CHAPTER 4. SYNTHESIS OF ALUMINIUM SELENIDE (Al 2 Se 3 ) NANO PARTICLES, DEPOSITION AND CHARACTERIZATION

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1 40 CHAPTER 4 SYNTHESIS OF ALUMINIUM SELENIDE (Al 2 Se 3 ) NANO PARTICLES, DEPOSITION AND CHARACTERIZATION 4.1 INTRODUCTION Aluminium selenide is the chemical compound Al 2 Se 3 and has been used as a precursor to hydrogen selenide, which is released when the solid is treated with acids. It should be stored away from moisture and air as it is hydrolytically unstable. Heterostructures combining III-VI semiconductors with silicon have attracted attention (Fritsche et al 2002, Ueno et al 2002, Zheng et al 1996) due to their close lattice matching and promising optoelectronic properties. Al 2 Se 3 is the least studied of the group III-VI chalcogenide (Schneider and Gattow 1954) when compared to other members of the III-VI family of semiconductors. The stable bulk form of Al 2 Se 3 is a defected wurtzite structure with 1/3 of the cation sites vacant. The first growth of aluminum selenide on silicon is reported by Adams et al (2004). Hence it is taken up aluminium selenide material for the above reasons and report the synthesize of aluminium selenide nano particles by chemical reaction method and preparation of thin films from the synthesised material by electron beam evaporation technique at different substrate temperatures. The prepared films are studied for their structural, optical and electrical properties.

2 SYNTHESIS As no report is available on the synthesis of aluminium selenide (Al 2 Se 3 ) from hydrothermal method, effort was made to synthesis aluminium selenide from selenium (Se) powder and aluminium chloride (AlCl 3 ) employing the hydrothermal reaction method. Many trial attempts were made to synthesis aluminium selenide from different ratios of the above source materials and finally the following ratio was selected to obtain aluminium selenide. 1 mol of AlCl 3 was dissolved in 200 ml distilled water and 1 mol of selenium metal powder was added into the solution. The solution was stirred well at room temperature for one hour with the help of magnetic stirrer. Then the solution was transferred to a Teflon lined autoclave and sealed well. The autoclave was kept in a hot oven at 200 C for 24 hours. After 24 hours the oven was allowed to cool to room temperature naturally. The product of aluminium selenide powder, black in colour, was found at the bottom of the autoclave and was separated by filtering the solution. The powder was washed with distilled water (5 times) and absolute methanol (3 times) and finally dried at 60 C in the oven. Figure 4.1 X-ray diffraction pattern of synthesised aluminium selenide powder

3 42 XRD peaks recorded for aluminium selenide compared well with standard JCPDS data file No (Monoclinic system, a=11.68 Å, b=6.730 Å and c=7.320 Å, space group Cc); thus confirm the synthesised material. The XRD peaks were indexed with the help of PowderX software and are shown in Figure 4.1. XRD pattern shows a preferential growth along (021) direction. The crystalline size of synthesised powder was calculated for some of the prominent peaks using Scherrer s formula given in equation (3.1) in the third chapter and these values are presented in the Table 4.1. The crystallite size is of the order of a few tens of nanometer. Table 4.1 Crystallite size of the synthesised aluminium selenide (hkl) 2 FWHM Crystallite Size (nm) (-111) (111) (021) (230) (-513) (-414) Thin Film Annealed at 150 C (111) DEPOSITION OF ALUMINIUM SELENIDE THIN FILMS The synthesised powder was used as a starting material to deposit aluminium selenide thin films. Electron beam evaporation technique was

4 43 employed to deposit the films on the well cleaned microslide glass substrates. ~0.0138g of aluminium selenide was cut from the pellet and placed in the graphite crucible for evaporation. The chamber pressure was pumped down to torr and 6kV energy was applied to the electron gun, to completely evaporate the material. The films were deposited at Room Temperature (RT) and then the films were annealed at different temperatures (50 C, 100 C and 150 C) under a low vacuum (10-3 torr) for 20 minutes. Annealed films were characterized to study the effect of annealing on their structural, electrical and optical properties. 4.4 CHARACTERIZATION X-ray Analysis Thickness of the deposited thin films was measured by stylus profilometry technique and the thickness of the films is ranging from 10 to 20 nm. XRD patterns recorded for the as deposited and annealed aluminium selenide thin films are presented in the Figure 4.2. The films deposited at RT are amorphous. Hence they are annealed at 50, 100 and 150 C to study the effect of annealing on their structural properties. Figure 4.2 shows that the film annealed at 150 C has a tendency of crystallising along the (111) direction with signature for (021) plane. Further it is observed that annealing the film at 200 C resulted in the evaporation of the films. The calculated crystallite size of the film annealed at 150 C is shown in the Table 4.1.

