Transmission electron microscopy assessment of the Si enhancement of Ti Al Ni Au
Ohmic contacts to undoped Al Ga N Ga N heterostructures
Vincent Desmaris, Jin-Yu Shiu, Chung-Yu Lu, Niklas Rorsman, Herbert Zirath, and Edward-Yi Chang
Citation: Journal of Applied Physics 100, 034904 (2006); doi: 10.1063/1.2218262 View online: http://dx.doi.org/10.1063/1.2218262
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/100/3?ver=pdfcov Published by the AIP Publishing
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Transmission electron microscopy assessment of the Si enhancement
of Ti/ Al/ Ni/ Au Ohmic contacts to undoped AlGaN / GaN heterostructures
Vincent Desmarisa兲
Microwave Electronics Laboratory, Microtechnology and Nanoscience, Chalmers University of Technology, SE 412 96 Gothenburg, Sweden
Jin-Yu Shiu
Microwave Electronics Laboratory, Microtechnology and Nanoscience, Chalmers University of Technology, SE 412 96 Gothenburg, Sweden and Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China
Chung-Yu Lu
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China
Niklas Rorsman and Herbert Zirath
Microwave Electronics Laboratory, Microtechnology and Nanoscience, Chalmers University of Technology, SE 412 96 Gothenburg, Sweden
Edward-Yi Chang
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China
共Received 4 January 2006; accepted 19 May 2006; published online 2 August 2006兲
The microstructure of Si/ Ti/ Al/ Ni/ Au was investigated using transmission electron microscopy and energy dispersive x-ray spectroscopy. The dependence of the contact resistance on the silicon layer thickness and the temperature was correlated to the microstructure of the alloyed contacts. The enhancement of the contact resistance by inserting a 30 Å thick Si layer under the Ti/ Al/ Ni/ Au metallization was attributed to diffusion of the contact into the AlGaN layer. Increasing the Si thickness and or the temperature resulted in the formation of Gold 共Au兲-based silicides, which prevent the formation of low interfacial TiN or AlN layers. © 2006 American Institute of Physics. 关DOI:10.1063/1.2218262兴
I. INTRODUCTION
AlGaN / GaN high electron mobility transistors共HEMTs兲 have demonstrated excellent performance1–5for microwave, high power, and high temperature applications due to the remarkable electrical and physical properties of the III-nitride semiconductors and the AlGaN / GaN heterojunction.6 However, high performance microwave and power devices for such applications rely on low resistance Ohmic contacts, which directly influence the knee voltage, transconductance, output current density, and resistive heating.7
Since the source and drain Ohmic contacts are formed at the top of the AlGaN / GaN epistructure in a HEMT, exten-sive research has been carried out on developing reliable and low resistance Ohmic contacts to AlGaN / GaN heterostures. Most of low resistance Ohmic contacts to AlGaN or AlGaN / GaN structure have been obtained using a Ti/ Al-based metallization, with a Ni,8 Ti,9 Pd,10 Pt,11 Cu,12 or Mo 共Ref. 13兲 presumed diffusion barrier overlayer, fol-lowed by a Au deposition to reduce the sheet resistance of the contact and limit the oxidation.
Desmaris et al.14 demonstrated further enhancement of the Ti/ Al/ Ni/ Au alloyed Ohmic contacts by inserting a thin
Si共30 Å兲 between the four-layer metallization and the semi-conductor, without the need of extra process steps such as regrowth,15 implantation,16 plasma preetching,17 adding an
n-GaN cap layer,18 or annealing prior to metallization.19 Contact resistance of less than 0.25⍀ mm was obtained on undoped AlGaN / GaN structure. However, the exact role of Silicon for the enhancement of the Ohmic contact was unde-fined. Youn and Kang,12 observed a similar enhancement of Ti/ Al/ Cu/ Au Ohmic contacts to Si-doped AlGaN / GaN het-erostructures. This was attributed to a local increase of the doping in the subcontact AlGaN region and the reduction of the potential offset at the interface due to the formation of a Al–Ti–Si–N intermetallic layer. Recently, Mohammed
et al.,20 using a Ti/ Si/ Al/ Si/ Mo/ Au metallization scheme attributed mainly the Si enhancement of the contact resis-tance to the formation of an Al–Ti–Si–N intermetallic layer based on Auger electron spectrometry; questioning the oc-currence of the local diffusion induced Si-doping mecha-nism. However, in their study Si was not directly deposited in contact to the semiconductor.
In this work, we investigate the microstructure of Si/ Ti/ Al/ Ni/ Au Ohmic contacts to undoped AlGaN / GaN heterostructure, by means of transmission electron micros-copy 共TEM兲 and energy dispersive x-ray spectroscopy 共EDX兲, in order to elucidate the role of silicon in the
a兲Electronic mail: [email protected]
0021-8979/2006/100共3兲/034904/4/$23.00 100, 034904-1 © 2006 American Institute of Physics
enhancement of the contact resistance and investigate its de-pendence on the silicon layer thickness and annealing tem-perature.
II. EXPERIMENTAL DETAILS
The AlGaN / GaN heterostructure was grown on sapphire by rf microdevices using metal-organic chemical-vapor deposition. The HEMT structure consisted of a 1m thick undoped GaN buffer layer followed by 25 nm undoped Al0.29Ga0.71N. The sheet carrier concentration and electron mobility obtained by Hall measurements were 1.2 ⫻1013cm−2 and 1100 cm2/ V s, respectively. Prior to pro-cessing, organic contaminants were removed in hot solvents before cleaning the samples using standard RCA methods.
Mesas were formed using a chlorine based inductively coupled plasma reactive ion etching共ICP-RIE兲 to isolate the active areas previous to contact deposition. Linear transfer length measurements 共TLMs兲 test structures were later pat-terned using photolithography. To eliminate surface oxide buildup, the samples were dipped into a buffered oxide etchant solution immediately before depositing the Ohmic contacts by electron beam evaporation and lifted-off to form a TLM patterns with spacings of 4, 8, 12, 16, 20, 25, and 30m. Two silicon layers of different thickness were depos-ited共30 and 90 Å兲 under the same Ti/Al/Ni/Au multilayer. A third sample without any silicon layer was also processed for reference. The samples were diced before one sample of each Si thickness was annealed at the same time in a N2 atmosphere at temperatures ranging from 700 to 850 ° C for 30 s. Based on the contact resistance measurements, four ad-ditional samples were prepared for TEM measurements.
The contact resistances of the different Ohmic metalliza-tions were calculated21 from the current-voltage measure-ments of the TLM test structure using a HP4145B semicon-ductor parameter analyzer. Although the material sheet resistance was assumed to be constant on a given mesa struc-ture, variations of the latter material property over the whole sample were considered.
The TEM specimens were prepared by using manual lapping before finely polishing by Ar-ion milling using Gatan precision ion polishing system. TEM measurements were carried out on a JEOL JEM-2010F FEG. In situ EDX spec-trometry was used in order to semiquantatively analyze the chemical compositions by means of an Oxford x-ray disper-sive spectrometer. The smallest electron beam spot size was 0.5 nm, allowing a very accurate spatial resolution of the analyzed areas.
III. RESULTS AND DISCUSSION A. Ohmic contact resistance
The temperature dependence of the contact resistances was first investigated for the three different metallizations. The different Ohmic contact resistances, extracted from the TLM measurements are presented in Fig. 1. The lowest Ohmic contact resistance of 0.23⍀ mm was observed for the Si共30 Å兲/Ti/Al/Ni/Au metallization revealing the electrical Si enhancement of the standard Ti/ Al/ Ni/ Au Ohmic metal-lization. However, increasing further the Si thickness did not
improve the contact resistance. Moreover, the contact resis-tances of the Si/ Ti/ Al/ Ni/ Au metallizations show a differ-ent temperature dependences to the annealing temperature than the standard Ohmic contacts. In fact, the contact resis-tance degenerated rapidly after reaching a minimum at about 780 ° C.
B. Microstructure of the Ohmic contacts
The microstructure of Ti/ Al/ Ni/ Au, Si共30 Å兲/Ti/ Al/ Ni/ Au and Si共90 Å兲/Ti/Al/Ni/Au Ohmic contacts to AlGaN / GaN undoped epistructures alloyed at 780 ° C are presented in Figs. 2–7. The microstructure observed for the Ti/ Al/ Ni/ Au metallization on the bright field TEM image 共Fig. 2.兲 is similar to the one described by Bright et al.22
Au diffused through the Ni layer and large islands made of Al–Au compounds were formed during annealing. TiN and Al–Ti–N were clearly identified at the interface and are com-monly believed to be responsible for the low resistance Ohmic contact to the nitrogen depleted AlGaN layer. In fact, the formation of the interfacial TiN and Al–Ti–N alloy23,24is
FIG. 1. Annealing temperature dependence of the contact resistance for the three metallization schemes.
FIG. 2. Bright field TEM picture of the microstructure of the Ti/ Al/ Ni/ Au contacts annealed at 780 ° C.
034904-2 Desmaris et al. J. Appl. Phys. 100, 034904共2006兲
supposed to consume some N of the AlGaN layer during alloying, leaving N vacancies in this layer which act as un-intentional dopants facilitating the Ohmic contact formation. An Al–Au phase was also observed between and above the TiN and Ti–Al–N grains, which was suspected to aid the Ohmic contact formation.22
Introducing the 30 Å thick Si layer resulted in the pen-etration of the contacts into the AlGaN and its partial con-sumption under almost the entire contact region, reducing the tunneling distance between the Ohmic alloyed metallization and the two dimensional electron gas25共2DEG兲 共Fig. 3兲. The contact inclusion in the AlGaN layer mainly consisted of small 共10–15 nm兲 Ti–Al–Ga–N and Ti–Al–Ga–Au grains. Similar inclusion was also observed by Fay et al. in the case of Ti/ Al/ Ti/ Au contacts, and the penetration of the contact in the layer was found to be correlated to the presence of Al–Au and being responsible for the low Ohmic contact.26 Moreover, relatively large Si共3%–4%兲–Al–Ga and Si共2%兲– Al–Ga–Ti–Au grains located just above the the original contact/AlGaN interface were identified, suggesting the Si enhanced the consumption and outdiffusion of Ga from Al-GaN layer, allowing the penetration of the Ohmic metalliza-tion. Similar Ga outdiffusion enhancements were earlier re-ported for Pt and Pd共Ref. 27兲 and we believe that Si might have the same effect. Very large grains made of Si–Al–Ni– Ga–Au共Fig. 3兲 were also observed on the top of the alloyed contacts, revealing the outdiffusion of Si and Ga up to the surface, unlike the published results about confinement of Si between the Ti and Mo layer by other authors.20
Furthermore, careful EDX analysis also the s axis on Fig. 3 of some remaining AlGaN show a gradient of Si con-centration from the surface into the semiconductor, showing the possible diffusion induced Si doping of the AlGaN Schottky layer, as suggested by Desmaris et al.14 and Youn and Kang.12Even though Si was present in small percentages 共2%兲 in the top of the AlGaN matrix, the resulting doping would be considerable, hence enhancing the Ohmic contact resistance.
