Characterization of ZnS:Cr films for Intermediate Band Solar Cells
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In this thesis samples of ZnS:Cr thin films have been made through physicalvapor deposition (PVD) with resistive heating and characterized by severalmethods. The thin films are of various thicknesses and contain differentconcentration of Cr. ZnS:Cr is a candidate to a bulk material for intermediateband solar cell, IBSC, and was therefore selected for this thesis. IBSC is aconcept for solar cells with a much higher effiency than todays common solarcell.The goal for the project was to put together the deposition system, make thesamples, measure the thicknesses and concentrations and characterize them.Methods like X-Ray Diffraction (XRD), Auger electron spectroscopy (AES),profilometer, four point measurement, Hall-measurement and optical transmissionmeasurements have been used to determine structure, concentrations,thicknesses, resistivity, number of carriers and transmission respectively.The samples and the characterization performed with XRD, AES, profilometer,Hall-measurement and four point measurement were done at NTNU. Thespectrum measurements were done at NTNU and the University in Vienna.Samples with thicknesses in the range of 200 Å to 3340 Å have been investigated.Some were deposited in room temperature, some were annealedafterwards for 2 hours at 4500C and some were deposited on substrates witha temperature of 3000C. The thicknesses were measured with a contact profilometer.The theoretical concentration of the samples were in the range of4% to 31%. There was impossible to measure samples with a lower concentrationthan 7%, even though these were made.Transmission spectra were made in five overlapping specrophotometerss. Twowere used in the visible region (UV-VIS), two in the near infra-red (NIR)region, and one Fourier Transform Infra-Red spectroscopy (FTIR) for thelong wavelength region. The range was from 250 nm up to 25000 nm. NoCr2+ absorption lines were found. This can indicate a cluster form of the Crand not a substitution of Cr2+ with Zn2+ ions. The band gap found fromthe transmission spectra were in the range of >4.5 eV to 2.95 eV. The highband gap imply insulating samples and inactive Cr states in cluster form.No peaks from lattice configuration were found in the XRD spectrum. This indicates an amorphous structure for both the samples deposited in roomtemperature, the annealed samples and the sample deposited on hot substrate.The lattice configuration of the sapphire substrate was found for 3samples. This was the  reflection for sapphire at 68.2 degrees.Four samples were investigated in AES. A 3340 Å 7% sample, a 800 Å <5%sample, a 370 Å 31% sample and a 830 Å >4% deposited on hot substrate.Less Cr than expected was found for the sample of 800 Å and 830 Å. However,all samples contained O and C. The sample deposited on hot substrate shouldcontain 80 Å Cr theoretically, but no traces were found. The same appliesfor the 800 Å sample with less than 5%. Both charged badly which implynon-conductive samples. The 31% and the 7% sample did not show the samecharging which implies conductivity. The samples showed a great amount ofCr, but did also contain O and C. However, from the Hall-measurement andfour probe measurement none of the samples showed conductive properties.This emphasize the suspicion of incorporation of Cr clusters in inactive statesmaking the samples insulating.