Defect States in Microcrystalline Silicon Probed

7/28/2019 Defect States in Microcrystalline Silicon Probed

http://slidepdf.com/reader/full/defect-states-in-microcrystalline-silicon-probed 1/5

Defect states in microcrystalline silicon probed

by photoluminescence spectroscopy

T. Merdzhanova a,b,*, R. Carius a , S. Klein a , F. Finger a , D. Dimova-Malinovska b

a Institut fur Photovoltaik, Forschungszentrum Julich GmbH, 52425 Julich, Germany b Central Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria

Available online 24 January 2006

Abstract

Photoluminescence (PL) spectroscopy is used to investigate defects and localized band tail states within the band gap of hydrogenated

microcrystalline silicon (Ac-Si:H) prepared by plasma enhanced chemical vapor deposition (PECVD) and hot wire chemical vapor deposition

(HWCVD). The effect of the substrate temperature (T S), which influences mainly the defect density, and silane concentration (SC), as the key

parameter to control the microstructure of the material were varied. In high quality Ac-Si:H films (T S=185–200 -C) a PL band (FAc_-Si-band) is

observed at ¨0.9–1.05 eV which is attributed to radiative recombination via localized band tail states in the microcrystalline phase. In Ac-Si:H

films prepared at higher T S (>300 -C), an additional PL band at ¨0.7 eV with a width of ¨0.17 eV is found for both PECVD and HWCVD

material. This band maintains its position at ¨0.7 eV with increasing SC in contrast to the observed shift of the FAc_-Si-band to higher energies.

Studies of the temperature dependences of the PL peak energy and intensity for the two bands show: (i) the PL band at 0.7 eV remains unaffected

upon increasing temperature, while the FAc_-Si-band shifts to lower energies, (ii) a much weaker quenching for the 0.7 eV band compared to the

FAc_-Si-band. It was also found that the PL band at 0.7 eV exhibits a slightly stronger temperature dependence of the PL intensity compared to

Fdefect _ band at 0.9 eV in a-Si:H suggesting similar recombination transition via deeper trap states. Due to a similar PL properties of the emission

band previously observed in Czochralski-grown silicon (Cz – Si), the 0.7 eV band in Ac-Si:H is assigned tentatively to defect-related transitions in

the crystalline phase.D 2005 Elsevier B.V. All rights reserved.

Keywords: Photoluminescence; Defect states; Silicon; Temperature-dependent measurements

1. Introduction

High-quality microcrystalline silicon (Ac-Si:H) for solar cell

applications can be prepared at low substrate temperatures T Son glass, stainless steel and plastics by using plasma enhanced

(PE-) and hot wire (HW-) chemical vapor deposition (CVD)

deposition techniques [1,2]. Studies of the PL properties of

such materials have identified a PL band located below the band gap of crystalline silicon that shifts to higher energies

with increasing silane concentration (SC), i.e. decreasing

crystalline volume fraction [3–6]. It was proposed that this

PL band originated from transitions between localized band tail

states similar to those in a-Si:H [7,8]. Previous studies on Ac-

Si:H thin films deposited at higher T S (>300 -C) by the two

techniques reveal higher spin densities (up to 1017 cmÀ3) and

higher optical sub-gap absorption resulting in a moderate

performance of solar cells prepared under such conditions

[2,5,9]. Here, we report on new results of a PL study on such

Ac-Si:H thin films.

2. Experimental

Intrinsic Ac-Si:H thin films were prepared on Corning glass

substrates or rough quartz using PECVD at 95 MHz at substrate temperatures (T S) of 200 -C and 325 -C, or HWCVD

at T S of 185 -C and 330 -C, with filament temperature (T F) of

1650 -C and 1850 -C. The film thickness of the samples

ranged from 0.4 Am to 1 Am (HWCVD), and from 0.5 Am to 3

Am (PECVD). The microstructure of the films was modified by

varying the dilution of the silane in hydrogen in the process

gas, i.e. silane concentration SC=[SiH4]/[SiH4+ H2]. Details

of the deposition parameters are published elsewhere [1,2,9].

