Preparation and characterization of pulsed laser deposition (PLD) SiC films

Si K-edge XAFS was used to characterize a stoichiometric SiC film prepared by pulsed KrF laser deposition. The film was deposited on a p-type Si(1 0 0) wafer at a substrate temperature of 250 °C in high vacuum with a laser fluence of ∼5 J/cm². The results reveal that the film contains mainly a SiC phase with an amorphous structure in which the Si atoms are bonded to C atoms in its first shell similar to that of crystalline SiC powder but with significant disorder.     

Internal (SiH)X groups,X = 1–4, in microcrystalline hydrogenated silicon and their IR spectra on the basis of periodic DFT modelling

Vibrational Si–H frequencies were calculated on the basis of density functional theory (DFT) using periodic boundary conditions for N-Si voids, N < 8, in microcrystalline hydrogenated silicon (MHS) and (100), (110), and (111) slabs of 8, 5, and 8 layers, respectively, with the dangling bonds being saturated with hydrogen atoms. The slabs are considered as the models of inter-grain boundaries (IGB) in MHS. The N-Si voids of different shapes have been obtained via random deleting N silicon atoms. It was shown that the high stretching modes (HSM) of Si–H vibrations, which are usually assigned to SiH_X, appear also due to (SiH)_X groups, X = 2–4, in the N-Si voids. No such (SiH)_X groups were formed with X > 1 at the IGB. The low stretching modes (LSM) are thus assigned to Si–H groups presented at both N-Si voids and IGB. Similar relative stability of the voids is obtained with two different DFT approaches, i.e., B3LYP with atomic basis set and Perdew-Burke-Ernzerhof (PBE) with plane wave basis set. This result allows a simple interpretation of usually small I_HSM/(I_LSM + I_HSM) intensity ratio as a consequence of minor concentration of any voids in device quality MHS.     

Ultrafast vibronic phase transitions induced in semiconductors by femtosecond laser pulses

The intermode anharmonic interaction in the theory of ultrafast (t∼10⁻¹³ s) vibronic phase transitions induced on semiconductor surfaces (Si, GaAs) by femtosecond laser pulses is calculated. The conditions for plasma-induced transitions either to a state of chaotic disorder in the positions of the atoms (“cold liquid”) or into a state with crystal symmetry different from the initial symmetry (a new crystalline phase) are determined. It is shown that a NaCl-type structure is realized in GaAs for a transition of the second type, the transition being due to the instability of the longitudinal optical phonon branch. The corresponding numerical estimates are made for Si and GaAs. Fiz. Tverd. Tela (St. Petersburg) 41, 1462–1466 (August 1999)     

Photographic diagnosis of crystalline silicon solar cells utilizing electroluminescence

The photographic surveying of electroluminescence (EL) under forward bias was proved to be a powerful diagnostic tool for investigating not only the material properties but also process induced deficiencies visually in silicon (Si) solar cells. Under forward bias condition, solar cells emit infrared light (wavelength around 1000 to 1200 nm) whose intensity reflects the number of minority carriers in base layers. Thus, all the causes that affect the carrier density can be detected, i.e., the minority carrier diffusion length (or in other words, lifetime), recombination velocity at surfaces and interfaces, etc. (intrinsic material properties), and wafer breakage and electrode breakdown, etc. (extrinsic defects). The EL intensity distribution can be captured by Si CCD camera in less than 1 s, and the detection area simply depends upon the optical lens system suitable to the wide range of 1 cm–1.5 m. This fast and precise technique is superior to the conventional scanning method such as the laser beam induced current (LBIC) method. The EL images are displayed as grayscale, which leads to the difficulty of distinguishing the sorts of those deficient areas. Since the intrinsic deficiency is more sensitive to temperature than the extrinsic deficiency, the change in solar cell temperature can offer the difference in EL intensity contrasts. These effects upon the measurement temperature can be applied to categorize the types of deficiency in the crystalline Si solar cell.     

