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What grown Amorphous gallium arsenide films? essay example
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What grown Amorphous gallium arsenide films?

Amorphous gallium arsenide (GaAs) films were grown on glass substrates by resistive heating technique under a vacuum 7.8×10-5 mbar at room temperature. Samples were annealed in an open atmosphere in tube furnace at 2500C, 2800C and 3200C temperatures. Annealed gallium arsenide deposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS or EDX) and Rutherford backscattering spectrometry (RBS). XRD analysis showed that the material have a polycrystalline textures.Surface morphological structures of all samples were shown at 5µm, 1µm, 0.5 µm, 0.2 µm ranges by SEM. Elemental compositions carried out by EDX. RBS carried out also the samples compositions, thickness and also showed that resistive heating evaporation is a non-uniform films technique.

Introduction

GaAs is a compound semiconductor in which gallium (Ga) element from group III and arsenic (As) from group V. In 1920, Goldschmidt first create GaAs and declared in 1929[1].Gallium is extract by heating of different materials, like zinc (Zn) ores and bauxite, and more extraordinary than gold (Au). Arsenic is toxicant but common material. GaAs has crystal shape known as cubic or zinc blended[2, 3].Areas with high application potentials are photovoltaic, high brightness LEDs and, eventually, all optical space switch technologies[4]. It is widely used in the semiconductor industry due to its wider direct band gap energy and higher electron mobility than Si[5].The band gap value of GaAs is 1.42eV which is very near to the optimum band gap energy 1.5 eV for solar energy conversion. For global applications GaAs cells are used as a solar concentrator PV because of higher radiation tolerance and higher efficiency than other solar cells[6].Due to this reason first GaAs microprocessor in 1980s introduced by Radio Corporation of America and USA States Department of Defense used it for Star Wars programs [7]. It is used in beam combiners, beam splitters and polarizer as substrates, and many other applications by variety of suitable coating[8]. The amorphous and crystalline thin film of GaAs on glass has been formed with various methods, such as pulsed laser deposition (PLD), flash evaporation, sputtering, electron beam evaporation, metal organic chemical vapor deposition, molecular beam epitaxy(MBE)[9]. Nowadays GaAs produces a large family of materials with vast ranges of properties which is well fitted with photo electronics, microelectronics, optoelectronics and thin films applications.

In this research work we investigate the effect of annealing in open atmosphere of amorphous GaAs films on glass substrates and also the non-homogeneity of films deposited by resistive heating evaporation technique.

Experimental details

To remove the impurities from the glass substrates kept it in acetone and then clean it by deionizer water into ultrasonic vessel for 15min for their purity. Dry clean glass substrates are kept in clean environment at room temperature before using it for the evaporation.In this experiment resistive heating evaporation unit “EdwardE306A, UK” is used. Glass slides used as substrates; three substrates kept inside the substrate rotor. The GaAs powder purity (99.999%, Beijing Goodwill Metal Technology Co.Ltd) kept on tantalum foil boat. The substrates rotor situated opposite to the boat and the distance between the substrate rotor and boat is approximately 5cm. GaAs thin film on glass substrates as grown under the vacuum pressure 7.8×10-5 mbar at room temperature and deposition have taken only 6min. After that, samples removed and kept it one by one in tube furnace in open atmosphere each sample annealed 20min and the annealing temperatures were 2500C, 2800C and 3200C.
Crystallinities of annealed films were evaluated by X-ray diffraction (XRD). A JEOL JDX3532 X-ray diffracto-meter, using CuKα radiations (λ=0.154 nm) operating voltage is 40 kV & current is 30 mA. Surface morphologies and structures of films were carried out by JEOL JSM5910 SEM, operating at 30 kV, with a resolution of 2.3 nm.The samples composition carried out by EDX. The samples thickness and samples elemental compositions analysis also were determined by Rutherford Backscattering Spectrometry (RBS) technique.

Results and discussion

1. Structural analysis

XRD is a non destructive technique used to determine the crystallite sizes, crystal structures, structural properties, and films thickness of the deposited thin films[10]. A JEOL JDX3532 X-ray diffracto-meter, using CuKα radiations (λ=0.154 nm) operating voltage is 40 kV &current is 30 mA.The crystallite grain size of film is determined from full width at half maximum (FWHM) of most intense diffraction line by Scherrer’s formula[11] as follows
D=Kλ/(∆(2θ)cos⁡θ )

Where, “D” is for crystallite grain size, “λ” is wavelength of X-ray used (λ= 0.1540556nm), ∆ (2θ) is full width at half maxima of peak (FWHM) in radians, “θ” is peak position in degrees or Bragg’s angle. “K” is a constant & normally value taken 0.9[12].

In XRD characterizations in figure diffracto-graph show the XRD pattern of the GaAs film on glass substrate and post annealed at 2500C gives a narrow peak at 27.350 indicated good crystallinity of the film [13][14]. Therefore, it is preferential orientation plane is (111)[15]. Crystal size of GaAs peaks of sample 2500C has 52.68 nm, determined with help of Scherrer’s formula. Similarly sample 2800C show a pronounced reflection at 27.420 corresponding to the (111) plane and crystal size has 57.35 nm approximately and sample 3200C peak at 27.430 has crystal size 57.58 nm [16].From the samples in the given figures we conclude that it depend upon annealing temperatures[17]. If the annealing temperature increases the films become more crystalline and size of the crystal also increase and the peak of samples having high annealing temperature slightly shift toward crystallinity as compare to the samples having lower annealing temperature[18].

2. Surface investigations

2.1. Scanning Electron Microscope (SEM) Analysis Technique

Scanning electron microscope is a technique used to investigate surface morphologically & topographically structures, grain sizes, porosities, and cracks etc of thin films successfully. SEM is also a destructive technique. In this research work morphological and topographical of samples characterizations were recorded using a JEOL JSM5910 SEM, operating at 30 kV, with a resolution of 2.3 nm.Glass substrate is non-conductive before SEM; gold is sputter to use as an electrode to work at conductive substrates because conductive substrate is compulsory for electron beam. As shown in figures 2.1, 2.2 and 2.3, the SEM images of the surfaces of samples 2500C, 2800C and 3200C in different ranges of magnification and resolution. SEM images of the surface morphology of GaAs thin films on glass substrates indicate that it depend on annealing temperature. GaAs has the defects mainly due to under coordinated sites, dangling bonds (db-As &db-Ga) and wrong bonds (Ga-Ga & As-As). In the amorphous case in the wrong bond films produce gap states and those bonds creates a lot of problems in electrical and optical properties of the GaAs film. These dangling and wrong bonds become lesser with the annealing temperatures and improved the surface of GaAs thin films and crystal sizes of the grins.In sample 2800C clearly shown in figure 2.2, to fill the empty spaces means rough and porous surfaces of films etc, and produced spherical crystals size in comparison with 2500C sample due to increasing annealing temperature and but in figure 2.3 , shows some cracks therefore to explain this problem two reasons are possible, first one is thehigh dose electron beam irradiation but the mechanism remains unknown [19] and the other one is the annealing of sample in open atmosphere in tube furnace.

 

2.2. Energy Dispersive X-ray Spectroscopy (EDS or EDX)Analysis Technique

It is used for elemental identification of film composition. EDX is non-destructive skill, in which all elements from Beryllium (Be) atomic number 4 to Uranium (U) atomic number 92 can be detected[20].An EDX detector is installed as an integrated portion of SEM. In this analysis, sample is bombarded with an electron beam inside the scanning electron microscope to knock out some electrons from sample surface. The vacancy in the sample atoms created by the ejection of an inner shell electron is occupied by a higher energy electron from an outer shell. During this transition, transferring of electron gives up its extra energy in the form of X-rays determined by energy dispersive spectrometer. Every peaks is special for atoms to represents a single element. A more dominant element in sample shows a higher peak in EDX spectrum. In this characterization a plot is not only determines elemental peaks but type of X-ray transition to which it emitted. An EDX graph is normally represented as X-ray counts as a function of its energy in keV. Elements can be identified from the graph by their narrow peaks at given energies[21].

