Journal of Non-Crystalline Solids 141 (1992) 162-165 North-Holland

]OURNA L OF

NON-CRYSTALLINE SOLIDS

Section 4. Metastable optical effects

Time-of-flight measurements of thermally induced defects in a-Si" H

R.A. Street and C.C. Tsai Xerox Palo Alto Research Center, Palo Alto, CA 94304, USA

Thermally induced defects in a-Si :H are studied by time-of-flight transient photoconductivity. The increase in defect density upon quenching from elevated temperatures is observed as a decrease in the mobility-lifetime product. The defect densities agree well with ESR data and also show that defects are induced approximately uniformly in the bulk. No thermally induced changes in the drift mobility are observed, from which it is deduced that there is no detectable change in the band tail states by the structural equilibration, by contrast with the situation in chalcogenides.

1. Introduction 2. Measurements

Defect and dopant equilibration occur in a- Si:H in the temperature range 100-350 °C [1,2]. The defect density in undoped a-Si:H, measured by electron spin resonance (ESR), increases with the equilibration temperature with an activation energy of 0.2-0.3 eV, which is interpreted as the average defect formation energy for the distribu- tion of creation sites [3-5]. Some of the reversible changes in ESR have been attributed to surface states [6]. There is also evidence that the equilib- rium defect density has a considerable variation through the material partly because of band bending effects [7]. ESR only measures the total number of paramagnetic defects (e.g., neutral dangling bonds); it is insensitive to diamagnetic defects and to the spatial distribution of states. It is therefore of interest to explore other tech- niques to measure the equilibrium defects. One such example described here is the time-of-flight technique, which measures the carrier drift mo- bility and the trapping rate into deep gap states. The drift mobility experiment also allows for the investigation of thermally induced changes in the band tail density of states distribution.

Undoped a-Si : H samples of thickness 4-8 Ixm and a 5 Ixm thick p-type sample were used in the study. The time-of-flight experiment used a 5 ns pulse from a nitrogen/dye laser combination. Measurements were made of both the drift mobil- ity, /x, and the charge collection, Q. The drift mobility is obtained from the transit time, which was evident at sufficiently high fields from its scaling with the inverse of the applied voltage. The mobilities were typical for a-Si:H with an electron mobility of approximately 1 cmZ/V s. The charge collection was obtained by integrating the transient photoconductivity pulse for a time period of 10 Ixs from the the excitation pulse. Figure 1 shows examples of the voltage depen- dence of the charge collection. The charge collec- tion saturates at sufficiently high voltages and this is the region in which the drift mobility is ob- tained. The loss of charge collection at low volt- age indicates deep trapping, and the value of/x~- can be obtained, where ~- is the deep trapping lifetime. The voltage dependence of the charge collection, Q, is given by the Hecht formula:

O = Q o ( t x r V / d 2 ) [ 1 - exp(-dZ/ txrV)] , (1)

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

R.A. Street, C.C. Tsai / TOF measurements of thermally induced defects in a-Si : H 163

where Q0 is the initial charge, and d is the thickness of the sample. The value of /xr is obtained by fitting the charge collection data to eq. (1), to obtain the voltage, VH, where

V H = d 2 / / J , T . ( 2 )

The values of V H are indicated in fig. 1 for the sets of data.

Different equilibrium defect densities were ob- tained by annealing and quenching at tempera- tures between 210 and 330 ° C, which is the range of temperatures at which equilibration can be readily achieved. The relaxation time rapidly in- creases below 200 °C such that equilibrium can- not be attained within a reasonable annealing time. By contrast, the relaxation time is so short above 330 °C that the quenching rate is not fast enough to freeze the high tempera ture equilib- rium state. The annealing times were chosen to ensure equilibrium and consequently were longer at the lower temperatures.

Figure 1 shows the charge collection data for some different anneal temperatures. Samples equilibrated from the higher tempera ture have a

100

v z O I.- 0 1 0 LLI

O O uJ [5 t~ ,¢ -t- O

>- , ¢ _ r _ . ~ - ~ - TA ...,.-,--/ , •

210°C ~ " ~ . . - I

• o ~ • • e l

• • 2 o 8 ~

o / ~ , , 320 ° C

I I 1 10

VOLTAGE (V)

Fig. 1. Voltage dependence of the room temperature electron charge collection after the sample has been equilibrated at the temperatures indicated. The arrows indicate the values of VI4 obtained by fitting to the Hecht formula. The data have

been offset vertically for clarity.

g "E o v :::L

ANNEAL TEMPERATURE (°C) 350 300 250 200

3x10 -7

10-~

3x10 "B

I f I 1.6

1 / / / O / / / /

. / ' ~ D O

i i

0 8gin [ ] 4gin

O 1 I- O 1 / /

1 O --6 I-

[]

I 2 . 0

A N N E A L T E M P E R A T U R E 1 0 0 0 / T A ( K )

Fig. 2. Dependence of the electron /xr on the anneal temper- ature for two undoped a-Si:H samples. The dashed line has

an activation energy of 0.3 eV.

reduced charge collection at low voltage. The value of /xr is correspondingly smaller, as indi- cated by the larger values of V H in the figure

101 i , ,

~'~ T v :

\ . \0NoopEo

10-2= o

- YI E (D

O -i-

P I i 1 0 100 200 300

ANNEAL TEMPERATURE T A (°C)

Fig. 3. The hole dr i f t mob i l i t y in undoped and p-type samples

after annealing and quenching from the indicated tempera- tures. The sample thickness is 5 Ixm and the applied field is 2 x 104 V/cm. The predictions from the theory of ref. [11] are

indicated.

