MBE grown type-II MWIR and LWIR superlattice photodiodes

We report on the status of GaSb/InAs type-II superlattice diodes grown and fabricated at the Jet Propulsion Laboratory designed for infrared absorption 2–5 μm and 8–12 μm bands. Recent LWIR devices have produced detectivities as high as 8 × 10¹⁰ Jones with a differential resistance–area product greater than 6 Ohm cm² at 80 K with a long wavelength cutoff of approximately 12 μm. The measured internal quantum efficiency of these front-side illuminated devices is close to 30% in the 10–11 μm range. MWIR devices have produced detectivities as high as 8 × 10¹³ Jones with a differential resistance–area product greater than 3 × 10⁷ Ohm cm² at 80 K with a long wavelength cutoff of approximately 3.7 μm. The measured internal quantum efficiency of these front-side illuminated MWIR devices is close to 40% in the 2–3 μm range at low temperature and increases to over 60% near room temperature.     

Structural and compositional analysis of InBixAsySb(1−x−y) films grown on GaAs(0 0 1) substrates by liquid phase epitaxy

The growth of epitaxial InBi_xAs_ySb_(1−x−y) layers on highly lattice mis-matched semi-insulating GaAs substrates has been successfully achieved via the traditional liquid phase epitaxy. Orientation and single crystalline nature of the film have been confirmed by X-ray diffraction. Scanning electron micrograph shows abrupt interface at micrometer resolution. Surface composition of Bi(x) and As(y) in the InBi_xAs_ySb_(1−x−y) film was measured using energy dispersive X-ray analysis and found to be 2.5 and 10.5 at.%, respectively, and was further confirmed with X-ray photoelectron spectroscopy. Variation of the composition with depth of the film was studied by removing the layers with low current (20 μA) Ar⁺ etching. It was observed that with successive Ar⁺ etching, In/Sb ratio remained the same, while the As/Sb and Bi/Sb ratios changed slightly with etching time. However after about 5 min etching the As/Sb and Bi/Sb ratios reached constant values. The room temperature band gap of InBi_0.025As_0.105Sb_0.870 was found to be in the range of 0.113–0.120 eV. The measured values of mobility and carrier density at room temperature are 3.1×10⁴ cm² V⁻¹ s⁻¹ and 8.07×10¹⁶ cm⁻³, respectively.     

Carrier relaxation in InGaAs

A modelocked Nd: YAG pumped optical parametric amplifier providing 35 ps pulses in the wavelength range λ = 1.45−2.1 μm has been used to study photoexcited carrier recombination in InGaAs. At carrier densities in excess of 10¹⁸ cm^-3 Auger processes are found to dominate the carrier recombination. An Auger rate of 2.5±0.5×10^-28 cm⁶ s^-1 is determined.     

Effects of substitution of Ti for Fe in BiFeO3 films prepared by sol–gel process

Ti substituted BiFe_1−xTi_xO_3+δ films have been prepared on indium–tin oxide (ITO)/glass substrates by the sol–gel process. The films with x=0.00–0.20 were prepared at an annealing temperature of 600 °C. X-ray diffraction patterns indicate that all films adopt R3m structure and the films with x=0 and 0.10 show pure perovskite phase. Cross-section scanning shows the thickness of the films is about 300 nm. Through 0.05 Ti substitution, the 2P_r increases to 8.30 μC/cm² from 2.12 μC/cm² of the un-substituted BiFeO₃ film and show enhanced ferroelectricity at room temperature. The 2P_r values are 2.63 and 0.44 μC/cm² for the films with x=0.01 and 0.2, respectively. Moreover, the films with x=0.05 and 0.10 show enhanced dielectric property since the permittivity increases near 150 at the same measuring frequency. Through the substitution of Ti, the leakage conduction is reduced for the films with x=0.05–0.20.     

