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J. Phys. Chem. 1995,99, 15262-15268
An Optically Detected Magnetic Resonance Study of Laser Ablation Grown PbI2 Epitaxial Films E. Lifshitz,* L. Bykov, and M. Yassen Department of Chemistry and Solid State Institute, Technion, Ha fa, 32000, Israel Received: June 13, 1995; In Final Form: August 7, 1 9 9 9 Epitaxial films of PbIz were grown by the laser ablation technique. The correlation between certain emission processes and lattice imperfections was examined, utilizing optically detected magnetic resonance (ODMR) spectroscopy. The present work focuses on the characterization of the nonexcitonic green and red emission bands. The results suggested that these emission processes are associated with a common donor state. Moreover, this donor state is participating also in nonradiative donor-acceptor recombination process. The quenching in luminescence intensity, pronounced in the ODMR measurement, indicates that the experiment detected actually the nonradiative recombination. Thus, the ODMR measurements identified a hole trapped at an isotropic nonradiative acceptor site with gh 2r05, that corresponds to the lead vacancy defect, [V-Ipb+2. The photogenerated electron is trapped in an anisotropic radiative donor site with gell x 2.34 and g e l x 1.73, associated with the iodine vacancy, [VOIidine.
I. Introduction Lead iodide, PbI2, is a direct bandgap semiconductor with a layered structure. Compounds exhibiting a structure of this kind possess strong intralayer bonding and only weak, so-called, van der Waals interlayer interactions. The layers can be stacked in a variety of ways to form different polytypes. The most common polytype is the 2H hexagonal structure. The bandedge optical properties of PbI2 single crystals have been studied extensively for about two decades'-4. Excitation of this material is dominated by the creation of an exciton with a cationic character. Previous electronic band structure calculations have implied that this cationic behavior originates from the nature of the band edge; the conduction-band edge arises predominantly from Pb, 6p atomic orbitals, while the valence band edge arises from Pb, 6s orbital^.^ The low-temperature luminescence of PbI2 single crystal consists of a series of exciton lines in the region of -2.5 eV and additional broad luminescent bands at lower energies centered at 2.44 eV (green) and 2.07 eV (red), respectively. While both the static and dynamic properties of the excitons have been investigated thoroughly, there is very little knowledge regarding the broad bands. Bibik et ale6have suggested that the green band consists of overlapping luminescing events of donor-acceptor recombination, associated with lattice imperfections. Goto et al.' have suggested that the latter band is associated with a self-trapped exciton, while Watanabe et aL8 have indicated that this band is associated with excitons trapped at a crystal imperfection, overlapping the donoracceptor recombination emission. Baltog et al.9 propose that the red band is associated with surface imperfections. In any event, to the best of our knowledge, there is no direct identification of the imperfection sites. Of late, the interest in PbI2 has been extended to the study of epitaxial films and nanometer-sized particles. As reported in recent publications, epitaxial films or superstructures(PbIDiI3) are prepared by thermal evaporation,'O while nanometer-sized particles embedded in various media (zeolite cages, Si02 films, or polymers) are achieved by sol-gel or deposition techniques.]' Although it has been shown that the absorption and photoluminescence spectra of epitaxial and nanometer-sized particles exhibit similar properties to those of the bulk,"*12 several *Abstract published in Advance ACS Abstracts, September 15, 1995.
0022-365419512099-15262$09.00/0
differences arise for the following reasons: (1) When the epitaxial film thickness or the particle radius approaches the size of the exciton Bohr radius, size quantization takes place.I3 The latter is pronounced as a blue shift in the optical transit i o n ~ . ' ~ (2) . ' ~Due to the large ratio difference between surface/ bulk atoms in the epitaxial and nanoparticles samples, there may be a proportionally larger amount of surface defects. These are capable of acting as traps for photogenerated excitons, holes, or electrons and may alter the recombination processes. The present work investigates the recombination processes of PbI2 epitaxial film prepared by the laser ablation technique. The research focuses on the identification of donor and acceptor sites which give rise to the green and red luminescence bands appearing below the exciton manifold. The research utilizes optically detected magnetic resonance spectroscopy (ODMR) in combination with the conventional luminescence technique. In the ODMR method, one measures change in luminescence intensity due to a magnetic resonance event at the excited state. This technique, therefore, enables us to correlate between certain recombination processes and local crystallographic defects. The identified sites are associated with intrinsic or surface defects. This will be elaborated on below.
