Youbong Lim

Dry thermal oxidation of a graded SiGe layer

A method for the dry thermal oxidation of a strained SiGe layer is proposed. By oxidation of a graded Si1−xGex layer, the effect of Ge pileup was significantly reduced and the undesirable strain relaxation by defect formation is prohibited. After oxidation, the oxidized SiGe layer was homogenized by postannealing process, and thereby a SiO2/SiGe interface with good structural properties was obtained. During postannealing, the homogenization was significantly enhanced by strain-induced diffusion, and it was clearly proved by the uphill diffusion. This result can propose an alternative oxidation method of strained SiGe/Si heterostructures.

Strain-induced diffusion in a strained Si1−xGex/Si heterostructure

Diffusivity of a strained heterostructure was theoretically investigated, and general diffusion equations with strain potential were deduced. There was an additional diffusivity by the strain potential gradient as well as by the concentration gradient. The strain-induced diffusivity was a function of concentration, and its temperature dependence was formulated. The activation energy of the strain-induced diffusivity was measured by high-resolution transmission electron microscopy. This result can be generally applied for the investigation of the diffusion in strained heterostructures.

Observation of a high-energy tail in ion energy distribution in the cylindrical Hall thruster plasma

A novel method is presented to determine populations and ion energy distribution functions (IEDFs) of individual ion species having different charge states in an ion beam from the measured spectrum of an E × B probe. The inversion of the problem is performed by adopting the iterative Tikhonov regularization method with the characteristic matrices obtained from the calculated ion trajectories. In a cylindrical Hall thruster plasma, an excellent agreement is observed between the IEDFs by an E × B probe and those by a retarding potential analyzer. The existence of a high-energy tail in the IEDF is found to be mainly due to singly charged Xe ions, and is interpreted in terms of non-linear ion acceleration.

Effect of multiply charged ions on the performance and beam characteristics in annular and cylindrical type Hall thruster plasmas

Plasma plume and thruster performance characteristics associated with multiply charged ions in a cylindrical type Hall thruster (CHT) and an annular type Hall thruster are compared under identical conditions such as channel diameter, channel depth, propellant mass flow rate. A high propellant utilization in a CHT is caused by a high ionization rate, which brings about large multiply charged ions. Ion currents and utilizations are much different due to the presence of multiply charged ions. A high multiply charged ion fraction and a high ionization rate in the CHT result in a higher specific impulse, thrust, and discharge current.

Strain effects in lattice-mismatched InxGa1−xAs/InyAl1−yAs coupled double quantum wells

Transmission electron microscopy (TEM) and Raman scattering spectroscopy measurements were performed to investigate strain effects in lattice-mismatched InxGa1−xAs/InyAl1−yAs modulation-doped coupled double quantum wells. The high-resolution TEM images showed that a 100-A In0.8Ga0.2As deep quantum well and a 100-A In0.53Ga0.47As shallow quantum well were separated by a 30-A In0.25Ga0.75As embedded potential barrier. The selected-area electron-diffraction pattern obtained from TEM measurements on the InxGa1−xAs/InyAl1−yAs double quantum well showed that the InxGa1−xAs active layers were grown pseudomorphologically on the InP buffer layer. The values of the strain and the stress of the InxGa1−xAs layers were determined from the electron-diffraction pattern. Based on the TEM results, a possible crystal structure for the InxGa1−xAs/InyAl1−yAs coupled double quantum well is presented.

Strain-induced anisotropic Ge diffusion in SiGe/Si superlattices

Anisotropic diffusion of Ge induced by nonuniform strain in SiGe/Si interfaces in the range of 700–850 °C is directly observed with medium-energy ion-scattering spectroscopy through its composition and strain profiles of atomic-layer depth resolution. For SiGe/Si interfaces with identical composition profiles but with different strain distributions, the anisotropic diffusion of Ge can be clearly correlated with the anisotropic relaxation of the nonuniform strain in the near-surface layer of several nm depth. The results suggest that atomic-scale strain control is critical to maintain abrupt SiGe/Si interfaces under thermal budget.

Effect of magnetic field configuration on the multiply charged ion and plume characteristics in Hall thruster plasmas

Multiply charged ions and plume characteristics in Hall thruster plasmas are investigated with regard to magnetic field configuration. Differences in the plume shape and the fraction of ions with different charge states are demonstrated by the counter-current and co-current magnetic field configurations, respectively. The significantly larger number of multiply charged and higher charge state ions including Xe4+ are observed in the co-current configuration than in the counter-current configuration. The large fraction of multiply charged ions and high ion currents in this experiment may be related to the strong electron confinement, which is due to the strong magnetic mirror effect in the co-current magnetic field configuration.

Magnetic field configurations on thruster performance in accordance with ion beam characteristics in cylindrical Hall thruster plasmas

Magnetic field configuration is critical in Hall thrusters for achieving high performance, particularly in thrust, specific impulse, efficiency, etc. Ion beam features are also significantly influenced by magnetic field configurations. In two typical magnetic field configurations (i.e., co-current and counter-current configurations) of a cylindrical Hall thruster, ion beam characteristics are compared in relation to multiply charged ions. Our study shows that the co-current configuration brings about high ion current (or low electron current), high ionization rate, and small plume angle that lead to high thruster performance.

Coexisting phenomena of the CuPt-type and the Cu3Au-type ordered structures near ZnTe/ZnSe heterointerfaces in ZnxCd1-xTe/ZnSySe1-y quantum wells

Abstract Transmission electron microscopy (TEM) measurements were carried out to investigate the ordered structures near the ZnTe/ZnSe heterointerfaces in ZnxCd1−xTe/ZnSySe1−y quantum wells. The high-resolution TEM micrographs and selective-area electron-diffraction patterns showed a CuPt-type ordered structure and a Cu3Au-type ordered structure near the ZnTe/ZnSe heterointerfaces in ZnxCd1−xTe/ZnSySe1−y quantum wells. The present results can help improve understanding for the application of ZnxCd1−xTe/ZnSySe1−y quantum wells in optoelectronic devices in the blue–green spectral region.

Multiply charged ions in hall thruster plasmas

Operation performance and ion beam characteristics related to multiply charged ions in 200 W class Hall thruster plasmas are studied. Our previous studies reported the propellant efficiency larger than unity1,2 in the cylindrical Hall thruster (CHT) plasma, which may be related to the presence of multiply charged ions. In this presentation, we report the measured result of the fraction of Xe2+ and Xe3+ ions in the CHT plasma (27-31%) that is significantly larger than that in the annular Hall thruster (AHT) plasma (5-8%) under the similar operating conditions3,4. The specific impulse and thrust are measured to be higher by 1.4 times in the CHT than those in the AHT at 300 V of the anode voltage. It is found that the high values of specific impulse and thrust are related to the high fraction of multiply charged ions and high ionization rate in the CHT. The details of the comparison of the overall performance and beam characteristics associated with multiply charged ions in the AHT and CHT are presented. The large effect on the fraction of multiply charged ions due to the different magnetic field configuration with different coil current directions in the CHT plasma will also be reported.