Surface Fermi Level Position Research Articles

Electronic structure of InSb (001), (110), and (111)B surfaces

The electronic structure of various (001), (110), and (111)B surfaces of n-type InSb was studied with scanning tunneling microscopy and spectroscopy. The InSb(111)B (3×1) surface reconstruction is determined to be a disordered (111)B (3×3) surface reconstruction. The surface Fermi-level of the In rich and the equal In:Sb (001), (110), and (111)B surface reconstructions was observed to be pinned near the valence band edge. This observed pinning is consistent with a charge neutrality level lying near the valence band maximum. Sb termination was observed to shift the surface Fermi-level position by up to 254±35 meV toward the conduction band on the InSb (001) surface and 60±35 meV toward the conduction band on the InSb(111)B surface. The surface Sb on the (001) can shift the surface from electron depletion to electron accumulation. We propose that the shift in the Fermi-level pinning is due to charge transfer from Sb clusters on the Sb terminated surfaces. Additionally, many subgap states were observed for the (111)B (3×1) surface, which are attributed to the disordered nature of this surface. This work demonstrates the tuning of the Fermi-level pinning position of InSb surfaces with Sb termination.

Probing electric properties of GaP nanowires with Kelvin probe force microscopy

Surface electronic properties of GaP nanowires were investigated using scanning probe force microscopy. I-V curves of individual free-standing NWs with different doping types were obtained. Surface Fermi level positions in the nanowires of different crystal phases and doping types were extracted using phase-modulated Kelvin probe force microscopy. The results indicate on weak Fermi level pinning in GaP nanowires. The difference between wurtzite and zinc blende GaP work function is observed.

Fermi level unpinning of metal/p-type 4H-SiC interface by combination of sacrificial oxidation and hydrogen plasma treatment

An ideal metal/p-type 4H-SiC interface with a “free-pinned” Fermi level has been achieved by the combination of sacrificial thermal oxidation (SO) and hydrogen plasma treatment (HPT) on the SiC surface. It is found that the Fermi level pinning could be attributed to the contaminants and defects of the p-type 4H-SiC surface. According to the X ray photoelectron spectroscopy and deep-level transient spectroscopy results, the oxygen and carbon contaminants decreased after SO. However, high-density carbon interstitial-related defects were generated close to the valance band during oxidation. With the subsequent HPT, the chemical residues and detrimental carbon-induced defects were eliminated by the reaction with hydrogen atoms. The p-type 4H-SiC surface was chemically and electrically well saturated with the surface Fermi level position close to the bulk position. An analytical model for the elimination of surface contaminants and defects was proposed to reveal the underlying mechanism of Fermi level depinning.

(Invited) Fundamental Differences Between the Traditional III-V Compounds and Nitride Semiconductors

In this talk I will address the fundamental question as to why the performance of emitters based on nitride semiconductors, grown heteroepitaxially on foreign substrates, is insensitive to high concentration of extended defects, which is not the case for traditional III-V compounds. I will discuss the following important differences between these two families of materials, which contribute to this finding: (a) the growth of Nitride semiconductors occurs at temperatures much lower than the half of their melting temperature, where the brittle to ductile transition occurs. In traditional III-V compounds the growth temperature is generally higher than ½ TM; (b) the chemical bonding in nitrides is strongly ionic while is mostly covalent in traditional III-V compounds. This leads to the bunching of the intrinsic surface states as well as the states associated with dangling bonds in edge dislocations near the band edges where they act as traps rather than recombination centers. Furthermore, the surface states have less effect on the surface Fermi level position; (c) the nitrides can exist in the wurtzite structure (equilibrium) and the cubic structure (metastable) and the enthalpy of formation of the two allotropic forms differs by only a few meV. Thus, conversion between the two phases occurs easily by creation of stacking faults along the close-packed (0001) and (111) planes, which due to the smaller energy gap of the cubic phase leads to strong band structure potential fluctuations in the wurtzite matrix; (d) additional band structure potential fluctuations exist in the nitride alloys due to various forms of partial alloy ordering or to compositional inhomogeneities introduced by phase separation or the growth mode, These potential fluctuations contribute to exciton localization and thus efficient radiative recombination.

