Photocurrent Peak Research Articles

Spatially resolved photoexcited charge-carrier dynamics in phase-engineered monolayer MoS2.

A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition-metal dichalcogenides (TMDs) is crucial for its integration into high performance semiconductor devices. Here, we investigate the transport properties of chemical vapor deposition (CVD) grown monolayer molybdenum disulfide (MoS2) under photoexcitation using correlated scanning photocurrent microscopy and photoluminescence imaging. We examined the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resolution. While maximum photocurrent generation occurs at the Schottky contacts of semiconducting (2H-phase) MoS2, after the metallic phase transformation (1T-phase), the photocurrent peak is observed toward the center of the device channel, suggesting a strong reduction of native Schottky barriers. Analysis using the bias and position dependence of the photocurrent indicates that the Schottky barrier heights are a few millielectron volts for 1T- and ∼ 200 meV for 2H-contacted devices. We also demonstrate that a reduction of native Schottky barriers in a 1T device enhances the photoresponsivity by more than 1 order of magnitude, a crucial parameter in achieving high-performance optoelectronic devices. The obtained results pave a way for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where ohmic contacts are necessary for achieving high-efficiency devices with low power consumption.

Interfacial barrier effects in blocked impurity band infrared detectors

Blocked impurity band (BIB) detectors, developed from extrinsic detectors, have long been employed for ground-based and airborne astronomical imaging and photon detections. They are the state-of-the-art choice for highly sensitive detection from mid-infrared to far-infrared radiation. In this work, we demonstrate the existence of an interfacial barrier in blocked impurity band structures by evidence of temperature-dependent dark currents, bias-dependent photocurrent spectra and corresponding theoretical calculations. The origin of the build-in field is studied. The temperature-dependent characteristics of space charge effects are also investigated in detail. It is found that at higher temperature (T 14 K), the space charge influence is negligible, and the interfacial barrier is mainly caused by bandgap narrowing effects. Based on interfacial barrier effects, a dual-excitation model is proposed to clarify the band structure of BIB detectors. The photocurrent spectra related to the two excitation processes, i.e., the direct excitation over the interfacial barrier and excitation to the band edge with subquent tunneling into blocking layer, are successfully extracted and agree reasonably well with the calculated band structure results. The effects of interfacial barrier on the photocurrent spectrum, peak responsivity and internal quantum efficiency of the devices are investigated. With the consideration of interfacial barrier effects, the calculated peak responsivity shows good agreement with the experimental result. It is suggested that interfacial barrier effects should be considered for successfully designing the BIB detectors. Additionally, the build-in field is found to equivalently lower the critical field for impact ionization. This study provides a better understanding of the working mechanism in BIB detectors and also a better device optimization.

Deep-level sensitization in Ge-SiO2 composite films observed by photocurrent spectroscopy

We studied the temperature dependence of the photocurrent spectra of a Ge-SiO2 composite thin film. We found that the spectral position of the photocurrent peak is determined by the competition between absorption and non-radiative recombination and that its temperature dependence is associated with the population variation of the energetically deep levels in the system under “thermal quenching” conditions. Combining these results with our previous deep-level transient spectroscopy data enables the association of these levels with the quantum confinement effect. We thus identify here a non-radiative recombination process associated with deep-level sensitization that stems from quantum confinement.

Enhanced charge photogeneration promoted by crystallinity in small-molecule donor-acceptor bulk heterojunctions

We examined sub-nanosecond time-scale charge carrier dynamics in crystalline films of a functionalized anthradithiophene (ADT) donor (D) with three different acceptor (A) molecules. A four-fold enhancement in ultrafast charge carrier separation efficiency was observed in D/A blends with a fullerene acceptor added at 7–10 wt. % concentrations, whereas a gradual decrease in peak photocurrent amplitude with acceptor concentration was observed with functionalized pentacene and indenofluorene acceptors. The results were directly correlated with the ADT-tri(ethylsilyl)ethynyl-F donor crystallinity. In the best-performing blends, the presence of crystalline acceptor domains was also established.

