Changes In Surface Band Bending Research Articles

Gap states induce soft Fermi level pinning upon charge transfer at ZnO/molecular acceptor interfaces

The deposition of strong molecular electron acceptors onto ZnO induces a substantial work function (\ensuremath{\phi}) increase due to electron transfer from the inorganic semiconductor to the molecules. The \ensuremath{\phi} increase results from two mechanisms: (i) a change of the surface band bending within ZnO and (ii) an interface dipole between the inorganic surface and the negatively charged acceptors. The molecule adsorption induced upward band bending in ZnO is, however, found to be limited to a few 100 meV, while the \ensuremath{\phi} increase is significantly larger (up to 2.8 eV). We elucidate the origin of limited upward band bending by revealing a notable gap state density-of-states (GDOS) using high-sensitivity photoemission spectroscopy. Upon acceptor-induced upward band bending, the GDOS with a wide energy distribution becomes increasingly unoccupied. This, in turn, makes the interface dipole dominant and limits the ZnO surface band bending changes due to a ``soft'' pinning at the Fermi level.

Size-dependent surface potential of Si-doped InN nanorods and the role of inhomogeneous free-electron distribution

We carry out the surface potential (SP) measurements of Si-doped InN nanorods (NRs) grown by plasma-assisted molecular beam epitaxy. Photoluminescence and photoemission spectroscopic studies reveal that the Si-doped InN nanorods possess surface electron accumulation. To estimate the SP value of the InN nanorods, a contact potential difference is measured using Kelvin probe force microscopy (KPFM). In order to avoid the influence of the surface adsorbed species, KPFM measurements were carried out at a high vacuum condition of ∼7.5 × 10−7 mbar. The SP value of the Si-doped InN nanorods is found to depend on the size of nanorods. The size-dependent SP value of the Si-doped InN nanorods is attributed to the variation in the downward surface band bending caused by the change in the sheet carrier density of surface electron accumulation. The change in surface band bending is the result of the variation in the free-electron distribution with a size of the NRs.

Surface State Density Determines the Energy Level Alignment at Hybrid Perovskite/Electron Acceptors Interfaces

Substantial variations in the electronic structure and thus possibly conflicting energetics at interfaces between hybrid perovskites and charge transport layers in solar cells have been reported by the research community. In an attempt to unravel the origin of these variations and enable reliable device design, we demonstrate that donor-like surface states stemming from reduced lead (Pb0) directly impact the energy level alignment at perovskite (CH3NH3PbI3-xClx) and molecular electron acceptor layer interfaces using photoelectron spectroscopy. When forming the interfaces, it is found that electron transfer from surface states to acceptor molecules occurs, leading to a strong decrease in the density of ionized surface states. As a consequence, for perovskite samples with low surface state density, the initial band bending at the pristine perovskite surface can be flattened upon interface formation. In contrast, for perovskites with a high surface state density, the Fermi level is strongly pinned at the conduction band edge, and only minor changes in surface band bending are observed upon acceptor deposition. Consequently, depending on the initial perovskite surface state density, very different interface energy level alignment situations (variations over 0.5 eV) are demonstrated and rationalized. Our findings help explain the rather dissimilar reported energy levels at interfaces with perovskites, refining our understanding of the operating principles in devices comprising this material.

Direct assessment of p–n junctions in single GaN nanowires by Kelvin probe force microscopy

Making use of Kelvin probe force microscopy, in dark and under ultraviolet illumination, we study the characteristics of p–n junctions formed along the axis of self-organized GaN nanowires (NWs). We map the contact potential difference of the single NW p–n junctions to locate the space charge region and directly measure the depletion width and the junction voltage. Simulations indicate a shrinkage of the built-in potential for NWs with small diameter due to surface band bending, in qualitative agreement with the measurements. The photovoltage of the NW/substrate contact is studied by analyzing the response of NW segments with p- and n-type doping under illumination. Our results show that the shifts of the Fermi levels, and not the changes in surface band bending, are the most important effects under above band-gap illumination. The quantitative electrical information obtained here is important for the use of NW p–n junctions as photovoltaic or rectifying devices at the nanoscale, and is especially relevant since the technique does not require the formation of ohmic contacts to the NW junction.

In Situ Conductance Analysis of Zinc Oxide Nucleation and Coalescence during Atomic Layer Deposition on Metal Oxides and Polymers.

