4.4 Report of project part 04: Epitaxial lead salt ... · experimentally from optical spectroscopy...

4.4 Report of project part 04: Epitaxial lead salt nanostructures

Epitaxial lead salt nanostructures for mid-infrared lasers and detectors

Principal investigator: Gunther Springholz

Institut für Halbleiter- und Festkörperphysik

Johannes Kepler Univeristät

Altenbergerstr. 69, A-4040 Linz

Phone: +43 (0)732 2468 9602

Fax: +43 (0)732 2468 8650

eMail: [email protected]

Report – Project description

4.4.1 Summary

The realization of self-assembled quantum dots with efficient emission in the mid-infrared

spectral region has remained a considerable challenge. In this project, therefore, novel self-

assembled semiconductor quantum dots were fabricated based on the lead salt compounds.

Due to their narrow band gaps and favourable electronic band structure as well as the low

non-radiative Auger recombination rates, these compounds are very well suited for mid-infra-

red devices. Self-assembled quantum dots were fabricated by strained-layer heteroepitaxy

using the Stranski-Krastanow growth mode as well as a novel alternative method based on

phase separation and nanoprecipitation. To obtain quantum dots with high emission efficien-

cy, strain-tuning and band gap engineering was applied. Detailed growth investigations were

performed for the PbSeTe quantum dot system for a wide range of compositions and strain

values ranging from 0 to 5.4%. By changing the buffer layer, tensile as well as compressively

strained dots were produced and their structural and electronic properties investigated in

detail with respect to their applicability for mid-infrared optoelectronic devices. Based on in

situ scanning tunneling microscopy, a strong intermixing between the dots and the matrix

material was found. This effect could be suppressed by alloying of the capping material. As

derived from optical spectroscopy investigations, PbSe dots exhibit a type II band alignment

to the surrounding barrier material due to the strong strain effects on the electronic band

structure. This arises from the large deformation potentials which were determined

experimentally from optical spectroscopy of strained-layer structures. As a consequence,

mid-infrared optical emission of these dots is too weak for obtaining stimulated emission.

To solve this problem, we have developed a new class of self-assembled quantum dots

based on the combination of zinc blende II-VI and rock salt lead salt compounds. Due to the

wide miscibility gap, phase separation and nanoprecipitation of highly symmetric PbTe

quantum dots in CdTe with extraordinary structural and optical properties occurs. Due to their

defect free and atomically sharp heterointerfaces as well as the excellent carrier confinement

arising from the large 1.5 eV difference of the PbTe/CdTe band gaps, intense room tempe-

rature photoluminescence emission in the range of 2 – 3.5 µm wavelength was obtained.

With respect to mid-infrared devices, we have developed improved Bragg mirrors and micro-

cavity structures, which were applied for fabrication of vertical cavity surface emitting lasers

and narrow band width mid-infrared detectors integrated interference filters were produced.

The project work involved successful collaborations with several other SFB partners, in

particular with P2 for transmission electron microscopy, P7 for x-ray diffraction, P5 and P12

for optical spectroscopy and P6 for ab initio calculations. This is documented by a large

number of common publications in scientific journals as well as presentations at conferences.

P04 Epitaxial lead salt nanostructures (Springholz)

4.4.2 Scientific Background – State-of-the-Art

Self-assembled semiconductor quantum dots fabricated directly by epitaxial growth have

attracted tremendous interest due to their novel physical properties as well as their great

potential for device applications. In most the most common scheme, these quantum dots are

synthesized by the Stranski-Krastanow growth mode of strained-layer material systems

(Leonhard1993) in which nanometer sized 3D islands spontaneously nucleate on the surface

of a thin 2D wetting layer. This results from the fundamental surface instability of strained

layers that is driven by the large reduction of strain within the islands due to lateral elastic

expansion or compression in the direction of the free side faces (Shchukin2003). The most

promising applications of self-assembled quantum dots are in the field of optoelectronics due

to the sharply peaked electronic density of states. Theoretically, quantum dot lasers have

been predicted to yield strongly increased material and differential gain (Asada1986), lower

threshold currents, higher modulation band widths and better temperature stability (Arakawa

1982). As demonstrated by recent work, indeed self-assembled InAs/GaAs quantum dot

lasers (Kirstaedter1994) outperform conventional quantum well lasers and ultra-low threshold

currents have been achieved (Huang2000). Up to now, however, quantum dot lasers have

been made exclusively from III-V semiconductors with emission wavelengths in the visible or

near infrared spectral regions (Grundmann2002).

