Pn Junction Devices Research Articles

Defects in Nitrogen-Doped ZnO Nanoparticles and Their Effect on Light-Emitting Diodes.

In this study, the effect of defects on the acceptor properties of nitrogen-doped ZnO nanoparticles (NPs) was investigated through the fabrication of light-emitting diodes (LEDs). Nitrogen-doped ZnO NPs were synthesized by an arc discharge in-gas evaporation method and post-annealed at 800 °C in an oxygen and nitrogen atmosphere. The annealed ZnO NPs were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. It was found that the annealing of nitrogen-doped ZnO NPs in a nitrogen environment increased the number of zinc vacancies, while annealing in an oxygen environment increased the number of oxygen vacancies due to nitrogen desorption. The output characteristics of LEDs fabricated with oxygen-annealed NPs were degraded, while those with nitrogen-annealed NPs were significantly improved. From these results, the contribution of zinc vacancies to acceptor formation in ZnO NPs was confirmed for the first time in actual pn junction devices.

Research on the electro-magnetic protection technique of SCB bridge based on vertical punch through PN junction

Pyrotechnic device is the key initiating component of weapon system, and its function reliability will directly affect the triggering of detonation and actuation mechanism. In order to enhance the survival ability of pyrotechnic device in intricate electromagnetic interference environment, it is necessary to carry out in-depth research on the electromagnetic protection technique of pyrotechnic device. In this paper, a new electromagnetic protection circuit based on vertical punch through PN junction device is proposed, which monolithically integrates the circuit with polysilicon semiconductor bridge on a single chip. The Electro-static Discharge (ESD) test was carried out according to the method specified by standard electrostatic standard 500 pF/500 Ω/25 kV, and the ESD capability of the proposed new chip was not less than 25 kV. Compared with the electromagnetic protection circuit by transient voltage suppressor diode based on lateral punch through junction device structure, the new chip increases the effective junction area of PN junction, reduces the current density, and increases the current conduction ability and electrostatic roughness of the diode. The new chip has a larger diode effective junction capacitance, which not only improves the ESD capability but also improves the RF protection capability at the same time.

Efficient Zn-based pn-junction thermoelectric device for energy harvesting

Efficient Zn-based pn-junction thermoelectric device for energy harvesting

The effect of silicon films impurity compensation on the performance of silicon drift detector

Silicon drift detector (SDD) fabricated using in-situ doping technology has been developed in recent years since its advantages of low process temperature and ultra-shallow junction. However, the N+ and P+ regions on the same substrate surface cannot be formed simultaneously during the process of fabricating SDD by using in-situ doping. To overcome these challenges, we proposed a simple method to form stacked silicon films with phosphorus doped layer and boron doped layer on the same surface of SDD by impurity compensation effect in this work. The stacked silicon films with thicker boron doped layer (stack-1, partially compensated) and thinner boron doped layer (stack-2, totally compensated) were prepared by impurity compensation. Then we further investigated the films properties and electrical properties of stack-1 and stack-2. The experimental results showed that the leakage current of stack-1 was larger than that of stack-2 in PN junction devices. The simulation results indicated that a tunnel junction was formed in the stack-1 which would cause a very large leakage current when a reverse bias was applied to the PN junction. Finally, stack-2 with completely compensated boron doped layer was used to fabricated SDD and the best energy resolution achieved at the temperature of 213 K was 180 eV @ the Mn-Kα line (5.9 keV). The good energy resolution of this SDD indicated that the stacked silicon films with fully compensated boron doped layer are suitable applied to fabricating the silicon drift detectors and can also obtain excellent device performance.

Gate-tunable Veselago interference in a bipolar graphene microcavity

The relativistic charge carriers in monolayer graphene can be manipulated in manners akin to conventional optics. Klein tunneling and Veselago lensing have been previously demonstrated in ballistic graphene pn-junction devices, but collimation and focusing efficiency remains relatively low, preventing realization of advanced quantum devices and controlled quantum interference. Here, we present a graphene microcavity defined by carefully-engineered local strain and electrostatic fields. Electrons are manipulated to form an interference path inside the cavity at zero magnetic field via consecutive Veselago refractions. The observation of unique Veselago interference peaks via transport measurement and their magnetic field dependence agrees with the theoretical expectation. We further utilize Veselago interference to demonstrate localization of uncollimated electrons and thus improvement in collimation efficiency. Our work sheds new light on relativistic single-particle physics and provide a new device concept toward next-generation quantum devices based on manipulation of ballistic electron trajectory.

