Solid Phase Epitaxial Recrystallization Research Articles

Highly 28Si enriched silicon by localised focused ion beam implantation

Solid-state spin qubits within silicon crystals at mK temperatures show great promise in the realisation of a fully scalable quantum computation platform. Qubit coherence times are limited in natural silicon owing to coupling to the 29Si isotope which has a non-zero nuclear spin. This work presents a method for the depletion of 29Si in localised volumes of natural silicon wafers by irradiation using a 45 keV 28Si focused ion beam with fluences above 1 × 1019 ions cm−2. Nanoscale secondary ion mass spectrometry analysis of the irradiated volumes shows residual 29Si concentration down to 2.3 ± 0.7 ppm and with residual C and O comparable to the background concentration in the unimplanted wafer. After annealing, transmission electron microscopy lattice images confirm the solid phase epitaxial re-crystallization of the as-implanted amorphous enriched volume extending over 200 nm in depth.

Atomistic simulations on the relationship between solid-phase epitaxial recrystallization and self-diffusion in amorphous silicon

Recent experimental results on self-diffusion (SD) in amorphous silicon (a-Si) [Kirschbaum et al., Phys. Rev. Lett. 120, 225902 (2018)] indicate that the atomic mechanism of this process is akin to that of solid-phase epitaxial recrystallization (SPER). In the present work, this relationship is investigated using classical molecular dynamics (MD) simulations with selected interatomic potentials. In the beginning, an overview of the status of the present knowledge on SPER and SD is given. Then, it is shown that the Stillinger–Weber (SW)-type and Tersoff (T)-type potentials considered yield structural data of a-Si, which are in rather good agreement with measurements. On the other hand, deviations are found for thermal properties. The results of partially extremely long MD calculations of SPER and SD yield that both processes can be described by a simple Arrhenius relation and that the activation enthalpies of SPER and SD are rather equal, which is in qualitative agreement with experiments. Obviously, the simulated atomic-level processes are very similar. However, for the known SW- and T-type interatomic potentials, a quantitative agreement with SPER and SD measurements cannot be found. This work demonstrates that significant improvements can be achieved if SW-type potentials with an increased value of the three-body parameter are used.

Effect of Solid-State Epitaxial Recrystallization on Defect Density in Ultrathin Silicon-on-Sapphire Layers

The main technological problem in the manufacture of electronics on silicon-on-sapphire (SOS) structures is the high density of defects in silicon-on-sapphire layers. The modern method of obtaining ultrathin SOS structures using solid-phase epitaxial recrystallization and pyrogenic thinning can significantly reduce the defectiveness in these layers. Nevertheless, the effect of the defectiveness of submicron SOS layers on the structural perfection of ultrathin layers remains unclear. In this work, ultrathin (100 nm) SOS structures have been obtained on submicron (300 nm) SOS structures with different structural quality. The crystallinity of 300 nm layers before the recrystallization process and ultrathin layers has been determined using X-ray diffraction and transmission electron microscopy. It has been found that the lowest values of the full width at half maximum (FWHM) of 0.19°–0.20° have been observed for an ultrathin SOS structure obtained based on the most structurally perfect SOS layers of 300 nm. It has been shown that a more perfect near-surface layer of the basic SOS structure of 300 nm and a double implantation regime make it possible to reduce the density of structural defects in the ultrathin Si layer by an order of magnitude to achieve ~1 × 104 cm–1.

Влияние твердофазной рекристаллизации с двойной имплантацией на плотность структурных дефектов в ультратонких слоях кремния на сапфире

The main technological problem in the manufacture of electronics on silicon-on-sapphire (SOS) wafers is the high density of defects in the epitaxial silicon layer. The modern method of obtaining ultrathin SOS wafers using solid-phase epitaxial recrystallization (SPER) and pyrogenic thinning that significantly reduce the defect density in these layers. Nevertheless, the influence of the defect density in submicron SOS layers on the structural quality of ultrathin SOS layers remains unclear. In this work, ultrathin (100 nm) SOS wafers were obtained by SPER of submicron (300 nm) SOS wafers with different structural quality. The crystallinity of 300 nm layers before the recrystallization process and ultrathin layers was determined by XRD and TEM. It was found that the smallest values of the FWHM 0.19-0.20° were observed for the ultrathin SOS layers obtained on the basis of 300 nm SOS wafers with the best structural quality. It was shown that the structural perfect near-surface Si layer, which serves as a seed layer in SPER process, and the double implantation regime allow to reduce the linear defect density in the ultrathin SOS layers by ~ 1×104 cm-1.

