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Studies on Cd1Se06Te04 Thin Films by Spectroscopic and Diffractometer Characterization
Cliff Orori Mosiori Duke Ateyh Oeba 2
1 Technical University of Mombasa
P. O. Box 90420-80100, Mombasa, Kenya
2 Kenyatta University
P. O. Box 43844-00100, Nairobi, Kenya
DOI: 10.22178/pos.26-3
LCC Subject Category: TP155-156, QC450-467, QD450-801
Received 12.07.2017 Accepted 12.09.2017 Published online 20.09.2017
Corresponding Author: [email protected]
© 2017 The Authors. This article is licensed under a Creative Commons Attribution 4.0 License
Abstract. Cadmium selenide tellurium is a compound containing cadmium, tellurium and selenium elements forming a combined solid. Hall measurements suggest that it is an n-type semiconductor. Related optical studies indicate that is transparent to infra-red radiation. Structural studies clearly show that it has a wurtzite, sphalerite crystalline forms. Cadmium is a toxic heavy metal, and selenium is only toxic in large amounts or doses. By this toxicity, cadmium selenide is a known to be carcinogen to humans; however, this does not stop investigating it for optoelectronic applications. Current research has narrowed down to investigating cadmium selenide when in the form of nanoparticles. Cadmium selenide finds applications has found applications in opto-electronic devices like laser diodes, biomedical imaging, nano-sensing, high-efficiency solar cells and thin-film transistors. By chemical bath deposition, Cd1Se0.6Te04 thin films were grown onto glass. Tellurium was gradually introduced as an impurity and its crystalline structure and optical properties were investigated by XRD and UV-VIS spectroscopy. The main Cd1Se0.6Te04/glass characteristics were correlated with the conditions of growing and post-growth treatment and it was found out that films were homogeneous films with controllable thickness onto the glass substrate and suitable for n-type "sandwich" heterostructures applications. Comparison of the intensities of equivalent reflexions provided a test for the internal consistency of the measurements. Equivalent reflexions in two specimens differed on average by 1.4 % and 0.6% from the mean measured intensity, attesting to the high internal consistency of measurements from extended-face crystals. By comparison from data obtained from all samples showed their average deviation from the mean to be 0.9 %.
Keywords: cadmium selenide tellurium; Cd1Se06Te04 thin films; glass; chemical bath deposition.
INTRODUCTION
Cadmium selenide compounds are used as n-type semiconducting layer in different kinds of "sandwiched" optoelectronic heterostructures due to their very interesting properties. They have a band gap of about 1.74 eV, good dielectric constants of about 10.2 [13] and varying electrical resistivity of about 1 H cm to 1012 H cm [3]. Pure crystal of CdSe has its band edge in the near infrared and show clear transmittance far into the infrared region [6]. Their long wavelength limits determined by the onset of lattice absorption are about 1.5 cm-1 at 24.3 [im [14] with a narrow impurity absorption centered at 18.5 um which varies from one crystal of CdSe to another. They also show a nonlinear optical behavior with
pulses tunable from 10 to 20 [im [1]. Tellurium equally is a rare, silvery-white, brittle, lustrous metalloid that can burn in air with a greenish-blue flame to form white tellurium dioxide (TeO2). When its present in certain compounds, tellurium exists mostly in the oxidation state IV and VI depending of other conditions [12]. Tellurium is therefore a semiconductor that is slightly photosensitive with radioactive isotopes. It is among the lightest element to exhibit alpha decay. Therefore, when investigating it, caution must be exercised. Using the Sellmeier equations [10] and where A is in microns, it has refractive index varying as:
2 , 1.858712 3.046U2
n20 = 4.1321 + —-+ —-, (1)
0 1 - 0.2187 1 - 3380
n2 = 4.0829 +
2.0038A2 3.5540A2 +
A2 - 0.2075 A2 - 3629
, (2)
where symbols have their conventional meanings.
