A. A. Lomov

Raman and X-ray studies of nanocrystals in porous stain-etched germanium

Abstract Using a combination of stain-etching with subsequent annealing in hydrogen, porous germanium films with a high concentration of germanium nanocrystals (NCs) were prepared for the first time. Structural studies of the films were performed by X-ray reflectometry and high-resolution triple-crystal diffractometry and Raman light spectroscopy methods. From a thorough analysis of the experimental data obtained by both methods, it was revealed that the films consist of germanium NCs with average sizes approximately 8–10 nm in annealed films. Other basic information about thickness, porosity, roughness, and lateral coherence length of the films is also presented.

Characterization of the structure of porous germanium layers by high-resolution X-ray diffractometry

The surface morphology and the structure of porous germanium layers obtained by chemical etching of n-type single-crystal Ge(111) substrates with their subsequent annealing in hydrogen atmosphere are studied by high-resolution X-ray diffractometry. It is established that upon etching a 1.5 to 2.0-μm-thick porous germanium layer is formed, which contains quasi-ordered microinhomogeneities in the form of elongated pits with characteristic dimensions of 1 μm and an average distance between them of 3–4 μm. The layer bulk has pores with radii ranging within 25–30 nm and nanocrystallites with an average size of 10 nm, with the average porosity being 56%.

Study of the amorphization of surface silicon layers implanted by low-energy helium ions

The structural changes in surface layers of Si(001) substrates subjected to plasma-immersion implantation by (2–5)-keV helium ions to a dose of D = 6 × 1015–5 × 1017 cm–2 have been studied by highresolution X-ray diffraction, Rutherford backscattering, and spectral ellipsometry. It is found that the joint application of these methods makes it possible to determine the density depth distribution ρ(z) in an implanted layer, its phase state, and elemental composition. Treatment of silicon substrates in helium plasma to doses of 6 × 1016 cm–2 leads to the formation of a 20- to 30-nm-thick amorphized surface layer with a density close to the silicon density. An increase in the helium dose causes the formation of an internal porous layer.

Application of X-ray diffraction methods in the study of micrometer-sized porous Si layers

An X-ray analysis of porous silicon layers (Sb-doped n+-Si(111)) obtained by anodic oxidation for different times with a current of 50 mA/cm2 is performed by the methods of double-crystal rocking curves and total external reflection. A nondestructive method for monitoring the stationary process of the formation of micrometer-sized porous silicon layers and estimating their porosity and thickness is proposed. The parameters obtained for porous silicon layers with a thickness of ∼6 μm are confirmed by the joint processing of diffraction curves for the 111 and 333 reflections on the basis of the developed model of dynamic scattering from layers while taking into account the strain profiles Δd(z)/d, the static Debye-Waller factor f(z), and the porosity P(z). The advantages and drawbacks of the proposed method are discussed.

Study of surface and interface roughnesses in porous silicon by high-resolution X-ray methods

The methods of triple-crystal X-ray diffractometry and the total external X-ray reflection are used to study porous silicon films on a p-type single crystal Si(111) substrate. For the first time, an increase of the pseudopeak intensity was experimentally observed for thick porous films. The following film characteristics are determined: thickness (1.8 μm), strain (3.8 × 10−3), porosity (70%), pore size (∼5 nm), roughness height of the surface (∼3 nm) and the film—substrate interface (∼7 nm), and correlation length (∼7–10 nm). It is shown that the main contribution to the pseudopeak intensity for thin films on single crystal substrates comes from angular broadening of the incident beam formed by the exit slit of a monochromator of a finite width. It is shown that the method is very sensitive to density inhomogeneity in subsurface crystal layers.

Reconstruction of the surface layer density profile by the X-ray reflectometry method

The method for reconstructing the profile of the material density distribution arbitrarily varying over the depth of the subsurface layer of the specimen from X-ray reflectometry data is suggested. For the first time, the recurrent relationships for the derivatives of the specular reflection coefficient with respect to the parameters of the subsurface layer are obtained, which reduce the volume of the necessary computations by one to two orders of magnitude. The class of functions for which such density-profile reconstruction can be made within a 5% error is characterized. The method is tested on thin tungsten and carbon films on silicon substrates and thin films of porous silicon films with p+-type conductivity.

The Formation of Helium Bubbles in Silicon Surface Layers via Plasma Immersion Ion Implantation

The surface layers of single-crystal silicon Si(001) substrates subjected to plasma-immersion implantation with 2- and 5-keV helium ions to a dose of 5 × 1017 cm–2 were probed via grazing incidence small-angle X-ray scattering and transmission electron microscopy. A surface layer formed by helium ions was found to possess a multilayer structure, wherein the upper layer is amorphous silicon, being on top of a sublayer with helium bubbles and a sublayer with a disturbed crystal structure. The in-depth electron density distribution, as well as the concentration and pore-size distribution, were established. The average pore sizes of bubbles at the above implantation energies are 4 nm and 8 nm, respectively.

Complementary study of the internal porous silicon layers formed under high-dose implantation of helium ions

The surface layers of Si(001) substrates subjected to plasma-immersion implantation of helium ions with an energy of 2–5 keV and a dose of 5 × 1017 cm–2 have been investigated using high-resolution X-ray reflectivity, Rutherford backscattering, and transmission electron microscopy. The electron density depth profile in the surface layer formed by helium ions is obtained, and its elemental and phase compositions are determined. This layer is found to have a complex structure and consist of an upper amorphous sublayer and a layer with a porosity of 30–35% beneath. It is shown that the porous layer has the sharpest boundaries at a lower energy of implantable ions.

Structure of the interfaces of the InxGa1 − xAs quantum well from X-ray diffraction data

The structure of the interfaces of a 10-nm-thick InxGa1 − xAs quantum well buried in the semiconductor GaAs matrix has been studied by the method of double-crystal X-ray diffractometry. It has been shown that, in comparison with the well-known photoluminescence method, the X-ray diffraction method has considerable advantages in characterization of multilayer systems. The detailed analysis of the rocking curves provided the reconstruction of the profiles of indium distributions in quantum wells for specimens with different indium concentrations.

Structural characteristics of multicomponent GaAs-InxGa1-xAs system from double-crystal X-ray diffractometry data

The article continues a series of publications on the technologically important multilayer InxGa1-xAs-GaAs/GaAs system with the 3-, 6-, and 9 nm-thick layers (quantum wells). The collimation system of the incident beam is improved. The dimensions of quantum wells and the interfaces between these wells are determined. The qualitative picture of quantum well “spreading” is described. The experimental diffraction reflection curves are measured from three different parts of the specimen. Their analysis shows how homogeneous the structure grown is.