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 419

multilayer. The huge PL intensity increase was only possible because RTA-treated unimplanted

samples suffered relaxation by dislocation formation and thus exhibited a very weak PL. The dis-

locations were then passivated by the implanted hydrogen. An additional intensity enhancement

mechanism was proposed due to an increase of the capture rate caused by an intermixing-induced

change in QD shape. Interestingly, the authors of [166] compared the time-integrated PL and the

time-resolved PL of several lattice-matched InGaAs/InP QWs intermixed either by ion implanta-

tion or by an impurity-free method. They have found that the carrier capture rates into QWs and

carrier relaxation from the wells depended on the type of intermixing used. Their results indicated

that the carrier lifetimes were signiﬁ cantly longer in samples intermixed by the impurity-

free method, while the carrier collection efﬁ ciency of the QWs was more efﬁ cient in samples

intermixed by ion implantation.

The inﬂ uence of the embedding material (InP, GaInAsP, InP and InGaAs) on the proton

implantation-induced intermixing of InAs QDs was studied by PL in [167] . The particle energy

of 40 keV placed the damage peak over the QD region. The QDs capped with InP layers exhibited

the highest implantation-induced energy shift due to strong group V interdiffusion whereas the

QDs grown on and capped with GaInAsP layers showed the least implantation-induced energy

shift due to weak group V and group III interdiffusion. The QDs capped with InP and InGaAs

layers showed intermediate implantation-induced energy shift and were less thermally sta-

ble as compared to the QDs grown on and capped with GaInAsP layers. The QDs capped with

InP layers demonstrated enhanced PL intensity when implanted with proton ﬂ uences less

than 5  1 0

c m

 2

. On the other hand, proton ﬂ uences above 1  1 0

c m

 2

reduced the PL

linewidth in all samples.

Whereas the PL measurements give only indirect information on the damage and intermix-

ing in irradiated QDs, such techniques as the Rutherford backscattering/channelling of light

ions (RBS/C), X-ray diffraction and X-ray diffuse scattering, high-resolution transmission elec-

tron microscopy (HRTEM), high-angle annular dark-ﬁ eld (HAADF) imaging in a scanning TEM

(STEM), and cross-sectional scanning tunnelling microscopy (XSTM) yield more direct evidence

on structural and compositional changes in the samples. Unfortunately, applications of these

techniques to the irradiation-induced intermixing of QDs are almost absent so far. The only work

we are aware of is [168] , where RBS/C was used to investigate the 1.0 MeV proton-induced inter-

mixing in InAs/GaAs QD structures. An activation energy as low as 0.2 eV for the In–Ga inter-

mixing was concluded from the experiments and ascribed to the presence of a non-equilibrium

concentration of point defects produced by proton irradiation. Besides, evidence for mass trans-

port of group III elements along or perpendicular to the growth  100  direction was offered.

In the implantation-induced intermixing experiments described above, the QD region was

directly damaged by the ion beam. However, in the case of the high-energy proton-induced inter-

mixing the concentration of point defects created at the site of the dots is low (cf., e.g., the defect

proﬁ les shown in Fig. 13.5a with typical layer thicknesses in QD structures), and the intermix-

ing occurs mainly due to defects that experience a long-range migration to the dots from the

adjacent barrier layers. This phenomenon is also exploited in intermixing experiments using

low-energy ion implantation or plasma etching, in which the defects are created in a subsurface

region. So, e.g., an exposure of InAs/Gas QDs to the 300 eV Ar plasma led to a degradation of

the PL intensity though the dots were situated at a depth free of primary displacement damage

[58] . In this sense, such intermixing experiments are similar to those were the defect ﬂ ux from

the surface is caused by a capping layer (see, e.g., [142, 169] ), with the important difference that

the irradiation technique allows the defect concentration and the heat treatment temperature

to be chosen separately. Thus, in [94] a blue shift of 160 nm and an increase in the PL intensity

of InAs/InGaAs/InP QD samples were observed immediately after Ar plasma exposure for 90 s,

and a further increase in the blue shift of 330 nm, accompanied by a 2.5 times increase in the PL

intensity and a narrowing in the PL linewidth, was achieved after subsequent RTA at 720°C. In

another work [170] , the use of an annealing temperature as low as 600°C after Ar plasma treat-

ment allowed a blue shift of the InAs/InP QD PL greater than 100 nm to be achieved, limiting

the thermal shift to only 10 nm. A halftoning of the band gap was performed using wet chemical

etching to remove defects partially from the InP cap so that defect creation, defect amount con-

trol, and intermixing promotion by defects became three thoroughly independent processes.

