The paper by Henry et al. In the very high doping range, the authors correlated success- fully the position of a broad nitrogen-related luminescence band with the doping concentration.
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The technique is almost exclusively used today to manufacture SiC wafers. The principle is simple. A graphite crucible is partially filled with SiC powder and a seed is attached on the lid of the crucible. The whole system is closed and heated up to temperatures above which SiC starts to sublime appreciably. A thermal gradient is applied such that the seed is slightly colder than the powder source. Material will thus transport from the source to the seed where it will condense. The principal constituents during sublimation are Si, Si2C, and SiC2 and the ratio between them is determined by the temperature.
The pressure is kept low to enhance the material transport, as is nicely illustrated by Maltsev et al. Typically, the pressure is kept below 50 mbar. In a study by Barrett et al.
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They could determine that convective effects were present at pressures above 20 Torr approximately Below 20 Torr the vapor transport is diffusive, according to their study. The temperature and temperature gradient are naturally very important factors for the growth. The growth rate is exponentially increasing with increasing tempera- ture. Typical trends are comprehensively illustrated by Vodakov et al. The diameter is rapidly increasing and 4-inch wafers have already been demonstrated.
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The increase in wafer diameter is significantly faster than that experienced for Si and GaAs, partly as a result of the knowledge base established for these technologies . There is no doubt that SiC, with the same diameter as silicon wafers, will be available in the not-too-distant future.
It was later discovered that the introduction of specific deep-level dopants into the SiC lattice created deep traps that captures free carriers . The dopant of choice for these compensated crystals was vanadium, which was found to compen- sate shallow acceptors. Therefore, during growth, the material needs to be doped p-type by the addition of boron or aluminum as well as introducing vanadium in larger amounts than the p-type dopant.
Unfortunately, this created some problems with yield and defects due to the large amounts of impurities present in the crystal. The activation energy of the vanadium trap was determined to be 1.
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The residual pressures are for curve: 1—10, 2—35, 3—50, 4—75, and 5— Torr. From: . It is particularly surprising in view of the fact that it was the same group that first introduced the vanadium doping. It is unfortunate that they did not realize that the most promising and much superior approach was to continue to improve the purity of the crystals as much as possible. Other than poor crystalline quality and yield problems, there was one more problem with the vanadium doped SI SiC wafers; they did not work for high- frequency applications due to excessive trapping of electrons.
A very elegant dem- onstration of this trapping can be seen in the publication by Sghaier et al. Then, in , the high-frequency community, in desperate need of better semi-insulating substrates, regained their confidence due to some interesting results where very pure, vanadium-free crystals had been grown that displayed SI behavior .
The authors found a deep level with an activation energy of 1. However, they specu- lated that it was intrinsic in nature. Another even more interesting development was occurring at the same time. This was the development of a new growth technique, called the High Temperature Chemical Vapor Deposition HTCVD technique , that produced crystals that were intrinsically semi-insulating. In a paper by Ellison et al.
Further work showed that several intrinsic defects can contribute to the semi- insulating properties of the crystal, such as Si-vacancies, C-vacancies, and C-antisites. Typically, the resistivity of semi-insulating crystals containing predomi- nately Si-vacancies is lower and the activation energy of the responsible defect is less than 1. Lately, photo-electron paramagnetic resonance EPR measurements revealed that the carbon vacancy acts as a deep donor with an activation energy of 1.
This provides to date the strongest evidence that C-vacancies play a role in SiC similar to the well-known EL2 deep level in GaAs for producing high-purity semi-insulating wafers. Now, 2-inch SI wafers without vanadium doping are sold commercially from several sources. Vanadium doped crystals will rapidly disappear and soon only be used as displays in museums or as book rests.
The material properties are improving steadily and there should be no reason for worries. Yet, one worry is the need for off-axis substrates for high-quality epitaxial At that time, only mm wafers were available commercially, and it was not a big problem to slice the wafers somewhat more off-axis. It was only a matter of 4-mm lost material in total from the grown boule.
