Table of contents

The engineering of tunnel barriers has not yet succeeded to the point where low cost high volume manufacture of tunnel devices can be undertaken. We analyse one overriding reason for the unacceptable variability in device performance and suggest that engineered tunnel barriers are unlikely to play a role in extending the CMOS Roadmap, as has recently been suggested.

We have designed, fabricated and measured the electroluminescence of InGaN-based green light-emitting diodes (LEDs) having composite omni-directional reflectors (ODRs) deposited on their backside. The composite ODR is composed of a stack of two individual ODRs, each of which is made of alternating layers of TiO₂ and SiO₂ with a thickness ratio that gives the largest possible 1D photonic bandgap. The lattice constants of these individual ODRs are chosen so that the effective bandgap of the resulting ODR completely covers the emission spectrum of the LEDs. The effective bandgap of our ODR extends from 498 nm to 548 nm. At a driving current of 300 mA, and with the LED emission peak at about 525 nm and a FWHM of about 35 nm, the light output powers of the LED with the composite ODR and the LED with a conventional Ti/Al metal reflector are found to be 52.9 mW and 40.7 mW, respectively. This 30% light extraction enhancement can be attributed to our composite ODR which has a higher reflectance, a lower optical absorption and a wider reflection angle compared with the Ti/Al reflector.

We demonstrate that photoelectrochemical (PEC) etching of GaN layers in KOH or H₃PO₄ solutions leads to the formation of specific surface morphologies which cause the material to exhibit different sensitivities to certain gases. PEC etching in a KOH solution results in a pyramidal morphology of the layer which exhibits a high sensitivity to methane gas, whereas etching in a H₃PO₄-based solution leads to the formation of individual nanoneedles with a high sensitivity to alcohol vapours. We also investigated the gas sensitivity of GaN structures with different morphologies as a function of temperature and the cross sensitivity to humidity. These results culminated in an integrated two-sensor array for methane detection in environments containing ethanol vapours. A new method of improving the recovery time characteristics of the sensor by applying high-voltage pulses is proposed.

Electrical transport properties of p-type ZnO:N films grown by thermal activation of the nitrogen dopant were investigated via the temperature-dependent Hall effect. The Hall mobility increases with decreasing temperature. Varied scattering mechanisms have been analysed including lattice vibration scattering, ionized impurity scattering and dislocation scattering. A fit of the theory to temperature-dependent hole mobility experimental data in p-type ZnO:N films gives dislocation densities in the order of 10¹² cm⁻². The analysis shows dislocation scattering is indeed important for the p-type ZnO films grown on the mismatched substrate. The thermal ionization energy of the nitrogen acceptor is estimated to be 170 meV in terms of the temperature-dependent hole concentration. On the other hand, the emission related to the acceptors is observed in PL spectra.

Electroluminescence (EL) and photoluminescence (PL) measurements were conducted on Si-implanted SiO₂ layers as a function of process and measurement parameters. Measurable light emission was observed from the metal oxide semiconductor light emitting diode (MOS-LED) when holes are injected from the substrate. It was shown that major PL and EL emissions have the same origin. However, two important differences were observed between EL and PL spectra. The first one is the light emission from the Si substrate due to the recombination of electrons supplied by the front contact and holes that were accumulated in the inversion region at the substrate/SiO₂ interface. This might be a factor reducing the contribution of Si nanocrystals to the EL emission of the MOS-LED structure as a result of decrease in the number of holes in the inversion layer. The second difference is that EL emission peaks stay at a slightly higher energy than PL peaks. It was observed that the EL peak shifts towards the PL peak with increasing bias voltage. This behaviour is explained by considering the size distribution of nanocrystals formed by ion implantation.

Single quantum dots have attracted much interest because they might act as hosts for future optical storage devices. We report hysteresis in the current flowing through bilateral p–i–n junctions incorporated with quantum dots under optical excitation at different wavelengths. The conductance of the n-type layer, a two-dimensional electron gas, can be modulated with optical excitation and gating quantum dots. With quasi-resonant optical excitation, the current shows a hysteresis due to the charging of the quantum dots. The device could potentially be used for single-photon detection.

The suitability of the thermometry based on transmission Fabry–Pérot interferences in semiconductor devices and ICs under working conditions is experimentally shown. Usually, they work at short time scales producing internal temperature increments, which are crucial for understanding their failure mechanisms. Thanks to this technique, the temperature profile in semiconductor devices and ICs is extracted with a very good depth resolution (below 35 µm). By means of versatile apparatus, transmission Fabry–Pérot and internal infrared laser deflection temperature measurements are simultaneously performed on a specific thermal test chip device. Concretely, the temperature evolution at various depths of the thermal test chip during the device heating and cooling processes is obtained. Experimental results are compared with the analytical model which thermally describes the thermal test chip behaviour during the heating process, achieving a very good agreement.

