Cd ratio on the optical constants and

Materials Science in Semiconductor Processing 41 (2016) 184–192

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Effect of Zn/Cd ratio on the optical constants and photoconductive gain of ZnO–CdO crystalline thin films A.M.M. Tanveer Karim a, M.K.R. Khan b,n, M. Mozibur Rahman b a b

Department of Physics, Rajshahi University of Engineering & Technology, Rajshahi 6204, Bangladesh Department of Physics, University of Rajshahi, Rajshahi 6205, Bangladesh

art ic l e i nf o

a b s t r a c t

Article history: Received 1 May 2015 Received in revised form 13 July 2015 Accepted 26 August 2015

Optical and photo-electrical properties of ZnO–CdO films with the ratio of Zn/Cd ¼1:0, 3:1, 1:1, 1:3 and 0:1 has been studied. XRD study confirms the combination of hexagonal ZnO and cubic CdO phase present in the polycrystalline sample. Atomic force microscopy results indicate that the crystal grains are agglomerated and surface roughness enhanced due to higher Cd concentration in ZnO. From optical studies, it is found that the transmittance and the band gap decreased as Cd content increased. Photoluminescence studies on ZnO–CdO films showed intense near-band edge emissions at room temperature and is attributed to recombination of excitons localized within band tail states likely caused by nonuniform Cd distribution in ZnO–CdO matrix. The dispersion of refractive index was analyzed by the Wemple–DiDomenico single-oscillator model. The third-order nonlinear polarizability is found high with higher concentration of cadmium at higher photon energies. Some other optical parameters such as dissipation factor, optical conductivity, interband transition strength, surface and volume energy loss have been calculated depending on dielectric constant evaluated from optical data. Finally, photoconductive gain and carrier lifetime have been calculated and found dependent on Zn/Cd ratio. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Thin films Optical Energy loss function Photoconductive gain

1. Introduction Thin film of transparent conducting oxide (TCO) which has high transparency through the visible spectrum attracts a lot of research interest due to their numerous applications such as light transparent electrodes, solar cells, thin-film photovoltaic and many other opto-electronic devices [1–5]. Among several of n-type TCOs, ZnO is regarded as a promising optoelectronic material due to its direct wide band gap (3.37 eV) and highest value of free exciton binding energy (60 meV) at room temperature (RT). This allows the excitonic transitions which leads higher radiative recombination efficiency for spontaneous emissions [6]. Also ZnO absorbs larger fractions of solar radiation [7] and the high thermal energy of ZnO (25 meV) leads to the extreme stability of excitons at RT to high temperatures [8]. CdO is also an n-type degenerate semiconductor of cubic structure with a direct band gap of 2.3 eV that combines many beneficial characteristics of both CdO and ZnO. So the band gap of ZnO can be modulated by substituting CdO which is one of the major requirements for the design of opto-electronics devices. Moreover, it is the fundamental and practical interest to study the Zn–Cd–O ternary system, since Zn n

Corresponding author. Fax: þ880 721750064. E-mail address: [email protected] (M.K.R. Khan).

http://dx.doi.org/10.1016/j.mssp.2015.08.037 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

and Cd belong to the same group in the periodic table and the ratio of Zn and Cd cations becomes important for obtaining a TCO film. These two attractive materials along with their derivatives have been fabricated by various methods, including molecular beam epitaxy [9], sol–gel [10], electrodeposition [11], thermal decomposition technique [12], pulsed laser deposition [13], sputtering [14], chemical vapor deposition [15] and spray pyrolysis [16]. Among these techniques, spray pyrolysis offers many advantages like low cost, easy handling, large area and nano-structured film productions. Many researchers concentrate on the preparation of alloy films made of ZnO and CdO and study the physical properties such as optical constants of these films [17]. The optical constants of ZnO– CdO films are key parameters to use these materials in optoelectronic devices. Also, in the field of material science, the energy loss process of single electron in the surface/bulk of the solid has attracted much attention. When the primary electron impinges and penetrates or single electron escapes it suffers certain energy loss through inelastic scattering process inside the solid. In general the energy loss function results from dielectric constant of the materials and the loss energy function is formed by the inter-band transitions [18]. The inter-band transitions involve two kinds of carriers and take place between the conduction band and the valence band. The process of intra-band transitions happen inside either the conduction or the valence band and involve only one

