Indium Segregation Research Articles

Structural and Passivative Behaviors of Cu(In) Thin Film

This investigation prepares a low-resistivity and self-passivated Cu(In) thin film. The dissociation behaviors of dilute Cu-alloy thin films, containing 1.5–5at.%In, were prepared on glass substrates by a cosputter deposition, and were subsequently annealed in the temperature range of 200–600 °C for 10–30 min. Thus, self-passivated Cu thin films in the form In2O3/Cu/SiO2 were obtained by annealing Cu(In) alloy films at an elevated temperature. Structural analysis indicated that only strong copper diffraction peaks were detected from the as-deposited film, and an In2O3 phase was formed on the surface of the film by annealing the film at an elevated temperature under oxygen ambient. The formation of In2O3/Cu/SiO2 improved the resistivity, adhesion to SiO2, and passivative capability of the studied film. A dramatic reduction in the resistivity of the film occurred at 500 °C, and was considered to be associated with preferential indium segregation during annealing, yielding a low resistivity below 2.92 μΩcm. The results of this study can be potentially exploited in the application of thin-film transistor–liquid crystal display gate electrodes and copper metallization in integrated circuits.

AlGaInP growth parameter optimisation during MOVPE for opto-electronic devices

For (Al x Ga 1− x ) 1− y In y P growth on GaAs for opto-electronic devices in metal-organic vapour phase epitaxy an accurate control of the aluminium content x and of the indium content y is crucial. Therefore, we applied an in situ curvature sensor to determine the lattice matching (i.e. indium content y) of the growing material. It was found that AlGaInP which is lattice matched at room temperature is tensile strained at growth temperature leading to a concave substrate bowing during growth. Overgrowing AlGaInP with GaAs an additional change in wafer bowing was observed presumably caused by indium segregation which is supported by a simulation of the measured data. Furthermore, for an accurate in situ determination of the aluminium content x and of the growth rate a high-temperature database for the optical constants n and k of lattice matched AlGaInP was established. Taking reflectance transients during growth of AlGaInP layers a numerical fit to the data enables a determination of the growth rate and the aluminium content.

Effect of indium segregation on optical and structural properties of GaInNAs∕GaAs quantum wells at emission wavelength of 1.3μm

We studied the effect of In segregation on the optical and structural properties of GaInNAs∕GaAs quantum wells (QWs). The segregation model developed by Muraki et al. [Appl. Phys. Lett. 61, 557 (1992)] is used to calculate the composition profiles of the QWs with different segregation efficiencies of In atoms. Confinement potentials of electron and hole are then derived, from which energies of electron and hole are numerically calculated by serving the Schrödinger equation. The effects of valence band mixing and strain are included in the calculations of the energies of electron and hole. The optical transition energy of the QWs is then obtained from the energy difference of electron and hole. It is found that the blueshift in transition energy due to segregation is mainly affected by strain rather than by composition in the studied QWs. Calculations using the segregation model together with the dynamical theory of x-ray diffractions are also carried out for the segregated QWs. The results indicate that the behavior of In segregation in Ga0.65In0.35N0.015As0.985∕GaAs QW can be resolved by both photoluminescence and x-ray diffraction for the segregation coefficients larger than 0.7.

Strain mediated reconstructions and indium segregation on InGaAs∕GaAs(001) alloy surfaces at intermediate lattice mismatch

In vacuo scanning tunneling microscopy is used to investigate the surface reconstructions of pseudomorphic InGaAs alloys at intermediate values of compressive strain. The coverage of different reconstructions varies with film thickness, concomitant with changes in composition and strain at the surface arising from In segregation and changes in surface morphology. Thin samples exhibit mainly disordered (1×3) reconstructions along with small regions of incommensurate (1×2). With increasing thickness, the (1×3) transforms into more regular (4×3) or c(4×6), whose coverage mirrors the increase and saturation of In surface composition. Regions of α2(2×4) reconstructions are also present, and their coverage initially increases with In surface composition, but later decreases upon saturation of In at the surface. This decrease is concurrent with the onset of surface roughening, suggesting that the α2(2×4) reconstruction is strain stabilized.

