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Growth of Inas Quantum Dot Without Introducing Wetting Layer by Alternate Deposition of Inas and Gaas with Quasi-Monolayer

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Growth of Inas Quantum Dot Without Introducing Wetting Layer by Alternate Deposition of Inas and Gaas with Quasi-Monolayer Journal of the Korean Physical Society, Vol. 42, February 2003, pp. S476 S479 ∼ Growth of InAs Quantum Dot without Introducing Wetting Layer by Alternate Deposition of InAs and GaAs with Quasi-monolayer Jong Su Kim∗ and Nobuyuki Koguchi Nanomaterials Laboratory, National Institute for Materials Science, Ibaraki 305-0047, Japan D. Y. Lee and I. H. Bae Department of Physics, Yeungnam University, Kyungsan 712-749 J. I. Lee, Gu-Hyun Kim, S. K. Kang and S. I. Ban Korea Research Institute of Standards and Science, Daejon 305-600 Jin Soo Kim Basic Research Laboratory, Electronic and Telecommunication Research Institute, Daejon 305-350 S. H. Lee School of Electronic and Electrical Engineering, Kyungpook National University, Daegu 702-710 H. K. Choi, Minhyon Jeon and Jae-Young Leem Department of Optical Engineering, Inje University, Kimhae 621-749 The structural and optical properties of non-wetting layer InAs quantum dots (QDs) have been investigated by transmission electron microscopy (TEM), photoreflectance (PR) and photolumines- cence (PL). By alternate depositing 0.83 1.2 ML InAs and 1.2 1.5 ML Ga(Al)As with different period on GaAs surface, the wetting layer∼ of InAs QDs was controlled.∼ TEM images clearly show the formation of QDs by using quasi monolayer (QML) deposition and non-wetting layer of InAs QDs. The QDs formed by using QML could not be grown by Stranski-Krastanov (S-K) growth. In PR measurement, the wetting layer transition is not observed for all the QML QDs. These QML QDs growth mechanisms are explained by adatom migration effect due to surface chemical potential. PACS numbers: 78.66.Fd, 81.05.Ea, 81.15.Hi Keywords: Quantum dot, Quasi-monolayer, InAs, Wettering layer, MBE I. INTRODUCTION device, there is need to isolate each quantum cell for electrically. In case of InAs QDs on GaAs, the self- assembling growth process of InAs islands occurs under Submonolayer deposition technique was demonstrated the Stranski-Krastanov (S-K) mode, which produces 3- to analyze surface energy and initial growth stages of D islands accompanied with a thin wetting layer (WL) highly strained lattice mismatched heteroepitaxial sys- at near the critical thickness to release the accumu- tem such as InAs/GaAs [1]. The starting point of this lated strain energy by large lattice mismatch between the growth technique is introduced in short period super- GaAs and InAs. To isolate each QD for electrically, it lattice system. Many research on quantum dots (QDs) is necessary to control the WL because the carrier mean have focused on optical properties of this structure and free path in the WL is about 30 nm in InAs [6] and the energy level engineering [2{4]. However, in these days, WL has also an important role for the carrier dynamics the interests of growing three-dimensional (3-D) islands in QD systems. as an active medium is enormously increased to real- In this work, we report a novel growth method of ize QD electrical device such as single electron transis- InAs QDs, in which any evidence for existence of WL tor and quantum communication quantum cellular au- has not been observed, in lattice-mismatched system. tomata [5]. However, to realize each QD electrical logic The structural and optical properties of these InAs QDs are analyzed by using transmission electron microscopy ∗E-mail: [email protected] -S476- Growth of InAs Quantum Dot without Introducing Wetting Layer { Jong Su Kim et al. -S477- ··· (TEM), photoluminescence (PL) and photoreflectance (PR). From TEM images and PR spectra, the WL re- lated features were not observed indicating that this kind of growth mode may not provide the WL. In addition, the emission peak position of InAs QDs obtained from PL is 1.32 µm at room temperature, which is quite im- portant region for fiber optic communications. II. EXPERIMENTAL The samples used in the present work were grown by a Riber 32P molecular beam epitaxy (MBE) on semi- insulating GaAs (100) using As4. Two different QDs growth techniques were adopted to investigate the wet- ting layer formation of InAs QDs on GaAs surface. The Fig. 1. Schematics of the conventional QD and quasi one is conventional InAs QDs formation technique us- monolayer deposited QDs structures. ing continuously InAs supply method. The other one is quasi monolayer deposition technique using migration effect. 