Transparent Wafer Research Articles

Multi-photon absorption enhancement by dual-wavelength double-pulse laser irradiation for efficient dicing of sapphire wafers

The evidence of multi-photon absorption enhancement by the dual-wavelength double-pulse laser irradiation in transparent sapphire was demonstrated experimentally and explained theoretically for the first time. Two collinearly combined laser beams with the wavelengths of 1064 nm and 355 nm, inter-pulse delay of 0.1 ns, and pulse duration of 10 ps were used to induce intra-volume modifications in sapphire. The theoretical prediction of using a particular orientation angle of 15 degrees of the half-wave plate for the most efficient absorption of laser irradiation is in good agreement with the experimental data. The new innovative effect of multi-photon absorption enhancement by dual-wavelength double-pulse irradiation allowed utilisation of the laser energy up to four times more efficiently for initiation of internal modifications in sapphire. The new absorption enhancement effect has been used for efficient intra-volume dicing and singulation of transparent sapphire wafers. The dicing speed of 150 mm/s was achieved for the 430 μm thick sapphire wafer by using the laser power of 6.8 W at the repetition rate of 100 kHz. This method opens new opportunities for the manufacturers of the GaN-based light-emitting diodes by fast and precise separation of sapphire substrates.

(Invited) Aspects of Vibrational Spectroscopy in Investigation of Low-Temperature Pigmented V-Oxide Films

Vanadium(V) oxide (V2O5) has attracted attention mainly due to its use in lithium secondary batteries. Consequently, various studies of its lithiated phases were made, leading to their identification as α, ε, δ, γ [1]. When prepared in the form of thin films V2O5 can be applied in a battery-type electrochromic (EC) devices as counter electrode, recognized for its multicolor electrochromism, electrochemical cycling stability in a specified potential range, good ion-storage capacity and relatively high transparency in the bleached state [2]. However, as for other transition metal oxides a temperature of above 300 °C is demanded for V2O5 formation, which rules out the possibility of its deposition on plastic substrates. With the aim to overwhelm this circumstance, a two-step procedure according to paint formulation procedures was developed in our laboratory. This preparation procedure has already been proven to be effective for Ni1-xO, TiO2, WO3 pigmented films [3,4]. During the first step, an oxide crystalline powder is prepared via sol-gel route and thermal treatment at elevated temperatures. As a second step, the milling process is performed in an appropriate solvent and with the use of dispersants to prevent the agglomeration of so-formed nanoparticles. The final dispersion can be used for spin-coating deposition of films on polymeric substrates, followed by thermal treatment at 150 °C. The first aim of this work is to investigate the possibility of the application of the described two-step procedure for the preparation of V-oxide pigmented films. The sol for the preparation of crystalline powder in the first step is made of vanadium(V) oxoisopropoxide diluted in 2-propanol. Other routes as hydrothermal synthesis will be investigated as well. In the second step, the size distribution of nanoparticles in dispersion is a factor that crucially determines the deposition possibilities of films. Therefore, the relation between crystalline particles and dispersant is studied via transmission electron microscopy (TEM). The structural and surface characterization of pigmented films is further made using infrared (IR) and Raman spectroscopies, scanning electron (SEM) microscopy, contact angles and surface energy. In situ UV-visible absorbance in a custom-made trielectrode cell is used to detect optical changes during intercalation/deintercalation processes. The possibilities of formation of EC devices with appropriate pigmented films will be considered and the performance compared to already existing EC systems. The second aim of this work is the study of possibilities of combined analytical tools for the investigation of intercalation/deintercalation in EC films and the developed V-oxide pigmented films will be used as an example. Especially spectroelectrochemistry, with a wide range of spectroscopic techniques can be advantageous. The above mentioned in situ UV-visible absorbance spectroelectrochemistry is widely used in investigation of EC films and devices for detection of optical modulation. More deep insight into the structural changes that occur in films can be performed using vibrational spectroscopies. Different techniques are characterized by certain advantages and disadvantages. For example, IR reflection-absorption spectroscopy can be coupled with electrochemical technique ex situ or in situ. While ex situ approach lack due to uncertainty during the transfer from the electrochemical cell to the spectrometer, the disadvantage of in situ cell is absorption of the electrolyte. Consequently, a cell with a very thin layer of electrolyte between the silicon window and EC film on reflective substrate should be designed. While ex situ technique offers usual IR RA spectra with regard to the uncovered background, only changes that occur in the structure during intercalation (or deintercalation) are detected in in situ spectra, which makes the identification much more difficult.IR absorbance spectroelectrochemistry can be performed ex situ after deposition of films on partly IR transparent silicon wafers. However, despite galvanostatic charging large overpotentials of silicon wafers may blur the direct comparison with electrochemical measurements of films on conductive substrates. With regard to IR spectroscopy, Raman spectroscopy is more complementary than competitive. As reported, the structural changes of polycrystalline V2O5 films can be significantly different than that of bulk materials [1]. From this reason, the ranking of the Raman spectra of novel V-oxide pigmented films is welcomed. With this aim there is constructed a new custom-made three-electrode in situ Raman cell and the V-oxide pigmented films studied for structural changes during coloration/bleaching processes. [1] Baddour-Hadjean et al., Chem. Rev. 2010, 110, 1278–1319.[2] Surca et al., J. Electrochem. Soc. 1999, 146, 232-242.[3] Mihelčič et al., Sol. Energy Mater. Sol. Cells 2012, 107, 175-187.[4] Mihelčič et al., Sol. Energy Mater. Solar Cells 2014, 120, 116-130.

