Structural properties of porous silicon/SnO2:F heterostructures

In this work we present structural studies made on SnO₂ deposited on macroporous silicon structures. The porous silicon substrates were prepared by anodization of p-type silicon wafers. The SnO₂ doped layers were synthesized by the sol-gel method from SnCl₄·5H₂O-ethanolic precursor, where the effect of fluorine doping level on structural properties was investigated. The fundamental structural parameters of tin oxide such as the lattice parameter and the crystallite size were studied in correlation with the dopant concentration. In addition, the effect of fluorine incorporation into the structure of tin oxide was analyzed on the basis of theoretical calculations that take into account the structural factor. The results obtained indicate that incorporation of fluorine occurs only at substitutional sites for SnO₂ deposited on porous silicon.     

纳米碳管/氧化锌异质结构的合成及发光性质

以纳米碳管（CNTs）为基体、铜为催化剂,采用催化碳热还原方法直接合成出具有异质结构的纳米碳管/氧化锌（CNT/ZnO）复合材料。利用扫描电镜、透射电镜及X射线衍射等手段研究了异质结构CNT/ZnO复合材料的形态和结构。发现氧化锌纳米线在纳米碳管表面的生长过程遵循催化剂诱导的汽-液-固（VLS）机制;氧化锌纳米线与铜催化剂和纳米碳管之间分别存在明显的界面,并且氧化锌纳米线与纳米碳管均保持了规整的晶体结构。同时也发现在大直径纳米碳管上易于形成高密的氧化锌纳米线;随沉积温度的升高ZnO的形态由线到棒最后形成颗粒。异质结构CNT/ZnO复合材料的诱导发光性能不同于氧化锌纳米线和纳米碳管,在蓝光区域的发光强度远大于紫外发光强度。     

Electrical and optical properties of p-SiC/n-GaN heterostructures

We report on the optical, electrical and structural properties of GaN films heteroepitaxially grown by low pressure chemical vapor deposition on 6H-SiC substrates. We employed photoluminescence (PL), Hall effect measurements, scanning tunneling microscopy (STM) and X-ray analysis to determine the quality of our films. Heterojunction diodes were fabricated on p-type SiC and characterized by temperature dependent current–voltage and capacitance–voltage techniques. The results are interpreted within the thermionic emission model and the barrier found is attributed to the conduction band offset between 6H-SiC and wurtzite GaN. The diodes show electroluminescence of the donor-acceptor pair recombination type of 6H-SiC at room temperature. By analysis of the injection behavior we can interpret our data, determining the high valence band offset between 6H-SiC and -GaN to 0.67 eV. This high valence band offset favors applications for hetero-bipolar transistors (HBT).     

Comparative study of electro-physical properties of heterostructures containing PECVD nanocrystalline and anodic porous silicon layers

We performed a comparative study of the electro-physical properties of heterostructures containing PECVD nanocrystalline silicon (nc-Si) and electro-chemically etched porous silicon (PS) layers over a wide range of thicknesses, in terms of their energy parameters. Based on the proposed analytical expressions and the experimental current-voltage and capacitance-voltage characteristics, we studied the characteristics of the surface states at the nc-Si(or PS) interfaces in Pd-nc-Si(or PS)-p-Si heterostructures. The results revealed that the surface states play an essential role in the carrier transport in both types of heterostructures that were investigated.     

Electrical properties of ZnTe-ZnSe heterostructures

ZnTe-ZnSe heterostructures obtained by vacuum epitaxy under various thermal conditions have been studied. The results of structural investigations and also the dependences of the current-voltage and capacitance-voltage characteristics on the layer growth conditions are described.

The electrical properties of heterojunctions obtained at substrate temperatures of 300°–500°C, i.e. when the ZnTe or ZnSe formed single-crystal films, have been investigated. It is shown that in forward biased heterojunctions the current flow may be described by the tunnel-recombination mechanism of Riben and Feucht. The soft breakdown observed in reverse biased heterojunctions is caused by impact ionization. The capacitance measurements indicate that the effective number of charged interface states is approximately 10¹²-10¹³ cm^-2. The band diagram of the ZnTe-ZnSe heterojunction is proposed.     

