What Are Semiconductors.docx

Submitted by : Tania Shahid Roll no. 504 Submitted to: DR. noor-ul-ain Programme: Msc physics (3 ) rd

Subject: project work Government sadiq college women university bahawalpur

What are Semiconductors? We can define semiconductor as a substance, usually a solid chemical compound or element, that can conduct an electric current under certain conditions, making it a good medium for the control of electricity. Types of Semiconductors Semiconductors are crystalline or amorphous solids with distinct electrical characteristics. Their electrical resistance is high but lower than that of insulators. They are mainly two types of semiconductors:  

Intrinsic Semiconductors Extrinsic Semiconductors

What are Intrinsic Semiconductors? The semiconductor material which does not have any impurities is known as intrinsic semiconductor or pure semiconductors.

Silicon and Germanium, which belong to the fourth group element, behave like a semiconductor. Each atom of silicon and germanium share an electron with their neighbours. A Silicon atom and its neighbours share a pair of electrons in covalent bonding. Whenever a covalent bond break, an electron-hole pair is formed. To remove the valence electrons from the outer shells a semiconductor atom needs the energy of the order 1.1 eV. The vacancy in the covalent bond is called a hole. Any other electron can fill this hole. In other words, a hole shifts from one covalent bond to another. We can assume that the hole is a positive charge carrier since the direction of the hole is opposite to that of the electron. In an intrinsic semiconductor, electrons and holes move in random directions and the number of free electrons (ne) and holes (nh) remain same. What are Extrinsic Semiconductors?

The introduction of the extrinsic semiconductor is due to the excess holes or excess electrons present in silicon. Pure semiconductors are of no use as there are very few charge carriers which can cause conduction process. By adding some impurities to the pure semiconductor the conductivity can be improved. This process is called doping. Depending on the type of doping material used, extrinsic semiconductors can be classified as: 

N-type semiconductors  P-type semiconductors N-type Semiconductors:

The N-type semiconductor has a large number of electrons in the conduction band and less number of holes in the valence band, so electrons are called majority carriers and holes are called minority carriers. A pentavalent impurity such as phosphorus or arsenic is added to the silicon crystal. Out of five valence electrons, four silicon atoms take part in covalent bonding with one arsenic/phosphorus atom. The fifth electron is loosely bound to the silicon atom. Such a silicon crystal is still electrically neutral as the extra electron does not show up as an additional charge in the atom. P-type Semiconductors:

The P-type semiconductor has a large number of hole in the conduction band and less number of electrons in the valence band, so holes are called majority carriers and electrons are called minority carriers. A trivalent impurity such as Boron is mixed with the silicon atoms. Boron can share three valence electrons with the silicon atom; the boron atom takes one electron from nearby covalent bonds with the silicon atom in order to complete eight electrons in its valence shell. As the trivalent impurity atoms accept electrons from the silicon atom, it is known as an acceptor impurity. The p-type silicon crystal so obtained is called p-type extrinsic semiconductor and the holes created are extrinsic carriers. Properties of Semiconductors Semiconductors possess specific properties. A substance that does not conduct electricity is called an insulator and a substance that conducts electricity is called a conductor. Semiconductors are substances with properties somewhere between them. Some properties of semiconductors are given below.       

Electrical Conductivity Excited electrons High thermal conductivity Light emission Heterojunctions Thermal energy conversion Photo Conductivity

Examples of Semiconductors Semiconductors are very common and are found in almost all electronic devices. Some examples of semiconductor materials are selenium, germanium, and silicon. (1)

Semiconductor Band Gaps From the band theory , of solids we see that semiconductors have a band gap between the valence and conduction bands. The size of the band gap has implications for the types of applications that can be made. A low band gap implies higher intrinsic conduction, and a high band gap implies a larger possible photon energy associated with a transition across the gap in light emitting diodes. Band gaps in electron volts are given for a few semiconductor materials in the table below. Material Band gap in eV PbSe

