In this paper, the Schottky behavior of Pt contact on n- GaN grown by RF-plasma assisted molecular beam epitaxy was investigated under different annealing temperatures

C, W Chin

F, K Yam

Hassan, Z

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Effects of thermal annealing of Pt Schottky contacts on n-GaN  C.W. Chin, Z. Hassan, F.K. Yam Nano-Optoelectronics Research and Tecnology Laboratory School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia takeshi_ccw@yahoo.com, zai@usm.my  Abstract In this paper, the Schottky behavior of Pt contact on n-GaN grown by RF-plasma assisted molecular beam epitaxy was investigated under different annealing temperatures.  The temperature dependence and structural evolution of the Schottky barrier heights of Pt contacts were characterized by using current- voltage (I-V) measurements and scanning electron microscopy (SEM). The results revealed that the Schottky barrier heights of the samples change with annealing temperature.  The high quality Schottky contact with a barrier height and ideality factor of 1.107 eV and 1.234 respectively can be obtained under 30 min annealing at 800oC in N2 ambience.   1. Introduction Gallium nitride (GaN) has a wide band gap of 3.4 eV which is useful for of UV/blue light emitting diodes and high temperature electronics applications.  GaN based electronic and optoelectronic devices including visible light emitting diode (LED) [1], ultraviolet photoconductive detectors [2], ultraviolet Schottky barrier photodetector [3], metal-semiconductor field effect transistors (MESFETs) [4], high electron mobility transistors (HEMTs) [5] and heterojunction bipolar transistors (HBTs) [6] have been reported.  In order to optimize the electrical behavior of these devices, it is crucial to develop stable and reliable ohmic and Schottky contacts.   Pt has a high work function that makes it ideal for use as Schottky contacts on n-type GaN and it also resistance to oxidation and corrosion [7].  The Schottky barrier height of Pt/n-GaN has been reported by several groups with value between 0.89 eV and 1.27 eV [8-10].  According to the Schottky-Mott model, the Schottky barrier height is dependent on the metal work function and electron affinity of semiconductor χ (GaN χ = 4.1eV) [11].  Metals with low functions are used in many ohmic contacts to n-GaN, while metals with high metal work functions have received the most attention in Schottky barrier contacts to n-GaN.  The work function of Pt metal is 5.65 eV and consequently expects to form a good Schottky contact on n-GaN [12].   In this paper, we report the experimental results of thermal annealing of Pt Schottky contacts on n-type GaN.  The structural and temperature dependence of the Schottky barriers heights of Pt contacts on n-GaN epilayer at different annealing temperature have been studied.   2. Experiment details The n-type GaN thin film used in this study was grown on sapphire substrate by using RF plasma-assisted molecular beam epitaxy (RF-MBE).  Active nitrogen was supplied by nitrogen plasma source while effusions cell were used for Ga and Si. Prior to the expitaxial growth, the sapphire substrate was nitridated for 45 min at 200oC.  This process consequently results in the formation of a relaxed AlN thin buffer layer [13].  After nitridation, a thin AlN buffer layer was grown at the substrate temperature of 780oC, and then GaN epilayer was grown at 760oC, followed by in situ doping using Si.  During the growth of n-type GaN, streaky reflection high energy electron diffraction (RHEED) patterns were observed.   In this experiment, only one wafer was used and it was divided into several small pieces to minimize possible variability between different growth runs.  The characteristics of Schottky barrier contacts are strongly influenced by the condition of the semiconductor surface prior to the metallization.  For III-V semiconductors, a few nanometers thin oxide layer grows rapidly on the surface when it exposed to air.  Hence, prior to the metal deposition, the n-type GaN film was cleaned by aqua regia solution to remove the native oxide on the surface.  Pt contacts with thickness of about 150nm are first deposited on the samples by using sputtering system.  To study the thermal annealing effects of Pt contacts on n-type GaN, samples were annealed in N2 atmosphere at temperature of 200oC, 400 oC, 600 oC, 800 oC for 30 min in tube furnace.  The current-voltage (I-V) measurements were performed with a Kiethley High-voltage-source-measure-unit model 237.  Scanning electron microscopy (SEM) was employed to investigate the surface morphology of the sample.   3. Results and discussion Fig. 1 shows the I-V characteristics for Pt Schottky contacts on n-GaN.  As-deposited, Pt contact shows ohmic characteristic, however after annealing at 200oC, a Schottky behavior is observed.  The metal contacts become more rectifying with increase in the annealing temperature.  The electrical parameters, i.e., Schottky barrier heights (SBHs), ФB, saturation current, Io, and ideality factor, η can be determined from the I-V measurements.  