Electronic Band Structure

We know it that the band structure of any semiconductor is essential in determining its potential effectiveness. An accurate knowledge about the band structure of a semiconductor is quite critical for exploring its applications and even improving the performance. This is why, an accurate knowledge about the band structure of a semiconductor is vital for culturing its applications and even filtering the performance. Furthermore, optical measurements and band-structure calculations depend on each other for understanding electronic band structures in semiconductors[58]. Since both conduction and valence bands contribute significantly to the energy range where the optical excitations fall in, it is impossible to give a detailed interpretation of optical reflectance without at least a semiquantitative band-structure calculation first. In like manner, the reliability of these calculations in turn depends on the correct interpretation of certain key features in the optical data. Obviously, the process of determining band structure is one of trial and error, but it often leads to a consistent, quantitative, detailed picture of the band structure of semiconductors in a limited range of energy around the fundamental gap. Considering that ZnO is a candidate semiconductor for optoelectronic device applications, a clear understanding of the band structure is of critical importance in explaining the optical and electrical properties. The most important factor responsible for a material to show a better optoelectronic property is the large exciton binding energy and this property is possessed by Zinc oxide having binding energy of 60mev which could be attended at and above room temperature due to excitonic recombination . The process of optical absorption and emission have been influenced by bound excitons which are extrinsic transition related to dopants or defects thereby usually responsible for creating discrete electronic states in the band gap. Theoretically, neutral or charged donors and acceptors are the members by which exciton could be bound with and it merely depends on the band structure of semiconductor material.

Several theoretical approaches of varying degrees of complexity have been employed to calculate the band structure of wurtzite ZnO. Such as Green’s functional method [59], Local Density Approximation (LDA) [60-61], GW approximation (GWA) [62-63] and First-principles (FP) [64-65], have been employed to calculate the band structure of wurtzite ZnO. Besides, a number of experimental data have also been published regarding the band structure of the electronic states in wurtzite ZnO, [66-71]. For example, D. Vogel et al further improved the LDA method by incorporating atomic self interaction corrected pseudo potentials (SIC-PP), in which Zn 3d electrons had been accurately taken into account to calculate the electronic band structure of ZnO. The corresponding results have been shown in figure 2.3.

The horizontal dashed lines indicate the measured gap energy and d-band width. SIC-PP is much more efficient at treating the d-bands than the standard LDA method. [Reprinted with permission from [60], Copyright 1995 by the American Physical Society].

The band gap of ZnO has been determined from the standard LDA calculations and it is only ~3 eV, as shown in Figure 2.4(a). This shriveled band gap was obtained because we see that 3d states have been treated as core levels in order to simplify the calculations in the standard LDA method. According to the calculation results from SIC-PP method as shown in Figure 2.4(b), the bottom 10 bands (occurring around -9 eV) correspond to Zn 3d levels. The next 6 bands from -5 eV to 0 eV correspond to O 2p bonding states. The first two conduction band states are strongly Zn localized and correspond to empty Zn 3s levels. In contrast to the left panel, the d-bands are shifted down in energy considerably and alongside the gap is opened drastically. Furthermore, the dispersion and bandwidth of the O 2p valence bands are changed significantly. The gap energy and the d-band position are grossly improved as compared to the standard LDA results. The band gap as determined from this calculation is 3.77 eV, which correlates reasonably well with the experimental value of 3.4 eV. Therefore, we can see that the band gap energy and d-band position have been significantly improved as compared to the standard LDA results.

As, both conduction and valence bands contribute considerably to the energy range where the optical excitations fall in, it is not possible to give a detailed analysis of optical reflectance without at least a semi quantitative band-structure calculation first. UV reflection/absorption or emission techniques can be used to measure the electronic core levels in solids. By applying these methods , we can measure the energy difference between the upper valence band states and the bottom conduction-band states.

D.W. Langer and C. J. Vesely used the x-ray induced photoemission measurement to determine the energy levels of core electrons in ZnO in 1970 [74]. In 1971, R. A. Powell et al carried out uv photoemission measurements on hexagonal ZnO cleaved in vaccum [75] . The results showed that the Zn 3d core level located at 7.5±0.2 eV below the valence-band maximum, which was ~3 eV lower than predicted by Rössler’s Green’s function band calculation. But this assignment is in excellent agreement with the result (7.6 eV) of previous x-ray photoemission measurements. Subsequently, in 1974, L. Ley at al presented the total valence-band x-ray photoemission spectra of 14 semiconductors including the hexagonal ZnO [76]. The results strongly proved that band-structure calculations in combination with x-ray photoemission spectra provide a powerful approach to establishing the total valence-band structure of semiconductors. Until now, some groups still used X-ray photoemission spectroscopy to investigate the band structure of ZnO. So far, the coherence between theoretical calculation and experiments for energy band structure has already been reached for a great number of semiconductors, including ZnO of course, since excellent and detailed optical data has been available in many cases.

