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

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Engineering of Quantum Dot Nanostructures for Photonic Devices 519

low (129 ⫾ 29 meV) that RT emission is inhibited. The QD emission is ﬁ rst red shifted by using a

1000 nm thick metamorphic LCL and then blue shifted when 21 ML thick In

0.31

0.69

As barriers

are added: the new design results in an activation energy of 191 ⫾ 10 meV and a RT light emis-

sion at 1.35 μ m.

On the basis of the combined approach of QD strain engineering and enhanced barriers, we

worked to extend the RT range of operating wavelengths of metamorphic QD nanostructures

towards the 1.55 μ m region. For this purpose, we reduced further the QD strain by using: (i)

x ⫽ 0.35 and f ⫽ 0.048 and (ii) x ⫽ 0.40 and f ⫽ 0.044, as shown in Fig. 16.12b . For the ﬁ rst

Figure 16.12 (a) Measured 10 K emission energy (open and closed circles, left scale) and RT emission wavelength

(squares, right scale) of structures without and with barriers (closed and open symbols, respectively) as functions of

QD–CL mismatch for structures with In

0.31

0.69

As CLs. The full and dashed arrows show the effects of QD strain

reduction and of the additional barriers, respectively. The ( x, z, t ) terns and the ( x, t ) pairs give the composition of

1 ⫺

As CL ( x ) and of In

1 ⫺

As barriers ( z ) and the thicknesses of LCL ( t ) in the case of structures with and

without 21 ML thick barriers, respectively. The inset schematically shows the conduction band proﬁ le in structures

without and with lower and upper barriers (LB and UB). (b) Same as panel (a) but for structures with x ⫽ 0.35 and

x ⫽ 0.40, aiming at long wavelength operation.

1.000

21 ML thick barriers

(0.31, 0.31, 1000)

(0.31, 190)

(0.31, 1000)

Wavelength @ RT

Energy @ 10 K

Without barriers

With barriers

LCL

UCL

with barriers

Without barriers

1.35

0.950

0.900

Emission energy (eV)

Emission wavelength (μm)

0.850

(a)

1.40

1.45

1.50

0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075

QD–CL mismatch

1.000

21 ML thick barriers

(0.35, 0.15, 500)

(0.40, 0.20, 1000)

(0.40, 0.25, 1000)

(0.40, 0.30, 1000)

(0.40, 1000)

(0.35, 50)

(0.35, 500)

Wavelength @ RT

Energy @ 10 K

Without barriers

With barriers

1.35

0.950

Emission energy (eV)

Emission wavelength (μm)

0.850

0.800

0.750

0.035 0.040 0.045 0.050 0.055

QD–CL mismatch

0.060 0.065 0.070 0.075

1.40

1.45

1.50

(b)

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

structure we obtained room temperature emission at 1.39 μ m by adding In

0.15

0.85

As barri-

ers; in the structures of the second type, upon the insertion of In

1 ⫺

As barriers RT emissions

were achieved at 1.43 μ m for z ⫽ 0.20, at 1.46 μ m for z ⫽ 0.25 and at 1.51 µm for z ⫽ 0.30, a

value rarely reported for InAs/InGaAs structures grown on GaAs substrates. The PL spectrum

at RT of this last structure is reported in Fig. 16.13 . It should be mentioned that in the structure

with z ⫽ 0.20 the 21 ML thick ( ⬃ 6 nm) barriers are very close to lattice relaxation, as the critical

thickness (depending on the mismatch between CLs and barriers) is 12 nm [41] . Nevertheless,

PL spectrum for this structure shows no degradation, indicating that strain relaxation has not

yet occurred.

1.0

0.8

1.51 µm

0.6

PL intensity (a.u.)

0.4

0.2

0.0

1.30 1.40 1.50

21 ML barriers

(0.40, 0.30, 1000)

Emission wavelength (µm)

1.60 1.70

Figure 16.13 PL emission spectrum at RT of the structure with 1000 nm thick In

0.40

0.60

As LCL and 21 ML

thick In

0.30

0.70

As barriers, peaked at 1.51 µm. The excitation power density is 5 W/cm

It is also interesting to notice how the In molar fraction z of InAlAs barriers can be regarded as

an additional parameter to tune the emission properties of QD structures, since the band discon-

tinuities due to additional barriers depend on z and determine the increase of the carrier conﬁ ne-

ment that inﬂ uences the QD emission energies and the activation energies for thermal emission

quenching, as discussed above. Indeed, it is possible to identify optimized barriers that minimize

the blue shift, while assuring an activation energy large enough to obtain QD emission at RT.

