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Demonstration of Submicron-Gate AlGaN/GaN High-Electron-Mobility Transistors on Silicon

with Complementary Metal–Oxide–Semiconductor-Compatible Non-Gold Metal Stack

View the table of contents for this issue, or go to the journal homepage for more

2013 Appl. Phys. Express 6 016501

(http://iopscience.iop.org/1882-0786/6/1/016501)

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Demonstration of Submicron-Gate AlGaN/GaN High-Electron-Mobility Transistors on Silicon

with Complementary Metal–Oxide–Semiconductor-Compatible Non-Gold Metal Stack

Subramaniam Arulkumaran1, Geok Ing Ng2, Sahmuganathan Vicknesh1, Hong Wang2, Kian Siong Ang1,

Chandramohan Manoj Kumar1, Khoon Leng Teo1, and Kumud Ranjan1

1Temasek Laboratories@NTU, Nanyang Technological University, Singapore 6375532School of Electrical and Electronics Engineering, Nanyang Technological University, Singapore 639798

Received October 13, 2012; accepted November 27, 2012; published online December 17, 2012

We have demonstrated 0.15-�m-gate-length AlGaN/GaN high-electron-mobility transistors (HEMTs) with direct-current (DC) and microwave

performances for the first time using a complementary metal–oxide–semiconductor (CMOS)-compatible non-gold metal stack. Si/Ta-based ohmic

contact exhibited low contact resistance (Rc ¼ 0:24��mm) with smooth surface morphology. The fabricated GaN HEMTs exhibited gmmax ¼250mS/mm, fT=fmax ¼ 39=39GHz, BVgd ¼ 90V, and drain current collapse <10%. The device Johnson’s figure of merit (J-FOM = fT � BVgd) is in

the range between 3.51 to 3.83 THz�V which are comparable to those of other reported GaN HEMTs on Si with a conventional III–V gold-based

ohmic contact process. Our results demonstrate the feasibility of realizing high-performance submicron GaN-on-silicon HEMTs using a Si CMOS-

compatible metal stack. # 2013 The Japan Society of Applied Physics

GaN high-electron-mobility transistors (HEMTs)have achieved record-high cut-off frequencies( fT � 370GHz),1) and microwave power density

(Pout � 2:5W/mm)@40GHz,2) which exceeded the existingSi-based device technology limits. Most reported GaNdevices with impressive performance were fabricated usingconventional III–V gold-based metal schemes.1–8) Now,many industries including existing silicon semiconductormanufacturers are showing increased interests in developingGaN HEMT technology on large-diameter Si substrates.3,8,9)

In order to utilize the existing Si fabrication line, non-goldohmic contacts with low contact resistance Rc are essential.To date, most of the ohmic contacts with low Rc valuesmake use of conventional III–V gold-based metal schemes.Table I summarizes some of the best reported Rc values for‘‘gold’’ and ‘‘non-gold’’ based ohmic contacts for AlGaN/GaN HEMTs on Si. The lowest Rc of 0.49��mm so farachieved is with a non-gold ohmic contact annealed at 870�C.10) Until now, researchers have only reported >1:5-�m-gate-length AlGaN/GaN HEMTs using ‘‘non-gold’’ or ‘‘Au-free’’ ohmic contacts for low-cost high-power electro-nics.8–10) However, for high frequency applications, sub-micron gate-length HEMTs are necessary. Thus far, to thebest of our knowledge, no report exists on the DC and micro-wave characteristics of submicron-gate-length AlGaN/GaNHEMTs using CMOS-compatible non-gold metal stack.In this letter, we report for the first time the DC and micro-wave characteristics of submicron-gate-length AlGaN/GaNHEMTs using CMOS-compatible non-gold metal stack.

