Showing papers in "MRS Proceedings in 1993"

The Role of Indentation Depth on the Measured Hardness of Materials

Abstract: Ultra micro-indentation tests on Ni and Cu samples showed increasing hardness with decreasing penetration depth over a range from 200 to 2000 nm. The results suggest increased strain hardening with decreased indentation depth. To establish that this is a real material effect, a series of tests were conducted on amorphous materials, for which strain hardening is not expected. The hardness of Metglas® was found to be independent of depth. A simple model of the dislocation densities produced under the indenter tip describes the data well. The model is based on the fact that the high density of dislocations expected under a shallow indentation would cause an increase in measured hardness. At large depths, the density of geometrically necessary dislocations is sufficiently small to have little effect on hardness, and the measured hardness approaches the intrinsic hardness of the material.

Preferred Orientations for Sol-Gel Derived Plzt Thin Layers

Abstract: PLZT thin layers were deposited onto various substrates by sol-gel methods, and crystallized under different conditions and substrate treatments. Relationships are given for the chemical characteristics of the substrate’s surface and the preferred orientations which develop on heat treatment. A preferred (111) orientation always developed for perovskite crystallized on Pt layers which contained Ti on the surface. This was attributed to the formation of Pt3Ti and the role of heteroepitaxial nucleation and growth sites. In addition, a preferred (100) orientation was also obtained on unarmealed Pt/Ti/SiO2/Si substrates which were free of Ti on the surface. This was attributed to self-textured growth with flat faces striving for minimum surface energy conditions. The results are discussed in terms of the importance of interfacial chemistry on the control of texture for crystallization of PLZT thin layers on coated substrates.

Models and Mechanisms for the Luminescence of Porous Si

Abstract: Explanations for the efficient, visible luminescence of porous Si fall broadly into three categories. At the extremes are the pure quantum-well point of view and the molecular agents hypothesis. We make here the case for a model in which the dominant absorption characteristics are those of a quantum well, but luminescence occurs via boundary states on the nanocrystalline particles. This so-called “smart quantum-well” mechanism accounts for the multiple emission bands of porous Si in a natural way. In particular, the infrared band is assigned as an electronic transition to a deep level. From this we determine the conduction band shift with particle size.

Diethyldiethoxysilane as a New Precursor for SiO2 Growth on Silicon

Abstract: Diethyldiethoxysilane (DEDEOS) is a potential single molecular precursor for SiO2 growth. Additionally, DEDEOS may be applicable in a deposition scheme that will allow precise atomic layer control of SiO2 film thicknesses. Deposition of SiO2 on silicon using DEDEOS was studied in three regimes. In the first regime, the adsorption and decomposition of DEDEOS were studied using temperature-programmed desorption (TPD) and Fourier-transform infrared spectroscopy (FTIR) under UHV conditions. TPD and FTIR results indicated that DEDEOS deposits ethyl and ethoxy species on the surface which decompose by a β-hydride elimination mechanism. In the second regime, step-wise SiO2 growth using DEDEOS was studied by laser-induced thermal desorption (LITD) and TPD as a function of repetitive growth cycles composed of DEDEOS deposition at 300 K followed by thermal annealing to 820 K. The amount of SiO2 growth per cycle initially decreased as a function of cycle until reaching a constant value after approximately 10 cycles. Finally, DEDEOS deposition of SiO2 was studied in a high pressure chamber at surface temperatures between 873 K and 1003 K and at a pressure of 0.5 Torr. The deposition of SiO2 as a function of exposure was found to display a fast initial growth step followed by a slower step which continued indefinitely. The activation energy for SiO2 growth by DEDEOS in the slower growth step was found to be 49 ± 6 kcal/mol. This activation energy is very similar to the activation energy of 45 kcal/mol observed for SiO2 growth by tetraethoxysilane (TEOS).

Microstructures and Mechanical Behavior of Nb-Ti Base Beta + Silicide Alloys

Abstract: Studies on Nb/Nb 5 Si 3 based in-situ composites have demonstrated an acceptable balance of low-temperature damage tolerance and high-temperature strength/creep resistance. However, catastrophic oxidation and embrittlement of these materials limit their usefulness in structural applications. Alloying studies were initiated at Wright Laboratory with the aim of obtaining incremental improvement in the overall oxidation response of the Nb/Nb 5 Si 3 system, while seeking microstructurally similar systems. The results showed that reduced metal recession rates and oxygen embrittlement can be obtained by Ti and Al additions to Nb-Si base alloys. This paper focuses on the effect of Ti and Al alloying additions to Nb-Si base alloys on phase equilibria, microstructures, temperature dependence of strength, low-temperature toughness, and environmental resistance.

