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Optical spectroscopy of hydrogenated amorphous silicon
Solar Cells, 24 (1988) 299 - 305
299
OPTICAL SPECTROSCOPY OF H Y D R O G E N A T E D AMORPHOUS SILICON* JACQUES I. PANKOVE
Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) University of Colorado, Boulder, CO (U.S.A.) (Received November 16, 1987; accepted December 22, 1987)
Summary Important properties of all semiconductors can be accessed via optical spectroscopy. The techniques described here are absorption, luminescence and Raman spectroscopies. These methods reveal the optical gap, the density of states in the gap, structural information and the carrier lifetime.
1. Introduction Characterization techniques based on optical measurements are nondestructive and yield a great variety of information. Optical measurements are widely used to study all the semiconductors. Here we are primarily interested in the use of these techniques in probing hydrogenated amorphous silicon (a-Si:H). The main properties that are accessed optically are the energy gap, information related to the distribution of states in the gap and to the carrier lifetime, and recombination processes. Microcrystallinity and structural quality can also be assessed optically. We shall n o t consider here those techniques that involve a combination of absorption and electric-field
© Elsevier Sequoia/Printed in The Netherlands
300 coefficient as a function of p h o t o n energy is shown in Fig. 1. In this semilogarithmic plot that spans about four decades, one can isolate three characteristic regions: the intrinsic absorption from which one derives the energy gap, the exponential Urbach tail and the sub-band-gap absorption region. We shall consider these in turn. 2.1. Intrinsic a b s o r p t i o n
At the highest photon energies, the absorption process consists in exciting valence band electrons to the conduction band. If the bands are assumed to be parabolic (the density of states increasing with the square root of the energy measured from the band edge) and if we consider that any state in the valence band can be linked to any state in the conduction band ( m o m e n t u m conservation is no problem in a-Si:H), then the absorption coefficient a is given by Tauc's empirical relationship [3] A = h---v (hv - - Eg) 2
where A is a constant and Eg is the energy gap. If one plots (ahv) 1/2 vs. hv, one obtains a straight line that intercepts the h v axis at Eg (Fig. 2). This is the standard m e t h o d for determining the energy gap of amorphous semiconductors. 2.2. T h e Urbach edge
In an amorphous semiconductor, the band edges are fuzzy. The lack of sharpness (Fig. 3) is due to defect states and to a great variation in bond energies from site to site, and to the variation in local stresses. Some bonds are stretched and some are bent; there are microvoids {tensile stress) and large impurities (compressive stress). Therefore, an electron from a highenergy spot in the valence band can reach a low-energy spot in the conducLog
ic
tion
Sub-bandgap absorption
hv Fig. 1. Typical absorption spectrum of a-Si:H.
301
/ Eg
O3
h.~
VB
Fig. 2. Tauc plot to determine the energy gap. Fig. 3. Perturbed band edges and tunneling-assisted absorption between tail states.
tion band if it is excited by a p h o t o n at an energy lower than the band gap with tunneling to the final state in the conduction band. It can be shown that the tunneling process leads to an exponential dependence; however, the exponential tail of states for each band dominates the tilt of the Urbach edge. The greater the perturbation of the band edges, the less abrupt is the absorption edge in the Urbach region; thus, adding germanium or carbon to a-Si:H broadens the Urbach region.
2.3. The sub-band-gap region At low p h o t o n energies (below 1.2 eV) an additional absorption process appears which involves states near the midgap. These states could be silicon dangling bonds forming a b u m p in the density of states as shown in Fig. 4. If the Fermi level is near the midgap, optical excitation can couple valence states to this midgap region and electrons from the midgap can be raised to the conduction band. Some of the midgap states are silicon dangling bonds that can be identified unambiguously by electron spin resonance (ESR) [4]. The sub-band-gap absorption occurs at low absorption coefficients where the usual transmission measurement is not sensitive. Fortunately, photothermal deflection spectroscopy (PDS) [5] is a very sensitive tech-
*5 c
v
E Fig. 4. Typical density of states distribution in a-Si:H.
