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Subbandgap absorption spectra of slightly doped a-Si:H measured with constant photocurrent method (CPM) and photothermal deflection spectroscopy (PDS)
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Solid State Communications, Vol. 85, No. 3, PP. 219-222, 1993. Printed in Great Britain.
0038-1098/9356.00+.00 Pergamon Press Ltd
SUBBANDGAP ABSORPTION SPECTRA OF SLIGHTLY DOPED a-Si:H MEASURED WITH CONSTANT PHOTOCURRENT METHOD (CPM) AND PHOTOTHERMAL DEFLECTION SPECTROSCOPY (PDS) Evelyne Sauvainl", Andreas Mettler, Nicolas Wyrsch, Arvind Shah Institute of Microtechnology, Rue Breguet 2, CH-2000 Neuchatel, Switzerland. i" presently with Division of Engineering, Brown University, Providence, R102912, USA. Received 21 September 1992 by J. Tauc The authors report on systematic measurements of the absorption spectra in the visible • near-infrared range performed on a series of slightly phosphorus- and boron-doped amorphous hydrogenated silicon (a-Si:H) samples. Optical transmission spectra, Constant Photocurrent Method (CPM) and Photothermal Deflection Spectroscopy (PDS) have been used to measure the absorption coefficient (c0 in differently doped samples. Variation of the gas phase doping levels in the ppm range is shown to lead to relevant differences between CPM and PDS spectra. The latter demonstrate that additional care has to be taken when evaluating deep defect density from CPM when the Fermi level is not constant, and especially so, if the Fermi level is below midgap.
1. INTRODUCTION Measurement of the absorption spectra in the visible and near infrared wavelength range is a commonly used technique to characterize the optoelectronic quality of aSi:H films for various device applications. The absorption spectra 0~(Ehv) of a-Si:H in this range shows two main features: an exponential decay and a residual shoulder (subbandgap absorption). In undoped a-Si:H, the value of the characteristic energy (Urbach slope) of the observed exponential decay is used to find the characteristic energy of the exponential valence bandtail states distribution. From the subbandgap absorption value, one can infer the value of !he deep defect density (dangling bond density). The latter is an important parameter for the quality of the film, since dangling bonds act as the main recombination centers, and therefore have a large effect on the steady-state transport properties. Although it has been noted that several problems appear when accurate evaluation of the deep defect density is attempted from the measured absorption spectra 1, the use of the different parts of the absorption spectra remains a common way to obtain information on the density of states (DOS). Optical transmission measurement is used to evaluate the absorption coefficient in the part of the spectra where absorption coefficient is high (1(34-103 cm-1). In order to measure low subbandgap absorption coefficient values (typically 0.1-10 cm-1) on thin a-Si:H films (typically a few micrometers), two sensitive indirect measurement techniques have been successfully developed: In the Photothermal Deflection Spectroscopy (PDS) 2 measurement technique, the absolute value of the absorption coefficient (O~PDS)is found from the heating of the absorbing film. The Constant Photocurrent Method (CPM) 3, on the other hand, evaluates the relative variation of the absorption coefficient using the photogeneration of free (mobile) carriers induced by light absorption. In evaluating the deep defect density from the value of the absorption coefficient in the subbandgap energy range, both PDS and CPM measurement methods have disadvantages. Neither of these methods deliver directly the true absorption spectra 0~a_Si:Hin this range, but rather give a "finger-print" of the absorption; in fact, the spectra we obtain by PDS and CPM are always strongly connected
with the measurement conditions and its limits. It is therefore necessary to distinguish between 0~PDS and otCpM. As the absorption coefficient ~PDS is related to all transitions producing heat (non-radiative recombination being the main recombination mechanism in a-Si:H at room temperature2), this measurement technique is affected by surface and interface defect states absorption, and should therefore be performed on relatively thick samples or on full thickness series, in order to give a correct value of the bulk deep defect density4. On the other hand, the CPM measurement technique is limited by the fact that it only probes the transitions that lead to the excitation of a charge carrier 9nto a cpn.ductive states 5. In addition, the value 0(CpM obtained is only valid if the lifetime of the majority carrier is constant; one attempts to establish this by keeping the photocurrent during the measurement constant, creating thereby a constant (quasi-) Fermi level or constant demarcation level for the recombination process. The various methods used for the evaluation of the deep defect density from the subbandgap absorption spectra in undoped a-Si:H have been summarized and discussed in a recent paperl: deconvolution, integrated excess, and the "single energy" techniques. For light-soaked undoped samples, the "single energy technique" at 1.2eV was found most useful in calibrating the spectra of CPM: aCPM(1.2eV) = 1 cm -1 corresponds then to a defect density of 2.4+5 x 1016 cm -3. In undoped a-Si:H