Real time in situ conductance is collected continuously during atomic layer deposition (ALD) of zinc oxide films, and trends are used to study ALD nucleation on polypropylene, nylon-6, SiO2, TiO2, and Al2O3 substrates. The detailed conductance change during the ALD cycle is ascribed to changes in surface band bending upon precursor/reactant exposure. Conductive pathways form earlier on the inorganic surfaces than on the polymers, with Al2O3 substrates showing more rapid nucleation than SiO2 or TiO2, consistent with the expected density of nucleation sites (e.g., hydroxyl groups) on these different materials. The measured conductance is ohmic, and both two- and four-electrode configurations show the same data trends. Detailed analysis of conductivity at deposition temperatures between 100 and 175 °C shows faster conductivity decay at higher temperature during the water purge step, ascribed to thermally activated water desorption kinetics. Analysis of real-time conductivity during ALD of other material systems could provide further insight into key aspects of film nucleation and nuclei coalescence.

Correlating the Local Defect-Level Density with the Macroscopic Composition and Energetics of Chalcopyrite Thin-Film Surfaces.

The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin-film solar cells. Especially, impurity-driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect-level density and unusual optoelectronic grain-boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provides evidence that Na diffusion from the glass substrate does not affect the surface defect passivation or grain-boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6 eV) due to Na. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat-induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n interface in standard chalcopyrite thin-film solar cells is dominated by band bending within the CIGSe absorber rather than the result of Na impurities or an n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as the origin for Cu depletion and band-gap widening at the surface of chalcopyrite thin films.

Photoluminescence and surface photovoltage of ethynyl derivative‐terminated Si(111) surfaces

AbstractThe electrochemical grafting of ethynyl derivatives on Si(111) surfaces has been investigated by means of pulsed photoluminescence (PL) and surface photovoltage (SPV) techniques. Ethynyl derivatives (ethynyl‐MgCl, ethynyl‐MgBr, propynyl‐MgBr, and phenylethynyl‐MgBr) from Grignard compounds were used. By SPV, a high surface photovoltage signal, UPh, is observed. The halogene in the Grignard reagents influences strongly the electronic properties and the polymerisation process, i.e. Br in the Grignard compound leads to better electronic properties than Cl. The propynyl‐terminated Si surface shows the highest PL intensity and at the same time the weakest UPh signal, i.e. this polymeric layer induces the smallest amount of recombination active defects at the interface but also the smallest change in surface band bending. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Contactless electroreflectance of InGaN layers with indium content ≤36%: The surface band bending, band gap bowing, and Stokes shift issues

Contactless electroreflectance (CER) supported by photoluminescence (PL) has been applied to study (i) the surface band bending, (ii) the band gap bowing, and (iii) the Stokes shift for InGaN layers grown by molecular beam epitaxy with 0.14≤In≤0.36. The type of surface band bending has been investigated on the basis of the shape of CER resonance. It has been found that the surface band bending changes from n-type for layers with low indium content (In<27%) to flatband (or weak p-type band) for layers with In∼35%. The band gap bowing has been determined to be 1.4±0.2 and 2.1±0.3 eV for CER data with and without strain corrections, respectively. From this analysis it has been concluded that the reliable value of the bowing parameter for unstrained InGaN should be between 1.4 and 2.1 eV. Comparing CER with PL data it has been found that the Stokes shift rises from 20 to 120 meV when the indium concentration increased from 14% to 36%. In addition, it has been observed that the intensity of PL from InGaN layers decreased exponentially with the increase in the indium content. The last two findings are attributed to an easier formation of native point defects and stronger indium segregation in InGaN alloys with higher indium concentrations.

Direct observation of Schottky to Ohmic transition in Al-diamond contacts using real-time photoelectron spectroscopy

Real-time photoelectron spectroscopy and in situ electrical measurements have been applied to the formation of Al contacts on p-type diamond. At 294K, an initially uniform Al film induces band bending in the diamond consistent with the measured (current-voltage) barrier height of 1.05V. The temperature-induced transition to an Ohmic contact has been monitored in real time revealing a direct correlation between the onset of surface bonding at 755K and an abrupt change in surface band bending. The reaction temperature is lower than previously believed, and there is a second transition point at 1020K where the rates of change of both reaction and band bending increase sharply.

