Pulsed sputtering selective epitaxial formation of highly degenerate n-type GaN ohmic contacts for GaN HEMT applications

This study describes the selective formation process of highly degenerate n-type GaN (d-GaN) ohmic contacts for the source and drain regions of GaN high electron mobility transistors (HEMTs) using pulsed sputtering deposition (PSD). The selective formation process using SiO2 masks and PSD epitaxial growth enabled the uniform formation of d-GaN in micron-meter size. The optimally formed d-GaN exhibited a minimum resistivity as low as 0.16 mΩ·cm, an electron concentration of 3.6 × 1020 cm−3, and a mobility of 108 cm2 V−1 s−1. Transmission-line-method measurements demonstrated that the contact resistance of GaN HEMTs with d-GaN regrowth contacts was remarkably low at 0.28 Ω·mm, leading to the reasonable DC output characteristics with an on-resistance of 2.8 Ω·mm and a maximum current density of 850 mA mm−1. These findings suggest that PSD epitaxial regrowth of d-GaN is a promising approach for the high-throughput formation of low-resistivity ohmic contacts on large-area GaN HEMT wafers.


W
ide bandgap GaN-based high electron mobility transistors (HEMTs) have expanded their applications to power switching 1) and high-power RF electronic devices 2) owing to their excellent properties, such as high electron mobility (∼2000 cm 2 V −1 s −1 ), high electron saturation velocity (2.5 × 10 7 cm s −1 ), and high electric breakdown field (3.3 MV cm −1 ). 3,4)A critical issue in the fabrication of these devices is forming effective ohmic contacts with a contact resistance (R c ) of ∼0.1 Ω•mm.[7] During RTA, Ti atoms diffuse through the AlGaN barrier, thereby forming metallic TiN by reacting with the AlGaN/ GaN layers.The typical R c of these metal alloy contacts is as low as 0.50-2.3[10][11][12] However, the lateral diffusion of metal atoms from the source and drain regions during RTA can deteriorate device performance, unsuitable for further scaling of GaN HEMTs for HF applications as it affects subsequent alignment steps to form short-gate electrodes. 13)n alternative method to form low-resistive ohmic contacts is the selective epitaxial formation of highly n-type doped GaN on the source and drain regions using SiO 2 masks.Conventional metal-organic CVD (MOCVD) techniques have faced challenges in growing highly n-type doped GaN with electron concentrations exceeding 10 20 cm −3 because of the heavy compensation effects. 14,15)In MOCVD processes, surface diffusion of precursors from mask regions compromises the uniformity of the regrown GaN. 16,17)Furthermore, MOCVD growth of highly Si-doped GaN often proceeds in the Stranski-Krastanov mode due to the antisurfactant effect of Si dopant, adversely degrading the structural and electrical properties of the material. 18,19)he MBE technique has yielded impressive results in the regrowth of highly n-type doped ohmic contacts for GaN HEMT applications.[22][23][24][25][26][27][28] Despite these advancements, MBE processes generally suffer from low productivity and have incompatible future largearea substrates.
Recently, our team demonstrated the potential of pulsed sputtering deposition (PSD) for the epitaxial growth of heavily n-type doped GaN, highlighting its promise as a high-throughput and low-temperature epitaxial growth technique for large-area substrates. 29)Notably, heavily Si-doped GaN grown by PSD exhibited record low resistivity of 1.6 × 10 −4 Ω•cm, an electron concentration of 3.9 × 10 20 cm −3 , and electron mobility of 100 cm 2 V −1 s −1 . 30)We discovered that Ge-doping and Sn-doping in GaN via PSD led to the formation of highly n-type doped GaN, with electron concentrations exceeding 10 20 cm −3 . 31,32)This highly n-type doped degenerate GaN (d-GaN) performed effectively as low-resistive tunneling junction contacts in UV-A LEDs 33) and in the tunneling-junction-based interconnection of cascaded LED structures. 34)Given these excellent electrical properties of d-GaN and the nature of PSD, which is wellsuited for large-area and high-throughput epitaxial growth, we believe that exploring the selective epitaxial formation process of d-GaN ohmic contacts for HEMT applications is a worthwhile endeavor.
In this letter, we present our investigation into the growth behaviors, structural and electrical properties of d-GaN regrowth contacts prepared via PSD, and the fundamental characteristics of GaN HEMTs with these d-GaN regrowth contacts.
The fabrication process of GaN HEMTs with d-GaN regrowth contacts is depicted in Figs.1(a)-1(f).We commenced with commercially available GaN HEMT wafers comprising epitaxial stacks: a 25 nm thick Al 0.25 Ga 0.75 N layer, a 1 nm thick AlN layer, a 1000 nm thick GaN layer, and a 300 nm thick buffer layer on a Si (111) substrate, as illustrated in Fig. 1(a).The sheet resistance (R SH ) of the twodimensional electron gas (2DEG) channel was 361 Ω sq −1 , with electron mobility of 1830 cm 2 V −1 s −1 and sheet carrier density of 9.5 × 10 12 cm −2 .An AFM image confirmed the atomically flat surface of the epitaxial stack in Fig. 1(a).
