Measurements

3.2 Deposition Technique
admixture of hydrogen (H 2 )offers a straightforward way to change the growth
conditions all the way from highly crystalline to amorphous growth. The silane
concentration SC defined as the ratio of silane gas flow and the total gas flow, is
therefore one of the main parameters varied in this work.
SC =
[SiH 4 ]
[SiH 4 ] + [H 2 ]
(3.15)
Besides the gas composition several of other parameters are significant in determining
the properties of the deposited films; the deposition pressure p, the substrate
temperature T S , and the plasma power density P. The plasma excitation frequency
ν ex is also very important for the film properties and classifies the process
into RF-PECVD (standard frequency of 13.56 MHz) and VHF-PECVD (higher
frequencies up to 150 MHz). Doping can be achieved by adding trimethylboron
(TMB) or diborane (B 2 H 6 ) and phosphine (PH 3 ) for p-type and n-type doping,
respectively 3 . Deposition parameters used throughout this work are listed in table
3.1
Table 3.1: Typical PECVD-Deposition Parameters used within this work.
Parameter
value
Excitation frequency ν ex 95MHz (VHF)
Plasma power density P 0.07 W/cm 2
Substrate temperature T Sub 200 ◦ C
Pressure p 40 Pa
Silane concentration SC 2 - 100%
Phosphor doping PC 0-20ppm
Boron doping BC 0-70ppm
3.2.2 Hot-Wire Chemical Vapor Deposition (HWCVD)
Hot-wire chemical vapor deposition (HWCVD), also known as catalytic chemical
vapor deposition (CAT-CVD) [151, 152, 153], is becoming increasingly popular
in the field of silicon thin film deposition, particulary since recently it was
3 Doping densities are typically measured in parts per million (ppm). Taking the density of
crystalline silicon a doping density of 1ppm corresponds to about 5 × 10 16 doping atoms per cm 3 .
However, the built-in factor as well as the doping efficiency of the dopant have to be taken into
account in order to determine the active doping density.
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