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.     

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.     

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.     

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.     

Shorter wavelength emission with InAs quantum dots growth directly on large bandgap quaternary (In0.68Ga0.32As0.7P0.3) barriers for high current injection efficiency

We present the growth of stacked layers of InAs quantum dots directly on high bandgap In_0.68Ga_0.32As_0.7P_0.3 (λ_g=1420 nm) barriers. The quaternary material is lattice matched to InP forming a double hetero-structure. Indium flux, number of InAs stacked layers and InGaAsP inner separation layer thickness were investigated. Photoluminescence (PL) and atomic force microscopy (AFM) analysis indicate the occurrence of gallium diffusion and the arsenic/phosphorus (As/P) exchange with the InGaAsP barriers. As a result, shorter wavelength emission is observed, making the structures suitable for telecom applications.     

Effect of Mn concentration and growth temperature on nanostructures and magnetic properties of Ge1−xMnx grown on Si

The nanostructures and magnetic properties of Ge_1−xMn_x thin films grown on Si substrates by molecular beam epitaxy, with different nominal Mn concentrations (1−4%) and different growth temperatures, have been systematically investigated by transmission electron microscopy and superconducting quantum interference device. It was discovered that when Ge_1−xMn_x thin films were grown at 70 °C, with increase in Mn concentration, Mn-rich tadpole shaped clusters started to nucleate at 1% Mn and become dominate in the entire film at 4% Mn. While for the thin films grown at 150 °C, tadpoles was firstly seen in the film with 1% Mn and subsequently Mn-rich secondary precipitates became dominant. The magnetic properties show specific features, which are mainly related to the nature and amount of Mn-rich clusters/precipitates within these thin films.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

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.     

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.     

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.     

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].     

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.     

Coexistence properties of phase separation and CuPt-ordering in InGaAsP grown on GaAs substrates by organometallic vapor phase epitaxy

CuPt-ordering and phase separation were directly investigated in In_1-xGa_xAs_yP_1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 °C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 °C showed less phase separation than the 690 °C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 °C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2×4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface.     

A study on structural formation and optical property of wide band-gap Be0.2Zn0.8O layers grown by RF magnetron co-sputtering system

Wide band-gap BeZnO layers were grown on Al₂O₃ (0 0 0 1) substrate using radio-frequency magnetron co-sputtering. The rate of Be_xZn_1−xO crystallized as a hexagonal structure was x=0.2. From the X-ray photoelectron spectroscopy measurement, the O–Zn bonds relating the crystal structure and the Be–O bonds related to the deviation of the stoichiometry in the BeZnO layer were caught at 530.4 and 531.7 eV in the O 1s spectrum, respectively. Thus, the observance on the Be 1s peak of 113.2 eV associated with the bonding Be–O indicates that the sputtered Be atoms are substituted for the host-lattice site in ZnO. This Be–O bonding shows a relatively low intense and broadening spectrum caused by large fluctuation of Be content in the BeZnO layer. From the photoluminescence and transmittance measurement, the free exciton and the neutral donor-bound exciton (D⁰, X) emissions were observed at 3.7692 and 3.7313 eV, respectively, and an average transmittance rate over 95% was achieved in a wide ultraviolet (UV)–visible region. Also, the binding energy for the (D⁰, X) emission was extracted to be 37.9 meV. Through the wide band-gap material BeZnO, we may open some possibilities for fabricating a ZnO-based UV light-emitting diode to be utilized as a barrier layer comprised of the ZnO/BeZnO quantum well structure and/or an UV light emitting material itself.     

The influence of the Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.     

Atomic ordering in GaAsSb (0 0 1) grown by metalorganic vapor phase epitaxy

Epitaxial GaAsSb (0 0 1) semiconductor alloys grown by metalorganic vapor phase epitaxy exhibit several spontaneously ordered structures. A superlattice structure with three-fold ordering in the [1 1 0] direction has been previously observed by different groups. CuAu structures with (1 0 0) and (0 1 0) ordering planes have also been reported. The physical origin of CuAu ordering in III–V semiconductors has not yet been explained. In this work we report the effect of growth conditions on CuAu ordering in GaAsSb, including miscut from (0 0 1), growth rate, bismuth surfactant concentration, and growth temperature. These data point to a surface kinetic mechanism not based on dimer strain, but possibly due to one-dimensional ordering at step edges.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.