Quantum Dots in Nanowires

CHAPTER FIVE

Quantum Dots in Nanowires Luca Francaviglia, Yannik Fontana, Anna Fontcuberta i Morral1 Laboratory of Semiconductor Materials, Institute of Materials, Ecole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland 1 Corresponding author: e-mail address: [email protected]

Contents 1. Introduction and Motivation 2. Axial QDs 2.1 Advantages 2.2 Growth 2.3 Properties 3. Radial QDs 3.1 InAs SK QDs 3.2 AlGaAs Segregation 4. Photonic Design References

159 161 161 162 165 170 171 172 174 179

1. INTRODUCTION AND MOTIVATION Thanks to their morphology and size, nanowires (NWs) offer several degrees of freedom for the formation of heterostructures at low dimensions, including quantum dots (QDs). Material composition or crystal phase in an NW can typically be varied in the direction across or along the axis. If the material transition is sharp, this leads to the formation, respectively, of core/ shell (radial) or axial heterostructures. These possibilities can be exploited to obtain quantum heterostructures, including quantum wells (QW), quantum wires (QWRs), and QDs embedded in NWs. For simplicity, here we refer to the latter as nanowire quantum dots (NWQDs). The shape and longitudinal size of NWs guarantee the link to the macroscopic world, either through electrical contacts or photon emission. One of the motivations for including QDs in NWs is that efficiency of light extraction can theoretically approach 100%, thanks to coupling with cavity modes (Purcell effect), waveguiding, and suppression of total internal

Semiconductors and Semimetals, Volume 94 ISSN 0080-8784 http://dx.doi.org/10.1016/bs.semsem.2015.07.005

#

2016 Elsevier Inc. All rights reserved.

159

160

Luca Francaviglia et al.

reflection (Borgstr€ om et al., 2005; Bulgarini et al., 2012b; Claudon et al., 2010; Munsch et al., 2013; Reimer et al., 2012). In agreement with this, NWQD-based single-photon subwavelength detectors were demonstrated (Reimer et al., 2011a; van Kouwen et al., 2010). At the same time, peripheral QDs may show higher sensitivity to external electromagnetic fields, thanks to the vicinity to the surface in comparison with bulk devices. QDs can also easily be addressed via electrical contacts. In turn, QDs can be embedded in an NW axial p–n junction, which allows the recombination of electron–hole pairs into photons emitted by the QD (Minot et al., 2007). The special shape of NWs also enables versatile manipulation of the density of states by the electrical fields through external gates. This has been used to induce QDs in NWs by localized distributed gating (Nadj-Perge et al., 2010; Pribiag et al., 2013). In this case, a QD is not defined as usual by a local compositional difference. The QD-like quantum confinement is rather obtained by local tuning of the Fermi level by applying a suitable voltage by means of narrow gate electrodes. An additional advantage of NWQDs is the wider choice in materials combination. Being the diameter in the few-tens-of-nanometer range, strain associated to lattice mismatch in heteroepitaxy is released in a more effective manner. Thus, the request for lattice mismatch between two materials is released. Last but not least, one should consider that the high surface-to-volume ratio of NWs allows the existence of other crystal phases not stable in the bulk (polytypism). In III–V NWs, zinc-blende and wurtzite alternating segments can be successfully controlled to confine charges and obtain type-II NWQDs within the same material (Akopian et al., 2010; Vainorius et al., 2014, 2015). Two opposite approaches exist for the fabrication of NWQDs: topdown and bottom-up. In the top-down approach, a substrate including QDs is etched with the exception of localized areas, leading to the formation of one-dimensional structures. As the substrate contains a layer of QDs, the resulting NWs can embed one or several QDs (Bleuse et al., 2011; Claudon et al., 2010; Yeo et al., 2014) depending on the initial density of QDs in the substrate and the NW diameter. In the bottom-up approach, QDs are included in the NW structure during the growth process, just by changing the precursors. This method offers additional degrees of freedom. It allows to obtain low-dimensional heterostructures both in radial and longitudinal direction (de la Mata et al., 2013). In particular, defect-free layers of different elements and compounds are possible, thanks to facile strain relief at the NW surfaces, even for strongly mismatched materials.

Quantum Dots in Nanowires

161

In general, bottom-up NW fabrication relies on the vapor–liquid–solid (VLS) method (Wagner and Ellis, 1964). A nanoscale metal droplet, typically gold, initiates the one-dimensional growth. Metal droplets, acting as a catalyst site, decompose or gather growth precursors in a preferential manner (Bj€ ork et al., 2004; Wagner et al., 2010). Ordered arrays of NWs can also be achieved after lithographic processing of deposited gold films (Diedenhofen et al., 2011; Fontana et al., 2012), showing the technological potential for upscaling. Since metal contamination from gold in the semiconductor could be detrimental for the optical properties of NWs and NWQDs, catalyst-free growth methods have been developed (Colombo et al., 2008; Motohisa et al., 2004). In particular, we discuss in this chapter NWQDs grown in GaAs NWs obtained by the Ga-assisted growth method (Colombo et al., 2008). In this case, gallium droplets drive the growth of GaAs NWs. This method can be used on both GaAs and Si substrates. Also in this case, NWs can be obtained in an ordered manner on a substrate after lithographic processing of the substrate (Foster et al., 2015). NWs and NWQDs can be obtained by a manifold of growth techniques ranging from metal–organic vapor-phase epitaxy (MOVPE) or metal–organic chemical vapor deposition (MOCVD) (Borgstr€ om et al., 2005; Bulgarini et al., 2012a; Dick, 2008; Ikejiri et al., 2007; Motohisa et al., 2004; Tatebayashi et al., 2012), to chemical beam epitaxy (CBE) (Ohlsson et al., 2002), and molecular beam epitaxy (MBE) (Fontcuberta i Morral et al., 2008). The area of NWQDs is extremely broad (Holmes et al., 2014, 2015; Tribu et al., 2008). In this chapter and for the sake of space, we have chosen to focus on NWQDs based on III-V NWs obtained by the bottom-up VLS method. QDs composed of III-V semiconductors are today an important part of semiconductor optoelectronics planar technology. For instance, roomtemperature lasing has been demonstrated in these structures (Huffaker et al., 1998; Newell et al., 1999). Many concepts of next-generation solar cells and spintronics include also planar III–V QDs. Still, there is the belief that NWQDs can bring this technology one step further, thanks to their particular properties and singularities. The focus of this chapter is to review some of them.

2. AXIAL QDs 2.1 Advantages The most popular configuration for NWQDs consists of QDs positioned on the NW axis, forming an axial heterostructure of the style A/B/A, where

162

Luca Francaviglia et al.

B shows quantum confinement. There are several reasons for using this configuration. First, this is the natural configuration for an NW as it is obtained just by changing growth precursors during the axial growth. Then, when the NW diameter is accurately chosen, this configuration allows to efficiently couple the emission of the QD to the fundamental waveguiding mode of the NW, as it will be described in more detail in Section 4. This is possible only when a fine control on the longitudinal and radial dimensions of the QD and NW can be achieved. NWQDs obtained on the NW axis by VLS are now mainstream. Excellent optical properties such as high brightness and the potential for their use in quantum information technology have been demonstrated. In addition, in axially defined NWQDs electron–hole pairs can be efficiently injected in the QDs. The electron and hole current flows from one NW-tip contact to the other and must entirely pass through the QD defined in between. This has been exploited, for instance, with optically active axial QDs positioned in the intrinsic region of an NW p–i–n junction to fabricate NWQD LEDs (Minot et al., 2007).

2.2 Growth The bottom-up growth of axially defined III–V NWQDs relies on the possibility to change the alloy composition during the axial growth with subnanometer precision. Regardless of the specific growth method (MBE, MOCVD, Au-assisted or not, etc.), the steps to define an NWQD are as follows. First, the NW axial growth is started with material A. In the VLS method, material A is dissolved by the metal droplet and precipitates at the droplet–NW interface. Layer by layer, this provides the longitudinal growth of the NW. After a certain growth time and for a chosen NW length, the material supply is changed to B. Then, the axial growth continues with a different composition B. The growth rate may change. At the desired length of the segment of material B, the material supply is changed back to A. Eventually, the growth is stopped and an NW of material A embeds a segment of material B. Figure 1B illustrates an example of alternation between two materials A and B. The composition is obtained by energy-filtered electron spectroscopy on a transmission electron micrograph. The characterization demonstrates that there is a segment of lower bandgap material (GaAsP) in a higher bandgap NW (GaP). If the dimensions of a lower bandgap material are small enough, the corresponding segment should show quantum confinement and should behave like a QD. After that, the growth conditions can be changed from

