Semiconductor MaterialsS. K. Tewksbury Microelectronic Systems Research Center Dept. of Electrical

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Semiconductor MaterialsS. K. Tewksbury Microelectronic Systems Research Center Dept. of Electrical

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A semiconductor material has a resistivity lying between that of a conductor and that of an insulator. However, in contrast to the granular materials used for resistors, a semiconductor establishes its conduction properties through a complex quantum mechanical behavior within a periodic array of semiconductor atoms, i.e., within a crystalline structure. For appropriate atomic elements, the crystalline structure leads to a disallowed energy band between the energy level of electrons bound to the crystal's atoms and the energy level of electrons free to move within the crystalline structure (i.e., not bound to an atom)....

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Nội dung Text: Semiconductor MaterialsS. K. Tewksbury Microelectronic Systems Research Center Dept. of Electrical

  1. Semiconductor Materials S. K. Tewksbury Microelectronic Systems Research Center Dept. of Electrical and Computer Engineering West Virginia University Morgantown, WV 26506 (304)293-6371 Sept. 21, 1995 Contents 1 Introduction 2 2 Crystalline Structures 3 2.1 Basic Semiconductor Materials Groups . . . . . . . . . . . . . . . . . . . . . 3 2.1.1 Elemental (IV-IV) Semiconductors . . . . . . . . . . . . . . . . . . . 3 2.1.2 Compound III-V Semiconductors . . . . . . . . . . . . . . . . . . . . 4 2.1.3 Compound II-VI Semiconductors . . . . . . . . . . . . . . . . . . . . 6 2.2 Three-Dimensional Crystal Lattice . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Crystal Directions and Planes . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Energy Bands and Related Semiconductor Parameters 8 3.1 Conduction and Valence Band . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2 Direct Gap and Indirect Gap Semiconductors . . . . . . . . . . . . . . . . . 12 3.3 Effective Masses of Carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.4 Intrinsic Carrier Densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.5 Substitutional Dopants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4 Carrier Transport 18 4.1 Low Field Mobilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.2 Saturated Carrier Velocities . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5 Crystalline Defects 23 5.1 Point Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.2 Line Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 5.3 Stacking Faults and Grain Boundaries . . . . . . . . . . . . . . . . . . . . . 26 5.4 Unintentional Impurities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.5 Surface Defects: The Reconstructed Surface . . . . . . . . . . . . . . . . . . 27 1
  2. 6 Summary 29 1 Introduction A semiconductor material has a resistivity lying between that of a conductor and that of an insulator. However, in contrast to the granular materials used for resistors, a semiconductor establishes its conduction properties through a complex quantum mechanical behavior within a periodic array of semiconductor atoms, i.e., within a crystalline structure. For appropriate atomic elements, the crystalline structure leads to a disallowed energy band between the energy level of electrons bound to the crystal’s atoms and the energy level of electrons free to move within the crystalline structure (i.e., not bound to an atom). This “energy gap” fundamentally impacts the mechanisms through which electrons associated with the crystal’s atoms can become free and serve as conduction electrons. The resistivity of a semiconductor is proportional to the free carrier density, and that density can be changed over a wide range by replacing a very small portion (about 1 in 106 ) of the base crystal’s atoms with different atomic species (doping atoms). The majority carrier density is largely pinned to the net dopant impurity density. By selectively changing the crystalline atoms within small regions of the crystal, a vast number of small regions of the crystal can be given different conductivities. In addition, some dopants establish the electron carrier density (free electron density) while others establish the “hole” carrier density (holes are the dual of electrons within semiconductors). In this manner, different types of semiconductor (n-type with much higher electron carrier density than the hole density and p-type with much higher hole carrier density than the electron carrier density) can be located in small but contacting regions within the crystal. By applying electric fields appropriately, small regions of the semiconductor can be placed in a state in which all the carriers (electron and hole) have been expelled by the electric field, and that electric field sustained by the exposed dopant ions. This allows electric switching between a conducting state (with a settable resistivity) and a non-conducting state (with conductance vanishing as the carriers vanish). This combination of localized regions with precisely controlled resistivity (dominated by electron conduction or by hole conduction) combined with the ability to electronically control the flow of the carriers (electrons and holes) leads to the semiconductors being the foundation for contemporary electronics. This foundation is particularly strong because a wide variety of atomic elements (and mixtures of atomic elements) can be used to tailor the semiconductor material to specific needs. The dominance of silicon semiconductor material in the electronics area (e.g., the VLSI digital electronics area) contrasts with the rich variety of semiconductor materials widely used in optoelectronics. In the latter case, the ability to adjust the bandgap to desired wavelengths of light has stimulated a vast number of optoelectronic components, based on a variety of technologies. Electronic components also provide a role for non-silicon semiconductor technologies, particularly for very high bandwidth circuits which can take advantage of the higher speed capabilities of semiconductors using atomic elements similar to those used in optoelectronics. This rich interest in non-silicon technologies will undoubtedly continue to grow, due to the rapidly advancing applications of optoelectronics, for the simple 2
