- Email: [email protected]
GaN heterostructures
Physica B 407 (2012) 2838–2840
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural and local electrical properties of AlInN/AlN/GaN heterostructures A. Minj, D. Cavalcoli, A. Cavallini n Dipartimento di Fisica, Universita di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Available online 18 August 2011
GaN layers and Al1 xInxN/AlN/GaN heterostructures have been studied by scanning probe microscopy methods. Threading dislocations (TDs), originating from the GaN (0 0 0 1) layer grown on sapphire, have been investigated. Using Current-Atomic Force Microscopy (C-AFM) TDs have been found to be highly conductive in both GaN and AlInN, while using semi-contact AFM (phase-imaging mode) indium segregation has been traced at TDs in AlInN/AlN/GaN heterostructures. It has been assessed that In segregation is responsible for high conductivity at dislocations in the examined heterostructures. & 2011 Elsevier B.V. All rights reserved.
Keywords: Al1 xInxN/AlN/GaN heterostructures Atomic Force Microscopy Current-AFM V-defects In segregation Local conductivity
1. Introduction High electron mobility transistor (HEMT) devices based on III–V nitrides are excellent candidates for high power and high frequency applications. GaN HEMTs are usually achieved using either AlGaN or AlInN as the barrier layer on top of a GaN buffer layer. Due to strong polarization effects and large amount of surface states, a two-dimensional electron gas (2DEG) is induced at the AlGaN/GaN or AlInN/GaN interfaces, without the need of any intentional doping in the materials. The lack of doping impurities in the active layer of the device dramatically reduces the impurity scattering, thus improving the carrier mobility. However, GaN and its heterostructures are usually epitaxially grown on a foreign substrate, such as sapphire, as a native substrate is still not available at a conveniently low cost. A high density of threading dislocations (TDs) is therefore generated in these materials as a result of large lattice and thermal expansion coefficient mismatches between GaN film and substrate. TDs are known to induce electron scattering in GaN, thus reducing the carrier mobility and increasing current leakage paths in GaNbased devices. These TDs induce the formation of pits with six {1 0 1 1} oriented sidewalls, which are called V-defects [1]. While the origin and the growth mechanisms of these V-defects are well known [2], their influence on the optical and electronic properties of GaN-based heterostructures and devices is still subject of investigation. In the case of In based alloys heterostructures the scenario becomes even more complex, as In atoms easily segregate at dislocations due to its high surface diffusivity. Moreover, the structural and electrical properties of dislocations sensitively
n
Corresponding author. Tel.: þ39 0512095108; fax: þ 39 0512095113. E-mail address: [email protected] (A. Cavallini).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.08.035
depend on the growth method, the growth conditions, as well as on the presence of impurities. The present contribution reports on the morphological and electrical characterization at the nanoscale of V-defects in GaN and AlInN/AlN/GaN heterostructures by scanning probe microscopy methods. Current-AFM (C-AFM) and phase contrast AFM have been used to map the electrical conductivity and phase inhomogeneities, respectively.
2. Experimental GaN and AlInN/AlN/GaN heterostructures have been characterized by scanning probe microscopy methods. 3 mm thick, (0 0 0 1) GaN layers have been grown at the 1050 1C using trimethylgallium (TMG) and ammonia by AIXTRON Metalorganic Chemical Vapor Deposition (MOCVD) on the c-plane sapphire, using low temperature GaN nucleation layer. This would lead to the formation of semi-insulating (non-intentionally doped) n-type GaN. AlInN/AlN/GaN heterostructures have been grown by AIXTRON MOCVD on the c-plane sapphire substrates. AlInN and AlN barrier layers have been grown at 750 1C, the estimated nominal thickness of AlInN is around 30 nm, of AlN around 1 nm. Indium content varies from 13% to 14% as assessed by high resolution X-ray diffraction [3]. Surface morphology and surface properties have been characterized by AFM (NT MDT-Solver PRO 47). Contact and semicontact modes have been used to obtain current and phase-contrast maps, respectively. Silicon-tips coated with Pt/Ir, diamond tips and conductive nanoneedles (Ag2Ga Nauganeedles) have been used to obtain the current-map. Current AFM has been operated in contact-mode with a constant bias voltage applied to the AFM tip with respect to the sample. Here, as the AFM tip scans
A. Minj et al. / Physica B 407 (2012) 2838–2840
the surface the current is measured, thus topographic and current images are obtained simultaneously. Phase contrast mode is operated in non-contact mode, i.e. the cantilever oscillates at its resonant frequency, driven by piezoelectric crystal. During the scan, the alterations in the oscillation amplitude act as a feedback signal to the electronic controller, which maintains the preset oscillation amplitude by adjusting the height of the scanner. This height is used to image the surface topography. Simultaneously, the phase difference between the cantilever oscillation and the driving signal, caused by the inelastic interaction between the tip and the surface, appears and it is monitored (phase-contrast image). This will carry additional and complimentary information relating to the surface properties. As the surface energy may spatially vary due to differences in the chemical composition, a phase contrast map can be seen as a map of compositional inhomogeneity on the sample surface [4].
