Showing papers in "Semiconductor Science and Technology in 2008"

Nonlinear optical properties of silicon waveguides

Abstract: Recent work on two-photon absorption (TPA), stimulated Raman scattering (SRS) and optical Kerr effect in silicon-on-insulator (SOI) waveguides is reviewed and some potential applications of these optical nonlinearities, including silicon-based autocorrelation detectors, optical amplifiers, high speed optical switches, optical wavelength converters and self-phase modulation (SPM), are highlighted. The importance of free carriers generated by TPA in nonlinear devices is discussed, and a generalized definition of the nonlinear effective length to cater for nonlinear losses is proposed. How carrier lifetime engineering, and in particular the use of helium ion implantation, can enhance the nonlinear effective length for nonlinear devices is also discussed.

Bipolar switching behavior in TiN/ZnO/Pt resistive nonvolatile memory with fast switching and long retention

Abstract: Highly stable bipolar resistive switching behaviors of TiN/ZnO/Pt devices were demonstrated for the first time. The excellent memory characteristics including fast switching speed (<20 ns for set and <60 ns for reset), long retention (in the order of 105 s) and non-electroforming process were demonstrated. The bipolar switching behaviors can be explained by formation and rupture of the filamentary conductive path consisting of oxygen vacancies. The excellent bipolar switching behavior can be attributed to the significant amount of oxygen vacancies in ZnO film and the effect of TiN layer serving as an oxygen reservoir.

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Abstract: Ga-doped ZnO (GZO) transparent conducting films and well-aligned ZnO nanotips were sequentially grown on a glass substrate using metal–organic chemical vapor deposition (MOCVD). The morphology control of ZnO from dense films to nanotips was realized through temperature-modulated growth. The ZnO nanotips/GZO structure was sensitized with dye N719 to form photoelectrochemical cells. It is found that the power conversion efficiency linearly increases with the length of ZnO nanotips. For the 1.0 cm2 dye-sensitized solar cell built from 4.8 µm ZnO nanotips, a peak incident photo-to-current conversion efficiency of 79% (at ~530 nm) and a power conversion efficiency of 0.77% under the illumination of one sun-simulated sunlight were achieved. UV light harvesting directly by ZnO was observed. The I–V characteristics of the cells were analyzed using a one-diode equivalent circuit model.

Effect of post-deposition annealing on the growth of Cu2ZnSnSe4 thin films for a solar cell absorber layer

Abstract: The effect of substrate temperature and post-deposition annealing on the growth and properties of Cu2ZnSnSe4 thin films, a potential candidate for a solar cell absorber layer, is investigated. The substrate temperature (Ts) is chosen to be in the range 523–673 K and the annealing temperature (Tpa) is kept at 723 K. Powder x-ray diffraction (XRD) patterns of as-deposited films revealed that the films deposited at Ts = 523 K and 573 K contain Cu2−xSe as a secondary phase. Single phase, polycrystalline Cu2ZnSnSe4 films are obtained at Ts = 623 K and films deposited at Ts = 673 K have ZnSe as a secondary phase along with Cu2ZnSnSe4. Direct band gap of as-deposited CZTSe films is found to lie between 1.40 eV and 1.65 eV depending on Ts. XRD patterns of post-deposition annealed films revealed that the films deposited at Ts = 523–623 K are single phase CZTSe and films deposited at Ts = 673 K still contain ZnSe secondary phase. CZTSe films are found to exhibit kesterite structure with the lattice parameters a = 0.568 nm and c = 1.136 nm. Optical absorption studies of post-deposition annealed films show that there is a slight increase in the band gap on annealing, due to decrease in the Cu content. Electrical resistivity of the films is found to lie in the range 0.02–2.6 Ω cm depending on Ts.

Strong exciton-photon coupling in semiconductor quantum dot systems

Abstract: An overview is given on strong coupling phenomena in semiconductor quantum dot systems by utilizing cavity-enhanced light–matter interaction. The basic theory on strong coupling, the quantum dot and cavity fabrication technologies are reviewed while mainly three approaches are highlighted, i.e., micropillar, photonic crystal and microdisc cavities. The first and recent strong coupling experiments and the impact for future work are discussed.

