Characterization of a 10-MHz quadrant APD for measuring frequency oscillations and tip displacements of microcantilevers

The room-temperature response of a 10-MHz quadrant avalanche photodiode (APD) is investigated for detection of high frequency oscillations and tip displacements of fabricated microcantilevers. Currently, no quadrant detectors with a response bandwidth in the megahertz range are available, and oscillations on the order of a few microseconds cannot be resolved. A comparison is made between optical and opto-mechanical measurements to characterize this detector by investigating the frequency response and signal-to-noise (SNR) of pulsed laser signals up to 10?MHz and reflected laser signals from freely vibrating microcantilevers up to 1.64?MHz. The power level of the minimum detectable signal incident on the APD is found to be 28.2?±?5.0?nW for optical measurements and 1.0???W?±?5.0?nW for opto-mechanical measurements.     

Efficient laser induced consolidation of nanoparticulate ZnO thin films with reduced thermal budget

Layers of ZnO nanoparticles with thicknesses of about 40 nm were prepared on Si substrates. It was shown that UV laser irradiation is suitable for consolidation and significant densification of the ZnO particle layers under ambient conditions. Both experiments and simulations show that an underlying SiO₂ particle layer has a beneficial effect in inhibiting heat transfer towards the substrate and thus enables the application of temperature-sensitive carrier substrates like polymer foils despite the extremely high melting temperature of ZnO.     

A laser system for the spectroscopy of highly charged bismuth ions

We present and characterize a laser system for the spectroscopy on highly charged ²⁰⁹Bi⁸²⁺ ions at a wavelength of 243.87?nm. For absolute frequency stabilization, the laser system is locked to a near-infra-red laser stabilized to a rubidium transition line using a transfer cavity based locking scheme. Tuning of the output frequency with high precision is achieved via a tunable rf offset lock. A?sample-and-hold technique gives an extended tuning range of several THz in the UV. This scheme is universally applicable to the stabilization of laser systems at wavelengths not directly accessible to atomic or molecular resonances. We determine the frequency accuracy of the laser system using Doppler-free absorption spectroscopy of Te₂ vapor at 488?nm. Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty of σ _{244 nm}=6.14?MHz (one standard deviation) over six days of operation.     

Optical characterization of thin silicon films deposited by CVD and annealed by pulsed laser

Polycrystalline Si layers 100 to 800 nm thick have been deposited on Si single crystal substrates by CVD and annealed with a Q-switched ruby laser at energies up to 1.5 J/cm². The optical characteristics of these layers have been measured by SEM and ellipsometry. The results can be attributed to a change in surface roughness with film thickness and laser energy.     

Passive mode locking of a Nd:YVO4 laser with an extra-long optical resonator

We describe a mode-locked, diode-pumped Nd:YVO4 laser with a very long optical cavity operating at 1064 nm. High-modulation, InGaAs quantum-well, semiconductor saturable-absorber mirrors were used for passive mode locking, providing a stable train of 13-ps pulses. A novel zero-q-transformation multipass cell provided a variable-length optical cavity as much as 100 m long. The output beam had M2 < 1.1 at average powers of 4.1, 3.9, and 3.5 W at repetition rates of 4.1, 2.6, and 1.5 MHz, respectively. To the best of our knowledge the last of these is the lowest repetition rate ever generated directly from a mode-locked nonfiber solid-state laser without cavity dumping.     

Effect of laser annealing on crystallinity of the Si layers in Si/SiO2 multiple quantum wells

We report on continuous-wave laser induced crystallisation processes occurring in Si/SiO₂ multiple quantum wells (MQW), prepared by remote plasma enhanced chemical vapour deposition of amorphous Si and SiO₂ layers on quartz substrates. The size and the volume fraction of the Si nanocrystals in the layers were estimated employing micro-Raman spectroscopy. It was found that several processes occur in the Si/SiO₂ MQW system upon laser treatment, i.e. amorphous to nanocrystalline conversion, Si oxidation and dissolution of the nanocrystals. The speed of these processes depends on laser power density and the wavelength, as well as on the thickness of Si-rich layers. At optimal laser annealing conditions, it was possible to achieve ∼100% crystallinity for 3, 5 and 10 nm thickness of deposited amorphous Si layers. Crystallization induced variation of the light absorption in the layers can explain the complicated process of Si nanocrystals formation during the laser treatment.     