For excitation of the luminescence, as well as for Raman

scattering experiment, a laser wavelength of 488 nm of an

Ar + laser with a power density of 5 W/cm2 was used.

0040-6090/$ - see front matter D

2005 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2005.12.114

* Corresponding author. Tel.: +49 2461 61 3177; fax: +49 2461 61 3735.

E-mail address: [email protected] (T. Merdzhanova).

Thin Solid Films 511 –512 (2006) 394 – 398

www.elsevier.com/locate/tsf



Therefore, a similar sample volume is probed by the two

experiments, except for a small variation due to the temper-

ature dependence of absorption coefficient. The integrated

intensity ratio I CRS= ( I 520+ I 500) / ( I 520+ I 500+ I 480) of the crys-

talline peaks (520 cmÀ1 and 500 cmÀ1) and the amorphous

peak (480 cmÀ1) was used as a semi-quantitative measure for

the crystalline volume fraction [10]. Photoluminescencespectra were recorded at temperatures of 10–200 K using a

Fourier transform (FT) spectrometer (Bruker FS66v) and a

liquid nitrogen (LN2) cooled Ge or an InAs detector, applying

lock-in technique to enhance signal-to-noise ratio.

3. Results

In Fig. 1(a, b), photoluminescence (PL) spectra (measured at

10 K) of Ac-Si:H thin films, prepared by HWCVD and PECVD

at high substrate temperatures (T S) of 330 -C and 325 -C with

various silane concentrations (SC) between 9% and 25% are

compared. All PL spectra are shifted vertically by 0.1 r.u. for clarity. In Fig. 1(a), the PL spectra of the HWCVD material with

SC (10–17.5%) reveal two emission bands, a high-energy PL

band at ¨0.97 eVand a full width at half maximum (FWHM) of

¨0.14 eV (FAc_-Si-band), attributed to the microcrystalline

phase [3–7] and a low-energy emission band centred at about

0.7 eV. The latter is very similar to the emission band previously

observed in poly-Si thin films on glass and denoted as Fdefect _

PL band [11]. The following can be considered as distinctive

features of the _defect _ band upon increasing SC: (i) the PL band

maintains the spectral maximum at about 0.7 eVand the FWHM

also holds a constant value of ¨0.17 eV, while the FAc_-Si-band

shifts to higher energies; and (ii) the PL intensity is slightly

higher than the intensity of the FAc-Si-band and it increases

continuously until the broader (¨0.3 eV) emission band at

¨1.26 eV, appears at SC = 17.5%. This well-known PL band in

a-Si:H thin films is related to radiative recombination between

the band tail states (Fa_-Si-band) [12]. The inset in Fig. 1(a)

shows similar increase of the PL intensities of the Fdefect _ Ac-Siand the FAc_-Si band upon increasing SC, i.e. almost the same

ratio between the two bands is kept until the contribution of the

Fa_-Si-band becomes significant at SC!18.5%, followed by a

substantial decrease of the PL intensity of the FAc_-Si-band and

the PL signal at 0.7 eV disappears.

The luminescence spectra of Ac-Si:H films, prepared by

PECVD are shown in Fig. 1(b), for comparison. All three

emission bands with maxima at ¨0.7 eV (Fdefect _ Ac-Si),

¨1.01–1.03 eV (FAc_-Si) and¨1.26 eV (Fa_-Si) were identified

even for films with lower SC between 9% and 13%. For

material with a SC of 15%, which reveals a PL spectrum

dominated by the signal at 1.26 eV, there is still a smallcontribution from the Fdefect _ Ac-Si-band and the FAc_-Si-band,

while for SC=17% the PL spectrum is entirely dominated by

the Fa_-Si-band. Similar to HWCVD films, the Fdefect _ Ac-Si-

band remains unchanged, while the FAc_-Si-band shifts to higher

energies, upon increasing SC. The dependence of the lumines-

cence intensity of the FAc_-Si- and the Fdefect _ Ac-Si bands on

the SC is also shown in the inset in Fig. 1(b). In contrast to

HWCVD, the PL intensities of the two emission bands do not

increase continuously with increasing SC and the intensity of

the Fdefect _ Ac-Si-band is slightly lower, while the FAc_-Si-band

reveals improved efficiency (compare inset in Fig. 1(a)).

