Transient photomodulation studies of phosphorus doped a-Si:H

292

Journal of Non-Crystalline Solids 114 (1989) 292-294 North-Holland

TRANSIENT PHOTOMODULATION STUDIES OF PHOSPHORUS DOPED a-Si:H T.X. ZHOU* and J. TAUC Department of Physics and Division of Engineering, Brown University, Providence, Rhode Island 02912 Z.V. VARDENY Department of Physics, University of Utah, Salt Lake City, Utah 84112 Time-resolved photomodulation spectra of P-doped a-Si:H were measured from 10-7 to 10-2 sec, at temperature between 80 to 220 K using probe wavelengths ranging from 0.75 to 5.5 ~tm. The spectra are compared with those in undoped a-Si:H and the origin of the differences in the recombination processes in the materials are discussed. 1. INTRODUCTION

a pump beam for photogeneration of carries and a probe

The transient photomodulation (TPM) spectroscopy

beam for measuring the photoinduced changes AT in the

technique has provided a wealth of information on the

sample transmission T. The pump beam was a dye laser

thermalization, trapping and recombination processes of photogenerated carries in undoped a-Si:H1,2. In this work

producing 10 ns pulses at 2.1 eV with more than 100 laJ per

we describe the application of the TPM technique to phosphorus (P-) doped a-Si:H and compare our results to those in undoped a-Si:H. Our main results are the following.

In undoped a-Si:H the. TPM spectra are

dominated by holes in the valence band (VB) tail which

pulse and a repetition rate of 20 Hz; the diameter of the beam was about lcm so that the maximum photogenerated carder density was more than 1019 cm -3. The probe wavelength was selected using a variety of incandescent light sources in combination with optical bandpass filters. The system

thermalize into the gap and therefore the onset of the

response was accounted for by taking the ratio AT/T. Since

photoinduced absorption (PA) band substantially shifts to

the photoinduced change in reflectivity can be neglected,

higher energies with time and temperature. However, the

AT/T is proportional to Aa, the photoinduced change in the

TPM spectra of P-doped a-Si:H (a-Si:H:P) depend very little on time and temperature indicating that the carrier

absorption coefficient ct, and correspondingly to N(t) the

distribution over the gap states is completed prior to the

density of photogenerated carders.

onset of recombination. The electron-hole recombination

The P-doped and undoped a-Si:H samples were 41xm

kinetics in both doped and undoped a-Si:H at all

thick films deposited by the glow discharge technique on

temperatures is governed by electron dynamics with

sapphire substrates. The optical interference fringes were

bimolecular kinetics as well as by monomolecular tunneling. In doped materials the tunneling involves band tails states

close enough in energy that the spectral range of the filters contained more than three fringe periods and provided an

close to the Fermi energy (EF) whereas in undoped materials

effective averaging. Data were taken at sample temperatures

the tunneling is into the dangling bond (DB) defects.

of 80, 150 and 220K with various pump intensities.

2. EXPERIMENTAL

3. RESULTS AND DISCUSSION

Two light sources are needed for the TPM spectroscopy2:

The TPM spectra at 80 K from 10-6 to 10-2 sec of

* Current address: Institute of Energy Conversion, University of Delaware, Newark, Delaware 19716. 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

293

T.X, Zhou et M. / Transient photomodulation studies

a-Si:H:P and a-Si:H are shown in Figs. 1 and 2. It is seen that the shape of the TPM spectra in a-Si:H:P do not change appreciably with time, whereas the low energy part of the PA in the undoped sample decays faster than the rest of the

and temperature because recombination involving the BT states4 is fast at high temperatures. The higher energy PA band is ascribed to transitions involving charged defects (D +

spectrum. The difference is much clearer when the TPM

and D-); the time and temperature dependence of this band are much weaker due to much slower recombination rates

spectra are compared at different temperatures in Figs. 3 and

associated with carders on DB defects 5. The low energy

4 where we show the spectra measured at 100 ~ts after the excitation. The spectra of a-Si:H:P do not change

onset in a-Si:H:P spectra, on the other hand, is due 2 to electrons trapped in impurity defects P4 +. Its weak time

appreciably with temperature while those of undoped a-Si:H

and temperature dependence indicate that electron

change significantly; the low energy part of the spectra

thermalization is completed prior to our transient

decays with temperature much more than the high energy part. It has been shown that the low energy PA band in the

measurements; the PA band onset is therefore associated with the steady-state quasi-Fermi level F N for the electrons.

undoped a-Si:H spectra is produced by holes trapped in the VB bandtail (BT) 3. Its strength depends strongly on time

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Figure 1 Transient photomodulation (TPM) spectra of a-Si:H:P at 80K at various time delays following pulsed excitation.

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Figure 3 TPM spectra at various temperatures at a fixed time delay (100kts) following the pulsed excitation in a-Si:H:P. i

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Figure 4 The same as in Fig. 3 but for undoped a-Si:H (from Ref. 1)

294

T.X. Zhou et M. / Transient photomodulation studies

The higher energy PA band in this material is associated with holes trapped at dopant related DB defects D-(p)2,6; its

I I ' l [

decay follows the decay of the lower energy counterpart because the electrons and holes decay together. In Fig.5 we show the decays of the integrated P4 + PA

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band in a-Si:H:P. The decays are not in the form of a single power law t-13with a constant [3 which is expected for dispersive recombination involving a single recombination channel 1,7. Stoddart et al. have shown 1 that an increase of

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13with time can be explained by a temperature independent monomolecular recombination by tunneling. However, the observed temperature dependence of the decays in a-Si:H'P is not in agreement with a temperature independent tunneling

Figure 6 The same as in Fig.5 but for various excitation intensifies in the range from 4 to 200 ~tJ per pulse at 220K.

process. Moreover, Fig.6 shows that the recombination rate depends on excitation intensity, indicating a process of bimolecular recombination kinetics. We are therefore

ACKNOWI.ErX3EMENTS

considering a model of the PA decay in a-Si:H:P based on

We thank W. Paul for providing the samples. We also

two recombination channels: The dominant channel at short

thank H.A. Stoddart, T.R. Kirst and L.Chen for technical

times and high carrier density is bimolecular, involving

assistance. This work was supported in part by NSF grant

thermal release of electrons from FN; this explains the

DMR 8706289.

temperature and excitation intensity dependences (Figs.5,6). The second recombination channel which prevails at longer times and low photocarrier density is the direct tunneling between trapped holes in doping related DB and the majority carrier electrons residing near EF6.

REFERENCES 1. H.A. Stoddart, Z. Vardeny, and J. Tauc, Phys. Rev. B38 (1988) 1362. 2. Z. Vardeny, T.X. Zhou, H.A. Stoddart and J. Tauc, Sol.State Commun. 65 (1988) 1049. 3. P. O'Connor and J. Tauc, Phys. Rev. B23 (1982) 2748. 4. D. Monroe, Phys. Rev. Lett.. 54 (1985) 146.

-2

5. R.A. Street, D.R. Biegelsen and R. L. Weisfield, Phys. Rev. B30 (1984) 5861. i

6. R.A. Street, J. Non-Cryst. Solids 77&78 (1985) 1. 7. J. Orenstein and M.A. Kastner, Sol. State Commun. 40 (1981) 85. I . . . .

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Figure 5 Integrated PA decay for the P4+ band in a-Si:H:P at various temperatures.

8. In a-Si:H, bimolecular recombination was observed by

Z. Vardeny, P.O'Connor, S. Ray, and J. Tauc, Phys. Rev. Lett. 44 (1980)1267;46 (1981)1108.