The present status of research and prospects for device applications of ferromagnetic (diluted magnetic) semiconductors (DMS) is presented. We review the nature of the electronic states and the mechanisms of the carrier-mediated exchange interactions (mean-field Zener model) in p-type Mn-based III-V and II-VI compounds, highlighting a good correspondence of experimental findings and theoretical predictions. An account of the latest progress on the road of increasing the Currie point to above the room temperature is given for both families of compounds. We comment on a possibility of obtaining ferromagnetism in n-type materials, taking (Zn,Mn)O:Al as the example. Concerning technologically important issue of easy axis and domain engineering, we present theoretical predictions and experimental results on the temperature and carrier concentration driven change of magnetic anisotropy in (Ga,Mn)As.

The manipulation of electron spin offers new dimensions for basic and applied research, and the potential for new capabilities for electronics technology. Device structures that are based on ferromagnetic materials and utilize spin-dependent transport have had a large technological impact on the magnetic recording industry, and are poised to impact integrated, digital electronics. The field of Spintronics is presented in the context of basic physical principles of carrier spin injection, transport and relaxation. Device families, which typically incorporate one or more magnetic component, are described. These include all-metal structures such as spin valves and magnetic tunnel junctions, hybrid ferromagnet - semiconductor devices that operate using magnetic fringe fields, and semiconductor based spin injection devices. Applications are focused on high density, high performance information processing and range from nonvolatile random access memory and reprogrammable logic to "system on a chip" integration of memory and logic.

This paper is a summary of experimental results obtained on spin-dependent transmission of hot electrons through thin ferromagnetic films. The goal of this contribution is to demonstrate that very simple devices can be used for measuring the spin polarization of a free electron beam. Both cases of free-standing magnetic metal foils and ferromagnetic metal/semiconductor Schottky diode are studied here and analyzed in terms of electron spin detection. In these experiments an electron beam is injected from vacuum into the magnetic thin film with controlled energy and spin polarization. The spin-dependence of the transmitted current, originating from the spin-filtering through the magnetic layer, yields a new and efficient way for measuring the incident electron beam polarization.

The usual charge carrier exploited in all electronic devices is the electron (and hole) and possesses hence, associated with its inertial masse, a spin degree of freedom. Manipulating spin-polarized electric current is the role devoted to Spintronics. Investigating the current induced magnetization switching (CIMS), consists in measuring the dynamics of the magnetization due to spin-polarized current injection. It is shown that two different mechanisms are operating. One can be described with a current dependent torque, and the other one is attributed to longitudinal spin-transfer.

The small size, high power, promise of access to any wavelength between 3.5 and 16 microns, substantial tuning range about a chosen center wavelength, and general robustness of quantum cascade (QC) lasers provide opportunities for new approaches to ultra-sensitive chemical detection and other applications in the mid-wave infrared. PNNL is developing novel remote and sampling chemical sensing systems based on QC lasers, using QC lasers loaned by Lucent Technologies. In recent months laboratory cavity-enhanced sensing experiments have achieved absorption sensitivities of 8.5 x 10^-11 cm^-1 Hz^-1/2, and the PNNL team has begun monostatic and bi-static frequency modulated, differential absorption lidar (FM DIAL) experiments at ranges of up to 2.5 kilometers. In related work, PNNL and UCLA are developing miniature QC laser transmitters with the multiplexed tunable wavelengths, frequency and amplitude stability, modulation characteristics, and power levels needed for chemical sensing and other applications. Current miniaturization concepts envision coupling QC oscillators, QC amplifiers, frequency references, and detectors with miniature waveguides and waveguide-based modulators, isolators, and other devices formed from chalcogenide or other types of glass. Significant progress has been made on QC laser stabilization and amplification, and on development and characterization of high-purity chalcogenide glasses, waveguide writing techniques, and waveguide metrology.

The promise, some fact and some fanciful, of nanotechnology has led to a well funded global race to develop new materials, components, and devices for use in a remarkably diverse range of applications. Towards the true realization of commercial- and defense-relevant devices, this paper focuses on passive and active optical detecting and sensing devices whose performance is markedly improved, with respect to traditional analogs, through the use of nanocomposite materials. Specifically to be discussed are efficient organic photovoltaics (OPVs) fabricated using doped and undoped carbon nanotube-containing conjugated polymers. All-organic photonic crystals based on ordered arrays of nanoparticles encapsulated in elastomeric matrices also are discussed. These nanocomposites exhibit bandstops that are highly tunable though stain generated by mechanical forces (mechano-chromism) or chemical affinity (chemo-chromism) which opens new doors for optical beam steering and chemical sensing.

Semiconductor lasers employing intersubband transitions in quantum boxes, so-called intersubband quantum-box (IQB) lasers are found, in theory, capable to have significantly lower threshold current densities and operating voltages than quantum cascade (QC) lasers. For that to happen, an enhancement factor of about 20 in the LO-phonon-assisted electron relaxation time is necessary, which is quite possible according to recent experimental data. The increased gain for the radiative stage in an IQB laser eliminates the need for a multiradiative-stage structure (typically 25 stages in QC lasers). Due to their inherently lower input power requirements, IQB lasers operating in the mid-IR wavelength range should be capable of much higher average-output powers than QC lasers at all temperatures, and continuous-wave (CW) operation at room temperature with high wallplug efficiency.

