Boundary effects on the optical properties of InGaN multiple quantum wells

We examine the issues of spontaneous and piezoelectric polarization discontinuity on the optical properties of 3.0-nm-thick indium gallium nitride (InGaN) multiple quantum wells (MQWs). A quench of band-edge emission from the cap GaN layer is observed when the photoexcitation source is changed from a 355- to a 248-nm laser. The interband transitions from the InGaN wells exhibit a linear dependence on the 1) spectral blue shift of /spl sim/8.5/spl times/10/sup -18/ meV /spl middot/ cm/sup 3/ and 2) change of the internal field of /spl sim/3/spl times/10/sup -14/ meV /spl middot/ cm/sup 2/ with the injected carrier density up to N/sub inj//spl sim/10/sup 19/ cm/sup -3/ at 77 K. These observations are attributed to the redistribution of photogenerated carriers in the InGaN wells due to the polarization discontinuity at the QW interface and the surface band bending effect. By incorporating an additional boundary condition of surface Fermi-level pinning into the Poisson equation and the band-structure analysis, it is shown the emission from the InGaN-GaN MQWs is dominant by the recombination between the high-lying subbands and the screening of internal field effects.     

High-speed InSb photodetectors on GaAs for mid-IR applications

We report p-i-n type InSb-based high-speed photodetectors grown on GaAs substrate. Electrical and optical properties of photodetectors with active areas ranging from 7.06/spl times/10/sup -6/ cm/sup 2/ to 2.25/spl times/10/sup -4/ cm/sup 2/ measured at 77 K and room temperature. Detectors had high zero-bias differential resistances, and the differential resistance area product was 4.5 /spl Omega/ cm/sup 2/. At 77 K, spectral measurements yielded high responsivity between 3 and 5 /spl mu/m with the cutoff wavelength of 5.33 /spl mu/m. The maximum responsivity for 80-/spl mu/m diameter detectors was 1.00/spl times/10/sup 5/ V/W at 4.35 /spl mu/m while the detectivity was 3.41/spl times/10/sup 9/ cm Hz/sup 1/2//W. High-speed measurements were done at room temperature. An optical parametric oscillator was used to generate picosecond full-width at half-maximum pulses at 2.5 /spl mu/m with the pump at 780 nm. 30-/spl mu/m diameter photodetectors yielded 3-dB bandwidth of 8.5 GHz at 2.5 V bias.     

II-VI semiconductors on InP for green-yellow emitters

Novel II-VI compound materials such as MgZnCdSe, BeZnCdSe, BeZnTe, and related superlattices grown on InP substrates have been investigated for yellow-green emitters employing molecular beam epitaxy. MgZnCdSe was grown in the Mg composition range of 0/spl sim/0.6 to clarify the compositional dependency of the bandgap and refractive index. MgSe-ZnCdSe and MgSe-ZnSeTe short-period superlattices were investigated; the superlattices behaved as quasi-quaternaries (QQs), so that their bandgap energies were controlled by the layer thickness combination of superlattices. For realizing strong lattice hardness, Be-contained H-VI compounds, such as BeZnCdSe and BeZnTe bulk crystals, and MgSe-BeZnCdSe, ZnCdSe-BeZnTe, and MgSe-BeZnTe short-period superlattices were investigated. The superlattices also behaved with QQ properties, by use of which multilayered heterostructures could be grown without growth interruption. Applying the superlattices, visible LEDs were fabricated emitting at the wavelengths from 554 (yellow-green) to 644 nm (red) at room temperature. For yellow (575 nm) LEDs, a long lifetime more than 3500 h was demonstrated even for defect densities as high as 10/sup 5/ cm/sup -2/. The BeZnTe buffers were effective in suppressing the defect density to less than 7 /spl times/ 10/sup 3/ cm/sup -3/. Finally, MgZnCdSe-based II-VI LDs were successfully operated with yellow-green lasing emissions around 560 nm at 77 K.     

