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GaSb/GaAs Quantum Nanostructures by Molecular Beam Epitaxy 289

height to diameter ratio ﬁ xed at 0.3. The resultant binding energy ranged from a negative value

of  12 to  19 meV. Although the two exciton energy shift relative to the exciton could qualita-

tively be attributed to the repulsion between the two conﬁ ned holes, the contributions from the

electron–hole attraction and electron–electron repulsion are not negligible. For example, for the

4.8 nm height QD, the two exciton binding energy of 19 meV includes 27 meV of the hole–hole

repulsion, 5 meV of the electron–electron repulsion, and 12 meV of the electron–hole attrac-

tion. The strong correlation exists between a pair of excitons in GaSb type II QDs having an anti-

bound nature due to these Coulomb interactions.

8.3.2.5 Exciton ﬁ ne structures of GaSb/GaAs QDs under a magnetic ﬁ eld

Magnetic ﬁ eld is one of most useful and controllable parameters for investigating the intrinsic

nature of the excitons. Figure 8.23 a shows the PL spectra of a single GaSb QD under a mag-

netic ﬁ eld with Faraday conﬁ guration. The sharp exciton emission line splits gradually into two

peaks with increasing magnetic ﬁ eld [83] . Figure 8.23a shows the centre energy of the two

peaks, which corresponds to a diamagnetic shift [83] . The energy positions after subtracting the

5 6 7

20

18

16

14

12

10

Height of QD (nm)

Binding Energy (meV)

GaSb QD

Calculated energy

Figure 8.22 Calculated two exciton binding energy as a function of lens-shaped GaSb QDs (height to diameter ratio

0.3) [74] . Reprinted with permission from K. Matsuda et al. , Appl. Phys. Lett. 90, 013101 (2007). Copyright

2007, American Institute of Physics.

600

400

200

200

400

600

012345

Magnetic field (T)

PL intensity (a.u.)

5 T

3 T

2 T

1 T

0 T

4 T

1.287 1.288 1.289

(a) (c)

012 345

(b)

Energy (eV)

Magnetic field (T)

Energy shift (µeV) Energy shift (µeV)

600

500

400

300

200

100

Figure 8.23 (a) A single GaSb QD PL spectra under a magnetic ﬁ eld. (b) Diamagnetic shift and Zeeman splitting of

Elsevier.

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290 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

contribution of the diamagnetic shift, due to Zeeman splitting, are also given in Fig. 8.23 c [83] .

The energies of the split peaks are given by

EE g B B

ex B



μβ+

(8.10)

where E

is the peak energy at 0 T, μ

is the Bohr magneton, g

is the g -factor of the exciton,

and β is the diamagnetic coefﬁ cient. From the experimental data, 25 μ eV/T

is obtained for the

diamagnetic coefﬁ cient and 3.7 for the g -factor. The derived diamagnetic coefﬁ cient is larger than

that typical of a type I In(Ga)As QD [84] , and is attributable to the weak electron conﬁ nement

outside the GaSb QD, because the diamagnetic coefﬁ cient is determined by the radius of the exci-

ton wavefunction and the effective mass. The g -factor of GaSb QD is also slightly larger than that

of In(Ga)As QDs. The exciton state in GaSb QD also has characteristic magneto-optical properties

due to the type II band structure.

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GaSb/GaAs Quantum Nanostructures by Molecular Beam Epitaxy 291

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292 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

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Growth and Characterization of ZnO Nano- and Microstructures

Marius Grundmann

Andreas Rahm ,

Thomas Nobis,

Michael Lorenz ,

Christian Czekalla

Evgeni M. Kaidashev,

Jörg Lenzner ,

Nikos Boukos,

and Anastasios Travlos

Universität Leipzig, Fakultät für Physik und Geowissenschaften, Institut für

Experimentelle Physik II, Linnéstr. 5, D-04103 Leipzig, Germany

Institut of Materials Science, National Center for Scientiﬁ c Research Demokritos,

