Mechanism of the 2H → 6H solid state transformation in SiC

Single crystals of 2H SiC transform directly into the 6H (ABCACB) structure when the transformation nucleates at temperatures above 2000°C. The 2H close-packed structure may be transformed to the 6H structure by displacing every third layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H to 6H structural transformation by such a layer displacement mechanism has been developed. The fact that all the observed solid state transformations in SiC crystals commence with the random insertion of stacking faults and then proceed further to create a statistically ordered structure, permits such a theory to be developed. Exact expressions for the diffracted intensity from such crystals have been obtained and the different diffraction effects observable on single crystal X-ray photographs predicted. A comparison of the theoretically predicted diffraction effects with those visible on the X-ray photographs of SiC crystals undergoing the 2H to 6H transformation shows that the structural transformations in SiC occur by the layer displacement mechanism.     

X-ray investigation of the mechanism of phase transformation in single crystals of ZnS,ZnxCd1−xS and ZnxMn1−xS (I). Calculation of diffraction effects by a three parameter model

Hexagonal close-packed (2H) single crystals of ZnS, Zn_xCd_1−xS and Zn_xMn_1−xS are known to undergo solid state transformation to the cubic close-packed (3C) and 6H-structures on annealing at elevated temperatures. The transformations occur by the non-random nucleation of stacking faults on individual close-packed layers parallel to (0001). The nature of the faults involved and their probability distribution during transformation determine the diffraction effects produced along the 10. L reciprocal lattice row by a crystal quenched in an intermediate state of transformation. We have investigated the mechanism of the transformation by comparing the diffraction effects recorded from such crystals on a single crystal diffractometer, with those calculated for an assumed model of the transformation. It is known that in these materials the faults involved in the transformation are deformation faults. To explain the observed diffraction effects we develop a three parameter theoretical model employing a fult probability α for the radom insertion of a deformation fault in the 2H structure, a fault probability β for the deformation faults to occur at three layer separations and a fault probability γ of their occcurrence at 2-layer separations. The probability α corresponds to the development of a fresh nucleus, the probability β to the growth of the 6H nucleus and the probability γ to the growth of a 3C nucleus. This paper develops the necessary theory of X-ray scattering for such a model of the transformation and predicts the diffraction effects for different values of α, β, and γ. The next paper compares these results with experimental observations.     

A model for the growth of anomalous polytype structures in vapour grown SiC

A number of polytype structures observed in vapour grown SiC crystals have a unit-cell which is an integral multiple of the unit-cell of the basic 6H, 15R or 4H structure. The growth of such anomalous structures cannot be understood in terms of spiral growth round a single screw dislocation in a basic matrix. However many of these polytype crystals display a single growth spiral on their (0001) face indicating that they have resulted from spiral growth round a single screw dislocation. It is shown that this anomaly can be resolved if the basic matrix is assumed to contain stacking faults near the surface at the time of the origin of the screw dislocation ledge. This possibility, overlooked in the earlier deduction of polytype structures, must be taken into consideration since vapour grown SiC crystals frequently contain a high concentration of random stacking faults, producing continuous streaks on their X-ray diffraction photographs. The most probable fault configurations that can occur in 6H, 15R and 4H structures of SiC have been deduced from a calculation of their stacking fault energy. These fault configurations are then considered to lie at different distances from the surface at the time of the origin of a screw dislocation ledge. Such a faulted ledge gives rise to polytype structures during subsequent spiral growth even if the screw dislocation has an integral Burgers vector. The most probable series of polytype structures that can result from such a faulted matrix model are deduced. It is shown that nearly all the polytype structures of SiC hitherto regarded as anomalous (such as 36H, 54H, 66H, 45R, 90R etc.) are among the expected structures and there is no need to postulate a complicated configuration of cooperating dislocations to account for their growth.     

