Formation Of SiGe Alloy Research Articles

High-speed Ge/Si electro-absorption optical modulator in C-band operation wavelengths.

We studied a high-speed electro-absorption optical modulator (EAM) of a Ge layer evanescently coupled with a Si waveguide (Si WG) of a lateral pn junction for high-bandwidth optical interconnect. By decreasing the widths of selectively grown Ge layers below 1 µm, we demonstrated a high-speed modulation of 56 Gbps non-return-to-zero (NRZ) and 56 Gbaud pulse amplitude modulation 4 (PAM4) EAM operation in the C-band wavelengths, in contrast to the L-band wavelengths operations in previous studies on EAMs of pure Ge on Si. From the photoluminescence and Raman analyses, we confirmed an increase in the direct bandgap energy for such a submicron Ge/Si stack structure. The operation wavelength for the Ge/Si stack structure of a Ge/Si EAM was optimized by decreasing the device width below 1-µm and setting the post-growth anneal condition, which would contribute to relaxing the tensile-strain of a Ge layer on a Si WG and broadening the optical bandwidths for Franz-Keldysh (FK) effect with SiGe alloy formation.

Formation of Si<sub>0.80</sub>Ge<sub>0.20</sub> Solid Solution by Mechanical Alloying

In general, semiconductor materials for thermoelectric generation prepared by vacuum metallurgy shows a relatively high value of figure-of-merit. However, differences in some properties of alloys elemental constituents can cause processing problems. Recently, Mechanical Alloying (MA) has been used to produce polycrystalline thermoelectric materials, such as (Bi,Sb)2 and (Te,Se)3(1). The industry is using this process since early 70’s to produce oxide dispersion strengthened alloys and those with widely different melting temperatures (2) In the present work, Si0.80Ge0.20 alloys were prepared via Mechanical Alloying (MA), using 99.9 % pure silicon and germanium powders, with a sieve size of 100 mesh. The MA has been performed, for several balls - to - powder ratio, in a SPEX 8000 vibratory high energy milling with tungsten carbide balls. Time for alloy formation was in a range from 3 to 9 hours, corresponding to charge ratio of 12:1 and 4:1, respectively. After two hours of processing time, the grinding temperature reached 80 0 C, and remained at this level until the end of the process. It was possible to follow the SiGe alloy formation by x-ray diffractometry, as the peak lines positions of elemental Si and Ge were continuously shifted, and end up to merge into a single broad peak. There was a convergence of the individual lattice parameters of Si and Ge to a single value of 5.470 A, measured within the limit of  0.005 A. For the Si0.80Ge0.20 system evaluated in this work, the alloying progress occurred continuously, and changed inversely with charge ratio.

Effect of composition modulation on the structural and optical properties of Si/Ge bilayers

We report structural as well as optical studies on Si/Ge bilayer structures having different individual layer thicknesses. The Raman spectrum of [Ge (5 nm)/Si (5 nm)] bilayer structure shows amorphous nature, while the [Si (5 nm)/Ge (5 nm)] bilayer structure shows a mixed nanocrystalline/amorphous behaviour of the layers. As the thickness of the individual layers increases to 10 nm, the introduction of large number of Si atoms at the interface results in reduction of Ge crystallization as well as higher intensity of interfacial SiGe alloy formation. This may be regarded as a consequence of the island growth induced surface roughening in the later case (i.e. in [Si (10 nm)/Ge (10 nm)] bilayer) as also revealed by corresponding atomic force microscopy (AFM) images. These results are also supported by Photoluminescence (PL) spectra recorded using two different photon energies of 300 and 488 nm along with the optical absorption measurements giving higher values of band gap as compared to their corresponding bulks, revealing the effect of quantum confinement in the deposited layers.

Determination of band gap in polycrystalline Si/Ge thin film multilayers

The valence band (VB) photoemission supported by ultraviolet–visible–near infrared spectroscopy techniques were used to determine the band gap values of polycrystalline Si and Ge single layers as well as of Si/Ge multilayer structures. The band gap values obtained from VB photoemission measurements for these structures were found to be much larger than their corresponding bulks and to match well with those determined from standard optical absorption measurements. In each case, the VB offset values were obtained by considering the corresponding VB maximum as a reference. The increase in band gap in case of thin single layers of Si and Ge with respect to bulks were interpreted in terms of quantum confinement effect, while in case of multilayer sample, the effect of various factors such as (i) intermixing leading to the formation of SiGe alloy, (ii) roughness at the interface, (iii) particle size, and (iv) strain seem to play an important role in the observed change in band gap.

Optical Characterization Of Self-Assembled Ge Dots On Silicon

AbstractSelf assembled quantum dots of Ge were obtained by magnetron sputter epitaxy of seven monolayers of Ge on a 33nm thick undoped Si buffer grown on top of a p-doped (100) Si substrate. The samples obtained in this manner were then capped with an increasing number of silicon layers in order to study the effect of Si deposition on the strain and the morphology of the dots. They were characterized “ex situ” by spectroscopic ellipsometry and Raman spectroscopy. The optical experiments revealed well defined differences between the capped and uncapped samples and among samples with different cap thicknesses.By monitoring the energy and the splitting of the E0', E1 and E2 interband optical transitions of Ge and the Ge-Si vibrational mode, the optical measurements evidence strain effects as well as the formation of SiGe alloy, in agreement with the ``in situ'' STM measurements.

Optical Characterization of Self-Assembled Ge Dots on Silicon

ABSTRACTSelf assembled quantum dots of Ge were obtained by magnetron sputter epitaxy of seven monolayers of Ge on a 33nm thick undoped Si buffer grown on top of a p-doped (100) Si substrate. The samples obtained in this manner were then capped with an increasing number of silicon layers in order to study the effect of Si deposition on the strain and the morphology of the dots. They were characterized “ex situ” by spectroscopic ellipsometry and Raman spectroscopy. The optical experiments revealed well defined differences between the capped and uncapped samples and among samples with different cap thicknesses.By monitoring the energy and the splitting of the E0', E1 and E2 interband optical transitions of Ge and the Ge-Si vibrational mode, the optical measurements evidence strain effects as well as the formation of SiGe alloy, in agreement with the “in situ” STM measurements.