Low on-resistance diamond field effect transistor with high-k ZrO2 as dielectric.

Although several high-k insulators have been deposited on the diamond for metal-insulator-semiconductor field effect transistors (MISFETs) fabrication, the k values and current output are still not fully satisfactory. Here, we present a high-k ZrO2 layer on the diamond for the MISFETs. The k value for ZrO2 is determined by capacitance-voltage characteristic to be 15.4. The leakage current density is smaller than 4.8 × 10(-5) A · cm(-2) for the gate voltage ranging from -4.0 to 2.0 V. The low on-resistance MISFET is obtained by eliminating source/drain-channel interspaces, which shows a large current output and a high extrinsic transconductance. The high-performance diamond MISFET fabrication will push forward the development of power devices.

Recently, wide band gap semiconductors such as SiC, GaN, and diamond have been developed in order to replace silicon partly for power devices due to their high carrier mobility and high breakdown field123. In particular, the diamond is considered to be an ideal material for the application of power devices due to its theoretical low power-loss at a high voltage4. However, there is a great issue for the development of the diamond-based electronic devices. Activation energies for the diamond dopants (such as 370 meV for boron) are much larger than thermal energy at room temperature (26 meV)5. Although boron δ-doped diamond was considered to be a promising channel layer for metal-insulator-semiconductor field effect transistors (MISFETs)678, its mobility was behind expectations. On the other hand, most of successful diamond-based FETs have been fabricated on hydrogenated-diamond (H-diamond) epitaxial layers, which accumulate two-dimensional hole gases (2DHGs) on the surface with sheet hole density of 1012–1013 cm−2 910.

Since the insulator with a higher-dielectric constant (higher-k) can provide the relatively large charge response at a smaller electrical field11, it is promising to fabricate the high-k insulator on the H-diamond. Several gate insulators such as SiO2, AlN, Al2O3, and CaF2 have been deposited on the H-diamond for fabricating the MISFETs12131415. However, the k values of them are not large. Although sputtering-deposited Ta2O511 and atomic-layer-deposited HfO216 had higher-k than other materials mentioned above, the high density of trapped and fixed charges in both the oxides made them difficult for the applications of the MISFETs. Our group demonstrated the normally-off HfO2-gated MISFET with k = 9.4 using the bilayer HfO2 structure prepared by atomic layer deposition (ALD) and sputtering deposition (SD) techniques17. This bilayer gate strategy was also applied to LaAlO3/Al2O3-gated MISFET with k = 9.118. Although these MISFETs showed good electrical properties, the k value was still not large. Recently, we have deposited SD-Ta2O5/ALD-Al2O3 on the H-diamond for MIS diode and MISFET19. The k value of the SD-Ta2O5/ALD-Al2O3 is around 12.7, which is larger than those of the SD-HfO2/ALD-HfO2 and SD-LaAlO3/ALD-Al2O3. However, the leakage current density (J) and fixed charge density were high. Thus, a new high-k insulator should be developed for the high-performance H-diamond-based MISFETs.

Since ZrO2 has a high-k (25), a wide band gap (5.8 eV), and a large breakdown field (15–20 MV·cm−1)20, it is possibly a good candidate for the H-diamond-based MISFETs. Several deposition techniques such as atomic layer deposition (ALD)21, molecular beam evaporation22, chemical vapor deposition23, pulsed laser deposition24, and sputtering deposition (SD)2526 have been used for the ZrO2 fabrication. The ZrO2 layer deposited on Si substrate by the SD technique was reported to show small leakage current and low trapped charge densities26. Thus, it is promising to deposit high-quality ZrO2 layer on the H-diamond by the SD technique. However, since the H-diamond surface is predicted to be damaged easily by plasma discharge during the SD deposition, a buffer layer is necessary to keep the 2DHG in the H-diamond. The Al2O3 deposited by the ALD technique was reported to be a large valence band offset27 against the H-diamond and was confirmed to be the effective buffer layer for the SD-LaAlO3 on the H-diamond18. Therefore, the SD-ZrO2/ALD-Al2O3 bilayer oxide insulator would be promising for the application of the H-diamond-based MISFET.

