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Review

Process of Au-Free Source/Drain Ohmic Contact to AlGaN/GaN HEMT

by
Lin-Qing Zhang
1,
Xiao-Li Wu
1,
Wan-Qing Miao
1,
Zhi-Yan Wu
1,*,
Qian Xing
2,* and
Peng-Fei Wang
3
1
College of Electronic and Electrical Engineering, Henan Normal University, No. 46 East of Construction Road, Xinxiang 453007, China
2
College of Intelligent Engineering, Henan Institute of Technology, No. 699 Pingyuan Road (East Section), Xinxiang 453003, China
3
State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, No. 220 Handan Road, Shanghai 200043, China
*
Authors to whom correspondence should be addressed.
Crystals 2022, 12(6), 826; https://doi.org/10.3390/cryst12060826
Submission received: 24 April 2022 / Revised: 27 May 2022 / Accepted: 6 June 2022 / Published: 10 June 2022
(This article belongs to the Special Issue Recent Advances in III-Nitride Semiconductors)
Browse Figures
Versions Notes

Abstract

:
AlGaN/GaN high electron mobility transistors (HEMTs) are regarded as promising candidates for a 5G communication system, which demands higher frequency and power. Source/drain ohmic contact is one of the key fabrication processes crucial to the device performance. Firstly, Au-contained metal stacks combined with RTA high-temperature ohmic contact schemes were presented and analyzed, including process conditions and contact formation mechanisms. Considering the issues with the Au-contained technique, the overview of a sequence of Au-free schemes is given and comprehensively discussed. In addition, in order to solve various problems caused by high-temperature conditions, novel annealing techniques including microwave annealing (MWA) and laser annealing (LA) were proposed to form Au-free low-temperature ohmic contact to AlGaN/GaN HEMT. The effects of the annealing method on surface morphology, gate leakage, dynamic on-resistance (RON), and other device characteristics are investigated and presented in this paper. By using a low-temperature annealing atmosphere or selective annealing method, gate-first Si-CMOS compatible AlGaN/GaN HEMT technology can be realized for high frequency and power application.