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

A very large blue shift up to 350 nm (280 meV) and a narrow emission linewidth down to

30 nm in intermixed InAs/InP quantum sticks (QS) were reported after low-energy (18 keV) P ion

implantation at 200°C and RTA at only 650°C [171] , see Fig. 13.25 . As in the case of Ar plasma

treatment, the QD layer was not directly damaged by the ions. The choice of a low RTA tempera-

ture allowed a very small thermal shift (less than 3 nm) to be kept. Importantly, the PL intensity

was not sacriﬁ ced up to an ion ﬂ uence of 5  1 0

c m

 2

at which the blue shift approached its

maximum and the linewidth its minimum.

1.0

0.5

0.0

1650150013501050

1200

Wavelength (nm)

As grown

only-annealed

x 10

2

5 x 10

2

5 x 10

2

5 x 10

2

Normalized PL intensity

400

300

200

100

Ion dose (cm

2

)

PL shift (nm)

(b)

(a)

Figure 13.25 (a) Normalized PL spectra at 20 K of intermixed InAs/InP quantum stick samples as a function of

the phosphorus ion implantation dose. The samples were annealed at 650°C for 120 s. (b) Photoluminescence peak

wavelength shift vs the ion dose [171] .

A direct comparison of proton and phosphorous ion implantation-induced intermixing of

InAs/InP QDs was reported in [172] . Proton irradiation induced less energy shift than P ion

implantation for a given concentration of atomic displacements, which was ascribed to the more

efﬁ cient dynamic annealing of the defects created by protons. The implantation-induced energy

shift reached a maximum value of about 260 meV for a ﬂ uence of 5  1 0

c m

 2

in the P ion

implanted QDs, which also showed narrower PL linewidths compared to the proton implanted

QDs. Defect production and annihilation processes evolved differently in samples with InGaAs

and InP top cap layers and varied with the implantation temperature. When the implantation

was performed at higher temperatures, the energy shift of the P ion implanted QDs capped with

an InP layer increased, probably due to the reduction in larger defect cluster formation at higher

temperatures, while the energy shift of the proton implanted QDs decreased due to increased

dynamic annealing irrespectively of their cap layers.

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Radiation Effects in Quantum Dot Structures 421

The intermixing also opens the way to the lowering of the dimensionality of semiconductor

heterostructures. So, e.g., ion implantation through lithographically deﬁ ned masks and focused ion

beams (FIB) [250] were used very early to implant quantum well structures and interdiffuse III–

V heterojunctions thus creating quantum wires, dots and antidotes [173–184] . This is now a

quickly developing area of technology [185–188] . At the present time, sub-5 nm FIB direct pat-

terning of nanodevices is possible [189] .

13.4.2 Self-organization upon irradiation

A very interesting and useful phenomenon is the self-organized creation of nanopatterns on

the surfaces of targets irradiated by ion beams at low and intermediate energies. So, submicron

ripples on various surfaces [190, 191] and nanometric dots on GaSb [192] can be produced by

ion-beam sputtering . The dots even form a highly ordered array with hexagonal symmetry [192]

( Fig. 13.26 ). Later on, nanodot production was reported in other materials such as InP [193] , Si

[194] , Ge [195] , InAs [196] , InSb [197] , MgO [198] , SiC [199] . The theoretical description of the

sputtering phenomenon is made by means of the molecular dynamics, Monte Carlo (e.g. TRIM),

and continuum methods. The effect of the sputtering treatment depends on the ion–target

combination, target temperature, ion energy and incidence angle, treatment time and ion ﬂ ux.

