Light Absorption Depth Research Articles

(Invited) Detection of Wavelength Information by Filter-Free Wavelength Sensor and Its Applications

This paper introduces the wavelength and light intensity detection method using the filter-free wavelength sensor without optical filters. Optical detection methods are widely used in medical, environmental, chemical, biofield, and agricultural analyses. In the biofield, the optical detection method makes it possible to predict, diagnose, and analyze diseases by detecting the wavelength information and light intensity, such as absorption, fluorescence, and spectral shift.In order to measure a specific wavelength and intensity of light, mainly used a detection method such as a filter cube of a fluorescence microscope or a monochromator of a spectrometer system. Even if this system has high sensitivity and selectivity, it is not easy to apply to POCTs that require portability due to the high price and increased system volume due to the integration of optical filters and other parts. Therefore, various studies have reported using semiconductor processes to detect the wavelength and light intensity with a high-performance and miniaturized system.An on-chip photodetection device has been reported, such as a fluorescence detection sensor to integrate interference or absorption filters on CMOS image sensors with high selectivity and sensitivity. Since the specific wavelength of light intensity detection is possible, it can apply to biofields such as fluorescence analysis. However, detecting different wavelength changes by fluorescent reagent is difficult because the optical filter is integrated into the CMOS image sensor. A method of multi-wavelength analysis with the single-pixel without an optical filter is reported using a CMOS-buried quad p-n junction photodetector. Since the light wavelength has a different absorption depth depending on the silicon absorption coefficient, it is possible to separate wavelength by measuring the current generated at each p-n junction. Therefore, it will provide a high-space-resolution image in the biofield. However, the possibility detection wavelength band is fixed according to the buried p-n junction numbers and depth.Previously, we reported a filter-free wavelength sensor with a photogate structure. Sensor can detect light intensity and wavelength without optical filters based on the dependence of the light absorption depth in silicon. The sensor fabricates using the 1-polysilicon, 2-metal process in the LSI facility of the Toyohashi University of Technology in Japan, as shown in Fig1(a). The sensor consists of a three-layer structure where deep n-well and p-well were formed on a p-type silicon substrate, as shown in Fig1(b). Applying a voltage to the photogate and n-well forms a potential peak W as a mountain shape below the sensing area. Therefore, the photocurrent generated by the incident light was divided into the surface side (I PG), and bottom side (I n-well) based on the potential peak W and wavelength identification is possible from the current ratio (I n-well/I PG). To detect the wavelength information of multi-peak light for applications in the biofield, compare the centroid wavelength and current ratio. We experimentally confirmed that the centroid wavelength was proportional to the current ratio by the light absorption depth of silicon. Since the light absorption depth depends on the light wavelength, the current ratio is constant even when the light intensity changes. Therefore, estimation of the light intensity is possible by the sum of each current.We proposed a wavelength, spectral shift, and absorbance measure method using a filter-free wavelength sensor to apply various fields. Figure 2(a) shows a schematic for detecting the fluorescence wavelength. Without using optical filters, fluorescence can be quantified by calculating the centroid wavelength of the light even when the excitation light is emitted simultaneously. In addition, it successfully identified fluorescence emitted from Legionella bacteria [1]. Figure 2(b) shows a schematic diagram that can identify the change in the spectrum using a sensor. Integrating an LSPR biosensor into a filter-free wavelength sensor makes it possible to measure the amount of spectrum changed by molecular binding. Therefore, miniaturization of the LSPR detection system becomes possible, and we succeeded in detecting and quantifying the S-protein of SARS-CoV-2 [2]. Figure 2(c) shows a schematic diagram of the absorbance measurements method for samples that absorb a specific wavelength range. By the proposed absorbance measurement method was possible to measure the ratio of chlorophyll content and chlorophyll a/b ratio contributing to plant photosynthesis [3].The filter-free wavelength sensor fabricated by the semiconductor process has been confirmed for its application as an optical detection system. In the future, we will continue research to develop an optical sensor system that is more compact, sensitive, and selective and can be applied to various fields.[1] Biosensors, vol.12, no.11, p.1033 (2022)[2] Applied Physics Express, vol.16, no.1, p.012012 (2023)[3] Jpn. J. Appl. Phys., vol.61, p.SD1041 (2022) Figure 1

(Invited) Detection of Wavelength Information by Filter-Free Wavelength Sensor and Its Applications

This paper introduces the wavelength and light intensity detection method using the filter-free wavelength sensor without optical components. Using a modified CMOS process, we developed a filter-free wavelength sensor with a photogate structure. The sensor can detect light intensity and wavelength without optical filters based on the dependence of the light absorption depth in silicon. Using a developed sensor, we demonstrated a wavelength, spectral shift, and light absorption detection method to apply various fields. In the future, we will continue research to realize an optical sensor system that is more compact, sensitive, and selective.