samples, ~PDS is about twice5 0~CpM at the photon energy involving electron transitions onto or from the deep defects: apDS(1.2eV) = 2 x otCPM(1.2eV). The study of a-Si:H samples with slightly varying doping levels (in 0.1 - 10ppmvol gas phase doping level range) is of special interest for a number of applications, like p-i-n solar cells, where one hopes thereby to improve the internal field profile and also the stabilized efficiency. Nevertheless, it is at In'st necessary to better understand the behaviour of uniform layers that are slightly doped, having different Fermi-level position, and therefore different occupation function of the gap states, especially of the dangling bond states. Note that, in the case of uniform layers, contrary to the case of a p-i-n solar cell, the material is locally electrically neutral. For the low doping concentrations presented here, the doping efficiency - the 219
Vol. 85, No. 3
SLIGHTLY DOPED a-Si:H
220
ratio of the active, fourfold coordinated atoms to the total dopant atoms - has been suggested6 to be close to 1. For higher doping levels the non-active, threefold coordinated dopant density starts to dominate, leading to a lower doping efficency and to a higher total defect density6. However, so far no systematic investigation of the optical absorption of samples with low doping levels has been carried out. 2. EXPERIMENTAL Our samples were deposited on a glass substrate (Corning 7059) at 220 o c with the VHF deposition technique (plasma frequency f=70 MHz) described elsewhere8. After deposition, all the films were annealed at 200oc for lh. These films have a thickness ranging between 1.7 p.m and 2.5 l.tm. Doping was achieved by mixing the dopant gases in Hydrogen (500ppm Diborane (B2H6) in H2 for p-type samples and 1000ppm Phosphine (PH3) in H2 for n-type samples). This mixture was then used with pure Silane for the film deposition. The gas phase doping level of the boron-doped films was varied between 0.2 and I0 ppmvol of B2H6 in Silane gas; for the phosphorus-doped films, it was varied from 0. I ppmvol to I ppmvol of PH3 in Silane gas. The range of gas phase doping levels used in this study was chosen to achieve a solid phase dopant atom concentration of the order of magnitude of the dangling bond density in undoped annealed a-Si:H. Figure I shows the values of the dark conductivity measured at room temperature after annealing as a function of the doping levels. The dark conductivity of the films was allowed to vary by several orders of magnitude. In all these films, the ratio of dark conductivity over photoconductivity (measured at 4 mW/cm 2 of red light) is larger or equal to 104 . Measurements of the absorption coefficient were performed at room temperature. Optical transmission measurements were performed with a standard UV-VIS spectrometer, and the absorption coefficient was calculated from the transmission spectra. Photothermal Deflection Spectroscopy (PDS) (as described in detail elsewere4 ) was performed on various pieces of the annealed films. CPM absorption measurements were performed on samples with coplanar Alurninium contacts separated.by 0.5 mm, thereby applying 10 V, using an cw monochromatic illumination level, and matching the photocurrent to a value several times larger than that of the dark current. The ohmic nature of the contacts was checked after the annealing procedure on all the films. A further check that contacts do not affect measurement was performed on the I0 ppmvol and 4
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ppmvol boron-doped samples, by using Chromium contacts: We thereby also observed ohmic behaviour and obtained the same CPM spectra. For the 10 ppmvo1 borondoped sample, as well as for the lppmvol phosphorusdoped sample, we observed no changes of the spectra by measuring them at temperatures up to 100"C. Further measurements with the lock-in technique (chopped light of 13 Hz, no bias light) gave identical results. The steady-state transport properties of the same doping series has recently been published seperatelyT: it could be evaluated from the measurements of steady-state photoconductivity and ambipolar diffusion length that for this series the ratio of the free photogenerated carrier densities nf/pf varies over 7 orders of magnitude. Thereby one can conclude that dangling bond states change over from being predominantly empty (D ÷ state) (on the p-type side of the series) to being predominantly doubly occupied (D- state) (on the n-type side of the series). 3. RESULTS Figure 2 shows the absorption spectra of the slightly doped a-Si:H samples. Transmission and PDS spectra give absolute values for a, and are observed to correlate well. When no PDS spectra is available, the relative ~CPM values can be calibrated by means of the transmission measurements: the Urbach slope can be extended and matched with the absolute absorption value at 3000cm -1 of 105
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Figure 1: Value of the dark conductivity (Gdark) measured at room temperature (after annealing) as a function of the gas phase doping level.
Figure2: Selected PDS, CPM and transmission spectra from the whole light doping series; "n-type" indicates phosphorus-doped, "p-type" boron-doped samples and "ppm" the gas phase doping level. The straight line indicates absorption values evaluated by an optical transmission spectrometer.
