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Applied Surface Science (v.254, #12)

Preface by Masao Takahashi Guest Editor; Viktor Bezák Guest Editor; Vladimír Gašparík Guest Editor (pp. 3613-3614).

Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application by H. Angermann; J. Rappich; L. Korte; I. Sieber; E. Conrad; M. Schmidt; K. Hübener; J. Polte; J. Hauschild (pp. 3615-3625).

Special sequences of wet-chemical oxidation and etching steps were optimised with respect to the etching behaviour of differently oriented silicon to prepare very smooth silicon interfaces with excellent electronic properties on mono- and poly-crystalline substrates. Surface photovoltage (SPV) and photoluminescence (PL) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were utilised to develop wet-chemical smoothing procedures for atomically flat and structured surfaces, respectively. Hydrogen-termination as well as passivation by wet-chemical oxides were used to inhibit surface contamination and native oxidation during the technological processing. Compared to conventional pre-treatments, significantly lower micro-roughness and densities of surface states were achieved on mono-crystalline Si(100), on evenly distributed atomic steps, such as on vicinal Si(111), on silicon wafers with randomly distributed upside pyramids, and on poly-crystalline EFG (Edge-definedFilm-fed-Growth) silicon substrates.The recombination loss at a-Si:H/c-Si interfaces prepared on c-Si substrates with randomly distributed upside pyramids was markedly reduced by an optimised wet-chemical smoothing procedure, as determined by PL measurements. For amorphous–crystalline hetero-junction solar cells (ZnO/a-Si:H(n)/c-Si(p)/Al) with textured c-Si substrates the smoothening procedure results in a significant increase of short circuit current I_sc, fill factor and efficiency η. The scatter in the cell parameters for measurements on different cells is much narrower, as compared to conventional pre-treatments, indicating more well-defined and reproducible surface conditions prior to a-Si:H emitter deposition and/or a higher stability of the c-Si surface against variations in the a-Si:H deposition conditions.

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; Interface state density; Recombination loss; Wet-chemical pre-treatment; a-Si:H/c-Si hetero-junction solar cells; Electrical characterisation; Surface photovoltage; Photoluminescence; Atomic force microscopy

Electrical and ellipsometry study of sputtered SiO₂ structures with embedded Ge nanocrystals by P. Basa; A.S. Alagoz; T. Lohner; M. Kulakci; R. Turan; K. Nagy; Zs.J. Horváth (pp. 3626-3629).

SiO₂ layer structures with a middle layer containing Ge nanocrystals were prepared by sputtering on n- and p-type Si substrates, and by consecutive annealing. Ge content in the middle layer was varied in the range of 40–100%. Most of the structures exhibited low breakdown voltages. The current through the structures became Schottky-like after breakdown. However, some p-type samples showed a considerable memory effect. It was obtained by spectroscopic ellipsometry that the middle layer contains amorphous Ge phase as well. The results also suggest intermixing of the layers during the sputtering and/or the annealing process.

Keywords: PACS; 81.15.Cd; 81.05−t; 78.67.Bf; 78.20.−eSputtering; SiO; ₂; structure; Annealing; Ge nanocrystals

Tunnelling through randomly inhomogeneous barriers as a special problem of the scattering theory by Viktor Bezák (pp. 3630-3634).

The tunnelling of electrons through an inhomogeneous delta barrier is considered. The strength of the barrier is defined as a function oscillating around a constant mean value along a plane. Owing to deviations from this value, the tunnelling through such a delta barrier has to be interpreted as a scattering. A simple model is discussed when circular windows of a given radius b representing themselves delta barriers of a given strengthγ0 are embedded in a homogeneous delta barrier defined with another strengthγ¯. When the centers of the windows are distributed randomly in the barrier plane, the potential energy of the electrons is a random function of two space coordinates. The perpendicular incidence is discussed with emphasis on the angular probability density of the tunnelled electrons. The derivation of the angular probability density proves that three basic quantum-mechanical phenomena can be described by one simple formula: tunnelling, diffraction and scattering.

