Indium Segregation Research Articles

DIFFERENCES BETWEEN THE INTERFACES OF InGaAs/GaAs AND GaAs/InGaAs IN SUPERLATTICES

The InyGa1-y As/GaAs superlattice with an InxGa1-x As (xyGA1-yAs interface was narrower than that at the InyGa1-y As/GaAs interface; in InyGa1-yAs alloy layer, the composition near the GaAs/InyGa1-y As interface was larger than that near the other interface. For the first time, we have explained the composition profile in these kinds of superlattices based on the indium segregation theory. In addition, a new strain relaxation model was presented to explain the differences in the smoothness between the two interfaces.

Determination of segregation, elastic strain and thin-foil relaxation in InxGa-1-x As islands on GaAs(001) by high resolution transmission electron microscopy

Lattice-parameter mismatch-induced strains in three-dimensional coherent InxGa1-xAs islands grown on GaAs(001) substrates have been determined experimentally on an atomic scale by a digital analysis of images obtained by high-resolution transmission electron microscopy. The strain distributions in the islands were simulated by finite-element calculations. The simulated strain distributions are found to be in good agreement with measured data, taking into account the thin-foil relaxation of electron-transparent specimens in addition to the well known elastic strain relief. Clear evidence is given for an indium segregation causing a misfit gradient between the interface and the islands' surface.

In situ XPS investigation of indium surface segregation for Ga 1− xIn xAs and Al 1− xIn xAs alloys grown by MBE on InP(001)

We present an in situ X-Ray photoemission spectroscopy (XPS) investigation of indium surface segregation for As-stabilized Ga 1− x In x As and Al 1− x In x As ternary bulk alloys grown by molecular beam epitaxy on InP( the following conditions: (i) lattice-matched Ga 0.47In 0.53As and Al 0.48In 0.52As grown at 300 and 525°C, and (ii) compressive strained and relaxed Ga 0.25In 0.75As grown at 525°C. Our results show, first, that a chemically shifted component indicative of an InAs surface layer appears within the In 4d core level spectrum. Second, below this InAs top layer, we found a composition gradient which increases with growth temperature and is greater for AlInAs than for GaInAs compounds. Finally, we show that strain seems to have no direct effect on the indium surface segregation at least for Ga 0.25In 0.75As at 525°C. We analyse and interprete the indium segregation phenomenon as induced by differences in In, Al and Ga surface mobilities during the layer by layer epitaxial growth process.

Influence of surface structure on segregation and alloy properties in (100)- and (311)-oriented [formula omitted] heterostructures

The influence of surface orientation and surface structure on indium segregation and alloy properties were systematically studied in InGaAs GaAs quantum well structures grown by molecular beam epitaxy. (100), (311)A and (311)B surface orientations and different approaches in the growth interruption at the interfaces were used in this investigation. The segregation process and alloy parameters were obtained by photoluminescence and RHEED measurements. We find significant differences in the optical properties and growth kinetic for the three orientations. Using growth interruption we were able to change the surface structure and reduce the segregation process for all orientations.

The Differences between the InyGa1−yAs/GaAs Interface and GaAs/InyGa1−yAs Interface in Superlattice over a InxGa1−xAs (x<y) Buffer Layer

AbstractThe InyGa1−yAs/GaAs superlattice on InxGa1−x.As (x<y) buffer layer was grown by MOCVD. The well layer is under compressive strain and the barrier layer is under tensile strain. However, both layers do not exceed the calculated critical value based on the M−yAs interface was smoother than the InyGa1−yAs/GaAs interface; the Indium composition gradual region at the GaAs/InyGal1−yAs interface was narrower than that at the InyGa 1−yAs/GaAs interface; in InyGa1−yAs alloy layer, the Indium composition near the GaAs/InyGa1-yAs interface was higher than that near another interface. For the first time, we explained the composition profile in this kind of superlattice based on the indium segregation theory. A new strain relaxation model, in which the 30-degree and 90-degree shockley partial dislocations were taken into account under both tensile and compressive strains, was presented to explain the difference of the smoothness between the GaAs/InyGa1−yAs interface and the InyGa 1−yAs/GaAs interface.