The depth-dependent electrical properties of InSb thin films grown on GaAs substrates result in an inherent magnetoresistance in the layers. For certain applications it is important to be able to manipulate this effect controllably. This paper demonstrates both experimentally and theoretically that the magnitude of the magnetoresistance can be dramatically changed by epilayer design. We show that the inclusion of a doped region in part of the layer structure allows the inherent magnetoresistance to be changed by a factor of 40.

New morphologies of InAs quantum dot (QD) ensembles forming on self-assembled GaAs nano-holed island templates are demonstrated. Droplet homoepitaxy (GaAs/GaAs) is used to generate holed nanoscale-sized mounds that appear to elongate along . Depending on the InAs monolayer (ML) coverages, subsequent InAs deposition forms different sizes and shapes of QD ensembles. While we initially observe the formation of the QDs at the hole sites when less InAs is deposited, QDs form around the edges of the mounds with greater InAs deposition. By varying the InAs depositions and growth temperatures, we demonstrate an ability to control the size and density of QDs. The observed decrease in the necessary critical thickness required for the InAs 2D–3D transition may be due to the higher density of monolayer steps on the sidewalls of the holes and on the edges of the mounds. This hybrid growth approach overcomes some limitations of typical QD growth on planar GaAs surfaces and may find applications in optoelectronics.

The structure of epitaxially grown Pr₂O₃ on Si(0 0 1) has been investigated by grazing incidence x-ray diffraction and x-ray reflectivity. We report experimental results from measurements made in situ, after growth, and during annealing to determine the stability of Pr₂O₃ at temperatures relevant for complementary metal oxide semiconductor (CMOS) processing. Pr₆O₁₁ is evaporated and converted to Pr₂O₃ using an effusion source operating at 1970 °C. Two different phases of Pr₂O₃ have been identified in the as-grown films in addition to a silicate layer that forms upon nucleation at the Si interface. The cubic Pr₂O₃ nucleates as a single crystal and grows in greater proportion to the polycrystalline hexagonal Pr₂O₃. The crystal structure of the as-grown film is stable during annealing up to about 800 °C at which point the Pr₂O₃ is consumed at the expense of the silicate phase.

Films of p-GaN (1 at% Be) and n-GaN (1 at% Si) in a nanocrystalline thin film form were deposited onto a fused silica substrate by a high pressure sputtering technique for realizing the GaN p–n homojunction structure: fused silica/Al/n-GaN/p-GaN/Au. The junction properties were evaluated by measuring I–V and C–V characteristics at different temperatures. Carrier concentrations of the above n-GaN and p-GaN layers were obtained from the C–V measurement of Al/n-GaN/Au and Au/p-GaN/Al Schottky diodes. The data were analysed by utilizing existing theories to generate information on the different parameters controlling the I–V characteristics of the p–n junction.

In the present paper, we propose a new scaling theory to model short channel effects (SCEs) in nanoscale double gate (DG) SOI MOSFETs, addressing two important technological issues—source/drain extension (SDE) region engineering and high-κ gate dielectrics. The impact of SDE region engineering through the optimization of lateral source/drain doping gradient and spacer width on SCEs is extensively analysed in DG devices with high-κ gate dielectrics, using the analytical model and 2D device simulations. Novel technology dependent scaling parameters, i.e., spacer-to-gradient ratio (ρ) and effective channel length (L_eff), are proposed for source/drain-engineered DG MOSFETs, and their significance in minimizing SCEs in high-κ gate dielectrics is discussed in detail. Results show that the optimal spacer-to-gradient ratio should be increased with the permittivity of high-κ dielectrics in order to maintain SCEs to an acceptable level. The results of the analytical model confirm well with simulated data over the entire range of spacer widths, doping gradients, high-κ gate dielectrics and effective channel lengths. The present work provides valuable design insights in the performance of nanoscale source/drain-engineered DG SOI devices with high-κ gate dielectrics and serves as an accurate tool to optimize important device parameters aiding technology development.

The impact of annealing on the dielectric performance of TiO₂ thin films synthesized by PECVD was investigated. Films annealed between 500 and 700 °C have an anatase crystal structure, while 800 °C annealed films display the rutile phase. The optimal annealing temperature was 600 °C, which both maximized the dielectric constant and minimized the leakage current density. The intrinsic dielectric constant of TiO₂ improved from 82 ± 10 in as-deposited films to 168 ± 30 after annealing. The leakage current of optimized films was superior to the SiO₂ control samples over a range of equivalent oxide thickness. Fowler–Nordheim tunnelling and Frenkel–Poole conduction were observed in the optimized films, while Schottky emission dominated leakage current at other conditions.

We report measurements of the energy loss rates and scattering times for two-dimensional electrons in a series of AlGaN/GaN heterostructures grown on different substrate materials. It was found that the device grown on a substrate having the largest lattice mismatch to GaN was within the dirty limit where strong carrier scattering gives rise to an enhanced energy loss rate, which is proportional to T⁴_e at low electron temperatures, T_e < 10 K. This is consistent with the measurements of the scattering times and shows that strong scattering by impurities and defects can play an important role in the energy relaxation of some GaN-based devices at low temperatures.

Using a transfer matrix method, we derive the dispersion equation for interface phonon–polariton modes (IPPMs) in a finite superlattice (SL) with a structural defect. The numerical results show that, in the nonradiative regime, there exist localized IPPMs (including retardation) inside or outside the Reststrahlen regions defined by the interval [ω_TO, ω_LO]. The localized IPPMs lie either in the minigaps or below and above the bulk bands, and their transversal electric field amplitudes are located in the vicinity of the defect layer or surface layers. The evolution of the IPPMs localized in the vicinity of a different interface can clearly be tracked. In addition, a brief analysis that radiative IPPMs evolve into nonradiative IPPMs is given.

Four different passivations are applied on AlGaN/GaN heterostructure field-effect transistors (HFETs) and their performance is compared. SiO₂ and SiN layers of different thicknesses and deposition temperatures, which induced different stresses, are used for passivation. The sheet charge density and the saturated drain current increased up to 27% and 37%, respectively, with increasing stress from compressive (−150 MPa) to tensile (50 MPa). The change of the stress-induced sheet charge density is 1.5 × 10¹¹ cm⁻² for 100 MPa. For non-stressed conditions, a passivation-induced sheet charge density of ∼1.3 × 10¹² cm⁻² is extrapolated. This indicates that the passivation-induced stress is only a partial effect of the HFET passivation, compared to the surface states reduction. The current collapse evaluation, by consecutive I–V sweeps with 20 ms and 416 µs integration time, shows that the devices with 30 nm thick SiN passivation exhibited the best performance. However, two processes with different time constants need to be considered. Trapping processes at the GaN/passivation interface as well as in the AlGaN(GaN) barrier layer are supposed to be responsible for the observed behaviour.

A reduction of Ni/Au ohmic contact resistivity on p-type GaN was obtained by surface treatment using N₂ plasma at room temperature. For p-type GaN with a hole concentration of about 1 × 10¹⁷ cm⁻³, the contact resistivity decreased from 5.0 × 10⁻² Ω cm² to 3.1 × 10⁻⁴ Ω cm² by the N₂ plasma treatment, compared to that of the HCl only-treated sample. The O 1s core-level peak in the x-ray photoemission spectra shows that the N₂ plasma treatment is effective in the removal of the surface oxide layer. Compared to the HCl only-treated surface, the surface Fermi level on the N₂ plasma-treated surface lies about 0.58 eV closer to the valence band edge, resulting in a much smaller surface barrier height to p-GaN than the HCl only-treated surface. The smaller surface barrier height of p-GaN treated with N₂ plasma can lead to a lower contact resistivity and can play an important role in lowering the metal contact resistivity to p-GaN.

Strain distribution in GaInAsP/InP compressively strained membrane quantum wires (with low refractive index polymer cladding layers) fabricated by electron-beam lithography, reactive-ion etching and two-step epitaxial growth is theoretically calculated using finite element analysis. Results are compared with those of its conventional counterpart in which InP cladding layers are used. It is found that the etching away of the InP cladding layers in membrane structures causes a redistribution of elastic strain. The normal strain along the growth direction is the most affected component during this redistribution. We have also studied the effects of varying wire width, barrier tensile strain and other parameters on the strain relaxation. The effective bandgap in the presence of strain relaxation is also estimated. Results show that owing to the redistribution of strain, membrane structures exhibit an increase in the effective bandgap.

The behaviour of localized states in Pd/nano- or mesoporous Si/p-Si heterojunctions is studied by the DLTS technique in a vacuum and different atmospheres: ambient air, Ar, N₂, CO₂, O₂. The complex DLTS spectra of both signs related to electron and hole traps in porous Si are detected. The intensity of DLTS peaks and activation energy is shown to be dependent on the morphology of the porous layers and ambient atmosphere in which DLTS measurements were carried out. The shift of activation energy with increase of the applied reverse voltage is interpreted from the point of view of a spatially inhomogeneous distribution of localized states in a porous layer. The cyclic transformation of DLTS spectra is observed for consecutive measurements in a vacuum and ambient atmosphere. Oxygen adsorption (at partial pressure of a few mbar) results in strong passivation of deep traps in mesoporous Si.