  3. reason that silicon is not suitable for producing an efficient optical source. This chapter provides an overview of many of the semiconductor materials in use. To organize the information, the topic is developed from the perspective of the factors contribut- ing to the electronic behavior of devices created in the semiconductor materials. Section 2 discusses the underlying crystalline structure and the semiconductor parameters which result from that structure. Section 3 discusses the energy band properties in more detail, extract- ing the basic semiconductor parameters related to those energy bands. Section 4 discusses carrier transport. Crystalline defects, which can profoundly impact the behavior of devices created in the semiconductors, are reviewed in Section 5. 2 Crystalline Structures 2.1 Basic Semiconductor Materials Groups Most semiconductor materials are crystals created by atomic bonds through which the va- lence band of the atoms are filled with 8 electrons through sharing of an electron from each of four nearest neighbor atoms. These materials include semiconductors composed of a single atomic species, with the basic atom having four electrons in its valence band (supplemented by covalent bonds to four neighboring atoms to complete the valence band). These elemental semiconductors therefore use atoms from group IV of the atomic chart. Other semiconduc- tor materials are composed of two atoms, one from group N (N < 4) and the other from group M (M > 4) with N + M = 8, filling the valence bands with 8 electrons. The major categories of semiconductor material are summarized below. 2.1.1 Elemental (IV-IV) Semiconductors Elemental semiconductors consist of crystals composed of only a single atomic element from group IV of the periodic chart, i.e., germanium (Ge), silicon (Si), carbon (C), and tin (Sn). Silicon is the most commonly used electronic semiconductor material, and is also the most common element on earth. Table 1 summarizes the naturally occurring abundance of some elements used for semiconductors, including non-elemental (compound) semiconductors. Table 1: Abundance (fraction of elements occurring on earth) of common elements used for semiconductors. Element Abundance Si 0.28 Ga 1.5 × 10−5 As 1.8 × 10−6 Ge 5 × 10−6 Cd 2 × 10−7 In 1 × 10−7 Figure 1a illustrates the covalent bonding (sharing of outer shell, valence band electrons 3
  4. by two atoms) through which each group IV atom of the crystal is bonded to four neighboring group IV atoms, creating filled outer electron bands of 8 electrons. In addition to crystals composed of only a single group IV atomic species, one can also create semiconductor crystals consisting of two or more atoms, all from group IV. For example, silicon carbide (SiC) has been investigated for high temperature applications. Six Ge1−x semiconductors are under present study to achieve bandgap engineering within the silicon system. In this case, a fraction x (0 < x < 1) of the atoms in an otherwise silicon crystal are silicon while a fraction 1 − x have been replaced by germanium. This ability to replace a single atomic element with a combination of two atomic elements from the same column of the periodic chart appears in the other categories of semiconductor described below (and is particularly important for optoelectronic devices). Group IV Group III Group V Group II Group VI Si Ga As Cd Se IV IV IV IV III V III V II VI II VI IV IV IV IV V III V III VI II VI II IV IV IV IV III V III V II VI II VI IV IV IV IV V III V III VI II VI II (a) (b) (c) Figure 1: Bonding arrangements of atoms in semiconductor crystals. (a) Elemental semi- conductor such as silicon. (b) Compound III-V semiconductor such as GaAs. (c) Compound II-VI semiconductor such as CdS. 2.1.2 Compound III-V Semiconductors The III-V semiconductors are prominent (and will gain in importance) for applications of optoelectronics. In addition, III-V semiconductors have a potential for higher speed opera- tion than silicon semiconductors in electronics applications, with particular importance for areas such as wireless communications. The compound semiconductors have a crystal lat- tice constructed from atomic elements in different groups of the periodic chart. The III-V semiconductors are based on an atomic element A from Group III and an atomic element B from Group V. Each Group III atom is bound to four Group V atoms, and each Group V atom is bound to four Group III atoms, giving the general arrangement shown in Figure 1b. The bonds are produced by sharing of electrons such that each atom has a filled (8 4
  5. electron) valence band. The bonding is largely covalent, though the shift of valence charge from the Group V atoms to the Group III atoms induces a component of ionic bonding to the crystal (in contrast to the elemental semiconductors which have purely covalent bonds). Representative III-V compound semiconductors are GaP, GaAs, GaSb, InP, InAs, and InSb. GaAs is probably the most familiar example of III-V compound semiconductors, used for both high speed electronics and for optoelectronic devices. Optoelectronics has taken advantage of ternary and quaternary III-V semiconductors to establish optical wavelengths and to achieve a variety of novel device structures. The ternary semiconductors have the general form (Ax , A1−x )B (with two group III atoms used to fill the group III atom positions in the lattice) or A(Bx , B1−x ) (using two group V atoms in the Group V atomic positions in the lattice). The quaternary semiconductors use two Group III atomic elements and two Group V atomic elements, yielding the general form (Ax , A1−x )(By , B1−y ). In such constructions, 0 ≤ x ≤ 1. Such ternary and quaternary versions are important since the mixing factors (x and y) allow the bandgap to be adjusted