Photoconductivity In Amorphous Silicon Research Articles

High efficiency light-induced dielectrophoresis biochip prepared using CVD techniques.

This article describes a high-efficiency light-induced dielectrophoresis biochip containing a thin film prepared through inductively coupled plasma chemical vapor deposition (ICPCVD). The biochip comprises two ITO glass substrates and a photoconductive amorphous silicon thin film. The biochip can effectively sort particular particles (or cells) by projecting visible light onto the surface of the silicon thin film. The sorting efficiency of biochips is highly associated with the quality of the deposited amorphous silicon thin films; therefore, the choice of deposition technique is extremely critical. However, no study has examined this problem. Hence, the current study thoroughly compared the efficiency of the biochip when films produced through plasma-enhanced chemical vapor deposition and ICPCVD are used.

Photoelectrochemical synthesis of DNA microarrays

Optical addressing of semiconductor electrodes represents a powerful technology that enables the independent and parallel control of a very large number of electrical phenomena at the solid-electrolyte interface. To date, it has been used in a wide range of applications including electrophoretic manipulation, biomolecule sensing, and stimulating networks of neurons. Here, we have adapted this approach for the parallel addressing of redox reactions, and report the construction of a DNA microarray synthesis platform based on semiconductor photoelectrochemistry (PEC). An amorphous silicon photoconductor is activated by an optical projection system to create virtual electrodes capable of electrochemically generating protons; these PEC-generated protons then cleave the acid-labile dimethoxytrityl protecting groups of DNA phosphoramidite synthesis reagents with the requisite spatial selectivity to generate DNA microarrays. Furthermore, a thin-film porous glass dramatically increases the amount of DNA synthesized per chip by over an order of magnitude versus uncoated glass. This platform demonstrates that PEC can be used toward combinatorial bio-polymer and small molecule synthesis.

Continuous optoelectrowetting for picoliter droplet manipulation

We report on a continuous optoelectrowetting mechanism enabling continuous high spatial resolution optical modulation of electrowetting effect on a featureless planar photoconductive amorphous silicon surface. Dynamically patterned “virtual electrode” switches voltage between the amorphous silicon layer and the dielectric layer for contact angle modulation. This device is particularly attractive for manipulating picoliter droplets. We have experimentally demonstrated trapping and moving of 10 and 50 pl droplets at a speed of 1 mm/s using a light beam.

An Amorphous Silicon Photoconductor for UV Detection

AbstractAn amorphous silicon photoconductor to detect wavelengths between 180 nm to 550 nm without scintillator is presented. The photoconductor is based on a coplanar configuration of the electrodes, similar to measurement structures to determine material characteristics of amorphous layers, e.g. for the Constant Photocurrent Method (CPM). After passing through a thin transparent passivation layer, the incident radiation is directly absorbed in the intrinsic a-Si:H material. The carrier collecting electrical field is applied perpendicular to the incoming light. Test structures have been fabricated with 80 nm thick sputtered chromium contacts on top of a 60 nm carbonized hydrogenated i-layer and a SixNx passivation layer with a thickness of about 36 nm. The spacing between the Schottky contacts is varied between 3 μm and 100 μm. They are deposited on top or below the a-SiC:H layer. First experiments with this simple coplanar design show that with an increasing voltage a shift towards UV wavelengths can be observed. The new UV detector is applicable in the field of TFA image sensors (Thin Film on ASIC) and in the new Lab-on-a-Chip concept presently under development at the institute for microsystem technologies.

Optimizing photodiode arrays for the use as retinal implants

The basic function of photoreceptors in the human eye is very similar to that of solar cells, namely delivering potential changes upon illumination. Solar cell or photodiode arrays may therefore serve for replacing natural photoreceptors, the progressive degeneration of which often results in a complete loss of vision in diseases like retinitis pigmentosa. Based on amorphous silicon, we have recently developed ultrathin and flexible microphotodiode arrays for subretinal implantation. We now report on the functional optimization of these devices, finally aiming at the recovery of vision in favour of the handicapped patients. The main topic of this study is the enhancement of stimulation power by the additional conversion of near infrared radiation, because the visible light alone will not be able to generate a sufficiently high charge transfer to the retina cells. Our concept is to use an amorphous silicon photoconductor for local light-induced enabling of the stimulation current, which is driven by a crystalline silicon solar cell acting as an infrared receiver. A proof of this concept is presented. First results indicate how the performance of our devices can be improved by minimizing the band discontinuities between amorphous and crystalline silicon, and by a proper optimization of doping levels and buffer layers.

Deformed Helix Ferroelectric Liquid Crystals with Large Tilt Angles in Optically Addressed Spatial Light Modulators for Dynamical Holography Applications

AbstractOptically addressed spatial light modulators (OASLMs) based on deformed helix ferroelectric liquid crystals (DHFLC) with high tilt angles on order of 40° and helical pitches less than 0.2μm were developed. The diffraction efficiency reached the order of 20%. The light induced deviation of the optical axis of the DHFLC layer was measured in sandwich structures consisting of photoconductors and liquid crystals. The photoelectric parameters of photoconductive amorphous silicon carbide a-SiC:H and a photoconductive polymeric films were measured with and without light blocking and reflecting layers. The application of the developed OASLMs in a holographic image corrector was demonstrated.

Transmissive liquid crystal light valve for parallelism optical thresholding operation

A transmissive liquid crystal valve for carrying out optical thresholding operations is proposed. It operates in parallel, that is, all the pixels of input are processed together simultaneously. The device is based on an amorphous silicon photoconductor and the features of liquid crystals. It does not require control architecture using very large scale integration (VLSI), so we can achieve a larger aperture ratio, higher resolution (more than 45 lines/mm), and a higher space-bandwidth prod- uct (more than 4310 5 pixels). Its response time is about 10 ms. Its processing capacity can reach approximately 2310 7 thresholding operations/s. The light valve does not break the beam propagation path, and can plug and play easily in optical processing and neural network systems. Its design, fabrication, and thresholding features are presented. © 1996 Society of Photo-Optical Instrumentation Engineers.

Modelling of Superalinearity and Thermal Quenching of Photoconductivity in Amorphous Silicon

Modelling of Superalinearity and Thermal Quenching of Photoconductivity in Amorphous Silicon

Near IR sensitive liquid crystal light valve with hydrogenated amorphous silicon photoconductor

An ITO/a-Si:H photosensor has been used in a near IR sensitive LCLV. In preparation of the photosensor, two main problems have been solved. One is the enhancement of IR transmittance of ITO; the other is improvement of the ratio of photo/dark conductivity in a-Si:H in the near IR range. As a result, IR transmittance as high as 80% and ratio of photo/dark conductivity larger than one order of magnitude are obtained. Images converted from near IR scenery by the LCLV are observed.

The concept of transport energy and its application to steady-state photoconductivity in amorphous silicon

A particular energy level in the band tail of a disordered semiconductor, called the ‘transport energy’, plays a crucial role in hopping transport of carriers in both equilibrium and non-equilibrium conditions and for both steady-state and transient phenomena. The transport energy is re-derived in a way that explains the universality of this energy level for different physical phenomena. It is shown that the transport energy determines each hopping event of an electron in the band tail and hence determines all phenomena dominated by the hopping. The concept of the transport energy is illustrated by comparing two recently suggested approaches to the description of the steady-state photoconductivity in amorphous semiconductors.

Neural network that incorporates direct optical imaging

A compact neural network architecture is described that can be trained to sense and classify an optical image directly projected onto it. The system is based on the combination of a two-dimensional amorphous silicon photoconductor array and a liquid-crystal spatial light modulator. Appropriate filtering of the incident optical image on capture is incorporated into the network training rules through a modification of the standard backpropagation training algorithm. Training of the network on two image-classification problems is described: the recognition of handprinted digits and facial recognition. The network, once trained, is capable of stand-alone operation, sensing an incident image, and outputting a final classification signal in real time.

120 × 100 pixel antiblooming array based on optically addressed ferroelectric liquid-crystal cells

We describe the principle and the operation of a 120 x 100 pixel antiblooming array. Each cell in the array is made up of a hydrogenated amorphous silicon photoconductor layer driving a ferroelectric liquid-crystal shutter. The device is designed to be coupled with a CCD camera to prevent high-blooming overload and image lag.