Correlations Among Degradations in Lead Zirconate Titanate thin Film Capacitors

Abstract: Many attempts have been made to reduce degradation properties of Lead Zirconate Titanate (PZT) thin film capacitors. Although each degradation property has been studied extensively for the sake of material improvement, it is desired that they be understood in a unified manner in order to reduce degradation properties simultaneously. This can be achieved if a common source(s) of degradations is identified and controlled. In the past it was noticed that oxygen vacancies play a key role in fatigue, leakage current, and electrical degradation/breakdown of PZT films. It is now known that space charges (oxygen vacancies, mainly) affect ageing, too. Therefore, a quantitative ageing mechanism is proposed based on oxygen vacancy migration under internal field generated by either remanent polarization or spontaneous polarization. Fatigue, leakage current, electrical degradation, and polarization reversal mechanisms are correlated with the ageing mechanism in order to establish guidelines for simultaneous degradation control of PZT thin film capacitors. In addition, the current pitfalls in the ferroelectric test circuit is discussed, which may cause false retention, imprint, and ageing.

Solubility and Dispersion Characteristics of Polyaniline

Abstract: We have demonstrated that the solubility characteristics of undoped polyaniline can be understood from the point of view of standard solubility parameters which measure the propensity for the polymer and the solvent to engage in dispersive, polar, and hydrogen bonding interactions. By empirical measures and group additive calculations, emeraldine base has been shown to be characterized by solvent interaction parameters: δd = 17.4 MPa1/2, δp = 8.1 MPa1/2, and δh = 10.7 MPa1/2, which together are equivalent to a total solubility parameter: δ = 22.2 MPa1/2. In the case of doped polyanilines, the solubility parameters can influence or even dominate the interactions of the doped polymer. Without the modifying influence of the dopant anion, the doped polyaniline compositions are more strongly polar and hydrogen bonding than the undoped polymer.

Multiple Pulse Irradiation Effects in Excimer Laser-Induced Crystallization of Amorphous Si Films

Abstract: We have experimentally Investigated the effects that are associated with multiple-pulse irradiation in the excimer laser processing of thin Si films on SiO2. Double-pulse irradiation experiments revealed results, which are consistent with that which is known from single-pulse crystallization experiments, and these experiments confirm the applicability of the transformation scenarios, which were derived from single pulse-induced crystallization experiments [1, 2]. The results from the multiple-pulse irradiation experiments clearly show that gradual and substantial grain enlargement can occur — and only occurs — when the irradiation energy density is close to but less than the level that is required to melt the film completely. Based on these findings, we argue that the grain enlargement effect is a near-complete melting phenomenon that is associated with polycrystalline Si films, and we present a grain boundary melting model to account for this phenomenon. A brief discussion on the apparent similarities and physical differences between the observed phenomenon and the solid state grain growth processes is provided herein.

Ion Beam Synthesis of Luminescent Si and Ge Nanocrystals in a Silicon Dioxide Matrix

Abstract: Ion beam synthesis of Si and Ge nanocrystals in an SiO 2 matrix is performed by precipitation from supersaturated solid solutions created by ion implantation. Films of SiO 2 on (100) Si substrates are implanted with Si and Ge at doses 1 × 10 16 /cm 2 - 5 × 10 16 /cm 2 . Implanted samples are subsequently annealed to induce precipitation of Si and Ge nanocrystals. Raman spectroscopy and high-resolution transmission electron microscopy indicate a correlation between visible room-temperature photoluminescence and the formation of diamond cubic nanocrystals approximately 2–5 nm in diameter in annealed samples. As-implanted but unannealed samples do not exhibit luminescence. Rutherford backscattering spectra indicate a steepening of implanted Ge profiles upon annealing. Photoluminescence spectra are correlated with annealing temperatures, and compared with theoretical predictions for various possible luminescence mechanisms, such as radiative recombination of quantum-confined excitons, as well as possible localized state luminescence related to structural defects in SiO 2 . Potential optoelectronic device applications are also discussed.