Increasing the thickness of the Si layer resulted in a dras-tic modification of the microstructure of the contacts. The
Ohmic contact did not penetrate the AlGaN layer and the formation of very large Ti–Ni–Si共10%兲–Al–Au segregation islands共Fig. 4兲 and pure Al crystallites in an AlN/Al matrix was observed共Fig. 5兲. Si and Ti were also found to diffuse out to the contact surface forming principally Al–Si共5%– 10%兲–Au, Ti–Al–Au, and Ti–Al compounds above the AlN / Al continuous layer. Nitrogen consumption from the AlGaN layer is thus observed, which would result in a local increase of the semiconductor doping and the enhancement of the Ohmic contact. We believe that in this case, the larger amount of Si diffused out to the surface, alloyed with the down diffusing Au forming the observed Si–Al–Au which hinders the formation of the Al–Au layer. The absence of this Al–Au alloy in turn allowed the outdiffusion of Ti through the Al layer.28,29Therefore Ti–Al–Au and Ti–Al alloys were observed above the AlN / Al layer.
Furthermore our results clearly show that the absence of the Al–Au layer is detrimental to the contact resistance, as suspected by Fay et al.26 and Wang et al.,29 who also ob-served the importance of the Al–Au diffusion front in the formation of the Ohmic contact to AlGaN and GaN, respec-tively.
FIG. 4. Bright field TEM picture of the segregated Si-rich islands formed when annealing the Si共90 Å兲Ti/Al/Ni/Au metallization 830 °C.
FIG. 5. Bright field TEM picture of the microstructure of the Si共90 Å兲Ti/Al/Ni/Au contacts annealed at 830 °C.
FIG. 3. Bright field TEM picture of the microstructure of Si共30 Å兲Ti/Al/Ni/Au contacts annealed at 780 °C.
After annealing the Si共30%兲/Ti/Al/Ni/Au Ohmic con-tacts at 830 ° C共Fig. 5兲, silicon could still be detected in the underlying AlGaN layer at a very low atomic percentage 共1%–2%兲, when performing an EDX scan along the s axis on Fig. 6. The contact penetration into the semiconductor was much scarcer than when annealed at 780 ° C. Formation of Si–Ti–Al–Au and Si–Al–Au islands with rather high silicon content 共5%–8%兲 was also observed in the Al–Au matrix, but no continuous thick Al or gold-silicide based layer was observed. This could be ascribed to the faster Si outdiffusion due to the higher temperature and limited Si supply because of the thin Si layer, which act as a getter for part of the down diffusing Au, hence preventing the formation of Al–Au alloy. Nevertheless a thin共1–2 nm兲 continuous contact layer with very low Si content consisting of Ti–N and Al–N was ob-served for the formation of the Ohmic contact 共Fig. 7兲.
IV. CONCLUSION
We demonstrated that the Si enhancement of Ti/ Al/ Ni/ Au Ohmic contacts to undoped AlGaN / GaN het-erostuctures was due to penetration of the Ohmic contact alloyed metallization into the AlGaN Schottky layer. The op-timal Si thickness resulted from the tradeoff between the Ga outdiffusion and consumption induced by the presence of Si and the prevention of the formation of the Ti encapsulating Al–Au layer due to the formation of Si–Al–Au and Si–Ti–
Al–Au compounds. Increasing the annealing temperature for the Si共30 Å兲/Ti/Al/Ni/Au is believed to result in a larger outdiffusion of silicon to the contact surface and the forma-tion on the undesired gold-silicide regions.
ACKNOWLEDGMENTS
The Swedish Foundation for Strategic Research 共SSF兲, The Chalmers Center for High Speed Technology共CHACH兲, and SAABTech AB are acknowledged for financial support. The authors would also like to thank M. Sc Kristina Dyne-fors at Chalmers University 共Sweden兲 for her help with the picture processing and the TEM center of the Department of Engineering and System Science, Taiwan National Tsing Hua Unversity 共R.O.C兲 for their assistance during the TEM imaging.
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共2005兲. FIG. 6. Bright field TEM picture of the microstructure of
Si共30 Å兲Ti/Al/Ni/Au contacts annealed at 830 °C.
FIG. 7. HRTEM picture of the Si共30 Å兲Ti/Al/Ni/Au contacts annealed at 830 ° C.
034904-4 Desmaris et al. J. Appl. Phys. 100, 034904共2006兲
