Effect of composition and forming parameter on evaporated CdSeTe films deposited at room temperature

doi:10.1016/S0022-3697(00)00029-9

Journal of Physics and Chemistry of Solids

Volume 63, Issue 1, January 2002, Pages 1-8

https://doi.org/10.1016/S0022-3697(00)00029-9 Get rights and content

Abstract

Amorphous films of the ternary compound CdSeTe, of composition in the range 0–10 at.% Cd and with a thickness of about 200 nm, have been prepared by thermal evaporation. The optical band gap was determined and found to be in the range 0.5–1.0 eV and arose from indirect transitions. A sharp decrease in the value of optical band gap (E_opt) is observed. The electrical properties of Cd thin films have been studied extensively. In general dc measurements have indicated Ohmic conductivity at low electric fields resulting from the thermal excitation of carriers from centers in the band gap. When the temperature is low enough so that carriers cannot be excited into one of the allowed bands, the dominant conduction may take place via hopping, whereby carriers hop from occupied to unoccupied sites located within a band of localized states situated within the band gap. The electrical and optical data were consistent and realized from the binding energy represented by the cohesive energy values. The generalized (8−n) rule was used to estimate the average co-ordination number. The obtained results were treated in the frame of the chemical bond approach proposed by Bicerano and Ovshinsky. The phase separation phenomena and the morphology were also studied for the prepared films.

Introduction

Amorphous semiconductors, in particular selenium alloys, exhibit the unique property of reversible transformations, which is useful in optical memory devices [1]. These glasses are of tremendous interest because of their wide range of transparency in the far-infrared region. The electrical properties of these glasses are influenced by the structural changes associated with thermal effects [2], [3]. An investigation of optical and electrical properties of the Se–Te–Cd system is reported in this paper for different Se:Cd ratios keeping the Te content at 30 at.%. Although the characterization of the different Se–Te binary systems has already been studied, the Se–Te–Cd system has not received much attention.

In chalcogenide glasses there are different conduction mechanisms that can be involved. Generally the conductivity (σ) in the chalcogenide can be written as follows [4] $σ(T)=σ_{0} exp (− Δ E_{σ_{0}} / K T)+σ_{1} exp (− Δ E_{σ_{1}} / K T)$ The two terms arise from two different conduction processes. The first term describes the high temperature region, where the dominant mechanism is the band conduction through the extended states. The pre-exponential factor (σ₀) depends on the compositions, ΔE_σ₀ is the electrical activation energy of conduction, K is the Boltzman constant, and T is the absolute temperature.

The second term is due to the hopping conduction via the localized states responsible for conduction in the second region. Here the conductivity arises from tunneling through unoccupied levels of the nearest neighboring center. The value of σ₁ is less than σ₀, partly because of the low density and mobility of the delocalized states.

Analysis of the optical absorption spectra is one of the most productive tools for understanding and developing the band structure and energy gap of both crystalline and amorphous systems. The absorption coefficient α has been calculated using the relation [5] $α(ω)=[1/d] ln [1−R)^{2} /T]$ where R is the reflectance, T the transmittance, α the absorption coefficient and d the sample thickness. The absorption coefficient α(ω) near the band edge in many amorphous semiconductors shows an exponential dependence on photon energy usually obeying Urbach's empirical relation [6]. $α(ω)=α_{0} exp (h ω/E_{e})$ where α₀ is a constant, ω the angular frequency of the incident photon, ℏ Planck's constant h divided by 2π and E_e (eV) the width of the band tails of the localized states in the band gap.

A model based on the electronic transitions between the localized states is not preferable. For higher values $(α≥10^{4} cm^{−1}),$ the absorption coefficient (where absorption is associated with interband transitions) takes the form [4], [7] $α h ω=β(h ω−E_{opt})^{n}$ where β⁻¹ is the band edge parameter, n is a number that characterizes the transition process, which takes values 1/2, 1, 2, or 3, 3/2 depending on the nature of the electronic transitions responsible for the absorption, and E_opt is the optical gap.