163

Quantum Dots in Nanowires

A

B P

As

Figure 1 (A) Dark-field scanning transmission electron microscopy (DF-STEM) image of the QD region in an InAs NW. TEM contrast allows to visualize as dark stripes the thin InP barriers that define the 10-nm InAs QD. The scale bar corresponds to 20 nm. (B) Energy-filtered transmission electron microscopy (EFTEM) images of axial GaAsP heterostructure in a GaP NW. The elemental distribution of phosphor (blue (lighter gray in the print version)) and arsenic (red (darker gray in the print version)) shows the position of the direct-bandgap absorption region, where a 15-nm long QD is embedded. The €rk et al. scale bar corresponds to 100 nm. Source: Adapted with permission from (A) Bjo €m et al. (2005), © 2005 ACS. (2004), © 2004 ACS and (B) Borgstro

axial to radial and an outer shell can be added to complete the NW structure. This can be useful to passivate the surface and avoid surface recombination, especially at the QD position; to prevent oxidation of the inner layers; and to match the NW thickness to the best conditions for coupling with the QD emission (Dalacu et al., 2012; Heinrich et al., 2010; Sanada et al., 2007). In principle, the shell deposition should not affect the lateral size of the already formed QD segment. This, instead, is defined by the NW diameter during the axial growth. Therefore, reliably tailoring the NW diameter through the growth conditions is essential. The overall NW diameter that takes into account also the possible presence of a shell defines the strength of coupling between a specific QD emission mode and the NW acting like a waveguide. The relationship between the QD emission wavelength and the optimal NW diameter has been both described theoretically and confirmed experimentally (Bulgarini et al., 2012b; Friedler et al., 2009). The NW diameter can also tune the spontaneous emission (SE) rate of axial QDs. Following this, Bulgarini et al. reported a quantum efficiency as high as 92%. In III–V semiconductors, there are two possibilities to switch from material A to B. One can change the group-III element (Ohlsson et al., 2002; Tatebayashi et al., 2012) or the group-V element (Bj€ ork et al., 2002, 2004). Some further combinations are available if either A or B is a ternary III–V alloy, with an additional group-III or group-V element. Some

164

Luca Francaviglia et al.

examples are GaAs QDs in AlGaAs NWs (Heinrich et al., 2010; Sanada et al., 2007), InAsP QDs in InP NWs (Bulgarini et al., 2012b; Kouwen et al., 2010; Minot et al., 2007; Reimer et al., 2011b), and InGaAs QDs in GaAs NWs (Tatebayashi et al., 2014). A further example is an optically active GaAs QD in a GaAsP section of a GaP NW (Borgstr€ om et al., 2005) (Fig. 1). The direct-bandgap GaAsP region surrounding the GaAs QD was meant to enhance the absorption of the laser excitation. The change in group-V element more easily leads to better results, as it is explained here below. In principle, the length of the low-bandgap segment simply depends on the growth time and ability to switch abruptly from one source to the other. In fact, many efforts have been devoted to define sharp interfaces and short segments. The challenge in controlling the length of the segments and abruptness of the interfaces is bound to the physics involved in the VLS method. Issues with the abruptness of the heterointerface are linked to what is known as the “reservoir effect” (Dick et al., 2012; Li et al., 2008): elements that should only compose the NW section A may not be completely consumed in the catalytic droplet when the growth of segment B starts. Thus, some contamination from A is found in the segment B. This results in a challenging control on the composition of the segments which can lead to shallow A/B interfaces. In addition, elemental supply at the NW–droplet interface often does not only come from the beams directly stemming from the crucibles. Sometimes, diffusion on the substrate and on the NW sidewalls may supply the droplet. Therefore, fine calibration of the growth time is necessary since the growth rate changes with the distance from the substrate, that is, with the height where the QD is positioned in the NW (Tatebayashi et al., 2014). The “reservoir effect” results in more constraints in the so-called catalystfree growth methods. In this case, the catalyst is also a component of the NW. The choice of the catalyst is not wide: only few elements have the right properties to catalyze the NW growth, like being liquid at the growth temperature. Typically, an element from group III is chosen, like in the Ga-assisted method for the growth of GaAs NWs (Colombo et al., 2008; Fontcuberta i Morral et al., 2008). Therefore, the formation of NWQDs by changing the group-III element is much more difficult if the catalyst-free approach is chosen. The whole droplet should be consumed and substituted by a new one with a different group-III element. Conversely, both in metalcatalyzed and catalyst-free approaches, sharp edges can be formed by changing the group-V material. This is due to the high vapor pressure and low

Quantum Dots in Nanowires

165

solubility of group-V elements in the droplet. In this way, sharper interfaces are then possible: as soon as one of the group-V sources is closed, the droplet is emptied and no “reservoir effect” effectively hinders the quality of the interface. After slightly more than a decade from the upswing in NW research in the 1990s, several authors reported the formation of sharp interfaces (Bj€ ork et al., 2002). Thanks to this progress, the formation of optically and electronically active QDs and the very fine control on the QD length (thus emission energy) of axial NWQDs could be achieved (Bj€ ork et al., 2004; Tatebayashi et al., 2014). Heterostructures formed by the group-III change were demonstrated already in 2002. It was shown that CBE VLS growth exhibits such a slow growth rate that the gas diffusion of different species on the substrate was negligible, favoring the formation of abrupt InAs–GaAs interfaces (Ohlsson et al., 2002). Axial heterostructures with atomically sharp interfaces were reported in the same year by Bj€ ork et al. using CBE (Bj€ ork et al., 2002). In this case, they obtained several InP sections in InAs NWs, with length ranging between 100 and 1.5 nm by the careful control of the growth time. Following this seminal work, axial InAs QDs within InP barriers were afterward demonstrated (Fig. 1A). In this case, thanks to their high vapor pressure, the group-V elements are expected to leave instantly the catalyst droplet upon switching of the source. Together with slow growth rate, this leads to monolayer sharpness of the interfaces. One should also mention that in addition, at each group-V change, the group-III source (indium) was turned off for 5 s, in order to slow down (ideally interrupt) the axial growth during the group-V switch. The described InP–InAs alternation was then used to obtain InAs NWQDs of different size within two InP barriers (Bj€ ork et al., 2004).

2.3 Properties In this section, we give a brief overview on the properties of axial bottom-up NWQDs. An example of the excellent properties that can be obtained is illustrated by the work of Reimer et al. (2012), which engineer the optical properties of axial NWQDs to obtain extremely bright single-photon emitters. In this case, the NWQDs were obtained by MOVPE. They consist of thin single InAs0.25P0.75 discs introduced in tapered InP NWs by the temporary introduction of As in the reactor (1–2 s). The QD height is about 7 nm, while the diameter is set by the size of the gold colloidal particles (
20 nm) used as a catalyst in the VLS process. After stopping the axial

166

Luca Francaviglia et al.

growth, an InP shell was grown with a tapering angle at the tip of about 2°. As a result, the QD is centered on the NW axis and efficiently surfacepassivated, thereby acting as an efficient tapered waveguide for the HE11 mode. This leads to potentially near-unity light extraction if a perfect mirror is positioned at the substrate level. A sketch and a scanning electron microscopy (SEM) image are presented in Fig. 2A and B, respectively. Almost 100% light extraction is a fundamental requirement for truly deterministic single-photon sources in quantum communication. Low-power photoluminescence (PL) shows the neutral exciton (X) recombination in the QD s-shell at 1.409 eV. At higher excitation power, the biexciton (XX) line appears at some meV from the X line (Fig. 2C). The XX line can be either above or below the X line, depending on the biexciton binding energy that, in turn, depends on the QD size. The distinction between X and XX emission was established according to a standard procedure: X- and XX-integrated intensities follow, respectively, a linear and a quadratic behavior in function of the excitation power (Fig. 2E). In other works (Kouwen et al., 2010), the same kind of InAsP QDs in InP

C

QD QD

240

180 20 120

10

60

30 0 1.38

0.5 0.0

30

NW

Gold mirror

g(2)(t)

W/cm XX

1.40

1.42

PL energy (eV)

1.44

–10

0 t (ns)

E 30 20

10

X 100 k

10

X 1 30

–

XX

Integrated counts (s–1)

11–12 mm

X X–

1.0

2

Extraction efficiency (%)

q = 2°

D 40

PL intensity (103 counts s–1)

B

PDMS

Tapered tip

A

10 k 100

2 Excitation power (W/cm )

Figure 2 (A) Sketch of the system in PDMS and on gold. (B) SEM image of an as-grown tapered InP NW. The arrows highlight the InAsP QD position. The tapering angle of the nanowire is θ¼2°. The dotted line is at the position where the NW in PDMS breaks for being transferred onto the gold mirror. The vertical and horizontal scale bars correspond to 1 μm. (C) PL spectra under pulsed laser excitation at increasing excitation power. The emission lines are labeled as exciton (X), charged exciton (X), and biexciton (XX). (D) Second-order correlation function (g2) of the X line showing the antibunching dip below 0.5. (E) Power dependence of the intensity of the X, X, and XX lines. The estimated collection efficiency at saturation is also reported for each line. Source: Adapted with permission from Reimer et al. (2012). © 2012 NPG.

