Isothermal close space sublimation for II-VI semiconductor filling of porous matrices
© Torres-Costa et al.; licensee Springer. 2012
Received: 9 May 2012
Accepted: 14 July 2012
Published: 23 July 2012
Isothermal close space sublimation, a simple and low-cost physical vapour transport technique, was used to infiltrate ZnTe and CdSe semiconductors in porous silicon. The structure of the embedded materials was determined by X-ray diffraction analysis while Rutherford backscattering spectra allowed determining the composition profiles of the samples. In both cases, a constant composition of the II-VI semiconductors throughout the porous layer down to the substrate was found. Resonance Raman scattering of the ZnTe samples indicates that this semiconductor grows in nanostructured form inside the pores. Results presented in this paper suggest that isothermal close space sublimation is a promising technique for the conformal growth of II-VI semiconductors in porous silicon.
Porous silicon (PS) has received a lot of interest in recent years due to its many peculiar properties. In particular, its porosity may reach up to 90% under appropriate conditions , and its internal surface may be as high  as 800 m2/cm3; this makes PS a promising material for catalytic applications and also as a matrix material for embedding different substances such as SnO2, C60 molecules  or even nematic liquid crystals . The interest in embedding materials in PS originates from different reasons. For example, luminescence stability of porous silicon is related to the passivation of its internal surfaces. Also, the small pores of silicon can be used as templates for nanocrystal formation. On the other hand, potential PS-based optoelectronic devices require electrical contacts not only in the external surface on the PS film, but also in the internal walls of the pores.
The filling of intricate and large aspect ratio pores as is the case of PS is a difficult task. This is because using most traditional vapour phase deposition techniques, the vapour transport is strongly favoured near the entrance of the pores; then they eventually become obstructed, and the inner part of the pores remain empty. The use of isothermal close space sublimation (ICSS) technique for growing epitaxial or polycrystalline films in dependence of the type of substrate has been verified in previous publications [6, 7]. This technique uses alternate exposure to the different elemental sources in a regime in which there is no temperature difference between the source and the substrate. This allows regulation of the growth process. For this reason we explore the possibility of filling PS layers using this technique. We presented the growth of CdSe and ZnTe semiconductors inside the pores of porous silicon using alternated evaporation of elemental Cd, Zn, Te and Se.
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 .
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)  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.
Results and discussion
According to these profiles, the sample can be divided in three regions. In region I (near surface region), the simulation of the spectrum indicates only a stoichiometric 1:1 CdSe film. This region represents only around 2% of the total thickness (in areal density of atoms units) of the films and can be related to the CdSe grains observed by SEM on the PS surface. In region II, the most part of the film (around 70% of the total thickness) shows a nearly constant composition of O, Si, Cd and Se. In this region, a stoichiometric Cd/Se proportion is observed. This supports the XRD measurements where no other phase different to CdSe is observed. In view of the CdSe stoichiometry, it can be assumed that oxygen present in the sample comes from SiO2 in the inner walls of the pores. From the compositional analysis, it can be concluded that CdSe has successfully infiltrated inside the PS layer down to the substrate in almost constant proportion, indicating a slow conformal CdSe growth inside the PS.
The laser wavelength of 514 nm (photon energy of 2.409 eV) is slightly above the ZnTe bandgap at room temperature. Therefore, most of the probing beam is absorbed; hence, the substrate Raman signal is barely visible in the spectrum. However, the Raman scattering from ZnTe is very strong. The longitudinal optical phonon mode of ZnTe appears at 204 cm−1. The peaks at multiples of this frequency correspond to the overtones of this fundamental phonon. The enhancement of the Raman signal for ZnTe and the appearance of the overtones may be attributed to a resonance effect indicating that the incident photon energy is very close to the semiconductor bandgap. In this case, the probing photon energy is 2,409 eV, which is 15 meV higher than the bulk ZnTe bandgap at room temperature. This enhancement of the ZnTe bandgap can be attributed to an electronic quantum confinement effect in ZnTe nanostructures. Assuming a spherical shape of the ZnTe clusters, a bandgap widening of 15 meV corresponds to cluster diameters of 13 nm.
In this work, ZnTe and CdSe compounds have been grown in porous silicon by ICSS. XRD and RBS measurements revealed that stoichiometric polycrystalline ZnTe and CdSe are formed. Moreover, in-depth compositional analysis of the RBS data showed that the stoichiometry and content of the semiconductor compounds remain almost constant throughout the porous layer down to the substrate. A resonance in the Raman scattering of the ZnTe phase has been observed in porous silicon layers embedded with ZnTe at photon energies above the bulk ZnTe bandgap, indicating that the semiconductor is growing in nanostructured form inside the pores which induce electronic quantum confinement effects. Results presented in this paper suggest that isothermal close space sublimation is a promising technique for the conformal growth of II-VI semiconductor nanostructures in porous silicon.
Osvaldo de Melo and Vicente Torres-Costa acknowledge the support given by the agreement between the University of Havana and the Autonomous University of Madrid. Research funding has been provided by projects MAT2008-06858-C02-02 (Spain) and Consolider FUNCOAT CSD2008-00023 (Spain).
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