302
nique for this measurement. PDS is illustrated in Fig. 5. The light from a monochromator is focused on a sample immersed in a transparent medium such as CC14. The absorbed energy is converted to heat, causing a temperature gradient to develop in the liquid. The refractive index of the liquid increases with temperature and therefore forms a gradient of refractive index. The beam from an He-Ne laser propagating near the sample surface is deflected by the refractive gradient. A position-sensitive detector converts this displacement into a signal that is related to the absorption constant. A comparison of PDS results with ESR data (Fig. 6) has shown a good correlation between the sub-band-gap absorption and the presence of silicon dangling bonds [6]. Light-induced metastability in a-Si:H creates silicon dangling bonds that can be detected more easily by PDS than by ESR. This is shown in Fig. 7. Light soaking increases the sub-band-gap absorption whereas annealing reduces it.
3. Photoluminescence Light at energies greater than the band gap is used to create electronhole pairs that relax to their lowest energy states (Fig. 8). From there, if the carriers recombine radiatively, photoluminescence is obtained, producing a peak at 1.2 + 0.1 eV. The luminescence process in a-Si:H has been attributed to tail-to-tail transitions [ 7 ]. However, one important observation points to a donor-to-acceptor transition [8]. It is true that it is difficult to separate the donors from the tail states of the conduction band or the acceptors from the valence band tail. However, the observation is that the emission spectrum does not depend on the excitation intensity. If the radiative transitions were from tail to tail, then the emission spectrum would shift to higher energies as the quasi-Fermi levels move deeper into the tails under increased excitation intensity. Since the spectrum does not depend on the excitation inten-
[f
Position Sensor .
.
.
.
T
"
K) 'e
Pump loser (or Broadband source)
Semple
~Chopper~ j,-Probe
p
~7
o
z~lO '6
laser {0'6
{0n'
Ns(ESR)
iO'e
Fig. 5. Experimental arrangement for photothermal deflection spectroscopy (courtesy N. M. Amer and W. B. Jackson). Fig. 6. Correlation between the number of defects deduced from PDS and ESR in undoped a-Si:H [6].
303
(38 cl cm-J
10 3
iO 2
I0
I 08
I 1.2 hv(eV)
I 1.6
Density of states lib
Fig. 7. Effects of light soaking and annealing on the PDS spectrum of a-Si:H (courtesy of N. M. Amer and W. B. Jackson). Fig. 8. Excitation and luminescence transitions in the photoluminescence of a-Si:H; D and A are donors and acceptors respectively.
sity, the emission must come from a strongly optically coupled set of states energetically located between the two quasi-Fermi levels. Donors and acceptors could be such states. Another useful luminescence technique is time-resolved spectroscopy wherein the luminescence decay is monitored as a function of time after pulsed excitation [7, 9]. A power law is usually obtained over many decades. This dependence has been interpreted in terms of a multiple-trapping model [10]. The efficiency of luminescence is dominated by non-radiative recombination processes. These cause a decrease in carrier lifetime. Hence, the luminescence efficiency has a bearing on the quality of a photovoltaic material where the survival of photogenerated carriers is extremely important. An example of lifetime reduction is that from light-induced metastable states that are silicon dangling bonds forming non-radiative recombination centers [11]. They cause a fatigue of the main luminescence peak and an enhancement at about 0.8 eV. The 0.8 eV luminescence is attributed to a small percentage of radiative transitions involving near-midgap states that could also be silicon dangling bonds [11].
