Nucleation and deposition of W on GaAs by excimer laser-assisted chemical vapor deposition

We have investigated laser-assisted chemical vapor deposition of W on GaAs to get insight into the deposition mechanism involved. Depositions were performed with an ArF excimer laser aligned parallel to the substrate and with WF6 and H2 as precursor and reducing agents, respectively. Samples were analyzed by Rutherford backscattering spectrometry and the sheet resistance was measured with a four-point probe. We investigated the temperature dependence of the deposition rate up to 400 °C for both laser-assisted and thermal deposition. At 350 °C and a laser repetition frequency of 20 Hz, we observed a deposition rate for laser-assisted deposition of 5.3 Å/s, whereas without laser irradiation no deposition occurred. At this temperature, the film resistivity was 11 µOmega cm. For deposition temperatures above 350 °C, anomalous behavior was observed in both deposition rate and film resistivity. Experiments in which W coverage was measured as a function of laser pulse repetition frequency and as a function of time revealed that laser-assisted deposition at 300 °C consists of a two-step process. The first phase consists of exclusively laser-induced deposition at a rate of 3.3×1015 atoms/cm2/min/Hz, until a W coverage of approximately 15×1015 atoms/cm2 is reached. Laser-independent growth was not observed probably through insufficient nucleation. The second phase consists of combined laser-induced and laser-independent deposition on the initial W layer. The laser-induced growth rate was 3.2×1015 atoms/cm2/min/Hz for repetition frequencies in excess of 5 Hz. At 20 pulses per second, laser-induced deposition formed 47% of the total deposition rate. Below 5 Hz, the laser-induced rate was higher than above 5 Hz but nonlinear with pulse frequency. The growth rate by laser-independent deposition was 72×1015 atoms/cm2/min at 300 °C, which corresponds well with theory for thermally activated deposition of W on W.