0.27

PbTe

0.29

PbS

0.37

InN

0.67*

Ge

0.67

GaSb

0.7

Si

1.11

InP

1.35

GaAs

1.43

CdTe

1.58

AlSb

1.6

CdSe

1.73

AlAs

2.16

ZnTe

2.25

GaP

2.26

CdS

2.42

AlP

2.45

ZnSe

2.7

SiC

2.86

GaN

3.4

ZnS

3.6

Diamond

5.5

AlN

6.2*

(2) The band gap is an intrinsic property of all solids. The following image should serve as good springboard into the discussion of band gaps.

This is an atomic view of the bonding inside a solid (in this image, a metal). As we can see, each of the atoms has its own given number of energy levels, or the rings around the nuclei of each of the atoms. These energy levels are positions that electrons can occupy in an atom. In any solid, there are a vast number of atoms, and hence, a vast number of energy levels. In solids, these atoms are packed tightly with one another, and thus, the energy levels of those atoms will be packed tightly onto one another.

A principle necessary to understand the concept of band gaps is the principle of convergence. Consider a single atom, for example. Atoms have a given number of energy levels, however, as an atom has more and more energy levels, the spacing between those energy levels decreases. Thus, the energy levels of an atom are not evenly spaced like the rungs of a ladder, but instead, have an unequal spacing that decreases as an atom has more and more energy levels. Now, go back to the previous image. The prior explanation of convergence only regarded a single atom. However, quite evidently, we understand that there are hundreds of billions of atoms in any solid. All of those energy levels will interact and pack onto one another, forming what are called bands. As the number of energy gaps approaches infinity (which is appropriate to consider in the context of a solid), two important energy bands are formed, the conduction band and the valence band.

If you really want to observe how energy bands are formed from the interaction of many energy levels in a solid, run the PhET simulation titled "Band Structures" run by the University of Colorado at Boulder. Watch how the conduction and valence bands are formed as you add more and more atoms (represented as quantum wells) to the simulation. Here is the link: Band Structure When these two types of bands are produced, there is a space (a certain quantity of energy) between these two bands. This is the band gap. This is the region where no electron states can exist (they can only exist in either of the two bands). Simply put, the band gap is the region that separates the valence and conduction bands. But how might the concept of band gaps apply to how we can define conductors, insulators, and semiconductors? We know that conductors conduct electricity, insulators don't, and that semiconductors normally don't, but can when they are doped with another element and placed in junctions. This following diagram explains the relation between band gaps and classifications of solids.

So what does this diagram demonstrate to us? In the case of a conductor, the valence and conductions band overlap. In the case of an insulator, the valence and conduction bands are very far away from one another. Semiconductors, likewise, have a sort of mix between both the band properties of conductors and insulators. The band gap of a semiconductor is small unlike an insulator, but there is no overlapping of bands like in a conductor. The energy gap, especially in the context of solid-state electronics, represents the amount of energy needed to move electrons across from one band to another. Considering this, this explains why conductors so readily conduct electricity, while insulators don't. Conductors have no band gap, so it is easy for electrons to move from one band to another. Insulators have a large band gap, making it difficult to move electrons from one band to the other.

Semiconductors, when undoped, will not conduct electricity readily because it has a band gap. However, the presence of dopants will increase the conductivity of the semiconductor. (3)

Annealing Annealing, in metallurgy and materials science, is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time, and then cooling. In annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness. As the material cools it recrystallizes. For many alloys, including carbon steel, the crystal grain size and phase composition, which ultimately determine the material properties, are dependent on the heating, and cooling rate. Hot working or cold working after the annealing process alter the metal structure, so further heat treatments may be used to achieve the properties required. With knowledge of the composition and phase diagram, heat treatment can be used to adjust between harder and more brittle, to softer and more ductile. In the cases of copper, steel, silver, and brass, this process is performed by heating the material (generally until glowing) for a while and then slowly letting it cool to room temperature in still air. Copper, silver[1] and brass can be cooled slowly in air, or quickly by quenching in water, unlike ferrous metals, such as steel, which must be cooled slowly to anneal. In this fashion, the metal is softened and prepared for further work—such as shaping, stamping, or forming. (4)

