Reference number of manuscript: H/PII.26

NEW INSIGHTS ON AMORPHOUS SILICON-NITRIDE

MICROCAVITIES

V. Ballarini1, G. Barucca2, E. Bennici1, C.F. Pirri1, C. Ricciardi1, E. Tresso1, F. Giorgis1,*

1INFM - Dipartimento di Fisica del Politecnico di Torino

Corso Duca degli Abruzzi 24, 10129 Torino, Italy

2INFM - Dipartimento di Scienza dei Materiali e della Terra, Università di Ancona

Via Brecce Bianche, 60131 Ancona, Italy

Abstract

All amorphous Silicon-nitride planar optical microcavities operating in the visible range have

been grown by Plasma Enhanced Chemical Vapor Deposition (PECVD). The luminescence intensity of

the N-rich Silicon-Nitride layer from a microcavity with 6 period DBR’s is two order of magnitude

higher than that of the luminescent layer without the cavity. Moreover, a strong directionality of the

microcavities emission can be observed. Such results can be ascribed to the anisotropic optical density

of states induced in the Fabry-Perot structure. Optical investigations on the Fabry-Perot resonators

yield quality factors strictly correlated to the number of periods of the Distributed Bragg Reflectors

(DBR’s).

Keywords: Amorphous Silicon based alloys, photoluminescence, optical properties, multilayers

PACS codes: 78.55.-m; 78.66.-w; 78.66.Jg

Corresponding Author: Tel. +39 011 5647362; Fax. +39 011 5647399; E-mail: giorgis@polito.it Dipartimento di Fisica del Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

1. Introduction

Among the silicon based amorphous semiconductors, a-Si1-xNx:H alloy shows an easy

tunability of the optical gap tunable in the range 1.9-5 eV and of the static refractive index, which

spans from 1.8 to 3.5, depending on nitrogen content [1]. Moreover, with increasing x values (the

atomic N content), the room temperature PL intensity increases by several order of magnitude with

respect to that of a-Si:H, reaching an internal radiative efficiency of some percent for N-rich films,

which is comparable with that of the porous Silicon . The last feature has been ascribed to the

reduced thermal excitation of photogenerated carriers from tail states to extended states through

which carriers diffuse and, eventually, recombine by non radiative processes, and to the increase of

the Coulomb interaction between electron-hole pairs [2]. The high radiative efficiency of silicon

nitride suggested us to realize the first optical microcavities with a N-rich a-Si1-xNx:H layer as

optical active medium, embedded in quarterwave multilayers with a certain refractive index contrast

(the DBR’s) obtained by alternating a-Si1-xNx:H thin films with different stoichiometry [3].

Aim of this work is to show and discuss the optical properties of microcavities entirely based on

amorphous silicon-nitride Fabry-Perot structures tuned in the visible range, which were grown by 13.56

MHz Plasma Enhanced Chemical Vapor Deposition (PECVD). Such properties are analyzed in terms

of photoluminescence efficiency and angular control of the radiation pattern, checking the influence of

the DBR efficiencies on the cavity performances.

2. Experimental

Several silicon-nitride-based microcavities (SNMs), with different number of layers in the DBR

were grown by 13.56 MHz PECVD on 7059 Corning glass substrates. In detail, the structures consist

in the the repetition of a-Si1-xNx:H quarterwave layer pairs of different composition (stoichiometric

a-Si3N4:H and understoichiometric a-Si1-xNx:H) separated by a λ-thick luminescent spacer made of N-

rich alloy (x∼ 0.5). The composition (and so the refractive index) of the a-Si1-xNx:H layers was

controlled by operating on the ammonia fraction present in a SiH4 + NH3 plasma, by fixing the total

flux at 75 sccm, while their thickness was estimated taking into account of the deposition rate

calculated by previous depositions of homogeneous samples. The substrate temperature, gas pressure

and electrode distance were fixed at 200 °C, 0.35 Torr and 20 mm respectively. The project of the

resonators was performed by using standard scattering matrix methods [4]. The interface of the

multilayer structures were analyzed by Transmission Electron Microscopy (TEM) performed by

Philips CM200 operating at 200 kV.

