Angle-dependent photoluminescence spectra of hydrogenated amorphoussilicon thin filmsD. C. Marra, E. S. Aydil, S.-J. Joo, E. Yoon, and V. I. Srdanov Citation: Appl. Phys. Lett. 77, 3346 (2000); doi: 10.1063/1.1326837 View online: http://dx.doi.org/10.1063/1.1326837 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v77/i21 Published by the American Institute of Physics. Related ArticlesSilicon nanocluster sensitization of erbium ions under low-energy optical excitation J. Appl. Phys. 111, 094314 (2012) Dopant effects on the photoluminescence of interstitial-related centers in ion implanted silicon J. Appl. Phys. 111, 094910 (2012) Above-room-temperature photoluminescence from a strain-compensated Ge/Si0.15Ge0.85 multiple-quantum-well structure Appl. Phys. Lett. 100, 141905 (2012) Capability of photoluminescence for characterization of multi-crystalline silicon J. Appl. Phys. 111, 073504 (2012) Investigation of defect states in heavily dislocated thin silicon films J. Appl. Phys. 111, 053706 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 21 20 NOVEMBER 2000

Angle-dependent photoluminescence spectra of hydrogenated amorphoussilicon thin films

D. C. Marra and E. S. Aydila)

Department of Chemical Engineering, University of California–Santa Barbara, Santa Barbara,California 93106

S.-J. Joo and E. YoonSchool of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea

V. I. Srdanova)

Center for Polymers and Organic Solids, University of California–Santa Barbara, Santa Barbara,California 93106

~Received 18 July 2000; accepted for publication 19 September 2000!

Multiple sharp peaks were observed in the visible photoluminescence spectra of amorphous siliconthin films, prepared by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition onoxidized silicon substrates. The angular dependence of the photoluminescence, measured by ahome-built fiber-optics device, revealed that the origin of these sharp features was due to Fabry–Perot cavity interference effects. The interference is enhanced by deposition on thermally grownoxide layers with relatively smooth surfaces. We also consider how thin-film interference effects canadd to the already existing confusion regarding the photoluminescence~PL! mechanism of porousand other luminescent forms of silicon and propose angle-dependent PL spectroscopy as a remedyfor identifying spectral features due to interference effects. ©2000 American Institute of Physics.@S0003-6951~00!00247-3#

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The synthesis of stable, luminescent silicon films hreceived renewed attention during the past decade.1,2 One ofthe driving forces behind this renewed interest is the pobility of combining mature silicon processing technolowith promising, but still controversial, optical propertiessilicon nanostructures. To date, researchers have prepnumerous photoluminescent and electroluminescent devfrom porous silicon and carefully studied their emissispectra.1,2 Despite prolific research, progress in this areabeen hindered by complications in identifying the originthe emission spectra.1,2 Although quantum confinement oexcitons seems consistent with much of the data, in minstances other emission sources could not be ruled out.situation can be further complicated by macroscopic tfilm interference effects3,4 that may be confused with molecular vibrations and/or lattice optical phonons coupledthe excitonic emission. In this letter, we report the obsertion of discrete emission peaks in the photoluminesce~PL! spectra of hydrogenated amorphous Si(a-Si:H) filmsdeposited on Si substrates with or without thermally groSiO2. Similar features in the PL spectra of porous anda-Si:Hhave been attributed to coupling of excitons to vibratiomodes of Si species,5,6 as well as to microcavityinterference.3,4 Herein, we report multiple sharp peaks in thvisible PL of a-Si:H thin films and show, through angleresolved PL spectroscopy, that these features are duFabry–Pe´rot cavity interference effects. We propose that agular dependence of the luminescence may help identifyorigin of features in PL spectra.

a!Authors to whom correspondence should be addressed; electronicAydil@engineering.ucsb.edu, srdanov@chem.ucsb.edu

