Surface Built-in Electric Field Research Articles

Surface states modulation via vacancy engineering for creating band bending on semiconductor BiOBr photocatalysts

In microelectronic devices, surface modification could be used to introduce acceptor or donor surface states by defect engineering method to modify the alignment of energy band for desired properties, which was not used in the catalytic performance enhancement of powder photocatalysts. In this work, surface oxygen vacancies are induced to adjust the surface electrical structures and characteristics of the semiconductor bismuth oxybromide (BiOBr). In addition to being defect states to trap photoexcited electrons, surface oxygen vacancies also bring acceptor surface states, which result in an enhanced upward band bending and attract more photoexcited holes to the surface. Significantly, with a high concentration of electrons and holes accumulated on the surface, the BiOBr with surface oxygen vacancies shows good activity in the activation of O2 and oxidation of water compared to the one with bulk oxygen vacancies. Especially, it can decompose Rhodamine B (RhB) under natural sunlight in one minute, exceeding the majority of previously reported photocatalytic materials under comparable conditions. This work demonstrates that the surface oxygen vacancies could induce strengthened band bending at the BiOBr surface and remarkably improve catalytic performance, which provides a scalable strategy to practically manipulate the surface built-in electric field of powder photocatalysts.

Fabrication of single crystalline WO3 nano-belts based photoelectric gas sensor for detection of high concentration ethanol gas at room temperature

WO3 gas sensing material was synthesized by hydrothermal process, exhibiting a belt-like surface morphology. By comparing and analysing the UV–vis absorption spectra, it is shown that the band gap of WO3 nano-belts (NBs) is 0.56 eV larger than that of WO3 nano-particles (NPs). Surface photo-voltage (SPV) spectra and SPV phase spectra indicate that the direction of the surface built-in electric field is from the surface to the bulk of WO3 NBs and the surface band bending is upwards. WO3 NBs based photoelectric gas sensor was assembled for detecting high concentration of C2H5OH at room temperature. The results demonstrate that the WO3 NBs based photoelectric sensor exhibits higher sensitivity and resolution to high concentration (1000 ppm–20000 ppm) of ethanol at room temperature than WO3 NPs based photoelectric sensor. WO3 NBs are promising candidates of gas sensing material used to fabricate photoelectric gas sensor for high concentration C2H5OH detection operating at room temperature.

Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

Polarization imaging of imperfect m-plane GaN surfaces

Abstract

Unravelling charge separation via surface built-in electric fields within single particulate photocatalysts.

Kelvin Probe Force Microscopy (KPFM) and spatially resolved surface photovoltage (SRSPV) techniques were employed to reveal built-in electric fields and surface photogenerated charge distribution on single particulate photocatalysts. The photogenerated holes and electrons spread over the whole surface of the particulate photocatalyst are imaged on n-type BiVO4 and p-type Cu2O single particles, respectively. It is demonstrated that the built-in electric field in the surface Space Charge Region (SCR) dictates the charge separation/transfer processes and allows the drift of one kind of the photogenerated carriers to the surface, while holding another kind of the carriers in the bulk. The results emphasize the role of the SCR played in the unidirectional charge transport between the bulk and surface in the particulate photocatalyst, which may be the crucial reason for low solar energy conversion efficiency.

Individual ZnO nanowires for photodetectors with wide response range from solar-blind ultraviolet to near-infrared modulated by bias voltage and illumination intensity

ZnO nanowires have relatively high sensitivity as ultraviolet (UV) photodetectors, while the bandgap of 3.37 eV is an important limitation for their applications in solar-blind UV (SBUV), visible (VIS) and near infrared (NIR) range. Besides UV response, in this study, we demonstrate the promising applications of individual undoped ZnO NWs as high performance SBUV-VIS-NIR broad-spectral-response photodetectors, strongly depended on applied bias voltage and illumination intensity. The dominant mechanism is attributed to the existence of surface states in nanostructured ZnO. At a negative bias voltage electrons can be injected into surface states from electrode, and moreover, under light illumination photogenerated electron-hole pairs can be separated efficiently by surface built-in electric field, resulting into a decrease of potential barrier height and depletion region width, and simultaneously accompanying a filling of oxygen vacancy and a rise of ZnO Fermi level.

Surface potential effect on excitons in AlGaN/GaN quantum well structures

AlGaN/GaN quantum well (QW) heterostructures grown by metalorganic chemical vapor deposition on sapphire and on free-standing GaN substrates have been studied by temperature dependent time-resolved photoluminescence. A dominant contribution of the exciton radiative lifetime is observed in homoepitaxial samples even at enhanced temperatures up to 100 K. The QW-related emission is found to be more sensitive to the near surface built-in electric field in the homoepitaxial samples, revealed as a red shift of the QW exciton energy with decreasing the cap layer thickness. Absence of such shift in the heteroepitaxial samples suggests, assuming a surface potential of 0.5 eV, an increased polarization field due to residual compressive stress.

