Report (phase 2) Abstract Introduction In this study, the optical properties of a thin layer deposited using a technique called PLD on top of other thin films of Cu and Ni is going to be examined using the PL (Photoluminescence technique), the confirmation of the processes of depositions of the thin film layers of Cu and Ni (through EBPVD) plus the deposition of ZnO (through PLD) on top of the Cu/Ni layers are to be examined through XRD which gives information about the elements present in the sample, SEM was used to give information about the surface and also to confirm if the correct thicknesses of the layers deposited were the correct values that were calculated.[3] 1. SAMPLE PREPARATION In this section, we focus on the preparation of the different layers of the sample that we are going to examine Si (100 wafer), SiO2, Ni, Cu, and ZnO. 1.1. Cutting and cleaning of the Si wafer The Si wafer was first cut into 1 x 1 mm pieces in respect of the crystal structure, Si of 100 structure was chosen since it is easy to cut into these required pieces without shattering or breaking into small fragments. Considering the effects of impurities, the Si wafer needed to be pure as possible, and lucky enough it was bought as pure Si from the manufacturer, the only problem was making sure that the sample was having fewer or preferably no contaminants at the surface since the techniques or machinery that was going to be used to analyze the sample are surface sensitive thus the Si needed to be cleaned. The water was then given an ultrasonic bath after it was cut into these 1 x 1 cm pieces. An ultrasonic bath uses high-frequency sound waves with a very high-intensity wave liquid (in our case liquid acetone and ethanol were used) to clean up foreign surface contaminants. [4] Two beakers were filled with ethanol and acetone such that the substrates that were cut were now firstly submerged in the acetone beaker first and then given the ultrasonic bath then moved to the beaker with ethanol and then given an ultrasonic bath also for one minute repeating the process for six batches containing six of the cut Si species. Plastic sample holders were firstly cleaned with ethanol and dried up with compressed Ni to make sure that the wafers are not contaminated when they are inserted inside the holders, also the samples themselves were dried up using this compressed Ni to make sure that they are dry before we put them in the containers to prevent watermarks on them, compressed Ni gas is chosen over compressed air since compressed air might contaminate the wafers since it contains oil fumes in it. Another important precaution during sample preparation and throughout the entire experiment is to make sure that the samples are always handled with tweezers (plastic or metal, preferably plastic to avoid etching of the deposited layers in later stages of the experiment) and to make sure that surgical gloves are always worn to avoid contaminations form the hands. 1.2. Growing of the SiO2 layer. The main aim of the project was to investigate diffusion between the Ni and Cu layers and the optical properties of ZnO thus one needs to make sure that they do not diffuse to the wafer, therefore a thin SiO2 layer having special properties such as: being very stable, having high resistivity, etching selectivity, was grown on top of the Si wafer using wet oxidation such that it acts as a barrier that prevents the Ni or Cu from diffusing into the Si wafer Wet oxidation uses both oxygen (Oβ) and vapor (HβO).[5] This is a process by which a water vapor that is very rich in oxygen (oxygen is introduced to boiling water via a tube) is tunneled to the sample in a quartz tube wrapped with aluminum thin foil to prevent the effects of condensation that might contaminate the wafers that are placed inside an aluminite ceramic. The aluminite ceramic with the Si wafers is placed inside a furnace (Lindberg furnace) with a temperature between 900 1000β for a certain period (t). As the water vapor enters the furnace the molecules break and form a hydroxide before reaching the surface of the Si, at the surface of the silicon reacts with the oxygen and forms silicon dioxide at a greater rate than in the dry oxidation process.[5] The Lindberg furnace was used in the deposition of SiO2. It has a control unit which controls the power going to the furnace. There is a flask of water on a hot plate with a stirring instrument inside, which ensures that water does not form bubbles as it boils. this flask is the source of oxygen in this experiment. The furnace consists of a thermocouple and there is a sensor in the middle, which measures the temperature of the furnace. Results 1. X-Ray Diffraction (XRD). Since the deposition process was done, now the main thing is to confirm if this process was done correctly by looking at the whole sample’s chemical compositions, since XRD is a technique that is mainly focused on the analysis of the bulk material by identifying the long-range order of crystalline materials. X-ray diffraction is a technique used to determine a sample's composition or crystalline structure. It sends x-ray beams through the sample, which bounce off the atoms in the structure and change the direction of the beam at some different angle, π, from the original beam. Constructive interference occurs when the xray beams that are whole number integers of the same wavelength add together to create a new beam with a higher amplitude. The angle of diffraction can then be used to determine the difference between atomic planes using Bragg's law, where π is the wavelength added, π is the angle of diffraction, and π(βππ) is the distance between atomic planes. The distance between atomic planes can then be used to determine the composition or crystalline structure.