More insights from CPM and PDS: Charged and neutral defects in a-Si:H

Solar Energy Materials and Solar Cells

ELSEVIER

Solar Energy Materials and Solar Cells 49 (1997) 7-12

More insights from CPM and PDS: Charged and neutral defects in a-Si:H Frank Siebke*, Helmut Stiebig, Reinhard Carius Forschungszentrum Jiilich, ISI-PV, P.O.Box 1913, D-52425 Jiilich, Germany

Abstract CPM and PDS spectra of annealed and degraded a-Si:H are analyzed. Numerical simulations of CPM and PDS data using occupation statistics yield information on the energy distribution and the charge state of the defects. The simulations reveal the coexistence of charged and neutral defects resembling the predictions of the defect-pool model. Charged states dominate the defect densities of annealed and degraded a-Si:H. In the case of spatial homogeneous defect densities, different sensitivities of CPM and PDS on charged and neutral defects cause different defect absorptions detected by both methods. Spatially inhomogeneous defect densities caused, e.g. by voids or columnar growth are detected by combining CPM and PDS since PDS detects the total defect density while CPM favors regions with low defect densities. Keywords. a-Si:H; Numerical simulation; CPM; PDS

1. Introduction The knowledge of the defect distribution in a-Si:H is important for design optimization of a-Si:H solar cells. Despite the tremendous efforts, the energy distribution and the charge state of defects in a-Si : H, as well as their changes upon light soaking, are still under discussion. The constant photocurrent method (CPM) and the photothermal deflection spectroscopy (PDS) are commonly used to measure the sub-band-gap absorption. Both methods yield identical spectra of the Urbach tail, but the defect absorption measured by PDS differs significantly from C P M data. Such differences

* Corresponding author. Fax: -49 2461 61 3735; e-mail: [email protected]. Present address: New Materials Research Center, SANYOElectric Co., Ltd., 1-18-13Hashiridani, Hirakata, Osaka 573, Japan. 0927-0248/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0927-0248(97)00 1 69-4

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are often ascribed to the influence of interface states [-1] or compensated by different calibration factors [-2], but the former is not proved and the latter still needs explanation. Here we present a detailed analysis of C P M and PDS data of doped and undoped a-Si:H before and after light soaking. Information on the distribution and the charge state of the defects are obtained by numerical simulations of C P M and PDS spectra.

2. Experimental The preparation conditions of the samples shown in this paper are the following: doped films were deposited on quartz by rf-glow discharge at 200°C and 0.3 mbar using silane. The rf power was 3 W~ For n- and p-type doping the concentrations in the gas phase were 4 ppm phosphine and 20 ppm diborane, respectively. In addition, an undoped hot-wire sample prepared at a substrate temperature of 370°C and a filament temperature of 1800°C at 0.04 mbar [-3] was investigated. Before measuring all films were annealed in vacuum at 180°C. A krypton laser was used for rapid light soaking (multiline red, 3 W/cm 2, 1 h). CPM and PDS spectra were taken by conventional set-ups at 300 K [4, 5].

3. Model The simulation of CPM and PDS spectra is based on density of localized states consisting of exponential band tails and a defect distribution on the basis of the defect-pool model [6]. A spatial homogeneous defect density without interface states is assumed. Our model includes the full set of optical transitions between localized and extended states, capture and emission processes of carriers into and out of localized states and the position of the Fermi level. For capture cross sections for holes into charged and neutral defect states we use a ratio of 16 in our simulation. The same set of parameters was also successfully used to calculate the temperature dependent photo conductivity of n-type a-Si:H as well as device characteristics [-7]. The consistent description of all three experiments allows a variation of these parameters only within a factor of 2. Detailed information on the occupation statistics are given elsewhere 1-8]. To simulate C P M spectra, we calculate the photon flux ~bcpM,required to keep the photocurrent constant, and obtain the absorption coefficient ~cPM ~c l/qScpM. Based on the defect distributions derived from C P M simulations, PDS spectra are calculated by summing up all optical transitions.

4. Results and discussion Fig. 1 shows CPM and PDS spectra of n-type a-Si:H in the annealed and in the degraded state. They show the features which all C P M and PDS spectra of doped and undoped a-Si:H have in common. Both methods yield identical spectra of the Urbach

