Showing papers in "Semiconductor Science and Technology in 1990"

Electrical properties of grain boundaries in polycrystalline compound semiconductors

Abstract: In polycrystalline semiconductors the trapping of charge at the grain boundaries has a decisive influence on the electrical transport properties through the formation of electrostatic potential barriers. By proper materials processing many interesting device applications can be realised, which exploit the electrical activity of these interfaces. In this review, the authors describe both the origin and the consequences of the charge capturing at grain boundaries. Special emphasis is given to polycrystalline compound semiconductors, where they summarise the present knowledge on the interface microstructure and its electrical properties. The model of a double Schottky barrier is shown to provide a quantitative basis for understanding the wide range of electrical phenomena in this class of materials. The steady-state current-voltage characteristic becomes highly non-linear through the interplay between the applied bias and the occupation of the defect states at the interface and in the depletion regions. For large potential barriers, high doping levels and elevated bias, large electric fields build up in the depletion regions. This triggers minority carrier generation through impact ionisation by hot majority carriers and strongly enhances the non-linearities in the charge transport. The dynamic electrical properties are probed by AC admittance or pulse measurements and can be traced back to the finite relaxation times of the trapped electron and hole charges. Comparing the experimental results with the theoretical predictions allows one to obtain valuable information on the electronic grain boundary parameters. The relationship between the observed electrical properties and the electronic structure of the junctions is discussed in detail, with ZnO varistors providing the majority of the experimental data. First indications for a general picture of the grain boundary electronic structure appropriate for all compound semiconductors are presented.

A new technique for growing large surface area SnO2 thin film (RGTO technique)

Abstract: A new technique for growing SnO2 thin films with high surface area is described, based on tin rheotaxial growth and its thermal oxidation (RGTO). Tin thin films, when grown, present a surface characterized by spheroidal agglomerates due essentially to the surface tension of the liquid metal; these agglomerates do not seem to have any electrical continuity. By means of a thermal oxidation, both the transformation of the metal into a semiconductor and the thin film continuity are obtained, owing to the volume increase during the above-mentioned phase transformation. After this annealing cycle, SnO2 thin films are slightly oriented in the (101) direction and present an electrical resistivity equal to about 102 Omega cm. So, the surfaces of SnO2 films appear to be formed by spongeous agglomerates, which are in electrical contact with the nearest agglomerates. SnO2 thin films grown by this method show a high sensitivity (defined as the relative per cent conductance variation) to H2; in fact, sensitivity to 200 PPM H2 in synthetic air at ambient pressure is equal to 400%. It seems to be possible to prepare other metal oxide semiconducting thin films with a high surface area using this technique.

Inter-subband scattering in a 2D electron gas

Abstract: A new technique, using the absolute amplitude of the low-field Shubnikov-de Haas oscillations, is presented which allows the direct measurement of inter-subband scattering in 2D heterojunction structures. When applied to a heterojunction with two subbands occupied the technique is used to show that inter-subband scattering is independent of temperature between 1 and 4 K. In agreement with previous reports frequency and amplitude intermodulation, which increases with temperature, is also observed. This is attributed to mixing, by thermal or impurity damping, between the two sets of oscillations in the Fermi energy. This explanation is supported by a model calculation which reproduces the features of the experimental results.

An FT-IR study of silicon dioxides for VLSI microelectronics

Abstract: SiO2 layers that are applied in VLSI (very large scale integration) microelectronics (gate dielectric, insulation, etc) are prepared in different ways: thermal, chemical vapour deposition (CVD), thermochemical. It has been investigated whether the composition, thickness, film stress and disorder of thin silicon dioxide can be characterised in a simple manner by Fourier-transform infrared absorption spectroscopy (FT-IR). In addition to the main absorption bands at 460 cm-1, 808 cm-1 and 1075 cm-1 (all transverse optical modes) spectral features at 1165 cm-1, 1200 cm-1 (LO4-TO4 pair) and 1230-1250 cm-1 (LO3 mode) have been analysed as a function of film thickness, preparation conditions, substrate and rapid thermal annealing. The position, full width at half maximum of the band at 1075 cm-1, the position of the LO3 mode and the intensity ratio of the LO4-TO4 pair and the LO3 mode can be related in a qualitative fashion to film stress, composition and disorder of the thin SiO2 films.

