Semiconductor growth and junction formation within nano-porous oxides

Article abstract of DOI:10.1002/1521-396X(200011)182:1<151::AID-PSSA151>3.0.CO;2-B

We have developed semiconductor growth techniques for the coating and filling of nanopores in ceramic-type substrates. The main idea behind this research is to use the large inner surface of ceramics as a template for the realization of semiconductor heterojunctions with extremely large interface area. As porous substrates we use lightly sintered nanocrystalline TiO₂ of 5-10 μm thickness. The pore volume in these substrates is approx. 50% and the average pore diameter is 30-50 nm. We are able to establish nanometer thick coatings on the inner surfaces of these substrates or - in a different technique - fill the pore volume with (100 ± 3)% efficiency. The growth techniques involve chemical and electrochemical methods from liquid solutions. Binary, ternary and, most recently, quaternary compounds of the II-VI and I-III-VI material systems were prepared.

Full text of DOI:10.1002/1521-396X(200011)182:1<151::AID-PSSA151>3.0.CO;2-B

R. KoÈnenkamp et al.: Semiconductor Growth and Junction Formation
151
phys. stat. sol. ꢀa) 182, 151 ꢀ2000)
Subject classification: 61.43.Gt; 68.35.Ct; 68.55.Jk; S8.11; S8.13; S8.16; S10.1
Semiconductor Growth and Junction Formation
within Nano-Porous Oxides
R. KoÈnenkamp1), K. Ernst, Ch.-H. Fischer, M. C. Lux-Steiner, and C. Rost
Hahn-Meitner Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany
ꢀReceived March 12, 2000)
We have developed semiconductor growth techniques for the coating and filling of nanopores in
ceramic-type substrates. The main idea behind this research is to use the large inner surface of
ceramics as a template for the realization of semiconductor heterojunctions with extremely large
interface area. As porous substrates we use lightly sintered nanocrystalline TiO2 of 5±10 mm
thickness. The pore volume in these substrates is ꢀ50% and the average pore diameter is
30±50 nm. We are able to establish nanometer thick coatings on the inner surfaces of these sub-
strates or ±± in a different technique ±± fill the pore volume with ꢀ100 Æ 3)% efficiency. The
growth techniques involve chemical and electrochemical methods from liquid solutions. Binary,
ternary and, most recently, quarternary compounds of the II±VI and I±III±VI material systems
were prepared.
Introduction Semiconductor heterojunctions with large interface area may be of inter-
est for several device applications, such as capacitors, sensors or solar cells. For a strong
area enlargement in micrometer thick films submicrometer structuring is needed. While,
in principle, this structuring can be achieved by lithographic techniques, it is clear that
large area applications require simpler and less expensive techniques. Porous materials
with a variety of pore topologies and sizes are available, but the deposition of materi-
als, particularly semiconductors, on to the inner surfaces of porous films has received
only limited attention. Here we show that deposition from solutions appears to be a
promising way to tackle this problem. We have found evidence that both, a filling of
nanopores and a coating on the wall surfaces, can be achieved by electrodeposition and
a modified chemical bath technique.
Preparation
Substrates Two different kinds of substrates were used in this study. Nanoporous TiO2
films of several micrometer thickness were prepared in a screen printing process using
a colloidal anatase precursor and subsequent sintering at 450 ꢁC [1]. The films consist
of anatase crystallites with diameter between 30 and 50 nm. The pore volume is ꢀ50%,
and the pore size is in the same range as the crystallite size. Gas adsorption measure-
ments ꢀBET method) indicate that the inner surface area in these films is 100 to 300
times larger than the geometric projection. The films are homogeneous on a micro-
meter scale. Accordingly, light scattering is very small for wavelengths in the visible
region. Figure 1 shows a transmission electron micrograph of these films [2]. A first
electrical characterization of this type of film is given in [3].
1) Tel.: 49 ꢀ30) 8062 2090; e-mail: koenenkamp@hmi.de

