5.3.
6.6Determination of defect concentration from absorption coef?cient
Using the subbandgap region of the absorption coef?cient(see Figure5.3a),the defect con-centration in a-Si:H is determined using a simple procedure.First,the absorption due to the defect states is determined as:
αdef=α?α0exp(E/E0)(5.17) whereα0and E0are obtained from the?t to the exponential absorption in region B.Assuming that the optical matrix element is constant in this region,αdef is related to the defect density concentration,N d,by[42]:
N d=K d
αdef·d E(5.18)
where K d is a correlation factor that depends on the measurement method used to deter-
mine the absorption coef?cient in region B and C.For two widely used approaches,pho-
tothermal de?ection spectroscopy and the photoconductivity methods,the correlation factor
has been determined to be K d=7.9×1015cm?2eV?1[42]and K d=1.6×1016cm?2eV?1 [47],respectively.Another approach often encountered in the literature is to correlate the
value of the absorption coef?cient at1.2eV to the density of states in the following way
[48]:
N d=2.4?5.0×1016αCPM(1.2eV)or N d=1.2?2.5×1016αPDS(1.2eV)(5.19)
Information about the energy dependence of the density of states in the mobility gap can be extracted from the absorption coef?cient using the deconvolution approach[43].In this approach,the energy distribution of the density of states is extracted by matching the simulated absorption coef?cient to the experimental one.
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5.3.
6.7Space charge methods
Several methods use the properties of a space charge region that is formed at a-Si:H interfaces. The?eld effect technique[49],deep level transient spectroscopy(DLTS)[50]and space charge limited current[51,52]are the main representatives of these techniques.
5.3.
6.8Deep level transient spectroscopy
Deep level transient spectroscopy(DLTS)is an extremely sensitive technique and different from ESR measurements in that it also detects energy levels from nonparamagnetic defects.It is routinely used to determine energy levels and defect concentrations in semiconductors,and it is perhaps the most common technique for measuring deep levels in crystalline semiconductors.It has been demonstrated[50,53]that in a-Si:H,DLTS is also a valuable method for evaluating the electronic density of states.Originally,a capacitance version of DLTS was used to characterize lightly doped a-Si:H?lms.Undoped a-Si:H?lms can be characterized using a charge version of DLTS(Q-DLTS)[54].
The Q-DLTS sample is usually a metal/oxide/semiconductor(MOS)structure consisting of a1μm thick a-Si:H layer deposited on a highly doped n+type crystalline silicon substrate, which acts as a back contact.For successful Q-DLTS experiments on undoped a-Si:H,a very thin insulating layer has to be created in the surface region of a-Si:H.This insulating layer strongly reduces the leakage current of the sample,which is then negligible with respect to charge transients,and enables shifting of the Fermi level in the a-Si:H?lm with an applied bias voltage.An Al(semitransparent)layer usually forms the top electrode.By applying bias voltage pulses to the MOS sample,the Fermi level is shifted towards the conduction band mobility edge and the states in the gap of a-Si:H are?lled with charge carriers.After each ?lling pulse,the transient current in the external circuit is measured as a function of temperature. The charge emitted from the occupied trap states is determined by integrating the measured current.The charge released at a speci?c temperature is proportional to the concentration of states at a speci?c energy in the mobility gap of the a-Si:H material.This technique is suitable for investigating the evolution of the gap states distribution due to light or particle induced degradation,providing new insights in the complex behaviour of a-Si:H under light or particle exposure[55].
Table5.1summarizes the criteria for device quality intrinsic amorphous silicon.
5.3.7Metastability
Inherent to a-Si:H are changes in its electronic properties under light exposure.This is known today as the Staebler–Wronski effect[24].Since the observation of the Staebler–Wronski effect, a large effort has been put into obtaining an understanding of the processes that cause the light induced structural and optoelectronic changes in a-Si:H[56–59].An essential feature of the light induced effects on a-Si:H?lms and solar cells is that most of the effects are‘metastable,’which means they are reversible and can be removed by annealing at temperatures above 150?C.
