A Study of N-type β-Ga2O3 Epitaxial Film

A Study of N-type β-Ga2O3 Epitaxial Film Grown on Sapphire Substrates via Ion Implantation and Its Device Characteristics

Professor Ray-Hua Horng

Hsin-Yin Tsai, Aproova Sood

Institute of Electronics, National Yang Ming Chiao Tung University

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At present, the semiconductor industry mostly makes its various components out of Si. However, with the advancement of process technologies and the improvement of component structures, related Si-based components have reached the limits of their material characteristics. Yet, as technology continues to develop, whether it is 5G and 6G communication, green energy, or electric vehicles, the related components all require higher speed and power. These demands are not ones that Si components can meet. Therefore, it is necessary to use other semiconductor materials to achieve these high power, high speed characteristics.

In recent years, the market demand and performance requirements for power components have continued to increase with the rapid development of electric vehicles and renewable energy. However, Si’s energy gap (Bandgap, E_g) is only 1.12 eV. It cannot withstand high voltages. Currently, more and more high voltage power components are making use of components made of the third generation wide-bandgap semiconductors silicon carbide (SiC) and gallium nitride (GaN). Components made of these materials can significantly increase the voltage and efficiency of power components, thereby improving the conversion efficiency in electric vehicles and renewable energy devices.

A semiconductor material’s power component characteristics are usually evaluated according to Baliga's Figure-Of-Merits (BFOM). BFOM is proportional to εμE_c^3. As a material’s E_g increases, the material’s critical electric field (E_c) also increases, which leads to a significant increase in its BFOM. The larger the BFOM, the greater the material’s potential in power components. As shown in the table in Figure 1, the E_g of SiC and GaN are 3.3 and 3.4 eV respectively, Therefore, in contrast to the BFOM of Si, they have BFOMs of 340 and 870 respectively. Moreover, the growth conditions for SiC and GaN’s substrates are severe and the processes costly, making the power components produced much more expensive than Si metal oxide semi-field effect transistors (MOSFET) and Si IGBTs. On the other hand, growth conditions for fourth generation wide-bandgap semiconductor Ga₂O₃ do not require high temperatures and pressures in addition to having lower costs. Moreover, Ga₂O₃ has an E_g as high as 4.8 eV, making its BFOM 4 and 10 times that of SiC and GaN respectively. If applied to power components, the breakdown voltage of the component could be effectively increased. However, theoretically, the on-resistance of this material is lower than that of Si, GaAs, SiC and GaN. A low on-resistance can reduce the power consumption of a component when it is in use, thereby improving conversion efficiency. Therefore, Ga₂O₃ is regarded as having the potential to replace SiC and GaN to become the semiconductor material for the next generation of high power components. Some have even said that Ga₂O₃ may replace SiC within the next ten years.

Figure 1.  Comparison of Semiconductor Material Properties

Ga2O3 has five Polymorphs: monoclinic (β-Ga2O3), rhombohedral (α- Ga2O3), defective spinel (γ-Ga2O3), cubic (δ-Ga2O3), and orthorhombic (ε-Ga2O3). Among them, β-Ga2O3 has the highest stability. The other four polymorphs all phase change into β-Ga2O3 at high temperatures. β-Ga2O3 has high chemical and thermal stability, as well as a bandgap of up to 4.8 eV, making it very suitable for application in power components.

Figure 2.  Diagram of the Transition Relationship Between Various Ga2O3 Polymorphs [1]

Although β-Ga₂O₃ has all the above advantages, it must be able to achieve a thin film with controllable conductivity before it can be used in components. Since β-Ga₂O₃ has a bandgap of 4.8 eV, its intrinsic carrier concentration is only 1.79 × 10^-23 cm^-3 [2]. As such, the resistance of un-doped β-Ga₂O₃ is very high like that of an insulator, making it unsuitable for use in semiconductor components. At present, the characteristics of the material make it extremely difficult to prepare P-type β-Ga₂O₃. N-type β-Ga₂O₃, on the other hand, may be achievable. According to theoretical calculations, the donor levels of Si, Ge and Sn in β-Ga₂O₃ are very close to the conduction band. Their activation energies are 30, 30, and 60 meV respectively for shallow donor levels [3]. Experiments have proven that the activation rates of these dopants are high. Several research teams have previously used molecular beam epitaxy (MBE) to grow β-Ga₂O₃ on β-Ga₂O₃ substrates while doping with Si[3], Ge[4], and Sn[5]. All of them can effectively increase the electron concentration of β-Ga₂O₃, reducing the sheet resistance value of the β-Ga₂O₃ so that it can be used to make power components.

