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TEM studies of as-grown, irradiated and annealed InN films
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Physica B 401–402 (2007) 646–649 www.elsevier.com/locate/physb
TEM studies of as-grown, irradiated and annealed InN films Z. Liliental-Webera,, R.E. Jonesa,b, H.C.M. van Genuchtena, K.M. Yua, W. Walukiewicza, J.W. Ager IIIa, E.E. Hallera,b, H. Luc, W.J. Schaffc a
Materials Sciences Division, Lawrence Berkeley National Laboratory, MS 62/203, Cyclotron Road, Berkeley, CA 94720, USA b Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA c Department of Electrical Engineering and Computer Science, Cornell University, Ithaca, NY 14853, USA
Abstract Transmission electron microscopy was applied to study structural changes of InN films grown by molecular beam epitaxy on c-sapphire substrates with a GaN buffer layer. The films were studied as-grown and also following by rapid thermal annealing, irradiation with 2 MeV He+ ions, and annealing after irradiation. Defects formed after each procedure were discussed. The results of these studies show that randomly distributed extended defects, formed upon irradiation, can come to uniform distribution throughout the samples upon annealing. An irradiation by 2 MeV He+ ions followed by thermal annealing at 425 and 475 1C leads to the unusual increase of the electron mobility of these films. Annealing at 500 1C led to the formation of In clusters and delamination of the film from the substrates. Published by Elsevier B.V. Keywords: InN; Transmission electron microscopy; Annealing; Irradiation; Defects
1. Introduction Indium nitride (InN) is an important III–V compound semiconductor with potential applications in microelectronics, optoelectronics and solar cells. Due to its narrow band gap (0.7 eV) [1], alloying with GaN and AlN ensures light emission from ultraviolet to red, covering in this way the whole solar spectrum. InN has the smallest effective electron mass of all the group-III nitrides, which leads to a potentially high mobility, saturation velocity and a large drift velocity at room temperature. It is, therefore, not surprising to observe rapidly increasing interest in the above-mentioned applications. The growth of high-quality epitaxial layers of InN has been established by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Preparation of high-quality InN is rather difficult and requires low growth temperatures due to the low InN dissociation temperature and the highequilibrium N2 vapor pressure over the InN film. Our films were grown by MBE and details of this growth procedure Corresponding author. Tel.: +1 510 486 6276; fax: +1 510 486 4995.
E-mail address: [email protected] (Z. Liliental-Weber). 0921-4526/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.physb.2007.09.042
are described elsewhere [2]. The majority of the films were grown on (0 0 0 1) sapphire wafers and some were also grown or r-plane sapphire substrates with a GaN buffer layer. Electron transport in the material is not fully understood. Our earlier studies of the effect of 2 MeV He+ irradiation followed by thermal annealing showed that this process creates films with high electron concentrations and high electron mobilities. There is a linear relationship between electron concentration and applied He ion fluences, which changes further upon annealing [3]. The irradiation fluences ranged from 1.1 1015 to 8.9 1015 cm2. The films were annealed in the temperature range of 375–500 1C. Transmission electron microscopy (TEM) has been used to study defects and their nature in the as-grown films and also in irradiated and annealed films. Cross-section and plan-view samples, transparent for electrons, have been prepared. A JEOL 3010 with an accelerating voltage of 300 keV and a sub-Angstrom CM 300 were used in these studies. Convergent beam electron diffraction (CBED) along the ½1 1 0 0 zone axis together with computer simulation was used to determine the growth polarity of the layers.
