Henini M. Handbook of Self Assembled Semiconductor Nanostructures for Novel devices in Photonics and Electronics

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Radiation Effects in Quantum Dot Structures 429

shape with sizes varying from 13 to 25 nm were detected. The cubic Fe

Ge is a ferromagnetic

compound with Curie temperature equal to 755 K. As the size of the precipitates was large,

no superparamagnetic relaxation effects were observed. With the ﬂ uence increasing up to

2  1 0

c m

 2

, the situation changed. As determined by XPS data, the majority of the implanted

Fe ions were in the metallic phase, and a small amount was in the Fe–Ge, Ge–O, and Fe–O phases.

From an analysis of the magnetometry results, the authors obtained a mean diameter of the

superparamagnetic clusters of ⬃ 4 nm. Negative magnetoresistance was observed and attributed

to spin-dependent scattering of charge carriers by these clusters. Ion-beam synthesis of Fe nano-

clusters in Ge was also reported in [335] .

Ge/Si: Epitaxially grown Ge/Si QD heterostructures were subjected to Mn implantation and

RTA [336] . Magnetization with T

 160 K was measured in a SQUID and ascribed to diluted

magnetic QDs. Some intermixing was acknowledged as a result of Raman scattering measure-

ments. However, no structural analysis in order to access the degree of intermixing and to prove

the existence of QDs after implantation and annealing was reported.

Si: Though the most important semiconductor is still silicon, the research efforts to pro-

duce magnetic phases in it have been surprisingly weak. In an analogy to Ge, an obvious way

to achieve magnetic functionalization of Si appears to be doping with transition metals (TMs).

When TMs react with Si, silicides are readily formed upon heating [337–339] . PtSi, TiSi

, CoSi

NiSi have metallic conductivity; CrSi

, β -FeSi

, ReSi

1.75

, Ru

, OsSi

, Ir

and MnSi

1.7

are sem-

iconductors. Manganese silicides are especially numerous: besides MnSi [340] and Mn

[341] ,

there is an entire family of semiconducting higher manganese silicides (HMS) MnSi

with a

tetragonal lattice and composition x varying from 1.67 to 1.75, which are usually labelled

MnSi

1.7

(Mn

, Mn

, and Mn

) [342] .

Unfortunately, most silicides either have a low Curie temperature T

or are not ferromagnetic at

all (iron silicide Fe

Si is an exception [339] ). In the absence of a magnetic ﬁ eld, MnSi orders mag-

netically with a helical structure below 30 K. However, in an applied ﬁ eld of 0.6 T the moments

align in an induced ferromagnetic state. Thus, in a ﬁ eld MnSi is often regarded as a good example

of a very weak itinerant ferromagnet [343–346] . In very thin layers (  60 Å of Mn deposited)

MnSi exhibits ferromagnetism at 10 K [347] . Mn

has a Curie temperature of 40–43 K [348,

349] . Mn

[350–352] and Mn

Si [353, 354] are antiferromagnetic at low temperatures.

So it was quite surprising when room temperature ferromagnetism was reported in Si doped

with Mn by ion implantation [355, 356] , though there had been reports on magnetic thin SiMn

ﬁ lms grown by other techniques [357–361] . However, it has not been shown which phase

was responsible for the effect. Other works followed where the existence of nanoclusters in the

implanted and annealed layers was proven with the most probable phase of the crystallites being

MnSi

1.7

[362, 363] . It was concluded from the XANES and EXAFS spectra that Mn ions implanted

into bulk silicon do not form either metallic or oxide inclusions [364] . Both XANES and EXAFS

spectra are in reasonable agreement with the model, which assumes the formation of clusters

with short-range order close to strained and defected Mn–Si compounds with ﬁ ve to eight near-

neighbour atoms. Clusters formed in the sample with Mn implantation performed on a heated

substrate showed the local order closer to that in the Mn–Si phases.

Thus, although the high-temperature ferromagnetism observed in Mn implanted Si is most

probably caused by clusters, it is not yet clear which is the crystallographic phase responsible for

the magnetism.

