Exciton - Light coupling in SiC nanocrystals
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Exciton - Light coupling in SiC nanocrystals Miguel Morales Rodriguez1 and Georgiy Polupan2 PCIM-Universidad Autónoma Metropolitana, Un. Azcapotzalco, 02200, Mexico, DF, 2 ESIME-National Polytechnic Institute, Mexico D.F. 07738,
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ABSTRACT The paper presents the results of the SiC:N nanocrystal characterization using the methods of photoluminescence (PL) at low temperatures and X-ray diffraction (XRD). Photoluminescence study of porous SiC:N (PSiC) layers with different PSiC thicknesses reveals the intensity stimulation for the high energy PL bands. The early investigation of temperature dependences of the high energy PL bands had shown that these PL bands related to free exciton emission in the different SiC polytypes. The SiC polytypes in the original n-type SiC:N wafers and in porous SiC layers were confirmed by XRD study. The intensity enhancement of excitonrelated PL bands in big size (50-250nm) SiC NCs is attributed to the realization of the exciton week confinement and exciton-light coupling in SiC NCs. The numerical simulation of exciton radiative recombination rates for the different exciton emissions has been done using a model of exciton – light coupling in SiC NCs. The experimental and numerically calculated results have been compared and discussed. INTRODUCTION The large band gap of SiC polytypes: from 2.36 eV in 3C-SiC up to 3.33 eV in 2H-SiC, makes SiC NCs as a perspective compound for the blue and ultraviolet (UV) light emitter diodes (LEDs) and full-color displays [1,2]. The porous SiC structures prepared on the n- and p-type bulk SiC substrates of different polytypes (3C-, 6H-, 4H- SiC) were studied intensively in the 90th mainly as the light emitting materials [3-9]. The emission intensity of SiC can be enhanced significantly when the crystallite size diminishes to the nanometer scale [5-7]. The mechanisms of intensity enhancement for variety of photoluminescence (PL) bands in the spectral range of 1.9-3.7 eV in PSiC are under discussion up to now. This paper presents the results of porous SiC characterization using X-ray diffraction (XRD) and photoluminescence spectroscopy techniques as well as the numerical simulation of radiative recombination rate for the different exciton related PL bands versus SiC NC sizes that has been performed using the model of exciton – light coupling in SiC NCs. EXPERIMENTAL DETAILS Porous SiC layers were formed by surface anodization of n-type 6H-SiC substrates doped with nitrogen with resistivity of 0.052 Ώ cm and orientation (0001) in 3% aqueous solution of HF at the dc current densities 8 (#1), 16 (#2), 24 (#3) and 48 (#4) mA/cm2 and an etching duration of 10 min without external illumination (Table 1). The X-ray diffraction experiments
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were made using the XRD equipment model of D-8 advanced (Bruker Co.) with Kα line from the Cu source (λ=1.5406Å). Table 1. Parameters of PSiC samples
Sample number #1 #2 #3 #4
Anodization current density, mA/cm2 8 16 24 48
Thickness, μm 2.1 4.3 6.2 12.1
Size of SiC NCs, nm 170-200 130-160 80-120 40-70
Photoluminescenc
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