Surface Fermi Level Pinning Research Articles

Origin of giant enhancement of phase contrast in electron holography of modulation-doped [formula omitted]-type GaN

The electron optical phase contrast probed by electron holography at n-n+ GaN doping steps is found to exhibit a giant enhancement, in sharp contrast to the always smaller than expected phase contrast reported for p-n junctions. We unravel the physical origin of the giant enhancement by combining off-axis electron holography data with self-consistent electrostatic potential calculations. The predominant contribution to the phase contrast is shown to arise from the doping dependent screening length of the surface Fermi-level pinning, which is induced by FIB-implanted carbon point defects below the outer amorphous shell. The contribution of the built-in potential is negligible for modulation doping and only relevant for large built-in potentials at e.g. p-n junctions. This work provides a quantitative approach to so-called dead layers at TEM lamellas.

Surface Charge: An Advantage for the Piezoelectric Properties of GaN Nanowires

The optimization of the new generation of piezoelectric nanogenerators based on 1D nanostructures requires a fundamental understanding of the different physical mechanisms at play, especially those that become predominant at the nanoscale regime. One such phenomenon is the surface charge effect (SCE), which is very pronounced in GaN NWs with sub-100 nm diameters. With an advanced nano-characterization tool derived from AFM, the influence of SCE on the piezo generation capacity of GaN NWs is investigated by modifying their immediate environment. As-grown GaN NWs are analysed and compared to their post-treated counterparts featuring an Al2O3 shell. We establish that the output voltages systematically decrease by the Al2O3 shell. This phenomenon is directly related to the decrease of the surface trap density in the presence of Al2O3 and the corresponding reduction of the surface Fermi level pinning. This leads to a stronger screening of the piezoelectric charges by the free carriers. These experimental results demonstrate and confirm that the piezo-conversion capacity of GaN NWs is favoured by the presence of the surface charges.

Investigation of the surface band structure and the evolution of defects in β-(AlxGa1−x)2O3

This study examines the electronic and luminescent properties of β-(AlxGa1−x)2O3 (0 ≤ x ≤ 0.42) thin films grown on (0001) sapphire using laser-MBE, with a focus on the evolution of defect energy levels and their impact on surface Fermi level pinning and luminescence. X-ray photoelectron spectroscopy (XPS) and cathodoluminescence (CL) have been employed to analyze surface band bending and defect evolution as a function of aluminum content. The results have revealed a pinned Fermi level at 3.6 eV above the valence band maximum despite the increase in the bandgap. The consequent upward band bending has been confirmed by a peak shift in the core level XPS. The defects that lead to the Fermi level pinning effect are attributed to E2*, which is related to a Ga vacancy or Ga vacancy-O vacancy complex. In addition, CL spectroscopy and depth-resolved CL have demonstrated consistent blue and ultraviolet emissions across the Al content range and a similar suppression of electron concentration on blue and ultraviolet emissions in β-(AlxGa1−x)2O3 and β-Ga2O3. Based on the observed evolution of defects with Al content, the blue band emission is attributed to electron transition in the donor–accepter pair.

Surface treatment of GaN nanowires for enhanced photoelectrochemical water-splitting

High-efficiency hydrogen production through photoelectrochemical (PEC) water splitting has emerged as a promising solution to address current global energy challenges. III-nitride semiconductor photoelectrodes with nanostructures have demonstrated great potential in the near future due to their high light absorption, tunable direct band gap, and strong physicochemical stability. However, several issues, including surface trapping centers, surface Fermi level pinning, and surface band bending, need to be addressed. In this work, enhanced photovoltaic properties have been achieved using gallium nitride (GaN) nanowires (NWs) photoelectrodes by adopting an alkaline solution surface treatment method to reduce the surface states. It was found that surface oxides on NWs can be removed by an alkaline solution treatment without changing the surface morphology through X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and other characterization methods. These findings provide new insights to the development of high-efficiency photoelectrodes for new energy source applications.

Subsurface Engineering Induced Fermi Level De-pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting is a promising approach for renewable solar light conversion. However, surface Fermi level pinning (FLP), caused by surface trap states, severely restricts the PEC activities. Theoretical calculations indicate subsurface oxygen vacancy (sub-Ov) could release the FLP and retain the active structure. A series of metal oxide semiconductors with sub-Ov were prepared through precisely regulated spin-coating and calcination. Etching X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and electron energy loss spectra (EELS) demonstrated Ov located at sub ~2-5 nm region. Mott-Schottky and open circuit photovoltage results confirmed the surface trap states elimination and Fermi level de-pinning. Thus, superior PEC performances of 5.1, 3.4, and 2.1 mA cm-2 at 1.23 V vs. RHE were achieved on BiVO4, Bi2O3, TiO2 with outstanding stability for 72 h, outperforming most reported works under the identical conditions.

