X. W. Lin

Synthesis of GaN by N ion implantation in GaAs (001)

Both the hexagonal and cubic GaN phases were synthesized in GaAs (001) by 50 keV N ion implantation at 380 °C and subsequent furnace annealing at 850–950 °C for 10 min–2 h. For a fluence of 1.5×1017 cm−2, transmission electron microscopy revealed that cubic GaN epitaxially crystallizes as precipitates in the GaAs matrix. A cubic‐to‐hexagonal GaN phase transition was observed for extended thermal anneals. By increasing the N fluence to 3×1017 cm−2, a continuous buried layer of randomly oriented hexagonal‐GaN nanocrystals was produced.

Sequential phase formation by ion‐induced epitaxy in Fe‐implanted Si(001). Study of their properties and thermal behavior

The epitaxial growth of FeSi2 silicides was studied by using ion‐beam epitaxial crystallization (IBIEC) of Fe‐implanted Si(001) samples. By employing Rutherford backscattering/channeling spectrometry and transmission electron microscopy it was possible to determine that the IBIEC process produces a γ‐, α‐, and β‐FeSi2 phase sequence, with increasing Fe concentration along the implantation profile. The critical concentrations for γ→α and α→β phase transitions are 11 and 21 at. %, respectively. A study of the thermal behavior of these phases shows that the γ‐ and α‐FeSi2 are metastable with respect to the β‐FeSi2 phase. The γ to β‐FeSi2 transition starts at 700 °C via an Ostwald ripening process. In addition a 800 °C, 1 h anneal of high Fe concentration samples produces a complete α and γ to β‐FeSi2 transformation. Finally, it is demonstrated that a regular or a rapid thermal annealing on Fe‐implanted Si samples induces only the formation of a β‐FeSi2 phase.

Morphological transition of InAs islands on GaAs(001) upon deposition of a GaAs capping layer

The interaction between a GaAs cap and InAs islands grown on vicinal GaAs(001) has been studied by transmission electron microscopy and atomic force microscopy. Samples were prepared by molecular beam epitaxy at 480 °C. Upon GaAs cap deposition, it was found that the previously grown InAs islands undergo a novel type of morphological transition, i.e., a transition from disk‐shaped to ring‐shaped islands. InAs becomes depleted or entirely absent in the central area of what had been a disk‐shaped InAs island. The GaAs cap was also shown to be virtually absent within the same central region, resulting in the formation of crater‐like surface depressions.

COARSENING AND PHASE TRANSITION OF FESI2 PRECIPITATES IN SI

FeSi2 precipitates were produced in Si(001) wafers by an ion‐beam induced epitaxial crystallization process and subsequently annealed at temperatures in the range 650–900 °C. The resulting precipitate coarsening and phase transition were studied by transmission electron microscopy. The coarsening process basically involves the evolution of plate‐shaped precipitates. The lengthening rate of the precipitates is considerably greater than the thickening rate, because the two broad faces of a plate are coherent or semicoherent, while the plate edges are incoherent. The lengthening kinetics was shown to be volume‐diffusion controlled and obey a cube power law. The corresponding activation energy was determined to be 3.55 eV, in excellent agreement with the value predicted by the classical Ostwald ripening model. In contrast, we demonstrated that the thickening process is interface controlled, which involves the migration of the interfaces via a ledge mechanism. Accordingly, an apparent activation energy of 2.18...

Ge/Si heterostructures grown by Sn‐surfactant‐mediated molecular beam epitaxy

Ge/Si heterostructures were grown on Si (001) by Sn‐submonolayer‐mediated molecular beam epitaxy (MBE) and characterized by a variety of techniques, in order to study the behavior of Sn surfactant during Ge and Si growth and its influence on Ge/Si interface quality. It was found that Sn strongly segregates to the growing surface of both Ge and Si and that the presence of Sn surfactant can effectively suppress Ge segregation into a Si overlayer and enhance the surface mobility of adatoms. These results suggest that Sn‐mediated epitaxy can be used as a viable method to produce Ge/Si superlattices, with an interface quality superior to those grown either by conventional MBE or with other types of surfactants.

