Diamond FET Using High-Quality Polycrystalline ... - IEEE Xplore

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IEEE ELECTRON DEVICE LETTERS, VOL. 27, NO. 7, JULY 2006

Diamond FET Using High-Quality Polycrystalline Diamond With fT of 45 GHz and fmax of 120 GHz K. Ueda, M. Kasu, Y. Yamauchi, T. Makimoto, M. Schwitters, D. J. Twitchen, G. A. Scarsbrook, and S. E. Coe

Abstract—Using high-quality polycrystalline chemical-vapordeposited diamond films with large grains (∼ 100 µm), field effect transistors (FETs) with gate lengths of 0.1 µm were fabricated. From the RF characteristics, the maximum transition frequency fT and the maximum frequency of oscillation fmax were ∼ 45 and ∼ 120 GHz, respectively. The fT and fmax values are much higher than the highest values for singlecrystalline diamond FETs. The dc characteristics of the FET showed a drain–current density IDS of 550 mA/mm at gate–source voltage VGS of −3.5 V and a maximum transconductance gm of 143 mS/mm at drain voltage VDS of −8 V. These results indicate that the high-quality polycrystalline diamond film, whose maximum size is 4 in at present, is a most promising substrate for diamond electronic devices. Index Terms—Field effect transistor (FET), hydrogen terminated, polycrystalline diamond, RF performance.

I. I NTRODUCTION

D

IAMOND is expected to be the most suitable material for high-power high-frequency electronic devices because of its high electric breakdown field (> 10 MV/cm), high carrier mobility (4500 cm2 /V · s for electrons, 3800 cm2 /V · s for holes [1]), high thermal conductivity (22 W/cm · K), and high saturation velocity (1.5 × 107 cm/s for electrons, 1.05 × 107 cm/s for holes [2]). Recently, using a single-crystal chemical-vapor-deposited (CVD) diamond, we achieved a maximum output power density of 2.1 W/mm at 1 GHz, which is the highest reported among diamond FETs [3]. This output power density is high enough for power amplifiers of base stations in wireless communication systems. However, the size of a single-crystal CVD diamond is limited to 4 mm, which is the size of a commercially available high-pressure-and-high-temperature (HPHT)-synthesized diamond substrate. From the viewpoint of semiconductor device processing, at least 4-in wafers are needed. One of the possible solutions is to use polycrystalline or highly oriented diamond films grown on large-area foreign substrates [4]–[6]. However, at present, the transition frequency fT and the maximum frequency of oscillation fmax were reported to be 2.7 and 3.8 GHz, respectively, for polycrystalline diamond FETs [4], and 9.6 and 17.3 GHz, respectively, for highly oriented diamond FETs Manuscript received February 28, 2006; revised April 14, 2006. This work was supported in part by the Ministry of Internal Affairs and Communications, Japan, under the SCOPE project. The review of this letter was arranged by Editor J. Sin. K. Ueda, M. Kasu, Y. Yamauchi, and T. Makimoto are with NTT Basic Research Laboratories, NTT Corporation, Atsugi 243-0198, Japan. M. Schwitters, D. J. Twitchen, G. A. Scarsbrook, and S. E. Coe are with Element Six Ltd., SL5 8BP Berkshire, U.K. Digital Object Identifier 10.1109/LED.2006.876325

Fig. 1.

Schematic cross section of a polycrystalline diamond FET.