InGaN-based ultraviolet emitting heterostructures with quaternary AlInGaN barriers

Nitride-based ultraviolet (UV) heterostructures with InGaN quantum wells and AlInGaN barrier layers have been grown by metal-organic chemical vapor deposition on sapphire substrates. The emission band was at 3.307 eV (375 nm) at room temperature (RT) and its full-width at half-maximum was /spl sim/82meV. In addition to the UV band, some blue emission admixture was found in a single-quantum-well (SQW) structure, which the authors attribute to recombination of injected electrons that are not captured into the SQW and enter the p-side of the structure. The authors demonstrate a significant advantage in utilizing multiple-quantum-well (MQW) structures that provide a more effective capture of injected carriers into wells and predominance of UV emission. Temperature-sensitive competition between two emission mechanisms in MQW structures has been observed. Below /spl sim/170 K, the blue impurity-related emission dominated. In the 170-190 K range, an anomalous temperature-induced "blue jump" by over /spl sim/340 meV to UV region occurred, with UV emission dominating above 190 K.     

Petawatt laser system and experiments

We have developed a hybrid Ti:sapphire-Nd:glass laser system that produces more than 1.5 PW of peak power. The system has produced up to 680 J of energy on target at 1054 nm in a compressed 440/spl plusmn/20-fs pulse by use of 94-cm diffraction gratings. A focused irradiance of up to 6/spl times/10/sup 20/ W/cm/sup 2/ was achieved using an on-axis parabolic mirror and adaptive optic wavefront control. Experiments with the petawatt laser system focused the beam on solid targets and produced a strongly relativistic interaction. Energy content, spectra, and angular pattern of the photon, electron, and ion radiations were diagnosed in a number of ways, including several nuclear activation techniques. Approximately 40-50% of the laser energy was converted to broadly beamed hot electrons, with an associated bremsstrahlung beam. High luminosity ion beams were observed normal to the rear surface of various targets with energies up to /spl ges/55 MeV, representing /spl sim/7% of the laser energy. These and other results are presented.     

Total reflection X-ray fluorescence spectrometry for the introduction of novel materials in clean-room production environments

In this paper, a number of case studies on the analysis of novel metallic contaminants on conventional and alternative substrates using the technique of total reflection X-ray fluorescence spectrometry (TXRF) is presented. Investigated materials include Si and Ge substrates, high-/spl kappa/ dielectric contaminants, and layers, and Si wafers contaminated with elements from metal gates and Cu interconnects. One focus is on the application and optimization of detection limits in direct TXRF. For the TXRF analysis of contaminants on Si wafers, a general conclusion is that a combination of three excitation sources is needed to cover the whole range of interest: a low-energy excitation (about 5 keV, e.g., WM/spl alpha/, Cr K/spl alpha/) for the low Z elements such as Na, Mg, and Al, a moderate-energy excitation (10-20 keV, e.g., WL/spl beta/, MoK/spl alpha/) for the 3d-transition elements, and a high-energy excitation (25-35 keV, e.g., W, continuum) for the analysis of elements such as Zr, Ru, Mo, and Pd. Also, for the analysis of novel substrates using direct TXRF, a careful selection of the excitation source results in better detection limits. In this way, detection limits at 10/sup 10/-10/sup 11/ at/cm/sup 2/ can be achieved, even for novel contaminants and substrates. As the International Technology Roadmap for Semiconductors (ITRS) requires control below 5/spl times/10/sup 9/ at/cm/sup 2/, the application of a preconcentration procedure such as vapor phase decomposition-droplet collection TXRF (VPD-DC-TXRF) is required. Proper use of this procedure allows the improvement of the detection limits by two to three orders of magnitude, depending on wafer size and chemical collection efficiency. The usability of this preconcentration procedure in combination with TXRF will be demonstrated for noble elements and germanium substrates.     

Negative thermal quenching in undoped ZnO and Ga-doped ZnO film grown on c-Al2O3 (0001) by plasma-assisted molecular beam epitaxy

Negative thermal quenching (NTQ) was observed in bound exciton emission line in undoped ZnO and the donor-to-valence-band emission in heavily Ga-doped ZnO thin films grown on c-Al₂O₃ (1000) through low temperature photoluminescence spectra. In both cases, the enhanced feature of PL peak intensity occurred in the temperature range of 35–45 K corresponding to the energies of either excitation to the vibrational/rotational resonance states or the involvement of B-valence band considering the activation energy of about 5 meV.     