A crucial aspect is the surface morphology (roughness). Topographical features may cause dif-

ferences in the energy deposition (e.g. shadow effect) and, thus, signiﬁ cant variations of the

local sputtering yield. Accordingly, small irregularities on a relatively smooth surface may be

enhanced by ion bombardment. This implies that microscopically ﬂ at surfaces are unstable under

Figure 13.26 (A) The extract of an SEM image of QDs forming on a GaSb surface during bombardment by 420 eV



ions at normal incidence and (B) the corresponding two-dimensional autocorrelation reveals the regularity and

hexagonal ordering of the dots, which extend over more than six periods. (C) Cross-sectional TEM image of the

nanostructures. The dots are crystalline without a disruption to the crystal lattice of the GaSb bulk. An amorphous

layer of ⬃ 2 nm, which is the penetration depth of the low-energy ions into the material, covers the dots. The base

and the height of the dots measure 30 nm [192] .

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

high-dose ion bombardment unless atom migration acts as a dominating smoothing effect [200] .

There are very recent reviews [191, 200] of the self-organized surface nanopatterning by ion

beam sputtering.

When a SiGe buffer layer is grown on (001)Si upon in situ irradiation by 1 keV As ions, the

implantation-induced defects represent effective channels for stress relaxation and create a nano-

patterned surface. When several layers of Ge QDs are then grown by MBE on top of such a buffer,

the nucleation of the Ge islands is enhanced in the strain ﬁ eld from the buried ion-induced pat-

tern, and the dots grow in a vertically correlated manner [201] . Creation of high-density arrays

of small-size Ge/Si QDs with a narrower size distribution of islands in comparison with conven-

tional MBE based on the use of pulsed irradiation with low-energy Ge



ions during the Ge/Si

heteroepitaxy was proposed in [202] and further investigated in [203–206] . It was concluded

that in situ high-temperature ion bombardment during the QD growth pushes the epitaxial layer

system towards equilibrium, thus enhancing the QD PL intensity.

13.4.3 Ion-beam synthesis

Another ample ﬁ eld is the ion-beam synthesis (IBS) of metal or semiconductor nanocrystals

(NCs) in amorphous and crystalline matrices (see, e.g., [207–214] ). These NCs possess unique

physical properties and can be used in novel devices, which exploit size effects on a nanome-

tre scale. They can be the basic unit in light-emitting diodes, photovoltaic devices and single

nano crystal–single electron transistors. The non-linear optical properties of metallic nanoclusters

in insulators are based on the size dependence of the plasmon frequency and third-order optical

non-linearity [215] . IBS of Si and Ge NCs in the gate oxide of MOS structures is now a stand-

ard technology [216, 217] . It was argued that Si NC non-volatile memory technology possesses

intrinsic radiation tolerance to both total ionizing doses of

Co gamma-irradiation and single

event effects, as well as built-in redundancy that is non-existent in ﬂ ash memory. This originates

from the ability of the cell to maintain functionality with only a fraction of intact charge storage

nodes, in contrast to a monolithic ﬂ oating gate [218] . Especially interesting is the achievement

of the optical gain in structures containing Si NCs in SiO

that emit strong luminescence [219,

220] . The IBS of NCs is a rapidly developing technology, so that a review can hardly be exhaus-

tive and up to date. Therefore only a bird’s-eye view of the research area will be provided in this

section. Magnetic NCs in semiconductors and isolating crystals will be discussed in more detail in

Section 13.4.4.

In the IBS technology, an impurity is usually implanted at a concentration far exceeding its sol-

ubility limit, and a phase separation occurs, leading to the formation of NCs. The latter can hap-

pen during the implantation process or during the subsequent heat treatment. Heat treatment

is usually necessary also for the damage annealing, when the irradiation temperature was not

too high. More than one impurity can be implanted in order to synthesize compound NCs [207,

209, 221–230, 438] or even doped compound NCs [209, 231, 232] . However, via implantation

of only one ion species in a compound or heavily doped target, NCs of compounds different from

the matrix can be synthesized [233–240] . Three different cluster morphologies can be observed:

separated systems, alloy clusters and core–shell clusters [241, 242] . Even rare gas nanocrystals

can be formed in solid matrices by ion implantation [243, 244] .

The reports on the formation of metal NCs in insulator crystals and glasses are especially

numerous. A few examples illustrating important physical processes will be cited here. There

are recent comprehensive reviews on the potentialities of ion implantation for the synthesis and

modiﬁ cation of metal nanoclusters in silica and silica glasses [211, 245] and in sapphire [246] .

Some recent results not mentioned above can be found in [247–251, 439, 440] .