Physics and applications of IV-VI compound semiconductor lasers

Abstract: Lasers made of lead salt compounds have proven to be best suited for coverage of the mid-infrared region from 3 to 30 mu m. Nowadays double-heterostructure (DH) lasers are the standard, using as active layers PbEuSSe for the short-wavelength region below 4 mu m, PbEuTeSe or PbEuSe for the 4-8 mu m range and PbSnTe or PbSnSe for wavelengths beyond 8 mu m. Originally liquid phase epitaxy but now molecular beam and hot wall epitaxy techniques are most commonly used to produce a wide variety of laser structures. While simple DH lasers with contact or mesa stripes for lateral confinement prevail in commercial devices, highly sophisticated prototypes with buried layer, corrugated distributed Bragg reflector (DBR) and distributed feedback (DFB) and single- and multi-quantum-well structures have been successfully manufactured. Gas spectroscopy has remained the main field of application where mode quality and tuning properties of the emitted radiation are most important. The dominant development goal has been the increase of the operating temperature to make the use of simple cooling equipment possible. Theoretical models for threshold current calculations have been established; however, the reliability of the results has been impaired by the fact that basic material properties of the ternary and quaternary compounds are not well know. In the course of this paper the state of the art and the development trends of lead salt lasers are described. The various fields of applications are summarised. Two examples-one dealing with car exhaust monitoring, the other with clinical isotopic ratio measurements of the exhaled breath-are discussed in some detail.

Non-equilibrium modes of operation of narrow-gap semiconductor devices

Abstract: Research work is reviewed which uses the phenomena of minority carrier extraction and minority carrier exclusion to reduce the carrier densities in narrow-gap Hg1-xCdxTe alloys to near-extrinsic values at temperatures where the materials are normally intrinsic. This is of particular interest for the suppression of Auger noise which otherwise severely limits the performance achievable from infrared detectors at near-ambient operating temperatures. Improved performance has been obtained from excluding photoconductive detectors at modulation frequencies above 10 kHz. Theoretical work predicts improved performance from heterojunction extracting diodes. Early experiments on 'proximity-extracting' diode structures show large reductions in leakage current at intermediate and ambient temperatures compared to non-extracted diodes, but the residual current is higher than that predicted from Shockley-Read generation. Bipolar transistor action is observed at temperatures where the material is near-intrinsic in equilibrium. The transistor characteristics are strongly influenced by electron impact ionisation.

Photoconductive bismuth sulphide thin films by chemical deposition

Abstract: A chemical bath deposition technique for obtaining photoconductive grade bismuth sulphide (Bi2S3) thin films of thickness approximately=0.05 to 0.3 mu m is described. These films show a photo-current to dark-current ratio of 100-500 and photoconductivity of approximately=1 Omega -1 cm-1 at 25 degrees C, under 1200 W m-2 tungsten-halogen illumination. Air annealing in the 100-250 degrees C range can drastically affect the photocurrent response of these films. The dark conductivity of the films can be increased by a factor of approximately=107 during such an annealing process. This suggests the possibility of post-deposition processing of the films for specific optical and optoelectronic applications.

On excitation and decay mechanisms of the Yb3+ luminescence in InP

Abstract: The excitation and decay mechanisms of the Yb3+ intra-4f-shell emission are studied in n-type MOVPE-grown and p-type LPE grown InP:Yb layers by photoconductivity measurements, time-resolved photoluminescence, photoluminescence excitation and emission spectroscopy. Assuming a pseudo-donor or pseudo-acceptor-like character of the isoelectronic Yb3+ Td centre the temperature dependences of the 4f-shell transition intensity and lifetime can be consistently explained. Models for the excitation and decay processes of the Yb3+ photoluminescence are proposed.

Surface phonons in GaAs cylinders

Abstract: Raman scattering from arrays of GaAs quantum cylinders with radii down to 30 nm has been observed. The spectra show an enhanced TO phonon intensity due to a change in the first-order selection rules. In addition, the smallest cylinders exhibit an extra spectral feature between the TO and LO phonons. The behaviour of this additional peak as a function of the scattering geometry, the cylinder size and the variation of the medium surrounding the cylinders is consistent with a theoretical model of the surface phonons of a cylinder.

