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0022-0248/$ - se

doi:10.1016/j.jc

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Journal of Crystal Growth 290 (2006) 176–179

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Improvement of luminescence from b-FeSi2 particles embedded insilicon, with high temperature silicon buffer layer

Cheng Lia,�, Hongkai Laia, Songyan Chena, T. Suemasub, F. Hasegawab

aResearch Center for Semiconductor Photonics, Department of Physics, Xiamen University, Xiamen, Fujian 361005, Peoples Republic of ChinabInstitute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

Received 10 August 2005; received in revised form 8 December 2005; accepted 18 January 2006

Available online 28 February 2006

Communicated by D.W. Shaw

Abstract

Optical and electrical properties of b-FeSi2 particles embedded in silicon matrix were improved by growing an un-doped silicon buffer

layer at high temperature. With silicon buffer layer, the formation defects were prevented in the silicon matrix during the growth of b-FeSi2 and the subsequent annealing, which results in the narrow photoluminescence spectra. The diodes with silicon buffer layer showed

smaller leakage current than those samples without silicon buffer layer. The thermal quenching of electroluminescence from such samples

with silicon buffer layer occurred at higher temperature and the intrinsic band gap energy shift between silicon and b-FeSi2 was measured

at about 0.23 eV.

r 2006 Elsevier B.V. All rights reserved.

PACS: 73.61.Cw; 78.20.�e; 78.60.Fi

Keywords: A1. Photoluminescence; A3. Molecular beam epitaxy; B2. Iron disilicide; B3. Light emitting diodes

1. Introduction

Semiconducting iron disilicide (b-FeSi2), used as activematerial for silicon-based light-emitting diodes, has beenattracting much attention with light emission at about1.55 mm, a minimum absorption window of silica opticalfibers [1,2].

Electroluminescence (EL) of b-FeSi2 acting as activeregion in a silicon p–n junction grown by ion-beamsynthesis and reactive deposition epitaxy has been reportedfrom low temperature to room temperature since 1997[3–5]. The optical properties and electrical characteristics ofb-FeSi2 grown by various methods have been investigatedin many groups [6–8]. However, all those samples showedcomplex optical properties, and so far, no conclusions weredrawn in good agreement with each other [1,6,8]. Furtherefforts are therefore in need to clarify the origin of theluminescence of the samples with b-FeSi2 embedded in

e front matter r 2006 Elsevier B.V. All rights reserved.

rysgro.2006.01.027

ing author. Tel.: +86592 2184220; fax:+86 592 2189426.

ess: lich@xmu.edu.cn (C. Li).

silicon matrix. Generally, owing to the large latticemismatch and difference of thermal expansion coefficientbetween silicon and b-FeSi2, defects and strain will beintroduced into b-FeSi2 and surrounding silicon duringgrowth and subsequent annealing processes. The defectsand strain might be responsible for the complex behaviorof the luminescence. In addition, the doping concentrationin silicon substrate and capped layer might also affect theproperties of the samples.In this paper, a high-temperature un-doped silicon buffer

layer was introduced before the growth of b-FeSi2 on n+-type silicon substrate. As a result, the luminescence andelectrical characteristics of the samples with the siliconbuffer layer were improved and the EL intensity wasrestrained from thermal quenching.

2. Experiment

The samples of b-FeSi2 used in this study were grown bymolecular beam epitaxy system equipped with silicon andiron electron gun evaporation sources. The substrates were

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Without Si buffer layer(a)8K

20K

30K

50K

80K

100K

120K

140K

160K

180K

200K

PL in

tens

ity (

arb.

uni

ts)

0.7 0.8 0.9 1.0 1.1 1.2

With Si buffer layer8K

30K

50K

70K

90K

120K

150K

180K

210k

(b)

PL in

tens

ity (

arb.

uni

ts)

Photon Energy (eV)

Fig. 1. Temperature dependence of PL spectra of the samples without

silicon buffer layer (a) and with silicon buffer layer (b).

C. Li et al. / Journal of Crystal Growth 290 (2006) 176–179 177

n-type epitaxial Si(1 0 0) wafers with the resistivity of0.02O cm. Two pieces of Si with the size of 2� 2 cm2 werecut from the same wafer. After the same clean process,250 nm un-doped silicon buffer layer was deposited on oneof the wafer at 850 1C. After that, 99.99% iron wasdeposited on both the wafers at 470 1C to form an b-FeSi2epitaxial layer of about 10–15 nm thick. Samples were thenannealed in situ at 850 1C for 1 h and the b-FeSi2 wereagglomerated into islands. Subsequently, a 0.4 mm thick,unintentionally doped silicon layer was grown at 500 1Cand a boron-doped silicon cap layer with doping concen-tration of about 5.0� 1018 cm�3 was grown at 700 1C. Boththe samples were then annealed at 900 1C in an Aratmosphere for 14 h to improve further the crystal quality,which results in the b-FeSi2 particles embedded in thesilicon matrix.

The mesa diodes were designed for electrical and ELmeasurements. By wet chemical etching, 1.5� 1.5mm2

mesa diodes were made Finger-type aluminum and AuSbwere deposited, respectively, on p+ cap silicon layer andthe backside of silicon substrate for contacts.

Photoluminescence (PL) and EL spectra of the sampleswere measured in a cryostat with the temperature rangefrom 8K to 300K. PL was measured using an He–Cd laser(442 nm) and EL spectra were measured by using a pulsecurrent source with 200Hz frequency and about 1

2duty

cycle. Luminescence was analyzed by a 25 cm focal lengthsingle monochromator, and detected by a liquid-nitrogen-cooled InP/InGaAs photomultiplier (Hamamatsu Photo-nics R5509-72) and was then amplified by the standardlock-in technique.