Vol. 85, No. 3 100
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the transmission measurement in low-doped a-Si:H. This procedure is most commonly used and has proven quite sucessful. Nevertheless, if there is a PDS spectra available, the best and simplest way is to calibrate the CPM spectra in the upper part of the Urbach slope with the PDS spectra and this was done so here. However, the latter calibration procedure is valid only as long as PDS and CPM are sensitive in the same way to the transitions implied in the energy range Used for the matching. In the energy range of the Urbach edge, this means that the excitation of an electron from the valence bandtail to the conduction band has to lead to the creation of a majority-type mobile carrier. In order to compare the different spectra, Figure 3 shows oq2pM and txpDS at the subbandgap photon energy of 1.2eV as a function of the gas phase doping level. First, we see that tXpDs(1.2eV) shows no significant trend with the changing doping level; furthermore, the variation is within less than 2 times the defect density in the undoped sample. We therefore interpret the almost constant value of tZPDS(1.2eV) as a function of the doping level as an indication that the bulk deep defect density did not change much with these very low doping levels (which are in the order of a few ppmvol gas dopant ratio): taking into acount the interface and surface state defects (see publication4 for thickness-dependent PDS measurements), we can estimate that the change of the bulk deep defects is less than one order of magnitude for 21.tm thick samples. This would, of course, no longer be the case for the higher doping levels6. The values of the deep defect density (Ndb) can be found from the value of aPDS(1.2eV) with help of the calibration formula1 mentioned above, applied to PDS:
221
case 3:
otCPM(1.2eV).
slightly p-type: 1 t~PDS(1.2eV) ~ ~- aCPM(1.2eV).
The first case has already been observed with undoped samples 5, and the factor 2 has been interpreted in the following way: whereas PDS is sensitive to all optical transitions, CPM detects just the transitions involving the creation of the mobile charge carriers contributing predominantly to the current (i.e. the type of carriers with the higher steady-state I.tx product)• For the presented "intrinsic" samples of case 1 (meaning here: phosphine gas concentration in silane < 0.2 ppmvol and diborane gas concentration in silane < 2 ppmvol), we believe that the CPM photocurrent is dominated by the electrons due to their higher mobility and that this remains valid even up to the lppmvol boron-doped sample• (In fact, one should remember that undoped a-Si:H layers are generally slightly n-tpye in nature, the Fermi-level lying above midgap, and that about lppmvol of boron-doping will just shift the Fermi level towards midgap. This corresponds also to the results published in7. For the layers to become p-type, they have to be sufficiently boron-doped, i.e. at least 2 ppmvol.) The predominant transition contributing to the CPM photocurrent in the subbandgap photon energy range should then be the transition from the neutral dangling bonds (DO state) to the conduction band. The excitation of an electron from the valence band onto a neutral dangling bond state does not contribute to the electron current of CPM, but can be detected by PDS. If most dangling bonds are in the D° state, this means that CPM is sensitive only to half of the transitions which can be measured by PDS, t~PDS/(XCPM=2. Experimentally, the factor of two between
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Ndb= aPDS(1.2eV) x (1.2 + 2.5) x 1016 cm-3. Note that the assumptions made for the interpretation of the steady state transport properties measurements (photoconductivity and ambipolar diffusion length) on the same series and recently published7 namely Ndb = constant are thereby confirmed. 4. DISCUSSION Here, we intend to discuss the observed differences between ~CPM and (~PDS for slightly doped samples and take a special look at the subbandgap energy range. First,
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Figure4: Ratio tzCPM(1.2eV ) / ~PDS(1.2eV)as a function of the gas phase doping level. The straight lines correspond to the three different cases: slightly n-type (ratio = 1) and slightly p-type (ratio = 4) as well as "intrinsic" / "near-intrinsic" (ratio = 1/2).
222
Vol. 85, No. 3
SLIGHTLY DOPED a-Si:H
0~CpM and 0~pDS is not observed to be very sharp, probably because several optical transitions in the subbandgap energy range may still influence the final value of ~CPM. For case 1, we can formulate our observation more prudently by saying that here ff,CPM is always smaller than 0~PDS in the subbandgap range : ~CPM(1.2eV) < etPDS(1.2eV). In the second case (n-type samples with Phosphine gas concentration in Silane > 0.2 ppmvol), there is no difference in the values of ~ C P M ( 1 . 2 e V ) and ~PDS(1.2eV). This can be interpreted in the following way: In the n-type samples, most dangling bonds are negatively charged (D- state). Excitation of electron from a D- state into the conduction band will generate a mobile electron, and therefore contributes to the CPM photocurrent of electrons. As most dangling bonds are negatively charged, these transitions are the only ones also for PDS, and therefore, ~CPM(1.2eV) = aPDS(l.2eV). In the third case (p-type samples with Diborane gas concentration in Silane > 2 ppmvol), o~CPM in the subbandgap energy range shows a larger value than ~PDS. Conceptually, one can easily understand that the interpretation of a CPM spectrum is going to be problematic for the p-type material. As long as the Fermi level is in the upper half of the gap, the CPM signal is principally based, for the whole range of the photon energies hv, i.e. for the bandtail (Urhach) edge and for the dangling bonds, on the same excitation and conduction mechanisms, i.e. excitation from gap states to the conduction band and conduction by free electrons. As soon as the layers are sufficiently p-type, this cannot hold any longer: On one hand, dangling bond states will be predominantely empty (D ÷) and the main mechanism by means of which they appear in the CPMsignal can only be excitation of an electron from the valence band to a dangling bond (the latter becoming thereby a D O state and the valence band obtaining a free hole, i.e. this transition leads to the generation of a majority type free carrier; This is the counterpart of case 2). On the other hand, as long as one can observe an exponential decay (Urbach edge) in the CPM spectrum (and this is clearly the case in Fig.2 for all samples), the CPM measurement has to be sensitive to the excitation of an electron from valence bandtail state onto the conduction band. The latter does not directly lead to the generation of a majority-type moblie carrier. It is therefore clear that for sufficiently boron-doped samples the transport mechanism (i.e. steady-state p.xproducts) dominating the CPM current can be quite different depending on the photon energy range. Thus, we cannot interprete the CPM spectra for boron-doped layers without developing a more elaborate model. The exact nature of the conduction mechanism involved in the CPM spectra on p-type layers is however difficult to assess, because the whole process is rather complex, and a quantitative interpretation has to be based on the exact values of the occupation function of the dangling bond state. It should furthermore also be taken into account how valence bandtail and valence band interact under steadystate subbandgap excitation (photogeneration and thermal emission from the bandtails and capture from the valence band being involved). Finally, more experimental data is also needed in order to avoid interpretation errors.