From crystalline germanium-silicon axial heterostructures to silicon nanowire-nanotubes

One-dimensional (1D) nanostructures have attracted considerable attention as a result of their exceptional properties and potential applications. Among them, 1D axial heterostructures with well-defined and controlled heterojunctions between different nanomaterials or between different 1D nanostructures (i.e., nanowire-nanotube heterojunctions) have recently become of particular interest as potential building blocks in future high-performance nano-optoelectronic and nanoelectronic devices. Here, we report on the preparation and characterization of crystalline silicon nanowire-nanotube (SiNW-NT) heterostructures with controlled geometry, kinked and unkinked, and composition using germanium-silicon nanowire heterostructures with abrupt heterojunctions (~2 nm wide) as a template via the VLS-CVD mechanism.     

Comparison of the top-down and bottom-up approach to fabricate nanowire-based silicon/germanium heterostructures

Silicon nanowires (NWs) and vertical nanowire-based Si/Ge heterostructures are expected to be building blocks for future devices, e.g. field-effect transistors or thermoelectric elements. In principle two approaches can be applied to synthesise these NWs: the ‘bottom-up’ and the ‘top-down’ approach. The most common method for the former is the vapour-liquid-solid (VLS) mechanism which can also be applied to grow NWs by molecular beam epitaxy (MBE). Although MBE allows a precise growth control under highly reproducible conditions, the general nature of the growth process via a eutectic droplet prevents the synthesis of heterostructures with sharp interfaces and high Ge concentrations. We compare the VLS NW growth with two different top-down methods: The first is a combination of colloidal lithography and metal-assisted wet chemical etching, which is an inexpensive and fast method and results in large arrays of homogenous Si NWs with adjustable diameters down to 50 nm. The second top-down method combines the growth of Si/Ge superlattices by MBE with electron beam lithography and reactive ion etching. Again, large and homogeneous arrays of NWs were created, this time with a diameter of 40 nm and the Si/Ge superlattice inside.     

Photoelectric properties of solar cells based on GaPNAs/GaP heterostructures

It is shown that photovoltaic converters (PVCs) can be based on GaPNAs/GaP heterostructures, which are of considerable interest for the creation of multijunction solar cells on silicon substrates. It is established that p-i-n structures with undoped GaPNAs layer provide for a more effective separation of charge carriers, which makes it possible to obtain a greater short-circuit current than that in p-n structures with an n-type base. A specific feature in spectral characteristics of the proposed PVCs is the presence of two peaks in the spectra of quantum efficiency, which is related to a complicated band structure of GaPNAs.     

Functional properties of silicon nanocrystals in oxygen-rich amorphous matrices formed by laser irradiation of substoichiometric silicon oxides

Silicon-based nanostructured thin films have been obtained through laser irradiation of amorphous silicon oxides. The optoelectronic properties of nanostructured films largely differ from their amorphous counterpart, exhibiting optical gap narrowing, wavelength-dependent spectral modification of the photoluminescence (PL) and conduction mechanism variations. In particular, following the hydrogen effusion and related defect density increase, a spectral red shift and PL intensity quenching is detected for λ_exc=514.5 nm, whereas PL enhancement and spectral blue shift is observed at λ_exc=632.8 nm. Different thermal activation regimes of conductivity are also detected by conductivity measurements.Such results are discussed in terms of microstructural changes from an hydrogenated amorphous network to a nanostructured two-phase material where wavelength-selective excitation of radiative recombination channels and temperature-dependent conductivity paths occurs.     

