Microsoft Word - Vol 6-2p4V2-press.doc IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 37 INVESTIGATION ON LAF3/Si STRUCTURE AS LIGHT- ADDRESSABLE POTENTIOMETRIC FLUORIDE (F‾) SENSOR ABU BAKAR MD. ISMAIL1,2, REZAUL ISLAM1, KOJI FURUICHI2, TATSUO YOSHINOBU2, HIROSHI IWASAKI2 1Department of Applied Physics and Electronics, Rajshahi University Rajshahi 6205, Bangladesh. Tel: 880-721-750041/4101, Fax: 880-721-750064, E-mail: ismail@ru.ac.bd; and ismail_abm@lycos.com 2The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. 8404 E-mail: iwasaki@sanken.osaka-u.ac.jp Abstract: In this article LaF3 films in LaF3/Si structure have been investigated for the measurement of fluoride ions in aqueous solution with higher accuracy using light- addressable potentiometric sensing technique. LaF3 films were directly grown on Si by vacuum evaporation. The effects of LaF3-thickness and annealing on the fluoride- sensitivity of the LaF3/Si structures have been studied. Accordingly four different LaF3/Si structures with various thickness of LaF3 (5/50/150nm) layer were fabricated. The structures are annealed at 400ºC for 10 minutes. The un-annealed LaF3/Si structures show very poor fluoride sensitivity. As predicted in the theoretical study structures with thinner LaF3 offer better steepness of the photocurrent characteristics that indicates better accuracy in fluoride concentration measurement. Among the annealed structures, the one prepared with 150nm LaF3 layer shows the best fluoride sensitivity of ≈ 54.5mV/pF in the range of pF1-pF4. Experimental results show a good promise for LaF3/Si structure as a light-addressable potentiometric sensor that can be used for the sensing and imaging the distribution of fluoride ion in aqueous medium with higher accuracy. Keywords: Light-addressable potentiometric sensor (LAPS), Fluoride sensor, Capacitive EIS structure 1. INTRODUCTION In recent years, new scientific evidence has emerged which suggests that there are significant risks and negligible benefits from ingesting low levels of fluoride [1]. Fluoridation of water supplies is harmful to bone, while providing negligible benefits when swallowed. Therefore, monitoring of fluoride has become very important for the environmental and the industrial processes. Generally detection of fluorine and fluorides are done by Electrochemical cells [2]. Almost all of them have a liquid electrolyte, which IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 38 is considered as their main disadvantage. Recently there are reports on the ionic conductor LaF3 thin-film as the sensitive layer in solidstate sensors for both fluorine and oxygen sensing. Researchers have investigated LaF3 in details for its various attributes such as its crystal structure, ionic conductivity and dielectric properties. As a candidate electrolyte for measuring oxygen gas concentration at room temperature use of LaF3 has been studied thoroughly [3-9] by some researchers. Most of these studies involved the LaF3 in polycrystalline form or chunks of single crystal. In those works LaF3 have been deposited with various deposition techniques as an epitaxial layer directly, or with additional insulating layer of SiO2 and/or Si3N4, on semiconductor. The interface between epitaxially grown LaF3 on silicon or gallium arsenide [10-12] has also been studied to obtain a heterostructure of low density of surface states. Development of fluoride sensitive field-effect transistors (pF-FET) have been carried out by Moritz [13], Hentsche et al. [14]. Baccar et al. [15] also investigated ion sensitive field-effect transistor (ISFET) and electrolyte-oxide-semiconductor (EOS) structure with a fluoride ion sensitive membrane obtained by fluoride and lanthanum ion implantation into the silica insulator layer. An overview of the above reports suggests that epitaxially grown LaF3 on Si <100> would give an interesting structure to be used for fluoride sensing. Apart from the choice of sensor materials, the choice of sensing technique is also important. Recently sensors based on light-addressable potentiometric sensing (LAPS technique has become popular [16-18] due to some of its advantages, such as selection of measurement point by a scanning light beam and spatial resolution. In our previous work [19] we first reported a Fluoride sensor using Si/LaF3 structure as light-addressable potentiometric sensor (LAPS). For potentiometric sensors high precision measurement is necessary since 1 mV in the potential shift corresponds to an error in concentration of 4% (exchange of one electron) [20]. Therefore the steepness of the LAPS IV characteristics is of great importance for achieving accurate measurements. This article presents the investigation on vacuum evaporated LaF3 on Si in a heterostructure of LaF3/Si/Au to be a high-accuracy fluoride- sensitive LAPS devices. The possibility of obtaining high accuracy in fluoride sensing is first theoretically studied, and then experimentally it is verified. 