Studies of InGaAs layers growth by metalorganic chemical vapor deposition for InP-HEMTs; Effects of trimethylindium and triethylindium
We studied an InGaAs epitaxial layers growth by metalorganic chemical vapor deposition (MOCVD) for InP-based high electron mobility transistors (HEMTs). The InGaAs channel layer of HEMTs was grown by a new material combination of triethylindium (TEI), triethylgallium (TEG) and arsine (AsH3). An electron mobility of the HEMTs structure sample was 20% higher than the one grown by trimethylindium (TMI), TEG and AsH3. And the DC characteristics of the HEMTs, the drain current and transconductance, were increased compared with the one. It is confirmed that this growth method is quite effective for the improvement of a characteristics of InP-based HEMTs
Photoluminescence spectra from δ-doped barrier layers in modulation-doped InAlAs/InGaAs field-effect transistor structures
The photoluminescence (PL) spectra from the Si δ-doped InAlAs barrier layers in modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structures with contact and recess-etch-stopper layers are investigated. A PL peak associated with the δ-doped layer (δ peak) is found. The δ-peak position shifts to shorter wavelength with increasing δ-doping concentration. The intensity reduction of the δ peak due to raising the temperature from 6 K is considerably slower than that of the exciton peak from the InAlAs buffer layer at about 8100 A, to which confined electrons and holes probably contribute. As a result, the δ-peak overwhelms the exciton peak and becomes the main peak of the InAlAs PL at temperatures higher than 40 K. The δ peak is not detected from HFET structures without the contact and recess-etch-stopper layers. The δ peak is attributed to the recombination of electrons from the δ-doped layer and photogenerated holes confined in the upper side of the barrier layer.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME