Showing papers in "Semiconductor Science and Technology in 2009"

Focused ion beam technology and ultimate applications

Abstract: In this topical review, the potential of the focused ion beam (FIB) technology and ultimate applications are reviewed. After an introduction to the technology and to the operating principles of liquid metal ion sources (LMIS), of ion optics and instrument architectures, several applications are described and discussed. First, the application of FIB for microcircuit inspection, metrology and failure analysis is presented. Then, we introduce and illustrate some advanced patterning schemes we propose as next generation FIB processing examples. These patterning schemes are (i) local defect injection or smoothing in magnetic thin film direct patterning, (ii) functionalization of graphite substrates to guide organization of clusters, (iii) local and selective epitaxy of III‐V semiconductor quantum dots and (iv) FIB patterned solid-state nanopores for biological molecules manipulation and analysis. We conclude this work by giving our vision of the future developments for FIB technology. (Some figures in this article are in colour only in the electronic version)

Characteristics of the ZnO thin film transistor by atomic layer deposition at various temperatures

Abstract: ZnO thin films were deposited by atomic layer deposition (ALD) at various temperatures and the resulting electrical and chemical properties were examined. The fraction of O–H bonds in ZnO films decreased from 0.39 to 0.24 with increasing processing temperatures. The O/Zn ratio decreased from 0.90 at 70 °C to 0.78 at 130 °C. The carrier concentration and resistivity changed sharply with decreasing temperature. The ZnO thin film transistors (TFTs) were fabricated at processing temperatures of 70 to 130 °C and the electrical properties of the TFT were as follows: the field-effect mobility ranged from 8.82 × 10−3 to 6.11 × 10−3 cm2 V−1 s−1, the on/off current ratio ranged from 1.28 × 106 to 2.43 × 106, the threshold voltage ranged from −12.5 to 14.7 V and the subthreshold swing ranged from 1.21 to 24.1 V/decade. The electrical characteristics of the ZnO TFT were enhanced as the processing temperature decreased.

Precursors' order effect on the properties of sulfurized Cu2ZnSnS4 thin films

Abstract: A dc magnetron sputtering-based method to grow high-quality Cu2ZnSnS4 (CZTS) thin films, to be used as an absorber layer in solar cells, is being developed. This method combines dc sputtering of metallic precursors with sulfurization in S vapour and with post-growth KCN treatment for removal of possible undesired Cu2?xS phases. In this work, we report the results of a study of the effects of changing the precursors' deposition order on the final CZTS films' morphological and structural properties. The effect of KCN treatment on the optical properties was also analysed through diffuse reflectance measurements. Morphological, compositional and structural analyses of the various stages of the growth have been performed using stylus profilometry, SEM/EDS analysis, XRD and Raman Spectroscopy. Diffuse reflectance studies have been done in order to estimate the band gap energy of the CZTS films. We tested two different deposition orders for the copper precursor, namely Mo/Zn/Cu/Sn and Mo/Zn/Sn/Cu. The stylus profilometry analysis shows high average surface roughness in the ranges 300?550 nm and 230?250 nm before and after KCN treatment, respectively. All XRD spectra show preferential growth orientation along (1 1 2) at 28.45?. Raman spectroscopy shows main peaks at 338 cm?1 and 287 cm?1 which are attributed to Cu2ZnSnS4. These measurements also confirm the effectiveness of KCN treatment in removing Cu2-xS phases. From the analysis of the diffuse reflectance measurements the band gap energy for both precursors' sequences is estimated to be close to 1.43 eV. The KCN-treated films show a better defined absorption edge; however, the band gap values are not significantly affected. Hot point probe measurements confirmed that CZTS had p-type semiconductor behaviour and C?V analysis was used to estimate the majority carrier density giving a value of 3.3 ? 1018 cm?3.