The IBS process forms a strongly non-equilibrium state that relaxes towards equilibrium dur-

ing annealing. The driving force is the supersaturation chemical potential. It is noteworthy that,

depending on the implanted species–host combination, supersaturation may also occur at near-

zero concentrations, so that as soon as the temperature is high enough for atomic mobility to

set in, the solution becomes unstable against very small concentration ﬂ uctuations and there

is no nucleation barrier [212] . Although the self-organization of NCs occurs during relaxation

towards equilibrium, it is difﬁ cult to achieve a process control which is sufﬁ cient for technological

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Radiation Effects in Quantum Dot Structures 423

applications [252] . Post-implantation annealing inevitably leads to a broad NC size distribution,

which is typical for conventional Ostwald ripening: the larger clusters grow at the expense of the

smaller ones. The process has been analytically described in a mean ﬁ eld approximation, in the

long-term limit of ripening, and under several other assumptions (LSW model) [253, 254] . A

variation of temperature and ion ﬂ ux during ion implantation can control the size distribution of

NCs, at least to a certain extent [255] .

On the other hand, the size and size distribution of NCs can be efﬁ ciently tailored by high-

energy ion or electron irradiation through a layer of NCs [256–258] , even when the irradiation

is made at a temperature as low as 93 K [259] or 77 K [260] . Under the steady-state condition of

an appropriate ion irradiation, an effective negative interface tension or capillary length appears

in the Gibbs–Thomson relation for NCs embedded in a matrix. Therefore, the system of NCs

evolves towards a maximum interface area, which is reached for a monodisperse size distribu-

tion (if nucleation of additional NCs is not allowed), i.e. inverse Ostwald ripening occurs [252] .

If collisional mixing at interfaces exceeds a certain intensity, then new NCs can nucleate during

ion irradiation [261] . The collisional mixing can be used to create well-controlled NC distribu-

tions also in ﬂ at layered structures. So, over-stoichiometric Si can be introduced to an SiO

layer

or layers from surrounding Si layers by intermixing with high-energy Si irradiation when the

implanted Si ions come to rest in the substrate well below the active device region [252, 262] .

Thus, sandwiched between the stable phases of SiO

and Si, unstable non-stoichiometric SiO

( x  2) phases are formed. Annealing restores the upper and lower SiO

/Si interfaces by phase

separation. However, the tails of the Si atom mixing proﬁ les do not reach the recovered inter-

faces by diffusion, so that the phase separation proceeds via nucleation and growth of Si NCs in

SiO

. The competition between interface restoration and nucleation self-aligns δ -layers of Si NCs

in SiO

along the two interfaces [262] . A selective dealloying in bimetallic Au

1 

nanoclus-

ters prepared by ion implantation in fused silica upon subsequent irradiation with light ions was

reported in [263] . In this process the irradiation with Ne ions promoted a preferential extraction

of Au from the alloy, resulting in the formation of Au-enriched “ satellite ” nanoparticles around

the original cluster.

Let us brieﬂ y consider also a few selected examples of irradiation-induced cluster precipitation .

Irradiation can promote nucleation and growth of elements already present in the substrate

(both introduced by a preceding implantation or during the growth). A phase separation lead-

ing to the formation of Si NCs was induced in silica by electron irradiation with energies above

150 keV without any need for introducing additional Si atoms by implantation or mixing [264] .

According to the authors the transformation of amorphous SiO

into crystalline Si takes place in

two steps. The ﬁ rst step involves transformation of amorphous SiO

into amorphous Si, while the

second step is the crystallization of amorphous Si. Valence electron ionization was determined

as the key factor for the transformation from SiO

to amorphous Si; beam heating and knock-on

displacement were found to be responsible for the transformation from amorphous Si to crystal-

line Si.

Room temperature MeV ion irradiation of a glass containing copper oxide can initiate nuclea-

tion of pure Cu clusters via the inelastic “ electronic ” component of the ion energy loss, when

the latter is above a threshold value [265] . Then the clusters grow under subsequent thermal

annealing, following the LSW kinetics. The authors claimed that the decoupling of nucleation

and growth allowed total control over the cluster density, average size, and size distribution.

A double-implantation technique was applied in [16] . First, Au was introduced into suprasil by

ion implantation at ﬂ uences below the threshold ﬂ uence for spontaneous cluster formation; then

the samples were irradiated with an MeV Si beam, thus assisting the formation of Au nanoclus-

ters. A relationship between the energy deposited in electronic excitations by the MeV Si ions in

the Au-implanted layer with the size of these nanoclusters as well as with the optical absorption

band due to formation of Au nanoclusters was demonstrated.

Nanocrystals of tin oxide were formed in e-beam evaporated amorphous ﬁ lms by high-energy

heavy ion irradiation without subsequent annealing [266] . The nucleation of nanocrystals was

ascribed to electronic excitation by the ions.

In the case of the ion bombardment of zircon (ZrSiO

), a phase decomposition occurs under irra-

diation at temperatures above 600°C. Between 600 and 750°C, zircon ﬁ rst becomes amorphous,

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

but with increasing dose, it gradually decomposes into an assemblage of randomly oriented cubic

or tetragonal ZrO

NCs that are embedded in a matrix of amorphous SiO

. The ZrO

precipitates are

approximately 3 nm in diameter. Irradiation above 750°C leads to direct decomposition of zircon

into the component oxides without the formation of an intermediate amorphous phase [267] .

The number of publications in this highly interesting area is rapidly growing, so that a sepa-

rate review would be necessary to give an account of the state of the art within it.

Gettering of metal impurities in semiconductors by implantation-induced defects has been

known for decades [268] and is widely used in the microelectronics technology. Furthermore,

the formation of voids or bubbles in silicon implanted with H or He is a well-known phenomenon

[269] . After annealing, He-implanted Si specimens exhibit voids that can be highly efﬁ cient get-

tering sites for metal impurities [270] . Self-assembled formation of spherically shaped nanometer-

sized voids in an Si/SiGe layered structure after Ge ion implantation followed by RTA was reported

[271, 272] . As we shall see, these processes are immediately related to the NC formation upon

ion implantation. It has been directly shown by high-angle annular dark-ﬁ eld (HAADF) imag-

ing in a scanning TEM (STEM) that the Ge and Er nanocrystal growth in ion-implanted SiC is

defect mediated: upon high-dose implantation at 700°C the dislocation loops are created, and

the precipitates segregate to the dislocation cores during subsequent annealing at 1600°C for

180 s [273] . A defect-mediated NC nucleation upon Pt implantation was implemented by pre-

irradiating thin SiO

ﬁ lms with high-energy Ge ions [274] . A striking feature was observed by

TEM in [275–279] : the formation of CdS and Cu nanoparticles in amorphous SiO

and of S, As,

Se, Zn, Cd and Ag nanocrystals in crystalline Si by ion implantation and subsequent annealing

was tightly related to the formation of intra- and interparticle voids in the implanted layers. The

voids were formed either at the implant temperature or when heated during annealing to tem-

peratures above the melting or boiling points of the implanted species. The authors indicated

four processes that may occur either individually or in concert and can lead to void formation: (i)

volume contraction of the precipitated material upon solidiﬁ cation from a liquid; (ii) evaporation

of previously existing particles to leave behind a large void; (iii) diffusion and aggregation of gas-

eous species to create voids in a process similar to that proposed for He-implanted Si [270] ; and

(iv) gettering of the implanted material to partially ﬁ ll voids that were previously formed in the

host crystal. A combination of these processes may result in different and complex morphologies

[277] . In situ TEM observations of Mn- and As-implanted silicon have proven even more clearly

that the voids are ﬁ rst formed upon annealing and then Mn and MnAs nanocrystals are formed

inside them [280] .

13.4.4 Creation of magnetic nanocrystals

Creation of magnetic NCs embedded in semiconductor matrices opens the way to diverse spin-

tronic applications. Ferromagnetic nanodots are basic elements for fabrication of various

devices, for detection of magnetic ﬁ eld and for information recording [281] . As in the other

cases described above, the ion implantation offers a versatile tool for nanofabrication. Despite the

importance of the technological area, it has never been reviewed. Therefore, it will be discussed

here in more detail.

