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Electrical and luminescent properties of ZnO:Bi,Er ceramics sintered at different temperatures
Journal of Luminescence 104 (2003) 103–114
Electrical and luminescent properties of ZnO:Bi,Er ceramics sintered at different temperatures J.C. Ronfard-Haret S.P.M.S., Ecole Centrale Paris, Grande voie des vignes, F 92295 Chatenay-Malabry cedex, France Received 14 June 2002; received in revised form 5 November 2002; accepted 3 December 2002
Abstract The electrical and luminescent properties of a series of polycrystalline sintered ZnO pellets containing small amounts of Bi and Er oxides have been studied as a function of the sintering temperature, between 950 C and 1100 C. The current–voltage curves of the pellets are non-linear; the conductivity of the pellets increases with the applied voltage in good agreement with the results of the literature reported for ZnO:Bi composites. They indicate the presence of electrically active grain boundaries, in which an excess charge of majority carriers is trapped at the interface. All the pellets are photoluminescent and electroluminescent, and in addition the pellets sintered at 1050 C and 1100 C are triboluminescent. The photoluminescence spectra under ZnO band-to-band excitation do not depend on the sintering conditions and show only the broad pattern characteristic of ZnO. The electroluminescence spectra are a mixture of the broad pattern attributable to ZnO and of sharp lines characteristic of the Er3+ ion. The relative intensity of the signals arising from ZnO and the rare earth depends on the sintering temperature. For a low sintering temperature, the broad ZnO spectrum predominates, whereas for a high sintering temperature, only the Er3+ sharp lines can be observed. The abrasion of pellets sintered at 1050 C and 1100 C results in an emission of green light. The triboluminescence spectrum of the pellet sintered at 1100 C is identical to its electroluminescence spectrum and shows only the emission of the Er3+ ion. The comparison between the electro- and the photoluminescence spectra shows that electroluminescence arising either from ZnO or Er3+ ions is due to direct hot electron impact excitation. Comparison between the tribo- and electroluminescence spectra shows that triboluminescence of the Er3+ ions has an electrical origin. The triboluminescence of Er3+ ions is proposed to result from a wrenching of the ZnO grains inducing intense electric fields across the grain boundaries cleavage in Er3+-rich regions in good accordance with the presence of electrically active grain boundaries. The Er3+ electroluminescence evidences the presence of hot electrons for applied voltages far below the breakdown voltage. The grain boundary barrier model usually put forward to account for the electric properties of ZnO-based varistors is unable to explain the electroluminescent properties of the ZnO:Bi,Er sintered pellets. Consequently, the electroluminescence of the Er3+ ions is proposed to result from an electric conduction along the grain boundaries parallel to the field, involving hot electrons in Er2O3-rich regions. r 2003 Elsevier Science B.V. All rights reserved. Keywords: ZnO; Electroluminescence; Triboluminescence; Electric properties; Hot electrons; Grain boundaries
E-mail address: [email protected] (J.C. Ronfard-Haret). 0022-2313/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00685-3
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1. Introduction Zinc oxide varistors are ZnO-based ceramic semiconductor devices with highly non-linear current–voltage characteristics [1–4]. They are fabricated by sintering ZnO powders with small amounts of additives such as Bi, Co, Mn, Sb and rare earth (RE) oxides. Among all the additives, some metal ions such as Mn2+ can substitute for Zn2+ ions in the lattice, whereas others such as Bi3+ or RE3+ remain located between the ZnO grains in the intergranular material because of their charge and size. The basic structure of ZnO varistors is formed by adding Bi2O3 to ZnO. Bi2O3 allows the trapping of electrons at the surface of the ZnO grains and makes potential barriers at the grain–grain contact. ZnO-based varistors, and therefore ZnO:Bi composites, are approximated as a stacking of good conductive cubic grains separated by grain boundaries [1–4]. Their electrical properties arise from the succession of grains and grain boundaries. A voltage applied to a device is divided into individual voltage drops, each corresponding to an elemental grain/grain boundary cell. Owing to the good conductivity of the grains, the entire applied voltage is sustained in narrow depletion layers at the surface of the positively biased grains. Electrons trapped at the boundary between the grains exchange with valence electrons. The conduction results from thermionic emission across the barriers and the non-ohmicity of the devices (non-linearity of the current–voltage curves) is a consequence of a lowering of the barriers when the applied voltage increases [1–5]. The electrical properties of varistors depend on the sintering temperature. An increase in the sintering temperature induces a decrease in the non-ohmicity of the current–voltage curves and an increase in the leakage current [1,3,6,7]. Trivalent RE ions (RE3+) exhibit a wide variety of luminescence phenomena [8]. They are commonly used in ZnS or ZnSe high-field electroluminescent devices [9]. RE3+ ions are inserted in ZnS or ZnSe where their excitation is due to a direct hot electron impact. Electrons, injected into the conduction band of the semiconductor, are accelerated to optical energies in the high electric field and impact-excite the RE3+ ions [9].
The presence of hot electrons in polycrystalline ZnO was predicted in theoretical studies [4]. Hot electrons were evidenced by both the ZnO bandgap and subband-gap electroluminescence (EL) in a ZnO-based varistor operating under breakdown conditions [10]. Electrons that cross a grain boundary are injected into the conduction band of the positively biased grain. They are accelerated in the depletion layer close to the boundary where they can gain enough kinetic energy to impactionize the semiconductor, leaving holes in the valence band. Due to the band bending, the holes move back to the boundary where they de-trap the trapped electrons responsible for the barrier and induce a further lowering of the barrier leading to a hole-induced breakdown. In addition, the holes left in the valence band are responsible for the observed luminescence. The subband-gap EL in a ZnO-based varistor operating under breakdown conditions appears under the microscope, as an array of luminescent centres [4]. The field is not uniform inside the device. The purpose of this paper is to correlate the luminescent properties of the material to its electrical properties and to its structure. It reports on a systematic study of the luminescence properties of a series of ZnO:Bi,Er ceramics sintered between 950 C and 1100 C with reference to the brick layer/grain boundary model of varistors. The EL is compared to the photoluminescence (PL) and to the triboluminescence (TL). The comparison between the PL and the EL evidences a highfield EL. The comparison between the EL and the TL evidences the electrical nature of the TL excitation mechanism. The EL of the Er3+ ion is used as a probe, which evidences the presence of hot electrons at low applied voltage. The location of the Er3+ ions between the ZnO grains evidences also the presence of current flows between the ZnO grains along the boundaries.
2. Experimental A mixture of ZnO (Aldrich 99.99% pure), Er2O3 # (Rhone-Poulenc 99.5% pure) and Bi2O3 (Aldrich 99.9% pure) powders was ground in an agate mortar in the presence of a small amount of
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ethanol. All the pellets were made from a single ground mixture of oxides (Er3+=0.12 at% of Zn2+ and Bi3+=0.75 at% of Zn2+) by pressing at 10 MPa, 500 mg of the dry powder for each pellet in a cylindrical matrix with a Specac press. Batches of three pellets were sintered at five different temperatures (950 C, 975 C, 1000 C, 1050 C and 1100 C) for 3 h under air in a Vecstar muffle furnace. The pellets are cylindrical. Before sintering, their diameter is 13 mm. Sintering reduces their size, and after calcination their diameter is equal to 10.9, 10.7, 10.7, 10.6 and 10.670.1 mm for the five above-mentioned temperatures, respectively. No significant difference is found in their thickness, which is always 170.05 mm. The PL and EL measurements were performed with a Perkin-Elmer MPF-44 spectrofluorimeter. The PL spectra were recorded from the flat circular face of a pellet located in the standard front surface accessory attached to the fluorimeter prior to any other measurement. A device, similar to the device described in Ref. [10], is next used for EL measurements. The pellets are electroded by covering both flat circular faces with In–Ga alloy before being mounted on a bakelite holder where the electric contacts are achieved by means of copper wires. Then, the edge of the pellets is abraded in order to obtain a flat rectangular surface (1 8 mm), perpendicular to the parallel circular surfaces of the pellets. The abrasion of the pellets reduces their circular surface (and volume) by ca. 10%. The bakelite holder is next put in the spectrofluorimeter in such a way that the image of this flat rectangular surface is focused on the entrance slit of the emission monochromator. For each experiment, the voltage, the current and the relative light intensity are measured simultaneously in order to allow accurate comparisons. The voltage and the current are measured with a digital voltmeter and microammeter, respectively. The TL spectrum of the pellet sintered at 1100 C was recorded during the abrasion of the pellet using a home-made spectrophotometer [11]. The scanning electron micrography analysis was performed with a LEO 1530 apparatus at the CECM, CNRS (Vitry, France).
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All experiments were performed at room temperature. Since the electrical conduction in ZnObased varistors and in RE3+-doped polycrystalline sintered ZnO is thermally activated [1–5,12,13], the pellets were maintained at room temperature by blowing a nitrogen flow in order to avoid an increase in their temperature during the EL measurements. For the highest applied voltages, an increase in the temperature results usually in a high current surging, which leads to an irreversible change in the electrical properties of the pellets (large decrease in resistance and loss of the electroluminescent properties).
3. Results and discussion 3.1. Scanning electron micrography The pellets are polycrystalline and were analysed using scanning electron micrography in order to measure the average grain size (/lS) and the number (ng ) of consecutive grains or grain boundaries in series through the thickness of the pellets. /lS is obtained using a classical intercept method with a correction factor equal to 1.56 [14] and ng is the ratio between the thickness of the pellets and the average grain size. The values of /lS and ng are reported in Table 1. /lS increases from 11.6 to 22 mm when the sintering temperature increases from 950 C to 1100 C. The grain size and its increase with the sintering temperature are in good agreement with the results reported in the literature for Bi-containing ZnO varistors. For instance, the grain size of ZnO-based varistors increases from 21.8 to 27.2 mm when the sintering temperature increases from 1150 C to 1350 C [6]. The grain growth inhibiting effect of RE2O3 reported in previous publications [12,13,15] is overwhelmed by the presence of Bi2O3, which is known as a liquid-phase sintering aid. In ZnO-based varistors, a breakdown occurs when the applied voltage reaches a value corresponding to the product of the ZnO band gap energy Eg by the number of grains or grain boundaries in series through the thickness of the device [1–3]. In the present case, a theoretical breakdown voltage VBr can be estimated for the
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Table 1 Characteristics of the pellets Sintering temperature ( C) /lS (mm) ng VBr (V) R0 (O) a100 mA V100 mA (V) FB0 (eV)
950 11.6 86 275 530 103 5 150 0.71
975 13.8 72 230 150 103 5 150 0.68
1000 15.5 65 210 65 103 4 110 0.66
1050 18.6 54 175 15.6 103 4 80 0.63
1100 22 45 145 9.5 103 7 65 0.62
/lSis the average grain size. ng is the number of grains or grain boundaries through the thickness of the 1 mm thick pellets VBr is the theoretical breakdown voltage.R0 is the zero-field resistance of the pellets. a is the non-linear coefficient. FB0 is the barrier height under zero bias.
five pellets under study. The VBr values calculated using Eg ¼ 3:2 eV are reported in Table 1. 3.2. Current–voltage characteristics The DC current–voltage characteristics (i2V curves) of the pellets are reported in Fig. 1. The measurements were restricted to current intensities lower than 100 mA owing to the power dissipated in the pellets. Consequently, the breakdown voltage could not be reached for all the pellets. In good agreement with the results of the literature, the curves are non-linear [1–7]. The resistance of the pellets decreases when the voltage increases. The curves can be characterized by the zero-field resistance of the pellets (R0 ) and a nonlinear coefficient a=d(ln i)/d(ln V) [1–4]. The R0 and a values are reported in Table 1 together with the voltage conditions corresponding to the determination of a: a increases with the voltage applied to the pellets, the a values reported in Table 1 are given for a current equal to 100 mA. The results are in good agreement with the results published by Bhushan et al. [6]. The maximum a value, obtained for ZnO:Bi composites operating under breakdown condition is usually close to 10. Bhushan et al. reported an increase in a (from 6 to 12) with the Bi content (from 0.5 to 5.0 mol%) for ZnO–Bi2O3 systems sintered between 900 C and 1100 C whereas Eda [3] stated that the a values never exceed 10 owing to the absence of a non-linearity enhancement agent such as Mn or Co oxide. In the present case, the values of Table 1 (a ¼ 427) are relatively low. But they are obtained at low (Bi+Er) concentra-
Fig. 1. Current–voltage characteristics of the pellets sintered at 950 C (m), 975 C (K), 1000 C (’), 1050 C (E) and 1100 C (.).
tions and at a voltage well below the theoretical breakdown voltage reported in Table 1. Within the frame of the double Schottky barrier/thermionic emission model, the current flow J across a grain boundary barrier submitted to a voltage drop U is given by [4,5] J ¼ A T 2 expðFB =kTÞ½1 expðqU=kTÞ ; ð1Þ where A and k are the Richardson’s and Boltzmann’s constants, T is the temperature, FB is the barrier height and q is the charge of the electron. For the lowest applied voltages, qU5kT; Eq. (1) reduces to J ¼ ðA TqU=kÞexpðFB0 =kTÞ;
ð2Þ
where FB0 is the barrier height under zero bias. The energy band diagram of a grain boundary
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under zero bias and under an applied polarization U is presented in Fig. 2a and b, respectively. The knowledge of both R0 and the number of barriers in series through the thickness of the pellets allows determination of FB0 : R0 decreases from 530 to 9.3 kO and the number of barriers from 86 to 45 when the sintering temperature increases from 950 C to 1100 C. The FB0 values are reported in Table 1. FB0 decays from 0.71 to 0.62 eV. Such a lowering in the barrier height when the sintering temperature increases has been already proposed to account for the sintering temperature dependence of the electrical properties of both ZnO:Bi composites and ZnO-based varistors. It is attributed to a volatilization of Bi during the sintering [1,3,6,7].
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At this point, it appears that the electrical properties of the pellets are in good agreement with former results obtained on Er-free ZnO:Bi composites [6]. They are correctly interpreted in terms of the existing model: a succession of ng grain boundary barriers separated by good conducting grains. There is apparently no special influence of the small amount of Er oxide on the electric properties of the pellets. The size of the Bi3+ ion is close to that of the Er3+ ion; the charge of both ions is identical. Bi2O3 and some RE2O3 such as La and Sm form solid solutions [16]. In ZnO-based varistors, both Bi3+ and RE3+ ions remain located in the boundaries at the surface of the ZnO grains [1,3,15,17]. The most probable is a dispersion of Er atoms in the Bi-rich intergranular material. 3.3. PL spectra and mechanism The PL spectrum of the pellet sintered at 1050 C, under ZnO band-to-band excitation at 365 nm, is reported in Fig. 3. The other pellets sintered at different temperatures give the same broad spectrum under the same excitation conditions. This broad spectrum, centred at 600 nm is characteristic of ZnO and has been widely studied [18]. Several PL mechanisms have been proposed recently for polycrystalline or powder samples: (i) a recombination between electrons from the
Fig. 2. Energy band diagram of a double Schottky barrier at a grain boundary under zero bias (upper, 2a) and under an applied polarization U close to the voltage corresponding to the ZnO band-gap energy (lower, 2b). The lower diagram shows the hot electron impact excitation of RE3+ ions dispersed in the depletion layer of the positively biased grain. Q represents the charges trapped at the interface. Ec is the bottom of the conduction band, Ev the top of the valence band and Ef is the fermi level energy. The other symbols are defined in the text.
Fig. 3. PL spectrum of a ZnO:Bi,Er pellet sintered at 1050 C under ZnO band-to-band excitation at 365 nm.
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bottom of the conduction band or shallow donor levels and holes trapped on deep levels [19,20], (ii) a recombination between holes in the valence band and electrons from the singly ionized level of oxygen vacancies [21], (iii) an electron–hole recombination involving a donor–acceptor complex consisted of zinc and oxygen vacancies [22]. Basically, the PL of ZnO is related to the presence of holes in the valence band. The band-to-band excitation of ZnO promotes electrons from the valence band to the conduction band and the holes left in the valence band induce the ZnO PL. The surface, or the depletion layers at the surface, of the ZnO particles, is involved in the PL mechanism [20–23]. For instance, the photogenerated holes can move to deep levels inside the gap in a two-step process via their trapping at O2 surface sites [20]. The spectrum shows a structure at 520 nm. This structure corresponds to the 4I15/2-2H11/2 transition of the Er3+ ion and is a consequence of a slight reabsorption by the Er3+ ions, of the light emitted by ZnO [24]. It cannot be due to an emission from the 2H11/2 or 4S3/2 level (2H11/2-4I15/2 or 4 S3/2-4I15/2 transition). The 2H11/2 and 4S3/2 levels are thermalized [25]; both emissions appear simultaneously in all the Er3+ emission spectra. In the present case, only a single peak is seen on the spectrum. The absence of a second peak shows unambiguously that the structure which appears at 550 nm does not correspond to an emission from the 2H11/2 or 4S3/2 level. The reabsorption of the ZnO luminescence by the RE3+ ions in ZnO:RE sintered samples has been demonstrated by the comparison between diffuse reflectance and emission spectra [24]. The reabsorption of emitted light occurs also for Ho3+ and Nd3+; it is not followed by any RE re-emission. In Er-doped polycrystalline sintered ZnO, the presence of holes in the valence band of ZnO is unable to induce any Er3+ luminescence [26]. There is no energy transfer from ZnO to the Er3+ ion.
the pellet sintered at 1100 C is the most intense. The spectrum recorded during the abrasion of this pellet is presented in Fig. 4 together with an EL spectrum of the same pellet recorded using the same apparatus. Both spectra are identical and characteristic of Er3+ ion. The emission spectra of the RE3+ ions have been widely studied and the attribution of the spectra of Fig. 4 can be readily performed [25]. They correspond to the 2H11/2-4I15/2 and 4S3/2-4I15/2 transitions of the Er3+ ion at 530 and 560 nm, respectively. The 2H11/2 and 4S3/2 levels of the Er3+ ion are thermalized, so that the relative intensity of the lines at 530 and 560 nm depends only on the energy difference between the 2 H11/2 and 4S3/2 levels and on the temperature. The TL mechanism locates the RE3+ ions between the grains in the intergranular material. It agrees with the presence of electrically active grain boundaries and the structure analysis of both sintered RE-doped ZnO [15] and ZnO-based varistors containing RE [17]. In semiconductors such as Ge and Si, bond ruptures are able to induce a variety of phenomena including generation of voltage and currents, and emission of light of various durations and wavelength [27] and current burst across the cleavage and luminescence occur also on cleaving III2V semiconductors [28,29]. The energy band diagram of a mated but not healed crack region of a semiconductor is
3.4. Er3+ TL spectrum and mechanism The pellets sintered at 1050 C and 1100 C are triboluminescent. They show an emission of green light when they are abraded. The light emitted by
Fig. 4. TL (upper) and EL (lower) spectra of the pellet sintered at 1100 C. Both spectra are recorded using the same apparatus. The EL spectrum is recorded under an applied voltage equal to 50 V.