( 200) CdO

CdO

( 200) CdO

Zn/Cd =1:3

28

32

( 200) CdO

( 101) ZnO ( 101) ZnO

( 102) ZnO

( 200) CdO

Zn/Cd =1:1

36

( 102) ZnO

( 101) ZnO

( 002) ZnO ( 002) ZnO

( 100) ZnO

( 100) ZnO

Intensity (a.u.)

ZnO, CdO and ZnO–CdO films were deposited on glass substrate by spray pyrolysis technique [19] at 360 °C in air ambient. The glass substrates were cleaned by using lukewarm aqueous solution of sodium carbonate, nitric acid and distilled water. The spray solution was prepared by mixing 0.1 M of zinc acetate dehydrate [Zn(COOCH3)2 2H2O] and cadmium acetate dehydrate [Cd(COOCH3)2 2H2O] diluted in ethanol and de-ionized water at 1:1 ratio. Then to obtain ZnO, ZnO–CdO and CdO thin films, the ratios of Zn/Cd were varied by 1:0, 3:1, 1:1, 1:3 and 0:1. The solution flow rate and gas pressure were kept constant at 1.0 ml/min and 105 N/m2, respectively. The distance between spray nozzle and substrate was kept at 0.21 m, spraying time was 20 min. The temperature was controlled through a copper–constantan thermocouple. The thicknesses of the films were (180720) nm measured using Newton's rings method. The x-ray diffraction study was performed by using Cu kα monochromatic radiation of wave length λ ¼0.154187 nm to record diffraction profile in between Bragg angle of (28–60)° by means of a x-ray diffractometer (SHIMADZU XRD 6000). The surface morphology of ZnO–CdO films was studied by non-contact mode atomic force microscopy using NCHR-tip (AFM-Model: XE70 PARK SYSTEMS). Optical transmission data was obtained using a UV–visible spectrophotometer (UV-1601 PC SHIJMADZU). The photo-conductivity was measured using an artificial solar simulator AM1.5 (Model: SET-140 F, SERIC Ltd. Japan).

( 100) ZnO ( 111) CdO ( 002) ZnO

2. Experimental

40 44 48 2θ (Degree)

Zn/Cd =3:1

ZnO

52

Ihkl Ir (hkl) 1 N

(

∑I

Ihkl

r (hkl)

)

(1)

where Ir(hkl) is the intensity of the reference (hkl) plane (JCPDS card nos. 361451 and 732245) I(hkl) is the observed intensity of the (hkl) plane and N is the number of diffraction peaks. The calculated Tc(hkl) values of different planes are included in Table 1. The value Tc(hkl) ¼1 represents films with randomly oriented crystallites while higher values indicate the abundance of grains oriented in a given (hkl) direction. From Table 1, it is clearly seen that the crystalline orientations are distributed among the planes without having any regular habit with the composition of the films. The 2D and 3D surface morphologies of ZnO–CdO crystalline films are studied by Atomic Force microscopy (AFM) as displayed in Fig. 2 (a–e). The AFM micrographs show that the films are composed of agglomerated grains perpendicular to the c-axis and the grain size

60

Table 1 The texture coefficient, Tc(hkl) of major (hkl) planes of ZnO–CdO thin films. Lattice parameters (nm)

Texture coefficients Tc(hkl)

a

c

(100)

(002)

(200)

0.3203 0.3260 0.3308 0.3382 0.3346

0.5135 0.5030 0.5024 0.5308

0.875 1.233 0.767 1.182 –

1.03 0.92 1.98 – –

– 1.49 0.452 1.21 1.22

3. Result and discussion

Tc (hkl) =

56

Fig. 1. XRD patterns of ZnO–CdO thin films with different Zn/Cd ratios.

