From 1409915a2449cdf810e0a88c5880b887766b3f28 Mon Sep 17 00:00:00 2001 From: hackbard Date: Thu, 15 Sep 2011 22:38:47 +0200 Subject: [PATCH] joerg changes (part 1) --- posic/thesis/intro.tex | 12 ++++++------ posic/thesis/sic.tex | 42 ++++++++++++++++++++++-------------------- 2 files changed, 28 insertions(+), 26 deletions(-) diff --git a/posic/thesis/intro.tex b/posic/thesis/intro.tex index 9b47705..36e6fae 100644 --- a/posic/thesis/intro.tex +++ b/posic/thesis/intro.tex @@ -6,21 +6,21 @@ Due to the large Si--C bonding energy SiC is a hard and chemical inert material Its radiation hardness allows the operation as a first wall material in nuclear reactors \cite{giancarli98} and as electronic devices in space \cite{capano97}. The realization of silicon carbide based applications demands for reasonable sized wafers of high crystalline quality. -Despite the tremendous progress achieved in the fabrication of high purity SiC employing techniques like the modified Lely process for bulk crystal growth \cite{tairov78,tsvetkov98} or chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) for homo- and heteroepitaxial growth \cite{kimoto93,powell90,fissel95}, available wafer dimensions and crystal qualities are not yet considered sufficient enough. +Despite the tremendous progress achieved in the fabrication of high purity SiC employing techniques like the modified Lely process for bulk crystal growth \cite{tairov78,tsvetkov98} or chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) for homo- and heteroepitaxial growth \cite{kimoto93,powell90,fissel95}, available wafer dimensions and crystal qualities are not yet sufficient. Another promising alternative to fabricate SiC is ion beam synthesis (IBS). -High-dose carbon implantation at elevated temperatures into silicon with subsequent annealing results in the formation of buried epitaxial SiC layers \cite{borders71,reeson87}. -A two-temperature implantation technique was proposed to achieve single crystalline SiC layers and a sharp SiC/Si interface \cite{lindner99,lindner99_2,lindner01,lindner02}. +High-dose carbon implantation at elevated temperatures into silicon with subsequent annealing results in the formation of buried SiC layers \cite{borders71,reeson87}. +A two-temperature implantation technique was proposed to achieve single crystalline, epitaxial SiC layers and a sharp SiC/Si interface \cite{lindner99,lindner99_2,lindner01,lindner02}. Although high-quality SiC can be achieved by means of IBS the precipitation mechanism is not yet fully understood. High resolution transmission electron microscopy studies indicate the formation of C-Si interstitial complexes sharing conventional silicon lattice sites (C-Si dumbbells) during the implantation of carbon in silicon. These C-Si dumbbells agglomerate and once a critical radius is reached, the topotactic transformation into a SiC precipitate occurs \cite{werner97,lindner01}. -In contrast, investigations of strained Si$_{1-y}$C$_y$/Si heterostructures form -ed by MBE~\cite{strane94,guedj98}, which incidentally involve the formation of SiC nanocrystallites, suggest an initial coherent precipitation by agglomeration of substitutional instead of interstitial C. +In contrast, investigations of strained Si$_{1-y}$C$_y$/Si heterostructures formed by MBE~\cite{strane94,guedj98}, which incidentally involve the formation of SiC nanocrystallites, suggest an initial coherent precipitation by agglomeration of substitutional instead of interstitial C. Coherency is lost once the increasing strain energy of the stretched SiC structure surpasses the interfacial energy of the incoherent 3C-SiC precipitate and the Si substrate. These two different mechanisms of precipitation might be attributed to the respective method of fabrication. While in CVD and MBE surface effects need to be taken into account, SiC formation during IBS takes place in the bulk of the Si crystal. -However, in another IBS study, Nejim et~al.