Two-Step GaN Layer Growth for High-Voltage Lateral AlGaN/GaN HEMT

Abstract

This paper presents reduced dislocation of the AlGaN/GaN heterostructure for high-voltage lateral high-electron-mobility transistor (HEMT) devices. AlGaN/GaN heterostructure was grown on sapphire substrate. Prior to the growth of the AlGaN layer, the GaN layer was grown via two-step growth. In the first step, the V/III ratio was applied at 1902 and then at 3046 in the second step. The FWHMs of the XRD (002) and (102) peaks of the GaN layer were around 205 arcsec ((002) peak) and 277 arcsec ((102) peak). Moreover, the surface of the GaN layer showed clear evidence of step flows, which resulted in the smooth surface of the layer as well as the overgrown of the AlGaN layer. Subsequently, the AlGaN/GaN heterostructure was fabricated into a lateral HEMT with wide gate-to-drain length (L_GD). The device exhibited a clear working HEMT characteristic with high breakdown voltages up to 497 V. In comparison to many reported AlGaN/GaN HEMTs, no AlGaN interlayer was inserted in our AlGaN/GaN heterostructure. By improving the growth conditions for the two-step growth, the performance of AlGaN/GaN HEMTs could be improved further.

1. Introduction

Over the past few decades, III–V nitride (AlGaInN) materials have driven significant advancements in light emitting diode (LED) and laser diode technologies. Following that, such materials are no doubt witnessing considerable attention in the development of field effect transistors (FETs), particularly high-electron-mobility transistors (HEMTs). Since nitride materials have a direct and wide bandgap, AlGaN/GaN HEMTs have the potential to exhibit major advantageous characteristics such as high breakdown voltage, fast switching, low switching loss, and high power conversion efficiency compared with silicon-based HEMTs [1]. Due to the expensive and limited availability of bulk GaN substrate, AlGaN/GaN HEMTs are mostly developed on non-native substrates such as sapphire and silicon. It is widely acknowledged that silicon is more appealing than sapphire for HEMTs because it can provide decent conductivity and is available in larger dimensions. Nonetheless, realizing high-performance AlGaN/GaN HEMTs on silicon substrate is a significant challenge. Without proper and intricate additional steps for the buffer layer, AlGaN/GaN HEMTs on silicon can easily suffer from extensive cracks, which seriously degrade the device’s performance. As such, there is still interest in using sapphire substrate in AlGaN/GaN HEMTs [2].

The difference in lattice parameters between sapphire and GaN always leads to the generation of threading dislocations. A clear trend for increased sheet resistance with increased threading dislocations was reported in [3]. Moreover, the report inferred that 2DEG mobility can be greatly impacted by the scattering from charged dislocation cores and dislocation strain fields when the dislocation density is high. According to a recent report, threading dislocations cause mobility fluctuation, resulting in poor resistance uniformity for AlGaN/GaN HEMT grown on a silicon substrate [4]. Several approaches have been proposed to counteract this issue, primarily by introducing interlayers of AlN interlayer [5] and AlInN interlayer [6]. Whereas improvement of growth conditions for GaN layers on sapphire substrates has been repeatedly reported across the literature, there have been fewer attempts to apply such work to AlGaN/GaN HEMTs, particularly for high-voltage application. As a matter of fact, the growth of the GaN layer for AlGaN/GaN HEMT also plays a significant role in enhancing the device’s performance. In III–V nitrides, epitaxy by metal organic chemical vapor deposition (MOCVD), two-step growth has proven to reduce the dislocations [7,8]. Such a technique introduced the transition from 3D to 2D growth. Upon the transition, more dislocation can be inclined and, hence, annihilated when they meet in the opposite directions. To date, there has been no attempt to demonstrate this reduced dislocation of the AlGaN/GaN heterostructure for high-voltage HEMT devices. Here, a focus is made on the growth of the GaN layer on sapphire through two-step growth prior to the AlGaN layer as an attempt to reduce the dislocations in the AlGaN/GaN heterostructure. No interlayer(s) was inserted into the structure. Excellent recent works show correctly engineered 1.2 kV HEMT devices [9], normally-off AlGaN/AlN/GaN HEMTs with recessed gate structures whose threshold voltage and saturation current are 4.5 V and 456 mA/mm [10], and multiple architectures including a SiN passivation layer, a notch structure, and a sandwich structure consisting of a gate field plate with a cut-off frequency of 24.1 GHz [11]. Although the growth of the AlGaN/GaN heterostructure was not thoroughly discussed, the growth was performed with interlayer insertion. This proposed two-step growth is undeniably highly feasible, even for different architectures depending on the desired performance. The idea is to assess the potential of the two-step growth for the GaN layer in simplifying the AlGaN/GaN HEMTs fabrication for high-voltage applications.

