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Materials Science and Engineering B 175 (2010) 150–158

Contents lists available at ScienceDirect

Materials Science and Engineering B

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icrostructural and optical characterization of germanium:indium tin oxideGe:ITO) nanocomposite films

.G. Allen ∗, G.H. Shih, B.G. Potter Jr.epartment of Materials Science and Engineering, University of Arizona, Tucson, AZ, United States

r t i c l e i n f o

rticle history:eceived 12 March 2010eceived in revised form 21 May 2010ccepted 3 July 2010

eywords:etal–oxide–semiconductor structures

ndium oxideermaniumuantum structureshin films

a b s t r a c t

The nanophase assembly and resulting optical and electronic properties of Ge:ITO composite thin films,produced by a sequential RF-sputtering deposition approach, were manipulated via deposition con-ditions and subsequent isochronal thermal anneals. The study examined the combined influences ofthermally induced changes in phase crystallinity, semiconductor-phase morphology, and Ge–ITO interfa-cial structure on properties of relevance to photovoltaic function. A range of Ge-phase spatial distributionswithin the ITO embedded phase were produced, including isolated Ge nanocrystals and two-dimensional-extended semiconductor structures, as evaluated using cross-sectional transmission electron microscopy.The magnitude of a quantum-confinement induced blue-shift in the Ge absorption onset increasedmonotonically with increased isochronal anneal temperature and was concomitant to the decrease inconnectivity of the as-deposited Ge-phase assembly. Raman spectroscopy, over the range of nanocompos-ite structures examined, confirmed the evolution of a germanium oxide interfacial structure anticipatedto affect both carrier confinement within the Ge and long-range charge transport in the nanocomposite.Shifts in the near-infrared transmission edge with anneal temperature were further correlated, usingHall-effect measurements, with a thermally equilibrated free carrier population. An increased free car-

rier density in composite films, over that of similarly treated single-phase ITO, was attributed to thepresence of the Ge semiconductor-phase. While a general reduction in carrier mobility accompaniedthe increased carrier density, resistivities of the composite films were found to be largely insensitiveto the nanostructure morphology changes and, moreover, were comparable to that of single-phase ITOfilms produced under similar preparation conditions. Finally, optical excitation at energies resonant withthe Ge absorption onset, but below the band-gap of the ITO, resulted in a photoconductive response

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. Introduction

Nanostructured semiconductors are widely investigated forheir quantum-size-related electrical and optical properties [1]. Theeduced dimensionality and limited spatial extent of the crystalattice in such systems (with crystallite sizes less than the Bohr exci-on diameter) [2] results in shifts in the allowed electronic energytates over those of the bulk crystals. This behavior thus enablesize-dependent tuning of the spectral absorption or emission char-

cteristics of nanoscale semiconductor systems used in a variety ofptoelectronic devices including light sources [3] or photosensors4,5]. In addition, quantum-size-dependent phenomena providehe potential to improve energy conversion efficiencies in photo-

∗ Corresponding author at: Arizona Materials Laboratory, 4715 Fort Lowell Rd.,ucson, AZ 85712, United States. Tel.: +1 520 322 2302.

E-mail addresses: cgallen@email.arizona.edu, carygallen@gmail.comC.G. Allen).

921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2010.07.018

nsfer from the Ge-phase to the ITO.© 2010 Elsevier B.V. All rights reserved.

voltaics (PV) by influencing single-photon processes for enhancedsolar spectral absorption as well as by accessing fundamentallynew operational modes within these systems, including multipleexciton generation, hot carrier extraction and intermediate bandabsorption [6–9].

These quantum-scale effects motivate examination ofnanophase semiconductor components (e.g. II–VI or Group IVquantum dots (QD’s)) supported by a variety of embedding phasesthat provide either passive mechanical support for the QD ensem-ble or, often, complementary optical and electronic function.The investigated systems include organic and nanostructuredhybrid solid-state phases, as well as, fluid electrolyte systems (e.g.dye-sensitized PV’s) (see Ref. [10] and Refs. within)). In the presentstudy a Ge nanophase absorber is introduced into a solid-state

inorganic transparent conductive oxide thin film, indium tin oxide(ITO), providing a medium for photocarrier transport.

Germanium offers a wide range of quantum-size-tuned band-gap energies, associated with the large Bohr exciton radius(24.3 nm) characteristic of the material [11]. Nanocrystalline Ge-

and Engineering B 175 (2010) 150–158 151

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C.G. Allen et al. / Materials Science

hases, previously examined in a variety of electrically insulatingmbedding mediums, including silica [12], titania [13] and alumina14], demonstrate quantum-size-tunable optical absorption overn energy range compatible with the solar spectrum. Regardlessf the specific composite systems used, the efficient applicationf semiconductor nanophases within PV device structures mustddress the inherent coupling between semiconductor quantum-cale processes, which define the optical response of the ensemble,nd the longer range charge transport characteristics of theseanoheterogeneous systems. The dramatic impact of interface-ediated collective phenomena on the electronic structure of a

uantum dot ensemble, in particular, will produce departures fromingle-QD optical behavior (e.g. spectral absorption) and, moreover,an significantly influence the potential for carrier generation andong-range transport.

In a previous work semiconductor–transparent conductivexide (Ge–ITO) nanocomposite thin films were produced using aultisource, sequential R.F. magnetron sputter deposition tech-

ique [15]. By controlling the relative exposure time to each of theputtering sources, it was possible to adjust the spatial distributionnd overall volume fraction of the semiconductor-phase within thelectrically active embedding matrix. The present article investi-ates the impact of semiconductor-phase structure on the opticalnd electronic properties of the composite through the manipula-ion of as-deposited phase assembly via isochronal annealing over aide range of temperatures. Structural characterizations, includingiffraction, vibrational spectroscopy and electron microscopy, weresed to monitor the influence of thermal processing on the evo-

ution of nanocomposite multi-length scale structure. Changes inhase crystallinity, morphology, and interfacial development werebserved. Additionally, correlation between these structural mod-fications and the resulting spectral absorption, carrier generationnd transport behavior of the composite were examined via opticalbsorption spectroscopy and Hall-effect techniques.

