FABRICATING NOVEL NANOSTRUCTURES BY GLANCING ANGLE DEPOSITION

  Yiping Zhao

Department of Physics and Astronomy, University of Georgia , Athens , GA 30602 , USA .

Introduction

One dimensional (1D) nanostructures with diameters of 1 – 200 nanometers and length up to several tens of micrometers are nanoscale building blocks that can provide break-through applications in nanoelectronics, photonics, and bioengineering . One key issue associated with those important applications is how to assemble the 1D nanostructures in an effective and controllable way. So far, there are four general approaches that have been employed to fabricate nanowire/nanorod structures: nanolithography, wet chemistry , and vapor- and template-based methods. Many materials have been fabricated into nanowire/rod form using the processes mentioned above [1]. Most pure elements, especially metals and semiconductors, as well as some binary materials, such as compound semiconductors and oxides, have already been fabricated into 1D nanowire/rod structures. However, in terms of material variety , and the control of fabrication, there are at least two challenges that present methods (except nanolithography) cannot meet: (1) There is still no general methodology to prepare 1D nanostructures from different materials, which makes the fabrication of hetero-nanostructure very difficult. (2) None of the above techniques is sophisticated enough to manipulate the diameters, orientations, and positions of the nanowires/rods.

Recently, it has been shown that by combining oblique angle deposition and substrate positional control, a technique called glancing angle de position (GLAD), one can produce different nano-sized columnar films with controlled porosity and shapes [2-4]. GLAD is a physical vapor deposition process where the deposition flux is incident at a large angle with respect to the surface normal and the substrate is rotating. GLAD produces columnar structures through the effect of shadowing during film growth, while substrate rotation controls the shape of the columns. In this technique, there are three parameters that determine the morphology of the columns: the incident angle, the growth rate, and the substrate rotational speed. By changing the angle of incidence, the columns can be sculptured into a C-shape, S-shape, and zigzag shape, while by changing the ratio of the de position rate to the rotational rate (which is defined as the pitch, p ), the column morphology can be varied between matchstick, helical, and vertical columns [2-4]. Since GLAD is a physical vapor deposition technique, it has many advantages in terms of controlling the growth of nanostructured thin films: 1. It can form a nano-column array naturally. 2. The porosity of the film can be controlled by changing the incident angle. 3. There is almost no restriction on materials since the growth process is a thermal evaporation. 4. The shape and in-plane alignment of columns can be easily modified. 5. It has the advantage of self-alignment due to the shadowing effect. 6. It can also generate three-dimensional nanostructures. These advantages make the GLAD technique very promising for nanostructure fabrications. In the following, we will give a brief review on the GLAD technique.

  Experimental Approach

  The experimental setup is shown in Fig. 1. During the deposition, the angle between the incoming vapor from the source and the surface normal of the substrate is set between 0 to 90 degrees , and the substrate is rotated azimuthally. The general experimental conditions are the following: the experiments were performed in a high vacuum chamber with a background pressure of 2x10^-4 Pa. The Si (99.9995%, from Alfa Aeser) was evaporated by electron beam bombardment or e-beam evaporation. The pressure during deposition was less than 1x10^-3 Pa, and the growth rate was monitored by a quartz crystal microbalance. The stepper motor was attached to a water-cooled stand and the circulating chilling water was held at 4 degree C to prevent the sample from heating up . The distance between the evaporation source and the substrate was about 30 cm. During the deposition, the deposition rate was fixed, while the rotation speed was controlled by a computer. The following SEM pictures show some typical structures that can be fabricated by GLAD:

(1) Vertically aligned nanorod arrays (Fig.2)

(2) Helical nanostructures (Fig. 3)

(3) Multilayered nanorod structures (Fig. 4)

(4) Formation of regular array of nanorods by templates (Fig.5)

Conclusion

  These examples demonstrate that the most intriguing advantage of GLAD is its ability to control the structure of the nanorods . This cannot be achieved by any other nanostructure fabrication techniques.

References

(1) P. Yang, Y. Wu, and R. Fan, Inorganic semiconductor nanowires . Inter. J. Nanoscience 1 , 1 (2002).

(2) K. Robbie and M. J. Brett, Sculptured thin films and glancing angle de position: growth mechanisms and applications. J. Vac. Sci. Technol. A 15 , 1460 (1997).

(3) Y.-P. Zhao, D.-X. Ye, G.-C. Wang, and T.-M. Lu, Sculpture aligned nano-column arrays and nano-flowers by glancing angle de position. Nano Letters 2 , 351 ( 2002) .

(4) Y.-P. Zhao, D.-X. Ye, P.-I. Wang, G.-C. Wang, and T.-M. Lu, Fabrication Si nano-columns and square springs on self-assembly colloid substrates . International Journal of Nanoscience 1 , 87 ( 2002) .