A Study On the Structure and Electrical Properties of Pb o.9 Sn 0.1 Se/Si Heterojunction

In this work, as Study on the structural and electrical properties of Pb o.9 Sn 0.1 Se/Si hetrojunction was made by depositing compound of Lead, Tin and Selenide film on Si by thermal evaporation. XRD diffraction analysis of the film, shows the dominant crystal orientation is (200) as well as (Pb o.9 Sn 0.1 Se) film deposited is polycrystalline structure. Electrical properties of Pb o.9 Sn 0.1 Se/Si heterojunction detector have been investigated. The electrical properties under dark condition show a rectifying behavior with low rectification factor, and exhibit soft breakdown reverse current. C-V characteristics suggest that the fabricated diode was abrupt type, built in potential determined by extrapolation from 1/C 2 -V curve to the point (V=0) and it was equal to (0.4V).


2-Expermental Work
Substrates of p-type single-crystal Si wafers of resistivity 3-5 ohm-cm and orientation (111) were used in the present study.After scribing these wafers into small pieces (typically 1cm x 0.6cm in size), with one surface polished with 2HF: 3HNO 3 : 3CH 3 COOH mixture (3:5:3) were cleaned ultrasonically by dipping in distilled water, acetone and isopropyl alcohol alternately.After cleaning, the samples were oxidized in dry oxygen.[7].The films of Pb o.9 Sn 0.1 Se were prepared by thermal evaporation in vacuum of the order of 10 -5 torr, the rate of evaporation was ≈1.6 nm/min, onto clean silicon mirror-like side substrates at room temperature (~300K).The average thicknesses of the deposits were determined by microbalance method.The maximum error in the determination of thickness was of the order of 10% estimated for the thinnest films (Pb o.9 Sn 0.1 Se/Si films of thickness 350 nm).Ohmic contacts of aluminum [8] were evaporated on the silicon side and Pb o.9 Sn 0.1 Se/Si side.

3-Result And Discussion
3-1 X-ray Diffraction Studies X-ray diffraction (XRD) studies have been carried out to identify the Pb o.9 Sn 0.1 Se phase present in the film.Fig. 1 shows the XRD pattern recorded on Pb o.9 Sn 0.1 Se film, coated on slide glass substrate.The film is polycrystalline and has an cubic crystal structure.Fig. 3 shows the dark and under illuminated I-V plots measured at 300˚ K for a typical n-Pbo.9Sn0.1Se/p-Sidiode.Notice that the magnitude of the photocurrent under reverse bias exhibits a flat dependence when the voltage value is higher than 2.5 V.At this voltage the structure is suitable to be used as an efficient photo-detector.where η is a constant of the order of (2-6), and A is another constant, of the order of (13-14.5)and is practically independent of temperature.Further analysis of these current characteristics with temperature shows that ln J varies approximately as -1/T in the low-bias region: in the high-bias region, ln J varies as T.

3-3 C-V characteristics
Junction capacitance measured as a function of bias voltage for the n-Pb o.9 Sn 0.1 Se/p-Si diodes shows C∝V - 1/2 dependence Fig. 5 which indicates an abrupt junction in that case.Abrupt (when relation between 1/C 2 and V is straight line) or graded (when relation between 1/C 3 and V straight line) according to the distances during which the transition from one region to the other is completed near the interfaces.Under these conditions, the C-V characteristics of the heterojunction can be explained on the basis of

Bias Voltage (V) Forward Current (A)
Anderson ' s model [10], according to which ( ) where q: is the electronic charge, ε 1 : and N A1 : are dielectric constant and concentration of donors in n-type semiconductor, ε 2 : and N D2 : are dielectric constant and concentration of acceptors in p-type semiconductor (i.e.Si) and V and V D are the applied bias and built-in is voltage, respectively.Value of V D estimated from 1/C 2 versus V plot obtained for heterojunction, the built -in potential (V bit. ) for the n -Pb o.9 Sn 0.1 Se/p-Si System was found to be (0.4V).

4-Conclusion
From what has been mentioned above, we can conclude that this type of n-Pb o.9 Sn 0.1 Se/p-Si heterojunction behaves as a poor rectifier due to the noise that comes from the narrow gap Pb o.9 Sn 0.1 Se film.The junction is abrupt type.

Fig
Fig. (3): Dark and illuminated I-V curves for a typical Pb o.9 Sn 0.1 Se/p-Si Fig. 4, shows typical semi logarithmic plot of the forward current characteristics of n-Pb o.9 Sn 0.1 Se/p-Si junction.The low Fig.(4): Typical Semi logarithmic plot of forward current as a function of bias voltage for n-Pb 0.9 Sn 0.1 Se/p-Si junction.