Lateral composition modulation in AlAs/InAs and GaAs/InAs short period superlattices structures: The role of surface segregation

The effect of In surface segregation on the microstructure of short period superlattices (SPSs) in two different material systems with nominally equivalent lattice misfit, AlAs/InAs and GaAs/InAs, has been investigated and compared. It was found that the quality of the SPSs and the appearance of lateral composition modulation are remarkably different in these two systems. For AlAs/InAs SPSs grown at temperatures of T=500 °C,T=500 °C, uniform structures devoid of lateral composition modulation were obtained. Samples grown at T>500 °CT>500 °C exhibit lateral composition modulation. Uniform and homogeneous SPS structures were not obtained in the GaAs/InAs structures over the entire temperature range examined in this study (475 °C⩽T⩽510 °C).(475 °C⩽T⩽510 °C). Instead, lateral composition modulation with varying degrees of regularity was always observed. It was found that In segregation and roughening determine the microstructure. A kinetic exchange model predicts that at an optimum temperature the SPS layers are more intermixed for the AlAs/InAs SPSs. Thus, the lattice mismatch is lower and the driving force for roughening is reduced, resulting in uniform SPS structures. Growing the structure away from the optimum temperature for smooth growth may induce roughening-driven composition modulation. The GaAs/InAs structures are less intermixed over the temperatures studied, which results in higher mismatch between the individual layers and a higher driving force for roughening and lateral composition modulation. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69963/2/JAPIAU-91-1-237-1.pd

Focused ion beam creation and templating of InAs and InAs/InP nanospikes

Ion beam irradiation has been examined as a method for creating nanoscale semiconductor pillar and cone structures, but has the drawback of inaccurate nanostructure placement. We report on a method for creating and templating nanoscale InAs spikes by focused ion beam (FIB) irradiation of both homoepitaxial InAs films and heteroepitaxial InAs on InP substrates. These 'nanospikes' are created as In droplets, formed due to FIB irradiation, act as etch masks for the underlying InAs. By pre-patterning the InAs to influence In droplet movement, nanospike locations on homoepitaxial InAs may be controlled with limited accuracy. Creating nanospikes using an InAs/InP heterostructure provides an additional measure of control over where the spikes form because nanospikes will not form on exposed regions of InP. This effect may be exploited to accurately control nanospike placement by pre-patterning an InAs/InP heterostructure to control the location of the InAs/InP interface. Using this heterostructure templating method it is possible to accurately place nanospikes into regular arrays that may be useful for a variety of applications.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90790/1/0957-4484_22_35_355302.pd

The Impact of The Initial Surface Reconstruction on Heteroepitaxial Film Growth and Defect Formation

While it is well known that growth conditions such as temperature greatly affect defect incorporation in thin films, less is known about the direct effects of the surface reconstruction. In this work, we examine the effect of the initial surface reconstruction on defect incorporation in GaSb/GaAs(001) lattice mismatched films. The stress built up in GaSb films grown on As-terminated and Sb-terminated GaAs was monitored during film growth and shows that the total relaxation is similar in both films along the [110], but lower on the Sb-terminated surface along the . These differences can be understood by examining the ability for the two surface terminations to accommodate strain. The resulting films show that the density of 3D islands is lower for the Sb-terminated surfaces, and that lattice mismatch strain is further accommodated by a 5° tilt. In contrast, the As-terminated surface contains both stacking faults and misfit dislocations. These results demonstrate the possibility to engineer specific defects into films by controlling the starting surface of film growth

Cooperative nucleation leading to ripple formation in InGaAs/GaAs films

In0.25Ga0.75AsIn0.25Ga0.75As epilayers were grown on GaAs (001) substrates (1.8% misfit strain) by molecular beam epitaxy to investigate the two-dimensional to three-dimensional transition as a function of thickness (t ⩽ 30 MLs).(t⩽30MLs). Tapping-mode atomic force micrographs show the evolution of the morphology as a function of thickness. As the film is deposited, the nucleation of 3D islands followed by cooperative nucleation of pits is observed. As the thickness increases, both islands and pits continue to nucleate and grow until they coalesce, resulting in a fully formed ripple morphology running along the [10].[11̄0]. The ripples also exhibit a secondary alignment roughly along the 〈310〉 which is attributed to the nucleation of islands with {136} faces. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70708/2/APPLAB-76-17-2382-1.pd