Optical properties of semiconductor

The first study about the optical properties of ZnO started many decades ago and it was around [77-95]. The new interest in ZnO is increased by its prospects in optoelectronics applications owing to its direct wide band gap of 3.37 eV at room temperature and it’s large exciton energy of 60 meV as well as it’s efficient radiative recombination. The strong exciton binding energy, which is much larger than that of GaN (25 meV), and the thermal energy at room temperature (25 meV) can ensure an efficient exciton emission at room temperature under low excitation energy. As a result, ZnO is familiar as a promising photonic material in the blue-UV region. Along with, we known that the optical properties of ZnO contain a lot of information, such as optical absorption, transmission, reflection, photoluminescence and so on.

ZnO nano materials has wide applications in the optical field, ZnO nanorods can be used in lasers for fast optical pumping and hence produce high power laser beams. In the near ultra violet and in visible regions, due to its wide band gap character, ZnO material displays luminescent properties. The Photoluminescence (PL) spectra of ZnO structures have been widely reported. Excitonic emissions have been observed from the PL spectra of ZnO materials. It has been also observed that on the confinement of quantum size of ZnO nanomaterials, the exciton binding energy increases but it has also been observed at the same time that the intensity of green emission increases very much due the decrease in the diameter of the ZnO nanorod diameters. This is due to large ratio of surface to volume factor of nanowires favoring higher level of defects and surface combinations. Quantum confinement was also reported to be responsible for blue shift in the near UV emission peak in the ZnO nanobelts. Other application include and are not limited to optical fibres, solar cells , surface acoustic devices etc.

Zinc oxide is generally crystal clear to visible light but strongly absorbs ultra violet light below 3655 A. The absorption is usually stronger than other white pigments. In the region of visible wavelengths, regular zinc oxide appears white, but, rutile and anatase titanium dioxide have a higher reactive index and thus has a superior opacity. The band gap energy (between valence and conducting bands) is 3.2 eV, this corresponds to the energy of 3655 A photons. Under ultra violet light zinc oxide is photoconductive. The combination of optical and semiconductor properties make doped zinc oxide a contender for new generations of devices. Solar cells require a transparent conductive coating, indium tin oxide and zinc oxide(doped) are the best materials. Intrinsic optical properties of ZnO nanostructures are being intensively studied for implementing photonic devices. Photoluminescence (PL) spectra of ZnO nanostructures have been extensively reported. Excitonic emissions have been observed from the photoluminescence spectra of ZnO nanorods. It is shown that quantum size confinement can significantly enhance the exciton binding energy. Strong emission peak at 380 nm due to band-to-band transition and green-yellow emission band related to oxygen vacancy are observed. PL spectra show that ZnO nanowire is a promising material for UV emission, while its UV lasing property is of more significance and interest. Due to its near-cylindrical geometry and large refractive index (˜2.0), ZnO nanowire/nanorod is a natural candidate for optical waveguide. The additional advantages of ZnO nanowire lasers are that the excitonic recombination lowers the threshold of lasing, and quantum confinement yields a substantial density of states at the band edges and enhances radiative efficiency. Optical wave guiding using dielectric nanowire also achieved considerable progress. Recently, ZnO nanowires were reported as sub-wavelength optical waveguide. Optically pumped light emission was guided by ZnO nanowire and coupled into SnO2 nanoribbon. These findings show that ZnO nanostructures can be potential building blocks for integrated optoelectronic circuits.

Photoluminescence properties of ZnO

Luminescence is a non-equilibrium process which requires external excitation and light emission through any process other than blackbody radiation can be called luminescence. Based on the excitation source, luminescence is referred to either as photoluminescence (PL) (caused by absorption of photons), electroluminescence (EL) (caused by electric current), cathodoluminescence (CL) (caused by an electron beam), chemoluminescence (caused by chemical reactions) or thermoluminescence (caused by heat).

Basic principles of PL and the possible emission lines in ZnO are described below. Luminescence in semiconductors is the direct result of electron transitions from higher to lower energy levels. Figure 2.7 shows the simplified band structure of a semiconductor near the centre of the first Brillouin zone, where a material with band gap energy Eg is irradiated by a laser with energy hν>Eg, resulting in the excitation of an electron into the conduction band (arrow 1) and leaving a hole behind in the valence band. An electron-hole (e-h) pair is thus generated. The electrons and holes thermalize to the lowest energy state of their respective bands via phonon emission (shown by the red-wavy arrows) before recombining (arrow 2) across the fundamental band gap or the defect levels within the band gap and emitting photons of the corresponding energies in two basic mechanism.

