Electron Accumulation Layer Research Articles

Nitrogen vacancies at InN (11¯00) surfaces: A theoretical study

We present an ab initio atomistic study of the effects of nitrogen vacancies both in InN bulk and at (11¯00) nonpolar surface and confirm that these defects act as n-type impurities. Based on the dependence of the vacancy formation energy on the distance from the surface, we predict that at thermodynamic equilibrium these defects tend to segregate in the outermost surface layers reaching a concentration which is few orders of magnitude higher than in the bulk phase. Considering out-of-equilibrium growth and in view of the large calculated energy barrier that characterize vacancy diffusion, we also predict that the N enriched layer will extend hundreds of angstroms from the surface for typical growth processes. This vacancy accumulation at surface is consistent with the observed high electron accumulation layer.

Probe and control of the reservoir density of states in single-electron devices

We present a systematic study of the density of states (DOS) in electron accumulation layers near a ${\text{Si-SiO}}_{2}$ interface. In the experiments we have employed two conceptually different objects to probe DOS, namely, a phosphorus donor and a quantum dot, both operating in the single-electron tunneling regime. We demonstrate how the peaks in reservoir DOS can be moved in the transport window independently of the other device properties. This method introduces a fast and convenient way of identifying excited states in these emerging nanostructures.

Fabrication of AC-coupled double-sided silicon strip sensor

The AC-coupled double-sided silicon microstrip sensors are designed and 14 masks are used to produce prototype strip sensors. The biasing resistors are made of p-doped polysilicons and the coupling capacitors are built by separating implanted strips and readout strips by a thin oxide layer. The p-stops in the atoll patterns are introduced to disrupt an electron accumulation layer at the Si–SiO2 interface on the ohmic side. The junction side has a double metal structure using two metal layers separated from each other by a thick insulator layer. The strip sensors are fabricated on a 5-in., high resistivity, 〈100〉-orientation, and 380μm thick n-type double-sided polished silicon wafer. The prototype sensor with an area of 2.8×2.8cm2 consists of 256(512) readout channels with a strip pitch of 100(50)μm. The leakage currents and the bulk capacitances as a function of the reverse bias voltage are measured to understand the fabrication process of the prototype sensors. The signal to noise ratios are measured by using a 90Sr radioactive source in order to evaluate performance of the sensors. The design of the AC-coupled double-sided silicon strip sensor and its various components are described, and measurements of the electric characteristics and the signal to noise ratio are presented.

Bulk transport measurements in ZnO: The effect of surface electron layers

Magnetotransport measurements and x-ray photoemission spectroscopy were used to investigate the surface conductivity of ZnO. Near-surface downward band bending, consistent with electron accumulation, was found on the polar and nonpolar faces of bulk ZnO single crystals. A significant polarity effect was observed in that the downward band bending was consistently stronger on the Zn-polar face and weaker on the O-polar face. The surface electron accumulation layer was found to significantly influence the electrical properties of high resistivity, hydrothermally grown bulk ZnO crystals at temperatures below 200 K, and is largely responsible for the anomalously low electron mobility reported for this material.

Electrical characterization of an interface n‐type conduction channel in cubic GaN/AlGaN heterostructures

AbstractWe report on the growth of non‐polar cubic GaN/AlxGa1‐xN heterostructures by plasma‐assisted molecular beam epitaxy on free standing (001) 3C‐SiC substrates. The samples consist of 600 nm thick GaN buffer and 30 nm AlxGa1‐xN layer. The growth was observed by in‐situ reflection high energy electron diffraction (RHEED). Layer thickness was determined by AlxGa1‐xN RHEED growth oscillations and by reflectance measurements after growth. The morphological and the structural properties are analyzed by high resolution x‐ray diffraction (HRXRD) and atomic force microscopy (AFM). Using a metal oxide semiconductor heterostructure (MOSH), capacitance voltage (CV) characteristics were measured. The oxide layer consists of SiO2, the thickness was varied between 15 nm and 30 nm. SiO2 layer was produced by plasma enhanced chemical vapour deposition (PECVD). The contact structure is lithographically deposited on top of the sample. Metal contacts have a diameter of 300 μm and were thermally evaporated consisting of 15 nm Ni and 50 nm Au. By CV measurements of the MOSH structures we obtain clear evidence for the existence of an electron accumulation layer at the GaN/AlGaN interface. The sheet carrier concentration at the hetero‐interface is calculated from experimental data (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface structure and chemical states of a-plane and c-plane InN films