to lie between the bandgaps of the simple compound crystals with only one type of Group III and one type of Group V atomic element. The adjustment of wavelength allows the material to be tailored for particular optical wavelengths, since the wavelength λ of light is related to energy (in this case the gap energy Eg ) by λ = hc/Eg , where h is Plank’s constant and c is the speed of light. Table 2 provides examples of semiconductor laser materials and a representative optical wavelength for each, providing a hint of the vast range of combinations which are available for optoelectronic applications. Table 3, on the other hand, illustrates the change in wavelength (here corresponding to color in the visible spectrum) by adjusting the mixture of a ternary semiconductor. Table 2: Semiconductor optical sources and representative wavelengths. Material layers used wavelength ZnS 454 nm AlGaInP/GaAs 580 nm AlGaAs/GaAs 680 nm GaInAsP/InP 1580 nm InGaAsSb/GaSb 2200 nm AlGaSb/InAsSb/GaSb 3900 nm PbSnTe/PbTe 6000 nm Table 3: Variation of x to adjust wavelength in GaAsx P1−x semiconductors. Ternary Compound Color GaAs0.14 P0.86 Yellow GaAs0.35 P0.65 Orange GaAs0.6 P0.4 Red 5
  6. In contrast to single element elemental semiconductors (for which the positioning of each atom on a lattice site is not relevant), III-V semiconductors require very good control of stoichiometry (i.e., the ratio of the two atomic species) during crystal growth. For example, each Ga atom must reside on a Ga (and not an As) site and vice versa. For these and other reasons, large III-V crystals of high quality are generally more difficult to grow than a large crystal of an elemental semiconductor such as Si. 2.1.3 Compound II-VI Semiconductors These semiconductors are based on one atomic element from Group II and one atomic el- ement from Group VI, each type being bonded to four nearest neighbors of the other type as shown in Figure 1c. The increased amount of charge from Group VI to Group II atoms tends to cause the bonding to be more ionic than in the case of III-V semiconductors. II-VI semiconductors can be created in ternary and quaternary forms, much like the III-V semi- conductors. Although less common than the III-V semiconductors, the II-VI semiconductors have served the needs of several important applications. Representative II-VI semiconductors are ZnS, ZnSe,and ZnTe (which form in the zinc blende lattice structure discussed below); CdS and CdSe, (which can form in either the zinc blende or the wurtzite lattice structure) and CdTe which forms in the wurtzite lattice structure. 2.2 Three-Dimensional Crystal Lattice The two-dimensional views illustrated in the previous section provide a simple view of the sharing of valence band electrons and the bonds between atoms. However, the full 3-D lattice structure is considerably more complex than this simple 2-D illustration. Fortunately, most semiconductor crystals share a common basic structure, developed below. FCC Lattice A FCC Lattice B (a) (b) (c) Figure 2: Three-dimensional crystal lattice structure. (a) Basic cubic lattice. (c) Face- centered cubic (fcc) lattice. (c) Two interpenetrating fcc lattices. In this figure, the dashed lines between atoms are not atomic bonds but instead are used merely to show the basic outline of the cube. The crystal structure begins with a cubic arrangement of 8 atoms as shown in Figure 2a. This cubic lattice is extended to a face-centered cubic (fcc) lattice, shown in 2b, by 6
  7. adding an atom to the center of each face of the cube (leading to a lattice with 14 atoms). The lattice constant is the side dimension of this cube. The full lattice structure combines two of these fcc lattices, one lattice interpenetrating the other (i.e., the corner of one cube is positioned within the interior of the other cube, with the faces remaining parallel), as illustrated in Figure 2c. For the III-V and II-VI semiconductors with this fcc lattice foundation, one fcc lattice is constructed from one type of element (e.g., type III) and the second fcc lattice is constructed from the other type of element (e.g., group V). In the case of ternary and quaternary semiconductors, elements from the same atomic group are placed on the same fcc lattice. All bonds between atoms occur between atoms in different fcc lattices. For example, all Ga atoms in the GaAs crystal are located on one of the fcc lattices and are bonded to As atoms, all of which appear on the second fcc lattice. The interatomic distances between neighboring atoms is therefore less than the lattice constant. For example, the interatomic spacing of Si atoms is 2.35 ˚ but A the lattice constant of Si is 5.43 ˚. A If the two fcc lattices contain elements from different groups of the periodic chart, the overall crystal structure is called the zinc blende lattice. In the case of an elemental semiconductor such as silicon, silicon atoms appear in both fcc lattices and the overall crystal structure is called the diamond lattice (carbon crystallizes into a diamond lattice creating true diamonds, and carbon is a group IV element). As in the discussion regarding III-V semiconductors above, the bonds between silicon atoms in the silicon crystal extend between fcc sublattices. Although the common semiconductor materials share this basic diamond/zinc blende lattice structure, some semiconductor crystals are based on a hexagonal close-packed (hcp) lattice. Examples are CdS and CdSe. In this example, all the Cd atoms are located on one hcp lattice while the other atom (S or Se) is located on a second hcp lattice. In the spirit of the diamond and zinc blende lattices above, the complete lattice is constructed by interpenetrating these two hcp lattices. The overall crystal structure is called a wurtzite lattice. Type IV-VI semiconductors (PbS, PbSe, PbTe, and SnTe) exhibit a narrow band gap and have been used for infrared detectors. The lattice structure of these example IV-VI semiconductors is the simple cubic lattice (also called an NaCl lattice). 2.3 Crystal Directions and Planes Crystallographic directions and planes are important in both the characteristics and the ap- plications of semiconductor materials since different crystallographic planes can exhibit sig- nificantly different physical properties. For example, the surface density of atoms (atoms/cm2 ) can differ substantially on different crystal planes. A standardized notation (the so-called Miller indices) is used to define the crystallographic planes and directions normal to those planes. The general crystal lattice defines a set of unit vectors (a,b, and c) such that an entire crystal can be developed by copying the unit cell of the crystal and duplicating it at integer offsets along the unit vectors, i.e., replicating the basis cell at positions na a + nb b + nc c, where na , nb , and nc are integers. The unit vectors need not be orthogonal in general. For 7
