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Absorption in amorphous silicon doping-modulated multilayers
Journal of Non-Crystalline Solids 97&98 (1987) 92%930 North-Holland, Amsterdam
927
ABSORPTION IN AMORPHOUSSILICON DOPING-MODULATED MULTILAYERS R. DURNY, S. DUCHARME, J. VINER and P.C. TAYLOR Department of Physics, University of Utah, Salt Lake City, Utah 84112, U.S.A. D. HANEMAN Department of Condensed Matter Physics, University of New South Wales, Kensington, Australia 2033 Photothermal deflection spectroscopy (PDS) measurementson doping-modulated multilayers of a-Si:H show that the measured width of the absorption due to band t a i l s increases as the relative i n t r i n s i c layer thickness decreases. This increased width is due to a wider distribution of band t a i l states in the n- and p-doped layers. A large absorption (~ 100 cm-1) well below the gap is observed in all samples. i . INTRODUCTION The high degree of control over the electronic and structural properties which can currently be achieved in hydrogenated amorphous silicon (a-Si:H) and related alloy systems suggests that i t may be possible to engineer new structures with novel electronic and optical properties. In analogy with recent advances in crystalline systems, many different types of multilayer structures based upon a-Si:H have recently been constructed. 1 A particularly interesting class of these multilayer structures is the doping superlattice where the host material remains unchanged from layer to layer but where the electronic properties of the layers can be changed dramatically by doping.
The
sequence most commonly employed is an n-doped layer, an undoped or i n t r i n s i c layer, a p-doped layer, and a second i n t r i n s i c layer.
To complete the multi-
layer structures this sequence is repeated a certain number of times and the resulting structures are often called n i p i ' s . One of the most interesting phenomenaobserved in these structures is persistent photoconductivity2-5 (PPC). The PPC in nipi structures based on a-Si:H is not well understood, but i t probably involves the activation of defects which may occur near the interfaces in the structure. For this reason information about defects introduced by doping, disorder or interfaces is of considerable importance for the ultimate understanding of the PPC effect. In the present paper we employ photothermal deflection spectroscopy (PDS) to measure optical absorption below the band gap in nipi structures based on a-Si:H. 0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R. Durny et a L / Amorphous silicon doping-modulated multilayers
928
2. EXPERIMENTAL The multilayer structure was prepared by radio frequency decomposition of silane doped with phosphine or diborane for n and p layers, respectively. 5 The deposition chamber was a 6 inch diameter stainless steel cross described elsewhere6 with a 2 inch diameter substrate heated uniformly to 220°C by a contacting oil bath.
The samples consisted of nipi amorphousmultilayers
with six periods (66 A thick n-layers, 33 A thick p-layers, and i-layers with thicknesses varying from 60 to 228 A).
The p- and n-doped layers were
produced with concentrations of approximately 0.01 at. % diborane and 0.1 at. % phosphine in silane, respectively.
Sampleswere deposited on glass substrates.
In order to check i f the samples exhibited the reported PPC phenomenonthe electrical conduction was measured in a coplanar configuration. To ensure contact to all the layers, the multilayered films were scratched before depositing the electrodes.
Each electrode was approximately 5 mmlong with a gap of
approximately I-2 mmbetween electrodes. The data were obtained at voltages for which the samples exhibited ohmic behavior. The absorption measurementswere made with a standard PDS system. In the PDS technqiue7 weak absorption is measured by illuminating the sample with the appropriate wavelength of l i g h t and detecting the absorbed power by observing the change in index of a l i q u i d which is in contact with the sample. For a sample approximately 1 pm thick on a glass substrate the sensitivity l i m i t was about 0.1 cm- I in the a-Si:H multilayer film.
Fringes in the original
absorption spectra have been removedby a suitable averaging procedure. Absorption in the glass substrate is negligible in the range over which the data were taken (~ 0.8-2.4 eV). 3. RESULTS AND DISCUSSION After a 30 s exposure at 300 K to 10 mW/cm2 heat-filtered, band-gap l i g h t , the dark conductivity of all of the multilayer structures increases from that in the annealed state A.