A floating gate negative capacitance MoS2 phototransistor with high photosensitivity.

Monolayer MoS2 exhibits interesting optoelectronic properties that have been utilized in applications such as photodetectors and light emitting diodes. For image sensing applications, improving the light sensitivity relies on achieving a low dark current that enables the detection weak light signals. Although previous reports on improving the detectivity have been explored with heterostructures and pn junction devices, some of these approaches lack CMOS compatibility processing and sufficient low dark current suppression. Steep slope transistors that overcome the Boltzmann tyranny can further enhance the performance in photodetectors by providing efficient extraction of photogenerated charges. Here, we report a monolayer MoS2 floating gate negative capacitance phototransistor with the integration of a hafnium-zirconium oxide ferroelectric capacitor. In this study, a SSmin of 30 mV dec-1, very low dark currents of 10-13-10-14 A, and a high detectivity of 7.2 × 1015 cm Hz1/2 W-1 were achieved under weak light illumination due to an enhancement in the photogating effect. In addition, its potential as an optical memory and as an optical synapse with excellent long-term potentiation characteristics in an artificial neural network was also explored. Overall, this device structure offers high photosensitivity to weak light signals for future low-powered optoelectronic applications.

A comparative simulation study on lateral and L-shaped PN junction phase shifters for single-drive 50 Gbps lumped Mach–Zehnder modulators

We present a comparative study of lateral and L-shaped PN junction Si optical phase shifters for Mach–Zehnder modulators in Si photonics based on TCAD simulation. First, we introduce an easy fabrication method for L-shaped PN junctions by inverting dominant dopant type in Si. Then, we present the quantitative comparison of Si optical phase shifters. The L-shaped PN junction device shows the larger modulation efficiency of VpiL and the lower optical phase shifter loss compared to the lateral one. The VpiL for the vertical one is 0.89 Vcm, nearly half that of the lateral one, 1.76 Vcm at the same optical phase shifter loss, 10.5 dB cm−1. Finally, taking advantage of single-drive configuration and optimizing input characteristic impedance, the large-signal simulation with 1 mW input power and a 500 μm phase shifter shows the dynamic optical modulation amplitude of 0.22 mW for the vertical one, which could be a promising solution for a compact device footprint without a traveling wave electrode.

Plasma-assisted molecular beam epitaxy of SnO(001) films: Metastability, hole transport properties, Seebeck coefficient, and effective hole mass

Transparent conducting or semiconducting oxides are important materials for (transparent) optoelectronics and power electronics applications. While most of these oxides can be doped n-type only with room-temperature electron mobilities on the order of 100cm^2/Vs p-type oxides are needed for the realization of pn-junction devices but typically suffer from exessively low (<<1cm^2/Vs) hole mobilities. Tin monoxide (SnO) is one of the few p-type oxides with a higher hole mobility, lacking a well-established understanding of its hole transport properties. Moreover, growth of SnO is complicated by its metastability with respect to SnO2 and Sn, requiring epitaxy for the realization of single crystalline material typically required for high-end applications. Here, we give a comprehensive account on the epitaxial growth of SnO, its (meta)stability, and its thermoelectric transport properties in the context of the present literature. Textured and single-crystalline, unintentionally-doped p-type SnO(001) films are grown by plasma-assisted molecular beam epitaxy. The metastability of this semiconducting oxide is addressed theoretically through an equilibrium phase diagram. Experimentally, the related SnO growth window is rapidly determined by an in-situ growth kinetics study as function of Sn-to-O-plasma flux ratio and growth temperature. The presence of secondary Sn and SnOx (1 < x <= 2) phases is comprehensively studied by different methods, indicating the presence of Sn3O4 or Sn as major secondary phases, as well as a fully oxidized SnO2 film surface. The hole transport properties, Seebeck coefficient, and density-of-states effective mass are determined and critically discussed in the context of the present literature on SnO, considering its anisotropic hole-effective mass.

