High Density Of Surface States Research Articles

Quantized Auger recombination of biexcitons in CdSe nanorods studied by time-resolved photoluminescence and transient-absorption spectroscopy

We studied the recombination dynamics of biexcitons in elongated CdSe nanocrystals (nanorods) using time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy. The decay times of the PL and TA signals decrease with increasing nanorod length. Under weak excitation, the PL decay is faster than the TA decay, and the nonradiative hole trapping determines the PL decay in several hundreds of picoseconds. Under intense excitation, the PL decay curves are similar to the TA decay curves, and the rapid biexciton decay is caused by nonradiative Auger recombination. A clear correlation is observed between the Auger recombination coefficient of the biexcitons and the average PL lifetime of the single excitons. Moreover, the Auger recombination lifetimes of the biexcitons are shorter in nanorods than in spherical nanocrystals of the same volume. Our study clarified that the Auger recombination rate is strongly affected by a high surface-state density.

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

This study investigated the effects of hydrogen (H 2) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current–voltage ( I– V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I– V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H 2-treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.

Magnetic Quantum Oscillations from Surface States of Bi Nanowires

ABSTRACTIn this work, we report the results of studies of the transverse magnetoresistance (MR) of single-crystal Bi nanowires with diameter d<80 nm. The single-crystal nanowire samples were prepared by the Taylor-Ulitovsky technique. Due to the semimetal-to-semiconductor transformation and high density of surface states with strong spin-orbit interactions, the charge carriers are confined to the conducting tube made of surface states. The non monotonic changes of transverse MR that are equidistant in a direct magnetic field were observed at low temperatures in a wide range of magnetic fields up to 14 T. The period of oscillations depends on the wire diameter d as for the case of longitudinal MR. An interpretation of transverse MR oscillations is presented.

Quantum Interference in Bismuth Nanowires: Evidence for Surface Charges

We report the results of studies of the transverse magnetoresistance (MR) of single-crystal Bi nanowires with diameter d<80 nm. The single-crystal nanowire samples were prepared by the Taylor-Ulitovsky technique. Due to the semimetal-to-semiconductor transformation and high density of surface states with strong spin-orbit interactions, the charge carriers are confined to the conducting tube made of surface states. The non monotonic changes of transverse MR that are equidistant in a direct magnetic field were observed at low temperatures in a wide range of magnetic fields up to 14 T. The period of oscillations depends on the wire diameter d as for the case of longitudinal MR. An interpretation of transverse MR oscillations is presented.

XPS study of palladium sensitized nano porous silicon thin film

Nano porous silicon (PS) was formed on p-type monocrystalline silicon of 2–5 Ω cm resistivity and (100) orientation by electrochemical anodization method using HF and ethanol as the electrolytes. High density of surface states, arising due to its nano structure, is responsible for the uncontrolled oxidation in air and for the deterioration of the PS surface with time. To stabilize the material PS surface was modified by a simple and low cost chemical method using PdCl2 solution at room temperature. X-ray photoelectron spectroscopy (XPS) was performed to reveal the chemical composition and the relative concentration of palladium on the nanoporous silicon thin films. An increase of SiO2 formation was observed after PdCl2 treatment and presence of palladium was also detected on the modified surface. I–V characteristics of Al/PS junction were studied using two lateral Al contacts and a linear relationship was obtained for Pd modified PS surface. Stability of the contact was studied for a time period of around 30 days and no significant ageing effect could be observed.

Fabrication and photoluminescence of ZnS:Mn2+ nanowires/ZnO quantum dots/SiO2 heterostructure

In this paper, we demonstrated the encapsulation of ZnS:Mn2+ nanowires (NWs) and ZnO quantum dots (QDs) with a layer of mesoporous SiO2 shell for the purpose of integrating dual emission property into one common nanostructure. The average diameter of ZnS:Mn2+ NWs, ZnO QDs, and ZnS:Mn2+/ZnO@SiO2 heterostructure was about 10 nm, 6 nm, and 22 nm, respectively. Within ZnS:Mn2+/ZnO@SiO2 nanocomposites, the intensity of the yellow-orange emission contributed by ZnS:Mn2+ NWs and the UV emission contributed by ZnO QDs was three and ten times higher than their individual components, respectively. The fluorescence intensity ratio of the dual emission can be tuned by adjusting the hydrolysis time of tetraethyl orthosilicate. The peak energy of the yellow-orange and UV emission showed blueshift and redshift as increasing the temperature, respectively. The anomalous enhancement of the integrated intensity for the UV emission with the temperature indicated that the high surface state density existing in ZnO QDs can overrun the influence of temperature quenching and even alter the photoluminescent properties.

