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Abstract
In the present study, a heterojunction made of an individual ZnO microwire via Ga incorporation (ZnO:Ga MW) with a p-Si substrate was constructed to develop a self-powered ultraviolet photodetector. When operated under an illumination of 370 nm light with a power density of ∼ 0.5 mW/cm2, the device exhibited an excellent responsivity of 0.185 A/W, a large detectivity of 1.75×1012 Jones, and excellent stability and repeatability. The device also exhibited a high on/off photocurrent ratio up to 103, and a short rising and falling time of 499/412 μs. By integrating the pyro-phototronic effect, the maximum responsivity and detectivity increased significantly to 0.25 A/W and 2.30×1012 Jones, respectively. The response/recovery time was drastically reduced to 79/132 μs without an external power source. In addition, the effects of light wavelength, power density, and bias voltage on the photocurrent response mediated by the pyro-phototronic effect were systematically characterized and discussed. Our work not only provides an easy yet efficient procedure for constructing a self-powered ultraviolet photodetector but also broadens the application prospects for developing individual wire optoelectronic devices based on the photovoltaic-pyro-phototronic effect.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Self-powered optoelectronic measuring devices and systems, such as those that can convert light signals into voltage and/or current signals as self-powered sensors or light energy harvesters without a power supply, are important, fundamental building blocks in common applications for environmental monitoring, light-wave communication, territorial intrusions, and imaging technology [1–9]. Self-powered photodetection devices with high reliability, high efficiency, and zero power consumption developed by utilizing the photovoltaic effect and/or integrating traditional photodetector devices without external power sources have been widely reported [10–14]. In particular, self-powered photodetectors based on the photovoltaic effect of the p-n junction and Schottky-type structures have attracted increasing attention owing to their advantages of easy fabrication, a large built-in electric field, and a free external power source. The relatively large depletion region of the p-n/Schottky junction, which originates from the inherent built-in potential, causes photogenerated carriers to be produced and separated by the built-in electric field to form a photon current without any external voltage [15–20].
To optimize device performance parameters in terms of responsivity, response speed, and detectivity, several experimental schemes, including composition engineering, interface engineering, passivation strategies, and so on, have been developed [21–28]. Recently, the pyro-phototronic effect, which is a current release phenomenon originating from non-centrosymmetric crystals due to temperature variation over time, has been observed in tourmaline materials. The pyro-phototronic effect is widely used in self-powered pyro-phototronic detectors to detect the intensity of light. Compared with other photon thermal detectors, pyro-phototronic-type detectors have superior response times and higher detection capability for low-level incident radiation [12,29–31]. The combination of photovoltaic and pyroelectric effects, called the pyro-phototronic effect, has been proposed as an interfacial physical effect for developing high-performance photodetectors by utilizing pyroelectric polar charges, which can effectively and quickly tune or control the generation, separation, diffusion, and recombination of local carriers within the p-n/Schottky junction [15,32–36]. ZnO, a typical wide bandgap semiconductor (direct bandgap $\sim$ 3.4 eV, high exciton binding energy $\sim$ 60 meV) that possesses a common pyroelectric character, has been widely used in the fabrication of pyro-phototronic effect-enhanced self-powered photodetectors owing to its non-centrosymmetric crystal structure and specific polar axis along the direction of spontaneous polarization [37–43]. Therefore, developing high-performance, low-dimensional photodetectors mediated by the pyro-phototronic effect, the preparation of well-crystallized ZnO micro/nanostructures with controlled doping, excellent electrical characteristics, and optical properties, especially for non-centrosymmetric crystal structures, is worthy of further effort [17,22,34,44,45].
In the present study, individual ZnO microwires with Ga incorporation (ZnO:Ga MWs) were successfully prepared using the chemical vapor deposition (CVD) method. An experimental scheme for fabricating self-powered ultraviolet photodetectors, which consisted of a single ZnO:Ga MW and p-type Si substrate heterostructure, is demonstrated. The light absorption at approximately 370 nm can be explained by the intermediate energy level transition of the ZnO:Ga MWs. The fabricated single MW heterojunction device exhibited maximum responsivity and detectivity, which reached 0.185 A/W and 1.75$\times$10$^{12}$ Jones for 370 nm light illumination, respectively. The corresponding response and recovery times were extracted to approximately 499/412 $\mu$s. Interestingly, by introducing the pyro-phototronic effect, the device performances were significantly enhanced, including photocurrent, responsivity, detectivity and the corresponding response times. The experimental results suggest that the incorporation of pyro-phototronic effect can effectively increase the device performance of the self-powered n-ZnO:Ga MW/p-Si heterojunction photodetector, which could provide great application demands for developing high-performance photodetection, light communication, and self-powered photovoltaic devices.
