Showing papers in "Semiconductor Science and Technology in 1991"

Heterojunction band offsets and effective masses in III-V quaternary alloys

Abstract: Estimates of valence-band and conduction-band offsets for lattice-matched and pseudomorphic strained heterostructures of six technologically important III-V quaternary alloys are presented. Valence-band offsets are obtained via interpolation of the theory-based results of Van de Walle's 'model-solid' approach for the binary constituents. Estimates for band gap differences are obtained via interpolation of the experimental band gap energies of the ternary constituents. Adding the valence-band offset and band gap difference gives an estimate of the conduction-band offset. Band-edge effective masses at Gamma are determined from a linear interpolation of the effective masses of the binary constituents, obtained from self-consistent ab initio band structure calculations. Results are shown to agree well with the outcome of experiments.

Simplified chemical deposition technique for good quality SnS thin films

Abstract: A chemical deposition technique, much simpler and more versatile than previously reported and capable of yielding good quality SnS films of thickness up to approximately=1.2 mu m under a choice of deposition conditions, is presented. The as-prepared films are polycrystalline with p-type dark conductivity in the range 10-5-10-4 Omega -1 cm-1 for the thicker ( approximately 1 mu m) films and showing a photocurrent to dark current ratio of 5-10 under 500 W m-2 tungsten halogen illumination. The optical transmittance and reflectance spectra and the photocurrent response curves of a series of SnS samples are explicitly presented to provide insight into possible applications of these films.

Structure, properties and applications of GexSi1-x strained layers and superlattices

Abstract: The first successful pseudomorphic GexSi1-x strained layers on Si were grown in Germany in 1975. The extensive work that has been done on the production, properties and application of GexSi1-x strained layers and superlattices over the last fifteen years is described in this review. Values of critical layer thickness of the strained layers predicted by equilibrium theories are discussed and compared with experimental results. For x<0.6 the observed values are much larger than the theoretical values. The possible causes of this discrepancy are investigated and it is concluded that the layers must be metastable due to an energy barrier for the production of dislocations. Critical layer thicknesses of superlattices are also discussed and symmetrically strained superlattices are described. Results of band structure calculations of the strained layers and superlattices are given. Strain causes a large reduction in the fundamental indirect band gap of the Si/Ge alloy. Calculated band line-ups at the strained layer-substrate interface are compared with experiment. Modifications in the band structure due to strain have considerable effect on the optical and transport properties of the strained layers. Observed luminescence, photoconductivity and mobility are compared with predictions, taking strain into account. In the case of superlattices, band structure is further modified by zone folding. Superlattice structures which can produce a direct band gap are discussed. Raman scattering has proved a valuable tool for characterizing strained layers and superlattices. In the case of GexSi1-x strained layers, the Raman spectrum shows phonon lines shifted by strain. In the case of superlattices, additional lines due to zone folded LA phonons are observed. Raman measurements may be used to determine strain, thickness of a period and quality of interfaces. This review places emphasis on the use of Si/Ge strained in layers in Si-technology-based integrated circuits. The devices which have been fabricated to exploit properties of these layers include double heterostructure bipolar transistors, optical detectors, modulation doped field-effect transistors and mixed tunnelling avalanche transit time diodes. Progress made in the last 18 months has been rapid; a transistor with a cut-off frequency of 75 GHz was recently fabricated and this device has considerable promise.

Contactless evaluation of semi-insulating GaAs wafer resistivity using the time-dependent charge measurement

Abstract: A method based on time-dependent measurement of charge transients has been developed to evaluate the specific resistivity of semi-insulating wafers quickly, non-destructively and with good lateral resolution. The material is inserted between capacitive electrodes. The time-dependent charge distribution after application of a voltage step allows evaluation of the resistivity with high accuracy in the range 106-109 Omega cm. The technique has been elaborated to allow rapid contactless scanning of wafers for routine measurement of the lateral variation of resistivity with a resolution of about 2 mm2. The results are in agreement with conventional Hall measurements. The mechanical and electronic systems are described in detail. Scans across wafers cut from as-grown as well as annealed ingots are presented.

Hydrogen in crystalline semiconductors

Abstract: The state of the current understanding of hydrogen in elemental and compound semiconductors is reviewed. The results of new experimental and theoretical studies of the microscopic structure of acceptor-hydrogen and donor-hydrogen complexes are presented. Questions regarding hydrogen tunnelling in some of the acceptor and donor complexes in elemental semiconductors, as well as the problem of electronic states of isolated hydrogen, are discussed in the light of the most recent experimental findings. Also, some promising areas of future research are indicated.