Different techniques could be used to grow impure CdSe thin films preparation such as chemical bath deposition [2], sputtering [9], chemical vapour deposition [8] or electrodeposition [4]. Among them, chemical bath deposition (CBD) is a simple and low-cost method and produces uniform, adherent and reproducible films. Moreover, CBD is a low temperature technique and can be used for CdSe deposition onto a wide range of substrates. Thin films of were grown by chemical bath deposition on glass using the multilayer technique is not new [6, 3]. Pure cadmium selenide films properties are extremely sensitive to preparation conditions [7, 14] and therefore the aim of this work is to study the influence of tellurium impurity on structural and optical properties.
chemical bath and only taken out after 1.0 hr, washed and re-introduced into a renewed hot chemical bath solution repeatedly. All other growth conditions were maintained.
Deposition parameters. For the two procedures in above, the deposition parameters were maintained as follows:
[Cd2+] = 3 x 10-3M; [C6H5O73-] = 1.2 x 10-1 M;
[NH3] = 3 x 10-1 M; [Se] = 3.1 x 10-2 M;
pH = 10.5; [Te] = 1.85 x 10-2 M.
All samples were washed, dried and annealed in air; at 350 °C to result into Cd1Se0.6Te0.4 thin films.
Characterization. The films were characterised by thickness using a profilometer and micro-weighing method and the film thickness was evaluated by averaging the resulting measurements, crystalline structure using diffractometer and obtained UV-Vis transmittance spectra from a photo Spectrometer.
METHODOLOGY Materials and Reagents
Cadmium acetate, NH3 aqueous solution, acetone, ethanol, sodium citrate and distilled water were purchased and used without purification. Chemical were bought from Sigma Aldrich while the glass pieces were purchased from Optical Filters Ltd.
Preliminary Procedures
Prior the deposition, the coated glass (50 mm x 25 mm x 1 mm) were ultrasonically cleaned with acetone/ethanol mixture and dried.
Procedure
Single-layer Growth. Cd1Se0.6Te0.4/ glass thin film structures were grown successively from renewed chemical bath (CB) using a precursor solution prepared from cadmium acetate, NH3 aqueous solution, sodium citrate and distilled water. The glass substrates were immersed vertically suspending them around the stirrer and the bath stirred was continuously while maintained at 70 °C. After attaining thermal equilibrium, Te impurities were introduced under stirring conditions.
Multi-layer Growth. To grow a multi-layer procedure, wet glass were immersed into the hot
RESULTS AND DISCUSSIONS
This work specifically investigated the structure and optical transmittance in the range of 300900 nm wavelength which is in the UV-Vis spec-troscopy (Figure 1 and Figure 2).
Figure 1 - Transmittance in multilayered Cd1Se06Te04 / glass film
Observed intensities were analyzed by crystallo-graphic least-squares program and modified to include dispersion corrections and to calculate the values of the atomic scattering factors at the appropriate value of sin (6/2) using the polynomial expansion [4].
Figure 2 - Transmittance in Cd1Se06Te04 / glass after the thermal treatment
acetate solutions. The chemical bath deposition of films involves the decomposition of alkaline solutions in the presence of a metal salt in the presences of chelating agents such as ammonia or sodium citrate, whose role is to control the CdiSeo6Teo4/glass film-growing rate as follows [8]:
Cd(CHsCOO)2 + Se+2 + 2OH- ^
^ CdSe + H2CN2 + 2H2O + 2CH3COO- (3)
For a structure consisting of light atoms and including low-angle data the effect of replacing the usual interpolation procedure by the polynomial expansion could well be more significant.
Chemical Reactions
CdiSeo.6Teo.4/glass samples were grown by chemical bath deposition method from cadmium
Film Thickness
Chemical Bath technique was adapted for multilayer CdiSeo.6Teo.4/glass and samples with 1, 2 and 4 consecutively deposited were prepared and characterised. Composition of the as prepared heterostructures, packing density, growing rate and thickness of CdiSeo.6Teo.4 films are presented in Table 1.
