Journal of Telecommunications and Information Technology, 2004, nr 1

kwartalni

Process technology for High Speed InP Based Heterojunction Bipolar Transistors

By the advances in high frequency communication systems, and particularly Internet, wide bandwidth and high-speed transistors became key devices for the circuits. Especially, optical fibres can transport data at high rates. Therefore, high-speed electronics is necessary for all kind of data processing. One limitation is the ultra high frequency modulation of the light for data transport. This requires high frequency and high voltage of operation. Second, high linearity of amplification is necessary for analog-to-digital conversion (ADCs). Indium Phosphide based Heterojunction Bipolar Transistors (InP HBTs) have the potential to provide high speed and high voltage for optoelectronic communication ICs. Moreover, since their energy band gap corresponds to the 1.3 and 1.55 µm wavelength, which are the wavelengths providing minimum optical loss in fibres, InP HBTs are the best choices for optical communication circuits. In this work, a process technology is developed for HBTs capable of 80Gbit/s. The theory and also careful analysis have shown that the main limitations for the device speed are the base-collector capacitances and base resistances. Various designs are proposed to lower RC time constant. The influence of emitter size is observed on RF performance and for optical lithography it is found out that emitters with area of 1x15 µm2 provides the best RF performance. Another important aspect is to lower any additional parasitic effects. Therefore directly contacted emitters are proposed to eliminate parasitic components caused by dummy pads and to dissipate the heat overall the emitter efficiently. Since the contact spacing between base and emitter plays an important role for base resistance, emitter contacts are patterned perpendicular to the major flat for low underetching. In addition to this, emitter mesa wet chemical etching process is optimised and 170 nm of underetching is achieved, which is sufficient to realize 1 µm width emitters. A mask set is designed offering directly contacted emitters and reliable processes. To reduce the base resistance, Pt/Ti/Pt/Au contact metal system providing 4x10?7 Ohm.cm2 contact resistance, is used on the base layer. Moreover, current density is also an important aspect for the RF performance. The well-known Kirk Effect is analysed and the collector layer is doped to improve the maximum current density. 1x1017 cm-3 of collector doping density has doubled the maximum current density in comparison to the HBTs with non-intentionally doped collector layers. For HBTs with an emitter area of 1x15 µm2 on the optimised layer structure, presented maximum oscillation frequency (fmax) of 330 GHz and cut-off frequency (fT) of 170 GHz at 1.2 mA/µm2. 1 µm is the minimum width achievable by wet chemical etching. But on the other hand, for ultra high speed HBTs, submicron emitters are dispensable. Therefore, an ICP-RIE process with Cl2/N2 chemistry providing less emitter underetching is optimised. According to this, dry etching processes offering etch rate of 120 nm/min up to 1200 nm/min are optimised. For HBTs lower etch rate, vertical sidewall profile less damage to the surface are important aspects. An etch rate of 120 nm/min is sufficient for emitter mesa etching. Therefore, this process is investigated in details. It is found out that, RMS surface roughness less than 5 nm, ± 5 % uniformity over 2” wafer and ± 5 % run-to-run stability can be achieved at this low etch rates. Since the dry etching with this chemistry is not selective for InP and InGaAs, a hybrid etching process is sufficient to complete the emitter mesa etching. With this hybrid etching, the selectivity problem is solved with an emitter underetching of 85 nm. HBTs with an emitter area of 0.5x7.5 µm2 processed with hybrid etching, has shown a maximum oscillation frequency of 370 GHz and a cut-off frequency of 165 GHz. Not only the RF performance but also the uniformity of the HBT properties is also an important aspect for circuit applications. Hbyrid etching process offers better homogeneity in terms of etching in comparison to the solely wet chemical etching. The samples etched with wet chemical etching have shown dc current gain 41 with 4 % standard deviation where the solely etched ones have presented dc current gain of 64 with 7 % standard deviation. The yield for both type of processing is more than 90 %.With the optimised layout, layer structure and processing, InP HBTs with high speed, high yield and high uniformity are now ready for 80 Gbit/s communication circuits

