The Materials Science of Semiconductors

Modern electronic materials and devices arguably are built upon nearly the entire periodic table (excluding only the actinides and a few other unusual or unstable elements). These diverse materials are required to meet the intense challenges which electronic device applications present. In their full extent, electronic applications range from simple copper wires, to high-performance magnetic materials for computer disks, to semiconductors for state-of-the-art microelectronic devices, and many more. Likewise, the critical properties of the materials range from electronic conductivity, to optical transmission, to diffusion-resistance or mechanical properties. It is not reasonable, nor is it particularly desirable, to cover all aspects of electronic materials in a single text. Consequently, the materials discussed here relate primarily to the most challenging applications, particularly with reference to microelectronic and optical devices. This volume is further restricted to the semiconducting materials used in active devices and leaves the metals, dielectrics, and other materials used in microelectronic processes to other texts. A wide range is considered including some traditional materials, such as silicon, and some in their infancy, such as organic semiconductors. Readers may also wish to consider books on epitaxial growth, and other processes relevant to microelectronics manufacturing as supplements to this text.

Before beginning a general discussion of electronic devices and the more complex aspects of semiconductors and other electronic materials, it is helpful to have an idea of their physics, especially their electronic structure. This chapter provides a partial review of the physics of solids. The nature of materials is determined by the interaction of their valence electrons with their charged nuclei and core electrons. This determines how elements react with each other, what structure the solid prefers, its optoelectronic properties and all other aspects of the material. The following sections describe the general method for understanding and modeling the energies of bands of electronic states in solids. A more detailed discussion of semiconductor bonding is provided in Chapter 5.

This Chapter reviews some of the basic physics and operation of selected circuit elements used in microelectronic devices. For a complete review, see the suggested readings. Basic resistance, capacitance, and inductance were covered in Chapter 2. We focus here on diodes, including clas-sic homojunctions, heterojunctions, and Schottky barriers, because they illustrate most of the important issues in microelectronic materials and because both field-effect and bipolar junction transistors are constructed from them. Once we have discussed diodes, a brief review of these two major classes of transistors is provided. Finally, we finish the review with some of the issues unique to light emitting and laser diodes.

This chapter provides a brief description of materials concepts that may be useful in understanding electronic materials. The review is not exhaustive but is intended to provide a minimum (and rather basic) level of familiarity with important concepts used in other chapters. As elsewhere, the reader is referred to the recommended readings for additional background and details.

Advanced devices place strong demands on semiconductor properties. To obtain the highest performance it is necessary to engineer the properties of constituent materials. In some devices, this means designing the electronic energy band structures. In other cases, the natures of defects in the materials are most critical. In this and the following chapter we consider band engineering and leave defect design to Chapter 7.

As with all materials, engineering semiconductors primarily involves formation of alloys and control of defects. Defect engineering is discussed in detail in Chapter 7. This chapter considers the basics of alloying. The objective is usually to control the optoelectronic properties of the semiconductor, primarily through its energy band structure. Other properties change as well and some of these, such as lattice constant, are important to producing a high-quality material, as we shall see in Chapter 7.

As with all other classes of materials, one of the primary keys (if not THE key) to engineering a semiconductor is control of defects in its structure. Defects can be divided into classes according to their dimensionality. Thus, zero (point), one (line), two (plane) and three (volume) dimensional defects occur in semiconductors and each is significant is considered in turn, although two and three-dimensional defects will be lumped together as they behave similarly. Furthermore, the behaviors of two and three-dimensional defects can be considered to be extensions of zero and one-dimensional behaviors. Therefore, we will spend more time on the latter two. In this chapter we will consider only defects in crystalline materials. Amorphous semiconductors, the ultimate in defective materials, are considered in the following chapter.

Amorphous semiconductors are somewhat of a niche area of electronic materials. However, they are critical to a number of important applications. It would be worth spending some time studying them based on these applications alone. More significantly, these materials are very distinct in their optical and electronic nature. Understanding their properties is highly instructive in a general sense.