Electron injection in "electron-only" devices based on a symmetric metal/silole/metal structure

We report on electron injection from two different metal electrodes into three silole derivatives, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PPSPP), 1,2-bis(1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl) ethane (2PSP) and 2,5-bis-(2', 2'-bipyridin-6-yl)-1, 1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), previously employed as emissive and electron transport materials in molecular organic light-emitting diodes (MOLEDs). Silole films were sandwiched between symmetric Mg:Ag or bilayer CsF-Al electrodes. The steady-state current density-voltage characteristics were measured as a function of the silole layer thickness for the two cathodes. The trap-free space-charge-limited current based on time-of-flight measurements compared with the injected electron current for PyPySPyPy indicated that Mg:Ag contacts limit the injected current, while CsF-Al contacts behave as quasi-ohmic contacts. Similar findings were obtained for 2PSP and PPSPP allowing steady-state derived electron mobility parameters to be extracted. Based on space-charge-limited conduction analysis of the measured current-voltage characteristics, PyPySPyPy is found to be a superior electron transporting silole with approximately an order of magnitude higher electron mobility (2.0/spl times/10/sup -4/ cm/sup 2//Vs) compared with those of 2PSP (2.4/spl times/10/sup -5/ cm/sup 2//Vs) and PPSPP (5.2/spl times/10/sup -5/ cm/sup 2//Vs), which is significantly higher than that of the prototype electron transport material tris (8-hydroxyquinolinolato) aluminum (III) (Alq/sub 3/) (6.5/spl times/10/sup -7/ cm/sup 2//Vs) at 0.6 MV/cm.     

High-power continuous-wave intracavity frequency-doubled Nd:GdVO/sub 4/-LBO laser under diode pumping into the emitting level

The continuous-wave high power laser emission of Nd:GdVO/sub 4/ at the fundamental wavelength of 1.06 /spl mu/m and its 531-nm second harmonic obtained by intracavity frequency doubling with an LBO nonlinear crystal is investigated under pumping by diode laser at 808 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 5/2/ transition) and 879 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 3/2/ transition). It is shown that, in spite of a lower absorption at 879 nm, the infrared emission is comparable under these two wavelengths of pump. The green emission performances were, however, improved by the 879 nm pump: 5.1 W at 531 nm with M/sup 2/=1.46 and 0.31 overall optical-to-optical efficiency was obtained from a 3-mm-thick 1-at.% Nd:GdVO/sub 4/ laser medium and a 10-mm-long LBO nonlinear crystal in a Z-type cavity for 16.5 W pump power. In similar conditions, the maximum green power for the 808 nm pump was 4.4 W, with 0.26 overall optical-to-optical efficiency and M/sup 2/=3.40 beam quality; at this pump wavelength the green emission shows evident saturation for pump power in excess of 9.9 W. This behavior is connected with the enhanced heat generation under 809-nm pumping, as evidenced by the increased thermal lensing of the fundamental emission. A careful alignment of the laser enables emission almost free of chaotic intensity fluctuations.     

Comparative study on the spectroscopic properties of Nd:GdVO/sub 4/ and Nd:YVO/sub 4/ with hybrid process

