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Applied Surface Science (v.254, #24)

No Title by Jacek Szuber Guest Editor (pp. 7999-7999).

Surface and near-surface passivation, chemical reaction, and Schottky barrier formation at ZnO surfaces and interfaces by L.J. Brillson; H.L. Mosbacker; M.J. Hetzer; Y. Strzhemechny; D.C. Look; G. Cantwell; J. Zhang; J.J. Song (pp. 8000-8004).

Using a combination of depth-resolved cathodoluminescence spectroscopy, electronic transport, and surface science techniques, we have demonstrated the primary role of native defects within ZnO single crystals as well as native defects created by metallization on metal–ZnO Schottky barrier heights and their ideality factors. Native defects and impurities resident within the ZnO depletion region as well as defects extending into the bulk from the intimate metal–ZnO interface contribute to barrier thinning of, carrier hopping across, and tunneling through these Schottky barriers. Chemical reactions at clean ZnO–metal interfaces lead to metal-specific eutectic or oxide formation with pronounced transport effects. These results highlight the importance of bulk crystal quality, surface cleaning, metal interaction, and post-metallization annealing for controlling Schottky barriers.

Keywords: ZnO; Defect; Chemical reaction; Schottky barrier; Interface; Plasma; Luminescence; Spectroscopy

Interface models and processing technologies for surface passivation and interface control in III–V semiconductor nanoelectronics by H. Hasegawa; M. Akazawa (pp. 8005-8015).

Interface models and processing technologies are reviewed for successful establishment of surface passivation, interface control and MIS gate stack formation in III–V nanoelectronics. First, basic considerations on successful surface passivation and interface control are given, including review of interface models for the band alignment at interfaces, and effects of interface states in nanoscale devices. Then, a brief review is given on currently available surface passivation technologies for III–V materials, including the Si interface control layer (ICL)-based passivation scheme by the authors’ group. The Si-ICL technique has been successfully applied to surface passivation of nanowires and to formation of a HfO₂ high-k dielectric/GaAs interfaces with low values of the interface state density.

Keywords: PACS; 81.05.Ea ;81.65.Rv ;73.20.At; 74.40.QvCompound semiconductors; Surface passivation; Band alignment; Fermi level pinning; Interface states

Passivation at semiconductor/electrolyte interface: Role of adsorbate solvation and reactivity in surface atomic and electronic structure modification of III–V semiconductor by Mikhail V. Lebedev (pp. 8016-8022).

These studies are focused on understanding the role played by a solvent in chemical and electronic processes occurred in the course of semiconductor surface passivation at semiconductor/electrolyte interface. It is shown that the chemical reactivity of the ionic adsorbate at a semiconductor/electrolyte interface can be changed considerably through interaction with solvent molecules. The reactivity of anions depends essentially on the solvating solvent: hydrated ions could be either slightly electrophilic or slightly nucleophilic, whereas the ions solvated by alcohol molecules are always strongly nucleophilic. Mechanism of interaction of such solvated ions with the semiconductor surface atoms depends on the solvent, as is demonstrated by the example of processes occurred at GaAs(100)/sulfide solution interfaces. It is found that on adsorption of HS⁻ ions from different solvents the AsS bonds with solvent-dependent ionic character are formed on a GaAs(100) surface. The surface obtained in such a way possesses different ionization energy and exhibit different electronic properties dependent on the solvent.

Keywords: PACS; 68.43.Fg; 73.40.Mr; 81.65.Rv; 82.20.Yn; 82.80.PvGaAs; Surface passivation; Reactivity; Solvated anions; Solvent effect; DFT; XPS; Raman scattering

Structural properties of GaAs surfaces nitrided in hydrazine-sulfide solutions by V.L. Berkovits; L. Masson; I.V. Makarenko; V.P. Ulin (pp. 8023-8028).

