Figure 32. Gas-phase structure of (H2GaN3)3. The N2 portion of the N3 group is omitted for clarity (adapted from [467]).

impurities. Heteroepitaxial growth (Fig. 33) of wurzite GaN on sapphire was achieved at 650 0C [467].

Although [H2GaN3]3 is a versatile CVD source for GaN, the high reactivity of this molecule, which is a significant advantage with respect to the formation of GaN, requires a careful manipulation of the pure product. The pronounced tendency of this precursor to decompose in an exothermic reaction liberating nitrogen is a typical feature of metal azide chemistry. Nevertheless, it is possible to control the pyrophoric nature of the azide precursors by using ligands capable of intramolecular coordination [468, 476]. Kim et al. have synthesized mononuclear [Et2Ga(N3)-MeHNNH2] as a Lewis acid-base adduct for low-temperature deposition of ^-GaN films on Si(111). The compound decomposes via loss of stable molecular species (Eq. (21)), yielding high-quality GaN films with little contamination [476].

In addition to the gas-phase synthesis, these precursors can be employed for the solution-based synthesis of GaN

Figure 33. High-resolution cross-sectional electron micrograph showing heteroepitaxial growth of wurtzite GaN on sapphire at 650 °C. Reprinted with permission from [467], J. McMurran et al., Appl. Phys. Lett. 74, 883 (1999). © 1999, American Institute of Physics.

nanocrystals. Gladfelter et al. have used a cyclotrigallazane, prepared by the reaction of [GaH3(NMe3)] in supercritical ammonia. This compound dehydrogenates in the solid state to form nanocrystalline GaN. The thermal instability of gallium azides can be used to obtain colloids of GaN. Thermolysis of [Et2Ga(N3)]3, [(N3)3Ga{NMe2(CH2)3}], and [(Et3N)Ga(N3)3] in triglyme has produced nanometric GaN particles [469] (Scheme 18). Furthermore, [(Et3N)Ga(N3)3] has been used to grow nanocrystals of GaN in a regular porous matrix. For this purpose, the mesoporous molecular sieve MCM-41 was impregnated by the precursor solution and heat-treated (500 °C) to achieve a spatially confined nucleation of GaN nanoparticles [469] (Scheme 18).

Scheme 18.

The above examples clearly illustrate the potential of molecular-level design in controlling the vapor pressure and reactivity of the precursor compounds. Furthermore, an appropriate selection of ligands can enforce a neat thermal decomposition mechanism to offer high-purity inorganic materials at low temperatures. Another advantage of suitable liquid precursors is the possibility of growing nanoparticles by spatially restricting the molecular species in ordered channels or pores of mesoporous matrices.

Metal pnictides are best represented by the 13-15 compounds, such as GaAs, and are well known for their electronic and optoelectronic applications. The single-source approach to the preparation of these materials has been intensively investigated and reviewed by Maury [422] and Cowley [464], who have also actively contributed to the field [134, 477-486]. The use of III-V adducts as substitutes for highly reactive group III alkyls by Benz et al. [487, 488] was among the pioneering steps in this direction. Maury et al. have used Lewis acid-base adducts of formulae ClR2Ga ER3 (E = As, P; R and R' = Me or Et) to overcome the problem of the high reactivity and toxicity of conventional dual sources [487, 489]. Films of GaP [490, 491] and InP [491] were successfully grown from the cyclic trimers of general formula [Et2M-P(Et)2]3 (M = Ga, In) with cova-lent metal-phosphorus bonds [487, 492]. Epitaxial growth of GaAs was achieved with ClR2Ga AsR3 (R = Me, Et). The thermal decomposition of two series of molecular precursors was achieved with the general formulae (C6F5)3-n MenGa AsEt3 (n = 0 or 2) and [ClR2Ga AsEt2]2CH2 (R = Me, Et). The relative stabilities of the central M-E bond with respect to peripheral M-ligand and E-ligand interactions was evaluated by two different approaches. The first case deals with the use of Lewis acid-base dative bonds, whose strength can be tuned by means of the more or less electron-donating nature of the alkyl groups, and varied admixture of halogen (Cl) or pseudo-halogen (C6F5) sub-stituents; the rather fragile M-E bonds are a major limitation in this approach, causing loss in volatility and surface mobility, whereas in the second approach complexes with covalent M-E bonds were used [289, 493, 494].

Cowley and Jones have used a variety of single-source organometallics that involve a-bonding between group III and group V elements [16, 487, 495]. They have tested different Ga-As precursors, such as [Me2Ga(^-AsBu2)]2 [484, 485, 487, 495-497], [Me2Ga(^-AsBu2)]2 [487, 487, 498], and [{Ga(AsBu2)}3] [487, 497]. The films deposited with the use of the methyl derivative [Me2Ga(^-AsBu2)]2 were arsenic-deficient [487, 496]. On the other hand, epitaxial films could be obtained with [Et2Ga(^-AsBu2)]2; however, the films exhibited anomalously low Hall mobilities due to the formation of galium islands.

The antimonides of group 13 elements are narrow direct bandgap semiconductors with small bandgaps (Eg (eV) = 1.60 (AlSb), 0.67 (GaSb), 0.16 (InSb)) and high electron mobility that render them very attractive for potential applications in optoelectronic devices. For instance, GaSb is used for the production of light-emitting and light-detecting devices operating in the 2-^m wavelength range (GaSb: 1), field effect transistors (GaSb: 2), and infrared detectors (GaSb: 3). The growth of thin films of group 13 nitrides, phosphides, and arsenides by CVD typically involves group 13 alkyls and group 15 hydrides; however, an analogous process is not possible for the preparation of group 15 antimonides because SbH3is thermally very unstable (decomposes below -60 °C), which is also true for primary (RSbH2) and secondary stibines (R2SbH). Furthermore, an excess of stibine during film growth may lead to the formation of elemental Sb on the substrate, which has to be strictly avoided because of the low volatility of elemental Sb. For these reasons, group 13 trialkyls (R3M) and tri-organostibines (R3Sb) are used as the common precursors (GaAs: 12). However, the inability to produce atomic hydrogen during pyrolysis, as well as the high stability of metal-carbon bonds, favor the incorporation of large amounts of carbon in the resulting material. GaSb has been grown from Sb(NMe2)3, which contains weaker Sb-N bonds [499, 500]. The alternative pathway is based on the use of heterocycles [R2MER2]X that are attractive SSPs, particularly because of their lower metal-carbon bond energies when compared with pure group 13 and 15 alkyls. Schulz et al. have used Bu3Ga-Sb(Bu) and Bu3Ga-Sb(Pr')3 as single-molecule precursors for the synthesis of highly oriented carbon-free GaSb nanocrystals and whiskers [97].

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