Wafer-level fabrication of GaN-based vertical light-emitting diodes using a multi-functional bonding material system

Abstract: We first report on the fabrication of 2 inch wafer-level GaN-based vertical light-emitting diodes (LEDs) by using a multi-functional bonding material system, which is composed of a thick Cu diffusion barrier and a bonding layer. The bonding material system superbly absorbs laser-induced stress and also effectively serves as a barrier to the indiffusion of Sn to the active region. Fully packaged vertical LEDs fabricated with indium tin oxide (ITO)/AgCu contact and the bonding material system give an operating voltage of 3.35 V at 350 mA. After over 1800 h, the operating voltages remain stable, and the reverse currents are in the range 3–8 × 10−7 A at −5 V.

Charge trapping and detrapping characteristics in amorphous InGaZnO TFTs under static and dynamic stresses

Abstract: We have investigated the static and dynamic bias stress-induced charge trapping and detrapping phenomenon in amorphous indium–gallium–zinc oxide thin film transistors. It is observed that the charges trapped after electron injection in the interface and bulk traps are unstable and slowly decay over time. The stretched-exponential equation, which can be derived based on the trapping/detrapping of charges to/from existing traps and continuous redistribution of charges in bulk dielectrics, is successfully applied in fitting the time dependence of the threshold voltage shift during the stress and recovery phases under dynamic stresses. The characteristic time constants decrease with increasing temperature and drain bias during the recovery phase. Under dynamic stresses with various frequencies, the threshold voltage shift strongly depends on the frequency of dynamic stresses, i.e., a high frequency stress results in a small threshold voltage shift and a long lifetime. The stress-induced threshold shift phenomenon is observed to be relieved after a long-time low temperature post thermal annealing process and device passivation with an aluminum oxide layer.

Trends in the electronic structure of dilute nitride alloys

Abstract: The band-anticrossing (BAC) model has been widely applied to analyse the electronic structure of dilute nitride III-V-N alloys such as GaNxAs1−x. The BAC model describes the strong band gap bowing observed at low N composition in GaNxAs1−x in terms of an interaction between the GaAs host matrix conduction band edge and a higher lying band of localized N resonant states. In practice, replacing As by N introduces a range of N-related defect levels, associated with isolated N atoms, N–N pairs and larger clusters of N atoms. We show that the effect of such defect levels on the alloy conduction band structure is strongly dependent on the relative energy of the defect levels and the host conduction band edge. We first consider GaNxAs1−x, where we show that the unexpectedly large electron effective mass and gyromagnetic ratio, and their non-monotonic variation with x, are due to hybridization between the conduction band edge and specific nitrogen states close to the band edge. The N-related defect levels lie below the conduction band edge in GaNxP1−x. We must therefore explicitly treat the interaction between the higher lying GaP host Γ conduction band minimum and defect states associated with a random distribution of N atoms in order to obtain a good description of the lowest conduction states in disordered GaPN alloys. Turning to other alloys, N-related defect levels should generally lie well above the conduction band minimum in InNSb, with the band dispersion of InNSb then well described by a two-level BAC model. Both InP and InAs are intermediate between InSb and GaAs. By contrast, we calculate that N-related defect levels lie close to the conduction band minimum in GaNSb, and will therefore strongly perturb the lowest conduction states in this alloy. Overall, we conclude that the BAC model provides a good qualitative explanation of the electronic properties of dilute nitride alloys, but that it is in many cases necessary to include the details of the distribution of N-related defect levels to obtain a quantitative understanding of the conduction band structure in dilute nitride alloys.