5. CONCLUSIONS In our present series of slightly boron- and phosphorus-doped samples (with gas phase doping levels in the range of a few ppmvol) we have observed an almost constant deep defect density as a function of the doping level. This has been established by evaluating the PDS subbandgap integrated excess absorption as well as the value of ~pDS(1.2eV), as measured on our 2gm thick samples. This observation itself is of importance for the understanding of doping effects at very low dopant concentrations. In addition, we found three different cases for CPM measurements on these slightly-doped samples; Between lppmvol boron-doped and 0.2ppmvol phosphorus doped films ("intrinsic" and "near-intrinsic" films) ~CPM is, at a photon energy of 1.2eV, roughly half the value of ~PDS, as generally expected 5. Above 0.5ppmvol phosphorus doping (slightly n-type) we found identical CPM and PDS spectra. Finally, above 2 ppmvol boron doping (slightly p-type), the value of txCPM shows an anomalous behaviour; it becomes clearly higher than o~pDS for the defect absorption range provided the exponential edges (band tails) are made to coincide. We have so far no straightforward and simple explanation for this observation. A final conclusion is that additional care has to be taken when evaluating deep defect densities from CPM subbandgap absorption, if the Fermi level is not constant, and especially so, if the Fermi level is below midgap. 6. ACKNOWLEDGMENTS The authors are very grateful for helpful discussions with Milan Vanecek, and for sample preparation by Sebastien Dubail. The authors acknowledge financial support from the Swiss Federal Renewable Energy Program under grant EF-REN90(045) and from the Swiss National Science Foundation under grant FN-27916.
7. REFERENCES 1 N. Wyrsch, F. Finger, T. J. McMahon, M. Vanecek, J. of Non-Crystalline Solids, 137&138, 347 (1991) 2 W.B. Jackson, N. M. Amer, A. C. Boccara and D. Fournier, Appl. Optics, 20, 1333 (1981) 3 M. Vanecek, J. Kocka, J. Stuchlik, Z. Kozisek, O. Stika and A. Triska, Solar Energy Mat. 8, 411 (1983) 4 H. Curtins, M. Favre, in "Amorphous Silicon and Related Materials" (pp.329-363) edited by H. Fritzsche, 1988 Word Scientific Publishing Company. 5 Z.E. Smith, V. Chu, K. Shepard, S. Aljishi, D. Slobodin, J. Kolodzey, and S. Wagner, Appl. Phys. Lett. 50, 1521 (1987) 6 M. Stutzmann, D.K. Biegelsen, and R.A. Street, Phys. Rev. B 35, 5666 (1987) 7 p. Pipoz, E. Sauvain, J. Hubin, A. Shah, to be published in the Proceedings of the MRS Spring Conf., San Francisco, (1992) 8 H. Curtins, N. Wyrsch, M. Favre, A. Shah, Plasma Chem. & Plasma Proc. 7, 267 (1987)
Solid State Communications, Vol. 85, No. 3, PP. 219-222, 1993. Printed in Great Britain.
0038-1098/9356.00+.00 Pergamon Press Ltd
SUBBANDGAP ABSORPTION SPECTRA OF SLIGHTLY DOPED a-Si:H MEASURED WITH CONSTANT PHOTOCURRENT METHOD (CPM) AND PHOTOTHERMAL DEFLECTION SPECTROSCOPY (PDS) Evelyne Sauvainl", Andreas Mettler, Nicolas Wyrsch, Arvind Shah Institute of Microtechnology, Rue Breguet 2, CH-2000 Neuchatel, Switzerland. i" presently with Division of Engineering, Brown University, Providence, R102912, USA. Received 21 September 1992 by J. Tauc The authors report on systematic measurements of the absorption spectra in the visible • near-infrared range performed on a series of slightly phosphorus- and boron-doped amorphous hydrogenated silicon (a-Si:H) samples. Optical transmission spectra, Constant Photocurrent Method (CPM) and Photothermal Deflection Spectroscopy (PDS) have been used to measure the absorption coefficient (c0 in differently doped samples. Variation of the gas phase doping levels in the ppm range is shown to lead to relevant differences between CPM and PDS spectra. The latter demonstrate that additional care has to be taken when evaluating deep defect density from CPM when the Fermi level is not constant, and especially so, if the Fermi level is below midgap.
