Tuning the structural properties of InAs nanocrystals grown by molecular beam epitaxy on silicon dioxide

Journal of Crystal Growth 321 (2011) 1–7

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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Tuning the structural properties of InAs nanocrystals grown by molecular beam epitaxy on silicon dioxide Moı¨ra Hocevar a,n, Gilles Patriarche b, Abdelkader Souifi a, Michel Gendry c a

Institut des Nanotechnologies de Lyon, UMR-CNRS 5270, INSA de Lyon, 7 avenue Jean Capelle, 69621 Villeurbanne Cedex, France CNRS-Laboratoire de Photonique et Nanostructures, Route de Nozay, FR-91460 Marcoussis, France c Institut des Nanotechnologies de Lyon, UMR-CNRS 5270, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully Cedex, France b

a r t i c l e i n f o

abstract

Article history: Received 20 October 2010 Received in revised form 17 January 2011 Accepted 24 January 2011 Communicated by H. Asahi Available online 1 February 2011

We report on InAs nanocrystals (nc-InAs) grown on silicon dioxide (SiO2) by solid-source molecular beam epitaxy. We show that the growth parameters influence the properties of the nc-InAs in terms of density, size, and crystallinity. The growth temperature influences mainly the density of the nc-InAs, whereas their size can be controlled by the number of deposited InAs monolayers. Using an adequate set of parameters, we show that the nc-InAs properties are tunable in a range where the crystal structure presents zero defects. These nc-InAs grown on SiO2 have high crystalline quality, making them perfectly suitable for advanced electronic devices. & 2011 Elsevier B.V. All rights reserved.

Keywords: A1. Nanocrystals A1. Desorption A1. Diffusion A3. Molecular beam epitaxy B1. Silicon Dioxide B2. Semiconducting III–V materials

1. Introduction The control of the matter at the nanoscale has been a warhorse for material scientists in the past decade, and concerns especially semiconducting nanocrystals that are subject of fundamental and practical interest. These nanocrystals are mainly studied for their potential application in future quantum devices for electronics, optics, and opto-electronics. For example, three-dimensional confined semiconducting nanocrystals embedded in silicon dioxide (SiO2) in metal-oxide-semiconductor (MOS) devices have been reported to show good memory properties as compared with conventional two-dimensional poly-silicon embedded in SiO2. Although silicon and germanium nanocrystals have been mostly studied since 1995 for non-volatile memory (NVM) applications [1], we demonstrated that the use of InAs nanocrystals (nc-InAs) embedded in SiO2 improves the retention properties of the memory devices [2]. Normally, nc-InAs are grown on III–V semiconductor substrates such as GaAs and InP. Some authors report nc-InAs grown on Si [3], but very few studies concern the integration of nc-InAs in SiO2. In 2001, Zheng et al. [4] have demonstrated the integration of nc-InAs and nc-InAsxP1 x in SiO2 by radio-frequency magnetron cosputtering. Yang et al. [5] have used a sol–gel process in order to

n

Corresponding author. E-mail address: [email protected] (M. Hocevar).

0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.01.067

obtain pure nc-InAs in SiO2 gel glasses. However, both techniques are not fully compatible with standard Si technologies, mainly because they do not meet the requirements for the control of the nc-NVM main parameters: the tunnel oxide thickness, both nanocrystal density and size, and the control oxide thickness. Our theoretical report [2] indicated a required diameter of minimum 8 nm to optimize data retention in MOS based NVMs. To achieve this goal, we studied the influence of the growth parameters on nc-InAs grown on SiO2 in a solid source molecular beam epitaxy (ss-MBE) reactor. Here we show the control of the density, and the size and the crystalline quality of the nc-InAs. The growth parameters we have mainly studied are the growth temperature TG, the arsenic pressure PAs, and the number of deposited InAs monolayers MLInAs. Finally, we determined the growth parameters for which the nanocrystal size and crystalline properties are optimal for memory applications.

