Demonstration of β-(Al_xGa_1-x)₂O₃/Ga₂O₃ double heterostructure field effect transistors

Beta-phase gallium oxide (β-Ga₂O₃) has emerged as a promising candidate for electronic device applications because of the wide bandgap energy (4.7 eV), the predicted high breakdown field (∼8 MV/cm),¹ and the feasibility in n-type conductivity.^2–4 Specifically, the availability of bulk substrates grown directly from the melt makes it a promising material compared to other conventional wide bandgap materials, such as GaN and SiC.^5,6 Recently, demonstrations of β-Ga₂O₃ field effect transistors^1,7–13 and Schottky diodes^14–19 have been reported. While promising results including high breakdown voltages^8,14 have been achieved, the device performance has been limited by the bulk-doped channels and the low channel mobility typically below 100 cm²/V·s.^1,7–13 These issues greatly limit the applications of β-Ga₂O₃ in both power and high-frequency devices. More recently, delta-doped β-Ga₂O₃ field effect transistor structures^20,21 were demonstrated as a promising candidate for electronic device applications, showing the ability for vertical scaling. Further introduction of β-(Al_xGa_1-x)₂O₃ makes it possible to achieve two-dimensional electron gas (2DEG) through modulation doping.^22–25 Benefiting from the spatial separation between the 2DEG channel and the dopants in the modulation-doped heterostructures, high channel mobilities of 180 cm²/V·s at room temperature and 2790 cm²/V·s at 50 K were achieved.²⁴ This led to the observation of Shubnikov-de Haas oscillations at cryogenic temperatures.²⁴ While the electron mobility was found to be fundamentally limited by optical phonon scattering at room temperature, high 2DEG charge density could enable better screening and therefore enhanced electron mobilities.^26,27 However, the confined 2DEG charge density in the modulation-doped field effect transistors (MODFETs) is limited by the onset of parallel conducting channels in the β-(Al_xGa_1-x)₂O₃ barrier layer.²⁴ In this work, we demonstrate β-(Al_xGa_1-x)₂O₃/Ga₂O₃ modulation-doped double heterostructures that are capable of increasing the confined 2DEG charge density.

The epitaxial stack of the modulation-doped β-(Al_xGa_1-x)₂O₃/Ga₂O₃ double heterostructure studied in this work is shown in Fig. 1(a). The structure was grown using oxygen plasma-assisted molecular beam epitaxy (MBE) on a (010)-oriented Fe-doped semi-insulating β-Ga₂O₃ substrate.²⁸ Prior to the growth, the substrate was exposed to oxygen plasma at 800 °C for 20 min to clean surface impurities.²⁹ Following that, a 300 nm un-intentionally doped (UID) β-Ga₂O₃ buffer layer was grown at 700 °C under slight oxygen-rich growth conditions. This was followed by the growth of 10 nm β-(Al_xGa_1-x)₂O₃ with 1.8 s of Si delta doping sandwiched at the center of this layer, 3 nm β-Ga₂O₃ quantum well (QW), 5 nm β-(Al_xGa_1-x)₂O₃, 3 s of Si delta doping, and 25 nm β-(Al_xGa_1-x)₂O₃ cap. Those layers were grown at 725 °C without any interruption, and the Si cell temperature was kept at 890 °C for both delta-doped layers.²⁰ The detailed growth conditions adopted in this study are similar to previous reports.^20,24 The high resolution X-ray diffraction (XRD) measurement of the (020) planes is shown in Fig. 1(b). The Al composition in the β-(Al_xGa_1-x)₂O₃ layers is estimated to be ∼17% based on the separation between the β-(Al_xGa_1-x)₂O₃ and the β-Ga₂O₃ peaks.³⁰ The diffraction fringes are good indication for sharp heterointerfaces, which are critical for the confinement of 2DEG channels. The atomic force microscopy (AFM) image confirmed the smooth surface after growth with a rms roughness of 0.57 nm as shown in Fig. 1(c).

The energy band diagrams of the modulation-doped structure under the free surface were calculated based on reasonable assumptions of the conduction band offset (ΔE_C = 0.4 eV) and surface pinning level (1.4 eV), while detailed studies are necessary for the extraction of those values. The deep acceptor level of Fe at 0.78 eV below the conduction band minimum was assumed to estimate the Fermi level in the Fe-doped β-Ga₂O₃ substrate.³¹ As shown in the energy band diagrams, electrons populate at two channels in the structure, with a major one in the β-Ga₂O₃ quantum well layer and a low density channel sitting at the bottom (Al_xGa_1-x)₂O₃/Ga₂O₃ interface. However, parallel conducting channels in the β-(Al_xGa_1-x)₂O₃ barrier layers could show up at high impurity doping levels as shown in the band diagrams, and they could lower the apparent channel mobility and impact the device performance.

