Germanium gate hydrogen-terminated diamond field effect transistor with Al2O3 dielectric layer

Abstract Investigation of germanium gate hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with Al2O3 dielectric layer has been successfully performed. The device demonstrates a normally-on characteristics, whose maximum drain-source current density, threshold voltage, maximum transconductance, on/off ratio, subthreshold swing, capacitance, carrier density, saturation carrier mobility, fixed charge density and interface state density are of −37.3 mA/mm, 0.22 V, 6.42 mS/mm, 108, 134 mV/dec, 0.33 μF/cm2, 9.83 × 1012 cm−2, 97.9 cm2/V·s, 7.63 × 1012 cm−2 and 2.56 × 1012 cm−2·eV−1, respectively. This work is significant to the development of H-terminated diamond FET.

It is worth noting that the high surface conductivity of H-terminated diamond may be commonly ascribed to the air adsorbates, which is sensitive to the environment, and this determines the thermal and chemical instability of H-terminated diamond [10]. In order to overcome this drawback, a variety of dielectric layers (such as SiN x /ZrO 2 [5], WO 3 [12], LiF/Al 2 O 3 [13], MoO 3 [14], Al 2 O 3 [15], V 2 O 5 [16], etc.) have been deposited by electron beam (EB) evaporation, atomic layer deposition (ALD) or magnetron sputtering technique, respectively. Notably, Al 2 O 3 /H-terminated diamond exhibits large valence band offset, which makes low leakage current density [24]. Thus, the Al 2 O 3 has been commonly used as the dielectric layer in H-terminated diamond FET [13,15,24].
As is known to all that the first transistor in the world is made of Germanium (Ge) material. Ge exhibits good stability and outstanding semiconductor properties, such as electron and hole mobility, etc., which has great application potential in the field of electronics industry [28]. Besides, as the semiconductor material polysilicon is often used as gate material for the current semiconductor processes, which can change the work function by doping impurities and then regulate the threshold voltage (V TH ). Based on this, we try a semiconducting Ge gate H-terminated diamond FET with Al 2 O 3 dielectric layer in this work. To the author's knowledge, few studies on Ge gate H-terminated diamond FET has been reported.
In this work, fabrication and characterization of Ge gate H-terminated diamond FET with Al 2 O 3 dielectric layer has been implemented.

Experimental
The fabrication process of Ge gate H-terminated diamond FET with Al 2 O 3 dielectric layer is demonstrated in Figure 1. A 3 × 3 × 0.5 mm 3 single crystal diamond synthesized by high pressure and high temperature (HPHT) technique was utilized as the substrate. Before homoepitaxy growth, the substrate was cleaned with a mixed solutions of H 2 SO 4 :HNO 3 = 1:1 at 250 °C for 1 h to remove the impurities on the diamond surface. The 200 nm homoepitaxy layer was grown by microwave plasma chemical vapor deposition system with gas flow, CH 4 /H 2 ratio, temperature, pressure and power of 500 sccm, 1%, 900 °C, 100 Torr and 1 kW, respectively, then the CH 4 flow was set to zero, and the substrate was maintained in hydrogen plasma to form H-terminated diamond surface for at least 20 min [23]. The device fabrication was started with 150 nm Au source/drain Ohmic electrodes obtained by photo-lithography and EB evaporation technique. After that, 15 min UV/ozone treatment was implemented in nonactive region for mesa isolation with 2DHG channel protected by photo-resist. Since H-terminated diamond demonstrates thermally and chemically instability [10], the ALD technique has been used in our experiment for the deposition of about 10 nm Al 2 O 3 dielectric layer to stabilize the H-terminated diamond surface at 80 °C. Finally, 50/50 nm Ge/Au gate material were evaporated sequentially. The electrical properties of Ge gate H-terminated diamond FET were measured by semiconductor analyzer Agilent B1505 A at room temperature.

