Structural and photoluminescence characteristics of molecular beam epitaxy-grown vertically aligned In0.33Ga0.67As/GaAs quantum dots

In this paper, we present the results of structural and photoluminescence (PL) studies on vertically aligned, 20-period In_0.33Ga_0.67As/GaAs quantum dot stacks, grown by molecular beam epitaxy (MBE). Two different In_0.33Ga_0.67As/GaAs quantum dot stacks were compared. In one case, the In_0.33Ga_0.67As layer thickness was chosen to be just above its transition thickness, and in the other case, the In_0.33Ga_0.67As layer thickness was chosen to be 30% larger than its transition thickness. The 2D–3D growth mode transition time was determined using reflection high-energy electron diffraction (RHEED). Structural studies were done on these samples using high-resolution X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM). A careful analysis showed that the satellite peaks recorded in X-ray rocking curve show two types of periodicities in one sample. We attribute this additional periodicity to the presence of an aligned vertical stack of quantum dots. We also show that the additional periodicity is not significant in a sample in which the quantum dots are not prominently formed. By analyzing the X-ray rocking curve in conjunction with RHEED and PL, we have estimated the structural parameters of the quantum dot stack. These parameters agree well with those obtained from XTEM measurements.     

Structure and optical properties of self-assembled InAs/GaAs quantum dots with In0.15Ga0.85As underlying layer

A high density of 1.02×10¹¹ cm⁻² of InAs islands with In_0.15Ga_0.85As underlying layer has been achieved on GaAs (1 0 0) substrate by solid source molecular beam epitaxy. Atomic force microscopy and PL spectra show the size evolution of InAs islands. A 1.3 μm photoluminescence (PL) from InAs islands with In_0.15Ga_0.85As underlying layer and InGaAs strain-reduced layer has been obtained. Our results provide important information for optimizing the epitaxial structures of 1.3 μm wavelength quantum dots devices.     

Evolution of ordered one-dimensional and two-dimensional InAs/InP quantum dot arrays on patterned InP (1 0 0) and (3 1 1)B substrates by self-organized anisotropic strain engineering

The formation of ordered InAs/InP quantum dot (QD) arrays is demonstrated on patterned InP (1 0 0) and (3 1 1)B substrates by the concept of self-organized anisotropic strain engineering in chemical beam epitaxy (CBE). On shallow- and deep stripe-patterned InP (1 0 0) substrates, depending on the stripe orientation, the linear one-dimensional InAs QD arrays are rotated away from their natural direction due to the presence of vicinal stepped sidewall planes modifying the self-organization process, coexisting with QD free steep side facets on the deep-patterned substrates. On shallow- and deep-patterned InP (3 1 1)B substrates only QD free side facets form with flat top and bottom areas, not affecting the natural ordering of the two-dimensional InAs QD arrays. On the deep-patterned substrates a row of dense QDs forms on top along the side facets due to their slow-growing behavior. The optical properties of the QD arrays on the patterned substrates are not degraded compared to those of arrays formed on planar substrates for both InP (1 0 0) and (3 1 1)B substrates showing the potential of self-organized anisotropic strain engineering combined with step engineering for the creation of advanced complex QD arrays and networks.     

Stacking pattern of multi-layer InAs quantum wires embedded in In0.53Ga0.47−xAlxAs matrix layers grown lattice-matched on InP substrate

Multi-layer InAs quantum wires were grown on, and embedded in In_0.53Ga_0.47−xAl_xAs (with x=0, 0.1, 0.3 and 0.48) barrier/spacer layers lattice matched to an InP substrate. Correlated stacking of the quantum wire arrays were observed with aluminum content of 0 and 0.1. The quantum wire stacks became anti-correlated as the aluminum content was increased to 0.3 and 0.48. The origin of such stacking pattern variation was investigated by finite element calculations of the chemical potential distribution for indium on the growth front surface of the capping spacer layer. It is shown that the stacking pattern transition is determined by the combined effect of strain and surface morphology on the growth front of the spacer layers.     

Characterization of InAs quantum dots on lattice-matched InAlGaAs/InP superlattice structures

Self-assembled InAs quantum dots (QDs) in an InAlGaAs matrix, lattice-matched to InP substrate, have been grown by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) are used to study their structural and optical properties. In InAs/InAlGaAs/InP system, we propose that when the thickness of InAs layer deposited is small, the random strain distribution of the matrix layer results in the formation of tadpole-shaped QDs with tails towards random directions, while the QDs begin to turn into dome-shaped and then coalesce to form islands with larger size and lower density to release the increasing misfit strain with the continuous deposition of InAs. XRD rocking curves showing the reduced strain with increasing thickness of InAs layer may also support our notion. The results of PL measurements are in well agreement with that of TEM images.     

