M. Vieira

Laser-scanned p-i-n photodiode (LSP) for image detection

Amorphous and microcrystalline glass/ZnO:Al/p(a-Si:H)/i(a-Si:H)/n(a-Si 1 - x C x :H)/Al imagers with different n-layer resistivities were produced by plasma-enhanced chemical vapor deposition technique (PE-CVD). The transducer is a simple, large area p-i-n photodiode; an image projected onto the sensing element leads to spatially confined depletion regions that can be readout by scanning the photodiode with a low-power modulated laser beam. The essence of the scheme is the analog readout and the absence of semiconductor arrays or electrode potential manipulations to transfer the information coming from the transducer. The effect of the image intensity on the sensor output characteristics (sensitivity, linearity, blooming, resolution, and signal-to-noise ratio) are analyzed for different material composition. The results show that the responsivity and the spatial resolution are limited by the conductivity of the doped layers. An enhancement of one order of magnitude in the image intensity and on the spatial resolution is achieved with a responsivity of 0.2 mW/cm 2 by decreasing the n-layer conductivity by the same amount. In a 4 x 4 cm 2 laser-scanned photodiode (LSP) sensor, the resolution was less than 100 μm and the signal-to-noise (S/N) ratio was about 32 dB. A physical model supported by electrical simulation gives insight into the methodology used for image representation.

Image capture devices based on p-i-n silicon carbides for biometric applications

Abstract The laser scanned photodiode (LSP) image sensor is optimized and used as a biometric (fingerprint) reader. A laser light illuminates the fingerprint placed on a glass surface in front of the sensor. The reflecting light coming from the glass is projected onto the active surface. The image is converted directly into a proportional electric current using the LSP as fingerprint scanner. Results show that a trade-off between read-out parameters (fingerprint scanner) and the biometric sensing element structure (p–i–n structure) is needed to minimize the cross talk between the fingerprint ridges (dark regions) and the fingerprint valleys (illuminated regions). In the optimized configuration and under reverse bias the user-specific information minutiae present a good contrast and a spatial resolution of 20 μm . An increased light-to-dark sensitivity, a flux range of two orders of magnitude and a responsivity lower than 6.5 μW cm −2 were obtained under slightly reverse voltage.

Transport mechanism in high resistive silicon carbide heterostructures

Glass/ZnO:Al/p (Si x C 1-x :H)/i (Si:H)/n (Si x C 1-x :H)/Al (0 < x < 1) heterojunctions were produced by PE-CVD at low temperature. Junction properties, carrier transport and photogeneration are investigated from illuminated current- and capacitance-voltage characteristics and spectral response measurements, in dark and under different illumination conditions. For the heterojunction high series resistance around 10 6 Ω and atypical J-V characteristics are observed leading to poor fill factors. It was also observed that the responsivity decreases with the increase of the light bias intensity. For the homojunction, the behaviour is typical of a non-optimised p-i-n cell and the responsivity has a slight variation with the light bias conditions. A numerical simulation gives insight into the transport mechanism suggesting that the potential drop across the low-conductive a-SiC:H contact causes a significant change in the drift-diffusion balance inside the i-layer bulk. In the homojunction even at high light fluxes the transport process remains drift dominated.

Wide spectral response in μc-Si:H photodiodes

Abstract Microcrystalline hydrogenated silicon based p-i-n photodiodes with an increased bias controlled IR sensitivity (above 800 nm) have been deposited successfully by a new cyclic chemical vapour deposition method. Details of preparation conditions, material characterization (structural, transport and optoelectronic properties), and device behaviour (response time, current-voltage and spectral photoresponse measurements) are reported. The results show that the microcrystalline device, compared with an amorphous device, presents a higher photocurrent with an increased collection efficiency in the red and near-IR due to an enhanced absorption.

