Showing papers in "Semiconductor Science and Technology in 1995"

A dye-sensitized nano-porous solid-state photovoltaic cell

Abstract: A photovoltaic cell was fabricated by sandwiching a monolayer of the pigment cyanidin adsorbed on nano-porous n-TiO2 film (deposited on conducting tin oxide glass) within a transparent polycrystalline film of p-CuI, filling the intercrystallite pores of the porous n-TiO2 film. Photoexcited dye is found to inject electrons into n-TiO2 and holes into p-Cul, generating photocurrents and photovoltages that are impressively high for a dye-sensitized solid-state photovoltaic cell.

Polycrystalline silicon thin film transistors

Abstract: During the past decade there has been a rapid growth of interest in poly-Si for the active device layer in thin film transistors (TFTS) for active matrix flat-panel displays. Whilst the early work, demonstrating the high carrier mobility of these devices, employed processing temperatures of approximately 1000 degrees C and quartz susbtrates, this was soon followed by the investigation of lower-temperature processes which were compatible with the use of glass substrates. Some of the key aspects of this work are reviewed in this article: the preparation of the material by direct deposition and by crystallization from a-Si precursors, the characterization of the defect-induced trapping states within the material and their passivation, and the present understanding of the TFT leakage current mechanisms. This work is put into the context of the requirements for active matrix liquid-crystal displays, and, with the understanding and control of poly-Si which has been achieved to date, its application in this area can be expected to increase rapidly in the coming years.

Current-voltage characteristics and barrier parameters of Pd2Si/p-Si(111) Schottky diodes in a wide temperature range

Abstract: Current-voltage characteristics of Pd2Si/p-Si(111) Schottky barrier diodes studied over a wide temperature range (60-201 K) are shown to follow a thermionic emission-diffusion mechanism under both the forward and the reverse bias conditions. The barrier parameters as evaluated from the forward I-V data reveal a decrease of zero-bias barrier height ( phi b0) but an increase of ideality factor ( eta ) and series resistance (Rs) with decrease in temperature. Moreover, the changes in phi b0, eta and Rs become quite significant below ~100 K. An In(Is/T2) versus 1/T plot is found to fit well with two straight lines in different temperatures regimes giving an activation energy of 0.33 eV (201-107 K) and 0.24 eV (below 107 K) and an effective Richardson constant of 33 A cm-2 K-2. However, the activation energy of 0.33 eV corresponds to the zero-bias barrier height at absolute zero. An In(Isf/T2) versus 1/ eta T plot is suggested to obtain the flat-band barrier height and the effective Richardson constant; the corresponding values obtained are 0.401 eV and 32.2 A cm-2 K-2 respectively. It is shown that the `T0 effect` cannot account for the apparent increase in ideality factor and decrease of barrier height at low temperatures. Finally, the decrease of barrier height with voltage under the reverse bias condition is attributed mainly to interfacial layer effects with a small contribution due to image force lowering.

The influence of film crystallinity on the coupling efficiency of ZnO optical modulator waveguides

Abstract: The coupling efficiencies of sputtered, c-axis-orientated zinc oxide (ZnO) films in guided wave resonance optical modulators have been measured. The ZnO planar waveguide is sandwiched between two 3 nm thick chromium layers on top of a SiO2/Si(100) substrate. This novel design facilitates application of a modulation voltage directly across the dielectric film in a prism coupler set-up and thus avoids voltage losses across the silicon dioxide (SiO2) optical isolation layer. We present a comparison of four differently prepared ZnO waveguides/modulators to investigate the influence of film crystallinity on the coupling efficiency and electro-optical parameters. The coupling efficiency of a ZnO waveguide is found to be dependent on both film thickness and average grain diameter measured in the plane of the film. A high efficiency (0.76) can be achieved for film thicknesses below 300 nm when the average grain diameter is 26 nm. This value is comparable to the maximum value of typically around 0.81 expected from model calculations for uniform couplers.