Quantum Dots in Nanowires

167

NWs are reported to show p-shell and d-shell recombination by PL at increasing excitation power, when the lower-energy recombinations are saturated. Generally speaking, the interest for X and XX recombination resides, among other reasons, in the possibility to obtain entangled photon pairs (Benson et al., 2000). This is thought to be promisingly true in axial NWQDs, since they can have a theoretically vanishing fine-structure splitting (Singh and Bester, 2009) and very small (8 μeV) splitting has been measured in InAsP QDs in InP NWs (Dalacu et al., 2012). In turn, this is due to the fact that axial NWQDs are spontaneously highly symmetric. Pulsed laser excitation was used to estimate the extraction efficiency of the QDNWs embedded in PDMS and transferred onto a gold mirror. In this condition, the expected collection efficiency of the NW waveguide is 71%. The excitation repetition rate was chosen to allow the QD to recombine before the successive pulse arrives. A single-photon flux of 24 MHz was estimated. One should note that a count rate in the order of MHz had also been reported in similar systems of NWQDs (Borgstr€ om et al., 2005), which demonstrates the real potential of NWQDs as single-photon sources. In this case, an overall collection efficiency of 42% is calculated. The main reason of difference from the estimated 71% is linked to the quality of the mirror (especially plasmon coupling with gold that would be efficiently avoided with a thin SiO2 insertion (Claudon et al., 2013; Friedler et al., 2009)). Best extraction efficiency (E) in NWQD emission was achieved by Claudon et al. (2010) (E¼72%) in needle-like etched waveguides and similarly by Munsch et al. (2013) (E¼75%) in trumpet-like etched waveguides. A Hanbury Brown and Twiss set-up was used to demonstrate the singlephoton emission on the exciton recombination. A second-order correlation function value at zero delay (g2(0)) of 0.12 was demonstrated. An antibunching dip below 0.5 constitutes a proof of single-photon emission (Fig. 2D). Finally, we would like to address one last characteristic concerning the optical properties of NWQDs: the highly anisotropic shape of the NWs and the dielectric environment around the QD, which can thoroughly screen the QD optical polarization properties of the NWQDs (van Weert et al., 2009). It has been shown that the polarization properties of the NWQDs are poorly accessed when the NWQD is lying perpendicularly to its axis. On the contrary, the QD polarization is conserved and can be measured when the excitation and collection are performed along the NW axis. Now we turn to the description of the electronic properties of axial NWQDs. Both n- and p-type doped NWs as well as pn junctions with

168

Luca Francaviglia et al.

embedded QDs can be obtained. For instance, the introduction of hydrogen sulfide and diethylzinc during the MOVPE growth of InP NWs leads, respectively, to n- and p-doping (Bulgarini et al., 2012a). This allows to produce NWQDs as building blocks for electronic devices, like LEDs (Minot et al., 2007), avalanche photodiodes (Bulgarini et al., 2012b), and singleelectron transistors (Thelander et al., 2003). For the fabrication of an NWQD device, the NWs are usually removed from the growth substrate. This is useful in optical applications, e.g., if one needs to add a mirror at the bottom end of an NW waveguide (Reimer et al., 2012) to enhance the light collection. It is also the usual procedure for electronic devices, like transistors, where an insulator must be added between the NW and the gate (often the substrate). With this regard, the NWs are typically transferred on a SiO2 substrate with markers. The position of the NWs is then identified by an SEM or an optical microscope. Afterward, the contacts are defined by electron beam lithography followed by metal evaporation (Thelander et al., 2004). Here, we use the seminal example of InAs NWs to illustrate electronic properties of QDs in NWs. InAs has a small bandgap (0.35 eV) and pinning of the Fermi level at the conduction band at the semiconductor surface is known for bulk InAs. As a result, ohmic contacts to InAs NWs are relatively straightforward to obtain. Bulk InAs has a low electron effective mass (0.023m0). Electron mobility in InAs NWs around 6500 cm2 V1 s1 has been demonstrated (Dayeh et al., 2007). This value is around one order of magnitude lower than what is currently reported for bulk InAs. Similar differences between bulk and NW mobilities are reported for other materials. The lowering of carrier mobility in NWs has been attributed to surface scattering. High mobility in NWs would be interesting for high-speed devices. In order to trace out the resistance of the electrodes, proper resistivity measurements are performed in a four-probe configuration, where two outer electrodes drive a known current through the tested NW. The voltage difference of a section of the NW is then measured by means of two inner noninvasive electrodes. Bj€ ork et al. (2004) and Thelander et al. (2003) reported on electronic properties of InAs NWs where a QD is defined as an InAs region within two 5-nm-thick InP barriers. For 3–4-μm-long NWs with a 55–65 nm diameter, they report resistance in the MΩ range. As expected, thicker InP barriers resulted in a higher NW resistance. Also, the resistivity significantly increased when the NW diameter was decreased. This is not only due to the reduction of the NW cross-section area but also to quantum confinement and surface scattering.

169

Quantum Dots in Nanowires

NWQD transistors should display discrete conductivity peaks and Coulomb blockade characteristics in function of the gate and source–drain bias, respectively, Vg and Vsd. This was studied for different QD lengths, in order to characterize different confinement regimes and demonstrate control over the electron population in single NWQDs (Bj€ ork et al., 2004). Perfect periodic oscillations of the conductance in function of the gate voltage are visible for 100-nm-long QDs. The period of the oscillations corresponds to the addition energy of the QD. This in turn depends on two contributions (constant interaction model): the charging energy EC, that is, the electrostatic energy necessary to add one electron to the QD, and the difference ΔE(N) in the single-particle energies between a system of N and N  1 electrons. The latter depends on the number N of electrons, and it has a negligible value if the NWQD is large enough. In the 100 nm QD, EC is around 5 meV and ΔE(N) less than 1 meV. Gradually, the shorter the QD becomes, the larger ΔE(N) is. The addition energy as a whole depends on N and the perfect periodicity of the conductance peaks is broken. Eventually, the strong confinement in a 10-nm long QD sets the QD ground state above the emitter and collector states at Vg ¼ 0, so no carriers are in the QD. By increasing Vg, up to 50 electrons are added to the QD (Bj€ ork et al., 2004). Characteristic Coulomb diamonds are shown in the dI/dV plot in function of Vg and Vsd (Fig. 3). Consecutive diamonds correspond to Coulomb blockade of the measured current while adding electrons one by one to the QD. Each diamond can be linked with an integer quantity of electrons starting from zero in the depleted 10 nm QD at Vg ¼ 0. The existence of Coulomb diamonds confirms the control on the NWQDs charge and their use as a single-electron switch.

V sd (mV)

20

0

0

2

6

8

12

16

22

–20 1.0

2.0

3.0

4.0

5.0

5.8

V g (V)

Figure 3 Plot of dI/dV as a function of Vsd and Vg. The numbers inside the large diamonds correspond to the shell structure of the QD. Source: Adapted with permission from €rk et al. (2004). © 2004 ACS. Bjo

170

Luca Francaviglia et al.

Odd-number diamonds exhibit all the same size. In fact, the addition energy is independent from N, since the spin degeneracy makes ΔE(N)¼ 0 for odd N. On the contrary, diamonds corresponding to even numbers have different dimensions. Some of them are larger and correspond to the so-called magic numbers. This is an indirect confirmation of the high symmetry of axial QDs, since the “magic numbers” correspond to shell-like QD energy levels. Such an ordered level structure would not be visible otherwise. The tunability of electron charging in NWQD states has also been reported for InAs0.25P0.75 QDs in InP NWs (Kouwen et al., 2010). In this case, PL and photocurrent spectroscopy were used on the same NWQD to study the charging state of the optically active embedded QD. In particular, charge control is reached by tuning both the source–drain bias and the backgate potential. The QD behavior was explained resorting the competition between the rate of electron charging of the QD (Γcharge) and the rate of radiative recombination in the QD (Γrad). In particular, Γcharge depends on the probability that an electron tunnels into the QD. This tunneling probability can be tuned by Vg. For Γcharge
Γrad, there is a certain probability that radiative recombination can be observed by PL before an electron tunnels into the QD. As a consequence, the charged exciton (X1) emission can be observed together with the exciton X0 recombination for several values of Vsd. If Vg is gradually increased toward positive values, eventually Γcharge overcomes Γrad. Then, the exciton recombination is less probable and two separated Vsd ranges for X0 and X1 recombination are observed. Spanning some tens of volts both in Vg and Vsd, the QD state ranges from an emptied QD to what is appointed as a neutral exciton (X0) and eventually to a negatively charged exciton with up to three electrons (X1, X2, X3). The ability to tune the charge state of NWQDs is essential for spin-readout protocols that involve the conversion of the electron spin into charge.