(Invited) Rapid In-Situ Carbon and Oxygen Cleaning of In0.53Ga0.47As(001) and Si0.5Ge0.5(110) Surfaces via a H2 RF Downstream Plasma

Channel passivation of SiGe and InGaAs requires removal of dangling bonds and formation of a thermally stable interface. On InGaAs, one solution is the deposition by self-limiting CVD or ALD of a few monolayer of silicon, silicon oxide, or silicon nitride. To form an Si-OH terminated In0.53Ga0.47As(001) surface, two monolayers of Si were deposited at 350°C with self-limiting CVD using Si2Cl6 dosing following by dosing anhydrous HOOH(g). Scanning tunneling microcopy (STM) shows and ordered surface is form while scanning tunneling spectroscopy (STS) shows the surface Fermi level position moves towards midgap due to a surface dipole formation from –OH groups and oxygen bonding to the surface. TMA was dosed at 250°C onto the Si-OH terminated InGaAs(001), and XPS shows the emergence of the Al 2p and C 1s peaks indicative of TMA surface nucleation. The TMA shifts the InGaAs Fermi level back towards the conduction band, consistent with unpinning. MOSCAPs were fabricated to further demonstrate Fermi level unpinning using the Si-OH termination of InGaAs. While this same Si deposition process could be employed on SiGe, a simple annealing process can be employed to form a silicon terminated SiGe surface. A saturation dose of H2O2(g) at 25oC chemisorbs to on Si0.5Ge0.5(110) and Si0.47Ge0.53(001) surfaces leaving the Fermi level on the surface unpinned, and the surface is functionalized with mainly Si-OH, Ge-OH and Si-O-Ge bond. After a subsequent TMA dose at 25oC and annealing at 300oC, XPS and STM verify that a thermally stable and well-ordered monolayer of Al2O3 is formed on SiGe(110) and (001) surfaces with only Al-O-Si bonds and no detectable Ge-O-Si bonds. The H2O2(g) functionalization provides 3 times higher O sites on the surface and 3 times higher TMA nucleation density than H2O(g) at 25°C and 120°C. A variety of strongly bound passivating atoms can induce formation of a nearly pure Si interfacial layer between high-k dielectrics and SiGe. This annealing procedure was employed to form MOSCAP on SiGe(001) using Al2O3 and HfO2 gate oxides after either (NH4)2S or NH3 plasma passivation. For NH3 plasma passivation, angle-resolved XPS showed that the annealing allows formation of nearly Ge-free SiON layer. For higher Ge concentrations, the annealing process is unlikely to form a nearly Ge-free SiON layer so a novel plasma-free chemistry has been develop to deposit SiON using ALD with N2H4(g) as a precursor. Figure 1

Positron annihilation and cathodoluminescence study on inductively coupled plasma etched GaN

AbstractEtching damage induced by inductively coupled plasma (ICP) with a mixture of Cl and BCl gases and the effect of subsequent annealing in Nwere studied through positron annihilation and cathodoluminescence (CL) for Mg‐doped and Si‐doped GaN. The changes in the concentration of nonradiative defects and in the shape of band bending caused by ICP etching or subsequent annealing were studied by comparing the CL intensities and the diffusion lengths of implanted positrons, respectively. For Mg‐doped GaN, CL spectra revealed that etching inactivated the Mg acceptors at the near‐surface region. After annealing at 900 °C, the Mg acceptors were reactivated. For Si‐doped GaN, etching introduced neutral vacancy‐type defects, and after annealing at 900 °C, the etching‐induced defects almost disappeared. However, there were residual nonradiative defects on the top surface that were associated with a change in the surface's Fermi level position.

A combined capacitance-voltage and hard x-ray photoelectron spectroscopy characterisation of metal/Al2O3/In0.53Ga0.47As capacitor structures

Capacitance-Voltage (C-V) characterization and hard x-ray photoelectron spectroscopy (HAXPES) measurements have been used to study metal/Al2O3/In0.53Ga0.47As capacitor structures with high (Ni) and low (Al) work function metals. The HAXPES measurements observe a band bending occurring prior to metal deposition, which is attributed to a combination of fixed oxide charges and interface states of donor-type. Following metal deposition, the Fermi level positions at the Al2O3/In0.53Ga0.47As interface move towards the expected direction as observed from HAXPES measurements. The In0.53Ga0.47As surface Fermi level positions determined from both the C-V analysis at zero gate bias and HAXPES measurements are in reasonable agreement. The results are consistent with the presence of electrically active interface states at the Al2O3/In0.53Ga0.47As interface and suggest an interface state density increasing towards the In0.53Ga0.47As valence band edge.