Comparative study on passivation of GaAs0.86P0.14/Al0.6Ga0.4As near-surface quantum well

The authors report a comparative study on ex situ passivation of a near-surface GaAs0.86P0.14/Al0.6Ga0.4As quantum well using various sulfide solutions and nitrogen plasma treatments. The built-in surface electric field is changed via band bending by applying various surface passivation conditions. The band bending is measured using x-ray photoelectron spectroscopy. Reduction in surface electric field in the range of 10–35 kV/cm is observed depending on different passivation conditions. The photoreflectance spectra show enhancement in intensity and blueshift of ∼3 meV accompanied by significant reduction in the broadening parameter of the observed e1-lh1 transitions. Among all the methods studied here, passivation by Na2S⋅xH2O is found to be most effective as it removes the native oxide layer completely leading to almost flat band condition.

Modulation mechanism of infrared photoreflectance in narrow-gap HgCdTe epilayers: A pump power dependent study

Modulation of built-in surface electric field was well established as a mechanism of photoreflectance (PR) processes in bulklike semiconductors, though pump light induced excess carrier concentration was once suggested theoretically as another possibility. In this work, pump power dependent infrared PR measurements were carried out on two arsenic-doped narrow-gap HgCdTe epilayer samples in a wide power range from about 1 mW to up to 480 mW. The relation between the integral intensity I of main PR features and pump power P can be described with I∝Pk, and the k is sensitive to temperature and pump power. While it takes a value of about 0.5 at 11 K or under considerably high pumping power at 77 and 150 K, it is close to 1.0 in a low pump power range at 77 and 150 K. The modulation mechanism is indicated as a combination of the modulation of built-in surface electric field and the pumping induced excess carrier concentration near the band edge. The modulation of built-in surface electric field takes place when the Debye screening length is larger than the pump light penetration depth, otherwise the pumping induced excess carrier concentration also comes into force. High pump power significantly enhances the signal and improves the signal-to-noise ratio of PR spectrum especially when the pumping induced excess carrier concentration plays a dominant role and can, therefore, serve as a primary choice for performing infrared PR measurements on narrow-gap HgCdTe epilayers efficiently.

Photoinduced charge transfer in ZnO/Cu2O heterostructure films studied by surface photovoltage technique

ZnO/Cu(2)O heterostructure films were prepared by a two-step electrodeposition method in aqueous solution on fluorine-doped tin oxide (FTO) substrates. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and UV-vis transmission measurements were utilized to characterize the films. Surface photovoltage (SPV) technique was used to investigate the process of photoinduced charge transfer. The results show that there is an electric field located at the interface between ZnO and Cu(2)O film and the photoinduced electrons in Cu(2)O film inject into ZnO under the effect of interfacial electric field with visible light irradiation. While under ultraviolet light illumination, the photoinduced electrons in Cu(2)O film accumulate at the surface of Cu(2)O film instead of injecting into ZnO under the action of surface built-in electric field of Cu(2)O film. The work function measurements confirm that the direction of interfacial electric field is from ZnO to Cu(2)O. These results are help to future design of high performance heterostructure photovoltaic devices.

Abnormal blueshift of UV emission in single-crystalline ZnO nanowires

Single-crystalline ZnO nanowires (NWs) were synthesized by a facile vapor transport method. The good orientation and high crystal quality were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) measurements. Excitation-power-dependence photoluminescence spectra of ZnO NWs show that the UV emission displayed an evident blueshift with increasing excitation power and the corresponding energy shift might be as large as 10 meV. This anomalous phenomenon correlates to the band bending level caused by the surface built-in electric field due to the existence of substantial oxygen vacancies. By increasing the excitation power, the enhanced neutralization effect near the surface will reduce the built-in electric field and lead to a reduction of band bending which triggers the blueshift of the UV emission.

Built-in surface electric field, piezoelectricity and photoelastic effect in GaN nanorods fornanophotonic devices

Novel behaviors arising from the coupling between the built-in surface electric field,piezoelectricity, electron–hole pairs and external light beam were observed in GaNnanorods. An increase in the optical excitation density resulted in a blueshiftin the photoluminescence spectra and a redshift in the frequency of the GaNA1(LO) phonon. The underlying mechanism was attributed to the screening of the built-in surfaceelectric field by photoexcited carriers and, through the converse piezoelectric effect, areduction in the internal strain. The existence of the built-in surface electric field in GaNnanorods was confirmed by scanning Kelvin probe microscopy. Our results firmly establishthe existence of the photoelastic effect in GaN nanorods. In addition to underpinningthe principle for applications in nanophotonic devices, this discovery also drawsattention to the novel effects arising from the inherent large surface-to-volume ratioof nanostructures, which is possibly applicable to many other nanomaterials.

Photoelastic effect in ZnO nanorods

A novel phenomenon called the photoelastic effect had been observed in ZnO nanorods, alongwith a number of intriguing anomalies. With increasing excitation power, it was found that theA1 (LO) phonon exhibited a red-shift in frequency, on top of a blue-shift in thephotoluminescence (PL) peak energy. In addition, the temperature-dependentphotoluminescence spectra behaved quite differently under high and low excitation power.All our results can be accounted for by the photoelastic effect, in which the built-insurface electric field was screened by photoexcited electrons and holes. Throughthe converse piezoelectric effect, the internal strain was therefore altered. Ourresults make possible a new thrust for manipulating the physical properties of ZnOnanorods, and should prove very useful in the application of optoelectric devices.

