Surface Fermi Level Pinning Research Articles

Electrical and Surface Properties of InAs/InSb Nanowires Cleaned by Atomic Hydrogen.

We present a study of InAs/InSb heterostructured nanowires by X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM), and in-vacuum electrical measurements. Starting with pristine nanowires covered only by the native oxide formed through exposure to ambient air, we investigate the effect of atomic hydrogen cleaning on the surface chemistry and electrical performance. We find that clean and unreconstructed nanowire surfaces can be obtained simultaneously for both InSb and InAs by heating to 380 ± 20 °C under an H2 pressure 2 × 10(-6) mbar. Through electrical measurement of individual nanowires, we observe an increase in conductivity of 2 orders of magnitude by atomic hydrogen cleaning, which we relate through theoretical simulation to the contact-nanowire junction and nanowire surface Fermi level pinning. Our study demonstrates the significant potential of atomic hydrogen cleaning regarding device fabrication when high quality contacts or complete control of the surface structure is required. As hydrogen cleaning has recently been shown to work for many different types of III-V nanowires, our findings should be applicable far beyond the present materials system.

Background and interface electron populations in InAs0.58Sb0.42

The unintentional background electron population and associated interface and surface conductivity in a heterostructure of InAs0.58Sb0.42 with a bandgap of 0.144 eV and AlInSb was studied with multi-carrier Hall-effect analysis. A free electron bulk concentration at 77 K was found with a density of 2.4 × 1015 cm−3and mobility of 140 000 cm2 V−1 s−1. A surface electron accumulation layer was observed with a density of 5.5 × 1011 cm−2 and mobility of 4500 cm2 V−1 s−1 that is consistent with predictions of surface Fermi level pinning. Another accumulation layer was identified at the interface with the AlInSb of 4 × 1011 cm−2 with a mobility of 37 000 cm2 V−1 s−1. The origin of the defects and the implications for device structures are discussed.

Understanding the role of Si doping on surface charge and optical properties: Photoluminescence study of intrinsic and Si-doped InN nanowires

In the present work, the photoluminescence (PL) characteristics of intrinsic and Si-doped InN nanowires are studied in detail. For intrinsic InN nanowires, the emission is due to band-to-band carrier recombination with the peak energy at $\ensuremath{\sim}$0.64 eV (at 300 K) and may involve free-exciton emission at low temperatures. The PL spectra exhibit a strong dependence on optical excitation power and temperature, which can be well characterized by the presence of very low residual electron density and the absence or a negligible level of surface electron accumulation. In comparison, the emission of Si-doped InN nanowires is characterized by the presence of two distinct peaks located at $\ensuremath{\sim}$0.65 and $\ensuremath{\sim}$0.73--0.75 eV (at 300 K). Detailed studies further suggest that these low-energy and high-energy peaks can be ascribed to band-to-band carrier recombination in the relatively low-doped nanowire bulk region and Mahan exciton emission in the high-doped nanowire near-surface region, respectively; this is a natural consequence of dopant surface segregation. The resulting surface electron accumulation and Fermi-level pinning, due to the enhanced surface doping, are confirmed by angle-resolved x-ray photoelectron spectroscopy measurements on Si-doped InN nanowires, which is in direct contrast to the absence or a negligible level of surface electron accumulation in intrinsic InN nanowires. This work elucidates the role of charge-carrier concentration and distribution on the optical properties of InN nanowires.

Tuning the Surface Charge Properties of Epitaxial InN Nanowires

We have investigated the correlated surface electronic and optical properties of [0001]-oriented epitaxial InN nanowires grown directly on silicon. By dramatically improving the epitaxial growth process, we have achieved, for the first time, intrinsic InN both within the bulk and at nonpolar InN surfaces. The near-surface Fermi-level was measured to be ∼0.55 eV above the valence band maximum for undoped InN nanowires, suggesting the absence of surface electron accumulation and Fermi-level pinning. This result is in direct contrast to the problematic degenerate two-dimensional electron gas universally observed on grown surfaces of n-type degenerate InN. We have further demonstrated that the surface charge properties of InN nanowires, including the formation of two-dimensional electron gas and the optical emission characteristics can be precisely tuned through controlled n-type doping. At relatively high doping levels in this study, the near-surface Fermi-level was found to be pinned at ∼0.95-1.3 eV above the valence band maximum. Through these trends, well captured by the effective mass and ab initio materials modeling, we have unambiguously identified the definitive role of surface doping in tuning the surface charge properties of InN.

