Nitride-based multiple-quantum-well (MQW) diodes are widely used as light-emitting diodes (LEDs) [1–4] to emit light and photodiodes [5–8] to detect light. Recent years, InGaN-based solar cells have been extensively investigated, because the energy gaps of InGaN alloys can be tuned to cover the entire solar spectrum [8–10]. Moreover, InGaN-based alloys have additional favorable physical properties, including high carrier mobility and drift velocity [11], strong optical absorption near the band edge [12], and superior resistance in high radiation conditions [8,13]. These characteristics enable the InGaN solar cell to operate in severe environments such as the desert or space, in which the performances of Si-based cells are degraded. In particular, the unique dual light-detecting and light-emitting modes of operation endow the MQW diodes with the ability to harvest energy from ambient light sources to power it for lighting. This means that, by integrating a simple circuit with a capacitor, the MQW diode can work as a solar cell at daytime and an LED at night. Compared with the double-heterojunction nanorod light-responsive LED [14], only a single MQW diode with the same device structure can lead to a multifunctional system without integrating single-functioning LED with a separate solar cell. Correspondingly, the fabrication process is simpler. Except for a self-powered light source, such self-harvesting systems open up new avenues toward varied applications of the Internet of Things (IoT) [15]. The stored energy can be employed to power the multifunctional system to emit or detect the modulated light for visible light communication or to charge the low-power-consumption intelligent terminals.

Here we propose fabricating and characterizing a multifunctional system made with suspended MQW diodes on an III-nitride-on-silicon platform. In connection with a capacitor to store the electrical energy, a tandem MQW diode structure charges the capacitor with an enhanced open-circuit voltage ($V_{\mathrm{oc}}$) [16–20], which is higher than the turn-on voltage of the MQW diode. The stored electrical energy can power the same MQW diode for lighting.

Advanced deposition schemes have been developed to grow III-nitride epitaxial layers on (111) silicon substrate by metalorganic chemical vapor deposition. Figure 1(a) shows a cross-sectional schematic of the epitaxial structure of the device. The Al-composition graded AlN/AlGaN buffer layers are employed to manage the lattice mismatch and the difference in thermal expansion coefficients between GaN and silicon substrate, whereby a 330 nm thick AlN layer is first grown; then a 600 nm thick AlGaN layer is deposited. Followed by a 400 nm thick undoped GaN, the 3400 nm thick n-type GaN is grown with a silicon concentration of $8\times {10}^{18}{\mathrm{cm}}^{-3}$. The active regions are formed by growing 9 pair 3 nm/10 nm InGaN/GaN MQW layers, and the p-n junction diode structure is finally obtained by depositing p-type Mg-doped GaN with a concentration of $3\times {10}^{19}{\mathrm{cm}}^{-3}$. Figure 1(b) shows the cross-sectional transmission electron microscopy (TEM) image of the 9 pair InGaN/GaN MQW active region. The inset is the magnified high-resolution TEM image of the interface between InGaN and GaN.

 

Fig. 1. (a) Cross-sectional schematic of the epitaxial structure. (b) TEM image of the 9-pair InGaN/GaN MQW active region, with the magnified high-resolution TEM image of the InGaN/GaN interface inset.

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From a manufacturing point of view, III-nitride-on-silicon platform offers a feasible approach to fabricate suspended MQW diode by a combination of silicon removal and III-nitride backside etch. Figure 2(a) shows a schematic diagram of the suspended MQW diode on silicon substrate. The ring-shaped electrode is proposed to balance light emission and detection. The multifunctional devices are produced by using a standard diode fabrication procedure. Photolithography is employed to define the isolation mesa, and inductively coupled plasma reactive ion etching (ICPRIE) is conducted to expose the n-type GaN by with ${\mathrm{Cl}}_2$ and ${\mathrm{BCl}}_3$ hybrid gases at the flow rates of 10 and 25 sccm, respectively. Followed by metal evaporation, liftoff, and rapid thermal annealing in ${\mathrm{N}}_2$ atmosphere, the 20 nm/400 nm Ni/Au bilayers are obtained for both p- and n-contacts. With a thick photoresist serving as a protective mask for the top device structures, silicon substrate beneath the device is removed by deep reactive ion etching from the backside. Subsequently, the suspended III-nitride membrane can be thinned from the silicon side, in which the ICP-RIE is performed without an additional protective mask. Like the scanning electron microscope (SEM) image that is shown in Fig. 2(b), a 2 μm thick and 1.5 mm in diameter MQW diode is obtained after finally removing the covered photoresist. Such suspended device architecture can be further mechanically transferred to plastic, glass, or other foreign substrates, offering the potential for wearable optoelectronic devices [21–23].