The GaN HEMT structure was grown by metal–organicchemical vapor deposition with a 2-nm-thick GaN cap layer,18-nm-thick Al0:26Ga0:74N barrier, 800-nm-thick GaN bufferand 1.4-�m-thick transition layer on 4-in. Si(111) (resistivity>6000��cm).6) The grown structure exhibited room tem-perature 2-dimensional electron gas (2-DEG) mobility of1450 cm2/V�s and sheet carrier density of 1:1� 1013 cm�2.This HEMT structure is chosen for this study to benchmarkwith our HEMTs using a conventional III–V gold-basedprocess.11) After the formation of mesa isolation using dryetching by BCl3/Cl2 plasma, Ta/Si/Ti/Al/Ni/Ta (8/2/15/140/30/25 nm) ohmic metal was deposited and annealedat 800 �C for 30 s in a N2 environment with a rapid ther-mal annealing (RTA) system. A typical Rc of 0.24��mm(standard deviation of 0.07��mm) is achieved out of 3

separate runs with an average specific contact resistivity (�c)of 1:25� 10�6 ��cm2. The achieved Rc is believed to be thelowest ever reported for non-gold ohmic contacts for con-ventional AlGaN/GaN HEMTs on Si and it is also lowerthan that of recessed ohmic contacts10) as shown in Table I.This ohmic metal stack also provides a smooth surfacemorphology and good edge definition [see Figs. 1(a) and1(b)]. This simple ohmic stack also avoids the need touse other complicated techniques such as an ohmic recess10)

or a regrown ohmic contact,1) which will complicate themanufacturing process. The ohmic contact details will bepublished elsewhere.

To form the T-gate, a first layer of 120-nm-thick SiN wasdeposited by plasma enhanced chemical vapor deposition(PECVD) after the ammonium sulfide [(NH4)2Sx] treatmentfor about 30min.11) The 0.15 �m footprint was opened andthe SiN was etched by CF4/O2 plasma. Then a 1.0 �m gatehead was subsequently formed with a CMOS-compatiblenon-gold metal stack of Ni/Al/Ta (100/400/30 nm). Finally,the non-gold metal-stack Ti/Al/Ta (50/800/30 nm) as aninterconnect metal was deposited followed by final passi-vation with 120-nm-thick PECVD-grown SiN. The metalschemes used for this study (i.e., ohmic contact, Schottkycontact, and interconnect metal lines) are commonly usedin the silicon fabrication process line and, hence, theywill not introduce any cross-contamination in the manu-facturing lines. The device dimensions used for this studyare Lsg=Lg=Lgd=Wg ¼ 0:8=0:15=1:7=ð2� 75Þ �m. The fab-ricated device DC and pulsed I–V characteristics of theHEMTs were measured using an Agilent B1500 semicon-ductor parameter analyzer and an Accent Diva D265,respectively. Microwave small-signal (from 6 to 40GHz)measurements were also carried out using an HP 8510Cvector network analyzer (VNA).

Figure 1(c) shows the typical IDS–VDS characteristics of0.15-�m-gate-length GaN HEMTs with a CMOS-compatiblenon-gold metal stack. The fabricated HEMTs exhibited amaximum drain current density (IDmax) of 830mA/mm, amaximum extrinsic transconductance (gmmax) of 250mS/mm,and a threshold voltage (Vth) of �3:75V. The Ni/Al/TaSchottky gate exhibited a barrier height of 0.88 eV and areverse gate-leakage current (�20V) of 3:8� 10�3 mA/mm.

Figure 2(a) shows the small-signal gain versus frequencyfor the GaN HEMT with CMOS compatible non-gold metal