Based upon the similar bulk electron affinity (Ea) values for thee and ITO components [16], only limited electron carrier con-nement is anticipated within the Ge-phase. However, spectralbsorption at energies higher than the absorption onset of bulk Geas previously shown to indicate the development of quantum-

onfinement conditions [15]. In this report, Raman spectroscopyesults confirm the formation of an interfacial oxide layer betweenhe Ge and ITO phases. Along with nanostructural modifications,his phase boundary is anticipated to influence the electronictructure of the semiconductor-phase, modifying spectral trans-ission as well as long-range carrier transport properties. The

bservation of a photoconductive response confirming extractionf carriers from the Ge-phase also supports the consideration ofuch nanocomposites for integration into thin film PV architecturesr as functional elements in other optoelectronic devices.

. Experimental

.1. Fabrication method

A schematic of the deposition system is shown in Fig. 1. Planarargets (SCI Engineered Materials, Ohio) of ITO (ITO: In2O3/SnO20/10 wt% and 99.99% purity) and poly Ge (99.99% purity) withwo-inch diameters were used as the sputtering sources. Thearget-to-substrate separation distance was 15 cm. The deposi-ion chamber was evacuated to a base pressure of l × 10−6 Torrnd then backfilled with argon (99.999%). The operating pres-

ure was maintained at 3.5 mTorr with an applied RF-power of5 and 40 W for the Ge and ITO sources, respectively. Films wereeposited to ambient-temperature substrates (T < 40 ◦C) at a depo-ition rate of 1 Å/s. Targets were pre-sputtered for 20 min before theeposition.

Fig. 1. A cross-sectional schematic of the dual source, RF sputter deposition systemused to create the nanocomposite specimens in this study.

The films were deposited at room temperature onto fused sil-ica substrates and (1 0 0) p-type silicon (specified 1–100 �-cm).Silicon substrates facilitated TEM cross-sectional specimen prepa-ration using focused ion beam (FIB) methods. Fused silica substrateswere used for all other characterizations. The nanocomposite filmswere fabricated by alternating exposure to the ITO and Ge sput-tering sources using a computer controlled stepper motor. Bafflinginside the chamber limited cross-contamination between the twosources. This technique allows the total volume fraction and distri-bution of Ge deposited between ITO layers to be altered by changingthe relative exposure times to each source. Here, the exposuretime to the ITO source was kept constant to produce layers witha thickness of 150 Å. The Ge exposure time resulted in layers 4and 15 Å thick, or volume fractions of 2.7% and 9%, respectively.These composite films will be referred to as 4Ge:ITO and 15Ge:ITOin the discussions that follow. The number of deposition cycleswas controlled to create 15Ge:ITO specimens with a total thick-ness of 500 nm. Films of 4Ge:ITO were produced with two overallthicknesses, 1760 and 500 nm. Film thicknesses were confirmedusing stylus profilometry across a shadow-masked step. The thicker4Ge:ITO specimens contain a total Ge content that is equivalentto that of the 15Ge:ITO specimens. These samples were used forXRD, Raman, and optical absorption measurements. The thinner(500 nm) 4Ge:ITO specimens were used for near-IR absorbance andelectronic characterization. As a point of comparison, single-phaseITO films (500 nm) were also generated.

In addition to altering the composition of the initial layers, post-deposition isochronal annealing was preformed over a temperaturerange of 250–550 ◦C. The anneals were performed for 5 min using amuffle tube furnace (fused silica reaction tube) and an inert atmo-sphere of flowing argon. Samples remained outside the hot zone ofthe furnace (room temperature) while purging and awaiting tem-perature stabilization of the heat zone. Specimens were containedin a fused silica boat that was introduced into the hot zone of the fur-nace for heat-treatment. The thermal anneal time was terminatedby removal of the sample from the hot zone.

2.2. Characterization methods

Visual inspection of composites films treated at 625 ◦C wassubject to roughening, signifying an upper temperature limit for

producing material suitable for large area characterizations on sil-ica. For this reason, the results reported below are primarily limitedto nanocomposite specimens subjected to thermal treatments ator below 550 ◦C although limited Raman data has been retained

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52 C.G. Allen et al. / Materials Science

i.e. 15Ge:ITO, 625 ◦C treatment) to provide additional insight intoicrostructural evolution with annealing.Transmission electron microscopy (TEM) (Jeol JEM 2000FX,

00 kV) was used to evaluate the microstructure of the com-osite films. Cross-sectional specimens (8 �m × 3 �m × 100 nm)ere fabricated by focused ion beam (FIB) (FEI Nova 200: Ga ion,

0 kV, 7 nA) using an OmniProbe nanomanipulator. Crystallinitynd phase identification of the nanocomposites were evaluatedsing x-ray diffraction (XRD) (Scintag XDS 2000, CuK� source).iffraction patterns of the 15Ge:ITO specimens were corrected for

he slight background from the silica substrates. This backgroundas subtracted from all XRD data collected.

Raman spectra were collected using a temperature-stabilizediode laser (� = 785 nm) as the excitation source. Measurementsere performed at room temperature, in air, in a back-scattering

onfiguration. The laser was focused onto the surface of the sam-le using a 10× microscope objective. The size of the illuminationpot on the surface of the sample was determined to be abouthree microns in diameter with an average intensity of 3 mW.igher incident intensities resulted in irreversible changes in the

ample spectra after prolonged exposure and such conditionsere avoided. The reproducibility of the spectra shown in this

eport was confirmed by taking measurements after prolongedaser exposure times (>30 min) and by evaluating multiple loca-ions on the films. In our experience, normalized spectra wereighly reproducible between sample sets prepared on differentccasions.