2839
3. Results and discussions Fig. 1a shows a typical topography map obtained in AFM contact mode of the GaN surface. The feature in the squared region corresponds to a V-defect, constituted by a threading dislocation opening at the surface as inverted pyramidal shape. Fig. 1b shows the C-AFM map obtained by applying 6.5 V at the conductive tip, while Fig. 1c shows the topography and current line profiles across the V-defect (dashed line in Fig. 1b). In n-type GaN V defects, the TD-facets are N-terminated due to surface reconstruction [5]; moreover, the authors of Refs. [6,7] reported the presence of excess negative charge in the region surrounding the dislocations by Scanning Tunnel Microscopy and Scanning Capacitance Microscopy. Our present results can be explained as follows: the current increase detected by C-AFM in correspondence to the V-defects can be related to the high excess negative charge at the TDs facets.
Fig. 1. AFM topography map obtained in contact mode (a) on the surface of a GaN layer showing a V-defect (squared), its C-AFM map (b) is obtained at an applied bias of 6.5 V. Topography and current line profiles across the V-defect are plotted in (c).
Fig. 2. Topography image of AlInN surface obtained in non-contact mode (a) phase-contrast image of AlInN surface (b) showing phase-contrast change around pits and threadlike features. There is dark contrast around the edges of threadlike features and pits whereas bright-contrast in their interior.
2840
A. Minj et al. / Physica B 407 (2012) 2838–2840
Fig. 3. AFM topography (a) and C-AFM current (b) maps obtained in contact mode (a) on the surface of a AlInN/AlN/GaN heterostructure. The C-AFM map (b) is obtained at an applied bias of 6.5 V.
Fig. 2a shows a typical topography of an AlInN/AlN/GaN heterostructure with the AlInN layer thickness of 30 nm, obtained by AFM in non-contact mode. The estimated average root meter square roughness is around 0.30 nm. The morphology shows the presence of grains and V-pits. These grains are typical of MOCVD grown nitrides and indicates 3D growth. The estimated density of V-pits is around 109 cm 2. Besides grains and V-pits, also threadlike features can be noted that are a series of TDs, which can or cannot merge with each other. Fig. 2b shows the phase contrast image of the same area. As phase contrast is a signature of compositional inhomogeneity at the sample surface, this contrast at threads and V-pits can be attributed to In segregation at V-defects. Due to its surface diffusivity, higher than that of the other chemical species present, In would segregate at TDs in AlInN [8,9]. This hypothesis is strongly confirmed by the topography and current AFM maps (Fig. 3a and b). Thread-like features and V-defects appear as high current regions in the C-AFM map (Fig. 3b). It must also be noted that the current flowing within these defects is significantly higher than the current flowing across similar defects in GaN. This means that, due to the highly conductive In segregated, the TDs behave as preferential current path creating shunts between the surface and the 2DEG.
4. Conclusions GaN layers and GaN based heterostructures are excellent candidates for high power microelectronic devices and high efficiency lighting systems. Nevertheless, these materials suffer for high dislocation density and strong compositional inhomogeneities. In segregation at TDs was demonstrated by AFM phasecontrast image. Using current-map, these In-rich regions have been found to be highly conductive, which would provide an
electrical path between the surface and the 2DEG. Hence, they would provide dominant assistance in leakage current in HEMT devices. The present results show that AFM in topographic, current and phase contrast mode is a very powerful technique for imaging, electrical characterization and compositional analyses at the nanoscale of threading dislocations in GaN and nitride based heterostructures.