2. Experimental section
2.1 Preparation of the individual ZnO:Ga MWs
The preparation of individual ZnO:Ga MWs with good crystallinity and high yield was achieved using a simple CVD method via a vapor-solid process [2,46,47]. The preparation of individual ZnO:Ga MWs was performed in a horizontal furnace using argon ($Ar$, 120 sccm) flow as the protecting gas. A mixture of ZnO (99.99%), Ga$_2$O$_3$ (99.999%), and C (99.99%) powders with a weight ratio of 10:1:11 was employed as the source material. After it had been well mixed, the precursor was placed in a corundum boat (of size 20 cm (length) $\times$ 2 cm (width) $\times$ 1.5 cm (depth)). Then, an Si crystal wafer (without any catalyst coating and of size 2 cm (length) $\times$ 2 cm (width)) was placed on top of the mixture to collect the products. The furnace temperature was set in advance, with a maximum of 1100 $^\circ$C at the location of the source material. Subsequently, the precursor mixture was placed in the hottest zone. The preparation procedure was performed for approximately 1 h. Finally, 10$\%$ oxygen ($O_2$) was introduced into the furnace chamber as the growth gas for about 30 min. The furnace was cooled naturally to room temperature. Wurtzite-crystallized ZnO:Ga MWs were obtained (Illustration in Fig. 1(a)).
Fig. 1. Morphology and structure of the as-synthesized ZnO:Ga MWs: (a) Optical photographs of as-synthesized wires. (b) SEM image of ZnO:Ga MWs, showing a near-perfect hexagonal cross-section; (c) XRD pattern of as-synthesized ZnO:Ga MWs; (d) Elemental content analysis of an individual ZnO:Ga MW examined by EDX mapping, and the corresponding elemental mapping of Zn, O, and Ga; (e) Low-magnification TEM image of the as-grown ZnO:Ga wire; (f) Corresponding high-resolution TEM image of the wire, which is marked in (e).
2.2 Fabrication of the n-ZnO:Ga MW/p-Si heterojunction
A heterostructured device composed of an n-ZnO:Ga MW and p-type Si template was fabricated [46,48,49]. For the device architecture, a commercially available p-type Si template was used for the hole-injecting layer. The device fabrication procedure was as follows: (1) Au ohmic contacts ($\sim$ 80 nm in thickness) were first evaporated on an activated Si layer using electron-beam heating evaporation. (2) A bilateral layer of MgO insulating films (approximately 1 $\mu$m in thickness) was prepared on the Si film using electron beam heating evaporation via a shadow mask. (3) An individual ZnO:Ga MW (the diameter was estimated to be approximately 10 $\mu$m) was subsequently placed across the slit of the p-Si film, with the bilateral MgO films serving as an insulating layer. (4) Finally, the ITO conductive glass was inversely bonded with MWs and acted as the top ohmic contact electrode of the n-ZnO:Ga MW/p-Si heterojunction device. Thus, the bilateral MgO films could be used to prevent the direct contact between the ITO electrode and the p-Si substrate. A typical device configuration is shown in Fig. 2(a), in which the ZnO:Ga MW lies on the Si substrate.
Fig. 2. Characterization of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector. (a) Schematic illustration of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector. (b) $I$-$V$ characteristic curve of the fabricated n-ZnO:Ga MW/p-Si heterojunction, accompanied by a logarithmic scale. (c) Light-dark $I$-$V$ characteristics curves of the as-constructed heterojunction under 370 nm monochromatic radiation. The light power density is 0.5 mW/cm$^2$. (d) Wavelength-dependent photoresponse spectra of a single n-ZnO:Ga/p-Si MW photodetector at biases of 0 and −1 V, respectively. (e) The schematic energy band diagram of the fabricated n-ZnO:Ga MW/p-Si heterojunction under thermal equilibrium at zero bias voltag. (f) An energy band diagram of the as-fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector under ultraviolet light illumination at zero bias voltage.
3. Results and discussions
As presented in the Experimental Section, ultralong ZnO:Ga MWs with uniformly hexagon-shaped cross sections and clean facets were prepared [2,46,47]. As seen in Fig. 1(a), the as-synthesized wires are well-aligned. These MWs can grow exceptionally long up to 2 cm. The morphological characterization of individual ZnO:Ga MWs was performed using scanning electron microscopy (SEM). According to the SEM images shown in Fig. 1(b), the as-synthesized sample exhibits hexagonal facets with very straight and smooth surfaces. Its diameter was measured as approximately 10 $\mu$m near the cross-sectional top. The crystalline quality and orientation of the as-synthesized ZnO:Ga microstructures were studied using X-ray diffraction (XRD). As illustrated in Fig. 1(c), the XRD patterns of the wires show that all diffraction peaks were extracted from crystalline ZnO:Ga MWs with the hexagonal wurtzite structure [50,51]. The strongest diffraction peak positioned at (100) indicates that the MWs were well aligned along the growth direction. Thus, the hexagonal shape of the base plane is plainly visible, suggesting their wurtzite structures [51–53]. To identify the chemical compositions of the elements Zn, O, and Ga, energy-dispersive X-ray spectroscopy measurements were performed. As shown in Fig. 1(d), the presence of Ga ions in the as-prepared ZnO matrix were confirmed [50,54,55].
