Owing to the enormous potential in high power and high frequency applications, the GaN based high electron mobility transistors (HEMTs) have attracted tremendous attention in the past two decades.^[1–3] One of the core issues in recent research is the realization and optimization of the enhancement-mode (E-mode) devices with positive threshold voltage (V_TH), which is crucial for the safe operation in the power electronic applications and various monolithic integrated digital circuits.^[4] Various methods have been tested to achieve the E-mode HEMTs, which include the gate recess,^[5,6] fluorine plasma ion implantation,^[7–9] and P-type GaN gate.^[10–12] However, all of these techniques require additional procedures during the device fabrication processes, which may adversely affect the technical repeatability and the device reliability.

Recently, the employment of quaternary InAlGaN/GaN heterostructure was proposed as a novel approach to fabricate the E-mode HEMTs, which only relies on the material growth techniques and is easy to control.^[13–17] By adjusting the alloy composition, the InAlGaN barrier can be polarization matched with the GaN channel, and thus the sheet carriers in the channel are depleted at zero gate bias. Jena et al. have calculated the polarization matched conditions of the InAlGaN/GaN heterostructures, which show great design freedom of this system.^[18] In addition, the previous reports revealed that the adoption of the InAlGaN barrier is beneficial to suppress the gate leakage and current collapse effects.^[19] However, some challenges still exist in current research. On one hand, the InAlGaN barrier with low indium content may not accomplish the polarization matched condition with the GaN channel, and leads to V_TH of the devices lower than 1 V which cannot meet the requirement of 3 V or even higher for practical applications. On the other hand, the current output characteristics of the InAlGaN based E-mode devices with the saturation drain current density below 54 mA/mm are unsatisfactory.^[13–17]

The main obstacle of the research lies in the difficulties in the growth of high quality InAlGaN material, as well as accurate control of the close polarization matched InAlGaN/GaN heterostructures. Recently, we proposed a pulsed metal organic chemical vapor deposition (MOCVD) growth technique for the epitaxy of InAlN and InGaN based heterostructures which possess excellent structural and transport properties.^[20–25] Moreover, the thickness and alloy component of the epitaxial film can be accurately controlled in the pulsed MOCVD process. These results suggest that pulsed MOCVD could be a promising method to address the issues in the growth of high quality InAlGaN based heterostructures. In this work, the nearly polarization matched InAlGaN/GaN heterostructures are grown using pulsed MOCVD technique and characterized with different measurements, and the fabrication of E-mode InAlGaN/GaN metal–oxide–semiconductor HEMTs (MOS-HEMTs) with enhanced V_TH and output performance is demonstrated.

InAlGaN/GaN heterostructures were grown on 2-inch sapphire (0 0 0 1) substrates in a vertical low pressure MOCVD system. Trimethylindium (TMIn), trimethylaluminum (TMAl), triethylgallium (TEGa), and ammonia (NH₃) were used as the precursors of In, Al, Ga, and N, respectively. The growth was initiated with a 20 nm low-temperature AlN nucleation layer and a 60 nm high-temperature AlN nucleation layer. The growth temperatures were 620 °C and 1070 °C, respectively. Then, GaN buffer layer was grown at 940 °C, followed by a 2 nm AlN interlayer. Finally, the growth temperature was decreased to 700 °C for growing a 18 nm InAlGaN barrier, which was carried out by using the pulsed MOCVD technique. As shown in Fig. 1, the unit pulses of 12-s TMIn, 6-s TMAl, 6-s TEGa, and 6-s NH₃ were introduced into the reactor successively, and the metal organic pulses were always followed by the NH₃ pulses. During the pulsed MOCVD process, the growth of InAlGaN barrier was actually achieved as a series of InN/AlN/GaN short period superlattice structures. The pulsed growth method can effectively reduce the defects induced by the parasitic clathrate generated in the pre-reaction, and it is also beneficial to increase the adatom migration and thus improve the growth homogeneity.^[20–25]

Fig. 1.

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	Fig. 1. Growth sequence of pulsed MOCVD for the InAlGaN barrier layer.