The surface structure of GaAs(100), (111)A, and (111)B substrates nitrided through the wet chemical treatment in hydrazine-sulfide solution have been studied by scanning tunneling microscopy (STM) under annealing in UHV. Such treatment has earlier been shown to produce a monolayer of gallium nitride on the (100)GaAs surface. The as-nitrided substrates of all surface orientations were found to be covered by an overfilm, which contains thioarsenic compounds and has a smooth relief. Thermal desorption of the overlfilm at about 530°C opens the own relief of the nitrided surfaces. For the (100) orientation such relief is not microscopically planar and consists of nano-scale vicinal hillocks. These hillocks occur due to surface microetching which proceeds simultaneously with the formation of the surface nitride layer. We have shown that the wet nitridation procedure forms a monolayer of surface nitride on the (111)B surface. During nitridation the (111)B surface, as well as the (100) one, is affected by the microetching in the hydrazine-sulfide solution. Therefore, it exhibits a characteristic relief formed by triangular vicinal pyramids. At the same time the nitride film is not formed on the (111)A surface, which is more chemically inert, and where the surface etching is almost absent.

Keywords: PACS; 68.55.Jk; 81.65.Rv; 82.45.Mp; 82.80.PvGallium arsenide; Chemical nitridation; Nitride monolayer; Surface morphology; Scanning tunneling microscopy; Auger electron spectroscopy

Study of S⁺ ion-assisted sulfurization of n-GaAs (100) surface by H.Y. Hu; Q. Zhao (pp. 8029-8034).

The chemical structure and site location of sulfur atoms on n-GaAs (100) surface treated by bombardment of S⁺ ions over their energy range from 10 to 100eV have been studied by X-ray photoelectron spectroscopy and low energy electron diffraction. The formation of Ga–S and As–S species on the S⁺ ion bombarded n-GaAs surface is observed. An apparent donor doping effect is observed for the n-GaAs by the 100eV S⁺ ion bombardment. It is found that the S⁺ ions with higher energy are more effective in the formation of Ga–S species, which assists the n-GaAs (100) surface in reconstruction into an ordered (1×1) structure upon subsequent annealing. The treatment is further extended to repair Ar⁺ ion damaged n-GaAs (100) surface. It is found that after a n-GaAs (100) sample is damaged by 150eV Ar⁺ ion bombardment, and followed by 50eV S⁺ ion treatment and subsequent annealing process, finally an (1×1) ordering GaAs (100) surface with low surface states is obtained.

Keywords: PACS; 41.75AK; 61.72. Vv; 81.65. Rv; 79.60.-IGaAs surface; Sulfur passivation; Low energy S; ⁺; ions

Analysis of mechanism of carbon removal from GaAs(100) surface by atomic hydrogen by P. Tomkiewicz; A. Winkler; M. Krzywiecki; Th. Chasse; J. Szuber (pp. 8035-8040).

Etching of carbon contaminations from the GaAs(100) surface by irradiating with atomic hydrogen, which is one of the key reactions to promote high-quality thin films growth by molecular beam epitaxy (MBE), has been investigated by mass spectrometry (MS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). It is shown that during the cleaning process at room temperature a total reduction of the Auger carbon signal, accompanied by desorption of methane as major reaction product, can be observed. The reaction pathways as well as the processes responsible for the observed carbon removal are discussed in detail to give a support for etching and growth quality enhancement not only in thin films epitaxy but in all atomic hydrogen promoted gas-phase III–V semiconductor processes.

Keywords: PACS; 81.05.Ea, 79.60.Dp, 81.65.Cf, 79.20.Rf, 81.70.JbGaAs; Cleaning; Etching; Atomic hydrogen; Auger electron spectroscopy; Mass spectrometry; X-ray photoelectron spectroscopy; Reaction products; Carbon species; Auger carbon pattern

New reconstruction-stoichiometry correlation for GaAs(001) surface treated by atomic hydrogen by K.V. Toropetsky; O.E. Tereshchenko; D.A. Petukhov; A.S. Terekhov (pp. 8041-8045).

