V. B. Shuman

Variation of the parameters and composition of thin films of porous silicon as a result of oxidation: Ellipsometric studies

The variation of the optical characteristics of thin films of oxidized porous silicon as a function of the preparation regime and subsequent heat treatment is investigated by ellipsometry. It is shown that the refractive index, optical thickness, and extinction coefficient of porous silicon films decrease monotonically, but the film thickness increases as the degree of oxidation of the silicon base layer increases. An analysis of the film thickness as a function of the degree of oxidation shows that it differs very little from the same dependence for the nonporous film. The composition of the films is determined from the measured refractive index at a wavelength λ=632.8 nm by means of curves calculated on the basis of the three-component Bruggeman model of the effective medium for layers with different initial porosities.

Features of the structure of a porous silicon carbide layer obtained by electrochemical etching of a 6H-SiC substrate

Transverse sections of the (11–20) cuts of a 6H-SiC substrate-porous SiC layer-epitaxial 6H-SiC layer structure were studied using electron microscopy. An intermediate layer is revealed between pores and unetched SiC which consists of a damaged region containing two-dimensional defects and a completely amorphous region. Energy-dispersive X-ray spectra measured within local (∼3 nm) areas in various regions of the transverse sections of the structure studied showed that the intermediate layer is enriched with carbon in comparison to the stoichiometric substrate composition. The excess carbon content is retained in the layer of epitaxial SiC contacting the porous layer.

Intense photoluminescence of porous layers of SiC films grown on silicon substrates

Luminescing porous layers have been prepared on SiC films grown on silicon substrates. The intensity of the photoluminescence increases very strongly as a result of electrolytic oxidation of porous layers. The spectrum of the pulsed photoluminescence consists of a series of overlapping bands from 1.8 to 3.3 eV. Investigations of the initial SiC films showed that they are nonstoichiometric and strongly disordered. Nonetheless, the intensity of the photoluminescence of the oxidized porous layers is much higher than can be obtained from correspondingly treated SiC crystals or crystalline films.

Properties of p+-n structures with a buried layer of radiation-induced defects

The p+-n structures based on n-type Si with dopant density 1.7×1013–1.2×1014 cm−3 were irradiated with 238Pu α particles. A layer containing radiation-induced defects with a density of the order of 3×1013 cm−3 was produced at a depth of 20 µm. This defect density gave rise to intense draining of nonequilibrium carriers in the injection-extraction regime with stationary injection as well as with pulsed generation by single particles. This makes it possible to treat the damaged layer as a plane, introduced into the bulk, with an infinite surface recombination rate. The radiation-induced defects also participated in decreasing the conductivity. A characteristic space charge distribution and, correspondingly, a bias dependence of the capacitance are observed in the structure under reverse bias. Despite the presence of formally three charge regions, four sections appear on the capacitance curve. This latter effect is due to the “additional” charge step arising in the contact potential difference field and is characteristic of compensated deep levels in semiconductors.

Use of direct wafer bonding of silicon for fabricating solar cell structures with vertical p-n junctions

A technology based on ion implantation and the direct wafer bonding of p+-p-n+ structures has been developed for multijunction silicon solar cells. The internal quantum efficiency of such structures is close to unity in the wavelength range 350–900 nm.

Electron-hole scattering in p-type silicon with a low charge-carrier injection level

A previously proposed method for determining the parameters of electron-hole scattering in indirect-gap semiconductors is used to investigate the properties of p-type silicon. Diode n+-p-p+ structures were used for the measurements. The results obtained by us indicate that complete dragging of the minority electrons by the majority holes is possible, even at room temperature, in p-type material with doping levels N>1018 cm−3.

Effect of anodic etching of heavily doped silicon on the location of the plasma minimum

It is shown experimentally that the minimum in the reflectance spectrum of heavily doped n-Si shifts strongly toward lower frequencies when porous Si layers form on it.

Characteristic features of silicon multijunction solar cells with vertical p-n junctions

A relatively simple technology (without photolithography) based on diffusion welding and ion-plasma deposition of an insulating coating has been developed for fabricating multijunction silicon solar cells with vertical p-n junctions. The effective collection factor for such structures is independent of the wavelength of the incident light in the wavelength range λ=340–1080 nm.

Nondestructive determination of the characteristics of porous silicon carbide layers

It shown that the porosity and thickness of a porous silicon carbide layer can be nondestructively monitored provided that the sample weight loss upon electrochemical etching is known and the reflectance spectrum exhibits interference.

Direct bonding of silicon wafers with a diffusion layer

The possibility of solid-phase direct bonding of silicon wafers having p+-or n+-type diffusion layers with a high surface dopant concentration has been demonstrated for the first time.