A review of advances in pixel detectors for experiments with high rate and radiation

The Large Hadron Collider (LHC) experiments ATLAS and CMS have established hybrid pixel detectors as the instrument of choice for particle tracking and vertexing in high rate and radiation environments, as they operate close to the LHC interaction points. With the High Luminosity-LHC upgrade now in sight, for which the tracking detectors will be completely replaced, new generations of pixel detectors are being devised. They have to address enormous challenges in terms of data throughput and radiation levels, ionizing and non-ionizing, that harm the sensing and readout parts of pixel detectors alike. Advances in microelectronics and microprocessing technologies now enable large scale detector designs with unprecedented performance in measurement precision (space and time), radiation hard sensors and readout chips, hybridization techniques, lightweight supports, and fully monolithic approaches to meet these challenges. This paper reviews the world-wide effort on these developments.Comment: 84 pages with 46 figures. Review article.For submission to Rep. Prog. Phy

Antimonide-based superlattice membranes for infrared applications

Semiconductor membranes offer an interesting materials and device development platform due to their ability to integrate dissimilar materials through a print, stamp and transfer process. There is a lot of interest in integrating antimonide based type-II superlattices (T2SL) onto inexpensive substrates, such as Si, to not only undertake fundamental studies into the optical, electronic and structural properties of the superlattices but also to fabricate wafer-level infrared (IR) photodetectors. An effective approach to transfer type-II superlattice membranes (T2SL-M) onto alternate substrates is based on membrane release from the native GaSb substrate followed by the transfer to a new host substrate. In this work, I have transferred InAs/GaSb and InAs/InAsSb T2SLs with different in-plane geometries from a GaSb substrate to a variety of host substrates, including Si, polydimethylsiloxane and metal coated surfaces. Electron microscopy shows structural integrity of transferred membranes with thicknesses ranging from 100 nm to 2.5 µm and lateral sizes from 24x24 µm2 v to 1x1 cm2 . Atomic force and electron microscopy reveal the excellent quality of the membrane interface with the new host. The crystalline structure of the membrane is not altered by the fabrication process, and minimal elastic relaxation occurs during the release step, as demonstrated by X-ray diffraction and mechanical modeling. I have also used the antimonide superlattice membranes to realize wafer level infrared detectors on silicon substrates without using conventional Indium-bump hybridization. In this approach, PIN superlattices are grown on top of a 60 nm Al0.6Ga0.4Sb sacrificial layer on a GaSb host substrate. Following the growth, I have transferred the individual pixels using an epitaxial lift-off technique, which consists of a wet-etch to undercut the pixels followed by a dry-stamp process to transfer the pixels to a silicon substrate prepared with a gold layer. I have done structural and optical characterization of the transferred pixels using an optical microscope, scanning electron microscopy, and photo luminescence. The interface between the transferred pixels and the new substrate is abrupt, and no significant degradation in the optical quality is observed. Next, I have fabricated an indium-bump-free membrane detector using this approach. Spectral response measurements and the current-voltage characteristics of an infrared photodetector, based on the InAs/InAsSb superlattice, bonded to Si demonstrates the functionality of transferred membranes in the infrared range. The performance of the membrane detector is compared to a control detector using the as-grown epitaxial material. The proposed approach to fabricate Indium-bump free detectors could pave the way for wafer-level integration of photonic detectors on silicon substrates, which could dramatically reduce the cost of these detectors. Since the release of T2SL-M is achieved using a high etch selectivity between the active region and the Al-containing sacrificial layers, a poor selectivity between the sacrificial layers and a variety of T2SL active regions will result in significant damage to the active layer of an IR detector. I have developed a novel two-step etching process to protect the T2SL-M while the sacrificial layer is etched away. In this process, both the top surface and the sidewalls of the membrane are coated with a hard-baked polymer film (i.e., photoresist), and therefore they are unexposed to the chemical etchant. For Al and Ga containing compounds, with no membrane isolation, this process leads to rough sidewalls which are expected to increase surface recombination in the membrane and therefore increases the dark current density of an IR detector. I have quantified this effect by characterizing T2SL IR detectors fabricated on isolated and non-isolated mesas. A comparative analysis of the dark current density measured for the two devices signify the effect of having exposed sidewalls during membrane release. These experimental results are consistent with theoretical calculations which show a relative enhancement of surface recombination with increasing roughness of the membrane sidewalls. The development of these Sb based T2SL membranes opens up new exciting prospects for material science studies and device architecture integration

