Boltovets N. S.1, Ivanov V. N.1, Sveshnikov Yu. N.2, Belyaev A. E.3, Avksentiev A. Yu.3, Konakova R. V.3, Kudryk Ya. Ya.3, Kurakin A. M.3, Milenin V. V.3 1 State Enterprise Research Institute “Orion”, tf, Eugene Pottier St., Kiev – 03057, Ukraine Tel.: +38044-465-0548, e-mail: bms@i.kiev.ua 2Closed Corporation “Elma-Malakhit”, Zelenograd, Russia 3V. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine

41, Nauki Prospect, Kiev – 03028, Ukraine Tel.: +38044-265-6182, e-mail: konakova@isp.kiev.ua

Abstract – The Au-TiBx(Ti)-AI-Ti-n-GaN ohmic contacts and Au-ZrBx-n-6H-SiC barrier contacts were studied before and after rapid thermal annealing at T = 700 and 800 °C. We measured the component concentration depth profiles and electrical characteristics of the contact systems. It was shown that TiBx buffer layers are promising for application in ohmic contacts to л-GaN and ZrBx in barrier contacts to / 7-6Н SiC.

I.  Introduction

During recent years wide experience has been gained in development of ohmic and barrier contacts to wide-gap semiconductors SiC and GaN that are best- investigated and promising for high-temperature microwave electronics. This experience is based on application of traditional (mostly vacuum) technologies. They conform well to the manufacturing technologies for various devices (microwave diodes and transistors of different types, ICs, etc.) when barrier contacts are made of polycrystalline layers of pure metals or their silicides or nitrides [1-7].

Such approach has many advantages. However, the problem of grain-boundary diffusion in such contacts in the course of their long-term work at high (> 500 °C) operating temperature [2-5] still remains unsolved. Our investigations showed that the above problem can be solved by use of nanostructure (quasi-amorphous) layers of TiB2– and ZrB2-type metal compounds as barrier- forming or buffer layers [8, 9]. Here we consider a possibility of formation of similar contacts to wide-gap semiconductors SiC and GaN.

II.    Sample preparation and experimental procedure

We studied samples of two types: Au-ZrB2-n-6H-SiC Schottky barriers and Au-TiB2(Ti)-AI-Ti-n-GaN ohmic contacts. Silicon carbide single crystals (thickness of -400 |im, concentration of noncompensated donor impurity (nitrogen) up to 1018 cm’3) Were grown using the Lely technique. Epitaxial layers of п-GaN (thickness of ~ 1 | im, donor concentration of ~ 1017 cm’3) were VPE-grown on the (0001) plane of Al203. The Ti, Al, TiB2, ZrB2 and Au contact layers were formed with magnetron sputtering. Ohmic contacts to SiC were formed by magnetron sputtering of Ni onto the С-face of SiC followed by firing at T = 1000 ° C for 90 s with further gilding.

Some part of samples was test structures. For them we measured Auger electron spectra before and after rapid thermal annealing (RTA) at T = 700 °C (for the GaN-based samples) or 800 °C (for the SiC-based samples). The Schottky-barrier device structures Au-ZrBx-n- 6H-SiC (100 |im in diameter) were made using planar technology [9].

l-V and C-V curves of the Schottky-barrier diode structures Au-ZrBx-n-6H-SiC, as well as contact resistance of Au-TiBx(Ti)-AI-Ti-n-GaN structures, were measured both before and after RTA.

III.  Results and discussion

Shown in Figs. 1 and 2 are the component concentration depth profiles for structures of both types taken after RTA. One can see that the layered structure of metallization survived. The component concentration depth profiles for contacts Au-Ti-AI-Ti-GaN after RTA at T = 700 °C are given in Fig. 3 for comparison.

One can see from these figures that after RTA the titanium metallization (a buffer layer formed between the Al and Au layers and hindering interactions between phases at the Au-Ti and Ti-AI interfaces) does not serve as a barrier for mass transfer in the Au-Ti-AI-Ti-GaN contact. This is evidenced by disruption of layered structure. Contrary to the above, the TiBx buffer layer in the Au-TiBx-AI-Ti-GaN ohmic contact and the barrier- forming ZrBx layer in the Au-ZrBx-SiC contact are not disrupted in the course of RTA.

