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Sunday, April 17, 2011
Friday, July 30, 2010
New Article in Thin Solid Films
Double subband occupation of the two-dimensional electron gas in InxAl1-xN/AlN/GaN/AlN heterostructures with a low indium content (0.064 < x < 0.140) barrier
We present a carrier transport study on low indium content (0.064 < x < 0.140) InxAl1 - xN/AlN/GaN/AlN heterostructures. Experimental Hall data were carried out as a function of temperature (33–300 K) and a magnetic field (0–1.4 T). A two-dimensional electron gas (2DEG) with single or double subbands and a two-dimensional hole gas were extracted after implementing quantitative mobility spectrum analysis on the magnetic field dependent Hall data. The mobility of the lowest subband of 2DEG was found to be lower than the mobility of the second subband. This behavior is explained by way of interface related scattering mechanisms, and the results are supported with a one-dimensional self-consistent solution of non-linear Schrödinger–Poisson equations.
cite: S. B. Lisesivdin, P. Tasli, M. Kasap, M. Ozturk, E. Arslan, S. Ozcelik, and E. Ozbay, "Double subband occupation of the two-dimensional electron gas in InxAl1-xN/AlN/GaN/AlN Heterostructures with Low Indium Barrier (0.064 < x < 0.139)" Thin Solid Films 518, 5572 (2010). DOI
We present a carrier transport study on low indium content (0.064 < x < 0.140) InxAl1 - xN/AlN/GaN/AlN heterostructures. Experimental Hall data were carried out as a function of temperature (33–300 K) and a magnetic field (0–1.4 T). A two-dimensional electron gas (2DEG) with single or double subbands and a two-dimensional hole gas were extracted after implementing quantitative mobility spectrum analysis on the magnetic field dependent Hall data. The mobility of the lowest subband of 2DEG was found to be lower than the mobility of the second subband. This behavior is explained by way of interface related scattering mechanisms, and the results are supported with a one-dimensional self-consistent solution of non-linear Schrödinger–Poisson equations.
cite: S. B. Lisesivdin, P. Tasli, M. Kasap, M. Ozturk, E. Arslan, S. Ozcelik, and E. Ozbay, "Double subband occupation of the two-dimensional electron gas in InxAl1-xN/AlN/GaN/AlN Heterostructures with Low Indium Barrier (0.064 < x < 0.139)" Thin Solid Films 518, 5572 (2010). DOI
Thursday, July 15, 2010
New Article in Journal of Applied Physics
Scattering analysis of two-dimensional electrons in AlGaN/GaN with bulk related parameters extracted by simple parallel conduction extraction method
We carried out the temperature (22–350 K) and magnetic field (0.05 and 1.4 T) dependent Hall mobility and carrier density measurements on Al0.22Ga0.78N/GaN heterostructures with AlN interlayer grown by metal-organic chemical-vapor deposition. Hall data is analyzed with a simple parallel conduction extraction method and temperature dependent mobility and carrier densities of the bulk and two-dimensional (2D) electrons are extracted successfully. The results for the bulk carriers are discussed using a theoretical model that includes the most important scattering mechanisms that contribute to the mobility. In order to investigate the mobility of two-dimensional electron gas, we used a theoretical model that takes into account the polar optical phonon scattering, acoustic phonon scattering, background impurity scattering, and interface roughness scattering in 2D. In these calculations, the values are used for the deformation potential and ionized impurity density values were obtained from the bulk scattering analysis. Therefore, the number of fitting parameters was reduced from four to two.
cite: S. B. Lisesivdin, A. Yildiz, N. Balkan, M. Kasap, S. Ozcelik, E. Ozbay "Scattering Analysis of Two-dimensional Electrons in AlGaN/GaN with Bulk Related Parameters Extracted by Simple Parallel Conduction Extraction Method" J. Appl. Phys. 108, 013712 (2010). DOI
We carried out the temperature (22–350 K) and magnetic field (0.05 and 1.4 T) dependent Hall mobility and carrier density measurements on Al0.22Ga0.78N/GaN heterostructures with AlN interlayer grown by metal-organic chemical-vapor deposition. Hall data is analyzed with a simple parallel conduction extraction method and temperature dependent mobility and carrier densities of the bulk and two-dimensional (2D) electrons are extracted successfully. The results for the bulk carriers are discussed using a theoretical model that includes the most important scattering mechanisms that contribute to the mobility. In order to investigate the mobility of two-dimensional electron gas, we used a theoretical model that takes into account the polar optical phonon scattering, acoustic phonon scattering, background impurity scattering, and interface roughness scattering in 2D. In these calculations, the values are used for the deformation potential and ionized impurity density values were obtained from the bulk scattering analysis. Therefore, the number of fitting parameters was reduced from four to two.
cite: S. B. Lisesivdin, A. Yildiz, N. Balkan, M. Kasap, S. Ozcelik, E. Ozbay "Scattering Analysis of Two-dimensional Electrons in AlGaN/GaN with Bulk Related Parameters Extracted by Simple Parallel Conduction Extraction Method" J. Appl. Phys. 108, 013712 (2010). DOI
Wednesday, June 30, 2010
New Article in Philosophical Magazine
Contributions of impurity band and electron-electron interactions to magnetoconductance in AlGaN
Low temperature electrical measurements of conductivity, the Hall effect and magnetoconductance were performed on a degenerate AlGaN sample. The sample exhibited negative magnetoconductance at low magnetic fields and low temperatures, with the magnitude being systematically dependent on temperature. The measured magnetoconductance was compared with models proposed previously by Sondheimer and Wilson [Proc. R. Soc. Lond. Ser. A 190 (1947) p. 435] and Lee and Ramakrishan [Rev. Mod. Phys. 57 (1985) p. 287]. Data were analyzed as the sum of the contribution of a two-band and electron-electron interactions to the magnetoconductance, applying these models to describe the observed behavior. Least-squares fits to the data are presented. In the sample, magnetoconductance can be explained reasonably well by assuming these contributions to the measured magnetoconductance. It was found that theoretical and experimental data were in excellent agreement.
cite: P. Tasli, A. Yildiz, M. Kasap, E. Ozbay, S. B. Lisesivdin and S. Ozcelik "Contributions of the impurity band and electron–electron interactions to the magnetoconductance of AlGaN" Philos. Mag. 90, 3591 (2010). DOI
Low temperature electrical measurements of conductivity, the Hall effect and magnetoconductance were performed on a degenerate AlGaN sample. The sample exhibited negative magnetoconductance at low magnetic fields and low temperatures, with the magnitude being systematically dependent on temperature. The measured magnetoconductance was compared with models proposed previously by Sondheimer and Wilson [Proc. R. Soc. Lond. Ser. A 190 (1947) p. 435] and Lee and Ramakrishan [Rev. Mod. Phys. 57 (1985) p. 287]. Data were analyzed as the sum of the contribution of a two-band and electron-electron interactions to the magnetoconductance, applying these models to describe the observed behavior. Least-squares fits to the data are presented. In the sample, magnetoconductance can be explained reasonably well by assuming these contributions to the measured magnetoconductance. It was found that theoretical and experimental data were in excellent agreement.
cite: P. Tasli, A. Yildiz, M. Kasap, E. Ozbay, S. B. Lisesivdin and S. Ozcelik "Contributions of the impurity band and electron–electron interactions to the magnetoconductance of AlGaN" Philos. Mag. 90, 3591 (2010). DOI
Monday, April 19, 2010
New Article in Journal of Materials Science: Materials in Electronics
The substrate temperature dependent electrical properties of titanium dioxide thin films
Titanium dioxide thin films were obtained by a dc sputtering technique onto heated glass substrates. The relationship between the substrate temperature and the electrical properties of the films was investigated. Electrical resistivity measurements showed that three types of conduction channels contribute to conduction mechanism in the temperature range of 13–320 K. The temperature dependence of electrical resistivity between 150 and 320 K indicated that electrical conduction in the films was controlled by potential barriers caused by depletion of carriers at grain boundaries. The conduction mechanism of the films was shifted from grain boundary scattering dominated band conduction to the nearest neighbor hopping conduction at temperatures between 55 and 150 K. Below 55 K, the temperature dependence of electrical resistivity shows variable range hopping conduction. The correlation between the substrate temperature and resistivity behavior is discussed by analyzing the physical plausibility of the hopping parameters and material properties derived by applying different conduction models. With these analyses, various electrical parameters of the present samples such as barrier height, donor concentration, density of states at the Fermi level, acceptor concentration and compensation ratio were determined. Their values as a function of substrate temperature were compared.