[6]. These values are much lower than our reported values for a single-crystal CVD diamond FET (fT = 25 GHz, fmax = 81 GHz [7]) because the crystal quality of these films is inferior to that of single-crystal CVD films. It is believed that grain boundaries in these films degrade the mobility and/or reduce the carrier concentration. We have successfully fabricated high-quality polycrystalline diamond and reported that, in a diamond radiation sensor, irradiation with 5.48-MeV alpha particles resulted in a wellresolved particle peak with a high charge collection efficiency of 15% [8], [9]. Here, using our high-quality polycrystalline diamond film, we report significant progress in fabricating FETs. The grain size of the polycrystalline film is ∼ 100 µm, which is comparable to our FET size. Thus, the effect of the grain boundary seems to be very small. Improvements in electron beam (EB) lithography and self-alignment technology enabled us to form 0.1-µm-long-gate FETs and reduce the source–gate resistance. As a result, we achieved high dc and radio frequency (RF) performance for polycrystalline diamond FETs. II. E XPERIMENTAL The polycrystalline diamond freestanding film used for the FETs was grown by the CVD method by Element Six [8], [9]. We used a polycrystalline diamond whose size is 10 × 10 × 0.5 mm instead of a 4-in one. This is because it is very difficult to cut a 4-in diamond wafer in pieces, and 10-mm samples are easy to handle. By controlling the growth environment, we have managed to minimize the incorporation of impurities such as nitrogen and boron to levels below ten parts per billion. From X-ray diffraction, we confirmed that the film had a predominately (110) orientation, and from scanning electron microscopy (SEM), we confirmed that the typical grain size was ∼ 100 µm. The FETs were fabricated and measured by an NTT group in the same way as in [3] and [7] (Fig. 1). The diamond surface was passivated with hydrogen (i.e., H-passivation) in a microwave plasma CVD system (1.3 kW, 2.45 GHz) to form a quasi two-dimensional hole channel, which

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UEDA et al.: DIAMOND FET USING HIGH-QUALITY POLYCRYSTALLINE DIAMOND

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Fig. 3. RF gain plot of the 0.1-µm-gate polycrystalline FET with WG = 100 µm at VGS = 0 V and VDS = −10 V. The maximum fT and fmax are also shown.

Fig. 2. (a) DC drain–current (IDS )–voltage (VDS ) characteristics for various gate biases (VGS = −3.5−2 V, ∆VGS = 0.5 V) for a polycrystalline diamond FET (LG = 0.1 µm, WG = 50 µm). (b) Gate bias dependence of transconductance gm and dc drain–current IDS of the polycrystalline FET measured at VDS of −8 V (LG = 0.1 µm, WG = 25 µm).

is ∼ 10 nm below the surface [7], [13]. Surface passivation (i.e., termination) with hydrogen denotes termination of the dangling bonds on the surface of the diamond with hydrogen atoms. Oxygen termination (i.e., O-termination) of the surface was used for device isolation. The source and drain Au ohmic contacts were formed on the H-terminated surface. An ohmic contact with a low contact resistance of ∼ 10−5 Ω · cm2 was obtained using Au without annealing [14]. EB lithography and self-alignment techniques enabled us to form 0.1-µm-long Al Schottky gate contacts. The device structure is a standard coplanar multifinger structure and uses neither a field plate nor a recess structure. The typical gate–drain and gate–source gap of the FETs is ∼ 0.5 µm. III. R ESULTS AND D ISCUSSION Fig. 2(a) shows the dc drain–current (IDS )–voltage (VDS ) characteristics for different gate–source voltages VGS for a