2. SENSOR FABRICATION AND EXPERIMENTAL SETUP The sensor chips were obtained from small pieces of silicon cut from an n-type Silicon wafer having a diameter of 8 cm, resistivity  of 1-10 Ω-cm and surface of (100) orientation. The back surface (unpolished) of silicon chips was first coated with AuSb (99.5%-Au and 0.5%-Sb) to establish the back metal contact of the sensor. The above- mentioned wafer was first degreased ultrasonically in acetone and deionized water. The pieces were briefly dipped in very dilute (2 %) HF solution to remove the native oxide just before insertion into the evaporation chamber. The LaF3 films were deposited in a home- assembled vacuum-coating unit. LaF3 anhydrous 99.99% purity was purchased from Kishida Chemical Co. Ltd., Japan, and was used without further purification. The vacuum pressure was maintained in the range of 10-8 Torr. The deposition rate was 5 nm/min. In this way, 5 nm (Sensor 2), 50 nm(Sensor 3), 150 nm (Sensor 4) -thick LaF3 layers were deposited on the bulk Si. The thicknesses were monitored in-situ as well as confirmed by ellipsometric measurements. The sensors (2, 3 and 4) thus constructed were annealed at 400ºC for 10 min. IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 39 3. THEORETICAL CONSIDERATION Theoretically it appears that the optimization of heterostructure of LaF3/Si to be used as a LAPS might provide the steeper photocurrent response and thereby provide better accuracy in the measurement of Fluoride concentration. The electrical equivalent circuit of the LaF3/Si structure to be used as LAPS can be given [21] as shown in Fig. 1. Fig. 1: AC equivalent circuit of LAPS device. The measured current is given by 1 ( ) j C RiI I p j R C Ci d      (1) Where, Ip = Current due to Light induced minority carriers I = Measured photocurrent Cd = Depletion capacitance Ci = Insulator capacitance R = Depletion resistance  = Angular frequency of AC photocurrent The term [2R(Ci + Cd)]-1 is defined as the cutoff frequency for the LAPS. The measured current, at a frequency below the cutoff, is given by i pI j C RI (2) Above the cutoff frequency the measured current is given by i p i d C I I C C   (3) V IR C C IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 40 Equations (2) and (3) show that, at a frequency below or above the cutoff frequency, the measured current can be increased by increasing the insulator capacitance Ci ( 1/d), and hence decreasing the thickness (d) of the insulating layer. Conventionally the insulating layer of LAPS is a combination SiO2 and Si3N4, as such the insulator capacitance of Ci(LaF3) >> Ci(SiO2+Si3N4) Therefore, using very thin LaF3 layer as the insulating layer in LAPS the measured photocurrent and hence the steepness of the I-V response can be increased. The steepness of the I-V characteristics is of great importance for achieving better resolution. The steepness of the photocurrent response is defined as the change of photocurrent for the unit change in surface potential, at the linear region of the photocurrent response characteristics around the inflexion point of the I-V characteristics. 4. RESULTS AND DISCUSSION For the investigation of LaF3 material as the fluoride ion sensitive material in the LAPS, sensors with two conditions, namely, as deposited (without annealing, Sensor 1) and, annealed at 400°C (Sensor 2, 3 and 4), were studied. The test solutions used in the characterization were prepared in the laboratory. The composition of the solutions was 2% tris (hydroxymethyl)-aminomethane solutions in distilled water, at a fixed pH value of 7.2. Constant ionic strength was maintained at 0.4 M of KNO3. Analytical grade KF was added to the solution in a concentration range of 10-1M to 10-7M to obtain the experimenting solutions having pF values from 1 to 7. As shown in Fig 2, the response for a particular solution is obtained by plotting the photocurrent for the corresponding bias. The typical response of the sensors for a pF2 solution is shown in Fig. 3. Fig. 2: Experimental set-up for the LAPS measurement. IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 41 Fig. 3: Typical LAPS response of the 50nm-LaF3/Si structure for pF4 solution. As shown in Fig. 3, the photocurrent for the LaF3/Si structures start flowing when the depletion region is created in the Si by the applied bias plus the surface potential at the insulator/electrolyte interface which corresponds to the Nernstian voltage of LaF3 for pF4 solution. The saturation level of the photocurrent is observed at the highly depleted (inverted) condition. As the bias is further decreased towards less positive values the photocurrent starts decreasing and then becomes nearly constant. The responses of various thickness LaF3/Si structures used as the Fluoride-LAPS are compared for a particular solution (pF4) and are shown in Fig. 4. As the theory predicts the amplitude of the photocurrent as well as the steepness of the response characteristics for the LaF3-based LAPS depends on the LaF3 thickness. Fig. 4: Photocurrent response of