Demonstration of Vertical GaN Schottky Barrier Diode With Robust Electrothermal Ruggedness and Fast Switching Capability by Eutectic Bonding and Laser Lift-Off Techniques

In this letter, we have successfully transferred the 4-inch crack-free GaN films from sapphire substrate to conductive silicon wafer by employing eutectic bonding and laser lift-off (LLO) techniques. The resultant 1-mm2 fully-vertical GaN Schottky barrier diodes (SBDs) exhibit a high current swing of 109, a low ideality factor of 1.03 and a high forward current of 10 A. Meanwhile, a decent breakdown voltage of 312 V is achieved, which is over 3 times higher than that of control device without performing epitaxial lift-off. Most importantly, such rectifiers show significantly enhanced electrothermal ruggedness, achieving a high surge-current density of 2.6 kA/cm2 and a low thermal resistance of 0.77 K·cm2/W. In addition, the excellent power rectification capability with a low reverse recovery time of 14 ns is obtained under high-speed switching condition with a high current ramp rate (<inline-formula> <tex-math notation="LaTeX">$di/dt$ </tex-math></inline-formula>) of 275 <inline-formula> <tex-math notation="LaTeX">${\mathrm {A/\mu s}}$ </tex-math></inline-formula>, implying the desired functionality of the LLO-vertical device architecture. These results thus present the great potentials of the substrate-transferred GaN SBDs for high-power and high-efficiency applications.


I. INTRODUCTION
GaN-BASED devices are considered as the most promising candidates for high-power-density and high-efficiency power electronic applications due to their superior material properties [1], [2], [3], [4], [5], [6], such as high breakdown electric field, high electron mobility and high thermal conductivity [7], [8]. In last decade, tremendous progresses have been witnessed toward the commercialization of lateral AlGaN/GaN power devices rated at 100-650 V [2]. Concurrently, vertical GaN devices are under active developments for miniaturized power switching applications [9], as the vertical structure is more favorable to realize high current capacity, high power density, and superior thermal performance [10], [11]. However, the commercialization of the vertical GaN-on-GaN power devices has been hindered by the high-cost and small-diameter of the bulk substrate materials [12]. Vertical GaN devices on low-cost and largescale foreign substrates such as silicon and sapphire, also known as quasi-vertical devices [13], [14], [15], [16], [17], are therefore highly desired. For GaN-on-Si quasi-vertical diodes, it is very challenging to grow high-quality thick GaN epilayers due to the large mismatch of lattice constant and thermal expansion coefficient between GaN and Si [13], [14], resulting in severe leakage and premature breakdown issues in such devices [15]. Alternatively, the high-quality GaN films with low dislocation density could be epitaxially grown on the mature sapphire substrate [16], [18]. However, the fundamental limitation of the sapphire substrate is its poor heat dissipation capability with a low thermal conductivity (k T ) [19], which would  seriously affect the device's electrostatic characteristics and electrothermal ruggedness, especially for thermal-resistance and surge-current capabilities [9], [20], [21]. Therefore, improving the thermal-related performances of GaN-onsapphire devices remain a formidable challenge.
Actually, such bottleneck problem has been well settled in the light-emitting diodes (LEDs) industries [22]. Based on the eutectic bonding and laser lift-off (LLO) technologies [23], the hetero-epitaxial GaN films of the LEDs can be directly removed from low-k T sapphire substrates and transferred to conductive Si carrier wafers. In this way, the fully-vertical LEDs after the LLO process exhibit evidently enhanced heat dissipation capability, enabling a high light-output power and wall-plug efficiency [24], thanks to the device-level thermal management. However, such powerful solution is rarely explored in GaN power devices, and the related research gaps on the electrostatic characteristics and electrothermal behaviors need to be filled.