Epitaxial growth of Ge thick layers on nominal and 6° off Si(0 0 1); Ge surface passivation by Si

Abstract: We have grown various thickness Ge layers on nominal and 6? off Si(0?0?1) substrates using a low-temperature/high-temperature strategy followed by thermal cycling. A combination of 'mounds' and a perpendicular cross-hatch were obtained on nominal surfaces. On 6? off surfaces, three sets of lines were obtained on top of the 'mounds': one along the 1?1?0 direction perpendicular to the misorientation direction and the other two at ~4.5? on each side of the 1?1?0 direction parallel to the misorientation direction. The surface root mean square roughness was less than 1 nm for 2.5 ?m thick nominal and 6? off Ge layers. Those slightly tensily strained Ge layers (R ~ 104%) were characterized by 5 ? 107 cm?2 (as-grown layers) ?107 cm?2 (annealed layers) threading dislocation densities, independently of the substrate orientation. We have then described the 550 ?C/650 ?C process used to passivate nominal Ge(0?0?1) surfaces with Si prior to gate stack deposition. An ~5 ? thick SiGe interfacial layer is self-limitedly grown at 550 ?C and then thickened at 650 ?C (5 ? min?1) thanks to SiH2Cl2 at 20 Torr. Such a Ge surface passivation yields state-of-the-art p-type metal oxide semiconductor field effect transistors provided that 15 ? Si layer thickness is not exceeded. For higher thickness, elastic strain relaxation (through the formation of numerous 2D islands) occurs, followed by plastic relaxation (for a 35 ? thick Si layer).

Sn-alloying as a means of increasing the optical absorption of Ge at the C- and L-telecommunication bands

Abstract: The optical properties of Ge1−ySny alloys (y ~ 0.02) grown by chemical vapor deposition on Si substrates have been studied using spectroscopic ellipsometry and photocurrent spectroscopy. The system shows a 10-fold increase in optical absorption, relative to pure Ge, at wavelengths corresponding to the C-telecommunication band (1550 nm) and a 20-fold increase at wavelengths corresponding to the L-band (1620 nm). Measurements on a series of samples with different thicknesses reveal nearly identical dielectric functions, from which the composition reproducibility of the growth method is estimated to be as good as 0.1%. It is shown that a model that includes excitonic effects reproduces the measured onset of absorption using the direct band gap E0 as essentially the only adjustable parameter of the fit.

Fabrication of the SnS/ZnO heterojunction for PV applications using electrodeposited ZnO films

Abstract: In the perspectives of fabricating a nontoxic and low-cost photovoltaic system, SnS has recently received considerable interest as a potential solar cell material. However, doped or undoped CdS has been extensively studied as a window material with p-SnS for the fabrication of a heterojunction solar cell. Because of the carcinogenic character of CdS, there is a prevailing need to search for an alternative window layer and scrutinize the performance of the heterojunction with p-SnS. In this work ZnO thin films were fabricated onto ITO using a galvanostatic electrodeposition approach from the dimethyl sulfoxide bath. The as-deposited ZnO films were subsequently annealed at 400 °C in argon ambiance to reduce the intermediate trap states and effective sintering. Finally, SnS thin films were grown onto the ZnO layer by thermally evaporating high-purity SnS powder. The photovoltaic properties of the SnS/ZnO heterojunction were investigated.

Tunneling current via dislocations in Schottky diodes on AlInN/AlN/GaN heterostructures

Abstract: The forward current–voltage–temperature characteristics of (Ni/Au)–Al0.83In0.17N/AlN/GaN heterostructures were studied in a temperature range of 80–375 K. The temperature dependences of the tunneling saturation current (It) and tunneling parameters (E0 )w ere obtained. Weak temperature dependence of the saturation current and the absence of temperature dependence of the tunneling parameters were observed in this temperature range. The results indicate that in the temperature range of 80–375 K, the mechanism of charge transport in the (Ni/Au)–Al0.83In0.17N/AlN/GaN heterostructure is performed by tunneling among dislocations intersecting the space-charge region. A model is used for nonuniform tunneling along these dislocations that intersect the space-charge region. The dislocation density that was calculated from the current–voltage characteristics, according to a model of tunneling along the dislocation line, gives the value 7.4 × 10 8 cm −2 . This value is close in magnitude to the dislocation density that was obtained from the x-ray diffraction measurements value of 5.9 × 10 8 cm −2 . These data show that the current flows manifest a tunneling character, even at room temperature. (Some figures in this article are in colour only in the electronic version)