Thermal annealing effects on vanadium pentoxide xerogel films ABSTRACT The effect of water molecules on the conductivity and electrochemical properties of vanadium pentoxide xerogel was studied in connection with changes of morphology upon thermal annealing at different temperatures. It was demonstrated that the conductivity was increased for the samples heated at 150oC and 270oC compared to the vanadium pentoxide xerogel. It was also verified a stabilization of electrochemical processes of the insertion and de-insertion of lithium ions the structure of thermally annealed vanadium pentoxide. Introduction Vanadium pentoxide xerogel can be produced by sol-gel process and exhibits a layered structure, which is suitable for intercalation chemistry with a variety of inorganic and organic species maintaining its basic structural integrity during the course of reactions [1-4]. In addition, the conductivity of vanadium pentoxide xerogel enables its utilization in many systems such as rechargeable cathodic material, electrochromic devices, and electrochemical sensors [5-7]. The conduction in this transition metal oxide can be explained by thermally activated electron hopping between metallic centers in different oxidation states: hopping of unpaired electrons between VIV and VV ions [8-11]. Moreover, such gels can also be considered as hydrated oxides,

and the ionic contribution to overall conduction arises from hopping of protons through the layered structure of vanadium pentoxide xerogel. Thus, the conduction within the xerogel is also determined by the intercalated water content [12-15]. Concerning the application of this material in secondary battery cathodes, vanadium pentoxide xerogel films allow the intercalation of lithium ions to maintain the electroneutrality of the system because electrons are also introduced during the intercalation reaction [6]. However, the process of lithium diffusion into the vanadium pentoxide structure is limited by solvent exchange [16], changes in volume and mechanical stress, as well as steric hindrance that induce the decrease of its charge-discharge capacity normally observed after some cycles, limiting the rechargeability [17]. In this context, the present work reports the effects of thermal annealing on electrochemical and conductivity properties of vanadium pentoxide xerogel films prepared by polymerizing decavanadic acid. In addition, the structural features as well as morphology of the samples were also invest Experimental Reagents and synthesis of V2O5.nH2O The vanadium pentoxide gel was prepared by ion exchange method as reported in the literature [1,8]. Sodium metavanadate (Fluka) was dissolved (4,25g) in 250 mL of deionized water and the resulting solution was eluted through an ion exchange column (H+ form, Dowex-50X). A pale yellow solution of polyvanadic acid was obtained and, after several days, a red polymerized V2O5.nH2O gel was formed by polycondensation at room temperature [1,8]. The films were obtained by the slow evaporation of the gel on a glass plate or on an ITO electrode at room temperature (25oC) and in air, leading to a xerogel. The thickness of the films ranged from 2.0 ìm to 5.0 ìm and it was estimated using the optical fringe interference method [18,19]. Thermal annealing was done in air at constant temperature (150oC, 270oC and 600oC) for 15 min. The compositions of the samples used in conductivity and electrochemical studies were determined by thermal analysis: 1- V2O5·2.1 H2O (without thermal treatment, sample A25); 2- V2O5·1.1 H2O (thermal treatment at 150ºC, sample A150); 3- V2O5·0.32 H2O (thermal treatment at 270ºC, sample A270); 4- V2O5 (thermal treatment at 600ºC, sample A600). Equipment and procedure The X-ray diffraction (XRD) data were recorded on a SIEMENS D5005 diffractometer using a graphite monochromator and CuKa emission lines. The samples, in film form, were obtained on a glass plate and the data were collected at room temperature over the range 2o £ 2q £ 50o. The transmission electronic spectra (Ultraviolet/Visible spectra) were recorded on a Varian Cary 50 spectrophotometer with the samples formed on a quartz plate (also used as the reference). Scanning electron microscopy (SEM) studies were carried out on a Zeiss DSM 940 microscope,