Subsurface Engineering Induced Fermi Level De‐pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting

AbstractPhotoelectrochemical (PEC) water splitting is a promising approach for renewable solar light conversion. However, surface Fermi level pinning (FLP), caused by surface trap states, severely restricts the PEC activities. Theoretical calculations indicate subsurface oxygen vacancy (sub‐Ov) could release the FLP and retain the active structure. A series of metal oxide semiconductors with sub‐Ovwere prepared through precisely regulated spin‐coating and calcination. Etching X‐ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and electron energy loss spectra (EELS) demonstrated Ovlocated at sub ∼2–5 nm region. Mott–Schottky and open circuit photovoltage results confirmed the surface trap states elimination and Fermi level de‐pinning. Thus, superior PEC performances of 5.1, 3.4, and 2.1 mA cm−2at 1.23 V vs. RHE were achieved on BiVO4, Bi2O3, TiO2with outstanding stability for 72 h, outperforming most reported works under the identical conditions.

Controlling Fermi level pinning in near-surface InAs quantum wells

Hybrid superconductor–semiconductor heterostructures are a promising platform for quantum devices based on mesoscopic and topological superconductivity. In these structures, a semiconductor must be in close proximity to a superconductor and form an Ohmic contact. This can be accommodated in narrow bandgap semiconductors, such as InAs, where the surface Fermi level is positioned close to the conduction band. In this work, we study the structural properties of near-surface InAs quantum wells and find that surface morphology is closely connected to low-temperature transport, where electron mobility is highly sensitive to the growth temperature of the underlying graded buffer layer. By introducing an In0.81Al0.19As capping layer, we show that we change the surface Fermi level pinning of the In0.81Al0.19As thin film as compared to the In0.81Ga0.19As, giving rise to a tuning of the Fermi level in the InAs layer. Experimental measurements show a strong agreement with Schrödinger–Poisson calculations of the electron density, suggesting the conduction band energy of the In0.81Ga0.19As and In0.81Al0.19As surface is pinned to 40 and 309 meV above the Fermi level, respectively.

Clean quantum point contacts in an InAs quantum well grown on a lattice-mismatched InP substrate

Indium arsenide (InAs) has a small effective mass, strong spin-orbit coupling, and surface Fermi level pinning. Together with improvements in the epitaxial growth of quantum well heterostructures, InAs presents an enticing platform for studying mesoscopic physics. Here, the authors investigate quantized conductance and magnetotransport through quantum point contacts (QPCs) fabricated on a deep InAs two-dimensional electron gas. These QPCs can serve as a robust building block for further experiments in quantum circuits and electron quantum optics.

Inhomogeneous Barrier Height Characteristics of n-Type AlInP for Red AlGaInP-Based Light-Emitting Diodes

For micro-light-emitting diode (LED)-based display applications, such as virtual reality and augmented reality, high-performance Ohmic contacts (namely, the improvement of current injection efficiency) is vital to the realization of high-efficiency micro-LEDs. The surface Fermi level pinning characteristics could be comprehended in terms of the relation between work function of metals (ΦM) and Schottky barrier height (SBH, ΦB). In this study, we have investigated the surface Fermi level pinning characteristics of (001) n-AlInP surfaces by employing Schottky diodes with different metals. With an increase in the temperature, ΦB increases linearly and ideality factors (n) decreases. This behavior is related to the barrier height inhomogeneity. Inhomogeneity-model-based ΦB is evaluated to be in the range of 0.86–1.30 eV, which is dependent on the metal work functions and are similar to those measured from capacitance-voltage relation. Further, The S-parameter, the relation between ΦB and ΦM (dΦB/dΦM), is 0.36. This is indicative of the partial pinning of the surface Fermi level at the surface states placed at 0.95 eV below the conduction band. Furthermore, it is also shown that (NH4)2S-passivation results in an increases the mean SBH and the S-parameter (e.g., 0.52).