MBE equipment requires an ultra-high vacuum, its deposition rate is slow, and it is costly. This makes it unsuitable for mass production. It is better suited to the early research and verification of materials. Metal-Organic Chemical Vapor Deposition (MOCVD), on the other hand, has a high output and low costs as well as good crystal quality, making it an excellent choice for the production of β-Ga₂O₃. Zixuan Feng and his team used the MOCVD to pass Triethylgallium (TEGa) and O₂ and used Silane (SiH₄) as the Precursor to prepare in-situ Si doped β-Ga₂O₃ thin films [6]. Furthermore, the doping concentration of the Si could be controlled by changing the chamber pressure. Based on Hall measurement, its carrier concentration can exceed 10¹⁶ cm^-3, and its carrier Mobility at room temperature can be as high as 184 cm²/Vs.

Although our research team has successfully grown β-Ga₂O₃ thin films on β-Ga₂O₃ substrates, β-Ga₂O₃ substrates are not currently very popular. It is a material with a high unit price. On the other hand, the c-plane Sapphire substrate has a lattice constant very close to that of β-Ga₂O₃. Furthermore, this substrate is affordable and much more competitive in the market. Our laboratory has been able to grow high quality, un-doped (UID) (-2 0 1) β-Ga₂O₃ on c-plane Sapphire substrates via MOCVD epitaxy. According to XRD analysis, its (-2 0 1) full width at half maximum (FWHM) can be as low as 400 arcsec. This means that β-Ga₂O₃ is highly crystalline.

Figure 3. (a) β-Ga2O3 on Sapphire Wafer Diagram; (b) XRD Diagram

In order to improve the conductivity of UID β-Ga₂O₃, we used Si ion implantation technology to implant Si into β-Ga₂O₃ thin films. We then used Rapid Thermal Annealing (RTA) to repair defects caused by Si ion impacts and activate the Si dopant. The Si concentration distribution can be adjusted by changing the dose and energy of the Si implantation. It can also be paired with the lithographic process to carry out doping in specific locations. In contrast to doping during epitaxy, ion implantation can change the electrical properties of the epitaxial film, providing greater flexibility for device manufacturing.

On the other hand, the Schottky barrier diode (SBD) is a common power component. The SBD utilizes the Schottky junction between metals and semiconductors to achieve rectification. SBDs have characteristics such as low on-voltages, high on-currents, and high switching speeds, etc.. During research, the SBD has been able to extract the carrier concentration of semiconductors using I-V, C-V and other methods. It also enables the use of temperature changes and large voltage measurements to determine the breakdown electric field, material stability and quality of semiconductors. Therefore, this paper will use doped SBDs to evaluate the feasibility of producing high power SBDs using Ga₂O₃ grown on Sapphire.

We used β-Ga₂O_3­ epitaxial films with 3 different Si Implantation concentrations: 1x10¹⁴, 6x10¹⁴, and 1x10¹⁵ cm^-2. The Si dopant was then activated via high temperature RTA processing. Since Si Implantation requires hitting the surface of the β-Ga₂O₃ with high energy Si ions to implant them into the epitaxial film, it is easy for the process to produce defects on the surface of the test piece. Therefore, we conducted Atomic Force Microscope (AFM) analysis on these three Si Implantation test pieces and the UID β-Ga₂O₃. The results are shown in Figure 4. Compared to the UID β-Ga₂O₃, the Ga vacancies are filled by Si after Si Implantation. This reduced defects, thereby decreasing the Root Mean Square (RMS) roughness of Si Implantation β-Ga₂O₃. As the implantation dose increases, however, the Si  will form Complexes, and the accumulation of defects causes the surface roughness to increase.

Figure 4. (a) UID β-Ga2O3 SBD AFM Diagram; AFM Diagram  of (b) 1x1014 cm-2, (c) 6x1014 cm-2, and (d) 1x1015 cm-2 Si Implantations

For further research, we made SBD with 3 different doses (1x1014, 6x1014, and 1x1015 cm-2) of Si Implantation β-Ga2O3 and compared them with the UID β-Ga2O3 SBD. In SBD development, Anode electrodes generally use Ni/Au for Schottky contacts, and Cathode electrodes generally use Ti/Au/Ti/Al for Ohmic contacts. The cross-sectional structure of the component and the electrode pattern as observed via a scanning electron microscope (SEM) are shown in Figure 5.

Figure 5. Cross-Section and Top-View SEM Image of β-Ga2O3 SBD

Once production of the β-Ga₂O₃ SBD was complete, I-V measurement was conducted via the Keysight B1505A  power component analyzer. The results are shown in Figure 6 [8]. After Si Implantation, the on-current of the SBD can be increased by 10⁸ times, and, as the Si Implantation dose increased, the current also increased significantly. This indicated that the Si replaced the Ga in providing electrons, increasing the carrier concentration and effectively reducing the resistance of β-Ga₂O₃, thus improving the performance of the components. In terms of differentiating the J-V and calculating the R_on,sp, as shown in Figure 7, the R_on,sp of the SBD drops significantly after Si Implantation. Use Equation 1 to perform Fitting on the J-V diagram to calculate the Ideality Factor (η) of the SBD. The closer the η value is to 1, the closer the component’s characteristics are to the ideal formula.  The η of UID β-Ga₂O₃ SBD is 8.05. After Si Implantation, the SBD can effectively reduce series resistance, thus causing the η to drop significantly. As the implantation dose increased, however, the η rose from 1.37 to 2.08. This was because, when the doping concentration (N_D) increased, the tunneling effect occurred, causing the η value to rise.