Epitaxial growth of μm-sized Cu pyramids on silicon

Triangular and quadratic Cu pyramids were epitaxially grown on Si(111) and Si(100) substrates, respectively, by pulsed laser deposition at elevated substrate temperatures above 200°C as well as by post-annealing of closed Cu layers prepared at room temperature. In both cases, three-dimensional pyramids with edge lengths of up to 9 μm were obtained, as observed by scanning electron microscopy and atomic force microscopy. Although the macroscopic shape is a pyramid, microscopically the islands consist of columnar grains (with lateral sizes of only about 50 nm at 260°C). The size and shape of the pyramids can be controlled by the substrate used, the amount of material deposited, and the temperature during deposition or annealing. Additionally, first hints were found that the pyramids can be aligned by structuring the substrate. The formation of such large pyramids is explained by a fast diffusion of Cu atoms on Si over distances of some μm and a high jump probability to higher pyramid layers.     

High-power broadband laser source tunable from 3.0 μm to 4.4 μm based on a femtosecond Yb:fiber oscillator

We describe a tunable broadband mid-IR laser source based on difference-frequency mixing of a 100?MHz femtosecond Yb:fiber laser oscillator and a Raman-shifted soliton generated with the same laser. The resulting light is tunable over 3.0?μm to 4.4?μm, with a FWHM bandwidth of 170?nm and maximum average output power up to 125?mW. The noise and coherence properties of this source are also investigated and described.     

ps-laser scribing of CIGS films at different wavelengths

Continuous growth of the thin-film electronics market stimulates the development of versatile technologies for large-scale patterning of thin-film materials on rigid and flexible substrates, and laser technologies are a promising method to accomplish the scribing processes. Lasers with picosecond pulse duration were applied in scribing of complex multilayered CuIn _x Ga_(1−x)Se₂ (CIGS) solar cells deposited on a polyimide substrate. The ablative properties of the films were examined as a function of the wavelength of laser radiation, pulse energy, and the irradiation dose. The selective removal of ITO and CIGS layers was achieved with 355 nm irradiation without any significant damage to the underlying layers in the ITO/CIGS/Mo/PI solar cell system. The 355 nm wavelength was also found to be favorable for scribing of absorber layer in a ZnO/CIGS/Mo/PI solar cell system. 266 nm radiation significantly modified the film structure due to high absorption. Extensive melt formation in the CIGS layer was found when 532 nm radiation was applied, though the trenches were smooth and crack-free.     

Fabrication and characterisation of microscale air bridges in?conductive gallium nitride

Fabrication and electrical characterisation of microscale air bridges consisting of GaN heavily doped with silicon is described. These were made from GaN–AlInN–GaN epitaxial trilayers on sapphire substrates, in which the AlInN was close to the composition lattice matched to GaN at ∼17% InN fraction. The start of the fabrication sequence used inductively coupled plasma etching with chlorine chemistry to define mesas. In situ monitoring by laser reflectometry indicated an AlInN vertical etch rate of 400 nm/minute, ∼70% of the etch rate of GaN. Processing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported between anchor posts at both ends. Deposition of Ti–Au contact pads onto the anchor posts allowed study of the electrical characteristics. At low applied voltages, vertical conduction through the undoped AlInN layers was minimal in comparison with the current path through the Si:GaN bridges. Typical structures showed highly linear current-voltage characteristics at low applied voltages, and had resistances of 1050 Ω. The observed resistance values are compared with the predicted value based on materials parameters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2×10⁵ A cm⁻².     

Integrated wavelength monitoring in a photonic-crystal tunable laser diode

A photonic-crystal tunable 1.55 μm laser diode is fitted with a wavelength monitor on its rear side. The 250-μm long laser based on a coupled-cavity design has approximately 15 nm tunability. The wavelength monitor collects and differentially feeds two-photodetecting areas, thanks to a mode conversion to a higher-order mode (a mini-stopband), followed by tunneling through a thin clad. The layout is numerically optimized to minimize unwanted reflections. Electrical cross-talk was prevented through guard rings and trenches. The correlation between wavelength and the monitor photocurrent ratio demonstrates a 10–20 GHz stabilization capability, or a 15 nm monitoring range.     