It has to be noted that density functional theory (DFT) calculations have predicted for the mon-

osilicides of Mn, Fe, Co, and Ni a CsCl-like structure that has not yet been observed as bulk com-

pound but may be stabilized epitaxially on Si(001). For very thin ﬁ lms of CoSi and MnSi grown in

this structure, a ferromagnetic ground state was obtained. It was found that the atomic structure

of MnSi ﬁ lms on Si(111) is close to the natural crystal structure of bulk MnSi (B20), and also

shows large magnetic moments of the Mn atoms at the surface and interface [365] . DFT calcu-

lations also predicted that Si-based heterostructures with one quarter to one layer δ -doping of

interstitial Mn display high spin polarization [366] .

Another approach was chosen in [280, 301, 367] . Mn and As ions were co-implanted at

350°C in (001)Si substrates that were subsequently subjected to RTA at various temperatures.

In ferromagnetic resonance (FMR) measurements, highly anisotropic spectra were detected

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

( Fig. 13.30 ) that could be interpreted as arising from hexagonal magnetic clusters with the hard

magnetization axis aligned along the four  111  Si axes. The spectra were strikingly similar to

those measured in MO CVD-grown, MnAs cluster-rich GaMnAs layers; these spectra had been

interpreted as arising from hexagonal MnAs clusters with MnAs[0001] 储 GaAs[111] [302] . FMR

spectra and magnetization measured in a SQUID persisted above RT. Ex situ and in situ energy-

ﬁ ltered cross-sectional TEM measurements revealed the existence of two main types of NCs.

(i) The smaller ones with diameters up to 20 nm consist of Mn and As and are crystalline; no phase

separation could be imaged. (ii) The larger ones are phase separated. They predominantly consist

of manganese silicides and are surrounded by a shell of amorphous As. During the annealing

voids are formed that are then ﬁ lled by diffusing Mn and As atoms [280] . The magnetism and the

FMR spectra are supposed to stem from the MnAs nanocrystals.

6000

3000

5000

1000

2000

4000

Magnetic field (gauss)

20 80

140

160

100

120

)

<110>

<111>

<001>

<111>

Figure 13.30 Angular dependence of the FMR line positions in an Mn- and As-implanted (001)Si sample after

RTA at 1100°C obtained for the magnetic ﬁ eld rotation in the (110) plane [301] .

13.4.4.3 Oxides

ZnO : Formation of magnetic nanoclusters was reported in ZnO implanted with Co [368–371,

445] , Fe [370, 372–374, 444] and Ni ions [375, 445] . In the case of Co implantation, X-ray dif-

fraction results indicated the presence of (110)-oriented hexagonal phase Co in the ZnO matrix.

The nanocrystal size was estimated to be ⬃ 3.5 nm. In-plane X-ray diffraction results showed that

the nanocrystals were epitaxial with respect to the ZnO host matrix [369] . The orientation rela-

tionship is as follows [445]: hcp-Co(0001)[10–10] || ZnO(0001)[10–10] and Ni (111) [112] ||

ZnO(0001)[10–10]. The existence of small superparamagnetic clusters in ZnO implanted with

Co at ⬃ 350°C was conﬁ rmed in [371] by magnetic measurements. The structure and magnetic

properties of Co or Ni NCs embedded inside ZnO could be tuned by postannealing. For the Co

case, 823 K annealing resulted in the co-existence of fcc-Co and hcp-Co. The magnetic anisot-

ropy was changing from out-of plane to in-plane. Especially for the Ni NCs, the magnetic anisot-

ropy was different from the bulk crystals [445].

Tiny bcc-Fe particles, formed inside the host matrix, were found to be responsible for the ferro-

magnetic properties of ZnO implanted with Fe at 350°C [372] . The particles were identiﬁ ed using

synchrotron X-ray diffraction and conversion electron Mössbauer spectroscopy (CEMS). Evidence

for the existence of other, unidentiﬁ ed, superparamagnetic NCs was brought by the zero ﬁ eld-

cooled (ZFC) and ﬁ eld-cooled (FC) magnetization measurements as a function of temperature in

Fe-implanted ZnO subjected to ﬂ ash lamp annealing [373] . In a subsequent work of the same

group [374] , crystallographically oriented Zn ferrite (ZnFe

) nanoparticles with the orienta-

tion relationship of ZnFe

(111)[110] 储 ZnO(0001)[11

0] were detected inside high ﬂ uence

Fe-implanted ZnO after a long-term annealing at 800°C. These ZnFe

nanoparticles showed

a hysteretic behaviour upon magnetization reversal at 5 K. The inﬂ uence of the implantation

parameters on the state of the implanted Fe and the appearance of the ferromagnetism was thor-

oughly investigated in [444].

