Lowering the density of dislocations in heteroepitaxial III-nitride layers: Effect of sapphire substrate treatment (review)

In this paper, different methods for lowering the dislocation density such as incorporation of the buffer layers, substrate patterning and nitridation, silan-ammonia treatment are reviewed and compared. Advantages and limitations of these methods as well as a specific mechanism to reduce the dislocation amount are discussed. Usually, high densities of threading dislocations within the range 10¹⁰…10¹¹ cm⁻² are present in typical thin nitride films that are directly grown on sapphire substrate. Using these methods for substrate preparation, the density of dislocations can be reduced to the value 1·10⁷ cm⁻². An important process that enables to obtain the high-quality GaN layers with the low dislocation density is patterning the sapphire substrate. The dislocation density of these substrates depends on the pattern shape and orientation of patterned strips on cplane sapphire. Layers of GaN grown on a cone-shaped pattern have the lowest dislocation density. In addition, patterning the sapphire substrate increases the external quantum efficiency of radiative structures and reduces the mechanical stresses in the nitride layers

Structural properties of nanocomposite SiO₂(Si) films obtained by ion-plasma sputtering and thermal annealing

The nanocomposite SiO₂(Si) films containing Si nanoclusters inside insulating SiO₂matrix are promising for many nanoelectronics applications. The ion-plasma sputtering of Si in O₂ containing gas mixture and following thermal annealing have been used to form nanocomposite SiO₂(Si) films. The structural properties of the obtained films have been studied using several methods. Among them, there were ellipsometry, IR spectroscopy, Raman spectroscopy, and AFM. Transition of SiOx matrix into insulating SiO₂ matrix has been revealed by IR spectroscopy. The shift of the transmittance spectra toward high frequency region and the increase in their intensity have been observed. The existence of amorphous and nanocrystalline phases into SiO₂(Si) films have been confirmed using Raman spectroscopy. Two material phases on the film surface, namely SiO₂ and Si, and surface density of silicon nanoclusters have been determined using AFM. It was shown that the size of silicon nanoclusters and their surface density depend on the level of enrichment with silicon of the initial SiOx film after ion-plasma sputtering and the temperature of subsequent annealing

Charge characteristics of the MOS structures with oxide films containing Si nanocrystals

The processes of charge accumulation in the MOS structures with SiO₂ films containing Si nanocrystals are investigated, depending on the conditions of their formation by pulsed laser deposition. High-frequency capacity-voltage characteristics of structures with the different thicknesses of films, sizes of Si nanocrystals, and their densities in the case of doping the films with gold and without it are measured. It is shown that the positive and negative charges are built-in, respectively, in the undoped films and those doped with gold. At the record of C-V curves, the accumulation of a positive charge is observed. The value of accumulated charge is higher in thin films and in the films doped with gold. The obtained results testify the possibility of the use of pulsed laser deposition for creation of memory structures based on the charge capture by Si nanocrystals

Transformation of SiOx films into nanocomposite SiO₂(Si) films under thermal and laser annealing

Oxide-assisted growth of Si nanocrystals includes deposition of a siliconenriched SiOx film at the first stage and annealing at the second one. The ion-plasma sputtering method has been used for deposition of the SiOx film. The influence of thermal and laser annealing on SiOx film properties has been investigated. Formation of silicon nanoislands on oxide film surface has been observed by AFM both after thermal and laser annealing. The height and surface density of the nanoislands depends both on the silicon content in the initial SiOx film and temperature of thermal annealing. The higher annealing temperature causes formation of large nanoislands, but their surface density decreases. Comparison of nanoislands created at thermal and laser annealing shows that in case of laser annealing the nanoislands are higher and their surface density is lower. The intensity of laser irradiation influences on nanoisland parameters significantly. The growth of electrical conductivity with thermal annealing temperature has been observed. The influence of gas atmosphere during annealing is also significant in case of higher temperatures. In case of laser annealing at the beginning at low laser irradiation intensities, the SiOx film conductivity increases, but the following growth of intensity causes the decrease in electrical conductivity

