Nikolai B. Zhitenev

National Institute of Standards and Technology, Maryland, United States

Are you Nikolai B. Zhitenev?

Claim your profile

Publications (40)158.56 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Electron beam induced current (EBIC) is a powerful technique which measures the charge collection efficiency of photovoltaics with sub-micron spatial resolution. The exciting electron beam results in a high generation rate density of electron-hole pairs, which may drive the system into nonlinear regimes. An analytic model is presented which describes the EBIC response when the {\it total} electron-hole pair generation rate exceeds the rate at which carriers are extracted by the photovoltaic cell, and charge accumulation and screening occur. The model provides a simple estimate of the onset of the high injection regime in terms of the material resistivity and thickness, and provides a straightforward way to predict the EBIC lineshape in the high injection regime. The model is verified by comparing its predictions to numerical simulations in 1 and 2 dimensions. Features of the experimental data, such as the magnitude and position of maximum collection efficiency versus electron beam current, are consistent with the 3 dimensional model.
    10/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The local collection characteristics of grain interiors and grain boundaries in thin film CdTe polycrystalline solar cells are investigated using scanning photocurrent microscopy. The carriers are locally generated by light injected through a small aperture (50-300 nm) of a near-field scanning optical microscope in an illumination mode. Possible influence of rough surface topography on light coupling is examined and eliminated by sculpting smooth wedges on the granular CdTe surface. By varying the wavelength of light, nanoscale spatial variations in external quantum efficiency are mapped. We find that the grain boundaries (GBs) are better current collectors than the grain interiors (GIs). The increased collection efficiency is caused by two distinct effects associated with the material composition of GBs. First, GBs are charged, and the corresponding built-in field facilitates the separation and the extraction of the photogenerated carriers. Second, the GB regions generate more photocurrent at long wavelength corresponding to the band edge, which can be caused by a smaller local band gap. Resolving carrier collection with nanoscale resolution in solar cell materials is crucial for optimizing the polycrystalline device performance through appropriate thermal processing and passivation of defect and surfaces.
    ACS Nano 10/2014; · 12.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Ideally, an EBIC measurement reflects the spatially resolved quantum efficiency of the device. In this work, a model for EBIC measurements is presented which applies when recombination within the depletion region is substantial. This model is motivated by cross-sectional EBIC experiments on CdS-CdTe photovoltaic cells which show that the maximum efficiency of carrier collection is less than 100 \% and varies throughout the depletion region. The model can reproduce experimental results only if the mobility-lifetime product $\mu\tau$ is spatially varying within the depletion region. The reduced collection efficiency is speculated to be related to high-injection effects, and the resulting increased radiative recombination.
    09/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Quantitative determination of electronic properties at high spatial resolution is crucial for the development of high-efficiency solar cells. Electron beam induced current (EBIC) is a powerful technique in which electron-hole pairs are created in proximity to an exposed surface, and the carrier collection efficiency is measured as a function of excitation position. Cross-sections of device are often created by focused ion beams (FIB) due to the flexibility of the patterning and milling processes. However, the irradiating Ga ions of the FIB fabrication may introduce unintended artifacts, affecting local electronic properties. In this study, we investigate the impact of the FIB process observed in EBIC measurements and two-dimensional finite element simulations.
    Microscopy and Microanalysis; 08/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: A thin film of quantum dots (QD) was used to visualize the local photo-response of polycrystalline CdTe solar cells by down-converting an electron beam of high energy to photons of visible light. The efficient photon generation in the QD film is compared to cathodoluminescence of the high-purity bulk semiconductors and phosphor.
    SID Symposium Digest of Technical Papers. 06/2014; 45(1).
  • [Show abstract] [Hide abstract]
    ABSTRACT: To boost the efficiency of thin-film polycrystalline solar cells that are microscopically inhomogeneous, it is imperative to understand how the grain interiors (GIs) and grain boundaries (GBs) within these materials affect its overall electronic properties. By using an apertured near-field scanning optical microscope in an illumination mode, we determined the local photocurrent that is generated within the GIs and at the GBs with nanoscale resolution and correlate the results with surface morphology and composition.
    IEEE Journal of Photovoltaics 01/2014; 4(1):311-316. · 3.00 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Modulated photocurrent spectroscopy was used to investigate the dynamic response of charge carrier transport in thin film CdTe/CdS solar cells. The impact of light bias and temperature over a broad excitation frequency range were measured. The observed features of the data, including a photocurrent ‘phase-lead’ and a ‘phase-lag’ over different regions of the frequency spectrum, were explored in the context of an equivalent circuit model. Comparisons between the model's predicted performance and the measured data suggest that charge carrier recombination at the cell's back metal/semiconductor contact is the main source of photocurrent loss in these devices.