4. Raman scattering To measure Raman scattering spectroscopy, a laser beam is incident on the sample at such an angle that there is no reflection of the incident beam into the spectrometer. A high-resolution spectrometer is used so that one can
304 measure the frequency-shifted scattered radiation while rejecting the laser light reflected by surface imperfections. Raman scattering is the technique of choice for assessing the structural quality of a material. The incident p h o t o n is scattered by optical phonons. These are vibrational modes of adjacent atoms. In a perfect crystal, every atom has a similar neighbor, therefore all the vibrating atoms produce a narrow range of p h o n o n modes. In a-Si:H, however, each atom has different sets of neighbors (some atoms are more distant, vibrating at a lower frequency) and some bonds are more stressed angularly. Hence, a broad range of modes is possible in an amorphous material. In the Raman scattering process, the incident p h o t o n s set in m o t i o n all the vibrational modes that are possible and escape with a reduced energy. Their energy is reduced by the p h o n o n energy. In a single crystal, the downshifting of the p h o t o n energy occurs over a narrow range, producing a sharp spectral line. However, in amorphous materials, where a broad range of p h o n o n s is possible, the Raman-shifted spectrum forms a broad band. Figure 9 shows the Raman spectra obtained with amorphous and microcrystalline silicon. In a-Si:H, only a broad spectrum is obtained. In the microcrystalline material, both a narrow line and a broad spectrum are obtained. The microcrystals produce the narrow line and the broad spectrum is attributable to a matrix of a-Si:H that surrounds the microcrystals. 478
523
I
I
t ,// , "-~
c
'
J5 /'
//
// . /
494
/
',
//
,
/
t
/
c //
c M
400
420
440
460
480
500
52O
54O
Raman shift (cm-~)
Fig. 9. Raman spectra of a-Si:H and of microcrystalline hydrogenated silicon: - - , DL164, p-type pc-Si:H; - - - - - - , DL158, p-type a-Si:H. (Data courtesy A. Mascarenhas using Chronar material [ 12 ].) 5. Conclusion Optical spectroscopy in its many forms (absorption, reflectance, luminescence and Raman scattering} is a powerful technique for determining fundamental properties of semiconductors. Reflectance spectroscopy was
305
not discussed in this paper because it is more useful in crystalline semiconductors (see the following paper by Kurtz et al. [13] ). Extending IR absorption spectroscopy to much longer wavelengths allows a determination of the vibrational modes, especially those that determine Sill, Sill 2 and SiO bonds [14].
References 1 R. S. Crandall, Hydrogenated amorphous silicon, in J. I. Pankove (ed.), Semiconductors and Semimetals, Vol. 21B, Academic Press, New York, 1984, pp. 245 - 297. 2 J. D. Cohen, A. V. Gelatos, K. K. Mahavadi and K. Zellama, Sol. Cells, 24 (1988) 287. 3 J. Tauc, R. Grigorovici and A. Vancu, Phys. Status Solidi, 15 (1966) 727. 4 R. A. Street, Phys. Rev. B, 24 (1981) 969. 5 W. B. Jackson, N. M. Amer, A. C. Boccara and D. Fournier, Appl. Opt., 20 (1981) 1333. 6 W. B. Jackson and N. M. Amer, Phys. Rev. B, 25 (1982) 5559. 7 R. A. Street, Hydrogenated amorphous silicon, in J. I. Pankove (ed.), Semiconductors and Semimetals, Vol. 21B, Academic Press, New York, 1984, pp. 197 - 244. 8 J. I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, New York, 1971, Dover, New York, 1975, p. 148. 9 S. Kurita, W. Czaja and S. Kinmond, Solid State Commun., 32 (1979) 879. 10 T. Tiedje and A. Rose, Solid State Commun., 37 (1981) 49. 11 J. I. Pankove and J. E. Berkeyheiser, Appl. Phys. Lett., 37 (1980) 705. 12 A. Delahoy, paper presented at the 8th Photovoltaic Advanced Research and Development Project Review Meeting, Denver, CO, U.S.A., 1987. 13 S. Kurtz, J. M. Olsen and A. Kibbler, Sol. Cells, 24 (1988) 307. 14 P. J. Zanzucchi, Hydrogenated amorphous silicon, in J. I. Pankove (ed.), Semiconductors and Semimetals, Vol. 21B, Academic Press, New York, 1984, pp. 113 - 140.