Quasi-monocrystalline silicon for thin-film devices

Thermal crystallization of a double layer porous Si film creates a monocrystalline Si film with a thin separation layer between the Si film and the reusable starting wafer. The process enables transfer of thin monocrystalline Si films to foreign substrates, whereby devices may be formed before or after separation of the film. Sub-micrometer thick films are almost compact, while films with a thickness of several μm contain voids, and are therefore termed “quasi-monocrystalline”. Internal voids strongly enhance optical absorption by light scattering. The hole mobility is 78 cm² V^-1 s^-1 at a p-type starting wafer resistivity of 0.05 Ω cm. Received: 24 March 1999 / Accepted: 29 March 1999 / Published online: 5 May 1999     

Hopping motion of chlorine atoms on Si(1 0 0)-(2 × 1) surfaces induced by carrier injection from scanning tunneling microscope tips

Injection of tunneling electrons and holes from the probe tips of a scanning tunneling microscope was found to enhance the hopping motion of Cl atoms between neighboring dangling-bond sites of Si dimers on Si(1 0 0)-(2 × 1) surfaces, featured by the rate of hopping linearly dependent on the injection current. The hopping rate formed peaks at sample biases of V_S∼+1.25 and −0.85 V, which agree with the peaks in the local density of states spectrum measured by scanning tunneling spectroscopy. The Cl hopping was enhanced at Cl-adsorbed sites even remote from the injection point. The Cl hopping by hole injection was more efficiently enhanced by sweeping the tip along the Si dimer row than by tip-sweeping along the perpendicular direction. Such anisotropy, on the other hand, was insignificant in the electron injection case. All of these findings can be interpreted by the model that the holes injected primarily into a surface band originated from the dangling bonds of Si dimers propagate quite anisotropically along the surface, and become localized at Cl sites somehow to destabilize the Si-Cl bonds causing hopping of the Cl atoms. The electrons injected into a bulk band propagate in an isotropic manner and then get resonantly trapped at Si-Cl antibonding orbitals, resulting in bond destabilization and hopping of the Cl atoms.     

On the interaction of halogen atoms with (111) and (100) surfaces of silicon

The chemisorption of F, Cl, Br, and I atoms on the (111) and (100) surfaces of silicon has been studied by the MNDO method and using clusters of Si atoms to simulate the substrate. In the case of the adducts F-Si₄H₉ and Cl-Si₄H₉ the MNDO results are in close agreement with previous ab-initio ones concerning both the equilibrium distances and the chemisorption energies. For all the cases considered, the binding energy decreases in the order F>Cl>Br>I. The most stabel adduct is always obtained upon chemisorption at the bridge position on the (100) surface. Chemisorption at on-top positions leads to slightly less stabel adducts and is nearly isoenergetic on (100) and (111) surfaces. All the results are essentially insensitive to the dimensions of the clusters used to simulate the substrate.     

F2 laser induced oxidation of inorganic material

Transparent SiO₂ thin films were selectively fabricated on Si wafer by 157 nm F₂ laser in N₂/O₂ gas atmosphere. The F₂ laser photochemically produced active O(¹D) atoms from O₂ molecules in the gas atmosphere; strong oxidation reaction could be induced to fabricate SiO₂ thin films only on the irradiated areas of Si wafer. The oxidation reaction was sensitive to the single pulse fluence of F₂ laser. The irradiated areas were swelled and the height was approximately 500-1000 nm at the 205-mJ/cm² single pulse fluence for 60 min laser irradiation. The fabricated thin films were analytically identified to be SiO₂ by the Fourier-transform IR spectroscopy. The SiO₂ thin films could be also removed by subsequent chemical etching to fabricate micro-holes 50 nm in depth on Si wafer for microfabrication.     