11. Experimental Section II.a. Materials. Epitaxial films of PbI2 were prepared by depositing the material on a glass or mica substrate, via the laser ablation technique. The deposition process was carried out in a special vacuum Torr) chamber containing an optical window, a deposition carrousel, a source-material crucible, and a temperature controller. The substrate temperature ranged from 50-80 "C, while the source crucible was kept at 80 "C. The ablation process was performed by focusing a 457.9 nm Ar+ laser with a 3.0 Wlcm2 power output on the surface of the source material (a 2H-PbI2 single crystal). The crystallographic properties of the prepared epitaxial films were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The thickness of the epitaxial film was determined by the a-step technique. II.b. Instrumental. The ODMR spectra were obtained by measuring the change in luminescence intensity, AIPL (or its circular component), induced by a magnetic resonance event at 0 1995 American Chemical Society
Laser Ablation Grown PbI2 Epitaxial Film the excited state. Thus, NPL was plotted versus the strength of the extemal magnetic field, Ho, leading to magnetic resonancelike spectra. The induced change was monitored either at a specific emission energy or through the detection of the total luminescence band. The magnetic resonance conditions were achieved by mounting the sample on a special sample probe, containing a “home made” TEIII microwave cavity, resonated at 10.8 GHz, with an unfilled Q of approximately 3000. The microwave source was a synthesized sweeper, amplified by solid-state components, and loop-coupled to the cavity via a low-loss cryogenic coaxial cable. The cavity itself was located at the center of a split Helmholtz superconductor magnet (Hobetween 0 and 25 kG), enabling optical access. The whole sample probe was immersed in a cryogenic dewar and was cooled to 1.4 K by superfluid helium. Samples were excited by a continuous (cw) 457.9 nm Ar+ laser with a power output of 0.5-550 mW/cm2. Emitted light was dispersed through a holographic grating monochromator and was detected by a photomultiplier or Si PIN photodiode. The green emission was selected either by the utilization of a band-pass filter (Oriel BP-520) or via the grating monochromator. The microwave power output was amplitude modulated at audio frequencies (30 Hz-17 KHz) and NPL was detected using a conventional lock-in operation at the microwave chopping frequency. The ODMR signal was detected in either of the following configurations: (a) in a direction, kern , parallel to the extemal magnetic field ($, II Ho,Faraday configuration), or (b) in a direction perpendicular to HO(kmI Ho, Voight configuration). In the Faraday configuration, the change in the circular polarization component of the luminescence intensity was detected. The ODMR spectra were recorded as a function of the angle between the c axis and the extemal magnetic field when the magnetic field was rotated in the (1 120) crystallographic plane. The ODMR spectra were also recorded at different audio frequencies and various laser excitation powers. 111. Results
The epitaxial films of PbI2 were grown by the laser ablation technique (section 11) and their crystallographic quality determined by SEM and XRD before any optical measurements were performed. The SEM measurement revealed an hexagonal morphology of the film surface, while XRD c o n f i i e d the crystallographic structure to be hexagonal (Dwsymmetry) with the c axis (the stacking axis of the layers) normal to the substrate. The symmetry of the films coincides mostly with that of the 2H polytype. However, occasionally some mixed polytypism has been observed. Even in the latter case the films maintain the direction of the c axis normal to the substrate plane. Relatively thick films (100-500 nm) were chosen for the present study. The photoluminescence (PL) spectra of three different PbI2 epitaxial films, recorded at 1.4 K and excited with 457.9 nm (2.706 eV) Ar+ laser, at laser power of 200 mW/cm2 are shown in Figure 1. Curve (a) demonstrates clearly the dominant luminescence bands. It comprises a free exciton (FE), a bound exciton (BE), and two broad bands (green and red) below the exciton manifold. The latter spectrum is similar to a bulk single crystal. Moreover, the central energies of the excitonic bands correspond to those of the bulk 2H-PbI2 polytype. As the thicknesses of the epitaxial films were > 100 nm, quantum size effect has been excluded in the present case. Although the spectrum (b) was recorded on an epitaxial film which was prepared under conditions identical to those of spectrum (a),
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a :
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Figure 1. Photoluminescence (PL) spectra of PbI2 epitaxial films. The corresponding samples of spectra (a) and (b) were prepared under identical conditions. The difference between them is discussed in the text. The sample corresponding to the spectrum (c) was subjected to postannealing in iodine vapors for a duration of about 6 h.