Influence of surface structure on surface segregation and alloy properties in (100)- and (311)-oriented [formula omitted] heterostructures

We have performed a comprehensive study of the effects of surface structure and orientation on indium segregation process and alloy properties in InGaAs GaAs quantum well. The (100), (311)A and (311)B surface orientations and different approaches in the growth interruption at the interfaces were used in this investigation. We find significant differences in the optical properties and growth kinetics for the three orientations. Our results show smaller indium surface segregation and alloy disorder in the (311)A orientations than in the (311)B and (100) orientations. Using growth interruptions we were able to change the surface structure and reduce the segregation process for all orientations.

Influence of indium segregation on exciton recombination dynamics in InGaAs/GaAs quantum wells

Low-temperature PL decay times, τPL, measured for a series of In x Ga1−x As/GaAs quantum wells (QWs) show an almost linear increase with increasing thickness (4≤L z ≤10 nm,x=0.15) and with increasing In composition (0.05≤x≤0.25,L z =8 nm). τPL also increases linearly with temperature up to 50K, as expected for free excitons and does not exhibit the interface effects seen for GaAs/AlGaAs QWs. Wells with different In compositions exhibit a similar temperature behaviour and there is a weak influence of strain on the decay time.

In situ observation of indium segregation by reflectance difference spectroscopy in single monolayer heterostructures grown by atomic layer epitaxy

Single monolayers of InAs in GaAs and GaAs in InAs have been grown by atomic layer epitaxy (ALE) at 50 Torr. In situ reflectance difference spectroscopy monitoring of the surface during each stage of the growth showed a strong asymmetry in the surface behavior between the two systems. Following insertion of an InAs monolayer in GaAs, approximately 20 ML of GaAs are required to recover an In-free, As-stabilized GaAs surface at 390 °C. On the other hand, following the insertion of 1 ML of GaAs in InAs, the spectrum returns to a Ga-free, As-stabilized InAs surface after only 1 ML of InAs deposition. This behavior shows clear evidence of the presence of segregation caused by thermodynamic factors even at the very low growth temperatures used for ALE.

Exciton recombination dynamics in InxGa1-xAs/GaAs quantum wells.

Low-temperature decay times ${\mathrm{\ensuremath{\tau}}}_{\mathrm{PL}}$ are reported for a series of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs quantum wells. These show a nearly linear increase with increasing thickness (4\ensuremath{\le}${\mathit{L}}_{\mathit{z}}$\ensuremath{\le}10 nm, x=0.15) but recombination in the widest well (12 nm) is dominated by nonradiative effects. The decay time increases almost linearly with temperature up to 50 K, as expected for free excitons. An increase in ${\mathrm{\ensuremath{\tau}}}_{\mathrm{PL}}$ with increasing In composition (0.05\ensuremath{\le}x\ensuremath{\le}0.25, ${\mathit{L}}_{\mathit{z}}$=8 nm) is also observed. Wells with different In compositions exhibit a similar temperature behavior and there is a weak influence of strain on the decay time. Additional peaks in the photoluminescence spectra occur to the low-energy side of the free-exciton peaks. These features, which exhibit longer decay times, are attributed to excitons localized in In-rich islands arising from indium segregation.

Optical investigations in (In,Ga)As/GaAs quantum wells grown by metalorganic molecular-beam epitaxy.

Photoluminescence, reflectivity, and thermally detected optical-absorption (TDOA) experiments have been performed at liquid-helium temperatures on two strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs quantum-well (QW) structures grown by metalorganic molecular-beam epitaxy (MOMBE). The QW thicknesses vary from 3--15 ML and 4--16 ML by two monolayer (ML) steps. It is demonstrated that the MOMBE technique allows the thickness to be controlled with an accuracy of 1 ML. The electron--heavy-hole excitonic transitions (${\mathit{e}}_{1}$${\mathrm{hh}}_{1}$) are detected for all the QW's. TDOA enables the light-hole excitonic transition to be observed for the thinnest well in the second sample. The QW excitonic absorption energies are compared with calculations within the framework of the envelope-function approximation by taking into account the strain effects and the indium segregation phenomenon. An accurate determination of the strain conduction-band offset is derived (${\mathit{Q}}_{\mathit{c}}$=0.64\ifmmode\pm\else\textpm\fi{}0.01) and it is found that the indium segregation is very weak at the QW interfaces compared to structures grown by MBE. The light-hole band configuration is type I for the evaluated alloy composition (x=0.21--0.22). Under low excitation intensity, intermediate photoluminescence emissions appear between the ${\mathit{e}}_{1}$${\mathrm{hh}}_{1}$ lines corresponding to nominal thicknesses; they can be associated with fluctuations of 1 ML thickness, which are reported, to our knowledge, for the first time, in this type of QW system. Efficient thermally activated interwell transfer of excitons is also in evidence from photoluminescence experiments. A simple model is used to account for the transfer from narrower to broader wells when the temperature is increased.

