Annealing Temperature Effect on Properties of Chemically Deposited PbTe Films and Bulk

The PbTe films were deposited onto glass substrate (microscopic slices) by a chemical bath method (CBD) at room temperature. The deposited films are dense, smooth, and uniform with silver gray metallic luster structure. Using XRD, it found that the structure of PbTe possesses stable face centered cubic (fcc) phase. The grain size of the PbTe bulk increased within the range of 33– 57 nm with annealing temperature increasing. AFM micrographs of surface of the prepared film are observed that horizontal distance in the rang (230– 395) nm. The band gaps of the PbTe are determined from UV-Vis spectrophotometer and are found to be within the range ( 0.39 0.95) eV. Energy band gab of PbTe which determined from FT -IR spectrophotometer is (0.36ev). The activation energy varied from 0.351.72 eV for films and from 0.11-0.34 eV for bulk with annealing temperature variations from 373-573K. Films and bulk exhibit p-type conduction and resistivity in the range (75×10 Ω. cm 146×10 Ω .cm). The carriers density and Hall mobility in PbTe bulk were in the rang 5.8 ×10 m and 4.004 m/Vs.


Introduction
Lead chalcogenide Semiconductor have attracted more attention in the past few decades, in both fundamental research and technological applications, because of their controllable size dependent electronic and optical properties. Recently, the synthesis of nanostructured semiconductor materials with controlled morphologies such as films , quantum dot , nano rod , nanowire, etc. It found that finds applications in optoelectronics, biotechnology, catalysis, etc. [1,2]. Lead chalcogenide materials (PbS , PbTe and PbSe) exhibit properties, which are unusual and possibly unique, relative to other semiconductors. Particularly, unusual feature of this group of materials is the relative stability of the lattice over a wide range of non-stoichiometry. Compared with values for other semiconductors [3]. As one of the important IV-VI semiconductor materials, the rock-salt (face centered cubic) structured lead telluride (PbTe), PbTe structure has been the object of particular attention, because of its narrow band gap of 0.32 eV (in the bulk at room temperature). The unusual characteristics of lead salt PbTe such as high carrier mobility, narrow band gap make them unique among polar compound and have important application in many fields, such as light emitting diodes and infrared laser in fiber optics, thermoelectric devices and infrared detection [4,5]. PbTe have potential applications in power generation and thermal sensing. Theoretical calculation sand experiments indicate that improvement in Te properties can be achieved as the dimensionality of materials is reduced [6,7]. Low dimensional Te materials such as films are of great interest for construction of high performance Te devices .In addition, PbTe films are also good candidates for optoelectronic applications in the mid-infrared range [8]. Various methods have been utilized to prepare PbTe thin films , such as vacuum evaporation [9,10], magnetron sputtering [11], molecular beam epitaxy [12], pulsed laser deposition [13], hot-wall epitaxy [14,15], and electro deposition [16]. All these methods need special equipment's, and the electro deposition method needs conductive substrates, although it is relatively low-cost. Chemical bath deposition (CBD) method does not have special requirement for substrate and does not need special equipment. This is the technique possesses a number of advantages such as low cost, low working temperature, and easy coating of large surfaces, over other film deposition methods [17]. CBD technique is very suitable method for deposition of polycrystalline PbTe films with good photoconductive properties [17]. The photoconductive effect of PbTe films may be attributed [18] to an amorphous to crystalline crystalline transformation during annealing process. It has been found that the properties of chemically deposited PbTe films depend strongly on the growth conditions such as PH deposition, temperature, deposition time, and annealing temperature [19]. In this work, PbTe films were deposited on glass substrate, at room temperature, in an alkaline aqueous solution and as a powder, by a CBD method. Optical properties of this film was studied via absorption spectrum. After preparation, PbTe samples were pressed in disk shape. Structural properties of PbTe has been studied by X-ray diffraction (XRD). DC and Hall effects measurements were carried out.

Research Paper
Open Access

Substrates Cleaning
The cleaning of the substrate surface is significant for the characteristic of the film structure. Microscopic slices glass substrates were cleaned using an oxidant mixture (K2Cr2O7:H2SO4 -1:10, HNO3, 1% EDTA) then rinsed with distilled water and dried.