3. RADIAL QDs On-axis precise positioning of NWQDs is fundamental for strong coupling with the dominant mode of a cylindrical waveguide (HE11). This is an advantage of bottom-up NWs over top-down defined NWs (Babinec et al., 2010). For the latter, accurate localization of the QD must be performed and followed by precise etching of the waveguide around it. Still, off-axis (or radial) QDs can offer interesting opportunities, too. First, their emission could couple to other waveguide modes, like peripheral

Quantum Dots in Nanowires

171

modes or air modes. As an example, a tangential dipole approaching the NW sidewalls would strongly couple to the continuum of radiation modes of the NW (Claudon et al., 2013). QDs in the vicinity of the NW surface could be used as QD-based sensors of external electromagnetic fields. Additionally, off-axis NWQDs can be used for enhanced monitoring of strain in NWs. In NWs clamped to the edge of a substrate, the emission of optically active radial QDs was observed to couple strongly with the mechanical modes of an NW oscillator (Montinaro et al., 2014). Here, the NWQDs could be used as probes of the NW motion. Vice versa, the QD state could be engineered/ modified by the NW position. Radial QDs still represent a limited research topic in comparison to axial QDs. Here, two different radial NWQD systems are discussed.

3.1 InAs SK QDs An interesting option for radial NWQDs consists of translating growth mechanisms of QDs on planar substrates to the facets of NWs. As an example, Stranski–Krastanov (SK) QD growth applied to NWs with hexagonal cross-section has been demonstrated both by MBE (Uccelli et al., 2010) and MOCVD (Yan et al., 2012, 2013). SK InAs QDs on GaAs NW facets were shown. SK InAs QDs are typically obtained on (001) GaAs films, while the NW facets of GaAs NWs belong to {110} or {112} crystal families, whose surface energy prevents the formation of SKQDs. In order to circumvent this issue, Uccelli et al. (2010) predeposited a 5-nm (110) AlAs layer on the GaAs facets prior to the InAs deposition. This allowed the formation of SK InAs QDs. A sketch of the system is presented in Fig. 4A and an STEM image with energy-dispersive X-ray (EDX) profile of a QD is reported in Fig. 4B and C. In this case, it seems that the QD is partially buried in the AlAs shell. Both single and flanked rows of QDs were found on the NW facets. When the amount of InAs growth was increased, some larger QD islands were also observed at the corner between two facets. The position of the QD on the NW surface may depend on the facet width and is thought to be due to the mechanism of stress relaxation of the strained InAs layer. Both X and XX emissions were recognized by the trend of PL intensity vs. excitation power. The optical activity and presence of both X and XX emission are encouraging for optical applications of SKQDs in NWs. Similarly, optically active InAs QDs on GaAs were also demonstrated by other groups (Yan et al., 2012, 2013) using MOCVD. The identification of a wetting layer (WL)

172

Luca Francaviglia et al.

A

B

C

Ga – As I

In Ga As

80

SiO2 GaAs (001)

Counts

Al – As II SiO2

60 40

GaAs (001)

In – As III

100 nm SiO2

GaAs (001)

20 0

0

100 50 Position (nm)

150

Figure 4 (A) Sketch of the fabrication of the NW heterostructures. (I) Ga-assisted GaAs NW growth on a GaAs (001)-oriented substrate. (II) Switch to radial growth of the AlAs shell on the NW side facets. (III) SK formation of InAs QDs on the AlAs shell. (B) Highangle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image (Z-contrast image) of an NW region with a radial QD. (C) Energy-dispersive X-ray spectroscopy (EDX) line scan corresponding to the dotted line in (B) (Uccelli et al., 2010).

in the spectra confirms the SK mechanism. The WL on the NW facets is thicker than on planar film, since the small dimensions and high surface curvature imply better relaxation of the lattice strain. The InAs QD formation and density are reported to depend on the indium supply at the NW sidewalls that is observed to depend on the NW density on the growth substrate (Yan et al., 2013).

3.2 AlGaAs Segregation SK QD growth is bound to combinations of materials with lattice mismatch sufficient to cause the formation of relaxed islands. This is mainly restricted to the case of InAs on GaAs. However, more freedom and potentially novel mechanisms are possible for QD formation in NWs, where strain is not the driving force. An interesting option has been shown by Heiss et al. for ternary alloys. Here, shell NWQDs in a GaAs–AlGaAs core-shell system were demonstrated (Heiss et al., 2013). The GaAs–AlGaAs combination had previously been explored for the intentional formation of axial QDs by MBE (Heinrich et al., 2010) and MOVPE (Sanada et al., 2007). Conversely, in Heiss et al. the natural segregation of aluminum on the prismatic surface of hexagonal GaAs NWs leads to the formation of optically active radial QDs. A sketch is reported in Fig. 5A. In this case, the GaAs NWs were grown by MBE in the (111) direction on Si wafers using the gallium-assisted method

173

Quantum Dots in Nanowires

A

Al30Ga70As

QD

Energy (eV)

Al60Ga40As 57% Al 48% Al

2

2.00

1.78

GaAs

B

C

Ec (Χ) e0 1.655

1.62 1.50

1.52

0

–0.25

Ec (Λ)

eQD1.738

0

–0.05 –0.15

20 nm

2

h0–0.028

hQD–0.095

–0.3

38% Al

Al10Ga90As

28% Al

Al30Ga70As

Ec (Γ) Ev (Γ)

–0.3

Al60Ga40As GaAs

Figure 5 (A) Sketch of an NW cross-section showing the hexagonal shape, the inner GaAs core, the AlGaAs shell, the Al-rich planes (stripes in cross-section view), and the ridge QD. (B) Zoom-in of an aberration-corrected HAADF STEM image of the crosssection of a multishell NW. A radial QD is shown and the experimental aluminum content of some points is reported. (C) Band-edge diagram from atomistic pseudopotential theory. From left to right: the Al30Ga70As shell matrix, the Al-rich (Al60Ga40As) barrier, the Al-poor QD kernel (Al10Ga90As), the barrier again, and the GaAs capping layer (Heiss et al., 2013).

(Colombo et al., 2008). In a subsequent step, the growth conditions were switched from axial to radial in order to deposit an Al33Ga66As shell, where the segregation process took place. Aluminum segregation with self-limiting profile had already been observed before on patterned planar substrates, and theoretical models have been proposed to explain this three-dimensional segregation both in MBE and MOCVD (Biasiol and Kapon, 1998; Biasiol et al., 2002). Segregation in AlGaAs NW shells is associated with two concurrent phenomena. First, aluminum adatoms have lower mobility than gallium and arsenic. Second, surface energy on the NW facets is not spatially homogeneous. This is due to the increased surface curvature at the ridge between two facets and because of the presence of {112} ridge nanofacets of higher Miller indices in addition to the broader {110} facets (Heiss et al., 2013; Zheng et al., 2013). As a consequence, Al-rich planes form in edges between the facets, along the whole length of the NW. These Al-rich planes have been observed in NWs grown by MOCVD, too (Zheng et al., 2013). In contrast with the theoretical conclusions drawn for the planar configuration (Biasiol and Kapon, 1998; Biasiol et al., 2002), the Al-rich planes in the NWs are not perfectly stable. For instance, in the sample grown

174

Luca Francaviglia et al.

by MOCVD, it was pointed out that the Al-rich stripes do not strictly follow a unique crystallographic direction. They rather develop an indented profile (Zheng et al., 2013). In Heiss et al., the Al-rich stripes are even reported to occasionally open into two branches, mostly parallel to the (1-1-2) and (-211) planes. The local bifurcation was also reported to be terminated by a (121) Al-rich plane. Such a structure is localized both in length and width. It results in a nanoscale pyramid surrounded by an Al-rich barrier and containing a Ga-rich kernel, as it is shown in the HAADF STEM image in Fig. 5B. As a consequence, these structures can show quantum confinement (simulated energy band structure in Fig. 5C) and were regarded as a novel type of self-assembled QDs. One of the interest in these new kind of NWQDs is that they exhibit an extremely bright single-photon emission and small linewidth, with g2(0)¼2% (Heiss et al., 2013), which is of great interest for a wide range of photonic applications. The shell QDs are elongated. The elongation direction has no specific relationship with the NW longitudinal axis (Corfdir et al., 2014; Fontana et al., 2014), confirming the segregation origin of the QDs. Charged exciton and biexciton lines were observed by PL in addition to exciton recombination (Fontana et al., 2014), which is a key point in view of exciton–biexciton cascade to produce entangled photon pairs. The g-factor directs the coupling between the QD carrier spins and an external magnetic field. It is an important parameter to accurately define and control spin states, which is in turn an undeniable step toward reliable qubit manipulation. Quantum information processing could take advantage of the fact that in these shell NWQDs, it was possible to tune the g-factor by simply varying the orientation of an external magnetic field (Corfdir et al., 2014). In future, the same segregation mechanism can be potentially exploited to obtain NWQDs in materials for which segregation phenomena are also known, like GaAs–AlInP core-shell NWs (Sk€ old et al., 2006; Wagner et al., 2010).