Nanoscale physics and defect state chemistry at amorphous-Si/In0.53Ga0.47As interfaces

Interface and surface passivation of thin layers of III–V compound semiconductors is a key enabler of many technological applications spanning from nano-electronics to nano-photonics. The interaction between thin amorphous Si (a-Si) layers and clean, group-III-rich reconstructed In0.53Ga0.47As interfaces is studied by combining high-resolution synchrotron radiation x-ray photoelectron spectroscopy and time-dependent surface photovoltage (SPV) experiments. From 0.6 to 2.4 monolayers (ML) of a-Si are deposited on non-intentionally doped, p- and n-type In0.53Ga0.47As. For each deposition step, the observed surface and interface chemistry is directly correlated to the measured surface Fermi level position, band bending and SPV.Group-III-reconstructed In0.53Ga0.47As surfaces are observed to be intrinsically unstable against reaction with Si and two different instability regimes have been identified. First, for low deposition temperature, Si reacts strongly and intermixes with the In0.53Ga0.47As surface inducing In and Ga out-diffusion even at a sub-monolayer amount. For 2.4 ML of a-Si, a net positive interface charge of 1.24 × 1012 #/cm2 and a band of defects close to the conduction band are detected. For post-annealing at temperatures lower than 380 °C, the interface rearranges. At temperatures higher than 380 °C, out-diffusion of As in the a-Si is found to be the main instability driver.

Optical bandgap and near surface band bending in degenerate InN films grown by molecular beam epitaxy

We study the surface charge accumulation on InN thin films that strongly effects mobility of charge carriers. The films are formed by MBE in the temperature range (400–470 °C) yielding films with different morphology, crystallinity, and optical properties. The band-gap values determined by optical absorption and PL studies are found to depend on the Hall carrier concentration as per the Moss-Burstein relation. The magnitude of the near surface band bending is measured on InN thin films by knowing the surface and bulk Fermi level position with respect to the valence band maximum using X-ray photoelectron spectroscopy and optical absorption measurements, respectively. The studies show that the surface charge occurs due to the In adlayer on the film causing band-bending is up to 1 eV for low band gap single crystalline films and nearly a flat band for the highly degenerate polycrystalline films. The absence of the band bending is seen to occur when the bulk carrier concentration related to the crystalline quality of the InN films, tends to equate with the surface charge density.

Electrical characterization of p-Si/SiGe/Si(100) surface covered by 9,10 Phenanthrenequinone over layer

Electrical characterization of the p-Si/SiGe/Si(100) surface covered by organic molecule 9,10 Phenanthrenequinone (PQ), have been evaluated via analysis of hole gas transferred in the SiGe quantum well at low temperature. The enhancement in the density of two dimensional hole gas (2DHG) formed in the SiGe quantum well in the structure covered by PQ thin film is attributed to electrical passivation of Si surface states. Finally, the density of Si surface states (state/eV cm2), and surface Fermi level position, have been evaluated via Modified Mid-gap Pinning Model (MMPM) applied to experimental results.

The role of extended defects on the performance of optoelectronic devices in nitride semiconductors

AbstractIn this paper I am addressing the fundamental question as to why the performance of optoelectronic devices based on nitride semiconductors is insensitive to high concentration of extended defects, which is not the case for traditional III–V compounds. There are three important differences between these two families of semiconductors, which contribute to this finding: (a) the chemical bonds in nitrides are strongly ionic while are mostly covalent in III–V compounds. This leads to the bunching of the surface states as well as the states associated with dangling bonds in edge dislocations near the band edges, which prevents them from being non‐radiative recombination centers. Furthermore, the surface states have less effect on the surface Fermi level position; (b) the nitrides can exist in the wurtzite structure (equilibrium) and the cubic structure (metastable) and the enthalpy of formation of the two allotropic forms differs only by a few meV. Thus, conversions between the two phases occur easily by creation of stacking faults along the closed packed (0001) and (111) planes. As a result basal plane stacking faults are abundant in both compounds and alloys. This leads to strong band structure potential fluctuations; (c) additional band structure potential fluctuations exist in InGaN and AlGaN alloys, due to phase separation and alloy ordering. Both types of potential fluctuations contribute to exciton localization and thus efficient radiative recombination even at room temperature.