Light soaking is thought to lead to the creation of additional dangling bond defects,which is regarded as the principal cause of the Staebler–Wronski effect.The increase of the density of
ADV ANCED AMORPHOUS SILICON SOLAR CELL TECHNOLOGIES
191
Table 5.1Requirements for device quality a-Si:H and a-SiGe:H ?lms for solar cells Property
a-Si:H a-SiGe:H Dark conductivity [ ?1cm ?1]<5×10?10<5×10?9AM1.5conductivity [ ?1cm ?1]>1×10?5
>5×10?6
Urbach energy [meV]<47<55Activation energy [eV]≈0.8≈0.7Bandgap,Tauc [eV]<1.8～1.45Bandgap,cubic [eV]
<1.6 1.32Absorption coef?cient (600nm)[cm ?1]≥3.5×104≥1×105Absorption coef?cient (400nm)[cm ?1]≥5×105≥6×105Density of defect states
(CPM,DBP)methods [cm ?3]≤1×1016≤1×1017ESR method [cm ?3]
≤8×1015Mobility-lifetime product (600nm)[cm 2/V]≥1×10?7
H content [at.%]
9–1110–15Microstructure parameter <0.1
<0.2Ge content [at.%]
40
defects in a-Si:H due to light soaking is demonstrated in Figure 5.4,which shows the change in the absorption coef?cient determined by the DBP technique.The sample was illuminated using a He–Ne red laser (λ=633nm)with an intensity of ～40mW cm ?2.Shown by Figure 5.4,the subbandgap absorption increases with illumination time in the photon energy range between 0.8and 1.4eV ,which re?ects an increase in the defect density.An important aspect of light induced defect generation in a-Si:H is that it saturates.The saturation value of the defect density near room temperature has been found to be ～2×1017cm ?3and is almost independent of illumination intensity and sample temperature up to 70?C [60].
1x10
-2
1x10-1
1x10
1x101
1x102
A b s o r p t i o n c o e f f i c i e n t (c m -1)
Energy (eV)
Figure 5.4The change in subbandgap absorption coef?cient of a-Si:H due to light soaking.
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0.00.5
1.0
ΔQ (n C )Temperature (K)
Figure 5.5The Q-DLTS signal of a-Si:H after light soaking for different exposure times.
Figure 5.5presents the evolution of the Q-DLTS signal measured after light soaking,which gives information about the energy distribution of the defect states.Light soaking was effected with a He–Ne red laser (λ=633nm)with an intensity of ～250mW cm ?2.The time evolution of the Q-DLTS spectrum shows complex behaviour.At a low temperature the signal disappears,while at 370K a peak grows signi?cantly with increasing illumination time.The Q-DLTS response around 450K does not seem to be in?uenced by moderate light soaking.These three components in the Q-DLTS signal are related to the positively charged,D h ,neutral,D z ,and negatively charged,D e ,defect state distributions,respectively,as predicted by the defect pool model [34].The arrows in Figure 5.5indicate the peak positions of D h ,D z ,and D e gap state distributions in the Q-DLTS spectra.The Q-DLTS measurements indicate that a substantial amount of positively charged defects is removed,the concentration of negatively charged defects remains unchanged and additional neutral defects representing dangling bonds [61]are c8ab0301a6c30c2259019ebcing the Q-DLTS technique,new insights in the origin and behaviour of different types of defects in a-Si:H have been obtained recently [55].In this work,it is proposed that in addition to dangling bonds,other types of defects exist in a-Si:H that play an important role in the Staebler–Wronski effect.Positively charged states above midgap are related to a complex formed by a Si dangling bond and a hydrogen molecule.The origins of negatively charged states below midgap are attributed to the ?oating bonds.Nevertheless,the Staebler–Wronski effect remains a complex phenomenon in a-Si:H and many unresolved issues still remain.The exact role of hydrogen,weak Si–Si bonds and Si–H bonds and complexes in the creation of the metastable defects is still under c8ab0301a6c30c2259019ebcputer simulations of an a-Si:H network support these investigations [62,63].Still,there is no commonly accepted model for the metastable defect creation in a-Si:H that is able to explain all experimental observations [58].
