Preparation And Characterization Of Epitaxial Tin Oxide Films On SiC And Sapphire Substrates

The wide band gap oxide semiconductors have potential applications in light-emitting diodes, laser diodes, thin-film solar cells, transparent thin-film transistors and ultraviolet (UV) detectors due to their excellent optical and electrical properties. SnO2film is a multi-functional transparent oxide semiconductor. Owing to its low preparation temperature, high thermal stability and stable chemical properties, SnO2film has been used in transparent conducting electrodes, solar cells, gas sensors and architectural glass. Moreover, SnO2film has a wide band gap and a high exciton binding energy, so it is also a promising UV luminescent material. SnO2films prepared by traditional methods are almost polycrystalline. They not only have poor crystallinity with many defects, but also suffer grain coarsening. Therefore, they are not suitable for high-performance optoelectronic devices. In contrast, epitaxial SnO2films show better structural uniformity, more outstanding optical and electrical properties, and higher physical and chemical stability. The high-quality epitaxial SnO2films can be used for fabricating high-performance semiconductor devices. Meanwhile, they can provide well-defined surfaces for theoretical research of SnO2surface and interface. In this paper, epitaxial SnO2films with different orientations have been prepared on SiC and sapphire substrates by metalorganic chemical vapor deposition. The structural, optical and electrical properties were investigated in detail. This study is important for the scientific research and applications.The key research work and results of this paper are as follows:1. SnO2films were deposited on6H-SiC (0001) substrates at temperatures of500,600,700and750℃, using high purity Sn(C2H5)4, O2and N2as Sn source, oxidant, and carrier gas, respectively. The measurements indicated all the films are rutile SnO2. SnO2films deposited at500and600℃were grown along single SnO2[100] orientation. At higher temperatures of700and750℃, SnO2films became polycrystalline. SnO2films deposited at600℃had the best crystallinity. As the substrate temperature increased, the oxygen vacancies in SnO2films were reduced and the lattice parameters were enlarged. The deposited SnO2films consisted of three domains rotated by120°due to the different crystal structure and symmetry between SnO2and6H-SiC. The in-plane orientation relationship between the film and substrate is SnO2[010]//SiC<1010> and SnO2[001]//SiC <1210>. The lattice mismatch is about-11.07%along the SnO2[010] direction, while it is about3.57%along the [001] direction of SnO2. The heteroepitaxy of SnO2on6H-SiC can be regarded as a nearly continuous extension of the hcp atomic arrangements with substituting O atoms for Si atoms. As the substrate temperature increased from500to750℃, the carrier concentration of SnO2films decreased monotonously from1.l×1020to1.6×1018cm-3, while the resistivity increased from0.01to0.57Qcm. The Hall mobility was12.7cm2V-1s-1for the film grown at600℃. In the visible range, the average transmittance of6H-SiC substrate was about60%. The average transmittance of SnO2samples was more than60%with the highest transparency of75%. This result indicated that SnO2films showed anti-reflectivity on6H-SiC substrate.2. Rutile SnO2films were deposited on r-cut sapphire substrates at temperatures of500,550,600and700℃. SnO2films grown at500and550℃were polycrystalline. As the substrate temperature increased to600and700℃, SnO2films were grown along a single orientation which is perpendicular to SnO2(101) plane. As the substrate temperature increased, the crystallinity of SnO2films improved and the lattice parameters increased. SnO2film deposited at700℃had a dense surface and contained no (101) twin. The in-plane orientation relationship between SnO2film and r-cut sapphire substrate is SnO2[010]//Al2O3[1210] and SnO2[101]//Al2O3[1011]. The lattice mismatch is-0.42%along the SnO2[010] direction and11.31%along the SnO2[101] direction. As the substrate temperature increased from500to700℃, the carrier concentration, Hall mobility and resistivity changed from7.7×1018to1.6×1016cm-3,7.4to28.1cm2V-1s-1and0.11to14.22Ωcm, respectively. The