Electronic states in vertically ordered Ge/Si quantum dots detected by photocurrent spectroscopy

We report on intraband photocurrent spectroscopy of sixfold stacked Ge/Si quantum dots embedded in a Si matrix and aligned along the growth direction. The dots are formed in a shape of pyramids with the average lateral size of 18 nm. The $n$-type heterostructures show broad spectral response ranging from 5 to 20 $\ensuremath{\mu}\mathrm{m}$, depending on the polarization of the incoming infrared light. The normal incidence photocurrent peak centered around 12--15 $\ensuremath{\mu}\mathrm{m}$ is attributed to the transitions from the electron states localized in the Si region adjacent to the dots to continuum states of the Si matrix. The electron confinement is caused by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge/Si quantum dots. Using the Ge content and dot shape determined by Raman and scanning tunneling microscopy analysis as input parameters for three-dimensional band structure simulations, a good agreement between measured and calculated electron binding energy is obtained. Photoluminescence spectroscopy and measurements of temperature dependence of dark conductance are used to correlate photocurrent results.

Topotactic transformation to mesoporous Co3O4 nanosheet photocathode for visible-light-driven direct photoelectrochemical hydrogen generation

Developing p-type photocathodes for direct hydrogen generation is an attractive challenge in photoelectrochemical (PEC) water splitting. In this paper, we report a development of mesoporous Co3O4 nanosheet photocathodes for visible-light-driven direct PEC hydrogen generation. The mesoporous Co3O4 nanosheet was fabricated through an in-situ topotactic transformation from solid hexagonal Co(OH)2 to porous spinel Co3O4. The specific surface area of the mesoporous Co3O4 nanosheet is extremely high as 112.8m2/g. Under visible light irradiation, the peak photocurrent value of the mesoporous Co3O4 nanosheet photocathode in S2− hole scavenger solution is 37 folds enhancement compared to that in only supporting electrolyte. The specific surface area effect on the PEC performance was also investigated. This demonstration of mesoporous Co3O4 nanosheet photocathode with high specific surface area could open up new opportunities for p-type metal oxide photocathodes in visible-light-driven direct PEC hydrogen generation.

Competing channels for hot-electron cooling in graphene.

We report on temperature-dependent photocurrent measurements of high-quality dual-gated monolayer graphene p-n junction devices. A photothermoelectric effect governs the photocurrent response in our devices, allowing us to track the hot-electron temperature and probe hot-electron cooling channels over a wide temperature range (4 to 300 K). At high temperatures (T > T(*)), we found that both the peak photocurrent and the hot spot size decreased with temperature, while at low temperatures (T < T(*)), we found the opposite, namely that the peak photocurrent and the hot spot size increased with temperature. This nonmonotonic temperature dependence can be understood as resulting from the competition between two hot-electron cooling pathways: (a) (intrinsic) momentum-conserving normal collisions that dominates at low temperatures and (b) (extrinsic) disorder-assisted supercollisions that dominates at high temperatures. Gate control in our high-quality samples allows us to resolve the two processes in the same device for the first time. The peak temperature T(*) depends on carrier density and disorder concentration, thus allowing for an unprecedented way of controlling graphene's photoresponse.

Resonant tunneling diode with a multiplication region for light detection

A resonant tunneling diode (RTD) with a multiplication region is designed for light detection in this paper. Via adding a n+–i–p+ multiplication region, we focus on promoting the photocurrent and light sensitivity of the detector. Through the calculation of the multiplication factor, the thickness of the multiplication region is determined. The influence factors of the electric field and potential distribution of the detector are investigated, thereby the thickness and doping concentration of the doped layers besides the double-barrier structure (DBS) are decided. Detectors with and without a multiplication region are fabricated from semiconductor heterostructures grown by molecular beam epitaxy. The current–voltage (I–V) and light sensitivity tests show that the detector with a multiplication region has better performance in peak photocurrent and light sensitivity.