1. INTRODUCTION Measurement of the absorption spectra in the visible and near infrared wavelength range is a commonly used technique to characterize the optoelectronic quality of aSi:H films for various device applications. The absorption spectra 0~(Ehv) of a-Si:H in this range shows two main features: an exponential decay and a residual shoulder (subbandgap absorption). In undoped a-Si:H, the value of the characteristic energy (Urbach slope) of the observed exponential decay is used to find the characteristic energy of the exponential valence bandtail states distribution. From the subbandgap absorption value, one can infer the value of !he deep defect density (dangling bond density). The latter is an important parameter for the quality of the film, since dangling bonds act as the main recombination centers, and therefore have a large effect on the steady-state transport properties. Although it has been noted that several problems appear when accurate evaluation of the deep defect density is attempted from the measured absorption spectra 1, the use of the different parts of the absorption spectra remains a common way to obtain information on the density of states (DOS). Optical transmission measurement is used to evaluate the absorption coefficient in the part of the spectra where absorption coefficient is high (1(34-103 cm-1). In order to measure low subbandgap absorption coefficient values (typically 0.1-10 cm-1) on thin a-Si:H films (typically a few micrometers), two sensitive indirect measurement techniques have been successfully developed: In the Photothermal Deflection Spectroscopy (PDS) 2 measurement technique, the absolute value of the absorption coefficient (O~PDS)is found from the heating of the absorbing film. The Constant Photocurrent Method (CPM) 3, on the other hand, evaluates the relative variation of the absorption coefficient using the photogeneration of free (mobile) carriers induced by light absorption. In evaluating the deep defect density from the value of the absorption coefficient in the subbandgap energy range, both PDS and CPM measurement methods have disadvantages. Neither of these methods deliver directly the true absorption spectra 0~a_Si:Hin this range, but rather give a "finger-print" of the absorption; in fact, the spectra we obtain by PDS and CPM are always strongly connected
with the measurement conditions and its limits. It is therefore necessary to distinguish between 0~PDS and otCpM. As the absorption coefficient ~PDS is related to all transitions producing heat (non-radiative recombination being the main recombination mechanism in a-Si:H at room temperature2), this measurement technique is affected by surface and interface defect states absorption, and should therefore be performed on relatively thick samples or on full thickness series, in order to give a correct value of the bulk deep defect density4. On the other hand, the CPM measurement technique is limited by the fact that it only probes the transitions that lead to the excitation of a charge carrier 9nto a cpn.ductive states 5. In addition, the value 0(CpM obtained is only valid if the lifetime of the majority carrier is constant; one attempts to establish this by keeping the photocurrent during the measurement constant, creating thereby a constant (quasi-) Fermi level or constant demarcation level for the recombination process. The various methods used for the evaluation of the deep defect density from the subbandgap absorption spectra in undoped a-Si:H have been summarized and discussed in a recent paperl: deconvolution, integrated excess, and the "single energy" techniques. For light-soaked undoped samples, the "single energy technique" at 1.2eV was found most useful in calibrating the spectra of CPM: aCPM(1.2eV) = 1 cm -1 corresponds then to a defect density of 2.4+5 x 1016 cm -3. In undoped a-Si:H samples, ~PDS is about twice5 0~CpM at the photon energy involving electron transitions onto or from the deep defects: apDS(1.2eV) = 2 x otCPM(1.2eV). The study of a-Si:H samples with slightly varying doping levels (in 0.1 - 10ppmvol gas phase doping level range) is of special interest for a number of applications, like p-i-n solar cells, where one hopes thereby to improve the internal field profile and also the stabilized efficiency. Nevertheless, it is at In'st necessary to better understand the behaviour of uniform layers that are slightly doped, having different Fermi-level position, and therefore different occupation function of the gap states, especially of the dangling bond states. Note that, in the case of uniform layers, contrary to the case of a p-i-n solar cell, the material is locally electrically neutral. For the low doping concentrations presented here, the doping efficiency - the 219
Vol. 85, No. 3
SLIGHTLY DOPED a-Si:H
220