1. Introduction

Traditional Si-based radio frequency (RF) and power devices, due to the physical limitation of Si material, cannot meet the requirements of higher-speed and higher-power in a 5G generation communication system. GaN-based HEMTs are regarded as promising candidates for high-frequency and high-power electronic devices due to their superior material properties [1,2,3,4]. The GaN-based materials are always heterogeneous and grown on other substrates, such as Si, SiC, and sapphire [5,6,7,8]. Among the heterojunction device structures, the AlGaN/GaN HEMT exhibits the best electronic characteristics [9,10,11]. Furthermore, the AlGaN/GaN-on-Si is the structure most commonly selected for power application for the following reasons [12,13,14,15]: the fabrication process is compatible with Si CMOS process flow, which helps to reduce the manufacturing cost. Realization of a large-size AlGaN/ GaN-on-Si wafer can also further reduce the cost. The regular epitaxial AlGaN/GaN layers are grown on a high-resistivity Si substrate by metal organic chemical vapor deposition (MOCVD), which is shown in Figure 1. From Si bottom to top, a GaN buffer layer with a thickness of about 3 μm is deposited for reducing stress caused by lattice mismatch. After that, the device layer is formed by depositing an unintentional doped (UID) GaN device layer (~100 nm), AlN interlayer (~1 nm), and AlxGa1-xN barrier layer (~25 nm, x varies from 0.15–0.4). The two-dimensional electron gas (2DEG) is formed at the AlGaN/GaN interface due to the polarization effect [16,17]. Finally, the GaN cap layer (~3 nm) is deposited for protecting the device surface.
For RF application, the gate of the fabricated device is always formed in a T-shape for smaller gate resistance [18,19,20]. The process flow, which can be seen in Figure 2, consists of mesa isolation, source/drain ohmic contact formation, SiN passivation, T-shape gate formation, gate electrode deposition, via formation, and testing electrode deposition.
For power application, the gate is fabricated in a regular shape, as shown in Figure 3. The fabrication process consists of mesa isolation, source/drain ohmic contact formation, gate electrode deposition, SiN passivation, via formation, and testing electrode deposition.
The quality of source/drain ohmic contact has a great impact on the performance of AlGaN/GaN HEMT, such as on-resistance (Ron), output current (IDS), extrinsic transconductance (Gm), and current gain cut-off frequency (fT) [11,21,22,23,24,25]. Always, the source/drain regions form ohmic contact to reduce RON and improve output current and fT characteristics. Thus, the source/drain ohmic contact technique should be investigated. Over the years, researchers have systematically investigated ohmic contact to n+-GaN and AlGaN/GaN for achieving low contact resistance (RC).
The most popular metallization schemes of ohmic contact in AlGaN/GaN HEMT is Ti/Al/X/Au, where X can be Ni, Mo, Pt, Ta, Ir, etc. [26,27,28,29,30]. Ti/Al/X/Au Au-contained schemes will be discussed in Section 2. However, the Au-contained metal scheme is not suitable for Si-based CMOS fabrication lines because Au has a high diffusion coefficient in silicon, which can lead to heavy metal contamination [31,32]. The use of Au will increase the fabrication cost, and long-term Au diffusion will lead to ohmic contact degradation [31,33]. Thus, an Au-free ohmic contact metal scheme should be proposed and investigated for achieving the purpose. In Section 3.1, the most advanced Au-free ohmic contact techniques for AlGaN/GaN HEMTs are displayed and discussed. Thermal budget is another issue that should be considered in AlGaN/GaN HEMT source/drain ohmic contact formation. Traditional rapid thermal annealing (RTA) usually needs a high-temperature atmosphere to form ohmic contact between a metal stack and an AlGaN/GaN heterojunction, which will lead to the oxidation of the AlGaN surface and create N defects [34], degrading the dynamic performance of AlGaN/GaN-based HEMTs. Also, TiN protrusions of random sizes take shape during the high-temperature annealing process [35]. Rough surface morphology and unideal ohmic metal edge sharpness can be observed to easily form short contact between the source/drain metal stack and gate metal electrode [36]. This phenomenon will be more severe for the Q band and Ka band small-sized AlGaN/GaN HEMT. In addition, the high-temperature process is incompatible with the gate-first fabrication technique [37]. Thus, instead of pursuing the optimized annealing temperature of the RTA technique, a novel annealing technique may be employed as an alternative to form low-temperature Au-free AlGaN/GaN HEMT ohmic contact with the existing Au-free metal schemes, including microwave annealing (MWA) and laser annealing (LA), etc. Details about MWA and LA techniques to form ohmic contact to AlGaN/GaN HEMTs will be discussed in Section 3.2, which is promising for the realization of gate-first technology and a Si-CMOS compatible GaN HEMT technique.