The aim of this work is to present a two-dimensional analysis for different gate stack dielectric structured n-MOSFETs with carrier quantization effects. The model is developed using Green's function for solving Poisson's equation, without implying the extensive effort required for a fully self-consistent solution of the Schrödinger and Poisson equations. Explicit results for potential distribution, threshold voltage and drain current, with different structural and bias parameters, have been presented, typical in the operation of modern devices. The model includes short channel, drain bias, and junction curvature effects. Based on extensive simulation and developed formulation, it is found that the conventional concept of a scaled transformation method for gate stack structures to replace silicon-dioxide (SiO₂) dielectric thickness with a thicker high dielectric does not predict the same characteristics. It has also been shown that using double-layer gate stack structures with low-k dielectric as the spacer material can well confine the electric fields within the channel, thereby enhancing gate controllability on the channel charge. Comparison of the results thus obtained is done with simulated results to justify the analysis.

ZnO and ZnO:Mn (at Mn incorporation rates between 1 and 5%) films were obtained at a substrate temperature of 350 ± 5 °C by the ultrasonic spray pyrolysis technique. Optical, structural and morphological properties of ZnO films as a function of Mn amount were discussed. Transmittance spectra, x-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) micrographs and energy dispersive x-ray spectroscopy (EDS) analyses were used to investigate the optical, structural and morphological properties and elemental analyses, respectively. All films have high transmittance, and the band gap changed with Mn incorporation from 3.32 to 3.18 eV. From Urbach tail analysis, the widths of localized states were calculated between 163 and 389 meV. XRD patterns indicated that all films have polycrystalline structures and the best effect on the structural properties of the films was obtained with 4% Mn incorporation. It was seen from SEM micrographs that all films have almost homogeneous surfaces. EDS analyses showed that the amount of Mn element in the solid film increased depending on the increasing Mn incorporation in the solution. As a result, Mn incorporation has a strong effect on the optical, structural and morphological properties of ZnO films and all films, especially the 4% Mn-incorporated one, can be used in optoelectronic industry or photovoltaic solar cells due to their suitable optical and structural properties.

A comparative study of quantum confinement of electrons and phonons in silicon nanocrystals produced by laser-induced etching on a silicon (Si) substrate and continuous wave (cw) laser-induced crystallization in a-Si:H film on a quartz substrate is presented here. Raman line shapes of optical phonons confined in Si nanocrystals were utilized using a phenomenological phonon-confinement model. It is appropriately modified by incorporating a Gaussian distribution of sizes for two-dimensional (columnar) and three-dimensional (spherical) confinement parameters for laser-induced etching and cw laser-induced crystallization processes, respectively. The calculated mean crystallite sizes were in consonance with those calculated from the bond-polarizability model. Confinement effects were found to be more prominent in Si nanocrystals prepared by laser-induced crystallization in comparison to laser-etched Si. Photoluminescence spectra of both the samples were also utilized to study the dimensionality aspect of nanocrystals.

Temperature dependences of dark current, i(T), thermally stimulated current, TSC, and current–voltage, I–V, characteristics have been investigated in p-type TlGaSe₂ single crystals having different technological origins, using different contacts, in a wide temperature range and at different heating rates. Two types of initially undoped crystals with high resistivity, differing in deviation from the stoichiometry (type 1 and type 2), have been investigated. It is shown that the mechanism of the current flow strongly depends on the technological origin of the sample. Negative TSC is observed in type 1 samples for the first time and attributed to the simultaneous release of holes and electrons from acceptor and donor states in the same temperature interval. In these types of crystals, the dark current appears due to the double injection of electrons and holes from the contacts and at high heating rates β⩾ 8–10 K min⁻¹ has a thermally activated character. The parameters of subsequent impurities have been obtained using the TSC theory. The shallow donor states with extremely low capture cross sections were shown to contribute to the thermally activated processes in these types of crystals. In type 2 crystals low-frequency current oscillations, LFCO, have been observed for the first time in the temperature range 120–180 K. These types of crystals are characterized by nonlinear N-type I–V characteristics. Mechanisms are proposed to explain the nonlinear I–V characteristics and current oscillations.

We report the effect of Al₂O₃ nanolayers on the efficiency of organic light-emitting devices. Several different Al₂O₃ nanolayers were introduced between 8-hydroxyquinoline aluminum (Alq₃) and N, N'-bis-(1-naphthyl)-N, N'-diphenyl-1, 1'-biphenyl-4, 4'-diamine (NPB) layers, and their influence on the device performance was investigated. When a 1.0 nm thick Al₂O₃ film was inserted, for an unoptimized device composed of indium–tin oxide (ITO)/NPB/Al₂O₃/Alq₃/LiF/Al, the current efficiency was as high as 3.75 cd A⁻¹. It was much higher than that of conventional devices. The mechanism of performance enhancement is discussed.

