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GaN heterostructures on chlorine plasma etched GaN templates without buried conductive layer
Materials Science in Semiconductor Processing 107 (2020) 104816
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Metalorganic vapour-phase epitaxy of AlGaN/GaN heterostructures on chlorine plasma etched GaN templates without buried conductive layer Mateusz Wo�sko a, *, Bogdan Paszkiewicz a, Andrzej Stafiniak a, Joanna Praz_ mowska-Czajka a, Andrej Vincze b, Kornelia Indykiewicz a, Michał Stępniak a, Bartosz Kaczmarczyk a, Regina Paszkiewicz a a b
Faculty of Microsystem Electronics and Photonic, Wroclaw University of Science and Technology, Janiszewskiego 11/17, 50-372, Wroclaw, Poland International Laser Center, Ilkovicova 3, 841 04, Bratislava, Slovak Republic
A R T I C L E I N F O
A B S T R A C T
Keywords: Metalorganic vapour phase epitaxy Nitrides RIE Regrowth HEMT Vertical device
In this work we present approach that allows regrowth of AlGaN/GaN heterostructures on plasma etched GaN templates without occurrence of buried conductive layer. Discussion about the influence of RIE (reactive ion etching) process on the properties of GaN surface is followed by presentation of experimental work results focused on reconstruction of GaN template surface after etching in chlorine plasma in order to growth AlGaN/ GaN heterostructure without parasitic channel. Analysis of GaN surface treatment after RIE process using 10% aqueus HF solution and low temperature GaN (LT-GaN) nucleation layer is carried out, including SEM (scanning electron microscopy) imaging and SIMS (secondary ion mass spectroscopy) profiling. The AlGaN/GaN hetero structures with the thickness as low as 250 nm deposited on plasma etched GaN templates with sheet resistance under 600 Ω/□ and good uniformity were fabricated using this approach. Presented method can be used in fabrication of current aperture vertical electron transistor (CAVET) structures with low leakage currents.
1. Motivation Fabrication of modern microelectronic devices, requires multistage epitaxy [1–5]. One example of such advanced devices is current aperture vertical electron transistor (CAVET) [6–10]. Typically, in the case of CAVET structures, growth of AIII-N materials is realized on substrates spatially structured in various plasma processes, including reactive ion etching. Epitaxial growth on GaN surface, etched by those techniques, becomes difficult due to surface deterioration after plasma treatment. AlGaN/GaN heterostructures grown directly on etched GaN show presence of unintentionally doped conductive layer located on the etched interface, with donor concentration higher than 1017 cm-3, that causes the GaN buffer resistivity degradation. Chlorine based RIE is used to decrease ohmic contact resistivity to n-GaN [11,12] and to reduce carrier concentration in p-GaN [13–15], however presence of parasitic conductive channel in regrown GaN buffer hinder the application of AlGaN/GaN heterostructures for HEMTs (high electron mobility tran sistors). The origin of donors in GaN after Cl-plasma etching are prob ably nitrogen vacancies [16] and chlorine complexes [17] on the GaN surface. This parasitic phenomenon is responsible for leakage currents
observed in CAVET structures [6,9,10]. In our work we focus on effective method, that could suppress the formation of buried conductive layer in AlGaN/GaN heterostructures grown on Cl plasma etched GaN templates. In our approach, application of low temperature GaN nucleation layer on the etched GaN surface preceded by the 10% aqueus HF solution treatment, gives smooth AlGaN/GaN heterostructure surface without electron accumulation at GaN/GaN interface. The detailed description of experimental scheme will be followed by presentation of characterisation results of examined AlGaN/GaN heterostructures. In particular, the role of etched GaN treatment in 10% HF solution in proper nucleation of subsequent epi layers will be explained. Moreover, based on SIMS and C-V profiling, the influence of plasma treatment on incorporation of donor-like impu rities/defects in GaN will be discussed. Finally, the positive influence of low temperature GaN layer in prohibiting the formation of parasitic conductive layer effect will be shown and explained.
* Corresponding author. E-mail address: [email protected] (M. Wo�sko). https://doi.org/10.1016/j.mssp.2019.104816 Received 9 July 2019; Received in revised form 27 October 2019; Accepted 30 October 2019 Available online 9 November 2019 1369-8001/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
M. Wo�sko et al.
Materials Science in Semiconductor Processing 107 (2020) 104816
Fig. 1. Layer scheme of AlGaN/GaN heterostructure grown in two step process, without plasma etching (reference) a), with plasma etching without cleaning prior to EPI II b), with plasma etching and rinsing in 10% aqueous solution of HF before EPI II c).