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to examine the precise structure of the as-grown ZnO:Ga samples. Figure 1(e) illustrates a typical TEM image of a ZnO:Ga wire, and the diameter was estimated to be approximately 500 nm. Figure 1(f) shows the HTEM image, which was taken at a random position marked at the edge of the wire. It is evident that the crystal lattice of individual ZnO:Ga MWs is well oriented, and no defects can be observed in the selected area. The lattice spacing was extracted to approximately 0.281 nm from the HRTEM figure, which is slightly larger than that of the undoped ZnO structures ($\sim$ 0.260 nm) [50,51]. In the present experimental scheme, the synthesis method via self-catalysis has unique advantages in comparison with conventional CVD methods, including growth to a large size and perfect hexagonal morphology, both of which are attributed to the special air flow effect, a specially designed corundum boat, and a quartz tube of appropriate size [50,56].
A heterojunction composed of an individual ZnO:Ga MW and a p-type Si substrate was constructed. The fabrication procedure is described in detail in the Experimental Section [46,48,49]. The illustration in Fig. 2(a) shows a schematic diagram of a photodetector device based on an n-ZnO:Ga MW/p-Si heterojunction. The electrical characterization of the current-voltage (I-V) curves and current-time (I–t) curves, and the photoelectric performances of the photodetection devices were measured using a photoelectric detection system that was composed of an Xe lamp, monochromator, chopper, and semiconductor analysis device (Keithley B1500A) [2,47,56]. The 370 nm light source was supported by an ultraviolet light lamp. The light intensity was measured using an ultraviolet A power meter. All measurements were performed at room temperature under an air atmosphere. The key parameters of responsivity ($R$) and detectivity ($D$) were employed to evaluate the photodetection performances of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector. The parameter $R$ indicates the efficiency of the detector in response to optical signals, while the parameter $D$ indicates the ability to detect weak signals from a noisy environment. $R$ can be expressed with the following formula [5,38,39,57]:
In this formula, $I_{ph}$ is the photocurrent, $I_\lambda$ is the illumination current, and $I_d$ is the dark current, $P_\lambda$ is the light power, $E_\lambda$ is the light power intensity, and $S$ is the effective illuminated area. The detectivity $D$ is described according to the following formula [27,41,44]: where $e$ is the electron charge.The photoelectric characters of the as-constructed n-ZnO:Ga MW/p-Si heterojunction device were studied at room temperature. Figure 2(b) exhibits current-voltage ($I$-$V$) characteristics in the dark. And the $I$-$V$ curve in a logarithmic plot is also displayed in the figure. Clearly, the plotted nonsymmetrical $I$-$V$ curves of the n-ZnO:Ga/p-Si heterojunction illustrate the rectifying behaviour, suggesting that p-n heterojunction between a single ZnO:Ga MW and p-Si layer was formed. Besides, the dark current of the fabricated n-ZnO:Ga MW/p-Si heterojunction device exhibits a relatively large on-state at the forward bias regium and a much smaller on-state at the reverse bias region. The heterojunction exhibits a high rectification ratio at $\pm$ 2V, which is approximate to the ZnO/Si heterojunction optoelectronic devices in previously reported literature [44,49,58]. When exposed to an ultraviolet light of 370 nm with an irradiant power of 0.5 mW/cm$^2$, significantly increased photocurrent is obtained and the photocurrent can reach about 200 nA under ultraviolet light illumination, which is much higher than the dark current at the reverse bias region, as shown in Fig. 2(c). The photocurrent of the n-ZnO:Ga MW/p-Si heterojunction device is much larger than that of its dark current at the reverse bias of −1.0 V, and the corresponding increased light-dark ratio is also larger than that of its current at the forward bias of 1.0 V.
