Showing papers in "Semiconductor Science and Technology in 1996"

Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits

Abstract: Local mechanical stress is currently an important topic of concern in microelectronics processing. A technique that has become increasingly popular for local mechanical stress measurements is micro-Raman spectroscopy. In this paper, the theoretical background of Raman spectroscopy, with special attention to its sensitivity for mechanical stress, is discussed, and practical information is given for the application of this technique to stress measurements in silicon integrated circuits. An overview is given of some important applications of the technique, illustrated with examples from the literature: the first studies of the influence of external stress on the Si Raman modes are reviewed; the application of this technique to measure stress in silicon-on-insulator films is discussed; results of measurements of local stress in isolation structures and trenches are reviewed; and the use of micro-Raman spectroscopy to obtain more information on stress in metals, by measuring the stress in the surrounding Si substrate is explained.

The theory of quantum-dot infrared phototransistors

Abstract: A novel device - the quantum-dot infrared phototransistor (QDIP) - is proposed and considered theoretically. The QDIP utilizes intersubband electron transitions from the bound states. The dark current and sensitivity are calculated using a proposed analytical model of the QDIP. It is shown that the QDIP can exhibit low dark current, high photoelectric gain and sensitivity surpassing the characteristics of other intersubband photodetectors.

Inhomogeneous line broadening and the threshold current density of a semiconductor quantum dot laser

Abstract: Theoretical analysis of the gain and threshold current of a semiconductor quantum dot (QD) laser is given which takes account of the line broadening caused by fluctuations in quantum dot sizes. The following processes are taken into consideration together with the main process of radiative recombination of carriers in QDs: band-to-band radiative recombination of carriers in the waveguide region, carrier capture into QDs and thermally excited escape from QDs, photoexcitation of carriers from QDs to continuous-spectrum states. For an arbitrary QD size distribution, expressions for the threshold current density as a function of the root mean square of relative QD size fluctuations, total losses in the waveguide region, surface density of QDs and thickness of the waveguide region have been obtained in an explicit form. The minimum threshold current density and optimum parameters of the structure (surface density of QDs and thickness of the waveguide region) are calculated as universal functions of the main dimensionless parameter of the theory developed. This parameter is the ratio of the stimulated transition rate in QDs at the lasing threshold to the spontaneous transition rate in the waveguide region at the transparency threshold. Theoretical estimations presented in the paper confirm the possibility of a significant reduction of the threshold currents of QD lasers as compared with the conventional quantum well lasers.

Self-organized growth of quantum-dot structures

Abstract: This paper reviews the current developments in the exciting field of self-assembled semiconductor quantum-dot structures during epitaxial growth of lattice mismatched systems. The formation of quantum-sized islands in the coherent Stranski - Krastanow growth mode, with focus on the possibilities of vertical and lateral ordering, and their electronic properties are described in detail. A overview is given of the self-organizing formation and optical properties of buried (InGa)As quantum discs found in a new growth mode in the metalorganic vapour-phase epitaxy of strained layers on high-index semiconductor surfaces.

Schottky barrier properties of various metals on n-type GaN

Abstract: Schottky barriers of Ti, Cr, Au, Pd, Ni and Pt on n-type GaN epitaxial layers grown by low-pressure metal-organic chemical vapour deposition on sapphire have been fabricated and characterized. Measurements were carried out using current - voltage (I - V), current - voltage - temperature (I -V - T) and capacitance - voltage (C - V) techniques. A modified Norde plot was used as one of the analysis tools for the I - V - T measurements. The barrier heights, ideality factors and effective Richardson constants are presented. Barrier heights of 0.88, 0.92, 0.99 and 1.08 eV for Au, Pd, Ni and Pt respectively were obtained from the modified Norde plot. Contacts of Ti and Cr exhibited only slightly rectifying characteristics. These results show that the barrier height on n-GaN increases monotonically, but does not scale proportionately, with increasing metal workfunction.