annealed-LaF3/Si structures for various thickness of LaF3. Figure 5 shows the linear portion of the photocurrent characteristics around the inflexion points. This plot is used to calculate the steepness of the photocurrent characteristics. The steepness is calculated as the change of photocurrent for unit change in surface potential. For our experimental setup, since we used a preamplifier to convert 0 0.1 0.2 0.3 0.4 0.5 -1.5 -1 -0.5 0 0.5 1 1.5 2 Un-annealed Annealed P ho to cu rr en t [ a. u] Bias [V] A B Linear region 0 0.2 0.4 0.6 0.8 1 1.2 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 50nm LaF 3 150nm LaF 3 5nm LaF 3 P ho to cu rr en t [ a. u] Bias [V] IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 42 the photocurrent into voltage by a factor of 106, the Steepness of the photocurrent response is defined as, )( )/(10)( 6 VBiasinChange AVxAntphotocurreinChange Steepness  Fig. 5. Linear portion around the inflexion point of the photocurrent characteristics of various thickness annealed structures. Figure 6 shows the calculated-stpeeness versus the LaF3 thickness for the annealed structures corresponding to the pF4 solution. The steepness of the annealed structures was always higher than the un-annealed structures. This is evident from the Fig. 3. Thus, we will focus on the behavior of annealed structures. As predicted theoretically, the steepness of the photocurrent characteristics was inversely proportional to the thickness of the LaF3 layer on Si. Fig. 6. Steepness versus LaF3 thickness for the annealed-structures. Figure 7 shows the typical photocurrent response of the annealed structures for varied concentration fluoride solutions in the range of pF1 to pF7. Here, each I-V curve is normalized at its peak value. The un-annealed (as-deposited) sensor showed indistinguishable or very low fluoride sensitivity. Therefore, the response of the annealed sensors will be discussed further. 0.1 0.15 0.2 0.25 0.3 0.35 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 5nm 150nm 50nm P ho to cu rr en t a ro un d th e ifl ex io n po in t [ a. u] Bias [V] 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 2 . 3 2 . 4 2 . 5 2 . 6 2 . 7 2 . 8 2 . 9 3 . 0 3 . 1 3 . 2 S te e p n e s s LaF3 IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 43 a) Un-annealed LaF3/Si structure b) Annealed LaF3/Si structure Fig. 7. Photocurrent response for varied concentration of Fluoride solution. As can be seen in Fig. 7(b), the photocurrent response for the pF5, pF6 and pF7 solution appears to be overlapping which suggests a loss in Fluoride sensitivity when the sensor is subjected to pF solution beyond pF4. The inflexion points for the photocurrent responses of various pF solutions are found by equating the second derivative of the response for each buffer to zero, and are plotted as shown in Fig. 8 for the annealed structures. The un-annealed structures did not show distinguishable inflexion voltages for various fluoride concentrations and are not plotted for simplicity in Fig. 8. The 50nm- LaF3/Si structure shows quite linear response for pF1 to pF5 whereas 5 nm-LaF3/Si and 150 nm-LaF3/Si structures show linear responses for pF1 to pF4 value of the solution. Fig. 8. Thickness dependency of the inflexion potentials. Figure 9 shows the plot of the fluoride sensitivity against the thickness of the sensor. We didn't find any ordered relationship between thickness of the LaF3 layer and Fluoride sensitivity. This is may be due to the fact that the fluoride sensitivity of LaF3 layer -500 0 500 1000 0 1 2 3 4 5 6 7 8 150nm LaF 3 50nm LaF 3 5nm LaF 3 In fle xi on v ol ta ge [m V ] pF 49.2mV/pF 53.3mV/pF 54.6mV/pF 12.635mV/pF 20.365mV/pF - 1 . 6 - 1 . 4 - 1 . 2 - 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 N o rm a li z e d p h o to c u rr e n t [a .u ] B i a s v o l t a g e [ V ] p F - 1 p F - 2 p F - 3 p F - 4 p F - 5 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 N o rm a li z e d p h o to c u rr e n t [a .u ] B i a s v o l t a g e [ V ] p F - 1 p F - 2 p F - 3 p F - 4 p F - 5 p F - 6 p F - 1 IIUM Engineering Journal, Vol. 6, No. 2, 2005 A. Ismail et al. 44 depends on the number of surface sites, which binds the fluoride ion and does not depend on the thickness of the layer. The minimum average sensitivity of 49.2 mV/pF was observed in this experiment with the 5 nm-LaF3/Si. The highest sensitivity of 54.6 mV/pF was obtained with 150nm-LaF3/Si. The minimum average sensitivity of the sensors was more than that obtained by Baccar et al. [15] for their sensor prepared with ion implantation technique (44 mV/pF). The detection limit for 5 nm-LaF3/Si structure was also higher than that obtained by Baccar et al (pF 2 - 4). But the detection limit of 50 nm- LaF3/Si and 150 nm-LaF3/Si structures is quite similar to that of what Baccar et a.l obtained. 49 50 51 52 53 54 55 0 20 40 60 80 100 120 140 160 pF s en si tiv ity [m V /p F ] Thickness [nm ] Fig. 9. Thickness versus sensitivity plot. 5. 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