Utilization of sputter depth profiling for the determination of band alignment at polycrystalline CdTe/CdS heterointerfaces

J. Fritsche, T. Schulmeyer, D. Kraft, A. Thißen, A. Klein, W. Jaegermann
2002 Applied Physics Letters  
The band alignment at polycrystalline CdS/CdTe heterointerfaces for thin-film solar cells is determined by photoelectron spectroscopy from stepwise CdTe deposition on polycrystalline CdS substrates and from subsequent sputter depth profiling. Identical values of 0.94Ϯ0.05 eV for the valence band offset are obtained. It is generally believed that the rectifying contact in the CdTe thin film solar cell is formed by a n-CdS/p-CdTe junction. 1 A large built-in voltage in the CdTe layer and a low
more » ... layer and a low barrier for the electrons at this interface are ideal for maximum conversion efficiency which requires a small conduction band discontinuity. The band alignment at semiconductor heterojunctions can be determined using electrical techniques, internal photoemission and photoelectron spectroscopy ͑PES͒. 2 Photoelectron spectroscopy is widely used for this application as it allows for parallel determination of chemical and electronic interface properties, which is the basis for studying fundamental properties and the possibilities of modifying semiconductor heterojunctions. 3 A typical PES experiment for the determination of band alignments is performed by stepwise deposition of the film material on a clean substrate surface. Both substrate and film preparation are done in ultrahigh vacuum ͑UHV͒ systems, which are directly connected to the photoelectron spectrometer to avoid sample contaminations. The preparation of semiconductor interfaces by stepwise deposition in UHV is often far from the actual manufacturing technology of empirically optimized devices. In the case of the CdTe thinfilm solar cell the CdTe and CdS films are usually prepared either by electrochemical deposition or by a very fast deposition at high substrate temperatures using close-spaced sublimation. 1,4 Sufficient efficiencies are only achieved after a chlorine treatment, the so-called activation. 1,5 To understand the effect of activation on the electronic properties of the CdTe/CdS heterojunction, an investigation of an activated interface is required. This is usually not feasible with surface sensitive techniques such as photoemission as the interface is buried below a thick CdTe layer. One possibility for the determination of band alignment at buried interfaces is sputter depth profiling. However, it is well known that sputtering is highly species sensitive leading to preferential sputtering of compound materials. This will generally modify the electronic structure of the materials and hence hinders the determination of electronic properties. Sputter depth profiling is therefore not an accepted procedure for the determination of band alignment at buried interfaces. In this letter the determination of the band alignment at the CdTe/CdS heterointerface by photoelectron spectroscopy is described. The interface properties are investigated by deposition of CdTe on an evaporated CdS film step by step on one sample without breaking the vacuum as well as by subsequent sputter depth profiling of the same sample. The experiments were performed using a Physical Electronics PHI 5700 electron spectrometer system equipped with a monochromated Al K␣ x-ray source, a He discharge lamp, and a focusing sputter gun. X-ray photoelectron spectra were taken with 45°off-normal emission. He I excited valence band spectra ͓ultraviolet photoelectron spectroscopy ͑UPS͔͒ were taken in normal emission with an applied sample bias voltage of Ϫ1.5 V. Sputtering was performed with 1 keV argon ions at 0.05 mA/cm 2 (ϳ3ϫ10 14 ions/cm 2 s) giving a typical sputter rate of ϳ6 Å/min. The deposition chamber for CdS and CdTe was directly attached to the surface analysis system. Films are deposited by thermal evaporation of materials with 99.99% purity from Al 2 O 3 crucibles using deposition rates of 3 Å/min at source temperatures of 500°C for CdTe and 600°C for CdS. Film thicknesses were determined using a quartz microbalance. As substrates indium tin oxide/SnO 2 coated glass as used for solar cell manufacturing were used. Prior to deposition of thick CdS films the substrates were cleaned by extensive rinsing in alcohol and deionized water and subsequent heating in UHV. All deposition steps were performed at room temperature. In Fig. 1 we show S 2 p and Te 4d core level lines obtained after stepwise deposition of CdTe on CdS up to 16 min corresponding to a CdTe film thickness of 48 Å. The S 2 p emission lines are gradually decreased with increasing film thickness. The attenuation with film thickness corresponds to a layer-by-layer growth mode. No chemical components other than those attributed to CdTe and CdS can be identified. The difference in S 2 p and Te 4d core level binding energy is given by E B (S 2p 3/2 )ϪE B (Te 4d 5/2 )ϭ121.57 Ϯ0.02 eV ͑see also Fig. 3͒ . S 2p and Te 4d core level lines obtained during sputter depth profiling of the CdTe/CdS interface are shown in Fig. 2 . The same sample as shown in Fig. 1 has been used. With increasing sputtering time the Te 4d intensity decreases and the S 2p intensity increases. There are no indications for chemically shifted components and the linewidths of the spectra are comparable to those shown in Fig. 1. a͒ Electronic
doi:10.1063/1.1507830 fatcat:q7vnx2apznbalnbh55tc67ep4u