The realization of self-assembled quantum dots with efficient mid-infrared emission at wave-

lengths beyond 2 µm has remained a considerable challenge. This is because for Stranski-

Krastanow quantum dots the high epitaxial strains imposed by the surrounding barrier

material drastically change the energy gaps and band alignments due to the deformation

potentials (Stier1998, Williamson1999, Pryor2005, Simma2006). As a result, the effective

band gaps of III-V semiconductor quantum dots are strongly increased and thus, even

narrow gap InAs or GaSb quantum dots emit in the near-infrared rather than the mid-infrared

spectral region (Leonhard1993, Moison1994, Glaser1996, Moltan2002). For the classical

InAs quantum dots, e.g., the compressive strain induced by the 7 % lattice mismatch to the

GaAs matrix doubles the 0.41 eV InAs band gap to more than 0.8 eV (Stier 1998,

Williamson1999), and with the additional quantum confinement shift, the dots typically emit at

wavelengths between 0.9 – 1.4 µm (Leonhard1993, Moison1994, Grundmann2002). For

antimonide GaSb or InSb quantum dots, the situation is complicated due to the staggered

type II band alignment to most III-V barrier materials (Van de Walle1989, Vurgaftman 2001).

As a result only one type of carrier is confined within the dots and due to the corresponding

spatially indirect optical transitions the luminescence emission is rather weak in these cases.

As revealed by a recent survey of band alignments in III-V nanostructures (Pyror 2005), a

type I band alignment for efficient mid-infrared emission is difficult to achieve with III-V


quantum dot materials and only very recently, InSb/GaSb quantum dots with infrared

emission beyond 2 µm have been obtained (Tasco2006). Therefore, up to now III-V quantum

dot lasers have been obtained only for visible or NIR emission (Grundmann2002).

The narrow band gap IV-VI semiconductors or lead salt compounds represent an attracttive

alternative class of materials for mid-infrared devices. This is due to the favourable electronic

properties of the lead salt compounds (Bauer2005) such as the mirror-like electronic band

structure with direct and narrow energy band gaps and the one order of magnitude lower

non-radiative Auger recombination rates (Findlay1998) as compared to narrow gap III-V or II-

VI compounds. As a consequence, lead salt diode lasers (Feit1996, Katzir1998,

Schiessel1999), covering a wide spectral region from 2 - 30 µm (Tacke1995, Katzier1989).

Due to their ultra-narrow line widths and their inherent wavelength tunability, lead salt lasers

have been used successfully in many high resolution spectroscopy applications (Tacke1995,

Katzir1989) such as molecular trace gas analysis (DeBacker1998), monitoring of gaseous

pollutants (Grisar1992), medical diagnostics (Sandstrom1999, Stepanov 2002) and time-

resolved analysis of industrial combustion processes. Apart from conventional edge emitters,

the development of lead-salt based ultra-high reflectivity Bragg mirrors (Schwarzl1999,

Springholz2006) has allowed the fabrication of the first mid-infrared vertical cavity surface

emitting lasers (VCSELs) by our group (Schwarzl1999) for which above room temperature

operation has been recently achieved (Heiss2001, Springholz2006, Schwarzl2007). On the

detector side, IV-VI infrared detector arrays have been successfully fabricated directly onto

silicon substrates with preprocessed integrated read-out electronics (Alchalabi2001], repre-

senting a new approach for monolithic integration of infrared photo detectors.