The sample exhibits a strong PPC as indicated by the
ratio of the conductivity 4 min. after illumination ~d to the dark conductivity in the annealed state A, ~A.
This ratio is greater than 100. Both the steady-
state photoconductivity ~p and the excess dark conductivity (~e = ~d - cA) exhibit the power law dependenceof current on incident l i g h t intensity which is usually observed. The PDS measurementsyield information on both the exponential band-tail absorption and the subgap defect (bulk and interface) absorption. Typical PDS spectra of two representative a-Si:H doping-modulated structures are shown in Fig. I.
From the f i t s to the exponential absorption in our samples, the widths
R. Durny et aL / Amorphous silicon doping-modulated multilayers of the exponential (Urbach) edge Eo were obtained. ness dependence of the Urbach energy Eo.
929
Figure 2 shows the thick-
This parameter sharply increases
with decreasing thickness of the i - l a y e r component of these multilayer structures. The absorption in the exponential or Urbach region is often taken as a measure of the disorder in an amorphous film.
Since Eo is a characteristic
energy which describes the width of the Urbach edge, this parameter provides a measure of the energy range over which the localized states at the edge of the band are significant.
There are three possible explanations for the increase
in the magnitude of Eo as the nipi layer thickness (or i - l a y e r thickness) decreases:
the increased importance in the structures with thinner i - l a y e r s of
(1) the n- and p-layers, (2) interfaces between layers, or (3) inhomogenieties in the i - l a y e r s near an interface, perhaps due to an increase in the H concentration.
Further experiments are necessary to determine which effect is the
dominant one. An estimate of the density of deep-level defects can be made by separating the exponential band-tail absorption from the subgap defect absorption.
It
has been suggested7 that the integrated absorption below the gap in a-Si:H
10 5
'~
10 4
i/II
..~ 103
.~
15o
/ lO 2
I
08
I
1.2
I
16
I
20
1
24
t 2
k 3
Energy (eV)
FIGURE 1 Dependence of the absorption in a-Si:H doping-modulated multilayer structures on the i - l a y e r thickness 1) di = 228 A, 2) di = 60 A.
FIGURE 2 Urbach edge parameter Eo vs. thickness of the i - l a y e r in dopingmodulated structures based on a-Si:H.
930
R. Durny et a L / Amorphous silicon doping-modulated multilayers
is proportional to the number of paramagnetic defects as measured by electron spin resonance (ESR). The proportionality constant appears to depend upon the preparation conditions.
Using a value determined by Amer and Jackson,7 the
absorption observed in all of the nipi layers reported in this paper corresponds to ~ 2 x 1018 defects cm-3.
Because the total thicknesses of the nipi struc-
tures varied by about a factor of three and the total i-layer thicknesses by about a factor of four while the measured absorption coefficient based on total sample thicknesses remained unchanged, we conclude that the below gap absorption is probably not due to the doped layers or to the interfaces. This absorption is relatively high for a typical i-layer of a-Si:H and i t may represent the diffusion of dopants into these regions. ACKNOWLEDGEMENTS One of us (R.D.) would l i k e to thank the University of Utah for their hospitality and the Slovak Technical University in Bratislava for granting a sabbatical leave.
The research at Utah was supported by the National Science
Foundation under grant number DMR-86-15217 and the Solar Energy Research Institute under contract number XM-5-05009-2. REFERENCES 1) B. Abeles and T. Tiedje, in Semiconductors and Semimetals, Vol. 21C (Academic Press, NY, 1984) p. 4D7. 2) J. Kakalios and H. Fritzsche, Phys. Rev. Lett. 53 (1984) 1602. 3) M. Hundhausen, L. Ley and R. Carius, Phys. Rev. Lett. 53 (1984) 1598. 4) S.C. Agarwal and S. Guha, Phys. Rev. B32 (1985) 8469. 5) D. Hanemanand D.H. Zhang, Phys. Rev. B35 (1987) 2536. 6) R.A. Street, J.C. Knights and D.K. Biegelsen, Phys. Rev. B18 (1978) 1880. 7) W.B. Jackson and N.M. Amer, Phys. Rev. B25 (1982) 5559.
