Influence of thermal treatment on the formation of ohmic contacts ...

Materials Science-Poland, 30(4), 2012, pp. 342-347 http://www.materialsscience.pwr.wroc.pl/ DOI: 10.2478/s13536-012-0051-y

Influence of thermal treatment on the formation of ohmic contacts based on Ti/Al/Ni/Au metallization to n-type AlGaN/GaN heterostructures∗ 1† , B. PASZKIEWICZ 1 , A. V INCZE 2,3 , R. PASZKIEWICZ 1 , M. T ŁACZAŁA 1 , ´ W. M ACHERZY NSKI J. KOV A´ Cˇ 3 1

Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, 11/17 Janiszewskiego Street, 50-372 Wroclaw, Poland 2 3

International Laser Centre, Ilkovicova 3, 841 04 Bratislava, Slovakia

Institute of Electronics and Photonics, FEI STU, Ilkovicova 3, 812 19 Bratislava, Slovakia

Interfacial reactions between Ti/Al/Ni/Au metallization and GaN(cap)/AlGaN/GaN heterostructures at various annealing temperatures ranging from 715 to 865 °C were studied. Electrical current-voltage (I-V) characteristics, van der Pauw Hall mobility measurements and surface topography measurement with atomic force microscopy (AFM) were performed. The ohmic metallizations were annealed at various temperatures in a rapid thermal annealing system and the annealing time of 60 seconds was kept for all samples. To study the influence of the parameters of annealing process on the properties of the 2 dimensional electron gas (2DEG) the van der Pauw Hall mobility measurement was used. Interfacial reactions between the contact metals and heterostructures were analyzed through depth profiles of secondary ion mass spectroscopy. It was observed that transition from nonlinear to linear I-V behavior occurred after the annealing at 805 °C. For the studied samples, the most promising results were obtained for the annealing temperature of 805 °C. This temperatue ensured not only low contact resistance but also made possible to preserve the 2DEG. Keywords: ohmic contact, AlGaN/GaN heterostructure, 2-dimensional electron gas (2DEG) © Wroclaw University of Technology.

1.

Introduction

AlGaN/GaN heterostructure sensors and fieldeffect transistors have been an area of intense interest for high temperature, high power and high frequency electronic devices applications [1, 2]. Many efforts have been dedicated to the development of fabrication processes of nitrides devices. Wide bandgap semiconductors are able to withstand a harsh environment and high temperature thus AlGaN/GaN devices could find many applications in the fields like: military, aerospace, automotive, petroleum, engine monitoring, flame detection and solar UV detection [3, 4]. Low resistance ohmic contacts to AlGaN/GaN are of great impor-

tance because an improvement of their electrical properties would lead to enhancement of the device performance. Fabrication of low resistance ohmic contacts is difficult because of the relatively high work functions of large amount of various metals in comparison with the electron affinity of Alx Ga1−x N materials [5]. Apart from the requirement of low resistance, the ohmic contacts to AlGaN/GaN heterostructures have to meet additional demands, like thermal and chemical stability, when they are dedicated to the operation in extremely harsh conditions.

2.

Experimental details

The GaN(cap)/AlGaN/GaN heterostructure applied in this study consisted of GaN(cap ∗ This paper was presented at the 35th International Microelec˚ ˚ 50 A)/Al µm) grown by 0.3 Ga0.7 N(250 A)/i-GaN(1 tronics and Packaging IMAPS-IEEE CPMT Conference, 21–24 metalorganic vapour phase epitaxy (MOVPE) on September 2011, Gda´nsk – Sobieszewo, Poland. † E-mail: [email protected] sapphire substrate Fig. 1.

Influence of thermal treatment on the formation of ohmic contacts based on Ti/Al/Ni/Au metallization to n-type AlGaN/GaN heterostructures

• Au metal layer was etched in iodinepotassium iodide solution, • Ni was etched in ferrous chloride, • Al was etched at 45 °C in the solution of H3 PO4 /HNO3 /CH2 COOH/H2 O (85:5:5:5), • Ti layer was etched in perhydrol warmed to 65 °C. Before the measurement, four indium contacts were made.

