Elsevier

Journal of Crystal Growth

Volume 512, 15 April 2019, Pages 219-222
Journal of Crystal Growth

Droplet etching with indium – Intermixing and lattice mismatch

https://doi.org/10.1016/j.jcrysgro.2019.02.030Get rights and content

Highlights

  • In-LDE is suited for etching of very deep nanoholes.

  • High thermal stability of In-etched nanoholes.

  • Intermixing between etching and substrate material.

  • Integration of In-LDE inside AlGaAs material-system possible.

Abstract

In contrast to Indium droplets the usage of Ga or Al droplets for the local droplet etching (LDE) technique is well established. It has been shown previously that In-droplets or In-alloys can be used for LDE with focus on small hole-depths at low etching temperatures. With our recent experiments we reveal that In-LDE is also well suited for the creation of very deep nanoholes (exceeding 100 nm) and very low densities in the range of 106 cm−2 for large average distances of several micrometers. The unexpected high thermal stability of the etching process itself as well as the formed wall-like structures clearly points to an intermixing between etching and substrate material during the LDE-process. This can be related to the lack of As during LDE. Additional experiments with capping layers also reveal high surface-roughness probably caused by a lattice mismatch, which can be also explained by intermixing. A second LDE step with Al can be applied to avoid roughening effects.

Introduction

Besides the fabrication of nanostructures based on droplet epitaxy [1], [2] or Stranski-Krastanov growth mode [3], [4] also structured templates can be used for the generation of nanostructures like Quantum Dots (QDs) or Quantum Rings (QRs). Aside from classical lithographic processing, which is quite demanding at the necessary resolution, self-assembled methods like local droplet etching (LDE) are an attractive alternative for the template fabrication. Especially because of the high-purity requirements for MBE, which would require recleaning of lithographically structured substrates.

The method of LDE [5] is based on the deposition of a group III component in the absence of As and leads to a formation of droplets on a semiconductor surface. In a second step, these droplets will etch into the substrate material caused by the As-gradient and form holes. The holes are surrounded by a “wall-like” structure formed from the crystallized initial droplet material. Droplet-etched nanoholes can be functionalized in an independent step through, e.g., a partial filling with GaAs and therefore used for the fabrication of strain-free GaAs QDs [6], plasmonic nanostructures [7] or more complex nanostructures like QD-molecules [8]. The low density of 107–108 cm−2 commonly achieved with the LDE technique allows single-QD spectroscopy just by using focused laser excitation. This and the correspondingly small crosstalk between neighboring QDs makes the method especially attractive for future use in single-photon emission [9] in the field of quantum information technologies. The method itself is completely MBE compatible, easy to integrate and widely tunable with respect to structural properties like hole depth and shape, densities or materials.

It was already shown that Ga [4], Al [6] or In [10] can be used for LDE. Here we focus on recent results using Indium for LDE indicating effects of intermixing and lattice mismatch. The usage of Indium seems especially interesting because of the low density as well as the formation of an InAs-wall. In contrast to previous reports we find that In-LDE is also suitable for generation of deep nanoholes.

The depth on the present In-etched nanoholes is extremely high compared to etching with Al or Ga and therefore well suited for the formation of more complex, vertically stacked nanostrucures while maintaining a very low density even at low etching temperatures. Also the formation of InAs-walls seems interesting for the fabrication of QRs with lower emission energies than GaAs and thus emission in the infrared or near infrared. Also the low thermal stability of InAs is assumed to be advantageous for the generation of “wall-free” hole-templates through additional heating/annealing steps.

Section snippets

Experimental

The following segment describes the sample fabrication using molecular beam epitaxy as well as the structural investigations with atomic force microscopy.

Results

In the following segment, the structural data for the In-etched hole-templates are presented and analyzed – first focusing on the attainable hole and wall-properties and the influence of the etching temperature, later on the influence of additional annealing and capping steps indicating intermixing and lattice-mismatch.

Conclusion

We demonstrate that local droplet etching (LDE) with In allows the creation of nanohole templates with very low densities in the range of 106 cm−2 and hole depth of more than 100 nm. The overall size and shape of the holes and surrounding walls can be tuned via etching temperature. Interestingly, even at temperatures of 550 °C and, therefore, already in the InAs evaporation-regime both, hole-etching and wall-formation take place. Additional annealing steps at 680 °C do not change the structural

Acknowledgement

The authors would like to thank S. Schnüll for the sample-growth as well as the “Deutsche Forschungsgemeinschaft” for financial support via HA 2042/8-1.

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    The deposited droplet can then intermix with liquefied In and Ga from the InGaAs buffer, potentially leading to the droplet becoming a dilute InGa alloy. This behavior of the droplet material intermixing with the underlying substrate has been observed in other material systems such as InAs/AlGaAs [16], GaAs/AlGaAs [18], and AlAs/AlGaAs [19]. As such, the In-droplets likely become a high-In-content InGa alloy.

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