2. Experimental details The substrates used in this study are p-type Si (1 0 0) wafers covered by a few nanometer-thick thermal SiO2 layer. The nc-InAs were grown using a ss-MBE reactor from Riber with valved cracking cells for arsenic. The InAs growth rate was kept constant at 0.22 mm/h using a constant beam equivalent pressure of In. Reflection high energy electron diffraction (RHEED) was used to

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follow in real time the growth of nc-InAs. The growth parameters we have studied are the growth temperature (TG) in the 290–530 1C range, the arsenic beam equivalent pressure (BEP) in the 0.1–1.6  10  5 Torr range, and the number of deposited InAs monolayers (MLInAs) in the 2–16MLInAs range. At the end of the growth, the samples were rapidly cooled down under arsenic pressure. On some samples, a SiO2 control layer was deposited at low temperature in an electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) reactor coupled with the MBE reactor. Properties of nc-InAs such as density, diameter, and height have been analyzed with atomic force microscopy (AFM) imaging. Complementary studies on the crystallinity and the chemical composition of the nanocrystals have been carried out with transmitted electron microscopy (TEM) coupled with energy dispersive X-ray spectrometry (EDX) detectors.

of nc-InAs was determined by energy dispersive X-ray spectrometry (EDX) along the dashed line crossing the nanocrystals. The spectra confirm that the nanocrystals contain both arsenic and indium, and are mostly InAs nanocrystals.

30 nm

O

SiO2 nc-InAs

As

Si 5 nm

In 3. The crystalline properties of nc-InAs grown on SiO2 Initial observations of InAs growth on SiO2 occurred on RHEED patterns. The SiO2 surface initially exhibited a diffuse pattern representative of its amorphous structure (Fig. 1(A)). Whatever the growth conditions be, well-defined ring patterns appear within the first instants as InAs grows. These rings are characteristics of polycrystalline InAs (Fig. 1(B)). The RHEED technique reveals the crystallinity of a population of nanocrystals but not of a single one; therefore microscopic imaging and chemical analysis are required to characterize the structural properties of single nc-InAs. Fig. 1(C) shows one plan view high resolution TEM (HRTEM) image of few nc-InAs grown on SiO2. Each nanocrystal is monocrystalline but the orientation differs from one to another. This is totally understandable since the amorphous nature of SiO2 cannot force any epitaxial relation between InAs and the SiO2 surface and subsequently orientate them in the same direction. As an evaluation of nc-InAs integration in devices, nc-InAs were embedded in a 20 nm-thick SiO2 layer with a PECVD reactor allowing SiO2 deposition at low temperature (200 1C) without any thermally related loss of arsenic species. Fig. 2(A) presents a cross-sectional high angle annular dark field (HAADF) TEM image of the nanocrystals embedded in SiO2. The chemical composition

2.12 Å 1.82 Å

3.48 Å

1Å

Fig. 2. Chemical and structural characterizations of the nc-InAs embedded in SiO2. (A) Cross-sectional HAADF image of the nc-InAs. The EDX scan is realized along the horizontal dashed line. The spectra of the oxygen (gray), arsenic (white), and indium (black) are displayed with arbitrary units. (B) Cross-sectional HRTEM image of a single nc-InAs embedded in SiO2. (C) Diffraction pattern of the whole sample.

10 nm

Fig. 1. RHEED patterns: (A) before the InAs growth, the RHEED presents a diffuse pattern of the silicon dioxide and (B) after the InAs growth, the RHEED presents polycrystalline rings of nc-InAs. (C) Plan-view HRTEM image of nc-InAs grown on amorphous SiO2.

M. Hocevar et al. / Journal of Crystal Growth 321 (2011) 1–7

The atomic layers of the nc-InAs are clearly observable on a cross-sectional HRTEM image (Fig. 2(B)) and oriented in the same direction. Each nanocrystal is monocrystalline. The Fourier transform of the image allows the precise measure of the interreticular distance, which is 0.345 70.007 nm [6]. We assumed this distance to be the (1 1 1) family planes of a cubic structure. The lattice parameter obtained is close to InAs crystallizing in the zinc-blend structure. The difference between this value, 0.59870.013 nm, and the theoretical value, 0.605 nm, of the lattice parameter of InAs can be explained by (1) The Fourier transform is obtained from a reduced number of atomic layers and the measure could be imprecise. (2) The embedding of the nc-InAs deforms them slightly by applying a compressive hydrostatic strain.

