Surface Carrier Concentration Research Articles

In induced reconstructions of Si(1 1 1) as superlattice matched epitaxial templates for InN growth

Indium induced surface reconstructions of Si(111)-7×7 are used as templates to grow high quality InN. We grow InN on Si(111)-7×7, Si(111)-4×1-In and Si(111)-1×1-In reconstructed surfaces and study the quality of the films formed using complementary characterization tools. InN grown on Si(111)-1×1-In reconstruction shows superior film quality with lowest band-edge emission having a narrow full width at half maximum, intense and narrow 0002 X-ray diffraction, low surface roughness and carrier concentration an order lower than other samples. We attribute the high quality of the film formed at 300°C to the integral matching of InN and super lattice dimensions, we also study the reasons for the band gap variation of InN in the literature. Present study demonstrates the proposed Superlattice Matched Epitaxy can be a general approach to grow good quality InN at much lower growth temperature on compatible In induced reconstructions of the Si surface.

Surface passivation of Cu(In,Ga)Se2 using atomic layer deposited Al2O3

With Al2O3 passivation on the surface of Cu(In,Ga)Se2, the integrated photoluminescence intensity can achieve two orders of magnitude enhancement due to the reduction of surface recombination velocity. The photoluminescence intensity increases with increasing Al2O3 thickness from 5 nm to 50 nm. The capacitance-voltage measurement indicates negative fixed charges in the film. Based on the first principles calculations, the deposition of Al2O3 can only reduce about 35% of interface defect density as compared to the unpassivated Cu(In,Ga)Se2. Therefore, the passivation effect is mainly caused by field effect where the surface carrier concentration is reduced by Coulomb repulsion.

Surfactantless photochemical growth of Ag nanostructures on GaN epitaxial films with controlled morphologies and their application for SERS

Ag nanostructures with controlled morphologies have been synthesized on GaN epitaxial films through a convenient, fast, and surfactantless photochemical method. On n-type GaN, uniform spheroidic Ag nanoparticles (NPs) of high density were obtained after photochemical reaction for a short time (30 s to 5 min) in AgNO3 aqueous solution. Reducing AgNO3 concentration yielded branched Ag nanostructures. The Ag NPs on the GaN substrates exhibit a Fabry–Perot-type surface plasmon resonance (SPR). On p-type GaN, Ag nanocrystals (NCs) with well-defined shapes ranging from cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedra were obtained through varying the concentration of AgNO3 solution. We propose that the formation of polyhedral Ag NCs is induced by the separation of the nucleation and growth processes by the GaN surface. Both surface states and carrier concentration are considered to play substantial roles during the growth process. This surfactantless photochemical method is general and enables the facile synthesis of Ag nanostructures at room temperature, which may be extended for the synthesis of other noble metal nanostructures, e.g.Au, Pt, and Pd, on GaN or other wide bandgap semiconductors, e.g.SiC. The obtained Ag/GaN hybrid systems show potential application as solid substrates for surface enhanced Raman scattering (SERS) analysis.

Effect of an Ultraviolet-Ozone Treatment on the Electrical Properties of Titanium-Oxide Thin-Film Transistors Fabricated by Using a Sol-Gel Process

Thin-film transistors (TFTs) utilizing titanium-oxide channel layers were fabricated by using a sol-gel process. The device characteristics of the channel layer for the TFTs were enhanced, resulting in a shift of the threshold voltage and in a decrease of the off-current. The on/off current ratio of the ultraviolet-ozone-treated TFTs was as large as 4.78 × 105. The surface carrier concentration of the titanium-oxide was decreased due to the decrease in the oxygen deficiency resulting from the negatively-charged oxygen.

Ortho-II band folding in YBa2Cu3O7−δfilms revealed by angle-resolved photoemission

We present an angle-resolved photoelectron spectroscopy study of YBa2Cu3O7-delta films in situ grown by pulsed laser deposition. We have successfully produced underdoped surfaces with ordered oxygen vacancies within the CuO chains resulting in a clear ortho-II band folding of the Fermi surface. This indicates that order within the CuO chains affects the electronic properties of the CuO2 planes. Our results highlight the importance of having not only the correct surface carrier concentration, but also a very well ordered and clean surface in order that photoemission data on this compound be representative of the bulk.

