Sequential nature of damage annealing and activation in implanted GaAs

Rapid thermal processing of implanted GaAs reveals a definitive sequence in the damage annealing and the electrical activation of ions. Removal of implantation-induced damage and restoration of GaAs crystallinity occurs first. Irrespective of implanted species, at this stage the GaAs is n-type and highly resistive with almost ideal values of electron mobility. Electrical activation is achieved next when, in a narrow anneal temperature window, the material becomes n- or p-type, or remains semi-insulating, commensurate to the chemical nature of the implanted ion. Such a two-step sequence in the electrical doping of GaAs by ion implantation may be unique of GaAs and other compound semiconductors

Laser pulse annealing of ion-implanted GaAs

GaAs single-crystals wafers are implanted at room temperature with 400-keV Te + ions to a dose of 1×10^15 cm^–2 to form an amorphous surface layer. The recrystallization of this layer is investigated by backscattering spectrometry and transmission electron microscopy after transient annealing by Q-switched ruby laser irradiation. An energy density threshold of about 1.0 J/cm^2 exists above which the layer regrows epitaxially. Below the threshold the layer is polycrystalline; the grain size increases as the energy density approaches threshold. The results are analogous to those reported for the elemental semiconductors, Si and Ge. The threshold value observed is in good agreement with that predicted by the simple model successfully applied previously to Si and Ge

Forms, properties and dissolution of controlled-release nitrogenous fertilisers

The forms, properties and dissolution rates of controlled-release nitrogenous fertilizers are reviewed. These materials belong to one of the categories: coated materials, uncoated sparingly-water-soluble inorganic and organic compounds. Physical and chemical properties of the fertilizer materials are described along with their dissolution characteristics. Information on indigenous materials proposed as controlled-release N sources is scarce and is required to evaluate their comparative efficacy

A Review of Fertilizer Use Research on Sorghum in India

This literature review, which covers the period 1960 to 1983, is concerned with the response of grain sorghum to all soil nutrients, related soil and climatic conditions, and the seasons in which the crop is grown. The main objective is to quantify the responses and their relationship to different environments. However, the results clearly demonstrate the widespread deficiency of nitrogen (N), phosphorus(P), and zinc(Zn) under both rainfed and irrigated conditions. High-yielding cultivars have shown greater responses than local cultivars and, invariably, both N and P have shown additive effect. Split application of N is generally more efficient than a single-dose application. When the, 5N technique is used it has been observed that about 62.5% fertilizer N is recovered by sorghum from Alfisols and 55% from Vertisols. Drilling of phosphate proved more efficient than broadcasting. The responses to potassium (K) are rather rare, except in long-term experiments. Responses to the application of Zn are reported, especially in Vertisols when the available Zn is about 1.0-1.2 ppm or less. In the postrainy-season crop the responses to N are dependent on the nature of the cultivar and the nitrate-N level of the soil. The optimum level of nitrogen for sorghum varies from 60 to 120 kg/ha N in the rainy season, 25 to 85 kg/ha N in the postrainy season, and 80 to 150 kg/ha N in the summer season. A finding of the review is that most of the publications reviewed report the results of the so-called rate-and-date type of agronomic research. Little effort appears to have been made to quantify the relationship of fertilizer responses to soil fertility and environments. Though responses to N and P, under both rainfed and irrigated cropping conditions, show that fertilizer use is the key for increasing production, few research data are available on nutrient x moisture interactions and their management

Management of Fertilizer Nutrients Other Than Nitrogen in the Semiarid Tropics of India

Available information on the management of fertilizer nutrients other than nitrogen in the semiarid tropics (SAT) of India (about 96 million ha) is reviewed. Crop responses to fertilizer nutrients are high under dryland conditions. Phosphorus and then zinc are of major importance for the nonirrigated SAT, but significant yield responses to potassium and sulfur, in addition to P and Zn. are also obtained under dryland conditions in the reddish brown lateritic soils of the Bangalore area. Iron chlorosis is increasingly being mentioned as a problem in the SAT. Available information on P, K, S, Zn. and Fe has been reviewed by continuously searching for information applicable to rainfed systems in irrigation-oriented overall research in nutrient management and soil fertility even for major dryland crops. Suggestions for further research have been made for the nutrients reviewed

Modeling the Growth and Development of Sorghum and Pearl Millet

The 1980s have witnessed substantial increases in food production. This has raised expectations that improved systems of farming will be rapidly adopted by small farmers in developing countries. The agroecological environment of such farms is fragile and the farmers are resource-poor. Therefore, strategies recommended for increasing food production must be ecologically sound and should result in sustainable agriculture. System modeling can greatly expedite the search for improved development strategies. Recent advances in crop modeling have made it possible to simulate yields and growth of several crops under varied soil and weather conditions with different management practices. This bulletin describes the framework of CERES (Crop Estimation Through Resource and Environment Synthesis) models developed by the International Benchmark Sites Network for Agrotechnology Transfer ( IBSNAT) and The International Crops Research Institute for the Semi- Ar i d Tropics ( ICRISAT) . Recent wor k on the simulation of nitrogen transformation in soils at the International Fertilizer Development Center ( IFDC) is discussed. A section on risk analyzes the cost-benefit implications of various inputs for increased crop production. RESCAP—a resource capture model developed at ICRISAT Center is presented. This publication is a cogent source book on the current status of development of CERES and RESCAP models, their data needs, outputs, and applications

Combined use of ion backscattering and x-ray rocking curves in the analyses of superlattices

Detailed compositional and structural analyses of superlattices have been carried out by mega-electron-volt He+ backscattering with channeling and with x-ray rocking curves. Through the combined use of the two techniques, depth profiles of strain, composition, and crystalline quality have been determined. An example of an AlxGa1-xAs/GaAs strained-layer-superlattice (SLS) is considered. The thicknesses of the individual periods in these SLS structures were accurately measured by backscattering spectrometry. The values so obtained were used in the detailed calculations of x-ray rocking curves. Excellent agreement between measured and calculated curves was achieved. Transition regions at the interfaces of the various layers in the SLS were also detected and measured by both techniques. The two techniques complement each other and together provide powerful quantitative tools to characterize SLS structures