Changes in surface band bending, surface work function, and sheet resistance of undoped ZnO films due to (NH4)2Sx treatment

In this study, the interaction of undoped ZnO films with (NH4)2Sx treatment have been investigated by x-ray photoelectron spectroscopy, photoluminescence, optical transmittance, and four-point probe. According to the experimental results, we find that the formation of Zn–S bonds and the reduction of oxygen vacancies (i.e., the S occupation of oxygen vacancies) near the ZnO surface might lead to an increase in the upward band bending, resulting in an increase in the sheet resistance and work function of ZnO.

Induced changes in surface band bending of n-type and p-type AlGaN by oxidation and wet chemical treatments

The surface chemistry and electrical properties of p-type and n-type AlGaN surfaces were studied via x-ray photoelectron spectroscopy before and after oxidation and wet chemical treatments. Shifts in the surface Fermi level were measured with the change in onset of the valence-band spectra. Oxidation and HF and (NH4)2Sx treatments on p-type AlGaN (n-type AlGaN) led to an increase (the reduction) in the surface band bending due to more N vacancies and N vacancies being occupied by S (i.e., donorlike states) than Al vacancies and Ga vacancies (i.e., acceptorlike states) near the p-type AlGaN (n-type AlGaN) surface region. The changes in surface chemistry indicate that oxidation and wet chemical treatments alter the surface state density through the formation of more donorlike states.

Nano-crystalline SnO 2 gas sensor response to O 2 and CH 4 at elevated temperature investigated by XPS

The electronic changes occurring at the surface of a nano-crystalline SnO 2 sensor upon exposure to O 2 and CH 4 at elevated temperature have been investigated by X-ray photoelectron spectroscopy (XPS). Gas exposures and subsequent XPS scanning were conducted at 120 and 250 °C to simulate the working conditions of the sensor. Exposure to O 2 at 120 °C resulted in a 0.2 eV upward band bending while subsequent exposure to CH 4 resulted in a 0.1 eV downward band bending, as expected from oxidising and reducing gases, respectively. Similar changes in surface band bending were observed at 250 °C, although quantitative analysis suggests that oxygen absorption might be enhanced. The results clearly indicate the electronic nature of the gas sensing mechanism when exposed to O 2 and CH 4 and the very high sensitivity of the sensor.

The interaction of carbon with the diamond surface: An in-situ photoelectron spectroscopy investigation

Abstract An investigation of the interaction of carbon with the diamond surface serves as a model experiment to study different aspects of diamond growth. The electronic structure of the surface, which is closely linked to the geometric arrangement of the atoms, is probed by photoelectron spectroscopy in the ultraviolet (UPS) and X-ray (XPS) regime. The carbon was supplied by electron beam evaporation of graphite and was deposited on polycrystalline diamond films. Carbon films with a thickness between 0.2 and 15 monolayers (ML) are deposited on the diamond surface at two different substrate temperatures (ambient and 700°C). Even at elevated substrate temperatures, diamond growth cannot be continued. In the course of carbon film formation we observe in the gradual emergence of the p - π band (ambient temperature), connected to the formation of an amorphous carbon film, or graphitic features (700°C) in the valence band (VB) spectra, which is accompanied by the disappearance of typical diamond features. The differences in film structure for the two temperatures are already apparent after the deposition of only one monolayer of carbon. The Cls core level peak reflects the changes in surface band bending and thereby formation of a reconstructed surface and/or a Schottky barrier. A comparison with the ion-irradiated diamond surfaces is made. The results are discussed with respect to diamond growth mechanisms and the applicability of this method to introduce sp 2 structures at the diamond surface.

Signature of the Weak Bond-Dangling Bond Conversion Process in a-Si:H as Seen by Total Photoelectron Yield Spectroscopy

