Showing papers on "Depletion region published in 2014"

Atomically thin p–n junctions with van der Waals heterointerfaces

Abstract: In heterostructures of the transition metal dichalcogenides MoS2 and WSe2, atomically thin p–n junctions are created that show gate-tunable rectifying and photovoltaic behaviour mediated by tunnelling-assisted interlayer recombination. Semiconductor p–n junctions are essential building blocks for electronic and optoelectronic devices1,2. In conventional p–n junctions, regions depleted of free charge carriers form on either side of the junction, generating built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes influenced by the spatial extent of this depletion region. With the advent of atomically thin van der Waals materials and their heterostructures, it is now possible to realize a p–n junction at the ultimate thickness limit3,4,5,6,7,8,9,10. Van der Waals junctions composed of p- and n-type semiconductors—each just one unit cell thick—are predicted to exhibit completely different charge transport characteristics than bulk heterojunctions10,11,12. Here, we report the characterization of the electronic and optoelectronic properties of atomically thin p–n heterojunctions fabricated using van der Waals assembly of transition-metal dichalcogenides. We observe gate-tunable diode-like current rectification and a photovoltaic response across the p–n interface. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic p–n junction between graphene layers enhances the collection of the photoexcited carriers. The atomically scaled van der Waals p–n heterostructures presented here constitute the ultimate functional unit for nanoscale electronic and optoelectronic devices.

Correction: Corrigendum: Ion concentration polarization-based continuous separation device using electrical repulsion in the depletion region

Abstract: We proposed a novel separation method, which is the first report using ion concentration polarization (ICP) to separate particles continuously. We analyzed the electrical forces that cause the repulsion of particles in the depletion region formed by ICP. Using the electrical repulsion, micro- and nano-sized particles were separated based on their electrophoretic mobilities. Because the separation of particles was performed using a strong electric field in the depletion region without the use of internal electrodes, it offers the advantages of simple, low-cost device fabrication and bubble-free operation compared with conventional continuous electrophoretic separation methods, such as miniaturizing free-flow electrophoresis (μ-FFE). This separation device is expected to be a useful tool for separating various biochemical samples, including cells, proteins, DNAs and even ions.

Dynamics of photogenerated holes in undoped BiVO4 photoanodes for solar water oxidation

Abstract: The dynamics of photogenerated holes in undoped BiVO4 photoanodes for water splitting were studied using transient absorption spectroscopy, correlated with photoelectrochemical and transient photocurrent data. Transient absorption signals of photogenerated holes were identified using electron/hole scavengers and applied electrical bias in a complete photoelectrochemical cell. The yield of long-lived (0.1–1 s) photogenerated holes is observed to correlate as a function of applied electrical bias with the width of the space charge layer, as determined by electrochemical impedance spectroscopy. The transient absorption decay time constant of these long-lived holes is also observed to be dependent upon the applied bias, assigned to kinetic competition between water oxidation and recombination of these surface accumulated holes with bulk electrons across the space charge layer. The time constant for this slow recombination measured with transient absorption spectroscopy is shown to match the time constant of back electron transfer from the external circuit determined from chopped light transient photocurrent measurements, thus providing strong evidence for these assignments. The yield of water oxidation determined from these measurements, including consideration of both the yield of long-lived holes, and the fraction of these holes which are lost due to back electron/hole recombination, is observed to be in good agreement with the photocurrent density measured for BiVO4 photoanodes as a function of bias under continuous irradiation. Overall these results indicate two distinct recombination processes which limit photocurrent generation in BiVO4 photoanodes: firstly rapid (≤microseconds) electron/hole recombination, and secondly recombination of surface-accumulated holes with bulk BiVO4 electrons. This second ‘back electron transfer’ recombination occurs on the milliseconds–seconds timescale, and is only avoided at strong anodic biases where the potential drop across the space charge layer provides a sufficiently large energetic barrier to prevent this recombination process.