Sample

The XRD patterns of the films are shown in Fig. 1. From XRD patterns, it is clear that the films are polycrystalline in nature and is a combination of hexagonal ZnO and cubic CdO phases. The lattice parameters estimated from most oriented planes are tabulated in Table 1. The micro-structural details depending on Zn/ Cd are given in [20,21]. The texture coefficient, Tc(hkl), of a film is a parameter that solely governed the preferential orientation of a crystal has been calculated for (100), (002) and (200) planes using the expression [22]

( 110) ZnO

( 111) CdO

( 100) ZnO

( 111) CdO

type of carrier. In this paper, we report the preparation and characterization of pure ZnO, CdO and a series of ZnO–CdO films. The main interest of the current work is to study the effect of Zn/Cd ratios on the structural, morphological, optical and photo-electrical properties of these films to use opto-electrical devices.

185

( 220) CdO

A.M.M.T. Karim et al. / Materials Science in Semiconductor Processing 41 (2016) 184–192

ZnO Zn/Cd ¼3:1 Zn/Cd ¼1:1 Zn:Cd ¼ 1:3 CdO

Roughness R a (nm)

0.311 0.788 0.417 0.541 0.627

increases with increasing Cd concentration. The average surface roughness, Ra of ZnO–CdO nano-crystalline films are shown in Table 1. The surface roughness varies with Zn/Cd ratio and the maximum roughness is found for film of Zn/Cd¼3:1. In order to evaluate different optical constants, transmittance spectra of the films were taken in the wavelength range 380– 800 nm as shown in Fig. 3. The transmittance spectra of the films show a fundamental absorption edge in the wavelength 400 nm. In the visible spectral range, the transmittance of the ZnO–CdO films varies from 78% to 50% depending on the Cd concentration. In the Cd content rich samples, comparatively higher grain size increases surface scattering and impurity centers which may reduce the transmission of the films. This behavior can be attributed to the higher value of the surface roughness of the films with higher Cd content due to the increase of optical scattering of incident light on the film surface. Moreover, free electron absorption due to oxygen deficiency may reduce the transmittance in the visible spectrum region and confirms the increases of carrier concentration with increasing Cd concentration [23]. However, the presence of transmission band indicates that the films are (band

186


Zn/Cd=1:0

Zn/Cd=3:1

(b)

Zn/Cd=1:1

Fig. 2. 2-D and 3-D AFM images of ZnO–CdO thin films.

pass/band stop) optical filter material depending on wave length. The optical band gap, Eg was obtained by extrapolating the linear part of Tauc's [24] plot of (αhν)2 as a function of hν of Fig. 4. Τhe intercept of the energy axis gives the value of Εg which decreased from 3.20 eV to 2.21 eV with increasing Cd concentration and the main effect of Cd concentration is a red shift of the absorption edge accounting for reduction of the band-gap. The slight

red shift observed in the emission peak could be attributed to the existence of some Cd phase [10]. Such a decrease in band gap has also been accounted for the fact that band gap of CdO (bulk value 2.23 eV) is less than that of ZnO (bulk value 3.37 eV) [25]. Since the band gap of CdO is lower compared to ZnO, some extra electrons may be present in the Fermi level which reduces the transparency of the films. Thus we have a decrease in transmission in the visible


187

Zn/Cd=1:3

Zn/Cd=0:1

Fig. 2. (continued)

1.6

80

2

1.28

60

2

40 ZnO Zn/Cd=3:1 Zn/Cd=1:1 Zn/Cd=1:3 CdO

30 20 10

ZnO Zn/Cd=3:1 Zn/Cd=1:1 Zn/Cd=1:3 CdO

0.96

14

50

(α h ν) x 10 (eV/m)

Transmittance (%)

70

400

500 600 700 Wavelength (nm)

800

Fig. 3. Transmission spectra of ZnO–CdO thin films with different Zn/Cd ratios.

region upon increasing Cd concentrations. Moreover, the decrease in band gap can also be explained with the existence of Cd impurities in the ZnO structure leading to lowering the conduction band towards Fermi energy that causes a decrease in the band gap. Thus, the excitation energy for the transition of electrons from the valence band to conduction band becomes small, i.e. the radiative recombination of these excitons may lead to the observed red shift [26].

