Research Article A Current Transport Mechanism on the Surface of Pd-SiO 2 Mixture for Metal-Semiconductor-Metal GaAs Diodes

Transcription

1 Advances in Materials Science and Engineering Volume 2013, Article ID , 4 pages Research Article A Current Transport Mechanism on the Surface of Pd-SiO 2 Mixture for Metal-Semiconductor-Metal GaAs Diodes Shih-Wei Tan and Shih-Wen Lai Department of Electrical Engineering, National Taiwan Ocean University, 2 Peining Road, Keelung 202, Taiwan Correspondence should be addressed to Shih-Wei Tan; tanshwei@mail.ntou.edu.tw Received 30 October 2012; Accepted 15 May 2013 Academic Editor: Markku Leskela Copyright 2013 S.-W. Tan and S.-W. Lai. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper presents a current transport mechanism of Pd metal-semiconductor-metal (MSM) GaAs diodes with a Schottky contact material formed by intentionally mixing SiO 2 into a Pd metal. The Schottky emission process, where the thermionic emission both over the metal-semiconductor barrier and over the insulator-semiconductor barrier is considered on the carrier transport of a mixed contact of Pd and SiO 2 (MMO) MSM diodes, is analyzed. The image-force lowering is accounted for. In addition, with the applied voltage increased, the carrier recombination is thus considered. The simulation data are presented to explain the experimental results clearly. 1. Introduction Schottkycontactofmetalonsemiconductorisessentialto MESFETs, HEMTs, optical sensors, and gas sensors. Recently, hydrogen has been widely used in hydrogen-fueled vehicles, medical treatment, chemical industries, and semiconductor fabrication. However, hydrogen-containing gases are explosive. Therefore, developing of hydrogen sensors for realtime in situ detection is highly necessary. Previous studies have demonstrated numerous palladium and platinumbased hydrogen sensors [1 14]. Amongthese sensors, metalsemiconductor (MS) diodes have been addressed as one of the most promising devices [2 7]. Hydrogen sensors employing metal-oxide-semiconductor (MOS) diodes have also been extensively studied [8 10]. In addition, Chiu et al. [11 14] reported a new metal-semiconductor-metal (MSM) hydrogen sensor with two multifinger Schottky contacts. Unlike conventional MS and MOS diodes, a mixture of palladium and silicon dioxides (MMO) is deposited upon the semiconductor layer. Compared to commonly used MS andmossensors,themmomsmsensorsachieveexcellent performance of high sensitivity. However, the current-voltage (I-V) characteristics in this paper [14] represent the diode current of the sensor operated in N 2.Thereasonthatcauses the two-step I-V curve is interesting. 2. Device Structure and Fabrication The process started with mesa isolation. HCl was used to remove the native oxide on the 0.8 μm n-gaaslayer with cm 3 doping concentration after a device mesa. Two multifinger Schottky electrodes forming a metalsemiconductor-metal (MSM) diode were implemented by thermally depositing a 30 nm mixture of Pd and SiO 2 with a weight ratio of 3. The area of the multifinger electrode was A cm 2. Another MSM diode with a 30 nm Pd directly deposited upon the GaAs layer was also fabricated for comparison. Figure 1 showsthemsmdiodewithtwo multifinger Schottky contacts. 3. Results Discussion I-V characteristics of MSM diode with and without a mixture of Pd and SiO 2 are shown in Figure 2. Becausethequality of the epitaxial wafer and evaporative mixture is excellent and uniform, all curves are bidirectional and symmetrical. Unlike the lowest curve representing the current of Pd MSM diodes, the upper curve with the two-step I-V curve is the current of MMO MSM diodes. Obviously, the I-V curve is the same as the published paper [14]; therefore, to deposit a 30 nm mixture of Pd and SiO 2 upon the GaAs layer is repeatable.