Acknowledgments This work has been supported by the EU under Project no. PITN-GA-2008-213238 (RAINBOW). Aixtron SE, Herzogenrath, Germany and RWTH Aachen, Aachen, Germany are gratefully acknowledged for sample growth and useful discussion. References [1] Z. Liliental-Weber, Y. Chen, S. Ruvimov, J. Washburn, Phys. Rev. Lett. 79 (1997) 2835. [2] A. Mouti, J.L. Rouvie re, M. Cantoni, J.F. Carlin, E. Feltin, Eric N. Grandjean, P. Stadelmann, Phys. Rev. B 83 (2011) 195309. [3] A. Vilalta-Clemente, M.A. Poisson, H. Behmenburg, C. Giesen, M. Heuken, P. Ruterana, Phys. Status Solidi A 207 (2010) 1105. [4] S. Kalinin, A. Gruverman, Scanning Probe Microscopy: Electrical and Electromechanical Phenomena at the Nanoscale, 2 volumes, Springer Science and Business Media, New York, 2007. [5] A. Lochthofen, W. Mertin, G. Bacher, L. Hoeppel, S. Bader, J. Off, B. Hahn, Appl. Phys. Lett. 93 (2008) 022107. [6] P.J. Hansen, Y.E. Strausser, A.N. Erickson, E.J. Tarsa, P. Kozodoy, E.G. Brazel, J.P. Ibbetson, U. Mishra, V. Narayanamurti, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 72 (1998) 2247. [7] Ya-Ping Chiu, Bo-Chih Chen, Bo-Chao Huang, Min-Chuan Shih, Li-Wei Tu, Appl. Phys. Lett. 96 (2010) 082107. [8] Th. Kehagias, G.P. Dimitrakopulos, J. Kioseoglou, H. Kirmse, C. Giesen, M. Heuken, A. Georgakilas, W. Neumann, Th. Karakostas, Ph. Komninou, Appl. Phys. Lett. 95 (2009) 071905. [9] A. Minj, D. Cavalcoli, A. Cavallini, Appl. Phys. Lett. 97 (2010) 132114.
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural and local electrical properties of AlInN/AlN/GaN heterostructures A. Minj, D. Cavalcoli, A. Cavallini n Dipartimento di Fisica, Universita di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Available online 18 August 2011
GaN layers and Al1 xInxN/AlN/GaN heterostructures have been studied by scanning probe microscopy methods. Threading dislocations (TDs), originating from the GaN (0 0 0 1) layer grown on sapphire, have been investigated. Using Current-Atomic Force Microscopy (C-AFM) TDs have been found to be highly conductive in both GaN and AlInN, while using semi-contact AFM (phase-imaging mode) indium segregation has been traced at TDs in AlInN/AlN/GaN heterostructures. It has been assessed that In segregation is responsible for high conductivity at dislocations in the examined heterostructures. & 2011 Elsevier B.V. All rights reserved.
Keywords: Al1 xInxN/AlN/GaN heterostructures Atomic Force Microscopy Current-AFM V-defects In segregation Local conductivity
1. Introduction High electron mobility transistor (HEMT) devices based on III–V nitrides are excellent candidates for high power and high frequency applications. GaN HEMTs are usually achieved using either AlGaN or AlInN as the barrier layer on top of a GaN buffer layer. Due to strong polarization effects and large amount of surface states, a two-dimensional electron gas (2DEG) is induced at the AlGaN/GaN or AlInN/GaN interfaces, without the need of any intentional doping in the materials. The lack of doping impurities in the active layer of the device dramatically reduces the impurity scattering, thus improving the carrier mobility. However, GaN and its heterostructures are usually epitaxially grown on a foreign substrate, such as sapphire, as a native substrate is still not available at a conveniently low cost. A high density of threading dislocations (TDs) is therefore generated in these materials as a result of large lattice and thermal expansion coefficient mismatches between GaN film and substrate. TDs are known to induce electron scattering in GaN, thus reducing the carrier mobility and increasing current leakage paths in GaNbased devices. These TDs induce the formation of pits with six {1 0 1 1} oriented sidewalls, which are called V-defects [1]. While the origin and the growth mechanisms of these V-defects are well known [2], their influence on the optical and electronic properties of GaN-based heterostructures and devices is still subject of investigation. In the case of In based alloys heterostructures the scenario becomes even more complex, as In atoms easily segregate at dislocations due to its high surface diffusivity. Moreover, the structural and electrical properties of dislocations sensitively
n
Corresponding author. Tel.: þ39 0512095108; fax: þ 39 0512095113. E-mail address: [email protected] (A. Cavallini).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.08.035
depend on the growth method, the growth conditions, as well as on the presence of impurities. The present contribution reports on the morphological and electrical characterization at the nanoscale of V-defects in GaN and AlInN/AlN/GaN heterostructures by scanning probe microscopy methods. Current-AFM (C-AFM) and phase contrast AFM have been used to map the electrical conductivity and phase inhomogeneities, respectively.