Observation of high mobility and cyclotron resonance in 20 Å silicon delta-doped GaAs grown by MBE at 480 °C

Abstract: The authors report on temperature-dependent Hall effect measurements on Si delta-doped GaAs samples grown by MBE at 480 °C, 530 °C and 620 °C. In the best sample grown at 480 °C the mobility is 6760 cm2 V-1 s-1 at 4.2 K. To the authors' knowledge this is the highest mobility ever reported in a delta-doped structure. From subband population measurements the spreading of the donors in the samples grown at low temperature is determined to be 20 A. On these high-mobility samples they were able to perform the first reported cyclotron resonance measurements. The electron effective mass is found to be considerably higher than that at the Gamma-conduction band minimum in GaAs.

Modelling of electroabsorption in coupled quantum wells with applications to low voltage optical modulation

Abstract: The authors model the room temperature electroabsorption of coupled double quantum wells using a transfer matrix technique to calculate carrier wavefunctions and their overlaps. Their behaviour with an electric field is compared with uncoupled multiple quantum wells with optical modulation in mind. Consideration of wavefunctions and energy levels allows them to choose a coupled well structure with good optical modulation at a low electric field. By fitting three parameters once, they predict absorption spectra. These are used to assess coupled quantum wells within an asymmetric Fabry-Perot reflection modulator. Very low voltage operating characteristics were calculated allowing for a total fluctuation in the coupling barrier width of one monolayer: a contrast ratio of 2.1:1, an insertion loss of 3.3 dB and an applied voltage of only 0.7 V; alternatively, a very high contrast with an insertion loss of 9.4 dB operating at 2.2 V. However, these predictions may be difficult to fulfil in devices grown at present because of the built-in field within the intrinsic region of a p-i-n diode.

Transformation steps of structure in flash-deposited films of a-InSe

Abstract: Conditions have been developed for the deposition of InSe semiconducting stoichiometric films using the flash evaporation technique. Structural identifications of the InSe films thus obtained have been determined using X-ray diffraction, and spectroscopic characterisations have been established by infrared and Raman scattering measurements. Optical absorption has been investigated over the range 0.8-1.5 eV. The crystallinity and morphology of films have been studied as a function of the starting material composition for which reproducible polycrystalline InSe films are found. The transformation steps of structure in flash-deposited films of a-InSe are consistent with the variation of vibrational measurements and with the changes of the absorption coefficient upon heat treatment.

High performance GaAs/GaInP heterostructure bipolar transistors grown by low-pressure metal-organic chemical vapour deposition

Abstract: The authors report the fabrication of high-quality GaAs/GaInP n-p-n heterojunction bipolar transistors grown by low-pressure metal-organic chemical vapour deposition on semi-insulating substrates. Various structures with homogeneous and graded bases have been fabricated. Doping profiles together with X-ray double diffraction patterns demonstrate the excellent control of growth parameters such as thicknesses, doping levels and interface quality. The static characteristics of the devices show current gains as high as 400, which is the highest value reported in that system.

Theoretical limit of mobility of two-dimensional electrons in GaAs

Abstract: The acoustic phonon limited mobility of two-dimensional electrons in an (001)AlGaAs/GaAs heterojunction is calculated. It exceeds the highest experimentally achieved value of mobility by approximately six times at the liquid helium temperature.

Physics and applications of IV-VI compound quantum well and superlattice structures

Abstract: The authors review the experimental investigations carried out on IV-VI artificially structured materials are summarised. The problems related to the determination of band offsets are discussed and special emphasis is given to strain effects and the consequences of interdiffusion. Deviations from the bulk magnetic behaviour reported in dilute magnetic IV-VI quantum well structures are presented. The unique structural properties of layered PbGeTe/PbTe which exhibit a stabilisation of the cubic phase are discussed.

Amphoteric behaviour of group V dopants in (Hg, Cd)Te

Abstract: The electrical behaviour of phosphorus in bulk Hg0.8Cd0.2Te crystals is compared with that in Hg0.7Cd0.3Te epitaxial films grown from Te-rich solutions. The mass action constants associated with the process of site transfer of phosphorus atoms from Hg to Te or interstitial lattice sites have been deduced for the bulk crystals and the epitaxial films. Additionally, the mass action constants for the lattice site transfer of As and Sb have also been deduced. The values of these constants indicate that the lack of site transfer from Hg to Te or interstitial lattice sites in the case of Sb and Bi, inferred experimentally in epitaxial films, is ascribed to a lower diffusivity of the Sb and Bi atoms in (Hg, Cd)Te. The implications of the amphoteric behaviour of the group V dopants on device fabrication are also discussed.