Silicon nanowire piezoresistance: Impact of surface crystallographic orientation

We investigate piezoresistance in lithographically defined silicon nanowires of various cross-sectional aspect ratios. Both ⟨110⟩- and ⟨100⟩-oriented nanowires are investigated under ⟨110⟩-oriented strain. The nanowire thickness is varied from 23 to 45 nm and the nanowire width is varied from 5 to 113 nm. Our data shows piezoresistance in silicon nanowires being a surface induced effect with {110} surfaces inducing a much larger piezoresistance than {100} surfaces. This is consistent with a higher density of surface states on {110} surfaces than on {100} surfaces. Our experimental findings support recent computational work pointing toward surface states being the source of giant piezoresistance in silicon nanowires.

Model for the electron distribution in modulation-doped heterostructures with high density of surface states

We study heterostructures, consisting of parallel layers of GaAs and ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ containing $n$-doped sheets and, on both sides, GaAs cap layers with a high density of surface states. Assuming thermal equilibrium between the electrons in the surface states, the donor-induced states, and the eigenstates near the $\text{GaAs}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ interface (IF), we obtain three different doping regimes. At low doping only the surfaces are charged with electrons, not the IF. At intermediate doping also a two-dimensional electron system (2DES) near the IF occurs. At high doping the electron densities of the surface layers and of the 2DES saturate, and a parallel channel occurs at the highly doped sheet, which accommodates the electrons resulting from further doping. Besides the results of a self-consistent Hartree-type calculation we present a quasiclassical approach, which gives very good results for the electron distribution over the sample in all three doping regimes. In the heavy-doping saturation regime also a simple electrostatic estimate is applicable.

Surface treatment of nanoporous silicon with noble metal ions and characterizations

A very large surface to volume ratio of nanoporous silicon (PS) produces a high density of surface states, which are responsible for uncontrolled oxidation of the PS surface. Hence it disturbs the stability of the material and also creates difficulties in the formation of a reliable electrical contact. To passivate the surface states of the nanoporous silicon, noble metals (Pd, Ru, and Pt) were dispersed on the PS surface by an electroless chemical method. GIXRD (glancing incidence X-ray diffraction) proved the crystallinity of PS and the presence of noble metals on its surface. While FESEM (field emission scanning electron microscopy) showed the morphology, the EDX (energy dispersive X-ray) line scans and digital X-ray image mapping indicated the formation of the noble metal islands on the PS surface. Dynamic SIMS (secondary ion mass spectroscopy) further confirmed the presence of noble metals and other impurities near the surface of the modified PS. The variation of the surface roughness after the noble metal modification was exhibited by AFM (atomic force microscopy). The formation of a thin oxide layer on the modified PS surface was verified by XPS (X-ray photoelectron spectroscopy).

Photoluminescence of rare earth3+ doped uniaxially aligned HfO2 nanotubes prepared by sputtering with electrospun polyvinylpyrolidone nanofibers as templates

