Self-assembled metal/molecule/semiconductor nanostructures for electronic device and contact applications

We report a fabrication approach in which we combine self-assembled metal/molecule nanostructures with chemically stable semiconductor surface layers. The resulting structures have well controlled dimensions and geometries (∼4 nm Au nanoclusters) provided by the chemical self-assembly and have stable, low-resistance interfaces realized by the chemically stable semiconductor cap layer (low-temperature grown GaAs passivated by the organic tether molecules). Scanning tunneling microscope imaging and current-voltage spectroscopy of nanocontacts ton-GaAs fabricated using this approach indicate high quality, ohmic nanocontacts having a specific contact resistance of ∼1 × 10⁻⁷Ω·cm² and a maximum current density of ∼1×10⁷ A/cm², both comparable to those observed in large area contacts. Uniform 2-D arrays of these nanocontact structures have been fabricated and characterized as potential cells for nanoelectronic device applications.     

Optical,electronic and magnetic properties of the dilute magnetic semiconductor Mn:CuInte2

CuInTe₂ and CuInSe₂ are direct bandgap ternary chalcopyrite (I-III-VI₂) semiconductors which are the simplest ternary analogs of the II-VI zincblende compounds. Some of the chalcopyrites exhibit nonlinear optical properties which make them good candidates for optical device technology. We have done preliminary studies on the influence of Mn²⁺ on the optical, transport and magnetic properties of CuInTe₂ in analog to previous stud-ies of Mn:CdTe etc. Bulk polycrystalline p-type samples, with up to 12% Mn substi-tution, have been examined with polarized light microscopy, x-ray energy dispersive analysis, optical spectrometry, van der Pauw technique and SQuID magnetometer mea-surements. From the metallurgical studies, Mn is found to substitute for Cu but not In. The bandgap of Mn:CuInTe₂ is larger than 0.92 eV. Most of our samples show phonon limited mobility, with the highest mobility occurring at ≈75K. Without Mn substitu-tion, CuInTe₂(Se₂) is diamagnetic. Samples with 3% Mn substitution in Cu sites are paramagnetic. At higher Mn concentration (3% < x < 12%), the (Cu_1-xMn_x)InTe₂ sam-ples show antiferromagnetic coupling and a measured effective magnetic moment up to ≈5.4 μB/Mn atom. Samples with attempted Mn substitution in In sites (from 3% to 9%) show anomalous magnetic behavior at low temperatures.     

Electron-optical masking of semiconductor structures

Electron-optical techniques were employed to fabricate planar silicon transistors having a 1-micron strip width and a combined dimensional and registration tolerance of 1000 Å. A new electron-sensitive positive resist was used to define a conventional oxide diffusion mask. The exposure equipment is described and electrical parameters of the complete device are given.     

Self-assembled metallic nanocrystal structures for advanced non-volatile memory applications

Nanocrystal (NC) based non-volatile memories are a leading candidate to replace conventional floating gate memory. Substituting the poly-silicon gate with a layer of discrete nanocrystals or nanodots provides increased immunity to charge loss. Metallic nanocrystals have been found to be advantageous over Si- or Ge-based approaches due to good controllability of the size distribution and the achievable NC densities as well as increased charge storage capacity of metallic nanocrystals. Sufficiently high NC densities have been achieved to demonstrate feasibility for sub-32 nm node non-volatile memory devices.     

四方晶相HfO2电子结构和光学性质研究

采用密度泛函理论(DFT)框架下的局域密度近似(LDA),计算了四方HfO2晶体的电子结构,包括能带结构和态密度.在此基础上计算了四方Hf02晶体的光学线性响应函数,包括复介电函数、吸收光谱、复折射率和光电导谱.通过比较发现,计算结果与实验结果吻合较好,说明采用密度泛函理论的局域密度近似来计算HfO2材料的光学性质是比较可靠的.     