curve (b) seems to be broadened at the high-energy side of the green band, resulting in partial overlapping of the exciton region. Spectrum (c) in Figure 1 was recorded on sample (b) after annealing under iodine vapor pressure (at 250 “C and inert conditions). It can be seen from spectrum (c) that the broadening at the high energy side of the green band has been reduced compared to curve (b), while the red band luminescence was almost quenched upon annealing in the iodine atmosphere. The contour shape of spectrum (c) is similar to that of spectrum (a). Moreover, postannealing at 250 “C without the iodine atmosphere did not change the spectral line shape. Evidently, the broadening of the high energy side of the green band and red band are associated with an iodine deficiency. The epitaxial films were excited with various laser excitation powers. The integrated intensity of the green band revealed a linear dependence on the laser power when excited with 457.9 nm Ar+ laser; the intensity of the band increased with increasing excitation power. However, the intensity of the red band increased more gently than that of the green band with increasing laser power. In other words, the relative intensity of this band with respect to the green diminished with increasing laser power. A typical ODMR spectrum of the PbI2 epitaxial film, recorded at the Voight (kern I Ho)configuration and monitoring the total green luminescence is shown in Figure 2. The spectrum consists of three resonance signals, labeled I, 11, and 111, associated with quenching of the luminescence intensity (negative ODMR signal). The most intense ODMR spectrum was obtained on the luminescence curve shown in Figure lb. The corresponding ODMR spectrum of the green band, recorded at the Faraday configuration (kern I I Ho) with circular polarization detection (not shown), appeared to be identical to that observed under Voight configuration. Moreover, the right (a+)and left (0-) circular polarization components of the ODMR in the latter configuration seem to be identical to each other. It should be
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Figure 2. Representative ODMR spectra of PbI2 epitaxial films, obtained under detection of the total green-band luminescence with = 457.9nm. The spectrum was recorded excitation wavelength of ,lex in the Voight configuration when the angle between the static magnetic field and the c axis was 0 = 50". Inset: electron and hole spin energy levels. Microwave induced and D-n.r.A recombination transitions are shown by the solid and dashed lines, respectively.
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Figure 4. Representative ODMR spectra, recorded in the Vioght configuration with dex= 457.9 nm, at three different orientations of the c axis with respect to Ho:(a) 0 = 75', (b) 0 = 50", (c) 0 = 0". The solid lines correspond to theoretical fits, as discussed in the text.
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Figure 3. Spectral dependence of resonance I (squares)and resonance I1 (triangles), respectively. The solid lines represent the corresponding luminescence band.
