Self optical gain in multilayered silicon-carbon heterostructures: A capacitive active band-pass filter model
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Self optical gain in multilayered silicon-carbon heterostructures: A capacitive active bandpass filter model M. A. Vieira1,3, M. Vieira1,2, P. Louro1,2, M. Fernandes1,2, J. Costa1,2, A. S. Garção2,3 1
Electronics Telecommunication and Computer Dept. ISEL, R. Conselheiro Emídio Navarro, 1949-014 Lisboa, Portugal Tel: +351 21 8317290, Fax: +351 21 8317114, [email protected] . CTS-UNINOVA, Quinta da Torre, Monte da Caparica, 2829-516, Caparica, Portugal. 3 DEE-FCT-UNL, Quinta da Torre, Monte da Caparica, 2829-516, Caparica, Portugal. 2
ABSTRACT This paper reports results on the use of a pi’n/pin a-SiC:H heterostructure as an active band-pass filter transfer function whose operation depends on the wavelength of the trigger light, on the applied voltage and on the wavelength of the additional optical bias. Results show that the device combines the demultiplexing operation with the simultaneous photodetection and self amplification of the signal. Experimental and simulated results show that the output signal has a strong nonlinear dependence on the light absorption profile. The device, modeled by a simple circuit with variable capacitors and interconnected phototransistors through a resistor, is a current-controlled device. It uses a changing capacitance to control the power delivered to the load acting as a state variable filter circuit. It combines the properties of active high-pass and low-pass filter sections into a capacitive active band-pass filter. INTRODUCTION There has been much research on semiconductor devices as elements for optical communication when a band or frequency needs to be filtered from a wider range of mixed signals. Amorphous silicon carbon tandem structures, through an adequate engineering design of the multiple layers’ thickness, absorption coefficient and dark conductivities can accomplish this function. In this paper, light-activated multiplexer/demultiplexer silicon-carbon devices are analyzed. Characteristics of tunable wavelength filters based on a-SiC:H multilayered stacked cells are studied both theoretically and experimentally. A capacitive active band-pass filter model supports the experimental data. An algorithm to decode the multiplexed signal is established. DEVICE CONFIGURATION The sensor element is a multilayered heterostructure based on a-Si:H and a-SiC:H produced by PE-CVD at 13.56 MHz radio frequency. The configuration of the device, shown in Figure 1, includes two stacked p-i-n structures (p(a-SiC:H)í'(a-SiC:H)-n(a-SiC:H)-p(a-SiC:H)-i(a-Si:H)-n(a-Si:H)) sandwiched between two transparent contacts. The thicknesses and optical gap of the front í'- (200nm; 2.1 eV) and back i- (1000nm; 1.8eV) layers are optimized for light absorption in the blue and red ranges, respectively [1]. As a
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ITO n i-Si:H (1000 nm)
P n i’-SiC:H (200 nm)
P ITO Glass
Figure 1 Device configuration.
result, both front and back pin structures act as optical filters confining, respectively, the blue and the red optical carriers to their active areas. SELF-OPTICAL BIAS AMPLIFICATION
Photocurrent (PA)
Th
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