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Liquid phase epitaxy (LPE) is a mature technology and has been used in the production of III-V compound semiconductor optoelectronic devices for some 40 years.
LPE has also been applied to silicon, germanium, SiC, and II-VI and IV-VI compound semiconductors, as well as magnetic garnets, superconductors, ferroelectrics, and other optical materials.
Many semiconductor devices including light-emitting diodes (LEDs), laser diodes, infrared detectors, heterojunction bipolar transistors and heterointerface solar cells were pioneered with LPE.
Further, LPE can produce epitaxial semiconductor layers of superior material quality with respect to minority carrier lifetime and luminescence efficiency.
Nevertheless, LPE has fallen into disfavor in recent years, especially for device applications requiring large-area uniformity, critical layer thickness and composition control, and smooth and abrupt surfaces and interfaces.
For making superlattices, quantum wells, strained layer structures, and heterostructures with large lattice mismatch or heterostructures comprised of materials with substantial chemical dissimilarity
(e.g. GaAs-on-silicon), LPE is often dismissed out of hand in favor of other epitaxy technologies such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD).
One might conclude that despite the long and venerable history of LPE, progress has stagnated with few prospects for new applications.
In particular, LPE suffers from widely held perceptions of poor reproducibility, and intrinsic difficulties in scale-up for large-area substrates and high-throughput operation.
Nevertheless, and as the chapters in this book describe in detail, on-going efforts and new developments in LPE continue to widen its scope of applications and circumvent its customary limitations.
New modes of liquid phase epitaxial growth can provide novel device structures. Some of the traditional shortcomings of LPE as mentioned above are addressed by new techniques and approaches based on novel melt chemistries, alternative methods of inducing growth
(e.g. through imposed temperature gradients, electric currents, mixing, Peltier cooling, or solvent evaporation), hybrid processes that combine LPE with other methods of epitaxy, and LPE on structured or masked substrates.
Further, unique attributes that distinguish LPE from competing semiconductor epitaxy technologies (MBE and MOCVD) enable it to serve important niches in semiconductor device technology.
To underscore some of the relative or unique advantages of LPE for the growth of semiconductor devices, a few of the more important features of LPE may be listed, and include:
1. High growth rates. Growth rates in the range of 0.1–1µm min−1 can be achieved with LPE. This is 10 to 100 times faster than MBE or MOCVD, and thus thick device structures are feasible with LPE.
2. The wide range of dopants available with LPE. Virtually any element added to the melt will be incorporated into the epitaxial layer to some finite degree.
Most of the periodic table can be utilized as dopants in LPE, and thus, LPE is an excellent tool for fundamental doping studies.
3. The preferential segregation of deleterious impurities to the liquid phase that results in low background impurities in the epitaxial layer. LPE can produce semiconductor material of extremely high purity.
4. The low point defect densities due to near-equilibrium growth conditions and/or favorable chemical potentials of crystal components in the liquid phase.
5. The absence of highly toxic precursors or by-products.
6. Low capital equipment and operating costs.
7. The feasibility of selective epitaxy and epitaxial lateral overgrowth (ELO) with high aspect ratios.
8. The ability to produce shaped or faceted crystals for novel device structures.
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