Semiconductor Infrared Devices and Applications

Infrared (IR) technologies—from Herschel’s initial experiment in the 1800s to thermal detector development in the 1900s, followed by defense-focused developments using HgCdTe—have now incorporated a myriad of novel materials for a wide variety of applications in numerous high-impact fields. These include astronomy applications; composition identifications; toxic gas and explosive detection; medical diagnostics; and industrial, commercial, imaging, and security applications. Various types of semiconductor-based (including quantum well, dot, ring, wire, dot in well, hetero and/or homo junction, Type II super lattice, and Schottky) IR (photon) detectors, based on various materials (type IV, III-V, and II-VI), have been developed to satisfy these needs. Currently, room temperature detectors operating over a wide wavelength range from near IR to terahertz are available in various forms, including focal plane array cameras. Recent advances include performance enhancements by using surface Plasmon and ultrafast, high-sensitivity 2D materials for infrared sensing. Specialized detectors with features such as multiband, selectable wavelength, polarization sensitive, high operating temperature, and high performance (including but not limited to very low dark currents) are also being developed. This Special Issue highlights advances in these various types of infrared detectors based on various material systems

Resonant tunnelling diode optoelectronic receivers and transmitters

This thesis describes the research work on double barrier quantum well (DBQW) resonant tunneling diode (RTD) based optoelectronic transmitters and receivers, focused on the design and characterization of resonant tunneling diode photodetectors (RTD-PD) implemented in the In53Ga47As/InP material system for operation at 1.55 μm and 1.31 μm wavelengths, and evaluate numerically the merits of the integration of an RTD/RTD-PD with a laser diode (LDs) to act as simple optoelectronic transmitters. The aim of the work was to investigate simple, low-cost, high-speed transmitter and receiver architectures taking advantage of RTDs properties such as the structural simplicity, high frequency (up to terahertz), and wide-bandwidth built-in electrical gain (roughly, from dc to terahertz). Also described are the preliminary studies of RTD-PDs operation as single photon detector at room temperature utilizing the excitability property. In this work, we evaluate which factors affect the bandwidth of RTD-PDs. Knowing the answer to this, we propose rules and optimizations necessary to achieving high bandwidth (>10 GHz) RTD-PDs. Furthermore, we show how to utilize the built-in amplification, arising from the RTD non-linear current-voltage (IV) curve and the presence of a negative differential resistance region (NDR) to building high responsivity photodetectors that can outperform current commercial technologies, particularly PIN photodiodes, in novel applications. The design and modeling work relied on numerical simulations utilizing the nonequilibrium Green’s function formalism (NEGF), which we implement using Silvaco ATLAS. We briefly introduce the NEGF method and Silvaco ATLAS and utilize them to do the design of the epitaxial structure of novel devices. The results of which are novel models which allow us to predict the effect that the RTD structural parameters (doping concentration and the lengths of both the emitter and collector) have on the peak voltage of the RTD. We study experimentally the factors affecting the bandwidth by optical characterization of several epitaxial layer stacks and propose hypotheses that help to explain the measured bandwidths. We show that for high-speed RTD-PDs (sub nanosecond), the light absorption layers should be confined to the locations where the electric field is sufficiently high and avoiding highly doped thick contact layers with band gap energies below the energy of the photons being detected. Additionally, we outline a set of rules for the design of RTD-PD detectors based on ni-n and p-i-n heterostructures, where the length, location, and doping level of the absorption regions are the relevant parameters to be considered in determining the bandwidth and responsivity of the devices. Moreover, we measure and report on the responsivity of RTDPDs under both DC and AC optical excitation. We show that RTD-PDs can have very high responsivity values reaching up to 1×107 A/W, and electrical bandwidth of around 1.26 GHz (1.75 GHz optical) that is limited by the lifetime of the photo-generated minority carriers (the holes). The last part of the thesis is dedicated to the study of RTD-PD circuits, where the integration between an RTD-PD and a laser diode (LD) is thoroughly examined. The LD acts as a load that is driven by the RTD-PD current. We derive and investigate the equivalent circuit for such a system incorporating the Schulman function for the RTD-PD IV, using the solution to study several operation regimes using MATLAB code. These regimes include the RTD-PD biased in the positive differential resistance region (PDR), when it is biased in the NDR region, and when induced to switch between the PDR and NDR regions. We also show how the excitability property of the RTD-PD can be used for detecting very low signal intensity levels, and the ability of RTDs to operate as voltage-controlled oscillators while biased in the NDR region

