Abstract
1. Introduction
III-oxide compound semiconductors hold great potential to pioneer new semiconductor-based technologies. This material system, including gallium oxide (Ga2O3) and aluminum oxide (Al2O3), has a bandgap range of 4–8 eV which is much larger than that of conventional semiconductors such as silicon (1.12 eV) and gallium arsenide (1.42 eV) and even wider bandgap (WBG) semiconductors such as GaN (3.4 eV) and 4H-SiC (3.25 eV). Among these III-oxide semiconductors, beta-phase Ga2O3 (β-Ga2O3) has garnered considerable attention for various optoelectronic and electronic applications[
Another advantage of β-Ga2O3 is the availability of cost-effective single-crystal substrates[
As shown in Fig. 1, β-Ga2O3 devices are capable of dramatically enhancing the efficiency of power electronics system by reducing the on-resistance. To access this low on-resistance region, high doping concentrations of β-Ga2O3 are needed. Currently, the majority of the β-Ga2O3 devices have a doping concentration below mid 1017 cm−3 range and the on-resistance is significantly higher than the theoretical limit[
Figure 1.(Color online) Theoretical benchmark plot of on-resistance versus breakdown voltage for power devices based on
2. Material characterization and device fabrication
The β-Ga2O3 single crystal substrates were purchased from the Tamura Corporation. The wafers were grown by the EFG method. Ga2O3 and tin oxide (SnO2) powder were used as the source material and the precursor for n-type Sn dopants, respectively. More details about the growth process can be found elsewhere[
The SBDs were then fabricated on the β-Ga2O3 substrate using standard photolithography. Before the metal depositions, the sample was cleaned in acetone and isopropyl alcohol (IPA) under ultrasonic to remove possible organic contaminants on the surface. For the ohmic contacts of the SBDs, Ti/Al/Ti/Au metal stacks were deposited on the Ga2O3 substrate using electron beam evaporation and subsequently annealed at 470 °C for 1 min in nitrogen ambient using rapid thermal annealing (RTA). For the Schottky contacts, Pt/Au metal stacks were deposited by electron beam evaporation. Fig. 2(c) shows the schematic top view and cross-section view of the β-Ga2O3 SBDs. The diameters of the left ohmic contact and right Schottky contact are 400 and 200 μm, respectively. The distance between the ohmic and Schottky contacts varied from 50 to 500 μm. A Keithley 2410 source meter and 4200-SCS parameter analyzer on a probe station with a controllable thermal chuck were used for the electrical measurement.
Figure 2.(Color online) (a) The rocking curve of the
3. Results and discussions
3.1. Forward J–V characterization and C–V characterization
Fig. 3 shows the temperature-dependent forward I–V characteristics of the β-Ga2O3 SBDs from 300 to 480 K (with steps of 20 degrees) in both linear scale and log scale. The upper current limit of the setup is 0.1 A. A high on/off ratio of ~109 was observed. To further discuss the electrical properties, on-resistance and turn-on voltage were then extracted. In Fig. 4(a), the on-resistance of the (
Figure 3.(Color online) Temperature-dependent forward
Figure 4.(Color online) (a) Comparision of on-resistance of previously reported
The current–voltage characteristics of the SBDs can be studied from the thermionic emission model[
where J is the current density, A* is the Richardson constant (for β-Ga2O3, it is calculated to be 41.1 A/cm2K2 using an effective electron mass of 0.34m0 and the equation
Fig. 5 shows the extracted ideality factor and the barrier height as a function of temperature. The values of n and Js are extracted from the measured J–V data in forward bias from 0.3 to 0.5 V. If the Schottky barrier is homogeneous and the thermionic emission model is valid, then the ϕeff should be temperature independent, ideality factor should be 1, and the y-intercept of ln(Js/T2) versus 1000/T graph should retrieve A*. However, the results obtained from Fig. (5) do not match this assumption. In Fig. 5(a), when the temperature is increased from 300 to 480 K, the ideality factor decreases from 1.39 to 1.13, and the Schottky barrier height increases from 0.94 to 1.10 eV. The correlation between the ideality factor and Schottky barrier height can be further observed by a well know linear relationship[
where ϕb0 and σ0 are the values at zero bias. The ideality factor thus becomes temperature-dependent and its value can exceed unity:
Fig. 5(c) shows the linear relationship of both n−1−1 and ϕeff with respect to 1000/T. The coefficients γ and ξ represent the voltage-induced deformation of the Schottky barrier distribution. Note that γ < 0 and ξ > 0, meaning larger voltage can decrease the mean Schottky barrier height and reduce the inhomogeneity of the barrier distribution, respectively [
Moreover, we can then combine the modified Schottky barrier Eq. (4) into the original thermionic emission Eq. (1) and obtain the following modified thermionic emission equation:
A modified Richardson plot using Eq. (8) is shown in the red line of Fig. 5(d). Form the fitting data (dashed line), the modified Richardson constant A* is ~41.26 Acm−2K−2 and
Figure 5.(Color online) (a) Ideality factor and Schottky barrier height as a function of temperature from 300 to 480 K. (b) Ideality factor versus Schottky barrier height. (c) Plot of effective barrier height and
Fig. 6(a) shows the C–V characteristics of the Ga2O3 SBDs at a frequency of 1 MHz at room temperature. By plotting the 1/C2 versus V in Fig. 6(b) and extracting the slope, the doping concentration can be calculated using the following equations[
Figure 6.(Color online)
where ε0 is vacuum permittivity, εr is the relative permittivity of β-Ga2O3, and Vbi is the built-in voltage. The doping concentration of the β-Ga2O3 substrate is about (2.9–3.5) ×1018 cm−3, which is a relatively high doping concentration. This result indicates that this sample is less resistive and explains the low on-resistance of the devices.
3.2. Reverse J–V characterization and the study of the surface current leakage
The temperature-dependent reverse J–V characteristics of the β-Ga2O3 SBDs is presented in Fig. 7. Note that the breakdown voltages were relatively low due to the high doping concentration of the substrates. The reverse current increased with the temperature, increasing from 300 to 480 K. Due to the ultra large bandgap of β-Ga2O3, the thermionic emission current over the barrier is very small compared to the measured current levels, hence it will be neglected in the following discussion[
Figure 7.(Color online) Temperature-dependent reverse
The second model is the one-dimensional variable-range-hopping conduction (1D-VRH) model[
where T0 is a characteristic temperature. Fig. 8(b) demonstrated the relationship of measured conductivity in log scale as a function of T−0.5 from 300 to 480 K. The good fitting between experiment data and fitting data suggest that both models play important roles in the reverse leakage mechanism. Further investigations are needed to decouple the two mechanisms and identify the primary mechanism.
Figure 8.(Color online) (a) Arrhenius plot of reverse leakage currents of the
Fig. 9 shows the leakage current for β-Ga2O3 SBDs as a function of contact distance from 50 to 500 μm at voltages of −6, −7, and −8 V. The reverse current decreased as the contact distance increased. This trend is opposite from the observation of some other material system such as AlN on sapphire[
Figure 9.(Color online) Leakage current as a function of contact distance between ohmic and Schottky contacts at different reverse voltages.
4. Conclusions
Lateral SBDs fabricated on highly doped β-Ga2O3 substrates by the EFG method were presented. The temperature-dependent J–V and C–V characteristics were analyzed rigorously. The C–V measurement indicated a high doping concentration of (2.9–3.5) × 1018 cm−3 of the β-Ga2O3 substrate. At the forward bias under room temperature, the SBDs exhibited a good rectifying behavior. At room temperature, the devices had a turn-on voltage of ~0.84 V, an on-resistance of ~0.9 mΩ·cm2, an on/off ratio of ~109, an ideality factor of 1.39, and a Schottky barrier height of 0.94 eV, respectively. In addition, the ideality factor showed a negative temperature dependence, and the Schottky barrier height had a positive temperature dependence. This is due to the inhomogeneous Schottky barrier interface caused by defects. Using the modified thermionic emission with the inhomogeneous Schottky barrier considered, the modified Richardson constant was found to be ~41.26 Acm−2K−2 and the mean Schottky barrier height ~1.36 eV. At the reverse bias, the device showed a relatively low breakdown voltage because of its high doping concentration. Two models including the two-step trap-assisted model and 1D-VRH model were used to fit the reverse leakage currents. Both play important roles in the reverse leakage current mechanism. The leakage current had a distinctive negative distance dependence, indicating it is bulk leakage dominated.
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