UHV-TEM

Ultra high vacuum transmission electron microscopy

Focused ion beam patterning Laterally controlled nucleation of Ge nano dots by in situ focused ion beam surface modification Self-assembly of islands during strained layer epitaxy is an established and successful method for forming nanoscale structures which can be exploited for their novel electronic and optical properties. The random nature of self-assembly, however, means that island positions are not easily controlled. The ability to choose nucleation sites, in combination with control of shape and size, will open new possibilities in device applications such as quantum cellular automata (QCA). Mesa structures are known to control nucleation, but the ability to laterally pattern islands without surface topography would be even more useful in device fabrication, and should also allow us to learn more about the parameters important in growth, independently of changes in surface step density or strain relief at mesa edges. Adressing this problem, we found that focused ion beam (FIB) patterning is a promising tool to control the nucleation sites of self-assembled Ge islands on Si(001) on the nanoscale. Experimental Setup The integration of a FIB column into the preparation chamber of our UHV-TEM allows us to pattern and subsequently study the annealing and growth process, in real time, during chemical vapor deposition of digermane carried out in the microscope polepiece. Using appropriate conditions, a small dose of implanted Ga+ ions is sufficient to control the lateral nucleation of the QDs, with precision of tens of nanometers, and at writing rates of ~104 features per second. Results After irradiating a specimen with a 25keV Ga+ ion beam (10pA beam current) diffraction patterns show that amorphous material is present in the irradiated spots and that some amorphous material is also redeposited between the spots (Fig. 1). The center of each irradiated spot has intensity with a ring of even higher intensity around the periphery. We believe that direct implantation damage causes the central intensity, while redeposited material from the sputter process causes the outer ring. Figure 1: QCA adder pattern written with the FIB using a 25keV Ga+ ion beam (10pA) and an irradiation time of 0.1msec/spot (6000 Ga ions per spot). The planview TEM image is recorded using a (g, 3g) weak beam condition (g=220) sensitive to strained or damaged regions of the Si lattice. Heating the FIB patterned sample above 600°C slowly anneals the damage from the Ga irradiation over a time frame of a few minutes while higher temperatures speed up the annealing process. During the annealing process the irradiated area anneales from the inside and the outside simulatiously forming a sharp ring of defects which vanishes by furhter heating, leaving only a few bright pinpoints below 6nm in size (Video 1). These spots may either be defect clusters or ultra-small dislocation loops left after annealing or small metastable g-Ga precipitates. AFM measurements, recorded ex situ after exposure to air, show no topography, and we can actually see surface steps running continously across the irradiated areas. Figure 2: Heating one cell of the QCA pattern slowly up to 600°C. The TEM video is speeded up and recorded using a (g, 3g) weak beam condition and, for a short sequence inbetween, a two beam bright field image. Still from Video 1 (annealing_of_fib_area.mp4) before annealing.> After annealing Ge island growth was performed at 600°C by introducing digermane into the sample area. FIB irradiation, annealing and growth were performed subsequently without exposure to air. Under these conditions, the first 2-3 monolayers of Ge form a flat layer, then further Ge deposition leads to spontaneous island nucleation followed by growth and coarsening. The Ge flux was stopped after islands started to form. Figure 3: (a) FIB pattern created using an irradiation time of 0.1ms, as-formed (left) and after annealing and Ge island growth (right). (b) Detail of irradiated areas after Ge growth. The arrows indicate single undislocated 8.5nm high Ge island, only 20nm in diameter, formed within each irradiated area. (c) Islands larger than 20nm are already dislocated. Conclusion Focused ion beam micropatterning is a succesful way to control the lateral nucleation of Ge islands. Irradiation with a dose as low as 6000 Ga+ ions per 100nm spot is enough to control the nucleation site of the islands without changing surface topography. In addition we observe a significant change in aspect ratio and size for Ge islands grown on irradiated areas compared to Ge islands grown on clean Si(001) due to apparent surfactant effects of the implanted Ga.