On a mm wafer, the amount of lost material is no less than 14 mm. The trend must be to reduce the off-axis angle. Fortunately, if we peek at what has happened previously in the Si and GaAs worlds, the standard was to use off-axis substrates initially, but as the material quality improved, the need for off-axis sub- strates was reduced and on-axis substrates are now used for epitaxy.
This is likely going to be the trend in SiC too. The need for better epitaxial procedures and higher-quality substrates than what is available today is important. Boule lengths are also likely to increase. More powder may be introduced into the growth chamber, however, thermal considerations become an issue as the boules increase too much in length. Even if the surface is kept at constant distance in the chamber, the distance from the source will vary as the material is depleted and the growth conditions change.
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The source material will release excess silicon in the beginning of the growth cycle and be more carbon-rich in the end due to preferential depletion of silicon. This is a known problem and it is a matter of detailed control and an understanding of the dynamic transport mechanisms in combination with thermodynamics. Never- theless, the result is invariably that SiC boules grown by seeded sublimation growth are Si-rich in the beginning and C-rich near the end, which creates yield issues.
Simu- lation of the process is necessary to improve the situation. The turnaround time for a boule growth run is several days. It takes time to pre- pare the source, load the crucible, attach the seed, evacuate the system carefully, gradually heat up the crucible under controlled conditions, and finally grow the boule and then cool down. The turnaround time itself is not a problem, however, the cost of manufacturing a wafer needs to decrease so the price of the wafers can be reduced in order for SiC to gain acceptance in the market.
Seeded sublimation growth is a mature and needed tool for the SiC industry today. There are still major challenges. Specifically, boules will need to be grown on off-axis substrates, or the off-axis angle needs to be eliminated, which will only be possible if a combined effort of improving wafer quality, polishing procedures, and epitaxial procedures is pursued. This technique uses gases instead of a powder as source material. More recent publications are available where the tech- nique is better described .
The gases used are mainly silane, ethylene, and a helium carrier. The carrier flow is very low.
Silane and ethylene are present at very high concentrations so that homogene- ous nucleation dominates the process. As the gases enter into the hot part of the injector, the silane will decompose and form small Si liquid droplets or solid micro- crystals, depending on the temperature. The ethylene will also take part in the reac- tion, forming microparticles of SixCy. It has been noted  that even a small addition of hydrocarbons converts the Si droplets to stable particles of SixCy or non- stoichiometric SiC.
The stability may, in a hand-waving circumstantial way, be intuitively understood from a solubility point of view. The solubility of carbon in silicon is very low, thus, when carbon is added to the Si droplets, the phase will be solid rather than liquid. The process can work without additions of a hydrocarbon, in which case the carbon is supplied through a reaction between the hot silicon and the graphite walls. This is usually not the preferred growth mode and additions of hydrocarbons are needed to obtain a high growth rate.
The formed microparticles of SixCy will move into the hot chamber or the subli- mation zone with the aid of the inert helium carrier gas. Once in the sublimation zone, the microparticles will sublime to form Si, Si2C, and SiC2, as in the case of seeded sublimation growth. A thermal gradient is applied, as illustrated in Figure 1. There are similarities between seeded sublimation growth and HTCVD in that solid particles sublimate in the reactor and the vapor condenses on a seed crystal maintained at a lower temperature.
However, the differences are quite dramatic and the outcome even more so. Take, for instance, the dynamics governing the growth Growth zone Sublimation zone Entrance zone Axial temperature Figure 1. Microparticles are formed in the inlet region of the system and trans- ported to the sublimation zone, where the particles sublime to finally condense on the growth sur- face.
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The heating of the particles in the sublimation zone must be efficient so that they sublime properly. This is obviously not a good scenario and material quality, as well as growth rate, will suffer. The growth rate is naturally influenced by the amount of input precursors, how- ever, too high concentrations will give rise to too large nuclei, which will be difficult to sublime.
As may have been deduced, if the thermal gradient between the seed and the sublimation zone is increased, the potential for sublimation will increase so that larger microparticles may be sublimed, thus larger amounts of precursors may be introduced and the growth rate will increase.