DAMAGE CAUSED BY NEUTRON IRRADIATION AND ARGON SPUTTERING

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INTRODUCTION

Gallium arsenide is an important material for devices to be used in space applications, owing to the increased speed of carriers in this material and its resistance to radiation damage. Gallium arsenide (GaAs) emerged as an optoelectronic material in the early 1960s and consolidated its position with the development, a decade later, of double heterostructure strip lasers. A more recent development is the first commercial single- mode diode laser with a continuous output power greater than 1 watt. Applications of this device include optical communication between satellites, telecommunication systems and thermal printing systems. High electron mobility transistors (HEMTs) have improved gain and noise performance over an extended frequency range.
A further advantage of this device is that it can be incorporated into either monolithic or hybrid microwave and millimeter-wave integrated circuits. GaAs Schottky barrier diodes (SBDs) are becoming more important in space applications; SBDs operating in the terahertz frequency range are used in radio astronomy and remote sensing of the earth’s atmosphere. The radiation environment experienced by earth-orbiting spacecraft is largely a manifestation of the interaction of charged particles with the earth’s magnetic field, forming the radiation- or Van Allen belts. Other sources of radiation include solar flares, cosmic rays, low energy plasma, other planets and secondary radiation. Typical particles in space are electrons, protons, alphas, neutrons and, to a lesser extent, heavier nuclei. The beginning of the space age in the 1950s led to interest in the effects of radiation on semiconductors. Thus, in the 1960s, extensive studies on radiation-induced defects were performed.
In addition to defect identification, the studies on radiation-induced defects addressed a number of fundamental aspects of defect physics, such as defect migration and lattice relaxation. It was discovered almost forty years ago that neutron irradiation profoundly affected the electrical characteristics of semiconductors such as germanium and silicon. Present-day interest has been stimulated by such semiconductor applications as solar cell power plants for space stations and satellites, and semiconductor particle andy-ray detectors. Particle irradiation also plays a significant role in various sectors of semiconductor technology. This irradiation is utilized to controllably modify the properties of electronic materials, such as during device isolation by proton implantation and transmutational doping of semiconductors.
It is a widely used processing step because, amongst others, it offers the capability of performing several processing steps in vacuum without exposing the semiconductor surface to atmosphere. Gallium arsenide may be doped with impurities resulting from nuclear reactions based on thermal neutrons, y-rays and charged particles. It has for instance been shown that nuclear doping of GaAs by a-particle irradiation is possible. However, it is well known that the use of energetic particles may result in unwanted changes in the surface and sub-surface. Hence, when designing electronic systems for operation in a radiation environment, it is imperative to know the effect of radiation on the properties of electronic components and materials comprising these systems.
A knowledge of the influence of radiation damage on the performance of these devices has become an active field of research. The effect of irradiation on an electronic material and the consequent degradation in performance of devices made from such material will depend upon, amongst other factors, the type and fluence of radiation. To evaluate the influence of radiation-induced defects on materials and devices, and to optimally utilize their beneficial effects in defect engineering, the characteristics of radiation-induced defects in different materials should be known. In the 1970s, a major breakthrough occurred in experimental techniques. In these new techniques, measurements were done on junctions such as rectifying metal-semiconductor interfaces.
A variant of these junction experiments was deep level transient spectroscopy (DLTS), which allows one to scan a temperature range and observe individual deep levels as peaks in a continuous spectrum, yielding, amongst other factors, the well-known DL TS « signature » of the defect. Further information regarding defect migration and the nature of the defect may be obtained from a knowledge of the thermodynamic properties of the defect under consideration. The technique used for defect characterization in this study, namely DLTS, is presented in section 1 of chapter 2. A general introduction into the theoretical aspects of defect production is considered in the second section of the same chapter. Finally, field-assisted emission and defect annealing are discussed in the last two sections of this chapter.

CONTENTS :

• CHAPTER 1: INTRODUCTION
• CHAPTER 2: THEORETICAL ASPECTS
  • 2.1 DEEP LEVEL TRANSIENT SPECTROSCOPY
    • 2.1.1 DL TS fundamentals
    • 2.1.2 DLTS « signature »
    • 2.1.3 Capture cross-section temperature dependence
  • 2.2 DEFECT CREATION IN SEMICONDUCTORS
    • 2.2.1 Defect structures
    • 2.2.1a Vacancies
    • 2.2.1 b Interstitials
    • 2.2.1c Impurities
    • 2.2.1 d Complex defects
    • 2.2.2 Aspects related to the theory of displacement of atoms in solids
  • 2.3 ELECTRIC FIELD EFFECT
    • 2.3.1 Poole-Frenkel mechanism
    • 2.3 .2 Tunnelling mechanisms
  • 2.4 DEFECT ANNEALING
    • 2.4.1 Annealing kinetics
    • 2.4.2 Particle-irradiation induced defects in GaAs
    • 2.4.3 Enhancement of defect migration

• CHAPTER 3: EXPERIMENTAL TECHNIQUES
  • 3.1 INTRODUCTION
  • 3.2 OHMIC CONTACT AND SBD FABRICATION
  • 3.3 1-V AND C-V MEASUREMENTS
  • 3.4 DLTS MEASUREMENTS
  • 3.5 VAN DE GRAAFF ACCELERATOR
  • 3.6 RADIONUCLIDE SOURCES
    • 3.6.1 Americium
    • 3.6.2 Strontium
  • 3.7 ANNEALING APPARATUS
• CHAPTER 4: RESULTS
  • 4.1 INTRODUCTION
  • 4.2 MATERIAL CHARACTERIZATION
  • 4.3 MATERIAL IRRADIATION
  • 4.4 ELECTRIC FIELD EFFECT
  • 4.5 DEFECT ANNEALING
  • 4.6 DAMAGE CAUSED BY NEUTRON IRRADIATION AND ARGON SPUTTERING
  • 4.7 SUMMARY OF RESULTS
    • 4.7.1 Electronic characteristics
    • 4. 7.2 Thermodynamic characteristics
    • 4.7.3 Comparative results
• CHAPTER 5: CONCLUSIONS
  • 5.1 INTRODUCTION
  • 5.2 MATERIAL CHARACTERIZATION
  • 5.3 MATERIAL IRRADIATION
  • 5.4 ELECTRIC FIELD EFFECT
  • 5.5 DEFECT ANNEALING
  • 5.6 DAMAGE CAUSED BY NEUTRON IRRADIATION AND ARGON SPUTTERING
  • 5.7 SUMMARY AND FUTURE STUDIES

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