Metalorganic chemical vapor deposition growth and characterization of InGaP/GaAs superlattices

Lattice-matched In0.49Ga0.51P/GaAs superlattices were grown on (001) GaAs substrates using metalorganic chemical vapor deposition. The interface properties were characterized by photoluminescence, transmission electron microscopy, and x-ray diffraction. By varying the growth temperature, the precursor flow rates, and the growth interruption at the interfaces, we found that, while arsenic and phosphorus carry over have some effect on the formation of a low-bandgap InGaAsP quaternary layer at the interfaces, the In surface segregation seems to play an important role in the formation of the interface quaternary layer. Evidence of this indium segregation comes from x-ray and photoluminescence studies of samples grown at different temperatures. These studies show that the formation of an interfacial layer is more prominent when the growth temperature is higher. Growing a thin (∼1 monolayer thick) GaP intentional interfacial layer on top of the InGaP before the growth of the GaAs layer at the P→As transition effectively suppresses the formation of the low-bandgap unintentional interface layer. On the other hand, the growth of a thin GaAsP (or GaP) layer before the growth of the InGaP layer, at the As→P transition increases the formation of a low-bandgap interfacial layer. This nonequivalent effect of a GaP layer at the two interfaces on the PL properties is discussed.

Nonconventional flash annealing on shallow indium implants in silicon

The diffusion behavior and the electrical characteristics of indium doped layers in silicon were studied. Indium was implanted in silicon at energies of 70 and 25 keV to doses of 5.8 and 3×1014, respectively. The implants were performed both in amorphous and crystalline silicon. The implants were submitted to a combination of thermal annealing, RTA, and flash annealing to regrow the implanted layers and activate the dopant. Four point probe sheet resistance measurements and Hall effect measurements were carried out to test the electrical properties of the implanted layers. The atomic concentration profiles were assessed using secondary ion mass spectrometry. A drastic increase in the dopant activation was observed following co-implanting with carbon. Moreover, the carbon presence inhibits the indium diffusion and segregation in damaged areas. The preamorphizing treatment affects the indium diffusion in two ways. For low thermal budget anneals the diffusion is suppressed, conversely the diffusion is enhanced under severe annealing conditions.

Observation of indium segregation effects in structural and optical properties of pseudomorphic HEMT structures

We present a detailed study on the influence of several growth related factors such as indium segregation, spacer layer thickness and Si δ doping on the structural, optical and electrical properties of molecular beam epitaxy grown pseudomorphic high electron mobility transistor (P-HEMT) structures. Simulation of a high resolution x-ray diffraction (HRXRD) rocking curve was performed and compared with the experimental data to determine the quantum well (QW) thickness and composition as well as the indium segregation related changes in the QW composition. It is shown that growth of a thin layer (6 Å) of GaAs on top of the pseudomorphic InGaAs layer followed by ‘flash-off’ at a higher temperature could minimize the indium segregation related degradation of the hetero interface. Photoluminescence (PL) and surface photo voltage (SPV) peaks corresponding to the sub-band transitions are analysed using a self-consistent solution of Schrödinger and Poisson equations. PL results showed the dependence of the transition energies on the spacer layer thickness and the effectiveness of the carrier transfer into the InGaAs well. SPV spectra showed characteristic peaks from all the layers in the HEMT structures and were correlated with the data obtained from PL and HRXRD.

Quantification of segregation and strain effects in InAs∕GaAs quantum dot growth

Reflection high-energy electron diffraction measurements of the critical thickness θcrit for quantum dot (QD) formation have been used to quantify the effects of indium segregation and strain on the growth of bilayer InAs∕GaAs(001) QD structures. These are not straightforward to deconvolute, because of the complex issues that arise during the growth and capping of the QDs. Segregation and out diffusion of In from buried QDs are shown to occur for GaAs thicknesses up to ∼6nm at 580°C. The existence of a floating In adlayer on the surface of the GaAs-capping layer as a result of In segregation is apparent at much lower substrate temperatures (510°C). The relative contribution of both segregation and strain on the reduction of θcrit during the growth of a second InAs layer is assessed. Compared with segregation, strain from the buried QDs can be measured through significantly larger capping thicknesses (∼30nm) under these conditions.

Local indium segregation and bang gap variations in high efficiency green light emitting InGaN/GaN diodes

High efficient green light emitting diodes (LED) on the basis of GaN/InGaN exhibit indium-rich nanoclusters inside the quantum wells (QW) due to InN–GaN phase decomposition. By direct measurements of the variations in the electronic structure, we show for the first time a correlation between indium-rich nanoclusters and local energy band gap minima. Our investigations reveal the presence of 1–3 nm wide indium rich clusters in these devices with indium concentrations x as large as x∼0.30–0.40 that narrow the band gap locally to energies as small as 2.65 eV. These clusters are able to act as local traps for migrating photon-emitting carriers and seem to boost the overall device performance.