Table 1 - Raw data for heterostructures Cd1Se06Te04 / glass
Samples code Heterostructures Type CdiSeo.6Teo.4 film type Total dep. time* Packing density (mg/cm2) Film thickness (nm) Growing rate (nm/min)
ITO-O Glass - 0 0 20** 0
ITO 1-0 CdiSeo.6Teo.4 / glass Mono-layer 1 h 0.098 17 0.2
ITO-2.1 CdiSeo.6Teo.4 / glass Multi-layer 2 h 0.470 89 0.7
ITO2.3 CdiSeo.6Teo.4 / glass 4 h 1.434 243 1.3
Notes: * n = number of layers, m = deposition time; ** - manufacturer measurement
The increase of CdiSeo.6Teo.4/glass film thickness with the total deposition time could be noticed. Moreover, for the same deposition time, the multilayer film is almost 7 times thicker then the corresponding monolayer one. The use of a high number of successively deposited layers (coatings) determines the increase of the film thickness. One can note that for various heterostructures, the growing rate is different, increasing with the number of coatings.
Optical Transmittance
The transmission spectra of the heterostructures that contain multilayer CdiSeo.6Teo.4/glass films illustrates the decrease in film transparency parallel with increases in film thickness (Figure i).
As opposed to post treated films, there seems to be an increase film transmittance (Figure 2). It can be noted that the glass substrate shows a high transparency on the entire visible domain.
X-ray diffraction
The X-ray diffraction were used to investigate the optical and structural properties of different CdiSeo.6Teo.4/glass hetero-structures (Figure 3 and Figure 4). The crystalline structure of thermally treated CdiSeo.6Teo.4/glass heterostructures was investigated by X-ray diffraction and characteristic bands of the hexagonal crystalline structure of the cadmium selenide could be noticed as depicted in Figure 3.
Diffraction angle 20 [degree]
Figure 3 - XRD spectra for Cd1Se06Te04 / glass after the thermal treatment
120 -
._. 100 -
=3 03
so-
't/j c
ü 60-
40 -
20 -
ITO
(222)
CdS
(100) CdS
ITO 2_1TT ITO 2 3TT
ITO
(400)
ITO
(411)
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Diffraction angle 20 [degree]
0
Figure 4 - XRD spectra for Cd^e^Te^/glass with multilayer CdS films
It was also noted that the crystallinity of the films were high as a result of post-growing thermal treatment. Therefore, in order to compare the effect of multiple layers on crystalline structure observed, the XRD spectra were first normalised in rapport with (222) peak of the indium-tin oxide as shown in Figure 4. The spectra normalisation was used to input in evidence of the increase of the three XRD characteristic bands observed in Cd1Se0.6Te0.4 / glass i.e. (100), (002), (101). This observation was attributed to the presence of a higher Cd1Se0.6Te0.4 crystal amount on the surface of glass.
CONCLUSION
Thin films of Cd1Se0.6Te0.4 were synthesized using special CBD. It proved to be a convenient deposition method since its films were well adhered to and were also homogeneous films with controllable thickness onto the glass substrate. Powder X-ray analysis confirmed the fundamental diffraction patterns of Cd1Se0.6Te0.4. UV-Vis and XRD investigation illustrated the quality of the as prepared Cd1Se0.6Te0.4 hetero-structures as determined, from X-ray intensity data obtained with extended-face crystals [11]. The wurtzite parameter u was found to be 0.37679 ± 0.00012.
Comparison of the intensities of equivalent reflexions provided a test for the internal consistency of the measurements. Equivalent reflexions in the samples differed on average by 1.4 % and 0.6 % from the mean measured intensity, attesting to the high internal consistency of measurements from extended-face crystals. By comparing data obtained from all its samples their average deviation from the mean to be 0.9 %.
ACKNOWLEDGEMENTS
The authors acknowledge the support given by Dr. Walter Kamande Njoroge of Kenyatta University in performing the XRD spectra and Mr Maera John of Maasai Mara University for performing transmittance measurements.
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