Fig. 1. Photoluminescence spectra of Ac-Si:H thin films, prepared by (a) HWCVD at T S=330 -C and T F=1850 -C and (b) PECVD at T S=325 -C with indicated SC.

Solid lines are measured PL data at 10 K with E x=2.54 eV, the dashed lines are the fitting curves with Gaussian line shape. Insets (a, b): dependence of the PLintensity of Fdefect _ Ac-Si-band and FAc_-Si-band on the SC.

T. Merdzhanova et al. / Thin Solid Films 511–512 (2006) 394–398 395



As the crystalline volume fraction has been shown to be a

valuable tool to parameterise Ac-Si:H thin films, the PL peak

energy of the FAc_-Si and Fdefect _ Ac-Si bands are shown in

Fig. 2 as a function of I CRS for the two sample series shown in

Fig. 1(a, b). Note, a few additional points have been added for

the HWCVD series, from PL spectra which were omitted for

clarity in Fig. 1(b). While the FAc_-Si-band shows an increase

of the PL peak energy with decreasing I CRS, the Fdefect _ Ac-Si-

band retains its spectral maximum at about 0.7 eV for all series

studied. The PECVD series (see Fig. 1(b)) exhibits slightly

higher values for the PL peak energy of the FAc_-Si-band

compared to the HWCVD series. In addition a higher PLintensity of the FAc_-Si-band is observed for the PECVD series

compared to HWCVD material.

A competition between radiative and non-radiative recom-

bination transitions as a function of measurement temperature

provides information on the recombination processes involved.

Therefore, the PL spectra in the temperature range (10–200 K)

for two samples, one with equal contributions of microcrystal-

line and amorphous phase I CRSå0.5 (SC=9%) (see Fig. 1(b))

and a second with fully amorphous microstructure (PECVD,

T S=200 -C) are displayed semi-logarithmically in Fig. 3(a, b).

Three emission bands at ¨0.7 eV (Fdefect _ Ac-Si), at ¨1.01 eV(FAc_-Si-) and at ¨1.26 eV (Fa_-Si-) characterize the PL

spectrum at 10 K for the film with I CRSå0.5 shown in Fig.

3(a). Referring to Fig. 3(a), there is a slight decrease in the PL

intensity of the FAc_-Si-band and slight increase in the intensity

of the Fdefect _ Ac-Si- and Fa_-Si band below 40 K. Further

increase of the temperature results in a much weaker quenching

of the Fdefect _ Ac-Si-band compared to the rapid quenching of

the FAc_-Si-band and the Fa_-Si-band. At 160 K the Fa_-Si-band

has disappeared while the FAc_-Si-band is visible up to 160 K.

Above 180 K, the Fdefect _ Ac-Si-band dominates the PL spectra.

In Fig. 3(b), the PL spectrum (at 10 K) for the amorphous

film consists of two emission bands, one at ¨

0.9 eV with half-width of ¨0.35 eV and the Fa_-Si-band. The low-energy band

has been attributed to recombination transitions via electron-

ically active defect states identified as dangling bonds, thus we

denote it here as (Fdefect _ a-Si-) ([12], and references therein).