Modeling of layered structures with stacking faults

A method for modeling unit cells of layered structures containing stacking faults is considered by the example of silicon carbide. A rotating-crystal pattern is calculated by the proposed method for silicon carbide with pseudorandom violation of the sequence of close-packed layers. The results of the calculation show that the intensity and shape of reflections of an X-ray diffraction pattern of a layered structure is determined by the configuration of stacking faults.     

Stacking faults in semi‐polar 6H‐SiC single crystals

6H‐SiC single crystals have been successfully grown on (1015) plane seed by sublimation method. High density stacking faults (SFs) were observed by transmission synchrotron radiation X‐ray topography. Based on the invisibility criteria of stacking faults, the displacement vectors of most SFs were determined to be the type of 1/6[1120]. Laser scanning confocal microscopy (LSCM) was used to observe the etching morphology of (0115) wafer. The etching steps of SFs were found and their density decreased from 3.6×10³ cm^‐1 to 2.0×10² cm^‐1 along the <0001> projection direction. The inclination angles of the SFs etching step plane to (10‐15) plane were measured by line scanning of LSCM. It was found that the inclination angles decreased from 20° to 10° along the <0001> projection direction. Different etching characteristics of SFs along radial direction of 6H‐SiC (1015) wafer should be attributed to different displacement vectors and different stacking fault energies for these stacking faults. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal growth of mixed AlN–SiC bulk crystals

Bulk AlN–SiC mixed single crystals are prepared by sublimation growth employing pure AlN or mixed AlN–SiC sources and 6H-SiC seed crystals. As the growth temperature is increased from 1900 to 2050 °C, using seeds with different off-axis orientations, inclined up to 42° from the basal plane toward the (0 1 –1 0)-plane, or using different source materials, crystals with different Si/C contents are obtained. Dependent on the Si and/or C content, crystal coloration changes from yellowish to greenish to blackish. Modification in crystals’ coloration and corresponding changes in below band-gap optical absorption and cathodoluminescence spectra are discussed.     

Modelling diffraction patterns from one‐dimensionally disordered structures of AIIBVI crystals by Monte Carlo Technique IV. The 6H structure with different kinds of stacking faults

To investigate the one‐dimensionally disordered structures (ODDS) in the close packed (cp) crystals, the Monte Carlo computer simulation technique has been applied. Calculations of the diffraction intensity distributions along the 10.L reciprocal lattice row from the 6H(33) structure with the four different kinds of the stacking faults (SFs): growth, deformation, layer displacement and extrinsic fault are presented. In particular, using the simple frequency functions of the fault to fault distances, both random and non‐random distributions of the SFs are considered. Distinctive features of the diffraction patterns corresponding to the chosen examples of the transformations from the parent 6H(33) structure into another small‐period polytypes are discussed in detail. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An X-ray investigation of growth and deformation faults in a vapour grown 2H SiC crystal

Single crystals of 2H SiC grown by hydrogen reduction of methyltrichlorosilane at 1400°C frequently contain a high concentration of random stacking faults in their hexagonal closepacked | AB | AB | … structure. This given rise to diffuse streaks along reciprocal lattice rows parallel to the c¹ axis for h ? k ≠ 0 mod 3. To investigate the nature of stacking faults in these crystals, the intensity distribution along the 10. l reciprocal lattice row of a 2H SiC crystal was recorded on a 4-circle computer-controlled single crystal diffractometer. The halfwidths of 10. l reflections with l even and l odd were found to be 0.36 and 0.24 reciprocal units respectively. It is observed (i) that the 10. l reflections with l even are highly broadened and (ii) that the halfwidths of l even and l odd reflections are in the ratio of 3 : 2. This suggests that the stacking faults present are predominantly growth and deformation faults. Since the fault concentration is very high, exact theoretical expressions for the halfwidths of 10. l reflections were used to calculate the growth and deformation fault probabilities (α and β) from the observed half widths, without neglecting the second and higher order terms in α and β. It is found that α = 0.11 and β = 0.20. The deformation fault probability (β) is surprisingly high for hard and brittle material like SiC which does not undergo plastic deformation easily. It is suggested that several deformation fault configurations have resulted from a clustering of growth faults.     