GR 15310 Ag. Paraskevi Attikis, POB 60228 Athens, Greece

9.1 Introduction

Zinc oxide (ZnO) was subject to intensive research in the 1950s [1] . Since then it had been rediscov-

ered several times. From the late 1990s of the last century there is again renewed interest in this

wide-band gap II–VI semiconductor. In particular, it is considered a promising candidate for use as

visible and UV light emitters. The ﬁ rst report of lasing in ZnO by optical pumping at 2 K dates back

to 1996 [2] . In 1997, lasing of the exciton was observed at room temperature by two groups inde-

pendently [3, 4] . The material exhibits tunable defect and dopant-related luminescence properties

in the visible region of the spectrum [1] . In contrast to gallium nitride (GaN), its main competitor

for ultraviolet (UV) applications, ZnO exhibits favourable properties such as a high exciton binding

energy of 60 meV (prospect of low laser threshold at room temperature, in contrast to 25 meV for

GaN), unproblematic wet-chemical etching, radiation hardness (space applications) and the avail-

ability of substrates for homoepitaxy. They are, for instance, available from Tokyo Denpa [5] . Other

advantages of ZnO include its biocompatibility [6] , a high piezoelectric constant [7] and large opti-

cal gain [8] . This fuels high device-related expectations, not only for optoelectronics but also for bio-

medical and biosensor applications.

ZnO is already in everyday use as UV blockers in sun tan lotions, in surface acoustic wave

devices (SAW), as gas sensors, in high-ﬁ eld varistors and as a catalyst, to name only a few of its

well-established applications.

Furthermore, it has been shown that the band gap of zinc oxide can be controlled between

2.8 eV and 4 eV by alloying with CdO and MgO, respectively [9–11] . ZnO/Mg

1 

O quantum

wells [12, 13] and superlattices [14] as well as Mg

1 

O/Cd

1  y

O multiquantum wells have

been studied [15] .

However, many potential applications are still hampered by the lack of control over p-type

conductivity. Typically, ZnO is n-type which is believed to be due to native point defects [16] , and

although there are several attempts to achieve p-type doping and conduction, reproducibility

and temporal stability are currently serious issues [17] . Recent progress includes the observation

of near UV to blue colour electroluminescence from a ZnO p-i-n homojunction light-emitting

diode (LED) grown by laser molecular-beam epitaxy as reported by Tsukazaki et al. [18, 19] .

Furthermore, ultraviolet electroluminescence from n-type ZnO/p-type polymer heterojunction

CHAPTER 9

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294 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

LEDs grown at low temperatures was demonstrated [20] . Also, it is expected that the spin degree

of freedom will play a signiﬁ cant role in 21st century information technology [21] . Due to its

predicted ferromagnetism above room temperature [22] , heavily doped p-type ZnMnO is consid-

ered a promising material for spintronics [23] which could pave the way to exploit spin in addi-

tion to charge in semiconductor devices. It was shown that the incorporation of 3D transition

metals can produce ferromagnetism in ZnO above room temperature [24] .

Especially nanoscale one-dimensional zinc oxide (ZnO) structures have stimulated great interest

during the past years because of their extraordinary structural diversity [25] . Nanowires, -tubes,

-belts, -ribbons, -sheets and many more complex ZnO structures can be fabricated. There are

reports on the synthesis of quasi-1D and 2D nanostructures [25] and combinations thereof

[26] , of tetrapod zinc oxide structures [27] , of tapered tip morphologies [28] and of ﬂ owerlike

ZnO [29] . At present, nanowires are the second hottest research topic in physics after carbon

nanotubes [30, 31] according to ranking scientiﬁ c ﬁ elds based on the “ Hirsch index ” [32] . Due

to its c-axis dominated growth behaviour, ZnO is of particular interest for self-assembled growth

of nanowire structures. Such bottom-up architecture approaches are believed to lead to applica-

tions which are, at least to a certain amount, defect tolerant. A considerable amount of work

has been done on catalyst-driven self-organization processes which are based on the vapour–

liquid–solid (VLS) growth mechanism proposed by Wagner and Ellis in 1964 [33] . Recent

progress in ZnO nanowires research is documented, for instance, in [34, 35] .