Applied Physics Express 6 (2013) 016501

016501-1 # 2013 The Japan Society of Applied Physics

http://dx.doi.org/10.7567/APEX.6.016501

stack. The devices exhibited a unit current gain cut-off fre-quency fT of 39GHz and a maximum oscillation frequencyfmax of 39GHz. This is believed to be the first reportedmicrowave performance of submicron AlGaN/GaN HEMTsusing CMOS-compatible non-gold metal stack. Figure 2(b)shows the pulsed I–V characteristics (pulse width = 200 ns;pulse period = 1ms) of AlGaN/GaN HEMTs with a CMOS-compatible non-gold metal stack. Very little current collapse(<10%) was observed for the gate- and drain-quiescentbiases (Vgs0 ¼ �8V, Vds0 ¼ 10V). The three terminal OFF-state breakdown voltage (BVgd) of the AlGaN/GaN HEMTsis 90V, which is measured by the drain current injectionmethod (see Fig. 3).12) The J-FOM13,14) ð¼ fT � BVgdÞ ofHEMTs are in the range between 3.51 to 3.83 THz�V fordifferent gate–drain spacing devices (Lgd ¼ 1:7 to 3.5 �m).The obtained values are comparable to that of the 0.15-�mgate-length GaN HEMTs (3.3 THz�V) on Si(111) fabricatedusing a conventional III–V gold-based metal stack.15)

In summary, we have demonstrated for the first time asubmicron-gate-length AlGaN/GaN HEMT on a high-resis-tivity Si substrate with DC and microwave performances

using a CMOS-compatible non-gold metal stack. Afterannealing at 800 �C, the ohmic contact exhibited a smoothsurface morphology with a low Rc value of 0.24��mm. The

Fig. 1. Surface morphology of (a) conventional III–V gold-based ohmic

contact, (b) CMOS-compatible non-gold ohmic contact, and (c) IDS–VDS

characteristics of 0.15-�m-gate-length AlGaN/GaN HEMTs with CMOS-

compatible non-gold metal stack.

(a) (b)

Fig. 2. (a) Small-signal characteristics of AlGaN/GaN HEMTs with

CMOS-compatible non-gold metal stack. Bias points: Vd ¼ 6V,

Vg ¼ �2:7V. (b) Pulsed IDS–VDS characteristics of 0.15-�m-gate-length

AlGaN/GaN HEMTs with CMOS compatible non-gold metal stack.

Vg ¼ þ1 to �5V, step �1V.

Table I. Contact resistance values for different GaN HEMT structures on Si substrate using conventional III–V gold-based ohmic stack and CMOS-

compatible non-gold ohmic stack.

ResearchAnnealing

groupHEMT on Si Metal stack temperature

Rc (��mm)

(�C) Gold Non-gold

Nitronex4) AlGaN/GaN Ti/Al/Ni/Au 8250.45

(Rough surface)—

NTU5) AlGaN/GaN Ti/Al/Ni/Au 8250.18

(Rough surface)—

ETH-Z6) AlGaN/GaN Ti/Al/Ni/Au 8500.45

(Rough surface)—

IEMN7) AlGaN/GaN Ti/Al/Ni/Au 9000.50

(Rough surface)—

IMEC8) AlGaN/GaN/AlGaN Ti/Al/W 800 — 0.65

IMEC9) AlGaN/GaN/AlGaN Ti/Al/Ti/TiN550

(Recess)— 1.25

MIT10) AlGaN/GaN Ti/Al/W870

(Recess)—

0.49

(Smooth surface)

This work AlGaN/GaN Ta/Si/Ti/Al/Ni/Ta 800 —0.24

(Smooth surface)

Fig. 3. Three-terminal OFF-state breakdown voltage characteristics of

HEMT with CMOS-compatible non-gold metal stack. The device has

dimensions of Lsg ¼ 0:8, Lg ¼ 0:15, Lgd ¼ 1:7, and Wg ¼ ð2� 75Þ �m.

S. Arulkumaran et al.Appl. Phys. Express 6 (2013) 016501

016501-2 # 2013 The Japan Society of Applied Physics

fabricated 0.15-�m-gate-length GaN HEMTs exhibitedgmmax ¼ 250mS/mm, fT=fmax ¼ 39=39GHz, BVgd ¼ 90Vand drain current collapse <10%. The device Johnson’sfigure of merit (J-FOM = fT � BVgd) is in the rangebetween 3.51 to 3.83 THz�V which are comparable to thoseof the other reported GaN HEMTs on Si substrate with aconventional III–V gold-based metal stack (3.3 THz�V).These results demonstrate the feasibility of non-gold metal-lization process to achieve submicron gate AlGaN/GaNHEMTs on Si substrate for high-frequency applications.