Temperature effects during the growth of InxGa1−xN films through the whole compositional range by plasma-assisted molecular beam epitaxy

Substrate temperature rises of over 200 °C have been observed for growth of InN and In-rich InGaN on GaAs substrates. We present a model to show that it is not the narrow bandgap that is responsible for the large temperature rises observed during growth of InN, but the large bulk background carrier concentration. We also show how the substrate temperature rise during growth increases as a function of increasing indium composition and the effects of controlling the substrate temperature on film quality.     

Shape transition from InAs quantum dash to quantum dot on InP(3 1 1)A

We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux.     

Formation of Ge-diffusion-suppressed GaAs layers and InAs quantum dots on Ge/Si substrates

Crystal growth of GaAs layers and InAs quantum dots (QDs) on the GaAs layers was investigated on Ge/Si substrates using ultrahigh vacuum chemical vapor deposition. Ga-rich GaAs with anti-site Ga atoms grown at a low V/III ratio was found to suppress the diffusion of Ge into GaAs. S-K mode QD formation was observed on GaAs layers grown on Ge/Si substrates with Ga-rich GaAs initial layers, and improved photoluminescence from 1.3 μm-emitting InAs QDs was demonstrated.     

Alloy phase separation in InAs/InAlAs/InP nanostructure superlattices studied by finite element calculation

The alloy phase separation effect in InAlAs spacer layer of InAs/InAlAs nanostructure superlattices was studied by two-dimensional finite element calculation. The calculation results showed that InAs islands with wide top would prefer to induce “V”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer, while InAs islands with narrow top would promote the formation of “I”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer which corresponded well with the experimental results reported in Ref. [9].     

Influence of cooling rate on the liquid-phase epitaxial growth of Zn3P2

We report the liquid-phase epitaxial growth of Zn₃P₂ on InP (1 0 0) substrates by conventional horizontal sliding boat system using 100% In solvent. Different cooling rates of 0.2–1.0 °C/min have been adopted and the influence of supercooling on the properties of the grown epilayers is analyzed. The crystal structure and quality of the grown epilayers have been studied by X-ray diffraction and high-resolution X-ray rocking measurements, which revealed a good lattice matching between the epilayers and the substrate. The supercooling-induced morphologies and composition of the epilayers were studied by scanning electron microscopy and energy dispersive X-ray analysis. The growth rate has been calculated and found that there exists a linear dependence between the growth rate and the cooling rate. Hall measurements showed that the grown layers are unintentionally doped p-type with a carrier mobility as high as 450 cm²/V s and a carrier concentration of 2.81×10¹⁸ cm⁻³ for the layers grown from 6 °C supercooled melt from the cooling rate of 0.4 °C/min.     

Anomalous temperature-dependent photoluminescence characteristic of as-grown GaInNAs/GaAs quantum well grown by solid source molecular beam epitaxy

The photoluminescence (PL) mechanisms of as-grown GaInNAs/GaAs quantum well were investigated by temperature-dependent PL measurements. An anomalous two-segmented trend in the PL peak energy vs. temperature curve was observed, which has higher and lower temperature-dependent characteristics at low temperature (5–80 K) and high temperature (above 80 K), respectively. The low and high-temperature segments were fitted with two separate Varshni fitting curves, namely Fit_low and Fit_high, respectively, as the low-temperature PL mechanism is dominated by localized PL transitions while the high-temperature PL mechanism is dominated by the e1–hh1 PL transition. Further investigation of the PL efficiency vs. 1/kT relationship suggests that the main localized state is located at 34 meV below the e1 state. It is also found that the temperature (80 K) at which the PL full-width at half-maximum changes from linear trend to almost constant trend correlates well with the temperature at which the PL peak energy vs. temperature curve changes from Fit_low to Fit_high.     

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO2 substrates

The anisotropic film properties of m-plane GaN deposited by metal organic vapour phase epitaxy (MOVPE) on LiAlO₂ substrates are investigated. To study the development of layer properties during epitaxy, the total film thickness is varied between 0.2 and 1.7 μm. A surface roughening is observed caused by the increased size of hillock-like features. Additionally, small steps which are perfectly aligned in (1 1 −2 0) planes appear for samples with a thickness of ∼0.5 μm and above. Simultaneously, the X-ray rocking curve (XRC) full width at half maximum (FWHM) values become strongly dependent on incident X-ray beam direction beyond this critical thickness. Anisotropic in-plane compressive strain is initially present and gradually relaxes mainly in the [1 1 −2 0] direction when growing thicker films. Low-temperature photoluminescence (PL) spectra are dominated by the GaN near-band-edge peak and show only weak signal related to basal plane stacking faults (BSF). The measured background electron concentration is reduced from ∼10²⁰ to ∼10¹⁹ cm⁻³ for film thicknesses of 0.2 μm and ∼1 μm while the electron mobilities rise from ∼20 to ∼130 cm²/V s. The mobilities are significantly higher in [0 0 0 1] direction which we explain by the presence of extended planar defects in the prismatic plane. Such defects are assumed to be also the cause for the observed surface steps and anisotropic XRC broadening.     