Effect of interfaces on the magnetoelectric properties of Co/PZT/Co heterostructures

We have studied Co (film)/PZT (substrate)/Co (film) heterostructures with plane-parallel interfaces, produced by direct growth of cobalt films 0.5 to 3.5 μm thick on PZT surfaces smoothed to a subnanometer level (where PZT stands for a ceramic PbZr_0.45Ti_0.55O₃ lead zirconate titanate ferroelectric substrate). The results demonstrate that they possess magnetoelectric properties comparable in magnitude to those characteristic of known structures but, in contrast to these latter, allow one to dispense with the condition that the volume fractions of the ferromagnetic and ferroelectric components be roughly equal. The interface is shown to play a key role in determining the magnetoelectric response of the heterostructures: above 9 mV/(cm Oe) (11.7 mV/A) in a magnetic field of 50 Oe (3980 A/m) at a frequency of 100 Hz and room temperature. The heterostructures are potentially attractive for use as nonvolatile sensors in household devices.     

The photosensitivity of amorphous-crystalline silicon heterostructures with an inversion channel

We investigate the effect of the resistivity of crystalline silicon (1.5–40 kΩ cm) and amorphousfilm thickness (200–2000 Å) on the photoelectric properties of (a-Si/c-Si) heterostructures based on high-resistance p-Si. The investigated heterostructures exhibit inversion surface-band bending in crystalline silicon. The presence of a conducting channel ensures accumulation of nonequilibrium carriers upon illumination of the areas separated from the electrode by distances that exceed by far their diffusion length. The heterostructures exhibit high photosensitivity, including in the UV spectral region. The spectral characteristics of such structures in the visible and near-IR regions are analogous to those of silicon tunnel MIS structures.     

Electronic properties of bottom gate silicon nitride/hydrogenated amorphous silicon structures

The electronic properties of silicon nitride/hydrogenated amorphous silicon (SiN/a-Si:H) interfaces are studied with complementary techniques: quasistatic capacitance measurements achieved on c-Si/SiN/a-Si:H/Al MIS structures, and dark conductivity, steady-state photoconductivity and modulated photocurrent (MPC) experiments performed on glass/a-Si:H and glass/SiN/a-Si:H samples fitted with two coplanar Al electrodes, using the same SiN and aSi:H layers as in the MIS structures. Results of bias annealing experiments on the MIS structures are explained in the framework of the defect-pool model taking account of a fixed positive charge in the insulator, which should yield a slight electron accumulation in the a-Si:H close to the SiN/a-Si:H interface under zero bias equilibrium conditions. This electron accumulation is clearly put into evidence from the experiments carried out on the coplanar samples, where we observe that the conductivities in the dark and under illumination are much higher in presence of the bottom SiN layer. The SiN layer also induces a significant decrease of the density of states above the Fermi level determined from MPC, which also confirms the changes in the defect density stated by the capacitance measurements and in agreement with the defect-pool model predictions.     

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications

Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley-Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.     

Photoelectric properties of n-TiO2/p-Co0.7Ni0.3O heterostructures

The main photoelectric parameters of n-TiO₂/p-Co_0.7Ni_0.3O film heterostructures obtained by sequential sputtering of corresponding metal targets in oxidizing atmosphere have been determined. The heterostructures are photosensitive in a wavelength range of 225–385 nm and exhibit no photoresponse in the visible spectral range. The proposed structures can be used for creating photodetectors with high spectral selectivity.     

Evaluation of AlGaN/GaN heterostructures properties by QMSA and AFM techniques

Atomic force microscopy and Quantitative Mobility Spectrum Analysis (QMSA) were applied for characterization and evaluation of the quality of AlGaN/GaN heterostructures. The structural uniformity, growth mode and electrical properties of the heterostructures were determined. The obtained results indicated that the time of growth of the low temperature GaN nucleation layer influenced the morphology and electrical properties of the AlGaN/GaN heterostructure.     

