Damage formation and annealing studies of low energy ion implants ...

of 3.5 keV until the sample was electron transparent. The sample was imaged in a JEOL
3010 TEM, operating at 300 keV. Images were taken in down-zone bright-field
conditions. Additionally energy filtered transmission electron microscopy (EFTEM)
was carried out on selected samples at CNR-IMM Catania (Italy). In EFTEM,
transmitted electrons are subject to energy filtering, providing the possibility of
elemental specific contrast.
4.4.3 Electrical measurements
Some Hall effect electrical measurements were carried out, using the van der
Pauw technique (7, 8, 45). From this technique, the sheet resistance Rs, the sheet carrier
density ns, and the mobility µ, can be determined, using a combination of resistance and
Hall effect measurements. A very simple description of the technique is given here.
The sheet resistance of a sample can be determined from several measurements
of voltage and current across the sample, in a set up illustrated in Figure 4.22a). A dc
current is applied between two terminals on one side and the voltage is measured across
the opposite side. This leads to a characteristic Resistance RA. The same types of
measurement are carried out using terminals rotated by 90° rotation, giving resistance
RB. The sheet resistance can be obtained from the equation:
( − R / R ) + exp(
− πR
/ R ) = 1
exp A s
B s
π (4.11)
To determine the activated concentration, Hall voltage measurements are
required. To measure the Hall voltage VH a current is applied between an opposing pair
of contacts and a constant magnetic field is applied perpendicular to the plane of the
sample, as shown in Figure 4.22b). The Hall voltage VH is measured across the
remaining pair of contacts. The sheet carrier density ns can be calculated using:
n = IB/
qV
(4.12)
s
H
a) b)
Figure 4.22 Set up for van der Pauw measurements. From (45).
99