ZnO whiskers can be utilized for the understanding of basic research phenomena [36] , but

primarily the unique physical properties were employed to demonstrate a variety of nanodimen-

sional device prototypes. They include nanoscale lasers [37] , nanocantilevers [38] , piezoelec-

tric nanogenerators based on ZnO nanowire arrays [39] , ZnO nanorod logic circuits [40] and

nanowire-ﬁ eld effect transistors [41] . Also, ﬁ eld-emission from ZnO nanowires was observed

[42] . They can further be employed to detect various chemical species like oxygen [43, 44] , eth-

anol [45] , NO

or NH

[46] . ZnO/Mg

1 

O nanorod multi-quantum-well structures (MQWs)

were fabricated and quantum conﬁ nement effects were observed [47, 48] . Room-temperature

ferromagnetism in well-aligned Zn

1 

O nanorods [49] was also detected. However, detailed

studies on the growth evolution and formation mechanisms, as well as on the lateral homogene-

ity of samples, are rare in the literature [50] .

Pulsed laser deposition (PLD) has been proven to be a versatile technique for the growth of

high-quality ZnO-based thin ﬁ lms [51, 52] . In this chapter, the growth of ZnO nanostructures

by a unique high-pressure PLD process will be studied in detail. Those structures will be char-

acterized structurally and optically with the main focus on so-called whispering gallery modes

in which our nanostructures act as dielectric resonators. Furthermore, we present stimulated

emission from ZnO microcrystals grown by carbothermal evaporation by spatially resolved pho-

toluminescence (PL) and high excitation spectroscopy (HES). Electrical characterization of those

crystals is also reported here.

9.2 Growth of nanowires PLD

9.2.1 Synthesis strategies for ZnO nanostructures

In the 1960s, the vapour–liquid–solid (VLS) mechanism was proposed by Wagner and Ellis as a

model to explain the morphology and growth of silicon whiskers [33, 53] . It was observed that

(a) an impurity was essential for whisker growth and (b) a small globule was present at the tip

of each whisker [33] . In the mechanism, the impurity is thought to form a liquid alloy droplet

which acts as a preferred site for deposition of particles from the vapour phase which then causes

the droplet to become supersaturated. For instance, a small particle of Au can be intentionally

used as such an impurity. At elevated temperature a small droplet of Si–Au formed in the case

investigated by Wagner and Ellis. This alloy has a high sticking coefﬁ cient for arriving atoms and

hence more Si enters the droplet, supersaturates it and ﬁ nally freezes out at the substrate. This

process results in whiskers with an alloy droplet on top. Typical more recent examples of semi-

conductor nanowire growth controlled by the VLS mechanism are Si [54, 55] and GaAs [56] .

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Growth and Characterization of ZnO Nano- and Microstuctures 295

For Si nanowires the VLS synthesis can be directly observed at different growth stages [57] . The

choice of the proper impurity/catalyst depends on a number of factors such as, the inertness to

the reaction products and the formation of a liquid alloy at the growth temperature [33] . Gold as

an impurity is very well suited to form an alloy with Zn at temperatures below 1000°C according

to their binary phase diagram.

A detailed study of the fundamental aspects of VLS growth, like kinetics, temperature effects

or the role of surface diffusion, was presented in 1975 by E.I. Givargizov [58] . Important results

include the observation that thin whiskers grow slower than thick ones and that there is a mini-

mum critical diameter at which growth stops completely due to a decrease of supersaturation

[58] . Experimental proof of this theory has been observed for GaAs, GaP, InAs and InP [59] . The

growth rate of whiskers depends on four mains steps [58] : (i) mass transport in the gas phase,

(ii) chemical reaction on the vapour–liquid interface, (iii) diffusion in the liquid phase and (iv) the

incorporation of the material in the crystal lattice.