On the other hand, since on-resistance (R) of the MISFET can affect the current density and power-loss, our effort is to reduce the R for the H-diamond-based MISFET. The R value is usually composed of source/drain electrode contact resistance (R), channel resistance beneath the oxide films (R), and source/drain-channel resistance (R)141718. The R is controlled by the metal contact to the 2DHG in the H-diamond, which is much smaller than the R and R. The R is directly related to the intrinsic properties (hole density and mobility) of the H-diamond. Some studies have focused on the increasing of hole density by the H-diamond surface treatment, which decreases the R and improve the electrical properties of the MISFETs1314. However, there are few studies on the topic of R decreasing. Almost all the H-diamond-based MISFETs have source/drain-channel interspaces (I)141718, which implies the existence of large R. Therefore, the H-diamond-based MISFET without the I is strongly required to be fabricated. Recently, we have fabricated SD-Ta2O5/ALD-Al2O3-gated H-diamond MISFET to reduce the R by eliminating I19. However, the current output was still not satisfactory.

In this reports, chemical and electrical properties of the high-k ZrO2 on the H-diamond are investigated. In addition, the SD-ZrO2/ALD-Al2O3-gated H-diamond MISFET with a low R and large output current is demonstrated. The k values for the SD-ZrO2/ALD-Al2O3 bilayer and single SD-ZrO2 layer are determined by capacitance-voltage (C-V) characteristic to be 12.8 and 15.4, respectively. The drain-source current maximum (I) for the MISFET without I is as large as −224.1 mA·mm−1.

Results

Chemical properties of ZrO2 layer

Figure 1 shows spectra of the Zr 3d, O 1 s, and O 1 s photoelectron energy loss peak for the amorphous SD-ZrO2 (32.5 nm) layer measured by x-ray photoelectron spectroscopy (XPS). The Zr 3d and O 1 s core level spectra were fitted by Voigt (mixed Lorenzian-Gaussian) lineshapes with Shirley background (dashed line). The solid lines matched to the dots were the sums of Voigt lineshapes and background. All the photoelectron spectra were calibrated relative to the reference C 1 s peak position (285.0 eV). The Zr 3d spectrum [Fig. 1 (a)] had the strong spin-orbit doublet Zr 3d5/2-Zr 3d3/2 with splitting of 2.3 eV. It was fitted by a single component (Zr-O) with the binding energy of 182.7 ± 0.2 eV for the Zr 3d5/2. To fit O 1 s spectrum [Fig. 1 (b)], two components (O-Zr and O-H) were required. The binding energies for them were 530.4 ± 0.2 and 532.2 ± 0.2 eV, respectively. Based on the intensity ratio of the Zr 3d to O 1 s and the inelastic mean free paths for them28, the atomic ratio between zirconium and oxygen can be calculated to be 1:2. Thus, we obtained the stoichiometric SD-ZrO2 layer on the H-diamond surface. The band gap energy (E) of the ZrO2 was determined by the O 1 s photoelectron energy loss peak [Fig. 1 (c)]. Threshold energy2930 of the O 1 s photoelectron energy loss peak was determined by extrapolating a linear fit of the leading edge to the baseline to be 536.0 ± 0.2 eV. According to the O 1 s core level binding energy (530.4 ± 0.2 eV) for the O-Zr bonds, the E of amorphous ZrO2 layer was determined to be 5.6 ± 0.4 eV, which was in good agreement with the other literature datum (5.8 eV)31.

Figure 1

XPS spectra of ZrO2 layer.

(a) Zr 3d, (b) O 1 s, and (c) O 1 s photoelectron energy loss spectra for the SD-ZrO2 (32.5 nm) layer. The Zr 3d and O 1 s core level spectra were fitted by Voigt (mixed Lorenzian-Gaussian) lineshapes after the application of Shirley background (dashed line). The solid lines matched to the dots are the sums of Voigt lineshapes and background.