Figure 6. (a) I-V Diagram of UID β-Ga2O3 SBD; I-V Diagram of Si implantation Doses of (b) 1x1014 cm-2, (c) 6x1014 cm-2, and (d) 1x1015 cm-2   Figure 7. Table of UID and Si Implantation β-Ga2O3 SBD Characteristics

Figure 8 shows the C-V measurements of the β-Ga₂O₃ SBD, which were analyzed using equations 2 to 7 of the semiconductor component physical formulas. It was calculated that the carrier concentrations of the 3 doses (1x10¹⁴, 6x10¹⁴, and 1x10¹⁵ cm^-2) of Si Implantation β-Ga₂O₃ are 1.08x10¹⁷, 4.31x10¹⁸, and 1.07x10¹⁹ cm^-3 respectively. This demonstrates that Si Implantation technology can exert great control over the carrier concentration and conductivity of β-Ga₂O₃. The resistance of  UID β-Ga₂O₃ SBD was too high, exceeding the machine’s analysis limit. Put the calculation results from equations 3 to 7 into equation 2 to calculate the Effective Schottky Barrier Height (ϕ_Bn). The ϕ_Bn of the 3 doses (1x10¹⁴, 6x10¹⁴, and 1x10¹⁵ cm^-2) of Si Implantation β-Ga₂O₃ are 0.82, 0.54, and 0.32V respectively. As the implantation dose increases, the Metal Induced Gap States (MIGS) cause fermi level pinning to become more serious, which in turn causes the ϕ_Bn to decrease and the reverse leakage current to increase. 

Figure 8. (a) UID β-Ga2O3 SBD C-V Diagram; C-V Diagram of Si implantation Doses (b) 1x1014 cm-2, (c) 6x1014 cm-2, and (d) 1x1015 cm-2

As high power components, β-Ga₂O₃ SBDs need to withstand high reverse bias voltages. So we conducted Breakdown voltage measurements on these four components. The Breakdown Voltage (V_BD) of UID β-Ga₂O₃ SBD was over 1030 V. As the Si Implantation dosage increased, the V_BD decreased, indicating the presence of a large number of defects after high dose implantation. As such, further processing and analysis are necessary after implantation to reduce the defect density, reduce the SBD reverse bias leakage current, and improve the V_BD.

Power components need to operate under large currents which generate large amounts of heat, causing component temperatures to rise and component characteristics to change. Therefore, we performed temperature-dependant I-V measurements on the Si Implantation β-Ga₂O₃ SBDs. As shown in Figure 9, as the component temperature increased, the forward current increased, and the on-resistance decreased. This is due to the electronic excitation and transition caused by the thermal lattice vibration effect brought on by the temperature increase, which cause the current to increase. [7]

Figure 9. Si implantation Doses (b) 1x1014 cm-2, (c) 6x1014 cm-2, and (d) 1x1015 cm-2 β-Ga2O3 SBD Temperature-Dependant I-V Diagrams

The E_G of β-Ga₂O₃ can be as high as 4.8 eV, and its BFOM is 3444 times that of Si, making it very suitable for use in power components. However, its high E_G­ means that the resistance of un-doped β-Ga₂O₃ is very high, so it cannot be used in the production of semiconductor components. As such, it requires moderate doping to increase its carrier concentration and reduce its resistance. N-type β-Ga₂O₃ often uses Si, Ge, or Sn as donors to increase the electron concentration. Our team used MOCVD to produce high quality, highly crystalline UID β-Ga₂O₃. Si Implantation can be used to control the doping concentration of the Si, which is conducive to the production of power components. Our team also successfully produced SBDs using Si Implantation β-Ga₂O₃ and used a variety of analytical techniques to explore both the material properties of Si Implantation β-Ga₂O_{3 ­}and the component properties of the SBDs. A portion of the content and data presented in this paper was published in the “Electrical performance study of Schottky barrier diodes using ion implanted Ga2O3 epilayers grown on sapphire substrates”, Materials Today Advanced, 17, 100346, 2023.

Reference: 

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[8] Apoorva Sood, Dong-Sing Wuu, Fu-Gow Tarntair, Ngo Thien Sao, Tian-Li Wu, Niall Tumilty, Hao-Chung Kuo, Singh Jitendra Pratap, Ray-Hua Horng, (2023)“Electrical performance study of Schottky barrier diodes using ion implanted b-Ga2O3 epilayers grown on sapphire substrates”, Materials Today Advanced, 17, 100346.