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Keywords:

  • field-effect transistors;
  • Ga2O3;
  • MESFET;
  • molecular beam epitaxy;
  • power devices;
  • Schottky barrier diodes

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

Gallium oxide (Ga2O3) is a strong contender for power electronic devices. The material possesses excellent properties such as a large bandgap of 4.7–4.9 eV for a high breakdown field of 8 MV cm−1. Low cost, high volume production of large single-crystal β-Ga2O3 substrates can be realized by melt-growth methods commonly adopted in the industry. High-quality n-type Ga2O3 epitaxial thin films with controllable carrier densities were obtained by ozone molecular beam epitaxy (MBE). We fabricated Ga2O3 metal-semiconductor field-effect transistors (MESFETs) and Schottky barrier diodes (SBDs) from single-crystal Ga2O3 substrates and MBE-grown epitaxial wafers. The MESFETs delivered excellent device performance including an off-state breakdown voltage (Vbr) of over 250 V, a low leakage current of only few μA mm−1, and a high drain current on/off ratio of about four orders of magnitude. The SBDs also showed good characteristics such as near-unity ideality factors and high reverse Vbr. These results indicate that Ga2O3 can potentially meet or even exceed the performance of Si and typical widegap semiconductors such as SiC or GaN for ultrahigh-voltage power switching applications.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

The worldwide quests for stable energy supplies and reduced greenhouse gas emissions in the near future have fueled demands for new energy sources to replace fossil fuels as well as ideas for revolutionary technologies to realize efficient energy generation and utilization. Power devices based on wide-bandgap semiconductors such as SiC and GaN are capable of delivering higher breakdown voltage (Vbr) and lower loss than Si devices, and have been intensively studied as alternative technologies for efficient power switching. However, both SiC and GaN power devices are unamenable to mass production since high-quality substrates are expensive, leaving ample room for new materials to enter the market. A new oxide semiconductor – gallium oxide (Ga2O3) – turns out to be an ideal material for power devices in ultrahigh-voltage switching applications. The superior material properties of Ga2O3, including a bandgap much larger than those of SiC and GaN, promise power devices with even higher Vbr and efficiency than their SiC and GaN counterparts. The other important feature of Ga2O3 is that native substrates can be fabricated from bulk single crystals synthesized by the same melt-growth methods employed for manufacturing sapphire substrates. As large and high-quality sapphire substrates are now being manufactured in a low-cost commercial process in numbers rivaling those for Si, it can be expected that melt-grown Ga2O3 substrates will reap the same benefits. Nevertheless, research and development (R&D) on Ga2O3 devices has lagged since most researchers have failed to exploit the material's excellent properties. With the vision that Ga2O3 power devices can potentially surpass SiC and GaN in not only device performance but also cost effectiveness, we began pioneering work on Ga2O3 power devices in 2010. We have already developed various elemental technologies and achieved several important milestones, most notably the world's first demonstration of single-crystal Ga2O3 transistors [1]. In this paper, we introduce the current status of our activities on Ga2O3 power devices and the future prospects of this new technology.

2 Crystal structure and material properties of β-Ga2O3

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

Ga2O3 crystals exhibit polytypism with five confirmed polytypes (α, β, γ, δ, ϵ). The β-polytype shown in Fig. 1 is believed to be the most stable, while the other polytypes are metastable. β-Ga2O3 crystallizes into the β-gallia structure belonging to the monoclinic system. The bandgap of β-Ga2O3 is 4.7–4.9 eV [2-4]. Semi-insulating Ga2O3 can be controllably doped with Sn or Si to obtain electron densities (n) from 1016 to 1019 cm−3 [5-8]. In contrast, there has been no clear evidence of hole conduction in Ga2O3. The extrapolated experimental bulk electron mobility of Ga2O3 reaches a relatively high value of about 300 cm2 V−1 s−1 for n = 1015–1016 cm−3, which is typical for the drift layers of vertical power transistors and diodes [9]. This relatively high mobility is in agreement with theoretical calculations showing that the electron effective mass of Ga2O3 is 0.23–0.34m0 (m0: free electron mass), which is comparable to those of conventional widegap (3–4 eV) semiconductors [4, 7, 10].

image

Figure 1. Atomic unit cell of β-Ga2O3.

