Molecular Beam Epitaxy

Molecular Beam Epitaxy (MBE) is a versatile technique for growing thin epitaxial structures made of semiconductors, metals or insulators [1.1–18]. In MBE, thin films crystallize via reactions between thermal-energy molecular or atomic beams of the constituent elements and a substrate surface which is maintained at an elevated temperature in ultrahigh vacuum. The composition of the grown epilayer and its doping level depend on the relative arrival rates of the constituent elements and dopants, which in turn depend on the evaporation rates of the appropriate sources. The growth rate of typically 1 µm/h (1 monolayer/s) is low enough that surface migration of the impinging species on the growing surface is ensured. Consequently, the surface of the grown film is very smooth. Simple mechanical shutters in front of the beam sources are used to interrupt the beam fluxes, i.e., to start and to stop the deposition and doping. Changes in composition and doping can thus be abrupt on an atomic scale.

The application of MBE to the growth of compound semiconductors of devices and monolithic circuits requires excellent film uniformity and reproducibility of growth conditions [2.1–7]. The uniformity in thickness, as well as in composition, of films grown by MBE depends on the uniformity of the molecular beams across the substrate. As already discussed (Sect. 1.1.2), the uniformity of the molecular beam patterns upon the substrate depends on the geometry of the “sources-substrate” system, and on the angular flux distribution of the individual sources in the system. The best uniformity of beam patterns is obtained with a sufficiently large source-to-substrate spacing, and with flux distributions at the source orifices which are isotropic in the solid angle subtended by the substrate [2.8]. The reproducibility of the growth process depends, on the other hand, on the long term stability of the beam fluxes, as well as on the flux transients resulting from the shutter operations, e.g., from cooling of the surface of the charge contained in the source upon shutter opening [2.9].

The maturity which the MBE technique has now achieved is reflected in the demand for high throughput, high yield MBE machines. A whole set of companies currently manufacture MBE-growth and MBE-related equipment that is sophisticated in design and reliable in application. Among the largest manufacturers which share the major part of the world market [3.1], the following may be listed: ISA Riber [3.2–4] and VG Semicon [3.5, 6] in Europe, Intevac MBE (formerly Varian) [3.7–9] and EPI (Epitaxial Products International) [3.10] in the USA (recently these US MBE suppliers merged [3.11] into EPI MBE Products Group [3.12], and ANELVA and ULVAC in Japan.

The experimental arrangement of MBE is unique among epitaxial thin film preparation methods in that it enables one to study and control the growth process in situ in several ways. In particular, Reflection High Energy Electron Diffraction (RHEED) allows direct measurements of the surface structure of the substrate wafer and the already grown epilayer. It also allows observation of the dynamics of MBE growth [4.1–30]. The forward scattering geometry of RHEED is most appropriate for MBE, since the electron beam is at grazing incidence (Fig. 3.29), whereas the molecular beams impinge almost normally on the substrate. Therefore, RHEED may be called an in-growth surface and analytical technique.

The MBE growth process occurs on the substrate crystal surface, i.e., in the third zone of the deposition chamber arrangement (Sect. 1.1.2). Many different experiments using, for example, modulated molecular beam mass spectrometry [5.1–3] or RHEED pattern intensity oscillations [5.3–13] have been devoted to study the growth mechanism in MBE. These experiments dealt mainly with the MBE growth process of GaAs and related compounds, however, some fundamental rules creating a physical picture of the MBE growth process in general may be concluded from the wealth of data which became available [5.3,14–17].

The possibility of growing high-quality epitaxial layers of different materials on lattice mismatched semiconductor substrates is a topic of considerable interest. The range of useful devices available with a given substrate is largely enhanced by this method. For example, GaAs and compounds related to it (AlGaAs, InGaAs, etc.) offer many advantages over Si in terms of increased speed and radiation resistance and its ability to process and transmit signals by light pulses. Si, on the other hand, is a well-established material for integrated circuits and exhibits superior mechanical and thermal characteristics. By growing epitaxial layers of GaAs on Si substrates, it is possible to combine the advantages of both materials. However, these materials are not matched together, neither by lattice constants (a _Si = 0.543 nm, a _GaAs = 0.565 nm), nor by thermal expansion coefficients (α_Si = 2.6 × 10^-6 K^-1, α_GaAs = 6.8 × 10^-6 K^-1). Therefore, dislocations and other lattice defects are usually present in GaAs-Si heterostructures. In order to reduce the defect densities in this mismatched heterostructures (in general, in other mismatched material heterostructure pairs, too), a basic understanding is needed on the interface effects emerging during the epitaxial growth of such material systems. The relevant theoretical and experimental data which may lead to this understanding will be presented and discussed in this chapter in a concise form. Further, for more complete information the reader is referred to the references listed at the end of the book, mainly in [6.1–3].

Different materials exhibit usually different properties from the point of view of growth peculiarities [7.1]. The characteristic features which distinguish the materials are usually connected with a specific chemical element, e.g., Si, As, P, or Hg. The presence of this element in the material to be grown demands special technological precautions because of, for example, high evaporation temperature, high volatility, or extraordinary chemical reactivity. The special properties of the constituent elements also frequently bring about quite different growth mechanisms of the compounds crystallized with MBE.

In the preceding chapters we have presented a consistent picture of the current status of MBE. Emphasis has been put on technological equipment, characterization methods and growth processes. This presentation exhibits some unavoidable shortcomings, especially concerning completeness of the list of references, and the detailed descriptions of the technological parameters used when growing specific materials by MBE.