We have proposed a hybrid procedure for determining spectroscopic parameters for uniaxial solid-state laser crystals. Using our procedure, the spectroscopic properties of Nd:GdVO/sub 4/ were evaluated and compared to those of Nd:YVO/sub 4/. As a result, the peaks of absorption and stimulated emission cross sections of Nd:GdVO/sub 4/ in /spl pi/-polarization were determined to be 2.6 and 10.3/spl times/10/sup -19/ cm/sup 2/, respectively, and were smaller than those of Nd:YVO/sub 4/. On the other hand, the fluorescence lifetime of 1 at% Nd:GdVO/sub 4/ was evaluated to be 83.4 /spl mu/s, and was similar to 84.1 /spl mu/s of 1 at% Nd:YVO/sub 4/. Therefore, the product of stimulated emission cross section and fluorescence lifetime (/spl sigma//sub em//spl tau//sub f/ product) of Nd:GdVO/sub 4/ was smaller than that of Nd:YVO/sub 4/ under 1 at% of Nd/sup 3+/ doping concentration. The radiative lifetime of spontaneous emission of Nd:GdVO/sub 4/ was 168 /spl mu/s and was 1.9 times longer than that of Nd:YVO/sub 4/. Because of the low value of radiative quantum efficiency of Nd:GdVO/sub 4/ (50%), careful cavity design is required for creating a well performing solid-state laser with Nd:GdVO/sub 4/, based on the larger /spl sigma//sub em//spl tau//sub f/ product rather than the /spl sigma//sub em//spl tau//sub f/ product of Nd:YAG.     

UV laser diode with 350.9-nm-lasing wavelength grown by hetero-epitaxial-lateral overgrowth technology

We have demonstrated a UV-laser diode with a lasing wavelength of 350.9 nm, which has a GaN-AlGaN multiquantum-well (MQW) active layer and was grown on low-dislocation-density Al/sub 0.18/Ga/sub 0.82/N template. The Al/sub 0.18/Ga/sub 0.82/N template was produced by the hetero-epitaxial lateral overgrowth technology on the low-cost sapphire substrate, and has partially low-dislocation density of approximately 2/spl times/10/sup 7/ cm/sup -2/. The lasing operation under pulsed current injection was achieved with the threshold current density of 7.3 kA/cm/sup 2/ and the operating voltage of 10.4 V.     

Electron spin relaxation in solids-an application to spontaneously occurring radical centre in phenol-formaldehyde resins

An outline of electron spin-relaxation is briefly presented with mechanisms and processes of spin-lattice relaxation in ionic and amorphous solids. Electron spin resonance (EPR) and pulsed EPR measurements were performed in the temperature range 4-300 K on a radical occurring in a dried resol phenol-formaldehyde resin. The radical appears in a small concentration of about 1/spl times/10/sup 16/ radicals/g in 30/spl deg/C dried samples and the number of the radicals increases continuously with time up to 2/spl times/10/sup 17/ radicals/g after 100 days. Computer simulations of the EPR spectrum show that breaking of the dimethylene ether linkage probably forms the radical during synthesis reactions. Electron spin-lattice relaxation rate increases relatively slowly on heating and at <80 K is determined by interactions with two-level tunnelling systems in the polymer structure. For higher temperatures the relaxation is governed by interactions with centres having energy levels split by 298 cm/sup -1/. Phase relaxation is described by the phase memory time with rigid lattice limit T/sub M//sup 0/=2 /spl mu/s. This time is shortened above 100 K by thermally activated radical molecule motions with activation energy 85 cm/sup -1/. The electron spin echo envelope modulation (ESEEM) spectrum shows unresolved peaks at /sup 1/H indicating strong molecular dynamics and delocalisation of the unpaired electron.     

Characteristics of a saturated 18.9-nm tabletop laser operating at 5-Hz repetition rate

We report the characteristics of a saturated high-repetition rate Ni-like Mo laser at 18.9 nm. This table-top soft X-ray laser was pumped at a 5-Hz repetition rate by 8-ps 1-J optical laser pulses impinging at grazing incidence into a precreated Mo plasma. The variation of the laser output intensity as a function of the grazing incidence angle of the main pump beam is reported. The maximum laser output intensity was observed for an angle of 20/spl deg/, at which we measured a small signal gain of 65 cm/sup -1/ and a gain-length product g/spl times/l>15. Spatial coherence measurements resulting from a Young's double-slit interference experiment show the equivalent incoherent source diameter is about 11 /spl mu/m. The peak spectral brightness is estimated to be of the order of 1/spl times/10/sup 24/ photons s/sup -1/ mm/sup -2/ mrad/sup -2/ within 0.01% spectral bandwidth. This type of practical, small scale, high-repetition soft X-ray laser is of interest for many applications.     

