The four-point bend test was used to measure the brittle-to-ductile transition (BDT) temperature in precracked samples of semi-insulating 4H—SiC at four different strain rates. As in other semiconductors, the BDT temperature TBDT was found to be very sharp, within ±15 °C, and to shift to higher temperatures with increasing rates of the applied load (or strain rate). The results appear to be consistent with a transition temperature Tc recently observed in the yield stress of the same material as measured by compression experiments. However, strain-rate measurements in four-point bend tests are not strictly equivalent to those in compression experiments, and therefore it is difficult to directly compare the measured BDT temperatures with the yield stress transitions. Nevertheless, it is believed that the reasonable agreement between TBDT and Tc supports the model recently proposed to explain these transition temperatures.

Together with sapphire, SiC is the most common substrate material for GaN epitaxial growth. In fact, SiC has advantages over sapphire because of its better thermal conductivity and lower film substrate lattice mismatch (∼3.5%). However, nucleation of GaN on SiC is rather difficult because of the low surface energy of SiC and the sensitivity of substrate preparation. This latter point makes it essential to use a very careful cleaning step, and also to pre-treat the substrate surface by growing a thick buffer layer of AlN at a relatively high temperature. In this study, several pre-treatment steps of SiC for GaN deposition were tested including (a) nitration with NH3 for 0.5-20 minutes, (b) pre-adsorption of trimethyl gallium (TMG) or trimethyl aluminum (TMA) for 0.5-5 minutes, and (c) deposition of an AlN buffer layer at ∼1150°C. After each pre-treatment, GaN was deposited by MOCVD using dilute H2(Ar+12%H2), NH3 and TMG. All the films were characterized by XRD and cross-sectional TEM. After nitration of SiC, the deposited GaN film was found to be polycrystalline. In case of pre-adsorption of TMG, epitaxial but island-like GaN formed on the substrate. In the third case, with an ultra-thin (∼1.5nm) coverage of AlN on SiC (by pre-adsorption of TMA or by 50 seconds deposition of AlN), GaN epilayers were successfully deposited on SiC. However, when AlN was deposited for longer than 3 minutes (up to 10 minutes), only polycrystalline GaN was obtained. With this technique of covering the surface with an ultra-thin layer of AlN, epitaxial GaN has been successfully deposited on 6H-SiC (0001), on 4H-SiC(0001), and on 3C-SiC/Si(111) substrates. The effect of the different pre-treatments of SiC on the quality of the deposited GaN films will be discussed and compared, and the optimal conditions for GaN deposition for each substrate will be presented.

Vickers hardness tests on {0001} and (1120) faces of a relatively thick GaN film grown on a (0001) sapphire substrate have been performed in the temperature range 25-1200°C. The microstructure of the plastic zone around the indentation sites has been investigated by transmission electron microscopy (TEM). At room temperature, the hardness was measured to be 12.3 GPa on the basal plane, and 11.1 GPa on the prism plane. The hardness decreases gradually with increasing temperature up to ∼800°C and then shows a plateau between ∼800 and ∼1050°C after which it decreases again above ∼1100°C. In contrast to the rather straight dislocations produced by room-temperature indentation of the (0001) face, the dislocations generated by indenting the (1120) face at room temperature were curved and in the shape of half-loops emanating from the indentation sites on the prism planes. Such dislocations were not dissociated within the resolution of weak-beam TEM.

Recent deformation experiments on semiconductors have shown the occurrence of a break in the variation of the critical resolved shear stress of the crystal as a function of temperature. These and many other examples in the literature evidence a critical temperature at which a transition occurs in the deformation mechanism of the crystal. In this paper, the occurrence of a similar transition in two polytypes of SiC is reported and correlated to the microstructure of the deformed crystals investigated by transmission electron microscopy, which shows evidence for partial dislocations carrying the deformation at high stresses and low temperatures. Based on these results and data in the literature, the explanation is generalized to other semiconductors and a possible relationship to their brittle–ductile transition is proposed.

Recent experiments on deformation of semiconductors show an abrupt change in the variation of the critical resolved shear stress, τy, with temperature, T. This implies a change in the deformation mechanism at a critical temperature Tc. In the cases examined so far in our laboratory and elsewhere, this critical temperature appears to coincide approximately with the brittle-ductile transition temperature, TBDT. In this paper, the deformation experiments performed on the wide bandgap semiconductor, 4H-SiC, over a range of temperatures and strain rates are described together with the characterization of induced dislocations below and above Tc by transmission electron microscopy. Based on these results, and those of Suzuki and coworkers on other compound semiconductors, some understanding of the different mechanisms operating at low and high temperatures in tetrahedrally coordinated materials has been gained, and a new model for their brittle-ductile transition has been proposed.

