JOURNAL OF MATERIALS SCIENCE 37 (2 0 0 2 ) 3953 – 3958 Properties of GaAs nanoclusters deposited by a femtosecond laser L. N. DINH, S. E. HAYES, A. E. WYNNE, M. A. WALL, C. K. SAW, B. C. STUART, M. BALOOCH Lawrence Livermore National Laboratory, Livermore, CA 94551, USA A. K. PARAVASTU, J. A. REIMER University of California, Berkeley, CA 94720, USA The properties of femtosecond pulsed laser deposited GaAs nanoclusters were investigated. Nanoclusters of GaAs were produced by laser ablating a single crystal GaAs target in vacuum or Ar gas. Atomic force and transmission electron microscopies showed that most of the clusters were spherical and ranged in diameter from 1 nm to 50 nm, with a peak size distribution between 5 nm and 9 nm, depending on the Ar gas pressure or laser fluence. X-ray diffraction, solid-state nuclear magnetic resonance, Auger electron spectroscopy, electron energy loss spectroscopy, and high-resolution transmission electron microscopy revealed that these nanoclusters were randomly oriented GaAs crystallites. An oxide outer shell of ∼2 nm developed subsequently on the surfaces of the nanocrystals as a result of transportation in air. Unpassivated GaAs nanoclusters exhibited no detectable photoluminescence. After surface passivation, these nanoclusters displayed photoluminescence energies less than that of bulk GaAs from which they were made. Our photoluminescence experiments suggest an abundance of sub-band gap surface states in these GaAs nanocrystals. C  2002 Kluwer Academic Publishers 1. Introduction GaAs nanocrystals are expected to exhibit novel size dependent properties due to quantum confinement ef- fects, such as the quantization of the electronic den- sity of states, and the blue shift of the optical absorp- tion and photoluminescence with smaller crystal sizes [1, 2]. Among the techniques employed in the produc- tion of GaAs, pulsed laser deposition (PLD) seems to be a suitable choice for the deposition of thin GaAs films or nanoclusters on different substrates. The main ad- vantages of PLD are its simple experimental setup and the ability to operate in a wide range of gas pressures. For complex materials with constituent elements hav- ing greatly different vapor pressures, laser ablation with short pulsed lasers has produced films with excellent stoichiometries [3]. There have been published studies concerning the formation of GaAs nanostructures by laser ablation with nanosecond lasers, the evolution of the GaAs target under ultrashort pulsed lasers, the prop- erties of the ablated plumes, and the generation of two and three dimensionally confined structures of GaAs by techniques other than PLD [4–17]. However, to our knowledge, there is no published report on GaAs nan- oclusters formed by femtosecond lasers. In this paper, we described the characteristics of GaAs nanoclusters synthesized via femtosecond laser deposition. 2. Experiments The output at 810 nm of a 150 femtosecond Ti-Sapphire laser operating at 1000 Hz, was employed in the growth of GaAs nanoclusters [18]. With a time averaged power of 2 W and a circular spot diameter of 0.85 mm at the ab- lation target, the laser beam had an average energy den- sity of 0.35 J/cm 2 per pulse. Samples were synthesized in a high vacuum chamber with a base pressure of about 1.3 × 10 -3 Pa. For the production of GaAs nanoclus- ters, the chamber gate valve connected to the pump- ing system was closed, and Ar gas was leaked into the chamber up to the desired pressure. During the ablation process, a p-type (100) GaAs single crystal wafer with a diameter of 5.08 cm was rotated while the laser beam was translated over the surface so that the whole wafer surface was utilized in a fairly uniform manner [19]. GaAs nanoclusters were deposited onto glass slides, metal foils, Si wafers, and highly oriented pyrolytic graphite (HOPG) substrates; all were at room tempera- ture and mounted on a holder located about 15 cm away and parallel to the GaAs target. After deposition, the samples were removed from the synthesis chamber and transported in air to the appropriate analysis stations for x-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), transmission electron mi- croscopy (TEM), electron energy loss spectroscopy (EELS), photoluminescence (PL) spectroscopy, and solid-state nuclear magnetic resonance (NMR). 3. Results Fig. 1 shows AFM images of less than one layer (a) and many layers (b) of nanoclusters deposited in a vacuum 0022–2461 C  2002 Kluwer Academic Publishers 3953