~\cite{nejim95} propose a topotactic transformation that is likewise based on the formation of substitutional C, which is accompanied by the emission of Si self-interstitial atoms that previously occupied the lattice sites and a compensating reduction of volume due to the lower lattice constant of SiC compared to Si. +However, in another IBS study \cite{nejim95} a topotactic transformation is proposed that is likewise based on the formation of substitutional C, which is accompanied by the emission of Si self-interstitial atoms that previously occupied the lattice sites and a compensating reduction of volume due to the lower lattice constant of SiC compared to Si. +The atomic migration involved in such a transformation is not clear. For several reasons, solving the controversial view of SiC precipitation in Si is of fundamental interest. A better understanding of the supposed SiC conversion mechanism and related carbon-mediated effects in silicon will enable significant technological progress in SiC thin film formation on the one hand and likewise offer perspectives for processes which rely upon prevention of precipitation events for improved silicon based devices on the other hand. diff --git a/posic/thesis/sic.tex b/posic/thesis/sic.tex index bdd9700..4320cbf 100644 --- a/posic/thesis/sic.tex +++ b/posic/thesis/sic.tex @@ -4,7 +4,7 @@ \section{Structure, properties and applications of silicon carbide} The phase diagram of the C/Si system is shown in Fig.~\ref{fig:sic:si-c_phase}. -In the solid state the stoichiometric composition of silicon and carbon termed silicon carbide (SiC) is the only chemical stable compound in the C/Si system \cite{scace59}. +In the solid state the stoichiometric composition of silicon and carbon termed silicon carbide (SiC) is the only chemical stable compound in the C/Si system~\cite{scace59}. \begin{figure}[t] \begin{center} \includegraphics[width=12cm]{si-c_phase.eps} @@ -18,10 +18,10 @@ Although they might have been considered \glqq diamonds from space\grqq{} Moissa In mineralogy SiC is still referred to as moissanite in honor of its discoverer. It is extremely rare and almost impossible to find in nature. -SiC is a covalent material in which both, Si and C atoms are sp$^3$ hybridized. +SiC is a mainly covalent material in which both, Si and C atoms are sp$^3$ hybridized. Each of the four sp$^3$ hybridized orbitals of a Si atom overlaps with one of the four sp$^3$ hybridized orbitals of the four surrounding C atoms and vice versa. -This results in fourfold coordinated covalent $\sigma$ bonds of equal length and strength for each atom with its neighbors. -Although the local order of Si and C next neighbor atoms characterized by the tetrahedral bonding is the same, more than 250 different types of structures called polytypes of SiC exist \cite{fischer90}. +This results in fourfold coordinated mostly covalent $\sigma$ bonds of equal length and strength for each atom with its neighbors. +Although the local order of Si and C next neighbor atoms characterized by the tetrahedral bonding is the same, more than 250 different types of structures called polytypes of SiC exist~\cite{fischer90}. The polytypes differ in the one-dimensional stacking sequence of identical, close-packed SiC bilayers. Each SiC bilayer can be situated in one of three possible positions (abbreviated a, b or c) with respect to the lattice while maintaining the tetrahedral bonding scheme of the crystal. \begin{figure}[t] @@ -69,9 +69,9 @@ Some of the key properties are listed in Table~\ref{table:sic:properties} and co Despite the lower charge carrier mobilities for low electric fields SiC outperforms Si concerning all other properties. The wide band gap, large breakdown field and high saturation drift velocity make SiC an ideal candidate for high-temperature, high-power and high-frequency electronic devices exhibiting high efficiency~\cite{wesch96,morkoc94,casady96,capano97,pensl93,park98,edgar92}. In addition the high thermal conductivity enables the implementation of small-sized electronic devices enduring increased power densities. -Its formidable mechanical stability, heat resistant, radiation hardness and low neutron capture cross section allow operation in harsh and radiation-hard environments~\cite{capano97}. +Its formidable mechanical stability, heat resistance, radiation hardness and low neutron capture cross section allow operation in harsh and radiation-hard environments~\cite{capano97}. -Despite high-temperature operations the wide band gap also allows the use of SiC in optoelectronic devices. +In addition to high-temperature operations, the wide band gap also allows the use of SiC in optoelectronic devices. Indeed, a forgotten figure, Oleg V. Losev discovered what we know as the light emitting diode (LED) today in the mid 1920s by observing light emission from SiC crystal rectifier diodes used in radio receivers when a current was passed through them~\cite{losev27}. Apparently not known to Losev, Henry J. Round published a small