Electrical properties of MIS diode

Figure 2 (a) shows the J for the SD-ZrO2/ALD-Al2O3/H-diamond MIS diode for gate voltage ranging from −4.0 to 2.0 V. The inset in the Fig. 2 (a) is the top view of the planar-type MIS diode. The J was obtained with dividing leakage current by area of the gate electrode (1.77 × 10−4 cm2). With the change of the gate bias from positive to negative, the J value increases to 4.8 × 10−5 A·cm−2 at −4.0 V. The obvious accumulation and depletion regions are observed in the C-V curve [Fig. 2 (b)]. The maximum capacitance is 0.310 μF·cm−2. Based on this value and the total oxide bilayer thickness (36.5 nm), the k value of the SD-ZrO2/ALD-Al2O3 bilayer can be calculated to be 12.8, which is larger than those of the SD-LaAlO3/ALD-Al2O3 (9.1), SD-HfO2/ALD-HfO2 (9.4), ALD-HfO2 (12.1), and SD-Ta2O5/ALD-Al2O3161718. According to the k value of 5.4 and the thickness of 4.0 nm for the ALD-Al2O3 on the H-diamond (Data shown in the supporting information), the k value for the single SD-ZrO2 can be calculated to be as large as 15.4. The C-V curve shows small flat band voltage (V) shift and small stretch-out32 in the depletion region, which implies low fixed and trapped charge densities in the SD-ZrO2/ALD-Al2O3/H-diamond structure3334. Based on the C-V curve and profiling technique1335, the 2DHG density as a function of depth from the ALD-Al2O3/H-diamond interface to the H-diamond epitaxial layer can be determined, which is shown in Fig. 2 (c). The peak hole density is 6.3 × 1019 cm−3. With increasing the depth, the hole density decreases quickly, which indicates the existence of 2DHG in the H-diamond epitaxial layer close to the ALD-Al2O3/H-diamond interface. Assuming the 2DHG region with 1 nm thickness, the sheet hole density (p) is evaluated to be 3.15 × 1012 cm−2.

Figure 2

Electrical properties of the MIS diode.

(a) The J-V and (b) the C-V curves for the SD-ZrO2/ALD-Al2O3/H-diamond MIS diode. (c) Hole concentration of the H-diamond channel layer as a function of depth from the ALD-Al2O3/H-diamond interface to the H-diamond epitaxial layer.

Electrical properties of MISFETs

Figure 3 shows schematic cross-sectional structures of the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs without [Fig. 3 (a)] and with [Fig. 3 (b)] I. The MISFET without I in Fig. 3 (a) has the T-shaped gate electrode with top and bottom lengths of 8 and 4 μm, respectively. The MISFET with I in Fig. 3 (b) has an interspace of 3 μm between gate and source/drain contacts. The gate length (L) and gate width (W) are 4 and 150 μm, respectively. Figs. 4 (a)0 and (b)0 show drain/source current versus voltage (I-V) characteristics for the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs without and with I, respectively. The gate voltage (V) was varied from -4.0 to 2.0 V in steps of +0.5 V. Both the MISFETs show p-type channel characteristics. The I for the MISFET without I is −224.1 mA·mm−1, which is much larger than that for the MISFET with I of −29.3 mA·mm−1. The R can be extracted from the linear region of the I-V characteristics. It is 29.7 Ω·mm for the MISFET without I. This value is much smaller than that of 208.4 Ω·mm for the MISFET with I. The transfer characteristics corresponding to the I-V curves are shown in the Figs. 4 (a, b)1,2. Threshold voltage (V) for the MISFET with I was 1.3 ± 0.1 V, which almost equals to the value of 1.4 ± 0.1 V for the MISFET without I. The extrinsic transconductance (g) was determined based on the slope of the linear portion for the I-V curves. The values of the g maximum (g) for the MISFETs without and with I are 70.4 ± 0.1 and 10.1 ± 0.1 mS·mm−1, respectively.

Figure 3

Schematic cross-sectional structures of the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs.

(a) Without and (b) with I.

Figure 4

Electrical properties of the MISFETs.

The I, I, , and g characteristics of the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs without [Figs. 4 (a)0,1,2] and with [Figs. 4 (b)0,1,2] I.