Wet etching and chemical polishing of InAs/GaSb superlattice photodiodes

Abstract: In this paper, we studied wet chemical etching fabrication of the InAs/GaSb superlattice mesa photodiode for the mid-infrared region. The details of the wet chemical etchants used for the device process are presented. The etching solution is based on orthophosphoric acid (H3PO4), citric acid (C6H8O7) and H2O2, followed by chemical polishing with the sodium hypochlorite (NaClO) solution and protection with photoresist polymerized. The photodiode performance is evaluated by current?voltage measurements. The zero-bias resistance area product R0A above 4 ? 105 ? cm2 at 77 K is reported. The device did not show dark current degradation at 77 K after exposition during 3 weeks to the ambient air.

Stability of thin film transistors incorporating a zinc oxide or indium zinc oxide channel deposited by a high rate sputtering process

Abstract: A novel rf sputtering technology in which a high density plasma is created in a remote chamber has been used to reactively deposit zinc oxide (ZnO) and indium zinc oxide (IZO) thin films at room temperature from metallic sputtering targets at deposition rates ∼50 nm min −1 , which is approximately an order of magnitude greater than that of rf magnetron sputtering. Thin film transistors have been fabricated using IZO with a maximum processing temperature of 120 ◦ C, which is defined by the curing of the photoresist used in patterning. Devices have a saturated field effect mobility of 10 cm 2 V −1 s −1 and a switching ratio in excess of 10 6 . Gate bias stress experiments performed at elevated temperatures show a consistent apparent increase in the field effect mobility with time, which is attributed to a charge trapping phenomenon.

Effects of tool edge radius on ductile machining of silicon: an investigation by FEM

Abstract: The submicron-level orthogonal cutting process of silicon has been investigated by the finite element approach, and the effects of tool edge radius on cutting force, cutting stress, temperature and chip formation were investigated. The results indicate that increasing the tool edge radius causes a significant increase in thrust force and a decrease in chip thickness. A hydrostatic pressure (~15 GPa) is generated in the cutting region, which is sufficiently high to cause phase transformations in silicon. The volume of the material under high pressure increases with the edge radius. Temperature rise occurs intensively near the tool–chip interface while the highest cutting temperature (~300 °C) is far lower than the necessary temperature for activating dislocations in silicon. As the edge radius is beyond a critical value (~200 nm), the primary high-temperature zone shifts from the rake face side to the flank face side, causing a transition in the tool wear pattern from crater wear to flank wear. The simulation results from the present study could successfully explain existing experimental phenomena, and are helpful for optimizing tool geometry design in silicon machining.

Analysis of the current-voltage characteristics of the Pd/Au Schottky structure on n-type GaN in a wide temperature range

Abstract: We report on the temperature-dependent electrical characteristics of the Au/Pd/n-GaN Schottky diode in the temperature range of 90–410 K. The barrier heights and ideality factors of Schottky diodes were found in the range 0.23 eV and 3.5 at 90 K to 0.97 eV and 1.9 at 410 K, respectively. It was observed that the zero bias barrier height Φbo decreases and the ideality factor n increases with a decrease in temperature. Such behavior is attributed to barrier inhomogeneities by assuming a Gaussian distribution of barrier heights at the interface. The estimated values of series resistance (RS) are in the range of 636 Ω at 90 K to 220 Ω at 410 K using Cheung's method. Based on the above observations, the Φbo, n and RS values are seen to be strongly temperature dependent. The flat-band barrier height Φbf(T = 0 K) and temperature coefficient α were found to be 0.67 eV and 2.81 × 10−3 eV K−1, respectively. Further, the homogeneous barrier height is estimated from the linear relationship between temperature-dependent experimental effective barrier heights and ideality factors and the value is approximately 1.31 eV. The effective Richardson constant is determined to be 20.43 A cm−2 K−2 and is in good agreement with the theoretical value. It is concluded that the temperature-dependent I–V characteristics of the Au/Pd/n-GaN Schottky diode can be successfully explained on the basis of thermionic emission (TE) mechanism with the Gaussian distribution of the barrier heights.

Conduction mechanism of leakage current due to the traps in ZrO2 thin film

Abstract: In this work, a metal-oxide-semiconductor capacitor with zirconium oxide (ZrO2) gate dielectric was fabricated by an atomic layer deposition (ALD) technique and the leakage current characteristics under negative bias were studied. From the result of current–voltage curves there are two possible conduction mechanisms to explain the leakage current in the ZrO2 thin film. The dominant mechanism is the space charge limited conduction in the high-electric field region (1.5–5.0 MV cm−1) while the trap-assisted tunneling due to the existence of traps is prevailed in the low-electric field region (0.8–1.5 MV cm−1). Conduction caused by the trap-assisted tunneling is found from the experimental results of a weak temperature dependence of current, and the trap barrier height is obtained. The space charge limited conduction is evidenced, for different temperatures, by Child's law dependence of current density versus voltage. Child's law dependence can be explained by considering a single discrete trapping level and we can obtain the activation energy of 0.22 eV.

Photoluminescence, electrical and structural properties of ZnO films, grown by ALD at low temperature

Abstract: We report the first results of the low-temperature photoluminescence study on polycrystal zinc oxide (ZnO) films obtained by atomic layer deposition at 100 °C, 130 °C and 200 °C. These ZnO films, when studied 'as-grown', show a strong excitonic emission even at room temperature. Low-temperature (T = 9 K) photoluminescence reveals lack of defect-related bands and a sharp photoluminescence peak at 3.36 eV with full width at half maximum of 6 meV which is comparable with the value reported for good quality bulk ZnO crystals. The energy position of the excitonic peak scales with temperature according to standard formulas and give the Debye temperature of 963 ± 26 K. We show that optical properties of low-temperature 'as-grown' ZnO films are correlated with structural and electrical ones and that optical study can be a valuable tool for evaluation of quality of ZnO films for novel electronic applications.

A compact drain current model of short-channel cylindrical gate-all-around MOSFETs

Abstract: A fully analytical potential model, valid in the weak inversion regime of short-channel cylindrical gate-all-around (GAA) MOSFET, is proposed. The model derivation is based on a previous analytical expression for tetragonal GAA MOSFET and the rotational symmetry of the tetragonal cross section. Device simulations were performed to verify that the potential distribution along the channel is properly described in all positions within the silicon body. Using the potential model, analytical expressions for the threshold voltage, subthreshold swing and drain-induced barrier lowering have been derived. Including the short-channel effects within an existing model for the subthreshold leakage current and an analytical drain current model of long-channel devices in strong inversion, a compact drain current model has been derived describing with good accuracy the transfer and output characteristics of short-channel GAA MOSFETs in all regions of operation.

Analysis of field-plate effects on buffer-related lag phenomena and current collapse in GaN MESFETs and AlGaN/GaN HEMTs

Abstract: A two-dimensional transient analysis of field-plate GaN MESFETs and AlGaN/GaN HEMTs is performed in which a deep donor and a deep acceptor are considered in a semi-insulating buffer layer, and quasi-pulsed current–voltage curves are derived from them. How the existence of a field plate affects buffer-related drain lag, gate lag and current collapse is studied. It is shown that in both MESFET and HEMT, the drain lag is reduced by introducing a field plate because electron injection into the buffer layer is weakened by it, and the buffer-trapping effects are reduced. It is also shown that the field plate could reduce buffer-related current collapse and gate lag in the FETs. The dependence of lag phenomena and current collapse on the field-plate length and on the SiN passivation layer thickness is also studied. The work suggests that in the field-plate structures, there is an optimum thickness of the SiN layer to minimize the buffer-related current collapse and drain lag in GaN MESFETs and AlGaN/GaN HEMTs.

