Tunable channel width of a UV-gate field effect transistor based on ZnO micro-nano wire

Abstract

A n-channel field effect transistor (FET) based on ZnO microwire has been fabricated for ultraviolet detection, where a PEDOT:PSS/ZnO wire junction serves as the gate. The sensitivity of the junction FET was enhanced by two orders of magnitude with a fast response time <1 s at 3 V compared with a Ag–ZnO–Ag detector under the illumination of UV light (325 nm). Such a great improvement in photoresponse is attributed to the introduction of a depletion layer, resulting in a lower dark current. The change of the junction FET channel width with various UV light intensities was calculated and discussed in terms of the carrier diffusion theory and the ideal I–V characteristics of the depletion mode JFET.

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Introduction

ZnO microwire has become a promising material for constructing ultraviolet (UV) photodetectors (PDs) due to extraordinary physical properties, including a direct band gap (E_g ∼ 3.4 eV), high electron mobility and excellent piezotronic effect.¹ In the past few years, photovoltaic type UV-PDs are based on various interface junctions such as p–n junction,^2–4 Schottky junction,^5–7 solid–liquid heterojunction,^8–10 which form a built-in electric field at the interface to drive photogenerated electron–hole pairs. Additionally, the photoconductive type UV-PDs is the other research hotspot, it was previously reported that a high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime.¹¹ Nevertheless the oxygen molecules adsorption–desorption is slow process resulting in a long response time. Surface functionalization of nanomaterial is the efficient method to promote the separation of the charge and shorten the response time in the photoconductive UV detectors.^12–18 Shengyi Yang et al. provides a new idea to fabricate a MOSFET-based photodetector with different gate-voltage and a wide spectral bandwidth.¹⁹ However, modulating electron transmission channel width based on p–n junction FET have not been applied for practical photoconductive detector. Moreover, the theoretical mechanism accounting for UV light modulation of the junction interface is still lacking and much desired.

In this work, we proposed a FET-detector with PEDOT:PSS/ZnO wire junction as the gate, which worked at zero gate voltage and showed a remarkable and fast UV photoresponse. The depletion width of ZnO/PEDOT junction under UV illumination with different light intensities was modulated to realize a strong and fast response. Furthermore, the excellent device performance was explained by the carrier diffusion theory and the ideal I–V characteristics of depletion mode JFET.

Experiential section

ZnO microwires investigated in this work were synthetized by a chemical vapor deposition (CVD) method without any catalyst, which is reported previously.²⁰ For device fabrication, some flexible polystyrene (PS) substrates of 30 mm length, 15 mm width, and 0.4 mm thickness were cleaned in ethanol, and the suitable ZnO microwire was placed on the PEDOT to form p–n junction, with silver paste acting as contact electrode to form ohmic contact.²¹ The morphology and structure of the ZnO wire and the fabricated devices were characterized by optical microscope, scanning electron microscope (FE-SEM, LEO1530). The photoluminescence spectrum was measured by a Raman Spectrometer (Jobin-Yvon, HR800) and excited by a 325 nm He–Cd laser source.

Results and physical mechanism

Fig. 1(a) depicts the morphology of ZnO microwire. The diameter and length of the wires are about several micrometers and thousands of micrometers, respectively. The PL spectrum of ZnO microwires excited by a 325 nm He–Cd Laser at room temperature is shown in the inset of Fig. 1(a). It is clearly seen that a sharp UV emission appears at 379 nm and a weak emission in the visible region corresponding to deep-level transition. The intense UV emission is attributed to the near-band-edge emission and the deep-level transition is related to the oxygen vacancies, surface states and some structural defects.²² Fig. 1(b) illustrates the schematic diagram of the n-channel FET hybrid device. In this FET-PDs device, the ZnO microwire was laid across the PEDOT to form pn junction, and the UV light illuminated on the pn junction acts as the V_g of the FET. For all I–V measurements, the potential was applied between the two silver electrodes.

Fig. 1 (a) SEM image of the as-prepared ZnO microwires. The inset shows the photoluminescence spectrum of ZnO micro/microwires with an excitation wavelength of 325 nm. (b) Schematic of the n-channel FET detector.

The ultraviolet light illuminated on ZnO/PEDOT p–n junction is seen to have gate effect on the ZnO n-channel. This p–n gate harvests an inherent built-in potential, which acts as driving force to separate the UV-light generated electron–hole pairs, so that the depletion layer width of the p–n junction decreases. At the same time, electronic transmission channel is broadened. Consequently, a great photoresponse was measured at 3 V bias versus the time when the gate was periodically exposed to the UV light (325 nm), which is shown in Fig. 2(b). In order to eliminate the influence of small barrier at the two end of ZnO microwire, the spot of UV light is about 2 μm. When the light is on, the current increases rapidly reaching a maximum, and then decreases gradually to a stable value. The decrease is ascribed to the recombination of the photogenerated charge carriers and with a slow diffusion process the accumulation of charge carriers can occur near the interface, which causes a higher rate of recombination under constant illumination. The rise time for a hybrid device is within 0.8 s, and decay response time constant of the device is within 3.8 s, which is shown in Fig. 2(c).

Fig. 2 (a) I–V characteristics of Ag–ZnO–Ag with PEDOT as the gate under 325 nm UV source and in dark. The inset shows the optical image of the device. (b) Photoresponse at 3 V when illuminating on the PEDOT/ZnO junction with 325 nm UV source. (c) Specific corresponding response time and recovery time. (d) The corresponding I–V behavior in dark or under UV illumination. The inset shows schematic of MSM structure and photo response at 3 V illuminated by 325 nm light source respectively.