Photoelectron Yield Spectroscopy is one of the most direct methods to study the density of gap states of amorphous semiconductors. We have made use of it to investigate the transient changes of the density of gap states of a-Si:H upon illumination with above-band-gap light. An excitation density of 50 mW/cm2 of 532 nm light from a frequency doubled Nd-YAG laser was modulated with frequencies between 0.1 Hz and 10 KHz and the transient response of the photoelectron yield signal was monitored by gated electron counting or a lock-in amplifier. From this we deduce a reversible increase of the occupied density of defect states under illumination with a maximum Δg ∼ 1017 cm3 eV-1 at about 0.3 eV below the Fermi energy and a decrease below EF− 0.75 eV, i. e. in the region of deep tail states. The effect exhibits a wide spectrum of time constants involved. Based on additional transient surface photo voltage measurements as well as numerical simulations we argue that the observed redistribution of states is not due to a light induced change in surface band bending or due to a mere excitation of electrons from occupied to empty states. Instead, we interpret the observed effect as a conversion of weak to dangling bonds, i.e. a precursor of the Staebler-Wronski Effect, lacking however the final step of hydrogen stabilisation necessary for the creation of metastable defects.

Effects of HF cleaning and subsequent heating on the electrical properties of silicon (100) surfaces

Changes in surface-band bending of both boron-doped and phosphorus-doped silicon (100) samples by exposure (40 s) to hydrofluoric acid (HF) with varying HF concentrations were studied by x-ray photoelectron spectroscopy. Effects of subsequent thermal annealing was investigated by in situ heating in vacuum. Hydrogen termination of the dangling bonds on silicon was found to be an effective means to reduce surface gap states on silicon. Near-flatband surfaces were observed on both n- and p-Si by the HF exposure when the doping concentration was not less than 1×1016/cm3, and when the HF concentration was not higher than 5%. A higher HF concentration promoted hydrogen diffusion and the formation of an H-B species in p-Si. As such, band bending increased on p-Si. However, the deactivation of boron could be recovered by annealing for less than 1 h at a temperature as low a 120 °C.

Gamma ray irradiated LED's: Surface emission and significant wavelength tuning via surface states

Wavelength tuning via shallow junction GaAs LED's as a result of gamma irradiation is increased significantly when the irradiated LED's are operated in vacuum. Vacuum operation is seen to be essentially equivalent to increased gamma ray dosage for low irradiation levels as a result of desorptive processes common to both phenomena. They give rise to decreased nonradiative and increased radiative components of surface recombination photon emission. It is this spectrum which is shifted according to changes in surface potential and forward voltage deriving from alterations in surface state populations. A mathematical model is developed to relate wavelength tuning with surface potential and forward voltage shift. This technique is, in principle, a general technique independent of semiconductor material. It suggests the possibility of wavelength tuning via surface band-bending changes deriving from surface electric field changes, as is done with MIS devices. Examination of irradiated diode properties in vacuum and under pressure permits greater insight into the basic nature of surface phenomena long suspected to play a significant role in the diode electronic property changes brought about by nuclear irradiation.

Wavelength tuning of GaAs LED's through surface effects

A novel wavelength tuning technique, applicable to shallow junction surface-emitting LED's, is shown to exhibit wavelength changes on the order of 50 nm, depending on current and gas environment, the tuning technique is external and is continuous, repeatable, reversible, controllable, and apparently nondestructive. Thermal and pressure changes in E g do not appear to be dominant mechanisms. The dominant mechanism is suspected to involve surface effects, such as desorption of adsorbed gases, which depend upon self-heating in vacuo. The one-to-one relationship between diode voltage and peak emission wavelength suggests photon-assisted tunneling as the tuning mechanism although surface band-bending changes may play some role. Surface effects alone, such as generated by UV illumination, are shown to yield noticeable LED wavelength shift, while bulk heating with red light of much higher average irradiance does not. This technique is, in principle a general technique independent of semiconductor material. The results described here should also have implications for optical-fiber communication, gas detection and identification, and space applications.

Spectral tuning and linewidth narrowing of shallow-junction surface emitting GaAs LED́s through γ-ray irradiation

Alterations of device characteristics as a result of γ-ray irradiation of shallow-junction surface emitting devices are different from those of deep-junction devices with respect to LED emission intensity reduction, I-V curve, line shape, and spectral shift. In particular, much larger spectral shifts in the opposite direction-toward longer wavelengths-are reported here than those found in the literature for deep-junction devices. A qualitative model based upon photochemical doping and changes in surface band bending is proposed to explain these phenomena. Changes in surface emitting shallow-junction optical radiation source device characteristics brought about by γ-ray irradiation are desirable ones for utilization in most optical fiber communication systems. These changes include linewidth narrowing, decreased time response, and decreased material dispersion because of the emission wavelength change.