GaN cap (5 nm) Al0.3 Ga0.7 N (25 nm) Un-doped GaN (1 µm) LT-GaN (100 nm) sapphire Fig. 1. AlGaN/GaN heterostructure used in this study.

For the research purposes three main groups of samples have been prepared: the first one with a photolithography mask designed for I-V measurement, the second one without photolithography mask dedicated for the van der Pauw Hall mobility measurement, and the third group of samples dedicated for depth profiles with secondary ion mass spectroscopy. Prior to metal deposition native oxide (Ga2 O3 ) was removed from all samples surfaces by etching in HCl:H2 O (1:1) solution, followed by a deionised water rinsing and drying in N2 flow. Then the samples were immediately loaded into the vacuum chamber of an evaporation system. The loading time was approximately 5 min, the time of evacuation to the vacuum level of 10 Pa was about 40 min and to the level lower than 10−4 Pa, approximately 24 hours. The metal stuck and the layers thicknesses of the Ti/Al/Ni/Au multilayer of the ohmic contact were selected on the basis of the authors previous unpublished study. The metallic contact consisting of Ti ˚ ˚ ˚ ˚ was (200 A)/Al (1000 A)/Ni (400 A)/Au (1500 A) deposited on the substrate under the vacuum conditions with a base pressure lower than 10−4 Pa. The Ti and Ni layers were deposited by using an electron beam evaporator whereas the Al and Au metallic layers were deposited with a resistance evaporator. The influence of annealing process on the contact properties was studied by I-V characterization, van der Pauw Hall mobility measurement and SIMS depth profiling. The Ti/Al/Ni/Au multilayer ohmic metallizations were annealed at various temperatures in a rapid thermal annealing (RTA) system [6] in the mixture of hydrogen/nitrogen (1:10) gasses. The temperature of each annealing process was changed over the range of 715 °C to 865 °C and the annealing time of 60 seconds was kept for all samples. The samples dedicated for the van der Pauw Hall mobility measurements were etched to remove the multilayer metallization in the following steps:

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To investigate the I-V characteristics of metalsemiconductor contacts, two contacts from the test structure intended for transfer length method (TLM) were used for measurement with an Agilent system. To study the influence of the parameters of annealing process on the condition of heterostructure, in particular on the 2-dimensional electron gas (2DEG), an indirect method using the van der Pauw Hall mobility measurement was employed [7]. To help in understanding of the significant difference in the surface morphology for ohmic contacts after annealing, AFM images were included. The AFM images and profiles were also used to find how deep the solid state reactions between the metallization and the semiconductor structure reach. In order to assess the depth of the reaction occurring during the thermal treatment between the metallization and the semiconductor structure, the metals forming the ohmic contact (Ti, Al, Ni, Au) were selectively etched. After the etching of the metal layers, the topography of the surface underneath the contact area was studied. It should image the morphology of the metal-semiconductor (m-s) interface. After the thermal treatment the metallic multilayer was etched according to the procedure described above.

3.

Results and discussion

Fig. 2 shows the current-voltage (I-V) characteristics between two contact pads for all samples annealed at various temperatures. The ohmic behaviour, which is represented by linear I-V characteristics, was obtained for annealing temperature above 805 °C. The van der Pauw Hall mobility measurement showed that the sheet density of carriers beneath the metal contacts remained on the same level (see Table 1), however, the Hall mobility of the carri-

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Fig. 3. Scheme of the possible location of metal contact in relation to 2DEG at the GaN(cap)/AlGaN/GaN heterostructure a) non annealed, b) after thermal annealing – 2DEG is not damaged, c) after thermal annealing – 2DEG is completely damaged.

Fig. 2. I-V characteristics of the contacts annealed at different temperatures.

