75μm, while the sputtering velocity is 0.3 nm/s. The following species have been monitored: 16O, 23Na,
32S, 63Cu, 69Ga, 80Se, 98Mo, 114Cd, 115In, with a mass resolution of 400. All the MCs+ ion-signal profiles are normalized to the Cs+ signal.
Raman Spectroscopy
The Ga content of CIGS thin films can be determined by micro-Raman spectroscopy. In fact, the frequency of the chalcopyrite A1 mode (representing the vibration of the anions in the x-y plane with cations at rest) has been observed to be linearly dependent from GGI: the phonon frequency shifted from 174 cm-1 for CIS to 184 cm-1 for CGS (99). Therefore, GGI of co-evaporated CIGS has been calculated by the Raman shift of the A1-mode peak (Rshift) as:
𝐺𝐺𝐼 = 0.1 ∙ 𝑅𝑠ℎ𝑖𝑓𝑡− 17.4
For this thesis, to investigate the CIGS composition along the growth axis, depth-resolved Raman measurements were made using a micro-Raman apparatus (Horiba Jobin–Yvon Labram) equipped with a confocal microscope (Olympus BH-4) with 4, 10, 50, ULWD 50 and 100 objectives (lateral spatial resolutions of approximately 25, 10, 2, 2 and 1µm). The spectrometer is fitted with a 20 mW He–Ne laser emitting at 632.8 nm, an edge filter, a 256×1024pixel CCD detector, a 1800 grooves/mm grating, and a density filter wheel. The spectrometer is calibrated at the silicon Raman peak of 520.6 cm−1 before each measurement. The spectra were recorded at different sample depths using the 100 objective: each datum is the mean of 4 repetitions lasting 30 s each, or the average of 30 measurements of 10 s repeated 2 times each obtained mapping a 20μm×40μm sample area. Peak fitting was carried out using a Lorentzian function. For the samples of this thesis, the GGI calculated by Raman and SIMS analyses did not match perfectly. PED-grown CIGS may be characterized by slightly different constants in the equation shown above. Moreover, the A1 mode is expected to shift in function of the copper content too.
The correlation between the composition of PED-grown CIGS and the Raman A1 peak is currently under investigation to optimize the GGI calculation.
UV-Vis spectroscopy
Optical transmittance/absorbance and reflectance of thin film can be determined by means of ultra-violet and visible (UV-Vis) spectroscopy. Absorbance measurements allows the calculation of the energy gap of the semiconductor by its absorption edge. Due to the electrons excitation from the VB to the CB by the incoming photons, absorbance occurs only for those photons with energy higher than EG, in first approximation. Hence, for a semiconductor with direct band-gap, EG can be derived by means of the Tauc plot from:
𝛼ℎ𝜈 = 𝐴 ∙ √ℎ𝜈 − 𝐸𝐺
Reflectivity measurements are indeed used to find out the thickness of the films. Interference phenomena occur when the film thickness has the same order of magnitude of the incident wavelength, giving rise to intensity fringes of the reflected light. Knowing the refractive index of the material, the film thickness can be estimated by the positions of maxima and minima in the spectrum of the reflected radiation.
In this thesis, a Jasco V-530 spectrophotometer was used working in the range 350nm-1100nm.
X-ray diffraction
The structural properties of thin films can be investigated by X-ray diffraction (XRD), to determine the crystal structure, the dimensions of the unit cell, the preferred crystallographic orientation of the grains and to detect the presence of unwanted phases. In a crystalline material, an incident X-ray beam is diffracted by the electrons of the lattice atoms in specific directions, thus constructive interference is obtained when the Bragg’s Law is satisfied. By scanning the intensity in function of the diffraction angle, the atomic structure and the crystalline lattice of the sample can be determined.
For this thesis, a Siemens D500 (Siemens, Berlin, Germany) system in Bragg–Brentano geometry was used, equipped with a Cu Kα X-ray source (λ=1.54 Å).
X-ray fluorescence
X-ray fluorescence (XRF) is an X-ray spectroscopy technique used for qualitative and quantitative analysis of materials. It is based on the excitation of electrons of the inner shells of an atom, obtained by bombarding with X-rays having a defined energy to excite only that element. After being promoted to excited states, the electrons recover their initial level emitting secondary (or fluorescent) X-rays, which have an energy unique for that element and an intensity related to its amount inside the sample.
By analysing the sample for several elements, it is possible to calculate the concentration ratio between these elements.
In this thesis, a XRF spectrometer model Epsilon 5 by PANalytical is used to determine the S content in Zn(O1-x,Sx) films.
Scanning electron microscopy
Scanning electron microscopy (SEM) can investigate the morphological properties of the samples:
surface roughness, grains dimension and shape, film thickness and density. SEM uses a low-current and high-voltage electron beam to scan a sample area and to produce images of it. The focused electrons interact with the atoms of the sample originating several kinds of signal (secondary electrons, X-rays, etc.), each one carrying information about surface topography and/or composition. Usually, the secondary electrons emitted by the surface are detected while the beam scans the sample. An image is reconstructed in function of the number of secondary electron reaching the detector: this number
possible to distinguish different materials, in function of their electrical characteristic (like electron conductivity), and morphological variations.
Here, two different SEM models were used:
Philips 515, working at 20kV, for cross-sectional images and energy dispersive X-ray spectroscopy;
Carl Zeiss Auriga Compact, working at 3 kV, for cross-sectional images.
Energy-dispersive X-ray spectroscopy
Energy-dispersive X-ray spectroscopy (EDS or EDX) allows to determine the elemental composition of the sample. It is mainly used in electron microscopes since it used an electron beam to cause the X-ray emission from the material. The highly-energetic electrons of the beam transfer energy to the electrons of the sample atoms promoting them to the outer shells. The following electron relaxation from the outer to the inner shell brings to the generation of a X photon. Thus, the detection of the X-ray emission of the material gives information about its chemical composition.
Profilometry
A profilometer is an instrument used for detecting small vertical variation of the sample surface, moving a stylus across the sample and applying a specified force. It can measure both the sample roughness or the film thickness. For the latter purpose, a sharp step is needed: it can be realized by using a mask, in contact with the substrate during the thin-film deposition, or by means of a controlled etching after the material growth.
Four-point probe
Four-point probe is a quick method to measure the sheet resistance of a semiconductor thin film. It uses four aligned contacts: current is flowing inside the sample through the two outer ones, while the two inner contacts measure the voltage. Knowing the film thickness, the resistivity can be easily calculated.
Hall measurement
Hall measurement is a characterisation method based on the Hall effect used to determine the carrier density and the carrier mobility in semiconductors. It consists in the application of a magnetic field on a semiconductor thin-film in which a current is flowing. The system setup is assembled so that the force due to the magnetic field is acting on the electrons perpendicular to the current flow and thus the electrons are driven towards one side of the sample. The electrons gradient along this direction builds up an electric field whose force opposes to the one of the magnetic field. Once steady conditions are reached, the intensity of the voltage between the two edges of the sample depends on the strength of the magnetic field and on the type, density and properties of the charge carriers in the semiconductor.