2 2 Advances in Materials Science and Engineering V +V Figure 1: Schematic diagram of the MSM diodes with two multifinger Schottky contacts. Mixture of Pd and SiO 2 Semiconductor I Pd Simulation results Figure 2: Current-voltage characteristics for Pd-mixture-GaAs and Pd-GaAs MSM diodes. Inset diagram is the schematic view of mixture of Pd and SiO 2 deposited upon the semiconductor layer. To consider the Schottky emission process and the imageforce lowering, the current of Pd MSM diodes (I Pd )canbe expressed as [15, 16] I Pd =A A T 2 e q{φ B q/4πε S E m }/kt, (1) where E m = (2qN D /εs )(V n +φ B + (kt/q) + V), A = 9.6 A/k-cm 2, A cm 2, T = 300K, q = C, k = J/K, φ B =0.83eV,N d = cm 3, ε S = 10.8, ε S =12.9, V n =0.05V,and V =0Vto 5 V are the maximal electric field, the Richardson constant, contact area, absolute temperature, unit electronic charge, Boltzmann constant, barrier height, doping concentration, permittivity of GaAs near the Pd, permittivity of GaAs, Fermi potential from conduction-band edge, and applied voltage, respectively. Figure 2 shows the simulation of I Pd as a dot symbol. The results of simulation and experiment match each other.however,thecontentofthepdandsio 2 in mixture is uniform, and the thermionic emission over the metalsemiconductor barrier and the insulator-semiconductor barrier is responsible for carrier transport. Therefore, the current of the MMO MSM diodes is discussed according to two components. The first is the designed in consideration of the thermionic emission over the metal-semiconductor barrier; the second is the designed in consideration of the thermionic emission over the insulator-semiconductor barrier. The inset of Figure 2 shows the schematic view of the mixture of Pd and SiO 2 deposited upon the semiconductor layer. To discuss the thermionic emission over the metalsemiconductor barrier, substituting for E m from (1), can be obtained [15, 16] as follows: =A A Pd T 2 e q{φ B [(q 3 N d /8π 2 (ε S )2 ε S )(φ B V n (kt/q) V)] 0.25 }/kt, where A Pd cm 2 is the effective Pd-contact area. φ B =0.81eVisnotthesameasI Pd because MMO MSM diodes (2)

3 Advances in Materials Science and Engineering 3 αv V 0.25 (a) (b) αv 0.5 V 0.5 Figure 3: Current of Pd-mixture-GaAs MSM diodes as a function of (a) V 0.25 with and (b) V 0.5 with for comparison. E Fm qφ B E C E F E i Figure 4: Band diagram under a larger applied voltage. E V αqv/ηkt Simulation results (a) (b) Figure 5: Current as a function of applied voltage for Pd-mixture-GaAs MSM diodes with (a) and (b) simulation result for comparison. do not fabricate simultaneously with Pd MSM diodes. Other parameters are the same as I Pd.Particularly,A Pd is given by (3/D A Pd = Pd ) 2/3 A, (3) (3/D Pd ) 2/3 2/3 +(1/D ox ) where D Pd =12.023g/cm 3 and D ox =2.648g/cm 3 are the density of Pd and the density of SiO 2,respectively.Figure 3(a) shows the I-V curve with V 0.25.AcurveoflnI against V 0.25 represents a straight line from V 0.25 =0Vto0.58V,meaning that is dominant from V =0Vto0.12V. In the discussion of thermionic emission over the insulator-semiconductor barrier on,weobtain[15] =A A ox T 2 e q{φ B qv/4πε i d}/kt, (4) where d =30nm,ε i = 3.7, and A ox cm 2 are the thickness of mixture, the permittivity of mixture, and the effective oxide-contact area. Other parameters are the same as.particularly,a Pd is given by (1/D A Pd = ox ) 2/3 A. (5) (3/D Pd ) 2/3 2/3 +(1/D ox )