Electron–Hole Recombination Time at TiO2 Single-Crystal Surfaces: Influence of Surface Band Bending

Abstract: Photocatalytic activity is determined by the transport property of photoexcited carriers from the interior to the surface of photocatalysts. Because the carrier dynamics is influenced by a space charge layer (SCL) in the subsurface region, an understanding of the effect of the potential barrier of the SCL on the carrier behavior is essential. Here we have investigated the relaxation time of the photoexcited carriers on single-crystal anatase and rutile TiO2 surfaces by time-resolved photoelectron spectroscopy and found that carrier recombination, taking a nanosecond time scale at room temperature, is strongly influenced by the barrier height of the SCL. Under the flat-band condition, which is realized in nanometer-sized photocatalysts, the carriers have a longer lifetime on the anatase surface than the rutile one, naturally explaining the higher photocatalytic activity for anatase than rutile.

Unitraveling-Carrier Photodiodes for Terahertz Applications

Abstract: Device design for unitraveling-carrier photodiodes (UTC-PDs) and their derivative structures is reconsidered from the point of view of terahertz (THz) applications. A key design procedure for maximizing their bandwidth is optimization by incorporating hybrid absorbers. The effect of quasi-field in p-type absorber is carefully examined. It has been shown that the initial velocity transient must be taken into account to evaluate the effective average velocity. Photomixers integrating a hybrid-absorber UTC-PD and a bow-tie antenna were fabricated and characterized. THz-wave generation by the photomixers in a frequency range of up to around 2.5 THz was confirmed. The observed THz-wave output exhibits significant changes with bias voltage, where the decrease in the output with increasing negative bias voltage is more pronounced at higher frequencies. This output behavior is due to the change in electron velocity in the diode depletion layer associated with the overshoot effect. From the dependence of the output power on frequency, effective electron velocity is found to be as high as $6 \times 10^{7}$ cm/s at optimum bias voltage of -0.4 V.

Hematite photoelectrodes for water splitting: Evaluation of the role of film thickness by impedance spectroscopy

Abstract: The electrochemical behavior of α-Fe2O3 photoelectrodes prepared by spray pyrolysis with different thicknesses was examined under dark and illumination conditions. The main charge transport phenomena occurring in the PEC cell photoelectrodes were characterized by electrochemical impedance spectroscopy (EIS) operating under dark conditions. The impedance spectra were fitted to an equivalent electrical circuit model for obtaining relevant information concerning reaction kinetics and charge transfer phenomena occurring at the semiconductor/electrolyte interface. A three-electrode configuration was used to carry out the electrochemical measurements allowing a detailed study concerning the double charged layer at the semiconductor/electrolyte interface that arises under dark conditions. The model parameters determined by EIS were then related to the film thickness to assess the role of electronic conduction in the performance of the cell. Moreover, by correlating the sample thickness differences with their electrochemical impedance spectroscopy response, it was possible to discriminate the two main phenomena occurring on semiconductor/electrolyte interfaces of photoelectrochemical systems under dark conditions: the space charge layer and the electrical double layer.

Effect of Drain Doping Profile on Double-Gate Tunnel Field-Effect Transistor and its Influence on Device RF Performance

Abstract: In this paper, we have investigated the effect of drain doping profile on a double-gate tunnel field-effect transistor (DG-TFET) and its radio-frequency (RF) performances. Lateral asymmetric drain doping profile suppresses the ambipolar behavior, improves OFF-state current, reduces the gate-drain capacitance, and improves the RF performance. Further, placing the high-density layer in the channel near the source-channel junction, a reduction in the width of depletion region, improvement in ON-state current (I ON ), and subthreshold slope are analyzed for this asymmetric drain doping. However, it also improves many RF figures of merit for the DG-TFET. Furthermore, lateral asymmetric doping effects on RF performances are also checked for the various channel length. Therefore, this paper would be beneficial for a new generation of RF circuits and systems in a broad range of applications and operating frequencies covering RF spectrum. So, the RF figures of merit for the DG-TFET are analyzed in terms of transconductance (g m ), unit-gain cutoff frequency (f T ), maximum frequency of oscillation (f max ), and gain bandwidth product. For this, the RF figures of merit have been extracted from the V-parameter matrix generated by performing the small-signal ac analysis. Technology computer-aided design simulations have been performed by 2-D ATLAS, Silvaco International, Santa Clara, CA, USA.

