Back Illumination Research Articles

Electron Diffusion Length in Mesoporous Nanocrystalline TiO2 Photoelectrodes during Water Oxidation

A long electron diffusion length (L) relative to film thickness is required for efficient collection of charge in photoelectrodes. L was measured in a nanocrystalline, mesoporous TiO2 electrode during photochemical water splitting by two independent methods: (1) analyzing the ratio of incident photon conversion efficiency (IPCE) measured under back and front side illumination, and (2) analyzing transient photovoltage rise and decay measurements. These gave values of L that agreed (L = 8.5−12.5 μm between −0.15 and 0.1 V vs Ag/AgCl 3 M KCl at pH 2). The transient measurements were consistent with trap limited transport and recombination of electrons as previously observed in dye-sensitized solar cells. L ∼ 10 μm is sufficient to collect separated electrons with minor electrolyte recombination losses in thin electrodes. However, charge collection may limit performance in doped mesoporous electrodes with weak visible light absorption where thicker films are required.

Enabling Wafer Level Processes for CIS Manufacturing

CMOS (Complimentary Metal Oxide Semiconductor) Image Sensors have become ubiquitous, appearing in cars, cell phones, toys and many other devices used in every day life. The primary reason for this increasing presence of CIS (CMOS Image Sensors) is the continual improvement of the performance to cost ratio of these devices. The drivers behind this are the advancements of CMOS image sensor technology such as improved signal to noise ratio as well as advancements in wafer level processing technology related to 3D packaging. Numerous process developments related to both the electrical and optical aspects of 3D packaging of CIS that have enabled this climb up the performance vs. cost curve will be reviewed in this paper with particular attention to:(1) Lens molding – The ability to mold lenses, both spherical and aspherical at the wafer level as well as make full size master stamps from partial masters for lens molding. These lenses can be molded on both sides of a wafer and the lenses aligned to each other;(2) Aligned wafer bonding for optical interconnects consisting of lens stacks and CIS wafer, to allow the thinning of a CIS for BSI (back side illumination), and for electrical interconnects. Together these processes allow the heterogeneous integration of optical and electrical elements at the wafer level and advance the CIS up the performance vs. cost curve.

Electrochemical differential photoacoustic cell to study in situ the growing process of porous materials

In order to study in situ the growing process of porous materials, a new electrochemical differential photoacoustic cell (DPC) was developed. This system allows to obtain the thermal signals coming from the growing process of the pores without the external noise component. The DPC is a good system to growth porous silicon and study their growing process with reproducibility. The porous silicon samples were obtained by using electrochemical etching of (100) n-type silicon wafers with different nominal resistivity values in the range of 1-25 Omega cm. The samples were formed in a solution of hydrofluoric acid and ethanol having a composition ratio of 1:1 in volume with etching voltage of 10 V and an etching time of 2 min using back illumination provided by a laser beam with a wavelength of 808 nm. The porous samples were characterized by means of Raman microscopy, x-ray diffraction, and scanning electron microscopy. The crystallite sizes of the samples were obtained through the analysis of the micro-Raman spectra using a phonon confinement model, and the analysis of the x-ray diffractograms.

In‐situ FFT impedance spectroscopy in new modes applied to pore growth in semiconductors

AbstractMajor progress in FFT impedance spectroscopy now allows in‐situ measurements during pore growth in three modes, two of which use modulation of the back side or front side illumination intensity, respectively; with the back side illumination mode being especially useful to n‐macro(bsi) pores in Silicon. The data obtained in all modes can be interpreted on the base of fully quantitative models which give perfect fits to the measured data and therefore allow to extract in‐situ data of many parameters, including the valence of the process and the actual pore depth. Based on this, macropore growth in Si has been re‐investigated, resulting in new growth modes and especially in substantially increased macropore growth rates as well as pore quality. It is now possible to grow macropores in n‐type Si far deeper and more than twice as fast compared to the present state of the art; the quality as expressed, e.g., in the pore wall roughness, can also be substantially improved. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of large and fast cmos aps cameras at latt