Very low threshold current density of 1.3-/spl mu/m-range GaInNAsSb-GaNAs3 and 5 QWs lasers

The dependence of the threshold current density on the number of wells for 1.3-/spl mu/m-range edge emitting lasers using GaInNAsSb novel material, at which the incorporation of the small amount of Sb make the two-dimensional growth condition wide, is studied. The lowest record ever reported for the threshold current density per well (Jth A/cm/sup 2//well@L=900 /spl mu/m) for 3 QWs lasers was achieved. GaInNAs-based 5 QWs lasers with the very low threshold current density per well of 160 A/cm/sup 2/ were successfully grown for the first time. Therefore, no significant deterioration of Jth is observed even though the number of wells increased up to 5. Since Jth of 5 QWs doesn't increased rapidly compared to SQW and 3 QWs as decreasing the cavity length, it is considered that lower Jth can be obtained by utilizing 5 QWs in devices such VCSELs which use short cavity length.     

1.3-/spl mu/m-range GaInNAsSb-GaAs VCSELs

1.3-/spl mu/m-range GaInNAsSb vertical-cavity surface-emitting lasers (VCSELs) with the doped mirror were investigated. GaInNASb active layers that include a small amount of Sb can be easily grown in a two-dimensional manner as compared with GaInNAs due to the suppression of the formation of three-dimensional growth in MBE growth. The authors obtained the lowest J/sub th/ per well (150 A/cm/sup 2//well) for the edge-emission type lasers due to the high quality of GaInNAsSb quantum wells. Using this material for the active media, the authors accomplished the first continuous wave operation of 1.3-/spl mu/m-range GaInNAsSb VCSELs. For the reduction of the threshold voltage and the differential resistance, they used the doped mirror grown by metal-organic chemical vapor deposition (MOCVD). By three-step growth, they obtained 1.3-/spl mu/m GaInNAs-based VCSELs with the low threshold current density (3.6 kA/cm/sup 2/), the low threshold voltage (1.2 V), and the low differential resistance (60 /spl Omega/) simultaneously for the first time. The back-to-back transmission was carried out up to 5 Gb/s. Further, the uniform operation of 10-ch VCSEL array was demonstrated. The maximum output power of 1 mW was obtained at 20/spl deg/C by changing the reflectivity of the front distributed Bragg reflector mirror. GaInNAsSb VCSELs were demonstrated to be very promising material for realizing the 1.3-/spl mu/m signal light sources, and the usage of the doped mirror grown by MOCVD is the best way for 1.3-/spl mu/m VCSELs.     

The Study of Ferroelectric La-Doped PbTiO3 Thin Films Prepared by RF Magnetron Sputtering

Ferroelectric La-modified lead titanate (PLT) thin film were grown on Pt/Ti/SiO2/Si by sputtering the Pb_0.93La_0.07TiO₃ targets containing an amounts of excess 8% PbO. The effects of sputtering and annealing conditions on the crystalline structures and the surface morphologies of the PLT thin films have been investigated. The remanent polarization (Pr) and the coercive field of the PLT film through rapid thermal annealing (RTA) was 10.2 μC/cm² and 45 kV/cm respectively. The maximum pyroelectric coefficient reached 19 nC/cm².K at 20°C.     

The effects of composition and doping on the response of GeC-Siphotodiodes

The spectral responses of a series of heterojunction diodes of p-type Ge_1-yC_y on n-type Si (100) substrates were measured by Fourier transform infrared (IR) spectroscopy. Alloy layers 0.5 μm thick were grown by molecular beam epitaxy at a substrate temperature of 400°C and were doped p-type with different C concentrations. With increasing C content, the diode dark current decreased, and the optical absorption band edge shifted toward higher energy by 70 meV for 0.12 at% of C. The increase in energy was attributed to the composition dependence of the bandgap rather than to strain relaxation, because the GeC layers were nearly relaxed with the same strain. The photoresponsivity was 0.07 A/W at a wavelength of 1.55 μm, and 0.2 A/W at a wavelength of 1.3 μm. These measurements show that GeC photodetectors have good properties and reasonable response at technologically important near-IR wavelengths and can be fabricated by heteroepitaxy for compatibility with Si integrated circuits     

