Chi Sheng

Thermal stability of a Si/Si1−xGex quantum well studied by admittance spectroscopy

The thermal stability of a SiGe/Si quantum well grown by molecular-beam epitaxy is studied by using the admittance spectroscopy technique. The values of activation energies of hole emission from the subbands in the SiGe/Si quantum well are derived from the admittance spectra. After annealing the sample at different temperatures, the activation energy varies in different behaviors. There is no significant change of the activation energy after annealing at 700 °C for 40 min. At the annealing temperature of 900 °C, the decrease of the activation energy with annealing time could be attributed to the interdiffusion of Ge, Si atoms at the heterointerfaces and the strain relaxation effect. An unexpected phenomenon is observed at the annealing temperature of 800 °C, i.e., the activation energy increases with the annealing time. This extraordinary phenomenon is supposed to be caused by the change of the well potential shape due to the B out-diffusion from the well to the Si barrier.

Near Band-Edge Photoluminescence in Strained and Relaxed Si1-xGex/Si Quantum Wells

Near band-edge photoluminescence was observed from strained and relaxed Si1-xGex/Si quantum wells grown by molecular beam epitaxy and annealed at 850–1100°C. Changes in the photoluminescence line energies were monitored, and the extent of both interdiffusion and relaxation in wells during annealing was calculated. Strain relaxation was observed only in quantum well structures annealed above 950°C. The experimental data confirmed the existence of an abrupt transition between stable and strain-relaxed Si1-xGex/Si quantum well structures, and it was also observed that photoluminescence was sensitive enough to detect the onset of strain relaxation in quantum well structures following rapid thermal annealing.

Oxygen δ-Doped Si Multi-Layers Grown by Molecular Beam Epitaxy

An oxygen δ-doped Si multilayer structure has been grown by molecular beam epitaxy (MBE). Thermal oxidation of a Si layer prepared by an MBE system at a low oxygen partial pressure resulted in a thin ordered oxide layer with a very flat surface. On the ordered oxide, Si was epitaxied through the solid phase epitaxy. Five periods of Si(2 nm)/SiOx(1 nm) multilayers were grown successfully. The O-doped Si layer (SiOx) was composed of 7 atomic layers and the crystalline structure of the SiOx layer was the same as that of the Si substrate but had a relatively transverse displacement. The in-plane stress restricted the number of the oxide layers grown to a thickness of less than 6 atomic layers. This structure may be the gateway for realizing Si-based quantum wells.

Hole confinement in boron δ-doped Si quantum wells studied by admittance spectroscopy

The admittance spectroscopy technique has been used to study the hole confinement in boron δ-doped Si quantum wells. A carrier thermal emission model is proposed to derive the activation energies of holes confined in the quantum wells from the measured conductance spectra. For the same peak doping concentration, the conductance peak shifts towards higher temperatures as the thickness of the δ-doped layer increases. The activation energies obtained from the measurements coincide well with the results of a self-consistent calculation of the subbands in the quantum wells. It verifies that the conductance peaks correspond to the hole emissions from the hole ground states in the δ-quantum wells to the Si valence band.

Interfacial Defects Related to the Substrate. Treatment in Molecular Beam Epitaxial Silicon.

Interfacial defects related to the residual carbon on the hydrogenterminated Si(100) surface have been studied using the deep-level transient spectroscopy (DLTS) technique. The defect level is found to be donorlike which compensates the acceptor impurity at the interface. With a fast load-in and a two-step annealing, the defect density can be suppressed below the DLTS detection limit of 1012 cm-3.

Ge Beam Treatment of Si Substrate for Molecular Beam Epitaxy

A new surface cleaning method for Si MBE is described in which the Si surface is exposed to Ge beams while the substrate is kept at certain temperature. It has been proved that the thin passivation layer of SiO 2 on the Si substrate will react with Ge at a relatively low temperature (620°C), and the products are volatile. The residual Ge on Si substrate can be reduced to less than 0.1 monolayer (ML). Ge beam treatment turns out to be an effective low temperature technique for preparing Si substrate, especially for the heteroepitaxial growth of Ge x Si 1-x /Si.

An investigation on the thermal stability of the GexSi1-x/Si superlattice grown by MBE

Abstract Strained GexSi1-x/Si superlattices with Ge content of 0.48 and 0.49 grown by MBE at 420°C (sample A) and 570°C (sample B) were annealed and studied by X-ray diffraction, Raman scattering and TEM. The cross section TEM image of the samples before annealing showed that the growth of sample A was two-dimensional, but for sample B, the growth of the GexSi1-x layer was three-dimensional. By the X-ray diffraction spectra before and after annealing for samples A and B, the strain relaxations from X-ray diffraction measurements were 27% and 35% for samples A and B, respectively. This difference might be from the three-dimensional growth of sample B, in which the strain was stronger than that in sample A. Raman scattering measurements showed that the Ge-Ge peak and the Ge-Si peak shifted towards the lower energy after annealing, while the Si-Si peak shifted towards the higher energy. The reason for the shift of these peaks could be explained by the common contribution of the strain relaxation and the internal diffusion of Ge atoms into the Si layer. In addition, the behavior of the strained GexSi1-x/Si superlattice annealed by rapid thermal annealing technology was also studied. A sample of superlattice with a Ge content of 0.46 was rapidly thermal annealed at the temperature range of 600 to 1000°C and at each temperature the sample was annealed twice for 40 s each time. From X-ray diffraction measurements it could be found that as the annealing temperature increased from 600 to 900°C, the strain relaxation would increase from 19% up to 57%. As the sample was annealed at 1000°C, no satellite peaks were in the X-ray diffraction spectra and no multilayer structure could be observed in the cross section TEM image, that is, in the sample the GeSi layers were completely mixed up with the Si layers.