Surface Fermi level pinning and carrier transport of indium-tin-oxide Ohmic contact to p-type GaN

The carrier transport mechanism of indium-tin-oxide (ITO) Ohmic contact to p-type GaN was investigated. The thermally annealed ITO contact to p-GaN produced a low specific contact resistance of 8.1×10−3Ωcm2, due to the low effective barrier height of 0.11eV, for which the carriers must overcome to flow from p-GaN to the metals via hopping conduction through deep-level defect states. The surface Fermi level of a highly Mg-doped p-GaN surface was also found to pin near the midgap states, i.e., ∼0.6 of bandgap above valence band, which are caused by deep-level defect states.

Investigation of polarization‐induced electric field in ultrathin InAlN layers on GaN by X‐ray photoelectron spectroscopy

AbstractThe polarization‐induced electric field in ultrathin InxAl1‐xN (0.17 ≤ x ≤ 0.30) layers on GaN was investigated by using X‐ray photoelectron spectroscopy (XPS). The core‐level energy position, ECL, and the full width at half maximum (FWHM) of the Al2p, In4d, and In3d spectra from 2.5‐nm‐thick InAlN layers increased with the increase in the photoelectron exit angle (elevation angle). These increases were well reproduced with numerical calculations assuming polarization‐induced internal fields combined with surface Fermi level pinning. The magnitudes of the internal field decreased as the In molar fraction increased. The Ga3d spectra from the host GaN layers markedly shifted by 530 meV depending on the molar fraction of InAlN layers, which was independent of the exit angle. This indicated that the Fermi level was unpinned at the interfaces, or GaN surfaces, and shifted due to the potential drop in the InAlN layers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong surface Fermi level pinning and surface state density in GaAs0.65Sb0.35 surface intrinsic-n+ structure

Room-temperature photoreflectance is employed to investigate the Fermi level pinning and surface state density of a GaAs0.65Sb0.35 surface intrinsic-n+ (SIN+) structure. Based on the thermionic emission theory and current-transport theory, the surface Fermi level VF and surface state density are determined experimentally from the dependence of the surface barrier height on the pump beam intensity. The surface state density Ds is estimated as approximately 1.91×1013 cm−2, and the Fermi level is located 0.63 eV below the conduction band edge at the surface. By sequential etching of the intrinsic layer, the Fermi level pinning in GaAs0.65Sb0.35 SIN+ structure is further demonstrated.

Photoreflectance spectroscopy for the qualification of GaAsSb alloys introduced in ultrafast heterojunction bipolar transistors on InP

AbstractGaAsSb is an important III–V semiconductor ternary alloy which is currently used as the thin base layer in high speed InP Heterojunction Bipolar Transistors (HBT). Up to now, very little is known about physical properties of GaAsSb alloys, such as surface Fermi level pinning and Schottky barrier heights with different contact metals. However such parameters impact directly the cut‐off frequency of HBT devices. Photoreflectance spectroscopy is applied to specific samples differing by their Sb concentration. The exploitation of Franz–Keldysh Oscillations (FKO) spectra allows determining the Fermi level pinning at the free surface of GaAsSb as a function of the alloy composition. Micro‐electroreflectance spectroscopy is also applied to Schottky diodes performed on the same structures, with Ti deposited at the GaAsSb surface. The Schottky barrier height between GaAsSb and Ti is derived from the FKO system recorded in the micro‐electroreflectance spectra. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-dimensional junction identification in multicrystalline silicon solar cells by scanning Kelvin probe force microscopy

We report on a two-dimensional investigation of the p-n junction in multicrystalline silicon solar cells using scanning Kelvin probe force microscopy (SKPFM). The junction location and depth were identified by SKPFM potential measurement and subsequent data analysis, where a procedure taking bias-voltage-induced changes in the potential and electric field was developed to avoid the effects of surface Fermi level pinning. Device simulation supported the junction identification procedure and showed a possible deviation of ∼40 nm in the junction identification. The two-dimensional electric-field images show that the shape of the junction follows the surface topography of the device, or, in other words, the junction depth is identical over the device.

Direct probe of the built-in electric field of Mg-doped a-plane wurtzite InN surfaces with time-resolved electric-field-induced second harmonic generation

The exceptionally large electron affinity of InN causes the pinning of surface Fermi level well above the conduction band minimum. This unique electronic property leads to the electron accumulation at InN surfaces and a large built-in electric field in the topmost few nanometers of InN surfaces. In this letter, we demonstrate that this surface electric field can be unambiguously determined and monitored in a-plane wurtzite InN surface via time-resolved electric-field-induced second harmonic generation. This finding makes it possible to directly probe and characterize the surface electronic properties of Mg-doped InN with an all-optical technique in ambient environment.

