Photoluminescence study of Si/Ge quantum dots

M. Larsson *, A. Elfving, P.O. Holtz, G.V. Hansson, W.-X. Ni

Department of Physics, Link€ooping University, SE-581 83 Link€ooping, Sweden

Abstract

Ge quantum dots embedded in Si are studied by means of photoluminescence (PL). The temperature dependent PL

measurements show two different types of recombination processes related to the quantum dots. We ascribe a peak near

0.80 eV to the spatially indirect recombination in the type-II band lineup where the electron is located in the sur-

rounding Si close to the interface and the hole in the Ge dot. Furthermore, a peak near 0.85 eV is attributed to the

spatially direct recombination. We observe a transition from the spatially indirect to the spatially direct recombination

as the temperature is increased. The measurements also show an up-shift of the Ge quantum dot emission energy with

increasing excitation power density. The blueshift is primarily ascribed to an enhanced confinement of the electron

associated with the increased band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made

with results, derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to

excitation power or temperatures are observed in contrast to the capped samples.

� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Photoluminescence; Self-assembly; Silicon–germanium

1. Introduction

Strain induced self-assembled Ge quantum dots

embedded in Si have attracted a large interest

during the last years due to the possibility to realize

Si based optoelectronics. For example, detectorapplications using Si/Ge quantum dot structures in

the active region have been suggested. Ge dots will

in this case serve as the active material due to the

smaller band gap. In spite of the fact that Si/Ge

interdiffusion will lead to some alloying of the dots

we will refer to them as Ge dots. Epitaxial growth

of the lattice mismatched Si/Ge material system has

demonstrated Stranski–Krastanov formation of

islands under certain conditions. This gives a sim-

ple and rather straightforward way to create Ge

quantum dots, compatible with the Si technology.

Unfortunately, both Si and Ge are indirect band

gap materials and phonons are normally requiredfor momentum conservation in optical transitions.

On the contrary, in SiGe alloys and Si/Ge quantum

structures, the symmetry of the lattice is broken,

which opens the possibility for optical transitions

without any phonon interaction. In quantum

structures, the spatial confinement of the carriers

will spread the wave functions in k-space and thus

increase the probability for a direct no-phononoptical transition [1,2]. In this work, the optical

properties of Ge quantum dots were investigated by

means of photoluminescence (PL) with tempera-

ture and excitation power density as variable pa-

rameters. We discuss the involved recombination

*Corresponding author. Tel.: +46-13-28-27-56; fax: +46-13-

28-89-69.

E-mail address: matla@ifm.liu.se (M. Larsson).

0039-6028/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0039-6028(03)00461-8

Surface Science 532–535 (2003) 832–836

www.elsevier.com/locate/susc

processes and compare the results with measure-

ments on uncapped samples.

2. Experimental

The two samples studied here were single layer

structures grown by solid-source molecular beam

epitaxy (MBE). At a growth temperature of 700

�C, eight monolayers Ge were deposited on

Si(1 0 0) substrates. The Ge quantum dots were

formed from this Ge layer via Stranski–Krastanov

growth mode. One of the samples was covered

with Si, forming a 160 nm capping layer. In bothsamples, the average dot diameter was about 200

nm and the typical height was 20–25 nm as de-

termined by atomic force microscopy (AFM)

studies. PL measurements were performed in a

variable temperature He-flow cryostat, and as ex-

citation source the 514 nm line of an Ar ion laser

was used. The PL signals were analyzed with a

double-grating monochromator, together with aliquid nitrogen cooled Ge detector, using standard

lock-in technique.