ratio of the active, fourfold coordinated atoms to the total dopant atoms - has been suggested6 to be close to 1. For higher doping levels the non-active, threefold coordinated dopant density starts to dominate, leading to a lower doping efficency and to a higher total defect density6. However, so far no systematic investigation of the optical absorption of samples with low doping levels has been carried out. 2. EXPERIMENTAL Our samples were deposited on a glass substrate (Corning 7059) at 220 o c with the VHF deposition technique (plasma frequency f=70 MHz) described elsewhere8. After deposition, all the films were annealed at 200oc for lh. These films have a thickness ranging between 1.7 p.m and 2.5 l.tm. Doping was achieved by mixing the dopant gases in Hydrogen (500ppm Diborane (B2H6) in H2 for p-type samples and 1000ppm Phosphine (PH3) in H2 for n-type samples). This mixture was then used with pure Silane for the film deposition. The gas phase doping level of the boron-doped films was varied between 0.2 and I0 ppmvol of B2H6 in Silane gas; for the phosphorus-doped films, it was varied from 0. I ppmvol to I ppmvol of PH3 in Silane gas. The range of gas phase doping levels used in this study was chosen to achieve a solid phase dopant atom concentration of the order of magnitude of the dangling bond density in undoped annealed a-Si:H. Figure I shows the values of the dark conductivity measured at room temperature after annealing as a function of the doping levels. The dark conductivity of the films was allowed to vary by several orders of magnitude. In all these films, the ratio of dark conductivity over photoconductivity (measured at 4 mW/cm 2 of red light) is larger or equal to 104 . Measurements of the absorption coefficient were performed at room temperature. Optical transmission measurements were performed with a standard UV-VIS spectrometer, and the absorption coefficient was calculated from the transmission spectra. Photothermal Deflection Spectroscopy (PDS) (as described in detail elsewere4 ) was performed on various pieces of the annealed films. CPM absorption measurements were performed on samples with coplanar Alurninium contacts separated.by 0.5 mm, thereby applying 10 V, using an cw monochromatic illumination level, and matching the photocurrent to a value several times larger than that of the dark current. The ohmic nature of the contacts was checked after the annealing procedure on all the films. A further check that contacts do not affect measurement was performed on the I0 ppmvol and 4
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ppmvol boron-doped samples, by using Chromium contacts: We thereby also observed ohmic behaviour and obtained the same CPM spectra. For the 10 ppmvo1 borondoped sample, as well as for the lppmvol phosphorusdoped sample, we observed no changes of the spectra by measuring them at temperatures up to 100"C. Further measurements with the lock-in technique (chopped light of 13 Hz, no bias light) gave identical results. The steady-state transport properties of the same doping series has recently been published seperatelyT: it could be evaluated from the measurements of steady-state photoconductivity and ambipolar diffusion length that for this series the ratio of the free photogenerated carrier densities nf/pf varies over 7 orders of magnitude. Thereby one can conclude that dangling bond states change over from being predominantly empty (D ÷ state) (on the p-type side of the series) to being predominantly doubly occupied (D- state) (on the n-type side of the series). 3. RESULTS Figure 2 shows the absorption spectra of the slightly doped a-Si:H samples. Transmission and PDS spectra give absolute values for a, and are observed to correlate well. When no PDS spectra is available, the relative ~CPM values can be calibrated by means of the transmission measurements: the Urbach slope can be extended and matched with the absolute absorption value at 3000cm -1 of 105
I
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,-
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I
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/
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t I 1 1.2 1.4 1.6 1.8 I
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i
CPM
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',
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0.8 1 1.2 1.4 1.6 1.8 2 2.2 Eh~ / eV
. . . . . .
(1
0 undoped
0.1
1 ppm ~ PH3/SiH4
Figure 1: Value of the dark conductivity (Gdark) measured at room temperature (after annealing) as a function of the gas phase doping level.
Figure2: Selected PDS, CPM and transmission spectra from the whole light doping series; "n-type" indicates phosphorus-doped, "p-type" boron-doped samples and "ppm" the gas phase doping level. The straight line indicates absorption values evaluated by an optical transmission spectrometer.
Vol. 85, No. 3 100
SLIGHTLY DOPED a-Si:H r . . . . . . .
•
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, ........
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ra
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we must point out that, in contrast to (XPDS,aCPM(1.2eV) varies much more as a function of the doping level. To highlight this, Figure 4 shows the ratio between (~q2PMand (XPDS, i.e. txCpM / t~PDS at the photon energy of 1.2eV. We can observe three different cases: case l:
"intrinsic" and "near-intrinsic": undoped and very low doping levels: aPDS(1.2eV) ~ 2 aCPM(1.2eV).
case 2:
slightly n-tye: t~PDS(1.2eV) ---
.