Cu-doped ZnO-based p–n hetero-junction light emitting diode

Abstract: Copper-doped p-ZnO thin films (Cu:ZnO) were grown on α-Al2O3(0 0 0 1) and 6H:SiC(0 0 0 1) single crystal substrates by plasma-assisted molecular beam epitaxy. A p–n hetero-junction with p-Cu:ZnO/n-6H:SiC was successfully fabricated and demonstrated as a greenish-blue light emitting diode (LED). The rectifying I–V curve along with the matching photoluminescence and electroluminescence emissions characterizes the fabricated p–n hetero-junction LED. The Cu cell temperature (TCu) and the post-deposition annealing environment greatly influence the Cu oxidation state, and hence the electrical conversion from n-type to p-type and carrier concentration in the films. The higher TCu and post-annealing in O-plasma were observed to be the favorable conditions for Cu2+ and hence the p-type nature of the films.

Magneto-gyrotropic effects in semiconductor quantum wells

Abstract: Magneto-gyrotropic photogalvanic effects in quantum wells are reviewed. We discuss experimental data, results of phenomenological analysis and microscopic models of these effects. The current flow is driven by spin-dependent scattering in low-dimensional structure gyrotropic media resulting in asymmetry of photoexcitation and relaxation processes. Several applications of the effects are also considered.

Barrier inhomogeneities of tungsten Schottky diodes on 4H-SiC

Abstract: Electrical properties of tungsten on silicon carbide (4H-SiC) Schottky diodes are investigated through the analysis of the forward current–voltage (I–V) characteristics measured at elevated temperatures within the range of 303–448 K. The subsequently derived Schottky barrier heights (SBHs) and ideality factors are found to be temperature dependent with distributions that are adequately explained within the framework of the model proposed by Tung in which he considers the barrier at a metal–semiconductor interface as consisting of locally non-uniform but interacting patches of different barrier heights embedded in a background of uniform barrier height. A uniform barrier height of 1.248 eV, a Richardson's constant of 129.95 A cm−2 K2 and a factor To of 23.92 K obtained agree very well with values published previously for similar Schottky barrier systems. Therefore, it has been concluded that the temperature-dependent I–V characteristics of the device can be successfully explained with lateral inhomogeneities distribution of the SBH.

The influence of series resistance and interface states on intersecting behavior of I–V characteristics of Al/TiO2/p-Si (MIS) structures at low temperatures

Abstract: In this study, we have investigated the intersection behavior of the forward bias current–voltage (I–V) characteristics of the Al/TiO2/p-Si (MIS) structures in the temperature range of 100–300 K. The intersection behavior of the I–V curves appears as an abnormality when compared to the conventional behavior of ideal Schottky diodes and MIS structures. This behavior is attributed to the lack of free charge at a low temperature and in the temperature region, where there is no carrier freezing out, which is non-negligible at low temperatures, in particular. The values calculated from the temperature-dependent forward bias I–V data exhibit unusual behavior, where the zero-bias barrier height (b0) and the series resistance (Rs) increase with increasing temperature. Such temperature dependence of b0 and Rs is in obvious disagreement with the reported negative temperature coefficient. An apparent increase in the ideality factor (n) and a decrease in the b0 at low temperatures can be attributed to the inhomogeneities of the barrier height, the thickness of the insulator layer and non-uniformity of the interfacial charges. The temperature dependence of the experimental I–V data of the Al/TiO2/p-Si (MIS) structures has revealed the existence of a double Gaussian distribution with mean barrier height values () of 1.108 eV and 0.649 eV, and standard deviations (σs) of 0.137 V and 0.077 V, respectively. Furthermore, the temperature dependence of the energy distribution of interface state density (Nss) profiles has been determined from forward bias I–V measurements by taking into account the bias dependence of the effective barrier height (e) and n. The fact that the values of Nss increase with increasing temperature has been attributed to the molecular restructuring and reordering at the metal/semiconductor interface under the effect of temperature.

