Self-assembled InP islands grown on GaP substrate

Solomon G.S., Trezza J.A., Marshall A.F., Harris, Jr. J.S.

Multilayer, vertically coupled, quantum dot structures are investigated using layers composed of InAs islands grown by molecular beam epitaxy in the Stranski-Krastanov growth mode. Single, 2, 5, and 10 InAs island layers are investigated in which the 40 \AA{} high InAs islands are separated by 56 \AA{} GaAs spacer layers. The InAs islands are vertically aligned in columns and are pseudomorphic. Between 1 and 10 layers of islands, 8 K photoluminescence shows a 25% reduction in PL linewidth, and a peak shift of 92 meV to lower energy, while transmission electron and atomic force microscopy show the island size in different layers remains constant. These effects are attributed to electronic coupling between islands in the columns, and a simple coupling model is used to simultaneously fit the spectral peak position shift and the linewidth changes.

Nabetani Y., Wakahara A., Sasaki A.

We grow InAs layers on (001) on-axis and misoriented GaAs substrates by molecular beam epitaxy, respectively. The critical thickness of each InAs layer is investigated with photoluminescence spectroscopy and transmission electron microscopy (TEM). We show that the critical thickness is significantly influenced by the substrate misorientation. The critical thickness of the InAs layer grown on the GaAs substrate misoriented toward the [11̄0] direction becomes thicker [5 monolayers (ML)] than that (3 ML) of the InAs layers grown on the GaAs substrates (001) on-axis or misoriented toward the [110] direction. The plan-view TEM images show that the islands grown coherently do not coalesce even beyond the on-axis critical thickness (3 ML). The strain energy is calculated based on valence-force-field model to investigate the interaction between dislocation and step in case on misoriented substrate. As results, we show that dislocation has the minimum length above which dislocation can stably exist and that the extra strain energy generated at the cross point of dislocation and step plays an important role in dislocation generation.

Carlsson N., Georgsson K., Montelius L., Samuelson L., Seifert W., Wallenberg R.

Coherent InP nano-sized islands, embedded into GaInP, have been grown by metal-organic vapour phase epitaxy using the Stranski-Krastanow growth mode. Photoluminescence, atomic force microscopy and transmission electron microscopy studies show that the insertion of a thin ∼ 4 monolayer thick GaP layer affects the critical thickness of the subsequently deposited two-dimensional InP wetting layer, increasing it from ∼ 1.5 monolayers (without an inserted GaP layer) to ∼ 2.5 monolayers (with an inserted GaP layer). We demonstrate that the inserted GaP layer affects also the island formation. The bimodal size distribution of Stranski-Krastanow islands, typical for low InP coverages, can be overcome without island coalescence by deposition on top of the thin GaP layer, where a coverage of InP of about 3.5–4.5 monolayers results in the formation of almost only the larger, fully developed, pyramidal islands. Annealing experiments at growth temperature of 580°C show that these islands (base area ≈ 40 × 50 nm2, height ≈ 10–15 nm, surface density ≈ (1−2) × 109 cm−2) are rather stable in a time-scale over several minutes before they slowly undergo an Ostwald ripening process.

Nabetani Y., Yamamoto N., Tokuda T., Sasaki A.

We observe the initial growth layer of InAs on a GaAs substrate by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The layer grows two-dimensionally until about 1.8 ML, and small islands which have a certain size are formed around 1.8 ML. Then, they grow to large sized islands after 2.0 ML, and the density of the small islands decreases. The sizes of large islands are not similar to each other. The plan-view TEM image reveals that the misfit dislocations are generated in the large islands. The growth mode of InAs on GaAs is explained by considering the surface energy and strain energy. Variations in the size and shape of the large islands are also interpreted.

Carlsson N., Seifert W., Petersson A., Castrillo P., Pistol M.E., Samuelson L.

