Appl. Phys. A 59, 23-27 (1994) Applied so,,,s PhysicsA e.' Surfaces © Springer-Verlag 1994 Study of the influence of native oxide layers on atomic force microscopy imaging of semiconductor surfaces H. Bluhm 1' *, U. D. Schwarz 2'**, F. Herrmann 1, P. Paufler 3 t Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany (Fax : + 49-40/4123-6368) 2 Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland 3 Institute of Crystallography and Solid State Physics, University of Technology, Mommsenstrasse 13, D-01069 Dresden, Germany Received 7 January 1994/Accepted 9 March 1994 Abstract. We have investigated the influence of the native oxide layer on semiconductor surfaces on the imaging properties of the atomic force microscope operated un- der ambient conditions by using epitaxial Inl_xGaxAs layers grown by Metal-Organic Chemical Vapour De- position (MOCVD) on (001) oriented InP substrates which have been kept under ambient conditions for two years. The thickness and composition of the native oxide layers were studied with ellipsometry and X-ray pho- toelectron spectroscopy, respectively. Subsequently, the sample surfaces were imaged by means of atomic force microscopy operated in air which revealed terrace struc- tures separated by monoatomic steps. The obtained data were compared with the surface morphology which can be expected from the MOCVD growth process. The results suggest that an accurate study of semiconductor layer growth by atomic force microscopy in air is pos- sible. PACS: 61.16, 68.65 During the past years, Scanning Tunnelling Microscopy (STM) developed into the most successful method for imaging semiconductor surfaces with high spatial resolu- tion [1]. STM investigations, however, require Ultra- High Vacuum (UHV) since native oxide layers are formed immediately upon exposing semiconductor sur- faces to air. Tunnelling through these oxides is difficult or even not possible [2]. In order to enable the imaging of poorly conducting samples, Binnig et al. [3] developed the Atomic Force Microscope (AFM) which is capable to map surfaces with high spatial resolution regardless of their electrical properties [4]. This versatile tool allows the study of semiconductor surfaces under ambient con- ditions. Some effort has been made, e.g., to investigate growth features [5-9], oxidation effects on cleavage * To whom all correspondence should be addressed ** Present address: Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany planes [10], growth of native oxide layers on GaAs (100) and InP (100) [11], roughness evaluation of oxidised Si (111) surfaces [12], gliding processes in heterolayers [13-15], surface damaging due to chemical-mechanical polishing [16], and cross-sectional imaging of multilayers [17, 18]. In addition, topographic micrographs showing clearly resolved surface step structures with monoatomic height on Si (111) [19, 20] and GaAs (100) [21-23] were present- ed recently. This was somewhat surprising since the nat- ive oxide layers on semiconductors do not only disturb the imaging of the semiconductor surfaces with the STM, they should also affect the capability of the AFM to resolve surface structures on semiconductors with high resolution. Such oxide layers are amorphous and can easily reach thickneses of several nanometers -- well in excess of the step height of 0.28 nm [for GaAs (100)]. Therefore, it is not obvious that the observed steps can be interpreted straightforward as growth steps with mo- noatomic height. To reduce the influence of the oxide layers as much as possible, up to now most of the authors used freshly prepared samples or applied special anneal- ing procedures prior to imaging with the AFM in air. For a more detailed investigation of the influence of the native oxide layer on the visibility of the as-grown semiconductor surface, we choose Inl_xGaxAs single heteroepitaxial layers grown by MOCVD on a (001) oriented InP substrate. The samples were kept under ambient conditions for two years to support the forma- tion of a rather thick native oxide layer. After this ex- posure to air the composition and thickness of the native oxide layer were determined by means of X-ray Pho- toelectron Spectroscopy (XPS) and ellipsometry, respec- tively. Subsequently, the (001) layer surfaces were imaged using an AFM operated under ambient conditions. The surface morphology obtained from the AFM measure- ments was compared with the MOCVD growth model [24], which allows to draw conclusions about the in- fluence of the native oxide layer on the capability of the AFM to image semiconductor surfaces under ambient conditions.