About 335 nm Ultraviolet Emissions Obtained from Simple Metal–Insulator–Semiconductor Light‐Emitting Tunnel Diodes

To overcome the low external quantum efficiency of ultraviolet light‐emitting diodes (UV LEDs) in the technologically significant wavelength range of 300–350 nm, a change of approach to device design may be required. Herein, room‐temperature electroluminescence (EL) at 335 nm is achieved from simple aluminum gallium nitride (AlGaN)‐based metal–insulator–semiconductor (MIS) light‐emitting diodes (LEDs), which do not contain any p‐doped material. Current–voltage and capacitance–voltage measurements indicate that electrons in the valence band of the n‐Al0.14Ga0.86 N layer efficiently tunnel via localized states in the thin, sputter‐deposited aluminum nitride (AlN) barrier of the MIS device to provide the source of holes needed for near‐band‐edge luminescence. The full width at half maximum (FWHM) in the UV emission peak is only around 18 nm and it is very close to the photoluminescence (PL) peak of the active n‐Al0.14Ga0.86N layer, confirming that the EL is derived from band‐to‐band radiative transitions in the simple non‐p‐doped MIS diode. This design is a potential option to overcome the problem of the poor thermal excitation of holes in the AlGaN‐based pn junction devices.

Silicon nanowire core–shell PN junction phototransistors by self-assembled monolayer doping

Nanoscale photoconductors often have extremely high gain in quantum efficiency but suffer from the difficulty to design the density of surface states that cause the high photogain. In this Letter, we created high-gain photoconductors by forming a core–shell PN junction in silicon nanowires via self-assembled molecular monolayer doping. The highly doped n-type shell deactivates all the surface states by filling with electrons so that the n-type shell as a well, instead of the surface states, captures and emits photogenerated minority electrons under ON/OFF light illumination. The corresponding excess majority holes are accumulated in the nanowire channel and thus modulate the channel width, resulting in the experimentally observed high photogain (∼108). The photoresponses of these phototransistors were systematically investigated as a function of the nanowire width and light illumination intensity. The results show that the nanowire channel is pinched off for the nanowires narrower than 73 nm due to the core–shell PN junction. We further derived analytical equations based on the PN junction device principle, finding the explicit gain equation that governs the photogain as a function of light intensity and other physical parameters of the nanowires. The explicit gain equations can fit well with the experimental data and allow us to design the core–shell nanowire phototransitors with desired performance.

Simudo: a device model for intermediate band materials

We describe Simudo, a free Poisson/drift-diffusion steady state device model for semiconductor and intermediate band materials, including self-consistent optical absorption and generation. Simudo is the first freely available device model that can treat intermediate band materials. Simudo uses the finite element method (FEM) to solve the coupled nonlinear partial differential equations in two dimensions, which is different from the standard choice of the finite volume method in essentially all commercial semiconductor device models. We present the continuous equations that Simudo solves, show the FEM formulations we have developed, and demonstrate how they allow robust convergence with double-precision floating point arithmetic. With a benchmark semiconductor pn junction device, we show that Simudo has a higher rate of convergence than Synopsys Sentaurus, converging to high accuracy with a considerably smaller mesh. Simudo includes many semiconductor phenomena and parameters and is designed for extensibility by the user to include many physical processes.

High sensitivity Si-based photodetection with nanoscale protective layer based on interface states

A large lateral photovoltaic effect has been observed at the interface of SiO2/p-Si/SiO2 structure. Different from the traditional Schottky junction or PN junction device, this photovoltage is mainly dominated by the interface states existing at SiO2/p-Si interface, where the covered nanoscale SiO2 layer brings this device a stable and high photoelectric performance. These interface states can be explained as built-in field caused by the band bending at interface, which regulates the generation and diffusion of photo induced carriers. In this study, we discuss clearly the factors that impact greatly on the photovoltage output and sensitivity, including the oxide thickness, resistivity and tunneling effect. We believe this simple but efficient device will be beneficial for the exploring in photoelectric detection.