Intentionally Carbon-Doped AlGaN/GaN HEMTs: Necessity for Vertical Leakage Paths

Abstract: Dynamic on-resistance (RON) in heavily carbon-doped AlGaN/GaN high electron mobility transistors is shown to be associated with the semi-insulating carbon-doped buffer region. Using transient substrate bias, differences in RON dispersion between transistors fabricated on nominally identical epilayer structures were found to be due to the band-to-band leakage resistance between the buffer and the 2-DEG. Contrary to normal expectations, suppression of dynamic RON dispersion in these devices requires a high density of active defects to increase reverse leakage current through the depletion region allowing the floating weakly p-type buffer to remain in equilibrium with the 2-DEG.

Potential-Induced Degradation (PID): Introduction of a Novel Test Approach and Explanation of Increased Depletion Region Recombination

Abstract: In recent years, a detrimental degradation mechanism of solar cells in large photovoltaic fields called potential-induced degradation (PID) has been intensively investigated and discussed. Here, the module efficiency is decreasing down to a fractional part of their original efficiency. In this study, we introduce a PID test at a solar-cell level and for individual module components applicable as a tool for process control in industries and root cause analyses in science departments. Using the proposed method, one example analysis of a solar cell that is degraded by the PID tester is presented. It is shown that PID of the shunting type influences both the parallel resistance (Rp) and the depletion region recombination behavior (J02) of the solar cell. Increased recombination in the depletion region is caused by Na decorated stacking faults crossing the depletion region. This strongly influences recombination behavior in the depletion region, leading to an increased J02 and an ideality factor n2 > 2. However, the defects leave the base of the solar cell primarily unaffected, and hence, J01 recombination remains rather low. Based on these findings, a model for the shunting and the increased depletion region recombination behavior is discussed.

Fundamental Oscillation up to 1.42 THz in Resonant Tunneling Diodes by Optimized Collector Spacer Thickness

Abstract: We report an increase in the oscillation frequency of terahertz oscillators using AlAs/InGaAs double-barrier resonant tunneling diodes (RTDs) by optimizing the collector spacer thickness. For high-frequency oscillation of RTDs, the electron delay time, which is composed of the dwell time in the resonance region and the transit time in the collector depletion region, must be reduced. Although the transit time is reduced by a thin collector spacer, the capacitance increases. Thus, an optimum thickness of collector spacer layer exists. In this report, we investigate the dependence of oscillation frequency on the collector spacer thickness. The RTDs were integrated with 20-μm-long slot antennas, and oscillations up to 1.1, 1.42, and 1.29 THz were obtained for spacer thicknesses of 25, 12, and 6 nm, respectively. The optimum spacer thickness for high-frequency oscillation was around 12 nm. The highest frequency in this experiment was 1.42 THz oscillation, with an output power of ~1 μW. We also extracted the electron velocity in the collector depletion region and the dwell time from the dependence of the delay time on the collector spacer thickness.

Direct observation of Cu, Zn cation disorder in Cu2ZnSnS4 solar cell absorber material using aberration corrected scanning transmission electron microscopy

Abstract: Chemical analysis of individual atom columns was carried out to determine the crystal structure and local point defect chemistry of Cu2ZnSnS4. Direct evidence for a nanoscale composition inhomogeneity, in the form of Zn enrichment and Cu depletion, was obtained. The lateral size of the composition inhomogeneity was estimated to be between ~1.5 and 5 nm. Photoluminescence confirmed the presence of a broad donor–acceptor transition consistent with the observed cation disorder. Areas of relatively high concentration of ZnCu + antisite atom donors locally increases the electrostatic potential and gives rise to band bending. Troughs in the conduction band and peaks in the valence band are ‘potential wells’ for electrons and holes, respectively. For a solar cell, these prevent minority carrier electrons from diffusing towards the edge of the space charge region, thereby reducing the carrier separation efficiency as well as reducing the carrier collection efficiency of majority carrier holes. Furthermore, electrons and holes ‘trapped’ within potential wells in close proximity have a high probability of recombining, so that the carrier lifetime is also reduced. High quality Cu2ZnSnS4 crystals free from composition inhomogeneities are therefore required for achieving high efficiency solar cell devices.