Real-time RHEED investigation of indium segregation in InGaAs layers grown on vicinal GaAs(001) substrates

The surface segregation of indium atoms was investigated in situ and in real time by reflection high-energy electron diffraction (RHEED) during molecular-beam epitaxy of InGaAs layers on misoriented GaAs(001) substrates. The strong damping of the RHEED oscillations was quantitatively related to the strength of the segregation process that was found to decrease with increasing miscut angle.

Internal electric-field and segregation effects on luminescence properties of quantum wells

Surface segregation of In atoms during molecular-beam epitaxy and its influence on the energy levels in strained piezoelectric InGaAs∕GaAs and InGaN∕GaN quantum wells (QWs) are investigated theoretically. It is shown that these effects modify the electronic states in the QW and the emission energy in the photoluminescence (PL) spectra. In this work, we solve analytically the Schrödinger equation in the absence of electric field, taking into account the shape changes in the QWs due to the segregation of In atoms during the growth process of the semiconductor heterostructures. Furthermore, the influence of the built-in electric field due to the piezoelectric effect on the PL emission is calculated by considering a variational electron wave function to calculate the ground-energy transitions inside the active region in the heterostructure. In particular, we apply this model to the case of indium segregation in InGaAs∕GaAs for moderate internal electric fields. The transition energy calculations between the confined electron and hole states as a function of the well width for different temperatures and In composition are in agreement with the measured PL energy peaks.

Investigation of microstructure and V-defect formation in InxGa1−xN/GaN MQW grown using temperature-gradient metalorganic chemical vapor deposition

Temperature-gradient metalorganic chemical vapor deposition (MOCVD) was used to deposit InxGa1−xN/GaN multiple quantum well (MQW) structures with a concentration gradient of indium across the wafer. These MQW structures were deposited on low defect density (2×108 cm−2) GaN template layers for investigation of microstructural properties and V-defect (pinhole) formation. Room temperature (RT) photoluminescence (PL) and photomodulated transmission (PT) were used for optical characterization, which show a systematic decrease in emission energy for a decrease in growth temperature. Triple-axis x-ray diffraction (XRD), scanning electron microscopy, and cross-sectional transmission electron microscopy were used to obtain microstructural properties of different regions across the wafer. Results show that there is a decrease in crystal quality and an increase in V-defect formation with increasing indium concentration. A direct correlation was found between V-defect density and growth temperature due to increased strain and indium segregation for increasing indium concentration.

Polycrystalline InGaN grown by MBE on fused silica glass

A series of polycrystalline InGaN films have been grown on fused silica glass using a plasma assisted molecular beam epitaxy (PAMBE) system. Films grown at 600 °C showed pronounced signs of indium segregation, with indium segregating to the substrate interface and film surface, to the point where an essentially In-free intermediate layer was formed. For high indium content InGaN and InN films, homogeneity was substantially improved upon reduction of the growth temperature. In these homogeneous films the PL peak position moves to lower energy with increasing In content, with a PL peak position of 0.8 eV measured for a pure InN film. For the InGaN films, the comparatively broad PL spectra could be separated into two Gaussian peaks. One peak shifted to high energy with increasing temperature, while the other exhibited a more complex behavior. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Integration of CdSe quantum dots with GaN optoelectronic materials

Material structures on the nanoscale can enable enhancement of optoelectronic device performance. For example, in the InGaN active layers of MOCVD grown blue light emitting diodes, indium segregation plays a critical role in the interplay between blue luminescent channels and non-radiative recombination centers such as crystal defects. Unfortunately, high efficiency luminescence of InGaN does not extend into the “deep green” spectral region, around the wavelength of peak human eye response. We are investigating whether commercially available luminescent nanostructures such as CdSe quantum dots can be incorporated into III-nitride devices to extend their high-efficiency performance into the “deep green”. Surfactant stabilized CdSe particles in liquid dispersions are drop-cast onto HVPE grown GaN. Physical properties of resultant CdSe surface structures are examined. Luminescence is reported before and after subsequent growth of GaN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence study of MBE grown InGaN with intentional indium segregation