4. PHOTONIC DESIGN We turn now to the discussion on the strategies for the optimization of the SE and of the optical properties of NWQDs. The interest is to engineer the electromagnetic environment of the emitter to obtain fast and bright single-photon sources with high fidelity and strong nonlinearity. A tailored electromagnetic environment for a QD can be represented by

Quantum Dots in Nanowires

175

the host NW. In turn, NWs themselves can be placed in cavities and photonic crystals (PCs) in order to tune the interaction with their surroundings. Semiconductor QDs generally provide radiative quantum yields near unity at cryogenic temperatures. Yet, when embedded in bulk matrix, their performance is affected by important drawbacks, like total internal reflection. Time-integrated measurements usually overcome the problem in macroscopic applications, while a high extraction efficiency is undeniable when timing and signal intensity are inevitably reduced. This is the case of quantum emitters, specifically when aiming at triggering single photons on demand. As a figure of merit, it is useful to consider the extraction efficiency η of the system that can be assessed as the probability to collect a photon into a singlemode channel, like a fiber. A strategy to enhance the brightness of QDs relies on placing them into a nanocavity in order to benefit from the Purcell effect. The coupling with a chosen cavity mode provides a higher density of final states for the corresponding radiative transition, enhancing the emission rate Γ(¼τ1) into that mode over the global rate γ of all the other radiative modes (Γ ≫ γ). Thus, a large fraction β of photons is funneled into the chosen mode ( β ¼ Γ Γ+ γ) and high-quality factors (Q) can be reached (exceeding 103). The emission enhancement is estimated by the Purcell factor FP that can be written as the ratio between the radiative lifetime in and out of the cavity and is proportional to Q/V, where V is the mode volume (around 102 in (λ/n)3 units). The reduction of Γ not only increases the brightness of a chosen mode but also extends the coherence of the emitted photons. This is required, for instance, to store information in photons as qubits. Yet, the operation linewidth is inversely proportional with Q, thus only almost monochromatic emission can take advantage of the Purcell effect. This would result in a severe limitation for the use of the exciton–biexciton cascade scheme and for broad linewidth room-temperature emitters. NWs offer a suitable alternative to nanocavities. A synthetic review of some NW designs can be found in Claudon et al. (2013). The driving idea is that the mismatch of refractive index between the semiconductor NW and the outer environment allows to use the NW as a waveguide. The outcoupling with a guided mode can result in a broadband enhancement of the emission. Simulations (Friedler et al., 2009) show that β for the HE11 mode remains above 90% for a wide interval of values of Dλ ranging from around 0.20 to 0.29, where D is the diameter of the NW and λ is the emission wavelength in vacuum of the dipole embedded in the waveguide. This result, almost wavelength independent, does not rely on any cavity effect.

176

Luca Francaviglia et al.

It only depends on the nearly perfect coupling of the SE with a single waveguide mode (fundamental HE11) and negligible coupling with the other radiative modes. The highest β is reached for Dλ
0:22. For lower Dλ , the optical modes are less confined in the waveguide and the emitters couple to a continuum of nonguided modes without a preferential direction of emission (Bleuse et al., 2011). For higher Dλ , higher waveguide modes can couple with the emission, decreasing β for the HE11 mode. Experimental confirmation was reported for several values of Dλ in InAsP QDs in InP NWs (Bulgarini et al., 2012b). In this work, the radiative lifetime τ is shown to decrease while increasing the Dλ ratio up to 0.245. Yet, photons strongly confined in a sub-wavelength-sized waveguide are expected to scatter at large angles while leaving the flat NW end facet, thus preventing the efficient collection by commercial optics of limited numerical aperture (NA). A solution is to increase the mode size of the emitted photon. This can be achieved by both reducing or increasing the waveguide diameter, in a needle-like (Claudon et al., 2010; Reimer et al., 2012) or trumpet-like (Munsch et al., 2013) shape. In both cases, the aim is to adiabatically guide the emission until the mode is mostly supported in air. The concept has been sketched in Fig. 6. The best adiabatic expansion of the mode in the waveguide happens for long tips slowly decreasing in diameter, that is, for very small tapering angles. Enhanced transmission and reduced far-field divergence have been theoretically predicted (Friedler et al., 2009) and experimentally proved both by top-down (Claudon et al., 2010) and bottom-up (Reimer et al., 2012) approaches. Bottom-up definition of the tapered tip is particularly attractive since very small opening angles are difficult to obtain in a top-down scheme. In particular, a simple Fabry–Perot model, in agreement with more complex fully vectorial calculations (Friedler et al., 2009), leads to the following Straight NW

⬙Needle⬙ NW

Figure 6 Sketch of different designs of NW waveguides.

⬙Trumpet⬙ NW

Quantum Dots in Nanowires

2

177

jrm jÞ ηðθÞ ¼ 12 β ð11 ++ βjr Tα ðsin θÞ, where rm accounts for the modal reflectivity of a mj bottom mirror and Tα is the transmission coefficient for tapering of angle α: the lower α is the closer to unity Tα becomes. The bottom mirror is meant to collect half of the photons that are emitted backward. One of the best options for implementing the bottom mirror, both from a theoretical and practical point of view (Claudon et al., 2010; Friedler et al., 2009), is a gold mirror separated from the NW by a thin silica (SiO2) layer in order to prevent detrimental losses due to plasmon coupling with the metal. The backemitted and reflected photons set a stationary wave in the NW. Thus, full exploitation of the mirror is only possible by positioning the QD at an antinode of the stationary wave. Indeed, poor control on the QD position has been regarded as a major contribution to a moderate performance (η¼42%) much lower than expectations (η¼71%) by Claudon et al. (2013) about Reimer et al. (2012) in spite of the small opening angle (1.5°0.2°). Alternatively, the trumpet shape is less demanding to grow by current top-down methods and prevents nonadiabatic losses also for relatively large tapering angles (Munsch et al., 2013). Single-photon emission has been demonstrated from InAs QDs at the base of the trumpet of a GaAs NW (Munsch et al., 2013). Extremely high external efficiency is also reported (η ¼ 0.75  0.1). Moreover, the trumpet shape naturally enhances the transmission of a Gaussian profile. The external coupling efficiency to a Gaussian beam ηg is evaluated to be 0.58  0.08. Further improvements of ηg would eventually enable very directive far-field emission and thus high collection also by optics of modest NA. NWs have also been reported to clearly dominate the polarization of the emitted light both in top-down (Munsch et al., 2012) and bottom-up (Foster et al., 2015) NWQDs. This effect is obtained for NW cross-sections whose aspect ratio is elongated. This is explained considering two electromagnetic modes, parallel and perpendicular with respect to the elongation axis. If the NW thickness is small enough not to support the perpendicular mode, then the emitted photons only couple with the mode parallelpolarized to the main axis. Also, the NW controls the preferential emission direction. Labeling as z the longitudinal axis of a single InP NW, it has been reported that the emission intensity peaks at a preferential angle of 49° from the z direction (Grzela et al., 2012), in a ring-like pattern. This has been explained by simulations resorting the role of the leaky modes supported by the NW waveguide. The NW behaves like an antenna and favors outcoupling of dipoles parallel to its longitudinal axis. This can be exploited to

178

Luca Francaviglia et al.

enhance the interaction with the NW but also makes it difficult to access the polarization of an embedded QD from a random direction (van Weert et al., 2009). Often WZ–ZB polytypism is observed along bottom-up III–V NWs. The type-II band alignment of WZ and ZB is known to dynamically trap and untrap carriers. In the surroundings of the QD, this sets a fluctuating Stark shift that broadens the QD emission linewidth. On the contrary, high phase control on pure ZB InP NWs is regarded as the origin of very narrow emission linewidth (30 μeV) from the embedded InAsP QDs (Dalacu et al., 2012). The addition of a passivation shell is reported to remove detrimental surface effects previously observed and the overall core-shell diameter is chosen to match the best outcoupling condition of the waveguide. In addition to the design of a single NW, PCs are reported to strongly change the NWQD emission properties. A PC shows a periodicity in the refractive index that interacts with the propagating electromagnetic waves. This sets a band structure of the photonic modes of the crystal, generally including forbidden gaps where the propagation of the waves is not possible. A way to form a PC is to grow NWs in an ordered pattern. The resulting photonic band structure changes the favorite emission direction of the NWQDs into non-Lambertian patterns. The refractive index of the immersion medium has been demonstrated to influence these patterns that are also shown to send in different directions the s and p polarized photons (Diedenhofen et al., 2011; Fontana et al., 2012). Therefore, controlling the geometry of the PC is a way to design directional sources and optimize the detection for a chosen direction. Optically pumped low-threshold lasing has been reported for bottom-up GaAs/InGaAs/GaAs NWs passivated by an InGaP shell and lithographically arranged to form a PC (Scofield et al., 2011). The NWs themselves provide the gain medium and the PC at a defined lattice constant forms the lasing cavity. An interesting aspect in this structure is the possibility to modulate the resonance wavelengths and the Q-factor by controlling the NW pitch and diameter, respectively. A different approach is to place single NWs into a PC. A groove, acting like a cavity, in a planar silicon PC was reported to efficiently enhance the emission from optically active InAsP discs in a InP NW through the Purcell effect (Birowosuto et al., 2014). Accurate atomic force microscopy manipulation is a key point that allows to position the NWs into the groove. An intense PL enhancement is observed at least at the frequency of the first cavity mode. The emission lifetime shortens by a factor of four in the groove and 91 ps is claimed as the shortest lifetime for III–V semiconductors. Eventually, a last proof of the flexibility of use of PCs comes from a theoretical

Quantum Dots in Nanowires

179

study that drew attention to NW PCs as promising systems to reach a photon-entangled state of two NWQDs (Angelatos and Hughes, 2015). In conclusion, it has been demonstrated that through the design of the environment surrounding the QDs and the NWs, one can master the brightness, the lifetime, the directionality, and the polarization of the emission, with the further possibility to set a state of entangled photons, to enhance the Q-factor, to improve the extraction efficiency, and to reach lowthreshold lasing at tunable resonances.

