Anomalous Photoresponse Research Articles

Light‐Triggered and Polarity‐Switchable Homojunctions for Optoelectronic Logic Devices

AbstractSemiconductor homojunctions based on 2D materials are promising candidates for optoelectronic logic devices due to their excellent electrostatic tunability and photoelectric characteristic. However, to reach programmable logic functions, the 2D homojunctions usually suffer from compulsory combinations of electrical and optical inputs. Here, a light‐triggered and polarity‐switchable homojunction made from the vertically stacked MoTe2/CuInP2S6/Au layers in a dual‐floating gate transistor configuration, is demonstrated. Utilizing the band alignment between MoTe2 and CuInP2S6, the photocarriers in MoTe2 can be excited to Au layer across the insulating CuInP2S6 efficiently, serving as floating gate to modulate the polarity of the MoTe2 homojunctions which produce photocurrents simultaneously in photovoltaic mode. A logic gate XOR is achieved in one single device without the need for any applied voltage. Moreover, the devices exhibit both anomalous photoresponse and optical memory behaviors in photoconductive mode. The results provide a potential route for the development of optoelectronic logic devices with fully light‐actuated sensing capability.

Anomalous Photoresponse of Heavily Doped GaAs/AlAs Superlattices with Electric Domains

The strong effect of weak interband illumination on tunneling transport in doped GaAs/AlAs superlattices was found under conditions of electric domain formation. The photoresponse at voltages below the threshold one (before the domain formation) did not observe. The phenomenon is referred to strong carrier depletion inside the triangular high-field domain. The domain modes transformations under the illumination were also found.

Two-Dimensional Tellurium Nanosheets Exhibiting an Anomalous Switchable Photoresponse with Thickness Dependence.

Two-dimensional (2D) tellurium (Te) was recently predicted to be promising for diverse electronic and optoelectronic applications. However, the synthesis of high-quality 2D Te structures remains challenging, which greatly hinders the exploration of its full properties. Herein, an anomalous photoresponse from negative to positive as a function of thickness in Te nanosheets is reported. Ultrathin Te layers with large size and clean interface were obtained through a topotactic transformation, in which the 2D Te structure was derived from a layered MTe2 (M=Ti, Mo, W) matrix by excessive lithiation. Prominently, the photoresponse in Te nanosheets exhibits negative behavior when the thickness is less than 5 nm, which turns positive as the thickness increases. This unusual photoresponse will shed light on the full exploration of 2D non-layered materials with exotic properties.

Two‐Dimensional Tellurium Nanosheets Exhibiting an Anomalous Switchable Photoresponse with Thickness Dependence

AbstractTwo‐dimensional (2D) tellurium (Te) was recently predicted to be promising for diverse electronic and optoelectronic applications. However, the synthesis of high‐quality 2D Te structures remains challenging, which greatly hinders the exploration of its full properties. Herein, an anomalous photoresponse from negative to positive as a function of thickness in Te nanosheets is reported. Ultrathin Te layers with large size and clean interface were obtained through a topotactic transformation, in which the 2D Te structure was derived from a layered MTe2 (M=Ti, Mo, W) matrix by excessive lithiation. Prominently, the photoresponse in Te nanosheets exhibits negative behavior when the thickness is less than 5 nm, which turns positive as the thickness increases. This unusual photoresponse will shed light on the full exploration of 2D non‐layered materials with exotic properties.

Topological Insulator Based Dual State Photo‐Switch Originating Through Bulk and Surface Conduction Channels

