On the development of ZnO nanorods on silicon substrate for light-emitting diode applications
- Djiokap, Stive Roussel Tankio
- Authors: Djiokap, Stive Roussel Tankio
- Date: 2018
- Subjects: Zinc oxide , Chemical reactions , Compound semiconductors
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/29973 , vital:30802
- Description: The interest in zinc oxide (ZnO), a promising material for blue/ultraviolet light emitting devices, arises from its large exciton binding energy (60 meV). The main challenge associated with this promising compound semiconductor, however, arises from the difficulty to achieve stable and/or reproducible p-type doping. Since silicon (Si) technology still dominates the semiconductor industry, the objective of this thesis is to probe into the possibility of using ZnO nanorods (NRs) on p-type silicon for opto-electronic devices. ZnO NRs have been grown on seeded Si, as well as on nickel oxide (NiO) and aluminum nitride (AlN) coated Si, using a two-step chemical bath deposition (CBD) process. Various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM), have been used to characterize the samples. The electrical characteristics of the heterojunction between the substrate and the ZnO nanostructures were evaluated by current-voltage (I-V) and capacitance-voltage (C-V) measurements. SEM and XRD studies have confirmed that, irrespective of the orientation of the Si substrate (Si (100) and Si (111)), the two-step CBD process yielded NRs that crystallised in the wurtzite structure and exhibited a hexagonal shape. Most of the rods developed perpendicularly to the surface of the substrate, with the orientation and distribution of the rods dictated by the seed layer density. Similarly, irrespective of the substrate, the luminescence of the ZnO nanostructures is dominated by near band edge (NBE) emission in the UV region (~ 3.29 eV) and deep level emission (DLE) in the visible region (2 eV to 2.6 eV). Annealing at moderate temperatures (~ 300 °C) increased the NBE emission and decreased the DLE. The removal of surface adsorbed impurities and enhanced defect passivation by hydrogen are responsible for these changes. The diode characteristics of the ZnO/Si heterojunction was studied by I-V and C-V measurements. Rectification was observed when the Si substrate had a relatively low acceptor density of ~1016 cm-3, while diodes produced on substrate with p ~1018 cm-3 were ohmic. From the C-V analysis the donor density in the ZnO was deduced to be ~1018 cm-3. In the case of rectifying junctions, thermionic emission did not dominate the charge transport. The carrier transport mechanism was therefore probed by the temperature dependent I-V xiii measurements (100 K to 295 K). Defect-assisted multistep tunneling was deduced to dominate in the n-ZnO/p-Si diodes at low forward bias. The band alignment between n-ZnO and p-Si predicts a much smaller barrier for electrons than for holes at the interface, which results in recombination on the Si side of the junction for a forward-biased diode. NiO intermediate layers (formed on Si by the thermal oxidation of Ni) were used to reduce electron injection from ZnO into Si. Scanning probe microscopy (SPM) and XRD analysis showed that while the grain size of the poly-crystalline NiO increased with NiO film thickness, the orientation and distribution of the subsequently grown ZnO nanorods were unaffected by the underlying NiO layer. Also, the photoluminescence response of the ZnO rods remained unchanged. I-V measurements did illustrate rectifying behaviour, with both the forward and reverse currents strongly decreased due to the resistive nature of the NiO. In another attempt at confining electrons to the ZnO side of the junction, AlN-coated Si (111) was used as a substrate for ZnO nanorods. CBD parameters that normally yield nanorods resulted in a plate-like architecture of the ZnO. By modifying the ZnO seed density on the AlN/Si substrate, the rod-like morphology could be recovered. Both the forward and reverse current decreased in these diodes. From studies aimed at identifying the transport mechanism it was concluded that trap-assisted tunnelling, resulting from a high density of defects in the seed layer, dominates in these devices. In conclusion, while no ZnO electroluminescence could be achieved from any of the devices, this study provides insight into the transport mechanisms in n-ZnO/barrier/p-Si heterostructures and highlights the importance of the heterointerface quality for light emitting devices.
