We investigated the electrical contact characteristics of indium tin oxide (ITO)/doped hydrogenated amorphous silicon (a-Si:H) junctions. For efficient collection of photo-generated carriers, photovoltaic and photodetector devices require good ohmic contacts with transparent electrodes. The amorphous-Si thin films were sputter deposited on ITO coated glass substrates. As-deposited p-type a-Si:H on ITO formed nearly ohmic type contacts and further annealing did not improve the contact characteristics. On the other hand, as-deposited n-type a-Si:H on ITO formed an ohmic contact, while further annealing resulted in a Schottky type contact. The ITO contact with p-type silicon semiconductor is a ro-bust ohmic contact for Si based optoelectronic devices.
Indium tin oxide (ITO) layers are frequently used as front contacts in thin film optoelectronic devices such as solar cells, light emitting diodes, laser diodes, and photodectors due to their high conductivity and transparency in the visible range of the solar spectrum [1,2]. In the case of thin film solar cells, the ITO contact is necessary to allow photons to reach the absorber layer and improve the photo-generated carrier collection [
The deposition of thin film solar cell structures on low cost and flexible substrates like plastic foil has necessitated the deposition of the thin films at relatively low temperature. Various low temperature schemes have been adapted to prepare a-Si:H films. They include low-power radio frequency (RF), direct current, electron cyclotron resonance and very high frequency—plasma-enhanced chemical vapor deposition [7,8]. However, these methods yield low growth rate. For inexpensive photovoltaic (PV) devices, high growth rate is required for depositing 0.5 µm thick absorber layers. Sputtering is a low cost, high deposition rate method that yields high purity films. Sputter deposited ITO on Si wafers or on Si films has been studied by various research groups [5,9], but there is not much work on the contact characteristics of the junction between ITO and sputter deposited amorphous Si films. In this article, we report on the electrical contact properties of ITO on pand n-type a-Si:H films. Our results shows that the ITO/p-type a-Si:H contact interface is better for silicon based optoelectronic device applications.
P-type and n-type Si:H films were deposited by a AJA Orion 1800F RF magnetron Sputter System (13.56 MHz) equipped with a load lock. The sputter deposition were carried out in an ultra pure argon (Ar) + hydrogen (H2) atmosphere, on commercially procured fluorinated indium tin oxide coated glass substrates. Four-inch diameter borondoped Si and phosphorus-doped Si targets were used for depositing n-type Si and p-type Si films respectively. The substrates were cleaned using acetone, methanol, isopropanol and de-ionized water before loading into the deposition chamber for plasma cleaning before deposition of the Si film. The system base vacuum was approximately 1 × 10–7 Torr, while the process pressure was 3 m Torr. A fixed RF power of 150 W was used for a 5 cm distance between the target and the sample. The substrates were kept at 200˚C during film deposition. The Ar and H2 flow rate was maintained at 10 and 1 sccm respectively.
Raman spectroscopy of the films was carried out using a Renishaw inVia confocal Raman spectrometer equipped with a research-grade Leica microscope, 20 × objective (numerical aperture of 0.40), and WiRE 2.0 software. A 785 nm laser light was utilized for excitation. The laser power on the sample was approximately 115 mW.
Cross-sectional transmission electron microscopy (TEM) specimens were prepared from the ITO/p-type a-Si:H/Al sample using a conventional ex-situ lift-out technique in a FEI Nova Nanolab 600 Dualbeam. The specimens were characterized by using a JEOL 2010F operated at 200 kV, equipped with an EDAX Si-Li energy dispersive X-ray spectroscopy detector. Currentvoltage (I-V) measurements were performed for ITO/p-Si and ITO/n-Si hetero-structures at room temperature using a Keithely 2400 SourceMeter. The contacts were made using Signatone 1160 series probe station.
All deposited films in this work were thoroughly characterized by Raman spectroscopy and TEM. Raman spectroscopy observation indicated that as-deposited films are amorphous silicon.
near 160 cm−1 and 480 cm−1, associated with transverse acoustic (TA) and transverse optic (TO) vibrational modes, respectively [
TEM analysis also confirms amorphous nature of the as-deposited p-type and n-type Si films and the as-deposited films are around 50 nm thick.
where Eg is the bandgap of the semiconductor, φM is the metal work function and χS is the semiconductor electron affinity. The work function for ITO is 4.7 eV, while the electron affinity for p-type a-Si:H is roughly 3.4 eV and it band gap is 1.8 eV. Hence, the calculated barrier height at the ITO/p-Si interface is around 0.5 eV. In addition, interface mixing also influences the contact properties. It is known, for instance that the sputter deposition of ITO on
Si can create an intermixed damaged layer to a depth of 2 - 3 nm even at room temperature [