  8. the cubic foundation of the diamond and zinc blende structures, however, the unit vectors are in the orthogonal x, y, and z directions. Figure 3 shows a cubic crystal, with basis vectors in the x,y, and z directions. Su- perimposed on this lattice are three planes (Figures 3a, b and c). The planes are defined relative to the crystal axes by a set of three integers (h, k, l) where h corresponds to the plane’s intercept with the x-axis, k corresponds to the plane’s intercept with the y-axis and l corresponds to the plane’s intercept with the z-axis. Since parallel planes are equivalent planes, the intercept integers are reduced to the set of the three smallest integers having the same ratios as the above intercepts. The (100), (010) and (001) planes correspond to the faces of the cube. The (111) plane is tilted with respect to the cube faces, intercepting the x, y, and z axes at 1, 1, and 1, respectively. In the case of a negative axis intercept, the corresponding Miller index is given as an integer and a bar over the integer,e.g., (¯ 100), i.e., similar to (100) plane but intersecting x-axis at -1. z y x (a) (b) (c) Figure 3: Examples of crystallographic planes within a cubic lattice organized semiconductor crystal. (a) (010) plane. (b) (110) plane. (c) (111) plane. Additional notation is used to represent sets of planes with equivalent symmetry and to represent directions. For example, {100} represents the set of equivalent planes (100), (¯ 100). ¯ (001), and (00¯ The direction normal to the (hkl) plane is designated [hkl]. (010), (010), 1). The different planes exhibit different behavior during device fabrication and impact electrical device performance differently. One difference is due to the different reconstructions of the crystal lattice near a surface to minimize energy. Another is the different surface density of atoms on different crystallographic planes. For example, in Si the (100), (110), and (111) planes have surface atom densities (atoms per cm2 ) of 6.78×1014 , 9.59×1014 , and 7.83×1014 , respectively. 3 Energy Bands and Related Semiconductor Parameters A semiconductor crystal establishes a periodic arrangement of atoms, leading to a periodic spatial variation of the potential energy throughout the crystal. Since that potential energy 8
  9. varies significantly over interatomic distances, quantum mechanics must be used as the basis for allowed energy levels and other properties related to the semiconductor. Different semi- conductor crystals (with their different atomic elements and different inter-atomic spacings) lead to different characteristics. However, the periodicity of the potential variations leads to several powerful general results applicable to all semiconductor crystals. Given these general characteristics, the different semiconductor materials exhibit properties related to the variables associated with these general results. A coherent discussion of these quantum mechanical results is beyond the scope of this chapter and we therefore take those general results as given. In the case of materials which are semiconductors, a central result is the energy- momentum functions defining the state of the electronic charge carriers. In addition to the familiar electrons, semiconductors also provide holes (i.e. positively charged particles) which behave similarly to the electrons. Two energy levels are important: one is the energy level (conduction band) corresponding to electrons which are not bound to crystal atoms and which can move through the crystal and the other energy level (valence band) corresponds to holes which can move through the crystal. Between these two energy levels, there is a region of “forbidden” energies (i.e., energies for which a free carrier can not exist). The separation between the conduction and valence band minima is called the energy gap or band gap. The energy bands and the energy gap are fundamentally important features of the semiconductor material and are reviewed below. 3.1 Conduction and Valence Band In quantum mechanics, a “particle” is represented by a collection of plane waves (ej(ωt−k·x) ) where the frequency ω is related to the energy E according to E = hω and the momentum ¯ p is related to the wave vector by p = hk. In the case of a classical particle with mass m ¯ moving in free space, the energy and momentum are related by E = p2 /(2m) which, using the relationship between momentum and wave vector, can be expressed as E = (¯ k)2 /(2m). h In the case of the semiconductor, we are interested in the energy/momentum relationship for a free electron (or hole) moving in the semiconductor, rather than moving in free space. In general, this E-k relationship will be quite complex and there will be a multiplicity of E-k “states” resulting from the quantum mechanical effects. One consequence of the periodicity of the crystal’s atom sites is a periodicity in the wave vector k, requiring that we consider only values of k over a limited range (with the E-k relationship periodic in k). Figure 4 illustrates a simple example (not a real case) of a conduction band and a valence band in the energy-momentum plane (i.e., the E vs k plane). The E vs k relationship of the conduction band will exhibit a minimum energy value and, under equilibrium conditions, the electrons will favor being in that minimum energy state. Electron energy levels above this minimum (Ec ) exist, with a corresponding value of momentum. The E vs k relationship for the valence band corresponds to the energy-momentum relationship for holes. In this case, the energy values increase in the direction toward the bottom of the page and the “minimum” valence band energy level Ev is the maximum value in Figure 4. When an electron bound to an atom is provided with sufficient energy to become a free electron, a 9