With increasing temperature, the PL intensity of the Fdefect _ a-

Si-band reveals a much weaker decrease compared to that

observed for the Fa_-Si-band, and dominates the PL spectra at

200 K. With increasing temperature the PL peak energy of the

Fdefect _ a-Si-band and the Fdefect _ Ac-Si band (see Fig. 3(a))

retain a spectral maximum at ¨0.9 eVand¨0.7 eV, in contrast

to the observed shift of the main PL bands (Fa_-Si- and FAc_-Si-

band) to lower energy due to the rapid decrease in the in the

high-energy part of the spectrum.The temperature dependence of the PL intensity of the main

and the defect related emission bands are summarized in Fig. 4

in an Arrhenius plot for different microcrystalline films and the

amorphous sample (see Fig. 3(b)). For the high quality Ac-Si:H

0 0.2 0.4 0.6 0.8 1

Raman intensity ratio IcRS

0.7

0.8

0.9

1

1.1

P L

e n e

r g y

( e V )

PECVD TS=325oCHWCVD TS=330oC

’defect’µc-Si-band

’µc’-Si-band

Fig. 2. PL peak energy of Fdefect _ Ac-Si-band and FAc_-Si-band vs. FRaman

crystallinity_, I CRS, for two sample series, prepared by PECVD (T S=325 -C)

and HWCVD (T S=330 -C, T F=1850 -C). The solid lines are guides to the eye.

Fig. 3. PL spectra at indicated temperatures for a samples, prepared by (a) PECVD at T S=325-

C; SC=9% ( I CRSå

0.5) as shown in Fig. 1(b) and (b) PECVD at T S=200 -C with fully amorphous microstructure. The luminescence was excited with photons of energy E x= 2.54 eV.

T. Merdzhanova et al. / Thin Solid Films 511–512 (2006) 394–398396



(HWCVD, T S=185 -C), Raman spectra indicate a predomi-

nant l y cryst al l ine st ructure w it h I CRSå0 .6 a nd th e

corresponding PL spectra exhibit only the FAc_-Si-band [5,6],

while for Ac-Si:H films (PECVD, T S=325 -C) the additional

PL band at 0.7 eV is found. The PL spectra of the fully

amorphous films reveal only the Fa_-Si-band (HWCVD,

T S=185 -C). To show the temperature dependence of the PLintensity of the Fdefect _ a-Si-band at ¨0.9 eV, a sample

deposited by PECVD was chosen (see Fig. 3(b)). The

temperature dependence of the PL intensity for the FAc_-Si-

band shows a rapid quenching by three orders of magnitude

starting at 40 K. The Fa_-Si-band exhibit an almost constant PL

intensity up to 80 K before it starts to decrease. Similar to

Fdefect _ band in a-Si:H, the PL intensity of Fdefect _ band in Ac-

Si:H does not show a significant decrease for temperatures

below 80 K but a stronger decrease as the Fdefect _ a-Si-band.

No single activation energy was found for Ac-Si:H thin films in

a manner similar to a-Si:H [12].

Fig. 5 shows the temperature dependence of the PL peak energy for the same sample series shown in Fig. 4. Upon

increasing temperature, the two Fdefect _ bands maintain at a

peak position of ¨0.7 eV for the Ac-Si:H films (at T S>300 -C)

and at ¨0.9 eV for the a-Si:H films studied. The half-width of

the Fdefect _ Ac-Si- and Fdefect _ a-Si-bands also holds constant

values of ¨0.17 eV and¨0.35 eV over a range of temperatures

(10– 200 K) (data not shown). For Ac-Si:H thin films with

I CRSå0.6, a continuous shift of the PL peak energy towards

lower energies is observed with increasing temperature. All

these films show a similar temperature coefficient of

¨1.1Â10À3 eV/K for the PL peak energy above 40 K. Above

80 K, the shift of the PL peak energy is much stronger for

highly amorphous films with a temperature coefficient of

¨1.6Â10À3 eV/K, which is in agreement with values reported

for a-Si:H ([12], and references therein).