2. Ti/Al/X/Au Au-Contained Ohmic Contact Technique

Considering the wide bandgap of (Al)GaN, two methods are adopted to form good quality ohmic contact: (1) reduce the barrier height(ΦB) by selecting the appropriate metal and (2) increase the possibility of tunneling by forming an n+-(Al)GaN surface. The energy band diagram of metal/(Al)GaN by two methods are shown in Figure 4.
For the first method, researchers selected Al, Ti, Au, Ni, Mo, Pd, Pt, Cr, etc. as contact metals and investigated their contact characteristics with (Al)GaN [38,39,40,41,42,43,44,45,46]. The work function (Wm), specific contact resistivity (ρc), and ΦB characteristics of different metal materials can be seen in Table 1.
The results indicate that it is challenging to form good quality ohmic contact between single layer metal and (Al)GaN due to the higher Wm of the selected metal material. Also, the Ti, Al, etc. single layers are prone to oxidation during the high-temperature annealing process. Thus, a multilayer metal stack combined with a high-temperature annealing scheme became the common choice for GaN-based devices’ source/drain ohmic contact formation [47]. The multilayer metal stack, as shown in Figure 5, always consists of four metal layers: the barrier layer, coating layer, diffusion barrier layer, and cap layer. The most popular metal scheme is Ti/Al/X/Au, where X can be Ni, Mo, Pt, Ta, Ir, etc. During the annealing process, the barrier layer Ti reacts with the AlGaN layer to form TiN [48]. TiN carries out lower work function [49], which lowers the Schottky barrier height and therefore helps in ohmic contact formation. Also, the created N vacancies formed in the reaction process make the AlGaN layer underneath the contact metal n+-doped, which makes electron tunneling easy for ohmic contact formation [50]. Coating layer Al reacts with Ti under the high-temperature atmosphere to form Al3Ti, which helps in ohmic contact formation [51]. The diffusion layer Ni, Mo, Pt, and Ta etc., which owns high melt point prevents Au indiffusion and Al outdiffusion [52]. Furthermore, the surface morphology of the contact metal can be affected by the metal layer. Au, which acts as a cap layer, prevents the Ti, Al layer oxidation within the high-temperature annealing atmosphere. Also, the Au layer improves the contact conductivity [53].
Ohmic contact resistance RC, ρc, surface morphology, and thermal stability are the important indexes of ohmic contact quality. The RC value can be affected by the selected metal stack, the metal thickness, ohmic recessing of the AlGaN layer, surface pre-treatment prior to ohmic metallization, annealing time and temperature, n-type doped in the semiconductor, etc. [52,54,55,56,57,58,59,60,61,62,63]. These affecting factors were investigated and optimized by universities and research institutions. Jacobs B et al. [54] have systematically studied the influence of the metal stack, Ti, Al, Ni thickness, Ti/Al ratio, and annealing condition on RC. In their results, the achieved optimized RC is 0.2 Ω·mm with Ti thickness of 30 nm, Ti/Al ratio of 6, Ni thickness of 40 nm, and annealing temperature of 900 °C for 30 s. Yan et al. [59] demonstrated that the RC can be further reduced by a multi-step annealing process. With the improvement of AlGaN/GaN material growth, ohmic contact formation for metal and AlGaN/GaN becomes challenging. Buttari D et al. [60] developed a low-damage Cl2 reactive ion etching (RIE) recess etch on an AlGaN layer (7nm), decreasing RC from 0.45 Ω·mm to 0.27 Ω·mm. By Si ion implantation in the source/drain region, Nomoto K et al. [61] proved that the on-resistance can be decreased dramatically. By etching holes that were 0.8 μm × 0.8 μm in size and etching a depth of 10 nm, Wang et al. [62] developed a pattern of square holes technique in the source/drain AlGaN layer and achieved an RC as low as 0.1 Ω·mm, as shown in Figure 6. The fabricated AlGaN/GaN HEMTs exhibits comparable output current, peak transconductance and threshold voltage characteristics with conventional AlGaN/GaN HEMTs. Before ohmic contact metal deposition, Fujishima T et al. [63] developed a SiCl4 and BCl3 plasma treatment on the source/drain surface and achieved low RC values of 0.41 Ω·mm and 0.17 Ω·mm, respectively. By optimizing the affecting factors of Ti/Al/X/Au schemes, the achieved RC can be as low as 0.3 Ω·mm, and even much lower. The obtained low RC can meet the requirements for the RF and power applications of AlGaN/GaN HEMT by using Ti/Al/Ni/Au metal schemes.
Universities and industries have conducted studies on the ohmic contact formation mechanism. Two methods are regarded as the mechanism to form ohmic contact to AlGaN/GaN HEMT: (1) a field emission (FE) tunneling mechanism [64,65,66] via the formation of a thinner high doping or a lower barrier; and (2) a spike mechanism [67,68] by TiN direct electron path formation along the dislocation. In the FE tunneling mechanism, Ti reacts with the GaN or AlGaN layer to form TiN. N should be extracted from GaN or AlGaN, generating a highly n-doped interface region, which is responsible for the occurrence of FE tunneling. The work function of TiN (3.74 eV) is lower than Ti (4.33 eV), which is believed to lower the Schottky barrier for low-resistance ohmic contact formation. In the spike mechanism, the TiN projections forming along the dislocation places penetrate through the AlGaN layer under a high temperature, which sets up a direct electron path connecting the 2DEG and the ohmic metal. This method is believed to be more efficient in electron transferring than the FE tunneling mode. With the improvement of AlGaN/GaN material growth, the spike phenomenon, which penetrates through the whole AlGaN layer, becomes less likely to happen. The cross-sectional diagrams of both physical models are shown in Figure 7, which reveals the ohmic contact formation mechanism.

