Negative Differential Photoconductivity Research Articles

Negative Differential Photoconductance as a Signature of Nonradiative Energy Transfer in van der Waals Heterojunction.

The physical proximity of layered materials in their van der Waals heterostructures (vdWhs) aids interfacial phenomena such as charge transfer (CT) and energy transfer (ET). Besides providing fundamental insights, CT and ET also offer routes to engineer optoelectronic properties of vdWhs. For example, harnessing ET in vdWhs can help to overcome the limitations of optical absorption imposed by the ultra-thin nature of layered materials and thus provide an opportunity for in situ enhancement of quantum efficiency for light-harvesting and sensing applications. While several spectroscopic studies on vdWhs probed the dynamics of CT and ET, the possible contribution of ET in the photocurrent generation remains largely unexplored. In this work, we investigate the role of nonradiative energy transfer (NRET) in the photocurrent through a vertical vdWh of SnSe2/MoS2/TaSe2. We observe an unusual negative differential photoconductance (NDPC) arising from the existence of NRET across the SnSe2/MoS2 junction. Modulation of the NRET-driven NDPC characteristics with optical power results in a striking transition of the photocurrent's power law from a sublinear to a superlinear regime. Our observations reveal the nontrivial influence of ET on the photoresponse of vdWhs, which offer insights to harness ET in synergy with CT for vdWh based next-generation optoelectronics.

Coexistence of room-temperature negative differential resistance, negative photoconductivity modulated by UV light intensity and optical switching in Cu/Ni:ZnO/InGa structure

Cu/Ni:ZnO/InGa structure has been fabricated by depositing the chemically synthesized Ni:ZnO nanoparticles on glass substrate. The device shows a negative differential resistance (NDR) effect and negative photoconductivity (NPC) at room temperature. The presence of NDR is attributed to the electrons trapped/detrapped by interface states at Cu/Ni:ZnO junction. The origin of NPC is understood in term of competition between carriers injection due to the external field and photocarriers due to the external optical signal. Thus, these two effects (NDR and NPC) contribute to the optical switching process found in our device.

Photo-Hall-Effect Spectroscopy with Enhanced Illumination in p−Cd1−xMnxTe Showing Negative Differential Photoconductivity

We studied deep levels (DLs) in p-type CdMnTe by photo-Hall effect spectroscopy with enhanced illumination. We showed that the mobility of minority and majority carriers can be deduced directly from the spectra by using proper wavelength and excitation intensity. Four deep levels with ionization energies 0.63 eV, 0.9 eV, 1.0 eV and 1.3 eV were detected and their positions in the bandgap were verified by comparison of photogenerated electron and hole concentrations. Deduced DL model was analyzed by numerical simulations with Shockley-Reed-Hall charge generation-recombination theory and compared with alternative DL models differing in the position of selected DLs relative to Ec and Ev. We showed that the consistent explanation of collected experimental data principally limits the applicability of alternative DLs models. We also demonstrated the importance of the extended operation photon fluxes used in the spectra acquisition for correct determination of DLs character. Negative differential photoconductivity was observed and studied by charge dynamic theoretical simulations.

Dual-wavelength photo-Hall effect spectroscopy of deep levels in high resistive CdZnTe with negative differential photoconductivity

Photo-Hall effect spectroscopy was used in the study of deep levels in high resistive CdZnTe. The monochromator excitation in the photon energy range 0.65–1.77 eV was complemented by a laser diode high-intensity excitation at selected photon energies. A single sample characterized by multiple unusual features like negative differential photoconductivity and anomalous depression of electron mobility was chosen for the detailed study involving measurements at both the steady and dynamic regimes. We revealed that the Hall mobility and photoconductivity can be both enhanced and suppressed by an additional illumination at certain photon energies. The anomalous mobility decrease was explained by an excitation of the inhomogeneously distributed deep level at the energy Ev + 1.0 eV, thus enhancing potential non-uniformities. The appearance of negative differential photoconductivity was interpreted by an intensified electron occupancy of that level by a direct valence band-to-level excitation. Modified Shockley-Read-Hall theory was used for fitting experimental results by a model comprising five deep levels. Properties of the deep levels and their impact on the device performance were deduced.