Oxidation of silicon: the VLSI gate dielectric

Abstract: Silicon dominates the semiconductor industry for good reasons. One factor is the stable, easily formed, insulating oxide, which aids high performance and allows practical processing. How well can these virtues survive as new demands are made on integrity, on smallness of feature sizes and other dimensions, and on constraints on processing and manufacturing methods? These demands make it critical to identify, quantify and predict the key controlling growth and defect processes on an atomic scale. The combination of theory and novel experiments (isotope methods, electronic noise, spin resonance, pulsed laser atom probes and other desorption methods, and especially scanning tunnelling or atomic force microscopies) provide tools whose impact on models is just being appreciated. We discuss the current unified model for silicon oxidation, which goes beyond the traditional descriptions of kinetic and ellipsometric data by explicitly addressing the issues raised in isotope experiments. The framework is still the Deal-Grove model, which provides a phenomenology to describe the major regimes of behaviour, and gives a base from which the substantial deviations can be characterized. In this model, growth is limited by diffusion and interfacial reactions operating in series. The deviations from Deal-Grove are most significant for just those first tens of atomic layers of oxide which are critical for the ultra-thin oxide layers now demanded. Several features emerge as important. First is the role of stress and stress relaxation. Second is the nature of the oxide closest to the Si, both its defects and its differences from the amorphous stoichiometric oxide further out, whether in composition, in network topology, or otherwise. Thirdly, we must consider the charge states of both fixed and mobile species. In thin films with very different dielectric constants, image terms can be important; these terms affect interpretation of spectroscopies, the injection of oxidant species and relative defect stabilities. This has added importance now that Pb concentrations have been correlated with interfacial stress. This raises further issues about the perfection of the oxide random network and the incorporation of interstitial species like molecular oxygen. Finally, the roles of contamination, particles, metals, hydrocarbons etc. are important, as is interface roughness. These features depend on pre-gate oxide cleaning and define the Si surface that is to be oxidized which may have an influence on the features listed above.

Infrared spectroscopy and transport of electrons in semiconductor superlattices

Abstract: Infrared spectroscopy is a powerful tool for revealing some key properties of electrons in the minibands of semiconductor superlattices. Some theoretical foundations of superlattice physics as well as the experimental work on infrared absorption will be reviewed in this article. After a discussion of the energy levels (minibands, donor states) and the effect of external electric and magnetic fields on the miniband structure, a framework for the description of optical transitions between and within minibands is developed. Experiments on inter- and intraminiband as well as on cyclotron resonance absorption are presented in some detail. The most striking phenomena observed in this context are the van Hove singularities in the miniband joint density of states, the thermal quenching of miniband conductivity, and magnetic miniband breakdown. Finally, transport and infrared emission experiments are summarized.

Activation energy in the early stage of electromigration in Al-1% Si/TiN/Ti bamboo lines

Abstract: The activation energy of electromigration in 'bamboo'-type, passivated Al-1% Si/TiN/Ti lines was determined by means of high resolution resistance measurements at wafer level. Both the very early stages, with a nonlinear behaviour of the resistance, and the subsequent stages, characterized by an approximately constant rate of resistance change, were analysed with an existing model that correlates electromigration and mechanical stress evolution. An activation energy could be extracted only in the phase of linear resistance increase, where a value of 0.95 eV was found. Diffusion at the interface between Al-Si and the barrier metal or the passivation could be responsible for this value of the activation energy.

Photosensitive ZnO thin films prepared by the chemical deposition method SILAR

Abstract: Structural, optical and photoresponse characteristics of ZnO thin films ( approximately 0.1 mu m) prepared by a relatively new chemical deposition method, SILAR (successive ion layer adsorption and reaction), are described. The films are deposited by successive immersion of glass substrate in a dilute solution of Zn2+-ammonia complex at room temperature and in hot water (96 degrees C) for up to 35 immersion cycles using a computerized electropneumatic deposition system. Film thickness varied from 0.02 mu m (6 cycles) to 0.11 mu m (35 cycles). The as-prepared films show hexagonal (zincite) structure with a preferred orientation-c-axis perpendicular to the plane of glass substrate-but lose this preferential orientation when annealed at 350 degrees C. An optical transmittance of 90% combined with a specular reflectance of 10% in the visible and near-infrared region is typical of these films. The as-prepared films show a photocurrent to dark current ratio of 105 under 900 W m-2 illumination from a solar simulator. The dark conductivity and photoconductivity are influenced by heat treatments in controlled atmospheres (O2, H2 and air).

