|Abstract: ||Abstract: A self-powered photodetector with ultrahigh sensitivity, fast photoresponse, and wide spectral detectivity covering from 1000 nm to 400 nm based on graphene/ZnO/Si triple junctions has been designed, fabricated, and demonstrated. In this device, graphene serves as a transparent electrode as well as an efficient collection layer for photogenerated carriers due to its excellent tunability of Fermi energy. The ZnO layer acts as an antireflection layer to trap the incident light and enhance the light absorption. Furthermore, the insertion of the ZnO layer in between graphene and Si layers can create build-in electric field at both graphene/ZnO and ZnO/Si interfaces, which can greatly enhance the charge separation of photogenerated electron and hole pairs. As a result, the sensitivity and response time can be significantly improved. It is believed that our methodology for achieving a high-performance self-powered photodetector based on an appropriate design of band alignment and optical parameters can be implemented to many other material systems, which can be used to generate unique optoelectronic devices for practical applications.
Hybrid nanocomposites consisting of multicomponent nanomaterials have attracted significant attention due to their uniqueness in multifunctionality and versatility, which cannot be found in one single component material. In this article, based on the appropriate band alignment and unique properties of different materials, we design, fabricate, and demonstrate a self-powered photodetector with ultra-high sensitivity and wide spectral response by combining graphene, zinc oxide (ZnO), and silicon heterostructures. Graphene, a monatomic-thick two-dimensional (2D) material with carbon atoms arranged in a hexagonal honeycomb lattice, has recently been studied intensively, mainly due to the atom-thick 2D structure and excellent properties of graphene sheets.1–7 Because of its high electron mobility (200 000 cm2/V s) and high electric current carrying capacity (>108 A/cm2), graphene is an excellent candidate for the transparent electrode. Furthermore, due to its distinct band structure, a small amount of extra electrons can cause a noticeable change in the conductance of graphene.8–10 Thus, graphene itself can serve as an absorber in detectors that are ultrafast and ultra-broadband. However, the sensitivity is relatively poor, which is not suitable for most practical applications.11–13 Even though the actual mobility of CVD grown and transferred graphene is only around 2000 cm2/V s, with its high transparency and environmentally friendly properties, CVD graphene still possesses several unique features for the exploration of next generation optoelectronic devices.4 In several previous articles, graphene-silicon heterojunction devices (GSHDs) have attracted much attention because they are a suitable candidate for applications in photo-detection,14–16 communication,17 solar cells,18,19 barristers,20 and chemical and biological sensors.21 In photo-detection, GSHDs are promising for their high responsivities over a broad spectral bandwidth near the visible region.14,22 However, in the ultraviolet region, the photo-response of the GSHD is not comparable to the visible region. In addition, the photo-response time is also an important factor for an ideal photodetector, but the GSHD does not possess a fast response time for practical applications.
On the other hand, zinc oxide (ZnO) has been considered as a promising candidate in ultraviolet photoelectric and gas sensing applications due to its wide direct band semiconductor behaviors (∼3.37 eV), high exciton binding energy, high UV absorption coefficient, and low cost.23–27 Comparing with other wide band-gap semiconductors, ZnO can be grown on various substrates, which makes it suitable for large-area photonic devices such as light emitting devices, solar cells, and photodetectors.28,29 Among various types of ZnO based photodetectors, such as pn junction, p-i-n junction, avalanche, and Schottky photodiodes, the Schottky photodiode contains multiple advantages, including high quantum efficiency, high response speed, low dark current, and high light sensitivity.30 Here, we provide a seminal attempt with the integration of graphene, ZnO, and highly doped silicon, forming a composite photodetector with ultrahigh gain and wide spectral response. With the additional ZnO layer, the photon could be trapped in between ZnO and silicon, and the built-in electric field can be created near the heterojunctions. As a result, the sensitivity of our device can be greatly improved from the near UV to the infrared region comparing with GSHD. In addition, the photodetector possesses a fast response time and it is self-powered, which are very useful for practical applications.
The device structure is illustrated in Fig. 1(a); a film consisting of single layer graphene was coated conformably onto a patterned ZnO/SiO2/n-Si substrate with a ZnO/n-Si half-opened window and pre-deposited Ti/Au contacts on top of the SiO2 area and on the corner of half-opened silicon area as contacts. The thickness of ZnO is optimized to 34 nm by the measurement of incident photon-to-electron conversion efficiency (IPCE) and the reflection spectrum. For comparison, the GSHD without the ZnO layer has also been made and measured as a control sample. Details on graphene synthesis, ZnO deposition, device fabrication, and measurements are as follows. The graphene/ZnO/silicon heterojunction devices were fabricated on commercially purchased highly n-doped (resistivity of 0.1 Ω cm) Si wafers with 300 nm SiO2 layer. First, these wafers were patterned by photolithography and etching in BOE solution to create a window to expose the n-doped silicon. Second, a thermal evaporation deposition technique was used to deposit Ti/Au (5 nm/100 nm) film contact pads on top of 300 nm SiO2 layer as well as on the corner of the exposed surface of the Si wafer. Third, a RF magnetron sputtering deposition technique was used to deposit the ZnO film on the exposed silicon window. The graphene films were grown at 1000 °C by using a mixed gas of CH4 (40 sccm) and H2 (20 sccm) via a CVD method in which 25 μm thick Cu foils were employed as the catalytic substrates. Fourth, to transfer graphene films onto the window, the as-grown graphene films (on metal foils) were spin-coated with PMMA, and the metal foils were dissolved in a dilute FeCl3 solution. The PMMA-coated graphene films were then transferred onto the top of the exposed window of ZnO/Si. After that, the devices were thoroughly rinsed with acetone and isopropanol to remove the PMMA and dried. A detailed characterization of the grown graphene sheet can be found in our previous publication.31 For the photocurrent measurement, we used an argon laser working at 488 nm as the excitation light source. A measurement system (Keithley 236) was utilized to supply the dc voltage and to record the photocurrent. The IPCE spectra were recorded using a setup consisting of a lamp system, a chopper, a monochromator, a lock-in amplifier, and a standard silicon photodetector (ENLI Technology). To determine the spectral response and time response of the photodetectors, we used a home-built system composed of a lamp system, a monochromator, an oscilloscope, and an optical chopper. Optical reflectance was carried out by UV/Vis/NIR spectroscopy with a PerkinElmer lambda 750 spectrophotometer using reflectance accessory.