13.4.4.1 III–V compounds

GaAs: Submicrometre magnetic structures were developed by FIB implantation of Mn in GaAs

epilayers and QWs in 1994 [282] . At the same time, formation of metal/semiconductor com-

posites by ion implantation of Fe and Ni into GaAs and a subsequent anneal to nucleate clus-

ters was tried [283]. By means of the electron diffraction and HRTEM the precipitates were

identiﬁ ed as hexagonal and metallic Fe

GaAs or Ni

GaAs. The orientation relationship to GaAs

was determined. However, magnetic measurements were not performed. After a while, mag-

netic nanoclusters were successfully synthesized by Mn implantation in GaAs [284, 285] . This

technology has been thoroughly explored in a series of forthcoming publications by various

groups [286–299] . Implantation of Mn ions into MBE-grown or semi-insulating GaAs with

subsequent RTA leads to the formation of GaMn precipitates from 200 to 400 nm in diameter,

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Radiation Effects in Quantum Dot Structures 425

composed of Ga and Mn at a ratio of about 60:40 [284, 286, 287] . The precipitates are ferro-

magnets with a Curie temperature well above 400 K. Considering that bulk GaMn phases at or

near this composition are ferromagnetic with lower Curie temperatures or simply non-magnetic,

an important role of the crystalline structure of the precipitates in their magnetism was con-

cluded [287] . The easy magnetization axes of 80% of the GaMn particles align preferentially

along three equivalent  001  directions of the GaAs host. The GaMn particles were identiﬁ ed

by electron diffraction and HRTEM as icosahedral quasicrystals having a deﬁ nite orientation

relationship to the GaAs host matrix. Nearly 30% of the measured particles were identiﬁ ed as

3D quasicrystals, and more than 60% of the remaining crystals showed quasicrystal-related fea-

tures such as quasicrystal approximants and incommensurate structures, from which one- or

two-dimensional Fibonacci chains could be seen in electron diffraction patterns [288] .

Low-temperature (LT) GaAs contains an excess of interstitial As, therefore implantation of Mn

in such layers with subsequent RTA leads to the formation of hexagonal MnAs precipitates with

particle sizes ranging from 10 to 25 nm ( Fig. 13.27a ) [289] . The diffraction pattern of these par-

ticles indicates an epitaxial relationship with the GaAs matrix ( Fig. 13.27b ): (0001)MnAs 储 (1

1 )

GaAs and (01

1)MnAs 储 (002)GaAs. The α -MnAs phase is ferromagnetic with NiAs-type hex-

agonal structure ( a  0.3724 nm, c  0.5706 nm). In the bulk, a reversible α -MnAs– β -MnAs

(paramagnetic, orthorombic) phase transition takes place at 318 K [300] . The maximum Curie

temperature obtained from SQUID measurements in [289] was T

 360 K, i.e. slightly higher

than that of bulk MnAs. This behaviour was attributed to an admixture of Ga to the MnAs

(a)

(b)

AIAs

Figure 13.27 (a) Cross-sectional TEM image of LT-GaAs:Mn annealed at 750°C (5 s) (Mn  1  1 0

c m

 2

(b) High-resolution TEM image of a single MnAs crystallite in LT-GaAs:Mn annealed at 750°C (5 s) ((Mn) 

1  1 0

c m

 2

) [289] .

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

precipitates, so that MnAs

alloy precipitates may have formed at high annealing tem-

peratures. Similar results were obtained in [293, 294] , where hexagonal MnAs nanoparticles

with diameters ranging from 9 nm to 13 nm were synthesized in (001)GaAs by implantation

of Mn alone and by co-implantation of Mn and As, in both cases followed by RTA. The cru-

cial role of As in the retention of Mn was shown. The same epitaxial relation as in [289] was

determined: (0002)MnAs 储 ( 1

–

1) GaAs and [1

10]MnAs 储 [011]GaAs, the highest attainable

through the association of these two materials. There are four different but equivalent orienta-

tions of the hexagonal prisms, as their basal plane can lie on any of the four { 111 } planes of

GaAs. The formation of MnAs nanocrystals in GaAs co-implanted with Mn and As is facilitated

by a pre-annealing at 600°C, just below the congruent temperature of GaAs [294] T

 630°C.

Ferromagnetic resonance (FMR) spectra measured on Mn and As co-implanted GaAs after

RTA treatment [301] were identical to those formerly reported in MO CVD-grown, cluster-rich

GaMnAs layers and interpreted as resulting from hexagonal MnAs nanocrystals with the [0001]

axis parallel to any of the  111  axes of the GaAs host [302] . Synthesis of MnAs nanocrys-

tals with a Curie temperature of approx. 320 K by Mn implantation in semi-isolating (001)GaAs

wafers with subsequent RTA in a nitrogen gas with a silicon proximity cap was reported in [291] .