The structure, stoichiometry and electronic properties of the GaAs(001)-(2×4)/ c(2×8) surface treated by cycles of atomic hydrogen (AH) exposure and subsequent annealing in UHV were studied with the aim of preparing the Ga-rich surface at low temperatures. Low energy electron diffraction showed reproducible structural transformations in each cycle: AH adsorption at the (2×4)/ c(2×8) surface led to the (1×4) structure at low AH exposure and to a (1×1) surface at higher AH exposure with subsequent restoration of the (2×4)/ c(2×8) structure under annealing at 450°C. The cycles of AH treatment preserved the atomic flatness of the GaAs(100) surface, keeping the mean roughness on to about 0.15nm. The AH treatment cycles led to the oscillatory behavior of 3dAs/3dGa ratio with a gradual decrease to the value characteristic for the Ga-rich surface. Similar oscillatory variations were observed in the work function. The results are consistent with the loss of As from the surface as a result of the desorption of volatile compounds which are formed after reaction with H. The prepared Ga-rich GaAs(001) surface showed the stability of the (2×4)/ c(2×8) structure up to the annealing temperature of 580°C.

Keywords: PACS; 68.35.Bs; 81.65; 61.14.HgGaAs; Atomic hydrogen; Surface structure; Stoichiometry; Low energy electron diffraction; X-ray photoelectron spectroscopy

Surface state density distribution at vacuum-annealed InP(100) surface as derived from the rigorous analysis of photoluminescence efficiency by P. Tomkiewicz; B. Adamowicz; M. Miczek; H. Hasegawa; J. Szuber (pp. 8046-8049).

The surface state density distribution N_SS( E) and surface Fermi level E_Fs position on a sequentially ultra-high vacuum-annealed n-InP(100) sample are investigated using rigorous computer analysis of dependences of the room temperature band-edge photoluminescence efficiency ( Y_PL) on the photon flux density ( Φ). We have found that the minimum density of a U-shaped N_SS( E) distribution as well as the donor-like surface state density are reduced by one order of magnitude after the annealing at 250 and 300°C. This can be assigned to the decrease in the disorder in the unintentionally formed InP native oxides. On the other hand, we demonstrate that the annealing simultaneously generates discrete surface states probably due to missing group V element (P) in the interface region which may account for electrical interface instabilities observed in InP-based devices. The results are discussed quantitatively and compared to other reports.

Keywords: PACS; 1. 07.05.Tp; 73.20.−r; 81.05.dz; 78.55.−m; 81.40.EfInP; Annealing; Passivation; Cleaning; Surface and interface states; Fermi level pinning; Photoluminescence; MISFET

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE by N. Kakuda; T. Yoshida; K. Yamaguchi (pp. 8050-8053).

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(001) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.

Keywords: PACS; 81.15Hi; 68.65Hb; 78.55CrQuantum dots; InAs; Sb; Molecular beam epitaxy; Photoluminescence; High-density

Properties of thick SiO₂/Si structure formed at 120°C by use of two-step nitric acid oxidation method by S. Imai; S. Mizushima; Asuha; W.-B. Kim; H. Kobayashi (pp. 8054-8058).

Thick (i.e., ∼10nm) SiO₂/Si structure has been formed at 121°C by immersion of Si in relatively low concentration HNO₃ followed by that in 68wt.% HNO₃ (i.e., two-step nitric acid (HNO₃) oxidation method of Si, NAOS) and spectroscopic properties and electrical characteristics of the NAOS SiO₂ layers are investigated. The SiO₂ thickness strongly depends on the concentration of HNO₃ aqueous solutions employed in the initial oxidation, and it becomes the largest at the HNO₃ concentration of 40wt.%. The MOS diodes with the ∼9nm SiO₂ layer formed by the NAOS method possess a relatively low leakage current density (e.g., 10⁻⁸A/cm² at the forward bias of 1V) and it is further decreased by more than one order of magnitude by post-metallization annealing (PMA) in hydrogen at 250°C. The good leakage characteristic is attributable to atomically flat SiO₂/Si interfaces and high atomic density of 2.30–2.32×10²²atoms/cm³ of the NAOS SiO₂ layers. High-density interface states are present in as-prepared SiO₂ layers and they are eliminated by PMA in hydrogen.

