This book presents the most recent important ideas and developments in the field of Hydrogenated Amorphous Silicon and related materials. Each contribution is authored by an outstanding expert in that particular area.
https://doi.org/10.1142/9789814434157_fmatter01
The following sections are included:
https://doi.org/10.1142/9789814434157_0001
Amorphous hydrogenated semiconductors are structurally heterogeneous. Nearly two decades of research, technological exploitation, and chemical reactor engineering have not produced materials which reflect the type of “random” structures that are described in solid state physics textbooks. Indeed, on the 0.5 to 2 nanometer length scale, these materials contain structural features unprecedented in semiconductor physics and solid state chemistry. The purpose of this paper is to demonstrate that nuclear magnetic resonance (NMR) techniques uniquely characterize these inhomogeneities. We suggest that the identification of common microstructural features in amorphous hydrogenated semiconductors is a first step toward understanding and predicting electro-optical properties.
https://doi.org/10.1142/9789814434157_0002
Information on the local environment of dopants in compensated and singly-doped amorphous Si has been obtained using NMR spin echo double resonance, a technique that is very sensitive to interatomic distance. It is found that there is a non-random local configuration around the dopants in these materials. In the compensated samples, almost half of the boron atoms have a phosphorus in the first neighbor shell. This is five times higher than the value for a random distribution and indicates that there is significant clustering of the dopants in compensated material. This clustering may account for the observation from the B NMR lineshape that a large fraction of B atoms is four-fold coordinated. In addition, the hydrogen is not directly bonded to these dopants. This implies that the H has a higher chemical affinity for Si than for B and P or that the clustering modifies the H bondin in compensated amorphous Si. For the singly-doped materials, the H-dopant structure is similar to that of the compensated samples for the P-doped samples but different for the B-doped material. For these, almost half of the B atoms have a hydrogen atom in the first neighbor shell. This is to be contrasted with the P-doped samples where the H atoms are in second or further neighbor shells. This local structure of P and B in hydrogenated amorphous Si is similar to that proposed for the dopant-H structures in hydrogen-passivated crystalline Si.
https://doi.org/10.1142/9789814434157_0003
The optoelectronic properties of a-SiGe:H alloys are generally conceded to be inferior to those of unalloyed a-Si:H. A brief review of properties justifying this conclusion is given. It is contended that the principal cause of this deterioration lies in the nature of the two-phase heterostructure. Three changes of emphasis are suggested for consideration (1) that the optimal conditions for preparation of good a-Ge:H be explored in more depth, instead of concentration on minor departures from the preparation conditions for optimized a-Si:H, (2) that the possibility of compositional fluctuations be considered, particularly those that might result in the installation of small volume. of small energy-gap material in a larger energy-gap matrix, and (3) that, in general, the two-phase structure of all films, including a-Si:H, be kept in mind when detailed band structures are deduced from the conventional property measurements.
https://doi.org/10.1142/9789814434157_0004
A method of field-assisted Ag+-Na+ ion exchange for determining the density of micropores in a-Si:H films deposited on glass is described. It is shown that the presence of micropores is an intrinsic feature in this material and that their density depends on the deposition conditions. It is suggested that the micropore density correlates with the morphological properties of the films: Films that appear more uniform in TEM micrographs have lower micropore densities.
https://doi.org/10.1142/9789814434157_0005
Among the structural properties of amorphous semiconductors, particular attention has been paid recently to medium range order (MRO) leading to a fast extension of this field. This paper will try to review the recent evolution of the concept of local order in relation with the development of Small Angle Scattering of thermal neutrons. A brief survey of the theory of Small Angle Scattering, first developed for X-rays, will be given, in order to explain the measurable parameters used to define Medium Range Order. Various examples of MRO studies on amorphous systems, will be given. Particular emphasis will be placed on MRO determination in a-Si:H and its relationship with electronic properties.