cite: A. Yildiz, S. B. Lisesivdin, M. Kasap, D. Mardare “The substrate temperature effect on the electrical properties of titanium oxide thin films” J. Mater. Sci.: Mater. Electron. 21, 692 (2010). DOI
Titanium dioxide thin films were obtained by a dc sputtering technique onto heated glass substrates. The relationship between the substrate temperature and the electrical properties of the films was investigated. Electrical resistivity measurements showed that three types of conduction channels contribute to conduction mechanism in the temperature range of 13–320 K. The temperature dependence of electrical resistivity between 150 and 320 K indicated that electrical conduction in the films was controlled by potential barriers caused by depletion of carriers at grain boundaries. The conduction mechanism of the films was shifted from grain boundary scattering dominated band conduction to the nearest neighbor hopping conduction at temperatures between 55 and 150 K. Below 55 K, the temperature dependence of electrical resistivity shows variable range hopping conduction. The correlation between the substrate temperature and resistivity behavior is discussed by analyzing the physical plausibility of the hopping parameters and material properties derived by applying different conduction models. With these analyses, various electrical parameters of the present samples such as barrier height, donor concentration, density of states at the Fermi level, acceptor concentration and compensation ratio were determined. Their values as a function of substrate temperature were compared.
cite: A. Yildiz, S. B. Lisesivdin, M. Kasap, D. Mardare “The substrate temperature effect on the electrical properties of titanium oxide thin films” J. Mater. Sci.: Mater. Electron. 21, 692 (2010). DOI
Thursday, April 1, 2010
New article in Current Applied Physics
Determination of the critical indium composition corresponding to the metal–insulator transition in InxGa1-xN (0.06 < x < 0.135) layers
The low-temperature conductivity of InxGa1-xN alloys (0.06 < x < 0.135) is analyzed as a function of indium composition (x). Although our InxGa1-xN alloys were on the metallic side of the metal–insulator transition, neither the Kubo-Greenwood nor Born approach were able to describe the transport properties of the InxGa1-xN alloys. In addition, all of the InxGa1-xN alloys took place below the Ioeffe–Regel regime with their low conductivities. The observed behavior is discussed in the framework of the scaling theory. With decreasing indium composition, a decrease in thermal activation energy is observed. For the metal–insulator transition, the critical indium composition is obtained as xc = 0.0543 for InxGa1-xN alloys.
cite: A. Yildiz, S. B. Lisesivdin, P. Tasli, E. Ozbay, and M. Kasap "Determination of the critical indium composition corresponding to the metal-insulating transition in InxGa1-xN (0.06 < x < 0.135) layers" Curr. Appl. Phys. 10, 838 (2010). DOI
The low-temperature conductivity of InxGa1-xN alloys (0.06 < x < 0.135) is analyzed as a function of indium composition (x). Although our InxGa1-xN alloys were on the metallic side of the metal–insulator transition, neither the Kubo-Greenwood nor Born approach were able to describe the transport properties of the InxGa1-xN alloys. In addition, all of the InxGa1-xN alloys took place below the Ioeffe–Regel regime with their low conductivities. The observed behavior is discussed in the framework of the scaling theory. With decreasing indium composition, a decrease in thermal activation energy is observed. For the metal–insulator transition, the critical indium composition is obtained as xc = 0.0543 for InxGa1-xN alloys.