operating at 20 kV. Electron paramagnetic resonance (EPR) spectra were obtained at room temperature using a computer interfaced Varian E-4 spectrometer operating at 9.5 GHz (X band) and the sample (in film form) was placed in a quartz sample-holder. The infrared spectra were recorded from 2000 cm-1 to 400 cm-1 on a BOMEM MB-100 FT-IR spectrometer. The films were dispersed in KBr and pressed into pellets. The thermogravimetric data were registered on a Thermal Analyst equipment model 2100-TA in air atmosphere and at a heating rate of 10oC min1 . Dc conductivity was measured as a function of temperature from 150K to 350K. Measurements were done with samples in film form and were performed in an evacuated chamber using a DC bias of 1 V between silver electrodes. When the samples were thermally treated, the conductivity measurements were performed as soon as the samples left the furnace in order to avoid re-hydration. Electrochemical experiments were carried out with an AUTOLAB (EcoChemie) model PGSTAT30 (GPES/FRA) potentiostat/galvanostat interfaced to a computer. The conventional three-electrode arrangement was used, consisting of an ITO supporting electrode, a platinum wire auxiliary electrode and saturated calomel electrode (SCE) as reference electrode. A 0.1 moldm-3 solution of LiClO4 in acetonitrile was used as electrolyte. Experiments were carried out in deoxigenated solutions and at room temperature. Results and Discussion X-ray diffraction (XRD) patterns for vanadium pentoxide xerogel in the as grown and thermally annealed states are shown in Figure 1. For samples treated at 150oC and 270oC (Figures 1b and 1c), the diffraction patterns present peaks, which are broad and have low intensities, suggesting a decrease in crystallinity after thermal treatments. The XRD pattern for the sample treated at 600oC (Figure 1d) is characteristic of crystalline V2O5phase. The shifts of 001 diffraction lines to higher 2q values, shown in Figure 1, indicate a gradual decrease of the interlayer spacing: V2O5.2.1H2O - 1.28 nm; V2O5.1.1H2O - 1.21 nm; V2O5.0.32H2O - 1.14 nm; V2O5 - 0.42 nm. The decrease in interlamellar spacing is related to the release of intercalated water. For the sample A600, it was verified a complete dehydration process. In addition, it should be noted that even with the thermal treatment at 270oC, the vanadium pentoxide lamellar structure is preserved; probably because of the remaining strongly bonded water molecules that are involved in the formation of the polyoxovanadat network.

The infrared spectra of the V2O5 xerogel and of the samples submitted to thermal annealing are illustrated in Figure 2. The strong and characteristic peaks around 1012 cm-1, 763 cm-1 and a broad one at 515 cm-1 have been ascribed to the stretching vibration of the vanadyl group, the in plane and out-of plane V-O-V vibrational modes associated with the V-O bridges, respectively [20-22]. The FTIR spectra of the samples A25 and A150(Figures 2a and 2b) present a weak peak at 923 cm-1 and two shoulders at 715 cm-1 and 678 cm-1 that are not observed in the spectrum of crystalline V2O5 solid. These bands are likely to be related to water molecules bonded to the vanadium pentoxide polymeric chain. The weaker peaks (715 cm-1 and 678 cm-1) could be related to hydrogen bonding with oxygen atoms of vanadyl group and, the band at 923 cm-1 can be assigned to the VOH2 stretching vibrational mode indicating the formation of coordination bonds with vanadium atoms of vanadyl groups in the interlamellar domain [23,24]. These results are in agreement with XRD data, i.e., when the sample is heated the interlayer water is released resulting in a smaller interlamellar distance until a new phase is formed, anhydrous V2O5