3340003-6951/2000/77(21)/3346/3/$17.00

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Thea-Si:H films were deposited using ultrahigh vacuuelectron cyclotron resonance chemical vapor deposition.details of the apparatus7,8 and sample preparation9 can befound in separate publications. In all cases, the SiH4 flowrate was diluted in H2 at a ratio of 50:1. The substrates weeither pristine or oxidized silicon~100! wafers with 100 nmthick SiO2. Room temperature PL spectra were excited w10 mW of 488 nm radiation from an Ar ion laser and rcorded in a backscattering geometry. Angle-resolvedspectra were collected using a 1 mmdiam optical fiber, at-tached to a 50 mm diam rotating stage with the luminescsample mounted at its center. The other side of the fibercoupled to a 0.5 m monochromator equipped with a gratblazed at 500 nm and a charge coupled device detectothis way, the angle-dependent PL spectra could be obtain the 0°–90° range with 2° resolution.

A typical PL spectrum of ana-Si:H film, deposited on aSi substrate with a thermally grown SiO2 layer, is shown inFig. 1. This spectrum was collected at 0° with respect tosurface normal and shows remarkably sharp features fluminescent Si PL spectrum. Although PL froma-Si:H andporous silicon films is typically broad and featureless, ocsionally there have been reports of discrete emission featin the luminescence spectra of these materials. For examusing confocal microscopy, Masonet al.5 observed well-resolved features separated by approximately the vibratiofrequency of the Si–O stretching mode in the PL spectrumsingle porous Si nanoparticles. The origin of these featuwas ascribed to coupling of confined excitons with Si–vibrational modes. Low temperature PL spectra, resembFig. 1, were also detected in thina-Si:H layers deposited byplasma deposition on the Si~100! substrate.6 The tempera-ture and power dependence of the PL intensity was foun

ail:

6 © 2000 American Institute of Physics

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3347Appl. Phys. Lett., Vol. 77, No. 21, 20 November 2000 Marra et al.

be consistent with exciton emission. Based on these repit would be prudent to consider the molecular origin of tdiscrete luminescence peaks in Fig. 1, particularly sinceO–Si vibrational modes have been identified in the infraspectra of these samples.9 Nevertheless, prompted by reporin the literature3,4 and the fact that some of our spectra rsembled those found in conjugated polymer microcavitie10

we examined the possibility that the spectrum in Fig. 1 cbe explained by interference phenomena occurring inFabry–Pe´rot etalon.

The Fabry–Pe´rot equation

ml52dn cosc, ~1!

defines conditions for constructive interference of electmagnetic radiation undergoing multiple internal reflectioin a solid thin film or an e´talon. In Eq.~1!, m is the interfer-ence order,l is the wavelength of the propagating radiatiod is the etalon thickness,n is the etalon index of refractionandc is the wave-front propagation angle with respect tosurface normal. For monochromatic light, a discrete sepropagation angles are allowed, each associated with aferent cavity modem; similarly, for a fixed angle and a whitelight source there will be a set of allowed wavelengths tsatisfy Eq.~1!, each associated with a cavity mode.

A thin film of luminescent a-Si:H deposited on asmooth, reflective substrate is not only a source of lightcan also act as a Fabry–Pe´rot etalon, and thus create opticainterference. The multiple reflection of the emitted ligarises from the contrast in refractive indices of the lumincent film with the substrate and vacuum. The waveguidingthe film is better and the cavity modes are sharper whendifferences in the refractive indices of the film and the srounding materials are larger. If the discrete luminescepeaks in Fig. 1 are due to macroscopic interference phenena, their wavelength should change with the propagaangle according to Eq.~1!. In contrast, the locations of thspectral features in the PL spectra should be independethe propagation angle if these luminescent peaks are duconfined excitons coupled to molecular vibrations or lattoptical phonons. If one observes the emitted light througmobile orifice, at some distance from the thin film surfaceis possible to detect changes in the PL spectrum withobservation angle. If the spectral features shift with the

FIG. 1. PL spectrum of ana-Si:H film on a SiO2-coated Si wafer. Thenarrow features are attributed to a Fabry–Pe´rot microcavity effect.