Growth and characterization of self-assembled Ge-rich islands on Si

Abstract: Ge-rich islands have been grown on Si (100) substrates by molecular beam epitaxy. Their density and size distribution are analysed by atomic force microscopy as a function of growth temperature, growth rate and Ge coverage. Overgrown islands have been studied by transmission electron microscopy, Raman scattering and photoluminescence. The first results of photocurrent spectroscopy on Si/Ge/Si pin diodes show the expected shift of the energy gap. Based on these results, novel device applications of Ge-rich islands in Si are proposed.

Evidence for the double distribution of barrier heights in Schottky diodes from I - V - T measurements

Abstract: The current - voltage (I - V) characteristics of palladium silicide-based Schottky diodes on n-type silicon have been measured over a wide temperature range (66 - 300 K). Their analysis on the basis of the thermionic emission - diffusion (TED) mechanism reveals an abnormal decrease of zero-bias barrier height and increase of ideality factor with decrease in temperature (T) and nonlinearity in the activation energy plot. Such behaviour is attributed to barrier inhomogeneities by assuming a Gaussian distribution of barrier heights at the silicide/silicon interface. Evidence is given for the existence of a double Gaussian distribution having mean barrier heights of 0.79 V and 0.64 V and standard deviations of 0.081 V and 0.057 V with ideality factors 1.064 and 1.363, and remain effective in the temperature range 134 - 300 K and 66 - 120 K respectively. Further, the effect of forward bias on the distribution parameters is discussed. A simple method, involving the use of a zero-bias barrier height versus inverse temperature plot, is suggested to deduce the presence of single/multiple distribution(s) of barrier heights and to determine the respective parameters.

Stresses and strains in epilayers, stripes and quantum structures of III - V compound semiconductors

Abstract: Stresses and strains in heterostructures have dominated semiconductor research during the last ten years. We review the theory and experimental work on stresses in III - V semiconductor heterostructures in this paper. First large-area lattice mismatched layers (InGaAs and InGaP layers grown on GaAs or InP substrates) and thermally strained layers (GaAs, GaP and InP layers grown directly on Si) are considered. The stresses in large-area epilayer - substrate structures are easy to model because substrate distortion can be neglected and strain in the epilayer is a simple biaxial tetragonal distortion. Calculated splitting of band edges, modification of bandgaps and shifts of Raman modes show good agreement with experiments. Edges of a stripe relax stress in the stripes and induce stress in the substrate. Since stresses in the stripe and the substrate are coupled, calculation of stresses in stripes and substrates is more involved. Recent finite element (FE) calculations of these stresses are discussed in detail and compared with the approximate analytical models and with luminescence and Raman measurements. FE calculations of stresses in buried quantum wires, stressor-induced quantum wires and quantum dots are also discussed. The results of these calculations are used to determine stability and luminescence of the quantum wires and dots and compared with the experimental results. Finally self-organized quantum dots consisting of islands formed during the 3D growth of InAs layers on GaAs are discussed. A possible explanation of the recent observation that they are formed in vertical columns embedded in GaAs is suggested. .

Electron states in a two-dimensional ring : an exactly soluble model

Abstract: An exactly soluble model for a two-dimensional ring is proposed. Using this model, we have obtained analytically the energy spectrum and wavefunctions for a ring in the presence of a uniform magnetic field and a thin magnetic flux. The model can also describe quantum dots, anti-dots, one-dimensional rings and straight two-dimensional wires, which provides an integrated picture for the electron states and their magnetic field response in these geometries. The simplicity and the flexibility of the model make it an ideal tool for the modelling of the Aharonov - Bohm effects and the persistent currents in quantum rings.

Crosslinked PMMA as a high-resolution negative resist for electron beam lithography and applications for physics of low-dimensional structures

Abstract: We present a novel technique which employs crosslinked PMMA as a high-resolution negative resist for electron beam lithography. The technique allows the patterning of submicrometre features in an insulating layer, thus simplifying the fabrication process of various multilayer devices. We demonstrate this by reference to specific devices and present simple experimental results which prove the usefulness of the technique.