Fabrication of Metal–Oxide–Diamond Field-Effect Transistors with Submicron-Sized Gate Length on Boron-Doped (111) H-Terminated Surfaces Using Electron Beam Evaporated SiO2 and Al2O3

A H-terminated surface conductive layer of B-doped diamond on a (111) surface was used to fabricate a metal–oxide–semiconductor field-effect transistor (MOSFET) using an electron beam evaporated SiO2 or Al2O3 gate insulator and a Cu-metal stacked gate. When the bulk carrier concentration was approximately 1015/cm3 and the B-doped diamond layer was 1.5 μm thick, the surface carrier mobility of the H-terminated surface on the (111) diamond before FET processing was 35 cm2/Vs and the surface carrier concentration was 1.5 × 1013/cm2. For the SiO2 gate (0.76 μm long and 50 μm wide), the maximum measured drain current at a gate voltage of −3.0 V was −75 mA/mm and the maximum transconductance was 24 mS/mm, and for the Al2O3 gate (0.64 μm long and 50 μm wide), these features were −86 mA/mm and 15 mS/mm, respectively. These values are among the highest reported direct-current (DC) characteristics for a diamond homoepitaxial (111) MOSFET.

Effect of surface carrier concentration on valence subbands in Si(111)p-type inversion layers: Angle-resolved photoemission spectroscopy

We investigated the effect of surface carrier concentration on the quantum levels and the in-plane effective masses of the subbands in Si(111) $p$-type inversion layers. Two inversion layers with different surface carrier concentrations were made as a result of metallic surface structures. The subband dispersion was measured by angle-resolved photoelectron spectroscopy. The energy levels of the observed subbands at $k=0.0\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ partially agree with the results of the calculation using the triangular approximation. The effective masses are not significantly affected by the surface carrier concentration. The spatial extension of the subband wave functions is discussed.

Effect of carrier recombination mechanisms on the open circuit voltage of n+-p GaInAsSb thermophotovoltaic cells

By analyzing the main recombination mechanisms in GaInAsSb materials, the dependences of the dark current density and open circuit voltage in n+-p GaInAsSb thermophotovoltaic cells on the recombination parameters, carrier concentration and cell thickness are calculated. The results show that the dark current mainly comes from p-region, and it is related with the surface and Auger recombinations in low and high carrier concentration ranges, respectively. The surface and Auger recombinations can be suppressed by reducing the surface recombination velocity and carrier concentration, respectively. The dark current density can be suppressed by optimizing material parameters and device surface passivation technique. So the high open circuit voltage can be obtained for GaInAsSb thermophotovoltaic cells.

Study of Low-Energy Doping Processes Using Continuous Anodic Oxidation Technique/Differential Hall Effect Measurements

Comparing with conventional spreading resistance profiling and differential Hall effect (DHE) methods, the continuous anodic oxidation technique/DHE (CAOT/DHE) technique may achieve more reasonable profiles of carrier concentration nh(x), mobility muh(x), and resistivity rho(x) and more reasonable carrier dose and xj in Si substrate. It has been successfully used to study ultralow energy doping techniques including B beam-line implant and B2H6 plasma doping (PLAD). CAOT/DHE data support the fact that the devices fabricated by PLAD achieve improvement to those fabricated by beam-line implant because PLAD offered higher surface carrier concentration and carrier dose. CAOT/DHE data quantitatively verify the so-called solid solubility limit activation theory - the carrier profiles and secondary ion mass spectrometry (SIMS) B impurity profiles under BSS are very well consistent on both beam-line and PLAD implants. As a cheaper and standard metrology, the SIMS/ARXPS method with the solid solubility limit activation theory may be used to quantitatively or semiquantitatively study the doping and activation processes.

Hole mobility enhancement by double-gate mode in ultrathin-body silicon-on-insulator p-type metal-oxide-semiconductor field-effect transistors

Hole mobility enhancement in double-gate (DG) ultrathin-body (UTB) silicon-on-insulator (SOI) p-type metal-oxide-semiconductor field-effect transistors (MOSFETs) is investigated systematically in comparison with single-gate (SG) UTB MOSFETs for various SOI thicknesses (TSOI) ranging from 2 to 30 nm. It is found that mobility in DG mode (μDG) is higher than that in SG mode (μSG) in all the measured ranges of TSOI and the surface carrier concentrations (Ns). In particular, enhancement of mobility of the sub-10-nm-thick TSOI devices is greater at higher Ns. As a result, it is demonstrated that μDG of the 8.1-nm-thick TSOI device is 12.9% greater than the universal mobility when Ns is 6×1012 cm−2. Higher μDG is attributed to the average effective mass reduction in DG mode because of the increased population in the light hole (LH) bands. Careful investigation also clarifies that mobility in the sub-10-nm-thick TSOI devices is enhanced greatly at higher Ns, since the DG mode-induced increase in the hole population in LH bands is enhanced at higher Ns; the reduction in the hole population in LH bands due to the high-Ns-enhanced carrier confinement in the normal direction is larger in SG mode than in DG mode.

Analysis and Modeling of Leakage Current for Four-Terminal MOSFET in Off-State and Low Leakage Switches

Leakage current for MOSFET in off-state is one of the serious problems in charge-based analog circuits under low power supply. To suppress the leakage current, a method that a slight voltage is applied to source to accomplish reverse bias between source and bulk is proposed. The proposed bias condition, also other bias conditions, is analyzed by injection carrier density in p-n junction and surface carrier concentration in MOS diode in four-terminal MOSFET. Leakage current is modeled by combining the characteristics of p-n junction with MOS diode in MOSFET. The characteristics of MOSFET fabricated with a standard 0.18µm n-well CMOS technology are measured to investigate the basic principle. Measured leakage current fits to the theoretical leakage current exactly. The proposed slight bias to source terminal in MOSFET is proved most efficient to reduce the leakage current. Based on the proposed source bias condition, MOSFET switches with low leakage current under a single power supply are proposed.

Effects of substrate temperature on the structural and electrical properties of Cu(In,Ga)Se 2 thin films

Polycrystalline Cu(In,Ga)Se 2 (CIGS) thin films were deposited onto soda-lime glass substrates using the three-stage process at the substrate temperature ( T sub) varying from 350 to 550 °C. The effect of T sub on the structural and electrical properties of CIGS films has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Hall effect measurement. We found that the surface roughness, constituent phases, film morphologies, resistivity ( ρ) and carrier concentration ( N P) of as-grown CIGS films indicated different change trends with increase in T sub. The higher T sub gives smooth surface, large grain size and single-phase CIGS films. The values of N P and ρ have two demarcated regions at T sub of 380 and 450 °C. At lower T sub of 380 °C, larger N P and lower ρ were dominated by the existence of secondary-phase Cu x Se with lower resistivity. In the case of 450 °C, the obvious changes in N P and ρ can be attributed to the sufficient Na incorporation diffused from the glass substrate. Finally, the correlation of cell parameters with T sub was analyzed.

Carrier-Transport-Enhanced Channel CMOS for Improved Power Consumption and Performance

An effective way to reduce supply voltage and resulting power consumption without losing the circuit performance of CMOS is to use CMOS structures using high carrier mobility/velocity. In this paper, our recent approaches in realizing these carrier-transport-enhanced CMOS will be reviewed. First, the basic concept on the choice of channels for increasing on current of MOSFETs, the effective-mass engineering, is introduced from the viewpoint of both carrier velocity and surface carrier concentration under a given gate voltage. Based on this understanding, critical issues, fabrication techniques, and the device performance of MOSFETs using three types of channel materials, Si (SiGe) with uniaxial strain, Ge-on-insulator (GOI), and III-V semiconductors, are presented. As for the strained devices, the importance of uniaxial strain, as well as the combination with multigate structures, is addressed. A novel subband engineering for electrons on (110) surfaces is also introduced. As for GOI MOSFETs, the versatility of the Ge condensation technique for fabricating a variety of Ge-based devices is emphasized. In addition, as for III-V semiconductor MOSFETs, advantages and disadvantages on low effective mass are examined through simple theoretical calculations.

High efficient terahertz emission from InN surfaces

AbstractTerahertz radiation was measured from InN and compared to p‐InAs excited by femtosecond optical pulses at 1060 and 800 nm. At 800 nm, atomically smooth InN with low background electron concentration exhibit higher THz emission than the highly effective material p‐InAs. The higher THz emission efficiency of InN is caused by the absences of any intervalley scattering, which in the case of InAs increases the effective mass of the photo generated electrons and, thus, reduces the Dember field, which is responsible for THz emission. Consequently, InN is a promising material for THz emission; however, further improvement of the material quality (surface roughness and carrier concentration) is needed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Morphological and electrical properties of InP grown by solid source molecular beam epitaxy

The properties of InP grown by solid source molecular beam epitaxy have been studied concerning electron mobility in correlation with surface morphology and carrier concentration. The surface roughness and carrier concentration exhibit pronounced dependences on growth temperature and V/III flux ratio. A growth regime for a smooth surface and a high electron mobility is determined with growth temperature from 364– 390 ∘ C and V/III flux ratio from 2.4 to 3.5. A InP film with maximum electron mobility of 4.57 × 10 4 cm 2 / V s at 77 K has been achieved.