Proper control of MBE growth conditions has yielded an In0.13Ga0.87N thin film sample with emission con- sistent with In-segregation. The photoluminescence (PL) from this epilayer showed multiple emission components. Moreover, temperature and power dependent studies of the PL demonstrated that two of the components were excitonic in nature and consistent with indium phase separation. At 15 K, time resolved PL showed a non-exponential PL decay that was well fitted with the stretched exponential solution ex- pected for disordered systems. Consistent with the assumed carrier hopping mechanism of this model, the effective lifetime, τ, and the stretched exponential parameter, β, decrease with increasing emission energy. Finally, room temperature micro-PL using a confocal microscope showed spatial clustering of low energy emission.

Indium segregation during multilayer InAs/GaAs(0 0 1) quantum dot formation

Indium segregation during the growth of multilayer InAs/GaAs(0 0 1) quantum dot (QD) structures has been studied using reflection high-energy electron diffraction (RHEED) measurements of the critical coverage ( θ crit) for second layer QD formation. A model bilayer structure was used in order to separate the effects of segregation and strain. The structure comprises an upper QD layer formed on top of a buried two-dimensional InAs layer. Growth temperature and the GaAs spacer layer thickness ( S) are both found to have a significant effect on θ crit. Indium segregation during growth of the capping layer leads to the presence of a surface In adatom population prior to deposition of the second InAs layer. Segregation occurs for S up to 8 nm at 510 °C, this value being reduced by ∼50% at 450 °C.

The influence of surface segregation on the optical properties of quantum wells

Segregation of column III atoms during molecular beam epitaxy of III-V semiconductor compounds result in nonabrupt interfaces and surface compositions different from the bulk. This effect modifies the electronic states in the quantum well and the emission energy in the photoluminescence spectrum. In this work, we have solved analytically the Schrödinger equation taking into account the shape changes in the quantum well due to the segregation of atoms during the growth process of the semiconductor heterostructures. We apply this model to the case of indium segregation in the InGaAs/GaAs system. The transition energy calculations between the confined electron and hole states as function of the well width for different In composition and growth temperature are in agreement with the measured photoluminescence energy peaks.

Investigations on V-defects in quaternary AlInGaN epilayers

The characteristics of V-defects in quaternary AlInGaN epilayers and their correlation with fluctuations of the In distribution are investigated. The geometric size of the V-defects is found to depend on the In composition of the alloy. The V-defects are nucleated within the AlInGaN layer and associated with threading dislocations. Line scan cathodoluminescence (CL) shows a redshift of the emission peak and an increase of the half width of the CL spectra as the electron beam approaches the apex of the V-defect. The total redshift decreases with decreasing In mole fraction in the alloy samples. Although the strain reduction may partially contribute to the CL redshift, indium segregation is suggested to be responsible for the V-defect formation and has a main influence on the respective optical properties.

Cobalt growth on Cu(111) in the presence of indium surfactant

The effect of a pre-deposited ultrathin film of indium on the deposition of cobalt on Cu(111) has been studied by an in situ combination of medium energy electron diffraction, scanning tunneling microscopy, and Auger electron spectroscopy. Pre-deposited indium allows cobalt to deposit in layer-by-layer growth, in contrast to the three-dimensional growth observed without the indium surfactant. The surfactant effect is connected to the surface alloys, Cu2In and Cu3In, that form upon indium pre-deposition. Initial cobalt nucleation processes and indium segregation during cobalt deposition are also discussed.

Combined x-ray diffraction/scanning tunneling microscopy study of segregation and interfacial bonding in type-II heterostructures

Appropriate control over the type-II band alignment between InAs and GaSb is important for a number of applications, including the further development of midinfrared (IR) semiconductor lasers and long-wavelength photodetectors. Accurate tailoring of interface structure and composition in such heterostructures grown by molecular beam epitaxy is nevertheless problematic for several reasons. Special challenges are posed by antimony segregation at the arsenide-on-antimonide interface, indium segregation at the antimonide-on-arsenide interface, and by the desire to selectively control the purity and types of interface bonds at either heterojunction. Here, we briefly review how cross-sectional scanning tunneling microscopy may be used to examine antimony and indium segregation with atomic-scale precision in type-II quantum wells, and then explain how such measurements suggest a unique structural interpretation for the residual strain exhibited by typical mid-IR semiconductor laser active regions in x-ray diffraction.