Characterization of Semiconductor Nanospikes Produced by Focused Ion Beam Erosion

Abstract

We report on a focused ion beam (FIB) method for creating III-V semiconductor spike nanostructures. By using a FIB to erode InAs films grown on InP substrates we have been able to create high aspect ratio heterostructure spikes, or "nanospikes," with average heights of 300 nm and maximum heights greater than 800 nm. The nanospike formation mechanism has been examined in situ by scanning electron microscopy (SEM) observation, and nanospike formation has been found to proceed via an ion-produced droplet masking process. Metallic droplets form on the surface due to preferential sputtering of the group V material [1,2]. These droplets initially move randomly across the surface but as film erosion proceeds they become stationary and act to shield the underlying material. In this way the droplets act as etch-masks that allow nanospikes to form as the surrounding material recedes due to FIB erosion. The droplet etch-masks are replenished by the addition of excess group III atoms produced by sputtering of the surrounding InAs film, and so are self-sustaining [3]. We have found that the InAs/InP film heterostructure is necessary for nanospike formation, as nanospikes will not form on bare InAs or InP substrates upon FIB irradiation. We have also found that the placement of nanospikes is dependent on the initial InAs film morphology and the recession of the InAs/InP interface as erosion proceeds. Nanospikes are constrained by that interface and form on regions of InAs only due to the different FIB responses and excess group III adatom production rates of InAs and InP [2]. Pre-patterning the InAs/InP heterostructure leads to well-ordered arrays of nanospikes. Using a hybrid pre-patterning and FIB erosion approach, it is possible to create ordered arrays of nanospikes in arbitrary patterns. Nanospike structure has been characterized by high resolution transmission electron microscopy (TEM), by scanning transmission electron microscopy (STEM), and by STEM energy dispersive spectrometry (EDS) compositional analysis. Characterization of the nanospike structure by TEM has revealed that they have an ion damaged outer layer and contain a single crystalline core that maintains the structure of the original film and is free of extended defects such as twin boundaries. The electrical I/V response of the nanospikes has been examined using a SEM/nanomanipulator system [4]. Annealing studies have also been conducted to examine the effect that annealing nanospikes under an arsenic overpressure has on their structure and electrical properties.