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3 Performance prospect of Ga2O3 power devices

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

With a bandgap wider than those of SiC and GaN, β-Ga2O3 promises power transistors and diodes with excellent characteristics including high Vbr, high power capacity, and low loss (i.e., high efficiency). Table 1 compares the important material properties of β-Ga2O3 with those of major semiconductors. The estimated breakdown electric field of Ga2O3 is 8 MV cm−1, which is three times larger than that of either SiC or GaN. This high breakdown field is the most attractive attribute of Ga2O3 for power devices, because Baliga's figure of merit (FOM) – the basic parameter to show how suitable a material is for power devices – is proportional to the cube of the breakdown field, but only linearly proportional to mobility. The theoretical limits of on-resistances as a function of Vbr for Ga2O3 and representative semiconductors as plotted in Fig. 2 suggest that the on-resistance of Ga2O3 devices can be one order of magnitude lower than those of SiC and GaN devices at the same Vbr. The thermal conductivity of Ga2O3 strongly depends on the crystal orientation due to its asymmetric crystal structure. In our experiments, the [010] direction has the highest value of 0.23 W cm−1 K−1, which is about twice as large as that in the [100] direction [11]. However, it is still much smaller than those of the other semiconductors and thus a clear weak point of Ga2O3 in terms of power device applications.

Table 1. Material properties of major semiconductors and β-Ga2O3
 SiGaAs4H-SiCGaNdiamondβ-Ga2O3
bandgap Eg (eV)1.11.43.33.45.54.7–4.9
electron mobility μ (cm2 V−1 s−1)14008000100012002000300
breakdown field Eb (MV cm−1)0.30.42.53.3108
relative dielectric constant ϵ11.812.99.79.05.510
Baliga's FOM inline image11534087024 6643444
thermal conductivity (W cm−1 K−1)1.50.552.72.1100.23 [010]
      0.13 [100]
image

Figure 2. Benchmarking the theoretical ideal performance limits of β-Ga2O3 power devices against other major semiconductors.

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4 Production of single-crystal Ga2O3 substrates

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

The superiority of Ga2O3 devices for mass production stems from the availability of affordable native substrates fabricated from melt-grown bulk crystals at low cost and with low energy consumption [12-14]. A melt-growth technology known as edge-defined film-fed growth (EFG) has a good track record of producing large sapphire wafers of over 6 inch in diameter. This method will be especially useful for high-volume production of Ga2O3 substrates using the same system configuration as for sapphire growth since it does not require a high-temperature or high-pressure environment, conserves source materials, and is easily scalable to large wafer diameters. In contrast, SiC and GaN bulk crystals and substrates are produced by energy-intensive and cost-prohibitive methods such as sublimation, vapor phase epitaxy, and high-pressure synthesis [15-17]. Figure 3 shows a 2-inch-diameter single-crystal Ga2O3 wafer produced in our laboratory. The crystal quality of the Ga2O3 wafer is very good, with a full-width at half-maximum of the X-ray diffraction rocking curve as narrow as 19 arcsec and a dislocation density on the order of 104 cm−2 as characterized by surface etch pits. The surface of the wafer was atomically flat after chemical-mechanical polishing with a small root-mean-square (RMS) surface roughness of 0.11 nm. Undoped Ga2O3 substrates show n-type conductivity due to unintentional Si incorporation from the Ga2O3 powder source (5 N). Compensation doping with deep acceptors such as Mg renders these materials semi-insulating.

image

Figure 3. Photograph of 2-inch-diameter single-crystal Ga2O3 wafer.

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5 Epitaxial growth of Ga2O3 thin films by ozone MBE

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

We have developed homoepitaxy of Sn-doped n-type β-Ga2O3 on native substrates by molecular-beam epitaxy (MBE) [9, 18]. The substrates used were unintentionally n-doped β-Ga2O3 (010) fabricated from a bulk synthesized by the floating-zone (FZ) method. Ga and Sn (n-type dopant) were, respectively, supplied from 7 N Ga metal and 4 N SnO2 powder heated in conventional Knudsen cells (K-cells). The oxygen source was an ozone(5%)–oxygen(95%) gas mixture. Growth temperatures ranging from 500 to 800 °C were attempted to investigate their effects on surface morphology and doping. The growth rate and film thickness were 600 nm h−1 and 600 nm, respectively.

