Photoelectrical properties of (Sb

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Photoelectrical properties of (Sb

₁₅

As

₃₀

Se

₅₅

)

_100x

Te

_x

(0 6 x 6 12.5 at.%) thin ﬁlms

K.A. Aly

^a,*

, A.M. Abousehly

^a

, A.A. Othman

^b

aPhysics Department, Al-Azhar University, Assiut, Egypt

bPhysics Department, Assiut University, Assiut, Egypt

Received 4 December 2006; received in revised form 26 July 2007 Available online 10 September 2007

Abstract

This paper reports photoelectrical properties of (As30Sb15Se55)_100xTexamorphous chalcogenide films (06x612.5 at.%) through measurements of ‘steady state’ and ‘transient’ photocurrents. The composition dependence of the steady state photocurrent at room temperature shows that the photoconductivity increases while the photosensitivity decreases with increasing Te content. A study of photoconductivity of (As30Sb15Se55)_100xTexat different levels of light intensity reveals that, the photoconductivity increases exponentially with increase in light intensity. The Photocurrent (I_ph) when plotted against light intensity (G) follows a power law (I_ph=G^c) the expo- nentcfor (As30Sb15Se55)_100xTexfilms has been found nearly 0.5 suggesting bimolecular recombination. The transient photoconductivity shows that the lifetime of the carrier decreases with increasing the light intensity. This decrease suggests that the photoconductivity mechanism in our samples was controlled by the transition trapping processes. The increase of Te content results in a monotonic decrease in the band gap and the free carrier life time of (As30Sb15Se55)_100xTexthin films. These results were interpreted on the basis of the chemical-bond approach.

PACS: 61.66.Dk; 73.50.Pz; 73.61.r

Keywords: Alloys; Transition metals; Solar cells; Photovoltaics; Electrical and electronic properties; Radiation eﬀects

1. Introduction

Chalcogenide glasses are an important class of materials that contain one or more chalcogen element, sulphur, sele- nium or tellurium in combination with elements from IVth, Vth or VIth group of the periodic system of elements [1].

Chalcogenide glasses have received a lot of attention due to their use in various solid state devices. Photoconductiv- ity measurements in these materials are very important from application point of view as well to understand the nature of localized states which are quite likely in these materials[2]. Photoconductivity studies on several chalcogenides have been carried out by many workers[3–6]. The addition of tellurium to the later glasses would be expected

to decrease their glass transition temperature and reduce their thermal stabilities [7]. According to Kastner [8], the addition of an element with a higher electropositive character than the elements in the host material will tend to decrease the activation energy of the electrical conductivity.

In the present study, the effect of the addition of tellurium on the steady state (dc) and the transient (ac) photoconductivity of thermally evaporated thin films of amorphous (As30Sb15Se55)100xTex (x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) is investigated. The effect of applied electric filed, light intensity, and temperatures on the ac- and dc-PC is also investigated.

2. Experimental details

The semiconducting (Sb15As30Se55)_100xTex(withx= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) chalcogenide glasses was

doi:10.1016/j.jnoncrysol.2007.07.022

* Corresponding author. Tel./fax: +20 88 2325647.

E-mail address:[email protected](K.A. Aly).

www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 354 (2008) 909–915

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prepared from their components of 99.999% high purity. The proper amount for each material was weighed, and then the weighed materials were intro- duced into cleaned silica tubes. To avoid the oxidation of the samples the tube was evacuated to 1.33·10³Pa, then put into a furnace at around 1250 K for 24 h. Dur- ing the course of heating the ampoule was shaken several times to maintain their uniformity. Finally, the ampoule was quenched into ice cooled water to avoid crystallization.

The amorphous state of the material was confirmed by a diffractometric X-ray scan (Philips diffractometer 1710) using Cu as target and Ni as filter (k= 1.542 A˚ ). Energy dispersive X-ray spectroscopy (Link analytical EDS) was used to measure the elemental composition and indicates that the investigated composition is correct up to

±0.3 at.%.

Thin films were prepared by thermal evaporation of small ingot pieces onto chemically cleaned glass substrates (microscope slides). The thermal evaporation process was performed by using a coating (Denton Vacuum 502 A) system, at a pressure of approximately 1.33·10³Pa. During the deposition process (at normal incidence), the substrates were suitably rotated in order to obtain films of uniform thickness. The thickness of the films lies in the range 900–940 nm. For electrical and photoelectrical measurements, two gold electrodes with a spacing of 0.001 m and length of 0.014 m were deposited. Silver paste is used for making the contacts; the electrode contacts had an ohmic character as determined by current–voltage characteristics.