Beyond those considerations for ideal case scenarios, there are other factors which can play

a role in catalysed nanowire growth. Every model has its limits: contrary to the calculations of

Givargizov [58] , there are predictions that the attainable VLS nanowire minimum size is not

limited a priori [60] . Also, there is a thermodynamic limit for the minimum radius of the metal–

liquid clusters at high temperature [61] . A further deviation from the VLS growth model was

observed by temperature dependent studies of the Au-assisted growth of InAs nanowires [62] .

In the particular case of ZnO, several growth processes occur in parallel for appropriate condi-

tions. It is observed that the whisker diameter can be far beyond the catalytic particle size [50]

which is due to an additional vapour–solid mechanism. The growth models for ZnO nanowires

and more complex two-dimensional ZnO nanostructures are large in number as is the struc-

tural diversity of ZnO. Next, we give a selective overview of the growth conditions for ZnO-based

nanostructures.

ZnO whiskers, platelets, and dendritic crystals have already been reported before renewed

interest arose. Earlier works include the vapour phase experiments by Sharma et al. (1971) [63] .

He and his co-workers synthesized mm-sized crystals by a modiﬁ ed vapour–phase method at

temperatures between 930 and 980°C. Furthermore, the growth of colourless and transparent

ZnO needles with a length of up to 200 mm by oxidation of zinc vapour was observed (1974)

[64] . In 1978, Iwanaga et al. [65] presented structures with different morphologies (hollow, nee-

dle, whisker, ribbon) depending on their starting material (ZnF

, ZnS, ZnSe and others) for the

reaction. They attributed the growth partly to the VLS mechanism. However, the typical size of

all those crystals was beyond several μ m. As already mentioned in the introduction, there is a

tremendous interest in nanometre-sized ZnO crystals which has led to hundreds of publications

during the past years.

Most commonly, physical vapour deposition or thermal evaporation are employed for the syn-

thesis of ZnO-based nanostructures. But also metal-organic vapour phase epitaxy (MOVPE), elec-

trochemical deposition or PLD was used. The growth temperatures span a very wide range: from

only 70°C to above 1400°C. The use of a catalyst for VLS growth is a feature which a large part

of the approaches share. Besides gold, which is most common, also liquid Zn droplets [66] , NiO

[67] , tin [68] or copper [69] were used as nucleation sites. ZnO nanostructures grow on almost

every substrate: sapphire, Si, glass, plastic, and so on. More special synthesis techniques are, for

instance, the production of ZnO by the oxidation of Zn wires [70] . All those methods can lead to

high-quality nm-sized crystals the shape of which depends strongly on the speciﬁ c growth con-

ditions. Some formation mechanisms are well understood while others need to be investigated

further. For instance, for the formation of ZnO nanorings and nanohelices there are theoretical

works on the necessary conditions upon which they form [71] . Also, the doping and alloying has

been shown to be practicable. ZnO has been combined with MgZnO to form coaxial nanowire

quantum wells (QW) [72] or multi-QWs [48, 73] . CdZnO [45, 74] , ZnCoO [75] and ZnMnO

[76] structures were presented. For a more comprehensive and complete overview, the reader is

referred to corresponding review articles [25, 34, 77] .

In our group, ZnO nanowires were grown by a special gold-catalysed high-pressure pulsed

laser deposition process which is described in detail in the next subsection. Furthermore, we

employ a vapour phase transport process proposed by Huang and Yao [61, 78] for the growth of

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296 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

ZnO microcrystals. This method is conventionally used to synthesize one- and two-dimensional

nanostructres [79, 80] . Here, our standard process was modiﬁ ed to yield whiskers with diam-

eters in th μ m range by thermal evaporation of a pressed ZnO–graphite (mass ratio 1:1) target

at ambient pressure. The growth temperature was 1100°C. An argon downstream was applied

as transport gas. The ZnO microwires were isolated on a carbon gluepad. SEM images of such a

microwire are shown in Fig. 9.1 .

25 µm

5 µm

Figure 9.1 SEM images of a ZnO microwire on a carbon gluepad.

