rus
Issue #2/2023
A.V.Nethentsev, K.A.Tsarik
ANALYTICAL REVIEW OF METHODS FOR PRODUCING ALLOYED AND NON-ALLOYED OHMIC CONTACTS TO GALLIUM NITRIDE NANOHETEROSTRUCTURES
ANALYTICAL REVIEW OF METHODS FOR PRODUCING ALLOYED AND NON-ALLOYED OHMIC CONTACTS TO GALLIUM NITRIDE NANOHETEROSTRUCTURES
DOI: https://doi.org/10.22184/1993-8578.2023.16.2.114.122
The paper considers the technological features of manufacturing ohmic contacts with resistances from 0,025 to 0,4 Ohm ∙ mm to nanoheterostructures based on gallium nitride. It has been established that non-burning ohmic contacts are the most suitable for mastering operating frequencies up to the terahertz range.
The paper considers the technological features of manufacturing ohmic contacts with resistances from 0,025 to 0,4 Ohm ∙ mm to nanoheterostructures based on gallium nitride. It has been established that non-burning ohmic contacts are the most suitable for mastering operating frequencies up to the terahertz range.
Теги: gallium nitride nanoheterostructure ohmic contact наногетероструктура нитрид галлия омический контакт
INTRODUCTION
Gallium nitride (GaN), due to its exceptional properties, is one of the most promising materials for power electronics [1]. Gallium nitride growing on a silicon substrate significantly improves heat dissipation and simplifies processing of structures. A number of papers are devoted to improve quality of gallium nitride epitaxial layers grown on silicon [2].
The main requirement for ohmic contacts is their low resistance (both resistance of the ohmic contact material and their contact resistance to the heterostructure active layers). Non-burning ohmic contacts meet this requirement. However, they require an additional epitaxy process to grow a thin layer of narrow-gap semiconductor. A more technologically advanced burnerable ohmic contact is more suitable. In general, resistance value of the ohmic contact is also influenced by concentration of charge carriers in the semiconductor layer [3]. Several approaches to ohmic contact formation technology are used by different research teams to obtain high quality output characteristics of transistors. In this paper the main features of metallization burning and doping technologies of highly alloyed films in the contact areas have been considered. Major advances in contact resistance and steepness values of transistors based on nitride gallium heterostructures are considered.
OPTIMISATION OF THE TECHNOLOGY FOR PRODUCING BURNABLE OHMIC CONTACTS FOR GALLIUM NITRIDE-BASED NANOSTRUCTURES
One way of decrease ohmic contacts resistance is to carry out a temperature treatment while metallisation components penetrate into the semiconductor layer. The resulting ohmic contacts are called burnt contacts. The most common metallisation is a titanium (Ti) and aluminium (Al) based metallisation system.
In [4] influence of the metal-semiconductor interface state on the ohmic contact resistance level (Ti/Al/Ni/Au) formed to GaN on Si (111) has been considered. It was found that current flow through the contact also depends on the structure and electrical properties of the reacted layer. The Ti/Al/Ni/Au metallization had thicknesses of 15/200/50/50 and 100/200/50/50 nm, and the contact based on the former metallization showed better electrical characteristics than the contact with 100 nm Ti. In particular, the contact resistivity values after annealing at 850 °C were 4.8 · 10–5 and 3.5 · 10–4 Ohm · cm2 respectively. However, the sample with 15 nm Ti layer has more developed surface morphology, with a RMS roughness of 30.6 and 22.3 nm, respectively. Also the cross-sectional TEM images of two samples with 15 nm Ti annealed at 800 and 850 °C were taken and are shown in Fig.1. As can be seen in Fig.1a in the first case the original layered structure is eroded and mixed.
An entirely different microstructure can be observed in a sample annealed at 850 °C (see TEM image in Fig.1b). The presence of clearly distinguishable layers is evident. The uppermost layer mainly contains the AlNi phase, below it there is a dark (due to the presence of Au) AlAu4 layer. The third layer, based on X-ray analysis, contains Ti-Al-Ni compounds. Also, in this case at the interface with the substrate a TiN layer with a thickness of 4–9 nm is formed. The electrical properties of ohmic contacts are determined precisely by the conductive TiN layer, formation of which depends on a number of process parameters, so in Jacobs and Kramer [5] a systematic approach to reduce resistance of Ti/Al/Ni/Au ohmic contacts in AlGaN/GaN structures is considered. All the ohmic contacts were of Ti/Al/Ni/Au type but differed in metal thickness, time and temperature during rapid thermal annealing.
