Defect distribution near abraded surface of III–V compound semiconductors

Microstructures of damaged layers near the abraded surface of GaP, GaAs_0.6P_0.4 and GaAs single crystals have been observed with a transmission electron microscope. The damage due to 0.3 m Al₂O₃ abrasion consists of dislocations. Dislocation densities in the abrasion band and other regions are larger than 5×10¹¹ cm^–2 and about 10¹¹ cm^–2, respectively. Dislocation density decreases with increasing distance (depth) from the abraded surface. The depths of the damage layers for GaP, GaAs_0.6P_0.4 and GaAs are about 0.4, 0.55, and 0.8 m, respectively. The Burgers vector of dislocations is a/2[1 1 0], and A and B dislocations are observed. During annealing, the dislocations are rearranged and eliminated by the reaction of the dislocations with different Burgers vectors and climbing motion. The rearrangement temperatures for dislocations in GaP and GaAs are about 500 and 450° C, respectively. Electrical resistance changes in the damaged layer of GaP are recognized by electrical resistance measurements.     

Field emission of Co nanowires in polycarbonate template

The Co nanowire arrays were synthesized by electrodeposition in polycarbonate template (PC) with 4 μm thickness. Electron field emission properties of cobalt nanowires were studied for wires with different aspect ratios, R ranged between 10 and 60, while the diameter of wires was fixed about 50 nm. The field emission properties of the samples showed low turn on electric field (E_to) with values varying between 2.9 and 11.3 V/μm showing a minimum value for R = 20 (E_to < 3 V/μm). On the other hand, the enhancement factor shows a peak for nanowires length about 1 μm. Field emission data using the Fowler-Nordhiem theory showed nearly straight-line nature confirming cold field emission of electrons. The fabricated field emitter arrays of cobalt nanowires in the PC templates opens the possibility of fabricating flexible flat panel displays.     

Microstructure and electric properties of (104)/(014)-oriented Bi3.15Nd0.85Ti3O12 films on Pt (111)/Ti/SiO2/Si by sol-gel method

Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) thin films with predominant (104)/(014) orientation were fabricated directly on (111)Pt/Ti/SiO₂/Si substrates through a sol-gel process. The volume fraction of (104)/(014)-oriented grains in the film was estimated to be about 65% according to X-ray pole figure. The BNdT film is dense and uniform and consists of columnar grains penetrating the whole film thickness. The (104)/(014)-oriented BNdT film capacitors showed excellent ferroelectric properties with 2P_r = 46.4 μC/cm² and E_c ≈ 140 kV/cm. The films also exhibit excellent piezoelectric property, with high piezoelectric coefficient d₃₃ ≈ 17 pm/V.     

Characterization of Si nanowaveguide line edge roughness and its effect on light transmission

The manufacture of low-loss silicon-on-insulator (SOI) photonic wires for telecommunication wavelengths (∼1.55 μm) is a challenge. Side wall and line edge roughness (LER) are the dominant sources of scattering loss. In this work the characterization of Si nanowaveguide (NW) LER was performed and its effect on light transmission was theoretically analyzed and measured. NW structures with a width of 0.5 μm and height of 0.22 μm were prepared using different thickness of hard-mask and plasma etch process technologies on SOI platforms. LER determination was performed by critical-dimension scanning electron microscope (CD-SEM), conventional atomic force microscope (AFM) and 3D-AFM. The characterization of LER quality was carried out by monitoring of its RMS deviation σ, main frequencies and correlation length L_c. The effect of sampling length was evaluated. The data for σ from 0.8 to 6 nm and L_c from 20 to 50 nm were found to be dependent mostly on etch process technology. Theoretical simulation of scattering loss due to LER based on the 2D analytical model for planar waveguides was performed. Finally, the correlation between NW optical transmission (scattering) losses (TL) and LER was obtained and can be applied to future technology development. Especially for the case of σ ∼ 0.85 nm and L_c ∼ 20 nm the lowest value of TL ∼ 1.2 dB/cm was measured at a wavelength of 1.55 μm. Good qualitative agreement between calculated and measured losses due to LER was found.     