9.2.2 High-pressure PLD process

PLD belongs to the class of physical vapour deposition techniques. It was ﬁ rst demonstrated by

Smith and Turner in 1965 [81] . However, it took more than two decades until PLD became a

relevant growth technique. The breakthrough came with the development of high-power lasers

and with new multi-element materials systems, such as high- T

oxide superconductors, for

which PLD is well suited. Nowadays, it is an established method for a variety of materials, espe-

cially oxides, but also organic polymers, metals and semiconductors [52] . Sankur et al. [82] ﬁ rst

reported on the epitaxial growth of ZnO using PLD.

At the University of Leipzig we have established a high-pressure PLD process designed for

nanostructure growth. The main parts of this system are a high-power pulsed KrF excimer laser

from Lambda Physik (LPX 305), the laser optics and the deposition chamber [51] . The chamber

consists of a T-shaped quartz tube with an outer diameter of 30 mm. The KrF excimer laser beam

(wavelength 248 nm) is guided to the deposition chamber inside a protection shielding, enters

along the centre bar of the T and is focused by a UV lens on the cylindrical surface of one of the

rotating PLD targets. Up to three different targets ﬁ t onto the linear rotary feedthrough bar thus

allowing in situ modulation of chemical composition. The rotation by the feedthrough ensures

homogeneous ablation all around the poly-crystalline targets. They were prepared by mixing,

pressing and sintering high-purity powders for 12 h at 1150°C. The growth from multi-component

targets (ZnMgO, ZnMnO and ZnCoO) is discussed in [51] and [76] .

The direction of the plasma plume is pointing above the substrate which is placed off-axis

on a ceramic slat. Its surface normal is perpendicular to the symmetry axis of the plume and

to the laser beam. The target to substrate distance was 5–35 mm. An encapsulated heater with

an arrangement of KANTHAL wire in ceramic tubes (patent pending DE 10255453.7) and

FIBROTHAL isolation material is built around the quartz tube. A type K thermoelement (Ni–

Cr–Ni) is used to measure the temperature at the outside wall of the tube. By this arrangement

the temperature at the growth zone can be varied in the range between room temperature and

950°C. With this conﬁ guration the gas ﬂ ow is directed from the target towards the substrate and,

hence, potentially supports the transport of material from the plasma plume. The ﬂ ow can be

controlled from 5 sccm to 30 slm by Qualiﬂ ow AFC 50D digital mass ﬂ ow controllers (MFCs). The

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Growth and Characterization of ZnO Nano- and Microstuctures 297

Ar background gas has a purity of 99.995%. In Table 9.1 the typical growth parameters for ZnO

nanostructures are summarized.

Due to the increased collision probability between the laser-produced plume and the back-

ground gas, the plume dimension for the high-pressure PLD case presented here is considerably

smaller than in conventional, i.e. high-vacuum, PLD. Furthermore, the ambient gas leads to a

broadening of the angular distribution of the plume [83] . Dyer and co-workers have shown that

the ratio between laser-pulse energy E and the background gas pressure p is the scaling param-

eter for the plume range L [84] :

∝

⎛

⎝

⎜

⎞

⎠

⎟

γ 3

where γ is the ratio of speciﬁ c heats of the elements in the plume [85] . For a typical laser pulse

energy of 500 mJ and a pressure of 0.1 mbar the plume dimension of an ablated ZnO target

is around 5 cm. Hence, the plume range for 100 mbar is well below 1 cm, a fact which can be

directly veriﬁ ed by the observation of the optical emission from the excited plume, as it is seen

through the viewport during ablation. The kinetic energies of species arriving at substrates

located beyond the dimension of the plume will be very low, i.e. they are expected to be ther-

malized. As will be shown in this work, this versatile set-up allows the fabrication of ZnO-based

nanostructures with tunable shape and chemical composition.

