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Mechanism of formation of highly conductive layer on ZnO crystal surface
Solid State Communications 136 (2005) 475–478 www.elsevier.com/locate/ssc
Mechanism of formation of highly conductive layer on ZnO crystal surface I.V. Markevich*, V.I. Kushnirenko, L.V. Borkovska, B.M. Bulakh V. Lashkarev Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 45 Prospect Nauky, 03028 Kyiv, Ukraine Received 5 August 2005; accepted 2 September 2005 by E.L. Ivchenko Available online 19 September 2005
Abstract The mechanism of formation of a thin highly conductive layer, which is known to be present on ZnO surface, has been proposed. This process has been assumed to consist in accumulation of mobile shallow donors at crystal surface due to their drift in bandbending electric field caused by adsorbed oxygen. Experimental results that confirm this mechanism have been obtained. q 2005 Elsevier Ltd. All rights reserved. PACS: 61.72.Ji; 66.30.Kh; 73.25.Ci Keywords: A. Semiconductors; C. Point defects; D. Diffusion in solids; D. Surface conductivity
1. Introduction Zinc oxide has many device applications in electronics, including transparent conducting films, piezoelectric transducers, phosphors, varistors and gas sensors; it is also a promising material for ultraviolet lasers and light-emitting diodes. These applications involved mainly thin layers and films, characteristics of which are strongly influenced by surface conditions. Therefore, detailed investigation of the change of ZnO surface properties under various external factors is important. One of these factors is atmospheric oxygen. It is known that ZnO is n-type and adsorbs oxygen from the air [1–9]. Oxygen atoms capture crystal electrons (i.e. act as acceptors), which results in creation of negative surface charge and formation of depletion layer with reduced conductivity. In undoped ZnO the thickness of this layer d1!1 mm [3,4]. The oxygen can be desorbed by both heating and light illumination. The thermal desorption, which occurs due to thermal ionization of electrons from oxygen, has activation energy of 1.1 eV and is essential at TR400 K [4]. The photodesorption, * Corresponding author. Tel./fax: C38 44 525 83 44. E-mail address: [email protected] (I.V. Markevich).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.09.001
which results from the capture of free photoholes by oxygen acceptors, is athermal process and can be observed at low temperatures [6]. After oxygen desorption the captured electrons return to the crystal and the prime conductivity of the near-surface region restores. It is evident that adsorption/desorption processes will strongly influence the conductance of the sample of the thickness d2%d1. In fact, oxygen desorption causes drastic increase in conductance of films, thin layers and powders [7–9]. The same effect, however, was also found in ZnO crystals with d2[d1. To explain this phenomenon it was supposed that a thin layer enriched with shallow donors was present on crystal surface, the prime conductance of such a layer exceeding the conductance of the rest of the sample [1–5]. This layer does not manifest itself while oxygen is adsorbed, but under oxygen desorption its conductance increases and becomes dominant. As a result, under illumination two components of photoresponse are observed: (i) usual photoconductance caused by photocarrier generation; (ii) conductance caused by accumulation of electrons in depletion layer due to oxygen photodesorption [6]. When the light is switched off, after relaxation of the photoconductance a residual conductance, which is the
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prime conductance of the near-surface layer, is observed [6]. When the sample is exposed to oxygen or to the air, the value of this residual conductance reduces because of oxygen readsorption [2–6]. The reduction process slows down with time due to the gradual increase of negative surface charge and, so, due to the rise of the potential barrier for electron capture by oxygen [2]. Increased surface conductivity of ZnO crystals was ascribed to the presence of excessive Zn [1,4,5,9]. However, the origin of Zn excess was not definitely established. Some authors believed that surface layer enriched with Zn was formed during crystal growth [9], others thought that some defect reactions, which resulted in appearance of nonstoichiometric Zn, took place on crystal surface under illumination [1,4,5]. At the same time, some earlier experimental data led to the conclusion that ZnO contained shallow donors, in all probability, Zn interstitials, which could migrate in crystal lattice at respectively low (about 400 K) temperatures [10–12]. If so, one can expect that these mobile donors, which are completely ionized and, thus, positively charged already below room temperature [10], will drift to negatively charged oxygen ions and will accumulate at the surface. Providing this mechanism realizes indeed, the layer enriched with donors must be unstable. It will dissolve with time after oxygen desorption when negative surface charge disappears and excessive donors are able to return to crystal bulk, and will appear again under subsequent oxygen adsorption. Both destruction and creation of the layer must be observed at temperatures, at which donors are mobile enough. To verify these suppositions, experiments described below were performed.