Figure 2(d) exhibits the wavelength-dependent responsivity of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector, which was measured at a zero bias and at the reverse bias of −1.0 V, confirming its excellent sensitivity in the ultraviolet band. The fabricated single MW-photodetector exhibits an outstanding responsivity in the ultraviolet region and a much lower responsivity in the visible region [29,41,44]. The strongest photoresponses peak at 370 nm, which corresponds to the optical bandgap of the ZnO:Ga MW ($\sim$ 3.37 eV). Meanwhile, the relatively weaker photoresponse in the visible light region may originate from the Si crystal wafer. The fabricated single MW heterojunction device exhibits a maximum responsivity value of 0.185 A/W, a detectivity value of 1.75$\times$10$^{12}$ Jones, and an $EQE$ of 83% under 370 nm ultraviolet illumination without any external power. Furthermore, the device exhibits an excellent detectivity of 7.0$\times$10$^{11}$ Jones, and a responsivity of 1.25 A/W at a reverse bias of −1.0 V. The experimental results of high detectivity and responsivity therefore suggest that the fabricated n-ZnO:Ga MW/p-Si heterojunction exhibits self-powered photodetection ability, which allow it to detect very weak signals and yield high efficiency without any external power [36,41,45].
To exploit the mechanism of the fabricated single MW based heterojunction device working at zero bias condition, an reconstructed energy band diagram is proposed. Figure 2(e) presents the energy band diagram of the fabricated p-n heterojunction between a single ZnO:Ga MW and p-Si substrate under thermal equilibrium at zero bias condition, according to the Anderson-Shockley model via the electron affinities of ZnO:Ga ($\chi$ $\sim$ 4.35 eV) and Si ($\chi$ $\sim$ 3.4 eV) [40,42,59]. Shown in the figure, the conduction/valence band offsets are determined to be $\Delta E_c$ = 0.95 eV and $\Delta E_v$ = 3.2 eV, respectively. It is conclusively determined that a classic type-II heterojunction could be produced in the fabricated n-ZnO:Ga MW/p-Si heterostructure device configuration. When the p-n heterojunction between a single ZnO:Ga MW and p-Si substrate produced, a space charge region is formed at the ZnO:Ga/Si heterointerface owing to the diffusion and drift movement of carriers. Thus, a built-in electric field is generated while maintaining the balance between the drift movement and diffusion movement of electrons and holes under zero bias voltage condition [19,22,45].
To explain the working principle of the self-powered n-ZnO:Ga MW/p-Si photodetector, the energy band structure of the heterojunction is illustrated in Fig. 2(f). Upon the ultraviolet illumination, electron-hole pairs can be generated, and then quickly separated by the built-in field within the depletion region, leading to break the dynamic balance and produce a photoinduced voltage. The holes will transfer from n-ZnO:Ga MW to the valence band of p-Si layer, while the electrons in the conduction band of n-ZnO:Ga MW will move to the ITO electrode, yielding a stable photocurrent. The carriers can be collected separately by the ITO and Ni/Au electrodes, forming the current loop driven by the photoinduced voltage, and then the device can act as a self-powered photodetector [5,19,40].
The repeatability and response speed are crucial parameters to developing high-performance photodetectors. The influence of the light power intensities on the charge carrier transport properties were studied. The $I$-$V$ characteristics of the device under a dark condition and different power intensities varied from 0.003 to 0.5 mW/cm$^2$, and they are plotted in Fig. 3(a). The rectification characteristic in the dark condition indicates typical p-n junction behavior. The device exhibited a good photoresponse at reverse bias voltages to ultraviolet light at 370 nm. In addition, it is evident that the photocurrent increases significantly with an increase in the incident light intensity. The fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector illustrated a observably high photocurrent, with the $I_{photo}/I_{dark}$ ratio being up to 10$^3$ [2,47]. Time-dependent photocurrent measurement was performed at a light density of 0.5 mW/cm$^2$ and a reverse bias of −1.0 V, and the corresponding $I$-$t$ curve is shown in Fig. 3(b). It is evident that the fabricated devices exhibit fast and stable switching features. Thus, the fabricated n-ZnO:Ga MW/p-Si heterojunction has great potential for the development of high-detection photo-switching ultraviolet optoelectronic devices. The rise and decay times under 370 nm light illumination are as short as 499 and 412 $\mu$s, respectively, which are faster than those of most reported ZnO/Si-based heterojunction photodetectors [38,43,44,58,60].
Fig. 3. (a) Photocurrent versus voltage ($I$-$V$) characteristics of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector under dark and ultraviolet illumination at a wavelength of 370 nm with a series of light power densities. (b) $I$-$T$ characteristics curve at a reverse bias of −1.0 V with ultraviolet light on/off switching. The light wavelength is 370 nm, and the corresponding power intensity is 0.5 mW/cm$^2$. (c) Transient photocurrent versus time ($I$-$t$) curves at a bias of 0 V with ultraviolet light on/off switching (370 nm, light power density of 0.5 mW/cm$^{2}$). (d) Amplified views of transient responses for the fabricated photodetector exhibiting an obvious four-stage current response behavior.