= (2.4 ÷5) 1016cm3(PDS) 1
10ppm
B2H6/SiH,I ~
•
0 0.I undoped ~
1 ppm PH3/SiH4
Figure 3: Value of the absorption coefficient at a photon energy of 1.2eV: aCPM(1.2 eV) and ~PDS(1.2 eV) as a function of the gas phase doping level. The straight line indicates the almost constant value of aPDS(1.2 eV). The corresponding deep defect densit~ was obtained by using the calibration factor of reference for PDS.
the transmission measurement in low-doped a-Si:H. This procedure is most commonly used and has proven quite sucessful. Nevertheless, if there is a PDS spectra available, the best and simplest way is to calibrate the CPM spectra in the upper part of the Urbach slope with the PDS spectra and this was done so here. However, the latter calibration procedure is valid only as long as PDS and CPM are sensitive in the same way to the transitions implied in the energy range Used for the matching. In the energy range of the Urbach edge, this means that the excitation of an electron from the valence bandtail to the conduction band has to lead to the creation of a majority-type mobile carrier. In order to compare the different spectra, Figure 3 shows oq2pM and txpDS at the subbandgap photon energy of 1.2eV as a function of the gas phase doping level. First, we see that tXpDs(1.2eV) shows no significant trend with the changing doping level; furthermore, the variation is within less than 2 times the defect density in the undoped sample. We therefore interpret the almost constant value of tZPDS(1.2eV) as a function of the doping level as an indication that the bulk deep defect density did not change much with these very low doping levels (which are in the order of a few ppmvol gas dopant ratio): taking into acount the interface and surface state defects (see publication4 for thickness-dependent PDS measurements), we can estimate that the change of the bulk deep defects is less than one order of magnitude for 21.tm thick samples. This would, of course, no longer be the case for the higher doping levels6. The values of the deep defect density (Ndb) can be found from the value of aPDS(1.2eV) with help of the calibration formula1 mentioned above, applied to PDS:
221
case 3:
otCPM(1.2eV).
slightly p-type: 1 t~PDS(1.2eV) ~ ~- aCPM(1.2eV).
The first case has already been observed with undoped samples 5, and the factor 2 has been interpreted in the following way: whereas PDS is sensitive to all optical transitions, CPM detects just the transitions involving the creation of the mobile charge carriers contributing predominantly to the current (i.e. the type of carriers with the higher steady-state I.tx product)• For the presented "intrinsic" samples of case 1 (meaning here: phosphine gas concentration in silane < 0.2 ppmvol and diborane gas concentration in silane < 2 ppmvol), we believe that the CPM photocurrent is dominated by the electrons due to their higher mobility and that this remains valid even up to the lppmvol boron-doped sample• (In fact, one should remember that undoped a-Si:H layers are generally slightly n-tpye in nature, the Fermi-level lying above midgap, and that about lppmvol of boron-doping will just shift the Fermi level towards midgap. This corresponds also to the results published in7. For the layers to become p-type, they have to be sufficiently boron-doped, i.e. at least 2 ppmvol.) The predominant transition contributing to the CPM photocurrent in the subbandgap photon energy range should then be the transition from the neutral dangling bonds (DO state) to the conduction band. The excitation of an electron from the valence band onto a neutral dangling bond state does not contribute to the electron current of CPM, but can be detected by PDS. If most dangling bonds are in the D° state, this means that CPM is sensitive only to half of the transitions which can be measured by PDS, t~PDS/(XCPM=2. Experimentally, the factor of two between
1G
l........
I. . . . . . . .
*
I! l I~. . . . . . .
i
. . . . . . .
I
4
1
"- -"
1/2 Q
•
Ndb= aPDS(1.2eV) x (1.2 + 2.5) x 1016 cm-3. Note that the assumptions made for the interpretation of the steady state transport properties measurements (photoconductivity and ambipolar diffusion length) on the same series and recently published7 namely Ndb = constant are thereby confirmed. 4. DISCUSSION Here, we intend to discuss the observed differences between ~CPM and (~PDS for slightly doped samples and take a special look at the subbandgap energy range. First,
0.1
hllllll
i
Iiiiill!
i ,,|
it
10 ppm 1 B2H6/SiH4 ~
..1
. 1111.1
.
..l*..J
tt
0 undoped
0.1 1 ppm ~ PH3/SiH4
Figure4: Ratio tzCPM(1.2eV ) / ~PDS(1.2eV)as a function of the gas phase doping level. The straight lines correspond to the three different cases: slightly n-type (ratio = 1) and slightly p-type (ratio = 4) as well as "intrinsic" / "near-intrinsic" (ratio = 1/2).