High-power, continuous-wave, mid-infrared optical parametric oscillator based on MgO:sPPLT

We report a stable, high-power, cw, mid-IR optical parametric oscillator using MgO-doped stoichiometric periodically poled LiTaO? (MgO:sPPLT) pumped by a Yb fiber laser at 1064 nm. The singly resonant oscillator (SRO), based on a 30 mm long crystal, is tunable over 430 nm from 3032 to 3462 nm and can generate as much as 5.5 W of mid-IR output power, with >4 W of over 60% of the tuning range and under reduced thermal effects, enabling room temperature operation. Idler power scaling measurements at ~3.3 μm are compared with an MgO-doped periodically poled LiNbO? cw SRO, confirming that MgO:sPPLT is an attractive material for multiwatt mid-IR generation. The idler output at 3299 nm exhibits a peak-to-peak power stability better than 12.8% over 5 h and frequency stability of ~1 GHz, while operating close to room temperature, and has a linewidth of ~0.2 nm, limited by the resolution of the wavemeter. The corresponding signal linewidth at 1570 nm is ~21 MHz.     

Highly efficient silver particle layers on glass substrate synthesized by the sonochemical method for surface enhanced Raman spectroscopy purposes

A fast method for preparing of silver particle layers on glass substrates with high application potential for using in surface enhanced Raman spectroscopy (SERS) is introduced. Silver particle layers deposited on glass cover slips were generated in one-step process by reduction of silver nitrate using several reducing agents (ethylene glycol, glycerol, maltose, lactose and glucose) under ultrasonic irradiation. This technique allows the formation of homogeneous layers of silver particles with sizes from 80 nm up to several hundred nanometers depending on the nature of the used reducing agent. Additionally, the presented method is not susceptible to impurities on the substrate surface and it does not need any additives to capture or stabilize the silver particles on the glass surface. The characteristics of prepared silver layers on glass substrate by the above mentioned sonochemical approach was compared with chemically prepared ones. The prepared layers were tested as substrates for SERS using adenine as a model analyte. The factor of Raman signal enhancement reached up to 5·10⁵. On the contrary, the chemically prepared silver layers does not exhibit almost any pronounced Raman signal enhancement. Presented sonochemical approach for preparation of silver particle layers is fast, simple, robust, and is better suited for reproducible fabrication functional SERS substrates than chemical one.     

Preparation and characterization of ferroelectric SrBi2Ta2O9 thin films on Si using Al2O3 buffer layers

SrBi₂Ta₂O₉ (SBT) thin films were prepared on p-type Si(100) substrates with Al₂O₃ buffer layers. Both the SBT films and the Al₂O₃ buffer layers were deposited by a pulsed laser deposition technique using a KrF excimer laser. An Al prelayer was used to prevent Si surface oxidization in the initial growth stage. It is shown that Al₂O₃ buffer layers effectively prevented interdiffusion between SBT and Si substrates. Furthermore, the capacitance–voltage (C-V) characteristics of the SBT/Al₂O₃/Si heterostructures show a hysteresis loop with a clockwise trace, demonstrating the ferroelectric switching properties of SBT films and showing a memory window of 1.6 V at 1 MHz. Received: 17 July 2000 / Accepted: 16 August 2000 / Published online: 30 November 2000     

Growth of zinc oxide nanostructures

Zinc oxide (ZnO) nanowhiskers have been prepared using a multilayer ZnO(50 nm)/Zn(20 nm)/ZnO(2μm) structure on a polished stainless steel (SS) substrate by high rate magnetron sputtering. The formation of uniformly distributed ZnO nanowhiskers with about 20 nm dia. and 2 to 5 μm length was observed after a postdeposition annealing of the prepared structure at 300–400° C. An array of highlyc-axis oriented ZnO columns (70–300 nm in dia. and up to 10 μm long) were grown on Si substrates by pulsed laser deposition (PLD) at a high pressure (1 Torr), and Raman studies showed the activation of surface phonon modes. The nanosized powder (15–20 nm) and nanoparticle ZnO films on glass substrate were also prepared by a chemical route. Nanowhiskers showed enhanced UV light detection characteristics, and the chemically prepared ZnO nanoparticle films exhibited good sensing properties for alcohol     

A 1.7-ps pulse mode-locked Yb3+:Sc2SiO5 laser with a reflective graphene oxide saturable absorber

By using a reflective graphene oxide as saturable absorber,a diode-pumped passively mode-locked Yb3+:Sc2Si O5(Yb:SSO)laser has been demonstrated for the first time.Without extra negative dispersion compensation,the minimum pulse duration of 1.7 ps with a repetition rate of 94 MHz was obtained at the central wavelength of 1062.6 nm.The average output power amounts to 355 m W under the absorbed pump power of 15 W.The maximum peak power of the mode-locking laser is up to 2.2 k W,and the single pulse energy is 3.8nJ.     

