Influence of a saline test environment on the Fatigue properties of laser metal deposition Ti-6al-4v specimens
- Authors: Botha, Sheldyn Jaye
- Date: 2021-04
- Subjects: Metal organic chemical vapor deposition , Salines
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/59203 , vital:60304
- Description: This study investigated the effects of the position of extraction and a corrosive environment on the fatigue properties of Ti-6Al-4V Laser Metal Deposition (LMD) specimens. Additive manufacturing (AM) processes allow for a reduction in manufacturing time, whilst constructing complex geometries. AM induces a complex thermal history which influences the microstructure in the material (1). The microstructure affects the fatigue properties where a finer microstructure is more favourable for increasing the fatigue life (2). Fatigue failure is one of the main modes of failure during a material’s service life (3). Thus, it is imperative to investigate the material’s fatigue behaviour. Major parameters of the LMD process were investigated, allowing bulk LMD coupons to be manufactured, from which fatigue specimens were extracted at three different heights. Static properties were attained through tensile testing and a Vickers Micro-Hardness evaluation. These properties allowed for calculating benchmark stresses that were utilised for fatigue testing. The biocompatibility of Ti-6Al-4V makes it desirable for medical implants – the core influence of this study. An implant is constantly subjected to a corrosive saline environment, introducing a corrosive mechanism of failure, known to decrease the life of a material. A rotary bending fatigue platform was utilised as it allowed for modifications to introduce a corrosive saline environment through means of a saline dosing system. Literature reports a higher possibility of corrosion fatigue occurring under rotary bending fatigue conditions for Ti alloy specimens (4,5). The fatigue data indicated no significant effect on the fatigue properties was evident when comparing the position of extraction, attributed to the interrupted build strategy utilised for manufacturing the bulk coupons. The specimens displayed a shift in fatigue life when subjected to a corrosive saline environment. Upon analysis, there was little variation in the microstructures at each position of extraction, attributed to the interrupted build strategy. , Thesis (MA) -- Faculty of Engineering, the Built Environment, and Technology, 2021
- Full Text:
- Date Issued: 2021-04
- Authors: Botha, Sheldyn Jaye
- Date: 2021-04
- Subjects: Metal organic chemical vapor deposition , Salines
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/59203 , vital:60304
- Description: This study investigated the effects of the position of extraction and a corrosive environment on the fatigue properties of Ti-6Al-4V Laser Metal Deposition (LMD) specimens. Additive manufacturing (AM) processes allow for a reduction in manufacturing time, whilst constructing complex geometries. AM induces a complex thermal history which influences the microstructure in the material (1). The microstructure affects the fatigue properties where a finer microstructure is more favourable for increasing the fatigue life (2). Fatigue failure is one of the main modes of failure during a material’s service life (3). Thus, it is imperative to investigate the material’s fatigue behaviour. Major parameters of the LMD process were investigated, allowing bulk LMD coupons to be manufactured, from which fatigue specimens were extracted at three different heights. Static properties were attained through tensile testing and a Vickers Micro-Hardness evaluation. These properties allowed for calculating benchmark stresses that were utilised for fatigue testing. The biocompatibility of Ti-6Al-4V makes it desirable for medical implants – the core influence of this study. An implant is constantly subjected to a corrosive saline environment, introducing a corrosive mechanism of failure, known to decrease the life of a material. A rotary bending fatigue platform was utilised as it allowed for modifications to introduce a corrosive saline environment through means of a saline dosing system. Literature reports a higher possibility of corrosion fatigue occurring under rotary bending fatigue conditions for Ti alloy specimens (4,5). The fatigue data indicated no significant effect on the fatigue properties was evident when comparing the position of extraction, attributed to the interrupted build strategy utilised for manufacturing the bulk coupons. The specimens displayed a shift in fatigue life when subjected to a corrosive saline environment. Upon analysis, there was little variation in the microstructures at each position of extraction, attributed to the interrupted build strategy. , Thesis (MA) -- Faculty of Engineering, the Built Environment, and Technology, 2021