With respect to lead salt nanostructures, the fabrication of self-assembled quantum dots was

first demonstrated by our group in Linz for the PbSe/PbEuTe material system (Pinczolits

1998). Detailed investigations revealed various aspects of growth such as facetation

(Pinczolits 1999), control of dot sizes and densities (Raab2002), thermal stability (Raab2000)

and ordering in superlattice structures (Springholz 1998, 2000). An important highlight was

the discovery of a novel self-organized fcc-like ABC… dot stacking in PbSe quantum dot

superlattices that gives rise to an extremely efficient lateral ordering process. As a result,

nearly perfect 3D quantum dot crystals with tunable lattice constants were made (Springholz

1998). This was explained by a theoretical model that allowed the prediction of different kinds

interlayer correlations for other material systems (Holy1999) as well as their dependence on

dot size and layer thicknesses (Springholz2000). Other groups have demonstrated the

fabrication of uniform PbSe quantum dots onto Si (111) substrates with predeposited lead

salt buffer layers (Alchalabi2003)], and IV-VI quantum dots were also synthesized on BaF2

(Ferreira2001) and InP substrates (Probrajenski2000). Also, self-assembled PbSe quantum


dots have been utilized for the fabrication of highly efficient thermoelectric devices as shown

by in a recent Science paper (Harman2002).

Up to the start of the project, most studies on self-assembled lead salt quantum dots had

focused on Stranski-Krastanow PbSe dots. Moreover, only their growth and structural

properties had been investigated and only little was known about the electronic and optical

properties of these dots which were found to yield only rather weak photoluminescence

emission. The objective of the project was therefore to study the electronic properties of the

dots in detail to clarify the nature of the optical transitions and complement these studies by

the investigation of the shape and composition of the buried dots embedded in various matrix

materials. In addition, strain engineering as well as the combination of new material systems

were planned to be investigated in order to increase the emission efficiency. With the

optimized dots emitter as well as photon detectors were to be realized. The investigations

thus encompassed a wide range of activities ranging from detailed growth studies, materials

development, structure characterization, optical spectroscopy as well as device fabrication.

4.4.3 Results and Discussion

As stated in the original application, the goal of this research project was to “develop novel

lead salt nanostructures for fabrication of mid-infrared quantum dot lasers and detectors”.

The latter requires quantum dot structures with strong carrier confinement and excellent

luminescence properties. The work packages of this project covered (i) the development of a

new material basis for mid-infrared active quantum dots and the investigation of their growth

properties, (ii) the determination of their structural and optical properties, (iii) the

development of ultra-high reflectivity Bragg mirrors and microcavity devices as well as the

realization of mid-infrared lasers and detectors. Several new material combinations were

investigated for fabrication of lead salt quantum dots such as strain-tuned ternary PbSeTe

quantum dots under compressive as well as tensile strain, and we have introduced Sr-based

instead of the Eu-based alloys as barrier material. As a completely new material system, the

growth of PbTe/CdTe quantum dots with very large quantum confinement and exceedingly

high luminescence efficiency was demonstrated. The results were published more than 20

papers in peer-reviewed scientific journals, among them one in Physical Review Letters and

six in Applied Physics Letters. The majority of the papers resulted from joint research work

with one or more SFB partners, which demonstrates the strength of the collaborations

developed in the IR-ON partnership. In the following, the most important results of the project

areas are summarized.


Shape transitions of buried PbSe quantum dots

Overgrowth of quantum dots leads to significant changes in shape and composition due to

intermixing with the surrounding capping material. In this project, a novel technique for inves-

tigation of the buried dot structure was developed based on scanning tunneling microscopy

measurements and analysis of the surface displacements induced by the buried dots as

shown in Fig. 1 (Abtin2006, Springholz2007). This was applied for PbSe dots as a model

system, focusing on the role of the chemical composition of the capping material on the over-

growth process. In particular, the surface evolution was found to differ drastically between

capping with PbTe or PbEuTe layers. For the former, the pristine PbSe pyramids rapidly

shrink and transform into flat and rounded dots, whereas for the latter the dot shape is com-

pletely preserved. This was explained by difference in bond energy between PbTe and EuTe,

where the presence of EuTe suppresses the intermixing between dots and matrix material.

The same was also found when SrTe is introduced into the capping material (Abtin2008).

