Indium Antimonide Nanowires: Synthesis and Properties

This article summarizes some of the critical features of pure indium antimonide nanowires (InSb NWs) growth and their potential applications in the industry. In the first section, historical studies on the growth of InSb NWs have been presented, while in the second part, a comprehensive overview of the various synthesis techniques is demonstrated briefly. The major emphasis of current review is vapor phase deposition of NWs by manifold techniques. In addition, author review various protocols and methodologies employed to generate NWs from diverse material systems via self-organized fabrication procedures comprising chemical vapor deposition, annealing in reactive atmosphere, evaporation of InSb, molecular/chemical beam epitaxy, solution-based techniques, and top-down fabrication method. The benefits and ill effects of the gold and self-catalyzed materials for the growth of NWs are explained at length. Afterward, in the next part, four thermodynamic characteristics of NW growth criterion concerning the expansion of NWs, growth velocity, Gibbs-Thomson effect, and growth model were expounded and discussed concisely. Recent progress in device fabrications is explained in the third part, in which the electrical and optical properties of InSb NWs were reviewed by considering the effects of conductivity which are diameter dependent and the applications of NWs in the fabrications of field-effect transistors, quantum devices, thermoelectrics, and detectors

Technology development for nanoscale InSb quantum split-gate structures

In this project some of the challenges of novel InSb based semiconducting material are investigated. Various features of electron transport in low dimensional semiconductors were studied for AlInSb/InSb quantum well (QW) two-dimensional electron gas (2DEG) heterostructures with an emphasis placed on realising onedimensional systems (which exhibit quantum phenomena where the conductance takes on a discrete ‘step-like’ nature). This material allows us to take advantage of very extreme material parameters such as light effective mass, the lowest binary material band gap, the highest electron mobility at room temperature, and an extremely large effective g-factor (with associated spin orbit coupling). However, this material still has significant challenges due to the large mismatch between the substrate GaAs and the QW, which produces threading dislocations that lead to limitations in mobility. Surface roughness has been investigated as a result of shallow etching for Ohmic contact deposition on the AlInSb/InSb wafer. Both dry and wet techniques have been investigated, and their effect on the electron transport as a function of roughness, primarily using Transmission Length Measurement (TLM). In addition, the Ohmic contact resistivity was investigated as a function of depth over a wide range of temperatures to extract an effective contact barrier. The contact potential barrier was found to have a strong effect at low temperatures, which leads to a non-linear I-V characteristic. Finally, this thesis studied different designs of nanoscale split gate structures that were fabricated on this state of the art InSb QW 2DEG material. This material was grown by collaborators at Sheffield University at the National Centre for III-V technologies. The devices were fabricated at Cardiff using photo-lithography and nanoscale electron beam lithography (EBL) using recipes tailored to this material

Accurate characterisation of Resonant Tunnelling Diodes for high-frequency applications

