Deep Defect Density Research Articles

Electrical and optical properties of glow-discharge a-SiOx : H (x

Hydrogenated amorphous silicon-oxygen alloy (a-SiO x : H) films were deposited by r.f. glow-discharge decomposition of a SiH 4 + CO 2 gas mixture at a substrate temperature of 300°C. The optical band gaps of the samples were found to change between 1.65 and 2.73eV by varying the oxygen content from 4.5 to 64.2 at %. While both the room-temperature photo- and dark-conductivity decrease with oxygen alloying, for oxygen contents below 11.9 at % the measured conductivities are comparable to those of unalloyed a-Si : H. In contrast, the deep-defect density and Urbach parameter continuously increase with oxygen alloying.

A Study of Electronic Defects in Hydrogenated Amorphous Silicon Prepared by the Expanding Thermal Plasma Technique

AbstractThe electronic properties of amorphous silicon films prepared by the expanding thermal plasma technique have been studied using steady-state and transient photoconductivity measurements. It is found that films deposited at a substrate temperature of 400°C have a conduction band tail slope of 29 meV, deep defect density of order 3×1016 cm-3, an Urbach tail slope of 65 meV, defect absorption of 5-10 cm-1, and a mobility-lifetime product of 1.3×10-7 cm2 V-1. Aslight increase in defect density and reduction in mobility-lifetime product is observed on moderate light-soaking. The overall optoelectronic quality is somewhat poorer than commercial PECVD material, but there is scope for improvement as deposition conditions are further optimised.

Channel Mobility in AlGaN/GaN HFETs on SiC and Sapphire Substrates

Drift mobility as a function of gate voltage has been measured in HFETs fabricated by MOVPE to a common layer structure, but with varying pinch-off voltage attributed to varying deep level defect densities. The mobility reached a peak of 2000 cm2/Vs for the best samples at room temperature. At high number density the layers showed a common mobility against gate voltage curve, which is attributed to the mobility being related to surface electric field rather than number density.

Boron doping of hydrogenated amorphous silicon prepared by rf-co-sputtering

This paper addresses the doping mechanism of amorphous semiconductors through the investigation of boron doped rf co-sputtered amorphous hydrogenated silicon. The activation energy and room temperature conductivity varied from 0.9 to 0.3 eV and from 10-12 to 10-4 Ohm-1 .cm-1, respectively, by ranging the boron concentration from 0 to 3 at.%. These ranges of electronic properties are of the same order of those reported for samples prepared by plasma enhanced chemical vapor deposition (PECVD). In spite of these similarities, there are some relevant differences. The doping effciency, at low boron concentrations, is much lower than that of PECVD samples. In addition, the creation of deep defects (dangling bonds) does not follow the square root power law dependence on the boron concentration as proposed by Street and observed in PECVD samples. These differences are associated with the density of defects, which is much higher in films prepared by sputtering. The increase in the deep defect density is more likely related to topological disorder, introduced by the presence of a high concentration of inactive impurities.

Transient Photoconductivity for the Identification of Spatial Inhomogeneities in the Defect Density in Amorphous Silicon

We achieved the probing of near-surface regions in hydrogenated amorphous silicon by directing green-pulsed laser light with a short absorption depth either through the air/silicon or the glass/ silicon interface. Transient photoconductivity under these conditions results in different decay behaviors of the photocurrent when the sample is illuminated from either side. By application of a Fourier transform technique on the photocurrent decay data we identify spatial inhomogeneities in the deep-defect density that are responsible for these differences. When compared with the defect density in the bulk of the film, obtained from probing the sample with homogeneously absorbed light, we can thus reveal a possible increase in the defect density towards either the air/silicon or silicon/substrate interface which remains undiscovered by other experimental techniques.

Semiinsulating CdTe

Experimental conditions for the growth of near stoichiometric high-resistivity CdTe single crystals with a minimal concentration of point defects are investigated. The position of the stoichiometric line in the pressure–temperature ( P– T) phase diagram is evaluated from high-temperature in situ galvanomagnetic measurements. Calculations based on a model of two major native defects (Cd vacancy and Cd interstitial) show, that a very small variation of Cd pressure P Cd results in a strong generation of uncompensated native defects. Modelling of room temperature carrier density dependence on the deep defect density N DD, P Cd, and annealing temperature T shows, that the range of optimal P Cd, at which high resistivity can be reached, broadens with increasing N DD or decreasing T. It is shown that at low T<450 oC the deep defect density <10 15 cm −3 is sufficient to grow the high-resistivity CdTe.