Piezotransistive III-V Nitride Microcantilever Based Mems/Nems Sensor for Photoacoustic Spectroscopy of Chemicals

Microcantilevers are highly attractive as transducers for detecting chemicals, explosives, and biological molecules due to their high sensitivity, micro-scale dimensions, and low power consumption. Though optical transduction of the mechanical movement of the microcantilevers into an electrical signal is widely practiced, there is a continuous thrust to develop alternative transduction methods that are more conducive to the development of compact miniaturized sensors. Piezoelectric and piezoresistive transduction methods are two of the most popular ones that have been utilized to develop miniaturized sensor systems. Piezoelectric cantilevers, which are commonly made of PZT film, have demonstrated very high sensitivity; however, they suffer from incompatibility with Si based circuitry and challenges with dc and low frequency measurements due to the problem of charge leakage. On the other hand, piezoresistive microcantilever, which are mostly made of Si, can be easily integrated with existing Si based process technologies, but suffer from low sensitivity. In addition, none of the above material systems are suitable for high temperature (\u3e300 °C) and harsh environment operation. III-V Nitride semiconductors are being extensively studied almost two decades for electronic and optoelectronic applications due to their exceptional physical and chemical properties, which include a wide bandgap, strong piezoelectric properties, high electron mobility, and chemical inertness. AlGaN/GaN heterostructures offer unique advantage over existing piezoresistive or piezoelectric materials, as it actually converts the piezoelectric response of these materials to piezoresistive response, since the two dimensional electron gas (2DEG) formed at the AlGaN/GaN interface gets modulated by the stress induced change in piezoelectric polarization. The epitaxial growth of III-V Nitride layers on a Si substrate enables direct integration of nitride microelectromechanical systems (MEMS) with mature Si based integrated circuits to develop miniaturized sensor systems. In spite of several technological advantages of III-V Nitride MEMS, of which a microcantilever is a simple example, only a handful of studies have been reported on their deflection characterization in static mode and none on dynamic bending mode. The effect of mechanical strain, on 2DEG density and output characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs), have been reported earlier. High gauge factors (\u3e100) have been reported for quasi-static and step bending response, however, the factors contributing to such high values, especially their deviation from much lower theoretical estimates, are poorly understood. Recently, very high gauge factor of -850 was reported for microcantilevers in transient condition, however, the corresponding dynamic response was not studied. Acoustic detection using microcantilevers have attracted interest in recent years, especially in photoacoustic spectroscopy, as they can offer up to two orders of higher sensitivity compared to existing acoustic sensors. III-V Nitride based ultrasonic microcantilevers sensors, offering high sensitivity, low noise, and harsh environment operation, are ideally suited for many demanding sensing applications that are not possible at present. This dissertation aims the theory and application of III-V Nitride microcantilevers and a novel electronic transduction scheme named as ‘Piezotransistive Microcantilever’ to transduce femtoscale excitation. A complete fabrication process, measurement techniques and several application aspects of this sensing technology specially acoustic wave detection generated in solid and air media with high sensitivity, have been demonstrated. This thesis reports on displacement measurement at the femtoscale level using a GaN microcantilever with an AlGaN/GaN Heterojuction Field Effect Transistor (HFET) integrated at the base that utilizes piezoelectric polarization induced changes in two dimensional electron gas (2DEG) to transduce displacement with very high sensitivity. With appropriate biasing of the HFET, an ultra-high Gauge Factor (GF) of 8700, the highest ever reported, was obtained, with an extremely low power consumption of \u3c1 \u3enW, which enabled direct electrical readout of the thermal noise spectra of the cantilever. The self-sensing piezotransistor was able to transduce external excitation with a superior noise limited resolution of 12.43 fm/Hz and an outstanding responsivity of 170 nV/fm, which is three orders higher that state-of-the-art technology, supported by both analytical calculations and laser vibrometry measurements. This extraordinary deflection sensitivity enabled unique detection of nanogram quantity of analytes using photoacoustic spectroscopy

Design and fabrication of MOMS-based ultrasonic probes for minimally invasive endoscopic applications

A Micro-opto-mechanical systems (MOMS) based technology for the fabrication of ultrasonic probes on optical fiber is presented. Thanks to the high miniaturization level reached, the realization of an ultrasonic system constituted by ultrasonic generating and detecting elements, suitable for minimally invasive applications or Non Destructive Evaluation (NDE) of materials at high resolution, is demonstrated. The ultrasonic generation is realized by irradiating a highly absorbing carbon film patterned on silicon micromachined structures with a nanosecond pulsed laser source, generating a mechanical shock wave due to the thermal expansion of the film induced by optical energy conversion into heat. The short duration of the pulsed laser, together with an appropriate emitter design, assure high frequency and wide band ultrasonic generation. The acoustic detection is also realized on a MOMS device using an interferometric receiver, fabricated with a Fabry-Perot optical cavity realized by means of a patterned SU-8 and two Al metallization levels. In order to detect the ultrasonic waves, the cavity is interrogated by a laser beam measuring the reflected power with a photodiode. Various issues related to the design and fabrication of these acoustic probes are investigated in this thesis. First, theoretical models are developed to characterize the opto-acoustic behavior of the devices and estimate their expected acoustic performances. Tests structures are realized to derive the relevant physical parameters of the materials constituting the MOMS devices and determine the conditions theoretically assuring the best acoustic emission and detection performances. Moreover, by exploiting the models and the theoretical results, prototypes of acoustic probes are designed and their fabrication process developed by means of an extended experimental activity

NEW PROCESSES FOR HETEROGENEOUS INTEGRATION OF III-NITRIDE OPTOELECTRONIC DEVICES: APPLICATION TO INGAN-BASED LIGHT EMITTING DIODES AND SOLAR CELLS GROWN ON 2D H-BN

Mechanical release and transfer of GaN-based heterostructures using 2D h-BN have undergone considerable development in van der Waals epitaxy of III-Nitride thin-films along with device fabrication and transfer onto various flexible and rigid substrates. The technique consists of a mechanical peeling-off of the epilayers from the native substrate, which allows a dry and fast release and transfer of optoelectronic and electronic III-N devices to arbitrary substrates. However, during the epitaxial growth and device fabrication of the epilayers, delaminations and cracks arise in the structures, which limits the size of crack-free devices to only a few hundreds of squared microns. The goal of this thesis is to develop new efficient, large-scale and low-cost new processes for heterogeneous integration of III-Nitride optoelectronic devices. These processes developed were used to fabricate lateral and vertical InGaN-based LEDs as well as nanopyramid-based InGaN solar cells grown on 2D h-BN. The outcomes of this thesis represent progress towards efficient, robust and low-cost 2D-hBN-assisted lift-off technology for heterogeneous integration of optoelectronic and electronic III-N devices.Ph.D

Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II

Wide bandgap (WBG) semiconductors are becoming a key enabling technology for several strategic fields, including power electronics, illumination, and sensors. This reprint collects the 23 papers covering the full spectrum of the above applications and providing contributions from the on-going research at different levels, from materials to devices and from circuits to systems

Ultra-thin and flexible CMOS technology: ISFET-based microsystem for biomedical applications

A new paradigm of silicon technology is the ultra-thin chip (UTC) technology and the emerging applications. Very thin integrated circuits (ICs) with through-silicon vias (TSVs) will allow the stacking and interconnection of multiple dies in a compact format allowing a migration towards three-dimensional ICs (3D-ICs). Also, extremely thin and therefore mechanically bendable silicon chips in conjunction with the emerging thin-film and organic semiconductor technologies will enhance the performance and functionality of large-area flexible electronic systems. However, UTC technology requires special attention related to the circuit design, fabrication, dicing and handling of ultra-thin chips as they have different physical properties compared to their bulky counterparts. Also, transistors and other active devices on UTCs experiencing variable bending stresses will suffer from the piezoresistive effect of silicon substrate which results in a shift of their operating point and therefore, an additional aspect should be considered during circuit design. This thesis tries to address some of these challenges related to UTC technology by focusing initially on modelling of transistors on mechanically bendable Si-UTCs. The developed behavioural models are a combination of mathematical equations and extracted parameters from BSIM4 and BSIM6 modified by a set of equations describing the bending-induced stresses on silicon. The transistor models are written in Verilog-A and compiled in Cadence Virtuoso environment where they were simulated at different bending conditions. To complement this, the verification of these models through experimental results is also presented. Two chips were designed using a 180 nm CMOS technology. The first chip includes nMOS and pMOS transistors with fixed channel width and two different channel lengths and two different channel orientations (0° and 90°) with respect to the wafer crystal orientation. The second chip includes inverter logic gates with different transistor sizes and orientations, as in the previous chip. Both chips were thinned down to ∼20m using dicing-before-grinding (DBG) prior to electrical characterisation at different bending conditions. Furthermore, this thesis presents the first reported fully integrated CMOS-based ISFET microsystem on UTC technology. The design of the integrated CMOS-based ISFET chip with 512 integrated on-chip ISFET sensors along with their read-out and digitisation scheme is presented. The integrated circuits (ICs) are thinned down to ∼30m and the bulky, as well as thinned ICs, are electrically and electrochemically characterised. Also, the thesis presents the first reported mechanically bendable CMOS-based ISFET device demonstrating that mechanical deformation of the die can result in drift compensation through the exploitation of the piezoresistive nature of silicon. Finally, this thesis presents the studies towards the development of on-chip reference electrodes and biodegradable and ultra-thin biosensors for the detection of neurotransmitters such as dopamine and serotonin