Diffraction anomalous fine structure: a new technique for probing local atomic environment

Abstract: Diffraction anomalous fine structure (DAFS) is the fine structure in the intensity of an X-ray diffraction peak in the vicinity of an absorption edge. DAFS is measured by monitoring the intensity of a diffraction peak as a function of the incident X-ray energy as it is scanned through an absorption edge. It combines the short range structural sensitivity of X- ray absorption spectroscopy with the long range periodicity of X-ray diffraction, and can provide structural information which is not available from these techniques alone, or in combination. We present a methodology which allows extraction of a specific X-ray absorbance spectrum from the DAFS spectrum, and illustrate it for KMnO4. We also demonstrate the ability of DAFS to separate the contributions to the X-ray absorption spectrum of the tetrahedral and octahedral cobalt sites in the spinel Co3O4

New Borate Structures for Nlo Applications

Abstract: By considering selected examples of new structure types, guidelines are set forth for the synthesis of new solid-state inorganic borates that are likely to have desirable properties for nonlinear optical applications. The structures of two new, noncentrosymmetric orthoborate fluorides BaCaBO3F and Ba7(BO3)3F5 demonstrate a feasibility for controlling linear optical properties and for producing noncentrosymmetric borates that melt congruently. The structure of SrLi(B3O5)3 represents an additional example of a noncentrosymmetric borate resulting from chirality of the B3O7 ring. In addition to potential practical value, crystals of the type AMOB2O5 (A = K, Rb, and Cs; M = Nb and Ta) provide a unique means for examining the structural dependent interrelationships of linear and nonlinear optical properties.

Scanning Probe Microscopy Study of Layered Dichalcogenide ReS2

Abstract: Single crystal ReS2 was characterized using atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The observed atomic-resolution AFM and STM images were interpreted by calculating the total, ρ(r0), and partial electron density distribution, ρ(r0, ef), respectively, of a single ReS2 layer. The experimental and theoretical results indicate that the basal plane surface state density is dominated by contributions from the sulfur layer This is in contrast to bulk band structure calculations which would predict that the major contribution to the STM images should be from the Re atoms. Furthermore, the surface-to-tip STM images are found to be quite different from tip-to-surface images. By interpreting STM and AFM data from ρ(r0) and ρ(r0, ef) calculations, it is shown that site specific structural and electronic details of the surface can be elucidated.

Optical Activation of Ion Implanted Rare-Earths

Abstract: A review of the main results concerning the ion implantation of the rare-earth elements is given. To obtain the best optical activation of rare-earths, we attempt to optimize the implantation (energy, dose) and annealing (temperature, duration) conditions. The studied materials are Si, II-VI binaries (ZnTe, CdS), III-V binaries (GaAs, InP), III-V ternaries (GaAlAs, GalnAs) and III-V quaternaries (GaInAsP).

Binding of copper and nickel to cavities in silicon formed by helium ion implantation

Abstract: Cavities formed in Si by He ion implantation and annealing are shown to be strong traps for Cu and Ni impurities. Experiments utilizing ion-beam analysis and transmission electron microscopy indicate that Cu is trapped at the internal surfaces of cavities up to {approximately}1 monolayer coverage with a binding energy of 2.2{plus_minus}0.2 eV relative to solution. This is greater than the heat of solution from the precipitated Cu{sub 3}Si phase, determined to be 1.7 eV in agreement with earlier work. Copper at cavity-wall sites is reversibly replaced by H during heating in H{sub 2} gas, indicating the relative stability of the two surface terminations. Initial results for Ni impurities indicate that trapping at cavities is again energetically preferred to silicide formation. The saturation coverage of Ni on the internal surfaces, however, is an order of magnitude smaller for Ni than Cu, consistent with published studies of external-surface adsorption. These results suggest that cavity trapping may getter metallic impurities in Si more effectively than methods based on silicide precipitation.

Solution Processable Conducting Polymer: Polyaniline-Polyelectrolyte Complexes

Abstract: Conducting polymers have novel electrical and optical properties that are potentially useful for applications in electroluminescence display, rechargeable batteries, electrochromic windows, chemical sensors, electromagnetic shielding and anti-electrostatic films. However, these new materials are less adaptable to industrial processes than the traditional synthetic polymers. One of the problems is that conducting polymers are difficult to be solubilized in any solvent, thus lacking solution processability during a manufacturing process. In this article we discuss a new molecular complex of polyaniline that is soluble in water and in polar organic solvents.