The present status and future prospects of compound semiconductor quantum nanostructures for sensing terahertz (THz) signals are reviewed. Various THz detectors reported in recent publications are reviewed first. They include infrared photodetectors based on quantum wells, superlattices and quantum dots, resonant tunneling diodes, single electron transistors and plasma resonators using high mobility electron transistor structures. Then, a novel approach by the authors utilizing planar arrays of quantum dots controlled by nano-scale Schottky wrap gates is presented and discussed. It is based on photon assisted resonant tunneling of single electrons. The novel device structure allows normal incidence of THz radiation as well as high density planar integration. A large responsivity of 0.3 A/W was obtained at 20 K for 2.5 THz laser beam.

In recent years, significant interest has emerged in the synthesis of magnetically-doped semiconductors. For computing, these materials provide a venue to exploit spin for computation. Direct bandgap spin-doped semiconductors are potential sources for polarized optical emission. In addition, quantum-entangled spins have been proposed as mediators in photon-based quantum communication schemes. In this paper, we discuss the properties of spin-doped ZnO. Mn-doped ZnO demonstrates ferromagnetism at relatively high temperatures as evidenced in hysteretic behavior in the magnetic flux versus field behavior. Transition metal doping was explored via ion implantation. Investigations of group V doping for p-type conduction are also discussed.

Recent measurements of photon tunneling through nanoholes in a gold film covered with a layer of the nonlinear polymer polydiacetylene have provided strong evidence for “photon blockade” effects similar to the Coulomb blockade phenomena observed in single-electron tunneling experiments. We will report on the control of photon tunneling at one wavelength through nonlinear nanoholes by a second, different, wavelength. Our observations suggest the possibility of building a new class of “gated” photon tunneling devices for all-optical signal processing and computation.

We present a review of the characteristics of several different types of high speed InGaAs/InP avalanche photodiode (APD)s that we have developed for different guard ring depth and for different main p-n junction shape. The APD structure that we propose consists of a greatly reduced width in InP multiplication layer and a high doping concentrated electric field buffer layer, where we also adopted a floating guard ring and a shaped main junction with recess etching for a reliable operation of an APD. We obtained high reliability APDs, which are tested for two-dimensional gain behavior and for accelerated life tests by monitoring dark current and breakdown voltage. The gain and bandwidth product of the best of our APDs was measured as high as 80 GHz.

The distribution of bound state energies of self-assembled quantum dots in quantum dot infrared photodetector structure is addressed using its temperature-dependent dark currents. The temperature-dependent dark currents have been fitted by the modified Arrhenius equation, which includes the distribution of activation energies. The fitting allows us to find the standard deviation and the average activation energy. It is found that the peak of photocurrent spectra, which correspond to intersubband transition energy, is coincided with the average of the activation energies. This was never achieved with the conventional Arrhenius equation.

Optical communications networks must be terminated by receiver circuitry capable of converting an optical circuit to an electrical one. While current III-V technology is capable of delivering high performance, it is costly and difficult to integrate with low-cost Si based technologies. In order to overcome these barriers, we are pursuing a Si-compatible technology for integrated photodetectors. Ge, monolithically integrated with Si, offers a low-cost, high-performance materials system for photodetector integration with existing Si technology. In this paper we discuss the performance requirements and figures of merit for integrated photodetectors. We then discuss the materials issues associated with the integration of Ge on Si and show that high quality Ge films can be grown directly on Si, despite the 4% lattice mismatch. By cyclic annealing after growth, the dislocation density can be reduced to 2.3x10⁷ cm^-2, and diodes fabricated on these films show a responsivity of 300 mA/W at 1300 nm without an AR coating. Finally, we discuss the integration of waveguides with photodetectors and propose an integration scheme we believe will be capable of delivering high-performance integrated photoreceivers on a Si platform.

The uni-traveling-carrier photodiode (UTC-PD) is a newly developed photodiode that utilizes only electrons as the active carriers. This unique feature enables a UTC-PD to achieve excellent high-speed and high-output characteristics simultaneously. Fabricated devices exhibit a record 3-dB bandwidth of 310 GHz, a very-short electrical output pulse of less than 1 ps, high-power millimeter-wave generation at 100 GHz with an output power of over 20 mW, and a sub-millimeter-wave emission at frequencies of up to 800 GHz. The superior capabilities of the UTC-PD for generating wideband millimeter/sub-millimeter waves and very-short electrical pulse signals can innovate various measurement and sensing systems, for instance, millimeter-wave imaging or network analysis. A waveguide-output UTC-PD module with a maximum output power of over 10 mW at 100 GHz is practically important for the photonic-local system in radio telescopes.

Evolution of devices for detecting smaller and smaller energy photons starting from the infrared region of the electromagnetic spectrum is described. A brief description of the extrinsic detector development for LWIR to FIR wavelengths is presented. Several other types of IR detectors including the cutoff wavelength extension into the FIR range for quantum well infrared photodetectors (QWIPs) are presented. Efforts in developing p-GaAs Homojunction Interfacial Workfunction Internal Photoemission (HIWIP) far-infrared detectors and the most recent development in GaAs/AlGaAs HEterojunction interfacial workfunction internal photoemission (HEIWIP) far-infrared detectors are also presented.

Silicon blocked impurity band (BIB) detectors rapidly became the state-of-the-art for photon detection in the near and mid IR range, improving device performance and increasing array size for satellite-based astronomical telescopes. The multiple advantages of the BIB device, in comparison to conventional extrinsic photoconductors, make them even more desirable for far IR detection, where photoconductors suffer from low absorption coefficients and complex transient behavior. This paper summarizes efforts to develop Ge-based and GaAs-based BIB materials and devices. Key challenges include the growth of the high purity blocking layer and the control of growth interfaces. Numerical modeling is presented that illustrates the effect on electric field profiles and device responsivity for variations in net blocking layer doping and extent of interface gradient.