Non-polar a-plane ZnMgO/ZnO quantum wells grown by molecular beam epitaxy

Abstract: Non-polar (Zn, Mg)O/ZnO quantum wells (QWs) have been grown on a r-plane sapphire by molecular beam epitaxy. The heterostructures are fully oriented and show a single wurtzite phase at least up to 40% Mg content, as evidenced by means of the x-ray pole figures analysis. The microstructure is dominated by stacking faults and related partial dislocations as shown by the transmission electron microscopy analysis. A series of QWs with different widths has then been studied, showing the absence of the quantum confined Stark effect. The photoluminescence energies of the QWs are satisfactorily simulated when taking into account the variation of the exciton binding energy with the QW width. Different approaches for the calculation of the QW exciton ground state energies are proposed and compared.

Spin relaxation of localized electrons in n-type semiconductors

Abstract: The mechanisms that determine spin relaxation times of localized electrons in impurity bands of n-type semiconductors are considered theoretically and compared with available experimental data. The relaxation time of the non-equilibrium angular momentum is shown to be limited either by hyperfine interaction, or by spin–orbit interaction in the course of exchange-induced spin diffusion. The energy relaxation time in the spin system is governed by phonon-assisted hops within pairs of donors with an optimal distance of about 4 Bohr radii. The spin correlation time of the donor-bound electron is determined either by exchange interaction with other localized electrons, or by spin-flip scattering of free conduction-band electrons. A possibility of optical cooling of the spin system of localized electrons is discussed.

Growth and characterization of free-standing zinc-blende (cubic) GaN layers and substrates

Abstract: In this paper, we describe bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy. We have grown GaN layers of up to 60 ?m in thickness. We present the data from characterization measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the material that is hexagonal in nature is not more than about 10% in the best thick samples. Cubic (0?0?1) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0?0?0?1) c-axis wurtzite GaN. Therefore, the free-standing GaN wafers we have grown would make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high-power and high-frequency electronic applications.

Spin orientation of holes in quantum wells

Abstract: This paper reviews the spin orientation of spin-3/2 holes in quantum wells. We discuss the Zeeman and Rashba spin splitting in hole systems that are qualitatively different from their counterparts in electron systems. We show how a systematic understanding of the unusual spin-dependent phenomena in hole systems can be gained using a multipole expansion of the spin density matrix. As an example we discuss spin precession in hole systems that can give rise to an alternating spin polarization. Finally, we discuss the qualitatively different regimes of hole spin polarization decay in clean and dirty samples.

Recent development in a high-speed silicon optical modulator based on reverse-biased pn diode in a silicon waveguide

Abstract: We review the recent development of a high-speed silicon optical modulator based on electric-field-induced carrier depletion effect in a silicon-on-insulator waveguide containing a reverse-biased p–n junction. The device design, fabrication and characterization are presented. To obtain efficient optical modulation, we design a sub-micrometer size silicon waveguide phase shifter based on both semiconductor device modeling and photonic circuit modeling. By employing traveling-wave drive that allows co-propagation of electrical and optical signals along the waveguide, we demonstrate a high-frequency modulator with 3 dB optical response bandwidth of 30 GHz and data transmission up to 40 Gb s−1. Such a high-speed silicon modulator will be a key component for silicon-photonic-integrated circuits for future computing I/O applications.

Hot-electron energy relaxation time in AlInN/AlN/GaN 2DEG channels

Abstract: A microwave noise technique has been used for experimental investigation, at room temperature, of power dissipation in the voltage-biased two-dimensional electron gas channel located in the GaN layer of a lattice-matched Al0.82In0.18N/AlN/GaN heterostructure. No saturation of the relaxation time is found in the investigated electron temperature range up to ~2800 K: the hot-electron energy relaxation time decreases from ~6 ps at near equilibrium to 75 ± 20 fs at ~200 nW/electron. The electron drift velocity reaches ~1.8 × 107 cm s−1 at 65 kV cm−1 electric field. The hot-phonon effect on power dissipation is discussed.