Also in ZnO implanted with Ni at 350°C crystalline fcc-Ni NCs were detected [375] . It was con-

cluded from magnetic measurements that around 27% of implanted Ni atoms were in the metal-

lic state.

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Radiation Effects in Quantum Dot Structures 431

MgO: In early experiments on the room temperature Fe implantation in MgO at ﬂ uences rang-

ing from 10

to 10

c m

 2

[376–379] , mainly CEMS supported by RBS/C, optical absorption,

electron microscopy and electrical conductivity measurements was implemented to investigate

the state of the implanted ions. It was found that implantation introduces iron in three charge

states: Fe

2 

, Fe

3 

, and metallic precipitates (Fe

) with the dominant role of Fe

3 

at low ﬂ uences,

2 

at medium ﬂ uences, and metallic iron clusters at the highest ﬂ uences. The isochronal ther-

mal annealings in air at temperatures between 300 and 700°C gradually caused the oxidation

and the nucleation of highly dispersed spinel-like clusters and then, at about 800–900°C, the

growth of magnesioferrite particles. In contrast, the heat treatment in vacuum converted all iron

into well-diluted Fe

2 

in MgO phase. In [380, 381] , a superparamagnetic behaviour of the mag-

netization was found after Fe implantation at RT until a ﬂ uence of 2.0  1 0

c m

 2

, suggesting

the precipitation of implanted Fe ions as nanogranules in the MgO matrix. The GMR effect with

signiﬁ cant MR ratio of 3.5% for ﬁ elds of 0–12 kOe was observed only for the sample implanted

at this dose and post-annealed at 300°C in argon. The superparamagnetic NCs changed into

ferromagnetic ones by annealing at 300°C and/or with increasing ﬂ uence due to the growth in

their size and the consequent blocking of superparamagnetic relaxation. CEMS, MOKE, XRD,

TEM including electron holography [320, 321] , and RBS/C were applied in [382] to investigate

the Fe NC formation in (001)MgO implanted with Fe in a wide temperature range (and also in

yttrium-stabilized zirconia ZrO

, see below). Non-magnetic metallic γ -phase Fe NCs were syn-

thesized inside MgO. The formation of the γ -Fe (fcc) phase was surprising, since it is usually not

stable at RT. The fraction of the γ -phase Fe

increased from 28% (nominally RT implantation) to

60% ( T

 800°C) as a function of implantation temperature T

at the expense of Fe

2 

states. At

 600°C and 800°C the Fe

2 

states were not present but were transformed into Fe

or Fe

3 

states. It was found that at T

 800°C the NCs show an exclusive orientation relationship of γ -

Fe(111) 储 MgO(111), respectively, γ -Fe [220] 储 MgO [220] . The mean diameter, as calculated by

the Scherrer formula, amounted to 5 nm. It was shown recently that ferromagnetic Fe NCs can

be synthesized by ion implantation in differently oriented MgO single crystals [(100), (110), and

(111)] [383] . They all exhibited ferromagnetic behaviour at 5 and 300 K. The coercive ﬁ eld H

strongly depended on the crystal orientations and obeyed the relation of H

(110)  H

(100)

 H

(111) at different measurement temperatures.