Effect of channel geometry on the operating limit of micro pulsating heat pipes

In the present study, the operating limit of micro pulsating heat pipes (MPHPs) was investigated with different channel geometries: cross-sectional shapes and hydraulic diameters. Five-turn closed-loop MPHPs, which have either circular or square channels with hydraulic diameters of 390, 480, and 570μm, were fabricated onto a silicon wafer using MEMS techniques. To allow flow visualization, a transparent glass wafer was bonded to the silicon wafer. FC-72 was used as the working fluid with a filling ratio of 50%. Experimental results show that the operating limit, where the thermal performance significantly deteriorates, increases with the hydraulic diameter in both cross-sectional shapes. At the same hydraulic diameter, the square-channel MPHP can handle approximately 70% higher maximum allowable heat flux than the circular-channel MPHP. Based on the results of flow visualization, a model for the operating limit of the MPHPs in a vertical orientation was proposed: It was postulated that the MPHPs reach the operating limit when the falling film flow rate is smaller than the evaporation rate at any instant the liquid film is formed. This postulate was experimentally confirmed by comparing two rates for various channel geometries. Finally, a correlation for predicting the maximum allowable heat flux of the MPHPs was developed using a scale analysis and experimental data. According to the model, the maximum allowable heat flux is found to be proportional to the third power of the hydraulic diameter regardless of the cross-sectional shape.

On the Origin of the 4.7 eV Absorption and 2.8 eV Emission Bands in Bulk AlN Substrates

