Tunable superlattice p-i-n photodetectors: characteristics, theory, and application

Extended measurements and theory on the recently developed monolithic wavelength demultiplexer consisting of voltage-tunable superlattice p-i-n photodetectors in a waveguide confirmation are discussed. It is shown that the device is able to demultiplex and detect two optical signals with a wavelength separation of 20 nm directly into different electrical channels at a data rate of 1 Gb/s and with a crosstalk attenuation varying between 20 and 28 dB, depending on the polarization. The minimum acceptable crosstalk attenuation at a data rate of 100 Mb/s is determined to be 10 dB. The feasibility of using the device as a polarization angle sensor for linearly polarized light is also demonstrated. A theory for the emission of photogenerated carriers out of the quantum wells is included, since this is potentially a speed limiting mechanism in these detectors. It is shown that a theory of thermally assisted tunneling by polar optical phonon interaction is able to predict emission times consistent with the observed temporal response

Development of high temperature gallium phosphide rectifiers

Large area high performance, GaP rectifiers were fabricated by means of Zn diffusion into vapor phase epitaxial GaP. Devices with an active area of 0.01 sq cm typically exhibit forward voltages of 3 volts for a bias current of 1 ampere and have reverse breakdown voltages of 300 volts for temperatures from 27 C to 400 C. Typical device reverse saturation current at a reverse bias of 150 volts is less than 10 to the minus 9th power amp at 27 C and less than 0.000050 amp at 400 C

Design and Analysis of a Discrete, PCB-Level Low-Power, Microwave Cross-Coupled Differential LC Voltage-Controlled Oscillator

Radio Frequency (RF) and Microwave devices are typically implemented in Integrated Circuit (IC) form to minimize parasitics, increase precision and tolerances, and minimize size. Although IC fabrication for students and independent engineers is cost-prohibitive, an abundance of low-cost, easily accessible printed circuit board (PCB) and electronic component manufacturers allows affordable PCB fabrication. While nearly all microwave voltage-controlled oscillator (VCO) designs are IC-based, this study presents a discrete PCB-level cross-coupled, differential LC VCO to demonstrate this more affordable and accessible approach. This thesis presents a 65 mW, discrete component VCO PCB with industry-comparable RF performance. A phase noise of -103.7 dBc/Hz is simulated at a 100 kHz offset from a 4.05 GHz carrier. This VCO achieves a 532 MHz (13.25%) tuning bandwidth. A figure of merit, FOMP, [1] value of -177.7 dB (includes phase noise and power consumption) is calculated at 4.05 GHz. This surpasses the performance of an industry standard VCO (HMC430LPx, Analog Devices), -176.5 dB, and four other commercially available VCOs. Furthermore, this study presents novel discrete design implementations to minimize both power consumption and capacitive loading effects, while optimizing phase noise. Finally, this project serves as a reference for analyzing and implementing low-level, complex RF and Microwave circuits on a PCB accessible to all students and independent engineers

Geiger-Mode Avalanche Photodiodes in Standard CMOS Technologies

Photodiodes are the simplest but most versatile semiconductor optoelectronic devices. They can be used for direct detection of light, of soft X and gamma rays, and of particles such as electrons or neutrons. For many years, the sensors of choice for most research and industrial applications needing photon counting or timing have been vacuum-based devices such as Photo-Multiplier Tubes, PMT, and Micro-Channel Plates, MCP (Renker, 2004). Although these photodetectors provide good sensitivity, noise and timing characteristics, they still suffer from limitations owing to their large power consumption, high operation voltages and sensitivity to magnetic fields, as well as they are still bulky, fragile and expensive. New approaches to high-sensitivity imagers tend to use CCD cameras coupled with either MCP Image Intensifiers, I-CCDs, or Electron Multipliers, EM-CCDs (Dussault & Hoess, 2004), but they still have limited performances in extreme time-resolved measurements. A fully solid-state solution can improve design flexibility, cost, miniaturization, integration density, reliability and signal processing capabilities in photodetectors. In particular, Single- Photon Avalanche Diodes, SPADs, fabricated by conventional planar technology on silicon can be used as particle (Stapels et al., 2007) and photon (Ghioni et al., 2007) detectors with high intrinsic gain and speed. These SPAD are silicon Avalanche PhotoDiodes biased above breakdown. This operation regime, known as Geiger mode, gives excellent single-photon sensitivity thanks to the avalanche caused by impact ionization of the photogenerated carriers (Cova et al., 1996). The number of carriers generated as a result of the absorption of a single photon determines the optical gain of the device, which in the case of SPADs may be virtually infinite. The basic concepts concerning the behaviour of G-APDs and the physical processes taking place during their operation will be reviewed next, as well as the main performance parameters and noise sources

III-nitride nanowire light-emitting diodes: design and characterization

III-nitride semiconductors have been intensively studied for optoelectronic devices, due to the superb advantages offered by this materials system. The direct energy bandgap III-nitride semiconductors can absorb or emit light efficiently over a broad spectrum, ranging from 0.65 eV (InN) to 6.4 eV (AlN), which encompasses from deep ultraviolet to near infrared spectrum. However, due to the lack of native substrates, conventional III-nitride planar heterostructures generally exhibit very high dislocation densities that severely limit the device performance and reliability. On the other hand, nanowire heterostructures can be grown on lattice mismatched substrates with drastically reduced dislocation densities, due to highly effective lateral stress relaxation. Nanowire light-emitting diodes (LEDs) with emission in the ultraviolet to visible wavelength range have recently been studied for applications in solid-state lighting, flat-panel displays, and solar-blind detectors. In this thesis, investigation of the systematic process flow of design and epitaxial growth of group III-nitride nanoscale heterostructures was done. Moreover, demonstration of phosphor-free nanowire white LEDs using InGaN/AlGaN nanowire heterostructures grown directly on Si(111) substrates by molecular beam epitaxy was made. Full-color emission across nearly the entire visible wavelength range was realized by controlling the In composition in the InGaN active region. Strong white-light emission was recorded for the unpackaged nanowire LEDs with an unprecedentedly high color rendering index of 98. Moreover, LEDs with the operating wavelengths in the ultraviolet (UV) spectra, with emission wavelength in the range of 280-320 nm (UV-B) or shorter wavelength hold tremendous promise for applications in phototherapy, skin treatments, high speed dissociation and high density optical recording. Current planar AlGaN based UV-B LEDs have relatively low quantum efficiency due to their high dislocation density resulted from the large lattice mismatch between the AlGaN and suitable substrates. In this study, associated with the achievement of visible LEDs, the development of high brightness AlGaN/GaN nanowire UV-LEDs via careful design and device fabrication was shown. Strong photoluminescence spectra were recorded from these UV-B LEDs. The emission peak can be tunable from 290 nm to 320 nm by varying the Al content in AlGaN active region which can be done by optimizing the growth condition including Al/Ga flux ratio and also the growth temperature. Such visible to UV-B nanowire LEDs are ideally suited for future smart lighting, full-color display, phototherapy and skin treatments applications

Design Of Silicon Controlled Rectifers Sic] For Robust Electrostatic Discharge Protection Applications

Electrostatic Discharge (ESD) phenomenon happens everywhere in our daily life. And it can occurs through the whole lifespan of an Integrated Circuit (IC), from the early wafer fabrication process, extending to assembly operation, and finally ending at the user‟s site. It has been reported that up to 35% of total IC field failures are ESD-induced, with estimated annual costs to the IC industry running to several billion dollars. The most straightforward way to avoid the ICs suffering from the threatening of ESD damages is to develop on-chip ESD protection circuits which can afford a robust, low-impedance bypassing path to divert the ESD current to the ground. There are three different types of popular ESD protection devices widely used in the industry, and they are diodes or diodes string, Grounded-gate NMOS (GGNMOS) and Silicon Controlled Rectifier (SCR). Among these different protection solutions, SCR devices have the highest ESD current conduction capability due to the conductivity modulation effect. But SCR devices also have several shortcomings such as the higher triggering point, the lower clamping voltage etc, which will become obstacles for SCR to be widely used as an ESD protection solutions in most of the industry IC products. At first, in some applications with pin voltage goes below ground or above the VDD, dual directional protection between each two pins are desired. The traditional dual-directional SCR structures will consume a larger silicon area or lead to big leakage current issue due to the happening of punch-through effect. A new and improved SCR structure for low-triggering ESD iv applications has been proposed in this dissertation and successfully realized in a BiCMOS process. Such a structure possesses the desirable characteristics of a dual-polarity conduction, low trigger voltage, small leakage current, large failing current, adjustable holding voltage, and compact size. Another issue with SCR devices is its deep snapback or lower holding voltage, which normally will lead to the latch-up happen. To make SCR devices be immunity with latch-up, it is required to elevate its holding voltage to be larger than the circuits operational voltage, which can be several tens volts in modern power electronic circuits. Two possible solutions have been proposed to resolve this issue. One solution is accomplished by using a segmented emitter topology based on the concept that the holding voltage can be increased by reducing the emitter injection efficiency. Experimental data show that the new SCR can posses a holding voltage that is larger than 40V and a failure current It2 that is higher than 28mA/um. The other solution is accomplished by stacking several low triggering voltage high holding voltage SCR cells together. The TLP measurement results show that this novel SCR stacking structure has an extremely high holding voltage, very small snapback, and acceptable failure current. The High Holding Voltage Figure of Merit (HHVFOM) has been proposed to be a criterion for different high holding voltage solutions. The HHVFOM comparison of our proposed structures and the existing high holding voltage solutions also show the advantages of our work

