This paper describes the recent progress made in developing wavelength tunable semiconductor light sources for WDM applications. Wide and quasi-continuous wavelength tunings were investigated for a wavelength-selectable laser and a wavelength tunable distributed Bragg reflector (DBR) laser having a super structure grating (SSG). A wavelength-selectable laser consisting of a DFB laser array, a multi-mode interferometer (MMI), and a semiconductor optical amplifier (SOA) demonstrated a quasi-continuous tuning range of 46.9 nm by using temperature control. A wavelength-tunable DBR laser with SSG exhibited a quasi-continuous tuning range of 62.4 nm by using three tuning current controls. Wavelength stabilization was also demonstrated under the temperature variations of 5.

Optical properties and growth of self-assembled quantum dots (SAQDs) for optoelectronic device applications are discussed. After briefly reviewing the history of research on QD lasers, we discuss growth of InAs SAQDs including the light emission at the wavelength of 1.52-µm with a narrow linewidth (22 meV) and the area-controlled growth which demonstrates formation of SAQDs in selected local areas on a growth plane using a SiO2 mask with MOCVD growth. Then properties of the InGaAs AQDs are investigated by the near-field photoluminescence excitation spectroscopy which reveals gradually increasing continuum absorption connected with the two-dimensional-like (2D-like) wetting layer, resulting in faster relaxation of electrons due to a crossover between 0D and 2D character in the density of states. In the coherent excitation spectroscopy, the decoherence time is determined to be about 15 ps, which is well explained by the phonon induced relaxation mechanism in the SAQDs. Finally, nitride-based SAQDs and perspective of QD optical devices are also discussed.

Fourier synthesis of ultrafast optical-pulse trains was demonstrated using a simplified experimental configuration consisting of three independent continuous-wave lasers and a semiconductor optical amplifier (SOA) used as a four-wave mixer. When the three lasers were phase-locked, ultrafast optical-pulse trains were successfully generated at repetition frequencies ranging from 504 GHz to 1.8 THz with high waveform stability.

This paper describes glyph representation, that is, shape representation of inheritance relationships between a superclass and subclasses in an object-oriented programming language. The inheritance relationships in object-oriented programming languages are usually represented in a visual programming environment by a diagram of a tree graph or a nested structure. That diagram is not integrated with a code view showing control and data flows. Using the proposed representation, one can understand the inheritance relationships of classes and the assignment compatibility or type conformance just by seeing the glyphs. One thus does not need to look at a hierarchy diagram in order to recognize them. The inheritance relationships are represented by inclusion relationships of glyphs. Methods for generating suitable glyphs from a class hierarchy are also described, as is a prototype system for glyph generation. Experiments using the Java 2 Standard Edition (J2SE), which has more than 1,500 classes, show that one can recognize inheritance relationships in the proposed representation faster than in the usual textual representation. Consequently the proposed representation can facilitate the understanding of inheritance in visual object-oriented programming environments.

A 2D photonic crystal surface-emitting laser using a triangular lattice is developed, and current-injected lasing oscillation is demonstrated. From consideration of the Bragg diffraction condition in the 2D triangular-lattice structure, it is shown that the 2D coupling phenomenon occurs in the structure. As a result of the 2D periodicity of the structure, the longitudinal mode and lateral mode can be controlled, and stable single-mode oscillation is possible over a large 2D area. The lasing mode of the structure is analyzed by calculating the photonic band diagram by the 2D plane-wave expansion method, and we show that four band edges at which the lasing oscillation can occur exist at the Γ point. Current-injected lasing oscillation is successfully demonstrated at room temperature under pulsed conditions. The threshold current density is 3.2 kA/cm2 and the lasing wavelength is 1.285 µm. From the near-field and far-field patterns, it is shown that large-area 2D (diameter 480 µm) lasing oscillation occurs in the device and the divergence angle is very narrow (less than 1.8). We also demonstrate the correspondence between the measured lasing wavelengths and calculated band diagram by comparing the polarization characteristics with the calculated distribution of the electromagnetic field. The results indicate that 2D coherent lasing oscillation occurs due to the multi-directional coupling effect in the 2D photonic crystal. Finally, we show that the polarization patterns of the lasers can be controlled by introducing artificial lattice defects from the theoretical calculation.