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similar to the energy band diagram of a grain boundary under zero bias presented in Fig. 2a [27]. In the case of polycrystalline sintered ZnO containing RE3+ ions, the grain boundaries form a lattice of mechanical defects throughout the material. The mechanical stress of the samples results in cleavages along the grain boundaries between the grains in the RE3+-rich regions. A consequence of the abrasion of the samples is a wrenching of ZnO grains producing an intense electric field across the grain boundary cleavage sufficient to cause the dielectric breakdown of the intergranular material. The TL of the RE3+ ions arises from their location in fragile regions between the grains. In addition, sheet conductivity at grain boundaries was reported for polycrystalline Ge and Si [30] showing that charged grain boundaries in semiconductors are able to present both conduction and TL properties. The identity between the TL and EL spectrum evidences the electrical origin of the excitation of the Er3+ TL, the location of Er3+ ions in the intergranular material between the grains and the presence of hot electrons in the boundaries between the grains. 3.5. EL spectra The EL spectra of the different pellets are shown in Fig. 5. They are a mixture of a broad pattern attributable to ZnO and of sharp lines attributable to the Er3+ ion. They show the emission corresponding to the 4F9/2-4I15/2 transition of the Er3+ ion at 660 nm in addition to the emission originating from the thermalized (2H11/2,4S3/2) levels at 530 and 560 nm [25]. The relative intensity of the emissions originating from ZnO and the Er3+ ion depends on the sintering temperature. The broad ZnO pattern predominates for the lowest sintering temperature whereas only the sharp lines of the Er3+ ion are detectable for the highest sintering temperature. No other pattern was detected, specially around 390 nm. The broad pattern attributable to ZnO in the EL spectrum of the pellets sintered at 950 C, 975 C and 1000 C shows few structures between 640 and 690 nm. These structures correspond to the reabsorption by the Er3+ ions (4I15/2-4F9/2 transi-
Fig. 5. EL spectra of a series of ZnO:Bi,Er sintered pellets. The electrical conditions (applied voltage, intensity) are: 160 V, 75 mA; 135 V, 70 mA; 110 V, 75 mA; 80 V, 80 mA; 70 V, 100 mA for the pellets sintered at 950 C, 975 C, 1000 C, 1050 C and 1100 C, respectively.
tion), of the light emitted by ZnO in addition to an emission of the Er3+ ions (4F9/2-4I15/2 transition). 3.6. Er3+ EL mechanism The voltage dependence of the EL intensity arising from the Er3+ ion and ZnO is presented in Fig. 6. The Er3+ EL is observed at 560 nm (4S3/2-4I15/2 transition) and 530 nm (2H11/24 I15/2 transition) for the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C. The ZnO EL is observed at 620 and 720 nm for the pellets sintered
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Fig. 6. Voltage dependence of the light B emitted by the Er3+ ion (filled symbols) at 530 nm (2H11/2-4I15/2 transition) for the pellet sintered at 975 C (K), 560 nm (4S3/2-4I15/2 transition) for the pellets sintered at 1000 C (’), 1050 C (E) and 1100 C (.), and ZnO (open symbols) at 620 nm for the pellet sintered at 950 C (n), and at 620 (J) and 720 nm (}) for the pellet sintered at 975 C.
below 1000 C. The EL voltage threshold decreases when the sintering temperature increases. For all the pellets, the EL arising either from ZnO or the Er3+ ion is observed for voltage ranges comprised between 25 and 150 V. The thickness of all the pellets is 1 mm; then the average field inside the pellets remains comprised between 250 and 1500 V cm1. EL in semiconductors occurs basically in two forms: injection and high-field EL [7]. In injection EL, the emission of light is consecutive to the recombination of majority carriers with injected minority carriers. In high-field EL, the excitation of luminescent centres results from an impact of majority charge carriers accelerated under high electric fields of the order of 106 V cm1. The injection of minority carriers in Er-doped polycrystalline sintered ZnO results in a ZnO characteristic EL spectrum similar to the PL spectrum: a broad pattern centred at 600 nm and a sharp emission at 380 nm; the spectrum shows no Er3+ emission [26]. In the present case, the Er3+ EL cannot result from an electron–hole pair recombination with a subsequent energy transfer from the semiconductor to the Er3+ ion since the PL spectrum which results from an electron–hole
pair recombination shows no Er3+ emission. Then, the EL is not an injection EL; it is rather a high-field EL despite the low value of the average electric field. In ZnO-based varistors, whose basic structure (sintered ZnO:Bi) is close to the present pellets, the field is inhomogeneous [1–7,10,17]. As pointed out before, ZnO:Bi composites and ZnO-based varistors are usually approximated as a stacking of good conducting ZnO grains separated by grain boundaries. Due to the high conductivity of the grains, the entire applied voltage is sustained across narrow regions in the depletion layer of the forward-biased grains close to the boundaries where the field is intense despite its low average value. Then, electrons injected in the forwardbiased grains can pick up a large amount of kinetic energy necessary to impact-excite the luminescent centres (Fig. 2b) and to impact-ionize ZnO. This model is proposed to account for the breakdown of ZnO-based varistors [10]. The application of this model to the EL of RE3+ ions inserted in polycrystalline sintered ZnO has been discussed in detail in previous articles [12,13]. A Destriau-type relationship has been proposed to account for the voltage dependence of the emitted light (B), owing to the energy distribution of hot electrons in semiconductors, which follows Boltzmann’s statistics [31]: lnðB=iÞp ng Eexc: =qV :
ð3Þ
Eexc: is an energy threshold for impact excitation. It is the energy of the emitting level in the case of a direct impact excitation. However, this mechanism is contradictory to some observations which have been presented in more recent articles and which have led to propose an excitation of the RE3+ ions between the grains, in the boundaries parallel to the field, in RE-rich regions [32,33]. The main observations dealing with the ZnO:Bi,Er systems are summarized below. The Er3+ ion EL occurs at low applied voltage, well below the breakdown voltage. In the present case, the voltage threshold of the Er3+ EL is 25, 50, 50 and 70 V for the pellets sintered at 1100 C, 1050 C, 1000 C and 975 C, respectively. From the ng values of Table 1, these values correspond to
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an average individual polarization /US close to 500 mV, well below the energy necessary to impact excite the Er3+ ion in its visible range emitting levels. The energy of the thermalized (2H11/2, 4S3/2) emitting levels of the Er3+ ion is close to 2.5 eV [25]. Then, either the barriers are strongly unequal in order to allow a few number of grain boundaries to sustain the entire applied voltage or the model of Fig. 2b does not apply; the excitation of the Er3+ ion does not occur in the depletion layer of forward-biased grains. The analysis, according to Eq. (3), of the Destriau plots (Fig. 7) of the light emitted by the Er3+ ion in the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C, gives unrealistic values for the product ng Eexc : This product, calculated from the data of Fig. 7, is close to 460 for the pellets sintered at 975 C, 1000 C and 1050 C and 230 for the pellet sintered at 1100 C. This leads to Eexc ¼ 6:3; 7.0, 8.5 and 10.2 eV for the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C, respectively. Such excitation energies are far above the energy of the thermalized (2H11/2, 4S3/2) emitting levels of the Er3+ ion. The model of Fig. 2b fails to give a satisfying quantitative explanation to the results. In contrast with the electrical properties, the luminescence properties of the pellet sintered at 1100 C are close to those already obtained with
Fig. 7. Destriau plot according to Eq. (4) of the Bn =i ratio for the pellets sintered at 975 C (K), 1000 C (’), 1050 (E) and 1100 C (.). The intensity of the Er3+ luminescence is measured at 530 (2H11/2-4I15/2 transition) or 560 nm (4S3/2-4I15/2 transition).