To infirm or confirm these hypothesis, we analyzed the nanocrystals by electronic diffraction to get a statistical and global measure of the lattice (Fig. 2(C)). The presence of the Si substrates allows a precise evaluation of the diffraction constant. The pattern exhibits three diffraction rings corresponding to the reticular planes (1 1 1), (2 2 0), and (3 1 1) of the InAs, which indicate a random orientation of the nanocrystals. As few other spots are visible, we assume that the nanocrystals are in majority nc-InAs. We extracted from the measure of the rings the interreticular distances that are 0.348 nm (1 1 1), 0.212 nm (2 2 0), and 0.182 nm (3 1 1). The precision is 70.001 nm. Some discrepancy exists between different spots of the same ring. However, it is possible to estimate a lattice parameter in the 0.599–0.603 nm range. These values are closer to the nominal value of bulk InAs. The hypothesis we made about a possible compressive strain during the embedding of the nanocrystals is thus valid, given the low discrepancy between the theoretical and the experimental values of the lattice parameter.

(1)

4. Influence of TG on the structural properties of the nc-InAs Crystal growth is a thermally activated process, thus nc-InAs morphology and properties are expected to depend on the growth temperature. Indeed, in a previous work we have shown that the growth temperature strongly influences the nanocrystals density [6]. We highlighted the existence of a critical growth temperature at around 420 1C, which defines two different regimes: (1) the low temperature regime, in which the growth is controlled by diffusion processes and (2) the high temperature regime, in which the growth is controlled by desorption processes. 4.1. Density, size, and volume Fig. 3 gathers the nc-InAs properties deduced from AFM imaging as a function of the growth temperature in the 290–530 1C range. For this study PAs was kept at a constant value of 4  10  6 torr and MLInAs was 4 ML. Fig. 3(A) shows that below 420 1C, the nc-InAs density (Dnc) slightly decreases from (7.771.1)  1011 to (1.671.1)  1011 cm  2, whereas above 420 1C, the density strongly decreases until (9.071.0)  108 cm  2 for TG ¼530 1C. The diffusion length of the indium adatoms on the thermal SiO2 can be determined from the nanocrystal density at the beginning of the nucleation process. Determining the diffusion length ld is of main importance to understand the physical interactions between adatoms and a surface. We used ld defined by

ld ¼ ðDnc Þ1=2 This relation is valid for diffusion limited 3D island growth [8]. We measured the density of the nanocrystals for growth temperatures of 350 and 410 1C when depositing 1MLInAs (at this

10

(2) hNC (nm)

DNC (cm-2)

1012 1011 1010

3

(1)

(2)

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TC

109

0 300

400 TG (°C)

300

500

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500

25 106 VNC (nm3/m2)

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(2)

5 300

TC

104

400 TG (°C)

500

(1)

102 1.2

1.4 1.6 1/TG (x10-3 K-1)

1.8

Fig. 3. Evolution of the nc-InAs properties with the growth temperature TG: (A) density, (B) height, (C) diameter, and (D) volume occupied by the nc-InAs (Arrhenius plot). The other growth parameters were kept constant at MLInAs ¼4 and PAs ¼ 4  10  6 Torr.

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stage, we assume all the nuclei are formed). We calculated ld E11 nm for a growth temperature of 350 1C (Dnc ¼7.7  1011 cm  2) and ld E38 nm for a growth temperature of 410 1C (Dnc ¼7.0  1010 cm  2). Note that the diffusion length of indium adatoms on thermal SiO2 at 410 1C is close to the one calculated in the same way for InAs quantum dots grown on InP at 530 1C [9]. We can conclude that, under the same conditions, indium adatoms diffuse longer on an oxide such as SiO2 than on a III–V semiconductor such as InP. In Fig. 3(B) and (C) are plotted the measured height (hnc) and diameter (dnc) of the nc-InAs versus the growth temperature. Below the critical temperature, the height and the diameter of the nanocrystals increase up to 5.570.9 and 1973.2 nm, respectively, whereas above 420 1C, hnc and dnc decrease until 2.570.6 and 15.3 72.6 nm, respectively. In Fig. 3(D) is plotted the volume occupied by the nanocrystals (Vnc). We observe that in the diffusion regime, the volume occupied by the nanocrystals is constant, as the increase in their size is compensated by the decrease in the density. In the regime controlled by desorption processes, Vnc strongly decreases as both the size and the density of the nanocrystals decrease.