2. Experimental 2.1. GaN surface treatment after Cl-based RIE All investigated samples were grown by MOVPE method in two step process using TMGa, TMAl, NH3 as precursors and H2 as a carrier gas. First, GaN(1900 nm)/sapphire templates were fabricated. Some of the samples were etched in RIE system (Oxford Instruments, Plasmalab 80 Plus) with parallel electrodes showerhead reactor. The RIE processes were conducted under the following conditions: RF-power (13.56 MHz) � - 200 W, pressure - 20 mTorr, temperature - 7 C, flow rate of reagents Cl2:Ar - 10:10 sccm. Next, the AlGaN/GaN heterostructures were deposited on all three samples in one epitaxial process. The composition of AlGaN barrier was determined on the basis of room temperature photoluminescence (PL) spectra, whereas thicknesses of subsequent layers (AlN/AlGaN/AlN/GaN) were calculated according to previous calibration procedures using 650 nm reflectometry. The PL peak of AlGaN layer was at 328.1 nm (FWHM: 12 nm) eV, what corresponds to the Al composition of 21%. In the first approach, three samples, that have different surface treatment before heterostructure epitaxy, were examined. The layer schemes of investigated samples are presented in Fig. 1. AlGaN/GaN epitaxy was performed on the GaN/sapphire template without plasma etching (as reference sample) and on two templates that were etched using RIE method with and without treating in 10% aqueous solution of
Fig. 2. SEM images of AlGaN/GaN heterostructures grown in two step process, without plasma etching (reference) a), with plasma etching without cleaning prior to EPI II b), with plasma etching and rinsing in 10% aqueous solution of HF before EPI II c).
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Fig. 3. Layer schemes a,c,e) and carrier profiles b,d,f) of AlGaN/GaN heterostructures grown in two step process, without plasma etching (reference) a,b), with plasma etching without LT-GaN c,d), with plasma etching and LT-GaN e,f).
HF. GaN etching in hydrofluoric acid is an effective method of removing residual impurities from the GaN surface [18]. Microscopic examination of the samples using SEM (Scanning Elec tron Microscope) leads to the conclusions that direct deposition of AlGaN/GaN heterostructure on etched in Cl-plasma GaN templates re sults in three dimensional growth of subsequent epilayers (Fig. 2b).
Arised hillocks have hexagonal shape with average dimensions from tens hundrets of nanometers to several micrometers. Wetting deterio ration is probably caused by the contamination of the GaN surface by Clplasma reaction products after RIE. This undesirable effect can be overcome by application of cleaning procedure on etched GaN surface, using 10% aqueous solution of HF, prior to AlGaN/GaN growth. There is 3
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Materials Science in Semiconductor Processing 107 (2020) 104816
no visible surface deterioration of AlGaN/GaN deposited on etched GaN template and treated by 10% HF (Fig. 2c), compared to the AlGaN/GaN deposited on nonetched GaN (Fig. 2a). 2.2. The origin of buried conductive layer in AlGaN/GaN heterostructures grown on chlorine plasma etched GaN Further research was aimed at the determination of the electrical parameters of the AlGaN/GaN heterostructures grown in two step epitaxy on etched GaN templates. The C-V profiling was used to deter mine the electron concentration in epistructure that are presented in Fig. 3c). The visible bump in carrier concentration at the depth of 250 nm (Fig. 3d) is located on GaN/etched GaN interface and can be explained by incorporation of chlorine residuals after RIE and/or strong surface deterioration and possible formation of nitrogen vacancies in GaN. Regardless of the source of donors, the buried conductive layer act as a parasitic channel for 2DEG in AlGaN/GaN heterostructure. In order to prevent this adverse effect, the low temperature GaN layer (LT-GaN), deposited prior to the growth of AlGaN/GaN on etched template, was used. LT-GaN is widely used as a nucleation layer in GaN epitaxy on sapphire [19] and as a defect-trap interlayer during deposition of high quality GaN [20]. Appropriate adjustment of LT-GaN layer growth and annealing conditions (temperature, pressure, time) can lead to enhanced incorporation of carbon atoms that acts as a acceptors in GaN [21] thus can compensate the donors at etched GaN interface. LT-GaN was � deposited at 530 C using TMGa and NH3 after thermal annealing of � etched GaN/sapphire template in H 2þ NH3 mixture at 1060 C. The layer scheme of investigated