It can be seen from Fig.2a that the results of studies of Ti/Al/Ni/Au metallisation-based contact systems show that electrical properties and surface morphology depend on the Ti layer thickness ratio to Al thickness. Increasing thickness of Al leads to decreasing of contact resistance. A thickness ratio of 6 shows the best results. Fig.2b presents the studied results of the influence of titanium thickness on contact resistance. From Fig.2c it can be seen that there is an optimum Ni thickness and that increasing or decreasing thickness of the nickel worsens the contact resistance value. Each sample was split into 4 pieces which were annealed at 700, 800, 900 and 1000 °C for 30 seconds in the nitrogen atmosphere. Each time the best results were obtained at 900 °C. Fig.2d shows a more detailed study of the annealing conditions for optimum metallisation pattern consisting of Ti/Al/Ni/Au (30/180/40/150 nm). The best results were obtained at 900 °C for 30 seconds in an N2 atmosphere. After a series of experiments, thicknesses and thermal annealing parameters were obtained to reduce the contact resistance value of Ti/Al/Ni/Au metal contacts. As a result, in this work, the optimized contact had a very low contact resistance of 0.2 Ohm · mm (7.3 · 10–7 Ohm·cm2) and the following parameters for 30/180/40/150 nm thicknesses, respectively [5].
As to Ti and Al based ohmic contacts temperature had to be above 800 °C in order to achieve this compromise. A more detailed study of the heat treatment effect process was carried out in [6], Ti/Al/Mo/Au contact metallization samples were annealed at different temperatures in a Rapid Thermal Annealing (RTA) system. Temperature of each annealing process changes between 825 °C and 855 °C and an annealing time of 60 seconds was maintained for all samples (Fig.3a).
As can be seen from Fig.3a, temperature treatment was carried out in the RTA mode, i.e. fast heating and fast cooling were realised. Figure 3b shows the volt-ampere characteristics of a Ti/Al/Mo/Au metallisation-based contact as a function of annealing temperature. Decreasing heat treatment temperature leads to non-linearity. This could mean that a barrier remains at the metal-semiconductor interface. The temperature of heat treatment effect process shows that the lowest total resistance at a given voltage is achieved at 855 °C. However, increasing annealing temperature above 855 °C leads to increasing the total resistance but the waveform remains slightly non-linear.
The metallisation compositions and heat treatment parameters for manufacturing the ohmic contacts to gallium nitride-based nanoheterostructures of a number of studies are shown in Table 1.
The peculiarities of the burning contact technology are: satisfactory level of contact resistance, high mechanical and temperature stability, and developed surface morphology. The burnable ohmic contact is promising for power semiconductor devices.
NON- BURNING OHMIC CONTACTS TO GALLIUM NITRIDE-BASED NANOSTRUCTURES
Nowadays, methods of non-burnable contacts manufacturing have been gaining ground. Figure 4 shows the concept of non-burnable manufacturing, selectively grown ohmic contacts. The nanoheterostructure surface is annealed through a SiO2 dielectric mask to a depth below the conductive channel shown with a dotted line, and then n+-GaN is grown in the resulting "windows". The introduction of Si admixture causes degeneration of GaN semiconducting layer, which should be in direct contact with the region of two-dimensional electron gas. Production of the contact is completed by the metallisation of the n+-GaN surface. There are a number of advantages to growing contacts over burning contacts. The re-growth was done by homoepitaxy, which ensures good adhesion of the deposited material. Gallium nitride and SiO2 mask have good temperature resistance, which ensures that the original contact shape is maintained and allows for more precise control of manufactured transistors geometry [13]. In recent years, reports have been published on obtaining resistivity of ohmic contacts up to 0.4 Ohm·mm for "Ga-face" HEMT [14] and up to 0.09 Ohm · mm [15] and then up to 0.025 Ohm·mm [16] for "N-face" HEMT. In addition, it has been reported about non-burnable build-up contacts to HEMT structures without etching "windows" for n+-GaN deposition [17], and also using selective etching the part of the AlGaN barrier layer, which does not effect on GaN layer [18].