152
R. KoÈnenkamp et al.
Fig. 1. Transmission electron micro-
graph of nanoporous TiO2 substrate, il-
lustrating the grain and pore morphol-
ogy of these films
10 nm
Micro-porous TiO2 substrates, typically 2±3 mm thick, were prepared in a spray
pyrolysis process at substrate temperatures of 450 ꢁC using tetra-isopropyl-titanate
solution as a precursor [4]. The crystallite size and shape in these films is similar to
that in the nanoporous material, but the overall film morphology is quite different,
as the films are highly inhomogeneous on the micrometer scale ꢀFig. 2a). Films of
this type give rise to strong light scattering in the visible region. The open morphol-
ogy of the films allows faster mass and charge transfer from gases and liquids. From
our scanning electron microscopy results we estimate the surface enlargement on the
micrometer scale to be a factor ten. Gas adsorption measurements indicate, how-
ever, that on a nano-meter scale, the surface enlargement is similar to that in the
nanoporous films.
Fig. 2. a) Scanning electron micrograph of bare microporous TiO2 substrate. b) Scanning electron
micrograph of the same substrate covered with electrodeposited CdTe

Semiconductor Growth and Junction Formation within Nano-Porous Oxides
153
Semiconductor electrodeposition CdTe and CuSCN were deposited on these porous
substrates using a standard three-electrode electrochemical reactor, and SnO2-coated
glass as the working electrode and Pt as a counter electrode. CdTe is deposited in an
aqueous solution containing 1M CdSO4 and 5 Â 10±±4 M TeO2 at pH 2.2 at a substrate
temperature of 90 ꢁC. Electrode potentials of 0.59 eV versus Ag/AgCl reference were
found to give near-stoichiometric films [4, 5]. After deposition the films are exposed to
CdCl2 solution and subsequently annealed at 400 ꢁC in air. This procedure is known to
increase the photosensitivity of CdTe and results in light p-type doping [6].
For CuSCN electrodeposition we used a dried and de-oxygenated solution of 0.05 M
CuꢀBF4)2 and 0.125 M KSCN in purified ethanol. Typical electrode potentials are 0.7 to
0.9 V versus Pt [7].
Ion-layer gas reaction ꢀILGAR) for the deposition of CuInS2 ILGAR deposition of
semiconducting sulfide and oxide compounds has recently been developed in our la-
boratory [8, 9]. The preparation of CuInS2 involves chemical bath deposition of CuꢀI)
and InꢀI) salts by dipping the porous substrates in appropriate precursor solutions. In a
subsequent gas phase reaction with pre-heated H2S vapour, the deposited ion salts are
converted into CuInS2 and annealed. The typical layer thickness after a single ILGAR
ꢀ
cycle is 10±20 A. Thicker layers can be obtained by repetitive cycling. A more detailed
description of the method is given in Ref. [9].
Experimental Results
Electrodeposition Figure 2b shows the coverage of CdTe films on the microporous
substrate. It is seen that CdTe grows in conformal manner, the thin film following
smoothly the micrometer scale morphology of the TiO2 surface. So far we could not
determine if the growth process also leads to a coating on the nanometer scale. Visual
inspection of the micrograph and additional X-ray diffraction ꢀXRD) experiments indi-
cate a crystallite size of 100±300 nm. The current±voltage characteristics of the
TiO2/CdTe interface show excellent rectification, indicating that a p±n±heterojunction
is formed. Photocurrent measurements show that quantum generation efficiencies close
to unity are obtained near the TiO2/CdTe interface region. These findings indicate that
the interface is of good electronic quality.
Figure 3 illustrates the growth process of CuSCN in the nanoporous TiO2 substrate.
Different from the case of CdTe deposition, we observe a directed growth process origi-
10µm
Fig. 3. Scanning electron micrographs of a nanoporous TiO2 film filled partially with CuSCN de-
posited in electrodeposition ꢀcross-sectional view). Directional advance of the CuSCN growth front
is observed after 10, 30, and 90 min deposition
11 physica ꢀa) 182/1