The effects of annealing on non-polar (1 1 2¯ 0) a-plane GaN films

Non-polar a-plane (1 1 2¯ 0) GaN films were grown on r-plane sapphire by metal–organic vapor phase epitaxy and were subsequently annealed for 90 min at 1070 °C. Most dislocations were partial dislocations, which terminated basal plane stacking faults. Prior to annealing, these dislocations were randomly distributed. After annealing, these dislocations moved into arrays oriented along the [0 0 0 1] direction and aligned perpendicular to the film–substrate interface throughout their length, although the total dislocation density remained unchanged. These changes were accompanied by broadening of the symmetric X-ray diffraction 1 1 2¯ 0 ω-scan widths. The mechanism of movement was identified as dislocation glide, occurring due to highly anisotropic stresses (confirmed by X-ray diffraction lattice parameter measurements) and evidenced by macroscopic slip bands observed on the sample surface. There was also an increase in the density of unintentionally n-type doped electrically conductive inclined features present at the film–substrate interface (as observed in cross-section using scanning capacitance microscopy), suggesting out-diffusion of impurities from the substrate along with prismatic stacking faults. These data suggest that annealing processes performed close to film growth temperatures can affect both the microstructure and the electrical properties of non-polar GaN films.     

Annealing effect on electrical and photoconductive properties of Si implanted GaSe single crystal

GaSe single crystals grown by Bridgman method have been doped by ion implantation technique. The samples were bombarded in the direction parallel to c‐axis by Si ion beam of about 100 keV to doses of 1 × 10¹⁶ ions/cm² at room temperature. The effects of Si implantation with annealing at 500 and 600 °C on the electrical properties have been studied by measuring the temperature dependent conductivity and photoconductivity under different illumination intensities in the temperature range of 100–320 K. It is observed that Si implantation increases the room temperature conductivity 10⁻⁷ to 10⁻³ (Ω‐cm)⁻¹ depending on the post annealing temperature. The analysis of temperature dependent conductivity shows that at high temperature region above 200 K, the transport mechanism is dominated by thermal excitation in the doped and undoped GaSe samples. At lower temperatures, the conduction of carriers is dominated by variable range hopping mechanism in the implanted samples. Annealing of the samples at and above 600 °C weakens the temperature dependence of the conductivity and photoconductivity. This indicates that annealing of the implanted samples activates Si‐atoms and increases structural deformations and stacking faults. The same behavior was observed from photoconductivity measurements. Hence, photocurrent‐illumination intensity dependence in the implanted samples obeys the power low I_pc ∝ Φⁿ with n between 1 and 2 which is an indication of continuous distribution of localized states in the band gap. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of one‐dimensionally disordered structures of AIIBVI crystals by Monte Carlo technique III. 4H structure with different kinds of stacking faults

To investigate one‐dimensionally disordered (ODD) structures in close packed (cp) crystals, the Monte Carlo computer simulation technique has been applied. Calculations of diffraction intensity distributions along the 10.L reciprocal lattice row from 4H structure with four different kinds of stacking faults (SFs): growth, deformation, layer displacement and extrinsic fault are presented. In particular, using simple frequency function of fault to fault distances, both random and non‐random distributions of SFs are considered. Distinctive features of the diffraction patterns corresponding to the chosen examples of transformations from the parent 4H structure into another small‐period polytypes are discussed in detail.     

Cr doping influence in ZnS single crystals on the complex disordered polytypical structure

The results of X-ray investigation of real structures of ZnS:Cr single crystals are presented. Doped ZnS single crystals were grown from the melt by Bridgman's high pressure method. Cr dopant converts the structure of ZnS crystals from 3C with individual stacking faults (for pure ZnS) to the complex polytypical structures disordered along one dimension and including specific orderings of cp layers that form polytype cells LH (L = 2l, l = 2, 3, 4 and 5). The structures are characterised by Farkas-Jahnkes statistical parameters π(m, p) and by parameters P_LH of the polytype unit cell formation probability (Kozielski, Tomaszewicz 1986; Palosz, Przedmojski).     