Optically detected nuclear magnetic resonance of nuclei within a quantum well

Abstract: Observations are reported of nuclear magnetic resonance (NMR) for nuclei in single GaAs/AlGaAs quantum wells, using an optical pumping and detection scheme. The technique gives signals only from nuclei in contact with the quantum-confined electron wavefunction and therefore discriminates against substrate and barrier nuclei. Some general factors which affect the strength of signals are discussed and preliminary results are presented which show the potential of NMR for measuring strain in pseudomorphic systems and for detecting and identifying impurities.

The p-to-n conversion of HgCdTe, HgZnTe and HgMnTe by anodic oxidation and subsequent heat treatment

Abstract: p-n junctions have been formed by anodic oxidation of HgCdTe, HgZnTe and HgMnTe followed by a low-temperature anneal. The dependence of junction depth on anneal time, temperature, concentration of native acceptors and material composition has been investigated. The results are interpreted as a diffusion of mercury into the underlying bulk from a finite source formed during anodisation at the interface between the oxide and the bulk of mercury-based semiconductor alloys. The converted layers exhibit high recommendation times and the method can be used for fabrication of photoconductors and photodiodes.

Preparation of ZnO films films by activated reactive evaporation

Abstract: High quality films of zinc oxide were prepared by an activated reactive evaporation technique onto 7059 Corning glass substrates. Film resistivities of approximately 10-3 Omega cm and optical transparency of more than 88% in the wavelength range 300-800 nm were prepared by adjusting depositing parameters. The electron carrier densities in the range 2-12*1019 cm-3 and mobilities of 10-45 cm2 V-1 s-1 were observed. The energy gap of zinc oxide film was evaluated as 3.3 eV from optical absorption measurements.

Hot electron transport in GaAs quantum wells: non-drifting hot phonons

Abstract: Experimental data concerning high-field parallel transport in GaAs/GaAlAs quantum well structures are reported. The results strongly suggest that (i) in modulation doped GaAs quantum wells the non-equilibrium LO phonons (hot phonons) are non-drifting, and (ii) in samples with similar 2D electron densities hot phonon effects increase with reduced dimensionality, and hence with increasing 3D electron concentration. Both observations are in excellent agreement with a recent model of high-field transport involving hot phonons.

The Au/CdTe interface: an investigation of electrical barriers by ballistic electron emission microscopy

Abstract: The authors describe the application of ballistic electron emission microscopy to measure electrical barriers with nm spatial resolution. They show that the interface between Au and chemically treated CdTe is non-uniform and that problems of reproducibility are associated with small patches at the interface where the barriers are low.

Electronic properties of band-inverted heterojunctions: supersymmetry in narrow-gap semiconductors

Abstract: Heterojunctions between semiconductors with mutually inverted bands contain massless spin-non-degenerate interface electron states independent of their profile. The universality of these states is due to the specific symmetry (the supersymmetry) of the effective Hamiltonian. The giant Landau splitting of interface states and selection rules for optical transitions in a magnetic field are predicted. Other types of 'supersymmetric' heterostructures-ferroelectric and antiferromagnetic domain walls-also possess interface states with unique properties.

Galvanomagnetic properties of single-crystal bismuth-antimony thin films

Abstract: Describes growth of the first thin ( approximately 1 mu m) epitaxial films of pure bismuth-antimony alloys using molecular beam epitaxy techniques. These structures were grown at elevated temperatures on single-crystal barium fluoride substrates of (111) orientation. Electron microscope observations show the films to be featureless and defect-free on the scale of 0.1 mu m. The films grow with their trigonal axis parallel to the (111) axis of the substrate, and Laue-backscattering pictures show that they are epitaxial. Mobilities of alloys with x=0 are of the order of 2 m2 V-1 s-1 at room temperature and increase to over 10 at 20 K and 100 at liquid helium temperatures. These values are far superior to those of other bismuth films grown to data, and approach mobilities observed in single-crystal bismuth. The dependence of energy gap and c axis lattice constant on x is different from that in bulk alloys, which may be due to the effects of strain arising from the 3.6% lattice mismatch between sample and substrate.

