Atomic configuration and charge state of hydrogen at dislocations in silicon

The effect of the introduction of hydrogen upon the vibration spectra and electrical characteristics of samples with dislocation networks at the interface of bonded silicon wafers was studied. In order to improve the sensitivity of measurements and to distinguish the signal from dislocation networks in Raman spectra, thin foils conventionally prepared for transmission electron microscopy were used as the sample under investigation. In the samples with dislocation networks, a Raman peak at 2000 cm^–1 was observed. This peak survived after annealing at a temperature of T = 500°C and was not observed in reference samples. Comparison of the experimental data with currently available theoretical calculations allowed one to attribute the observed peak to neutral hydrogen atoms H⁰ at the center of Si–Si bonds. The peak is metastable in the ideal lattice, but becomes stable in the vicinity of dislocations.     

Characterization and Formation Mechanism of Six Pointed Star-Type Stacking Faults in 4H-SiC

Synchrotron white-beam x-ray topography (SWBXT) studies of defects in 100-mm-diameter 4H-SiC wafers grown using physical vapor transport are presented. SWBXT enables nondestructive examination of thick and large-diameter SiC wafers, and defects can be imaged directly. Analysis of the contrast from these defects enables determination of their configuration, which, in turn, provides insight into their possible formation mechanisms. Apart from the usual defects present in the wafers, including micropipes, threading edge dislocations, threading screw dislocations, and basal plane dislocations, a new stacking fault with a peculiar configuration attracts our interest. This fault has the shape of a six-pointed star, comprising faults with three different fault vectors of Shockley type. Transmission and grazing topography of the fault area are carried out, and detailed contrast analysis reveals that the outline of the star is confined by 30° Shockley partial dislocations. A micropipe, which became the source of dislocations on both the basal plane slip system and the prismatic slip system, is found to be associated with the formation of the star fault. The postulated mechanism involves the reaction of 60° dislocations of a/3 〈 $ \bar{2}110 $ 〉 Burgers vector on basal plane and pure screw dislocations of a/3 〈 $ 11\bar{2}0 $ 〉 Burgers vector on prismatic plane and cross slip of the partial dislocation from prismatic plane to basal plane leading to expansion of the faults.     

Point defects generated by direct-wafer bonding of silicon

High-purity float zone (FZ) silicon p- and n-type wafers were directly bonded hydrophobically at 700°C and 1050°C. Electrical I-V and capacitance-voltage (C-V) characterization was performed on p-n junctions of 2-mm square chips cut out from properly bonded areas of the wafers. The point defects were investigated by Deep Level Transient Spectroscopy (DLTS). As expected, annealing at 700°C resulted in high void density, while at 1050°C void-free bonded structures were formed. The I-V characteristics were dominated by recombination at the bonded interface and by the resistance of the n-type wafer. The point defects identified by DLTS are divacancy, vacancy-phosphor, and vacancy-oxygen complexes. These defects explain qualitatively the observed characteristics of the samples.     

Dislocation arrangements in gaas/ga1−xlnxas multilayers grown on (001), (111) and (112) substrates

GaAs/Ga_1−xIn_xAs strained layer superlattices with well-widths of 7 nm, barrier widths of 14 nm and periods of 10 to 30 have been examined by transmission electron microscopy in plan view and in cross-section for (001), (111) and (112) substrates. Individual layers are below the critical thickness while the overall SLS's are above the critical thickness for dislocation generation. (001) substrates give rise to square grids of 60° dislocations lying parallel to 〈110〉 directions with inclined 1/2 〈101〉 Burgers vectors, resulting from dislocation motion on 8 slip systems. (111) substrates give triangular networks of 60° dislocations lying parallel to 〈110〉 directions resulting from motion on 6 slip systems. (112) substrates have two sets of primary dislocations lying along (132) directions, along with secondary 60° dislocations lying along [110]. Long Lomer-Cottrell dislocations with Burgers vectors lying parallel to the substrate/SLS interface are occasionally observed for (001) substrates, while short segments are observed for (112) substrates; these are formed by reaction between conjugate dislocations. These dislocations arrangements are discussed in terms of the resolved shear stresses resulting from epitaxy for the various substrates orientations. Dislocation densities are much less than those required for complete strain relief. This is analyzed in terms of the reduction in the stress acting on dislocations from partial strain relief, along with a friction stress due to a combination of the Peierls stress and solution hardening from the In substitution. A friction stress ∼10^-3μ is required to explain the observed dislocation densities (μ, is the shear modulus).     