Negative photoconductivity in films of alloys of II–VI compounds

Various negative photoelectric effects in films of alloys of II–VI compounds deposited from a solution are studied depending on the deposition mode and heat treatment. A combined electronic-molecular mechanism of the negative photocapacitance effect, which is for the first time found in Cd1 − xZnxS and CdS1 − xSex films, and negative slowly relaxing photoelectric effects is established. The latter are caused by the transition of electrons arranged in a nanoscale surface layer from shallow energy levels of attachment centers to deeper levels with lower polarizability and by the presence of nanoscale clusters playing the role of a “reservoir” for minority carriers in these materials. A model, which makes it possible to interpret the main regularities of negative photoconductivity in Cd1 − xZnxS and CdS1 − xSex films deposited from a solution, is suggested. It is established that the negative residual photoconductivity is explained on the basis of a double barrier profile, while the negative differential photoconductivity is explained by the presence of nanoscalel electric domains.

Photocurrent saturation and negative differential photoconductivity in Mn4Si7-Si〈Mn〉-Mn4Si7 and Mn4Si7-Si〈Mn〉-M heterojunctions

A mechanism behind the saturation of the photocurrent and occurrence of negative differential photoconductivity in Mn4Si7-Si〈Mn〉-Mn4Si7 and Mn4Si7-Si〈Mn〉-M heterojunctions is found. Mn4Si7-Si〈Mn〉-Mn4Si7 and Mn4Si7-Si〈Mn〉-M structures are studied with a model of back-to-back diodes. Photocurrent-voltage characteristics are taken at high constant and pulsed applied biases. It is found that the nonlinearity of the photocurrent-voltage characteristics and photoconductivity kinetics are due to the quenching of photoconductivity by Joule self-heating.

Negative Differential Photoconductance in Gold Nanoparticle Arrays in the Coulomb Blockade Regime

We investigate the photoconductance of gold nanoparticle arrays in the Coulomb blockade regime. Two-dimensional, hexagonal crystals of nanoparticles are produced by self-assembly. The nanoparticles are weakly coupled to their neighbors by a tunneling conductance. At low temperatures, the single electron charging energy of the nanoparticles dominates the conductance properties of the array. The Coulomb blockade of the nanoparticles can be lifted by optical excitation with a laser beam. The optical excitation leads to a localized heating of the arrays, which in turn gives rise to a local change in conductance and a redistribution of the overall electrical potential in the arrays. We introduce a dual-beam optical excitation technique to probe the distribution of the electrical potential in the nanoparticle array. A negative differential photoconductance is the direct consequence of the redistribution of the electrical potential upon lifting of the Coulomb blockade. On the basis of our model, we calculate the optically induced current from the dark current-voltage characteristics of the nanoparticle array. The calculations closely reproduce the experimental observations.

Photo and dark current noise in self-assembled quantum dot infrared photodetectors

Current noise is investigated in InAs/GaAs self-assembled quantum dot infrared photodetectors in the dark and under irradiation at T = 4.2 K . At low temperature, the noise is consistent with a mechanism of fluctuations driven by the electric field, related to field- and photon-assisted tunneling rather than trapping–detrapping of charge carriers from the quantum dots. In particular, a strong noise suppression effect determines the decrease of the fluctuation intensity as the voltage increases in the negative differential photoconductivity region of the I– V characteristics. The noise suppression mechanism acts in the dark and under irradiation. The noise intensity decreases consistently with the occurrence of the nonlinear differential photoconductivity effect.

Theoretical Study on the Photocurrent of a ZnO-based Quantum Dot Photodetector

ZnO has great potential for an UV photodetector. Many researchers have tried to fabricate a ZnO-based quantum dot (QD) photodetector. A theoretical model is proposed to evaluate the photocurrent of a ZnO-based quantum dot photodetector. The results predict that a ZnO QD photodetector outputs a increasing photocurrent with increasing the voltage and a maximum photocurrent at a voltage. For higher voltage, it exhibits a negative differential photoconductivity. Also, the non-uniformity of the QDs and QDs of too small size will reduce the performance of the photodetector.