4.2. Structural properties in the diffusion regime

are 3.870.4 nm in height and 6.370.6 nm in diameter. At 400 1C, the nc-InAs height and diameter are 5.971.3 and 9.8 72.0 nm, respectively. At this temperature, the contrast between nc-InAs and the amorphous SiO2 surface is higher than at 350 1C, which is consistent with a higher atomic density of the nanocrystals and a better crystallinity. The nc-InAs shape is also better defined when they are grown at higher temperature, indicating a better surface diffusion of the adatoms at a higher temperature and thus a better structural organization. Finally, we observed that the nanocrystals are hemispherical, whereas after embedding they present a circular shape (Fig. 2(B)). This change in the shape of the nanocrystals could be due to a slightly faster oxidation in the angles of the nanocrystals during SiO2 deposition. The value of the contact angle tells us a lot about the chemical interactions between materials grown on a surface. As suggested in the insets of Fig. 4, no wetting layers are present on both samples grown at 350 and 410 1C. It is thus possible to extract experimentally the contact angle y of the nc-InAs on a surface of thermal SiO2. The measure is a bit imprecise as uncapped nanocrystals oxidize with air as soon as they are removed from the UHV equipment. The angle measured on several nanocrystals is about (90 720)1, as shown in Fig. 4(C). Using the Young–Dupre´ equation applied for nanocrystals [10]: cos y ¼ ðgs gi Þ=gnc

Fig. 4 presents plane view TEM images of two uncapped samples with nc-InAs grown at 350 1C (a) and 400 1C (b) (with 4MLInAs and PAs ¼4  10  6 Torr). The nanocrystals are clearly visible on the light SiO2 matrix. As shown on the cross section views of the samples (see insets Fig. 4(A) and (B)), for both temperatures we did not notice any wetting layer, indicating a Volmer–Weber growth mode. The nanocrystals grown at 350 1C

100 nm

100 nm

10 nm

with gs, gi, and gnc, the surface energy of the substrate, the interface, and the nanocrystal, respectively, allowing the estimation of the interface energy between the nanocrystal and the thermal SiO2. Using y ¼(90720)1, gs(SiO2)¼450 mJ/m2, and gnc(InAs) ¼650 mJ/m2, we find an InAs/SiO2 interface energy of 4507220 mJ/m2. This value is much higher than the Ge/SiO2 interface energy (150 mJ/m2) obtained for nc-Ge grown on SiO2 [10]. The Ge nanocrystals wet better the SiO2 surface than the nc-InAs. We can note that these calculated values (for Ge or InAs) are relatively high and therefore representative of a difficult wetting of Ge and InAs on SiO2 and thus of the observed Volmer–Weber growth mode. To complete the description, we can notify that below 400 1C, we observed some In2O3 nanocrystals among the nc-InAs (see HRTEM image and Fourier transform of one nanocrystal in Fig. 5). Such nc-In2O3 have been already observed among the ncInP grown on SrTiO3 substrates at low growth temperatures [7]. We think that the same mechanisms take place during the growth of nc-InAs on SiO2 surface. Therefore, the incoming indium adatoms can either react with As atoms present at the surface, or with O atoms of the SiO2 surface. The In–O complexes diffusing on the SiO2 surface can nucleate and participate to the nc-In2O3 growth, which number is relatively low as few oxygen atoms are available on the SiO2 surface. The nc-InAs would start to grow afterwards, as soon as all the possible nc-In2O3 are grown. Above 400 1C, no nc-In2O3 was found on our sample: we assume that the In-O species desorb at these temperatures.

5. Influence of the arsenic pressure PAs on the structural properties of the nc-InAs

70°

110°

Fig. 4. Plan-view TEM images of uncapped nc-InAs grown on SiO2 at 350 1C (A) and 410 1C (B). The insets on the bottom show cross-sectional view of the samples (size of the picture 25 nm  200 nm). (C) Cross-sectional HRTEM image of a single nc-InAs and the corresponding contact angles.