Investigation of the aging processes of the p-n junction based on a single dislocation

An investigation was made of the effects of aging on diodes based on 60° dislocations. The electrical properties of the dislocation p-n junctions showed good stability and were unaffected by the Cottrell impurity atmosphere. This property suggests that there are practical uses for semiconductor devices based on the principle of dislocations. A proposed assembly of the diode based on a single 60° dislocation is described.     

Dislocation nucleation mechanism and doping effect in p-type ZnSe/GaAs

Nitrogen doped ZnSe/GaAs heterostructures grown at 150 and 250°C were studied by transmission électron microscopy (TEM). The density of threading dislocations and the interfacial dislocation structure in ZnSe/GaAs heterostructures are related to the N-doping concentration. In addition, in-situ TEM heating studies show that Frank partial dislocations formed below critical thickness in N-doped ZnSe/GaAs are the sources for nucleation of a regular array of misfit dislocations at the ZnSe/GaAs interface. By the dissociation of the Frank partial dislocations and interaction reactions between the dislocations, the 60° misfit dislocations form. The Frank partial dislocations bound stacking faults which usually form in pairs at the film/substrate interface. The density of stacking faults increases with increasing N-doping concentration. Thus, at high N-doping levels, the dislocation nucleation sources are close together and not all of the Frank partial dislocations dissociate, so that a high density of threading dislocations results in samples with high N-doping concentrations. The high density of threading dislocations in the ZnSe film are found to be associated with a reduction or saturation of the net carrier density.     

位错间的相互作用对应变外延层应变释放的影响

利用D-T模型考虑了界面位借间的相互作用,计算了外延层与材底为不同失配、不同厚度时,应变的释放。发现由于失配产生的位错,在外延层应变释放的初期,主要是靠产生60°位错来释放应变;而在产生较多的位错后,位错间的相互作用将导致外延展的应变释放过程缓慢,此时要考虑90°位错对应变释释放所造成的影响;并对已有的实验结果进行了一些解释。     

Two series of “dislocation” photoluminescence bands in cadmium telluride crystals

Generation of dislocations in CdTe crystals induces new lines in the radiative recombination spectra, so-called “dislocation” photoluminescence bands. The spectral distribution of the “dislocation” photoluminescence bands and the spatial distribution profiles of their intensity in the vicinity of the points of indentation on the (111) and (001) faces are obtained. From the comparison of the profiles with the crystallographic structure of dislocations, the types of defects responsible for two groups of emission bands are identified. One of the groups (with the main peak at 841 nm) is controlled by the electron states of 60° dislocations with extra half-planes framed by tellurium atoms, referred to as Te(g) dislocations. The emission lines of the other group (with the peak at 806 nm) correspond to the ordered structures of point defects generated by the steps on the screw segments of dislocation half-loops with the Cd(g) head dislocations.     

Investigation of the reversibility of deformation in silicon sheets

We have investigated the degree to which plastic deformation is reversible in silicon by bending and re-flattening initially defect-free single-crystal Czochralski silicon samples using four-point bending to simulate the deformation experienced during ribbon crystal growth. Optical and scanning electron microscopy of etched sample cross-sections after single bending and bending and re-flattening at 1100° C and 1200° C revealed that the dislocation densities in the re-flattened samples were lower than in singly-bent samples by 60–90%, indicating that dislocations are either being annihilated within the silicon bulk or are exiting the silicon at the free surfaces. There was little evidence of dislocation interaction in the singly-bent single-crystal samples investigated with transmission electron microscopy, so the latter mechanism is more likely. Although the re-flattened specimens have a lower dislocation density, there is little improvement in the minority-carrier diffusion length, measured by electron-beam induced current, which in all cases ranged from 10–20 μm. Since the minority-carrier diffusion length changed little, even with a change in dislocation density from 10⁶ dislocations/cm²–10⁸ dislocations/cm², other defects must be controlling the diffusion length. There is some correlation between dislocation density and minority-carrier diffusion length within a given sample, but this may be due to indirect effects such as generation of point defects by moving dislocations.     

