Mechanical Properties of Semiconductors

Two main types of semiconductors are the n-type and p-type semiconductors, but other classification is usually used to define the wide range of commonly known types. One way to classify semiconductors is as intrinsic semiconductor material and extrinsic material.

Generally speaking, elasticity is concerned with the deformation under applied force which may be results in non-permanent dimension or/and shape change while the force is acting.

The concept of plastic deformation in its various forms was extensively covered in the books Mechanical Properties of Material (Pelleg) and Mechanical Properties of Nanomaterials (Pelleg).

Although the term defects in crystals include point defects (vacancies and interstitials), dislocations stacking faults and grain boundaries, the emphasis of this chapter is mainly on point defects and dislocations, also known as line defects.

Generally, deformation refers to alteration of the shape and/or size of a material due to applied force. Also, modification can occur from a change in temperature. The acting force can be either tensile, compressive or shear [1]. One can also indicate the change in terms of: (i) displacement-referring to an absolute change in position of a point in the object under consideration; (ii) deflection-a relative change in external displacement of an object (In engineering, deflection is the degree or angle to which a part of a structural element is displaced under a load).

Basic concepts of the time dependent deformation-more commonly known as creep-has been considered earlier, generally in material ( J. Pelleg, Mechanical Properties of Materials (Springer, 2013, Chapter 5), p. 259.) and specifically in ceramics ( J. Pelleg, Mechanical Properties of Ceramics (Springer, 2014. Chapter 6), p. 417). Creep, the time dependent deformation, occurs under constant stress.

Weakening of a material caused by cyclic loading resulting in structural damage leading to catastrophic failure is known as fatigue. The majority of failures of engineering materials are caused by fatigue. Brittle material at room temperature-such as silicon-would not be expected to be susceptible to fatigue under applied cyclic stresses. Under applied stress brittle material fail almost completely, fracture at the site of the highest stress concentration which is generally the site of a flaw. The flaws are invariably processing-induced and randomly distributed, leading to scatter in the measured strength, which makes it difficult to predict a stress level near the fracture strength. Localized structural damage manifests itself in cracks. Once a crack has initiated, each loading cycle will grow the crack a small amount, even when repeated alternating or cyclic stresses are of an intensity considerably below the normal strength. The stresses could be due to vibration or thermal cycling. Fatigue damage is caused by the action of cyclic stress. No deformation occurs when a sample is exposed to fatigue because it’s brittle nature. The danger in fatigue that it occurs without warning at stress levels considerably below the yield stress. Over the years much experience has been accumulated by exploring the possible reasons for this occurrence and test have been suggested to evaluate the propensity for failure of machine elements which are exposed to pulsating or vibrational stresses. Nevertheless, the difficulty in predicting fatigue failure produces a wide spread of statistical results and often a deviation of ~ 50% from the average value is observed. In contrast other mechanical tests, do not deviate from an average value more than ~ 2–3%. This explains why many test specimens are used in fatigue experiments to reach a meaningful average value, below which the probability for fatigue fracture is quite low. A commonly used test, having repeated stress with reverse loading against the number of cycles in order to evaluate the endurance of a specimen is the S–N plot, where S represent stress and N stands for the number of cycles. It is not only alternating stress that may cause fatigue failure, but also the duration of exposure, as in material such as glass, that may fracture after long-term exposure to stress without undergoing any plastic deformation. The term for such behavior is ‘static fatigue’. Thermal fatigue is the term assigned to material failure caused by repeated changes in stress doe to the rise and fall of thermal gradients for various reasons, involving restrictions in thermal expansion or contraction.

Most semiconductors are crystalline inorganic solids, and usually they are classified according to their location in the periodic table. Generally, metallic elements are known for their ductility at room temperature while ceramics such as semiconductors are mostly insulators and almost to nonductile features. Compared to the large deformation strain before fracture in metals even in the range of 5–100% strain, in brittle inorganic semiconductors the strain is very small of about 0.1–0.2%. One could recall the reason for this difference, namely, semiconductors (like ceramic insulators) are primarily held by ionic and/or covalent bonds, and they are not flexible, thus in general they have poor (or none) ductility and low electrical conductivity. Therefore, almost all semiconductors, elemental (Si. Ge) or binary (SiC. GaAs) are brittle and insulating or very low in their conductivity. The electrical conductivity in semiconductors can vary in a wide range from 10⁻⁷ Sm⁻¹ to 10⁵ Sm⁻¹ (S stands for Siemenns and defined as [S] = [W⁻¹] = [A/V], where A stands for ampere, V for volt and W for Ohm). On the other hand, the ductility in metals is attributed to the intrinsic defects present in them with unique character of dislocations and the chemical bonding characterized by a cloud of electros surrounding the nuclei to form electrostatic interactions the so-called metallic bond.

Reducing the size of materials -and semiconductors are no exception- to nanoscale dimensions, changes extremely their bulk properties. These semiconductors exhibit unique optoelectronic and magnetic properties and at a small size of about 20 nm or below, high surface area and quantum size effects characterize them. Recall, the so-called quantum size effect describes the physics of electron properties in solids with great reductions in particle size. This effect does not come into play by going from macro to micro dimensions. However, it becomes dominant when the nanometer size range is reached, particularly at the lower end of the nanoscale, affecting as indicated, optical, electrical, and magnetic behavior of materials. It is expected that the mechanical properties of the nanoscale materials -the subject of this book- also, will be much affected as compared to their bulk counterpart. Materials can be produced as nanoscale size in one dimension (for example, very thin surface coatings), in two dimensions (for example, nanowires and nanotubes) or in all three dimensions (for example, nanoparticles and quantum dots).

It is well documented that the semiconductor industry has enormous effect on the environment, and more recently the flagship of the environmental problem is global climate change. The globe is gradually getting warmer with an estimated increase of 1.5–2 ^cC per year in temperature of the globe. Scientists claim that all man-made, self-destructing acts might set in within the next 20–30 years. This is an optimistic approximation, but people of realism expect it to occur much sooner.