Wear resistance of ZrC/TiN and ZrC/ZrN thin multilayers grown by pulsed laser deposition

ZrC/TiN and ZrC/ZrN multilayers thinner than 350 nm were grown on (100) Si substrates at a temperature of 300 °C by the pulsed laser deposition (PLD) technique using a KrF excimer laser (λ=248 nm, pulse duration τ=25 ns, 8.0 J/cm² fluence and 40 Hz repetition rate). Cross-sectional transmission electron microscopy, Auger electron spectroscopy depth profiling and simulations of X-ray reflectivity curves indicated that there was intermixing between the deposited layers at the interfaces as well as between the first layer and the substrate. Nanoindentation investigations found hardness values between 35 and 38 GPa for the deposited multilayers. Linear unidirectional sliding wear tests were conducted using a ball-on-plate tribometer under 1 N normal force. Wear tracks were produced in a Hysitron nanoindenter with 1 μm radius diamond tip under a 500 μN load. High-resolution cross-sectional transmission electron microscopy studies of the wear tracks showed that the multilayers withstood these tests without significant damage. The results could be explained by the use of a high laser fluence during deposition that resulted in very dense and strongly adherent nanocrystalline layers.     

Micro-Raman investigation of stress distribution in laser drilled via structures

Through-wafer vertical electrical interconnects (vias) with diameters varied from 15 to 80 μm were formed on Si substrates using a UV diode-pumped solid state laser (355 nm). Micro-Raman spectroscopy was employed for the investigation of stress and structural changes induced in silicon within the heat-affected zone due to laser machining. A maximum stress of ∼300 MPa, as a result of laser drilling, was observed close to the via edge. It was found that the stress decays within a distance of 1-3 μm from the via’s side-wall and that the laser machining did not lead to the formation of amorphous silicon around the via structures.     

Light induced adsorption of Si nano-composites in LiF crystals at 157 nm

Si nano-composites were precipitated on LiF crystals following ablation from Si targets with laser light at 157 nm. The LiF/Si interface was analyzed with scanning electron microscopy, atomic force microscopy and energy dispersive X-ray microanalysis. It was found that Si composites were strongly attached to LiF ionic sites to form inhomogeneous structures consisted of small isotropic crystals 0.1-1 μm long, rich in Si and fluorine, which eventually further agglomerate to form larger structures. The thickness of the LiF/Si interface was increased from 50 nm to 2 μm following laser irradiation at 157 nm, due to accelerated adsorption of Si in the LiF interface by VUV light.     

Two-dimensional clusters in SiGe/Si heterostructures and their effect on field effect transistor transport characteristics

Heterostructures of Si_0.80Ge_0.20/Si (100) were grown by pulsed laser deposition consisting of alternating 10 nm SiGe and silicon layers for a total of 10 periods. The presence of alloy clustering at SiGe/Si interfaces was investigated by parallel transport in transistor structures. Photoluminescence line broadening indicated the presence of Ge-rich clusters (20–30 nm) in layers grown at a substrate temperature of 600 ^○C using a excimer laser at 248 nm wavelength.The 2D clusters were modeled, assuming a spherical shape and a potential of the form δΔU where δ is the composition variation and ΔU is the disorder potential. A scattering matrix element was derived which included intraband scattering. Transport effects were calculated using Monte Carlo techniques. Of interest was lateral transport in a field effect transistor configuration. Therefore, velocity versus field curves were calculated where electron motion is confined to the 2D-like Si/SiGe interface layers. Using concepts of charge control, current versus voltage relationships were derived. The derived models showed that for the case of a 250 μm conducting channel lateral length with 2D clusters evenly placed at every 10 nm, the transconductance increase is 25% and is a function of both cluster size and separation.In order to measure the effect of clustering, high electron mobility transistors were fabricated with two conducting 2D channels consisting of Si/SiGe/Si grown on high resistivity (100) silicon. Using 0.5 μm gates, a transconductance of 125 mS/mm was obtained in transistors without alloy clustering which decreased to less than 80 mS/mm when alloy clustering was present. The present investigation has related the presence of 2D nano-clusters to device performance and has also shown agreement between the experimental results and theoretical calculations.