Doping and temperature dependence of paramagnetic states in microcrystalline silicon

Journal of Non-Crystalline Solids 227–230 Ž1998. 1026–1030

Doping and temperature dependence of paramagnetic states in microcrystalline silicon ) J. Muller , F. Finger, C. Malten, H. Wagner ¨ Forschungszentrum Julich, ISI-PV, D-52425 Julich, Germany ¨ ¨

Abstract Continuous wave and pulsed electron spin resonance ŽESR. in the dark and under light illumination are used to study the doping and temperature dependence of defect, conduction electron and dopant states in microcrystalline silicon. Two resonances are found, which are attributed to silicon dangling bond states and located in different structural environments of the material. Light-induced ESR suggests that these two states have different energies, while in dark ESR, both show a wide and flat distribution in energy. The intensity of a third resonance from conduction electrons shows an almost 1:1 correlation with conductivity over three orders of magnitude. All resonances exhibit a Curie-like temperature dependence and the linewidth of the conduction electron resonance increases with temperature due to a decrease in the spin lattice relaxation time. Hyperfine interaction with phosphorus in n-type material and indications of hole states in p-type material are reported. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Temperature dependence; Paramagnetic states; Microcrystalline silicon

1. Introduction Electron spin resonance ŽESR. experiments give valuable information on type, energy, microscopic environment, and involvement in charge carrier recombination processes of the electronic states, in particular defect states, in microcrystalline silicon Ž m c-Si:H.. A complication for the interpretation of ESR measurements in m c-Si:H arises from the heterogeneous structure of the material m c-Si:H w1x which consists of coherent crystalline regions of a few 10 nm with smaller defect density forming larger size columnar clusters which are surrounded by hydrogen- and defect-rich zones. The crystalline )

Corresponding author. Tel.: q49-2461-61-3932; fax: q492461-61-3735; e-mail: [email protected].

regions can be intermixed with amorphous material or areas of smaller density Žvoids.. Contributions from the different structural domains may be resolved by using material with largely different structural properties Žsuch as grain size and crystalline volume fractions. or Fermi level positions. This problem has been addressed in the past and several ESR signals with different g-values have been identified and attributed to defect states Ž g s 2.0052, 2.0043. and conduction electrons ŽCE, 1.9978 F g F 1.9983. w2x. The resonance at g s 2.0052 is due to Si dangling bond ŽDB. states ŽPSi ' Si., which has been concluded from electron irradiation experiments w3x. The line at g s 2.0043 could possibly be related to DB states in silicon-rich Si–O layers w4,5x, as oxygen take-up is frequently observed in this granular material ŽP. Hapke, A. Muck, ¨ private com-

0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 3 4 7 - 0

J. Muller ¨ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1026–1030

munication.. Conclusive evidence for such an identification is still missing. The dependence of these resonances upon light illumination ŽLESR. and the recombination of charge carriers were investigated and discussed in a schematic band diagram w6,7x, and the spin lattice relaxation times, T1 , were measured by pulsed ESR w8x. In the present study, the emphasis is on Ž1. the variation in intensity of the three dominant ESR and LESR signal contributions upon a shift of the Fermi level by doping—to determine the electronic states on an energy scale, Ž2. the temperature dependence of intensity and linewidth of the resonances, in particular the CE signal, and Ž3. the investigation of donor and acceptor states.

2. Experiment Samples were prepared with very high frequency Ž49 to 115 MHz. plasma enhanced chemical vapour deposition at 2008C from mixtures of silane and hydrogen on glass and aluminium foil. The material has a crystallinity ) 90% and coherent crystalline regions typically 20 to 30 nm in diameter w1x. For pand n-type doping, diborane or phosphine Ž1–60 ppm in the gas phase. were added, resulting in room temperature ŽRT. dark conductivities of 3 P 10y2 Srcm for both p-type and n-type material. CW- and pulsed-ESR studies ŽX-band. were performed on powder samples Ž40 to 70 mg. which were sealed in quartz tubes under He atmosphere. The temperature T was varied between 4.5 K and 300 K. LESR experiments were carried out with the heat filtered light of a 100-W halogen lamp.

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Fig. 1. CW-ESR spectra of microcrystalline silicon with similar n-type and p-type doping levels recorded at 40 K in the dark Žsolid lines. and numerical fits to the data Ždotted lines..

tional exponential wing to simulate the asymmetry of the CE line, the spectra of samples with a wide range of p- and n-type dark conductivities have been deconvoluted and the resulting intensities are plotted in Fig. 2. Included are samples with no, or only residual n-type doping. The two resonances at g s 2.0052 and g s 2.0043 are found with little change in intensity over the entire doping range, i.e., upon shifts of the Fermi level. Both resonances decrease for the largest p-type doping and increase for the largest n-type doping. In contrast to the resonances at g s 2.0052 and g s 2.0043, only the ESR spectra of n-type material exhibit the CE resonance in the dark. There is an almost 1:1 correlation between its intensity Ždetermined at 40 K. and the room temperature dark conductivity which is in turn related to the dopant

3. Results 3.1. Doping dependence— dark ESR In Fig. 1, the dark ESR spectra Žat 40 K. of two samples with similar n- and p-type doping levels are shown. The three prominent components at g s 2.0052 " 0.0004, g s 2.0043 " 0.0002 and 1.9978 F g F 1.9983 can be seen. The g-value of the CE resonance is found to decrease with increasing n-type doping. Using Gaussian line shapes with an addi-

Fig. 2. Spin density of the observed resonances in n-type Žright hand side. and p-type Žleft hand side. material with different doping levels as a function of room temperature dark conductivity.