This combined approach looks to be a very promising way towards InAs QDs emitting in the

1.55 μ m window, widening the spectral window of operation of such nanostructures. As a mat-

ter of fact, the usefulness of this approach is conﬁ rmed by very recent results obtained on long

wavelength QD lasers based on metamorphic structures that were obtained in a very much

device-orientated approach to structure design [37, 89, 90] .

16.4 QD nanostructures for 0.98–1.04 μ m emission

The preparation of QD structures based on GaAs substrates emitting in the 0.98–1.04 μ m range

for telecom and medical application requires the blue shift of emission as compared to that of

InAs/(In)GaAs QD structures; since the earliest results reported in the literature [91, 92] , this is

usually achieved by using InGaAs QDs embedded in AlGaAs CLs. This result is attributed to the

increase in both the energy gap of QDs and the QD–CL band discontinuities (and, then, in the

conﬁ nement energies of carriers in the QDs) due to the introduction of Ga and Al in QDs and

CLs, respectively.

In these structures the lattice mismatch between the GaAs substrates and the AlGaAs conﬁ n-

ing layers is negligibly small, so that the QD strain engineering approach cannot be followed, as

it is done in the case of metamorphic InAs/InGaAs QD structures. Instead, the main parameters

that can be used to engineer both the emission wavelength and efﬁ ciency are the compositions of

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Engineering of Quantum Dot Nanostructures for Photonic Devices 521

QDs and CLs and QD dimensions that can be controlled, for instance, by means of the In(Ga)As

coverage.

As it will be shown below, the emission properties are also determined by QD shapes and com-

position proﬁ les that are related in a subtle way to nominal compositions and growth conditions.

The last considerations explain why simple models such as the one described in section 16.3.2

cannot generally be used for InGaAs/AlGaAs QD nanostructures.

16.4.1 InGaAs/AlGaAs QDs: the effect of CL composition

The effect of the CL composition on the emission energy of InAs/AlGaAs and In

0.50

As/

AlGaAs QD structures is presented in Fig. 16.14 . Unlike the LCLs that were deposited at 600°C,

the QDs and UCLs were grown by ALMBE at relatively low temperatures (460 and 360°C, respec-

tively), with the aim of reducing interdiffusion and segregation of constituent elements. In the

Figure, the blue shift of emission with increasing Al composition in CLs is clearly evidenced for

both the In

0.50

As (circles) and InAs (squares) QD systems. Moreover, we observe that,

by using In

0.50

As QDs and Al

0.30

0.70

As CLs, we obtained efﬁ cient RT emission (empty

circles) at the 0.98 µm wavelength of optoelectronic interest.

1.400

0.50

As/AIGaAs QDs

1.350

1.300

0.950

0.900

1.000

1.250

1.200

Emission energy (eV)

Wavelength (µm)

1.150

1.100

1.050

0.00 0.05 0.10 0.15

CL composition

InAs/AIGaAs QDs

0.20 0.25 0.30

10 K

300 K

Constant QD size and r

AFM QD size and constant r

AFM QD size and fitted r

10 K

Figure 16.14 10 K (full symbols) and RT (empty symbols) PL emission energies (left vertical scale) and respective

wavelengths (right vertical scale) as functions of the CL composition in In

0.50

As (circles) and InAs (squares)

QDs embedded in Al

1 ⫺ x

As. The solid lines are guides for the eye, while the dotted, dashed and chained lines

represent the results of model calculations.

In order to gain a deeper insight on the effect of CL composition, we compare the emission

energy of InAs/AlGaAs QD structures (squares in Fig. 16.14 ) with the results of the single-

band effective-mass model calculation discussed in section 16.3.2 (lines). QD heights and base

dia meters were evaluated by means of the statistical analysis of AFM measurements performed

on uncapped structures grown under the same conditions. The morphology investigations

revealed that, by increasing the Al composition in CLs from 0 to 0.30, the most frequent value of

QD heights decreases from 4.9 to 2.8 nm, while that of base diameters remains almost constant

(24 nm); the dependence of QD sizes on the composition of the underlying AlGaAs surfaces is

usually attributed to the larger reactivity of Al as compared to that of Ga [93] .