Rare earth (RE) ions (Eu3+,Tb3+) doped uniaxially aligned HfO2 nanotubes were prepared by radio frequency sputtering with electrospun polyvinylpyrolidone (PVP) nanofiber templates. The as-sputtered samples were annealed at different temperatures (500–1000 °C) in O2 ambient in order to remove their PVP cores and make the HfO2 shells well crystallized. Morphologies and crystal configuration of the samples were investigated by optical microscope, scanning electron microscopy, transmission electron microscopy, x–ray diffraction, and Raman spectroscopy. The nanotubes have uniform intact structure with an average diameter of 200 nm and a wall thickness of about 25 nm. Photoluminescence (PL) properties of the RE doped nanotubes have been studied in detail. The emission peaks of the aligned HfO2:Eu and HfO2:Tb nanotubes could correspond to the D50→F7J (J=0–2) transitions of Eu3+ and the D54→F7J (J=3–6) transitions of Tb3+, respectively. The PL intensities of the HfO2:RE3+ nanotubes were higher by several orders of magnitude than that of the films. This enhancement in the PL could be ascribed to the high density of surface states of HfO2:RE3+ nanotubes.

Magnetic Quantum Oscillations in Single Bi Nanowires

Our study presents the experimental measurements of Aharonov-Bohm (AB) oscillations on single Bi nanowires with diameter d<80 nm. The single nanowire samples prepared by the Ulitovsky technique were cylindrical single crystals with (10 $\bar{1}$ 1) orientation along the wire axis. Due to semimetal-to-semiconductor transformation and high density of surface states, Bi nanowire (d∼50 nm) effectively turns into a conducting tube. The oscillations of longitudinal magnetoresistance (MR) of Bi nanowires with two periods Δ B 1∼Φ 0 and Δ B 2∼Φ 0/2 were observed, where Φ 0=h/e is the flux quantum. From B∼8 T down to B=0 the extremums of Φ 0/2 oscillations are shifted up to 3π at B=0 which is the manifestation of the Berry phase shift due to electron moving in nonuniform magnetic field. The derivative of MR for a 55 nm bismuth nanowire was measured at various inclined angles. An interpretation of the MR oscillations is presented.

Improved Performances of InGaN Schottky Photodetectors by Inducing a Thin Insulator Layer and Mesa Process

The performances of InGaN Schottky photodetectors with varied fabrication processes were investigated. The photoresponse and dark current of InGaN Schottky photodetectors can be obviously improved by inserting a thin Si3N4 passivation layer between the InGaN layer and the Schottky metal. Furthermore, a mesa process gives not only a further increase in the photoresponse but also a pronounced reduction in the reverse leakage current of about two orders of magnitude. A lateral surface leakage current mechanism associated with the 2-D variable-range hopping conduction through high-density surface states in InGaN is proposed to explain the reduction of the reverse leakage current after etching the mesa.

Effects of an anodic oxide passivation layer on mesa-type InGaAs (PIN) photodetectors

Because of the high density of surface states and large surface recombination velocity found in III–V materials, surface passivation remains a crucial processing step for the fabrication of mesa-type InGaAs photodetectors. It is required to find an easy and economical method to form an effective passivation layer on the surface of the detector. In this work, an oxide passivation layer formed by electrochemical anodization was proposed and a 256-element In0.78Ga0.22As photodetector array was fabricated. The dark current performance achieved is as low as 2 × 10−7 A (bias voltage of −0.1 V) at 300 K. The reduction of dark current indicates that the anodic oxide passivation layer is effective in reducing recombination at the exposed mesa surface.

Growth mechanism and optical properties of ZnO nanotube by the hydrothermal method on Si substrates

Well-faceted hexagonal ZnO nanotubes have been synthesized on Si substrates by a simple hydrothermal method and the subsequent aged process at relative low temperature without any catalysts or templates. The samples have been characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) measurement. XRD pattern confirmed that the as-prepared ZnO was the single-phase wurtzite structure. SEM results showed that the samples were tubular textures, with the average lengths of 3–5 μm, inner diameters of 300–350 nm. The surface-related optical properties have been investigated by photoluminescence (PL) spectrum and Raman spectrum. The PL implied that fairly high surface state density existed in ZnO nanotubes. And the green emission in Raman measurement might be related to surface states. A possible growth mechanism on the formation of crystalline ZnO nanotubes has been presented.

