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- Vasiliauskas, Remigijus
(författare)
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Sublimation Growth and Performance of Cubic Silicon Carbide
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon carbide (SiC) is a wide band gap semiconductor satisfying requirements to replace silicon in devices operating at high power and high frequency at high temperature, and in harsh environments. Hexagonal polytypes of SiC, such as 6H-SiC and 4H-SiC are available on the power device markets. However, the cubic SiC (3C-SiC) polytype is still not industrially used, essentially due to the lack of 3CSiC substrates. This is mainly because of a high density of defects appearing in the crystals. Thus, it is critical to understand material growth and defect formation, and learn to control their appearance. Ensuring, that growth methods capable of large scale industrial production can be applied.The aim of this work was to develop operation conditions for fabrication of 3C-SiC crystals via understanding fundamentals of the growth process and to explore structural and electrical properties of the grown material, including its suitability for substrate applications. The physical vapor transport or sublimation process has already shown a capability to produce substantial quantities of large area and high quality hexagonal SiC substrates. In the present study a similar growth principle, but in a different geometry, namely sublimation epitaxy, was applied. Using this method very high growth rates (up to 1 mm/h) can be achieved for hexagonal polytypes while maintaining high material quality. Additionally, the growth process does not require expensive or hazardous materials, thus making the method very attractive for industrial use.When growing 3C-SiC directly on 6H-SiC, the substrate roughness does not have significant influence on the yield and quality of 3C-SiC. This is mostly due to the growth of homoepitaxial 6H-SiC which appears before the 3C-SiC. Structural characterization showed that 3C-SiC grown directly on 6HSiC exhibited the highest quality as compared with other substrate preparation, such as annealing or deposition of a 3C-SiC buffer layer. Thus, further investigation was devoted to the growth of 3C-SiC on 6H-SiC substrates.The parameter window for the growth of 3C-SiC is quite narrow. The temperature interval is from ~1675oC, where the material starts to nucleate, to ~1850oC, where an uncontrolled growth process begins. Si-rich conditions (high Si/C ratio) and high supersaturation are needed in the growth chamber for preferable 3C-SiC nucleation. Deviation from these parameters leads to the growth of homoepitaxial 6HSiC in spiral or 2D island mode along with cubic SiC with high defect density.Nucleation is the most important step in the growth process. The growth on 6H-SiC substrates commences from homoepitaxial 6H-SiC growth in spiral mode, which makes the surface perfect for 3CSiC nucleation. At temperature of ~1675oC the supersaturation is high enough and the 3C-SiC nucleation initiates in two-dimensional islands on the 6H-SiC spiral terraces. Control of the homoepitaxial 6H-SiC growth is a key element in the growth of 3C-SiC.SiC is a polar material having surfaces terminated by either silicon or carbon atoms, called Si- and C-face, respectively. The growth is different on both faces due to the different free surface energies. The lower surface free energy on the C-face causes more uniform nucleation of 3C-SiC and thereafter more uniform twinned domain distribution. Additionally, calculations showed that increase of growth temperature from 1675oC to 1775oC does not change the supersaturation ratio on the C-face due to a much higher surface diffusion length. This results in appearance of pits in the 3C-SiC layer with a 6H-SiC spiral. The pits were not observed on Si-face material as the supersaturation ratio was much higher. Pits formed in the early stages of growth were overgrown more effectively during the later stages.Characterization by transmission electron microscopy showed that transformation from 6H-SiC to 3C-SiC is not abrupt and can appear in two different modes. The first one is forming a few micrometers of polytypic transition zone consisting predominantly of 15R-, 6H- and 3C-SiC. The second one appears due to a competition between 3C-SiC and 6H-SiC resulting in a step-like intermixing zone between these polytypes. Four-fold twins were observed, which resulted in depressions at the surface of 3C-SiC. These defects expand proportionally to the layer thickness, thus drastically reducing usable area of thick layers.Electrical measurements revealed carrier mobility ~200 cm2/Vs at room temperature and the dominant charge carrier scattering is by neutral centers and phonons. The neutral centers originate from extended defects, such as 6H-SiC inclusions, stacking faults and twin boundaries. By growing 3C-SiC on atomically flat and vicinal substrates a preferential orientation of twin boundaries (TBs) was achieved. The mobility was higher in the material with twin boundaries parallel to the current flow, and lower when twin boundaries were perpendicular to the current flow. This was less pronounced at higher temperature as relatively fewer carriers have to overcome barriers created by TBs.Finally, the substrate capability of the 3C-SiC (111) was demonstrated by growth of a monolayer graphene, which was compared with graphene grown on hexagonal SiC poytypes. The quality of the graphene in terms of thickness uniformity and pit appearance was the best when grown on 3C-SiC. The lower quality on hexagonal substrates was attributed to a more difficult process control which is due to the more complex crystal structure.
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| 2. |
- Yazdi, Gholamreza, 1966-
(författare)
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Growth and Characterization of AlN : From Nano Structures to Bulk Material
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aluminum nitride (AlN) exhibits a large direct band gap, 6.2 eV, and is thus suitable forsolid state white-light-emitting devices. It is capable in spintronics because of its high Curietemperature if doped with transition metals. AlN can also be used as a buffer layer for growth ofdevice-grade GaN as well as for application in sensors, surface acoustic wave devices, and hightemperatureelectronics. AlN shows excellent field-emission performance in vacuummicroelectronic devices due to its small electron affinity value, which is from negative to 0.6 eV.In this sense, nanostructured AlN, such as AlN nanowires and nanorods, is important forextending our knowledge on the potential of nanodevice applications. For growth of bulk AlN thesublimation- recondensation (a kind of physical vapor transport growth) method is the mostsuccessful and promising crystal growth technique.In thesis the physical vapor transport (PVT) principle has been implemented for synthesisof AlN on 4H-SiC in sublimation epitaxy close space configuration. It has been shown that theAlN crystal morphology is responsive to the growth conditions given by temperature (1650-1900oC) and nitrogen pressure (200-800 mbar) and each morphology kind (platelet-like, needles, columnar structure, continuous layers, and free-standing quasi bulk material) occurs within anarrow window of growth parameters. Controlled operation conditions for PVT growth of wellaligned perfectly oriented arrays of AlN highly symmetric hexagonal microrods have beenelaborated and the mechanism of microrod formation has been elucidated. Special patterned SiCsubstrates have been created which act as templates for the AlN selective area growth. Themicrorods revealed an excellent feature of boundary free coalescence with growth time,eventually forming ~120 μm thick AlN layer which can be easily detached from the SiC substratedue to a remarkable performance of structural evolution. It was discovered that the locally grownAlN microrods emerge from sharp tipped hexagonal pyramids, which consist of the rare 2H-SiCpolytype and a thin AlN layer on the surface. Two unique consequences appear from the finding,the first is that the 2H-SiC polytype facilitates the nucleation of wurtzite AlN, and the second isthat the bond between the low angle apex of the pyramids and the AlN layer is very week, thusallowing an easy separation to yield free standing wafers. AlN nanowires with an aspect ratioas high as 600 have been grown with a high growth rate. Again, they have perfect alignmentalong the c-axis of the wurtzite structure with small tilt given by the orientation of the SiCsubstrate. The nanowires possess a single crystal structure with high perfection, since neitherdislocations nor stacking faults were revealed.The proposed growth concept can be further explored to enlarge the free standing AlNwafers up to a size provided by commercially available SiC four inch wafers. Also, AlN wafersfabricated by the present method may be used as seeds for large boule growth. AlN nanowires, asobtained in this study, can be used for creating a piezoelectric generator and field emitters withhigh efficiency.
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