Keywords: PACS; 03.65.Lm; 03.65.Xp; 61.14.Lj; 68.4; 73.40.Gk; 85.30.HiTunnelling; Inhomogeneous delta barrier; Scattering probability density; Diffraction

Preparation and structural properties of MgO films grown on GaAs substrate by Štefan Chromik; Marianna Španková; Ivo Vávra; Jozef Liday; Peter Vogrinčič; Peter Lobotka (pp. 3635-3637).

Epitaxial MgO thin films have been grown on semiinsulating GaAs (001) substrates using electron beam (e-beam) evaporation. X-ray diffraction indicates c-axis oriented MgO with (002) reflection only and rocking curve widths ∼2.2–3°. Transmission electron microscopy (TEM) analyses confirm an epitaxial growth of the MgO films. We study the microstructure and the defects at the interface between the MgO film and the GaAs substrate. Auger electron Spectroscopy (AES) concentration depth profiles reveal no contamination of the MgO films by As and Ga at different temperatures of the deposition process.

Keywords: PACS; 68.55.−a; 61.10.Nz; 68.37.Lp; 81.15.Ef; 77.55.+fElectron beam evaporation; MgO thin films; X-ray diffraction; Transmission electron microscopy; Auger electron spectroscopy

Hg-based cuprate superconducting films patterned into structures for ultrafast photodetectors by Š. Chromik; M. Valeriánová; V. Štrbík; Š. Gaži; P. Odier; X. Li; Y. Xu; R. Sobolewski; F. Hanic; G. Plesch; Š. Beňačka (pp. 3638-3642).

We present results which describe procedures of the preparation and patterning of Hg,Re–Ba–Ca–Cu–O superconducting films suitable for coplanar structures usable as possible photodetectors and for microbridges. We compare structural and electrical properties of the final films from the point of their applicability for photodetectors. Our prepared Re-doped Hg-based films on LaAlO₃ substrate are continuous with a sufficient adhesion to the substrate and with a maximum zero resistance critical temperature T_C0∼122K. The X-ray diffraction of all the films confirmed the Hg-1212 phase beside the minor Hg-1223 phase and intergrowth phases in some cases. In spite of epitaxial character of the final Hg-based films, the microwave and magnetic measurements suggest a possible existence of nonstoichiometric material between the grains which we register as a presence of weak links near the transition into the superconducting state. We show some electrical properties of the prepared superconducting structures, too. The prepared coplanar structures show ultrafast photoresponse signal to incident laser pulse.

Keywords: PACS; 74.72.Jt; 74.78.Bz; 74.62.BfHg-based superconductors; Thin films; Superconducting structures; Patterning

Structural and optical properties of sputtered ZnO thin films by S. Flickyngerova; K. Shtereva; V. Stenova; D. Hasko; I. Novotny; V.Tvarozek; P. Sutta; E. Vavrinsky (pp. 3643-3647).

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈001〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400–800nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.

Keywords: PACS; 81.05−t; 81.15.CdRF diode sputtering; ZnO:Al; Structural and optical properties

Study and applications of plasma-modified Si and porous Si surfaces by N. Gabouze; N. Benzekkour; Be Mahmoudi; S. Belhousse; H. Cheraga; N. Ghellai (pp. 3648-3652).

In this work we have studied the electrical, chemical and physical properties of CH_x/silicon and CH_x/porous silicon (PS). The hydrocarbon (CH_x) layer has been deposited by plasma of methane under argon atmosphere. Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) Spectroscopy and photoluminescence have been used to characterize the CH_x/p-Si, CH_x/PS interfaces and, electrolyte/(CH_x/p-Si) and CH_x/PS/p-Si structures. The results show that in the case of bare silicon, the CH_x layers act as a resistant film to HF electrolyte and can be used as a potential tool for ultra-low thickness for masking and patterning. The deposition of CH_x layer on PS shows that CH_x/PS/p-Si structure presents a rectifying behaviour and can be used for detecting low concentration of large variety of gases. In addition CH_x coated PS samples exhibit more intense luminescence than that observed from an uncoated PS surface where red luminescence is shown .In conclusion, the results clearly demonstrate the interest and applications of Si and PS electrodes coated with hydrocarbon groups.