4. Discussion

In Ac-Si:H thin films prepared at substrate temperatures

higher than 300 -C, an emission band at about 0.7 eV with ahalf-width of about 0.17 eV was observed in addition to the

main band located at ¨0.9–1.05 eV (FAc_-Si-band), irrespec-

tively of the preparation method used (see Fig. 1(a, b)). This

emission band was not detected for Ac-Si:H prepared at lower

T S, which is identified as high quality material. Fig. 2 shows

clearly that the new emission band maintains a spectral

maximum at about 0.7 eV, while the FAc_-Si-band shifts to

higher energies with changing the structural composition from

crystalline ( I CRSå0.74) to predominantly amorphous, i.e.

decreasing crystalline volume fraction, I CRS. The main band

in Ac-Si:H is attributed to the radiative recombination via

localized band tail states in the microcrystalline phase in amanner similar to a-Si:H [7,8]. A reduction of the density of the

band tail states due to structural relaxation of the Ac-Si:H

network by hydrogenated amorphous silicon surrounding the

crystalline phase was proposed as a cause for the shift of the

FAc_-Si-band to higher energies [6,7]. The PL band at 0.7 eV

remains unaffected upon decreasing I CRS (see Fig. 2) and in

addition the PL peak energy was found to be independent of

the measurement temperature (Fig. 3(a)) as was observed for

the Fdefect _ band at ¨0.9 eV in a-Si:H (see Fig. 3(b)). This

suggests that band tail states are very likely not involved in the

recombination transition at 0.7 eV in Ac-Si:H. In agreement

with this is the weaker temperature dependence of the PL

intensity for the 0.7 eV band as compared to the main emission

Fig. 4. Temperature dependence of the PL intensity of the FAc_-Si-, Fa_-Si-,

Fdefect _ Ac-Si-, Fdefect _ a-Si-band for samples, prepared by different deposition

methods and T S. The PL intensity for the FAc_-Si- (at 40 K) and Fa_-Si-bands (at

80 K) are normalized with respect to the highest PL intensity achieved for highquality films (HWCVD, T S=185 -C).

Fig. 5. Temperature dependences of the PL peak energy of the FAc_-Si-, Fa_-Si-,

Fdefect _ Ac-Si-, Fdefect _ a-Si-bands for the same samples as shown in

Fig. 4. The temperature coefficients for the shift of the FAc_-Si-band

(¨1.1Â10À3 eV/K) and the Fa_-Si-band (¨1.6Â10À3 eV/K) is given.

T. Merdzhanova et al. / Thin Solid Films 511–512 (2006) 394–398 397



band (see Fig. 4). In a manner similar to a-Si:H, the

temperature quenching of the main band in Ac-Si:H is very

likely due to thermally enhanced emission of carriers trapped in

the band tail states and subsequent non-radiative recombination

via capture at defect states. The more rapid quenching of the

PL intensity of the FAc_-Si-band in comparison to the Fa_-Si-

band (Fig. 4) points to shallower trap states, i.e. steeper bandtails in Ac-Si:H. This is in line with the weaker shift of the FAc_-

Si-band with temperature coefficient of ¨1.1Â10À3 eV/K,

while the Fa_-Si-band is shifted stronger (1.6Â10À3 eV/K)

upon increasing temperature (see Fig. 5).

The Fdefect _ band in a-Si:H at about 0.9 eV found in the

material of high defect density in addition to the main Fa_-Si-

band at ¨1.3–1.4 eV is considered to be due to recombination

transitions from the band edge to intrinsic defects (dangling

bonds). The weaker temperature dependence of the PL

intensity as compared to the Fa_-Si-band is related to one of

the carriers being deeply trapped at a dangling bond [12]. The

similar but slightly stronger temperature dependence of the PLintensity of the 0.7 eV band in Ac-Si:H suggests that a similar

recombination process could be involved, i.e. transition into

deep trap states.

An emission band with similar PL properties that maintains

spectral maximum at about 0.65 eV between 4.2 K and 300 K

and quenched from 90 K has been previously observed for

hydrogenated poly-Si on glass deposited at 625 -C or 550 -C,

which is similar to Ac-Si:H thin films studied in this work [11].

