Planar and Perpendicular Conductivity of Doping Modulated Amorphous Silicon Multilayers
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PLANAR AND PERPENDICULAR CONDUCTIVITY OF DOPING MODULATED AMORPHOUS SILICON MULTILAYERS H. Steemers, I. Chen, J. Mort, F. Jansen, M. Morgan, N. Kachonkin and
D. Kuhman Xerox Webster Research Center, Webster, NY 14588
ABSTRACT
The conductivity of a-Si:H multilayers consisting of alternating boron and phosphorus doped layers has been studied as a function of sub-layer thickness. The planar and perpendicular dark conductivity is
compared with theoretical
analysis of space-charge doping in these structures and this effect is found to dominate the transport as the sub-layer thickness is reduced below a critical value. INTRODUCTION There has been a growing interest in amorphous multilayers over the last few years. Compositionally modulated multilayers, such as the a-Si:H/a-SiN:H [1] and a-Si:H/a-Ge:H [2] superlattices have been studied for quantum-size effects, whereas in the doping modulated or npnp multilayers a persistent photoconductivity [3] has been investigated in the plane of the layers. In this paper we report both the conductivity in, and perpendicular to, the plane of a doping modulated amorphous silicon multilayer as a function of the sub-layer thickness, to study the effect of space-charge doping in these structures. Space-charge doping has been previously discussed [4]. Figure (1) is an energy band diagram of a npnp multilayer, indicating the modulation of the valence and conduction
bands in this structure for two regimes of sub-layer
thicknesses, ds. The solid line is for d.much larger than the depletion width 8 and represents an idealized system resulting in the full modulation of the energy levels. As d. is reduced to - 8, space-charge doping, due to the transfer of excess carriers across the interface, reduces the modulation of the band edges as indicated by the broken lines. In this paper we will report on the effect of space-charge doping on the planar and perpendicular conductivtiy of npnp multilayers. After describing the sample preparation, evaluation, and experimental techniques, the conductivity results are reported. Finally, the potential relevance to devices is discussed.
n
--
p E-
n
p
n
•---
.
-
p -
-Fig.
1. Energy band diagram
a
.of
doping
modulated
with
sub-layer
multilayer E__-_
thickness ds. Solid line is for
ds greater than the depletion
width whereas the broken line illustrates
the
space-charge doping.
Mat. Res. Soc. Symp. Proc. V.
70. 190 Mainals Research Socity
effect
of
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A
P+
n3+
Glass
(a)A
(b) Fig. 2. Sample configuration for perpendicular conductivity measurements.
(a)
planar
conductivity
and
(b)
EXPERIMENTAL DETAILS
Sample preparation The multilayers were prepared in a computerized hot-wall rf plasma deposition system. The total pressure in the system was maintained at 208 mtorr, the samples deposited at 2380 C with a power density of 8.66 Wcm. 2 . The multilayers were deposited simultaneously on glass and aluminum substrates. The doping levels of 9 ppm phosphorus and 108 ppm boron in the n and p-type layers respec
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