https://doi.org/10.1142/9789814434157_0006
This paper discusses the effect of hydrogen in the plasma deposition of amorphous and crystalline silicon. The way hydrogen controls the defect reduction, film formation process, atomic structure, hydrogen incorporation, electronic properties, and grain growth is examined systematically by deliberately adding hydrogen to the silane plasma to enhance its ‘etching’ effect. By promoting ‘etching’ hydrogen plays an important role in producing low-temperature crystalline silicon. It is also the key for making amorphous silicon a successful low-temperature technology.
https://doi.org/10.1142/9789814434157_0007
The use of ion implantation to form amorphous Si layers will be reviewed. The unique features of implantation in terms of cleanliness and controllability have allowed us to explore some of the kinetic and thermodynamic properties of amorphous Si. Solid phase epitaxy has been observed over the temperature range 500-1200°C with a single activation energy of 2.7 eV. Recent measurements of ion-beam-induced crystallization and impurity segregation will be discussed. Diffusion of impurities in amorphous Si shows some similarities with diffusion in crystalline Si. The enthalpy of crystallization has been measured by calorimetry techniques to be 11.9KJ/mole. There is little evidence of relaxation phenomena in either the diffusion or heat release measurements. We have established that amorphous Si undergoes a first order melting transition at a temperature some 250°C beneath the crystal melting temperature.
https://doi.org/10.1142/9789814434157_0008
Recent data which suggests that a major part of the defect density in doped hydrogenated amorphous silicon is in thermal equilibrium at elevated temperatures is reviewed. The time to reach equilibrium is thermally activated, so that cooling establishes a slowly relaxing nonequilibrium state resembling a glass. At the equilibrium temperature which separates the slowly relaxing “frozen” state from the high temperature equilibrium state, there is no observed change in the transport mechanism; thus the changes in the electronic properties arise from changes in the band tail electron density alone. Numerical modeling of the transport for both the equilibrium and frozen-in states are in good agreement with the data.
https://doi.org/10.1142/9789814434157_0009
Evidence that the bonded hydrogen network in a-Si:H can be considered a separate material which has the properties of a glass is reviewed. The motion of hydrogen underlies the metastable changes in the defect structure, as reflected in measurements of the electronic equilibration. The equilibration temperature is shown to satisfy various definitions of a glass transition temperature. The influence of the microstructure of the bonded hydrogen on the equilibration effects, and microscopic mechanisms for film growth and for thermal equilibrium processes are examined.
https://doi.org/10.1142/9789814434157_0010
In this chapter, the kinetics of carrier-induced metastable defect formation is described. Results from electrical measurements on metal-insulator-hydrogenated amorphous silicon (MIS) structures are used to measure the creation and annealing of defects in the hydrogenated amorphous silicon (a-Si:H) as a function of time, temperature, and carrier density. A rate equation which relies on the dispersive diffusion of hydrogen as the rate limiting step explains both the generation and annealing of defects. The equation yields stretched exponential type decays, and the characteristic decay time of the metastable changes is found to be inversely related to the hydrogen diffusion coefficient. This relation is experimentally verified for three different types of metastable changes: carrier-induced changes, light-induced, and changes due to rapid thermal quenching in doped material. It is suggested that a possible origin of the rate equation is a model in which the metastable changes occur by the dispersive diffusion of H atoms to defect sites or occupied band tail states.
https://doi.org/10.1142/9789814434157_0011
The density of states (DOS) in the mobility gap is briefly reviewed. Constant photocurrent method (CPM), experimental details and deconvolution of the optical absorption coefficient are presented and discussed in details. On the basis of CPM results on doped and undoped a-Si:H, supporting experiments and from the results of other authors (photo-deflection spectroscopy, photoyield spectroscopy, DLTS, photocapacitance, photoluminiscence) two models of DOS are emerging : the original street's model but with greatly reduced effective correlation energy Ueff(≤ 0.1 eV) or the model based on Coulombic interaction of dangling bonds with dopant atoms. Supporting arguments and internal consistency of both of them are discussed.
https://doi.org/10.1142/9789814434157_0012
Photothermal Deflection Spectroscopy (PDS) is used to study the deep defect related and shallow state density in hydrogenated amorphous silicon. Films with thicknesses ranging from 0.01-100 μm were employed in order to separate surface/interface and bulk defect densities. It is shown that surface/interface states may strongly influence the PDS spectra and that special care has to be taken as far as the interpretation is concerned. A defect density model accounting for surface/interface and bulk effects is proposed and discussed along with experimental data. Based on this model we suggest the presence of considerable film inhomogeneities along the film growth axis.