By analysing Fig. 16.14 , we observe that the composition-dependent increase in the QD–CL

band discontinuities alone (chained line) cannot justify the observed blue shift of PL emission.

In order to reproduce the experimental data, the model calculations have to take into account

not only the decrease in QD sizes as observed by AFM (dotted line) but also the change (from 4

to 2) in the major-to-minor base–diameter ratio ( r ) of the assumed truncated-conical QD shape

(dashed line). We note that r may have a more general meaning than that of the ratio between

diameters: indeed, it can be considered as a phenomenological parameter that accounts for non-

uniform composition proﬁ les induced by segregation and mixing effects at the QD–CL interfaces

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

and for other effects not explicitly considered by the model. These considerations suggest that the

CL composition in the InAs/AlGaAs QD system affects not only band discontinuities and QD sizes

but also other details of the QD band structure; these details are related to QD shapes and com-

position proﬁ les that: (i) cannot be directly controlled even under our low-growth temperature

conditions and (ii) are not easily modelled.

Our considerations are supported by a few experimental observations; by means of scanning

tunnelling microscopy and Raman scattering measurements performed on nominal InAs QDs

embedded in AlGaAs CLs it has been shown that the Al composition in CLs affects both the sizes

[93] and composition proﬁ les [94, 95] of QDs.

The CL composition is known to affect not only the QD emission energy but also the PL emis-

sion efﬁ ciency [96] . By analysing with Eq. 16.6 the temperature dependence of the integrated

PL intensity of In

0.50

As QDs embedded in Al

1 ⫺

As CLs, we derived the high- T activa-

tion energy E

for thermal quenching of photoluminescence (full circles in Fig. 16.15 ), which is

generally attributed to the thermal escape of carriers from QDs [51, 52] . The interesting feature

of the plot is that the activation energy increases with the Al composition in CLs, which linearly

determines the QD–CL band discontinuities.

300

250

200

Activation energy (meV)

150

100

0.0 0.05 0.10 0.15

4.0ML In

0.50

As/AI

1⫺x

As QDs

CL composition

0.20 0.25 0.30

Adapted from [97]

Figure 16.15 Measured activation energy for carrier thermal escape from In

0.50

As/Al

1 ⫺

As QD

structures (full circles, present work) and energy separations between WL and QD conﬁ ned levels (empty symbols),

as derived from [97], represented as functions of the CL composition. The solid line is a guide for the eye.

Structures similar to the ones considered here were previously studied by means of PL and

resonant PL techniques [53, 97] ; the authors were able to deduce the energy of the WL levels

(deﬁ ned as the difference between the energy of electron and hole conﬁ ned levels in WLs) by:

(i) ﬁ tting the temperature dependence of some PL features with the model developed in [51] and

(ii) analysing differences in resonant and non-resonant PL spectra of the structures. The results of

the two methods are very close to each other and compare favourably with the values calculated by

describing WLs in terms of quantum wells of suitable thicknesses [53, 97] . The empty symbols in

Fig. 16.15 represent the energy separations Δ E

QD–WL

between QD (obtained from PL measure-

ments [97] ) and WL levels (deduced by methods (i) (empty squares) and (ii) (empty circles)).

We observe that Δ E

QD–WL

agrees satisfactorily with the activation energy E

for thermal escape

of carriers from QDs (full circles), thus showing that our results are consistent with those of [53,

97] , and that the ﬁ nal states of the carrier escape process are the WL levels.

We note that E

and Δ E

QD–WL

increase by increasing the Al composition in the CLs since the

less conﬁ ned and, therefore, shallower WL levels are more sensitive to the increase in band dis-

continuities with respect to more conﬁ ned and, therefore, deeper QD levels.

16.4.2 InGaAs/AlGaAs QDs: the effect of QD composition

As for the dependence of QD emission energy on QD composition, we analysed the emission

of MBE In

1 ⫺

As QDs embedded in Al

0.30

0.70

As CLs with y ranging from 0.40 to 0.70.