Acknowledgments The authors would like to acknowledge the support

from SERC-A�STAR under the TSRP program grant No. 102-169-030. The

authors are thankful to the MTDC team, in particular to Bryan, Foo, and Anand

for their assistance.

1) Y. Yue, Z. Hu, J. Guo, B. S. Rodriguez, G. Li, R. Wang, F. Faria, T. Fang,

B. Song, X. Gao, S. Guo, T. Kosel, G. Snider, P. Fay, D. Jena, and H. Xing:

IEEE Electron Device Lett. 33 (2012) 988.

2) F. Medjdoub, M. Zegaoui, B. Grimbert, D. Ducatteau, N. Rolland, and

P. A. Rolland: IEEE Electron Device Lett. 33 (2012) 1168.

3) S. Arulkumaran, G. I. Ng, S. Vicknesh, H. Wang, K. S. Ang, J. P. Y. Tan,

V. K. Lin, S. Todd, G.-Q. Lo, and S. Tripathy: Jpn. J. Appl. Phys. 51

(2012) 111001.

4) J. W. Johnson, E. L. Piner, A. Vescan, R. Therrien, P. Rajagopal, J. C.

Roberts, J. D. Brown, S. Singhal, and K. J. Linthicum: IEEE Electron

Device Lett. 25 (2004) 459.

5) S. Arulkumaran, S. Vicknesh, G. I. Ng, S. L. Selvaraj, and T. Egawa: Appl.

Phys. Express 4 (2011) 084101.

6) H. F. Sun, A. R. Alt, H. Benedickter, and C. R. Bolognesi: Electron. Lett.

45 (2009) 376.

7) D. Ducatteau, A. Minko, V. Hoel, E. Morvan, E. Delos, B. Grimbert, H.

Lahreche, P. Bove, C. Gaquiere, J. C. De Jaeger, and S. Delage: IEEE

Electron Device Lett. 27 (2006) 7.

8) M. Van Hove, S. Boulay, S. R. Bahl, S. Stoffels, X. Kang, D. Wellekens,

K. Geens, A. Delabie, and S. Decoutere: IEEE Electron Device Lett. 33

(2012) 667.

9) B. De Jaeger, M. Van Hove, D. Wellekens, X. Kang, H. Liang, G.

Mannaert, K. Geens, and S. Decoutere: 24th Int. Symp. Power

Semiconductor Devices (ISPSD), 2012, p. 49.

10) H. S. Lee, D. S. Lee, and T. Palacios: IEEE Electron Device Lett. 32

(2011) 623.

11) S. Vicknesh, S. Arulkumaran, and G. I. Ng: Proc. IEEE MTT-S Int.

Microwave Symp. Dig., 2012, p. 1.

12) J. A. del Alamo and M. H. Somerville: IEEE J. Solid-State Circuits 34

(1999) 1204.

13) F. A. Marino, N. Faralli, D. K. Ferry, S. M. Goodnick, and M. Saraniti:

J. Phys.: Conf. Ser. 193 (2009) 012040.

14) M. J. Rosker, J. D. Albrecht, E. Cohen, J. Hodiak, and T. H. Chang: Proc.

IEEE MTT-S Int. Microwave Symp. Dig., 2010, p. 1214.

15) S. Yoshida, M. Tanomura, Y. Murase, K. Yamanoguchi, K. Ota, K.

Matsunaga, and H. Shimawaki: Proc. IEEE MTT-S Int. Microwave Symp.

Dig., 2009, p. 665.

S. Arulkumaran et al.Appl. Phys. Express 6 (2013) 016501

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