Multilayer nanostructures based on porous silicon for optoelectronics
The interest in porous silicon as an optoelectronic material is primarily due to its large surface area and the availability of nanoscale crystals in its pores. These features make it a promising material for use both in photosensitive [1–4] and in luminescent structures [5–6]. However, the extensive use of porous silicon in the electronic devices is constrained by the difficulty in controlling the properties of the resulting porous layer, especially in the possibility of obtaining a low-resistance material. To make the process of creating porous silicon well reproducible, the surface is used as the initial surface with predetermined pore formation centers. For example, on the textured surface of single-crystal silicon, which is a surface filled with regular tetrahedral pyramids, the pore formation occurs mainly at the points of contact of the pyramids’ bases. At the same time, if we use structures with an already formed p-n junction, the original type and level of doping are preserved at the pyramid tops and in their scope, thus, the resulting structure is an array of micro diodes on a common single-crystal substrate separated from each other by high-resistance regions of porous silicon. Such structures are more stable, they have good electrical properties and can be used in various applications as photosensitive or luminescent materials [7–8].
At present, the greatest efficiency in converting the solar energy into electrical energy in silicon solar cells (SC) (25.6% for laboratory samples, 22% for industrial samples) was achieved based on a-Si: H / c-Si heterostructure. In these structures, a layer of amorphous hydrogenated silicon (a-Si: H) of different types of conductivity serves as a "wide-band gap window", since the width of the a-Si: H bandgap is approximately 1.7–1.9 eV. It exceeds the bandgap of the single crystal silicon (1.12 eV), and extends the range of the SC photosensitivity to shorter wavelengths of the solar spectrum . In this paper, a layer of nanocrystalline porous silicon and silicon carbide on the textured surface was used as a "wide-band gap" of photosensitive structures. Because the procedure of creating micro-relief leads to an increase in the surface recombination rate, then dielectric coating of the rare earth element fluoride (REE), which has antireflection and passivation properties on silicon surface, can be used to reduce the negative effect [10–12]. It has been shown that the structures with a porous layer formed on a textured surface exhibit markedly higher photovoltaic characteristics than the structures without a porous layer, and the use of coatings from rare earth element fluorides can reduce the rate of surface recombination.
Based on oxidized nanocrystalline porous silicon ions implanted into the pores of the rare earth elements (REE), effective luminescent structures integrated into the silicon optoelectronics can be created, which will increase the speed, the information recording density, noise immunity and other parameters of the silicon-substrate electronic circuits. Such structures have good luminescence properties in the near-IR spectral region at room temperature [13, 14].
2. EXPERIMENTAL PROCEDURE
To investigate the spectral characteristics of semiconductor structures and to use the samples with a porous layer created on those to the simulated surface of single-crystal silicon wafers. To create a p-n junction, the upper working layer of the photosensitive structures was doped with phosphorus or the wafers with pre-formed p-n junction. A layer of silicon carbide on the photovoltaic structures was created by chemical transport in the open system of solid-state silicon and carbon with a carrier gas (hydrogen) in the epitaxy zone, followed by deposition of porous silicon on the surface. To create a luminescence with centric structures, the porous layer was saturated with erbium from an aqueous solution. Manufacturing process of the samples is described in [3–5, 8, 10, 12, 13]. The structure and composition of the samples was studied by Carl Zeiss EVO 50 electron microscope with X–Max 80 detector for microelement analysis (Oxford Instruments). The spectral photosensitivity studies of the PVC samples in the visible region were performed using a МДР‑3 monochromator. Photosensitivity R was calculated as a ratio of photocurrent I to the power of incident radiation P:
R = I / P.
Luminescent studies of samples in the near-infrared spectral region were carried out using a high-resolution Fourier spectrometer (BOMEM DA‑3). The luminescence was excited by the Ar-laser radiation with a wavelength of 532 nm. All measurements were carried out at room temperature.
3. RESULTS AND DISCUSSION
3.1. Morphology and composition of the samples
The structure of the textured surface of the nSiC / p-porSi sample is shown in Fig. 1. Porous silicon can be seen at the boundary of the pyramids as a dark contrast area, separate large pores with diameters of over 200 nm can be seen both at the boundary and on the pyramids’ walls.
Quantitative analysis of composition of the nSiC / p-porSi structure surface (Fig.2) shows that the atomic ratio of carbon (55.61%) is greater than the atomic ratio of silicon (28.36%). It can be assumed that, in addition to silicon carbide, the carbon is found in the pores in the form of nanowires, which is observed in the structures fabricated by this technology .
Figure 3 (a, b) shows the images of transverse shear of the porSi: Er structure and the x-ray fluorescence spectrum of the isolated regions. Analysis of the spectra indicates that erbium content in the porous layer region is 0.07 at.% in the average.
3.2 Spectral characteristics of samples
Figures 4 and 5 show spectra of reflection coefficients of surfaces with a carbidized and non- carbidized porous layer, and nSiC / p-porSi heterostructures photosensitivity. It can be seen that carbidization slightly increases the reflection coefficients of a porous layer in the region from 400 to 700 nm, where the reflection coefficient decreases in the range from 200 to 300 nm. The spectral sensitivity markedly increases in the short-wave part of the spectrum, which is explained by the presence of silicon and silicon carbide nanocrystals in the porous layer.
Fig. 6 shows photoluminescence spectrum of a set of samples, cut from a single wafer on which a luminescent porSi: Er structure was formed. A porous layer of different thicknesses, from 5 to 10 µm, was locally created on the wafer. Erbium was penetrated over the entire surface of the wafer. The most intense luminescence of photons with a maximum at a wavelength of 1.55 micron (4B curve) corresponding to the radiation of the Er3+ ion was registered on the sample with the greatest thickness of the porous layer. There is practically no luminescence beyond the porous layer (5A curve). This confirms the assumption that the most favorable conditions for the excitation of erbium ions luminescence are created in the porous layer.
The studies allow to conclude about the prospects of using porous silicon in optoelectronics. n-SiC / p-porSi heterostructures with enhanced photosensitivity in the wavelength region of the solar spectrum will expand the PVC photosensitivity range and hence will increase its effectiveness. The porSi: Er structure samples have good luminescence properties with their maxima at a wavelength of 1.55 µm at room temperature and can be used as a basis for creating IR LEDs.