Electron Transfer at the n-InP Poly(Pyrrole) Interface
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707 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
EXPERIMENTAL Diode fabrication The n-InP Ipoly(pyrrole) diodes were fabricated as follows. A section of n-InP (dopant density: 6.2 x 1015 cm3, thickness = 2701Rm, typical area = 0.1 cm2 ) was ohmically back contacted using Ga/In eutectic. The n-InP was then etched (30s 0.05% Br2/methanol, methanol rinse, 30s 30% NH 4OH(aq), water rinse) and blown dry with N 2. Immediately following the etch, poly(pyrrole) was solution cast on the n-InP sample using the aforementioned method of Freund and coworkers [ 10]. Poly(pyrrole) was also cast onto a piece of platinum foil using the same polymerization solution. The electrochemical potential of the poly(pyrrole) fabricated in this manner was measured to be 0.2V vs. SCE in a 0.1M Bu 4NBF 4 (Bu = butyl) / CH 3CN solution using a non-aqueous Ag/Ag+ reference electrode (+0.3V vs. SCE) and its conductivity to be 1020 Q2-cm!. After rinsing with methanol and allowing to dry, the poly(pyrrole) side of the nInP I poly(pyrrole) diode was ohmically contacted by press contacting it with the poly(pyrrole) covered platinum foil. Materials Methanol (J.T. Baker), tetrahydrofuran (Mallinckrodt), phosphomolybdic acid hydrate (Aldrich), and n-InP (Crystacomm) were used as received. Pyrrole (Aldrich) was vacuum distilled prior to use. Measurements Current voltage measurements were performed using a Solartron 1287 electrochemical interface and capacitance-voltage measurements were performed using a Solartron 1260 impedance analyzer over the frequency range 1Hz - 1MHz using a 1OmV waveform amplitude. RESULTS Figure 1 shows the current density (J) - voltage (V,,pp) behavior of a typical n-InP I poly(pyrrole) diode. As shown in Fig. lb, the forward bias current (Vapp< 0) is modeled well by the diode equation [Il] (with the sign convention as described in Fig. 1): J=JO l- exp~ qnkT I nk"
)J
(1)
where Jo is the equilibrium exchange current density, n is the diode quality factor, T is temperature, q is the elementary charge, and k is the Boltzmann constant. The very slight "tailing off" at high applied potential is due the series resistance of the junction. Table I summarizes the best fit parameters of the forward bias (Vapp< 0) data to Eq. 1 for three different n-InP I poly(pyrrole) diodes. The barrier height of these interfaces were measured using capacitance-voltage (MottSchottky) techniques as described by the following relation for an n-type semiconductor [11]:
708
11ý -2 -
C
10-4
E 0 10- 5
E 0
(-4-
E
10"7 0
-6-.
A
'•-8 -0.4
0.0
10"87 1 0-"9
0.4
0.2
0.0
0.4
0.6
IVapp IV
Vapp I V
Figure 1: (A)Current density-voltage behavior for an n-lnP I poly(pyrrole) interface (sample m140 in Table I). Negative applied potentials correspond to higher electron energy in the n-lnP relative to the PP, and negative currents correspond to net electron flow from the n-lnP to the PP. The scan rate for in (A) these measurements was 20 mV s1 (B) Semi-logarithmic representation of the forward bias data 10'9 A cm°2.
(note ab
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