Trapping phenomena in intrinsic hydrogenated amorphous silicon like materials studied using current transient spectroscopies

Journal of Non-Crystalline Solids 352 (2006) 1130–1133 www.elsevier.com/locate/jnoncrysol

Trapping phenomena in intrinsic hydrogenated amorphous silicon like materials studied using current transient spectroscopies Vibha Tripathi a

a,*

, Y.N. Mohapatra a, P. Roca i Cabarrocas

b

Department of Physics, Indian Institute of Technology, Kanpur 208016, UP, India b LPICM, Ecole Polytechnique, 91128 Palaiseau cedex, France

Abstract Transient spectroscopies such as time analyzed transients spectroscopy (TATS) provide powerful means of comparing density of states in new forms of amorphous like materials. These spectroscopies were utilized to study hydrogenated amorphous silicon (a-Si:H) and hydrogenated polymorphous silicon (pm-Si:H) grown at different pressures using PECVD. The results reveal marked differences between the two materials. In case of a-Si:H, as expected characteristic emission from a broad density of states in the form of stretched exponentials is observed. The corresponding spectra for pm-Si:H, on the other hand are dominated by nearly exponential fast current decay processes with discrete energies between 0.25 eV and 0.36 eV. The spectra of pm-Si:H grown at different pressures show contributions from crystallite inclusions and the medium in varying degree. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Amorphous semiconductors; Silicon; Solar cells; Nanocrystals; Defects

1. Introduction Recently, the ability to tune the electronic properties of silicon based disordered materials by including nanocrystals in different forms has given rise to a new class of materials with a better suit of electronic properties [1]. Pm-Si:H belongs to the class of mixed phase nanocrystalline material, which may be roughly described as amorphous silicon with silicon crystallites embedded in it. Pm-Si:H is deposited under plasma conditions close to powder formation in a PECVD reactor [2]. Though there have been studies on pm-Si:H comparing carrier mobility, and stability of the material, a comparative study of defect controlling electrical properties have not been done. In this paper, we use reverse current transient in PIN diode structure to compare electrically active defects in these materials.

*

Corresponding author. Tel.: +91 512 2598530; fax: +91 512 256621. E-mail addresses: [email protected] (V. Tripathi), [email protected] (Y.N. Mohapatra). 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.12.050

Since methods appropriate to crystalline semiconductors such as DLTS cannot be used to characterize these materials, we use time domain analysis of transients to infer defect states from current isothermal transients. 2. Experimental details Several PIN devices incorporating amorphous and polymorphous silicon were prepared with variation in deposition pressure [3]. The devices had a glass/SnO2/p-type a-SiC/SiC buffer/intrinsic a-Si:H or pm-Si/n-type a-Si:H/ Al structure. The metallic-contact areas are 0.12 cm2. The thickness of p-, i- and n-layers are about 12, 4000 and 25 nm, respectively. The details of the samples are given in Table 1. The diodes were forward biased for about 40 ms at 2 V and then reverse biased to record reverse current transients (single-shot). The experiments were performed in a temperature-controlled cryostat under dark conditions. Details of the experiments are given elsewhere [4]. Steady state I–V characteristics were carried out at each temperature before transient measurements commenced. The variation in the

V. Tripathi et al. / Journal of Non-Crystalline Solids 352 (2006) 1130–1133 Table 1 The deposition parameters of different diodes

110221 (C) 110101 (B) a-Si:H (A)

Sample structure SnO/PIN/Cr SnO/PIN/Cr SnO/PIN/Cr

0.0

Deposition pressure (mTorr)

Deposition temperature (°C)

Thickness (l)

1760 1200 Std

175 175 175

3.5 2.5 2.5

-0.1

-0.2

-0.3 3

-0.4

2

Signal

ambient temperature of the diode was controlled to less than 102 K.

