Nanyang Research Programme Investigating Photoconductivity of Heterostructures (SPMS06A) Student Name: Zhang Yiwen School of Student: Nanyang Junior College Supervisor: WANG Xiao, Renshaw (Asst Prof) School of Supervisor: Nanyang Technological University Table of Contents 1 ABSTRACT ................................................................................................... 2 2 INTRODUCTION ............................................................................................. 2 3 LITERATURE REVIEW .................................................................................... 3 4 METHODOLOGY ............................................................................................ 5 5 RESULTS AND DISCUSSION ........................................................................... 7 6 CONCLUSION ............................................................................................... 9 7 REFERENCES ............................................................................................. 10 1 Abstract Nowadays, photodiodes are widely utilized in both industrial and daily basis, such as spectroscopy, analytical instrumentation, optical position sensors, laser range finders and smoke detectors.1 However, there are limitations such as corrosion, poor sunlight absorption and external bias requirement.2 In order to improve the sensitivity and flexibility of photodiodes, this project aims to find a suitable two-dimensional MoS2 material which can be fabricated onto the device and investigate its photoconductivity. Methods such as mechanical exfoliation, atomic force microscopy (AFM) and electron beam lithography (EBL) were carried out. The results shown by the microscopic images proved the effectiveness of these methods to obtain suitable MoS2 monolayers. Hence, the methods can be used to make other heterostructure material for photodiodes as well. 2 Introduction A photodiode is a semiconductor device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes can be used to detect the presence or absence of minute quantities of light and can be calibrated for extremely accurate measurements from intensities below 1 pW/cm2 to intensities above 100 mW/cm2.3 It is also a semiconductor device that has p–n junction. In this project, the kind of photodiode with a reverse biased p-n junction is focused. The working principles of photodiodes are similar to regular semiconductor diodes. When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced(Fig.2.1). 4,5,6 Fig.2.1. It is a PN junction photodiode. Blue arrows are the incident photons; green arrows are the movement of both types of electrons; the red spheres are the electrons while the sliver spheres are the (positive) electron holes. In addition, MoS2 material is used in this project. Due to its decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike 7 the bulk material, the MoS2 monolayer emits light strongly. And this can enhance the photoconductivity of the device. Therefore, this project will present practical procedures of fabricating desirable MoS2 monolayers onto the device and test the device to investigate the photoconductivity of the simplified photodiodes configuration. 3 Literature Review A p–n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. The "p" (slightly positive) side contains an excess of holes, while the "n" (slightly negative) side contains an excess of electrons(Fig.3.1). Fig.3.1. It is a PN junction. The green circles are the electrons while the white circles are electron holes. The blue area is the n-type semiconductor material while the red area is the p-type semiconductor material. This is a diode with a p-n junction works in a reverse biased condition(Fig.3.2). The applied reverse bias adds to the built-in voltage and results in a wider depletion region. Fig.3.2. It is a diode with a p-n junction under reverse biased condition. The white electron holes are filled with the green electrons and the electron flow is to the left. In a photodiode, it is discovered that the light is absorbed exponentially with distance and is proportional to the absorption coefficient. The absorption coefficient is very high for shorter wavelengths in the ultraviolet region and is small for longer wavelengths. Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while silicon becomes transparent to light wavelengths longer than 1200 nm.8 MoS2 has been one of the most studied layered transition metal dichalcogenides (TMDCs). Monolayer MoS2 is a semiconductor with a direct bandgap of 1.8 eV. This property of MoS2 is motivating, making it possible for 2D materials to be used in switching and optoelectronic devices.9 Therefore, MoS2 is an optimal material to be used for photodiodes. To further facilitate the process of obtaining suitable MoS2 monolayers, atomic force microscopy (AFM) was adopted. AFM is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit.10 AFM has three major functions such as measurement, imaging and manipulation. AFMs can be used to measure the forces between the probe and the sample as a function of their mutual separation.11 For imaging, by scanning the position of the sample with the tip and recording the height of the probe that corresponds to a constant probe-sample interaction, a three-dimensional shape (topography) of a sample surface at a high resolution can be obtained. This is achieved by raster scanning the position of the sample with respect to the tip and recording the height of the probe that corresponds to a constant probe-sample interaction. 12 The surface topography is commonly displayed as a pseudocolor plot. In manipulation, the forces between tip and sample can also be used to change the properties of the sample in a controlled way.13 Apart from a three-dimensional shape (topographical images) obtained, AFM can also measure other properties such as stiffness and electrical properties of the sample and display them as images with a rather high resolution. 4 Methodology The practical procedures of finding a piece of suitable MoS2 monolayers include three sections: material preparation, material characterisation and device fabrication. 4.1 Material preparation In this project, with the use of mechanical exfoliation14 (scotch-tape method), a suitable thickness of MoS2 piece can be obtained. Then, use tweezers to adhere the MoS2 piece to the brighter surface of the SiO2/Si substrate (with 285 nm thick of SiO2 on top of 0.7mm thick of Si). The SiO2/Si substrate is a 10mm×10mm chip cut by diamond pen for the convenience in the location of MoS2 monolayer. 4.2 Material characterisation Next, optical microscope is used to locate the position of a suitable single layer MoS2 material. Usually, a blue rectangle-shaped layer is desired. Additionally, by removing the impurities on the surface of the chip in a vacuum condition which makes the SiO2/Si chip become hydrophilic, there is a higher possibility to obtain a suitable single-layered MoS2. In order to confirm the layer number, AFM was used to measure the thickness of the two-dimensional MoS2 material. 4.3 Device fabrication As the device made is rather small, a layer of polymer ‘mask’ helps facilitate the process of making electrodes on the device. The formation of a layer of Poly(methyl methacrylate) (PMMA) can be done by spin coating. To locate the position of the single-layered MoS2 material more precisely, the method of electron-beam lithography (EBL) is conducted to make cross bar array (each cross is 2.5 µm apart). Hence, with the record of cross bar array, the position of the single-layered MoS2 can be determined more accurately and more efficiently. In order to dissolve PMMA in organic solvents, the electron beam is used to break the intermolecular bonds in PMMA. Then, during the ‘develop’ process, the solvent of a mixture of methyl isobutyl ketone (MIBK) and isopropanol (IPA) with a ratio of 1:3 is used to dissolve the broken bonds parts of PMMA. The 1:3 proportion of MIBK and isopropanol (IPA) is optimal for the use of a solvent as pure IPA cannot dissolve PMMA and low concentration of MIBK is not able to dissolve PMMA fast enough. (Fig.4.1) Fig.4.1. Device with a polymer mask. Then, supply sufficient power to meltdown the metal such as gold and keep raising the temperature until the molten gold vapourise and adhere to the device with the polymer mask. After cooling down, the electrodes are formed. (Fig.4.2) Fig.4.2. Device with a polymer mask coated with gold. Next, the process of ‘lift-off’ will dissolve the device coated with gold solid into a second solvent such as Acetone, PMMA can thus be removed as well as the gold coated on PMMA, leaving gold electrodes. (Fig.4.3) Fig.4.3. Device with gold electrodes installed. 4.4 Device Testing Test the photoconductivity of the device made with voltage applied at the neighbouring gold electrodes under both dark and light condition. Dark condition refers to no light source is used while the light condition refers to the environment with light source used. Collect and record the data (current) obtained and display it using I-V graph. 5 Results and Discussion Through mechanical exfoliation, the monolayered MoS2 is obtained and the microscopic images are shown in Fig 5.1. The regular shape and purple or blue colour indicate the suitable thickness of MoS2. Fig.5.1.