Tin sulphide films deposited by plasma-enhanced chemical vapour deposition

Abstract: Tin sulphide thin films have been prepared by the plasma enhanced chemical vapour deposition technique, using and as source materials. For given values of the deposition pressure, relative gas flow rates and deposition time, the plasma power density and the substrate temperature were varied in the ranges from 0.023 to 0.080 W and from 100 to 300 respectively. The deposited films show an orthorhombic crystalline structure for the entire plasma power range and for substrates temperatures higher than 150. From the studied optical properties, considering that this is an indirect energy bandgap material, the energy bandgap is calculated to have a value of 1.16 eV; the phonon involved in the electronic transition has an energy of 0.18 eV. From the measurements of electrical conductivity as a function of temperature an activation energy of 0.3 eV was determined with a p-type electrical conductivity.

Single-electron phenomena in semiconductors

Abstract: The study of single-electron phenomena associated with tunnelling in semiconductor nanostructures has emerged in recent years as a major forefront of condensed matter physics, whose implications range from fundamental physics to electronic device applications. This paper presents a tutorial review of the subject, with emphasis on the role of single-electron charging in such semiconductor `quantum dots'. The main purpose is to describe the various phenomena observed in these experiments and to present the theoretical understanding of these phenomena in an introductory fashion. The paper attempts to explain the underlying physics at the intuitive level and tries to draw, as much as possible, a unifying perspective on a relatively large body of knowledge acquired within a short time by the conjunction of many individual contributions.

Free and bound excitons in thin wurtzite GaN layers on sapphire

Abstract: Free and bound excitons have been studied by photoluminescence in thin () wurtzite-undoped GaN, n-type GaN:Si as well as p-type GaN:Mg and GaN:Zn layers grown by metal-organic chemical vapour phase deposition (MOCVD). An accurate value for the free A exciton binding energy and an estimate for the isotropically averaged hole mass of the uppermost valence band are deduced from the data on undoped samples. The acceptor-doped samples reveal recombination lines which are attributed to excitons bound to and respectively. These lines are spectrally clearly separated and the exciton localization energies are in line with Haynes' rule. Whenever a comparison is possible, it is found that the exciton lines in these thin MOCVD layers are ultraviolet-shifted by 20 to 25 meV as compared to quasi-bulk () samples. This effect is interpreted in terms of the compressive hydrostatic stress component which thin GaN layers experience when grown on sapphire with an AlN buffer layer.

Rashba spin splitting in a gated HgTe quantum well

Abstract: Metal - oxide - semiconductor field-effect transistors (MOSFETs) on CdTe/HgTe/CdTe heterostructures are fabricated with silicon dioxide gate insulators. In these devices, the density of the quasi two-dimensional electron gas in the HgTe quantum well can be tuned in a wide range. In low magnetic fields we observe beating patterns in the Shubnikov - de Haas oscillations that render possible the determination of the coefficient of the Rashba term in the Hamiltonian as a function of electron density. This coefficient consistently describes the splittings observed in cyclotron resonance in low magnetic fields.

Thin gate oxide damage due to plasma processing

Abstract: Plasma processes cause current to flow through the thin oxide and the resultant plasma-induced damage can be simulated and modelled as damage produced by constant-current (or voltage) electrical stress. Plasma processing causes MOSFET parameter degradation, from which one can deduce the plasma charging current. Since the scattering of post-damage device parameters is due to a reproducible variation of stress current across the wafer, one can easily analyse the effect of device geometry on damage by comparing test structures in the same die rather than the averages over a wafer. We have developed a quantitative model for thin oxide plasma charging damage by examining the oxide thickness dependence of the charging current. The model successfully predicts the oxide thickness dependence of plasma charging. It is shown that plasma acting on a very thin oxide during processing may be modelled essentially as a current source. Thus the damage will not be greatly exacerbated as the oxide thickness is further reduced in the future. Although annealing in forming gas can passivate the traps generated during plasma etching, subsequent Fowler-Nordheim stressing causes more traps to be generated in these devices than in devices that have not been through plasma etching. The protection diode should be forward biased during processing to safely protect the gate oxide. In CMOS circuits, the drains of the driver circuit can generally act as adequate protection diodes for the oxide regardless of N or P substrate and the polarity of the plasma charging current. The plasma stress current can be reduced by reducing the ion density, which is unfortunately linked to the etch rate or directionality, or by reducing the electron temperature. Maintaining a very uniform plasma over the surface of the wafer, reducing the plasma charging current during the over-etch time and judicious use of protection diode and antenna design rules will reduce plasma damage to an acceptable level for ULSI production even for very thin gate oxides.

