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Applied Surface Science (v.256, #19)
Surface passivation of III–V semiconductors for future CMOS devices—Past research, present status and key issues for future by H. Hasegawa; M. Akazawa; A. Domanowska; B. Adamowicz (pp. 5698-5707).
Currently, III–V metal–insulator–semiconductor field effect transistors (MISFETs) are considered to be promising device candidates for the so-called “More Moore Approach” to continue scaling CMOS transistors on the silicon platform. Strong interest also exists in III–V nanowire MISFETs as a possible candidate for a “Beyond CMOS”-type device. III–V sensors using insulator–semiconductor interfaces are good candidates for “More Moore”-type of devices on the Si platform. The success of these new approaches for future electronics depends on the availability of a surface passivation technology which can realize pinning-free, high-quality interfaces between insulator and III–V semiconductors.This paper reviews the past history, present status and key issues of the research on the surface passivation technology for III–V semiconductors. First, a brief survey of previous research on surface passivation and MISFETs is made, and Fermi level pinning at insulator–semiconductor interface is discussed. Then, a brief review is made on recent approaches of interface control for high-k III–V MIS structures. Subsequently, as an actual example of interface control, latest results on the authors’ surface passivation approach using a silicon interface control layer (Si ICL) are discussed. Finally, a photoluminescence (PL) method to characterize the interface quality is presented as an efficient contactless and non-destructive method which can be applied at each step of interface formation process without fabrication of MIS capacitors and MISFETs.
Keywords: PACS; 71.20.Nr; 71.55.Eq; 73.21.Hb; 73.40.Gk; 73.40.KpIII–V semiconductor; Surface passivation; MISFET; High-k dielectric; Photoluminescence; GaAs
High-k Al2O3 MOS structures with Si interface control layer formed on air-exposed GaAs and InGaAs wafers by M. Akazawa; H. Hasegawa (pp. 5708-5713).
This paper attempts to realize unpinned high-k insulator–semiconductor interfaces on air-exposed GaAs and In0.53Ga0.47As by using the Si interface control layer (Si ICL). Al2O3 was deposited by ex situ atomic layer deposition (ALD) as the high-k insulator. By applying an optimal chemical treatment using HF acid combined with subsequent thermal cleaning below 500°C in UHV, interface bonding configurations similar to those by in situ UHV process were achieved both for GaAs and InGaAs after MBE growth of the Si ICL with no trace of residual native oxide components. As compared with the MIS structures without Si ICL, insertion of Si ICL improved the electrical interface quality, a great deal both for GaAs and InGaAs, reducing frequency dispersion of capacitance, hysteresis effects and interface state density ( Dit). A minimum value of Dit of 2×1011eV−1cm−2 was achieved both for GaAs and InGaAs. However, the range of bias-induced surface potential excursion within the band gap was different, making formation of electron layer by surface inversion possible in InGaAs, but not possible in GaAs. The difference was explained by the disorder induced gap state (DIGS) model.
Keywords: PACS; 73.40.Ty; 73.20.At; 73.61.EyAl; 2; O; 3; MIS; Si ICL; GaAs; InGaAs; Hydrofluoric acid
Molecular self-assembly and passivation of GaAs (001) with alkanethiol monolayers: A view towards bio-functionalization by J.J. Dubowski; O. Voznyy; G.M. Marshall (pp. 5714-5721).
Properties of as prepared or nanoengineered III–V semiconductor surfaces provide attractive means for photonic detection of different adsorbants from surrounding gaseous or liquid environments. To be practical, this approach requires that the surface is made selectively sensitive (functionalized) to targeted species. In addition, such surface has also to stay stable over extended period of time to make it available for rapid testing. Numerous reports demonstrate attractive properties of GaAs for sensing applications. One of the most fundamental issues relevant to these applications concerns the ability to functionalize chemically, or biologically, the surface of GaAs. The most studied method of GaAs surface functionalization is based on formation of self-assembled monolayers (SAMs) of various n-alkanethiols, HS-(CH2) n-T (T=CH3, COOH, NH2, etc.). In spite of multi-year research concerning this issue, it has only been recently that a comprehensive picture of SAMs formation on GaAs and an understanding of the natural limitation of the SAM–GaAs interface in some bio-chemical sensing architectures has begun to emerge.