REFERENCES Akopian, N., Patriarche, G., Liu, L., Harmand, J.C., Zwiller, V., 2010. Crystal phase quantum dots. Nano Lett. 10 (4), 1198–1201. http://dx.doi.org/10.1021/nl903534n. Angelatos, G., Hughes, S., 2015. Entanglement dynamics and Mollow nonuplets between two coupled quantum dots in a nanowire photonic-crystal system. Phys. Rev. A 91, 051803. http://dx.doi.org/10.1103/PhysRevA.91.051803. http://link.aps.org/ doi/10.1103/PhysRevA.91.051803. Babinec, T.M., Hausmann, B.J.M., Khan, M., Zhang, Y., Maze, J.R., Hemmer, P.R., Loncar, M., 2010. A diamond nanowire single-photon source. Nat. Nanotechnol. 5, 195–199. http://dx.doi.org/10.1038/nnano.2010.6. Benson, O., Santori, C., Pelton, M., Yamamoto, Y., 2000. Regulated and entangled photons from a single quantum dot. Phys. Rev. Lett. 84, 2513–2516. http://dx.doi.org/10.1103/ PhysRevLett.84.2513. http://link.aps.org/doi/10.1103/PhysRevLett.84.2513. Biasiol, G., Kapon, E., 1998. Mechanisms of self-ordering of quantum nanostructures grown on nonplanar surfaces. Phys. Rev. Lett. 81, 2962–2965. http://dx.doi.org/10.1103/ PhysRevLett.81.2962. http://link.aps.org/doi/10.1103/PhysRevLett.81.2962. Biasiol, G., Gustafsson, A., Leifer, K., Kapon, E., 2002. Mechanisms of self-ordering in nonplanar epitaxy of semiconductor nanostructures. Phys. Rev. B 65, 205306. http://dx.doi. org/10.1103/PhysRevB.65.205306. http://link.aps.org/doi/10.1103/PhysRevB.65. 205306. Birowosuto, M.D., Yokoo, A., Zhang, G., Tateno, K., Kuramochi, E., Taniyama, H., Takiguchi, M., Notomi, M., 2014. Movable high-Q nanoresonators realized by semiconductor nanowires on a Si photonic crystal platform. Nat. Mater. 13, 279–285. http:// dx.doi.org/10.1038/nmat3873. Bj€ ork, M.T., Ohlsson, B.J., Sass, T., Persson, A.I., Thelander, C., Magnusson, M.H., Deppert, K., Wallenberg, L.R., Samuelson, L., 2002. One-dimensional heterostructures in semiconductor nanowhiskers. Appl. Phys. Lett. 80 (6), 1058–1060. http://dx.doi.org/ 10.1063/1.1447312. http://scitation.aip.org/content/aip/journal/apl/80/6/10.1063/1. 1447312. Bj€ ork, M.T., Thelander, C., Hansen, A.E., Jensen, L.E., Larsson, M.W., Wallenberg, L.R., Samuelson, L., 2004. Few-electron quantum dots in nanowires. Nano Lett. 4 (9), 1621–1625. http://dx.doi.org/10.1021/nl049230s. Bleuse, J., Claudon, J., Creasey, M., Malik, N.S., Ge´rard, J.M., Maksymov, I., Hugonin, J.P., Lalanne, P., 2011. Inhibition, enhancement, and control of spontaneous emission in photonic nanowires. Phys. Rev. Lett. 106, 103601. http://dx.doi.org/10.1103/ PhysRevLett.106.103601. http://link.aps.org/doi/10.1103/PhysRevLett.106.103601. Borgstr€ om, M.T., Zwiller, V., Mu¨ller, E., Imamoglu, A., 2005. Optically bright quantum dots in single nanowires. Nano Lett. 5, 1439–1443. http://dx.doi.org/10.1021/ nl050802y.

180

Luca Francaviglia et al.

Bulgarini, G., Reimer, M.E., Hocevar, M., Bakkers, E.P.A.M., Kouwenhoven, L.P., Zwiller, V., 2012. Avalanche amplification of a single exciton in a semiconductor nanowire. Nat. Photonics 1749-48856, 455–458. http://dx.doi.org/10.1038/nphoton.2012.110. Bulgarini, G., Reimer, M.E., Zehender, T., Hocevar, M., Bakkers, E.P.A.M., Kouwenhoven, L.P., Zwiller, V., 2012. Spontaneous emission control of single quantum dots in bottom-up nanowire waveguides. Appl. Phys. Lett. 100 (12), 121106. http://dx. doi.org/10.1063/1.3694935. http://scitation.aip.org/content/aip/journal/apl/100/12/ 10.1063/1.3694935. Claudon, J., Bleuse, J., Malik, N.S., Bazin, M., Jaffrennou, P., Gregersen, N., Sauvan, C., Lalanne, P., Ge´rard, J.M., 2010. A highly efficient single-photon source based on a quantum dot in a photonic nanowire. Nat. Photonics 4, 174–177. http://dx.doi.org/ 10.1038/nphoton.2009.287. Claudon, J., Gregersen, N., Lalanne, P., Grard, J.M., 2013. Harnessing light with photonic nanowires: fundamentals and applications to quantum optics. ChemPhysChem 1439764114 (11), 2393–2402. http://dx.doi.org/10.1002/cphc.201300033. Colombo, C., Spirkoska, D., Frimmer, M., Abstreiter, G., Fontcuberta i Morral, A., 2008. Ga-assisted catalyst-free growth mechanism of GaAs nanowires by molecular beam epitaxy. Phys. Rev. B 77, 155326. http://dx.doi.org/10.1103/PhysRevB.77.155326. http://link.aps.org/doi/10.1103/PhysRevB.77.155326. Corfdir, P., Fontana, Y., Van Hattem, B., Russo-Averchi, E., Heiss, M., Fontcuberta i Morral, A., Phillips, R.T., 2014. Tuning the g-factor of neutral and charged excitons confined to self-assembled (Al,Ga)As shell quantum dots. Appl. Phys. Lett. 105 (22), 223111. http://dx.doi.org/10.1063/1.4903515. http://scitation.aip.org/content/aip/journal/ apl/105/22/10.1063/1.4903515. Dalacu, D., Mnaymneh, K., Lapointe, J., Wu, X., Poole, P.J., Bulgarini, G., Zwiller, V., Reimer, M.E., 2012. Ultraclean emission from InAsP quantum dots in defect-free wurtzite InP nanowires. Nano Lett. 12 (11), 5919–5923. http://dx.doi.org/10.1021/ nl303327h. Dayeh, S.A., Aplin, D.P.R., Zhou, X., Yu, P.K., Yu, E.T., Wang, D., 2007. High electron mobility InAs nanowire field-effect transistors. Small 1613-68293 (2), 326–332. http:// dx.doi.org/10.1002/smll.200600379. de la Mata, M., Zhou, X., Furtmayr, F., Teubert, J., Gradecak, S., Eickhoff, M., Fontcuberta i Morral, A., Arbiol, J., 2013. A review of MBE grown 0D, 1D and 2D quantum structures in a nanowire. J. Mater. Chem. C 1, 4300–4312. http://dx.doi.org/10.1039/ C3TC30556B. Dick, K.A., 2008. A review of nanowire growth promoted by alloys and non-alloying elements with emphasis on Au-assisted III-V nanowires. Prog. Cryst. Growth Charact. Mater. 0960-897454 (3–4), 138–173. http://dx.doi.org/10.1016/j.pcrysgrow.2008.09. 001. http://www.sciencedirect.com/science/article/pii/S0960897408000181. Dick, K.A., Bolinsson, J., Borg, B.M., Johansson, J., 2012. Controlling the abruptness of axial heterojunctions in III-V nanowires: beyond the reservoir effect. Nano Lett. 12 (6), 3200–3206. http://dx.doi.org/10.1021/nl301185x. Diedenhofen, S.L., Janssen, O.T.A., Hocevar, M., Pierret, A., Bakkers, E.P.A.M., Urbach, H.P., Rivas, J.G., 2011. Controlling the directional emission of light by periodic arrays of heterostructured semiconductor nanowires. ACS Nano 5 (7), 5830–5837. http://dx.doi.org/10.1021/nn201557h. Fontana, Y., Grzela, G., Bakkers, E.P.A.M., Rivas, J.G., 2012. Mapping the directional emission of quasi-two-dimensional photonic crystals of semiconductor nanowires using Fourier microscopy. Phys. Rev. B 86, 245303. http://dx.doi.org/10.1103/PhysRevB.86.245303. http://link.aps.org/doi/10.1103/PhysRevB.86.245303. Fontana, Y., Corfdir, P., Van Hattem, B., Russo-Averchi, E., Heiss, M., Sonderegger, S., Magen, C., Arbiol, J., Phillips, R.T., Fontcuberta i Morral, A., 2014. Exciton footprint