Growth and properties of strained Si1-x-yGexCy layers

Abstract: Advances made in the growth and properties of CSi and CSiGe pseudomorphic strained layers are reviewed. The solubility of C in Si is small (3.5*1017 atoms/cm3 near the melting point). However, high-quality strained layers of the alloys with considerably larger C concentrations have been grown using MBE, CVD and solid-phase epitaxy methods. A careful control of the growth rate and temperature is necessary to avoid formation of silicon carbide. In high-quality layers, most of the C atoms occupy lattice positions of the Si or SiGe host crystals and a substitutional alloy is formed although the equilibrium volume of C atoms is only 30% of that of Si. The formation and stability of alloys of atoms with large differences in size is a topic of fundamental interest. Experimental and theoretical investigations have focused on the microscopic structure of substitutional Si1-x-yGexCy alloys. C compensates the compressive strain produced by Ge in the pseudomorphic layers grown on a Si substrate. From Raman studies of the microscopic strain in substitutional Si1-x-yGexCy alloys it has been concluded that Si-Si bonds experience a considerable local deformation even in strain-compensated alloys. The pair interaction of substitutional C atoms in an Si lattice and the possibility of forming ordered alloys have been studied theoretically. It has been found that the interaction of pairs of substitutional C atoms is attractive for special atomic configurations. Information available on electronic properties is rather meagre. Recent theoretical work shows that the bandgap of the alloy should decrease with C concentration. Experiments to confirm this have not yet been performed. Using strain-compensated alloys it is possible to grow symmetrically strained superlattices without the need of growing buffer layers. Si1-x-yGexCy strained layers are likely to be very useful for passive applications such as buffer layers. Considerably more work is required to determine their utility for active device applications.

Hot-carrier degradation in submicrometre MOSFETs: from uniform injection towards the real operating conditions

Abstract: An overview is given of the present understanding of the hot carrier degradation problem in submicrometre MOSFETs. First we discuss the degradation mechanisms observed under, for circuit operation, somewhat artificial but well-controlled uniform-substrate hot electron and substrate hot hole injection conditions. Then the more realistic case of static channel hot carrier degradation is treated, and some important process-related effects are illustrated, followed by the behaviour under the most relevant case for real operation, namely dynamic degradation. Finally, the strategies for improving hot carrier reliability and a forecast of the hot carrier reliability problem for sub-0.25 mu m technologies are briefly discussed.

Ferroelectric thin film technology for semiconductor memory

Abstract: Recent advances in ferroelectric thin film technology have generated significant interest for use as capacitor dielectrics in semiconductor memories. Ferroelectric non-volatile memories (NVRAMS) have several unique features which set them apart from floating-gate non-volatile memories, including high-speed write operation (intrinsic switching times below several nanoseconds), high write endurance (>1013 read/write cycles) and low operating voltages (as low as 1.5 V). These characteristics make ferroelectric NVRAMs excellent candidates for the non-volatile memory in portable electronic products such as RF tags, smart cards and pagers. This paper describes the operation of a ferroelectric NVRAM cell and the scaling methodology, reviews the device physics and reliability mechanisms and summarizes the challenges posed by the integration of ferroelectric capacitors with CMOS technology. The high dielectric constant ( epsilon r approximately 300-1000) of the same class of materials used for ferroelectric NVRAMS can be exploited for high-density DRAMs. Capacitors fabricated with these materials have been shown to meet the charge storage specifications of gigabit-scale DRAMs (equivalent SiO2 thickness as low as 2.3 AA) without the extremely severe geometries and exceedingly complex processes required with conventional trench and stacked SiO2/Si3N4 or Ta2O5 capacitor cell technologies. Recent progress in this area is summarized.

Spectroscopy of lattice defects in tetrahedral II-VI compounds

Abstract: The properties of lattice defects in the tetrahedral Il-VI compounds, as revealed by optical and electron spin resonance spectroscopy, are reviewed. Particular attention is given to the A-centre, namely a complex of a cation vacancy and a shallow donor, as a model system. Cation and anion vacancies are treated but information about interstitials is scarce. Some of the older data are reinterpreted. The amount of firmly based information is small but is sufficient for tentative general ideas to be put forward.

The II-VI semiconductor blue-green laser: challenges and solution

Abstract: Recent advances in the area of ZnSe-based blue-green laser diodes are reviewed. We will discuss the solutions to some long-standing problems which for many years stood in the way of successful development of these devices, such as the choice of the quantum well structure for the active region, doping of ZnSe and ohmic contacts. We also address the problems that are responsible for the still very short lifetimes of blue-green laser diodes at the present stage of development, which still need to be overcome before these devices can be applied in practical situations.