3. Results and discussion

3.1. Band alignment

Since the growth mode is strain induced and the

dot formation is a result of elastic relaxation, the

sandwiching Si above and below the islands ex-

hibits tensional strain [3]. It is known that in ten-

sile strained Si, the D(2) valleys of the conductionband is downshifted, which results in a type-II

band alignment at the interface between the Ge

dots and the surrounding Si (see Fig. 1) [3,4].During the formation of the dot, the Ge rich

wetting layer surrounds the base of the dot and

prevents it to expand laterally, while the upper

part of the Ge dot is expected to be more relaxed

than the base. This will cause an asymmetric strain

through the structure. Consequently, the Si layer

above the Ge dot will exhibit a higher strain than

in the Si material below the dot.As a result, the band offset in the conduction

band at the top interface will be larger than at the

bottom interface (Fig. 1). In Fig. 1a, the two

transitions observed in the PL measurements are

labeled A and B for the spatially indirect and di-

rect transition, respectively. The spatially indirecttransition is expected to be rather weak due to the

spatial separation of the carriers. The probability

for this indirect transition is related to the mag-

nitude of the overlap of the wave functions leaking

into the potential barriers. For the spatially direct

transition, when both the electrons and the holes

are located in the quantum dot, the alloy disorder

and quantum confinement effects may relax the k-conservation condition enough to increase the

probability for optical recombination. The third

possible transition (labeled C in Fig. 1a) is close in

energy to the B transition and can not be separated

from the spatially direct transition in PL mea-

surements on capped structures.

3.2. Excitation power dependence

The PL dependence of the excitation power

density was examined in order to investigate the

mechanisms of the carrier recombination. Fig. 2a

and b show the PL spectra for varied excitation

CB

VB

A C

(b)

CB

VB

Low carrier density Growthdirection

High carrier density

Si SiGe(a)

B

Fig. 1. (a) A schematic illustration of the band edge alignment

along the growth direction of a Ge quantum dot. Spatially in-

direct transitions are marked A and C, while the spatially direct

transition is marked B. (b) An illustration of the band bending

effects in the structure at two different carrier concentrations.

M. Larsson et al. / Surface Science 532–535 (2003) 832–836 833

power at two different temperatures, 10 and 30 K,

respectively. Typical emissions from the Si sub-

strate and the thin Ge wetting layer are observed

above 0.95 eV, while the quantum dot related

emission occurs below 0.95 eV [2,5–7]. At lowtemperature (10 K), a significant blueshift of 25

meV of the Ge dot related emission is observed as

the excitation power is increased from 3 to 100

mW. Other groups have earlier shown a similar

excitation power dependence of the Ge quantum

dot emission [2,5,6]. The shift can be explained in

terms of a type-II band lineup, where the electrons

are located in the Si, while the holes are trapped inthe Ge dot. The Coulomb interaction between

these spatially separated electrons and holes will

bend the energy bands at the interface to form a

Hartree potential on each side of the interface

[5,8]. At high carrier concentrations, the band

bending will shift the electron and hole levels to

higher energies due to an increased confinement,

resulting in higher transition energies (Fig. 2a).State filling could also cause an effective blueshift

of the PL peak at sufficiently high carrier densities

[6]. When the temperature is increased to 30 K, the

emission spectra look quite different for all exci-

tation powers (Fig. 2b). At low excitation power,

the dot luminescence is divided into two branches,

centered at approximately 0.80 and 0.85 eV, re-spectively. When the excitation power is increased

from 1 to 150 mW, the intensity of the high energy

peak increases faster than the low energy peak, to

totally dominate the spectrum at high excitation

power. It should be noted that the energy position

of this peak (at 30 K) is always higher than the

blueshifted low energy peak observed at high ex-

citation power at 10 K, strongly indicating thatthese two contributions have different origins. It

should also be pointed out, that the high energy

peak does not shift with increasing excitation

power, implying that band bending is not affecting

this transition.

3.3. Temperature dependence

Fig. 3a and b show the temperature dependence

of the luminescence at two different excitation

0.7 0.8 0.9 1.0 1.1 1.2

100mW

50mW

10mW

3mW

T=10K

0.7 0.8 0.9 1.0 1.1 1.2

T=30K

1mW

3mW

10mW

50mW

100mW

150mW

(a) (b)

Fig. 2. Normalized PL spectra of Ge dots. The excitation power dependence is shown for two different temperatures (a) 10 K and (b)

30 K. The quantum dot related emission is below 0.95 eV, while the emissions above this energy are contributions from the substrate

and the wetting layer.