Contrary to [289] , the authors of [291] concluded from their magnetic force microscopy (MFM)

measurements that the precipitates ’ [0001] axis (which is the hard magnetization axis) is normal

to the (001) wafer surface, and the [11

0 ]

MnAs

(easy magnetization axis) is parallel to [110]

GaAs

When the RTA was performed in a forming gas, GaMn precipitates were formed. MnAs and

MnAs

precipitates were simultaneously observed after Mn implantation in semi-insulating

GaAs and RTA in a nitrogen atmosphere [33] .

Though the MnAs nanoclusters are embedded in the GaAs matrix, they possess a residual

degree of freedom caused by lattice defects surrounding them, so that the ﬁ rst-order magneto-

structural phase transition at approximately 40°C, from the hexagonal–ferromagnetic α -phase

to the orthorhombic–paramagnetic β -phase, could be observed [298] . The nanoclusters ’ lattice

parameter presents a thermal hysteresis, i.e. the phase transition occurs at higher temperatures

during heating as compared to a cooling cycle, and is not very abrupt, which was interpreted as

due to the coexistence of the two phases. By analogy with literature results on thin MnAs ﬁ lms

grown on GaAs, both the coexistence and the hysteresis were attributed to strain effects.

Granular ﬁ lms, consisting of Co granules embedded in GaAs, were prepared by recoil implan-

tation of Co in n-type (100)GaAs single crystals [303] . The GaAs surface, on which a 30 nm

thick Co layer had been deposited prior to the implantation by RF magnetron sputtering, was

bombarded with Xe ions. The samples showed large extraordinary Hall effect.

Giant magnetoresistance [290] and enhanced positive magnetoresistance [292, 304] were

observed in GaAs layers containing MnAs NCs synthesized by Mn implantation. Several other

interesting effects were reported in GaAs/MnAs and other hybrid semiconductor–nanomagnet

systems fabricated by epitaxial techniques [305–312] . Obviously, similar results can be obtained

using ion implantation, especially FIB.

Though magnetic nanocluster synthesis by ion implantation is best studied in GaAs, there are

numerous reports on the implantation-induced formation of magnetic precipitates in other semi-

conductors and isolators.

GaP: Very small FM NCs containing only ten spins were detected in Mn-implanted p -type GaP

[313] . The magnetism was suppressed when n -type GaP substrates were used. The presence of

ferromagnetic clusters and hysteresis to temperatures of at least 330 K was attributed to disorder

and proximity to a metal-insulating transition. The experimental data suggest a percolation type

of picture in which isolated ferromagnetic clusters, immersed in a background of paramagnetic

moments, grow in size as the temperature is lowered until, at T  T

, long-range order extends

through the whole system.

InP: MnP and InMn

crystallites with a size of ⬃ 20 nm and Curie temperatures of 291 K and

well above RT, respectively, were observed in Mn-implanted InP:Zn [314, 315] .

GaN: Mn and Fe were also implanted in GaN [316, 317] . After appropriate annealing, the

samples showed signatures of ferromagnetism with Curie temperatures  250 K for Mn and

 150 K for Fe implantation. The structural analysis of the Mn-implanted GaN showed regions

consistent with the formation of Ga

1 

N platelets occupying ⬃ 5% of the implanted volume.

In [443] RT ferromagnetism was observed in GaN after Fe implantation at 623 K with different

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Radiation Effects in Quantum Dot Structures 427

ﬂ uences and was mainly ascribed to α-Fe precipitations. The formation of secondary phases such

as Mn

or Mn

was excluded by careful diffraction analysis. Creation of Co or CoGa mag-

netic clusters as a result of the Co implantation in GaN was reported in [318] .

13.4.4.2 Group IV semiconductors

SiC: SmSi

, Sm

, Co

Si and SmCo nanoinclusions were created by Sm and Co implantation in

4H–SiC [319] (see Fig. 13.28 ). Various types of epitaxial relationship between the nanocrystals

and the SiC matrix were found. The magnetism of the Co-related nanocrystals was checked by

the Lorentz microscopy, see Fig. 13.29 (as to the measurement technique, see the recent reviews

[320, 321] ). Magnetic ﬁ elds ranging from 2 to 4 T were measured at RT.

