Porous silicon was prepared by electrochemical etching of monocrystalline p+
(100) silicon wafers, in a 1:1 ethanol and HF (48 wt.%) electrolyte. The process took place in a Teflon cell under illumination provided by a 150-W halogen lamp to increase final porosity. Anodisation current was provided by a computer-controlled EG&G 263 galvanostat/potentiostat (Princeton Applied Research, Oak Ridge, TN, USA). Applied current density was 10 mA/cm2
for low porosity layers and 150 mA/cm2
for high porosity ones. This setup is known to produce homogeneous, spongelike porous silicon samples [8
CdSe and ZnTe semiconductors were grown onto PS surface by ICSS technique. General details of this growth setup and process can be found elsewhere [6
]. A graphite crucible, schematically shown in Figure
, was placed in a plate-shaped temperature profile at 390°C for ZnTe growth and 290°C for CdSe growth. PS samples (1
) were alternately exposed to Zn (99.99%) and Te (99.999%) and to Cd (99.99%) and Se (99.999%) elemental sources (provided by Goodfellows Cambridge Ltd., Huntingdon, England) by cyclically moving the sliding part of the boat back and forth. In between expositions to the sources, a vapour purge step was allowed, locating the substrate in an intermediate purge hole in the graphite boat. The process was performed under an Ar gas flow of about 4 cm3
/s at near atmospheric pressure. A programmed linear actuator LA12-PLC from LINAK (Nordborg, Denmark) was used to perform the deposition cycles. Previous to the growth experiments, the PS substrates were degreased in acetone and alcohol and in some cases dipped for 15 s in HF. Exposure time to the elements and purge time were varied. The temperatures were selected taking into account previous growth experiments of thin films; they represent the lower limits of the temperature range in which CdSe and ZnTe can be grown by ICSS. The distance between sources and substrates was around 5 mm, and the growth surface was a circle of 7 mm of diameter.
Sketch of the graphite boat used for the infiltration of the PS layers.
Rutherford backscattering spectroscopy (RBS) analyses for the characterization of the samples were performed with a 3,035-keV α-particle beam provided by the Cockcroft-Walton tandem accelerator at the Centre for Micro-Analysis of Materials at Universidad Autónoma de Madrid. A main Si detector was placed at 170.5° scattering angle position and a second one with a variable scattering angle position was placed at 165°, giving information of the depth profile of the sample. For the analysis, RBS spectra were simulated using the SIMNRA code (Max-Plank-Institut für Plasmaphysik, Garching, Germany) [9
] in order to determine the composition profiles. High-resolution scanning electron microscopy (SEM) images of the samples were obtained using a JEOL Microscope mod. JSM 6335 F (JEOL Ltd., Akishima, Tokyo, Japan). Grazing incidence X-ray diffraction (XRD) scans were taken using a Siemens D-5000 powder diffractometer (Siemens AG, Munich, Germany). Raman spectra were measured with a Renishaw Ramascope 2000 microspectrometer (Renishaw, Wotton-under-Edge, UK) and a
100 microscope objective.