2. Experimental GaN and AlInN/AlN/GaN heterostructures have been characterized by scanning probe microscopy methods. 3 mm thick, (0 0 0 1) GaN layers have been grown at the 1050 1C using trimethylgallium (TMG) and ammonia by AIXTRON Metalorganic Chemical Vapor Deposition (MOCVD) on the c-plane sapphire, using low temperature GaN nucleation layer. This would lead to the formation of semi-insulating (non-intentionally doped) n-type GaN. AlInN/AlN/GaN heterostructures have been grown by AIXTRON MOCVD on the c-plane sapphire substrates. AlInN and AlN barrier layers have been grown at 750 1C, the estimated nominal thickness of AlInN is around 30 nm, of AlN around 1 nm. Indium content varies from 13% to 14% as assessed by high resolution X-ray diffraction [3]. Surface morphology and surface properties have been characterized by AFM (NT MDT-Solver PRO 47). Contact and semicontact modes have been used to obtain current and phase-contrast maps, respectively. Silicon-tips coated with Pt/Ir, diamond tips and conductive nanoneedles (Ag2Ga Nauganeedles) have been used to obtain the current-map. Current AFM has been operated in contact-mode with a constant bias voltage applied to the AFM tip with respect to the sample. Here, as the AFM tip scans
A. Minj et al. / Physica B 407 (2012) 2838–2840
the surface the current is measured, thus topographic and current images are obtained simultaneously. Phase contrast mode is operated in non-contact mode, i.e. the cantilever oscillates at its resonant frequency, driven by piezoelectric crystal. During the scan, the alterations in the oscillation amplitude act as a feedback signal to the electronic controller, which maintains the preset oscillation amplitude by adjusting the height of the scanner. This height is used to image the surface topography. Simultaneously, the phase difference between the cantilever oscillation and the driving signal, caused by the inelastic interaction between the tip and the surface, appears and it is monitored (phase-contrast image). This will carry additional and complimentary information relating to the surface properties. As the surface energy may spatially vary due to differences in the chemical composition, a phase contrast map can be seen as a map of compositional inhomogeneity on the sample surface [4].
2839
3. Results and discussions Fig. 1a shows a typical topography map obtained in AFM contact mode of the GaN surface. The feature in the squared region corresponds to a V-defect, constituted by a threading dislocation opening at the surface as inverted pyramidal shape. Fig. 1b shows the C-AFM map obtained by applying 6.5 V at the conductive tip, while Fig. 1c shows the topography and current line profiles across the V-defect (dashed line in Fig. 1b). In n-type GaN V defects, the TD-facets are N-terminated due to surface reconstruction [5]; moreover, the authors of Refs. [6,7] reported the presence of excess negative charge in the region surrounding the dislocations by Scanning Tunnel Microscopy and Scanning Capacitance Microscopy. Our present results can be explained as follows: the current increase detected by C-AFM in correspondence to the V-defects can be related to the high excess negative charge at the TDs facets.
Fig. 1. AFM topography map obtained in contact mode (a) on the surface of a GaN layer showing a V-defect (squared), its C-AFM map (b) is obtained at an applied bias of 6.5 V. Topography and current line profiles across the V-defect are plotted in (c).
Fig. 2. Topography image of AlInN surface obtained in non-contact mode (a) phase-contrast image of AlInN surface (b) showing phase-contrast change around pits and threadlike features. There is dark contrast around the edges of threadlike features and pits whereas bright-contrast in their interior.