Photoelectrochemical water splitting in a tungsten oxide - nickel oxide thin film material library

A combinatorial thin film library containing WO3 and NiO was produced using thermal co-evaporation of WO3 and Ni followed by high temperature reoxidation. Microstructure and crystallographic particularities of the WO3-NiO library investigated by SEM and XRD revealed three distinct compositional zones: A low Ni concentration zone containing crystalline WO3 and amorphous NiO, a high Ni concentration zone containing crystalline NiO and amorphous WO3 and a middle range amorphous zone connecting the extremes. Photo Electrochemical Scanning Droplet Cell Microscopy PE-SDCM was used for locally investigating the photoelectric response of the library as a function of Ni concentration. A substantial photocurrent peak was identified at 6.2 at.% Ni with values in excess of 2.5 mA cm−2 while Ni amounts above 9 at.% resulted in a completely photoinactive thin film. XPS surface analysis indicated a surface composition different than the bulk with WO3 being present up to 45 at.% Ni in the bulk. For bulk Ni amounts above this composition, the surface contained no WO3 anymore, only an amorphous Ni sub-oxide being suggested.

Infrared photoresponse of GeSn/n-Ge heterojunctions grown by molecular beam epitaxy.

Heterojunction devices of Ge(1-x)Sn(x) / n-Ge were grown by solid source molecular beam epitaxy (MBE), and the mid-infrared (IR) photocurrent response was measured. With increasing Sn composition from 4% to 12%, the photocurrent spectra became red-shifted, suggesting that the bandgap of Ge(1-x)Sn(x) alloys was lowered compared to pure Ge. At a temperature of 100 K, the wavelengths of peak photocurrent were shifted from 1.42 µm for pure Ge (0% Sn) to 2.0 µm for 12% Sn. The bias dependence of the device response showed that the optimum reverse bias was > 0.5 volts for saturated photocurrent. The responsivity of the Ge(1-x)Sn(x) devices was estimated to be 0.17 A/W for 4% Sn. These results suggest that Ge(1-x)Sn(x) photodetectors may have practical applications in the near/mid IR wavelength regime.

PA-MBE Grown p-n (p-ZnO:(As+Sb)/&lt;i&gt;n&lt;/i&gt;-GaN) and p-i-n (p-ZnO:As/HfO&lt;sub&gt;2&lt;/sub&gt;/n-GaN) Heterojunctions as a Highly Selective UV Detectors

Zinc oxide is a promising candidate for application in UV photodetectors due to the large direct band gap and the high absorption coefficient in the UV. The high quality p-n and p-i-n structures consist of single or dual acceptor doped ZnO:(As,Sb) films grown by MBE, thin HfO2 layer grown by ALD method and n-type GaN templates. The As and Sb concentrations is 1020 cm-3.The maximum forward-to-reverse current ratio IF/IR in the obtained p-n diodes is of about 105 at ±4 V and in the case of p-i-n diodes is of about 106, which are very good results for this type of heterojunctions. The UV photodetectors are highly selective. The maximum of the detection wavelength was found at about 365 nm (FWHM of the photocurrent peak is ~17 nm). In the case of p-i-n detectors, the maximum of detection was found at 376, 360, and 341 nm. Additionally, it is possible to control the detection range by the applied reverse voltage. The dark to light current ratio in both cases is ~104.

Hot Wall Epitaxy(HWE)법에 의한 BaIn2Se4에피레어 성장과 가전자대 갈라짐에 대한 광전류 연구

A stoichiometric mixture of evaporating materials for BaIn2Se4 epilayers was prepared from horizontal electric furnace. To obtain the single crystal thin films, BaIn2Se4 mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were 620 o C and 400 o C, respectively. The crystalline structure of the epilayers was investigated by the photoluminescence and double crystal X-ray diffraction (DCXD). The carrier density and mobility of BaIn2Se4 epilayers measured from Hall effect by van der Pauw method are 8.94×10 17 cm -3 and 343 cm 2 /vs at 293 K, respectively. The temperature dependence of the energy band gap of the BaIn2Se4 obtained from the absorption spectra was well described by the Varshni's relation, Eg(T)=2.6261 eV-(4.9825×10 -3 eV/K)T 2 /(T+558 K). The crystal field and the spin-orbit splitting energies for the valence band of the BaIn2Se4 have been estimated to be 116 meV and 175.9 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the Δso definitely exists in the states of the valence band of the BaIn2Se4/ GaAs epilayer. The three photocurrent peaks observed at 10 K are ascribed to the A1-, B1-exciton for n = 1 and C21-exciton peaks for n=21.