Recent scientific advancements regarding the generation and detection of terahertz (THz) radiation have led to a rapid increase in research interest in this frequency band in the context of its numerous potential applications including high-speed wireless communications, biomedical diagnostics, security screening and material science. Various proposed solutions have been investigated in the effort to bridge this relatively unexplored region of the electromagnetic spectrum, and thus exploit its untapped potential. Among them, the resonant tunnelling diode (RTD) has been demonstrated as the fastest electronic device with its room temperature operation extending into the THz range. The RTD exhibits a negative differential resistance (NDR) region in its I-V characteristics, with this feature being key to its capabilities. Even though the unique capabilities of RTD devices have been experimentally proven in the realisation of compact NDR oscillators and detectors, with fundamental frequencies of about 2 THz, and high-sensitivity detectors up to 0.83 THz, the reliable design procedures and methodologies of RTD-based circuits are yet to be fully developed. In this regard, significant effort has been devoted primarily to the accurate theoretical description of the high-frequency behaviour of RTDs, using various small-signal equivalent circuit models. However, many of these models have had either limited or no experimental validation, and so a robust and reliable RTD device model is desirable. The aim of this thesis is to describe a systematic approach regarding the design, fabrication and characterisation of RTD devices, providing a universal methodology to accurately determine their radio-frequency (RF) behaviour, and so this way enable a consistent integrated circuit design procedure for high-frequency circuits. A significant challenge in the modelling of RTD devices is represented by the presence of parasitic bias oscillations within the NDR region. This has been identified as one of the main restricting factors with regards to the accurate high-frequency characterisation of this operating region. The common approach to overcoming this limitation is through a stabilising technique comprising of an external shunt-resistor network. This approach has been successfully demonstrated to suppress bias oscillations in RTD-based circuits which require operation within the NDR region. However, the introduction of the additional circuit component associated with this method increases the complexity of the de-embedding procedure of the extrinsic parasitic elements, rendering the overall device characterisation generally difficult at high-frequencies. In this work, a novel on-wafer bond-pad and shunt resistor network de-embedding technique was developed in order to facilitate the characterisation of RTDs throughout the complete bias range, without limitation to device sizing or frequency, under a stable operating regime. The procedure was demonstrated to accurately determine the circuit high-frequency behaviour of the RTD device from S-parameter measurements up to 110 GHz. The universal nature of this procedure allows it to be easily adapted to accommodate higher complexity stabilising networks configuration or different bond-pad geometries. Furthermore, the de-embedding method has also enabled the development of a novel quasi-analytical procedure for high accuracy extraction of the device equivalent circuit parameters, which is expected to provide a strong experimental foundation for the further establishment of a universal RTD RF model. The applicability of the developed high-frequency model, which can be easily scaled for various device sizes, together with the measured RTD I-V characteristics was further demonstrated in the development of a non-linear model, which was integrated in a commercial simulator, the Advanced Design Systems (ADS) software from Keysight Technologies. From an application perspective, the model was used in the design of an RTD as a square-law detector for high-frequency data transmission systems. The simulated detector performance was validated experimentally using an RTD-based transmitter in the W-band (75 – 110 GHz) up to 4 Gbps (error free transmission: BER < 10-10 in a waveguide connection), and in the Ka-band (26.5 – 50 GHz) up to 2.4 Gbps (error free transmission in a wireless data link), which demonstrated the accuracy of the developed RTD modelling approach. Lastly, a sensitivity analysis of the RTD-based detector within the Ka-band showed a superior RTD performance over commercially available solutions, with a peak (corrected) detector responsivity of 13.48 kV/W, which is a factor of >6 better compared to commercially available Schottky barrier diode (SBD) detectors

Characterization of Plasma-Enhanced Atomic Layer Deposited Ga2O3 using Ga(acac)3 On GaN

abstract: This research has studied remote plasma enhanced atomic layer deposited Ga2O3 thin films with gallium acetylacetonate (Ga(acac)3) as Ga precursor and remote inductively coupled oxygen plasma as oxidizer. The Ga2O3 thin films were mainly considered as passivation layers on GaN. Growth conditions including Ga(acac)3 precursor pulse time, O2 plasma pulse time, N2 purge time and deposition temperature were investigated and optimized on phosphorus doped Si (100) wafer to achieve a saturated self-limiting growth. A temperature growth window was observed between 150 ℃ and 320 ℃. Ga precursor molecules can saturate on the substrate surface in 0.6 s in one cycle and the plasma power saturates at 150 W. A growth rate of 0.31 Å/cycle was observed for PEALD Ga2O3. Since the study is devoted towards Ga2O3 working as passivation layer on GaN, the band alignment of Ga2O3 on GaN were further determined with X-ray Photoemission Spectroscopy and Ultraviolet Photoemission Spectroscopy. Two models are often used to decide the band alignment of a heterojunction: the electron affinity model assumes the heterojunction aligns at the vacuum level, and the charge neutrality level model (CNL) which considers the presence of an interface dipole. The conduction band offset (CBO), valence band offset (VBO) and band bending (BB) of PEALD Ga2O3 thin films on GaN were 0.1 ±0.2 eV, 1.0±0.2 eV and 0.3 eV respectively. Type-I band alignments were determined. Further study including using PEALD Ga2O3 as passivation layer on GaN MOS gate and applying atomic layer etching to GaN was described.Dissertation/ThesisDoctoral Dissertation Physics 201