Section snippets

Experimental procedure

Bulk amorphous Se_70−xTe₃₀Cd_x with 0≤x≤10 were prepared by fusing a mixture of the appropriate quantities of the elements in evacuated fused silica ampoules up to 900°C for 8 h and shaken several times to ensure complete homogeneity. The molten materials were quenched in ice water. Films were prepared by the thermal evaporation technique using a standard unit Edward 306 E coating unit with a conventional rotary and oil diffusion pump maintaining residual pressure in the order of $1.3×10^{−4} Pa .$ High

X-ray diffraction patterns of SeTeCd thin film

Fig. 1 shows the X-ray diffraction patterns of Se_70−xTe₃₀Cd_x with Cd=1, 3, 5, 7 and 10 at.%. The patterns reveal no sharp diffraction lines except x=0, which is characterized by a few patterns.

Effect of composition on the activation energy

The temperature variation of the dc conductivity of Se_70−xTe₃₀Cd_x 0≤x≤10 is shown in Fig. 2. It is clear that at the higher temperatures ln σ shows linear dependence on 1000/T, i.e. it exhibits semiconductor behavior and the slope of solid lines drawn in Fig. 2 decreases from 0.43 eV for undoped material to

Discussion

According to Fig. 1 we observed that the non-crystalline structure is the general form for the studied films except for x=0, which is characterized by a minute of crystallinity

By following Table 2, we noticed that σ_RT varies slightly on incorporation with Cd content. It has a maximum value of $12.30×10^{−4} Ω^{−} 1 cm^{−1}$ for Cd 10 at.%, which corresponds to minimum metallic conduction. Moreover the value of E_σ decreases monotonically with an increasing concentration of Cd.

Substitution of Cd for Se causes

Conclusion

We conclude that when Cd(10 at.%) increased both E_σ and E_opt decreased. This shift may be attributed to the formation of Cd–Se bonds and the decrease in concentrations of other bonds that exist in the glass. This may result in a perturbation in the system that will broaden the valence and conduction band edges in the mobility gap. In other words increasing Cd content decreased E by the generation of excess electronic delocalization. It is seen that the optical gap closely correlated with the

References (15)

J.R. Bosnell et al.
Solid State Electronics
(1972)
Jc. Phillips
J. Non-Cryst. Solids
(1979)
A.L. Allred
J. Inorg. Nucl. Chem.
(1961)
P. Predeep et al.
Phys. Status Solidi (a)
(1996)
G.B. Thomas et al.
Philos. Mag.
(1972)
N.F. Mott et al.
Electronic Process in Non-Crystalline Materials
(1979)
F.D. Emichellis et al.
Appl. Opt.
(1987)

There are more references available in the full text version of this article.

Cited by (30)