note~\cite{round07} reporting a bright glow from a SiC diode already in 1907. However, it was Losev who continued his studies providing comprehensive knowledge on light emission of SiC (entitled luminous carborundum) and its relation to diode action~\cite{losev28,losev29,losev31,losev33} constituting the birth of solid-state optoelectronics. @@ -79,7 +79,7 @@ And indeed, the first significant blue LEDs reinvented at the start of the 1990s Due to the indirect band gap and, thus, low light emitting efficiency, however, it is nowadays replaced by GaN and InGaN based diodes. However, even for GaN based diodes SiC turns out to be of great importance since it constitutes an ideal substrate material for GaN epitaxial layer growth~\cite{liu_l02}. As such, SiC will continue to play a major role in the production of future super-bright visible emitters. -Especially substrates of the 3C polytype promise good quality, single crystalline GaN films~\cite{takeuchi91,yamamoto04,ito04}. +Especially substrates of the 3C polytype promise good quality, single crystalline GaN films~\cite{takeuchi91,yamamoto04,ito04,haeberlen10}. The focus of SiC based applications, however, is in the area of solid state electronics experiencing revolutionary performance improvements enabled by its capabilities. These devices include ultraviolet (UV) detectors, high power radio frequency (RF) amplifiers, rectifiers and switching transistors as well as microelectromechanical system (MEMS) applications. @@ -131,7 +131,7 @@ Excellent reviews of the different SiC growth methods have been published by Wes \subsection{SiC bulk crystal growth} The industrial Acheson process \cite{knippenberg63} is utilized to produce SiC on a large scale by thermal reaction of silicon dioxide (silica sand) and carbon (coal). -The heating is accomplished by a core of graphite centrally placed in the furnace, which is heated up to a maximum temperature of \unit[2700]{$^{\circ}$C}, after which the temperature is gradually lowered. +The heating is accomplished by a core of graphite centrally placed in a furnace, which is heated up to a maximum temperature of \unit[2700]{$^{\circ}$C}, after which the temperature is gradually lowered. Due to the insufficient and uncontrollable purity, material produced by this method, originally termed carborundum by Acheson, can hardly be used for device applications. However, it is often used as an abrasive material and as seed crystals for subsequent vapor phase growth and sublimation processes. @@ -141,7 +141,7 @@ The obtained polycrystalline material consists of small crystal grains with a si A significant breakthrough was made in 1955 by Lely, who proposed a sublimation process for growing higher purity bulk SiC single crystals \cite{lely55}. In the so called Lely process, a tube of porous graphite is surrounded by polycrystalline SiC as gained by previously described processes. -Heating the hollow carbon cylinder to \unit[2500]{$^{\circ}$C} leads to sublimation of the material at the hot outer wall and diffusion through the porous graphite tube followed by an uncontrolled crystallization on the slightly cooler parts of the inner graphite cavity resulting in the formation of randomly sized, hexagonally shaped platelets, which exhibit a layered structure of various alpha polytypes with equal \hkl{0001} orientation. +Heating the hollow carbon cylinder to \unit[2500]{$^{\circ}$C} leads to sublimation of the material at the hot outer wall and diffusion through the porous graphite tube followed by an uncontrolled crystallization on the slightly cooler parts of the inner graphite cavity resulting in the formation of randomly sized, hexagonally shaped platelets, which exhibit a layered structure of various alpha (non-cubic) polytypes with equal \hkl{0001} orientation. Subsequent research \cite{tairov78,tairov81} resulted in the implementation of a seeded growth sublimation process wherein only one large crystal of a single polytype is grown. In the so called modified Lely or modified sublimation process nucleation occurs on a SiC seed crystal located at the top or bottom of a cylindrical growth cavity. @@ -170,14 +170,14 @@ In the case of SSMBE atoms are provided by electron beam evaporation of graphite The following review will exclusively focus on CVD and MBE techniques. The availability and reproducibility of Si substrates of controlled purity made it the first choice for SiC epitaxy. -The heteroepitaxial growth of SiC on Si substrates has