INDEPENDENT CONTROL OF SIZE AND SHAPE OF GAAS NANOSTRUCTURES DURING DROPLET EPITAXY USING ULTRA-LOW ARSENIC FLUX

Balakirev S.V.; Lakhina E.A.; Kirichenko D.V.; Chernenko N.E.; Shandyba N.A.; Eremenko M.M.; Solodovnik M.S.

i l St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.3 Научно-технические ведомости СПбГПУ. Физико-математические науки. 15 (3.3) 2022

Conference materials UDC 538.9.

DOI: https://doi.org/10.18721/JPM.153.362

Independent control of size and shape of GaAs nanostructures during droplet epitaxy using ultra-low arsenic flux

S. V. Balakirev \ E. A. Lakhina 1H, D. V. Kirichenko \ N. E. Chernenko \ N. A. Shandyba \ М. М. Eremenko \ M. S. Solodovnik 1

1 Southern Federal University, Taganrog, Russia H [email protected]

Abstract. GaAs nanostructures are promising candidates for use in future nanoelectronics and quantum photonics. However, technology of their controllable fabrication with precisely predefined size, shape and surface density still requires further improvement. In this paper, we reveal a possibility to reduce a size of gallium droplets using exposure to the arsenic flux of ultra-low values. The control of size and shape of droplets is implemented independently of their surface density that enables formation of low-density arrays of small-sized quantum dots. Based on droplet arrays with trimodal size distribution, we demonstrate that droplets with larger sizes are less influenced by the low arsenic flux whereas smaller droplets may reduce in volume or decay completely resulting in the formation of nanoholes. The technique under consideration can be used for the fabrication of single quantum dot devices with specified characteristics.

Keywords: droplet epitaxy, GaAs, nanostructures, arsenic flux

Funding: This study was supported by the Russian Science Foundation Grant No. 21-7900310, https://rscf.ru/project/21-79-00310/, at the Southern Federal University.

Citation: Balakirev S. V., Lakhina E. A., Kirichenko D. V., Chernenko N. E., Shandyba N. A., Eremenko M. M., Solodovnik M. S., Independent control of size and shape of GaAs nanostructures during droplet epitaxy using ultra-low arsenic flux. St. Petersburg State Polytechnical University Journal. Physics and Mathematics, 15 (3.3) (2022) 315—319. DOI: https://doi. org/10.18721/JPM.153.362

This is an open access article under the CC BY-NC 4.0 license (https://creativecommons. org/licenses/by-nc/4.0/)

Материалы конференции УДК 538.9.

DOI: https://doi.org/10.18721/JPM.153.362

Независимое управление размером и формой наноструктур GaAs при капельной эпитаксии с помощью ультрамалого потока мышьяка

С. В. Балакирев 1, Е. А. Лахина 1Н, Д. В. Кириченко 1, Н. Е. Черненко 1, Н. А. Шандыба 1, М. М. Еременко 1, М. С. Солодовник 1

1 Южный федеральный университет, Таганрог, Россия н [email protected]

Аннотация. В данной работе демонстрируется возможность уменьшения размера капель галлия за счет воздействия на них потока мышьяка ультрамалых значений. Управление размером и формой капель осуществляется независимо от их поверхностной плотности, что позволяет формировать массивы квантовых точек малого размера с низкой поверхностной плотностью. Основываясь на массивах капель с тримодальным распределением по размерам, мы демонстрируем, что капли большего размера в меньшей степени подвержены влиянию малого потока мышьяка, в то время как капли меньшего размера могут уменьшаться в объеме или полностью распадаться, приводя к формированию наноуглублений.

Ключевые слова: капельная эпитаксия, GaAs, наноструктуры, поток мышьяка

Финансирование: Исследование выполнено за счет гранта Российского научного фонда № 21-79-00310, https://rscf.ru/project/21-79-00310/, в Южном федеральном университете.

Ссылка при цитировании: Балакирев С. В., Лахина Е. А., Кириченко Д. В., Черненко Н. Е., Шандыба Н. А., Ерёменко М. М., Солодовник М. С. Независимое управление размером и формой наноструктур GaAs при капельной эпитаксии с помощью ультрамалого потока мышьяка // Научно-технические ведомости СПбГПУ. Физико-математические науки. 2022. Т. 15. № 3.3. C. 315-319. DOI: https://doi.org/10.18721/ JPM.153.362

Статья открытого доступа, распространяемая по лицензии CC BY-NC 4.0 (https:// creativecommons.org/licenses/by-nc/4.0/)

Introduction

Semiconductor nanostructures, including A3B5 quantum dots, have recently attracted increasing attention due to the intensification of the transition of micro- and optoelectronic devices to principles based on quantum effects [1]. GaAs/AlGaAs quantum dots have proven themselves particularly well as potential quantum light sources because of their negligible fine structure splitting and short exciton lifetimes [2]. However, quantum dots in lattice-matched systems are impossible to obtain via the traditional Stranski-Krastanov growth mechanism [3]. An alternative method that has been widely used recently is a droplet epitaxy which makes it possible to form Ga droplets and then transform them into GaAs nanostructures independently in two main stages [4].

In order to fabricate high-efficiency emitters of single photons and entangled photon pairs, single quantum dots are usually required [5], which can be realized by the formation of low-density arrays of nanostructures with their further division into separate device elements [6]. A difficulty lies in the fact that achieving a low surface density requires a high substrate temperature leading to the droplet enlargement or a decrease in the amount of deposited material (deposition thickness) [7]. However, the deposition thickness cannot be reduced to near-zero values because of the presence of a critical thickness of droplet formation which increases with decreasing temperature [8, 9].

Previously, we demonstrated that large In droplets obtained at temperatures providing their low surface density can be reduced in volume due to the phenomenon of diffusion decay under a low arsenic flux [10, 11]. In this paper, we reveal the same process concerning Ga droplets on the GaAs(001) surface. Although nanorings are observed at the place of original perimeters of droplets, droplets shrink by almost 1.3 times, which can be extended by further alterations of technological parameters. We also demonstrate that small droplets reduce in volume more intensively than large droplets. This fact can be used as well to provide a flexible control of the droplet size with their surface density kept at the same level.

Materials and Methods

SemiTEq STE35 molecular beam epitaxy equipment was used to grow samples on epi-ready GaAs(001) substrates. Calibration of the growth rates and in situ monitoring of the growth processes were carried out using a reflection high-energy electron diffraction system.

After standard procedure of the native oxide removal at a substrate temperature (T) of 600 °C under an As4 pressure P = 4-10-5 Pa, 250 nm of GaAs buffer layer was grown at T = 580 °C with a growth rate of 1 monolayer (ML) per second. Then, a deposition temperature T = 500 °C was set on the substrate in the absence of the arsenic vapor in the growth chamber.

After the background pressure was reduced below 2-10-7 Pa, 3 equivalent ML of gallium was deposited on the GaAs surface at a nominal growth rate of 0.25 ML/s. Then, the substrate temperature was decreased to a value TULF = 300 °С at which the exposure of droplets to the ultra-low arsenic flux was carried out. The flux values corresponded to increments ДР of the pressure in the chamber which ranged from 3-10-8 to 5-10-7 Pa. An additional exposure to the flux

increment AP = 6-10-8 Pa at TULF = 400 °C was carried out on one of the samples. In 5 minutes after closing the arsenic valve, each of samples was transferred out of the chamber and delivered to FEI Nova Nanolab scanning electron microscope (SEM) and NT-MDT NTEGRA atomic force microscope (AFM) for the nanostructure characterization.

Results and Discussion

It is well-known that Ga droplets formed on the Ga(Al)As(001) surface can be converted into GaAs dots [12] or transformed into GaAs rings and ring-hole complexes [13] when exposed to the arsenic flux. However, a very low arsenic flux which is traditionally used for droplet etching of the surface [14] can lead to a different phenomenon at a relatively low temperature, namely the diffusion decay of droplets [10, 11]. This phenomenon consists in the outflow of atoms from the droplet due to the metal concentration gradient between the droplet and the surface enriched in arsenic. If the rate of this event is higher than the rate of crystallization or etching, the droplet can reduce in volume before its arsenization while the surface density of droplets retains its initial value. As one can see in Fig. 1 and 2, an increase in the arsenic pressure increment AP from 3-10-8 to 5-10-7 Pa leads to a decrease in the average diameter of droplets obtained after deposition of 3 ML of gallium on GaAs surface from 71 nm to 56 nm.

a) b)

300 nm I

300 nm

Fig. 1. SEM images of nanostructures obtained after deposition of 3 ML of gallium on the GaAs surface and subsequent exposure to the low arsenic flux at TULF = 300 °C: AP = 3-10-8 Pa (a), AP = 6-10-8 Pa (b), AP = 5-10-7 Pa (c), AP = 1-10-6 Pa (d)

A further increase in the pressure increment AP to 1 10-6 leads to a larger change in the droplet diameter as a percentage of the original droplet diameter (26% for the sample with AP = 1-10-6 and 24% for the sample with AP = 5-10-7), determined by the droplet boundaries or the diameter of the crystallized ring retained around the droplet. Although the change in the droplet size is not very significant, it can be increased by various techniques including the alterations of exposure time and flux values, substrate temperature during the exposure, multistage exposure with interruptions etc. The surface density of droplets in a range of the pressure increments from 3-10-8 to 1-10-6 Pa is around 4.5-108 cm-2.

