Defect reduction and surface passivation of SiO2/Si by heat treatment with high-pressure H2O vapor

Heat treatment with high-pressure H₂O vapor was applied to improve interface properties of SiO₂/Si and passivate the silicon surface. Heat treatment at 180–420 °C with high-pressure H₂O vapor changed SiO_x films, 150 nm thick formed at room temperature by thermal evaporation in vacuum, into SiO₂ films with a Si-O-Si bonding network similar to that of thermally grown SiO₂ films. Heat treatment at 130 °C with 2.8×10⁵ Pa H₂O for 3 h reduced the recombination velocity for the electron minority carriers from 405 cm/s (as-fabricated 150-nm-thick SiO_x/Si) to 5 cm/s. Field-effect passivation was demonstrated by an additional deposition of defective SiO_x films on the SiO₂ films formed by heat treatment at 340 °C with high-pressure H₂O vapor. The SiO_x deposition reduced the recombination velocity from 100 cm/s to 48 cm/s. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Effective surface passivation of crystalline silicon using ultrathin Al2O3 films and Al2O3/SiNx stacks

We measure surface recombination velocities (SRVs) below 10 cm/s on p‐type crystalline silicon wafers passivated by atomic–layer–deposited (ALD) aluminium oxide (Al₂O₃) films of thickness ≥10 nm. For films thinner than 10 nm the SRV increases with decreasing Al₂O₃ thickness. For ultrathin Al₂O₃ layers of 3.6 nm we still attain a SRV < 22 cm/s on 1.5 Ω cm p‐Si and an exceptionally low SRV of 1.8 cm/s on high‐resistivity (200 Ω cm) p‐Si. Ultrathin Al₂O₃ films are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is extremely low compared to the frequently used plasma‐enhanced chemical vapour deposition of silicon nitride (SiN_x). Our experiments on silicon wafers passivated with stacks composed of ultrathin Al₂O₃ and SiN_x show that a substantially improved thermal stability during high‐temperature firing at 830 °C is obtained for the Al₂O₃/SiN_x stacks compared to the single‐layer Al₂O₃ passivation. Al₂O₃/SiN_x stacks are hence ideally suited for the implementation into industrial‐type silicon solar cells where the metal contacts are made by screen‐printing and high‐temperature firing of metal pastes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-assembly of InAs quantum dots on GaAs(001)by molecular beam epitaxy

Currently, the nature of self-assembly of three-dimensional epitaxial islands or quantum dots (QDs) in a lattice-mismatched heteroepitaxial growth system, such as InAs/GaAs(001) and Ge/Si(001) as fabricated by molecular beam epitaxy (MBE), is still puzzling. The purpose of this article is to discuss how the self-assembly of InAs QDs in MBE InAs/GaAs(001) should be properly understood in atomic scale. First, the conventional kinetic theories that have traditionally been used to interpret QD self-assembly in heteroepitaxial growth with a significant lattice mismatch are reviewed briefly by examining the literature of the past two decades. Second, based on their own experimental data, the authors point out that InAs QD self-assembly can proceed in distinctly different kinetic ways depending on the growth conditions and so cannot be framed within a universal kinetic theory, and, furthermore, that the process may be transient, or the time required for a QD to grow to maturity may be significantly short, which is obviously inconsistent with conventional kinetic theories. Third, the authors point out that, in all of these conventional theories, two well-established experimental observations have been overlooked: i) A large number of “floating” indium atoms are present on the growing surface in MBE InAs/GaAs(001); ii) an elastically strained InAs film on the GaAs(001) substrate should be mechanically unstable. These two well-established experimental facts may be highly relevant and should be taken into account in interpreting InAs QD formation. Finally, the authors speculate that the formation of an InAs QD is more likely to be a collective event involving a large number of both indium and arsenic atoms simultaneously or, alternatively, a morphological/structural transformation in which a single atomic InAs sheet is transformed into a three-dimensional InAs island, accompanied by the rehybridization from the sp²-bonded to sp³- bonded atomic configuration of both indium and arsenic elements in the heteroepitaxial growth system.     