Long-wavelength 128*128 GaAs quantum well infrared photodetector arrays

Abstract: The authors discuss the device physics and detector performance of quantum well infrared photodetectors (QWIPS). On the basis of these results, large two-dimensional arrays of 128*128 pixels were fabricated and measured. The excellent long-wavelength imaging results (99% of the pixels working and a noise-equivalent temperature difference NE Delta T=10 mK) demonstrate the potential of this new GaAs QWIP technology.

Subband dependent mobilities and carrier saturation mechanisms in thin Si doping layers in GaAs in the high density limit

Abstract: Shubnikov-de Haas and persistent photoconductivity measurements have been performed as a function of hydrostatic pressure to study the saturation of the free electron concentration and the mobilities of the individual subbands at high doping densities in very thin sheets (2, 5, 10 nm) of silicon donors in MBE GaAs. The samples were grown at very low temperature (400 degrees C) in order to limit dopant diffusion, and silicon concentrations were close to the solubility limit at this temperature. As has been shown previously with spike-doped GaAs(Si), the relative occupancies and the mobilities of the lower subbands are very sensitive to the spreading of the dopant distribution. A routine was developed for the analysis of the Fourier transforms of the complex pattern of Shubnikov-de Haas peaks in order to provide quantitative values for the mobilities of the individual subbands. The results of this analysis are compared with values deduced from the magnetic field dependence of the resistivity and Hall effect. On applying hydrostatic pressures of the order of 15 kbar in the dark, a decrease of the free electron concentration of a factor of two was observed. This was accompanied by an increase in the mobility of all the subbands due to the change in the charge state of the silicon donors in the doping slab. With the two thinnest slabs the mobility at ambient pressure is so low in the i=0 subband that Shubnikov-de Haas peaks from this subband could not be detected at fields up to 15 T, although strong peaks could be observed from the higher order subbands. After illumination of the thinnest sample at high pressure the measured free electron concentration is not restored to the zero pressure value. One possibility is that the missing electrons occupy a localized non-metastable Si state resonant with the conduction band rather than a DX centre. The pressure coefficient of the carrier density yields an extrapolated position for the energy level for the Si localized state of 270+or-10 meV above the Gamma -conduction band edge at ambient pressure.

Photovoltaic infrared sensor arrays in monolithic lead chalcogenides on silicon

Abstract: MBE growth of epitaxial IV-VI lead salt layers on Si (111) substrates and fabrication of photovoltaic infrared devices in the layers is reviewed. IV-VI on Si IR sensors have potential as a low-cost technique of fabrication of large IR focal plane arrays for both the 3-5 mu m and 8-12 mu m ranges because of the easy fabrication procedure and because uniformity problems are much less severe in IV-VIs due to the weaker composition dependence of the bandgap compared with Cd1-xHgxTe. Sensor arrays are fabricated in 2-4 mu m thick PbTe, PbS1-xSex and Pb1-xEuxSe for 3-5 mu m and in Pb1-xSnxSe for 8-12 mu m cut-off. An intermediate epitaxial stacked 0.2 mu m thick CaF2-BaF2 bilayer serves for compatibility and helps to overcome the large lattice and thermal expansion mismatch between the Si substrate and the IV-VI layer. Perfectly smooth surfaces with surface defect concentrations down to 103 cm-2, and X-ray rocking-curve linewidth of approximately=150 arcsec are obtained. Sensor arrays with 66 and 256 elements are described, the latter having been grown on standard Si chips with Al metallization.

Photoluminescence of GaSb grown by metal-organic vapour phase epitaxy

Abstract: Low-temperature photoluminescence of epitaxial GaSb grown by MOVPE from TMGa and TMSb on various substrates is studied and compared with existing results for GaSb grown by other techniques. The effects of growth conditions are considered. It is found that a growth temperature of 650 degrees C is too high, and the layers are of very poor quality, while below the optimum temperature of 600 degrees C the growth rate slows, although the optical quality appears unaffected. Investigations into the range of III/V ratios over which good quality material could be grown indicated that this factor was more critical for GaSb than for GaAs; Sb-rich conditions produced samples with poor radiative efficiency, while samples grown under Ga-rich conditions were covered in excess Ga droplets. In addition, the authors found that, in common with other growth techniques, the concentration of the native defect in GaSb could be controlled using the III/V ratio, and an excellent correlation was found between electrical results and features in the photoluminescence spectra. For layers not lattice-matched to the substrate, the spectrum is red-shifted. They surmise that this is due to differential thermal contraction of the epilayer and substrate. A homoepitaxial sample was chosen for detailed study and from the dependence of the spectra on temperature and excitation intensity, a previously observed bound exciton was confirmed and an acceptor of 15 meV binding energy was found.

