A. Ponchet

Instabilities in crystal growth by atomic or molecular beams

Abstract When growing a crystal, a planar front is desired for most of the applications. This plane shape is often destroyed by instabilities of various types. In the case of growth from a condensed phase, the most frequent instabilities are diffusion instabilities , which have been studied in detail by many authors but will be briefly discussed in simple terms in Section 2. The present review is mainly devoted to instabilities which arise in ballistic growth, especially molecular beam epitaxy (MBE). The reasons of the instabilities can be geometric, but they are mostly kinetic (when the desired state cannot be reached because of a lack of time) or thermodynamic (when the desired state is unstable). The kinetic instabilities which will be studied in detail in Sections 4 and 5 result from the fact that adatoms diffusing on a surface do not easily cross steps (Ehrlich–Schwoebel or ES effect). When the growth front is a high symmetry surface, the ES effect produces mounds which often coarsen in time according to power laws. When the growth front is a stepped surface, the ES effect initially produces a meandering of the steps, which eventually may also give rise to mounds. Kinetic instabilities can usually be avoided by raising the temperature, but this favours thermodynamic instabilities of the thermodynamically unstable materials (quantum wells, multilayers …) which are usually prepared by MBE or similar techniques. The attention will be focussed on thermodynamic instabilities which result from slightly different lattice constants a and a +δ a of the substrate and the adsorbate. They can take the following forms. (i) Formation of misfit dislocations, whose geometry, mechanics and kinetics are analysed in detail in Section 8. (ii) Formation of isolated epitaxial clusters which, at least in their earliest form, are ‘coherent’ with the substrate, i.e. dislocation-free (Section 10). (iii) Wavy deformation of the surface, which is presumably the incipient stage of (ii) (Section 9). The theories and the experiments are critically reviewed and their comparison is qualitatively satisfactory although some important questions have not yet received a complete answer. Short chapters are devoted to shadowing instabilities, twinning and stacking faults, as well as the effect of surfactants.

Relationship between self‐organization and size of InAs islands on InP(001) grown by gas‐source molecular beam epitaxy

Using the strained‐induced 2D–3D transition, InAs dots have been grown on InP(001) and examined by transmission electron microscopy. Two different modes of island size and spatial distribution have been identified. For deposit of 1.5 and 1.8 monolayers, the islands are about 7 nm high and randomly distributed. Above 2 monolayers, they are about five times smaller in volume and locally self‐organized, with a typical distance of 40 nm independent of the island density. It is suggested that the strong dependence of the island size on the total amount of deposited InAs is mainly due to long range interactions through the substrate.

Lateral modulations in zero‐net‐strained GaInAsP multilayers grown by gas source molecular‐beam epitaxy

Compressive GaInAsP multiple quantum wells (MQW) grown by gas source molecular‐beam epitaxy present altered structural and optical characteristics when tensile GaInAsP barriers are used instead of lattice‐matched ones. An alternate tensile/compressive GaInAsP MQW has been examined by transmission electron microscopy. A strong lateral modulation of thickness, strain, and probably chemical composition was shown. This modulation exhibits pronounced anisotropy, with a periodicity of about 50 nm along the [110] direction. Although its origin is not fully accounted for yet, it seems to allow partial elastic relaxation of tensile layers. Based on this analysis, a schematic description of distortion modulation is proposed.

Wavelength tuning of InAs quantum dots grown on (311)B InP

We report on the synthesis of InAs quantum dots on (311)B InP substrates. It is found that the use of such high index surfaces allows the formation of a high density (5×1010 islands/cm2) of small InAs islands (diameter≈350 A) on InP. Moreover, a large improvement of the size uniformity is obtained in comparison with deposition on (100) surface. The standard height deviations are ±13% and ±50% for islands grown on (311)B and (100) surfaces, respectively. Then, we show that the modification of the As/P flux sequences, after the island formation, permits the control of the quantum dot emission wavelength. The achievement of quantum dots emitting at 1.55 μm at 300 K indicates that this method is promising for telecom device making.

Influence of stress and surface reconstruction on the morphology of tensile GaInAs grown on InP(001) by gas source molecular beam epitaxy

Tensile/compressive and tensile/lattice matched Ga1 − xInxAs multilayers have been grown by gas source molecular beam epitaxy (GS-MBE) on InP(001) and examined by transmission electron microscopy (TEM). The tensile layers exhibit undulations parallel to [110] for a lattice mismatch of −0.5% and prismatic mesas for lattice mismatches of −1 and −1.7%. The facets making up the mesas are mainly of the (114)A and (113)A types. These growth modes are accounted for by elastic relaxation mechanisms. The relationship between the various morphologies and lattice mismatch is discussed by reviewing the competition between elastic energy and surface energy. It is shown that the relaxation can be partially or fully frozen by decreasing the growth temperature or increasing the V/III flux ratio. The dependence upon the growth technique is also underlined by comparison with metalorganic vapor phase epitaxy (MOVPE). The modulation anisotropy and the frequent occurrence of (114)A facets are examined in terms of surface reconstruction. It is suggested that the crystallographic morphology of (114)A facets facilitates a reconstruction which is very similar to the regular 2 × 4 reconstruction of the (001) GaAsAs surface. Therefore the exact nature of steps and facets and their surface reconstruction should be taken into account in elastic relaxation mechanisms.

Strain in InAs islands grown on InP(001) analyzed by Raman spectroscopy

Raman scattering has been used to investigate strained InAs islands grown on InP(001), in correlation with transmission electron microscopy. Symmetry arguments, selective resonance at the InAs E1‐like transition and comparison with a single‐quantum‐well structure allowed to distinguish between the islands and remaining wetting layer signals. Whereas the vibrational modes of the 2D thin layers are greatly affected by interface roughness and confinement, strain effects mainly account for the phonon frequency shifts in the islands.

SELF-INDUCED LATERALLY MODULATED GAINP/INASP STRUCTURE GROWN BY METAL-ORGANIC VAPOR-PHASE EPITAXY

Zero‐net strained multilayer alternating tensile GaInP and compressive InAsP have been grown on (001)InP by metal‐organic vapor‐phase epitaxy. A structural analysis using transmission electron microscopy (TEM) is reported. A remarkably regular laterally modulated structure has been observed. GaInP‐ and InAsP‐rich vertical zones alternate with a periodicity of 0.28 μm along the lateral [110] direction, thus balancing the mismatch along the [110] rather than the [001] growth direction. TEM experiments suggest that each vertical zone is partially elastically relaxed.

Gas source molecular beam epitaxy of alternated tensile / compressive strained GaInAsP multiple quantum wells emitting at 1.5 μm

Abstract Compressive and tensile strained GaInAsP layers as well as zero-net strain multiple quantum wells, grown by gas source molecular beam epitaxy, have been investigated. Reflection high energy electron diffraction patterns show two-dimensional growth for compressive strained layers ( Δa a . Three-dimensional growth is observed after a few nanometers for tensile strained layers even for low tensile value ( Δa a . Transmission electron microscopy shows that three-dimensional growth of a tensile strained layer is related to a quasi-periodic composition modulation along the [110] in the epitaxial plane.

Structural and optical analyses of GaP/Si and (GaAsPN/GaPN)/GaP/Si nanolayers for integrated photonics on silicon

We report a structural study of molecular beam epitaxy-grown lattice-matched GaP/Si(0 0 1) thin layers with an emphasis on the interfacial structural properties, and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown onto the GaP/Si pseudo substrates, through a complementary set of characterization tools. Room temperature photoluminescence at 780 nm from the (GaAsPN/GaPN) quantum wells grown onto a silicon substrate is reported. Despite this good property, the time-resolved photoluminescence measurements demonstrate a clear influence of non-radiative defects initiated at the GaP/Si interface. It is shown from simulations, how x-ray diffraction can be used efficiently for analysis of antiphase domains. Then, qualitative and quantitative analyses of antiphase domains, micro-twins, and stacking faults are reported using complementarity of the local transmission electron microscopy and the statistical x-ray diffraction approaches.

ELASTIC ENERGY OF STRAINED ISLANDS : CONTRIBUTION OF THE SUBSTRATE AS A FUNCTION OF THE ISLAND ASPECT RATIO AND INTER-ISLAND DISTANCE

The finite element method is applied to strain-induced islands. The distribution of the elastic energy in the island and the substrate is determined as a function of the island aspect ratio and inter-island distance. When the height-over-base ratio increases, the total elastic energy density decreases and the relative contribution of the substrate increases. When the inter-island distance decreases, the elastic energy density increases and the relative contribution of the substrate decreases. The influence of the aspect ratio on the relaxation rate is amplified for short inter-island distances.