927
ABSORPTION IN AMORPHOUSSILICON DOPING-MODULATED MULTILAYERS R. DURNY, S. DUCHARME, J. VINER and P.C. TAYLOR Department of Physics, University of Utah, Salt Lake City, Utah 84112, U.S.A. D. HANEMAN Department of Condensed Matter Physics, University of New South Wales, Kensington, Australia 2033 Photothermal deflection spectroscopy (PDS) measurementson doping-modulated multilayers of a-Si:H show that the measured width of the absorption due to band t a i l s increases as the relative i n t r i n s i c layer thickness decreases. This increased width is due to a wider distribution of band t a i l states in the n- and p-doped layers. A large absorption (~ 100 cm-1) well below the gap is observed in all samples. i . INTRODUCTION The high degree of control over the electronic and structural properties which can currently be achieved in hydrogenated amorphous silicon (a-Si:H) and related alloy systems suggests that i t may be possible to engineer new structures with novel electronic and optical properties. In analogy with recent advances in crystalline systems, many different types of multilayer structures based upon a-Si:H have recently been constructed. 1 A particularly interesting class of these multilayer structures is the doping superlattice where the host material remains unchanged from layer to layer but where the electronic properties of the layers can be changed dramatically by doping.
The
sequence most commonly employed is an n-doped layer, an undoped or i n t r i n s i c layer, a p-doped layer, and a second i n t r i n s i c layer.
To complete the multi-
layer structures this sequence is repeated a certain number of times and the resulting structures are often called n i p i ' s . One of the most interesting phenomenaobserved in these structures is persistent photoconductivity2-5 (PPC). The PPC in nipi structures based on a-Si:H is not well understood, but i t probably involves the activation of defects which may occur near the interfaces in the structure. For this reason information about defects introduced by doping, disorder or interfaces is of considerable importance for the ultimate understanding of the PPC effect. In the present paper we employ photothermal deflection spectroscopy (PDS) to measure optical absorption below the band gap in nipi structures based on a-Si:H. 0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R. Durny et a L / Amorphous silicon doping-modulated multilayers
928
2. EXPERIMENTAL The multilayer structure was prepared by radio frequency decomposition of silane doped with phosphine or diborane for n and p layers, respectively. 5 The deposition chamber was a 6 inch diameter stainless steel cross described elsewhere6 with a 2 inch diameter substrate heated uniformly to 220°C by a contacting oil bath.
The samples consisted of nipi amorphousmultilayers
with six periods (66 A thick n-layers, 33 A thick p-layers, and i-layers with thicknesses varying from 60 to 228 A).
The p- and n-doped layers were
produced with concentrations of approximately 0.01 at. % diborane and 0.1 at. % phosphine in silane, respectively.
Sampleswere deposited on glass substrates.
In order to check i f the samples exhibited the reported PPC phenomenonthe electrical conduction was measured in a coplanar configuration. To ensure contact to all the layers, the multilayered films were scratched before depositing the electrodes.
Each electrode was approximately 5 mmlong with a gap of
approximately I-2 mmbetween electrodes. The data were obtained at voltages for which the samples exhibited ohmic behavior. The absorption measurementswere made with a standard PDS system. In the PDS technqiue7 weak absorption is measured by illuminating the sample with the appropriate wavelength of l i g h t and detecting the absorbed power by observing the change in index of a l i q u i d which is in contact with the sample. For a sample approximately 1 pm thick on a glass substrate the sensitivity l i m i t was about 0.1 cm- I in the a-Si:H multilayer film.
Fringes in the original
absorption spectra have been removedby a suitable averaging procedure. Absorption in the glass substrate is negligible in the range over which the data were taken (~ 0.8-2.4 eV). 3. RESULTS AND DISCUSSION After a 30 s exposure at 300 K to 10 mW/cm2 heat-filtered, band-gap l i g h t , the dark conductivity of all of the multilayer structures increases from that in the annealed state A.