2840
A. Minj et al. / Physica B 407 (2012) 2838–2840
Fig. 3. AFM topography (a) and C-AFM current (b) maps obtained in contact mode (a) on the surface of a AlInN/AlN/GaN heterostructure. The C-AFM map (b) is obtained at an applied bias of 6.5 V.
Fig. 2a shows a typical topography of an AlInN/AlN/GaN heterostructure with the AlInN layer thickness of 30 nm, obtained by AFM in non-contact mode. The estimated average root meter square roughness is around 0.30 nm. The morphology shows the presence of grains and V-pits. These grains are typical of MOCVD grown nitrides and indicates 3D growth. The estimated density of V-pits is around 109 cm 2. Besides grains and V-pits, also threadlike features can be noted that are a series of TDs, which can or cannot merge with each other. Fig. 2b shows the phase contrast image of the same area. As phase contrast is a signature of compositional inhomogeneity at the sample surface, this contrast at threads and V-pits can be attributed to In segregation at V-defects. Due to its surface diffusivity, higher than that of the other chemical species present, In would segregate at TDs in AlInN [8,9]. This hypothesis is strongly confirmed by the topography and current AFM maps (Fig. 3a and b). Thread-like features and V-defects appear as high current regions in the C-AFM map (Fig. 3b). It must also be noted that the current flowing within these defects is significantly higher than the current flowing across similar defects in GaN. This means that, due to the highly conductive In segregated, the TDs behave as preferential current path creating shunts between the surface and the 2DEG.
4. Conclusions GaN layers and GaN based heterostructures are excellent candidates for high power microelectronic devices and high efficiency lighting systems. Nevertheless, these materials suffer for high dislocation density and strong compositional inhomogeneities. In segregation at TDs was demonstrated by AFM phasecontrast image. Using current-map, these In-rich regions have been found to be highly conductive, which would provide an
electrical path between the surface and the 2DEG. Hence, they would provide dominant assistance in leakage current in HEMT devices. The present results show that AFM in topographic, current and phase contrast mode is a very powerful technique for imaging, electrical characterization and compositional analyses at the nanoscale of threading dislocations in GaN and nitride based heterostructures.
Acknowledgments This work has been supported by the EU under Project no. PITN-GA-2008-213238 (RAINBOW). Aixtron SE, Herzogenrath, Germany and RWTH Aachen, Aachen, Germany are gratefully acknowledged for sample growth and useful discussion. References [1] Z. Liliental-Weber, Y. Chen, S. Ruvimov, J. Washburn, Phys. Rev. Lett. 79 (1997) 2835. [2] A. Mouti, J.L. Rouvie re, M. Cantoni, J.F. Carlin, E. Feltin, Eric N. Grandjean, P. Stadelmann, Phys. Rev. B 83 (2011) 195309. [3] A. Vilalta-Clemente, M.A. Poisson, H. Behmenburg, C. Giesen, M. Heuken, P. Ruterana, Phys. Status Solidi A 207 (2010) 1105. [4] S. Kalinin, A. Gruverman, Scanning Probe Microscopy: Electrical and Electromechanical Phenomena at the Nanoscale, 2 volumes, Springer Science and Business Media, New York, 2007. [5] A. Lochthofen, W. Mertin, G. Bacher, L. Hoeppel, S. Bader, J. Off, B. Hahn, Appl. Phys. Lett. 93 (2008) 022107. [6] P.J. Hansen, Y.E. Strausser, A.N. Erickson, E.J. Tarsa, P. Kozodoy, E.G. Brazel, J.P. Ibbetson, U. Mishra, V. Narayanamurti, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 72 (1998) 2247. [7] Ya-Ping Chiu, Bo-Chih Chen, Bo-Chao Huang, Min-Chuan Shih, Li-Wei Tu, Appl. Phys. Lett. 96 (2010) 082107. [8] Th. Kehagias, G.P. Dimitrakopulos, J. Kioseoglou, H. Kirmse, C. Giesen, M. Heuken, A. Georgakilas, W. Neumann, Th. Karakostas, Ph. Komninou, Appl. Phys. Lett. 95 (2009) 071905. [9] A. Minj, D. Cavalcoli, A. Cavallini, Appl. Phys. Lett. 97 (2010) 132114.