Built-in electric field enhancement/retardation on intermixing

The built-in electric field may impose a drift on charged point defects and may thus enhance or retard the intermixing during annealing. Electric field is built-in near the surface due to the pinning of surface Fermi level after argon plasma treatment on InP surfaces of InP∕InGaAs quantum well samples. Subsequent annealing leads to different intermixing results due to the different field directions on InP cap layers in different doping types. Experiments also showed different influences of the built-in field on the two sublattices largely due to different charge numbers of point defects on the respective sublattices.

Two‐dimensional simulation of type‐II InP/GaAsSb/InP double heterojunction bipolar transistors

Two-dimensional (2D) numerical simulations of self-aligned sub-micron InP/GaAsSb double heterojunction bipolar transistors (DHBTs) were performed to investigate the effects of base band gap narrowing and surface recombination. Base energy band gaps of 0.72 eV and 0.67 eV for GaAs0.51Sb0.49 bases with doping levels of 5×1018 and 5×1019 cm–3 were extracted from the comparison between the measurement data and simulation results. We took into account the surface Fermi level pinning and introduced a surface state model for the emitter side wall and extrinsic base surface. To the best of our knowledge, a good match between measured and simulated InP/GaAsSb DHBTs characteristics, from low to high current densities has not been achieved prior to the present work. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fermi level pinning on Si0.83Ge0.17 surface by inductively coupled plasma treatment

Effects of inductively coupled plasma (ICP) treatment on Fermi level pinning on the surface of Si0.83Ge0.17 was studied by current–voltage and x-ray photoemission spectroscopy measurements. ICP treatment induced the growth of silicon oxide, suggesting that Si vacancies are generated under the oxide. From linear fitting of Schottky barrier heights with metal work functions, it was found that surface state density increased from 6.60×1012to1.13×1013∕cm2eV by the ICP treatment, leading to the pinning of surface Fermi level about EC∼0.53eV. From this, it is suggested that the Si vacancies are the main surface states in pinning Fermi level on the ICP-treated surface.

Electrical characterization of SiO2/n-GaN metal–insulator–semiconductor diodes

We have investigated the interface properties of SiO2/n-GaN metal–insulator–semiconductor (MIS) diodes by using capacitance–voltage (C–V) and capacitance transient techniques. The MIS diodes were fabricated by SiO2 sputtering onto an n-GaN epitaxial layer grown by atmospheric pressure metalorganic chemical-vapor deposition on a sapphire substrate. C–V characteristics show a total interface trap density of ∼2.2×1012 eV−1 cm−2 and display capacitance saturation in deep depletion (>15 V). The capacitance in deep depletion is found to significantly increase by incident white light. A capacitance transient is also seen after applying reverse voltages, reflecting thermal emission of carriers from the SiO2/GaN interface. Deep-level transient spectroscopic measurements reveal a dominant interface trap with an activation energy of ∼0.77 eV from the conduction band, corresponding to the capacitance transient. Therefore, this interface trap is considered to induce surface Fermi-level pinning, which results in the capacitance saturation in the measured C–V characteristics.

Effect of InAs quantum dots on the Fermi level pinning of undoped-n+ type GaAs surface studied by contactless electroreflectance

Self-assembled InAs quantum dots (QDs) have been fabricated by depositing 1.6, 1.8, 2.0 and 2.5 monolayer (ML) InAs on surfaces of the undoped-n+ (UN+) type GaAs structure. Room temperature contactless electroreflectance (CER) was employed to study the built-in electric field and the surface Fermi level pinning of these QD-covered UN+ GaAs samples. The CER results show that 1.6 ML InAs QDs on GaAs do not modify the Fermi level, whereas for samples with more than 1.6 ML InAs coverage, the surface Fermi level is moved to the valence band maximum of GaAs by about 70 meV (which is independent of the InAs deposition thickness) compared to bare GaAs. It is concluded that the modification of InAs coverage on the Fermi level on the GaAs surface is due to the QDs, rather than to the wetting layer.