Influence of indium segregation on the emission from InGaAs/GaAs quantum wells

Indium segregation in InxGa1−xAs/GaAs (0.05<x<0.25) quantum wells grown by molecular beam epitaxy has been investigated using low temperature photoluminescence. Additional features at low energy are evident in some of the spectra that are consistent with trapping of free excitons by In-rich islands at the top interface, which occurs as the result of In segregation.

Kinetic model of element III segregation during molecular beam epitaxy of III-III′-V semiconductor compounds

Segregation of column III atoms during molecular beam epitaxy of III-III′-V semiconductor compounds causes nonabrupt interfaces and a surface composition different from the bulk one. To derive concentration profiles, a thermodynamical equilibrium model has been used for a long time. This model applies well to describe segregation processes at high growth temperatures, but fails in predicting concentration profile variations with substrate temperature. We have thus developed a kinetic model which correctly takes into account the evolution with the growth temperature. We apply this model to the case of indium segregation in the GaxIn1−xAs/GaAs system. The calculated indium concentration profiles are compared to those obtained with the thermodynamical equilibrium model. A kinetic limitation of segregation is shown to appear at low substrate temperatures and sufficiently high growth rates. This limitation is predicted to arise below 400 °C for a growth rate of 1 monolayer/s for In segregation in the GaxIn1−xAs/GaAs system.

Optical and Structural Properties of AP-MOVPE GaInP/GaAs Heterostructures

Lattice matched GaInP/GaAs heterostructures were grown by atmospheric pressuremetal organic vapor phase epitaxy (AP-MOVPE). Compositional intermixing of As/P and Ga/In near the heterointerfaces was studied by photoluminescence (PL) spectroscopy. Indium segregation, memory effect of In into GaAs and the carry-over of As in the GaInP layer during the growth process were considered as three major factors giving rise to the anomalous emissions in the PL spectra. Both thermal annealing and zinc doping strongly enhanced the compositional interdiffusion near the heterointerfaces.

Composition effects in ‘‘flash’’ evaporated CuIn(SexTe1−x)2 films

Thin films with composition around CuInSeTe and CuInSe1.5Te0.5 have been deposited by ‘‘flash’’ evaporation. The structural, electrical, optical, and surface [x-ray photoelectron spectroscopy (XPS) measurements] properties have been determined. Chalcogen content is always lower than 50% and crystallinity is higher when Se content increases. The samples always show p-type conduction even when the defect of chalcogen is 5% with resistivities in the 0.5–8.6 Ω cm range. The energy gap is in the 0.81–1.11 eV range. XPS measurements indicate that, after exposure to air, only the Te species presents two oxidation states preventing oxidation of the remaining elements. These properties are analyzed in terms of indium segregation at grain boundaries, anion vacancies, and cation disorder in the samples.

Indium desorption from strained InGaAs/GaAs quantum wells grown by molecular beam epitaxy

The desorption and segregation of indium during molecular beam epitaxy growth of strained InGaAs/GaAs multiple quantum wells have been investigated by secondary ion mass spectroscopy (SIMS). The quantum wells were grown at various substrate temperatures, and the growths were interrupted at specified interfaces. Results indicate that indium desorption and segregation increase as the growth temperature is increased regardless of the type of growth interruption. However, the desorption was relatively high in the case of wells whose growths were interrupted at the top of the wells compared to the cases of bottom and no interruption, probably due to longer annealing of the exposed well surfaces at a high temperature. Similar results were obtained for the indium segregation. In all the cases, appreciable desorption starts only above 550 °C. SIMS results were analyzed using a simple rate-equation model for indium desorption, and the activation energy for indium desorption was found to be constant for both the top and bottom interrupted samples.

Interface segregation and clustering in strained-layer InGaAs/GaAs heterostructures studied by cross-sectional scanning tunneling microscopy.

Interface roughness due to segregation and clustering has been studied in atomic detail for the first time, using a cross-sectional scanning tunneling microscope and its spectroscopic ability to distinguish In and Ga atoms in GaAs/${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As/GaAs strained layers. In the ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As layers, InAs is found to cluster preferentially along the growth direction with each cluster containing 2-3 indium atoms. Indium segregation induced an asymmetrical interface broadening. The interface of GaAs grown on ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As is found to be broadened to about 5--10 atomic layers, while that of InGaAs on GaAs is about 2--4 layers broad.

