Terahertz radiation induced by the Wannier-Stark localization of electrons in a natural silicon carbide superlattice

Intense terahertz electroluminescence from SiC structures with a miniband electron spectrum caused by the natural superlattice has been observed. The shape of the terahertz radiation line, the linear dependence of the position of its maximum on the bias voltage, the typical value of the field required to induce the radiation, and the prevailing polarization of the radiation along the superlattice axis indicate that the observed radiation results from to the excitation of stationary Bloch oscillations of electrons in the natural silicon carbide superlattice.     

Wannier-Stark resonances under strong localization conditions in natural silicon-carbide superlattices

A detailed experimental investigation is made of the electronic transport under conditions of Wannier-Stark localization of carriers in a natural superlattice of hexagonal polytypes of silicon carbide. The 4H and 6H polytypes, which possess different superlattice and miniband spectrum parameters, are employed. Direct measurements of the electronic current versus the average electric field in the active region of the sample revealed a series of regions of negative differential conductivity in fields ranging from 500 to 2100 kV/cm. Analysis of the results shows that the observed current resonances are associated with the development of the Wannier-Stark quantization process and are due to conduction mechanisms such as hopping conduction, induced between the levels of a Wannier-Stark ladder by a resonant electron-phonon interaction, and the resonant interminiband tunneling from the first into the second miniband. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 105–109 (25 July 1996)     

Collective electronic excitations in a semiconductor superlattice in the Landau and Wannier-Stark ladder regime

Using a mean-field approximation, we have developed a systematic treatment of collective electronic modes in a semiconductor superlattice (SL) in the presence of strong electric and magnetic fields parallel to the SL axis. The spectrum of collective modes with zero wavevector along the SL axis is shown to consist of a principle magnetoplasmon mode and an infinite set of Bernstein-like modes. For non-zero wavevector along the SL axis, in addition to the cyclotron modes, extra collective modes are found at the frequencies |Nω _c±Mω _s|, which we call cyclotron-Stark modes (ω _c and ω _s are respectively the cyclotron and Stark frequencies, N and M are integer numbers). The frequencies of the modes propagating in “oblique” direction with respect to the SL axis show oscillatory behavior as a function of electric field strength. All the modes considered have very weak spatial dispersion and they are not Landau damped. The specific predictions made for the dispersion relations of the collective excitations should be observable in resonant Raman scattering experiments. Received 29 August 2002 / Received in final form 25 February 2003 Published online 4 June 2003 RID="a" ID="a"e-mail: 612033@inbox.ru     

Recrystallization Phase in He-Implanted 6H-SiC

The evolution of the recrystallization phase in amorphous 6 H-SiC formed by He implantation followed by thermal annealing is investigated. Microstructures of recrystallized layers in 15 keV He~+ ion implanted 6 H-SiC(0001)wafers are characterized by means of cross-sectional transmission electron microscopy(XTEM) and high-resolution TEM. Epitaxial recrystallization of buried amorphous layers is observed at an annealing temperature of 900℃. The recrystallization region contains a 3 C-SiC structure and a 6 H-SiC structure with different crystalline orientations.A high density of lattice defects is observed at the interface of different phases and in the periphery of He bubbles.With increasing annealing to 1000℃, 3 C-SiC and columnar epitaxial growth 6 H-SiC become unstable, instead of[0001] orientated 6 H-SiC. In addition, the density of lattice defects increases slightly with increasing annealing.The possible mechanisms for explanation are also discussed.     

Boron-related deep centers in 6H-SiC

6H-silicon carbide layers are grown by a liquid phase epitaxy (LPE) process. The layers are doped with boron either by ion implantation or during the LPE process from a B-doped silicon melt. Deep-level transient spectroscopy (DLTS), admittance spectroscopy and photoluminescence (PL) are used to investigate deep impurity centers. Two electrically active defect centers are detected: the isolated boron acceptor at E _B=E _v+0.3eV and the boron-related D-center at E _D=E _v+0.58eV. The yellow luminescence observed in these layers is proposed to be due to pair recombination via D-center and nitrogen donor. Formation and origin of the D-center are discussed.     