https://doi.org/10.1142/9789814434157_0013
Recent progress in our understanding of the near-surface electronic properties of a-Si:H is reviewed with emphasis on the results of total yield photoelectron spectroscopy in combination with the Kelvin probe. The large dynamic range and high energy resolution of the total yield method allow the effects of bulk doping or oxidation on the distribution of occupied band gap states near clean a-Si:H surfaces to be monitored in detail.
https://doi.org/10.1142/9789814434157_0014
The concept of the Defect Pool—that an amorphous network has associated with it a broad spectrum of local environments at which defects (atomic coordination other than the desired number) may occur—and the fact that the energy cost of forming a defect at any particular site can depend on the charge state, together imply that the characteristic properties of defects in an “equilibrated” amorphous semiconductor will depend on the position of the Fermi-level during equilibration. After first reviewing the evidence for the proposal that undoped hydrogenated amorphous silicon is a system to which such chemical equilibrium concepts can be applied, this chapter uses data on both doped and undoped films to pin down the nature of this defect pool. An experiment is proposed which should distinguish whether the apparent shift in defect levels with doping is due to ‘initimate pair’ or ‘defect pool’ effects. It is pointed out that the same Fermi-level-dependent site selection rules which apply in the case of doping operate in the distinctions between intrinsic (‘stable’) and metastable defects. The implications for devices such as solar cells and thin-film transistors are discussed, and an approach for integrating surface states into this scheme is outlined.
https://doi.org/10.1142/9789814434157_0015
Low levels of optical absorption in thin films can be measured using photoluminescence (PL) generated in the films as a light source. This technique, which is useful for measuring the temperature dependence of the optical absorption within the band gap of a-Si:H and related alloys, employs PL traveling in waveguide modes along the film. Experimental details and selected results are presented.
https://doi.org/10.1142/9789814434157_0016
In this paper we review the application of the transient junction photocapacitance method to the study of gap state transitions in a-Si:H. In n-type samples these techniques disclose several previously unobserved transitions of the dominant deep (D) defect. In particular, these studies have indicated a minimal role for lattice relaxation in these transitions, and allow us to estimate the correlation energy for this defect. In undoped films we have compared the transient photocapacitance and junction photocurrent spectra to unambiguously separate electron from hole processes and thus help distinguish between possible deep defect models. These measurements also provide a new method to determine the μτ product for holes. Finally, we assess the advantages of the transient photocapacitance method compared to other sub-band-gap optical spectroscopies employed in the study of a-Si:H.
https://doi.org/10.1142/9789814434157_0017
The steady state and transient photomodulation spectra have been studied in phosphorous doped and undoped a-Si:H. The spectra in undoped a-Si:H are analyzed in terms of photocarriers with unthermalized distribution trapped in band-tail states, and carriers trapped in dangling bond defects. We have also found photo-induced changes in the Si-H stretching vibrations which are associated with the trapped photocarriers in band tail states. The spectra in doped a-Si:H are due to photocarriers trapped in impurity related defects. The apparent differences between the dangling bond energy levels in the gap of doped and undoped a-Si:H is pointed out and discussed.
https://doi.org/10.1142/9789814434157_0018
A thermally stimulated current (TSC) method to determine the density of gap states is described. Some basic theory used by other authors is given. A possible extension of the method is presented, which results in the determination of the density of states (DOS) as well as the μΤ product and values of μ and Τ separately. Results of measurements on a-Si:H are shown as an illustration of this method.
https://doi.org/10.1142/9789814434157_0019
Traditionally, stationary dangling bonds have been the only intrinsic defects in a-Si. Many observed properties cannot be explained satisfactorily on the basis of such premises. It is shown, that there are in principle two primitive intrinsic defects, namely dangling bonds (threefold-coordinated atoms) and floating bonds (fivefold-coordinated atoms) and that the latter are mobile. The mobility of the intrinsic defect is shown to be the key property which leads to a systematic and elegant description of a broad range of observed phenomena.