Implantation of Co and Ni in (001)MgO crystals with subsequent heat treatment up to 1280 K

in vacuum was used to create colloidal dispersions of superparamagnetic Co and Ni particles

[384–387] . The number and size of the aggregates depended on the atomic concentration of the

implanted ions and the subsequent annealing treatments. From magnetic measurements, aver-

age diameters of fcc-Co NCs from 2.3 to 6.8 nm and that of fcc-Ni NCs from 4.1 to 6.7 nm as

well as the anisotropy constants were calculated; the particles behaved as single domains. The

presence of an antiferromagnetic layer surrounding the Ni aggregates, possibly a nickel oxide

layer, was concluded from the asymmetric shape of the hysteresis curves and the non-saturating

behaviour of the high-ﬁ eld magnetic susceptibility [385] . As evidenced by the Mie scattering

from the colloids, Co implanted at an energy of 250 keV diffuses out of the precipitates, when the

annealing temperature is increased from 1250 to 1600 K, and probably occupies substitutional

sites in the MgO lattice [387] . For the implantation energy of 100 keV, the outdiffusion starts at

1050 K, and at 1250 K the precipitates are already dissolved. Very recently, the metal and alloy

NC formation in MgO, LaAlO

and YSZ (see below) targets covered by Pt and FePt thin ﬁ lms by

means of ion-induced mixing upon Pt ion irradiation was achieved [446].

TiO

: Titanium dioxide occurs in several forms, such as rutile, anatase and brookite. Rutile

is tetragonal (a  4.59 Å and c  2.96 Å) with a band gap of 3 eV, while the anatase phase also

crystallizes in a tetragonal structure ( a  3.78 Å and c  9.52 Å) with a band gap of 3.2 eV [388,

389] . Thin ﬁ lms of cobalt-doped anatase were reported as the ﬁ rst semiconductor exhibiting fer-

romagnetism at RT [390] . The nucleation of cobalt or cobalt-rich magnetic NCs was claimed to

be responsible for this ferromagnetic behaviour [391, 392] . Thus, most of the publications on

the transition metal ion implantation in TiO

were dedicated to cobalt [393–400] and only a few

to nickel [395, 401] .

Anatase ﬁ lms implanted with Co showed superparamagnetic properties due to the presence of

magnetic NCs consisting of Co or a ferromagnetic Co compound, whose existence was proved by

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

scanning TEM. Upon annealing, the particle size and the blocking temperature were increased

by raising the annealing temperature and/or decreasing the oxygen pressure [393] . In Co-

implanted rutile, the ordered magnetic behaviour due to the presence of Co NCs was observed

even above RT [395] .

Strongly anisotropic FMR spectra were observed in the Co-implanted (100) and (001) rutile

samples and attributed to the metallic cobalt NCs synthesized during the ion implantation pro-

cedure. Dominant four-fold in-plane anisotropy of the FMR signal was interpreted as indicating

the cubic (probably, fcc) phase of metal Co, while the additional axial anisotropy was ascribed to

the inﬂ uence of the host rutile structure. The annealing of implanted samples resulted in disap-

pearance of the ferromagnetism and formation of paramagnetic Co

2 

centres [394] . Magnetic

anisotropy of similarly prepared samples was studied by means of MOKE and SQUID magnetom-

etry. Room temperature magnetism was observed. A strong angular dependence of the remanent

magnetization and coercive ﬁ eld in the plane of the implanted surface was observed: two-fold

anisotropy for the (100) and (110) substrate and four-fold anisotropy for the (001) substrate

samples [397, 398] . Upon varying the Co ﬂ uence in the range from 0.15–1.50  1 0

 2

sequentially paramagnetic, superparamagnetic, weak ferromagnetic and, eventually, strongly

anisotropic ferromagnetic response at RT was found [399, 400] . The thermomagnetic analy-

sis revealed in the ferromagnetic samples two magnetic transitions with ordering temperatures

⬃ 700 K and T

⬃ 850 K. Annealing of the samples in air strongly suppressed the ferromag-

netic phases if the heating temperature exceeded the corresponding transition temperature.

Subsequent high-vacuum annealing restored only the low-temperature ferromagnetic phase.

The following model suggesting coexistence of two different magnetic phases in Co-implanted

rutile was proposed to explain the temperature dependence of the magnetization and the anisot-

ropy of the hysteresis loops [400] . Cobalt in a form of NCs in the very near-surface layer (the ﬁ rst

magnetic phase) determines the superparamagnetic behaviour of the samples implanted with

low and medium ﬂ uences. Cobalt dopant in the form of Co

2 

ions substituting the host Ti

4 

ions

is the base of the second magnetic phase. The oxygen vacancies formed by implantation provide

the charge compensation and serve as mediators for the exchange interaction between the Co

2 

ions. At high ﬂ uences, the bulk-like ferromagnetic order is developed due to ion accumulation

and indirect exchange interaction between the Co

2 

ions.