(001) silicon surfacial grain boundaries obtained by direct wafer bonding process: accurate control of the structure before bonding

Ultra-thin (001) silicon films bonded onto (001) silicon wafers, which form ‘surfacial grain boundaries’, are analysed by transmission electron microscopy and X-ray diffraction. The aim of this study is to find a way of controlling precisely, before direct wafer bonding, the structure of the Si/Si (001) interface. Two kinds of dislocation networks are found at the bonded interface. A square array of screw dislocations accommodates the twist between the two crystals, whereas a linear network of mixed dislocations accommodates the tilt resulting from the residual vicinality of the initial surfaces. A theoretical study shows that knowing and choosing before bonding the characteristics of these interfacial dislocations depend on the control of the ‘twist’ angle during the hydrophobic molecular bonding process. Recently, a new bonding method allows us to obtain an accuracy of ±?0.005° for the ‘twist’ angle from patterned grooves without any crystallographic measurement. The precision of this technique is compared with three different measurement of the disorientation between the two grains taken after bonding. The first one is deduced from the periodicity of the dislocation networks located at the interface. The second one is calculated from large angle convergent beam electron diffraction patterns. The last one is obtained by synchrotron X-ray diffraction. The possibility of choosing precisely the characteristics of the Si (001) ‘surfacial grain boundaries’ before direct wafer bonding process is then discussed in light of an experimental study.     

The optical transition in porous Si: The effects of quantum confinement,surface states and hydrogen passivation

Summary We present a theoretical study of two infinite wires of Si with a different lateral size. The analysis is based on the linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA). We consider free, partially and totally H-covered [001] Si quantum wires with rectangular cross-section. The results of this investigation prove the quantum wire nature of porous Si and interpret many of its physical features. In particular we show thata) as expected quantum confinement originates the opening of the LDA gap;b) the gap opening effect is asymmetric: 1/3 of the widening is in the valence band, while 2/3 in the conduction band;c) the near band gap states originate from Si atoms located at the center of the wire;d) the confinement is enhanced in the case of free surfaces;e) the imaginary part of the dielectric function shows a low-energy side structure strongly anisotropic, identified as responsible of the luminescence transition;f) the presence of dangling bonds destroys the luminescence properties;g) in spite of featurec), all Si atoms are collectively involved in the luminescence transition;h) the shift detected by the Si L_{2, 3}VV Auger signal is due to H-interaction effect and is not a measure of the quantum confinement effect;i) the Si atoms probed by the Si L_{2, 3}VV Auger are bonded with H and H₂. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995.     

Excimer-laser etching on silicon

Studies have been made of poly- and single Si etching induced by excimer-laser irradiation of the silicon surfaces in halogenated gases. Etching was investigated for different conduction types, impurity concentrations and crystallographic planes. Chlorine atoms accept electrons generated in photoexcited, undoped p-type Si, thus becoming negative ions which are pulled into the Si. However, the n⁺-type Si is etched spontaneously by Cl^– as a result of the availability of conduction electrons. Fluorine atoms, with the highest electronegativity, take in electrons independent of whether the material is n- or p-type. And thus, the easy F^– ion penetration into Si causes spontaneous etching in both types. New anisotropic etching for n⁺ poly-Si is investigated because of its importance to microfabrication technology. Methyl methacrylate (MMA) gas, which reacts with Cl atoms, produces a deposition film on the n⁺ poly-Si surface. The surface, from which the film is removed by KrF (5 eV) laser irradiation, is etched by Cl atoms, while the film remains on the side wall to protect undercutting. However, with the higher photon energy for the ArF (6.4 eV) laser, the Si-OH bonds are broken and electron traps are formed. These electrontrapping centers are easily annealed out in comparison to the plasma-induced centers. Pattern transfer etching for n⁺ poly-Si has been realized using reflective optics. The problems involved in obtaining finer resolution etching are discussed.     