There the defect-related emission band at ¨0.7 eV in Ac-Si:H

is tentatively assigned to recombination transitions via defect

states in the crystalline phase. Similar PL properties of the

emission band were also observed in Cz – Si annealed at 450 -C

and attributed to thermal donors related to oxygen precipitatesin silicon [13,14]. We therefore propose that similar deep

defects exist in Ac-Si:H. It is interesting to note that no

indication for a contribution of the Fdefect _ a-Si-band in the

defect-rich Ac-Si:H films is found, even in samples where a

significant contribution of the Fa_-Si-band in addition to the

Fdefect _ Ac-Si-band is observed.

5. Conclusions

In addition to a main emission band at ¨0.9–1.05 eV, a

new emission band at ¨0.7 eV was found for Ac-Si:H thin

films (T S>300-

C) with high spin density (1017

cmÀ3

) andhigher optical sub-gap absorption. Since the 0.7 eV emission

band remains unaffected, while the main PL band shifts to

higher energies upon decreasing crystalline volume fraction

and, in addition the PL peak energy, was found to be

independent of the measurement temperature, we conclude

that localized band tail states are not involved in the

recombination transition at ¨0.7 eV. The similar but slightly

stronger temperature dependence of the PL intensity of

the 0.7 eV band in Ac-Si:H, compared to Fdefect _ PL band at ¨0.9 eV in a-Si:H originating from recombination transitions

from the band edge into dangling bonds, suggests that a similar

recombination process is involved, i.e. transition of one of the

carriers into deep trap states. The PL band at ¨0.7 eV is

tentatively assigned to defect-related transitions in the crystal-

line phase, by comparison with a PL band previously observed

in Cz– Si attributed to recombination processes involving

defects connected to thermal donors which are related to

oxygen precipitates.

Acknowledgments

We are grateful to Markus Hulsbeck and Josef Klomfass for

their contributions to this paper.

References

[1] O. Vetterl, A. Gross, T. Jana, S. Ray, A. Lambertz, R. Carius, F. Finger,

J. Non-Cryst. Solids 299–302 (2002) 772.

[2] S. Klein, F. Finger, R. Carius, T. Dylla, B. Rech, M. Grimm, L. Houben,

M. Stutzmann, Thin Solid Films 430 (2003) 202.

[3] G. Yue, J.D. Lorentzen, J. Lin, D. Han, Q. Wang, Appl. Phys. Lett. 75

(1999) 492.

[4] D. Han, K. Wang, Sol. Energy Mater. Sol. Cells 78 (2003) 181.

[5] R. Carius, T. Merdzhanova, F. Finger, S. Klein, O. Vetterl, J. Mater. Sci.,

Mater. Electron. 14 (2003) 625.[6] R. Carius, T. Merdzhanova, F. Finger, Mater. Res. Soc. Symp. Proc. 762

(2003) 321.

[7] R. Carius, T. Merdzhanova, S. Klein, F. Finger, J. Optoelectron. Adv.

Mater. 7 (1) (2005) 121.

[8] A.U. Savchouk, S. Ostapenko, G. Nowak, J. Lagowski, L. Jastrebski,

Appl. Phys. Lett. 67 (1) (1995) 82.

[9] A.L. Baia Neto, A. Lambertz, R. Carius, F. Finger, J. Non-Cryst. Solids

299– 302 (2002) 274.

[10] L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, H. Wagner,

Phila. Mag. A 77 (1998) 1447.

[11] S.S. Ostapenko, A.U. Savchuk, G. Nowak, J. Lagowski, L. Jastrebski,

Mat. Sci. Forum 196 – 201 (1995) 1897.

[12] R.A. Street, Adv. Phys. 30 (1981) 593.

[13] M. Tajima, J. Cryst. Growth 103 (1990) 1.

[14] M. Tajima, H. Takeno, M. Warashina, T. Abe, Mat. Sci. Forum 143 – 147(1994) 147.

T. Merdzhanova et al. / Thin Solid Films 511–512 (2006) 394–398398

Documents

Defect States in Microcrystalline Silicon Probed