770 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 49, NO. 9, SEPTEMBER 2013 In-Situ I–V Measurements of ZnS:TiO 2 /p-Si Quantum Dot Heterojunction Photodiode Under 120 MeV Au 9+ Ion Irradiation Prabha Sana, Shammi Verma, Kumsi Chandrashekharappa Praveen, and Manzar Mohammad Malik Abstract—The 120 MeV Au 9+ ion induced modifications in a ZnS:TiO 2 /p-Si quantum dot (QD) heterojunction has been stud- ied by In-Situ current-voltage ( I –V ) measurements. This paper shows that the process of ion irradiation is a novel technique in modifying the electronic properties of QDs based semiconductor heterojunctions. After Au 9+ ion irradiation, there is an increase in ideality factor for ZnS:TiO 2 /p-Si QDs heterojunction at lower fluences and further at higher fluences the ideality factor decreases. In addition, the other parameters like leakage current, series resistance, and reverse saturation current are also affected under the SHI irradiation. In pristine samples, the dependence of the currents under the light illumination shows high photo responsivity. The observed responsivity is 2.7 × 10 10 A/W for ultraviolet light (wavelength 376 nm) at reverse bias voltage 1.5 V. The increase in responsivity to 4.9 × 10 10 A/W is observed after irradiation up to fluence of 3 × 10 13 ions/cm 2 . Index Terms—Swift heavy ion, heterojunction, photo diode, quantum dots, photoluminescence. I. I NTRODUCTION S EMICONDUCTOR based photo detectors are frequently being developed for advancement in the specialty to sense both ultraviolet (UV) and visible photons. In this framework, a wide band gap material is used as active layer for absorbing the UV photons and transmission of low energy photons are collected by a narrow band gap material. Most of the UV-visible photo detectors are being fabricated on Si wafers using, p-n, p-i -n and metal-semiconductor-metal structures [1], [2]. Recently, considerable efforts have been made by several researchers for developing ZnS based photo detec- tors [3]–[5]. ZnS is a very promising material due to its wide and direct band gap of 3.68 eV. ZnS absorbs light near the UV region and is highly transparent in the visible wavelength region. As-grown ZnS is always behaving as intrinsic n-type semiconductor due to native defects, which Manuscript received March 29, 2013; revised June 12, 2013; accepted July 16, 2013. Date of publication July 19, 2013; date of current version July 31, 2013. P. Sana is with the Department of Physics, Maulana Azad National Institute of Technology, Bhopal 461051, India (e-mail: prabhasana@yahoo.co.in). S. Verma is with the Inter University Accelerator Centre, New Delhi 110067, India (e-mail: shammiktl@gmail.com). K. C. Praveen is with the Department of Physics, University of Mysore, Mysore 570006, India (e-mail: kcpavi@gmail.com). M. M. Malik is with the Department of Physics, Maulana Azad National Institute of Technology, Bhopal 461051, India (e-mail: manzar. malik@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2013.2273946 donates the electrons or assists charge transport processes. There are a few reports on the ZnS/ p-Si heterojunctions, where the ZnS film is grown by different physical tech- niques [6], [7]. However these physical deposition methods are relatively expensive when compared to the solution based synthesis technique. ZnS quantum dots (QD’s) have a leading edge to the next generation technology due to its distinguished performance and efficiency for device fabrication. ZnS QD’s can be synthesized successfully with size- and dimensionality- dependent functional properties by solution based synthesis methods [8]. Surface passivation of ZnS QD’s also offers many advantages such as substantial reduction of surface defect den- sities, prevention of the surface from adsorption of unwanted species, prevention of unnecessary charge injection, and partial screening of the external fields. In this context, ZnS-core:TiO 2 shell has good electrical and optical properties, and reliability superior to ZnS QD’s [9], [10]. Therefore the fabrication of ZnS:TiO 2 /p-Si QD’s heterojunction could be an appropriate choice for device fabrication specially to detect the ultraviolet light, suitably for space applications. However in radiation atmosphere, the electronic devices involuntarily expose to radiations and malfunctioning of devices could be occurred. The extent of damage formation degrades the performance of the device over year of operation. The extent of damage formation and property modification depend strongly on ions energy and mass. Therefore in present work, the high energy swift heavy ion (SHI) irradiation on ZnS:TiO 2 / p-Si QD’s het- erojunction have been studied to understand the modification in the QD interface properties which lead to changes in the current transport process across the barrier. Irradiation of the semiconductors devices with ion beam introduces electrically active defects. The defects change the property of material at the surface or deep into the surface depending upon the energy of the ion [11]. The electronic energy loss S e , of swift heavy ions, due to inelastic collisions is two to three orders of magnitude larger when compared to nuclear energy loss S n due to elastic collisions. The large S e may lead to phenomena like, mixing at interface, modification or introduction of the microscopic inhomogeneities at the interface and annealing of the interface defects that can alter the electronic structure at the interface. Present paper emphasizes the importance of understanding the phenomenon of ion induced modifications in heterojunction, as well as its application in some fields such as the testing of radiation hardness for aerospace industry and the development of photo detectors [12], [13]. It is also very important to correlate the SHI induced effects on the material 0018-9197 © 2013 IEEE