Molecular-dynamics simulations of solid phase epitaxy in silicon: Effects of system size, simulation time, and ensemble

Solid phase epitaxial (SPE) recrystallization of amorphous Si on a Si(001) substrate was examined by large-scale (6144–129024 Si atoms), long-time (up to 2000 ns) molecular-dynamics (MD) simulations using the empirical Tersoff interatomic potential. We particularly focused on the effects of the MD cell size, simulation time, and ensemble on the SPE growth rate. We found that the simulations under the isothermal–isochoric conditions (NVT ensemble) show a higher crystallization rate than those under the isothermal–isobaric conditions (NPT ensemble). The system size dealt with in the present MD simulation, i.e., >6144 Si atoms, was enough to estimate the SPE growth rate. The Arrhenius plot of the growth rate between 1300 and 1600 K exhibited a single activation energy, ∼2.4 eV, which is in agreement with the experimental value (∼2.7 eV). However, the growth rate at temperatures below 1300 K deviated from the extrapolated ones from 1300 to 1600 K, which is because recrystallization does not reach a steady state: long-time MD simulations are required to estimate the growth rate at low temperature. The structure analysis of amorphous/crystalline interfaces suggested that the braking of atomic bonds parallel to the interface becomes a rate-limiting step of the SPE growth.

Self-Diffusion in Amorphous Silicon by Local Bond Rearrangements.

Experiments on self-diffusion in amorphous silicon (Si) were performed at temperatures between 460 to 600° C. The amorphous structure was prepared by Si ion implantation of single crystalline Si isotope multilayers epitaxially grown on a silicon-on-insulator wafer. The Si isotope profiles before and after annealing were determined by means of secondary ion mass spectrometry. Isothermal diffusion experiments reveal that structural relaxation does not cause any significant intermixing of the isotope interfaces whereas self-diffusion is significant before the structure recrystallizes. The temperature dependence of self-diffusion is described by an Arrhenius law with an activation enthalpy Q=(2.70±0.11) eV and preexponential factor D_{0}=(5.5_{-3.7}^{+11.1})×10^{-2} cm^{2} s^{-1}. Remarkably, Q equals the activation enthalpy of hydrogen diffusion in amorphous Si, the migration of bond defects determining boron diffusion, and the activation enthalpy of solid phase epitaxial recrystallization reported in the literature. This close agreement provides strong evidence that self-diffusion is mediated by local bond rearrangements rather than by the migration of extended defects as suggested by Strauß etal. (Phys. Rev. Lett. 116, 025901 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.025901).

Study on Ultra Shallow Junction n-MOS with 350°C Microwave Annealing for Activation of Phosphorus Dopants in Germanium

In this study, low-energy microwave annealing is used to fabricate ultra-shallow junctions for less than 20 nm IC node applications in a potential material: Germanium (Ge). Germanium is a new promising material and might replace silicon in the future. A novel microwave annealing technique with two steps for the solid phase epitaxial recrystallization (SPER) and phosphorus dopants activation was applied to Germanium. The purpose of the first step, microwave annealing of 1.2 kW for 75 s, is to re-grow the amorphous layer and repair the destroyed crystal lattices caused by ion implantation. In the second-step of annealing, low-energy microwave, 0.9 kW (≈350°C) for 300 s, is used to activate the phosphorus dopants effectively without diffusion and de-activation. The target dopant activation concentration level will achieve 10²⁰ cm^-3. The sheet resistance will decrease to 78 ohm/sq. The low resistance after activating is reflected to the performance of the n-MOS device fabricated by the Ge substrate. The S. S. factor is 900 mV/dec, and the I_on/off ratio rise to 2.6×10¹. This has demonstrated that the two-step MWA has excellent control, significantly regarding leakage current.

Atomic transport during solid-phase epitaxial recrystallization of amorphous germanium

The atomic mixing of matrix atoms during solid-phase epitaxy (SPE) is studied by means of isotopically enriched germanium (Ge) multilayer structures that were amorphized by Ge ion implantation up to a depth of 1.5 μm. Recrystallization of the amorphous structure is performed at temperatures between 350 °C and 450 °C. Secondary-ion-mass-spectrometry is used to determine the concentration-depth profiles of the Ge isotope before and after SPE. An upper limit of 0.5 nm is deduced for the displacement length of the Ge matrix atoms by the SPE process. This small displacement length is consistent with theoretical models and atomistic simulations of SPE, indicating that the SPE mechanism consists of bond-switching with nearest-neighbours across the amorphous-crystalline (a/c) interface.

Defect formation and recrystallization mechanisms in silicon-on-sapphire films under ion irradiation

The effect of the parameters (energy, dose) of the irradiation of silicon-on-sapphire (SOS) structures with ions Si+ ions on the quality of the silicon-film crystal structure after solid-phase epitaxial recrystallization and annealing is studied. It is shown that the most efficient mechanism of crystal-structure recovery is recrystallization from the silicon surface layer which is a seed.