Measurements of dark and photoconductivity of (Sb₁₅As₃₀Se₅₅)_100xTe_x thin films were carried out in the temperature range 300–400 K under vacuum of about 1.33·10³Pa. The measuring system consists of a regu- lated dc power supply (Pasco, model 1030A) in series with the sample and Keithley electrometer (model 610C). The photocurrent (Iph) was determined as the difference between the currents measured with and without illumination. The light source was a 200 W tungsten lamp. Small fluctuations in the measured photocurrent (0.2%) of stud- ied films.

The intensity of the light source was measured with the help of a photocell. The current of the photocell was directly proportional to the intensity of the light. We used the relative light intensity (G) as the ratio between the light intensity and the maximum light intensity.

For ac-PC, SR 540 Stanford mechanical chopper and load resistance R equal to 106X are used. The variation across the load resistance due to the modulated PC was measured by a lock-in ampliﬁer (SR 830 Stanford Research System).

The temperature was controlled and stabilized with an accuracy of 0.1 K by using cryogenic system which consists of ITC-4 temperature controller and DN 1714 Oxford cryostat. Monochromatic light was obtained by using Zeiss monochromator (model MQ3).

3. Results

3.1. Steady state photoconductivity

In most of the chalcogenides, the dc dark conductivity (rdc) can be expressed by the following relation[4]:

rdc¼r0expðDEdc=KTÞ; ð1Þ wherer0is the pre-exponential factor, DEdcis the activation energy for dark conduction,kis Boltzmann’s constant andTis the absolute temperature. When ln[rdc] is plotted versus 1/Ta straight line is obtained whose slope isDEdc/K and intercept is ln(r0) as shown inFig. 1.

Fig. 1 represents the linear fit for temperature dependence of dc dark conductivity for different compositions (Sb15As30Se55)100xTex (with x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin films at applied electric field 4·10³V cm¹ (the average regression coefficientR= 0.997 ± 0.002).

On the other hand, Eq.(1)can be rewritten in the form of

rph¼r0exp DEph=KT

ð2Þ with the same constants as shown in Eq.(1)for the deter- mination of the activation energy for photoconduction DEph [4]. When ln(rph) plotted versus (1/T) a straight line is obtained whose slope isDEph/K and intercept is ln(r0).

The temperature dependence of dc-photoconductivity (rph) for (Sb15As30Se55)100xTex (with x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin films at applied electric field 4·10³V cm¹ and relative light intensity (G= 1.0) is shown in Fig. 2. (The average regression coefficient R= 0.997 ± 0.001).

2.4 2.6 2.8 3.0 3.2 3.4

-22 -20 -18 -16 -14 -12 -10

Te content 0 at. % 2.5 at. % 5 at. % 7.5 at. % 10 at. % 12.5 at. %

ln [σdark (Ω-1cm-1)]

1000/ T (K^-1)

Fig. 1. Temperature dependence of dc dark conductivity of (As30Sb15- Se55)100xTex(06x612.5 at.%) thin ﬁlms.

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The photosensitivity of the material is an important parameter in PC measurements and it expressed by the (rph/rdc) ratio at a particular temperature and light intensity. Therefore, the deducted values ofr0,rdc,DEdc,DEph

and the calculated values of rph/rdc of (As30Sb15-

Se55)_1xTex(x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin ﬁlms at room temperature andG= 1.0 were listedTable 1.

The dependence of photocurrent on the light intensity has been carried out in order to investigate the nature of the recombination process occurs in (As30Sb15Se55)_100xTex

thin films. It was found that the intensity dependence of photocurrent found to obeys to the power law (Iph aG^c) where the exponent cdetermines whether the recombination process is bi- or monomolecular[9]. The slope of linear relation between ln(Iph) and the relative light intensity (G) yields (c).Fig. 3 shows the variation in the photocurrent at room temperature and applied electric field 4·10³V cm¹and at different illumination levels is shown inFig. 3.

Fig. 4shows the composition dependence of spectral distribution of dc-photoconductivity of (As30Sb15Se55)_100x-

Tex (x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin films measured at room temperature, G= 1.0 and applied electric field 4·10³V cm¹. This figure investigate that, the addition of Tellurium shifts the photocurrent peak into the low-energy side of the spectrum that leads to the decrease in the energy gap. The energy gap (Eg) of (As30Sb15Se55)_100xTex thin films was calculated by using the relation[10]:

E_gðeVÞ ¼hc=k1=2; ð3Þ

wherehis the Plank’s constant,cis the velocity of light and k1/2is the wavelength (nm) at which the photocurrent falls to half its maximum value. The obtained values ofE_gare plotted as function of Te content as shown in Fig. 5(a).

