Design and fabrication of a portable spectrometer to investigate optical properties of thin films
Morumbwa, Benard Riro
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Optical spectroscopy is a non-destructive and contactless method for characterizing optical and electrical properties of thin films; reflectance, transmittance, refractive index, permittivity and band gap. These properties determine the application of a thin film as a polarizer, laser, solar cell or photodiode among others. To measure accurately optical properties of thin films, the common types of optical spectrometers used are ellipsometers and spectrophotometers. However these instruments are expensive and there is need to device a cost effective optical spectrometer. In this study, a portable laboratory spectrometer that uses reflectance measurement to determine the optical and electrical properties of thin films was designed. The spectrometer consists of a fluorescent lamp (250V, 25W) as the source of visible light, a holographic grating (1200 lines/mm) which is placed in focus to the light generating electromagnetic spectrum in the 380 - 780 nm excitation range. A concave mirror of focal length 5 cm mounted on a shaft of a gear system (reduction ratio 50:1) driven by a stepper motor of resolution 3.75o focuses monochromatic beams onto thin film sample and to the first photo-detector, through computer controlled small angular rotations of the stepper motor. The reflected monochromatic beam from the probed thin film sample is directed to the second photo-detector and the intensities of both the incident and reflected beam are then converted to digital form by analog to digital converter (ADC0809CCN) connected to enhanced parallel port used in computer interfacing. Visual C++ programming language was used in developing and implementing the driver program that controls the designed instrument. The program enables the computer to record reflectance measurement of the probed sample in order to determine other optical properties. To test the accuracy and reliability of the portable spectrometer designed in this work, reflectance of semiconductor thin film samples of CuO and SnSe were measured using both the designed and a standard spectrometer (SolidSpec-3700). The reflectance measurement for CuO and SnSe had an error of ± 1.88 % and ± 2.17 % respectively, in reference to the standard spectrometer (SolidSpec-3700) reflectance spectra. This error margin is within tolerance level for reliable measurement. In addition the portable spectrometer designed in this work has a resolution of 2 nm ± 5%, cheaper compared to commercially available spectrometers measuring in the same range and portable.