164 R.A. Street, C.C. Tsai / TOF measurements o f thermally induced defects in a-Si : H

obtained by fitting to eq. (1). No change in the drift mobility is observed for the different anneal- ing temperatures, with an experimental uncer- tainty of about 25%. It is also found that the charge collection results are reversible in that a lower temperature anneal restores the larger val- ues of/x~- after a prior high temperature anneal.

Figure 2 shows the data of the dependence of /x~- on anneal temperature for the two samples studied. The data indicate a reduction in/x~- by a factor 3-5 as the anneal temperature is in- creased. The results for the two samples are within experimental error and show no significant thickness dependence. The dashed line in the figure has an activation energy of 0.3 eV, with a measurement uncertainty of _+ 0.1 eV.

Figure 3 shows further drift mobility data, giv- ing the dependence of the hole drift mobility on the annealing temperature for undoped and p- type material [8]. Equilibration in p-type a -Si :H occurs at a lower temperature and so these mea- surements are performed in the range 80-210 ° C. No change in the drift mobility is observed in either sample.

3. Discussion

The loss of charge collection in the time-of- flight measurement is caused by deep trapping of carriers. The value of /x~ is inversely propor- tional to the density, N D, of deep traps. Analysis of a trapping model appropriate for transport at a mobility edge, for a defect with capture cross-sec- tion, o-, gives [9]

txT"N D = e a / 6 o ' k r ~ 2.5 × 10 8 cm-1 V-1 (3)

The numerical value in eq. (3) is the experimental result for capture of electrons in undoped a-Si : H, obtained from ESR measurements of the defect density [9], and corresponds to a defect cross-sec- tion of 3 × 10-is cm 2, which is typical for many deep states in semiconductors. According to this expression, a measured /xr of 10 -7 cm 1 V - ! corresponds to a defect density of 2.5 × 1015 e m - 3.

The reduction in /x~- at increasing anneal tem- peratures Clearly indicates the formation of a

larger equilibrium defect density, and therefore confirms the ESR measurements of an increasing dangling bond spin density. The magnitude and temperature dependence of the results also agrees well with the earlier measurements which found an activation energy of 0.2-0.3 eV [3-5]. The TOF data further indicate that the defects are in the bulk and uniformly distributed, ruling out the possibility of a surface effect. Only bulk defects cause deep trapping in the time-of-flight experi- ment because carriers are transported through the film. The fit to the Hecht formula of eq. (1) demonstrates that the defects are approximately uniformly distributed. At low voltage, when the charge collection is about 0.1Q0, the carriers only move through a small fraction of the sample, while they traverse the whole sample when the voltage is large. A non-uniform defect distribu- tion would be exhibited as a deviation from a fit to eq. (1). A higher defect density in the first 1000-2000 A due to the band bending is possible, because the charge collection measurement is relatively insensitive to defects this close to the surface.

The lack of any measurable change in the drift mobility indicates that there is no detectable ef- fect on the band tail density of states distribution by defect equilibration. The hole drift mobility is similarly unchanged in undoped and p-type a- S i :H as is shown in fig. 3 [8]. The absence of an effect on the drift mobility seems consistent with the models of the conversion of weak bonds into dangling bonds [1,3,10]. Although the increase of the equilibrium dangling bond density implies a corresponding reduction in the weak bonds, the loss of a small density of states from the band tails would probably not be observable in the drift mobility. The data in fig. 2 indicate a change in defect density of less than 1016 cm -3. The density of states at the demaraction energy in a typical drift mobility experiment is estimated to be con- siderably larger than this value.

Bar-Yam et al. proposed that there is an equi- librium band tail density of states distribution of amorphous semiconductors [11]. This model pre- dicts that the exponential band tail slope in- creases proportionally with anneal temperature, and would have a very large effect on the carrier

R.A. Street, C.C. Tsai / TOF measurements of thermally induced defects in a-Si : H 165

drift mobility, as indicated in fig. 3. The absence of a detectable change in the electron or hole mobility indicates that the band tails do not equi- librate in a-Si : H. By contrast, a reversible change in the carrier mobility is observed in chalcogenide glasses when these materials are quenched from different temperatures near the glass transition [12]. This comparison of the two materials sug- gests that full structural equilibration occurs in the glasses but not in a-Si:H. The conclusion is consistent with the hydrogen glass model for a- Si:H, which proposes that the reversible equilib- rium structural changes in a-Si:H are associated with the hydrogen bonding, and that the silicon network is largely unchanged [2]. The glasses are in a liquid-like state above the equilibrium tem- perature, while a-Si:H remains solid.

4. Conclusions

The time-of-flight data support the conclusion that defect equilibrium occurs uniformly within the bulk of a-Si:H. The defect density obtained from the charge collection agrees with ESR data, and increases with temperature with an activation energy of about 0.3 eV. No change in the drift mobility is observed for different quenching temperatures.

The research is supported by the Solar Energy Research Institute.

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[4] R.A. Street and K. Winer, Phys. Rev. B40 (1989) 6236. [5] S. Zafar and E. Schiff, Phys. Rev. Lett. 66 (1991) 1493. [6] C. Lee, W.D. Ohlsen and P.C. Taylor, Phys. Rev. B36

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