Advanced Silicon & Semiconducting Silicon-Alloy Based Materials & Devices

Jo Nijs, Jo Nijs

• Editore: CRC Press
• Anno: 2021
• ISBN:
  9781000447798
• Edizione: 1
• Argomento: Physical Sciences
• Categoria: Physical & Theoretical Chemistry

One of the first books to cover advanced silicon-based technologies, Advanced Silicon and Semiconducting Silicon Alloy-Based Materials and Devices presents important directions for research into silicon, its alloy-based semiconducting devices, and its development in commercial applications. The first section deals with single/mono crystalline silicon, focusing on the effects of heavy doping; the structure and electronic properties of defects and their impact on devices; the MBE of silicon, silicon alloys, and metals; CVD techniques for silicon and silicon germanium; the material properties of silicon germanium strained layers; silicon germanium heterojunction bipolar applications; FETs, IR detectors, and resonant tunneling devices in silicon, silicon germanium, and d-doped silicon; and the fascinating properties of crystalline silicon carbide and its applications. The second section explores polycrystalline silicon. It examines large grain polysilicon substrates for solar cells; the properties, analysis, and modeling of polysilicon TFTs; the technology of polysilicon TFTs in LCD displays; and the use of polycrystalline silicon and its alloys in VLSI applications. With contributors from leading academic and industrial research centers, this book provides wide coverage of fabrication techniques, material properties, and device applications.

PART ONE:

Single crystalline silicon and its alloys

1

Heavy Doping Effects in Silicon

P. Van Mieghem and R.P. Mertens

1.1 INTRODUCTION

The doping of a pure semiconductor modifies the material fundamentally. A large number of impurities is inserted in the lattice of the pure semiconductor. These structural changes have important consequences interesting as well from a theoretical as practical point of view. Semiconductors can be doped n-type (p-type) by adding typical donors (acceptors). The type of doping determines the mechanism of conduction. Indeed, donors will add electrons in the conduction band of the pure material and the conduction (at sufficiently high temperature) will be dominated by these donor-electrons. On the other hand, acceptors add holes to the valence band and cause a conduction of holes. Thus, the role of doping is twofold: the high resistivity of the pure semiconductor can be reduced and the type of doping distinguishes between electron (n-type) or hole (p-type) conduction.

The doping concentration can be varied over a large region. When the semiconductor is heavily doped, differences from a lowly doped semiconductor in more than only resistivity are observed. About twenty years ago, anomalies in the current gain of bipolar transistors resulting from a heavily doped emitter were measured. The mechanisms responsible for these supplementary differences, called heavy doping effects (HDE), found an increasing interest. Apart from the bipolar transistor, also the semiconductor lasers seemed to be influenced by HDE. Currently, HDE on semiconductor devices are generally accepted to modify electronic properties. Techniques to dope selected areas very precisely are impressively improved during the last years as demonstrated by the high performance of epitaxial bipolar transistors and primarily by the hetero-junction devices. In research laboratories, a new generation of components, called quantum devices (such as the high electron mobility transistor (HEMT), the resonant tunnel diode (RTS) and superlattices) are investigated. In these devices, quantummechanical effects dominate the operation mechanisms. As the sensitivity of quantum processes exceeds those of the current semi-classical processes, HDE are believed to play an increasing role. HDE can introduce new, remarkable phenomena such as the recently observed Fermi-edge singularities in 2D structure at low temperatures [Schmitt-Rink, 1986; Livescu, 1988; Zhang, 1990].

We will show that HDE consist in both a bandgap narrowing (BGN) and a distortion of the density of states (DOS), called bandtailing. This work confines itself to these two effects. Other topics that involve HDE are omitted. The influence of heavy doping on other semi- conductor parameters such as mobility, lifetime, diffusion coefficients and recombination rate¹ are not considered because the effect of heavy doping on these latter parameters can be related to the important many-body quantity, the self-energy (explained in 1.4.2) [Mahan, 1986]. Numerical techniques for device simulators that include heavy doping models are not discussed but can be found in the work of Bennett and Lowney [1990]. Finally, the presented theoretical discussion is mainly limited to a zero temperature description.

The effects of BGN and the distortion in the DOS have been introduced as a consequence of doping. However, BGN also occurs in non-doped material where a large amount of carriers is piled up. Actually, we will see below that BGN is a many-body effect.

The first chapter of this book is oulined as follows. Section 1.2 explains the physical mechanisms of HDE. Before discussing the theories of HDE in section 1.4, we briefly overview the milestones and most refined theories and experiments in the field of HDE in section 1.3. The last section 1.5 investigates the influence of HDE on some important silicon devices.

1.2 THE PHYSICS OF HEAVY DOPING EFFECTS

Heavy doping effects change the bandstructure of a semiconductor in two particular aspects. First, the bandgap which is a characteristic quantity for each crystalline material, narrows. This effect is called bandgap narrowing (BGN). Second, HDE introduce energy levels in the forbidden energy zone resulting in a modification of the density of states (DOS). The latter effect is known as bandtailing.

Let us concentrate on a n-type material. Each donor will create an energy level in the forbidden zone that forms the ground state for its weakly bound outer electron. This donor electron circles around the donor atom at a distance of the order of the effective Bohr radius a_B. As long as the donor doping concentration is low, the donor atoms will on the average be distributed far enough from each other so that the donor electron wave functions will neglibly overlap. Thus the impurity levels are well isolated and at sufficiently low temperature, no electronic conduction occurs: the lowly doped semiconductor is an isolator.

Increasing the donor concentration augments the number of impurity states. At a critical doping concentration - the Mott critical doping concentration- the donor electron wave functions start to overlap significantly such that electronic conduction does not vanish at low temperatures. At and above the Mott critical concentration, the conduction becomes metallic in nature. The overlap of wave functions gives rise to the formation of an impurity band that merges with the conduction band. For still higher doping concentrations, we arrive in the heavily doped regime, where now more than one donor electron moves in the effective Bohr sphere. This large number of donor electrons closely packed to each other will introduce different kinds of interactions that distort both the conduction and valence band.

1.2.1 Electron-impurity interactions

In the heavy doping regime, the large number of electrons will screen the electrostatic Coulomb interaction exerted by the impurity ion on its surrounding donor electron so that the latter becomes more or less unbounded. However, the electron motion is not really unobstructed since the donor ions remain locally electrically active. They polarize the environment. Due to this interaction the individual electron energy is lowered by an amount æΣ^ei, the electron-impurity self-energy.

The electrons traveling through the crystal can be regarded as balls rolling over ...