• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.13 no.8
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• pp.643-650
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• 2000

Density-of-states effective mass(m*$_{d}$) and conductivity mass(m*$_{c}$)for Ge and Ge$_{0.8}$/Sn$_{0.2}$ are obtained by using 8$\times$8 k.p and strain Hamiltonians. It is shown that m*$_{d}$ and m*$_{c}$ for electrons in Ge/Ge$_{0.8}$/Sn$_{0.2}$(001) and Ge$_{0.8}$/Sn$_{0.2}$/Ge(001) are much smaller than those for electrons in relaxed Ge mainly due to the increase of interaction caused by the strain between the conduction band and valence bands at the $\Gamma$ point. The lift of degeneracy in Ge/Ge$_{0.8}$/Sn$_{0.2}$(001) and Ge/Ge$_{0.8}$/Sn$_{0.2}$(001) makes m*$_{d}$ and m*$_{c}$ for holes smaller than those in relaxed Ge and results in the decrease of the interband scattering as well as interband scattering. The decrease of the interband scattering is more obvious in Ge/Ge$_{0.8}$/Sn$_{0.2}$(001) because of its large splitting energy between the heavy hole and light hole band. Therefore, Ge/Ge$_{0.8}$/Sn$_{0.2}$(001) is expected to be good candidate for the development of ultra high-speed CMOS device.CMOS device.eed CMOS device.CMOS device.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.175.1-175.1
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• 2014

Despite growing interest in Ge as a possible alternative to Si, reliable data on Ge surface has been relatively scarce. Using low temperature scanning tunneling microscopy (STM), we investigate band-bending effects of localized charge traps at Ge(001) surface at 78 K. For this investigation, we prepared nearly defect-free Ge(001) surface by keeping the background pressure to < $1{\times}10^{-10}$ mbar during outgassing. Ge(001) surfaces this obtained exhibit a flat-band condition, and deposition of charge traps induce a distinct, sharp boundary between pinned and depinned surface area in the constant current mode STM images. We will show the tip-surface interaction plays an essential role in producing the boundary, and discuss about the conditions that enable the pinning effect.

• Proceedings of the Korean Magnestics Society Conference
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• 2009.12a
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• pp.190-190
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• 2009

We report the epitaxial growth of MgO and CoFe/MgO on Ge (001) substrates using molecular beam epitaxy. It was found that the epitaxial growth of a MgO film on Ge could be realized at a low growth temperature of $125{\pm}5^{\circ}C$ and the MgO matches the Ge with a cell ratio of $\sqrt{2}$:1 which renders MgO rotated by $45^{\circ}$ relative to Ge. In-situ and ex-situ structural characterizations reveal the epitaxial crystal growth of bcc CoFe/MgO on Ge with the in-plane crystallographic relationship of CoFe(001)[100] || MgO(001)[110] || Ge(001)[100], exhibiting sharp interfaces in the (001) matching planes. The saturation magnetization of the sample is $1430{\pm}20$ emu/cc, which is comparable to the value of bulk CoFe.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.13 no.12
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• pp.989-995
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• 2000

G $e_{1-x}$ S $n_{x}$G $e_{1-y}$S $n_{y}$ is a very promising material for the high-speed device due to the fact that electron and hole mobilities for the strained G $e_{1-x}$ S $n_{x}$G $e_{1-y}$S $n_{y}$ are greatly enhanced. Because G $e_{1-x}$ S $n_{x}$G $e_{1-y}$S $n_{y}$ has a direct band gap for the proper combination of x and y, it can be applied to the optoelectronic device. Therefore, the study of the electrical property for G $e_{1-x}$ S $n_{x}$G $e_{1-y}$S $n_{y}$(001) with a direct energy gap is needed. G $e_{1-x}$ S $n_{x}$ layer can not be grown thickly due to the large difference of lattice constants. This fact prefers the structure of the device where electrons and holes move in the plane direction. The transverse mobilities of electron and hole for G $e_{0.8}$S $n_{0.2}$Ge(001) are 2~3 times larger than those for Ge/Ge/ sub0.8/S $n_{0.2}$(001). Therefore, G $e_{0.8}$S $n_{0.2}$Ge(001) is expected to be better than Ge/G $e_{0.8}$S $n_{0.2}$(001) for the development of the high-speed device.h-speed device.device.h-speed device. device.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.11 no.4
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• pp.267-273
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• 1998

The band structures of $Si_{1-x}Ge_x$ layers grown on $Si_{1-y}Ge_y$ substrate are calculated using k$\cdot$p and strain Hamiltonians. The hole drift mobilities in the plane direction are then calculated by taking into account the screening effect and the density-of-states of the impurity band. When $Si_{1-x}Ge_x$ is grown on Si substrate, the mobilities of (110) and (111) $Si_{1-x}Ge_x$ layers are larger than that of (001) $Si_{1-x}Ge_x$. However, due to the large defect and surface scattering, (110) and (111) $Si_{1-x}Ge_x$ layers may not be useful for the development of the fast device. Meanwhile, when Si is grown on $Si_{1-y}Ge_y$ substrate, the mobilities of (001) and (110) Si layers are greatly enhanced. Based on the amount of defect and the surface scattering, it is expected that Si grown on (001) $Si_{1-y}Ge_y$ substrate, where the Ge contents is larger than 10%(y>0.1), has the highest mobility.

• Korean Journal of Optics and Photonics
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• v.10 no.4
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• pp.306-310
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• 1999

Electron intersubband absorption in Sb $\delta$-doped Si(110)/SiGe multiple quantum well structures is observed. Normally incident light can excite electrons in Si(110) quantum wells, which is not possible for Si(001) or GaAs quantum wells. The influence of Ge composition in SiGe barries is investigated. As the Ge composition in SiGe barriers increases, the absorption strength is decreased and the transition energy is increased. It is verifired by comparing the calculated and experimental results obtained at various incident and polarization angles that normally incident light and parallel incident light are absorbed in different processes.

• Journal of the Korean Vacuum Society
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• v.16 no.2
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• pp.145-152
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• 2007

The evolution dynamics of nanoscale Ge islands on both Si (001) and (113) surfaces is explored using ultraviolet photoelectron emission microscopy (UV-PEEM). Real-time monitoring of the in-situ growth of the Ge island structures can allow us to study the variation of the size, the shape and the density of the nanostructures. For Ge depositions greater than ${\sim}4$ monolayer (ML) with a growth rate of ${\sim}0.4\;ML/min$ at temperatures of $450-550^{\circ}C$, we observed island nucleation on both surfaces indicating the transition from strained layer to island structure. During continuous deposition the circular islands grew larger via ripening processes. AFM measurements showed that the islands grown on Si (001) were dome-shaped while the islands on Si (113) were multiple-side faceted with flat tops of (113)-orientation. In contrast, for Ge deposition with a lower growth rate of ${\sim}0.15\;ML/min$ on Si(113), we observed the shape transition from circular into elongated island structures. The elongated islands grew longer along the [$33\bar{2}$] during continuous Ge deposition. The shape evolution of the islands is discussed in terms of strain relaxation and kinetic effects.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.135-135
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• 2010

In Mn-Ge equilibrium phase diagram, many Mn-Ge intermetallic phases can be formed with difference structures and magnetic properties. The MnGe has the cubic structure and antiferromagnetic(AFM) with Neel temperature of 197 K. The calculation predicted that the $MnGe_2$ with $Al_2Cu$-type is hard to separate between the paramagnetic(PM) states and the AFM states because this compound displays PM and AFM configuration swith similar energy. Mn-doped Ge showed the FM with Currie temperature of 285 K for bulk samples and 116 K for thin films. In addition, the $Mn_5Ge_3$ compound has hexagonal structure and FM with Curie temperature around 296K. The $Mn_{11}Ge_8$ compound has the orthorhombic structure and Tc is low at 274 K and spin flopping transition is near to 140 K. While the bulk $Mn_3Ge_2$ exhibited tetragonal structure ($a=5.745{\AA}$;$c=13.89{\AA}$) with the FM near to 300K and AFM below 150K. However, amorphous $Mn_3Ge_2$ ($a-Mn_3Ge_2$) was reported to show spin glass behavior with spin-glass transition temperature (Tg) of 53 K. In addition, the transition of crystalline $Mn_3Ge_2$ shifts under high pressure. At the atmospheric pressure, $Mn_3Ge_2$ undergoes the magnetic phase transition from AFM to FM at 158 K. The pressure dependence of the phase transition in $Mn_3Ge_2$ has been determined up to 1 GPa. The transition was found to occur at 1 GPa and 155 K with dT/dP=-0.3K/0.1 GPa. Here report that Ferromagnetic $Mn_3Ge_2$ thin films were successfully grown on GaAs(001) and GaSb(001) substrates using molecular beam epitaxy. Our result revealed that the substrate facilitates to modify magnetic and electrical properties due to tensile/compressive strain effect. The spin-flopping transition around 145 K remained for samples grown on GaSb(001) while it completely disappeared for samples grown on GaAs(001). The antiferromagnetism below 145K changed to ferromagnetism and remained upto 327K. The saturation magnetization was found to be 1.32 and $0.23\;{\mu}B/Mn$ at 5 K for samples grown on GaAs(001) and GaSb(001), respectively.

• Journal of the Korean Magnetics Society
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• v.25 no.2
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• pp.31-38
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• 2015

Recently, a first-principles study led to a prediction that quaternary Heusler compounds, CoFeCrZ (Z = Ga, Ge) are excellent half-metallic ferromagnets. In this study, we investigate the electronic and the magnetic properties at the (001) surfaces of CoFeCrGa and CoFeCrGe by means of the full-potential linearized augmented plane wave (FLAPW) method within generalized gradient approximation. We considered two types of surface termination: CoFe-terminated and CrZ-terminated surfaces, Z being either Ga or Ge. From the calculated total magnetic moments and the local density of states, we found that half-metallicity is not preserved for all the surfaces. But the calculated atomic density of states showed that CrGa-terminated surface of the CoFeCrGa is almost half-metallic. The magnetic moment of the Co, Fe, or Cr atoms at the surface or subsurface layers in each system had very different values.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.15 no.9
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• pp.770-775
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• 2002

We present the band lineups of G $e_{1-}$x S $n_{x}$ G $e_{1-}$y S $n_{y(001)}$ heterostructures for the new devices. The energy gap of the bulk G $e_{1-}$x S $n_{x}$ alloy is calculated by taking into account the Vegard's law. The change of the energy gap due to the strain is understood in terms of the deformation Potential theory The valence band offset is obtained from the average bond energy model, where the changes of the band offset due to alloy compositions and strain are included. It is found that Ge/G $e_{1-}$y S $n_{y(001)}$ heterostructure has a staggered lineup type for 0$\leq$0.06 and a straddling one for 0.06$\leq$0.26. Meanwhile, Ge/G $e_{l-y}$ S $n_{y(001)}$ heterostructure has a staggered lineup type for 0$\leq$0.19 and a broken-gap one for 0.19$\leq$0.26. As a result, the various type of the G $e_{1-}$x S $n_{x}$ G $e_{1-}$y S $n_{y(001)}$ heterostructure can be applied for the useful device.evice.