In [19] epitaxial AlGaN/GaN HEMT heterostructure with silicon ion implantation with a dose of 1 · 1016 cm-3 was considered (Fig.4).
The structures resistance were measured using the long line method. The following data were obtained: contact resistance Rк = 0,96 Ohm · mm and surface resistance RПП = 383 Ом/м2.
In addition to selective doping, the ion doping method is increasingly being used. In [20] evaluation of Cr/Pt/Au non-alloyed ohmic contacts to GaN epitaxial structures and conventional Ti/Al/Ni/Au alloyed contacts to AlGaN/GaN heterostructures with ion-doped contact layers were performed. Contact resistance was 2.8 · 10–6 and 3.5 · 10–7 Ohm · cm2 respectively. Thus it can be concluded: technology of non-burning ohmic contacts makes it possible to achieve low resistances, which makes such contacts the most suitable for microwave transistors.
INFLUENCE OF CONTACT RESISTANCE VALUE ON THE WAVELENGTH SLOPE OF GALLIUM NITRIDE FIELD-EFFECT TRANSISTORS
It is important to assess the impact of contact resistance values on semiconductor device parameters. One such parameter is the steepness. In recent years, along with improvements in plasma chemical processing technology, formation of metallization layers and epitaxial methods of growing nitride layers, improving the parameters of microwave transistors based on gallium nitride.
Figure 5 shows the dependence between the slope of the HEMT field effect transistor and the contact resistance. The steepness decreases smoothly with increasing contact resistance. Both fluxed and non-burning ohmic contacts have been considered. The highest value of steepness is achieved in the work on non-burning contacts [16] and is 1105 mS/mm. In general, the above data shows significantly higher steepness, for HEMT transistors with non-burning contacts. For transistors with burnt ohmic contacts the highest steepness value is 400 mS/mm. Moreover, these values confirm prospective of the contact resistance reduction problem.
CONCLUSIONS
This paper shows what can be expected from a nitride gallium microwave transistor at different values of ohmic contact resistance. In general, this is a promising challenge for power semiconductor devices. As can be seen from the above, non-burning ohmic contact technology has made it possible to achieve low resistances, making such contacts most suitable for microwave transistors, especially in the nanometer range of topological dimensions.
ACKNOWLEDGEMENTS
This work was carried out with the financial support of the Ministry of Education and Science as a part of state task FSMR-2022-0004.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Gallium nitride (GaN), due to its exceptional properties, is one of the most promising materials for power electronics [1]. Gallium nitride growing on a silicon substrate significantly improves heat dissipation and simplifies processing of structures. A number of papers are devoted to improve quality of gallium nitride epitaxial layers grown on silicon [2].
The main requirement for ohmic contacts is their low resistance (both resistance of the ohmic contact material and their contact resistance to the heterostructure active layers). Non-burning ohmic contacts meet this requirement. However, they require an additional epitaxy process to grow a thin layer of narrow-gap semiconductor. A more technologically advanced burnerable ohmic contact is more suitable. In general, resistance value of the ohmic contact is also influenced by concentration of charge carriers in the semiconductor layer [3]. Several approaches to ohmic contact formation technology are used by different research teams to obtain high quality output characteristics of transistors. In this paper the main features of metallization burning and doping technologies of highly alloyed films in the contact areas have been considered. Major advances in contact resistance and steepness values of transistors based on nitride gallium heterostructures are considered.
OPTIMISATION OF THE TECHNOLOGY FOR PRODUCING BURNABLE OHMIC CONTACTS FOR GALLIUM NITRIDE-BASED NANOSTRUCTURES
One way of decrease ohmic contacts resistance is to carry out a temperature treatment while metallisation components penetrate into the semiconductor layer. The resulting ohmic contacts are called burnt contacts. The most common metallisation is a titanium (Ti) and aluminium (Al) based metallisation system.
