Showing papers on "p–n junction published in 2014"

Doping against the native propensity of MoS2: degenerate hole doping by cation substitution.

Abstract: Layered transition metal dichalcogenides (TMDs) draw much attention as the key semiconducting material for two-dimensional electrical, optoelectronic, and spintronic devices. For most of these applications, both n- and p-type materials are needed to form junctions and support bipolar carrier conduction. However, typically only one type of doping is stable for a particular TMD. For example, molybdenum disulfide (MoS2) is natively an n-type presumably due to omnipresent electron-donating sulfur vacancies, and stable/controllable p-type doping has not been achieved. The lack of p-type doping hampers the development of charge-splitting p–n junctions of MoS2, as well as limits carrier conduction to spin-degenerate conduction bands instead of the more interesting, spin-polarized valence bands. Traditionally, extrinsic p-type doping in TMDs has been approached with surface adsorption or intercalation of electron-accepting molecules. However, practically stable doping requires substitution of host atoms with dopa...

Lateral MoS2 p-n junction formed by chemical doping for use in high-performance optoelectronics.

Abstract: This paper demonstrates a technique to form a lateral homogeneous 2D MoS2 p–n junction by partially stacking 2D h-BN as a mask to p-dope MoS2. The fabricated lateral MoS2 p–n junction with asymmetric electrodes of Pd and Cr/Au displayed a highly efficient photoresponse (maximum external quantum efficiency of ∼7000%, specific detectivity of ∼5 × 1010 Jones, and light switching ratio of ∼103) and ideal rectifying behavior. The enhanced photoresponse and generation of open-circuit voltage (VOC) and short-circuit current (ISC) were understood to originate from the formation of a p–n junction after chemical doping. Due to the high photoresponse at low VD and VG attributed to its built-in potential, our MoS2 p–n diode made progress toward the realization of low-power operating photodevices. Thus, this study suggests an effective way to form a lateral p–n junction by the h-BN hard masking technique and to improve the photoresponse of MoS2 by the chemical doping process.

Enhancement of Photovoltaic Response in Multilayer MoS2 Induced by Plasma Doping

Abstract: Layered transition-metal dichalcogenides hold promise for making ultrathin-film photovoltaic devices with a combination of excellent photovoltaic performance, superior flexibility, long lifetime, and low manufacturing cost. Engineering the proper band structures of such layered materials is essential to realize such potential. Here, we present a plasma-assisted doping approach for significantly improving the photovoltaic response in multilayer MoS2. In this work, we fabricated and characterized photovoltaic devices with a vertically stacked indium tin oxide electrode/multilayer MoS2/metal electrode structure. Utilizing a plasma-induced p-doping approach, we are able to form p–n junctions in MoS2 layers that facilitate the collection of photogenerated carriers, enhance the photovoltages, and decrease reverse dark currents. Using plasma-assisted doping processes, we have demonstrated MoS2-based photovoltaic devices exhibiting very high short-circuit photocurrent density values up to 20.9 mA/cm2 and reasonab...

Fast and broadband photoresponse of few-layer black phosphorus field-effect transistors

Abstract: Few-layer black phosphorus, a new elemental 2D material recently isolated by mechanical exfoliation, is a high-mobility layered semiconductor with a direct bandgap that is predicted to strongly depend on the number of layers, from 0.35 eV (bulk) to 2.0 eV (single-layer). Therefore, black phosphorus is an appealing candidate for tunable photodetection from the visible to the infrared part of the spectrum. We study the photoresponse of field-effect transistors (FETs) made of few-layer black phosphorus (3 nm to 8 nm thick), as a function of excitation wavelength, power and frequency. In the dark state, the black phosphorus FETs can be tuned both in hole and electron doping regimes allowing for ambipolar operation. We measure mobilities in the order of 100 cm2/V s and current ON/OFF ratio larger than 103. Upon illumination, the black phosphorus transistors show response to excitation wavelengths from the visible up to 940 nm and rise time of about 1 ms, demonstrating broadband and fast detection. The responsivity reaches 4.8 mA/W and it could be drastically enhanced by engineering a detector based on a PN junction. The ambipolar behavior coupled to the fast and broadband photodetection make few-layer black phosphorus a promising 2D material for photodetection across the visible and near-infrared part of the electromagnetic spectrum.

p–n junction CuO/BiVO4 heterogeneous nanostructures: synthesis and highly efficient visible-light photocatalytic performance

