A study of thermal stability of different ZnO/p-Si diode structure toward application of radiation detectors

Radiation hard detectors are difficult and costly to design. Moreover silicon based detectors are not superior in-term of high temperature environment compared to wide band semiconductor materials such as zinc oxide (ZnO). ZnO semiconductor are actively studied for the past decade due to its versatile properties such as high transparency, ability to be deposited at low temperatures, and high resistance towards radiation. Radiation hard materials must be able to withstand high temperature operation. A nuclear power plant demands high temperature up to 673 K operation especially near reactor pressure vessel. Not many work are done on the effect of temperature on radiation hard semiconductor material. The aims of this research is to remodel and simulating two different ZnO/p-Si heterojunction diode structure material and to study the temperature effect of diode parameters such as barrier height, apparent barrier height, ideality factor (n), series resistance, shunt resistance, simulation of Spice ZnO/p-Si heterojunction diode model and subsequently to optimize thermal stability for high temperature application such as radiation detector and electrical power plant. Modeling and simulation was analyzed and executed in Matlab and LTSpice. During characterization, the temperature were ramped-up from room temperature to 673 K. Furthermore, this analyzed result were compared to experimental result that have been published on high ranking journal. For each temperature, the parameters extracted were tabulated. Subsequently, all electrical characteristic obtained have been plotted on graph. On top of that, the effect of barrier inhomogeneity have been carried out and there were three prime prove of barrier inhomogeneity which is small barrier height value obtain from gradient of l n ( T .) versus V-1 , low Richardson constant value, and linearly lines on plotted barrier height against ideality factor. Result shows the structure 1 ideality factor of 2.07 were achieved at 673 K temperature with semi-empirical structure 2 hole concentration and calculated structure 1 electron concentration of 7.05x106cm-3 and 2.59x1013cm-3 respectively. Beside that, the activation energy was found at 0.35 eV. In conclusion, structure 1 ZnO/p-Si heterojunction diode shows good thermal stability compared to structure 2 but structure 2 show ability to fabricate high current and low turn on voltage of 0.8 V at room temperature. It has been observed that structure 1 ZnO/p-Si can withstand up to 673 K temperature thus proven that ZnO/p-Si as an substitute or alternative to high temperature environment operation

Broadband luminescence in defect-engineered electrochemically produced porous Si/ZnO nanostructures

The fabrication, by an all electrochemical process, of porous Si/ZnO nanostructures with engineered structural defects, leading to strong and broadband deep level emission from ZnO, is presented. Such nanostructures are fabricated by a combination of metal-assisted chemical etching of Si and direct current electrodeposition of ZnO. It makes the whole fabrication process low-cost, compatible with Complementary Metal-Oxide Semiconductor technology, scalable and easily industrialised. The photoluminescence spectra of the porous Si/ZnO nanostructures reveal a correlation between the lineshape, as well as the strength of the emission, with the morphology of the underlying porous Si, that control the induced defects in the ZnO. Appropriate fabrication conditions of the porous Si lead to exceptionally bright Gaussian-type emission that covers almost the entire visible spectrum, indicating that porous Si/ZnO nanostructures could be a cornerstone material towards white-light-emitting devices

On the development of ZnO nanorods on silicon substrate for light-emitting diode applications

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

Fabrication and characterization of p-Si/n-ZnO heterostructured junctions

In this paper ZnO nanorods and nanodots (with and without a SiO2 buffer layer) were grown on p-Si, forming p-n heterojunctions. The nanorod devices showed no visible electroluminescence (EL) emission but showed rectifying behavior. Covering around 60% of the length of the nanorods with PMMA produced an ideality factor of 3.91 +/- 0.11 together with a reverse saturation current of 6.53 +/- 4.2 x 10(-8) A. Up to two orders of magnitude rectification was observed for the current at bias -3 and 3 V. The nanodot devices showed EL emission under forward bias conditions. It seems that the buffer layer increased both the stability and efficiency of the devices, since the buffer layer device could operate at larger applied voltage and showed EL emission under reverse bias

Growth and characterization of ZnO nanorods using chemical bath deposition

Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices

Growth and characterization of ZnO nanorods using chemical bath deposition

Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices