Changtao Peng

RELAXED SI0.7GE0.3 LAYERS GROWN ON LOW-TEMPERATURE SI BUFFERS WITH LOW THREADING DISLOCATION DENSITY

Si0.7Ge0.3 epilayers with low threading dislocation density have been grown on Si (001) substrates by introducing a low temperature Si buffer. Such a structure can be used as the buffer for the growth of device structures. In comparison with the conventional compositionally graded buffer system, it has the advantages of having lower threading dislocation density, smaller thickness for required degree of relaxation, and smoother surface. Experimental evidence suggests that an anomalous relaxation mechanism has been involved.

EVOLUTION OF MOSAIC STRUCTURE IN SI0.7GE0.3 EPILAYERS GROWN ON SI(001) SUBSTRATES

In this article, we report a study of mosaic structures in partially relaxed Si0.7Ge0.3 epilayers grown on Si(001) substrates by x-ray double- and triple-axis diffractometry. The samples have different layer thicknesses and hence different degrees of strain relaxation. Our results show that, at early stages of strain relaxation, the films contain mosaic regions laterally separated by perfect regions. This is because the mosaic structure caused by a misfit dislocation is effectively localized in a lateral range of the layer thickness. Therefore, far from the dislocations, the film is virtually a perfect crystal. With the increase in the degree of strain relaxation, and consequently in the dislocation density, the mosaic regions of the layer expand while the perfect regions shrink and finally vanish completely. Moreover, our results indicate that the conventional method of estimating dislocation density from the x-ray rocking curve width fails in our case.

Relaxed Ge0.9Si0.1 alloy layers with low threading dislocation densities grown on low-temperature Si buffers

Relaxed GexSi1−x epilayers with high Ge fractions but low threading dislocation densities have been successfully grown on Si (001) substrate by employing a stepped-up strategy and a set of low-temperature GeySi1−y buffers. We show that even if the Ge fraction rises up to 90%, the threading dislocation density can be kept lower than 5×106 cm−2 in the top layers, while the total thickness of the structure is no more than 1.7 μm.

As-doped p-type ZnO films by sputtering and thermal diffusion process

As-doped p-type ZnO films were grown on GaAs by sputtering and thermal diffusion process. Hall effect measurements showed that the as-grown films were of n-type conductivity and they were converted to p-type behavior after thermal annealing. Moreover, the hole concentration of As-doped p-type ZnO was very impressible to the oxygen ambient applied during the annealing process. In addition, the bonding state of As in the films was investigated by x-ray photoelectron spectroscopy. This study not only demonstrated an effective method for reliable and reproducible p-type ZnO fabrication but also helped to understand the doping mechanism of As-doped ZnO.

Improvement of Ge self-organized quantum dots by use of Sb surfactant

A Sb-mediated growth technique is developed to deposit Ge quantum dots (QDs) of small size, high density, and foe of dislocations. These QDs were grown at low growth temperature by molecular beam epitaxy. The photoluminescence and absorption properties of these Ge QDs suggest an indirect-to-direct conversion, which is in good agreement with a theoretical calculation

Liquid-phase-epitaxy-grown InAsxSb1-x/GaAs for room-temperature 8-12 mu m infrared detectors

High-quality InAsxSb1−x (0<x⩽0.3) films are grown on GaAs substrates by liquid phase epitaxy and electrical and optical properties of the films are investigated, revealing that the films exhibit Hall mobilities higher than 2×104cm2V−1s−1 and cutoff wavelengths longer than 10μm at room temperature (RT). Photoconductors are fabricated from the films, and notable photoresponses beyond 8μm are observed at RT. In particular, for an InAs0.3Sb0.7 film, a photoresponse of up to 13μm with a maximum responsivity of 0.26V∕W is obtained at RT. Hence, the InAsxSb1−x films demonstrate attractive properties suitable for room-temperature, long-wavelength infrared detectors.

Strain relaxation of GeSi alloy with low dislocation density grown on low-temperature Si buffers

We have developed a low-temperature (LT) growth technique. Even with Ge fraction x upto 90%, the total thickness of fully relaxed GexSi1-x buffers can he reduced to 1.7 mu m with dislocation density lower than 5 x 10(6) cm(-2). The surface roughness is no more than 6 nm. The strain relaxation is quite inhomogeneous From the beginning. Stacking faults generate and form the mismatch dislocations in the interface of GeSi/LT-Si

The formation of dislocations in the interface of GeSi/low-temperature Si buffer grown on Si (001)

We have studied the mechanism of strain relaxation of Ge0.3Si0.7 alloy layers grown on low-temperature (LT) Si buffers by high-resolution X-ray diffraction and transmission electron microscopy (TEM). X-ray rocking-curve analyses show that the strain relaxation of an alloy layer is quite inhomogeneous at its early stage, and becomes homogeneous when the layer approaches becomes fully relaxed. High-resolution cross-section TEM analyses indicate that stacking faults hale formed in the LT buffer near the interface of LT-Si/GeSi and have separated the mismatch dislocations in the interface of GeSi/LT-Si into Shockley partials. We propose that this could be due to the aggregation of the vacant defects ill the LT-SI layer. The mechanism of this aggregation is the osmotic force created by the vacancy super-saturation in the LT-SI layer and/or the tension stress propagating from the interface of GeSi/LT-Si

Study of Ge0.96Si0.04 epilayers grown on Si (001) at high temperature

We have studied the propcrties of Ge0.96Si0.04 epilayers grown at high temperature on Si (0 0 1) substrates by X-ray diffraction, Raman spectra, and high-resolution transmission electron microscopy (TEM). Raman spectra show that there is an obvious transition region between the Ge0.96Si0.04 layer ang the Si substrate. The thickness of this region is about 100nm. Towards the Ge0.96Si0.04 layer, the Si fraction decreases rapidly from about 30 to 4%. In X-ray diffraction, there is a sharp peak of Ge0.96Si0.04 and a broad peak of the GeSi alloy region with lower Ge fraction. High resolution of cross-sectional TEM images show that the interface of GcSi/Si substrate is :1 1 Ij faceted

Critical Ge concentration for 2×n reconstruction appearing on GeSi covered Si(100)

Using reflection high-energy electron diffraction (RHEED), we monitored in situ the appearing of 2 x n reconstruction with different Ge compositions of GeSi alloys deposition, at growth temperature 600 degrees C. A relation between the thickness and Ge composition has been determined. To understand the relation, we perform a Ge segregation simulation using the two-stale exchange kinetic model. As the Ge concentration in the top layer reaches about 0.8 for all Ge compositions. due to Ge segregation, the 2 x n reconstruction begins to appear to release the accumulated misfit strain. For a GeSi alloy with very low Ge composition, such as Ge0.05Si0.95 alloy, there is no 2 x n reconstruction shown even when the deposited GeSi alloy film thickness exceeds its critical thickness, because the saturated Ge concentration of the top layer in such alloys is lower than 0.8