Donor-related levels in GaAs and AlxGa1-xAs

Abstract: It is now widely accepted that DX levels in n-type AlxGa1-xAs are ground states of isolated substitutional donors in distorted configurations which are stabilized by trapping two electrons. The leading model is that the distortion occurs when the group IV donor substituted on a group III lattice site, moves along a axis towards an interstitial site. In the case of a group VI donor which substitutes on a group V lattice site, one of the group III neighbours moves toward the interstitial site. Recent work suggests that localized excited states of both donor configurations play an important role in electron transitions between DX levels and the conduction band. The research leading to this view of substitutional donors is reviewed. Some implications of this model for technological applications of AlxGa1-xAs and related alloys are discussed.

Type II superlattices for infrared detectors and devices

Abstract: Superlattices consisting of combinations of III-V semiconductors with type II band alignments are of interest for infrared applications because their energy gaps can be made smaller than those of any 'natural' III-V compounds. Specifically, it has been demonstrated that both InSb/InAsxSb1-x superlattices and Ga1-xInxSb/InAs superlattices can possess energy gaps in the 8-14 mu m range. The efforts have focused on the Ga1-xInxSb/InAs system because of its extreme broken gap band alignment, which results in narrow energy gaps for very short superlattice periods. The authors report the use of in situ chemical doping of Ga1-xInxSb/InAs superlattices to fabricate p-n photodiodes. These diodes display a clear photovoltaic response with a threshold near 12 mu m. They have also attained outstanding structural quality in Ga1-xInxSb/InAs superlattices grown on radiatively heated GaSb substrates. Cross-sectional transmission electron microscope images of these superlattices display no dislocations, while high resolution X-ray diffraction scans reveal sharp high-order superlattice satellites and strong Pendellosung fringes.

Physics and technology of thin film electroluminescent displays

Abstract: The underlying physics involved in the industrially produced yellow emitting thin-film electroluminescent devices (TFELD) with ZnS:Mn as the active material is reviewed and the aspect of newly emerging solutions for full colour TFELDS is considered. This review concentrates on the importance of the concept of loss-free acceleration for the understanding of high light generation efficiency. Some implications for the basics of high-field electronic transport are mentioned: the range of field strengths and times involved seems to be difficult to treat within the idea of band structures. Other basic features of any understanding of the device are the impact excitation of inner shell transitions, energy transfer and cross relaxation within the dopant system, and tunnel emission from interface states, required to occur with high densities at a suitable depth. From formulae, based on the so-called simple model of TFELD, the operating parameters, and their limits and expected margins are derived. A bright future for full colour TFELDS is forecast.

Optical and electro-optical properties of II-VI quantum dots

Abstract: The paper summarizes recent studies on absorption, luminescence, nonlinear optical effects and electro-absorption on wide-gap II-VI nano-crystals embedded in a glass matrix.

Operation and properties of narrow-gap semiconductor devices near room temperature using non-equilibrium techniques

Abstract: Narrow-gap semiconductors have many unique properties which are advantageous for optical and electronic applications These properties and the principles of the non-equilibrium techniques which are required to access them at or near ambient temperature are reviewed The application of these techniques to raise the operating temperature of cadmium mercury telluride infrared detectors is discussed The growth of InSb and In1-xAlxSb to demonstrate the operation of other electronic devices in a narrow-gap semiconductor is described Heterostructure diodes incorporating a thin In1-xAlxSb layer have values of R0A several orders of magnitude greater than conventional devices at ambient temperature Negative differential resistance predicted to accompany Auger suppression has been observed Enhancement-mode MISFETS have near-classical output characteristics at 294 K with a typical dynamic range of 23dB and transconductance of 34 mS mm-1

HgCdTe infrared diode arrays

Abstract: This paper gives a description of the infrared photovoltaic detector technology developed by LETI and now industrialized by SOFRADIR. Results obtained on several kinds of arrays both in the 3-5 mu m and 8-12 mu m wavelength ranges are presented. The technological choices made by the laboratory to achieve components with increased complexities are also given.