Erbium in silicon–germanium quantum wells

Strained Si_1−xGe_x/Si quantum wells have been doped with erbium by implantation. A comparison is made with strained Si_1−xGe_x/Si quantum wells and relaxed Si_1−xGe_x, with x between 10% and 25%, doped with erbium during MBE growth. The erbium concentration was between 1×10¹⁸ and 5×10¹⁸ cm⁻³ throughout the active regions. Transmission electron microscopy, X-ray diffraction, and photoluminescence studies indicate that good regrowth can been achieved after full amorphisation by implantation of the strained quantum wells. The erbium luminescence is more intense in the Si_1−xGe_x/Si layers, but erbium-implanted samples containing Si_1−xGe_x exhibit defect luminescence in the region of 0.9–1.0 eV. These defects are also present when Si_1−xGe_x/Si quantum wells are implanted with an amorphising dose of silicon, and then regrown. They are attributed to small germanium-rich platelets, rather than to erbium-related defects. Electroluminescence is presented from a forward biased erbium-implanted Si_0.87Ge_0.13/Si structure at a drive current density of only 1.8 mA/cm².     

EXAFS analysis of Er sites in Er–O and Er–F co-doped crystalline Si

We present extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) analyses of Er–O and Er–F co-doped Si. Samples were prepared by multiple implants at 77 K of Er and co-dopant (O or F) ions resulting in the formation of a2 μm thick amorphous layer uniformly doped with 1×10¹⁹ Er/cm³ and 3×10¹⁹ O/cm³, 1×10²⁰ O/cm³ or 1×10²⁰ F/cm³. EXAFS measurements show that the local environment of the Er sites in the amorphous layers consists of 6 Si first neighbors. After epitaxial regrowth at 620°C for 3 h, Er is fully coordinated with 8 F ions in the Er–F samples, while Si and O ions are concomitantly present in the first shell of O co-doped samples. Post regrowth thermal treatments at 900°C leave the coordination unchanged in the Er+F, while the Er+O (ratio 1 : 10) doped samples present Er sites with a fully O coordinated shell with an average of 5 O atoms and 4 O atoms after 30 s and 12 h, respectively. We have also found that the fine structure and intensity of the high-resolution PL spectra are strongly dependent on the Er-impurity ratio and on thermal process parameters in the Er–O co-doped samples, while this is not observed for the F-doped samples. The most intense PL response at 15 K was obtained for the 1 : 3 E : O ratio, suggesting that an incomplete O shell around Er is particularly suitable for optical excitation.     

Determination of different oxygen transport mechanisms and measurement of the oxygen ion conductivity of Y1Ba2Cu3O7−x

By undertaking AC electrochemical impedance experiments on yttria stabilised zirconia electrolytes with polished Y₁Ba₂Cu₃O_7−x electrodes, the activation energy for oxygen ion transport within the bulk of Y₁Ba₂Cu₃O_7−x, in air, over the temperature range 823 K–1043 K, was determined to be 1.50 ± 0.05 eV. At 1000 K the oxygen ionic conductivity was calculated to be around one order of magnitude lower than that in yttria stabilised zirconia. Typical calculated values were σ=5×10⁻⁵ (ω cm)⁻¹ and 6×10⁻³ (ω cm)⁻¹ at the respective temperatures 823 K and 1043 K. By employing a similar cell but with Y₁Ba₂Cu₃O_7−x paste electrodes, oxygen transfer between the Y₁Ba₂Cu₃O_7−x and the electrolyte was found to occur via a surface diffusional processes. Over the temperature range 873 K–1098 K, in air, the activation energy for in-diffusion at the surface was found to be 1.4±0.1 eV and that for out-diffusion at the surface to be 1.76±0.05 eV.     

Fabrication of Y123 disk by the seeded infiltration and growth method

The seeded infiltration and growth process was used to prepare disks of Y123. This process produces a net-shape product with a uniform distribution of Y211 with a majority of the particles under 2 μm. The Y211 area fraction remained constant at about 27% for different cooling rates in the range 0.5–4°C/h. The critical current density at 77 K was determined to be in excess of 100 000 A/cm² under zero field and about 20 000 A/cm² under 1.5 T magnetic field, which indicates that this method is a good replacement for the conventional melt-texturing process.     