It is also worth noting that the average diameter of droplets obtained after deposition of 3 ML of gallium and exposed to the ultra-low arsenic flux AP = 6-10-8 Pa at TlLF = 300 °C have a trimodal distribution (Fig. 3, a, c) with one peak corresponding to large droplets and two small peaks related to smaller droplets appearing on the surface due to the secondary nucleation [10]. The peak positions representing the average diameter of droplets of each group are at 37, 49 and 69 nm (Fig. 3, c).

The exposure of droplets to the arsenic flux at a higher temperature of 400 °C leads to the complete decay of smaller droplets with their transformation into rings and holes whereas larger droplets retain on the surface (Fig. 3, b). The peak position of large droplets shifts from 69 to 64 nm (Fig. 3, d), which indicates a decrease of the droplet size as a result of the arsenic exposure.

Fig. 2. Arsenic pressure dependences of the average diameter of nanostructures obtained after deposition of 3 ML of gallium on the GaAs surface at T = 500 °C and subsequent exposure to the low arsenic flux at Tttt „ = 300 °C

ULF

Droplets of smaller sizes transform into nanorings with a peak position of their diameter at 50 nm.

Within some of the rings, small holes with an average diameter of 14 nm are also observed (Fig. 3, b, d). Holes are formed on the surface because of the droplet etching which is a typical process occurred at relatively high temperatures and low arsenic fluxes [15]. However, large droplets reduce in volume without the ring and hole formation which indicates that droplets with near-critical sizes are more influenced by the arsenic flux. This phenomenon can be used to decrease the surface density of nanostructures after initial formation of high-density droplet arrays.

Fig. 3. SEM images (a, b) and histograms of size distributions (c, d) of nanostructures obtained after deposition of 3 ML of gallium and subsequent exposure to the ultra-low arsenic flux AP = 6*10-8 Pa

at: (a, c) TULF = 300 °C; (b, d) TULF = 400 °C

Conclusion

We reported, for the first time, the possibility of reducing the gallium droplet size using the ultra-low arsenic flux to obtain low-density arrays of small-sized nanostructures. The droplet diameter decreases with increasing arsenic flux whereas the surface density remains approximately at the same value. Droplet arrays obtained after deposition of 3 ML of gallium at T = 500 °C with subsequent exposure to the arsenic flux are shown to have a trimodal size distribution. We revealed that gallium droplets of a large size exposed to the arsenic flux at TULF = 400 °C reduce in volume or transform into dot-and-ring complexes while droplets with smaller sizes decay completely leaving behind nanorings or nanoholes formed because of the droplet etching.

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THE AUTHORS

BALAKIREV Sergey V. SHANDYBA Nikita A.

[email protected] [email protected]

ORCID: 0000-0003-2566-7840 ORCID: 0000-0001-8488-9932

LAKHINA Ekaterina A. EREMENKO Mikhail M.

[email protected] [email protected]

ORCID: 0000-0002-9326-2418 ORCID: 0000-0002-7987-0695

KIRICHENKO Danil V. SOLODOVNIK Maxim S.

[email protected] [email protected]

ORCID: 0000-0001-7476-2778 ORCID: 0000-0002-0557-5909

CHERNENKO Natalia E.

[email protected]

ORCID: 0000-0001-8468-7425

Received 19.07.2022. Approved after reviewing 27.07.2022. Accepted 31.07.2022.

INDEPENDENT CONTROL OF SIZE AND SHAPE OF GAAS NANOSTRUCTURES DURING DROPLET EPITAXY USING ULTRA-LOW ARSENIC FLUX Текст научной статьи по специальности «Физика»

Аннотация научной статьи по физике, автор научной работы — Balakirev S.V., Lakhina E.A., Kirichenko D.V., Chernenko N.E., Shandyba N.A.

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