Atomic force microscopy studies on sulfur-, selenium- and tellurium-based metal chalcogenide thin films: A review

Sulfur, selenium and tellurium-based metal chalcogenide films have been prepared using various deposition methods. Investigation of morphological properties of the generated surface structures on chalcogenide thin films using atomic force microscopy technique was reported. The purpose of this work is to describe past important research findings that are related to atomic force microscopy technique.  
 
   
 
 Key words: Atomic force microscopy, surface roughness, film thickness, grain size.


ATOMIC FORCE MICROSCOPY
The first atomic force microscopy was operated in contact mode by Binnig et al. (1986) in Fiorani and Sberveglieri (1994).The AFM technique has been used for problems in a wide range of fields of the natural sciences such as semiconductor, medicine, molecular biology, solid state physics, polymer chemistry and nanomaterials (Xie et al., 2011).Applications of the AFM technique include the identification of atoms at a surface (Sergei and Whangbo, 1996), the study of changes in physical properties (Gernot and Henning, 2011), the evaluation of interactions between a specific atom and its neighboring atoms (Jaroslaw and Kash, 2005).
Atomic force microscopy works by bringing an atomically sharp tip close to a surface (Franz and Calvin, 2006).AFM tips were made from silicon, silicon nitride (Alan, 2015), silicon oxide (Enrico and Ernst, 2015), and quartz (Rebecca and Lisa, 2000).Basically, there are few main parts in AFM.Microscope stage is used for moving AFM tip (Maurice et al., 2007), sample holder and force sensor.Controls electronics consist of optical microscope (Peter and Paul, 2010) and vibration controller.The control electronics usually takes the form of a large box interfaced to both the computer and microscope stage.The computer is employed for setting the scanning parameters such as scan speed and scan size.Lastly, frame supports the whole AFM microscope.It must be very rigid (Georg et al., 2006), so that, it does not let vibrations between the surface and tip.

Modes of AFM
There are different imaging modes of AFM, namely, contact mode, non-contact mode and tapping mode (Nikodem and Kuan, 2011).The first and original mode of operation is contact mode (Bharat et al., 2008).It has the ability to produce high resolution images.Tip of the probe always touching the sample (less than a few angstroms) in this mode.In this mode, the deflection of the cantilever is sensed (Seungbum, 2004) and compared in a DC feedback amplifier to some desired value of deflection.
The first non-contact mode was developed by Martin et al. (1987) in Baldeschwieler et al. (1996).In non-contact mode, a sharp probe is moved close (ranging from tens of angstroms to hundreds of angstroms) to the surface under study.It uses detection of a cantilever resonant frequency as an indirect measure of sample topography.Finally, the image is constructed from the force interactions during the scan (Kantorovich et al., 2000).
Tapping mode is used by researcher when operating in ambient conditions or in liquids (Yang, 2009).In this mode, the cantilever is driven to oscillate up and down at or near its resonance frequency.This mode allows high resolution topographic imaging of sample surfaces including on surfaces that are easily damaged and loosely held to their substrate.Furthermore, it overcomes some problems associated with friction, adhesion and electrostatic forces (Fernando et al., 2004) that can plague other scanning methods.

Scanning electron microscope (SEM)
Scanning electron microscope (SEM) is a type of electron microscope scans, a focused electron beam over a surface to create an image (Nidal et al., 2017).The electrons in the beam interact with the sample, producing various signals that can be used to collect useful information (Patrick, 2009) such as surface topography and composition (Ahmad et al., 2015).Transmission electron microscopy (TEM) is employed to observe the features of very small specimens such as structure and morphology (Jozef, 2017).It uses an accelerated beam of electrons (Olga and Dieter, 2012), which passes through thin specimen to produce images.Comparison between scanning electron microscopy, atomic force microscopy and transmission electron microscopy as listed in Table 1.
Here, this review will focus upon morphology applications of AFM technique in sulphur-, selenium-, tellurium-based metal chalcogenide films.In this article, author discusses the analysis results (published literature), advantages and limitations of AFM.

