Thin CuInS 2 Films Prepared by MOMBE: Interface and Surface Properties
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Thin CuInS2 Films Prepared by MOMBE: Interface and Surface Properties C. Pettenkofer, C. Lehmann, W. Calvet Hahn-Meitner-Institut SE6 Glienickerstr 100 D-14109 Berlin ABSTRACT CuInS2 films were prepared on Si(111) by MOMBE using standard MBE Cu and In sources and Ditertbutyldisulfide (TBDS) as an organic sulfur precursor. The films were analyzed in situ by XPS, LEED and UPS. Deposition at 250°C yields chalcopyrite films [1] with admixtures of CuSi2 and CuIn alloys. Deposition at higher temperatures up to 550°C was used to clarify the feasibility of the process. High quality LEED diffraction patterns show epitaxial growth but CuSi2 precipitations are still observed for deposition on Si(111). A buffer layer of indiumsulfide showed no considerable effect on the interface morphology. The obtained LEED pattern are in accordance with the epitaxial relation Si{111}||CuInS2{112}. Even for temperatures as high as 550°C no incorporation of Carbon or residual hydrocarbons in the film or adsorbed at the surface were detected. INTRODUCTION CuInS2 films were technologically prepared by sputtering the metals Cu and In on substrates with a subsequent rapid thermal annealing process in a sulfur atmosphere. Epitaxial films are deposited by co-evaporation of the three constituents [2,3,4]. In an new approach we tried to circumvent the problem of high sulfur background pressures by introducing a MOMBE growth process [1], which yielded stoichiometric epitaxial films. But in the interface CuSi2 admixtures were found. The low reactivity of the TBDS was found to be responsible for the silicide formation. To increase the reactivity it was concluded to rise the substrate temperature to 550°C by increasing the TBDS partial pressure. EXPERIMENTAL Preparation took place in a specialized MBE System [3] equipped with a bubbler for TBDS. Analysis is performed in situ in our UHV coupled surface analysis system. Photoelectron spectra were taken with MgKα and HeI radiation in normal emission. Si(111) substrates were Hterminated [3] and introduced in the MBE system. The deposition process is controlled by a computer routine for ramping the Knudsen cells, the substrate, monitoring deposition rates of the cells, and actuating the cell shutters. Standard parameters are 800°C for the In single filament cell, 1200°C for the Cu Dual filament cell. TBDS is dosed from an UHV bubbler via a leak valve through a nozzle to the sample. TBDS pressure was in the range of 10-3mbar. Deposition is performed in several deposition steps to follow the interface bulk material formation. Due to the UHV environment no sample degradation is observed between consecutive deposition steps. For ex situ analysis (SEM and XRD) samples were brought up to air after examination by the in situ techniques. RESULTS AND DISCUSSION
Intensity [a.u.]
Intensity [a.u.]
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93 8
9 36
9 34
9 32
9 30
9 28
448 446 4 44 B in ding E ne rgy [eV ]
4 42 16 6
1 64
162
1 60
10
9
8
7
6 5 4 3 Binding Energy [eV]
2
1
0
-1
Cu 2p spectrum In 3d spectrum S2p spectrum HeI v
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