Journal of Crystal Growth 378 (2013) 450–453

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Journal of Crystal Growth

0022-02

http://d

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journal homepage: www.elsevier.com/locate/jcrysgro

Fabrication of low-density self-assembled InAs quantum dotson InP(311)B substrate by molecular beam epitaxy

Kouichi Akahane n, Naokatsu Yamamoto

National Institute of Information and Communications Technology (NICT), 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan

a r t i c l e i n f o

Available online 10 January 2013

Keywords:

A1. Nanostructures

A3. Molecular beam epitaxy

B2. Semiconducting III–V materials

48/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.jcrysgro.2012.12.174

esponding author. Tel.: þ81 42 327 6804; fa

ail address: akahane@nict.go.jp (K. Akahane).

a b s t r a c t

We developed a method of fabricating low-density InAs self-assembled quantum dots (QDs) on

InP(311)B substrates by controlling the substrate temperature, and it was found that the lateral size

and height of the InAs QDs increased with increasing substrate temperature. The density of the InAs

QDs decreased to 1.28�1010/cm2, which corresponded to one QD per mesa structure, with a diameter

of 100 nm. The photoluminescence (PL) property was also investigated, and it showed a clear spectrum

even at room temperature at a 1.55 mm fiber-optic communications system band. A blueshift of the PL

peak wavelength was observed as the size of the InAs QDs increased. This unusual PL property can be

explained by In segregation and In re-evaporation at relatively high growth temperature.

& 2013 Elsevier B.V. All rights reserved.

Table 1Summary of sample names and growth temperatures of

InAs QDs.

Sample name Tsub (1C)

Sample (a) 470

1. Introduction

Semiconductor quantum dots (QDs) are attracting significantresearch interest because of their potential application in high-performance optical devices such as quantum dot lasers and semi-conductor optical amplifiers. Further, new applications of QDs inquantum information processing, quantum communications, andsolar cells have also been developed. Although self-assembled QDshave been generally used in these applications, the required densityof the QDs depends on the application. For example, quantumdot lasers and semiconductor optical amplifiers [1–6] need highdensity for sufficient gain and a wide spectrum in the optical-communication band. On the other hand, quantum cryptographyrequires high-quality sources for single photons and entangledphoton pairs; for QD photon sources to fulfill these needs, we haveto fabricate low-density QDs on the surface to avoid generation ofother photons from neighboring QDs. The fabrication technique forhigh-density InAs QD grown on InP(311)B substrates operating at1.55 mm wavelength band using the strain compensation method iswell advanced [7,8], and the resultant QDs shows laser operation andhigh-gain characteristics [6,9,10]. On the other hand, a fabricationtechnique for low-density QDs has not been developed yet. We havepreviously obtained photoluminescence (PL) from a single QD inthe 1.55 mm wavelength band, which is suitable for fiber-opticcommunications systems, by using high-density QD samples withsubmicrometer-size tapered cones fabricated by an accurate deviceprocess [11,12]. However, low-density QD samples are required fora simple and flexible device process. In this study, we developed

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x: þ81 42 327 6941.

a technique for fabricating low-density InAs QDs grown on anInP(311)B substrate by molecular beam epitaxy (MBE).

2. Experiments

The samples were fabricated using MBE (Veeco GEN II) onInP(311)B substrates. After thermal cleaning of the substrate, a150-nm-thick lattice-matched In1�x�yGaxAlyAs buffer layer wasgrown at 470 1C. Then, 5-monolayer (ML) InAs QDs were grown atdifferent substrate temperatures (Tsub), which are summarized inTable 1. After the growth of the QDs, the substrate temperaturewas again decreased to 470 1C to prevent changes to the size andshape of the QDs by annealing effects. Then, a second layer of150-nm-thick In1�x�yGaxAlyAs spacer layers were grown at470 1C. Finally, the 5-ML InAs QDs layer was grown again at Tsub

for the analysis of surface morphology. To realize the sameconditions as before capping the first layer, the substrate tem-perature was set to 470 1C, using the same ramp rate and waitingfor 1 min before decreasing the substrate temperature to roomtemperature. The postgrowth surface morphology was observed

Sample (b) 490

Sample (c) 510

Sample (d) 530

Sample (e) 550

Fig. 1. AFM images of InAs QDs grown on InP(311)B substrates at (a) 470, (b) 490, (c) 510, (d) 530, and (e) 550 1C. The scan ranges are 1�1 mm2.

K. Akahane, N. Yamamoto / Journal of Crystal Growth 378 (2013) 450–453 451

K. Akahane, N. Yamamoto / Journal of Crystal Growth 378 (2013) 450–453452

using an atomic force microscope (AFM) under normal atmo-spheric conditions. PL measurements were performed using the532 nm second-harmonic line of a Nd:YVO4 laser excited using asemiconductor laser diode, a 250 mm monochromator, and anelectrically cooled PbS detector at room temperature.

Fig. 3. Variation of the density of InAs QDs grown on InP(311)B substrate with

respect to the growth temperature.

