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The experimental thicknesses of AD-ZnTe layers were obtained by using Microfocus Optical Thickness Profilometer measurement system while the theoretical thicknesses

30 40 50 60 70

1500 1550 1600 1650 1700

Atomic Composition (%)

Cathodic Potential (mV)

% of Zn atom

% of Te atom

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were estimated using Faraday's law of electrolysis. Thickness (T) of the layer in cm is given by Equation (5.3).

 nF

T  JtM (5.3)

where M is the molecular weight of ZnTe thin film (193.01 gmol^-1), t is the growth time in seconds, J is the average current density observed during deposition in Acm^-2, F is Faraday’s constant (λθ48η Cmol^-1), ρ is the density of ZnTe (6.34 gcm^-3) and n is the total number of electrons required in the deposition of 1 mole of ZnTe (n=6 as given by Equations (5.1) and (5.2)).

The samples used for this measurement were grown at 1600 mV for different duration (0.5–4.0 hours). This experiment was carried out in a ZnTe electrolyte containing excess Te. The composition of the ZnTe bath contains 0.015 M ZnSO4.7H2O and 10 ml of dissolved TeO2 in 800 ml of de-ionised water. The experimentally measured values and theoretically estimated values are shown in Figure 5.13. As expected, the thickness of AD-ZnTe layers increase with increase in deposition time. As illustrated in Figure 5.13, an approximate linear variation of thickness with growth time was observed in both theoretical and experimental curves. The theoretically estimated thickness is generally higher than the experimentally measured thickness because not all the electronic charges used in the theoretical estimation are actually utilised in the deposition of ZnTe thin films. Some of these charges flow through the electrolyte and are used for electrolysis of water thus making the experimentally measured thicknesses to be less than the theoretically estimated values.

Figure 5.13. Experimental and theoretical estimation of thickness of as-deposited ZnTe layers as a function of deposition time.

-100 400 900 1400 1900 2400

0 1 2 3 4 5

Thickness (nm)

Time (hours) Theoretical

Experimental

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5.4.1 Effect of thickness on electrical properties of ZnTe layers

The electrical properties of ZnTe layers grown at 1600 mV was studied at different thicknesses using techniques such as PEC cell measurement, I-V and C-V.

5.4.1.1 Thickness effect on PEC signals

PEC cell measurements were carried out so as to determine the effect of thickness variation on the magnitude of PEC cell signals of AD- and HT-ZnTe layers grown at 1600 mV. It should be recalled that at this V_g, a positive PEC signal was obtained as earlier explained in section 5.3.4. Table 5.3 shows the PEC signals obtained for AD- and HT-ZnTe layers at different growth durations ranging from 0.50 to 4.00 hours while Figure 5.14 is the diagrammatic illustration of PEC signals given in Table 5.3. As seen in Figure 5.14, the highest and lowest PEC signals were observed at the growth time of 2.00 and 4.00 hours respectively. For the HT-ZnTe layers, a progressive increase was observed from 0.50 to 2.00 hours. Beyond 2.00 hours of thin films deposition, a drastic reduction took place in the magnitude of the PEC signals.

Table ‎5.3. PEC signals of AD- and HT-ZnTe at different growth duration ranging from 0.50 to 4.00 hours.

5.4.1.2 Thickness effect on mobility and Fermi level position

I-V and C-V techniques were both used to investigate how thickness variation affects the resistivity, acceptor density, mobility and position of Fermi level in p-ZnTe layers.

0

Figure 5.14. Typical PEC signals as a function of deposition time for AD- and HT-ZnTe layers grown at a cathodic potential of 1600 mV.

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The device structures glass/FTO/p-ZnTe/Au were fabricated to investigate the material resistivity using I-V technique while C-V technique was used to investigate the acceptor density and Fermi level position of glass/FTO/p-ZnTe/Al device structures.

Approximately same thicknesses of ZnTe layers were used for both I-V and C-V experiments. Table 5.4 gives the summary of results obtained from I-V and C-V measurement analyses. It was noticed that as the thickness of the ZnTe layer increases, the depletion width (W) increases while the acceptor density (NA) decreases. The highest conductivity and mobility occurred at growth time of 2.00 hours. It was also observed that as the material thickness increases, the Fermi level position moves away from the valence band maximum towards the mid-gap position. These experimental results show that the thickness of the semiconductor material influences the Fermi level position of electroplated ZnTe layers.

Table ‎5.4. Effect of thickness of ZnTe layers on electrical parameters obtained from I-V and C-V techniques.

The experimental results summarised in Table 5.4 explain the possibility of having both degenerate and non-degenerate p-type ZnTe semiconductors. For a p-type semiconductor to be termed degenerate, one or both of the following conditions must be met; the first condition is that the acceptor density (NA) must be greater than the effective density of states in the valence band edge (NV) while the second condition is that (EF-EV) is kT [34]. By substituting mp* = 0.20mo which is the effective hole mass of ZnTe into Equation 3.43 of Chapter 3, the effective density of states in the valence band edge of ZnTe thin film was calculated to be 2.24×10¹⁸ cm^-3. As revealed in Table 5.4, the ZnTe layer grown for 30 minutes duration belong to the degenerate p-type semiconductor since (EF-EV) is kT while the ZnTe layers deposited within the range

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1.00 to 4.00 hours belong to the non- degenerate p-type semiconductor since N_A is <

NV and (E_F-E_V) is kT.

