"Influence of Individual Atoms in Binary and Ternary Semiconductors

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1 Volume 119 No , ISSN: (on-line version) url: ijpam.eu "Influence of Individual Atoms in Binary and Ternary Semiconductors Abstract: R.VELAVAN 1,MYVIZHI.P 2, 1 Professor, 2 AssistantProfessor,Dept. of Physics, 2 Department of Bioinformatics, 1 BIST, BIHER, Bharath university,chennai-73, 1 velavan.phy@bharathuniv.ac.in On the basis of the band structure, crystals can be classified into three types metals, insulators and semiconductors. Introduction: A band which is completely full carriers no electric current, even in the presence of an electric field. It follows therefore that a solid behaves as a metal only when some of the bands are partially occupied. The sodium (Na) for example since the inner bands 1s, 2s, 2p are fully occupied, they do not contribute to the current. Therefore concern only with the topmost occupied band, the valence band[1-9]. In Na, this is 3s band. This band can accommodate 2Nc electrons, where Ncis the total number of primitive unit cells. Now in Na, a Bravais lattice, each cell has one atom, which contributes one valence (or 3s) electron. Therefore the total number of valence electrons is Nc, and as these electrons occupy the band, only half of it is filled. Thus sodium behaves like a metal because its valence band is only partially filled [10-16]. In a similar fashion, the other alkalis, Li, K, etc., are also metals because their valence bands the 2s, 4s, etc., respectively are only partiallyfull. In some crystalline solids, the forbidden energy gap between the valence band and the conduction band is very large. Diamond is a good example of an insulator. Here the top band originates from a hybridization of the 2s and 2p atomic states, which gives rise to two bands split by an energy gap[17-21]. Since these bands arise from s and p states, and since the unit cell here contains two atoms, each of these bands can accommodate 8Nc electrons. Now in diamond each atom contributes 4 electrons, resulting in 8 valence electrons per cell [22-29]]. The gap in diamond is about 7 ev. Thus valence band here is completely full, and the substance is an insulator,as stated above. There are substance which fall in an intermediate position between metals and insulators. If the gap between the valence band and conduction band is small, then electrons are readily excitable thermally from the former to latter band. Both bands become only partially filled and both contribute to the electric conduction. Such a substance is known as a semiconductor [30-36]]. Examples are Silicon (Si) and Germanium (Ge), in which the gaps

2 are about 1 and 0.7 ev, respectively. The energy band diagram of a metal, an insulator and a semiconductor is given in fig.1.1. Figure.1.1 Energy band diagram of a metal, An insulator and asemiconductor. A semiconductor is a material which has electrical conductivity between that of a conductor such as copper and an insulator such as glass. The conductivity of a semiconductor increases with increasing temperature[37-45], behaving opposite to that of a metal. Also it has resistivity between conductors and insulators. The resistivity of a semiconductor lies approximately between 10-2 and 10 4 m at room temperature. The resistance of a semiconductor decreases with increase in temperature over a particular temperaturerange. Current conduction in a semiconductor occurs via free electron and holes [46-50], which are collectively known as charge carriers. As temperature is increased, electrons are thermally excited from the valence band to the conduction band. Both the electrons in the conduction band and the holes left behind in the valence band contribute to the electrical conductivity. CLASSIFICATION OFSEMICONDUCTORS:

3 The conductivity of semiconductors may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity or dopant added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are referred to as extrinsic semiconductors. By adding impurity to pure semiconductors, the electrical conductivity may be varied by factors of thousands or millions.the materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. Semiconductor doped with donor impurities are called n-type (when a doped semiconductor contains excess free electrons it is known as n-type semiconductor), while those doped with acceptor impurities are known as p-type semiconductor (when a doped semiconductor contains excess holes it is called p-type semiconductor).they are classified as shown in the fig.1.2. Intrinsicsemiconductors: An intrinsic semiconductor is a pure semiconductor without any significant dopant species present. The electrical conductivity of intrinsic semiconductors can be due to crystallographic defects or electron excitation. In an intrinsic semiconductor the number of electrons in the conduction band is equal to the number of holes in the valence band, n = p. Example for intrinsic semiconductorsare Silicon (Si) and Germanium (Ge). Semiconductor Intrinsic Extrinsic N-type P-type

4 Figure1.2 Classification ofsemiconductor Extrinsicsemiconductors: Doped semiconductors are referred to as extrinsic semiconductors. Doping alters the electrical properties of the semiconductor and improves its conductivity. Doping process produces the following two types of semiconductors N-typesemiconductors P-typesemiconductors Depending upon the type of impurity atoms added, an extrinsic semiconductor can be classified as N-type or P-type as shown in fig.1.3. N-typesemiconductors: The addition of pentavalent impurities such as Antimony (Sb), Arsenic (As) or Phosphorous (P) contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. P-typesemiconductors: The addition of trivalent impurities such as Boron (B), Aluminium (Al) or Gallium (Ga) to an intrinsic semiconductor creates deficiencies of valence electrons, called holes

