Picosecond laser ablation of SiO2 layers on silicon substrates

In this work, we report on laser ablation of thermally grown SiO₂ layers from silicon wafer substrates, employing an 8–9 ps laser, at 1064 (IR), 532 (VIS) and 355 nm (UV) wavelengths. High-intensity short-pulse laser radiation allows direct absorption in materials with bandgaps higher than the photon energy. However, our experiments show that in the intensity range of our laser pulses (peak intensities of <2×10¹² W/cm²) the removal of the SiO₂ layer from silicon wafers does not occur by direct absorption in the SiO₂ layer. Instead, we find that the layer is removed by a “lift off” mechanism, actuated by the melting and vaporisation of the absorbing silicon substrate. Furthermore, we find that exceeding the Si melting threshold is not sufficient to remove the SiO₂ layer. A second threshold exists for breaking of the layer caused by sufficient vapour pressure. For SiO₂ layer ablation, we determine layer thickness dependent minimum fluences of 0.7–1.2 J/cm² for IR, 0.1–0.35 J/cm² for VIS and 0.2–0.4 J/cm² for UV wavelength. After correcting the fluences by the reflected laser power, we show that, in contrast to the melting threshold, the threshold for breaking the layer depends on the SiO₂ thickness.     

Heteroepitaxial GaN films on silicon substrates with porous buffer layers

New complex buffer layers based on a porous material have been developed for epitaxial growth of GaN films on Si substrates. The characteristics of gallium nitride heteroepitaxial layers grown on silicon substrates with new buffer layers by metal-organic vapor phase epitaxy are investigated. It is shown that the porous buffer layers improve the electric homogeneity and increase the photoluminescence intensity of epitaxial GaN films on Si substrates to the values comparable with those for reference GaN films on Al₂O₃ substrates. It is found that a fianite layer in a complex buffer is a barrier for silicon diffusion from the substrate into a GaN film.     

Surface quantum oscillations in n-type (100) silicon inversion layers on sapphire

Measurements of Shubnikov-de Haas oscillations in (100) Si/($\text{1}$012)Al₂O₃ MOSFET's were performed in magnetic fields up to 10 Tesla for different tilt angles between the magnetic field direction and the surface normal. The experimental results show that the lowest electric subband in this system is twofold degenerate and is formed by the “heavy” cyclotron mass valleys. This can be explained by a large lateral stress present in the SOS (silicon on sapphire) system.     

High resolution X-ray studies of porous PbTe layers on silicon substrates

The structure of PbTe films after anodic electrochemical etching in Norr electrolyte is studied by high resolution X-ray diffractometry and reflectometry. Lattice defects before and after etching are estimated. The quantitative parameters of the pores are determined. The advantage of the complex application of high resolution X-ray methods for the determination of the real structure of lead-telluride porous films is shown.     

Effect of growth conditions on photoluminescence of erbium-doped silicon layers grown using sublimation molecular-beam epitaxy

Erbium-doped epitaxial silicon layers were grown using two different growth techniques, namely, commonly used molecular-beam epitaxy (MBE) and solid-phase epitaxy (SPE). It is shown that an erbium-doped silicon epitaxial layer deposited through SPE on a cold substrate and subsequently annealed displays amore intense photoluminescence at a wavelength of 1.54 μm than do MBE-grown layers. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 102–104. Original Russian Text Copyright ? 2004 by Shengurov, Svetlov, Chalkov, Andreev, Krasil’nik, Kryzhkov.     

Crystallization features of silicon layers on thermo-insulated substrates under nanosecond laser radiation

P-Si layers on c-Si substrate with a thermo-insulating coating (TIC) were irradiated by the 50-ns second harmonic of an Nd-glass laser. Time-resolved measurements, optical microscopy (OM) and scanning electron microscopy (SEM) data for samples with a 0.1–0.9 m p-Si layer and a 0.1 m SiO₂ or 0.16 m Si₃N₄ TIC layer show that if the p-Si layers melt throughout the depth the solidification starts from the surface as well as from the rear. The latent heat released upon coarse grain growth from the rear prevents further crystallization near the top surface and results in partial or complete remelting of the solid phase on the surface.     

Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD

We investigate the structural and electrical properties of Al_xIn_1–xN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested Al_xIn_1?xN/AlN/GaN/Al_0.04Ga_0.96N heterostructures when compared to the standard Al_xIn_1–xN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al_0.04Ga_0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the Al_xIn_1?xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.     

