Wang Zh.M. One-Dimensional Nanostructures

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12 Ordering of Self-Assembled Quantum Wires on InP(001) Surfaces 315

Fig. 12.25 Temperature de-

pendence of the PL peak

energies of the thick wires

(a) and thin wires (b)inthe

sample

which is the common character of the QDs (QWRs) structures and can be attributed

to the carriers redistribution among the QWRs with different sizes. As a compari-

son, the lower energy peak is almost ﬁxed at 0.68 eV. This unusual shift of PL peak

is assumed to be related to the thermal strain. Because the linear thermal expansion

coefﬁcient of In

0.52

0.48

As (4.85×10

−6

A/K at 300 K) is larger than that of InP

(4.5×10

−6

A/K at 300 K) [53], a net thermal compressive strain will be introduced

in the InAlAs layers as the temperature increases. This compressive strain in the

InAlAs layers is passed to the InAs nanostructures buried in the InAlAs matrix.

As shown in the inset of Fig. 12.24, the thick wires, which contribute to the lower

energy PL peak, are usually centered at the places where two or three thin wires

joint. It means the thick wires can serve probably as the concentrations of force,

i.e. the stress transferred through the thin InAs wires can be ampliﬁed at the thick

wires. In addition, this thermal strain in the thick wires should have a multiaxial

nature due to the asymmetric distribution of the surrounding thin InAs wires. It has

been reported that the multiaxial asymmetric lattice distortion does not change lin-

early with temperature and the linear thermal expansion coefﬁcient should also be

considered in precise treatment [51, 54]. Therefore, the ampliﬁed multiaxial com-

pressive strain can induce a large blue-shift for the PL peak of the thicker wires,

which compensates the red shift induced by the increase in temperature.

12.4.3 Phonon Vibration Property

Phonons are important for the carrier relaxation process of QWRs (QDs), espe-

cially when the splitting of the energy levels in QWRs (QDs) matches a multiple

316 W. Lei et al.

Fig. 12.26 Raman spectra of

the samples grown with 2,

4, and 6 ML InAs measured

under the z(x



)z conﬁg-

uration. The inset shows the

QWR LO phonon peaks after

subtraction of the InAs-like

background

of the QWR (QD) phonon energy. Because of the two dimensional conﬁnement,

QWRs may show vibrational properties different from those of QDs and quantum

wells (QWs). So, the knowledge of the phonons in QWRs will be helpful for the

understanding of fundamental physics in low dimensional system and their device

applications.

Figure 12.26 shows the Raman spectra of InAs/InAlAs/InP(001) QWR sam-

ples grown with 2, 4, and 6 ML InAs deposited thickness measured under the

z(x,x+y)

z conﬁguration ( x



[100], y



[010], z



[001], x





10], y





[110])[55]. The

samples consist of six layers of InAs separated by 15 nm In

0.52

0.48

As spacer

layers. As shown in Fig. 12.26, all the Raman spectra display one intense peak

centered at 235cm

−1

, which corresponds to the InAs-like LO mode in the In-

AlAs matrix [56–58]. Above the InAs-like LO mode peak, the Raman spectra of

the samples all display an additional peak whose frequency depends on the sample

(249, 250, and 249cm

−1

for the sample with 2, 4, and 6 ML InAs deposited thick-

ness, respectively). These three peaks can be attributed to the LO phonon of the

InAs QWRs [59]. The QWR LO phonon peak increases in intensity with increas-

ing the InAs deposited thickness due to the increase of In-As bonds in the QWRs.

The different frequency shift of the QWR LO phonon peak can be understood by

the different strain in the QWRs grown with different InAs deposited thickness [55].

The polarized Raman spectra of the samples are shown in Fig. 12.27 [55]. The

QWR LO phonons show an obvious polarization dependence as they are observed

in the z(x



)z conﬁguration, but not in the z(x



)z conﬁguration, as shown in

Fig. 12.27a and b. This demonstrates that the QWR LO phonons follow the selection

rule of the LO phonons in the bulk zinc-blende semiconductors, which is similar to

the Raman scattering results on InAs/GaAs QDs [59]. As shown in Fig. 12.27c, the

QWR LO phonons and the InAs-like LO phonons in InAlAs matrix of the sample

with 6 ML InAs deposited thickness also appear in the z(x



)z polarization con-

ﬁguration, which can be ascribed to the threading dislocations in the sample. The

threading dislocations may relax the selection rule of the phonons in both the InAs

wires and InAlAs matrix, leading to the appearance of LO phonons in the z(x



polarization conﬁguration [55].

