Si/SiGe Nanostructures Fabricated by Atomic Force Microscopy Oxidation

In this work, local AFM oxidation technique in a controlled humidity environment has been used to create small features in strained SiGe alloys. When directly oxidizing SiGe alloys, minimum line widths of 20nm were achieved by adjusting parameters such as the bias voltage on the microscope tip and the tip writing speed. It was found that when bias voltage increases, and/or when the tip writing speed decreases, the oxidation height of silicon-germanium increases. In contrast to conventional thermal oxidation, the oxide height on SiGe alloys is slightly less than that on Si. Finally, this method was used to successfully cut conducting SiGe quantum well lines with high resolution. INTRODUCTION Si/SiGe nanostructures are of great interest for future Si-based nanodevices, such as quantum dot transistors. Conventional fabrication techniques, such as electron beam lithography and reactive ion etching (RIE), are high-energy processes which can cause radiation and etching damage, leading to the possibility of interface states in quantum devices. Thus a low energy nanolithography process is an important technological challenge for the fabrication of nanostructure devices. Recently, atomic force microscopy (AFM) with low tip voltages (~20 V) under a controlled humidity environment has been used to locally oxidize silicon on a scale of 10’s of nanometer [1]. Nanoelectronic devices using AFM oxidation have been fabricated on silicon, metal and gallium arsenide [2-4]. When oxidizing silicon, the nano-scale pattern of the oxide can be transferred into silicon substrate using the generated oxide as a mask [5]. However, AFM oxidation patterning of Si/SiGe heterostructures has never been demonstrated to the best of our knowledge, though strained Si/SiGe heterostructures give superior carrier transport mobility compared with silicon [6]. In this work, we reported the AFM oxidation of strained SiGe layers grown on a silicon substrate. Varying oxidation conditions, such as bias voltage and writing speed, height and width of SiGe oxide were studied. The AFM oxidation of SiGe was compared with that of silicon. Finally, the method was used to pattern the electrical conducting small SiGe wires. AFM oxidation was performed at room temperature in tapping mode on a Digital Instruments Nanoscope III. While scanning tunneling microscopy (STM) offers the possibility of fine-features and AFM in contact mode has the advantage of a high writing speed, a trade-off between them is achieved in AFM tapping mode [7]. A heavily-doped silicon tip with curvature radius smaller than 10 nm was used (MikroMasch Inc.). In tapping mode, the AFM cantilever is driven by a piezoelectrode to oscillate at or near the resonant frequency (~ 300 kHz) of the tip cantilever. Before oxidation, the sample was first submerged in acetone for 10 min in an ultrasonic bath, and then dipped into a 10% diluted aqueous HF solution for 1 min. A subsequent deionized water rinse for 1 min led to Si or SiGe surface being passivated with a hydrogen. During AFM writing, a feedback loop kept the tapping amplitude (and thus the average distance Mat. Res. Soc. Symp. Proc. Vol. 686 © 2002 Materials Research Society