4 4 Advances in Materials Science and Engineering Figure 3(b) shows the I-V curve with V 0.5.Acurveof ln is proportional to V 0.5 from V 0.5 = 1.2 V to 1.9 V, meaning that is dominant from V = 1.4 V to 3.6 V. Furthermore, when a larger voltage is applied (>4 V), the bands bend even more downward so that the intrinsic level E i at the surface crosses over the Fermi level E F. Figure 4 shows the band diagram under a larger applied voltage. At this point, the number of holes (minority carriers) at the surface is larger than the number of the electrons (majority carrier), and thermionic emission of electrons is recombined by holes. The current ( )isproportionaltoqv/ηkt. can be expressed as [15] =S e qv/ηkt, (6) where S = Aisthesaturationcurrentof recombination and η = 8.1istheidealityfactor.Figure 5(a) shows the plot of ln against V, representing a straight line when the applied voltage is greater than 4 V. Figure 5(b) shows the summation of,,and as a dot symbol. The results of simulation and experiment match each other. 4. Conclusions This study examined the current transport mechanism for Pd MSM GaAs diodes with a new Schottky contact material formed by intentionally mixing SiO 2 into a Pd metal. A mechanism concept has successfully explained the influence of the mixed SiO 2 on the current for MMO MSM diodes under the applied voltage. The effectively simulated data for the Schottky emission process, image-force lowering, and carrier recombination clearly explain the experimental results. Acknowledgment ThisworkisfinanciallysupportedbytheNationalTaiwan Ocean University of the Republic of China under the contract nos. NTOU and 101B [6] A. Salehi, A. Nikfarjam, and D. J. Kalantari, Pd/porous-GaAs Schottky contact for hydrogen sensing application, Sensors and Actuators B,vol.113,no.1,pp ,2006. [7] C.Hung,T.Tsai,H.Chen,Y.Tsai,T.Chen,andW.Liu, Further investigation of a hydrogen-sensing Pd/GaAs heterostructure field-effect transistor (HFET), Sensors and Actuators B, vol. 132, no. 2, pp , [8] C.W.Hung,K.W.Lin,R.C.Liuetal., Onthehydrogensensing properties of a Pd/GaAs transistor-type gas sensor in a nitrogen ambiance, Sensors and Actuators B, vol. 125, no. 1, pp , [9]T.H.Tsai,H.I.Chen,K.W.Linetal., Hydrogensensing characteristics of a Pd/AlGaN/GaN schottky diode, Applied Physics Express,vol.1,no.4,pp ,2008. [10] C. H. Huang, J. H. Tsai, T. M. Tsai et al., Hydrogen sensor with Pd nanoparticles upon an interfacial layer with oxygen, Applied Physics Express,vol.3,no.7,ArticleID075001,2010. [11] S. Y. Chiu, H. W. Huang, T. H. Huang et al., High-sensitivity metal-semiconductor-metal hydrogen sensors with a mixture of Pd and SiO 2 forming three-dimensional dipoles, IEEE Electron Device Letters,vol.29,no.12,pp ,2008. [12] S.Y.Chiu,H.W.Huang,K.C.Liangetal., GaNsensorswith metal-oxide mixture for sensing hydrogen-containing gases of ultralow concentration, Electronics Letters, vol. 45, no. 4, pp , [13] S. Y. Chiu, K. C. Liang, T. H. Huang et al., GaN Sensors with metal-oxide mixture for sensing hydrogen-containing gases of ultralow concentration, Japanese Applied Physics, vol. 48, no. 4, Article ID , [14] S. Chiu, J. Tsai, H. Huang et al., Integrated hydrogen-sensing amplifier with Schottky-type diode and InGaP-GaAs heterojunction bipolar transistor, IEEE Electron Device Letters, vol. 30, no. 9, pp , [15] S.M.SzeandK.K.Ng,Physics of Semiconductor Devices, 3rd ed,johnwiley&sons,newjersey,nj,usa,2007. [16] V. L. Rideout and C. R. Crowell, Effects of image force and tunneling on current transport in metal-semiconductor (Schottky barrier) contacts, Solid State Electronics, vol.13,no. 7, pp , References [1] T. Usagawa and Y. Kikuchi, Air-annealing effects for Pt/Ti Gate Si-metal-oxide-semiconductor field-effect transistors hydrogen gas sensor, Applied Physics Express, vol. 3, no. 4, ArticleID , [2] M. Eriksson and L. Ekedahl, Hydrogen adsorption states at the Pd/SiO 2 interface and simulation of the response of a Pd metal-oxide-semiconductor hydrogen sensor, Applied Physics,vol.83,no.8,pp ,1998. [3] I. Lundström, S. Shivaraman, C. Svensson, andl. Lundkvist, A hydrogen-sensitive MOS field-effect transistor, Applied Physics Letters,vol.26,no.2,pp.55 57,1975. [4] D. E. Aspnes and A. Heller, Barrier height and leakage reduction in n-gaas-platinum group metal Schottky barriers upon exposure to hydrogen, Vacuum Science and Technology B,vol.1,no.3,pp ,1983. [5] H. Y. Nie and Y. Nannichi, Pd-on-GaAs Schottky contact. Its barrier height and response to hydrogen, Japanese Applied Physics,vol.30,no.5,pp ,1991.

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