Amorphous Silicon Pin Research Articles

Electrically Detected Magnetic Resonance on a Chip (EDMRoC) for Analysis of Thin-Film Silicon Photovoltaics

Electrically detected magnetic resonance (EDMR) is a spectroscopic technique that provides information about the physical properties of materials through the detection of variations in conductivity induced by spin-dependent processes. EDMR has been widely applied to investigate thin-film semiconductor materials in which the presence of defects can induce the current limiting processes. Conventional EDMR measurements are performed on samples with a special geometry that allows the use of a typical electron paramagnetic resonance (EPR) resonator. For such measurements, it is of utmost importance that the geometry of the sample under assessment does not influence the results of the experiment. Here, we present a single-board EPR spectrometer using a chip-integrated, voltage-controlled oscillator (VCO) array as a planar microwave source, whose geometry optimally matches that of a standard EDMR sample, and which greatly facilitates electrical interfacing to the device under assessment. The probehead combined an ultrasensitive transimpedance amplifier (TIA) with a twelve-coil array, VCO-based, single-board EPR spectrometer to permit EDMR-on-a-Chip (EDMRoC) investigations. EDMRoC measurements were performed at room temperature on a thin-film hydrogenated amorphous silicon (a-Si:H) pin solar cell under dark and forward bias conditions, and the recombination current driven by the a-Si:H dangling bonds (db) was detected. These experiments serve as a proof of concept for a new generation of small and versatile spectrometers that allow in situ and operando EDMR experiments.

High‐speed focus‐induced photoresponse in amorphous silicon photodetectors for optical distance measurements

The Focus-Induced Photoresponse (FIP) enables 3D sensing capabilities by evaluating the irradiance dependent non-linear detector response in defect-based materials. Since this advantage is intricately associated to a slow response, the electrical bandwidth of previous FIP sensors is limited to a few kHz only. We report the FIP in amorphous silicon pin photodiodes and propose a sensor read out based on a harmonics analyses. We achieve modulation frequencies of 500 kHz and a non-linear beat frequency detection up to at least 3.5 MHz, surpassing the bandwidth of state-of-the-art architectures by at least a factor of 175. The FIP sensors further achieve signal-to-noise ratios of ∼50 dB, depth resolutions of at least 5.4 mm at 126 cm and a DC FIP detection limit of 1.3 µW/mm2.

71‐2: Novel Optical Image Sensor Array Using LTPS‐TFT Backplane Technology as Fingerprint Recognition

A novel photo sensor by LTPS TFT backplane technology is developed, which amorphous silicon (a‐Si) layer stacks on poly silicon (p‐Si) layer to form a vertical hybrid poly and amorphous silicon PIN photo diode (HPAS‐PIN). This photo diode shows good photosensitivity as traditional a‐Si photo sensor. An optical fingerprint sensor array is realized based on the novel HPAS‐PIN photo diode and successfully capturing fingerprint image.

Few-Layer MoS2/a-Si:H Heterojunction Pin-Photodiodes for Extended Infrared Detection

Few-layer molybdenum disulfide (FL-MoS$_2$) films have been integrated into amorphous silicon (a-Si:H) pin photodetectors. To achieve this, vertical a-Si:H photodiodes were grown by plasma-enhanced chemical vapor deposition (PE-CVD) on top of large-scale synthesized and transferred homogeneous FL-MoS$_2$. This novel detector array exhibits long-term stability (more than six month) and outperforms conventional silicon-based pin photodetectors in the infrared range (IR, $\lambda$ = 2120 nm) in terms of sensitivities by up to 50 mAW$^{-1}$. Photodetectivities of up to 2 x 10$^{10}$ Jones and external quantum efficiencies of 3 % are achieved. The detectors further feature the additional functionality of bias-dependent responsivity switching between the different spectral ranges. The realization of such scalable detector arrays is an essential step towards pixelated and wavelength-selective sensors operating in the IR spectral range.

Integrated thin film Si fluorescence sensor coupled with a GaN microLED for microfluidic point-of-care testing

An integrated fluorescence sensor consisting of a SiO2/Ta2O5 multilayer optical interference filter and hydrogenated amorphous silicon (a-Si:H) pin photodiode was coupled with a GaN microLED to construct a compact fluorescence detection module for point-of-care microfluidic biochemical analysis. The combination of the small size of the GaN microLED and asymmetric microlens resulted in a focal spot diameter of the excitation light of approximately 200 µm. The limit of detection of the sensor was as high as 36 nM for fluorescein solution flowing in a 100 µm deep microfluidic channel because of the lack of directionality of the LED light. Nevertheless, we used the GaN microLED coupled with the a-Si:H fluorescence sensor to successfully detect fluorescence from a streptavidin R-phycoerythrin conjugate that bound to biotinylated antibody-coated microbeads trapped by the barrier in the microfluidic channel.