device with gate length LG of 0.1 µm and gate width WG of 50 µm. The dc characteristics show IDS of 550 mA/mm at VGS = −3.5 V. IDS is approximately 50% higher than our value obtained for single-crystal CVD diamond FETs (350 mA/mm for LG = 0.1 µm) [3]. This is because the source resistance of the FET is reduced by decreasing the source–gate gap from 1.4 to 0.5 µm. The drain bulk leakage and the gate current, which is often observed in FETs using a single-crystal CVD diamond [3], [10], were not observed in the polycrystalline FETs. There is a slight increase of drain–current due to the short-channel effect because a good pinch-off was observed in the case of 1-µm-gate FETs. Fig. 2(b) shows the gate bias VGS dependence of the dc extrinsic transconductance gm and IDS at VDS of −8 V for the 0.1-µm-gate FET. gm stays high (> 130 mS/mm) in a relatively wide VGS range from −0.5 to −2.0 V, and the maximum gm is 143 mS/mm. This value is comparable to the highest gm reported for a single-crystal CVD diamond FET (150–165 mS/mm) [11], [12]. The small-signal parameters for a device with LG = 0.1 µm and WG = 100 µm were measured in a frequency range from 1 to 20 GHz. fT and fmax were extracted from the frequency dependence of the short circuit current gain (|h21|2 ) and the unilateral power gain (U ) as shown in Fig. 3. The maximum fT and fmax are 45 and 88 GHz at VGS = 0 V and VDS = −10 V, respectively. These gain values were obtained after subtracting the parasitic components obtained from the open and short structures on the same sample. The fT value is much higher than that of single-crystalline diamond FETs, which showed a value for fT of 25 GHz [7]. This is because the LG of the FETs was reduced from 0.2 to 0.1 µm by the improvements in EB lithography and self-alignment technology, and the FET was fabricated using the high-quality polycrystalline diamond with high gm . Fig. 4 shows the RF gain plot of a polycrystalline FET with LG = 0.1 µm and WG = 50 µm at VGS = 0.5 V and VDS = −18 V. The maximum fT and fmax are 38 and 120 GHz, respectively. The fmax value is also the highest among the diamond FETs including single-crystal diamond FETs. The high fmax /fT ratio (= ∼ 3.2) results from the low channel resistance

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IEEE ELECTRON DEVICE LETTERS, VOL. 27, NO. 7, JULY 2006

ACKNOWLEDGMENT The authors would like to thank Dr. T. Tamamura for the electron beam lithography, Dr. T. Enoki (NTT Photonics Laboratories) for the discussions on RF devices, Dr. K. Torimitsu and Dr. H. Takayanagi (NTT Basic Research Laboratories) for the encouragement and support, and Prof. E. Kohn (University of Ulm, Germany) for the continuous support and advice throughout the NTT’s diamond research. R EFERENCES

Fig. 4. RF gain plot of a polycrystalline FET with extracted transition frequency fT and maximum frequency of oscillation fmax (LG = 0.1 µm, WG = 50 µm) at VGS = 0.5 V and VDS = −18 V.

and the low drain conductance in the polycrystalline FET. The transition frequency fT for WG = 100 µm was 45 GHz, and that for WG = 50 µm was slightly low (38 GHz). One possible reason why the gate resistance of the FET is high is because the gate is not T shaped. We will investigate this further. We performed dc and small-signal RF measurements on 25 FET devices. There is a small difference in the electronic properties among them. Therefore, we think that the effect of the grain boundary is small for these measurements. The possible reason for these superior characteristics of polycrystalline diamond FETs is that the grain size (∼ 100 µm) is comparable to the FET size. The crystal quality inside the grains of the high-quality polycrystalline diamond may be comparable to that of a single-crystal CVD diamond. IV. C ONCLUSION The high RF and dc performance of diamond FETs using high-quality polycrystalline diamond films with large grains (∼ 100 µm) has been demonstrated. The maximum fT of ∼ 45 GHz and the maximum fmax of ∼ 120 GHz were obtained in polycrystalline FETs with LG of 0.1 µm. The fT and fmax values are the highest among diamond FETs including those that are based upon a single-crystal CVD diamond. The dc characteristics of the FET show a value for IDS of 550 mA/mm at VGS = −3.5 V and a maximum gm of 143 mS/mm at VDS = −8 V. These results show that the high-quality polycrystalline freestanding diamond, whose maximum size is 4-in in at present, is a most promising substrate material for active diamond device.