Room temperature photoreflectance of different electron concentration GaN epitaxial layers

Wurtzite-structure GaN epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates were studied by photoreflectance (PR) spectroscopy performed at room temperature. Several nominally undoped samples with different electron concentrations (5×10 15–5×10 18 cm −3), deposited under different growth conditions, were investigated. For low electron concentration we have observed three well-resolved excitonic transitions related to A, B, and C excitons. In this case, extremely small broadening of PR lines (few milielectronvolts) has been found. With the increase of carrier concentration in epilayers, we have observed an increase of broadening of the transitions. This has led to the disappearance of excitons in PR spectra and to the observations of one broad non-excitonic feature for highest electron concentration. Additionally, in some cases the Franz–Keldysh oscillations (FKO) have been also observed reflecting the existence of a surface built-in electric field in the epilayers, which do not destroy the excitons.

Strong enhancement of THz radiation intensity from semi-insulating GaAs surfaces at high temperatures

The temperature dependence of THz radiation from semi-insulating GaAs surfaces excited by ultrashort laser pulses has been studied above room temperature up to 900 K. It is found that the radiation power increases with an increase in temperature and has a peak at ∼500 K, and then decreases with a further increase in temperature. The power radiated at 500 K is more than 30 times stronger than that at room temperature. The enhancement mechanism is discussed in relation to the temperature dependence of the built-in surface electric field.

PHOTOREFLECTANCE SPECTROSCOPY OF EXCITONS IN β-ZnP 2

Photoreflectance (PR) spectroscopy has been performed for the singlet exciton system of g -ZnP 2 in order to investigate fine structure of the higher exciton states. The PR spectrum shows clear structure at each exciton level with large modulation amplitude. It is shown that the mechanism by which the PR signal is produced is electroreflectance due to the periodic cancellation of the built-in surface electric field by photoexcited carriers. A high-resolution PR spectrum obtained by using a Ti:sapphire laser as a probe light reveals the higher members of singlet excitons up to n =6.

Magnetic field and temperature dependence of terahertz radiation from InAs surfaces excited by femtosecond laser pulses

The magnetic field (0–5 T) and temperature (10–300 K) dependence of terahertz radiation from InAs surfaces excited by femtosecond laser pulses has been studied in detail. The radiation intensity is strongly enhanced under the magnetic field and at low temperatures, which is explained based on simple classical motion of photoexcited electrons under a built-in surface electric field and Lorentz force undergoing scattering. The radiation spectra are broad even at low temperatures under high magnetic fields, which suggests that strong electron scattering mechanisms exist for photoexcited electrons at InAs surfaces.

Electrooptical Properties of InGaAs/GaAs Strained Single Quantum Wells

The electrooptical properties of (100) and (111)B InGaAs/GaAs strained single quantum wells have been studied by the photoreflectance spectroscopy at various temperatures and under different biases. There is a Si δ-doping sheet under the GaAs surface to screen the surface built-in electric field from the quantum well. The spectral features consist of the excitonic interband transitions in the quantum well, and the Franz-Keldysh oscillations from the band edge transitions of the GaAs barrier near the surface. The piezoelectric field tilts the (111)B single quantum well toward the δ-doping and causes subband filling by the electrons from the δ-doping centers. With a proper external bias to repel electrons from the tilted quantum well, the excitonic interband transition in the (111)B system is enhanced.

Characteristics of three-beam photoreflectance in ZnTe/GaAs with deep traps

The characteristics of surface built-in electric field in ZnTe/GaAs with deep traps was studied by using three-beam photoreflectance (PR). Ar-ion laser-modulated PR intensity was very weak. When Ar-ion laser was used as modulation source, PR intensity under He–Ne laser bias was stronger than that under no bias irradiation, whereas He–Ne laser was used as modulation source, PR intensity under Ar-ion laser bias was weaker than that of under no bias irradiation. These results could be explained by competitive process in direction of modulated electric field between deep trap levels and band-to-band. As the temperature is increased from 15 K to about 90 K above band gap excitation, PR intensity was increased, whereas below band gap excitation, PR intensity was decreased. The increase in temperature results in increasing of probability of thermal emission rate of deep trap levels.

Photoellipsometry Analysis of n-AlGaAs/GaAs Heterojunction Structures

Photoellipsometry, a new contactless spectroscopic method, was applied to n-AlGaAs/GaAs heterojunction structures. Two samples were measured and analyzed, each having an epitaxially grown AlGaAs layer of a thickness of about 100 nm, with a different Al composition and a different doping density, on top of an undoped GaAs substrate. The objective of this research was to determine surface built-in electric field strength, depletion width, broadening, and critical point energies of AlGaAs for each given sample. The measured spectra were analyzed using the Franz-Keldysh theory with the inclusion of field inhomogeneity and nonuniform broadening effects. Good agreement between the measured and calculated spectra indicates that theories and models used were appropriate for the samples investigated and that the calculated results were reliable.