Topological insulators are novel electronic materials, where time reversal symmetry protects the spin‐polarized surface states from backscattering. Although, the bulk offers a trivial semiconducting response to incident light, but the metallic surface states exhibit interesting electrical response towards the incident radiation, such as polarization dependent surface photocurrent and topological phase transitions. Here, we study the temperature dependent near‐infrared photoresponse in bismuth selenide (Bi2Se3) nano‐flakes. A very good photo‐sensitivity to near‐infrared 800 nm wavelength is observed for the temperature range 300–2 K. Voltage responsivity at 2 K (1.481 × 1010 VW−1) is estimated to be four‐order greater than at 300 K (2.095 × 106 VW−1) and is comparatively much higher than the previously reported values of voltage responsivity in other materials. Interestingly, we also find the temperature dependent existence of both positive and negative photoresponse in our device. An anomalous photoresponse reversal is observed at 2 K, where resistance of the nano‐flake increases under light illumination. We propose that this phenomenon is due to the topological surface states, which have dominant transport contribution at very low temperatures. These observations establish the fact that topological insulators can be further engineered to develop a dual state photo‐switch, by manipulating the bulk and surface state conduction contribution.

Photoresponse of composites of zinc oxide and poly(3-hexythiophene) under selective UV and white-light illumination

We investigate charge transport in UV sensing devices based on organic-inorganic semiconductor composites with the metal-semiconductor-metal (MSM) structure. Composite materials of zinc oxide (ZnO) nanoparticles and poly(3-hexylthiophene) (P3HT) were prepared by drop-casting their colloidal mixture in chloroform onto low-cost interdigitated copper electrodes. The current-voltage characteristics of the devices were investigated under both dark and illuminated conditions in the UV–visible range. The highest photoresponse was observed for an optimal P3HT:ZnO ratio of 1:8 w/w in the wavelength range between 310 and 380 nm. The dynamic response was investigated by pulsing a 365 nm UV light with a long period to reveal the response time of 4 s and the recovery time of less than 1 s. The photoresponse of the materials was also investigated for a shorter period of UV pulsing, using a rotating chopper. The response time and recovery time for the short UV pulse were found to be approximately 20 m and 25 m, respectively. The dual response times should stem from the presence of two types of semiconductor materials, namely ZnO with a high electron mobility and P3HT with a moderate hole mobility. To probe the charge generation and transport mechanisms, we further investigate the photoresponse using UV pulsing under background white light of different intensities, and vice versa. The background white light was found to deteriorate the UV photoresponse of the materials. On the other hand, the background UV illumination produced an anomalous photoresponse pattern with the white light pulsing. Understanding the charge transport mechanisms for composite materials is highly important for future applications in low-cost UV sensors and tunable optoelectronic devices.

Anomalous photoresponse in the deep-ultraviolet due to resonant excitonic effects in oxygen plasma treated few-layer graphene

Graphene holds great promise for novel optoelectronic devices due its exceptional optical properties. Here we report an anomalous photoresponse due to resonant excitonic effects in oxygen functionalized epitaxial graphene (EG) with gain exceeding 104 in the deep-ultraviolet (DUV) region. The method used to introduce the oxygen adatoms is via mild oxygen plasma treatment. Upon oxygen adsorption, spectral weight transfer occurs and a photoresponse emerges. The photoresponse in the oxygen plasma treated EG (OPTEG) is attributed to the enhancement of the resonant exciton in the DUV and suppression of optical conductivity below it, as evidenced in our spectroscopic ellipsometry measurements. Our result shows that the OPTEG produces large photoconductive gain, high selectivity, and detectivity in excess of 1012 Jones in the DUV spectral range and can be harnessed to fabricate tunable graphene-based high-gain DUV photodetectors.

Visible light initiated and collapsed resistive switching in TbMnO3/Nb:SrTiO3 heterojunctions

Simple oxide heterostructures have been fabricated by depositing TbMnO3 thin films on Nb:SrTiO3 substrates at different temperatures. Remarkable switching with an on/off resistance ratio of ~5000 is found in the sample grown at 720 °C, while only tiny resistive hysteresis can be observed in the sample grown at 650 °C. A jump switching in the I-V loop at the lower resistance state with negative bias is initiated by a visible light pulse in the sample grown at 650 °C, whereas a drop switching can be observed in the sample grown at 720 °C. A trapping-detrapping process along the TbMnO3/Nb:SrTiO3 interfaces is proposed to explain the anomalous photoresponse.