Channel mobility and on-resistance of vertical double implanted 4H-SiC MOSFETs at elevated temperatures

Abstract: Temperature and gate voltage dependences of the channel electron mobility were studied in short-channel high-voltage vertical double implanted 4H-SiC MOSFETs (VDMOSFETs). With increasing gate voltage, field effect electron mobility, μFE, increased, tending to saturate at large Vg values reaching a maximum of ~4 cm2 V−1 s−1 at room temperature. With the temperature increase, μFE increased monotonically and reached a value of ~16 cm2 V−1 s−1 at 510 K. These trends are explained by the high density of the interface traps, which was extracted from the temperature dependence of the threshold voltage. The electron mobility in the drift region decreased with temperature increase. As a result, the contribution of the drift region to the on-resistance was dominant at elevated temperatures limiting the VDMOSFETs performance at temperatures above ~420 K. The on-resistance of VDMOSFETs was only weakly dependent on temperature within the temperature range from 300 K to 510 K.

Fermi level pinning and effects on CuInGaSe2-based thin-film solar cells

Abstract: This paper briefly summarizes the knowledge accumulated in the literature on the copper indium gallium diselenide (CIGS) material and solar cells based on CIGS. After reviewing the present use of solid-state physics principles to describe thin-film solar cells based on CIGS, a new concept is proposed with the aid of latest findings on electrical contacts to the CIGS material. It has been shown that the Fermi level pinning takes place at one of the few experimentally observed defect levels. The main levels observed to date are at 0.77, 0.84, 0.93 and 1.03 eV with a ±0.02 eV error and are situated above the top of the valence band edge. As a result, discrete values of open circuit voltages are observed, and the situation is very similar to the recent work reported on CdS/CdTe solar cells. Based on these new observations, different ways for further development of CIGS solar cells are proposed.

Radiation-induced defects in Czochralski-grown silicon containing carbon and germanium

Abstract: Formation processes of vacancy-oxygen (VO) and carbon interstitial-oxygen interstitial (CiOi) complexes in electron-irradiated Czochralski-grown Si crystals (Cz–Si), also doped with Ge, are investigated. IR spectroscopy measurements are employed to monitor the production of these defects. In Cz–Si with carbon concentrations [Cs] up to 1 × 1017 cm−3 and Ge concentrations [Ge] up to 1 × 1020 cm−3 the production rate of VO defects as well as the rate of oxygen loss show a slight growth of about 10% with the increasing Ge concentration. At high concentrations of carbon [Cs] around 2 × 1017 cm−3 the production rate of VO defects is getting larger by ~40% in Cz–Si:Ge at Ge concentrations around 1 × 1019 cm−3 and then at [Ge] ≈ 2 × 1020 cm−3 this enlargement drops to ~13%, thus approaching the values characteristic of lesser concentrations of carbon. A similar behavior against Ge concentration displays the production rate of CiOi complexes. The same trend is also observed for the rate of carbon loss, whereas the trend for the rate of oxygen loss is opposite. The behavior of Ge atoms is different at low and high concentrations of this isoelectronic impurity in Cz–Si. At low concentrations most isolated Ge atoms serve as temporary traps for vacancies preventing them from indirect annihilation with self-interstitials. At high concentrations Ge atoms are prone to form clusters. The latter ones are traps for vacancies and self-interstitials due to the strain fields, increasing the importance of indirect annihilation of intrinsic point defects. Such a model allows one to give a plausible explanation for the obtained results. A new band at 994 cm−1 seen only in irradiated Ge-doped Cz–Si is also studied. Interestingly, its annealing behavior was found to be very similar to that of VO complexes.

