O. I. Olusola

One-sided rectifying p–n junction diodes fabricated from n-CdS and p-ZnTe:Te semiconductors

The fabrication of a one-sided p-n hetero-junction (HJ) diodes have been successfully carried out using both p-type ZnTe and n-CdS semiconductors. Chemical bath deposition (CBD) and electrodeposition (ED) techniques have been used in the deposition of n-CdS and p-ZnTe layers respectively. Before the fabrication of the one-sided p-nHJ diodes, the electrical properties of glass/ FTO/p-ZnTe/Al and glass/FTO/n-CdS/Au rectifying structures were separately studied using capacitance-voltage (C-V ) technique so as to determine the doping density of each of the thin films. The results from C-V analyses showed that p-ZnTe is moderately doped with an acceptor density of 3.55×1015 cm3 while n-CdS is heavily doped with a donor density of 9.00×1019 cm3. The heavy doping of n-CdS and moderate doping of p-ZnTe will make the interface between n-CdS and p-ZnTe thin films a one-sided n+p diode. Therefore, to fabricate the CdS/ZnTe hetero-structure, it was ensured that approximately same thickness of CdS and ZnTe thin films being used in the initial experiment to study the electrical properties of glass/FTO/n-CdS/Au and glass/FTO/p-ZnTe/Al were also used in the development of the one-sided n+p junction diodes to obtain more accurate results. The electronic properties of the device structure were studied using both current-voltage (IV ) and C-V measurement techniques. The I-V results show that the one-sided n+pHJ diodes possess good rectifying quality with a series resistance (Rs) of35 and rectification factors exceeding 102.7 under dark condition. The results of theC-Vanalyses showed that the acceptor density of the onesided n+pHJ diode is of the order of 1015 cm3 while the donor density is of the order of 1018 cm3. The results obtained from this analysis still showed the moderate doping of p-ZnTe and the degenerate nature of n-CdS.

Growth of n- and p-type ZnTe semiconductors by intrinsic doping

Using intrinsic doping, n- and p-type ZnTe thin films have been electrodeposited (ED) on glass/fluorine-doped tin oxide (FTO) conducting substrate in aqueous solutions of ZnSO4·7H2O and TeO2. The intrinsic doping was achieved by simply varying the deposition potential. The films have been characterised for their structural, optical, electrical, morphological and compositional properties using X-ray diffraction (XRD), optical absorption, photoelectrochemical (PEC) cell measurements, scanning electron microscopy and energy-dispersive X-ray analysis techniques, respectively. The XRD results reveal that the electroplated films are polycrystalline and have hexagonal crystal structures. Optical absorption measurements have been used for the bandgap determination of as-deposited and heat-treated ZnTe layers. The bandgap of the as-deposited ZnTe films are in the range (1.70–2.60) eV depending on the deposition potential. PEC cell measurements reveal that the ED-ZnTe films have both n- and p-type electrical conductivity. Using the n- and p-type ZnTe layers, a p-n homo-junction diode with device structure of glass/FTO/n-ZnTe/p-ZnTe/Au was fabricated. The fabricated diode showed rectification factor of 102, ideality factor of 2.58 and threshold voltage of ~0.25 V.