Development of a turning mid turbine frame with embedded design-part I: Design and steady measurements

© 2014 by ASME. The paper presents a new setup for the two-stage two-spool facility located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology. The rig was designed in order to simulate the flow behavior of a transonic turbine followed by a counter-rotating low pressure (LP) stage like the spools of a modern high bypass aeroengine. The meridional flow path of the machine is characterized by a diffusing S-shaped duct between the two rotors. The role of turning struts placed into the mid turbine frame is to lead the flow towards the LP rotor with appropriate swirl. Experimental and numerical investigations performed on the setup over the last years, which were used as baseline for this paper, showed that wide chord vanes induce large wakes and extended secondary flows at the LP rotor inlet flow. Moreover, unsteady interactions between the two turbines were observed downstream of the LP rotor. In order to increase the uniformity and to decrease the unsteady content of the flow at the inlet of the LP rotor, the mid turbine frame was redesigned with two zero-lifting splitters embedded into the strut passage. In this first part of the paper the design process of the splitters and its critical points are presented, while the time-averaged field is discussed by means of five-hole probe measurements and oil flow visualizations. The comparison between the baseline case and the embedded design configuration shows that the new design is able to reduce the flow gradients downstream of the turning struts, providing a more suitable inlet condition for the low pressure rotor. The improvement in the flow field uniformity is also observed downstream of the turbine and it is, consequently, reflected in an enhancement of the LP turbine performance. In the second part of this paper the influence of the embedded design on the time-resolved field is investigated.

Study of GaN epilayers growth on freestanding Si cantilevers

Five-micron thick freestanding Si cantilevers were fabricated on bulk Si (1 1 1) substrates with surface/bulk micromachining (SBM) process. Then 1-mu m thick GaN layers were deposited on the Si cantilevers by metal-organic chemical vapor deposition (MOCVD). Epilayers on cantilever areas were obtained crack-free, and the photoluminescence (PL) spectra verified the stress reduction and better material quality in these suspended parts of GaN. Back sides of the cantilevers were also covered with GaN layers, which prevented the composite beams from bending dramatically. This paper had proved the feasibility of integrating high-quality GaN epilayers with Si micromechanical structures to realize GaN-based micro electro-mechanical system (MEMS). (C) 2009 Elsevier Ltd. All rights reserved.

An experimental study about the influence of well thickness on the electroluminescence of InGaN/GaN multiple quantum wells

The influence of well thickness on the electroluminescence (EL) of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition is investigated. It is found that the peak wavelength of EL increases with the increase of well thickness when the latter is located in the range of 3.0-5.1 nm. The redshift is mainly attributed to the quantum confined Stark effect (QCSE). As a contrast, it is found that the EL intensity of InGaN/GaN MQWs increases with the increase of well thickness in spite of QCSE. The result of X-ray diffraction demonstrates that the interface become smoother with the increase of well thickness and suggests that the reduced interface roughness can be an important factor leading to the increase of EL intensity of InGaN/GaN MQWs. (C) 2009 Elsevier B.V. All rights reserved.

Impact of thickness of GaN buffer layer on properties of AlN/GaN distributed Bragg reflectors grown by metalorganic chemical vapor deposition

We studied the impact of the thickness of GaN buffer layer on the properties of distributed Bragg reflector (DBR) grown by metalorganic chemical vapor deposition (MOCVD). The samples were characterized by using metallographic microscope, transmission electron microscope (TEM), atomic force microscopy (AFM), X-ray diffractometer (XRD) and spectrophotometer. The results show that the thickness of the GaN buffer layer can significantly affect the properties of the DBR structure and there is an optimal thickness of the GaN buffer layer. This work would be helpful for the growth of high quality DBR structures.

Role of Ga vacancies in enhancing the leakage current of GaN Schottky barrier ultraviolet photodetectors

The leakage current of GaN Schottky barrier ultraviolet photodetectors is investigated. It is found that the photodetectors adopting undoped GaN instead of lightly Si-doped GaN as an active layer show a much lower leakage current even when they have a higher dislocation density. It is also found that the density of Ga vacancies in undoped GaN is much lower than in Si-doped GaN. The Ga vacancies may enhance tunneling and reduce effective Schottky barrier height, leading to an increase of leakage current. It suggests that when undoped GaN is used as the active layer, it is necessary to reduce the leakage current of GaN Schottky barrier ultraviolet photodetector.