Electronic, lattice vibration and mechanical properties of CdTe, ZnTe, MnTe, MgTe, HgTe and their ternary alloys

Abstract: The main representative telluride family, CdTe, ZnTe, MnTe, MgTe and HgTe, and their mixed ternary alloys have been extensively studied, but some points bearing on phonon frequencies properties have never been previously reported or are still not clear. In this paper, we report results on the electronic, lattice dynamical and mechanical properties for this family in the zinc?blende (ZB) structure. Numerical calculations based on the empirical pseudo-potential method under the virtual crystal approximation combined with the Harrison bond orbital model are used to investigate these properties. Our results are found to be generally in good agreement with the available experimental data. Other case, our results are predictions and may serve as a reference. The information derived from the present study demonstrates the potential of the materials of interest for optoelectronic and photovoltaic device applications from the ultraviolet to infrared and mid-infrared.

Optical properties of CuIn1−xGaxSe2 quaternary alloys for solar-energy conversion

Abstract: The optical properties of CuIn1−xGaxSe2 epitaxial single-crystal layers were determined by spectroscopic ellipsometry (SE) and complementary photoreflectance spectroscopy (PR) in dependence of composition. Accurate values of refractive index n and extinction coefficient κ and values of the fundamental and higher band-gap energies of quaternary selenides were obtained for six different Ga concentrations: x = 0.08, 0.19, 0.22, 0.50, 0.55, 0.82. In addition, for CuIn1−xGaxSe2 with x = 0.08 and 0.55, variable-angle ellipsometric measurements were performed. Only a very small contribution of the extraordinary component to the measured effective dielectric function was found, which implies that, in chalcopyrite single-crystal layers, the extraordinary component cannot be separated by altering the angle of incidence.

Thermally induced voltage shift in capacitance–voltage characteristics and its relation to oxide/semiconductor interface states in Ni/Al2O3/InAlN/GaN heterostructures

Abstract: The Ni/Al2O3/InAlN/AlN/GaN metal-oxide-semiconductor heterostructure (MOS-H) is investigated using capacitance–voltage and capacitance–time characteristics in the temperature range of 25–300 °C. An anomalous positive voltage shift of the capacitance–voltage curve with increasing temperature was observed and attributed to the hole emission from the oxide/semiconductor interface states. Distribution of the interface states density, Dit(E), at the Al2O3/InAlN interface was evaluated using a modification of the constant-capacitance deep-level transient spectroscopy. The MOS-H capacitor threshold voltage shift under negative bias was repetitively recorded as a function of time at elevated temperatures. Dit in the range of 0.1–3 × 1013 eV−1 cm−2 was determined.

Multi-interface roughness effects on electron mobility in a Ga0.5In0.5P/GaAs multisubband coupled quantum well structure

Abstract: We analyse the effect of interface roughness scattering on low temperature electron mobility μn mediated by intersubband interactions in a multisubband coupled Ga0.5In0.5P/GaAs quantum well structure. We consider a barrier δ-doped double quantum well system in which the subband electron mobility is limited by the interface roughness scattering μIRn and ionized impurity scattering μimpn. We analyse the effect of the intersubband interaction and coupling of subband wavefunctions through the barrier on the intrasubband and intersubband transport scattering rates. We show that the intersubband interaction controls the roughness potential of different interfaces through the dielectric screening matrix. In the case of lowest subband occupancy, the mobility is mainly governed by the interface roughness of the central barrier. Whereas when two subbands are occupied, the interface roughness of the outer barrier predominates due to intersubband effects. The influence of the intersubband interaction also exhibits interesting results on the well width up to which the interface roughness dominates in a double quantum well structure.

