X-Git-Url: https://hackdaworld.org/gitweb/?p=lectures%2Flatex.git;a=blobdiff_plain;f=posic%2Fthesis%2Fsic.tex;h=9926ccdf2176339cb39ae674b19c4dd7a0e128f0;hp=7228cbe228b001dfaac4e84931fe25af39eb961d;hb=9a540409168f421fb8eee3861a1fe8d6e3a28b38;hpb=d6517743fe4eea98817d22188ad7596af22e7da0 diff --git a/posic/thesis/sic.tex b/posic/thesis/sic.tex index 7228cbe..9926ccd 100644 --- a/posic/thesis/sic.tex +++ b/posic/thesis/sic.tex @@ -243,13 +243,13 @@ The authors assumed that due to the auxiliary heating rather than ion beam heati 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 \ac{IBS} of SiC. -Further studies revealed the possibility to form buried layers of SiC by IBS at moderate substrate and anneal temperatures \cite{lindner95}. +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}. For a critical dose, which was found to depend on the Si substrate orientation, the formation of a stoichiometric buried layer of SiC exhibiting a well-defined interface to the Si host matrix was observed. In case of overstoichiometric C concentrations the excess C is not redistributed. -These investigations demonstrate the presence of an upper dose limit, which corresponds to a \unit[50]{at.\%} C concentration at the implantation peak, for the thermally induced redistribution of the C atoms from a Gaussian to a box-shaped depth profile upon annealing. -For higher concentrations the formation of strong C-C bonds is expected. -Increased temperatures are necessary for the dissociation of these C clusters. +These investigations demonstrate the presence of an upper dose limit, which corresponds to a \unit[53]{at.\%} C concentration at the implantation peak, for the thermally induced redistribution of the C atoms from a Gaussian to a box-shaped depth profile upon annealing. +This is explained by the formation of strong graphitic C-C bonds for higher C concentrations \cite{calcagno96}. +Increased temperatures exceeding the Si melting point are expected to be necessary for the dissociation of these C clusters. Furthermore, higher implantation energies were found to result in layers of variable composition exhibiting randomly distributed SiC precipitates. In another study \cite{serre95} high dose C implantations were performed at room temperature and \unit[500]{$^{\circ}$C} respectively. Implantations at room temperature lead to the formation of a buried amorphous carbide layer in addition to a thin C-rich film at the surface, which is attributed to the migration of C atoms towards the surface. @@ -257,17 +257,50 @@ In contrast, implantations at elevated temperatures result in the exclusive form Annealing at temperatures up to \unit[1150]{$^{\circ}$C} does not alter the C profile. Instead defect annihilation is observed and the C-rich surface layer of the room temperature implant turns into a layer consisting of SiC precipitates, which, however, are not aligned with the Si matrix indicating a mechanism different to the one of the direct formation for the high-temperature implantation. -.. lindner limit in dose -> 1250 -... two temp implantation ... sharp interface -By understanding some basic processes (32-36), \ac{IBS} nowadays has become a promising method to form thin SiC layers of high quality exclusively of the 3C polytype embedded in and epitactically aligned to the Si host featuring a sharp interface \cite{lindner99,lindner01,lindner02}. +Based on these findings, a recipe was developed to form buried layers of single-crystalline SiC featuring an improved interface and crystallinity \cite{lindner99,lindner01,lindner02}. +Therefore, the dose must not exceed the stoichiometry dose, i.e. the dose corresponding to \unit[50]{at.\%} C concentration at the implantation peak. +Otherwise clusters of C are formed, which cannot be dissolved during post-implantation annealing at moderate temperatures below the Si melting point \cite{lindner96,calcagno96}. +Annealing should be performed for \unit[5-10]{h} at \unit[1250]{$^{\circ}$C} to enable the redistribution from the as-implanted Gaussian into a box-like C depth profile \cite{lindner95}. +The implantation temperature constitutes the most critical parameter, which is responsible for the structure after implantation and, thus, the starting point for subsequent annealing steps. +Implantations at \unit[400]{$^{\circ}$C} resulted in buried layers of SiC subdivided into a polycrystalline upper and an epitaxial lower part. +This corresponds to the region of randomly oriented SiC crystallites and epitaxially aligned precipitates surrounded by thin amorphous layers without crystalline SiC inclusions in the as-implanted state. +However, an abrupt interface to the Si host is observed after annealing. +As expected, single-crystalline layers were achieved for an increased temperature of \unit[600]{$^{\circ}$C}. +However, these layers show an extremely poor interface to the Si top layer governed by a high density of SiC precipitates, which are not affected in the C redistribution during annealing and, thus, responsible for the rough interface. +Hence, to obtain sharp interfaces and single-crystalline SiC layers temperatures between \unit[400]{$^{\circ}$C} and \unit[600]{$^{\circ}$C} have to be used. +Indeed, reasonable results were obtained at \unit[500]{$^{\circ}$C} \cite{lindner98} and even better interfaces were observed for \unit[450]{$^{\circ}$C} \cite{lindner99_2}. +To further improve the interface quality and crystallinity a two-temperature implantation technique was developed \cite{lindner99}. +To form a narrow, box-like density profile of oriented SiC nanocrystals \unit[93]{\%} of the total dose of \unit[$8.5\cdot 10^{17}$]{cm$^{-2}$} is implanted at \unit[500]{$^{\circ}$C}. +The remaining dose is implanted at \unit[250]{$^{\circ}$C}, which leads to the formation of amorphous zones above and below the SiC precipitate layer and the desctruction of SiC nanocrystals within these zones. +After annealing for \unit[10]{h} at \unit[1250]{$^{\circ}$C} a homogeneous, stoichiometric SiC layer with sharp interfaces is formed. + +To summarize, by understanding some basic processes, \ac{IBS} nowadays has become a promising method to form thin SiC layers of high quality exclusively of the 3C polytype embedded in and epitaxially aligned to the Si host featuring a sharp interface. +Due to the high areal homogeneity achieved in \ac{IBS}, the size of the layers is only limited by the width of the beam-scanning equipment used in the implantation system as opposed to deposition techniques, which have to deal with severe wafer bending. +This enables the synthesis of large area SiC films. \section{Substoichiometric concentrations of carbon in crystalline silicon} -diffusion mechanism, lattice distortion, hmm ... extra section needed? +The C solid solubility in bulk Si is quite low +% carbon as an impurity / solubility / lattice distortion / diffusion +% agglomeration phenomena +% suppression of transient enhanced diffusion of dopant species +% strained silicon / heterostructures +% -> skorupa 3.2: c sub vs sic prec +% -> my own links: strane etc ... +% -> skorupa 3.5: heterostructures + +% hmm ... extra section needed? \section{Assumed cubic silicon carbide conversion mechanisms} \label{section:assumed_prec} +Although much progress has been made in 3C-SiC thin film growth in the above-mentioned growth methods during the last decades, there is still potential +.. compatible to the established and highly developed technology based on silicon. + +Although tremendous progress has been achieved in the above-mentioned growth methods during the last decades, available wafer dimensions and crystal qualities are not yet statisfactory. + +... \cite{lindner99_2} ... + on surface ... md contraction along 110 ... kitabatake ... and ref in lindner ... rheed from si to sic ... in ibs ... lindner and skorupa ... @@ -278,3 +311,4 @@ nejim however ... also indictaed by other direct synthesis experiments like martin90 and conclusions of reeson8X ... eichhornXX, koegler, lindner ... +