[4] The X-ray involved in XRD is monochromatic X-rays that have been created by a cathode ray tube using the characteristic process rather than the Bremsstrahlung process. To make the x-rays to be monochromatic, a Ni filter is used since during the characteristic process the x-rays can either come in Kalpha or K-beta peak. The Ni filter was chosen since……………………….. ππ = 2π sin π π = 1,2,3 … … π(βππ) = π0 √(β)2 +(π)2 +(π)2 (eq 1) (eq 2) Data analysis. A spectrum of intensity of a signal against its 2π is plotted, where the positions of 2π correspond to a certain spacing between the atoms in the samples, determined by the angle of diffraction from the incident x-ray beam sent into the sample. The intensity of the peaks gives information about the number of atoms with that certain spacing between them, the greater the intensity of the peak, the greater the number of atoms with that distinct spacing.[4] Using equation 1 & 2, the necessary parameters can be calculated using the following steps to obtain the necessary spacings or lattice parameters to identify the chemical compositions in that corresponding peak: A. Calculate π(βππ) interplanar distancing. - ππ π(βππ) = 2π πππ from Braggs law. B. Calculate π0 lattice parameter. - π0 = (√(β)2 + (π)2 + (π)2 ) × π(βππ) for BCC and FCC. - π0 = 3 [(π(βππ) )2 4 × (β2 + π 2 + π 2 )] for HCP. In the above method one needs to be aware that different crystals or materials have different atomic arrangements (FCC, BCC & HCP) and it plays a huge role in calculating the interplanar distances and lattice parameters. During our sample analysis through XRD the following parameters were set to a computer that is connected to the XRD machine to control how is the machine going to operate: Diffraction angle - 8° < π < 100 ° Voltage – 40000 V. Beam current – 40 mA. Time steps – 0.5 seconds. Position interval – 0.01986° The following table shows the calculated values of the lattice parameters from the different elements present in those samples, one may notice that SiO2 is not present in the table because it is amorphous and shows no diffraction patterns under XRD : Samples 1A 2A 3A 1C 2C 3C Powder Zinc Oxide Si (100) C. SEM. D. PHOTOLUMINESCENCE. ZnO is one of the most interesting SC of the 21st century, it is a very stable and environmentally friendly material that can be utilized in various forms (powder, thin films, single crystals, and nanostructures) with a wide band gap of ~ 3.37 eV at room temperature, making it a very good candidate for photonic applications in the UV or blue spectral range, while the high exciton-binding energy of ~ 60 meV [1] allows efficient excitonic emission even at room temperature and that has various practical applications such as: optical waveguides, surface acoustic wave devices, varistors, phosphors, transparent conductive oxides, chemical and gas sensors (because it shows sensitivity at different gas species like carbon monoxide (CO), ethanol (C2H5OH) and acetylene(C2H2) ), spin functional devices, and UV-light emitters, due to its electronic and optical properties under various environments.[1] Based on the concert of light-matter interaction, emission in the UV, visible or infrared photons from an electronically excited species. - Different types of luminescence corresponding to different types of exciting sources: TYPE OF LUMINESCENCE Photoluminescence Radioluminescence Cathodoluminescence Electroluminescence Thermoluminescence EXCITATION SOURCE Photons Ionized radiation Cathode rays Electric fields Heating after prior storage of energy Atoms in a solid In solid, only energy levels available for occupation by the electrons occur in bands of discrete, very closely spaced levels (bands); energy values between these bands are forbidden. This model leads to the familiar classification of the electronic properties of metals, semiconductors, and insulators. This band gap theory can is an effect of bringing atoms closer together such that the electronic levels start to overlap each other making it impossible to distinguish which is which then you have a band instead of energy levels, the discrete energy levels from the individual atoms then split into molecular levels that are defined by the number of bonding atoms and the orbital degeneracy of the level. Figure 1: The creation of energy bands and the splitting of energy levels of N atoms brought close to each other. In most solids, the atoms are bound together by forces such as ionic, covalent, and metallic forces. In semiconductors, the energy gap between the valence band and conduction is not constant for all values of the electron’s wavevector k, some materials have a direct gap( momentum of the top of the valence band and bottom of the conduction gap has the same value of in a graph of energy and momentum) and some have indirect band gap ( momentum of the top of the VB and the bottom of the CB are not of the same momentum value). To move electrons from the VB to the CB such that the semiconductor start showing some interesting electronic properties, the electrons must first be given some energy or must be excited. For a direct band gap, the electrons only require a photon such that it moves from VB-CB leaving a hole in the VB this is often referred to as an electron-hole pair while for an indirect gap, the electron requires photons energy and phonon energy to give it momentum