Materials Science Forum Online: 2011-07-27 ISSN: 1662-9752, Vol. 695, pp 569-572 doi:10.4028/www.scientific.net/msf.695.569 2011 Trans Tech Publications, Switzerland DEFECTS STUDY BY ACTIVATION ENERGY PROFILE FOR LOWERING LEAKAGE CURRENT IN P-N JUNCTION Itsara Srithanachai a, Surada Ueamanapong a, Poopol Rujanapich a, Narin Atiwongsangthong a, Surasak Niemcharoen a, Amporn Poyai b, and Wisut Titiroongruang a a Electronics Research Center and Department of Electronics Faculty of Engineering, King Mongkut s Institute of Technology Ladkrabang, Bangkok 10520, Thailand b Thai Microelectronics Center (TMEC), Chachoengsao 24000, Thailand E-mail: Srithanachai@gmail.com, Usurada@gmail.com, poopolr@yahoo.com, kanarin@kmitl.ac.th, knsurasa@kmitl.ac.th, amporn.poyai@nectec.or.th, ktwisut@kmitl.ac.th Keywords: Activation Energy, Silicon, P-N Junction, Leakage Current, Defect Abstract. Diode leakage current consists of diffusion (I d ) and generation current (I g ), which is strongly sensitive to the residual defect density. These defects can be studied by activation energy (E a ). Therefore, this paper presents a method for calculating activation energy of silicon p-n junctions from volume generation current. It combines temperature-dependent current voltage (I V) and capacitance voltage (C-V) measurements of diodes. The I g can be found from the volume leakage current by subtraction of the volume diffusion current, which is calculated while the depletion width is zero. The activation energy (E a ) is derived from slope of an Arrhenius plot of I g. To derive the correct slope the temperature dependence of the depletion width, which is obtained from the corrected volume capacitance has been applied. The E a profile below junction has been shown. The lower E a value has been found near the junction, which may relate to the junction implantation. Introduction Requirements for present-day high-quality silicon substrates are extremely tight with respect to control of grown-in or processing induced defects. Besides, it may still be a marked effect of the starting material on electrical characteristics of simple device structures, like a p-n junction [1]. The defects can be a source of the leakage current in p-n junction which is one of main parameter that affects device performance. This leakage current is related to electrically active defects in the silicon [2]. Therefore, one way to study defects is analyzing E a of the leakage current in p-n junction. Usually, it can be extracted from the generation current [3]. To yield information on energy level of generation centers in depletion region of a diode we study reverse current (I R ) as a function of temperature. E a is derived from slope of an Arrhenius plot of ln(i R ) versus 1/kT, whereby only exponential 1/T dependence is taken into account; where k and T are the Boltzmann constant and the absolute temperature, respectively [1]. Here, the method using current-voltage (I- V) and capacitance-voltage (C-V) characteristics and analyses of the leakage current E a will be shown. Experiment Sample preparation For this study, p + -n junction diodes were fabricated on the <111> orientation 134-140 Ω-cm 300 µm of n-type silicon substrate. The wafers were sent into photolithography and etch processes to open 4 mm 2 for active area and 10 µm for guard ring of 1 µm thickness photoresist window. Then wafers were implanted boron with dose of 1x10 16 cm -2 at energy of 120 kev and implanted phosphorous with the same condition on backside wafer for ohmic contact and followed by an 1050 o C, 60 min thermal anneal. After that wafers were sent into metallization process to create 1 µm thickness of aluminum layer at both sides. The second photolithography step and etch processes were conducted to create aluminum patterns then anneal at 400 o C for 30 min. Wafers were sawed and assembled on PCB before finished with wire bond process. Finally, the chip is ready to connect to test circuit outside. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-09/05/16,04:14:33)
570 Eco-Materials Processing and Design XII Measurement procedure HP4156B was used to measure electrical properties of diode. The current-voltage (I-V) characteristics were measured on wafer with bias step of 0.025 V from reverse (V R ) to forward (V F ) voltage, in the range of -10 to +1 V, whereby the bias was applied to the back n-type substrate and the current was measured at the top p-type. Temperature dependent measurements were done at 30, 40, 50, 60, 70, and 80 o C, respectively, allowing extraction of the E a of the reverse current. Capacitance-voltage (C-V) measurements were performed on the same diode at a frequency of 100 khz, as a function of temperature, to extract the volume depletion width (W). Results and Discussion The current-voltage of p-n junction diode presented in Figure 1, [4,5]. For the case of reverse bias, the saturation current or leakage current includes the contributions of the diffusion current (I d ) and the generation current (I g ), which generate in the depletion region. The leakage current is presenting in equation (1) and the generation current is described in equation (2). I 0 = I d + I g I 0 = I d + Aqn i W/τ g (1) (2) Where A is the area of p-n junction, n i is the intrinsic carrier concentration, W is the width of depletion