Customized nanostructures MBE growth: from quantum dots to quantum rings

Journal of Crystal Growth 251 (2003) 213–217

Customized nanostructures MBE growth: from quantum dots to quantum rings D. Granados*, J.M. Garc!ıa ! Instituto de Microelectronica de Madrid (IMM-CNM-CSIC) Fabricacion y Caracterazacion de Nanostructuras, c Isaac Newton 8 (PTM), 28760 Tres Cantos, Madrid, Spain

Abstract When self-assembled InAs/GaAs(0 0 1) quantum dots (QD) are overgrown by a thin (2 nm) GaAs cap under different growth conditions, morphological changes occur. The effects of growth conditions on the final structural properties are analyzed by atomic force microscopy. Under As4, thin cap deposition at B5401C produces elongated dash-like nanostructures, whereas at 5001C two humps are obtained from each QD. When As2 is used and the thin cap is deposited at 5001C, quantum rings are obtained. Ensemble photoluminescence (PL) spectroscopy and polarization PL at 15 K show drastic changes on confinement properties. Shape control results in PL emission tuning from 1110 nm (dots) to 920 nm (rings). r 2003 Elsevier Science B.V. All rights reserved. PACS: 61.46.+w; 81.16.
c; 81.16.Dn; 78.67.Hc Keywords: A1. Nanostructures; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials

1. Introduction Studies of growth control methods of selfassembled quantum dots (QD) size, for tuning QD optical properties, have particular interest nowadays. Morphological shape-surface changes of molecular beam epitaxy (MBE) grown QD due to thin capping overgrowth with GaAs have been reported by different authors [1–5] as a powerful technique to obtain low dimensional nanostructures. In this work, we show that the final morphological structure and optical properties of QD

overgrown by a thin GaAs cap strongly depend on the details of the growth parameters such as substrate temperature for the capping process (TCAP ) and As type and pressure. Controlling the growth parameters, it is possible to transform the original ensemble of QD into dash-like, camellike or quantum ring (QR) nanostructures. These drastic changes take place for the same amount of deposited material, emphasizing the importance of growth details in the formation of these nanostructures.

2. Growth details and characterization setup *Corresponding author. Tel.: +34918060700; fax: +34918060701. E-mail address: [email protected] (D. Granados).

A home made MBE machine was used to grow two different sets of samples: atomic force

0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(02)02512-5

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microscopy (AFM) samples and photoluminescence (PL) samples. Commercial epiready GaAs(0 0 1) substrates were used. The substrate temperature (T) is carefully calibrated during the growth by determining oxide desorption temperature and surface reconstruction phase transitions by means of reflection high energy electron diffraction (RHEED). QD are grown by MBE deposition of 1.7 InAs monolayers (ML) in pulses of 0.1 ML at 5401C, using an As2 pressure of B3–4  10
6 mbar. RHEED was used to monitor the 2D–3D transition. The QD are annealed at the same temperature for 1 min, to improve size distribution. Low density ensembles (108–109 cm
2) are obtained, which are required for unmistakable AFM morphological characterization. After QD annealing, a 2 nm thin GaAs capping layer is grown at B1 ML/s at different temperatures (TCAP ) and annealed during 1 min; different nanostructures are obtained depending on the TCAP ; and As type and pressure used during the thin cap process. AFM samples are immediately cooled down. PL samples are obtained overgrowing the nanostructures by a thick GaAs layer where the first 20 nm of GaAs are grown at the same TCAP and then T is increased rapidly to 5901C, in order to obtain high-quality GaAs. Contact mode AFM characterization has been performed ex situ in a home made AFM using commercial tips. PL and polarization PL (PPL) are done at 15 K, using the 514.5 nm Ar laser line for excitation and a 0.22 m monochromator with 1200 lines/mm grating (blaze at 750 nm) with a liquid nitrogen cooled Ge diode for detection. For PPL measurements the [1 1 0] direction of the samples is oriented at 451 with respect to the polarization direction of the laser to get both balanced pumping along [1 1 0] and ½1 1% 0 directions and to compensate the effective transfer function of the grating. A Glann–Thomson polarizer at the entrance of the monochromator is used to measure PL intensity along both principal directions (PL110, PL11% 0). Polarization degree (PD) is defined as PD ¼ ðPL110 %
PL110 Þ=ðPL110 % þ PL110 Þ:

3. Results and discussion Fig. 1a shows an AFM image of the initial QD ensemble, with a narrow size distribution (79%) of large QD (LQD), 10 nm high. LQD are necessary to transform QD into the different nanostructures presented in this work. Under As4 (4–5  10
6 mbar), deposition of a 2 nm thin cap at TCAP ¼ 5401C (Fig. 1b) produces from each initial QD, sharp (160  40 nm) dashlike nanostructures, 2 nm high; whereas if the cap is deposited at a lower temperature, TCAP ¼ 5001C; produces two-humped ‘‘camel-like’’ nanostructures (100  50 nm), 2.5 nm high (Fig. 1c). When the growth is performed under As2 flux (3–4  10
6 mbar) at TCAP ¼ 5001C a depleted region in the centre of the initial position of each QD appears and QR (100  90 nm), 1.5 nm high, are formed (Fig. 1d). These morphological changes let us understand more about self-assembled QR formation. Some authors [6,7] have suggested a wetting droplet instability (called de-wetting) of the InAs QD as a possible mechanism for QR formation. Although this would imply a liquid InAs QD which seems to be unexpected, there are experimental [8] and

Fig. 1. AFM images (250  250 nm2) of: (a) QD, (b) QD capped with 2 nm of GaAs at 5401C, (c) QD capped with 2 nm of GaAs at 5001C, (d) QD capped with 2 nm of GaAs at 5001C and using As2.

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theoretical results [9] proving that when a small amount (o2 ML) of InAs is deposited on top of GaAs(0 0 1), the presence of biaxial epitaxial stress leads to a mixture of coexisting elemental and compound stress-free matter, including phases of liquid In and liquid InAs. Moreover, during InAs deposition, it has been observed [10,11] that for a wide range of temperatures (from 1701C to 5201C) there is as much as B50% of liquid InAs which is not contributing to the stress. This material is incorporated in subsequent GaAs cap, forming a graded In1
xGaxAs layer. All these considerations picture the actual wetting layer more like an embedding layer (EL) connecting all the QD. These factors are consistent with cross section scanning tunnelling microscope (XSTM) observations [12]. It has been suggested that the driving force for this ‘‘melting’’ is the elastic (stress/strain) energy [8]. Thus, we expect that when QD are partially covered, and consequently compressed, part of the InAs shaping the QD undergo a phase transition into liquid phase. When cap growth is stopped, before completely covering the QD, an imbalanced state of tensions around the InAs droplet produces ejection of material out of the centre of each QD [6]. It is crucial to take into account that In(Ga)As alloying process is also present. The competition of these two processes at different TCAP can explain our experimental observations. Meanwhile, InAs liquefaction is an almost T independent process, In(Ga)As alloying is strongly T dependent. Although at 5401C compressed InAs is liquid, strong In(Ga)As alloying processes [13] take place at this T; resulting in the formation of an immobile central InGaAs [14], giving rise to dashed-like nanostructures (Fig. 1b). For TCAP ¼ 5001C; Fig. 1c, the migration of Ga atoms into the central region is reduced and In(Ga)As alloying takes place mainly on the outside edge of the QD. De-wetting of the liquid central InAs region leads to the formation of a depleted region surrounded by immobile InGaAs resulting on camel-like shape. Under As4 flux, migration of group III atoms is longer for [1 1% 0] direction, giving rise to asymmetrically elongated nanostructures; indeed, as it is shown in Fig. 1b, for TCAP ¼ 5401C the dash-like nanostructures are