As depicted in Fig. 1(a), low resistance ohmic contacts were realized through MBE regrowth of degenerately doped β-Ga₂O₃ contact layers with a high Si doping concentration above 10²⁰ cm⁻³.^21,24 Prior to the MBE regrowth, the ohmic contact regions were recessed to a thickness of 55 nm to expose both of the electron channels. This was followed by the growth of 80 nm n⁺-Ga₂O₃ in the ohmic contact regions. The contact regrowth procedure was described in previous reports.^21,24 Ohmic contacts were then achieved through evaporation of a Ti/Au (= 30/130 nm) metal stack and a subsequent rapid thermal annealing at 470 °C for 1 min under N₂ ambient. The device mesa isolation was realized using BCl₃-based plasma etching. A Pt/Au (= 30/130 nm) metal stack was deposited to form Schottky gate contacts for capacitance measurements and MODFET devices. Based on transfer length measurements (TLMs) shown in Fig. 2(a), the sheet resistance and the contact resistivity at room temperature are extracted to be 4.3 kΩ/□ and 1.1 Ω mm, respectively. Ohmic contacts were further confirmed at cryogenic temperatures, which enabled temperature-dependent Hall analysis.

The temperature-dependent Hall measurements of the modulation-doped double heterostructure were carried out using a Van der Pauw configuration with regrown contacts. The measured charge concentration and Hall mobility in the temperature range of 15 K to 300 K are shown in Fig. 2(b). At room temperature, the measured Hall mobility and carrier density are 123 cm²/V·s and 1.14 × 10¹³ cm⁻², respectively, giving a sheet resistance of 4.5 kΩ/□, which matches with the value extracted from TLM measurements. Upon lowering the measurement temperature, the Hall carrier density showed a substantial drop till 70 K and then remained nearly constant at ∼3.85 × 10¹² cm⁻² at lower temperatures. This is different from the case with pure 2DEG, where the electron density is relatively insensitive to temperature,²⁴ and we attribute this to the formation of parasitic conducting channels in the β-(Al_xGa_1-x)₂O₃ barriers. The electron mobility in the parasitic channels drops significantly with reducing temperature because of ionized impurity scattering,^32,33 and therefore, the contribution to the Hall voltage and conductivity of these parasitic channels is expected to be negligible at low temperatures.³⁴ As a result, the total 2DEG density confined in the β-Ga₂O₃ quantum well can be estimated from the low-temperature Hall measurements to be 3.85 × 10¹² cm⁻². This is higher than that can be confined in the modulation-doped single heterostructure without the β-(Al_xGa_1-x)₂O₃ back barrier,²⁴ and it demonstrates that the double heterostructure provides a viable solution to increase the 2DEG charge density.

While the Hall mobility was reduced due to parallel conduction at room temperature, it showed a significant increase with decreasing temperatures. The peak Hall mobility of 1775 cm²/V·s was obtained at 40 K, and it dropped off slightly to 1584 cm²/V·s at 15 K. Similar behavior was observed in single channel modulation doped structures²⁴ and was attributed to background impurity scattering and interface roughness scattering. The Hall mobility values at low temperature are substantially higher than the reported values for bulk-doped β-Ga₂O₃.¹⁵ This is attributed to the spatial separation between the dopant impurities and the 2DEG channels.

Capacitance-voltage (CV) measurements at the excitation frequency of 10 kHz, 100 kHz, and 1 MHz were carried out at room temperature under Pt Schottky contact, as shown in Fig. 3(a). Low hysteresis and frequency dispersion were observed, indicating low density of interface charge or traps. The threshold voltage was extracted to be −6.4 V. The CV characteristics showed a nearly flat capacitance profile, suggesting a high density of electrons accumulated in the β-Ga₂O₃ quantum well channel. The integrated charge density from the flat part of the CV profile (−2.4 V to −4.3 V) is 3.28 × 10¹² cm⁻², which is close to the Hall charge density at low temperature. The further increase in the capacitance from −2.4 V to 0 V is attributed to electron accumulation in the parasitic channel in the β-(Al_xGa_1-x)₂O₃ cap layer.^35–37 The extracted apparent charge profiles from the CV characteristics assuming a dielectric constant of 10 are shown in Fig. 3(b), where a 2DEG channel is evident at 31.0 nm below the top surface. This matches well with the position of the β-Ga₂O₃ quantum well channel and further confirms the formation of high density 2DEG through modulation doping. The difference between the experimental charge profile and the calculated profile [shown as a dashed line in Fig. 3(b)] is attributed to the resolution limits of capacitance-voltage measurements.^38,39