Fabrication of 12 µm pixel-pitch 1280 × 1024 extended short wavelength infrared focal plane array using heterojunction type-II superlattice-based photodetectors

We present an intial demonstration of a 1280 × 1024 extended short-wavelength infrared focal plane array (FPA) imager with 12 μm pixel-pitch based on type–II InAs/AlSb/GaSb superlattice heterojunction photodetectors, with a novel bandstructure-engineered photo-generated carrier extractor as the window layer in the hetero structure to efficiently extract the photo-generated carriers. This heterostructure with a larger bandgap top window/contact layer leads to the device having lower dark current density compared to conventional pn junction devices. The large format FPA was fabricated with 12 μm pixel-pitch using a developed fabrication process. Test pixels fabricated separately exhibit 100% cut–off wavelengths of ∼2.22, ∼2.34 μm, and ∼2.45 μm at 150, 200 K, and 300 K. The test devices achieve saturated quantum efficiency values under zero bias of 54.3% and 68.4% at 150 and 300 K, under back-side illumination and without any anti-reflection coating. At 150 K, these photodetectors exhibit dark current density of 1.63 × 10−7 A cm−2 under −20 mV applied bias providing a specific detectivity of 1.01 × 1011 cm · Hz1/2/W at 1.9 μm.

Highly-efficient thermoelectric pn-junction device based on bismuth telluride (Bi2Te3) and molybdenum disulfide (MoS2) thin films fabricated by RF magnetron sputtering technique

Layered structure bismuth telluride and molybdenum disulfide thin films were successfully deposited on different substrates using radio-frequency magnetron sputtering technique. The structural, morphological, and thermoelectric transport properties of bismuth telluride and molybdenum disulfide thin films have been investigated systematically to fabricate high-efficient thermal energy harvesting thermoelectric device. The magnitude of the Seebeck coefficient of bismuth telluride thin films decreases with increase in film thickness. Bismuth telluride grown at 350 °C for 10 min, which is approximately 120 nm, displays a maximum Seebeck coefficient of −126 μV K−1 at 435 K. The performance shows strong temperature dependence when the films were deposited at 300 °C, 350 °C, and 400 °C. The power factor increases from 0.91 × 10−3 W/mK2 at 300 K to about 1.4 × 10−3 W/mK2 at 350 K. Molybdenum disulfide films show the positive Seebeck coefficient values and their Seebeck coefficient increases with film thickness. The AFM images of bismuth telluride thin films display a root-mean-square (rms) roughness of 32.3 nm and molybdenum disulfide thin films show an rms roughness of 6.99 nm when both films were deposited at 350 °C. The open-circuit voltage of the pn-junction thermoelectric generator (TEG) device increases with increase in ΔT to about 130 mV at ΔT = 120 °C. We have demonstrated a highly efficient pn-junction TEG device for waste heat recovery applications.

Thin film of guest-free type-II silicon clathrate on Si(111) wafer

Thin films of guest-free type-II Si clathrate (Si136) were fabricated on Si(111) wafers in two steps: NaxSi136 thin-film formation by thermal decomposition of NaSi precursor films and Na removal from the NaxSi136 film by a heat treatment with iodine. Cross-sectional TEM observation and XRD and Raman measurements verified the formation of 1-μm-thick Si136 films on the Si wafer. Since the prepared films showed n-type conduction, pn junction devices were developed by a Si136/p-type Si structure. This device showed a photovoltaic (PV) response under white light illumination. The thin film formation and the PV response of Si136 indicated this Si allotrope to be the next-generation platform for semiconductor technology.

(Invited) Silicon-Based Nanocomposites for Thermoelectric High Temperature Waste Heat Recovery

Thermoelectric generators convert heat flows directly into useable electrical energy. In an appealing future scenario of application, waste heat could be partly recovered by thermoelectric generators. It is typically argued that the heat comes without additional costs. Hence, every contribution of a thermoelectric generator results in an increase of the over-all process efficiency. A thermoelectric converter material with sufficient performance at competitive costs is hereby one precondition. A pragmatic way to optimize the thermoelectric performance of a material is the introduction of nanostructural elements into bulk materials. Hereby, the availability of high quality nanopowder fosters the development of novel thermoelectric materials: Materials which have per se an insufficient thermoelectric figure of merit can be optimized in a range that they become interesting substitute materials like nanocrystalline silicon or composites of nanocrystalline silicon and metal silicides. High hot side temperatures require innovative device concepts. Some years ago, a thermoelectric generator concept was proposed, which uses a large area pn junction aligned in parallel to the temperature gradient. This device concept is especially designed for high hot side temperatures and large ΔTs of at least a few hundreds of degree C. All electrical and mechanical contacts on the pn junction device are on the cold side which is beneficial with respect to stability against degradation of the device. Using nanocrystalline silicon as thermoelectric converter material, we have shown that for temperature differences larger than 300°C, the pn junction device performs competitively with an ideal traditional device and outperforms this device if non-ideal contact resistances of a real device are included.