Effect of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge

Single-walled carbon nanotubes (SWCNTs) have been synthesized in high yield by the dc arc discharge technique under heat-pretreatment of the graphite rod conditions. Before executing arc discharge, the graphite rods containing the catalysts were heat treated at 600, 700, 800 and 900 °C for 1–3 h, respectively. Effects of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge were investigated. The heat-treatment temperature and time were found to be crucial for a high yield of high-purity SWCNTs. Optimum parameter was found to be at the heat-treatment temperature of 800 °C for 2 h. The SWCNTs synthesized under the optimum condition have better field-emission characteristics. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.9 V/μm and the threshold field where current density reaches 10 mA/cm² is 3.9 V/μm.     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Spin split-off transition based IR detectors operating at high temperatures

GaAs/AlGaAs based Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors were used to demonstrate experimental split-off response that is based on hole transitions between light/heavy hole bands and the split-off band (spin-orbit). Preliminary results indicate that, this detection mechanism is more efficient than free carrier mechanism for NIR operation. An unoptimized, GaAs/AlGaAs detector with a free carrier threshold wavelength of 20 μm showed a maximum operating temperature of 130 K for split-off response in the range 1.5–5 μm with a peak D^* of 1.0 × 10⁸ Jones. By adjusting the free carrier threshold to match the split-off threshold, it should be feasible to further increase the operating temperature. Analysis indicates that practical devices with properly optimized parameters are capable of achieving room temperature operation with higher specific detectivity. The possible ways to tailor the threshold, for the split-off response to different wavelength rangers using different materials such as phosphides and nitrites are also discussed.     

Modal dispersion phase-matched frequency doubling in composite planar waveguide using ion-exchanged glass and optically nonlinear poled polymer

We report on the fabrication and characterization of composite planar waveguides for frequency doubling. Modal dispersion phase matching is obtained for TM₀ fundamental and (TM₁, TM₂) harmonic wavelength. The experimental conversion efficiency was η=0.45% with an SH wavelength of 0.516 μm for TM₀^ω–TM₂^2ω mode conversion. The phase-matching condition can be fulfilled by adjusting the diffusion time of ion-exchange and the thickness of the polymer layer.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

EPR study of erbium-impurity complexes in silicon

Electron paramagnetic resonance (EPR) measurements have been used to characterise Er complexes formed in FZ silicon by the implantation of erbium together with either oxygen or fluorine. The samples have a 2 μm thick layer containing 10¹⁹ Er/cm³ alone or in addition 3×10¹⁹ O/cm³, 10²⁰ O/cm³ or 10²⁰ F/cm³. Various post-implantation anneals were carried out. Several different erbium centres, which have either C_1h monoclinic or trigonal symmetry, are observed and the way in which the type of centre depends on the implantation and annealing conditions is reported.     

Realization and properties of ramp-type YBa2Cu3O7−δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa₂Cu₃O_7−δ and Nb have been fabricated. This procedure allows connections between high-T_c and low-T_c superconductors at different crystal sides of the high-T_c superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa₂Cu₃O_7−δ to the Nb. Critical current densities up to 600 A/cm² are obtained at T=4.2 K, with typical R_nA values of 0.8 μΩ cm². The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa₂Cu₃O_7−δ crystal axes is explained by considering a predominant d_x²−y² order parameter symmetry of the YBa₂Cu₃O_7−δ. The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high-T_c superconductor.     

Increased surface roughness by oxygen plasma treatment of graphite/polymer composite

The technology of selective plasma etching was applied to increase the surface roughness of graphite/polymer composite. Etching was performed with a low pressure weakly ionised oxygen plasma created with a RF generator of the output power of 200 W and frequency of 27.12 MHz. The density of charged particles, density of neutral oxygen atoms and the electron temperature was about 1×10¹⁶ m⁻³, 4×10²¹ m⁻³, and 5 eV, respectively. The effects of plasma treatment were observed by scanning electron microscope (SEM), electron microprobe (EMPA) and Talysurf. It was found that the surface roughness was increased by approximately 15 times, from a virgin sample at the roughness of R_a=0.27 μm to a very rough surface with R_a=4 μm. The roughness increased with increasing plasma exposure time. The EMPA results showed that the amount of sulphur in the surface layer decreased with increasing etching time indicating that PPS polymer was the material etched preferentially.     