For the selective formation of the d-GaN contacts, 200 nm thick SiO 2 masks were deposited by e-beam evaporation [Fig.1(b)].Source and drain regions were defined by opening the SiO 2 mask using standard photolithography and fluorinebased reactive ion etching (RIE) [Fig.1(c)].The barrier and channel layers were etched using chlorine-based inductively coupled plasma (ICP) RIE with an ICP power of 30 W and bias of 200 W, reaching an etched depth of 100 nm.The AFM image in Fig. 1(c) depicts that the etched GaN surfaces were smooth, with a typical root mean square surface roughness of 0.28 nm.After wet wafer cleaning, the patterned wafers were introduced into the PSD chamber for the growth of heavily Si-doped layers in an Ar/N 2 ambient.
The scanning electron microscopy (SEM) image in Fig. 1(d) reveals polycrystalline GaN with fine grains typically <100 nm on the SiO 2 mask, whereas flat GaN films grew on the etched GaN surfaces.Electron backscattering diffraction patterns of this region displayed a clear six-fold symmetry, indicating selective epitaxial formation of d-GaN on the etched surfaces.Subsequently, SiO 2 masks and polycrystalline GaN were removed using HF acid, as depicted in Fig. 1(e).The SEM image in Fig. 1(e) confirms the complete removal of polycrystalline GaN and SiO 2 mask and the uniform formation of high-quality d-GaN.In Fig. 1(f), the source and drain metal stacks of Ti/Al/Ti/Au (30/70/30/50 nm) were deposited by e-beam evaporation, and mesa isolation was defined by ICP-RIE.Lastly, another metal stack of Ni/Au (100/200 nm) was deposited as gate electrodes.
Figures 2(a) and 2(b) present the electrical properties of selectively formed degenerate n-type GaN (d-GaN) using the described processes.Hall-effect measurements conducted via the van der Pauw method on 100 μm × 100 μm clover-leaf patterns revealed an electron mobility of 100 cm 2 V −1 s −1 at an electron concentration of 3.6 × 10 20 cm −3 .This mobility is considerably higher than previously reported values for highly Si-doped GaN prepared by MOCVD 35) or MBE. 22)urthermore, the correlation between electron concentration and mobility, as depicted in Fig. 2(a), was comparable to that of nonpatterned d-GaN films previously reported. 30)Notably, no deterioration in electrical properties was observed in the selectively formed d-GaN with SiO 2 masks.The minimum resistivity reached as low as 0.16 mΩ•cm at an electron concentration of 3.6 × 10 20 cm −3 , corresponding to an extremely low R SH of 8 Ω sq −1 .This low resistance is effective in reducing the access resistance from the sourcedrain metals to the 2DEG channel.
Subsequently, the contact resistance of the GaN HEMT with d-GaN regrowth contacts was assessed using transfer length method (TLM) measurements.The electrode spacing for the TLM patterns was meticulously determined through SEM observations.According to the literature, 21,24) the overall contact resistance R C can be divided into three components as shown in Fig. 3(a): the contact resistance between the metal and GaN (R 1 ), the access resistance of GaN itself (R 2 ), and the contact resistance between GaN and 2DEG (R 3 ) (R C = R 1 + R 2 + R 3 ).Figure 3(b) and its inset illustrate the TLM pattern and TLM result for evaluating the R 1 .From the vertical intercept in the figure, the R C between d-GaN and ohmic metal stacks was determined to be 0.06 Ω•mm even without RTA process.Similarly, Fig. 3(c) and its inset display the TLM results and schematic of the TLM patterns for evaluating the total R C of the HEMT.The total R C of the HEMT was determined to be as low as 0.28 Ω•mm, 011006-2 © 2024 The Author(s).Published on behalf of ][22][23][24][25][26][27][28] The R 2 was calculated to be 0.008 Ω•mm using the R SH of d-GaN and the spacing between the edges of d-GaN and the 2DEG channel.The R 3 can be estimated to be 0.21 Ω•mm from the equation: R The quantum limit of the minimum interface resistance between a large contact and a 2DEG depends on the 2DEG concentration (n s ) near the interface, which is expressed by the following equation 36,37) R q n 2 , where ℏ is the Dirac's constant, q is the elementary charge and n s is sheet charge density.From the n s of 9.5 × 10 12 cm -2 in our samples, the minimum interface contact resistance should be 0.027 Ω•mm, which was smaller than our experimental R 3 value.Typically, the R 3 values of MBE grown n + -GaN contacts were slightly higher than the quantum limit. 21,24)The further optimization of the regrowth process can improve the total R C of the GaN HEMT with the regrown d-GaN and contribute to the highly scalable regrowth ohmic contact technology via sputtering process.011006-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 1 .
Fig. 1.(a) Schematic structure of GaN HEMT epitaxial stack and its AFM surface image.(b) Deposition of 200 nm thick SiO 2 mask by e-beam evaporation for regrowth of d-GaN on the source-drain region.(c) Chlorine-based inductively coupled (ICP) RIE process for the removal of the AlGaN and AlN barrier layers in the source-drain region with the etching depth of 100 nm.AFM image of the etched GaN surface is displayed as well.(d) Regrowth process of a highly Si-doped 200 nm thick GaN (d-GaN) layer by PSD and its surface image measured by SEM.The {11-21} EBSD pole figure for the d-GaN regrown area was measured.(e) Lift-off process of SiO 2 mask using HF acid.SEM image exhibited a clear boundary between the regrowth region and the AlGaN barrier.(f) Formation of source and drain metal stacks Ti/Al/Ti/Au(30/70/30/50 nm) and gate metal stack Ni/Au(100/200 nm) by e-beam evaporation.Mesa isolation was defined by the ICP-RIE process.