Raman spectra of Si-implanted silicon on sapphire

Raman scattering from Si-implantation-amorphized and subsequently thermally recrystallized silicon on sapphire (SOS) shows that optical phonon frequencies of silicon well correlate with the recrystallization temperature rather than the epitaxial temperature. This implies that the strain in as-epitaxial SOS has been relieved and replaced by the one which depends upon the recrystallization temperature.     

Electrical measurements of boron implanted silicon on sapphire and bulk silicon

Hall effect and sheet resistivity measurements have been performed on boron implantations in 1μm silicon layers on sapphire (SOS), and in bulk silicon. The doses used were 10¹⁴, 10¹⁵ and 10¹⁶ ions/cm², and implantation energies were 150 and 300 keV. The samples were annealed at temperatures between 300 and 800°C. As a rule the effective number of carriers in SOS was found to be about twice the number of carriers in bulk silicon. However, the mobility is lower in bulk silicon, resulting in a sheet resistivity almost the same in boron implanted SOS and bulk silicon.     

Fabrication of Al/AlO_x/Al Josephson junctions on silicon and sapphire substrates using a cold-development technique

In order to obtain high-quality superconducting qubits, we employed a cold-development technique, using temperatures down to-20°C, to fabricate Al/AlOx/Al Josephson junctions. Cold development greatly reduced the sensitivity of the electron-beam resist to the developer, eliminated molecules of the electron-beam resist at trench edges, and improved the repeatability and reliability of the nanopatterning process. The fabricated samples have well-defined geometries and increased dose margins, with lateral sizes of 100 nm×100 nm on both silicon and sapphire substrates. Together with the bridge-free fabrication method we used in these experiments, we believe that the cold-development technique can play an important role in quantum information technology that employs superconducting qubits.     

Buried channel waveguides in plasma-enhanced chemical vapour deposited silicon oxynitride layers on silicon substrates,formed by electron-beam irradiation

It is shown that buried channel waveguides may be formed from a bilayer planar guide (made by plasma-enhanced chemical vapour deposition of silicon oxynitride) by bombardment with low-energy electrons. This results in an expansion of the material, and hence a decrease in the refractive index. Consequently, irradiation around a narrow stripe will induce lateral confinement. Data are presented for the electron-induced refractive index and volume changes, together with preliminary results for single-mode channel guides operating at 1.52m wavelength.     

Excimer-laser-induced micropatterning of silicon dioxide on silicon substrates

Highly resolved micropatterns induced on SiO₂-coated Si sample surfaces have been investigated using a KrF excimer laser (λ: 248 nm and τ: 23 ns). Uniform micropatterns were observed to form in the oxide layer after laser-induced melting of interfaces. The pattern size can be controlled either by the laser parameters or even by the oxide layer thickness. SEM analysis identified that the micropatterns were virtually initiated at the molten interface and the oxide layer followed the interface patterning to change its profile. Simulation of laser interaction with double-layered structures indicated that the oxide layer could melt or be ablated due to interface superheating when it was deposited on a highly absorbing Si substrate. IR analysis has demonstrated that the structural properties of the SiO₂ layer undergo no appreciable changes after laser radiation. This process provides a possible basis for its application in micropatterning of transparent materials using excimer lasers. Received: 4 September 2000 / Accepted: 13 September 2000 / Published online: 30 November 2000     

Infrared electroluminescence from erbium-doped spark-processed silicon

The infrared (IR) electroluminescence (EL) of erbium-doped spark-processed silicon (sp-Si) was investigated. For this, a device was constructed which consisted of a silicon wafer on which an erbium layer was vapor deposited, followed by spark-processing and rapid thermal annealing for 15 min at 900 °C in air. The metallization consisted of a 200 nm Ag layer (above the spark-processed area) and a 50 nm thick Al film (on the “back side”), containing a window through which the light could escape. Maximal light emission occurred near 1.55 μm, that is, at a wavelength where commonly used fiber optical materials have their minimum in energy loss. The processing parameters for most efficient light emission were an Er thickness of 200-300 nm, a spark-processing time of about 30 s, an n-type Si wafer having a low (3-5 Ω cm) resistivity, an operating temperature near room temperature, and an operating voltage between 25 and 40 V under reverse bias. The results are interpreted by postulating an energy transfer from sp-Si to the Er³⁺ ions involving the first excited state ⁴I_13/2 to ground state ⁴I_15/2. Further, impact excitation and hot electrons that are accelerated into the erbium doped sp-Si by the applied field (100 kV/cm) are considered.     