For thermionic emission and V > 3kT/q, the general diode equations are [14]:  I  = Ioexp{qV/(ηkT)}                                        (1)  Io= AA*T2exp{-qФB/(kT)}                               (2)  Where Io is the saturation current density, η is the ideality factor, q is the electron charge, k is Boltzmann’s constant, T is the absolute temperature, A* is the effective Richardson coefficient, A is the contact area, and ФB is the barrier height.  The theoretical value of A* can calculated using  A*= 4pim*qk2/h3                                                   (3)  where h is plank’s constant. For n-type GaN, m* = 0.20mo is the effective electron mass for GaN and the value of A* is determined to be 26.4 Acm-2K-2 [15].   The extrapolated value of current to zero voltage for the straight line portion of the plot of lnI vs V gives the saturation current Io and give a straight line with a slope of q/(ηkT).  The barrier height of the Schottky contact was determined from the Eq. (2).  For sample involving two Schottky contacts, representing two diodes connected back-to-back, the I-V characteristics of the Schottky contact are more appropriate to be analyzed in the more general form of equation, where it can be used under reverse bias conditions [16,17]  I = Ioexp{qV/(ηkT)}[1-exp{-qV/(kT)}]            (4)  The equation can be written as  )/exp(1)/exp()/exp( kTqVIkTqVkTqVIo η=−                  (5)  The y-intercept of a linear curve fits yields the saturation current, Io which is then plotted as ln{Iexp{qV/kT}/[exp{qV/kT}-1]} versus V.  From this plot, ФB the barrier height can be extracted by using Eq. (2).   The results of the Schottky barrier height and ideality factor of the Pt Schottky contacts at different annealing temperatures are summarized in Table 1.  It can be seen clearly from Table 1 that the ideality factor and Schottky barrier height of the samples change with annealing temperature.  A slight increase of Schottky barrier height of Pt contacts were observed for 400oC annealed and increased greatly after annealing above 800oC.  At 800oC annealing temperature, the Schottky barrier height and the ideality factor of the Pt contact are 1.107 eV and 1.234 eV respectively.  The increase of the Schottky barrier height can be correlated with the reduction of nonstoichiometric defects in the metallurgical interface [18,19].          Table 1 The comparison of SBH and ideality factor of Pt Scottky contact for various annealing temperature      and duration   Sample Annealing temperature Schottky barrier height (eV) Ideality factor,n 1 As deposited N.A N.A 2 200oC 0.451 1.030 3 400oC 0.677 1.050 4 600oC 0.876 1.156 5 800oC 1.107 1.234     -4.00E-02-3.00E-02-2.00E-02-1.00E-020.00E+001.00E-022.00E-023.00E-024.00E-02-6.00E+00 -4.00E+00 -2.00E+00 0.00E+00 2.00E+00 4.00E+00 6.00E+00voltagecurrentas-deposited200C400C600C800C Fig. 1. The I-V characteristics of the samples with Pt Scottky contact under different annealing temperatures.                                                   (a) as grown                                                                                    (b) 200oC                                                (c) 400oC                                                                                        (d) 600oC    (e) 800oC  Fig. 2. SEM observation of the metal surfaces after annealing at different temperatures.   The morphology of the surface of Pt contacts after annealing was observed by using SEM. The results of SEM are shown in Fig. 2.  As seen in the figure, the as-deposited and annealed samples (200oC) show a smooth and featureless surface.  However, the rough surfaces were observed at annealing temperature of 400oC.   Increasing the annealing temperature at 600oC brings more dramatic changes.  The rough surfaces become more obvious.  When the annealing    temperature reached 800oC, the particles of Pt become larger and reveal dark areas of the surface.  Metal island formation was observed at high temperatures.  Island formation during the high annealing of thin metal contacts on n-GaN is due to the difference of surface energies between thin metal films and GaN that causes dewetting [20]. 4. Conclusion Thermal annealing effects of Pt Schottky contact on n-GaN were studied by annealing samples in N2 ambience for 30 min at various temperatures. The Schottky barrier height and the surface of Pt contact change with various annealing temperatures.  The behavior of Pt on n-GaN shows that they are suitable Schottky contacts at temperature as high as 800oC.  Acknowledgement The support from Universiti Sains Malaysia is gratefully acknowledged.   References [1] Nakamura. S., Mukai. T., and Senoh. M., jpn. J. Appl. Phys. 30, (1991), pp. L1998-L2001. [2] Khan. M.A., Kuznia. J.N., Olson. D.T., Van Hove. M.J. and Blasingame. M., Appl. Phys. Lett. 60, (1992), pp. 2917-2919. [3] Khan. M.A., Kuznia. J.N., Olson. D.T.,  Blasingame. M. and Bhattarai. A.R., Appl. Phys. Lett. 63, (1993), pp. 2455-2456. [4]   Khan. M.A., Kuznia. J.N., Bhattarai. A.R. and            Olson. D.T., Appl. Phys. Lett. 62, (1993), pp.           1786-1787. [5] Khan. M.A., Bhattarai. A.R., Kuznia. J.N. and Olson. 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Effects Of Thermal Annealing Of Pt Schottky Contacts On n-GaN.

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