The influence of PAs during the InAs growth on the density and size of the nc-InAs has been studied both in the diffusion regime (at 385 1C) and in the desorption regime (at 440 1C) for MLInAs ¼4. At 385 1C, an increase in the PAs from 2  10  6 to 1.6  10  5 Torr leads to a slight increase in the height of the nanocrystals from 4.571 to 5.5 71 nm, whereas the density is almost constant at a maximum value of 1.5  1011 cm  2 (Fig. 6(A) and (B)). On the contrary, in the desorption regime at 440 1C, the effect of the PAs is more effective. The nc-InAs density increases from 7.6  109 to

M. Hocevar et al. / Journal of Crystal Growth 321 (2011) 1–7

5

5.530 5.56

5.48

5 nm-1

2 nm

Fig. 5. (A) Plan-view HRTEM image of one nc-In2O3 and (B) the Fourier transform of the image allowing the measurement of inter-reticular distances.

1011

1010

hNC (nm)

DNC (cm-2)

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385 °C

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440 °C

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8

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106

104 385 °C 102

440 °C 0

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PAS (10-6 torr)

Fig. 6. Evolution of the nanocrystal properties with PAs at 385 1C (full square) and 440 1C (empty circle): (A) density, (B) height, (C) diameter, and (D) volume occupied by nc-InAs. The number of deposited InAs monolayers was kept constant at 4 ML.

6.2  1010 cm  2 and the height increases from 3.370.6 to 4.871.0 nm. While the nanocrystal diameter seems to be constant with PAs in the diffusion regime, a slight increase is observed in the desorption regime when PAs is increased (Fig. 6(C)). We will note however that AFM, which enlarges the lateral dimension, do not allow to measure accurately the diameter of the nanocrystals. Consequently to the increase in density and size, the equivalent volume increases also with PAs (Fig. 6(D)). Concerning Stranski– Krastanov III–V quantum dots grown by MBE, the pressure of the V elements (PV) influences the III elements migration and consequently the dot density [11]. Generally, an increase in PV results in a drop of the diffusion length of III element adatoms, and thus in an increase in the dot density. In this case of Volmer–Weber type InAs dots grown on SiO2, the PAs has no evident effect in the diffusion regime at 385 1C. As density and size rise being observed with PAs increase in the desorption regime at 440 1C, we think this is due to a decrease of the indium desorption and not to a decrease of the indium diffusion length. From these results, PAs

seems to have a limited influence on the nc-InAs properties. It is then possible to use a constant PAs during the nc-InAs growth while tuning their structural properties with TG and MLInAs. 6. Influence of MLInAs on the structural properties of nc-InAs 6.1. Density, size, and volume The influence of MLInAs on the characteristics of the nc-InAs was carefully studied for two different temperatures in the diffusion regime (TG ¼350 and 410 1C), as at these temperatures Vnc is constant (Fig. 3(D)). Fig. 7(A)–(D) presents the evolution of Dnc, of the size (hnc and dnc), and of Vnc, with an increase in MLInAs. These values were extracted from 1 mm  1 mm AFM pictures taken just after the growth of each sample. For this study PAs was kept at a constant value of 4  10  6 Torr. The density (Fig. 7(a)) does not depend on MLInAs at 350 1C. Dnc is typically (5.571.0)  1011 cm  2 whatever be the MLInAs. On

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1012

hNC (nm)

DNC (cm-2)

10

1011 TG = 350°C 1010

5

TG = 410°C 0 0

4

8 MLInAs

12

16

0

4

8 MLInAs

12

16

25 106 VNC (nm3/m2)

dNC (nm)

20 15 10

104

TG = 350°C TG = 410°C

5 0

4

8 MLInAs

12

16

102

0

4

8 MLInAs

12

16

Fig. 7. Evolution of the nc-InAs properties with MLInAs at 350 1C (full square) and 410 1C (empty circles): (A) density, (B) height, (C) diameter, and (D) volume occupied by the nc-InAs. The As partial pressure was kept constant at 4  10  6 Torr.