AlGaN alloys are the building blocks for deep UV optoelectronics and high-power devices. It has been demonstrated that the highest crystalline quality AlGaN films with high Al content are obtained on AlN single crystal substrates. The resulting mismatch between AlGaN and AlN causes a compressive strain within the AlGaN layers, which varies with composition. Nevertheless, pseudomorphic AlGaN films with Al content higher than 50%, and dislocation densities lower than 104 cm-2 have been achieved, sustaining compressive stresses with thicknesses exceeding 3 µm. Such results demonstrate the advantages of using AlN substrates for this technology, and at such some have been realized on several deep-UV optoelectronics applications. Kinoshita et al. have demonstrated UV LEDs emitting at 265 nm with output powers exceeding 80 mW. UV LEDs grown on these native substrates have higher reliabilities and higher output powers. In addition, optically pumped lasers emitting at wavelengths between 230 nm and 280 nm that display cavity modes and single polarized-state emission with low lasing thresholds have been developed. Nevertheless, there are several limitations related to the performance and further improvement of the active regions as related to their quantum efficiencies and the polarization of their emission is desired. These limitations will be classified in two main categories: (1) identification and control of point defects, and (2) efficient doping. One of the main limitations based on point defects is the one related to the UV absorption band present at 265 nm in AlN. This relatively broad absorption band limits the use of the substrate within the deep UV range and several complicated fabrication procedures were devised to overcome this limitation. In this work, the origin of this absorption band will be discussed, along with other related optical features and the use of co-doping as a way of removing this undesired absorption. Experimental and theoretical results that explore the optical properties of native and impurity point defects as well as point-defect complexes in AlN with respect to the 265 nm absorption band will be presented. Theoretical results, based on DFT, are compared to photoluminescence and absorption measurements of AlN samples grown by physical vapor transport, metal-organic chemical vapor deposition, and hydride vapor-phase epitaxy . Our results demonstrate that there are multiple sources of a deep-UV absorption between 4.5 and 5.0 eV. The presence of the isolated carbon substitutional CN - produces an absorption band centered at 4.7 eV and emission at 3.9 eV, thus its reduction leads to the reduction of the absorption band. In addition, one mechanism that contributes to the apparent UV transparency when the material is co-doped with Si is discussed. These schemes produced transparent AlN wafers suitable for deep UV LED production. The other limitation relates to obtaining technologically useful n-type conductivities in high Al-content AlGaN within the low and high doping regimes. Besides dislocation density reduction, identification and elimination of compensating defects are necessary to achieve this goal. AlGaN films with high Al mole fractions (0.65<x<1) were grown by MOCVD on single crystalline AlN and sapphire. It is found that with an increase in Al content, there was both a decrease in the free carrier concentration and mobility. The reduction in free carrier concentration for x>0.8 was primarily caused by an increase in activation energy. This is due to the formation of a Si DX-center, thus inspiring the search for alternative dopants. Preliminary data on Ge doping of AlGaN will be presented to assess its effectiveness as a donor throughout the whole composition regime. In addition, for samples grown with different Si doping levels it is observed that up to a critical Si concentration, the carrier concentration was always proportional to the Si source flow. Any further increase led to a free carrier concentration reduction. PL spectra exhibited deep defect-related peaks corresponding to transitions between Si and deep acceptor states. For Si concentrations less than an Al-content dependent critical concentration, a higher energy peak dominates the spectra. This peak was related to a (VAl-complex)2- based on DFT calculations. For Si concentrations greater than the critical concentration, a (VAl)3- related lower energy peak dominates the spectra. The energy levels associated with these defects increased with Al composition where the energy increase was the same as the increase in band gap. Acknowledgement: Partial financial support from NSF (DMR-1312582 and ECCS-1508854), DARPA (W911QX-10-C-0027), ARPA-E (DE-AR0000299), ARO (W911NF14C0008) is greatly appreciated. All PVT AlN wafers used for homoepitaxial growth were supplied by HexaTech, Inc., Morrisville, NC.

Cation Exchange Synthesis and Unusual Resistive Switching Behaviors of Ag2Se Nanobelts

Ag2Se nanobelts are prepared through employing ZnSe nanobelts as templates via a facile cation exchange approach. The templates are derived from precursor ZnSe·0.5N2 H4 nanobelts, which are synthesized by a simple hydrothermal method. As-synthesized precursor nanobelts are with 200 nm in width and several hundreds of micrometers in length. Annealed in N2 , they are transformed into ZnSe nanobelts with preserving their initial morphology. Following with a complete replacement of Zn(2+) by Ag(+), Ag2Se nanobelts with single crystalline are obtained via a cation-exchange reaction. Combined with the Langmuir-Blodgett assembly technique, regular films of ZnSe nanobelts can be achieved on transparent glass substrates and Si wafers with interdigital Au electrode arrays. Further, the optical and electrical evolutions are investigated from ZnSe nanobelts to Ag2 Se nanobelts. Finally, the resistive switching characteristic are carefully explored for Ag2Se nanobelts regularly arranged on interdigital Au microelectrodes. The results indicate that it is analogous to complementary resistive switching behaviors, which is different from that of traditional two terminal devices about previously reported Ag2Se. In order to clarify this phenomenon, a possible mechanism has been proposed and indirectly demonstrated through in situ SEM (scanning electron microscropy) observation.