-40 -30 -20 -10 0 10 20 30

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d = 60 ÅxSe = 55%

d = 40 Å

Dep

th (Å

)

(a) Varying base width

60°

(d) Varying composition(c) Varying facet angle

xSe = 55%

240Å 280Å 320Å 360Å 400Å 440Å 480Å520Å

(b) Varying base widthd = 60 Å

Lateral direction (nm)

STM

d = 60 Åb = 340Å

15°

xSe=

STM

STM STM

30%

30°20°

10°

100% 90% 80% 70% 60% 50% 40%

b = 240Å 280Å 320Å 360Å 400Å 440Å 480Å520ÅxSe = 55%

Dep

th (Å

)

Figure 1: Left: STM images of PbSe dots covered with PbTe cap thicknesses of 30, 40, 80 and 120 Å from (a) to (d), respectively. The inserts show the facetted pyramidal shape of the uncapped dots (a) and the zoomed-in image of a single surface depression induced by a buried dot (b). Right: Calculated surface depression profiles for different dot shapes (gray lines) for different dot parameters as indicated in the figures plotted in comparison with the STM profiles (blue lines). d is the cap thickness, xSe the Se concentration, α the side wall angle, and b the dot base width. Best agreement was found for xSe = 55%, b = 340 Å, α = 20° and a dot height of 30Å (Springholz2007).

For quantitative determination of the dot structure, we have developed a structure model

combined with elasticity calculations. By this novel approach the shape and chemical compo-

sition of the buried dots was deduced. The influence of the capping process on the dots was

independently investigated by synchrotron x-ray scattering experiments performed at the

ESRF in Grenoble (Holy2005, Schülli2004). The obtained results are in good agreement with

the STM measurements. We have also performed a series of studies on surface exchange


reactions in mixed anion IV-VI heterostructures, demonstrating an almost complete Se for Te

surface exchange under chalcogen-rich MBE conditions. Moreover, we showed that this

effect can be also utilized for synthesis of self-assembled PbSe quantum dots.

Self-assembled PbTe quantum dots

The growth of compressively PbTe quantum dot on PbSe (111) was studied for the first time.

The critical thickness for dot formation was found to be 1.5 monolayers and it significantly

decreases with decreasing temperature. With increasing PbTe thickness and in-creasing dot

size, a shape transformation of the dots from {100} facetted pyramids to truncated pyramids

and finally truncated hexagons was found as shown in detail by Fig. 2. Correspondingly, the

aspect ratio of the dots decreases. For a given growth temperature, the PbTe dot size is

much larger and the density much lower compared to those of PbSe dots. While this could

be expected from the much larger surface diffusion lengths of PbTe growth due lower binding

energy, the Arrhenius analysis of the density as a function of temperature shows that the

activation energy for dot formation are quite similar in both systems but only the pre-

exponential factor is drastically reduced for PbTe dots. Due to the larger dot size, a very slow

surface planarization occurs during overgrowth of PbTe dots. Therefore, for multilayer dot

structures much larger spacer thicknesses are required and therefore, no interlayer corre-

lations are formed in great contrast to PbSe quantum dot superlattices.

Figure 2: AFM images of PbTe dots on PbSe (111) at coverages of 1, 2, 3 and 8 monolayers from (a) to (d). (e) PbTe dot height h versus dot width w evaluated for four different PbTe coverages from 2 to 8 monolayers. The shape evolution of the dots is shown schematically in (f) and consists of three stages, namely, (i) small dots with pyramidal shape defined by {100} side facets (ii) intermediate dots with truncated pyramidal shape and lower aspect ratio, and (iii) large dots with truncated hexagonal shape arising from the additional {110} side facets.


Self-organized ordering and stacking quantum dot superlattices

Stacking of quantum dot layers in multilayer structures was investigated a method for

improving the ordering and uniformity of the dots. In particular, we have performed studies on

the lateral ordering process of PbSe and ternary PbSeTe quantum dots separated by

PbEuTe and PbSrTe spacers (Lechner2004) and have shown that the ordering is strongly

influenced by the chemical composition of the spacer layers. This was explained by the

strong intermixing between the dots and matrix material, which modifies the surface strain

distributions responsible for interlayer correlations. The ordering process was also modeled

by Monte Carlo growth simulations, by which the transitions between different dot stacking

types as a function of thickness and composition of the spacer layers could be quantitatively

explained (Springholz2006, 2008).

Strain tuned PbSeTe quantum dots

Strain is a crucial prerequisite for formation of Stranski-Krastanow quantum dots but also

strongly influences their electronic properties due to strain-induced changes in the band

structure. Therefore, we have investigated in detail the dependence of the growth and optical

properties of dots as a function of strain by using ternary PbSeTe alloys with varying

composition as dot material and PbTe or PbSe buffer layer as pseudo-substrates.