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2. Results and discussion 2.1. As-grown samples The InN layers studied are 0.7–2 mm thick. Bright field TEM studies of as-grown films (grown on c-plane Al2O3) show that the InN layer makes an abrupt interface with an underlying GaN buffer layer. In high-resolution images, this interface appears as a straight line, similar to the interface between sapphire and GaN. However, there are some areas where this interface is slightly undulated. Some additional dislocations are formed at this interface leading to a higher density of dislocations in the InN layer above this interface (2 1010 cm2). The majority of dislocations (2/3) have a screw/mixed character and only 1/3 are of an edge character. The density of dislocations decreases toward the sample surface (8 109 cm2) (Fig. 1). The distribution of defects in different samples grown under similar growth condition differs. There are areas (sometimes in the same sample) where many stacking faults are formed on basal and prismatic planes in addition to dislocations. The basal stacking faults are delineated either by prismatic stacking faults (as found earlier in a-plane GaN [4,5] grown on a-plane SiC or r-plane Al2O3) or by partial dislocations. However, for InN samples grown on r-plane Al2O3, clear columnar growth is observed (Fig. 2). The column size is in a range of 100–250 nm. The density of defects in these samples reaches as high as 1011 cm2. Basal and prismatic stacking faults are also present in these samples, but the distribution of defects is similar to those observed in a-plane grown GaN [4]. The surface of the samples grown on a-plane Al2O3 is much more corrugated than the samples grown on c-plane Al2O3 (Fig. 2). The differences between the valleys and tips of columns can be
Fig. 2. Columnar growth of a-plane InN. Note narrow columns and substantial surface corrugation.
Fig. 3. An experimental (a) and calculated (b) CBED pattern for 100 nm thick InN sample (c) grown on c-plane GaN. An arrow shows the growth direction and In polarity.
as large as 80 nm; while in the samples grown on c-plane, this is never larger than 5–10 nm. CBED was applied to study the growth polarity of InN layers grown on the c-plane of sapphire using a JEOL 3010 TEM. The experimental patterns were taken for different sample thicknesses. CBED patterns for the same zone axis and sample thickness were simulated for the accelerating voltage (300 keV), as used in the experiment. A good agreement between experimental and calculated patterns was obtained. Based on these experiments and taking into account the rotation angle between the image and a diffraction pattern in our microscope, it was determined that the layers were grown with In polarity (Fig. 3). 2.2. Irradiated samples
Fig. 1. TEM micrograph of an as-grown InN sample.
The samples were irradiated with 2 MeV He+ particles in order to generate point defects. The irradiation fluences ranged from 1.1 1015 cm2 to 8.9 1015 cm2. Only small structural changes are observed in the samples that were irradiated with lower fluences (Fig. 4a). Formation of small
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Fig. 5. TEM micrograph of the annealed as-grown sample revealing formation of voids (three arrows) along the dislocation lines and voids associated with a precipitate. Another arrow indicates an In precipitate with Moire´’ fringes.
Fig. 4. TEM micrographs showing irradiated samples with a fluence of (a) 1.1 1015 cm2 and (b) with 8.9 1015 cm2. Note high density of planar defects (SF) in (a). Examples of dislocation loops (l) and nanopipes (n) are shown.