The optical band gap Egwas fitted as second polynomial function in the form (Eg= 1.490.0085(Te)4.12· 10⁴(Te)²) and the regression coefficientR is 0.991. Also, the cohesive energies (CE) were estimated by summing the bond energies over all the bonds expected in the material. The calculated values of the cohesive energies were plotted as a function of Te content and also fitted to second polynomial function in the form (CE = 2.2

2.4 2.6 2.8 3.0 3.2 3.4

-19 -18 -17 -16 -15 -14

-13 Te content

0 at. % 2.5 at. % 5 at. % 7.5 at. % 10 at. % 12.5 at. %

ln [σphoto (Ω-1cm-1)]

1000/ T (K^-1)

Fig. 2. The variation of dc-PC with temperature atG= 1.0 and applied electric ﬁeld 4·10³V cm¹of Tex(As30Sb15Se55)100xthin ﬁlms.

Table 1

The dc dark conductivity (rdc), the pre-exponential factor (r0), the photosensitivity (rph/rdc), the activation energy for dc conduction (DEdc) and the activation energy of photoconduction (DEph) of Tex(As30Sb15Se55)_100xthin ﬁlms

Te content at.% rde·10⁸± 1% (X¹cm¹) at 300 K r0·10³± 1 (X¹cm¹) rph/rdcat 300 K DEdc(eV) ± 1% DEph(eV) ± 1%

0 0.805 3.15 8.41 0.742 0.299

2.5 1.648 1.81 5.321 0.717 0.328

5 2.238 1.44 4.46 0.700 0.346

7.5 3.458 1.13 3.38 0.688 0.362

10 8.057 0.836 1.96 0.655 0.374

12.5 14.56 0.472 1.27 0.626 0.39

-1.8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0

-21.3 -21.0 -20.7 -20.4 -20.1 -19.8

Te content 0 at. % 2.5 at. % 5 at. % 7.5 at. % 10 at. % 12.5 at. %

ln [photocurrent (A)]

ln [G (A.U.)]

Fig. 3. The dependence of photocurrent on relative light intensity (G) measured at room temperature and applied electric ﬁeld 4·10³V cm¹of (As30Sb15Se55)_100xTexthin ﬁlms.

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0.0062(Te)6·10⁵(Te)²) with regression coeﬃcient (R= 0.99) as shown inFig. 5(b).

The dependence of the photocurrent on the relative light intensity of (As30Sb15Se55)_100xTex where (x= 0) at room temperature and applied electric ﬁeld 4·10³V cm¹, is presented graphically as shown inFig. 6.

3.2. Transient photoconductivity

The composition dependence of ac-PC (rac) of (As30Sb15Se55)_100xTex (where x= 0, 7.5 and 12.5) thin

ﬁlms at room temperature, relative light intensity (G= 1.0) and at applied electric ﬁeld 4·10³V cm¹ is shown in Fig. 7(a). The observed decrease of rac/r_ph with the increasing the chopping frequency (F) was found to obey the relation[11]:

rac=rph¼tanh½1=ð4FtÞ; ð4Þ where t is the time. Using the above relation, the carrier lifetime (s) could be evaluated. This can be done by draw- ing a straight line parallel to the frequency axis at the height of 0.76 from the maximum (where tanh(1) = 0.76) and dropping a normal from the point of intersection on the frequency axis, to cut oﬀ a segment equal to 1/(4s) from the frequency axis, thenscan be estimated. The observed increase of therac/rph with the increase in Te content as shown inFig. 7(a) leads to the decreases of the free carrier life time (s) (seeFig. 7(b)).

The dependence of carrier lifetime on the applied electric ﬁeld as a function of Te content of (As30Sb15Se55)_100xTex

(x= 0, 2.5, 5, 7.5, 10, and 12.5 at.%) thin ﬁlms at room temperature and at relative light intensity (G= 1) is shown inFig. 8.

7800 800 820 840 860

1 2 3

Wavelength (nm) Photocurrent x 10-9 (A)

G = 0.8 G = 1.6 G = 2.4 G = 3.2 G = 4.0 X = 0

Fig. 6. The dependence of the spectral distribution of steady state photocurrent on the relative light intensity G measured at room temperature and at applied electric ﬁeld 4·10³V cm¹for As30Sb15Se55

thin ﬁlms.

0 20 40 60 80 100

0.3 0.6 0.9

0 2 4 6 8 10 12 14

1.16 1.20 1.24 1.28 1.32

Te content 0 at.%

7.5 at.%

12.5 at.%

σac / σph

F (Hz)

Te conten at.%

τ (10-2 sec)

Fig. 7. (a) The chopping frequency dependence of ac-PC and (b) the composition dependence of carrier lifetime of (As30Sb15Se55)100xTex

(x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin ﬁlms at room temperature, G= 1.0 and at applied electric ﬁeld 4·10³V cm¹.