0.64

0.32

0 1.5

2

2.5 hν (eV)

3

3.5

Fig. 4. Band gap for ZnO–CdO film with different Zn/Cd ratios.

The optical emission characteristics of ZnO–CdO were studied by photo luminescence (PL) spectra using excitation energy 4.133 eV (λ ¼ 300 nm) as shown in Fig. 5. A strong emission peak observed at wavelength 393 nm resulted from excitonic recombination corresponding to the band-edge emission of the ZnO–CdO nano-crystals. Besides, a relatively strong green band centered at about 500 nm occurred due to deep-level or trap-state emission [27]. Van Heusden et al. [28] found that only the single ionized oxygen vacancies are responsible for the green

188


transition strengths can be provided from the M 1 and M 3 moments of the optical spectrum. The M 1 and M 3 moments are expressed as [31]:


PL Intensity (a.u)

680

510

M−1 =

170

400

450 500 Wavelength (nm)

550

Fig. 5. Room temperature photoluminescence spectra of ZnO–CdO thin films.

χ (3) luminescence in the ZnO–CdO nano-films. Green band may also be attributed to zinc interstitials (Zni). Compared with the PL spectrum of ZnO, Zn/Cd ¼1:1 and CdO films exhibit blue shift because more electrons contributed by cadmium dopants would take up the energy levels located at the bottom of conduction band. Thus radiative recombination of these excitons will lead to a blue shift [29]. The refractive index of the films has been calculated using the relation [30]: 2 ⎤1/2 ⎛ 1 + R ⎞ ⎡ ⎛ (1 + R) ⎞ ⎟ + ⎢⎜ n=⎜ ⎟ − 1 − k 2⎥ ⎝ 1 − R ⎠ ⎢⎣ ⎝ (1 − R)) ⎠ ⎥⎦

(2)

The dispersion of refractive index below the interband absorption edge can be analyzed using the single-oscillator model [31]:

n2 − 1 =

E02

Ed E0 − (hυ)2

(3)

where n is the refractive index of the films, (hυ) is the photon energy, E0 is the oscillator energy which is related to the lowest direct gap, and Ed is the dispersion energy and is independent of both the absorption threshold band gap and the lattice constant. The experimental verification of Eq. (3) can be obtained by plotting (n2 1) 1 versus (hυ)2 as shown in Fig. 6. The oscillator energy E0 and dispersion energy Ed are obtained from the slope (E0Ed) 1 and intercept E0/Ed on the vertical axis of the straight line portion of (n2 1) 1 versus (hυ)2 plot, respectively. The obtained E0 and Ed values are given in Table 2. The oscillator energy E0 can be considered as an average energy gap and was found to be in proportion to the optical energy gap as E0 E 2.0Eg. A measure of interband

1.8

2

(n -1)

1.2

0.9

0.6 4.5

5

5.5

6

6.5

7

7.5

8

8.5

2 2 (hν) (eV) 2

Fig. 6. Plot of (n 1)

1

(4)

(

(

))

⎤4 ⎥ ⎥ ⎥⎦

(5)

where A is a constant and equal to 1.7 10 10 esu. The optical susceptibility, χ(3) as a function of energy is shown in Fig. 7. This is an important parameter which indicates the possibility of using ZnO–CdO thin films as an optical switching. From Fig. 7, it is seen that the third-order nonlinear optical susceptibility increases with photon energy may be due to the decrease of M–O bond length of the films. It is also clear that the optical susceptibility increases slowly with the photon energy in the range up to 2.8 eV after that (near band edge energy) it increases sharply for the films with higher concentration of Cd. This means that Cd enriched films are more suitable for applying as optical switch after 2.8 eV. The source of dielectric constant is partially due to free electrons and to bound electrons as represented by the following relation [33]:

⎡ ⎛ e2 ⎞ N ⎤ n2=εL −⎢ ⎜ ⎥ λ2 ⎟ ⎣ ⎝ π ϵ 0 c2 ⎠ m* ⎦

(6)

where εL is the lattice dielectric constant, ϵ0 is the permittivity of free space, e is the charge of electron, c is the speed of light and (N/ m*) is the ratio of carrier concentration to its effective mass. The values of εL and (N/m*) are determined from the extrapolation of relation between n2 and λ2to λ2=0 and from the slopes of the curves of Fig. 8 for ZnO–CdO films and the values are listed in Table 2. The loss factor is the ratio of the imaginary part to the real part of the dielectric constants. It measures the loss rate of power of a mechanical mode, such as an oscillation in a dissipation system. The dissipation factor [34] can be calculated by the following relation:

ε2 ε1

(7)

The variation of dissipation factor ( tan δ ) of the investigated film with photon energy is shown in Fig. 9. From the figure, it is seen that the energy dissipation is increased with increasing Zn/Cd ratio. It is also seen that the dissipation factor decreases sharply in between 380 and 430 nm range and becomes almost flat at higher wavelength regions. This suggests that interactions between photons and electrons are taken place in the films at lower wavelength region. Optical conductivity, sopt is an important parameter to observe the optical response of ZnO–CdO films given by the relation [35]:

-1

1.5

Ed E03

⎡ Ε0 Εd ⎢ = Α⎢ 2 2 4 E π ⎢⎣ 0 − (hν )

tan δ = ZnO Zn/Cd=3:1 Zn/Cd=1:1 Zn/Cd=1:3 CdO

M−3 =

The values of E0, Ed and moments M 1 and M 3 are given in Table 2. From the table it is clear that the oscillator energy and the dispersion energy decreased with increased Cd content subsequently moments M 1 decreased while the reverse process occurs for M 3 moment. From the values of E0 and Ed, the third-order nonlinear optical susceptibility, χ(3), which is also called nonlinear polarizability parameter [32] is calculated using Eq. (5)

340

0 350

Ed , E0

2

with (hυ) for ZnO–CdO thin films.

σopt =

αnc 4π

(8)

where c is speed of light. The optical conductivity of the films was


189

Table 2 Band gap and different optical parameters of ZnO–CdO thin films. Sample

Eg (eV)

E0 (eV)

Ed (eV)

M1

M 3 (eV) 2

εL

(N/m*)x1048 cm 3 gm 1

m* (gm)

ZnO Zn/Cd ¼ 3:1 Zn/Cd ¼ 1:1 Zn:Cd ¼ 1:3 CdO

3.20 2.97 2.70 2.43 2.21

6.62 6.26 5.44 4.76 4.24

12.12 11.36 9.44 6.81 5.24

1.83 1.81 1.73 1.43 1.23

0.041 0.046 0.058 0.063 0.068

2.49 2.14 2.00 1.71 1.56

2.22 2.29 1.80 2.04 2.10

1.18 10 31 1.96 10 31 4.46 10 31 1.56 10 29 3.71 10 28

2.5 ZnO Zn/Cd=3:1 Zn/Cd=1:1 Zn/Cd=1:3 CdO

op

σ x 1014 (Sec)-1

2.0

1.5

1.0

0.5

0 1.5

Fig. 7. Optical susceptibility χ(3) as a function of hν for ZnO–CdO thin films.

3.5

3

n

2

2.5


1.5

1

0

1

2

3 2

4 -13

λ x 10

5

6

7

2

0.5 ZnO Zn/Cd=3:1 Zn/Cd=1:1 Zn/Cd=1:3 CdO

0.4

tanδ

0.3

2.5 hν (eV)

3

3.5

Fig. 10. Variation of optical conductivity with hν for ZnO–CdO thin films.

found in the order of 1014 s 1 and this conductivity substantially increased with increasing Zn/Cd ratio in whole spectrum range as observed from Fig. 10. It is also seen that sopt increases with increasing photon energy up to 2.8 eV after which increases sharply towards saturation. This high optical conductivity is due to increase of electron extinction by photon energy and the order of magnitude of the optical conductivity confirms very high photo response of the film leads to an increased electronic transfers through the materials [36]. The complex interband transition strength can be calculated from the real and imaginary parts of the dielectric constant data by Eq. (9). In general, the interband transitions originate from the excitation of electrons in the valence band to an empty state in the conduction band, so these can be identified as transition in a band structure model [37]:

m

Fig. 8. Variation of n2 with λ2 for ZnO–CdO thin films.