Ni NCs with dimensions ranging from 3 to 20 nm were observed by high-angle annular dark-

ﬁ eld (HAADF) Z-contrast scanning TEM and HRTEM in Ni-implanted rutile [401] . Accordingly,

superparamagnetic behaviour was observed in the magnetometry measurements, exhibiting a

blocking temperature of 50 K [395] or 85 K [401] .

: To realize perpendicular magnetic recording media with a high density, much atten-

tion has been paid to L 1

-type FePt (fct phase). One of the reasons is its high magnetocrystalline

anisotropy (above 10

erg/cm

), enabling a small fct-FePt particle to overcome thermal instabil-

ity that causes superparamagnetism [402] . Formation of oriented ferromagnetic FePt NCs was

achieved in Al

also by ion implantation [403–405] . The NCs had coercivity values in excess

of 20 kOe.

The production of buried granular magnetic layers was implemented through implantation

of Fe [406–408] and Fe  Co ions [251, 409] into α -Al

single crystals. Upon annealing in

vacuum Fe ions precipitate as oriented α -Fe NCs, and the orientational relations are of (111)

储 (0001)Al

and [1

0 ] F e 储 [11

0]Al

, or (110)Fe 储 (0001)Al

[406] . Magnetization

curves exhibited superparamagnetic or ferromagnetic behaviours depending on the diameters of

the iron NCs. The granular layers exhibited an eminent giant magnetoresistance (GMR) [251,

407, 410] .

A full bibliography on metal-implanted sapphire till early 2005 can be found in [246] .

SiO

: There are numerous reports also on the synthesis of magnetic NCs in SiO

by implanta-

tion of Co [411–415] , Fe [416–418] , Ni [419] , Co  Ni [420, 421] , and Fe  Pd [422] ions.

In contrast to the IBS of magnetic NCs out of the implanted components, ion irradiation of

silicon-based gels with the stoichiometry SiO

1.5

, prepared from triethoxysilane, and con-

taining Fe

3 

or Ni

2 

ions in solid solution, was used to obtain the precipitation of metallic NCs

in [423–426] . The ﬁ lms were irradiated with 1.5 MeV He or 3 MeV Au ions. It was concluded

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Radiation Effects in Quantum Dot Structures 433

that electronic excitations were responsible for the reduction and precipitation processes, while

atomic displacements induced a partial re-dissolution by mixing. The nucleation occurred for a

critical value of the energy density deposited by ions in electronic excitation. The NCs exhibited a

narrower range of sizes than in ﬁ lms submitted to heat treatments in vacuum. The hammering

effect of ions on the silica matrix caused a tilt of easy magnetization axis when the volume frac-

tion of particles was of a few per cents, but this axis remained in the ﬁ lm plane for over 10%.

The synthesis and modiﬁ cation of metal nanoclusters in silica and silica glasses through ion

implantation has recently been reviewed in [211, 245] .

Yttrium-stabilized ZrO

: Precipitated layers of ferromagnetic α -Fe or ferrimagnetic Fe

embedded in the near-surface region of (100)-oriented yttrium-stabilized zirconia ZrO

(YSZ)

and exhibiting a magneto-optical activity were created by using Fe ion implantation and ther-

mal processing [382, 427, 428] . TEM and XRD studies established that each α -Fe or Fe

was a single crystal that was crystallographically aligned with respect to the YSZ host. The for-

mation of either α -Fe (bcc) or Fe

(or even γ -Fe) NCs depended on the annealing conditions.

The α -Fe precipitates exhibited two different, but symmetry related, orientations relative to the

YSZ host. The predominant orientation was: α -Fe(110) 储 YSZ(001) out-of-plane and α -Fe[001] 储

YSZ[100] in-plane. The less common relative orientation was found to be: α -Fe(001) 储 YSZ(001)

out-of-plane and α -Fe[100] 储 YSZ[110] in-plane. The particles of the small γ -Fe component had

only one relative orientation that was determined to be: γ -Fe(001) 储 YSZ(001) out-of-plane and

γ -Fe[100] 储 YSZ[100] in-plane [382, 427] . Electron holography [320, 321] clearly indicated a

magnetic dipole associated with a single Fe nanoparticle [382] .