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Engineering of Quantum Dot Nanostructures for Photonic Devices 523

The structures were prepared by using: (i) a relatively high Al composition in the CLs in order to

enhance the QD-to-WL energy separation, as a result of the previous discussion, and (ii) growth

temperatures of 470°C for QDs (by MBE) and the lower part of UCLs (by ALMBE) and of 600°C

for the uppermost part of UCLs (by MBE), being the relatively high values of the last temperature

a requirement to improve the material quality so as to allow the preparation of optimized QD

laser structures [98] .

The results of the 10 K emission energy as a function of the InGaAs coverage θ for different

values of y are shown in Fig. 16.16 , where we observe that the blue shift of emission energy is

achieved by: (i) decreasing θ at constant In composition in QDs or (ii) decreasing y at constant

InGaAs coverage.

1.400

1.350

y ⫽ 0.70

y ⫽ 0.60

y ⫽ 0.45

y ⫽ 0.40

10 K

1.300

Emission energy (eV)

1.250

1.200

1.150

345678

Covera

e (ML)

1⫺y

As/AI

0.30

0.70

As QDs

9 10111213

Figure 16.16 10 K PL emission energies of In

1 ⫺

As/Al

0.30

0.70

As QD structures as functions of the QD

coverage for different QD compositions. The lines are guides for the eye.

As for the effect of coverage, we note that, for constant In composition in QDs, the decrease in

θ yields a reduction of QD sizes that results in higher carrier conﬁ nement energies and, then, in

the blue shift of the emission energy. It should be noted that the lower limit for θ at any given y

is represented by the critical coverage θ

for the two-dimensional to three-dimensional growth

transition for self-assembled QDs; as a consequence, the emission energy cannot be indeﬁ nitely

blue shifted by decreasing θ at constant y , as shown in Fig. 16.16 .

As for the change in QD composition at constant θ , four effects are to be considered that act in

different ways on the band structure and, therefore, on the emission energy: by decreasing the In

composition in QDs at constant θ : (1) the energy gap of the QD material E

widens and, there-

fore, since the CL band edges are unaffected by the change in y (2) the band discontinuities Δ E

diminish, (3) owing to the dependence of θ

on y [99–102] , the effective amount of InGaAs that

self-assembles into QDs is reduced and, hence, the QD sizes decrease, and (4) the QD–CL lattice

mismatch decreases thus leading to the formation of larger QDs, as observed by Orr [103] and Li

[101] by means of kinetic Monte Carlo simulations and experiments, respectively. As it can be seen

in Fig. 16.16 , the combined effect of the four mechanisms is the blue shift of the emission energy.

It remains to be explained the apparently surprising observation that for higher In composition

y it is possible to have higher emission energies: in this connection, it should be recalled that θ

decreases for increasing y [99–102] ; therefore, ranges of smaller coverages can be accessed, thus

resulting in blue shifted emission ( Fig. 16.16 ).

As a result of these mechanisms, 0.98 μ m RT emission has been obtained from the low-coverage

MBE structures with In

0.70

0.30

As QDs embedded in Al

0.30

0.70

As CLs. We observe that struc-

tures characterized by both blue shifted emission energy and high emission efﬁ ciency at RT may

be obtained by means of the simultaneous increase in: (i) carrier conﬁ nement energies and/or

QD material energy gap and (ii) energy separation between WL and QD states. Both conditions

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

can be fulﬁ lled when the Al composition in CLs is increased (section 16.4.1); instead, we note

that the approach of intentionally decreasing the QD sizes and/or enhancing E

in order to blue

shift the emission (section 16.4.2) should not be followed since the increase in the transition

energy would be obtained concomitantly to the decrease in the energy separation between WL

and QD states, which would give rise to enhanced thermal escape of carriers from QDs.