299
OPTICAL SPECTROSCOPY OF H Y D R O G E N A T E D AMORPHOUS SILICON* JACQUES I. PANKOVE
Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) University of Colorado, Boulder, CO (U.S.A.) (Received November 16, 1987; accepted December 22, 1987)
Summary Important properties of all semiconductors can be accessed via optical spectroscopy. The techniques described here are absorption, luminescence and Raman spectroscopies. These methods reveal the optical gap, the density of states in the gap, structural information and the carrier lifetime.
1. Introduction Characterization techniques based on optical measurements are nondestructive and yield a great variety of information. Optical measurements are widely used to study all the semiconductors. Here we are primarily interested in the use of these techniques in probing hydrogenated amorphous silicon (a-Si:H). The main properties that are accessed optically are the energy gap, information related to the distribution of states in the gap and to the carrier lifetime, and recombination processes. Microcrystallinity and structural quality can also be assessed optically. We shall n o t consider here those techniques that involve a combination of absorption and electric-field
© Elsevier Sequoia/Printed in The Netherlands
300 coefficient as a function of p h o t o n energy is shown in Fig. 1. In this semilogarithmic plot that spans about four decades, one can isolate three characteristic regions: the intrinsic absorption from which one derives the energy gap, the exponential Urbach tail and the sub-band-gap absorption region. We shall consider these in turn. 2.1. Intrinsic a b s o r p t i o n
At the highest photon energies, the absorption process consists in exciting valence band electrons to the conduction band. If the bands are assumed to be parabolic (the density of states increasing with the square root of the energy measured from the band edge) and if we consider that any state in the valence band can be linked to any state in the conduction band ( m o m e n t u m conservation is no problem in a-Si:H), then the absorption coefficient a is given by Tauc's empirical relationship [3] A = h---v (hv - - Eg) 2
where A is a constant and Eg is the energy gap. If one plots (ahv) 1/2 vs. hv, one obtains a straight line that intercepts the h v axis at Eg (Fig. 2). This is the standard m e t h o d for determining the energy gap of amorphous semiconductors. 2.2. T h e Urbach edge
In an amorphous semiconductor, the band edges are fuzzy. The lack of sharpness (Fig. 3) is due to defect states and to a great variation in bond energies from site to site, and to the variation in local stresses. Some bonds are stretched and some are bent; there are microvoids {tensile stress) and large impurities (compressive stress). Therefore, an electron from a highenergy spot in the valence band can reach a low-energy spot in the conducLog
ic
tion
Sub-bandgap absorption
hv Fig. 1. Typical absorption spectrum of a-Si:H.
301
/ Eg
O3
h.~
VB
Fig. 2. Tauc plot to determine the energy gap. Fig. 3. Perturbed band edges and tunneling-assisted absorption between tail states.
tion band if it is excited by a p h o t o n at an energy lower than the band gap with tunneling to the final state in the conduction band. It can be shown that the tunneling process leads to an exponential dependence; however, the exponential tail of states for each band dominates the tilt of the Urbach edge. The greater the perturbation of the band edges, the less abrupt is the absorption edge in the Urbach region; thus, adding germanium or carbon to a-Si:H broadens the Urbach region.
2.3. The sub-band-gap region At low p h o t o n energies (below 1.2 eV) an additional absorption process appears which involves states near the midgap. These states could be silicon dangling bonds forming a b u m p in the density of states as shown in Fig. 4. If the Fermi level is near the midgap, optical excitation can couple valence states to this midgap region and electrons from the midgap can be raised to the conduction band. Some of the midgap states are silicon dangling bonds that can be identified unambiguously by electron spin resonance (ESR) [4]. The sub-band-gap absorption occurs at low absorption coefficients where the usual transmission measurement is not sensitive. Fortunately, photothermal deflection spectroscopy (PDS) [5] is a very sensitive tech-
*5 c
v
E Fig. 4. Typical density of states distribution in a-Si:H.
302
nique for this measurement. PDS is illustrated in Fig. 5. The light from a monochromator is focused on a sample immersed in a transparent medium such as CC14. The absorbed energy is converted to heat, causing a temperature gradient to develop in the liquid. The refractive index of the liquid increases with temperature and therefore forms a gradient of refractive index. The beam from an He-Ne laser propagating near the sample surface is deflected by the refractive gradient. A position-sensitive detector converts this displacement into a signal that is related to the absorption constant. A comparison of PDS results with ESR data (Fig. 6) has shown a good correlation between the sub-band-gap absorption and the presence of silicon dangling bonds [6]. Light-induced metastability in a-Si:H creates silicon dangling bonds that can be detected more easily by PDS than by ESR. This is shown in Fig. 7. Light soaking increases the sub-band-gap absorption whereas annealing reduces it.
3. Photoluminescence Light at energies greater than the band gap is used to create electronhole pairs that relax to their lowest energy states (Fig. 8). From there, if the carriers recombine radiatively, photoluminescence is obtained, producing a peak at 1.2 + 0.1 eV. The luminescence process in a-Si:H has been attributed to tail-to-tail transitions [ 7 ]. However, one important observation points to a donor-to-acceptor transition [8]. It is true that it is difficult to separate the donors from the tail states of the conduction band or the acceptors from the valence band tail. However, the observation is that the emission spectrum does not depend on the excitation intensity. If the radiative transitions were from tail to tail, then the emission spectrum would shift to higher energies as the quasi-Fermi levels move deeper into the tails under increased excitation intensity. Since the spectrum does not depend on the excitation inten-
[f
Position Sensor .
.
.
.
T
"
K) 'e
Pump loser (or Broadband source)
Semple
~Chopper~ j,-Probe
p
~7
o
z~lO '6
laser {0'6
{0n'
Ns(ESR)
iO'e
Fig. 5. Experimental arrangement for photothermal deflection spectroscopy (courtesy N. M. Amer and W. B. Jackson). Fig. 6. Correlation between the number of defects deduced from PDS and ESR in undoped a-Si:H [6].
303
(38 cl cm-J
10 3
iO 2
I0
I 08
I 1.2 hv(eV)
I 1.6
Density of states lib
Fig. 7. Effects of light soaking and annealing on the PDS spectrum of a-Si:H (courtesy of N. M. Amer and W. B. Jackson). Fig. 8. Excitation and luminescence transitions in the photoluminescence of a-Si:H; D and A are donors and acceptors respectively.
sity, the emission must come from a strongly optically coupled set of states energetically located between the two quasi-Fermi levels. Donors and acceptors could be such states. Another useful luminescence technique is time-resolved spectroscopy wherein the luminescence decay is monitored as a function of time after pulsed excitation [7, 9]. A power law is usually obtained over many decades. This dependence has been interpreted in terms of a multiple-trapping model [10]. The efficiency of luminescence is dominated by non-radiative recombination processes. These cause a decrease in carrier lifetime. Hence, the luminescence efficiency has a bearing on the quality of a photovoltaic material where the survival of photogenerated carriers is extremely important. An example of lifetime reduction is that from light-induced metastable states that are silicon dangling bonds forming non-radiative recombination centers [11]. They cause a fatigue of the main luminescence peak and an enhancement at about 0.8 eV. The 0.8 eV luminescence is attributed to a small percentage of radiative transitions involving near-midgap states that could also be silicon dangling bonds [11].