such that is dislocated to the CB. Recombination of the electrons and holes created is possible to produce a photon, this recombination also follows the same process as in the direct and indirect band gaps, but direct bandgap materials (GaAs for LEDs) are more efficient in producing optical devices because during the recombination no additional phonon energy is needed as in indirect band gaps materials (such as Si). Element Si (semiconductor) ZnO (semiconductor) SiO2 (insulator) Ge (semiconductor) Direct/ indirect Band gap Indirect Direct Direct Direct Eg at room temperature (eV) 1.12 eV 3.35 eV 9.70eV 0.66 eV The following figure shows direct and indirect band gaps and the associated emissions due to recombination’s of the electrons with holes. Figure 2: representation of direct and indirect bandgap energy as a function of wave number k and the relative emission due to them. For radiative emission processes electrons at the bottom of the conduction bottom transfer to empty states at the top of the valence band, releasing energy as a photon of radiation. In the direct-gap semiconductor, the transition is just a direct one resulting in a hole-electron combination that results in luminescence with energy βπ ≈ πΈπ . The indirect semiconductors the radiative transition is indirect with a small transition probability thus raising the probability that this transition will be non-radiative, but this probability of radiative transition in the indirect gap can be enhanced by doping the material such that the recombination takes place at the impurity center where the k selection rule may be broken. This action of doping then causes a shift in the luminescence spectra in a predictable way or variation that is roughly linear however there’s a sudden change of slope of the transition at the crossover from direct to indirect gap material. The spectral variation of the optical constraints gives detailed information about the band structure, the absorption rate at any photon energy above the bandgap is determined by the density β of states in the valence and conduction bands separated by the energyπΈ ≈ π. Singularities in the 2π density of states produce edges in the absorption spectra, which may increase or decrease with an increase in frequency. Sharp peaks are observed when upward and downward edges almost coincide. ZnO optical properties The basis of their physical properties as their electronic and vibrational states, the existence and nature of defects and impurities, and semiconductor materials have provided a lot of information on such different aspects. Intrinsic properties of semiconductors are based upon the interaction between holes in the valence band (VB) and electrons in the conduction band (CB), as well as excitonic causes due to the Coulomb interaction. Extrinsic optical characteristics are dependent on the dopants/defects initiated in the semiconductor, which constructs discrete electronic states between VB and CB. PL spectra of ZnO’s different nanostructures demonstrate UV emission, one/ two bands of visible emissions induced by defects, interstitials, and anti-sites, including vacancies and complex defects [60,61]. ZnO reveals a band gap of 3.37 eV with higher exciton energy of 60 meV at room temperature. The larger value of exciton energy than GaN (25 meV) makes it efficient exciton emission at room temperature below low excitation energy. So, ZnO is one of the most encouraging photonic materials in the blue-UV region [62]. PL spectroscopy has been employed to study the optical properties of ZnO nanorods, which gives details about band gaps, defects, and the feature of the crystal. What are excitons. Electrons in a solid can be free to move around and it can be bounded by forces such as Coulomb’s forces. Bounded electrons oscillate with a specific angular frequency corresponding to the energy given to that oscillating system, as these dipoles oscillate, they can then emit radiation of a certain wavelength corresponding to the energy that the system is losing as It oscillates. A variety of spectroscopic phenomena associated with both impurities and defects have optical absorption and emission bands at energies below the bandgap. Discussion Conclusion References. [1] Özgür, Ü., Alivov, Y.I., Liu, C., Teke, A., Reshchikov, M., DoΔan, S., Avrutin, V.C.S.J., Cho, S.J. and Morkoç, A.H., 2005. A comprehensive review of ZnO materials and devices. Journal of applied physics, 98(4), p.11. [2] DjurišiΔ, A.B. and Leung, Y.H., 2006. Optical properties of ZnO nanostructures. small, 2(8β9), pp.944-961. [3] Khan, H., Yerramilli, A.S., D'Oliveira, A., Alford, T.L., Boffito, D.C. and Patience, G.S., 2020. Experimental methods in chemical engineering: Xβray diffraction spectroscopy—XRD. The Canadian journal of chemical engineering, 98(6), pp.1255-1266. [4] Fuchs, F.J., 2015. Ultrasonic cleaning and washing of surfaces. In Power ultrasonics (pp. 577609). Woodhead Publishing. [5] Anon., 2017. semiconductor. Samsung. [Online] Available at: https://semiconductor.samsung.com/support/tools-resources/fabrication-process/eight-essentialsemiconductor-fabrication-processes-part-2-oxidation-to-protect-thewafer/#:~:text=Dry%20oxidation%20uses%20pure%20oxygen%20%28O%E2%82%82%29%2C%20and%2 0consequen [Accessed 12-06-2023]. [6] Anon., 2019. LibreTexts chemistry. [Online] Available at: https://chem.libretexts.org/Courses/Franklin_and_Marshall_College/Introduction_to_Materials_Charac terization__CHM_412_Collaborative_Text/Diffraction_Techniques/Xray_diffraction_(XRD)_basics_and_application [Accessed 13 May 2023].