12 Ordering of Self-Assembled Quantum Wires on InP(001) Surfaces 317

Fig. 12.27 Polarized Raman

spectra of the samples: (a)

thesamplegrownwith2ML

InAs, (b) the sample grown

with 4 ML InAs, (c)the

sample grown with 6ML

InAs. The insets show the

partially zoom-in images of

the Raman spectra of the

samples

12.4.4 Intraband Photocurrent

Theoretically, QWRs allow the absorption of normal-incidence radiation due to their

special selection rule of intraband transition [1, 60]. So, QWRs can be used for

fabricating infrared photodetectors with normal incidence, which takes advantage

of the intraband transition in QWRs. Here, the intraband transition in QWRs is

investigated with photocurrent (PC) spectroscopy [61]. Two kinds of infrared light

sources (tungsten lamp and globar lamp) are used in the PC measurements. Lat-

eral electrodes with 1.5 mm spacing in between are obtained by alloying the Indium

stripe in the sample surface. The electrical ﬁelds are applied along the [110] direc-

tion. When the PC measurements are performed with a globar lamp, a solid-state

laser (532 nm) is used as the pump light source for optical pumping. All photocur-

rent spectra are measured in a normal incidence conﬁguration.

Figure 12.28 shows the 14 K PL and PC spectra of an InAs/InAlAs/InP(001)

QWR sample measured with tungsten lamp. The sample consists of six layers of

InAs QWRs (3 ML) separated by 15 nm In

0.52

0.48

As barrier layers. The 15 nm

InAlAs barriers are composed of 4 nm undoped InAlAs, 2 nm Si-doped InAlAs

(1×10

−3

) and 9 nm undoped InAlAs. The inset of Fig. 12.28a shows the typ-

ical AFM image of a sample grown under the same growth condition, in which

wire-like InAs structures are observed. Comparing with PL spectra and AFM im-

age, the higher energy PL peak is attributed to the thin wire-like structures, while

the lower energy PL peak is attributed to the thick wire-like structures.

Compared with the PL spectrum, more features are revealed in the PC spectrum,

as shown in Fig. 12.28b. The PC features centered at 1.55 and 1.42 eV are from the

318 W. Lei et al.

Fig. 12.28 PL (a)andPC

(b) spectra of the sample

measured at 14 K. The

inset in panel (a)shows

the typical AFM image of a

sample grown under the same

condition without cap layer.

The size of the AFM image

is 0.5µm ×0.5µm

Fig. 12.29 Temperature de-

pendent intraband PC spectra

of the sample measured with

a globar lamp under a steady

interband excitation

interband transition of the InAlAs matrix and InP substrate, respectively. The PC

signal ranging from 0.6 to 1.4 eV can be ascribed to the interband transitions of

the wire-like InAs structures. Because of the large size distribution of the nanos-

tructures, a large Stokes shift is observed between PL and interband PC peaks of

the nanostructures [62]. Besides these PC peaks, there is a broad PC peak located

around 0.41 eV, which can be attributed to the intraband transition from the bound

states of the InAs wire-like structures to the continuum states of the InAlAs barri-

ers [61]. It should be pointed out that the intraband PC signal in Fig. 12.28b can be

observed only when there is an interband excitation (the illumination of the tung-

sten lamp serves as the interband excitation in Fig. 12.28b), which may be due to the

possible deep traps in the sample structure and can be improved by growing high

quality sample [61].

