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This paper presents a model of the microporous silicon formation process which is based on hole depletion due to quantum confinement in the porous structure. This model is compared with the formation of larger porous structures (meso-, macroporous) where hole depletion is generated by a space charge region.
This detailed electron microscope study of porous silicon compares the different structures of macro-, meso- and microporous material. Mesoporous silicon of high porosity (∼-80%) exhibits efficient red photoluminescence at room temperature. Transmission electron microscopy provides strong direct evidence that this visible luminescence arises from dramatic carrier confinement in quantum-size, crystalline silicon structures. Images of undulating, interconnected ‘quantum wires’ of widths <3nm are shown.
Raman spectra from a thick porous silicon film (∼100 μm) that strongly emits in the visible (∼ 6350 Å) at room temperature are obtained. An asymmetric peak with a Raman shift of ∼ 508 - 510 cm−1 and a width of ∼ 40 cm−1 is seen in every spectrum. This Raman feature resembles that of μc-Si, suggesting that the local structure of the porous silicon is a network of interconnected crystalline silicon islands with the island size in the nanometer range., and that the, shape of the islands is more sphere-like than rod-like. The characteristic dimension of the islands in these porous silicon films is estimated to be ∼ 2.5 - 3.0 nm on the basis of an empirical model calculation of phonon confinement.
Transmission Fourier Transform Infrared (FTIR) Spectroscopy was utilized to monitor the effect of surface coverage on photoluminescent porous silicon. These experiments were performed in situ in an ultrahigh vacuum (UHV) chamber to correlate simultaneously surface coverage and photoluminescence intensity. The goal of these FTIR and photoluminescence studies was to clarify the mechanism of the photoluminescence from porous silicon.
A novel immersion scanning technique for making microporous silicon has been successfully applied to blank and lithographically patterned Si substrates. The advantages of the method lie in its simplicity, speed and adaptability to large and odd-size substrates. The photoluminescence (PL) spectra of microporous Si show a continuous decrease in intensity between 200K and 2K, but are fully reversible. Thermal desorption spectroscopy on microporous Si shows a classic hydrogen desorption spectrum which coincides with a quenching of the PL intensity. Under constant excitation, a degradation of PL Intensity occurs in oxygen and wet nitrogen but is only partially reversible in dry N2. Microporous Si PN junctions exhibiting normal I-V characteristics have been successfully fabricated with standard Si VLSI processes. Visible light emission under forward bias is detected which increases linearly In Intensity with Input current. This is the first observation of electroluminescence in the visible region from microporous SI PN junctions.
We demonstrate for the first time that chemical etching of Si in HF-HNO3-based solution without applying bias can produce a room temperature photoluminescent porous Si layer. Scanning electron microscope studies reveal a surface morphology similar to that of the conventionally anodized porous Si. The formation mechanism of the chemically etched (CE) film can be explained by a local anodization concept. X-ray diffraction studies on the luminescent CE porous Si show a broad amorphous peak.
We have made structural and compositional studies of luminescent laterally anodized porous Si. Scanning electron microscopy reveals a surface with a network of cracks, while transmission electron microscopy shows a dual porous Si structure in which the upper layer is amorphous and the lower layer is either amorphous or crystalline, depending on anodization conditions. X-ray diffraction verified the presence of the amorphous layer. Secondary ion mass spectroscopy reveals very high concentrations of H, B, C, N, O, and F in the amorphous layer. Our results indirectly suggest that the amorphous layer is primarily responsible for luminescence.
Orange-red light emission has been observed for the first time from crystalline silicon nanoparticles produced by gas phase synthesis in a non-thermal microwave plasma. The size and crystalline nature of the particles have been confirmed by transmission electron microscopy and X-ray diffraction. Photoluminescence at 300 K and 77 K has been measured and analyzed. The emission spectra are consistent with quantum mechanical calculations based on a quantum box.
Using thermal effusion of H together with a weighing of the remaining Si we determine the composition of luminescing porous Si. From studies of the IR vibration spectrum and its change with effusion we. argue that the H of freshly prepared microporous Si is bound in the dihydride form SiH2. We thus obtain a surface to volume ratio and a characterisicic linear dimension of the porous surface layer. The paper concludes with some observations on photo- and electroluminescence.
We have used electron microscopy to examine the microstructure of porous silicon films over a wide range of doping levels, and photoluminescence spectroscopy to study their optical properties. We discuss the impact of our experimental results on models from the literature which were proposed to explain visible luminescence from porous silicon.
Porous silicon samples photoluminous in orange, pink or greenish-yellow were formed by anodization. The shapes and wave length positions of their photoluminescence peaks were not influenced by the excitation wavelength. The photoluminescence intensity at any wavelength between 520nm and 800nm varied as a function of the excitation wavelength In accordance with the imaginary part of the spectral refractive index of bulk silicon. Thus the excitation process responsible for the visible photoluminescence of porous silicon seems to be bulk-silicon-like.
Photoluminescence from porous silicon was studied in a helium atmosphere and at high magnetic fields. In a magnetic field, the spectral peak had a negligible shift, amounting to +0.2 ±0.5 meV at B= 18T. If quantum confinement is the major effect, then the conduction electron would be confined to a diameter of less than 3.5 nm.
Microporous silicon layers contain an enormous surface area (> 500 m2 cm−3) that influences their structural, optical and electrical properties. When freshly etched the pore wall surface can be extremely clean and composed primarily of hydrogen and fluorine. Extended storage in ambient air however will convert this clean hydride surface into that of a contaminated native oxide.
Using dynamic SIMS profiling we demonstrate here that slow oxidation at room temperature by ambient air is accompanied by impregnation with atmospheric boron and sulphur but that levels of calcium and sodium for example, remain exceedingly low. We conclude that the pore wall surface is very efficiently protected from particulate - related airborne species but is susceptible to contamination from small molecules present in the atmosphere at trace levels.
Porous silicon of various porosity has been prepared by electrochemical etching of silicon with different doping levels. Room temperature photoluminescence in the visible range is observed from the powder scraped from the top layer of the etched samples. In this paper we use Raman scattering to characterize the source of the high efficiency photoluminescence. We have also studied microcrystalline silicon prepared by thermal annealing of hydrogenated amorphous silicon/amorphous silicon oxide multilayers.
PLE spectroscopy was performed on samples of porous Si at temperatures of 300 and 4.2 K. PLE provides information about the absorption coefficient in the limit of optically thin samples and is an alternative method for determining absorption data on porous Si without removal of the substrate. The spectra obtained correspond closely to the relative changes in absorption coefficient for bulk Si between 2.0 and 3.5 eV, with a strong increase above 3.0 eV which is due to the direct bandgap transition.
We report piezoresistance studies in microcrystalline silicon films produced by reactive sputtering from a silicon target in an atmosphere of hydrogen and argon. The microcrystalline silicon films are two phase materials consisting of 50-100Å diameter silicon crystallites embedded in an amorphous Si-Hx matrix. The conductivity of the films was found to decrease significantly when the films were put under compression. Conductivity decreases of up to 100% were observed; this large conductivity changes with strain indicate that microcrystalline silicon is ideally suited for highly sensitive strain gauge applications. The results can be qualitatively accounted for in a model which assumes quantum confinement of carriers in 50Å diameter silicon crystallites separated by tunnelable amorphous Si-Hx barriers.
P-i-n doped short-period SimGen strained layer superlattices (SLS) are grown on (100) silicon substrates by low temperature molecular beam epitaxy (300C°<∼Tg<∼400C°). The SLS's are grown with period lengths around 10 monolayers (ML) to a thickness of 250nm on a rather thin (50nm) homogeneous Si1−ybGeyb alloy buffer layer serving as strain symmetrizing substrate. Photoluminescence at T=5K is observed for various SimGen SLS samples, the strongest signal was found for a Si5 Ge5 SLS. Samples with identical SLS's but different buffer layer composition and thicknesses are grown to study the influence of strain on the PL. Electroluminescence (EL) at the same energy range is observed from mounted SimGen SLS mesa and waveguide diodes up to T=130K – for the first time reported in strain symmetrized short-period SimGen SLS. The intensity and peak positon of the EL signal was found to be dependent on the injected electrical power.
We report the first demonstration of visible light emission from an all solid-state n-p heterojunction diode based on porous silicon. The p-type silicon was electrochemically etched in a hydrofluoric acid solution to form a porous silicon region; the n-p heterojunction diode was fabricated by depositing a wide bandgap n-type semiconductor, indium-tin-oxide (ITO), onto the surface of the porous silicon. With positive bias applied, electroluminescence was observed with a relatively narrow peak at about 580 nm. The device showed strong rectifying properties and no light emission was observed under reverse bias condition. Photoluminescence in the red, orange, yellow, and green was also observed in separate sample preparations.
We have made studies on the TRANSVERSE transport properties of the porous Si made from a novel P/N junction structure. The structures of porous Si were examined for various electrochemical etching conditions and they were correlated with the electrical data. The junciton was fabricated by shallow diffusion, with porous Si formed perpendicular to the junction and between two indium ohmic contacts. This structure confines currents to the direction parallel to the surface. Distinct feature on I–V curves have been observed, including sudden rise of currents and the existence of negative differential resistances (NDR). The characteristics appeared stable and depended on the polarity of bias. Suggestions are made that the porous Si could be composed of microcrystals, and feasibility of carrier transport through quantum boxes responsible for the electrical behaviors are discussed.