A detailed study of Auger recombination in 1.3?m InGaAsP/InP quantum wells and quantum well wires

Abstract: A detailed study of direct Auger processes in 1.3 mu m InGaAsP/InP quantum wells and quantum well wires is presented. CHCC, CHLH, and CHSH Auger processes are considered, as are transitions involving confined states only, and transitions in which the excited carrier is in an unbound state of the well, with the other carriers confined. The effects of using realistic confined state wavevectors and the inclusion of evanescent tails for the confined states are examined. The approximations of Boltzmann statistics and parabolic subbands are examined, and it is found that these assumptions do not unduly affect the results (in contrast to the bulk case). A comparison of numerical results with present experimental estimates of Auger coefficients is given.

Determination of acceptor ionisation energies in CdxHg1-xTe

Abstract: Variable temperature Hall measurements have been performed on p-type CdxHg1-xTe(CMT) with 0.18

Investigation of photons in HgCdTe using Raman scattering and far-infrared reflectivity

Abstract: The authors have measured and analyzed the Raman-scattering and far-infrared spectra from unoriented n- and p-type Hg1-xCdxTe with x approximately=0.21. Measurements were made in the 80-180 cm-1 frequency region at temperatures of 10 and 80 K. Comparison of the n- and p-type spectra leads to the conclusion that the electronic contribution to the Raman spectra is small. In particular, the data show that the 135 cm-1 feature originates in lattice effects and may be related to the Te-3Hg-Cd combination as previously suggested. The weak but distinct fine structure present in the CdTe-like reststrahlen band at approximately 153 cm-1 is tentatively assigned to be a result of localised oscillations of Cd ions in the alloy lattice. A study of the spectral changes between 10 and 80 K clearly indicates a softening of the HgTe-like TO phonon.

Electron trapping instabilities in polycrystalline silicon thin film transistors

Abstract: The stability of polycrystalline silicon thin film transistors formed at low temperatures has been studied. Charge trapping in the deposited gate oxide is observed over a wide range of conditions. At high gate biases this is due to the passage of a tunnel current, and at lower biases to direct tunnelling into the traps. Trap concentrations, cross sections and energy levels have been deduced based upon a simple trapping model, and an empirical device lifetime model is presented.

Pressure dependence of electron concentration and mobility in GaAs:Si-effects of on-site and inter-site interactions within a system of DX centres

Abstract: A novel special experimental procedure, where a helium gas pressure cell is employed to vary the hydrostatic pressure at low temperatures, is used to study the freeze-out of carriers on the metastable states of Si donors in GaAs. This technique separates the modifications of the mobility originating from alterations in the band structure from those related to the electron transfer to DX centres. The procedure allows a more precise evaluation of the ability of high-pressure experiments to distinguish between models of positive and negative U for the DX centre. The mobility increase with pressure is theoretically shown to occur for both repulsive (positive U) and attractive (negative U) on-site electron-electron interactions, provided that the inter-site Coulomb interactions are taken into account.

Carbon-related radiation damage centres and processes in p-type Si

Abstract: The paper is concerned with carbon interstitial (Ci), Ev+0.28 eV, and a carbon-related centre, Ev+0.37 eV, growing almost simultaneously with the removal of the Ci atoms in electron-irradiated p-type Cz-grown Si. An observed inverse annealing stage of the former defect at approximately 275 K most likely indicates the existence of internal processes taking place below room temperature which liberate Ci as a final product. These processes are in accord with other experimental findings showing that the concentration of the Ev+0.37 eV defect is larger than that of the Ci. The identity of the two centres found to contribute to the creation of the Ev+0.37 eV defect state is discussed.

Quantised dissipative states at breakdown of the quantum Hall effect

Abstract: The authors report the breakdown of the nearly dissipationless quantum Hall effect into a set of distinct, quantised dissipative states in a wide, high-quality GaAs/AlGaAs sample. They found 35 dissipative stages on one plateau and nine on another plateau which have longitudinal voltage drops accurately quantised in units of h(cross) omega c/e to within our +or-0.6% measurement uncertainty. This voltage quantisation implies that the energy dissipation per carrier is quantised in units of the Landau level spacing h(cross) omega c.