  10. Energy E Conduction band minimum (free electrons) Ec Energy gap Eg Ev Valence band minimum (free holes) L [111] direction Γ [100] direction Κ k k Figure 4: General structure of conduction and valence bands. hole is left behind. Therefore, the energy gap Eg = Ec − Ev represents the minimum energy necessary to generate an electron-hole pair (higher energies will initially produce electrons with energy greater than Ec , but such electrons will generally lose energy and fall into the potential minimum). The details of the energy bands and the bandgap depend on the detailed quantum mechanical solutions for the semiconductor crystal structure. Changes in that structure (even for a given semiconductor crystal such as Si) can therefore lead to changes in the energy band results. Since the thermal coefficient of expansion of semiconductors is non- zero, the band gap depends on temperature due to changes in atomic spacing with changing temperature. Changes in pressure also lead to changes in atomic spacing. Though these changes are small, the are observable in the value of the energy gap. Table 4 gives the room temperature value of the energy gap Eg for several common semiconductors, along with the rate of change of Eg with temperature (T ) and pressure (P ) at room temperature. The temperature dependence, though small, can have a significant impact on carrier densities. A heuristic model of the temperature dependence of Eg is Eg (T ) = Eg (0o K) − αT 2 /(T + β). Values for the parameters in this equation are provided in Table 5. Between 0K and 1000K, the values predicted by this equation for the energy gap of GaAs are accurate to about 2 × 10−3 eV (electron volts). 10
  11. Table 4: Variation of energy gap with temperature and pressure. Adapted from [5]. Semiconductor Eg (300K) dEg /dT (meV/o K) dEg /dP (meV/kbar) Si 1.110 -0.28 -1.41 Ge 0.664 -0.37 5.1 GaP 2.272 -0.37 10.5 GaAs 1.411 -0.39 11.3 GaSb 0.70 -0.37 14.5 InP 1.34 -0.29 9.1 InAs 0.356 -0.34 10.0 InSb 0.180 -0.28 15.7 ZnSe 2.713 -0.45 0.7 ZnTe 2.26 -0.52 8.3 CdS 2.485 -0.41 4.5 CdSe 1.751 0.36 5. CdTe 1.43 -0.54 8 Table 5: Temperature dependence parameters for common semiconductors. Adapted from [4]. Eg (0o K) α(×10−4 ) β Eg (300K) GaAs 1.519 eV 5.405 204 1.42 eV Si 1.170 eV 4.73 636 1.12 eV Ge 0.7437 eV 4.774 235 0.66 eV 11
  12. 3.2 Direct Gap and Indirect Gap Semiconductors Figure 5 illustrates the energy bands for Ge, Si and GaAs crystals. In Figure 5b, for silicon, the valence band has a minimum at a value of k different than that for the conduction band minimum. This is an indirect gap, with generation of an electron-hole pair requiring an energy Eg and a change in momentum (i.e., k). For direct recombination of an electron-hole pair, a change in momentum is also required. This requirement for a momentum change (in combination with energy and momentum conservation laws) leads to a requirement that a phonon participate with the carrier pair during a direct recombination process generating a photon. This is a highly unlikely event, rendering silicon ineffective as an optoelectronic source of light. The direct generation process is more readily allowed (with the simultaneous generation of an electron, a hole, and a phonon), allowing silicon and other direct gap semiconductors to serve as optical detectors. Germanium Silicon GaAs 6 5 4 Energy (eV) 3 Conduction band 2 minimum 1 Energy gap 0 Valence band minimum -1 -2 -3 L [111] Γ [100] K L [111] Γ [100] K L [111] Γ [100] K Wave Vector (a) (b) (c) Figure 5: Conduction and valence bands for (a) germanium, (b) silicon, and (c) GaAs. Adapted from [4]. 12
  13. In Figure 5c, for GaAs, the conduction band minimum and the valence band minimum occur at the same value of momentum, corresponding to a direct gap. Since no momentum change is necessary during direct recombination, such recombination proceeds readily, pro- ducing a photon with the energy of the initial electron and hole (i.e., a photon energy equal to the bandgap energy). For this reason, direct gap semiconductors are efficient sources of light (and use of different direct gap semiconductors with different Eg provides a means of tailoring the wavelength of the source). The wavelength λ corresponding to the gap energy is λ = hc/Eg . Figure 5c also illustrates a second conduction band minimum with an indirect gap, but at a higher energy than the minimum associated with the direct gap. The higher conduction band minimum can be populated by electrons (which are in an equilibrium state of higher energy) but the population will decrease as the electrons gain energy sufficient to overcome that upper barrier. 3.3 Effective Masses of Carriers For an electron with energy close to the minimum of the conduction band, the energy vs momentum relationship is approximately given by E(k) = E0 +a2 (k −k ∗ )2 +a4 (k −k ∗ )4 +. . . . Here, E0 = Ec is the “ground state energy” corresponding to a free electron at rest and k ∗ is the wave vector at which the conduction band minimum occurs. Only even powers of k − k ∗ appear in the expansion of E(k) around k ∗ due to the symmetry of the E-k relationship around k = k ∗ . The above approximation holds for sufficiently small increases in E above Ec . For sufficiently small movements away from the minimum (i.e., sufficiently small k − k ∗ ), the terms in k−k ∗ higher than quadratic can be ignored and E(k) ≈ E0 +a2 k 2 , where we have taken k ∗ = 0. If, instead of a free electron moving in the semiconductor crystal, we had a free electron moving in free space with potential energy E0 , the energy-momentum relationship would be