5.4.2 Effect of thickness on morphological properties of ZnTe layers

SEM technique was used to study how variation in growth duration affects the morphology of ED-ZnTe layers. Figures 5.15 (a-g) show the obtained micrographs of ED-ZnTe layers deposited within the explored growth duration. The SEM images revealed that the grain size increases as the deposition time increases. These results agree with the experimental results reported by Shaaban et al. that the crystallites size of ZnTe thin films increase with the thickness of thin film [35].

The summary of range of grain sizes obtained for ED-ZnTe layers deposited between the growth times of 0.25 to 4.00 hours is given in Table 5.5. The value of theoretical thicknesses obtained from Faraday’s equation is also shown in Table η.η for easy comparison. As seen from Table 5.5, the smallest grain size is denoted as G.Sminimum

while the largest grain size is depicted as G.Smaximum. The average grain size (G.Saverage) was obtained by finding the mean of G.Sminimum and G.Smaximum. The smallest range of grain sizes (21.6-101.5) nm occurred at deposition time of 15 minutes while the largest range of grain sizes (725.5-3091.5) nm was obtained at growth time of 4.00 hours. The graphical relationship between the measured average grain sizes (G.S_average) and growth time is described in Figure 5.15 (h). Figure 5.15 (h) also includes the plot of theoretical thickness obtained from Faraday’s equation for easy comparison. A good correlation exists between Figure 5.15 (h) and Figure 5.13.

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Figure 5.15. (a-g) Typical SEM micrographs of ED-ZnTe layers grown within duration of 0.25 to 4.00 hours and (h) Thickness of ED-ZnTe layers measured from SEM technique and theoretically estimated from Faraday’s equation.

(a) tg = 15 mins (b) tg= 30 mins

(c) tg= 1 hour (d) tg= 1.5 hours

(e) tg= 2 hours (f) tg= 3 hours

(g) tg= 4 hours

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Table ‎5.5. Summary of thickness results obtained from SEM technique and Faraday’s law of electroplating.

5.4.3 Effect of thickness on optical absorption

The optical absorption spectra of AD- and HT-ZnTe layers for selected growth duration are illustrated in Figure 5.16. The results of optical absorption measurements carried out at different growth time are presented in Table 5.6. The optical results showed that by increasing the thickness of the ZnTe layers grown in a Te-rich ZnTe electrolyte, the bandgap of the ZnTe layer can be modified. It was observed that the energy bandgaps of the thin films decrease with increase in growth time and film thickness. Researchers working on thin films have also described and explained how variation in thickness of thin films affects the energy bandgap of semiconductor materials [36,37].

Figure 5.16. Optical absorption spectra showing the effect of thickness variation on energy bandgap of (a) As-deposited ZnTe layers and (b) Heat-treated ZnTe layers.

0.0

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Table ‎5.6. Energy bandgaps for AD- and HT-ZnTe layers at different growth duration.

Growth time (hours) 0.25 0.50 1.00 1.50 2.00 3.00 Bandgap (eV)

± 0.02

AD-ZnTe 2.60 2.20 1.68 1.55 1.30 0.55

HT-ZnTe 2.35 2.10 1.65 1.55 1.20 0.55

As shown in Figure 5.17, the visual appearance of ZnTe layers differ from each other due to the variation in growth duration. Figure 5.17 (a) and 5.17 (b) show the visual appearance of ZnTe layers grown for 0.50 and 2.00 hours respectively. The energy bandgap of as-deposited Te-rich p-ZnTe (ZnTe:Te) layer grown for 0.50 hours as stated in Table 5.6 falls in the range of bandgap for bulk ZnTe thin films while the energy bandgap of as-deposited Te-rich p-ZnTe (ZnTe:Te) layer grown for 2 hours is ~1.30 eV;

this value deviates from the bandgap of stoichiometric ZnTe which is reported to be in the range (2.10–2.26) eV [25]. In Figure 5.17 (b), the Te-rich ZnTe layer appears very dark in appearance thus making it to be highly light absorbing. Te being a semi-metal has a very low bandgap of 0.37 eV [38] and since the ZnTe layers were grown in a Te-rich ZnTe electrolyte for longer duration, more Te easily comes to the surface of the ZnTe layer due to the redox potential of Te atom. This phenomena causes a reduction in the bandgap from ~2.20 to ~1.30 eV providing a suitable method for bandgap grading.

The optical absorption curves in Figure 5.16 thus show that the bandgap of ZnTe is tunable by controlling the deposition time, the amount of Te in the ZnTe electrolyte or simply by changing the deposition voltage. It must be noted that the deposition time also determines how much Te and Zn is deposited on the cathode.

Figure 5.17. Visual appearance of electroplated ZnTe layers grown at same cathodic potential of 1600 mV for different duration of (a) 30 minutes and (b) 2 hours.

(a) (b)

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