AlGaN/GaN heterostructure with LT-GaN layer is presented in Fig. 3e) and the carriers concentration profile is shown in Fig. 3f). For comparison purposes, the electron profile in heterostructure grown on non-etched GaN template (Fig. 3a) is pre sented in Fig. 3b). It is clearly visible, that application of LT-GaN layer prior to AlGaN/GaN epitaxy on plasma treated GaN is efficient method of avoiding conductive layer formation on RIE etched interface. The SIMS measurements explained the possible source of donors in plasma etched GaN as well as acceptors in heterostructures grown on LTGaN. Composition profiles of samples from Fig. 3 are presented in Fig. 4. In plasma etched samples (Fig. 4b and c), the Cl-peak is visible at the interface between GaN template and heterostructure, what is unam biguously contamination after chlorine treatment in RIE process. The presence of C- near the surface of the samples (Fig. 4a and b) is probably caused by the organic contamination after GaN template (Epi I) or/and AlGaN/GaN heterostructure (Epi II) growth. However, the clearly visible bump of carbon concentration (over two times of magnitude compared to background) on the interface comes from the LT-GaN. Therefore, the assumption about the compensatory role of carbon is highly probable. 2.3. Influence of GaN buffer thickness in AlGaN/GaN on the sheet resistance of 2DEG Due to strong deterioration of surface quality after reactive ion etching of GaN, the question, how the material quality of subsequently deposited AlGaN/GaN heterostructure will affect the electrical proper ties of 2DEG (two dimensional electron gas) is crucial for possible de vices applications of regrown heterostructures. Because the surface reconstruction occurs during epilayer growth, the material quality improvement should be observed for thicker GaN buffer layers. Using the previously described method of buried conductive layer elimination AlGaN/GaN heterostructures growth on plasma treated GaN, three samples with different GaN buffer thickness (Epi II) were fabricated and their properties were compared. The layer scheme of investigated het erostructures was identical to the previously shown, however the thicknesses of u-GaN buffers (Epi II) were 150 nm, 250 nm and 500 nm for subsequent samples. Furthermore, the GaN/sapphire samples were selectively etched using SiO2 mask in order to investigate differences between heterostructures grown on etched and non-etched areas of the
Fig. 4. SIMS profiles of composition of AlGaN/GaN heterostructures grown in two step process, without plasma etching (reference Fig. 3a) a), with plasma etching and without LT-GaN (Fig. 3c) b), with plasma etching and LT-GaN (Fig. 3e) c).
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Fig. 5. Schematic work flow of AlGaN/GaN samples preparation in two step epitaxy on selectively etched GaN templates.
Fig. 6. Layer scheme of AlGaN/GaN heterostructures with different GaN buffer thicknesses (Epi II) deposited on selectively etched GaN/sapphire templates.
samples. The schematic workflow of samples preparation is presented in Fig. 5 and the layer scheme of the final structures in Fig. 6. As a result, half of the one inch area of the sample was etched and another half nonetched. Samples were characterized by GHz contactless reflectance spec troscopy. This nondestructive method allows fast measurement of sheet resistance of thin semiconductor layers. Since both: buffer layer in GaN template and buffer layer in AlGaN/GaN heterostructure have low car rier concentration, the lowest resistivity in the structure occurs in 2DEG, therefore the sheet resistance measured by this method is equal to the sheet resistance of 2DEG. The sheet resistance of 2DEG is an indicator of heterostructure quality in regard to high power applications. The sheet resistivity maps of investigated AlGaN/GaN heterostructures with different GaN buffer thicknesses are presented in Fig. 7. As can be ex pected, the average resistance changes with buffer thickness and has the lowest value for sample with 500 nm GaN (Fig. 7). With the decrease of GaN thickness, the average sheet resistance value increase. Moreover, the sample asymmetry (between etched and non-etched part) becomes more evident in samples with thinner GaN layer. It can be assumed that the limit thickness of GaN buffer at which there is no distinguish dif ference between the two parts of the sample is equal to 250 nm. For thinner buffer, there is still visible existence of 2DEG, however re sistivity in some areas is above 1000 Ω/□.
Fig. 7. Sheet resistance maps of AlGaN/GaN heterostructures deposited on selectively etched GaN templates with different GaN buffer thickness: 150 nm a), 250 nm b), 500 nm c). 5
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Materials Science in Semiconductor Processing 107 (2020) 104816
3. Conclusions [5]
The source of buried conductive layer in nitrides structures grown on Cl-plasma etched GaN templates is chlorine residuals contamination origin from RIE process. The carrier concentration at the etched inter face can be as high as 1018 cm-3. This parasitic phenomenon can be effectively suppressed by deposition of LT-GaN layer on etched surface prior to the high temperature epitaxy of AlGaN/GaN heterostructure. Application of LT-GaN increases the incorporation of carbon that acts as an acceptor, and reduces the conductivity of parasitic channel in AlGaN/ GaN overgrowth structure. In order to obtain two dimensional growth of epitaxial structures on Cl-treated GaN, the post process treatment in 10% aqueous solution of HF should be applied. Using this approach it is possible to growth AlGaN/GaN heterostructures with GaN buffer thickness as low as 250 nm and very good uniformity of 2DEG sheet resistance (600–700 Ω/□). Fabrication of thinner heterostructures is possible but results in nonuniformity of sheet resistance.
[6] [7]
[8] [9]
[10] [11]
Acknowledgement
[12]
This work was co-financed by the National Center for Research and Development grants TECHMATSTRATEG No.1/346922/4/NCBR/2017, the National Science Centre grant No. DEC-2015/19/B/ST7/02494, Wroclaw University of Technology statutory grants and by the SlovakPolish International Cooperation Program. This work was accomplished thanks to the product indicators and result indicators achieved within the projects co-financed by the European Union within the European Regional Development Fund, through a grant from the Innovative Economy (POIG.01.01.02-00-008/08-05) and by the National Centre for Research and Development through the Applied Research Program Grant No. 178782 and Grant LIDER No. 027/533/L-5/13/NCBR/2014, Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic and of the Slovak Academy of Sciences VEGA 1/0929/17 and the Polish National Agency for Academic Ex change under the contract PPN/BIL/2018/1/00137.
[13] [14] [15]
[16]
[17] [18]
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Metalorganic vapour-phase epitaxy of AlGaN/GaN heterostructures on chlorine plasma etched GaN templates without buried conductive layer Mateusz Wo�sko a, *, Bogdan Paszkiewicz a, Andrzej Stafiniak a, Joanna Praz_ mowska-Czajka a, Andrej Vincze b, Kornelia Indykiewicz a, Michał Stępniak a, Bartosz Kaczmarczyk a, Regina Paszkiewicz a a b
Faculty of Microsystem Electronics and Photonic, Wroclaw University of Science and Technology, Janiszewskiego 11/17, 50-372, Wroclaw, Poland International Laser Center, Ilkovicova 3, 841 04, Bratislava, Slovak Republic
A R T I C L E I N F O
A B S T R A C T
Keywords: Metalorganic vapour phase epitaxy Nitrides RIE Regrowth HEMT Vertical device
In this work we present approach that allows regrowth of AlGaN/GaN heterostructures on plasma etched GaN templates without occurrence of buried conductive layer. Discussion about the influence of RIE (reactive ion etching) process on the properties of GaN surface is followed by presentation of experimental work results focused on reconstruction of GaN template surface after etching in chlorine plasma in order to growth AlGaN/ GaN heterostructure without parasitic channel. Analysis of GaN surface treatment after RIE process using 10% aqueus HF solution and low temperature GaN (LT-GaN) nucleation layer is carried out, including SEM (scanning electron microscopy) imaging and SIMS (secondary ion mass spectroscopy) profiling. The AlGaN/GaN hetero structures with the thickness as low as 250 nm deposited on plasma etched GaN templates with sheet resistance under 600 Ω/□ and good uniformity were fabricated using this approach. Presented method can be used in fabrication of current aperture vertical electron transistor (CAVET) structures with low leakage currents.