800 850 900 950

0 2 4 6 8

10 Te content 0 at. % 2.5 at. % 5 at. % 7.5 at. % 10 at. % 12.5 at. %

Wavelength (nm) Photocurrent x 10-9 (A)

Fig. 4. The composition dependence of spectral distribution of photocurrent of Tex(As30Sb15Se55)_100xthin ﬁlms at room temperature,G= 1.0 and applied electric ﬁeld 4·10³V cm¹.

0 2 4 6 8 10 12 14

1.30 1.35 1.40 1.45 1.50 1.55

2.10 2.12 2.14 2.16 2.18 Eg 2.20

Te content. % Eg (eV)

CE

CE (eV)

a b

Fig. 5. The variation of both optical band gap (Eg) and the cohesive energy (CE) as a function of Te content Tex(As30Sb15Se55)_100xthin ﬁlms.

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Fig. 9 represented the effect of light intensity on the carrier lifetime as a function of Te content for (As30Sb15- Se55)_100xTex (x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin films at room temperature and at applied electric field 4·10³V cm¹. Finally, the temperature dependence of the carrier lifetime as a function of Te content of for (As30Sb15Se55)_100xTex (x= 0, 2.5, 5, 7.5, 10, and 12.5 at.%) thin films at relative light intensity (G= 1.0) and at applied electric field 4·10³V cm¹ is shown in Fig. 10.

4. Discussion

4.1. Steady state photoconductivity

The obtained results shown in Fig. 1indicate that, the conduction in (As30Sb15Se55)1xTex ﬁlms is through an activated process with single activation energy in the temperature range 300–400 K. According to Mott and Davis [12]the value ofr0in the range l0⁵–10⁶X¹m¹in chalcogenide glasses indicates that the conduction is mostly in extended states. Therefore, according to the calculated values ofr0shown inTable 1we can concluded that, the conduction in (As30Sb15Se55)_1xTex thin ﬁlms takes place in the extended states and it decrease with increasing Te content.

In Table 1 one notice that, the deduced values of the activation energy of photoconductionDEph is much smal- ler than that of dark conductionDEdcand it increases with the increase in Te content. This result is in good agreement with Mathur and Kumar[2], they found that the replace- ment of Se by Te in the Ge–Se system leads to the increase in DEph. Also, Table 1 shows that, the photosensitivity decreases with increasing Te content. Although the dark current increases with increasing Te content, the photocurrent does not increase by the same proportion. This behavior of photosensitivity reﬂects the increase in the density of defect states in (As30Sb15Se55)_100xTex thin ﬁlms with increasing Te content. These results are in good agreement with those obtained by Mehra et al.[13].

In the single trap analysis[14], the exponentcis equal to 1.0 for monomolecular recombination and 0.5 for bimolecular recombination while in case of the continuous

1 2 3 4

1.2 1.4 1.6 1.8 2.0

Te content 0 at.%

2.5 at.%

5 at.%

7.5 at.%

10 at.%

12.5 at.%

τ (10-2 sec)

E

Fig. 8. The carrier lifetime versus the applied electric ﬁeld as a function of Te content of Tex(As30Sb15Se55)100x(x= 0, 2.5, 5, 7.5, 10, and 12.5 at.%) thin ﬁlms at room temperature and atG= 1.

0.8 1.6 2.4 3.2 4.0

1.2 1.4 1.6 1.8 2.0

Te content 0 at.%

2.5 at.%

5 at.%

7.5 at.%

10 at.%

12.5 at.%

τ (10-2 sec)

G

Fig. 9. The carrier lifetime dependence on the light intensity as a function of Te content of (As30Sb15Se55)_100xTexthin ﬁlms at room temperature and at applied electric ﬁeld 4·10³V cm¹.

300 320 340 360 380 400

1.2 1.4 1.6 1.8

2.0 ^{0 at.%}

2.5 at.%

5 at.%

7.5 at.%

10 at.%

12.5 at.%

τ (10-2 sec)

Temperature (K)

Te content

Fig. 10. The carrier lifetime versus temperature as a function of Te content (As30Sb15Se55)_100xTex(x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin ﬁlms atG= 1 at applied electric ﬁeld 4·10³(V cm¹).

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distribution of traps, the value ofclies between 0.5 and 1.0 [9]. The deduced values ofcaccording to the power low law (I_ph aG^c) (see Fig. 3) have been found close to 0.5 for all the samples. This square root dependence of photocurrent on light intensity indicates the existence of bimolecular recombination in which recombination rate of electrons is proportional to the number of holes.