2. Materials and Methods

The GaN layer was grown on a 2″ c-plane (0001) sapphire substrate in a metal–organic chemical vapour deposition (MOCVD) horizontal flow Taiyo Nippon Sanso (SR4000KS–HT) reactor. Trimethylgallium (TMGa) and ammonia (NH₃) were used as precursors to grow the GaN layer, whereas hydrogen was used as the gas carrier. The growth pressure was maintained at atmospheric pressure. Prior to the growth, the sapphire substrate was thermally cleaned at 1100 °C for 10 min. After the substrate was cleaned, a low temperature (LT) GaN buffer layer was grown at 520 °C for ~80 s. This was followed by the growth of the GaN layer in two steps. In the first step, a 1.2 µm thick GaN layer was grown at 1100 °C for 25 min with a V/III ratio of 1902. In the second step, a GaN layer was grown for 60 min at 1150 °C with a thickness of 2.2 µm and a V/III ratio of 3046. The V/III ratio was lower in the first step than the second step in order to introduce 3D growth, which was beneficial to promote dislocation inclinations. After that, a 45 nm thick AlGaN layer was grown on the GaN layer at 1030 °C for 15 min for two-dimensional electron gas (2DEG) formation. To grow the AlGaN layer, trimethylaluminum (TMAl) was used together with TMGa and NH₃. From an X-ray diffraction (XRD) ω-2θ scan, the percentage of aluminum (Al) content in the AlGaN layer was determined. The GaN layer was measured by XRD ω-scan in (002) and (102) directions in order to obtain an insight into the crystalline quality of the layer. The surface roughness of the layer was measured by atomic force microscopy (AFM). The AFM measurement was also performed on the AlGaN layer.

In the next stage, the AlGaN/GaN heterostructure was fabricated for realizing a working high-voltage AlGaN/GaN lateral HEMT with wide gate-to-drain length (L_GD) architectures. First, mesa isolation was performed for the active areas by inductively coupled plasma (ICP) etching using chlorine (Cl₂) gases. Ti/Al/Ti/Au contacts for the source and drain were formed by sequential deposition of the bilayer metals on the AlGaN/GaN surface and lift off, followed by rapid thermal processing (RTP) in nitrogen ambient for 1 min at 800 °C. After removing the photoresist, the sample was dipped in diluted hydrofluoric acid (HF) for removing native oxide. Then, a 200 nm Al₂O₃ layer was deposited for surface passivation. In the subsequent step, post-deposition annealing (PDA) was carried out at 450 °C in an oxygen ambient to improve the gate insulator quality. To allow electrical contact, the Al₂O₃ layer on top of the source and drain contacts was then removed and a Ni/Au gate electrode was eventually deposited on the Al₂O₃. In principle, Al₂O₃ is one of the most attractive candidates for high-k gate dielectrics with its high dielectric constant, high breakdown field, and wide bandgap. Furthermore, it is most widely implemented for AlGaN/GaN HEMT devices [12]. The gate-to-drain length (L_GD) and gate width (W_G) were 14 μm to 23 μm and 25 μm, respectively. Room temperature measurements of current–voltage (IV) and breakdown characteristics were conducted using a High-Power Parameter Analyzer System.

3. Results and Discussion

Figure 1a,b show the XRD rocking curve ω-scan in (002) and (102) directions for the GaN layer grown with two-step growth, respectively. The full width at half maximum (FWHM) for the (002) peak is 205 arcsec, whereas the (102) peak is 277 arcsec. The FWHMs for the (002) peak and the (102) peak mainly lead to the estimation of screw dislocation density and edge dislocation density, respectively. The density for screw dislocation was 8.47 × 10⁷ cm⁻², whereas for edge dislocation it was 4.08 × 10⁸ cm⁻² [13]. The total threading dislocation density was 4.93 × 10⁸ cm⁻². This value is lower than a reported GaN layer [7], which was also grown in two steps.

The result indicates that the first step with the V/III ratio at 1902 successfully formed the 3D growth. The 3D growth transformed into 2D growth in the second step by applying the V/III ratio at 3046. Such a transformation enabled lateral growths, which led to the inclination and annihilation of the dislocations. Figure 1c shows the 2D AFM images for the surface of the GaN layer at different magnifications. The wavy surface implies that the layer was grown in step-flow mode, which typically results in a smooth surface. The RMS roughness with the corresponding magnification was also included in the figure. Apparently, the RMS roughness is lower than 1 nm. This shows that through the 3D to 2D growth transformation (two-step growth), a smoother surface was obtained for the GaN layer.

The ω-2θ XRD measurement for the AlGaN/GaN heterostructure is shown in Figure 2a. The peak for the AlGaN was observed at around 17.87°. Through a simulation fitting, the Al content in the AlGaN layer was estimated to be 20%. The AlGaN layer with 20% Al content is a typical layer for AlGaN/GaN HEMTs, e.g., [14]. Since the thickness of the AlGaN layer was very thin (45 nm), the density for the screws and edge dislocations were difficult to measure. Nonetheless, from the XRD data, it is clear that the AlGaN peak is broader than the GaN peak. Such a result is expected because the incorporation of Al into the GaN material always leads to deterioration of the crystalline structure. This was due to the difference in lattice parameters between AlN and GaN. The crystalline deterioration of the AlGaN layer is more significant for higher percentages of Al content [15].

Figure 2b shows the AFM images for the surface of the AlGaN layer of the AlGaN/GaN heterostructure at different magnifications. The wavy surface is still observable. Moreover, evidence of hillocks with spiral growth appears on the surface. Such hillocks are commonly observed in MOCVD-grown AlGaN. The formation of the hillocks is generally related to screw dislocation decoration [16]. Due to the nature of the screw dislocations, there are already pinned steps at the surface. The pinned steps can locally prevent the monolayer steps from moving, resulting in hillocks. The RMS roughness with corresponding magnification was also included. Apparently, the RMS roughness was almost similar to the underneath GaN layer, as seen in Figure 2 earlier. This may be due to the small thickness of the AlGaN layer, which rendered insignificant change to the surface with respect to the GaN layer.

Through Hall effect measurement, sheet resistance and carrier density were determined to be 522.2 Ω/sq and 4.11 × 10¹³ cm⁻², respectively. Figure 3a depicts a 3D diagram of the Al₂O₃/AlGaN/GaN HEMT that was fabricated with a W_G of 25 µm. Meanwhile, Figure 3b shows a device of 17 μm L_GD in log scale I_DS–V_GS, and Figure 3c shows I_DS–V_DS. For the transfer characteristics, the V_GS was swept from −50 V to 50 V with a V_DS of 50 V. At the off state, the current density in log scale remains approximately 10¹². As the V_GS surpasses the threshold voltage (V_TH), the current increases as the V_GS increases linearly until 8 V and remains nearly constant at 10⁴ toward the V_GS of 50 V. The purple line in the linear fit that intercepts with the voltage axis indicates that the V_TH is 18 V. From this characteristic, it is determined that the I_ON/I_OFF ratio was in the order of 10⁸, which is relatively high for an AlGaN/GaN HEMT. As for the I_DS–V_DS characteristics, the gate-source voltage (V_GS) was varied from 30 V to 0 V in 10 V steps for the Al₂O₃/AlGaN/GaN HEMT. The normalized maximum current density (I_Dmax) measured was 9.2 mA/mm, which is relatively low due to the thick 200 nm Al₂O₃ layer and wide L_GD. These, however, can be further improved by reducing the thickness for the Al₂O₃ layer or the length of L_GD.