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Bi-free ZnO:Er sintered pellets [11]. Bi2O3 has apparently no influence on the luminescent properties of the pellet. The PL spectrum is the emission spectrum of ZnO (Fig. 3). The EL and TL spectra are the emission spectrum of the Er3+ ion (Fig. 4). The Er3+ EL is observed at low voltage (Fig. 6). The identity between the EL and the TL spectrum evidences an EL excitation mechanism which does not involves an electron impact of Er3+ ions in the depletion layer of a forward-biased grain in a strongly polarized grain boundary (Fig. 2b). Then, identical conclusions can be drawn for Bi-free and Bi-doped ZnO:Er pellets. The Er3+ EL is a consequence of current flows along the boundaries in an Er-rich material. This current flowing along the boundaries was proposed by Levinson to account for the discrepancies between the model and the results observed during the pre-breakdown of ZnO-based varistors [34]. The efficiency Z of an electroluminescent system is defined as the ratio of the number of emitted photons by the number of transferred charges [35]. Then, the variations of Z are given by the variations of the B=i ratio. Z is the product of the excitation efficiency Zexc by the luminescence efficiency Zlum and by the optical efficiency Zopt [35]: Z ¼ Zexc Zlum Zopt :
ð4Þ
The optical efficiency is related to the experimental conditions used to observe the luminescence; it remains constant for a device operating under identical optical conditions. The luminescence efficiency is mainly related to the deactivation process of the emitting centres; it also remains constant for a given device. Only the excitation efficiency, which is defined as the ratio of the number of excited luminescent centres by the number of transferred charges, can depend on the voltage applied to the pellets. From Fig. 7, it appears that Z; and consequently Zexc ; increase with the applied voltage for the Er3+ EL arising from the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C. Usually, such an increase in Zexc with the voltage applied to an electroluminescent thin film device is a consequence of an increase in the average energy of the hot electrons. For instance, the fraction of hot electrons able to impact-excite
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the luminescent levels of the Er3+ ion in ZnS:Cu: Cl:Er thin films increases exponentially with this average energy [36]. This leads to a voltagedependent spectrum. In the present case, the same explanation cannot be proposed. The Destriau plots of Fig. 7 show that the B=i ratio increases by a factor higher than 102 when the applied potential increases by a factor close to 2 for the pellets sintered at 1000 C, 1050 C and 1100 C. According to Eq. (3), an increase in the B=i ratio close to 102 would be a consequence of an increase in the average energy of the hot electrons, and consequently in the applied voltage, by a factor close to 4.5 and not 2. Since the energy of the (2H11/2, 4S3/2) thermalized levels of the Er3+ ion is close to 2.5 eV, such an increase in the average energy of the hot electrons would lead to the presence of unrealistic voltage drops higher than 10 V, owing to the neglect of the cooling of the hot electrons, inside the pellets when they are submitted to the highest applied voltages (110, 80 and 70 V for the pellets sintered at 1000 C, 1050 C and 1100 C, respectively). Clearly, such voltage drops cannot be present inside the pellets owing to the possible ionization of ZnO. This ionization of ZnO is predicted by the classical grain boundary model of varistors; it is evidenced by the ZnO band-gap EL observed during the breakdown of varistors and restricts the voltage drop at a grain boundary to a value corresponding to the ZnO band-gap energy [10]. However, the presence of current flows along the boundaries, responsible for the impact excitation of the Er3+ ions, can account for both the apparent discrepancies between the electrical and luminescent properties and the large increase in the luminescence intensity with the applied voltage. The measured current i is the overall current, which crosses the pellets. It is responsible for the observed electrical properties. The current flowing along the boundaries, which is related to the electroluminescent properties, is only a fraction of i: Then, Zexc ; which represent the average energy of the hot electrons, is not simply related to the B=i ratio since the overall current should be replaced by the value of the current flowing along the boundaries.
In addition, the examination of the visible range EL under the microscope, during the breakdown of a ZnO-based varistor, shows that the light arises from an array of localized luminescent centres [4]. An increase in the number of these luminescent centres with the applied voltage can also be responsible for the increase in the luminescence intensity. But in any case, the low voltage threshold of EL remains unexplained within the frame of a succession of ng consecutive luminescent centres with identical electrical properties. Either the number of luminescent centres is not related to the number of successive grains or grain boundaries in series through the thickness of the pellets, or these luminescent centres differ in their electrical properties in order to allow an increase in their number with the applied voltage. 3.7. ZnO EL mechanism The broad pattern attributed to ZnO in the EL spectrum of the samples sintered at 950 C, 975 C and 1000 C corresponds to electron–hole pair recombination. The different ZnO PL mechanisms involve the surface, or the depletion layer at the surface, of the ZnO particles [19–22], for instance, a hole migration from the conduction band to a deep level in the bulk in a two-step process via a hole trapping at an O2 surface site [20]. In the present case, the absence of the exciton emission band at 390 nm tends to disregard a mechanism involving holes in the valence band [21]. Then, the spectrum corresponds more likely to a recombination between conduction band electrons and holes trapped on deep levels, which can arise either from an impact ionization of the deep levels (direct excitation), or from an impact ionization of the O2 surface site (indirect excitation). An impact ionization of the O2 site at the surface of the grains agrees well with the presence of hot electrons in the intergranular material between the grains. Within the frame of an excitation of the ZnO luminescence by hot electrons accelerated in the depletion layer of the positively biased grains, Eq. (3) can describe the mechanism for either a direct or an indirect excitation. Only the value of Eexc: changes with the excitation mechanism. The value of Eexc: is the value of the band gap energy in
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the case of an impact of valence electrons, and the value necessary for the excitation of the O2 site or for the ionization of the deep levels. An analysis of the Destriau plots of the ZnO luminescence (Fig. 8) according to Eq. (3) gives also unrealistic results, as in the case of the Er3+ luminescence: Eexc: ¼ 7:7 and 5.3 eV for the pellets sintered at 950 C and 975 C, respectively. At this point, it is impossible to ascribe the observed ZnO EL either to an excitation of ZnO inside the depletion layer of the positively biased grains or to an excitation of ZnO inside the intergranular material between the grains. However, as in the case of the Er3+ luminescence, both the large increase in the B=i ratio (102 for the pellet sintered at 950 C and 20 for the pellet sintered at 975 C) and the high Eexc: values favour the hypothesis of an excitation by current flows along the boundaries inside the intergranular material. The change in the EL spectrum with the sintering temperature shows that an increase in the sintering temperature favours the excitation of the Er3+ ion compared to that of ZnO. Depending on the model (excitation inside the intergranular material by current flows along the boundaries or excitation inside the depletion layers of the positively biased grains), this can be a consequence of either a better segregation of the Er3+ ions to the boundaries or a better insertion of the Er3+
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ions inside the ZnO lattice. No firm conclusion can be drawn from the change in the EL spectra.
4. Conclusion The present results show that the structure and the electrical behaviour of ZnO:Bi composites containing a small amount of Er oxide are close to that of the Er-free ZnO:Bi composites whereas their electroluminescent behaviour is close to that of the Bi-free ZnO:Er composites. The classical brick layer/grain boundary model put forward for ZnO-based varistors accounts correctly for both the electrical and the triboluminescent properties of the ZnO:Bi,Er composites but is unable to account for their electroluminescent properties. The Er3+ ions, which act as a probe owing to their luminescent properties, evidence the presence of hot electrons for applied voltages well below the breakdown voltage. The conditions of observation of the Er3+ ion EL, which cannot be explained by the classical grain boundary model, lead to propose an excitation of the Er3+ ions outside the ZnO grains, in the intergranular material, by current flows between the grains in good agreement with the TL of the Er3+ ions, which shows that an excitation mechanism that locates the Er3+ ions outside the ZnO grains in the intergranular material does exist. However, the amount of current in the boundaries between the grains and its voltage dependence cannot yet be pre! cised. In addition, the study shows that the surface of the grains is involved in all the luminescence processes. Acknowledgements The author thanks Dr. J.L. Pastol from CECM, CNRS (Vitry, France) for the SEM measurements and Dr. P. Valat and V. Wintgens for their help in the TL experiments. References
Fig. 8. Destriau plot according to Eq. (4) of the Bn =i ratio for the pellets sintered at 950 C (n) and 975 C (J and }). The intensity of the ZnO luminescence is measured at 620 (n and J) or 720 nm (}).
[1] T. Gupta, J. Am. Ceram. Soc. 73 (1990) 1817. [2] L.M. Levinson, H.R. Philipp, Ceram. Bull. 65 (1986) 639.
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[3] K. Eda, IEEE Electrical Insulation Mag. 5 (1989) 28. [4] F. Greuter, G. Blatter, Semicond. Sci. Technol. 5 (1990) 111. [5] G.E. Pike, C.H. Seager, J. Appl. Phys. 50 (1979) 3414. [6] B. Bhushan, S.C. Kashyap, K.L. Chopra, J. Appl. Phys. 52 (1981) 2936. [7] S.N. Bai, T.Y. Tseng, J. Appl. Phys. 74 (1993) 695. [8] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [9] H.E. Gumlich, A. Zeinert, R. Mauch, Electroluminescence, in: D.R. Vij (Ed.), Luminescence of Solids, Plenum Press, New York, 1998. [10] G.E. Pike, S.R. Kurtz, P.L. Gourley, H.R. Philipp, L.M. Levinson, J. Appl. Phys. 57 (1985) 5512. [11] J.C. Ronfard-Haret, P. Valat, V. Wintgens, J. Kossanyi, J. Lumin. 91 (2000) 71. [12] S. Bachir, J. Kossanyi, C. Sandouly, P. Valat, J.C. Ronfard-Haret, J. Phys. Chem. 99 (1995) 5674. [13] S. Bachir, C. Sandouly, J. Kossanyi, J.C. Ronfard-Haret, J. Phys. Chem. Solids 57 (1996) 1869. [14] M.I. Mendelson, J. Am. Ceram. Soc. 52 (1969) 443. [15] J.C. Ronfard-Haret, K. Azuma, S. Bachir, D. Kouyate, J. Kossanyi, J. Mater. Chem. 4 (1994) 139. [16] E.M. Levin, C.R. Robbins, H.E. McMurdie, in: M.K. Reser (Ed.), Phase Diagrams for Ceramists, The American Ceramic Society, Columbus, OH, 1964. [17] K. Mukae, K. Tsuda, I. Nagasawa, Jpn. J. Appl. Phys. 16 (1977) 1361. [18] S. Bhushan, Zinc oxide, in: D.R. Vij, N. Singh (Eds.), Luminescence and Related Properties in II–VI Semiconductors, Nova Science Publishers, New York, 1998. [19] D.C. Reynolds, D.C. Look, B. Joogai, J.E. Van Nostrand, R. Jones, J. Jenny, Solid State Commun. 106 (1998) 701.
[20] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelenbergh, A. Meijerink, J. Phys. Chem. 104 (2000) 1715. [21] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [22] S.A. Studenikin, M. Cocivera, J. Appl. Phys. 91 (2002) 5060. [23] M. Anpo, Y. Kubokawa, J. Phys. Chem. 88 (1984) 5556. [24] J. Kossanyi, D. Kouyate, J.C. Ronfard-Haret, P. Valat, J. Pouliquen, U. Mammel, D. Oelkrug, G.P. Kelly, F. Wilkinson, J. Lumin. 46 (1990) 17. [25] S. Hufner, . Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, 1978. [26] D. Kouyate, J.C. Ronfard-Haret, J. Kossanyi, J. Electroanal. Chem. 319 (1991) 145. [27] D. Haneman, N.S. Mc Alpine, E. Busch, C. Kaalund, Appl. Surf. Sci. 92 (1996) 484. [28] D.G. Li, N.S. Mc Alpine, D.G. Haneman, Surf. Sci. Lett. 281 (1993) L315. [29] D.G. Li, N.S. Mc Alpine, D.G. Haneman, Surf. Sci. 303 (1994) 171. [30] L.K. Fionova, A.V. Artemyev, Grain Boundaries in Metal and Semiconductors, Les Editions de Physique, Les Ulis, France, 1993. [31] G.A. Baraff, Phys. Rev. 128 (1962) 2507. [32] J.C. Ronfard-Haret, J. Kossanyi, Chem. Phys. 241 (1999) 339. [33] J.C. Ronfard-Haret, J. Kossanyi, J.L. Pastol, J. Phys. Chem. Solids 62 (2001) 565. [34] L.M. Levinson, Ceram. Bull. 68 (1989) 866. [35] R. Mach, G.O. Mueller, Phys. Stat. Sol. 81a (1984) 609. [36] A. Krier, F.J. Bryant, Phys. Stat. Sol. 83a (1984) 315.
Electrical and luminescent properties of ZnO:Bi,Er ceramics sintered at different temperatures J.C. Ronfard-Haret S.P.M.S., Ecole Centrale Paris, Grande voie des vignes, F 92295 Chatenay-Malabry cedex, France Received 14 June 2002; received in revised form 5 November 2002; accepted 3 December 2002
Abstract The electrical and luminescent properties of a series of polycrystalline sintered ZnO pellets containing small amounts of Bi and Er oxides have been studied as a function of the sintering temperature, between 950 C and 1100 C. The current–voltage curves of the pellets are non-linear; the conductivity of the pellets increases with the applied voltage in good agreement with the results of the literature reported for ZnO:Bi composites. They indicate the presence of electrically active grain boundaries, in which an excess charge of majority carriers is trapped at the interface. All the pellets are photoluminescent and electroluminescent, and in addition the pellets sintered at 1050 C and 1100 C are triboluminescent. The photoluminescence spectra under ZnO band-to-band excitation do not depend on the sintering conditions and show only the broad pattern characteristic of ZnO. The electroluminescence spectra are a mixture of the broad pattern attributable to ZnO and of sharp lines characteristic of the Er3+ ion. The relative intensity of the signals arising from ZnO and the rare earth depends on the sintering temperature. For a low sintering temperature, the broad ZnO spectrum predominates, whereas for a high sintering temperature, only the Er3+ sharp lines can be observed. The abrasion of pellets sintered at 1050 C and 1100 C results in an emission of green light. The triboluminescence spectrum of the pellet sintered at 1100 C is identical to its electroluminescence spectrum and shows only the emission of the Er3+ ion. The comparison between the electro- and the photoluminescence spectra shows that electroluminescence arising either from ZnO or Er3+ ions is due to direct hot electron impact excitation. Comparison between the tribo- and electroluminescence spectra shows that triboluminescence of the Er3+ ions has an electrical origin. The triboluminescence of Er3+ ions is proposed to result from a wrenching of the ZnO grains inducing intense electric fields across the grain boundaries cleavage in Er3+-rich regions in good accordance with the presence of electrically active grain boundaries. The Er3+ electroluminescence evidences the presence of hot electrons for applied voltages far below the breakdown voltage. The grain boundary barrier model usually put forward to account for the electric properties of ZnO-based varistors is unable to explain the electroluminescent properties of the ZnO:Bi,Er sintered pellets. Consequently, the electroluminescence of the Er3+ ions is proposed to result from an electric conduction along the grain boundaries parallel to the field, involving hot electrons in Er2O3-rich regions. r 2003 Elsevier Science B.V. All rights reserved. Keywords: ZnO; Electroluminescence; Triboluminescence; Electric properties; Hot electrons; Grain boundaries
E-mail address: [email protected] (J.C. Ronfard-Haret). 0022-2313/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00685-3
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1. Introduction Zinc oxide varistors are ZnO-based ceramic semiconductor devices with highly non-linear current–voltage characteristics [1–4]. They are fabricated by sintering ZnO powders with small amounts of additives such as Bi, Co, Mn, Sb and rare earth (RE) oxides. Among all the additives, some metal ions such as Mn2+ can substitute for Zn2+ ions in the lattice, whereas others such as Bi3+ or RE3+ remain located between the ZnO grains in the intergranular material because of their charge and size. The basic structure of ZnO varistors is formed by adding Bi2O3 to ZnO. Bi2O3 allows the trapping of electrons at the surface of the ZnO grains and makes potential barriers at the grain–grain contact. ZnO-based varistors, and therefore ZnO:Bi composites, are approximated as a stacking of good conductive cubic grains separated by grain boundaries [1–4]. Their electrical properties arise from the succession of grains and grain boundaries. A voltage applied to a device is divided into individual voltage drops, each corresponding to an elemental grain/grain boundary cell. Owing to the good conductivity of the grains, the entire applied voltage is sustained in narrow depletion layers at the surface of the positively biased grains. Electrons trapped at the boundary between the grains exchange with valence electrons. The conduction results from thermionic emission across the barriers and the non-ohmicity of the devices (non-linearity of the current–voltage curves) is a consequence of a lowering of the barriers when the applied voltage increases [1–5]. The electrical properties of varistors depend on the sintering temperature. An increase in the sintering temperature induces a decrease in the non-ohmicity of the current–voltage curves and an increase in the leakage current [1,3,6,7]. Trivalent RE ions (RE3+) exhibit a wide variety of luminescence phenomena [8]. They are commonly used in ZnS or ZnSe high-field electroluminescent devices [9]. RE3+ ions are inserted in ZnS or ZnSe where their excitation is due to a direct hot electron impact. Electrons, injected into the conduction band of the semiconductor, are accelerated to optical energies in the high electric field and impact-excite the RE3+ ions [9].
The presence of hot electrons in polycrystalline ZnO was predicted in theoretical studies [4]. Hot electrons were evidenced by both the ZnO bandgap and subband-gap electroluminescence (EL) in a ZnO-based varistor operating under breakdown conditions [10]. Electrons that cross a grain boundary are injected into the conduction band of the positively biased grain. They are accelerated in the depletion layer close to the boundary where they can gain enough kinetic energy to impactionize the semiconductor, leaving holes in the valence band. Due to the band bending, the holes move back to the boundary where they de-trap the trapped electrons responsible for the barrier and induce a further lowering of the barrier leading to a hole-induced breakdown. In addition, the holes left in the valence band are responsible for the observed luminescence. The subband-gap EL in a ZnO-based varistor operating under breakdown conditions appears under the microscope, as an array of luminescent centres [4]. The field is not uniform inside the device. The purpose of this paper is to correlate the luminescent properties of the material to its electrical properties and to its structure. It reports on a systematic study of the luminescence properties of a series of ZnO:Bi,Er ceramics sintered between 950 C and 1100 C with reference to the brick layer/grain boundary model of varistors. The EL is compared to the photoluminescence (PL) and to the triboluminescence (TL). The comparison between the PL and the EL evidences a highfield EL. The comparison between the EL and the TL evidences the electrical nature of the TL excitation mechanism. The EL of the Er3+ ion is used as a probe, which evidences the presence of hot electrons at low applied voltage. The location of the Er3+ ions between the ZnO grains evidences also the presence of current flows between the ZnO grains along the boundaries.
2. Experimental A mixture of ZnO (Aldrich 99.99% pure), Er2O3 # (Rhone-Poulenc 99.5% pure) and Bi2O3 (Aldrich 99.9% pure) powders was ground in an agate mortar in the presence of a small amount of
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ethanol. All the pellets were made from a single ground mixture of oxides (Er3+=0.12 at% of Zn2+ and Bi3+=0.75 at% of Zn2+) by pressing at 10 MPa, 500 mg of the dry powder for each pellet in a cylindrical matrix with a Specac press. Batches of three pellets were sintered at five different temperatures (950 C, 975 C, 1000 C, 1050 C and 1100 C) for 3 h under air in a Vecstar muffle furnace. The pellets are cylindrical. Before sintering, their diameter is 13 mm. Sintering reduces their size, and after calcination their diameter is equal to 10.9, 10.7, 10.7, 10.6 and 10.670.1 mm for the five above-mentioned temperatures, respectively. No significant difference is found in their thickness, which is always 170.05 mm. The PL and EL measurements were performed with a Perkin-Elmer MPF-44 spectrofluorimeter. The PL spectra were recorded from the flat circular face of a pellet located in the standard front surface accessory attached to the fluorimeter prior to any other measurement. A device, similar to the device described in Ref. [10], is next used for EL measurements. The pellets are electroded by covering both flat circular faces with In–Ga alloy before being mounted on a bakelite holder where the electric contacts are achieved by means of copper wires. Then, the edge of the pellets is abraded in order to obtain a flat rectangular surface (1 8 mm), perpendicular to the parallel circular surfaces of the pellets. The abrasion of the pellets reduces their circular surface (and volume) by ca. 10%. The bakelite holder is next put in the spectrofluorimeter in such a way that the image of this flat rectangular surface is focused on the entrance slit of the emission monochromator. For each experiment, the voltage, the current and the relative light intensity are measured simultaneously in order to allow accurate comparisons. The voltage and the current are measured with a digital voltmeter and microammeter, respectively. The TL spectrum of the pellet sintered at 1100 C was recorded during the abrasion of the pellet using a home-made spectrophotometer [11]. The scanning electron micrography analysis was performed with a LEO 1530 apparatus at the CECM, CNRS (Vitry, France).
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All experiments were performed at room temperature. Since the electrical conduction in ZnObased varistors and in RE3+-doped polycrystalline sintered ZnO is thermally activated [1–5,12,13], the pellets were maintained at room temperature by blowing a nitrogen flow in order to avoid an increase in their temperature during the EL measurements. For the highest applied voltages, an increase in the temperature results usually in a high current surging, which leads to an irreversible change in the electrical properties of the pellets (large decrease in resistance and loss of the electroluminescent properties).
3. Results and discussion 3.1. Scanning electron micrography The pellets are polycrystalline and were analysed using scanning electron micrography in order to measure the average grain size (/lS) and the number (ng ) of consecutive grains or grain boundaries in series through the thickness of the pellets. /lS is obtained using a classical intercept method with a correction factor equal to 1.56 [14] and ng is the ratio between the thickness of the pellets and the average grain size. The values of /lS and ng are reported in Table 1. /lS increases from 11.6 to 22 mm when the sintering temperature increases from 950 C to 1100 C. The grain size and its increase with the sintering temperature are in good agreement with the results reported in the literature for Bi-containing ZnO varistors. For instance, the grain size of ZnO-based varistors increases from 21.8 to 27.2 mm when the sintering temperature increases from 1150 C to 1350 C [6]. The grain growth inhibiting effect of RE2O3 reported in previous publications [12,13,15] is overwhelmed by the presence of Bi2O3, which is known as a liquid-phase sintering aid. In ZnO-based varistors, a breakdown occurs when the applied voltage reaches a value corresponding to the product of the ZnO band gap energy Eg by the number of grains or grain boundaries in series through the thickness of the device [1–3]. In the present case, a theoretical breakdown voltage VBr can be estimated for the
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Table 1 Characteristics of the pellets Sintering temperature ( C) /lS (mm) ng VBr (V) R0 (O) a100 mA V100 mA (V) FB0 (eV)
950 11.6 86 275 530 103 5 150 0.71
975 13.8 72 230 150 103 5 150 0.68
1000 15.5 65 210 65 103 4 110 0.66
1050 18.6 54 175 15.6 103 4 80 0.63
1100 22 45 145 9.5 103 7 65 0.62
/lSis the average grain size. ng is the number of grains or grain boundaries through the thickness of the 1 mm thick pellets VBr is the theoretical breakdown voltage.R0 is the zero-field resistance of the pellets. a is the non-linear coefficient. FB0 is the barrier height under zero bias.
five pellets under study. The VBr values calculated using Eg ¼ 3:2 eV are reported in Table 1. 3.2. Current–voltage characteristics The DC current–voltage characteristics (i2V curves) of the pellets are reported in Fig. 1. The measurements were restricted to current intensities lower than 100 mA owing to the power dissipated in the pellets. Consequently, the breakdown voltage could not be reached for all the pellets. In good agreement with the results of the literature, the curves are non-linear [1–7]. The resistance of the pellets decreases when the voltage increases. The curves can be characterized by the zero-field resistance of the pellets (R0 ) and a nonlinear coefficient a=d(ln i)/d(ln V) [1–4]. The R0 and a values are reported in Table 1 together with the voltage conditions corresponding to the determination of a: a increases with the voltage applied to the pellets, the a values reported in Table 1 are given for a current equal to 100 mA. The results are in good agreement with the results published by Bhushan et al. [6]. The maximum a value, obtained for ZnO:Bi composites operating under breakdown condition is usually close to 10. Bhushan et al. reported an increase in a (from 6 to 12) with the Bi content (from 0.5 to 5.0 mol%) for ZnO–Bi2O3 systems sintered between 900 C and 1100 C whereas Eda [3] stated that the a values never exceed 10 owing to the absence of a non-linearity enhancement agent such as Mn or Co oxide. In the present case, the values of Table 1 (a ¼ 427) are relatively low. But they are obtained at low (Bi+Er) concentra-
Fig. 1. Current–voltage characteristics of the pellets sintered at 950 C (m), 975 C (K), 1000 C (’), 1050 C (E) and 1100 C (.).
tions and at a voltage well below the theoretical breakdown voltage reported in Table 1. Within the frame of the double Schottky barrier/thermionic emission model, the current flow J across a grain boundary barrier submitted to a voltage drop U is given by [4,5] J ¼ A T 2 expðFB =kTÞ½1 expðqU=kTÞ ; ð1Þ where A and k are the Richardson’s and Boltzmann’s constants, T is the temperature, FB is the barrier height and q is the charge of the electron. For the lowest applied voltages, qU5kT; Eq. (1) reduces to J ¼ ðA TqU=kÞexpðFB0 =kTÞ;
ð2Þ
where FB0 is the barrier height under zero bias. The energy band diagram of a grain boundary
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under zero bias and under an applied polarization U is presented in Fig. 2a and b, respectively. The knowledge of both R0 and the number of barriers in series through the thickness of the pellets allows determination of FB0 : R0 decreases from 530 to 9.3 kO and the number of barriers from 86 to 45 when the sintering temperature increases from 950 C to 1100 C. The FB0 values are reported in Table 1. FB0 decays from 0.71 to 0.62 eV. Such a lowering in the barrier height when the sintering temperature increases has been already proposed to account for the sintering temperature dependence of the electrical properties of both ZnO:Bi composites and ZnO-based varistors. It is attributed to a volatilization of Bi during the sintering [1,3,6,7].
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At this point, it appears that the electrical properties of the pellets are in good agreement with former results obtained on Er-free ZnO:Bi composites [6]. They are correctly interpreted in terms of the existing model: a succession of ng grain boundary barriers separated by good conducting grains. There is apparently no special influence of the small amount of Er oxide on the electric properties of the pellets. The size of the Bi3+ ion is close to that of the Er3+ ion; the charge of both ions is identical. Bi2O3 and some RE2O3 such as La and Sm form solid solutions [16]. In ZnO-based varistors, both Bi3+ and RE3+ ions remain located in the boundaries at the surface of the ZnO grains [1,3,15,17]. The most probable is a dispersion of Er atoms in the Bi-rich intergranular material. 3.3. PL spectra and mechanism The PL spectrum of the pellet sintered at 1050 C, under ZnO band-to-band excitation at 365 nm, is reported in Fig. 3. The other pellets sintered at different temperatures give the same broad spectrum under the same excitation conditions. This broad spectrum, centred at 600 nm is characteristic of ZnO and has been widely studied [18]. Several PL mechanisms have been proposed recently for polycrystalline or powder samples: (i) a recombination between electrons from the
Fig. 2. Energy band diagram of a double Schottky barrier at a grain boundary under zero bias (upper, 2a) and under an applied polarization U close to the voltage corresponding to the ZnO band-gap energy (lower, 2b). The lower diagram shows the hot electron impact excitation of RE3+ ions dispersed in the depletion layer of the positively biased grain. Q represents the charges trapped at the interface. Ec is the bottom of the conduction band, Ev the top of the valence band and Ef is the fermi level energy. The other symbols are defined in the text.
Fig. 3. PL spectrum of a ZnO:Bi,Er pellet sintered at 1050 C under ZnO band-to-band excitation at 365 nm.
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bottom of the conduction band or shallow donor levels and holes trapped on deep levels [19,20], (ii) a recombination between holes in the valence band and electrons from the singly ionized level of oxygen vacancies [21], (iii) an electron–hole recombination involving a donor–acceptor complex consisted of zinc and oxygen vacancies [22]. Basically, the PL of ZnO is related to the presence of holes in the valence band. The band-to-band excitation of ZnO promotes electrons from the valence band to the conduction band and the holes left in the valence band induce the ZnO PL. The surface, or the depletion layers at the surface, of the ZnO particles, is involved in the PL mechanism [20–23]. For instance, the photogenerated holes can move to deep levels inside the gap in a two-step process via their trapping at O2 surface sites [20]. The spectrum shows a structure at 520 nm. This structure corresponds to the 4I15/2-2H11/2 transition of the Er3+ ion and is a consequence of a slight reabsorption by the Er3+ ions, of the light emitted by ZnO [24]. It cannot be due to an emission from the 2H11/2 or 4S3/2 level (2H11/2-4I15/2 or 4 S3/2-4I15/2 transition). The 2H11/2 and 4S3/2 levels are thermalized [25]; both emissions appear simultaneously in all the Er3+ emission spectra. In the present case, only a single peak is seen on the spectrum. The absence of a second peak shows unambiguously that the structure which appears at 550 nm does not correspond to an emission from the 2H11/2 or 4S3/2 level. The reabsorption of the ZnO luminescence by the RE3+ ions in ZnO:RE sintered samples has been demonstrated by the comparison between diffuse reflectance and emission spectra [24]. The reabsorption of emitted light occurs also for Ho3+ and Nd3+; it is not followed by any RE re-emission. In Er-doped polycrystalline sintered ZnO, the presence of holes in the valence band of ZnO is unable to induce any Er3+ luminescence [26]. There is no energy transfer from ZnO to the Er3+ ion.
the pellet sintered at 1100 C is the most intense. The spectrum recorded during the abrasion of this pellet is presented in Fig. 4 together with an EL spectrum of the same pellet recorded using the same apparatus. Both spectra are identical and characteristic of Er3+ ion. The emission spectra of the RE3+ ions have been widely studied and the attribution of the spectra of Fig. 4 can be readily performed [25]. They correspond to the 2H11/2-4I15/2 and 4S3/2-4I15/2 transitions of the Er3+ ion at 530 and 560 nm, respectively. The 2H11/2 and 4S3/2 levels of the Er3+ ion are thermalized, so that the relative intensity of the lines at 530 and 560 nm depends only on the energy difference between the 2 H11/2 and 4S3/2 levels and on the temperature. The TL mechanism locates the RE3+ ions between the grains in the intergranular material. It agrees with the presence of electrically active grain boundaries and the structure analysis of both sintered RE-doped ZnO [15] and ZnO-based varistors containing RE [17]. In semiconductors such as Ge and Si, bond ruptures are able to induce a variety of phenomena including generation of voltage and currents, and emission of light of various durations and wavelength [27] and current burst across the cleavage and luminescence occur also on cleaving III2V semiconductors [28,29]. The energy band diagram of a mated but not healed crack region of a semiconductor is
3.4. Er3+ TL spectrum and mechanism The pellets sintered at 1050 C and 1100 C are triboluminescent. They show an emission of green light when they are abraded. The light emitted by
Fig. 4. TL (upper) and EL (lower) spectra of the pellet sintered at 1100 C. Both spectra are recorded using the same apparatus. The EL spectrum is recorded under an applied voltage equal to 50 V.
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similar to the energy band diagram of a grain boundary under zero bias presented in Fig. 2a [27]. In the case of polycrystalline sintered ZnO containing RE3+ ions, the grain boundaries form a lattice of mechanical defects throughout the material. The mechanical stress of the samples results in cleavages along the grain boundaries between the grains in the RE3+-rich regions. A consequence of the abrasion of the samples is a wrenching of ZnO grains producing an intense electric field across the grain boundary cleavage sufficient to cause the dielectric breakdown of the intergranular material. The TL of the RE3+ ions arises from their location in fragile regions between the grains. In addition, sheet conductivity at grain boundaries was reported for polycrystalline Ge and Si [30] showing that charged grain boundaries in semiconductors are able to present both conduction and TL properties. The identity between the TL and EL spectrum evidences the electrical origin of the excitation of the Er3+ TL, the location of Er3+ ions in the intergranular material between the grains and the presence of hot electrons in the boundaries between the grains. 3.5. EL spectra The EL spectra of the different pellets are shown in Fig. 5. They are a mixture of a broad pattern attributable to ZnO and of sharp lines attributable to the Er3+ ion. They show the emission corresponding to the 4F9/2-4I15/2 transition of the Er3+ ion at 660 nm in addition to the emission originating from the thermalized (2H11/2,4S3/2) levels at 530 and 560 nm [25]. The relative intensity of the emissions originating from ZnO and the Er3+ ion depends on the sintering temperature. The broad ZnO pattern predominates for the lowest sintering temperature whereas only the sharp lines of the Er3+ ion are detectable for the highest sintering temperature. No other pattern was detected, specially around 390 nm. The broad pattern attributable to ZnO in the EL spectrum of the pellets sintered at 950 C, 975 C and 1000 C shows few structures between 640 and 690 nm. These structures correspond to the reabsorption by the Er3+ ions (4I15/2-4F9/2 transi-
Fig. 5. EL spectra of a series of ZnO:Bi,Er sintered pellets. The electrical conditions (applied voltage, intensity) are: 160 V, 75 mA; 135 V, 70 mA; 110 V, 75 mA; 80 V, 80 mA; 70 V, 100 mA for the pellets sintered at 950 C, 975 C, 1000 C, 1050 C and 1100 C, respectively.
tion), of the light emitted by ZnO in addition to an emission of the Er3+ ions (4F9/2-4I15/2 transition). 3.6. Er3+ EL mechanism The voltage dependence of the EL intensity arising from the Er3+ ion and ZnO is presented in Fig. 6. The Er3+ EL is observed at 560 nm (4S3/2-4I15/2 transition) and 530 nm (2H11/24 I15/2 transition) for the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C. The ZnO EL is observed at 620 and 720 nm for the pellets sintered
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Fig. 6. Voltage dependence of the light B emitted by the Er3+ ion (filled symbols) at 530 nm (2H11/2-4I15/2 transition) for the pellet sintered at 975 C (K), 560 nm (4S3/2-4I15/2 transition) for the pellets sintered at 1000 C (’), 1050 C (E) and 1100 C (.), and ZnO (open symbols) at 620 nm for the pellet sintered at 950 C (n), and at 620 (J) and 720 nm (}) for the pellet sintered at 975 C.
below 1000 C. The EL voltage threshold decreases when the sintering temperature increases. For all the pellets, the EL arising either from ZnO or the Er3+ ion is observed for voltage ranges comprised between 25 and 150 V. The thickness of all the pellets is 1 mm; then the average field inside the pellets remains comprised between 250 and 1500 V cm1. EL in semiconductors occurs basically in two forms: injection and high-field EL [7]. In injection EL, the emission of light is consecutive to the recombination of majority carriers with injected minority carriers. In high-field EL, the excitation of luminescent centres results from an impact of majority charge carriers accelerated under high electric fields of the order of 106 V cm1. The injection of minority carriers in Er-doped polycrystalline sintered ZnO results in a ZnO characteristic EL spectrum similar to the PL spectrum: a broad pattern centred at 600 nm and a sharp emission at 380 nm; the spectrum shows no Er3+ emission [26]. In the present case, the Er3+ EL cannot result from an electron–hole pair recombination with a subsequent energy transfer from the semiconductor to the Er3+ ion since the PL spectrum which results from an electron–hole
pair recombination shows no Er3+ emission. Then, the EL is not an injection EL; it is rather a high-field EL despite the low value of the average electric field. In ZnO-based varistors, whose basic structure (sintered ZnO:Bi) is close to the present pellets, the field is inhomogeneous [1–7,10,17]. As pointed out before, ZnO:Bi composites and ZnO-based varistors are usually approximated as a stacking of good conducting ZnO grains separated by grain boundaries. Due to the high conductivity of the grains, the entire applied voltage is sustained across narrow regions in the depletion layer of the forward-biased grains close to the boundaries where the field is intense despite its low average value. Then, electrons injected in the forwardbiased grains can pick up a large amount of kinetic energy necessary to impact-excite the luminescent centres (Fig. 2b) and to impact-ionize ZnO. This model is proposed to account for the breakdown of ZnO-based varistors [10]. The application of this model to the EL of RE3+ ions inserted in polycrystalline sintered ZnO has been discussed in detail in previous articles [12,13]. A Destriau-type relationship has been proposed to account for the voltage dependence of the emitted light (B), owing to the energy distribution of hot electrons in semiconductors, which follows Boltzmann’s statistics [31]: lnðB=iÞp ng Eexc: =qV :
ð3Þ
Eexc: is an energy threshold for impact excitation. It is the energy of the emitting level in the case of a direct impact excitation. However, this mechanism is contradictory to some observations which have been presented in more recent articles and which have led to propose an excitation of the RE3+ ions between the grains, in the boundaries parallel to the field, in RE-rich regions [32,33]. The main observations dealing with the ZnO:Bi,Er systems are summarized below. The Er3+ ion EL occurs at low applied voltage, well below the breakdown voltage. In the present case, the voltage threshold of the Er3+ EL is 25, 50, 50 and 70 V for the pellets sintered at 1100 C, 1050 C, 1000 C and 975 C, respectively. From the ng values of Table 1, these values correspond to
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an average individual polarization /US close to 500 mV, well below the energy necessary to impact excite the Er3+ ion in its visible range emitting levels. The energy of the thermalized (2H11/2, 4S3/2) emitting levels of the Er3+ ion is close to 2.5 eV [25]. Then, either the barriers are strongly unequal in order to allow a few number of grain boundaries to sustain the entire applied voltage or the model of Fig. 2b does not apply; the excitation of the Er3+ ion does not occur in the depletion layer of forward-biased grains. The analysis, according to Eq. (3), of the Destriau plots (Fig. 7) of the light emitted by the Er3+ ion in the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C, gives unrealistic values for the product ng Eexc : This product, calculated from the data of Fig. 7, is close to 460 for the pellets sintered at 975 C, 1000 C and 1050 C and 230 for the pellet sintered at 1100 C. This leads to Eexc ¼ 6:3; 7.0, 8.5 and 10.2 eV for the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C, respectively. Such excitation energies are far above the energy of the thermalized (2H11/2, 4S3/2) emitting levels of the Er3+ ion. The model of Fig. 2b fails to give a satisfying quantitative explanation to the results. In contrast with the electrical properties, the luminescence properties of the pellet sintered at 1100 C are close to those already obtained with
Fig. 7. Destriau plot according to Eq. (4) of the Bn =i ratio for the pellets sintered at 975 C (K), 1000 C (’), 1050 (E) and 1100 C (.). The intensity of the Er3+ luminescence is measured at 530 (2H11/2-4I15/2 transition) or 560 nm (4S3/2-4I15/2 transition).
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Bi-free ZnO:Er sintered pellets [11]. Bi2O3 has apparently no influence on the luminescent properties of the pellet. The PL spectrum is the emission spectrum of ZnO (Fig. 3). The EL and TL spectra are the emission spectrum of the Er3+ ion (Fig. 4). The Er3+ EL is observed at low voltage (Fig. 6). The identity between the EL and the TL spectrum evidences an EL excitation mechanism which does not involves an electron impact of Er3+ ions in the depletion layer of a forward-biased grain in a strongly polarized grain boundary (Fig. 2b). Then, identical conclusions can be drawn for Bi-free and Bi-doped ZnO:Er pellets. The Er3+ EL is a consequence of current flows along the boundaries in an Er-rich material. This current flowing along the boundaries was proposed by Levinson to account for the discrepancies between the model and the results observed during the pre-breakdown of ZnO-based varistors [34]. The efficiency Z of an electroluminescent system is defined as the ratio of the number of emitted photons by the number of transferred charges [35]. Then, the variations of Z are given by the variations of the B=i ratio. Z is the product of the excitation efficiency Zexc by the luminescence efficiency Zlum and by the optical efficiency Zopt [35]: Z ¼ Zexc Zlum Zopt :
ð4Þ
The optical efficiency is related to the experimental conditions used to observe the luminescence; it remains constant for a device operating under identical optical conditions. The luminescence efficiency is mainly related to the deactivation process of the emitting centres; it also remains constant for a given device. Only the excitation efficiency, which is defined as the ratio of the number of excited luminescent centres by the number of transferred charges, can depend on the voltage applied to the pellets. From Fig. 7, it appears that Z; and consequently Zexc ; increase with the applied voltage for the Er3+ EL arising from the pellets sintered at 975 C, 1000 C, 1050 C and 1100 C. Usually, such an increase in Zexc with the voltage applied to an electroluminescent thin film device is a consequence of an increase in the average energy of the hot electrons. For instance, the fraction of hot electrons able to impact-excite
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the luminescent levels of the Er3+ ion in ZnS:Cu: Cl:Er thin films increases exponentially with this average energy [36]. This leads to a voltagedependent spectrum. In the present case, the same explanation cannot be proposed. The Destriau plots of Fig. 7 show that the B=i ratio increases by a factor higher than 102 when the applied potential increases by a factor close to 2 for the pellets sintered at 1000 C, 1050 C and 1100 C. According to Eq. (3), an increase in the B=i ratio close to 102 would be a consequence of an increase in the average energy of the hot electrons, and consequently in the applied voltage, by a factor close to 4.5 and not 2. Since the energy of the (2H11/2, 4S3/2) thermalized levels of the Er3+ ion is close to 2.5 eV, such an increase in the average energy of the hot electrons would lead to the presence of unrealistic voltage drops higher than 10 V, owing to the neglect of the cooling of the hot electrons, inside the pellets when they are submitted to the highest applied voltages (110, 80 and 70 V for the pellets sintered at 1000 C, 1050 C and 1100 C, respectively). Clearly, such voltage drops cannot be present inside the pellets owing to the possible ionization of ZnO. This ionization of ZnO is predicted by the classical grain boundary model of varistors; it is evidenced by the ZnO band-gap EL observed during the breakdown of varistors and restricts the voltage drop at a grain boundary to a value corresponding to the ZnO band-gap energy [10]. However, the presence of current flows along the boundaries, responsible for the impact excitation of the Er3+ ions, can account for both the apparent discrepancies between the electrical and luminescent properties and the large increase in the luminescence intensity with the applied voltage. The measured current i is the overall current, which crosses the pellets. It is responsible for the observed electrical properties. The current flowing along the boundaries, which is related to the electroluminescent properties, is only a fraction of i: Then, Zexc ; which represent the average energy of the hot electrons, is not simply related to the B=i ratio since the overall current should be replaced by the value of the current flowing along the boundaries.
In addition, the examination of the visible range EL under the microscope, during the breakdown of a ZnO-based varistor, shows that the light arises from an array of localized luminescent centres [4]. An increase in the number of these luminescent centres with the applied voltage can also be responsible for the increase in the luminescence intensity. But in any case, the low voltage threshold of EL remains unexplained within the frame of a succession of ng consecutive luminescent centres with identical electrical properties. Either the number of luminescent centres is not related to the number of successive grains or grain boundaries in series through the thickness of the pellets, or these luminescent centres differ in their electrical properties in order to allow an increase in their number with the applied voltage. 3.7. ZnO EL mechanism The broad pattern attributed to ZnO in the EL spectrum of the samples sintered at 950 C, 975 C and 1000 C corresponds to electron–hole pair recombination. The different ZnO PL mechanisms involve the surface, or the depletion layer at the surface, of the ZnO particles [19–22], for instance, a hole migration from the conduction band to a deep level in the bulk in a two-step process via a hole trapping at an O2 surface site [20]. In the present case, the absence of the exciton emission band at 390 nm tends to disregard a mechanism involving holes in the valence band [21]. Then, the spectrum corresponds more likely to a recombination between conduction band electrons and holes trapped on deep levels, which can arise either from an impact ionization of the deep levels (direct excitation), or from an impact ionization of the O2 surface site (indirect excitation). An impact ionization of the O2 site at the surface of the grains agrees well with the presence of hot electrons in the intergranular material between the grains. Within the frame of an excitation of the ZnO luminescence by hot electrons accelerated in the depletion layer of the positively biased grains, Eq. (3) can describe the mechanism for either a direct or an indirect excitation. Only the value of Eexc: changes with the excitation mechanism. The value of Eexc: is the value of the band gap energy in
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the case of an impact of valence electrons, and the value necessary for the excitation of the O2 site or for the ionization of the deep levels. An analysis of the Destriau plots of the ZnO luminescence (Fig. 8) according to Eq. (3) gives also unrealistic results, as in the case of the Er3+ luminescence: Eexc: ¼ 7:7 and 5.3 eV for the pellets sintered at 950 C and 975 C, respectively. At this point, it is impossible to ascribe the observed ZnO EL either to an excitation of ZnO inside the depletion layer of the positively biased grains or to an excitation of ZnO inside the intergranular material between the grains. However, as in the case of the Er3+ luminescence, both the large increase in the B=i ratio (102 for the pellet sintered at 950 C and 20 for the pellet sintered at 975 C) and the high Eexc: values favour the hypothesis of an excitation by current flows along the boundaries inside the intergranular material. The change in the EL spectrum with the sintering temperature shows that an increase in the sintering temperature favours the excitation of the Er3+ ion compared to that of ZnO. Depending on the model (excitation inside the intergranular material by current flows along the boundaries or excitation inside the depletion layers of the positively biased grains), this can be a consequence of either a better segregation of the Er3+ ions to the boundaries or a better insertion of the Er3+
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ions inside the ZnO lattice. No firm conclusion can be drawn from the change in the EL spectra.
4. Conclusion The present results show that the structure and the electrical behaviour of ZnO:Bi composites containing a small amount of Er oxide are close to that of the Er-free ZnO:Bi composites whereas their electroluminescent behaviour is close to that of the Bi-free ZnO:Er composites. The classical brick layer/grain boundary model put forward for ZnO-based varistors accounts correctly for both the electrical and the triboluminescent properties of the ZnO:Bi,Er composites but is unable to account for their electroluminescent properties. The Er3+ ions, which act as a probe owing to their luminescent properties, evidence the presence of hot electrons for applied voltages well below the breakdown voltage. The conditions of observation of the Er3+ ion EL, which cannot be explained by the classical grain boundary model, lead to propose an excitation of the Er3+ ions outside the ZnO grains, in the intergranular material, by current flows between the grains in good agreement with the TL of the Er3+ ions, which shows that an excitation mechanism that locates the Er3+ ions outside the ZnO grains in the intergranular material does exist. However, the amount of current in the boundaries between the grains and its voltage dependence cannot yet be pre! cised. In addition, the study shows that the surface of the grains is involved in all the luminescence processes. Acknowledgements The author thanks Dr. J.L. Pastol from CECM, CNRS (Vitry, France) for the SEM measurements and Dr. P. Valat and V. Wintgens for their help in the TL experiments. References
Fig. 8. Destriau plot according to Eq. (4) of the Bn =i ratio for the pellets sintered at 950 C (n) and 975 C (J and }). The intensity of the ZnO luminescence is measured at 620 (n and J) or 720 nm (}).
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