Fabrication of diamond MISFET with micron-sized gate length on boron-doped (111) surface

Diamond & Related Materials 14 (2005) 2043 – 2046 www.elsevier.com/locate/diamond

Fabrication of diamond MISFET with micron-sized gate length on boron-doped (111) surface Takeyasu Saito a,*, Kyung-ho Park a, Kazuyuki Hirama b, Hitoshi Umezawa b, Mitsuya Satoh b, Hiroshi Kawarada b, Hideyo Okushi a a

Diamond Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan b Department of Electrical Engineering and Bioscience, Waseda University, Shinjyuku-ku, Tokyo 169-8555, Japan Available online 17 August 2005

Abstract A hydrogenated surface conductive layer of B-doped diamond on (111) was employed to fabricate a metal insulator semiconductor field effect transistor (MISFET) using a CaF2 and Cu stacked gate. The carrier mobility and concentration of the hydrogenated surface on (111) before FET processing were 35 cm2/V s and 1013/cm2, respectively, when bulk carrier concentration and film thickness of the B-doped underlaying diamond was 3
1015/cm3 and 1.5 Am, respectively. The DC characteristics of the gate with 1.1 Am length and 50 Am width showed that the maximum measured drain current was 240 mA/mm at  3.0 V gate voltage, and the maximum transconductance ( g m) was 70 mS/mm. The cut-off frequency of 4 GHz was obtained, which is one of the best values for the RF characteristics of a diamond homoepitaxial (111) MISFET. D 2005 Elsevier B.V. All rights reserved. Keywords: Surface conductive layer; Field effect transistor; Transconductance; Cut-off frequency

1. Introduction Diamond semiconductor devices are expected to be of importance for high-power, high-frequency and harsh environmental (radiation, high temperature, toxic chemicals) applications based on its excellent properties, such as wide-band-gap energy (5.5 eV), high breakdown electric field (10 MV/cm) and the maximum thermal conductivity (20 W/cm K) and low dielectric constant (5.7) [1]. By introducing boron and phosphorous into the gas phase during chemical vapor deposition (CVD), p-type and n-type diamonds have been developed, respectively [2,3]. Boron-doped diamond field effect transistors (FETs) were reported in the 1980s [4 – 6]; however, the fabrication of low-resistivity semiconductive diamond thin film is still difficult, particularly n-type ones, due to the deep donor level being as deep as 0.6 eV. Consequently, diamond device technology is limited to those with the p-type H-terminated surface conductive layer which shows a high surface carrier density. This H-terminated diamond surface exhibits unique

* Corresponding author. Tel.: +81 29 861 5202; fax: +81 29 861 2774. E-mail address: [email protected] (T. Saito). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.08.044

characteristics such as high carrier concentration of more than 1013/cm2, shallow carrier profiles within almost 10 nm from the surface and stability in ambient atmosphere [7 –10]. Investigations to elucidate the origin of such properties are still in progress, and further comprehension of the hydrogen-terminated surface structure is necessary. However, most of such activities have been carried out for the (100)-oriented surface for two reasons: availability of the substrate and its smooth surface morphology. For FETs with H-terminated surface conductive layers, device performance has improved rapidly since they were first reported by Kawarada et al. [11]. Metal semiconductor FETs (MESFETs) and metal insulator semiconductor FETs (MISFETs) on H-terminated surfaces on (100) substrates attained cut-off frequencies over 20 GHz with the gate length of 200 nm [12,13]. Again, most of the studies were dedicated to (100) single-crystal diamond. However, in a recent report on the electrical properties of a hydrogenated and oxidized (111) surface [14], it was clearly presented that the oxidized (111) surface is still conductive. This suggests that the (111) surface has potential and practical advantage for use in harsh environment without dehydrogenation-induced resisitivity increase, compared with the hydrogenated (100) surface.

