Solid Phase Epitaxial Crystallization Research Articles

Twinning microstructure in the solid-phase epitaxial crystallization of BaTiO3

Amorphous BaTiO3 layers deposited on SrTiO3 (001) substrates at room temperature were subsequently crystallized using solid phase epitaxy (SPE). Heating an initially amorphous BaTiO3 layer in air at 650 °C for 3 h resulted in crystallization with components in two distinct crystallographic orientation relationships with respect to the substrate. Part of the volume of the BaTiO3 layer crystallized in a cube-on-cube relationship with the substrate. Other volumes crystallized in four variants of a 70.5° rotation about ⟨110⟩, resulting in a ⟨221⟩ surface normal in each case. Each of these four variants forms a Σ = 3 coincident site lattice with respect to the SrTiO3 substrate and the cube-on-cube oriented BaTiO3. Heating for the same duration and temperature in a reducing gas atmosphere resulted in the formation of polycrystalline BaTiO3 with no preferred crystallographic orientation. The dependence on the gas atmosphere indicates that it may be possible to tune the annealing time, temperature, and atmosphere to produce a single crystalline BTO on STO by SPE or produce a desired distribution of orientations.

Crystallographic Rotation during Solid-Phase Epitaxy of SrTiO3 from Nanoscale Seed Crystals

The crystallization of amorphous complex oxide layers from isolated seed crystals presents an opportunity to remove geometric constraints posed by thin-film epitaxial growth methods employing single-crystal substrates. The crystallization processes initiated by a distribution of isolated nanoscale seeds occur in a state of mechanical stress that is different from planar thin-film epitaxy. The effects of this stress were probed in the crystallization of the model perovskite oxide SrTiO3 nucleated by isolated nanoscale SrTiO3 seeds. Synchrotron nanobeam scattering and diffraction were used to probe the spatial distribution of crystalline and amorphous SrTiO3. Contributions to the diffraction patterns from these components were identified using non-negative matrix factorization. Individual SrTiO3 crystallites exhibit a lattice rotation resulting from the density difference between amorphous and crystalline SrTiO3. Stress at the crystal–amorphous interface advancing from the seeds produces a rotation of the crystal lattice at a rate of tens of degrees per micron of crystallization in the plane of the film. The rate of the lattice rotation provides insight into the crystallization mechanism. The lattice rotation indicates that nanoscale morphological control during solid-phase epitaxial crystallization from nanocrystal seeds can be achieved by manipulating the interface stress between the amorphous and crystalline phases to control dislocation dynamics.

Solid-phase epitaxial crystallization of β-Ga2O3 thin film by KrF excimer laser irradiation from backside of NiO (111)-buffered α-Al2O3 (0001) substrate at room temperature

We have investigated the solid-phase epitaxial crystallization of β-Ga2O3 thin films on NiO (111) (3 nm thick)-buffered α-Al2O3 (0001) substrates prepared by pulsed KrF excimer laser irradiation onto amorphous Ga2O3 thin films from the backside of the substrate at room temperature. The results of x-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy indicated that the solid-phase crystallization of the amorphous Ga2O3 thin film to epitaxial β-Ga2O3 film started from the film/substrate interface toward the film surface with an average growth rate of about 0.1 nm thickness per one pulse irradiation. The optical bandgap of the epitaxial β-Ga2O3 thin films (∼80 nm thick) was estimated to be 4.9 eV from the UV/Vis transmittance measurement.

Solid phase epitaxy of silicon thin films by diode laser irradiation for photovoltaic applications

High temperature solid phase epitaxial crystallization of amorphous silicon layers prepared by electron beam evaporation is investigated. By using a continuous wave diode laser for heating the films rapidly (in milliseconds to seconds) this method is suitable on glass substrates with low temperature resistance. Therefore, the method is an economically advantageous technique of producing absorber layers for thin film solar cells. For the experiments 500nm of amorphous silicon was deposited on two different configurations of substrates. In the first one monocrystalline wafers of three different crystallographic orientations were used. In the second one a polycrystalline seed layer prepared on borosilicate glass served as substrate. The crystallization process was monitored in situ by time resolved reflectivity measurements. Depending on the crystal orientation 2s to 3s was needed for complete solid phase epitaxial crystallization of the amorphous films. The evolution of temperature during crystallization was simulated numerically.