dislocation loops could be observed as a result of point defect agglomeration. These loops could be found in the areas free of dislocations as well as along dislocation lines. The density of dislocation loops in the areas free of linear or planar defects was estimated to be 8.6 109 cm2. High densities of planar defects have been found in these samples, the majority of which are bands of basal stacking faults surrounded by prismatic stacking faults or dislocation lines (Fig. 4a). It is believed that the presence of these defects is rather characteristic of as-grown material to which the irradiation was applied and not a result of irradiation, despite that the density of planar defects in this particular sample was higher than in the other samples. For higher fluences, the size of these loops and their density slightly increases and in some areas, their density can be measured as 2.2 1010 cm2 (Fig. 4b). 2.3. Annealed samples Rapid thermal annealing was performed on as-grown and also on earlier irradiated samples using a Heatpulse 10T-02 Rapid Thermal Pulsing System with flowing N2
gas. The samples were annealed at 375 or 425 1C for different times varying from 10 to 300 s. Some samples were also annealed at 475 and 500 1C. There were no observable structural changes in the as-grown samples annealed at 375 1C. However, annealing of samples at 475 1C showed some changes (Fig. 5). The main defects were small voids distributed along screw and edge dislocations. Their size did not exceed more than 5 nm and the distance between voids was about 20 nm. These voids were not observed in the as-grown samples. Occasionally, one could find areas where Moire´’ fringes indicated the presence of spherical In precipitates but their density was rather low (only three precipitates were found in whole sample) and their size did not exceed 15 nm. In one area of the sample, an elongated precipitate was found with a length of 90 nm and a width of 25 nm. This precipitate did not show any Moire´’ fringes (might be located close to the sample surface), but it was connected with void formed at one end of the precipitate of a size of 15 20 nm2. Studies are underway to determine the composition of such precipitates and frequency of their appearance. Annealed samples also show surface deterioration (formation of triangular funnels), which mainly occurs in the areas where two or three dislocations are close to each other. Samples that were irradiated with a lower fluence (1.1 1015 cm2) and subsequently annealed show the formation of similar defects as annealed as-grown samples, but the funnels formed at the sample surface enter deeper into the sample. Many nanopipes were also formed along the dislocations. Similarly to irradiated samples (with the same fluence), lamellas of stacking faults were often present in this sample. Samples that were irradiated with a higher fluence (8.9 1015 cm2) and subsequently annealed show substantial structural changes (Fig. 6). High densities of
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c-plane sapphire were studied, but some information was also given for the samples grown on r-Al2O3. The influence of irradiation by 2 MeV He+ ions followed by rapid thermal annealing of these samples was characterized. This process creates films with high electron concentrations and high electron mobilities. TEM studies showed that small dislocation loops were created upon irradiation. Annealing alone leads to the formation of voids along dislocation lines and nanotubes. Some groves were formed along grain boundaries. Annealing of irradiated samples leads to enhanced motion of point defects. As a result, dislocation loops become larger and more uniformly distributed throughout the sample. The path of existing dislocations becomes undulated. Fig. 6. Formation of high density of dislocation loops in the irradiated sample followed by annealing at 475 1C. 11
Acknowledgments
2
dislocation loops (2.8 10 cm ) are formed through the samples. Their sizes vary from 5 to 20 nm. An interaction of the loops with dislocation lines leads to some undulation of dislocations. The formation of larger loops and an increase of their density might be due to the agglomeration of individual vacancies created by the irradiation. Large voids are also formed at the interface with GaN. Large groves are formed at the sample surface at grain boundaries. Annealing at 500 1C led to the formation of In clusters and delamination of the film from the substrates. 3. Conclusions In this paper, structural defects formed in the InN layers grown by MBE on GaN/Al2O3 substrates were characterized using TEM methods. Mostly samples grown on
This work is supported by the US Department of Energy under Contract no. DE-AC02-05CH11231. The use of electron microscopes in the NCEM of the LBNL is greatly appreciated. References [1] W. Walukiewicz, J.W. Ager III, K.M. Yu, Z. Liliental-Weber, J. Wu, S.X. Li, R.E. Jones, J.D. Denlinger, J. Phys. D 39 (2006) R85. [2] H. Lu, W.J. Schaff, J. Hwang, H. Wu, et al., Appl. Phys. Lett. 77 (2000) 2548. [3] R.E. Jones, H.C.M. van Genuchten, K.M. Yu, W. Walukiewicz, S.X. Li, J.W. Ager III, Z. Liliental-Weber, E.E. Haller, H. Lu, W.J. Schaff, Appl. Phys. Lett. 90 (2007) 162103. [4] D.N. Zakharov, Z. Liliental-Weber, B. Wagner, Z.J. Reitmeier, E.A. Preble, R.F. Davis, Phys. Rev. B 71 (2005) 2353345. [5] Z. Liliental-Weber, X. Ni, H. Morkoc, Mater. Res. Soc. Proc. 955-E (2006) 0955-I07-5.