Development, Demonstration, and Device Physics of FET-Accessed One-Transistor GaAs Dynamic Memory Technologies

The introduction of digital GaAs into modem high-speed computing systems has led to an increasing demand for high-density memory in these GaAs technologies. To date, most of the memory development efforts in GaAs have been directed toward four- and six-transistor static RAM\u27s, which consume substantial chip area and dissipate much static power resulting in limited single-chip GaAs storage capacities. As it has successfully done in silicon, a one-transistor dynamic RAM approach could alleviate these problems making higher density GaAs memories possible. This dissertation discusses theoretical and experimental work that presents the possibility for a high-speed, low-power, one-transistor dynamic RAM technology in GaAs. The two elements of the DRAM cell, namely the charge storage capacitor and the access field-effect transistor have been studied in detail. Isolated diode junction charge storage capacitors have demonstrated 30 minutes of storage time at room temperature with charge densities comparable to those obtained in planar silicon DRAM capacitors. GaAs JFET and MESFET technologies have been studied, and with careful device design and choice of proper operating voltages experimental results show that both can function as acceptable access transistors. One-transistor MESFET- and JFET-accessed DRAM cells have been fabricated and operated at room temperature and above with a standby power dissipation that is only a small fraction of the power dissipated by the best commercial GaAs static RAM cells. A 2 x 2 bit demonstration array was built and successfully operated at room temperature to demonstrate the addressable read/write capability of this new technology

Advanced characterisation of novel III-nitride semiconductor based photonics and electronics on polar and non-polar substrates

Advanced characterisation has been carried out on a number of novel III-nitride based photonics and electronics, including micro-LED arrays achieved by a direct epitaxy approach, high performance c-plane HEMTs structure achieved by a novel growth method and non-polar GaN/AlGaN HEMTs. In this work, a systematic study has been conducted to understand the electrical properties of these novel devices, demonstrating their excellent properties. Furthermore, the electrical properties are directly related to epitaxial growth, which provides useful information for further improving device performance, such as 2D growth mode for GaN on a large lattice-mismatched substrate which plays an important in obtaining high breakdown and minimised leakage current for HEMTs. Micro-LEDs are the key elements for a microdisplay system, where electrical properties are extremely important. Potentially, any leakage current can trigger to turn on any neighbouring microLEDs which are supposed to be off. Instead of using conventional fabrication methods which normally enhances leakage current, our team developed a direct epitaxy approach to achieving microLED arrays. In this work, detailed I-V characteristic and capacitance measurements have been conducted on these novel microLED devices, demonstrating leakage currents as low as 14.1 nA per LED and a smooth negative capacitance curve instead of odd positive capacitance performances. Furthermore, a comparison study between our microLEDs and the microLEDs prepared using the conventional method indicates our device shows a large reduction of size-dependent inefficiency while such a behaviour is never observed on the microLEDs fabricated by the conventional methods. Unlike the classic two-step method for GaN growth on large lattice-matched sapphire, our team developed a high-temperature AlN buffer technology, where a 2D growth mode, instead of an initial 2D and then 3D growth mode that typically happens for the growth of conventional GaN growth, takes place through the whole growth process. This method allows us to achieve a breakdown electric field strength of 2.5 MV/cm, a leakage current of as low as 41.7 pA at 20 V and saturation current densities as high as 1.1 A/mm. In this work a systematic study has conducted in order to establish a relationship between the excellent device performance and material properties, where a very low screw dislocation density plays a critical role, while our 2D growth method can provide an excellent opportunity for achieving such a low screw dislocation density. This demonstrates the major advantage over the classic two-step method in the growth of power and RF devices. In our case, we have obtained an unintentional doping as low as 2×10^14 cm-3 and screw dislocation densities of 2.3×10^7 cm-2. Compared with c-plane GaN based HEMTs due to its intrinsic polarisation, non-polar GaN/AlGaN HEMTs on r-plane sapphire yields potential advantages in terms of the fabrication of normal-off devices which are particularly important for practical applications. However, it is a great challenge to achieve high quality non-polar GaN on sapphire. Some initial work has been conducted, where the detailed characterisation indicates an electron mobility of 43 cm2 V-1 s-1 has been initially obtained. Furthermore, instead of using an AlGaN/GaN heterostructure with a modulation doping, we deliberately use a quantum well structure as an electron channel, leading to a mobility of 76 cm2 V-1 s-1. Our simulations as well as measurements also provide a guideline for optimising the general epitaxial structure