Solid phase epitaxial regrowth of amorphous silicon is not affected by structural relaxation

A piece of amorphous silicon made by self-ion implantation was structurally relaxed by thermal annealing at 650°C; subsequently, half of the sample was re-implanted to bring it back to a non-relaxed state. The sample was next submitted to a series of low-temperature anneals (425–435°C) in order to induce solid phase epitaxial crystallization and the growth rate was measured on both halves of the sample. No difference was found within an uncertainty of ±3%, which would imply that the activation energy for solid phase epitaxial regrowth remains constant to within 2 meV under structural relaxation.

A high-temperature radiation-resistant rectifier based on p+-n junctions in 4H-SiC ion-implanted with aluminum

A combination of a high-dose (5 s- 1016 cm-2) implantation of Al ions into epitaxial n-type 4H SiC layers grown by chemical deposition from th e vapor phase and rapid (15 s) thermal annealing at 1700–1750°C has been used to form layers with a rectangular impurity profile according to the mechanism of solid-phase epitaxial crystallization. The combined effects of enhanced diffusion of radiation defects after implantation and gettering of defects during annealing bring about an improvement in the quality of the initial material, which ensures an increase in the diffusion length of the minority charge carriers by several times. Metastable states annealed within different temperature ranges are formed in SiC under the effect of irradiation with various particles. Low-temperature annealing of radiation defects increases the radiation and temporal lifetime of devices under irradiation. High-temperature annealing of radiation defects makes it possible to vary the lifetime of nonequilibrium charge carriers, i.e, vary the frequency range of devices. The radiation resistance of SiC-based devices increases as the operation temperature is increased to 500°C.

Stressed multidirectional solid-phase epitaxial growth of Si

The study of the solid-phase epitaxial growth (SPEG) process of Si (variously referred to as solid-phase epitaxy, solid-phase epitaxial regrowth, solid-phase epitaxial crystallization, and solid-phase epitaxial recrystallization) amorphized via ion implantation has been a topic of fundamental and technological importance for several decades. Overwhelmingly, SPEG has been studied (and viewed) as a single-directional process where an advancing growth front between amorphous and crystalline Si phases only has one specific crystallographic orientation. However, as it pertains to device processing, SPEG must actually be considered as multidirectional (or patterned) rather than bulk in nature with the evolving growth interface having multiple crystallographic orientations. Moreover, due to the increasingly ubiquitous nature of stresses presented during typical Si-based device fabrication, there is great interest in specifically studying the stressed-SPEG process. This work reviews the progress made in understanding the multidirectional SPEG and, more importantly, stressed multidirectional SPEG process. For the work reviewed herein, (001) Si wafers with ⟨110⟩-aligned, intrinsically stressed Si3N4/SiO2 patterning consisting of square and line structures were used with unmasked regions of the Si substrate amorphized via ion implantation. It is revealed that the stresses generated in the Si substrate from the patterning, both in line and square structures, alter the kinetics and geometry of the multidirectional SPEG process and can influence the formation of mask-edge defects which form during growth to different degrees as per differences in the substrate stresses generated by each type of patterning. Likewise, it is shown that application of external stress from wafer bending during SPEG in specimens with and without patterning can also influence the geometry of the evolving growth interface. Finally, the effect of the addition of SPEG-enhancing impurities during multidirectional stressed growth is observed to alter the evolution of the growth interface, thus suggesting that stress influences on growth are much less than those from dopants. Within the context of prior work, attempts are made to correlate the prior observations in single-directional stressed SPEG with the observations from patterned stressed SPEG reviewed herein. However, as is argued in this review, it ultimately appears that much of the research performed on understanding the single-directional stressed-SPEG process cannot be reasonably extended to the multidirectional stressed-SPEG process.