E(k) = E0 + (¯ k)2 /(2m0 ), where m0 is the mass of an electron. By comparison of h these results, it is clear that we can relate the curvature coefficient a2 associated with the parabolic minimum of the conduction band to an effective mass m∗ , i.e., a2 = (¯ 2 )/(2m∗ ) or e h e 1 2 ∂ 2 Ec (k) = 2· . m∗ e h ¯ ∂k 2 Similarly for holes, an effective mass m∗ of the holes can be defined by the curvature of h the valence band minimum, i.e., 1 2 ∂ 2 Ev (k) = 2· . m∗ h h ¯ ∂k 2 Since the energy bands depend on temperature and pressure, the effective masses can also be expected to have such dependencies, though the room temperature and normal pressure value is normally used in device calculations. The above discussion assumes the simplified case of a scalar variable k. In fact, the wave vector k has three components (k1 , k2 , k3 ), with directions defined by the unit vectors of the underlying crystal. Therefore, there are separate masses for each of these vector components 13
  14. of k, i.e., masses m1 , m2 , m3 . A scalar mass m∗ can be defined using these directional masses, the relationship depending on the details of the directional masses. For cubic crystals (as in the diamond and zinc blende structures), the directions are the usual orthonormal directions and m∗ = (m1 ·m2 ·m3 )1/3 . The three directional masses effectively reduce to two components if two values are equal (e.g., m1 = m2 ), as in the case of longitudinal and transverse effective masses (ml and mt , respectively) seen in silicon and several other semiconductors. In this case, m∗ = [(mt )2 · ml ]1/3 . If all three values of m1 , m2 , m3 are equal, then a single value m∗ can be used. An additional complication is seen in the valence band structures in Figure 5. Here, two different E-k valence bands have the same minima. Since their curvatures are different, the two bands correspond to different masses, one corresponding to heavy holes with mass mh and the other to light holes with mass ml . The effective scalar mass in this case is m∗ = (mh +ml )2/3 . Such light and heavy holes occur in several semiconductors, including 3/2 3/2 Si. Values of effective mass are given in Tables 8 and 13. 3.4 Intrinsic Carrier Densities The density of free electrons in the conduction band depends on two functions. One is the density of states D(E) in which electrons can exist and the other is the energy distribution function F (E, T ) of free electrons. The energy distribution function (under thermal equilibrium conditions) is given by the Fermi-Dirac distribution function −1 E − Ef F (E) = 1 + exp kB T which, in most practical cases, can be approximated by the classical Maxwell-Boltzmann distribution. These distribution functions are general functions, not depending on the specific semiconductor material. The density of states D(E), on the other hand, depends on the semiconductor material. A common approximation is √ 2 (E − Ec )1 /2 ∗ 3 Dn (E) = Mc 2 (me ) /2 π h3 ¯ for electrons and √ 2 (Ev − E)1 /2 ∗ 3 Dp (E) = Mv (mh ) /2 π2 h3 ¯ for holes. Here, Mc and Mv are the number of equivalent minima in the conduction band and valence band, respectively. Note that necessarily E ≥ Ec for free electrons and E ≤ Ev for free holes due to the forbidden region between Ec and Ev . The density n of electrons in the conduction band can be calculated as ∞ n= F (E, T )D(E)dE. E=Ec 14
  15. For Fermi levels significantly (more than a few kB T ) below Ec and above Ev , this integration leads to the results n = Nc e−(Ec −Ef )/kb T and p = Nv e−(Ef −Ev )/kb T where n and p are the densities of free electrons in the conduction band and of holes in the valence band, respectively. Nc and Nv are effective densities of states which vary with temper- ature (slower than the exponential in the equations above), effective mass, and other condi- tions. Table 6 gives values of Nc and Nv for several semiconductors. Approximate expressions for these densities of state are Nc = 2(2πm∗ kB T /¯ 2 )3/2 Mc and Nv = 2(2πm∗ kB T /¯ 2 )3/2 Mv . e h e h These effective densities of states are fundamental parameters used in evaluating the elec- trical characteristics of semiconductors. The equations above for n and p apply both to intrinsic semiconductors (i.e., semiconductors with no impurity dopants) as well as to semi- conductors which have been doped with donor and/or acceptor impurities. Changes in the interrelated values of n and p through introduction of dopant impurities can be represented by changes in a single variable, the Fermi level Ef . The product of n and p is independent of Fermi level and is given by n · p = Nc · Nv e−Eg /kB T where the energy gap Eg = Ec − Ev . Again, this holds for both intrinsic semiconductors and for doped semiconductors. In the case of an intrinsic semiconductor, charge neutrality requires that n = p ≡ ni , where ni is the intrinsic carrier concentration and n2 = Nc · Nv e−Eg /kB T . i Since, under thermal equislibrium conditions np ≡ n2 (even under impurity doping condi- i tions), knowledge of the density of one of the carrier types (e.g., of p) allows direct deter- mination of the density of the other (e.g., n = n2 /p). Values of ni vary considerably among i semiconductor materials: 2 × 10−3 /cm3 for CdS, 3.3 × 106 /cm3 for GaAs, 0.9 × 1010 /cm3 for Si, 1.9 × 1013 /cm3 for Ge, and 9.1 × 1014 for PbS. Since there is appreciable temperature dependence in the effective density of states, the equations above do not accurately represent the temperature variations in ni over wide temperature ranges. Using the approximate expressions for Nc and Nv above, ni = 2(2πkB T /¯ 2 )3/2 (m∗ m∗ )3/4 Mc Mv e−Eg /kB T , h e h exhibiting a T 3/2 temperature dependence superimposed on the exponential dependence on 1/T . For example, at 300K, ni (T ) = 1.76 × 1016 T 3/2 e−4550/T cm−3 for Ge and ni (T ) = 3.88 × 1016 T 3/2 e−7000/T cm−3 for Si. 15