New and novel far-infrared photon detector concepts based on modern epitaxial growth and fabrication techniques are discussed. Simple analyses are carried out to compare the relative merits among three cases based on quantum wells, quantum dots, and free carrier absorptions. Typical results obtained on quantum dot detectors are presented and areas that need improvements are pointed out. Finally some comments are made on the realization of two-dimensional imaging devices.

This letter focuses on the first-principles model of the photocurrent in quantum well infrared photodetectors (QWIPs). The model explores the absorption coefficient measured experimentally. Then we move to show how the responsivity and quantum efficiency vary with α. The work in this paper shows that the device response indeed increases as QWIPs are designed and fabricated to detect longer wavelengths despite the fall in device performance as a result of the growing effect of the dark (noise) current.

We review our recently combined study of temperature-dependent photoluminescence, absorption, and photocurrent measurements with theoretical models on PbSrSe thin films grown by molecular beam epitaxy for the key properties of PbSrSe thin films and their microstructures. The derived empirical equations for band gaps, effective masses, and refractive indices have been employed successfully in PbSrSe/PbSe multiple quantum well (MQW) mid-infrared laser systems, which opens the way for the design of IV-VI MQW mid-infrared lasers. The infrared detection of PbSrSe thin films has also been demonstrated at different temperatures, where the spectral intensity and wavelength coverage are determined by the band gap and the film thickness.

II-V-based magnetic semiconductors are gathering great interest because of their potential utility as new functional materials that are expected to lead to the introduction of spin degree-of-freedom for semiconductor devices. New magnetic semiconductor heterostructure, InMnAsSb/InSb, was proposed and grown by low temperature molecular beam epitaxy (MBE). Mid-infrared (2-6 μm) light-induced ferromagnetism was observed at temperatures lower than 40-50K and this light-induced magnetization remained even after the stop of light irradiation. Observed characteristics are applicable to the mid-infrared sensor/memory devices. New magnetic semiconductor GaCrN layers were grown by electron-cyclotron-resonance (ECR) plasma-assisted MBE and the ferromagnetic characteristics were observed at 7-400K. Clear hysteresis and clear saturation were observed in the magnetization versus magnetic field (M-H) curves at all measuring temperatures. We also observed the photoluminescence (PL) emission from GaCrN layers. Applications to the novel devices controlling charges (electrons and holes), spins and photons are expected.

InAs/InP self-assembled quantum dots (SAQDs) are promising active layers for optical devices for fiber-optic communication. Furthermore, they may be used for the fabrication of uncooled mid and far infrared detectors. InAs/InP SAQDs were grown by low pressure-metalorganic chemical vapor deposition, where As/P exchange reaction and growth interruption step play an important role. The InAs quantum dot (QD) stacks were successfully grown on (001) InP substrate and their optical properties were characterized. Far-infrared absorption peaks were observed at 819 cm^-1 (12.20 μm) and 518 cm^-1 (19.35 μm) at room temperature by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Raman analysis showed that the peak at 819 cm^-1 was attributed to a plasmon related peak in the n-type InP substrate. On the other hand, the absorption peak at 518 cm^-1 was regarded as a peak related with intersubband transition in the InAs QDs, suggesting that room temperature operating quantum dot infrared photodetectors (QDIPs) can be fabricated. In situ monitoring of the QD evolution and stacking sequences were also discussed.

Recently it has been shown that aqueous solutions of sodium dodecyl sulfate (SDS) encapsulated and polymer wrapped single-walled carbon nanotubes (SWNTs) fluoresce in the near infrared (NIR) in the regime of the E₁₁ van Hove transitions for semiconducting SWNTs. For bundled SWNTs, fluorescence is observed to be quenched along with a shift and broadening of the absorbance spectrum. Here, we study two other commercially available surfactants, BRIJ-97 and Triton-X-100, by analysis of carbon nanotube fluorescence and absorptivity in the NIR. It is found that changing the surfactant alters the corresponding optical properties of the solubilized carbon nanotubes. The NIR absorbance spectra of BRIJ-97 and Triton-X-100 carbon nanotube solutions are also compared with the absorbance spectrum of NaCl destabilized SDS-SWNT solutions. By controlling the amount of NaCl added to an aqueous solution of SDS-SWNTs, the optical absorbance spectrum can be made to match that of BRIJ-97 and Triton-X-100 solutions. Lastly, a correlation is drawn between the amount of shift in the absorbance spectrum and the fluorescence intensity, independent of surfactant used. This shift and decrease in fluorescence intensity may be due to carbon nanotube bundling.

We studied the photoreflectance (PR) and photoluminescence (PL) spectroscopies of self-assembled InAs quantum dots grown on n⁺-GaAs (100) by molecular beam epitaxy. The PL spectroscopy of self-assembled Ge quantum dots (Ge-QDs) grown on n⁺-Si (100) by metal organic chemical vapor deposition epitaxy was also investigated. PL spectra show the optical transitions from the ground state and excited states in the InAs quantum dots (InAs-QDs) and a transition from the ground state in the Ge-QDs at the temperature 20 K. PR spectra show the energy features of the transitions of the ground state and four excited states in the InAs-QDs, InAs wetting layer, and GaAs band-gap. The fitted results of the transition energies and the broadening parameters are reported. The results demonstrate that low growth rate of the InAs-QDs, the submonolayer deposited of the alternating beam, and covered with the overgrowth InGaAs methods, have improved the nano-structure quality of the InAs-QD, grown at 485ºC and V/III ratio of 2. The high quality Ge-QDs were made, and characterized by PL experiments.