Ga0.5In0.5P/InP quantum-sized structures, grown by metalorganic vapor phase epitaxy, have been optically characterized by photoluminescence, cathodoluminescence, and photoluminescence excitation spectroscopy. Additional structural information has been obtained by atomic force microscopy. We find that the two-dimensional layer-by-layer growth mode is limited to the growth of 1-ML-thick and, in part, 2-ML-thick quantum wells. The transition towards three-dimensional Stranski–Krastanow island growth occurs before the second monolayer of InP is completed. To further study the dynamics of the island formation, growth interruptions were introduced between the InP deposition and the subsequent growth of the upper GaInP barrier. The two types of coherent islands show a quantum confinement in vertical direction, corresponding to about 2- and 3-ML-thick and about 9- and 10-ML-thick InP strained quantum wells.

Oshinowo J., Nishioka M., Ishida S., Arakawa Y.

We report the direct deposition of strained InGaAs-dot structures with a diameter of about 15 nm on GaAs surfaces by metalorganic chemical vapor deposition growth. High resolution scanning electron micrographs show highly uniform quantum-sized dots formed by the Stranski–Krastanow growth mode. The sharp photoluminescence emission band of buried dot structures indicates efficient carrier capture and a homogeneous heterointerface. The average dot size and area dot density can be controlled accurately by growth temperature, and InGaAs deposition thickness, respectively.

Nabetani Y., Ishikawa T., Noda S., Sasaki A.

A few mololayers of InAs is heteroepitaxially grown on GaAs substrate by molecular-beam epitaxy. Structure and optical properties are investigated. Reflection high-energy electron-diffraction observation reveals that an InAs layer forms a three-dimensional structure with specific facets after two-dimensional growth. The transmission electron microscope observation shows that these structures have structural anisotropy in the growth plane. Photoluminescense spectroscopy shows that the luminescence from the InAs structures exhibits the polarization property caused by the quantum dot effect of the structural anisotropy.

Moison J.M., Houzay F., Barthe F., Leprince L., André E., Vatel O.

The deposition of InAs on GaAs proceeds first by two-dimensional (2D) growth and above a 1.75-monolayer coverage by the formation of single-crystal dots on a residual 2D wetting layer. By atomic force microscopy measurements, we show that the first dots formed are in the quantum size range (height 30 Å, half-base 120 Å), that the dispersion on their sizes is remarkably low (±10%), and that they are located fairly regularly (interdot distance 600 Å). Upon further growth, density and shapes do not change but sizes increase up to double values before coalescence occurs. Self-organized growth in strained structures is then shown to be a simple and efficient way of building regular quantum dots.

Petroff P.M., DenBaars S.P.

Abstract In this paper, we review our latest developments on the growth and properties of self-assembling quantum dot structures. The self-assembling growth technique which was initially developed using molecular beam epitaxy (MBE), has now been extended to metalorganic chemical vapor deposition (MOCVD). The paper first presents structural results based on atomic force and transmission electron microscopy studies of the quantum dot arrays which were obtained by MBE and MOCVD growth. From the detailed structural analysis we have observed that the formation of coherently strained dots of InAs, InAlAs, and InP dots on various cladding layer surfaces. MBE growth of InAs self-assembled dots has achieved the smallest size distribution, with dots as small as 12nm in diameter. For the MOCVD growth of InP dots we have found that the surface morphology and growth temperature of lower cladding layer growth has a profound influence on island size and density. Recent results on the optical and transport properties of the MBE grown self-assembling dot (SAD) arrays are also presented.

Leonard D., Krishnamurthy M., Reaves C.M., Denbaars S.P., Petroff P.M.

The 2D–3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum-sized dot structures. Transmission electron micrographs reveal that when the growth of In0.5Ga0.5As is interrupted exactly at the onset of this 2D–3D transition, dislocation-free islands (dots) of the InGaAs result. Size distributions indicate that these dots are ∼300 Å in diameter and remarkably uniform to within 10% of this average size. The areal dot densities can be varied between 109 and 1011 cm−2. The uniformity of the dot sizes is explained by a mechanism based on reduction in adatom attachment probabilities due to strain. We unambiguously demonstrate photoluminescence at ∼1.2 eV from these islands by comparing samples with and without dots. The luminescent intensities of the dots are greater than or equal to those of the underlying reference quantum wells.

Wang X., Wakahara A., Sasaki A.