Without the need for controlled conditions for high-voltage measurements (e.g., Flourinert emersion) in a vacuum probe system, what stands out in this figure, Figure 4a, at V_GS = 30 V, the average breakdown voltages of over 300 V for devices with five different L_GD values; devices with L_GD values of 14 μm, 16 μm, 18 μm, 20 μm, and 22 μm showed breakdown voltages of 368 V, 497 V, 262 V, 311 V, and 168 V, respectively. Likewise, Figure 4b shows the average breakdown voltages of over 300 V for devices with five different L_GD values; devices with L_GD values of 15 μm, 17 μm, 19 μm, 21 μm, and 23 μm showed breakdown voltages of 408 V, 226 V, 254 V, 340 V, and 302 V, respectively. Ideally, the breakdowns should increase as the L_GD increases. To briefly analyze this behaviour, as the possibilities of breakdown mechanisms have been thoroughly presented [17,18], here are the two aspects that are often taken into consideration: the electric field distribution of L_GD and the dielectric material reliability. From the electric field perspective, the distribution of electric field along the side of the drain and gate-edge point are correlated to the behaviour of the Al₂O₃/AlGaN/GaN HEMT breakdown voltages. This occurred because highly concentrated electric field spikes (with the absence of field plate structures) were located at the interfaces of the gate/Al₂O₃ when V_DS high voltage was applied [19]. As can be seen in Figure 4a,b, for the devices with 19 μm L_GD and 22 μm L_GD, the breakdown voltages were 168 V and 254 V, respectively. On the other hand, for 15 μm L_GD and 16 μm L_GD devices, the breakdown voltages were 408 V and 497 V, respectively. This may be due to high applied voltages which lead to high rates of impact ionization, which subsequently leads to early breakdown of the device, caused by enhanced carrier generation [20]. Not to negate the fact of gate dielectric reliability and vertical leakage due to undoped GaN grown on sapphire consists of background donor impurities [21]. These are also possible, as the current can possibly flow into the Al₂O₃ and GaN layers instead of the 2DEG channel. As can be seen with the device with the 17 μm L_GD, the device exhibits high leakage currents that increase rapidly until the device reaches its breakdown voltage of 226 V.

Nevertheless, the true breakdown for this Al₂O₃/AlGaN/GaN HEMT device, which has yet to be investigated further, is expected to be much higher [22]. As demonstrated previously, GaN-based heterojunction field effect transistors (HFETs) (with an AlN nucleation layer in between the C-doped GaN buffer layer and c-plane sapphire substrate) were measured using Fluorinert FC-40 for high-voltage measurements to avoid complications of environmental conditions, including arcing and tracking [23]. The maximum breakdown voltage obtained for the L_GD 16 m was 1350 V. This is in line with the classical Paschen’s Law, as the breakdown voltage is proportional to the L_GD when the air pressure is constant. Although, it should be noted that the breakdown voltage will reach a saturation point with the increase of L_GD and this has yet to be discovered. The results from the electrical measurements clearly indicate that our fabricated AlGaN/GaN HEMT exhibited a working HEMT characteristic, although without the AlGaN interlayer. The two-step growth of the GaN layer is the possible reason behind this behaviour. With improvements in the two-step growth, the performance of the HEMT can be increased further.

4. Conclusions

Two-step growth, which was applied in the growth of the GaN layer, resulted in an excellent crystalline quality and smooth surface to the AlGaN/GaN heterostructure. When the AlGaN/GaN heterostructure was fabricated into a lateral AlGaN/GaN HEMT, a working HEMT characteristic was clearly observed with breakdown voltages up to 497 V. This work showed that without any interlayers, which are commonly introduced prior to the growth of the AlGaN/GaN heterostructure, a working high-voltage AlGaN/GaN HEMT can be achievable. Whereas the device performance of AlGaN/GaN HEMTs could be improved by the structure or architecture, such as open-gate, dual-gate, etc., this work paves the way towards the improvement and simplification of heterostructures by further optimizing the conditions of two-step growth.