The polarization dependence of phase modulation in a semiconductor optical amplifier (SOA) has negative impacts on some pump/probe applications of SOAs. We propose the necessity of fabricating SOA with polarization-insensitive gain and polarization-insensitive phase modulation and study the possibility. The investigation reveals that polarization sensitivity of phase modulation depends on the product of an optical confinement factor and a material differential refractive index. We study strain effects on polarization dependences of the differential refractive index and gain in a strained multiple-quantum well; the results demonstrate that by conducting the proper design of an active region and waveguide structure, reduction of SOA polarization sensitivity to gain and phase modulation can be achieved. An optimally designed example of a 1.55 µm tensile-strained MQW SOA is given.

An alternative streaming transport regime in wurtzite InN is predicted by ensemble Monte Carlo simulations. In polar semiconductors the conventional streaming transport, characterized by a cyclic motion of electrons in the momentum space, is caused by electron energy relaxation via optical phonon emission. It is shown that under certain conditions a different mechanism, namely impact ionization, can be responsible for the streaming transport as well. In the present paper, we observe transient drift velocity oscillations in InN with remarkably higher frequencies (23 THz at an electric field of 400 kV cm⁻¹) induced by impact ionization. Moreover, these oscillations occur even at room temperature and at high impurity concentrations in contrast to the conventional streaming transport. The frequency of the drift velocity oscillations depends on the strength of an applied static electric field and can be tuned over a wide frequency range.

A systematic study of radiation effects on the major parameters of ohmic and Schottky contacts based on n-ZnO is introduced. Al and Au metals were used as contact elements in order to fabricate the ohmic and Schottky structures, respectively. The transmission line method (TLM) measurements on Al/n-ZnO have revealed that high-energy (6, 9, 12 MeV) and relatively low-dose (3 × 10¹² e⁻ cm⁻²) electron irradiation produced lower specific ohmic contact resistivity values as compared with the reference sample. The current–voltage (I–V) and capacitance–voltage (C–V) measurements on the Au/n-ZnO structures are shown to increase in ideality and to decrease in the Schottky barrier heights with increasing electron energy. These findings have been interpreted based on the assumption that the atoms of the contact elements diffused into the semiconductor material, thus turning the rectifying character to ohmic behaviour with the influence of radiation–matter interaction and subsequent annealing effects.

Zinc selenide thin films were deposited onto well-cleaned glass and silicon substrates using the vacuum evaporation technique under a vacuum of 3 × 10⁻⁵ Torr. These films were implanted with mass-analysed 75 keV N⁺ ions at different doses ranging from 10¹⁴ to 10¹⁶ ions cm⁻². The composition, microstructure, surface roughness, optical bandgap and electrical properties of the as-deposited and nitrogen-implanted films were studied by Rutherford backscattering (RBS), grazing incidence x-ray diffraction (GIXRD), atomic force microscopy (AFM), Raman scattering, optical transmittance and I–V measurements. The RBS analysis indicates that the composition of the as-deposited and nitrogen-implanted films is nearly stoichiometric. The thickness of the as-deposited film is calculated as 230 nm. The structure of the as-deposited and nitrogen-implanted thin films is cubic. In the AFM analysis, the surface roughness is found to increase with an increasing dose of nitrogen ions. In the optical studies, the optical bandgap value decreases with increase of the N⁺ dose. The prepared device exhibits a very good response in the visible region.

We have studied the selective etching of Si_1−xGe_x versus silicon in a reduced pressure–chemical vapour deposition apparatus. Such a tool, normally used to deposit Si_1−xGe_x epitaxial layers, can indeed be called upon to conduct some lateral etching experiments as it is equipped with gaseous hydrochloric acid. Achieving a high enough selectivity in Si/Si_1−xGe_x/Si 'silicon-on-nothing' (SON) like stacks requires a germanium concentration of at least 30% (with our process conditions). (1 1 6) and (1 1 10) facets are formed at the tunnel entrance for [Ge] = 30%, however. Those facets will be more or less marked, depending on the etching time and etching temperature. A high selectivity (i.e. >100) between Si and Si_1−xGe_x can be achieved for higher Ge concentrations (i.e. >30%), without any facets. A Si_0.5Ge_0.5 layer can be removed with practically an infinite selectivity versus silicon (and also versus silicon dioxide). The tunnel depth has apparently some influence on the etching kinetics. Indeed, for etching to occur, (i) some incoming HCl molecules must reach the Si_1−xGe_x layer surface at the end of the tunnel and (ii) the etching by-products must be able to get out of the tunnel and thus reach the vent. Deeper tunnels will thus lead to lower etching rates. This HCl etching technique is thus usable for the fabrication of SON-like transistors when the Ge concentration of the sacrificial Si_1−xGe_x layer is higher than 30%.