Keywords: Self-assembled monolayers; n-Alkanethiols; passivation of GaAs; Bio-chemical sensing architectures
Composition and morphology of fluorinated anodic oxides on InAs (111)A surface by N.A. Valisheva; O.E. Tereshchenko; I.P. Prosvirin; T.A. Levtsova; E.E. Rodjakina; A.V. Kovchavcev (pp. 5722-5726).
The composition and morphology of fluorinated anodic oxide (FAO) films grown on InAs (111)A in alkaline aqueous (pH 11.5) and acid waterless (pH 1.5) electrolytes are studied by means of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) in order to reveal the passivation mechanism of fluorine on the FAO/InAs(111)A interface. The formation of the highest oxidation form of As+5 and passivation of defects in the FAO layers during the fluorination process explain the reduction of the density of surface states and unpinning of the Fermi level on the fluorinated AO/InAs(111)A interface.
Keywords: PACS; 68.47.Fg; 73.20.−r; 81.65.Rv; 68.55.Ng, 79.60.−iInAs; Interface; Anodic oxide; Phase composition; Surface states; Passivation
Surface and interface issues in wide band gap semiconductor electronics by F. Roccaforte; F. Giannazzo; F. Iucolano; J. Eriksson; M.H. Weng; V. Raineri (pp. 5727-5735).
Wide band gap (WGB) materials are the most promising semiconductors for future electronic devices, and are candidates to replace the conventional materials (Si, GaAs, …) that are approaching their physical limits. Among WBG materials, silicon carbide (SiC) and gallium nitride (GaN) have achieved the largest advancements with respect to their material quality and device processing. Clearly, the devices performances depend on several surface and interface properties, which in turn are often crucially determined by the quality of the available material, as well as by the device processing maturity. In this paper, some surface and interface issues related to SiC and GaN devices processing are reviewed. First, the control of metal/SiC barrier uniformity and surface preparation will be discussed with respect to the performance of Schottky-based devices. Moreover, the impact of high-temperature annealing required for high-voltage Schottky diodes and MOSFETs fabrication, on the surface morphology and device performances will also be briefly presented. In the second part, it will be shown that for GaN the material quality is still the main concern, since dislocations have a severe influence on the current transport and barrier homogeneity of metal/GaN interfaces. Other practical implications of thermal annealing and surface passivation during GaN-based devices fabrication will also be addressed.
Keywords: PACS; 73.40.−c; 73.40.Kp; 73.61.EySiC; GaN; Surfaces; Interfaces; Schottky; HEMT
Surface defects in semiconductor lasers studied with cross-sectional scanning tunneling microscopy by R.J. Cobley; K.S. Teng; M.R. Brown; P. Rees; S.P. Wilks (pp. 5736-5739).
Cross-sectional scanning tunneling microscopy is used to study defects on the surface of semiconductor laser devices. Step defects across the active region caused by the cleave process are identified. Curved blocking layers used in buried heterostructure lasers are shown to induce strain in the layers above them. Devices are also studied whilst powered to look at how the devices change during operation, with a numerical model that confirms the observed behavior. Whilst powered, low-doped blocking layers adjacent to the active region are found to change in real time, with dopant diffusion and the formation of surface states. A tunneling model which allows the inclusion of surface states and tip-induced band bending is applied to analyze the effects on the tunneling current, confirming that the doping concentration is reducing and defect surface states are being formed.
Keywords: PACS; 07.79.Cz; 68.37.Ef; 68.37.Yz; 73.20.FzScanning tunneling microscopy (STM); Semiconductor laser; Passivation; AlGaAs; InP
GaN/LiNbO3 (0001) interface formation calculated from first-principles by Simone Sanna; Wolf Gero Schmidt (pp. 5740-5743).
