A Growth Technique to Make Extensive Atomically Flat Silicon Surfaces
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A Growth Technique to Make Extensive Atomically Flat Silicon Surfaces Doohan Lee, Todd Schroeder, James Engstrom and Jack Blakely Cornell Center for Materials Research, Ithaca, NY, 14853, U.S.A. ABSTRACT We have carried out a series of experiments aimed at producing arrays of mesas on both Si(001) and Si(111) which are free from atomic steps. These are of interest in CMOS technology and for quantum well structures. They also provide interesting substrates for fundamental surface science experiments. In previous work we have created atomically flat regions surrounded by ridges through an evaporation method. The present work ’inverts’ the previous process by using a pattern of trenches to define the mesas and then depositing Si to grow the atomic steps off the edges. The mesas are created on Si wafers, which are ~1° from the (111) and (001) plane by lithography and reactive ion etching. Step-free mesas were formed on Si(111) but not yet on Si(001). Both the evaporation and this new growth technique rely on step flow to move the steps to the edges of the flat areas. Although the evaporation method is simpler, an advantage of the growth technique is that it can be carried out at lower temperature. The maximum size of mesa that can be made free of atomic steps depends on the combination of temperature and deposition rate. On very large step-free terraces nucleation of islands and concentric arrays of mono-atomic steps are observed; these correspond to the vacancy pits observed with the evaporation method. INTRODUCTION The spacings between the atomic steps due to 'miscut' from the nominal orientation are about 8 nm on Si(001) and 18 nm on (111) orientation at a typical miscut angle of 1°. Fabrication of step-free surfaces on semiconductors is of increasing importance for device applications where atomically smooth, sharp interfaces are required such as CMOS technology and substrates for quantum well structures. In previous work we have created arrays of atomically flat regions, on both Si(001) and (111) by an evaporation process[1,2]. The process involves starting with a wafer surface patterned with a series of ridges and then annealing to move the atomic steps into the ridges[1,2,3]. The annealing has to be done at a temperature that is high enough for evaporation to be quite fast but below the roughening temperature of the terrace. In the experiments described here we have 'inverted' the previous process by using a pattern of trenches to define arrays of mesas and then depositing Si to grow the atomic steps off the edges by the step flow growth process. We have produced arrays of stepfree mesas on Si(111) with dimensions that range from the quantum dot limit to several microns. As shown below, simply depositing the required amount of material does not, however, ensure the formation of step-free mesas; barriers to step flow and nucleation on the terraces can become limiting factors. Closely related experiments have previously been carried out to create step-free mesas on GaAs using metal organic vapor phase epitaxy as th
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