This work investigates the superior high-temperature and high-linearity characteristics of a double δ-doped AlGaAs/In_xGa_1−xAs/AlGaAs pseudomorphic high electron mobility transistor (pHEMT) with a symmetrically linearly graded In_xGa_1−xAs channel and a wide energy gap AlGaAs barrier. Distinguished high-temperature device characteristics are presented, including an extrinsic transconductance (g_m,max) of 182 (223) mS mm⁻¹, a drain–source saturation current density (I_DSS) of 428 (524) mA mm⁻¹, an output conductance of 0.334 (0.352) mS mm⁻¹, a gate-voltage swing (GVS) of 1.45 (1.5) V, a voltage gain (A_v) of 505 (658) and a reverse breakdown voltage (BV_GD) of −24.1 (−31.2) V at 500 (300) K, respectively, with gate dimensions of 0.65 × 200 µm². In addition, the device demonstrates a superior stable thermal threshold coefficient (∂V_th/∂T) of −0.55 mV K⁻¹, a thermal GVS coefficient (∂GVS/∂T) of −0.25 mV K⁻¹ and a wide gate-bias range of 1.25 V for a unity-gain cut-off frequency (f_t) of over 20 GHz. Consequently, the proposed device shows good potential for high-temperature and high-linearity circuit applications.

We measured resistivity in the range of 30–390 K on four semi-insulating low-temperature grown molecular-beam epitaxy GaAs samples. The growth temperature range was from 215 °C to 315 °C. Arrhenius fittings with T⁻¹ and hopping fitting with T^−1/4 do not permit us the definition of the temperature ranges controlled by band and hopping conduction, respectively. This leads to major errors in the calculation of both activation energies and hopping parameters. We have used the differential activation energy in order to clearly identify the temperature range for the different transport mechanisms. Hopping dominates at low temperatures and band conduction at high temperatures. In-between, a mixed conduction regime is observed. We introduce a criterion to clearly define the temperature range of hopping, band and mixed conduction. The lower temperature at which mixed conduction is identified decreases for samples with increasing growth temperature. Only the sample grown at 215 °C presents both forms of hopping conduction before entering the mixed conduction regime. Hopping parameters were obtained from the fittings of the differential activation energy and the values are in good agreement with the usual method of calculating them if the correct temperature range is used.

It is shown how the total electric current in a semiconductor is modified due to heating of the carrier systems by a dc electric field. The change of the carrier concentration in the conduction and valence bands due to heating plays an important role in the transport of the electron and hole systems. The dependence of the capture factor on hot electron temperature leads to extra nonlinearity of the kinetic coefficient in the current–voltage characteristic curve even when the electron mobility is independent of the electron temperature. This additional nonlinear term in current density has never been considered in standard classical theory of hot electron transport. This extra nonlinear term is of the same order as those of the nonequilibrium energy of carrier contribution.

In spite of several papers, the origin of visible luminescence from germanium nanocrystals in a SiO₂ matrix is controversial even today. Some authors attribute the luminescence to quantum confinement of charge carriers in these nanocrystals. On the other hand, surface or defect states formed during the growth process have also been proposed as the source of luminescence in this system. We have addressed this long-standing query by simultaneous photoluminescence and Raman measurements on germanium nanocrystals embedded in the SiO₂ matrix, grown by two different techniques: (i) low-energy ion implantation and (ii) atom beam sputtering. Along with our own experimental observations, we have summarized some of the works done by other authors and proposed a hybrid model to explain the visible photoluminescence from nanocrystalline germanium in the SiO₂ matrix.

A thin-film a-Si:H pin detector was developed for selective detection of UVA (320–400 nm) radiation. In order for the fabrication technology to be transferable onto flexible substrates, all of the processing steps were conducted at temperatures less than 125 °C. The measured saturation current as low as 2 pA cm⁻² and the ideality factor of 1.47 show that the pin diodes have a good quality i-layer as well as p–i and n–i interfaces. The film thicknesses were optimized to suppress the detector sensitivity in the visible spectral range, and the peak of spectral response was observed at 410 nm. The selectivity estimated from the ratio of the photocurrent generated by UVA absorption to the total photocurrent is 21%.

We report the observation of ultrafast dynamics in interferometric switching using an InGaAs quantum-well laser amplifier. The switching windows, measured in a three-beam pump–probe interferometric set-up using sub-picojoule pulses of 800 fs duration at 1.57 µm wavelength, reveal an ultrafast (∼2 ps) feature. This reshaping of the switching window edge was found to change substantially with the pump pulse energy, and is attributed to refractive index changes caused by the heating of the carrier distribution in the amplifier. A rate-equation model is used to analyse these results, showing very good agreement with the experimental evidence.