3. Au-Free Ohmic Contact Technique

3.1. Existing Advanced Au-Free Ohmic Contact Technology

The cap layer of Au is proposed to prevent the oxidation of ohmic metal under a high-temperature atmosphere. Au is also proposed to form Ga vacancies in AlGaN or GaN layers to decrease the contact resistance [31]. It is challenging to form high-quality ohmic contact with low contact resistance by using Au-free metal schemes. However, as mentioned in the introduction section, Au-contained metal schemes suffer from Si-based CMOS technology non-compatibility, rough surface morphology, and degradation of dynamic device performance. Also, the use of Au will increase the fabrication cost. Thus, Au-free ohmic contact metal schemes should be proposed and investigated for achieving the purpose. Over the years, groups from all over the world have studied and proposed a number of metal schemes, such as Ti/Al/W [33], Ta/Al/Ta [69], Ti/Al/Ti/TiN [70,71], Ta/Si/Ti/Al/Ni/Ta [72], Ti/Al/Ni/Pt [73], Ti/Al/TiN [74], Ti/TiN [75], Ti/Al/NiV [76], Ti/Al/Ti/W [77,78], TixAly [79], and Ti/Al/Ni/Ti [80]. In these schemes, the cap layer of Au is replaced by W, TiN, and Ta, etc. For the Ti/Al-based method, Lee H S et al. [81] used a fully recessed AlGaN structure in the source/drain region, followed by a Ti/Al/W deposition and 870 °C annealing process, reducing the RC to 0.49 Ω·mm. This method enables the direct contact between Ti/Al/W and 2 DEG. The Ti/Al/W metal stacks also exhibit much smoother morphology and a much lower voltage drop property for the same output current level, which is a very important characteristic for high-voltage power electronics. The source/drain AlGaN layer can be partially recessed to achieve ohmic contact with an RC of 0.21 Ω·mm using Ti/Al/Ti/TiN at the low temperature of 550 °C [71]. By optimizing the Ti thickness (2.5 nm) and annealing temperature (550 °C), the low RC ohmic contact can be realized. High-resolution transmission electron microscopy (HR-TEM) and electron dispersive X-ray spectroscopy (EDS) were adopted to investigate alloy distribution and the ohmic contact formation mechanism. From the TEM and EDS results as shown in Ref. [71], they deduced that Ti plays a catalytic role for the Al-N reaction and N extraction. The formation of a continuous AlN thin layer at the contact interface accounts for the low resistance of Au-free ohmic contacts on AlGaN/GaN at a low annealing temperature.
Yoshida T et al. [33] reduced the RC to 0.358 Ω·mm at the low temperature of 500 °C by optimizing the thickness (a thin Ti layer of 2.7 nm) of Ti using Ti/Al/W, as shown in Figure 8. The fabricated devices have a maximum drain current density (IDS), RON, and peak transconductance (Gm) of 0.422 A/mm, 9.7 Ω·mm and 73.2 mS/mm, respectively. These characteristics are comparable to Au-contained AlGaN/GaN HEMTs.
Furthermore, the cap layer can be replaced by Pt, Ni, and NiV. Liu et al. [73] used a Ti/Al/Ni/Pt Au-free metal stack and achieved an RC of 0.6 Ω·mm at 975 °C annealing temperature. The maximum drain current and Gm achieved 700 mA/mm and 140 mS/mm, respectively. Tham W H et al. [76] used a Ti/Al/NiV source/drain metal stack scheme and achieved an RC of 0.8 Ω·mm featuring the low temperature of 500 °C. The AlGaN/GaN MIS-HEMTs were fabricated and measured. The achieved specific on-state resistivity and maximum saturation drain current were 2.3 mΩ cm2 and 0.21 A/mm. Gao et al. [80] demonstrated a two-step Ti/Al/Ni/Ti Au-free ohmic contact technique (Ti/Al deposited by EBE and Ni/Ti grown by magnetron sputtering) and achieved an ρc of 5.59 × 10−5 Ω/cm2. The achieved maximum saturation drain current and RON reached around 0.4 A/mm and 15.6 Ω·mm.
Ta-based schemes were also proposed to form ohmic contact to AlGaN/GaN heterojunctions. Li et al. [82] deposited a Ta/Si/Ti/Al/Ni/Ta metal stack and achieved a low RC of 0.22 Ω·mm by optimizing the annealing temperature. The ohmic contact formation was also investigated by using X-ray diffraction (XRD), TEM, and EDS analysis, as shown in Figure 9. It was deduced that the formation of TixSiy alloy with a low work function and intermixing with Ta results in a low ohmic contact resistance RC. Johnson D W et al. [69] used a Cl2 plasma etch to recess the whole AlGaN barrier layer in the source/drain region and deposit a Ta/Al/Ta layer with optimized thickness. The achieved RC can be as low as 0.2 Ω·mm with the annealing temperature lower than 600 °C. Maximum drain current is found to be 236 mA/mm at VDS = 10 V. Maximum transconductance and RON were 55 mS/mm and 41.3 Ω·mm, respectively. Considering the recess ohmic contact technique increased the fabrication complexity, in 2021, Fan et al. [79] developed a non-recessed Ti-Al alloys Au-free scheme instead of Ti/Al-based multilayers. By optimizing the ratio of Ti in TixAly and annealing temperature, the low RC of 0.063 Ω·mm can be obtained with Ti5Al1 under the optimal annealing condition. The EDS and TEM results as shown in Ref. [79] show that the Al outdiffusion can be suppressed and Ti can deeply diffuse into the AlGaN barrier layer by using TixAly alloy. The extracted N atoms increase with an increasing Ti ratio of TixAly. Therefore, a higher Ti ratio TixAly is beneficial in forming a highly doped n-type AlGaN region, resulting in a much lower ohmic contact RC. Furthermore, their group proposed an Au-contained Ta0.83Al0.17/Au metal scheme to realize a low ohmic contact of 0.14 Ω·mm at 900 °C [83]. Unlike the spike mechanism in the Ti/Al/Ni/Au method, the Ta-Al-Au mixed more uniformly with each other so that Au-dominated alloy fully penetrated the AlGaN barrier layer for electron path formation. Thus, the RC could be decreased drastically.
Au-free ohmic contact formation can also be realized by intentional ion implantation in the AlGaN/GaN within the source/drain region [84]. In addition, selective regrowth of the source/drain region after the partial AlGaN etching can also realize Au-free ohmic contact formation [85,86]. However, these methods require strict process conditions, increase the fabrication cost, and make the fabrication process more complex. Therefore, only few universities and institutions investigate these techniques.
The AlGaN/GaN HEMTs devices’ electrical characteristics, with the existing advanced Au-free ohmic contact technique, can be summarized in Table 2. The results show that AlGaN/GaN HEMTs with Au-free ohmic contacts exhibit similar electrical characteristics comparable to Au-contained AlGaN/GaN HEMTs. The surface morphology of Au-free ohmic metals is more flat than the Au-contained ones due to the smaller root mean square (RMS) surface roughness.