Fabrication and properties of V-doped semi-insulating 6H-SiC photoconductive semiconductor switch

High power photoconductive semiconductor switches (PCSSs) were fabricated on V-doped semi-insulating 6H-SiC single crystal. The 6H-SiC PCSS were measured by applying a bias voltage from 1 kV to 14 kV. The triggered laser was a 355 nm pulse laser with an energy density of 150 J/mm2. The peak photocurrent shows an increasing trend with improving the applied voltage. The peak photocurrent running through the PCSS and the calculated on-state resistance are 185 A and about 22 respectively when the applied voltage reaches 14 kV, and the corresponding peak power is 2.59 MW.

Effect of phosphorus doping concentration on n-type Ge layer growth

The n-type Ge layers with various phosphorus doping concentrations were grown on p-type Si (100) wafers by using Rapid Thermal Chemical Vapor Deposition (RTCVD). The root-mean-square (RMS) surface roughness of the n-type Ge layer increased from 0.167 to 14.131 nm when the phosphorus doping concentration was increased from 3 × 1016 to 3 × 1019 cm−3. High-Resolution X-ray Diffraction was used to evaluate the in-plane lattice constants and the tensile strain of the n-type Ge layer as functions of the phosphorus doping concentration. The presence of tensile strain was further confirmed from the Raman measurements, which revealed a strain reduction from 0.071 to 0.065% when the phosphorus doping concentration was increased from 3 × 1016 to 3 × 1019 cm−3. Moreover, a considerable increase in the photocurrent peak energy was observed.

Surface activation behavior of negative-electron-affinity exponential-doping GaAs photocathodes

In view of the effect of doping structure on the surface activation behavior, the Cs–O activation experiments under the same preparation condition were performed on the exponential-doping and uniform-doping GaAs photocathodes. The activation results show that, compared with the uniform-doping photocathode, the exponential-doping photocathode presents a lower photocurrent peak after the initial Cs activation and a higher photocurrent peak after the Cs–O alternate activation. Based on the double dipole model, the parameters of surface potential barrier are obtained from the calculation of longitudinal energy distribution of emitted electrons according to the ratio of photocurrent peak after Cs–O activation to that after Cs activation. The discrepancy in the photocurrent evolution during the entire activation process is related to the different variations in surface potential barrier profile.

Doping-concentration dependence of a boron-doped p-type Ge layer grown on a Si (100) substrates by using RTCVD

Boron-doped p-type Ge layers were grown on n-type Si (100) wafers at various boron doping concentrations by using Rapid Thermal Chemical Vapor Deposition. The root-mean-square was surface roughness of p-type Ge layer increase from 0.708 nm to 19.980 nm as the doping concentrations increase from 3 × 1016 cm−3 to 4 × 1022 cm−3. From High Resolution X-ray diffraction, the in-plane lattice constants and tensile strains of the p-type Ge layer were evaluated as function of boron doping concentration the Raman shift of the for each location indicate a tensile strain from the p-type Ge layer. The tensile strain of the p-type Ge layer decreases from 0.071% to 0.032% as the boron doping concentrations increases from 3 × 1016 cm−3 to 4 × 1022 cm−3. Moreover, the photocurrent peak energies increased. The temperature dependence of the direct band-gap energy, E, of the p-type Ge layer could be described by using Varshnis empirical expression: EΓ(T) = 0.86 − 5.82 × 10−4 T2/(T + 296).

V掺杂6H-SiC光导开关制备与性能研究

High power photoconductive semiconductor switches (PCSSs) were fabricated on V-doped semi-insulating 6H-SiC single crystal. The 6H-SiC PCSS were measured by applying a bias voltage from 1 kV to 14 kV. The triggered laser was a 355 nm pulse laser with an energy density of 150 J/mm2. The peak photocurrent shows an increasing trend with improving the applied voltage. The peak photocurrent running through the PCSS and the calculated on-state resistance are 185 A and about 22 respectively when the applied voltage reaches 14 kV, and the corresponding peak power is 2.59 MW.