In particular, the $I$-$t$ curve of the as-constructed n-ZnO:Ga MW/p-Si heterojunction device was further plotted in Fig. 3(c), which operated at zero bias with a periodic 370 nm light illumination. In addition to the reproducibility and a rapid photoresponse, the $I$-$t$ curve also exhibits distinct four-stage photoresponse behavior to 370 nm ultraviolet light without an external bias voltage, with a single cycle being extracted from the Fig. 3(c) [29,36,41]. The unique configuration and excellent light response performance of this n-ZnO:Ga MW/p-Si heterostructure make the device an ideal model for the subsequent investigation of pyro-phototronic effects for improving electrical and photoelectric performances [41,45,59]. Further, Fig. 3(d) shows the expanded dynamic behavior of the single-cycle transient response, and the contribution of both the photovoltaic and pyroelectric output current signals [17,37,45]. In the plotted $I$-$t$ curve, the relative contributions of the photovoltaic current ($I_{ph}$) and the pyroelectric current ($I_{py}$) were estimated for the fabricated photodetectors. As illustrated in the figure, the current increases steeply as the light is turned on and reaches a maximum value of 0.85 nA. Nevertheless, the current decreases exponentially over time and reaches a stable current value of $\sim$ 0.50 nA. As the light is turned off, the current reaches $\sim$ −0.085 nA, and then begins to increase again. The current in the dark condition attains a stable base value of 0 nA. Stable current values in the dark and under ultraviolet illumination were used to calculate the photocurrent density ($I_{ph}$), which was found to be approximately 0.50 nA.
The emblematic transient characteristic of the device in response to a typical 355 nm laser (the light power of 13.65 mW) was further measured by extracting the photovoltaic effect and pyro-phototronic effect, and its ON/OFF cycle is illustrated in Fig. 4(a). It is clearly shown that the whole transient response procedure can be divided into four stages (labeled I, II, III and IV, respectively). Prior to studying the working mechanism of the phenomenon, a staggered type-II energy band alignment was formed at the interface of the n-ZnO:Ga/p-Si heterojunction [41,49,52,59]. In the zero bias condition, a built-in electric field ($\mathbf {E}$) may form, as shown in Fig. 4(b). Under reverse bias operation, the overall band structure of p-Si shifts upward, leading to the bending of the barrier region. However, because the extra voltage is in accord with the built-in $\mathbf {E}$, which exists within the depletion region of the p-n junction, the thickness of the depletion layer at the p-n junction increases (see Fig. 4(b)) [52,59]. Under forward bias operation, the overall band structure of p-Si shifts downward, leading to bending in the barrier region. As the extra voltage is in contrast to the built-in $\mathbf {E}$, it may result in a reduction of the depletion region toward the n-ZnO:Ga/p-Si heterojunction, as illustrated in Fig. 4(c) [52,59].
Fig. 4. Time response characteristics of the pyro-phototronic-effect-based n-ZnO:Ga MW/p-Si heterojunction photodetector under light-on and light-off illumination. (a) Typical transient photocurrent response of self-powered single-MW photodetector induced by photo-pyroelectric effect via a combination of photovoltaic and pyroelectric effects. (b) Built-in electrical potential ($\mathbf {E}$) of the p-n heterojunction and its direction in the device in the absence of light. (c) At a reverse bias voltage, the combination of built-in $\mathbf {E}$ and extra electric potential can broaden the thickness of the depletion layer toward the n-ZnO:Ga/p-Si heterojunction. (d) At forward bias voltage, the compensation of built-in $\mathbf {E}$ caused by the extra electric potential can lead to a thinner depletion layer toward the n-ZnO:Ga/p-Si heterojunction. (e) Evolution of the p-n heterojunction width and generation of photocurrent induced by the photovoltaic effect during illumination. (f) Evolution of the forward pyro-phototronic effect induces an electric potential in a direction identical to the built-in potential under light-on illumination. (g) Evolution of the reverse pyroelectric potential in a direction opposite to the built-in potential under light-off illumination.
The working principle of the pyro-phototronic effect on the fabricated photodetector was interpreted, and the corresponding dynamic processes are illustrated in Figs. 4(e)–(g). At zero bias, there is no dark current $I_d$ flowing in the circuit under dark conditions at the initial stage (in the first stage of I). When the incident light illuminates the device (the second stage $\sim$ II), a sharp current peak that is attributed to both the pyro-phototronic effect and the photovoltaic effect induces an instantaneous temperature rise ($dT/dt$ $>$ 0) within the ZnO:Ga MWs and photon-generated carriers under ultraviolet light irradiation (Fig. 4(g)) [22,30,61]. As a result, the transient current $I_{py+ph}$ is composed of the photovoltaic effect and the pyro-phototronic effect, and the steady-state current $I_{ph}$ is mainly caused by the photovoltaic effect. Then, the output current decreases substantially and returns to a steady value $I_{ph}$ because the transient pyroelectric polarization degrades to drive the temperature down to a stable state quickly (the third stage $\sim$ III). In this case, the pyroelectric current disappears owing to the absence of temperature variation ($dT/dt$ = 0), and the output current $I_{ph}$ originates only from the photovoltaic effect (see Fig. 4(e)) [17,30,37].