Lateral composition modulation in mixed anion multilayers

Lateral composition modulation on the group V sublattice has been observed in GaAs/GaSb short period superlattices. Cross sectional transmission electron microscopy and x-ray diffraction reciprocal space maps reveal that all structures are phase-separated with Sb compositions for the strongest modulated structure of x = 0.73x=0.73 in the Sb-rich regions, x = 0.55x=0.55 in the As-rich regions, and wavelengths 15 ⩽ Λ ⩽ 20 nm.15⩽Λ⩽20nm. The composition modulation observed in these films is not due to spinodal decomposition, because an alloy grown at the same conditions results in a homogeneous layer, but may be related to vertical stacking of quantum dots that nucleate during the growth of the structure. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69861/2/APPLAB-81-18-3368-1.pd

Focused ion beam modification of surfaces for directed self-assembly of InAs/GaAs(001) quantum dots

Controlled nucleation of InAs quantum dots has been achieved by Ga+ focused ion beam modification of GaAs(100) surfaces. Quantum dots may be induced in irradiated regions despite the fact that the deposited thickness is less than the critical thickness for their formation under typical growth conditions when the ion dose is greater than 1013 ions cm−2. We also find that the dot density increases with increasing ion dose, and reaches saturation for D>1014 ions cm−2. Parameters such as dot height and diameter are unaffected by the dose level. Thus, we show that the increase in dot density is a result of diffusion of adatoms from outside the patterned region. The mechanism for enhanced quantum dot formation is due to the formation of monolayer deep holes created in the substrate by the ion beam, which may be used to form regular arrays of quantum dots.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58138/2/nano7_45_455303.pd

Step instability and island formation during annealing of pseudomorphic InGaAs/GaAs layers

The morphological stability of compressively strained In0.27Ga0.73As/GaAsIn0.27Ga0.73As/GaAs pseudomorphic layers has been investigated during annealing. Large three-dimensional islands form at the beginning of annealing on initially flat surfaces, likely to relieve strain energy. The islands disappear with increasing annealing, being reabsorbed into the terraces. At the same time, the step line destabilizes forming cusps that inject two-dimensional vacancy islands into the terrace. At high temperatures, this process leads to a severe deterioration of the morphology that is not due to decomposition. The island dissolution and the development of the step instability are likely alternative paths towards the reduction of surface energy. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71229/2/APPLAB-83-22-4518-1.pd

Elastically Induced Coexistence of Surface Reconstructions

Scanning tunneling microscopy of Sb-capped GaAs shows the coexistence of different surface reconstructions. The majority of the surface consists of an α2(2×4) reconstruction typically observed for GaAs(001) surfaces. At step edges, an α(4×3) reconstruction, common for GaSb(001), is observed. We argue that strain couples the surface reconstruction to the film morphology. Density functional theory calculations show that the (2×4) reconstruction is stabilized in GaSb films when the lattice parameter is constrained to that of GaAs, as happens in the middle of a terrace, while the (4×3) reconstruction is stabilized when the lattice parameter is allowed to relax toward that of GaSb at step edges. This result confirms the importance of elastic relaxation in the coexistence of surface reconstructions

Lateral composition modulation in short period superlattices: The role of growth mode

The role of the growth mode on lateral composition modulation is studied in short period superlattices of AlAs/InAs and GaAs/InAs. Reflection high energy electron diffraction and scanning tunneling microscopy are used to monitor the growth mode and the quality of the interfaces. Cross-sectional transmission electron microscopy indicates that samples that grow via the layer-by-layer growth mode do not exhibit lateral composition modulation and the superlattice structure is well defined. Lateral composition modulation forms when roughening occurs during growth. However, too much roughening, i.e., three-dimensional island nucleation destroys the regularity of the composition modulation in both the lateral and vertical directions. These results are in general agreement with theoretical predictions. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70098/2/APPLAB-79-25-4118-1.pd