The Spectroscopic Ellipsometry and the Time Resolved Microwave Conductivity (TRMC) are efficient tools for in-situ non invasive characterizations during the growth of semiconductors and interfaces. From ellipsometry, one estimates the optical absorption, structural composition of the material in the bulk and near the interface. The TRMC measures the transient microwave reflectivity induced by carriers photogenerated by a pulsed laser. From TRMC, one may estimate the mobility of the carriers in a thin film or in bulk materials, the carrier lifetime in the bulk or near the surface. Particularly, we characterize microcrystalline silicon: electron and hole mobility, electron mobility inside the grain, trapping. We also analyze the semiconductor/dielectric interface, particularly for c-Si/SiO₂. Using various UV laser fluxes, we can characterize the surface recombination, estimate the interface field and compare with the density of states obtained from capacitance measurement. The results are compared with simulation.

Band diagram of GaN-based p-i-n structures containing InGaN/GaN multiple quantum wells is discussed in the presence of an external electric field. Carrier lifetime, photocurrent, and photoluminescence intensity as a function of applied voltage can be understood based on the potential diagram. These structures effectively generate “spatially localized strain pulse,” which is not present in other materials such as in GaAs-based p-i-n structures. The mechanisms of the generation and propagation of the strain pulse and their relevance with the piezoelectricity of GaN are discussed. These structures with InGaN/GaN multiple quantum wells also produce electromagnetic radiation of 0.11 THz, which is absent in GaN-based double heterostructures.

This paper summarizes some of the recent advances made by our group on the growth, characterization and applications of AlGaN alloys with high Al contents. Recently, our group has achieved highly conductive n-type Al_xGa_1-xN for x as high as 0.7 (a resistivity value as low as 0.15 ohm-cm has been achieved). Prior to this, only insulating Al_xGa_1-xN (x > 0.5) can be obtained. Our success is largely attributed to our unique capability for monitoring the optical qualities of these layers -- the development of the world's first (and presently only) deep UV picosecond time-resolved optical spectroscopy system for probing the optical properties of III-nitrides [photoluminescence (PL), electro-luminescence (EL), etc.] with a time-resolution of a few ps and wavelength down to deep UV (down to 195 nm). Our time- resolved PL results have shown that we must fill in the localization states (caused by alloy fluctuation) by doping before conduction could occur. The density of states of localization states is about 10¹⁸/cm³ in this system. It was also shown that Al_xGa_1-xN alloys could be made n-type for x up to 1 (pure AlN). Time-resolved photoluminescence (PL) studies carried out on these materials have revealed that Si-doping reduces the effect of carrier localization in Al_xGa_1-xN alloys and a sharp drop in carrier localization energy as well as a sharp increase in conductivity occurs when the Si doping concentration increases to above 1 x 10¹⁸ cm^-3. For the Mg-doped Al_xGa_1-xN alloys, p-type conduction was achieved for x up to 0.27. The Mg acceptor activation energy as a function of Al content has been deduced. Mg-δ-doping in GaN and AlGaN epilayers has been investigated. We have demonstrated that δ-doping significantly suppresses the dislocation density, enhances the p-type conduction, and reduces the non-radiative recombination centers in GaN and AlGaN. AlN epilayers with high optical qualities have also been grown on sapphire substrates. Very efficient band-edge PL emission lines have been observed for the first time with above bandgap deep UV laser excitation. We have shown that the thermal quenching of the PL emission intensity is much less severe in AlN than in GaN and the optical quality of AlN can be as good as GaN. From the low temperature (10 K) emission spectra, as well as the temperature dependence of the recombination lifetime and the PL emission intensity, the binding energies of the bound excitons and free excitons in AlN were deduced to be around 16 meV and 80 meV, respectively. From this, the energy bandgap of AlN epilayers grown on sapphire was found to be around 6.11 eV at 10 K. The observed large free exciton binding energy implies that excitons in AlN are extremely robust entities. This together with other well-known physical properties of AlN may considerably expand future prospects for the application of III-nitride materials.

Effect of chemical dry cleaning (CDC) and pre-treatment (NH₃ annealing) on the interface property of high-K gate dielectrics (NiTiO₃) in MOS device have been investigated. A surface layer is damaged due to oxide dry etching (CHF₃/CF₄/Ar) process in active region formation. This damaged layer was generally removed by wet etching (HF) however it creates a rough layer on the substrate surface. It appears that the surface roughness by plasma dry etching is better then that by HF wet oxide etching. When the metal is directly deposited on the silicon substrate, a poor interface quality leads to a high leakage current. A damage-free, smooth surface, and thin interfacial layer formed on substrate surface are important for high-k gate dielectric process. Therefore, a method using Chemical Dry Cleaning (CF₄/O₂) is adopted after dry oxide etching (CHF₃/CF₄/Ar). In addition, CDC can then be adopted to effectively remove fluorocarbon residue and Si-F and Si-C bonds on the silicon surface etched in fluorocarbon plasma chemistry. After CDC, pre-deposition treatment can be employed to inhibit the growth of the interfacial layer. The pre-treatment strategies (NH₃ annealing) are used to prevent silicide formation and to elevate the interface property of the devices. Finally, post deposition annealing (N₂ annealing) can be performed to reduce the leakage current. These approaches not only strengthen the structure of Si-N bonds, but also improve the smoothness and uniformity at the interface of the metal oxide/silicon substrate.