  16. Table 6: Nc and Nv at 300K. Adapted from [5]. Nc (×1019 /cm3 ) Nv (×1019 /cm3 ) Ge 1.54 1.9 Si 2.8 1.02 GaAs 0.043 0.81 GaP 1.83 1.14 GaSb 0.021 0.62 InAs 0.0056 0.62 InP 0.052 1.26 InSb 0.0043 0.62 CdS 0.224 2.5 CdSe 0.11 0.74 CdTe 0.13 0.55 ZnSe 0.31 0.87 ZnTe 0.22 0.078 3.5 Substitutional Dopants An intrinsic semiconductor material contains only the elemental atoms of the basic material (e.g., silicon atoms for Si, gallium and arsenic atoms for GaAs, etc.). The resistivity is quite high for such intrinsic semiconductors and doping is used to establish a controlled lower resistivity material (and to establish pn junctions for active devices). Doping concentrations are generally in the range 1014 to 1017 /cm3 , small relative to the density of atoms in the crystal (e.g., to the density 5 × 1022 atoms/cm3 of silicon atoms in Si crystals). Table 7 lists a variety of dopants and their energy levels for Si and GaAs. Table 7: Acceptor and donor impurities used in Si and GaAs. Adapted from [1]. Donor Ec − Ed (eV) Acceptor Ea − Ev (eV) Si crystal Sb 0.039 B 0.045 P 0.045 Al 0.067 As 0.054 Ga 0.073 GaAs crystal S 0.006 Mg 0.028 Se 0.006 Zn 0.031 Te 0.03 Cd 0.035 Si 0.058 Si 0.026 Figure 6a illustrates acceptor dopants and donor dopants in silicon. In the case of acceptor dopants, group III elements of the periodic chart are used to substitute for the group IV silicon atoms in the crystal. This acceptor atom has one fewer electron in its outer shell than the silicon atom, and readily captures a free electron to provide the missing 16
  17. electron needed to complete the outer shells (8 electrons) provided by the covalent bonds. The acceptor atom, with a captured electron, becomes a negative ion. The electron captured from the outer shell of a neighboring silicon atom leaves behind a hole at that neighboring silicon atom (i.e., generates a free hole when ionized). By using acceptor doping densities NA substantially greater than ni , a net hole density p ni is created. With np = n2 a constant, i the electron density n decreases as p increases above ni . The resulting semiconductor material becomes p-type. In the case of donor dopants, group V elements are used to substitute for a silicon atom in the crystal. The donor atom has one extra electron in its outer shell, compared to a silicon atom, and that extra electron can leave the donor site and become a free electron. In this case, the donor becomes a positive ion, generating a free electron. By using donor doping densities ND substantially greater than ni , a net electron density n ni 2 is created, and p decreases substantially below ni to maintain the np product np = ni . An n-type semiconductor is produced. Figure 6b illustrates the alternative doping options for a III-V semiconductor (GaAs used as an example). Replacement of a Group III element with a Group II element renders that Group II element an acceptor (one fewer electron). Replacement of a Group V element with a Group VI element renders that Group VI element a donor (one extra electron). Group IV elements such as silicon can also be used for doping. In this case, the Group IV element is a donor if it replaces a Group III element in the crystal and is an acceptor if it replaces a Group V element in the crystal. Impurities which can serve as either an acceptor or as a donor within a crystal are called amphoteric impurities. Acceptor and donor impurities are most effectively used when the energy required to generate a carrier is small. In the case of small ionization energies (in the crystal lattice), the energy levels associated with the impurities lie within the bandgap close to their respective bands (i.e., donor ionization energies close to the conduction band and acceptor ionization energies close to the valence band). If the difference between the ionization level and the corresponding valence/conduction band is less than about 3kB T (≈ 0.075 eV at 300K), then the impurities (called shallow energy level dopants) are essentially fully ionized at room tem- perature. The dopants listed in Table 7 are such shallow energy dopants. A semiconductor doped with NA ni acceptors then has a hole density p ≈ NA and a much smaller electron density n ≈ ni /NA . Similarly, a semiconductor doped with ND 2 ni donors has an electron density n ≈ ND and a much smaller hole density p = ni /ND . From the results given earlier 2 for carrier concentrations as a function of Fermi level, the Fermi level is readily calculated (given the majority carrier concentration). Most semiconductors can be selectively made (by doping) either n-type or p-type, in which case they are called ambipolar semiconductors. Some semiconductors can be selectively made only n-type or only p-type. For example, ZnTe is always p-type while CdS is always n-type. Such semiconductors are called unipolar semiconductors. Dopants with energy levels closer to the center of the energy gap (i.e., the so called deep energy level dopants) serve as electron-hole recombination sites, impacting the minority carrier lifetime and the dominant recombination mechanism in indirect band gap semicon- ductors. 17
  18. Si Group IV Group Semiconductor IV Substitutes III Group IV V Acceptor Donor Accepts electron Forfeits electron to mimic Si to mimic Si (a) Ga Group III-V As Group Semiconductor Group V III Substitutes Substitutes Group III Group V II IV IV VI Acceptor Donor Acceptor Donor Accepts electron Forfeits electron Forfeits electron to mimic Ga to mimic Ga Accepts electron to mimic As to mimic As (b) Figure 6: Substitution of dopant atoms for crystal atoms. (a) IV-IV semiconductors (e.g., silicon). (b) III-V semiconductors (e.g., GaAs). 4 Carrier Transport Currents in semiconductors arise both due to movement of free carriers in the presence of an electric field and due to diffusion of carriers from high, carrier density regions into lower, carrier density regions. Currents due to electric fields are considered first. In earlier discussions, it was noted that the motion of an electron in a perfect crystal can be represented by a free electron with an effective mass m∗ somewhat different than the e 18