We describe a simple all-optical wavelength converter based on a Fabry-Perot semiconductor optical amplifier (FPSOA). We measure its static characteristics in detail and successfully demonstrate its dynamic wavelength-conversion operation (both inverted and non-inverted) at 2.5 Gbit/s. This is the first demonstration of FPSOA-based wavelength conversion. Quasi-digital response is also observed. Low input power, ease of fabrication and good compatibility with WDM networks are important advantages of FPSOA.

The low bias region of the base current has been studied in SiGe HBTs and shown to arise from tunneling at the emitter periphery. Tunneling also describes the reverse bias base-emitter current, which we believe is enhanced by mid-gap states. The reverse bias causes damage to the base-emitter region, increasing the base current. We also show that after a short period of severe reverse bias stress the base current displays random telegraph signals. These phenomena are often observed in silicon bipolar transistors, confirming that the incorporation of SiGe has not produced any other undesirable characteristics.

Characteristics of the optical feedback noise in semiconductor lasers under superposition of the HF (High Frequency) current were experimentally examined and theoretically analyzed. The feedback noise was mostly suppressed by superposition of HF current, but still remained when frequency of the HF current coincided with a rational number of the round trip time period for the optical feedback in experimental measurement. Theoretical analysis was also given to explain these characteristic based on the mode competition theory of the semiconductor laser.

Current-voltage characteristics of Cr-doped hydrogenated amorphous silicon-V (Cr/p+a-Si:H/V) analogue memory switching devices have been measured over a wide range of device resistance from several kilo-ohms to several hundred kilo-ohms, and over a temperature range from 13 K to 300 K. Both the bias and temperature dependence of the conductance show similar characteristics to that of metal-insulator heterogeneous materials (i.e. discontinuous or granular metallic films), which are analysed in terms of activated tunnelling mechanism. A modified filamentary structure for the Cr/p+a-Si:H/V switching devices is proposed. The influence of embedded metallic particles on memory switching is analysed and discussed.

We analyze electron transport of silicon single-electron transistors (Si SETs) with an ultra-small quantum dot using a master-equation model taking into account the discreteness of quantum levels and the finiteness of scattering rates. In the simulated SET characteristics, aperiodic Coulomb blockade oscillations, fine structures and negative differential conductances due to the quantum mechanical effects are superimposed on the usual Coulomb blockade diagram. These features are consistent with the previously measured results. Large peak-to-valley current ratio of negative differential conductances at room temperature is predicted for Si SETs with an ultra-small dot whose size is smaller than 3 nm.

Negative differential conductance based on lateral interband tunnel effect is demonstrated in a planar degenerate p+-n+ diode (Esaki tunnel diode). The device is fabricated with the current silicon ultralarge scale integration (Si ULSI) process, paying attention to the processing damage so as to reduce an excess tunnel current that flows over some intermediate states in the tunnel junction. I-V characteristics at a low temperature clearly show an intrinsic electron transport, indicating phonon-assisted tunneling in Si as in the case of the previous Esaki diodes fabricated by the alloying method. In addition, a simple circuit function of bistable operation is demonstrated by connecting the planar Esaki diode with conventional Si metal-oxide-semiconductor field effect transistors (MOSFETs). The planar Esaki diode will be a promising device element in the functional library for enhancing the total system performance for the coming system-on-a-chip (SoC) era.

Si single-electron transistors with a high voltage gain at a considerably high temperature have been fabricated by vertical pattern-dependent oxidation. The method enables the automatic formation of very small tunnel junctions having capacitances of less than 1 aF. In addition, the use of a thin (a few ten nanometers thick) gate oxide allows a strong coupling of the island to the gate, which results in a gate capacitance larger than the junction capacitances. It is demonstrated at 27 K that an inverting voltage gain, which is governed by the ratio of the gate capacitance to the drain tunnel capacitance, exceeds 3 under constant drain current conditions.