Keywords: PACS; 82.80.Gk; 78.55.Mb; 78.20.Ci; 73.40.Qv; 07.07.DfSilicon; MIS structures; Porous materials; Gas sensors

Mechanism and control of current transport in GaN and AlGaN Schottky barriers for chemical sensor applications by Hideki Hasegawa; Masamichi Akazawa (pp. 3653-3666).

Strong interests are recently emerging for development of integrated high-performance chemical sensor chips. In this paper, the present status of understanding and controlling the current transport in the GaN and AlGaN Schottky diodes is discussed from the viewpoint of chemical sensor applications. For this purpose, a series of works recently carried out by our group are reviewed in addition to a general discussion. First, current transport in GaN and AlGaN Schottky barriers is discussed, introducing the thin surface barrier (TSB) model to explain the anomalously large leakage currents. Following this, attempts to reduce the leakage currents are presented and discussed. Then, as an example of gas-phase sensors using Schottky barriers, a Pd/AlGaN/GaN Schottky diode hydrogen sensor developed recently by our group is presented with a discussion on the sensing mechanism and related current transport. On the other hand, in liquid-phase sensors, contact is made between liquid and semiconductor which is regarded as a kind of Schottky barrier by electrochemists. As one of such liquid-phase sensors, open-gate AlGaN/GaN heterostructure field effect transistor (HFET) pH sensor developed recently by our group is presented. Finally, a brief summary is given together with some remarks for future research.

Keywords: PACS; 73.40.Ty; 73.40.Ns; 73.40.Gk; 85.30.De; 85.30.Hi; 81.05.EaGroup-III nitrides; GaN; AlGaN; Chemical sensors; Schottky diodes

Nitric acid oxidation method to form SiO₂/3C–SiC structure at 120°C by Sung-Soon Im; Sumio Terakawa; Hitoo Iwasa; Hikaru Kobayashi (pp. 3667-3671).

3C–SiC(001) surfaces are considerably rough with the roughness root mean square value ( R_ms) of 1.3nm, but the surfaces become considerably smooth (i.e., R_ms of 0.5nm) by heat treatment in pure hydrogen at 400°C. Two-step nitric acid (HNO₃) oxidation (i.e., immersion in ∼40wt% HNO₃ followed by that in 68wt% HNO₃) performed after the hydrogen treatment can oxidize 3C–SiC at extremely low temperature of ∼120°C, forming thick SiO₂ (e.g., 21nm) layers. With no hydrogen treatment, the leakage current density of the 〈Al/SiO₂/3C–SiC〉 metal-oxide-semiconductor (MOS) diodes is high, while that for the MOS diodes with the hydrogen treatment is considerably low (e.g., ∼10⁻⁶A/cm² at the forward gate bias of 1V) due to the formation of uniform thickness SiO₂ layers. The MOS diodes with the hydrogen treatment show capacitance–voltage curves with accumulation, depletion, and deep-depletion characteristics.

Keywords: PACS; 81.40.Cp; 73.61.Ng; 85.40.Hp; 73.40.QvSiC; MOS; Nitric acid oxidation; Hydrogen treatment; Low temperature oxidation; Silicon oxide; Gate oxide

Solution of the optical parameters of the thin film systems and interfaces by S. Jurečka; E. Pinčík; R. Brunner (pp. 3672-3676).

We report on the optical parameters of the semiconductor thin films determination. The method is based on the dynamical modeling of the spectral reflectance function combined with the genetic optimization of the initial model. The spectral dependency of the thin film optical parameters computation is based on the optical transitions modeling. The combination of the dynamical modeling and the genetic optimization enable comfortable analysis of the spectral dependences of the optical parameters and incorporation of the microstructure effects on the multilayer system optical properties. The results of the optical parameters of i-a-Si thin films determination are presented.

Keywords: PACS; 68.55 Jk; 78.66 Jg; 81.05 Gc; 81.15 GhOptical parameters; Semiconductor; Thin film; Roughness; Numerical methods

Optical inhomogeneity model for evaporated Y₂O₃ obtained from physical thickness measurement by A. Kasikov (pp. 3677-3680).

A multiparameter fitting with additional parameters for film inhomogeneity based on transmission results is used to get film inhomogeneity information and to compare different models for film structure. For a number of evaporated materials similar results from transmission fitting have been obtained by using a model consisting of two sublayers with a constant difference in refractive indices between them, either with a thin sublayer in the contact with a substrate or with air. As additional information, we obtained the film physical thickness result from step profile measurements for an oxygen-doped Y₂O₃ film on a fused silica and we compared it with the fit results for this coating. The result closest to the profilometry one has been achieved for a model with a thinner sublayer in contact with the substrate. The differences are great enough to assert that Y₂O₃ films on a fused silica possess a higher refractive index in the first stages of growth and then, after some transition, the main material with smaller refractive index grows on it.

Keywords: PACS; 42.79.Wc; 78.20.Ci; 78.67.Pt; 81.15.Aa; 81.15.EfY; ₂; O; ₃; Vacuum evaporation; Refractive index; Transmission; Inhomogeneity

Near-field scanning optical microscopy studies of thin film surfaces and interfaces by Petr Klapetek; Jiří Buršík (pp. 3681-3684).

In this article, the results of the modeling of topography related artifacts appearing in near-field scanning optical microscopy measurements are presented. The results obtained for near-field scanning optical microscope operation in reflection mode with off-axis far field detector position are compared with experimental results. It is shown that the chosen numerical method – Finite Difference in Time Domain method (FDTD) – can be used for efficient modeling of main topography related artifact. It is also seen that the far field detector position can have large influence on the resulting reflection mode optical images.

Keywords: PACS; 68.37.Uv; 78.68.+mNSOM; FDTD; Image artifacts

Nitric acid method for fabrication of gate oxides in TFT by Shigeaki Mizushima; Shigeki Imai; Asuha; Masato Tanaka; Hikaru Kobayashi (pp. 3685-3689).

We have developed low temperature formation methods of SiO₂ layers which are applicable to gate oxide layers in thin film transistors (TFT) by use of nitric acid (HNO₃). Thick (>10nm) SiO₂ layers with good thickness uniformity (i.e., ±4%) can be formed on 32cm×40cm substrates by the two-step nitric acid oxidation method in which initial and subsequent oxidation is performed using 40 and 68wt% (azeotropic mixture) HNO₃ aqueous solutions, respectively. The nitric acid oxidation of polycrystalline Si (poly-Si) thin films greatly decreases the height of ridge structure present on the poly-Si surfaces. When poly-Si thin films on 32cm×40cm glass substrates are oxidized at azeotropic point (i.e., 68wt% HNO₃ aqueous solutions at 121°C), ultrathin (i.e., 1.1nm) SiO₂ layers with a good thickness uniformity (±0.05nm) are formed on the poly-Si surfaces. When SiO₂/Si structure fabricated using plasma-enhanced chemical vapor deposition is immersed in 68wt% HNO₃, oxide fixed charge density is greatly decreased, and interface states are eliminated. The fixed charge density is further decreased by heat treatments at 200°C, and consequently, capacitance–voltage characteristics which are as good as those of thermal SiO₂/Si structure are achieved.

Keywords: PACS; 81.40.Cp; 73.61.Ng; 85.40.Hp; 73.40.QvMOS; Nitric acid oxidation; Silicon oxide; Silicon; Thin film transistor; Low temperature oxidation; Gate oxide

Microstructure of hydrogenated silicon thin films prepared from silane diluted with hydrogen by J. Müllerová; P. Šutta; G. van Elzakker; M. Zeman; M. Mikula (pp. 3690-3695).

We report results obtained from FTIR and TEM measurements carried out on silicon thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) from silane diluted with hydrogen. The hydrogen content, the microstructure factor, the mass density and the volume per Si–H vibrating dipoles were determined as a function of the hydrogen dilution. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (μc-Si:H). With increasing dilution the transition from amorphous to microcrystalline phase appears faster and the average mass density of the films decreases. The μc-Si:H films are mixed-phase void-rich materials with changing triphasic volume fractions of crystalline and amorphous phases and voids. Different bonding configurations of vibrating Si–H dipoles were observed in the a-Si:H and μc-Si:H. The bonding of hydrogen to silicon in the void- and vacancy-dominated mechanisms of network formation is discussed.