The surface electron accumulation layer, surface structure, and surface chemical states of a-plane (nonpolar) and c-plane (polar) InN epitaxial films were investigated. Electrical measurements indicated electron accumulation layers on the surface of both the InN films. Angle-resolved x-ray photoelectron spectroscopy (XPS) measurements indicated a strong band bending at both surfaces, thus confirming the surface electron accumulation. Further XPS analysis of the near-surface chemical states indicated an In adlayer at the surface of c-plane InN and an oxygen adsorbed layer on the a-plane InN. These results suggest different ad-layers to cause the surface electron accumulation on c-plane and a-plane InN.

InAs Nanowire Transistors as Gas Sensor and the Response Mechanism

We report a study of the response of InAs nanowire field-effect transistor sensor devices to various gases and alcoholic vapors. It is concluded that the change in conductance of the device in response to chemical vapors is a combined result of both the charge transfer and modified electron mobility effects. In particular, we found that surface adsorption of most chemical molecules can reduce electron density in nanowires from approximately 10(4) to approximately 10(3)/microm and enhance the electron mobility greatly (from tens to a few hundred of cm(2)/(V s)) at the same time. These effects are attributed to the interactions between adsorbed molecules and the electron accumulation layer and rich surface states on the InAs nanowire surface.

Growth of M ‐plane (10‐10) InN on LiAlO2 (100) substrate

AbstractIn this study, we report the growth and characterization of M ‐plane InN films on LiAlO2 (100) substrates by radio‐frequency plasma assisted molecular beam epitaxy (RF‐MBE). InN films were grown at various temperatures and under various V/III ratios on the substrates. Pure M ‐plane InN films were successfully grown at a high temperature of 450 °C and under a slightly In‐rich condition, while the incorporation of C ‐plane phase was observed in M ‐plane InN films grown at low temperatures of less than 400 °C or under a N‐rich condition. These indicate that controls of growth temperature and V/III ratio are important for the growth of pure M ‐plane InN films. The in‐plane epitaxial relationships of M ‐plane InN on LiAlO2 (100) were [0001]InN // [010]LiAlO2 and [1‐210]InN // [001]LiAlO2. A surface electron accumulation layer on the obtained M ‐plane InN film is also discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of InN nanocolumns: Electron accumulation at InN non‐polar surfaces and dependence on the growth conditions

AbstractInN nanocolumns grown by plasma‐assisted molecular beam epitaxy have been studied by photoluminescence (PL) and photoluminescence excitation (PLE). The PL peak energy was red‐shifted with respect to the PLE onset and both energies were higher than the low temperature band‐gap reported for InN. PL and PLE experiments for different excitation and detection energies indicated that the PL peaks were homogeneously broadened. This overall phenomenology has been attributed to the effects of an electron accumulation layer present atthe non‐polar surfaces of the InN nanocolumns. Variations in the growth conditions modify the edge of the PLE spectra and the PL peak energies evidencing that the density of free electrons can be somehow controlled by the growth parameters. It was observed that In‐BEP and substrate temperature leading to shorter In diffusion lengths diminished the effects of the electron accumulation layer on the optical properties. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of a two-dimensional electron gas in ZnO/MgZnO single heterostructures and quantum wells

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112̄0) and ZnO (0001̄) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance–voltage ( C– V) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C– V measurements the sheet carrier density in a Zn 0.8 Mg 0.2O/ZnO/Zn 0.8 Mg 0.2O quantum well (QW) structure with a well width of about 5 nm is calculated to be only about 9.0 × 10 10 cm − 2 . For the films deposited on sapphire 2D growth is observed in the Burton–Cabrera–Frank mode, as confirmed by atomic force microscopy. Step flow growth mode was achieved for the homoepitaxial thin films. Quantum confinement effects have been confirmed by photoluminescence (PL) measurements. Homoepitaxial QWs are more homogeneous (smaller inhomogeneous recombination broadening) than heteroepitaxial QWs.