Al x Ga 1− x P(0≤x≤1) epilayers are grown by OMVPE using TBP as a phosphorus source. GaP epilayers with a specular surface are obtained at growth temperatures higher than 740°C with a V/III ratio of 20. The conduction type of the grown layers is n-type, and the carrier concentrations of the GaP layers are as low as 7.4 × 10 14 cm −3 . In the growth of Al x Ga 1− x P, the growth rate, the Al distribution coefficient, and the electrical properties of the Al x Ga 1− x P epilayers exhibit very different behavior compared with those of AlGaAs grown by OMVPE.

Wakahara A., Wang X., Sasaki A.

AlGaP layers and AlP/GaP superlattices were grown by atmospheric pressure OMVPE at low temperature on GaP substrates using tertiarybutylphosphine (TBP) as a phosphorus source for safety and using (NH 4 ) 2 S x solution for surface stabilization. High crystalline quality AlGaP epi-layers with specular surfaces were obtained at relatively low growth temperatures ( 4 ) 2 S x surface treatment. The AlP/GaP superlattices were grown at 725°C with a V/III ratio of 20 by using the (NH 4 ) 2 S x treatment. The X-ray diffraction profile showed satellite reflection peaks up to as high as fifth order for the (AlP) 17 /(GaP) 17 superlattice.

Tabuchi M., Noda S., Sasaki A.

When a grown crystalline layer is as thin as one or two monolayers, the lattice constant in a grown layer is coherent with the substrate lattice even in a highly lattice-mismatched crystalline system. In this study, we calculate the strain energy in deviated bond lengths and bond angles for individual atoms by valence force field (VFF) method and find the atom position with the minimum strain energy. The critical thickness below which no dislocation occurs is derived. The thickness is thinner as compared with those predicted by previous conventional theories. However, the experimental results agree fairly well with our theoretical results.

Keller D.

A local, nonlinear transform is derived that allows an STM or AFM image which has been distorted by a nonideal tip to be reconstructed. The image reconstruction transform is related to the Legendre transforms of the distorted image surface and the tip surface. Two simple examples are solved analytically. It is shown that the local curvature of the true, undistorted sample surface is the sum of the curvatures of the tip and the distorted image surface. Some distorted images are, strictly speaking, not invertible, that is, cannot be completed reconstructed. The reason for this is discussed, and it is shown that in these cases the transform correctly reconstructs the parts of the image that are invertible, and leaves “holes” whenever a noninvertible section of an image is encountered. Numerical calculations are presented in which several artificially generated images and one experimentally obtained STM image of a photoresist grating are reconstructed.

Eaglesham D.J., Cerullo M.

We show that the islands formed in Stranski-Krastanow (SK) growth of Ge on Si(100) are initially dislocation free. Island formation in true SK growth should be driven by strain relaxation in large, dislocated islands. Coherent SK growth is explained in terms of elastic deformation around the islands, which partially accommodates mismatch. The limiting critical thickness, ${\mathit{h}}_{\mathit{c}}$, of coherent SK islands is shown to be higher than that for 2D growth. We demonstrate growth of dislocation-free Ge islands on Si to a thickness of \ensuremath{\approxeq}500 \AA{}, 50\ifmmode\times\else\texttimes\fi{}higher than ${\mathit{h}}_{\mathit{c}}$ for 2D Ge/Si epitaxy.

Piedra-Lorenzana J.A., Yamane K., Hori A., Wakahara A.

Abstract The necessity for improved renewable energy sources has increased in recent years, particularly solar cells have been continuously improving. This study proposes a type-II quantum dot (QD) structure using InP and GaP-based III–V–N alloys to enhance electron/hole spatial separation for photovoltaic applications. With appropriate size and thickness, InP QD/GaAsPN enables type-II band alignment. Additionally, it has a tunable bandgap of approximately 1.7 eV with strain compensation conditions on a Si substrate, which enables dislocation-free III–V/Si tandem cells. Self-assembled nanostructures of InP were fabricated on GaP, and two types of islands were observed. Growth parameters were investigated to ensure better control over the morphology of islands. Subsequently, the optimized parameters were employed for fabricating a 30-period good quality InP/GaP stacked QD structure without any strain compensation layers. These results may help in designing more efficient GaP-based III–V–N solar cells on Si substrates.