1. Motivation Fabrication of modern microelectronic devices, requires multistage epitaxy [1–5]. One example of such advanced devices is current aperture vertical electron transistor (CAVET) [6–10]. Typically, in the case of CAVET structures, growth of AIII-N materials is realized on substrates spatially structured in various plasma processes, including reactive ion etching. Epitaxial growth on GaN surface, etched by those techniques, becomes difficult due to surface deterioration after plasma treatment. AlGaN/GaN heterostructures grown directly on etched GaN show presence of unintentionally doped conductive layer located on the etched interface, with donor concentration higher than 1017 cm-3, that causes the GaN buffer resistivity degradation. Chlorine based RIE is used to decrease ohmic contact resistivity to n-GaN [11,12] and to reduce carrier concentration in p-GaN [13–15], however presence of parasitic conductive channel in regrown GaN buffer hinder the application of AlGaN/GaN heterostructures for HEMTs (high electron mobility tran sistors). The origin of donors in GaN after Cl-plasma etching are prob ably nitrogen vacancies [16] and chlorine complexes [17] on the GaN surface. This parasitic phenomenon is responsible for leakage currents
observed in CAVET structures [6,9,10]. In our work we focus on effective method, that could suppress the formation of buried conductive layer in AlGaN/GaN heterostructures grown on Cl plasma etched GaN templates. In our approach, application of low temperature GaN nucleation layer on the etched GaN surface preceded by the 10% aqueus HF solution treatment, gives smooth AlGaN/GaN heterostructure surface without electron accumulation at GaN/GaN interface. The detailed description of experimental scheme will be followed by presentation of characterisation results of examined AlGaN/GaN heterostructures. In particular, the role of etched GaN treatment in 10% HF solution in proper nucleation of subsequent epi layers will be explained. Moreover, based on SIMS and C-V profiling, the influence of plasma treatment on incorporation of donor-like impu rities/defects in GaN will be discussed. Finally, the positive influence of low temperature GaN layer in prohibiting the formation of parasitic conductive layer effect will be shown and explained.
* Corresponding author. E-mail address: [email protected] (M. Wo�sko). https://doi.org/10.1016/j.mssp.2019.104816 Received 9 July 2019; Received in revised form 27 October 2019; Accepted 30 October 2019 Available online 9 November 2019 1369-8001/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
M. Wo�sko et al.
Materials Science in Semiconductor Processing 107 (2020) 104816
Fig. 1. Layer scheme of AlGaN/GaN heterostructure grown in two step process, without plasma etching (reference) a), with plasma etching without cleaning prior to EPI II b), with plasma etching and rinsing in 10% aqueous solution of HF before EPI II c).
2. Experimental 2.1. GaN surface treatment after Cl-based RIE All investigated samples were grown by MOVPE method in two step process using TMGa, TMAl, NH3 as precursors and H2 as a carrier gas. First, GaN(1900 nm)/sapphire templates were fabricated. Some of the samples were etched in RIE system (Oxford Instruments, Plasmalab 80 Plus) with parallel electrodes showerhead reactor. The RIE processes were conducted under the following conditions: RF-power (13.56 MHz) � - 200 W, pressure - 20 mTorr, temperature - 7 C, flow rate of reagents Cl2:Ar - 10:10 sccm. Next, the AlGaN/GaN heterostructures were deposited on all three samples in one epitaxial process. The composition of AlGaN barrier was determined on the basis of room temperature photoluminescence (PL) spectra, whereas thicknesses of subsequent layers (AlN/AlGaN/AlN/GaN) were calculated according to previous calibration procedures using 650 nm reflectometry. The PL peak of AlGaN layer was at 328.1 nm (FWHM: 12 nm) eV, what corresponds to the Al composition of 21%. In the first approach, three samples, that have different surface treatment before heterostructure epitaxy, were examined. The layer schemes of investigated samples are presented in Fig. 1. AlGaN/GaN epitaxy was performed on the GaN/sapphire template without plasma etching (as reference sample) and on two templates that were etched using RIE method with and without treating in 10% aqueous solution of
Fig. 2. SEM images of AlGaN/GaN heterostructures grown in two step process, without plasma etching (reference) a), with plasma etching without cleaning prior to EPI II b), with plasma etching and rinsing in 10% aqueous solution of HF before EPI II c).
2
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Materials Science in Semiconductor Processing 107 (2020) 104816
Fig. 3. Layer schemes a,c,e) and carrier profiles b,d,f) of AlGaN/GaN heterostructures grown in two step process, without plasma etching (reference) a,b), with plasma etching without LT-GaN c,d), with plasma etching and LT-GaN e,f).