Growth of low defect density GaP layers on Si substrates within the critical thickness by optimized shutter sequence and post-growth annealing

We have developed a growth procedure for realizing a low defect density GaP layer on an Si substrate. The growth procedure consists of two parts. One is the post-growth annealing for the annihilation of stacking faults (SFs). We have investigated an annihilation mechanism with molecular beam epitaxy grown GaP layers. 1-monolayer-thick SFs typically generate from the GaP/Si interface in a non-annealed GaP layer. In a 700 °C annealed GaP layer, generation points of these SFs tend to shift toward the GaP surface. In a 730 °C annealed GaP layer, SFs density is effectively decreased. These results suggest that SFs are annihilated through the climb motion of two partial dislocations during the post-growth annealing. Another one is the optimized shutter sequence for migration enhanced epitaxy. We have revealed that it is effective for the suppression of both three-dimensional growth and melt-back etching to increase in a stepwise manner the number of supplied Ga atoms per cycle. As a result, the generation of threading dislocations and pits is remarkably suppressed. A root mean square surface roughness of 0.13 nm is obtained within the critical thickness. We have estimated etch pit density (EPD) to be ∼7×10⁵ cm⁻² with a GaPN/GaP/Si structure. To the best of our knowledge, this value is same as that of commercially available GaP substrates and is the lowest one in the EPD of GaP/Si heteroepitaxy.     

Stacking faults in deformed zinc and magnesium single crystals

Using the transmission electron microscopy dissociated dislocations containing stacking faults were observed in Mg single crystals, deformed in (0001) <1120> slip system, and in Zn single crystals deformed in {1122} <1123> slip system. The overlapping flat stacking faults lying in {1102} planes and dissociated triple nodes in (0001) planes are observed in Mg. In Zn flat stacking faults are found in {112 2} planes. The value of stacking fault energy γ has been determined in Mg crystals (γ ⋍ 10 erg/cm²). Such a low value of γ is attributed to the segregation of impurities on dislocations.     

Isothermal crystallization kinetic of ZnO thin films

Zinc oxide (ZnO) thin films deposited by DC magnetron sputtering were annealed in nitrogen atmosphere at different temperatures ranging from 100 to 500 °C with a step of 100 °C; the annealing time was 6 h. In order to study the film’s crystallization kinetic, their structures were monitored by means of X-ray diffraction (XRD) analysis each hour. Variation in grain size, calculated from the XRD patterns, with annealing time and temperature, obeys the classical parabolic law of grain growth. Exponent n was found to be dependent on the annealing temperature; it ranged from 5.13 to 3.8 with increase in annealing temperature. From the obtained exponent n values we inferred that the grain growth mechanism is mainly governed by the atom jumping across the grain boundary. We have found that the grain growth is characterized by a low activation energy ranging from 22 to 24 kJ/mol.     

TEM Investigation of Dislocations in RNi2B2C (R = Y,Tb, Ho,Er, Y0.6Tb0.4, Er0.8Tb0.2)

To evaluate the influence of different rare earth metals on dislocation type and stacking fault occurrence in RNi2B2C compounds samples with R = Y, Tb, Ho, Er, Y_0.6Tb_0.4 and Er_0.8Tb_0.2 were investigated. As a result of the TEM analysis the parameter decisive for the lattice defects in these compounds is not the type of components but the annealing treatment of the samples. In well homogenized material the dominant Burgers vector of perfect dislocations was <100] and stacking faults are hardly found. In inhomogeneous samples the dominant Burgers vector was <110] and the perfect dislocations often lie in the planes of the numerous stacking faults. In both types of materials the experimental results hint at the existence of two different slip systems.     