Transparent conductive tungsten-doped tin oxide polycrystalline films prepared on quartz substrates

Abstract: Tungsten-doped tin oxide transparent conductive films were fabricated on quartz substrates by the method of pulsed plasma deposition with post-annealing. Doping of tungsten can effectively enhance the conductivity of the films while maintaining high transparency. The structural, electrical and optical properties of the films have been investigated with different annealing temperatures and tungsten-doping contents. X-ray diffraction measurement (XRD) confirmed that tungsten-doped tin oxide films annealed above 500 °C are polycrystalline and have a tetragonal rutile structure. Chemical state analysis revealed that tungsten atoms were fully oxidized to the valence of 6+. The lowest resistivity of 6.84 × 10−4 ohm cm was reproducibly obtained, with a carrier mobility of 66.4 cm2 V−1 s−1 and a carrier concentration of 1.38 × 1020 cm−3 in 3 wt% tungsten-doping films annealed at 800 °C in air. The average optical transmittance is about 87% in the visible region from 400 to 700 nm, and about 90% in the near-infrared region from 700 to 2500 nm, with the optical band gap ranging from 4.07 eV to 4.22 eV.

Electrical and optical properties of molybdenum-doped ZnO transparent conductive thin films prepared by dc reactive magnetron sputtering

Abstract: Molybdenum-doped ZnO (ZMO) transparent conductive thin films were prepared by dc reactive magnetron sputtering on glass substrates from metallic targets. The structure, surface morphology, chemical state, optical and electrical properties of ZMO films were studied. The XRD pattern confirmed that ZMO thin films were polycrystalline with the hexagonal crystal structure, and the surface morphology measured by AFM demonstrated that the surface was smooth and compact. Chemical state analysis revealed that molybdenum atoms existed mainly in Mo6+ and Mo5+ ions but not in only single oxidation states. The minimum resistivity of 7.9 × 10−4 Ω cm is obtained with a carrier mobility of 27.3 cm2 V−1 s−1 and a carrier concentration of 3.1 × 1020 cm−3, and the average transmittance is more than 85% in the visible light region. The refractive index and extinction coefficient at the wavelength of 550 nm are 1.853 and 7.0 × 10−3, respectively. The energy bands increase from 3.37 eV to 3.8 eV with the increase in carrier concentrations and the carrier effective mass m* is 0.33 times the electron mass.

A new mechanism of electric dipole spin resonance: hyperfine coupling in quantum dots

Abstract: A recently discovered mechanism of electric dipole spin resonance, mediated by the hyperfine interaction, is investigated experimentally and theoretically. The effect is studied using a spin-selective transition in a GaAs double quantum dot. The resonant frequency is sensitive to the instantaneous hyperfine effective field, revealing a nuclear polarization created by driving the resonance. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively. An unexplained additional signal at half the resonant frequency is presented.

Fabrication, performance and parasitic parameter extraction of 850 nm high-speed vertical-cavity lasers

Abstract: High-speed, oxide-confined, polyimide-planarized 850 nm vertical-cavity surface emitting lasers (VCSELs) with different aperture sizes were fabricated and characterized. Comprehensive small signal measurements and analysis were conducted. The VCSELs exhibit intrinsic, parasitic and thermal maximum bandwidth limitations of 42.3 GHz, 21.5 GHz and 17.6 GHz, respectively. Devices with a 10 μm oxide aperture exhibited a maximum modulation bandwidth of 15.3 GHz, limited by thermal effects, and a modulation current efficiency factor (MCEF) of 9.2 GHz mA −1/2 . A VCSEL equivalent circuit model which incorporates the frequency dependence of the polyimide dielectric permittivity and loss is presented. A genetic algorithm (GA) was utilized to extract the parasitic circuit elements from measured microwave reflection coefficients (S11) over a frequency range of 50 MHz to 20 GHz for different device sizes at different bias currents. Good agreement between extracted and calculated parasitic circuit element values was obtained. Several modifications to the device’s fabrication steps and structure are suggested to further improve the device’s output power and modulation bandwidth. (Some figures in this article are in colour only in the electronic version)