Thermopower and 1/f Noise Measurements in Amorphous Silicon-Carbon Alloys
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e S
C
F + A]
and
o = o0 exp I
j)
where the heat of transport term A is of order unity.5 However, the activation energy Es found from thermopower and that found from conductivity (termed Eo) are not equal. Because Es occurs linearly in the thermopower expression, the high and low energy
regions in the fluctuating energy landscape near the mobility edge contribute more equally to the thermopower than in the conductivity where FU appears in an exponential, leading to a greater weighting of the high energy regions. Beyer and Overhof introduced the Qfunction to characterize this difference Eu - Es, which they interpreted as a measure of the magnitude of the energy fluctuations:
=lna+-S
=
o-
T
where EQ = Ea - Es and Q0 = lno0 + A.3 In this paper we study the relationship between the 1/f noise statistics and the disorder at the mobility edge measured by EQ for a series of
577 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
hydrogenated amorphous silicon carbon (a-SiCx:H) films, where the carbon concentration is systematically varied. EXPERIMENTAL TECHNIQUES The n-type amorphous silicon carbon alloys studied here were made by the glow discharge decomposition of silane (SiH 4 ), phosphine (PH3) and methane (CH4). A film without carbon was synthesized at the University of Minnesota, while the other films were grown at Xerox PARC. All the films were n-type doped (1000 ppm PH3 in SiH4 ) with the concentration xc of CH4 defined as [CH 4 ]/[CH4 + SiH 4] ranging from xc = 0 to xc = 0.65. All films were 1 gm thick and deposited onto Coming 7059 glass substrates. The substrate temperature was 250'C with an incident r.f. power of 2 W. After deposition of the aSiCx:H co-planar chromium electrodes - 500 A thick were deposited onto the sample. The chromium and film were scratched to yield two regions each having electrodes - 1 mm by 2 mm, with a separation between each electrode of 0.9 mm.
In the thermopower experimental configuration shown in Figure 1, two copper blocks with resistor heaters are - 4 mm apart in a shielded vacuum chamber. The film is mounted across the two heated blocks with two chromium electrodes on each block. Two type T thermocouples (wire thickness - 0.1 mm) TC1 and TC2 measure the temperature of the sample at the chromium electrodes above each heated block. The thermoelectric voltage generated by the amorphous material when the blocks are at different temperatures, SSi-Cu is measured with a Keithley 617 electrometer using the copper leads of TC1 and TC2. The absolute thermopower is determined from Ssi-Cu by adding the absolute thermopower of SCu which is taken from the literature. The thermopower and conductivity are measured after the film is annealed at 470 K for one hour and then slowly cooled TC1
I(-1 C/min) to room
TC2
temperature.
The
thermopower at a given temperature is obtained by taking the slope of a plot of the measured
thermoelectric voltage against temperature.
Electre
hin film
Bbck
A
Block 4mm 4mm
Each thermoelectric voltage is generated by varying t
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