Quantum Dots in Nanowires

181

of self-assembled AlGaAs quantum dots in core-shell nanowires. Phys. Rev. B 90, 075307. http://dx.doi.org/10.1103/PhysRevB.90.075307. http://link.aps.org/ doi/10.1103/PhysRevB.90.075307. Fontcuberta i Morral, A., Colombo, C., Abstreiter, G., Arbiol, J., Morante, J.R., 2008. Nucleation mechanism of gallium-assisted molecular beam epitaxy growth of gallium arsenide nanowires. Appl. Phys. Lett. 92, 063112. http://dx.doi.org/10.1063/1.2837191. http:// scitation.aip.org/content/aip/journal/apl/92/6/10.1063/1.2837191. Foster, A.P., Bradley, J.P., Gardner, K., Krysa, A.B., Royall, B., Skolnick, M.S., Wilson, L.R., 2015. Linearly polarized emission from an embedded quantum dot using nanowire morphology control. Nano Lett. 15 (3), 1559–1563. http://dx.doi.org/10.1021/ nl503933n. Friedler, I., Sauvan, C., Hugonin, J.P., Lalanne, P., Claudon, J., Ge´rard, J.M., 2009. Solidstate single photon sources: the nanowire antenna. Opt. Express 17 (4), 2095–2110. http://dx.doi.org/10.1364/OE.17.002095. http://www.opticsexpress.org/abstract. cfm?URI¼oe-17-4-2095. Grzela, G., Paniagua-Domnguez, R., Barten, T., Fontana, Y., Snchez-Gil, J.A., Rivas, J.G., 2012. Nanowire antenna emission. Nano Lett. 12 (11), 5481–5486. http://dx.doi.org/ 10.1021/nl301907f. Heinrich, J., Huggenberger, A., Heindel, T., Reitzenstein, S., H€ ofling, S., Worschech, L., Forchel, A., 2010. Single photon emission from positioned GaAs/AlGaAs photonic nanowires. Appl. Phys. Lett. 96 (21), 211117. http://dx.doi.org/10.1063/1.3440967. http://scitation.aip.org/content/aip/journal/apl/96/21/10.1063/1.3440967. Heiss, M., Fontana, Y., Gustafsson, A., Wu¨st, G., Magen, C., O’Regan, D.D., Luo, J.W., Ketterer, B., Conesa-Boj, S., Kuhlmann, A., Houel, J., Russo-Averchi, E., Morante, J.R., Cantoni, M., Marzari, N., Arbiol, J., Zunger, A., Warburton, R.J., Fontcuberta i Morral, A., 2013. Self-assembled quantum dots in a nanowire system for quantum photonics. Nat. Mater. 12, 439–444. http://dx.doi.org/10.1038/nmat3557. Holmes, M.J., Choi, K., Kako, S., Arita, M., Arakawa, Y., 2014. Room-temperature triggered single photon emission from a III-nitride site-controlled nanowire quantum dot. Nano Lett. 14 (2), 982–986. http://dx.doi.org/10.1021/nl404400d. Holmes, M.J., Kako, S., Choi, K., Podemski, P., Arita, M., Arakawa, Y., 2015. Probing the excitonic states of site-controlled GaN nanowire quantum dots. Nano Lett. 15 (2), 1047–1051. http://dx.doi.org/10.1021/nl503949u. Huffaker, D.L., Park, G., Zou, Z., Shchekin, O.B., Deppe, D.G., 1998. 1.3 μm Roomtemperature GaAs-based quantum-dot laser. Appl. Phys. Lett. 73 (18), 2564–2566. http://dx.doi.org/10.1063/1.122534. http://scitation.aip.org/content/aip/journal/apl/ 73/18/10.1063/1.122534. Ikejiri, K., Noborisaka, J., Hara, S., Motohisa, J., Fukui, T., 2007. Mechanism of catalyst-free growth of GaAs nanowires by selective area MOVPE. J. Cryst. Growth 00220248298, 616–619. http://dx.doi.org/10.1016/j.jcrysgro.2006.10.179. http://www. sciencedirect.com/science/article/pii/S0022024806010773. Kouwen, M.P.V., Reimer, M.E., Hidma, A.W., van Weert, M.H.M., Algra, R.E., Bakkers, E.P.A.M., Kouwenhoven, L.P., Zwiller, V., 2010. Single electron charging in optically active nanowire quantum dots. Nano Lett. 10 (5), 1817–1822. http://dx. doi.org/10.1021/nl100520r. osele, U., 2008. Transition region width of nanowire hetero- and Li, N., Tan, T.Y., G€ pn-junctions grown using vapor-liquid-solid processes. Appl. Phys. A 0947839690 (4), 591–596. http://dx.doi.org/10.1007/s00339-007-4376-z. Minot, E.D., Kelkensberg, F., van Kouwen, M., van Dam, J.A., Kouwenhoven, L.P., Zwiller, V., Borgstr€ om, M.T., Wunnicke, O., Verheijen, M.A., Bakkers, E.P.A.M., 2007. Single quantum dot nanowire LEDs. Nano Lett. 7, 367–371. http://dx.doi. org/10.1021/nl062483w.

182

Luca Francaviglia et al.

Montinaro, M., Wu¨st, G., Munsch, M., Fontana, Y., Russo-Averchi, E., Heiss, M., Fontcuberta i Morral, A., Warburton, R.J., Poggio, M., 2014. Quantum dot optomechanics in a fully self-assembled nanowire. Nano Lett. 14 (8), 4454–4460. http:// dx.doi.org/10.1021/nl501413t. Motohisa, J., Noborisaka, J., Takeda, J., Inari, M., Fukui, T., 2004. Catalyst-free selective-area MOVPE of semiconductor nanowires on (111)B oriented substrates. J. Cryst. Growth 0022-0248272 (1–4), 180–185. http://dx.doi.org/10.1016/j.jcrysgro.2004.08.118. http://www.sciencedirect.com/science/article/pii/S0022024804010899. Munsch, M., Claudon, J., Bleuse, J., Malik, N.S., Dupuy, E., Ge´rard, J.M., Chen, Y., Gregersen, N., Mørk, J., 2012. Linearly polarized, single-mode spontaneous emission in a photonic nanowire. Phys. Rev. Lett. 108, 077405. http://dx.doi.org/ 10.1103/PhysRevLett.108.077405. http://link.aps.org/doi/10.1103/PhysRevLett. 108.077405. Munsch, M., Malik, N.S., Dupuy, E., Delga, A., Bleuse, J., Ge´rard, J.M., Claudon, J., Gregersen, N., Mørk, J., 2013. Dielectric GaAs antenna ensuring an efficient broadband coupling between an InAs quantum dot and a Gaussian optical beam. Phys. Rev. Lett. 110, 177402. http://dx.doi.org/10.1103/PhysRevLett.110.177402. http://link.aps.org/ doi/10.1103/PhysRevLett.110.177402. Nadj-Perge, S., Frolov, S.M., Bakkers, E.P.A.M., Kouwenhoven, L.P., 2010. Spin-orbit qubit in a semiconductor nanowire. Nature 468, 1084–1087. Newell, T., Bossert, D., Stintz, A., Fuchs, B., Malloy, K., Lester, L., 1999. Gain and linewidth enhancement factor in InAs quantum-dot laser diodes. IEEE Photonics Technol. Lett. 1041-113511 (12), 1527–1529. http://dx.doi.org/10.1109/68.806834. Ohlsson, B., Bj€ ork, M., Persson, A., Thelander, C., Wallenberg, L., Magnusson, M., Deppert, K., Samuelson, L., 2002. Growth and characterization of GaAs and InAs nano-whiskers and InAs/GaAs heterostructures. Phys. E 1386-947713 (2–4), 1126–1130. http://dx.doi.org/10.1016/S1386-9477(02)00318-1. http://www.science direct.com/science/article/pii/S1386947702003181. Pribiag, V.S., Nadj-Perge, S., Frolov, S.M., van den Berg, J.W.G., van Weperen, I., Plissard, S.R., Bakkers, E.P.A.M., Kouwenhoven, L.P., 2013. Electrical control of single hole spins in nanowire quantum dots. Nat. Nanotechnol. 8 (3), 170–174. http://dx.doi. org/10.1038/nnano.2013.5. Reimer, M.E., van Kouwen, M.P., Barkelid, M., Hocevar, M., van Weert, M.H.M., Algra, R.E., Bakkers, E.P.A.M., Bj€ ork, M.T., Schmid, H., Riel, H., Kouwenhoven, L.P., Zwiller, V., 2011. Single photon emission and detection at the nanoscale utilizing semiconductor nanowires. J. Nanophotonics 5 (1), 053502. http://dx.doi.org/ 10.1117/1.3562279. Reimer, M.E., van Kouwen, M.P., Hidma, A.W., van Weert, M.H.M., Bakkers, E.P.A.M., Kouwenhoven, L.P., Zwiller, V., 2011. Electric field induced removal of the biexciton binding energy in a single quantum dot. Nano Lett. 11 (2), 645–650. http://dx.doi.org/ 10.1021/nl1037424. Reimer, M.E., Bulgarini, G., Akopian, N., Hocevar, M., Bavinck, M.B., Verheijen, M.A., Bakkers, E.P., Kouwenhoven, L.P., Zwiller, V., 2012. Bright single-photon sources in bottom-up tailored nanowires. Nat. Commun. 3, 737. http://dx.doi.org/10.1038/ ncomms1746. Sanada, H., Gotoh, H., Tateno, K., Nakano, H., 2007. Exciton and biexciton emissions from single GaAs quantum dots in (Al,Ga)As nanowires. Jpn. J. Appl. Phys. 46 (4S), 2578. http://stacks.iop.org/1347-4065/46/i¼4S/a¼2578. Scofield, A.C., Kim, S.H., Shapiro, J.N., Lin, A., Liang, B., Scherer, A., Huffaker, D.L., 2011. Bottom-up photonic crystal lasers. Nano Lett. 11 (12), 5387–5390. http://dx. doi.org/10.1021/nl2030163.