Bulk single crystals of 4H-SiC have been deformed in compression in the temperature range 550–1300°C, whereas a GaN thin film grown on a (0001) sapphire substrate was deformed by Vickers indentation in the temperature range 25–800°C. The TEM observations of the deformed crystals indicate that deformation-induced dislocations in 4H-SiC all lie on the (0001) basal plane but depending on the deformation temperature, are one of two types. The dislocations induced by deformation at temperatures above ∼1 100°C are complete, with a Burgers vector, b, of but are all dissociated into two partials bounding a ribbon of stacking fault. On the other 3 hand, the dislocations induced by deformation in the temperature range 550<T<∼ 1100°C were predominantly single leading partials each dragging a stacking fault behind them. From the width of dissociated dislocations in the high-temperature deformed crystals, the stacking fault energy of 4H-SiC has been estimated to be 14.7±2.5 mJ/m2. Vickers indentations of the [0001]-oriented GaN film produced a dense array of dislocations along the three 〈1120〉 directions at all temperatures. The dislocations were slightly curved with their curvature increasing as the deformation temperature increased. Most of these dislocations were found to have a screw nature with their b parallel to 〈1120〉. Also, within the resolution of the weak-beam method, they were not found to be dissociated. Tilting experiment show that these dislocations lie on the {1100} prism plane rather than the easier (0001) glide plane.

By scratching the (0001)Si surface of 6H-SiC followed by annealing, dislocations were introduced in the crystal that were subsequently characterized by Transmission Electron Microscopy (TEM). Schottky diodes were then manufactured from the dislocated crystal and their electrical properties were studied by capacitance-voltage (C−V), current-voltage (I−V), and thermally-stimulated capacitance (TSCap) measurements. It was found that the deformation introduces deep traps Maynly located in the upper third of the bandgap promoting a significant increase in the series resistance of the diodes. The as-introduced dislocations were predominantly 30° partials and their core nature was determined to be Si(g) by the technique of Large Angle Convergent Beam Electron Diffraction (LACBED). The compensation effects observed after deformation are presumed to be caused not only by Si(g) dislocations but also by the other defects generated during the deformation step.

Several problems related to the dynamics of dislocation sources and the plasticity of silicon crystals are investigated with the help of a mesoscopic simulation. The questions successively examined are the dynamics of a source of perfect dislocations and the conditions under which perfect or partial dislocations are emitted by a source. This leads to a discussion of the initial steps of the model proposed by Pirouz for mechanical twinning and, further, to the suggestion that a relation may exist between several transitions experimentally observed at low temperatures in elemental or compound semi-conductors: a change in the slope of the yield stress vs. temperature curves, a brittle-to-ductile transition and a change in the nature of the mobile dislocations. Finally, simulations are presented of the yield point phenomenon that is a well-known feature of Si and Ge crystals. The results are discussed in terms of evolutionary laws for the total dislocation density during straining.

Micro/nanopipes are linear defects along the c-axis of hexagonal polytypes of SiC and GaN that are currently the focus of much attention. It has been shown that these defects can be very detrimental to the electronic properties of devices manufactured from, at least, 6H-SiC. In this paper, the origin of these defects is discussed in terms of Frank's theory [1] that dislocations will have a hollow core when their Burgers vector is large. Two fundamental issues about such dislocations are addressed: their formation along the c-axis of the crystal, and their stability despite their large Burgers vectors [2]. The proposed model is based on the mosaic structure of sublimation-grown 6H- or 4H-SiC, and VPE-grown 2H-GaN on sapphire substrates. The presence of unit c-axis screw dislocations is attributed to the accommodation of low-angle twist boundaries in the mosaic structure. The formation of superscrew dislocations with large Burgers vector, which empty their cores to reduce the excessive strain energy there, is shown to be the result of 3c screw dislocations in the axis of triple junctions which “getter” the neighboring unit dislocations and simultaneously increase their diameter. The predictions of the model are compared with available data in the literature, and suggestions are made for the decrease of nano/micropipe density.