https://doi.org/10.1142/9789814434157_0020
Electron spin resonance results are used to review the present understanding of the microscopic structure of localized defects in the elementary amorphous semiconductors a-Si:H, a-Ge:H, and a-C:H, as well as in the silicon alloys a-SiGe, a-SiC, a-SiN, and a-SiO. It is described how structural parameters can be obtained from spin resonance spectra, and as an example the dangling bond defect in a-Si:H is analyzed in detail. In addition, other intrinsic or impurity-related defects in the bulk of these materials and at their interfaces or surfaces are discussed.
https://doi.org/10.1142/9789814434157_0021
We discuss recombination processes and nature of the tail and gap states in a-Si:H and a-Si:H/a-SiNx:H multilayers on the basis of our ODMR, luminescence, photoinduced absorption and ENDOR measurements. We present other results relevant to this subject and attempt to interpret them in terms of our model.
https://doi.org/10.1142/9789814434157_0022
Nature of defects and the incorporation scheme of hydrogen in a-Si:H, a-Si1−xCx:H and a-Si1−xNx:H are investigated by combined measurements of dark ESR. light-induced ESR, low photon energy absorption by constant photocurrent method (CPM) and NMR for films with various thicknesses. Both the surface density of dangling bonds in the disordered surface layer and the density of dangling bonds in the bulk region are obtained discriminating the neutral and charged dangling bonds. In a-Si:H, a defective layer with about 20 nm exists on the free surface side of the film and a large fraction of dangling bonds in the bulk region is in charged states. The incorporation of C decreases and the incorporation of N increases the fraction of charged defects. The charged defects in a-Si1−xNx:H are concluded to be pairs of N+4 and Si−3. It is possible that the charged defects in a-Si:H are also pairs of N+4 and Si−3 originating from a small amount of N contamination. The content of dispersed H atoms evaluated from NMR measurements decreases with an increase in x for a-Si1−xNx:H and a-Si1−xCx:H for x < 0.2, and correlates well with the increase in the bulk density of neutral dangling bonds. The density of charged dangling bonds does not have a correlation with it. As x increases above 0.2, the dispersed H atoms become more clustered leaving H-poor region with a large amount of dangling bonds.
https://doi.org/10.1142/9789814434157_0023
We describe a systematic study on thermally and light-induced changes in the gap states of P-doped a-Si:H using several variations of isothermal capacitance transient spectroscopy (ICTS). Two different features are found out in photo-induced changes; (1) gap states (NCT) were created in the range of 0.25-0.35 eV below the conduction band the EC, which are attributed to 31P-related-hyperfine-ESR centers, and (2) the conversion from deeper defect states located 1.0-1.2 eV below EC (*D−) to shallower defect located 0.4-0.6 eV below EC(D−). On the other hand, no suggestive changes in the density-of-states distribution at least in the energy range 0.25-1.5 eV below EC were observed after thermal quenching, although shallow states above Fermi level EF increased. It suggests that thermally and light-induced effects are independent of each other from the viewpoints of defect creation.
https://doi.org/10.1142/9789814434157_0024
Electron irradiation experiments have contributed to the extensive study of metastable defects in a-Si:H during the past few years. This chapter gives a survey of the experimental techniques (EBIC-measurements, keV- and MeV-electron irradiation) and the results of these experiments. Different creation processes enable a more effective and much faster production of the metastable defects compared to light soaking. Under keV-electron irradiation a saturation of the number of metastable defects in the order of 1018 cm–3 was observed. A model explaining the high efficiency and the kinetics of the defect creation by kev-electron irradiation is proposed.