Result and discussion
The output characteristics of Ge gate H-terminated diamond FET with nominal dimensions of 4 μm gate length (L G ), 100 μm gate width (W G ) and 20 μm source drain gap (L SD ) is shown in Figure 2. The actual dimensions of the device are measured to be 4.96 μm, 99.37 μm and 19.38 μm by SEM technique. The absolute value of drain-source current density (I DS ) increases with the increased absolute value of V GS , indicating a p-type channel with hole carriers under the Al 2 O 3 dielectric layer. The V GS changes from 2 to −6 V with a step of −1 V. The maximum drain-source current density (I DSmax ) is −37.3 mA/mm at V GS of −6 V and drain-source voltage (V DS ) of −20 V. The I DSmax is comparably high than that of our previous work, and this may be ascribed to the well protected 2DHG channel [25,26].
The transfer characteristics at the saturation region of V DS = −20 V is shown in Figure 3. The V TH is extracted to be 0.22 V based on the relationship of |I DS | 1/2 and V GS , illustrating a normally-on characteristics [17]. Moreover, we have compared the V TH with another device with similar structure, and the unique difference is without Ge. The V TH for the H-terminated diamond FET with and without Ge are of 0.22 V and 1.45 V, respectively. Obviously, the V TH shifts negatively for the Ge gate H-terminated diamond FET compared with that without Ge. Namely, Ge may adjust the V TH of the device, and the details will be discussed in our future work. The maximum transconductance (G m ), subthreshold swing (SS) and on/off ratio are 6.42 mS/ mm, 134 mV/dec and 10 8 [23]. Here, J TFE is the |I GS | induced by TFE model, J S shows the saturation current, and P is a parameter related to the temperature and carrier tunneling probability [23].
To further understand the channel transport characteristics, the interface and the Al 2 O 3 layer, the capacitance-voltage (C-V) characteristics of the device are studied at frequency of 1 MHz. As demonstrated in Figure 4, the depletion and accumulation region of hole carriers are distinctly observed. The gate oxide capacitance (C OX ) is 0.33 μF/cm 2 at V GS of −6 V. The flat band capacitance (C FB ) is calculated to be 0.29 μF/cm 2 based on the reference [18]. The flat band voltage (V FB ) is evaluated to be −3.6 V [18]. In addition, the C-V curve shifts to the negative direction corresponding to the V GS of 0 V, indicating the existence of fixed positive charge (Q f ) in Al 2 O 3 gate material [18]. The Q f is calculated to be 7.63 × 10 12 cm −2 based on following expression (1) [18]. Moreover, the carrier density (ρ) is evaluated to be 9.83 × 10 12 cm −2 obtained at V GS of −6 V by ∫ C V GS d [27]. (1) Moreover, the saturation carrier mobility (μ sat ) is also calculated with formula (2) [11]. As demonstrated in Figure 5, the μ sat of Ge gate H-terminated diamond FET varies from 97.9 cm 2 /V·s to 23.7 cm 2 /V·s with V GS varies from −1 V to −6 V. (2) The interface state density (D it ) is 2.56 × 10 12 cm −2 ·eV −1 refer to formula (3) [19]. And, k is Boltzmann constant, T is the temperature, C D is the depletion capacitance that may be negligible as its value is much smaller than C OX [21]. (3)

Conclusion
In conclusion, we have preliminarily realized a Ge gate H-terminated diamond FET with Al 2 O 3 dielectric layer. The output characteristics demonstrates a typical p-type channel with I DSmax of −37.3 mA/mm. The transfer characteristics shows V TH of 0.22 V, indicating a normally-on characteristics. The G m, SS and on/off ratio are 6.42 mS/mm, 134 mV/dec and 10 8 , respectively. The C-V characteristics show the C OX of 0.33 μF/cm 2 , Q f of 7.63 × 10 12 cm −2 and ρ of 9.83 × 10 12 cm −2 . In addition, the μ sat and D it characteristics are also evaluated to be 97.9 cm 2 /V·s and 2.56 × 10 12 cm −2 ·eV −1 . Accordingly, this work indicates that Ge is a promising material to be used in H-terminated diamond FET. In our future work, we will try to regulate the V TH by doping impurities on Ge.

Disclosure statement
No potential conflict of interest was reported by the authors.