Abstract: A new strategy via coupling a polyol route with an oxidation process has been developed to successfully synthesize p–n junction CuO/BiVO4 heterogeneous nanostructures. The experimental results reveal that the as-prepared p–n junction CuO/BiVO4 heterogeneous nanostructures exhibit much higher visible-light-driven photocatalytic activity for the degradation of model dye rhodamine B (RhB) than the pure BiVO4 nanocrystals. The photocatalytic degradation rate (C/C0) of the RhB for p–n junction CuO/BiVO4 heterogeneous nanostructures is about two times higher than that of pure BiVO4 nanocrystals. The enhanced photocatalytic efficiency is attributed to a large number of p–n junctions in CuO/BiVO4 heterogeneous nanostructures, which effectively reduces the recombination of electrons and holes by charge transfer from n-type BiVO4 to the attached p-type CuO nanoparticles. This work not only provides an efficient route to enhance the visible-light-driven photocatalytic activity of BiVO4, but also offers a new strategy for fabricating p–n junction heterogeneous nanostructure photocatalysts, which are expected to show considerable potential application in solar-driven wastewater treatment and water splitting.

Potential gradient and photocatalytic activity of an ultrathin p-n junction surface prepared with two-dimensional semiconducting nanocrystals.

Abstract: The creation of p–n junction structure in photocatalysts is a smart approach to improve the photocatalytic activity, as p–n junctions can potentially act to suppress the recombination reaction. Understanding the surface conditions of the junction parts is one of the biggest challenges in the development of photocatalyst surface chemistry. Here, we show a relationship between the photocatalytic activity and potential gradient of the junction surface prepared from two-dimensional crystals of p-type NiO and n-type calcium niobate (CNO). The ultrathin (ca. 2 nm) junction structure and the surface potential were analyzed using low energy ion scattering spectroscopy and Kelvin probe force microscopy. The photocatalytic H2 production rate for the n–p (CNO/NiO) junction surface was higher than those for p–n (NiO/CNO) junction, p, and n surfaces. The surface potential of the CNO/NiO junction part (surface: CNO) was lower than that of the CNO crystals in the same CNO crystal surface. These potential gradients resul...

Optimizing performance of silicon-based p-n junction photodetectors by the piezo-phototronic effect.

Abstract: Silicon-based p–n junction photodetectors (PDs) play an essential role in optoelectronic applications for photosensing due to their outstanding compatibility with well-developed integrated circuit technology. The piezo-phototronic effect, a three-way coupling effect among semiconductor properties, piezoelectric polarizations, and photon excitation, has been demonstrated as an effective approach to tune/modulate the generation, separation, and recombination of photogenerated electron–hole pairs during optoelectronic processes in piezoelectric-semiconductor materials. Here, we utilize the strain-induced piezo-polarization charges in a piezoelectric n-ZnO layer to modulate the optoelectronic process initiated in a p-Si layer and thus optimize the performances of p-Si/ZnO NWs hybridized photodetectors for visible sensing via tuning the transport property of charge carriers across the Si/ZnO heterojunction interface. The maximum photoresponsivity R of 7.1 A/W and fastest rising time of 101 ms were obtained fro...

Direct imaging of p-n junction in core-shell GaN wires.

Abstract: While core–shell wire-based devices offer a promising path toward improved optoelectronic applications, their development is hampered by the present uncertainty about essential semiconductor properties along the three-dimensional (3D) buried p–n junction. Thanks to a cross-sectional approach, scanning electron beam probing techniques were employed here to obtain a nanoscale spatially resolved analysis of GaN core–shell wire p–n junctions grown by catalyst-free metal–organic vapor phase epitaxy on GaN and Si substrates. Both electron beam induced current (EBIC) and secondary electron voltage constrast (VC) were demonstrated to delineate the radial and axial junction existing in the 3D structure. The Mg dopant activation process in p-GaN shell was dynamically controlled by the ebeam exposure conditions and visualized thanks to EBIC mapping. EBIC measurements were shown to yield local minority carrier/exciton diffusion lengths on the p-side (∼57 nm) and the n-side (∼15 nm) as well as depletion width in the r...

