations at higher frequencies, the signal will exhibit noticeable fluctuations due to the inherent swaying of the platform itself.
Fig. 5. (a) Diagram of the vibration test system; (b) 50 Hz vibration signal measured at different spacings; (c) Vibration signal diagram of different frequencies measured at the same distance.
3.4 Force sensor
Furthermore, we assembled a sensor with four 1.5 cm long springs and a silver mirror to form the force sensor shown in Fig. 6(a). When force is applied to the silver mirror, the springs compress, resulting in a change in the distance between the mirror and the sensor, thus altering the intensity of the reflected light. The generated photocurrent increases with increasing force, as shown in the results of Fig. 6(b). The measurement range of the sensor is 0.7-3.4 N, which is limited by the precision of the force gauge and the distance detection capabilities. Due to the presence of the spring groove, complete compression of the springs is not achievable. When the applied force exceeds 3.4 N, the spring deformation becomes nonlinear, leading to significant errors in the photocurrent data. Thus, data beyond 3.4 N were not included in the fitting. The response time (tres) of the sensor, as shown in Fig. 6(c), is 295 ms to reach 10% of the minimum value when pressed. In contrast, when the force is released, the recovery time (trec), indicating the duration required for the photocurrent to reach 90% of the maximum value, is measured at 300 ms. Upon fitting the data, as depicted in Fig. 6(d), a linear relationship between the force magnitude and photocurrent was observed. Moreover, the force measurement range can be adjusted by using springs with different stiffness coefficients.
Fig. 6. (a) Structure diagram of the force sensor; (b) Diagram of different force sizes and resulting photocurrents; (c) Magnified view of the instantaneous photocurrent response after normalization. (d) The fitting diagram of photocurrents measured at different forces.
Improving the linearity of pressure sensing is crucial for the accuracy and reliability of pressure measurement systems. We can improve the linearity of force sensing in the following ways. Installing a stop around the sensor: By installing a stop mechanism around the force sensor, we can significantly reduce the lateral play or jitter that occurs during the compression and rebound of the spring element. This lateral movement can introduce non-linearities into the force reading because it allows for variable contact points and forces acting on the sensor. A stop ensures that the spring is compressed and decompressed in a more controlled and linear fashion, directly aligning with the intended axis of measurement. This results in a more consistent and linear pressure-to-output relationship. Using a shorter spring groove: A longer spring groove can increase the obstruction effect, as the spring may not move as freely due to increased friction and potential for binding along the groove walls. By using a shorter spring groove, we reduce the obstruction effect, allowing the spring to deform more smoothly and consistently. This, in turn, leads to a more uniform and linear response during the measurement process.
4. Conclusion
We monolithically integrated two identical MQW diodes into a single chip. These diodes were separately used as a light transmitter and a light detector. Due to the spectral overlap between the EL and responsivity spectra, a wireless light communication architecture was thereby established. Accordingly, changes in the external environment serve as modulators that pulse the reflected light. This modulated optical information is finally converted into electrical signals via the on-chip MQW detector, realizing a noncontact optical sensing system. The resulting sensor, which his tiny and reflective, can detect the movement of objects within 17 mm separation, with a detection precision below 1 mm. We have further extended the sensor's application to vibration and pressure monitoring. These proof-of-concept experiments enable fascinating application prospects for miniaturized optical sensing systems using MQW technology.
Funding
Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX23_1007); Natural Science Research of Jiangsu Higher Education Institutions of China (22KJA510003); National Natural Science Foundation of China (62005130, 62274096); National Key Research and Development Program of China (2022YFE0112000).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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