Figure 3: Comparison of AFM images of tensile (top) and compressively (bottom) PbSeTe dots grown on PbTe, respectively PbSe buffer layers. In both cases the coverage was 3, 5, and 7 monolayers for composition of x = 100, 60 and 50 %, from (a) to (c), and (d) to (f), respectively. Note the different scale of the AFM images: Top 1x1 µm2, bottom 3x3 µm2.


This results in tensile, respectively, compressively strain dots and the magnitude and sign of

strain can be varied over the whole range from 0 to ± 5.4%. From systematic growth studies

(see AFM images of Fig. 3), the complete growth phase diagrams were established in both

cases as a function of composition and layer thickness as shown in Fig. 4. In particular, the

critical wetting layer thickness for dot formation was shown to increases rapidly with

decreasing misfit strain and a transition from 3D island growth to 2D layer-by-layer growth

was found at a critical strain value of about 2 %. At lower values, the strain is relaxed

exclusively by misfit dislocations. For the different material composition and sign of strain, the

evolution of dot sizes, shapes and size distributions was also determined quantitatively by

atomic force microscopy measurements.

Tensile strained PbSeTe on PbTe (111) Compressive PbSeTe on PbTe (111)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1

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AFM-2D AFM-Transiton AFM-3D RHEED Θc2 RHEED Θc1 AFM 2D-MD :Transition hMDMatthews-Blakeslee

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Figure 4: Phase diagram of critical thickness versus layer composition for growth of (a) tensile strained PbSexTe1-x quantum dots on PbTe buffer and (b) compressively strained PbTexSe1-x dots on PbSe buffer layers. Open symbols indicate the layer thickness where 2D growth is observed, full symbols the layer thicknesses where 3D dots are found by AFM. Full circles indicate the critical thickness observed by in situ RHEED.

PbTe/CdTe quantum dots

To overcome the weak carrier confinement in strained PbSeTe SK quantum dots, we have

fabricated quantum dots from the PbTe/CdTe material system. This combines materials with

large difference in band gap and crystal structure (rock salt versus zinc blende type), but with

nearly matched lattice constant. As revealed by our detailed growth and electron microscopy

studies, due to the absence of strain quantum dot formation relies on a completely different

mechanism that for the usual SK dots. In buried PbTe/CdTe heterostructures, thin 2D PbTe

layers transform into isolated octo-cubo-octahedral quantum dots after thermal treatment.

This transformation results from the immiscibility of the material system and is driven by the

lowering of the inter-face energy by phase transition. As a result, these nanoprecipitate

quantum dot exhibit completely different properties compared to the usual SK dots, namely,

the formation of highly symmetrical shapes and atomically sharp interfaces without any signs

2D+MD

Selenium concentration xSe

2D3D

AFM-2D AFM-Transiton AFM-3D RHEED Θc2 RHEED Θc1 AFM 2D-MD :Transition hMDMatthews-Blakeslee

PbSe

xTe 1

-x th

ickn

ess

(ML)

2D+MD2D+MD

3D 3D


of alloy intermixing. Moreover, the emission of such PbTe/CdTe quantum dots is extremely

bright and can be tuned over a wide wavelength region from about 3.5 to 2 µm by changing

the size of the dots. This can be accomplished by control of the initial PbTe layer thickness

as well as by the post growth annealing conditions.

(c)

(b)

(a)

inte

nsity

(arb

. uni

ts)

Figure 5: (a) Cross sectional TEM image of a PbTe/CdTe multilayer sample containing four PbTe layers with 1, 2, 5 and 10 nm thicknesses subjected to 10 min post-growth annealing at 360°C to transform the PbTe layers into isolated quantum dots. (b) Dot height histograms for the 1, 2 and 5 nm layers, indicating a substantial increase of dot size with increasing PbTe layer thickness. (c) Room temperature photoluminescence spectra of PbTe/CdTe quantum dots for the multilayer as well as single layers with 1, 3 and 10 nm thicknesses (Groiss 2007).

