Creation of Defects in Germanium

Get Complete Project Material File(s) Now! »

Introduction

Germanium is once again at the forefront of the semiconductor revolution as it holds the promise of even faster devices for the not too distant future. However, from a materials point of view, it is the ideal material to study ex- tremely small changes both on its surface and in its bulk due to the availability of ultra-pure germanium. Element 32, Ge, was used during the pioneering years of semiconductor research to formulate theories and demonstrate physical principles that are still applicable today. The McWhorter theory for 1/f noise, plasticity in a diamond lattice semiconductor and Frank-Read dislocation sources were first studied in Ge (Claeys and Simoen 2007). The discovery of the transistor at Bell Labs in 1948 on a Ge slab was the starting point for the Microelectronics Industry with a market estimated to exceed US $304 billion in 2013 (report of the World Semiconductor Trade Statistics organization). This enterprise is the largest worldwide and is predicted to continue growing for the immediate future but it faces unique challenges. With the ever increasing need for faster and smaller devices the limit in miniaturization is approaching for Si integrated circuits(ICs). Ge, strained SiGe and strained Si with their high mobility charge carriers are now seen as good candidates for devices operating at increased speeds. The instability of the GeO2 layer that was always a major disadvantage has been annulled by the need for high-κ dielectrics on Si at the 65 nm complementary metal-oxide-semiconductor (CMOS) technology node and beyond.
These fundamentals are very encouraging but many challenges remain as Ge has never been subjected to the level of investigation that Si has experienced. Material availability may be the biggest drawback against the widespread use of Ge in microelectronics which highlights the importance of using only a thin layer in schemes like germanium-on-insulator (GOI). Defects studies shed light on the most interesting properties of semiconductors. Point defects and impurities influence the local resistivity, doping type and doping concentra- tion. Devices in the future, as a consequence of miniaturization, will have extremely shallow junctions with high conductivity, where the role of defects that enhance diffu- sion of the implanted dopant species will have to be better understood (Law et al. 2008). Not all defects are electrically active which further complicates analysis. The most obvi- ous example of such a defect is one that has been passivated using hydrogen, a common practice during material growth and device fabrication. The ability of defects to influ- ence the carrier lifetime can be used constructively but must be carefully controlled as they can also become efficient carrier lifetime killers (Simoen, Claeys et al. 2007). Defect studies on Ge are of particular interest as very little has been done when compared to similar research on Si considering that Ge could be utilized in technologically superior devices if its potential is actualized.