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four times more elongated in the [1 1% 0] direction than in the [1 1 0]. At the same TCAP ¼ 5001C; the use of As2 produces formation of QR (Fig. 1d). The effects of As2 use on the final isotropical distribution of material are discussed next. During GaAs homoepitaxy is well known that the migration distance on both principal directions is different due to an anisotropy on the chemical reactivity of the steps [15]. Meanwhile, arsenic terminated B-steps (perpendicular to [1 1% 0] direction) provide highly active sites for gallium adsorption and gallium terminated A-steps are less reactive to group III migrating atoms. The commonly found elongated structures along [1 1% 0] direction grown by MBE and step flow growth mode in B-type vicinal surfaces are attributed to this chemical reactivity anisotropy. Step flow of Atype facets can be achieved for higher As pressure [15]. If we assume a similar chemical behavior of the original InAs island steps, an increase on the effective As pressure will produce and enhance of the reactivity of A-steps to Ga atoms. The use of As2 has similar effects to employ a higher As4 flux [16]. These morphological changes induce drastic modifications on the confining potential leading to modified energy levels as can be confirmed by means of PL measurements. QD have a highly efficient PL emission both at room (not shown) and low temperatures (LT) (Fig. 2, squares). Interband transitions between ground and first excited state of electrons and holes are observed at low B20 W cm
2 power excitation (Fig. 2, squares). PL spectra of dash-like nanostructure (Fig. 2, circles) is B85 meV blue-shifted with respect to QD emission, accordingly with AFM observation of a vertical reduction of initial QD size. Although camel-like nanostructures are slightly higher (0.5 nm) than dash-like ones, LT PL (Fig. 2, triangles) shows a larger blue-shift emission of the former ones (178 meV). LT PL of QR (Fig. 2, stars) shows the largest blue-shift (220 meV). This larger blue-shift confirms a reduction of the effective vertical confinement at the center of the nanostructure. The polarization dependence of the emitted light for high pumping power on a QD sample is shown in Fig. 3. As the pumping

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Fig. 2. Low temperature PL spectra at Pexc ¼ 20 W/cm2. PL samples are obtained overgrowing the nanostructures appearing in Fig. 1 by a thick GaAs layer.

Fig. 3. QD PPL measurements at 20 K using Pexc ¼ 1000 W/ cm2.

power has been increased with respect to Fig. 2, several transitions between ground and excited states are observed in the PL spectrum. PPL energy dependence and intensity are in excellent agreement with the work of Noda [17], where it is shown theoretically and experimentally that polarization properties of the transitions of slightly asymmetrical QD can be assigned to transitions between excited states of electrons and holes with the same quantum number. Noda et al. explain the positive PPL signal (stronger PL along [1 1% 0]) due to an overlap of the emission modes and

Fig. 4. QR PPL measurements at 20 K using Pexc ¼ 1000 W/ cm2.

predominance of the lower energy states due to band-filling effect [17]. Recently [18] it has been shown that a giant vertical permanent dipole moment with sign opposite to the observed one in QD is present in similar QR nanostructures. This change of the sign of the dipole is associated with the strongly different InAs content profile in the vertical direction. Whereas QD have a highly stressed InAs contents at the top, the QR present the opposite InAs strain distribution, with higher InAs contents at the base. But as this In(Ga)As is asymmetrically distributed around the crater, it is expected that they also present some on-plane permanent dipole which could contribute to the PPL anisotropy. In spite of the asymmetrical shape of the rings, the PPL (Fig. 4) is just 6%, somewhat smaller than the QD PPL value (9%). It also presents a negative (black line) character on the lower energy states, contrary to the observed measurements on QD, suggesting complicated spatial distributions of the electron and hole wave function envelopes. Further studies must be performed.

4. Conclusion Several conclusions on the mechanisms involved during transformation of self-assembled nanostructures via thin GaAs cap can be listed. The

D. Granados, J.M. Garc!ıa / Journal of Crystal Growth 251 (2003) 213–217

highly strained InAs QD are destabilized by deposition of GaAs, driving a redistribution of the material in an imbalanced manner where two matter transport processes compete. Meanwhile, high migration of liquid phase In and InAs enhances mass transport out of the central region of the QD, In(Ga)As alloying effects reduce mobility and trends to avoid the formation of a crater at high substrate temperatures (5401C). QR formation is achieved for low substrate temperatures (5001C). The use of As2 is required for a successfully isotropic redistribution of the InGaAs forming the rim. High QD with more than 7 nm high are necessary to obtain efficiently a depleted central region. This on-will modifications of nanostructures size and shape lead to controlled changes on their energy levels as shown by the drastic changes observed on their optical properties as PL wavelength emission and polarization response.

Acknowledgements We would like to thank Axel Lorke and Richard Warburton for fruitful discussions. This work was partially supported by project number 07T/0062/ ! 2000 of Comunidad Autonoma de Madrid, by Spanish MYCT under project TIC99-1035-C02 and by Nanomat project of the EC Growth Programme, contract no. G5RD-CT-2001-00545.

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