Space-Charge-Limited Charge Injection From ITO/PPV Into a Trap-Free Molecularly Doped Polymer

Abstract: We describe bilayer structures comprised of a poly (p-phenylene vinylene) (PPV) layer and a trap-free diaryldiamine (TPD) doped in polycarbonate (PC) layer, sandwiched between indium-tin-oxide (ITO) and aluminum (Al) contacts. Two critical phenomena in the operation of polymer based electroluminescent devices, interface injection and carrier range, are investigated. It is established that the ITO/PPV contact is capable of sustaining dark current under trap-free space-charge-limited (TFSCL) conditions into a hole transporting TPD:PC layer. TFSCL currents are not observed in devices without the PPV layer. Upon increasing the thickness of the PPV layer a deviation from the TFSCL regime is observed which is attributed to trapping of the injected holes within PPV. These observations suggest a novel method for estimating the trapping Mobility-lifetime product μτ for holes in PPV. By this means we estimate μτ ∼ 10−9 cm2/V.

Water Soluble Polyanilines: Properties and Applications

Abstract: A new class of water soluble conducting polyanilincs have been developed. This is accomplished by oxidatively polymerizing aniline monomers on a template such as a polymeric acid. The resulting polyanilines readily dissolve in water. These materials can be applied as removable discharge layers for electron-beam (e-beam) lithography and for SEM mask metrology. Cross-linkable functionality can be incorporated on the polyanilinc backbone resulting in water soluble polyanilines that are radiation curable. Upon irradiation these materials cross-link and become insoluble and thus can be utilized as permanent conducting coatings for electrostatic discharge (ESD) applications. In addition, the cross-linkable polyanilines can be made into water developable conducting resists. 1.0μm conducting lines have been patterned with e-bcam irradiation.

Colloidal Au Nanoclusters Formed in Fused Silica by MeV Ion Implantation and Annealing

Abstract: MeV implantation of Au has been used to create a high density of Au nanoclusters in the near surface of fused silica. Measurements of the nanocluster size and size distribution under various implantation/annealing conditions are presented and correlated with measurements of optical absorption arising from surface plasmon resonance absorption by the Au nanoclusters in fused silica. Preliminary measurements of the nonlinear refractive index are included.

The role of texture on the reliability of Aluminum-based interconnects

Abstract: Preferred crystallographic orientation has long been recognized to play an important role in interconnect reliability where a strong (111) texture improves electromigration lifetime. Detailed microstructural analyses have enabled the role of texture to be better understood. Although Bragg-Brentano scans are often used to characterize texture, it is shown that this technique gives incomplete and sometimes misleading information. The pole figure technique provides a complete description of the texture. The measurement and presentation of textures consider experimental aspects unique to thin film analysis as a prerequisite to developing quality data. Five textural archetypes are identified, and metrics presented for their quantification. Processing effects on texture are complex and depend on all facets of deposition conditions, on substrate/underlayers, and on annealing. General trends and specific examples of the impact of each aspect are given where it will be shown that deposition conditions and the presence of underlayers have the greatest influence. The role of texture on reliability is considered for four failure modes: thermal hillocks, grain collapse, stress voiding, and electromigration. Electromigration results are emphasized where texture must be considered in the context of grain structure, in general, and, more specifically, the ratio of line width to grain size. Most measures of microstructure and reliability are statistical. The importance of local microstructural analysis will be emphasized in terms of both the arrangement of grains relative to the line dimension and microtexture characterization of the grain-to-grain misorientations.