5 N-type P-type Figure1.3 Classification of extrinsicsemiconductor DIRECT & INDIRECT BANDGAP SEMICONDUCTORS: Direct bandsemiconductors: In direct band gap semiconductors, the minimum of the conduction band and the maximum of the valence band lie along the same k point as in fig.1.4, where k is the wave vector of an electron in momentum space. A direct optical transition isdrawn vertically with no significant change of, because the absorbed photon has a very small wave vector. The threshold frequency of the absorbed photon is. The direct transition determines the energy gap. Indirect band gapsemiconductors: In indirect band gap semiconductor, indirect transition involves both a photon and a phonon because the band edges of the conduction and valence bandsare widely separated in space as in fig.1.4. The threshold energy for the indirect process in figure is greater than the true band gap. The absorption threshold for the indirect transition between the band edges is at, where is the frequency of an emitted phonon. At higher temperature phononsare alreadypresent [12] ;ifaphononisabsorbedalongwithaphoton,thethresholdenergy. Note: The figure shows only the threshold transitions. Transitions occur generally between almost all points of the two bands for which the wave vectors and energy can be conserved [12]

6 Figure1.4 Direct and Indirect band gapsemiconductors INTRODUCTIONOFELEMENTALANDCOMPOUND SEMICONDUCTORS: Semiconductors include a large number of substances of widely different chemical and physical properties. These are grouped into several classes of similar behavior, the classification being on the position in the periodic table of the elements [11]. Elemental Semiconductors: These are single element semiconductors, from the group IV of the periodic table: Silicon (Si), Germanium (Ge). The elemental semiconductors all crystallize in the diamond structure. The diamond structure has an fcc lattice with a basis composed of two identical atoms, and is such that each atom is surrounded by four neighboring atoms, forming a regular tetrahedron gives a planar view of this coordinational environment in Si, with three dimensional tetrahedron projected on a plane [11]. The tetrahedron bond in Si as shown infig

7 Figure1.5The tetrahedron bond insilicon Compound Semiconductors: A compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table. These semiconductors are typically from group III V, for example elements from group III- Boron, Aluminium, Gallium, Indium and from group V- Nitrogen, Phosphorous, Arsenic, Antimony and Bismuth. The range of possible formula is quite broad because these elements can form binary (two elements, e.g., GaAs), ternary (three elements, e.g., InGaAs) and quaternary (four elements, e.g., AlInGaP) alloys. Compound semiconductors offer high performance (optical characteristics, higher frequency, and higher power) than elemental semiconductors and greater device design flexibility due to mixing of materials. Compound semiconductors allow us to perform Band gap Engineering by changing the energy band gap as a function of position. Binary Semiconductors: A semiconductor compound consisting of two elements is known as binary semiconductor. Some of the examples of binary semiconductors are SiC, GaAs, andcds. IV III V II VI These are formed from group III and V or II and VI elements of the periodic table. These compounds crystallize mainly into two structures, Zincblende and Wurtzite. Though

8 the bonding is more ionic, it retains much of the properties of the covalent bond which is characterized by high strength and stability. The Zincblende and Wurtzite structure of the GalliumNitride (GaN) compound is given in fig.1.6,1.7. Figure 1.6 Crystal structure of GaN Figure1.7 Crystal structure ofgan Ternary Semiconductors: A semiconductor compound consisting of three elements is known as Ternary

9 semiconductors. Ternary compounds can be formed by one of the following methods IV (Si) II - VI (Zn) (Se) I II VI2 (Cu) (In) (Se2) (Chalcopyrites) Chalcopyrit e Ternary chalcopyrite semiconductor ABC2 is isoelectronic analogues of the II-VI binaries. The possibility of semi conductivity in I-III-VI2 compounds can be obtained by ordered substitutions of groups I and III for group II according to Grimm-Sommerfeld rule, i.e., there must be an average of four valence atoms per atomic site. Some of the ternary chalcopyrites are CuInSe2, CuGaS2, and AgInS2 etc. The example of chalcopyrite crystal structure is shown in fig.1.8. Figure1.8 Crystal structure of CuInSe2chalcopyrite

10 IV (Si) III - V (Ga) (As) II IV V2 (Zn) (Si) (P2) (P- nictides) P-nictides Ternary p-nictides semiconductor ABC2 is isoelectronic analogues of the III-V binaries. The possibility of semi conductivity in II-IV-V2 compounds can be obtained by ordered substitutions of groups II and IV for group III according to Grimm-Sommerfield rule, i.e., there must be an average of four valence atoms per atomic site. Some of the ternary pnictides are ZnSiP2, CdGeP2, and CdSnAs2 etc. The example of pnictides crystal structure is shown in fig.1.9. Figure1.9 Crystal structure ofznsnp2pnictide. REFERENCE 1. Ramamoorthy, R., Kanagasabai, V., Kausalya, R., Impact of celebrities' image on brand,, V-116, I-18 Special Issue, PP ,

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