Directional Carrier Transport in Micrometer-Thick Gallium Oxide Films for High-Performance Deep-Ultraviolet Photodetection.

Incorporating emerging ultrawide bandgap semiconductors with a metal-semiconductor-metal (MSM) architecture is highly desired for deep-ultraviolet (DUV) photodetection. However, synthesis-induced defects in semiconductors complicate the rational design of MSM DUV photodetectors due to their dual role as carrier donors and trap centers, leading to a commonly observed trade-off between responsivity and response time. Here, we demonstrate a simultaneous improvement of these two parameters in ε-Ga2O3 MSM photodetectors by establishing a low-defect diffusion barrier for directional carrier transport. Specifically, using a micrometer thickness far exceeding its effective light absorption depth, the ε-Ga2O3 MSM photodetector achieves over 18-fold enhancement of responsivity and simultaneous reduction of the response time, which exhibits a state-of-the-art photo-to-dark current ratio near 108, a superior responsivity of >1300 A/W, an ultrahigh detectivity of >1016 Jones, and a decay time of 123 ms. Combined depth-profile spectroscopic and microscopic analysis reveals the existence of a broad defective region near the lattice-mismatched interface followed by a more defect-free dark region, while the latter one serves as a diffusion barrier to assist frontward carrier transport for substantially enhancing the photodetector performance. This work reveals the critical role of the semiconductor defect profile in tuning carrier transport for fabricating high-performance MSM DUV photodetectors.

Residual Stress Control in Mid-infrared Laser Rounding of Glass Plate Edges by Reciprocal Scanning Assist Laser Irradiation

Light absorption depth of glasses in a mid-infrared wavelength of 2.8 μm is the same scale as thickness of glass plates used in flat panel displays. Such an absorption property allows relatively uniform heating in the thickness direction of glass plates when a mid-infrared laser is irradiated vertically to the glass plate surface. Therefore, the mid-infrared laser is convenient for edge rounding of complex shape glass plates. Application of the mid-infrared laser to the edge finishing process has been developed. However, residual tensile stress in rounded edges which weakens final products has been a problem to be solved. Simultaneous assist laser irradiation with the mid-infrared laser rounding is a candidate solution because of its controllability of residual stress. In this study, residual stress control has been examined by high-frequency reciprocal scanning assist laser irradiation. In addition, relationship between residual stress and assist heating width has been investigated by numerically and experimentally. Existence of an optimum assist heating width which minimize residual stress was confirmed. Both increase and decrease in assist heating width from the optimum width increased residual stress.

Fe2TiO5/Fe2O3 (Shell/Shell) and (Shell/Core) Heterostructured for Efficient Oxygen Evolution.

A delicate synthesis strategy of Fe2TiO5/Fe2O3 (shell/shell (s/s)) and (shell/core (s/c)) heterostructures was proposed to realize efficient photocatalytic water oxidation. The hierarchical structure increases the light absorption depth of material, the strongly coupled components promoted the separation of photoinduced excitons, and the high surface area with adequate porosity allowed the diffusion of electrolyte to adsorb at rich active sites. Furthermore, Fe2TiO5/Fe2O3 (s/s) was coated by graphitic carbon nitride (g-CN) subunits and exhibited good photocatalytic water reduction, resulting from the contact interface accelerating the effective separation of electrons and holes.

3D-printed Cu2O photoelectrodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting is an alternative to fossil fuel combustion involving the generation of renewable hydrogen without environmental pollution or greenhouse gas emissions. Cuprous oxide (Cu2O) is a promising semiconducting material for the simple reduction of hydrogen from water, in which the conduction band edge is slightly negative compared to the water reduction potential. However, the solar-to-hydrogen conversion efficiency of Cu2O is lower than the theoretical value due to a short carrier-diffusion length under the effective light absorption depth. Thus, increasing light absorption in the electrode–electrolyte interfacial layer of a Cu2O photoelectrode can enhance PEC performance. In this study, a Cu2O 3D photoelectrode comprised of pyramid arrays was fabricated using a two-step method involving direct-ink-writing of graphene structures. This was followed by the electrodeposition of a Cu current-collecting layer and a p–n homojunction Cu2O photocatalyst layer onto the printed structures. The performance for PEC water splitting was enhanced by increasing the total light absorption area (Aa) of the photoelectrode via controlling the electrode topography. The 3D photoelectrode (Aa = 3.2 cm2) printed on the substrate area of 1.0 cm2 exhibited a photocurrent (Iph) of −3.01 mA at 0.02 V (vs. RHE), which is approximately three times higher than that of a planar photoelectrode with an Aa = 1.0 cm2 (Iph = −0.91 mA). Our 3D printing strategy provides a flexible approach for the design and the fabrication of highly efficient PEC photoelectrodes.

Effect of conductive substrate on the photoelectrochemical properties of Cu2O film electrodes for methyl viologen reduction

Cuprous oxide (Cu2O) is a candidate for low-cost p-type semiconductor oxides for photoelectrochemical solar energy conversion. The Cu2O crystalline films electrodeposited on conductive substrates have been extensively studied as photocathodes because of their moderate photoelectrochemical performance, but their incident photon-to-current conversion efficiency (IPCE) was still limited because of the incompatibility between minority carrier diffusion length and light absorption depth in planar structures. One of the strategies to solve this issue is to decrease the thickness of the Cu2O films with keeping the loading amount. Herein, we used a sintered titanium microfiber felt as a three-dimensional macroporous scaffold for Cu2O crystalline thin films because it exhibits a large surface area compared with the conventional flat substrate. The effect of the Cu2O film thickness on the photocathode properties was investigated using methyl viologen as an acceptor of the photoexcited electrons. The Cu2O thin films deposited on Ti microfibers were superior to that of the Cu2O films deposited on dense planar substrates such as fluorine-doped tin oxide (FTO)-coated glass and titanium sheets. It was revealed that the large Cu2O–electrolyte and Cu2O–substrate interfaces resulted in the enhancement of IPCE for the photoelectrochemical cathodic reaction.

Extreme radiation resistance in InN

Carrier dynamics were studied in InN, which is a promising material for radiation-resistant optoelectronic devices. InN epilayers with different background electron densities were prepared by metal-organic chemical vapor deposition and irradiated with protons. Application of the light-induced transient grating technique in the sub-picosecond domain allowed to study the carrier lifetime and diffusion coefficient as a function of the total carrier density (intrinsic, photoexcited, and irradiation-induced). The dominance of the direct Auger process in carrier recombination was revealed. The irradiation of InN by protons, such as those encountered in space applications, decreases the carrier lifetime while simultaneously increasing the diffusion coefficient (up to 190 cm2/s at the highest total carrier density studied). As a result, the average carrier diffusion length does not decrease below ∼140 nm, and exceeds the light absorption depth of InN. The lifetime damage factor was determined, and the lifetime dependence on the displacement damage dose was measured. The radiation resistance of InN surpasses that of GaAs, InGaAs, and InGaN. InN-based photovoltaic devices may work for long space missions.

Direct Auger recombination and density-dependent hole diffusion in InN

Indium nitride has a good potential for infrared optoelectronics, yet it suffers from fast nonradiative recombination, the true origin of which has not been established with certainty. The diffusion length of free carriers at high densities is not well investigated either. Here, we study carrier recombination and diffusion using the light-induced transient grating technique in InN epilayers grown by pulsed MOCVD on c-plane sapphire. We show that direct Auger recombination governs the lifetime of carriers at densities above ~1018 cm−3. The measured Auger recombination coefficient is (8 ± 1) × 10−29 cm−3. At carrier densities above ~5 × 1019 cm−3, we observe the saturation of Auger recombination rate due to phase space filling. The diffusion coefficient of holes scales linearly with carrier density, increasing from 1 cm2/s in low-doped layers at low excitations and up to ~40 cm2/s at highest carrier densities. The resulting carrier diffusion length remains within 100–300 nm range, which is comparable to the light absorption depth. This feature is required for efficient carrier extraction in bipolar devices, thus suggesting MOCVD-grown InN as the material fit for photovoltaic and photonic applications.

Role of Laser Power, Wavelength, and Pulse Duration in Laser Assisted Tin-Induced Crystallization of Amorphous Silicon

This work describes tin-induced crystallization of amorphous silicon studied with Raman spectroscopy in thin-film structures Si-Sn-Si irradiated with pulsed laser light. We have found and analyzed dependencies of the nanocrystals’ size and concentration on the laser pulse intensity for 10 ns and 150 μm duration laser pulses at the wavelengths of 535 nm and 1070 nm. Efficient transformation of the amorphous silicon into a crystalline phase during the 10 ns time interval of the acting laser pulse in the 200 nm thickness films of the amorphous silicon was demonstrated. The results were analyzed theoretically by modeling the spatial and temporal distribution of temperature in the amorphous silicon sample within the laser spot location. Simulations confirmed importance of light absorption depth (irradiation wavelength) in formation and evolution of the temperature profile that affects the crystallization processes in irradiated structures.