1.31 μm GaAs-based heterojunction p-i-n photodetectors using InGaAsNSb as the intrinsic layer grown by molecular beam epitaxy

GaAs-based double-heterojunction p-i-n photodetectors using In_zGa_1−zAs_1−x−yN_xSb_y in the i-layer are fabricated for the first time using the solid source molecular beam epitaxy growth method. A peak responsivity of ∼ 0.29 A/W, corresponding to quantum efficiencies (QE) of 38% is attained from the best p-i-n device between 0.9 and 1.1 μm. The dark current is ∼ 70 nA at a reverse bias of 2 V and the cutoff wavelength reaches ∼ 1.4 μm. The surfactant effect generated by the presence of Sb in this material allows thick high quality dilute nitride material growth. A Sb-free p-i-n device consisting of InGaAsN/GaAs is also fabricated to compare the device performance with the InGaAsNSb/GaAs p-i-n devices.     

Preparation of Magnéli phases of Ti27O52 and Ti6O11 films by laser chemical vapor deposition

Magnéli phases of Ti₂₇O₅₂ and Ti₆O₁₁ films were prepared by laser chemical vapor deposition using Ti(dpm)₂(O-i-Pr)₂ as a precursor. Ti₆O₁₁ film was obtained at a laser power (P_L) of 200 W and a deposition temperature (T_dep) of 1270 K. Ti₂₇O₅₂ film was obtained at P_L = 150 to 200 W and T_dep = 1120 to 1250 K. Ti₆O₁₁ and Ti₂₇O₅₂ films had a faceted texture about 2 μm in grain size and a columnar cross section. The deposition rate of Ti₂₇O₅₂ and Ti₆O₁₁ films were 90 and 70 μm h^− 1, respectively.     

Dielectric response of AlP by in-situ ellipsometry

We present and analyze pseudodielectric function data <ε> = <ε₁> + i<ε₂> of AlP from 0.75 to 5.05 eV. The sample is a 1.0 μm thick AlP film grown on (001) GaAs by molecular beam epitaxy (MBE). Spectroscopic ellipsometric data were obtained before removing the sample from the MBE chamber to avoid oxidation and related artifacts. Analysis of interference oscillations and corrections for overlayer effects with a multilayer parametric model yield the closest representation to date of the intrinsic bulk dielectric response ε of AlP. From this analysis we obtain the energies of the E₀′ and E₁ critical points of AlP.     

Magnetotransport and THz-Optical Investigations at Devices with HgTe Quantum Wells

We investigated the magnetoconductivity and the Terahertz (THz) photo-conductivity of devices with HgTe quantum wells embedded in barrier layers of HgCdTe. For the photoconductivity measurements, a THz laser system (p-Ge-Laser) is applied. This laser uses transitions between Landau levels of light holes in Ge and emits laser pulses in the wavelength range 120 μm < λ < 180 μm. The THz laser radiation is used in order to excite charge carriers over the Landau-gap. The response of the sample to the laser impulses is in 2D-samples in the quantum Hall (QH) regime measured in order to receive data of the relaxation of the charge carriers. In this presentation we present photoconduction measurements of the HgTe-quantum well in Hall bar, Corbino as well as combined Corbino-Hall bar geometry in the QH-regime. The material system HgTe/HgCdTe is characterized by a small effective mass (compared to GaAs, in our case 0.023 m ₀) and accordingly smaller magnetic fields for the appropriate cyclotron energy. Thus, this material combination provides the opportunity to make THz detectors using magnetic fields below 2 T for operation.     

Carrier dynamics in coalescence overgrowth of GaN nanocolumns

Coalescence overgrowth of pattern-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of metal organic chemical vapor deposition layers, overgrown on the columnar structure with varying diameters of columns. Nano-imprint lithography was applied to open circular holes of 250, 300, 450, and 600 nm diameter on the SiO₂ layer, deposited on the GaN layer on the c-plane sapphire template. After the growth of ~ 1 μm high GaN nanocolumns, the further coalescence conditions led to an overgrown layer ~2 μm thickness. Photoelectrical and optical properties of the overgrown layers and a reference sample were investigated by time-resolved picosecond transient grating and time-integrated photoluminescence. We note a 3-4 fold increase in carrier lifetime in the overgrown epilayers when the diameter of columns increased from 250 to 450 nm. This feature is a clear indication of an ~4-fold reduced defect density.     