Photoemission from valence bands of GaAs(001) grown by molecular beam epitaxy

Angle-resolved photoemission measurements for GaAs(001) prepared by MBE have been carried out using synchroton radiation. Observed bulk features in the spectra can be explained by a direct transition model using free-electron-like final states down to energies as low as 16 eV above the top of the valence bands. Spectral bulk valence band features can be distinguished from surface features by an appropriate choice of photon energies and polar angles. This is demonstrated using calculated valence bands.     

Evolution of InAs islands in the Stranski-Krastanow mode of InAs/GaAs (001) fabricated using molecular beam epitaxy

Based on step-by-step observation using atomic force microscope, two distinctive successive phases were distinguished in accordance with evolution of the three-dimensional InAs islands during the Stranski-Krastanow mode of the InAs/GaAs (001) system fabricated using molecular-beam epitaxy. The initial phase is consistent with a power law, and the latter phase is a comparatively gradual one.     

Evolution of InAs islands in the Stranski-Krastanow mode of InAs/GaAs(001) fabricated using molecular beam epitaxy

Based on step-by-step observation using atomic force microscope, two distinctive successive phases were distinguished in accordance with evolution of the three-dimensional InAs islands during the Stranski-Krastanow mode of the InAs/GaAs(001) system fabricated using molecular-beam epitaxy. The initial phase is consistent with a power law, and the latter phase is a comparatively gradual one.      

GaSb layers with low defect density deposited on (001) GaAs substrate in two-dimensional growth mode using molecular beam epitaxy

We report on the growth of fully relaxed and smooth GaSb layers with reduced density of threading dislocations, deposited on GaAs substrate. We prove that three parameters have to be controlled in order to obtain applicable GaSb buffers with atomically smooth surface: interfacial misfit (IMF), the etch pit density (EPD) and the growth mode.The GaSb/GaAs interfacial misfit array and reduced EPD ≤1.0 × 10⁷ cm^?2 were easily obtained using As-flux reduction for 3 min and Sb-soaking surface for 10 s before the GaSb growth initiation. The successive growth of GaSb layer proceeded under the technological conditions described by the wide range of the following parameters: r_G ∈ (1.5 ÷ 1.9) Å/s, T_G ∈ (400 ÷ 520)°C, V/III ∈ (2.3 ÷ 3.5). Unfortunately, a spiral or 3D growth modes were observed for this material resulting in the surface roughness of 1.1 ÷ 3.0 nm. Two-dimensional growth mode (layer by layer) can only be achieved under the strictly defined conditions. In our case, the best quality 1-μm-thick GaSb buffer layer with atomically smooth surface was obtained for the following set of parameters: r_G = 1.5 Å/s, T_G = 530 °C, V/III = 2.9. The layer was characterized by the strain relaxation over 99.6%, 90° dislocations array with the average distance of 5.56 nm, EPD ～8.0 × 10⁶ cm^?2 and 2D undulated terraces on the surface with roughness of about 1 ML. No mounds were observed. We belive that only thin and smooth GaSb layer with reduced EPD may be applied as the buffer layer in complex device heterostructures. Otherwise, it may cause the device parameters deterioration.     

GaAs(001)衬底上分子束外延生长InNSb单晶薄膜

利用射频氮等离子辅助分子束外延(RF-MBE)技术在GaAs(001)衬底上生长稀氮 InNSb半导体薄膜,并通过原子力显微镜(AFM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和拉曼散射光谱等测量手段对样品的微结构和N组分等进行了表征.结果显示样品有较好的晶体质量,N组分可高达0.84%(XRD的结果).本文还对样品的输运性质进行了表征,结果显示样品在室温下具有较低的载流子浓度和较高的迁移率.另外,初步研究表明在InSb中掺入N可导致其室温磁阻明显下降. 关键词： 分子束外延 稀氮半导体 X射线衍射 拉曼光谱     