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J. Muller ¨ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1026–1030

a linewidth of approximately 50 mT is found at g s 2.1, superimposed on the narrower lines at g s 2.0052rg s 2.0043 ŽFig. 4.. 3.3. Doping dependence— light-induced ESR

Fig. 3. CW-ESR spectrum of an n-type sample Ž17 ppm. recorded at 20 K with high microwave power, so that only the CE line is visible. The shoulders in the magnified view are due to hyperfine interaction.

concentration. Assuming a build-in and doping efficiency of unity, the density of phosphorus dopants calculated from the gas phase doping ratio is about twice the CE spin density. 3.2. Donor and acceptor states In highly n-doped samples, the hyperfine Žhf. doublet of 31 P can be observed between 4.5 K and 80 K ŽFig. 3.. The spectrum was recorded at 20 K with a microwave power of 25 mW, where all lines other than CE are saturated. The hyperfine splitting DHFS is approximately 11 mT. The spin density of the hf-lines is 2 P 10 16 cmy3 , which is about 10% of the overall conduction electron spin density of 2.4 P 10 17 cmy3 . Using the electron spin echo detected field sweep technique, which has much smaller base line distortion, in p-type samples, an additional resonance with

Fig. 4. Electron spin echo detected field sweep spectrum of a highly p-doped sample Ž57 ppm. showing the presence of a broad line superimposed on the resonances at g s 2.0052r g s 2.0043.

While in the dark spectra the two dangling bond ŽDB. states are observed over the entire doping range, upon illumination the resonances show different dependences depending on type of doping and doping level. In Fig. 5, the difference spectra ŽLESR–dark ESR. of two p-type and one n-type sample are shown Žnormalized to the same peak-topeak intensity.. In p-type samples, the light increases the resonance at g s 2.0043 and the CE line at 1.9978 F g F 1.9983 but there is no change in intensity at g s 2.0052 within errors of measurement. With increasing p-doping, the increase of the CE line becomes less while the resonance at g s 2.0043 increases further. The overall ŽLESR–dark ESR.-intensity also increases with p-doping. In n-type samples, LESR response is only found for the DB Ž g s 2.0052. and the CE line, while the line at g s 2.0043 does not change. The ŽLESR–dark ESR.-intensity becomes smaller with increasing doping level in contrast to the case of p-doping, and for the largest doping level, no increase of the CE line is observed. 3.4. Temperature dependence The lines at g s 2.0043 and g s 2.0052 show no deviation from the Curie-law Žintensity proportional to 1rT . down to 40 K Žnot shown.. For temperatures

Fig. 5. Difference spectra ŽLESR–dark ESR. of one n-type and two p-type samples with different doping levels. Spectra were recorded at 40 K.

J. Muller ¨ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1026–1030

Fig. 6. Intensity of the CE resonance in three n-type samples with different doping levels as a function of 1000r T for temperatures between 4.5 K and 150 K.

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ing temperature due to the strong increase in linewidth as a result of the decrease in T1. The latter effect is demonstrated in Fig. 7, where the temperature dependence of the linewidth ŽFWHM. of the asymmetric CE-line is plotted between 20 K and 300 K Žnote: this is not the peak-to-peak linewidth.. To observe the CE line at RT, we use a minimally doped n-type sample Ždoping- 1 ppm. and apply high microwave power levels. The linewidth increases strongly between 120 K and 300 K, while for T - 120 K, it only shows a slight temperature dependence.

4. Discussion less than 40 K, a decrease in the signal intensity even for the lowest microwave power levels is due to saturation. For the CE line saturation sets in at lower temperatures due to a smaller spin lattice relaxation time T1 w8x, so that the temperature dependence can be investigated to 4.5 K. In Fig. 6, the normalized intensities of the CE line for three different doping levels are shown between 150 K and 4.5 K. Within experimental uncertainty, no deviation from a Curie-like behaviour is observed. At higher temperatures, measurements of the CE resonance are more difficult. RT conductivities ) 5 P 10y4 Srcm lead to a strong reduction of the ESR cavity quality factor, while for material with lower conductivity, the CE signal height is small even at low temperatures and decreases with increas-

Fig. 7. Linewidth Žfull width at half maximum. of the CE resonance in a residual doped n-type sample as a function of temperature between 20 K and 300 K.