222
Vol. 85, No. 3
SLIGHTLY DOPED a-Si:H
0~CpM and 0~pDS is not observed to be very sharp, probably because several optical transitions in the subbandgap energy range may still influence the final value of ~CPM. For case 1, we can formulate our observation more prudently by saying that here ff,CPM is always smaller than 0~PDS in the subbandgap range : ~CPM(1.2eV) < etPDS(1.2eV). In the second case (n-type samples with Phosphine gas concentration in Silane > 0.2 ppmvol), there is no difference in the values of ~ C P M ( 1 . 2 e V ) and ~PDS(1.2eV). This can be interpreted in the following way: In the n-type samples, most dangling bonds are negatively charged (D- state). Excitation of electron from a D- state into the conduction band will generate a mobile electron, and therefore contributes to the CPM photocurrent of electrons. As most dangling bonds are negatively charged, these transitions are the only ones also for PDS, and therefore, ~CPM(1.2eV) = aPDS(l.2eV). In the third case (p-type samples with Diborane gas concentration in Silane > 2 ppmvol), o~CPM in the subbandgap energy range shows a larger value than ~PDS. Conceptually, one can easily understand that the interpretation of a CPM spectrum is going to be problematic for the p-type material. As long as the Fermi level is in the upper half of the gap, the CPM signal is principally based, for the whole range of the photon energies hv, i.e. for the bandtail (Urhach) edge and for the dangling bonds, on the same excitation and conduction mechanisms, i.e. excitation from gap states to the conduction band and conduction by free electrons. As soon as the layers are sufficiently p-type, this cannot hold any longer: On one hand, dangling bond states will be predominantely empty (D ÷) and the main mechanism by means of which they appear in the CPMsignal can only be excitation of an electron from the valence band to a dangling bond (the latter becoming thereby a D O state and the valence band obtaining a free hole, i.e. this transition leads to the generation of a majority type free carrier; This is the counterpart of case 2). On the other hand, as long as one can observe an exponential decay (Urbach edge) in the CPM spectrum (and this is clearly the case in Fig.2 for all samples), the CPM measurement has to be sensitive to the excitation of an electron from valence bandtail state onto the conduction band. The latter does not directly lead to the generation of a majority-type moblie carrier. It is therefore clear that for sufficiently boron-doped samples the transport mechanism (i.e. steady-state p.xproducts) dominating the CPM current can be quite different depending on the photon energy range. Thus, we cannot interprete the CPM spectra for boron-doped layers without developing a more elaborate model. The exact nature of the conduction mechanism involved in the CPM spectra on p-type layers is however difficult to assess, because the whole process is rather complex, and a quantitative interpretation has to be based on the exact values of the occupation function of the dangling bond state. It should furthermore also be taken into account how valence bandtail and valence band interact under steadystate subbandgap excitation (photogeneration and thermal emission from the bandtails and capture from the valence band being involved). Finally, more experimental data is also needed in order to avoid interpretation errors.
5. CONCLUSIONS In our present series of slightly boron- and phosphorus-doped samples (with gas phase doping levels in the range of a few ppmvol) we have observed an almost constant deep defect density as a function of the doping level. This has been established by evaluating the PDS subbandgap integrated excess absorption as well as the value of ~pDS(1.2eV), as measured on our 2gm thick samples. This observation itself is of importance for the understanding of doping effects at very low dopant concentrations. In addition, we found three different cases for CPM measurements on these slightly-doped samples; Between lppmvol boron-doped and 0.2ppmvol phosphorus doped films ("intrinsic" and "near-intrinsic" films) ~CPM is, at a photon energy of 1.2eV, roughly half the value of ~PDS, as generally expected 5. Above 0.5ppmvol phosphorus doping (slightly n-type) we found identical CPM and PDS spectra. Finally, above 2 ppmvol boron doping (slightly p-type), the value of txCPM shows an anomalous behaviour; it becomes clearly higher than o~pDS for the defect absorption range provided the exponential edges (band tails) are made to coincide. We have so far no straightforward and simple explanation for this observation. A final conclusion is that additional care has to be taken when evaluating deep defect densities from CPM subbandgap absorption, if the Fermi level is not constant, and especially so, if the Fermi level is below midgap. 6. ACKNOWLEDGMENTS The authors are very grateful for helpful discussions with Milan Vanecek, and for sample preparation by Sebastien Dubail. The authors acknowledge financial support from the Swiss Federal Renewable Energy Program under grant EF-REN90(045) and from the Swiss National Science Foundation under grant FN-27916.
7. REFERENCES 1 N. Wyrsch, F. Finger, T. J. McMahon, M. Vanecek, J. of Non-Crystalline Solids, 137&138, 347 (1991) 2 W.B. Jackson, N. M. Amer, A. C. Boccara and D. Fournier, Appl. Optics, 20, 1333 (1981) 3 M. Vanecek, J. Kocka, J. Stuchlik, Z. Kozisek, O. Stika and A. Triska, Solar Energy Mat. 8, 411 (1983) 4 H. Curtins, M. Favre, in "Amorphous Silicon and Related Materials" (pp.329-363) edited by H. Fritzsche, 1988 Word Scientific Publishing Company. 5 Z.E. Smith, V. Chu, K. Shepard, S. Aljishi, D. Slobodin, J. Kolodzey, and S. Wagner, Appl. Phys. Lett. 50, 1521 (1987) 6 M. Stutzmann, D.K. Biegelsen, and R.A. Street, Phys. Rev. B 35, 5666 (1987) 7 p. Pipoz, E. Sauvain, J. Hubin, A. Shah, to be published in the Proceedings of the MRS Spring Conf., San Francisco, (1992) 8 H. Curtins, N. Wyrsch, M. Favre, A. Shah, Plasma Chem. & Plasma Proc. 7, 267 (1987)