折射面入光探测器的制作方法

一种折射面入光探测器的制作方法，制作过程包括如下步骤:步骤1:在衬底上采用MOCVD的方法生长探测器材料结构;步骤2:在探测器材料结构的上面制作p型欧姆接触;步骤3:在p型欧姆接触的周边向下刻蚀，形成探测器台面结构;步骤4:在步骤3形成的器件的上表面生长第一SiO2层;步骤5:在探测器台面结构的一侧腐蚀掉部分第一SiO2层，形成条形掩模图形结构;步骤6:在掩膜图形结构上，采用化学腐蚀的方法，向下腐蚀出燕尾槽;步骤7:腐蚀掉器件表面的第一SiO2层，重新在整个器件的表面生长第二SiO2层;步骤8:在燕尾槽中填入聚合物，使得材料表面平整;腐蚀掉p欧姆接触上的第二SiO2层;步骤9:在探测器台面结构的一侧制作p型金属电极;步骤10:将衬底减薄，在衬底的背面制作金属电极;步骤11:清除掉燕尾槽中的聚合物，解理芯片，完成整个器件的制作。

High Al-content AlInGaN epilayers with different thicknesses grown on GaN/sapphire templates

We studied the structural and optical properties of high Al-content AlInGaN epilayers with different thicknesses grown on GaN/sapphire templates by metalorganic chemical vapor deposition (MOCVD). Direct evidences of the gradual evolution of the content of Al, Ga and In along the growth direction were obtained. When the film thickness was over a certain value, however, the AlInGaN epilayer with constant element contents began to form. These results were also supported by the blue shift and splitting of the photoluminescence (PL) peak. For the thinnest epilayer, the surface was featured with outcrops of threading dislocations (TDs) which suggested a spiral growth mode. With increase in thickness, step-flow growth mode and V-shaped pits were observed, and the steps terminated at the pits. (C) 2008 Elsevier B. V. All rights reserved.

Epitaxial Semiconductor Quantum Wires

The investigation on the direct epitaxial quantum wires (QWR) using MBE or MOCVD has been persuited for more than two decades, more lengthy in history as compared with its quantum dot counterpart. Up to now, QWRs with various structural configurations have been produced with different growth methods. This is a reviewing article consisting mainly of two parts. The first part discusses QWRs of various configurations, together with laser devices based on them, in terms of the two growth mechanisms, self-ordering and self-assembling. The second part gives a brief review of the electrical and optical properties of QWRs.

A simple route of morphology control and structural and optical properties of ZnO grown by metal-organic chemical vapour deposition

Employing the metal-organic chemical vapour deposition (MOCVD) technique, we prepare ZnO samples with different morphologies from the film to nanorods through conveniently changing the bubbled diethylzinc flux (BDF) and the carrier gas flux of oxygen (OCGF). The scanning electron microscope images indicate that small BDF and OCGF induce two-dimensional growth while the large ones avail quasi-one-dimensional growth. X-ray diffraction (XRD) and Raman scattering analyses show that all of the morphology-dependent ZnO samples are of high crystal quality with a c-axis orientation. From the precise shifts of the 2 theta. locations of ZnO (002) face in the XRD patterns and the E-2(high) locations in the Raman spectra, we deduce that the compressive stress forms in the ZnO samples and is strengthened with the increasing BDF and OCGF. Photoluminescence spectroscopy results show all the samples have a sharp ultraviolet luminescent band without any defects-related emission. Upon the experiments a possible growth mechanism is proposed.

Fabrication and characterization of GaN-based LEDs grown on nanopatterned sapphire substrates

Sapphire substrates were nanopatterned by dry (inductively coupled plasma, ICP) etching to improve the performance of GaN-based light-emitting diodes (LEDs). GaN-based LEDs on nanopatterned sapphire substrates (NPSS) were fabricated by metal organic chemical vapor deposition (MOCVD). The characteristics of LEDs fabricated on NPSS prepared by dry etching were studied. The light output power and wall-plug efficiency of the LEDs fabricated on NPSS were greater than those of the conventional LEDs fabricated on common planar sapphire substrates when the injection currents were the same. The LEDs on NPSS and common planar sapphire substrates have similar I-V characteristics.

An evidence of defect gettering in GaN

The effect of Neon ion implantation on the structural and optical properties of MOCVD grown GaN was studied. X-ray diffraction and low temperature photoluminescence measurements were carried out on the implanted samples annealed at 900 degrees C. The peak at 3.41 eV exhibited an interesting behavior in as-grown and the implanted samples. Annealing has enhanced the intensity of this peak in as-grown samples, but suppressed in all the implanted samples. Capturing of defects by cavities during gettering process is interpreted as the reason for the observed behavior of this luminescence peak. Implantation dose of 5 x 10(15) ions/cm(2) caused the complete quenching of yellow band luminescence. (C) 2008 Elsevier B.V. All rights reserved.

High-performance 2 mm gate width GaNHEMTs on 6H-SiC with output power of 22.4 W @ 8 GHz

Optimized AlGaN/AlN/GaN high electron mobility transistors (HEMTs) structures were grown on 2-in semi-insulating (SI) 6H-SiC substrate by metal-organic chemical vapor deposition (MOCVD). The 2-in. HEMT wafer exhibited a low average sheet resistance of 305.3 Omega/sq with a uniformity of 3.85%. The fabricated large periphery device with a dimension of 0.35 pm x 2 nun demonstrated high performance, with a maximum DC current density of 1360 mA/mm, a transconductance of 460 mS/mm, a breakdown voltage larger than 80 V, a current gain cut-off frequency of 24 GHz and a maximum oscillation frequency of 34 GHz. Under the condition of continuous-wave (CW) at 9 GHz, the device achieved 18.1 W output power with a power density of 9.05 W/mm and power-added-efficiency (PAE) of 36.4%. While the corresponding results of pulse condition at 8 GHz are 22.4 W output power with 11.2 W/mm power density and 45.3% PAE. These are the state-of-the-art power performance ever reported for this physical dimension of GaN HEMTs based on SiC substrate at 8 GHz. (c) 2008 Elsevier Ltd. All rights reserved.

Improvement of the performance of GaN-based LEDs grown on sapphire substrates patterned by wet and ICP etching

Sapphire substrates patterned by a selective chemical wet and an inductively coupled plasma (ICP) etching technique was proposed to improve the performance of GaN-based light-emitting diodes (LEDs). GaN-based LEDs were fabricated on sapphire substrates through metal organic chemical vapor deposition (MOCVD). The LEDs fabricated on the patterned substrates exhibit improved device performance compared with the conventional LED fabricated on planar substrates when growth and device fabricating conditions were the same. The light output powers of the LEDs fabricated on wet-patterned and ICP-patterned substrates were about 37% and 17% higher than that of LEDs on planar substrates at an injection current of 20 mA, respectively. The enhancement is attributable to the combination of the improvement of GaN-based epilayers quality and the improvement of the light extraction efficiency. (c) 2008 Elsevier Ltd. All rights reserved.

Study of GaN growth on ultra-thin Si membranes

10 mu m-thick ultra-thin Si (111) membranes for GaN epi-layers growth were successfully fabricated on silicon-on-insulator (SOI) substrate by backside etching the handle Si and buried oxide (BOX) layer. Then 1 mu m-thick GaN layers were deposited on these Si membranes by metal-organic chemical vapor deposition (MOCVD). The crack-free areas of 250 mu m, x 250 mu m were obtained on the GaN layers due to the reduction of thermal stress by using these ultra-thin Si membranes, which was further confirmed by the photoluminescence (PL) spectra and the simulation results from the finite element method calculation by using the software of ANSYS. In this paper, a newly developed approach was demonstrated to utilize micromechanical structures for GaN growth, which would improve the material quality of the epi-layers and facilitate GaN-based micro electro-mechanical system (MEMS) fabrication, especially the pressure sensor, in the future applications. (C) 2008 Elsevier Ltd. All rights reserved.

The influence of 1 nm AlN interlayer on properties of the Al0.3Ga0.7N/AlN/GaN HEMT structure

The sheet carrier concentrations, conduction band profiles and amount of free carriers in the barriers have been determined by solving coupled Schrodinger and Poisson equation self-consistently for coherently grown Al0.3Ga0.7N/GaN and Al0.3Ga0.7N/AlN/GaN structures on thick GaN. The Al0.3Ga0.7N/GaN heterojunction structures with and without 1 nm AlN interlayer have been grown by MOCVD on sapphire substrate, the physical properties for these two structures have been investigated by various instruments such as Hall measurement and X-ray diffraction. By comparison of the theoretical and experimental results, we demonstrate that the sheet carrier concentration and the electrons mobility would be improved by the introduction of an AlN interlayer for Al0.3Ga0.7N/GaN structure. Mechanisms for the increasing of the sheet carrier concentration and the electrons mobility will be discussed in this paper. (C) 2007 Elsevier Ltd. All rights reserved.