834 M. Larsson et al. / Surface Science 532–535 (2003) 832–836

powers. At low excitation power densities and low

temperatures, the low energy peak at 0.80 eV

dominates the spectrum (Fig. 3a). As the temper-

ature is raised, a redistribution of the emission

intensities from the low energy peak to the high

energy peak at 0.85 eV is observed. When the ex-citation power is increased to 50 mW (Fig. 3b), the

temperature behavior is quite similar as for the

lower excitation power, but with one notable dif-

ference: The low energy peak is up-shifted due to

band bending, as described in the previous section.

This behavior is consistent with the high energy

peak as being due to the spatially direct recombi-

nation, while the low energy peak at 0.80 eV isrelated to indirect transitions across the dot in-

terface. At low temperatures, the only possible

recombination channel is the spatially indirect

transition across the interface for which the elec-

tron is located in the Si surrounding the Ge dot,

while the hole is confined inside the Ge dot. A

higher temperature results in an increased proba-

bility for the electrons to populate the higher en-ergy level inside the dot, which opens the

possibility for an alternative recombination chan-

nel in addition to the low energy emission at 0.80

eV. In our case, the probability for the spatially

direct transition at 0.85 eV increases. For this

transition, the overlap of the wave functions is

considerably larger than for the spatially indirecttransition, resulting in a more efficient lumines-

cence.

3.4. Uncapped structures

The PL from uncapped dots is detectable, but

the intensity is significantly reduced due to the

strong non-radiative surface recombination. Fig. 4shows that the shape of the spectra of uncapped

dots differ significantly from those measured on

the capped dots. The position of the quantum dot

related emission band is down shifted to a peak

energy at 0.77 eV. It is established that the capping

of self-assembled Ge islands will result in increased

intermixing of Si and Ge [7]. The intermixing will

accordingly up-shift the band gap of the dot. As aresult, the emission of the capped dots is expected

0.7 0.8 0.9 1.0 1.1 1.2

P=10mW

42K

35K

30K

25K

20K

15K

11K

8K

5K

0.7 0.8 0.9 1.0 1.1 1.2

P=50mW

150K

100K

75K

50K

40K

30K

20K

15K

10K

6K

(a) (b)

Fig. 3. Temperature dependence of the PL spectra from the Ge dots at two different excitation power densities, (a) 10 mW and (b) 50

mW, respectively. The spectra shown in (b) are normalized with respect to the quantum dot emission.

M. Larsson et al. / Surface Science 532–535 (2003) 832–836 835

to occur at higher energies than in uncapped

structures, which is in consistence with our ex-

perimental results. Furthermore, no shift of the

transition energy is observed as the excitation

power is increased. This behavior is expected, since

the small band offset at the bottom of the dot isnot sufficient to capture enough electrons to give

rise to any significant band bending. As mentioned

above, the intensity of the emission is lower than

that for the capped samples, due to competing

recombination via non-radiative surface states.

This fact implies that the surface recombination

will ensure low carrier densities and consequently

no detectable energy shifts due to band bending.At increased temperatures, no second peak at

higher energy is observed in contrast to the capped

dot sample, in consistence with our band lineup

picture. The energy position for the emission

should be practically the same for the spatially

direct (B in Fig. 1a) and indirect transition in un-

capped dots (C in Fig. 1a) due to the small band

offset described above.

4. Conclusion

The present PL study of Ge quantum dots

embedded in Si shows two different dot related

transitions. Based on the experimental results, we

suggest a type-II energy band lineup that gives

possibility for one spatially indirect transition,

which is blueshifted with increasing excitation

power together with one spatially direct transitioninside the dots. Temperature dependent measure-

ments show that the direct transition is a more

efficient recombination channel, as expected.

Measurements on uncapped structures show only

one quantum dot related emission band, which has

excitation power and temperature dependencies

that are consistent with our model proposed on the

recombination processes.

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Fig. 4. Normalized PL spectra from an uncapped dot structure

at 30 K with a varied excitation power.

836 M. Larsson et al. / Surface Science 532–535 (2003) 832–836