Quantum dots (QDs) are unique nano structures that have received special attentions in recent years because these are strong candidates for advanced semiconductor quantum devices. From recently reports, 3-dimensional (3D) islands (between InAs QDs and 1ML high InAs 2-dimensional (2D) islands) were formed during the QD formation [1, 2]. But, the precise mechanism for forming the 3D islands was not well understood yet. In this report, we observed the InAs 3D islands grown by the STMBE system [3] which equipped with a scanning tunneling microscope (STM) inside a molecular beam epitaxy (MBE) growth chamber, performing true in situ imaging during the MBE growth [4].
 An experimental procedure is as follows. We used undoped-GaAs(001) just substrates. After forming a GaAs buffer layer at 580 ^oC, a substrate temperature was decreased from 580 ^oC to 500 ^oC under an As₄ irradiation. When the substrate temperature was stabilized, we started in situ STM observation with the As₄ irradiation (a tip bias was -1.0 V, a tunneling current was 0.6 nA, and a scan speed was 6000 nm•s^-1). A tip scanned from left to right and moved from bottom to top of the image. Fig.1(a) show a STM image of GaAs buffer layer before supplying In and Fig.1(b) shows a line profile indicated as a white line. During the observation, we grow 2.6 ML InAs (growth rate was 2.8×10^-3 MLs^-1). Fig.2(a) show a STM image of InAs WL during In supply and Fig.2(b) shows a line profile indicated as a white line. The amount of the InAs supply at the bottom of Fig.2(a) was 0.18 ML and that of its upper part was 1.27 ML. As the amount of the InAs increased, the number and volume of the InAs 2D island gradually increased.  Moreover, a InAs 3D island (height: 0.4 nm, widhth: 20 nm) on the 2D island shown in Fig.2(b). InAs 3D islands appeared from the early stage of InAs supply (0.18 ML ~). The density of the 3D islands was same extent regardless of the amount of the InAs supply shown as Fig.3.

Fig.1 STM image and line profile. (a) STM image of before InAs supply, (b) line profile indicated as a white line.

Fig.2 STM image and line profile. (a) STM image of during InAs supply, (b) line profile indicated as a white line.

Fig.3 Densities of InAs 3D islands on GaAs(001) as a function of InAs supply.

[1] D.Tex and I.Kamiya, Phys. Rev. B. 83, 081309 (2011).
[2] T.R.Ramachandran, R.Heitz, P.Chen and A.Madhukar, Appl. Phys. Lett. 70, 640 (1997).
[3] S.Tsukamoto and N.Koguchi, J. Cryst. Growth 201/202, 118 (1999).
[4] S.Tsukamoto, T.Honma, G.R.Bell, A.Ishii, and Y.Arakawa, Small 2, 368(2006).

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