Minority carrier effects in nanoscale Schottky contacts

We report the current–voltage behavior for nanoscale point contacts to Si(111)obtained in ultrahigh vacuum using scanning tunneling microscopy. EpitaxialCoSi2 islands provide single-crystal contacts with well-defined size and shape.The zero bias conductance is found to be independent of the island size (102–104 nm2) and shape, but varies strongly with the surface Fermi level position. This behavior isexplained by the recombination–generation current from minority carriers at the freesurface, which may be orders of magnitude larger than the majority carrier thermionic ortunnel currents across the contact interface. This can give rise to large shifts ofthe apparent ideality factor and Schottky barrier height for the point contact.

Surface states and accumulation nanolayer induced by Ba and Cs adsorption on the n-GaN(0 0 0 1) surface

The Ba and Cs adsorption on the n-GaN(0 0 0 1) surface has been studied in situ by the threshold photoemission spectroscopy using s- and p-polarized light excitation. Two surface bands induced by Ba (Cs) adsorption are revealed in surface photoemission spectra below the Fermi level. The surface-Fermi level position is found to be changed from significantly below the conduction band minimum (CBM) at clean n-GaN surface to high above the CBM at Ba, Cs/n-GaN interfaces, with the transition from depletion to electron accumulation occurring at low coverages. Photoemission from the accumulation nanolayer is found to excite by visible light in the transparency region of GaN. Appearance of an oscillation structure in threshold photoemission spectra of the Ba, Cs/n-GaN interfaces with existing the accumulation layer is found to originate from Fabry–Perot interference in the transparency region of GaN.

Surface and bulk properties of sputter deposited undoped and Sb-doped SnO 2 thin films

Electronic surface and bulk properties of sputter deposited polycrystalline intrinsic and Sb-doped SnO 2 thin films have been investigated by a combination of in situ photoelectron spectroscopy, electrical four-point conductivity, and optical transmission measurements. The work function and ionization potential of the polycrystalline films increase with increasing oxygen content in the sputter gas by ∼ 1.4 and ∼ 1 eV, respectively. The changes are explained by the different surface terminations known for single crystalline SnO 2. Comparison of surface and bulk Fermi level positions indicates flat band situation for most cases but the presence of a depletion layer for Sb-doped films deposited under oxidizing conditions. Large changes of electrical conductivity depending on the oxygen content in the sputter gas were observed for undoped SnO 2 which can be understood in terms of different concentrations of oxygen vacancies. In contrast, literally no changes occur for SnO 2:Sb, which is attributed to the too high formation energy of compensating defects like oxygen interstitials or Sn vacancies.

The influence of conduction band plasmons on core-level photoemission spectra of InN

The core-level lineshape in photoemission spectra of InN is studied by high-resolution X-ray photoemission spectroscopy. The In 3d and N 1s core-levels are asymmetric, displaying a high binding energy tail which is attributed to inelastic losses to and/or screening by conduction band plasmons in the accumulation layer present at InN surfaces. The extent of the asymmetric tail decreases with decreasing surface Fermi level position associated with a lower density of electrons in the accumulation layer.

Formation of ultrathin SiNx∕Si interface control double layer on (001) and (111) GaAs surfaces for ex situ deposition of high-k dielectrics