We have manufactured large-scaled highly reliable MNOS EEPROMs over the last twenty years. In particular, at the present time, the smart-card microcontroller incorporating an embedded 32-kB MNOS EEPROM is rapidly expanding the markets for mobile applications. It might be said that we have established the conventional MNOS nonvolatile semiconductor memory technology. This paper describes the device design concepts of the MNOS memory, which include the optimization and control of the tunnel oxide film thickness (1.8 nm), and the scaling guideline that considers the charge distribution in the trapping nitride film. We have developed a high-performance MONOS structure and have not found any failure due to the MONOS devices in high-density EEPROM products during 10-year data retention tests after 105 erase/write cycles. The future development of this highly reliable MNOS-type memory will be focussed on the high-density cell structure and high-speed programming method. Recently, some promising ideas for utilizing an MNOS-type memory device, such as 1-Tr/bit cell for byte-erasable full-featured EEPROMs and 2-bit/Tr cell for flash EEPROMs have been proposed. We are convinced that MNOS technology will advance into the area of nonvolatile semiconductor memories because of its high reliability and high yield of products.

This paper reviews device technologies of flash memories, whose market has grown explosively due to the advantages of: (1) their low cost provided by availability of the single-transistor type cell with adoption of block-erase operation; (2) high functionality as electrically erasable and programmable non-volatile memories; and (3) high reliability with the mature floating gate technology. As for fast-random-access flash memories, their scaling issue, including a multi-level-cell technology, is discussed, and technologies for low power consumption, which is highly demanded for mobile electronic equipment, their major application, are described. Furthermore, device technologies of serial-access flash memories, which have achieved low cost with cell-size reduction, are also reviewed. Finally, a future promising technology of the NROM concept, which achieves a multi-storage-cell with low voltage operation and a simple process, is introduced.

We demonstrate all-optical clock recovery at 160 Gbit/s by injection locking of a 10 GHz mode-locked laser diode. Effective locking in a range of 10 MHz is observed for average input powers around -10 dBm. The timing jitter is analyzed for data rates between 10 Gbit/s and 160 Gbit/s. Beyond 40 Gbit/s, the high frequency timing jitter of the slave laser becomes of prime importance and has to be taken into account since it degrades the performance of a subsequent receiver. Increasing power penalties are found, especially beyond 80 Gbit/s.

A novel temperature insensitive wavelength filter consisting of GaAlAs/GaAs distributed Bragg reflectors (DBRs) has been demonstrated. This micromachined DBR is mechanically tuned by differential thermal expansion. The strain-induced displacement of one mirror can generate wavelength tuning and trimming functions with an adjustable temperature dependence. We succeeded in the control of temperature dependence in this micromachined semiconductor filter by properly designing a vertical cavity structure. The achieved temperature dependence was as small as +0.01 nm/K, which is one-tenth of that of conventional semiconductor based optical filters. Also, a wavelength trimming of over 20 nm was demonstrated after completing the device fabrication. In addition, we demonstrated a 4 4 multiple wavelength micromachined vertical cavity filter array. The multi-wavelength filter array with a wavelength span of 45 nm was fabricated by partially etching off a GaAs wavelength control layer loaded on the top surface of device.

We have measured the temperature dependence of the gain characteristics in 1.3-µm AlGaInAs/InP strained multiple-quantum-well (MQW) semiconductor lasers using Hakki-Paoli method. By measuring the temperature dependences of the peak gain value and the gain peak wavelength, we evaluated the temperature dependences of the threshold current and the oscillation wavelength, respectively. The small temperature dependence of the threshold current in AlGaInAs/InP lasers is caused by the small temperature dependence of the transparency current density, which is represented by the characteristic temperature TJtr of 116 K. In AlGaInAs/InP high T0 lasers, the temperature dependence of the oscillation wavelength is slightly larger than that in GaInAsP/InP lasers because of the larger temperature dependence of bandgap wavelength 0.55 nm/K.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.

We have measured the temperature dependence of the gain characteristics in 1.3-µm AlGaInAs/InP strained multiple-quantum-well (MQW) semiconductor lasers using Hakki-Paoli method. By measuring the temperature dependences of the peak gain value and the gain peak wavelength, we evaluated the temperature dependences of the threshold current and the oscillation wavelength, respectively. The small temperature dependence of the threshold current in AlGaInAs/InP lasers is caused by the small temperature dependence of the transparency current density, which is represented by the characteristic temperature TJtr of 116 K. In AlGaInAs/InP high T0 lasers, the temperature dependence of the oscillation wavelength is slightly larger than that in GaInAsP/InP lasers because of the larger temperature dependence of bandgap wavelength 0.55 nm/K.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.