Fixed-gain CMOS differential amplifiers with no external feedback for a wide temperature range

We present original CMOS amplifiers designed for the DC to 10 MHz frequency range and operating in the 70-380 K temperature range. Aimed applications concern readout circuitry to be associated with THz bolometric pixels (either high-T_c superconducting or uncooled semiconducting), which require accuracy, low noise and low power consumption. Two designs are described that both exhibit high fixed-gain (40 dB) in a feedback-free architecture, which is based on a new low-transconductance composite transistor for an accurate control of this gain. Both amplifiers have been realized in a regular 0.35 μm CMOS process and tested in the 4.2-380 K temperature range, exhibiting good agreement between designed and measured characteristics.     

Erbium-activated silica-zirconia planar waveguides prepared by sol-gel route

Er³⁺ doped (100 − x)SiO₂ − xZrO₂ planar waveguides were prepared by the sol-gel route, with x ranging from 10 up to 30 mol%. Multilayer films doped with 0.3 mol% Er³⁺ ions were deposited on fused quartz substrates by the dip-coating technique. The thickness and refractive index were measured by m-line spectroscopy at different wavelengths. The fabrication protocol was optimized in order to confine one propagating mode at 1.5 μm. Photoluminescence in the near and visible region indicated a crystalline local environment for the Er³⁺ ion.     

UNCD electron emitters using Si nanostructure as a template

The electron field emission (EFE) properties of silicon nanostructures (SiNSs) coated with ultra-nanocrystalline diamond (UNCD) were characterized. The SiNS, comprising cauliflower-like grainy structure and nanorods, was generated by reaction of a Si substrate with an Au film at 1000 °C, and used as templates to grow UNCD. The UNCD films were deposited by microwave plasma-enhanced chemical vapour deposition (MPECVD) using methane and argon as reaction gases. The UNCD films can be grown on the SiNS with or without ultrasonication pretreatment with diamond particles. The EFE properties of the SiNS were improved by adding an UNCD film. The turn-on field (E₀) decreased from 17.6 V/μm for the SiNS to 15.2 V/μm for the UNCD/SiNS, and the emission current density increased from 0.095 to 3.8 mA/cm² at an electric field of 40 V/μm. Ultrasonication pretreatments of SiNS with diamond particles varied the structure and EFE properties of the UNCD/SiNS. It is shown that the ultrasonication pretreatment degraded the field emission properties of the UNCD/SiNS in this study.     

Synthesis and structures of chloride thiosilicates with lanthanides Ln3Cl[SiS4]2 (Ln = La, Ce, Pr)

Single crystals of Ln₃Cl[SiS₄]₂ (Ln=La, Ce, Pr) were prepared by the reaction of lanthanide metal, sulfur, silicon and chlorine. Data collection was carried out using a STOE imaging plate detector at 293 K. The homologue compounds crystallize in the space group C2/c of the monoclinic system isotypically to Ln₃X[SiS₄]₂ (X=Br, I) and the A-type of the halide oxosilicates Ln₃X[SiO₄]₂ (X=Cl, Br; Ln=La, Ce, Pr) with four formula units in cells of dimensions:

•

La₃Cl[SiS₄]₂: a=1567.2(3) pm, b=777.8(2) pm, c=1101.5(2) pm, β=96.88(2)°

•

Ce₃Cl[SiS₄]₂: a=1559.4(3) pm, b=770.2(2) pm, c=1096.9(2) pm, β=97.07(2)°

•

Pr₃Cl[SiS₄]₂: a=1555.9(3) pm, b=764.2(1) pm, c=1093.2(2) pm, β=97.40(2)°

The corresponding residuals (all data) for the refined structures are 2.28% (La), 2.15% (Ce), and 3.17% (Pr), respectively. In the crystal structures, the chloride ions form chains along [0 0 1] with trigonal coordination by the lanthanide ions.     

