Digital Object Identifier 10.1109/JEDS.2017.2653419
Characterizations of Metal-Oxide-Semiconductor
Field-Effect Transistors of ZnGaO
Grown on Sapphire Substrate
YI-SIANG SHEN1, WEI-KAI WANG2, AND RAY-HUA HORNG1,3(Fellow, IEEE)1 Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung 402, Taiwan 2 Department of Materials Science and Engineering, Da-Yeh University, Changhua 515, Taiwan
3 Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan CORRESPONDING AUTHOR: R.-H. HORNG (e-mail: rhh@nctu.edu.tw)
This work was supported by the Ministry of Science and Technology, Taiwan, under Contract MOST104-2221-E-009-199-MY3.
ABSTRACT Zinc gallate (ZnGaO) epilayers were grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition and fabricated into metal-oxide-semiconductor field-effect tran-sistors (MOSFETs). The ZnGaO MOSFETs exhibited a complete channel pinch-off of the drain current for VGS< −4.43 V, high off-state breakdown voltage of 378 V, high ION/IOFFratio of 106, and low gate leakage current.
INDEX TERMS Zinc gallate, metal-oxide-semiconductor field-effect transistors (MOSFETs), channel pinch-off.
I. INTRODUCTION
Wide-band gap oxides have attracted interests as new materials and possess unique properties for application in next-generation semiconductor power devices and deep ultra-violet (UV) detectors [1], [2]. Among these oxides, ZnGa2O4 has been paid great attention as a transparent conducting oxide in the UV region due to its high chemi-cal stability and outstanding optichemi-cal properties [3]–[5]. The material ZnGa2O4consists of ZnO and Ga2O3, which crys-tallizes in the spinel structure and has an energy gap of about 5.2 eV [6]. In previous report, ZnGa2O4 is a phos-phor and is a promising application in field emission displays operating at low accelerating voltage [7]. Up to now, most of the reports are relative to the synthesis of ZnGa2O4 nanostructures with different morphologies, such as nanowires, nanorods and nanocrystal [8]–[10]. The cor-responding electronic transport and optical properties of the ZnGa2O4 nanostructure were also studied [11]. However, there are still very few works to investigate the film and epilayer properties of ZnGa2O4. In addition, as far as we know, no relevant research about MOSFET based on ZnGa2O4 epilayer has been reported yet. In this work, the material qualities of zinc gallate (ZnGaO; ZGO) were described. Moreover, MOSFETs made of ZGO epilayers
grown on c-plane sapphire by MOCVD were also studied. The device performances indicated that the ZGO MOSFET has potential for application in transparent power electronic devices.
II. DEVICE STRUCTURE AND FABRICATION
ZGO MOSFETs were fabricated using 200-nm-thick single-crystal ZGO epilayers grown on c-plane (0001) sapphire substrates at 600◦C by metalorganic chemical vapor depo-sition. Diethylzinc (DEZn) and triethylgallium (TEGa) were employed as the Zn and Ga precursors, respectively. Ar (99.999 %) and purified oxygen (99.999 %) were adopted as carrier gases and oxidizer, respectively. During the growth of GZO thin films, Ar passed through the bubblers to deliver the DEZn and TEGa vapors to the reactor. After growth, the intrinsic ZGO is a n-type epilayer confirmed by Hall measurement. In order to demonstrate the film was ternary epilayer, the x-ray diffractemery was used to measure the crystal structure. The electrical mobility, resistivity, and elec-tron concentration of ZGO used for the MOSFET study were 2.2 cm2/V.s, 47.5 .cm, and 5.9× 1016 cm−3, respectively, and were obtained through Hall measurement at room tem-perature (RT). The process commenced with mesa isolation
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in an inductively coupled plasma reactive ion etching sys-tem by using BCl3/Cl2/Ar. Ti/Al/Ti/Au (25/125/50/60 nm) was evaporated to form the metal contacts for the source (S) and drain (D) electrodes. Notably, the Ti metal contacting to ZGO exhibited ohmic contact behavior without any thermal annealing. The 40-nm Al2O3 dielectric layer was deposited through ALD at 300◦C. Finally, Ni/Au (150/50 nm) gate metals were deposited on the Al2O3 by e-beam evapora-tion. The lengths of the device channel, gate, and the access region were 20 μm, LG = 3 μm, and LGS = LGD = 8.5 μm, respectively. The current-voltage (I-V) characteristics of these samples were measured using an Agilent 1505B parameter analyzer at RT. In order to evaluate the crys-tal structure of the ZGO epilayer, the X-ray diffraction (XRD, PANalytical, Cu Kα radiation) was used to measure the crystalline quality. Microstructure of the ZGO film was investigated by transmission electron microscopy (TEM). III. RESULTS AND DISCUSSION
Fig. 1 shows the XRD diffraction pattern of ZGO film grown on sapphire substrate. The XRD pattern shows strong peak at around 42◦ which was identified to be the Al2O3 (0006) plane. Moreover, the monoclinic β-Ga2O3 film with very small peak located at diffraction angles of 38.4◦ corre-sponding to the (-402) planes reflections was obtained. It was also found that there were additional features of diffraction intense peaks observed at 18.57◦, 37.61◦, and 57.82◦. These peaks very closed to the 18.40◦, 37.34◦, and 57.40◦ charac-teristic reflections of (111), (222) and (333) crystal planes of ZnGa2O4(JCPDS card 381240). These results indicated that the epilayer has transferred most of Ga2O3 into the ZGO epilayer. The measured XRD data are also shown in Table 1.