Properties of high growth rate amorphous silicon deposited by MC-RF-PECVD

Hydrogenated amorphous silicon (a-Si : H) thin films have been deposited on glass and crystalline silicon substrates by magnetically confined RF-PECVD (MC-RF-PECVD) at different RF power densities in order to verify the influence of this deposition parameter on the density of states (DOS) and growth rate (RG). It was found that the highest growth rate, 7.8 ( A is obtained for a-Si : H films deposited with an RF power density of 14.3 mW/cm 3 . For the DOS calculation, constant photocurrent method (CPM) data have been used. The lowest value of DOS is approximately 8 � 10 15 /eV/cm 3 and was obtained for a-Si : H films produced with an RF power density in the range of 10–20 mW/cm 3 . Infrared spectroscopy shows that when the RF power density increases, the concentration of SiH3 groups decreases and the concentration of SiH groups increases. At 8 mW/cm 3 , a maximum of the SiH2 concentration is obtained. At this point, a maximum of the optical gap (1.9 eV) is observed and a minimum of the dark conductivity is verified. We conclude that the best films are achieved in an RF power density range (7.1–21.4 mW/cm 3 ) for which an increase of SiH and a decrease of both SiH2 and SiH3 are simultaneously obtained. Thereafter, for higher power densities, an inversion of DOS and growth rate behaviour are observed due to ion bombardment. r 2002 Elsevier Science Ltd. All rights reserved.

Influence of the band offset on the performance of photodevices based on the c-Si/a-Si:H heterostructure

Abstract Recent studies on amorphous-crystalline silicon p-i-n heterojunctions (HIT, Heterojunction with Intrinsic Thin layer) have indicated the potential for these materials to be used as low cost, high efficiency solar cells. Moreover, the HIT structure is also expected to be suitably applied to various semiconductor junctions for micro and optoelectronic devices. Despite this potential, the exact operation of these cells is not yet fully understood and an S-shaped J–V characteristic curve is observed under certain conditions, leading to a poor collection efficiency. We present in this paper some results, obtained using the computer simulator ASCA, about the internal electric configuration of a p a-Si:H/a-Si:H/n c-Si heterostructure under different illumination conditions. The obtained results are related to the corresponding simulated J–V characteristic curve and compared with our experimental results, in order to explain the S-shaped J–V curve featured by these device. The band discontinuities at the amorphous-crystalline interface are shown to be responsible for such a behavior. We find that the conduction band offset is the most limiting parameter for the optimal collection of photogenerated carriers.

A two-dimensional visible/infrared detector based on μc-Si:H p–i–n structures

We present preliminary results on the analysis and application of an optical detector for imaging light contours. The sensor has a two-dimensional structure and uses a TCO/μc-p-i-n Si:H configuration as the active device and two lateral ohmic contacts to convert the projected light image into a lateral output photovoltage. A steady-state light pattern was used to simulate the image, in addition to a smallest intensity scanner beam which moved across the device creating an ac output signal. This signal depends on the position, intensity and homogeneity of the light pattern. A bidimensional model to explain the sensor performance is presented based on a small-signal ac circuit analysis.

Photocurrent multiplication in ITO/SiOx/Si optical sensors

Abstract A visible/near-infrared optical sensor based on an ITO/SiOx/n-Si structure with internal gain is presented. This surface-barrier structure was fabricated by a low-temperature processing technique. The interface properties and carrier transport were investigated from dark current–voltage and capacitance–voltage characteristics. Examination of the multiplication properties was performed under different light excitation and reverse bias conditions. The spectral and pulse response characteristics are analyzed. The current amplification mechanism is interpreted by the control of electron current by the space charge of photogenerated holes near the SiOx/Si interface. The optical sensor output characteristics and device applications are discussed.

Uv-visible ITO/GaP photodiodes: characterization and modeling

The electrical and optical properties of the indium-tin-oxide (ITO)/n-n + GaP structure have been investigated. Current-voltage, capacitance-voltage, spectral and pulse photoresponse characteristics are presented. A barrier height of 1.52 eV has been determined from the C-V characteristics analysis. The transmittance spectra of the ITO/GaP interface in the wavelength range 250-500 nm have been calculated for different film thicknesses for device optimization. The internal quantum efficiency is analyzed as a function of the n-GaP epitaxial layer parameters and reverse bias. The results show that the hole diffusion length in the epitaxial layer of the n-n + GaP substrate can be determined from the measurements of the photocurrent variation on reverse bias.

Image processing in a μc-Si:H p–i–n image transducer

Abstract A two-dimensional p–i–n imager based on μc-Si:H material is analysed. The basic building block for the sensor element is a transparent conductive oxide (TCO)/μc-p–i–n Si:H photodiode with front metal contacts and a TCO back contact. A scan-out process based on the photovoltage signal induced by a modulated HeNe laser is used to acquire the image. An analysis of the geometric image distortion and image restoration is given. Basic image processing algorithms are applied for image enhancement and pattern recognition.