Several measurements were performed to investigate the material quality of the InAlGaN/GaN heterostructures. Figure 2 shows the high-resolution x-ray diffraction (HRXRD) ω–2θ scan of the InAlGaN/GaN heterostructures from the symmetric (0 0 0 6) reflection. Three dominant peaks are located at around 126.0°, 127.1°, and 137.3°, which correspond to the GaN buffer, InAlGaN barrier, and AlN nucleation layers, respectively. No additional diffraction peak can be observed, suggesting the good crystal quality of the quaternary InAlGaN materials without phase separation. In addition, the scanning electron microscope (SEM) measurement was also performed to investigate the surface morphology of the InAlGaN/GaN heterostructures. As shown in Fig. 3(a), the surface of the InAlGaN barrier is uniform and smooth, and there is no obvious macroscopic defect on it. The more detailed morphology feature was characterized by the atomic force microscopy (AFM) and the result is shown in Fig. 3(b). Clearly an atomic terrace with a few micro-pits can be observed, indicating a two-dimensional step-flow growth mode. Furthermore, the root mean square (RMS) roughness is as low as 0.29 nm in a scan area. The surface morphology of the InAlGaN material in our experiment is much better than the previously reported one grown by the standard MOCVD process,^[26] suggesting that pulsed MOCVD is a promising method to overcome the difficulties in the growth of high quality InAlGaN films.

Fig. 2.

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	Fig. 2. HRXRD (0 0 0 6) 2θ–ω scan of the InAlGaN/GaN heterostructures grown by pulsed MOCVD.

Fig. 3.

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	Fig. 3. (a) SEM and (b) AFM surface image of the InAlGaN/GaN heterostructures grown by pulsed MOCVD.

The precise control of the alloy composition is a key factor for the growth of InAlGaN/GaN heterostructures. Secondary ion mass spectroscopy (SIMS) was employed to accurately investigate the alloy composition of the InAlGaN barrier. Figure 4 illustrates the variation of the relative atom fractions of In, Al, and Ga with the depth from surface. It can be observed that the element content of the InAlGaN barrier layer is stable and uniform, with the In, Al, and Ga concentrations of 17%, 42%, and 41%, respectively. It can be observed that Al atoms slightly diffuse into the GaN channel layer, which may affect the transport properties of the heterostructures due to the enhanced alloy disorder scattering. This phenomenon can be suppressed by the optimization of the growth process, such as decreasing the growth rate and adding pause time between different layers. According to the previously reported results,^[18] the In_0.17Al_0.42Ga_0.41N barrier is nearly polarization matched with the GaN channel. The channel two-dimensional electron gas (2DEG) density of the In_xAl_yGa_(1−x−y)N/GaN system can be calculated based on the model proposed by Ambacher,^[27,28] and the result is shown in Fig. 5. It can be observed that the 2DEG density is very sensitive to the alloy composition of the InAlGaN barrier, and the corresponding result in our experiment is 2.0 ×10¹² cm⁻², which is much lower than that of the conventional depletion-mode heterostructures. The Hall effect measurement with the van der Pauw configuration was also carried out to further investigate the transport properties of the InAlGaN/GaN heterostructures. The 2DEG density in the channel layer is determined to be 1.8×10¹² cm⁻², which shows a good agreement with the calculated result. Meanwhile, the electron mobility is as high as , and this result is much better than the previous ones.^{[13,15,17,19]} Both of the calculation and experiment results indicate that the InAlGaN/GaN heterostructures achieve the nearly polarization matched condition, and the superior transport properties lay a good foundation for the E-mode MOS-HEMTs fabrication.

Fig. 4.

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	Fig. 4. SIMS depth profiles of In, Al, and Ga in the InAlGaN/GaN heterostructures grown by pulsed MOCVD.

Fig. 5.

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	Fig. 5. Calculated 2DEG density of the InxAlyGa(1−x−y)N/GaN heterostructures with different alloy composition in the barrier layer.

