Qimiao Chen

Detailed Study of the Influence of InGaAs Matrix on the Strain Reduction in the InAs Dot-In-Well Structure.

InAs/InGaAs dot-in-well (DWELL) structures have been investigated with the systematically varied InGaAs thickness. Both the strained buffer layer (SBL) below the dot layer and the strain-reducing layer (SRL) above the dot layer were found to be responsible for the redshift in photoluminescence (PL) emission of the InAs/InGaAs DWELL structure. A linear followed by a saturation behavior of the emission redshift was observed as a function of the SBL and SRL thickness, respectively. The PL intensity is greatly enhanced by applying both of the SRL and SBL. Finite element analysis simulation and transmission electron microscopy (TEM) measurement were carried out to analyze the strain distribution in the InAs QD and the InGaAs SBL. The results clearly indicate the strain reduction in the QD induced by the SBL, which are likely the main cause for the emission redshift.

Vibrational properties of epitaxial Bi4Te3 films as studied by Raman spectroscopy

Bi4Te3, as one of the phases of the binary Bi–Te system, shares many similarities with Bi2Te3, which is known as a topological insulator and thermoelectric material. We report the micro-Raman spectroscopy study of 50 nm Bi4Te3 films on Si substrates prepared by molecular beam epitaxy. Raman spectra of Bi4Te3 films completely resolve the six predicted Raman-active phonon modes for the first time. Structural features and Raman tensors of Bi4Te3 films are introduced. According to the wavenumbers and assignments of the six eigenpeaks in the Raman spectra of Bi4Te3 films, it is found that the Raman-active phonon oscillations in Bi4Te3 films exhibit the vibrational properties of those in both Bi and Bi2Te3 films.

Structural properties of GeSn thin films grown by molecular beam epitaxy

GeSn thin films on Ge (001) with various Sn concentrations from 3.36 to 7.62% were grown by molecular beam epitaxy and characterized. The structural properties were analyzed by reciprocal space mapping in the symmetric (004) and asymmetric (224) planes by high resolution X-ray diffraction (XRD). The lateral correlation length (LCL) and the mosaic spread (MS) were extracted for the epi-layer peaks in the asymmetric (224) diffraction. With the increase of Sn concentration, the LCL reduces while the MS increases, indicating degrading crystalline quality. Dislocations were observed in the sample with 7.62% Sn concentration by transmission electron microscope, consistent with the strain relaxation found in XRD mapping. Besides, the surface morphologies were investigated.

A new route toward light emission from Ge: tensile-strained quantum dots

The tensile-strained Ge quantum dot (QD) is proposed as a new route for the realization of direct band gap conversion in Ge. Ge QDs were successfully grown on an InP substrate by molecular beam epitaxy. The strain field in the QDs were analyzed by high resolution transmission electron microscopy and simulated by the finite element method based on the measured geometries. The strain field in the QDs is found to be non-uniform and the shear component plays a significant role in the energy band structure, leading to larger required hydrostatic strain than that in the Ge thin films under biaxial strain to become a direct band gap.

Bi2Te3 photoconductive detectors on Si

The peculiar properties of the gapless surface states with a Dirac cone shaped energy dispersion in topological insulators (TIs) enable promising applications in photodetection with an ultra-broad band and polarization sensitivity. Since many TIs can be easily grown on silicon (Si) substrates, TIs on Si could make an alternative route for photon detection of Si photonics. We present good device performances of a Si-based single-crystal bismuth telluride (Bi2Te3) photoconductive detector. Room temperature photo responses to 1064 nm and 1550 nm light illumination were demonstrated. Linear dependences of the photocurrent on both the incident light power and the bias voltage were observed. The main device parameters including responsivity and quantum efficiency were extracted.

InPBi Quantum Dots for Super-Luminescence Diodes

InPBi thin film has shown ultra-broad room temperature photoluminescence, which is promising for applications in super-luminescent diodes (SLDs) but met problems with low light emission efficiency. In this paper, InPBi quantum dot (QD) is proposed to serve as the active material for future InPBi SLDs. The quantum confinement for carriers and reduced spatial size of QD structure can improve light emission efficiently. We employ finite element method to simulate strain distribution inside QDs and use the result as input for calculating electronic properties. We systematically investigate different transitions involving carriers on the band edges and the deep levels as a function of Bi composition and InPBi QD geometry embedded in InAlAs lattice matched to InP. A flat QD shape with a moderate Bi content of a few percent over 3.2% would provide the optimal performance of SLDs with a bright and wide spectrum at a short center wavelength, promising for future optical coherence tomography applications.

Novel group IV nano- and micro-structures for light sources on silicon

We present our recent researches on novel group IV nano- and micro-structures for potential light sources on Si, including the tensile strained Ge quantum dots (QDs), GeSn thin films and microstructures, and Ge(Sn) nanowires. Tensile-strained Ge QDs were grown by SS-MBE, and photoluminescence was achieved. The GeSn thin films were demonstrated with Sn concentration above the bandgap transition critical point, and partially suspended GeSn microstructures were fabricated for relaxing the compressive strain.

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.

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.