Laterally coupled periodic semiconductor laser structures: Blochfunction analysis

The Bloch function analysis is applied to the problem of periodical phase-locked semiconductor laser array structures. This method makes it possible to treat in the same manner both evanescent and leaky modes of such structures and to optimize these structures in order to get maximum content in the central lobe of emission far-field pattern. If one places high gain in the low-index array regions, the preferred array modes are leaky modes coupled either in-phase or out-of-phase depending on parameters of structure. The linear and nonlinear problems of the propagation of finite aperture beams in such laser structures are considered. Propagation of such a beam is described by the Schrodinger equation with `photon mass' determined by a curvature of photon dispersion     

Radiative efficiency in low-dimensional semiconductor structures

The ratio of the spontaneous emission and Auger recombination rates for quantum wells and wires is compared theoretically with that for bulk structures. Dramatic improvements in radiative efficiency are not predicted for these novel structures unless carrier densities are greatly reduced.     

Self-heating effects in basic semiconductor structures

Investigates the effects of self-heating on the high current I -V characteristics of semiconductor structures using a fully coupled electrothermal device simulator. It is shown that the breakdown in both resistors and diodes is caused by conductivity modulation due to minority carrier generation. In isothermal simulations with T=300 K, avalanche generation is the source of minority carriers. In simulations with self-heating, both avalanche and thermal generation of minority carriers can contribute to the breakdown mechanism. The voltage and current at breakdown are dependent on the structure of the device and the doping concentration in the region with lower doping. For all structures, except highly doped resistors with poor heating sinking at the contacts, the temperature at thermal breakdown ranged from 1.25T_i to 3T_i , where T_i is the temperature at which the semiconductor goes intrinsic. Hence, it is found that T=T_i is not a general condition for thermal (or second) breakdown. From these studies, an improved condition for thermal breakdown is proposed     

Transport boundary condition for semiconductor structures

Boundary conditions (BCs) to the Poisson and transport equations for stationary transport processes of non-equilibrium carriers in semiconductor structures are formulated. The applicability of the resulting BCs for several materials (metals, bipolar semiconductors, including ones in the quasineutrality approach, QNA) and their structures are analyzed for both closed and open circuits. As a first step, general BCs for different types of contacts between two materials and between a solid and the vacuum under thermodynamic equilibrium conditions are thoroughly discussed. Later on, BCs are presented for the general case of a contact between a solid (this contact is considered as a “free-surface” in a broad sense) and the vacuum under non-equilibrium conditions. The concept of “free-surface”, and its BCs, can be extended and applied to other interfaces such as semiconductor–insulator that is relevant in semiconductor devices. Finally, the case of a hetero-contact between two conducting media under a current flow is addressed. BCs for closed-circuit conditions are presented and it is shown that BCs for open-circuit can be obtained as a limit of the proposed ones when the electric current tends to zero. In all the considered cases the changes operated in the BCs if the QNA is invoked are carefully studied.     

Optical bistability in semiconductor periodic structures

A theoretical demonstration of optical bistability in periodic layered media, particularly in long-period GaAs/AlAs superlattices is presented. The proposed structure consists of a periodic multilayer system, as opposed to previously demonstrated nonlinear bistable devices which employed Fabry-Perot etalons. The optical resonance effect which is essential for bistable devices is, in this case, induced by a refractive index modulation. The nonlinear active medium is distributed in the whole structure rather than placed between the two mirrors of a Fabry-Perot cavity. It is shown by a complete calculation of wave propagation in the periodic nonlinear medium that a multiple valued feature appears in the structure's nonlinear reflectivity spectrum. The input/output characteristics of the structure exhibit bistable hysteresis similar to that of a nonlinear Fabry-Perot etalon     

Anomalous Kossel effect in semiconductor structures

A theoretical analysis is made of the intensity distribution within a Kossel line from a semiconductor crystal in an extremely asymmetric x-ray diffraction scheme. Crystals of thickness comparable with the x-ray penetration depth are evaluated as a potential means of bringing about the anomalous Kossel effect, which is predicted to occur in the vicinity of a degenerate point. The possibility is investigated of using a thin amorphous film to enhance the effect. Dispersion-surface geometry, excitation points, and Poynting-vector directions are examined in detail for the anomalous Kossel effect.     