In this work, for the first time, we have demonstrated the large-are (1-mm 2 ) fully-vertical GaN Schottky barrier diodes (SBDs) achieved by transferring a 4-inch substrate from sapphire to silicon via Au/In eutectic bonding and LLO techniques. Benefiting from the combined advantages of high-quality epitaxial material and fully-vertical structure configuration, the LLO-SBD exhibits superior device performances, such as a high current swing of 10 9 and a high forward current of 10 A. In particular, the LLO-SBDs show a robust electrothermal ruggedness, presenting a high surgecurrent density of 2.6 kA/cm 2 and a low thermal resistance of 0.77 K·cm 2 /W. Moreover, the required circuit-level operating capability of the device is verified in the double-pulse switching circuit. These results show that the eutectic bonding and LLO processes pave a viable way for fabricating high-performance fully-vertical GaN rectifiers. Fig. 1(a) and (b) show the schematic profiles of the reference device and the LLO-SBD, respectively, where the GaN epitaxial structure of both devices was grown on the 4-inch low-cost sapphire substrates, consisting of a 700-nm buffer layer, a 3-μm n + -GaN current spreading layer (N D ∼ 1 × 10 18 cm −3 ) and a 8-μm n − -GaN drift layer (N D -N A ∼ 1 × 10 16 cm −3 ) from the bottom to the top. Here, the drift layer used to enhance the reverse blocking capability is thicker than that current GaN-on-Si diodes (1.3 ∼ 3 μm [14], [17]) while maintaining good crystal quality. For material characterizations, high-resolution X-ray diffraction (XRD) and atomic force microscopy (AFM) are commonly used to evaluate the quality of the epitaxial structures [14], [25]. The full-width-half-maximums (FWHMs) of GaN (002) and (102) planes extracted from the rocking curves are 93.6 arcsec and 154.8 arcsec, corresponding to the screw and edge dislocations of 1.78 × 10 7 and 1.27 × 10 8 m −2 [4], respectively. Furthermore, the AFM image of the n − -GaN drift layer show the atomically flat surface with a stepped-and-terraced structure [18], exhibiting a small root mean square (RMS) roughness of 0.17 nm ( Fig. 1(d)) [14].

II. MATERIAL CHARACTERIZATION AND DEVICE FABRICATION
The fabrication process flow of the Ref-SBDs has been widely reported in the previous work [6], [16], as shown in the inset of Fig. 1(a). The most critical step for the fabrication of quasi-vertical devices is to remove the GaN drift layer in the cathode region by deep-trench etching, which inevitably introduces the leakage paths caused by sidewall damages [26]. Notably, the fully-vertical structure implemented here ingeniously circumvents this issue. The fabrication of the LLO-SBDs begins with the formation the front-side Schottky contact (anode) of Ni/Pt (300/100 nm) by electron beam evaporation, followed by thermal annealing at 450 • C in nitrogen ambient for 5 min. Subsequently, a 500-nm SiO 2 passivation layer is deposited on the anode region prior to the opening of electrical contact window, which also serves as the field plate [10]. By performing 200 • C Au/In eutectic bonding, the Schottky anode is tightly bonded onto a 650-μm thick conductive Si carrier wafer [27]. Thereafter, the back-side sapphire substrate is thinned to 200 μm and ultimately removed by LLO technique using a KrF excimer laser (266 nm) [23]. After removing residual Ga droplets by dilute HCl solution [28], the GaN buffer layer is further etched by the Cl 2 /BCl 3 -based inductively coupled plasma (ICP), followed by the 85 • C TMAH wet treatment for 10 min to smooth the etched surface [25]. Finally, a Ti/Al/Ni/Au (30/150/50/100 nm) stack is deposited on the highly doped n + -GaN layer to form the ohmic contact. The cross-sectional scanning electron microscope (SEM) image of the LLO-SBD, and 4-inch crack-free GaN film after LLO process, are shown in Fig. 1(c) and 1(e), respectively. To facilitate circuit-level measurements, the fabricated devices are sealed in a standard TO-220 package ( Fig. 1(e)).

III. RESULTS AND DISCUSSION
The forward I−V curves of the Ref-and LLO-SBDs in log-and linear-scales are shown in Fig. 2(a) and (b), respectively, which are measured under pulsed conditions with a pulse width of 50 μs and a duty cycle of 0.5%. The fully-vertical LLO-SBD exhibits significantly enhanced rectification behaviors as compared to the quasi-vertical Ref-SBD, including a high current swing of 10 9 , a near-unity ideality factor (η) of 1.03, an extracted Schottky barrier height (Φ B ) of 1.08 eV [13] and a low turn-on voltage of 0.65 V (at a forward current density of 1 A/cm 2 [13]), which are comparable with the state-of-the-art GaN-on-GaN SBDs [2]. Furthermore, such device presents an excellent current handling capability, achieving high forward current in excess of 10 A with good linearity in the I-V curve, thanks to the vertical current flow and efficient thermal spreading in full-vertical structure [29], [30], while the lower forward current and higher on-resistance (R on ) in the Ref-SBD should be attributed to the non-uniform current distribution caused by the severe current crowding effect at the mesa edge [12]. In comparison, the obtained R on in the LLO-SBD is 4.3 m ·cm 2 , which consists of Si substrate resistance (R Sub ), contact resistance (R C ) and epitaxial layer resistance (R Epi ) [2]. The R Sub of 0.04 m ·cm 2 , which can be extracted from the conductivity and thickness of the conductive Si carrier wafer, only accounts for ∼0.1% of the R on . By using transfer length method (TLM) [2], the R C of 1.38 m ·cm 2 (∼32.1%) can be obtained. Therefore, most of the R on in the LLO-SBD is contributed by the R Epi ∼ 67.8%. By optimizing the structural parameters of the GaN epitaxial layer [2], it is expected that the on-resistance of LLO-SBD could be further reduced. Fig. 3(a) shows the reverse breakdown characteristics of the two type of diodes. Compared with the Ref-SBD, the 1-mm 2 LLO-SBD exhibits more than one order of magnitude lower reverse leakage current and 3-times higher breakdown voltage (BV). Such notable improvement in reverse blocking capability could be ascribed to the construction of fully-vertical structure by the LLO process without the need for deep trench etching, thus avoiding etch-related sidewall damage [16]. As a result, the LLO-SBD shows lower leakage current compared to the quasi-vertical Ref-SBD which inevitably suffers severe etching damage [26]. Meanwhile, the anode field-plate structure naturally formed in the eutectic bonding process can alleviate the E-field crowding effect at the Schottky contact edge ( Fig. 1(b)) [15], [32], thereby enhancing the breakdown ruggedness.
To further investigate the reverse-blocking characteristics of LLO-vertical devices, the temperature-dependent breakdown experiments have been conducted. As shown in Fig. 3(b), with the temperature varying from 25 to 150 • C, the leakage current of the device increases with different magnitudes at a certain bias voltage, which should be dominated by the various leakage mechanisms. It has been reported that the typical leakage mechanisms elucidated in fully-vertical GaN SBDs mainly include variable range hopping (VRH), Poole-Frenkel emission (PFE), trapassisted tunneling (TAT) and space-charge-limited conduction (SCLC) [14], etc. In this work, with an initial gradually increasing bias voltage, a strong temperature-dependent leakage behavior is observed, suggesting that the current transport is related to a thermally stimulated conduction process [33]. Combined with the linear relationship between ln(J/E) and E 1/2 (Fig. 3(c)), the leakage current should be dominated by the PF emission, possibly originating from the trap-assisted E-field enhanced the thermal emission of carriers from a trap state into a continuum trap state [34]. When the bias voltage exceeds ∼30 V, the leakage mechanism can  be explained by the VRH through dislocations or defectmediated conduction, as supported by the E-field dependence (LnJ ∝ E) (Fig. 3(d)). Furthermore, it is found that the TAT mechanism is responsible for the increased leakage current at high bias voltages (> 70 V here), as shown in Fig. 3(e). The schematic illustration of these leakage mechanisms is present in Fig. 3(f).