Optical absorbance measurements of the nanocomposite filmsere collected in 1 nm increments over a wavelength range

f 200–3300 nm (PerkinElmer Lambda 950). Measurements oflectron concentration, carrier mobility and resistivity wereetermined from Hall-effect measurements (Ecopia HMS-3000,

.5 Tesla) that were performed using the van der Pauw method.hmic contacts were provided by gold-plated probes. Repro-ucibility was established through measurements of identicallyrepared nanocomposite specimens deposited on different occa-ions. Photoconductivity measurements were performed using an

ig. 2. Transmission electron micrographs of 4Ge:ITO (top) and 15Ge:ITO (bottom) specic and d), and at 550 ◦C (e and f) which also has circles distinguishing Ge-phase domains.

ngineering B 175 (2010) 150–158

in-line, four-point universal probe station (Jandel RM3) allow-ing optical access to the specimens. Illumination was providedusing a photodiode operating at � = 475 nm (10 nm FWHM) and35 mW/cm2.

3. Results

3.1. Microstructure development

Cross-sectional TEM micrographs of the nanocomposites areshown in Fig. 2. In these images the Ge-phase is associated withthe brighter features due to the reduced z-contrast of Ge comparedto the heavier elements which constitute the ITO. High resolutionexamination of the as-deposited 4Ge:ITO and 15Ge:ITO (Fig. 2a andb, respectively) show no evidence of lattice fringing, consistent withthe deposition of amorphous material. The as-deposited films areuniform with a periodicity of the alternating Ge and ITO layersconsistent with the deposition conditions described above. Com-posites treated at 310 ◦C (Fig. 2c and d) show evidence of atomiclattice fringing within the ITO layers (the darker fields) indicat-ing crystallization of the ITO embedding phase (see XRD below). Inboth composite multilayer designs the Ge-phase appears to haveremained largely interconnected. (Specimens annealed at 250 ◦Cappear nearly identical to as-deposited specimens and are not pre-sented here.)

A “break-up” of this Ge-phase morphology and the developmentof more isolated, equiaxed Ge domains are observed in compos-ite specimens treated at 550 ◦C. Evaluating the micrograph of the550 ◦C 4Ge:ITO specimen (Fig. 2e), many regions show the devel-opment of apparently isolated Ge domains embedded within theITO. The 15Ge:ITO specimen (Fig. 2f) treated at the same temper-ature retains what is apparently a more interconnected Ge-phase

assemblage after the anneal, although, there is some uncertaintyin the assessment of connectivity within the Ge-phase due to theanticipated overlap of Ge-related contrast features arising fromforeshortening in the cross-sectional image. Generally, the asso-ciated Ge domains have equiaxed features with dimensions of

mens; as deposited (a and b), annealed in an argon atmosphere for 5 min at 310 ◦C

C.G. Allen et al. / Materials Science and Engineering B 175 (2010) 150–158 153

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ig. 3. X-ray diffraction patterns of as-deposited and thermally treated 4Ge:ITOgrey) and 15Ge:ITO composite films (black) are shown. Thermal treatment time forll samples was 5 min.

0–80 Å. More elongated features with lengths of 100–120 Å andidths of about 50–60 Å are also observed. The composite with

hinner Ge layers exhibit smaller, more equiaxed domains, whilehe elongated features are more apparent in the 15Ge:ITO speci-

en.The XRD patterns of nanocomposite films deposited on silica

re shown in Fig. 3. These results confirm that the as-depositedpecimens are amorphous. The crystalline ITO phase transitionindicated by TEM) was also confirmed for specimens treated at10 ◦C or above. In addition to the reflections that are associ-ted with the cubic ITO phase [JCPDS No. 44-1087], an additionaleflection is observed at 2� = 27.6◦. The location of this peak is inood agreement with previously reported values for a strainedanocrystalline Ge-phase [17] and is close to the bulk Ge (1 1 1)eflection at 27.3◦ [JCPDS No. 04-0545]. The Ge reflection is greatern magnitude in patterns taken from the composites treated at50 ◦C.

To further evaluate the evolution of the embedded Ge-phase,aman scattering spectra from 4Ge:ITO and 15Ge:ITO specimensre shown in Fig. 4. As-deposited 4Ge:ITO (Fig. 4a) films exhibit aroad feature peaked at 238 cm−1. Spectra from a 310 ◦C 4Ge:ITOpecimen show reduced peak intensity at this energy relative tohe broad scattering observed between 325 and 650 cm−1. Thisower energy feature (238 cm−1) is no longer apparent in the 550 ◦CGe:ITO specimen. These spectra, instead, have a relatively weakeak at 298 cm−1 against the now more intense higher energy scat-ering.

The annealing-induced changes in the Raman spectra of the5Ge:ITO are distinguishable from those observed from theGe:ITO film structure. The as-deposited 15Ge:ITO specimenFig. 4b) exhibits a very broad resonance feature that is centeredear 264 cm−1. In contrast to the thermally induced (310 ◦C anneal)hanges observed in the films composed of thinner Ge layers (i.e.Ge:ITO), the spectrum of the 15Ge:ITO specimen annealed at thisemperature is largely unchanged from that obtained in its as-eposited condition. The 15Ge:ITO composite annealed at 550 ◦C

ontinues to exhibit broad, low energy scattering, though the peaks now located at higher energy (275 cm−1). When treated at 625 ◦C,

relatively sharp feature (again near 298 cm−1) evolves. In thispecimen, the higher energy vibrational bands are also apparent,hough relatively less intense compared to the peak at 298 cm−1.