Physica B 401–402 (2007) 646–649 www.elsevier.com/locate/physb
TEM studies of as-grown, irradiated and annealed InN films Z. Liliental-Webera,, R.E. Jonesa,b, H.C.M. van Genuchtena, K.M. Yua, W. Walukiewicza, J.W. Ager IIIa, E.E. Hallera,b, H. Luc, W.J. Schaffc a
Materials Sciences Division, Lawrence Berkeley National Laboratory, MS 62/203, Cyclotron Road, Berkeley, CA 94720, USA b Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA c Department of Electrical Engineering and Computer Science, Cornell University, Ithaca, NY 14853, USA
Abstract Transmission electron microscopy was applied to study structural changes of InN films grown by molecular beam epitaxy on c-sapphire substrates with a GaN buffer layer. The films were studied as-grown and also following by rapid thermal annealing, irradiation with 2 MeV He+ ions, and annealing after irradiation. Defects formed after each procedure were discussed. The results of these studies show that randomly distributed extended defects, formed upon irradiation, can come to uniform distribution throughout the samples upon annealing. An irradiation by 2 MeV He+ ions followed by thermal annealing at 425 and 475 1C leads to the unusual increase of the electron mobility of these films. Annealing at 500 1C led to the formation of In clusters and delamination of the film from the substrates. Published by Elsevier B.V. Keywords: InN; Transmission electron microscopy; Annealing; Irradiation; Defects
1. Introduction Indium nitride (InN) is an important III–V compound semiconductor with potential applications in microelectronics, optoelectronics and solar cells. Due to its narrow band gap (0.7 eV) [1], alloying with GaN and AlN ensures light emission from ultraviolet to red, covering in this way the whole solar spectrum. InN has the smallest effective electron mass of all the group-III nitrides, which leads to a potentially high mobility, saturation velocity and a large drift velocity at room temperature. It is, therefore, not surprising to observe rapidly increasing interest in the above-mentioned applications. The growth of high-quality epitaxial layers of InN has been established by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Preparation of high-quality InN is rather difficult and requires low growth temperatures due to the low InN dissociation temperature and the highequilibrium N2 vapor pressure over the InN film. Our films were grown by MBE and details of this growth procedure Corresponding author. Tel.: +1 510 486 6276; fax: +1 510 486 4995.
E-mail address: [email protected] (Z. Liliental-Weber). 0921-4526/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.physb.2007.09.042
are described elsewhere [2]. The majority of the films were grown on (0 0 0 1) sapphire wafers and some were also grown or r-plane sapphire substrates with a GaN buffer layer. Electron transport in the material is not fully understood. Our earlier studies of the effect of 2 MeV He+ irradiation followed by thermal annealing showed that this process creates films with high electron concentrations and high electron mobilities. There is a linear relationship between electron concentration and applied He ion fluences, which changes further upon annealing [3]. The irradiation fluences ranged from 1.1 1015 to 8.9 1015 cm2. The films were annealed in the temperature range of 375–500 1C. Transmission electron microscopy (TEM) has been used to study defects and their nature in the as-grown films and also in irradiated and annealed films. Cross-section and plan-view samples, transparent for electrons, have been prepared. A JEOL 3010 with an accelerating voltage of 300 keV and a sub-Angstrom CM 300 were used in these studies. Convergent beam electron diffraction (CBED) along the ½1 1 0 0 zone axis together with computer simulation was used to determine the growth polarity of the layers.