Figure 6 shows the schematic cross-section view of the InAlGaN/GaN MOS-HEMTs, which were prepared on the as-grown nearly polarization matched heterostructures. The fabrication procedures of the MOS-HEMTs can be summarized as follows. The Ti/Al/Ni/Au (220 Å/1400 Å/550 Å/450 Å) metal stacks were firstly deposited and annealed at 85 °C for 30 s to form the Ohmic contacts. Then, the mesa isolation was achieved by etching to a depth of 300 nm with the inductively coupled plasma in a Cl₂-based chemistry, followed by a 3 nm Al₂O₃ gate dielectric layer. The gate dielectric is crucial to suppress the gate leakage voltage of the E-mode devices, which usually works in the positive gate bias conditions. Finally, a gate area with length (L_G) of was defined by the photolithograph, and a Ni/Au (450 Å/2000 Å) gate electrode was deposited. The gate-source and gate-drain distances are and , respectively.

Fig. 6.

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	Fig. 6. Cross-section of the InAlGaN/GaN MOS-HEMTs.

The output and transfer characteristics of the MOS-HEMTs are shown in Fig. 7. The device exhibits a maximum drain current density (I_D,max) of 669 mA/mm at V_GS = 6 V and V_DS = 7 V. The drain current I_DS slightly decreases with the increase of V_DS when , which is related to the self-heating effects. By the method of extracting the x-axis intercept of the linear extrapolation of the I_DS curve at V_GS (G_m,max), the V_TH of the devices is determined to be 3.1 V. This is the first report of V_TH higher than 3 V for the quaternary InAlGaN based HEMTs, demonstrating their practical application value. The performance of our E-mode MOS-HEMTs is benchmarked against some state-of-the-art InAlGaN based E-mode devices in the plot of V_TH versus I_D,max in Fig. 8. Our InAlGaN/GaN MOS-HEMTs grown by pulsed MOCVD possess the best V_TH–I_D,max performance among the InAlGaN based E-mode devices, showing the great potential for power electronic applications. Moreover, the output characteristics available are somewhat conservative mainly resulting from the rather high Ohmic contact resistance of , which results from the large bandgap of the InAlGaN barrier and the long gate length. Further improvement of device performance can be expected by optimizing the Ohmic contact and adopting advanced device processing technology like the electron beam lithography.

Fig. 7.

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	Fig. 7. (a) DC output and (b) transfer characteristics of the InAlGaN/GaN MOS-HEMTs.

Fig. 8.

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	Fig. 8. The VTH versus ID,max of some state-of-the-art InAlGaN based E-mode devices.

In addition, the current collapse and frequency characteristics of the InAlGaN/GaN MOS-HEMTs were investigated. Figure 9(a) shows the pulsed output characteristics (using 500 ns pulses and 10 ms cycle for two quiescent points: V_GS = 0 V, V_DS = 0 V and V_GS = −8 V, V_DS = 10 V) for the InAlGaN/GaN MOS-HEMTs. It can be observed that the pulsed current is higher than the DC current, which results from the suppression of the self-heating effect. In addition, the current collapse from (0, 0) state to (−8, 10) state is related to the trapping effect.^[29] The current collapse can be expressed as (I_{(0, 0)state}−I_{(−8, 10)state})/I_(0,0)state, which is only 6% for the InAlGaN/GaN MOS-HEMTs in our experiment. These results demonstrate the high quality of the InAlGaN material with low trap concentration, indicating the superiority of the pulsed MOCVD for growing InAlGaN quaternary alloy. As shown in Fig. 9(b), the MOS-HEMTs exhibit a current gain cutoff frequency f_t of 12.6 GHz and a unilateral power gain maximum oscillation frequency f_max of 23.4 GHz at a bias of V_DS=10 V and V_GS=4.5 V. Taking the gate length of into consideration, this result is satisfactory and can be further improved by scaling down the device size.

Fig. 9.

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	Fig. 9. (a) DC and pulsed output characteristics and (b) frequency performance of the InAlGaN/GaN MOS-HEMTs.

In summary, nearly polarization matched InAlGaN/GaN heterostructures were grown by pulsed MOCVD, and the good morphological, structural, and electrical characteristics were demonstrated in detail. In addition, high-performance E-mode MOS-HEMTs were fabricated on the prepared heterostructures, which exhibited the highest V_TH of 3.1 V and I_D,max of 669 mA/mm of the InAlGaN based E-mode devices. The results strongly indicate that the InAlGaN/GaN heterostructure grown by pulsed MOCVD is a promising candidate for the fabrication of novel group-III nitride electronic devices.