In the fourth stage, a strong reversed current peak $I_{py}$ is observed owing to the reversed pyrocharges, which are induced by the instantaneous temperature decrease ($dT/dt$ $<$ 0) as the light is turned off. The corresponding procedure corresponds to the status indicated in Fig. 4(f). Afterwards, the temperature of the device decreases to room temperature and remains steady without light irradiation. Thus, the disappearance of the pyroelectric current ($dT/dt$ = 0) accompanied by the output current returns to a stable value near zero (Fig. 4(e)). That is, the current of the fabricated device recovers to a dark current $I_d$ (Fig. 4(b)) [12,30,61].
By varying the power densities from 0.002 to 0.5 mW/cm$^2$, the dynamic response of the fabricated n-ZnO:Ga MW/p-Si heterojunction device at 370 nm illumination is systematically investigated and summarized in Fig. 5(a). It is evident that four-stage photocurrent dynamic behaviors induced by the pyro-phototronic effect can be observed at all power densities under a zero-bias voltage. The corresponding currents $I_{py+ph}$ and $I_{ph}$ are extracted and respectively plotted in Fig. 5(b) as blue dots. The output signals of the fabricated heterojunction devices are significantly enhanced by the comprehensive pyro-phototronic effect. In addition, the enhancement ratio of $I_{py+ph}$/$I_{ph}$ increases with an increase in the power density (Shown in Fig. 5(b), solid violet line). The visibly enhanced ratio indicates that the comprehensive pyro-phototronic effect greatly increases and/or optimizes the photocurrent of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector [17,22,37,45].
Fig. 5. Pyro-phototronic effect enhanced ultraviolet photodetection performances of the self-powered ZnO:Ga MW/p-Si heterojunction photodetector. (a) Transient $I$-$t$ characteristics of the photodetector mediated by pyro-phototronic effect at a wavelength of 370 nm with different power densities from 0.02 to 0.5 mW/cm$^{2}$. (b) Comparison of photocurrent $I_{ph}$ and pyro-phototronic effect induced $I_{py+ph}$ as a function of the radiated light power density, and the enhancement of the photocurrent calculated from $I_{py+ph}$ to $I_{ph}$ under different power densities, showing that the $I_{py+ph}$/$I_{ph}$ ratio increases with the increase of light power density. (c) Comparison of responsivity $R_{ph}$ and pyro-phototronic effect induced $R_{py+ph}$ as a function of the radiated light power density, and the calculated enhancement of responsivity from $R_{py+ph}$ to $R_{ph}$ under different power densities, showing that the $R_{py+ph}$/$R_{ph}$ ratio increases with the increase of light power density. (d) Comparison of detectivity $D_{ph}$ and pyro-phototronic effect induced $D_{py+ph}$ as a function of the radiated light power density, and the calculated enhancement of responsivity from $D_{py+ph}$ to $D_{ph}$ under different power densities, showing that the $D_{py+ph}$/$D_{ph}$ ratio increases with the increase of light power density.
To understand the role of the comprehensive pyro-phototronic effect in enhancing the performance of the fabricated heterojunction photodetector, the corresponding photoresponsivity was calculated according to Eq. (1). The corresponding photoresponsivities $R_{py+ph}$ and $R_{ph}$ were calculated, and they are shown in Fig. 5(c) (blue dotted lines). At the presence of pyro-phototronic effect, the responsivity can be increased up to 0.25 A/W. Besides, the transient $R_{py+ph}$ and stable $R_{ph}$ demonstrate a monotonous reduction trend for various power densities. As illustrated in Fig. 5(c) (violet dotted lines), the enhancement ratio of $R_{py+ph}$/$R_{ph}$ increases by varying the power density. In particular, the enhancement ratio increases sharply at a light power intensity of approximately 0.1 mW/cm$^2$, suggesting that the photoresponsivity $R_{py+ph}$ is dramatically enhanced by introducing a comprehensive pyro-phototronic effect. Therefore, the device performance of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector, especially in terms of responsivity, can be obviously enhanced by a comprehensive pyro-phototronic effect [17,22,37,45].