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2. Results and discussion 2.1. As-grown samples The InN layers studied are 0.7–2 mm thick. Bright field TEM studies of as-grown films (grown on c-plane Al2O3) show that the InN layer makes an abrupt interface with an underlying GaN buffer layer. In high-resolution images, this interface appears as a straight line, similar to the interface between sapphire and GaN. However, there are some areas where this interface is slightly undulated. Some additional dislocations are formed at this interface leading to a higher density of dislocations in the InN layer above this interface (2 1010 cm2). The majority of dislocations (2/3) have a screw/mixed character and only 1/3 are of an edge character. The density of dislocations decreases toward the sample surface (8 109 cm2) (Fig. 1). The distribution of defects in different samples grown under similar growth condition differs. There are areas (sometimes in the same sample) where many stacking faults are formed on basal and prismatic planes in addition to dislocations. The basal stacking faults are delineated either by prismatic stacking faults (as found earlier in a-plane GaN [4,5] grown on a-plane SiC or r-plane Al2O3) or by partial dislocations. However, for InN samples grown on r-plane Al2O3, clear columnar growth is observed (Fig. 2). The column size is in a range of 100–250 nm. The density of defects in these samples reaches as high as 1011 cm2. Basal and prismatic stacking faults are also present in these samples, but the distribution of defects is similar to those observed in a-plane grown GaN [4]. The surface of the samples grown on a-plane Al2O3 is much more corrugated than the samples grown on c-plane Al2O3 (Fig. 2). The differences between the valleys and tips of columns can be
Fig. 2. Columnar growth of a-plane InN. Note narrow columns and substantial surface corrugation.
Fig. 3. An experimental (a) and calculated (b) CBED pattern for 100 nm thick InN sample (c) grown on c-plane GaN. An arrow shows the growth direction and In polarity.
as large as 80 nm; while in the samples grown on c-plane, this is never larger than 5–10 nm. CBED was applied to study the growth polarity of InN layers grown on the c-plane of sapphire using a JEOL 3010 TEM. The experimental patterns were taken for different sample thicknesses. CBED patterns for the same zone axis and sample thickness were simulated for the accelerating voltage (300 keV), as used in the experiment. A good agreement between experimental and calculated patterns was obtained. Based on these experiments and taking into account the rotation angle between the image and a diffraction pattern in our microscope, it was determined that the layers were grown with In polarity (Fig. 3). 2.2. Irradiated samples
Fig. 1. TEM micrograph of an as-grown InN sample.
The samples were irradiated with 2 MeV He+ particles in order to generate point defects. The irradiation fluences ranged from 1.1 1015 cm2 to 8.9 1015 cm2. Only small structural changes are observed in the samples that were irradiated with lower fluences (Fig. 4a). Formation of small
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Fig. 5. TEM micrograph of the annealed as-grown sample revealing formation of voids (three arrows) along the dislocation lines and voids associated with a precipitate. Another arrow indicates an In precipitate with Moire´’ fringes.
Fig. 4. TEM micrographs showing irradiated samples with a fluence of (a) 1.1 1015 cm2 and (b) with 8.9 1015 cm2. Note high density of planar defects (SF) in (a). Examples of dislocation loops (l) and nanopipes (n) are shown.
dislocation loops could be observed as a result of point defect agglomeration. These loops could be found in the areas free of dislocations as well as along dislocation lines. The density of dislocation loops in the areas free of linear or planar defects was estimated to be 8.6 109 cm2. High densities of planar defects have been found in these samples, the majority of which are bands of basal stacking faults surrounded by prismatic stacking faults or dislocation lines (Fig. 4a). It is believed that the presence of these defects is rather characteristic of as-grown material to which the irradiation was applied and not a result of irradiation, despite that the density of planar defects in this particular sample was higher than in the other samples. For higher fluences, the size of these loops and their density slightly increases and in some areas, their density can be measured as 2.2 1010 cm2 (Fig. 4b). 2.3. Annealed samples Rapid thermal annealing was performed on as-grown and also on earlier irradiated samples using a Heatpulse 10T-02 Rapid Thermal Pulsing System with flowing N2
gas. The samples were annealed at 375 or 425 1C for different times varying from 10 to 300 s. Some samples were also annealed at 475 and 500 1C. There were no observable structural changes in the as-grown samples annealed at 375 1C. However, annealing of samples at 475 1C showed some changes (Fig. 5). The main defects were small voids distributed along screw and edge dislocations. Their size did not exceed more than 5 nm and the distance between voids was about 20 nm. These voids were not observed in the as-grown samples. Occasionally, one could find areas where Moire´’ fringes indicated the presence of spherical In precipitates but their density was rather low (only three precipitates were found in whole sample) and their size did not exceed 15 nm. In one area of the sample, an elongated precipitate was found with a length of 90 nm and a width of 25 nm. This precipitate did not show any Moire´’ fringes (might be located close to the sample surface), but it was connected with void formed at one end of the precipitate of a size of 15 20 nm2. Studies are underway to determine the composition of such precipitates and frequency of their appearance. Annealed samples also show surface deterioration (formation of triangular funnels), which mainly occurs in the areas where two or three dislocations are close to each other. Samples that were irradiated with a lower fluence (1.1 1015 cm2) and subsequently annealed show the formation of similar defects as annealed as-grown samples, but the funnels formed at the sample surface enter deeper into the sample. Many nanopipes were also formed along the dislocations. Similarly to irradiated samples (with the same fluence), lamellas of stacking faults were often present in this sample. Samples that were irradiated with a higher fluence (8.9 1015 cm2) and subsequently annealed show substantial structural changes (Fig. 6). High densities of
ARTICLE IN PRESS Z. Liliental-Weber et al. / Physica B 401–402 (2007) 646–649
649
c-plane sapphire were studied, but some information was also given for the samples grown on r-Al2O3. The influence of irradiation by 2 MeV He+ ions followed by rapid thermal annealing of these samples was characterized. This process creates films with high electron concentrations and high electron mobilities. TEM studies showed that small dislocation loops were created upon irradiation. Annealing alone leads to the formation of voids along dislocation lines and nanotubes. Some groves were formed along grain boundaries. Annealing of irradiated samples leads to enhanced motion of point defects. As a result, dislocation loops become larger and more uniformly distributed throughout the sample. The path of existing dislocations becomes undulated. Fig. 6. Formation of high density of dislocation loops in the irradiated sample followed by annealing at 475 1C. 11
Acknowledgments
2
dislocation loops (2.8 10 cm ) are formed through the samples. Their sizes vary from 5 to 20 nm. An interaction of the loops with dislocation lines leads to some undulation of dislocations. The formation of larger loops and an increase of their density might be due to the agglomeration of individual vacancies created by the irradiation. Large voids are also formed at the interface with GaN. Large groves are formed at the sample surface at grain boundaries. Annealing at 500 1C led to the formation of In clusters and delamination of the film from the substrates. 3. Conclusions In this paper, structural defects formed in the InN layers grown by MBE on GaN/Al2O3 substrates were characterized using TEM methods. Mostly samples grown on
This work is supported by the US Department of Energy under Contract no. DE-AC02-05CH11231. The use of electron microscopes in the NCEM of the LBNL is greatly appreciated. References [1] W. Walukiewicz, J.W. Ager III, K.M. Yu, Z. Liliental-Weber, J. Wu, S.X. Li, R.E. Jones, J.D. Denlinger, J. Phys. D 39 (2006) R85. [2] H. Lu, W.J. Schaff, J. Hwang, H. Wu, et al., Appl. Phys. Lett. 77 (2000) 2548. [3] R.E. Jones, H.C.M. van Genuchten, K.M. Yu, W. Walukiewicz, S.X. Li, J.W. Ager III, Z. Liliental-Weber, E.E. Haller, H. Lu, W.J. Schaff, Appl. Phys. Lett. 90 (2007) 162103. [4] D.N. Zakharov, Z. Liliental-Weber, B. Wagner, Z.J. Reitmeier, E.A. Preble, R.F. Davis, Phys. Rev. B 71 (2005) 2353345. [5] Z. Liliental-Weber, X. Ni, H. Morkoc, Mater. Res. Soc. Proc. 955-E (2006) 0955-I07-5.