Optically programmed neural network capable of stand-alone operation

A twa-dimensional amorphous silicon photoconductor array and a liquid-crystal display form the core components of a hardware system for the implementation of a multilayer perceptron neural network. All connections between layers, as well as the nonlinear transfer characteristics associated with the hiddenand output-layer neurons, are implemented in analog circuitry so that the network, once trained, behaves as a stand-alone processor. Subject to a standard backpropagation training algorithm, the network is shown to train very successfully. Training of the network is studied under different levels of weight quantization, neuron output resolution, and random weight-defect probability. A computer simulation of the hardware network is also performed, and excellent agreement is shown between the results of the hardware network and those of the computer simulation. It is concluded that the training capability of the present hardware network is very little degraded by its nonidealities, including the level of weight quantization and limit in neuron output resolution.

Tunneling recombination and the photoconductivity of amorphous silicon in the temperature region around 100 K.

This paper proposes an interpretation of the photoconductivity of a-Si:H in the temperature region between $app 50--- and $app 150--- K, where the photoconductivity increases rapidly with temperature and usually exhibits a thermally activated behavior. In this model, the recombination takes place by direct tunneling between electrons and holes trapped in the band tails, whereas the photoconductivity is due to electron conduction in extended states. It is shown that this model can account for all the general features exhibited by the photoconductivity of a-Si:H in the low-temperature region, although the magnitude of the photoconductivity predicted by the model tends to be larger than that measured.

Time dependence of the low-temperature hopping photoconductivity in amorphous silicon

Abstract Studies of the time evolution of the d.c. and a.c. photoresponse of hydrogenated amorphous silicon samples at low temperatures are reported. Both real and imaginary parts of the a.c. loss rise linearly with time to a steady state and then, on removal of the light, show a long-lasting signal due to trapped carriers. The initial rate of rise is proportional to illuminating intensity. The d.c. response is found to grow and decay much faster than that at finite frequencies. No sign of any transition from geminate to non-geminate regimes is seen. Current models are unable to explain the details of this behaviour.

Influence of Plasma Density and Energy on the Characteristics of Amorphous Silicon Films Prepared by ECR Microwave Plasma CVD

ABSTRACTThe influence of magnetic field on Plasma parameters of ECR plasma were measured by the Langmuir probe technique. And it was shown that the plasma parameters can be controlled by magnetic field. Amorphous silicon films were prepared by ECR plasma CVD method under various magnetic field conditions for the purpose to investigate the effect of plasma density and energy on the property of amorphous silicon films. High plasma density improved the photoconductivity of amorphous silicon but the optical gap and activation of dopant depended on the substrate temperature rather than plasma density.

Interface Effects on Photocarrier Transport in a-Si:H

ABSTRACTWe discuss the absorption-length and film thickness dependence of photoconductivity in amorphous silicon using measurements of the ambipolar diffusion length and the Debye screening length.

Spatially addressable light transducer using an organic electroluminescent diode combined with amorphous silicon carbide film as an electron photoinjecting electrode

A new type of light transducer has been designed consisting of an organic electroluminescent (EL) diode on a photoconductive amorphous silicon carbide (a-SiC:H) film as an electron photoinjecting electrode. Input light received by a photosensitive a-SiC:H film is transduced to the output light with a different wavelength emitted from an EL diode in the layered configuration. The input-light pattern was well kept spatially in the output. The EL output light was controllable with dc voltage applied to the device and input-light intensity. These features are suggested to be potentially applicable for a new type of the spatial light modulator.

The origin of persistent photoconductivity in amorphous silicon

The persistent photoconductivity (PPC) effect in doping modulated amorphous silicon multilayers can result from the thermal equilibration of the defect structure in the doped a-Si:H layers. The magnitude, exposure and doping dependence of the PPC effect can be accounted for using the thermal equilibrium model.

Characterization of highly photoconductive amorphous silicon germanium alloys prepared by DC glow discharge

Amorphous SiGe alloys have been deposited in different dc glow discharges (diode, triode configuration) from SiH 4/GeH 4 mixtures with and without H 2 dilution of the neutral gas. Triode reactor and/or H 2 dilution has brought about excellent AMI photoconductivities in the range 1.7eV – 1.4eV of (2×10 −4 − 10 −5)1/ωcm. The incorporation of Ge causes only small increases in Urbach energies as well as some rise in midgap density of states (PDS and CPM). Analyses of transport behaviour from temp. dependent stationary and transient photoconduction experiments (TOF and TPC) however point towards a considerable increase in conduction band tailing and therefore a reduced effective mobility of photoexcited electrons in comparison to a-Si:H.