Acceptor binding energy in GaN and related alloys

Abstract: The recent achievement of p-type doping in GaN epitaxial films emphasizes the importance of the acceptor ionization energy with relevance to the application of group III nitrides in visible-light-emitting devices. Measured values of approximately 200 meV suggest that doping efficiencies at room temperature may be no more than 1%, resulting in undesirable series resistance in LEDs and lasers. Consideration of the effective mass hydrogen model of the acceptor ground state in GaAs, GaP and GaN suggests that this large value of GaN results from the strong electronegativity of the nitrogen atom and the corresponding reduction in dielectric constant, compared with the other compounds. This is further emphasized by the need to use the high-frequency dielectric constant rather than the static value when the ionization energy is greater than the TO phonon energy. Using in ( infinity )=5.35 and an effective mass of 0.4 m provides good agreement with the measured ionization energy of GaN, this value of mn being consistent with estimates based on recent hole mobility measurements. It is also consistent with smoothly increasing hole mass in the sequence GaAs (0.33 m), GaP (0.37 m), GaN (0.40 m). Finally, the opportunity is taken to speculate about likely values of acceptor ionization energies in AlGaN and AlGaAsN alloys.

Electroabsorption modulators for high-bit-rate optical communications: a comparison of strained InGaAs/InAIAs and InGaAsP/InGaAsP MQW

Abstract: Conventional high-speed (>10 GHz) external modulators at 1.55 mu m exist in various types and on several kinds of substrate material. Only electroabsorption modulators on InP have so far demonstrated polarization-independent operation at high speeds. In this paper, the properties of two strained multiple quantum well (MQW) modulators are reviewed in the context of optical fibre communications in terms of loss, drive voltage, bandwidth (>40 GHz), eye diagram, bit error rate measurements and optical power handling capacity. An effective modulation polarization sensitivity of 0.6 dB is reported. The two components differ mainly in their saturation optical power levels. A new power saturation mechanism is proposed to explain the nonlinearity of the InGaAs/lnAIAs modulator.

Temperature-dependent interface reactions and electrical contact properties of titanium on 6H-SiC

Abstract: In this study the system Ti-Si-C was investigated in terms of two aspects, metallurgical and electrical, in order to understand the formation and the properties of Ti electrical contacts on n-type 6H-SiC. For the metallurgical investigation bulk diffusion couples were prepared from monocrystalline 6H-SiC and Ti and annealed between 700 and 1200 degrees C for different lengths of time. The reaction zones were investigated using a SEM (secondary electron and backscattered electron images as well as energy-dispersive X-ray analysis). For the investigation of the electrical properties Ti contacts were sputter-deposited onto 6H-SiC wafer stripes and annealed at similar temperatures. The contact properties were measured in terms of current-voltage characteristics. We discovered that, over the whole temperature range investigated, the reaction layer growth follows a parabolic growth law which is thermally activated. Above 1200 degrees C the diffusion path from SiC to Ti is SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti5Si3/Ti. The contacts show ohmic behaviour. Between 1000 and 800 degrees C the diffusion path is: SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are also ohmic. Below 700 degrees C the diffusion path is SiC/TiC1-y/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are of Schottky type. It is concluded that the phase in contact with the SiC determines the electrical properties of the junction and the possibility of manufacturing a complete Schottky diode using n-type 6H-SiC and Ti only is demonstrated.

Electro-optic imaging of potential distributions in the quantum Hall regime

Abstract: The potential distribution in Hall effect experiments on two-dimensional electron systems (2DES) is imaged with a scanning polarization optical microscope utilizing the internal electro-optic effect. Outside the quantum Hall regime (QHR), Hall potential profiles vary linearly between the two edges. In the QHR, the observed Hall potential profile is nonlinear. Its shape depends on magnetic field and current, indicating inhomogeneous current transport in the interior of the 2DES. In the plateau centre, the Hall potential drop is concentrated in the middle of the channel. The results are interpreted in terms of electron density inhomogeneities. These lead to the observed potential distributions because of the very strong dependence of the conductivity on the local electron density.