Since 2004, at the Laboratoire d'Astrophysique de Toulouse Tarbes (LATT), we work with CMOS APS detectors, firstly to develop a large-field,high-resolution camera for the observation of the solar supergranulation, secondly to develop a fast camera for an adaptive optics test bench. In these two projects, we use detectors from FillFactory, now continued by Cypress Semiconductor Corporation: IBIS4-14000, IBIS-16000, LUPA-4000, 14 Mpixels, 16 Mpixels, 4 Mpixels respectively. The last one just reads in a 240 × 240 pixels window to obtain readout rate of 1000 Image/s. For these purposes we developed dedicated controllers to follow the high pixel rate and multi-output readout of this type of detectors. We also studied the characterization methods and measured the main parameters of these CMOS detectors to know their behaviour. Using these kinds of APS detectors in these two particular projects proves that we can already find niches to use CMOS detectors in astronomy taking advantage of their present specificities. Recent improvements like back illumination, noise reduction, should rapidly open news possibilities.

Wavelength extended InGaAs/InAlAs/InP photodetectors using n-on- p configuration optimized for back illumination

Using n-on- p InGaAs/InAlAs/InP heterojunction structure with relative thin Be doped linearly graded In x Al 1− x As as buffer layer, wavelength extended InGaAs photodiodes with 50% cutoff wavelength of 2.0 μm and 2.4 μm at room temperature have been grown by using gas source MBE with a convenient but reliable correlative ramping procedure, and their dark current performance in a wide temperature range have been investigated. For 300 μm diameter detectors at 290 K, typical R 0 A and dark current at 10 mV reverse bias are 760 Ωcm 2/11.3 nA for cutoff wavelength of 2.0 μm, and 104 Ω cm 2/75.1 nA for cutoff wavelength of 2.4 μm. Room temperature peak detectivity D λ p ∗ reaches 1.3E11 cm Hz 1/2/W and 6.5E10 cm Hz 1/2/W, respectively measured using 900 K black body source. For back illumination the parameter optimization of n-on- p configuration is much easier than those of p-on- n, also the structure quality of the n-on- p is better than those of p-on- n because of the high Be doping in the linear graded buffer layer instead of Si.

New modes of FFT impedance spectroscopy applied to semiconductor pore etching and materials characterization

AbstractA systematic approach for the application of fast Fourier transform (FFT) impedance spectroscopy to semiconductor photo‐electrochemistry is given. In particular, photo‐impedance spectroscopy in two novel modes is used in conjunction with conventional current–voltage impedance spectroscopy, and applied to some aspects of pore etching in InP and Si as well as to the characterization of multicrystalline Si or other solar cell material. The technique, applicable in situ with high time resolution, employs optimized hardware and software, extensive mathematical modeling of carrier transport, and the numerical implementation of fast parameter extraction algorithms. The complete system with a fully integrated impedance spectrometer delivers a wealth of new data including key parameters of pore etching. Examples given include the in situ tracking of the fast switch‐over between pore growth modes in InP as well as following a pore etching process over long time scales in Si. FFT impedance spectroscopy in the novel back side illumination mode, in combination with the conventional mode, allows not only some active control of macropore etching in n‐type Si by supplying crucial real‐time data like the valence of the dissolution process and the pore depth, but also provides new fundamental insights into the electrochemical processes occurring at the (porous) electrode. FFT photo‐impedance spectroscopy in the front side illumination mode used in conjunction with a scanned laser beam allows fast local measurements of all semiconductor parameters relevant for solar cell applications, like doping concentration, minority carrier lifetime, diffusion constant, or surface recombination velocity with high spatial resolution; this is demonstrated for large multicrystalline Si wafers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MEMS-based optical limiter

We propose the design of an optical limiter based on a microelectromechanical systems deformable mirror. The design is based on aperturing focused light reflected out of an optically driven deformable mirror, deformed in a parabolic form. We derive an expression for the reflected light intensity, and we show that the reflected light saturates as a function of back illumination light intensity.

In-Situ Assessment of Macropore Growth in Low-Doped n-Type Silicon

Understanding pore growth in semiconductors in detail needs some in-situ information about the processes occurring in a growing pore. It is shown that dual-mode in-situ FFT impedance spectroscopy can provide useful data for macropore etching in n-type Si under back side illumination. A new illumination mode is used alternating with the conventional current-voltage mode, and data evaluation is based on an advanced model developed for this kind of pore etching. The principles of the technique and the basics of the model are introduced and illustrated by a number of examples, some of which show new kinds of pore growth modes.

Growth Modes of Macropores in n-Type Silicon

The growth of macropores in Silicon under galvanostatic conditions always terminates at some maximum depth by some growth mode transition, e.g. from pore growth to cavity formation. It will be shown that macropore growth in lightly doped n-type Si under back side illumination may experience several different kinds of growth mode transitions, including some new ones; in particular if new and improved electrolytes are used. Existing models of macropore growth cannot account for these mode transitions and need to be augmented.

Rough conical-shaped TiO2-nanotube arrays for flexible backilluminated dye-sensitized solar cells

Conical-shaped anodic TiO2-nanotube (TiNT) arrays with length of a few tens of micrometer and rough tube walls were fabricated for use in flexible backilluminated dye-sensitized solar cells (DSSCs) which exhibited conversion efficiency of 4.3% under AM1.5 back side illumination. As compared to TiO2 nanoparticles/Ti-based DSSC, TiNT/Ti-based DSSC showed enhanced light-harvesting efficiency, rapid electron-transport rate, prolonged electron lifetime, and reduced dark current, leading to an increase in efficiency by ∼30%.

Enhanced optical responsivity of InAlAs∕InGaAs metamorphic high mobility electron mobility using indium tin oxide transparent gate technology

The high optical responsivity of a transparent gate metamorphic high electron mobility transistor (TG-MHEMT) on GaAs substrate is demonstrated. The transmittance of a 1.3μm emission wavelength for sputtered indium tin oxide TG-MHEMT gate fingers was 90% after 2min of annealing at 300°C and the sheet resistance was 31.3Ωmm. Compared to the back side illumination of a MHEMT phototransistor, which is beneficial for improving photoresponse, the TG-MHEMT can avoid the inconvenient back side thin-down and polish processes. Experimental results demonstrate that TG-MHEMT has good optical performance at a wavelength of λ=1.3μm and comparable to that of the conventional metal gate MHEMT. The increase in drain current and decrease in threshold voltage caused by the illumination can be explained by the photovoltaic effect beneath the transparent gate. The responsivity of TG-MHEMT is roughly 20.6A∕W for a wavelength of λ=1.3μm.

Fabrication of Grätzel solar cell with TiO 2/CdS bilayered photoelectrode

In this investigation, cadmium sulfide (CdS)-sensitized stable Grätzel solar cell has been fabricated, where nano-crystalline titanium oxide (TiO 2) photoelectrode has been deposited on SnO 2:F coated glass substrate by facing target reactive sputtering technique and CdS sensitizing layer has been deposited by chemical bath deposition technique. The X-ray diffraction measurement reveals that both the TiO 2 and CdS layer show polycrystalline nature. The optical absorbance edge of the photoelectrode extends up to 540 nm, as apparent from optical measurements. This solar cell shows good photocurrent–photovoltage characteristics with fill factor 59% and maximum efficiency 1.04% with the use of a polysulfide electrolyte under back side illumination.

STUDY OF OPTICAL PROPERTIES OF POROUS SILICON PRODUCED BY METAL-AID

We presented fluorescence spectra and cathode-luminescence spectra of as-prepared porous silicon under different preparation conditions. The luminescence of porous silicon may be related to the luminescent centers on porous silicon surface. The luminescent efficiency depended on the porosity of porous silicon, which further depended on the oxidative level of porous silicon with metal assistance. Under back illumination, the higher the oxidative degree of the metal is, the higher is the porosity and the luminescent efficiency of porous silicon. But under front illumination, the results exhibited an opposite tendency, which can be explained by the formation theory of PS.