For infrared photodetectors, one important parameter is their operation tem-

perature. Figure 12.29 shows the temperature dependent intraband PC spectra of

the sample measured with a globar lamp under steady interband excitation. It is

12 Ordering of Self-Assembled Quantum Wires on InP(001) Surfaces 319

observed that the intraband PC signal decreases with increasing temperature, which

can be attributed to the decrease of the lifetime and mobility of electrons and the

thermal escape of electrons. In addition, the linewidth of the intraband PC signal

decreases and the low energy cut-off of the inraband PC peak shifts to higher energy

gradually when the temperature is increased. These changes mainly arise from the

thermal ionization of electrons from the thinner InAs wires. Clearly, the intraband

PC signal of the InAs QWRs can be observed up to about 160 K, indicating their

great potential applications in infrared photodetectors.

12.4.5 Quantum Wire Lasers

Lasers based on self-assembled InAs QWRs or quantum dashes (QDash) grown

on InP(001) substrates can extend the lasing wavelength up to 1.55µm and even

above, which is important for the long-wavelength optical ﬁber communication.

The InP-based InAs QWRs or QDashes laser was ﬁrst reported by Wang et al. in

2001 [63]. In this device the InAs QDash layers were embedded in a compressively

strained InAlGaAs waveguides, and then surrounded by InAlAs cladding layers lat-

tice matched to InP substrate. The laser worked in a pulsed mode at room tempera-

ture with a ground state lasing wavelength of 1.6µm and the threshold current den-

sity of 410A/cm

. By using a dash-in-well structure, in which the InAs QDashes

were inserted in a compressively strained InGaAs QWs, the lasing wavelength was

extended up to 2.03µm at room temperature [64]. Room temperature continuous

wave (CW) working of the InAs QDash laser was achieved in 2004 [65]. The thresh-

old current density of the laser was below 1.2kA/cm

, and the lasing wavelength

was about 1.57µm with a output power of 50 mW per facet and no degradation was

found at 10 mW/facet output for over 6500 hours in the aging test [66]. The room

temperature CW operating of InAs QWR laser was demonstrated in 2006 [67]. In

this QWR laser device, the ground state lasing wavelength was about 1.73µm and

the threshold current showed obvious dependence on the laser cavity orientation.

The threshold current of the laser with cavity along the [1–10] direction was larger

than that with cavity along the [110] direction, which could be attributed to the

different optical gain for different electric ﬁeld polarization.

12.5 Summary

In this chapter, we brieﬂy present the recent advance in the ordering of InAs QWRs

on InP(001) substrates. As reviewed earlier, considerable progress has been made in

this ﬁeld. By tuning the LCM effect in InAlAs spacer layer, the island shape, island

density, and spatial correlation can be controlled to a certain extent. And, uniform

InAs QWRs with high quality can be achieved by using the strain compensating

technology. However, the growth control and optical properties of these QWRs are

320 W. Lei et al.

still not satisfying and need to be improved further for their device applications,

which presents a big challenge to us. To achieve the ideal QWRs, we have a lot of

work to do!

Acknowledgments This work was supported by the 973 program (2006CB604908) and the

National Natural Science Foundation of China (60390074, 60625402).

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Index

AAM. See Anodic alumina membranes

AAO template, 34

Al-decorated nanotubes, 235

Amorphous materials, 167

Anisotropic crystal growth, 276

Anisotropic scatterers, 135

Anisotropic surface energy, 194

Anodic alumina membranes, 34

Antenna-like plasmon resonances, 177

Antenna resonances, 176, 183, 200–202. See

also Particle excitations

Arc discharge process, 21–23

Atomic conﬁgurations

of GaN nanotube, 114–115, 118, 119

slabs, 104

of stretching tube 2, 113

Au-Ga alloy, 6

Au-In alloy

liquid-like migration of, 7, 8

oxidation of, 4–5

Au-In eutectic temperature, 2

Au-Intip,2,4,7

Au nanoparticles

in InAs nanowire growth, 8, 10

in optical lithography-patterned substrate,

in Si nanowire growth, 3

on SiO

surface, 86

Auxiliary differential equation method

for studying dispersion properties, 150–151

Ballistic electron transport, 218

BDT temperature, 113

of cubic GaN, 118

and nanotube thickness, 116

and rate of applied loading, 117

Binary system, metallic

Ga(In) content in Au-Ga(Au-In), 10

melting point depression of, 7–8

Birefringence, 134

angular dependent reﬂection and transmis-

sion, 137

long nanowires, 139–141

parameter estimation

in ensembles of aligned nanowires, 137

of layers of GaP nanowires, 142

Maxwell–Garnett approximation,

anisotropic media, 136

short nanowires, 141–143

Birefringent layer, single transmission, 138

BN nanotube sheath, 46

Bond length alternation (BLA), 267

Bottom-up chemical synthesis, 275

Boundary element method (BEM)