Operation of resonant-tunneling diodes with strong back injection from the collector at frequencies up to 1.46 THz

Abstract: In search for possibilities to increase the operating frequencies of resonant-tunneling diodes (RTDs), we are studying RTDs working in an unusual regime. The collector side of our diodes is so heavily doped that the collector depletion region is fully eliminated in our RTDs and the ground quantum-well subband stays immersed under (or stays close to) the collector quasi-Fermi level. The electron injection from the collector into the RTD quantum well is very strong in our diodes and stays comparable to that from the emitter in the whole range of RTD operating biases. Our RTDs exhibit well pronounced negative-differential-conductance region and peak-to-valley current ratio around 1.8. We demonstrate operation of our diodes in RTD oscillators up to 1.46 THz. We also observe a fine structure in the emission spectra of our RTD oscillators, when they are working in the regime close to the onset of oscillations.

p‐i‐n Heterojunction Solar Cells with a Colloidal Quantum‐Dot Absorber Layer

Abstract: A quantum-dot (QD) p-i-n heterojunction solar cell with an increased depletion region is demonstrated by depleting the QD layer from both the front and back junctions. Due to a combination of improved charged extraction and increased light absorption, a 120% increase in the short-circuit current is achieved compared with that of conventional ZnO/QD devices.

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.

CuO–ZnO Micro/Nanoporous Array-Film-Based Chemosensors: New Sensing Properties to H2S

Abstract: CuO-ZnO micro/nanoporous array-films are synthesized by transferring a solution-dipped self-organized colloidal template onto a device substrate and sequent heat treatment. Their morphologies and structures are characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectrum analysis. Based on the sensing measurement, it is found that the CuO-ZnO films prepared with the composition of [Cu(2+)]/[Zn(2+)]=0.005, 0.01, and 0.05 all show a nice sensitivity to 10 ppm H2S. Interestingly, three different zones exist in the patterns of gas responses versus H2S concentrations: a platform zone, a rapidly increasing zone, and a slowly increasing zone. Further experiments show that the hybrid CuO-ZnO porous film sensor exhibits shorter recovery time and better selectivity to H2S gas against other interfering gases at a concentration of 10 ppm. These new sensing properties may be due to a depletion layer induced by p-n junction between p-type CuO and n-type ZnO and high chemical activity of CuO to H2S. This work will provide a new construction route of ZnO-based sensing materials, which can be used as H2S sensors with high performances.

Semiconductor devices including superlattice depletion layer stack and related methods

Abstract: A semiconductor device may include an alternating stack of superlattice and bulk semiconductor layers on a substrate, with each superlattice layer including a plurality of stacked group of layers, and each group of layers of the superlattice layer including a plurality of stacked base semiconductor monolayers defining a base semiconductor portion and at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. The semiconductor device may further include spaced apart source and drain regions in an upper bulk semiconductor layer of the alternating stack of superlattice and bulk semiconductor layers, a gate on the upper bulk semiconductor layer between the spaced apart source and drain regions, an STI trench extending through the bulk and superlattice layers into the substrate, and the bulk layers may be doped with alternating dopant conductivity types.

Theoretical study of time-resolved luminescence in semiconductors. II. Pulsed excitation

Abstract: In the second part of this series, we studied TRL decay on semiconductor layers and thin film homostructures after a pulsed excitation by simulation with Synopsys TCAD® and by mathematical approximation. Again, our working example is Cu(In,Ga)Se2. We investigate the influence of the excitation pulse length, axial diffusion, bulk-defects, and defects at the contacts, as well as space charge on the TRL-decay separately by quasi one-dimensional simulations of semiconductor layers and semiconductor homostructures. Material parameters like defect density, carrier mobility, and surface recombination velocity are varied in a wide range, such that the calculations are applicable to other semiconductors. We further study the influence of multi-pulse excitation. We show how material parameters such as carrier lifetime and carrier mobility can be extracted from the TRL transients and how the samples can be characterized by excitation dependent measurements in the open circuit case. We can explain some effects found in luminescence experiments, like an increased decay in semiconductor junctions due to the electric field in the space charge region. However, we also discuss the effect of charge storage which may lead to decreased decay. It is revealed that under high injection conditions single layers within a semiconductor stack can be characterized in terms of carrier lifetime.