Dielectrics for passivation of planar InP/InGaAs diodes

We have investigated dielectrics for passivating planar InP or InGaAs photodiodes: thermally evaporated Al₂O₃ and SiO, sputtered Si₃N₄ and SiO₂ and also SiO₂ using chemical vapour deposition. The measured bulk and field-effect properties of all dielectrics excluding sputtered SiO₂ were suitable for this application. In planar InGaAs diodes with Cd diffused or Mg implanted p⁺-region a disordered dielectric/semiconductor surface led to high reverse current densities above 1 mA/cm². In InP diodes with p⁺-diffusion and dielectrics exhibiting positive flatband voltages, e.g. Si₃N₄ and Al₂O₃, reverse current densities of 10 μA/cm² were measured probably caused by a slight inversion of the semiconductor surface. With a SiO or CVD-SiO₂ passivating layer on n-InP lowest leakage current densities (10 nA/cm²) were achieved. Very low dark-current planar photodiodes InP/InGaAsP/InGaAs have been fabricated using SiO passivation (30 nA/cm²).     

InAs/GaAs quantum dot infrared photodetectors with different growth temperatures

InAs/GaAs quantum dot infrared photodetectors were fabricated with quantum dots grown at three different temperatures. Large detection wavelength shift (5–14.5 μm) was demonstrated by changing 40 degrees of the epitaxy temperature. The smaller quantum dots grown at lower temperature generate 14.5 μm responses. The detectivity of the normal incident 15 μm QDIP at 77 K is 3 × 10⁸ cm Hz^1/2/W. A three-color detector was also demonstrated with quantum dots grown at medium temperature. The three-color detection comes from two groups of different sizes of dots within one QD layer. This new type of multicolor detector shows unique temperature tuning behavior that was never reported before.     

The electrical characterization of Zn(Phen)q/p-type Si/Al diode with interfacial layer by current–voltage characteristics

The current–voltage characteristics of Zinc (II) [(8-hydroxyquinoline)(1,10-phenanthroline)] complex (Zn(phen)q)/p-type Si/Al diode with interfacial layer have been investigated. The barrier height and ideality factor of the diode were found to be 0.71 eV and 2.05. Zn(phen)q/p-type Si/Al diode shows a metal–insulator–semiconductor structure resulted from presence of series resistance and an interfacial layer. The n and φ_B values obtained the presence of interfacial layer are 1.02 and 0.70 eV, respectively. The effect of series resistance was evaluated using a method developed by Cheung. The R_s and n values were determined from the d ln(I)/dV plot and were found to be 30.43 kΩ and 2.16, respectively. The barrier height and R_s values were calculated from the H(I)–I plot and were found to be 0.70 eV and 30.99 kΩ. The density of the interface states of the Zn(phen)q/p-type Si/Al diode was calculated and was found to be an order of 10¹³ eV⁻¹ cm⁻².     

Domain wall pinning in polycrystalline perovskite La0.7Sr0.3MnO3

Reversible and irreversible domain wall (DW) motions have been investigated in La_0.7Sr_0.3MnO₃ ceramic samples using frequency-response complex permeability with various amplitudes of AC field. We also examine the effects of temperature in the range from 293 to 368 K and transverse DC magnetic field with a maximum of 4.40×10⁵ A/m on the real part of permeability (μ′). Two relaxations corresponding to reversible wall motions and domain rotations occur in low and high frequency regions, respectively. The irreversible DW displacements can be activated as the amplitude larger than the pinning field of 3 A/m, leading to an increase in μ′. The μ′ obeys a Rayleigh law at the temperature below 343 K or under DC field of less than 4.22×10⁴ A/m. The Rayleigh constant η increases from 5.45×10⁻² to 1.54×10⁻¹ (A/m)⁻¹ as the temperature rises from 293 to 343 K, and η decreases from 5.58×10⁻² to 3.67×10⁻² (A/m)⁻¹ with increasing DC field from 1.99×10³ to 4.22×10⁴ A/m.