- Full Text:
- Date Issued: 2018
- Authors: Djiokap, Stive Roussel Tankio
- Date: 2018
- Subjects: Zinc oxide , Chemical reactions , Compound semiconductors
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/29973 , vital:30802
- Description: The interest in zinc oxide (ZnO), a promising material for blue/ultraviolet light emitting devices, arises from its large exciton binding energy (60 meV). The main challenge associated with this promising compound semiconductor, however, arises from the difficulty to achieve stable and/or reproducible p-type doping. Since silicon (Si) technology still dominates the semiconductor industry, the objective of this thesis is to probe into the possibility of using ZnO nanorods (NRs) on p-type silicon for opto-electronic devices. ZnO NRs have been grown on seeded Si, as well as on nickel oxide (NiO) and aluminum nitride (AlN) coated Si, using a two-step chemical bath deposition (CBD) process. Various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM), have been used to characterize the samples. The electrical characteristics of the heterojunction between the substrate and the ZnO nanostructures were evaluated by current-voltage (I-V) and capacitance-voltage (C-V) measurements. SEM and XRD studies have confirmed that, irrespective of the orientation of the Si substrate (Si (100) and Si (111)), the two-step CBD process yielded NRs that crystallised in the wurtzite structure and exhibited a hexagonal shape. Most of the rods developed perpendicularly to the surface of the substrate, with the orientation and distribution of the rods dictated by the seed layer density. Similarly, irrespective of the substrate, the luminescence of the ZnO nanostructures is dominated by near band edge (NBE) emission in the UV region (~ 3.29 eV) and deep level emission (DLE) in the visible region (2 eV to 2.6 eV). Annealing at moderate temperatures (~ 300 °C) increased the NBE emission and decreased the DLE. The removal of surface adsorbed impurities and enhanced defect passivation by hydrogen are responsible for these changes. The diode characteristics of the ZnO/Si heterojunction was studied by I-V and C-V measurements. Rectification was observed when the Si substrate had a relatively low acceptor density of ~1016 cm-3, while diodes produced on substrate with p ~1018 cm-3 were ohmic. From the C-V analysis the donor density in the ZnO was deduced to be ~1018 cm-3. In the case of rectifying junctions, thermionic emission did not dominate the charge transport. The carrier transport mechanism was therefore probed by the temperature dependent I-V xiii measurements (100 K to 295 K). Defect-assisted multistep tunneling was deduced to dominate in the n-ZnO/p-Si diodes at low forward bias. The band alignment between n-ZnO and p-Si predicts a much smaller barrier for electrons than for holes at the interface, which results in recombination on the Si side of the junction for a forward-biased diode. NiO intermediate layers (formed on Si by the thermal oxidation of Ni) were used to reduce electron injection from ZnO into Si. Scanning probe microscopy (SPM) and XRD analysis showed that while the grain size of the poly-crystalline NiO increased with NiO film thickness, the orientation and distribution of the subsequently grown ZnO nanorods were unaffected by the underlying NiO layer. Also, the photoluminescence response of the ZnO rods remained unchanged. I-V measurements did illustrate rectifying behaviour, with both the forward and reverse currents strongly decreased due to the resistive nature of the NiO. In another attempt at confining electrons to the ZnO side of the junction, AlN-coated Si (111) was used as a substrate for ZnO nanorods. CBD parameters that normally yield nanorods resulted in a plate-like architecture of the ZnO. By modifying the ZnO seed density on the AlN/Si substrate, the rod-like morphology could be recovered. Both the forward and reverse current decreased in these diodes. From studies aimed at identifying the transport mechanism it was concluded that trap-assisted tunnelling, resulting from a high density of defects in the seed layer, dominates in these devices. In conclusion, while no ZnO electroluminescence could be achieved from any of the devices, this study provides insight into the transport mechanisms in n-ZnO/barrier/p-Si heterostructures and highlights the importance of the heterointerface quality for light emitting devices.
- Full Text:
- Date Issued: 2018
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
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
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