- Full Text:
- Date Issued: 2021-04
Atmospheric pressure metal-organic vapour phase epitaxial growth of InAs/GaSb strained layer superlattices
- Authors: Miya, Senzo Simo
- Date: 2013
- Subjects: Gallium arsenide semiconductors , Organometallic compounds , Compound semiconductors , Metal organic chemical vapor deposition , Superlattices as materials , Epitaxy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10557 , http://hdl.handle.net/10948/d1020866
- Description: The importance of infrared (IR) technology (for detection in the 3-5 μm and 8-14 μm atmospheric windows) has spread from military applications to civilian applications since World War II. The commercial IR detector market in these wavelength ranges is dominated by mercury cadmium telluride (MCT) alloys. The use of these alloys has, however, been faced with technological difficulties. One of the materials that have been tipped to be suitable to replace MCT is InAs/InxGa1-xSb strained layer superlattices (SLS’s). Atmospheric pressure metal-organic vapour phase epitaxy (MOVPE) has been used to grow InAs/GaSb strained layer superlattices (SLS’s) at 510 °C in this study. This is a starting point towards the development of MOVPE InAs/InxGa1-xSb SLS’s using the same system. Before the SLS’s could be attempted, the growth parameters for GaSb were optimised. Growth parameters for InAs were taken from reports on previous studies conducted using the same reactor. Initially, trimethylgallium, a source that has been used extensively in the same growth system for the growth of GaSb and InxGa1-xSb was intended to be used for gallium species. The high growth rates yielded by this source were too large for the growth of SLS structures, however. Thus, triethylgallium (rarely used for atmospheric pressure MOVPE) was utilized. GaSb layers (between 1 and 2 μm thick) were grown at two different temperatures (550 °C and 510 °C) with a varying V/III ratio. A V/III ratio of 1.5 was found to be optimal at 550 °C. However, the low incorporation efficiency of indium into GaSb at this temperature was inadequate to obtain InxGa1-xSb with an indium mole fraction (x) of around 0.3, which had previously been reported to be optimal for the performance of InAs/InxGa1-xSb SLS’s, due to the maximum splitting of the valence mini bands for this composition. The growth temperature was thus lowered to 510 °C. This resulted in an increase in the optimum V/III ratio to 1.75 for GaSb and yielded much higher incorporation efficiencies of indium in InxGa1-xSb. However, this lower growth temperature also produced poorer surface morphologies for both the binary and ternary layers, due to the reduced surface diffusion of the adsorbed species. An interface control study during the growth of InAs/GaSb SLS’s was subsequently conducted, by investigating the influence of different gas switching sequences on the interface type and quality. It was noted that the growth of SLS’s without any growth interruptions at the interfaces leads to tensile strained SLS’s (GaAs-like interfaces) with a rather large lattice mismatch. A 5 second flow of TMSb over the InAs surface and a flow of H2 over GaSb surface yielded compressively strained SLS’s. Flowing TMIn for 1 second and following by a flow of TMSb for 4 seconds over the GaSb surface, while flowing H2 for 5 seconds over the InAs surface, resulted in SLS’s with GaAs-like interfacial layers and a reduced lattice mismatch. Temperature gradients across the surface of the susceptor led to SLS’s with different structural quality. High resolution x-ray diffraction (HRXRD) was used to determine the thicknesses as well as the type of interfacial layers. The physical parameters of the SLS’s obtained from simulating the HRXRD spectra were comparable to the parameters obtained from cross sectional transmission electron microscopy (XTEM) images. The thicknesses of the layers and the interface type played a major role in determining the cut-off wavelength of the SLS’s.