Effect of oxygen concentration in the sputtering ambient on the microstructure, electrical and optical properties of radio-frequency magnetron-sputtered indium tin oxide films

Abstract: Indium tin oxide (ITO) films have been prepared by radio-frequency (rf) magnetron sputtering. The effect of oxygen concentration in the sputtering ambient on the structure and properties of ITO films are investigated. The films have a preferred orientation in the (440) plane. The presence of oxygen during sputtering enhances the crystallization of the film. Film grain size increases as more oxygen is added. According to the XPS measurements, the addition of oxygen reduces the oxygen-deficient region of the film. Hence the optical transmittance of the film is improved while film conductivity decreases.

Generalized approach to the parameter extraction from I - V characteristics of Schottky diodes

Abstract: Two approaches for Schottky barrier parameter evaluation are presented and compared. The first method extracts the barrier height, the ideality factor and the series resistance also for the structures that have no linear part in the forward direction of the curve. This enables one to take into consideration also the reverse part of I - V curves that is normally omitted and the information lost. The second, more general, approach takes into account an inhomogeneity of the Schottky barrier and extracts the parameters of the barrier height distribution. It is shown that for this case it is possible to substitute the ideality factor, which is a non-physical parameter, by a barrier height distribution with the mean value and the standard deviation . Using this method a single I - V measurement is sufficient for determining the barrier height distribution.

Electrical conductivity, optical absorption and structural studies in thin films

Abstract: Electrical, optical and structural properties of thin films deposited on glass substrates are studied for different substrate and post-deposition annealing temperatures. The substrate temperature is varied in the range 300 - 773 K. The activation energy has been calculated for the films using the Arrhenius plot and the bandgap has been obtained from the optical transmission spectra. The activation energy varied from 76 meV to 330 meV for in the impurity scattering region and for the value is around 700 meV in the intrinsic region. Optical absorption spectra show variation in the bandgap, varying from 1.18 to 1.42 eV as the substrate temperature varied from 300 - 773 K. Films deposited at higher substrate temperatures give a = 5.93 A and c = 11.22 A for the tetragonal phase. The mean grain size of the films is for polycrystalline films.

Type conversion of p-(HgCd)Te using and Ar reactive ion etching

Abstract: Hydrogen/methane gas mixtures and pure argon were used for reactive ion etching (RIE) of ( and 0.28). The effect of the ratio on the depth of the etched surface and the depth of the p - n junction created under the etched surface were studied for the RIE process. It was found that the etch depth reaches a maximum at an ratio and the depth of the p - n junction decreases with increasing fraction in the mixture. The roughness of the etched surface is smallest using a gas mixture with a small amount of (20 - 30%). For the pure Ar RIE process the etch and p - n junction depths were studied as functions of etch time, Ar pressure and rf power. Clear evidence for the creation of p - n junctions using various kinds of Ar RIE processes is found.

A new design for submicron double-barrier resonant tunnelling transistors

Abstract: A new fabrication technique is described for a submicron vertical AlGaAs/GaAs double-barrier resonant tunnelling transistor which is ideal for investigating single electron charging of, and single electron tunnelling through, a quantum dot containing just a `few' electrons. The electrical characteristics and figures of merit are presented to demonstrate the improved `squeezing' capability of the Schottky gate which is located around, and partially on, the etched side wall of the device mesa after a combined dry and wet etch to a point below the two barriers, and contrasted with those of an earlier design where etching is stopped above the two barriers.