Semiconductor nanostructures in an alumina template matrix: micro- versus macro-scale photoelectrochemical behavior

We show herein that the photoelectrochemical behavior of a given semiconductor nanodot (p-CuSCN or n-TiO 2) in an alumina template matrix, is remarkably different than that of its macro-sized counterpart. Three separate examples of this distinct difference in behavior are presented. It is shown how the photoresponse (e.g. photocurrent) may be amplified (from a low level typical of the signal emanating from a ∼10 −11 cm 2 region corresponding to a semiconductor nanodot) by using a large number of electrically inter-connected Au nanowires to support the overlying semiconductor nanodots. The anomalous photoresponse of p-CuSCN nanodots in the template matrix was also numerically simulated by a simple parallel equivalent circuit consisting of a semiconductor and a photocapacitor. Possible practical application scenarios are finally presented for these nanostructures.

Anomalous photoresponse of thick bismuth films to pulsed laser excitation

Current-biased 4.3-μm Bi films give an anomalous photoresponse when they are illuminated with 20-ns 1.064-μm laser pulses. The films resistivity decreases during irradiation. This observation excludes a simple bolometric process as a cause for the induced transients and supports the prediction of a previously reported model which required the optical pumping of carriers into a metastable band.

Connection between the average hexagonality and anomalous photoresponse of faulted ZnS polytypes

Connection between the average hexagonality and anomalous photoresponse of faulted ZnS polytypes

Anomalous photoresponse of metal films

Current biased samples are illuminated by short pulses of laser light and voltages are measured for a range of temperature 7 K<T<350 K. Classically, no response is expected for times greater than 10 ps after the pulse arrival. However, transient signals with temperature-independent decay times are observed in Ni and Nb samples, and a similar signal with diminished amplitude is observed in a Au sample.

Defect Heterojunction Model for Anomalous Photoresponse of P/N GaAs Grown on N-Ge Substrates

ABSTRACTA GaAs p/n junction grown on a n-Ge substrate has an anomalous photoresponse that includes a quantum yield response starting at the 0.7-eV band edge of Ge. In terrestrial sunlight, its I-V characteristic has an anomalous “notched” shape that is progressively removed by increasing the infrared light content. The I-V is well modeled by a space-charge-limited-current diode theory with a GaAs/Ge interface defect density of 1.92 (1012) cm−2 eV−1 at 0.4 eV below the conduction band edge. Similar defect densities have been reported for GaAs/oxide interfaces and for a GaAs p/n junction interface specially selected for high defect densities.

Some physical properties of the TiO 2 semiconductor electrode

Some physical properties of n-TiO 2 single crystals have been studied by electrochemical and photoelectrochemical techniques. X-ray diffractometry confirmed the TiO 2 basic rutile type structure. For the TiO 2/0.5 N NaOH system in the dark a Tafel slope of 0.11 V/decade was observed between 0.50 and 0.61 V versus SCE, suggesting a one-electron process. The potential, at which the gaseous oxygen evolution was confirmed, was well over 2.0 V versus SCE. On the other hand the potential for oxygen evolution was markedly decreased to −0.75 V versus SCE, when the TiO 2 was illuminated. From these facts and the observation of passivation, transpassivation and current saturation an oxygen-pumping mechanism is proposed for the polarization behaviour of the TiO 2 electrode in the dark. The spectral responses of photocurrent generated from TiO 2 at 0.0 V bias versus SCE showed that the peak illumination current at 360 nm is larger than the dark current by three orders of magnitude. The band gap of TiO 2 was determined as 3.1 eV by assuming that the photoelectrolysis of water involves an indirect energy transition. An anomalous photoresponse was observed at 1.8 eV, which is due to the presence of surface states at the TiO 2/electrolyte interface.

Photothermal effect at TiO2 electrodes in a photoelectrochemical cell

Anomalous photoresponse of n-TiO2 in a photoelectrochemical cell was previously observed to be induced by light of energy lower than the bandgap. This photoresponse was attributed to surface states at the TiO2-electrolyte interface. We studied this effect at stationary and rotating disk TiO2 electrodes and we show that the anomalous photoresponse is a thermal effect due to heating of the crystal surface by the illumination.