The effective mobility (μ) for the H-diamond channel layer can be calculated by the following equation with conditions of R ≪ R, where the W, L, C, I, and V have been mentioned above to be 150 μm, 4 μm, 0.310 μF·cm−2, −224.1 mA·mm−1, and 1.4 ± 0.1 V, respectively. The μ was calculated to be 217.5 ± 0.5 cm2·V−1·s−1 at the V of −7.0 V. Based on the μ value, the sheet hole density (p) for the H-diamond channel can be calculated by the following equation, where the q is the charge of one electron (1.6 ×10−19 C). The R is the sheet resistance of the H-diamond channel layer, which was determined to be 7.46 × 103 Ω/□. Thus, the p can be calculated to be 3.74 ×1012 cm−2, which is close to the p and reasonable for the common H-diamond.

Discussion

The electrical properties of the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs without and with I were summarized in the table 1. The R for the MISFET without I is almost seven times smaller than the one with I. Therefore, the I and g values for the MISFET without I are almost seven times larger than those with I. The reduction of R has obviously decreased the R and significantly improved the electrical properties of the H-diamond-based MISFETs. Recently, the highest I was reported by Hirama et al. to be −600 mA·mm−1 at L = 4 μm for the ALD-Al2O3-gated H-diamond MISFET14, which is larger than our present value of −224.1 mA·mm−1. This is attributed to the higher sheet hole density of 4 × 1013 cm−2 for the H-diamond epitaxial layer treated by NO2 ambient than the as-grown one of 3.74 ×1012 cm−2 in this report. It is believed that the increment of the hole density will possibly provide the highest I in the future.

Table 1

Electrical properties of the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs

MISFET	RON [Ω·mm]	IDSmax [mA·mm−1]	VTH [V]	gm,max [mS·mm−1]
Without IS/D-CH	29.7	−224.1	1.3 ± 0.1	70.4
With IS/D-CH	208.4	−29.3	1.4 ± 0.1	10.1

Table 2 summarized our recent reports for the electrical properties of the insulator/diamond MISFETs. The SD-Ta2O5 and ALD-HfO2 on the diamond showed k values larger than 12.11116. However, they were difficult to operate the MISFETs because the high trapped and fixed charge densities were existed in both the oxides. Although the SD-LaAlO3/ALD-Al2O3/H-diamond and SD-HfO2/ALD-HfO2/H-diamond MISFETs showed good electrical properties131718, the k values of them were smaller than 9.4. Recently, the SD-Ta2O5/ALD-Al2O3 bilayer has been deposited on the H-diamond for the MISFETs. The k values of SD-Ta2O5/ALD-Al2O3 and single SD-Ta2O5 were as large as 12.7 and 16.5, respectively19. The I was −97.7 mA·mm−1, which was larger than those of the SD-LaAlO3/ALD-Al2O3/H-diamond and SD-HfO2/ALD-HfO2/H-diamond MISFETs due to the eliminating of I. However, the J of SD-Ta2O5/ALD-Al2O3/H-diamond MIS diode was one order larger than that of the SD-ZrO2/ALD-Al2O3 MIS diode. In addition, the fixed charge density in the SD-Ta2O5/ALD-Al2O3 was higher than that in the SD-ZrO2/ALD-Al2O3.

Table 2

Summary of our recent reports for the electrical properties of the insulator/diamond MISFETs1116171819.

Oxide insulator	k	WG (μm)	LG (μm)	IS/D-CH (μm)	RON (Ω·mm)	IDSmax (mA·mm−1)
ALD-HfO2	12.1	-	-	-	-	-
SD-Ta2O5	16.0	-	-	-	-	-
SD-LaAlO3/ALD-Al2O3	9.1	150	10	15	637	−7.5
SD-HfO2/ALD-HfO2	9.4	150	4	10	230	−38.7
SD-Ta2O5/ALD-Al2O3	12.7	150	4	0	42.9	−97.7
SD-ZrO2/ALD-Al2O3	12.8	150	4	5	208.4	−29.3
				0	29.7	−224.1