Fig. 4. Normalized Raman spectra collected from (a) 4Ge:ITO composite films, alongwith a GeO2 spectrum (dots) collected by others (see Ref [30]) and (b) 15Ge:ITOcomposite films.

3.2. Optical absorbance

The optical absorbance results of the composite films can bedescribed in terms of the two primary spectral features observed:a higher energy absorbance edge (typically located in the visibleand near-UV) and a lower energy transmission drop observed inthe near-infrared (see Figs. 5 and 6, respectively).

Fig. 5a and b shows optical absorbance collected in the visibleand near ultraviolet spectral regions from 4Ge:ITO and 15Ge:ITOspecimens, respectively. Since absorbance in this spectral region isinfluenced by the incorporated Ge-phase, studies were performedon 1.76 �m thick 4Ge:ITO and 500 nm 15Ge:ITO specimens. Theuse of different film thicknesses for each specimen type provideda consistent Ge content within the optical beam path in each case.This allowed the effect of Ge nanostructure on optical response tobe isolated for study.

Generally, the UV-absorption onsets of 4Ge:ITO specimens areat higher energies than similarly treated 15Ge:ITO (Fig. 5a and b,respectively) specimens. 5-min thermal treatments at higher tem-peratures generally shift the absorption onset to higher energies(blue-shift) compared to the as-deposited film, with the excep-tion of the 250 ◦C anneal specimens that exhibit no modification inabsorption spectrum in this wavelength range. A blue-shift in theabsorbance onset (approximately 0.3 eV) is noted for the 4Ge:ITOspecimens (Fig. 5a) when the annealing temperature is 310 ◦C. Asimilar shift of the absorption onset is apparent in 15Ge:ITO spec-imens only after annealing at 550 ◦C, as shown in Fig. 5b. The4Ge:ITO composite, after the same 550 ◦C treatment, has a greatlydiminished visible light absorbance and the absorption onset isblue-shifted by about 1 eV. Here the absorbance associated withthe ITO phase (Eg ∼ 4.1 eV) is also apparent.

Fig. 6a and b shows representative absorbance spectra in thenear-infrared spectral region for the 4Ge:ITO and 15Ge:ITO as-deposited and thermally treated composite films, respectively.Increased absorbance in this spectral range is produced byincreased reflectance near the free carrier plasma resonance (ωp)associated with the majority (ITO) phase of the composite. In thiscase, the absorption from samples of similar thickness (500 nm)

is compared. The near-IR absorbance edge shifts to higher ener-gies after annealing. The association of the near-IR transmissionedge feature with ωp is supported by Fig. 6c that shows a correla-tion between the frequency of the effective zero-transmission point

154 C.G. Allen et al. / Materials Science and Engineering B 175 (2010) 150–158

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Fig. 6. Absorbance spectra from 500 nm thick (a) 4Ge:ITO and (b) 15Ge:ITO com-

ig. 5. Absorbance spectra collected from (a) 4Ge:ITO (1.76 �m) and (b) 15Ge:ITO500 nm) composite specimens.

btained in the optical spectra and the electron carrier concentra-ion (ne) measured via the Hall-effect (discussed below).

.3. Electrical and optoelectric properties

Average values for the Hall-effect parameters of composite filmsre provided in Fig. 7. The carrier transport properties measuredrom single-phase ITO films are also included. When comparing theesults obtained for the 4Ge:ITO and 15Ge:ITO films, it is clear thatanocomposite specimens annealed at the same temperature haveery similar electrical properties, despite the different as-depositede layer thicknesses. Fig. 7a depicts carrier concentrations, ne

Ge:ITO

nd neITO, of the Ge:ITO composite and single-phase ITO films,

espectively, processed at the various temperatures. It is observedhat nanocomposite films with the incorporated Ge-phase havearger free carrier concentrations than those obtained for similarlyreated single-phase ITO specimens. Notably, ne

Ge:ITO values areonsistently about twice the ne

ITO values when comparing filmsrocessed at the same temperature. In contrast, Hall mobilitiesFig. 7b) of the composite films (�e

Ge:ITO) are significantly lowerhan the mobilities measured for single-phase ITO films (�e

ITO).n this case, the �e

ITO values remain relatively constant (around0 cm2/(V s)), while mobilities in the nanocomposites decreaseith increased annealing temperature. The as-deposited compos-

te specimens have mobilities (� Ge:ITO = 23 ± 3 cm2/(V s)) that are

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educed by nearly 50% after annealing at 550 ◦C. The contrastingehaviors of the carrier density and mobility with annealing pro-uce Hall resistivities (Fig. 7c) for the nanocomposite films thatre generally within the range of the annealed single-phase ITO

posites specimens are shown, along with, (c) the near-infrared zero transmissionplotted vs. the carrier concentration of the films. The line in (c) shows a square-rootdependence of frequency with carrier density.

films generated in this study (1–7) × 10−4 �-cm. This is despitethe nanostructural changes that occur with annealing (see earlierdiscussion).

Of significance for optoelectronic applications is the observationof photoconductivity in the 550 ◦C 4Ge:ITO specimen. The photoac-tivated reduction in resistivity is 7.8 ± 2.3% after prolonged opticalexposure. The normalized sheet resistance is shown as a function ofillumination time in Fig. 8. The photoexcitation illumination energy(2.6 eV) is chosen because it is within the transparency region ofsingle-phase ITO, but within a spectral range associated with Geabsorption. Additionally, it is observed that the reduced resistivity

(increased conductivity) is retained upon removal of the excitationlight. This persistent photoconductivity (PPC) lasts for hours, withthe initial dark resistivity recovering in a time-frame of ∼10 h. ThePPC effect is demonstrated in the inset of Fig. 8 which shows the

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ig. 7. Results from Hall-effect measurements of 4Ge:ITO (squares), 15Ge:ITO (cir-les), and single-phase ITO (triangles) films; (a) electron carrier concentrations, (b)obility values, and (c) resistivities.

ormalized resistance of a similar sample with illumination on for0 min and off (dark conditions) for 30 min (as noted by the arrows).o other composite specimens, nor single-phase ITO films (Fig. 8),

how a photoconductive response using identical illumination con-itions.

ig. 8. Measured sheet resistances of 4Ge:ITO and single-phase ITO films annealed at50 ◦C for 5 min (normalized by respective dark resistances) as a function of illumi-ation (475 nm, 35 mW/cm2) time. The inset shows a plot generated from a similarGe:ITO specimen with the illumination turned off after 1800 s (30 min) and theneasured in the dark.

ngineering B 175 (2010) 150–158 155

4. Discussion

4.1. Microstructure

Using the sequential RF-sputtering deposition technique, a Genanophase can be incorporated into an ITO embedding medium. Byaltering the initial Ge layer thickness and through post-depositionannealing, it is possible to influence the morphology, crystallinity,and spatial distribution of the Ge-phase. In the 4Ge:ITO films, theas-deposited Ge layers are not expected to be completely continu-ous, as layer growth is interrupted by subsequent ITO deposition.The growth of the Ge layer is expected to have proceeded fromnucleated islands. Composites produced using longer Ge layerdeposition times (i.e. 15Ge:ITO) are expected to form larger islandsand a more continuous 2-D Ge layer. Decreased dimensionality(e.g. from 2-D (layers) to 0-D (islands)) allows access to quantum-confinement conditions which increases the energy gap of thenanophase.

TEM micrographs of the as-deposited composite films (Fig. 2aand b) show clearly distinct regions of amorphous Ge (a-Ge) phaseassemblage embedded within amorphous ITO layers. Though someinter-diffusion of the layers at the interface is expected using RF-sputtering (see Raman discussion below), the solubility of Ge inITO is expected to be low (the solubility limit for Ge in In2O3 isreportedly <0.5 atomic% [18]) indicating only a limited potentialfor incorporation of Ge into the ITO lattice near Ge-phase regionsof the as-deposited nanocomposite structure. Evaluation of the spa-tial morphology of the Ge-phase after thermal processing indicatesthat the solid-state diffusion of Ge atoms, required to produce mor-phological change of the Ge-phase in these films, is also limitedat 310 ◦C. The comparison of Fig. 2c and d to the respective as-deposited films indicates that the primary differences in imagecontrast are attributable to the crystallization of the ITO phase (alsoconfirmed via XRD, Fig. 3). This process occurs, however, indepen-dent of significant morphological evolution of the Ge-phase, whichretains a largely continuous layer appearance.

Annealing at 550 ◦C results in the development of more iso-lated Ge domains, consistent with the activation of Ge diffusionat this temperature. Comparing the micrographs of the 4Ge:ITOcomposite (Fig. 2e) to that of the 15Ge:ITO specimen (Fig. 2f), thin-ner layers developed smaller, more spatially isolated Ge domains,while thicker layers retain a more interconnected appearance. Itis expected that only the initial stages of the Ge morphologicaldevelopment are being accessed during the short annealing timesused. In this context, the thinner Ge layers require less time to com-plete diffusional processes necessary to spatially isolate the Ge intoquantum dot or disk-like morphologies within the embedding ITOlayers.

The amorphous, as-deposited ITO phase crystallizes uponannealing at temperatures at or exceeding 310 ◦C (Fig. 3). Thisagrees with the reported crystallization temperature (Tc) of single-phase RF-sputtered ITO films [19]. The appearance of a crystallineGe (c-Ge) reflection using this annealing temperature, however, isunexpected. Bulk, single-phase, a-Ge thin films do not readily crys-tallize at this temperature [20] and limited anneal time. It is notedthat Tc’s less than the bulk have been widely reported when an a-(Ge or Si) phase is annealed in contact with a metal (e.g. Cu, Ag, Au,or Pb) [21–23]. “Metal-induced” crystallization is often attributedto the development of more mobile (“liquid-like”) semiconductoratoms at the metallic interface. A reduced Tc (to our knowledge) hasnot been previously reported for a-(Ge or Si) contacted with a metal

oxide and requires further evaluation. The coincidence of the ITOand Ge crystallization, however, does suggest that changing inter-facial energies may contribute to the thermodynamics of this Gecrystallization process [22]. The 550 ◦C temperature is approach-ing the Tc of a-Ge [20]. The increased intensity of the Ge (1 1 1)

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eflection in the XRD patterns suggests an increased c-Ge-phaseithin the composites coincident, as would be expected, with the

ctivation of the solid-state diffusional processes at the interface.Raman analysis also provides insight into the amorphous-to-

rystalline Ge-phase transition with thermal treatment. It is widelyeported that c-Ge has a narrow resonance peak, located near00 cm−1 [24], which corresponds to an optical phonon mode ofhe semiconductor lattice. With a-Ge, the corresponding Ge–Geibrational mode is typically broader and the peak is shifted topproximately 270–275 cm−1 [25]. This is consistent with Ramannalysis of single-phase a-Ge films (thickness = 500 nm) generatedn this study (not shown). It is noted that, indium oxide Raman shiftsave been previously evaluated [26], but the scattering efficiency isery low and is not detected from single-phase ITO films generatedn this study (not shown). Additionally, given the large interfacialrea present in these nanoheterogeneous materials, analysis of theibrational spectra provides insight into the nature of the Ge:ITOnterface and its evolution with thermal treatment.