Nanometer Probing of Operating Nano-Photonic Devices

The external performance of quantum optoelectronic devices is governed by the three-dimensional profiles of electric potentials determined by the distribution of charge carriers (electrons and holes) within the active regions of the devices. Charge carrier dynamics play a vital role in active photonic quantum/nano devices, such as electrically-pumped semiconductor lasers. As an example, in quantum cascade lasers (QCLs) the Electric Field Domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially non-uniform and temporally unstable. Until now, there are no experimental means of probing the inner potential profile directly and as a result the mechanisms responsible for sub-par device performance of QCLs remain the subject of speculation. Another example is interband cascade lasers (ICLs), in which the distribution of gain-providing charge carrier governs the operation and performance of the devices, but has not been experimentally measured prior to this study. This work presents a systematic experimental study of gain-providing charge carrier distribution in a lasing interband cascade laser and electric potential distribution in THz QCLs. The unique charge carrier distribution profile in the quantum-well active region is quantitatively measured at nanometer scales by using the non-invasive scanning voltage microscopy (SVM) technique. Experimental results clearly confirm the accumulation and spatial segregation of holes and electrons in the core of the ICL device. The measurement also shows that the charge carrier density is essentially clamped in the presence of stimulated emission in ICLs, thus conclusively differentiating the lasing from non-lasing devices. The SVM technique has been applied to lasing THz QCLs to verify the hypothesis of electric field domains in semiconductor quantum structures. The experimental results reveal that the multi-quantum-well active region is divided into multiple sections having distinctly different electric fields. The electric field across these serially-stacked quantum cascade modules are observed not to continuously increase in proportion to the gradual increase of the applied device bias, but rather jumps between discrete values related to tunneling resonances. Also in the THz QCLs the progression of the observed EFDs are carefully probed. Experimental evidences reveal that an incremental change in device bias leads to a hopping-style shift in the EFD boundary – the higher electric field domain expands at least one module each step at the expense of the lower field domain within the active region. The SVM findings in THz QCLs indicate the importance of quantum active region design for intrinsically more uniform and stable electric field profiles. The two showcase study examples demonstrate that the cryogenic-temperature SVM is an enabling technique, being able to measure and resolve nanometer scale features non-destructively on operating devices. This experimental approach allows directly mapping the electric field distribution as well as the charge carrier distribution inside operating semiconductor quantum devices at nanometer scales, thus connecting the inner workings with the external measures of the devices. The experimental approach is expected to facilitate a deeper understanding of fundamental processes that are governing the operation and performance of a wide range of nanoelectronic and nanophotonic devices.4 month

Indium Antimonide Nanowires:Synthesis and Properties

This article summarizes some of the critical features of pure indium antimonide nanowires (InSb NWs) growth and their potential applications in the industry. In the first section, historical studies on the growth of InSb NWs have been presented, while in the second part, a comprehensive overview of the various synthesis techniques is demonstrated briefly. The major emphasis of current review is vapor phase deposition of NWs by manifold techniques. In addition, author review various protocols and methodologies employed to generate NWs from diverse material systems via self-organized fabrication procedures comprising chemical vapor deposition, annealing in reactive atmosphere, evaporation of InSb, molecular/chemical beam epitaxy, solution-based techniques, and top-down fabrication method. The benefits and ill effects of the gold and self-catalyzed materials for the growth of NWs are explained at length. Afterward, in the next part, four thermodynamic characteristics of NW growth criterion concerning the expansion of NWs, growth velocity, Gibbs-Thomson effect, and growth model were expounded and discussed concisely. Recent progress in device fabrications is explained in the third part, in which the electrical and optical properties of InSb NWs were reviewed by considering the effects of conductivity which are diameter dependent and the applications of NWs in the fabrications of field-effect transistors, quantum devices, thermoelectrics, and detectors

Nanocrystalline diamond thin film integration in AlGaN/GaN high electron mobility transistors and 4H-SiC heterojunction diodes

The extremely high thermal conductivity and mechanical hardness of diamond would make it the natural choice for device substrates when large area wafer production becomes possible. Until this milestone is achieved, people could utilize nanocrystalline diamond (NCD) thin films grown by chemical vapor deposition (CVD). A topside thermal contact could be pivotal for providing stable device characteristics in the high power, high temperature, and high switching frequency device operating regime that next-generation power converter circuits will mandate. This work explores thermal and electrical benefits offered by NCD films to wide bandgap semiconductor devices. Reduction of self-heating effects by integrating NCD thin films near the device channel of AlGaN/GaN high electron mobility transistors (HEMTs) is presented. The NCD layers provide a high thermal conductivity path for the reduction of hot electron dispersion, a phenomenon caused by self-heating and detrimental to the continuous operation of GaN devices in power switching circuits. Recent advances in diamond doping have made it possible to think of this material as a very wide bandgap semiconductor (5.5 eV for ideal diamond). A few unique properties, such as negative electron affinity (&chi; = -0.2 eV for H-terminated diamond), make this material very interesting. Using H-terminated NCD, a heterojunction with 4H-SiC has been developed. Undoped and B-doped NCD were deposited on both n- and p- 4H-SiC epilayers. Different metals were studied to provide an Ohmic contact to the NCD layer. I-V measurements on p+ NCD / n- 4H-SiC p-n junctions indicated Schottky rectifying behavior with a turn-on voltage of around 0.2 V. The current increased over 8 orders of magnitude with an ideality factor of 1.17 at 30 &deg;C. Ideal energy-band diagrams suggested a possible conduction mechanism for electron transport from the SiC conduction band to either the valence band or Boron acceptor level of the NCD film. Cathodoluminescence and thermally stimulated current methods were employed to study the deep level assisted conduction in this heterojunction. Applications as a simultaneous UV-transparent optical and Schottky electrical contact to 4H-SiC are discussed

1300-nm InAs/GaAs quantum-dot lasers monolithically grown on silicon substrates

To imitate the way electrical components evolved from discrete devices to devices integrated on Si platform, the next stage for integrated circuits is to integrate photonic components with electrical components on one chip, with active devices known as optoelectronic integrated circuits (OEIC). An ideal solution for this would be to have an all-Si laser. However due to the indirect bandgap of Si this is difficult to achieve. Therefore attention has been focused on trying to integrate the existing and mature III-V laser technology with Si. The difference in lattice constant between GaAs and Si makes direct, monolithic growth of GaAs on Si difficult due to the generation of high defect densities. But the advances in quantum dot (QD) technology and in III-V buffer layer techniques have led to the improvements of direct growth integration. In this thesis an AlAs nucleation layer (NL) in the place of a GaAs nucleation layer was found to increase the photoluminescence intensity and reduce defect density in active layers. Lasers were fabricated with lower threshold current densities than similar devices with GaAs NL. Lasing operation at 1.28 μm was achieved up to 63 °C with a threshold current density of 675 A/cm2 at room temperature. In addition, Ge-on-Si substrates have been used to demonstrate the lasers on Si substrates with a very low pulsed threshold current density of 64 A/cm2, which is significantly lower than any other laser integrated with Si substrates. Also this was the first demonstration of a CW laser on Si with a threshold current density of 163 A/cm2. Lasers were operated up to 30 °C for CW devices and 84 °C for pulsed devices. The difference in threshold currents and temperature performance between CW and pulsed operation is due to high device resistances caused by a combination of poor contact resistance and the introduction of defects from the Si/Ge interface. In conclusion, lasers on Si substrates have been fabricated with low threshold current densities. A very low threshold current density of 64 A/cm2 has been achieved with a Ge-on-Si device and is the lowest result for any type of Si laser at the time of writing, which shows good potential for future integration with Si electronics