    Solar Energy Materials and Solar Cells 09/2013; 116:126–134. · 5.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate local electronic properties of cadmium telluride solar cells using electron beam induced current (EBIC) measurements with patterned contacts. EBIC measurements are performed with a spatial resolution as high as ≈20 nm both on the top surface and throughout the cross-section of the device, revealing a remarkable degree of electrical inhomogeneity near the p–n junction and enhanced carrier collection in the vicinity of grain boundaries (GB). Simulation results of low energy EBIC suggest that the band bending near a GB is downward, with a magnitude of at least 0.2 eV for the most effective current-collecting GBs. Furthermore, we demonstrate a new approach to investigate local open-circuit voltage by applying an external bias across electrical contact with a point electron-beam injection. The length scale of the nanocontacts is on the length scale of a single or a few grains, confining current path with highly localized photo-generated carriers.
    Solar Energy Materials and Solar Cells 08/2013; 117:499. · 5.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The power conversion efficiency of commercial solar modules based on thin-film chalcogenide materials is well below the theoretical limits. To understand the underlying physical mechanisms limiting the efficiency, we investigate local photovoltaic properties isolating the difference between the grain bulk (0.5-2 mkm in size) and the grain boundary in CdTe absorber. Local current-voltage measurements are performed using nano-contacts in conjunction with local electron-hole pairs generation comparing multiple injection techniques. First, the carriers are excited using variable energy electron beam enabling measurements with a spatial resolution down to 20 nm. Second, we have developed a novel approach for high-resolution and high-throughput photocurrent imaging downconverting electron beam into a near-field optical source using a thin film (50 nm) of phosphors. The electron beam is fully absorbed in the phosphors layer, and the cathodoluminescence is used as a local photon source. Third, we generate carriers using a near-filed optical microscope varying the excitation wavelength. The results show that, in a well-optimized material, a large fraction of grain boundaries displays higher photocurrent as compared to grain bulk effectively serving as a three-dimensional distributed photocurrent collector.
    03/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We used the modulated photocurrent spectroscopy technique based on sinusoidal excitation of high-powered LEDs to investigate the dynamic response of charge carrier transport in thin film solar cells based on CdTe. The impact of light bias, voltage bias and the temperature over a broad excitation frequency bandwidth were studied. The observed features of the data, including a photocurrent phase-lead and a phase-lag over different regions of the frequency spectrum, were explored in the context of an equivalent circuit model. Comparisons between the model's predicted performance and the measured data suggest that charge carrier recombination at the cell's back metal/semiconductor contact is the main source of photocurrent loss in the cells that were investigated by our group.
    03/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a fast, versatile photocurrent imaging technique to visualize the local photo response of solar energy devices and optoelectronics using near-field cathodoluminescence (CL) from a homogeneous quantum dot layer. This approach is quantitatively compared with direct measurements of high-resolution Electron Beam Induced Current (EBIC) using a thin film solar cell (n-CdS / p-CdTe). Qualitatively, the observed image contrast is similar, showing strong enhancement of the carrier collection efficiency at the p-n junction and near the grain boundaries. The spatial resolution of the new technique, termed Q-EBIC (EBIC using quantum dots), is determined by the absorption depth of photons. The results demonstrate a new method for high-resolution, sub-wavelength photocurrent imaging measurement relevant for a wide range of applications.
    AIP Advances. 01/2013; 3.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In graphene, as in most metals, electron-electron interactions renormalize the properties of electrons but leave them behaving like noninteracting quasiparticles. Many measurements probe the renormalized properties of electrons right at the Fermi energy. Uniquely for graphene, the accessibility of the electrons at the surface offers the opportunity to use scanned probe techniques to examine the effect of interactions at energies away from the Fermi energy, over a broad range of densities, and on a local scale. Using scanning tunneling spectroscopy, we show that electron interactions leave the graphene energy dispersion linear as a function of excitation energy for energies within �200 meV of the Fermi energy. However, the measured dispersion velocity depends on density and increases strongly as the density approaches zero near the charge neutrality point, revealing a squeezing of the Dirac cone due to interactions.
    Physical Review Letters 09/2012; 109:116802. · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We determined the electromechanical properties of a suspended graphene layer by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, as well as computational simulations of the graphene-membrane mechanics and morphology. A graphene membrane was continuously deformed by controlling the competing interactions with a STM probe tip and the electric field from a back-gate electrode. The probe tip-induced deformation created a localized strain field in the graphene lattice. STS measurements on the deformed suspended graphene display an electronic spectrum completely different from that of graphene supported by a substrate. The spectrum indicates the formation of a spatially confined quantum dot, in agreement with recent predictions of confinement by strain-induced pseudomagnetic fields.
    Science 06/2012; 336(6088):1557-61. · 31.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The relation between macroscopic charge transport properties and microscopic carrier distribution is one of the central issues in the physics and future applications of graphene devices (GDs). We find strong conductance enhancement at the edges of GDs using scanning gate microscopy. This result is explained by our theoretical model of the opening of an additional conduction channel localized at the edges by depleting accumulated charge by the tip.