9.2.3 Growth morphology

In order to observe the nanowhisker growth at different stages, a series of ﬁ ve samples with dif-

ferent growth times or, correspondingly, a different number of laser pulses (1000–16 000) with

a constant repetition frequency of 10 Hz has been produced ( Fig. 9.2 ). Note that there is always

a factor of two between the growth times of successive samples. All other PLD parameters

remained unchanged. Typical as-grown ZnO nanowire structures are shown in the SEM images

in Fig. 9.2 . For the purpose of accurate determination of the nanostructure dimensions, the

substrate has been cut along the axis of symmetry ( cf . PLD chamber geometry [51] ) by scratch-

ing the sapphire with a diamond needle tip. This facilitates an open view down to the nanowire

base and the substrate cleaving edge. For better comparability, all SEM pictures shown in this

section were taken exactly at the centre of the samples. Figure 9.2a–c represents tilted views for

the samples grown from 10 nm Au dots with 1000, 4000 and 16 000 PLD pulses, respectively.

Figure 9.2d is the corresponding top view of the sample in Fig. 9.2c . Nanowires with hexagonal

cross-section are clearly resolved mimicking the wurtzite crystal structure of ZnO. The prism side

faces are oriented parallel to each other indicating that there is a very good in-plane epitaxial

Table 9.1 Key parameters of the PLD system for nanowire growth at the

University of Leipzig

PLD parameter Value

Background gas 100% Ar or Ar  O

with up to 50% O

Background gas pressure 10–200 mbar

Gas ﬂ ow 10–500 sccm

Growth temperature 400–950 ° C

Laser pulse energy 350–650 mJ

Laser wavelength 248 nm (KrF)

Laser focus size at target 2  5 m m

Pulse number 10–36 000

Pulse frequency 1–20 Hz

Target–substrate distance 5–35 mm

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298 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics

relationship. It is apparent from the pictures that the length of the whiskers increases upon

increase of deposition time. The growth evolution is shown in more detail in Fig. 9.3a and b .

In the ﬁ rst growth steps the aspect ratio (length/diameter) increases almost linearly with depo-

sition time ( Fig. 9.3b ). Note that the x -scale in the ﬁ gure is logarithmic for better visualization.

Most of the whiskers are between 20 and 70 nm in diameter ( Fig. 9.3c and d ). Figure 9.3c shows

a typical corresponding histogram (sample with 8000 pulses). The sample with 16 000 pulses

plays an exceptional role. Most of its nanowires are thicker than 70 nm while the diameter distri-

bution functions ( Fig. 9.3d ) are similar for all other samples. In this last growth step the length

and the diameter increase by similar relative amounts, resulting in a nearly constant aspect ratio.

The lateral nanowire density of the ﬁ rst four samples scattered between 56 and 80 wires per

μ m

and for the last sample (16 000 pulses) it was as low as 23 wires/ μ m

. It is evident from

Fig. 9.3c and d that at later growth stages the whiskers tend to grow together thus forming struc-

tures with larger diameter but reduced lateral density.

For comparison, a sample based on colloids with 50 nm diameter was synthesized using

8000 laser pulses. The substrate in the PLD chamber is oriented in such a way that one side faces

the PLD target, i.e. the surface is parallel to the symmetry axis of the plasma plume. In Fig. 9.4

the mean whisker diameter is plotted against the relative position on the sample. The inset of

Fig. 9.4 shows schematically ﬁ ve different positions on a substrate where position 1 corresponds

to the side with the smallest distance to the target and position 5 is 10 mm further apart from

it. The diameter decreases systematically with increasing distance from the target. An analo-

gous ﬁ nding was presented earlier for different substrate positions [51] . The narrowest diam-

eter variation (represented by the bars in Fig. 9.4 ) can be found in the middle of the sample,

at position 3. Also the height of the nanowires varies over the substrate surface ( Fig. 9.5 ).

This difference of growth morphology on a single sample arises from the expansion dynamics

of the plasma plume. The energy and density of the ablated species depend strongly on the

distance from the target. The availability of ZnO whiskers with different and deﬁ ned sizes on a

single sample is of great use for basic research, as, for example, for optical cavity experiments

(see section 9.3).

(b)

500 nm

400 nm

(a)

1m

Figure 9.2 Typical SEM images of PLD-grown ZnO nanowires taken at different growth stages after application of:

(a) 1000, (b) 4000 and (c) 16 000 laser pulses. (d) represents the top view of the sample shown in (c).

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