The output and transfer characteristics of the double heterostructure MODFETs were measured as shown in Fig. 4. For the device with a gate length of L_G = 0.6 μm, a gate-drain spacing of L_GD = 2.2 μm, and a source-drain spacing of L_SD = 3.1 μm, a maximum drain current of I_DS = 257 mA/mm was obtained. The drain current showed an on/off ratio of 10⁸, and the subthreshold slope was extracted to be 150 mV/decade. The extracted threshold voltage of −7.0 V is in close agreement with the pinch-off voltage obtained from CV measurements. The transconductance (g_m) showed a peak value of 39 mS/mm at V_DS = 10 V and V_GS = −1.1 V but dropped substantially under more forward gate bias. This is attributed to charge transfer into the parasitic channel in the β-(Al_xGa_1-x)₂O₃ barrier layer. The field effect mobility was extracted from a fat transistor with a gate length/width of L_G/W = 100/100 μm.³⁹ Based on the measurement of the g_m profile under a small drain bias of V_DS = 0.1 V and the CV profile (C_G) under the gate, the field effect mobility can be extracted using $μFE=LGW*gmCG*VDS$⁠, and its dependence on the depletion width is shown in Fig. 4(c). The field effect mobility peaks at 150 cm²/V·s in the β-Ga₂O₃ quantum well layer; however, it shows a significant reduction to 35 cm²/V·s in the β-(Al_xGa_1-x)₂O₃ layers, which could contribute to the reduced Hall mobility.

As shown in Fig. 5, three-terminal off-state breakdown measurements were carried out on the double heterostructure MODFET devices. The devices were biased at V_GS = −20 V and immersed in Fluorinert solution. For the device with a gate-drain spacing of L_GD = 1.55 μm, a breakdown voltage of 428 V was obtained, corresponding to an averaged breakdown field of 2.8 MV/cm. While aggressive lateral scaling of the devices is critical especially for high frequency device applications, the breakdown voltage could be limited by the small gate-drain spacing. Here, the breakdown voltage for a highly scaled gate-drain spacing of 196 nm, as confirmed from scanning electron microscopy (SEM) imaging, was also evaluated as shown in Fig. 5(b). To avoid electric breakdown of the airgap, 150 nm SiO₂ was deposited on the device as a dielectric layer using plasma-enhanced chemical vapor deposition (PECVD). At the gate bias of −20 V, the device showed catastrophic breakdown at V_DS = 42.4 V, corresponding to a breakdown voltage of 62.4 V. This translates to an average breakdown field of 3.2 MV/cm. The off-state current in both devices is dominated by the gate leakage current prior to the catastrophic breakdown, indicating the breakdown of the Schottky diode at the gate edge on the drain side. Further improvements in the breakdown voltage are expected through the incorporation of edge termination techniques. This demonstrates the feasibility for aggressive device scaling while maintaining high power output using β-Ga₂O₃ transistors.

In summary, a modulation-doped β-(Al_xGa_1-x)₂O₃/Ga₂O₃ double heterostructure was demonstrated using Si delta-doping in the barrier layers with an Al composition of 17%. Even though parasitic conduction was observed through the temperature dependent Hall measurement, a high Hall mobility of 123 cm²/V·s was measured for the effective carrier density of 1.14 × 10¹³ cm⁻² at room temperature. The confined 2DEG charge density was estimated from the low temperature Hall measurement to be 3.85 × 10¹² cm⁻², which is higher than what can be achieved in a single heterostructure with a similar Al composition in the barrier layer. This provides a viable solution to increase the carrier density of the 2DEG in the modulation doped structures without the introduction of the parasitic channel. The existence of the 2DEG channel was further confirmed from the CV measurement. The double heterostructure MODFETs showed a maximum drain current of I_DS = 257 mA/mm, a peak g_m of 39 mS/mm, and a pinch-off voltage of −7.0 V. The average breakdown field was estimated to be 3.2 MV/cm for the device with a small gate-drain spacing of 196 nm based on the three-terminal off-state breakdown measurement. The demonstrated modulation-doped β-(Al_xGa_1-x)₂O₃/Ga₂O₃ double heterostructure field effect transistor could be a promising candidate for high power and high frequency device applications.

The project or effort depicted was or is sponsored by the Department of the Defense, Defense Threat Reduction Agency (Grant No. HDTRA11710034). The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. We acknowledge funding from The Ohio State University Institute of Materials Research (IMR) Multidisciplinary Team Building Grant.