cite: A. Yildiz, S. B. Lisesivdin, P. Tasli, E. Ozbay, and M. Kasap "Determination of the critical indium composition corresponding to the metal-insulating transition in InxGa1-xN (0.06 < x < 0.135) layers" Curr. Appl. Phys. 10, 838 (2010). DOI
Tuesday, March 16, 2010
New Article in Semiconductor Science and Technology
Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel
The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)-1 with corresponding sheet carrier densities of 2.3–1.3 × 1013 cm-2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)-1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm-2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.
cite: S. Gokden, R. Tulek, A. Teke, J. H. Leach, Q. Fan, J. Xie, U. Ozgur, H. Morkoc, S. B. Lisesivdin, E. Ozbay "Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with InGaN channel" Semicond. Sci. Technol. 25, 045024 (2010). DOI
The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)-1 with corresponding sheet carrier densities of 2.3–1.3 × 1013 cm-2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)-1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm-2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.
cite: S. Gokden, R. Tulek, A. Teke, J. H. Leach, Q. Fan, J. Xie, U. Ozgur, H. Morkoc, S. B. Lisesivdin, E. Ozbay "Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with InGaN channel" Semicond. Sci. Technol. 25, 045024 (2010). DOI
Saturday, January 23, 2010
Oracle+Sun
Bizi şoka sokan son büyük balığın küçük balığı yeme olayı Oracle'ın Sun'ı almasıydı. Veritabanlarıyla ilgilenmesemde, MySQL ile pek işim olmadıysa da, yine de OpenOffice kullanıcısıyız dedik, birazda eski Sun donanımlarını sevmek işin içine girince, Sun'ın alınmasına üzülmüştük.
Oracle'ın satın almasından bu yana herkes MySQL ve Solaris'e ne olacak onun merakındaydı. Bu ürünler Oracle'ın mevcut ürünlerinin rakipleri denebilir. Sun'da satın almadan bu yana pek göze batan işlerde yapmadı.
Tüm bu konular, 27 Ocakta konuşulacak gibi görünüyor. Çünkü Oracle'ın CEO'su Larry Ellison, bu tarihte Oracle+Sun stratejisini açıklayacak. Bir yıl kaybettiler, artık ses çıkmasının zamanıydı.
Gerçi o gün Apple'ın toplantısıda var. Güzel bir gün olacağa benziyor.
Sunday, January 3, 2010
New Article in Applied Physics A: Materials Science & Processing
Investigation of low-temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with Simple Parallel Conduction Extraction Method
The electrical conduction mechanisms in various highly resistive GaN layers of Al x Ga1−x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal–insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln (ρ) is proportional to T −1/4 for the insulating sample and proportional to T −1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.
cite: A. Yildiz, S. B. Lisesivdin, M. Kasap, S. Ozcelik, E. Ozbay and N. Balkan "Investigation of low temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with SPCEM" Appl. Phys. A 98, 557 (2010). DOI
The electrical conduction mechanisms in various highly resistive GaN layers of Al x Ga1−x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal–insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln (ρ) is proportional to T −1/4 for the insulating sample and proportional to T −1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.
cite: A. Yildiz, S. B. Lisesivdin, M. Kasap, S. Ozcelik, E. Ozbay and N. Balkan "Investigation of low temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with SPCEM" Appl. Phys. A 98, 557 (2010). DOI
Monday, December 28, 2009
New Article in Crystal Research and Technology
Well parameters of two-dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD
Resistivity and Hall effect measurements were carried out as a function of magnetic field (0-1.5 T) and temperature (30-300 K) for Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two-dimensional electron gas (2DEG) channel located at the Al0.88In0.12N/GaN interface with an AlN interlayer and a two-dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al0.88In0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al0.88In0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA.
Resistivity and Hall effect measurements were carried out as a function of magnetic field (0-1.5 T) and temperature (30-300 K) for Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two-dimensional electron gas (2DEG) channel located at the Al0.88In0.12N/GaN interface with an AlN interlayer and a two-dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al0.88In0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al0.88In0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA.
cite: P. Tasli, S. B. Lisesivdin, A. Yildiz, M. Kasap, E. Arslan, E. Özbay, and S. Özcelik, Cryst. Res. Tech. 45, 133 (2010).
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