Browsing by Author "Tekeli, Z."
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Item Open Access The behavior of the I-V-T characteristics of inhomogeneous (Ni∕Au)-Al0.3Ga0.7N∕AlN∕GaN heterostructures at high temperatures(EDP Sciences, 2007-09-12) Tekeli, Z.; Altındal, S.; Çakmak, M.; Özçelik, S.; Çalıikan, D.; Özbay, EkmelWe investigated the behavior of the forward bias current-voltage-temperature (I-V-T) characteristics of inhomogeneous (Ni/Au)-Al0.3Ga0.7N/AlN/GaN heterostructures in the temperature range of 295-415 K. The experimental results show that all forward bias semilogarithmic I-V curves for the different temperatures have a nearly common cross point at a certain bias voltage, even with finite series resistance. At this cross point, the sample current is temperature independent. We also found that the values of series resistance (R-s) that were obtained from Cheung's method are strongly dependent on temperature and the values abnormally increased with increasing temperature. Moreover, the ideality factor (n), zero-bias barrier height (Phi(B0)) obtained from I-V curves, and R-s were found to be strongly temperature dependent and while Phi(B0) increases, n decreases with increasing temperature. Such behavior of Phi(B0) and n is attributed to Schottky barrier inhomogeneities by assuming a Gaussian distribution (GD) of the barrier heights (BHs) at the metal/semiconductor interface. We attempted to draw a Phi(B0) versus q/2kT plot in order to obtain evidence of the GD of BHs, and the values of (Phi) over bar (B0)=1.63 eV and sigma(0)=0.217 V for the mean barrier height and standard deviation at a zero bias, respectively, were obtained from this plot. Therefore, a modified ln(I-0/T-2)-q(2)sigma(2)(0)/2(kT)(2) versus q/kT plot gives Phi(B0) and Richardson constant A(*) as 1.64 eV and 34.25 A/cm(2) K-2, respectively, without using the temperature coefficient of the barrier height. The Richardson constant value of 34.25 A/cm(2) K-2 is very close to the theoretical value of 33.74 A/cm(2) K-2 for undoped Al0,3Ga0,7N. Therefore, it has been concluded that the temperature dependence of the forward I-V characteristics of the (Ni/Au)-Al0.3Ga0.7/AlN/GaN heterostructures can be successfully explained based on the thermionic emission mechanism with the GD of BHs.Item Open Access Effects of thermal annealing on the morphology of the AlxGa(1x)N films(Elsevier, 2010-01-08) Corekci, S.; Tekeli, Z.; Cakmak, M.; Ozcelik, S.; Dinc, Y.; Zeybek, O.; Özbay, EkmelEffects of thermal annealing on the morphology of the AlxGa(1-x)N films with two different high Al-contents (x=0.43 and 0.52) have been investigated by atomic force microscopy (AFM). The annealing treatments were performed in a nitrogen (N-2) gas ambient as short-time (4 min) and long-time (30 min). Firstly, the films were annealed as short-time in the range of 800-950 degrees C in steps of 50-100 degrees C. The surface root-mean-square (rms) roughness of the films reduced with increasing temperature at short-time annealing (up to 900 degrees C), while their surface morphologies were not changed. At the same time, the degradation appeared on the surface of the film with lower Al-content after 950 degrees C. Secondly, the Al0.43Ga0.57N film was annealed as long-time in the range of 1000-1200 degrees C in steps of 50 degrees C. The surface morphology and rms roughness of the film with increasing temperature up to 1150 degrees C did not significantly change. Above those temperatures, the surface morphology changed from step-flow to grain-like and the rms roughness significantly increased.Item Open Access On the profile of frequency dependent dielectric properties of (Ni/Au)/GaN/Al0.3Ga0.7N heterostructures(Elsevier, 2010-10-12) Tekeli, Z.; Gökçen, M.; Altindal, Ş.; Özçelik, S.; Özbay, EkmelThe voltage (V) and frequency (f) dependence of dielectric characteristics such as dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tan δ) and real and imaginary part of electrical modulus (Μ′ and M″) of the (Ni/Au)/GaN/Al0.3Ga 0.7N heterostructures have been investigated by using experimental admittance spectroscopy (capacitance-voltage (C-V) and conductance-voltage (G/w-V)) measurements at room temperature. Experimental results show that the values of the ε′, ε″, tan δ and the real and imaginary parts of the electric modulus (M′ and M″) obtained from the C and G/w measurements were found to be strong function of frequency and applied bias voltage especially in depletion region at low frequencies. These changes in dielectric parameters can be attributed to the interfacial GaN cap layer, interface polarization and a continuous density distribution of interface states and their relaxation time at metal/semiconductor interface. While the values of the ε ′ decrease with increasing frequencies, tan δ,M′ and M″ increase with the increasing frequency. Also, the dielectric loss (ε″) have a local maximum at about frequency of 100 kHz. It can be concluded that the interface polarization can occur more easily at low frequencies with the number of interface states located at the metal/semiconductor interface.Item Open Access The profile of temperature and voltage dependent series resistance and the interface states in (Ni/Au)/Al0.3Ga0.7N/AlN/GaN heterostructures(Elsevier BV, 2008-11) Tekeli, Z.; Altındal, Ş.; Çakmak, M.; Özçelik, S.; Özbay, EkmelThe temperature dependence of capacitance–voltage (C–V) and the conductance–voltage (G/w–V) characteristics of (Ni/Au)/Al0.3Ga0.7N/AlN/GaN heterostructures were investigated by considering the effect of series resistance (Rs) and interface states Nss in a wide temperature range (79–395 K). Our experimental results show that both Rs and Nss were found to be strongly functional with temperature and bias voltage. Therefore, they affect the (C–V) and (G/w–V) characteristics. The values of capacitance give two peaks at high temperatures, and a crossing at a certain bias voltage point (∼3.5 V). The first capacitance peaks are located in the forward bias region (∼0.1 V) at a low temperature. However, from 295 K the second capacitance peaks appear and then shift towards the reverse bias region that is located at ∼−4.5 V with increasing temperature. Such behavior, as demonstrated by these anomalous peaks, can be attributed to the thermal restructuring and reordering of the interface states. The capacitance (Cm) and conductance (G/w–V) values that were measured under both reverse and forward bias were corrected for the effect of series resistance in order to obtain the real diode capacitance and conductance. The density of Nss, depending on the temperature, was determined from the (C–V) and (G/w–V) data using the Hill–Coleman Method.Item Open Access Surface morphology of Al0.3Ga0.7N/Al2O3-high electron mobility transistor structure(American Scientific Publishers, 2008-02) Çörekçi, S.; Usanmaz, D.; Tekeli, Z.; Özçelik, S.; Özbay, EkmelWe present surface properties of buffer films (AlN and GaN) and Al0.3Ga0.7N/Al2O3-High Electron Mobility Transistor (HEMT) structures with/without AlN interlayer grown on High Temperature (HT)-AlN buffer/Al2O3 substrate and Al2O3 substrate. We have found that the GaN surface morphology is step-flow in character and the density of dislocations was about 108–109 cm−2. The AFM measurements also exhibited that the presence of atomic steps with large lateral step dimension and the surface of samples was smooth. The lateral step sizes are in the range of 100–250 nm. The typical rms values of HEMT structures were found as 0.27, 0.30, and 0.70 nm. HT-AlN buffer layer can have a significant impact on the surface morphology of Al0.3Ga0.7N/Al2O3-HEMT structures.