As another critical parameter of the photodetector, the specific detectivity $D$ was calculated under different incident light intensities according to Eq. (2). As shown in Fig. 5(d), the steady-state $D_{ph}$ corresponding to the steady current decreases with increasing power density of the illuminated ultraviolet light. Particularly, the detectivity $D_{py+ph}$ obtained from the transient current also exhibits a significant decrease with an increase in incident power density. Compared with steady-state $D_{ph}$ ($\sim$ 1.75$\times$10$^{12}$ Jones), the detectivity $D_{py+ph}$ is increased up to 2.30$\times$10$^{12}$ Jones. Additionally, the corresponding ratio between $D_{py+ph}$ and $D_{ph}$ was also calculated, and it is plotted in Fig. 5(d) (the violet solid line). This demonstrates that the ratio of $D_{py+ph}$/$D_{ph}$ is clearly increased by introducing a comprehensive pyro-phototronic effect.
It has previously been suggested that the pyro-phototronic effect is greatly influenced by the application of external biasing [17,22,37]. Therefore, to account for this behavior, the existence of pyro-phototronic signals in the as-fabricated n-ZnO:Ga MW/p-Si heterojunction photodetectors was investigated under different bias voltages (light power density of 0.5 mW/cm$^2$). The $I$-$t$ characteristics of the fabricated heterojunction device reversely biased at voltages ranging from 0 to −1.0 V are presented in Fig. 6(a). At a reverse bias voltage smaller than −0.5 V, the output current demonstrates obvious four-stage responses in each cycle, clearly showing the pyro-phototronic effect in the as-grown ZnO:Ga MWs. As the reverse bias voltage increases in the range of −0.5– −1.0 V, the dark-state current increases, and the transient currents $I(t)$ and $I^\prime (t)$ gradually disappear because of the compensation of the pyro-phototronic effect by Joule heating. These results indicate that the pyro-phototronic effect is controllable and highly dependent on the bias voltage [33,38,43].
Fig. 6. Pyro-phototronic effect enhanced ultraviolet photodetection performances of the self-powered ZnO:Ga MW/p-Si heterojunction photodetector. (a) Transient $I$-$t$ characteristics of the pyro-phototronic effect enhanced photodetector at a wavelength of 370 nm with different reverse voltages from 0 V to −1.0 V; the light power density is set to 0.5 mW/cm$^{2}$. (b) By varying the applied bias, the comparison of photocurrent $I_{ph}$, pyro-phototronic induced the current $\sim$ $I_{py}$ and the pyro-phototronic effect enhanced the photocurrent $I_{py+ph}$. (c) Comparison of responsivity $R_{ph}$ and pyro-phototronic effect induced $R_{py+ph}$ as a function of the applied voltage, and the calculated ratio of $R_{py+ph}$/$R_{ph}$ under different reverse biases, showing that the enhancement decreases with the applied bias. (d) Comparison of detectivity $D_{ph}$ and pyro-phototronic effect induced $D_{py+ph}$ as a function of the applied bias, and the calculated ratio of $D_{py+ph}$/$D_{ph}$ under different reverse biases, showing that the enhancement decreases with the applied bias.
Furthermore, the differences between the transient and stable currents $I_{py+ph}$, $I_{ph}$, and $I_{py}$ were calculated as a function of the reverse bias voltages. Shown in Fig. 6(b), the currents $I_{py+ph}$ and $I_{ph}$ exhibit an increasing trend with an increase in the applied reverse bias voltage. Additionally, the transient $I_{py}$ first increases and then decreases with an increase in the applied reverse bias voltage, showing a maximum value at the reverse bias voltage of −0.5 V. Therefore, as the bias voltage increases, the photocurrent and dark current of the device increases but the pyro-phototronic effect gradually disappears when the reverse bias voltage is larger than −0.5 V. This suggests that the pyro-phototronic effect gradually weakens until it disappears because of the Joule heat arising from the applied bias voltage [43,45,61].
By varying the applied reverse bias voltage, the corresponding photoresponsivities $R_{py+ph}$ and $R_{ph}$ were calculated. As shown in Fig. 6(c), the transient $R_{py+ph}$ and $R_{ph}$ exhibit a monotonous increasing trend with the reverse bias voltage (blue dotted lines). The enhancement ratio $R_{py+ph}$/$R_{ph}$ decreases with an increase in the reverse bias voltage. In particular, the enhancement ratio decreases sharply to 1.0 at a reverse bias of −1.0 V, suggesting that the pyro-phototronic effect on the photoresponsivity disappears [34,44,45].