the contrary, at 410 1C, Dnc increases with MLInAs from 7.5  109 cm  2 for 1MLInAs to 2  1011 cm  2 for 16MLInAs. Concerning the size, it increases with MLInAs for both temperatures (Fig. 7(B) and (C)). At 410 1C, the nc-InAs height increases from 1.870.4 nm for 1MLInAs to 9.4 72.0 nm for 16MLInAs, and the diameter increases from 1273.1 nm for 1MLInAs to 20.874.5 nm for 16MLInAs. Consequently, the nc-InAs equivalent volume increases with MLInAs (Fig. 7(D)). According to the theory of nucleation and growth kinetics, the density of stable nucleı¨, and (less accurately) of nanocrystals, is expected to increase with the deposition time, and to level off at a saturation value. In this regime, and before coalescence, the size of the stable nucleı¨ linearly increases with the growth time, the slope depending on the growth temperature [12]. The results on the nc-InAs density dependence with increase in growth time are coherent with the nucleation and growth kinetics theory. At low growth temperature, the density reaches almost immediately a saturation value, here (5.5 71.0)  1011 cm  2. In this case, the nucleation rate is almost infinite as desorption processes are not dominant. At higher growth temperatures, the nucleation rate increases with a lower slope, as evaporation of adatoms can also occur. As a conclusion, the height of the nc-InAs can be tuned with the growth time from 2 to 9 nm in the saturation regime where the density is constant. 6.2. Structural properties The majority of the nc-InAs present crystalline structures whatever be the MLInAs. However, a careful study of the TEM images highlighted structural defects and growth anomalies appearing with MLInAs increase. These defects are mainly ‘‘coupled nanocrystals’’, coalesced nanocrystals, and twins. Fig. 8(A) shows two nanocrystals grown independently and appearing ‘‘coupled’’. Additional defects of the nanocrystals are twins. These defects appear in two ways. First, nanocrystals growing close-by can end

up touching each other. The plane bonding the nanocrystals is a twin (Fig. 8(B)). Twins can also be observed in a ‘‘single’’ nanocrystal. In this case, the twins form intrinsically in the nanocrystal (Fig. 8(C)). Partial dislocations associated to twins are catastrophic defects for nanocrystal based memory devices, as they can serve as trap for carriers and, as in two-dimensional layers, they reduce strongly the free carrier concentration. Here for 4 ML, they are relatively isolated ( o1%), but the number of twins increases with the increase of MLInAs, which makes possible the coalescence of nanocrystals. We attempted to quantify the number of ‘‘defective’’ nanocrystals versus their size from HRTEM images taken from different samples. In total we have looked at 200 nanocrystals. The objective is to find out the limit size for monocrystalline and defect free nc-InAs grown on SiO2. Fig. 8(D) displays three statistical bar charts: the bottom one gives the distribution of diameter of 0-defect nanocrystals, the middle one, the distribution for 1-defect nanocrystals, and the upper chart displays the distribution of diameters for observed nanocrystals with two or more visible defects. As expected, the frequency of faulty nanocrystals increases with their size. As the diameter increases, coalescence or coupling can occur. According to Fig. 8(D), below a critical diameter of 6 nm, the nanocrystals have no defects. However, above 6 nm, the frequency of faulty nanocrystals increases. In conclusion, growing monocrystalline and defect-free nc-InAs on SiO2 are only possible until 6 nm (4MLInAs at 350 1C). In order to get larger nanocrystals, engineering the growth sequence becomes mandatory to prevent coalescence and nanocrystal coupling.

7. Conclusion We extensively studied the influence of the growth parameters of nc-InAs on SiO2. By this study, we defined the best way to tune the structural properties of nc-InAs grown on SiO2 in

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Frequency

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0 15

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10 5 0

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2 and more

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Fig. 8. Plan-view HRTEM images of defects encountered in the nc-InAs/SiO2. (A) Coupled nc-InAs. (B) Two coalesced nc-InAs. (C) Single twinned nc-InAs (growth parameters: TG ¼ 410 1C, PAs ¼ 4  10  6 Torr and MLInAs ¼ 8). (D) Frequency of observed faulty nanocrystals versus their diameter (bottom ¼0-defect, middle ¼ 1-defect, and top¼ 2 or more defects). We observe a critical diameter of 6 nm above which one or more defects appear.