Silicon-germanium heterostructures — advanced materials and devices for silicon technology

The continuing massive investment in silicon technology and the unique physical and chemical properties of the Si-SiO₂ system will ensure the dominance of silicon in microelectronics well into the 21st century. This momentum stimulates development of new materials which should further enhance the performance of silicon microelectronic circuitry. Such materials must, however, be compatible with silicon processing technologies. Major advances in silicon technology are now in prospect due to breakthroughs in molecular beam epitaxy (MBE) growth which have occurred over the last decade and which have enabled silicon to be alloyed to its nearest neighbours in the periodic table — Ge, C, and Sn. The Si/Si_1–xGe_x heteroepitaxial material system in particular is emerging as a strong candidate to form a silicon-based heterojunction technology. The incorporation of thin, strained, (pseudomorphic) layers of Si_1–xGe_x in silicon allows significant valence band and conduction band edge misalignments to be realized along with appreciable reductions in bandgap energies. Bandgap engineering-such a powerful tool for modifying semiconducting properties (and previously the reserve of compound semiconductors) — thus becomes accessible to the mainstream microelectronics material. This review considers the dramatic impact SiGe could have on future silicon microelectronics.     

Multilayer AlN/AlGaN/GaN/AlGaN heterostructures for high-power field-effect transistors grown by ammonia MBE on silicon substrates

We report preliminary results on the transfer of the ammonia MBE technology of AlN/AlGaN/GaN/AlGaN heterostructures to silicon substrates. Optimization of the growth conditions allowed the number of macroscopic cracks in the epilayers to be reduced and ensured the growth of heterostructures with two-dimensional electron gas, which are suitable for the creation of field-effect transistors. The saturation current of prototype devices based on the heterostructures grown on silicon substrates are comparable with the analogous parameter of devices grown on sapphire and exhibits no decrease related to thermal scattering at high bias voltages.     

InAsSbP/InAs heterostructures for thermophotovoltaic converters: Growth technology and properties

A liquid phase epitaxy (LPE) technique for growing indium arsenide (InAs)based narrow-bandgap semiconductor compounds for thermophotovoltaic (TPV) applications has been developed. InAs-based multicomponent solid solutions and InAs/InAsSbP heterostructures with E _g = 0.35–0.6 eV are promising materials for TPV converters that operate at an emitter temperature of 1000–2000°C. The sensitivity of new TPV elements is extended toward longer wavelengths (up to 3.8 μm), which ensures effective conversion of low-energy photons. The epitaxial films of quaternary InAsSbP solid solutions were obtained by LPE from a supercooled solution melt and by the liquid phase electroepitaxy technique with controlled doping of a growth solution from a single liquid source of components. The films have homogeneous compositions and highly perfect crystal structures. The values of reverse saturation currents in n-InAs/p-InAsSbP heterostructures are close to theoretical predictions.     

Ferroelectric/superconductor heterostructures

This review covers the fabrication and characterization of ferroelectric/superconductor heterostructures such as Pb(Zr_xTi_1−x)O₃/YBa₂Cu₃O_7−δ (YBCO), BaTiO₃/YBCO and Ba_xSr_1−xTiO₃/YBCO etc. on various single crystal substrates. Pulsed laser deposition, laser molecular beam epitaxy, and magnetron-sputtering methods are compared. This report shows that pulsed laser deposition equipped with in situ reflection high-energy electron diffraction is a good method to control the growth mode of YBCO thin films. Furthermore, laser molecular beam epitaxy is a superb method for research of complex oxide films and their superlattices. Atomic force microscopy and transmission electron microscopy showed the ferroelectric films grown on the rough surface of the YBCO films produced high-density planar defects in the film and is detrimental to the ferroelectric/dielectric properties of the heterostructures. Therefore, for device usage, it is more advantageous to use SrRuO₃ than YBCO as the bottom electrode material. For growing atomically smooth surface films step-flow mode is highly recommended. Prospects of microwave device application of the ferroelectric/superconductor heterostructures are discussed, and proposed the BSTO films as the best candidate for passive microwave components.     

Facile fabrication and enhanced gas sensing properties of ZnSnO3/NiO heterostructures

Journal of Materials Science: Materials in Electronics - The sensing performance of gas sensitive materials can be improved by adjusting their microstructure and electronic structure. In this work,...