Fig.1 shows the I-V characteristic of the cylindrical Langmuir probe. The plasma potential
is about 20V; this result is consistent with the plasma potential shown in Fig. 2
for the silicon dominated plasma. The insert in Fig.1 shows the ion saturation current (1.8 mA/cm2) and the floating potential (-40V). These values have been confirmed using a planar
disc probe; the ion saturation current measured in that case is 3.2 mA/cm2. The electron temperature estimated from the I-V characteristic is 12 eV, so the
ion saturation current gives a plasma density of 3-5 x 1010 cm-3.
The optical emission spectrum plotted in Fig. 2 shows the main emission lines of the
excited states of neutral silicon, and two emission lines due to silicon ions (634
and 637nm). No other significant emission lines are observed, showing that the plasma
is not contaminated with any other gas.
One of the characteristics of the HARE system is that, due to the spherical expansion
of the flux of evaporant, there is a strong gradient in the density of silicon neutrals
along the axis of the system9. The values of the neutral density for our evaporating conditions
(3.6kW) have been calculated from the measured deposition rate of silicon on a sample
located in the top chamber, assuming a directed flux of silicon with an energy of 0.11eV
(1300K). For an ideal gas at a temperature of 1300K the silicon atom density of 1012 cm-3 in the plasma source region would correspond to a pressure of 0.14 mTorr. This is
slightly less than 0.2 mTorr, which is the minimum pressure at which we are able
to maintain an argon plasma.