Influence of the misfit-dislocation screw component on the formation of threading dislocations in semiconductor heterostructures

In heterostructures with the (001) interface and diamond and sphalerite crystal lattices, the total relief of mismatch stresses by introducing two mutually perpendicular arrays of 60° misfit dislocations (MDs) was shown to be possible only if their screw components were of the same type. In the opposite case, it was necessary to introduce additional MD arrays that increased the probability of formation of threading dislocations in an epitaxial film. When the process is nonoptimal and two mutually perpendicular arrays are introduced with opposite types of screw components, excess energy of long-range shear stresses is accumulated. Examples of nonoptimal introduction of misfit dislocations are the operation of the Frank-Read and Hagen-Strunk modified dislocation sources. The relaxation process was simulated and investigated experimentally.     

Influence of annealing on the dislocation-related electrical conductivity of germanium

Germanium n-type single crystals with a donor concentration of 3×10¹² cm^?3 were deformed at 760°C to strains of δ≤71% with a rate of 6×10^?3 s^?1, cooled to room temperature, and then annealed for t≤20 h at 900°C. Low-temperature static electrical conductivity due to holes trapped by dislocations and transported along a branching dislocation network was measured before and after annealing of the deformed samples. It was found that annealing enhances the dislocation-related electrical conductivity in the samples with δ<50% and diminishes this conductivity in the samples with δ>60%. Selective etching and X-ray diffraction analysis showed that the main structural distinction of the samples with δ>60% is the presence of recrystallized regions. The influence of annealing on dislocation-related electrical conductivity is explained by an increase in connectedness of the dislocation network for δ<50% and by a decrease in this connectedness in the case of δ>60%.     

Mechanical properties of nitrogen-doped CZ silicon crystals

We investigated the effect of the nitrogen doping level on plastic properties of 300 mm silicon material in the temperature range between 650°C and 1000°C. Undoped, low N-doped, and high N-doped materials were examined. The dependence of the upper and lower yield point and the size dependence of indentation related dislocation rosettes on temperature were obtained.An increase in the nitrogen concentration leads to enhanced upper and lower yield points. Possible mechanisms of interaction of dislocations with impurities resulting in hardening of silicon single crystals are discussed. Finite element method (FEM) calculated data of bearing stress observed in 300 mm Si wafers annealed in a vertical furnace are compared with measured upper yield point values. At high nitrogen concentration the tolerable process temperatures, where plastic flow of Si wafers under load can be avoided, are significantly increased.     

Room temperature wafer bonding of silicon, oxidized silicon, and crystalline quartz

The use of plasma immersion as preparation for room temperature wafer bonding has been investigated. Silicon wafers have been successfully bonded at room temperature after exposure to oxygen or argon plasma. Oxidized silicon wafers and crystalline quartz have been bonded after exposure to oxygen plasma. The bonded interfaces exhibit very high surface energies, comparable to what can be achieved with annealing steps in the range of 600–800°C using normal wet chemical activation before bonding. The high mechanical stability obtained after bonding at room temperature is explained by an increased dynamic in water removal from the bonded interface allowing covalent bonds to be formed. Electrical measurements were used to investigate the usefulness of plasma bonded interfaces for electronic devices.     

Temperature and duration effects on microstructure evolution during copper wafer bonding

Interfacial morphologies during Cu wafer bonding at bonding temperatures of 300–400°C for 30 min followed by an optional 30-min or 60-min nitrogen anneal were investigated by means of transmission electron microscopy (TEM). Results showed that increased bonding temperature or increased annealing duration improved the bonding quality. Wafers bonded at 400°C for 30 min followed by nitrogen annealing at 400°C for 30 min, and wafers bonded at 350°C for 30 min followed by nitrogen annealing at 350°C for 60 min achieve the same excellent bonding quality.     

Microstructure and Sn Crystal Orientation Evolution in Sn-3.5Ag Lead-Free Solders in High-Temperature Packaging Applications

Understanding the reliability of eutectic Sn-3.5Ag lead-free solders in high-temperature packaging applications is of significant interest in power electronics for the next-generation electric grid. Large-area (2.5 mm × 2.5 mm) Sn-3.5Ag solder joints between silicon dies and direct bonded copper substrates were thermally cycled between 5°C and 200°C. Sn crystal orientation and microstructure evolution during thermal cycling were characterized by electron backscatter diffraction in the scanning electron microscope. Comparisons were made between the observed initial texture and microstructure and its evolution during thermal cycling. Gradual lattice rotation and grain boundary misorientation evolution observed due to thermal cycling suggested a continuous recrystallization mechanism. Recrystallization behavior was correlated with dislocation slip activities.     