Fig. 1. Exciton luminescence spectra of ZnO crystal at 77 K measured for the same near-electrode sample region after action of electric field when the adjacent electrode acts as the cathode (1) and the anode (2); EdZ70 V/cm, TdZ600 K, DtdZ20 min. Schematic diagram of the sample measuring is shown in inset; shaded region indicates where exciton luminescence was measured.
intensity of donor-bound exciton luminescence band. Exciton luminescence was excited by N2-laser (lz337 nm) at 77 K. 2. Experimental details The effect of preliminary illumination on conductance and photoluminescence of bulk ZnO crystals at low temperatures was studied. Intentionally undoped ZnO single crystals were grown by a vapor-phase technique. The crystals were transparent colorless needles of hexagonal cross section of about 0.2–0.3 mm in diameter and of 10– 15 mm in length with rZ0.1–10 U cm. The most highly resistive crystals were selected for measurements. Ohmic indium electrodes were melted on both crystal ends (inset in Fig. 1). Focused light of xenon lamp was used for the sample illumination. Preliminary illumination was carried out in air, and after switching off the light the sample was immersed in liquid nitrogen. About 10 samples have been investigated, the results for all of them being similar. To make sure that mobile shallow donors were present in the crystals indeed, a technique based on space redistribution of charged mobile defects due to their drift in external electric field was used [13,14]. It is obvious that mobile donors must depart from the anode and accumulate at the cathode. The change of shallow donor density in near-electrode regions after external electric field action was controlled by the change in
3. Results and discussion Luminescence spectra were typical for undoped ZnO crystals and consisted of free exciton zero-phonon band I0 (lz368 nm), its phonon replicas I1 (lz374 nm) and I2 (lz383 nm), as well as of donor-bound exciton band ID (lz369 nm) [15] (Fig. 1). The sample was heated to TdZ 600–650 K and electric field EdZ50–100 V/cm was applied to it for time interval DtdZ20–30 min. Then the sample was cooled, the electric field was switched off and exciton luminescence was measured in near-electrode region (inset in Fig. 1). Next these procedures were repeated using opposite electric field polarity. Thus, each electrode acted alternately as both the cathode and the anode. The luminescence spectra for the same crystal region after application of electric field of different polarities were compared. As Fig. 1 shows, the action of electric field resulted in the rise of ID intensity at the cathode and its drop at the anode. These results indicate that after the action of external electric field the density of shallow donors
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responsible for ID band increased at the cathode and decreased at the anode. To reveal the prime conductance of the near-surface layer, oxygen photodesorption was carried out. For this purpose illumination of the sample with focused light of xenon lamp was used. Fig. 2 shows the temperature dependences of ZnO crystal dark conductance measured in optical cryostat before and after oxygen photodesorption. In the former case, the measurements were performed after keeping of the sample for several hours in the air in dark at 300 K (the initial state, curve 1). In the latter case, the sample was subjected to preliminary illumination (PI) at TPIZ300 K for time interval DtPIZ20 min and then was cooled in dark (curve 2). One can see that PI caused considerable increase of sample conductance at T!180 K. It means that at TO180 K surface conductance is less than bulk one, but under cooling the latter drops sharply and the former becomes dominant. In fact, it was found for different samples that the higher the crystal conductivity the lower the temperature at which surface conductance manifested itself. The initial state restored when the sample was kept in the air in dark at 300 K. To control the behavior of highly conductive layer the influence of oxygen desorption/ adsorption on the value of this layer prime conductance was investigated. The latter was determined from photoresponse kinetics curves measured at 77 K. The sample was immersed in liquid nitrogen both after keeping in dark in the air at 300 K (initial state) and after PI at different TPI (Fig. 3). The dark conductance at 77 K was too
low to be measured. In the initial state, after light switching on at first a sharp rise of conductance was observed and then its slow rise to saturation due to oxygen photodesorption occurred. After light switching off, the signal reduced to some steady-state value that was much higher than the value of the conductance before illumination (Fig. 3, curve 1). This residual conductance sr induced by illumination represents the prime conductance of the near surface layer (see above). It was found that short-term (DtPIZ20–30 min) PI at 300 K did not have any effect on sr value, while prolonged (DtPIZ 6–8 h) light action at this temperature resulted in its considerable decrease (Fig. 3, curve 2). The process of sr decrease accelerated with TPI rise (Fig. 3, curve 3) and at TPIZ 750 K to destroy sr completely DtPIZ5 min is already sufficient (Fig. 3, curve 4). When after that the sample was kept in dark in the air, sr restored; the higher the temperature the faster restoration process (Fig. 4). At the same time, the restoration did not occur when the sample with destroyed sr was kept in helium gas ambient. On the contrary, keeping of the sample with restored sr in helium gas ambient resulted in sr decrease. In the latter experiment the sample was illuminated for 20 min to desorb oxygen and then was kept in dark for several hours at 300 K. After such a procedure considerable reduction of sr was observed (Fig. 4, curve 5). The same sr reduction took place when the illumination of the sample kept in He gas was continuous. The destruction and creation of sr could be made many times one after the other in the same sample.
Fig. 2. Temperature dependence of ZnO crystal dark conductance measured after keeping in air in dark for several hours (1) and after preliminary illumination for 20 min (2) at 300 K.
Fig. 3. Relaxation kinetics of ZnO crystal photoresponse at 77 K in the initial state (1) as well as after preliminary illumination for 6 h at 300 K (2), for 5 min at 550 K (3) and for 5 min at 750 K (4).
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leads to the formation of highly conductive layer. It should be noted that highly conductive layer instability, which manifests itself already at room temperature, can contribute to device degradation and, so, must be taken into account in ZnO device development.
4. Conclusion In summary, the mechanism of formation of highly conductive layer on ZnO crystal surface has been proposed. The mechanism is based on redistribution of mobile donors in near-surface region due to their drift in depletion bandbanding field created by adsorbed oxygen. Mobile shallow donors were revealed by means of the technique based on charged defect drift in external electric field. Accumulation of these donors at the surface results in the rise of Fermi level in this region and, so, in the increase of surface conductivity. Experimental results that confirm this mechanism have been obtained. It has been shown that highly conductive layer can be destroyed and created over and over again. Both processes are observed at TR300 K and are controlled by oxygen adsorption/desorption. Found instability of highly conductive layer must be taken into account in ZnO device operation and development. Fig. 4. Relaxation kinetics of ZnO crystal photoresponse at 77 K after preliminary illumination for 5 min at 750 K (1) and following keeping in dark for 6 h at 300 K (2), for 1 min at 550 K (3) and for 5 min at 550 K (4). Curve 5-relaxation kinetics of the sample in the initial state (curve 4) after PI for 20 min and following keeping in dark for 8 h at 300 K in He gas ambient.
Acknowledgements
Above results show that highly conductive surface layer is unstable indeed. When the sample is in the air, this layer disappears under illumination and appears in dark, while in He gas ambient it disappears in dark as well as under illumination. Both destruction and creation processes were found to be thermally activated and to occur in the same temperature range. Thus one can conclude that the layer enriched with donors does not result from neither stationary doping of the surface with Zn during crystal growth [9], nor creation of excess Zn as a result of defect reactions induced by illumination [1,4,5]. Obtained results prove that mechanism based on donor drift in depletion band-bending field takes place in fact. Accumulation of shallow donors at the surface leads to the increase of their density and, so, to the rise of Fermi level in this region. As curve 2 in Fig. 2 shows, the prime conductance of near surface layer is almost independent of temperature, i.e. Fermi level in this region is close to c-band, and, so, practically all donor electrons contribute to nearsurface layer conductance. In crystal bulk Fermi level is much deeper (Fig. 2, curve 1), and when excess donors return to the bulk, only some quantity of their electrons remains in c-band, the lower the temperature the smaller this quantity. So donor redistribution by itself, without the increase of donor density,
References
The financial support of the National Academy of Sciences of Ukraine is kindly acknowledged.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15]
R.J. Collins, D.G. Thomas, Phys. Rev. 112 (1958) 388. H. Van Hove, A. Luyckx, Solid State Commun. 4 (1966) 603. W. Bauer, G. Heiland, J. Phys. Chem. Solids 32 (1971) 2605. W. Go¨pel, Surf. Sci. 62 (1977) 165. W. Go¨pel, U. Lampe, Phys. Rev. B 22 (1980) 6447. R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. p. 559. N. Golego, S.A. Studenikin, M. Cocivera, J. Electrochem. Soc. 147 (2000) 1592. V.L. Rapoport, L.L. Golego, Sov. Phys. Solid State 7 (1965) 1124. I.P. Kuz’mina, V.A. Nikitenko, Zinc Oxide. Production and Optical Properties, Nauka, Moskow, 1984. p. 166. D.G. Thomas, J. Phys. Chem. Solids 3 (1957) 229. T. Gupta, J. Am. Ceram. Soc. 73 (1990) 1817. D. Gerthsen, D. Litvinov, Th. Gruber, C. Kirchner, A. Waag, Appl. Phys. Lett. 81 (2002) 3972. N.E. Korsunskaya, I.V. Markevich, T.V. Torchinskaya, M.K. Sheinkman, J. Phys. C: Solid State Phys. 13 (1980) 2975. N.E. Korsunska, I.V. Markevich, L.V. Borkovska, L.Yu. Khomenkova, M.K. Sheinkman, O. Yastrubchak, Physica B 308–310 (2001) 967. R.L. Weiher, W.C. Talt, Phys. Rev. 166 (1968) 791.
Mechanism of formation of highly conductive layer on ZnO crystal surface I.V. Markevich*, V.I. Kushnirenko, L.V. Borkovska, B.M. Bulakh V. Lashkarev Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 45 Prospect Nauky, 03028 Kyiv, Ukraine Received 5 August 2005; accepted 2 September 2005 by E.L. Ivchenko Available online 19 September 2005
Abstract The mechanism of formation of a thin highly conductive layer, which is known to be present on ZnO surface, has been proposed. This process has been assumed to consist in accumulation of mobile shallow donors at crystal surface due to their drift in bandbending electric field caused by adsorbed oxygen. Experimental results that confirm this mechanism have been obtained. q 2005 Elsevier Ltd. All rights reserved. PACS: 61.72.Ji; 66.30.Kh; 73.25.Ci Keywords: A. Semiconductors; C. Point defects; D. Diffusion in solids; D. Surface conductivity
1. Introduction Zinc oxide has many device applications in electronics, including transparent conducting films, piezoelectric transducers, phosphors, varistors and gas sensors; it is also a promising material for ultraviolet lasers and light-emitting diodes. These applications involved mainly thin layers and films, characteristics of which are strongly influenced by surface conditions. Therefore, detailed investigation of the change of ZnO surface properties under various external factors is important. One of these factors is atmospheric oxygen. It is known that ZnO is n-type and adsorbs oxygen from the air [1–9]. Oxygen atoms capture crystal electrons (i.e. act as acceptors), which results in creation of negative surface charge and formation of depletion layer with reduced conductivity. In undoped ZnO the thickness of this layer d1!1 mm [3,4]. The oxygen can be desorbed by both heating and light illumination. The thermal desorption, which occurs due to thermal ionization of electrons from oxygen, has activation energy of 1.1 eV and is essential at TR400 K [4]. The photodesorption, * Corresponding author. Tel./fax: C38 44 525 83 44. E-mail address: [email protected] (I.V. Markevich).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.09.001
which results from the capture of free photoholes by oxygen acceptors, is athermal process and can be observed at low temperatures [6]. After oxygen desorption the captured electrons return to the crystal and the prime conductivity of the near-surface region restores. It is evident that adsorption/desorption processes will strongly influence the conductance of the sample of the thickness d2%d1. In fact, oxygen desorption causes drastic increase in conductance of films, thin layers and powders [7–9]. The same effect, however, was also found in ZnO crystals with d2[d1. To explain this phenomenon it was supposed that a thin layer enriched with shallow donors was present on crystal surface, the prime conductance of such a layer exceeding the conductance of the rest of the sample [1–5]. This layer does not manifest itself while oxygen is adsorbed, but under oxygen desorption its conductance increases and becomes dominant. As a result, under illumination two components of photoresponse are observed: (i) usual photoconductance caused by photocarrier generation; (ii) conductance caused by accumulation of electrons in depletion layer due to oxygen photodesorption [6]. When the light is switched off, after relaxation of the photoconductance a residual conductance, which is the
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prime conductance of the near-surface layer, is observed [6]. When the sample is exposed to oxygen or to the air, the value of this residual conductance reduces because of oxygen readsorption [2–6]. The reduction process slows down with time due to the gradual increase of negative surface charge and, so, due to the rise of the potential barrier for electron capture by oxygen [2]. Increased surface conductivity of ZnO crystals was ascribed to the presence of excessive Zn [1,4,5,9]. However, the origin of Zn excess was not definitely established. Some authors believed that surface layer enriched with Zn was formed during crystal growth [9], others thought that some defect reactions, which resulted in appearance of nonstoichiometric Zn, took place on crystal surface under illumination [1,4,5]. At the same time, some earlier experimental data led to the conclusion that ZnO contained shallow donors, in all probability, Zn interstitials, which could migrate in crystal lattice at respectively low (about 400 K) temperatures [10–12]. If so, one can expect that these mobile donors, which are completely ionized and, thus, positively charged already below room temperature [10], will drift to negatively charged oxygen ions and will accumulate at the surface. Providing this mechanism realizes indeed, the layer enriched with donors must be unstable. It will dissolve with time after oxygen desorption when negative surface charge disappears and excessive donors are able to return to crystal bulk, and will appear again under subsequent oxygen adsorption. Both destruction and creation of the layer must be observed at temperatures, at which donors are mobile enough. To verify these suppositions, experiments described below were performed.
Fig. 1. Exciton luminescence spectra of ZnO crystal at 77 K measured for the same near-electrode sample region after action of electric field when the adjacent electrode acts as the cathode (1) and the anode (2); EdZ70 V/cm, TdZ600 K, DtdZ20 min. Schematic diagram of the sample measuring is shown in inset; shaded region indicates where exciton luminescence was measured.
intensity of donor-bound exciton luminescence band. Exciton luminescence was excited by N2-laser (lz337 nm) at 77 K. 2. Experimental details The effect of preliminary illumination on conductance and photoluminescence of bulk ZnO crystals at low temperatures was studied. Intentionally undoped ZnO single crystals were grown by a vapor-phase technique. The crystals were transparent colorless needles of hexagonal cross section of about 0.2–0.3 mm in diameter and of 10– 15 mm in length with rZ0.1–10 U cm. The most highly resistive crystals were selected for measurements. Ohmic indium electrodes were melted on both crystal ends (inset in Fig. 1). Focused light of xenon lamp was used for the sample illumination. Preliminary illumination was carried out in air, and after switching off the light the sample was immersed in liquid nitrogen. About 10 samples have been investigated, the results for all of them being similar. To make sure that mobile shallow donors were present in the crystals indeed, a technique based on space redistribution of charged mobile defects due to their drift in external electric field was used [13,14]. It is obvious that mobile donors must depart from the anode and accumulate at the cathode. The change of shallow donor density in near-electrode regions after external electric field action was controlled by the change in
3. Results and discussion Luminescence spectra were typical for undoped ZnO crystals and consisted of free exciton zero-phonon band I0 (lz368 nm), its phonon replicas I1 (lz374 nm) and I2 (lz383 nm), as well as of donor-bound exciton band ID (lz369 nm) [15] (Fig. 1). The sample was heated to TdZ 600–650 K and electric field EdZ50–100 V/cm was applied to it for time interval DtdZ20–30 min. Then the sample was cooled, the electric field was switched off and exciton luminescence was measured in near-electrode region (inset in Fig. 1). Next these procedures were repeated using opposite electric field polarity. Thus, each electrode acted alternately as both the cathode and the anode. The luminescence spectra for the same crystal region after application of electric field of different polarities were compared. As Fig. 1 shows, the action of electric field resulted in the rise of ID intensity at the cathode and its drop at the anode. These results indicate that after the action of external electric field the density of shallow donors
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477
responsible for ID band increased at the cathode and decreased at the anode. To reveal the prime conductance of the near-surface layer, oxygen photodesorption was carried out. For this purpose illumination of the sample with focused light of xenon lamp was used. Fig. 2 shows the temperature dependences of ZnO crystal dark conductance measured in optical cryostat before and after oxygen photodesorption. In the former case, the measurements were performed after keeping of the sample for several hours in the air in dark at 300 K (the initial state, curve 1). In the latter case, the sample was subjected to preliminary illumination (PI) at TPIZ300 K for time interval DtPIZ20 min and then was cooled in dark (curve 2). One can see that PI caused considerable increase of sample conductance at T!180 K. It means that at TO180 K surface conductance is less than bulk one, but under cooling the latter drops sharply and the former becomes dominant. In fact, it was found for different samples that the higher the crystal conductivity the lower the temperature at which surface conductance manifested itself. The initial state restored when the sample was kept in the air in dark at 300 K. To control the behavior of highly conductive layer the influence of oxygen desorption/ adsorption on the value of this layer prime conductance was investigated. The latter was determined from photoresponse kinetics curves measured at 77 K. The sample was immersed in liquid nitrogen both after keeping in dark in the air at 300 K (initial state) and after PI at different TPI (Fig. 3). The dark conductance at 77 K was too
low to be measured. In the initial state, after light switching on at first a sharp rise of conductance was observed and then its slow rise to saturation due to oxygen photodesorption occurred. After light switching off, the signal reduced to some steady-state value that was much higher than the value of the conductance before illumination (Fig. 3, curve 1). This residual conductance sr induced by illumination represents the prime conductance of the near surface layer (see above). It was found that short-term (DtPIZ20–30 min) PI at 300 K did not have any effect on sr value, while prolonged (DtPIZ 6–8 h) light action at this temperature resulted in its considerable decrease (Fig. 3, curve 2). The process of sr decrease accelerated with TPI rise (Fig. 3, curve 3) and at TPIZ 750 K to destroy sr completely DtPIZ5 min is already sufficient (Fig. 3, curve 4). When after that the sample was kept in dark in the air, sr restored; the higher the temperature the faster restoration process (Fig. 4). At the same time, the restoration did not occur when the sample with destroyed sr was kept in helium gas ambient. On the contrary, keeping of the sample with restored sr in helium gas ambient resulted in sr decrease. In the latter experiment the sample was illuminated for 20 min to desorb oxygen and then was kept in dark for several hours at 300 K. After such a procedure considerable reduction of sr was observed (Fig. 4, curve 5). The same sr reduction took place when the illumination of the sample kept in He gas was continuous. The destruction and creation of sr could be made many times one after the other in the same sample.
Fig. 2. Temperature dependence of ZnO crystal dark conductance measured after keeping in air in dark for several hours (1) and after preliminary illumination for 20 min (2) at 300 K.
Fig. 3. Relaxation kinetics of ZnO crystal photoresponse at 77 K in the initial state (1) as well as after preliminary illumination for 6 h at 300 K (2), for 5 min at 550 K (3) and for 5 min at 750 K (4).
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leads to the formation of highly conductive layer. It should be noted that highly conductive layer instability, which manifests itself already at room temperature, can contribute to device degradation and, so, must be taken into account in ZnO device development.
4. Conclusion In summary, the mechanism of formation of highly conductive layer on ZnO crystal surface has been proposed. The mechanism is based on redistribution of mobile donors in near-surface region due to their drift in depletion bandbanding field created by adsorbed oxygen. Mobile shallow donors were revealed by means of the technique based on charged defect drift in external electric field. Accumulation of these donors at the surface results in the rise of Fermi level in this region and, so, in the increase of surface conductivity. Experimental results that confirm this mechanism have been obtained. It has been shown that highly conductive layer can be destroyed and created over and over again. Both processes are observed at TR300 K and are controlled by oxygen adsorption/desorption. Found instability of highly conductive layer must be taken into account in ZnO device operation and development. Fig. 4. Relaxation kinetics of ZnO crystal photoresponse at 77 K after preliminary illumination for 5 min at 750 K (1) and following keeping in dark for 6 h at 300 K (2), for 1 min at 550 K (3) and for 5 min at 550 K (4). Curve 5-relaxation kinetics of the sample in the initial state (curve 4) after PI for 20 min and following keeping in dark for 8 h at 300 K in He gas ambient.
Acknowledgements
Above results show that highly conductive surface layer is unstable indeed. When the sample is in the air, this layer disappears under illumination and appears in dark, while in He gas ambient it disappears in dark as well as under illumination. Both destruction and creation processes were found to be thermally activated and to occur in the same temperature range. Thus one can conclude that the layer enriched with donors does not result from neither stationary doping of the surface with Zn during crystal growth [9], nor creation of excess Zn as a result of defect reactions induced by illumination [1,4,5]. Obtained results prove that mechanism based on donor drift in depletion band-bending field takes place in fact. Accumulation of shallow donors at the surface leads to the increase of their density and, so, to the rise of Fermi level in this region. As curve 2 in Fig. 2 shows, the prime conductance of near surface layer is almost independent of temperature, i.e. Fermi level in this region is close to c-band, and, so, practically all donor electrons contribute to nearsurface layer conductance. In crystal bulk Fermi level is much deeper (Fig. 2, curve 1), and when excess donors return to the bulk, only some quantity of their electrons remains in c-band, the lower the temperature the smaller this quantity. So donor redistribution by itself, without the increase of donor density,
References
The financial support of the National Academy of Sciences of Ukraine is kindly acknowledged.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15]
R.J. Collins, D.G. Thomas, Phys. Rev. 112 (1958) 388. H. Van Hove, A. Luyckx, Solid State Commun. 4 (1966) 603. W. Bauer, G. Heiland, J. Phys. Chem. Solids 32 (1971) 2605. W. Go¨pel, Surf. Sci. 62 (1977) 165. W. Go¨pel, U. Lampe, Phys. Rev. B 22 (1980) 6447. R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. p. 559. N. Golego, S.A. Studenikin, M. Cocivera, J. Electrochem. Soc. 147 (2000) 1592. V.L. Rapoport, L.L. Golego, Sov. Phys. Solid State 7 (1965) 1124. I.P. Kuz’mina, V.A. Nikitenko, Zinc Oxide. Production and Optical Properties, Nauka, Moskow, 1984. p. 166. D.G. Thomas, J. Phys. Chem. Solids 3 (1957) 229. T. Gupta, J. Am. Ceram. Soc. 73 (1990) 1817. D. Gerthsen, D. Litvinov, Th. Gruber, C. Kirchner, A. Waag, Appl. Phys. Lett. 81 (2002) 3972. N.E. Korsunskaya, I.V. Markevich, T.V. Torchinskaya, M.K. Sheinkman, J. Phys. C: Solid State Phys. 13 (1980) 2975. N.E. Korsunska, I.V. Markevich, L.V. Borkovska, L.Yu. Khomenkova, M.K. Sheinkman, O. Yastrubchak, Physica B 308–310 (2001) 967. R.L. Weiher, W.C. Talt, Phys. Rev. 166 (1968) 791.