Characteristics Of Heterojunctions Research Articles

Solution-processed, hybrid 2D/3D MoS2/Si heterostructures with superior junction characteristics

We report a theoretical and experimental investigation of the hybrid heterostructure interfaces between atomically thin MoS2 nanocrystals (NCs) on Si platform for their potential applications towards next-generation electrical and optical devices. Mie theory-based numerical analysis and COMSOL simulations based on the finite element method have been utilized to study the optical absorption characteristics and light–matter interactions in variable-sized MoS2 NCs. The size-dependent absorption characteristics and the enhancement of electric field of the heterojunction in the UV-visible spectral range agree well with the experimental results. A lithography-free, wafer-scale, 2D material on a 3D substrate hybrid vertical heterostructure has been fabricated using colloidal n-MoS2 NCs on p-Si. The fabricated p-n heterojunction exhibited excellent junction characteristics with a high rectification ratio suitable for voltage clipper and rectifier applications. The current–voltage characteristics of the devices under illumination have been performed in the temperature range of 10–300 K. The device exhibits a high photo-to-dark current ratio of ∼3 × 103 and a responsivity comparable to a commercial Si photodetector. The excellent heterojunction characteristics demonstrate the great potential of MoS2 NC-based hybrid electronic and optoelectronic devices in the near future.

Interface engineering for enhancement in performance of organic/inorganic hybrid heterojunction diode

Interface engineering through self-assembled monolayer (SAM) is an efficient way for tailoring the work function and electronic property of surface; enhancing the charge injection efficiency and device performance for microelectronics applications. Despite this, there is lack of study on effect of interface engineering of organic/inorganic hybrid heterojunction diode through SAM. Here, we have reported the surface engineering for tailoring the surface work function, electronic property, enhancement in injection efficiency and device performance. Therefore, Zinc Oxide (ZnO) film surface was modified with SAM before formation of hybrid ZnO/poly(3-hexylthiophene) (P3HT) heterojunction diode and compared with unmodified ZnO/P3HT diode. Prior to measurement of J-V heterojunction characteristics, both interfaces were characterized using absorption spectra, grazing incidence X-ray diffraction (GIXD), scanning electron microscopy (SEM), atomic force microscopy (AFM), kelvin probe force microscopy (KPFM). The modification of ZnO with SAM prior to heterojunction formation allows the better fabrication of diodes featuring ∼10 fold enhancement in rectification ratio at ±3 V and ∼32 fold enhancement in forward current density at 3 V with advancement in electronic device parameter. The enhancement in electrical characteristics are also discussed taking into account the absorption spectra, structural analysis, surface morphology, topography, surface potential, barrier height, and the energy band diagram of SAM's modified and unmodified diodes. Our study has cemented a path to further improve the device performance and parameter for electronic applications.

Enhanced photoluminescence and heterojunction characteristics of pulsed laser deposited ZnO nanostructures

We report on the growth of ZnO nanostructures in different gas ambient (Ar and N2) using pulsed laser deposition technique. Despite the similar growth temperature, use of N2 ambient gas resulted in well-aligned nanorods with flat surface at the tip, whereas, nanorods grown with Ar ambient exhibited tapered tips. The Nanorods grown under N2 ambient exhibited additional Raman modes corresponding to N induced zinc interstitials. The nanorods are c-axis oriented and highly epitaxial in nature. Photoluminescence spectroscopy reveals that the UV emission can be significantly enhanced by 10 times for the nanorods grown under Ar ambient. The enhanced UV emission is attributed to the reduction in polarization electric field along the c-axis. n-ZnO nanorods/p-Si heterojunction showed rectifying I–V characteristics with a turn of voltage of 3.4V.