Nanostructuring the SiOx layers by using laser-induced self-organization

The processes of laser-induced transformation of SiOx oxide layers into the nanocomposite ones were studied. The possibility of phase separation in the form of Si nanocrystals surrounded by corresponding SiO2 oxide matrix under irradiation by nanosecond pulses of YAG:Nd+3-laser were shown. Laser radiation at the fundamental wavelength, λ1 = 1064 nm, and second harmonic, λ2 = 532 nm, were applied at researches. The size and surface concentration of nanofragments dependences on the intensity and wavelength of the laser irradiation have been determined from experimental data based on atomic force microscopy, infrared transmission spectra and electro-physical measurements. SiOx nanocomposite layers containing Si nanoparticles, the size of which depends on laser beam intensity and wavelength, have been obtained. The processes of nanoparticles formation occur mainly through generation and mass transfer of interstitial atoms in the solid mode (before the melting point threshold) due to the effect of laser thermal shock

Electron transport through nanocomposite SiO₂(Si) films containing Si nanocrystals

The current transport through insulating SiO₂ films with silicon nanocrystals in Si/SiO₂(Si)/Al structures has been investigated in the wide range of temperatures (82…350 K). The nanocomposite SiO₂(Si) films containing the silicon nanoclusters embedded into insulating SiO₂ matrix have been obtained by ion-plasma sputtering of silicon target and subsequent high-temperature annealing. Based on the detailed analysis of current-voltage characteristics, calculation of some electrical parameters has been performed and the mechanism of electron conductivity of nanocomposite SiO₂(Si) films has been ascertained. The electrical conductivity of the films is based on the mechanism of hopping conductivity with variable-range hopping through the traps near the Fermi level

The influence of H₂S and H₂ adsorption on characteristics of MIS structures with Si porous layers

The adsorboelectric effect arising in multilayered semiconductor structures based on the porous Si with catalytically active Pd electrodes due to action of low concentrations of hydrogen containing gases (Н₂, H₂S) at the room temperature is studied. The kinetic dependences of the change in output signals of the samples upon action of different concentrations of gas molecules are studied using the capacitancevoltage characteristic method. The isotherms of adsorption are derived. A physical model of the adsorption of hydrogen containing gases in these structures is proposed to explain the observed phenomena

Increase of planar homogeneity of multi-silicon structures by gettering treatments

Two types of gettering treatments are considered and compared from the viewpoint of their usefulness to decrease LD scatter over the wafer in multi-silicon photovoltaic structures. It was found that in both cases high degree of homogeneity in LD distribution over the sample surface and cleaning of the samples from recombination active impurities are achieved. Possible mechanisms of the homogenization are briefly discussed

Peculiarities of nanocrystalline silicon films growth on porous anodic alumina surface

The influence of porous alumina template morphology on silicon films growth at deposition by PE CVD has been investigated. As it was shown, the structural properties of silicon phases depend on the pore geometry and surface morphology of the anodized porous alumina substrate. In case of porous alumina formation in one stage, ripple-like morphology takes place. The growth of a-Si:H film is observed at deposition. After two-stage anodization, the porous alumina has tipped/ribbed morphology. In this case, usually a-Si:H film grows on the bottom of the pores, and nc-Si:H/a-Si:H one grows on the tips. In the case of deep pores, the nanocrystalline nc-Si:H film grows only above the top of the pores. The obtained results could be used when developing new types of photocells, sensors, nanophotonics and ionics devices

Peculiarities of the Field Emission with Porous Si Surfaces, Covered by Ultrathin DLC Films

Field emission of electrons from silicon tips with porous silicon layers on their surface has been investigated. The silicon tip arrays were formed by wet chemical etching of n-type Si. The wafers with piramidal emitters were then anodized to form on the surface a porous silicon layer. The conditions of the anodic etching of silicon in ethanol solution of HF under the illumination have been changed widely. The investigation of influence of the thin diamond-like carbon film on different porous silicon layers on electron field emission have been performed. The parameters of the emission efficiency such as field enhancement factors, emitting area factors and threshold voltages for comparison and characterization of different layer structures have been estimated from current-voltage dependences. The results show significant influence of preparation technology of layer structures on their emission properties