Motivation

A summarized yet remarkably complete history of Ge, from a semiconductor perspective, was written by Haller 2006. It emphasizes the important results that were achieved and mentions the niche applications that this interesting material is used for. The future use of Ge for microelectronics applications looks promising but many challenges remain. The property of Ge that has rekindled interest in this material is that when compared to bulk Si, it exhibits electron and hole mobilities that are higher by factors of ~2 and 4, respectively. Promising results in the development of Ge metal-oxide- semiconductor field effect transistors(MOSFETs) have been obtained but major obstacles remain (Simoen and Claeys 2007). The way forward is further complicated by industry’s need to translate existing Si device manufacturing technology across when considering novel devices. Possible future directions for the semiconductor industry and the role that Ge can play is discussed in Claeys and Simoen 2007. Additional information on the successes and the challenges of using Ge as a mainstream semiconductor is covered in a number of chapters of this excellent resource. While the semiconductor properties of Ge are of great importance, one should not lose sight of the possibility to use these same properties to shed light on fundamental physics problems. This study is primarily concerned with the introduction of point defects in Ge during processing and the impact that these have on the electrical properties of Ge. The term defect will always refer to an electrically active point defect unless specified differently.
The creation of extended defects was not investigated, although possible to measure electrically, as our focus on creating devices with exceptional rectifying characteristics would be lost. To advance our current electronic technology it will be necessary to not only build faster devices but these will have to be smaller too. Only by understanding how the electrical and physical properties of semiconductors relate to each other can we hope to further develop the manufacturing and spectroscopic techniques that will result in better devices. Defects in semiconductors may influence device performance adversely but have also been shown to improve the switching speed of select devices thus ensuring the relevance of defect studies. Deep level transient spectroscopy (DLTS), based on work started more than forty years ago (Lang 1974), was chosen as the most suitable tool to investigate defects in Ge. A number of studies in the early years of DLTS reported on defects in Ge but as the technique was still in its infancy, many of the values obtained varied significantly from measurements taken later (Fage-Pedersen et al. 2000). A vast quantitative improvement to conventional DLTS arrived with the idea to use the Laplace transform method to analyse the decay transient (Dobaczewski et al. 1994). Not only was the resolution of DLTS improved by more than an order of magnitude, but for the first time fine structure could be discerned from broad peaks that were obtained conventionally. This technique, known as Laplace-DLTS (L-DLTS), has had a profound effect on the spectroscopy of electrical defects (Peaker, Markevich, Hawkins et al. 2012) but requires a dedicated experimentalist to analyse the data. Many defects have been observed in Ge and a small number of these have been identified (E-center, A-center and V2-H, as examples). From a survey of the available literature it is clear that the majority of the research is decades old and that a great deal of it needs to be revisited (Vanhellemont, Simoen et al. 2007). Process-induced defects is an area that has received very little attention. Not only are the majority of these defects unidentified at present but also the defect causing mechanisms are, in many cases, not well understood. These mechanisms are manifestations of physical phenomena that play a role in many systems and deserve detailed examination, both experimentally and theoretically. The future device that will form the backbone of the semiconductor industry is un- decided presently. The Schottky barrier diode (SBD) is a candidate worth considering and efforts to improve this devices’ properties yield results applicable to other rectifying junctions while providing the ideal window through which to observe defects in the Ge bulk.

READ  Electrochemical Deposition and Electrocatalytic Properties of Multilayered Nanoclusters of Platinum and Gold

Contents :

  • List of Figures
  • List of Tables
  • Glossary
  • Acronyms
  • 1 Introduction
    • 1.1 Motivation
    • 1.2 Objectives
    • 1.3 Contributions
    • 1.4 Thesis Outline
  • 2 Theoretical Overview
    • 2.1 Physical Vapour Deposition
      • 2.1.1 Resistive Evaporation
      • 2.1.2 Electron Beam Deposition
      • 2.1.3 Sputter Deposition (SD)
    • 2.2 Creation of Defects in Germanium
      • 2.2.1 Grown-in Defects
      • 2.2.2 Process-induced Defects
      • 2.2.3 Radiation-induced Defects
    • 2.3 Electrical Characterization
      • 2.3.1 C-V, I-V and I-T Measurements
      • 2.3.2 Deep Level Transient Spectroscopy
    • 2.4 Annealing of Defects
    • 2.5 Summary
  • 3 Experimental Techniques
    • 3.1 Introduction
    • 3.2 Device Fabrication
      • 3.2.1 Ohmic Contacts
      • 3.2.2 Schottky Barrier Diodes
    • 3.3 Defect Introduction
      • 3.3.1 Electron Beam Deposition
      • 3.3.2 Plasma Processing
      • 3.3.3 Radionuclide Sources
    • 3.3.4 Van de Graaff Accelerator
      • 3.4 Device Measurement
      • 3.4.1 I-V and C-V Measurements
    • 3.4.2 DLTS Measurements
    • 3.5 Annealing of Defects
    • 3.6 Summary
  • 4 Results
    • 4.1 Introduction
    • 4.2 Material Characterization
    • 4.3 Electron Beam Deposition
      • 4.3.1 EBD Induced Defects
      • 4.3.2 EBE Defects
    • 4.4 Sputter Deposition Defects
    • 4.5 ICP Induced Defects
    • 4.6 Radiation induced defects
    • 4.7 Defect Annealing
    • 4.8 Summary
    • 5 Conclusions
  • 5.1 Summary of Conclusions
    • 5.2 Future Work
    • Bibliography
    • A Ge defects analysed
    • B Derived Publications
    • C EBD introduced defects in n-Si
    • C.1 Conclusions

GET THE COMPLETE PROJECT

Related Posts