Band offset formation at semiconductor heterojunctions through density-based minimization of interface energy

It is well known that the magnitude of band offset (BO) at any semiconductor heterojunction is directly derivable from the distribution of charge at that interface and that the latter is decided by a minimization of total energy. However, the fact that BO formation is governed by energy minimization has not been explicitly used in theoretical BO models, likely because the equilibrium charge densities at heterojunction interfaces appear difficult to predict, except via explicit calculation. In this paper, electron densities at a large number of (100), (110), and (111) oriented heterojunctions between lattice-matched, isovalent semiconductors with the zinc blende (ZB) structure have been calculated by first-principles methods and analyzed in detail for possible common characteristics among energy-minimized densities. Remarkably, the heterojunction electron density was found to largely depend only on the immediate, local atomic arrangement. In fact, it is so much so that a juxtaposition of local electron-densities generated in oligo-cells (LEGOs) accurately reproduced the charge densities that minimize the energy for the heterojunctions. Furthermore, the charge distribution for each bulk semiconductor was found to display a striking separability of its electrostatic effect into two neutral parts, associated with the cation and the anion, which are approximately transferrable among semiconductors. These discoveries form the basis of a neutral polyhedra theory (NPT) that approximately predicts the equilibrium charge density and BO of relaxed heterojunctions from the energy minimization requirement. Well-known experimentally observed characteristics of heterojunctions, such as the insensitivity of BO to heterojunction orientation and the identity of interface bonds, the transitivity rule, etc., are all in good agreement with the NPT. Therefore, energy minimization, which essentially decides the electronic properties of all other solid and molecular systems, also governs the formation of the charge density at these heterojunction interfaces. In particular, the approach presented here eliminates the need to invoke mechanisms that are specific to semiconductor interfaces.

Photovoltaic and impedance spectroscopic characteristics of heterojunction of graphene-PEDOT:PSS composite and n-silicon prepared via solution-based process

Recent investigations involving nanoscale energy conversion using two-dimensional nanomaterials such as graphene and transition metal dichalcogenides (TMDs) hold promise to mitigate future energy challenges. The heterojunction devices designed by interfacing 2D layers with 3D bulk semiconductors (2D/3D heterojunctions) including graphene/silicon and TMDs/silicon are widely explored for the photovoltaic characteristics. Developing a thorough understanding of the interfacial chemistry via impedance spectroscopic analysis will leverage the potential of these 2D/3D junctions for large-scale integrations. Here, the non-vacuum solution-processed reduced graphene oxide (rGO), poly(3,4-ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and their composite (PEDOT:PSS(rGO)) were spin coated on the silicon (n-Si) substrates for fabrication of heterojunctions, with the device construct of rGO/n-Si, PEDOT:PSS/n-Si and PEDOT:PSS(rGO)/n-Si having Ohmic metal contacts in both top and bottom. The significant efficiency improvement of three orders for PEDOT:PSS(rGO)/n-Si device over the rGO/n-Si and PEDOT:PSS/n-Si heterojunction devices can be attributed to (i) increased conducting channels in the active region and (ii) reduced series resistance. Further, the impedance spectroscopy is employed to understand the interfacial chemistry of PEDOT:PSS(rGO)/n-Si solid-state junction, which is explained by a parallel resistance–capacitance circuit model.

Characterization of n-Type Nanocrystalline Iron Disilicide/Intrinsic Ultrananocrystalline Diamond/Amorphous Carbon Composite/p-Type Silicon Heterojunction Photodiodes

We prepared n-type nanocrystalline iron disilicide (NC-FeSi2)/intrinsic (i) ultrananocrystalline diamond/amorphous carbon composite (UNCD/a-C)/p-type Si heterojunctions and evaluated as photodiodes. UNCD/a-C and NC-FeSi2 films were deposited by coaxial arc plasma deposition and pulsed laser deposition, respectively. The capacitance-voltage and current-voltage characteristics of heterojunctions were measured at room temperature. The inserted i-UNCD/a-C layer to form pin heterojunctions reduced the capacitance and dark current as compared with those in the case of pn heterojunctions. The build-in potential of heterojunctions was estimated to be 1.2 eV. The prepared heterojunctions showed typical rectifying action and a response for an illumination with a 6 mW, 1.31 μm laser. The recombination process is the predominant mechanism of current transport in the heterojunctions. The dynamic resistance area product and detectivity were 1.54 × 103 Ω cm2 and 5.0 × 108 cmHz1/2/W at-1 V. The evident improvement in the device performance was demonstrated, which should be due to the reduction of dark current by i-UNCD/a-C layer.