Gunn oscillations in planar heterostructure diodes

Abstract: Gunn oscillations have been observed and modelled, using a Monte Carlo method, in planar semiconductor GaAs/AlGaAs heterostructure diodes. Our simulation results support an interpretation of experimental results whereby the Gunn domains travel parallel to the semiconductor layers, as opposed to perpendicular to the layers in traditional vertical devices. Fabricated devices with contact separations of 4 µm down to 1.3 µm have been found to oscillate over a range of frequencies from 24.5 GHz to 108 GHz. These structures offer the prospect of generating frequencies further into the terahertz range and an increased ease of integration and flexibility over equivalent traditional vertical structures.

Properties of gas source molecular beam epitaxy grown wavelength extended InGaAs photodetector structures on a linear graded InAlAs buffer

Abstract: The properties of gas source molecular beam epitaxy grown wavelength extended (2.4 µm) InGaAs photodetector structures on a linear graded InAlAs buffer with different grading rates have been investigated by means of XRD and PL techniques in conjunction with optical and atomic force microscopy. Results show that full relaxation and favorable optical characteristics of the active layers only occur for the wafers with a mismatch grading rate of about 1.2% µm−1 or lower, whereas moderate morphology and structural quality could be achieved for a mismatch grading rate up to 2.4% µm−1. A thin GSMBE grown linear graded InAlAs buffer layer of 1.4 µm is sufficient to relax the strain of an InGaAs layer with 1.7% mismatch to the InP substrate and reach a good quality of the wafer. The relaxation mechanisms of the buffer at different grading rates were also discussed.

Effect of body doping on double-gate MOSFET characteristics

Abstract: This paper presents a systematic study of the body doping effect on symmetric double-gate (DG) MOSFETs. Two-dimensional simulation tools are used to investigate the doping effect on long-channel and short-channel devices. Both n-type and p-type doping are studied. For long-channel devices, the threshold voltage shift due to body doping is proportional to the total dopant when the device is fully depleted. Thus, it is straightforward to include the doping effect in compact models. When the device is partially depleted, in addition to the threshold voltage shift, the subthreshold slope can be degraded. For short-channel devices, improvement of the short-channel effects is observed when the body doping concentration is close to the level that makes the device partially depleted.

A 1.33 µm InAs/GaAs quantum dot laser with a 46 cm−1 modal gain

Abstract: We report on 1.33 µm quantum dot (QD) lasers grown on GaAs substrates that show a modal gain of 45 cm−1, low threshold current density of 150 A cm−2 and room-temperature continuous wave output power of 2.5 W. The active region is based on ten InAs/InGaAs/GaAs quantum dot layers formed by activated phase separation. High structural quality of the active region is achieved, owing to minimization of the total amount of strained material per QD layer. The optical confinement factor is increased by exploiting high Al composition (80%) in the cladding layers. A modal gain over 20 cm−1 in the 1315–1345 nm wavelength range is revealed by the Hakki–Paoli technique at a low current density of 500 A cm−2.

Photoconductivity of self-assembled ZnO nanoparticles synthesized by organometallic chemistry

Abstract: We report on the photoconductivity properties of devices based on ZnO nanoparticles for UV detection. The nanoparticles were synthesized by organometallic chemistry, self-assembled on Si substrates covered by inter-digited metallic electrodes and annealed at 200 °C. The photoconductivity of these devices was measured for wavelengths ranging from 300 to 600 nm. The sensitivity of our samples at 350 nm excitation wavelength is about 1 A cm2 W−1, or 1200 A W−1, with a visible rejection of 150. The photoconductivity strongly depends on pressure and is one order of magnitude larger under vacuum than at atmospheric pressure. Transient photoconductivity characteristics at atmospheric pressure and under vacuum are fitted using stretched exponential functions and quantitatively analysed. These results show the potential use of chemically prepared ZnO nanoparticles for UV and/or gas detection.