Compared with the quasi-vertical Ref-SBDs, one of the unparalleled advantages of fully-vertical LLO-SBDs is their significantly enhanced heat dissipation capability [35]. As shown in Fig. 4(a), by using a T3Ster thermal transient tester, the thermal resistance characteristics from junction to case (R θJC ) of two type of diodes have been first evaluated according to the transient-dual-interface method (TDIM) (i.e., JEDEC 51-14 standard) [36], [37]. Based on TDIM, the R θJC can be determined by the separation point between two different interfaces (with and without thermal grease) in transient thermal impedance curves (Z th ∼ t) [35]. Fig. 4(b) presents the Z th ∼ t thermal characteristics of different diodes. Apparently, the LLO-SBD exhibits an evidently lower R θJC of 0.77 K·cm/W at the separation point, as compared to 2.52 K·cm/W of the Ref-SBD, confirming the effectiveness of the LLO-vertical structure configured for thermal management. Indeed, with the on-state time increasing from the microsecond to second timescales [38], the transient values are consistent with the steady state R θJC (0.77 K·cm/W) as the device reaches thermal equilibrium, regardless of the pulsed duty cycle [38] (Fig. 4(c) and (d)).
In particular, the electrothermal ruggedness of the LLO-SBD has been further demonstrated by performing 0.1-ms forward surge-current characterizations [9], [39]. Fig. 5(a) shows the schematic circuit of surge current measurements, which includes a 1700 V/45 m SiC MOSFET, a device under test (DUT) and a resonant circuit module consisting of a capacitor (C surge ) and a load inductor (L surge ). By turning on the SiC MOSFET, a half-sine surge current pulse would be generated in the resonant circuit and go through the DUT, and the corresponding surge current and voltage are recorded by the oscilloscope. It is worth noting that the peak surge current (I peak ) can be changed by adjusting the C surge and L surge in the resonant circuit to evaluate the current surging capability of the diodes, according to the relationship of I peak = V CC (C surge /L surge ) 1/2 . Fig. 5(b) and (c) show the surge current and voltage waveforms of the 1-mm 2 LLO-SBD and Ref-SBD, respectively. With the I peak increasing from 7 A (0.7 kA/cm 2 ) to 26 A (2.6 kA/cm 2 ), the corresponding surge voltage of the fully-vertical LLO-SBD increases from the 3 V to 18 V, leading to a maximum power density of 46.8 kW/cm 2 and a dynamic on-resistance of 0.69 . However, catastrophic thermal destruction featuring voltage collapse occurs in the quasi-vertical Ref-SBD at a low surge current of 5 A, resulting in self-heating-induced transient on-resistance as high as 6 . These results clearly present the efficient heat dissipation and robust electrothermal ruggedness of the fullyvertical structure enabled by Au/In eutectic bonding and laser lift-off (LLO) techniques.
Furthermore, based on the decent reverse blocking and forward conducting characteristics, high-speed dynamic switching capability has been demonstrated in LLO-SBD. By implementing a widely used double pulse test (DPT) (Fig. 6(a)) [31], the device shows a fast reverse recovery behavior, verifying the required circuit-level functionality of the LLO-SBD for power applications. As shown in Fig. 6(b), under a high-speed switching condition from a forward current of 1 A to a reverse bias of 300 V with a fast di/dt of 275 A/μs [18], the LLO-SBD exhibits a low reverse recovery time (t rr ) of 14 ns and a switching charge (Q rr ) of 12 nC, outperforming the value obtained in the Si fast-recovery diode (RF305BM6S). This superior switching performance should be attributed to the high electron mobility of the unipolar GaN SBD, and the relative immunity to surface-related charge trapping effects in the fully-vertical structure [31].

IV. CONCLUSION
With the combined advantages of high-quality 4-inch epitaxial materials and substrate transfer technology, the superior electrostatic characteristics and fast switching capability have been demonstrated in the LLO-vertical GaN SBDs. Most importantly, the achieved fully-vertical device exhibits robust electrothermal ruggedness, presenting a high surge-current density of 2.6 kA/cm 2 and a low thermal resistance of 0.77 K·cm 2 /W. This work thus paves the way for improving the electrostatic performances and electrothermal ruggedness of the hetero-epitaxial GaN SBDs via substrate engineering technique.