2

Re Jcv + Im Jcv =

m24π 2 (hν )2 ( ε2 + iε1) 2 e2h2

(9)

where mo is the mass of the electron, h is Plank's constant, e is the charge of electron and (hν) is the photon energy. From the equation it is clear that the real part of interband transition strength ReJcv is proportional to the imaginary part of the dielectric function and the imaginary part of interband transition strength ImJCV cvis proportional to the real part of the dielectric function. For computational convenience we take the factor 6

0.2

0.1

0

400

500 600 700 Wavelength (nm)

800

Fig. 9. Variation of tan δ with wavelength for ZnO–CdO thin films.

3

m24π 2 e2h2

in Eq. (9),

whose value in CGS units is 8.289 10 g cm eV 2. Thus, the variation of calculated ReJcv and ImJcv with photon energy is shown in Figs. 11 and 12. From these figures it is clear that both ReJcv and ImJcv increase with increasing photon energy and denotes that the probability of electronic transition increases with increasing photon energy. Fig. 11 also shows that the interband transition strength ReJcv grows significantly when the photon energy is around 2.8 eV, which indicates most of the high absorption happens in the ZnO–CdO films that increases the excitation of the electrons. From Fig. 12, some noticeable peaks are observed after 2.8 eV that leads to single electron excitation process occurs in the films. The energy loss functions of transparent ZnO–CdO thin films

190


each photon absorbed per second that creates an electron–hole pair. Here G is calculated from the ratio between the photocurrent to the dark current at the different bias voltages and can be expressed as [39]:

25

cv

-6

Re J x 10 (eV)

2

20

15


G = τ /Tr

where τ is the carrier life time, Tr is the transition time and it is expressed [39] by:

10

Tr =

5.0

0 1.5

2

2.5 hν (eV)

3

3.5

Fig. 11. Real part of interband transition strength as a function of hν for ZnO–CdO thin films.

9

cv

-5

Im J x 10 (eV)

2

8 7

(12)


6

l2 μVB

(13)

where μ is the Hall mobility of the films, distance between two current leads, l¼ 1 cm and VB is the applied voltages. The variation of photo-conductive gain and photo-carrier lifetime with different bias voltages is shown in Fig. 14 and Fig. 15, respectively. From Fig. 14, ZnO–CdO nano-crystals show low photoconductive gain, in general, due to high photo-current of the films. However, the gain of CdO is high compared to other samples because of its low photocurrent. Under illumination, photo-generated holes migrate to the surface and are trapped, leaving behind unpaired electrons in the films that contribute to the photocurrent. The carrier lifetime as shown in Fig. 15, of the unpaired electrons are decreased with increasing applied voltages due to the failure of oxygen molecules desorption from the surface at higher applied voltages when holes neutralize the oxygen ions [40].

5 4

4. Conclusions

3

In this work, a series of ZnO–CdO thin films successfully prepared by a very simple cost effective spray pyrolysis method. XRD results show that the synthesized ZnO–CdO samples are polycrystalline with combination of hexagonal ZnO and cubic CdO phases. The lattice parameters a increased and c decreased with increasing Cd content in ZnO. The AFM study reveals that the crystal grains are agglomerated in the c-axis. The surface of the films changes with Zn/Cd ratio and observed to increase roughness with increased Cd content. The optical band gap of ZnO–CdO film was found slightly red shifted to longer wavelength due to the effect of increased Cd concentration. A room temperature PL spectrum shows near-band-edge emissions governed by recombination of excitons localized within the band tail. The dispersion of the films has been estimated using the Wemple–DiDomenico single oscillator model and dispersion parameters E0, Ed and moments of the optical spectrum M 1 and M 3 were calculated. The nonlinear polarizability parameter found in the range 10 13 esu and increases with Cd concentration after 2.8 eV for ZnO–CdO films. The optical conductivity of the films was found very high of the order of 1014 that attributed to the increase of electron excitations. The complex interband transitions increases with increasing photon energies suggesting higher probability of electronic transitions in these films. The energy loss function VELF found higher than SELF and maximum excitation occurs at 3.3 eV. The high optical conductivity and photo-conductive gain reveal high photo-response of these films. Considering above results, ZnO–CdO crystalline films seem useful to make the materials a prominent one for switching and photovoltaic applications.