Figure 13.28 Three typical types of nanocrystals created in 4H–SiC after Sm–ion implantation and annealing viewed

along the [112

–

0]-4H–SiC direction; experimental HRTEM images (a–c), corresponding lattice banding maps (d–f),

diffractograms (g–i) and corresponding calculations: HRTEM images (j–l) and diffraction patterns (m–o) for [100]-

SmSi

-nanocrystal (a, d, g, j, m), [110]-SmSi

-nanocrystal (b, e, h, k, n) and [110]-Sm

-nanocrystal (c, f, i, l, o).

Reﬂ ections of the nanocrystals are encircled; reﬂ ections of the 4H–SiC matrix are boxed [319] .

Ge: In Ge, ferromagnetic nanoparticles were synthesized by implantation of Mn and Fe. After

an Mn implantation in (100)Ge substrates at elevated temperatures (240–300°C), with and

without subsequent annealing at 400°C, X-ray diffraction (XRD), TEM, X-ray photoemission

(XPS), electron energy loss spectroscopy (EELS), scanning tunnelling spectroscopy (STM) and

magneto-optical Kerr effect (MOKE) measurements revealed the coexistence of Mn dilution into

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

the crystalline Ge matrix and of MnGe precipitates embedded in it [214, 322–325] . The pre-

cipitates with smaller size (3–10 nm) were amorphous, whereas the bigger ones (8–15 nm) were

mainly composed of the metallic Mn

hexagonal crystalline phase ( T

⬃ 296 K, [326] ). The

small amorphous precipitates have semiconducting properties with a band gap of 0.45  0.05 eV

[327] , and their contribution is believed to be predominant in determining the magnetic proper-

ties of the system [328, 329] .

Mn implantation in Ge at RT at ﬂ uences used for magnetic functionalization (  10

c m

 2

)

leads to amorphization of the implanted layer. The topmost part of the implanted layer is also

porous and oxidized [330] . Subsequent annealing at 400°C leaves this layer amorphous, whereas

the deeper regions reconstruct into Ge nanocrystals ( ⬃ 10 nm in diameter). Though magnetic

hysteresis was observed at the annealed samples up to above 250 K, such a structure appears less

promising for electronic applications.

An important step in implementing a spin ﬁ eld-effect transistor [331] was done in [332] . The

RT Mn implantation was undertaken through a periodic array of 20 nm diameter holes in an

SiO

layer deposited on top of n- and p-type (111)Ge wafers. Following implantation, RTA was

performed on samples at 450°C in an N

atmosphere for 20 min. The TEM pictures showed that

the Mn

1 

DMS nanodots were coherently incorporated in the Ge matrix. XRD revealed the

presence of the Mn

and Mn

( T

⬃ 270 K, [326] ) ferromagnetic phases, the latter in a

negligible concentration. More importantly, the structures formed on n- and p-type substrates

exhibited different magnetic properties. Hysteresis was observed only in p-type, suggesting the

existence of hole-mediated exchange coupling. Using gate-biased MOS structures, the authors

succeeded in controlling the hysteresis by varying the hole concentration in the Mn

1 

channel.

Fe nanoclusters were also successfully incorporated in p-type (110)Ge by ion implantation

[333, 334] . At the lower ﬂ uence used (2  1 0

c m

 2

) dilute Fe

Ge precipitates of irregular

Figure 13.29 (a) Reconstructed phase image of Co–ion implanted 4H–SiC. Nanocrystals marked 1–4 show

magnetic activity, those marked 5 show electric activity. (b) Bright ﬁ eld images showing the same sample area as

in (a). (c) HRTEM image of the nanocrystal marked 2. (d) Plot of the phase of the electron wave along the region

marked 2. The distance of 13 nm between minimum and maximum phase ﬁ ts exactly to the diameter of the round-

shaped nanocrystal 2. (e) Magniﬁ ed and ampliﬁ ed phase images of the magnetic dipoles marked 1–4 and the

monopoles marked 5 in (a), the dotted line in 2 marks the trace of the plot of the dipole shown in (d) [319] .

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