Self-formation of dielectric layer containing CoSi2 nanocrystals by plasma-enhanced atomic layer deposition

Dielectric layer containing CoSi₂ nanocrystals was directly fabricated by plasma-enhanced atomic layer deposition using CoCp₂ and NH₃ plasma mixed with SiH₄ without annealing process. Synchrotron radiation X-ray diffraction and X-ray photoelectron spectroscopy results confirmed the formation of CoSi₂ nanocrystal. The gate stack composed of dielectric layer containing CoSi₂ nanocrystals with ALD HfO₂ capping layer together with Ru metal gate was analyzed by capacitance–voltage (C–V) measurement. Large hysteresis of C–V curves indicated charge trap effects of CoSi₂ nanocrystals. The current process provides simple route for the fabrication of nanocrystal memory compatible with the current Si device unit processes.     

Arsenic cross-contamination in GaSb/InAs superlattices

We have investigated the cross-contamination of As in GaSb/InAs superlattices. We demonstrate a method of varying the lattice constant of the superlattice. By controlling the As background pressure in the growth chamber, the strain can be controlled to about 0.01%, corresponding to As cross-incorporation variations of about ±1%. The distribution of As is investigated by X-ray diffraction and cross-sectional scanning tunneling microscopy, and the critical thickness is obtained.     

Improvement of GaInNAsSb films fabricated by atomic hydrogen-assisted molecular beam epitaxy

The effect of incorporation of antimony in GaInNAs films grown by atomic hydrogen-assisted molecular beam epitaxy (MBE) has been investigated. We show that the rate of incorporation of N and In forming GaInNAs do not depend on the Sb beam flux. However, the incorporation of Sb is strongly dependent on the Sb/As₂ flux ratio. Introducing a small amount of Sb (<∼1%) significantly improves the photoluminescence (PL) emission efficiency of GaInNAs, but Sb concentration of >1% rapidly degrades the PL intensity, though a large redshift can still be achieved. Therefore, there is an optimum amount of Sb for the growth of low-strained GaInNAs films to improve the overall optical quality.     

AlGaAs/GaAs/AlGaAs quantum wells as a sensitive tool for the MOVPE reactor environment

We present in this work a simple quantum well (QW) structure consisting of GaAs wells with AlGaAs barriers as a probe for measuring the performance of arsine purifiers within a metalorganic vapour phase epitaxy system. Comparisons between two different commercially available purifiers are based on the analysis of low-temperature photoluminescence emission spectra from thick QWs, grown on GaAs substrates misoriented slightly from (1 0 0). Neutral excitons emitted from these structures show extremely narrow linewidths, comparable with those that can be obtained by molecular beam epitaxy in an ultra-high vacuum environment, suggesting that purifications well below the 1 ppb level are needed to achieve high quality quantum well growth.     

Droplet epitaxy of zinc-blende GaN quantum dots

Zinc-blende GaN quantum dots were grown on 3C-AlN(0 0 1) by a vapor–liquid–solid process in a molecular beam epitaxy system. We were able to control the density of the quantum dots in a range of 5×10⁸–5×10¹² cm⁻². Photoluminescence spectroscopy confirmed the optical activity of the GaN quantum dots in a range of 10¹¹–5×10¹² cm⁻². The data obtained give an insight to the condensation mechanism of the vapor–liquid–solid process in general, because the GaN quantum dots condense in metastable zinc-blende crystal structure supplied by the substrate, and not in the wurtzite crystal structure expected from free condensation in the droplet.     

Growth of height-controlled InGaN quantum dots on GaN

InGaN height-controlled quantum dots (HCQDs) were grown by alternately depositing In_0.4Ga_0.6N QD and In_0.1Ga_0.9N spacer layers on a seed In_0.4Ga_0.6N QD layer. Structural and optical studies showed that the height of the InGaN QDs was controlled by the deposition cycle of In_0.4Ga_0.6N/In_0.1Ga_0.9N layers. Photoluminescence studies showed that the In_0.4Ga_0.6N HCQDs provided deep potential wells and the piezoelectric field-induced quantum-confined Stark effect was negligibly small. These phenomena are attributed to variation in quantum confinement energy in the electronically coupled InGaN HCQDs providing deep potential wells.