A self-assembled monolayer (SAM) molecule is attractive as an active region of an electron device because of its inherent small thickness (~1-2 nm) between each electrode. We reported processes to fabricate small Au/SAM/Au junctions by using electron beam lithography. As a SAM molecule, we used benzene-1,4-dithiol on Au. To obtain an atomically flat Au electrode without deformation of shape, lower deposition rate, lower sample temperature, and adequate annealing temperature were required. By using a SiO₂ pattern as a shadow mask, twice oblique evaporations made small Au/SAM/Au junctions. A minimum feature size of slit of a SiO₂ pattern was 160 nm by using electron beam lithography. Si substrate isolated by SiO₂ works as a gate electrode of three terminal devices by the Au/SAM/Au junctions. Observed current-voltage characteristics between the drain and the source showed nonlinear characteristics and weak modulation by gate bias was observed. The processes to improve device characteristics are also discussed.

The design, fabrication and performance of low threshold selectively oxidized infrared vertical cavity surface emitting lasers (VCSELs) for operation at 0.89μm and 1.55μm wavelengths using optimized graded Bragg mirrors, is reported. The devices are based on III-V ternary (AlGaAs/GaAs) and quaternary (AlInGaAs/GaInAsP/InP) graded semiconductor alloys and quantum wells and are grown by Molecular Beam Epitaxy. The VCSEL arrays are processed using inductively coupled plasma (ICP) etching with BCl₃ gas mixtures to achieve vertical walls and small geometries, and the fabrication of the devices proceeds by using conventional Ohmic contacts (Ti-Pt-Au and Ni-Au-Ge-Ni) and indium tin oxide (ITO) transparent contacts. The theoretical investigation of the optical properties of the quaternary compound semiconductor alloys allows us to select the optimum materials for highly reflective Bragg mirrors with less periods. The simulation of the designed VCSEL performance has been carried out by evaluation of the important laser characteristics such as threshold gain, threshold current density and external quantum efficiency.

Conventional wisdom suggests that, for low avalanche noise, avalanche photodiodes should operate at low electric fields, where electron and hole ionisation coefficients can differ widely. However, the associated weak ionization requires long multiplication regions, which in turn demand high bias voltages and result in long carrier transit times, reducing device speed. Moreover, multiplication is particularly sensitive to temperature in this region. In this paper we discuss the effects of dead space on reducing noise in short devices and on the associated benefits in predicted response time and reduced temperature sensitivity. The paper is illustrated with work from the Sheffield group.

Resonant cavity enhanced photodetectors (RCE-PDs) are promising candidates for applications in high-speed optical communications and interconnections. In these high-speed photodetectors, both high bandwidth and high external quantum efficiency can be achieved simultaneously because of the multipaths of the incident light due to the presence of the Fabry-Perot cavity into which the photodetector is inserted. In this paper, state-of-the-art RCE-PDs are discussed. Different structures of the RCE-PDs such as RCE-PIN, RCE-APD, and RCE-MSM PDs are presented and discussed. The material requirements for the RCE PDs with different material system compositions for the different structures and different wavelengths of the incident light that the photodetectors are sensitive to, are discussed. An overview of the analysis and a SPICE model of the RCE-PDs will be presented. These analyses include the calculations of quantum efficiency, (QE), impulse response and frequency response of RCE-PDs. These analyses are sensitive to the standing wave effect (SWE) and to carrier diffusion, and both effects are studied. Optimization procedures for the design of ultrafast RCE-PDs will be presented, showing how the QE, bandwidth and speed of these photodetectors can be improved by adjusting the parameters of both the cavity and the photodetector itself. Finally, comparisons to experimental results and a survey of the performance of state-of-the-art of RCE-PDs will be presented.

The electronic features of semiconductor nanostructures, such as zero-dimensional states, are usually inferred from macroscopic optical and transport experiments. Although, direct probing of electrical features in semiconductor nanostructures looks very attractive, it is very difficult for a conventional semiconductor structure. However, direct probing becomes possible through a combination of low-temperature scanning tunneling microscopy and InAs(111)A surface in an ultra-high vacuum, where conductive electrons automatically accumulate near the clean surface. The clear observation of a Friedel oscillation pattern around a dislocation demonstrates successful mapping of the local-density-of-states (LDOS) of the conductive electrons. Inverted pyramidal defects are naturally formed during molecular beam epitaxial growth of InAs thin films on GaAs(111)A substrates and they operate as well-defined quantum dots. The measured LDOS pattern inside the quantum dots clearly changes as a function of energy, i.e. a sample bias, reflecting the LDOS pattern of each zero-dimensional state. A resonant concentration of the LDOS to the zero-dimensional energy levels is also demonstrated in these experiments. The LDOS measurements of a series of inverted pyramidal quantum dots with different side lengths and their comparison with theoretical calculations suggest a unique feature of the quantum dot system examined in this study.