Keywords: PACS; 78.66.Jg; 68.55.−a; 78.30.−j; 81.40.−zThin-film silicon; Amorphous phase; Microcrystalline phase; Hydrogen

In situ measurement of plasma charging on SiO₂ hole bottoms and reduction by negative charge injection during etching by T. Ohmori; T. Makabe (pp. 3696-3709).

Charging damage in the fabrication of a micro- and nanoelectronic device is one of the electrical damages during plasma etching and caused basically by a huge difference of the flux velocity distribution between positive ions and electrons toward the wafer to be processed. Beam-like positive ions are accumulated on the bottom of a miniaturized structure during etching. With the evolution of the technology node, charging damage will increase due to several factors, increase of plasma exposure time, decrease of annealing temperature, and narrow process window, etc., caused by the increase of the number of metal layers and the introduction of new materials such as low-k and high-k instead of SiO₂. The progress of a top-down nanotechnology depends on the development of in situ diagnostics regarding plasma damage to lower-level elements and on the development of charging-free plasma process. In this paper, in situ charging measurements by using a test chip and negative charge injection to the wafer by optical computerized tomography are first demonstrated. Second, we discuss the characteristics of the charging potential on the bottom of SiO₂holes during etching in a two-frequency capacitively coupled plasma (2f-CCP), and refer to the procedure to reduce the positive potential by utilizing the negative charge acceleration to the hole bottom under the artificial formation of a double-layer close to the wafer. In addition, the charging’s effect on the aspect ratio of the hole and the antenna ratio are discussed.

Keywords: Charging-free process; Charging damage; Negative charge injection

Photoluminescence of very thin oxide/a-Si:H structures passivated in HCN solutions by E. Pinčík; R. Brunner; H. Kobayashi; M. Takahashi; M. Kučera (pp. 3710-3714).

In this contribution we present new experimental facts concerning evolution of the a-Si:H photoluminescence (PL) spectra, recorded at 6K, induced by both formation of very thin oxide layer in the surface region of the semiconductor by nitric acid solutions and passivation of a-Si:H defect states using HCN aqueous solutions. a-Si:H layers were deposited on both n-type of crystalline Si and the Corning glass. The analysis of the set of PL spectra – their interpretation – indicates that two explanations of blue shifts of the PL band maxima are possible. The first is connected with formation of several structurally different a-Si:H-based phases inside the amorphous matrix and/or with the Street model of recombination of localized electron-hole pairs coupled with the optical phonon in Si.Additionally, as it was stated, the wet chemical oxidation of 1μm thick a-Si:H layers deposited on the glass can induce formation of hydrogenated micro-sized a-Si grains. Passivation procedure performed in the HCN solution can transform an equivalent part of the a-Si:H semiconductor to a-Si:CN. If the multiphase model is accepted for interpretation of the PL spectra then the following main PL transitions related with different phases were observed: 1.20, 1.25, 1.38, 1.41, 1.44, and 1.48eV.

Keywords: PACS; 78.55.Qr; 78.66.Jg; 81.16.Pr; 85.40.LsSilicon; Amorphous materials; HCN solution; PL spectra

Si cleaning method without surface morphology change by cyanide solutions by Masao Takahashi; Yueh-Ling Liu; Hiroaki Narita; Hikaru Kobayashi (pp. 3715-3720).

Hydrogen cyanide (HCN) aqueous solutions can remove copper contaminants from Si surfaces more effectively than hydrochloric acid/hydrogen peroxide mixture (HPM) and sulfuric acid/hydrogen peroxide mixture (SPM). When pH of the HCN solutions is adjusted at 9, Si surface morphology is not changed, while when pH exceeds 10, the Si surfaces are considerably roughed. AFM measurements show that Cu contaminants are present in the form of particles on the bare Si surfaces. XPS measurements show that the particles consist of metallic Cu. The Cu particle height decreases almost linearly with the cleaning time, and the Cu surface concentration decreases exponentially with it. It is concluded that Cu particles gradually dissolve into the HCN aqueous solutions by the direct reaction with cyanide ions at the surface of the Cu particles.

Keywords: PACS; 81.65.Cf; 68.43.−hSurface cleaning; Silicon; Cyanide solutions; Copper; Etching; Morphology

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Applied Surface Science (v.254, #12)

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Applied Surface Science (v.254, #12)