Fabrication of SnS thin films by the successive ionic layer adsorption and reaction (SILAR) method

Abstract: Tin sulfide films of 0.20 µm thickness were grown on glass and ITO substrates by the successive ionic layer adsorption and reaction (SILAR) method using SnSO4 and Na2S solution. The as-grown films were well covered and strongly adherent to the substrate. XRD confirmed the deposition of SnS thin films and provided information on the crystallite size and residual strain of the thin films. FESEM revealed almost equal distribution of the particle size well covered on the surface of the substrate. EDX showed that as-grown SnS films were slightly rich in tin component. High absorption in the visible region was evident from UV–Vis transmission spectra. PL studies were carried out with 550 nm photon excitation. To the best of our knowledge, however, no attempt has been made to fabricate a SnS thin film using the SILAR technique.

A silicon compatible resonant cavity enhanced photodetector working at 1.55 µm

Abstract: In this paper, the design of a novel photodetector at 1.55 µm, working at room temperature and completely silicon compatible, is reported. The device is a resonant cavity enhanced (RCE) structure incorporating a silicon photodetector based on the internal photoemission effect. In order to quantify the performance of photodetector, quantum efficiency including the image force effect, bandwidth and dark current as a function of bias voltage is numerically calculated. A comparison among three different Schottky barrier silicon photodetectors, having as metal layers gold, silver or copper respectively, is proposed. The highest efficiency (0.2%) but also the highest dark current is obtained with metal having the lowest barrier, while for all devices, values of the order of 100 GHz and 100 MHz were obtained, respectively, for the carrier transit time limited 3 dB bandwidth and bandwidth efficiency.

Effects of substrate temperature on the optical and electrical properties of Al:ZnO films

Abstract: Al-doped ZnO films were grown on glass substrates by the pulsed-laser deposition technique with varying substrate temperatures. The optical band gap decreases from 3.64 to 3.46 eV as the substrate temperature increases from 350 to 450 °C, illustrating the increase in Al content in the context of a degenerate semiconductor, and can be explained in the framework of the Burstein–Moss effect. All films show optical transparency greater than 85%. Al:ZnO films show a metal–semiconductor transition to metal-like behavior as the substrate temperature increases from 350 to 450 °C. The observed metal-like and metal–semiconductor transitions are explained by taking into account the Mott phase transition and localization effects due to defects. The resistivity decreases from 896 to 470 µΩ cm as the substrate temperature increases from 350 to 450 °C. In addition, the competition between the thermally activated carriers and scattering effects of free carriers in a degenerate semiconductor can also explain the metal–semiconductor transition.

Temperature-dependent Hall effect studies of ZnO thin films grown by metalorganic chemical vapour deposition

Abstract: The electrical properties of zinc oxide (ZnO) thin films of various thicknesses (0.3–4.4 µm) grown by metalorganic chemical vapour deposition on glass substrates have been studied by using temperature-dependent Hall-effect (TDH) measurements in the 18–300 K range. The high quality of the layers has been confirmed with x-ray diffraction, transmission electron microscopy, scanning electron microscopy and photoluminescence techniques. TDH measurements indicate the presence of a degenerate layer which significantly influences the low-temperature data. It is found that the measured mobility generally increases with increasing layer thickness, reaching a value of 120 cm2 V−1 s−1 at room temperature for the 4.4 µm thick sample. The lateral grain size of the layers is also found to increase with thickness indicating a clear correlation between the size of the surface grains and the electrical properties of corresponding films. Theoretical fits to the Hall data suggest that the bulk conduction of the layers is dominated by a weakly compensated donor with activation energy in the 33–41 meV range and concentration of the order of 1017 cm−3, as well as a total acceptor concentration of mid-1015 cm−3. Grain boundary scattering is found to be an important limiting factor of the mobility throughout the temperature range considered.

