Papers by Keyword: Heterostructures

Abstract: The most common technological solution for increasing the light efficiency of the LEDs based upon AlGaInP heterostructures are discussed in the paper. The creation of LEDs with the inclusion of quantum wells and quantum dots in the active region, removing the original base and placing the LED on a new substrate, the replacement of the absorbent substrate by the reflective, using light-reflective surfaces such as Bragg reflectors or a mirror base (substrate), the list of new based materials for the LEDs based upon AlGaInP heterostructures as same as sapphire, glass, gallium phosphide, silicon and silicon carbide are presented. Therefore, new advanced methods of emission power are emerging.

Abstract: In the last decade, silicon carbide (SiC) has gained a remarkable position among wide bandgap semiconductors due to its high temperature, high frequency, and high power electronics applications. SiC heterostructures, based on the most prominent polytypes like 3C-SiC, 4H-SiC and 6H-SiC, exhibit distinctive electrical and physical properties that make them promising candidates for high performance optoelectronic applications. The results of simulations of nn-junction 3C-4H/SiC and 6H-4H/SiC heterostructures, at the nanoscale and microscale, are presented in this paper. Nanoscale devices are simulated with QuantumWise Atomistix Toolkit (ATK) software, and microscale devices are simulated with Silvaco TCAD software. Current-voltage (IV) characteristics of nanoscale and microscale simulated devices are compared and discussed. The effects of non-ideal bonding at the heterojunction interface due to lattice misplacements (axial displacement of bonded wafers) are studied using the ATK simulator. These simulations lay the groundwork for the experiments, which are targeted to produce either a photovoltaic device or a light-emitting diode (working in the ultraviolet or terahertz spectra), by direct bonding of SiC polytypes.

Abstract: Based on a Kronig-Penney model the electronic properties of heteropolytypic superlattices consisting of lamellas of 3C-and 4H-SiC polytypes with thicknesses below ten nanometers are analysed. Due to the large difference in the electron negativity and the resulting high barriers between the different polytypic lamellas an increased number of minizones compared to other materials are formed. The field strength for the appearance of the negative differential resistance in the heteropolytypic superlattice is lower than the critical fields in the silicon carbide polytypes.

Abstract: This paper presents a study of the vertical current transport in a graphene (Gr) heterostructure with Al_xGa_1-xN/GaN, which represent the main building block of a novel high frequency device, the hot electron transistor (HET) with Gr base. The morphological and electrical properties of this heterostructures have been investigated at nanoscale by atomic force microscopy (AFM) and conductive atomic force microscopy (CAFM). In particular, local current-voltage measurements by the CAFM probe revealed the formation of a Schottky contact with low barrier height (∼0.41 eV) and excellent lateral uniformity between Gr and AlGaN. Basing on the electrical parameters extracted from this characterization, the theoretical performances of a HET formed by a metal/Al₂O₃/Gr/AlGaN/GaN stack have been evaluated.

Abstract: AlGaN/GaN heterostructures are important materials for the fabrication of high power and high frequency devices. However, the mechanisms of Ohmic contacts formation on these systems are continuously under scientific debate. In this paper, a structural and electrical investigation of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures is reported. In particular, the behavior of Ti/Al/Ni/Au multilayers was monitored at different annealing temperatures. The contacts became Ohmic after annealing at 750°C and showed a decreasing temperature behavior of the specific contact resistance R_C, described by a thermionic field emission mechanism. On the other hand, annealing at 850°C led to a further reduction of R_C , with a slightly increasing dependence of R_C on the measurement temperature (here regarded as a “metal-like” behavior). The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.

Abstract: It is known that the rate of introduction of radiation defects into the Schottky barrier of GaAs and InP diodes depends on the presence of the electric field. The main aim of this research is to study the influence of the built-in electric field at the p-n junction on the resistance of dual AlGaAs heterostructure IR-LEDs under gamma rays irradiation. It is determined that the rate of defects introduction in the presence of built-in electric field or reverse bias at p-n junction is considerably less than the rate of defects introduction into the neutral region without electric field. Then the possible ways of improving the radiation resistance of infrared LEDs are considered.

Abstract: In this paper we present the results of investigation of the power mode influence on the resistance to fast neutron irradiation of IR-LEDs based on AlGaAs heterostructures. The investigation shows that there are 2 stages of LED emissive power lowering. At the first stage the emissive power decreases due to reorganization of existing defective structure. At the second stage it happens as the result of radiation defects introduction. The rate of defects introduction that influences the emissive power lowering at the first stage in the space charge region of the embedded p-n junction is higher than in the neutral region. The forward current flowing under irradiation results in partial annealing of introduced defects and consequently to resistance growth at the first stage of emissive power lowering. The LED power mode doesn’t contribute essentially to the power lowering at the second stage while observed difference is due to decrease of contribution of the first stage to the whole process of emissive power lowering.

Abstract: Micro Raman characterization has been used to determine the stress status of 3C-SiC epilayer grown on pseudomorphic-Si thin layer on Si_1-xGe_x/Si(001). The strain conditions of the Si_1-xGe_x films grown on Si(001) have been determined by the analysis of additional Silicon Raman peaks, which Raman shifts are related to the lattice parameter. Through the analysis of the Raman spectra, the correlation between the Si_1-xGe_x film, the crystal quality and the stress relaxation of the 3C-SiC as a function of the Germanium fraction (x), have been evaluated. The increase of Germanium fraction determines the reduction of the voids density located at the 3C-SiC/Si interface and the relaxation of the stress within the epilayer. Moreover, the 3C-SiC crystal quality, monitored by the Full Width at Half Maximum of the TO Raman mode, remains unchanged for any Germanium fraction values.

Abstract: The presented radiation model allows evaluating the contribution of ohmic contacts metal-semiconductor into LED radiation resistance and, thus, can be used when developing the design and the production technology of LED with required radiation resistance.

Abstract: Crystalline germanium nanoclusters (NCs) are grown by a molecular-beam epitaxy technique on chemically oxidized Si (100) surface at 700oC. Deposition of silicon on the surface with Ge nanoclusters leads to surface reconstruction and formation of polycrystalline diamond-like Si coverage, while nanoclusters core becomes tetragonal SiGe alloy. Possible mechanisms for nanoclusters growth are discussed. Selective photoexcitation of Ge or SiGe nanoclusters or space-charge layer of underlying Si allows to observe two non-equilibrium steady-states with higher and lower conductivity values as compared to the equilibrium one. The persistent photoconductivity (PPC) behaviour was observed after excitation of electron-hole pairs in Si (001) substrate. This effect may be attributed to spatial carrier separation by macroscopic fields in the depletion layer of the near-surface Si. Decreasing of surface conductivity, driven by optical recharging of NCs and Si/SiO₂ interface states, is observed in the spectral range from 0.6 to 1.0 eV. Conductivity drop is discussed in the terms of hole accumulation by Ge-NC states enhancing the local-potential variations and, therefore, decreasing the surface conductivity of p-Si.