High-density-plasma (HDP)-CVD oxide to thermal oxide wafer bonding for strained silicon layer transfer applications

Direct wafer bonding between high-density-plasma chemical vapour deposited (HDP-CVD) oxide and thermal oxide (TO) has been investigated. HDP-CVD oxides, about 230 nm in thickness, were deposited on Si(0 0 1) control wafers and the wafers of interest that contain a thin strained silicon (sSi) layer on a so-called virtual substrate that is composed of relaxed SiGe (∼4 μm thick) on Si(0 0 1) wafers. The surfaces of the as-deposited HDP-CVD oxides on the Si control wafers were smooth with a root-mean-square (RMS) roughness of <1 nm, which is sufficiently smooth for direct wafer bonding. The surfaces of the sSi/SiGe/Si(0 0 1) substrates show an RMS roughness of >2 nm. After HDP-CVD oxide deposition on the sSi/SiGe/Si substrates, the RMS roughness of the oxide surfaces was also found to be the same, i.e., >2 nm. To use these wafers for direct bonding the RMS roughness had to be reduced below 1 nm, which was carried out using a chemo-mechanical polishing (CMP) step. After bonding the HDP-CVD oxides to thermally oxidized handle wafers, the bonded interfaces were mostly bubble- and void-free for the silicon control and the sSi/SiGe/Si(0 0 1) wafers. The bonded wafer pairs were then annealed at higher temperatures up to 800 °C and the bonded interfaces were still found to be almost bubble- and void-free. Thus, HDP-CVD oxide is quite suitable for direct wafer bonding and layer transfer of ultrathin sSi layers on oxidized Si wafers for the fabrication of novel sSOI substrates.     

Filling the voids in silicon single crystals by precipitation of Cu3Si

This article presents a method to decorate all open volume defects in a silicon single crystal by Cu₃Si precipitation. Si single crystals are being used for the determination of Avogadro's number with sufficient accuracy (1 part in 10⁸) to allow the establishment of a physical standard for the kilogram. The method described here can be used to certify that the open volume in such crystals is sufficiently small. The voids in this work were formed artificially by annealing He-implanted silicon wafers. Annealing after application of a copper nitrate solution to the surface followed by slow cooling produced Cu₃Si precipitates in the η′ phase in the voids. To fill all the voids completely it proved necessary to coat the surfaces, after application of the nitrate, with a pure Cu layer as well. Slow cooling keeps the supersaturation of Cu small but sufficient to precipitate Cu into the voids. Formation of dislocation loops and stacking faults, often seen during Cu silicide precipitation in silicon after fast cooling, can be minimized. Other transition metals, such as Au, Ni, and Fe, were found to be less suitable than Cu for filling voids.     

Mixed Si-Ge nanoparticle quantum dots: a density functional theory study

We present a systematic study of small mixed composition Si and Ge nanoparticle quantum dots performed using density functional theory (DFT) and time-dependent density functional theory (TDDFT) with real space grids and norm-conserving pseudopotentials. Quantum dot models are obtained by distributing Si and Ge atoms within basic models containing a specified number of atomic sites, tetrahedrally-coordinated about a central site, with H atoms added to terminate the surfaces. We consider models containing up to 35 semiconductor atoms, and a number of subsets of the total number of possible configurations (e.g. alternating elements in successive shells). Following calculation of structural and energetic properties, the optical absorption spectra are determined using time-dependent DFT within Casida’s linear-response approach. With respect to Si, the inclusion of Ge moves spectral weight to lower energies. We observe that the relative fractions of Si and Ge have stronger effects on the absorption spectra than the structural distribution of those atoms within the nanoparticle, which tends to have quite small effects.     