noted that the ODMR spectrum monitored at the redluminescence band (either in the Voight or Faraday configurations) appeared to be identical to the spectrum shown in Figure 2. However, the ODMR of the red band could be observed only under low excitation power (550 mW/cm2. The intensities of the ODMR resonances, AZPL(I)and AZPL(II) were monitored at several different emission energies. These values were plotted versus the emission energy for the resonance signals in Figure 3 (squares and triangles, respectively). These plots exhibit the spectral dependence of the ODMR signals. As a comparison, Figure 3 includes also the PL spectrum, as a solid line. Due to the weakness of resonance 111, it was impossible to plot its spectral dependence accurately. Because of the similarity of the ODMR spectra of the green and red luminescence bands, the following paragraphs will represent other experimental evidence associated with the green luminescence alone. However, the common mechanism of the green and red emission bands will be explained in the Discussion. The anisotropy of the resonance signals was examined by recording the ODMR spectra at various angles (0) between the
c-crystallographicaxis and the direction of the external magnetic field (Ho). The ODMR spectra recorded in the Voight configuration monitoring the green band with 0 = 75" , 0 = 50", and 0 = 0" are shown in Figure 4a-c, respectively. It is seen from these spectra that resonance I1 is isotropic, relatively narrow with a full width half-maximum (fwhm) of 20 G and a g factor of 2.05. Resonance I is rather broad (fwhm w 650 G) and shows anisotropic behavior. The g factor of resonance I ranges between 2.22 and 2.34. Resonance I11 in Figure 4 also appears to be broad and anisotropic, with a g factor of about 1.73. The smooth dashed curves in the Figure correspond to theoretical simulation of resonances I III, as will be discussed in section IV. The ODMR spectra of the PbI2 epitaxial film were recorded at various audio frequency modulation of the microwave power, ranging from 30 Hz to 17 kHz. Representative ODMR spectra of the green band, recorded at the Voight configuration with audio frequencies of 4.7 kHz and 3 11 Hz, are shown in Figure 5a,b, respectively. It is seen from the figure that the three resonance signals respond differently to the changes in the microwave power modulation frequency. Figure 6 shows representative ODMR spectra of the green band recorded in the Voight configuration, under different laser excitation powers. While resonance I1 seems to appear as a negative ODMR signal in the studied laser power range, the signals of resonances I and I11 altered from negative, to derivative-like, to positive, with decreasing laser excitation power. The results show some correlation between the anisotropy of resonances I and III. As will be discussed in section IV,these resonances are associated with the same defect site. It should be indicated that the change of sign from negativeto-positive of resonance I, when recorded in the red band occurs at laser power range 20-100 W/cm2.
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IV. Discussion The luminescence spectra l a and l b were recorded on samples grown under identical conditions. Thus, slight varia-
Laser Ablation Grown PbI2 Epitaxial Film
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Figure 5. Representative ODMR spectra of PbIz epitaxial films = 457.9 nm, under recorded in the Voight configuration with lex different microwave power modulation frequencies: (a) 7.14 kHz,(b) 311 Hz.
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V.6. Figure 7. Schematic drawing of several radiative and nonradiative recombination states. n.r. corresponds to the nonradiative recombination process, while G and R correspond to the green and red radiative transitions, respectively.
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Figure 6. Representative ODMR spectra of the PbI2 epitaxial films recorded at various laser excitation powers as indicated on the figure.