2.5 ML single InAs QDs (QD1) was grown as a Structural characterization of the InAs QDs was done reference by using conventional method. Using the quasi by TEM measurement. The dot densities in reference monolayer deposition technique, three different samples sample (QD1) and 5 periods QML QD sample (QD2) are 10 2 10 2 found to be 6.5 10 cm− and 3.4 10 cm− , re- were grown. The sample structures are 5 periods 1.5 ML × × GaAs/0.83 ML InAs (QD2), 5 periods 1.2 ML AlAs/ 1.2 spectively. Figure 2(a) and 2(b) show the cross-sectional ML InAs (QD3) and 10 periods 1.2 ML GaAs/1.2 ML TEM images of reference QDs and QML QDs with 5 pe- InAs (QD4). The schematics of sample structures are riods of 1.5 ML GaAs/0.83 ML InAs. A dramatic change shown in Fig. 1. Before depositing the InAs QDs, the in the InAs arrangement can be observed in Fig. 2. First, the lateral size of the QML QDs is about 1.5 times big- substrate temperature was set to 580 ◦C for the growth of GaAs buffer layer and the substrate temperature was ger than that of the conventional QDs. Second, the dark contrast due to the InAs wetting layer in Fig. 2(a) is ob- lowered to 480 ◦C for the QMLs and InAs QDs layer. The formation of QDs was monitored by in situ reflec- served, but we did not observed the InAs wetting layer tion high-energy electron diffraction (RHEED) pattern. in Fig. 2(b). One may speculate that the wetting layer The structural properties of QDs were studied by us- is either absent or negligibly small. A high resolution ing a high-resolution transmission electron microscopy TEM image of QML InAs QD with 10 periods of 1.2 ML (HRTEM). GaAs /1.2 ML InAs, which was observed with (100) re- In order to analyze the wetting layer of QDs, we used flection, is shown in Fig. 2(c). It is apparent that the photoreflectance (PR) measurement. The modulation in dot is more disk or sphere-like than oval or pyramidal, reflectivity was produced by mechanically chopping the in shape. This led them to believe that these dots could laser beam, which created the electron-hole pairs. A 5 not be formed by S-K growth mode. mW He-Ne laser (wavelength with 632.8 nm) chopped Figure 3 shows the PR spectra obtained from the InAs at frequency of 800 Hz was used for this purpose. The QDs on GaAs at room temperature. The PR spectra probe light from a quartz tungsten halogen lamp pass- of conventional QDs (QD1) show two transitions from ing through the monochromater was irradiated on the GaAs due to heavy-hole (HH) and light-hole (LH) split- surface of the samples. The reflected light signal was de- ting [8]. There are also PR spectra below the GaAs tected with an InGaAs photodiode and fed into a Lock- band-edge transition from the InAs wetting layer (WL). In amplifier. The PR signal (∆R/R) was recoded as a From these results, we know that the evidence of wetting function of energy. The schematic diagram for the ex- layer could be interpreted by PR measurement. The PR perimental setup was shown in our previous work [7]. spectra obtained from the QML QD with different InAs In PL measurement, the Ar+ laser with wavelength of QML thickness (0.83, 1.2 ML), barrier material (AlAs, 514.5 nm was used as an excitation source to generate GaAs), and period of QML. There is only one main fea- electron-hole pairs and the temperature range was from ture for each spectrum, that is, from the GaAs layer. The 10 K to 300 K. The luminescent signal was detected with PR features associated with the fundamental band-edge a liquid nitrogen cooled Ge detector. transition of GaAs layer dominate the spectra shown in Fig. 3 and the wetting layer related transitions are not observed. From these PR results, we carefully suggest that InAs QDs grown by alternating QML GaAs and III. RESULTS AND DISCUSSION InAs have no wetting layer. This can be also observed -S478- Journal of the Korean Physical Society, Vol. 42, February 2003 Fig. 3. Photoreflectance spectra of the conventional QD and quasi monolayer deposited QDs. Fig. 2. Cross sectional TEM images of (a) QD1, (b) QD2, and (c) Lattice image of QD4. assume that the surface is separated by two-domain face in TEM images of Fig. 2. The wetting layer formation InAs and Al(Ga)As. The next introduced InAs is de- mechanism in QML deposition technique is explained by posited on the InAs domain as same manner. adatom migration effect due to surface chemical poten- Figure 4 shows PL spectra from reference sample tial. The strain field modulates the surface chemical po- (QD1) and the 10 periods QML QD (QD4) at room tem- tential for Ga adatoms as follows [9]: perature. As shown in Fig. 4, the PL emission peaks Y"2(r)Ω µGa(r) = µGa + t + γΩκ(r) (1) h 0 2 i Ga Here µ0 is the chemical potential of Ga adatoms on the reference flat surface with the lattice parameter equal to that of the bulk GaAs.
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