Imaging the electrostatic landscape of unstrained self-assemble GaAs quantum dots

Unstrained GaAs quantum dots are promising candidates for quantum information devices due to their optical properties, but their electronic properties have remained relatively unexplored until now. In this work, we systematically investigate the electronic structure and natural charging of GaAs quantum dots at room temperature using Kelvin probe force microscopy (KPFM). We observe a clear electrical signal from these structures demonstrating a lower surface potential in the middle of the dot. We ascribe this to charge accumulation and confinement inside these structures. Our systematical investigation reveals that the change in surface potential is larger for a nominal dot filling of 2 nm and then starts to decrease for thicker GaAs layers. Using k · p calculation, we show that the confinement comes from the band bending due to the surface Fermi level pinning. We find a correlation between the calculated charge density and the KPFM signal indicating that k · p calculations could be used to estimate the KPFM signal for a given structure. Our results suggest that these self-assembled structures could be used to study physical phenomena connected to charged quantum dots like Coulomb blockade or Kondo effect.

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Photoreflectance is a contactless type of modulation optical spectroscopy. It is used to study the band structure features of monocrystalline semiconductors, their doping level, the composition of alloys, as well as surface and interface band bending. Using high-quality GaAs as an example, the possibilities of describing the photoreflectance lineshape by one-electron and exciton models are demonstrated. The spectra of ultrapure samples of this material exhibit an oscillating structure well described by excitonic effects. For III-V alloys, a review of photoreflectance results concerning the effect of composition and temperature on the band gap and spin-orbit splitting is carried out. Determination of the position of the Fermi level on the surface (Fermi level pinning) for III-V crystals is considered. The currently developing technique for measuring photoreflectance in the mid-infrared range (photomodulation Fourier transform infrared spectroscopy) is described in detail. It is shown that phase correction plays a decisive role in such measurements. Original results demonstrate the capabilities of this method in a wide wavelength range.

Analysis of simultaneous occurrence of shallow surface Fermi level pinning and deep depletion in MOS diode with Mg-ion-implanted GaN before activation annealing

The characteristics of a MOS diode with Mg-ion-implanted GaN before activation annealing were investigated. Mg ion implantation onto n-GaN with slightly high Si doping concentration (5 × 1017 cm–3) was performed with a moderate dosage (1.5 × 1012 cm–2). The completed MOS diode showed n-type features. The capacitance–voltage (C–V) and capacitance–frequency (C–f) characteristics of the MOS diode indicated that shallow surface Fermi level pinning and deep depletion occurred simultaneously. By applying the conductance method to the measured C–f characteristics, a discrete level at 0.2–0.3 eV below the conduction band edge was detected. On the basis of the simulation of the high-frequency-limit C–V curve, the detected discrete level distributed in the bulk of n-GaN rather than at the insulator/semiconductor interface, so that it caused surface Fermi level pinning at a relatively shallow energy level and deep depletion owing to its acceptorlike nature simultaneously.

Ultrafast and Sensitive Self-Powered Photodetector Featuring Self-Limited Depletion Region and Fully Depleted Channel with van der Waals Contacts.

Self-powered photodetectors with great potential for implanted medical diagnosis and smart communications have been severely hindered by the difficulty of simultaneously achieving high sensitivity and fast response speed. Here, we report an ultrafast and highly sensitive self-powered photodetector based on two-dimensional (2D) InSe, which is achieved by applying a device architecture design and generating ideal Schottky or ohmic contacts on 2D layered semiconductors, which are difficult to realize in the conventional semiconductors owing to their surface Fermi-level pinning. The as-fabricated InSe photodiode features a maximal lateral self-limited depletion region and a vertical fully depleted channel. It exhibits a high detectivity of 1.26 × 1013 Jones and an ultrafast response speed of ∼200 ns, which breaks the response speed limit of reported self-powered photodetectors based on 2D semiconductors. The high sensitivity is achieved by an ultralow dark current noise generated from the robust van der Waals (vdW) Schottky junction and a high photoresponsivity due to the formation of a maximal lateral self-limited depletion region. The ultrafast response time is dominated by the fast carrier drift driven by a strong built-in electric field in the vertical fully depleted channel. This device architecture can help us to design high-performance photodetectors utilizing vdW layered semiconductors.

