L.W. Guo

Low‐temperature buffer layer for growth of a low‐dislocation‐density SiGe layer on Si by molecular‐beam epitaxy

A method using a low‐temperature Si (LT‐Si) buffer layer is developed to grow a SiGe epilayer with low density of dislocations on a Si substrate by molecular‐beam epitaxy. In this method, a LT‐Si layer is used to release the stress of the SiGe layer. The samples have been investigated by x‐ray double‐crystal diffraction and transmission electron microscopy. The results indicate that the LT‐Si is effective to release the stress and suppress threading dislocations.

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.

White light-emitting diodes based on a single InGaN emission layer

White light-emitting InGaN∕GaN diode with an InGaN underlying layer grown on the (0001) sapphire substrate was fabricated by low pressure metal-organic vapor phase epitaxy. The electroluminescence measurements show that the emitted white light is composed of blue and yellow lights, centered at around 440 and 570nm, respectively, for an injection current of 20mA. Cross-sectional transmission electron microscopy reveals that In-rich quantum dots were formed in InGaN wells due to phase separation of indium. It is suggested that the yellow and blue lights come from In-rich quantum dots and the low-indium regions, respectively, in InGaN quantum wells.

Influence of AlN buffer layer thickness on the properties of GaN epilayer on Si(111) by MOCVD

The effect of thickness of the high-temperature (HT) AlN buffer layer on the properties of GaN grown on Si(111) has been investigated. Optical microscopy (OM), atomic force microscopy (AFM) and X-ray diffraction (XRD) are employed to characterize these samples grown by metal-organic chemical vapor deposition (MOCVD). The results demonstrate that the morphology and crystalline properties of the GaN epilayer strongly depend on the thickness of HT AlN buffer layer, and the optimized thickness of the HT AlN buffer layer is about 110nm. Together with the low-temperature (LT) AlN interlayer, high-quality GaN epilayer with low crack density can be obtained.

Effects of barrier growth temperature ramp-up time on the photoluminescence of InGaN∕GaN quantum wells

The influence of ramp-up time of barrier growth temperature on optical properties is investigated for InGaN∕GaN quantum wells deposited on sapphire substrate by metal organic chemical vapor deposition. Three ramp-up times are used from the low and high growth temperatures for the well and barrier, respectively. The results indicate that increasing the ramp-up time leads to a blueshift of the photoluminescence (PL) peak position and a broadening of the PL emission linewidth. Similarly, “S-shaped” temperature dependences of the PL peak energy are observed in all the samples. However, very different temperature dependences of PL linewidth, such as the conventional shaped, “U-shaped,” and S-shaped, are observed in the samples with different ramp-up time. These effects are attributed to the redistribution of the In-rich clusters in the wells. Small quantum-dot-like In-rich clusters with high density are considered to be formed in the wells for the sample with a long ramp-up time, leading to the unconventional ...

Anomalous tunneling effect on photoluminescence of asymmetric coupled double InGaN/GaN quantum wells

Tunneling-assisted carrier transfer in coupled double InGaN∕GaN quantum wells (QWs) has been studied by temperature-dependent photoluminescence (PL) at varied excitation density. It is found that the carriers captured by the wide (“deep”) well are efficiently transferred to the adjacent narrow (“shallow”) one by resonant tunneling, which results in anomalous temperature dependence of PL intensity and significantly enhanced luminescent efficiency for the narrow well. This is disparate from those conventional tunneling-assisted behaviors in coupled double QWs constructed by zinc-blende materials without polarization effect, where the carriers are always tunneling from the narrow (“shallow”) well to the wide (“deep”) one.

Structural characteristic and magnetic properties of Mn oxide films grown by plasma-assisted MBE

We report on the structural characteristics and magnetic properties of Mn oxide films grown on MgO (001) substrates by plasma-assisted molecular beam epitaxy (MBE). Under well-controlled growth conditions, three types of Mn oxide films were obtained. They are Mn3O4, lambda -MnO2 and their mixed structure (Mn3O4 + lambda -MnO2). A structural phase diagram about the films is depicted as a function of growth rate and substrate temperature. It is found that at lower growth rate, a new phase of Mn oxide lambda -MnO2 was obtained, and two transition temperatures were observed in its magnetization. At a higher growth rate, the Mn3O4 film was obtained. Its coercive force and transition temperature were different from its bulk sample. The atomic force microscopy (AFM) images exhibited the surfaces of the three films are composed of pyramids, flat surfaces and truncated-pyramids, respectively

Low temperature step-graded InAlAs/GaAs metamorphic buffer layers grown by molecular beam epitaxy

Low-temperature step-graded InAlAs metamorphic buffer layers on GaAs substrate grown by molecular beam epitaxy were investigated. The strain relaxation and the composition of the top InAlAs layer were determined by high-resolution triple-axis x-ray diffraction measurements, which show that the top InAlAs layer is nearly fully relaxed. Surface morphology was observed by reflection high-energy electron diffraction pattern and atomic force microscopy. Under a selected range of growth parameters, the root mean square surface roughness of the sample grown at 380 °C is 0.802 nm, which has the smallest value compared with those of other samples.Furthmore, The ω-2θ and ω scans of the triple-axis x-ray diffraction, and photoluminescence show the sample grown at 380 °C has better crystalline quality. With decreasing As overpressure at this growth temperature, crystalline quality became poor and could not maintain two dimensional growth with increasing overpressure. The carrier concentrations and Hall mobilities of the InAlAs/ InGaAs/GaAs MM-HEMT structure on low-temperature step-graded InAlAs metamorphic buffer layers grown in optimized conditions are high enough to make devices.

Structural characteristics and magnetic properties of λ-MnO2 films grown by plasma-assisted molecular beam epitaxy

High-quality λ-MnO2 single-crystal films were fabricated on MgO (001) substrates by plasma-assisted molecular beam epitaxy. The structural characteristics and magnetic properties of the films were studied. Two magnetic transition temperatures, T1 and T2, were observed from the films. Their magnetic behavior differed from that of bulk polycrystalline samples synthesized by removing Li ions from the parent LiMn2O4. The first, T1, setting at around 20 K, corresponded to long-range antiferromagnetic ordering similar to that observed in bulk at 32 K. T2, set at 43 K, corresponded to weak long-range ferromagnetic ordering among net magnetization of clusters, in which short-range antiferromagnetic ordering existed. T1 shifted to lower temperature with decreasing film thickness, which indicated that the proximity of the interface and strain played a key role in determining the transition temperature.

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