Flexible semi-transparent a-Si:H pin solar cells for functional energy-harvesting applications

We report on the fabrication and characterization of penetration-type semi-transparent hydrogenated amorphous silicon (a-Si:H) pin solar cells on flexible plastic substrates. Device-grade solar cell layers were designed and fabricated at a maximum process temperature of 150 °C, which is suitable for plastic substrate application. The solar cell layers were characterized using electrical, structural, and optical measurements, while optical and electrical measurements were carried out for the solar cells along with mechanical durability test. The flexible semi-transparent a-Si:H solar cells showed an average visible range (500–800 nm) optical transmittance of 34.2% and a power conversion efficiency (PCE) of ∼5.0%. The fabricated solar cells also showed good flexibility and durability from rigid bending cycling test. The device performance demonstrated the effectiveness of semi-transparent a-Si:H solar cells enabling low-cost, light-weight functional energy-harvesting applications.

Biomimetic Radial Tandem Junction Photodetector with Natural RGB Color Discrimination Capability

Imitating natural rod or cone cells in human eyes can inspire a biomimetic design of filter‐free color sensing photodetector with the aid of three‐dimensional (3D) nanophotonic engineering. Here, a novel radial tandem junction (RTJ) super‐rod structure is proposed and demonstrated, which consists of coaxially stacking hydrogenated amorphous silicon PIN junctions upon silicon nanowires, resembling to the live retinal cells in terms of both geometry and dimension. As a consequence, these RTJ units enjoy a unique wavelength‐selective and cavity‐mode‐assisted light absorption/responses in the inner and the outer radial PIN junctions, which are readily tunable to achieve natural RGB color discrimination in the RTJ rod‐cells without the need of any filter system. This unique RTJ design can indicate a new 3D implementation of color sensing photodetectors enabled by advanced nanophotonic structuring and engineering.

Effect of n + Layer Deposition Power on Light Sensing of Amorphous Silicon pin Diodes

The effect of the plasma-enhanced chemical vapor deposition power of the phosphorus-doped n + layer on the performance of the hydrogenated amorphous silicon pin diode for the light sensing, i.e., under solar, red, green, and blue light exposures, has been investigated. With all other deposition parameters fixed, the plasma power influenced the n + film deposition rate, resistivity, and defects, which are critical to the light sensing characteristics. The optimized n + layer deposition power for the light sensing, i.e., based on the external quantum efficiency, has been identified and discussed.

Substrate and Passivation Techniques for Flexible Amorphous Silicon-Based X-ray Detectors.

Flexible active matrix display technology has been adapted to create new flexible photo-sensing electronic devices, including flexible X-ray detectors. Monolithic integration of amorphous silicon (a-Si) PIN photodiodes on a flexible substrate poses significant challenges associated with the intrinsic film stress of amorphous silicon. This paper examines how altering device structuring and diode passivation layers can greatly improve the electrical performance and the mechanical reliability of the device, thereby eliminating one of the major weaknesses of a-Si PIN diodes in comparison to alternative photodetector technology, such as organic bulk heterojunction photodiodes and amorphous selenium. A dark current of 0.5 pA/mm2 and photodiode quantum efficiency of 74% are possible with a pixelated diode structure with a silicon nitride/SU-8 bilayer passivation structure on a 20 µm-thick polyimide substrate.

Towards 12% stabilised efficiency in single junction polymorphous silicon solar cells: experimental developments and model predictions

We have combined recent experimental developments in our laboratory with modelling to devise ways of maximising the stabilised efficiency of hydrogenated amorphous silicon (a-Si:H) PIN solar cells. The cells were fabricated using the conventional plasma enhanced chemical vapour deposition (PECVD) technique at various temperatures, pressures and gas flow ratios. A detailed electrical-optical simulator was used to examine the effect of using wide band gap P-and N-doped μ c-SiOx :H layers, as well as a MgF2 anti-reflection coating (ARC) on cell performance. We find that with the best quality a-Si:H so far produced in our laboratory and optimised deposition parameters for the corresponding solar cell, we could not attain a 10% stabilised efficiency due to the high stabilised defect density of a-Si:H, although this landmark has been achieved in some laboratories. On the other hand, a close cousin of a-Si:H, hydrogenated polymorphous silicon (pm-Si:H), a nano-structured silicon thin film produced by PECVD under conditions close to powder formation, has been developed in our laboratory. This material has been shown to have a lower initial and stabilised defect density as well as higher hole mobility than a-Si:H. Modelling indicates that it is possible to attain stabilised efficiencies of 12% when pm-Si:H is incorporated in a solar cell, deposited in a NIP configuration to reduce the P/I interface defects and combined with P- and N-doped μ c-SiOx :H layers and a MgF2 ARC.

Heterogeneously integrated laser-induced fluorescence detection devices: Integration of an excitation source

We have proposed and developed a fluorescence detector in which a thick SiO2/Ta2O5 multilayer optical interference filter is monolithically integrated on a hydrogenated amorphous silicon (a-Si:H) PIN photodiode. Combined with the optical transparency of the detector’s glass substrate, the annular shape of the detector will enable us to construct a module containing a detector and an excitation source. Based on a detection platform that comprises a fluorescence-collecting microlens and an annular fluorescence detector while using an external laser source, we have demonstrated single molecule DNA detection capability when combined with a polymerase chain reaction (PCR). We also describe our work towards the integration of an excitation source to fabricate a heterogeneously-integrated laser-induced fluorescence detection (LIF) device comprising a laser diode, micro-lenses and the integrated a-Si:H fluorescence detector.

Color sensing ability of an amorphous silicon position sensitive detector array system

The color sensing ability of a data acquisition prototype system integrating a 32 linear array of 1D amorphous silicon position sensitive detectors (PSD) was analyzed. Besides being used to reproduce a 3D profile of highly reflective surfaces, here we show that it can also differentiate primary red, green, blue (RGB) and derived colors. This was realized by using an incident beam with a RGB color combination and adequate integration times taking into account that a color surface mostly reflects its corresponding color. A mean colorimetric error of 25.7 was obtained. Overall, we show that color detection is possible via the use of this sensor array system, composed by a simpler amorphous silicon pin junction.

Parameter determination for device simulations of thin film silicon solar cells by inverse modeling based on temperature and spectrally dependent measurements

Opto-electronic device simulation of thin-film silicon solar cells requires a set of many input parameters for the electronic properties of amorphous and microcrystalline silicon. In this paper, a systematic determination of these is performed by inverse modeling and particle swarm optimization with experimental and simulated current-voltage characteristics of amorphous and microcrystalline silicon pin diodes, both in the dark and under illumination. The parameters are validated by additional experiments and simulations of the temperature-dependent dark current, of spectrally selective illumination, and of the external quantum efficiency of the amorphous and microcrystalline silicon diode, respectively. The validation of the input parameters is complemented by simulation of a tandem solar cell with the amorphous and the microcrystalline silicon input parameters. Simulation and experiment (spectrally dependent current-voltage characteristics and external quantum efficiency) of such a tandem solar cell show good agreement. Sensitivity studies on different sets of input parameters for the inverse modeling process are also presented.

Analysis of Optical and Morphological Properties of Aluminium Induced Texture Glass Superstrates

Texturing the glass surface is a promising method for improving the light trapping properties of superstrate thin-film silicon solar cells, as it enables thinner absorber layers and, possibly, higher cell efficiencies. In this paper we present the optical and morphological properties of borosilicate glass superstrates textured with the aluminium induced texture (AIT) method. High haze values are achieved without any reduction in the total optical transmission of the glass sheets after the AIT process. Scanning electron microscope and atomic force microscope (AFM) measurements reveal a laterally uniform surface morphology of the AIT texture. We demonstrate that the surface roughness and thus the transmission haze can be controlled by adjusting the AIT process parameters. From the AFM images, we extract histograms of the local height and angle distributions of the texture. Samples with a wide angle distribution are shown to produce the highest optical haze. The results of this analysis provide a better understanding of the correlation between the AIT process parameters and the resulting surface morphology. This analysis is further extended to an amorphous silicon pin solar cell deposited onto the textured glass substrate.

24.2: A 1300‐dpi Optical Image Sensor Using an a‐Si:H Photo Diode Array Driven by LTPS TFTs

AbstractWe have developed a 1300‐dpi optical image sensor using a hydrogenated amorphous silicon (a‐Si:H) pin diode array driven by low‐temperature polycrystalline silicon (LTPS) thin film transistors (TFT) on a glass substrate. This paper describes a new architecture for achieving a high‐resolution optical image sensor using LTPS TFT production equipment.

Effect of long-term He–Ne laser light exposure and subsequent annealing on hydrogenated amorphous silicon pin photodiodes

Changes in the current–voltage (IV) characteristics were monitored on as prepared a-Si:H pin photodiodes during 60 min He–Ne laser illumination (every 15 min in situ measurements). After illumination was over, the pin diodes were subjected to 1 h annealing at 200 °C and IVs were taken again. Results showed that the short circuit current (Isc) decreased with the He–Ne exposure time which is known as a Stabler–Wronski effect of degradation and increased after the annealing. In order to describe the origin of this phenomenon, a 1 µm thick a-Si:H film was deposited on Mo-coated glass sheets by plasma enhanced chemical vapour deposition (PECVD) using the same procedure as that of the i-type layer in the pin diodes. Fourier transform infrared spectra (FTIR) were taken on the sample before and after the 1 h He–Ne laser exposure as well as after annealing for 1 h at 200 °C in air. Hydrogen (H) concentration in the film was estimated from the typical Si–H vibration absorbance peak at 2000 cm−1. The results showed that the H-concentration decreases from the initial value of about 10%, to about 7% after the 1 h He–Ne laser exposure. After the thermal annealing, the hydrogen concentration increased to about 9%. The above hydrogen concentration change was directly related to the change in the IV characteristics of the pin a-Si:H diode.

