Influence of annealing temperatures on nonlinear optical dielectrical semiconducting results and Fermi Essay

Influence of annealing temperatures on nonlinear optical, dielectrical, semiconducting results and Fermi level position for CdP0.03Te0.97 thin filmAbstractCdP0.03Te0.97 thin films were deposited at room using thermal evaporation and annealed at 100 and 200°C. The effect of annealing temperature (Tann) on both of dispersion energy (Ed) and oscillating energy (Eo) were studied. The lattice dielectric constant (µL) and free carrier concentration/effective mass (N/m*) were calculated for these samples. The values of first order of moment (M-1), the third order of moment (M-3) and static refractive index (no) were determined.

The both of dielectric loss (µ) and dielectric tangent loss (µ\) for these films increased with photon energy (hЅ) and had a highest value higher than the energy gap (Eg). Also, the same behavior was noticed for the real part of optical conductivity (1) and imaginary part of optical conductivity (2), the relation between Volume Energy Loss (VEL) and Surface Energy Loss (SEL) (VEL/SEL) were estimated. The linear optical susceptibility ((1)) increases with (Tann).

The nonlinear optical parameters such as, nonlinear refractive index (n2), the third-order nonlinear optical susceptibility ((3)), non-linear absorption coefficient (Іc) , were calculated theoritically. Both of the electrical susceptibility (e) and relative permittivity (µr) increase with (Tann) and had a highest value higher than energy gap. The semiconducting results such as, density of the valence band, conduction band and Fermi level position (Ef) were calculated.Key words: CdP0.03Te0.97 thin film, thermal annealing, dielectric results, linear optical susceptibility, nonlinear optical parameters and semiconducting results. 1. IntroductionCdTe is II”VI crystalline compound with zinc blende crystal structure and has direct band gap of 1.44 eV[1] which is suitable for electronic applications such as photovoltaic devices [2] light emitting diodes [3,4] solar cells [5,6] X-ray and gamma detectors [7] Field Effect Transistors (FETS) [8] Lasers [9] and nonlinear integrated optical devices[10]. The structure of CdTe thin films were carried out by many authors [11-15] it was found that CdTe thin films had a polycrystalline structure [11-12] while the annealing process increases the crystallinity of CdTe films [15, 16]. The optical properties of CdTe thin films were studied [17-23] it was found that the gap values were (1.44-1.60 eV)[17] (1.45-1.52 eV)[18] 1.534 eV [19] and (1.43- 1.50 eV)[21] and also CdTe had a high absorption coefficient (104cm-1) [22,23]. The annealing effect on optical properties were investigated [24-28] it was found that the annealing process decreases energy gap of CdTe[27] and reduces both of transmission and reflection for CdTe [28]. On the other hand the doping effect on the optical properties of CdTe thin films were studied [29-35]. It was found that Sn dopant increases absorption coefficient [29] Al, Sb dopant ratio decreases energy gap[31] band gap increases, refractive index and extinction coefficient decrease with increasing Ni ratio in Cd1€’xNixTe films[34]. The transmittance spectra decrease by addition Cu [35]. The increase in band gap energy and the effect of annealing temperature on structure and optical properties of CdPTe thin films were studied [36] it was found that the optical energy gap increases with annealing temperature. In this work we investigate the annealing temperatures effect on dilectrical, semiconducting results, Fermi level position, first order optical susceptibility and nonlinear optical results for CdTe thin film doped with phosphorus. 3. Results and discussions3.1. Dielectric, optical conductivity and linear optical susceptibility results The structure of these films with different annealing temperatures is illustrated in previous work [36]. The optical transmittance (T) and reflectance (R) were measured and discussed in previous work [36]. The single oscillator theory was expressed by Wemple”DiDomenico relationship [37]: (1)Where n is the refractive index values of these samples which is determined in previous work [36] E is the photon energy. The values of Eo and Ed for all samples are shown in table 1. Fig. 1 shows the relation between (n2) and (“2) to determine the ratio of carrier concentration to effective mass (N/m*) using the following equation [38]: (2)Where µL is lattice dielectric constant, µo is permittivity of free space, e is the charge of electron, n and k are linear refractive index and absorption index of these films respectively which were determined in previous work [36] N is the free carrier concentration for CdP0.03Te0.97 film with different annealing temperatures (Tann) and c is the speed of light so the values of (N/m*) are shown in table 1. From this table it was noticed that the (Tann) affected on the ratio (N/m*) the high annealing temperature, the access of electrons. The values of the first order of moment (M-1) and the third order of moment (M-3) derived from the relations [38]: (3) (4)Table 1 shows the values of M-1 and M-3 for these thin films. The oscillator strength f which was calculated as follow [39]: (5)The values of f are shown in table 1. Another important parameter depending on both of Eo and Ed is that static refractive index no which was determined using following equation [40]: (6)The values of no for all these samples are shown in table 1. Fig.2 represents the dependence of (n2-1)-1 on (hЅ) for these thin films. From this figure it was seen that all these samples had the same behavior and the values of (n2-1)-1 increased with (Tann). The dielectric loss (µ) and dielectric tangent loss (µ\) for these films were calculated as follow [41]: (7) (8) Figs. 3(a,b) show the relation between both of (µ) and (µ\) and (hЅ) for these films. From this figure it was seen that at energy less that (Eg) the values of both of (µ) and (µ\) decreased with (Tann) for all studied samples and the peak maximum values position decreased with increasing (Tann) this is due to the increasing of electron motilities with annealing temperatures. The optical conductivity was calculated from the following equations [42]: (9) (10) Figs 4(a,b) show the dependence of (1) and (2) on (hЅ) for these films. The behavior of both (1) and (2) for all these studied films is the same with (hЅ) and increase with (hЅ) for all these samples. The values of Volume Energy Loss Function (VELF) and Surface Energy Loss Function (SELF) for these films were determined optically as follow [38]: (11) (12)The relation between VELF/SELF for these thin films is shown in Fig. 5. Linear optical susceptibility ((1)) describes the response of the material to an optical wave length, ((1)) was determined using the following relation [43]: (13) The relation between ((1)) and (hЅ) for CdP0.03Te0.97 thin film with different annealing temperatures is shown in Fig.6. From this figure it was seen that the linear optical susceptibility ((1)) increased with (Tann) and the values of ((1)) had a maximum values higher than (Eg) this means that there is a possibility of wide change in optical properties by a thermal annealing.3.2. Nonlinear optical propertiesAn important parameter of the non-linear optical parameters is that the nonlinear refractive index (n2) which can be explained as when light with high intensity propagates through a medium this causes nonlinear effects[44] n2 was determined from the following simple equation [45-46]: (14)The dependence of n2 on wavelength for CdP0.03Te0.97 thin film with different annealing temperatures is shown in Fig. 7. The values of n2 decrease with wavelength for all these studied samples and also n2 values decrease with (Tann); this is due to the increase of transmittance with (Tann) [36] which leads to decrease of propagated light. An important parameter to assess the degree of nonlinearities is the third-order nonlinear optical susceptibility ((3)) which was determined using the following equation [47]: (15)Where A is a quantity that is assumed to be frequency independentand nearly the same for all materials =1.7 x 10-10 e.s.u [45]. The dependance of ((3) ) on (hЅ) for CdP0.03Te0.97 thin film with different (Tann) is shown in Fig.8. It was noticed that the behavior of ((3)) is the same for all studied samples the values of ((3)) increase with (hЅ) this is due to when (hЅ) increases the defliction of the incident ligth beam increases but the values of ((3)) decrease with (Tann) this could attributd to the variation of free carrier concentration which leads to the increase of electrons mobility with (Tann). On the other hand another important nonlinear parameter such was non-linear absorption coefficient (Іc) which determined as follows [48]: (16)Fig. 9 shows the influence of hЅ on (Іc). It is observed that the values of (Іc) increase with (hЅ) for all these samples as shown in Fig. 9. Because of the higher values of (hЅ) the large number number of excited electron which overcome the band gap.3.3. Electrical results Electrical susceptibility ((e)) was determined using the following relation [49]: (17) Fig. 10 shows ((e)) vs. (hЅ) of these investigated samples. From this Fig. it is clear that the values of ((e)) increase with (Tann) this is due to the increasing of electron mobility. The relative permittivity (µr) was calculated using the following relation [50]: (18) The relation between relative permittivity (µr) and wavelength for CdP0.03Te0.97 thin film with different (Tann) is shown in Fig.11. It is clear that the values of (µr) increase with (hЅ) for all these samples; this could be attributed to the increasing of electron mobility with (hЅ).3.4. Semiconducting and electronic results The density of states (DOS) of a system describes the number of states per interval of energy at each energy level available to be occupied. Nv and Nc play very important rule of examination the linear optical transition and non-linear optical properties. The Nv and Nc are calculated as follow [51]: (19) (20)Where Nv and Nc are the density of states for both valence and conduction bands respectively effective mass of electrons in (CdTe) m*e = 0.11 [52] effective mass of holes in (CdTe) m*h = 0.18 [52] effective mass of holes in (CdP) m*h = 0.373 [53] effective mass of holes in (CdP) m*h = 0.128[53] and K is a Boltzmann constant. The determined values for both Nv and Nc are shown in table 1. Another important factor was determined theoretically is the position of Fermi level [47]: (21)The values of Fermi level position for these investigated thin films are shown in table 1.4. Conclusion The values of Ed and Eo increased with (Tann) for CdP0.03Te 0.97 (Ed from 6.50 to 7.10 eV) and also Eo had the values from (3.4 to 3.75 eV). The values of (N/m*) increased with (Tann) which increased free carrier. The values of M-1 and M-3 also increased with increasing (Tann). The both of (µ) and (µ\) increased with (hЅ) the maximum values decreased with increasing (Tann) due to the increase of electron mobility’s with increasing (Tann). Both of (1) and (2) decreased with (Tann). ((1)) increased with (Tann) due to increase carrier concentration. The values of (n2) increased with (“) for all these samples while ((3)) increased with (hЅ). This means that these samples had a high ability to changing its optical properties by changing wavelength and applied field. The non-linear absorption coefficient (Іc) decreased with (Tann) while both of ((e)) and (µr) increase with (Tann) and had a highest value higher than the energy gap. The annealing temperatures affected on the values of both of Nv and Nc while Ef affected slightly with (Tann). Table 1: The determined values of CdP0.03Te 0.97 film with different annealing temperatures such as lattice dielectric constant µL, Oscillation energy Eo, Dispersion energy Ed, first order of moment M-1, third order of moment M-3, Field strength (f), static refractive index (no), (N/m*), density of conduction band Nc (cm-3), density of valence band Nv (cm-3) and Fermi level position .Fermi Level Position (eV) NV NC N/m* no Field strength (f) (eV)2 M-3 (eV) M-1 (eV) Dispersion energy Ed (eV) Oscillation energy Eo (eV) lattice dielectric constant µL Sample0.11 1.1E+22 2.2E+22 2.1E+49 1.70 22.10 2.55 4.70 6.50 3.40 16.00 As-deposited0.15 1.7E+22 3.5E+22 2.8E+49 1.69 24.48 2.61 4.95 6.80 3.60 18.00 Annealed at 100 oC0.19 2.4E+22 5.0E+22 1.5E+50 1.71 26.63 2.66 5.16 7.10 3.75 21.50 Annealed at 200 oC