Scaling in back-illuminated GaN avalanche photodiodes

Avalanche p-i-n photodiodes of various mesa areas were fabricated on AlN templates for back illumination for enhanced performance through hole-initiated multiplication, and the effects of increased area on device performance were studied. Avalanche multiplication was observed in mesa sizes up to 14063μm2 under linear mode operation. Uniform gain and a linear increase of the dark current with area were demonstrated.

Geiger-mode operation of back-illuminated GaN avalanche photodiodes

The authors report the Geiger-mode operation of back-illuminated GaN avalanche photodiodes. The devices were fabricated on transparent AlN templates specifically for back illumination in order to enhance hole-initiated multiplication. The spectral response in Geiger-mode operation was analyzed under low photon fluxes. Single photon detection capabilities were demonstrated in devices with areas ranging from 225to14063μm2. Single photon detection efficiency of 20% and dark count rate <10kHz were achieved in the smallest devices.

The optical properties of porous silicon produced by metal-assisted anodic etching

Porous silicon (PS) was obtained from n-type (100) mono-crystalline silicon wafers with different metal using two different illumination conditions. The visible photoluminescence (PL) may come from defect-related radiative centers on PS surface and adsorbed hydrogen atoms may be associated to the elimination of irradiative centers on PS surface, which can be proved by the infrared absorption spectra. The metal can be used as catalytic role to increase the etching rate under back illumination, but under front illumination, the metal can cancel light-generated carrier leading to the decrease of etching rate during anodic etching. Furthermore, the change of minority carrier lifetime is opposite to the change of PL efficiency of PS, which can be Confirmed by the results of μ-PCD measurements.

Bifacial configurations for CdTe solar cells

We present a different back contact for CdTe solar cell by the application of only a transparent conducting oxide (TCO), typically ITO, as a back electrical contact on all-PVD CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells, which can be illuminated from either or both sides. Also devices with thin CdTe layers (from ∼2 μm down to 1 μm) have been prepared to improve the conversion efficiency on the back side illumination, which is limited by the collection of carriers far away from the junction and to reduce the amount of material in the CdTe device. Reproducible solar cells exceeding 10% efficiency on the front side illumination and exceeding 3% on the back side illumination are reported.

Hole-initiated multiplication in back-illuminated GaN avalanche photodiodes

Avalanche p-i-n photodiodes were fabricated on AlN templates for back illumination. Structures with different intrinsic layer thicknesses were tested. A critical electric field of 2.73MV∕cm was estimated from the variation of the breakdown voltage with thickness. From the device response under back and front illumination and the consequent selective injection of holes and electrons in the junction, ionization coefficients were obtained for GaN. The hole ionization coefficient was found to be higher than the electron ionization coefficient as predicted by theory. Excess multiplication noise factors were also calculated for back and front illumination, and indicated a higher noise contribution for electron injection.

A Novel Self-Aligned Etch-Stopper Structure With Lower Photo Leakage for AMLCD and Sensor Applications

In this letter, the authors introduce a novel self-aligned etch-stopper sidewall-contact hydrogenated amorphous silicon (a-Si : H) thin-film transistor (ESSC-TFT), which reduces the photo leakage current by more than one order of magnitude and increases the on-off ratio to seven orders of magnitude under back light illumination. Such a TFT will enable high-resolution and high-brightness liquid-crystal displays (LCDs) for next-generation TV, monitor, notebook, and mobile-phone applications. This ESSC-TFT design reduces the volume of a-Si film in which the active region can totally be shielded by the gate metal resulting in the prevention from direct back light illumination. With the sidewall contact, the hole current is reduced due to the smaller contact area between drain/source and a-Si layer. As well as the source, drain parasitic intrinsic resistance of a-Si can be also lessened by the ESSC-TFT structure. Although the defects between etched a-Si and n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+ </sup> a-Si film may degrade the on current, the ESSC-TFT still exhibits higher on-off ratio and lower leakage than the one in traditional etch-stopper (ES)-TFT structure. The ESSC-TFT structure can be used not only for TFT-LCD application but also for the applications that demand high on-off ratio and low-leakage device, such as X-ray image sensor