ZnO Subwavelength Wires For Fast-response Mid-infrared Detection

Mid infrared (IR) detection, typically relying on thermal or photo response, has wide applications in the many fields. Generally, IR photon detectors offer advantages of fast response and high sensitivity, but usually require low temperature operation with complex cooling equipments. Thermal detectors provide the possibility for room temperate operation with broadband response, but usually suffer from slow response times due to relatively large thermal inertia of the sensitive elements.Zinc oxide (ZnO) micro- and nanowires show great commercial potential owing to their low cost and easy fabrication. Utilizing the relatively strong absorption in mid-IR regime, the small dimension and the excellent chemical and thermal stabilities of ZnO microwire, we propose mid-IR thermal detection based on ZnO subwavelength wires. In the paper, we explore the photothermal response of ZnO subwavelength wires, and demonstrate a room-temperature-operation thermal detector.ZnO subwavelength wires with smooth surface and hexagonal section were synthesized via a chemical vapor transport process. The wire was placed across two Ti/Au electrodes sputtered on the plate. To improve the electrical contact properties, a microdrop of In/Ga liquid alloy was used to cover the contact area to ensure an ohmic contact. The middle part of the wire was suspended above the groove to get isolation from the substrate. Light from a Coherent K-250 CO₂ laser, centered at the wavelength of 10.6μm, was used to irradiate the ZnO wire. The laser beam was focused by a ZnSe lens to a 220-μm-diameter spot on the ZnO wire. Since the dark resistance of the ZnO wire is very large, a constantly illumination from a halogen lamp (about 6800 lx) is applied on the ZnO wire for stable and reliable measurement of the response of the ZnO wire.When a ZnO wire absorbs mid-IR light, its temperature rises, leading to the change in the resistance that can be used to retrieve the intensity of the incident light. We measured some typical I-V characteristics of ZnO wires under irradiation of 10.6-μm-wavelength light at various intensities. It is proved the current decreases with the increase of irradiation power and the resistance increases linearly with intensity of theirradiation. We obtain the noise intensity of 700μV/Hz^1/2 (at 1kHz), which corresponds to a noise equivalent power (NEP) of 5.8μW/Hz^1/2.To investigate the influence of ambient on ZnO wire for mid-IR detection, we measured resistance of a ZnO wire in typical ambient gases including air, argon, nitrogen, and oxygen. Though the background resistance varies in different atmospheres, the amplitude and response time are insensitive to the ambient gases. The response time of the ZnO wire mid-IR detection is about 1.3 ms, which is much faster than other types of room-temperature-operated microbolometers or thermocouples. The time constantτis theoretically estimated to be 1.1 ms, which coincides well with the measured value of 1.3 ms.