The Raman spectra of the as-deposited composite films (Fig. 4and b) exhibit broad peaks centered around 238 and 264 cm−1

4Ge:ITO and 15Ge:ITO specimens, respectively). It is suspectedhat the Raman peaks are shifted to lower wavenumbers because ofontributions to the vibrational spectrum from Ge structures nearr at the Ge:ITO interface. It is noted, for example, that the 15Ge:ITOs-deposited spectrum (Fig. 4b) is very similar to a Raman spectrumollected from an a-Ge:silica (13:22 Å) multilayer composite [27].n that study, the authors also varied the Ge layer thickness (from38 to 8 Å) and observed inhomogeneous broadening of a spec-ral peak at 267 cm−1. The increased relative intensity of scatteringt lower wavenumbers (200–250 cm−1) compared to the spectralail (400–800 cm−1) (called “broadening” in that work [27]) wasttributed to an increased distribution of Ge bond angles (disorder)s the Ge layer thickness is decreased. Noting the increasing rela-ive contribution of interfacial structure to the spectra, the behavioras attributed to a mixed interfacial layer.

Similarly, though the matrix phase is different in the presentork (i.e. ITO vs. SiO2), a mixed-component interfacial layer is also

xpected in the present system. Considering that the atomic massesf In and Sn are greater than Ge, any interfacial alloying with thesetoms would be expected to result in lower energy vibrationalodes associated with the Ge-phase (e.g. Raman studies of Ge–Sn

lloys [28]). In this case, the vibrational signature associated withhe Ge-phase in the nanocomposite 4Ge:ITO films is anticipated toe more affected by interfacial effects due to the larger surface areao volume ratio in these specimens. Such an interpretation is consis-ent with the red-shift in vibrational resonance energy associatedith the Ge-phase observed in the 4Ge:ITO spectra (more discon-

inuous Ge-phase structure) compared to that of the 15Ge:ITO filmsmore interconnected phase structure).

Also apparent in the Raman spectra, and especially prevalentn 4Ge:ITO specimens, are broad, higher energy (325–650 cm−1)esonances. The Raman spectrum of bulk GeO2 glass previouslyeasured by others [29,30] is shown at the bottom of Fig. 4a.

aman signatures of Ge–O–Ge stretches in the oxide glass includemain peak located at 420 cm−1, a shoulder at 520 cm−1, andodes between 500 and 600 cm−1 associated with the symmet-

ic stretches of bridging oxygens in 6-membered rings, a defectD2) assigned to the breathing motion of oxygen in 3-memberedings, and bending modes, respectively. Related studies [31] havehown that pressure-quenched germania has permanent struc-ural defects in the glass network that can modify the annealed

lass vibrational structure. In this case, there is a reduction in theontribution from the main peak (420 cm−1) and increased con-ribution of the D2 peak, at 550 cm−1. This is consistent with thenalysis of Raman spectra from the nanocomposites generated inhis study which exhibit a resonance structure at 452 cm−1 and

ngineering B 175 (2010) 150–158

a shoulder at 551 cm−1. Raman scattering in this spectral region(between 325 and 650 cm−1) is, therefore, assigned to the pres-ence of strained bonds associated with germanium oxide (GeOx)formation. The high spatial correlation between Ge and oxygen(from the ITO matrix) at the interface between the two phases andthe greater relative strength of these oxide-related features in the4Ge:ITO films compared with the 15Ge:ITO specimens supports aninterpretation of oxide formation at the Ge:ITO interface.

Thus, the three primary processes that contribute to the evolu-tion of the Ge nanophase in these nanocomposite thin films arecrystallization (of both ITO and Ge), morphological change (viasolid-state diffusion), and interfacial structure development (viaatomic rearrangement and oxidation). Given the inherent timedependence of these thermally activated processes (via, for exam-ple, diffusional motion and structural rearrangement), studies arebeing conducted on specimens subjected to different isothermalannealing times. Preliminary results show that a 4Ge:ITO specimensubject to a longer annealing time (80 min) at 310 ◦C, for exam-ple, exhibits a Raman spectrum (not shown) that is nearly identicalto that of the corresponding 4Ge:ITO specimen annealed for only5 min, as shown in Fig. 4a. In contrast, the same film type annealedat 550 ◦C for the 80 min time period has a more substantially mod-ified film nanostructure and associated Raman behavior comparedto the specimen held at 550 ◦C for only 5 min. Moreover, the delayin the thermally mediated structural modification observed in the15Ge:ITO specimens (in which the tendency toward a more inter-connected Ge-phase assembly yielding a lower Ge:ITO interfacialarea than the corresponding 4Ge:ITO films), suggests the mediat-ing contribution of the ITO interface in promoting crystallizationand related structural modification in the Ge-phase. Additionalisochronal and isothermal annealing studies between 310 and550 ◦C temperature range are now under investigation to betteridentify different kinetic regimes of morphological developmentwithin the composites and to provide further insight into the ener-getics of the mechanisms contributing to these structural changes.

4.2. Optical properties

Based on the microstructure studies described above, and thecorresponding optical absorption measured, a clear correlationbetween the nanostructural characteristics (connectivity, spatialextent) of the Ge-phase and its optical behavior is observed.Quantum-size effects on the energy level structure of semiconduc-tors and the associated optical transition energies (e.g. band-gapenergy) have been well documented by others [1–11,13–15,32,33]and were discussed earlier. However, as mentioned in the Intro-duction, the embedding ITO matrix is not expected to providesignificant energetic confinement for electrons in the conductionband. Though bulk valence band offsets are substantial, the conduc-tion band energy barrier, based upon bulk electron affinity values(Ea (ITO) = 4.1–4.5 eV; Ea (Ge) = 4.0 eV [16]) is minimal.