200 nm–1000 nm spectra of light emitted in the impact of <Superscript>40</Superscript>Ar<Superscript>10+</Superscript> upon Al and Si solid surfaces

This paper reports the measured results of the 200 nm–1000 nm characteristic spectral lines of Al, Si and Ar atoms when highly charged ions ⁴⁰Ar¹⁰⁺ are incident upon Al and P-type Si surfaces. The ion ⁴⁰Ar¹⁰⁺ is provided by the ECR ion source of the National Laboratory of the Heavy Ion Accelerator in Lanzhou. The results show that when the low-speed ions in the highly charged state interact with the solid surfaces, the characteristic spectral lines of the target atoms and ions spurted from the surfaces can be effectively excited. Moreover, because of the competition of the non-radiation de-excitation of the hollow atom by emitting secondary electrons with the de-excitation process by radiating photons, the spectral intensity of the characteristic spectral lines of Ar atoms on the P-type Si surface is, as a whole, greater than that of Ar atoms on the Al surface.     

共振离子化质谱在高纯材料表面杂质分析中的应用

用激光熔出－共振离子化质谱法测定纯铁中及高纯硅片表面的铬。测定含有１０１２原子／ｃｍ２Ｃｒ的信号强度表明本方法可测定低至１０９原子／ｃｍ２的样品，其空间分辨率为０．２ｍｍ，还可进行二维杂质元素分析。     

Nature of contrast in Ge/Si(111) layers in scanning tunneling microscopy in the presence of Bi and Sb surfactants

It is known that the use of Bi surfactant (unlike Sb) upon the growth of Ge layers on Si(111) increases the contrast between Ge and Si atoms in a scanning tunneling microscope. This makes it possible to distinguish the Ge and Si surfaces. This effect is studied using computer simulation based on the density functional theory. To explain the observed difference between the Ge and Si layers, both structural and electronic effects are considered. The local density of electronic states, as well as the corresponding decay length to vacuum, has been calculated for each of the surfaces. The simulation results have been compared to the previous scanning tunneling microscopy data.     

Desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips

We investigated desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips. The hole-induced desorption of chlorine atoms had a threshold bias voltage corresponding to the energy position of the S3 surface band originated in Si backbonds. The chlorine atom desorption rate was almost proportional to the square of the tunneling current. We have discussed possible mechanisms that two holes injected into Si surface states get localized at the backbonds of chlorinated Si adatoms, which induces the rupture of Cl-Si bonds to result in chlorine atom desorption.     

Flow pattern defects in germanium-doped Czochralski silicon crystals

The distribution and etching rate of flow pattern defects (FPDs) in germanium- doped Czochralski (GCZ) silicon (Si) wafers with light and heavy dopants—either boron (B) or phosphorus (P)—have been investigated. In the lightly doped (both B and P) Czochralski (CZ) Si crystals, the FPD densities in GCZ Si decrease with the increase of Ge concentration. In the heavily B-doped GCZ Si crystals, the FPDs are denser compared with the heavily B-doped CZ Si, whereas the reverse is true in the heavily P-doped GCZ Si and CZ Si crystals. It is also shown that the etching rates in the lightly doped CZ Si crystals can be slightly enlarged by the Ge doping. It is proposed that, in lightly doped GCZ Si, Ge doping could consume free vacancies and thus form high-density but small-sized voids, while the stress compensation induced by B and Ge atoms could increase the vacancy concentration in heavily B-doped GCZ Si, leading to sparse and large-sized voids.     

Ⅶ族元素在Si(111)和Ge(111)表面上的吸附

本文用原子集团模型和电荷自洽的EHT方法研究了Ⅶ族元素在Si(111)和Ge(111)表面上的化学吸附。利用能量极小的原则确定了各元素的化学吸附构型。对于Cl,在这两个表面均是顶位吸附。对于Ⅰ,都呈三度空位吸附。对Br,顶位及三度空位吸附均能发生,但是在Si(111)表面顶位吸附要优于三度空位,而在Ge(111)表面则三度空位吸附优于顶位。最后对Ⅶ族元素原子在这两种表面的吸附行为作了讨论,并与实验作了比较。 关键词：