InGaN/GaN tunnel junctions for hole injection in GaN light emitting diodes

Abstract: InGaN/GaN tunnel junction contacts were grown using plasma assisted molecular beam epitaxy (MBE) on top of a metal-organic chemical vapor deposition (MOCVD)-grown InGaN/GaN blue (450 nm) light emitting diode. A voltage drop of 5.3 V at 100 mA, forward resistance of 2 × 10−2 Ω cm2, and a higher light output power compared to the reference light emitting diodes (LED) with semi-transparent p-contacts were measured in the tunnel junction LED (TJLED). A forward resistance of 5 × 10−4 Ω cm2 was measured in a GaN PN junction with the identical tunnel junction contact as the TJLED, grown completely by MBE. The depletion region due to the impurities at the regrowth interface between the MBE tunnel junction and the MOCVD-grown LED was hence found to limit the forward resistance measured in the TJLED.

Ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer

Abstract: This paper reports the proposal, design, and demonstration of ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer to optimize light management and minimize non-radiative recombination. According to our recently developed semi-analytical model, this design offers one of the highest potential achievable efficiencies for GaAs solar cells possessing typical non-radiative recombination rates found among commercially available III-V arsenide and phosphide materials. The structure of the demonstrated solar cells consists of an In0.49Ga0.51P/GaAs/In0.49Ga0.51P double-heterostructure PN junction with an ultra-thin 300 nm thick GaAs absorber, combined with a 5 μm thick Al0.52In0.48P layer with a textured as-grown surface coated with Au used as a reflective back scattering layer. The final devices were fabricated using a substrate-removal and flip-chip bonding process. Solar cells with a top metal contact coverage of 9.7%, and a MgF2/ZnS anti-reflective coating demonstrated...

Direct Laser Writing of Air-Stable p–n Junctions in Graphene

Abstract: Photo-oxidation of spin-cast films of 6,13-bis(triisopropylsilylethynyl) pentacene has been exploited to develop a novel means of spatially modulating doping in graphene. The degree of n-doping of initially p-type graphene can be varied by laser irradiation time or intensity with carrier density change up to ∼7 × 1012 cm–2. This n-doping approach is demonstrated as an effective means of creating p–n junctions in graphene. The ability to direct-write arbitrary shapes and patterns of n-doped regions in graphene simply by scanning a laser source should facilitate the exploitation of p–n junctions for a variety of electronic and optoelectronic device applications.

Magnetic photocatalysts with a p–n junction: Fe3O4 nanoparticle and FeWO4 nanowire heterostructures

Abstract: Magnetic n-type semiconductor Fe3O4 nanoparticle and p-type semiconductor FeWO4 nanowire heterostructures were successfully synthesized without any surfactants or templates via a facile one-step hydrothermal process at 160 °C. The heterojunction structure and morphology were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Magnetic measurements indicated the coexistence of ferrimagnetic behavior of Fe3O4 and weak antiferromagnetic behavior of FeWO4. The degradation of methylene blue (MB) under UV-Visible light irradiation was studied as a model experiment to evaluate the catalytic activity of the Fe3O4/FeWO4 heterostructure p–n junctions. The decomposition efficiency was 97.1% after one hour UV-Visible irradiation. This magnetic photocatalyst can be easily recovered from the solution using a permanent magnet and redispersed by removing the magnet.

Interface investigation of planar hybrid n-Si/PEDOT:PSS solar cells with open circuit voltages up to 645 mV and efficiencies of 12.6 %

Abstract: We have studied interface formation properties of hybrid n-Si/PEDOT:PSS solar cells on planar substrates by varying the silicon substrate doping concentration (N D). Final power conversion efficiencies (PCE) of 12.6 % and open circuit voltages (V oc) comparable to conventional diffused emitter pn junction solar cells have been achieved. It was observed, that an increase of N D leads to an increase of V oc with a maximal value of 645 mV, which is, to our knowledge, the highest reported value for n-Si/PEDOT:PSS interfaces. The dependence of the solar cell characteristics on N D is analyzed and similarities to minority charge carrier drift-diffusion limited solar cells are presented. The results point out the potential of hybrid n-Si/PEDOT:PSS interfaces to fabricate high performance opto-electronic devices with cost-effective fabrication technologies.

Photoresponse of an Electrically Tunable Ambipolar Graphene Infrared Thermocouple

Abstract: We explore the photoresponse of an ambipolar graphene infrared thermocouple at photon energies close to or below monolayer graphene’s optical phonon energy and electrostatically accessible Fermi energy levels. The ambipolar graphene infrared thermocouple consists of monolayer graphene supported by an infrared absorbing material, controlled by two independent electrostatic gates embedded below the absorber. Using a scanning infrared laser microscope, we characterize these devices as a function of carrier type and carrier density difference controlled at the junction between the two electrostatic gates. On the basis of these measurements, conducted at both mid- and near-infrared wavelengths, the primary detection mechanism can be modeled as a thermoelectric response. By studying the effect of different infrared absorbers, we determine that the optical absorption and thermal conduction of the substrate play the dominant role in the measured photoresponse of our devices. These experiments indicate a path towa...