optical band gaps of SnO2films prepared at500,550,600and700℃were3.88,3.80,3.75and3.96eV, respectively. The average transmittance of SnO2samples was78%in the visible range. At room temperature, SnO2film grown at500℃only showed a broad defect-related luminescence peak near530nm. As the substrate temperature increased, the intensity of this peak decreased and the peak position had red shift. For SnO2film grown at700℃, the defect-related luminescence peak shifted to605nm. In addition, a intense band edge luminescence peak appeared at333nm in the UV region. At low temperatures, the UV peak had blue shift while the intensity did not increase remarkably. This UV emission was ascribed to the recombination of local excitons in SnO2film. Moreover, an intense luminescence peak associated with oxygen vacancies in deep levels appeared near480nm at13K.3. Rutile SnO2films were deposited on a-cut sapphire substrates at temperatures of500,600and700℃. The SnO2film grown at500℃was polycrystalline with low crystallinity. At600and700℃, SnO2films were grown along a single orientation which is perpendicular to SnO2(101) plane. The in-plane orientation relationship between SnO2film and a-cut sapphire substrate is SnO2[010]//Al2O3[0001] and SnO2[101]//Al2O3[1100].The lattice mismatch is calculated to be9.47%along the SnO2[010] direction, and3.81%along the SnO2[101] direction. Three types of{101} twins were observed in SnO2films grown on a-cut sapphire. The (101) twin boundaries were parallel to the substrate surface, while the (101) twin boundaries were68°inclined to the substrate surface. These twins caused slight misorientation of the growth plane and many steps and inclinations in the film surface.4. Rutile SnO2films were deposited on m-cut sapphire substrates at temperatures of500,600,700and750℃. The film grown at500℃was (301) preferred with poor crystallization. As the substrate temperature increased to600and700℃, SnO2films were grown along single SnO2[001] orientation. At750℃, the film became polycrystalline again. SnO2film grown at700℃has the best crystallinity. The in-plane orientation relationship between SnO2film and m-cut sapphire substrate is SnO2[010]//Al2O3[0001] and SnO2[100]//Al2O3[1210].The lattice mismatch is calculated to be-0.42%along the SnO2[100] direction, and9.47%along the SnO2 [010] direction. The average transmittance of Sno2samples was80%in the visible range. Sno2film grown at700℃showed a broad luminescence peak near600nm at room temperature. At13K, a new broad luminescence peak appeared near480nm. The two luminescence peaks were attributed to the recombination of electrons and holes through oxygen vacancies with different energy levels in the band gap.According to the measurements and analysis of the structure of SnO2films, we proposed an epitaxial growth model of SnO2films on r, a and m-cut sapphire substrates. The reason why SnO2films can be epitaxially grown on sapphire substrates is that they have similar oxygen atomic arrangement.5. On the basis of the study above, we prepared SnO2:Sb films on6H-SiC (0001) and r-cut sapphire substrates at600℃using high purity Sn(C2H5)4, Sb(CH3)3, O2and N2as Sn source, Sb source, oxidant, and carrier gas, respectively. All the Sb-doped SnO2films were rutile and grown along the same orientation to undoped SnO2films. Sb was substitutionally doped in SnO2films. SnO2:Sb films were degenerate semiconductor. The ionized impurity scattering was the main scattering mechanism at low temperature, while the lattice vibration scattering became dominant at high temperature in SnO2:Sb films. As Sb concentration increased, the (101) twins in SnO2:Sb films on r-cut sapphire were reduced. The SnO2:Sb film with the lowest resistivity of1.3×10-3Ω cm and the highest carrier concentration of2.5×1020cm-3was obtained at5%Sb-doping on r-cut sapphire. The samples showed high transparency of～80%in the visible range. The band gaps were3.77,3.86,3.96,4.10and4.11eV for the0,1%,3%,5%and7%Sb-doped SnO2films on r-cut sapphire, respectively. As Sb concentration increased, the optical band gap of SnO2:Sb films were enlarged. For the SnO2:Sb film on6H-SiC (0001) substrate, it showed the lowest resistivity of9.3×10-4Ω cm and the highest carrier concentration of3.4×1020cm-3at3%Sb-doping.