Study the hall effect and DC conductivity of CdSe and Te doped CdSe thin films prepared by RF magnetron sputtering method
2023, Materials Letters: X
CdSe and CdSe:Te thin films were grown on Si p-type substrates by RF magnetron sputtering method. The doping percentage of Tellurium (Te) in CdSe was 7% for the CdSe:Te thin film. The results show that after the doping of Te in the CdSe thin film, the conductivity changes from n-type to p-type and the mobility of the CdSe thin film increased. The conductivity of Te doped CdSe was found to be in order of 10⁻⁶ Ω⁻¹ cm⁻¹, while without doping it was 10⁻⁵ Ω⁻¹ cm⁻¹. X-ray diffraction (XRD) confirmed the presence of CdSe, Te and Si.
Fabrication of CdSexTe1-x thin films by sequential growth using double sources
2021, Physica B: Condensed Matter
CdSe_xTe_(1-x) (CST) ternary thin films were fabricated by stacking thermally evaporated CdSe and electron beam evaporated CdTe layers. The final structure was achieved in a stoichiometric form of approximately Cd:Se:Te = 50:25:25. The post-annealing processes at 300, 400, and 450 °C were applied to trigger the compound formation of CST thin films. The X-ray diffraction (XRD) profiles revealed that CdTe and CdSe have major peaks at 23.9° and 25.5° corresponds to (111) direction in cubic zinc-blend structure. Raman modes of CdTe were observed at 140 and 168 cm⁻¹, while Raman modes of CdSe films were detected at 208 and 417 cm⁻¹. The post-annealing process was found to be an effective method in order to combine both diffraction peaks and the vibrational modes of CdTe and CdSe, consequently to form CST ternary alloy. Transmission spectroscopy analysis revealed that CST films have direct band gap value of 1.6 eV.
Physical, linear and nonlinear optical properties of amorphous Se<inf>90-x</inf>Te<inf>10</inf>M<inf>x</inf> (M = Zn, In, Pb, x = 0, 5) chalcogenide thin films by electron-beam deposition
2021, Journal of Non-Crystalline Solids
Amorphous Se₉₀Te₁₀ and Se₈₅Te₁₀M₅ (M= Zn, In, Pb) were deposited as thin films on glass substrates by electron-beam evaporation. The effect of Zn, In and Pb incorporated at the expense of Se on optical and physical properties of chalcogenide Se₉₀Te₁₀ were investigated and results were correlated. The transmission and reflectance spectra of the samples were used to deduce some dispersive linear optical constants which were studied from 250 to 2500 nm. The optical gap was evaluated through extrapolation of the Tauc plot in the strongly absorbing region and based on the Wemple-DiDomenico single oscillator model. Ideas from the chemical bond approach and consideration of disorders emanating from localized states in the mobility gap were employed to explain the optical trends. Some dielectric and nonlinear optical parameters including nonlinear refractive index and susceptibility of the thin films were calculated.
Effect of bismuth incorporation on thermal properties of quaternary chalcogenide glass Se<inf>80</inf>Te<inf>15-x</inf>Cd<inf>5</inf>Bi<inf>x</inf> (x=0,5,10) alloys
2020, Ceramics International
Citation Excerpt :
We have used Cd because chalcogenide glasses containing Cd are of great importance in semiconductor applications [15–17], furthermore doping of Cd in chalcogenide glasses has favorable effects on glass forming ability and specific heat of glasses [58]. It has been reported [59] that Cd incorporation to binary Se–Te chalcogenide glass has significant impact on the electrical conductivity, optical band gap and thermal activation energy. Moreover, Bi incorporated chalcogenide glasses have been studied by various researchers and studies revealed that Bi doping effectively enhances the semiconducting properties of chalcogenide glasses [18,19].
Differential scanning calorimetry has been employed to investigate the thermal properties of Se₈₀Te_15-xCd₅Bi_x (x = 0, 5, 10) glass system. The Variation of characteristic temperatures such as glass transition temperature (T_g), onset crystallization temperature (T_c), peak crystallization temperature (T_p) and melting temperature (T_m) are studied as a function of composition and heating rate. A stepwise increase in values of characteristic temperatures has been observed with the increase in heating rate. Further, obtained values of characteristic temperatures display decreasing trend with increase in bismuth concentration. The compositional and heating rate dependence of Thermal parameters such as glass forming ability (ΔT), thermal stability (H_g), enthalpy (ΔH) and entropy (ΔS) has been investigated. It has been observed that ΔT and H_g have lowest and highest values for Se₈₀Te₅Cd₅Bi₁₀ and Se₈₀Te₁₅Cd₅ compositions respectively. Avrami index (n) and activation energy for crystallization (E_c) have been calculated for all the samples. Values of n obtained for Se₈₀Te₁₅Cd₅, Se₈₀Te₁₅Cd₅ Bi₅ and Se₈₀Te₅Cd₅Bi₁₀ compositions are found to be 1.44, 2.16 and 2.16 respectively. Based on the obtained values of n, the nucleation process in alloys is discussed. It is noteworthy to mention that least value of Ec is observed for the alloy Se₈₀Te₅Cd₅Bi₁₀ containing the highest amount of bismuth. Furthermore, calculated thermal parameters are used to anticipate the electrical switching behavior of the alloys.