The subsurface damage generated by mechanical polishing of silicon carbide wafers was investigated and quantified by plan view transmission electron microscopy (TEM) and atomic force microscopy (AFM). Damage generated during polishing using diamond abrasives with 0.5 µm particle size consists of dislocation loops with length up to 400 nm from the scratches. The total dislocation density was estimated at 5 × 10¹⁰ dislocations cm⁻². TEM analysis of the Burgers vectors indicates that the initial perfect dislocations have a Burgers vector of b = a/3 ⟨11–20⟩-type with many dislocation dissociated into two partials with b = a/3 ⟨1–100⟩. The depth of damage was estimated to be up to 50 nm. 4H–SiC homoepitaxial layers grown on mechanically polished substrates without further surface treatment exhibit threading dislocation density along scratches in the order of 10⁵ cm⁻¹.

In this study, we have investigated the time scale of the excitation of electrons leading to a transition from the quantum Hall (QH) state to the dissipative state in the two-dimensional electron system (2DES) of GaAs/AlGaAs heterostructures with Corbino geometry. The real-time-resolved measurements were performed by applying rectangular electrical pulses of 90 ns ⩽ t_p ⩽ 180 ns pulse widths to the contacts of QH Corbino discs with various electron mobilities (0.1 × 10⁶ cm² V⁻¹ s⁻¹ ⩽ μ_H ⩽ 1.6 × 10⁶ cm² V⁻¹ s⁻¹) at the integer filling factor of ν = 2. The breakdown of the quantum Hall effect (QHE) occurs after a certain time (3 ns ⩽ τ_exc ⩽ 30 ns, τ_exc ≪ t_p), which is a function of applied voltage (pulse amplitude), magnetic field, electron mobility and temperature.

Generation–recombination processes in a narrow band gap active region of infrared photodiodes are studied theoretically and experimentally. Thanks to an analysis of the transport in InAsSb photodiodes as a function of temperature, we demonstrate that these processes can be reduced by controlling the doping of the active region. The first Auger-dominated detector in this spectral range is shown, with negligible diffusion and SRH generation–recombination processes. This leads to the highest detectivity ever reported for a high-temperature antimonide-based detector in this spectral range: D* = 2.5 × 10¹⁰ cm Hz^1/2 W⁻¹ at 250 K and 1.3 × 10¹¹ cm Hz^1/2 W⁻¹ at 180 K and λ = 3.39 µm.

We demonstrate a tunable wavelength-selective photodetector which operates at a long wavelength. The device was fabricated by bonding a GaAs-based Fabry–Perot filter, which can be tuned via thermal-optic effects, with an InP-based p–i–n absorption structure. Device performance was theoretically investigated by considering the finite-size diffracting-beam input which is the normal case in practical conditions. We show that the size of the input diffracting-beam has a significant impact on device external quantum efficiency as well as the response linewidth. An integrated device with spectral linewidth as narrow as 0.7 nm (FWHM), wavelength tuning range of 9.0 nm (1540.9 nm ∼ 1549.9 nm), 3 dB bandwidth of 10 GHz and external quantum efficiency of about 18% was demonstrated.

In this paper, we report a collector-up npn heterojunction bipolar transistor (C-up HBT) which employs a p-type doping buried layer inserted between an extrinsic emitter and a subemitter for current confinement. Fabrication of C-up AlGaAs/GaAs HBTs with a selectively buried layer by metalorganic chemical vapour deposition (MOCVD) regrowth is described. The fabricated C-up AlGaAs/GaAs HBT demonstrates good common-emitter I–V characteristics and a current gain of 18. A systematic analysis is performed to verify the functionality of the p-type doping buried layer using a two-dimensional device simulator. It is found that the p-type doping buried layer should be biased properly to achieve the high efficiency of current confinement. And the HBT with a large ΔE_V at the base-emitter heterojunction is preferred for the proposed C-up HBTs to reduce hole back-injection.

The temperature-dependent dc characteristics and noise performance of an interesting InGaP/GaAs heterojunction bipolar transistor (HBT) with emitter ledge passivation are demonstrated. Experimentally, due to the emitter ledge passivation, higher current gains and wider collector current over the measured temperature range (300–400 K) are observed as compared to a conventional device. In addition, the studied device exhibits lower base current ideality factors, better thermal stabilities on dc current gains, lower base surface recombination current densities and improved device reliability. Therefore, the studied device is suitable for low-power and high-temperature electronic applications.