been stimulated for a long time due to the lack of suitable large substrates that could be adopted for homoepitaxial growth. +The heteroepitaxial growth of SiC on Si substrates has been stimulated for a long time due to a lack of suitable large substrates that could be adopted for homoepitaxial growth. Furthermore, heteroepitaxy on Si substrates enables the fabrication of the advantageous 3C polytype, which constitutes a metastable phase and, thus, can be grown as a bulk crystal only with small sizes of a few mm. -The main difficulties in SiC heteroepitaxy on Si is due to the lattice mismatch of Si and SiC and the difference in the thermal expansion coefficient of \unit[8]{\%}. +The main difficulties in SiC heteroepitaxy on Si arise due to the lattice mismatch of Si and SiC by \unit[20]{\%} and the difference in the thermal expansion coefficient of \unit[8]{\%}. Thus, in most of the applied CVD and MBE processes, the SiC layer formation process is split into two steps, the surface carbonization and the growth step, as proposed by Nishino~et~al. \cite{nishino83}. Cleaning of the substrate surface with HCl is required prior to carbonization. -During carbonization the Si surface is chemically converted into a SiC film with a thickness of a few nm by exposing it to a flux of C atoms and concurrent heating up to temperatures about \unit[1400]{$^{\circ}$C}. +During carbonization the Si surface is chemically converted into a SiC film with a thickness of a few nm by exposing it to a flux of C atoms and concurrent heating up to temperatures of about \unit[1400]{$^{\circ}$C}. In a next step, the epitaxial deposition of SiC is realized by an additional supply of Si atoms at similar temperatures. -Low defect densities in the buffer layer are a prerequisite for obtaining good quality SiC layers during growth, although defect densities decrease with increasing distance of the SiC/Si interface \cite{shibahara86}. +Low defect densities in the buffer layer are a prerequisite for obtaining good quality SiC layers during growth, although defect densities decrease with increasing distance to the SiC/Si interface \cite{shibahara86}. Next to surface morphology defects such as pits and islands, the main defects in 3C-SiC heteroepitaxial layers are twins, stacking faults (SF) and antiphase boundaries (APB) \cite{shibahara86,pirouz87}. APB defects, which constitute the primary residual defects in thick layers, are formed near surface terraces that differ in a single-atom-height step resulting in domains of SiC separated by a boundary, which consists of either Si-Si or C-C bonds due to missing or disturbed sublattice information \cite{desjardins96,kitabatake97}. However, the number of such defects can be reduced by off-axis growth on a Si \hkl(0 0 1) substrate miscut towards \hkl[1 1 0] by \unit[2]{$^{\circ}$}-\unit[4]{$^{\circ}$} \cite{shibahara86,powell87_2}. @@ -194,10 +194,10 @@ Investigations indicate that in the so-called step-controlled epitaxy, crystal g This growth mechanism does not require two-dimensional nucleation. Instead, crystal growth is governed by mass transport, i.e.\ the diffusion of reactants in a stagnant layer. In contrast, layers of the 3C polytype are formed on exactly oriented \hkl(0 0 0 1) 6H-SiC substrates by two-dimensional nucleation on terraces. -These films show a high density of double positioning boundary (DPB) defects, which is a special type of twin boundary arising at the interface of regions that occupy one of the two possible orientations of the hexagonal stacking sequence, which are rotated by \unit[60]{$^{\circ}$} relative to each other, respectively. +These films show a high density of double positioning boundary (DPB) defects, which is a special type of twin boundary arising at the interface of regions that occupy one of the two possible orientations of the hexagonal stacking sequence, which are rotated by \unit[60]{$^{\circ}$} relative to each other. However, lateral 3C-SiC growth was also observed on low tilt angle off-axis substrates originating from intentionally induced dislocations \cite{powell91}. Additionally, 6H-SiC was observed on clean substrates even for a tilt angle as low as \unit[0.1]{$^{\circ}$} due to low surface mobilities that facilitate arriving molecules to reach surface steps. -Thus, 3C nucleation is assumed as a result of migrating Si and C containing molecules interacting with surface disturbances by a yet unknown mechanism, in contrast to a model \cite{ueda90}, in which the competing 6H versus 3C growth depends on the density of surface steps. +Thus, 3C nucleation is assumed as a result of migrating Si and C containing molecules interacting with surface disturbances, in contrast to a model \cite{ueda90}, in which the competing 6H versus 3C growth depends on the density of surface steps. Combining the fact of a well defined 3C lateral growth direction, i.e.