Three-dimensional localization of atoms in the polychromatic optical superlattice

Quasiclassical kinetic theory of the light pressure force has been applied to describe the localization of atoms (or ions) with the transition F=1→F=0 in the three-dimensional (3D) dissipative optical superlattice of a new type. Its action is based on the effect of the gradient force rectification in the polychromatic field: superposition of the three color far-off-resonant field and partially coherent resonant field. An approximate explicit solution of the kinetic equation for 3D motion of atoms in such a (multicolor) field has been achieved. This solution demonstrates the capability of the polychromatic superlattice to provide efficient cooling and strong spatial localization of the particles and to form an atomic (or ionic) grating with highly controllable characteristics.     

Compensating defect centres in semi-insulating 6H-SiC

Photoinduced transient spectroscopy (PITS) has been applied to study electronic properties of point defects associated with charge compensation in semi-insulating (SI) 6H-SiC substrates. The photocurrent relaxation waveforms were digitally recorded in a wide temperature range of 20–800 K and in order to extract the parameters of defect centres, a two-dimensional analysis of the waveforms as a function of time and temperature has been implemented. As a result, the processes of thermal emission of charge carriers from defect centres were seen on the spectral surface as the folds, whose ridgelines depicted the temperature dependences of emission rate for detected defect centres. The new approach was used to compare the defect levels in vanadium-doped and vanadium-free (undoped) SI 6H-SiC wafers.     

Anisotropic Carrier Transport in n-Doped 6H-SiC

Physics of the Solid State - In this paper, a study is presented on the charge transport in n-type doped semiconductor 6H-SiC (in both transient and steady state) using a nonequilibrium quantum...     

Wannier-Stark states in finite superlattices

Individual Wannier-Stark states are resolved in a current experiment over a wide electric-field range for a 5 and 4 period finite superlattice utilizing a hot-electron transistor. The observed field dependence of the tunneling transmission through the various states directly resembles the progressive localization of the wave functions. The basic transport through Wannier-Stark states in short-period superlattices is identified to be coherent. By tuning the Wannier-Stark state splitting with electric field into the optical phonon energy, the opening of new LO-phonon mediated transport paths is observed.     

Ion-implantation doping of crystalline 6H-SiC

The feasibility of ion implantation for p- and n-type doping of 6H-SiC has been studied. Single crystals were implanted at room temperature with 10¹⁷ ions/cm³ of B and Al, and of N and P, respectively, and step-annealed at temperatures up to 1900 K. The state of the crystal order was monitored by ion-beam-scattering techniques. After annealing at 1800 K, at a backscattering yield of about 1% in 0001-direction, maximum electrical activity of all dopants was observed within the range of 3–80% at room temperature. Impurity ionization levels were derived from conductivity measurements in the temperature range between 300–80 K, which also indicate the presence of compensating defects.     

Mn掺杂6H-SiC的第一性原理研究

本文通过密度泛函理论第一性原理平面波超软赝势计算方法计算了Mn掺杂6H-SiC的电子结构与光学性质。计算结果显示掺杂Mn后的6H-SiC为间接带隙p型半导体,且带隙较本征体有所降低,带隙由2.022 eV降为0.602 eV,电子从价带跃迁所需能量减少。掺杂后的Mn的3d能级在能带结构中以杂质能级出现,提高了载流子浓度,导电性增强。光学性质研究中,掺杂Mn后的介电函数虚部在低能处增加,电子激发态数量增多,跃迁概率增大。掺杂后的光吸收谱能量初值也较未掺杂的3.1 eV扩展到0 eV,反射谱发生红移。由于禁带宽度的降低使得光电导率起始范围得到扩展。