Electronic transport properties of graphene/Al2O3 (0001) interface

The electronic structure and transport properties of a single layer of graphene (Gr) on α-Al₂O₃ surface are studied using the density functional theory (DFT). We present three models that take into account the atom at the termination of the alumina surface: a) Al atoms, with the center of the Gr hexagon directly over an Al atom; b) Al atoms, with a carbon directly positioned above an Al atom; c) oxygen atoms. Two processes of geometric optimization were used: (i) All the atoms of the supercell were allowed to move in accordance with the BFGS quasi-Newton algorithm; (ii) The atoms of the three topmost layers of the α-Al₂O₃ (0001) slab, including the C atoms, were allowed to move, whereas the atoms of the remaining layers were frozen in their respective atomic bulk positions. The first two models preserve qualitatively the electronic structure of the pristine Gr using the geometric optimization process (i) whereas, in the third model this structure was lost due to a significant charge transfer between the carbon and oxygen atoms irrespective of the optimization procedure. However, models (a) and (b) with the optimization (ii) reveal a p-type semiconducting behavior.     

An investigation of misfit dislocations in Al on (001) GaAs grown by chemical beam epitaxy

We report a transmission electron microscope study of the morphology and interfacial structure of Aluminium grown on (001) GaAs by chemical beam epitaxy (CBE). The Al grows in islands for all thicknesses deposited, and exhibits four distinct orientation relationships with respect to the substrate. One of these orientation relationships becomes dominant as growth progresses, with (011)_Al parallel to (001)_GaAs. Misfit dislocations can be seen in the interface between this orientation and the substrate with Burgers vector 1/4(110)_GaAs, and a crystallographic analysis shows that these dislocations are associated with interfacial steps of height 1/2[001]_GaAs. In (001)Al on (001)GaAs, the existence of these dislocations has in the past been regarded as evidence for the existence of a rigid-body shift of the Al in the interfacial plane. Using cross-sectional high-resolution TEM, it is shown that this shift is not present in the (011) orientation. The similarity in the microstructure and crystallography of the (001) and (011) orientations leads us to suggest that there is also no shift in (001) Al on (001)GaAs. This is in conflict with previous investigations of this system using a wide variety of techniques.     

Efficient second-harmonic generation in birefringently phase-matched GaAs/Al(2)O(3) waveguides

We report efficient second-harmonic generation of femtosecond pulses in birefringently phase-matched GaAs/Al(2)O(3) waveguides pumped at 2.01mum. By use of pump pulses of ~200-fs duration and type I interaction, practical second-harmonic average powers of up to ~650muW were obtained, with an average input power of ~50muW. Waveguides of four different widths and two different lengths were investigated, and a normalized conversion efficiency of greater than 1000%W(-1)cm(-2) was obtained for a 1-mm waveguide. Measurements of pump and second-harmonic spectra provided clear evidence of phase matching and depletion of the pump spectrum. The measured bandwidth of the second harmonic was ~1.3nm. From the measurements of transmitted pump power at the phase-matching wavelength, pump depletions of more than 80% were recorded.     

Thermal instability of HfO2 on InP structure with ultrathin Al2O3 interface passivation layer

We studied the thermal stability of HfO₂ on an InP structure when an Al₂O₃ interface passivation layer (PL) was introduced. In contrast to the thick (～4 nm) Al₂O₃‐PL, an almost complete disappearance of the thin (～1 nm) Al₂O₃‐PL was observed after a post‐deposition anneal at 600 °C. Based on various chemical and electrical analyses, this was attributed to the intermixing of the thin Al₂O₃‐PL with HfO₂, which might have been accompanied by the out‐diffusion of a substantial amount of substrate elements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reconstruction dependence of the etching and passivation of the GaAs(001) surface

The microscopic nature of the selective interaction of iodine with an As- and Ga-stabilized GaAs(001) surface has been investigated by the photoelectron emission and ab initio calculations. The adsorption of iodine on the Ga-stabilized (4 × 2)/c(8 × 2) surface leads to the formation of the prevailing chemical bond with gallium atoms; to a significant redistribution of the electron density between the surface Ga and As atoms; and, as a result, to a decrease in their binding energy. Iodine on the As-stabilized (2 × 4)/c(2 × 8) surface forms a bond predominantly with surface arsenic atoms. Such a selective interaction of iodine with the reconstructed surfaces gives rise to the etching of the Ga-stabilized surface and the passivation of the As-stabilized surface; this explains the layer-by-layer (“digital”) etching of GaAs(001) controlled by the reconstruction transitions on this surface.     