Study of high-temperature Smart Cut™: Application to silicon-on-sapphire films and to thin foils of single crystal silicon

Two novel processes of elaboration of silicon thin films and silicon foils are proposed, based on the knowledge brought from the Smart Cut™. The first relies on the laser-beam annealing of an implanted silicon wafer in order to induce a separation layer within the implanted wafer and a transfer upon a transparent wafer. The second consists in depositing a layer of liquid silicon upon an implanted silicon wafer and to form a silicon foil by liquid phase epitaxial growth that can be separated from the substrate by Smart Cut™. The separation kinetics of an implanted silicon wafer is characterized for temperature between 450°C and 700°C, considering doses of implantation from 3.5∗1016Hcm−2 to 1.0∗1017Hcm−2. The out-diffusion of hydrogen is studied by Energy Recoil Detection Analysis and a model of diffusion of hydrogen in implanted silicon is proposed. Based on this analysis, a model for the kinetics of splitting at high temperature is established. Smart Cut™ separation is demonstrated for temperature up to 1250°C, considering an implanted silicon wafer bonded with a sapphire wafer, through which a laser beam anneals the structure. The kinetics of separation by laser beam annealing is characterized and compared to the kinetics established between 450°C and 700°C. The roughness of the silicon on sapphire film is characterized by Atomic Force Microscopy and a transfer is realized considering an implanted silicon bonded with a glass wafer of 200mm of diameter. Finally, this article presents results of liquid silicon deposition onto an implanted silicon substrate. These results demonstrate the possibility to detach the film grown by liquid phase epitaxy and the upper part of the implanted substrate by Smart Cut™. Electron Backscattering Diffraction Pattern analysis is considered in order to demonstrate the occurrence of epitaxy of the deposited liquid onto the implanted substrate.

Environmentally benign processing of YAG transparent wafers

Transparent yttrium aluminum garnet (YAG) wafers were successfully produced via aqueous tape casting and vacuum sintering techniques using a new environmentally friendly binder, a copolymer of isobutylene and maleic anhydride with the commercial name ISOBAM (noted as ISOBAM). Aqueous YAG slurries were mixed by ball-milling, which was followed by de-gassing and tape casting of wafers. The final YAG green tapes were homogenous and flexible, and could be bent freely without cracking. After the drying and sintering processes, transparent YAG wafers were achieved. The microstructures of both the green tape and vacuum-sintered YAG ceramic were observed by scanning electronic microscopy (SEM). Phase compositions were examined by X-ray diffraction (XRD). Optical transmittance was measured in UV–VIS regions with the result that the transmittance is 82.6% at a wavelength of 800nm.

Spray pyrolysis deposition and characterization of highly c‐axis oriented hexagonal ZnS nanorod crystals

Here we demonstrate that highly c‐axis oriented vertically aligned hexagonal ZnS nanorod‐like crystals have been synthesized via a facile and low‐cost spray pyrolysis method. ZnS layers were grown onto soda‐lime glass (SGL), glasses covered with transparent conductive oxides (TCOs: ITO and FTO) and silicon wafer substrates using zinc chloride and thiocarbamide precursors solution and air as carrier gas. The effect of main technological parameters on formation and properties of ZnS nanorods are studied. SEM, XRD, PL and UV‐VIS were applied to characterize the ZnS layers. Dimensions of the ZnS rods are mainly controlled by the deposition temperature, precursor concentration and substrate type. For instance, rods with d = 80–100 nm and l = 450 nm were obtained using ZnCl2 with c = 0.1 mol/L; whereas rods with d = ca. 80 nm and l = 250 nm were produced from ZnCl2 with c = 0.05 mol/L on glass. According to PL study, such defects as sulphur vacancies (Vs) are present in the ZnS samples, and concentration of Vs decreases with increasing the sulphur amount in solution.According to UV‐VIS, all ZnS layers possess high optical transmittance ca. 80% in the visible spectrum region; the Eg of ZnS is 3.7 eV independent of the deposition conditions, corresponding to wurtzite ZnS. ZnS rods produced at 550 and 580 °C show high haze factor values of ca. 90% at 320–350 nm revealing high light scattering ability. Vertically aligned, uniform in size ZnS rods can be directly grown onto glass, TCO and silicon substrates.

