Thin Solid Films S18 (2010) 1744–1746

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Optical property of silicon quantum dots embedded in silicon nitride bythermal annealing

Baek Hyun Kim a,⁎, Robert F. Davis a, Seong-Ju Park b

a Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United Satesb Nanophotonic Semiconductors Laboratory, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 500-712, Republic of Korea

⁎ Corresponding author.E-mail address: bhkim@andrew.cmu.edu (B.H. Kim)

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.12.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 June 2009Received in revised form 1 December 2009Accepted 1 December 2009Available online 6 December 2009

Keywords:SiliconQuantum dotsHydrogen passivationThermal annnealingPhotoluminescence

We present the effects on the thermal annealing of silicon quantum dots (Si QDs) embedded in siliconnitride. The improved photoluminescence (PL) intensities and the red-shifted PL spectra were obtained withannealing treatment in the range of 700 to 1000 °C. The shifts of PL spectra were attributed to the increase inthe size of Si QDs. The improvement of the PL intensities was also attributed to the reduction of point defectsat Si QD/silicon nitride interface and in the silicon nitride due to hydrogen passivation effects.

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1. Introduction

There has been increasing interest in using silicon quantum dots(Si QDs) embedded in silicon nitride as light sources for integratedsilicon-based electronics [1]. Silicon-based light sources would permitdevices containing both optical and electrical components to befabricated in a single processing sequence, eliminating the need forsubsequent alignment and bonding of discrete components. Becausenanosize semiconductor materials typically have band gaps that arestrongly dependent on cluster size or layer thickness, Si QDs can beused to produce light of greater intensity over a wider wavelengthrange than devices fabricated from bulk silicon [2–4]. However, theemission from Si QDs is diminished by the presence of non-radiativecenters in the matrix film and at the interface between the Si QDs andthe matrix materials. The passivation of non-radiative centers is anessential requirement for increasing the radiative yield. In the case ofSi nanocrystals, there are many reports of improved radiativeefficiency after hydrogen passivation using either mixtures ofhydrogen and nitrogen or nitrogen gas alone [5–10]. The annealingof Si nanocrystals also increases their size, shifting the emission tolonger wavelength. In this paper, we describe the change to theoptical properties of Si QDs resulting from passivation of non-radiative centers at QD/silicon nitride interface and in the siliconnitride matrix and size changes occurring after conventional thermalannealing.

2. Experimental details

The silicon nitride films were grown using plasma-enhancedchemical vapor deposition (PECVD) with nitrogen-diluted 5% SiH4 andN2 reactant gases. The substrate was a lightly doped p-type Si wafer(100) with a hole concentration of approximately 1015 cm−3. The flowrate of silane was fixed at 10 sccm and the flow rate of additionalnitrogen was 100 sccm. The pressure was 3.75×10−3 Pa and thesubstrate temperature was maintained at 300 °C. The plasma powerwas 5W. After deposition, the films were heated for 60 min in N2 tobetween 700 and 1000 °C. H2 plasma treatment was performed usinginductively coupled plasma (ICP) in a He atmosphere. The plasma wasgenerated using a H2 flow rate of 30 sccm and a chamber pressure of3.75×10−2 Pa. The ICP power was 1500W and the RF power wasmaintained at 50W. The samples were cooled to 20 °C and treated for 5or 7 min. PL measurements were obtained at room temperature using aspectra analyzer system (PR-704, Photo Research Co.) with excitationprovided by a 325 nmHe–Cd laser. The chemical bonding in the samplewas studied using Fourier-transform infrared spectroscopy (FTIR) from400 to 4000 cm−1 with a resolution of 4 cm−1.

3. Results and discussion

Fig. 1 depicts the variation in PL spectra and peak positions for Si QDsembedded in silicon nitride films as the annealing temperature wasvaried between 700 and 1000 °C. As the annealing temperature wasincreased, the PL peaks were shifted to lower energy values (Fig. 1(b)).The PL intensity of the Si QD layer annealed at 700 °Cwas enhanced by afactor of 2 compared to theas-grownfilm(Fig. 1(a)).Webelieve that the

Fig. 1. (a) PL spectra of Si QD layers as a function of the annealing temperature. (b) PLspectra of Si QD layers as functions of the annealing temperature and PL peak positions.

Fig. 2. (a) FTIR spectra of Si QD layers as a function of the annealing temperature. (b) FTIRabsorption peak intensities of Si QD layers as a function of the annealing temperature. Allpeak intensities are normalized with the peak intensity values at an as-grown sample.

Fig. 3. PL spectra of annealed Si QD layers as a function of time of H2 plasma treatment.Annealed Si QD layers were prepared by thermal annealing at 1000 °C for 60 min in N2

ambient.

1745B.H. Kim et al. / Thin Solid Films S18 (2010) 1744–1746

enhanced PL intensity of Si QDs annealed at 700 °C is due to passivationof defects by hydrogen atoms dissociated from Si and N atoms [11]. Toestimate theoptical properties of samples annealed below700 °C, the PLfor Si QDs were investigated in the temperature range from 400 to1000 °C for 15 min in N2 ambient. The PL intensity annealed at 600 °Cwas increased to approximately 2 times of that at the as-grown sampleand the PL peak positions were continuously moved from 683.60 to692.39 nm, indicating the reduction of the defects due to hydrogenpassivation.

The bonding configuration of the film varied with the annealingtemperature (Fig. 2(a)). The intense absorption band at 840–870 cm−1

was assigned to the Si–N stretching mode. The band at 3350 cm−1

corresponds to the N–H stretchingmode. The band at 2180–2200 cm−1

is the result of the Si–H stretching mode. The relative intensities of thethree peaks varied as the annealing temperature was increased from700 to1000 °C. Fig. 2(b) is a plot of that peak intensities normalizedwithrespect to the as-grown film. When the annealing temperature wasincreased, the N–H and Si–H peaks diminished in intensity as theincreased temperature promoted the release of hydrogen atoms fromN–H and Si–H bonds. The Si–N peak intensities decreased slightly withincreasing annealing temperature and the peak location was shiftedfrom 841 to 835 cm−1. The reduced Si–N peak intensity suggests thatthe Si and N atoms released from Si–H and N–H bonds do not rearrangeto formSi–Nbonds. The changes in the Si–Npeakmost likely result fromdecomposition of the silicon nitride matrix [12].

In order to investigate the red-shifted PL, Si QD layers annealed at1000 °C for 90 min in N2 were subjected to H2 plasma treatment. Fig. 3contains the PL spectra of annealed Si QD layers after various H2

1746 B.H. Kim et al. / Thin Solid Films S18 (2010) 1744–1746

plasma treatment times. After annealing, the PL emission was red-shifted and lower in intensity. The emission intensities of theannealed Si QD layer increased as a function of H2 plasma treatmenttime, while the PL peak wavelength was unchanged. H2 plasmatreatment was effective in passivating non-radiative defects at the SiQD/silicon nitride interface and in the silicon nitride film. Thediffusion of hydrogen increases the size of the QDs by enabling theformation of new Si–Si bonds between silicon atoms previouslybonded to hydrogen [13,14], leading to longer emission wavelengths.

4. Summary

We observed an increase in PL intensity and a shift in PL peakposition following thermal annealing of PECVD-grown Si QDs. Afterannealing at a temperature of 700 °C, the PL intensity was doubledand the emission was red-shifted compared to as-grown Si QDs. Theshift in the PL peak with increasing annealing temperature wasattributed to an increase in the size of the Si QDs. The improvement inthe PL intensity was the result of hydrogen passivation of point

defects at the Si QD/silicon nitride interface and in the silicon nitridematrix.

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