Band-structure analysis of the conduction-band mass anisotropy in 6Hand 4HSiC

Abstract

The band structures of 6H SiC and 4H SiC calculated by means of the full-potential linear-muffin-tin orbital method are used to determine the effective mass tensors for their conduction-band minima. The results are shown to be consistent with recent optically detected cyclotron resonance measurements which find the ratio of cyclotron masses for B ⊥ c to B c to be larger (smaller) than unity for the 6H (4H) polytype. However, contrary to previous suggestions, appreciable anisotropies in the c-plane are found. For 6H SiC, a strong dependencies on band-filling is predicted because of the occurence of a double well minimum along the ML-axis. The calculated mass tensors for 3C and 2H are also reported. The technological potential of SiC for high-temperature, high-power and high-frequency electronic devices as well as the intrinsic scientific interest of the polytypism of SiC have been recognized for several decades. Only recently, however, has it become possible to obtain electronic grade quality single crystal single polytype material. Recently, it was found that 4H SiC exhibits a much smaller (and opposite) anisotropy of the electron mobility with respect to the c-axis than does 6H SiC. 1,2 It has been unclear up to now whether this could be attributed to a corresponding anisotropy in the effective mass tensors. Rather conflicting results have been reported on the effective masses as determined by a fitting of IR absorption spectra of the excited shallow donor states 3,4 and by optically detected cyclotron resonance (ODCR). 5,6 In this letter, calculated band structures of 6H SiC and 4H SiC are used to determine the effective mass tensors at the conduction-band minimum. Our results are in good agreement with the ODCR results on the anisotropies with respect to the c-axis for 6H and 4H SiC masses and hence support a mass-anisotropy as the cause for the mobility anisotropies. Our calculations also predict appreciable anisotropies of the mass tensors in the c-plane. In addition, they predict a strong dependency of the masses on band-filling in the case of 6H SiC due to a doublewell-like conduction-band minimum.Our band-structure calculations were performed within the density functional theory in the local density approximation (LDA) 7 using the full-potential (FP) linear muffin-tin orbital method. 8,9 They differ in some small, but for the present purpose relevant details, from our earlier results 10 which used the atomic sphere approximation (ASA). 11 The previous calculations were already found to provide a satisfactory account of the UVreflectivity spectra. 12 In the present work, we performed calculations on a fine mesh of k-points in order to examine the details of the energy-band surfaces close to the conduction-band minima.In 6H SiC, the minimum is found about halfway between M and L along the M L ≡ U -axis of the 6H-Brillouin zone. 13 This leads to a 6-valley model in which the valleys occur in 3 pairs with each pair being separated by a small barrier (of 9.5 meV) at the three equivalent M points. The lowest b...

Ballistic electron emission microscopy study of Schottky contacts on 6H- and 4H-SiC

“…In fact, it turns out that band structure calculations indicate that a second CBM does exist in 4H-SiC at about 0.11-0.13 eV above the first, 9,10 in relatively good agreement with our BEEM measurements. Our BEEM measurements hence provide the first direct experimental confirmation of these calculations.…”

Phase stability, electronic structure, and optical properties of indium oxide polytypes

“…Not only the band gap, but also the band dispersion comes out incorrectly and this effect is well pronounced in transition metal oxides (see, e.g., Ref. 55,56,58,[61][62][63][64][65][66] Analysis shows that calculated effective masses for In 2 O 3 -I and -II are almost the same, while those for In 2 O 3 -III are much smaller. Hence, carrier mobility in In 2 O 3 -III is expected to be larger than that in In 2 O 3 -I and -II.…”

Free electron density and mobility in high-quality 4H–SiC

“…They established that the nitrogen donor has two different ionization energies, depending on the substitutional position of the donor ͑cubic or hexagonal͒ in the carbon sublattice. Concerning the structure of the conduction band, it is generally agreed ͑Lambrecht and Segall, 4 Persson and Lindefelt 5 ͒ that there exist three equivalent minima ͑six half-minima͒ at the M points of the Brillouin zone. The electron mobility was investigated by many groups.…”