Conduction band-edge States associated with the removal of d-state degeneracies by the Jahn-Teller effect

X-ray absorption spectroscopy (XAS) is used to study band edge electronic structure of high-/spl kappa/ transition metal (TM) and trivalent lanthanide rare earth (RE) oxide gate dielectrics. The lowest conduction band d/sup */-states in TiO/sub 2/, ZrO/sub 2/ and HfO/sub 2/ are correlated with: 1) features in the O K/sub 1/ edge, and 2) transitions from occupied Ti 2p, Zr 3p and Hf 4p states to empty Ti 3d-, Zr 4d-, and Hf 5d-states, respectively. The relative energies of d-state features indicate that the respective optical bandgaps, E/sub opt/ (or equivalently, E/sub g/), and conduction band offset energy with respect to Si, E/sub B/, scale monotonically with the d-state energies of the TM/RE atoms. The multiplicity of d-state features in the Ti L/sub 2,3/ spectrum of TiO/sub 2/, and in the derivative of the O K/sub 1/ spectra for ZrO/sub 2/ and HfO/sub 2/ indicate a removal of d-state degeneracies that results from a static Jahn-Teller effect in these nanocrystalline thin film oxides. Similar removals of d-state degeneracies are demonstrated for complex TM/RE oxides including Zr and Hf titanates, and La, Gd and Dy scandates. Analysis of XAS and band edge spectra indicate an additional band edge state that is assigned Jahn-Teller distortions at internal grain boundaries. These band edges defect states are electronically active in photoconductivity (PC), internal photoemission (IPE), and act as bulk traps in metal oxide semiconductor (MOS) devices, contributing to asymmetries in tunneling and Frenkel-Poole transport that have important consequences for performance and reliability in advanced Si devices.     

Lasing at 1.28 /spl mu/m of InAs-GaAs quantum dots with AlGaAs cladding layer grown by metal-organic chemical vapor deposition

We report the device characteristics of stacked InAs-GaAs quantum dot (QD) lasers cladded by an Al/sub 0.4/Ga/sub 0.6/As layer grown at low temperature by metal-organic chemical vapor deposition. In the growth of quantum dot lasers, an emission wavelength shifts toward a shorter value due to the effect of postgrowth annealing on quantum dots. This blueshift can be suppressed when the annealing temperature is below 570/spl deg/C. We achieved 1.28-/spl mu/m continuous-wave lasing at room temperature of five layers stacked InAs-GaAs quantum dots embedded in an In/sub 0.13/Ga/sub 0.87/As strain-reducing layer whose p-cladding layer was grown at 560/spl deg/C. From the experiments and calculations of the gain spectra of fabricated quantum dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots. We also discuss the device characteristics of fabricated quantum dot lasers at various growth temperatures of the p-cladding layer.     

Integrated optical 2 /spl times/ 2 switch for wavelength multiplexed interconnects

A highly compact integrated optical switch is proposed and demonstrated for broadband optical switching applications. Routing of 8 /spl times/ 10 Gb/s data channels is demonstrated using a low-cost 1250-Mb/s control scheme. The advantages of lossless operation, broad optical bandwidth, and nanosecond switching times are leveraged. Multichannel wavelength is exploited for reduced latency, enhanced capacity, and functionality, while retaining compatibility with existing off-the-shelf electronics and transceiver technology. The requirements for optical header processing, wavelength translation, and optical buffering are avoided. Low-penalty multiwavelength transmission is demonstrated for a highly compact sub-mm/sup 2/ amplifying 2 /spl times/ 2 switch. Pattern dependent gain and amplified spontaneous emission are minimized to facilitate 0.0-0.4 dB penalty. Mitigation techniques compatible with the architecture are deployed to reduce the penalty under adverse operating conditions. Control schemes are proposed and demonstrated to facilitate 8 /spl times/ 10 Gb/s optically switched networking.