The EDX study of the GaAs films on glass substrates as shown in figures. Since all samples have deposited under same conditions, and annealed at 2500C, 2800C and 3200C temperatures in tube furnace in open atmosphere. The composition of the samples 2500C, 2800Cand 3200Chave obtained from energy dispersive X-ray (EDX) measurements & shown different concentration of As & Ga and seems no contamination (Carbon C concentration due to SEM grid) [22].It means that annealing of GaAs in open atmosphere is not contaminated. In comparisons all samples peaks clearly mention the difference in figures and samples 2800C and 3200C shows slightly shift peak due to increased in annealed temperature. Generally X-rays transitions of K series occur only in light elements, L series or K & L series X-raystransitions occur in intermediate elements, M series or L & M series X-rays transitions emit from heavy elements. From EDX spectrum and composition table it’s clearly finding that GaAs is an intermediate material[23][24]. The energy dispersive spectrum is displayed in form with the x-axis representing X-ray energy in channels 0-20 keV and y-axis representing the number of counts per channel as shown in figures.

 

2.3. Rutherford Backscattering Spectrometry (RBS) Analysis Technique

It is a non destructive technique used for compositional analysis sample like area concentration measurements (atoms/cm2), and quantitative depth profiling[25]. It is also used to study crystal structures and to measure contaminants[26]. In this technique normally high energy ion beams like He or H ion with the energies of few MeV are used as probes[27]. It has the capability to penetrate thousands of angstroms or even microns into samples. Number of target atoms in film can be obtaining from the peak yields & film thickness from peak width[28].RBS is more sensitive for heavy elements than for light elements more than hundred times. It means that RBS has good mass resolution for light elements as compared to heavy elements[29]. GaAs is also an intermediate material.The film thickness analysis of GaAs/Glass have determined by the following parameters with the help of RBS technique[30].

Experimental details, Rutherford Backscattering Spectrometry (RBS), software used (RC43, XRUMP, SIMNRA), accelerator(5MV Pelletron Tandem Accelerator), type of detector (Solid State Barrier (SSB)), current (26 nA), beam (He++), energy (2.084 MeV), charge (15 uC), incident angle (00), backscattering angle (1700), resolution of detector (20 KeV).

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Allsampleshave shown slightly difference in plotted figures. Comparisons of samples 2500C, 2800C and 3200C, in figures seem slightly difference due to area concentration of atoms/cm2. GaAs/Glass composition presented in table 2.3.1, we can deduce that thin film of GaAs/Glass content no impurity in this work. In RBSanalysis it shown that resistive heating evaporation is a non-uniform films technique because each sample has different film thickness.

 

Conclusions

In this research, GaAs powder were deposited on glass substrates by resistive heating evaporation technique under vacuum pressure 7.8×10-5 mbar at room temperature. Samples were annealed each 20min in an open environment in tube furnace at 2500C, 2800C and 3200C temperatures. Each sample was characterize with differenttechniques such that XRD, SEM, EXD, RBS.In XRD we were found the crystal size with help of Scherrer’s formula which is excellent match literatures.SEM have good result but in each sample figures resolution range 0.2 µm shows some cracks may be used of high dose electron beam irradiation but the mechanism remains unknown. The composition of the sampleshave obtained from EDX measurements and shown different concentration of As & Ga and seems no contamination. We can say that in open environment annealing of GaAs were not contaminated. We have also found that GaAs is an intermediate compound because of X-rays transitions of K and L series. We have also confirmed with the help of RBS the elemental composition and thickness of samples. All sampleshave shown slightly difference in peaks and different values of thickness but no contamination found. It means that resistive heating evaporation technique is a non-uniform film technique due to different thickness of samples

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