3.2. Low-Temperature Au-Free Ohmic Contact Technology

The benchmark of RC values against annealing temperatures of state-of-the-art Au-free ohmic contacts for the AlGaN/GaN HEMT can be seen in Figure 10, including recess and non-recess schemes [33,69,70,71,72,73,74,75,76,77,78,79,80,81,82]. It can be seen that low RC values (<0.3 Ω·mm) always occur with high annealing temperature (>600 °C) conditions. Only a few methods meet the low RC and low annealing temperature requirements. Thermal budget is another issue that should be considered in AlGaN/GaN HEMT source/drain ohmic contact formation.
A high-temperature atmosphere will lead to the oxidation of AlGaN surfaces and create N defects [34], degrading the dynamic performance of AlGaN/GaN-based HEMTs. Also, TiN protrusions of random sizes take shape during the high-temperature annealing process [35]. Rough surface morphology and unideal ohmic metal edge sharpness can be observed, easily forming short contact between the source/drain metal stack and gate metal electrode [36]. This phenomenon will be more severe for the Q band and Ka band small-sized AlGaN/GaN HEMT. In addition, the high-temperature process is incompatible with the gate-first fabrication technique [37]. Thus, the Au-free low-temperature ohmic contact technique should be investigated and proposed for AlGaN/GaN HEMT. Au-free ohmic contact with AlGaN/GaN HEMT always needs a high temperature for contact formation using traditional RTA. With the existing optimized Au-free metal schemes, the annealing method might be another element which affects the ohmic contact quality. Therefore, the novel annealing technique should be introduced for low-temperature Au-free ohmic contact to AlGaN/GaN HEMT. A novel annealing technique may be employed and investigated for AlGaN/GaN HEMT ohmic contact formation: microwave annealing (MWA) and laser annealing (LA), etc.
MWA: as a novel annealing technique, MWA has special characteristics (including selective heating of specific materials, rapid heating rates and temperature gradients, the elimination of wall effects, and the superheating of solvents) and non-thermal effects [87]. Ohmic metals and 2DEG have strong ability to absorb microwaves. Thus, MWA can be an alternative to AlGaN/GaN HEMT source/drain ohmic contact formation. Using an Au-contained Ti/Al/Ni/Au metal stack, Zhang et al. [88] demonstrated that a low temperature of approximately 600 °C can be used to achieve good quality of ohmic contact (RC = 0.6 Ω·mm) with an MWA power of 4200 W and heating time of 20min, as shown in Figure 11 [89]. High-resolution atomic force microscope (AFM) results show that the MWA-HEMT source/drain ohmic metal surface, after annealing process, is relatively flat compared to the RTA one. RC values and root mean square (RMS) surface roughness characteristics for various MWA conditions are compared with the RTA method and shown in Table 3 [90].
TEM and EDS analysis are employed to learn the ohmic contact formation mechanism and reasons for surface roughness [88]. From the TEM and EDS results, as shown in Figure 12a,b, a direct current path is also formed in MWA-HEMT, which facilitates the flow of electrons in both directions, indicating that the spike mechanism is still the major mechanism of forming carrier conduction paths. From the EDS results as shown in Table 4, we deduce that less Ni-Au blocks were formed under a low-temperature atmosphere, which accounts for the superior surface morphology formation.
The MWA-HEMT also exhibits good gate leakage and output current characteristics as compared to RTA-HEMT. The maximum drain current value reaches 850 mA/mm. The MWA method was also employed to form ohmic contact to AlN/GaN HEMT [89] and InAlN/GaN HEMT [91]. For InAlN/GaN HEMT, Chou et al. [91] fabricated an MWA-HEMT, which exhibits better RON, subthreshold swing (SS), RF performance, and current gate leakage characteristics than the RTA one. Furthermore, MWA-HEMT ohmic contact has a much smoother surface morphology, which results in high reliability. For AlN/HEMT [89], MWA also delivers better surface morphology and a lower gate leakage current. The ohmic contact formation mechanism was also studied. The experimental results fit well with the field emission (FE) model, as shown in Figure 13.
