Photocurrent and Photovoltaic of Photodetector Based on Porous Silicon
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Available online at www.worldscientificnews.com WSN 77(2) (2017) 314-325 EISSN 2392-2192 Photocurrent and Photovoltaic of Photodetector based on Porous Silicon Hasan A. Hadi Department of Physics, Education Faculty, AL Mustansiriyah University, Baghdad, Iraq E-mail address: [email protected] ABSTRACT We have studied the dependence of photodetector photocurrent on incident power density of light with anodization current and time. The fabrication of Al/PS/p-Si photodetector heterojunction PDH by electrochemical etching method ECE and semi-transparent Al films in thickness range of 80 nm are deposited by thermal evaporation on porous silicon layers to investigate the photocurrent - voltage characteristics of the PDH. When the anodization current varied from 20 to 60 mA, the photocurrent PC was increase according to the anodization parameters at 1.2 mw/cm2 power density. The results also show that the short current Isc and open circuit voltage Voc saturate at high power density. The difference in the value of Voc and Isc at different etching current density is related to the Si nano crystallites layer thickness and the porosity which itself is greatly affected by the etching current density. Keywords: Porous Silicon, ECE, Photovoltaic, PDH, PC 1. INTRODUCTION Nanostructured porous silicon PS was received on bulk Si wafers by the method of anodic electrochemical etching in hydrofluoric acid solution .The electrochemical etching of silicon has been utilized as a structuration technique to obtain nano and micro porous surfaces [1] .Electrochemical etching is one of the simplest and most reliable method used to synthesis porous silicon [2]. The material has since been the subject of various investigations and reviews of its physical properties, including the nature of its bandgap and the presence of World Scientific News 77(2) (2017) 314-325 quantum confinement in the nano crystallites contained in the material, for their potential applications in electroluminescent devices ,photo-sensors [3]. Porous silicon has attracted much attention among technologists for developing optical and electronic devices [4]. The direct band gab semiconductor offer much stronger absorption coefficient; therefore, the porous silicon has better optoelectronic properties than bulk silicon [5]. Porous silicon is a very promising material due to its excellent properties and compatibility with silicon based microelectronics with reduced fabrication cost. It is important to deposit metals and change chemical composition of the PS surface with metal atoms to form a good electrical contact for microelectronics and photo electronics [6]. Porous silicon photoconductors are commonly fabricated by depositing aluminum film on top of oxidized porous silicon structure. The passivation of the surface by oxidation could improve the external quantum efficiency of a porous silicon photodiode [7]. Any system used to make electrical contact must be transparent to the optical emission and have a low resistivity. It must not react with the silicon which might destroy the optical activity and must also passivate the surface [8]. PS plays an important role in photovoltaic characteristics through the improvement of light absorption [9] and exhibits a set of unique properties such as direct and wide modulated band gap, high resistivity, large surface area to volume ratio, and almost the same monocrystalline structure as bulk silicon. These valuable properties make PS as attractive and promising material for superior optoelectronic device fabrication [10]. Photodetectors are used for accurate measurement of light intensity in science and industry [11]. Among several types of photodetectors like p-n junctions, p-i-n diodes, p-i-n diodes, schottky barrier detectors and metal–semiconductor–metal (MSM) photodetectors, the advantages of MSM devices such as simplicity of fabrication, high response speed and reduction in noise are unique [12]. The study of the mechanisms of charge carrier transport, have gain great importance according to its wide applications such as light-emitting devices, photodetectors, solar cells, chemical, and other fields of science [13]. Normally, photo-detectors are reverse biased. Without illumination, the current flowing through a PS/p-Si junction is called dark current. This current depends upon the junction characteristics and is super imposed on the photocurrent across the junction. Photocurrent represented an important parameter which effected on spectral responsivity, and the linearity of detector properties also quantum efficiency [5]. Under illumination, the current of each junction has two components: a photo-generated component and a dark component. The photo-generated