Keywords: Low-temperature oxidation; Nitric acid oxidation; Silicon; SiO; ₂; Interface states; Hydrogen treatment; Metal-oxide-semiconductor (MOS)

Passivation of defect states in Si-based and GaAs structures by E. Pinčík; H. Kobayashi; R. Brunner; M. Takahashi; Yueh-Ling Liu; L. Ortega; K. Imamura; M. Jergel; J. Rusnák (pp. 8059-8066).

Formation of defect states on semiconductor surfaces, at its interfaces with thin films and in semiconductor volumes is usually predetermined by such parameters as semiconductor growth process, surface treatment procedures, passivation, thin film growth kinetics, etc. This paper presents relation between processes leading to formation of defect states and their passivation in Si and GaAs related semiconductors and structures. Special focus is on oxidation kinetics of yttrium stabilized zirconium/SiO₂/Si and Sm/GaAs structures. Plasma anodic oxidation of yttrium stabilized zirconium based structures reduced size of polycrystalline silicon blocks localised at thin film/Si interface. Samarium deposited before oxidation on GaAs surface led to elimination of EL2 and/or ELO defects in MOS structures. Consequently, results of successful passivation of deep traps of interface region by CN⁻ atomic group using HCN solutions on oxynitride/Si and double oxide layer/Si structures are presented and discussed. By our knowledge, we are presenting for the first time the utilization of X-ray reflectivity method for determination of both density of SiO₂ based multilayer structure and corresponding roughnesses (interfaces and surfaces), respectively.

Keywords: PACS; 71.20.Nr; 71.20.Mq; 73.20.At; 73.40.Ty; 73.30.+ySilicon; Gallium arsenide; Ultrathin dielectrics; Double layer; Interface roughness

Passivation of structured p-type silicon interfaces: Effect of surface morphology and wet-chemical pre-treatment by H. Angermann (pp. 8067-8074).

The effect of both surface morphology and wet-chemical pre-treatment on electronic surface and interface properties was investigated for mono- and polycrystalline silicon substrates with special surface structures. Surface charge, energetic distribution, and density of rechargeable states on these surfaces were determined by surface photovoltage (SPV) measurements. These results were correlated to previously reported findings on atomically flat Si(111) and Si(100) surfaces of monocrystalline wafers. In this paper, a specially optimised sequence of cleaning, wet-chemical oxidation, and oxide removal procedures is described in detail for the first time. This method was successfully applied in order to remove contaminations and damaged surface layers and to obtain atomically flat areas on substrates with evenly distributed atomic steps, polycrystalline and monocrystalline substrates with randomly distributed pyramids. A significant reduction in surface micro-roughness, interface state density, and recombination loss was achieved. Using passivation by wet-chemical oxidation or H-termination, respectively, the optimised surface state can be preserved by the time of following preparation steps and during subsequent a-Si:H plasma enhanced chemical vapour deposition (PECVD).

Keywords: PACS; 73.20.Hb; 81.65.Cf; 81.65.Rv; 78.55.−m; 68.37.Ps; 68.37.−d; 84.60.Jt; 84.37.+q; 89.30.Cc 73.40.Lq 73.40.−cSilicon substrates; Wet-chemical pre-treatment; Interface state density; Recombination loss; a-Si:H/c-Si hetero-junction solar cells; Electrical characterisation; Surface photovoltage

Nanostructure formation aided by self-organised Bi nanolines on Si(001) by G.P. Srivastava; R.H. Miwa (pp. 8075-8082).