The stable adsorption sites for both Ga and N ions on the ideal and on the reconstructed LiNbO3 (0001) surface are determined by means of first-principle total energy calculations. A single N layer is found to be more strongly bound to the substrate than a single Ga layer. The adsorption of a GaN monolayer on the polar substrate within different orientations is then modeled. On the basis of our results, we propose a microscopic model for the GaN/LiNbO3 interface. The GaN and LiNbO3 (0001) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [101¯0]GaN‖ [112¯0]LiNbO3. In this way the (0001) plane lattice mismatch between GaN and LiNbO3 is minimal and equal to 6.9% of the GaN lattice constant. The adsorbed GaN and the underlying LiNbO3 substrate have parallel c-axes.
Keywords: PACS; 68.35.bg; 68.43.Bc; 81.10.AjGaN; LiNbO; 3; Growth; Theory; Interfaces; Ferroelectrics
Nitric acid oxidation of Si (NAOS) method for low temperature fabrication of SiO2/Si and SiO2/SiC structures by H. Kobayashi; K. Imamura; W.-B. Kim; S.-S. Im; Asuha (pp. 5744-5756).
We have developed low temperature formation methods of SiO2/Si and SiO2/SiC structures by use of nitric acid, i.e., nitric acid oxidation of Si (or SiC) (NAOS) methods. By use of the azeotropic NAOS method (i.e., immersion in 68wt% HNO3 aqueous solutions at 120°C), an ultrathin (i.e., 1.3–1.4nm) SiO2 layer with a low leakage current density can be formed on Si. The leakage current density can be further decreased by post-metallization anneal (PMA) at 200°C in hydrogen atmosphere, and consequently the leakage current density at the gate bias voltage of 1V becomes 1/4–1/20 of that of an ultrathin (i.e., 1.5nm) thermal oxide layer usually formed at temperatures between 800 and 900°C. The low leakage current density is attributable to (i) low interface state density, (ii) low SiO2 gap-state density, and (iii) high band discontinuity energy at the SiO2/Si interface arising from the high atomic density of the NAOS SiO2 layer.For the formation of a relatively thick (i.e., ≥10nm) SiO2 layer, we have developed the two-step NAOS method in which the initial and subsequent oxidation is performed by immersion in ∼40wt% HNO3 and azeotropic HNO3 aqueous solutions, respectively. In this case, the SiO2 formation rate does not depend on the Si surface orientation. Using the two-step NAOS method, a uniform thickness SiO2 layer can be formed even on the rough surface of poly-crystalline Si thin films. The atomic density of the two-step NAOS SiO2 layer is slightly higher than that for thermal oxide. When PMA at 250°C in hydrogen is performed on the two-step NAOS SiO2 layer, the current–voltage and capacitance–voltage characteristics become as good as those for thermal oxide formed at 900°C.A relatively thick (i.e., ≥10nm) SiO2 layer can also be formed on SiC at 120°C by use of the two-step NAOS method. With no treatment before the NAOS method, the leakage current density is very high, but by heat treatment at 400°C in pure hydrogen, the leakage current density is decreased by approximately seven orders of magnitude. The hydrogen treatment greatly smoothens the SiC surface, and the subsequent NAOS method results in the formation of an atomically smooth SiO2/SiC interface and a uniform thickness SiO2.
Keywords: PACS; 81.6.Cp; 73.40.Qv; 73.61.Ng; 85.40.HpNitric acid oxidation; Low temperature oxidation; Si; Ultrathin oxide; Metal-oxide-semiconductor (MOS); Gate oxide
On ultra-thin oxide/Si and very-thin oxide/Si structures prepared by wet chemical process by E. Pincik; H. Kobayashi; J. Rusnak; W.B. Kim; R. Brunner; L. Malinovsky; T. Matsumoto; K. Imamura; M. Jergel; M. Takahashi; Y. Higashi; M. Kucera; M. Mikula (pp. 5757-5764).