for solving Maxwell’s equations, 183

Brittle to ductile transition temperature. See

BDT temperature

Buckling behavior, GaN nanostructures

critical stress and strain, 121–122

stress-strain response, 118–119

temperature and strain rate effects on,

119–120

tube length effect on, 121

Bulk GaN, potential energy variation, 102

Carbon-coated copper TEM grid, 285

Carbon nanotube (CNT), 217. See also

Multi-walled nanotubes (MWNTs);

Single-walled nanotubes (SWNTs)

conﬁned reaction, 20–21

323

324 Index

fabrication with contact engineering,

227–231

metal clusters effects on, 233–238

metal oxidation effects on, 219–223

pressure and interfacial contact effects on,

223–227

projected density of states of, 223–224

quantum interference patterns observed in,

218

semiconducting, 220

Channel-decorated devices, 233

Chemical sensors, 233

Chemical vapor deposition

for formation of nanostructures, 25

for growth of semiconductor nanowires, 128

Si substrate

β-SiC nanowires, 25–26

SiC nanorods, 26

Coaxial nanocables, 38

Copper nanowires. See also Nanowire

electrical transport properties of, 192

electrochemically deposition of, 191

Core shell heterostructures, 94

Core-shell p-n junctions, fabrication of, 69

Core-shell SiC/SiO2 nanowires

ﬁeld-emission currents, 42

Young’s modulus for, 48

Critical points models, 153

used in conjunction with Drude model,

155–156

Cu-silicide/Si/oxide nanochains, 67

CVD. See Chemical vapor deposition

Data storage devices, performance of, 287

Debye model, 154

Density-functional theory (DFT) method, 219

Diameter ﬂuctuations, SiC nanowires

as fractal, 73–74

mono-afﬁnity and multiafﬁnity, 74–75

as random walk, 75–76

Diluted magnetic semiconductor (DMS)

nanowires, 248

Dipolar resonance, 179

Dipole discrete dipole approximation (DDA),

183

Dispersion models, combinations of, 156–157,

159

Dispersive materials

FDTD method for modeling, 148

studied using Debye, Drude, or Lorentz

models, 153

Drude–Lorentz model, 166, 205

Drude model, 154

combination with Lorentz model, 156–157

for studying dispersion properties, 150

Electric-ﬁeld assisted assembly technique, 82

Electromagnetic boundary conditions, 131

Electromagnetic nanowire resonances, 177

Electromagnetic waves scattering, by metal

objects, 179

Electron-beam lithography (EBL), 187,

194–196

Electron-ion interactions, 219

Energy dispersive X-ray spectrometry (EDX),

278, 295

Fermi level, 220, 225

Ferromagnetic semiconductor nanostructures,

248

Fe-silicide nanowires

β-FeSi

nanowires

Au/Si templates, annealing, 72

cooling process, 73

fabrication of, 71

α-FeSi

and ε-FeSi nanowires, 71

Finite density of states (DOS), 248

Finite-Difference Time-Domain (FDTD)

method

antenna modes, existence of, 179

calculation of SERS gains using, 205

elecromagnetic phenomena, for modeling

of, 148

for plasmonic structures’ design and

optimization, 147

Fowler–Nordheim relationship, 41–42

GaAs nanowires, growth of, 6–7

Gallium nitride, 98

SW potential for, 101

GaN nanotubes

buckling behavior of, 118–122

construction, 98–100

hexagonal structure of, 100

melting processes, simulation of, 101–102

melting temperature dependence on

thickness, 106

potential energy variation of, 104–105

snapshots of, 105

tensile behavior of, 112–118

thermal conductivity of, 106–111

GaN nanowires, 250

GaP nanowires

birefringence parameter, 142, 143

cross-sectional SEM images of, 130, 142