Corrosion-induced degradation of microelectronic devices

Abstract: Corrosion of the metallization used for interconnecting the various circuit elements on a microelectronic device continues to to cause concern regarding reliability despite the fact over 500 papers have been published on this subject representing 30 years of effort by industry. Controversy still remains in a number of technologically important areas, including the functional form and fitting parameters of the acceleration functions. In this paper we briefly summarize the current understanding of corrosion-induced degradation and its effect on the reliability of microelectronic circuits

Spray pyrolysis deposition of thin films

Abstract: Ternary compound thin films have been prepared on Pyrex glass substrates by the spray pyrolysis process using tin chloride and n, n-dimethylthiourea as starting materials. The depositions were carried out at a substrate temperature of . The identification of the phase was achieved by means of x-ray diffraction measurements. The optical reflectance and transmittance of the prepared films were used to obtain the variation of the refractive index and the extinction coefficient as a function of the wavelength. These calculated values were used to find the absorption coefficient and the optical bandgap and gave . From measurements of the conductance as a function of , a dark activation energy was determined with a value of 1.02 eV.

Biased percolation and abrupt failure of electronic devices

Abstract: We propose a new percolation model as an aid to understand abrupt failure of electronic devices. It is called biased percolation because we assume that local Joule heating determines the probability of generating defects causing percolative breakdown of the device. We take as a simple geometry a homogeneous thin film, modelled as a two-dimensional resistor network. By carrying out Monte Carlo simulations we investigate the evolution of the system including: the damage pattern, current distribution, resistance degradation, resistance relative fluctuations and its power spectrum associated with 1/f noise. Our results show that biased percolation efficiently simulates degradation of thin films in good agreement with available experiments and predicts several features that should take place close to the abrupt failure of most devices.

Landau subbands generated by a lateral electrostatic superlattice - chasing the Hofstadter butterfly

Abstract: Magnetoresistance studies on two-dimensional electron systems in GaAs - AlGaAs heterojunctions exposed to voltage-tunable square lateral superlattices with periods in the 100 nm range reveal at low temperatures a characteristic splitting of individual Landau bands. Around magnetic field values at which p/q flux quanta penetrate the unit cell of the superlattice, with p and q being small integers, p subbands can be observed. A discussion of which conditions have to be met to reveal such Hofstadter-type splitting of Landau bands illuminates the experimental difficulties that have to be overcome in chasing the Hofstadter butterfly.

Low-frequency noise used as a lifetime test of LEDs

Abstract: Low-frequency noise (1/f noise) has been measured in light emitting diodes (LEDs) which have been subjected to an accelerated life test by means of large forward bias current pulses Over a large range of stress pulses the electrical and functional LED properties remain unaltered but an increase in the 1/f noise level was seen and this was correlated with the device reliability The product `initial noise X initial rate of noise increase' correlated best with the LED lifetime

Low-temperature luminescence study of GaN films grown by MBE

Abstract: We report the results of low-temperature photoluminescence measurements on GaN films grown by molecular beam epitaxy on (0001) sapphire substrates. Samples were either nominally undoped or doped with Si. The spectra are generally dominated by a sharp peak at 3.47 eV which is attributed to excitons bound to neutral donors. A much weaker peak (or shoulder) near 3.45 eV probably arises from excitons bound to neutral acceptors. On raising the temperature to 50 K, in some samples free exciton peaks can be partially resolved on the high-energy side of the main line. In others we believe that these free excitons are recaptured onto neutral acceptors, thus enhancing the low-energy side of the line. A broader emission line appears in many samples at an energy near 3.42 eV which shows significant variation in position between samples. Our data show that it represents a free-to-bound, probably a free hole-to-donor, transition. This donor has previously been associated with oxygen. Of particular interest is the fact that some samples show a second sharp peak at 3.27 eV, together with a second broader peak at about 3.17 eV (also variable in energy). The sharp peak is energetically consistent with its being either a donor - acceptor or a free electron-to-bound hole transition, but subsidiary measurements rule out both these possibilities. We suggest that it may represent an exciton bound to a deep donor or a shallow donor-bound exciton in zinc blende GaN inclusions contained within the mainly wurtzite material. We tentatively interpret the 3.17 eV line as a phonon replica of this zinc blende line, the phonon energy being perturbed by the small size of the inclusions and by strain effects within these inclusions.