Gap state distribution in amorphous hydrogenated silicon carbide films deduced from photothermal deflection spectroscopy

The density of gap states distribution in silicon (Si) rich hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films with varying carbon (C) fraction (x) is investigated by the photothermal deflection spectroscopy (PDS). The films are grown using the Electron Cyclotron Resonance Chemical Vapor Deposition (ECR-CVD) technique. By using different methane-to-silane gas flow ratios, a-Si1−xCx:H with x ranging from 0 to 0.36 are obtained. A deconvolution procedure is performed based on a proposed DOS model for these Si rich a-Si1−xCx:H. Good fits between the simulated and experimental spectra are achieved, thus rendering support to the model proposed. Deduction of the DOS enables us to obtain various parameters, including the optical gap and the valence band tail width. The fitted mobility gap Eg is found to be well correlated to the Tauc gap Etauc and E04 gap deduced from the optical absorption spectra. A correlation is also seen between the fitted valence band tail width Evu, the Urbach energy Eu and the defect density. All these parameters are seen to increase with C alloying. A shift in the defect energy level in the midgap with increasing C incorporation is observed, together with a broadening of the defect distribution and a stronger correlation between the defect bands, which can be accounted for in terms of the influence of C dangling bonds on the deep defect density distribution.

Role of interstitial hydrogen and voids in light-induced metastable defect formation in hydrogenated amorphous silicon: A model

Densities of states of a large set of samples have been measured. Samples were deposited by radio-frequency powered glow discharge under various conditions of temperature, power, gas pressure, and dilution of silane. Density of states was studied in the as-deposited, light-soaked, and annealed states. For all the samples light soaking resulted in an increase of both the deep defect density and of the conduction-band tail states. For samples deposited on the edge of crystallinity and polymorphous materials irreversible modifications of the density of states were observed after light soaking followed by annealing. Since none of the existing models of the metastability can account for this behavior we propose a model. In this model, light induced creation of dangling bonds is mediated by interstitial hydrogen. Hydrogen coming from the breaking of Si-H bonds is trapped into voids or platelets during light soaking and released in the lattice during annealing. This model fully explains our experimental results and also many other experimental observations found in the literature.

Spectral photoresponses and transport properties of polymorphous silicon thin films

The electronic properties of low temperature (150 °C) deposited hydrogenated polymorphous silicon thin films are studied in both coplanar and sandwich configurations using a number of complementary characterisation techniques. It is shown that these films have essentially enhanced transport properties for the holes and lower deep defect densities than standard hydrogenated amorphous silicon. From the photovoltaic point of view, this leads to a better normalised collection efficiency in the red part of the spectrum in Schottky diodes made of these films due to a lower recombination rate and a wider electric field distribution. The influence of light-soaking and subsequent annealing are described.

On the doping mechanism of boron-doped hydrogenated amorphous silicon deposited by rf-co-sputtering

The doping mechanism of boron-doped a-Si:H deposited by rf-co-sputtering was investigated and compared with results obtained by plasma enhanced chemical vapor deposition (PECVD). The activation energy and room temperature conductivity could be varied by approximately the same range as that reported for PECVD films. However, there are some relevant discrepancies between the results obtained by these two techniques. First, one needs a much larger amount of boron in the sputtered films, in comparison with PECVD ones, in order to get samples with almost the same activation energy. This implies that the doping efficiency is much lower in films prepared by sputtering. Second, the relation between the dangling bonds and the impurity concentration does not appear to follow the square root power law function as proposed by Street and observed in PECVD films indeed. These differences are likely to be related to the density of defects, which is much higher in films prepared by sputtering. The increase in the deep defect density is more likely to be related to topological disorder introduced by the presence of a large number of inactive impurities.

Effect Of Increasing Growth Rates on Film Properties and Device Performance for DC Glow Discharge Amorphous Silicon

AbstractWe have examined the electronic properties of intrinsic amorphous silicon films as a function of deposition rate, and compared these with the performance of companion solar cell p-i-n devices. The device efficiency in the light-soaked state was strongly inversely correlated with growth rate. Film properties were evaluated in both the as-grown and light soaked states using drive-level capacitance profiling and transient photocapacitance spectroscopy. Although deep defect densities measured by drive-level capacitance profiling did not vary significantly between samples, the magnitude of the defect band deduced via transient photocapacitance spectroscopy was well correlated with device performance. Possible reasons for this discrepancy are discussed. Urbach energies were also correlated with film growth rate.

Transient Photocurrents in Microcrystalline Silicon Films

AbstractTransient photoconductivity in microcrystalline silicon thin films prepared by VHF PECVD over a range of [silane]: [silane + hydrogen] concentrations γ from 3 to 6.3% has been measured as a function of temperature (210 K - 390 K) and laser pulse density (1013 – 1016 cm3). The behavior of highly crystalline films (γ ∼ 3%) bears certain similarities to n-type amorphous silicon, where the power law decay at long times is controlled by hole rather than electron kinetics. As γ approaches 6% the decay begins to resemble that of intrinsic amorphous silicon, consistent with structural studies which indicate a transition from crystalline to amorphous character. Transient photocurrent density of states spectroscopy on transitional films suggests a similar deep defect density to high-quality amorphous silicon, but a larger conduction band tail energy of some 30 meV and a tenfold increase in localised state density at a depth of about 0.4 eV.

Reverse bias annealing of Schottky diodes: evidence for the lower defect density and better stability of polymorphous silicon compared to amorphous silicon

Reverse bias annealing (RBA) experiments on high quality Schottky diodes formed on both hydrogenated amorphous silicon (a-Si:H) and hydrogenated polymorphous silicon (pm-Si:H) are presented. A strong increase of the deep defect density after RBA is revealed by capacitance measurements. The spatial location of created defects is shown to be limited within the space charge region established by the reverse bias applied during annealing. Changes are metastable and the structure can recover after annealing under short circuit. It is shown that pm-Si:H has a much lower deep defect density and is more robust against defect creation than a-Si:H. Results are discussed in terms of possible mechanisms for defect creation related to the microstructure of the materials.

Influence of the a-Si:H structural defects studied by positron annihilation on the solar cells characteristics

The influence of the power density, P D , on the density and structure of defects of undoped a-Si:H thin films, deposited by r.f.-PECVD, is studied by the constant photocurrent method, (CPM), and by slow positron beam spectroscopy, respectively. Deep defect density, N DD , remains approximately constant at 10 16 cm −3, typical of device quality material, for P D in the range 7–20 mW·cm −3, as calculated from CPM. Out of this range, N DD increases roughly one order of magnitude for both lower and higher power densities. Positron annihilation spectroscopy reveals the predominance of two kinds of vacancy type defects in the films: large vacancy clusters or voids for P D ∼7 mW·cm −3 and small vacancy type defects as P D increases to ∼30 mW·cm −3. The behaviour of solar cells, prepared with a-Si:H intrinsic layers with similar deep defect densities, varies with the S parameter of the a-Si:H film, S film: as S film increases, which can be related to a lower material density, the solar cell characteristics degrades.

Experimental evidence indicating a global mechanism for light-induced degradation in hydrogenated amorphous silicon

Using the modulated photocurrent method we studied the deep defect creation and annealing kinetics of amorphous silicon-germanium alloys with Ge fractions below 10 at. %. Two distinct defect bands are observed corresponding to neutral Si and neutral Ge dangling bonds. During annealing the Si and Ge defects directly compete with each other, implying a global reconfiguration mechanism. The creation kinetics indicate the usual nearly 1/3 power dependence on exposure time for the total deep defect density, but not for either type individually. The details of the Ge vs Si defect creation is used to discriminate between some proposed defect creation models.

New experiments on the relationship between light-induced defects and photoconductivity degradation

ABSTRACTWe report experimental results that help settle apparent inconsistencies in earlier work on photoconductivity and light-induced defects in hydrogenated amorphous silicon (a-Si:H) and point toward a new understanding of this subject. After observing that light-induced photoconductivity degradation anneals out at much lower T than the light-induced increase in deep defect density, Han and Fritzsche[1] suggested that two kinds of defects are created during illumination of a-Si:H. In this view, one kind of defect degrades the photoconductivity and the other increases defect sub-bandgap optical absorption. However, the light-induced degradation model of Stutzmann et al.[2] assumes that photoconductivity is inversely proportional to the dangling-bond defect density. We observe two kinds of defects that are distinguished by their annealing activation energies, but because their densities remain in strictlinear proportionduring their creation, the two kinds of defects cannot be completely independent.In our measurements of photoconductivity and defect absorption (constant photocurrent method) during 25°C light soaking and during a series of isochronal anneals between 25 &lt; T &lt; 190°C, we find that the absorption measured with E ≤1.1 eV, first increases during annealing, then exhibits the usual absorption decrease found for deeper defects. The maximum in this absorption at E ≤1.1eV occurs simultaneously with a transition from fast to slow recovery of photoconductivity. The absorption for E ≤1.1eV showstwodistinct annealing activation energies: the signal rises with about 0.87 eV and falls with about 1.15 eV. The 0.87 eV activation energy roughly equals the activation energy for the dominant, fast, recovery of photoconductivity. The 1.15 eV activation energy roughly equals the single activation energy for annealing of the light-induced dangling bond absorption.