- Full Text:
- Date Issued: 2013
- Authors: Miya, Senzo Simo
- Date: 2013
- Subjects: Gallium arsenide semiconductors , Organometallic compounds , Compound semiconductors , Metal organic chemical vapor deposition , Superlattices as materials , Epitaxy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10557 , http://hdl.handle.net/10948/d1020866
- Description: The importance of infrared (IR) technology (for detection in the 3-5 μm and 8-14 μm atmospheric windows) has spread from military applications to civilian applications since World War II. The commercial IR detector market in these wavelength ranges is dominated by mercury cadmium telluride (MCT) alloys. The use of these alloys has, however, been faced with technological difficulties. One of the materials that have been tipped to be suitable to replace MCT is InAs/InxGa1-xSb strained layer superlattices (SLS’s). Atmospheric pressure metal-organic vapour phase epitaxy (MOVPE) has been used to grow InAs/GaSb strained layer superlattices (SLS’s) at 510 °C in this study. This is a starting point towards the development of MOVPE InAs/InxGa1-xSb SLS’s using the same system. Before the SLS’s could be attempted, the growth parameters for GaSb were optimised. Growth parameters for InAs were taken from reports on previous studies conducted using the same reactor. Initially, trimethylgallium, a source that has been used extensively in the same growth system for the growth of GaSb and InxGa1-xSb was intended to be used for gallium species. The high growth rates yielded by this source were too large for the growth of SLS structures, however. Thus, triethylgallium (rarely used for atmospheric pressure MOVPE) was utilized. GaSb layers (between 1 and 2 μm thick) were grown at two different temperatures (550 °C and 510 °C) with a varying V/III ratio. A V/III ratio of 1.5 was found to be optimal at 550 °C. However, the low incorporation efficiency of indium into GaSb at this temperature was inadequate to obtain InxGa1-xSb with an indium mole fraction (x) of around 0.3, which had previously been reported to be optimal for the performance of InAs/InxGa1-xSb SLS’s, due to the maximum splitting of the valence mini bands for this composition. The growth temperature was thus lowered to 510 °C. This resulted in an increase in the optimum V/III ratio to 1.75 for GaSb and yielded much higher incorporation efficiencies of indium in InxGa1-xSb. However, this lower growth temperature also produced poorer surface morphologies for both the binary and ternary layers, due to the reduced surface diffusion of the adsorbed species. An interface control study during the growth of InAs/GaSb SLS’s was subsequently conducted, by investigating the influence of different gas switching sequences on the interface type and quality. It was noted that the growth of SLS’s without any growth interruptions at the interfaces leads to tensile strained SLS’s (GaAs-like interfaces) with a rather large lattice mismatch. A 5 second flow of TMSb over the InAs surface and a flow of H2 over GaSb surface yielded compressively strained SLS’s. Flowing TMIn for 1 second and following by a flow of TMSb for 4 seconds over the GaSb surface, while flowing H2 for 5 seconds over the InAs surface, resulted in SLS’s with GaAs-like interfacial layers and a reduced lattice mismatch. Temperature gradients across the surface of the susceptor led to SLS’s with different structural quality. High resolution x-ray diffraction (HRXRD) was used to determine the thicknesses as well as the type of interfacial layers. The physical parameters of the SLS’s obtained from simulating the HRXRD spectra were comparable to the parameters obtained from cross sectional transmission electron microscopy (XTEM) images. The thicknesses of the layers and the interface type played a major role in determining the cut-off wavelength of the SLS’s.