terms of density, size, and crystalline properties. We demonstrated that nc-InAs have good crystalline quality, although they are randomly oriented on the SiO2 surface. The density and the size can be both tuned on a large scale. On the one hand, the density is controlled with the growth temperature on a scale ranging from 108 to 1012 cm  2, and on the other hand, the size is controlled with the growth time on a scale ranging from 2 to 10 nm. We showed that perfectly monocrystalline nanocrystal samples are limited in size (6 nm). Finally, we advice to use growth temperatures just below TC ¼410 1C, for the highest densities and better crystallinity, as these parameters influence the quality of nanocrystal based devices, especially nanocrystal based memory devices.

Acknowledgements The authors would like to gratefully thank J.-B. Goure, C. Botella, P. Regreny, A. Descamps, and D. Albertini for technical supports. This work has been partly funded by Re´gion RhˆoneAlpes, in the frame of the cluster project ‘‘InAs nanomemories’’. References [1] T. Baron, B. Pelissier, L. Perniola, F. Mazen, J.M. Hartmann, G. Rolland, Chemical vapor deposition of Ge nanocrystals on SiO2, Applied Physics Letters 87 (7) (2003) 1444–1446.

[2] M. Hocevar, N. Baboux, M. Gendry, A. Souifi, Improvement of data retention in nanocrystal-based memories using InAs QDs embedded in SiO2 on silicon, IEEE Transactions on Electron Devices 56 (11) (2009) 2657–2663. [3] G.E. Cirlin, et al., Formation of InAs quantum dots on a silicon (1 0 0) surface, Semiconductor Science and Technology 13 (11) (1998) 1262–1265. [4] M. Zheng, X. Zhang, L. Yang, C. Liang, L. Zhang, Preparation and optical properties of InAs0.4P0.6 nanocrystal alloy embedded in SiO2 thin films, Semiconductor Science and Technology 16 (6) (2001) 507–510. [5] H. Yang, B. Zhang, X. Wang, X. Wang, T. Li, S. Xi, X. Yao, Sol–gel synthesis and optical properties of size controlled indium arsenide nanocrystals embedded in silica glasses, Journal of Crystal Growth 280 (2005) 521–529. [6] M. Hocevar, P. Regreny, A. Descamps, D. Albertini, G. Saint-Girons, G. Patriarche, M. Gendry, A. Souifi, InAs nanocrystals on SiO2/Si by molecular beam epitaxy for memory applications, Applied Physics Letters 91 (2007) 133114. [7] G. Saint-Girons, P. Regreny, J. Cheng, G. Patriarche, L. Largeau, M. Gendry, G. Xu, Y. Robach, C. Botella, G. Grenet, G. Hollinger, Competition between InP and In2O3 islands during the growth of InP on SrTiO3, Journal of Applied Physics 104 (2008) 033509. [8] T.R. Ramachandran, A. Madhukar, I. Mukhametzhanov, R. Heitz, A. Kalburge, Q. Xie, P. Chen, Nature of Stranski–Krastanow growth of InAs on GaAs (0 0 1), J. Vac. Sci. Technol. B 16 (3) (1998) 1330–1333. [9] A. Michon, Study of the growth of InAs/InP(0 0 1) quantum dots by metalorganic vapor phase epitaxy for 1.55 mm applications. PhD dissertation, Universite´ Pierre et Marie Curie—Paris VI, 2007. [10] A. Karmous, I. Berbezier, A. Ronda, Formation and ordering of Ge nanocrystals on SiO2, Physical Review B 73 (2006) 75323. [11] A. Yamashiki, T. Nishinaga, Arsenic pressure dependence of incorporation diffusion length on (0 0 1) and (1 1 0) surfaces and inter-surface diffusion in MBE of GaAs, Journal of Crystal Growth 198–199 (1999) 1125–1129. [12] V.N.E. Robinson, J.L. Robins, Nucleation kinetics of gold deposited onto UHV cleaved surfaces of NaCl and KBr, Thin Solid Films 20 (1974) 155–175.