Surface Band Bending Research Articles

Probing the long-lived photo-generated charge carriers in transition metal dichalcogenides by time-resolved microwave photoconductivity

Abstract Understanding the dissociation of excitons into long-lived free charge carriers is a crucial issue when considering the applications of transition metal dichalcogenides (excitonic semiconductors) oriented toward the use of solar energy (such as photovoltaics or photocatalysis). In our work, long-lived carriers have been observed by time-resolved microwave photoconductivity (TRMC) for the first time in both atomically thin and bulk MoS2, MoSe2, WS2, and WSe2 crystals. The lifetime of majority carriers is close to microseconds and can even reach several microseconds due to different contribution of surface and defect states, as well as surface band bending (bulk). The three components depend on the material and vary from sample to sample, therefore determining the dynamics of the TRMC signal. The rise time of TRMC signal was found to be in the range of 0.1–0.2 μs and as it depends on the studied material it can be speculated that it is related to the dissociation time of excitons captured by traps.

Kelvin probe force microscopy for material characterization.

Kelvin probe force microscopy is a scanning probe method for imaging the surface potential by atomic force microscopy. The surface potential is one of the most important surface properties and is correlated to e.g. the work function, surface dipoles, localized surface charges and structural properties. It gives detailed information on the electrical properties and can be combined with optical and electrical excitation mechanisms providing additional properties like surface band bending and charge carrier mobilities. We will introduce the main concept and will briefly describe the major methods of operation. Based on the analysis of a Si superjunction device, structures dopant profiling and the concept of surface photovoltage measurements will be introduced. The influence of local charge accumulation on these devices will be presented and the effect on the measured contact potential values will be discussed.

Convertible Green Luminescence Determined by Surface Band Bending in ZnO

Convertible Green Luminescence Determined by Surface Band Bending in ZnO

Study of high–low KPFM on a pn-patterned Si surface

Comparative measurements between frequency modulation Kelvin probe force microscopy (FM-KPFM) using low frequency bias voltage and heterodyne FM-KPFM using high frequency bias voltage were performed on the surface potential measurement. A silicon substrate patterned with p- and n-type impurities was used as a quantitative sample. The multi-pass scanning method in the measurements of FM-KPFM and heterodyne FM-KPFM was used to eliminate the effect of the tip–sample distance dependence. The measured surface potentials become lower in the order of the p-type region, n-type region and n+-type region by both FM-KPFM and heterodyne FM-KPFM, which are in good agreement with the order of the work functions of the pn-patterned Si sample. We observed the difference in the surface potentials due to the surface band bending measured by FM-KPFM and heterodyne FM-KPFM. The difference is due to the fact that the charge transfer between the surface and bulk levels may or may not respond to AC bias voltage.

Surface-dominant transport properties in MoSe2 nanosheets

The electronic transport and photoconduction properties of the molybdenum diselenide (MoSe2) nanosheets were investigated. The transfer length method analysis suggests the electric current following an anomalously two-dimensional (2D) flow rather than the general 3D mode. The 2D current implies a higher electron concentration and downward band bending at the surface. A broad spectral photoresponse from ultraviolet to near-infrared wavelengths was also observed for the MoSe2 nanosheets. The optimal responsivity and photoconductive gain respectively at 58 A/W and 179 under the wavelengths at 405 nm were achieved. A surface-controlled photoconduction mechanism based on the surface band bending and electron accumulation is proposed to explain the long carrier lifetime and its dependence of light intensity and environment in MoSe2 nanostructures.

Local Tuning of the Surface Potential in Silicon Carriers by Ion Beam Induced Intrinsic Defects

The immobilization of biomaterials on a carrier is the first step for many different applications in life science and medicine. The usage of surface-near electrostatic forces is one possible approach to guide the charged biomaterials to a specific location on the carrier. In this study, we investigate the effect of intrinsic defects on the surface potential of silicon carriers in the dark and under illumination by means of Kelvin probe force microscopy. The intrinsic defects were introduced into the carrier by local, stripe-patterned ion implantation of silicon ions with a fluence of 3 × 1013 Si ions/cm2 and 3 × 1015 Si ions/cm2 into a p-type silicon wafer with a dopant concentration of 9 × 1015 B/cm3. The patterned implantation allows a direct comparison between the surface potential of the silicon host against the surface potential of implanted stripes. The depth of the implanted silicon ions in the target and the concentration of displaced silicon atoms was simulated using the Stopping and Range of Ions in Matter (SRIM) software. The low fluence implantation shows a negligible effect on the measured Kelvin bias in the dark, whereas the large fluence implantation leads to an increased Kelvin bias, i.e. to a smaller surface work function according to the contact potential difference model. Illumination causes a reduced surface band bending and surface potential in the non-implanted regions. The change of the Kelvin bias in the implanted regions under illumination provides insight into the mobility and lifetime of photo-generated electron-hole pairs. Finally, the effect of annealing on the intrinsic defect density is discussed and compared with atomic force microscopy measurements on the 2nd harmonic. In addition, by using the Baumgart, Helm, Schmidt interpretation of the measured Kelvin bias, the dopant concentration after implantation is estimated.