The MWA GaN-based HEMTs devices’ electrical characteristics compared to the RTA ones can be summarized in Table 5 as follows: the results show that MWA-HEMT exhibits comparable RC, output current, GM characteristics, and better RON and surface morphology. Thus, considering the excellent advantages of MWA, combined with Ti/Al-based, Ta-based, and TixAly metal stack schemes, the MWA method is promising to realize low-temperature Au-free ohmic contact formation to AlGaN/GaN HEMT instead of pursuing an optimized record-low temperature for the RTA method.
LA: Tzou A J et al. [92] demonstrated the incorporation of a Si implantation in the GaN HEMTs and non-alloyed ohmic contact process by using the LA method. The surface morphology, contact resistance, on-resistance, and surface trap density can be improved by LA instead of the RTA technique. Ferreyra R A et al. [93] achieved a non-alloy ohmic contact using a Hf/Al/Ti metal stack with the picosecond LA method, featuring the RC of 0.17 Ω·mm. Without source/drain n+ GaN regrowth, Sheng’s group proposed a laser annealing (LA) for AlGaN/GaN HEMT ohmic contact formation with a Ti/Al/Ni/Au metal stack [94]. The fabricated LA-HEMT exhibits smooth morphology, low contact resistance, high breakdown voltage and low dynamic on-resistance. The wafer is not damaged during the LA period due to the larger energy bandgap of GaN than LA photon energy. By applying a wavelength of 532 nm, 20 ns of a pulse duration, 100 kHz of pulse frequency, and energy density up to 2.99 J/cm2, the achieved RC is 2.18 Ω·mm [95]. The ohmic contact formation mechanism by LA has been studied and proposed. The experiment’s data fit the thermionic field emission (TFE) model closely. From their results they deduce that the current transport mechanism of the LA sample is dominated by thermionic field emission (TFE). The energy band diagram for the contacts annealed by LA can be seen in Figure 14.
The LA method results in a much smoother surface morphology due to less metal diffusion and alloy aggregation for the ultra-short LA time. They also studied the surface damage condition by LA and compared it with the RTA method [96]. The X-ray photoelectron spectroscopy (XPS) results, as shown in Figure 15, show that the oxidation reaction with a high-temperature atmosphere can be eliminated by the LA method. Also, Au-free ohmic contact using an Au-free Ti/Al/Ni/W metal stack combined with the LA annealing technique was proposed and studied. The LA-HEMT exhibits better isolation leakage current, low-density surface states, and suppressed RON [97].
Instead of a whole device region under a high-power laser pulse, Liu et al. [98] proposed a micro-scale LA approach to form ohmic contact to AlGaN/GaN HEMT. The schematic illustration of the micron-scale laser annealing is shown in Figure 16. Due to the selective source/drain region heating by LA, the gate electrode, which is temperature sensitive, is not thermally affected. Therefore, gate-first devices is realized using this micro-scale LA technique. The extracted RC is as low as 0.3 Ω·mm when the laser power setting as 0.9 W. The high heating rate tends to form a thicker TiN layer, leaving a high density of N vacancy in AlGaN which makes tunneling much easier. In addition, the LA-HEMT shows better output current and smaller gate leakage.
The LA GaN-based HEMTs devices’ electrical characteristics compared to the RTA ones can be summarized in Table 6 as follows: from the testing results [94], the laser annealing reveals smooth morphology, low contact resistance, high breakdown voltage, and low dynamic on-resistance for the fabricated devices combined with the existing Au-free metal stack technique; this method is beneficial for gate-first technology and Si-CMOS compatible GaN HEMT technique.