The sample exhibits a strong PPC as indicated by the
ratio of the conductivity 4 min. after illumination ~d to the dark conductivity in the annealed state A, ~A.
This ratio is greater than 100. Both the steady-
state photoconductivity ~p and the excess dark conductivity (~e = ~d - cA) exhibit the power law dependenceof current on incident l i g h t intensity which is usually observed. The PDS measurementsyield information on both the exponential band-tail absorption and the subgap defect (bulk and interface) absorption. Typical PDS spectra of two representative a-Si:H doping-modulated structures are shown in Fig. I.
From the f i t s to the exponential absorption in our samples, the widths
R. Durny et aL / Amorphous silicon doping-modulated multilayers of the exponential (Urbach) edge Eo were obtained. ness dependence of the Urbach energy Eo.
929
Figure 2 shows the thick-
This parameter sharply increases
with decreasing thickness of the i - l a y e r component of these multilayer structures. The absorption in the exponential or Urbach region is often taken as a measure of the disorder in an amorphous film.
Since Eo is a characteristic
energy which describes the width of the Urbach edge, this parameter provides a measure of the energy range over which the localized states at the edge of the band are significant.
There are three possible explanations for the increase
in the magnitude of Eo as the nipi layer thickness (or i - l a y e r thickness) decreases:
the increased importance in the structures with thinner i - l a y e r s of
(1) the n- and p-layers, (2) interfaces between layers, or (3) inhomogenieties in the i - l a y e r s near an interface, perhaps due to an increase in the H concentration.
Further experiments are necessary to determine which effect is the
dominant one. An estimate of the density of deep-level defects can be made by separating the exponential band-tail absorption from the subgap defect absorption.
It
has been suggested7 that the integrated absorption below the gap in a-Si:H
10 5
'~
10 4
i/II
..~ 103
.~
15o
/ lO 2
I
08
I
1.2
I
16
I
20
1
24
t 2
k 3
Energy (eV)
FIGURE 1 Dependence of the absorption in a-Si:H doping-modulated multilayer structures on the i - l a y e r thickness 1) di = 228 A, 2) di = 60 A.
FIGURE 2 Urbach edge parameter Eo vs. thickness of the i - l a y e r in dopingmodulated structures based on a-Si:H.
930
R. Durny et a L / Amorphous silicon doping-modulated multilayers
is proportional to the number of paramagnetic defects as measured by electron spin resonance (ESR). The proportionality constant appears to depend upon the preparation conditions.
Using a value determined by Amer and Jackson,7 the
absorption observed in all of the nipi layers reported in this paper corresponds to ~ 2 x 1018 defects cm-3.
Because the total thicknesses of the nipi struc-
tures varied by about a factor of three and the total i-layer thicknesses by about a factor of four while the measured absorption coefficient based on total sample thicknesses remained unchanged, we conclude that the below gap absorption is probably not due to the doped layers or to the interfaces. This absorption is relatively high for a typical i-layer of a-Si:H and i t may represent the diffusion of dopants into these regions. ACKNOWLEDGEMENTS One of us (R.D.) would l i k e to thank the University of Utah for their hospitality and the Slovak Technical University in Bratislava for granting a sabbatical leave.
The research at Utah was supported by the National Science
Foundation under grant number DMR-86-15217 and the Solar Energy Research Institute under contract number XM-5-05009-2. REFERENCES 1) B. Abeles and T. Tiedje, in Semiconductors and Semimetals, Vol. 21C (Academic Press, NY, 1984) p. 4D7. 2) J. Kakalios and H. Fritzsche, Phys. Rev. Lett. 53 (1984) 1602. 3) M. Hundhausen, L. Ley and R. Carius, Phys. Rev. Lett. 53 (1984) 1598. 4) S.C. Agarwal and S. Guha, Phys. Rev. B32 (1985) 8469. 5) D. Hanemanand D.H. Zhang, Phys. Rev. B35 (1987) 2536. 6) R.A. Street, J.C. Knights and D.K. Biegelsen, Phys. Rev. B18 (1978) 1880. 7) W.B. Jackson and N.M. Amer, Phys. Rev. B25 (1982) 5559.