Höfling C., Schneider C., Forchel A.

Hatami F., Bierwagen O.

The growth of low-dimensional semiconductor nanostructures, that is, quantum-wells (QWs), quantum wires (QWrs), and quantum-dots (QDs), is reviewed with emphasis on the self-assembled growth of QDs and QWrs by the Stranski–Krastanow (SK) growth mode in traditional III–V material systems (In,Ga,Al)–(As,P,Sb). The growth of QWs, and alternative routes to the SK growth for the preparation of QWrs and dots are also presented.

Umeno K., Furukawa Y., Urakami N., Mitsuyoshi S., Yonezu H., Wakahara A., Ishikawa F., Kondow M.

The authors have investigated the growth and luminescence properties of InPN alloys grown by solid-source molecular-beam epitaxy (MBE). The N composition increases with decreasing growth rate, P2∕In flux ratio, and growth temperature. In this work, the highest N composition obtained is 0.56% for the InPN sample. The appropriate growth temperature is around 400°C. However, the growth-temperature window of the InPN alloys having a smooth surface is very narrow. In order to obtain photoluminescence (PL) emission from the InPN samples grown by solid-source MBE, InPN alloys must be grown under the condition of lower-plasma power since the grown-in point defects induced by N plasma are reduced. Thermal treatment is effective to improve the luminescence efficiency of InPN alloys, and the appropriate annealing temperature is around 700°C. However, the S-shape behavior is observed only for the annealed InPN samples by atomic rearrangements during thermal treatment, which is attributed to the weaker bond strength of In–N than that of In–P. In addition, the PL peak energy corresponding to the near-band edge emission redshifts with increasing annealing temperature. These results indicate that the luminescence properties of InPN alloys are unique in contrast to other dilute nitrides such as GaAsN and GaPN alloys.

Gerhard S., Baumann V., Höfling S., Forchel A.

We present a study of the growth, morphology and optical properties of Ga(x)In(1-x)P quantum dots (QDs) grown by molecular beam epitaxy (MBE) for various Ga concentrations x. QD areal densities up to 10(11) cm(-2) have been achieved showing strong dependence on the amount of gallium supplied. Structural properties are evaluated using scanning electron microscopy (SEM) and atomic force microscopy (AFM) and are related to photoluminescence properties of the QDs. Both structural and optical properties are promising for future applications of the herein reported QDs in visible wavelength optoelectronic devices.

Hatami F., Masselink W.T., Schrottke L., Tomm J.W., Talalaev V., Kristukat C., Goñi A.R.

The optical emission and dynamics of carriers in Stranski-Krastanow self-organized InP quantum dots embedded in a GaP matrix are studied. InP deposited on GaP (001) using gas-source molecular-beam epitaxy forms quantum dots for InP coverage greater than 1.8 monolayers. Strong photoluminescence from the quantum dots is observed up to room temperature at about 2 eV; photoluminescence from the two-dimensional InP wetting layer is measured at about 2.2 eV. Modeling based on the ``model-solid theory'' indicates that the band alignment for the InP quantum dots is direct and type I. Furthermore, low-temperature time-resolved photoluminescence measurements indicate that the carrier lifetime in the quantum dots is about 2 ns, typical for type-I quantum dots. Pressure-dependent photoluminescence measurements provide further evidence for a type-I band alignment for InP/GaP quantum dots at normal pressure with the GaP X states lying about 30 meV higher than the $\ensuremath{\Gamma}$ states in the InP quantum dots, but indicate that they become type II under hydrostatic pressures of about 1.2 GPa.

Datta S., Bhattacharya A., Gokhale M.R., Pai S.P., John J., Arora B.M.

We discuss the growth of In x Ga 1− x P quantum dots on nominally (1 0 0)-oriented GaP substrates by low-pressure metal organic vapor phase epitaxy. Parameters like the growth temperature, thickness of the InGaP layer, strain, via the indium composition of In x Ga 1− x P, and the post-growth ripening time have been varied to study their effect on the growth of self-assembled InGaP quantum dots in Stranski–Krastanow mode. Under optimized conditions quantum dots with lateral dimension ∼50 nm and height ∼5 nm and a density >10 10  cm 2 have been achieved. Surface photovoltage spectroscopy is shown to be a convenient and sensitive technique to monitor the various stages in the growth of quantum dot structures.