The sensitivity R was deduced from the formula R = (I_UV − I₀)/I₀, where I₀ is the dark current and I_UV is the current under UV illumination. According to the current–time relationship shown in Fig. 2(b), the sensitivity of the FET-detector is 2752%.

For comparison, we fabricated the Ag–ZnO–Ag Ohmic-contact UV detector, which is depicted in the inset of Fig. 2(d).

The highly linear I–V characteristic curve of the device is shown in Fig. 2(d). Under the UV source (325 nm) with intensity of 17.4 mW, the current changed slowly and had no obvious increase. The current was not saturated until 140 s continuous illumination. The recovery time was 482 s, and the current could not reset to its initial state even after 700 s. Based on the formula R = (I_UV − I₀)/I₀, the sensitivity of Ag–ZnO–Ag detector is 22.46%. Here, the p–n junction gate plays a significant role in enhancing the performance of the FET detector.

A physical model based on band energy theory to account for the origin of the enhanced performance for FET-PDs is shown in Fig. 3.

Fig. 3 (a) Schematic illustration of electron–hole generation process and transmitting procedure coated by PEDOT:PSS. (b) The difference of depletion layer width in dark or under UV illumination with 325 nm wavelength.

The PEDOT on the surface of ZnO could result in the formation of a localized p–n junction, which creates a charge depletion region in the microwire. The role of PEDOT is similar to that of the gate in field-effect transistor. When the PEDOT:PSS/ZnO NR interface is illuminated by UV light, the electron–hole pairs are generated, which are efficiently separated by the built-in potential due to the depletion region at the interface. The electrons get transferred to the ZnO side, and the holes move to the PEDOT side, which changes the interface charge distribution and dramatically reduces the capacitance of the interface region. As a consequence, the width of depletion layer decreases, resulting in a great increase in the photoconductance of FET UV detector, shown in Fig. 3(a).

As can be seen in Fig. 3(b), when the device was taken to the dark condition, a broad depletion layer exists at the surface of ZnO. The blocked electronic transmission channel can reduce the dark current. When the UV light was illuminated on the ZnO/PEDOT junction, the built-in potential reduces and the channel broadens out, resulting in a high sensitivity and fast response.

To further confirm the scaling effect of the depletion channel on the improved performance, the representative photoresponse curves with increasing power were measured from 0.17 mW to 17.4 mW at 5 V, shown in Fig. 4(a). The light power density differed by changing the decay mode of a Raman Spectrometer (Jobin-Yvon, HR800). When the device was exposed to the UV light or not, a sharp on/off switching behavior appeared.

Fig. 4 (a) The source–drain current of the FET-PDs as the gate voltage varies from 0.17 mW to 1.74 mW. (b) Typical I_ds–V_ds characteristic of the device by varying light power from 0.17 mW to 17.4 mW with (325 nm). (c) The linear relation for ΔB with the square of Δlight intensity (m² W²). The inset shows the increase of sensitivity with Δlight intensity.

To gain the insight of UV gate effect on the electronic transmission channel, the depletion layer width is calculated based on the carrier diffusion theory and the ideal I–V characteristics of JFET depletion mode.

According to the current–voltage relationship for one-sided n-channel JFET:

where

The space charge region width between ZnO/PEDOT junction can be plotted by the equation below:

where I_DS is the current between the source and the drain terminals, V_DS is the drain-to-source voltage, V_PO is the internal pinchoff voltage for JFET, V_bi is the built-in potential barrier between the ZnO/PEDOT junction, V_GS is the gate voltage, a is the channel thickness, L is the channel length, e is the electronic charge, μ_n is carriers mobility of ZnO, G₀₁ is the conductance of the channel, is the dielectric constant of ZnO, V_R is the magnitude of the applied reverse-biased voltage, N_d and N_a are the concentration of electron and hole respectively.

Combined the equation above the fitting formula is given by:

where , , C = V_bi − V_GS

Therefore

where ΔW is the change value of depletion layer width at the contact interface between ZnO and PEDOT, μ_n and μ_p are carriers' mobility of ZnO and PEDOT, ε_p and ε_n is the dielectric constant of PEDOT and ZnO respectively.

The source to drain I–V curve was measured by using Keithley 4200SCS with sweeping voltage from 0 to 5 V at room temperature, as shown in Fig. 4(b). According to the fitting curve, we can get the values of ΔB by varying optical signal, shown in Fig. 4(c). The ΔW is negative because of the negative ΔB based on the formula (4), it is indicated that the W would be narrow down with the increase of light intensity. In other words, the depletion layer width decreases and at the same time the electronic transmission channel broadens out by increasing the light power. The opened channel of junction FET bring about the improvement of sensitivity shown in the inset of Fig. 4(c). The results of the validation study showed that the performance of UV PDs can be enhanced by modulating the electronic transmission channel, and a high-performance of UV detector can be expected when the structure integrated with the conventional UV detectors.

Conclusions

In summary, ZnO wire based n-channel junction FET has been successfully fabricated for UV detection. The sensitivity of the device increased from 22.46% to 2752% and the response time decreased from 140 s to 0.8 s owing to the p–n junction. The channel width of junction FET can be modulated under various UV light powers. The junction FET channel can be determined to linearly broaden with the increasing of light intensity according to the carrier diffusion theory and the ideal I–V characteristics of JFET depletion mode. This work provides a novel insight into the fundamental physics for the optoelectronics.

Acknowledgements

This work was supported by the National Major Research Program of China (2013CB932601), Major Project of International Cooperation and Exchanges (2012DFA50990), NSFC (51232001, 51172022, 51372023), the Research Fund of Co-construction Program from Beijing Municipal Commission of Education, the Fundamental Research Funds for the Central Universities, the Program for Changjiang Scholars and Innovative Research Team in University.

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