Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines.
KeywordsQuantum dots Molecular beam epitaxy Droplet etching Photoluminescence Atomic force microscopy
Crystalline semiconductor quantum dots (QDs) can be regarded as artificial atomic-like entities, which intrigue from a fundamental point of view . But semiconductor QDs are also very attractive for device applications where QDs turned out to be superior to bulk material. This has been demonstrated for instance by the first QD-based laser that exhibits a lower threshold current density compared to QW lasers . Further advanced applications for QDs are proposed, such as qubits in quantum computing  or single-photon sources in quantum cryptography [4, 5].
Quantum dot fabrication techniques that are based on self-assembling mechanisms during epitaxial growth allow the integration of QD layers into semiconductor heterostructures. In this field, a very prominent example is strain-induced InAs QDs grown on GaAs in the Stranski–Krastanov mode [6–9]. A further interesting method for self-assembled QD generation is the droplet epitaxy in Volmer–Weber mode. The method was first demonstrated by Koguchi and Ishige  in 1993. In comparison with the Stranski–Krastanov technique, droplet epitaxy is more flexible regarding the choice of the QD material. For instance, the fabrication of strain-free GaAs QDs [11–13], InGaAs QDs with controlled In content [14, 15], and InAs QDs  has been demonstrated.
During droplet epitaxial QD fabrication , first liquid metallic droplets are generated on semiconductor surfaces, e.g., by Ga deposition without As flux. The growth temperature T = 100–350° typically is kept very low compared to usual MBE growth conditions. After Ga droplet formation, an As pressure is applied in order to crystallize the droplets and transform them into GaAs QDs. Interestingly, deposition of Ga droplets on GaAs at significantly higher temperatures T = 450–620° results in the formation of deep nanoholes in the substrate surface. This effect was first observed by Wang et al.  in 2007 and represents a local removal of material from semiconductor surfaces without the need of any lithographic steps. As an important advantage compared to conventional lithography processes, this local droplet etching (LDE) is fully compatible with usual MBE equipment and can be easily integrated into the MBE growth of heterostructure devices. LDE was demonstrated in addition on AlGaAs [19, 20] and AlAs  surfaces as well as etching with InGa [19, 22–24] and Al  droplets.
After droplet etching, the nanohole openings are surrounded by walls that are crystallized from droplet material and may act as quantum rings [19, 22–25]. The crystallization of the walls  and the time evolution of the transformation from the initial droplets into nanoholes with wall  were studied in previous publications. A first functionalization of the nanoholes, the fabrication of a novel type of very uniform, strain-free GaAs QDs by filling of LDE nanoholes in AlGaAs with GaAs, has been demonstrated . In the present paper we describe the influence of the LDE process and sample design on the optical properties of such GaAs QDs.
Local Droplet Etching and Nanohole Filling
We fabricate LDE nanoholes using solid-source molecular beam epitaxy (MBE) on (001) GaAs wafers. Two different sample designs will be discussed in the following, denoted as type I and type II. After growth of a GaAs buffer layer, a 200-nm-thick Al0.36Ga0.64 As barrier layer was deposited. For the samples of type II, an additional 5-nm-thick AlAs layer was grown before LDE. Type I samples have no such AlAs layer. Afterward, the As shutter and valve were closed and droplet formation was initiated at a temperature T1 by opening the Al shutter for a time t1 = 6 s. We used Al droplets for etching in order to avoid an additional carrier confinement by the wall. The temperatures were T1 = 620° for the type I samples and T1 = 650° for the type II samples with the additional AlAs layer. During this stage, a strongly reduced arsenic flux is important . The As flux in our experiments was approximately hundred times lower compared to typical GaAs growth conditions. The Al flux F corresponded to a growth speed of 0.47 ML/s, and droplet material was deposited onto the surface with coverage θ = F t1. After droplet deposition, the temperature was set to a value T2, and a thermal annealing step of time t2 was applied in order to remove liquid etching residues. For the present samples, we have used T2 = T1 and t2 = 180 s.
Optical Properties of LDE QDs
The local droplet etching of nanoholes in semiconductor surfaces represents a powerful new degree of freedom for the design of novel semiconductor heterostructures and devices. This method allows to tune the structural properties over a wide range by adjusting the materials and the process parameters. Self-assembled quantum dots are created by filling of nanoholes in AlGaAs with GaAs. Dependent on the sample design and the LDE process parameters, these QDs show either broadband optical emission or discrete sharp lines. Broadband light sources are very attractive because of their wide range of applications, which include fiber-optic gyroscopes, fiber-optic sensors, optical coherence tomography, and wavelength-division multiplexing transmission . On the other hand, self-assembly of strain-free quantum dots with very uniform size distribution may help to overcome some limitations of the widely used Stranski–Krastanov InAs QDs.
The authors would like to thank the “Deutsche Forschungsgemeinschaft” for financial support via SFB 508 and GrK 1286.
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