 

Fig. 2. (a) Schematic diagram of the suspended MQW diode on silicon substrate. (b) SEM image of the fabricated structure.

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As an LED, the emission spectrum from the InGaN/GaN MQW diode is inherently broadband. On the other hand, the MQW diode can detect light when it functions as a photodiode. The overlap between the emission and detection spectra endows the MQW diode with the unique ability to detect light emitted by it, which is the fundamental mechanism of multifunctional III-nitride system. The chip is separated from the processed wafer and wire-bonded to a test pad for device characterization. The spectral responsivity is measured by illuminating the MQW diode using an Xe lamp combined with a monochromator iHR320. Figure 3 shows the electroluminescence (EL) spectrum and spectral responsivity of the InGaN/GaN MQW diode. When the MQW diode is turned on, the dominant EL peak locates at 448 nm. The absorption edge of the MQW diode is approximately 450 nm.

 

Fig. 3. EL spectrum and spectral responsivity of the InGaN/GaN MQW diode.

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Figure 4(a) shows the typical current density versus voltage (J-V) and power density versus voltage (P-V) characteristics of the MQW diode under an incident laser centering at 410 nm with power of 1.7 mW. The measured $V_{\mathrm{oc}}$, short-circuit current density ($J_{\mathrm{sc}}$) and fill factor (FF) of the MQW diode are 2.39 V, $8.9\mathrm{mA}/{\mathrm{cm}}^2$, and 40%, respectively. The power conversion efficiency (PEC) is calculated to be 8.76%. The crystal quality of MQWs grown on silicon substrate is limited and, thus, affects the efficiency of MQW diodes. With advances in the epitaxial growth of GaN-based MQWs on silicon, the crystal quality will be improved, and the MQW structures can be optimized, leading to the improvement in the conversion efficiency. The $V_{\mathrm{oc}}$ is logarithmically dependent on the $J_{\mathrm{sc}}$. Figure 4(b) shows the measured J-V of the MQW diode as a function of the illumination power of the laser. The generated electrical energy within the MQW diode is proportional to the number of absorbed photons. Therefore, the $J_{\mathrm{sc}}$ is linearly increased with increasing of the illumination power. When the illumination power is increased from 757 μW to 15.5 mW, Fig. 4(c) shows that the $J_{\mathrm{sc}}$ is linearly increased from 5.72 to $44.1\mathrm{mA}/{\mathrm{cm}}^2$. However, the $V_{\mathrm{oc}}$ inherently has a maximum value and is increased from 2.36 to 2.44 V.

 

Fig. 4. (a) Room-temperature J-V and P-V characteristics of the InGaN/GaN MQW diode under 1.7 mW laser irradiation. (b) Measured J-V of the MQW diode as a function of the illumination power of the laser. (c) Summarized $J_{\mathrm{sc}}$ and $V_{\mathrm{oc}}$ under laser illumination with different powers.