[1] J. Isberg, J. Hammersberg, E. Johansson, T. Wikstrom, D. J. Twitchen, A. J. Whitehead, S. E. Coe, and A. Scarsbrook, “High carrier mobility in single-crystal plasma-deposited diamond,” Science, vol. 297, no. 5587, pp. 1670–1672, Sep. 2002. [2] L. Reggiani, S. Bosi, C. Canai, and F. Nava, “Hole-drift velocity in natural diamond,” Phys. Rev. B, Condens. Matter, vol. 23, no. 6, pp. 3050–3057, Mar. 1981. [3] M. Kasu, K. Ueda, H. Ye, Y. Yamauchi, S. Sasaki, and T. Makimoto, “2 W/mm output power density at 1 GHz for diamond FETs,” IEE Electron. Lett., vol. 41, no. 22, pp. 1249–1250, Oct. 2005. [4] H. Umezawa, T. Arima, N. Fujihara, H. Taniuchi, H. Ishizaka, M. Tachiki, C. Wild, P. Koidl, and H. Kawarada, “RF performance of high transconductance and high-channel-mobility surface channel polycrystalline diamond metal–insulator–semiconductor field-effect-transistors,” Jpn. J. Appl. Phys., vol. 41, no. 4B, pp. 2611–2614, Apr. 2002. [5] M. Kubovic, K. Janischowsky, and E. Kohn, “Surface channel MESFETs on nanocrystalline diamond,” Diamond Relat. Mater., vol. 14, no. 3–7, pp. 514–517, Mar.–Jul. 2005. [6] M. Kubovic, A. Aleksov, M. Schreck, T. Bauer, B. Stritzker, and E. Kohn, “Field effect transistor fabrication on hydrogen-terminated diamond grown on SrTiO3 substrate and iridium buffer layer,” Diamond Relat. Mater., vol. 12, no. 3–7, pp. 403–407, Mar.–Jul. 2003. [7] M. Kubovic, M. Kasu, I. Kallfass, M. Neuburger, A. Aleksov, G. Koley, M. G. Spencer, and E. Kohn, “Microwave performance evaluation of diamond surface channel FETs,” Diamond Relat. Mater., vol. 13, no. 4–8, pp. 802–807, Apr.–Aug. 2004. [8] P. J. Sellin, M. B. H. Breese, A. P. Knights, L. C. Alves, R. S. Sussmann, and A. J. Whitehead, “Imaging of charge transport in polycrystalline diamond using ion-beam-induced charge microscopy,” Appl. Phys. Lett., vol. 77, no. 6, pp. 913–915, Aug. 2000. [9] R. S. Sussmann, J. R. Brandon, G. A. Scarsbrook, C. G. Sweeney, T. J. Valentine, A. J. Whitehead, and C. J. H. Wort, “Properties of bulk polycrystalline CVD diamond,” Diamond Relat. Mater., vol. 3, no. 4–6, pp. 303–312, Apr. 1994. [10] M. Kasu, M. Kubovic, A. Aleksov, N. Teofilov, Y. Taniyasu, R. Sauer, E. Kohn, T. Makimoto, and N. Kobayashi, “Influence of epitaxy on the surface conduction of diamond film,” Diamond Relat. Mater., vol. 13, no. 2, pp. 226–232, Feb. 2004. [11] H. Matsudaira, S. Miyamoto, H. Ishizaka, H. Umezawa, and H. Kawarada, “Over 20-GHz cutoff frequency submicrometer-gate diamond MISFETs,” IEEE Electron Device Lett., vol. 25, no. 7, pp. 480–482, Jul. 2004. [12] A. Aleksov, A. Denisenko, U. Spitzberg, T. Jenkins, W. Ebert, and E. Kohn, “RF performance of surface channel diamond FETs with sub-micron gate length,” Diamond Relat. Mater., vol. 11, no. 3–6, pp. 382–386, Mar.–Jun. 2002. [13] A. Kubovic, A. Denisenko, W. Ebert, M. Kasu, I. Kallfass, and E. Kohn, “Electronic surface barrier characteristics of H-terminated and surface conductive diamond,” Diamond Relat. Mater., vol. 13, no. 4–8, pp. 755–760, Apr.–Aug. 2004. [14] H. Ye, M. Kasu, K. Ueda, Y. Yamauchi, N. Maeda, S. Sasaki, and T. Makimoto, “RF performance of diamond MESFET at elevated temperatures and its equivalent circuit,” Jpn. J. Appl. Phys., in press.