Electrical characterisation of p-doped distributed Bragg reflectors in electrically pumped GaInNAs VCSOAs for 1.3 μm operation

The high resistivity that is encountered in p-type DBRs is an important problem in vertical cavity surface emitting lasers and optical amplifiers (VCSELs and VCSOAs). This is because the formation of potential barriers at the interfaces between layers of high and low refractive index inhibits the carrier flow, thus increasing the DBR series resistance. In this work, the electrical characteristics of two p-type doped DBR structures grown on undoped and p-type doped GaAs substrates have been investigated. The DBRs are designed for VCSOAs operating at 1.3 μm and consist of 14-periods of alternating GaAs and Al_0.9Ga_0.1As in the first sample and 14-periods of GaAs and Al_0.3Ga_0.7As/Al_0.9Ga_0.1As in the second one. For the longitudinal transport sample, Hall mobility and sheet carrier density were measured in the temperature range from 77 to 300 K. In the vertical transport sample, current–voltage (I–V) measurements across the DBR layers were carried out at different temperatures in the range between 15 and 300 K. We achieved resistivity reduction in our samples by using an interface composition grading technique aimed at improving the VCSOA characteristics.     

Influence of the contacting scheme in simulations of radial silicon nanorod solar cells

Silicon nanorod solar cells were simulated using the Silvaco Technical Computer Aided Design (TCAD) software suite. For reasons of speed optimization the simulations were performed in cylinder coordinates taking advantage of the model's symmetry. Symmetric doping was assumed with a dopant density of 10¹⁸ cm⁻³ in the p-type core and in the n-type shell, and the location of the pn-junction was chosen such that the space charge region was located adjacent to the shell surface. Two contact configurations were explored. In configuration A the cathode contact was wrapped around the semiconductor nanorod, while in configuration B the cathode was assumed just on top of the nanorod. In both cases the anode was located at the bottom of the rod. Cell efficiency was optimized with regard to rod radius and rod length. Optimization was performed in a three-step procedure consisting in radius optimization, length optimization and again radius optimization. A maximum in efficiency with respect to rod length L was visible in configuration A, leading to an optimum value of L = 48 μm. This maximum is explained by the combination of an increase of short-circuit current density J_sc and a decrease of open-circuit voltage U_oc with L. In configuration B, J_sc also increases with L, but U_oc stays rather constant and the maximum in efficiency only appears at very large values of L ≈ 12 mm. We restricted the rod length to L ≤ 100 μm for further optimization, in order to stay in an experimentally feasible range. During the optimization of rod radius R in configuration A the open circuit voltage increased continuously, while short circuit current density stayed rather constant. This leads to an increase in efficiency with R, which only stops at very large radii, where R starts to be comparable with L. In configuration B efficiency is almost independent of R, provided that the radius is large enough to comprise a well-formed space charge region, here only a shallow maximum can be estimated. With the demand of rod lengths being smaller than 100 μm, optimum parameters L = 48 μm, R = 32 μm and L = 96 μm, R = 2 μm were extracted for configuration A and B, respectively.     

Long-range order of cylinders in diblock copolymer thin films using graphoepitaxy

Topographically patterned substrates are known to induce long-range lateral order in spherical diblock copolymers, but it is not clear that similar confinement will also order cylindrical diblock copolymers across the whole surface. The role of graphoepitaxial parameters including trough width and mesa height on the ordering process of cylindrical domains in diblock copolymers thin films is monitored in this study. The quantification of order was achieved by the calculation of an order parameter of the hexagonally packed cylinders. These results demonstrate that graphoepitaxy is an effective method to induce long-range order in cylindrical domain diblock copolymer systems. An increase in order was observed in samples prepared on the mesas and in troughs of widths up to 20 μm, and mesa heights greater than 1.0 but less than 5.0 L₀ The role of molecular weight on the kinetics of the ordering process of cylindrical domains in diblock copolymers thin films is also monitored in this study, where ordering is readily observed for lower molecular weight copolymers (number average molecular weight, M_n = 63,000), but not for larger copolymers (M_n = 230,000). The reduction of the rate of formation of long-range order is attributed to the impeded diffusion of higher molecular weight polymers. These results demonstrate that there will exist upper limits on the molecular weights of diblock copolymers that can be used to create nanoscale templates with long-range order, which also translates to an upper limit in pore size and spacing in these templates.     