Correlation between film and cell properties for DC plasma deposited amorphous silicon

ABSTRACTWe have carried out measurements to try to correlate amorphous silicon film properties with companion solar cell device performance. The dc plasma deposited intrinsic films were prepared with various hydrogen dilution levels, and increasing power levels to increase growth rate. The electronic properties were determined using admittance spectroscopy and drive-level capacitance profiling (DLCP) techniques as well as transient photocapacitance and photocurrent spectroscopy. Cell and film performance were explored in both as-grown and light-soaked states. We observed that, although cell performance decreased systematiclly with increasing growth rate, it depended on factors other than the deep defect density in the matched films. On the other hand, we did observe that increases in defect density caused by the light-induced degradation led to fairly predictable decreases in the cell fill factors.

Study of Defects in Hydrogenated Amorphous Silicon by Constant Photocurrent Method and Positron Annihilation

ABSTRACTThe influence of the power density, PD, on the density and structure of defects of undoped a-Si:H thin films, deposited by rf-PECVD, is studied by constant photocurrent method, CPM, and by slow positron beam spectroscopy, respectively. Deep defect density, NDD, remains approximately constant at 10-16 cm-3, typical of device quality material, for PD in the range 7 - 20 mW.cm-3 calculated from CPM. NDD increases roughly one order of magnitude for both low and for high power densities. Positron annihilation spectroscopy reveals the presence of two kinds of vacancy type defects in the films: large vacancy clusters or voids for PD ∼ 7 mW.cm-3 and small vacancy type defects as PD increases to ∼30 mW.cm-3. The relative abundance of the structural defects is shown to be controlled by the power density used during the deposition without affect the electronic deep defect density, significantly.

Materials optimization for thin film transistors fabricated at low temperature on plastic substrate

In this paper, we describe the optimization of the electronic properties and the mechanical stress in the amorphous hydrogenated silicon (a-Si:H) and amorphous silicon nitride (a-SiN x :H) layers. The goal is to obtain smaller defect density and smaller mechanical stress materials that provide low-leakage current and to prevent the peeling of the multilayer structure off the substrate. The developed fabrication process used a 90% H 2 + 10% SiH 4 source gas mixture for the deposition of a-Si:H and He-diluted 2.5% SiH 4 + 97.5% (NH 3 + N 2) gas mixture for the deposition of a-SiN x :H at a substrate temperature of 120°C. The properties of the deposited film samples were measured by infrared spectroscopy, constant photocurrent method (CPM), and electron spin resonance. Optimized a-Si:H films have 11 at.% of predominantly monohydride-bonded hydrogen, CPM defect density about 4 × 10 16 cm −3, and an Urbach energy of a 53 meV. In the a-SiN x :H films, He dilution results in the removal of hydrogen from SiH bonds, and in the increase of film density assisted by mechanical stress transition from tensile to compressive, while maintaining a low deep defect density.

Comparative studies of the influence of hydrogen incorporation on the electronic properties of a-Ge:H films prepared by two different techniques

We present the results of a detailed investigation of hydrogen incorporation and optical absorption in the 0.6-1.3 eV range for two series of hydrogenated amorphous germanium films (a-Ge:H) deposited by reactive sputtering (series A) and by plasma-enhanced chemical vapor deposition from germane (series B). Our results clearly show that the series A samples are characterized by a larger bonded hydrogen concentration (CH), a more rapid gap variation with increasing CH, a smaller refractive index, and a lower density than the series B samples. We also compare in detail the energy distribution of the localized states in the pseudo-gap and the deep-defect states density as deduced from a decomposition of the optical absorption spectra based on a theoretical model for the gap states density in amorphous tetracoordinated semiconductors.PACS No.: 71.90