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servation angle, then the origin of the sharp features are cclusively due to Fabry–Pe´rot interference. If no such shift isobserved, then explanations of the features with mechaniinvolving coupling of excitons to molecular or lattice vibrations should be considered. Figure 2 shows the angularpendence of the PL spectra of ana-Si:H film grown on anoxidized Si substrate. Clearly, angular dependence of thespectraconclusively rules out the molecular nature of thsharp luminescence peaks. The sharp features in Figs. 1 an2 are due to Fabry–Pe´rot interference.

The propagation anglec and the observation angleu inour experiment are related through Snell’s law,n sinc5sinu. Substitution of Snell’s law in Eq.~1! yields

ml52dAn22sin2 u, ~2!

a relationship between the observation angleu and the cavitymode wavelengthl. The refractive index itself also dependon the wavelengthl.11 From the experimental data, it is possible to determine both the thickness of the e´talon and theindex of refraction of the material, as well as the correcavity mode numbering. In practice, one finds that all thparameters are strongly correlated in a least square fit,12 re-quiring some of the parameters to be determined in indepdent experiments. The film thickness of thea-Si:H film inFig. 2 is 1.3mm, as determined by scanning electron microcopy. The index of refraction is modeled using

n5A1B

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whereA, B, andC are constants andn is 3.1 at 632 nm. Thismodel provides a good fit of the refractive index in the sptral region of interest 500–900 nm. Using these data in~2! reveals unambiguously the correct cavity mode numbing as shown in Fig. 3.

The sharpness of the observed cavity modes is demined by the cavity quality factorQ, which in turn is deter-mined by the smoothness of the film–substrate and filvacuum interfaces. The PL spectra of twoa-Si:H samples,grown under identical experimental conditions but deposi

FIG. 2. Angular dependence of the PL spectra. The baselines of the sphave been offset for clarity. The shift in emission to lower wavelength wincreasing detection angle obeys the Fabry–Pe´rot equation. Detection anglemeasured with respect to the surface normal decreases in increments ofrom 60° to 10° in the direction of the arrow.

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3348 Appl. Phys. Lett., Vol. 77, No. 21, 20 November 2000 Marra et al.

on different substrates, are shown in Fig. 4. Sample~a! isgrown on a smooth, oxidized silicon wafer, whereas sam~b! is deposited on a pristine Si wafer cleaned by hydrogplasma prior to deposition. Even though the films wereposited using identical conditions, the difference in theirspectra is striking. As suggested by Curtiset al.,3 thin orpoor quality optical cavities would produce broad and fewinterference fringes that could be incorrectly attributed

FIG. 3. PL emission wavelength as a function of detection angle forcavity modes. The experimental data are shown as symbols while thelines represent the model predictions based on Eq.~2!.

FIG. 4. The PL spectra of twoa-Si:H films deposited on different substratmaterials using identical plasma conditions. Sample~a! was deposited on100 nm of SiO2 on Si, while sample~b! was grown on a pristine Si wafer

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luminescence properties inherent to the material. Also,very thin films exhibiting strong microcavity effects, thsharp intrinsic luminescence may be completely quenchecertain observation angles. Hence, the microcavity interence effects must be considered with great caution wheninterpretation of the luminescence spectra of thin filmsattempted.

In conclusion, hydrogenated amorphous silicon filmgrown on a smooth and oxidized silicon substrate, can acoptical microcavities. The sharp interference pattern,tained in the luminescence spectra of such films, can be cfused with narrow excitonic or vibronic spectral featureSilicon films grown on rough interfaces with no oxide laygive rise to poor microcavities characterized by broad cavmodes that resemble PL spectra with broad spectral featuPeak shifts in film-thickness dependent weakly modulaPL spectra of luminescent Si films could be erroneouslytributed to quantum confinement effects and broad featumay be interpreted as PL arising from different sources.ing a simple fiber-optic device, which allows detectionspectrally and angle-resolved luminescence, it is possiblrecognize artifacts arising from the microcavity interfereneffects.

This research was supported by the NSF/DoE Partnship for Basic Plasma Science and Engineering~Award No.DMR 97-13280! and the Camille and Henry Dreyfus Foundation. V.I.S. acknowledges the NSF~Grant No. DMR-9520970! for support.

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