Blue inorganic light emitting diode on flexible polyimide substrate using laser lift-off process.

The fabrication process for the blue GaN inorganic light emitting diode (ILED) on flexible polyimide (PI) substrate by laser lift off (LLO) method was demonstrated. The GaN epi-structure was grown on patterned sapphire wafer. GaN samples were temporary bonded with polyimide substrate by flexible silver epoxy. Separation of the whole GaN LED film from GaN/sapphire wafer was accomplished using a single KrF excimer (248 nm) laser pulse directed through the transparent sapphire wafer. Device fabrication was carried out on both rigid silicon and flexible polyimide substrate, and I-V performance for both devices was measured. The optimized LLO process for the whole GaN LED film transfer would be applicable in flexible LED applications without compromising electrical properties.

All‐epitaxial fabrication of a nanowire plasmon laser structure

AbstractAn all‐epitaxial approach was demonstrated to create coaxial plasmon laser structures composed of an aluminum plasmonic metal/SiNx dielectric/InGaN quantum well shell surrounding a p‐GaN nanowire core. Strong UV luminescence was observed from as‐grown vertically‐aligned arrays as well as horizontally‐aligned nanowires transferred to a transparent carrier wafer. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Radiative cooling by light down conversion of InGaN light emitting diode bonded to a Si wafer

Using the recently proposed process of radiative cooling by light down conversion, we demonstrate cooling of about 5 K for InGaN light emitting diode (39 mg thermal load) that is self-heated up to 450 K and bonded to a cooler, a 15 × 15 × 4 mm3 Si wafer pumped with an above bandgap excitation from a 1.09-μm diode laser. Cooling occurs due to the enhancement of thermal emission in an initially transparent Si wafer when the overall energy of multiple (about 20) below bandgap photons escaping the wafer exceeds the energy of the single pumped photon. The cooling efficiency amounts to 93%.

Self-assembled self-cleaning broadband anti-reflection coatings

Anti-reflection (AR) coatings are widely used in a spectrum of optical and optoelectronic devices, such as monitors, car dashboards, optical lenses, photodiodes, and solar cells. Narrowband quarter-wavelength single-layer and broadband multilayer dielectric AR coatings are typically fabricated by expensive and low-throughput vapor deposition processes (e.g., sputtering). Inspired by the subwavelength-structured cornea of some nocturnal moths, nanostructured broadband moth-eye AR coatings that can significantly suppress optical reflection over a wide range of wavelengths and light incident angles have been extensively exploited by both top-down and bottom-up approaches. Among many available bottom-up technologies, colloidal self-assembly is a promising approach as it is simple, fast, and inexpensive. In this review article, we will discuss two scalable colloidal self-assembly technologies based on Langmuir–Blodgett assembly and spin-coating for fabricating quarter-wavelength and moth-eye AR coatings with unique self-cleaning functionalities on transparent substrates (e.g., glass) and semiconductor wafers (such as crystalline silicon and GaAs).