Mechanical properties

ZnO is a relatively very soft material with approximate hardness just 4.5. Its elastic constants are relatively smaller than those of other III-V semiconductors, e.g. GaN. The high heat capacity and high heat conductivity, low values of thermal expansion and high melting points are some of the characteristics of ZnO. ZnO has been proposed to be a more promising UV emitting phosphor than GaN because of its larger exciton binding energy (60 meV). Among the semiconductors bonded tetrahedrally, its found that ZnO has the highest piezoelectric tensor. This makes it an important material for many piezoelectric applications, which require a high degree of electromechanical coupling among them. Its high heat capacity and heat conductivity with low thermal expansion and high melting temperatures makes it beneficial for ceramics. It has one of the highest piezoelectric tensor compared to other III-V semiconductors such as GaN and An. This property makes it important for many piezoelctircal applications, which require a large electromechanical coupling.

Electrical Properties

The fundamental study of the electrical properties of ZnO nanostructures is crucial for developing their future applications in nanoelectronics. ZnO has a quite large band gap. The advantages of a large band gap include higher values of breakdown voltages, sustaining large electric fields, high-temperature and high-power operations. ZnO has n-type character, in the absence of doping. Non-stoichiometry is usually the origin of n-type character. Due to defects such as oxygen vacancies and zinc interstitials, ZnO nanowires are reportedly show n-type semiconductor behavior. The main impediment of ZnO for wide-ranging applications in electronics and photonics rests with the difficulty of p-type doping. Successful p-type doping for ZnO nanostructures will greatly enhance their future applications in nanoscale electronics and optoelectronics. P-type and n-type ZnO nanowires can serve as p-n junction diodes and light emitting diodes (LED).

Having a relatively large direct band gap of ~3.7 eV and large excitation binding energy of 60 meV compared to themal energy which is of the order of ~ 26 meV at room temperature. Due to this very important property of large band gap, it has an advantage to sustain at large electric fiels, and can have lower electronic noise and will operate at high temperatures. Usually most of the ZnO materials are of n- type character, p-type doping of ZnO is relatively difficult. But these limitations of p-type doping does not restrict the applications of ZnO in electronic and optoelectronic cases which usually require junctions of n-type and p-type materials.

Chemical Properties

ZnO occurs as white powder known as zinc white. It is usually orange or red in color due to manganese impurity.. The mineral usually contains a certain amount of manganese and other elements and is of yellow to red color.

Crystalline zinc oxide is thermochromic, which changes from white to yellow colour when heated and reverting to white colour on cooling. This change in colour is caused by a very small loss of oxygen at high temperatures.

Zinc oxide is amphoteric, that is it reacts with both acids and alkalis. With acid it reacts to form familiar compound such as zinc sulfate. With alkali it forms zincates.

ZnO + 2 HCl → ZnCl2 + H2O

ZnO + 2 NaOH + H2O → Na2(Zn(OH)4)

ZnO reacts slowly with fatty acids in oils to produce the corresponding carboxylates, such as oleate or stearate. ZnO forms cement-like products when mixed with a strong aqueous solution of zinc chloride and these are best described as zinc hydroxy chlorides. This cement was used in dentistry.

ZnO decomposes to form zinc vapor and oxygen at about 1975 ˚C, indicating its considerable stability. Heating with carbon converts ZnO into Zn, which is more volatile.

ZnO + C → Zn + CO

ZnO also forms cement-like products when reacted with phosphoric acid, and this forms the basis of zinc phosphate cements used in dentistry. A major component of zinc phosphate cement produced by this reaction is hoplites, Zn3(PO4)2·4H2O.

ZnO decomposes into zinc vapor and oxygen only at around 1975 °C, reflecting its considerable stability. Heating with carbon converts the oxide into zinc vapor:

ZnO + C → Zn + CO

The following reaction is extremely important in zinc pyro metallurgy :

ZnO + CO → Zn + CO2

Zinc oxide reacts violently with aluminum and magnesium powders, with chlorinated rubber and linseed oil on heating causing fire and explosion hazard.

It reacts with hydrogen sulfide to give the sulfide: this reaction is used commercially in removing H2S using ZnO powder (e.g., as deodorant).

ZnO + H2S → ZnS + H2O

When ointments containing ZnO and water are melted and exposed to ultraviolet light, hydrogen peroxide is produced.

Commercial zinc oxide shows a measurable but low level of water solubility, 0.005 g/litre.

Zinc oxide exposed to air absorbs both water vapour and carbon dioxide.