The structural situation in the nanocomposites of the presentstudy, however, involves a separate interfacial phase. As previouslydiscussed, study of the Raman behavior, over the range of nanos-tructural variants accessed here, supports the presence of an oxidelayer at the Ge:ITO interface. The presence of such an interfacialstructure would serve as a low electron affinity (high band-gap)blocking layer, modifying the local electronic structure at the inter-face over bulk-like behavior and confining carriers within the Ge.The correlation observed between the Ge-phase nanostructure and

the high-energy absorption edge of the nanocomposites is there-fore consistent with the effects of carrier confinement, in this casedue to the presence of the oxide interface. The trends observed inthe higher energy (near UV) optical absorption onset with nanos-tructure can be explained in this context.

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Qualitative trends in absorption behavior can be observedhrough examination of the high-energy absorbance onset. Com-aring different thin film designs (i.e. different absolute Geputtering exposure times used to generate the Ge layer dur-ng deposition), the absorbance onset of the as-deposited 4Ge:ITOpecimens are at higher energies than the onset of identically pro-essed 15Ge:ITO specimens (Fig. 5a and b) This is consistent withn increased energy gap associated with a smaller, more discon-inuous Ge-phase observed for composites with the 4 Å Ge layer.uch an interpretation of the as-deposited film behavior, however,ust also infer the presence of an interfacial structure (even before

nnealing) that would enhance carrier confinement.The correlation between nanostructure and optical absorption

dge position is further illustrated in films annealed at 250 ◦C.n this case no change in structure was observed via TEM oraman spectroscopy and correspondingly the absorption onset

s unchanged. Thermal treatments did produce blue-shifts in thebsorption onset if the thermal treatment is performed at a tem-erature sufficient to influence the nanostructural morphology thee-phase (as indicated by TEM and Raman shown in Figs. 2 and 4,

espectively).The lowest temperature observed to result in a blue-shift in

bsorption onset of the 4Ge:ITO (Fig. 5a) is 310 ◦C. While crystalliza-ion (see XRD, Fig. 3) of ITO and Ge may contribute to a reductionn structural defects and a general sharpening of the absorptiondge [34,35], the primary contributor to the blue-shift in this cases associated with the changing interfacial structure of the Ge-phasend its impact on carrier confinement conditions within the film.lthough the TEM micrograph (Fig. 2c) does not indicate a dra-atic modification in the germanium phase assembly with anneal

t this temperature, the Raman analysis of these films does indicatehe development of resonances associated with the GeOx structure,onfirming the increased development of the associated interfacialxide. While more time-dependent thermal treatments and mea-urements are needed to confirm this trend, it is likely that theonsumption of the Ge during oxidation would also tend to reducehe effective size of the confining volume available within the Geanocrystal domain. Moreover, oxidation in regions bridging adja-ent, larger Ge islands could result in oxidation-induced breaks inhe Ge-phase, promoting a more discontinuous Ge extended struc-ure. The lack of this abrupt change in geometry (dimensionality)f the confining volume would explain why the absorption onsetf the 15Ge:ITO specimen is not impacted by the identical ther-al treatment at this temperature (Fig. 5b), even though an oxide

f the same thickness is expected to have formed. In this case thehicker initial, as-deposited Ge layers (thickness = 15 Å) require anncreased extent of oxidation to fully severe Ge bridges betweendjacent Ge-phase domains within the layer.

When annealed at 550 ◦C, the higher temperature also promotesignificant solid-state diffusion of Ge and ITO components, addingo the oxidation process. The resulting more dramatic break-up ofhe Ge morphology (see Fig. 2) is accompanied by a large blue-hift in absorption onset of the 4Ge:ITO specimen. This is consistentith the formation of isolated, carrier-confining regions producingconcomitant change in the electronic structure of the Ge. The blue-hift is not as significant in the 15Ge:ITO specimens treated at thisemperature as a larger degree of continuity within the extendede-phase assembly is retained in this case.

.3. Electrical and optoelectronic properties

The near-IR reflection edge (Fig. 6c) qualitatively agrees withhe measured free electron carrier density of the composite films.

ithin a Drude model framework [36], the relationship of ωp to ne

s given by: ωp2 = nee2/meε, where e, me, and ε is the charge of an

lectron, electron effective mass, and material permittivity, respec-

ngineering B 175 (2010) 150–158 157

tively. Since the reflection onset is proportional to the square rootof ne

Ge:ITO (the line in Fig. 6c represents an ideal carrier densitydependence), the Drude interpretation applies.

The general trend of increasing n-type carrier density observedin both nanocomposite and single-phase ITO specimens with ther-mal anneal is attributed to the general improvement in crystallinitywith heat-treatment (see Fig. 3) and a corresponding reduction indefect trapping centers within the ITO. In this case, the matrix mate-rial is expected to serve as the primary charge transport mediumfor the composite. However, Fig. 7a shows that the compositefilms have significantly greater carrier concentrations than simi-larly treated single-phase ITO films. Germanium is a known dopantof ITO or In2O3 [37,38]. While Ge atomistic donors have not beenruled out, the “doping scheme” in the present study is expected tobe different since the matrix material (ITO) and dopant (Ge) are notsimultaneously co-deposited. Also, given that the Ge content is wellabove the solubility limit of Ge in ITO (provided the Ge solubilityis similar to that reported for In2O3 (<0.5 atomic% [18])), the for-mation of a separate Ge material phase is expected to be favoredupon thermal annealing. Considering incomplete confinement ofthermally equilibrated electrons within the Ge electronic energystructure (due to electron tunneling through the oxide interfacialphase described above), electron movement from the Ge to the ITOis anticipated to lead to an increase in the equilibrium carrier pop-ulation within the ITO resulting in an increased free carrier densityin the nanocomposites compared to similarly treated single-phaseITO films.

