Chapter 5 Fabrication and analysis of photodetector devices based on surface modified
5.2. Experimental Details
5.2.1. Synthesis of graphene oxide, reduced graphene oxide, and graphene quantum dots
The previous chapters explain the synthesized large lateral size GO and further reduced the oxygen functional groups from the drop-casted GO using thermal treatment. Again, GQDs Table 5.1: Description of the samples with sample codes.
Sample code Descriptions
GO-RT GO dried at room temperature GO-50 GO dried at 50 ˚C
GO-85 GO dried at 85 ˚C GO-120 GO dried at 120 ˚C GO-230 GO dried at 230 ˚C
Ar-GO Ar plasma treated on GO-85 SO Nanoparticles (NPs) of SnO2
SO-GO Nano hybrid of SnO2 NPs on GO layer SO-RGO Nano hybrid of SnO2 NPs on RGO layer GQDs-SO Nano hybrid of GQDs on SnO2 NPs
are synthesized from the GO solution using a hydrothermal method assisted by a tip-sonication process that was discussed in chapter 2.
5.2.2. Fabrication of Inter digitated electrode (IDE)
The 10 µm channel-IDE electrodes are fabricated using UV lithography techniques shown in Fig. 5.1. A 300 nm thick SiO2 on Si substrate is cleaned using three steps: (a) the substrate is deep in the Piranha solution (8:1 of H2SO4 in H2O2 solution) for 15 minutes to remove the unwanted particles present on the substrate. The substrate is rinsed in the de-ionized water and then dry it using N2 gas. (b) The substrate is sonicated for 15 minutes in the ethanol
Fig. 5.1: Schematic illustration for fabrication of IDE pattern on SiO2/Si substrate.
solution. Finally, (c) the substrate is sonicated in acetone solution for 15 minutes and dried after being rinsed in water. Positive photoresist (MicroChemicals GmbH: S1813) is deposited on the cleaned SiO2/Si at 3500 RPM for 60 s by using spin coater (Apex instruements:
spinNXG-M1). Moreover, the positive photoresist (PR) thickness is at ̴ 1 µm. The deposited substrate is baked on a hot plate for 3 minutes at 130 ˚C and slowly reduced to 60 ˚C after 3 minutes. The substrate is placed inside the UV exposer stage, just below the microscopic optical lens (20x) for exposing the UV light. The desired IDE pattern is designed in K-layout (software tool) and executed program in the system software (DilaseSoft).
After completion of the exposure, the substrate is removed from the system to develop the pattern. The substrate is dipped in the developer solution for 60 s, and then rinsed with de- ionized water. Once the patterns are developed, the metallization process is done in the
thermal/E-beam evaporator system. The substrate is placed on the sample holder inside the evaporator chamber; the inside pressure of the chamber is evacuated up to 5.8 × 10-6 mbar using a turbo-molecular vacuum pump (Pfeiffer vacuum) supported by a rotary vacuum pump.
The wanted vacuum pressure is achieved after 6 hours of a continuous run of the vacuum system. The substrate is deposited by 10 nm thick Cr using an E-beam evaporator and then by 20 nm of Au using a thermal evaporator. After metal deposition, the substrate is taken for the metal lift-off process; the 130 ˚C baked substrate is dipped into the acetone solution for 3 minutes and shaken in the container gently. Thus, the 10 µm channel IDE pattern is fabricated using UV lithography.
Now, the IDE pattern is composed of two parts: fringes and two metal pads. Fringes are made in dimensions of 10 µm width of a metal fringe and 10 µm of the minimum gap between two adjacent metal fringes. Each dimension of the metal pad is 2.5×1 mm2 in width and length, sufficient for electrical probing with metal tips.
5.2.3. Synthesis of nano hybrid of SnO2 on graphene oxide
A composite of GO and NPs of SnO2 (Aldrich) is synthesized using drop-casting NPs of SnO2 on the GO sheets. At first, 2 µl of GO solution is drop-casted on the pattern and dried in an open environment. 3mg of ̴ 100 nm NPs of SnO2 are dispersed in the 20 ml of de-ionized water using the tip-sonication process for 30 minutes. 2 µl of dispersed NPs are drop-casted on the GO sheets where they are placed on the IDE pattern. The device is dried for 45 minutes at 85 ˚C on the hot plate.
5.2.4 Characterization Techniques
The morphology of the GO, annealed GO, and IDE pattern on the SiO2/Si was observed using a field emission scanning electron microscope (FESEM) (JEOL, JSM-7610F). The thickness of the IDE patterns was studied using an atomic force microscope AFM (Bruker, Innova), and data were analyzed using analysis software (NanoScope Analysis 1.5). The structural analysis of the GO and annealed GO was studied using XRD (Rigaku, RINT 2500 TTRAX-III), with Cu Kα radiation as a source of x-ray and a scanning speed of 3 °/min. The structural defects and functional groups attached to the graphitic carbons of GO and annealed GO were studied using Raman spectroscopy (Horiba, LabRam HR) with the wavelength of 532 nm as a laser source using a 100× optical lens. The functional groups attached to the GO and annealed GO were studied using an X-ray photoelectron spectroscopy (XPS) (PHI X-tool, ULVAC-PHI INC) with Al Kα as an X-ray energy source at 20 kV and 54 W. The functional groups of the GO/annealed
GO were studied using the transmittance mode of the Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer, spectrum BX). The photoluminescence (PL) of GO solution was measured using the fluorimeter (Horiba, Fluromax-4) using 360 nm excitation.
5.2.5 Photodetector set-up
A customized photodetector system is used for the photo-response measurement, shown in Fig. 5.2. It consists of a microprobe station (ECOPIA, EPS-500), a source-measure unit (Keithley 2401), a 405 nm diode laser (CNI Laser), a monochromator (Oriel Instruments), a pulse generator (Scientific), and a focusing lens.
Fig. 5.2: Illustrated schematic diagram of a photodetector measurement system.
At first, the device is placed on the micro-probe station, and two gold-coated micro-tips are engaged on both sides of the metal pads of the device. The metal probes are connected to a source-measure unit (Keithley 2401) which is further connected to the personal computer through a GPIB cable. A laser source is focused on the device to measure its photo-response;
it has an adjustable laser power. A pulse generator is connected to the laser source to provide a pulse form of laser to the device to measure the ON/OFF response. All the response activities are controlled and acquired using KickStart software.