\ the tilt direction, and an intentionally induced dislocation enables the controlled growth of a 3C-SiC film mostly free of DPBs \cite{powell91}. Lower growth temperatures, a clean growth ambient, in situ control of the growth process, layer-by-layer deposition and the possibility to achieve dopant profiles within atomic dimensions due to the reduced diffusion at low growth temperatures reveal MBE as a promising technique to produce SiC epitaxial layers. @@ -235,24 +235,26 @@ The ion beam synthesis (IBS) technique, i.e.\ high-dose ion implantation followe A short chronological summary of the IBS of SiC and its origins is presented in the following. High-dose carbon implantation into crystalline silicon (c-Si) with subsequent or in situ annealing was found to result in SiC microcrystallites in Si \cite{borders71}. -Rutherford backscattering spectrometry (RBS) and infrared (IR) spectroscopy investigations indicate a \unit[10]{at.\%} C concentration peak and the occurrence of disordered C-Si bonds after implantation at room temperature (RT) followed by crystallization into SiC precipitates upon annealing demonstrated by a shift in the IR absorption band and the disappearance of the C profile peak in RBS. +Rutherford backscattering spectrometry (RBS) and infrared (IR) spectroscopy investigations indicate a \unit[10]{at.\%} C concentration peak and the occurrence of disordered C-Si bonds after implantation at room temperature (RT) followed by crystallization into SiC precipitates upon annealing. +This is demonstrated by a shift in the IR absorption band and the disappearance of the C profile peak in RBS. Implantations at different temperatures revealed a strong influence of the implantation temperature on the compound structure \cite{edelman76}. -Temperatures below \unit[500]{$^{\circ}$C} result in amorphous layers, which is transformed into polycrystalline 3C-SiC after \unit[850]{$^{\circ}$C} annealing. +Temperatures below \unit[500]{$^{\circ}$C} result in amorphous layers, which are transformed into polycrystalline 3C-SiC after annealing at \unit[850]{$^{\circ}$C}. Otherwise single crystalline 3C-SiC is observed for temperatures above \unit[600]{$^{\circ}$C}. Annealing temperatures necessary for the onset of the amorphous to crystalline transition have been confirmed by further studies \cite{kimura81,kimura82}. Overstoichiometric doses result in the formation of clusters of C, which do not contribute to SiC formation during annealing up to \unit[1200]{$^{\circ}$C} \cite{kimura82}. The amount of formed SiC, however, increases with increasing implantation temperature. The authors, thus, concluded that implantations at elevated temperatures lead to a reduction in the annealing temperatures required for the synthesis of homogeneous layers of SiC. -In a comparative study of O, N and C implantation into Si, the absence of the formation of a stoichiometric SiC compound layer involving the transition of a Gaussian into a box-like C profile with respect to the implantation depth for the superstoichiometric C implantation and an annealing temperature of \unit[1200]{$^{\circ}$C} in contrast to the O and N implantations, which successfully form homogeneous layers, has been observed \cite{reeson86}. +In a comparative study of O, N and C implantation into Si, the absence of the formation of a stoichiometric SiC compound layer involving the transition of a Gaussian into a box-like C depth profile with respect to the implantation depth for the superstoichiometric C implantation and an annealing temperature of \unit[1200]{$^{\circ}$C} in contrast to the O and N implantations, which successfully form homogeneous layers, has been observed \cite{reeson86}. This was attributed to the difference in the enthalpy of formation of the respective compound and the different mobility of the respective impurity in bulk Si. Thus, higher annealing temperatures and longer annealing times were considered necessary for the formation of homogeneous SiC layers. Indeed, for the first time, buried homogeneous and stoichiometric epitaxial 3C-SiC layers embedded in