Ryou J.H., Dupuis R.D., Walter G., Holonyak N., Mathes D.T., Hull R., Reddy C.V., Narayanamurti V.

We have studied the properties of InP self-assembled quantum dots embedded in various In0.49(AlxGa1−x)0.51P matrix layers to optimize the growth condition of the quantum dots and structures for III-phosphide quantum-dot-based lasers operating in visible spectral regions. Self-assembled quantum dot-related structures are grown by low-pressure metalogranic chemical vapor deposition and characterized by atomic-force microscopy, high-resolution transmission-electron microscopy, and photoluminescence. High density (∼1010 cm−2) and conveniently sized (∼5×20 nm) quantum dots are produced by growth condition optimization. We find that the quantum-dot heterostructure with a In0.49(AlxGa1−x)0.51P matrix layer having the largest direct band gap produces the most efficient luminescence at room temperature. Laser structures are prepared using optimized growth conditions and matrix materials. Laser operation with lasing wavelengths λ=650–680 nm are demonstrated at 77 and 300 K by optical pumping.

Sasaki A.

Three-dimensional formation occurring at the initial stage of the heteroepitaxial growth of a lattice-mismatched system is reviewed. It is experimentally proved that the formation of InAs islands on the GaAs substrate is due to the Stranski–Krastanov (S–K) growth mode. This formation has been proposed to realize a mesoscopic island structure which can be utilized as a quantum dot (QD). They are naturally formed with the S–K growth mode. They have been called naturally- or self-formed QDs. The strain energy of the QD is calculated with the valence-force field model, and then the possible formation of dislocation-free QDs is described. The experimental results of InAs/GaAs and InP/GaP QDs are presented. In the stacked layer of InAs/GaAs QDs, the GaAs spacer thickness for the vertical ordering of QDs is investigated. The ordering, dependent on the spacer thickness, is found by the transmission electron microscope images of cross-sectional stacked layers. The QD can be neither stacked nor vertically ordered when the thickness is greater than 2 h where h denotes the QD height. The QDs are crushed when the spacer is thinner than h. The vertically ordered QDs are stacked when the GaAs spacer thickness is between h and 2 h.

Masselink W.T., Hatami F., Mussler G., Schrottke L.

We describe the growth and optical emission from strained InP quantum wells and quantum dots grown on GaP substrates using gas-source molecular beam epitaxy. Self-organized quantum dot formation takes place for InP coverage greater than 1.8 monolayers on the (1 0 0) GaP surface. Atomic force and scanning-electron microscopy studies indicate that unburied dots have a lateral size of 60– 100 nm and are about 20 nm high, with dot densities in the range of 2– 6×10 8 cm −2 for InP coverage between 1.9 and 5.8 MLs . Intense photoluminescence is emitted from both the quantum wells and the quantum dots at energies of about 2.2 and 2.0 eV , respectively. Time-resolved measurements indicate rather long carrier lifetimes of about 19 ns in the quantum wells and about 3 ns in the quantum dots. The data indicate that the InP/GaP quantum wells form a type-II band system, with electrons in the X valleys of the GaP recombine with holes in the InP. Furthermore, in the InP/GaP quantum dot system, the conduction band edge in the X valley of the GaP is nearly aligned with that in the Γ valley of the InP. Rapid thermal annealing of the quantum dots results in at least a six-fold enhancement of integrated emission intensity as well as some Ga-In interdiffusion. The low interdiffusion activation energy indicates that the material near the interface between the GaP matrix and the InP dots is not free of defects.

Wallart X., Deresmes D., Mollot F.

