CMOS compatible Si/SiO 2 multilayers for Light Emitting Diodes
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CMOS compatible Si/SiO2 multilayers for Light Emitting Diodes Z. Gaburro1, L. Pavesi1, G. Pucker2, P. Bellutti2 1 INFM and Department of Physics, University of Trento, via Sommarive 14, I-38050 Povo (Tn) Italy 2 ITC-IRST, via Sommarive 16, I-38050 Povo (Tn) Italy ABSTRACT We report photoluminescence and electroluminescence at room temperature in diodes based on Si/SiO2 multilayers. The multilayers are fabricated by alternating Si and SiO2 layers, whose thickness is, respectively, 2 and 5 nanometers. In photoluminescence, a single band is observed, centered at 800 nm, which is due to electron-hole pair recombination under quantum confinement. On the other hand, in electroluminescence, two bands are reported. The first band is in the infrared spectrum, and is blackbody radiation. The second band is visible, and is originated by relaxation of a single type of electrical carrier (electrons), as suggested by a fast decay time (less than 0.1 µs). Possible mechanisms can be hot-electron relaxation or coupling with surface plasmon-polaritons. INTRODUCTION Adding optical functionality to ultra-large scale integrated (ULSI) circuits is not a trivial task, due to the indirect band gap in Si [1]. It is possible to exploit hybrid approaches, such as integration of III-V active devices, or complex technologies such as strained Si-Ge [2]. However, there are other possibilities. Recently, a low cost and simple potential solution has been indicated by the demonstration of electroluminescence (EL) from metal-SiO2-Si tunnel diodes [3]. The physical origin of the light generation is electron-hole plasma recombination, which gives an observable emission in 1.0-1.1 eV [3]. The spectrum of the emission is concentrated in a region close to the Si band gap [3], and therefore cannot be exploited for applications requiring visible light. An alternate solution can be the insertion of Si low dimensional systems. Indeed, it is well known that low dimensionality can overcome the limitations on the photoluminescence (PL) efficiency by exploiting quantum confinement (QC) of the electrons, which also shifts the emission in the visible region [4]. Such approach avoids insertion of hybrid materials. It has been extensively demonstrated that single and multiple quantum wells (QW’s) of Si separated by wide band gap barriers (e.g., SiO2, CaF2, or Al2O3) show enhanced PL efficiency, as compared to crystalline bulk Si [5-15]. Recently, we have fully fabricated Si/SiO2 multilayers in a standard and thus inexpensive complementary metal- SiO2-Si (CMOS) process, and showed QC effects in PL emission in our structures [16]. In this work, beside PL, we report the observation of two simultaneous bands of EL – the first in the IR region, the second in the visible region - from diodes based on these Si/SiO2 multilayers. While the PL is based on QC, both EL mechanisms are not due to QC. EXPERIMENTAL The starting substrate was a p/p+ epitaxial Si wafer. The epitaxial material was used to ensure a high gate oxide quality. Thickness of the epitaxial layer was about 10 µ
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