Infrared spectroscopy and band offsets of PbSeTe quantum dots

From photo-luminescence investigations of highly strained PbSe quantum dots the emission

was found to be rather weak compared to unstrained quantum wells. To resolve this issue,

photoconductivity experiments on PbSe/PbEuTe quantum dot multilayers were performed as

shown in Fig. 6. We found a significant blue shift of the photoconductivity onset with

decreasing dot size, but the absence of a photocurrent freeze out at low temperatures

indicates a type II band offset between the dots and the matrix material. According to our

model calculations (see Fig. 6(d)), this is due to the strong downward shift of the PbSe

valence band edges by the tensile dot strain, which means that photo-excited holes are not

confined in the dots. For quantitative determination of the band offsets and deformation

potentials of PbSe, we have fabricated series of PbSe/PbEuSeTe multi quantum well

structures in which the strain in the PbSe wells was varied over a wide range. From FTIR

spectroscopy studies, we found a type I – type II transition at a strain value of ~1% and from

the shifts of the energy levels as a function of strain, the deformation potentials were

deduced. While this configuration yields an excellent photo response signal for detector

applications, for high luminescence emission a type I band alignment is required. Therefore,

we have investigated ternary quantum dots with reduced strain values and increased barrier


height. Although a significant improvement in luminescence efficiency was obtained in this

way, the limitation due to insufficient hole confinement could not be resolved.

unstrained case

strained case

(a)

(b)

(c)

(d)

Figure 6: Lateral photocurrent measurements (solid line) at PbSe QDs with different dot size (dotted line: simulation of PC signal). The arrows mark the transition energies in the different structures of the QD superlattice. E1

s/p (E2s/p) transition to dot ground (first excited) s/p-states, E1

l/o transition to longitu-dinal and oblique wetting layer ground states, Eg,b barrier band gap. Insets: AFM images of the QDs (Simma2006).

Devices

High reflectivity Bragg mirrors constitute one of the crucial elements of high finesse micro-

cavities and vertical cavity emitting lasers (VCSELs). For the infrared spectral region,

dielectric layers with large refractive index contrast are needed in order to keep the total

thickness of epitaxial Bragg mirrors at a value feasible for MBE growth. In this project we

have developed new types of Bragg mirrors based on the ternary PbSeTe/EuSe/BaF2

material system which can be lattice matched for a certain PbSeTe ternary composition and

which features a very high refractive index contrast of up to 100% (Springholz2005). The

Bragg mirrors and microcavity structures were characterized by AFM, x-ray diffraction and

FTIR spectroscopy and samples were provided to P5-Heiss for fabrication of nanocrystal

devices. Due to the high refractive index contrast, these mirrors exhibit unique properties like

omni-directionality (Baumgartner2006) and tunability of the sharp cavity resonance by

changing of the incidence angle as shown by Fig. 7(a) and (b). With these novel Bragg

mirrors, vertical cavity surface emitting lasers with record performance values were produced

with cw emission at wavelengths between 5.9 and 8 µm and operation temperatures up to

140K, as shown by the emission spectra displayed in Fig. 7(d) and (e). Currently, we aim at


further increasing the operation temperatures and to incorporate PbTe/CdTe quantum dots

into the active region of the VCSEL structure.

(c) (d)

(e)

Figure 7: (a) Transmittance spectra of a high-finesse BaF2/PbEuTe microcavity structure plotted as a function of incidence angle, showing the broad stop band region with the central sharp cavity reso-nance peak. The λ/2 cavity was fabricated from high-reflectivity BaF2/PbEuTe Bragg mirrors grown by MBE. (b) Plot of the stop band edges and cavity resonance as a function of incidence angle for TM and TE polarizations obtained from experiments (open symbols = TE, filled = TM) compared to model calculations (solid/dashed lines) (Baumgartner2006). (c) and (d) MIR cw laser emission recorded for a VCSEL made from such mirrors at 54 and 135 K with the layer structure indicated by the cross-sectional electron microscopy image shown in (e) (Schwarzl2007).

Apart from the VCSELs we have also started to produce PbTe/CdTe micropillar lasers, but

up to now only strong spontaneous but no stimulated emission was obtained due to the not

optimized etching conditions, resulting in a too high side wall roughness of the micropillars.

For the infrared detectors, the photo response of type II PbSe/PbEuTe quantum dots was

characterized in detail in collaboration with partner P12 (Simma2006), indicating a high photo

conductive gain even at room temperature. Antireflection coatings were developed in order to

minimize the multiple interference fringes arising from the substrate reflection. Adding an

epitaxial narrow band interference filter structure allowed to fabricate mid infrared detectors

with a narrow spectral response (Böberl 2006) that was tuned exactly to the O-C-H stretching

vibration as required, e.g., for the optical detection of acetaldehyde.