The surface morphologies of the epitaxial films were evaluated by atomic force microscopy (AFM). Figure 4(a) shows the AFM images of Ga2O3 epitaxial films grown at various temperatures. Step bunching along the [100] direction is observed for growth temperatures higher than 700 °C, and the effect becomes more pronounced with increasing growth temperatures. On the other hand, a low growth temperature of 500 °C results in a rough surface due to three-dimensional growth. The RMS surface roughness values of the Ga2O3 epitaxial films estimated from 1 × 1 μm2 AFM scans are plotted as a function of growth temperature in Fig. 4(b). The smoothest films were grown at 550–650 °C.

image

Figure 4. (a) Surface morphologies of MBE Ga2O3 thin films grown at various temperatures as observed by AFM and (b) RMS surface roughness of the Ga2O3 film as a function of growth temperature.

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Electrochemical capacitance–voltage (CV) measurements were employed to evaluate the dependence of effective donor concentration (NdNa) on growth parameters. Figure 5(a) shows the NdNa depth profiles in three Sn-doped Ga2O3 epitaxial films grown at 540, 570, and 600 °C, respectively. The NdNa in Sn-doped Ga2O3 as a function of SnO2 K-cell temperature is shown in Fig. 5(b). Uniform doping concentrations along the growth direction that corresponded to the SnO2 vapor pressure were achieved for the samples grown at 540 and 570 °C. In contrast, a delay in doping at the initial stage of the 600 °C growth due likely to Sn segregation was revealed in Fig. 5(a), causing the lower-than-expected average NdNa in the sample according to Fig. 5(b). These results indicate that a growth temperature below 570 °C is necessary for accurate control of carrier density.

image

Figure 5. (a) Depth profiles of NdNa in Sn-doped Ga2O3 epitaxial films grown at various temperatures estimated by electrochemical CV measurements and (b) NdNa of Sn-doped Ga2O3 films as a function of SnO2 K-cell temperature.

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We have optimized the structural and electrical properties of MBE-grown Sn-doped β-Ga2O3 homoepitaxial films. High-quality epilayers with both smooth surfaces and uniform doping profiles were obtained at growth temperatures of 540–570 °C.

6 Ga2O3 MESFETs

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

Transistor action for single-crystal Ga2O3 devices was demonstrated using simple metal-semiconductor field-effect transistor (MESFET) structures [1]. Sn-doped n-Ga2O3 MESFET channel layers were grown on Mg-doped semi-insulating β-Ga2O3 (010) FZ substrates by ozone MBE. Figure 6(a) shows a cross-sectional schematic illustration of the n-Ga2O3 MESFET. To form Ohmic contacts, reactive ion etching (RIE) using a gas mixture of BCl3 and Ar was first performed to reduce the contact resistance substantially, followed by evaporation of Ti/Au. Then, Schottky gates were fabricated by Pt/Ti/Au deposition and lift off. The surface was left unpassivated. The gate length, source-drain spacing, and diameter of the inner circular drain electrode were 4, 20, and 200 μm, respectively. We employed a circular FET pattern as shown in the optical micrograph of a fabricated device in Fig. 6(b), since a device isolation technique has not yet been developed.

image

Figure 6. (a) Cross-sectional schematic illustration and (b) optical micrograph of Ga2O3 MESFET.

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Figure 7(a) and (b) show the DC output current–voltage (IV) and transfer characteristics, respectively, of the Ga2O3 MESFET. The drain current (Id) was effectively modulated by a gate voltage (Vg) with a sharp pinch-off characteristic. The device exhibited a maximum Id of 16 mA at a drain voltage (Vd) of 40 V and a Vg of +2 V. The three-terminal destructive breakdown in the off-state, which resulted in burned gate electrodes, occurred at a Vd of over 250 V. Transconductance peaked at 1.4 mS for Vd = 40 V. The off-state drain leakage current (Ioff) was as small as 3 μA, resulting in a high Id on/off ratio of about 104. The Ioff was comparable to the gate leakage current, indicating that leakage through the semi-insulating Ga2O3 substrate was negligible. Furthermore, most gate leakage current could be attributed to the large 100-μm-diameter gate pad [Fig. 6(b)], and the actual leakage through the gate finger should be lower by at least one order of magnitude. Therefore, the Ioff can be effectively suppressed simply by optimizing the device configuration. While our first Ga2O3 MESFETs are inferior to the state-of-the-art SiC and GaN devices, their performance was comparable to or better than that of GaN MESFETs in the early 1990s [19, 20]. The high Vbr and low leakage current of these transistors indicate their great potential as power devices.

image

Figure 7. (a) DC output IV curves and (b) transfer characteristics of Ga2O3 MESFET.