3. Results and discussion

Temperature dependences of PL spectra of the sampleswithout and with silicon buffer layer are shown in Figs.1(a) and (b), respectively. The PL spectra reveal that thefull-width at half-maximum (FWHM) at about 0.8 eV peakenergy of the sample with silicon buffer layer is narrowerand the PL spectra of the sample without silicon bufferlayer have a shoulder at about 0.87 eV. As it is well known,D1(0.807 eV) and D2(0.87 eV) luminescence peaks havebeen observed in the deformed bulk silicon with defects [9],we should attribute the peak at about 0.81 eV to theluminescence from b-FeSi2 and defects in silicon. Inaddition, the peaks of D1 and D2 lines in deformed siliconalways appear simultaneously. It is reasonable to suggestthat the broadening of PL spectra of the samples withoutsilicon buffer layer is the mixture of b-FeSi2 and the defectsin the surrounding silicon, while the narrow PL spectra ofthe samples with silicon buffer layer mainly originate fromthe b-FeSi2 particles. It indicates that defects wereintroduced in the surrounding silicon during the processof b-FeSi2 growth on doped silicon substrates and thesubsequent annealing. High-temperature undoped siliconbuffer layer prevents the formation of defects in thesurrounding silicon, and thus improves the PL spectra.

Fig. 2 shows the temperature dependence of PL peakenergy of the samples with and without the silicon bufferlayer. The PL peak energy of the sample without the siliconbuffer layer is slightly lower than that with the siliconbuffer layer. The reason is not yet clear. But combined withthe defects appeared in the silicon of the sample withoutsilicon buffer layer, the PL peak energy of the samplewithout silicon buffer layer might be affected by both theD1 line and b-FeSi2 luminescence, which is ascribed to thisdisagreement.Fig. 3 shows the current–voltage characteristics of the

diodes with and without silicon buffer layers. Both theforward and reverse current of the diode with silicon bufferlayer are reduced by about 5 orders of magnitude.Although the different current–voltage characteristicsbetween the two samples can be explained by the formationof p–n heterojunction between p-type b-FeSi2 and n-typesilicon substrate could increase the current for the sample

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0 50 100 150 200 2500.78

0.79

0.80

0.81

With Si buffer

Without Si buffer

PL p

eak

Ene

rgy

(eV

)

Temperature (K)

Fig. 2. PL peak energy of the samples with and without the silicon buffer

layer.

-2.0 -1.5 -1.0 -0.5 0.0 0.5

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

With Si buffer

Without Si buffer

Cur

rent

(A

)

Voltage (V)

Fig. 3. Current–voltage characteristics of the diodes with and without

silicon buffer layers; the current of the diodes with silicon buffer layer was

reduced by about 5 orders of magnitude.

0 20 40 60 80 100 120 140

With Si buffer

Without Si buffer

Ea=64meV

Ea=230meV

Inte

grat

ed E

L in

tens

ity (

arb.

uni

ts)

1000/T (1/K)

Fig. 4. Dependence of the integrated EL intensity on temperature for the

diodes with and without silicon buffer layer. The activation energy for the

sample with silicon buffer layer is about 230, and 64meV for the sample

without silicon buffer layer, which was ascribed to the defects in the

samples.

C. Li et al. / Journal of Crystal Growth 290 (2006) 176–179178

without silicon buffer layer, the low leakage current of thesamples with silicon buffer layer suggests that the defectdensity at the interface between silicon buffer layer and b-FeSi2 is low enough to affect the optical properties of thosesamples.

The integrated EL intensity dependence on temperatureis shown in Fig. 4. The EL intensity quenches more slowlyfor the sample with silicon buffer layer until roomtemperature, while the EL intensity decreased quickly forthe sample without silicon buffer layer. The activationenergy of thermal quenching for the samples with siliconbuffer layer was fitted at about 230meV, which is in good

agreement with the band gap energy shift between siliconand b-FeSi2 as reported in Refs. [5,10]. While, theactivation energy for the sample without silicon bufferlayer is much smaller, about 64meV. As mentioned above,the luminescence of the samples without silicon buffer layercan be contributed to the mixture radiation of the defects insilicon and b-FeSi2. Although much more carriers areinjected through the heterojunction between silicon and b-FeSi2 (as shown in Fig. 3, the larger current is shown forthe diode without silicon buffer layer) in the samplewithout silicon buffer layer, the EL quenches quickly. Areasonable explanation is that most of the injected carriersrecombine via defects non-radiatively with the increase oftemperature. This results in the quick thermal quenching ofEL intensity and small activation energy. The high-temperature silicon buffer layer improves the intensity ofluminescence of the light-emitting diodes by preventing theformation of defects or dislocations during the growth of b-FeSi2 and subsequent annealing process.

4. Conclusion

High-temperature undoped silicon buffer layer wasintroduced in the silicon-based b-FeSi2 light-emittingdiodes to improve the luminescence and electrical proper-ties. The undoped silicon buffer layer prevented theformation of defects and dislocations in the surroundingsilicon, which gave rise to the low leakage current and slowthermal quenching of electroluminescence intensity. Thecomplex optical properties of b-FeSi2 were demonstrateddue to the defects and dislocations in the samples, and

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0.23 eV band energy shift between silicon and b-FeSi2 wasrealized once again.

Acknowledgements

This work was partially supported by Innovationprojects of Fujian province for young scientific researchers(contact no. 2004J021) and Scientific Research Foundationfor the Returned Overseas Chinese Scholars, State Educa-tion Ministry and state key laboratory on integratedoptoelectronics (Institute of semiconductors, Chineseacademy of sciences).

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