Molecular Beam Epitaxy

From Research to Mass Production

Mohamed Henini, Mohamed Henini

• Publisher: Elsevier Science
• Year: 2012
• ISBN:
  9780123918598
• Topic: Tecnologia e ingegneria
• Subtopic: Scienza dei materiali

This multi-contributor handbook discusses Molecular Beam Epitaxy (MBE), an epitaxial deposition technique which involves laying down layers of materials with atomic thicknesses on to substrates. It summarizes MBE research and application in epitaxial growth with close discussion and a 'how to' on processing molecular or atomic beams that occur on a surface of a heated crystalline substrate in a vacuum.MBE has expanded in importance over the past thirty years (in terms of unique authors, papers and conferences) from a pure research domain into commercial applications (prototype device structures and more at the advanced research stage). MBE is important because it enables new device phenomena and facilitates the production of multiple layered structures with extremely fine dimensional and compositional control. The techniques can be deployed wherever precise thin-film devices with enhanced and unique properties for computing, optics or photonics are required. This book covers the advances made by MBE both in research and mass production of electronic and optoelectronic devices. It includes new semiconductor materials, new device structures which are commercially available, and many more which are at the advanced research stage.

• Condenses fundamental science of MBE into a modern reference, speeding up literature review
• Discusses new materials, novel applications and new device structures, grounding current commercial applications with modern understanding in industry and research
• Coverage of MBE as mass production epitaxial technology enhances processing efficiency and throughput for semiconductor industry and nanostructured semiconductor materials research community

Chapter 1

Molecular beam epitaxy
fundamentals, historical background and future prospects

Secondo Franchi

CNR-IMEM Institute, Parco delle Scienze, Parma, Italy

Chapter Outline

1.1 Introduction

1.2 Basics of MBE

1.3 The technology of MBE

1.3.1 MBE machines

1.3.2 MBE growth chambers

1.3.3 Sources of molecular beams

1.3.3.1 Thermal-evaporation cells

1.3.3.2 Valved cracker cells

1.3.4 Variants of the MBE process

1.3.4.1 Gas source MBE, metalorganic MBE and chemical beam epitaxy

1.3.4.2 Nitride MBE and reactive MBE

1.3.4.3 Group-IV MBE

1.3.4.4 Migration enhanced epitaxy (MEE) and atomic layer MBE (ALMBE)

1.3.4.5 Droplet epitaxy

1.3.5 Measurements of molecular beam fluxes

1.3.6 Control of composition and doping profiles along growth direction

1.4 Diagnostic techniques available in MBE systems

1.4.1 RHEED and surface reconstructions

1.4.2 STM and surface reconstructions

1.4.3 Other techniques (AES, SIMS, XPS and UPS)

1.5 The physics of MBE

1.5.1 Group-V and group-III species on III–V surfaces

1.5.2 2D layer-by-layer growth mechanism

1.5.2.1 Monte Carlo simulations of the MBE 2D growth process

1.5.2.2 Evidence by RHEED of the 2D layer-bylayer growth mechanism

1.5.2.3 Other evidences of the 2D layer-by-layer two-dimensional growth mechanism

1.5.2.4 Consequences of the layer-by-layer growth mechanism

1.5.2.5 Effects of interface roughness on device properties

1.5.2.6 Layer-by-layer growth mechanism in lattice-matched or lowly latticemismatched structures

1.5.3 Three-dimensional growth mechanism

1.5.3.1 Three-dimensional growth of selfassembled nanoislands

1.5.3.2 Strain relaxation as the driving force for 3D growth

1.5.3.3 Metamorphic structures and quantum dot strain engineering

1.5.4 MBE-assisted growth of one-dimensional structures

1.6 Historical background

1.7 Future prospects

1.8 Conclusions

References

1.1 Introduction

In the last four decades, it has been largely proved that epitaxial technologies for material growth have unique advantages over simpler counterparts, in spite of their generally higher technological costs. Interesting epitaxial technologies developed since the end of the 1960s are vapour-phase epitaxy (VPE), metalorganic VPE (MOVPE) and molecular beam epitaxy (MBE), along with its metalorganic variant (MOMBE).

In this chapter it will be shown that MBE and variants are particularly well suited for the growth of advanced epitaxial structures; indeed, MBE allows for the growth of (i) materials with reduced concentrations of thermodynamical defects, due to the relatively low growth temperatures; (ii) structures where composition or doping profiles in the growth direction can be modulated in abrupt or continuous ways, a feature that has opened the way to (a) the preparation of new epitaxial structures, (b) the fabrication of innovative devices, (c) researches that were awarded the 2000 Nobel prize in physics (e.g., Alferov [1]; Nobel lecture and Kroemer [2]; Nobel lecture); and, finally, (iii) quantum structures, where engineered composition and doping profiles confine carriers in two- or three-dimensional regions with sizes smaller or comparable to the de Broglie wavelength of carriers (Esaki [3]; von Klitzing [4]; Tsui [5] and Störmer [6]; Nobel lectures).