Spontaneous emission in multilayer semiconductor structures

A theoretical analysis of spontaneous emission from a quantum well in dielectric multilayer structures, especially the influence of dielectric geometry on the relative intensities of guided and radiation modes and on polarization, is presented. We first discuss a relatively simple three-layer case, and subsequently a technologically more interesting five-layer structure that has been proposed for a high-power laser. The expression for the partial as well as total, emission rates is derived within a broader framework of coupled Heisenberg equations of motion for charge carriers and the quantized electromagnetic field. Thereby, the explicit mode decomposition of the Green tensor is avoided. Still, the beta factor for individual guided modes, which is a relevant quantity for the lasing threshold of a device, can be identified and a competition between modes is shown to exist in specific cases     

ZnS:Er电子结构和光学性质的第一性原理计算

采用第一性原理平面波赝势方法,分析了ZnS及掺杂Er3+之后的电子结构,进而预测了其光学性质。结果表明,掺杂之后体系的带隙变窄,同时价带与导带之间形成一个新的中间带。随着掺杂Er3+浓度的增加,体系的绝缘性呈下降趋势,吸收光谱则发生红移现象。这一结论与前人报道的实验结果相符。     

Influence of ionizing radiation in electronic and optoelectronic properties of III–V semiconductor compounds

The interaction between radiation and matter is very important in the study of materials used in the aerospace industry. The improvement of the resistance of various devices is crucial. In the present work we have produced simulation results of damages induced in electronic devices of III–V semiconductor compounds, using SRIM-TRIM, CASINO and GEANT4 programs. The energies used for ⁺ particles, SRIM-TRIM, were from 500 keV to 4 MeV for both FETs and HEMTs and the energies used for β⁻ particles, CASINO, were from 1 to 500 keV.     

The effect of electronic feedback on semiconductor lasers

Negative electronic feedback (EFB) has a strong effect on the performance of a bistable laser diode amplifier and on injection-locked lasers. Negative EFB drastically reduces the switching-up input power level and the hysteresis in the input-output power characteristic and in the tuning curves of the bistable laser amplifier. Furthermore, negative EFB leads to a reduction in the time delay associated with optical switching in diode laser amplifiers. This provides a means of enhancing the versatility of the proposed system in some potential applications. For an injected-locked laser, negative EFB achieves a broadening in the locking bandwidth and its dynamically stable region     

Trapped plasma oscillations in unipolar semiconductor structures

Based on the results of a large-signal computer simulation, a trapped plasma mode of oscillation is described in an n+-n-n+ silicon structure. The mechanism is similar to TRAPATT in many ways, but relies upon space-charge injection of majority carriers rather than a depletion region for the necessary initial electric field profile. A necessary design criteria ofN_{d}L < 10^{12}cm^-2is established.     

Size-quantization stark effect in quasi-zero-dimensional semiconductor structures

A theory of the size-quantization Stark effect in semiconductor nanocrystals under conditions in which the polarization interaction of an electron (hole) with a nanocrystal surface plays the dominant role is developed. It is shown that, in the region of interband absorption, the shifts of electron (hole) size-quantization levels in a nanocrystal subjected to an external homogeneous electric field are governed by the quadratic Stark effect. A new electrooptical method is proposed furnishing an opportunity to determine the critical radii of nanocrystals in which bulk excitons can appear.     

Digital architectures: a rhetoric of electronic document structures

As multimedia communication continues to grow, online technologies have dramatically changed the ways we use and present information-so much so, that we need new theories and models for understanding how technology and content are related in the new communication environment. The paper presents a theory of digital architecture, explains how SGML, HTML, and information architectures are related in the creation of a new online literacy and rhetoric, and discusses concepts, skills, and resources needed for educating tomorrow's technical communicators     

Some properties of semiconductor superlattices

We present here some of the results obtained on semiconductor superlattices from optical and magneto-optical experiments. These investigations give interesting informations on the electronic properties of these two-dimensional systems which have opened up a new field of investigations in semiconductor physics.