Imaging space charge regions in Sm-doped ceria using electrochemical strain microscopy

Abstract: Nanocrystalline ceria exhibits a total conductivity several orders of magnitude higher than microcrystalline ceria in air at high temperature. The most widely accepted theory for this enhancement (based on fitting of conductivity data to various transport and kinetic models) is that relatively immobile positively charged defects and/or impurities accumulate at the grain boundary core, leading to a counterbalancing increase in the number of mobile electrons (small polarons) within a diffuse space charge region adjacent to each grain boundary. In an effort to validate this model, we have applied electrochemical strain microscopy to image the location and relative population of mobile electrons near grain boundaries in polycrystalline Sm-doped ceria in air at 20–200 °C. Our results show the first direct (spatially resolved) evidence that such a diffuse space charge region does exist in ceria, and is localized to both grain boundaries and the gas-exposed surface.

Origin of attendant phenomena of bipolar resistive switching and negative differential resistance in SrTiO3:Nb/ZnO heterojunctions

Abstract: Epitaxial ZnO thin films were grown on SrTiO3:Nb (NSTO) substrates by pulsed laser deposition. The NSTO/ZnO heterojunctions exhibit a typical rectification characteristic under a small voltage, while two attendant behaviors of bipolar resistive switching and negative differential resistance appear under a large voltage. The NSTO/ZnO heterojunctions show extremely weak resistance switching hysteresis without applying a forward bias. However, when the forward bias increases to some extent, the hysteresis becomes more and more prominent and negative differential resistance gradually appears. Furthermore, the high resistance state is obtained when sweeping from negative to positive voltage bias, and vice versa. We propose a model for these behaviors at NSTO/ZnO interface, in which the space charge region in ZnO is wide in high resistance state when the interface state is unoccupied, while the space charge region becomes narrower in low resistance state due to Fermi pinning when the interface state is completely occupied, and the low resistance state is remained until electrons are detrapped from the interface state.

Imaging Charge Separation and Carrier Recombination in Nanowire p-i-n Junctions Using Ultrafast Microscopy

Abstract: Silicon nanowires incorporating p-type/n-type (p-n) junctions have been introduced as basic building blocks for future nanoscale electronic components. Controlling charge flow through these doped nanostructures is central to their function, yet our understanding of this process is inferred from measurements that average over entire structures or integrate over long times. Here, we have used femtosecond pump–probe microscopy to directly image the dynamics of photogenerated charge carriers in silicon nanowires encoded with p-n junctions along the growth axis. Initially, motion is dictated by carrier–carrier interactions, resulting in diffusive spreading of the neutral electron–hole cloud. Charge separation occurs at longer times as the carrier distribution reaches the edges of the depletion region, leading to a persistent electron population in the n-type region. Time-resolved visualization of the carrier dynamics yields clear, direct information on fundamental drift, diffusion, and recombination processes ...

Schottky contacts to In2O3

Abstract: n-type binary compound semiconductors such as InN, InAs, or In2O3 are especial because the branch-point energy or charge neutrality level lies within the conduction band. Their tendency to form a surface electron accumulation layer prevents the formation of rectifying Schottky contacts. Utilizing a reactive sputtering process in an oxygen-containing atmosphere, we demonstrate Schottky barrier diodes on indium oxide thin films with rectifying properties being sufficient for space charge layer spectroscopy. Conventional non-reactive sputtering resulted in ohmic contacts. We compare the rectification of Pt, Pd, and Au Schottky contacts on In2O3 and discuss temperature-dependent current-voltage characteristics of Pt/In2O3 in detail. The results substantiate the picture of oxygen vacancies being the source of electrons accumulating at the surface, however, the position of the charge neutrality level and/or the prediction of Schottky barrier heights from it are questioned.