Property control of expanding thermal plasma deposited textured zinc oxide with focus on thin film solar cell applications

Property control of expanding thermal plasma deposited textured zinc oxide is demonstrated considering intrinsic, i.e. bulk, and extrinsic transparent conducting oxide quality relevant for application in thin film amorphous silicon pin solar cells. Particularly the interdependence of electrical conductivity, film composition and film morphology, i.e. structure, feature shape and roughness of the surface, is addressed. Control of film composition is mainly governed by plasma production and gas phase chemistry inherently inducing a significant contribution to film morphology, whereas control of film morphology solely is governed by near-substrate conditions. Especially the ratio of zinc to oxygen and the reactor chamber pressure appear to be determinative in obtaining zinc oxide exhibiting the appropriate intrinsic and extrinsic quality, i.e. a high electrical conductivity, a high transmittance, a textured rough surface morphology and a strong hydrogen plasma resistance. The solar cell performance of appropriate undoped and aluminium doped textured zinc oxide inherently obtained during deposition is comparable with respect to Asahi U-type fluorine-doped tin oxide.

Fusion of a-Si:H sensor technology with microfluidic bioanalytical devices

Hydrogenated amorphous silicon (a-Si:H) pin photodiodes are used as fluorescence sensors for microfluidic analysis devices. A hybrid-integrated sensor consisting of a half-ball lens, a ZnS/YF 3 multilayer optical interference filter with a pinhole, and an annular a-Si:H photodiode is developed that allows the laser excitation to pass up through the central aperture in the photodiode. Using this integrated device, the limit of detection for fluorescein flowing in a 50-μm deep channel is 17 nM (S/N=3) and DNA fragment sizing and chiral analysis of glutamic acid are successfully performed. The a-Si:H sensor exhibits high sensitivity at the emission wavelengths of commonly used fluorescent dyes and is readily microfabricated and integrated at low cost making it ideal for portable microfluidic bioanalyzers.

Fusion of a-Si:H sensor technology with microfluidic bioanalytical devices

Hydrogenated amorphous silicon (a-Si:H) pin photodiodes are used as fluorescence sensors for microfluidic analysis devices. A hybrid-integrated sensor consisting of a half-ball lens, a ZnS/YF 3 multilayer optical interference filter with a pinhole, and an annular a-Si:H photodiode is developed that allows the laser excitation to pass up through the central aperture in the photodiode. Using this integrated device, the limit of detection for fluorescein flowing in a 50-μm deep channel is 17 nM (S/N=3) and DNA fragment sizing and chiral analysis of glutamic acid are successfully performed. The a-Si:H sensor exhibits high sensitivity at the emission wavelengths of commonly used fluorescent dyes and is readily microfabricated and integrated at low cost making it ideal for portable microfluidic bioanalyzers.

Experimental study and modeling of reverse-bias dark currents in PIN structures using amorphous and polymorphous silicon

Polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film, with a lower defect density of states and better electronic properties than standard amorphous silicon. We have studied the reverse-bias dark current in PIN structures using this material as the intrinsic layer and compared the results to amorphous silicon PIN devices. All the structures were grown using a standard plasma enhanced chemical vapor deposition process. For thick pm-Si:H devices, we have achieved reverse-bias dark current densities about ten times lower than those obtained using amorphous silicon as the intrinsic layer. This is consistent with the lower defect density of states in polymorphous silicon, which is about 7×1014 cm−3 against 5×1015 cm−3 for amorphous silicon. For a 2.5-μm-thick pm-Si:H diode, the current density obtained is as low as 3 pA cm−2 at −3 V. However, for thinner structures (∼0.5 μm), polymorphous and amorphous silicon show nearly the same reverse-bias leakage current. The experimental dark as well as illuminated characteristics of the diodes have been simulated using a model that incorporates field enhanced thermal generation under reverse-bias conditions (Poole–Frenkel effect). Results reveal that in pm-Si:H diodes, the P/I interface is much more defective than in standard a-Si:H PIN diodes. This fact is shown to completely mask the advantage of the lower defect density of pm-Si:H, in thin PIN diodes. However, in thick samples the electric field in the device and, therefore, also the Poole–Frenkel enhancement of thermal generation are smaller. The effect of the lower density of states in polymorphous silicon is then dominant, and we have achieved a dark current density of 3 pA cm−2 at −3 V for a 2.5-μm-thick diode, as already stated.