SiO

: The implantation of Fe ions at RT into forsterite (Mg

SiO

) also leads to the forma-

tion of metallic Fe NCs, especially at high implantation ﬂ uences [437] . The subsequent anneal-

ing in air at T  350–700°C gradually causes the oxidation of iron and then, at about 800°C, the

growth of γ -Fe

particles with a size of about 15 nm. At higher temperatures (900–1200°C)

the transformation of γ -Fe

into α -Fe

was observed simultaneously with the large outdif-

fuison of iron.

In this chapter, we have only reviewed the cases where magnetic NCs had been deliberately

synthesized. However, it has to be considered that in many reports claiming the creation of true

diluted magnetic semiconductors (DMS) the real source of magnetism were magnetic nanocrys-

tals, so the spectrum of existing hybrid nanomagnet/semiconductor systems is certainly much

broader than described in this chapter.

13.4.5 Other nanotechnological applications of radiation

Swift heavy ions (SHI, i.e. ions moving at a velocity comparable to the Bohr velocity of the elec-

tron) lose energy in materials mainly through inelastic collisions with the atomic electrons.

Along the trajectory, a trail of defects (point defects, defect clusters, structural phase transforma-

tion) known as latent track may be formed depending on the type of ion and its energy as well as

the physical properties of the target material. This damage is always created in the close vicin-

ity of the trajectory of the projectile [2] . The latent tracks can be transformed via etching into

open nanochannels with a high aspect ratio ( ⬃ 1000), which allows the controlled production of

nanoporous membranes that can be ﬁ lled with the required material, thus producing nanowires,

nanotubules and chains of nanoparticles in insulators, semiconductors and polymers [429] . To

date, the tracks have been used as molecular ﬁ lters, bio-sensors (e.g. for DNA detection) or to pro-

duce metallic nanowires, but single ion irradiation opens the door even more for the creation of

well-ordered semiconductor nanostructures.

An effect similar to that produced by SHI in insulators, though predominantly potential

energy driven, is provoked by the impact of slow highly charged ions [430] . There is a well-

deﬁ ned threshold of potential energy carried into the collision of about 14 keV for nano-sized

hillock formation on the (111) surface of CaF

single crystals. Estimates of the deposited energy

density suggest that the threshold is linked to a solid–liquid phase transition ( “ melting ” ) on the

nano scale. This method makes it possible for regular nanostructures to form on surfaces without

damaging deeper-lying crystal layers.

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

13.5 Conclusions

In this chapter, several radiation effects and their applications to quantum-size semiconductor

structures, ante omnia quantum dots, have been considered. Numerous instances have been

cited illustrating the main topics of interest of radiation effect studies as formulated at the end

of the Introduction (Section 13.1). The QD heterostructures and QD lasers are generically more

resistant to radiation damage ( “ radiation hard ” ) than their bulk and 2D counterparts, which is

caused not only by the localization of the wavefunction of the conﬁ ned carriers but also by the

expulsion of the mobile defect components to the surface/interface of the nanocrystals. There

are many exciting applications of particle irradiation to QD technology, such as intermixing,

self-organized formation of surface nanostructures, ion-beam synthesis of nanocrystals in solid

matrices, as well as cluster precipitation in irradiated media. An emphasis has been given to the

ion-beam synthesis of magnetic nanocrystals in solid hosts.

Acknowledgements

I had the luck to work with many outstanding colleagues and gifted students from Belarus,

Germany, Portugal, Russia and other countries. They are too numerous to be particularized here,

but I am deeply grateful to all of them for fruitful collaboration and for inspiration and impulses

they have given me. This work was supported in part by the SANDiE Network of Excellence of

the EU and by the project POCI/FIS/61462/2004 of the FCT of Portugal. The TRIM simulations

made by N.M. Santos are gratefully acknowledged.

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Radiation Effects in Quantum Dot Structures 435

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