In conclusion, the chemical and electrical properties of the high-k SD-ZrO2/ALD-Al2O3 layer on the H-diamond were investigated. The atomic ratio between zirconium and oxygen of the SD-ZrO2 layer and E were determined by XPS to be 1:2 and 5.6 ± 0.4 eV, respectively. The J of the MIS diode was smaller than 4.8 × 10−5 A·cm−2. The k values of the SD-ZrO2/ALD-Al2O3 bilayer and single SD-ZrO2 layer were determined to be as large as 12.8 and 15.4, respectively. The R, I, g, μ, and p for the MISFET without I were 29.7 Ω·mm, −224.1 mA·mm−1, 70.4 ± 0.1 mS·mm−1, 217.5 ± 0.5 cm2·V−1·s−1, and 3.74 × 1012 cm−2, respectively.

Methods

H-diamond, Al2O3, and ZrO2 depositions

The H-diamond homoepitaxial layer was deposited by a microwave plasma-enhanced chemical vapor deposition on the Ib-type single crystalline diamond (100) substrate with dimension of 2.5 × 2.5 × 0.5 mm3. In order to remove remaining contaminations on the surface of the diamond, the substrate was firstly annealed at 1000°C for 20 min in an ambient of H2. The deposition temperature and chamber pressure for the H-diamond were 900 ~ 940°C and 80 Torr, respectively. The H2 and CH4 flow rates were 500 and 0.5 sccm, respectively. Thickness of the H-diamond epitaxial layer was about 200 nm. The ALD-Al2O3 buffer layer with thickness of 4.0 nm was deposited using precursors of Al(CH3)4 and water vapor at 120°C. The pulse and purge times for them were 0.1 and 4.0 s, respectively. The SD-ZrO2 film with thickness of 32.5 nm was deposited on the ALD-Al2O3/H-diamond by the radio-frequency (RF) SD technique in a pure argon-atmosphere at room temperature. The RF power was 30 W and the chamber pressure was kept at 1 Pa. Thicknesses of the ALD-Al2O3 and SD-ZrO2 was determined by ellipsonmeter. The XPS measurements for the SD-ZrO2 were performed by a monochromated Al Kα X-ray source (hv = 1486.6 eV) with a vacuum pressure of 1.0 × 10−9 Torr. Each spectrum was recorded with a 0.05 eV step and a 55 eV pass energy.

MIS diode fabrication

The gate and ohmic contact electrodes were deposited on the surfaces of the SD-ZrO2/ALD-Al2O3 and H-diamond using an electron-beam evaporator, respectively. The diameter of the circular SD-ZrO2/ALD-Al2O3 film and gate electrode was 150 μm with an interspacing of 10 μm between the SD-ZrO2/ALD-Al2O3 and the ohmic Au/Ti/Pd contact (Schematic cross-sectional structure of the MIS diode shown in the supporting information). The current-voltage and C-V measurements for the MIS diode were performed at room temperature under dark condition. The frequency for the C-V measurement was 50 kHz with voltage steps of 0.1 V.

MISFETs fabrication

The fabrication of the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs was based on the combinations of laser lithography, dry-etching, an ALD technique, a SD technique, electrodes metallization, and lift-off technique. There are mainly three steps (Fabrication process shown in the supporting information): I. mesa-structure fabrication, II. source/drain ohmic contact fabrication, and III. gate oxide and contact fabrication. The photoresists of LOR5A and AZ5214E were used for all the patterning processes. After coating the photoresist, the sample was exposed by 405 nm illumination using the laser lithography system and then was developed in a tetramethylammonium hydroxide solution. The H-diamond channel layer was etched under O2 atmosphere with pressure of 10 Pa by reactive ion etching system in order to fabricate the mesa-structure. The ALD-Al2O3 buffer layer and SD-ZrO2 layer were subsequently deposited on the H-diamond surface. Note that the oxide insulators were patterned by the photoresists, which made the MISFET without I fabricated successfully. After the finishing each fabrication step, the photoresists were removed by an N-methylpyrrolidone solution at room temperature. The electrical properties of the SD-ZrO2/ALD-Al2O3/H-diamond MISFETs were measured under a dark condition using high performance MX-200/B prober and B1500A parameter analyzer.