2044

T. Saito et al. / Diamond & Related Materials 14 (2005) 2043 – 2046 200

Table 1 Properties of hydrogenated diamond (111) surface and bulk properties after wet oxidation

Wet oxidized surface

Sheet resistivity 8.1 – 12.0 kV/sq Conductivity 40 – 74 V cm

Mobility 31.2 – 43.7 cm2/V s Mobility 40 – 120 cm2/V s

150

Id (mA/mm)

Hydrogen surface

Surface carrier concentration 1.7 – 2.4E+13/cm2 Carrier concentration 0.71 – 3.8E+15/cm3

-2.05V

50

-1.05V

-0.05V

0

1

evaporated on the H-terminated region was employed as the ohmic contact for the source and the drain. Inductively coupled plasma oxidation was used for the elimination the Hterminated layer to establish isolation. Self-aligned gate formation was carried out to form 1.1– 2.1-Am-long gates by employing CaF2 (50 nm) and Cu (50 nm) stacked GOI structure deposition and subsequent lift-off processes, where CaF2 and Cu were prepared by resistively heated evaporation at 100 W. A schematic diagram of the entire process was depicted in our previous works [16,17]. DC and RF characteristics were examined using a parameter analyzer (Agilent 4156C) and network analyzer (Agilent 8720ES), respectively. 3. Results and discussion Table 1 shows that the carrier mobility and concentration of the hydrogenated surface conductive layer on (111) before FET processing in this study and those of a wet oxidized surface. Surface carrier mobility and concentration were approximately 35 cm2/V s and 2
1013/cm2, respectively, when CH 4 concentration was 0.1%. The surface mobility was still onequarter of that obtained on (100) samples. The deposition conditions in the work by Kasu et al. were 1% CH4 concentration with 1.3 kW microwave power and substrate temperature in the range from 650 to 685 -C [15]. The mobility of our film is slightly lower than that of the (111) samples of 150

300

-Id (mA/mm)

-2.5V -2.0V

150

-1.5V

100

Vg=-3.0V

(b)

Vg=-3.0V

200

-2.5V

100 -2.0V -1.5V

50

-1.0V

50

-1.0V -0.5V

-0.5V

0

0

1~0V

0

2

3

Fig. 2. I d – V g characteristics of gate with 1.1 Am length and 50 Am width measured in ambient atmosphere. The gate and drain voltages were changed every 0.05 V from +1 to 3 V and every 1.0 V from 0.05 to 8.05 V, respectively.

Homoepitaxial diamond growth was carried out in a quartzwall reactor by ASTEX 1.5 kW using high-temperature and high-pressure synthesized Ib-type diamond (111) which was cleaned in acid solutions in advance. The source gas was CH4 diluted in H2 (0.1%). Microwave power and total pressure were 1200 W and 50 Torr, respectively. After growth was completed, the diamond sample was oxidized in HNO3/H2SO4 solution and then van-der-Pauw-configured Ti(30 nm)/Pt(30 nm)/ Au(100 nm) electrodes were formed by electron beam evaporation. AC Hall measurement was performed using the TOYO Corporation Resi Test 8300 system in ambient atmosphere. Electrodes were removed with acid solution, and subsequently, plasma hydrogenation for 30 min under the same conditions as for the deposition was carried out. DC Hall measurement was also performed using the Accent HL5500 system in ambient atmosphere before MISFET fabrication. Au

250

2

-Vg (V)

2. Experimental

-Id (mA/mm)

-3.05V

100

0 -1

However, only a very limited number of surface properties of H-terminated devices on homoepitaxially grown (111) diamond crystals have been examined so far. Kasu et al. reported that surface carrier mobility and concentration were 74 cm2/V s and 1.7
1013/cm2, respectively, and also fabricated an Al gate MESFET with 11-Am gate length [15]. However, MESFET has disadvantages such as instability at high-power operation as well as high gate leakage current at high forward bias. Further studies with more precise gate dimensions are necessary. In the current study, the p-type surface conductive layer of homoepitaxially grown B-doped diamond on (111) was employed to fabricate MISFET with a micrometer-sized gate length, and the device properties were evaluated.

(a)

-7.05V -6.05V -5.05V -4.05V

Vd =-8.05V

4

-Vd (V)

6

8

1~0V

0

2

4

6

8

-Vd (V)

Fig. 1. I d – V d characteristics of gates with (a) 1.1 Am length and 50 Am width and (b) 2.1 Am length and 50 Am width. Both are measured in ambient atmosphere. The gate and drain voltages were changed every 0.5 V from +1 to 3 V and every 1.0 V from 0 to 8 V, respectively.