FIGURE 1. XRD pattern of ZGO film grown on c-plane sapphire substrate.
Fig. 2(a) shows the high-resolution TEM bright field image focused on the interface between ZGO film and c-plane sapphire substrate. According to our analysis, the d-spacing of 2.42 Å of ZGO film was obtained in this image, which was very close to the standard d-spacing of 2.406 Å of ZnGa2O4(222) plane. In addition, the high-resolution TEM bright field image taken at the middle region of ZGO film
TABLE 1. XRD diffraction measured results of ZGO film.
is displayed in Fig. 2(b). Based on our observation, the most lattice features shown in Fig. 2(b) belonged to that marked with green parallel lines. These lattices possess the d-spacing value of 4.809 Å, where the standard d-spacing of ZnGa2O4(111) plane is 4.808 Å. On the other hand, a frac-tion of lattice features presented in this image was also labeled with yellow parallel lines, and its d-spacing was much similar to the standard one of Ga2O3(-402) plane. Obviously, the TEM observations are in well agreement with the XRD result, as shown in Fig. 1. In other words, the main crystal structure formed in the ZGO film is the ZnGa2O4 phase with the (111)-family planes. Besides, a small portion of Ga2O3(-402) phase existed in the ZGO film also can be confirmed by TEM. Fig. 2(c) shows the selected area elec-tron diffraction pattern of the ZnGa2O4 region shown in Fig. 2(b). The diffraction dots with a regular arrangement indicates the microstructure is single crystalline. Moreover, this single crystalline diffraction pattern reveals the ZnGa2O4 phase of ZGO film is formed along the [111] direction (with the [1-21] zone axis).
In order to demonstrate the epilayer has become the ZGO film, the energy gap of ZGO was evaluated by cathodelu-minescence (CL) measurement. Fig. 3 shows CL spectrum of ZGO film at RT. The spectrum consists of two ultravi-olet (UV) luminescence emission bands at the wavelength peaks position of 330 nm (3.75 eV; ECL1) and 242 nm (5.12 eV), respectively. In the visible region, there exists a weak intensity at peak of 500 nm (2.48 eV; ECL2). The weaker CL intensity at peak 242 nm could be due to the conduction band to the valence band transition of ZGO. Theoretically, the Eg of ZnGa2O4is about 5.2 eV. In this work, the ZGO is not a perfect ZnGa2O4. Nevertheless, it has demonstrated again that the epilayer is the ZGO and not Zn-dopant Ga2O3. On the other hand, a strong UV emission band (330 nm) could be attributed to the radia-tive carriers transition from the donor level (Ed) to the valence band. Although the ZGO is the dominated struc-ture, there exists a few part of β-Ga2O3 (demonstrated by XRD and TEM, shown in Figs. 1 and 2). Obviously, sup-press of intrinsic green emission band centered at around 500 nm which is induced by donor-acceptor-pair transition in β-Ga2O3 through Zn-incorporation, and further contri-bution on the UV emission band was obtained. Based on above analysis, the Ed was estimated to be approximately about 1.37 eV by a formula Ed = Eg− ECL1, which is consists with previous reported by Varley et al. [12]. An acceptor level (Ea) for the ZnGawas calculated to be 1.27 eV,
FIGURE 2. HR-TEM images taken at (a) the interface between ZGO and sapphire and (b) the middle region of ZGO film. (c) Selected area electron diffraction pattern of ZnGa2O4phase shown in Fig. 2(b).
(Ea= ECL1−ECL2). Schematic diagram of the energy levels in the ZGO film is also illustration in the inset of Fig. 3.
Fig. 4 shows the DC output I-V (IDS-VDS) characteristics of ZGO MOSFET at gate voltage (VGS) from -5 to 15 V
FIGURE 3. CL spectrum of ZGO film grown on sapphire substrate measured at RT. Insets are the magnified 5.12 eV, 2.48 eV spectra and the energy levels diagram in the ZGO film.