In [4] influence of the metal-semiconductor interface state on the ohmic contact resistance level (Ti/Al/Ni/Au) formed to GaN on Si (111) has been considered. It was found that current flow through the contact also depends on the structure and electrical properties of the reacted layer. The Ti/Al/Ni/Au metallization had thicknesses of 15/200/50/50 and 100/200/50/50 nm, and the contact based on the former metallization showed better electrical characteristics than the contact with 100 nm Ti. In particular, the contact resistivity values after annealing at 850 °C were 4.8 · 10–5 and 3.5 · 10–4 Ohm · cm2 respectively. However, the sample with 15 nm Ti layer has more developed surface morphology, with a RMS roughness of 30.6 and 22.3 nm, respectively. Also the cross-sectional TEM images of two samples with 15 nm Ti annealed at 800 and 850 °C were taken and are shown in Fig.1. As can be seen in Fig.1a in the first case the original layered structure is eroded and mixed.
An entirely different microstructure can be observed in a sample annealed at 850 °C (see TEM image in Fig.1b). The presence of clearly distinguishable layers is evident. The uppermost layer mainly contains the AlNi phase, below it there is a dark (due to the presence of Au) AlAu4 layer. The third layer, based on X-ray analysis, contains Ti-Al-Ni compounds. Also, in this case at the interface with the substrate a TiN layer with a thickness of 4–9 nm is formed. The electrical properties of ohmic contacts are determined precisely by the conductive TiN layer, formation of which depends on a number of process parameters, so in Jacobs and Kramer [5] a systematic approach to reduce resistance of Ti/Al/Ni/Au ohmic contacts in AlGaN/GaN structures is considered. All the ohmic contacts were of Ti/Al/Ni/Au type but differed in metal thickness, time and temperature during rapid thermal annealing.
It can be seen from Fig.2a that the results of studies of Ti/Al/Ni/Au metallisation-based contact systems show that electrical properties and surface morphology depend on the Ti layer thickness ratio to Al thickness. Increasing thickness of Al leads to decreasing of contact resistance. A thickness ratio of 6 shows the best results. Fig.2b presents the studied results of the influence of titanium thickness on contact resistance. From Fig.2c it can be seen that there is an optimum Ni thickness and that increasing or decreasing thickness of the nickel worsens the contact resistance value. Each sample was split into 4 pieces which were annealed at 700, 800, 900 and 1000 °C for 30 seconds in the nitrogen atmosphere. Each time the best results were obtained at 900 °C. Fig.2d shows a more detailed study of the annealing conditions for optimum metallisation pattern consisting of Ti/Al/Ni/Au (30/180/40/150 nm). The best results were obtained at 900 °C for 30 seconds in an N2 atmosphere. After a series of experiments, thicknesses and thermal annealing parameters were obtained to reduce the contact resistance value of Ti/Al/Ni/Au metal contacts. As a result, in this work, the optimized contact had a very low contact resistance of 0.2 Ohm · mm (7.3 · 10–7 Ohm·cm2) and the following parameters for 30/180/40/150 nm thicknesses, respectively [5].
As to Ti and Al based ohmic contacts temperature had to be above 800 °C in order to achieve this compromise. A more detailed study of the heat treatment effect process was carried out in [6], Ti/Al/Mo/Au contact metallization samples were annealed at different temperatures in a Rapid Thermal Annealing (RTA) system. Temperature of each annealing process changes between 825 °C and 855 °C and an annealing time of 60 seconds was maintained for all samples (Fig.3a).
As can be seen from Fig.3a, temperature treatment was carried out in the RTA mode, i.e. fast heating and fast cooling were realised. Figure 3b shows the volt-ampere characteristics of a Ti/Al/Mo/Au metallisation-based contact as a function of annealing temperature. Decreasing heat treatment temperature leads to non-linearity. This could mean that a barrier remains at the metal-semiconductor interface. The temperature of heat treatment effect process shows that the lowest total resistance at a given voltage is achieved at 855 °C. However, increasing annealing temperature above 855 °C leads to increasing the total resistance but the waveform remains slightly non-linear.