Electronically reconfigurable metal-on-silicon metamaterial

Reconfigurable metamaterial-based apertures can play a unique role in both imaging and in beam-forming applications, where current technology relies mostly on the fabrication and integration of large detector or antenna arrays. Here, we report the experimental demonstration of a voltage-controlled, silicon-based electromagnetic metamaterial operating in the W-band (75-110 GHz). In this composite semiconductor metamaterial, patterned gold metamaterial elements serve both to manage electromagnetic wave propagation while simultaneously acting as electrical Schottky contacts that control the local conductivity of the semiconductor substrate. The active device layers consist of a patterned metal on a 2-{\mu}m-thick n-doped silicon layer, adhesively bonded to a transparent Pyrex wafer. The transmittance of the composite metamaterial can be modulated over a given frequency band as a function of bias voltage. We demonstrate a quantitative understanding of the composite device through the application of numerical approaches that simultaneously treat the semiconductor junction physics as well as wave propagation.

Sorting and lysis of single cells by BubbleJet technology

We demonstrate that chip-integrated BubbleJet technology can be used for cell sorting as well as for single-cell lysis. Vapor bubbles on the millisecond timescale are generated by titanium microheaters manufactured on transparent glass wafers. A PDMS layer is bonded on top of the glass wafer and defines microfluidic channels. Cell sorting is demonstrated with mouse fibroblasts which are hydrodynamically focused and travel at velocities of about 500μm/s. For cell sorting the bubble generators are placed outside the microfluidic channel but are connected to it via a 20μm nozzle. Randomly selected fibroblasts have been deflected laterally up to 60μm from their original streamline to a collector outlet by actuation. Single-cell lysis is performed by bubble generators placed inside a flow channel directly below the cells’ path. Bubble generation pushes the cells towards the channel ceiling or the channel wall and cells are lysed due to the large shear force at an efficiency of 100%. Cycle times for sorting and lysis have experimentally been determined to be 5ms and 20ms, respectively.

Probing Particle Movement in CMP with Fluorescence Technique

Chemical mechanical polishing (CMP) is now widely used in semiconductor manufacturing. Characterizing the particle movement in CMP is essential to understanding of the complicated material removal mechanisms for CMP. In this study, an experimental system based on fluorescence technique has been designed and developed. The particle displacement and velocity in the interface between a transparent wafer and a polishing pad were observed in-situ with the system. It can be found that some particles were embedded on the polishing pad and rotated with the pad. And the others were able to move free in slurry flow. All the particles in the experiments could be classified into four groups. The counting procedure suggested that the percentage of free particles in the field of view was between 10 and 50%. By analyzing the velocity of free particles at various rotation speeds and applied pressure, it was found that some of the free particles were able to travel at a speed 12–18 times the linear speed. The uncertainty in the velocity measurements was less than 8%. The objective of this study was to offer deep insights into the material removal mechanisms by measuring the real-time particle movement in CMP.

Cu-Cu Thermo-Compression Bonding for TSV Integration

We developed total solutions for the optically aligned Cu-Cu direct bonding process. The quantitative analyses of the interfacial adhesion energy and seam voids of Cu-Cu bonds performed with varying process parameters showed that bonding temperature and post-bond annealing have the most significant influence on bond properties. By optimizing experimental parameters, we could achieve, even with a short bonding time, the sufficient interfacial adhesion energy ({greater than or equal to}5 J/m2) with no interfacial voids. Post-bond annealing performed at 250-300ºC drastically improves the interfacial adhesion energy. SmartView alignment enables the face-to-face Cu-Cu bonding of non-IR transparent wafers while maintaining less than 0.2µm (3σ) and 1.0µm (3σ) of the pre-bond and post-bond alignment accuracy, respectively. Temporary bonding and debonding based on a new spin-on adhesive has such benefits as temperature stability up to 250ºC, compatibility with bumped structures, short debonding time, low-temperature (<200ºC) slide-off release, easy cleanup with polar solvents, etc.