N-type conversion of SnS by isovalent ion substitution: Geometrical doping as a new doping route.

Tin monosulfide (SnS) is a naturally p-type semiconductor with a layered crystal structure, but no reliable n-type SnS has been obtained by conventional aliovalent ion substitution. In this work, carrier polarity conversion to n-type was achieved by isovalent ion substitution for polycrystalline SnS thin films on glass substrates. Substituting Pb2+ for Sn2+ converted the majority carrier from hole to electron, and the free electron density ranged from 1012 to 1015 cm−3 with the largest electron mobility of 7.0 cm2/(Vs). The n-type conduction was confirmed further by the position of the Fermi level (EF) based on photoemission spectroscopy and electrical characteristics of pn heterojunctions. Density functional theory calculations reveal that the Pb substitution invokes a geometrical size effect that enlarges the interlayer distance and subsequently reduces the formation energies of Sn and Pb interstitials, which results in the electron doping.

Microstructure, wettability and electrical properties of n-ZnO/ZnO-SL/p-Cu2O heterojunction

ZnO/ZnO-SL/Cu2O heterojunction was successfully fabricated on Cu foil by hydrothermal and sol–gel method. Crystalline phase, surface morphology and photoluminescence spectrum of the heterojunction were measured by X-ray diffraction, scanning electronic microscopy and fluorescence spectrometer, respectively. Contact angles of ZnO thin films were measured using a contact angle apparatus. The current–voltage characteristics of heterojunction in dark were investigated at room temperature. The results indicated that the heterojunction was composed of Cu2O phase and ZnO phase. The post-heat temperature of 250 °C was beneficial to preferred growth of ZnO thin film along the c-axis direction. The Sample 2 was composed of the uniform, compact and vertically aligned ZnO nanorods and has the most obvious hydrophobic-to-hydrophilic transition. The transport characteristics of the Sample 2 may be related to the space-charge-limited current in region II and a trapped-charge-limited current in region III.

ANNEALING TIME EFFECT ON NANOSTRUCTURED n-ZnO/p-Si HETEROJUNCTION PHOTODETECTOR PERFORMANCE

In this work, n- ZnO /p- Si heterojunction photodetectors were prepared by drop casting of ZnO nanoparticles (NPs) on single crystal p-type silicon substrates, followed by (15–60) min; step-annealing at 600∘C. Structural, electrical, and optical properties of the ZnO NPs films deposited on quartz substrates were studied as a function of annealing time. X-ray diffraction studies showed a polycrystalline, hexagonal wurtizte nanostructured ZnO with preferential orientation along the (100) plane. Atomic force microscopy measurements showed an average ZnO grain size within the range of 75.9 nm–99.9 nm with a corresponding root mean square (RMS) surface roughness between 0.51 nm–2.16 nm. Dark and under illumination current–voltage (I–V) characteristics of the n- ZnO /p- Si heterojunction photodetectors showed an improving rectification ratio and a decreasing saturation current at longer annealing time with an ideality factor of 3 obtained at 60 min annealing time. Capacitance–voltage (C–V) characteristics of heterojunctions were investigated in order to estimate the built-in-voltage and junction type. The photodetectors, fabricated at optimum annealing time, exhibited good linearity characteristics. Maximum sensitivity was obtained when ZnO / Si heterojunctions were annealed at 60 min. Two peaks of response, located at 650 nm and 850 nm, were observed with sensitivities of 0.12–0.19 A/W and 0.18–0.39 A/W, respectively. Detectivity of the photodetectors as function of annealing time was estimated.

Investigation of band-offsets at monolayer-multilayer MoS₂ junctions by scanning photocurrent microscopy.