From Fig. 5one can notice that, the optical band gap decreases with the increase of Te content. The observed decrease of (Eg) with the increase in Te content can be interpreted on the basis of the chemical-bond approach [15].

The bond energiesD(A–B) for heteronuclear bonds have been calculated by using the empirical relation

DðA–BÞ ¼ ½DðA–AÞ DðB–BÞ²þ30ðv_A–v_BÞ² ð5Þ proposed by Pauling [16], where D(A–A) andD(B–B) are the energies of the homonuclear bonds (in units kcal/

mol) [17], vA and vB are the electronegativity values for the involved atoms[15]. Bonds are formed in the sequence of decreasing bond energy until the available valence of atoms is satisfied [18]. In the present compositions, the Se–Te bonds with the highest possible energy (44.197 kcal mol¹) are expected to occur first. Since the Sb–Se (43.981 kcal mol¹) followed by As–Se (41.706 kcal mol¹) to saturate all available valence of Se. There are still unsatisfied as which must be satisfied by As–As defect homopolar bonds. Based on the chemical-bond approach, the bond energies are assumed to be additive.

Thus, the cohesive energies (CE) were estimated by summing the bond energies over all the bonds expected in the material. The calculated values of the cohesive energies were plotted as a function of Te content as shown in Fig. 5. These results indicate that, the cohesive energies of these glasses show a decrease with increasing Te content.

Therefore, it can be concluded that the decrease of (Eg) with increasing Te (Fig. 5) content is most probably due to the reduction of the average stabilization energy by Te content. It should be mentioned that the approach of the chemical bond neglects dangling bond and other valence defects as a ﬁrst approximation. Also van der Walls inter- actions are neglected, which can provide a means for fur- ther stabilization by the formation of much weaker links than regular covalent bonds.

The observed increase of the photocurrent with increasing of the light intensity shown inFig. 6as a result to the increase of the incident photons on the sample.

4.2. Transient photoconductivity

The observed increase of therac/rphwith the increase in Te content (Fig. 7(a)) leads to the decreases of the free carrier life time (s) as shown in Fig. 7(b). The observed decrease of the free carrier lifetime reﬂects the increase of the density of defect states with increasing Te content in

(As30Sb15Se55)100xTex thin ﬁlms. The lifetime of excess carriers depends upon the density of localized states, when the density of localized states increases the lifetime decreases.

From Fig. 8one can notice that, as the applied electric ﬁeld increases the carrier lifetime decreases. This behavior is due to the increase of carrier velocity which leads to a decrease in the lifetime according to[19]:

s¼ ðS V nÞ¹; ð6Þ

whereVis the carrier velocity,Sis the capture cross section of free electron recombination centers andnis the electron concentration.

The rac/rph was found to increase with the increasing light intensity leading to the decrease of carrier life time as shown inFig. 9. This behavior ofrac/rphsuggests that, the PC mechanism in our case was controlled by the transition trapping processes. In the transition trapping processes the probability of trapping decreases with increasing the intensity of the exciting light. That can be interpreted on the basis of the traps which capture free carriers for a time release them back to the Free State by thermal re-excitation. This process could continue after the light had been removed. This slow emptying of the traps can maintain the PC decay after the light has been switched oﬀ for a long time compared with the decay time when the recombination process only was considered.

Finally, the observed decrease of the carrier lifetime with the increase of temperature (Fig. 10) can be ascribed to the increase in the carrier density in the free states due to the thermal re-excitation of the trapped carriers, which leads to a decrease of carrier lifetime according to the Eq.(5).

5. Conclusion

The temperature dependence of dark and photoconductivity of (As30Sb15Se55)_100xTex (x= 0, 2.5, 5, 7.5, 10 and 12.5 at.%) thin ﬁlms in the temperature range 300–400 K reveals that the conduction is through an activated process with a single activation energy. The activation energy for electrical conduction decrease with the increase in Te content while the photosensitivity decrease with increasing Te content. The intensity dependence of photocurrent indicates the existence of bimolecular recombination in (As30Sb15Se55)_100xTex thin ﬁlms in which recombination rate of electrons is proportional to the number of holes.

The transient photoconductivity measurements suggest that the photoconductivity mechanism in our samples is controlled by transition trapping processes. The free carrier life time decrease with the increase of Te content, Temper- ature, Applied electric ﬁeld, and light intensity. The chemical-bond approach can be applied successfully to interpret the decrease in the band gap of (As30Sb15Se55)100xTexthin ﬁlms with increasing Te content.

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