https://doi.org/10.1142/9789814434157_fmatter02
The following sections are included:
https://doi.org/10.1142/9789814434157_0025
This article reviews recent work on the transport of excess carriers in hydrogenated amorphous silicon over a temperature range from 10K to 450K. Particular emphasis is placed on carrier-tail state interactions which determine the nature of the transport mechanism. Most of the results are obtained from fast, transient drift mobility experiments and this approach will briefly be discussed. Two main transport regimes will then be investigated. The first is the multi-trapping range, extending down to about 150K for electrons, in which the excess carriers propagate in the extended states, but interact strongly with tail states during transit. The problem of deducing extended state mobilities will be discussed. In the second, the low temperature regime, detailed new results on the hopping transport above a thermalisation limit near the bottom of the tail states will be presented. The interpretation of the results is supported by model calculations which also lead to information on the tail state distribution. The analysis is extended to the σ(ϵ) distribution under steady state conduction and the interpretation of recent low temperature photoconductivity results is discussed.
https://doi.org/10.1142/9789814434157_0026
The photoluminescence (PL) and photoconductivity (PC) of a-Si:H is studied concomitantly as a function of the electric field in the temperature range 20–300K. High electric fields are found to quench the photoluminescence and enhance the photoconduction. This anticorrelation of PL and PC is typical of geminate pair recombination. The analysis in terms of such a model leads to a determination of the quantum efficiency for generation of mobile carriers. Below 50K this quantity amounts to about 3·10−3 and increases at higher temperatures approaching 1 near room temperature. The paper discusses important consequences of the presented results for the interpretation of PL and PC–data.
https://doi.org/10.1142/9789814434157_0027
The spectral response of photoluminescence (PL), its temperature dependence, and the distribution of PL lifetimes in single layers of a-Si:H which are intrinsic as well as in layers doped with phosphorus or boron will be compared with the same PL properties in pnpn modulation-doped multilayers and in a–Si:H/a–SiNx:H multilayers. Our results will be compared with present recombination models. A comparison of an a–SiNx:H alloy film with silicon-siliconnitride multilayers suggests that the blue-shift of the PL peak energy with decreasing a–Si:H sublayer thickness is due to quantum well effects and not due to nitrogen contamination of the silicon layer.
https://doi.org/10.1142/9789814434157_0028
This paper describes the experimental techniques, analytical methods, and computer modelling procedures which are employed in the study of conduction mechanisms, localised state distributions, electric field distributions, and other characteristics of amorphous semiconductor films. It is shown that, with sufficient care in both experimental technique and interpretation, detailed information can be extracted. Such data are considered to be an essential requirement, if the properties of the emerging generation of thin-film semiconductors are to be fully optimised for various commercial applications.
https://doi.org/10.1142/9789814434157_0029
Electron drift-mobility measurements in hydrogenated amorphous silicon (a-Si:H) specimens far from thermal equilibrium are reviewed. An introduction to experimental techniques is given, and results obtained with negative space-charge, under optical bias, and with single and double-injection are described. Quasi-Fermi level motion, charged defect limitation of transport, and defect metastability models are summarized.
https://doi.org/10.1142/9789814434157_0030
We describe a new, purely optical technique to perform time-of-flight experiments with subpicosecond time resolution. The technique is demonstrated on very thin silicon layers which are separated by barriers forming an a–Si:H/a–SiNx:H multilayer. The current is detected by measuring the difference in the absorption coefficient between the carriers in the silicon layers and the carriers that have arrived at the interfaces. The transport of electrons is shown to be dispersive over the whole picosecond time range.
https://doi.org/10.1142/9789814434157_0031
The Steady State Photocarrier Grating Technique is briefly reviewed, and compared to other techniques for measuring the diffusion length. New results of the light intensity and temperature dependence of the ambipolar diffusion length in amorphous hydrogenated silicon are presented.
https://doi.org/10.1142/9789814434157_0032
In this article we review optical and electronic transport data measured in amorphous silicon-germanium alloys with the goal of identifying the density of states as a function of alloy composition. The results show that while alloying a-Si:H with germanium has little effect on the valence band tail, the conduction band tail density of states is increased dramatically. Defect distributions both above and below midgap are detected and identified with the dangling bond D+/° and D°/− states. The density of deep defects below midgap increases exponentially with germanium content. Above midgap, a large concentration of defects lying between 0.3 and 0.5 eV below the conduction band edge has a strong effect on transient electron transport.