The formation, stability, and suitability of n-type junctions in germanium formed by solid phase epitaxial recrystallization

Design and optimization of n-type doped regions in germanium by solid phase epitaxial recrystallization (SPER) have been studied by the authors. A systematic study is presented of process variables that influence activation and thermal stability, including preamorphization, coimplants, recrystallization temperature, and postrecrystallization thermal treatments. Unlike silicon, activation after recrystallization in germanium is not optimum where the postrecrystallization thermal budget is kept to a minimum. With the aid of modeling, a maximum peak activation of 7×1019 cm−3 was extracted. A steady increase in sheet resistance during postrecrystallization anneals confirms the formation of metastable activation by SPER. It is predicted that active concentrations of 6–8×1019 cm−3 are sufficient to meet targets for sub-20 nm technologies.

Amorphization and graphitization of single-crystal diamond — A transmission electron microscopy study

The amorphization and graphitization of single-crystal diamond by ion implantation were explored using transmission electron microscopy (TEM). The effect of ion implantation and annealing on the microstructure was studied in (100) diamond substrates Si + implanted at 1 MeV. At a dose of 1 × 10 15 cm − 2, implants done at 77 K showed a damage layer that evolves into amorphous pockets upon annealing at 1350 °C for 24 h whereas room temperature implants (303 K) recovered to the original defect free state upon annealing. Increasing the dose to 7 × 10 15 Si +/cm 2 at 303 K created an amorphous-carbon layer 570 ± 20 nm thick. Using a buried marker layer, it was possible to determine that the swelling associated with the amorphization process was 150 nm. From this it was calculated that the layer while obviously less dense than crystalline diamond was still 15% more dense than graphite. Electron diffraction is consistent with the as-implanted structure consisting of amorphous carbon. Upon annealing, further swelling occurs, and full graphitization is achieved between 1 and 24 h at 1350 °C as determined by both the density and electron diffraction analysis. No solid phase epitaxial recrystallization of diamond is observed. The graphite showed a preferred crystal orientation with the (002)g//(022)d. Comparison with Monte Carlo simulations suggests the critical displacement threshold for amorphization of diamond is approximately 6 ± 2 × 10 22 vacancies/cm 3.

Atomistic simulation of amorphous germanium and its solid phase epitaxial recrystallization

Amorphous Ge and its recrystallization are investigated by molecular-dynamics simulations using a Stillinger-Weber-type interatomic potential. Unlike previously used parametrizations of this potential the parameter set employed in this work yields a reasonable description of all condensed phases of Ge. The preparation of amorphous Ge is performed by cooling from the molten state. Structural and thermal properties of the amorphous phase such as the pair correlation function, the atomic density, as well as the melting temperature are calculated and a good agreement with experimental data is found. In order to obtain the initial atomic configuration for the simulation of recrystallization of amorphous Ge, a simulation cell that contains an amorphous and a crystalline layer is carefully prepared by melting a part of a primarily crystalline simulation cell and by cooling the liquid in a similar manner as in the preparation of bulk amorphous Ge. The recrystallization is simulated in the temperature range between 600 and 950 K. The simulation cell is built in such a manner that the main regrowth direction is parallel to [100]. Using an efficient characterization method the configuration of the current amorphous-crystalline interface, its average position with respect to the [100] direction, and its roughness given by the rms deviation of this position are determined throughout the simulations. Consistently with former models for Si it is found that recrystallization of amorphous Ge occurs mainly at small {111} facets and is characterized by a sequential local rearrangement of atomic bonds and positions. In very good agreement with experiments the dependence of the velocity of solid phase epitaxial recrystallization on temperature can be approximated by a straight line in an Arrhenius plot. However, the absolute value of the velocity is too high compared to the experimental data. The main reason for this discrepancy may be the overestimation of the flexibility of atomic bonds by the present interatomic potential which leads to an underestimation of the activation energy. Similar to the state of the art in atomistic simulations of solid phase epitaxial regrowth in Si, there is not yet a suitable interatomic potential which allows a consistent quantitative modeling of both the condensed phases and the solid phase epitaxial recrystallization.

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.

High-resolution X-ray diffraction by end of range defects in self-amorphized Ge

In this study we detected the positive perpendicular strain ( ɛ ⊥) due to end of range (EOR) defects formed in Ge amorphized with 300 keV, 2.5 × 10 15 Ge/cm 2 at liquid nitrogen temperature by means of high-resolution X-ray diffraction. We found that, after complete solid phase epitaxial recrystallization of the amorphous layer (about 1 h at 340 °C), only 2% of the original ɛ ⊥ survives. This strain completely disappears after 270 min at 405 °C. On the other hand, after this more aggressive annealing, a thin negatively strained layer appears just below the surface. The whole set of data is discussed and compared with existing literature.