GaN has been intensively studied because of its potential applications for the fabrication of blue- or ultraviolet-light emitting devices. Sapphire (α-Al2O3) is generally used as the substrate for growth of GaN film. However, the large lattice mismatch between GaN and Al2O3is a possible cause of the large defect density in the GaN films. Consequently, alternative substrates are being studied with the aim of growing films of lesser defect densities and improved opto-electronic properties. In this paper, we report a transmission electron microscopy (TEM) study of a GaN film grown on cubic SiC which has been obtained by carbonization of the top silicon layer of a SIMOX substrate, i.e. the system GaN/SiC/Si/SiO2/Si.

Cross-sectional TEM specimens were prepared by the conventional sandwich technique with the foil surface normal to the Si[l10] direction. The composite sample was ground and dimpled to a thickness of ∼ 10μm, and subsequently ion thinned to electron transparency.

SiC was grown on the top Si layer of SIMOX by carbonization followed by chemical vapor deposition (CVD). Subsequently, GaN was deposited on the SiC by metalorganic (MO)CVD to produce a GaN/SiC/Si/SiO2/Si multilayer structure. This multilayer film was investigated by conventional transmission electron microscopy (TEM) and high resolution (HR)TEM using cross-sectional thin foils. The GaN layer was found to consist of predominately hexagonal gallium nitride (h-GaN), and a smaller fraction of cubic GaN (c-GaN) crystallites. The orientation relationship between most of the h-GaN grains and cubic SiC (3C-SiC) was found to be (0001)GaN//(1I1)sic; [1120]GaN//[110]SiC, while most of the c-GaN grains had an orientation relationship (001)GaN//(001)SiC; [110]GaN//[110]SiC with respect to the 3C-SiC substrate. The hexagonal grains of GaN were found to grow as two variants. In this paper, the defects in both h-GaN and c-GaN are characterized and discussed.

With the current availability of large-area 3C-SiC films, it is imperative that stable high temperature contacts be developed for high power devices. By comparing the existing data in the literature, we demonstrate that the contact behavior on each of the different polytypes of SiC will vary significantly. In particular, we demonstrate this for 6H-SiC and 3C-SiC. The interface slope parameter, S, which is a measure of the Fermi-level pinning in each system varies between 0.4–0.5 on 6H-SiC, while it is 0.6 on 3C-SiC. This implies that the barrier heights of contacts to 3C-SiC will vary more significantly with the choice of metal than for 6H-SiC.

Aluminum, nickel and tungsten were deposited on 3C-SiC films and their specific contact resistance measured using the circular TLM method. High temperature measurements (up to 400°C) were performed to determine the behavior of these contacts at operational temperatures. Aluminum was used primarily as a baseline for comparison since it melts at 660°C and cannot be used for very high temperature contacts. The specific contact resistance (ρc) for nickel at room temperature was 5 × 10−4 Ω-cm2, but increased with temperature to a value of 1.5 × 10−3 Ω-cm2 at 400°C. Tungsten had a higher room temperature × 10−3 Ω-cm2, which remained relatively constant with increasing temperature up to 400°C. This is related to the fact that there is hardly any reaction between tungsten and silicon carbide even up to 900°C, whereas nickel almost completely reacts with SiC by that temperature. Contact resistance measurements were also performed on samples that were annealed at 500°C.

Dislocations produced by 1300 °C indentation of the silicon-terminated (111) face of 3C-SiC were investigated by transmission electron microscopy. They were all found to be either widely separated partial dislocation pairs, or else, arrays of single partial dislocation half-loops on neighboring parallel slip planes and having the same Burgers vector. It was concluded that in the latter case, each array consisted of leading partial dislocations which had nucleated without accompanying trailing partial dislocations. The core nature of both dissociated dislocations and arrays of single partial dislocations has been determined by the technique of large angle convergent beam electron diffraction. The results indicate that the core of all single partial dislocation half-loops constituting an array consists of silicon atoms. It is concluded that, with the present deformation geometry, the Si-core partial dislocations are preferentially nucleated before the C-core partial dislocations. In the case of a dissociated dislocation, when a pair of partials was present, electron microscopy observations revealed that the morphology of the two partial dislocations was very different; while the Si-core partials were smooth, the C-core partial dislocations had a zig-zag morphology.

Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire. A high density of threading dislocations parallel to the c-axis crossed the film from the interface to the film surface. They were found to have a predominantly edge character with a Burgers vector. In addition, dislocation hal-loops, elongated along the c-axis of GaN, were also found on the prism planes. These dislocations had a mostly screw character with a [0001] Burgers vector. Substrate surface steps with a height of were found to be accommodated by localized elastic bending of GaN (0001)GaN planes in the vicinity of the film/substrate interface. Observations show that the region of the film, with a thickness of ∼100 nm, adjacent to the interface is highly defective. This region is thought to correspond to the low-temperature GaN “buffer” layer which is initially grown on the sapphire substrate. Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed. The analysis of the results leads us to conclude that the film is under residual biaxial compression.

We have grown silicon carbide (SiC) on ultrathin Si (about 300Å) and on thick Si (about 2000Å) on commercial SIMOX (from IBIS Corp and SOITEC, Inc.), and bulk Si. Electron diffraction and Rutherford backscattering spectroscopy (RBS)/channeling studies indicate epitaxial growth of singlecrystal β-SiC even at growth temperatures as low as 1100°C.

We have already demonstrated the fabrication of ultrathin Si, as thin as 140Å on SiO2 by using the low-energy SIMOX (LES) (20 to 30 keV) process to produce films of lower cost and excellent integrity compared to thinned commercial SIMOX. Based on these results, ultrathin Si-on-insulator (SOI) substrates appear to have great potential for device quality SiC films. However, the carbonization and/or growth of SiC on ultrathin Si requires further optimization because the processes for surface cleaning and growth of SiC on bulk Si substrates cannot be applied because of the thinness of the substrate layers. Additional carbonization work at higher temperatures has indicated the possibility of converting the entire Si top layer.

The characteristic defects of 4H-SiC homoepitaxial thin films grown on bulk substrates using chemical vapor deposition (CVD) are described based on transmission electron microscopy (TEM), atomic force microscopy (AFM) and surface decoration studies. Emphasis is placed on understanding the formation mechanism of surface triangular defects. Cross-sectional TEM observations revealed the existence of two variants of 3C-SiC inclusions in 4H epitaxial films. In the plan-view orientation, g4H = 3304 type reflections were found useful for distinguishing the two variants of 3C-SiC platelets that are present in the 4H epilayer. A decoration technique was employed to reveal the relationship between the 3C platelets and surface features, e.g., surface steps. A formation mechanism for surface triangular defects is proposed, which is partially confirmed by the etch pit patterns obtained on the epilayer surfaces after a molten KOH etch.

Attempts were made to produce bands of hexagonal diamond (Lonsdaleite) by high-temperature indentation of cubic diamond. (011) wafers of type I diamond were indented over the temperature range 1000–1300°C and the microstructure of the indentation plastic zone investigated by transmission electron microscopy (TEM). No hexagonal diamond was produced; instead, the plastic zone consisted of arrays of dislocations lying on the {001} planes. A possible mechanism for the generation of such dislocations, and reasons for the absence of hexagonal diamond, are discussed.

Multilayers of vanadium and MgO with different layer thicknesses were grown by molecular beam epitaxy (MBE) and the V/MgO and MgO/V interfaces were investigated by cross-sectional high resolution electron microscopy (HREM). For the smallest layer thickness of 5 nm, the vanadium and MgO films were coherent with each other and no misfit dislocations were observed. For the larger layer thicknesses of 10 nm and 50 nm, the vanadium surface exhibited undulations, and for the largest thickness, interdiffusion had occurred between the MgO film and the underlying V substrate and an interfacial spinel phase had formed. These observations are discussed in terms of the morphological instabilities of the strained vanadium film as it grows on the MgO substrate.

When a 6H-SiC single crystal is deformed under indentation or uniaxial compression in orientations not favorable for the activation of the 1/3[1120](0001) easy glide system, the secondary slip system is activated. Additionally, for low- temperature deformations, “kinks” and/or micro-cracks form in the crystal. In this paper, experimental results on relatively lowtemperature compression and indentation tests of single crystal 6H-SiC, and the microstructure of the deformed crystals, are presented. Based on the results, the secondary slip system in 6HSiC has been determined to be 1/3[1120](1100), which may actually be a combination of alternate glide of 1/3[1120] dislocations on the (1102) and (1102) planes. Further, dislocation mechanisms for the nucleation of prism-plane and basal-plane cracks, and for the process of kinking, in deformed 6H-SiC are proposed.

High-quality single crystals of 6H–SiC have been indented at 1170 °C in vacuum. A TEM study of the indented regions shows that a 6H → 3C polytypic transformation has occurred, further confirming that this phase transformation can be induced by an applied stress.