Table 1. Influence of thermal annealing (t = 60 s) of metal contacts on the electrical parameters of AlGaN/GaN heterostructures underneath the metal contacts. Temperatures Sheet density Hall I-V ofhcarriers of thermal characteristics mobility i h 2i 1 cm annealing [°C] 2 Non annealed 715 °C 745 °C 775 °C 805 °C 835 °C 865 °C

cm

V ·s

9 · 1012

1769 1562 1411 1379 1447 538 506

9.1 · 1012 9.24 · 1012 9.19 · 1012 9.51 · 1012 9.42 · 1012 9.47 · 1012

Nonlinear Nonlinear Nonlinear Nonlinear Linear Linear Linear

ers decreased from 1769 cm2 /V·s for non annealed samples to 506 cm2 /V·s for samples annealed at the highest (865 °C) temperature. This resulted in a significant increase in sheet resistance of the AlGaN/GaN heterostructures. The thermal annealing at the elevated temperature affected the whole structure and also the 2DEG just underneath the metal contacts, which led to the degradation of the heterostructure up to complete damaging of AlGaN/GaN properties after annealing at temperature above 835 °C (the Hall mobility decreased below 538 cm2 /V·s). The process of the degradation can be explained by the solid state reaction phenomenon of chemical elements between

the metal contact layers and GaN(cap)/AlGaN layers (Fig. 3). This could lead to the consumption of GaN(cap)/AlGaN layer into the GaN layer. The critical AlGaN thickness of 3 nm is required to enable 2DEG accumulation at the AlGaN/GaN interface [8]. To confirm the structural changes in the m-s junction during the thermal treatment (as a result of reactions occurring in the bulk substrate) and to image the effect of the reactions on the structure of m-s ohmic junction to AlGaN/GaN heterostructure we have investigated the structural changes after the thermal treatment in the temperatures above 835 °C, where the electron mobility falls below 538 cm2 /V·s (Table 1), showing a complete degradation of the heterostructure. After etching off the metallization, the AFM images of surface topography (Fig. 4) show apparent agglomerates whose height exceeds the height of the semiconductor structure. A selective etching in HCl:H2 O (1:1) solution confirmed that these were gallium islands, which probably originated from decomposition of the semiconductor material in the area of the m-s interface during the thermal treatment. In order to analyse the influence of the reactions occurring in the bulk during the thermal treatment on the m-s interface the AFM topography of sample surface was studied. Fig. 4b shows a topography profile of the semiconductor surface after etching off the metallization (measured along the path marked in Fig. 4a) with the indicated characteristic features. The most important conclusion following the analysis of the profile is the fact that the reactions occurring between the metallization and the semiconductor structure during the thermal treatment may reach the AlGaN/GaN junction, resulting


(a)

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(b)

Fig. 4. AFM topography image of etched ohmic contact (a) and topography profile (b). The Ti/Al/Ni/Au ohmic contact was annealed in RTP process above 835 °C.

in a complete degradation of the heterostructure in the layer beneath the metal contact. To study the chemical reactivity of the ohmic contact, SIMS depth profiling method was used. Emphasis was given to the observation of chemical composition of the heterostructure under metallization, to find the differences between structures with linear and nonlinear I-V characteristics (Fig. 2). Because of very poor morphology and high roughness after thermal annealing process (Fig. 5) and front-side SIMS sputtering we were not able to accurately define the depth resolution which rendered the data useless to precisely evaluate the diffusion of ohmic contact metals into the underlying heterostructures. The thermal annealing strongly affected the surface morphology and caused migration of metals, finally resulting in the formation of agglomerates. The particular mechanisms causing the agglomerates height increase during the thermal annealing at various temperatures were discussed in our previous paper [9]. For the annealing temperature of 745 °C and 775 °C the height of the agglomerates approaches up to 0.58 µm and up to Fig. 5. 1.2 µm, respectively (Fig. 5). On the basis of the SIMS profiles we observed characteristic changes in the ohmic contact structure. The intermetallic diffusion process can be clearly observed in the SIMS depth profiles of the

AFM images of surface morphology of the ohmic contact after thermal annealing for 60 s at various temperatures. The height of agglomerates increased from 0.67 µm for 715 °C to 1.16 µm for 865 °C.

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Fig. 6. SIMS depth profiles of the ohmic contacts annealed at various temperatures.

annealed contact (Fig. 6). After 800 sec (715 °C) the interface of metals/GaN(cap) begins. It is indicated by decreasing the Au signal, high AlN intensity and constant signal of GaN starting at this point. Although the metal with high melting point like Ni was used in the ohmic contacts as a diffusion barrier, due to its low bulk diffusion, the migration of Au atoms deep into the metal interfaces (reaching the cap GaN layer) is evident. On the other hand, outdiffusion of the Ga atoms from the substrate toward the metallic layers takes place during the annealing treatment, as suggested by the intensity of Ga lines at the interface – metals/GaN(cap). It could be caused by the decomposition of GaN cap layers during the RTA treatment. Linear I-V characteristics were obtained, after annealing at temperature of 805 °C and higher. What is characteristic, at the SIMS profiles, a complete consumption of GaN cap layer by solid state reaction was observed in this range of temperatures.

4.

Conclusions

It has been demonstrated that the application of appropriate parameters of thermal annealing process of Ti/Al/Ni/Au ohmic contacts to GaN(cap)/AlGaN/GaN heterostructures can lead to the preservation of 2DEG beneath the metal contacts without substantial impairment of the 2DEG properties. There is a temperature, which allows one to obtain the linear I-V characteristics of evaporated ohmic contacts of good electrical properties after annealing with simultaneous preservation of 2DEG beneath the metallization. For the examined samples, the best results – linear I-V characteristic and Hall mobility equal to 1447 cm2 /V·s, were achieved for the temperature of thermal annealing of 805 °C. Rapid thermal annealing in temperatures above 835 °C caused the degradation of the heterostructure underneath the metallization of ohmic contacts (the Hall mobility decreased below 538 cm2 /V·s). Also, the investigations of m-s in-


terface confirmed the complete degradation of the heterostructures underneath the ohmic contact after thermal annealing above 835 °C because of solid state reactions which reached deeper than the AlGaN/GaN interface. The SIMS depth profiling analysis was unable to achieve high depth resolution due to poor surface topography of metallization. The height of the agglomerates reached up 1.1 µm and in fact could lead to intermixing of the layers. Nevertheless, high heterogenity of the metal-semiconductor interface of annealed ohmic contact was observed. The complete consumption of GaN cap layer with simultaneous linearity of I-V characteristics of ohmic contact was perceived.

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References

[1] A MANO H., K ITO M., H IRAMATSU K., A KASAK I., Jpn. J. Appl. Phys., 28 (1989), 1705. [2] NAKAMURA S., M UKAI T., S ENOH M., Appl. Phys. Lett., 64 (1994), 1687. [3] F ERGUSON I., T RAN C.A., K ARLICEK R.F., F ENG Z.C., S TALL R., L IANG S., Mater. Sci. Eng., 50 (1997), 311. [4] WALKER D. et al, Appl. Phys. Lett., 68 (1996), 2100. [5] S CHMITZ A.C., P ING A.T., K HAN M.A., C HEN Q., YANG J.W., A DESIDA I., J. Electron. Mat., 27 (1998), 255. ´ [6] M ACHERZY NSKI W., T ŁACZAŁA M., Elektronika, 7 (2007), 37. [7] S CHRODER D.K., Semiconductor material and device characterization, third edition, John Wiley & Sons, New Jersey, 2006. [8] I BBETSON J.P., F INI P.T., N ESS K.D., D ENBAARS S.P., S PECK J.S., M ISHRA U.K., Appl. Phys. Lett., 77 (2000), 250.. Acknowledgements [9] M ACHERZYNSKI W. et al., Optica Applicata, XIXXX, (2009), 673. This work was co-financed by European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08-04), National Centre for Science under the grant no. N N515 495740, N N 518424036, N Received 2011-11-17 N515 607539, R02 018 02 by Wroclaw University of TechnolAccepted 2012-10-14 ogy statutorys grants no. S10019, B10010 and Slovak-Polish International Cooperation Program no. SK-PL-0017-09.