Physical behaviour of CdO films prepared by activated reactive evaporation

Abstract: High transparent and conductive CdO films were prepared using the activated reactive evaporation technique on Corning 7059 glass substrates. The film resistivity and transmittance were found to be influenced by the glow power of the discharge. Electron mobilities of 4-28 cm2 V-1 s-1 and carrier density in the range 7.8*1018-6.97*1019 cm-3 were observed. The energy gap of the films calculated from the optical absorption measurements was 2.42 eV.

Lateral position control of an electron channel in a split-gate device

Abstract: A new mode of operation with a split-gate device has been studied theoretically. A simple model for quantitative description of the pinch-off voltage and channel position is proposed. Independent control of voltages applied to disconnected parts of a split gate enables the electron channel to be shifted laterally. The shift was calculated for both narrow and wide channels. The method can be used to study electron scatterers in the area defined by the slit in the gate.

Lattice misfit and relative tilt of lattice planes in semiconductor heterostructures

Abstract: High resolution X-ray diffraction has been used to investigate the relative tilt between epilayer and substrate lattice planes of different semiconductor heterostructures. All epilayers were grown on (001) GaAs substrates misoriented by 2 degrees towards the next (011) direction. Results on the amount of the relative tilt and the direction of the maximum relative tilt are presented. The authors find that for heterostructures with small misfit (f 0.05) an angle of about 90 degrees between the direction of maximum relative tilt and the direction of the miscut substrate has been observed.

LVM spectroscopy of carbon and carbon-hydrogen pairs in GaAs grown by MOMBE

Abstract: Cryogenic (4.2 K) infrared absorption measurements of GaAs films grown by MOMBE and containing a high concentration of carbon acceptors ( approximately 2*1020 cm-3) show an asymmetric Fano resonance at the frequency of the CAs local mode (582 cm-1). After rapid transient annealing at 950 degrees C for 12 s in an atmosphere of helium and arsine, there is a reduction in the hole concentration, and localized vibrational mode (LVM) spectroscopy reveals the presence of nearest neighbour H-CAs passivated pairs. Previously unreported lines at 452.8 cm-1 and 563.0 cm-1 may be due to the vibrations of the passivated carbon atoms, or possibly CGa donors.

Optical properties of II-VI semiconductor nanocrystals

Abstract: An empirical tight-binding framework and the recursion method are used to calculate some electronic properties of II-VI semiconductor nanocrystals. First, the ground-state energy of the exciton is calculated. Results are given for six different II-VI compounds. A simple evaluation of the optical absorption peak of the excitons is favourably compared with experimental data. Finally, a calculation of the density of states is presented.

MBE growth of the (Zn,Cd)(Se,Te) system for wide-bandgap heterostructure lasers

Abstract: The authors address the principal materials for the design of a ZnSe-based visible injection laser: the controlled p-type doping of ZnSe and the development of heterostructures for optical and carrier confinement. Diode I-V characteristics of ZnSe p-n homojunctions are presented to demonstrate the existence of p-type ZnSe layers doped with lithium. These diodes exhibit electroluminescence in the visible range. Growth of the II-VI quaternary Zn1-yCdySe1-xTex is reported for the first time. Along with tuning of the bandgap energy with composition, these quaternary alloys are expected to provide tuning of the band offsets with ZnSe, thus having great potential for the design of practical visible heterostructure lasers.

Electron−LO-phonon scattering rates in GaAs−AlxGa1−xAs quantum wells

Abstract: The authors calculate the electron-LO-phonon scattering rates for confined phonon modes associated to intrasubband and intersubband transitions for GaAs-AlxGa1-xAs quantum wells with finite depth. The Huang and Zhu approximation for the confined LO phonon modes, based on a microscopic lattice-dynamic approach, is used and the results are compared with corresponding calculations for infinitely deep quantum wells and some experimental results. The authors found a remarkably good agreement with the experimental results.

The MOVPE growth and doping of ZnTe

Abstract: Zinc telluride layers have been grown on (100) GaAs, GaSb and ZnTe substrates at 350 degrees C with atmospheric pressure MOVPE. Diisopropyl-telluride, dimethylzinc-triethylamine and diethylzinc were chosen as metallorganic precursors. The samples were characterized by photoluminescence at 2 K and the Hall effect. In the PL spectra the light hole, heavy hole and bound exciton transitions are well resolved. Most of the transitions have been assigned. The partial pressure ratios of the alkyls were optimized. The influence of composition and stoichiometry of the substrates on the purity of the ZnTe is revealed. Various alkyls as sources of As, Bi, Ga, In and I were investigated for their suitability as p- or n-type dopants. It is shown that tetraethylbiarsine and ethyliodide are promising alkyls for p- and n-type doping of ZnTe.