The thickness-dependent band structure of MoS2 implies that discontinuities in energy bands exist at the interface of monolayer (1L) and multilayer (ML) thin films. The characteristics of such heterojunctions are analyzed here using current versus voltage measurements, scanning photocurrent microscopy, and finite element simulations of charge carrier transport. Rectifying I-V curves are consistently observed between contacts on opposite sides of 1L/ML junctions, and a strong bias-dependent photocurrent is observed at the junction. Finite element device simulations with varying carrier concentrations and electron affinities show that a type II band alignment at single layer/multilayer junctions reproduces both the rectifying electrical characteristics and the photocurrent response under bias. However, the zero-bias junction photocurrent and its energy dependence are not explained by conventional photovoltaic and photothermoelectric mechanisms, indicating the contributions of hot carriers.

Observation of a photoinduced, resonant tunneling effect in a carbon nanotube-silicon heterojunction.

A significant resonant tunneling effect has been observed under the 2.4 V junction threshold in a large area, carbon nanotube–silicon (CNT–Si) heterojunction obtained by growing a continuous layer of multiwall carbon nanotubes on an n-doped silicon substrate. The multiwall carbon nanostructures were grown by a chemical vapor deposition (CVD) technique on a 60 nm thick, silicon nitride layer, deposited on an n-type Si substrate. The heterojunction characteristics were intensively studied on different substrates, resulting in high photoresponsivity with a large reverse photocurrent plateau. In this paper, we report on the photoresponsivity characteristics of the device, the heterojunction threshold and the tunnel-like effect observed as a function of applied voltage and excitation wavelength. The experiments are performed in the near-ultraviolet to near-infrared wavelength range. The high conversion efficiency of light radiation into photoelectrons observed with the presented layout allows the device to be used as a large area photodetector with very low, intrinsic dark current and noise.

Pn-Heterojunction effects of perylene tetracarboxylic diimide derivatives on pentacene field-effect transistor.

We investigated the heterojunction effects of perylene tetracarboxylic diimide (PTCDI) derivatives on the pentacene-based field-effect transistors (FETs). Three PTCDI derivatives with different substituents were deposited onto pentacene layers and served as charge transfer dopants. The deposited PTCDI layer, which had a nominal thickness of a few layers, formed discontinuous patches on the pentacene layers and dramatically enhanced the hole mobility in the pentacene FET. Among the three PTCDI molecules tested, the octyl-substituted PTCDI, PTCDI-C8, provided the most efficient hole-doping characteristics (p-type) relative to the fluorophenyl-substituted PTCDIs, 4-FPEPTC and 2,4-FPEPTC. The organic heterojunction and doping characteristics were systematically investigated using atomic force microscopy, 2D grazing incidence X-ray diffraction studies, and ultraviolet photoelectron spectroscopy. PTCDI-C8, bearing octyl substituents, grew laterally on the pentacene layer (2D growth), whereas 2,4-FPEPTC, with fluorophenyl substituents, underwent 3D growth. The different growth modes resulted in different contact areas and relative orientations between the pentacene and PTCDI molecules, which significantly affected the doping efficiency of the deposited adlayer. The differences between the growth modes and the thin-film microstructures in the different PTCDI patches were attributed to a mismatch between the surface energies of the patches and the underlying pentacene layer. The film-morphology-dependent doping effects observed here offer practical guidelines for achieving more effective charge transfer doping in thin-film transistors.

Correlation of Interfacial Transportation Properties of CdS/CdTe Heterojunction and Performance of CdTe Polycrystalline Thin-Film Solar Cells

The light and dark output performances of CdS/CdTe solar cells made by close-spaced sublimation (CSS) were investigated to elucidate the transportation properties of carriers at CdS/CdTe heterojunction interface. It has been found that the interfacial transportation properties were relatively sensitive to variations of the characteristics of heterojunction due to the series resistance and shunting effects. For the high quality cell with 12.1% efficiency, narrow depletion region of ~1.1 microns and large electric field intensity of ~1.3 V/μm allow the sufficient energy-band bending close to CdS layer at CdS/CdTe heterojunction, which changes the carrier transportation mechanism from emission to diffusion and leads to the optimal rectifying characteristics with small dark saturation current density ~6.4 × 10−10 A/cm2. As a result, the schematic diagram of heterojunction band structure corresponding to various performances of solar cells has also been presented.