Modeling two-dimensional solid-phase epitaxial regrowth using level set methods

Modeling the two-dimensional (2D) solid-phase epitaxial regrowth (SPER) of amorphized Si (variously referred to as solid-phase epitaxial growth, solid-phase epitaxy, solid-phase epitaxial crystallization, and solid-phase epitaxial recrystallization) has become important in light of recent studies which have indicated that relative differences in the velocities of regrowth fronts with different crystallographic orientations can lead to the formation of device degrading mask edge defects. Here, a 2D SPER model that uses level set techniques as implemented in the Florida object oriented process simulator to propagate regrowth fronts with variable crystallographic orientation (patterned material) is presented. Apart from the inherent orientation dependence of the SPER velocity, it is established that regrowth interface curvature significantly affects the regrowth velocity. Specifically, by modeling the local SPER velocity as being linearly dependent on the local regrowth interface curvature, data acquired from transmission electron microscopy experiments matches reasonably well with simulations, thus providing a stable model for simulating 2D regrowth and mask edge defect formation in Si.

Dopant enhanced H diffusion in amorphous silicon and its effect on the kinetics of solid phase epitaxy

Dopant enhanced H infiltration into surface amorphous Si ( a-Si) layers formed by ion implantation is reported. Dopant enhanced H in-diffusion is observed for both B and P implant profiles in a-Si and the effect of this enhanced in-diffusion on the kinetics of solid phase epitaxial (SPE) crystallization is quantified. Near-surface dopant profiles were formed by multiple energy ion implantation. Anneals were performed in air over the temperature range 500–640 ° C. Secondary ion mass spectrometry (SIMS) analysis of partially annealed samples reveals that implanted dopants enhance the in-diffusion of H thereby increasing its concentration at depths well beyond the dopant implanted region. The effect of an enhanced H concentration on crystallization rates via SPE is monitored using time resolved reflectivity. Boron is found to enhance both H diffusion and SPE to a much greater extent than P. Possible links between the SPE and H diffusion mechanisms are discussed and the possibility of applying SPE models to H diffusion is explored. Implications for the formation of shallow junctions are also considered.

Stressed solid-phase epitaxial growth of ion-implanted amorphous silicon

The kinetics of stressed solid-phase epitaxial growth (SPEG), also referred to as solid-phase epitaxy, solid-phase epitaxial regrowth, solid-phase epitaxial recrystallization, and solid-phase epitaxial crystallization, of amorphous (α) silicon (Si) created via ion-implantation are reviewed. The effects of hydrostatic, in-plane uniaxial, and normal uniaxial compressive stress on SPEG kinetics are examined in intrinsic (0 0 1)Si. Particular emphasis is placed on unifying the results of different experiments in a single-stress-dependent SPEG model. SPEG kinetics are observed to suffer similar exponentially enhanced growth rates in hydrostatic and normal uniaxial compressive stress. However, there are discrepancies between researchers in terms of the influence of in-plane stress on growth rates. Two different stress-dependent SPEG models are thus advanced, each with different physical bases. The model advanced by Aziz et al. proposes SPEG can be modeled as a single-atomistic process while the model advanced by Rudawski et al. suggests that stress influences the nucleation and migration processes of growth differently and that SPEG cannot be modeled as a single step. The basis for the Rudawski et al. model is based on the crystal island and ledge migration model of SPEG advanced by others. Morphological instabilities of the growing α/crystalline interface with in-plane compression are also addressed within the context of both the Aziz et al. and Rudawski et al. models. Finally, using the Rudawski et al. model, it is possible to examine, characterize, and isolate the different atomistic processes during growth. Calculation of the activation energies for nucleation and migrations processes suggests that the activation energy of 2.7 eV observed for the growth rate in stress-free SPEG by Olsen and Roth is representative of the activation energy for the single-atomistic process of crystal island nucleation. Thus, the study of stressed SPEG provides a new atomistic picture of the nature of growth.

The effect of oxygen on segregation-induced redistribution of rare-earth elements in silicon layers amorphized by ion implantation

A model of segregation-induced redistribution of impurities of rare-earth elements during solid-phase epitaxial crystallization of silicon layers amorphized by ion implantation is developed. This model is based on the assumption that a transition layer with a high mobility of atoms is formed at the interphase boundary on the side of a-Si; the thickness of this layer is governed by the diffusion length of vacancies in a-Si. The Er concentration profiles in Si implanted with both erbium and oxygen ions are analyzed in the context of the model. It shown that, in the case of high doses of implantation of rare-earth ions, it is necessary to take into account the formation of Rm clusters (m = 4), where R denotes the atom of a rare-earth element, whereas, if oxygen ions are also implanted, formation of the complexes ROn (n = 3–6) should be taken into account; these complexes affect the transition-layer thickness and segregation coefficient.

