Photoconductivity

Art: Science & Technology

N V Joshi

• Publisher: Routledge
• Year: 2017
• eBook ISBN: 9781351424875
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Featuring detector technology capable of sensing even a few photons, this valuablereference guide provides criteria for selecting techniques and equipment appropriate tovarious types of faint signals. It highlights many important facets of photoconductivityand photodetection, including the measurement of weak photosignals in the presence ofnoise... statistics relating to the creation, annihilation, and transport of charge carriers... and time-dependent behavior, photoquenching, negative photoconductivity, andphotosensitivity.Complete with more than 125 diagrams and tables, Photoconductivity: Art, Science, and Technology gives special attention to modem two-dimensionalphotodetectors... describes various configurations for experimental techniques inphotoconductivity measurements... surveys band structure properties, with usefulreference to such contemporary structures as n-i-p-i and modulation doped materials...illustrates the concept of noise in photoconductors and its role in detector technology...and observes unusual photoconducting properties in diluted magnetic semiconductors.Photoconductivity: Art, Science, and Technology serves as an indispensableresource for optical, electrical, laser, and aerospace engineers, physicists, materialsscientists, photonic scientists, and graduate students interested in these disciplines.

Photoconductivity

1

Introduction

1.1. INTRODUCTION

Photoconductivity is an important property of semiconductors by means of which the bulk conductivity of the sample changes due to incident radiation. When the conductivity decreases, then “negative photoconductivity” is the term used. There are a few experimental situations where such an effect is observed, but in general the conductivity increases when the radiation is incident on the sample. This property has been used advantageously in several branches of science and technology.

Photoconductivity is not an elementary process in solids. Photoconduction, as the name suggests, includes the generation and recombination of charge carriers and their transport to the electrodes. Obviously, the thermal and hot carrier relaxation process, charge carrier statistics, effects of electrodes, and several mechanisms of recombination are involved in photoconduction. Above all, every mechanism mentioned here is a complicated one, and therefore photoconductivity in general is a very complex process. For interpretation purposes, complementary information from other experimental techniques such as photoluminescence, optical absorption, and electroreflectance is always needed.

In spite of the complexity of the photoconductance process, it provides useful and valuable information about physical properties of materials and offers applications in photodetection and radiation measurements. Recent advances in thin-film technology, particularly in excellent quality crystal growth (doped and undoped) by metal organo chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) gave a new dimension (in fact, a reduced dimension) to the photodetectors, and now quantum well and superlattice photodetectors are a reality. We will see in Chapter 6 some of the details of modern photodetectors. Here we want to note that all these features make photoconductivity a fast-growing field.

Detection of weak photosignals needs a careful selection of several parameters. It is necessary to know the change in the resistance of the sample caused by the incident radiation, the impedance of the measuring circuit, the response time, the capabilities and limitations of the photodetector, and sources of noise with their magnitudes. The combined knowledge helps in selecting the proper instruments. The handling of such information and its use for low-level radiation detection requires a conceptual foundation, and therefore they are certainly as much an art as a science.

In modern life, many applications demand a time domain of a short interval (of the order of picoseconds). This demands that photodetectors have ultrafast response. To achieve this, the distance between two contacts must be small (on the order of a micrometer), a technological requirement. In short, photoconductivity is science, art, and technology combined.

Historically, the first photoconductivity effect was recorded in 1873 by W. Smith [1], who observed that the resistivity of selenium was decreased by radiation shining on it. According to the literature, this is the very first experimental detection of photoconductivity (in fact, it was a combination of photoconductivity and the photovoltaic effect).

Understanding the origin of the observed photoconductivity was very difficult at that time. At the end of the nineteenth century, it was known experimentally that when radiation (visible or ultraviolet) is incident on a metal surface, electrons are ejected from the metal. This observation was not so unexpected, but what was surprising was that the kinetic energy of the ejected electrons is proportional to the frequency of the radiation and independent of its intensity. This phenomenon could not be understood on a classical basis.

In 1905, Einstein explained this puzzling observation. A beam of monochromatic light consists of packages of energy of magnitude hυ where h is Planck’s constant and υ is the frequency of the radiation. During the interaction process this quantum of energy is completely transferred to the electron. Thus, the electron acquires energy hυ. The kinetic energy of the electron, therefore, is equal to hυ minus the work function of the metal.

Considering the equivalence of mass and energy, the momentum of each quantum (or photon) can also be calculated as hυ/c or h/λ.

Einstein’s explanation of the photoelectric effect thus opened the way to interpreting the interaction of radiation with matter, particularly optical absorption, photoconductivity, the photovoltaic effect, and other related phenomena. This effect was considered an interesting property of selenium. Soon, more and more materials having this property were discovered, and today we know that there exist more than 1000 semiconducting compounds that have a reasonably good photoresponse. In fact, all nonmetals have a certain degree of photoresponse, even though in many cases it is difficult to detect.

From about 1920 to 1940 the photovoltaic effect was also investigated. In a short time, a close relationship between rectifier and photovoltaic cell was realized, and around the beginning of 1930 photocells of copper and selenium became commercially available. Immediately the importance of photocells became clear, particularly for defense; and the intensive and systematic investigation of radiation detection was accelerated. During World War II, attention was focused on infrared detectors, but that work was classified. After the end of the war, somewhere around 1947, this work was declassified. The major development in this period was carried out on lead compounds such as PbS, PbSe, and PbTe.

Following World War II, a rapid development of several branches of solid state physics, among them photoconductivity, is observed.

The basic principle involved in photoconductivity is often stated in a very simple way. When photons of energy greater than that of the band gap of the semiconductor are incident upon a photoconductive material, electrons and holes are created in the conduction and valence ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• About the Series
• Preface
• 1 Introduction
• 2 Techniques for Photoconductivity Measurements
• 3 Transient Photoconductivity
• 4 Band Structure Details and Photoconductivity
• 5 Noise
• 6 Modern Photodetectors
• 7 Special Topics
• Appendix
• List of Symbols
• Index