Thermal conductivity of bulk ZnO after different thermal treatments

Thermal conductivities (κ) of melt-grown bulk ZnO samples thermally treated under different conditions were measured using scanning thermal microscopy. Samples annealed in air at 1050°C for 3 h and treated with N-plasma at 750°C for 1 min. exhibited κ=1.35±0.08 W/cm-K and κ=1.47±0.08 W/cm-K, respectively. These are the highest values reported for ZnO. Atomic force microscopy (AFM) and conductive-AFM measurements revealed that surface carrier concentration as well as surface morphology affected the thermal conductivity.

Analysis of current flow in polycrystalline TFTs

A model for predicting the change of currents at the surface of polycrystalline materials for both ohmic and blocking contacts is developed. The model includes electron/hole traps within grain boundaries that are comparable in thickness to that of the dielectric on the surface of the polycrystalline semiconductor. The grains and their interfaces with the dielectric are assumed to be trap free. Account is also taken of the reducing carrier Debye Length as the surface carrier concentration is increased, from its intrinsic value, by the field effect. The net surface conductance is obtained by integrating the carrier density across the surface region through to the back of the material. Four regimes are identified: quasi-drift and quasi-diffusion for the high and low current regimes when there is a good supply of carriers and generation and quasi-diffusion when there is a limited supply of carriers. The analytical relationships are found to give satisfactory agreement with results for the temperature and field dependence of surface conductance in polycrystalline silicon in these regimes. The dependence of surface conductance on field effect voltage is found, at lower currents, to be a means of determining the energy distribution of electron/hole traps.

Tunnel charge transport within silicon in reversely-biased MOS tunnel structures

The features of the electrical behaviour of a MOS tunnel structure, which arise from the tunnel carrier transport in semiconductor, are considered. For the explicitely given band diagram, the total current increases due to the contribution of electrons in energy range where the only-oxide tunneling is impossible. The resonance transport via the discrete levels in the quantum well may introduce steps in the reverse current–voltage characteristic. The band-to-band tunneling, which is to be treated as semiconductor tunneling, perturbates the balance of minority carriers in the inversion layer, modifying the charge state of a MOS structure. The stationary non-equilibrium support of a large surface carrier concentration becomes therefore possible, and the voltage partitioning in the MOS structure is distorted.

The Tail of Localized States in the Band Gap of the Quantum Well in the In[sub 0.2]Ga[sub 0.8]N∕GaN System and Its Effect on the Laser-Excited Photoluminescence Spectrum

It is established experimentally that the peak in the photoluminescence spectrum of the In0.2Ga0.8N/GaN heterostructure with a quantum well shifts by ∼150 meV as the power density of a nitrogen laser used for excitation is increased from 10 to 1000 kW/cm2. The large blue shift is interpreted as a manifestation of the tail of the density of localized states in the band gap of the quantum well. It is shown that, in the model of the ideal quantum well in which there is no tail of localized gap states, it is impossible to account for the large blue shift. A phenomenological expression is suggested for the density of states. The expression involves the adjustable parameter, namely, the Urbach energy that characterizes the tails of the density of localized states. By this means, both the long-wavelength edge of the luminescence spectrum and the large blue shift can be described. The quasi-Fermi level of photoelectrons depends on the pump intensity and is selected to fit each experimental curve. A qualitative agreement between the theoretical and experimental spectra is obtained, demonstrating that the model is adequate. From the condition for electrical neutrality, the surface carrier concentration is determined. It is found to be several orders of magnitude higher than the pyroelectric concentration in narrow quantum wells.

Characterization of copper indium ditelluride/electrolyte interface utilizing electrochemical impedance spectroscopy

Copper indium telluride (CIT) is a promising semiconductor material for use in thin film solar cells, especially photoelectrochemical solar cells (PESC). Impedance measurements were used to evaluate the relative band edge positions of polycrystalline n-CuInTe 2 electrodes in various aqueous electrolytes, by measuring the extrapolated flat-band potentials, V fb. By varying applied dc potential, frequency and pH of the electrolyte, the semiconductor–electrolyte interfaces are studied through capacitance measurements and Mott–Schottky analysis. The flat-band potential and carrier concentrations were determined from the Mott–Schottky plots. It was found that V fb could be shifted depending on the pH of the electrolyte, while it is independent of the testing frequency. The changes in V fb with pH could be due to surface states and other modifications through etching and polishing. The surface carrier concentrations were found to increase with pH. The different surface chemistry studied by X-ray photoelectron spectroscopy might account for this change.