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7 Ga2O3 SBDs

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

Another important component for power electronics is the Schottky barrier diode (SBD) [21]. We fabricated simple SBDs on unintentionally n-doped single-crystal β-Ga2O3 FZ substrates with a thickness of 600 μm. The n was uniform along the thickness of the substrate but showed in-plane variation from 3 × 1016 to 1 × 1017 cm−3 as evaluated by CV measurements. Figure 8 shows a cross-sectional schematic illustration of the Ga2O3 SBD structure. Circular Schottky contacts with a diameter of 100 μm were fabricated on the front side of the substrate as anode electrodes by standard photolithographic patterning, Pt/Ti/Au evaporation, and lift off. The cathode electrode (Ti/Au) was evaporated onto the backside of the substrate following an RIE treatment using a mixture of BCl3 and Ar gases to decrease the Ohmic contact resistance.

image

Figure 8. Cross-sectional schematic illustration of Ga2O3 SBD.

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Figure 9(a) and (b) plot the forward current density–voltage (JV) characteristics of two different Ga2O3 SBDs fabricated at different locations on the same substrate with n = 3 × 1016 and 5 × 1016 cm−3, respectively. Note that the J value simply corresponds to the current divided by the anode electrode area. The near-unity ideality factors of 1.04–1.06 indicated the high crystal quality of the Ga2O3 substrate and good Schottky interface property. A Schottky barrier height of 1.3–1.5 eV was extracted for the Pt/β-Ga2O3 interface. Due to the low unintentional n and hence low substrate conductivity, the on-resistances (Ron) of the Ga2O3 SBDs, which was determined from the slope of the linear regions in Fig. 9(a), were relatively high at 7.85 and 4.30 mΩ cm2 when compared with those of state-of-the-art SBDs based on other semiconductors. However, they can be readily improved by incorporating an n+-Ga2O3 contact layer. Figure 9(c) shows the reverse JV characteristics of the Ga2O3 SBDs. The reverse Vbr values were about 150 and 115 V for n = 3 × 1016 and 5 × 1016 cm−3, respectively, which were reasonably high for these carrier densities considering the lack of surface passivation or edge termination.

image

Figure 9. (a, b) Forward (linear and semilog plot, respectively) and (c) reverse JV characteristics of Ga2O3 SBDs.

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8 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

We propose a new oxide compound semiconductor Ga2O3 as a promising candidate for power device applications because of its excellent material properties and suitability for mass production. Homoepitaxy of n-type Ga2O3 thin films on β-Ga2O3 (010) substrates by MBE with precise control of carrier density over the range of 1016–1019 cm−3 was demonstrated. Ga2O3 MESFETs and SBDs were fabricated on single-crystal β-Ga2O3 substrates. The MESFETs showed excellent device performance such as a three-terminal off-state Vbr of 250 V and an Id on/off ratio of about four orders of magnitude. The SBDs also exhibited good characteristics such as near-unity ideality factors and high reverse Vbr. These results indicate that Ga2O3 can potentially meet or even exceed the performance of Si and typical widegap semiconductors such as SiC or GaN for ultrahigh-voltage power switching applications.

The R&D on Ga2O3 power devices is at its early stage. Large-area Ga2O3 wafers of over 4 inch in diameter are yet to be produced. Practical power switching equipment is best designed using more advanced transistors architectures such as normally-off vertical devices, which calls for further developments in epitaxial growth, doping, and device processing technologies. Our pioneering development of Ga2O3 transistors and diodes paves the way for new high-performance devices that will advance the power semiconductor industry and reduce global energy consumption.

Acknowledgement

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References

This work was partially supported by “The research and development project for innovation technique of energy conservation” of the New Energy and Industrial Technology Development Organization (NEDO). It also made use of a research grant from the PRESTO program of the Japan Science and Technology Agency (JST).

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Crystal structure and material properties of β-Ga2O3
  5. 3 Performance prospect of Ga2O3 power devices
  6. 4 Production of single-crystal Ga2O3 substrates
  7. 5 Epitaxial growth of Ga2O3 thin films by ozone MBE
  8. 6 Ga2O3 MESFETs
  9. 7 Ga2O3 SBDs
  10. 8 Conclusions
  11. Acknowledgement
  12. References