Selective epitaxy

An understanding of gas-phase and surface chemistry is particularly important to the next generation of MOVPE processes involving selective epitaxy [18] and atomic layer epitaxy (ALE) [19]. In the first process, the compound semiconductor is deposited selectively on substrate areas opened in a suitable masking material (e.g., SiOz). This is achieved by operating under conditions where nucleation occurs only on the substrates. Slight variations in processing environment and the presence of impurities can cause nucleation on the mask and result in loss of selectivity. 

New directions of SiC LPE include (1) multilayer structure growth with the layers thinner than 0.1 mm, (2) growth of epitaxial layers with the Nd-Na concentration less than 1015cm 3, (3) low temperature (1000 - 1200 °C) LPE from alternative melts, (4) selective epitaxial growth, (5) heteropolytype epitaxy, and (6) SiC-AIN p-n structure growth. 

Tseng, H.-C., Chang,. C.Y, Pan, F.M., Chen, J.R., and Chen, L.J. (1997). Effects of isolation materials on facet formation for silicon selective epitaxial growth, Appl. Phys. Lett. 71, 2328-2330. 

A powerful feature of wet etching is the abiUty to achieve excellent etch selectivities of one material over another. This can be extremely useful in the fabrication of epitaxial devices with different material layers. Because selective etching allows the removal of specific layers, the final accuracy of the etch can approach that of the epitaxial layers. Etch selectivities of >100 1 have been achieved for citric acid H202 etching of GaAs—AlGaAs and InGaAs—InP stmctures (133). 

ALLOYS, PURE silicon) and in epitaxial siUcon deposition (see Electronic materials Integrated circuits Semiconductors) as selective reducing agents as monomers and as elastomer intermediates (see Elastomers, SYNTHETIC). Not least is the use of these materials as intermediates for production of other silanes and sihcones. 

For construction of suitable samples molecular beam epitaxy was selected, the method of choice for the production of complicated epitaxial layer systems with different materials. As substrates Si wafer material (about 20x20 mm-, thickness 1 mm) and SiO, discs (diameter 30 mm, thickness 3 mm) were used. Eight layered structures (one, two and three layers) were built up with Al, Co, and Ni, with an indicated thickness of 70 nm, each. 

A ZnSe-on-GaAs epitaxial layer required a sensitive survey of near-surface contamination. PAI was selected for ZnSe analysis because its major constituents and many of the expected impurities are elements that have poor ion yields in conventional LIMS. Figures 8 and 9 are two mass spectra acquired from the ZnSe epitaxial layer. 

High catalytic activity and selectivity of silicalite-l/H-ZSM-5 composites must be caused by the direct pore-to-pore connection between H-ZSM-5 and silicalite-l as revealed by Fe-SEM and TEM [43]. The silicalite-l crystals were epitaxially grown on the surface of the H-ZSM-5 crystals. 

In fact, different techniques revealed cadmium segregation and decrease of the Pb/Se ratio near the InP/PbSe interface, indicating that during the first steps of deposition a CdSe layer is formed on InP prior to the PbSe growth. It was suggested that selective adsorption of Cd(0) on the InP surface gives rise to an epitaxial CdSe monolayer, which facilitates an ordered PbSe growth on account of the small lattice mismatch (0.7%) at the CdSe/(rock salt)PbSe interface. Importantly, it was found... 

The primary goal of the researchers has been to produce Q-dots possessing all of the attributes of the Q-dots prepared using liquid-phase synthetic methods (that is adjustability of the nanocrystal identity and diameter and size monodispersity) and also the technological utility of Q-dots prepared by MBE (specifically, the deposition of nanocrystals with a defined orientation and an electrical output contact). It was shown that the E/C-synthesized 5-CuI and CdS Q-dots were indeed epitaxial with narrow size distribution and strong photoluminescence tunable by the particle size. Qne of the advantages of the E/C method is that it can be made size selective. The key point is that the size as well as the size dispersion of product nanoparticles are directed actually by the corresponding properties of the metal nanoparticles therefore the first deposition step assumes special importance. 

Gorer S, Ganske JA, Hemminger JC, Penner RM (1998) Size-selective and epitaxial electrochemical/chemical synthesis of sulfiir-passivated cadmium sulfide nanocrystals on graphite. J Am Chem Soc 120 9584-9593... 

The kinetics and mechanisms of the C —> G transition in a concentrated solution of PS-fr-PI in the PS-selective solvent di-n-butyl phthalate was studied [137,149]. An epitaxially transformation of the shear-oriented C phase to G, as previously established in melts [13,50,150], was observed. For shallow quenches into G, the transition proceeds directly by a nucleation and growth process. For deeper quenches, a metastable intermediate structure appears, with scattering and rheological features consistent with the hexag-onally perforated layer (PL) state. The C -> G transition follows the same pathways, and at approximately the same rates, even when the initial C phase is not shear-oriented. 

The electrochemical atomic layer epitaxy (ECALE) technique, also known as electrochemical atomic layer deposition (EC-ALD), is based on layer-by-layer electrodeposition. Each constituent of the thin him are deposited separately using underpotential deposition (UPD) of that element. UPD is a process wherein an atomic layer of one element is deposited on the surface of a different element at a potential under that needed to deposit the element on itself. ECALE has been used to grow mainly II-VI and III-V compounds. A thorough review of ECALE research has been published by Stickney.144 A summary of the materials deposited using ECALE are given in Table 8.4, with a more detailed discussion for a few select examples given below. 

In this chapter we discuss the measurement and analysis of simple epitaxial stractures. After showing how to select the experimental conditions we show how to derive the basic layer parameters the composition of ternaries, mismatch of quaternaries, misorientation, layer thickness, tilt, relaxation, indications of strain, curvature and stress, and area homogeneity. We then discuss the hmitations of the simple interpretation. 

Polymorph selectivity can be achieved through two-dimensional epitaxy which allows efficient screening of substrates for polymorph control through geometric lattice modeling prior to performing experiments with actual libraries. (Adapted from Mitchell et ah, 2001)... 

---