# The Charge Transport Properties of a HWCVD a-Si:H Thin Film under Bending Pressure

Authors: M. Boshta; V. Morchshakov; K. Bärner; R. Braunstein
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

The transient thermoelectric effects (TTEs) method  is used to measure the ambipolar space charge built up in a low-pressure hot wire chemical vapor deposition (HWCVD) technique a-Si:H layer deposited on a glass substrate. The stage 2 TTE-transients yield the trap state density difference with and without bending pressure up to 9 bars. The a-Si:H sample shows a reduction of the negative storage peaks at 0.045 eV and 0.026 eV with increasing pressure, while the positive (hole trap) peak and the zero crossing practically do not change with the pressure. At the maximum bending pressure, the negative peaks are almost zero and shifted into the band gap or toward the conduction band. Our result shows that it is necessary to produce and mount hydrogenated thin film solar cell stress-free.


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## Body

## 1. Introduction

Amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H)
thin films are already widely used in
microelectronics and solar cell technology. Their application in thin film solar cells promises
considerable cost reduction as a result of low material consumption and low-temperature
process as well as the possibility of a monolithic series connection of cells
[1, 2]. The low consumption of the raw material and the low-temperature process
during the production result also in low energy consumption during the
production of the thin film solar cells. Among the materials used for thin film
solar cells, amorphous silicon is the most important material in the commercial
production [1]. Microcrystalline silicon combines the advantages of the a-Si:H
technology and the extended absorption of crystalline silicon at long
wavelengths. Thin film tandem cell structures comprising amorphous a-Si:H
and microcrystalline silicon absorber layers promise conversion efficiencies as
high as those achieved with a-Si:H technology alone. In the case
of silicon thin film technology, the reduction of interfacial and mechanical
strains in the substrate, due to mounting of the solar panels, has become an
important research topic.This work deals with the controlled
variation of the mechanical strains in the substrate/thin film structure.
Assuming a strong cohesion between thin film and substrate, we can create expansion
or compression via concave or convex bending of the substrate. Only few such
experiments have been reported [3, 4], and even these results are somewhat inconclusive.
Nevertheless, the current accepted knowledge is that uniaxial strains mainly
change the bond angles, while bending pressure mainly affects the bond
distances. The bond distance changes in turn are supposed to strongly influence
the activation and mobility of the H-atoms. They are expected to partially
reposition under bending pressure. Another, but completely irreversible, effect
is the collapse of microvoids under pressure [4]. As such internal surfaces are
considered to be defect sources, this way the defect density can be reduced
too. In this work, we combine the TTE-flash method with the application of
bending pressure.


## 2. Experimental

The sample is manufactured using a low-pressure hot wire  chemical vapor deposition technique
(HWCVD) [5–7]. Here, silane (SiH4) decomposes at the hot surface
of a meandering tungsten wire whereupon the Si deposits on a nearby substrate
[8, 9]. The sample THD84 was prepared on a glass (1737Corning) substrate at a
substrate temperature of 535 K. The sample thickness is 2.16 μm, and the hydrogen
content is between 12 and 14 at. %.The time-resolved thermoelectric power (TTE) has been proposed as a transient method for determining carrier
lifetime, carrier diffusivity, carrier mobility, and trap levels in crystalline
and amorphous semiconductors, but it also allows to determine the Seebeck
voltageS(T) and the heat diffusivity D(T).A light pulse which is limited in
space and time falls on the one end of a rectangular sample. Voltages appear at
the end faces and decay with time. These voltage transients sometimes overlap
and are empirically described using a superposition of exponentials,(1)V(t)=V∞+∑aiexp⁡(−tτ1). Here, ai=V(0)i−V(∞)i is the relaxation amplitude
of ith relaxation process or the ith carrier type in case of a multicarrier
system.Specifically, the observed TTE decay
curves consist of three stages, each with a characteristic relaxation timeτi.(1)
Stage 1 (photodiffusion decay Dember effect): it is related to the recombination of
electron hole pairs generated by the light and diffusing into the dark zone of
the sample.(2)
Stage 2 (transient Seebeck effect): it is due
to the diffusion of thermally generated carriers in pair, that is,
electron-hole pair from higher-
to lower-temperature regions of the material. In doped or defect containing
semiconductors, Stages 1 and 2 can build up an ambipolar space charge.(3)
Stage 3 (quasistatic Seebeck effect): it is due to the phonon diffusion from the hot
point (illuminated section) to the cold point of the material.
Figure 1
shows the design of the bending pressure arrangement that has been used in this
experiment. It basically consists of two fixed
points (edges of the sample) and a mechanical force which is applied at the
center of the sample. The a-Si:H film and the glass substrate have thicknesses
of hf and hs and Young's moduli of Yf and Ys,
respectively. When an external force is applied, it causes the substrate to
bend elastically. The bending curvature depends on the thickness and Young's
modulus of the substrate. We assume that the substrate is elastically isotropic
in the plane. Under a specific tensile or compressional force, a film-substrate couple
bends with a constant curvature R. The bending momentum elongates the substrate
in the upper section of the film-substrate couple
and compresses the substrate in the lower section. This bending produces a
torque or bending moment, Mf, about the y axis which passes through
the center of the substrate for convenience. Thus,(2)Mf=Ffz=σfhf(hs2⁢).
This moment must be balanced by the moment generated by the substrate bending force.
Regardless of the prevailing stress distribution, maintenance of mechanical
equilibrium requires that the net force (F) and the bending moment be
eliminated in the film-substrate couple.
Thus, the condition that required for analysis is(3)Ms+Mf=0.From
these requirements, the stress (σf)
of the film is expressed as(4)σf=[Y1−v]Rhs26hf,
where R(m-1) is the curvature, being positive on a convex film face and
negative on a concave one. ν is Poisson’s ratio, which for most materials
lies around v=0.3±0.1.Figure 1
The bending pressure set-up.The curvatureR can be measured by the change in the angle of a reflected laser
beam as it is scanned across the substrate. In this case, the beam is acting as
an optical lever. Alternatively, optical interference fringes against a
reference flat can be measured, in which case also any variation in R over the
substrate is readily detected [10].