Microstructure examination of copper wafer bonding

The microstructure morphologies and oxide distribution of copper bonded wafers were examined by means of transmission electron microscopy (TEM) and energy dispersion spectrometer (EDS). Cu wafers exhibit good bond properties when wafer contact occurs at 400°C/4000 mbar for 30 min, followed by an anneal at 400°C for 30 min in N₂ ambient atmosphere. The distribution of different defects showed that the bonded layer became a homogeneous layer under these bonding conditions. The oxidation distribution in the bonded layer is uniform and sparse. Possible bonding mechanisms are discussed.     

Improvement of heteroepitaxial growth by the use of twist-bonded compliant substrate: Role of the surface plasticity

In a first step, we have investigated the structure and the mechanical behavior of twist-bonded GaAs-compliant substrates. For a twist angle lower than 15°, the bonded interface was observed to contain a dense network of pure screw dislocations. For a larger twist angle, no dislocations were observed at the bonding interface. The mechanical behavior of these compliant substructures were investigated using nanoindentation, and the results were compared to those obtained on standard bulk GaAs. It appears that the nature of the interface has a great influence on the mechanical properties of these compliant substrates. In a second step, InGaAs alloys were grown on such compliant structures, and their quality was compared to that of layers grown on standard substrates. A promising improvement was observed on compliant structures.     

Electron levels and luminescence of dislocation networks formed by the hydrophilic bonding of silicon wafers

Local energy levels produced by dislocations at the interface between bonded n- and p-Si wafers are studied by deep level transient spectroscopy and by a new technique for the detection of impurity luminescence, induced by the occupation of electron states upon the application of electric pulses (the pulsed trap-refilling-enhanced luminescence technique). It is established that only the shallow levels of the dislocation network, with activation energies of about 0.1 eV, are responsible for the D1 dislocation-related luminescence band in both n- and p-type samples. The occupation of deep levels has no effect on the D1-band intensity. A model of coupled neutral trapping centers for charge carriers is proposed. In this model, the difference between the energy position of the D1 band (0.8 eV) and the corresponding interlevel energy spacing (0.97 eV) is attributed to the Coulomb interaction between charge carriers trapped at the levels.     

InP and Si metal-oxide semiconductor structures fabricated using oxygen plasma assisted wafer bonding

In this paper, InP metal-oxide-semiconductor (MOS) structures are fabricated by transferring thermally grown SiO₂ to InP from oxidized Si wafers using oxygen plasma assisted wafer bonding followed by annealing at either 125°C or at 400°C. Well-defined accumulation and inversion regions in recorded capacitance-voltage (C-V) curves were obtained. The long-term stability was comparable to what has been previously reported. The structures exhibited high breakdown fields, equivalent to thermally grown SiO₂-Si MOS structures. The transferring process was also used to fabricate bonded Si MOS structures.     

Structural characterization of ultra-thin (0 0 1) silicon films bonded onto (0 0 1) silicon wafers:: a transmission electron microscopy study

Ultra-thin (0 0 1) silicon films (thickness less than 25 nm) directly bonded onto (0 0 1) silicon wafers have been investigated by transmission electron microscopy. Twist interfacial dislocations accommodate the twist between the two crystals, whereas tilt interfacial dislocations accommodate the tilt resulting from the residual vicinality of the initial surfaces. In low-twist angle grain boundaries, twist interfacial dislocations are dissociated and no precipitates are detected. In high-twist angle grain boundaries, there is no dissociation and a high density of silicon oxide precipitates is observed at the interface. Tilt interfacial dislocations are pinned by these precipitates, they are more mobile than precipitates. Without precipitates, their lines are straighter than those with precipitates, and this is especially when the bonded wafers are annealed at a high temperature. When no precipitates are present, tilt interfacial dislocations are associated by pairs, and we demonstrate that each tilt interfacial dislocations introduce a diatomic interfacial step at the interface.