Gate Controlled Negative Differential Resistance and Photoconductivity Enhancement in Carbon Nanotube Addressable Intra-connects

ABSTRACTWe have observed gate-controlled N-shaped negative differential resistance (NDR) and photoconductivity enhancement in carbon nanotube (CNT) based addressable intra-connects. The intra-connects – bridges spanning across planar electrodes – were measured at room temperature. Individual single-walled CNT (SWCNT) channels were grown using chemical vapor deposition (CVD) precisely between very sharp metal tips on the pre-fabricated electrodes without post-processing. The electrodes were made of cobalt. Methane and H2 gas mixture were introduced into quartz tube for an hour at 900 C, with flow rate of 1900 sccm for methane and 20 sccm for H2. We have investigated two different cases: in one case, the source-drain current-voltage, Ids-Vds, characteristics were linear. The other case exhibited nonlinear Ids-Vds characteristics. Raman scattering of the intra-connects indicated that each were made of SWCNT with radial breathing mode (RBM) at 191.9 cm-1 and 176.2 cm-1, respectively. Current-voltage Ids-Vds characteristics were measured for various Vgs from -10 V to +10 V. Negative differential resistance (NDR) was found in the Ids-Vgs curves for gate bias in the region of -3>Vgs>-6 V. The NDR peak was shifted to the negative side as the source-drain voltage was increased from Vds=0 to 0.75 V. Otherwise, the intra-connects exhibited characteristics of an ordinary p-type channel. The experiments were repeated under white light illumination. The light increases the carrier density in the channel but not in the metal electrodes and allowed us to study the effective doping of the channel without affecting the work function of the SWCNT/metal contact. The overall channel conductance increased under the light irradiation. Under illumination, the devices became more stable, as well. In summary, we have investigated contact properties between a SWCNT intra-connect and metal electrodes in a well controlled layout settings.

Carrier transport in Ge nanowire/Si substrate heterojunctions

Low impedance and negligible conductivity temperature dependence are found for micron-long Ge nanowires (NWs) grown on (p+)Si substrates. In contrast, Ge NW/(n+)Si substrate samples exhibit many orders of magnitude higher impedance, an exponential dependence of conductivity on temperature, current instabilities, and negative differential photoconductivity. Our experimental results are explained by a model that considers energy-band alignment and carrier transport in abrupt Ge NW/Si substrate heterojunctions.

Wannier-Stark ladder conditions in 4H-SiC p-n junctions grown on off axis substrates

4 H - Si C p+-n−-n+ junctions have been prepared on 8° off oriented plane aiming to investigate conditions of Wannier-Stark localization (WSL), and respectively the occurrence of a negative temperature coefficient of avalanche breakdown voltage (TCABV). A method is proposed that can enable obtaining important results without reaching the regime of destructive avalanche breakdown. By analyzing the photocurrent of light with different wavelengths and at strong electric fields, a negative differential photoconductance has been discovered testifying that the WSL is not suppressed at the field inclination in 8° off axis, and a negative TCABV is expected in commercial p-n junctions.

The Wannier-Stark effects in the 6H-SiC planar junction field-effect transistors with a p-n junction as the gate

Dependence of the short-circuit photocurrent on the voltage V g applied to the gate of the 6H-SiC planar field-effect transistor is studied. The negative differential photoconductivity appeared at a certain value of V g; the parameters of this photoconductivity corresponded to those of the Wannier-Stark ladders in the natural 6H-SiC super lattice. At the same value of V g, a fairly abrupt decrease to zero of the source-drain current I sd is observed, which is indicative of cutoff at the voltage that is much lower than the expected cutoff voltage for this structure. The effect is attributed to a decrease in mobility in the mode of the Wannier-Stark ladders, a decrease in the rate of ionization of the donor atoms, and a reduction in the screening of the field.