- Full Text:
- Date Issued: 2013
MOCVD growth and electrical characterisation of InAs thin films
- Authors: Shamba, Precious
- Date: 2007
- Subjects: Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10527 , http://hdl.handle.net/10948/706 , Metal organic chemical vapor deposition
- Description: In this work, a systematic study relating the surface morphologies, electrical and structural properties of both doped and undoped InAs and InAsSb epitaxial films grown by metalorganic chemical vapour deposition (MOCVD) was undertaken. A comparative study using TBAs and AsH3 as the group V source in the growth of InAs revealed a considerable improvement, primarily in the electrical properties of InAs grown using TBAs with no significant difference in the surface morphology. InAs layers grown using TBAs, exhibited superior 77 K mobilities of up to 46 000 cm2/Vs, exceeding the best MOCVD data to date. The feasibility of tetraethyl tin (TESn) as an n-type dopant in InAs was to our knowledge investigated for the first time. The incorporation efficiency of this dopant was extensively studied as a function of substrate temperature, V/III ratio, substrate orientation and TESn flow rate. Results from this study show that the doping efficiency is temperature dependent and is not influenced by a variation of the V/III ratio or substrate orientation. Furthermore, Sn doping concentrations could be controlled over 2 orders of magnitude ranging between 2.7 x 1017 and 4.7 x 1019 cm-3 with 77 K mobilities ranging from 12 000 to 1300 cm2/Vs. The electrical properties of zinc doped InAs employing dimethyl zinc (DMZn) as the ptype dopant, were studied as a function of V/III ratio and substrate orientation. The effect of a variation of these parameters on the structural properties and surface morphology of InAs is also reported. The substrate orientation appears to have no influence on the Zn incorporation. An increase in Zn incorporation resulted in a deterioration of both the surface morphology and structural quality of the InAs layers. The incorporation efficiency of DMZn in InAsSb was studied as a function of growth temperature, V/III ratio and DMZn flow rate. A higher Zn incorporation was observed in InAsSb epitaxial layers grown at a lower temperature and V/III ratio as opposed to the layers grown at a higher temperature and V/III ratio. This study also revealed that the use of DMZn caused a dopant memory effect. A two-layer model proposed by Nedoluha and Koch (1952) was used to simulate the Hall measurements of Zn doped InAs and InAsSb in order to correct the shortcomings of conventional Hall measurements in determining the electrical properties exhibited by these materials.
- Full Text:
- Date Issued: 2007
- Authors: Shamba, Precious
- Date: 2007
- Subjects: Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10527 , http://hdl.handle.net/10948/706 , Metal organic chemical vapor deposition
- Description: In this work, a systematic study relating the surface morphologies, electrical and structural properties of both doped and undoped InAs and InAsSb epitaxial films grown by metalorganic chemical vapour deposition (MOCVD) was undertaken. A comparative study using TBAs and AsH3 as the group V source in the growth of InAs revealed a considerable improvement, primarily in the electrical properties of InAs grown using TBAs with no significant difference in the surface morphology. InAs layers grown using TBAs, exhibited superior 77 K mobilities of up to 46 000 cm2/Vs, exceeding the best MOCVD data to date. The feasibility of tetraethyl tin (TESn) as an n-type dopant in InAs was to our knowledge investigated for the first time. The incorporation efficiency of this dopant was extensively studied as a function of substrate temperature, V/III ratio, substrate orientation and TESn flow rate. Results from this study show that the doping efficiency is temperature dependent and is not influenced by a variation of the V/III ratio or substrate orientation. Furthermore, Sn doping concentrations could be controlled over 2 orders of magnitude ranging between 2.7 x 1017 and 4.7 x 1019 cm-3 with 77 K mobilities ranging from 12 000 to 1300 cm2/Vs. The electrical properties of zinc doped InAs employing dimethyl zinc (DMZn) as the ptype dopant, were studied as a function of V/III ratio and substrate orientation. The effect of a variation of these parameters on the structural properties and surface morphology of InAs is also reported. The substrate orientation appears to have no influence on the Zn incorporation. An increase in Zn incorporation resulted in a deterioration of both the surface morphology and structural quality of the InAs layers. The incorporation efficiency of DMZn in InAsSb was studied as a function of growth temperature, V/III ratio and DMZn flow rate. A higher Zn incorporation was observed in InAsSb epitaxial layers grown at a lower temperature and V/III ratio as opposed to the layers grown at a higher temperature and V/III ratio. This study also revealed that the use of DMZn caused a dopant memory effect. A two-layer model proposed by Nedoluha and Koch (1952) was used to simulate the Hall measurements of Zn doped InAs and InAsSb in order to correct the shortcomings of conventional Hall measurements in determining the electrical properties exhibited by these materials.