## 3. Results

The transient thermoelectric voltages have been recorded at pressures between 0 and 9 bar for
the HWCVD a-Si:H sample on glass substrate. The correction of the rough
transient voltage data were done according to the procedure described in [11].
We observe some structures in the stage 2 transient voltage versus temperature
curve V20(T), that is, the TTE-voltage curve versus temperature does
not have an exponential trend but changes at certain temperature values.  This implies that there is a significant
structure of the trap density of states at these temperature values when one
moves deeper into the optical gap, starting from the conduction (or valence)
band edge. Since this is not observed in nonhydrogenated a-Si thin films [11],
it suggests that the high H-content is responsible for most of the trap state
density in some way.Simultaneously,
we measure the relaxation timeτ20 of the a-Si:H sample. There is a significant structure also in the relaxation
time. However, usually we calculate the trap state density difference from the
derivative of the corrected voltage with respect to the temperature and not
from the correlated relaxation time [11]. It has been shown in [12] that the
derivative of the corrected transient voltage is approximately proportional to
the difference trap density of states, (NT−−NT+),
that is, the proportionality constant, g, is a weakly temperature dependent
function,(5)dV20c/dT=g(NT−−NT+). According to (5),
the space charges originating from the hole and electron traps can compensate
each other. This behavior is found very often with intrinsic or compensated
material [7].Figure2 shows
the trap state density of a-Si:H sample under different bending pressures as a
function of energy.Figure 2
Temperature derivativedV20c/dT versus energy (ε=2kT) and T (upper
scale) showing the difference density of trap states as they evolve into the
gap under various bending pressures.


## 4. Discussion

With increasing
pressure, the sharp negative (doping) peaks (at 45 meV and 26 meV) are reduced
while the positive (hole trap) peak and the zero crossing practically do not
change position. However, both negative peaks shift inside the energy gap
toward the valence band. At the maximum bending pressure (9 bars), the negative
peaks have almost disappeared.The slope of the
negative peak decreases with the increase of the bending pressure as shown in
Figure3. This means that the peak width and position change with increasing
bending pressure. In contrast, the positive peak intensity and shape show
virtually no change with the bending pressure. The strong shift in the negative
defect density of state peaks suggests a pressure-induced repositioning of the
H-atom related trap states, by shifting them either  toward the middle of the
band gap or very close to the conduction band edge. In both positions, the
electron trap levels contribute less to the negative space charge build up and
decay. We propose that the high photoconductivity found in this sample (THD 84)
has to do with that negative “doping” peak. Obviously, it is just
positioned right in energy for an intermediate storage of the photogenerated
excess electrons, which would enhance the overall lifetime. As the application
of pressure is expected to shift these energetic positions out of their
optimum, this concept would also explain the high-pressure sensitivity of that
peak. It also tells us that mechanical stresses are to be avoided if the
carrier lifetime in thin films is optimized through critically positioned trap
states. The data are obtained from a-Si:H sample. However, the results could be
applied also to poly-Si:H thin films, since the transport properties of
poly-Si:H thin films are mainly governed by their interface amorphous phases—which are usually left to optimize the light absorption.Figure 3
The slope of the negative
peak of difference trap state density of a-Si:H sample versus bending pressure.



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*Source: 643743-2008-03-11.xml*