Engineering topics

Wicks, G. W.

Publications and source records attributed to Wicks, G. W..

7 records

NASA NTRS1988

Influence of substrate misorientation on defect and impurity incorporation in GaAs/AlGaAs heterostructures grown by molecular-beam epitaxy

GaAS/AlGaAs heterostructures have been grown by molecular-beam epitaxy on GaAs substrates intentionally oriented (tilted) a few degrees (0-6.5) off the (001) plane towards either (111)A, (111)B, or (011). It was observed that the 4-K photoluminescence and low-field electron transport properties of these structures may be functions of the substrate tilt angle and tilt direction, depending on the concentration of impurities incorporated during growth. A substrate tilt during molecular-beam epitaxy is observed to have the largest effect on these properties when the background impurity concentration in the molecular-beam epitaxial machine is high. This supports the contention that the observed changes in material characteristics are due to differences in the incorporation of defects and impurities. The incorporation of defects and impurities are reduced by using substrates tilted toward (111)A, in comparison to nominally flat (001) substrates or substrates tilted toward (111)B.

Radulescu, D. C.↗

NASA NTRS1987

High speed semiconductor lasers and integrated receivers for microwave and millimeter wave signal transmission

This project was proposed to be a three-year contract, but was cut off after one year (1987) of support. Sufficient progress was made to allow continuation for a year on minimal internal funds before proper support levels were received from a combination of ONR, RADC, and IBM contracts at about the start of 1989. A major DARPA grant, expected to start August 1, 1990, will sharply expand this effort to include several faculty members. During the course of this project, two students did thesis research, one Ph.D. on lasers and one M.S. on high speed photodetectors. These theses were finished in September 1988, nine months after the contract was cut off. This report covers excerpts from those theses as well as information obtained on other programs since that time.

Eastman, L. F.↗

NASA NTRS1987

Effects of substrate misorientation and background impurities on electron transport in molecular-beam-epitaxial grown GaAs/AlGaAs modulation-doped quantum-well structures

The effects of substrate misorientation off the (001) plane and of background impurities on electron transport in MBE-grown GaAs/AlGaAs modulation-doped superlattice-buffered quantum-well structures were investigated. Low-field transport data were obtained on GaAs/AlGaAs structures grown on substrates oriented 0, 2, 4, and 6.5 deg off the (001) plane towards either (111)A or (111)B. It is shown that the low-field two-dimensional electron gas (2DEG) mobility is a function of the angle and direction of the substrate orientation, and that the 2DEG mobility is a function of the direction of the applied electric field in the GaAs quantum well. The anisotropy in the 2DEG mobility is also a function of the tilt angle and tilt azimuth direction of the substrate from the (001) plane. In addition, it is shown that the amount of interface scattering from the inverted interface is a sensitive function of the amount of background impurities in the MBE machine.

Radulescu, D. C.↗

NASA NTRS1987

Anisotropic transport in modulation-doped quantum-well structures

Anisotropic electron transport has been observed in GaAs modulation-doped quantum wells grown by molecular-beam epitaxy on a thick (001) Al(0.3)Ga(0.7)As buffer grown at 620 C. Thicker quantum wells (150, 200, and 300 A) show progressively less anisotropy, which vanishes for a 300-A quantum well. The degree of anisotropy is also reduced or eliminated by suspending growth of the Al(0.3)Ga(0.7)As for a period of 300 s prior to growing the GaAs quantum well. Growing the Al(0.3)Ga(0.7)As buffer at higher temperatures (680 C) also reduces the degree of anisotropy. Higher two-dimensional electron gas sheet densities result in less anisotropy.The anisotropy is eliminated by replacing the thick Al(0.3)Ga(0.7)As buffer with a periodic multilayer structure comprising 15 A of GaAs and 200 A of Al(0.3)Ga(0.7)As. The degree of anisotropy is related to the thickness and growth parameters of the Al(0.3)Ga(0.7)As layer grown just prior to the growth of the GaAs.

Radulescu, D. C.↗

NASA NTRS1987

Anisotropic transport in modulation doped quantum well structures

The degree of anisotropy in the anisotropic electron transport that has been observed in GaAs modulation-doped quantum wells grown by MBE on Al(0.3)Ga(0.7)As is related to the thickness and growth parameters of this substrate, which is grown just prior to the inverted interface. It is presently observed that the inverted interface has an anisotropic roughness which affects the 77 K low field electron transport parallel to the interface, and gives rise to anisotropic electron scattering in the GaAs modulation-doped quantum well.

Radulescu, D. C.↗

NASA NTRS1984

Calculation of the conduction band discontinuity for Ga(0.47)In(0.53)As/Al(0.48)In(0.52)As heterojunction

Photoluminescence studies at 4 K on Ga(0.47)In(0.53)As/Al(0.48)In(0.52)As single quantum wells exhibit emission ranging from 1.318 eV for a 15-A well to 0.82 eV for thick wells. The emission energy of each single quantum well is compared to theoretical curves which are generated from a finite potential square well model. The closest agreement between the experimental curves and the theoretical curves occurs when the conduction band discontinuity is taken to be 70 percent of the bandgap discontinuity or 0.52 eV.

Welch, D. F.↗

NASA NTRS1983

GaInAs-AlInAs heterostructures for optical devices grown by MBE

The band gap of Ga(0.47)In(0.53)As corresponds to an emission wavelength of about 1.65 microns. Lasers have been produced with Al(0.48)In(0.52)As as cladding layers operating at room temperature. The peak emission of Ga(0.47)In(0.53)As can be continuously varied from 1.65 to 1.2 microns by the use of the multiquantum well structures. This range of wavelengths covers the minimum loss and dispersion in optical fibers and will be applicable to integrated optics. Double heterostructure broad area lasers have been fabricated using AlInAs as cladding layers to the GaInAs active layer. Room temperature threshold current densities of 4.3 kA/sq cm have been obtained for lasers with a 4500 A active region. The first data on GaInAs/AlInAs quantum well emitters will be presented. Photoluminescence of 4 K from quantum well layers of 100, 150, and 180 A with 150 A AlInAs barrier layers produced emission at 1.27, 1.35, and 1.41 microns, respectively. Ga(0.47)In(0.52)As quantum well LEDs have also been produced which emit at 1.34 microns.

Welch, D. F.↗