Nanocrystalline diamond microelectrode arrays fabricated on sapphire technology for high-time resolution of quantal catecholamine secretion from chromaffin cells

The quantal release of oxidizable molecules can be successfully monitored by means of polarized carbon fiber microelectrodes (CFEs) positioned in close proximity to the cell membrane. To partially overcome certain CFE limitations, mainly related to their low spatial resolution and lack of optical transparency, we developed a planar boron-doped nanocrystalline diamond (NCD) prototype, grown on a transparent sapphire wafer. Responsiveness to applied catecholamines as well as the electrochemical and optical properties of the NCD-based device were first characterized by cyclic voltammetry and optical transmittance measurements. By stimulating chromaffin cells positioned on the device with external KCl, well-resolved quantal exocytotic events could be detected either from one NCD microelectrode, or simultaneously from an array of four microelectrodes, indicating that the chip is able to monitor secretory events (amperometric spikes) from a number of isolated chromaffin cells. Spikes detected by the planar NCD device had comparable amplitudes, kinetics and vesicle diameter distributions as those measured by conventional CFEs from the same chromaffin cell.

Planar-Integrated Optics and Microfluidics for Lab-on-a-Chip Systems

A planar integration platform for lab-on-a-chip systems is proposed. It evolves from planar-integrated free-space optics and includes planar microfluidics. Optical systems are folded into transparent wafers and can be fabricated with mask-based techniques. Fluidic actuation is achieved electroacoustically by means of Rayleigh-type surface acoustic waves that are generated with interdigital transducers on a piezoelectric wafer. Simple proof-of-principle demonstrators were constructed and experimentally tested. They contain a ring-shaped flow channel in which an arbitrary fluid can be set into circulation, and integrated free-space optical systems for microscopic brightfield and darkfield illumination, respectively. As piezoelectric material 128° y-cut LiNbO3 was used, the flow channel and the optical functions were realized in poly(methyl methacrylate) (PMMA).

Modelocked integrated external-cavity surface emitting laser

A modelocked integrated external-cavity surface emitting laser (MIXSEL) is a novel type of ultrafast semiconductor laser that integrates a saturable absorber directly into a vertical external cavity surface-emitting laser. The saturable absorber requirements and integration challenges to obtain self-starting and stable pulse formation are discussed. One single quantum dot absorber layer was optimised for this application. Since the first feasibility demonstration of an optically pumped MIXSEL, the authors have further improved the average output power to 185 mW with 32 ps pulses at around 3 GHz pulse repetition rate at a centre wavelength of ≃957 nm. The authors analyse and discuss the challenges for further power scaling and pulse shortening. The MIXSEL concept appears suitable for cost-efficient wafer-scale mass production when the external cavity is defined by a transparent wafer into which the curved output coupler can be etched. The semiconductor MIXSEL structure would then be glued to such a transparent wafer. The potential for electrically pumped MIXSELs will make this laser technology even more attractive.

Optical properties and structure of HfO2 thin films grown by high pressure reactive sputtering

Thin films of hafnium oxide (HfO2) have been grown by high pressure reactive sputtering on transparent quartz substrates (UV-grade silica) and silicon wafers. Deposition conditions were adjusted to obtain polycrystalline as well as amorphous films. Optical properties of the films deposited on the silica substrates were investigated by transmittance and reflectance spectroscopy in the ultraviolet, visible and near infrared range. A numerical analysis method that takes into account the different surface roughness of the polycrystalline and amorphous films was applied to calculate the optical constants (refractive index and absorption coefficient). Amorphous films were found to have a higher refractive index and a lower transparency than polycrystalline films. This is attributed to a higher density of the amorphous samples, which was confirmed by atomic density measurements performed by heavy-ion elastic recoil detection analysis. The absorption coefficient gave an excellent fit to the Tauc law (indirect gap), which allowed a band gap value of 5.54 eV to be obtained. The structure of the films (amorphous or polycrystalline) was found to have no significant influence on the nature of the band gap. The Tauc plots also give information about the structure of the films, because the slope of the plot (the Tauc parameter) is related to the degree of order in the bond network. The amorphous samples had a larger value of the Tauc parameter, i.e. more order than the polycrystalline samples. This is indicative of a uniform bond network with percolation of the bond chains, in contrast to the randomly oriented polycrystalline grains separated by grain boundaries.