5 44 Figure 4.2 Recorded XRD patterns of the aluminium selenide thin films prepared at RT, Annealed at 50 C, 100 C and 150 C UV-Vis-NIR spectral Analysis Optical transmittance spectra of the as deposited and annealed films were recorded in the range of nm and are presented in the Figure 4.3. The film prepared at RT has relatively less transmittance (T) in the visible region when compared with the transmittance in IR region. Annealing the films at various substrate temperatures modifies the transmittance remarkably by increasing the transmittance in the nm and reducing the transmittance in the region of nm. The lower

6 45 cut-off of the film prepared at RT is at ~600 nm and this value is shifted to ~730 nm for the film annealed at 50 C. The shift further moves to longer wave length side for increase in the annealing temperature (100 C 850 nm, 150 C 1046 nm). The shift in the cut off value is also known as red shift. Figure 4.3 UV-Vis-NIR spectra of aluminium selenide thin film prepared at RT and annealed at 50 C, 100 C and 150 C Optical Band Gap Figure 4.4 shows the variation of ( h ) 2 as a function of the photon energy (h ) in the vicinity of the fundamental absorption. The optical band gap of the aluminium selenide thin film prepared at RT and annealed at 50 C, 100 C and 150 C was determined by extrapolating the liner line of the plot drawn between photon energy (h ) and ( h ) 2. It is observed that the value of direct optical band gap E g lies between 0.5 and 0.75 ev.

7 46 Figure 4.4 Plot between photon energy (h ) and ( h ) 2 of aluminium selenide thin film prepared at RT and annealed at 50 C, 100 C and 150 C Atomic Force Microscopy (AFM) Analysis The surface morphology of the as deposited and annealed films was studied with the help of Atomic Force Micrographs. The samples were scanned for 2 2 µm 2 area. The roughness root mean square (RMS) value for this film is presented in the Table 4.2. The film annealed at 150 C possesses the relatively larger roughness RMS value. Table 4.2 Obtained roughness RMS value from AFM analysis Films RT annealed at 50 C annealed at 100 C annealed at 150 C Roughness RMS value nm nm nm nm

8 47 (a) (b) (c) (d) Figure 4.5 AFM images of aluminium selenide thin films (a) prepared at RT, annealed at (b) 50 C, (c) 100 C and (d) 150 C Scanning Electron Microscopy (SEM) Analysis Surface modifications of the aluminium selenide thin films due to annealing were captured and presented in Figure 4.6. From this it is clear that the annealing process modifies the morphology of the aluminium selenide thin films. Holes produced on the surface of the film prepared at RT disappeared due to annealing at 50 C. Further annealing the as deposited films at 100 and 150 C produced track like structures on the surface.

9 48 Figure 4.6 SEM micrographs of aluminium selenide thin films (a) prepared at RT, annealed at (b) 50 C, (c) 100 C and (d) 150 C Electrical Studies The electrical properties of the aluminium selenide thin films prepared at RT and annealed at 50 C, 100 C and 150 C were studied at room-temperature Hall measurements (ECOPIA HMS-3000) in van der Pauw configuration. The carrier concentration, mobility and resistivity of the films are presented in the table 4.3. The positive sign of the bulk concentration indicates that the aluminium selenide belongs to class of p-type semiconductor. The aluminium selenide thin films possess high resistivity.

10 49 Table 4.3 Electrical parameters of aluminium selenide thin films prepared at RT and annealed at 50 C, 100 C and 150 C Substrate Bulk Concentration Mobility Resistivity Temperature /cm 3 cm 2 /Vs cm 50 C C C CONCLUSION Aluminium selenide nano particles were synthesised by hydrothermal method at 200 C temperature. The formation of aluminium selenide was confirmed by the powder XRD pattern. The peaks obtained were indexed using PowderX software. XRD peaks of the synthesised material compare well with the standard JCPDS card file No , thus confirming the synthesised material. The synthesised aluminium selenide crystallizes in monoclinic crystal system. The thin films were deposited from synthesised nano particles and were annealed at different temperature (50 C, 100 C and 150 C) for 20 minutes under a low vacuum. The deposited and annealed films were subjected to XRD analysis. The as prepared films, annealed at 50 C and 100 C are amorphous in nature whereas the film annealed at 150 C acquires crystalline nature. Annealing the films relatively increases the transmittance in the nm and reduces the transmittance

11 50 in the nm range. The optical band gap of prepared and annealed films lies in the range of ev. AFM study reveals that the films have smooth surface and the roughness RMS value lie in the range of nm. SEM micrographs show that the process of annealing removed the holes from the surface of the as deposited films. The electrical studies carried out show that the aluminium selenide thin films belong to p-type semiconductor and have high resistivity.