Simulation of carbon nanotubes/GaAs hybrid photovoltaic using AMPS-1D

The performance and characteristics of a hybrid heterojunction single-walled carbon nanotube/GaAs solar cell were modelled and numerically simulated using the AMPS-1D device simulation tool. The device physics and performance parameters with different junction parameters were analysed. The results suggest that the open-circuit voltage changes very slightly by changing the work function, acceptor and donor density, while the other electrical parameters reach an optimum value. Increasing the concentration of a discrete defect density in the absorber layer decreases the electrical parameters. The current–voltage characteristics, quantum efficiency, band gap and thickness variation of the photovoltaic response will be quantitatively considered.

An analytical model for analyzing the current-voltage characteristics of bulk heterojunction organic solar cells

An analytical model for analyzing the current-voltage (J-V) characteristics of bulk heterojunction (BHJ) organic solar cells is developed by incorporating exponential photon absorption, dissociation efficiency of bound electron-hole pairs (EHPs), carrier trapping, and carrier drift and diffusion in the photon absorption layer. Modified Braun's model is used to compute the electric field-dependent dissociation efficiency of the bound EHPs. The charge carrier concentrations and hence the photocurrent are calculated by solving the carrier continuity equation for both holes and electrons in the organic layer. The overall load current is calculated considering the actual solar spectrum and voltage dependent forward dark current. The model is verified by published experimental results. The efficiency of the P3HT:PCBM based solar cells critically depends on the dissociation of bound EHPs. On the other hand, cells made of a blend of the conjugated polymer (PCDTBT) with the soluble fullerene derivative (PCBM) show nearly unity dissociation efficiency, and their cell efficiency strongly depends on the charge collection efficiency. The effects of carrier lifetimes on the performance of PCDTBT solar cells have also been studied. The model is also used to investigate the effect of titanium oxide (TiOx) layer (at the back contact) on the J-V characteristics of PCDTBT solar cells. The results of this paper indicate that improvement of charge carrier transport in PCDTBT:PCBM blend and dissociation of bound EHPs in P3HT:PCBM blend are extremely important to increase the power conversion efficiency of the respective BHJ solar cells.

Sol‐gel undoped and Al‐doped ZnO films crystallized under an electric field and the ZnO/Si heterojunction characteristics

AbstractEffect of electric field on the crystallization of ZnO sol‐gel films was investigated by measuring x‐ray diffraction (XRD) patterns, photoluminescence (PL) spectra, and current‐voltage (I‐V) characteristics of n‐ZnO/p‐Si heterojunctions. When sol‐gel ZnO films (undoped and Al‐doped) were crystallized at 800 °C in air for 10 min, an electric field was applied between 0 and 2.0 kV/cm. As the electric field was increased, the XRD (002) peak intensity increased and had a maximum value at 1.5 and 2.0 kV/cm for the films on quartz and silicon substrates, respectively. The Hall carrier mobility increased and the carrier concentration decreased for the crystallized undoped and Al‐doped films when the electric field was 1.0 kV/cm and 1.5 kV/cm, respectively. As the electric field was increased from 0 to 2.0 kV/cm, the forward current increased and the reverse current decreased for n‐ZnO/p‐Si heterojunctions. These results indicated that application of electric fields was an effective means to reduce the defect density of ZnO sol‐gel films and to improve their heterojunction characteristics.

Improving working lifetime and efficiency of phosphor doped organic light-emitting diodes

Long working lifetime and high efficient phosphorescent organic light-emitting diode (PHOLED) in which mixed host composed of wide-band-gap based 4, 7-diphenyl-1, 10-phenanthroline (Bphen) and (4,4'-bis(carbazol-9-yl)-biphenyl) (CBP) was demonstrated. The PHOLED with structure of ITO/MoO(3)/CBP:MoO(3) (15 v%, 30 nm)/CBP(10 nm)/([50v%:50v% CBP:Bphen]: 6v% Ir(ppy)(3))(30 nm)/Bphen (40 nm)/LiF (1 nm)/Al offers a peak power efficiency of 41.6 lm/W (a peak current efficiency of 39.8 cd/A)) at a low driving voltage of 3 V which increases by 55% and 27% compared to that of corresponding single-host (SH) and double emitting layer (DML) devices, respectively. Especially very long work lifetime (3530 hs) at an initial luminance of 500 cd/m(2) of the mixed hosted device is exhibited, rising by about 4.1 and 2.46 times relative to that of corresponding SH and DML devices. High efficiency and longer working lifetime was attributed to the absence of heterojunction and balanced charge carrier transport characteristics in the mixed host based OLED structure. The more detail mechanism was also presented.