Metallurgy of Al–Ni–Ge ohmic contact formation on n‐GaAs

Al–Ni–Ge ohmic contacts on n‐GaAs were prepared by sequential vapor deposition and furnace annealing at 500 °C. The metallurgical properties of the contacts were studied by transmission electron microscopy. It was found that while Al–Ni–Ge as a whole is relatively stable against GaAs, extensive interfacial reactions readily occur within the contact layers, resulting in a very stable layered structure of the type Al3Ni/Ni–Ge/GaAs, with e′‐Ni5Ge3 being the major phase in the Ni–Ge layer. GaAs twins and Ni–As precipitates were found in a thin layer immediately below the metallization, suggesting that the ohmic behavior can be accounted for in terms of a GaAs regrowth mechanism.

Sequential phase formation by ion-induced epitaxy in fe-implanted si(001)

Ion‐beam‐induced epitaxial crystallization (IBIEC) of Fe‐implanted Si(001) was studied by transmission electron microscopy and Rutherford backscattering spectrometry. For sufficiently high Fe doses, it was found that IBIEC at 320 °C results in sequential epitaxy of Fe silicide phases in Si, with a sequence of γ‐FeSi2, α‐FeSi2, and β‐FeSi2 with increasing Fe concentration along the implantation profile. The critical concentrations for the γ‐α and α‐β phase transitions were determined as ≊11 and 21 at. % Fe, respectively. The observed sequential phase formation can be correlated to the degree of lattice mismatch with the Si matrix and the stoichiometry of the silicide phases.

Molecular beam epitaxy of InAs and its interaction with a GaAs overlayer on vicinal GaAs (001) substrates

GaAs/InAs/GaAs heterostructures were grown by molecular beam epitaxy on vicinal GaAs (001) substrates. Effects of substrate misorientation on the early stage of InAs epitaxy, as well as the interaction between InAs and a GaAs overlayer, were studied by transmission and scanning electron microscopies and by photoluminescence measurements. The formation of InAs islands were observed after a few monolayer InAs deposition. Two major results were obtained in this study: (a) Upon deposition of a crystalline GaAs overlayer, InAs islands undergo a novel type of morphological transition, i.e., from disk‐shaped to ring‐shaped ones. (b) Substrate misorientation results in anisotropic effects on InAs island formation. In comparison with on‐axis or [110] tilted samples, substrate misorientation toward [110] by up to 5° leads not only to reduction in InAs island density by a factor of 2, but also to the formation of InAs quantum dots. These results were found to be consistent with photoluminescence experiments.

Comparison Between Ion-Beam and Thermal-Annealing Induced Solid Phase Epitaxy in Fe-Implanted Si

Rutherford backscattering spectrometry and transmission electron microscopy were used to compare thermally induced solid phase epitaxy (SPE) with ion-beam induced epitaxial crystallization (IBIEC) of Fe-implanted Si (001). It was found that thermal annealing leads to both Si SPE and β-FeSi 2 precipitation at 520°C, but has no visible effect at 320°C. In contrast, Si SPE and FeSi 2 precipitation occur at both 320 and 520°C, when ion irradiation is introduced. The precipitates grow epitaxially as γ-FeSi 2 at 320°C, but consist of both β-FeSi 2 and γ-FeSi 2 at 520°C. It was also found that thermal annealing at 520°C results in Fe segregation toward the surface, while IBIEC basically retains the as-implanted Fe profile.

Effects of Al2O3 cap on the structural and electrical properties of Au/Te/Au contacts on an n‐type GaAs substrate

Au/Te/Au contacts to n‐type GaAs were prepared by sequential vapor deposition and subsequently annealed at temperatures in the range 420–480 °C. The structural and electrical characteristics of the contacts were characterized by transmission electron microscopy and current–voltage measurements. We found that the electrical behavior of the contacts depends dramatically on whether they are covered with an Al2O3 cap during annealing. While a cap‐annealed Au/Te/Au contact remains rectifying, annealing without the cap results in ohmic behavior. In conjunction with the observed structural properties, this phenomenon can be understood in terms of the doping model for ohmic contact formation.