Molecular beam epitaxy of III–V semiconductors

Molecular beam epitaxy (MBE) has been instrumental in the advancement of the physics and technology of semiconductors that has occurred over the last few decades. The III-V material system has led the way in these new developments. This article discusses the technology of III-V MBE, highlights selected topics in its development, discusses growth mechanisms, and mentions possible future directions.     

Superfine structures of semiconductors grown by molecular‐beam epitaxy

We initiated the present study to investigate experimentally a fundamental quanturn-mechanical effect—the resonant transmission of electrons through potential barriers in semicon- ductors. The phenomenon of such resonance is a familiar one. In the simple case of a double barrier, the incident electrons are able to tunnel through both barriers without attenuation if their energies coincide with the resonant energies.¹ In the caSe of a series of equally spaced barriers, e.g., that of the KronigPenney model of a one-dimensional crystal, the electronic structure exhibits forbidden bands of attenuation that are separated by allowed bands where perfect transmission of electrons would occur.     

Nonconservative liquid-phase epitaxy of semiconductors

The kinetics of nonconservative liquid-phase epitaxy of semiconductors with replenishment of the solution in a melt under the action of different applied fields and owing to the internal energy of the system itself are analyzed. The physical-chemical laws governing these processes, which are useful for growing high-quality semiconductor epitaxial layers with given electrophysical and geometric properties, ae determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 51–56, January, 1988.I thank V. N. Lozovskii for useful discussions and interest in this work.     

Molecular beam epitaxy

Molecular beam epitaxy (MBE) is a process for growing thin, epitaxial films of a wide variety of materials, ranging from oxides to semiconductors to metals. It was first applied to the growth of compound semiconductors. That is still the most common usage, in large part because of the high technological value of such materials to the electronics industry. In this process beams of atoms or molecules in an ultra-high vacuum environment are incident upon a heated crystal that has previously been processed to produce a nearly atomically clean surface. The arriving constituent atoms form a crystalline layer in registry with the substrate, i.e., an epitaxial film. These films are remarkable because the composition can be rapidly changed, producing crystalline interfaces that are almost atomically abrupt. Thus, it has been possible to produce a large range of unique structures, including quantum well devices, superlattices, lasers, etc., all of which benefit from the precise control of composition during growth. Because of the cleanliness of the growth environment and because of the precise control over composition, MBE structures closely approximate the idealized models used in solid state theory.

This discussion is intended as an introduction to the concept and the experimental procedures used in MBE growth. The refinement of experimental procedures has been the key to the successful fabrication of electronically significant devices, which in turn has generated the widespread interest in the MBE as a research tool. MBE experiments have provided a wealth of new information bearing on the general mechanisms involved in epitaxial growth, since many of the phenomena initially observed during MBE have since been repeated using other crystal growth processes. We also summarize the general types of layered structures that have contributed to the rapid expansion of interest in MBE and its various offshoots. Finally we consider some of the problems that remain in the growth of heteroepitaxial structures, specifically, the problem of mismatch in lattice constant between layers and between layer and substrate. The discussion is phenomenological, not theoretical; MBE has been primarily an experimental approach based on simple concepts.     

Kinetics of nonconservative liquid phase epitaxy of semiconductors

An expression is obtained for the stationary growth rate for two- and three-component systems in the diffusion mode on the basis of a detailed analysis of the components in the liquid phase and on its boundaries. It is shown that the solution-melt experiences a hydrodynamic flow opposite to the crystallization boundary at a velocity close to the growth rate in value. Also revealed are the singularities of the kinetics of the process in three-component systems that are associated with unbalanced identically directed diffusion fluxes of components forming a stoichiometric semiconducting compound.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 15–19, August, 1988.     

Electron transport in high quality undoped ZnO film grown by plasma-assisted molecular beam epitaxy

High quality ZnO films were grown on c-plane sapphire substrate using low temperature ZnO buffer layer by plasma-assisted molecular beam epitaxy. The film deposited at 720 °C showed the lowest value of full-width at half maximum for the symmetric (0002) diffraction peak of about 86 arcsec. The highest electron mobility in the films was about 103-105 cm²/V s. From temperature-dependent Hall effect measurements, the mobility strongly depends on the dislocation density at low temperature region and the polar optical phonon scattering at high temperature, respectively. Moreover, by obtaining the activation energy of the shallow donors, it was supposed that hydrogen was source of n-type conductivity in as-grown ZnO films.     

Growth kinetics of Si-molecular beam epitaxy

Growth mode, surface morphology, crystal perfection and growth rate of siliconmolecular beam epitaxy films were observed as function of temperature (450°–950°C), silicon flux density (8×10¹⁴–8×10¹⁵cm²/s) and surface orientation (111, 110, 100). Within the varied parameters growth proceeds by the two-dimensional growth mode via the lateral motion of atomic steps originating from the slight misorientation of commercially available substrates (typically 0.25°). Single crystalline films with high lattice perfection and smooth surfaces result from this growth mode. The growth rate — linearly dependent on Si-flux density and independant of temperature and orientation — indicates a condensation coefficient near unity. An atomic step flow model on the basis of the Burton-Cabrera-Frank theory explains this behaviour by mobile adatoms with low activation energy of diffusion.     

Optimal doping control of magnetic semiconductors via subsurfactant epitaxy

"Subsurfactant epitaxy" is established as a conceptually new approach for introducing manganese as a magnetic dopant into germanium. A kinetic pathway is devised in which the subsurface interstitial sites on Ge(100) are first selectively populated with Mn, while lateral diffusion and clustering on or underneath the surface are effectively suppressed. Subsequent Ge deposition as a capping layer produces a novel surfactantlike phenomenon as the interstitial Mn atoms float towards newly defined subsurface sites at the growth front. Furthermore, the Mn atoms that failed to float upwards are uniformly distributed within the Ge capping layer. The resulting doping levels of order 0.25 at. % would normally be considered too low for ferromagnetic ordering, but the Curie temperature exceeds room temperature by a comfortable margin. Subsurfactant epitaxy thus enables superior dopant control in magnetic semiconductors.     

Electron affinity for intrinsic semiconductors

An approach for estimating the electron affinity using the method of dielectric formalism is developed. It is shown that the volume component of the electron affinity is related to the formation of exchange-correlation holes in the valence band. The interaction of an electron with this hole on the surface is responsible for the surface component. The relation obtained agrees satisfactorily with the rather meager experimental data available for semiconductors, and enables the electron affinity to be estimated for polycrystalline semiconductors for which there is no reference data. The calculation is carried out for 14 well-known semiconductors. In the case of metals, the relations obtained give the work function of the electron, which agrees, with a relative error of up to ±30%, with experimental data for the majority of elements.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 76–80, 1982.     

Electron propagation in magnetic semiconductors

A simple decoupling scheme is suggested and analyzed for the problem of the electron propagation in magnetic semiconductors treated in the s-d model. In most important cases, the method is shown to reduce to the Kubo form of the coherent potential approximation. It is then used to assess the role of higher order effects in the red shift of the optical absorption edge.     

Electron transport in disordered semiconductors

The density matrix of an electron in an amorphous semiconductor, written as a double path integral, is averaged over random disorder. For small disorder the golden rule result is recovered. When the mean disorder energy approaches k_BT, the electronic mobility goes to zero signalling disorder induced localization.     

Molecular beam epitaxy and reconstructed surfaces

In the last years molecular beam epitaxy (MBE) has become a very important method to create new materials. Scattering and channeling of high-energy ions is a mass-dispersive, surface-sensitive crystallographic technique, particularly suited for the investigation of epitaxial systems. The combination of both techniques allows new insight into some of the fundamental processes at interfaces and surfaces. As an example we discuss the influence of substrate reconstruction on epitaxial growth. We show for the Si/Ge and Si/Si systems that the reordering of the reconstruction-induced displacements at the substrate surface, a necessary condition for epitaxial growth, is critically dependent on the type of reconstruction: Deposition of Ge or Si on Si(100)2×1 at room-temperature relieves the reconstruction, whereas Si(111)7×7 appears unaffected. This difference is discussed in terms of structural models for these surfaces. We shall also discuss the implications of these results with respect to MBE and, in particular, to Si homoepitaxial temperatures.     

Molecular beam epitaxy growth of quantum devices

The inherent fragility and surface/interface-sensitivity of quantum devices demand fabrication techniques under very clean environment. Here, I briefly introduces several techniques based on molecular beam epitaxy growth on pre-patterned substrates which enable us to directly prepare in-plane nanostructures and heterostructures in ultrahigh vacuum. The molecular beam epitaxy-based fabrication techniques are especially useful in constructing the high-quality devices and circuits for solid-state quantum computing in a scalable way.     

Laser-induced molecular beam epitaxy of group-III nitrides

nitrides by laser-induced molecular beam epitaxy (LIMBE). The requirements for the formation of high-quality, monocrystalline layers are much stronger than those for polycrystalline films. We have modified and improved the conventional pulsed laser deposition. In our process, we use metallic targets in a nitrogen environment instead of ceramic or pressed powder nitrides and we employ picosecond laser pulses with high energy (>1 mJ) and high repetition rate (2 kHz). We have grown GaN, AlN, InN, InGaN and Mg-doped GaN on sapphire (0001).     

Molecular beam epitaxy with synchronization of nucleation

A new MBE technique based on the possibility to maintain undamping RHEED oscillations by periodic increase of the surface supersaturation synchronized with the oscillations is presented. Surface supersaturation may be increased by any way that is suitable for a particular case. The technique has been referred to by the authors as MBE with nucleation synchronization and allows epistructures of any thickness to be grown with in situ control of their properties by RHEED or automatic ellipsometry. It can be applied both to elementary semiconductors and compounds like A_IIIB_V, A_IIB_VI, etc.     

Electron states in diluted magnetic semiconductors

One of the remarkable properties of the II–VI diluted magnetic semiconductor (DMS) quantum dot (QD) is the giant Zeeman splitting of the carrier states under application of a magnetic field. This splitting reveals strong exchange interaction between the magnetic ion moment and electronic spins in the QD. A theoretical study of the electron spectrum and of its relaxation to the ground state via the emission of a longitudinal optical (LO) phonon, in a CdSe/ZnMnSe self-assembled quantum dot, is proposed in this work. Numerical calculations showed that the strength of this interaction increases as a function of the magnetic field to become more than 30 meV and allows some level crossings. We have also shown that the electron is more localized in this DMS QD and its relaxation to the ground state via the emission of one LO phonon is allowed.