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.

Maximum Active Concentration of Ion-Implanted Phosphorus During Solid-Phase Epitaxial Recrystallization

In this paper, we showed that the maximum active P concentration of approximately 2 times1020 cm-3 exists during solid-phase epitaxial recrystallization (SPER). This maximum active concentration is close to the reported values for other active impurity concentrations during SPER. We introduced the concept of an isolated impurity that has no neighbor impurities with a certain lattice range. Assuming that impurities interact with three or four neighbor impurities, we can explain the activation phenomenon during SPER. According to our model, the isolated P concentration N iso has a maximum value of approximately 2 times1020 cm-3 at a total impurity concentration of approximately 1021 cm-3, and it decreases with a further increase in total impurity concentration. Deactivation occurs after the completion of SPER with increasing annealing time, and the active impurity concentration decreases with time but is always higher than the maximum diffusion concentration N Diff max. We also observed that N Diff max is independent of the annealing time despite nonthermal activation in the high-concentration region. We evaluated the dependence of N Diff max on annealing temperatures. We think that this N Diff max can be regarded as the electrical solid solubility N Esol that the active impurity concentration reaches in thermal equilibrium. We observed the transient enhanced diffusion (TED) after the completion of SPER, and that, the deactivation process continues during and after TED, and the corresponding diffusion coefficient is still much higher than that in thermal equilibrium even after TED has finished, which suggests that the deactivation process releases point defects.

Electron-beam induced recrystallization in amorphous apatite

Electron-beam induced recrystallization of irradiation-induced amorphous Sr2Nd8(SiO4)6O2 is investigated in situ using transmission electron microscopy with 200keV electrons at room temperature. Epitaxial recrystallization is observed from both the amorphous/crystalline interface and the surface, and the recrystallization is more pronounced with increasing electron-beam flux. Since the temperature increase induced by electron-beam irradiation is estimated to be less than 7K and maximum energies transferred to target atoms are below the displacement energies, ionization-induced processes are considered to be the primary mechanisms for the solid-phase epitaxial recrystallization observed in the present study.

HRXRD studies of strain relaxation in ion-implanted strained Si on relaxed Si1−xGex

The relaxation process of ion-implanted strained silicon films grown on silicon-rich relaxed Si1−xGex alloys was studied to determine the critical strain regime necessary for the breakdown of solid phase epitaxial recrystallization. Experimental structures were grown via molecular beam epitaxy (MBE) and contained a 50nm strained silicon capping layer on relaxed Si1−xGex. The relaxed Si1−xGex alloy compositions range from 0 to 30at.% germanium. A 12keV Si+ implant at a fluence of 1×1015atoms/cm2 was used to generate an amorphous layer ∼30nm thick, confining it to the strained silicon cap. The degree of relaxation of the silicon cap layer was quantified by high-resolution X-ray diffraction (HRXRD) omega–2theta rocking curves and reciprocal space maps. Maps were acquired for the (004) and (113) reflections to obtain the in and out-of-plane lattice parameter of the layers. Upon annealing, the solid phase regrowth (SPER) process broke down for the highest level of strain. Additionally, regrowth related defects were observed in these samples using cross-sectional transmission electron microscopy (XTEM). These results indicate a reduction of strain in the Si0.7Ge0.3 samples occur as a result of SPER breakdown that generated dislocations and stacking faults throughout the silicon capping layer.

Solid Phase Recrystallization and Strain Relaxation in Ion-Implanted Strained Si on SiGe Heterostructures

AbstractThe relaxation process of strained silicon films on silicon-rich relaxed SiGe alloys has been studied. Experimental structures were grown via Molecular Beam Epitaxy (MBE) growth techniques and contain a strained silicon capping layer approximately 50 nm thick. The relaxed SiGe alloy compositions range from 0 to 30 at.% germanium. A 12 keV Si+ implant at a dose of 1×1015 atoms/cm2 was used to generate an amorphous layer ∼30 nm thick, which was confined within the strained silicon capping layer. Upon annealing at 500 °C, it was found that the solid phase epitaxial regrowth process of the amorphous silicon breaks down for high strain levels and regrowth related defects were observed in the regrown layer. In addition, high-resolution X-Ray diffraction results indicate a reduction in strain for the silicon capping layer. This study addresses the critical strain regime necessary for the breakdown of solid phase epitaxial recrystallization in silicon.