Mechanism of silicon epitaxy on porous silicon layers

The mechanism of silicon epitaxy on porous Si(111) layers is investigated by the Monte Carlo method. The Gilmer model of adatom diffusion extended to the case of arbitrary surface morphology is used. Vacancies and pendants of atoms are allowed in the generalized model, the activation energy of a diffusion hop depends on the state of the neighboring positions in the first and second coordination spheres, and neighbors located outside the growing elementary layer are also taken into account. It is shown that in this model epitaxy occurs by the formation of metastable nucleation centers at the edges of pores, followed by growth of the nucleation centers along the perimeter and the formation of a thin, continuous pendant layer. Three-dimensional images of surface layers at different stages of epitaxy were obtained. The dependence of the kinetics of the epitaxy process on the amount of deposited silicon is determined for different substrate porosities. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 512–517 (10 April 1998)     

Nanoindentation characterization of GaN epilayers on A-plane sapphire substrates

Gallium nitride (GaN) epilayers was deposited on a-axis sapphire substrate by means of metal-organic chemical vapor deposition (MOCVD) method. The GaN epilayers has been investigated in their repetition pressure-induced impairment events from nanoindentation technique and, the relative deformation effect was observed from atomic force microscopy (AFM). From the morphological studies, it is revealed that none of crack and particle was found even after the indentation beyond the critical depth on the residual indentation impression. The ‘pop-in’ event was explained by the interaction of the deformed region, produced by the indenter tip, with the inner threading dislocations in the GaN films. Pop-in events indicate the generation and motion of individual dislocation, which is measured under critical depth and, no residual deformation of the GaN films is observed.     

Optical properties of erbium-doped porous silicon waveguides

Planar and buried channel porous silicon waveguides (WG) were prepared from p⁺-type silicon substrate by a two-step anodization process. Erbium ions were incorporated into pores of the porous silicon layers by an electrochemical method using ErCl₃-saturated solution. Erbium concentration of around 10²⁰ at/cm³ was determined by energy-dispersive X-ray analysis performed on SEM cross-section. The luminescence properties of erbium ions in the IR range were determined and a luminescence time decay of 420 μs was measured. Optical losses were studied on these WG. The increased losses after doping were discussed.     

Constructing self-organized structures on silicon and sapphire surfaces

We investigated methods to fabricate distinctive structures on silicon and sapphire substrates to grow a carbon nanotube (CNT) network using a solution from the Belousov-Zhabotinsky (BZ) reaction. The BZ reaction is a chemical system where chemical reactions and material diffusion coexist in a nonequilibrium state and generate spatiotemporal patterns in a petri dish. Precipitates from the reaction should also produce distinctive structures after being piled on the substrates. The structures have metal particles that act as catalysts for growing CNTs or quantum dots of nanodot devices. Therefore, such structures should be suitable to fabricate three-dimensional CNT networks or nanodot devices. To confirm this, we investigated the fabrication of distinctive structure using a BZ reaction solution. Results indicated that the BZ reaction solution produced interesting structures on the substrates. Moreover, we confirmed that the shape of the structure changed when the substrate used was changed. We believe that the developed methods are suitable to fabricate nanodevices, especially CNT network devices.     

Room-temperature electroluminescence from erbium-doped amorphous hydrogenated silicon

We have studied electroluminescence (EL) in the amorphous silicon-based erbium-doped structures under reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 μm exhibits a maximum near the room temperature. The excitation of erbium ions occurs by an Auger process which involves the capture of conduction electrons by neutral dangling bonds (D⁰-centers) located close to erbium ions. The stationary current through the structure is kept by a reverse process of thermally activated tunnel emission of electrons from negatively charged dangling-bond defects (D⁻-centers) to the conduction band of the amorphous matrix. A theoretical model proposed explains consistently all of our experimental data.     

Infrared angular spectroscopy characterization of epitaxial layers of n-type silicon grown on N+ or P+ substrates

Summary In this paper the technique of infrared angular spectroscopy applied to the characterization of epitaxial layers ofn-type silicon grown on N⁺ or P⁺ substrates is illustrated. Some results are reported and discussed concerning films having a free-carrier concentration ranging from 10¹⁴ cm⁻³ to 10¹⁷ cm⁻³ and thickness of the order of 10 μm. A significant comparison with results obtained by other techniques (four-point probe, spreading resistance,C−V plots, etc.) is performed and a few simple conclusions are drawn. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.