Advancements in High Indium Content Alinn Grown Via Metal Modulated Epitaxy and Application Towards Polar/Non-Polar Optical Devices

AlInN has been a topic of recent study due to its unique property among III-nitrides of lattice matching to GaN at a composition of 18% indium, making it a strong candidate for power electronic and optoelectronic applications. A widely unexplored area is the application of AlInN towards visible/Near-IR optical devices. AlInN has a tunable bandgap, spanning the infrared to ultraviolet range, of 0.7 to 6.1eV. At a composition of about 70% indium AlInN has a perfect bandgap, 1.7eV, for tandem solar cells with silicon and for various optical communications applications. Challenges exist with the growth of AlInN as a result of the large lattice parameter mismatch and differences in growth regimes between the two binary components. At the low temperatures required for the growth of AlInN the aluminum adatoms have a low mobility, often leading to lateral phase separation in the film. Metal Modulated Epitaxy (MME) offers a good solution to the growth issues of AlInN. This flux modulated technique allows for growth under metal rich conditions, increasing surface diffusion lengths of the Aluminum while limiting droplet formation and terminating in a dry surface suitable for devices. For comparison ~100 nm thick high indium content AlInN samples were grown using nitrogen rich (0.8 III/V ratio) MBE and MME using a III/V ratio of 1.3 with a dose designed to prevent surface segregation (details supplied at the conference). Both films were grown cold at 375 degrees C to limit phase separation The MME sample showed improvement over the MBE sample in crystal quality and surface roughness, with the XRD (0002) reflection FWHM improving from 783 arcsec to 184 arcsec, the XRD (10-15) reflection FWHM improving from 2456 arcsec to 1421 arcsec, and the AFM surface roughness improving from 1.52nm rms to 0.882nm rms between the MBE and MME samples respectively. The carrier concentrations of both samples were found to be about 1E19 cm-3 via hall measurement implying that like InN, In-rich AlInN may suffer from excessive residual electron concentrations and/or surface fermi-level pinning. Optical measurements will be presented at the conference. The same MME growth conditions were used to grow 100nm of indium rich AlInN directly on a (111) p-silicon substrate. Simple indium contacts were used to contact the Si and AlInN. Vertical conduction and rectification between the silicon and AlInN layers were observed in this large area (~0.25x0.25 cm2) device. Understanding of the limitations of and physics governing this promising vertical polar/non-polar heterojunction will be given at the conference. Additionally, results from various growth conditions will be detailed. Figure 1

Mechanical stress dependence of the Fermi level pinning on an oxidized silicon surface

A combination of micro-Raman spectroscopy and micro-XPS (X-ray photo-electron spectroscopy) mapping on statically deflected p-type silicon cantilevers is used to study the mechanical stress dependence of the Fermi level pinning at an oxidized silicon (001) surface. With uniaxial compressive and tensile stress applied parallel to the ⟨110⟩ crystal direction, the observations are relevant to the electronic properties of strain-silicon nano-devices with large surface-to-volume ratios such as nanowires and nanomembranes. The surface Fermi level pinning is found to be even in applied stress, a fact that may be related to the symmetry of the Pb0 silicon/oxide interface defects. For stresses up to 240 MPa, an increase in the pinning energy of 0.16 meV/MPa is observed for compressive stress, while for tensile stress it increases by 0.11 meV/MPa. Using the bulk, valence band deformation potentials the reduction in surface band bending in compression (0.09 meV/MPa) and in tension (0.13 meV/MPa) can be estimated.

Anchoring MWCNTs to 3D honeycomb ZnO/GaN heterostructures to enhancing photoelectrochemical water oxidation

Gallium nitride (GaN) is one of the ubiquitously known photoanode for photoelectrochemical water splitting (PEC-WS) due to its tunable band gap and favorable band edge positions. However, the unavoidable surface defects in GaN induces surface Fermi level pinning and surface band bending which severely reduces its PEC conversion efficiency. Constructing heterostructure is the key to approaching better charge separation efficiency and light harvesting ability for PEC-WS. Considering this, we have fabricated ternary heterostructure of GaN/ZnO/MWCNTs photoanode by combining metal organic chemical vapour deposition (MOCVD), hydrothermal and ‘dip and dry’ methods. FE-SEM results showed the formation of 3D hierarchical honeycomb structure of ZnO on GaN thin film surface when MWCNTs are added into hydrothermal reaction. We investigate the advantage of ZnO honeycomb structure in enhancing the solar PEC-WS performance of GaN photoanode. The synergy of incorporating MWCNTs has resulted into improvement in surface morphology, electron transportation and light harvesting capability. The as obtained ternary heterostructure exhibits photocurrent (Jph) of 3.02 mA/cm2 at 0 V versus Pt electrode under 1-sun light illumination which is about 2.58 times higher than that of pristine GaN photoanodes (Jph = 1.14 mA/cm2).