- Full Text:
- Date Issued: 2007
Metalorganic vapour phase epitaxial growth and characterisation of Sb-based semiconductors
- Authors: Vankova, Viera
- Date: 2005
- Subjects: Compound semiconductors , Epitaxy , Organometallic compounds , Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10548 , http://hdl.handle.net/10948/d1019678
- Description: This study focuses on the growth and characterization of epitaxial InAs and InAs1-xSbx. Layers are grown on InAs, GaAs and GaSb substrates by metalorganic vapour phase epitaxy, using trimethylindium, trimethylantimony and arsine as precursors. The growth parameters (V/III ratio, Sb vapour phase compositions) are varied in the temperature range from 500 ºC to 700 ºC, in order to study the influence of these parameters on the structural, optical and electrical properties of the materials. The layers were assessed by X-ray diffraction, electron and optical microscopy, photoluminescence and Hall measurements. Furthermore, the influence of hydrogenation and annealing on the electrical and optical properties of GaSb was investigated. It is shown that the growth temperature and the V/III ratio play a vital role in the resulting surface morphology of homoepitaxial and heteroepitaxial InAs layers. Growth at low temperatures is found to promote three-dimensional growth in both cases, with improvements in the surface morphologies observed for higher growth temperatures. All the investigated epilayers are n-type. It is shown that the electrical properties of heteroepitaxial InAs epilayers are complicated by a competition between bulk conduction and conduction due to a surface accumulation and an interface layer. The low temperature photoluminescence spectra of homoepitaxial InAs are dominated by two transitions. These are identified as band-to-band/excitonic and donor-acceptor recombination. The incorporation efficiency of antimony (Sb) into InAs1-xSbx is dependent on the growth temperature and the V/III ratio. Under the growth conditions used in this study, the incorporation efficiency of Sb is controlled by the thermal stability of the two constituent binaries (i.e. InAs and InSb). Changes in the low temperature photoluminescence spectra are detected with increasing x. From temperature and laser power dependent measurements, the highest energy line is attributed to band-to-band/excitonic recombination, while the peak appearing approximately 15 meV below this line is assigned to donor-acceptor recombination. The origin of an additional “moving” peak observed for higher Sb mole fraction x is tentatively attributed to quasi-donor-acceptor-recombination, arising from increased impurity/defect concentrations and a higher compensation ratio in the material. However, the unusual behaviour of this peak may also be ascribed to the presence of some degree of ordering in InAsSb. The exposure of a semiconductor to a hydrogen plasma usually leads to the passivation of shallow and deep centres, thereby removing their electrical and optical activity. In this study, the passivation and thermal stability of the native acceptor in p-type GaSb is also investigated. It is shown that this acceptor can be passivated, where after improvements in the electrical and optical properties of GaSb are observed. Upon annealing the passivated samples above 300 °C, the acceptor is reactivated.
- Full Text:
- Date Issued: 2005
- Authors: Vankova, Viera
- Date: 2005
- Subjects: Compound semiconductors , Epitaxy , Organometallic compounds , Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10548 , http://hdl.handle.net/10948/d1019678
- Description: This study focuses on the growth and characterization of epitaxial InAs and InAs1-xSbx. Layers are grown on InAs, GaAs and GaSb substrates by metalorganic vapour phase epitaxy, using trimethylindium, trimethylantimony and arsine as precursors. The growth parameters (V/III ratio, Sb vapour phase compositions) are varied in the temperature range from 500 ºC to 700 ºC, in order to study the influence of these parameters on the structural, optical and electrical properties of the materials. The layers were assessed by X-ray diffraction, electron and optical microscopy, photoluminescence and Hall measurements. Furthermore, the influence of hydrogenation and annealing on the electrical and optical properties of GaSb was investigated. It is shown that the growth temperature and the V/III ratio play a vital role in the resulting surface morphology of homoepitaxial and heteroepitaxial InAs layers. Growth at low temperatures is found to promote three-dimensional growth in both cases, with improvements in the surface morphologies observed for higher growth temperatures. All the investigated epilayers are n-type. It is shown that the electrical properties of heteroepitaxial InAs epilayers are complicated by a competition between bulk conduction and conduction due to a surface accumulation and an interface layer. The low temperature photoluminescence spectra of homoepitaxial InAs are dominated