Domain pinning in GaAs∕AlGaAs quantum well infrared photodetectors

We have analyzed the spatial distribution of electric field domains induced by negative differential photoconductivity in n-type GaAs∕AlGaAs quantum well infrared photodetectors. We find strong evidence of two different domain configurations, with the high-field domain and the low-field domain, respectively, adjacent to the emitter contact. A distinctive signature of these domain configurations is provided by the observed total current, which is observed to be close to either the valley current or the peak current. We also discuss the emergence of the two configurations.

Effects of electron correlation on the photocurrent in quantum dot infrared photodetectors

The effect of electron correlation on the photocurrent of self-assembled InAs/InGaAs quantum-dot infrared photodetector is studied. It is found that Coulomb interaction and level mixing in the many-body open system lead to double peaks associated with the intraband transitions involving two lowest levels of the quantum dot. Furthermore, the photocurrent displays a negative differential photoconductance due to Coulomb blockade.

Negative differential infrared photoconductivity in quantum-dot structures

Using a simple model for monopolar quantum-dot structures which takes into account the heating of mobile (unbound) electrons, we calculated the current–voltage characteristics of these structures under infrared illumination and revealed the origin of the effect of negative differential infrared photoconductivity observed in recent experiments. It is clarified also why decreasing photocurrent–voltage characteristics coexist with steep rising dark current–voltage characteristics.

Negative differential photoconductivity in quantum-dot infrared photodetectors

We present a simple model for quantum-dot infrared photodetectors (QDIPs) describing nontrivial (decreasing) dependences of the photocurrent on the QD density and the applied voltage. It is shown that recent experiments demonstrating a negative differential photoconductivity in QDIPs can be interpreted in terms of this model. The effects under consideration can be attributed to the repulsive potential of charged quantum dots and heating of mobile electrons influencing the rate of the electron capture. Qualitative distinctions between the QDIP photocurrent–voltage and dark current–voltage characteristics are explained as well.

Characteristics of InGaAs quantum dot infrared photodetectors

A quantum dot infrared photodetector (QDIP) consisting of self-assembled InGaAs quantum dots has been demonstrated. Responsivity of 3.25 mA/W at 9.2 μm was obtained for nonpolarized incident light on the detector with a 45° angle facet at 60 K. The QDIPs exhibit some unique electro-optic characteristics such as a strong negative differential photoconductance effect and blueshift of the response peak wavelength.

Bistability and negative photoconductivity in optically induced real-space transfer.

Optically induced charge transport in a modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructure is studied theoretically. Intersubband excitation of electrons in the GaAs quantum well due to infrared absorption leads to their real-space transfer into the adjacent ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As layer. We derive a system of nonlinear model equations considering as relevant physical mechanisms the generation-recombination processes in GaAs, longitudinal and transverse dielectric carrier relaxation, as well as resonant and nonresonant tunneling across the interface, and analyze its behavior with the methods of nonlinear dynamics. We predict optoelectronic bistability associated with different photoconductive responses between the two locally stable steady states corresponding to different types of tunneling prevalent: the regime of dominant resonant-tunneling currents shows negative differential photoconductivity, whereas the nonresonant tunneling state is practically invariant with respect to changes of the IR intensity.

Superlattice surface-normal asymmetric Fabry-Perot reflection modulators: optical modulation and switching

The surface-normal asymmetric Fabry-Perot reflection modulators discussed are based on the electroabsorption stemming from the effective absorption edge blue shift caused by Wannier-Stark localization in superlattices. Due to the modulator's normally off characteristics at the Fabry-Perot resonance, they exhibit negative differential photoconductivity. When the modulator is connected to a similar modulator or a simple photodiode, the negative differential photoconductance at resonance enables the modulator to operate as a self-electrooptic effect device (SEED) that shows clear bistable loops, and also has very large on-off ratios at its output. The operating principle of these modulators and SEEDs, and their sensitivity to variations of operating wavelengths, layer thickness and composition, and temperature are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>