Electrically Conducting Compatible Blends of Polyaniline / Poly(Vinyl Pyrrolidone)

Abstract: Blending polyaniline (PAn) with a host polymer has the advantage of reducing the amount of polyaniline necessary for conductivity, increasing the stability of this conduction, and enhancing the overall mechanical integrity, versatility, and processability of PAn. PAn blends have generally been observed to be immiscible, reducing many of the above mentioned advantages. In this work, PAn was solution blended with poly(vinyl pyrrolidone) (PVP) in either NMP or DMAc, both polar solvents with an amide linkage. We have found PAn to be miscible with PVP under certain processing conditions. The extent of this compatibility and resulting mechanical properties were assessed by DMA, DSC, TGA, SAXS, and light microscopy. DSC thermograms exhibit a single Tg for all blends ranging from 3 to 30 % by weight PAn. Light microscopy and SAXS also suggest no phase separation. TGA results show that NMP binds very tightly to both PAn and PVP, with solvent cast blends retaining up to 20 wt% NMP, even after relatively vigorous drying. Electrical conductivity measurements of cast films show conductivity to range from 10−3 to 2 S/cm over the same composition range.

Molecularly Engineered Polymer Leds

Abstract: Polymeric light emitting devices may be fabricated from a simple structure consisting of a low work function cathode (typically calcium or magnesium), a conjugated semiconducting polymer and a transparent anode (typically indium-tin oxide). Optimum device efficiencies require the balanced injection of electrons and holes. This paper describes the application of molecular engineering in the design of a family of poly(cyanoterephthalylidenene)s which show increased electron affinity over the unsubstituted analogue [poly(p-phenylenevinylene) PPV]. In particular these polymers as the emissive layer in a bilayer device with indium tin oxide (ITO, positive transparent contact) and aluminum (stable negative contact) and PPV as a hole transporting layer exhibit internal efficiencies up to 4%.

Observation and Modelling of Electromigration-Induced Void growth in Al-Based Interconnects

Abstract: Accelerated electromigation tests on unpassivated, pure aluminum interconnects were performed. The failure mechanisms were observed by interrupting the tests and examining the conductor lines using an SEM. Because the metal thin film was subjected to a so-called laser reflow process before patterning, grain boundaries were visible in the SEM as thermal grooves. Voids were observed to move along the line and to grow in a transgranular manner, and a characteristic asymmetric void shape was identified which seems to be related to the failure mechanism. It is argued that substantial progress in modelling and understanding of electromigration failure can be made by consideration of such void shape effects.

Light-Emitting Diodes Using Semiconducting Oligothiophenes

Abstract: The light-emitting diodes (LEDs) using semiconducting oligothiophenes, dimethylquaterthiophene (DMQtT, tetramer), dimethylquinquethiophene (DMQqT, pentamer) and dimethylsexithiophene (DMSxT, hexamer), have been investigated. These oligomers were deposited on ITO-coated glass in ultra high vacuum and an aluminum electrode was subsequently vacuum-deposited on top of the oligomers. These structures have the diode configuration with Schottky barrier between the oligomers and aluminum. The LED using DMSxT shows good rectifying feature with the rectifying ratio of 1500 at ±10V. Red-orange emission is clearly observed above 4V bias. In this LED, DMSxT acts not only as an emitting layer but also as a hole transport layer. We have also fabricated and studied alternate layered structures of DMSxT/DMQtT and DMSxT/DMQqT as the emitting layer. In these configurations, the carrier recombination can be modulated because both DMQtT and DMQqT have energy gaps wider than that of DMSxT. The quantum efficiencies (photons emitted per carriers injected) of the LEDs using DMSxT/DMQtT and DMSxT/DMQqT are about one hundred times and one thousand times larger than that of the LED solely based on DMSxT, respectively. These results indicate that the layered structures are advantageous in increasing quantum efficiency of the emission.

Influence of Structural Parameters of Conducting Polymers on their Microwave Properties

Abstract: The complex dielectric constant e* (e* = e′-ie″) of conducting polymers (polyaniline and poly(3-alkyl thiophene)) is studied over a frequency range spanning from 130 MHz to 20 GHz. The effects of structural parameters (counter-anion size, molar mass, length of the alkyl chain on the substituted monomer) on electrical properties (σdc and e* values) have been investigated.

Reactive ion Etching of Pt/PbZrxTi1−xO3/Pt Integrated Ferroelectric Capacitors

Abstract: Dry etching of a Pt/PbZrxTi1−xO3/Pt (Pt/PZT/Pt) ferroelectric capacitor stack with CF4/Ar plasmas with a reactive ion etching process for the fabrication of micrometer-sized integrated ferroelectric capacitors is described. The etch rate for both Pt and PZT is determined as a function of the process settings: Power, pressure and CF4-Ar gas flow ratio. A chemical enhancement of the etch rate is found for PZT. It is shown that it is possible to etch the Pt/PZT/Pt ferroelectric capacitor stack in a CF4/Ar plasma in a single lithographic process using patterning by photoresist masking. Redeposition processes occurring during etching are described.