One of the most exciting opportunities in optoelectronics currently is devices based on organic materials. These have many advantages, primarily: lower-technology processing with less sensitivity to processing environment (but many are very air sensitive), flexibility, and the opportunity to apply the enormous power of organic synthesis to tailoring the properties of the materials to specific applications. Furthermore, organics can emit light directly as do conventional cathode-ray-tubes and plasma display panels, rather than relying on back-lighting systems such as are used in liquid-crystal displays. One can imagine these technologies leading to poster-sized televisions which can be rolled up and stored in mailing tubes, or unrolled and thumb-tacked to a wall. The materials are already being applied in compact lightweight, power-efficient light emitting devices in small areas such as cell-phone displays. The primary problem with all organic devices is stability. When carriers are injected into these materials, sometimes a molecule falls apart. This does not need to be very common for the device to degrade significantly over relatively short operating times. This chapter considers the options for organic semiconductors and how they are applied.

Up to this point this text has focused primarily on materials themselves and not how to produce them. A major aspect of materials science is the control of the kinetic and thermodynamic conditions under which materials are produced to yield specific properties. This chapter and the ones that follow describe some of the ways semiconductor electronic materials are created as thin films. For comparison, the most popular method of production of bulk materials was covered in Chapter 4. Bulk wafers are useful as substrates but are impractical for many applications, especially where alloys are needed. In current technology, thin films constitute most of the active and passive layers that are used in electronic devices.

This chapter covers the common features of all vapor phase thin film growth techniques – the processes by which atoms land on surfaces, move about, leave the surface, and how surface atoms go on to produce complete films. As with other chapters in this book, whole texts have been written on the subject so this treatment reviews only the highlights. Following chapters will cover specific classes of processes. Subjects of this chapter and include adsorption, desorption, surface structure and energy and how they are related to surface diffusion and the evolution of morphology, and adhesion.

Physical vapor deposition refers to vacuum deposition methods that produce the source gas by evaporation, sputtering, or a related nonchemical method. Broadly, these methods transfer kinetic energy to atoms in a solid or liquid sufficient to overcome their binding energy. Evaporation refers to heating a material until the source atoms vaporize. Sputtering is a process of physical impacts transferring kinetic energy to atoms in a target. There are many related methods such as laser-ablation, which is similar to conventional evaporation but supplies energy to the surface locally by a laser beam rather than heating the entire material in an oven. Likewise, cathodic arc deposition is a sputtering-based process that uses a more localized, higher intensity glow discharge to bombard the target in a small region, rather than “sputtering” which refers to a more general bombardment of the target. This chapter describes the most common conventional evaporation and sputtering methods. Descriptions of the related techniques may be found in the recommended readings.

Chemical vapor deposition (CVD) refers to a class of methods in which a solid is grown by reaction of gaseous source materials and yielding a product effluent gas. There are a number of variants on the process based on the pressure range at which it is conducted, the type of reactants, and whether some method to activate the reaction is used. CVD can also be conducted in an atomic layer deposition (ALD) mode in which single layers of atoms are produced one at a time. CVD has a number of advantages over physical vapor deposition. For example, the reaction can often be arranged to be selective more easily, depositing material only in certain regions of the substrate rather than covering it with a blanket layer. CVD is generally more conformal than physical vapor deposition, meaning that it covers a rough surface relatively uniformly, tracking the morphology rather than resulting in thin, lowquality coatings on vertical walls of the substrate, as is the case for physical vapor deposition methods. Other advantages include that CVD uses source materials that flow into the process chamber from external reservoirs that can be refilled without contamination of the growth environment, it does not require very high vacuum levels, it can generally process substrates in larger batches than evaporation, and is more forgiving in terms of its tolerance for precision in the process conditions. Counterbalancing these advantages, CVD source materials are generally highly toxic or flammable, requiring great care in the design and operation of a CVD process system. CVD also frequently requires high temperatures. For microelectronics manufacturing the benefits generally outweigh the problems. Thus, most device makers use CVD when possible rather than, for example, MBE.