Journal of ELECTRONIC MATERIALS, Vol. 45, No. 12, 2016
DOI: 10.1007/s11664-016-5023-2 2016 The Minerals, Metals & Materials Society
Fourier Transform Infrared Spectroscopy Measurements of Multi-phonon and Free-Carrier Absorption in ZnO POONEH SAADATKIA,1,2 G. ARIYAWANSA,3 K.D. LEEDY,3 D.C. LOOK,3 L.A. BOATNER,4 and F.A. SELIM 1,2,5 1.—Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA. 2.—Department of Physics and Astronomy, Bowling Green State University, Bowling Green, OH 43403, USA. 3.—Air Force Research Laboratory, Sensors Directorate, WrightPatterson Air Force Base, Dayton, OH 45433, USA. 4.—Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. 5.—e-mail:
[email protected]
Fourier transform infrared (FTIR) measurements were carried out on thin films and bulk single crystals of ZnO over a wide temperature range to study the free-carrier and multi-phonon infrared absorptions and the effects of hydrogen incorporation on these properties. Aluminum-doped ZnO thin films were deposited on quartz substrates using atomic-layer deposition (ALD) and sol–gel methods. Hall-effect measurements showed that the ALD films have a resistivity of q = 1.11 9 103 X cm, three orders of magnitude lower than sol– gel films (q = 1.25 X cm). This result is consistent with the significant difference in their free-carrier absorption as revealed by FTIR spectra obtained at room temperature. By reducing the temperature to 80 K, the free carriers were frozen out, and their absorption spectrum was suppressed. From the FTIR measurements on ZnO single crystals that were grown by the chemical vapor transport method, we identified a shoulder around 3350 cm1 and associated it with the presence of two or more hydrogen ions in a Zn vacancy. After reducing the hydrogen level in the crystal, the measurements revealed the multi-phonon absorption of ZnO in the range of 700–1200 cm1. This study shows that the multi-phonon absorption bands can be completely masked by the presence of a large concentration of hydrogen in the crystals. Key words: FTIR spectroscopy, ZnO, free-carrier absorption, multi-phonon absorption, hydrogen vibrational modes
INTRODUCTION ZnO is considered to be one of the most promising oxides in the optoelectronics field.1–4 It has a direct wide band gap (3.3 eV)5 and emits light in the blue and near-ultra violet (UV) regions. Because of higher 60 meV binding energy of excitons in ZnO, compared to a value of 25 meV in GaN, ZnO is thought to be more favorable for applications in UV light-emitting and laser diodes.6 Additionally, ZnO is one member of a family of transparent conducting oxides, which are materials that are widely used in the fabrication of numerous devices including (Received August 8, 2016; accepted October 1, 2016; published online October 21, 2016)
transparent electrodes, sensors, and transparent contacts on solar cells.7,8 Un-doped, as-grown ZnO typically exhibits n-type conductivity, while p-type ZnO with reproducible properties and low resistivity has not yet been developed.9 Large acceptor activation energies of typical p-type dopants such as nitrogen10,11 or phosphorous, as well as high n-type background doping due to hydrogen impurities12 and Zn interstitials,13,14 are considered to be the main issues in hindering p-type doping of ZnO. In fact, hydrogen is one of the most common impurities in oxide materials due to the strong bonding of H atoms to an O atom,15 and is also a significant origin of natural n-type conductivity in ZnO.12,16 Theoretical12,16,17 and experimental18,19 studies have indicated that hydrogen can diffuse into the bulk of ZnO 6329
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where it acts as a shallow donor at the interstitial sites20 and oxygen vacancy sites instead of simply passivating dopants.12,21,22 In addition, hydrogen can passivate native defects and greatly alter the ZnO optical and electronic properties. The study of hydrogen location and its effects on the properties of ZnO are, therefore, of major interest.23,24 Infrared (IR) spectroscopy has been extensively applied to measure the vibrational modes of hydrogen in ZnO.25,26 McClusky et al. have measured an O–H vibrational line at 3326 cm1 for H2-annealed commercial cermet ZnO samples and associated this line with hydrogen in an antibonding configuration (AB^) with a transition dipole moment oriented at an angle of 110 to the c axis.27,28 By increasing the temperature, the mode became broader and shifted to a higher frequency of 3336.8 cm1 at room temperature.27 Later, this interpretation was challenged in Ref. 29 and we will discuss it later in this paper. Three other IR absorption lines at 3611.3 cm1, 3349.6 cm1 and 3312 cm1 were identified at T = 10 K in hydrogen-treated chemical vapor transport (CVT)-grown ZnO samples by Lavrov et al.,30 of which the first line was assigned to a center containing an O–H bond aligned with respect to the c axis of the crystal (BCk), and the other two modes were identified as local vibrational modes (LVMs) of a zinc vacancy having two hydrogen atoms.30 Both the LVM lines of 3326 cm1 and 3611 cm1 were observed to be unstable with regard to thermal annealing,26,28,30,31 and they are also sample-dependent, i.e. they can generally be observed in H-annealed samples but the ratios are related to the type of sample. For instance, the 3611 cm1 line is strong in commercial Eagle-Picher samples while Cermet samples reveal a strong 3326 cm1 peak implying that one of the peaks is coming from H paired with another impurity.26,32 Another strong and stable IR hydrogen-related defect has been reported at 3577 cm1 at T = 10 K in hydrothermally grown ZnO; this defect consists of a single O–H bond aligned with the c axis of the crystal.33 With respect to un-doped ZnO nanoparticles, six hydrogen-related LVMs were observed at 2990 cm1, 3315 cm1, 3351 cm1, 3329 cm1, 3351 cm1, 3425 cm1, and 3572 cm1; all of these disappeared after annealing at 800C.34 Two of these features (i.e., 2990 cm1 and 3425 cm1) were found to be associated with the symmetric and antisymmetric stretching modes of C–H and also hydrogen stretching modes in ammonia molecules. The lines at 3315 cm1 and 3351 cm1 had been previously reported as a hydrogenated Zn vacancy saturated with two hydrogen atoms, and the LVMs at 3329 cm1 and 3572 cm1 were suggested to be related to a hydrogen donor in an anti-bonding configuration and hydrogen located at a H-I* defect.34 These previous studies have provided valuable information about the local vibrational modes of hydrogen in ZnO. In the present work, we focus on the results of FTIR measurements of the
free-carrier and multi-phonon infrared absorption of ZnO, and we show that hydrogen concentration in the crystal plays a significant role in the spectra. In comparison with IR spectroscopy, FTIR is the preferred method for this type of study due to its faster scanning process,35 higher sensitivity, greater optical throughput than that offered by grating monochromators,36 and internal calibration. For the free-carrier absorption studies, measurements were carried out on conductive ZnO films doped with Al and grown by atomic layer deposition and sol–gel methods. Hall-effect measurements were performed on the same films to provide a comparison with the FTIR data. Multi-phonon and vibrational spectroscopy studies were mainly performed on high-quality ZnO single crystals that were grown by the chemical vapor transport method to increase the signal intensity and enhance the identification of vibrational modes. EXPERIMENTAL Aluminum-doped ZnO films were deposited on 3in (c.7.6-cm)- and 2-in (c.5.1-cm)-diameter quartz substrates using atomic layer deposition (ALD) and sol–gel methods. Details of the growth processes are given elsewhere.37,38 In order to improve the conductivity, a few films were annealed in a low flow rate of hydrogen at 400C for 1 h, since hydrogen is expected to passivate deep acceptors such as Zn vacancies. This process only improved the conductivity in sol–gel films whereas the ALD films were highly conductive even before hydrogen annealing and no further improvement was observed. The film thickness was 163 nm for the ALD films and about 600 nm for the sol–gel films. Most of the ZnO single crystals used in this study were grown by the chemical vapor transport (CVT) method. Chemical vapor transport is a suitable and simple technique to grow bulk single crystals39–41 and is especially useful for materials with a low vapor pressure like ZnO, where high temperature is essential for crystal growth in the absence of an intermediate reaction. In this work, the crystal growth is performed through the reduction of polycrystalline spheres of ZnO that are heated to 1250C in an alumina tube in a mixture of flowing hydrogen and either argon or nitrogen carrier gases. The initial interaction with hydrogen gas produces zinc vapor and the nitrogen or argon carrier gas transports the Zn vapor to a cooler region of the growth chamber where the crystals are formed. Prior to the FTIR measurements, a number of CVT samples were annealed in various atmospheres as follows: (1) in air at 1100C for 1 h, (2) at low-rate H2 flow at 400C for 2 h, and (3) at high-rate H2 flow at 400C for 2 h. For comparison purposes, a few measurements were also carried out on commercial ZnO single crystals that were grown from the melt and purchased from Cermet. The thickness of the bulk single crystals was about 0.5 mm.
Fourier-Transform Infrared Spectroscopy Measurements of Multi-phonon and Free-Carrier Absorption in ZnO
FTIR measurements were carried out using a Thermo-scientific spectrometer with a KBr beam splitter. The spectra were recorded over a wide temperature range from liquid helium to room temperature. For low temperatures, the samples were placed in a Janis continuous-flow liquidhelium cryostat. The spectral range varied from 700 cm1 to 5000 cm1 and two different detectors were used in the experiments, a Deuterated TriGlycine Sulfate (DTGS) detector and a MCT (mercury cadmium telluride) detector which is more sensitive to IR light in the range of 600– 5000 cm1. RESULTS AND DISCUSSION In Fig. 1, we present the full range of the FTIR spectra as measured under ambient conditions for three types of samples: a ZnO thin film on a quartz substrate annealed in low concentration of hydrogen, a ZnO single crystal grown by the CVT method, and a ZnO single crystal grown from the melt. We
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will focus on specific regions in the spectra in subsequent figures to discuss the free-carrier and multi-phonon absorptions and the associated effects of hydrogen concentration. The y axis of the graphs represents the percentage of light absorbed in the sample. The single-crystal spectra show the ZnO absorption edge at about 1100 cm1; it is sharper in the CVT samples as compared to the Cermet samples due to the higher impurity levels present in the latter which lead to reduced transparency. The peaks at 1600 cm1 and 3350 cm1 are background-related lines since the room-temperature measurements were carried out in air. The spectrum of the film in the region from 700 cm1 to 2000 cm1 is dominated by the strong absorption of the quartz substrate, which absorbs all light in this region, as illustrated below. It would be interesting to remove the effect of the quartz substrate and reveal the features of the IR spectrum of the films in this region. In future experiments, one can consider depositing ZnO films on different substrates that are transparent in this IR range.
Fig. 1. The full range of FTIR spectra measured at room temperature: (a) Al:ZnO ALD film annealed in low concentration of H2; (b) ZnO single crystal grown by the chemical vapor transport technique; (c) commercial ZnO single crystal grown from the melt and purchased from Cermet.
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Fig. 2. (a) FTIR spectra of Al:ZnO films from 2000 to 5000 cm1 at room temperature. (b) Comparison of the FTIR spectra of Al:ZnO film grown by the sol–gel method at room temperature and at 80 K.
Study of Free-Carrier Absorption The first measurement of the free-carrier absorption in un-doped ZnO single crystals was carried out by Thomas in 1959 using a contact-free technique in order to probe the free-carrier concentration during his annealing experiments.42 Later, the correlation between the conductivity and free-carrier absorption was investigated in hydrogenated ZnO nanoparticles by McCluskey et al.25 In their work, a significant increase in the conductivity of ZnO pellets was observed with a large increase in the free-carrier absorption after a hydrogen anneal confirming the fact that hydrogen donors diffuse into the ZnO nanoparticles. However, similar annealing treatments did not significantly influence the conductivity of ZnO bulk single crystals.25 These infrared absorption measurements indicated the presence of free-carrier absorption in the 1111– 3333 cm1 spectral region for the higher conductivity bulk ZnO samples. Free-carrier absorption measurements were also used to verify that adding lithium to a ZnO single crystal makes it into a semiinsulating material.43 Here, we have used FTIR methods to compare the free-carrier absorption in two conductive ZnO films that were grown by two different methods, i.e., ALD and sol–gel techniques. The films were intentionally doped with Al to induce additional donors into the ZnO and thereby increase the conductivity. We are not aware of any study of the free-carrier absorption in ZnO films. Figure 2a shows the room-temperature IR absorbance of Aldoped ZnO films in the range of 2000–5000 cm1; this shows the absorption of a sol–gel film, an asgrown ALD film and an annealed ALD film. To reveal the effect of the substrate, the IR spectra of the quartz substrate were also measured and are plotted in Fig. 2a. It can be seen that all of the ALD films have a higher free-carrier absorption than the sol–gel film despite the three times thicker sol–gel films compared to the ALD films. This result is in
agreement with the resistivity obtained from Halleffect measurements that yielded a value of q = 1.11 9 103 X cm for the as-grown ALD film and q = 1.25 X cm for the sol–gel film. The wavelength dependence in all of the ZnO films represents the longitudinal optical phonon scattering and ionized dopants scattering.44 It is represented by kz where the exponent z equals around 1.1 for asgrown ALD film, 1.3 for H2-annealed ALD film, and 1.8 for sol–gel film. For the ALD films, the sample annealed in hydrogen shows a slightly smaller freecarrier absorption than that of the as-grown sample, a result that is also consistent with the resistivity measurements which is q = 1.20 9 103 X cm for the hydrogen-annealed ALD film in comparison with 1.11 9 103 X cm for the as-grown ALD film. This result illustrates that the conductivity in these films is dominated by Al donors and not by hydrogen impurities, as in the case of un-doped ZnO.26 Figure 2b compares the FTIR absorption spectra at 80 K and room temperature for the sol–gel film and shows the ‘‘freeze out’’ of donors at low temperature and the suppression of their absorption. Multiphonon Infrared Absorption of ZnO and Hydrogen Effect The multi-phonon absorption bands originate from the fundamental frequencies of pure crystals and have been studied previously in ZnO and other similar materials with the wurtzite structure such as GaN and AlN42,45–49 These materials have strong carrier-phonon coupling and strong Restrahlen-related absorption of radiation due to the interaction with optical phonons.49 For ZnO, there are two different infrared-active lattice vibrations due to the polar c axis of ZnO including xk when the two sublattices move against each other in the c-direction and x^ which belongs to a vibration perpendicular to the c axis.47 The anisotropy of the crystal that is related to the restrahlen frequency is also expected
Fourier-Transform Infrared Spectroscopy Measurements of Multi-phonon and Free-Carrier Absorption in ZnO
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to take place in the multi-phonon absorption spectrum.47 Figure 3 shows the FTIR absorbance spectra in the range from 700 cm1 to 1500 cm1 at 6 K for two ZnO single crystals annealed in different atmospheres. One sample was annealed in a high concentration of H2 gas flowing at 400C and the other sample was annealed in an O2 atmosphere at 1100C. The figure reveals a significant difference in the FTIR peaks in the region of 700–1100 cm1. The multi-phonon absorption bands in the oxygen-annealed sample are obvious while they are completely obscured in the H-annealed sample. Many of the peaks identified in the oxygen-annealed sample are consistent with the reported values for the multi-phonon absorption of ZnO.47,48 At room
temperature (not shown in the figure), the peaks are shifted by