Surface photovoltage characterisation of metal halide perovskite on crystalline silicon using Kelvin probe force microscopy and metal-insulator-semiconductor configuration

In this study we analysed halide perovskite films deposited directly on crystalline silicon by means of two set-ups using different operating modes of the surface photovoltage (SPV) methods, i.e., the Kelvin probe force microscopy (KPFM) and the metal-insulator-semiconductor (MIS) technique. The KPFM allowed to visualize surface potential distribution on a microscale while MIS technique allowed to study SPV spectral dependence. We studied wavelength dependent SPV of these samples, which allowed us to effectively vary the probe depth in the sample and discern the contribution from each interface to the overall effect measured under white light illumination. Depending on where the photocarriers are generated, different SPV signals are observed: at the perovskite/Si interface, the signal depends on Si doping type, while at the surface the SPV is always negative indicating downward surface band bending. This is confirmed by analysing SPV phase measured in the AC MIS mode. In addition, distinction between slow and fast processes contributing to measured SPV was possible. It has been observed, that with decreasing the illumination wavelength, the processes causing SPV become slower, which can indicate that high energy photons not only generate electronic photocarriers but can also induce chemical changes with creation of defects or ionic species that also modify the measured SPV.

Revealing of InP multi-layer stacks from KPFM measurements in the dark and under illumination

Solar cells are complex devices, being constituted of many layers and interfaces. The study and the comprehension of the mechanisms that take place at the interfaces is crucial for efficiency improvement. This paper applies Kelvin probe force microscopy (KPFM) to study materials and interfaces with nanometer scale imaging of the surface potential in the dark and under illumination. KPFM measurements are highly sensitive to surface states and to the experimental measurement environment influencing the atomic probe operating conditions. Therefore, in order to develop a quantitative understanding of KPFM measurements, we have prepared a dedicated structured sample with alternating layers of InP:S and InP:Fe whose doping densities were determined by secondary-ion mass spectroscopy. We have performed KPFM measurements and shown that we can spatially resolve 20 nm thick InP layers, notably when performed under illumination which is well-known to reduce the surface band-bending.

Influences of ALD Al2O3 on the surface band-bending of c-plane, Ga-face GaN

The recently demonstrated approach of grafting n-type GaN with p-type Si or GaAs, by employing ultrathin Al2O3 at the interface, has shown the feasibility to overcome the poor p-type doping challenge of GaN. However, the surface band-bending of GaN that could be influenced by the Al2O3 has been unknown. In this work, the band-bending of c-plane, Ga-face GaN with ultrathin Al2O3 deposition at the surface of GaN was studied using X-ray photoelectron spectroscopy. The study shows that the Al2O3 can help suppress the upward band-bending of the c-plane, Ga-face GaN with a monotonic reduction trend from 0.48 eV down to 0.12 eV as the number of Al2O3 deposition cycles increases from 0 to 20. The study further shows that the band-bending can be mostly recovered after removing the Al2O3 layer, concurring that the introduction of ultrathin Al2O3 is the main reason for the surface band-bending modulation.

Solution–processed Cu2O/ZnO/TiO2/Pt nanowire photocathode for efficient photoelectrochemical water splitting

State–of–the–art Cu2O photocathodes are mostly fabricated by means of electrodeposition and have been reported to be flooded with the copper vacancies (VCu), into which the photoexcited electrons are rapidly trapped and thereby hardly injected into the electrolyte for photoelectrochemical water splitting. To address this issue, an alternative approach consisting of simple wet chemistry combined with post–thermal treatment is employed in the present contribution to grow the Cu2O nanowires (NWs) on the Cu foil, which provides the Cu ions to fill VCu. An important result of the depletion of VCu is the space charge layer extending deep into the Cu2O NWs, wherein the photoexcited charge carriers are rectified by the surface band bending for efficient separation. The charge separation is further reinforced by using ZnO to first extract the photogenerated electrons from Cu2O, TiO2 to further glean the electrons from ZnO, and the Pt co–catalyst to facilitate their injection into the electrolyte. These materials are deposited sequentially on the Cu2O NWs by means of chemical bath deposition, which is put forward in the present contribution as a cost–effective and readily scalable alternative to the conventional atomic layer deposition technique. The synergistic effect leads to a photocurrent density of the Cu2O/ZnO/TiO2/Pt NWs of −8.2 mA cm−2, well outperforming that of the pristine Cu2O NWs by more than 2.5–fold and, moreover, being comparable to that of the electrodeposited counterparts reported in the literature.

