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Electrical and optical characterizations of heterostructures and thin films based on group III-V compound semiconductors are presented. Optical properties of GaMnN thin films grown by Metalorganic Chemical Vapor Deposition (MOCVD) on GaN/Sapphire templates were investigated using IR reflection spectroscopy. Experimental reflection spectra were fitted using a non - linear fitting algorithm, and the high frequency dielectric constant (ε∞), optical phonon frequencies of E1(TO) and E1(LO), and their oscillator strengths (S) and broadening constants (Γ) were obtained for GaMnN thin films with different Mn fraction. The high frequency dielectric constant (ε∞) of InN thin films grown by the high pressure chemical vapor deposition (HPCVD) method was also investigated by IR reflection spectroscopy and the average was found to vary between 7.0 - 8.6. The mobility of free carriers in InN thin films was calculated using the damping constant of the plasma oscillator. The terahertz detection capability of n-type GaAs/AlGaAs Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) structures was demonstrated. A threshold frequency of 3.2 THz (93 µm) with a peak responsivity of 6.5 A/W at 7.1 THz was obtained using a 0.7 µm thick 1E18 cm−3 n - type doped GaAs emitter layer and a 1 µm thick undoped Al(0.04)Ga(0.96)As barrier layer. Using n - type doped GaAs emitter layers, the possibility of obtaining small workfunctions (∆) required for terahertz detectors has been successfully demonstrated. In addition, the possibility of using GaN (GaMnN) and InN materials for terahertz detection was investigated and a possible GaN base terahertz detector design is presented. The non - linear behavior of the Inter Pulse Time Intervals (IPTI) of neuron - like electric pulses triggered externally in a GaAs/InGaAs Multi Quantum Well (MQW) structure at low temperature (~10 K) was investigated. It was found that a grouping behavior of IPTIs exists at slow triggering pulse rates. Furthermore, the calculated correlation dimension reveals that the dimensionality of the system is higher than the average dimension found in most of the natural systems. Finally, an investigation of terahertz radiation efect on biological system is reported.

In this work, homojunction interfacial workfunction internal photoemission (HIWIP) detectors based on GaAs, and heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors based mainly on the Gallium Arsenide/Aluminum Gallium Arsenide material system are presented. Design principles of HIWIP and HEIWIP detectors, such as free carrier absorption, photocarrier generation, photoemission, and responsivity, are discussed in detail. Results of p-type HIWIPs based on GaAs material are presented. Homojunction detectors based on p-type GaAs were found to limit their operating wavelength range. This is mainly due to band depletion arising through carrier transitions from the heavy/light hole bands to the split off band. Designing n-type GaAs HIWIP detectors is difficult as it is strenuous to control their workfunction. Heterojunction detectors based on Gallium Arsenide/Aluminum Gallium Arsenide material system will allow tuning their threshold wavelength by adjusting the alloy composition of the Aluminum Gallium Arsenide/Gallium Arsenide barrier, while keeping a fixed doping density in the emitter. The detectors covered in this work operate from 1 to 128 micron (300 to 2.3 THz). Enhancement of detector response using resonance cavity architecture is demonstrated. Threshold wavelength extension of HEIWIPs by varying the Al composition of the barrier was investigated. The threshold limit of approximately 3.3 THz (92 micron), due to a practical Al fraction limit of approximately 0.005, can be overcome by replacing GaAs emitters in Gallium Arsenide/Aluminum Gallium Arsenide HEIWIPs with Aluminum Gallium Arsenide/Gallium Arsenide emitters. As the initial step, terahertz absorption for 1 micron-thick Be-doped Aluminum Gallium Arsenide epilayers (with different Al fraction and doping density) grown on GaAs substrates was measured. The absorption probability of the epilayers was derived from these absorption measurements. Based on the terahertz absorption results, an Aluminum Gallium Arsenide/Gallium Arsenide HEIWIP detector was designed and the extension of threshold frequency (f0) to 2.3 THz was successfully demonstrated. In a different study, switching in Gallium Arsenide/Aluminum Gallium Arsenide heterostructures from a tunneling dominated low conductance branch to a thermal emission dominated high conductance branch was investigated. This bistability leads to neuron-like voltage pulses observed in some heterostructure devices. The bias field that initiates the switching was determined from an iterative method that uses feedback information, such as carrier drift velocity and electron temperature, from hot carrier transport. The bias voltage needed to switch the device was found to decrease with the increasing device temperature.

Developing methods to control the chemistry of surfaces on the 1—100 nm length scale is a fundamental and exciting challenge in many disciplines of science today because it opens new possibilities in fields ranging from molecular electronics (ME) to biomedicine to catalysis. Scanning probe microscopy techniques have been a key goal in achieving this challenge. This work begins with an exhaustive review of scanning probe lithography techniques using self-assembled monolayer (SAM) systems reported in the literature to date. Experimental study describes the development of a scanning probe lithography technique (termed replacement lithography) in which an STM tip selectively desorbs organothiolate SAMs in a predefined pattern, allowing a replacement thiol to adsorb onto the exposed gold in the patterned region. The replacement parameters were investigated using electroactive containing replacement thiol species because they displayed large apparent height contrasts in STM images, allowing the efficacy of the pattern to be easily ascertained. These data were subsequently employed to create mesoscale chemical gradients with replacement lithography. The electronic properties of redox active SAMs were also shown to display negative differential resistance in current-voltage measurements, a behavior that has possible utility in the development of ME devices. Temporal investigations monitoring the stochastic variation in apparent height contrasts in STM images of electroactive containing SAM guest species isolated within insulating host SAM matrices was also studied. The observations from these data may lend insight to how ME candidates might behave when sandwiched in a two-electrode configuration (metal-SAM-metal junction).

This dissertation presents research on the application of resonant-tunneling diodes (RTDs) in multivalued-logic circuits and very-high-speed digital circuits. An RTD-based signed-digit full adder cell is presented in which carry propagation along a chain of adders is eliminated by means of a redundant arithmetic algorithm. RTDs enable efficient implementation of sophisticated multivalued-logic functions. The two-peak negative-differential-resistance (NDR) characteristics of two RTDs connected in series are used to easily detect four voltage threshold levels of a multivalued input signal. The proposed concept was successfully demonstrated via a prototype in which RTDs were connected as external devices to a custom-designed CMOS integrated circuit. A second, fully integrated, prototype was developed using MOS-NDR in a standard CMOS process. MOS-NDR is a new prototyping technique for circuits that combine MOS transistors and NDR devices where NDR characteristics are emulated using only enhancement-mode n-channel MOSFETs. This was the first demonstration of the MOS-NDR technique in multivalued logic applications. A simulation-based study is presented in which performance of bistable logic circuits combining RTDs and III--V transistors is measured as experiment parameters vary. The circuit topology, the type and speed of transistors, and the driven output load are some of the parameters used in the experiments. There are two basic types of topologies for RTD bistable logic circuits: quantum bistable logic, where the state acquired after each clocking event is determined by the interaction between RTDs and transistors, and balanced pair logic, where the logic state depends on the interaction of two or more RTDs. Two types of compound-semiconductor transistors were considered in the study, namely, heterostructure bipolar transistors and high-electron-mobility transistors. The results of the study indicate that the best circuit configuration combines the balanced-pair logic with high-electron-mobility transistors. The experimental simulation framework developed for this study is not tied to any particular technology or circuit technique, and it can thus be used as a general-purpose tool for circuit evaluation and comparison.

As CMOS technology advances to its physical limits of feature size shrinking, it is important to investigate technologies that employ alternative device physics and transport phenomena. Among a host of nascent technologies, resonant-tunneling diodes (RTDs) are the most mature and promising for commercial introduction for the following reasons. First, RTDs can operate at room temperature with large peak-to-valley current ratio (PVCR) which enables a large noise margin in RTD-based logic circuits. Second, RTDs can be monolithically integrated with conventional technologies such as heterojunction bipolar transistors (HBTs), and high-electron mobility transistors (HEMTs). Third, the self-latching property of RTDs makes it possible to realize complex logic functionality with a few devices (RTDs and transistors). This can have a tremendous impact on logic circuit designs and greatly increases the IC packing density. This thesis encompasses many aspects of research on InP-based RTD-HBT circuits. Device properties were first studied in this work. The RTD characteristics can be designed by optimizing the layer structures. The fabricated RTDs showed a wide range of peak current density from 5.7 x 103 A/cm 2 to 1.27 x 105 A/cm2 on different layer structure designs. The smallest peak voltage obtained was 0.25V on the RTD with an InAs subwell design. The highest PVCR found was 30 at room temperature. The InAlAs/InGaAs material system was used for fabricating HBTs. The highest fT and fMAX obtained in this work were 60GHz and 87GHz, respectively on a 2.5 x 2.5 mum 2-HBT. A breakdown voltage greater than 5V has been obtained on the HBT with a 7500A-thick collector. Semiconductor PIN diodes for photodetectors have also been fabricated. The measured responsivity was 0.7A/W. Si-based tunnel diodes were also studied in this work because of the possibility of integrating tunnel diodes with the CMOS process. Negative differential resistance was measured on MBE-wafers. These wafers had delta-doping planes to provide extremely high doping concentration and form triangular potential wells. These wells helped confine electrons and increase the tunneling probability. The maximum PVCR found was 1.18 at room temperature and 1.2 at 80K. The PVCR should be improved by optimizing the growth conditions. Based on the measured device characteristics, RTD-HBT logic gates were designed and fabricated. Two different processes, the via-hole process and the air-bridge process, were developed and improved for this work. Both processes featured all wet-etching processing and self-aligned base contacts. Several digital circuits including a static inverter, concensus element, minority gate and the Monostable-Bistable transition Logic Element (MOBILE) inverting gate have been implemented with RTDs and HBTs. The logic function of these RTD-based circuits has been confirmed up to 10Gb/s which was the limit of our test instrumentation. Photoreceivers based on RTDs and PIN diodes have also been successfully fabricated and tested. Compared to a commercially available photoreceiver, the fabricated photoreceiver consumes very low power (only 0.21mW) and has a high conversion gain of 3000V/W. Optimization of the circuit design should greatly improve the sensitivity of the photoreceivers. These results indicate the potential of RTD-HBT circuits of digital gates and optical communications.

Device physics limitations of conventional complementary metal-oxide semiconductor (CMOS) transistors are likely to cause diminishing integrated circuit performance improvement in the sub 100-nm regime. At these dimensions, quantum effects become prominent leading to realization of devices utilizing quantum-mechanical tunneling transport mechanisms for obtaining picosecond device switching speeds. The negative differential-resistance (NDR) current-voltage (I-V) characteristic of such devices, achieved due to resonant tunneling, is also ideally suited for the design of compact self-latching logic circuits. While resonant tunneling devices have been demonstrated using III-V materials, their large absolute current values and low integration levels have limited their use to niche high-performance small-scale circuits. Given the advantages of resonant tunneling devices, it is attractive to envision compact, high-functionality NDR circuits implemented in a technology such as CMOS that offers low power dissipation and very high integration levels. Even as research in cointegration of resonant tunneling devices in Silicon, referred to as quantum MOS (QMOS), is ongoing, this dissertation presents novel combinational and bistable logic families, and compact flip-flop circuits using resonant tunneling diodes (RTDs) and MOS transistors. Analytical studies of static and dynamic QMOS performance parameters yield expressions for theoretical circuit comparison with CMOS, and also for optimizing RTD characteristics. QMOS circuits are characterized using SPICE simulation, and simulation-based comparison of QMOS and CMOS circuits highlights potential area-power-delay savings of this new circuit technique. The folded I-V characteristic of RTDs allows gate-level bistable-clocked-mode system operation. Performance improvement of such fine-grained QMOS pipelines arises from compactness of logic design, elimination of pipeline latch area, delay and power overhead, and high switching speed of the RTD. A pipelined carry-save multiplier and a 32-bit parallel correlator are designed to study the system-level advantages of QMOS logic. In particular, signal processing systems and communication systems benefit from fine-grained pipelining due to minimal data dependence and large volume of similar computations at each cycle. In the absence of a fabrication process that cointegrates RTDs in Silicon, a study of various QMOS prototyping schemes is presented that identifies the best means to verify system-level behavior of QMOS circuits while research in RTD-CMOS cointegration continues.