LITERATURE SURVEY: METAL CHALCOGENIDE THIN FILMS
The author performed extensive searches in Google Scholar, Scopus, ISI and international refereed journals, using a combination of the search terms: atomic force microscope, film thickness, surface roughness, grain size, and metal chalcogenide thin films.
Atomic force microscopy (AFM) is a highly sensitive technique (Shivprasad et al., 2005) can give the images on the nanometer scale (Lehr, 2000).AFM has found increasing use in the material sciences and engineering fields (Last et al., 2010).Since 1990, the number of research publications making use of AFM to investigate the morphology of samples has increased (Ho, 2014;Mukherjee et al., 2016b;Soumya et al., 2014;Daniel et al., 2016;Siang et al., 2011;Kelvin et al., 2011).The characterization of obtained films is an important step in order to identity and improve the quality of films.AFM has been traditionally evaluated as a powerful tool to carry out an accurate measurement of surface roughness of films (Victor, 2012).Surface roughness can affect the optical properties of films (reflectivity and scattering) and electrical conductivity as well.
Sulphur is a chemical element (S) and has a pale yellowish color.It has atomic number 16, with a chemical formula S 8 .In pure form, it is completely tasteless and odorless.Abundant multivalent non-metal sulphur happens in many sulfide and sulfate minerals.Nontoxically enhanced sulfur reaction for formation of chalcogenide thin films and have been widely employed for high performance optoelectronic devices.Table 2 shows the surface roughness, film thickness and grain size of sulfur-based metal chalcogenide films measured by AFM technique.
Selenium is a non-metal, with symbol Se and atomic number 34 (Valadabadi et al., 2010).It is rare and in the Group 16 of the period table.Generally, it is a mineral found in the soil.The used selenium has good photovoltaic and photoconductive properties (Christopher et al., 2016); the most important uses of selenium in optoelectronic devices and solar cells (David, 2013).This is due to the fact that it has good photovoltaic and photoconductive properties.For example, the electrical conductivity increases more than 100 fold in illumination conditions.Additionally, the electrical resistivity of selenium varies over a tremendous range, depending upon experimental conditions.Table 3 indicates the surface roughness, film thickness and grain size of selenium-based metal chalcogenide films measured by AFM technique.
Tellurium has an atomic number of 52 and the symbol Te.Tellurium has a silvery-white appearance and is found in native form as elemental crystals.Tellurium is an element in Group VI of the Periodic Table, which has been intensively studied because of its unique optical and electrical properties.Tellurium films show p-type conduction (Begona et al., 2015), because of lattice defects acting as acceptors.The band gap about 0.34 eV (Chen et al., 2017) and the carrier concentration at room temperature is in the range of (1-5) × 10 18 cm -3 . Tellurium is used in cadmium telluride (CdTe) solar panels, a rapidly growing and increasingly important market.Massive commercial production of CdTe by First Solar Company has significantly increased tellurium demand.However, CdTe is classified as harmful if inhaled and harmful to aquatic life with long lasting effects.Table 4 displays the surface roughness, film thickness and grain size of tellurium-based metal chalcogenide films measured by AFM technique.
Researchers expressed some challenges (Ampere, 2011) of atomic force microscopy technique.For instance, there is need for sharp probe (Dufrene, 2011) for high resolution in order to achieve good results.The feedback controller should have a fast control in order for adjust topographic film to be produced.A high speed computer is needed to produce the images in real time.Lastly, the force between probe and sample should be 1 nN or less because of noise and stability considerations.

CONCLUSION
Current research literature suggests that atomic force microscopy is one of the most widely used instruments in research in the material sciences and engineering.Characterization of morphological metal chalcogenide thin films could be done using this instrument effectively.Three-dimensional AFM images of topography provide nanoscale information on film structure including surface roughness, film thickness, and grain size.

CONFLICT OF INTERESTS
The author has not declared any conflict of interests.

Close space sublimation
The grain sizes of CdTe films deposited at 450 and 620°C were 0.3 µm and 2 to 5 µm.Due to the higher surface mobility during deposition.

Molecular beam epitaxy
Atomic force microscopy image of 7.09 µm SnTe layer grown on (111) BaF2 surface revealed spirals with monolayer steps formed around threading dislocations.There are some small black pits, called holes (left during growth) could be detected.

CONFLICT OF INTERESTS
The author has not declared any conflict of interests.

Table 1 .
Comparison between SEM, TEM and AFM.

Table 2 .
Surface roughness, film thickness and grain size of sulfur-based metal chalcogenide films measured by AFM technique.

Table 3 .
Surface roughness, film thickness and grain size of selenium-based metal chalcogenide films measured by AFM technique.
Jung et al. (2006) epitaxy RMS value is 2.9 nm and rough surface is seen.Jung et al. (2006)

Table 4 .
Surface roughness, film thickness and grain size of tellurium-based metal chalcogenide films measured by AFM technique.