In contrast to the carrier density trends observed with anneal-ing, the mobility of the ITO films is not significantly impacted bythe short (5 min) thermal treatment. The mobility of the nanocom-posite films, however, exhibits a general reduction in free carriermobility below that of the single-phase ITO. Thermal annealingfurther decreases carrier mobility in the nanocomposites. Thereduced carrier mobility in the nanocomposites, and the downwardtrend with annealing, is consistent with increased carrier scatter-ing within the evolving, high interfacial area, nanoheterogeneouscomposite system.

The opposing trends observed in carrier density and mobilitywith annealing result in a relatively consistent total resistivity inthe thermally treated nanocomposite materials. Resistivity val-ues are of the same order as single-phase ITO films, indicatingthat these materials offer an opportunity to manipulate spectralabsorbance while maintaining electrical conductivity levels com-parable to the single-phase transparent oxide conductors. In thisrespect, the present nanocomposite strategy enables the decou-pling of optical behavior from electronic characteristics, a key pointof compromise in photovoltaic heterojunction materials based onsingle-phase components.

These important optical and electronic characteristics are com-bined with a measurable photoconductive response. Referring toFig. 8, the 475 nm (2.6 eV) photoexcitation is used in the presentstudy because it is lower in energy than the band-gap of ITO, yetwithin the Ge absorption spectral range. Moreover, excitation atthis photon energy does not initiate a photoconductive responsein single-phase ITO films (Fig. 8). This excitation energy is near theE1 direct band-edge transition of bulk Ge and it has been used todemonstrate optoelectronic behavior in other Ge nanophase con-taining systems [39,5]. While only observed in a limited range ofsample configurations (4Ge:ITO, 550 ◦C anneal), a photoconductiveresponse in this excitation energy range supports the developmentof a photoexcited, non-equilibrium electron density associated

with photoexcitation of electrons within the Ge-phase and thetransfer of carriers to a continuous ITO phase for transport to elec-trical contacts.

The persistent photoconductivity (PPC) of the 4Ge:ITO specimensubjected to a 550 ◦C anneal is also of interest in understanding the

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ontribution of nanostructure to photoconductivity response. It isoted that a similar photoconductive response with illuminationime has been observed in titania:Ge nanocomposites [40] using ahite light source. In this earlier work, the PPC response was asso-

iated with spatial charge separation of electrons from holes intohe two different material phases. PPC, in fact, has been described inerms of macroscopic potential barriers that can occur at interfaces41].

In the present study, the energy structure of the Ge–ITO interfacebased on bulk parameters) will likely favor electron transfer fromhe Ge to the ITO phase. Even if tunneling through an oxide-basednterfacial layer is inferred, the lower effective mass of the electronhould still result in its preferential promotion (over the holes) tohe ITO matrix. The microstructure of the 4Ge:ITO specimen afterhe 550 ◦C anneal (Fig. 2e), indicates that the Ge nanocrystallitesre isolated within the embedding ITO matrix. The presence of PPCithin these specimens therefore suggests that the probability for

ecombination of the photocarriers within the Ge-phase (antici-ated to be a fast process given the finite spatial extent of thetructure and associated overlap in photocarrier wavefunctions)s limited once the photoexcited electron moves into the ITO. Inhis case, other mechanisms will also contribute to the relaxationf excited carriers and the associated recovery of dark conductivityevels. This assessment is, however, preliminary. Careful analysisf the photoconductivity decay kinetics in specimens with var-ed Ge extended structure (and carrier confinement conditions)s now underway to further assess the impact of spatial chargeeparation.

. Conclusions

The influence of post-deposition isochronal annealing on thes-deposited nanostructure and corresponding optical and elec-ronic behavior of semiconductor–transparent conductive oxideGe-ITO) nanocomposite thin films deposited via sequential RF-puttering was examined. The technique was successful in theroduction of semiconductor-phase spatial distributions rangingrom isolated Ge nanocrystals to two-dimensional-extended struc-ures of semiconductor embedded in ITO, dependent on both thes-deposited nanocomposite structure and the thermal processingonditions used. A clear correlation between the high-energy opti-al absorption onset and the Ge-phase length scale and extendedssembly within the composite was attributed to quantum-sizeffects within the semiconductor. Carrier confinement in thee:ITO system, in which the component phases have similar bulklectron affinity values, is attributed to an oxide-based interfa-ial structure at the Ge:ITO boundary that is enhanced by thermalreatment.

A high structural sensitivity was also observed in the carrierransport properties of the thin films. An increasing free carrierensity and decreasing mobility was observed with increasing

sochronal heat treatment temperature (and, hence, nanostructuralevelopment) for both of the as-deposited composite film designsxamined. These opposing trends resulted in a relatively consistentesistivity over the range of nanostructures developed. Moreover,he examined nanocomposite films retained an overall resistiv-ty that was similar to that of a single-phase ITO films, despitehe high interfacial area characteristics of this nanoheterogeneous

aterial system. These results thus indicate the potential to manip-

late the optical behavior of the nanocomposite independent of

ts electrical properties through control of the semiconductor-hase extended structure. These properties, coupled with theemonstration of a photoconductive response in such systems,ncourage future efforts to explore these types of composite mate-

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ngineering B 175 (2010) 150–158

rials for use in a variety of optoelectronic applications, includingphotovoltaics.

Acknowledgements

The authors acknowledge the support of the State of ArizonaTRIF Solar Initiative, the Arizona Research Institute for Solar Energy(AzRISE), and Science Foundation Arizona’s Solar Technology Ini-tiative. We also thank J. Madocks, M. George, and H. Chandra atGeneral Plasma, Inc. and the LeRoy Eyring Center for Solid StateScience at Arizona State University for their assistance. One of us(G.H.S.) acknowledges the support of the DoD Science, MathematicsAnd Research for Transformation (SMART) program.

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