Growth and Electrical Properties of ZnAl2Se4 Single Crystal Thin Film by Hot Wall Epitaxy

A stoichiometric mixture of evaporating materials for <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin films was prepared in a horizontal electric furnace. These <TEX>$ZnAl_2Se_4$</TEX> polycrystals had a defect chalcopyrite structure, and its lattice constants were <TEX>$a_0=5.5563{\AA}$</TEX> and <TEX>$c_0=10.8897{\AA}$</TEX>.To obtain a single-crystal thin film, mixed <TEX>$ZnAl_2Se_4$</TEX> crystal was deposited on the thoroughly etched semi-insulating GaAs(100) substrate by a hot wall epitaxy (HWE) system. The source and the substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$400^{\circ}C$</TEX>, respectively. The crystalline structure of the single-crystal thin film was investigated by using a double crystal X-ray rocking curve and X-ray diffraction <TEX>${\omega}-2{\theta}$</TEX> scans. The carrier density and mobility of the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film were <TEX>$8.23{\times}10^{16}cm^{-3}$</TEX> and <TEX>$287m^2/vs$</TEX> at 293 K, respectively. To identify the band gap energy, the optical absorption spectra of the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film was investigated in the temperature region of 10-293 K. The temperature dependence of the direct optical energy gap is well presented by Varshni's relation: <TEX>$E_g(T)=E_g(0)-({\alpha}T^2/T+{\beta})$</TEX>. The constants of Varshni's equation had the values of <TEX>$E_g(0)=3.5269eV$</TEX>, <TEX>${\alpha}=2.03{\times}10^{-3}eV/K$</TEX> and <TEX>${\beta}=501.9K$</TEX> for the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$ZnAl_2Se_4$</TEX> were estimated to be 109.5 meV and 124.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$ZnAl_2Se_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{21}$</TEX>-exciton peaks for n = 21.

Localized Photoelectrochemistry on a Tungsten Oxide–Iron Oxide Thin Film Material Library

A WO3-Fe2O3 thin film combinatorial library was fabricated using a vapor phase co-deposition method followed by a combined thermal annealing and oxidation process. The scanning electron microscopy (SEM) analysis of the library microstructure combined with X-ray diffraction (XRD) investigations suggested that α-Fe2O3 grains preferentially grow from boundaries of domains, containing finer grains of WO3 and Fe2WO6, forming filiform networks on the surface. The surface density of the hematite networks depends on the amount of Fe present in the library. Photocurrents measured at different applied biases using Photo Electrochemical Scanning Droplet Cell Microscopy (PE-SDCM) were analyzed and mapped along the entire compositional spread. A distinctive photocurrent peak was detected at 21.9 atom % Fe, and its appearance was correlated to the higher amount of hematite present in the library at this specific composition together with a specific WO3 crystallographic orientation ((222) orthorhombic or (400) monoclinic). This finding is confirmed by qualitative and quantitative XPS surface analysis at the photocurrent peak position in the material library. Thus the enhancement of the photocurrent cannot be exclusively attributed to certain surface modifications since only hematite was found on the library surface at the peak composition.

Layer-by-Layer Assembly of Graphene Oxide and a Ru(II) Complex and Significant Photocurrent Generation Properties

The multilayer films were fabricated by layer-by-layer electrostatically coassembling graphene oxide and a ruthenium complex of [Ru(bpy)2L](ClO4)2 {L = 2-(2,6-di(pyridin-2-yl)pyridine-4-yl)-1H-imidazo[4,5-f]-1,10-phenanthroline} and characterized using UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and cyclic voltammetry. The dependence of redox properties and cathodic photocurrents on the number of layers deposited and the photocurrent generation mechanism and polarity were studied in detail. The homogeneous growth and close packing of the two film-forming components, linear relationships of the dark cyclic voltometry peak currents and photocurrents vs number of layers deposited, and large cathodic photocurrent density of 4.1 μA/cm(2) for a four-layer film make this novel hybrid thin film promising applications ranging from molecular photovoltaic and photocatalytic molecular devices to photoelectrochemical sensing.