Indium segregation and misorientation effects on the optical properties of MBE grown In0.35Ga0.65As/GaAs quantum wells

Photoluminescence (PL), reflectivity and thermally-detected optical absorption (TDOA) experiments have been carried out, at liquid helium temperatures, On In0.35Ga0.65As/GaAs quantum wells (QWs) with different thicknesses of 4, 6, 8 and 10 monolayers (MLs), grown by molecular beam epitaxy (MBE) on substrates oriented exactly on (100) (nominal growth) or misoriented by 4-degrees or 6-degrees towards (111)Ga (vicinal growth). Indium segregation effects are considered in the comparison of die calculated fundamental (e1hh1) transition energies of the QWs with the experimental results. Within the framework of a simple model, a segregation energy of 0.35 eV is used to fit the experimental data. It is shown that the misorientation improves the optical properties of QWs: diminution of the spectral broadenings and reduction of the blue shift between reflectivity structures or TDOA peaks and luminescence lines. This suggests a better quality of the interfaces and an improvement of the strain homogeneity. A step edge effect on the fundamental transition energy of the 6 and 8 ML quantum wells is shown but the blue shift observed for a growth on a 4-degrees off misoriented surface can also be due to a modification of the growth mode.

Interfacial defects and morphology of InGaAs epitaxial layers grown on tilted GaAs substrates

Systematic transmission electron microscopy studies of In0.2Ga0.8As layers grown by molecular-beam epitaxy on [001] GaAs substrates and on substrates tilted up to 10° toward [110], [120], [100], [1̄10], and [1̄20] are described. Three different layer thicknesses were investigated: 6, 20, and 40 nm. In the 40 nm layers misfit dislocations were formed which partially relaxed the elastic strain. Three-dimensional growth with an indication of microscale In segregation (dendritic growth) occurred for substrates tilted toward [1̄10] and [1̄20] directions. Smooth growth surfaces were observed when the substrates were tilted toward [110] and [120] showing that the chemistry of the predominant surface growth steps can play an important role in the growth mode and quality of the epitaxial layer. Indium segregation to misfit dislocation cores, the presence of In-rich platelets near the interface, and diffuseness of the interface are considered as evidence for short range migration of In during and after growth.

In situ core-level photoelectron spectroscopy study of indium segregation at GaInAs/GaAs heterojunctions grown by molecular-beam epitaxy

In situ ultraviolet photoelectron spectroscopy (UPS) measurements have been performed at GaInAs/GaAs (100) interfaces. The high depth resolution of the UPS technique applied to the specific case of GaInAs/GaAs heterostructures [core-level photoelectron escape depth λ≊2.1 monolayers (MLs)] is shown, yielding quantitative informations on the Ga/In atomic profiles at the ML scale. A surface segregation model is developped and the phenomenological segregation energy is obtained by UPS. This energy is used to generate potential energy profiles in a calculation of the energy of photoluminescence (PL) lines in GaInAs/GaAs quantum wells; the quantitative agreement with measured PL lines is very good.

Detection and reduction of indium segregation during molecular-beam epitaxial growth of InGaAs/GaAs using in situ reflection mass spectrometry

The use of reflection mass spectrometry (REMS) as an in situ diagnostic for surface segregation of indium during InGaAs layer growth is reported. InGaAs growth at substrate temperatures above 500 °C yields a REMS signature which indicates a thickness-dependent surface In content. Adapting a simple segregation model [K. Muraki, S. Fukatsu, Y. Shiraki, and R. Ito, Appl. Phys. Lett. 61, 557 (1992)], the segregation ratio R was extracted under various deposition conditions in real time. The segregation ratio obtained during InGaAs/GaAs multiple quantum well growth at 500–530 °C suggests the presence of InGaAs composition grading near interfaces, and agrees qualitatively with ex situ characterization by x-ray diffraction. The high values of R (0.7–0.8) observed under normal device-layer growth produces a surface layer with high In content (i.e., InAs). The segregation ratio was not sensitive to unrelaxed layer strain, but showed a large increase under conditions which probably produce island (three-dimensional) growth. Methods for preparation of high-quality InGaAs layers with more abrupt AlGaAs/InGaAs/GaAs heterointerfaces are discussed, such as use of a two-step growth procedure or the incorporation of thin ‘‘cap layers’’ prior to high temperature AlGaAs growth.