Unravelling Photocarrier Dynamics beyond the Space Charge Region for Photoelectrochemical Water Splitting

Semiconductor photoelectrodes for photoelectrochemical (PEC) water splitting require efficient carrier generation, separation, and transport at and beyond the space charge region (SCR) formed at the aqueous interface. The trade-off between photon collection and minority carrier delivery governs the photoelectrode design and implies maximum water splitting efficiency at an electrode thickness equivalent to the light absorption depth. Here, using planar ZnO thin films as a model system, we identify the photocarriers beyond the SCR as another significant source to substantially enhance the PEC performance. The high-quality ZnO films synthesized by pulsed laser deposition feature very few deep trap states and support a long photocarrier lifetime. Combined with photoelectrochemical characterization, ultrafast spectroscopy, and numerical calculations, it is revealed that engineering the exciton concentration gradient by film thickness facilitates the inward diffusion of photocarriers from the neighboring illumi...

Light Trapping for Solar Fuel Generation with Mie Resonances

The implementation of solar fuel generation as a clean, terawatt-scale energy source is critically dependent on the development of high-performance, inexpensive photocatalysts. Many candidate materials, including for example α-Fe2O3 (hematite), suffer from very poor charge transport with minority carrier diffusion lengths that are significantly shorter (nanometer scale) than the absorption depth of light (micrometer scale near the band edge). As a result, most of the photoexcited carriers recombine rather than participate in water-splitting reactions. For this reason, there is a tremendous opportunity for photon management. Plasmon-resonant nanostructures have been employed to effectively enhance light absorption in the near-surface region of photocatalysts, but this approach suffers from intrinsic optical losses in the metal. Here, we circumvent this issue by driving optical resonances in the active photocatalyst material itself. We illustrate that judiciously nanopatterned photocatalysts support optical Mie and guided resonances capable of substantially enhancing the photocarrier generation rate within 10-20 nm from the water/photocatalyst interface.

Spatial Distribution of Photocurrent in Si Stripes under Tilted Illumination Measured by Multimode Scanning Probe Microscopy

Spatial distribution of photocurrent in Si stripes of 50–1000 nm in width were investigated as a function of optical excitation wavelength by multimode scanning probe microscopy (MSPM). Inhomogeneous distribution of the MSPM photocurrent in the Si stripe interior was attributed to the light intensity profile. A model that included light absorption depth profile and the probe-induced band-bending region reproduced the photocurrent profile for tilted illumination of the stripes. An effective spatial resolution of ∼10 nm was deduced from the photocurrent measurements with multimode SPM in the constant-force mode.

Rationally Designed, Three‐Dimensional Carbon Nanotube Back‐Contacts for Efficient Solar Devices

Structure matters in solar-to-fuel conversion efficiency, demonstrated for TiO2 coated onto designed three-dimensional (3-D) carbon nanofiber scaffolds. Performance enhancement of up to three times compared to flat films can be mostly attributed to structurally-enhanced carrier collection and photon management processes. Using 3-D templates to balance light absorption depth and carrier collection can lead to a new class of high efficiency and cheap energy conversion devices.

Plasmon Enhanced Solar-to-Fuel Energy Conversion

Future generations of photoelectrodes for solar fuel generation must employ inexpensive, earth-abundant absorber materials in order to provide a large-scale source of clean energy. These materials tend to have poor electrical transport properties and exhibit carrier diffusion lengths which are significantly shorter than the absorption depth of light. As a result, many photoexcited carriers are generated too far from a reactive surface and recombine instead of participating in solar-to-fuel conversion. We demonstrate that plasmonic resonances in metallic nanostructures and multilayer interference effects can be engineered to strongly concentrate sunlight close to the electrode/liquid interface, precisely where the relevant reactions take place. On comparison of spectral features in the enhanced photocurrent spectra to full-field electromagnetic simulations, the contribution of surface plasmon excitations is verified. These results open the door to the optimization of a wide variety of photochemical processes by leveraging the rapid advances in the field of plasmonics.