Enhanced piezoelectric output of NiO/nanoporous GaN by suppression of internal carrier screening

The efficiency of piezoelectric nanogenerators (PNGs) significantly depends on the free carrier concentration of semiconductors. In the presence of a mechanical stress, piezoelectric charges are generated at both ends of the PNG, which are rapidly screened by the free carriers. The screening effect rapidly decreases the piezoelectric output within fractions of a second. In this study, the piezoelectric outputs of bulk- and nanoporous GaN-based heterojunction PNGs are compared. GaN thin films were epitaxially grown on sapphire substrates using metal organic chemical vapor deposition. Nanoporous GaN was fabricated using electrochemical etching, depleted of free carriers owing to the surface Fermi-level pinning. A highly resistive NiO thin film was deposited on bulk- and nanoporous GaN using radio frequency magnetron sputter. The NiO/nanoporous GaN PNG (NPNG) under a periodic compressive stress of 4 MPa exhibited an output voltage and current of 0.32 V and 1.48 μA cm−2, respectively. The output voltage and current of the NiO/thin film-GaN PNG (TPNG) were three and five times smaller than those of the NPNG, respectively. Therefore, the high-resistivity of NiO and nanoporous GaN depleted by the Fermi-level pinning are advantageous and provide a better piezoelectric performance of the NPNG, compared with that of the TPNG.

Unveiling the Influence of Surface Fermi Level Pinning on the Piezoelectric Response of Semiconducting Nanowires

AbstractZnO nanowires (NWs) are excellent candidates for the integration of energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. However, ZnO NWs are usually nonintentionally n‐doped during their growth. Thus, it can be expected theoretically that their piezoelectric response be degraded and mostly geometry‐independent as a result of strong screening effects by free carriers, while experimentally many NW‐based piezoelectric transducers demonstrate quite reasonable performance. In this paper, this apparent contradiction is explained by surface Fermi level pinning (SFLP). Simulations based on the finite element method are developed to investigate the influence of SFLP on the electrical response of ZnO piezogenerators based on arrays of vertical ZnO NWs (the so‐called VING) operated in compression or bending mode. The results show that the nonsymmetrical response with respect to loading direction, which has been observed experimentally when the VING is bent, can be explained when account is taken of the reverse piezoelectric effect associated with SFLP. Influence of geometry and comparison to thin film are also discussed. It is found that the performance improvement, which is observed experimentally at scales much larger than predicted by ab initio models when NWs are compared to thin films, can be partly related to SFLP effects rather than larger piezoelectric coefficients.

FTIR photoreflectance of narrow-gap heterostructures based on AlxIn1-xSb alloys

Photoreflectance (PR) spectra of AlxIn1-xSb-based heterostructures have been obtained by using a novel photomodulation Fourier-transform infrared (FTIR) spectroscopy method. The studied samples - bulk AlxIn1-xSb epilayers and InSb/AlxIn1-xSb heterostructures - were grown by molecular beam epitaxy on semi-insulating GaAs substrates via AlSb buffer layers. The critical-point energies E0 and E0+Δ0 of AlxIn1-xSb alloys of various direct-gap compositions were defined from the PR spectra features. It was found that the E0 values are greater than the observed AlxIn1-xSb photoluminescence peak energy, with the difference increasing with x. For the InSb/AlxIn1-xSb heterostructures, PR signals corresponding to Franz-Keldysh oscillations in the alloy barrier have been observed. Analysis of their period has allowed one to determine the intensity of the internal electric field in AlxIn1-xSb layers. This result enables evaluation of the surface Fermi level pinning, and elucidation of the effect of doping in such heterostructures.

Ge nanopillar solar cells epitaxially grown by metalorganic chemical vapor deposition

Radial junction solar cells with vertically aligned wire arrays have been widely studied to improve the power conversion efficiency. In this work, we report the first Ge nanopillar solar cell. Nanopillar arrays are selectively patterned on p-type Ge (100) substrates using nanosphere lithography and deep reactive ion etching processes. Nanoscale radial and planar junctions are realized by an n-type Ge emitter layer which is epitaxially grown by MOCVD using isobutylgermane. In situ epitaxial surface passivation is employed using an InGaP layer to avoid high surface recombination rates and Fermi level pinning. High quality n-ohmic contact is realized by protecting the top contact area during the nanopillar patterning. The short circuit current density and the power conversion efficiency of the Ge nanopillar solar cell are demonstrated to be improved up to 18 and 30%, respectively, compared to those of the Ge solar cell with a planar surface.