Physics of semiconductor microcavity lasers

Abstract: This review summarizes recent developments and successes in the theoretical modelling of the characteristics of semiconductor microcavity lasers. After a discussion of the basic laser properties, results of a quasi-equilibrium many-body theory are presented which are very useful for the understanding of microcavity laser operation not too far above the laser threshold. Non-equilibrium phenomena, such as spectral and kinetic hole burning as well as plasma heating effects, are analysed using a quantum kinetic approach. Comparisons with experimental observations are discussed, before open problems and future challenges are outlined.

Composition dependence of the unit cell dimensions and the energy gap in Zn1-xMgxSe crystals

Abstract: Zn1-xMgxSe mixed crystals were grown by the high-pressure Bridgman method in the x range 0.06 to 0.285. X-ray investigations show that with increasing Mg content the transition from a sphalerite structure to wurtzite occurs at x=0.185+or-0.03. After annealing in liquid zinc or zinc vapour, investigated crystals exhibit n-type conductivity as well as blue-violet and orange photoluminescence in the temperature range from 40 K to room temperature.

4-11 mu m infrared emission and 300 K light emitting diodes from arsenic-rich InAs1-xSbx strained layer superlattices

Abstract: Arsenic-rich InAs/lnAs1-xSbx strained layer superlattices (SLSs) grown on GaAs substrates by molecular beam epitaxy (MBE) are studied for their potential application as infrared emitters. The long-wavelength emission (4-11 mu m) is highly sensitive to superlattice design parameters and is accounted for by a large type-II band offset, greater than in previously studied antimony-rich InSb/lnAs1-xSbx SLSs. High internal PL efficiencies (>10%) and intense luminescence emission were observed at these long wavelengths despite large dislocation densities. Initial unoptimized InAs/lnAs1-xSbx SLS light emitting diodes gave approximately=200 nW of lambda =5 mu m emission at 300 K.

Modelling the temperature dependence of threshold current, external differential efficiency and lasing wavelength in QW laser diodes

Abstract: The temperature behaviour of GaAs/GaAlAs DQW-GRINSCH high-power laser diodes is calculated by means of a numerical model. The model includes a microscopic description of gain and spontaneous radiative recombination, a phenomenological description of interface and Auger recombination, and includes a pumping-current-dependent leakage. Based on the model, the temperature dependences of the macroscopic parameters of threshold current, external differential efficiency and wavelength are calculated. The resulting numerical values for these parameters are in excellent agreement with our experiments. Spontaneous radiative recombination is shown to be the dominant loss mechanism.

Self-consistent analysis of electric field effects on Si delta -doped GaAs

Abstract: We theoretically study the subband structure of single Si delta -doped GaAs inserted in a quantum well and subject to an electric field applied along the growth direction. We use an efficient self-consistent procedure to solve simultaneously the Schrodinger and Poisson equations for different values of electric field and temperature. We thus find the confining potential, the subband energies and their corresponding envelope functions, the subband occupations, and the oscillator strength of intersubband transitions. Opposite to what is usually the case when dealing with the quantum-confined Stark effect in ordinary quantum wells, we observe an abrupt drop of the energy levels whenever the external field reaches a certain value. This critical value of the field is seen to depend only slightly on temperature. The rapid change in the energy levels is accompanied by the appearance of a secondary well in the confining potential and a strong decrease of the oscillator strength between the two lowest subbands. These results open the possibility to design devices for use as optical filters controlled by an applied electric field.

Temperature dependence of the electron impact ionization coefficient in silicon

Abstract: We have used a Monte Carlo simulation to study the temperature dependence of the electron impact ionization coefficient in silicon. On the basis of the results of the Monte Carlo calculations, we propose an empirical model for the impact ionization coefficient as a function of electric field or average electron energy, which is applicable over a wide range of lattice temperatures.

Ballistic electron transport through a quantum point contact defined in a Si/Si0.7Ge0.3 heterostructure

Abstract: We have studied the low-temperature (T=25 mK) ballistic transport of electrons in a split-gate device fabricated from a Si/Si07Ge03 heterostructure In the absence of a magnetic field the conductance is quantized in units of i*4e2/h as the gate voltage is tuned where i is the number of occupied subbands In this system, both the spin and the valley degeneracies have to be taken into account These can be lifted by applying a perpendicular magnetic field Magnetic depopulation of the one-dimensional subbands is observed and can be simulated using a simple square well potential model