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The current-voltage (I-V) and capacitance-voltage (C-V) plots for one MQW diode and two tandem MQW diodes are shown in Figs. 5(a) and 5(b), respectively. For one MQW diode, the turn-on voltage is about 2.32 V, and the capacitance is about 1.2 nF. For two tandem MQW diodes, the turn-on voltage and capacitance are 4.8 V and 0.5 nF, respectively. In connection with an external capacitor of 1000 μF, a simple charging circuit is formed. When we shine the 410 nm laser light on one MQW diode, light photons that carry energy are absorbed by the MQW diode and then converted into the electrical energy, which charges the capacitor. The charging voltage is measured using an oscilloscope with an input impedance of 1 MΩ. In this case, the MQW diode exchanges energy with its environment by absorbing light photons. Figure 5(c) shows the power-dependent charging process as a function of the charging time. It takes about 23 s to reach a steady-state voltage of 2.43 V at an illumination power of 34.8 mW. With increasing of the illumination power, more light photons will be absorbed to generate electron-hole pairs. It can be seen that it will take a shorter time to reach the steady-state voltage when the illumination power is increased. Only about 8.5 s is taken for one MQW diode to reach a steady-state voltage of 2.45 V at an illumination power of 78.2 mW, indicating that a faster charging process can be obtained with higher illumination power. The steady-state voltage is slightly increased with increasing of the illumination power. However, it has a maximum limit. An effective way to improve energy harvesting efficiency is to use a tandem MQW diode structure, as shown in Fig. 5(d). Two MQW diodes are linked in a series to form a tandem structure, which sums the charging process. A conversion from the energy of the incident light to the electrical energy is implemented under an illumination power of 78.2 mW, and a steady-state voltage of 4.78 V is finally achieved after charging the capacitor for 32 s. Compared with one MQW diode, the steady-state voltage is greatly increased, which is high enough to turn on one MQW diode for lighting. The tandem MQW diodes structure endows the multifunctional system with the ability to obtain higher steady-state voltage for a self-powered light source or the low-power consumption charging system.

 

Fig. 5. (a) I-V plots (b) C-V plots for one MQW diode and two tandem MQW diodes. Charging voltages as functions of time, measured across a 1 MΩ resistor for several optical irradiate power levels: (c) one MQW diode and (d) two MQW diodes.

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In association with a switch circuit, the tandem MQW diode structure is implemented for the demonstration of a self-powered light source, as shown in Fig. 6(a). When the light is incident on the MQW diodes and the switch is opened, the energy of the incident light is converted into the electrical energy, and the capacitor is charging. Since the steady-state voltage of the tandem MQW diode structure is higher than the turn-on voltage of the MQW diode, a self-powered lighting occurs when the switch is closed, in which the electrical energy stored in the capacitor is converted into the energy of the emitted light. In this case, the MQW diodes exchange energy with their environment by emitting light. For a given illumination power and a tandem MQW diode structure, a larger capacitor requires longer charging time to reach the steady-state voltage and also provides longer self-powered time for lighting, as shown in Fig. 6(b). Such multifunctional systems can convert the energy of ambient light sources into the electrical energy during the day when the sun shines the system, and power them for lighting during the night by converting the stored electrical energy into the energy of the emitted light, indicating a promising use as multifunctional power station for charging intelligent terminals.

 

Fig. 6. (a) Charging and self-powering setup. (b) Charging and powering voltages as functions of time for different capacitors.

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In association with silicon removal and III-nitride backside thinning processes, suspended MQW diodes are implemented on an III-nitride-on-silicon platform. The energy of the incident light is converted into electrical energy with a PEC of 8.76% under a 410 nm laser illumination with power of 1.7 mW, where the measured $V_{\mathrm{oc}}$, $J_{\mathrm{sc}}$ and FF of the MQW diode are 2.39 V, $8.9\mathrm{mA}/{\mathrm{cm}}^2$, and 40%, respectively. When we use a 410 nm laser beam to shine light on the MQW diodes, the light photons that carry energy are absorbed by the MQW diodes to produce electron-hole pairs. The generated electrical energy charges an external capacitor, which is connected with the MQW diodes. The electrical energy stored in the capacitor, in turn, is employed to power the same MQW diode for lighting, which leads to a conversion from the electrical energy to the energy of the emitted light. Such multifunctional systems open up new avenues for harnessing ambient light sources to power themselves for lighting or charge intelligent terminals for IoT applications.