New developments on diamond photodetector for VUV solar observations

Abstract: A new large-size metal–semiconductor–metal photoconductor device of 4.6 mm in diameter based on diamond material has been reprocessed and characterized in the vacuum-ultraviolet (VUV) wavelength range. The metal finger contacts have been processed to 2 µm in width with spacing between the contacts of 5 µm for a bias voltage of 5 V. The responsivity, stability, linearity and homogeneity have been tested. Solutions and progresses on diamond processing are identified and are reported. In the VUV wavelength range of interest, the diamond photodetector is sensitive with a maximum response of 48 mA W−1 at 210 nm with a corresponding external quantum efficiency of 42%, homogenous and stable under short irradiation. It indicates a 200–400 nm rejection ratio of more than four orders of magnitude and demonstrates the advantages of diamond-based detectors in terms of high rejection ratio and high output signal for VUV solar observation missions.

Studies on fabrication and characterization of a high-performance Al-doped ZnO/n-Si (1?1?1) heterojunction photodetector

Abstract: ZnO:Al/c-Si (1 1 1) isotype heterojunction photodetectors were fabricated by a chemical spray pyrolysis technique. High responsivity and good junction characteristics were obtained after post-deposition rapid thermal annealing (RTA). Dark and illuminated I–V characteristics were measured and analyzed. The ideality factor was deduced from I–V characteristics and found to be 1.3 after RTA. C–V measurements revealed that the junction was abrupt type. The energyband diagram, based on the Anderson model, was constructed from the electrical properties of the junction. Good photoresponses in UV and visible regions with responsivity were around 0.1 A W−1 and 0.47 A W−1, respectively. The rise time of the detector was improved after RTA and found to be 50 ns. These results suggest that the Al dopant could be a good choice to fabricate the doped ZnO/Si devices for photodetection and other optoelectronic applications. We describe here the fabrication process and optoelectronic characteristics of the photodetector.

Solution-processed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene thin-film transistors with a polymer dielectric on a flexible substrate

Abstract: The authors report the fabrication of solution-processed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene thin-film transistors with a cross-linked poly-4-vinylphenol (PVP) dielectric on a polyethersulphone (PES) substrate. The device exhibited useful electrical characteristics, including a saturation field effect mobility of 2.08 × 10−2 cm2 V−1 s−1, a current on/off ratio of 105, a threshold voltage of −2 V and an excellent subthreshold slope of 0.86 V/dec. It was demonstrated that the significant improvement in the subthreshold slope of TIPS-pentacene TFTs could be attributed to a decreased carrier trap density at the PVP/TIPS-pentacene film interface. Furthermore, a 1,2,3,4-tetrahydronaphthalene (Tetralin) solvent used in this study had a high boiling point, which had a positive effect on the morphology and the molecular ordering of the TIPS-pentacene film.

Fabrication and electrical properties of Al/Safranin T/n-Si/AuSb structure

Abstract: We have fabricated an Al/Safranin T (ST)/n-Si/AuSb device and have investigated its current?voltage (I?V), capacitance?voltage (C?V) and capacitance?frequency (C?f) characteristics at room temperature. The barrier height and ideality factor values of 0.78 eV and 3.52 have been obtained from the forward bias current?voltage plot. The value of the barrier height was compared with the barrier height value of 0.50 eV of a conventional Al/n-Si diode. This was attributed to the ST organic film modifying the effective barrier height by affecting the space charge region of the inorganic Si semiconductor substrate. A modified Norde's function combined with the conventional forward I?V method has been used to extract the parameters including barrier height and series resistance. The barrier height and series resistance obtained from Norde's function have been compared with those from Cheung functions, and it has been seen that there is a good agreement between the barrier height values from both methods. It has also been seen that the values of capacitance are almost independent of frequency up to a certain value of frequency, whereas at high frequencies the capacitance has decreased. The higher values of capacitance at low frequencies have been attributed to the excess capacitance resulting from the interface states in equilibrium with the n-Si that can follow the alternating current (ac) signal.