The conductivity of Bi(111) investigated with nanoscale four point probes

The room temperature conductance of Bi(111) was measured using microscopic four point probes with a contact spacing down to 500 nm. The conductance is remarkably similar to that of the bulk, indicating that surface scattering is not a major mechanism for restricting the mobility at this length scale. Also, the high density of electronic surface states on Bi(111) does not appear to have a major influence on the measured conductance. The lower limit for the resistivity due to electronic surface states is found to be around 5 Ω. With such a value for the surface resistivity, surface conduction should not be a significant factor to inhibit the observation of the predicted semiconductor to semimetal transition for thin films of Bi.

Spectroscopic and Photoelectrochemical Properties of DNA/CdS Nanoparticle Composites

DNA/CdS nanoparticle composites (DNA/CdS NPC) containing single or double strands and different concentrations of DNA were constructed in aqueous solutions. The effect of DNA on the photoelectrochemical properties of DNA/CdS NPC was investigated and the spectroscopic properties of DNA/CdS NPC were characterized by TEM, UV-Vis, IR, and fluorescent spectrometry. The results showed that CdS nanoparticles (CdS NPs) were combined with DNA strands through the electrostatic interaction; DNA templates did not affect the band gap of CdS NPs; DNA-templated CdS NPs had a higher density of surface states than that stabilized by Na4P2O7, which enhanced the photoluminescence (PL) intensity whereas restrained the photocurrent response of CdS NPs. Besides, there was certain dependency of both the increase of PL intensity and the decrease of photocurrent response on DNA concentrations in DNA/CdS NPC. The composites were hopeful for applications in both fluorescent tagged detections and quantitative analysis of DNA.

Strong broadband optical absorption in silicon nanowire films

The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found to be higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

Effects of Rapid Thermal Annealing Treatment on the Surface Band Bending of n-type GaN Studied by Surface Potential Electric Force Microscopy

It is generally accepted that the Schottky barrier height (SBH) is affected by the initial band bending at the bare nGaN surface as well as by an additional contribution following metal deposition. In this work the effect of processing used for device fabrication on the surface band bending of bare c-plane nGaN was studied by surface potential electric force microscopy (SP-EFM). An increase of the initial upward band bending from 1.0 ± 0.1eV for the as-grown GaN to 1.9 ± 0.1eV after RTA treatment in N2 ambient was observed. No significant dependence of band bending on N2 or Ar as ambient gas during the RTA treatment was observed. The increase of the initial upward band bending was also confirmed by photoluminescence (PL) measurements. We suggest that the RTA treatment causes a high density of surface states, possibly as a result of high temperature reaction of ambient gas and remnant contamination.

Growth and Optical Properties of Faceted Hexagonal ZnO Nanotubes

Well-faceted hexagonal ZnO nanotubes were synthesized by a simple hydrothermal method and the subsequent aging process without any catalysts or templates. The formation of the tubular structure is closely linked to the polarity of ZnO and the selective adsorption behavior of Zn2+ amino complexes. The surface-related optical properties were studied with use of Raman and photoluminescence spectra. It was found that the oxygen vacancy-related visible emission intensity decreased while surface defect-related visible emission intensity increased when the nanotubes were annealed in oxygen ambient. The anomalous enhancement of PL integrated intensity with the temperature shows fairly high surface state density existing in ZnO nanotubes.

Effect of KOH treatment on the schottky barrier height and reverse leakage current in Pt/n-GaN

The effect of potassium hydroxide (KOH) treatment was investigated for Pt/n-GaN Schottky diodes using current-voltage (I-V) and capacitance-voltage (C-V) characteristics. Atomic force microscopy (AFM) showed that the surface profile did not change significantly as a result of KOH treatment. The Schottky barrier height increased for both I-V and C-V measurements. The reverse leakage current was also reduced by KOH treatment. By fitting these data to the reverse current based on the thermionic field emission (TFE) model, it was shown that the experimental results are consistent with the presence of high densities of surface states, which were reduced appreciably by KOH treatment. The similar behavior of Pt/n-GaN samples grown by hydride vapor phase epitaxy (HVPE) suggests that the KOH treatment is effective in improving the rectifying characteristics of Schottky barriers to n-GaN grown by both organometallic vapor phase epitaxy (OMVPE) and HVPE.