Development of high-performance, cost-effective quantum dot lasers for data-centre and Si photonics applications

Photonic technologies have been considered new methods to achieve high bandwidth data communication and transmission. Si-photonics was proposed to address the discrepancy between bulky photonic devices and advanced electronics and create high-density integrated photonics. One of the challenges is integrating all the components necessary for full-functionality photonic integrated circuits (PIC). Great efforts have been devoted to overcoming the inherent limitations of Group-IV materials to provide sufficient gain, efficient modulation and sensitive detections. Making Si the host material for efficient light emission poses the most stringent requirements and is the primary missing component in the Si-photonics platform. Incorporating III-V materials with the Si photonics platform and quantum dot (QD) structure is a promising solution to the problem of a fully-integrated and high-functioning PIC. High-performance QD lasers on III-V substrate or epitaxially on silicon have been developed in the last few decades with low threshold current density, low-temperature sensitivity, great reliability and large injection efficiency. Moreover, from the dynamic aspect, the intrinsic frequency of direct modulated laser and noise intensity is important for its applications in a data centre. QD is considered an alternative to quantum wells (QWs); however, the demonstrated QD laser has not fulfilled initial expectations, mainly due to its high gain compression and low differential gain. Another feature that needs to be noticed is feedback sensitivity, as the properties of semiconductor lasers are greatly degraded by reflection from external reflectors, such as the fibre connects and facets of integrated devices. QD devices are predicted to have stronger feedback resistance due to their large damping and small linewidth enhancement factor (LEF). These properties have attracted much research, and high-performance QD devices have been developed. In this thesis, we comprehensively investigated QD laser performance and applied our QD laser in the optical module instead of the commercial QW distributed feedback (DFB) laser. The background of Si photonics, the development of QD devices, and the fundamentals of QD lasers are presented in Chapter 1. The basic static and dynamic performances are demonstrated in Chapters 2 and 3. The GaAs-based QD laser provides a low threshold, high-temperature stability, and low noise operation with a limited small signal bandwidth. Chapter 4 provides a comprehensive study of the feedback resistance of the QD laser. The onset of coherence collapse is determined as -14 dB, verified by the static optical and electrical spectra and small signal response. Based on previous measurements, the QD laser is proven to be a high-performance, low-cost candidate for the Si-photonics module. In Chapter 5, the QD laser is used in practical applications, including a large signal transmission system with and without feedback and a commercial optical module. Although the intrinsic bandwidth of the QD laser is limited to around 5GHz due to the large damping and unoptimised capacitance, 30 Gbps data transmission has been demonstrated by a directly modulated QD laser. Large, high-speed signal modulation is achieved due to its high gain compression factor. Regarding the laser with intentional feedback, there is little degradation in the eye diagram under the whole feedback level up to -8dB. We also replaced the commercial QW DFB laser in 100G data-centre reach (DR)-1 optical module with our QD Fabry Perot (FP) laser without an isolator which gives a clear eye diagram under 53 Gbps 4-level pulse amplitude modulation (PAM4) with an extinction ratio (ER) of 4.7 dB. In conclusion, this thesis verifies the feasibility of adopting the QD laser as a light source for the Si-photonics module. The QD laser is selected over other lasers because of its low threshold, high-temperature stability and maximum operating temperature, and strong tolerance to unintentional feedback. This is the first project to measure critical feedback levels with different characteristics and to theoretically analyse the inconsistent value. More importantly, this thesis’ most original contribution is investigating the commercial applications of QD lasers in a Si-photonics module in an isolator-free state. In summary, the QD laser has been proven to be a feasible solution for the next-generation optical system