Disclosing the structural, phase transition, elastic and thermodynamic properties of CdSe<inf>1−x</inf>Te<inf>x</inf>(x = 0.0, 0.25, 0.5, 0.75, 1.0) using LDA exchange correlation
2017, Results in Physics
Citation Excerpt :
Their experimental results showed that the physical and photoelectrical properties depend on the preparation method, deposition time, and the temperature [21–23]. The CdSe1−xTex formed in amorphous film for small thickness less than 200 nm, and for higher value of thickness films were found to be polycrystallized in nature of a hexagonal phase [24]. The band gap was found to be affected by the thickness of the film and the concentration of Te, where the energy band gap decreases by increasing the Te concentration and by increasing the film thickness.
The phase transition and structural, elastic, thermodynamic characteristics of CdSe_1−xTe_x alloys for all compositions x (x = 0, 0.25, 0.5, 0.75, 1) in both hexagonal wurtzite (WZ) and cubic zinc-blende (ZB) are studied at zero K and zero pressure in emphasis of the Full Potential Linearized Augmented Plane Wave (FP-LAPW) approach, in accordance with the Density Functional Theory (DFT). This was inserted within the WIEN2k code, alongside a local density approximation (LDA) in order to consider the exchange-correlation functional. For all compositions the CdSe_1−xTe_x alloys were found to be mechanically stable for both phases ZB and WZ, and the strongest material among all structures is CdSe. Our findings reveal that the relation between elastic constants and the Te concentrations is not linear. The induced phase transition from ZB to WZ is studied at zero K, and the corresponding volume collapses at the phase transition boundary are calculated for all compositions x (x = 0.0, 0.25, 0.50, 0.75). Our results show that for all compositions of the CdSe_1−xTe_x alloys, the stable phase is zinc-blende. Furthermore, the elastic characteristics of ZB and WZ phases of CdSe_1−xTe_x alloys, alongside elastic constants, bulk modulus and shear modulus were determined and assessed in comparison with other theoretical and experimental findings available. A positive relationship was observed.
Estimation of some physical characteristics of chalcogenide bulk Cd<inf>50</inf>S<inf>50 - X</inf>Se<inf>x</inf> glassy systems
2015, Journal of Non-Crystalline Solids
This work reports the determination of many of the physical properties of ternary chalcogenide bulk Cd₅₀S_50 − xSe_x (30 ≤ x ≤ 50) glasses as well as the study of their dependence upon the Se-content. Mechanical milling technique was used to get a solid-state solution from elemental powders of Cd, S and Se. The phase formation of Cd–S–Se mixture was obtained by using a high energy ball milling for 48 h. The product of the milling process was pressed as pellets using a compressor of pressure about 6.5 MPa. The amorphous nature of bulk pellet samples has been checked using X-ray diffraction. It was found that, the values of mass density, molar volume, excess volume, field strength, compactness and the atomic density were increased, whereas the values of polaron radius and the average spacing of Se–Se, average heat of atomization, average single bond energy, mean bond energy, cohesive energy and glass transition temperature were decreased with increasing the Se concentration percent in CdSSe matrix. Heat of atomization was correlated to cohesive energy of the present Cd₅₀S_50 − xSe_x glasses. Chemical bond approach model was used to analyze the heat of atomization, the mean bond energy and the glass transition temperature values.

View all citing articles on Scopus

View full text

Effect of composition and forming parameter on evaporated CdSeTe films deposited at room temperature

Abstract

Introduction

Section snippets

Experimental procedure

X-ray diffraction patterns of SeTeCd thin film

Effect of composition on the activation energy

Discussion

Conclusion

Solid State Electronics

J. Non-Cryst. Solids

J. Inorg. Nucl. Chem.

Phys. Status Solidi (a)

Philos. Mag.

Electronic Process in Non-Crystalline Materials

Appl. Opt.