Electrical I-V measurement

Effect of thickness vs. Voltage-Current (V-I) characterization of prepared Zn/Te thin films were also studied. The measurements were taken at room temperature for without annealing thin films, after annealed at 200°C and 300°C for 500 nm Zn/Te thin films. By Figure 4 observed that I-V curve is measured at room temperature for without annealed Zn/Te thin films which indicate linear I-V curve with ohmic contact variation at the interface between of Zn/Te and Si substrate contact which shows non-rectifying barrier potential at the interface of P-N junction contact [9]. At ohmic contact current is relative to applied voltage magnitude accordance with ohm’s law. I-V curve is obtained Linear at room temperature for without annealed thin films shows ohmic contact with typically metallic nature of Zn/Te thin films. This ohmic behaviour increase with increasing the temperature of films. At 200°C after annealing thin films clearly indicate that the thin films shows nonlinear curves after heat treatment appropriate with granular uniformity and demonstrate Schottky contact at the interface which shows rectifying behaviour of potential barrier between P-N junctions with tunnelling effect. After annealing at 300°C tellurium diffuse in Zinc and readily available Schottky barrier junction in the interface that makes alloy compound of Zinc Telluride (ZnTe). Thin films attained semiconductor nature after heat treatment [10].

Mobility measurement

Mobility of charge carriers which is called “Hall mobility” is measured using “Hall Effect” measurement. Mobility of charge carriers exponentially decreases with increase the temperature (Figure 6). It is also observed that the mobility of charge carriers at without annealed thin films is less than the mobility of charge carriers comparatively after annealed at 200°C and 300°C. It is also observed that mobility of conduction electrons is more than the valance electrons and conduction electrons are more easily affected than valence electrons. It is also observed that with increasing the temperature, relaxation time or mean free time between charge carriers collision also increases. In XRD analysis observed that after annealing the crystal grain particle size increases and there would be better contact at the interface of Zn/Te and Si substrate due to it mean free life time of charge carriers decreases and mobility decreases with increases the annealing temperature of thin films [10].

Electrical switching characterization

A P-N junction at the interface of contact Zn/Te on Si substrate works as a diode can neither turn ‘ON’ nor turn “off” instantly. It takes finite time for turning “ON” and turning “Off” which is called switching times of diode [11]. Diode has two switching time: Forward recovery time and Reverse Recovery Time. Test unit for the measurement of switching time shown in Figure 7.

(a) Turn on time (forward recovery time) in Figure 8 shows that when diode is forward biased (turned on), voltage across it reduces. Turn on time is defined as time interval in which voltage across diode changes from 90% maximum voltage to 10% of minimum voltage.

(b) Turn off Time (Reverse Recovery Time) shown in Figure 9 which indicate that when a forward biased diode is suddenly reverse biased, current does not drop to zero instantly. It takes finite time to drop to zero. This time is known as Turn off time. Reverse recovery time plays a major role in high speed switching application. For fast switching from “ON” state to “OFF” state, the reverse recovery time should be as small as possible. In Figure 9 (ts) is the storage time, when the diode is in reversed biased the positive terminal of battery repels injected minority holes and negative terminal of battery repels injected minority electrons. Therefore, current flows whose direction is from N-region to P-region (reverse current). This current is constant till all injected holes return back to their parent region P and excess electrons to N-region. This time interval (t1-to) is known as storage time (ts). After this instant current falls to reverse saturation current (I0) (I0 is due to thermally generated minority charge carriers).The time interval (t2-t1) is called as transition time (tt).

Sum of these two time intervals (ts and tt ) equals the turn off time or reverse recover time (trr) of P-N junction diode [1,8].

Structural properties

The Zn/Te thin films characterized with XRD with and without annealing. XRD pattern for 200 nm shown in Figure 10 and 500 nm shown in Figure 11. Results point out XRD peaks analogous to Tellurium Zinc at specific 2θ angle for orthorhombic face of thin films. This work confirmed that corresponding materials have been successfully deposited on substrate, Silicon and Zinc peaks are exposed whereas no peak related to “Te” appeared because of non-crystalline performance of Tellurium. Subsequent to annealing Tellurium has been merged in Zinc and relative peaks of composites of Zinc telluride starts appear. Observed peaks analysed using standard XRD pro software [12]. By XRD analysis it is also observed that with higher annealing temperature peak height increases and the peak width decreases. It has been observed that grain and crystals size increases due to growth of the peak height is the function of annealing temperature which could grow up the crystalline phase of thin films.

SEM analysis

The without annealing and annealing result measured by SEM images for dissimilar magnification which has decreases as of 10 µm to 1 µm which is exposed in Figure 13. These thin films visibly point out that Tellurium and Zinc full-grown on Silicon which results that large granular clusters starts to be appear with the decrease in magnification from 10 µm to 1 µm later than annealing surface morphology of thin films recrystallize. After annealing rough lumps and large bunch starts to appear in spherical shape and distributed on the whole surface. These clusters formed due to the agglomeration of particles during to heat treatment. In accordance to the SEM images it may bring to a close that thin films which are as-deposited have partially amorphous structure even as after annealing it becomes polycrystalline in nature. Without annealed thin film becomes smoother in outside morphology on the other hand after annealing minute gapes starts appear in between the granules due to diffusion of Tellurium in Zinc particles which supports our electrical studies. After annealing the amplitude of atomic vibration increases due to it crystal grain size shows large interatomic spacing between the grain particles. By SEM analysis it also concluded that grain size is increases when the annealing temperature is increasing [6,7].

CONCLUSION

Zn/Te Thin films hetero structure of variety of thickness has been fabricated on silicon under the vacuum of about 10^-5 torr, by vacuum coating unit. Energy band gap (direct) has been obtained 2.22 eV to 2.30 eV at without annealing through Four point Probe scheme which results that as we increase the material thickness, Energy band gap decreases exponentially. To find out the effect of recrystallization thin films of Zn/Te were annealed at 200°C and 300°C. After annealing it has been observed that energy band gap increases till 750 nm and suddenly decreases due to “Quantum Confinement Effect” and higher size of the grain particles and energy band gap obtained in range of 2.85 eV to 2.97 eV. Atomic force microscopy, X-ray diffraction, and scanning electron microscopy analysis are done to get complete and reliable micro structural in sequence. I-V Electrical measurements done for find out the nature of thin films. Electrical switching times study also done in forward and reverse bias after set up the devices to find out the effect of forward recovery time and reverse recovery time on P-N junction Zn/Te heterostructure which shows that reverse recovery time is product of transition time and storage time that is desirable measurement in high speed switching applications.

CONFLICT OF INTEREST

The authors declare that they have no conflict of interest.