Reconfigurable p-n junction diodes and the photovoltaic effect in exfoliated MoS2 films

Abstract: Realizing basic semiconductor devices such as p-n junctions are necessary for developing thin-film and optoelectronic technologies in emerging planar materials such as MoS2. In this work, electrostatic doping by buried gates is used to study the electronic and optoelectronic properties of p-n junctions in exfoliated MoS2 flakes. Creating a controllable doping gradient across the device leads to the observation of the photovoltaic effect in monolayer and bilayer MoS2 flakes. For thicker flakes, strong ambipolar conduction enables realization of fully reconfigurable p-n junction diodes with rectifying current-voltage characteristics, and diode ideality factors as low as 1.6. The spectral response of the photovoltaic effect shows signatures of the predicted band gap transitions. For the first excitonic transition, a shift of >4kBT is observed between monolayer and bulk devices, indicating a thickness-dependence of the excitonic coulomb interaction.

Analysis of the electrical properties of Cr/n-BaSi2 Schottky junction and n-BaSi2/p-Si heterojunction diodes for solar cell applications

Abstract: Current status and future prospects towards BaSi2 pn junction solar cells are presented. As a preliminary step toward the formation of BaSi2 homojunction diodes, diodes with a Cr/n-BaSi2 Schottky junction and an n-BaSi2/p-Si hetero-junction have been fabricated to investigate the electrical properties of the n-BaSi2. Clear rectifying properties were observed in the current density versus voltage characteristics in both diodes. From the capacitance-voltage measurements, the build-in potential, VD, was 0.53 V in the Cr/n-BaSi2 Schottky junction diode, and the Schottky barrier height was 0.73 eV calculated from the thermoionic emission theory; the VD was about 1.5 V in the n-BaSi2/p-Si hetero-junction diode, which was consistent with the difference in the Fermi level between the n-BaSi2 and the p-Si.

Melt growth of bulk Bi2Te3 crystals with a natural p–n junction

Abstract: Single crystals of Bi2Te3 were grown from Bi–Te melts using the modified Bridgman method. It was shown for the first time that solidification of 61 and 62 mol.% Te melts provides a built-in p–n junction on the cleaved plane of as grown crystals without any post growth treatment. The formation of a p–n junction along the growth crystal was explained by Te segregation. Both p- and n-parts of the ingot have shown high carrier concentrations n ≈ p ≈ 1 × 1019 cm−3 and high carrier mobility ~104 cm2 V s−1 at 4 K. In the transition p–n region, Hall carrier concentration is decreased by two orders of magnitude as a result of intrinsic compensation of carriers.

An Analytical Model With 2-D Effects for 4H-SiC Trenched Junction Barrier Schottky Diodes

Abstract: This paper presents an analytical model with 2-D effects for 1200 V 4H-silicon carbide trenched junction barrier Schottky (TJBS) diodes. Energy band diagrams of junction barrier Schottky and TJBS diodes in reverse biases are analyzed in detail. An analytical model of potential and electric field distributions with 2-D effects is proposed. Based on the modeling results of the surface electric field, a physical model accounting for three main mechanisms, namely, thermionic emission, tunneling, and avalanche multiplication is developed for the device reverse I-V characteristics. The models are verified by experimental results. Based on the physical model, optimization of the TJBS parameters including barrier height, junction depth, and junction spacing can be carried out with the target of achieving the lowest device ON-state voltage while limiting the reverse leakage current to a reasonable level. As a result, for a given breakdown voltage, an optimum set of parameters can be obtained.

High Temperature Thermoelectric Device Concept Using Large Area PN Junctions

Abstract: A new high temperature thermoelectric device concept using large area nanostructured silicon p-type and n-type (PN) junctions is presented. In contrast to conventional thermoelectric generators, where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we experimentally demonstrate a device concept in which a large area PN junction made from highly doped densified silicon nanoparticles is subject to a temperature gradient parallel to the PN interface. In the proposed device concept, the electrical contacts are made at the cold side eliminating the hot side substrate and difficulties that go along with high temperature electrical contacts. This concept allows temperature gradients greater than 300 K to be experimentally applied with hot side temperatures larger than 800 K. Electronic properties of the PN junctions and power output characterizations are presented. A fundamental working principle is discussed using a particle network model with temperature and electric fields as variables, and which considers electrical conductivity and thermal conductivity according to Fourier’s law, as well as Peltier and Seebeck effects.