Self-organized nanohole array formed by anodic oxidation of aluminum has been intensively studied to use them as molds to form nano wires or dots. Recently we established a formation of porous alumina film on solid substrates such as Si, and found a formation of ordered aluminum hexagonal dot array after finishing of anodic oxidation on a SiO₂/Si substrate. However, the size and the geometrical arrangement of well-ordered nanoholes are limited by a narrow window of self-organization condition of anodic oxidation. We investigated AFM nano-indentation to control the initial position of nanoholes during anodic oxidation, in order to realize nano-holes and Al dots with desired sizes and densities. Arrays of nanoholes with a nearest neighbor distance from 50 to 120 nm were successfully formed. Al dot teragonal arrays were formed after selective wet chemical etching of porous alumna film. Electron transport through Al nanodot array with an adequate inter-dot distance exhibited a clear Coulomb blockade at liquid He temperature.

Our effort to form reliable tunnel barriers in single and multi-wall carbon nanotubes is presented. As well as the standard method which uses a simple deposition of contact metal on nanotubes, we use a narrow SiO₂ deposition and Ar ion beam irradiation to form the tunnel barrier in nanotubes. Such devices as a single electron transistor, double coupled quantum dots and a single electron inverter have been fabricated. Their performance has been measured from 20 mK up to liquid helium temperature. The unique microwave response of coupled quantum dots is also presented. We show that the carbon nanotubes are attractive material for the building block of quantum-dot based nanodevices with extremely small dimensions, and our method may be useful to realize the devices and cirucits.

For the high temperature operation of a single electron transistor, the nano fabrication process is the most important key technology. For the complete room temperature characteristics of the single electron transistor, such as a room temperature Coulomb diamond characteristics, 1 to approximately 2 nm feature size is indispensable. The conventional nano fabrication technology, however, could not attain such a small feature size of 1 to approximately 2 nm. A carbon nanotube is one of the best candidates for the nano-scale devices because of its small features size, e.g., the single wall carbon nanotube has a diameter of 1 to approximately 2 nm. Application of such a small feature of a carbon nanotube to a single electron transistor could be expected to increase the Coulomb energy and also to raise the operation temperature up to 300 K. However, the control of the precise positioning of the carbon nanotube for the device application has been so far a quite hard work and has been almost impossible. Only some primitive techniques were succesfully achieved. In the present paper, we proposed the new technology that could easily control the position of the carbon nanotube by using the patterned chemical catalyst. The carbon nanotube can be grown between the patterned chemical catalyst using the thermal CVD process. The single electron transistor was fabricated using the position controlled grown carbon nanotube as a channel and its electrical properties were examined. The single electron transistor showed the Coulomb diamond characteristics even at room temperature. The Coulomb energy is 400 meV and corresponding Coulomb temperature is as high as approximately 5000 K.

This paper surveys recent work in several photodetector areas including high-speed, low-noise avalanche photodiodes, solar-blind ultra-violet PIN photodiodes, and quantum dot infrared photodetectors (QDIPs).

Quantum dots are one of the fascinating quantum structures in broad device applications. The progress made in III-V quantum dots due to self-assembled growth technique was incredibly rapid to realize practical device applications such as lasers and detectors. In this review, the growth issues of quantum dots in self-assembling technique and the progress made in optoelectronic devices is addressed. The 1.3 μm lasers have been successfully demonstrated on GaAs substrate with InGaAs multi-stacked QDs. The low threshold, high power laser characteristics and its carrier dynamics in QD lasers is discussed. More than 10GHz of 3dB-modulation bandwidth was obtained from well-designed 1.3 μm QD lasers. The potential application of high speed and tunable light emitting sources is reviewed. The long wavelength quantum dot photodetectors (QDIP) is another promising application of fine quantum structure. The physical properties and characteristics of QDIP are briefly addressed. The applications of quantum dots are not limited to the optoelectronic devices and currently they have received increasing attention for bio-sensing applications due to their small fine structures. We briefly discuss a model for bio-sensing mechanism of QD fine structure as interacting with bimolecular at the end.

HgCdTe infrared (IR) detectors have been intensively developed since the first synthesis of this ternary alloy in 1958. The paper summarizes the fundamental properties of this versatile alloy, and relates the material properties to its successful applications as an IR photoconductive and photovoltaic detector material. An emphasis is put on key developments in the crystal growth and their influence on device evolution. Recent advances of backside illuminated HgCdTe heterojunction photodiodes have enabled third generation of multispectral instruments for remote sensing applications and have led to the practicality of multiband IR focal plane array technology.

Thermal detectors, while typically less sensitive than quantum detectors, are useful when the combination of long wavelength signals and relatively high temperature operation makes quantum detectors unsuitable. Thermal detectors are also appropriate in applications requiring flat spectral response over a broad wavelength range. JPL produces thermopile detectors and linear arrays to meet space science requirements in these categories. Thermopile detectors and arrays are currently being fabricated for two space applications. The first is the Mars Climate Sounder (MCS) instrument, to fly on the Mars Reconnaissance Orbiter mission, scheduled to launch in 2005. MCS is an atmospheric limb sounder utilizing nine 21-element thermopile arrays. The second application is the Earth Radiation Budget Suite (ERBS), part of the National Polar Orbiting Environmental Satellite System (NPOESS). This instrument measures upwelling radiation from the earth in the spectral range 0.3-100 μm.