HF. GaN etching in hydrofluoric acid is an effective method of removing residual impurities from the GaN surface [18]. Microscopic examination of the samples using SEM (Scanning Elec tron Microscope) leads to the conclusions that direct deposition of AlGaN/GaN heterostructure on etched in Cl-plasma GaN templates re sults in three dimensional growth of subsequent epilayers (Fig. 2b).
Arised hillocks have hexagonal shape with average dimensions from tens hundrets of nanometers to several micrometers. Wetting deterio ration is probably caused by the contamination of the GaN surface by Clplasma reaction products after RIE. This undesirable effect can be overcome by application of cleaning procedure on etched GaN surface, using 10% aqueous solution of HF, prior to AlGaN/GaN growth. There is 3
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no visible surface deterioration of AlGaN/GaN deposited on etched GaN template and treated by 10% HF (Fig. 2c), compared to the AlGaN/GaN deposited on nonetched GaN (Fig. 2a). 2.2. The origin of buried conductive layer in AlGaN/GaN heterostructures grown on chlorine plasma etched GaN Further research was aimed at the determination of the electrical parameters of the AlGaN/GaN heterostructures grown in two step epitaxy on etched GaN templates. The C-V profiling was used to deter mine the electron concentration in epistructure that are presented in Fig. 3c). The visible bump in carrier concentration at the depth of 250 nm (Fig. 3d) is located on GaN/etched GaN interface and can be explained by incorporation of chlorine residuals after RIE and/or strong surface deterioration and possible formation of nitrogen vacancies in GaN. Regardless of the source of donors, the buried conductive layer act as a parasitic channel for 2DEG in AlGaN/GaN heterostructure. In order to prevent this adverse effect, the low temperature GaN layer (LT-GaN), deposited prior to the growth of AlGaN/GaN on etched template, was used. LT-GaN is widely used as a nucleation layer in GaN epitaxy on sapphire [19] and as a defect-trap interlayer during deposition of high quality GaN [20]. Appropriate adjustment of LT-GaN layer growth and annealing conditions (temperature, pressure, time) can lead to enhanced incorporation of carbon atoms that acts as a acceptors in GaN [21] thus can compensate the donors at etched GaN interface. LT-GaN was � deposited at 530 C using TMGa and NH3 after thermal annealing of � etched GaN/sapphire template in H 2þ NH3 mixture at 1060 C. The layer scheme of investigated AlGaN/GaN heterostructure with LT-GaN layer is presented in Fig. 3e) and the carriers concentration profile is shown in Fig. 3f). For comparison purposes, the electron profile in heterostructure grown on non-etched GaN template (Fig. 3a) is pre sented in Fig. 3b). It is clearly visible, that application of LT-GaN layer prior to AlGaN/GaN epitaxy on plasma treated GaN is efficient method of avoiding conductive layer formation on RIE etched interface. The SIMS measurements explained the possible source of donors in plasma etched GaN as well as acceptors in heterostructures grown on LTGaN. Composition profiles of samples from Fig. 3 are presented in Fig. 4. In plasma etched samples (Fig. 4b and c), the Cl-peak is visible at the interface between GaN template and heterostructure, what is unam biguously contamination after chlorine treatment in RIE process. The presence of C- near the surface of the samples (Fig. 4a and b) is probably caused by the organic contamination after GaN template (Epi I) or/and AlGaN/GaN heterostructure (Epi II) growth. However, the clearly visible bump of carbon concentration (over two times of magnitude compared to background) on the interface comes from the LT-GaN. Therefore, the assumption about the compensatory role of carbon is highly probable. 2.3. Influence of GaN buffer thickness in AlGaN/GaN on the sheet resistance of 2DEG Due to strong deterioration of surface quality after reactive ion etching of GaN, the question, how the material quality of subsequently deposited AlGaN/GaN heterostructure will affect the electrical proper ties of 2DEG (two dimensional electron gas) is crucial for possible de vices applications of regrown heterostructures. Because the surface reconstruction occurs during epilayer growth, the material quality improvement should be observed for thicker GaN buffer layers. Using the previously described method of buried conductive layer elimination AlGaN/GaN heterostructures growth on plasma treated GaN, three samples with different GaN buffer thickness (Epi II) were fabricated and their properties were compared. The layer scheme of investigated het erostructures was identical to the previously shown, however the thicknesses of u-GaN buffers (Epi II) were 150 nm, 250 nm and 500 nm for subsequent samples. Furthermore, the GaN/sapphire samples were selectively etched using SiO2 mask in order to investigate differences between heterostructures grown on etched and non-etched areas of the
Fig. 4. SIMS profiles of composition of AlGaN/GaN heterostructures grown in two step process, without plasma etching (reference Fig. 3a) a), with plasma etching and without LT-GaN (Fig. 3c) b), with plasma etching and LT-GaN (Fig. 3e) c).