The metallisation compositions and heat treatment parameters for manufacturing the ohmic contacts to gallium nitride-based nanoheterostructures of a number of studies are shown in Table 1.
The peculiarities of the burning contact technology are: satisfactory level of contact resistance, high mechanical and temperature stability, and developed surface morphology. The burnable ohmic contact is promising for power semiconductor devices.
NON- BURNING OHMIC CONTACTS TO GALLIUM NITRIDE-BASED NANOSTRUCTURES
Nowadays, methods of non-burnable contacts manufacturing have been gaining ground. Figure 4 shows the concept of non-burnable manufacturing, selectively grown ohmic contacts. The nanoheterostructure surface is annealed through a SiO2 dielectric mask to a depth below the conductive channel shown with a dotted line, and then n+-GaN is grown in the resulting "windows". The introduction of Si admixture causes degeneration of GaN semiconducting layer, which should be in direct contact with the region of two-dimensional electron gas. Production of the contact is completed by the metallisation of the n+-GaN surface. There are a number of advantages to growing contacts over burning contacts. The re-growth was done by homoepitaxy, which ensures good adhesion of the deposited material. Gallium nitride and SiO2 mask have good temperature resistance, which ensures that the original contact shape is maintained and allows for more precise control of manufactured transistors geometry [13]. In recent years, reports have been published on obtaining resistivity of ohmic contacts up to 0.4 Ohm·mm for "Ga-face" HEMT [14] and up to 0.09 Ohm · mm [15] and then up to 0.025 Ohm·mm [16] for "N-face" HEMT. In addition, it has been reported about non-burnable build-up contacts to HEMT structures without etching "windows" for n+-GaN deposition [17], and also using selective etching the part of the AlGaN barrier layer, which does not effect on GaN layer [18].
In [19] epitaxial AlGaN/GaN HEMT heterostructure with silicon ion implantation with a dose of 1 · 1016 cm-3 was considered (Fig.4).
The structures resistance were measured using the long line method. The following data were obtained: contact resistance Rк = 0,96 Ohm · mm and surface resistance RПП = 383 Ом/м2.
In addition to selective doping, the ion doping method is increasingly being used. In [20] evaluation of Cr/Pt/Au non-alloyed ohmic contacts to GaN epitaxial structures and conventional Ti/Al/Ni/Au alloyed contacts to AlGaN/GaN heterostructures with ion-doped contact layers were performed. Contact resistance was 2.8 · 10–6 and 3.5 · 10–7 Ohm · cm2 respectively. Thus it can be concluded: technology of non-burning ohmic contacts makes it possible to achieve low resistances, which makes such contacts the most suitable for microwave transistors.
INFLUENCE OF CONTACT RESISTANCE VALUE ON THE WAVELENGTH SLOPE OF GALLIUM NITRIDE FIELD-EFFECT TRANSISTORS
It is important to assess the impact of contact resistance values on semiconductor device parameters. One such parameter is the steepness. In recent years, along with improvements in plasma chemical processing technology, formation of metallization layers and epitaxial methods of growing nitride layers, improving the parameters of microwave transistors based on gallium nitride.
Figure 5 shows the dependence between the slope of the HEMT field effect transistor and the contact resistance. The steepness decreases smoothly with increasing contact resistance. Both fluxed and non-burning ohmic contacts have been considered. The highest value of steepness is achieved in the work on non-burning contacts [16] and is 1105 mS/mm. In general, the above data shows significantly higher steepness, for HEMT transistors with non-burning contacts. For transistors with burnt ohmic contacts the highest steepness value is 400 mS/mm. Moreover, these values confirm prospective of the contact resistance reduction problem.
CONCLUSIONS
This paper shows what can be expected from a nitride gallium microwave transistor at different values of ohmic contact resistance. In general, this is a promising challenge for power semiconductor devices. As can be seen from the above, non-burning ohmic contact technology has made it possible to achieve low resistances, making such contacts most suitable for microwave transistors, especially in the nanometer range of topological dimensions.
ACKNOWLEDGEMENTS
This work was carried out with the financial support of the Ministry of Education and Science as a part of state task FSMR-2022-0004.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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