by two transitions. These are identified as band-to-band/excitonic and donor-acceptor recombination. The incorporation efficiency of antimony (Sb) into InAs1-xSbx is dependent on the growth temperature and the V/III ratio. Under the growth conditions used in this study, the incorporation efficiency of Sb is controlled by the thermal stability of the two constituent binaries (i.e. InAs and InSb). Changes in the low temperature photoluminescence spectra are detected with increasing x. From temperature and laser power dependent measurements, the highest energy line is attributed to band-to-band/excitonic recombination, while the peak appearing approximately 15 meV below this line is assigned to donor-acceptor recombination. The origin of an additional “moving” peak observed for higher Sb mole fraction x is tentatively attributed to quasi-donor-acceptor-recombination, arising from increased impurity/defect concentrations and a higher compensation ratio in the material. However, the unusual behaviour of this peak may also be ascribed to the presence of some degree of ordering in InAsSb. The exposure of a semiconductor to a hydrogen plasma usually leads to the passivation of shallow and deep centres, thereby removing their electrical and optical activity. In this study, the passivation and thermal stability of the native acceptor in p-type GaSb is also investigated. It is shown that this acceptor can be passivated, where after improvements in the electrical and optical properties of GaSb are observed. Upon annealing the passivated samples above 300 °C, the acceptor is reactivated.
- Full Text:
- Date Issued: 2005
On the mocvd growth of ZnO
- Authors: Pagni, Olivier Demeno
- Date: 2004
- Subjects: Zinc oxide thin films , Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:11075 , http://hdl.handle.net/10948/382 , Zinc oxide thin films , Metal organic chemical vapor deposition
- Description: Zinc oxide (ZnO) is a II-VI semiconductor material that offers tremendous potential as a light emitter in the blue-to-UV range. It has a wurtzite structure, and a direct band gap that can be tuned from 3.0 to 4.0 eV by alloying with Cd or Mg, respectively. In this work, ZnO thin films were grown by metalorganic chemical vapor deposition (MOCVD) on n-Si 2 ° off (100), amorphous glass, n-GaAs (100), and c-plane sapphire substrates. Diethyl zinc (DEZn) and tert-butanol (TBOH) were chosen as precursors. For the first time, Second Harmonic Generation Imaging was applied to the mapping of ZnO epilayers. The images obtained highlighted the polycrystalline character of the thin films, and provided insight as to the growth mode of ZnO on Si. The influence of substrate temperature on the structural properties of the epilayers was investigated by X-ray diffraction and optical microscopy. Grain sizes as high as 54 nm were measured. The optimum temperature range for this system proved to be 450 – 500 °C. The influence of the VI:II ratio during growth on the optical properties of the epilayers was studied by UV-vis-near IR spectroscopy. The lowest Urbach tail E0 parameter was measured for material grown at a VI:II ratio of 18:1. The films’ free electron concentration was shown to decrease by over two orders of magnitude, from 1019 to 1017 cm-3, as the VI:II ratio increased from 10 to 60:1. This decrease in carrier concentration with rising VI:II ratio was paralleled to the surge at 12 K of a photoluminescence (PL) emission band characteristic of p-type ZnO. The band gap energies extracted from room temperature transmission spectra ranged between 3.35 and 3.38 eV, in agreement with the value of 3.35 eV measured by room temperature PL. Moreover, variable temperature PL spectra were recorded between 12 and 298 K on ZnO grown on Si. The 12 K spectrum was dominated by a donor-bound exciton (D°X) at 3.36 eV, while the 298 K scan displayed strong free exciton emission (FX) at 3.29 eV. The width of the D°X band proved to be as narrow as 7 meV. The intensity ratio between the room temperature near-band edge emission and the defect-related green band was as high as 28:1, highlighting the optical quality of the layers deposited in this work. The electrical properties of the thin films were studied by Hall measurements (van der Pauw configuration), and a maximum room temperature mobility of 11 cm2/Vs was recorded. Furthermore, a palladium (Pd) Schottky barrier diode on ZnO was fabricated. The barrier height and ideality factor were calculated from current–voltage measurements to be 0.83 eV and 1.6, respectively. The capacitance–voltage curve of the diode yielded a carrier concentration in the depletion region of 8·1017 cm-3. This study has shown that the optical and electrical properties of ZnO depend strongly on the growth conditions employed. A suitable choice of growth parameters can yield high quality ZnO that may be used for various devices. Keywords: Hall, MOCVD, optical spectroscopy, photoluminescence, Schottky barrier diode, SH Imaging, X-ray diffraction, ZnO.