TATS signal

Sample number

1131

3. Results and discussion

1

-0.5

3.1. Time analyzed transient spectroscopy (TATS)

0

TATS is an isothermal spectroscopic technique of analyzing transients, where time window is varied while keeping the temperature fixed [5,6]. As in deep level transient spectroscopy (DLTS), a better known spectroscopic technique, information about defect level and defect capture cross-section may be obtained by TATS. The difference between TATS and DLTS lies in the fact that in DLTS technique, data is scanned as a function of temperature keeping the rate window fixed, whereas in TATS we vary rate window and keep temperature fixed. Isothermal spectroscopic techniques are easier to implement experimentally, and further enable a more intensive line-shape analysis. The TATS signal S(t) is given by SðtÞ ¼ Iðt; T Þ  Iðt þ ct; T Þ;

ð1Þ

where I, t and T are respectively given by, current, time, temperature and c is an experimentally selectable parameter which has been set to one in our analysis. In case, the signal I(t, T) is composed of a sum of exponentials, each appears in TATS signal S(t) as a peak with the height being proportional to the corresponding strength of the exponentials. The sign of the peak is dependent on whether the signal is a decaying or rising exponential. The analysis of signals by this method makes it easy to track discernible features. Further, the line-shape is amenable to fitting and analysis to recover parameters or test more complicated signal models than exponentials. There are several advantages of TATS analysis over techniques such as DLTS. TATS is a spectroscopy in the time domain and therefore possible temperature dependence of the transient pre-factor containing occupancy etc does not occur. Additionally in methods involving temperature scanning such as DLTS, the line shape is dependent upon trap parameters and the temperature regime of observation. In contrast, the width of the TATS peak depends only on the parameter c, which is chosen to optimize resolvability of signal to noise ratio. In order to illustrate the analytic procedure, we generate a transient decay data as a combination of two exponentials given as

-0.6 1E-3

0.005

0.05 0.1

0.5

5

10

time (s) Fig. 1. Simulation of TATS spectrum obtained from the transient I(t) constituting of two exponentials of time constants 41.28 ms and 1.2 s in the amplitude ratio of 2:1, respectively. Note that height reflects the amplitude ratio. The transient in linear scale is shown in the inset.

    t t IðtÞ ¼ A0  A1 exp   A2 exp  . s1 s2

ð2Þ

Fig. 1 is computer generated transient data using summation of two exponentials with different time constants s1 and s2 (0.04128 ms and 1.2 ms) and amplitudes A1 and A2 (2 and 1). TATS, shown in inset of Fig. 1 displays the features in proportion to the amplitude which may not be the case in some other spectroscopic techniques. In many cases of practical importance, the non-exponential current transients can be represented by a stretched exponential of the form [7]  b t DI  D exp  ; b 6 1; ð3Þ se where, b is the stretching factor. In disordered materials, electrical transient with origin in continuously distributed defect states in the gap typically give rise to such stretched exponentials. TATS can be gainfully utilized to distinguish between closely spaced multiple traps and continuum of traps which leads to stretched exponential. In order to illustrate this point, a typical TATS spectrum of transient for polymorphous silicon sample at T = 289 K has been shown in Fig. 2. This spectrum has been fitted with both stretched exponential as well as to the sum of two decaying exponentials. As we increase b the TATS spectra stretches out and becomes broader. Such a spectra is clearly unable to provide a good fit as shown in Fig. 2. Moreover the spectra which is summation of two or more exponentials is very different from a stretched exponential spectra and it is easy to distinguish between the two. In rest of the paper, we go

1132

V. Tripathi et al. / Journal of Non-Crystalline Solids 352 (2006) 1130–1133

0.00

Temp=309K

0.0

Sample#110101 Temp= 289

0.05

-0.5 -0.10

0.00

I(t) nA

-0.15

-0.20

data TATS_two peaks fit Stretched_TATS (beta= 0.7) 1E-3

0.01

-0.05

TATS signal

TATs signal

-0.05

-1.0

0.1

a-Si:H 110101 -0.25 110221

time (s) Fig. 2. TATS spectrum at temperature T = 289 K. Pm-Si:H sample #110101 has been fitted with two discrete exponentials as well as stretched exponential. The stretching factor b = 0.7 for stretched exponential.

1E-3

0

1

0.01

0.11

time (s)

2

time (s) Fig. 3. A typical set of isothermal reverse current transients taken in various a-Si:H and pm-Si:H samples for comparison. The inset shows TATS spectra of the three isothermal reverse transients shown in the figure.