Quantum Dots in Nanowires

183

Singh, R., Bester, G., 2009. Nanowire quantum dots as an ideal source of entangled photon pairs. Phys. Rev. Lett. 103, 063601. http://dx.doi.org/10.1103/PhysRevLett.103. 063601. http://link.aps.org/doi/10.1103/PhysRevLett.103.063601. Sk€ old, N., Wagner, J.B., Karlsson, G., Herna´n, T., Seifert, W., Pistol, M.E., Samuelson, L., 2006. Phase segregation in AlInP shells on GaAs nanowires. Nano Lett. 6 (12), 2743–2747. http://dx.doi.org/10.1021/nl061692d. Tatebayashi, J., Ota, Y., Ishida, S., Nishioka, M., Iwamoto, S., Arakawa, Y., 2012. Site-controlled formation of InAs/GaAs quantum-dot-in-nanowires for single photon emitters. Appl. Phys. Lett. 100 (26), 263101. http://dx.doi.org/10.1063/1.4731208. http://scitation.aip.org/content/aip/journal/apl/100/26/10.1063/1.4731208. Tatebayashi, J., Ota, Y., Ishida, S., Nishioka, M., Iwamoto, S., Arakawa, Y., 2014. Highly uniform, multi-stacked InGaAs/GaAs quantum dots embedded in a GaAs nanowire. Appl. Phys. Lett. 105 (10), 103104. http://dx.doi.org/10.1063/1.4895597. http:// scitation.aip.org/content/aip/journal/apl/105/10/10.1063/1.4895597. ork, M.T., Ohlsson, B.J., Larsson, M.W., Wallenberg, L.R., Thelander, C., Ma˚rtensson, T., Bj€ Samuelson, L., 2003. Single-electron transistors in heterostructure nanowires. Appl. Phys. Lett. 83 (10), 2052–2054. http://dx.doi.org/10.1063/1.1606889. http://scitation.aip. org/content/aip/journal/apl/83/10/10.1063/1.1606889. Thelander, C., Bj€ ork, M., Larsson, M., Hansen, A., Wallenberg, L., Samuelson, L., 2004. Electron transport in InAs nanowires and heterostructure nanowire devices. Solid State Commun. 0038-1098131 (9–10), 573–579. http://dx.doi.org/10.1016/j.ssc.2004.05. 033. http://www.sciencedirect.com/science/article/pii/S0038109804004156. Tribu, A., Sallen, G., Aichele, T., Andre´, R., Poizat, J.P., Bougerol, C., Tatarenko, S., Kheng, K., 2008. A high-temperature single-photon source from nanowire quantum dots. Nano Lett. 8 (12), 4326–4329. http://dx.doi.org/10.1021/nl802160z. Uccelli, E., Arbiol, J., Morante, J.R., Fontcuberta i Morral, A., 2010. InAs quantum dot arrays decorating the facets of GaAs nanowires. ACS Nano 4 (10), 5985–5993. http:// dx.doi.org/10.1021/nn101604k. Vainorius, N., Jacobsson, D., Lehmann, S., Gustafsson, A., Dick, K.A., Samuelson, L., Pistol, M.E., 2014. Observation of type-II recombination in single wurtzite/zinc-blende GaAs heterojunction nanowires. Phys. Rev. B 89, 165423. http://dx.doi.org/10.1103/ PhysRevB.89.165423. http://link.aps.org/doi/10.1103/PhysRevB.89.165423. Vainorius, N., Lehmann, S., Jacobsson, D., Samuelson, L., Dick, K.A., Pistol, M.E., 2015. Confinement in thickness-controlled GaAs polytype nanodots. Nano Lett. 15 (4), 2652–2656. http://dx.doi.org/10.1021/acs.nanolett.5b00253. van Kouwen, M.P., van Weert, M.H.M., Reimer, M.E., Akopian, N., Perinetti, U., Algra, R.E., Bakkers, E.P.A.M., Kouwenhoven, L.P., Zwiller, V., 2010. Single quantum dot nanowire photodetectors. Appl. Phys. Lett. 97 (11). http://dx.doi.org/10.1063/ 1.3484962 http://scitation.aip.org/content/aip/journal/apl/97/11/10.1063/1.3484962. van Weert, M.H.M., Akopian, N., Kelkensberg, F., Perinetti, U., van Kouwen, M.P., Rivas, J.G., Borgst€ orm, M.T., Algra, R.E., Verheijen, M.A., Bakkers, E.P.A.M., Kouwenhoven, L.P., Zwiller, V., 2009. Orientation-dependent optical-polarization properties of single quantum dots in nanowires. Small 1613-68295 (19), 2134–2138. http://dx.doi.org/10.1002/smll.200900423. Wagner, R.S., Ellis, W.C., 1964. Vapor-liquid-solid mechanism of single crystal growth. Appl. Phys. Lett. 4 (5), 89–90. http://scitation.aip.org/content/aip/journal/apl/4/5/ 10.1063/1.1753975. old, N., Wallenberg, L.R., Samuelson, L., 2010. Growth and segregation of Wagner, J.B., Sk€ GaAs-AlxIn1-xP core-shell nanowires. J. Cryst. Growth 0022-0248312 (10), 1755–1760. http://dx.doi.org/10.1016/j.jcrysgro.2010.02.009. http://www.sciencedirect.com/science/ article/pii/S0022024810000837.

184

Luca Francaviglia et al.

Yan, X., Zhang, X., Ren, X., Lv, X., Li, J., Wang, Q., Cai, S., Huang, Y., 2012. Formation mechanism and optical properties of InAs quantum dots on the surface of GaAs nanowires. Nano Lett. 12 (4), 1851–1856. http://dx.doi.org/10.1021/nl204204f. Yan, X., Zhang, X., Li, J., Cui, J., Wang, Q., Huang, Y., Ren, X., 2013. Growth of InAs quantum dots on Si-based GaAs nanowires by controlling the surface adatom diffusion. J. Cryst. Growth 0022-0248384 (0), 82–87. http://dx.doi.org/10.1016/j.jcrysgro.2013.09.014. http://www.sciencedirect.com/science/article/pii/S0022024813006155. Yeo, I., de Assis, P.L., Gloppe, A., Dupont-Ferrier, E., Verlot, P., Malik, N.S., Dupuy, E., Claudon, J., Ge´rard, J.M., Auffe`ves, A., Nogues, G., Seidelin, S., Poizat, J.P., Arcizet, O., Richard, M., 2014. Strain-mediated coupling in a quantum dot-mechanical oscillator hybrid system. Nat. Nanotechnol. 9, 106–110. http://dx.doi.org/10.1038/ nnano.2013.274. Zheng, C., Wong-Leung, J., Gao, Q., Tan, H.H., Jagadish, C., Etheridge, J., 2013. Polaritydriven 3-fold symmetry of GaAs/AlGaAs core multishell nanowires. Nano Lett. 13, 3742–3748. http://dx.doi.org/10.1021/nl401680k.