Resonant tunnelling diode epitaxial wafer design manufacture and characterisation

Resonant tunnelling diodes realised using the AlAs/InGaAs lattice match to InP substrates have demonstrated promising performance as THz sources. The main limitations to deployment are imposed by the device output power, which is critically dependent on the structural quality of the epitaxial material. Future design and growth optimization require tools to characterize the thin RTD active region on different length scales and to create a link between design variables and device performance. This thesis reports a combined non-destructive characterisation scheme based on photoluminescence spectroscopy (PL), X-ray diffraction (XRD), and photoluminescence excitation (PLE) spectroscopy. The scheme improves the accuracy and reproducibility of all the RTD design parameters to provide accurate feedback for future epitaxy optimization both in R&D and in future manufacturing. A new PL technique is also proposed to investigate RTD structural imperfection on a length scale comparable with the RTD device mesa area, allowing an investigation into important growth imperfections affecting the device performance and reproducibility. Alternative RTD designs made by substituting the ternary InGaAs well with an InAs/GaAs superlattice are proposed and demonstrated in the last part of the thesis. Design criteria and simulations are reported and improvements in the structure epitaxial quality are highlighted by PL

Optoelectronics – Devices and Applications

Optoelectronics - Devices and Applications is the second part of an edited anthology on the multifaced areas of optoelectronics by a selected group of authors including promising novices to experts in the field. Photonics and optoelectronics are making an impact multiple times as the semiconductor revolution made on the quality of our life. In telecommunication, entertainment devices, computational techniques, clean energy harvesting, medical instrumentation, materials and device characterization and scores of other areas of R&D the science of optics and electronics get coupled by fine technology advances to make incredibly large strides. The technology of light has advanced to a stage where disciplines sans boundaries are finding it indispensable. New design concepts are fast emerging and being tested and applications developed in an unimaginable pace and speed. The wide spectrum of topics related to optoelectronics and photonics presented here is sure to make this collection of essays extremely useful to students and other stake holders in the field such as researchers and device designers

Wide Bandgap Based Devices

Emerging wide bandgap (WBG) semiconductors hold the potential to advance the global industry in the same way that, more than 50 years ago, the invention of the silicon (Si) chip enabled the modern computer era. SiC- and GaN-based devices are starting to become more commercially available. Smaller, faster, and more efficient than their counterpart Si-based components, these WBG devices also offer greater expected reliability in tougher operating conditions. Furthermore, in this frame, a new class of microelectronic-grade semiconducting materials that have an even larger bandgap than the previously established wide bandgap semiconductors, such as GaN and SiC, have been created, and are thus referred to as “ultra-wide bandgap” materials. These materials, which include AlGaN, AlN, diamond, Ga2O3, and BN, offer theoretically superior properties, including a higher critical breakdown field, higher temperature operation, and potentially higher radiation tolerance. These attributes, in turn, make it possible to use revolutionary new devices for extreme environments, such as high-efficiency power transistors, because of the improved Baliga figure of merit, ultra-high voltage pulsed power switches, high-efficiency UV-LEDs, and electronics. This Special Issue aims to collect high quality research papers, short communications, and review articles that focus on wide bandgap device design, fabrication, and advanced characterization. The Special Issue will also publish selected papers from the 43rd Workshop on Compound Semiconductor Devices and Integrated Circuits, held in France (WOCSDICE 2019), which brings together scientists and engineers working in the area of III–V, and other compound semiconductor devices and integrated circuits