The optical characterizations performed on the microcavities consist in spectral transmittance and

reflectance (T-R) spectroscopy and stationary PL measurements. A dispersive grating Perkin-Elmer

Lambda 9 spectrophotometer was used for T-R measurements, from which refractive indexes of

silicon-nitride films with different composition were extracted. PL measurements were performed at

room temperature by exciting the samples with the 514.5 line of an Ar-Kr laser. The emitted light was

collected in a single monocromator with a focal length of 30 cm (Acton Spectra 300i ) coupled to a

cooled back illuminated CCD camera (Rooper/Princeton GS/120B-UV).

The composition (x=N/Si+N) of the alloys has been determined by considering their refractive indexes

n, through a calibration curve x(n) obtained by Rutherford backscattering spectrometry performed on a-

Si1-xNx:H thin films previously grown [1].

3. Results and Discussion

The abruptness of the interface between a-Si1-xNx:H layers with different N content is shown in

fig.1. The interface roughness of the deposited SNMs can be estimated in the range of few Ă,

warranting the structural quality of the multilayers.

The normal-incidence T-R spectra of a cavity tuned at λC=642 nm with two specular DBR with six

periods (refractive index contrast n1/n2=2.2/1.8) are shown in fig. 2(a). The reflectance spectrum

presents a wide stop band with high reflectivity (R≈1) and a narrow dip, corresponding to the

resonance of the λ-thick F-P structure (its second interference order). Such a resonance is observed in

the PL spectrum collected perpendicularly to the sample surface (fig.(b)).

The intensity of the PL resonant peak shows a noticeable quenching for collecting angles far from the

normal emission, as shown in the polar plot. In the same figure, the emission of a λ-thick single layer

with the same composition as that of the spacer embedded in the microcavity (N/(Si+N)=0.5) is shown

for comparison. The cavity signal present for λ > 700 nm is essentially due to the luminescence yield of

the high refractive index layers (with composition x=0.41) belonging to the DBRs.

Three effects can be noticed in the PL spectra of the microcavity: (a) a strong narrowing of the spacer

emission band (with a width ∆λ = 5 nm), (b) a noticeable luminescence yield enhancement at the

resonance wavelength (more than two order of magnitude with respect to the PL intensity of the spacer

without the cavity), (c) a marked directionality of the emitted light around a cone of 20°-30° with

respect to the axis normal to the stack surface (see the inset of fig. 2 (b)). This marked directionality

could explain the luminescence enhancement even in absence of an altered spontaneous emission rate,

never verified for such a kind of resonators [5]. Actually, the high efficiency of the DBRs which embed

the spacer surely induce an anisotropic density of the optical modes, thus allowing a spatial

redistribution of the spontaneous emission.

To check the influence of the DBRs on the SNMs performances, several symmetrical cavities (with

specular Bragg mirrors surrounding the emitting layer) with different number of periods in the DBR

were fabricated. Fig. 3 (a),(b) show the transmittance and the PL spectra of SNMs with DBR periods

ranging from 3 up to 5 and a refractive index contrast a little bit reduced with respect to that of the

SNM previously analyzed (n1/n2=2.1/1.8).

First of all, it is worth to note that all the cavities compared in fig.3 show the same resonance

wavelength, within a dispersion of few nm, as a proof of the excellent control of the thickness and of

compositional homogeneity obtainable by PECVD technique. By increasing the number of periods, that

is the DBRs reflectance, the transmittance and the luminescence intensity at the resonance wzavelength

enhance. In particular, the stop band of the cavities (the spectral region with low transmittance close to

the resonance peak) becomes sharper by increasing the efficiency of the DBRs, that is the number of

periods. Contemporary, the transmittance within such a band approaches values close to zero,

demonstrating an improvement of SNMs performances. Fig. 4 shows their quality factors Q=λ/∆λ

(extracted by the transmittance spectra) as a function of the number of periods in the DBRs. The Q

factor grows exponentially with an increasing mirror multilayers periods, in agreement with other

experimental and theoretical analysis on a-Si:H/a-SiO:H and GaAs/AlAs microcavities [6].