Electrical characterization and modelling of n–n Ge-Si heterojunctions with relatively low interface state densities

We investigated the electrical behavior of n–n Ge-Si isotype heterojunction diodes prepared by surfactant-mediated epitaxy of relaxed n-Ge layers on (100) n-Si substrates. Current-voltage characteristics were measured at different temperatures between 10 °C and 90 °C. The experimental results were interpreted with a new heterojunction model based on Shockley-Read-Hall kinetics for electron and hole capture/emission at the interface traps, which describes the bias dependent interface and semiconductor charges, the trap-mediated currents, and the thermionic electron transmission current. The modeled thermionic electron emission current was in excellent agreement with the experimental current-voltage characteristics in the whole temperature range for negative (≥−0.5 V) and positive (≤0.1 V) Ge biases. Trap-mediated currents were much smaller for reasonable trap capture cross sections σ ≤ 10−14cm2. From the experimental data, we extracted an electron barrier height of 0.59 eV at room temperature and an effective density of interface traps of only 5·1012cm−2eV−1 near the Si midgap. The charge carrier exchange between these traps with the Ge side was found to be much more efficient than with the Si side. The presence of a hole inversion layer at the interface proved to be essential for the interpretation of the heterojunction characteristics.

Modification of optical and electrical properties of zinc oxide-coated porous silicon nanostructures induced by swift heavy ion

Morphological and optical characteristics of radio frequency-sputtered zinc aluminum oxide over porous silicon (PS) substrates were studied before and after irradiating composite films with 130 MeV of nickel ions at different fluences varying from 1 × 1012 to 3 × 1013 ions/cm2. The effect of irradiation on the composite structure was investigated by scanning electron microscopy, X-ray diffraction (XRD), photoluminescence (PL), and cathodoluminescence spectroscopy. Current–voltage characteristics of ZnO-PS heterojunctions were also measured. As compared to the granular crystallites of zinc oxide layer, Al-doped zinc oxide (ZnO) layer showed a flaky structure. The PL spectrum of the pristine composite structure consists of the emission from the ZnO layer as well as the near-infrared emission from the PS substrate. Due to an increase in the number of deep-level defects, possibly oxygen vacancies after swift ion irradiation, PS-Al-doped ZnO nanocomposites formed with high-porosity PS are shown to demonstrate a broadening in the PL emission band, leading to the white light emission. The broadening effect is found to increase with an increase in the ion fluence and porosity. XRD study revealed the relative resistance of the film against the irradiation, i.e., the irradiation of the structure failed to completely amorphize the structure, suggesting its possible application in optoelectronics and sensing applications under harsh radiation conditions.

Heterojunction characteristics of ZnO and CuO substrates formed by direct bonding

AbstractThe n‐ZnO/p‐CuO heterojunction characteristics have been investigated by direct bonding of ZnO and CuO substrates at room temperatures, and by post‐annealing at 800ºC. The ZnO substrate was fabricated by mixing of ZnO and Al2O3 (2%) powders, pressing at 50 MPa, and sintering at 1400 °C while the CuO substrate was fabricated by mixing of CuO and Li2CO3 (1%) powders, pressing at 300 MPa, and sintering at 700 °C. Rectifying behaviour with an ideality factor of 126 was observed after bonding of these substrates. Post‐annealing of the heterojunction, however, significantly increased both the forward and the reverse currents, and the rectifying behaviour was lost. Symmetrical I‐V curves with threshold voltages of about ± 1 V were observed and this degradation could be explained by impurity (Al and Li) segregation at the junction interface. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)