Fig. 4. PL spectra of InAs QDs grown at (a) 470, (b) 490, (c) 510, (d) 530, and

(e) 550 1C.

3. Results and discussion

Fig. 1(a)–(e) shows AFM images of the InAs QDs grown onInP(311)B substrates at 470, 490, 510, 530, and 550 1C, respec-tively. Obviously, the density of the QDs decreased with increas-ing substrate temperature during the growth process. The averagelateral size along the [�233] and the average height and densityof samples in Fig. 1(a)–(e) were as follows: Fig. 1(a) 40 nm,3.1 nm, and 9.06�1010/cm2; 1(b) 45 nm, 3.3 nm, and 9.08�1010/cm2; 1(c) 48 nm, 4.2 nm, and 6.16�1010/cm2; 1(d) 57 nm,5.6 nm, and 3.40�1010/cm2; and 1(e) 70 nm, 3.3 nm, and 1.28�1010/cm2. Fig. 2 shows the summary of sizes of the QDs. Thecircles show the average lateral size along the [�233] plane andthe squares show the average height of the InAs QDs. The lateralsize increased with increasing growth temperature. Although theheight showed similar tendency trend up to the growth tempera-ture of 530 1C, the height of the QDs grown at 550 1C was lessthan that of those grown at 530 1C. Although the change in thelateral size and height was minimal at growth temperaturesranging from 470 to 510 1C, it became significant above 510 1C.This phenomenon is related to the enhancement of surfacediffusion and the re-evaporation of In atoms that occurs in thistemperature range. The enhancement of surface diffusion of Inatoms usually increases the lateral size and height of QDs [13].Therefore, we considered that the re-evaporation of In atoms wassignificant for the QDs grown at 550 1C.

The density of the QDs decreased monotonically with increas-ing substrate temperature, as shown in Fig. 3. The density of theInAs QDs grown at 550 1C was 128/mm2, so that one quantum dotoccupied an area of 7800 nm2. This area corresponds to that of acircle with a diameter of 100 nm, which can be well defined byelectron beam lithography and dry etching [11,12]. In this case, itis important that one mesa structure includes one QD to avoidgeneration of other photons from neighboring QDs.

Fig. 4 shows the PL spectra for each of the QD samples. We canobserve clear PL emissions around 1.55 mm from all samples atroom temperature, even in spite of the fact that the QD density wasdecreased, which means that the QD quality is very high. The peak

Fig. 2. Variation of the lateral size [�233] and height of InAs QDs grown on

InP(311)B substrate with respect to the growth temperature.

wavelengths of QD samples shown in Fig. 4(a)–(e) were 1662, 1654,1665, 1575, and 1541 nm, respectively. Although the size of theQDs increased with increasing growth temperature, the PL peakwavelength showed a blueshift, which is not a normal behaviorfrom the view point of quantum confinement. This PL property canbe attributed to two possible reasons. One reason is the change inlateral-coupling state from a dense QD ensemble to an isolated QDensemble. We have previously shown the lateral-coupling proper-ties of a densely formed QD stacked structure using time-resolvedPL measurements [14,15]. With the increase in the growth tem-perature of the QDs, the distance between the neighboring QDs isincreased, so that the lateral coupling of the QDs vanishes. Thisdisappearance of lateral-coupling QDs should lead to the blueshiftof the PL peak wavelength. Another reason for the blueshift of thePL peak wavelength is the change in the barrier height of the QDsresulting from In segregation and re-evaporation. On the (311)Bsubstrates, mass transport occurs easily, which has been confirmedin AlGaAs [16] and InGaAlAs [17] material systems. Therefore, Insegregation occurred during the growth at relatively high substratetemperatures. At the surface, In re-evaporation occurred, as men-tioned before, and therefore the barrier energy level increasedwhen compared to that obtained during the growth at lower

K. Akahane, N. Yamamoto / Journal of Crystal Growth 378 (2013) 450–453 453

substrate temperatures. Thus, it might be possible to control Insegregation either by using a higher Al composition in the under-lying InGaAlAs layer, which would prevent In segregation, and/orby starting with a slightly higher In composition in InGaAlAs, whichcan compensate for the loss in In due to In segregation. Never-theless, the emission wavelengths remained in the fiber-opticcommunications band. Therefore, these techniques can be appliedto fabricate high-performance sources for single photons andentangled photon pairs.

4. Summary

We investigated a technique for fabricating low-density InAsself-assembled QDs on InP(311)B substrates by controlling thesubstrate temperature. The lateral size and height of the InAs QDsincreased with increasing growth temperature. The density of theInAs QDs could be reduced to 1.28�1010/cm2, which is lowenough to fabricate a mesa structure with a diameter of 100 nmon a device. Such a device can act as a source for single photons orentangled photon pairs because the mesa structure will containonly one QD. The PL property was also investigated: a strongspectrum was observed even at room temperature in the 1.55 mmfiber-optic communications system band. Blueshift of the PL peakwavelength was observed with increasing size of the InAs QDs.This unusual PL property can be explained by In segregation andIn re-evaporation at relatively high growth temperatures.

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