carriers cause a photocurrent, which opposes the diode current under forward bias. Therefore, the diode can be used as a photodetector - using a reverse or even zero bias voltage - as the measured photocurrent is proportional to the incident light intensity. The photovoltaic effect is observed and the photocurrent increase with power density and bias dependence [14]. We can also recognize the linear relation between ISC and VOC to a maximum value, beyond which both values tend to saturate and become constant, this made it usefull to use as adetector. This occurs due to the total separation of the photo-generated electron-hole pairs at the deplation region at the interfase between the PS and p-Si. This normally results since the open circuit voltage is linearly proportional to the generated photocurrent and also, depends on the thickness and the porosity of the silicon nano crystallites layer [15]. The photo-generated carriers cause a photocurrent, which opposes the diode current under forward bias. Therefore, the diode can be used as a photodetector - using a reverse or even zero bias voltage - as the -315- World Scientific News 77(2) (2017) 314-325 measured photocurrent is proportional to the incident light intensity. We present in this paper, survey optoelectronic effects in PS with experimental measurements of photocurrent current- voltage, photovoltaic of PS samples prepared with increasing anodization current and anodization time. The photocurrent of porous layers induced an increase in anodization current and anodization time that were studied as a function of the incident power density of light. 2. EXPERIMENTAL The porous silicon PS surfaces were synthesized by electrochemical etching of (100) Si oriented as shown in previous work [16]. PS films were produced using monocrystalline silicon wafers p-type, with resistivity of 10 Ω.cm etched in a mixture of 40% HF: purity Methanol (99.9%) solutions at ratio of (1:1) for different anodization time and different anodization current densities. Cleaning is necessary to remove any traces of organic, metallic and ionic contaminants from samples. Methanol and alcohol are used commonly to clean the wafer by immersing it in these chemicals in turn in the ultrasonic bath for few minutes. Finally, they are rinsed in distilled water treated ultrasonically followed by drying in a hot air stream. Thin Al films of 800 nm thick, were thermal evaporation deposited on the back side of the wafers. The samples are prepared in sandwich configuration, Al/PS/c-Si/ Al, the top one semitransparent electrode thermally evaporated with 88 nm. The evaporation is performed in a vacuum pressure of torr, using an evaporation plant model “E306 A manufactured by Edwards high vacuum”. Dark current – voltage in forward and reverse directions Al/PS/p- Si/Al measurements are carried out by applying voltage supplied to the sample from a stabilized d.c. Power supply, type LONG WEI DC PS-305D 30 ranges of (0-6) V. The current passing through the device is measured using a UNI-T UT61E Digital Multimeters. The cross-sectional view of PS/c-Si heterojunction photodetector is presented in previous studies [17-18]. Measurements of photocurrent of heterojunction were done under white light of different illumination power densities supplied by a halogen lamp with power of 150 W, which was connected to a Variac and calibrated by power meter [5]. 3. RESULT AND DISCUSSION The current–voltage characteristics of a Al/PS/p-Si heterojunction photodetector under white light illumination of constant power densities and different etching current are shown in Figures (1-4). It has been suggested that the light is absorbed at both PS and Si. The generation of photoelectrons, via both porous silicon and silicon exciton intermediate, is followed by electron transfer from Si into PS through the potential barrier at the interface. This is a result of a difference in energy gab between the two semiconductors. When the sample illuminated with light of varying intensity power. The results show reduced resistance with increasing photon energy of the illuminating light for all samples, likely due to increased generation of electron hole-pairs. It can be seen from these figures that the current value at a given voltage for PS/c-Si HJ under illumination is higher than that in the dark. This indicates that the light generates carrier-contributing photocurrent due to the production of electron– -316- World Scientific News 77(2) (2017) 314-325 hole pairs as a result of the light absorption. This behavior yields useful information on the electron–hole pairs, which are effectively generated in the junction by incident photons. Under the influence of the electric field at the junction, electrons are accelerated towards the PS, while the generated holes are swept towards the p-Si along the potential