These results agree with those of measurements of

l-       V and C-V curves of the Au-ZrBx-n-6H SiC Schottky diodes and contact resistance in the Au-TiBx(Ti)-AI-Ti-GaN structures before and after RTA. For instance, the Schottky barrier height ерь and ideality factor n calculated from the forward branch of lV curve of Au-ZrBx-n-6H-SiC Schottky diode remained practically the same (0.79-0.80 V and -1.2, respectively) after RTA. Estimation of <pb from C-V curves taken before and after RTA gave 0.87 V, at invariable slope of the 1/C2= F (V) curve (here З is the barrier capacitance and \ / is the applied voltage) that corresponded to the concentration of noncompensated donors in n-6H-SiC equal to ~ 1018 cm’3

Contact resistance in the starting Au-TiBx(Ti)-AI- Ti-GaN structures was (7-^-8)-10′6 Q-cm2 After RTA its value remained practically the same in structures with TiBx buffer layer, while in the Au-Ti-AI-Ti-GaN contacts it increased by more than two orders of magnitude.

The above results may be explained if one invokes our earlier results concerning structure of sputtered TiBx and ZrBx layers [8]. The mode of TiBx and ZrBx sputtering was chosen in such a way as to provide amorphous structure of the deposited layers. In this case boundary- grain diffusion in the layers contacting with titanium and zirconium borides is considerably retarded, and the layered structure of metallization is retained.

IV.  Conclusion

The results of our investigation enable us to conclude that the buffer and barrier-forming layers on the basis of titanium and zirconium borides are promising for application in ohmic contacts to GaN and barrier contacts to SiC.

[1]    Morkoc H. Nitride Semiconductors and Devices.

Fig. 1. Component concentration depth profiles in Au-ZrBx-n-6H-SiC barrier contact after RTA at T= 80(fC

Springer. 1999.

[2]    Danilin V. N.. Dokuchaev Yu. P., Zhukova T. A., Komarov M. A. High-power High-frequency and Radiation- stable Microwave Devices of New Generation on the Basis of Wide-gap AIGaN/GaN Heterojunction Structures. GUP NPP “Pulsar”. 2001 (in Russian).

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[5]    Porter L., Davis R. A critical review of ohmic and rectifying contacts for silicon carbide // Mater. Sci. Eng. 1995. v. B34. No 1. pp. 83-105.

[6] Jacob C., Pirouz P., Kuo H. I. Mehregany M. High temperature ohmic contacts to ЗС-silicon carbide films / / Sol. St. Electron. 1998. v. 42. No 12. pp. 2329-2334.

[7]    DavydovS. Yu., Lebedev A. A., Tikhonov S. K. On Schot- tky barrier at a contact with silicon carbide // Fiz. Tekhn. Poluprov. 1997. v. 31. No 5. pp. 597-599 (in Russian).

Fig. 2. Component concentration depth profiles in Au-TiBx(Ti)-AI-Ti-n-GaN ohmic contact

[8] Boitovets N. S., Ivanov V. N.. Konakova R. V., Kudryk Ya. Ya., Lytvyn O. S., Lytvyn P. М., Milenin V. V. Interactions between phases and the features of structure relaxation in TiBx-n-GaAs (InP, GaP, 6H-SiC) contacts exposed to active treatments // Fiz. Tekhn. Poluprov. 2004. v. 38. No 7. pp. 769-774 (in Russian).

[9] Boitovets N. S., Ivanov V. N.. Konakova R. V., Kudryk Ya. Ya., Milenin V. V., AgueevO. A., Svetlichny A. М., Soloviev S., Sudarshan T. S. Technological aspects of development of planar microwave diodes with Au-Ti(ZrBx)-n-n+-4H SiC Schottky barrier// In: 13th Intern. Crimean Conf. “Microwave & Telecommunication Technology”, Conf. Proc. Sevastopol. Weber Co. 2003. pp. 159-160 (in Russian). ISBN 966-7968-12X. IEEE Cat. Number 02EX570.

Провідникові і бар’єрних КОНТАКТИ до мікрохвильових діоди на основі SiC І GaN

Fig. 3. Component concentration depth profiles in Au-Ti-AI-Ti-GaN contacts after RTA atT= 700 °C

Болтовец Н. С., Іванов В. Н., Свєшніков Ю. Н.,

Бєляєв А. Е., Авксентьєв А. Ю., Конакова Р. В., Кудрик Я. Я., Куракін А. М., Міленін В. В.

Анотація – Омічні контакти Au-TiBx(Ti)-AI-Ti-n-GaN і бар’єрні контакти Au-ZrBx-N-6H-SiC досліджувалися до та після швидкого термічного відпалу при Г = 700 і 800 ° С. Вимірювалися профілі розподілу компонентів та електричні характеристики контактних систем. Показана перспективність використання буферних шарів TiBx в омічних контактах до л-GaN і ZrBx в бар’єрних контактах к /?-6Н SiC.

Джерело: Матеріали Міжнародної Кримської конференції «СВЧ-техніка і телекомунікаційні технології»