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Physica E 16 (2003) 476–480

www.elsevier.com/locate/physe

Luminescence study of Si/Ge quantum dotsM. Larsson∗, A. Elfving, P.-O. Holtz, G.V. Hansson, W.-X. Ni

Department of Physics, Link�oping University, SE-581 83 Link�oping, Sweden

Abstract

We present a photoluminescence (PL) study of Ge quantum dots embedded in Si. Two di5erent types of recombinationprocesses related to the Ge quantum dots are observed in temperature-dependent PL measurements. The Ge dot-relatedluminescence peak near 0:80 eV is ascribed to the spatially indirect recombination in the type-II band lineup, while ahigh-energy peak near 0:85 eV has its origin in the spatially direct recombination. A transition from the spatially indirect tothe spatially direct recombination is observed as the temperature is increased. The PL dependence of the excitation powershows an upshift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is ascribedto band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results derived frommeasurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperaturesare observed in contrast to the capped samples.? 2002 Elsevier Science B.V. All rights reserved.

PACS: 73.20.D; 78.55

Keywords: Quantum dots; Photoluminescence; Band alignment; Silicon–Germanium

1. Introduction

Self-assembled Ge quantum dots embedded in Simatrices have attracted a large interest during thepast years due to the possibility to realize Si-basedoptoelectronics. Unfortunately, both Si and Ge areindirect band gap materials and phonons are normallyrequired for momentum conservation in optical tran-sitions. On the contrary, it has been demonstrated thatthe optical properties of Si and Ge can be improvedusing quantum structures. In SiGe alloys and Si/Gequantum structures, the symmetry of the lattice isbroken, which opens the possibility for optical tran-sitions without any phonon interaction. While Si/Ge

∗ Corresponding author. Tel.: +46-13-28-27-56; fax: +46-13-28-89-69.

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

interdi5usion will lead to some alloying of the dotswe will refer to them as Ge dots. Epitaxial growthof the lattice mismatched Si/Ge material system hasbeen demonstrated to proceed via Stranski–Krastanovmode under certain growth conditions [1]. This givesa simple and rather straightforward way to create Gequantum dots, compatible with the Si technology. Inthis work, the optical properties of Ge quantum dotswere investigated by means of photoluminescence(PL) with temperature and excitation power densityas variable parameters. We discuss the involved re-combination processes and compare the results withmeasurements on uncapped samples.

2. Experimental

The two samples studied here were single layerstructures grown by solid-source molecular beam

1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved.PII: S1386 -9477(02)00652 -5

M. Larsson et al. / Physica E 16 (2003) 476–480 477

epitaxy (MBE). At a growth temperature of 700◦C, 8monolayers Ge were deposited on Si(1 0 0) substrates.The Ge quantum dots were formed from this Ge layervia Stranski–Krastanov growth mode. One of thesamples was covered with Si, forming a 160 nmcapping layer. In both samples, the average dotdiameter was about 200 nm and the typical height was20–25 nm as determined by atomic force microscopystudies. PL measurements were performed in a vari-able temperature He-Iow cryostat, and as excitationsource the 514 nm line of an Ar ion laser was used.The PL signals were analyzed with a double-gratingmonochromator, together with a liquid nitrogencooled Ge detector, using standard lock-in techniques.

3. Results and discussion

3.1. Band alignment

Since the growth mode is strain induced and thedot formation is a result of elastic relaxation, the Siabove and below the islands exhibits tensional strain[1]. It is known that in tensile strained Si, the �(2)valleys of the conduction band is downshifted, whichresults in a type-II band alignment at the interfacebetween the Ge dots and the surrounding Si (seeFig. 1) [1,2]. Due to the asymmetry of these structures,where the base of the dot is surrounded of a Ge-richwetting layer, the upper part of the dot is expected tobe more relaxed than the base. As a result, the bando5set in the conduction band at the top interface willbe larger than at the bottom interface (Fig. 1). A highconcentration gradient of the interdi5used Si materialinside the dot may further a5ect the band o5set. How-ever, we expect this e5ect to be small compared to theenergy shift originated from the strain. In Fig. 1a, thetwo transitions observed in the PL measurements arelabeled A and B for the spatially indirect and directtransition, respectively. Considering the optical tran-sitions, the spatially indirect transition is excepted tobe of rather weak nature due to the separation of thecarriers. The probability for this indirect transition isrelated to the magnitude of the overlap of the wavefunctions leaking into the potential barriers. For thespatially direct transition, when both the electrons andthe holes are located in the quantum dot, the alloydisorder and quantum conJnement e5ects may relax