region and τ g is generation lifetime. Fig 1. The current-voltage characteristics of the silicon p-n junction diode Fig 2. The capacitance-voltage characteristics of the silicon p-n junction diode The leakage current increases at higher reverse bias (larger depletion width higher reversed bias voltage), which can be surely explained by equation (2). The diffusion current is independent of applied bias because all of a fixed amount of the minority carriers generated by thermal energy within a diffusion length of the depletion region will essentially get swept across the junction due to the field, regardless of the applied bias and the strength of the electric field. Therefore, increasing of leakage current at higher reverse bias is mostly the result of generation current and also depends on the depletion width which can obtain from the capacitance that was shown in Figure 2. The relation between depletion width and capacitance can explain in equation (3), where ε si is permittivity of silicon. From equation (2), the increment of generation current is result of the increase in depletion width. W = A ε si / C In case of varied temperature, as shown in Figure 3, the leakage current increases as temperature increases due to diffusion current increases with temperature. Table 1 shows the summary of the diffusion current. To derive the diffusion current, it can be calculated from the relation of leakage (3)
Materials Science Forum Vol. 695 571 current and depletion width. Figure 4 shows how to calculate the diffusion current and in this example it is 0.65 na. From equation (2), the generation current was calculated after we derived the diffusion current for difference temperature. Table 1 The volume diffusion current at various temperature Temperature [ o C] 30 40 50 60 70 80 I d [na] 0.002 0.043 0.209 0.650 2.405 7.903 Fig 3. The current-voltage characteristics of the silicon p-n junction diode at various temperatures Fig 4. The leakage current versus the depletion width of the silicon p-n junction diode A general relation between a physical variable and its activation energy can be applied to the leakage current of a p n junction, according to I R (T) exp(-e a /kt) With I R is reverse current. The slope of an Arrhenius plot, I R (T) vs 1/kT yields the activation energy E a [6]. Figure 5 shows the effect of difference temperature corrections on the activation energy from Arrhenius plot of generation current versus the temperature of the difference bias. Figure 6 shows the approximation of E a is 0.69 ev. The lower E a value has been found near the junction, which may relate to the boron implantation. While the defects related to phosphorus implantation may dominate at deeper. This can be expected the higher leakage current near the junction. In order to reduce the leakage current, these kinds of defects have to be controlled. (4) Fig 5. Arrhenius plot of generation current versus the temperature of the difference bias Fig 6. The value of the activation energy versus bias voltage
572 Eco-Materials Processing and Design XII Summary This paper clearly shows the method to obtain the activation energy profile below the junction from the temperature dependence I-V and C-V characteristics. This is a useful method to study the behavior of electrically active defects. Lower activation energy has been found near the junction. Acknowledgment This work has been supported by Thailand Graduate Institute of Science and Technology (TGIST) under scholarship No. TG-44-22-53-014D. References [1] A. Poyai, E. Simoen, C. Claeys, A. Czerwinski, and E. Gaubas, Improved extraction of the activation energy of the leakage current in silicon p n junction diodes, Appl. Phys. Lett., Vol. 78(14), pp. 1997-1999, 2001. [2] W. Pengchan, T.Phetchakul, and A. Poyai, Implantation-induced Defects Analysis Base on Activation Energy Diagnostics, Integrated Circuits, ISIC, pp. 518 52,1 Issue 14-16 Dec 2009. [3] A. Poyai, C. Claeys and E. Simoen, Improved extraction of carrier concentration and depletion width from capacitance-voltage characteristics of silicon n+-p-well junction diodes Appl. Phys. Lett., Vol. 80(7), pp. 1192-1194, 2002. [4] S.M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, New York, 1981. [5] Gerold W. Neudeck, The pn junction diode, 2nd Modular series on solid state devices, Eds Gerold W. Neudeck and Robert F. Pierret, Addison-Wesley Publishing Company, 1989. [6] A. Czerwinski, E. Simoen, A. Poyai, and C. Claeys, Activation energy analysis as a tool for extraction and investigation of p n junction leakage current components, J. Appl. Phys., Vol. 94(2), pp. 1218-1221, 200
Eco-Materials Processing and Design XII 10.4028/www.scientific.net/MSF.695 Defects Study by Activation Energy Profile for Lowering Leakage Current in P-N Junction 10.4028/www.scientific.net/MSF.695.569 DOI References [1] A. Poyai, E. Simoen, C. Claeys, A. Czerwinski, and E. Gaubas, Improved extraction of the activation energy of the leakage current in silicon p n junction diodes, Appl. Phys. Lett., Vol. 78(14), pp.1997-1999, (2001). 10.1063/1.1359487 [3] A. Poyai, C. Claeys and E. Simoen, Improved extraction of carrier concentration and depletion width from capacitance-voltage characteristics of silicon n+-p-well junction diodes, Appl. Phys. Lett., Vol. 80(7), pp.1192-1194, (2002). 10.1063/1.1435809