FIGURE 4. DC I–V characteristics of ZGO MOSFET measured at RT.
in steps of 2 V, while the VDS was swept from 0 to 40 V
at RT. The device exhibited a clear pinch-off behavior and maximum IDSof 0.135 mA/mm at VGS= +15 V. In addition,
the transistor shows favorable gate-modulation performance with Id saturation, whereas the channel mobility and
sub-threshold slope of ZGO MOSFET were calculated about 2.46 cm2/V.s and 40 mV/decade, respectively. Moreover, the insert plot in Fig. 4 shows the linear I-V characteristic of Ti/Al/Ti/Au on ZGO without thermal annealing. Obtaining ohmic contacts was thus easy in this study because Ti metal layer can be ohmic contact with the ZGO. Similar reports on the ohmic contact through Ti/Al/Ti/Au multiple metals were found by Wang et al. [13].
Fig. 5 shows the transfer characteristics at a VDS of 13 V. The threshold voltage on ZGO MOSFET was -4.43 V, which suggests that the device presented the depletion mode characteristic. Clearly, the ZGO was n-type and could enhance the conductivity through formation oxy-gen vacancy due to Zn atom compensation effect [14]. However, it is necessary to attract more electrons by applying
FIGURE 5. Transfer characteristics of ZGO MOSFET at VDS=13 V measured at RT.
FIGURE 6. (a) Breakdown voltage characteristic of ZGO MOSFET and (b) IDS-VGScurve of ZGO MOSFET measured at 13 V of VDS.
positive voltage on gate electrode to switch the transis-tor on. The peak intrinsic maximum transconductance (gm) was 9.46×10−3 mS/mm at a V
DS of 13 V, which could be attributed to the both of substitutional defects (from Zn-incorporated into β-Ga2O3) and the lattice mismatch between the sapphire substrate and ZGO film. These defects resulted in the reduction of electron density and mobility in the ZGO film. Moreover, the carriers’ scattering effect,
attributable to defects and impurities, reduced the mobility of the ZGO MOSFET. These results are consistent with those of Dang et al. [15].
The three-terminal breakdown behavior of the ZGO MOSFET device was evaluated and is shown in Fig. 6 (a). The breakdown voltage could be attributed to the leakage current through the Al2O3 layer or ZGO epilayer break-down. However, a low gate leakage current was less than
1.45 × 10−5 mA/mm (data not shown) from the ZGO film device. Therefore, the breakdown voltage was as high as 378 V at VGS of 0 V, indicating that ZGO MOSFET with Zn-incorporated exhibits enhanced off-state breakdown volt-age. Nevertheless, the measured reverse gate leakage is low enough for device application. Fig. 6 (b) presents the IDS -VGS curve of ZGO MOSFET at a VDS of 13 V. The low off-state leakage current (10−7 mA/mm) through the ZGO film was a result of the Al2O3 gate dielectric. At a VDS of 13 V, IDS on/off ratio of approximately six orders of magnitude were achieved.
IV. CONCLUSION
We have fabricated ZGO MOSFET on a c-plane sapphire substrate and obtained improved device characteristics, such as a high ION/IOFF drain current ratio of six orders of mag-nitude, clear pinch-off behavior, and breakdown voltage of 378 V. Moreover, we have demonstrated the strong poten-tial of the cost-effective growth of ZGO-on-sapphire for application in future transparent electronic power devices. REFERENCES
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YI-SIANG SHEN received the B.S. degree in
electrical engineering from National Normal University, Kaohsiung, Taiwan, in 2014, and the M.S. degree from the Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung, Taiwan, in 2016. His major research focuses on nitride-based electric power device.
WEI-KAI WANG received the B.S. degree
in manufacturing engineering from Yuan Ze University, Chungli, Taiwan, in 2000, the M.S. degree in electrical engineering from the University of Chung Hua, Hsinchu, Taiwan, in 2002, and the Ph.D. degree from the Department of Materials Engineering, University of Chung Hsing, Taiwan, in 2006. He is currently an Assistant Professor with the Department of Materials Science and Engineering, Da-Yeh University, Changhua, Taiwan. His research interests include development of GaN-based optoelectronic semiconductors and electric devices.
RAY-HUA HORNG (M’07–SM’11–F’02) received
the B.S. degree in electrical engineering from National Cheng Kung University, Tainan, Taiwan, in 1987, and the Ph.D. degree in electrical engi-neering from National Sun Yat-sen University, Kaohsiung, Taiwan, in 1993.
She is currently a Distinguished Professor with the Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan. Her current research interests include solid-state lighting devices, solar cells, power device, HEMT, flexible electronics, optoelectronics, and nitride and oxide semiconductor MOCVD growths.