- Full Text:
- Date Issued: 2004
- Authors: Pagni, Olivier Demeno
- Date: 2004
- Subjects: Zinc oxide thin films , Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:11075 , http://hdl.handle.net/10948/382 , Zinc oxide thin films , Metal organic chemical vapor deposition
- Description: Zinc oxide (ZnO) is a II-VI semiconductor material that offers tremendous potential as a light emitter in the blue-to-UV range. It has a wurtzite structure, and a direct band gap that can be tuned from 3.0 to 4.0 eV by alloying with Cd or Mg, respectively. In this work, ZnO thin films were grown by metalorganic chemical vapor deposition (MOCVD) on n-Si 2 ° off (100), amorphous glass, n-GaAs (100), and c-plane sapphire substrates. Diethyl zinc (DEZn) and tert-butanol (TBOH) were chosen as precursors. For the first time, Second Harmonic Generation Imaging was applied to the mapping of ZnO epilayers. The images obtained highlighted the polycrystalline character of the thin films, and provided insight as to the growth mode of ZnO on Si. The influence of substrate temperature on the structural properties of the epilayers was investigated by X-ray diffraction and optical microscopy. Grain sizes as high as 54 nm were measured. The optimum temperature range for this system proved to be 450 – 500 °C. The influence of the VI:II ratio during growth on the optical properties of the epilayers was studied by UV-vis-near IR spectroscopy. The lowest Urbach tail E0 parameter was measured for material grown at a VI:II ratio of 18:1. The films’ free electron concentration was shown to decrease by over two orders of magnitude, from 1019 to 1017 cm-3, as the VI:II ratio increased from 10 to 60:1. This decrease in carrier concentration with rising VI:II ratio was paralleled to the surge at 12 K of a photoluminescence (PL) emission band characteristic of p-type ZnO. The band gap energies extracted from room temperature transmission spectra ranged between 3.35 and 3.38 eV, in agreement with the value of 3.35 eV measured by room temperature PL. Moreover, variable temperature PL spectra were recorded between 12 and 298 K on ZnO grown on Si. The 12 K spectrum was dominated by a donor-bound exciton (D°X) at 3.36 eV, while the 298 K scan displayed strong free exciton emission (FX) at 3.29 eV. The width of the D°X band proved to be as narrow as 7 meV. The intensity ratio between the room temperature near-band edge emission and the defect-related green band was as high as 28:1, highlighting the optical quality of the layers deposited in this work. The electrical properties of the thin films were studied by Hall measurements (van der Pauw configuration), and a maximum room temperature mobility of 11 cm2/Vs was recorded. Furthermore, a palladium (Pd) Schottky barrier diode on ZnO was fabricated. The barrier height and ideality factor were calculated from current–voltage measurements to be 0.83 eV and 1.6, respectively. The capacitance–voltage curve of the diode yielded a carrier concentration in the depletion region of 8·1017 cm-3. This study has shown that the optical and electrical properties of ZnO depend strongly on the growth conditions employed. A suitable choice of growth parameters can yield high quality ZnO that may be used for various devices. Keywords: Hall, MOCVD, optical spectroscopy, photoluminescence, Schottky barrier diode, SH Imaging, X-ray diffraction, ZnO.
- Full Text:
- Date Issued: 2004
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