    Nano Letters 03/2012; 12(4):1839-44. · 13.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Realization of the intrinsic electronic properties of graphene devices has been limited by charge scattering and surface roughness found when graphene is placed on SiO2 substrates. Recently, graphene devices fabricated on hexagonal boron-nitride (h-BN) dielectrics have shown superior device performance compared with graphene placed directly on SiO2 substrates. We have performed scanning tunneling microscopy / spectroscopy (STM / STS) measurements to investigate the local electronic structure of graphene devices on h-BN substrates as a function of charge density and magnetic field. The disorder potential is significantly reduced compared with graphene in direct contact with SiO2. Correspondingly, the widths of Landau levels (LLs) are much narrower becoming comparable to those measured in epitaxial graphene on SiC. The energy and the spatial dispersion of LLs is used to analyze the Fermi velocity of the Dirac particles at different charge densities, an electron-hole asymmetry, and discrete splittings of LLs due to residual spatially varying disorder potential.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have performed scanning tunneling spectroscopy measurements on gated-graphene devices in the quantum Hall regime under varying disorder potential landscapes. Relatively thin hexagonal boron-nitride (h-BN) crystals are mechanically exfoliated on SiO2/Si substrates and single-layer graphene films are later transferred on pre-located h-BN crystals. In this device scheme, we can investigate the interactions of Dirac particles with local impurities ranging from strongly disordered to weakly perturbed environments by adjusting the thickness of h-BN crystals, while varying both the Fermi-energy with respect to a Dirac point and magnetic field. In the h-BN devices, we have observed that the electron-hole puddles are larger in lateral size than those observed on SiO2 devices, and resonance scatterings are significantly reduced due to weakened disorder potentials. Accordingly, we start observing well-defined Landau levels (LLs) as early as 0.5 T and the width of individual LLs, broadened by the scattering of charged carriers, is much narrower than those from graphene on SiO2. In high magnetic fields, we observe the electronic structure of graphene devices is significantly altered by the electron-electron interactions and the formation of large interaction energy gaps. We will discuss the spatial, orbital quantum number, and magnetic field dependence of the observed interaction gaps.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate local electronic properties of CdTe solar cells using electron beam to excite electron-hole pairs and evaluate spatially resolved photocurrent characteristics. Standard semiconductor processes were used to fabricate Ohmic metal contacts on the surface of p-type CdTe / n-type CdS device extracted from a commercial solar panel. An ion milling process was used to prepare cross-sections of the devices. Local injection of carriers was controlled by an acceleration voltage of electron beam (1 kV to 30 kV) in a scanning electron microscope, and the results were correlated with the local morphology, microstructure, and chemical composition of the devices.
    Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: When graphene is close to charge neutrality, its energy landscape is highly inhomogeneous, forming a sea of electron-like and hole-like puddles, which determine the properties of graphene at low carrier density. However, the details of the puddle formation have remained elusive. We demonstrate numerically that in sharp contrast to monolayer graphene, the normalized autocorrelation function for the puddle landscape in bilayer graphene depends only on the distance between the graphene and the source of the long-ranged impurity potential. By comparing with available experimental data, we find quantitative evidence for the implied differences in scanning tunneling microscopy measurements of electron and hole puddles for monolayer and bilayer graphene in nominally the same disorder potential.
    Physical review. B, Condensed matter 12/2011; 84(23). · 3.77 Impact Factor
  • Source
    Yuxiang Liu, Hua Xu, Felix Stief, Nikolai Zhitenev, Miao Yu
    [Show abstract] [Hide abstract]
    ABSTRACT: We present experimental demonstration of light superfocusing by using an optical fiber based surface plasmonic (SP) lens with nanoscale concentric annular slits. A far-field, sub-diffraction-limit sized focus was achieved with an optical fiber based device. The performance of SP lenses with three and four annular slits was experimentally characterized. Guidelines and suggestions on designing the SP lens are provided. As a microscale device with nanoscale features, the fiber-based SP lens can provide a solution to bridging nanophotonics and conventional optics.
    Optics Express 10/2011; 19(21):20233-43. · 3.55 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Despite recent progress in understanding geometric structure, electronic structure, and transport properties in a graphene device (GD), role of point defects, edges, traps in a GD or a gate insulator has been poorly defined. We have studied electronic and geometric structures of these defects using scanning probe microscopy and try to link those with the transport properties of the GD. We perform scanning gate microscopy study to understand the local carrier scattering. It was found that geometric corrugations, defects and edges directly influence the local transport current. This observation is linked directly with a proposed scattering model based on macroscopic transport measurements. We suggest that dangling bonds in insulator-material SiO2 mainly used in GDs produce charge puddles and they work as scattering centers.
    Microelectronic Engineering 07/2011; 88(7):1211-1213. · 1.22 Impact Factor