Preparation of highly photosensitive CdSe thin films by a chemical bath deposition technique

Abstract: Thin films of CdSe were deposited by a chemical bath technique from citratocadmium(II) or tartratocadmium(II) complex ions and N,N-dimethylselenourea. The final thicknesses of the films were from the citrate bath and from the tartrate bath, when the depositions were made at room temperature. The films are uniform and adherent to glass substrates. The as-prepared films are only marginally photosensitive with a photocurrent to dark current ratio of <10 under white light of intensity . Annealing the films in air for 30 min - 2 h at results in high photosensitivities, , depending on the film thickness, bath composition, and the duration and temperature of annealing. The photocurrent rise and decay time are short, typically of the order of milliseconds. The combination of film thickness, annealing temperature and duration of annealing that presents the best photosensitivity for the films is discussed.

Strain relaxation in tensile-strained layers on Si(001)

Abstract: We investigated in detail the strain relaxation behaviour of metastable tensile-strained epilayers on Si(001) by comparing the layers before and after an annealing step using a variety of different diagnostic methods. The dominant strain-relieving mechanism is the formation of carbon-containing interstitial complexes and/or silicon carbide nanoparticles, similar to the behaviour of carbon in silicon under thermodynamical equilibrium conditions (concentrations below the solid bulk solubility limit). We did not observe any carbon out-diffusion. To grow material suitable for device applications, all carbon atoms should be incorporated substitutionally. There is only a very narrow temperature window for perfect epitaxial growth of such layers, limited on one side by the possible formation of interstitial carbon complexes and on the other side by the deterioration of epitaxial growth at low temperatures. The carbon concentration should not exceed a few per cent to avoid strain-driven precipitation.

Vacancy- and acceptor-H complexes in InP

Abstract: It has been suggested that iron in InP is compensated by a donor, related to the local vibrational mode and previously assigned to the fully hydrogenated indium vacancy, . Using AIMPRO, an ab initio local density functional cluster code, we find that acts as a single shallow donor. It has a triplet vibrational mode at around this value, consistent with this assignment. We also analyse the other hydrogenated vacancies , and determine their structure, vibrational modes, and charge states. Substitutional group II impurities also act as acceptors in InP, but can be passivated by hydrogen. We investigate the passivation of beryllium by hydrogen and find that the hydrogen sits at a bond-centred site and is bonded to its phosphorus neighbour. Its calculated vibrational modes are in good agreement with experiment.

Tunnelling spectroscopy of low-dimensional states

Abstract: In this review a survey of tunnelling processes between barrier-separated two-dimensional (2D) systems and systems of different dimensionality is given. Tunnelling between barrier-separated 2D systems can be studied on very different samples such as triple-barrier structures, double-barrier structures with a two-dimensional emitter, double-barrier structures under hydrostatic pressure, double heterostructures, coupled quantum wells and also coupled 2D electron - hole systems. Pure 2D - 2D tunnelling processes with individual contacts on both 2D systems, however, are only reported on double heterostructures and coupled quantum wells. Using a transfer Hamiltonian formalism, it is shown that all resonances in the tunnelling current have their origin in density of states effects, transmission coefficients or the overlap integrals between the initial and final states. 2D subband energies, background impurity concentrations, the effective mass and also non-parabolicity effects can be determined quantitatively in terms of the transfer Hamiltonian formalism. By nanofabrication, tunnelling processes between 2D systems and states of lower dimensionality (1D, 0D) can also be investigated. Here, the tunnelling processes are mainly influenced by the overlap integral between the initial and final states. The corresponding resonance positions in the tunnelling current strongly depend on the shape of the confining potential and, moreover, the current - voltage characteristics turn out to be the Fourier transform of the 1D (0D) wavefunction of the final state. A brief survey of 1D - 1D and 1D - 0D tunnelling experiments is also given.