https://doi.org/10.1142/9789814434157_0033
The photoluminescence properties of amorphous hydrogenated silicon/germanium alloys have been investigated covering the whole composition range. Whereas the peak energy of the main photoluminescence band parallels the linear decrease of the optical band gap, the energy of the defect photoluminescence band remains constant up to 32% germanium in the film and decreases upon doping. Most of the results, e.g. the dependence of the photoluminescence energy on the composition, and on temperature can be interpreted within the models developed for a-Si:H. The behavior of the defect band suggests a constant energy difference between the conduction band edge and the negatively charged silicon-like dangling bond up to 32% germanium in the film. Moreover, the data suggest that the nonradiative recombination process is not controlled by the density of germanium dangling bonds.
https://doi.org/10.1142/9789814434157_0034
Optical transitions between two-dimensional (2D) subband states in hydrogenated amorphous silicon (a-Si:H) quantum well (QW) structures have been extensively investigated by using differential absorption technique. The temperature and wavelength-derivatives of absorption spectrum clearly show a staircase form for a-Si:H QW structures of the well width below 40-50Å. These results suggest that 3D-parabolic band splits into a series of subbands. Analysis of the observed energies of subband transitions in terms of a square QW model leads to a conclusion that the electron and hole effective masses are 0.3 m0 and m0, where m0 denotes free electron mass.
https://doi.org/10.1142/9789814434157_0035
The following sections are included:
https://doi.org/10.1142/9789814434157_0036
Recent in situ and spectroscopic ellipsometry investigations of hydrogenated amorphous silicon (a-Si:H) nucleation behavior, microstructural evolution, and interface formation are reviewed. An outline of the commonly applied experimental techniques and data analysis is also presented. In situ ellipsometry reveals a nuclei formation and convergence sequence in the first 50Å of a-Si:H growth by rf plasma deposition from silane on c-Si and metal substrates. This sequence provides evidence of favorable growth chemistry that results in material with a low density of structural defects. The influence of deposition parameters and processes on the nucleation and subsequent microstructural evolution of a-Si:H is covered in detail. Among the other topics discussed include: nucleation of microcrystalline Si, evolution of surface roughness on a-Si:H, inert and reactive gas plasma modification of a-Si:H, and formation of a-Si:H heterostructures with SiO2, wide band gap alloys, and Bdoped a-Si:H.
https://doi.org/10.1142/9789814434157_0037
We review recent theoretical and experimental progress in Raman scattering which make it a powerful probe of the structure of ultrathin amorphous layers and interfaces. Special attention is given to quantitative experimental analysis of multilayer samples. We also review recent results on hydrogenated amorphous silicon layered with other amorphous materials including silicon nitride, germanium, and silicon oxide.
https://doi.org/10.1142/9789814434157_0038
In this contribution we review some experimental results on acoustic phonons in superlattices based on amorphous semiconductors. These phonons have coherence lengths far exceeding the superlattice period and as in the crystalline superlattices their dispersion relation is folded within the mini-Brillouin-Zone defined by the periodicity. As a consequence, new lines with frequencies that depend on the acoustic properties and on the thicknesses of the layers appear in the low-frequency Raman spectrum. In addition, frequency gaps which correspond to frequency ranges where no propagating phonon mode exists appear as minima in the phonon transmission spectrum of the superlattice. These results indicate that amorphous multilayer structures are potential candidates for application in filters for high-frequency phonons.
https://doi.org/10.1142/9789814434157_0039
The electron and hole transport in a-Si:H,F/a-Si,Ge:H,F multilayers is studied in detail. The experimental techniques used to study these structures include dark σd and photo σph conductivities, the constant photocurrent method (CPM), electron time-of-flight (TOF) and the voltage dependence of photogenerated hole collection in Schottky barriers. The results show clearly the asymmetry between carrier transport parallel and perpendicular to the layers. Both parallel and perpendicular transport differ from that in non-stratified bulk samples. Small, quantitative differences are observed for parallel transport, but large and qualitative differences for perpendicular transport.
https://doi.org/10.1142/9789814434157_bmatter02
The following sections are included: