Three and Six Channel Sensors: Realization and Colorimetric Characterization
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279 Mat. Res. Soc. Symp. Proc. Vol. 558 02000 Materials Research Society
EXPERIMENT The 3-channel sensor allows the simultaneous detection and read-out of three sensor signals. The detector is realized on a glass substrate coated with flat TCO (transparent conductive oxide) of 155 nm thickness and consists of a pin/TCO/nip/TCO/pin layer sequence. The amorphous layers have been deposited in a multichamber PECVD system at 200'C. The i-layer thickness of the top, middle and bottom diode is 85nm, 205nm and 700 nm, respectively. The optical bandgap of the intrinsic layers decreases from 2.0eV, 1.9eV to 1.74eV. The 315nm thick highly transparent and conductive layers (TCO) are realized by sputtered ZnO using an rfmagnetron sputter system [5]. Finally, the back contact was produced by thermally evaporated Al. For the 6-channel sensor the pin-diodes between the TCO-layers are replaced by pinipstructures. The total thickness of this structure is around 1700 nm and the bandgap of the employed materials is reduced from 2.2 eV in the first diode in the top structure down to 1.6 eV in the rear diode of the bottom structure. The optical bandgap of the absorption layers are alloyed with carbon or germanium. In order to contact the single terminals of the device, the structures
were patterned using photolithography and reactive ion etching (RIE). RESULTS Three-Channel Sensor
The spectral response curves of the 3-color detector, measured under short circuit conditions, exhibit maxima at 450nm, 560nm and 640nm and a full width at half maximum (FWHM) of 185nm, 125nm and 120nm, respectively (Fig. 1). The preferential absorption of blue, green and red light is detected in the top, middle and bottom diode, respectively, by adjusted bandgap and thickness of the absorption layers. The amplitudes of the spectral response curves differ by less than a factor of 2 and the ratio of the absolute spectral response values between the top/middle/bottom diode is 0.9/0.55/1, respectively. The good separation between the three diodes is also confirmed by measured monochromatic IN-curves [4]. As an indicator, the photocurrent under 450nm blue light (650nm red light) decreases (increases) from the top to the bottom diode by a factor of 1430 (16.5). Beside the color information, the dynamic is a further optimization criterion. The dynamic range for all three diodes exceeds 75dB in the bias voltage range from OV down to -0.5V. The dynamic is defined as the logarithm of the ratio between the photocurrent under 1000lx white light and the dark current. For the middle and the bottom diode, the dynamic is above 90dB, since the dark current is lower than 2xl0"° A/cm under reverse bias voltage of -0.5V. 250 B
Top
1'200
Middle Bottom
150
~100
sensor pixel
300
preprocessing color processing electronic unit (3x3 matrix)
Fig. 1: Schematic configuration of a 3channel sensor in combination with a preprocessing electronic and color processing unit (3x3 matrix).
400
600 500 wavelength [nm]
700
800
Fig. 2: Measured spectral response of the 3channel sen
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