Kelvin probe force microscopy for the characterization of semiconductor surfaces in chalcopyrite solar cells Ch. Sommerhalter, S. Sadewasser * , Th. Glatzel, Th.W. Matthes, A.J ager-Waldau, M.Ch. Lux-Steiner Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin, Germany Abstract Kelvin probe force microscopy in ultrahigh vacuum is a powerful technique for the quantitative characterization of structural and electronic properties of semiconductor surfaces and interfaces on a nanometer scale. In chalcopyrite heterojunction solar cells the interfaces play a crucial role for the performance of the device. We studied chalcopyrite heterostructuresbasedonepitaxialCuGaSe 2 thin®lmspreparedbyMOVPE.Lateralvariationsofthecontactpotential dierenceandthesurfacephotovoltageSPV)wereinvestigatedafterdierentprocesssteps,includingthedepositionof n-CdSorn-ZnSebuerlayersandthen  -ZnO window layer. Measurements on the CuGaSe 2 absorber material show terraces with preferential orientation in the [110] direction in the topographic image. A negative SPV of 300mVon theas-grownCuGaSe 2 absorbercouldbeattributedtoahighlydopedp  -Cu 2x Sesurfacelayerofafewnmthickness, whichwasremovedbyaKCNetch,resultingina¯atbandcondition.Thedepositionofthebuerlayeralonedoesnot leadtoasigni®cantbandbendingattheCuGaSe 2 /buerinterfaceandthedepositionoftheZnOwindowlayerseemsto be crucial for the development of the band bending within the absorber. Ó 2001 Elsevier Science B.V. All rights re- served. Keywords: Atomic force microscopy; Work function measurements; Surface photovoltage; Heterojunctions; Semiconducting surfaces 1. Introduction KelvinprobeforcemicroscopyKPFM)hasbe- comeawidelyusedtechniquefortheinvestigation of the contact potential of surfaces on the nano- meter scale by atomic force microscopy AFM) [1±4]. However, to obtain quantitative results ul- trahigh vacuum UHV) conditions are required sincetheworkfunctionofasurfaceisverysensitive to adsorbates and oxidation. Recently, two ap- proachesforUHV-KPFMwerepresented.Kitam- ura et al. [2] employed the frequency modulation FM) technique for the detection of both, the to- pographyaswellasthecontactpotentialdierence CPD).Kikukawaetal.[3]employedtheamplitude modulationAM)techniquefortheimagingofthe CPD.Recently,wedemonstratedtheimagingofthe CPD variation at monolayer steps of highly ori- ented pyrolytic graphite and GaAs110) by AM- KPFM in UHV [4]. Even the screened coulomb potentialofsinglechargesationizeddopantsitesin p-WSe 2 0001)couldberesolved. Within the chalcopyrite system CuIn 1x ,Ga x ) S y ,Se 1y ) 2 , with 0 6 x; y 6 1, a variety of com- positions have been used for solar cell device Surface Science 482±485 2001) 1362±1367 www.elsevier.nl/locate/susc * Corresponding author. Tel.: +49-30-8062-2164; fax: +49- 30-8062-3199. E-mail address: sadewasser@hmi.de S. Sadewasser). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)00878-0