In order to realize pinning-free high-k dielectric metal-insulator-semiconductor (MIS) gate stack on (001) and (111)B oriented GaAs surfaces using the Si interface control layer (Si ICL) concept, formation of a SiNx∕Si ICL double layer was investigated as a chemically stable structure on (001) and (111)B surfaces which allows ex situ deposition of HfO2 high-k dielectric films without losing the benefit of Si ICL. First, Si ICLs grown by molecular beam epitaxy (MBE) on (001) and (111)B GaAs surfaces with various initial surface reconstructions were investigated in detail by reflection high energy electron diffraction and x-ray photoelectron spectroscopy (XPS) investigations at each step of the interface formation. Large shifts of the surface Fermi level position toward unpinning were observed after Si ICL growth on appropriately formed Ga-stabilized surfaces. It was found that Si layers grow epitaxially with Si–Ga bonds at the Si∕GaAs interface and Si–As termination on top, suggesting surfactant roles played by As atoms. Then, an ultrathin SiNx buffer film was formed on the Si ICL by its in situ partial nitridation in the MBE chamber. An XPS analysis of the resultant SiNx∕Si ICL double layer formed on (001) and (111)B surface indicated that the structure is chemically stable against air exposure on both surfaces in the sense that it prevents the host GaAs surface from subcutaneous oxidation, although SiNx film itself partially turns into SiOxNy. Finally, high-k MIS capacitors were formed by ex situ deposition of HfO2 on the SiNx∕Si ICL/GaAs structure after transferring the sample through air. The capacitance-voltage (C-V) analysis indicated that the MIS interface is completely pinning-free with a minimum interface state density in the range of low 1011cm−2eV−1.

In‐situ X‐ray photoelectron spectroscopy characterization of Si interlayer based surface passivation process for AlGaAs/GaAs quantum wire transistors

AbstractDetailed properties of the Si interface control layer (Si ICL)‐based surface passivation structure are characterized by in‐situ X‐ray photoelectron spectroscopy (XPS) in an ultra‐high vacuum multi‐chamber system. Si ICLs were grown by molecular beam epitaxy (MBE) on GaAs and AlGaAs(001) and (111)B surfaces, and were partially converted to SiNx by nitrogen radical beam. Freshly MBE‐grown clean GaAs and AlGaAs surfaces showed strong Fermi level pinning. Large shifts of the surface Fermi level position corresponding to reduction of pinning took place after Si ICL growth, particularly on (111)B surface (around 500 meV). However, subsequent surface nitridation increased pinning again. Then, a significant reduction of pinning was obtained by changing SiNx to silicon oxynitride by intentional air‐exposure and subsequent annealing. This has led to realization of a stable passivation structure with an ultrathin oxynitride/Si ICL structure which prevented subcutaneous oxidation during further device processing under air‐exposure. The Si‐ICL‐based passivation process was applied to surface passivation of quantum wire (QWR) transistors where anomalously large side‐gating phenomenon was completely eliminated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of high spatial resolution scanning work function spectroscopy to semiconductor surfaces and interfaces

Work function can be measured from the onset energy of secondary electron (SE) emission spectra in a scanning Auger microscope. In this article, the authors demonstrate the potential of such high spatial resolution work function spectroscopy in probing the Fermi-level position at semiconductor surfaces and the band bending at semiconductor interfaces. Using in situ cleaved cross-sectional GaAs and SiC p-n junctions, they show that the SE onset energy is very sensitive to the surface Fermi-level position and can also be used to measure electron affinity. By measuring the widths of the transition regions, the doping densities and built-in potential can be quantitatively obtained at a spatial resolution of ∼10nm. This technique can also be applied to a wide range of semiconductor heterojunctions. The influences of air cleaving, Ar ion sputtering, contamination, and primary electron beam are studied. They also discuss the advantages, limitations, and possible future improvements of this technique.

Transition from electron accumulation to depletion at InGaN surfaces

The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0⩽x⩽1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x=0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.

Computer analysis of an influence of oxygen vacancies on the electronic properties of the SnO2 surface and near‐surface region

AbstractAn influence of the surface stoichiometric defects on the electronic and electrical properties of SnO2 layers has been studied by means of computer simulation. The surface potential and in‐depth potential profiles in the accumulation region have been rigorously computed at various temperatures (from 400 to 900 K). The behaviour of both the surface and bulk Fermi level position versus temperature has been analysed. Furthermore, in‐depth profiles of carrier concentration and the surface potential as well as the SnO2 layer conductance for different surface donor density of state have been calculated. It has been found that the surface reduction and bulk donor concentration (in the range from 3 × 1014 to 3 × 1018 cm–3) are strongly correlated with the development of near‐surface accumulation region and thus modification of the layer electrical parameters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)