Scalable tight-binding model for graphene.

Artificial graphene consisting of honeycomb lattices other than the atomic layer of carbon has been shown to exhibit electronic properties similar to real graphene. Here, we reverse the argument to show that transport properties of real graphene can be captured by simulations using "theoretical artificial graphene." To prove this, we first derive a simple condition, along with its restrictions, to achieve band structure invariance for a scalable graphene lattice. We then present transport measurements for an ultraclean suspended single-layer graphene pn junction device, where ballistic transport features from complex Fabry-Pérot interference (at zero magnetic field) to the quantum Hall effect (at unusually low field) are observed and are well reproduced by transport simulations based on properly scaled single-particle tight-binding models. Our findings indicate that transport simulations for graphene can be efficiently performed with a strongly reduced number of atomic sites, allowing for reliable predictions for electric properties of complex graphene devices. We demonstrate the capability of the model by applying it to predict so-far unexplored gate-defined conductance quantization in single-layer graphene.

From high-Tc superconductors to highly correlated Mott insulators—25 years of pulsed laser deposition of functional oxides in Leipzig

Pulsed laser deposition (PLD) in Leipzig started in 1989 with Bi2Sr2Ca1Cu2O8 high-Tc superconducting thin films grown in a laser-ionization mass spectrometer. Here, we briefly review 25 years of development of advanced PLD processes in Leipzig and their application to high-Tc superconducting, wide-bandgap semiconducting and multiferroic thin films, nanostructures and composites. The first two decades were devoted to large-area and double-sided YBa2Cu3O7−δ thin films and hetero- and homoepitaxial ZnO-based films and nanowires, respectively. Based on that, transparent, oxide-based electronic devices are processed with epitaxial n-ZnO:Mg, Ga2O3, In2O3, or TiO2 films. Amorphous oxide films of p-ZnCo2O4 and p-NiO provide p-type counterparts in highly rectifying pn-junction devices and are an environmentally friendly alternative. Magnetoelectric multiferroic composites, and highly correlated iridate thin films are other current hot research topics. PLD appears as one of the most flexible growth techniques for functional oxides on research and demonstrator level.

A new method for polychromatic X-ray μLaue diffraction on a Cu pillar using an energy-dispersive pn-junction charge-coupled device.

μLaue diffraction with a polychromatic X-ray beam can be used to measure strain fields and crystal orientations of micro crystals. The hydrostatic strain tensor can be obtained once the energy profile of the reflections is measured. However, this remains a challenge both on the time scale and reproducibility of the beam position on the sample. In this review, we present a new approach to obtain the spatial and energy profiles of Laue spots by using a pn-junction charge-coupled device, an energy-dispersive area detector providing 3D resolution of incident X-rays. The morphology and energetic structure of various Bragg peaks from a single crystalline Cu micro-cantilever used as a test system were simultaneously acquired. The method facilitates the determination of the Laue spots' energy spectra without filtering the white X-ray beam. The synchrotron experiment was performed at the BM32 beamline of ESRF using polychromatic X-rays in the energy range between 5 and 25 keV and a beam size of 0.5 μm × 0.5 μm. The feasibility test on the well known system demonstrates the capabilities of the approach and introduces the "3D detector method" as a promising tool for material investigations to separate bending and strain for technical materials.

Observation of Tunneling Effects in Lateral Nanowire pn Junctions

As electronic device dimensions are continuously reduced, applied bias conditions significantly change and the transport mechanisms must be reconsidered. Tunneling devices are promising for scaled-down electronics because of expected high-speed operation and relatively low bias. In this work, we investigated the tunneling features in silicon-oninsulator lateral nanowire pn junction and pin junction devices. By controlling the substrate voltage, tunneling features can be observed in the electrical characteristics. We found that the minimum substrate voltage required for tunneling to occur in pn junctions is higher as compared with pin junctions. The main cause of these effects relies in the difference between the doping profiles, since the pn junctions contain a co-doped region, while the pin junctions contain an i-layer.