Focused Ion Molecular Beam Epitaxy Research Articles

Lateral n–p junction for acoustoelectric nanocircuits

We report the experimental realization of a device comprising a lateral n–p junction grown by focused-ion molecular-beam epitaxy and a transducer to generate a surface acoustic wave. Acoustic charge transport across the junction and the accompanying photon emission are demonstrated. This type of light-emitting diode is suitable for integration into acoustoelectric nanocircuits in which quasione-dimensional semiconductor channels serve as “wires” through which packets of charge are transported by surface acoustic waves. The diode provides a means by which to extend the functionality of acoustoelectric nanocircuits into the optical domain.

Electroabsorption in a GaAs n-i-p-i structure with selective contacts fabricated by Focused Ion Molecular Beam Epitaxy

Optical modulators fabricated using doping superlattice structures have been of interest recently. To enable the optical absorption coefficient of these structures to be modulated by an external electrical bias, selective ohmic contacts to the p and n-type layers are essential. This was first demonstrated by Doehler et al. (Appl. Phys. Lett. 49 (12), 1986), using the technique of shadow masking with Molecular Beam Epitaxial (MBE) growth. They successfully fabricated selective contacts to a GaAs n-i-p-i superlattice, which showed good p-n junction characteristics and strong electroluminescence at room temperature. In this paper, we describe the use of the Focused Ion Molecular Beam Epitaxial (FIMBE) method to fabricate similar structures in a single-step process with minimal ex-situ processing. Using dopant ion beams of Si and Be from a Au Si Be liquid metal ion source, 2mm doped squares were patterned during MBE growth with a 0.5mm lateral overlap. After ex-situ wet etching for mesa definition and evaporation of AuBe and AuGeNi for the p and n-type contacts respectively, the electrical characteristics were measured at room temperature. A turn-on voltage of 0.6V and a reverse saturation current of 10nA was measured in the dark at room temperature. Optical transmission experiments showed a peak change in absorption for 900nm wavelength light when an AC bias from −2V to +2V around a DC bias of 1V was applied to the ohmic contacts.

Independently contacted electron–hole gas heterostructures fabricated with focused ion beam doping during molecular beam epitaxial growth

The successful combination of two mature technologies, molecular beam epitaxy (MBE) and focused ion beams into a single, versatile semiconductor growth system has recently been established. It has been shown that the focused ion MBE (FIMBE) system is able to produce spatially selected doping profiles of high quality material in both bulk-doped GaAs and modulation doped GaAs/AlGaAs heterostructures. With the versatility of a Au–Si–Be liquid metal ion source (LMIS), the range of devices that can be produced with three-dimensional topologies has been greatly increased. One such structure is the closely spaced, electron–hole gas heterostructure, which can be easily fabricated, and independently contacted using the FIMBE method. We have fabricated such structures with different barrier widths between the gases, and studied their low-temperature transport properties at various carrier densities.

Focused ion molecular-beam epitaxy – a novel approach to 3D device fabrication using simultaneous p- and n-type doping

The combination of molecular-beam epitaxial (MBE) growth and in situ focused ion beam (FIB) technology, has attracted considerable research interest in recent years since early studies in this area by Hirayama and Okamoto (Jpn. J. Appl. Phys. 24 (1985) L965) and Miyauchi et al. (J. Vac. Sci. Technol. B 4 (1986) 189) on III–V semiconductor material systems. In this paper, we describe a novel fabrication technique that directly integrates a FIB column into an MBE growth chamber. This “FIMBE” system allows the generation of novel III–V device structures that would otherwise be impossible to fabricate via conventional lithographic techniques. The use of a Au 70Si 15Be 15 liquid metal ion source (LMIS) in our FIMBE system offers for the first time both n and p-type selective-area doping during MBE growth. A mass filtered 2 kV FIB column was used to selectively dope regions of a single GaAs epilayer using both Be and Si, with landing energies of 30 and 25 eV, respectively. High-quality bulk GaAs was grown with 77 K mobilities of 0.3 m 2 V −1 s −1 at a carrier concentration of 5×10 23 m −3 using the Si 2+ species, and 0.065 m 2 V −1 s −1 at a carrier concentration of 1×10 23 m −3 for Be 2+. The epilayer quality is thus comparable to that obtained from conventional thermally doped material. Low-energy ion-doped 2D hole gases (2DHGs) over selective areas have also been fabricated for the first time using 30 eV Be 2+ ions as the dopant species in a GaAs/Al x Ga 1− x As heterostructure. This resulted in a 2DHG with an unilluminated 1.7 K mobility of 8.8 m 2 V −1 s −1 at a carrier concentration of 2.6×10 15 m −2. A Si 2+ ion doped HEMT was also grown, attaining a 1.7 K 2DEG mobility of 60.7 m 2 V −1 s −1 at a carrier concentration of 1.9×10 15 m −2 after illumination. This compares favourably with a thermally doped control structure grown immediately afterwards that attained a 1.7 K 2DEG mobility of 80.7 m 2 V −1 s −1 at a similar carrier concentration after illumination.