Coarsening and Slope Selection During Crystal Growth and Etching of Ge(001)
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ABSTRACT The evolution of surface morphology during low temperature crystal growth by molecular beam epitaxy and etching by low-energy ions is characterized by in-situ scanning tunneling microscopy. Epitaxial growth of Ge(001) at low temperatures in the vicinity of 155°C produces a pattern of growth mounds while etching at temperatures near 270'C produces a pattern of low aspect ratio pits. The characteristic in-plane length scale of the surface roughness d increase with a power law dependence on time but the exponent depends on temperature and process. Prior to the onset of amorphous growth, the amplitude of the surface roughness G 1/ 2(d/2) increases more rapidly than d; i.e. the slope of the sides of the growth mounds increases with time. By contrast, the ratio of G'/2(d/2) to d remains nearly constant during ion etching for a wide range of etching times. INTRODUCTION Large scale surface structures patterns of growth mounds or growth pyramids, see for example Fig. la - have now been observed in the homoepitaxial growth of several materials: GaAs(001) [1, 2], Ge(001), [3], Si(001) [4], Fe(001) [5], Cu(001) [6], and Rh(l1l) [7]. Asymmetric kinetics for the attachment of adatoms at ascending versus descending steps, originally observed directly by field-ion microscopy on refractory metal surfaces [8], are thought to be responsible for this instability in the growth of a singular surfaces [1, 9]. Similarly, asymmetric kinetics for step edge attachment of surface vacancies created by low energy ion etching has been used to explain surface roughening dominated by multilayer pits on Pt(111)[10, 11] and Ge(001) [12], see Fig. lb. In earlier publications [3, 12], we reported our studies of surface roughening of Ge(001) during molecular beam epitaxy (MBE) and ion-etching for a wide range of processing temperature. Surface morphologies are quantified using in-situ scanning tunneling microscopy (STM) and also by atomic force microscopy (AFM) when the aspect ratio of the surface roughness becomes large. In this paper, we emphasize the evolution of the surface morphology as a function of processing time. Previously unpublished data and analysis are presented for the evolution of morphology during ion etching at temperatures near 2700C where well-organized patterns of pits are observed. Data for the evolution of MBE growth morphologies are extended to a greater range of film thickness and growth temperatures than reported previously [13]. EXPERIMENTAL DETAILS Ge(001) wafers are initially cleaned using using repeated UV-ozone oxidation and removal of the oxide in water [14]. After introduction into the vacuum chamber, the final oxide is 221 Mat. Res. Soc. Symp. Proc. Vol. 399 0 1996 Materials Research Society
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Figure 1: (a) STM image of a growth mound observed at the surface of a 100 0 nm thick film of Ge(001) grown by MBE at 155 C. (b) STM image of etch pit produced by the etching of 130 monolayers of Ge(001) using 240 eV Xe+ ions at 295°C. In both cases, the morphology is dominated by narrow (001)2 terraces separat
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