Large spin accumulation voltages in epitaxial M n 5 G e 3 contacts on Ge without an oxide tunnel barrier

Abstract: Spin injection in high-quality epitaxial $\mathrm{M}{\mathrm{n}}_{5}\mathrm{G}{\mathrm{e}}_{3}$ Schottky contacts on $n$-type Ge has been investigated using a three-terminal Hanle effect measurement. Clear Hanle and inverted Hanle signals with features characteristic of spin accumulation and spin precession are observed up to 200 K. Strikingly, the observed spin voltage is several orders of magnitude larger than predicted by the theory of spin injection and diffusive spin transport. Since the devices have no oxide tunnel barrier, the discrepancy between theory and experiments cannot be explained by the often-invoked spin accumulation in localized states associated with the oxide or oxide/semiconductor interface. The observed spin voltages therefore must originate from the Ge itself, either from spins in the Ge bulk bands or its depletion region.

Impact of GaN cap on charges in Al2O3/(GaN/)AlGaN/GaN metal-oxide-semiconductor heterostructures analyzed by means of capacitance measurements and simulations

Abstract: Oxide/semiconductor interface trap density (Dit) and net charge of Al2O3/(GaN)/AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor (MOS-HEMT) structures with and without GaN cap were comparatively analyzed using comprehensive capacitance measurements and simulations. Dit distribution was determined in full band gap of the barrier using combination of three complementary capacitance techniques. A remarkably higher Dit (∼5–8 × 1012 eV−1 cm−2) was found at trap energies ranging from EC-0.5 to 1 eV for structure with GaN cap compared to that (Dit ∼ 2–3 × 1012 eV−1 cm−2) where the GaN cap was selectively etched away. Dit distributions were then used for simulation of capacitance-voltage characteristics. A good agreement between experimental and simulated capacitance-voltage characteristics affected by interface traps suggests (i) that very high Dit (>1013 eV−1 cm−2) close to the barrier conduction band edge hampers accumulation of free electron in the barrier layer and (ii) the higher Dit cen...

Study of photocurrent generation in InP nanowire-based p + -i-n + photodetectors

Abstract: We report on electrical and optical properties of p+-i-n+ photodetectors/solar cells based on square millimeter arrays of InP nanowires (NWs) grown on InP substrates The study includes a sample series where the p+-segment length was varied between 0 and 250 nm, as well as solar cells with 93% efficiency with similar design The electrical data for all devices display clear rectifying behavior with an ideality factor between 18 and 25 at 300 K From spectrally resolved photocurrent measurements, we conclude that the photocurrent generation process depends strongly on the p+-segment length Without a p+-segment, photogenerated carriers funneled from the substrate into the NWs contribute strongly to the photocurrent Adding a p+-segment decouples the substrate and shifts the depletion region, and collection of photogenerated carriers, to the NWs, in agreement with theoretical modeling In optimized solar cells, clear spectral signatures of interband transitions in the zinc blende and wurtzite InP layers of the mixed-phase i-segments are observed Complementary electroluminescence, transmission electron microscopy (TEM), as well as measurements of the dependence of the photocurrent on angle of incidence and polarization, support our interpretations

Modeling of electric field in silicon micro-strip detectors irradiated with neutrons and pions

Abstract: Edge-TCT method was used to extract velocity profiles in heavily irradiated silicon micro-strip detectors. Detectors were irradiated up to 1016 c -2 with reactor neutrons, 200 MeV pions and a combination of both. A simple electric field model assuming two space charge regions at each side of the detector and neutral bulk in-between was found to describe the field profile. It was observed that after heavy irradiation a sizeable electric field is present in the entire detector volume. For pion-irradiated detectors strikingly different profiles were obtained and attributed to the large oxygen concentration in the detector bulk. The model parameters were also studied during the long term annealing. The space charge region near the strips was found to shrink which in turn leads to larger electric field and impact ionization. The model parameters extracted from the measurements were fed to the device simulation program which showed reasonable agreement between simulated and measured data at lower fluences.

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.