Synthesis, characterization and magnetic properties of hexagonal (VO)0.09V0.18Mo0.82O3 · 0.54H2O microrods

(VO)_0.09V_0.18Mo_0.82O₃ · 0.54H₂O microrods have been synthesized for the first time via a hydrothermal treatment of aqueous peroxomolybdic acid and vanadyl sulfate. The compound crystallizes in hexagonal rods with space group P6₃, and lattice constants a = 10.586 Å, and c = 3.698 Å. The single crystalline rods exhibit diameters of 1-2 μm and lengths up to 45 μm. A variety of techniques, including X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscope, Fourier-transform infrared spectroscopy, differential scanning calorimetry and static magnetometry were used to characterize the product.     

Reactively sputtered GaAsxN1−x thin films

Thin films of GaAs_xN_1−x alloys were deposited by reactive rf magnetron sputtering of GaAs target with a mixture of argon and nitrogen as the sputtering gas. Growth rate was found to decrease from ∼ 7 μm/h to ∼ 2 μm/h as the nitrogen content increased from 0% to 40%. XRD and TEM studies of the films reveal the presence of hexagonal GaN with a significant increase of the lattice parameters in a narrow range of composition of the sputtering gas (5-10% nitrogen), which is attributed to the incorporation of arsenic. The limited availability of nitrogen in the sputtering atmosphere is found to encourage the incorporation of arsenic in the alloy films. Optical absorption coefficient spectra of the films were obtained from reflection and transmission data. The effect of arsenic incorporation is seen in the optical absorption spectra of the films, which show a continuous shift of the absorption edge to lower energies with respect to that of gallium nitride.     

Size-dependent field-emission characteristics of ZnO nanowires grown by porous anodic aluminum oxide templates assistance

The field-emission electrical properties of hydrothermally synthesized zinc-oxide nanowires grown on SiO₂-Si substrate are reported. Vertically aligned single-crystalline emitters with distinct length L and diameter D are realized by controlling the assisted growth of porous anodic aluminum oxide (AAO) templates. Field-emission measurement revealed that these field emitters exhibited controllable turn-on field E_to and the field enhancement factor β. Sample with feature size of L = 500 nm and D = 80 nm was prepared and then used as a basis for examining the size effect. Lower E_to and higher β were observed consistently for increasing the aspect ratio L/D. The enhanced properties of E_to = 1.48 V/μm and β up to 6100 are achieved for L/D = 53 (L = 1600 nm and D = 30 nm). Optimal characterizing parameters of E_to and β will be reached while pursuing extreme L/D practically. Factors such as the geometric limit of AAO template manufacture and the filling efficiency of ZnO into AAO pores will exert influence on the size-dependent effect.     

Improved critical current densities in thick YBa2Cu3O7 − δ multilayer films interspaced with non-superconducting YBa2Cu3Ox nanodots

We have fabricated and studied quasi-multilayered thick YBa₂Cu₃O_7 − δ (YBCO) films composed of several YBCO layers interspaced with quasi-layers of non-superconducting YBa₂Cu₃O_x nanodots, grown by Pulsed Laser Deposition on SrTiO₃ (100) substrates. Magnetization J_c(B) at 77.3 K for these thick films showed significant improvement as compared to pure YBa₂Cu₃O_7 − δ films of same or even smaller thickness. A high J_c(B) in our quite thick films (1 μm to 6 μm) provides a very high total critical current per centimetre of the film width, I_c − w. Critical current as high as 830 A per cm width in self field and 77.3 K was achieved in 5 μm thick quasi-multilayer film with non-superconducting YBa₂Cu₃O_x nanodots. Frequency-dependent susceptibility measurements showed also an increase in the pinning potential. The angular dependence of I_c − w at 86.5 K, in 3 T shows a clear indication of anisotropic pinning centres aligned along the c-direction.