Electrostatic Potentials for Metal Oxide Surfaces and Interfaces

Abstract: We discuss the development of interaction potentials which explicitly allow for charge transfer in metallic oxides The charge transfer is calculated self-consistently using a charge equilibration approach, which allows the amount of charge transferred to respond to the electrostatic environment We model the metal-metal, metal-oxygen, and oxygen-oxygen interactions with Rydberg function pair potentials By fitting the Rydberg potential parameters to the elastic and structural constants of the material, we arrive at an efficient model for the simulation of metallic oxides We demonstrate the applicability of the model by describing some preliminary results on the rutile phase of titanium dioxide

Conducting Polymers: Past, Present and Future…

Abstract: Since their discovery 16 years ago, the field of intrinsically conducing polymers — “synthetic metals” — has developed at an unexpectedly rapid rate. The concept of “doping” is the unifying theme which distinguishes this class of organic polymers — “conducting polymers” — from all others. Doping results in dramatic electronic and magnetic changes with a concomitant increase in conductivity to, or approaching, the metallic regime. Doping phenomena and the chief types of dopable organic polymers are described with particular emphasis on polyaniline, which is now probably the most actively-studied conducting polymer. It has been commercialized on a relatively large scale and presently appears to be the leading conducting polymer for technology. It shows considerable promise for electromagnetic interference (EMI) shielding and as a gas separation membrane, and is currently used in commercial rechargeable batteries. Polypyrrole is used commercially in capacitors and as an electrically conductive coating on conventional fabrics. Additional potential uses of conducting polymers such as light-emitting diodes, electrochromic windows, chemical sensors, etc. are also described briefly.

Optical Absorption, Luminescence, And Redox Switching Properties Of Polyphenylene Derivatives

Abstract: A series of polyphenylene derivative polymers with a variety of heterocyclic units along the main chain, and various pendant groups, have been synthesized and their optoelectrochemical properties examined. Polyphenylenes containing electronically isolated emitter centers have optical absorption and luminescence characteristics which are a function of the length of the conjugated unit. A water soluble electroactive rigid-rod poly (p-phenylene), specifically Poly[2,5-bis (propoxy-3-sulfonate)-1,4-phenylene- alt ,4-phenylene], has been prepared. Solution cast films of this polymer exhibit optical evidence for bipolarons via both p-type and n-type electrochemical doping. Further poly ( p -phenylene) (PPP) derivatives are being pursued, including PPP's bearing pendant donor molecules for charge transfer complex formation. A series of polymers containing alternating p-phenylene units with bithienylene and bifuranylene units, specifically poly[1,4-bis (2-heterocycle)-2,5-disubstituted-1,4-phenylenes], have been synthesized with both alkyl and alkoxy pendant groups. Steric interactions disrupt the ability of the redox doped polymers to attain planarity, strongly affecting the polymer's oxidation potentials and DC conductivities. The substituent length has little effect on the electronic properties of the polymer, but greatly affects the polymer's solubility and transport properties during redox switching.

Advances in Dye-Doped Sol-Gel Lasers

Abstract: The sol-gel process is a solution synthesis technique which provides a low temperature chemical route for the preparation of rigid transparent matrices. A number of laser dyes have been incorporated in different sol-gel matrices and tunable laser action has been demonstrated with these materials. This paper extends the sol-gel laser field into two significant areas, infrared dyes and pyrromethenes. The work with the tricarbocyanine dyes shows the versatility of sol-gel chemistry as organic modifications produce a favorable environment for the dye molecules. The results with the pyrromethene system show a considerable increase in output energy and offer the promise of longer laser lifetimes.

Fundamentals of Semiconductor Detectors for Ionizing Radiation

Abstract: The modes of operation of semiconductor detectors are reviewed, together with the influence of charge carrier collection in developing a signal pulse for spectroscopic applications. Because of the importance of charge trapping in many semiconductors of interest in the fabrication of room temperature radiation detectors, the effects of incomplete charge collection are quantified. Calculated results are presented for the expected pulse height and energy resolution under a variety of charge collection conditions.