CB

VB

A C

(b)

CB

VB

Low carrier densityGrowth

direction

High carrier density

Si SiGe

(a)

B

Fig. 1. (a) Schematic illustration of the band edge alignmentalong the growth direction of a Ge quantum dot. Spatially indirecttransitions are marked A and C, while the direct is marked Band (b) illustration of band bending e5ects in the structure at twodi5erent carrier concentrations.

the k-conservation condition enough to increase theprobability for optical recombination. The third possi-ble transition (labeled C in Fig. 1a) is close in energyto the B transition and cannot be separated from thisspatially direct transition in PL measurements oncapped structures.

3.2. Excitation power dependence

The PL dependence of the excitation power densitywas examined in order to investigate the mechanismsof the carrier recombination. Figs. 2a and b show thePL spectra for varied excitation power at two di5erenttemperatures, 10 and 30 K, respectively. The struc-tures above 0:95 eV are attributed to contributionsfrom the substrate and the wetting layer, while thequantum dot-related emission occurs below 0:95 eV.

478 M. Larsson et al. / Physica E 16 (2003) 476–480

0.7 0.8 0.9 1.0 1.1 1.2

T=30K

1mW

3mW

10mW

50mW

100mW

150mW

PL

inte

nsi

ty (

a.u)

Energy (eV)

0.7 0.8 0.9 1.0 1.1 1.2

100mW

50mW

10mW

3mW

PL

inte

nsi

ty (

a.u)

Energy (eV)

T=10K

(a) (b)

Fig. 2. Normalized photoluminescence of Ge dots. Excitation power dependence at (a) 10 K and (b) 30 K. The quantum dot-relatedemission is below 0.95, while the emissions above 0:95 eV are contributions from the substrate and the wetting layer.

At a low temperature (10 K), a signiJcant blueshiftof 25 meV is observed as the excitation power is in-creased from 3 to 100 mW, a behavior that has beenreported earlier [3–5]. This shift can be explained interms of a type-II band lineup, where the electrons arelocated in the Si, while the holes are trapped in the Gedot. The Coulomb interaction between these spatiallyseparated electrons and holes will bend the energybands at the interface to form a Hartree potential oneach side of the interface [4,6]. At high carrier concen-trations, the band bending will shift the electron andhole levels to higher energies due to increased con-Jnement, resulting in higher transition energies (Fig.2a). State Jlling could also cause an e5ective blueshiftof the PL peak at suKciently high carrier densities[5]. When the temperature is increased to 30 K, theemission spectra look quite di5erent for all excitationpowers (Fig. 2b). At low excitation power, the dot

luminescence is divided into two branches, centered atapproximately 0.80 and 0:85 eV, respectively. Whenthe excitation power is increased from 1 to 150 mW,the intensity of the high-energy peak increases fasterthan the low-energy peak to totally dominate the spec-trum at high excitation power. It should be noted thatthe energy position of this peak (at 30 K) is alwayshigher than the up-shifted peak observed at high ex-citation power at 10 K, strongly indicating that thesetwo contributions have di5erent origins. It should alsobe pointed out that the high-energy peak does not shiftwith increasing excitation power, implying that bandbending is not a5ecting this transition.