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Fig. 1. Measured(solidlines)and simulated(dashedlines)CPM and PDS spectra of P-doped a-Si:H. tail but the defect absorption measured by PDS differs significantly from CPM. PDS values exceed the corresponding CPM data below ~ 1.4 eV. Analysis of the interference fringes using front and back side illumination shows that a possible contribution from defect-rich interface layers to the PDS data can be neglected [9]. Furthermore, PDS spectra of the annealed state show a fiat region at low photon energies while CPM data increase over orders of magnitude. Light soaking causes a Fermi-level shift towards midgap and an increase of the defect absorption at about 1.2 eV in CPM and PDS. Below 0.9 eV the defect absorption shows no increase or even a decrease after degradation for various n-type films. These results cannot be explained by the standard defect model which assumes one defect peak at midgap. Fig. 2 shows the densities of localized states derived from simulation of the CPM and PDS spectra of Fig. 1. In the annealed state only D- states contribute significantly to the CPM defect absorption whereas the minority carriers are not observed because of their small pz-product. A fit of the corresponding PDS spectrum requires the assumption of a narrow band of D Ostates close to the Fermi level. It turns out that electrons excited from these states are immediately recaptured by the created D ÷ states, thus contributing only weakly to the photocurrent. Therefore, CPM underestimates the density of D O states, whereas in PDS spectra all absorption processes are equally weighed. Upon light soaking the defect density increases (Fig. 1) and the Fermi level shifts towards midgap. Assuming that light soaking does not alter the density of active dopants and taking into account charge neutrality, the simulation reproduces the experimental data only if D +, D- and D Ostates are of the same order of magnitude. But there are still more charged than neutral defect states. Furthermore,

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it is evident that the intersection of the C P M spectra of the annealed and the degraded sample at 0.85 eV has to be attributed to a charge transfer from D - states due to the Fermi-level shift after degradation. Minor deviations between measured and simulated spectra may have their origin in the simplified description of the defect density by the use of Gaussian distributions [8]. In addition, small differences between the position of the sample before and after light soaking in combination with thickness inhomogenieties cause a shift of the interference fringes. Measured and simulated C P M and PDS spectra of a p-type a-Si:H film in the annealed and in the degraded state are shown in Fig. 3. Above a photon energy of about 1.1 eV the sub-band-gap absorption spectra of C P M and PDS coincide, but below 1.1 eV both methods yield different spectra. For both the annealed and the degraded states very good agreement could be achieved between measured and simulated PDS and C P M spectra. The simulations reveal that above 1.t eV the defect absorption is dominated by charged defects but below that the presence of D o states, which are detected by PDS but not by CPM, causes the differences. Upon light soaking the Fermi level shifts towards midgap and the defect absorption at photon energies of about 1.2 eV increases. In the case of good agreement between measured and simulated PDS and C P M spectra the distribution of charged defects can be derived from C P M while PDS detects the total defect density, giving additional information on D o states. A case in which no defect distribution is found to simulate both, C P M and PDS, is discussed in the following for the example of an undoped hot wire sample (Fig. 4). In the annealed state the PDS defect absorption at ~-- 1.2 eV exceeds that of C P M by more than one order of magnitude. Using defect distributions derived from CPM, the simulated PDS spectra do not fit the measured data. Increasing the density of D O states in the simulation results in a better agreement between simulated and measured PDS data but leads to a clear mismatch between calculated and experimental C P M spectra. To explain the origin of such a deviation between C P M

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a n d P D S two effects are considered: (i) the presence of a defect-rich layer at the film surface o r at the s u b s t r a t e - f i l m interfaces. As stated above, this can be excluded as a source of these differences from analysis of the interference fringes. (ii) M i c r o s c o p i c i n h o m o g e n e i t i e s , i.e. voids a n d c o l u m n a r growth, with a high defect density at inner

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F. Siebke el al,/Solar Energy Materials and Solar (:ells' 49 (1997) 7 12

surfaces. This can explain the observed behavior. Samples showing voids and/or columnar growth in transmission electron microscopy exhibit a large difference between the C P M and P D S data [3]. Carriers generated in the proximity of these defects have reduced lifetimes. They hardly contribute to the C P M signal but show up in PDS data. After light soaking the increased defect density is spatially more uniform. We believe that as long as the ratio of P D S and C P M signals at 1.2 1.3 eV is larger than a factor of about 3 no c o m m o n distribution describing P D S and C P M can be found and the sample has to be regarded as inhomogeneous.

5. Conclusions C P M and PDS provide complementary inR~rmation of the defect distribution in a-Si:H. The determination of the distribution of charged and neutral states in a-Si:H requires detailed analysis of P D S and C P M data by numerical simulations. In the case of a spatially h o m o g e n e o u s defect density the analysis of C P M gives information on the distribution of charged defects, while additional P D S data are required to determine the density of neutral defects. Since P D S detects the total defect density and C P M favors transport paths with low defect densities, a combination of P D S and C P M also detects inhomogeneities caused by voids or c o l u m n a r growth. Analysis of material properties by only one of these methods means an incomplete characterization. This may be one reason for the often stated lack of a correlation between material properties and device performance.

Acknowledgements The authors wish to thank J. KlomfaB for technical assistance and L. Zanzig for providing hot-wire samples and T E M data. The work was supported by the BMBF.

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