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Fig. 5. Schematic work flow of AlGaN/GaN samples preparation in two step epitaxy on selectively etched GaN templates.
Fig. 6. Layer scheme of AlGaN/GaN heterostructures with different GaN buffer thicknesses (Epi II) deposited on selectively etched GaN/sapphire templates.
samples. The schematic workflow of samples preparation is presented in Fig. 5 and the layer scheme of the final structures in Fig. 6. As a result, half of the one inch area of the sample was etched and another half nonetched. Samples were characterized by GHz contactless reflectance spec troscopy. This nondestructive method allows fast measurement of sheet resistance of thin semiconductor layers. Since both: buffer layer in GaN template and buffer layer in AlGaN/GaN heterostructure have low car rier concentration, the lowest resistivity in the structure occurs in 2DEG, therefore the sheet resistance measured by this method is equal to the sheet resistance of 2DEG. The sheet resistance of 2DEG is an indicator of heterostructure quality in regard to high power applications. The sheet resistivity maps of investigated AlGaN/GaN heterostructures with different GaN buffer thicknesses are presented in Fig. 7. As can be ex pected, the average resistance changes with buffer thickness and has the lowest value for sample with 500 nm GaN (Fig. 7). With the decrease of GaN thickness, the average sheet resistance value increase. Moreover, the sample asymmetry (between etched and non-etched part) becomes more evident in samples with thinner GaN layer. It can be assumed that the limit thickness of GaN buffer at which there is no distinguish dif ference between the two parts of the sample is equal to 250 nm. For thinner buffer, there is still visible existence of 2DEG, however re sistivity in some areas is above 1000 Ω/□.
Fig. 7. Sheet resistance maps of AlGaN/GaN heterostructures deposited on selectively etched GaN templates with different GaN buffer thickness: 150 nm a), 250 nm b), 500 nm c). 5
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3. Conclusions [5]
The source of buried conductive layer in nitrides structures grown on Cl-plasma etched GaN templates is chlorine residuals contamination origin from RIE process. The carrier concentration at the etched inter face can be as high as 1018 cm-3. This parasitic phenomenon can be effectively suppressed by deposition of LT-GaN layer on etched surface prior to the high temperature epitaxy of AlGaN/GaN heterostructure. Application of LT-GaN increases the incorporation of carbon that acts as an acceptor, and reduces the conductivity of parasitic channel in AlGaN/ GaN overgrowth structure. In order to obtain two dimensional growth of epitaxial structures on Cl-treated GaN, the post process treatment in 10% aqueous solution of HF should be applied. Using this approach it is possible to growth AlGaN/GaN heterostructures with GaN buffer thickness as low as 250 nm and very good uniformity of 2DEG sheet resistance (600–700 Ω/□). Fabrication of thinner heterostructures is possible but results in nonuniformity of sheet resistance.
[6] [7]
[8] [9]
[10] [11]
Acknowledgement
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This work was co-financed by the National Center for Research and Development grants TECHMATSTRATEG No.1/346922/4/NCBR/2017, the National Science Centre grant No. DEC-2015/19/B/ST7/02494, Wroclaw University of Technology statutory grants and by the SlovakPolish International Cooperation Program. This work was accomplished thanks to the product indicators and result indicators achieved within the projects co-financed by the European Union within the European Regional Development Fund, through a grant from the Innovative Economy (POIG.01.01.02-00-008/08-05) and by the National Centre for Research and Development through the Applied Research Program Grant No. 178782 and Grant LIDER No. 027/533/L-5/13/NCBR/2014, Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic and of the Slovak Academy of Sciences VEGA 1/0929/17 and the Polish National Agency for Academic Ex change under the contract PPN/BIL/2018/1/00137.
[13] [14] [15]
[16]
[17] [18]
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