Finally, the specific detectivity $D$ was also calculated. As shown in Fig. 6(d), the steady-state $D_{ph}$ corresponding to the steady current first decreases and then increases with an increasing reverse bias voltage. Particularly, the detectivity $D_{py+ph}$ obtained from the transient current also exhibits significantly similar variation with an increase in the reverse bias voltage. The corresponding ratio between $D_{py+ph}$ and $D_{ph}$ was also calculated and is plotted in Fig. 6(d) (violet solid line). This demonstrates that the ratio of $D_{py+ph}$/$D_{ph}$ dramatically decreases as the comprehensive pyro-phototronic effect gradually disappears.
As described above, not only do the pyro-phototronic signals change with external bias, but there is also a significant change in the response speed of the photodetectors. To further confirm the relationship between the ultrafast response speed and the pyro-phototronic effect, a single-cycle transient response under different bias voltages (0 and −1.0 V) should be examined. The response time was defined as the time from 10% to 90% of the maximum photocurrent, and the recovery time was defined as the falling time from 90% to 10% of the maximum photocurrent. The enhancing the pyro-phototronic effect on the response time of the self-powered n-ZnO:Ga MW/p-Si heterojunction photodetector is carefully demonstrated by comparing the performances of the device operating under the biases of 0 V and −1.0 V when applying a power density of 0.5 mW/cm$^2$ illumination. As illustrated in Fig. 7(a), the $I$-$t$ characteristic obviously indicates the disappearance of the pyro-phototronic effect-induced sharp peak at the reverse bias of −1.0 V [41,57]. Meanwhile, at a reverse bias voltage of −1.0 V, the rising time and falling time were evaluated as 499 and 412 $\mu$s, respectively.
Fig. 7. (a) Enlarged single-period transient $I$-$t$ characteristic of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector at a bias of −1.0 V; the corresponding response times are extracted as 499/412 $\mu$s. (b) Enlarged one-period transient $I$-$t$ characteristic of the fabricated n-ZnO:Ga MW/p-Si heterojunction photodetector at zero bias, and the corresponding response times are extracted as 79/132 $\mu$s.
The increased influence of the pyro-phototronic effect on the response time of the fabricated single-MW photodetector was revealed by comparing the rise and fall times under bias voltages of 0 V and −1.0 V at a power density of 0.5 mW/cm$^2$. When operated at zero bias condition, the short-circuit current response to ultraviolet light illumination demonstrates the typical four-stage behavior of the pyro-phototronic effect at zero bias voltage, and the $I$-$t$ curve is illustrated in Fig. 7(b). By calculating the response times obtained from the $I$-$t$ curves, the rising time and falling time were extracted to be 79 and 132 $\mu$s for the self-powered n-ZnO:Ga MW/p-Si heterojunction photodetector under zero bias, respectively. Considering the system response to the optical chopper switched light signals, the actual response time of these devices could be faster. Therefore, the response time of the constructed n-ZnO:Ga MW/p-Si heterojunction photodetector was reduced from several hundred microseconds to tens of microseconds, which was accelerated by the pyro-phototronic effect [44,57]. Interestingly, the behavior of the peaks in the photoresponse changes, and the rise and fall times increase as the applied field varies from 0 V to −1.0 V, which confirms that the main governing phenomenon behind the ultrafast response is the pyro-phototronic effect. These results also indicate the need to configure such photodetectors in a self-powered manner without applying any bias voltages to maximize the enhancement by the pyroelectric effect. With the pyro–phototronic effect, the present device architecture opens a new avenue for designing an ultraviolet ultrafast detector for future advanced optoelectronic devices [12,36,57].
4. Conclusion
To summarize, a heterojunction composed of an individual ZnO:Ga MW and p-Si substrate was designed, and its photoresponse to ultraviolet light illumination was systematically checked and investigated. The experimental results suggest that the photocurrents, responsivities, and detectivities of the fabricated ZnO:Ga/Si heterojunction devices at all light intensities increased by over one order of magnitude from the pyro-phototronic and photovoltaic effects. Meanwhile, the rise and fall times were dramatically reduced from 499 $\mu$s to 79 $\mu$s and from 412 $\mu$s to 132 $\mu$s, respectively. The working principle behind the different photoresponse behaviors of the photodetector under ultraviolet light illumination was carefully studied. In addition, the biased voltages and light power densities also played a significant role in the photovoltaic-pyroelectric coupled effects on device performances. This work not only offers a simple and effective scenario for fabricating a high-performance self-powered ultraviolet photodetector, but also provides an in-depth interpretation of the photovoltaic-pyroelectric coupled effect on a single wire-based p-n heterojunction structure.
Funding
Postgraduate Research Practice Innovation Program of Jiangsu Province (KYCX20\_0171); Open Fund of Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education (INMD-2020M03); Fundamental Research Funds for the Central Universities (NT2020019); National Natural Science Foundation of China (11774171, 11874220, 11974182, 21805137).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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