Trap centers and minority carrier lifetimes are investigated in InAs/(GaIn)Sb superlattices used for photodetectors in the far-infrared wavelength range. In our InAs/(GaIn)Sb superlattice photodiodes, trap centers located at an energy level of ~1/3 band gap below the effective conduction band edge could be identified by simulating the current-voltage characteristics of the diodes. The simulation includes diffusion currents, generation-recombination contributions, band-to-band coherent tunneling, and trap assisted tunneling. By including the contributions due to trap-assisted tunneling, excellent reproduction of the current voltage curves is possible for diodes with cut-off wavelength in the whole 8-32 μm spectral range at temperatures between 140 K and 25 K. The model is supported by the observation of defect-related optical transitions at ~2/3 of the band-to-band energy in the spectra of the low temperature electroluminescence of the devices. With the combination of Hall- and photoconductivity measurements, minority carrier lifetimes are extracted as a dependence of temperature and carrier density.

New infrared (IR) detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/In_xGa_1-xSb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. The infrared properties of various compositions and designs of these type-II superlattices have been studied. The infrared photoresponse spectra are combined with quantum mechanical modeling of predicted absorption spectra to provide insight into the underlying physics behind the quantum sensing in these materials. Results for superlattice photodiodes with cut-off wavelengths as long as 25 microns will be presented.

In this paper, we present a brief overview of the most common electromagnetic model used to analyze the properties and performance of photonic crystal structures, namely the plane wave method. This method is commonly used to determine the electromagnetic dispersion properties of a photonic crystal structure and thereby provides key insight into the nature of the band diagram, which ultimately reveals the presence of a band gap. Following the overview, we then apply this tool to estimate the effects of fabrication errors on the confinement properties of photonic crystal slab waveguides. As we show in this paper, fundamental issues related to a shift in the allowable states in a photonic crystal wave are show to be a source of significant propagation loss.

HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors have been demonstrated with cutoff wavelengths λ_c up to 92 μm. One method of increasing the response in a desired range is to employ the cavity effect to create resonant maxima. Results are reported here confirming the presence of cavity enhancements in both the absorption and the detector response of HEIWIP structures at the 3λ/4 resonance. The detectors consisted of 13, 19 and 30 Be doped GaAs emitter and undoped Al_0.02Ga_0.98As barrier layers. Transmission and reflection spectra for multilayer GaAs/AlGaAs IR detectors in the range 2000-100 cm^-1 at room temperature are presented. Comparisons with the calculated results based on free carrier absorption and interaction with optical phonons model are reported. It is shown that the absorption can be maximized by using the resonant cavity effect. The use of the resonant cavity effect should allow the design of detectors with increased response in the desired wavelength ranges.

We discuss a growth technique for producing a high quality quantum dot (QD) ensemble that has both high QD density and low defect density. The growth conditions under which the QDs nucleate, are optimized to form high density QDs with few defects, however, it is very difficult to prevent coalescence completely. Pausing the AsH₃ flow for a few seconds after nucleation allows surface adatoms to migrate from poly-crystalline defect sites where the bond strengths are weak to crystalline QD sites. We will discuss statistical analysis based on atomic force microscope (AFM) images, high-resolution transmission electron microscopy (HRTEM), and electroluminescence (EL) to characterize the effects of AsH₃ pauses on the QD density and crystallographic shape. Ground state PL from capped QDs is measured at 1.38 μm with a 40 meV linewidth. We demonstrate lasing from the first excited state under room-temperature pulsed characteristics.

A theoretical model for the dependence on temperature of the carrier behavior in a semiconductor structure containing InAs quantum dots grown inside a Ga_0.85In_0.15As quantum well is presented. The conditions, that have to be imposed in order to obtain analytical solutions with obvious physical interpretation are kept to minimum. Two temperature domains are approached in this model. In the low temperature case the equation system that describes the carrier behavior can be reduced to a cubic equation. One of the solutions of the equation represents the quantum dot photoluminescence yield. Also, a solution is obtained for the dot emission yield in the high temperature domain, where the carrier thermal escape from dots cannot be neglected. The solution depends, on the probabilities for electron and hole capture and reemission, and on the number of dot states occupied by electrons and holes. Temperature dependent measurements of the quantum dot photoluminescence are performed and the results are fit with the theoretical model.

The migration of carriers in optically pumped semiconductor structures with quantum dots is investigated using a confocal setup. Experiments are performed in order to test the performances of the optical setup. The samples studied by optical confocal microscopy are the following: semi-insulating GaAs, InAs quantum dots grown directly in GaAs, InAs quantum dots grown inside a Ga_0.85In_0.15As quantum well and a Ga_0.85In_0.15As quantum well with GaAs barriers. Measurements are done in a range of temperatures starting from 78 K up to 295 K. A theoretical model for the migration length is presented. The solution of a diffusion-type equation is used to fit the experimental data.

We have studied the influence of overgrowth procedure and a few monolayer-thick AlAs capping layers on the properties of self-assembled InAs quantum dots (QDs) using transmission electron microscopy (TEM), scanning electron microscopy, and photoluminescence (PL). PL spectroscopy was used to study and optimize optical properties of the QDs by shape engineering (QD truncation) through adjustment of the thickness of overlayers and temperature of the subsequent heating. QDs with 6 nm-thick overlayer with heating step at 560°C was found to have the highest PL intensity at room temperature and the lowest FWHM, 29 meV. Ground state energy of the truncated QDs is very stable against variations of growth parameters. TEM measurements show that the capping AlAs layer covers the QDs entirely even though the dots are truncated by the heating step. 1.22 μm edge-emitting laser with triple-layer truncated QD gain medium demonstrated room temperature minimum threshold current density, 56 A/cm², and high saturated modal gain, 16 cm^-1. Extremely high characteristic temperature, T_o = 304 K in the 20 - 60°C interval, and maximum lasing temperature of 219°C were measured for this laser diode.