16.5 Conclusions

In this work we reviewed methods allowing the extension of the spectral window of QD emission,

through the engineering of the InAs/GaAs QD structure, taking advantage of predictions pro-

vided by a simple effective-mass model. In order to match the 1.3–1.6 μ m window we considered

InAs/InGaAs metamorphic QD nanostructures, where band discontinuities and energy gap of

the QD material are controlled by means of two design parameters that are the composition of

the InGaAs CLs and the QD–CL mismatch (which determines the QD strain). We demonstrated

how by this approach, termed QD strain engineering , we can substantially red shift the emission

wavelength of InAs QDs embedded in InGaAs CLs and optimize the activation energy for thermal

escape of conﬁ ned carriers, reaching 1.44 μ m emission at RT, under excitation power densities as

low as 5 W/cm

. Moreover, the careful analysis of model calculations led us to conclude that the

proposed approach yields two degrees of freedom, the CL composition and the QD–CL mismatch,

which allow us to control the QD–CL band discontinuities and QD energy gap, respectively; the

availability of such parameters permits us not only to tune the QD emission wavelength at pre-

determined values, but also to maximize the activation energy for thermal quenching of emission.

To overcome the reduction of the barrier discontinuities and, therefore, of the activation energy,

which is a consequence of red shifting the emission, we combined the use of QD strain engineer-

ing with enhanced additional barriers, achieving RT emission up to 1.51 μ m, thus proving that

this approach is of great interest to extend the QD emission wavelengths to 1.55 μ m and beyond.

To match the 0.98–1.04 μ m window, AlGaAs conﬁ ning layers and InGaAs QDs have been suc-

cessfully used in order to increase the conﬁ nement energies and the QD energy gap, respectively.

Experimental results and model calculations allowed us to discuss the effect of QD and CL com-

position on the QD morphology, highlighting its impact on the QD emission blue shift and on the

activation energy for conﬁ ned carrier thermal escape. In particular, we showed how the CL com-

position, besides band discontinuities, affects also the QD dimensions and other details of the QD

band structure, resulting in a complex interplay of different effects. Furthermore, by studying the

effect of the change in composition of InGaAs QDs, we identiﬁ ed different mechanisms contribut-

ing to the blue shift of the emission and achieved 0.98 μ m RT emission from different structures

consisting of InGaAs QDs embedded in Al

0.30

0.70

As CLs.

By comparing the results of the model in the case of InAs/InGaAs and of In(Ga)As/AlGaAs

structures we can conclude that the very satisfactory predictions of the model (on emission

energy, activation energy for thermal quenching of emission, as well as on approaches that

result in the substantial improvement of the emission efﬁ ciency) rely not only on the soundness

of the assumptions but also on the circumstances that: (i) important quantities such as composi-

tions of QDs and CLs are very close to the nominal ones and (ii) the QD sizes evaluated by AFM

are conceivably very little modiﬁ ed during the subsequent steps of growth, such as the deposi-

tion of UCLs. These results are related to the low-temperature growth conditions that have been

used throughout our experiments and that limit activated alloy phase separation [19, 26, 29,

32, 33] , intermixing and segregation phenomena. On the other hand, the relatively high reactiv-

ity of Al gives rise to important changes in QD size, shape and composition in In(Ga)As/AlGaAs

structures, which would require the use of speciﬁ c techniques to measure these quantities in the

complete structures to be modelled.

In conclusion, we demonstrated how by a careful design of structures grown under well-

deﬁ ned conditions it is possible to cover the 0.98–1.04 μ m and 1.3–1.6 μ m windows with QD

nanostructures based on GaAs substrates, a result that could open the path to the realization of

QD photonic and optoelectronic devices for a wide range of applications.

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Engineering of Quantum Dot Nanostructures for Photonic Devices 525

Acknowledgements

This work has been supported by the MIUR-FIRB project “ Nanotecnologie e Nanodispositivi

per la Società dell’Informazione ” and by the SANDiE Network of Excellence of EU, contract no.

NMP4-CT-2004-500101.

Note added in proof

Very recently, emissions upto 1.59 μm at room temperature were obtained under excitation

power densities as low as 5 W/cm

from metamorphic QD nanostructures on GaAs substrates; the

structures consist of InAs QDs, In

0.45

0.55

AsCLs and In

0.35

0.65

As additional barriers. These

results demonstrate how by using the combined approach of QD strain engineering and bar-

rier enhancing it is possible to: (i) tune the emission wavelength in the whole 1.3–1.6 μm range,

(ii) increase the activation energy of PL thermal quenching with the consequent improvement of

RT emission efﬁ ciency [104].

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