Figure 4 (
a) presents a schematic image of the GaN HEMT structure with regrown d-GaN and its device dimensions.The source-drain length (L SD ) and gate length (L G ) were 5.0 μm and 2.5 μm, respectively.Figures 4(b) and 4(c) summarize the DC output characteristics of the GaN HEMT with regrown d-GaN.In Fig. 4(b), the drain current to drain voltage (I DS -V DS ) curves of the GaN HEMT are plotted with gate voltage (V GS ) steps of −1 V, ranging from 2 to −5 V.The linear rise of I DS without V DS offset demonstrates the perfect ohmic characteristics of the d-GaN regrowth contacts.The maximum I DS reached as high as 850 mA mm −1 owing to the low R C of the d-GaN contacts, and the device on-resistance (R ON ) was measured at 2.8 Ω•mm.This R ON value was almost comparable to the sum of the source and drain contact, and the channel resistance (2.4 Ω•mm) calculated from the TLM results.In Fig. 4(c), the I DS ) is plotted against the V GS at a V DS of 8 V.A threshold voltage (V th ) of −4.7 V was extracted from the linear extrapolation of the transfer characteristics.These results indicate that the d-GaN regrowth contacts prepared

Fig. 2 .
Fig. 2. (a) Resistivity and (b) electron mobility of selectively regrown d-GaN as a function of the electron concentration.

Fig. 3 .
Fig. 3. (a) Schematic of TLM patterns on GaN HEMT structure and the overall contact resistance R C with three components of R 1 , R 2 and R 3. (b) TLM fitting for determining the contact resistance between d-GaN and ohmic metal electrode.Inset: schematic of TLM patterns on d-GaN.(c) TLM fitting for determining the contact resistance of the GaN HEMT with regrown d-GaN contacts.Inset: schematic image of TLM patterns on GaN HEMT structure.