154
R. KoÈnenkamp et al.
nating from the SnO2 conducting layer and resulting in a complete penetration of the
porous TiO2. A horizontal CuSCN growth front is clearly discernible in these micro-
graphs. Volumetric and weight measurements show that the void volume of the nano-
porous substrate is filled to ꢀ100 Æ 3)% in the process. The electrical characteristics of a
completely filled TiO2 film show excellent rectification. Since the TiO2/SnO2, CuSCN/SnO2
and CuSCN/graphite are ohmic, it can be concluded that the rectification occurs at the
large area TiO2/CuSCN interface [10].
The directed growth mode of the CuSCN is a consequence of an accurate adjustment
of the electrode potential during deposition. In electrodeposition the voltage applied at
the working electrode provides the energy for a change of charge state in the ionic
species in solution. In this case the positively charged CuꢀII)SCN+ complexes are re-
duced to neutral CuꢀI)SCN which is deposited. The threshold potential for the electro-
chemical reaction is ±±0.6 V versus Pt. For the directed growth process the electrode
potential is only slightly more cathodic than this value, lying at ±±0.7 V. This assures
that the deposition threshold is reached only in the immediate vicinity of the conduct-
ing SnO2 layer, while the potential in the TiO2 bulk remains below threshold due to
ohmic resistance and space charge layers in the TiO2.
The CdTe deposition occurs at potentials around ±±0.6 V versus Ag/AgCl, which is
much more cathodic than the deposition threshold of ±±0.3 V. Due to the open mor-
phology of the films it is thought that for these conditions the voltage drop in the
microporous TiO2 is not sufficient [11] to prevent growth on the TiO2 surface, such that
conformal coverage is obtained.
ILGAR Efficient and controllable semiconductor deposition in porous structures can
also be achieved in electroless deposition techniques. Figure 4 ꢀupper part) illustrates
the deposition of the ternary compound
CuInS2 on a microporous substrate using
the ILGAR technique. Conformal cover-
age with a similar morphology as shown
in Fig. 2b for electrodeposited CdTe is
observed. XRD measurements show that
the predominant phase is roquesite,
which is of interest for most photoelec-
Fig. 4. Upper part: ILGAR deposition of
CuInS2 on microporous TiO2 showing confor-
mal coverage with ꢀ10 nm grain size. Lower
part: EDX line scans across a nanoporous
TiO2 film coated with CuInS2 using the
ILGAR technique. Homogeneous distribution
of Cu and In, and accumulation of S near the
SnO2 interface are observed. Additional XRD
measurements confirm that the deposit is the
roquesite phase of CuInS2

Semiconductor Growth and Junction Formation within Nano-Porous Oxides
155
tric applications. Electrical measurements indicate an intimate electrical contact be-
tween TiO2 and CuSCN. At the present stage of development, however, rectification
and the photoelectrical response is not yet as excellent as in the case of the CdTe/TiO2
interface.
Perhaps even more interesting is our finding that ILGAR is also suited for deposi-
tion in nanoporous substrates. Our results show that the deposition occurs as a confor-
mal coating on the inner surface of the porous substrate [12], but so far we could not
establish the homogeneity of the coating on the nanometer scale. A first estimate indi-
ꢀ
cates that in one cycle of the ILGAR process, a 10±20 A thick layer of CuInS2 is
produced in the nanoporous material. With an increasing number of cycles the layer
thickness grows, until pore filling is obtained. Fig. 4 ꢀlower part) shows an EDX line
scans across a 8 mm thick nanoporous TiO2 film coated with CuInS2. In conjunction
with other experiments we interpret the line scan in terms of a more or less homoge-
neous deposition throughout the nanoporous TiO2 substrate with some S accumulation
near the interface to the underlying SnO2 layer.
Conclusions We have shown that solution growth techniques can be used to establish
semiconductor growth inside micro- and nanoporous thin films. Intimate high-quality
electrical contact could be established in electrodepostion and ILGAR. p±n heterojunc-
tions showing excellent rectification and charge separation were obtained with CdTe
grown on TiO2. In electrodeposition two growth modes were observed. At electrode
potentials near the deposition threshold we find a directed growth process which origi-
nates from the conducting surface of the electrode and leads to penetration and com-
plete filling of the nanoporous material. Deposition of CuSCN was shown as an exam-
ple of this growth mode. For electrode potentials significantly more cathodic than the
deposition threshold, conformal growth can be achieved, as demonstrated here for the
case of CdTe. ILGAR deposition was shown to give extremely thin conformal coating
in nano- and microporous substrates.
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