Extraction of the specific contact resistance, ρ_c, using the established circular transmission line measurement (c-TLM) and a series resistance measurement across mesa-isolated diodes is compared for a non-alloyed Pd/Ag contact to p-GaN. The limitations of the c-TLM technique are discussed and it is shown that for ρ_c values below 10⁻⁴ Ω cm² both unintentional submicron errors in the actual radii and uncertainty in the resistance measurements can lead to order of magnitude changes in the extracted ρ_c. An additional current–voltage measurement across a mesa-isolated diode is proposed. The accuracy of the extracted ρ_c in this case requires consistency of the intrinsic diode characteristic, namely the ideality, at the bias voltages used for extraction, which is in turn related to the carrier transport and recombination properties. From a comparison with the c-TLM, we conclude that the resistance should be extracted at current densities <10 A cm⁻² as junction heating changes the diode ideality.

A 3.6 THz quantum cascade detector is studied under the influence of a strong magnetic field applied perpendicularly to the plane of the layers. Modulation of lifetimes based on elastic intersubband scattering processes is observed, leading to an increase of the responsivity for particular values of the applied magnetic field.

Indium tin oxide (ITO) thin films have been deposited by rf magnetron sputtering on glass substrates at different substrate temperatures. The structural, electrical and optical properties of these films have been investigated to obtain optimum values for resistivity, optical transmittance and surface smoothness. The film deposited at a substrate temperature of 300 °C shows good conductivity, optical transmittance, crystallinity and surface smoothness. These ITO films were used to fabricate organic light emitting diodes (OLEDs). The dc current–voltage (I–V) studies on ITO/PEDOT:PSS/MEH-PPV/Al test structures show better rectifying behaviour on a smoother ITO substrate.

The effect of thermal annealing temperature on electrical and structural properties of Rh/Au Schottky contacts to n-type GaN (∼4 × 10¹⁷ cm⁻³) has been investigated by current–voltage (I–V), capacitance–voltage (C–V), x-ray diffraction (XRD) and Auger electron microscopy (AES). Calculations showed that the Schottky barrier height of the as-deposited Rh/Au contact was 0.57 eV (I–V) and 0.62 eV (C–V), respectively. However, the Schottky barrier height increased with annealing temperature up to 500 °C, reaching maximum values of 0.84 eV (I–V) and 1.05 eV (C–V). Based on the Auger electron microscopy and x-ray diffraction results, the formation of gallide phases at the Rh/Au/n-GaN interface could be the reason for the increase of Schottky barrier heights after annealing at temperatures 400 °C and 500 °C.

We report results from experimental studies on the current driven breakdown of the quantum Hall effect in an InAs/GaSb-based electron–hole system. We find that the critical currents observed in this system are typically much smaller than those reported for single carrier-type systems. Width dependence measurements show two regimes of behaviour. For sufficiently wide samples, the critical current has a linear dependence on the channel width, while for narrower samples, the critical current has a strong tendency to be vastly reduced. There is a critical width at which this crossover occurs, which is found to depend on the magnetic field, Fermi energy and the ratio between the electron and hole concentrations. We refer to the two regimes as the 'linear regime' and the 'fragile regime'. For samples in the linear regime, critical electric fields or macroscopic critical current densities can be defined. Although the absolute values seem small compared to single carrier-type systems, the values are found to be comparable when scaled by the activation energies. In the fragile regime, the critical current is disproportionately small compared with single carrier-type systems.

In a quantum dot in which the spin degeneracy of a carrier is lifted, the Fano effect may be used as an effective means to generate spin polarization of transmitted carriers. In this work, we propose a new and more effective design of a spin-dependent polarizer. The proposed device consists of a quantum wire with two side-coupled quantum dots. A detailed analysis of the spin-dependent polarized current is carried out, and we find some improvements as compared to more conventional designs.

The PDF file provided contains web links to all articles in this volume.

As we close volume 21 of Semiconductor Science and Technology it is interesting to reflect on the achievements of the year. Notable this year has been the considerable increase in submissions to the journal. As of the end of October we had received a 26% increase in submissions compared to the same time last year. This has naturally had a considerable impact on the number of referees we need in order to maintain the high quality standards of the journal. In the first ten months of this year Semiconductor Science and Technology approached 1468 referees based in 36 different countries. Of the 1468 referees asked to review an article just under half, 723, wrote a report.

Excluding members of the Editorial Board, who often review one or more articles a month, our `most hard-working' referee reviewed 6 articles for us in the first ten months of this year. In order to provide a timely publication decision for our authors we ask referees to prepare a review within 21 days. The average time for a referee to report is just under this at 18 days. Our fastest review this year so far was an impressive 2 hours and 5 minutes.

Managing the peer review for a growing journal like Semiconductor Science and Technology would be a far more difficult task if it were not for the many conscientious referees who review articles for us. We would like to take this opportunity to express our thanks to all of our referees from this and previous years. We value your careful and well constructed reports which are of great assistance to us in maintaining the rigorous quality standards of Semiconductor Science and Technology.