Bidirectional Control of the Band Bending at the (2̅01) and (010) Surfaces of β-Ga2O3 Using Aryldiazonium Ion and Phosphonic Acid Grafting

Beta gallium oxide (β-Ga2O3) is an ultrawide-bandgap semiconductor with one of the highest-known breakdown strengths (∼8 MV/cm) making it a strong candidate for high-efficiency power electronics, deep-ultraviolet photodetectors, and transparent electronic devices. Bare β-Ga2O3 surfaces exhibit a strong upward band bending and electron depletion that is reduced by the formation of a hydroxyl termination on exposure to the atmosphere, although this effect varies with exposure conditions and the processing history of different samples. This work investigates the covalent modification of (2̅01) and (010) β-Ga2O3 surfaces with organic layers, as a mechanism for more effectively controlling their surface band bending. We compare the different electronic effects of grafted layers formed by the electrochemical reduction of 4-nitrobenzenediazonium ions and the spontaneous grafting of octadecylphosphonic acid (ODPA). Atomic force microscopy, synchrotron X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy confirmed the presence of few-nanometer layers of covalently attached nitrophenyl (NP) and ODPA molecules. Valence band XPS showed that NP modification produced upward band bending shifts of +0.51 and +0.53 eV on (2̅01) and (010) β-Ga2O3, respectively, with further increases of +0.35 and +0.20 eV after X-ray-induced reduction of the nitro substituent to amino-like moieties. In contrast, ODPA modification produced downward shifts in surface band bending of −0.36 and −0.07 eV on (2̅01) and (010) β-Ga2O3, respectively. Our study demonstrates that covalently bound NP and ODPA molecules can be used to significantly modify the electronic properties of β-Ga2O3 surfaces, a finding that should prove useful for electronic applications of this material.

Ultrasensitive gas sensor based on nanocube In2O3-CNH composite at low operating temperature

A facile solvothermal route prepared In2O3/CNH nanocomposites with different CNH concentrations. All nanocomposites' structural and morphological properties were investigated by XRD, FESEM, TEM, XPS, and BET. The H2S sensing performance of the sensors based on them was tested, operated between 40 °C and 300 °C. The results showed that the optimal composition is In2O3/CNH (2 wt%) with a sensor signal to 2 ppm H2S, at an operating temperature of 70 °C, of almost 3000. The sensors demonstrated excellent selectivity, low impact of humidity and good reproducibility and stability. The superior sensor performance is related to the p-n junction formation between the CNH and In2O3, which determines the increase of the surface band bending and the sensor response. The gas sensing mechanism is thoroughly discussed, and the In2O3/CNH nanocomposite sensor can be considered an ideal candidate for high-performance detection of H2S with low-power consumption.

Transparently Passivating Catalyst of Hydrated Manganese Phosphate for Photoelectrochemical O2 Generation

A transparently hydrated manganese phosphate (“Mn–Pi”) catalyst is coated onto a Ti-doped hematite (Ti:Fe2O3) photoanode by the photo-assisted electrochemical deposition. Under a visible light illumination of 100 mW cm–2, the photocurrent density of Ti:Fe2O3/Mn–Pi reaches 0.28 mA cm–2 at 1.2 VRHE, which is around two times higher than that obtained with the pristine Ti:Fe2O3 electrode in a neutral electrolyte, and the “onset” potential reduces by 100 mV. The passivating role of the Mn–Pi catalyst is revealed experimentally. The surface states of hematite are significantly passivated after Mn–Pi deposition, which relieves the degree of Fermi level pinning of hematite and enhances the surface band bending of hematite. Therefore, the photovoltage and the charge separation are improved. Moreover, due to the valence change of the Mn element in Mn–Pi, the charge transfer through the Mn–Pi layer is favorable, which has a low charge transfer resistance for photogenerated holes. The suitability of this transparently passivating catalyst for concentrating light is also identified. This work demonstrates that Mn–Pi can be applied for photoelectrochemical water splitting as a transparently passivating catalyst.

(Invited) Semiconductor Surface Passivation by ALD and CVD Prepared Nanolayers

For (opto)electronic devices, it is generally key to reduce the trapping and recombination of charge carriers at the semiconductor surfaces and interfaces. This is becoming more and more challenging due to the ongoing diversification in semiconductor materials to be used in the devices (extending from Si to (Si)Ge and from group-IV to group-III-V semiconductors) but also due to the increasing surface-to-volume ratio of the nanostructures employed. Surface passivation can be achieved by employing thin films of semiconductor or dielectric materials which often serve other functionalities in the devices too. The underlying mechanism of the surface passivation can be the reduction of surface defect states (i.e. so-called chemical passivation) and/or band bending due to so-called field-effect passivation effect (e.g. due to fixed charges in the film).In this contribution the surface passivation of semiconductors by recent innovative nanolayer approaches will be presented. These nanolayers include materials such as Al2O3, POx, a-Si:H and (doped) ZnO as well as stacks thereof. The preparation of the materials by (plasma-enhanced) chemical vapor deposition (CVD) and atomic layer deposition (ALD) will be discussed as well as the application of the nanolayers on semiconductors such as Si, SiGe, Ge and InP. Results will be presented for both wafers and nanowires. Special attention will be given to the underlying mechanism of the surface passivation achieved as well as its relation to the properties of the nanolayers employed. Figure 1

(Invited) Recent Progress on Anisotropic Electrical and Optical Studies in Ultrawide Bandgap β-Ga2O3 Electronics and Photonics

Ultrawide bandgap semiconductor beta-phase gallium oxide (β-Ga2O3) has recently garnered significant research interests in various electronic and photonic devices such as power devices, RF electronics, photodetectors, self-powered sensors, and high temperature and radiation-hardened devices. This is mainly driven by two factors. The first factor is β-Ga2O3’s advantageous material properties, including large bandgap (4.8 eV), high critical electric field (8 MV/cm), high device figure of merits, and excellent material stability in harsh environments. The second factor is that cost-effective bulk β-Ga2O3 substrates are commercially available, and high-quality β-Ga2O3 epilayers can be grown using various methods (e.g., MOCVD, MBE, and HVPE) for electronic and photonic device fabrication. One of the most important aspects to consider for β-Ga2O3 device design and fabrication is crystal orientation. Since β-Ga2O3 crystalizes in a highly asymmetric monoclinic structure, there are many selections available in terms of crystal orientations such as (010), (-201), (001), and (100). These different crystal orientations may lead to anisotropic electrical and optical properties in β-Ga2O3 electronics and photonics, which warrants a systematic study. This talk will discuss our recent studies of electrical and nonlinear optical properties in β-Ga2O3 with different crystal orientations and their effects on β-Ga2O3 devices. In the first part of the talk, a comparative investigation will be presented for vertical (010) and (-201) β-Ga2O3 Schottky barrier diodes, where their temperature-dependent forward and reverse I-V and C-V characteristics are compared. At forward bias, they showed different on-resistances, turn-on voltages, electron mobilities, and Schottky barrier heights. At reverse bias, different leakage currents, breakdown voltages, and leakage mechanisms were observed in the two devices. These discrepancies are ascribed to different atomic configurations and dangling bonds, and distinct surface band bending revealed by x-ray photoelectron spectroscopy (XPS) on the two surfaces of different crystal orientations. In addition, we also examined the deep-level defects in β-Ga2O3 with different crystal orientations using deep level transient spectroscopy (DLTS). The second part of the talk will focus on the nonlinear optical properties of (010) and (-201) β-Ga2O3, including two-photon absorption coefficient, Kerr nonlinear refractive index, and their polarization dependence. (-201) β-Ga2O3 has a smaller two-photon absorption coefficient, a larger Kerr refractive index, and strong in-plane nonlinear optical anisotropy. Interestingly, the two-photon absorption coefficient of β-Ga2O3 is 20x smaller than wide bandgap GaN, indicating its potential for low-loss waveguides, stable resonators, and integrated photonics. We recently demonstrated β-Ga2O3 optical waveguides with a low loss of 3.7 dB/cm in the ultraviolet to near infrared spectral region. More investigations are expected in this area as β-Ga2O3 electronics and photonics research progresses. And the choice of crystal orientations for the mass production of β-Ga2O3 devices rests on applications, device designs, and costs of substrate of a certain crystal orientation.

Spectroscopic insight of low energy electron emission from diamond surfaces

Low work function materials are desirable in many applications such as electron emission and photocatalysis. We have studied low energy electron emission from low work function hydrogen terminated diamond surfaces via electron spectroscopy to gain insight into the mechanisms involved during electron excitation and emission. Electron emission was found to be dominated by electrons within the band gap energy region, allowed due to the negative electron affinity diamond surface, while sub-bandgap illumination was able to significantly increase emission current. Substantial upward surface band bending greater than 2 eV was observed for the diamond samples, which affect electron accumulation at the surfaces. Intra-bandgap states are shown to strongly influence electron emission behavior, which can have great implications for various energy conversion devices.

Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins

Due to their intrinsically large surface-to-volume ratio, nanowires and nanofins interact strongly with their environment. We investigate the role of the main air constituents nitrogen, oxygen and water on the efficiency of radiative recombination in GaN nanostructures as a function of different surface treatments and at temperatures up to 200 °C. Oxygen and water exposures exhibit a complex behavior as they can both act quenching and enhancing on the photoluminescence intensity dependent on the temperature. For oxygen, these characteristics are already observed for low concentrations of below 0.5% in nitrogen. While the photoluminescence intensity changes induced by oxygen occur independently of illumination, the influence of water is light-induced: it evolves within tens of seconds under ultraviolet light exposure and is heavily influenced by the nanostructure pre-treatment. In contrast to observations in dry atmospheres, water prevents a recovery of the photoluminescence intensity in the dark. Combined measurements of the electrical current through GaN nanofins and their photoluminescence intensity reveal the environmental influence on the interaction of non-radiative recombination processes and changes in the surface band bending of the nanostructures. Several investigated solvents show an enhancing effect on the PL intensity increase, peaking in c-hexane with a 26-fold increase after 6 min of light exposure. Stabilization of the PL intensity was achieved by a passivation of the GaN surface with Ga x O y , and ZnO shells. Surprisingly, Al2O3 coatings resulted in a highly instable PL intensity during the first minutes of illumination. Our findings reveal the high importance of controlled environmental conditions for the investigation of nanostructures, especially when aimed at their applications in the fields of environmental sensing, photo-catalysis and light-emitting diodes.

Design rules of pseudocapacitive electrode materials: ion adsorption, diffusion, and electron transmission over prototype TiO2

The development of a high-performing pseudo-capacitor requires a comprehensive understanding of electrode materials from the aspects of electron transfer and electrolyte ion adsorption and diffusion. Herein, these factors are considered over the prototype TiO2, and a high pseudo-capacitance is achieved via the introduction of various defects, i.e., oxygen defect (VO) and co-doped defect (VO+NO). The study is based on joint explorations of first-principle calculations and the transfer matrix method. Relative to pristine TiO2 (300 F g−1), defective TiO2 produces pseudocapacitance as high as 1700 F g−1. Moreover, defects induce small barriers for electron transmission caused by surface band bending. The climbing image nudged elastic band diffusion of H ions displays a much higher barrier in TiO2−VO than in TiO2−VO+NO. Such a result indicates easy H diffusion in the co-doped system. This work provides insights into the adsorption and diffusion of electrolyte ions and the influence of defects on electron transfer. The results are also significant for the design and optimization of electrode materials for the next generation of supercapacitors.

Interfacial Magnetic Behaviors and Chemical States of Fe Grown on MoS2

Magnetic and chemical properties of ultrathin Fe films on molybdenum disulfide (MoS2) are investigated using magneto‐optic Kerr effect (MOKE) and X‐ray photoemission spectroscopy (XPS). A magnetization hysteresis loop is found after nominal 10 Å of Fe deposition on MoS2 at room temperature. The thickness‐dependent MOKE measurement evaluates that the magnetic dead layer (MDL) exists up to ≈2 Å of Fe on MoS2 due to the interaction between Fe and MoS2. XPS shows small peak shifting for Mo 3d and S 2p peaks toward lower binding energies with initial Fe deposition, indicating the modification of surface band bending. Fe 2p peaks for lower thickness of Fe are significantly broader than those for higher thickness, which is attributed to the coexistence of metallic and ionic Fe in the film. The presence of ionic Fe, localized at the interface, indicates strong interactions between Fe and MoS2, and the existence of chemical bonding at the interface is corroborated with the formation of MDL.

Schottky contact formation by an insulator: Lithium fluoride on silicon

The low work function material lithium fluoride (LiF) facilitates electron-selective contacts to n-type silicon and is frequently used in dopant-free heterocontacts for silicon solar cells. Our photoelectron spectroscopy (PES) data show that LiF deposition on n-Si leads, indeed, to Fermi-level crossing of the conduction band minimum. Furthermore, PES reveals intrinsic surface band bending on hydrogen-terminated p-Si(100). LiF deposition on p-Si leads to a rigid shift of the Si core-levels and the vacuum-level pointing to ideal Schottky contact formation. This is further supported by the open-circuit voltage of Al/LiF/p-Si/Al solar cells, which corresponds to the photoelectron spectroscopy (PES)-measured band bending magnitude.