T. Saito et al. / Diamond & Related Materials 14 (2005) 2043 – 2046 200

ft(this study) fmax(this study) ft(MESFET) fmax(MESFET) ft(MISFET) fmax(MISFET) ft(poly-MISFET)

60

ft/fmax (GHz)

gm (mS/mm)

70

MESFET(Cu) MESFET(Al) MISFET(100) MISFET(poly) MISFET(111), this study

150

2045

100

50

50 40 30 20 10

0

0 0

1

2

3

4

0

1

Fig. 3. g m values as a function of gate length (L g) for Cu gate MESFET (0), Al gate MESFET (?), Cu/CaF2 gate MISFET on single-crystalline (100) diamond (‚), Cu/CaF2 gate MISFET on polycrystalline diamond (g) and Cu/CaF2 gate MISFET on single-crystalline (111) diamond (>, this study).

Kasu et al.; however, they also mentioned that the maximum mobility in their films was 74 cm2/V s. Generally, hole mobility (l), transconductance ( g m), and f t have the following relationship [18]: gm ¼

ft ¼

2

W lCG ðVG  VT Þ L

ð1Þ

gm ; 2pCGS

ð2Þ

where C GS, W, L and C G are the gate-source capacitance, the gate width, the gate length and the CaF2 capacitance per unit gate area, respectively. Therefore, increase of the mobility is essential to achieve higher g m or f t. However, the carrier mobility on the hydrogenated (111) surface acquired in this study did not show clear dependence on CH4/H2 concentration in the range from 0.1% to 1.5% in air. Conductivity and mobility, measured in ambient atmosphere, after wet oxidation with HNO3/H2SO4 solution were 60 V cm and about 70 cm2/V s when carrier concentration was 3
1015/cm3. The conductivity of our film was higher than those reported for B-doped (100) diamond with B2H6 by Kiyota et al. [19]. Mobility was

more than one magnitude lower than those conventionally obtained from B-doped (100) diamond. Fig. 1(a) and (b) shows the I d –V d characteristics of gate with 1.1 Am length and 50 Am width, and 2.1 Am length and 50 Am width, respectively, measured in ambient atmosphere. The gate and drain voltage changed from + 1 to  3 V and from 0 to  8 V, respectively. The CaF2 gate with 1.1 Am length and 50 Am width used here has the gate leakage current of the order of 10 1 A/mm2 when V d =  0.05 V and V g = T 1 V, which is three orders higher than in the case of a pulsed-laser ablation Al2O3-gate diamond MISFET [20]. Fig. 1 shows clear pinch-off characteristics; however, a local maximum current at the end of the linear region and a gradual current decrease are observed when gate voltage is more than  2.0 V. The reason for this trend is not clear at this moment. The maximum normalized drain current is 240 mA/mm at  3.0 V gate voltage. Fig. 2 shows the I d – V g characteristics of the gate with 1.1 Am length and 50 Am width measured in ambient atmosphere. The maximum transconductance ( g m) can be determined from Fig. 2 to be 67 mS/mm. Fig. 3 shows the 20

(a)

(b)

15

15

MSG/MAG

10

5

Gain (dB)

Gain (dB)

4

Fig. 5. f t and f max values as functions of gate length (L g). f t of Cu/CaF2 gate MISFET on polycrystalline (111) diamond (
), f t of Cu/CaF2 gate MISFET on single-crystalline (100) diamond (q), f max of Cu/CaF2 gate MISFET on singlecrystalline (100) diamond (r), f t of Al gate MESFET (g), f max of Al gate MESFET (n), f t of Cu/CaF2 gate MISFET on single-crystalline (111) diamond (>, this study) and f max of Cu/CaF2 gate MISFET on single-crystalline (111) diamond (?, this study).

20

10

MSG/MAG

5

|H21|2

2

|H21|

0 1.E+08

3

Lg (um)

Lg (um)

1.E+09

1.E+10

Frequency (Hz)

1.E+11

0 1.E+08

1.E+09

1.E+10

1.E+11

Frequency (Hz)

Fig. 4. (a) Current gain (|H21|2), maximum stable gain (MSG), and maximum available gain (MAG) as functions of frequency for CaF2 MISFET with 1.1 Am gate length and 100 Am gate width at V g = 2.0 V and V d = 8.05 V. (b) Current gain (|H21|2), and maximum stable gain (MSG), maximum available gain (MAG) as functions of frequency for CaF2 MISFET with 2.1 Am gate length and 100 Am gate width at V g = 2.0 V and V d = 8.05 V.

2046

T. Saito et al. / Diamond & Related Materials 14 (2005) 2043 – 2046

maximum transconductance ( g m) as a function of gate length. The average maximum transconductance ( g m) of several devices on diamond chips with 1.1 Am gate length and 2.1 Am gate length were estimated to be 56 mS/mm and 39 mS/mm, respectively. The g m values from a Cu-gate MESFET on single-crystalline diamond [21], from an Al gate MESFET on single-crystalline diamond [12,22,23], from a Cu/CaF 2 gate MISFET on single-crystalline diamond [13,16,17], and from a Cu/CaF2 gate MISFET on polycrystalline diamond [24] are also plotted in the same figure. The diamond (111) MISFET with a CaF2 gate exhibited comparable I d and g m characteristics to those of a CaF2 gate MISFET on diamond (100), suggesting that there is no major difference in the devices with different H-terminated surfaces at this moment. Fig. 4 shows that the small-signal RF gain of the diamond (111) MISFETs with 1.1 Am and 2.1 Am gate lengths resulted in S-parameter measurements from 50 MHz to 20 GHz. The cut-off frequency ( f t) can be calculated from the point where current gain (h 21) crosses the 0-dB gain axis. The maximum oscillation frequency ( f max) can also be calculated from the point where maximum stable gain and maximum available gain (MSG/MAG) cross the 0-dB gain axis. The f t and f max are 4.3 GHz and 7.0 GHz for the 1.1-Am MISFETs, and 2.1 GHz and 2.9 GHz for the 2.1-Am MISFETs, respectively. These are the first results for the RF performances from (111) diamond MISFETs, and they indicate the capability of high-frequency function. Fig. 5 shows the average f t and f max as a function of gate length under various conditions. The cut-off frequency of 3.6 GHz and f max of 10 GHz were obtained from the (111) devices at the 1.1-Am gate length. The f t and f max values from the Al gate MESFET on single-crystalline diamond [12,21], the Cu/ CaF 2 gate MISFET on single-crystalline diamond [13,15,17,19] and the Cu/CaF2 gate MISFET on polycrystalline diamond [24] were also plotted in the same figure. The diamond (111) MISFET with the CaF2 gate also exhibited RF performance comparable to that of the diamond (100) MISFET. 4. Conclusions In summary, sheet resistivity, surface mobility and carrier concentration of H-terminated surfaces on B-doped diamond (111) in air were 10 kV/sq, 35 cm2/V s and 2
1013/cm2, respectively, when bulk carrier concentration was 3
1015/cm3. The MISFET with the H-terminated diamond surface and a micrometer-sized gate length on B-doped (111) was successfully fabricated using CaF2 gate insulators. The maximum drain current and transconductance of 240 mA/mm and 70 mS/ mm, respectively, were obtained with a 1.1-Am gate length. The cut-off frequency of 4 GHz and f max of 10 GHz were obtained, which indicate comparable characteristics, I d, g m and RF performance, to those of the diamond (100) MISFET, suggesting that there is no major difference between devices with different H-terminated surfaces at this moment.