Characterization of thin film ZnCdS/CdTe solar cells

Abstract: n-ZnCdS/p-CdTe polycrystalline thin film solar cells were fabricated by laser evaporating CdTe onto sprayed ZnCdS films The cells were characterized by studying current-voltage, capacitance-voltage and spectral response measurements A maximum efficiency of 76% was observed for a cell area of 1 cm2

Metastable defects in silicon: hints for DX and EL2?

Abstract: A review is given of defects that display metastability in silicon, with emphasis on those that have been identified and the various mechanisms that they reveal for the phenomenon. Pair defects described include interstitial-iron-substitutional-group-III-acceptors and ones formed by interstitial carbon with substitutional group V donors or substitutional carbon. Interstitial hydrogen, boron and silicon and substitutional nitrogen and oxygen are taken as examples of isolated single-atom defects that display on-centre to off-centre instabilities. It is argued that this single-atom instability can be understood in terms of a predictable Jahn-Teller effect and that this concept may provide useful insight into the DX and EL2 phenomena in the III-V materials and their alloys.

Tellurium-based II-VI compound semiconductors and heterostructures under strain

Abstract: Reviews work on some of the electronic and structural properties of CdTe-(CdZn)Te and CdTe-(CdMn)Te strained-layer quantum wells and superlattices under high pressure. The hydrostatic pressure dependence of the band gap of CdTe is deduced to be Eg=Ego+76P (meV GPa-1)-4P2(meV GPa-2) consistent with all the available data, and for ZnTe is Eg=Ego+105P-2P2. Results obtained by piezomodulation of the subband structure of such systems are also presented. The authors determine the values of the CdTe shear deformation potentials: b=-1.17 eV and d=-3.20 eV.

An efficient Monte Carlo particle technique for two-dimensional transistor modelling

Abstract: A novel very efficient technique for charge current calculations in two-dimensional Monte Carlo particle simulation is proposed. The technique obtains accurate results by using a significantly smaller number of particles than the commonly accepted technique. The proposed technique permits the two-dimensional Monte Carlo particle simulation to run on personal computers instead of on supercomputers. The results of field-effect transistor simulation are presented.

Physical properties of undoped and doped hydrogenated amorphous silicon carbide

Abstract: A systematic study on the basic characteristics of p-type a-SiC:H films doped with B2H6 and n-type doped with PH3 is presented. The effect of doping on the optical and electrical properties of the films has been examined. The results obtained on energy gap, spin density, Urbach energy and integral of excess absorption suggest that the phosphorus dopant atoms introduce fewer additional defects than does boron. The difference in the behaviour of the two types of doping has been attributed to the chemistry of boron and phosphorus.

InSb-based materials for detectors

Abstract: The development of new epitaxial techniques has given rise to a variety of novel material combinations Pseudomorphic combinations where the partners have lattice constants which differ by more than 1% are currently being extensively studied The built-in strain can alter the symmetry and magnitudes of the bandgaps concerned The growth and properties of narrow-gap semiconductor systems are reviewed together with their use as components for strained-layer structures The materials discussed are InSb, InAs and the alloys of these two compounds The alloy system InAs1-xSbx is prone to metallurgical problems such as ordering and phase separation in the mid-alloy range but high-mobility samples have been grown Other alloy systems are also affected by similar problems Spike-doped and n-i-p-i structures are studied in InSb and InAs Minimal dopant diffusion is found The control of the material properties achieved has enabled the fabrication of a number of prototype device structures including n-i-p-i photodetectors and resonant interband tunnelling structures with inversion barriers

Photovoltaic intersubband detectors for 3-5 mu m using GaAs quantum wells sandwiched between AlAs tunnel barriers

Abstract: The authors report on a new type of GaAs quantum well intersubband photodetector operating at wavelengths of 3-5 mu m. In these structures, the intersubband spacing is considerably enlarged by using ultrathin AlAs barriers on either side of the GaAs quantum wells followed by a thicker Al0.3Ga0.7As layer. These AlAs layers act as tunnel barriers allowing both photoconductive and photovoltaic detection of the intersubband excitation. Photovoltaic detector operation is possible because of an asymmetry between the confining AlAs layers, which is attributed to the epitaxial growth process. They also discuss the low-signal detector noise which is determined from the dark-current behaviour. The noise behaviour strongly favours photovoltaic detectors over photoconductive ones.