Electrical and structural characterization of metal germanides

Metal-semiconductor contacts have been widely studied in the past 60 years. These structures are of importance in the microelectronics industry. As the scaling down of silicon-based complementary metal-oxide-semiconductor (CMOS) devices becomes more and more challenging, new material and device structures to relax this physical limitation in device scaling are now required. Germanium (Ge) has been proposed as a potential alternative to silicon. In this thesis a systematic study of the thermally induced reaction of transition metals with the n-Ge substrate is outlined. Investigations in the change of the electrical properties of the metal germanide structures is studied in a wide range of temperatures. Current-voltage (I-V), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS) and high-resolution Laplace-DLTS (L-DLTS) techniques have been used for the electrical characterization of the fabricated Schottky contacts. Results obtained indicate the variation of the electrical properties of these Schottky contacts can be attributed to combined effects of interfacial reactions and phase transformation during the annealing process. The barrier height distribution in identically prepared Schottky contacts on n-Ge (100) showed that the barrier heights and ideality factors varied from diode to diode even though they were identically fabricated. The properties of the n-Ge Schottky contacts have revealed a strong dependence on temperature. The current transport mechanism has been shown to be predominantly thermionic emission at high temperatures while at low temperatures, the Schottky contacts have exhibited the dominance of the generation-recombination current mechanism. The variation of the Schottky barrier heights at low temperatures have been attributed to barrier inhomogeneities at the metal-semiconductor (MS) interface. Results from defect characterization by DLTS show that the E-centre is the dominant defect introduced in n-Ge by electron beam deposition during contact fabrication and substitutional related defects are induced during the annealing process. The identification of some of the defects was achieved by using defect properties, defect signature, annealing mechanisms and annealing behaviour and comparing these properties to the results from theoretical defect models. Annealing showed that defects in Ge can be removed by low thermal budget of between 250–350°C. Finally, structural characterization of these samples was performed by scanning electron microscopy (SEM) and Rutherford backscattering spectrometry (RBS) techniques. From the SEM images it can be observed that the onset temperature for agglomeration in the 30 nm Ni/n-Ge (100), and Pt/-, Ir/- and Ru/n-Ge (100) systems occur at 500–600°C and 600–700°C, respectively.Thesis (PhD)--University of Pretoria, 2010.Physicsunrestricte

Historical climate impact attribution of changes in river flow and sediment loads at selected gauging stations in the Nile basin

The Nile basin is the second largest basin in Africa and one of the regions experiencing high climatic diversity with variability of precipitation and deteriorating water resources. As climate change is affecting most of the hydroclimatic variables across the world, this study assesses whether historical changes in river flow and sediment loads at selected gauges in the Nile basin can be attributed to climate change. An impact attribution approach is employed by constraining a process-based model with a set of factual and counterfactual climate forcing data for 69 years (1951–2019), from the impact attribution setup of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP3a). To quantify the role of climate change, we use the non-parametric Mann-Kendall test to identify trends and calculate the differences in long-term mean annual river flow and sediment load simulations between a model setup using factual and counterfactual climate forcing data. Results for selected river stations in the Lake Victoria basin show reasonable evidence of a long-term historical increase in river flows (two stations) and sediment load (one station), largely attributed to changes in climate. In contrast, within the Blue Nile and Main Nile basins, there is a slight decrease of river flows at four selected stations under factual climate, which can be attributed to climate change, but no significant changes in sediment load (one station). These findings show spatial differences in the impacts of climate change on river flows and sediment load in the study area for the historical period

Representation of seasonal land use dynamics in SWAT+ for improved assessment of blue and green water consumption

This research article was published by Hydrology and Earth System Sciences, 2022In most (sub)-tropical African cultivated regions, more than one cropping season exists following the (one or two) rainy seasons. An additional cropping season is possible when irrigation is applied during the dry season, which could result in three cropping seasons. However, most studies using agro-hydrological models such as the Soil and Water Assessment Tool (SWAT) to map blue and green evapotranspiration (ET) do not account for these cropping seasons. Blue ET is a portion of crop evapotranspiration after irrigation application, while green ET is the evapotranspiration resulting from rainfall. In this paper, we derived dynamic and static trajectories from seasonal land use maps to represent the land use dynamics following the major growing seasons to improve simulated blue and green water consumption from simulated evapotranspiration in SWAT+. A comparison between the default SWAT+ set-up (with static land use representation) and a dynamic SWAT+ model set-up (with seasonal land use representation) is made by a spatial mapping of the ET results. Additionally, the SWAT+ blue and green ET were compared with the results from the four remote sensing data-based methods, namely SN (Senay), EK (van Eekelen), the Budyko method, and soil water balance method (SWB). The results show that ET with seasonal representation is closer to remote sensing estimates, giving higher performance than ET with static land use representation. The root mean squared error decreased from 181 to 69 mm yr−1, the percent bias decreased from 20 % to 13 %, and the Nash–Sutcliffe efficiency increased from −0.46 to 0.4. Furthermore, the blue and green ET results from the dynamic SWAT+ model were compared to the four remote sensing methods. The results show that the SWAT+ blue and green ET are similar to the van Eekelen method and performed better than the other three remote sensing methods. It is concluded that representation of seasonal land use dynamics produces better ET results, which provide better estimations of blue and green agricultural water consumption

Electrical characterisation of ruthenium Schottky contacts on n-Ge (1 0 0)

Please read abstract in the article.The South African National Research Foundationhttp://www.elsevier.com/locate/physbnf201

Deep level transient spectroscopy (DLTS) study of defects introduced in antimony doped Ge by 2 MeV proton irradiation

Please read the abstract in the article.The South African National Research Foundation and Monash University, South Africa.http://www.elsevier.com/locate/physbnf201

How Can We Represent Seasonal Land Use Dynamics in SWAT and SWAT+ Models for African Cultivated Catchments?