The effect of potassium on the rate of solid phase epitaxy in silicon

Rutherford backscattering spectroscopy and ion channeling and secondary ion mass spectroscopy have been used to study the evolution of a potassium profile in amorphous silicon during solid phase epitaxial crystallisation. The potassium profile exhibits some propensity for refinement in front of the crystallising interface for the concentrations used in this experiment. Also, the potassium is partially substitutional in the crystallised layer. Preliminary studies on the influence of potassium on the rate of solid phase epitaxy (SPE) in silicon are presented. Surface amorphous films doped with potassium were crystallised and the rate of epitaxy was monitored using time resolved reflectivity. The inclusion of potassium was found to retard the rate of SPE. These results indicate that potassium doped amorphous films may be helpful in efforts to further refine present models of SPE in silicon.

Study of ion beam induced epitaxial crystallization of SrTiO3

Amorphous SrTiO3 on single crystal SrTiO3 (100) has been crystallized by He+, Ne+, or Ar+ ions with energy of 200 keV–2 MeV at a substrate temperature of 100–250 °C. Rutherford backscattering spectrometry in channeling geometry and x-ray diffraction were used to evaluate the crystallization. Ion-beam-induced epitaxial crystallization (IBIEC) of SrTiO3 was confirmed and the activation energy of IBIEC observed was about 0.1–0.3 eV, a value about 1/10 relative to thermal solid phase epitaxial crystallization. The observed IBIEC seems to be consistent with previously proposed models in which IBIEC is dominated by point defects produced by ion irradiation and their migration to the amorphous/crystal (a/c) interface. The IBIEC mechanism and point defect behavior are discussed by the use of simple models taking into account the rate limiting processes of IBIEC for both point defect diffusion and atomic rearrangement at a/c interface.

Holmium redistribution upon solid-phase epitaxial crystallization of amorphized silicon layers

Concentration profiles of Ho were investigated after annealing at a temperature of 620°C of silicon layers implanted with 1-MeV Ho+ ions to doses of 1−3×1014 cm−2 exceeding the amorphization threshold, as well as with O+ ions with energies that ensure the coincidence of the concentration maxima of implanted impurities and doses that are greater by an order of magnitude than those of Ho+. The crystallization of the amorphized silicon layer occurs by the mechanism of solid-phase epitaxy. The main features of the segregation redistribution of Ho are shown to be similar to the previously studied segregation behavior of Er. An increase in the Ho concentration at the initial stage of the solid-phase epitaxial crystallization is explained by the small rate of mass transfer through the amorphous layer-single crystal interface. An analytical expression was obtained to describe the variation of the segregation coefficient in the process of solid-phase epitaxial crystallization, including its initial stage, which allows the calculation concentration profiles of rare-earth elements.

Influence of the orientation of the silicon substrate on the properties of avalanche Si:Er:O light-emitting structures

The influence of the orientation of silicon on the structural and luminescence properties of avalanche light-emitting diodes fabricated by the coimplantation of erbium and oxygen followed by solid-phase epitaxial (SPE) crystallization of the amorphized layer is considered. The luminescence properties are a consequence of the formation of various structural defects during the SPE crystallization: V-shaped dislocations and erbium precipitates form in (100) Si:Er:O layers, and larger structural defects, i.e, twins, are observed in (111) Si:Er:O layers along with an increase in the dislocation density by more than four orders of magnitude in comparison with the (100) orientation. The luminescence properties of avalanche and tunnel light-emitting diodes are also compared. In contrast to tunnel diodes, in avalanche diodes erbium ions are excited in the entire space-charge layer, and the effective excitation cross section of the Er3+ ions and their lifetime in the excited state are 3–4 times larger.

Redistribution of erbium during the crystallization of buried amorphous silicon layers

The redistribution of Er during its implantation in silicon at doses close to the amorphization threshold and its subsequent solid-phase epitaxial (SPE) crystallization is investigated. The formation of a buried amorphous (a) layer is discovered at Er doses equal to 5×1013 and 1×1014 cm−2 using Rutherford backscattering. The segregation of Er in this case takes place inwardly from the two directions corresponding to the upper and lower boundaries of the buried αlayer and leads to the formation of a concentration peak at the meeting place of the two crystallization fronts. A method for calculating the coordinate dependence of the segregation coefficient k from the distribution profiles of the erbium impurity before and after annealing is proposed. The k(x) curve exhibits a drop, whose width increases with decreasing Er implantation dose. Its appearance is attributed to the nonequilibrium nature of the segregation process at the beginning of SPE crystallization.

Effect of ion implantation on redistribution of erbium during solid-phase epitaxial crystallization of silicon

It is known that Er-ion implantation and O-ion coimplantation into an amorphized Si layer affect the final Er concentration profile when the layer is subjected to solid-phase epitaxial (SPE) crystallization. This paper discusses how this effect depends on dose, energy, temperature, and the parameters of the segregation model, i. e., the transition layer width L and the coordinate dependence of the segregation coefficient k(x). Increasing the Er implantation dose as well as decreasing the implantation energy and temperature cause L to decrease and the segregation coefficient k to increase more rapidly at the initial stage of SPE crystallization. These phenomena could be due to increased defect formation in the amorphous implanted layer. Additional O coimplantation leads to similar changes in L and k(x), due to Er-O complex formation.

Nickel distribution in crystalline and amorphous silicon during solid phase epitaxy of amorphous silicon

Solid phase epitaxy of nickel-doped amorphous silicon (a-Si) films on crystalline silicon (c-Si) substrates has been investigated. The crystallization mode of the a-Si films depends strongly on the nickel concentration. Below ∼5×1018 Ni/cm3, redistribution of nickel into the c-Si substrate occurs and the regrowth process is controlled by an “ordinary” thermally activated solid phase epitaxial crystallization (SPEC) process. In contrast, above 5×1018 Ni/cm3 segregation of Ni in the a-Si films is observed and the silicon crystallization is driven by impurity enhanced SPEC and/or silicide mediated crystallization in the bulk of the amorphous film. The redistribution of Ni during crystallization can be described in terms of an effective distribution coefficient accounting for the actual crystallization rate of a-Si and the Ni diffusivity in a-Si. Further, dissolution of implantation-induced (311) defects is observed in the samples exhibiting Ni diffusion into the crystalline phase during ordinary SPEC. This indicates annealing and /or suppression of the (311) defects due to nickel accumulating in the region of end-of-range defects.

Model of the redistribution of erbium during the solid-phase epitaxial crystallization of silicon

A quantitative model of the redistribution of rare-earth-ion impurities during the solid-phase epitaxial crystallization of Si layers amorphized by implantation is developed. The parameters of the model include the segregation coefficient k and the width of the transition layer. The movement of the crystallization front toward the surface is accompanied by an increase in the segregation coefficient at a rate which can be characterized by the ratio of the thickness of the recrystallization layer to the width of the transition layer. The increase in k is attributed to defect accumulation in the transition layer. In the case of a thin Er-containing amorphous layer, the segregation coefficient does not reach k=1, because the impurity is driven back toward the surface. In the case of a thicker Er-containing layer, the segregation coefficient exceeds k=1 and prevents the accumulation of impurity atoms near the surface.

Hydrogenation of amorphous silicon prepared by ion implantation

Hydrogenation effects on structural relaxation and crystallization processes of non-hydrogenated amorphous silicon (a-Si) prepared by ion implantation are investigated using Raman scattering and positron lifetime measurements. It has been found that hydrogenation enhances agglomeration of vacancy-type defects which in turn accelerates the crystallization processes. Based on these results, we propose a model in which larger vacancy-type defects, such as microvoids, formed during relaxation processes of a-Si are dissociated into mono- and divacancies at the amorphous/crystalline interface resulting in enhancement of solid phase epitaxial crystallization.