In this chapter it will be emphasised that MBE growth takes place according to two mechanisms that allow the fabrication of structures where carriers may undergo two- (2D) or three- (3D) dimensional quantum confinement; indeed, most of quantum structures (such as quantum wells, superlattices, selectively doped heterostructures, structures for resonant tunnelling, as well as quantum dots) and related devices have been demonstrated by MBE.

Many classes of materials have been prepared by MBE: semiconductors, such as III–Vs, II–VIs, IV–VIs and IV–IVs, and also oxides, magnetic materials and metals; however, most of the work on the development of technology and physics of MBE reviewed in this chapter has been done on III–Vs.

In this chapter, after the short introduction (Section 1.1), the basic elements of MBE technology are considered in Section 1.2. Section 1.3 deals with different aspects of MBE such as (i) layouts of MBE machines used for research purposes (Section 1.3.1), (ii) growth chambers for research (Section 1.3.2), and (iii) sources of molecular beams (Section 1.3.3); in Section 1.3.4 a description is given of the numerous variants of MBE, developed either for growing different materials or for taking advantages of particular features of MBE growth mechanisms. Then, two points are considered, which are particularly relevant for the growth of structures with engineered composition and doping profiles: (i) the measurement of molecular beam fluxes (Section 1.3.5) and (ii) the accurate control of such profiles (Section 1.3.6). One of the most interesting features of MBE is the availability of diagnostic techniques in growth or in analysis chambers, all connected under ultra-high vacuum (UHV). The use of these techniques, that in a few instances are available in-situ (in the growth chamber) and in real time (during the growth), accounts for the deep understanding of growth mechanisms, which, in turn, explains why MBE processes can be controlled very accurately and with high yields. Techniques such as reflection high-energy electron diffraction (RHEED), scanning tunnelling microscopy (STM), Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS), X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) are briefly reviewed in Section 1.4. Section 1.5.1 deals with surface phenomena of group-III and group-V species on III–Vs substrates, which determine the kinetic mechanisms of MBE growth of III–Vs and which have been supposedly extended also to other systems; in Section 1.5 the 2D layer-by-layer growth mechanism is discussed, which results in the growth of interfaces between lattice-matched (or lowly mismatched) materials smooth on the atomic scale; in particular, proofs of the occurrence of such a mechanism are considered, which were obtained both experimentally and by Monte Carlo simulation of the growth process. Section 1.5.3, instead, deals with the 3D growth of self-assembled nanoislands, which takes place when the materials in structures are lattice-mismatched by more than 2–3%; it will be shown that this mechanism is the natural candidate for the preparation of quantum dot structures, characterised by the 3D confinement of carriers; in Section 1.5.3 it is shown how strain can be considered as a degree of freedom to engineer the band structure of metamorphic structures. In Section 1.5.4 a short consideration is given to the MBE-assisted growth of nanowires based on the vapour–liquid–solid mechanism. A résumé of the historical background of MBE is given in Section 1.6, while in Section 1.7 some prospects of MBE are presented; finally, in Section 1.8 a few conclusions are drawn.

1.2 Basics of MBE

Molecular beam epitaxy (MBE) is an epitaxial process by which growth of materials takes place under UHV conditions on a heated crystalline substrate by the interaction of adsorbed species supplied by atomic or molecular beams [7]. The layers or deposits have: (i) the same crystalline structure of the substrate or a structure with a similar symmetry and (ii) a lattice parameter differing from that of the substrate by no more than ∼10%. The beams generally have thermal energy and are produced by evaporation or sublimation of suitable materials contained in ultra-pure crucibles.

In the archetypal case of III–V semiconductors grown on III–V substrates, such as InGaAsP on GaAs, adsorbed group-III atoms and group-V molecules migrate on the heated substrate surface until they interact in proximity of suitable vacant lattice sites, where they are incorporated into the solid phase (Section 1.5.1); in InGaAsP solid solutions, In and Ga, as well as As and P, are randomly distributed in the group-III and group-V sublattices, respectively. Atomic beams of elements such as Si and Be provide n-type and p-type doping in III–V compounds.

As it will be discussed in more detail below, beams may also be produced by injection in the growth environment of gaseous species, such as AsH₃ and PH₃ (gas source MBE (GSMBE)) or of volatile metalorganic compounds carried by hydrogen flows (metalorganic MBE, (MOMBE)).

Cells producing ionised beams with non-thermal energy have been proposed: they may generate Zn-ion beams to dope III–Vs [8] or As- and Sb-ion beams for Si ([9] [11], respectively) (Section 1.3.4.3); the ions are accelerated by electric fields (a few hundreds eV) towards growing layers in order to improve the sticking of dopants. Another example of beams with non-thermal energy is that of supersonic beams (a few tens eV) which are used to enhance surface migration of adsorbed species and, then, to improve the morphology of deposits [12].

Interesting examples of epitaxial relationships between layers and substrates are those between (i) III–V (100) zinc-blende layers and substrates (such as GaAs/GaAs, AlGaAs/GaAs, InGaAs/GaAs, InGaAsP/InP ...