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Electrical characterization of growth-induced defects in bulk-grown ZnO
Superlattices and Microstructures 39 (2006) 17–23 www.elsevier.com/locate/superlattices
Electrical characterization of growth-induced defects in bulk-grown ZnO F.D. Aureta,∗, J.M. Nela, M. Hayesa, L. Wua, W. Weschb, E. Wendlerb a Physics Department, University of Pretoria, Pretoria, 0002, South Africa b Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Jena, Germany
Available online 19 September 2005
Abstract We have investigated and compared the defects in ZnO grown by two methods, namely seeded chemical vapour transport (SCVT) and melt-growth (MG), using conventional deep level transient spectroscopy (DLTS) and high resolution (Laplace) DLTS with Au and Ru Schottky barrier diodes. Both materials contained two prominent defects, E1, at E C –0.12 eV and E3 at E C –0.29 eV. The SCVT ZnO has E1 as the main defect with a concentration of about 1016 cm−3 , while the MG ZnO has E3 as the main defect with a concentration of above 1016 cm−3 . It has been speculated that this defect is the oxygen vacancy in ZnO. High resolution Laplace DLTS suggests that this level could consist of two closely spaced levels but with different capture cross-sections. The relative concentrations of these defects were found to vary across the region probed by DLTS. The E1 and E3 defects also showed opposite trends in an electric field: an increase in electric field enhanced emission from E1 whereas it slowed down emission from E3. The peak splitting and field dependence may, however, be a consequence of a non-exponential transient. Finally, etching in HCl:H2 O did not affect the defect concentrations or introduce additional defects in ZnO in the MG ZnO. © 2005 Published by Elsevier Ltd
1. Introduction The diversity of ZnO can be seen in the many products that use it, including amongst others, facial powders, phosphors, paints, piezoelectric transducers, varistors, and ∗ Corresponding author. Tel.: +27 12 420 2684; fax: +27 12 362 5288.
E-mail address: [email protected] (F.D. Auret). 0749-6036/$ - see front matter © 2005 Published by Elsevier Ltd doi:10.1016/j.spmi.2005.08.021
18
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
transparent conducting films, the latter being very important for the photovoltaic industry. From a review, where the properties of ZnO are summarized [1], it is clear that ZnO can be used for several other, more sophisticated, electro-optical applications. Zinc oxide has recently become the focus of many studies since a wide range of applications are possible due to its direct wide bandgap of 3.37 eV [2]. Devices such as detectors, lasers and diodes operating in the UV and blue regions of the spectrum have been reported [3], but are not very efficient yet. Furthermore, the large bandgap of ZnO renders it suitable as window or buffer layers in the fabrication of solar cells and as a substrate or buffer layer for the group III—nitride based devices. Further practical advantages of ZnO include bulk-growth capability, amenability to conventional wet chemistry etching, which is compatible with Si technology [2] (unlike the case for GaN), and convenient cleavage planes. Essential elements in the study of electrical properties of ZnO, as for many sensor applications, are reliable rectifying contacts. The rectifying contacts can be in the form of p–n junctions, metal–oxide–semiconductor (MOS) structures or Schottky barrier diodes (SBDs). As-grown ZnO has intrinsically n-type conduction and to form a p–n junction a high enough electrically activated p-type dopant concentration is required. Further, to the best of our knowledge no DLTS studies have yet been reported using p–n junctions on ZnO. Additionally, defects can be introduced during the high temperature processing of a p–n junction. Alternative approaches for space charge junctions to ZnO are the formation of a p–n heterojunction on the normally n-type ZnO, or the fabrication of a metal–oxide–semiconductor (MOS) structure. Although successful DLTS studies can be performed using these rectifying junctions, it is also reported [4] that an additional defect, not yet found in ZnO, was introduced during the fabrication of the MOS structure. The SBD processing usually involves a simple fabrication process at low temperatures, and the introduction of defects into the semiconductor can be kept to a minimum when special precautions are taken. Defects can firstly be introduced into the ZnO during the growth process of the bulk material, secondly, as mentioned above, during the processing steps of the contact fabrication or thirdly, during typical device operation. For possible use of ZnO in space (e.g. as a UV detector) the following should be borne in mind. Sometimes, these devices have to operate at elevated temperatures, typically above 200 ◦C, in harsh radiation conditions comprising energetic particles. An important consideration then is that the material should be as radiation hard as possible for reliable operation over extended periods. Presently, the main wide band gap materials for space applications are considered to be the III–V nitrides, SiC and diamond. It has been reported [5,6] that the effect of irradiation particles on ZnO is significantly lower than the effect on GaN, GaAs and Si. However, in this paper we will restrict ourselves to defects introduced during growth as well as during the contact fabrication. 2. Experimental We have investigated and compared the defects present in bulk ZnO grown by two methods, namely seeded chemical vapour transport (SCVT) and melt-grown (MG), by conventional deep level transient spectroscopy (DLTS) and high-resolution (Laplace)
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
19
DLTS. The ZnO was cleaned using organic solvents (acetone, methanol) prior to contact formation. For the SCVT ZnO, Au Schottky contacts, 0.7 mm in diameter and 200 nm thick, were resistively deposited onto the (0001) Zn face of the ZnO crystal through a mechanical mask. Thereafter, InGa ohmic contacts were applied to the opposite side (O face) of the sample. For the MG ZnO, an ohmic contact consisting of Ti/Al/Pt/Au of 20/80/40/80 nm thick was electron beam evaporated onto the O face of the sample. After the sample was annealed at 200 ◦C for 30 min in a nitrogen overpressure, Ru Schottky contacts, 0.48 mm in diameter and 50 nm thick, were electron beam evaporated onto the (0001) Zn face of the ZnO crystal through a mechanical mask. The latter process was also reversed, i.e. the Schottky contact formed on the O face and the ohmic on the Zn face of MG ZnO. We have experimentally verified that the 200 ◦C annealing did not alter the defect properties in the ZnO but this ohmic contact was found to be superior to the previously used InGa contact. MG ZnO was also etched for 15 s in a 1:1 solution of HCl:H2 O prior to metallization. The ZnO was then rinsed in acetone and methanol before contact deposition. 3. Results and discussion The electrical properties of SBDs on the SCVT ZnO, and of the defects therein, have been reported before [6]. For the Ru SBDs on the MG ZnO it was found that the rectification properties of these diodes improved with storage time after metallization. About one week after fabrication the reverse current at 1 V bias reached 5 × 10−5 A or better, i.e. sufficiently low for reliable DLTS measurements. It has been shown previously [6] that in the SCVT ZnO the most prominent electron traps were the E1 (0.12 eV) and E3 (0.29 eV) with concentrations of 1016 cm−3 and 1014 cm−3 , respectively. In the case of the MG ZnO the E3 level is the dominant defect, Fig. 1, with a concentration of above 1016 cm−3 . Note that at room temperature the free carrier concentration of the MG ZnO was about 5 × 1016 cm−3 . The slightly asymmetric E3 peak shape suggests that the E3 peak may be the result of overlapping peaks of more than one defect with closely spaced energy levels. Alternatively, it may be a consequence of the large trap concentration of E3 that results in a non-exponential transient. In the MG ZnO the E1 defect is present in a concentration of about 1014 cm−3 . In Fig. 2 we present the Arrhenius plots from which the activation enthalpies and apparent cross sections were calculated. If the E3 is assumed to be one level then its activation energy and capture cross section are 0.29 eV and 1.0 × 10−15 cm2 , respectively. This is in good agreement with the values reported for the E3 level in SCVT ZnO. This value of 0.29 eV is also the same as that reported by Simpson et al. [7] for a level that they assigned to the oxygen vacancy in hydrothermally grown ZnO. To investigate the possibility that E3 actually consists of more than one level, we have employed high-resolution Laplace DLTS [8]. This technique is based on the analysis of the transient via an inverse Laplace transform and is capable of separating defects with emission rate ratios as close as 2:1 if the signal to noise ratio is sufficiently high. LaplaceDLTS using bias and pulse conditions of Vr = −2 V and V p = 0 V is unable to clearly deconvolute the E3 peak into more than one component (inset in Fig. 1) but suggests that it may consist of two peaks. However, when we record a signal using the double-DLTS
20
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
Fig. 1. DLTS spectrum of MG ZnO recorded using a rate window of 80 s−1 , a quiescent reverse bias of −2 V and a filling pulse amplitude of 2 V. The inset shows the Laplace DLTS spectrum of the E3 defect recorded at a temperature of 190 K.
Fig. 2. Arrhenius plots for defects in SCVT and MG ZnO. The filled circles are the data points obtained from Laplace DDLTS on the MG ZnO.
(DDLTS) method [9] and analyse it with the Laplace technique, we find two well-separated peaks. In the DDLTS method DLTS signals recorded under the same quiescent bias, Vr , but with different filling pulse amplitudes (V p1 and V p2 ), are subtracted. In this way defects are sampled in a well-defined narrow region below the SBD interface. In our case we have found that for V p1 − V p2 = 0.1 V in the range −1.4 V ≤ V p1 ≤ 0.3 V for Vr = −2 V
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
21
Fig. 3. Laplace DDLTS spectra recorded from the E3 defect pair in MG ZnO at 190 K. A quiescent reverse bias of −2 V was used and the difference in filling pulse amplitudes was maintained at 0.1 V.
the E3 signal could be split into two signals indicating the presence of two defect levels, E3a and E3b (Fig. 3). Actually, the energy levels of these two defects differ by only a few meV but their capture cross-sections are different by a factor of two. For E3a we have measured the apparent capture cross section as σ = 7×10−16 cm2 and for E3b we obtained σ = 1.5 × 10−15 cm2 from the Arrhenius plots. The Laplace analysis further indicated that the relative magnitudes of these two peaks change with V p1 (always = V p2 + 0.1 V) (Fig. 3). Another interesting observation was made regarding the emission behaviour of the E3 level in an electric field. Normally, the potential well of the defect is skewed by the electric field so that defects can escape more easily from the shallow side of the well, for example in the Poole–Frenkel model. This results in an apparently shallower level which, at a given rate window, would give rise to a DLTS peak at lower temperature when increasing the electric field. In our case when using DDLTS we can probe defects at increasing electric fields by increasing the value of V p1 (but always = V p2 + 0.1 V). For the E1 defect this produces the expected result (Fig. 4), i.e. its peak shifts to lower temperatures when analysing defects in a higher electric field region. However, E3 shows just the opposite effect: if we monitor the emission of defects in regions of increasing field, its peak shifts to higher temperatures. In order to explore the origin of this anomalous field effect we considered the fact that there may be interaction between the applied electric field and the lattice via the high ¯ piezo-electric coupling coefficient. We therefore deposited SBDs on the [0001] and [0001] faces of the ZnO for which the effect of the electric field should be opposite. However, the effect of the electric field was identical in SBDs on both surfaces. The most likely explanation for this field effect of the E3 that goes opposite to what is expected from the Poole–Frenkel model is that it is an effect produced because of a non-exponential transient. Such an effect has indeed been observed during the modelling of emission and capture from the DX center in AlGaAs [10]. It was shown that in that case the non-exponentiality was
22
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
Fig. 4. DDLTS spectra that illustrate the effect of sampling in different regions below the SBD interface on the DLTS peak positions of E1 and E3.
due to the capture processes taking place while the thermal emission is produced. We are currently further investigating this matter. Finally, we have investigated, using DLTS, the effect of chemical etching of MG ZnO in HCl:H2 O. By applying filling pulses that facilitated DLTS probing up to the metal–ZnO interface we have found that this etching did not influence the existing defect concentrations. More importantly, it also did not introduce any additional levels in the energy region scanned by DLTS. 4. Summary and conclusions We have investigated and compared, using conventional as well as Laplace DLTS, the defects in ZnO grown by seeded chemical vapour transport (SCVT) and melt-growth (MG). Both materials contained two prominent defects. The main defect, E1, in SCVT ZnO has a shallow level at 0.12 eV below the conduction band and a concentration of about 1016 cm−3 . In MG ZnO the main defect, E3, has a level at 0.29 eV below the conduction band and its concentration is above 1016 cm−3 . It has been speculated that this defect is the oxygen vacancy in ZnO. High-resolution Laplace DLTS suggested that this level may consist of two closely spaced levels but with different capture cross-sections. The relative concentrations of these defects were found to vary across the region probed by DLTS. Another interesting observation was that the emission enhancement from the E1 and E3 showed opposite trends in an electric field: an increase in electric field enhanced emission from E1 whereas it slowed down emission from E3. Note, however, that both the peak splitting of the E3 and its field dependence may be the consequence of a nonexponential signal arising from, among other things, its high concentration relative to the
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
23
free carrier concentration of the MG ZnO. Finally, etching in HCl:H2 O did not affect the defect concentrations or introduce additional defects in MG ZnO. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
D.C. Look, Mater. Sci. Eng. B 80 (1–3) (2001) 383. C. Kligshirn, Phys. Status Solidi b 71 (1975) 547. T. Aoki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. J.M. Nel, F.D. Auret, L. Wu, M.J. Legodi, W.E. Meyer, M. Hayes, Sensors Actuators B 100 (2004) 270. D.C. Look, D.C. Reynolds, J.W. Hemsky, R.L. Jones, J.R. Sizelove, Appl. Phys. Lett. 75 (1999) 811. F.D. Auret, S.A. Goodman, M. Hayes, M.J. Legodi, H.A. van Laarhoven, D.C. Look, J. Phys.: Condens. Matter 13 (2001) 8989. J.C. Simpson, J.F. Cordaro, J. Appl. Phys. 63 (5) (1988) 1781. L. Dobaczewski, P. Kaczor, I.D. Hawkins, A.R. Peaker, J. Appl. Phys. 76 (1994) 194. H. Lefevre, M. Schultz, Appl. Phys. 12 (1977) 45. I. Izpura, E. Munoz, E. Calleja, Semicond. Sci. Technol. 8 (1993) 1258.
Electrical characterization of growth-induced defects in bulk-grown ZnO F.D. Aureta,∗, J.M. Nela, M. Hayesa, L. Wua, W. Weschb, E. Wendlerb a Physics Department, University of Pretoria, Pretoria, 0002, South Africa b Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Jena, Germany
Available online 19 September 2005
Abstract We have investigated and compared the defects in ZnO grown by two methods, namely seeded chemical vapour transport (SCVT) and melt-growth (MG), using conventional deep level transient spectroscopy (DLTS) and high resolution (Laplace) DLTS with Au and Ru Schottky barrier diodes. Both materials contained two prominent defects, E1, at E C –0.12 eV and E3 at E C –0.29 eV. The SCVT ZnO has E1 as the main defect with a concentration of about 1016 cm−3 , while the MG ZnO has E3 as the main defect with a concentration of above 1016 cm−3 . It has been speculated that this defect is the oxygen vacancy in ZnO. High resolution Laplace DLTS suggests that this level could consist of two closely spaced levels but with different capture cross-sections. The relative concentrations of these defects were found to vary across the region probed by DLTS. The E1 and E3 defects also showed opposite trends in an electric field: an increase in electric field enhanced emission from E1 whereas it slowed down emission from E3. The peak splitting and field dependence may, however, be a consequence of a non-exponential transient. Finally, etching in HCl:H2 O did not affect the defect concentrations or introduce additional defects in ZnO in the MG ZnO. © 2005 Published by Elsevier Ltd
1. Introduction The diversity of ZnO can be seen in the many products that use it, including amongst others, facial powders, phosphors, paints, piezoelectric transducers, varistors, and ∗ Corresponding author. Tel.: +27 12 420 2684; fax: +27 12 362 5288.
E-mail address: [email protected] (F.D. Auret). 0749-6036/$ - see front matter © 2005 Published by Elsevier Ltd doi:10.1016/j.spmi.2005.08.021
18
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
transparent conducting films, the latter being very important for the photovoltaic industry. From a review, where the properties of ZnO are summarized [1], it is clear that ZnO can be used for several other, more sophisticated, electro-optical applications. Zinc oxide has recently become the focus of many studies since a wide range of applications are possible due to its direct wide bandgap of 3.37 eV [2]. Devices such as detectors, lasers and diodes operating in the UV and blue regions of the spectrum have been reported [3], but are not very efficient yet. Furthermore, the large bandgap of ZnO renders it suitable as window or buffer layers in the fabrication of solar cells and as a substrate or buffer layer for the group III—nitride based devices. Further practical advantages of ZnO include bulk-growth capability, amenability to conventional wet chemistry etching, which is compatible with Si technology [2] (unlike the case for GaN), and convenient cleavage planes. Essential elements in the study of electrical properties of ZnO, as for many sensor applications, are reliable rectifying contacts. The rectifying contacts can be in the form of p–n junctions, metal–oxide–semiconductor (MOS) structures or Schottky barrier diodes (SBDs). As-grown ZnO has intrinsically n-type conduction and to form a p–n junction a high enough electrically activated p-type dopant concentration is required. Further, to the best of our knowledge no DLTS studies have yet been reported using p–n junctions on ZnO. Additionally, defects can be introduced during the high temperature processing of a p–n junction. Alternative approaches for space charge junctions to ZnO are the formation of a p–n heterojunction on the normally n-type ZnO, or the fabrication of a metal–oxide–semiconductor (MOS) structure. Although successful DLTS studies can be performed using these rectifying junctions, it is also reported [4] that an additional defect, not yet found in ZnO, was introduced during the fabrication of the MOS structure. The SBD processing usually involves a simple fabrication process at low temperatures, and the introduction of defects into the semiconductor can be kept to a minimum when special precautions are taken. Defects can firstly be introduced into the ZnO during the growth process of the bulk material, secondly, as mentioned above, during the processing steps of the contact fabrication or thirdly, during typical device operation. For possible use of ZnO in space (e.g. as a UV detector) the following should be borne in mind. Sometimes, these devices have to operate at elevated temperatures, typically above 200 ◦C, in harsh radiation conditions comprising energetic particles. An important consideration then is that the material should be as radiation hard as possible for reliable operation over extended periods. Presently, the main wide band gap materials for space applications are considered to be the III–V nitrides, SiC and diamond. It has been reported [5,6] that the effect of irradiation particles on ZnO is significantly lower than the effect on GaN, GaAs and Si. However, in this paper we will restrict ourselves to defects introduced during growth as well as during the contact fabrication. 2. Experimental We have investigated and compared the defects present in bulk ZnO grown by two methods, namely seeded chemical vapour transport (SCVT) and melt-grown (MG), by conventional deep level transient spectroscopy (DLTS) and high-resolution (Laplace)
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
19
DLTS. The ZnO was cleaned using organic solvents (acetone, methanol) prior to contact formation. For the SCVT ZnO, Au Schottky contacts, 0.7 mm in diameter and 200 nm thick, were resistively deposited onto the (0001) Zn face of the ZnO crystal through a mechanical mask. Thereafter, InGa ohmic contacts were applied to the opposite side (O face) of the sample. For the MG ZnO, an ohmic contact consisting of Ti/Al/Pt/Au of 20/80/40/80 nm thick was electron beam evaporated onto the O face of the sample. After the sample was annealed at 200 ◦C for 30 min in a nitrogen overpressure, Ru Schottky contacts, 0.48 mm in diameter and 50 nm thick, were electron beam evaporated onto the (0001) Zn face of the ZnO crystal through a mechanical mask. The latter process was also reversed, i.e. the Schottky contact formed on the O face and the ohmic on the Zn face of MG ZnO. We have experimentally verified that the 200 ◦C annealing did not alter the defect properties in the ZnO but this ohmic contact was found to be superior to the previously used InGa contact. MG ZnO was also etched for 15 s in a 1:1 solution of HCl:H2 O prior to metallization. The ZnO was then rinsed in acetone and methanol before contact deposition. 3. Results and discussion The electrical properties of SBDs on the SCVT ZnO, and of the defects therein, have been reported before [6]. For the Ru SBDs on the MG ZnO it was found that the rectification properties of these diodes improved with storage time after metallization. About one week after fabrication the reverse current at 1 V bias reached 5 × 10−5 A or better, i.e. sufficiently low for reliable DLTS measurements. It has been shown previously [6] that in the SCVT ZnO the most prominent electron traps were the E1 (0.12 eV) and E3 (0.29 eV) with concentrations of 1016 cm−3 and 1014 cm−3 , respectively. In the case of the MG ZnO the E3 level is the dominant defect, Fig. 1, with a concentration of above 1016 cm−3 . Note that at room temperature the free carrier concentration of the MG ZnO was about 5 × 1016 cm−3 . The slightly asymmetric E3 peak shape suggests that the E3 peak may be the result of overlapping peaks of more than one defect with closely spaced energy levels. Alternatively, it may be a consequence of the large trap concentration of E3 that results in a non-exponential transient. In the MG ZnO the E1 defect is present in a concentration of about 1014 cm−3 . In Fig. 2 we present the Arrhenius plots from which the activation enthalpies and apparent cross sections were calculated. If the E3 is assumed to be one level then its activation energy and capture cross section are 0.29 eV and 1.0 × 10−15 cm2 , respectively. This is in good agreement with the values reported for the E3 level in SCVT ZnO. This value of 0.29 eV is also the same as that reported by Simpson et al. [7] for a level that they assigned to the oxygen vacancy in hydrothermally grown ZnO. To investigate the possibility that E3 actually consists of more than one level, we have employed high-resolution Laplace DLTS [8]. This technique is based on the analysis of the transient via an inverse Laplace transform and is capable of separating defects with emission rate ratios as close as 2:1 if the signal to noise ratio is sufficiently high. LaplaceDLTS using bias and pulse conditions of Vr = −2 V and V p = 0 V is unable to clearly deconvolute the E3 peak into more than one component (inset in Fig. 1) but suggests that it may consist of two peaks. However, when we record a signal using the double-DLTS
20
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
Fig. 1. DLTS spectrum of MG ZnO recorded using a rate window of 80 s−1 , a quiescent reverse bias of −2 V and a filling pulse amplitude of 2 V. The inset shows the Laplace DLTS spectrum of the E3 defect recorded at a temperature of 190 K.
Fig. 2. Arrhenius plots for defects in SCVT and MG ZnO. The filled circles are the data points obtained from Laplace DDLTS on the MG ZnO.
(DDLTS) method [9] and analyse it with the Laplace technique, we find two well-separated peaks. In the DDLTS method DLTS signals recorded under the same quiescent bias, Vr , but with different filling pulse amplitudes (V p1 and V p2 ), are subtracted. In this way defects are sampled in a well-defined narrow region below the SBD interface. In our case we have found that for V p1 − V p2 = 0.1 V in the range −1.4 V ≤ V p1 ≤ 0.3 V for Vr = −2 V
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
21
Fig. 3. Laplace DDLTS spectra recorded from the E3 defect pair in MG ZnO at 190 K. A quiescent reverse bias of −2 V was used and the difference in filling pulse amplitudes was maintained at 0.1 V.
the E3 signal could be split into two signals indicating the presence of two defect levels, E3a and E3b (Fig. 3). Actually, the energy levels of these two defects differ by only a few meV but their capture cross-sections are different by a factor of two. For E3a we have measured the apparent capture cross section as σ = 7×10−16 cm2 and for E3b we obtained σ = 1.5 × 10−15 cm2 from the Arrhenius plots. The Laplace analysis further indicated that the relative magnitudes of these two peaks change with V p1 (always = V p2 + 0.1 V) (Fig. 3). Another interesting observation was made regarding the emission behaviour of the E3 level in an electric field. Normally, the potential well of the defect is skewed by the electric field so that defects can escape more easily from the shallow side of the well, for example in the Poole–Frenkel model. This results in an apparently shallower level which, at a given rate window, would give rise to a DLTS peak at lower temperature when increasing the electric field. In our case when using DDLTS we can probe defects at increasing electric fields by increasing the value of V p1 (but always = V p2 + 0.1 V). For the E1 defect this produces the expected result (Fig. 4), i.e. its peak shifts to lower temperatures when analysing defects in a higher electric field region. However, E3 shows just the opposite effect: if we monitor the emission of defects in regions of increasing field, its peak shifts to higher temperatures. In order to explore the origin of this anomalous field effect we considered the fact that there may be interaction between the applied electric field and the lattice via the high ¯ piezo-electric coupling coefficient. We therefore deposited SBDs on the [0001] and [0001] faces of the ZnO for which the effect of the electric field should be opposite. However, the effect of the electric field was identical in SBDs on both surfaces. The most likely explanation for this field effect of the E3 that goes opposite to what is expected from the Poole–Frenkel model is that it is an effect produced because of a non-exponential transient. Such an effect has indeed been observed during the modelling of emission and capture from the DX center in AlGaAs [10]. It was shown that in that case the non-exponentiality was
22
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
Fig. 4. DDLTS spectra that illustrate the effect of sampling in different regions below the SBD interface on the DLTS peak positions of E1 and E3.
due to the capture processes taking place while the thermal emission is produced. We are currently further investigating this matter. Finally, we have investigated, using DLTS, the effect of chemical etching of MG ZnO in HCl:H2 O. By applying filling pulses that facilitated DLTS probing up to the metal–ZnO interface we have found that this etching did not influence the existing defect concentrations. More importantly, it also did not introduce any additional levels in the energy region scanned by DLTS. 4. Summary and conclusions We have investigated and compared, using conventional as well as Laplace DLTS, the defects in ZnO grown by seeded chemical vapour transport (SCVT) and melt-growth (MG). Both materials contained two prominent defects. The main defect, E1, in SCVT ZnO has a shallow level at 0.12 eV below the conduction band and a concentration of about 1016 cm−3 . In MG ZnO the main defect, E3, has a level at 0.29 eV below the conduction band and its concentration is above 1016 cm−3 . It has been speculated that this defect is the oxygen vacancy in ZnO. High-resolution Laplace DLTS suggested that this level may consist of two closely spaced levels but with different capture cross-sections. The relative concentrations of these defects were found to vary across the region probed by DLTS. Another interesting observation was that the emission enhancement from the E1 and E3 showed opposite trends in an electric field: an increase in electric field enhanced emission from E1 whereas it slowed down emission from E3. Note, however, that both the peak splitting of the E3 and its field dependence may be the consequence of a nonexponential signal arising from, among other things, its high concentration relative to the
F.D. Auret et al. / Superlattices and Microstructures 39 (2006) 17–23
23
free carrier concentration of the MG ZnO. Finally, etching in HCl:H2 O did not affect the defect concentrations or introduce additional defects in MG ZnO. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
D.C. Look, Mater. Sci. Eng. B 80 (1–3) (2001) 383. C. Kligshirn, Phys. Status Solidi b 71 (1975) 547. T. Aoki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. J.M. Nel, F.D. Auret, L. Wu, M.J. Legodi, W.E. Meyer, M. Hayes, Sensors Actuators B 100 (2004) 270. D.C. Look, D.C. Reynolds, J.W. Hemsky, R.L. Jones, J.R. Sizelove, Appl. Phys. Lett. 75 (1999) 811. F.D. Auret, S.A. Goodman, M. Hayes, M.J. Legodi, H.A. van Laarhoven, D.C. Look, J. Phys.: Condens. Matter 13 (2001) 8989. J.C. Simpson, J.F. Cordaro, J. Appl. Phys. 63 (5) (1988) 1781. L. Dobaczewski, P. Kaczor, I.D. Hawkins, A.R. Peaker, J. Appl. Phys. 76 (1994) 194. H. Lefevre, M. Schultz, Appl. Phys. 12 (1977) 45. I. Izpura, E. Munoz, E. Calleja, Semicond. Sci. Technol. 8 (1993) 1258.