Abstract
Differential reflectance spectroscopy (DRS) is a powerful tool to study processes during thin-film growth, especially that of transition metal dichalcogenides and organic thin films. To satisfy the requirements for in situ and real-time monitoring of film growth, including spectral resolution and sensitivity at the level of monolayers and even sub-monolayers, the most challenging technical task in DRS is to reduce noise to an extremely low level so that the best possible signal-to-noise ratio can be achieved. In this paper, we present a simplified and cost-effective DRS apparatus, with which we show that the measurement noise is mainly composed of thermal drift noise and explore the temperature-dependence of the DRS signal. Based on the results obtained, we propose an easily realized and effective scheme aiming to reduce the noise. Experimental results demonstrate that this scheme is effective in stabilizing reliable signals for a long period of several hours. Significant noise reduction is achieved, with the typical average noise of the DRS system being decreased to 0.05% over several hours. The improved DRS system is applied to study the growth of an organic semiconductor layer for an organic field-effect transistor device. The results indicate that the apparatus proposed in this paper has potential applications in fabrication of devices on the nanoscale and even the sub-nanoscale.
Highlights
Differential reflectance spectroscopy (DRS) measures a normalized relative change in the reflectance of a surface.1 This technique is especially suitable for in situ studies in high-vacuum environments, because the reflectance of a sample surface changes with the thickness of the deposited film and synchronously with the period of film growth
It can be seen that our scheme is effective in that the typical noise of the DRS system has been decreased to 0.05% during 2 h at all the wavelengths typically employed in DRS experiments
We have studied the origin of DRS noise and have compared various methods for reducing it
Summary
Differential reflectance spectroscopy (DRS) measures a normalized relative change in the reflectance of a surface. This technique is especially suitable for in situ studies in high-vacuum environments, because the reflectance of a sample surface changes with the thickness of the deposited film and synchronously with the period of film growth. DRS has been successfully applied to observe the evolution of optical features during the growth of thin films, including films of organic semiconductors and transition metal dichalcogenide materials, such as pentacene, bis-pyrene,4 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), dicyanovinyl quaterthiophenes, and molybdenum disulfide.8,9 These studies indicate that the typical DRS signal induced by a single monolayer is of the order of 10−3, which is quite small.. To achieve a small detection area on the sample surface without loss of light intensity, a long-work-distance achromatic lens with wavelength range 193–1000 nm is used to focus the light at normal incidence on the target area This lens is essential for fluorescence measurements, these will not be discussed in this paper. We rarely use the iris, because the lens alone usually provides a suitable reflected intensity and a sufficiently small detected area for most common experiments
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