Film preparation
The lead telluride films were deposited by the (CBD) method. For deposition of PbTe films, lead acetate was used as Pb 2+ and telluride oxide as Te 2− source in an alkaline medium. solution, prepared by taking 1mmol of lead acetate Pb(CH3COO)2 .3H2O dissolved in 20 ml of distilled water, stirred for 10 min, 1 mmol of TeO2 , 0.02 mol of KOH and 2 mmol of trisodium citrate (TSC) dissolved in 100 ml of deionized water then it stirred for 15 min, all constituents were mixed together. While the solution was stirred we added 8 mmol of KBH4 , within a few seconds color of the solution turned dark brown indicating complete dissolution of KBH4, and the solution was continuously stirred for 30 min .pH (11.5) and temperature (407K) were kept constant for all depositions. The solution was diluted to 200 ml in a beaker and then was placed at room temperature without stirring .Two Microscope glass slide was used as the substrate after being cleaned by the method mentioned above. The slide was put in the solution at an angle of 30̊ to the bottom of the beaker .The solution gradually turned dark. About 72 h later. The reaction process of deposition of PbTe as following: Pb 2+ + TeO3 2-→ PbTeO3 (4) 2PbTeO3 + 6BH4 -→ 2PbTe + 6OH -+ 3B2H6↑ (5) The Pb (Ac)2 and TeO2 were dissolved in excess alkali and form HPbO 2and TeO3 2ions, according to reactions (1) and (2), respectively. The TeO3 2-, HPbO -2 and BH4 -(from KBH4) all are negative charges; therefore, the reduction reactions related to them are difficult. Some HPbO 2ions are hydrolyzed into Pb 2+ (reaction (3)). The Pb 2+ combines with the TeO3 -2 to form PbTeO3 colloidal particles (reaction (4)). The PbTeO3 colloidal particles are reduced into PbTe by BH4 -(reaction(5)) and PbTe nuclei form. The PbTe nuclei attach to the surface of the substrate, and on the wall of the beaker grow up and form films, and the PbTe nuclei in the solution grow up slowly as powder and precipitate at the bottom of the beaker. The deposition procedure was carried out at room temperature; therefore, the growth of the PbTe nuclei should be very slow.
The silver gray film deposited on the downward side of the substrate was more strongly adhered while that on the upward side was weakly adhered. Therefore, hereinafter, only the film deposited on the downward side of the substrate was further studied. It was heated at 473K temperature for 30 minutes and kept at room temperature for further measurements. Black powder precipitated on the bottom of the beaker was collected. The powder was washed with distilled water and ethanol several times, then dried at 343K for 4h , The formed powder was milled by a ceramic mortar, pressed under 7 ton/cm 2 as 3 disks of 22.3 mm diameter and 1.5mm thickness. Then disk samples were annealed at 373, 473, 573K temperature for 30 minutes. Structural characteristics of the bulk materials were determined by X-ray diffraction (XRD) method using Philips X-pert Pro diffract meter at room temperature with Cu-Kα radiation (λ=1.54 °A). Optical absorption spectra of the films were taken via a UV spectrophotometer in the wavelength range (300 to 900) nm. Disks were polished and washed by alcohol. In addition, an air-dry silver pastes were applied to the samples surfaces as electrodes, to ensure a good electrical contact and introduce them for further characterization

XRF spectrum
Using XRF device, Unisantis® type Si-PIN with molybdenum (Mo) target operated at 50kV and 1mA, XRF spectrum was taken for the prepared sample before heat treatment.   Figure 2 Shows the XRD spectrum of three samples of PbTe in discs form, annealed at 373, 437, and 573 K for 1h. Fig.2 shows several diffraction peaks at 2θ values of 24.1, 27.5, 39.6, 49.5 and 64.3. Which correspond to (111), (200), (220), (222) and (420) planes of the face-centeredcubic (fcc) rock-salt structure of PbTe. Where some of orientation plane are missing in spectrum of sample treated at 373K (Fig.2a) Scherrer's formula was used to calculate crystallite size [20,21]:

Structural study
Where K is a constant taken to be 0.94, λ is the wavelength of X-ray of Cu-Kα radiation (λ = 1.54Aᵒ) and β2θ is the full width at half maximum (FWHM) of the diffraction peak corresponding to a particular crystal plane. The strain (ε) was calculated by the following expression [22]: The dislocation density (δ), defined as the length of dislocation lines per unit volume of the crystal, was evaluated from the formula [22]: Figure 2. The XRD profile of PbTe sample at various temperatures at 373K B) at 473K C) at 573K Table 2 summarizes the calculated values of crystallite size (D nm), strain (ε), dislocation (δ), and lattice constant (a).  (2) that the crystallite size increases with increasing annealing temperature. In contrast, the strain and dislocation decrease with increasing annealing temperature. This means that the thermal treatment increases the relaxation. Figure 3 Shows the morphology of films deposited at30 • C and annealed at 373, 473, and 573K taken by AFM. We note that the surface of the film have a form of granules with different sizes vary between (230 -395) nm. Also one can observe that the grain size increases with heat treatment temperature. It can be observed that the surface of the films is not very compact. Small crystallites or grains compose them. Round shaped clusters packed together constitute the films. This cluster appearance caused by size of the clusters increases with annealing temperature and many empty spaces can be seen between these clusters. The average grain size is about 287 nm.

UV-Visible spectrum:
The optical absorption of PbTe thin films was studied in the wavelength range 350 to 900 nm. As it shown in Figure 5, PbTe film shows higher absorption at shorter wavelength side.
Energy band gab can be calculated using Tauc formula [23]: Where A is constant, Eg is the band gap of the film. The exponent n depends on transition type. The values of n for direct allowed, indirect allowed, and direct forbidden transmissions are n = 1/2 , 2, and 3/2, respectively. The direct band gaps were obtained from the linear portion of (αhυ) 2 vs. hυ plot as shown in Fig. 6, which lie in the range 0.39 -0.99 eV. The indirect band gaps were obtained from the (αhυ) 1/2 vs. hυ plot as shown in Fig. 7. Its value is 1.55eV. While there is no direct forbidden transition. It found that direct band gap is higher than that of bulk value of PbTe (0.34 eV) because of the grain size effect of PbTe.

AC measurements
AC measurement carried out using GAIN PHASE ANALYZER (Schlumberger -SI1253 ). Thin film complex impedance spectrum was taken for thin film in frequency range (1Hz -20KHz ) at constant voltage (v = 5V) and at different temperature (373, 473 and 473K). Fig.  (8) shows the relationship between the imaginary part X(ω) and the real part R(ω) of the complex impedance.
We note that impedance spectrum has a semi-cycle due to Debye model which indicates that the grains are deviated from homogeneity and the equivalent circuit consists of capacitance in parallel with resistance. It's noticeable when R(ω)=0, X(ω)≠0, that mean at frequency equals to 0 Hz, free charges presented at the surface of the sample.

(I -V) characteristics
(I -V) characteristics of three samples thin film of PbTe annealed at 373 -473 -573 K for 1h were carried out. Fig 9. Shows that the relationship between I and V is approximately linear.

For PbTe thin film
The variation of electrical conductivity of semiconductor in function of 1/T is given by: Where, σ is conductivity at temperature T, σ0 is a constant, k is Boltzmann constant and Ea. is the activation energy. Fig 10. Shows ln σ in function of 1/T for the three thin film samples annealed at three different temperatures. Table 2. Shows that the calculated activation energy for the three samples decrease from (0.35 to 1.72) eV as annealing temperatures increase. This means that the change in activation energies may refer to the structural improvement due to eliminate defect through annealing.  The change of activation energy Ea attributed to the variation of barrier height when the film samples annealed at different temperature.    It can be noticed that activation energy decrease with annealing tempuratuer increasing. This reduction can take the same interpretation of the thin film.

Hall effect measurements
The variation of Hall voltage (VH) with current for PbTe bulk samples annealed at 373, 473, 573 K is shown in fig 12. Hall effect reveals that the prepared thin PbTe films are p-type due to sulfide ions vacancies [24] that means the conduction is dominated by holes [25].  The variation of carrier concentration (PH) and Hall mobility (µH) with annealing temperature for PbTe bulk are shown in fig 13. It was found that the carriers concentration decrease with increasing annealing temperature while Hall mobility increases with the annealing temperature increasing are shown in table 5.

Conclusions
CBD chemical method was an effective, easy and cheap way to deposit PbTe Thin films. XRF showed that the prepared samples were closed to stoichiometry. XRD showed that the higher the annealing temperature, the less strain and dislocations. FT-IR revealed the existence ofsome toxicity in the prepared samples. Activation energy showed decrease with increasing annealing temperature both for thin films and bulk sample. AFM micrograph and Impedance spectrum showed that there is no homogeneity in the bulk and thin film samples. Hall measurements confirmed p-type conduction for PbTe films deposited on the glass substrate. The carrier concentration was estimated to be about 5.8×10 23 m -3 and the mobility is about 4 m 2 /V s.