Using reflection high energy electron diffraction and atomic force microscopy, the growth of Ga 1− x In x P alloys on GaP (0 0 1) with x varying from 0.2 to 1 (InP) is investigated and compared to that of arsenides on GaAs (0 0 1) or InP (0 0 1). At 520°C, the evolution of the critical thickness for 3D growth versus In content is rather similar to that observed in the GaInAs/GaAs system. For x ⩽0.5, 3D growth leads to the development of wire-like structures along the [1 1 0] direction which can be related to recent results on the phosphide surface reconstructions. Finally, for the growth of InP on GaP at 520°C, the critical thickness is 2.1 MLs and we observe a small density of very large islands, in contrast to the InAs/GaAs case. At 400°C, the critical thickness decreases (1.7 MLs) as well as the island mean size whereas the density increases. We discuss this behavior in terms of surface energy.

Wallart X., Deresmes D., Mollot F.

We study the growth of strained Ga1−xInxP layers on GaP (001) by gas-source molecular-beam epitaxy for x varying from 0.25 to 1. At a growth temperature of 520 °C, we find two main differences with respect to the well known GaInAs/GaAs system. First, for 0.25⩽x⩽0.5, we observe the development of wire-like structures oriented along the [110] direction and on the other hand, the growth of InP on GaP leads to the formation of huge dots in small density. The influence of the growth parameters such as the growth temperature or the phosphine flow rate is presented. The whole set of results is discussed in light of recent work on the phosphide surface reconstructions with a particular emphasis on the role of the cation-rich one.

Ryou J., Dupuis R.D., Reddy C.V., Narayanamurti V., Mathes D.T., Hull R., Mintairov A., Merz J.L.

We report the characteristics of InP self-assembled quantum dots embedded in In0.5Al0.5P on GaAs substrates grown by metalorganic chemical vapor deposition. The InP quantum dots show increased average dot sizes and decreased dot densities, as the growth temperature increases from 475°C to 600°C with constant growth time. Above the growth temperature of 600°C, however, dramatically smaller and densely distributed self-assembled InP quantum dots are formed. The small InP quantum dots grown at 650°C are dislocation-free “coherent” regions with an average size of ∼20 nm (height) and a density of ∼1.5 × 108 mm−2. These InP quantum dots have a broad range of luminescence corresponding to red or organge in the visible spectrum.

Dupuis R.D., Ryou J.H., Heller R.D., Walter G., Kellogg D.A., Holonyak N., Reddy C.V., Narayanamurti V., Mathes D.T., Hull R.

We describe the operation of lasers having active regions composed of InP self-assembled quantum dots embedded in In0.5Al0.3Ga0.2P grown on GaAs (100) substrates by MOCVD. InP quantum dots grown on In0.5Al0.3Ga0.2P have a high density on the order of about 1-2x10 cm-2 with a dominant size of about 10-15 nm for 7.5 ML growth.[1] These In0.5Al0.3Ga0.2P/InP quantum dots have previously been characterized by atomic-force microscopy, high-resolution transmission electron microscopy, and photoluminescence.[2] We report here the 300K operation of optically pumped red-emitting quantum dots using both double quantum-dot active regions and quantum-dot coupled with InGaP quantum-well active regions. Optically and electrically pumped 300K lasers have been obtained using this active region design; these lasers show improved operation compared to the lasers having QD-based active regions with threshold current densities as low as Jth ∼ 0.5 KA/cm2.

Dupuis R.D., Ryou J.H., Heller R.D., Walter G., Kellogg D.A., Holonyak N., Reddy C.V., Narayanamurti V., Mathes D.T., Hull R.

We describe the operation of lasers having active regions composed of InP selfassembled quantum dots embedded in In0.5Al0.3Ga0.2P grown on GaAs (100) substrates by MOCVD. InP quantum dots grown on In0.5Al0.3Ga0.2P have a high density on the order of about 1–2×10 cm−2 with a dominant size of about 10–15 nm for 7.5 ML growth.[1] These In0.5Al0.3Ga0.2P/InP quantum dots have previously been characterized by atomic-force microscopy, high-resolution transmission electron microscopy, and photoluminescence.[2] We report here the 300K operation of optically pumped red-emitting quantum dots using both double quantum-dot active regions and quantum-dot coupled with InGaP quantum-well active regions. Optically and electrically pumped 300K lasers have been obtained using this active region design; these lasers show improved operation compared to the lasers having QD-based active regions with threshold current densities as low as Jth ∼ 0.5 KA/cm2.