This research article published by MDPI, 2020In SWAT and SWAT+ models, the variations in hydrological processes are represented by Hydrological Response Units (HRUs). In the default models, agricultural land cover is represented by a single growing cycle. However, agricultural land use, especially in African cultivated catchments, typically consists of several cropping seasons, following dry and wet seasonal patterns, and are hence incorrectly represented in SWAT and SWAT+ default models. In this paper, we propose a procedure to incorporate agricultural seasonal land-use dynamics by (1) mapping land-use trajectories instead of static land-cover maps and (2) linking these trajectories to agricultural management settings. This approach was tested in SWAT and SWAT+ models of Usa catchment in Tanzania that is intensively cultivated by implementing dominant dynamic trajectories. Our results were evaluated with remote-sensing observations for Leaf Area Index (LAI), which showed that a single growing cycle did not well represent vegetation dynamics. A better agreement was obtained after implementing seasonal land-use dynamics for cultivated HRUs. It was concluded that the representation of seasonal land-use dynamics through trajectory implementation can lead to improved temporal patterns of LAI in default models. The SWAT+ model had higher flexibility in representing agricultural practices, using decision tables, and by being able to represent mixed cropping cultivations

Annealing and surface conduction on hydrogen peroxide treated bulk melt grown, single crystal ZnO

We report on the studies carried out on hydrogen peroxide treated melt grown, bulk single crystal ZnO samples. Results show the existence of two shallow donors in the as-received ZnO samples with energy levels (37.8 ± 0.3) meV that has been suggested as Zni related and possibly H-complex related and (54.5 ± 0.9) meV which has been assigned to an Al-related donor. Annealing studies performed on the hydrogen peroxide treated samples reveal the existence of a conductive channel in the samples in which new energy levels have been observed, Zn vacancies, related to the Group I elements, XZn. The surface donor volume concentration of the conductive channel was calculated from theory developed by D. C. Look [1]. Results indicate an increase in surface volume concentration with increasing annealing temperature from 6.0´1017 cm-3 at 200°C to 4.37´1018 cm-3 at 800°C.South African National Research Foundationhttp://www.elsevier.com/locate/physbhb2013ai201

Current-voltage temperature characteristics of Au/n-Ge (1 0 0) Schottky diodes

The variation in electrical characteristics of Au/n-Ge (1 0 0) Schottky contacts have been systematically investigated as a function of temperature using current-voltage (I-V) measurements in the temperature range 140-300 K. The I-V characteristics of the diodes indicate very strong temperature dependence. While the ideality factor n decreases, the zero-bias Schottky barrier height (SBH) ( ɸʙ) increases with the increasing temperature. The I-V characteristics are analysed using the thermionic emission (TE) model and the assumption of a Gaussian distribution of the barrier heights ɸʙ vs. ½ kT plot has been used to show the evidence of a Gaussian distribution of barrier heights and values of ɸʙ = 0.615 eV and standard deviation σs0 = 0.00858 eV for the mean barrier height and zero-bias standard deviation have been obtained from this plot, respectively. The Richardson constant and the mean barrier height from the modified Richardson plot were obtained as 1.37 A cm-2 K-2 and 0.639 eV, respectively. This Richardson constant is much smaller than the reported of 50 A cm-2 K-2. This may be due to greater inhomogeneities at the interface.The South African National Research Foundationhttp://www.elsevier.com/locate/physbnf201

Determination of the laterally homogeneous barrier height of palladium Schottky barrier diodes on n-Ge (1 1 1)

We have studied the experimental linear relationship between barrier heights and ideality factors for palladium (Pd) on bulk-grown (1 1 1) Sb-doped n-type germanium (Ge) metal-semiconductor structures with a doping density of about 2.5×1015 cm−3. The Pd Schottky contacts were fabricated by vacuum resistive evaporation. The electrical analysis of the contacts was investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements at a temperature of 296 K. The effective barrier heights from I–V characteristics varied from 0.492 to 0.550 eV, the ideality factor n varied from 1.140 to 1.950, and from reverse bias capacitance–voltage (C−2–V) characteristics the barrier height varied from 0.427 to 0.509 eV. The lateral homogenous barrier height value of 0.558 eV for the contacts was obtained from the linear relationship between experimental barrier heights and ideality factors. Furthermore the experimental barrier height distribution obtained from I–V and (C−2−V) characteristics were fitted by Gaussian distribution function, and their mean values were found to be 0.529 and 0.463 eV, respectively

Effect of thermal treatment on the characteristics of iridium Schottky barrier diodes on n-Ge (1 0 0)

Iridium (Ir) Schottky barrier diodes were deposited on bulk grown (100) Sbdoped n-type germanium by using the electron beam deposition system. Electrical characterization of these contacts using current-voltage (I-V) and capacitance-voltage (C-V) measurements was performed under various annealing conditions. The variation of the electrical properties of these Schottky diodes can be attributed to combined effects of interfacial reaction and phase transformation during the annealing process. Thermal stability of the Ir/n-Ge (100) was observed up to annealing temperature of 500oC. Furthermore, structural characterization of these samples was performed by using a scanning electron microscopy (SEM) at different annealing temperatures. Results have also revealed that the onset temperature for agglomeration in a 20 nm Ir/n-Ge (100) system occurs between 600-700oC.This work has been made possible by financial assistance from the South African National Research Foundation.http://www.elsevier.com/locate/jallcomnf201