Electrical properties of plasma-grown oxide on MBE-grown SiGe

Abstract: The oxidation of semiconductors by RF (radio-frequency) plasma anodization offers several advantages over conventional thermal furnace oxidation, such as low process temperature and faster oxidation rate. This paper investigates the electrical properties of the plasma-grown oxide on SiGe at room temperature and compares them with thermally grown oxides of SiGe. An interface state peak has been observed at around Ev+0.75 eV. This peak is found to be caused by the presence of the SiGe layer and we present evidence that it is related to a silicon dangling bond. We have also observed that the magnitude of the voltage pulse necessary to induce avalanche electron injection increases through the presence of the SiGe layer. We believe that this is related to the increased ionization rate associated with the smaller bandgap of SiGe. The presence of Ge atoms in the plasma oxide has introduced electron traps with capture cross sections of the order of 10-15 and 10-16 cm2. Negative bias temperature stress indicates that the bonding between Ge and H is weak. This aging test also supports the theory that the interface states are acceptor-like in the upper half and donor-like in the lower half of the energy bandgap.

Variable range hopping transport in the tails of the conductivity peaks between quantum Hall plateaus

Abstract: We measure the electrical conductivity in the dissipative regime between adjacent integral quantum Hall plateaus. Variable-range-hopping transport is found, in accordance with a recent theory. The characteristic temperature for hopping conduction follows a power-law dependence on filling factor. Conductivity-peak broadening with increasing current is studied and a power-law dependence is observed.

Non-local aspects of breakdown in pin diodes

Abstract: Measurements of breakdown voltage in p-i-n diodes with thin i regions are compared with local and non-local theoretical calculations. It is found that overshoot effects compensate for the dead space at high fields close to breakdown but that non-local aspects become stronger at lower fields.

Wear-out of ultra-thin gate oxides during high-field electron tunnelling

Abstract: Ultra-thin (<6 nm) SiO2 wear-out is characterized by time-dependent dielectric breakdown and stress-induced leakage current (SILC) measurements on n+ poly-Si/SiO2/n-Si capacitors, stressed by high-field tunnel injection of electrons from the Si substrate. A drastic increase of the charge to breakdown (QBD) and a strong decrease of the SILC are observed for thinner oxide layers and lower tunnel current densities. This is explained by the corresponding reduction of the hot-electron energy during stressing. With the decrease in gate oxide thickness from 6 nm to 3 nm, a transition from Fowler-Nordheim to direct electron tunnelling is observed in the current-voltage characteristics of the capacitors. It is demonstrated that no significant wear-out occurs in a 3.5 nm oxide layer for direct tunnelling of electrons from the Si substrate.

Formation of a stable ohmic contact to CdTe thin films through the diffusion of P from Ni-P

Abstract: Thin films of CdTe used in electronic devices often have problems with electrical contacts. This is due to the non-availability of a contacting material with a large work function for proper matching with p-CdTe. Moreover, the method of doping the thin film is the real problem. There are several possible solutions in this connection, one of which is the formation of a p+ layer on the CdTe surface by the reaction or indiffusion of dopant materials. Another approach is to engineer the barrier height prior to the metal contact deposition by depositing a layer of Cu-doped ZnTe onto the CdTe. However, all of the above methods have their own limitations. In this paper a method is presented which can overcome these limitations. Using an autocatalytic reduction process, NbP composite material has been deposited successfully on the p-CdTe surface. On annealing at an optimum temperature of 250 degrees C, the contact resistivity comes down to 0.1-0.08 Omega cm2. XRD and EDAX studies reveal that the lowering of the contact resistance is due to the diffusion of P into the CdTe with the formation of a p+ layer. A model for this has also been presented in the text.

Fatigue mechanisms in thin film PZT memory materials

Abstract: The recent evolution of high-quality thin films of the PZT family has brought closer the use of ferroelectrics in non-volatile memory applications. While the inherent properties of the films are more than acceptable for most memory applications, the reduction in switched charge under repeated ferroelectric switching remains a problem. This so-called fatigue leads to changes of phonon frequencies and Raman cross sections in PbZr0.48Ti0.52O3 (PZT). Simultaneously, a characteristic IR absorption band near 4000 cm-1 shifts towards higher energies. This absorption signal is discussed in terms of polaronic states related to carriers generated by oxygen loss from the samples. It will be shown that the fatigue in PZT can be related to an increase of the local lattice distortion and a formation of defects both caused by oxygen loss from the samples.