Measurement of Built-in Electrical Potential in Cu(In, Ga)Se2 Solar Cells

AbstractThe built-in electrical potential on cross sections of Cu(In, Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. In the conditions of open and short circuits, no significant potential variation on the p-n junction was probed due to the surface Fermi-level pinning. With an external reverse-bias voltage applied to the device, we were able to probe the potential on the junction; the potential profiles demonstrate that the p-n junction is a buried homojunction, located 30-80 nm from the CIGS/CdS interface in the CIGS film. The potential measurement over the CdS and ZnO layers, which is consistent with the band diagram calculations, indicates that the CdS and ZnO layers are inactive for the collection of photoexcited carriers.

InAs-based bipolar transistors grown by molecular beam epitaxy

Large mobilities and electron saturation velocity make InAs a promising material for high speed devices. Investigations into materials characteristics of doped InAs show nonideal behavior with standard molecular beam epitaxy dopants, silicon, and beryllium. Critical thicknesses for cracking of AlxIn1−xAs on InAs were empirically determined as a function of x. Mesa pn junctions in InAs show no effects of surface Fermi level pinning and exhibit good rectification with low reverse leakage. Bipolar junction transistor and heterojunction bipolar transistor devices are presented, along with their dc electrical characteristics. Common emitter current gains of 100 have been achieved in these bipolar devices.

Chemistry and electrical properties of surfaces of GaN and GaN/AlGaN heterostructures

Chemical and electrical properties of the surfaces of GaN and GaN/AlGaN heterostructures were systematically investigated by x-ray photoelectron spectroscopy (XPS), capacitance–voltage, and current–voltage measurements. From in situ XPS study, relatively smaller band bending of 0.6 eV was seen at the GaN (2×2) surface grown by radio frequency-assisted molecular beam epitaxy on the metalorganic vapor phase epitaxy GaN template. After exposing the sample surface to air, strong band bending took place at the surface. The surface treatment in NH4OH solution and N2 plasma was found to reduce the surface Fermi level pinning. Surface passivation process of GaN utilizing SiNx film by electron-cyclotron-resonance assisted plasma chemical vapor deposition (ECR–CVD) achieved low interface state density, 2×1011cm−2 eV−1. No pronounced stress remained at the SiNx/GaN interface, which was confirmed by Raman spectroscopy. The present NH4OH/ECR–N2 plasma treatment was also found to be effective in realizing well-ordered and nearly oxide-free surface of a GaN/AlGaN heterostructure. The subsequent passivation process using the ECR–CVD SiNx film enhanced the drain current in the gateless GaN/AlGaN high electron mobility transistor. A surface passivation process utilizing an ultrathin Al-oxide layer reduced leakage current and improved gate controllability of two-dimensional electron gas in the Schottky gate contact fabricated on the GaN/AlGaN heterostructures.

Electrochemical capacitance voltage profiling of the narrow band gap semiconductor InAs

InAs is a narrow band gap compound semiconductor with potential applications in infra-red detectors and high speed transistors. In order to facilitate device design using this material, it is essential that carrier concentration profiles be accurately known. Capacitance-voltage (CV) profiling is often employed for this purpose. Due to surface Fermi level pinning, it is difficult to form metal Schottky contacts to InAs layers, making conventional CV profiling difficult. Electro-chemical CV (ECV) measurements have been successfully performed on InAs epilayers grown by molecular beam epitaxy on GaP. A solution of 0.2 M EDTA with 0.2M NaOH and 10–20% by volume of ethylenediamine acts both as an etchant and as a Schottky contact to InAs. The profiles obtained for undoped InAs layers were compared to Hall effect data, and showed good agreement. ECV profiling of thick layers with p- and n-type doped regions is also demonstrated.

Reflection anisotropy spectroscopy study of the near-surface electric fields in undoped,n- andp-doped low-temperature grown GaAs (001)

We have evaluated the effective surface electric fields, effective depletion widths, and the sign (n-type/upward band bending) of the near-surface electric fields in undoped as well as $n\ensuremath{-}(5\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ and $p\ensuremath{-}(5\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ doped low-temperature grown (LTG) GaAs (001) using the optical method of reflection-anisotropy (RA) spectroscopy in the vicinity of the spin-orbit split ${E}_{1},$ ${E}_{1}+{\ensuremath{\Delta}}_{1}$ optical features. Even for the highest p doping the surface band bending is still n type, indicating the strong influence of the large donor trap densities. Our results are in good agreement with a self-consistent Poisson's calculation of the near-surface electric fields assuming deep donor $(1\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ and acceptor $(1\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ trap densities and midgap surface Fermi-level pinning. This experiment provides valuable information about the defect density and position in LTG:GaAs. Our results will be compared to the recent RA spectroscopy experiment and interpretation of Chen et al. [Phys. Rev. B 55, R7379 (1997)].