3.3. Temperature dependence

Figs. 3a and b show the temperature depen-dence of the luminescence at two di5erent excitation

M. Larsson et al. / Physica E 16 (2003) 476–480 479

0.7 0.8 0.9 1.0 1.1 1.2

Exc. pow.=10mW

42K

35K

30K

25K

20K

15K

11K

8K

5K

PL

inte

nsi

ty (a

.u)

Energy (eV)

0.7 0.8 0.9 1.0 1.1 1.2

Exc. pow.=50mW

PL

inte

nsi

ty (

a.u)

Energy (eV)

150K

100K

75K

50K

40K

30K

20K

15K

10K

6K

(b)(a)

Fig. 3. Temperature dependence of the photoluminescence from the Ge dots at two di5erent excitation power densities: (a) 10 mW and(b) 50 mW, respectively. For 50 mW, the spectra are normalized with respect to the quantum dot emission.

powers. At low excitation power densities and lowtemperatures, the low-energy peak at 0:80 eV dom-inates the spectrum (Fig. 3a). As the temperatureis raised, a redistribution of the emission intensitiesfrom the low-energy peak to the high-energy peak at0:85 eV is observed. When the excitation power isincreased to 50 mW (Fig. 3b), the temperature be-havior is quite similar as that for the lower excitationpower with one notable di5erence: The low-energypeak is upshifted due to band bending, as describedin the previous section. We attribute the high-energypeak to inter-dot recombination, while the low-energypeak at 0:80 eV is related to recombination via thedot/Si interface. At low temperatures, the only pos-sible recombination channel is the spatially indirecttransition across the interface for which the electronis located in the Si surrounding the Ge dot, while

the hole is conJned inside the Ge dot. A higher tem-perature results in an increased probability for theelectrons to populate the higher energy level insidethe dot, which opens the possibility for an alternativerecombination channel in addition to the low-energyemission at 0:80 eV. In our case, the probability forthe spatially direct transition at 0:85 eV increases.For this transition, the overlap of the wave functionsis considerably larger than for the spatially indirecttransition, resulting in a more eKcient PL emission.

3.4. Uncapped structures

The PL from uncapped dots is also observed, butthe intensity is signiJcantly reduced due to the strongnon-radiative surface recombination. Fig. 4 showsthat the shape of the spectra of uncapped dots di5er

480 M. Larsson et al. / Physica E 16 (2003) 476–480

0.7 0.8 0.9 1.0 1.1 1.2

3mW

10mW

30mW

100mW

PL

inte

nsi

ty (

a.u)

Energy (eV)

T=30K

Fig. 4. Normalized photoluminescence from an uncapped structureat 30 K with varied excitation power.

signiJcantly from those measured on the capped dots.The position of the quantum dot-related emissionband is downshifted with a peak energy at 0:77 eV.It is established that the capping of self-assembledGe islands will result in increased intermixing of Siand Ge [7]. The intermixing will accordingly upshiftthe band gap of the dot. As a result, the emission ofthe capped dots is expected to occur at higher ener-gies than in uncapped structures, which is consistentwith our experimental results. Further on, no shiftof the transition energy is observed as the excita-tion power is increased. This behavior is expected,since the small band o5set at the bottom of the dotis not suKcient to capture enough electrons to giverise to any signiJcant band bending. As mentioned

above, the intensity of the emission is lower thanthat for the capped samples, due to recombination vianon-radiative surface states. This fact implies that thesurface recombination will ensure low carrier densi-ties and consequently no detectable energy shifts dueto band bending. At increased temperatures, no secondpeak at higher energy is observed in contrast to thecapped dot sample, in consistent with our band lineuppicture. The energy position for the emission shouldbe practically the same for the spatially direct (B inFig. 1a) and indirect transition in uncapped dots (C inFig. 1a) due to the small band o5set described above.

4. Conclusion

The present PL study of Ge quantum dots em-bedded in Si shows two di5erent dot-related transi-tions. Our experimental results have been explainedby a type-II energy band lineup that gives possi-bility for one spatially indirect transition, which isblueshifted with increasing excitation power togetherwith one spatially direct transition inside the dots.Temperature-dependent measurements show that thedirect transition is a more eKcient recombinationchannel, as expected. Measurements on uncappedstructures show only one quantum dot-related emis-sion band, which has excitation power and tempera-ture dependencies that are consistent with our modelproposed on the recombination processes.

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