2 1.5

2

2.5 hν (eV)

3

3.5

Fig. 12. Imaginary part of interband transition strength as a function of hν for ZnO– CdO thin films.

are related to optical properties and important for optical communication applications. Also the energy loss function has an advantage of covering the complete energy range including valance interband transitions and core level excitations. The volume energy loss function (VELF) and surface energy loss function (SELF) describes the electron transitions of ZnO–CdO films at low and high energy and proportional to the characteristic energy loss of fast electrons traveling the bulk and surface of the material, respectively. These quantities were estimated using real and imaginary parts of the dielectric constant by the following equations [38]:

⎛ 1⎞ ε VELF = Im ⎜ ⎟ = 2 2 2 ⎝ ε* ⎠ ε1 + ε2

(10)

⎛ 1 SELF = − Im ⎜ ⎝ ε* +

(11)

⎞ ε2 ⎟= 1 ⎠ ( ε1 + 1)2 + ε 2 2

where ε* is the complex dielectric constant. The variation of VELF and SELF with photon energy is shown in Fig. 13 for different Zn/Cd ratio. From the figure it is clear that the energy loss functions by the free charge carriers have almost same behavior when they traverse through the bulk material and surface. It is also seen that the volume energy loss is greater than surface energy loss with photon energies for all samples and the maximum extinction occurs around 3.3 eV. The photocurrent gain G is an alternative way to define the photo-sensitivity of ZnO–CdO nano-crystals in terms of the number of charge carriers which pass between the films per second for

Acknowledgments One of the authors A.M.M. Tanveer Karim is thankful to the Ministry of Science and Information & Communication Technology (MOSICT) (Grant No. 39.012.002.01.30.014.2010-93(101)) of the Peoples Republic of Bangladesh for providing financial support.


0.2

0.14 VELF SELF

0.12

ZnO

VELF SELF

Zn/Cd=3:1

0.15

VELF & SELF

0.1 VELF & SELF

191

0.08 0.06 0.04

0.1

0.05

0.02 0 1.5

2

2.5 hν (eV)

3

0 1.5

3.5

2.5 hν (eV)

3

3.5

3

3.5

0.3

0.16 VELF SELF

0.14

Zn/Cd=1:1

VELF SELF

0.25

VELF & SELF

0.12

VELF & SELF

2

0.1 0.08

Zn/Cd=1:3

0.2 0.15 0.1

0.06

0.05

0.04 0.02 1.5

2

2.5 hν (eV)

3

0 1.5

3.5

2

2.5 hν (eV)

0.4 VELF SELF

0.35

CdO

VELF & SELF

0.3 0.25 0.2 0.15 0.1 0.05 0 1.5

2

2.5

3

3.5

hν (eV) Fig. 13. Variation of volume and surface energy loss with hν for ZnO–CdO thin films.

0.3


0.25

Gain

0.2 0.15


25 20 15

0.1

10

0.05

5 0 2

4

6

8

10

12

14

Voltage (Volt) Fig. 14. Photo-conductive gain as a function of applied voltage for ZnO–CdO thin films.

0 2

4

6

8 10 Voltage (Volt)

12

14

Fig. 15. Carrier lifetime as a function of bias voltage for ZnO–CdO thin films.

192


We gratefully acknowledge Dr. M. Faruk Hossain, Department of Electrical and Electronic Engineering, Rajshahi University of Engineering and Technology, Bangladesh for providing photo-electrical measurement.

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