4.4.4 Collaboration within and beyond the SFB

To achieve the wide range of innovative results, intense collaborations have been estab-

lished within this SFB program as well as several groups outside of Austria. These collabora-

tions involved the exchange of samples, sharing of measurement results, experimental set-

ups and instrumentation, exchange of expertise and know-how, common publications as well


as joint supervision of diploma and PhD students. Specifically, the project was supported by

partners P2-Schäffler, P5-Heiss, P7-Stangl and P12-Fromherz with material analysis and

characterization using transmission electron microscopy, x-ray diffraction, optical spectros-

copy, modeling as well as design and processing of devices. Samples were provided to P2

for detailed TEM investigations as well as to P5 for further device fabrication. Know-how

concerning laser fabrication was obtained from partner P3-Strasser. Theoretical input was

received from partners P6-Bechstedt/Kresse by ab initio calculations of the electronic band

structure of the lead salt compounds as well as of the interface structure of PbTe/CdTe dots.

The latter strongly contributed to the development of an improved understanding of the novel

quantum dot system. Our group provided partners P2, P5, P7 and P12 with AFM expertise

and instrumentation for characterization of self-assembled quantum dots, nanocrystals as

well as patterned nanostructures. Evidently, the strong links and intense collaborations within

the IR-ON research program have generated tremendous synergies and have thus resulted

in a large number of common publications.

P4-Springholz

P2-TEM investigations

P6-ab initio calcuations

P5-spectroscopy & microcavities P12-detectors

spectroscopy P7-X-ray

diffraction

Figure 8: Integration of the project P4 within the SFB. Joint research work was performed with partners P2-Schäffler, P5-Heiss, P6-Kresse/ Bechstedt, P7-Stangl, and P12-Fromherz. This involved exchange of samples, sharing of measurement results, use of instrumentation, exchange of expertise and ideas, and common publications and joint supervision of diploma & PhD students.

Concerning the collaboration with external groups, the modeling and analysis of the structure

properties and growth of self-assembled quantum dots was strongly supported by the group

of Prof. Vaclav Holy at the University of Prague, mid-infrared laser spectroscopy measure-

ments were performed in the group of Prof. Harald Pascher, the group of Dr. T. Wojtowicz at

the Institute of Physics at the Polish Academy of Sciences in Warzawa has supplied us with

very high quality virtual substrates for II-VI / IV-VI heteroepitaxy and with the group of Prof.

Yano at the Osaka Institute of Technology we have developed the novel method for

synthesis of PbTe/CdTe quantum dots by the nanoprecipitation process.


4.4.5 References

Abtin L., G. Springholz, and V. Holy (2006), Surface Exchange and Shape Transitions of PbSe Quantum Dots during Overgrowth, Phys. Rev. Lett. 97, 266103-6.

Abtin L. and G. Springholz (2008), Stabilization of PbSe quantum dots by ultra-thin EuTe and SrTe barrier layers, submitted to Appl. Phys. Lett.

Alchalabi K., D. Zimin, G. Kostorz, H. Zogg (2003), Self-Assembled Semiconductor Quantum Dots with Nearly Uniform Sizes, Phys. Rev. Lett. 90, 026104.

Alchalabi, K. Zimin, D. Zogg, H. Buttler, W. (2001), Monolithic heteroepitaxial PbTe-on-Si infrared focal plane array with 96×128 pixels, IEEE Electron. Device Lett. 22, 110.

André R. and Le Si Dang (1997), Low-temperature refractive indices of Cd1-xMnxTe and Cd1-yMgyTe , J. Appl. Phys. 82, 5086.

Arakawa Y., and H. Sakaki (1982), Multidimensional quantum well laser and temperature dependence of its threshold current, Appl. Phys. Lett. 40, 939.

Asada M., Y. Miyamoto, Y. Suematsu (1986), Gain and the threshold of three-dimensional quantum-box lasers, IEEE J. Quantum Electron. 22, 1915.

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4.4 Report of project part 04: Epitaxial lead salt ... · experimentally from optical spectroscopy...

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