tions in their appearance may be associated with intrinsic defects. Moreover, post treatment with iodine vapors resulted in the conversion of spectrum l b into IC. Thus, a part of the green luminescence and the red band are associated with iodine deficiency. Previous work with the layered iodides15 suggests the consideration of external impurities such as 02-in substitutional position or 13- complex. The existence of 02-can be excluded by carrying the growth and postannealing treatment either in vacuum or under inert conditions, respectively. The 13-, found as a byproduct in iodine chemistry, may act as an acceptor site in the present case. However, its location in the interstitial site is excluded because of the large size of this complex, while its location on the surface will be excluded in the discussion below, showing that the acceptor site is mainly intrinsic and located at a centrosymmetric site. The latter arguments suggest that the recombination centers, identified in the ODMR experiments, are associated with intrinsic defects alone. It was mentioned in section 111that the contour shape as well as the sign of the ODMR resonances detected at the green and red bands were identical. The coincidence in the contour shape may suggest that the green and red bands are associated with radiative recombination processes, sharing at least one common state. The luminescence spectra shown in Figure 1 suggests
that this common state is an iodine vacancy, acting as a donor site in PbI2 epitaxial films. If the observed ODMR spectrum is associated with one of the radiative emission processes, detection of the change at one luminescence band, upon magnetic resonance conditions, was supposed to show enhancement of luminescence intensity, while the other band was supposed to show quenching of this intensity. However, the experimental evidences exclude this possibility, since the ODMR spectra of both bands are associated with quenching of the luminescence intensity (see Figure 2). This suggests that a third nonradiative process is connected to both the green and red luminescence emission. In other words the ODMR method in the present case, detected a radiative and nonradiative recombination centers. Previous observations of the quenching of the luminescence intensity in ODMR experiment on various other materials were also correlated with nonradiative processes.l6 On the basis of the above arguments, an altemative model, presented in Figure 7, can be proposed: The common radiative single donor state corresponds to iodine vacancy, [V0],dine,while the common nonradiative (n.r.) state may be an acceptor site. Confirmation of these donor and acceptor sites associated with the nonradiative recombination will be discussed through the description of the ODMR spectrum analysis. As mentioned in the introduction, the green and red emission bands are mainly associated with radiative donor-acceptor recombination, and thus the radiative processes (labeled G and R in Figure 7) involve the existence of additional radiative acceptor sites (vide infra). The latter sites may be diamagnetic and therefore will be not pronounced in the ODMR spectra. Indeed, several reports in the past have suggested that PbI2 is a p-type semiconductor, containing an excess acceptor site^.'^.'^ The paragraphs below discuss the analysis of the ODMR spectrum and identify the radiative donor (D) and nonradiative acceptor (n.r.A) sites, participating in the nonradiative recombination process (D-n.r.A). The ODMR spectra presented in section I11 exhibit three resonance transitions. The experimental results discussed above indicate that resonance 11 always exhibits unique behavior in angle dependence (Figure 4), audio frequency dependence (Figure 5 ) , and spectral dependence (Figure 3), while resonances I and 111 show some correlated dependence (anisotropy and audio frequency dependence). These results reveal that ODMR signals are associated with electrons and holes trapped at two different defect sites with relatively weak exchange interaction between them. Generally, a weak electron-hole exchange interaction indicates a spin manifold in the excited state containing independent spin quantum numbers for the electrons and holes and excludes the possibility of a triplet manifold. Moreover, the observation
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of an identical spectrum when monitored in Faraday configuration with u+ and u- polarization excludes the possibility of the existence of AM, = f l transitions. Thus, the ODMR spectrum detected in the present work, whether monitored in the Voight or Faraday configuration, corresponds to the Ah4, = 0 (n)transitions and requires that each one of the trapped carriers (electron and hole) have a spin quantum number Ms= l/2. The relevant spin manifold is drawn at the inset of Figure 2. The solid arrows correspond to the spin resonance transitions, while the dashed arrows represent the donor-acceptor nonradiative transitions. The detection of the ODMR spectra at various emission energies and the corresponding spectral dependence indicate that resonance I is associated with virtually the entire green luminescence band, barring only a small energy region at the low-energy site, while it does cover the entire red band. On the contrary, the spectral dependence of resonance I1 coincide with the entire green and red bands. The green band under investigation may contain additional emission processes overlapping the D-n.r.A pair recombination. The dependence of resonance (I) on the excitation intensity supplies further support of its participation in D-n.r.A pair nonradiative recombination. The spin manifold represented in the inset of Figure 2 considers the simplest case in which the donor and n.r. acceptor sites are separated by a specific distance, rl. In the sample, however, there is an ensemble average of D-n.r.A distances (rI- ~ i ) ,which means an ensemble average of recombination times ( z D . ~ , ~ . A ) .Thus, under relatively high excitation power ('200 mW/cm2) the ODMR method mostly detects the nearest D-n.r.A pairs. In the latter case ZD.".?.A