3.2. Comparison and defect spectra 1x10

SAMPLE 110221 TATS results

4

Tau3_TATS Tau2_TATS Tau1_TATS

3

1x10

2

2

1x10

TauT

Fig. 3 shows a set of normalized transients recorded at T = 309 K for all the three samples listed in Table 1. We see a distinct change in the nature of transients for the three different samples incorporating active material deposited under different processing conditions. The change in the reverse current, DI, with respect to time in a-Si:H sample becomes significant only after 0.01 s. Note that the decay is highly non-exponential in nature and continues to decay even after few seconds. The reverse current due to polymorphous silicon sample 110221, deposited at 1760 mTorr, shows an initial rise, peaks at about 0.1 s and then starts decaying again. Pm-Si:H sample 110101, which was deposited at 1200 mTorr, shows strongest rise which continues till about 0.8 s. Note that a simple room temperature reverse current transient experiment is able to display the differences in the material properties very clearly. The inset in Fig. 3 shows the corresponding TATS spectra. The three diodes give rise to very distinct signatures in the spectra. Amorphous silicon shows weak and broad positive peak. Pm-Si:H sample #110101 deposited at lower pressure shows a dominant negative peak. Pm-Si:H sample (#110221) deposited at higher pressure shows negative peak, similar to sample number 110101 at lower temperatures, but at higher temperatures, there is a positive component as well as in the case of a-Si:H. The clearly observed changes in the spectra are indicators of our ability to distinguish between different materials, and hence demonstrating the power of transient based techniques for characterization.

a-Si:H 110101 110221

-0.15

-0.20

1

on to apply this technique in order to find out differences in electronic properties of amorphous and polymorphous silicon material.

-0.10

2.7

3.0

3.3

3.6

3.9

1000/T Fig. 4. The Arrhenius plot of sample #110221 (pm-Si:H deposited at 1760 mTorr) obtained from fitting of TATS spectra. We get signal of three distinct states at 0.25 ± 0.01 eV, 0.37 ± 0.01 eV, and 0.42 ± 0.01 eV.

Arrhenius plot obtained from the fitting of TATS spectrum at different temperatures for sample #110101 and sample #110221 are shown in Figs. 4 and 5, respectively. Arrhenius plot from nearly exponential transients in both the polymorphous silicon sample lead to distinct activation energies in the range of 0.25 ± 0.01 eV to 0.42 ± 0.01 eV. Sporadic observation of discrete states in polycrystalline or microcrystalline material is often linked to states at the interface. However this appears improbable in our case as it has been noted that silicon crystallites in polymorphous

V. Tripathi et al. / Journal of Non-Crystalline Solids 352 (2006) 1130–1133

crystallite and communicate with the bands of surrounding matrix. The details of this model will be published elsewhere.

Sample# 101101

TATS1 TATS2

0.26eV

4. Conclusion

10000

TauT

2

We have demonstrated TATS of reverse current transient in PIN devices as an efficient transient spectroscopic method for investigating defect states in amorphous and nanocrystalline materials. By using TATS, we were able to show that crystallites present in amorphous silicon matrix cause discrete states to occur in the band gap of amorphous silicon. The activation energies of the discrete states were found to lie in the band of 0.25 ± 0.01 eV to 0.42 ± 0.01 eV.

0.21eV

1000

2.8

2.9

3.0

3.1

3.2

3.3

3.4

1133

3.5

3.6

3.7

3.8

3.9

1000/T Fig. 5. Arrhenius plot corresponding to two discrete traps inferred from current transient analysis for sample #110101. We get signal of two distinct states at 0.21 eV and 0.26 eV.

silicon are embedded in relaxed amorphous matrix [1]. Clearly the states we observe are localized states within the nanocrystallites. Since the transients that we analyze are fairly slow it is unlikely that these correspond to discrete states within the conduction band as in the quantum well formed with band states. It is well known that deep states such as those associated with a vacancy or an ill-fitting impurity can give rise to states within crystalline semiconductors. We propose that the observed discrete states in polymorphous material owe their origin to defects in the

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