We provide a mini review of recent theoretical investigations of nanostructure formation aided by self-organised Bi nanolines on the Si(001) surface. It is suggested that hydrogen-passivated single-domain Si(001) produced by the formation of defect-free, hundreds of nm long, and 1.2–1.5nm wide Bi nanolines provides an appealing template with preferential sites for adsorption of other elements. Based on ab initio pseudopotential calculations it is suggested that using the Bi nanoline template it should be possible to grow the following structures: mixed Ge–Si dimer structures on the Si(001) terrace between two neighbouring Bi nanolines; small In nanoclusters along the nanoline; and line and cluster structures of Fe atoms with novel electronic and magnetic properties.

Keywords: Nanostructure; Si(0; 0; 1) surface; Bi nanolines; Density functional theory

Electronic and geometric investigations of the Ca/Si(111)-(5×2) surface by A.Z. AlZahrani; G.P. Srivastava (pp. 8083-8088).

The atomic and electronic structures of the Ca/Si(111)-(5×2) surface have been theoretically investigated by using the pseudopotential method and the local density approximation (LDA) of the density functional theory. The geometrical model is based on the deposition of three Ca lines on the top of a combination of a honeycomb chain channel (HCC) and the Seiwatz (SZ) chain formed by Si-adatoms. The Ca adatoms lie on bothH3 andT4 symmetry sites. This structural model produces a semiconducting surface band structure with a clear LDA energy gap of 0.56eV. Ca-derived band gap states have been identified by comparing the band structures for the clean Si(111)-(5×2) and Ca/Si(111)-(5×2) surfaces. Significant charge transfer from the Ca adatoms to neighbouring Si atoms has been concluded by analysing electronic charge density and STM simulations. A discussion of the role of Ca adatoms in improving the chemical passivation of the Si(111) surface has been attempted by examining the orbital nature of states in the fundamental band gap of bulk Si.

Keywords: Surface passivation; Si(1; 1; 1) surface; Ca adsorption; Density functional theory; Pseudopotential method

XPS study of the surface chemistry of Ag-covered L-CVD SnO₂ thin films by M. Kwoka; L. Ottaviano; M. Passacantando; G. Czempik; S. Santucci; J. Szuber (pp. 8089-8092).

In this paper, we present the results of X-ray photoelectron spectroscopy characterization of SnO₂ thin films prepared by laser chemical vapour deposition (L-CVD) and subsequently covered by Ag atoms just after deposition and after long-term exposed to dry air, subsequent annealing in ultra high vacuum at 400°C and dry air oxidation at 400°C. Using the standard analytical procedure based on atomic sensitivity factors, the variation of surface chemistry defined in terms of the relative concentration of the main components of the films after the above-mentioned procedures has been determined. It was confirmed that after dry air exposure as well as dry air oxidation, the layers undergo an oxidation reaching almost SnO₂ stoichiometry. Besides, during ultra high vacuum annealing, the films undergo reduction to almost SnO stoichiometry. At the same time, Ag atoms deposited at the top of layers diffuse into the subsurface layers. This was confirmed by X-ray photoelectron spectroscopy depth profiling analysis.

Keywords: PACS; 68.35 Dv; 68.55a; 79.60.-I; 81.15Cd; 81.15Gh; 81.65Mq; 81.70Jb; 82.80.PvTin dioxide; L-CVD thin films; Ag-doping; XPS surface chemistry; Depth profiling

Surface chemistry study of Mn-doped germanium nanowires by V. Grossi; P. Parisse; M. Passacantando; S. Santucci; G. Impellizzeri; A. Irrera; L. Ottaviano (pp. 8093-8097).