  19. real mass me of an electron. In this model, once the effective mass has been determined, the atoms of the perfect crystal can be discarded and the electron viewed as moving within free space. If the crystal is not perfect, however, those deviations from perfection remain after the perfect crystal lattice has been discarded and act as scattering sites within the “free space” seen by the electron in the crystal. Substitution of a dopant for an element of the perfect crystal leads to a distortion of the perfect lattice from which electrons can scatter. If that substitutional dopant is ionized, the electric field of that ion adds to the scattering. Impurities located at interstitial sites (i.e., between atoms in the normal lattice sites) also disrupt the perfect crystal and lead to scattering sites. Crystal defects (e.g., a missing atom) disrupt the perfect crystal and appears as a scattering site in the “free space” seen by the electron. In useful semiconductor crystals, the density of such scattering sites is small relative to the density of silicon atoms. As a result, removal of the silicon atoms through use of the effective mass leaves a somewhat sparsely populated space of scattering sites. The perfect crystal corresponds to all atomic elements at the lattice positions and not moving, a condition which can occur only at 0K. At temperatures above absolute zero, the atoms have thermal energy which causes them to move away from their ideal site. As the atom moves away from the nominal, equilibrium site, forces act to return it to that site, establishing the conditions for a classical oscillator problem (with the atom oscillating about its equilibrium location). Such oscillations of an atom can transfer to a neighboring atom (by energy exchange), leading to the oscillation propagating through space. This wave-like disturbance is called a phonon and serves as a scattering site (phonon scattering, also called lattice scattering) which can appear anywhere in the crystal. 4.1 Low Field Mobilities The dominant scattering mechanisms in silicon are ionized impurity scattering and phonon scattering, though other mechanisms such as mentioned above do contribute. Within this “free space” contaminated by scattering centers, the free electron moves at a high velocity (the thermal velocity vtherm ) set by the thermal energy (kB T ) of the electron, with 2kB T /3 = 0.5m∗ vtherm and therefore vtherm = 4kB T /3m∗ . At room temperature in Si, the thermal e 2 e velocity is about 1.5 × 107 cm/sec, substantially higher than most velocities which will be induced by applied electric fields or by diffusion. Thermal velocities depend reciprocally on effective mass, with semiconductors having lower effective masses displaying higher thermal velocities than semiconductors with higher effective masses. At these high thermal velocities, the electron will have an average mean time τn between collisions with the scattering centers during which it is moving as a free electron in free space. It is during this period between collisions that an external field acts on the electron, creating a slight displacement of the orbit of the free electron. Upon colliding, the process starts again, producing again a slight displacement in the orbit of the free electron. This displacement divided by the mean free time τn between collisions represents the velocity component induced by the external electric field. In the absence of such an electric field, the electron would be scattered in random directions and display no net displacement in time. With then applied electric field, the 19
  20. electron has a net drift in the direction set by the field. For this reason, the induced velocity component is called the drift velocity and the thermal velocities can be ignored. By using the standard force equation F = eE = m∗ dv/dt with velocity v = 0 at time e t = 0 and with an acceleration time τn , the final velocity vf after time τn is then simply vf = eτn E/m∗ and, letting vdrif t = vf /2, vdrif t = eτn E/(2m∗ ). e e The drift velocity vdrif t in an external field E is seen above to vary as vdrif t = µn E, where the electron’s low field mobility µn is given approximately by µn ≈ eτn /(2m∗ ). Similarly, e holes are characterized by a low field mobility µp ≈ eτp /(2m∗ ), where τp is the mean time h between colisions for holes. This simplified mobility model yields a mobility which is inversely proportional to the effective mass. The effective electron masses in GaAs and Si are 0.09me and 0.26me , re- spectively, suggesting a higher electron mobility in GaAs than in Si (in fact, the electron mobility in GaAs is about 3 times greater than that in Si). The electron and hole effective masses in Si are 0.26me and 0.38me , respectively, suggesting a higher electron mobility than hole mobility in Si (in Si, µn ≈ 1400 cm2 /V · sec and µp ≈ 500 cm2 /V · sec). The simplified model for µn and µp above is based on the highly simplified model of the scattering con- ditions encountered by carriers moving with thermal energy. Far more complex analysis is necessary to obtain theoretical values of these mobilities. For this reason, the approximate model should be regarded as a guide to mobility variations among semiconductors and not as a predictive model. Table 8: Conductivity effective masses for common semiconductors Ge Si GaAs m∗ e 0.12 m0 0.26 m0 0.063 m0 m∗ h 0.23 m0 0.38 m0 0.53 Table 9: Mobility and temperature dependence at 300K. Mobility is in cm2 /V · sec. Adapted from [11]. Ge Si GaAs µn µp µn µp µn µp Mobility 3900 1900 1400 470 8000 340 Temperature dependence T −1.66 T −2.33 T −2.5 T −2.7 – T −2.3 The linear dependence of the mobility µn (µp ) on τn (τp ), suggested by the simplified development above, also provides a qualitative understanding of the mobility dependence on impurity doping and on temperature. As noted earlier, phonon scattering and ionized impurity scattering are the dominant mechanisms controlling the scattering of carriers in most semiconductors. At room temperature and for normally used impurity doping con- centrations, phonon scattering typically dominates ionized impurity scattering. As the tem- perature decreases, the thermal energy of the crystal atoms decreases, leading to a decrease 20
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