Acknowledgements This work was carried out under the Advanced Diamond Devices Project, The New Energy and Industrial Technology Development Organization (NEDO), Japan. A part of this work was supported by the AIST-Nano-Processing Facility (AISTNPF), Nanotechnology Research Institute (NRI) of the National Institute of Advanced Industrial Science and Technology (AIST), a member of the Nano-foundry Group, conducts the Nanoprocessing Partnership Program (NPPP), as a part of the Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). References [1] J.E. Field, Properties of Diamond, Academic Press, London, 1979. [2] S. Koizumi, M. Kamo, Y. Sato, H. Ozaki, T. Inuzuka, Appl. Phys. Lett. 71 (1997) 1065. [3] N. Fujimori, H. Nakahata, T. Imai, Jpn. J. Appl. Phys. 29 (1990) 824. [4] J.F. Prins, Appl. Phys. Lett. 41 (1982) 950. [5] M.W. Geis, D.D. Rathman, D.J. Ehrlich, R.A. Murphy, W.T. Lindley, IEEE Electron Device Lett. EDL-8 (1987) 341. [6] H. Shiomi, Y. Nishibayashi, N. Fujimori, Jpn. J. Appl. Phys. 28 (1989) L2153. [7] S.A. Grot, G.S. Gildenblat, C.W. Hatfield, C.R. Wranski, A.R. Badzian, T. Badzian, R. Messier, IEEE Electron Device Lett. EDL-11 (1990) 100. [8] T. Maki, S. Shikama, M. Komori, Y. Sakaguchi, K. Sakuta, T. Kobayashi, Jpn. J. Appl. Phys. 31 (1992) L1446. [9] K. Hayashi, S. Yamanaka, H. Okushi, K. Kajimura, Appl. Phys. Lett. 68 (1996) 376. [10] K. Tsugawa, K. Kitatani, H. Noda, A. Hokazono, K. Hirose, M. Tajima, H. Kawarada, Diamond Relat. Mater. 8 (1999) 927. [11] H. Kawarada, M. Aoki, M. Ito, Appl. Phys. Lett. 65 (1994) 1563. [12] M. Kubovic, M. Kasu, I. Kallfass, M. Neuburger, A. Alekov, G. Koley, M.G. Spencer, E. Kohn, Diamond Relat. Mater. 13 (2004) 802. [13] H. Matsudaira, S. Miyamoto, H. Ishizaka, H. Umezawa, H. Kawarada, IEEE Electron Device Lett. EDL-25 (2004) 480. [14] S.G. Ri, C.E. Nebel, D. Takeuchi, B. Rezek, H. Kato, M. Ogura, T. Makino, S. Yamasaki, H. Okushi, Appl. Phys. Lett. (submitted for publication). [15] M. Kasu, M. Kubovic, A. Alekov, N. Teofilov, R. Sauer, E. Kohn, M. Makimoto, Jpn. J. Appl. Phys. 43 (2004) L975. [16] H. Umezawa, H. Taniuchi, T. Arima, M. Tachiki, K. Tsugawa, S. Yamanaka, D. Takeuchi, H. Okushi, H. Kawarada, Jpn. J. Appl. Phys. 39 (2000) L908. [17] H. Matsudaira, T. Arima, H. Umezawa, S. Miyamoto, H. Ishizaka, M. Tachiki, H. Kawarada, Diamond Relat. Mater. 12 (2003) 1814. [18] S. Miyamoto, H. Matsudaira, H. Ishizaka, K. Nakazawa, H. Taniuchi, H. Umezawa, M. Tachiki, H. Kawarada, Diamond Relat. Mater. 12 (2003) 399. [19] H. Kiyota, E. Matsushima, K. Sato, H. Okushi, T. Ando, J. Tanaka, M. Kamo, Y. Sato, Diamond Relat. Mater. 6 (1997) 1753. [20] K. Hirama, S. Miyamoto, H. Matsudaira, H. Umezawa, T. Chikyo, A. Hasegawa, H. Koinuma, H. Kawarada, Jpn. J. Appl. Phys. (submitted for publication). [21] H. Umezawa, H. Taniuchi, T. Arima, M. Tachiki, H. Kawarada, Diamond Relat. Mater. 10 (2001) 1743. [22] A. Alekov, A. Denisenko, U. Spitzberg, T. Jenkins, W. Ebert, E. Kohn, Diamond Relat. Mater. 11 (2002) 382. [23] P. Gluche, A. Alekov, A. Vescan, W. Ebert, E. Kohn, IEEE Electron Device Lett. EDL-18 (1997) 547. [24] H. Umezawa, T. Arima, N. Fujiwara, H. Taniuchi, H. Ishizaka, M. Tachiki, C. Wild, P. Koidl, H. Kawarada, Jpn. J. Appl. Phys. 41 (2002) 2611.