Single crystal germanium nanowires have been grown by vapour–liquid–solid deposition onto silicon oxide substrates with Au catalyst nanoparticles. They have been doped by two different techniques: Ge and Mn co-evaporation during growth and post-growth Mn implantation. Scanning electron microscopy images show that Mn-implanted nanowires have a lower surface density and a smaller average diameter (18.8nm) than the un-doped ones and those Mn doped by co-deposition. The effectiveness of Mn doping has been verified by X-ray photoemission spectroscopy and by energy-dispersive X-ray measurements, indicating in the two cases significant Mn atomic concentration in the nanowire. X-ray diffraction indicates that the nanowires are single crystals and that they do not contain precipitates of Mn extrinsic phases. Both SEM and XPS experimental evidences are in line to indicate that the Mn doping by ion implantation is preferable with respect to that one performed by co-evaporation as it reduces the thickness of the outer oxide sheath of the nanowires and their diameter.

Keywords: PACS; 81.07.−b; 61.72.Tt; 79.60.−I; 79.60.Jv; 68.37.Hk; 81.65.MqGermanium; Manganese; Nanowires; Vapour–liquid–solid growth mechanism; Ion implantation; SEM; XPS; XRD

Investigation of the 4H–SiC surface by O.J. Guy; M. Lodzinski; K.S. Teng; T.G.G. Maffeis; M. Tan; I. Blackwood; P.R. Dunstan; O. Al-Hartomy; S.P. Wilks; T. Wilby; N. Rimmer; D. Lewis; J. Hopkins (pp. 8098-8105).

The silicon carbide (SiC) surface is more complex than that of silicon and can be carbon-terminated or silicon-terminated, and can exist as several reconstructions. Investigations of the surface structure as a function of temperature, under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), are presented. The 4H–SiC surface can be passivated using a silicon deposition/evaporation technique to reconstruct the surface. This has a significant effect on the electrical behaviour of metal contacts to the silicon carbide surface, critical in any electronic device. Atomic resolution STM studies of the 4H–SiC surface have revealed step features and micropipe defects in unprecedented detail. STM has also been used to image clusters of metal deposited on the 4H–SiC surface. The effect of annealing on the behaviour of these nickel clusters is also presented. The surface of the silicon carbide is extremely important in the fabrication of silicon carbide electronic devices and this paper presents a discussion of the SiC surface with particular reference to its impact on SiC device applications in power electronics.

Keywords: PACS; 85.30.−z; 68.47.Fg4H–SiC; Surface; Interface; Contact; XPS; STM

Influence of surface cleaning effects on properties of Schottky diodes on 4H–SiC by N. Kwietniewski; M. Sochacki; J. Szmidt; M. Guziewicz; E. Kaminska; A. Piotrowska (pp. 8106-8110).

Ir/4H–SiC and IrO₂/4H–SiC Schottky diodes are reported in terms of different methods of surface pretreatment before contact deposition. In order to find the effect of surface preparation processes on Schottky characteristics the SiC wafers were respectively cleaned using the following processes: (1) RCA method followed by buffered HF dip. Next, the surface was oxidized (5.5nm oxide) using a rapid thermal processing reactor chamber and circular geometry windows were opened in the oxide layer before metallization deposition; (2) the same as sequence (1) but with an additional in situ sputter etching step before metallization deposition; (3) cleaning in organic solvents followed by buffered HF dip. The I– V characteristics of Schottky diodes were analyzed to find a correlation between extracted parameters and surface treatment. The best results were obtained for the sequence (1) taking into account theoretical value of Schottky barrier height. The contacts showed excellent Schottky behavior with ideality factors below 1.08 and barrier heights of 1.46eV and 1.64eV for Ir and IrO₂, respectively. Very promising results were obtained for samples prepared using the sequence (2) taking into account the total static power losses because the modified surface preparation results in a decrease in the forward voltage drop and reverse leakage current simultaneously. The contacts with ideality factor below 1.09 and barrier height of 1.02eV were fabricated for Ir/4H–SiC diodes in sequence (2).

Keywords: PACS; 73.30.+y; 73.40.Cg; 73.40.Ei; 73.40.Sx; 79.40.+z; 81.65.Cf; 85.30.HiSiC; Ir; IrO; ₂; Schottky; Surface cleaning; Surface etching

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Applied Surface Science (v.254, #24)

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Applied Surface Science (v.254, #24)