Effective silicon surface passivation by atomic layer deposited Al2O3/TiO2 stacks

In this work atomic layer deposition of Al₂O₃ and TiO₂ has been used to obtain dielectric stacks for passivation of silicon surfaces. Our experiments on n‐ and p‐type silicon wafers deposited by thin Al₂O₃/TiO₂ stacks show that a considerably improved passivation is obtained compared to the Al₂O₃ single layer. For Al₂O₃ films thinner than 20 nm the emitter saturation current density decreases with increasing TiO₂ thickness. Especially the passivation of ultrathin (～5 nm) Al₂O₃ is very effectively enhanced by TiO₂ due to a decreased interface defect density as well as an increased fixed negative charge in the stacks. Hence, the thin Al₂O₃/TiO₂ stacks developed in this work can be used as a passivation coating for Si‐based solar cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Midgap interface states at epitaxial Al/AlAs(001) heterojunctions

Using ab initio pseudopotential calculations, we have investigated the nature of the electronic states with energies within the semiconductor band gap in abrupt, defect-free As-terminated Al/AlAs(001) contacts. Resonant interface states, not accounted for by commonly accepted models, occur at the J-point of the interface Brillouin zone near the Fermi energy in the semiconductor midgap region. They correspond to intermetallic bonds between Al atoms of the semiconductor and those of the metal. The new interface states derive from an interaction between localized states of the Al(001) surface and AlAs conduction band states, mediated by localized states of the non-reconstructed As-terminated AlAs(001) surface.     

Growth of SrTiO3 on Si(001) by hybrid molecular beam epitaxy

We report the heteroepitaxial growth of SrTiO₃ thin films on Si(001) by hybrid molecular beam epitaxy (hMBE). Here, elemental strontium and the metal‐organic precursor titanium tetraisopropoxide (TTIP) were co‐supplied in the absence of additional oxygen. The carbonization of pristine Si surfaces during native oxide removal was avoided by freshly evaporating Sr into the hMBE reactor prior to loading samples. Nucleation, growth and crystallization behavior as well a structural properties and film surfaces were characterized for a series of 46‐nm‐thick SrTiO₃ films grown with varying Sr to TTIP fluxes to study the effect of non‐stoichiometric growth conditions on film lattice parameter and surface morphology. High quality SrTiO₃ thin films with epitaxial relationship (001)SrTiO₃ || (001)Si and [110]SrTiO₃ || [100]Si were demonstrated with an amorphous layer of around 4 nm thickness formed at the SrTiO₃/Si interface. The successful growth of high quality SrTiO₃ thin films with atomically smooth surfaces using a thin film technique with scalable growth rates provides a promising route towards heterogeneous integration of functional oxides on Si. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Switching of GaAs(001) termination by action of molecular iodine

This paper presents experimental results of an ultrahigh vacuum study of 4 × 2/c(8 × 2) → 2 × 4/c(2 × 8) structural transition on GaAs(001) caused by the thermal removal of the saturated iodine monolayer formed at GaAs(001)-4 × 2/c(8 × 2). It has been found out that the original c(8 × 2) low energy electron diffraction pattern transforms into 4 × 1 at 0.6 ML of iodine coverage and then keeps up to its saturation at 1.0 ML. We have determined that GaI is the only chemical product of the iodine action, its double peak was observed in the thermal desorption spectra at T = 150–370°C. The explanation of surface processes underlying 4 × 2/c(8 × 2) → 2 × 4/c(2 × 8) phase transition is presented below.     

Activation of Al2O3 passivation layers on silicon by microwave annealing

Thin aluminum oxide layers deposited on silicon by thermal atomic layer deposition can be used to reduce the electronic recombination losses by passivating the silicon surfaces. To activate the full passivation ability of such layers, a post-deposition annealing step at moderate temperatures (≈400 ^°C, duration≈30 min) is required. Such an annealing step is commonly done in an oven in air, nitrogen, or forming gas atmosphere. In this work, we investigate the ability to reduce the duration of the annealing step by heating the silicon wafer with a microwave source. The annealing time is significantly reduced to durations below 1 min while achieving effective minority carrier lifetimes similar or higher to that of conventionally oven-annealed samples.