Figure 3 shows scanning electron micrograph images of the V2O5 xerogel before and after the thermal annealing at different temperatures. The SEM image of the hydrated vanadium pentoxide matrix (Figure 3a) indicates the presence of a network of chains interconnected randomly [3,25]. An interesting point is that, despite the thermal treatment at 150oC and 270oC (Figures 3b and 3c, respectively), the samples retained the initial morphology of the V2O5 xerogel with no drastic distortions in the polymeric chains. On the other hand, the SEM image of the sample submitted at 600oC shows how the morphology changed dramatically from a network of chains to a non-continuous surface formed by stick-like microcrystallites with 20

mm - 30 mm of length.

Figure 4 shows the evolution of EPR spectra obtained at room temperature for the samples before and after thermal treatment at different temperatures. For the sample without thermal treatment (Figure 4a), the EPR spectrum shows the characteristic profile that arises from the unpaired 3d electron (V(IV), S=1/2, I=7/2) localized around vanadium centers that results in a hyperfine structure in an axially distorted crystal field [8,26]. After thermal treatment, a broadening of the signal can be observed up to 270oC (Figures 4b and 4c). This effect can be explained by the reduction of VV to VIV which enhances the spin-spin exchange interaction, i.e., as the content of VIV increases the hyperfine EPR interactions tend to disappear. By comparison with previous results reported in literature [24], it can be inferred that after the thermal treatment at 270oC the ratio of VIV/(VV + VIV) is at least 16%. It should be noted that for the highest annealing temperature no EPR signal was observed (Figure 4d), which is an indication of the oxidation of VIV centers during the crystallization process.

The electronic spectra of the vanadium pentoxide xerogel and of the samples submited to heating are shown in Figure 5. The UV/Vis spectra of the vanadium pentoxide xerogel and of the sample heated at 270oC shows an absorption band at 384 nm attributed to vanadium (V)-oxide charge transfer (CT) transition [4,8]. This transition intensity decreases upon thermal treating, suggesting that upon heating VV centers are reduced to VIV. This result in is agreement with that reported in the literature, i.e., dehydration of V2O5.nH2O gels leads to some reduction of vanadium ions [1,27]. Whereas, for the sample treated at 600oC, the intensity of CT band increased with a bathochromic shift, indicating the oxidation of vanadium VIV sites, in good agreement with the

EPR results. Thermal annealing affects the electrical conductivity, as shown in Figure 6. The conductivity increases upon thermal treatment up to 270oC: the room temperature conductivity changed from 2.23±0.04 x10-2 (W cm)-1 to 3.80±0.07 x10-2 (W cm)-1 for the sample treated at 270oC. The activation energy also changed from 0.29±0.03 eV in V2O5.2.1H2O to 0.24±0.02 in V2O5.0.32H2O. On the other hand, it was verified a decrease in conductivity for anhydrous V2O5 (4.07±0.08 x10-3 (W cm)-1). The changes observed in conductivity (Figure 6) may arise from different factors: from the small polaron model [28], the activation energy (W) of the

conductivity at higher temperatures reflects the contributions from the polaron binding energy, the structural disorder, as well as the transfer integral (the coupling potential between two hopping sites). In the case of V2O5 xerogel, it is especially difficult to separate these contributions to the activation energy, which can vary from 0.17 eV to 0.65 eV [29]. Higher activation energy is normally attributed to the disorder term. In our system, the disorder has indeed increased with the heating as shown by XRD data (Figure 1) and SEM images (Figure 3), which clearly does not explain the decrease in activation energy for samples heated by 270oC. Thus, assuming that the small polaron hopping model is valid, which may not be the case since our samples are cl. (5) Reference: 1. https://byjus.com/physics/semiconductors-and-insulators/. 2. http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/bandgap.html. 3. https://www.quora.com/Why-do-semiconductors-have-a-band-gap. 4. https://en.wikipedia.org/wiki/Annealing_(metallurgy). 5. http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-46702005000200001.