single crystalline Si were obtained by the same group consequently applying annealing temperatures of \unit[1405]{$^{\circ}$C} for \unit[90]{min} and implantation temperatures of approximately \unit[550]{$^{\circ}$C} \cite{reeson87}. -The necessity of the applied extreme temperature and time scale is attributed to the stability of substitutional C within the Si matrix being responsible for high activation energies necessary to dissolve such precipitates and, thus, allow for redistribution of the implanted C atoms. +The necessity of the applied extreme temperature (a few degrees below the Si melting point) and time scale is attributed to the stability of substitutional C within the Si matrix being responsible for high activation energies necessary to dissolve such precipitates and, thus, allow for redistribution of the implanted C atoms. In order to avoid extreme annealing temperatures close to the melting temperature of Si, triple-energy implantations in the range from \unit[180--190]{keV} with stoichiometric doses at a constant target temperature of \unit[860]{$^{\circ}$C} achieved by external substrate heating were performed \cite{martin90}. It was shown that a thick buried layer of SiC is directly formed during implantation, which consists of small, only slightly misorientated but severely twinned 3C-SiC crystallites. The authors assumed that due to the auxiliary heating rather than ion beam heating as employed in all the preceding studies, the complexity of the remaining defects in the synthesized structure is fairly reduced. Even better qualities by direct synthesis were obtained for implantations at \unit[950]{$^{\circ}$C} \cite{nejim95}. Since no amorphous or polycrystalline regions have been identified, twinning is considered to constitute the main limiting factor in the IBS of SiC. +Layers obtained by direct synthesis are characterized by rough surfaces of the buried layer and the substrate originating from the dendritic growth of SiC crystals at these temperatures \cite{lindner06}. Further studies revealed the possibility to form buried layers of SiC by IBS at moderate substrate and anneal temperatures \cite{lindner95,lindner96}. Different doses of C ions with an energy of \unit[180]{keV} were implanted at \unit[330--440]{$^{\circ}$C} and annealed at \unit[1200]{$^{\circ}$C} or \unit[1250]{$^{\circ}$C} for \unit[5--10]{h}. @@ -414,7 +416,7 @@ While in CVD and MBE, surface effects need to be taken into account, SiC formati However, in another IBS study Nejim et~al. \cite{nejim95} propose a topotactic transformation that is likewise based on substitutional C, which replaces four of the eight Si atoms in the Si unit cell accompanied by the generation of four Si interstitials. Since the emerging strain due to the expected volume reduction of \unit[48]{\%} would result in the formation of dislocations, which, however, are not observed, the interstitial Si is assumed to react with further implanted C atoms in the released volume. The resulting strain due to the slightly lower Si density of SiC compared to Si of about \unit[3]{\%} is sufficiently small to legitimate the absence of dislocations. -Furthermore, IBS studies of Reeson~et~al. \cite{reeson87}, in which implantation temperatures of \unit[500]{$^{\circ}$C} were employed, revealed the necessity of extreme annealing temperatures for C redistribution, which is assumed to result from the stability of substitutional C and consequently high activation energies required for precipitate dissolution. +Furthermore, IBS studies of Reeson~et~al.~\cite{reeson87}, in which implantation temperatures of \unit[500]{$^{\circ}$C} were employed, revealed the necessity of extreme annealing temperatures for C redistribution, which is assumed to result from the stability of substitutional C and consequently high activation energies required for precipitate dissolution. The results support a mechanism of an initial coherent precipitation based on substitutional C that is likewise valid for the IBS of 3C-SiC by C implantation into Si at elevated temperatures. The fact that the metastable cubic phase instead of the thermodynamically more favorable hexagonal $\alpha$-SiC structure is formed and the alignment of these cubic precipitates within the Si matrix, which can be explained by considering a topotactic transformation by C atoms occupying substitutionally Si lattice sites of one of the two fcc lattices that make up the Si crystal, reinforce the proposed mechanism. -- 2.20.1