Fast and efficient silicon thermo-optic switching based on reverse breakdown of pn junction

Abstract: We propose and demonstrate a fast and efficient silicon thermo-optic switch based on reverse breakdown of the pn junction. Benefiting from the direct heating of silicon waveguide by embedding the pn junction into the waveguide center, fast switching with on/off time of 330 and 450 ns and efficient thermal tuning of 0.12 nm/mW for a 20 μm radius microring resonator are achieved, indicating a high figure of merit of only 8.8 mW·μs. The results here show great potential for application in the future optical interconnects.

Origin of electrons emitted into vacuum from InGaN light emitting diodes

Abstract: The mechanism responsible for efficiency droop in InGaN light-emitting diodes (LEDs) has long been elusive due to indirect measurement techniques used for its identification. Auger recombination is unique among proposed efficiency droop mechanisms, in that it is the only mechanism capable of generating hot carriers. In a previous study [J. Iveland et al., Phys. Rev. Lett. 110, 177406 (2013)], we performed electron energy analysis of electrons emitted into vacuum from a forward biased InGaN LED that had been brought into negative electron affinity by cesiation. Three peaks were observed in the energy spectrum of vacuum emitted electrons. In this Letter, we unambiguously identify the origin of the peaks. The two higher energy peaks correspond to accumulation of electrons transported to the surface in the bulk Γ and side L conduction band valleys. The L-valley peak is a direct signature of a hot Auger electron population. The lower energy peak results from surface photoemission induced by the internal LED light emitted from the InGaN quantum wells. Two control experiments were performed. In the first, a simple GaN pn junction generated only a single Γ peak in electroemission. In the second, selective detection of the photoemission from an LED under modulated light excitation and DC electrical injection confirms that only the low energy peak is photogenerated and that LED light is incapable of generating Γ or L-valley peaks, the latter only occurring due to the Auger effect in the LED active region.

Ferroelectric-semiconductor photovoltaics: Non-PN junction solar cells

Abstract: Traditional positive-negative (PN) junction based solar cells have many limitations. Herein, we introduce ferroelectric-semiconductor solar cells that use the bound surface charges of the ferroelectric for achieving charge separation in the semiconductor. The feasibility of the new concept cells was verified both experimentally and theoretically in detail. The new cells are unique in that free charge carriers and fixed charge carriers are physically separated from each other. The feature allows us to go beyond traditional junction-based structures and have more freedom in material selection, device design, and fabrication.

Band mapping across a pn-junction in a nanorod by scanning tunneling microscopy.

Abstract: We map band-edges across a pn-junction that was formed in a nanorod. We form a single junction between p-type Cu2S and n-type CdS through a controlled cationic exchange process of CdS nanorods. We characterize nanorods of the individual materials and the single junction in a nanorod with an ultrahigh vacuum scanning tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determine the conduction and valence band-edges at different points across the junction and the individual nanorods. We could map the band-diagram of nanorod-junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in the nanoscale.

Synthesis of hierarchical Bi2O3/Bi4Ti3O12 p–n junction nanoribbons on carbon fibers from (001) facet dominated TiO2 nanosheets

Abstract: A novel hierarchical photocatalyst of Bi2O3/Bi4Ti3O12 p–n junction nanoribbons (NRs) on carbon fibers has been successfully fabricated from the precursor of (001) facet dominated TiO2 nanosheets (NSs), which provide well-shaped templates. The dominant (001) facets of the TiO2 precursor diminished slower than the (101) facets during the hydrothermal process. Single crystallized p-type Bi2O3 and n-type Bi4Ti3O12 coexist, forming p–n junctions within one NR. This hierarchical nanostructure exhibits markedly improved photocatalytic activity for degradation of methyl orange. The improvement can be attributed to enhanced absorption in the visible light region due to the ultrathin NRs, more efficient charge separation/transportation owing to the formation of a p–n junction, and high exposure of reactive (001) facets resulting from the formation of a hierarchical structure. Moreover, this hierarchical photocatalyst shows high stability and excellent recycling properties.