Photoinduced Ag deposition on periodically poled lithium niobate: Wavelength and polarization screening dependence

This research addresses the wavelength dependence of the fabrication of Ag nanostructures through photoinduced deposition using single crystal ferroelectric lithium niobate as a template. The photoinduced deposition involves ultraviolent light illumination of polarity patterned lithium niobate while immersed in a AgNO3 solution. The results focus on the differences of the Ag nanostructure formation process on the positive and negative domains and domain boundaries. The results indicate that for below-band-gap excitation, a very low density of nanostructures is observed. However, for all above-gap-excitation wavelengths, deposition occurs on both polarity surfaces and at the domain boundaries. The density is greatest at the domain boundaries and reduced densities of smaller nanostructures are observed to form on both the positive and negative domains. The deposition on the domain surfaces is greatest for the shortest wavelengths, whereas the domain selectivity is increased for wavelengths just above the band gap. The external screening and weak band bending of single crystal lithium niobate introduces an enhanced electric field at the domain boundary. The enhanced electric field leads to migration of electrons to the domain boundary and consequently enhanced formation of Ag nanoparticles along the boundary. The variation in the reduction rate versus illumination wavelength is attributed to the light absorption depth and the competition between the photochemical and photoelectric deposition processes. To explore the transition from surface to bulk screening of the polarization charge, oxygen implanted PPLN surfaces were prepared and used for the Ag photoinduced deposition. Consistent with the transition to internal (bulk) screening, the Ag nanoparticle formation on the oxygen implanted PPLN surfaces showed suppressed boundary nanowire formation.

Determining lifetime in silicon blocks and wafers with accurate expressions for carrier density

In recent years, many studies of silicon minority-carrier lifetime have been performed with quasi-steady-state photoconductance measurements. The method has been used to characterize the defect levels, surface passivation, and trapping effects in wafers by using absolutely calibrated data for minority-carrier lifetime versus minority-carrier injection level. This paper generalizes the quasi-steady-state photoconductance technique for use in thick wafers or blocks of silicon where the diffusion length or light absorption depth is much less than the sample thickness. The photogeneration and carrier diffusion profiles are calculated for special cases of interest. The measured effective lifetimes can then be used to estimate the bulk lifetime in the material, and report this lifetime at an appropriate average minority-carrier density for the measurement conditions. In this way, the results and measurement methodologies previously developed for use on wafers can be applied to single- or multi-crystalline silicon ingots or thick wafers. In thick silicon samples, long-wavelength weakly absorbed light can be used to reduce the effects of surface recombination on the measurement giving important advantages compared to the case of measuring unpassivated wafers. The generalization presented here offers advantages for accurately determining the bulk lifetime of silicon material prior to sawing into wafers and without requiring surface passivation.

Mobility spatial distribution function: Comparative method for conjugated polymers/molecules

The authors report improved method for extracting mobility spatial distribution function that accounts for the finite light-absorption depth. They employ step function light excitation and use the photocurrent temporal response to extract the electrons’ mobility spatial distribution function (MDF). This directly measures the relevant electronic disorder in amorphous conjugated polymers. Comparing two different polymers shows that qualitative analysis of field effect transistor data is in good agreement with the quantitative analysis provided by the MDF concept/method. The better uniformity of the electronic properties found in green polypnenylene vinylene is a clear indication for its better performance and lifetime.

The Effects of Post Excimer Laser Annealing on (Ba,Sr)TiO3 Thin Films at Low Substrate Temperatures

The amorphous (Ba,Sr)TiO3 (α-BST) films sputtered onto the Pt/TiN/Ti/Si substrates are greatly improved using the excimer laser annealing (ELA) technique of wavelength 248 nm at low substrate temperature 300°C. The dielectric constant of the α-BST film is remarkably enhanced from 80 to over 250 after ELA treatment. The heat conduction analysis indicates that a very shallow light absorption depth of the wavelength 248 nm for the BST film behaves an excellent thermal property, which doesn't damage the underlayer films during ELA treatment. Besides, the leakage current of ELA-BST films is strongly influenced by the laser energy fluence (LEF). The detailed mechanisms of the crystallinity and the electric properties are systematically studied in this report.

A high-speed, high-sensitivity silicon lateral trench photodetector

We report a novel silicon lateral trench photodetector that decouples the carrier transit distance from the light absorption depth, enabling both high speed and high responsivity. The photodetector, fabricated with fully VLSI compatible processes, exhibits a 6-dB bandwidth of 1.5 GHz at 3.0 V and an external quantum efficiency of 68% at 845 nm wavelength. A photoreceiver with a wire-bonded lateral trench detector and a BiCMOS transimpedance amplifier demonstrates excellent operation at 2.5 Gb/s data rate and 845 nm wavelength with only a 3.3 V bias.