For what concerns the external radiative efficiency of the SNMs, it can be directly extracted by the PL

spectra. Actually, the spectra shown in fig. 4 (b) can be compared each other with a good reliability,

since differences in sample mounting were corrected by normalizing the PL spectra to that of a

reference sample mounted on the same sample holder. This procedure allowed for the comparison of

the PL intensities within a 10% experimental error. Interestingly, despite the increase of the DBR

periods enhanced the light absorptance of the top mirrors, there is an enhancement of the emission

efficiency in the SNMs by increasing the Q factor of the resonators. In this case, the enhanced light

extraction efficiency imposed by the coherent scattering overcomes the absorption losses due to the

high refractive index layers used in the Bragg mirrors and the possible diffuse scattering at the

multilayer interfaces. This fact is very promising in view of application of SNMs in large area and high

brightness light emitting devices with high spectral purity and tunable emission obtainable simply

varying the DBR and spacer thickness.

4. Conclusions

a-SiN:H based optical microcavities operating in the visible range showed appealing optical

properties such as enhancement of the luminescence intensity (two order of magnitude wit 6 period

DBRs), reduction of the PL linewidth (few nm) and directionality along the normal to the cavity

surface. Such features are strictly correlated to the efficiency of the Bragg mirrors embedding the

emitting spacer, and look similar to other F-P microcavities based on III-V semiconductors [7]

The discussed results represent a deeper insight dealing with planar microcavities fabricated with the

amorphous Silicon technology, demonstrating their high potential for Si based optoelectronics.

References

[1] F. Giorgis, C.F. Pirri, E. Tresso, Thin Solid Films 307 (1997) 298.

[2] F. Giorgis, C. Vinegoni, L. Pavesi, Phys. Rev. B 61 (2000) 4693.

[3] F. Giorgis, Appl. Phys. Lett. 77 (2000) 522.

[4] E. Hecht, Optics (Addison-Wesley Publishing, 1987) p. 373.

[5] F. Giorgis, S. Ferrero, P. Mandracci, C. F. Pirri, M. Cazzanelli and L. Pavesi, Mat. Research Soc.

Proc., Vol. 637 (2001) E 2.7.

[6] V. G. Golubev, A. A. Dukin, A. V. Medvev, A. B. Pevtsov, A. V. Sel’kin, N. A. Feoktistov,

Semiconductors 35 (2001) 1213.

[7] G. Björk, S. Machida, Y. Yamamoto, K. Igeta, Phys. Rev. A 44, 669 (1991).

Figure captions

Fig. 1 (a) Cross sectional TEM viewgraph showing a typical microcavity structures. (b) Vievgraph

with higher magnification aimed at evidence the high quality of the multilayer interfaces.

Fig. 2 (a) Reflectance (continuous line) and transmittance (dashed line) spectra at normal incidence

of the microcavity with 6 DBR periods with a resonance at λC=642 nm. (b) Room temperature PL

spectra of the cavity and of the λ-thick emitter. The layers compositions are a-Si0.59N0.41:H / a-

Si3N4:H for the DBR layers and a-Si0.5N0.5:H for the emitter. The spectrum of the single layer (a-

Si0.5N0.5:H) has been multiplied by a factor 40 for clarity purposes. The inset shows the polar plot of

the PL intensity at the resonance versus the emission angle (θ = 0 corresponds to the light emission

normal to the SNM surface).

Fig. (3) (a) Transmittance spectra at normal incidence of the microcavities with different DBR

periods. (b) Room temperature PL spectra of the SNMs and the reference emitter. The layers

compositions are a-Si???N???:H / a-Si3N4:H for the DBR layers and a-Si???N???:H for the emitter.

Fig. (4) Q-factor of the SNMs versus the DBR periods. The inset shows the PL intensity obtained by

calculating the PL peak area around the resonance wavelength as a function of the quality factor.

Fig. 1

50 nm

a) b)

500 600 700

b)

Reference number of manuscript: H/PII.26"NEW INSIGHTS ON AMORPHOUS ….." by F. Giorgis et al.Fig. 2

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Reference number of manuscript: H/PII.26"NEW INSIGHTS ON AMORPHOUS ….." by F. Giorgis et al.Fig. 3

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