Effect of Heavy Doping on the Photoluminescence and Photoreflectance Spectra of Silicon and SiGe Layers.
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CHROBOCZEK**, * LPM (URA CNRS) INSA Bdt.502, 69621 Villeurbanne Cedex, FRANCE, [email protected] ** France Telecom CNET-CNS, BP98, 38243 Meylan, FRANCE ABSTRACT PL and PR measurements have been performed on a set of silicon samples p-type doped 18 -3 20 between 10 cm and 10 cm 3 . Both optical methods are shown to be complementary as the bandgap narrowing effect is clearly evidenced from PL results, whereas low temperature PR results rather show the filling effect of the valence band, in good agreement with theoretical determinations. We observe an increase of E'0 transition as p-type doping level is increased. PR spectra of Si and SiGe alloys at low temperature are presented together with theoretical fitting using the third derivative functional form of the unperturbed dielectric function of both direct optical transitions involved in the main PR feature : E'0 and E,. This characterisation work will be discussed by pointing out that PL and PR results are closely connected, allowing the measurement of the effect of heavy doping on the Si and SiGe band structures.This study is fully relevant to high speed bipolar transistor development. INTRODUCTION Continued interest in heavily-doped semiconductors stems from the fact that practically all the electronic devices used today contain heavily doped components. Heavy doping is used either for diminishing the resistance of device constituents or for increasing electric fields in junctions. A few years ago, heavy doping was used in order to obtain a reduction of the hole flow towards the emitter by the valence band offset [1] . This has not only improved the current gain but also permitted for higher base doping to be used, which improved high frequency performances of the transistors. The device with doping-induced band gap narrowing, called the pseudo-heterojunction transistor [2] - to distinguish it from the heterojunction bipolar transistor (HBT) with a SiGe base - is simpler to fabricate and still attracts much attention. However, potentialities of HBTs are considerable as the emitter/base valence band offset can reach 0.3 eV. However, heavy doping of semiconductors induces a deformation of the band structure and leads to band filling, band gap narrowing, many body effects, which can influence device properties. Theoretical treatments of such physical effects are very complicated and usually neglect the many-body effect [3]. Experimental study of such effects have already been reported by photoluminescence (PL) and photoluminescence excitation as well as by electrical characterization techniques (see for instance ref [4], [5]). We present in this paper complementary results obtained by PL and PR on different Si and SiGe samples with heavy p-type doping. PR measurements are sensitive to direct optical transitions and allow to check the band structure of the materials not only near the Brillouin zone center, but also around secondary extrema of the valence or conduction bands. EXPERIMENTS We have studied two sets of boron doped samples : one silicon set with differen
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