Electrical Conductivity of InN Nanowires and the Influence of the Native Indium Oxide Formed at Their Surface

The electrical properties of InN nanowires were investigated in four-point probe measurements. The dependence of the conductance on the wire diameter allows distinguishing between "core" bulk (quadratic) and "shell" sheet (linear) contributions. Evidence of the formation of a thin In(2)O(3) layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In(2)O(3) interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.

Optical studies of MBE-grown InN nanocolumns: Evidence of surface electron accumulation

Vertically self-aligned InN nanocolumns have been investigated by means of scanning electron microscopy, Raman scattering, and photoluminescence spectroscopy. Different nanocolumn morphologies corresponding to different molecular beam epitaxy growth conditions have been studied. Raman spectra revealed strain-free nanocolumns with high crystalline quality for the full set of samples studied. Longitudinal optical modes both uncoupled and coupled to an electron plasma coexist in the Raman spectra pointing to the existence of two distinctive regions in the nanocolumn: a surface layer of degenerated electrons and a nondegenerated inner core. The characteristics of the low-temperature photoluminescence and its dependence on temperature and excitation power can be explained by a model considering localized holes recombining with degenerated electrons close to the nonpolar surface. The differences observed in the optical response of different samples showing similar crystalline quality have been attributed to the variation in the electron accumulation layer with the growth conditions.

Transport Coefficients of InAs Nanowires as a Function of Diameter

Wire transport: The diameter-dependent transport coefficients in InAs nanowires (see picture) are studied at equal vertical and lateral fields and at equal gate–source voltage (VGS) sweep rates. Simulations with a one-dimensional Schrödinger–Poisson solver support the presence of an electron accumulation layer near the positive interface-state charges that lead to the observed trends.

Organic Light‐Emitting Diodes with Field‐Effect‐Assisted Electron Transport Based on α‐bi;,ω‐bi;‐Diperfluorohexyl‐quaterthiophene

AbstractMaterials commonly used in the carrier transport layers of organic light‐emitting diodes, where transport occurs through the bulk, are in general very different from materials used in organic field‐effect transistors, where transport takes place in a very thin accumulation channel. In this paper, the use of a high‐performance electron‐conducting field‐effect transistor material, diperfluorohexyl‐substituted quaterthiophene (DFH‐4T), as the electron‐transporting material in an organic light‐emitting diode structure is investigated. The organic light‐emitting diode has an electron accumulation layer in DFH‐4T at the organic hetero‐interface with the host of the light‐emitting layer, tris(8‐hydroxyquinoline) aluminum (Alq3). This electron accumulation layer is used to transport electrons and inject them into the active emissive host‐guest layer. By optimizing the growth conditions of DFH‐4T for electron transport at the organic hetero‐interface, high electron current densities of 750 A cm−2 are achieved in this innovative light‐emitting structure.

Separation of bulk and surface electron transport in metamorphic InAs layers using quantitative mobility spectrum analysis

Electron transport in low dislocation density, strain-relaxed InAs layers grown on metamorphic InAsyP1−y∕InP substrates by molecular beam epitaxy was characterized using quantitative mobility spectrum analysis (QMSA) of Hall effect measurements. QMSA applied to systematically varied metamorphic InAs samples reveals high bulk electron mobilities of ∼20000cm2∕Vs at 300K at a Si doping concentration of 1×1017cm−3, simultaneously with a separate population of much slower electrons having an average mobility of ∼2400cm2∕Vs due to parallel conduction within the InAs surface electron accumulation layer. Measurements made on higher doped samples reveal only a single electron population participating in transport due to lowered surface band bending that reduces surface accumulation of electrons in conjunction with the high conductivity of the high mobility metamorphic InAs bulk that overwhelms any remaining surface conductivity in the Hall effect measurements.