Novel Materials and Fabrication Techniques for Enhanced Current Spreading and Light Extraction in High Efficiency Light-emitting Diodes

Although solid-state lighting based on III-nitride light-emitting diodes (LEDs) is on track to become the dominant technology for lighting our world, we have yet to reach the efficacy limit in these devices. Many challenges remain to be solved to achieve this goal. In this thesis, we discuss novel materials and fabrication techniques which can be used to enhance the efficiency of LEDs through improving lateral current spreading through p-type GaN and increasing light extraction. Presented in the first part of this thesis is the growth and characterization of hydrothermal ZnO thin films. The effect of growth conditions on the morphology and optoelectronic properties of the films will be discussed. In particular, we studied the effects that group III dopants (Al, Ga, and In) have when introduced into the hydrothermal ZnO thin films. The Ga doped film showed the lowest resistivity, with a resistivity of 1.94 mΩ cm for films doped with 0.4 at.% Ga. Undoped ZnO films showed the lowest optical absorption coefficient of 441 cm-1 at 450 nm. This study represents the first time all three dopants have been systematically compared to one another. The second part of the thesis focuses on an alternative approach to forming transparent contacts to p-GaN. This involved the regrowth of highly doped n-type GaN using ammonia molecular beam epitaxy (NH3 MBE) to form epitaxial tunnel junction contact. We discuss several aspects of the growth and performance of regrown TJ contacts on p-n diode structures as well as InGaN/GaN LEDs. Improved turn-on voltages and reduced series resistances have been realized by depositing highly doped Si-doped n-type GaN using MBE on polarization enhanced p-type InGaN contact layers grown using MOCVD. We compared the effects of different Si doping concentrations, and the addition of p-type InGaN on the forward voltages of p-n diodes and LEDs. It was found that increasing Si concentrations from 1.9x1020 to 4.6x1020 cm-3 and including a highly doped p-type InGaN at the junction both contribute to the narrowing of the depletion width, lowering series resistance from 4.2x10-3 to 3.4x10-3 Ω cm2 and decreasing turn-on voltages of the diodes.The third, and last part, of this thesis, will focus device results utilizing the alternative current spreading contacts discussed in the previous chapters. LED devices fabricated using doped ZnO films will be analyzed as well as LEDs utilizing MBE n-GaN TJ contacts on blue InGaN flip-chip triangular LEDs grown on both free-standing GaN and patterned sapphire substrates. The performance of LEDs with Ga-doped ZnO (Ga:ZnO) and Sn-doped In2O3 (ITO) current-spreading layers (CSLs) has been evaluated at high injection current densities. LEDs with electron beam-hydrothermally deposited Ga:ZnO transparent CSLs showed improved performance compared to electron beam deposited ITO at all current densities. External quantum efficiency and wall plug efficiency were both higher for blue emitting LEDs with ZnO. Luminous efficacy increased greatly for the ZnO-based CSL with a peak value of 113 lm/W compared to 82 lm/W for the ITO-based CSL, a 37% improvement. Issues with metal organic chemical vapor deposition (MOCVD), device processing, and device performance are discussed as well