Zhenpu Zhang

Effect of nanoclusters induced by Si implantation on total dose radiation response of a SOI wafer

Si ion implantation with subsequent annealing was used to improve the irradiation hardness of SOI wafers. The reduced negative shifts in the threshold voltage of the n-channel transistors showed that the total dose radiation hardness was remarkably enhanced by this modification process, and the pseudo-MOS method was used to test the Id–VG characteristics of the SOI samples before and after irradiation. Photoluminescence (PL) spectra demonstrated that Si nanoclusters were formed during annealing in the buried oxide of the implanted samples, which were possibly the main candidates for charge compensation. X-ray photoelectron spectroscopy (XPS) results also suggested that reduction of the threshold voltage shifts was accompanied with the formation of Si nanoclusters near the peak of the implants. Furthermore, the effect of positive charge buildup can be partly reduced as a result of the improvement on proton trapping in the bulk of the oxide due to the implantation induced nanoclusters.

Effect of rapid thermal annealing on InP1-xBix grown by molecular beam epitaxy

The effect of post-growth rapid thermal annealing on structural and optical properties of InP1-xBix thin films was investigated. InPBi shows good thermal stability up to 500 °C and a modest improvement in photoluminescence (PL) intensity with an unchanged PL spectral feature. Bismuth outdiffusion from InPBi and strain relaxation are observed at about 600 °C. The InPBi sample annealed at 800 °C shows an unexpected PL spectrum with different energy transitions.

Raman scattering studies of dilute InP1−xBix alloys reveal unusually strong oscillator strength for Bi-induced modes

Room-temperature Raman scattering studies of new InP1-xBix alloys grown by molecular beam epitaxy are reported. Two new Bi-induced vibrations observed at 149 and 171 cm-1 are assigned to InBi-like TO and LO phonon modes, respectively, and exhibit an unusually strong intensity for the dilute regime. Two additional modes at 311 and 337 cm-1 are resolved as well with unknown origins. The Raman intensities of the InBi-like TO and LO bands, as well as the new mode at 337 cm-1, exhibit strong and linear dependence on the Bi concentration for the composition range studied, 0.003 ≤ x ≤ 0.023. This correlation may serve as a fast and convenient means of characterizing bismuth composition not only in the ternary alloy InP1-xBix but also in the quaternaries such as In1-yGayP1-xBix and In1-yAlyP1-xBix.

GeSn/Ge dual-nanowire heterostructure

A dual-nanowire heterostructure with a GeSn layer laterally laying on Ge nanowires is demonstrated by MBE. The strain field analyzed by FEM shows that the novel proposal can significantly release the compressive strain in GeSn for potential direct bandgap conversion as a Si-based light source.

Theoretical Investigation of Biaxially Tensile-Strained Germanium Nanowires

We theoretically investigate highly tensile-strained Ge nanowires laterally on GaSb. Finite element method has been used to simulate the residual elastic strain in the Ge nanowire. The total energy increment including strain energy, surface energy, and edge energy before and after Ge deposition is calculated in different situations. The result indicates that the Ge nanowire on GaSb is apt to grow along 〈100〉 rather than 〈110〉 in the two situations and prefers to be exposed by {105} facets when deposited a small amount of Ge but to be exposed by {110} when the amount of Ge exceeds a critical value. Furthermore, the conduction band minima in Γ-valley at any position in both situations exhibits lower values than those in L-valley, leading to direct bandgap transition in Ge nanowire. For the valence band, the light hole band maxima at Γ-point is higher than the heavy hole band maxima at any position and even higher than the conduction band minima for the hydrostatic strain more than ∼5.0%, leading to a negative bandgap. In addition, both electron and hole mobility can be enhanced by owing to the decrease of the effective mass under highly tensile strain. The results suggest that biaxially tensile-strained Ge nanowires hold promising properties in device applications.

Vapor-solid-solid grown Ge nanowires at integrated circuit compatible temperature by molecular beam epitaxy

We demonstrate Au-assisted vapor-solid-solid (VSS) growth of Ge nanowires (NWs) by molecular beam epitaxy at the substrate temperature of similar to 180 degrees C, which is compatible with the temperature window for Si-based integrated circuit. Low temperature grown Ge NWs hold a smaller size, similar uniformity, and better fit with Au tips in diameter, in contrast to Ge NWs grown at around or above the eutectic temperature of Au-Ge alloy in the vapor-liquid-solid (VLS) growth. Six growth orientations were observed on Ge (110) by the VSS growth at similar to 180 degrees C, differing from only one vertical growth direction of Ge NWs by the VLS growth at a high temperature. The evolution of NWs dimension and morphology from the VLS growth to the VSS growth is qualitatively explained by analyzing the mechanism of the two growth modes.

Photoluminescence from tensile-strained Ge quantum dots

Summary form only given. It has been theoretically predicted that 1.9% biaxial tensile strain can convert Ge, which is compatible with Si CMOS technology, into a direct band-gap semiconductor, making it a candidate material for light sources on Si. Combining the advantage of tensile strain with quantum dot (QD), we proposed that tensile-strained QD is a new route toward light emission from Ge. In this work, we chose In0.52Al0.48As, which is lattice matched to InP, as barrier layer and grew the structure by molecular beam epitaxy (MBE). Photoluminescence (PL) was successfully achieved at room temperature.

Highly tensile-strained Ge quantum dots on GaSb by MBE for light sources on Si

It is theoretically predicted that biaxial tensile strain as much as 1.4% can make up the 136 meV gap between the Γ and L valley in Ge [1], thereby converting Ge from an indirect-bandgap semiconductor into a direct-bandgap one that can emit light efficiently covering the telecom band. The mobility of both carriers is dramatically increased simultaneously. Therefore, tensile-strained Ge has drawn large interest in the potential for high speed transistors and light sources for Si photonics. We have proposed and demonstrated that tensile-strained Ge quantum dot (QD) on InP is a better solution for the realization of light sources on Si than thin films since it can hold large strain to convert the bandgap and insensitive to structural defects at the same time [2]. In this work, the molecular beam epitaxy (MBE) of tensile-strained Ge QDs on GaSb(001) with thickness ranging from sub-monolayer (ML) to a few MLs is studied. The formation and evolution of the deposited Ge QDs are investigated by the reflection high-energy electron diffraction (RHEED), and the surface morphology is measured by atomic force microscopy (AFM). In FIG. 1, it is shown that the RHEED pattern changed to a dotty one after 1.7 ML of the Ge deposition indicating a Stranski-Krastanov (SK) growth mode with the existence of a wetting layer. FIG. 2 are AFM images of the samples with different Ge thickness. It can be found that when the thickness is below one ML, the Ge atoms nucleate randomly on the GaSb atomic steps, forming sub-ML islands. The two dimensional growth continues to a full coverage of the GaSb surface and up to 1.7 ML. A few QDs can be found before 1 ML, probably due to surface defects. These sub-ML islands and the one ML thick Ge films are fully strained (7.2% tensile strain). When the thickness is larger than 1.7 ML, clear formation of QDs is observed. The QDs are mostly rectangular shape with the edges along the (110) directions. The evolution observed from RHEED and AFM is consistent. Later, samples of the Ge with different thicknesses capped by GaSb were also grown. Further analysis including optical properties are under implement.