4. Raman scattering To measure Raman scattering spectroscopy, a laser beam is incident on the sample at such an angle that there is no reflection of the incident beam into the spectrometer. A high-resolution spectrometer is used so that one can
304 measure the frequency-shifted scattered radiation while rejecting the laser light reflected by surface imperfections. Raman scattering is the technique of choice for assessing the structural quality of a material. The incident p h o t o n is scattered by optical phonons. These are vibrational modes of adjacent atoms. In a perfect crystal, every atom has a similar neighbor, therefore all the vibrating atoms produce a narrow range of p h o n o n modes. In a-Si:H, however, each atom has different sets of neighbors (some atoms are more distant, vibrating at a lower frequency) and some bonds are more stressed angularly. Hence, a broad range of modes is possible in an amorphous material. In the Raman scattering process, the incident p h o t o n s set in m o t i o n all the vibrational modes that are possible and escape with a reduced energy. Their energy is reduced by the p h o n o n energy. In a single crystal, the downshifting of the p h o t o n energy occurs over a narrow range, producing a sharp spectral line. However, in amorphous materials, where a broad range of p h o n o n s is possible, the Raman-shifted spectrum forms a broad band. Figure 9 shows the Raman spectra obtained with amorphous and microcrystalline silicon. In a-Si:H, only a broad spectrum is obtained. In the microcrystalline material, both a narrow line and a broad spectrum are obtained. The microcrystals produce the narrow line and the broad spectrum is attributable to a matrix of a-Si:H that surrounds the microcrystals. 478
523
I
I
t ,// , "-~
c
'
J5 /'
//
// . /
494
/
',
//
,
/
t
/
c //
c M
400
420
440
460
480
500
52O
54O
Raman shift (cm-~)
Fig. 9. Raman spectra of a-Si:H and of microcrystalline hydrogenated silicon: - - , DL164, p-type pc-Si:H; - - - - - - , DL158, p-type a-Si:H. (Data courtesy A. Mascarenhas using Chronar material [ 12 ].) 5. Conclusion Optical spectroscopy in its many forms (absorption, reflectance, luminescence and Raman scattering} is a powerful technique for determining fundamental properties of semiconductors. Reflectance spectroscopy was
305
not discussed in this paper because it is more useful in crystalline semiconductors (see the following paper by Kurtz et al. [13] ). Extending IR absorption spectroscopy to much longer wavelengths allows a determination of the vibrational modes, especially those that determine Sill, Sill 2 and SiO bonds [14].
References 1 R. S. Crandall, Hydrogenated amorphous silicon, in J. I. Pankove (ed.), Semiconductors and Semimetals, Vol. 21B, Academic Press, New York, 1984, pp. 245 - 297. 2 J. D. Cohen, A. V. Gelatos, K. K. Mahavadi and K. Zellama, Sol. Cells, 24 (1988) 287. 3 J. Tauc, R. Grigorovici and A. Vancu, Phys. Status Solidi, 15 (1966) 727. 4 R. A. Street, Phys. Rev. B, 24 (1981) 969. 5 W. B. Jackson, N. M. Amer, A. C. Boccara and D. Fournier, Appl. Opt., 20 (1981) 1333. 6 W. B. Jackson and N. M. Amer, Phys. Rev. B, 25 (1982) 5559. 7 R. A. Street, Hydrogenated amorphous silicon, in J. I. Pankove (ed.), Semiconductors and Semimetals, Vol. 21B, Academic Press, New York, 1984, pp. 197 - 244. 8 J. I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, New York, 1971, Dover, New York, 1975, p. 148. 9 S. Kurita, W. Czaja and S. Kinmond, Solid State Commun., 32 (1979) 879. 10 T. Tiedje and A. Rose, Solid State Commun., 37 (1981) 49. 11 J. I. Pankove and J. E. Berkeyheiser, Appl. Phys. Lett., 37 (1980) 705. 12 A. Delahoy, paper presented at the 8th Photovoltaic Advanced Research and Development Project Review Meeting, Denver, CO, U.S.A., 1987. 13 S. Kurtz, J. M. Olsen and A. Kibbler, Sol. Cells, 24 (1988) 307. 14 P. J. Zanzucchi, Hydrogenated amorphous silicon, in J. I. Pankove (ed.), Semiconductors and Semimetals, Vol. 21B, Academic Press, New York, 1984, pp. 113 - 140.