退火处理对DKDP晶体光学均匀性的影响研究

分别对不同生长速度的DKDP晶体进行了退火处理.结果表明,适当温度的退火处理能有效地提高晶体的光学均匀性,尤其是对快速生长的晶体质量提高更为显著.在实验温度范围内,退火温度越高,质量改善越明显.对退火机理进行了初步讨论.     

Dielectric properties of Zr-Sn-Ti-O thin films prepared by pulsed laser deposition

Zr_0.26Sn_0.23Ti_0.51O₂ (ZSTO) films with a dielectric constant of about 40 have been prepared directly on silicon substrates by pulsed laser deposition at 600 °C. TEM observation showed that the as-deposited films are amorphous. Differential thermal analysis showed that the ZSTO films crystallize at about 620 °C. Capacitance-voltage (C-V) characteristics of metal-oxide-semiconductor (MOS) composed of Pt/ZSTO/Si prepared at different deposition temperature have been measured. The EOT of the MOS structures with the same ZSTO physical thickness increased slightly when the deposition temperature increased. The EOT is about 4.2 nm for the 40 nm ZSTO deposited at 600 °C. The leakage current characteristics of ZSTO films for the as deposited, post-annealed in oxygen ambient and post-annealed in nitrogen ambient by rapid thermal annealing have been studied comparatively. The films post-annealed in nitrogen ambient have the lowest leakage current and the as-deposited films have the largest leakage current characteristics. It is proposed that amorphous Zr-Sn-Ti oxide stabilized at 600 °C is a potential dielectric material for dynamic random access memory and high k dielectric gate applications.     

Growth of epitaxial TmFeCuO4 thin films by pulsed laser deposition

Epitaxial thin films of TmFeCuO₄ with a two-dimensional triangular lattice structure were successfully grown on yttria-stabilized-zirconia substrates by pulsed laser deposition and ex situ annealing in air. The films as-deposited below 500 °C showed no TmFeCuO₄ phase and the subsequent annealing resulted in the decomposition of film components. On the other hand, as-grown films deposited at 800 °C showed an amorphous nature. Thermal annealing converted the amorphous films into highly (0 0 1)-oriented epitaxial films. The results of scanning electron microscopic analysis suggest that the crystal growth process during thermal annealing is dominated by the regrowth of non-uniformly shaped islands to the distinct uniform islands of hexagonal base.     

Metastable SiCN glass matrices studied by energy-filtered electron diffraction pattern analysis

Bulk SiCN glasses, derived from polymer precursors with tailored architectures, were characterized upon pyrolysis at 1000 °C and subsequent annealing at 1400 °C and 1540 °C. The characterization tools employed were high-resolution transmission electron microscopy (HRTEM) imaging and energy-filtered selected area electron diffraction (EF-SAED). Main emphasis of this work was to verify as to whether the intrinsic amorphous structure of polymer-derived bulk materials exhibits different structural features upon pyrolysis, that depend on the functionalities of the pre-ceramic polymer, and whether such characteristic features are maintained upon high-temperature anneal. HRTEM imaging did not reveal any pronounced variation between the different SiCN glass structures after the organic–inorganic transition at 1000 °C or even upon heat treatment at 1400 °C. However, EF-SAED analysis showed differences in the amorphous structure of the four polymer-derived SiCN glasses studied even upon thermolysis at 1000 °C. The evolution of the electron diffraction intensity profiles of the EF-SAED patterns after subsequent thermal treatment at elevated temperatures showed clear evidence of structural rearrangements within the SiCN glass networks upon annealing at temperatures exceeding the pyrolysis temperature. The experimental results provide evidence that the metastable glass structure of polymer-derived SiCN ceramics depends on the starting polymer employed and, in addition, that the intrinsic glass architecture affects the thermal stability (onset of crystallization) of the amorphous matrix.