Electroreflectance Study of Porous Silicon made from Substrates with Different Resistivities
- PDF / 1,006,277 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 26 Downloads / 215 Views
Electroreflectance Study of Porous Silicon Made From Substrates With Different Resistivities
Toshihiko Toyama, Yasuharu Nakai and Hiroaki Okamoto Department of Physical Science, Graduate School of Engineering Science, Osaka University Toyonaka, Osaka 560-8531, Japan ABSTRACT Employing electroreflectance (ER) spectroscopy, we have studied optical transitions in porous Si (PSi) with a thickness of 100±50 nm made from crystalline Si (c-Si) substrates with different resistivities, 4–10 Ωcm (p-), 0.1–1 Ωcm (p), and < 0.018 Ωcm (p+). The ER features observed at 1.1–2.8 eV in p+ PSi are analyzed with a simple effective mass approximation (EMA) model for confined electron-hole (e-h) pairs in a spherical quantum dot as we have previously done for those observed at 1.2–3.1 eV in p- PSi. From the ER analysis with the EMA model, the effective crystal size is estimated without destruction, and the kinetic energy and the Coulomb attraction energy of the confined e-h pairs are also deduced. Furthermore, the ER features corresponding to the optical transitions at E1(E0') critical point (CP) are observed at 2.8–3.3 eV in p+ PSi. With an increase in the crystal size, the transition energy of E1(E0') CP in p+ PSi is decreased, while that in p- and p PSi is unchanged from that of 3.4 eV found in c-Si including the p+ c-Si. From the Raman results, strain- and disorder-induced spectral changes are found to be negligible, so that the high doping induced effect is the most acceptable mechanism for the red-shift in E1(E0') transitions of Si nanocrystals with the crystal size of 2–3 nm.
INTRODUCTION Since the first discovery of strong photoluminescence (PL) at room temperature with emission energies being larger than the bandgap of crystalline Si (c-Si) [1], porous Si (PSi) has been intensively studied as a physical phenomenon arising from quantum confinement (QC) effects found in low-dimensional semiconductors [2]. From early research days, it has been known that the crystal size of Si nanocrystals embedded in PSi made from low resistivity or highly doped substrate (p+ PSi) tends to be large comparing with that in PSi made from high resistivity substrate (p- PSi) [3]. The larger crystal size would correspond to a relative small blue shift in optical absorption edge from fundamental indirect gap Eg(Γ–X) of c-Si [4]. Besides, larger size-gradient in growth direction has been found in p+ PSi, which would induce broad and featureless spectral lineshapes in dielectric functions at critical points (CPs) observed by reflectance spectroscopy [5] or spectroscopic ellipsometry (SE) [6]. In addition, anomalous features are found in dielectric function from blue to violet region, however, which are also interpreted as the size-gradient or structural anisotropy related effects [6]. Thus there has been no report on experiments using p- and p+ PSi with an equivalent crystal size (and with small sizegradient) to find changes in the optical properties of PSi based on the doping level. Recently we have reported results the optical properties of p- PSi meas
Data Loading...
