A silicon atom representing this?oating bond defect is indicated in Figure5.1b by a dotted circle.
In pure a-Si(amorphous silicon that contains no other atoms than silicon),there is a large concentration of about1021defects per cm3in the amorphous atomic structure.Material with such a large defect density cannot be used for a functional device.When amorphous silicon is deposited in such a way that hydrogen can be incorporated in the atomic network (as by glow discharge deposition from silane),then hydrogen atoms bond with most of the silicon dangling bonds.Strong silicon–hydrogen bonds are formed,which are illustrated in Figure5.1b.Hydrogen passivation of dangling bond defects reduces the defect density from about1021cm?3in pure a-Si to1015–1016cm?3in a-Si:H,i.e.less than one dangling bond per one million silicon atoms.It is this material,an alloy of silicon and hydrogen,in which substitutional doping has been demonstrated and which is suitable for electronic applications.
5.3.1.1Electron spin resonance
An experimental technique that can provide information on the microscopic structure of defects in semiconductors,including the amorphous ones,is electron spin resonance(ESR)[27]. Electron spin resonance measurements on a-Si:H have identi?ed a single type of defect that is associated with a neutral dangling bond[26,28].The ESR is regarded as an experimental standard for determining defects in a-Si:H,and the ESR results are considered unambiguous. However,the sensitivity of this method is limited for thin?lms with a low spin density and the method only gives information about paramagnetic defects,i.e.defects with an unpaired electron.For this reason,ESR can underestimate the defect density,because the charged dangling bonds do not possess an unpaired spin signal.Therefore,the results from ESR strongly depend on the Fermi level position,which determines the electron occupation of defects. 5.3.1.2Hydrogen characterization in hydrogenated amorphous silicon
Since hydrogen plays an important role in defect passivation,the incorporation and stability of hydrogen in a-Si:H has been the topic of intensive research.Infrared absorption spectroscopy
ADV ANCED AMORPHOUS SILICON SOLAR CELL TECHNOLOGIES179 is widely used to provide information about Si–H x bonding con?gurations in a-Si:H[26]. Three characteristic infrared absorption bands are observed in a-Si:H:a peak at640cm?1,a doublet at840–890cm?1,and absorption peaks in the range of2000–2200cm?1.The peak at640cm?1re?ects the rocking mode of hydrogen covalently bonded in all possible bonding con?gurations,i.e.silicon mono(x=1),di(x=2),and trihydride(x=3)and polymeric (Si–H2)n bonding con?gurations,and therefore this peak is used to determine the hydrogen content in a-Si:H[18].The doublet at840–890cm?1is assigned to the dihydride wagging mode.A peak around2000cm?1is assigned to the stretching mode of the isolated Si–H bonds(also referred to as the low stretching mode(LSM)in the literature)and a peak in the range of2060–2160cm?1includes contributions from the stretching mode(referred to as the high stretching mode(HSM))of Si–H bonds at internal surfaces,e.g.voids,dihydride and trihydride bonds.A‘microstructure parameter,’denoted as R?,is determined from the LSM and HSM absorption peaks.The R?is widely used to characterize the microstructure in the a-Si:H network as it roughly indicates two different‘phases’,namely a dense network and a fraction of the network containing voids.The microstructure parameter is de?ned as:
R?=
I HSM
I LSM+I HSM
(5.1)
where I HSM and I LSM are the integrated absorption strength of the LSM and HSM,respec-tively.In general,device quality a-Si:H contains less than10atomic%of hydrogen and is characterized by R?<0.1.
Hydrogen diffusion and evolution measurements help to characterize hydrogen motion, trapping and evolution in a-Si:H[29].Nuclear magnetic resonance(NMR)gives information about the local atomic environment in which the hydrogen atoms reside[30].Recently,it has been reported,based on NMR experiments,that molecular hydrogen forms up to40%of the total hydrogen content in a-Si:H[31].
5.3.2Density of states
An essential component for determining the distributions and concentrations of charge carriers in a semiconductor material is information about the energy distribution of states,often called the density of states.For an ideal intrinsic silicon crystal,the valence band and the conduction band are separated by a well de?ned bandgap,E g,and there are no allowed energy states in the bandgap.Due to the long range disorder in the atomic structure of a-Si:H,the energy states of the valence band and the conduction band spread into the bandgap and form regions of states that are called band tails.In addition,the defects introduce allowed energy states that are located in the central region between the valence band and conduction band states.This means that there is a continuous distribution of density of states in a-Si:H and that there is no well de?ned bandgap between the valence band and the conduction band.
The energy states in which the charge carriers can be considered as free carriers are described by wave functions that extend over the whole atomic structure.These states are nonlocalized and are called extended states.The disorder in a-Si:H causes the wave functions of the tail and defect states to become localized within the atomic network.These states are called localized states.Consequently,mobility that characterizes transport of carriers through the localized states is strongly reduced.This feature of a sharp drop in the mobility of carriers
180THIN FILM SOLAR CELLS
in the localized states in comparison to the extended states is used to de?ne the bandgap in a-Si:H.This bandgap is denoted by the term mobility gap,E mob,because the presence of a considerable density of states in the mobility gap is in con?ict with the classical concept of a bandgap without any allowed energy states.The energy levels that separate the extended states from the localized states in a-Si:H are called the valence band,E V,and the conduction band, E C,mobility edges.The mobility gap of a-Si:H is larger than the bandgap of single crystal silicon and has a typical value between1.7eV and1.8eV.
5.3.3Models for the density of states and recombination–generation
statistics
In general,the energy distribution of states in a-Si:H is characterized by three different regions: (i)extended states above the mobility edge of the conduction band,(ii)extended states below the mobility edge of the valence bandand(iii)localized states between the mobility edges.The continuous distribution of the localized states is a superposition of the conduction and valence band tail states and the defect states.
In Figure5.2,we present a standard model of the density of states distribution.In this model,the valence and conduction band states are represented by a parabolic dependence on energy that merges with exponentially decaying valence and conduction band tail states. The defect states are represented by two equal Gaussian distributions,which are shifted from each other by the correlation energy,U.The correlation energy is assumed to be constant and positive.As mentioned earlier,dangling bonds are considered the dominant defect in a-Si:H.A dangling bond can be in three charge states:positive(D+),neutral(D0)and negative(D?).An imperfection with three possible charges,such as a dangling bond,is represented in the band diagram by two energy levels E+/0and E0/?,which,depending on the position of the Fermi level,characterize the charge occupation of the imperfection.These two energy levels are
Figure5.2The standard model for density of states in a-Si:H.
ADV ANCED AMORPHOUS SILICON SOLAR CELL TECHNOLOGIES181 called the transition energy levels.The two Gaussian distributions,D+/0and D0/?,represent the energy distributions of states corresponding to+/0and0/?charge transitions of dangling bonds,respectively.Since the dangling bonds are represented by both the donor like(+/0) and acceptor like(0/?)states,dangling bonds are called amphoteric defects.