Optical properties and material microstructures of three InGaN/GaN quantum well (QW) samples with various silicon-doping concentrations in barriers were measured. From the high-resolution transmission electron microscopy images, quantum dots (QDs) of a few nm in size were observed in silicon-doped samples. The regularities of QDs in size, shape and distribution increased with doping concentration up to 5 x 10¹⁸ cm^-3. Such observations implied that the reduction of quantum-confined Stark effect in such a sample was due to the relaxation of strain energy in QDs with silicon doping, besides the carrier screen effect. In other words, the microstructures were crucially changed with silicon doping in barriers. Also, the carrier localization effect was actually enhanced although potential fluctuation indeed became less randomly distributed. The calibrated radiative lifetimes in both silicon-doped samples showed the consistent trend of the formation of 0-D structure upon silicon doping.

Quantum dot (QD) is one of the most perspective candidates to be used as an active region of temperature-insensitive 1.3-micron GaAs based lasers for optical networks. However, the limited optical gain achievable in QD ground state hindered their practical use. In this work we have demonstrated that using of high number of QDs stacks grown under proper conditions by MBE is an effective way to considerably increase the optical gain of QD lasers. Ridge waveguide laser diodes with width of 2.7 μm and 4.5 μm based on various numbers of QD layers (N=2, 5, 10) were fabricated and studied in this work. Ultra-low threshold current of 1.43 mA was achieved for 2-stack QD. Regime of simultaneous lasing at ground- and excited-states was discovered. This effect was accounted for the finite time of carriers capture to the ground-state in QD. Multi-stack QD structures enabled to maintain continuous work ground-state lasing up to the current density of 10 kA = 100xJ_th. Enhanced optical gain allowed us to unite very high differential efficiency (>75%) with low threshold current (<100 A/cm²) and characteristic temperature (T₀>100K). For example, laser diode of 1-mm cavity length has shown single mode output power of 100mW at operating current of 195 mA and at high operation power demonstrated insensibility to the changes of temperature. The combination of parameters achieved is quite competitive to all technologies currently used for 1.3-micron lasers including traditional InP-based lasers and makes QD gain medium very promising for VCSEL and telecom laser applications.

Most quantum sensing is limited by fundamental 1/f noise, and is described by the author’s quantum 1/f noise theory. The present review paper summarizes this new aspect of quantum mechanics and displays its applications in spin valves and related devices, in the classical forms of infrared detectors, and in various other nanodevices, such as MEMs. In spintronic devices, the quantum 1/f effect causes macroscopic quantum fluctuations of the electronic spin flip rate. They translate into 1/f noise in the leakage current, given by the conventional quantum 1/f formula. In infrared detectors, the dark current exhibits similar quantum 1/f fluctuations that are present in the elementary cross sections and process rates that limit and determine the dark current. In n⁺-p junction infrared detectors, for instance, at low temperatures, these are mainly the recombination cross sections present in the space charge region and on the surface in the junction region. At higher temperatures, they are mainly the scattering cross sections in the diffusion tail in the p region. Conventional quantum 1/f effect in these scattering cross sections will be noticed as mobility fluctuations, and therefore also as 1/f fluctuations in the diffusion constant, causing again 1/f noise in the dark current.

Most sensors traduce various environmental, industrial, or laboratory parameters into electrical signals. Quantum sensing pushes this to the ultimate limit of achievable sensitivity, detectivity, resolution, and stability. Quantum mechanics sets these limits through the uncertainty principle. It causes the ubiquitous 1/f noise, as we show here for LiNbO₃. Quantum 1/f theory, known as the quantum theory of the fundamental 1/f noise phenomenon, is a new aspect of quantum mechanics, that governs the nonlinear interaction of particle and field, introducing the new notions of "physical cross sections" (PCS) and "physical process rates" (PPR). These contain the fundamental macroscopic quantum 1/f fluctuations. They yield the usually defined PCS and PPR with infrared radiative corrections, when the expectation value is taken over the low-frequency photon states of negative entropy. The physical charged particle, such as an electron, contains both the bare particle and its electromagnetic field. The latter is in a coherent state, where the phase of the electromagnetic field oscillators is determined. Heisenberg’s uncertainty principle then requires that the energy must be uncertain, leading to nonstationary states describing the particle. The particle’s resulting fractional current fluctuations in large devices always have the spectrum 2α/πf, where α=1/137 is the fine structure constant.

Very high average power QCLs are demonstrated within the 5.8 - 9 μm wavelength range. At longer wavelengths, scaling of the power is demonstrated by increasing the number of emitting regions in the waveguide core. At λ = 9 μm, over 3.5 W of peak power per facet has been demonstrated at room temperature for a single 25 μm by 3 mm diode, with an average power of 150 mW at 6% duty cycle. At shorter wavelengths, highly strain-balanced heterostructures are used to create a high coduction band offset and minimize leakage current. At λ = 6 μm, utilizing a high reflective coating and epilayer-down mounting of the laser, we demonstrate 225 mW of average power from a single facet at room temperature. Increasing the conduction band offset further and optimizing the doping in the injector region has led to demonstration of > 250 mW average power (λ = 5.8 μm) at > 50% duty cycle for a 20 μm by 2 mm HR coated diode bonded epilayer-down to a copper heatsink. Also at room temperature, use of Au electroplating and wider ridges has allowed us to further demonstrate without epilayer-down bonding, 0.67 W average power at 17% duty cycle from a single 40 μm by 2 mm HR coated laser.