Y. H. Luo

Controlled arrangement of self-organized Ge islands on patterned Si (001) substrates

We report the ability to arrange self-organized Ge islands on patterned Si (001) substrates. Selective epitaxial growth of Si is carried out with gas-source molecular beam epitaxy to form Si mesas followed by subsequent Ge growth. Self-aligned and regularly spaced Ge islands are formed on the 〈110〉-oriented ridges of the Si stripe mesas. A mono-modal size distribution of the islands has been observed on the ridges. Using preferential nucleation sites allows us to place Ge islands at predetermined positions. The controlled arrangement of self-organized nanostructures offers the potential applications of island arrays for the implementation in nanoelectronics and quantum computation.

Band alignments and photon-induced carrier transfer from wetting layers to Ge islands grown on Si(001)

Temperature- and excitation-power-dependent photoluminescence measurements were carried out for the multilayer structure of Ge islands grown on a Si(001) substrate by gas-source molecular-beam epitaxy. When the excitation power increases from 10 to 400 mW, the photoluminescence peak from the Ge islands showed a large linear blueshift of 34 meV while that of the wetting layers did not change much. These two different power dependences are explained in terms of type-II and type-I band alignments for the islands and the wetting layers, respectively. When the sample temperature increased from 8 to 20 K, an anomalous increase of photoluminescence intensity for islands was accompanied by a rapid decrease of that from the wetting layers, implying that a large portion of photon-induced carriers in the wetting layer was transfered to the neighboring islands and the Si layer, respectively, thus resulting in an increase of photoluminescence intensity of the islands.

A SURFACTANT-MEDIATED RELAXED SI0.5GE0.5 GRADED LAYER WITH A VERY LOW THREADING DISLOCATION DENSITY AND SMOOTH SURFACE

A method to grow a relaxed Si0.5Ge0.5 graded layer with a very smooth surface and a very low threading dislocation density using solid-source molecular-beam epitaxy is reported. This method included the use of Sb as a surfactant for the growth of a 2 μm compositionally graded SiGe buffer with the Ge concentration linearly graded from 0% to 50% followed by a 0.3 μm constant Si0.5Ge0.5 layer. The substrate temperature was kept at 510 °C during the growth. Both Raman scattering and x-ray diffraction were used to determine the Ge mole fraction and the degree of strain relaxation. Both x-ray reflectivity and atomic force microscopy measurements show a surface root mean square roughness of only 20 A. The threading dislocation density was determined to be as low as 1.5×104 cm−2 as obtained by the Schimmel etch method. This study shows that the use of a Sb surfactant and low temperature growth is an effective method to fabricate high-quality graded buffer layers.

High-quality Ge films on Si substrates using Sb surfactant-mediated graded SiGe buffers

High-quality Ge films were grown on Si substrates by solid-source molecular beam epitaxy using SiGe graded layer and Sb surfactant-mediation technique. Transmission electron microscopy measurements show that samples grown using this method have a lower threading dislocation density than those grown by other typical methods, such as grading at high temperature (700 °C) only, grading at intermediate temperature (510 °C) only, and the use of low temperature Si buffer. A relaxed Ge film on a 4-μm-thick graded buffer was grown and shown to have a threading dislocation density of 5.4×105 cm−2 and surface roughness of 35 A. Ge p–i–n diodes were fabricated and tested. Under a reverse bias of 1 V, the p–i–n Ge mesa photodiodes exhibit a very low dark current density of 0.15 mA/cm2.

Optical and acoustic phonon modes in self-organized Ge quantum dot superlattices

Raman scattering measurements were carried out in self-organized Ge quantum dot superlattices. The samples consisted of 25 periods of Ge quantum dots with different dot sizes sandwiched by 20 nm Si spacers, and were grown using solid-source molecular-beam epitaxy. Optical phonon modes were found to be around 300 cm−1, and a dependence of the Raman peak frequency on the size of dots was evidenced in good agreement with a prediction based on phonon confinement and strain effects. Acoustic phonons related to the Ge quantum dots have also been observed.

High-quality strain-relaxed SiGe films grown with low temperature Si buffer

High-quality strain-relaxed SiGe templates with a low threading dislocation density and smooth surface are critical for device performance. In this work, SiGe films on low temperature Si buffer layers were grown by solid-source molecular beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, photoluminescence spectroscopy, and Raman spectroscopy. Effects of the growth temperature and the thickness of the low temperature Si buffer were studied. It was demonstrated that when using proper growth conditions for the low temperature Si buffer the Si buffer became tensily strained and gave rise to the compliant effect. The lattice mismatch between the SiGe and the Si buffer layer was reduced. A 500 nm Si0.7Ge0.3 film with a low threading dislocation density as well as smooth surface was obtained by this method.

Compliant effect of low-temperature Si buffer for SiGe growth

Relaxed SiGe attracted much interest due to the applications for strained Si/SiGe high electron mobility transistor, metal-oxide-semiconductor field-effect transistor, heterojunction bipolar transistor, and other devices. High-quality relaxed SiGe templates, especially those with a low threading dislocation density and smooth surface, are critical for device performance. In this work, SiGe films on low-temperature Si buffer layers were grown by solid-source molecular-beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, and photoluminescence spectroscopy. It was demonstrated that, with the proper growth temperature and Si buffer thickness, the low-temperature Si buffer became tensily strained and reduced the lattice mismatch between the SiGe and the Si buffer layer. This performance is similar to that of the compliant substrate: a thin substrate that shares the mismatch strain in heteroepitaxy. Due to the smaller mismatch, misfit dislocation and threading dislocation de...

Effective compliant substrate for low-dislocation relaxed SiGe growth

An effective compliant substrate for Si1-xGex growth is presented. A silicon-on-insulator substrate was implanted with B and O forming 20 wt % borosilicate glass within the SiO2. The addition of the borosilicate glass to the buried oxide acted to reduce the viscosity at the growth temperature of Si1-xGex, promoting the in situ elastic deformation of the thin Si (∼20 nm) layer on the insulator. The sharing of the misfit between the Si and the Si1-xGex layers was observed and quantified by double-axis X-ray diffraction. In addition, the material quality was assessed using cross-sectional transmission electron microscopy, photoluminescence and etch pit density measurements. No misfit dislocations were observed in the partially relaxed 150-nm Si0.75Ge0.25 sample as-grown on a 20% borosilicate glass substrate. The threading dislocation density was estimated at 2×104 cm-2 for 500-nm Si0.75Ge0.25 grown on the 20% borosilicate glass substrate. This method may be used to prepare compliant substrates for the growth of low-dislocation relaxed SiGe layers.

Observation of inter-sub-level transitions in modulation-doped Ge quantum dots

The inter-sub-level transitions in modulation-doped Ge quantum dots are observed. The dot structure is grown by molecular-beam epitaxy, and consists of 30 periods of Ge quantum dots sandwiched by two 6 nm boron-doped Si layers. An absorption peak in the midinfrared range is observed at room temperature by Fourier transform infrared spectroscopy, which is attributed to the transitions between the first two heavy-hole states of the Ge quantum dots. This study suggests the possible use of modulation-doped Ge quantum dots for improved infrared detector applications.

Effects of interdiffusion on the band alignment of GeSi dots

The interdiffusion effects on the band alignment of the GeSi dots embedded in Si matrix were studied by temperature- and excitation-power-dependent photoluminescence measurements. A different power-dependent behavior of the photoluminescence for the as-grown and the annealed samples was observed. It was suggested that the band alignments of the dots changed from type II to type I after annealing due to the Ge/Si interdiffusion. The decrease of the valence band offset, which was also induced by the Ge/Si interdiffusion, was observed from the temperature-dependent photoluminescence measurements.