The properties of ultra-thin oxide/Si and very-thin oxide/Si structures prepared by wet chemical oxidation in nitric acid aqueous solutions (NAOS) and passivated in HCN aqueous solutions were investigated by electrical, optical and structural methods. n- and p-doped (100) crystalline Si substrates were used. There were identified more types of interface defect states in dependence on both post-oxidation treatment and passivation procedure. On samples prepared on n-type Si, continuous spectrum of defect states of 0.05–0.2eV range and discrete defect traps, ∼ ECB−0.26eV and ∼ ECB−0.39eV, were found. All mentioned defects are related with various types of Si dangling bonds and/or with SiO x precipitates. Post-metallization annealing of investigated MOS structures reduced the interface defect density and suppressed the leakage currents. It did not change spectral profile of interface defect states in the Si band gap. In addition, there are presented following two optical phenomena: relation between amplitude of photoluminescence signal of NAOS samples and parameters of chemical oxidation process and quantum confinement effect observed on samples containing Si grains of size less as ∼2nm.
Keywords: PACS; 81.65; 82.30; 78.66; 81.60Silicon; Very-thin oxide; Passivation; Ellipsometry; Deep levels
Effect of deuterium on passivation of Si surfaces by J. Mizsei; A.E. Pap; K. Gillemot; G. Battistig (pp. 5765-5770).
A properly passivated silicon surface is chemically stable, and all interface properties are constant. The freshly etched Si surface is full of dangling bonds resulting in high surface activity and instability. Hydrogen treatment is a proper procedure to decrease the number of dangling bonds. We demonstrated that deuterium adsorbs on Si surface at room temperature much stronger than hydrogen. Moreover, in case of deuterium-passivated wafers the vacuum storage can be omitted without risking the non-controlled native oxidation of silicon for up to 5h or more. It could be a suitable and more robust surface cleaning and passivation process for the industry, but heavy water is expensive. As a cheaper procedure, we showed that 1min vapor phase treatment at 65°C of heavy water (D2O)+50% HF (e.g. 20:1) mixture was enough to remove the native oxide and to passivate the Si surface without any degradation of the atomic surface flatness. The surface evolution of the D-passivated surface was followed by contact angle measurements, spectroscopic ellipsometry (SE), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), infrared absorption spectroscopy (IR), surface potential and surface photovoltage (SPV) mapping and light induced potential transient method. Qualification and the results were compared to the H-passivated, bare and native oxide covered Si surface. Our passivation test results confirmed that using D-passivation on Si surface is a promising method in the MOS technology and the interface engineering processes.
Keywords: Deuterium-passivation; Surface voltage; Vibrating capacitor; SPV; Native oxide; Silicon surface; Tunnel current; Surface charge
Influence of Si substrate preparation on surface chemistry and morphology of L-CVD SnO2 thin films studied by XPS and AFM by M. Kwoka; L. Ottaviano; N. Waczyńska; S. Santucci; J. Szuber (pp. 5771-5775).
Results of experimental studies of the influence of substrate preparation on the surface chemistry and surface morphology of the laser-assisted chemical vapour deposition (L-CVD) SnO2 thin films are presented in this paper. The native Si(100) substrate cleaned by UHV thermal annealing (TA) as well as thermally oxidized Si(100) substrate cleaned by ion bombardment (IBA) have been used as the substrates. X-ray photoemission spectroscopy (XPS) has been used for the control of surface chemistry of the substrates as well as of deposited films. Atomic force microscopy (AFM) has been used to control the surface morphology of the L-CVD SnO2 thin films deposited on differently prepared substrates. Our XPS shows that the L-CVD SnO2 thin films deposited on thermally oxidized Si(100) substrate after cleaning with ion bombardment exhibit the same stoichiometry, i.e. ratio [O]/[Sn]=1.30 as that of the layers deposited on Si(100) substrate previously cleaned by UHV prolonged heating. AFM shows that L-CVD SnO2 thin films deposited on thermally oxidized Si(100) substrate after cleaning with ion bombardment exhibit evidently increasing rough surface topography with respect to roughness, grain size range and maximum grain height as the L-CVD SnO2 thin films deposited on atomically clean Si substrate at the same surface chemistry (nonstoichiometry) reflect the higher substrate roughness after cleaning with ion bombardment.
Keywords: Tin dioxide; L-CVD thin films; Si substrate preparation; Surface chemistry; Surface morphology; Atomic force microscopy (AFM); X-ray photoelectron spectroscopy (XPS)
Structural, electronic, and magnetic properties of pristine and oxygen-adsorbed graphene nanoribbons by R.H. Miwa; R.G.A. Veiga; G.P. Srivastava (pp. 5776-5782).
The structural, electronic and magnetic properties of pristine and oxygen-adsorbed (3,0) zigzag and (6,1) armchair graphene nanoribbons have been investigated theoretically, by employing the ab initio pseudopotential method within the density functional scheme. The zigzag nanoribbon is more stable with antiferromagnetically coupled edges, and is semiconducting. The armchair nanoribbon does not show any preference for magnetic ordering and is semiconducting. The oxygen molecule in its triplet state is adsorbed most stably at the edge of the zigzag nanoribbon. The Stoner metallic behaviour of the ferromagnetic nanoribbons and the Slater insulating (ground state) behaviour of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. However, the local magnetic moment of the edge carbon atom of the ferromagnetic zigzag ribbon is drastically reduced, due to the formation of a spin-paired C–O bond.
Keywords: Graphene nanoribbons; Pseudopotential theory; Density functional theory
Structural and electronic properties of H-passivated graphene by A.Z. AlZahrani; G.P. Srivastava (pp. 5783-5788).
The atomic and electronic structures of graphane ( hydrogen-passivated graphene) are theoretically investigated using the local density approximation (LDA) of the density functional theory (DFT) and the pseudopotential method. Our total energy calculations suggest that the chairlike configuration for graphane is more energetically stable than the boatlike and tablelike configurations by approximately 0.129eV/cell and 0.655eV/cell, respectively. Our calculations suggest that the LDA band gap of the chairlike structure is approximately 3.9eV. The equilibrium geometry and the band structure of the chairlike conformer are investigated and compared with the available experimental and theoretical data. We further present total and partial charge density to reveal the orbital nature of the highest occupied and the lowest unoccupied states.
Keywords: Graphene; Graphane; Density functional theory; Local density approximation; Pseudopotential method; Hydrogen passivation
Molecular gated transistors: Role of self-assembled monolayers by O. Shaya; E. Halpern; B. Khamaisi; M. Shaked; Y. Usherenko; G. Shalev; A. Doron; I. Levy; Y. Rosenwaks (pp. 5789-5795).
In order to understand the biosensing mechanism of field-effect based biosensors and optimize their performance, the effect of each of its molecular building block must be understood. In this work the gating effect of self-assembled linker molecules on field-effect transistor was studied in detail. We have combined Kelvin probe force microscopy, current–voltage measurements, capacitance–voltage measurements, equivalent circuit modeling and device simulations in order to trace the mechanism of silicon-on-insulator biological field-effect transistors. The measurements were conducted on the widely used linker molecules (3-aminopropyl)-trimethoxysilane (APTMS) and 11-aminoundecyl-triethoxysilane (AUTES), which were self-assembled on ozone activated silicon oxide surface covering the transistor channel. In a dry environment, the work function of the modified silicon oxide decreased by more than 1.5eV, and the transistor threshold voltage increased by about 30V following the self-assembly. A detailed analysis indicates that these changes are due to negative induced charges on the top dielectric layer, and an effective dipole due to the polar monolayer. However, the self-assembly did not change the silicon flat-band voltage when in contact with an electrolyte. This is attributed to electrostatic screening by the electrolyte.
Keywords: PACS; 73.61.-rMolecular gated transistors; Polar self-assembled monolayers