Liquíd Phase Epitaxy
In this section an account of the processes used to deposit epitaxial semiconductor films is given. We start with LPE, a process in which melts rather than vapors are in contact with the growing films. Introduced in the early 1960s, LPE is still used to produce heterojunction devices. However, for greater layer uniformity and atomic abruptness, it has been supplanted by CVD and MBE techniques. LPE involves the precipitation of a crystalline film from a super-saturated melt onto the parent substrate, which serves as both the témplate for epitaxy and the physical support for the heterostructure. The process can be understood by referring to the GaAs binary-phase diagram on p. 31. Consider a Ga-rich melt containing 10 at% As. When heated above 950°C, all of the As dissolves. If the melt is cooled below the liquidus temperature into the two-phase field, it becomes supersaturated with respect to As. Only a melt of lower than the original As content can now be in equilibrium with GaAs. The excess As is, therefore, rejected from solution in the form of GaAs that grows epitaxially on a suitably placed substrate. Many readers will appreciate that the crystals they grew as children from supersaturated aqueous solutions essen-tially formed by this mechanism.
Through control of the cooling rates, different kinetics of layer growth apply. For example, the melt temperature can either be lowered continuously together with the substrate (equilibrium cooling) or separately reduced some 5-20 °C and then brought into contact with the substrate at the lower temperature (step cooling). Theory backed by experiment has demonstrated that the epitaxial layer thickness increases with time as /3/í2 for equilibrium cooling and as /l/'2 for step cooling (Ref. 10). Correspondingly, the growth rates or time derivatives vary as r1/2 and t~l/2, respectively. These diffusion-controlled kinetics respectively indicate either an increasing or decreasing film growth rate with time depending on mechanism. Typical growth rates range from ~ 0.1 to 1 ^m/min. A detailed analysis of LPE is extremely compli-cated in ternary systems because it requires knowledge of the thermodynamic equilibria between solid and solutions, nucleation and interface attachment kinetics, solute partitioning, diffusion, and heat transfer. LPE offers several advantages over other epitaxial deposition methods, including low-cost appara-tus capable of yielding films of controlled composition and thickness, with lower dislocation densities than the parent substrates.
To grow múltiple GaAs-AlGaAs heterostructures, one translates the seed substrate sequentially past a series of crucibles holding melts containing various amounts of Ga and As together with such dopants as Zn, Ge, Sn, and Se as shown in Fig. 7-17. Each film grown requires a sepárate melt. Growth is typically carried out at temperatures of ~ 800 °C with máximum cooling rates of a few degrees Celsius per minute. Limitations of LPE growth include poor thickness uniformity and rough surface morphology particularly in thin layers. The CVD and MBE techniques are distinctly superior to LPE in these regards.
Seeded Lateral Epitaxial Film Growth over Insulators
The methods we describe here briefly have been successfully implemented in Si but not in GaAs or other compound semiconductors. The use of melts suggests the inclusión of this subject at this point. Technological needs for three-dimensional VLSI and isolation of high-voltage devices have spurred the development of techniques to grow epitaxial Si layers over such insulators as Si02 or sapphire. In the recently proposed LEGO (lateral epitaxial growth over oxide) process (Ref. 17), the intent is to form isolated tubs of high-quality Si surrounded on all sides by a moat of SiOz. Devices fabricated within the tubs require the electrical insulation provided by the Si02. As a result they are also radiation-hardened or immune from radiation-induced charge effects originating in the underlying bulk substrate. The process shown schematically in Fig. 7-18 starts with patterning and masking a Si wafer to define the tub regions followed by etching of deep-slanted wall troughs. A thick Si02 film is grown and seed windows are opened down to the substrate by etching away the Si02. Then a thick polycrystalline Si layer (~ 100 fim thick) is deposited by CVD methods. This surface layer is melted by the unidirectional radiant heat flux from incoherent light emitted by tungsten halogen are lamps (lamp furnace). The underlying wafer protected by the thermally insulating Si02 film does not melt except in the seed windows. Crystalline Si nucleates at each seed, grows vertically, and then laterally across the Si02, leaving a single-crystal layer in its wake upon solidification. Lastly, mechanical grinding and lapping of the solidified layer prepares the structure for further microdevice processing. Conventional dielectric isolation processing also employs a thick CVD Si layer. But the latter merely serves as the mechanical handle enabling the bulk of the Si wafer to be ground away.
An account of the most widely used VPE methods—chloride, hydride, and organometallic CVD processes—has been given in Chapter 4. Here we briefly address a couple of novel VPE concepts that have emerged in recent years. The first is known as vapor levitation epitaxy (VLE), and the geometry is shown in Fig. 7-19. The heated substrate is levitated above a nitrogen track cióse to a porous frit through which the hot gaseous reactants pass. Upon impingement on the substrate, chemical reactions and film deposition occur while product gases escape into the effluent stream. As a function of radial distance from the center of the circular substrate, the gas velocity increases while the gas concentration profile exhibits depletion. These effects cancel one another, and uniform films are deposited. The VLE process was designed for the growth of epitaxial III-V semiconductor films and has certain advantages worth noting:
1. There is no physical contact between substrate and reactor.
2. Thin layer growth is possible.
3. Sharp transitions can be produced between film layers of multilayer stacks.
4. Commercial scale-up appears to be feasible.
5. The second method, known as rapid thermal CVD processing (RTCVD), is an elaboration on conventional VPE. Epitaxial deposition is influenced through rapid, controlled variations of substrate temperature. Source gases (e.g., halides, hydrides, metalorganics) react on low-thermal-mass substrates heated by the radiation from externa! high-intensity lamps (Fig. 7-19). The latter enable rapid temperature excursions, and heating rates of hundreds of degrees Celsius per second are possible. For IH-V semiconductors, high-quality epitaxial films have been deposited by first desorbing substrate impurities at elevated temperatures followed by immediate lower temperature growth (Ref. 18).
6. Very high quality lattice-matched heteroepitaxial films can be grown by CVD methods. This is particularly true of OMVPE techniques where atomi-cally abrupt heterojunction interfaces have been demonstrated in altemating AlAs-GaAs (superlattice) structures. Only molecular-beam epitaxy, which is considered next, can match or exceed these capabilities.
Epitaxial Film Growth and Characterization
Film Growth Mechanisms
Irrespective of whether homo- or heteroepitaxy is involved, it is essential to grow atomically smooth and abrupt epitaxial layers. This implies a layer growth mechanism, and thermodynamic approaches to layer growth based on surface energy arguments have been presented in Chapter 5. Ideally, the desired layer-by-layer growth depicted in Fig. 7-22 is achieved through lateral terrace, ledge, and kink extensión by adatom attachment or detachment. In this case the new layer does not grow until the prior one is atomically complete. One can also imagine the simultaneous coupled growth of both the new and underlying layers.
In this section we explore the interactions of molecular beams with the surface and the steps leading to the incorporaron of atoms into the growing epitaxial film. Although MBE is the focus, the results are, of course, applica-ble to other epitaxial film growth sequences. The first step involves surface adsorption—the process in which impinging particles enter and interact within the transition región between the gas phase and substrate surface. Two kinds of adsorption—namely, physical (physisorption) and chemical (chemisorption) — can be distinguished. If the particle (molecule) is stretched or bent but retains its identity, and van der Waals forces bond it to the surface, then we speak of physisorption. If, however, the particle loses its identity through ionic or covalent bonding with substrate atoms, chemisorption is involved. The two can be quantitatively distinguished on the basis of heats of adsorption—//^ and Hc, for physisorption and chemisorption, respectively. Typically, Hp ~ 0.25 eVand Hc~ 1-10 eV.
In Situ Film Characterization
This section deals with techniques that are capable of monitoring the structure and composition of epitaxial films during in situ growth. Both LEED and RHEED have this ability. They are distinguished in Fig. 7-25. An ultrahigh-vacuum environment is a necessity for both methods because of the sensitivity of diffraction to adsorbed impurities and the need to elimínate electron-beam scattering by gas molecules. In LEED a low-energy electrón beam (~ 10-1000 eV) impinges normally on the film surface and only penetrates a few angstroms below the surface. Bragg's law for both lattice periodicities in the surface plañe results in cones of diffracted electrons emanating along forward and backscat- tered directions. Simultaneous satisfaction of the diffraction conditions means that constructive interference occurs where the cones intersect along a set of lines or beams radiating from the surface. These backscattered beams are intercepted by a set of grids raised to different electric potentials. The first grids encountered retard the low-energy inelastic electrons from penetrating. The desired diffracted (elastic) electrons of higher energy pass through and, accelerated by later grids, produce illuminated spots on the fluorescent screen.
In RHEED the electrón beam is incident on the film surface at a grazing angle of a few degrees at most. Electrón energies are much higher than for LEED and range from 5 to 100 keV. An immediate advantage of RHEED is that the measurement apparatus does not physically interfere with deposition sources in an MBE system the way LEED does. This is one reason why RHEED has become the preferred real-time film characterization accessory in MBE systems.
Both LEED and RHEED parteras of the (7 X 7) structure of the Si(lll) surface are shown in Fig. 7-26. To obtain some feel for the nature of these diffraction pátterns, we think in terms of reciprocal space. Arrays of reciprocal lattice points form rods or columns of reciprocal lattice planes shown as vertical lines pointing normal to the real surface. They are indexed as (10), (20), etc., in Fig. 7-27. Consider now an electrón wave of magnitude 2ir/X propagating in the direction of the incident radiation and terminating at the origin of the reciprocal lattice. Following Ewald, we draw of sphere of radius 2ir/Xabout the center. A property of this construction is that the only possible directions of the diffracted rays are those that intersect the reflecting sphere at reciprocal lattice points as shown. To prove this, we note that the normal to the reflecting plañe is the vector connecting the ends of the incident and diffracted rays.
Ronellys Flores---CRF---libro the materials science of thin films
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