Assuming the same band alignment for all interfaces between crystalline and disordered regions, we conclude from Fig. 2 that both of the observed defect states are distributed over a range of energies. This distribution is different from the situation in amorphous silicon Ža-Si:H. where a peak of the defect spin density is found in the mobility gap center. Therefore, a location of the two resonances in the amorphous phase of the m c-Si:H material, similar to dangling bonds in a-Si:H, is not likely. Probably, the defects are located in different environments of the material. However, if large potential fluctuations between adjacent phases occur, these fluctuations could also explain the missing doping dependence and no conclusions concerning the energy distribution of the defects can be drawn. While the intensity of the dark ESR signals at g s 2.0052 and g s 2.0043 does not change upon doping, LESR ŽFig. 5. indicates a different energy of the two corresponding microscopic states. The state with g s 2.0043 should be located deeper in the gap Žcloser to the valence band., which, if the state is in the same microscopic environment, would be surprising as we would expect a larger g-value Žstronger bonding.. This discrepancy remains at present. The value of the 31 P hyperfine splitting Ž DHFS s 11 mT, Fig. 3. in n-type m c-Si:H lies between the one found in crystalline Ž4.2 mT at 4.2 K w9x. and amorphous Ž24.5 mT at 40 K w10x. silicon, indicating that the localisation of the donor electrons in microcrystalline silicon is also in between a-Si:H and crystalline Si.

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J. Muller ¨ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1026–1030

The broad resonance observed in p-type material ŽFig. 4. does not appear in n-doped or intrinsic samples, and we tentatively ascribe it to hole states in the crystalline regions w8x. Finally, we comment on the temperature behaviour of defect and CE signals ŽSection 3.4.. The saturation properties of the defect resonances are consistent with the long spin lattice relaxation times T1 for low temperatures w8x. T1 shows a similar T-dependence as for the dangling bond resonance in amorphous silicon. The strong increase of the CE linewidth with temperature ŽFig. 7. is similar to what has been reported for phosphorus-doped crystalline silicon w11,12x and confirms our earlier interpretation of the disappearance of the CE line at high T w2x. It also agrees qualitatively with the temperature dependence of T1 found for the CE resonance w8x.

5. Summary In m c-Si:H, the small change in intensity of the dark ESR signals at g s 2.0043 and g s 2.0052 upon doping indicates a wide distribution in energy, probably in different regions of the material, or strong potential fluctuations. In contrast, LESR suggests distinctly different energy positions of the two states. The intensity of the CE resonance shows a clear correlation with conductivity which in turn scales with the maximum available dopant electron concentration. For the doping concentrations investigated, it exhibits Curie-like behaviour between 150 K and 4.5 K and little saturation due to short T1 times. The proposed increase of the CE linewidth at high temperatures w2x was confirmed.

Resonances of phosphorus dopant states with a hyperfine splitting of DHFS s 11 mT in between the values for crystalline and amorphous silicon are found. In boron doped material, electron spin echo detected field sweep measurements reveal the presence of a broad line Žlinewidth 50 mT. attributed to hole states.

Acknowledgements We thank D. Steinbacher and J. Wolff for technical assistance. This work is supported by the BMBF.

References w1x M. Luysberg, P. Hapke, R. Carius, F. Finger, Philos. Mag. A 75 Ž1997. 31. w2x F. Finger, C. Malten, P. Hapke, R. Carius, R. Fluckiger, H. ¨ Wagner, Philos. Mag. Lett. 70 Ž1994. 247. w3x C. Malten, F. Finger, P. Hapke, T. Kulessa, C. Walker, R. Carius, R. Fluckiger, H. Wagner, Mater. Res. Soc. Symp. ¨ Proc. 358 Ž1995. 757. w4x E. Holzenkampfer, F.W. Richter, J. Stuke, U. Voget-Grote, J. ¨ Non-Cryst. Solids 32 Ž1979. 327. w5x C.F. Young, E.H. Pointdexter, G.J. Gerardi, J. Appl. Phys. 81 Ž1997. 7468. w6x J. Muller, C. Malten, F. Finger, H. Wagner, Proc. of the 23rd ¨ Int. Conf. on the Physics of Semiconductors, 23 Ž1996. 2697. w7x J. Muller, F. Finger, C. Malten, H. Wagner, Mater. Res. Soc. ¨ Symp. Proc. 452 Ž1997. 827. w8x C. Malten, J. Muller, F. Finger, Phys. Stat. Sol. Žb. 201 ¨ Ž1997. R15. w9x R.C. Fletcher, W.A. Yager, G.L. Pearson, A.N. Holden, W.T. Read, F.R. Merrit, Phys. Rev. 94 Ž1954. 1392. w10x M. Stutzmann, D.K. Biegelsen, R.A. Street, Phys. Rev. B 35 Ž1987. 5666. w11x A.M. Portis, A.F. Kip, C. Kittel, Phys. Rev. 90 Ž1953. 988. w12x H. Kodera, J. Phys. Soc. Jpn. 27 Ž1969. 1197.