Surface electronic properties of undoped InAlN alloys

The variation in surface electronic properties of undoped c-plane InxAl1−xN alloys has been investigated across the composition range using a combination of high-resolution x-ray photoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys, electron accumulation layers, accompanied by a downward band bending, are present at the surface, with a decrease to approximately flatband conditions with increasing Al composition. However, for the Al-rich alloys, the undoped samples were found to be insulating with approximate midgap pinning of the surface Fermi level observed.

InN nanotips as excellent field emitters

Unidirectional single crystalline InN nanoemitters were fabricated on the silicon (111) substrate via ion etching. These InN nanoemitters showed excellent field emission properties with the threshold field as low as 0.9V∕μm based on the criterion of 1μA∕cm2 field emission current density. This superior property is ascribed to the double enhancement of (1) the geometrical factor of the InN nanostructures and (2) the inherently high carrier concentration of the degenerate InN semiconductor with surface electron accumulation layer induced downward band bending effect that significantly reduced the effective electron tunneling barrier even under very low external field.

Nonparabolic coupled Poisson-Schrödinger solutions for quantized electron accumulation layers: Band bending, charge profile, and subbands at InN surfaces

The one-electron potential, carrier concentration profile, quantized subband state energies, and parallel dispersion relations are calculated for an accumulation layer at a semiconductor surface by solving Poisson's equation within a modified Thomas-Fermi approximation and numerically solving the Schrodinger equation for the resulting potential well. A nonparabolic conduction band, described within the Kane k.p approximation, is incorporated in the model. Example calculations are performed for a typical clean InN surface and for a variety of surface state densities and bulk carrier concentrations. Agreement is found between the model calculations and experimental measurements of the subband energies and dispersions at c-plane InN surfaces from electron tunneling spectroscopy and angle resolved photoemission spectroscopy.

Mechanism of Electron Accumulation Layer Formation at the MBE-grown InAs(111)A Surface

Electronic properties, related to the correlation between quantum states and Fermi level (EF) at the semiconductor boundaries, are discussed as one of the essential issues for understanding nanometer-scale physics and its technological application. To understand the mechanism of surface EF stabilization, the origin of two-dimensional electrons in the native electron accumulation layer was explored using a low-temperature scanning tunneling microscope in ultra-high vacuum at the clean (111)A surface of the undoped InAs thin film grown by molecular beam epitaxy. According to direct comparison of densities between surface defects and accumulated electrons, it was found that the electron accumulation layer was introduced by the two-dimensionally distributed native donor point defects composed of In adatoms and In antisites at the density of 1012 cm−2. The mechanism of the surface EF pinning was explained using a model based on the shallow defect band formation due to highly formed surface donors. It was shown that this surface band was composed of the hydrogenic donor bound states and delocalized due to many-body effects induced by the Mott transition in correlated donor-electron system at the clean surface, and was not composed of the atomic orbitals of point defects or the dangling bonds of surface atoms.

Examining the Anomalous Electrical Characteristics Observed in InN Nanowires

Research interest in InN has intensified in recent years because of its unique material properties and promising applications in electronic and photonic devices. Measurements on InN nanowires presented by Chang et al., [J. Electron. Mater. 35, 738 (2006)] showed an anomalous resistance behavior in InN nanowires with diameters less than 90 nm. We examine possible theories presented in literature to explain this intriguing observation. We propose that the presence of a high density electron accumulation layer at the surface of thin InN nanowires is the most probable cause for the uncharacteristic relationship between the total measured resistance and the ratio of length-to-area. High density surface electron accumulation layer, characteristic of InN films and nanowire, promotes a surface conduction path distinct from the bulk conduction. For large diameter nanowires, bulk conduction is likely to be the dominant mechanism while surface conduction is proposed to play a major role for small diameter InN nanowires.