4. Conclusions

In this work, the conventional AlGaN/GaN HEMT Ti/Al/Ni/Au Au-contained ohmic contact technique was discussed comprehensively, including the selected metal stack, the metal thickness, ohmic recessing the AlGaN layer, surface pre-treatment prior to ohmic metallization, annealing time and temperature, n-type doped in the semiconductor, etc. After that, the existing Au-free ohmic contact was briefly introduced. In addition, to lower the thermal budget for the annealing process, novel MWA and LA annealing techniques were proposed for low-temperature Au-free ohmic contact to AlGaN/GaN HEMT, which is beneficial for gate-first technology and Si-CMOS compatible GaN HEMT technique realization.

Author Contributions

L.-Q.Z.; writing—original draft, investigation, data curation, conceptualization. X.-L.W., W.-Q.M., Z.-Y.W.; writing—original draft, investigation, data curation, conceptualization. Q.X., P.-F.W.; writing—review & editing, visualization, validation. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported in part by the Doctoral Scientific Research Start-Up Foundation of Henan Normal University under Grant 5101239170008, Henan Province Natural Science Foundation of Youth Fund No. 212300410185, Key scientific research projects of higher education institutions in Henan Province No. 21A510005, 22B510009 and in part by Key Laboratory of Optoelectronic Sensing Integrated Application of Hennan Province.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic AlGaN/GaN-on Si device structure.
Figure 1. Schematic AlGaN/GaN-on Si device structure.
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Figure 2. Schematic process flow of T-shaped AlGaN/GaN HEMT.
Figure 2. Schematic process flow of T-shaped AlGaN/GaN HEMT.
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Figure 3. Schematic process flow of regular AlGaN/GaN HEMT.
Figure 3. Schematic process flow of regular AlGaN/GaN HEMT.
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Figure 4. Energy band diagram of metal/(Al)GaN by reducing the barrier height(ΦB) (a) and increasing the possibility of tunneling by forming n+-(Al)GaN surface (b).
Figure 4. Energy band diagram of metal/(Al)GaN by reducing the barrier height(ΦB) (a) and increasing the possibility of tunneling by forming n+-(Al)GaN surface (b).
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Figure 5. Schematic structure of metal stacks used for forming ohmic contact to AlGaN/GaN HEMT.
Figure 5. Schematic structure of metal stacks used for forming ohmic contact to AlGaN/GaN HEMT.
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Figure 6. Schematic structure of AlGaN/GaN HEMT with holes etching in the ohmic region.
Figure 6. Schematic structure of AlGaN/GaN HEMT with holes etching in the ohmic region.
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Figure 7. Physical models of Ti/Al/Ni(x)/Au ohmic contact to AlGaN/GaN HEMT: FE tunneling mechanism (a) and spike mechanism (b).
Figure 7. Physical models of Ti/Al/Ni(x)/Au ohmic contact to AlGaN/GaN HEMT: FE tunneling mechanism (a) and spike mechanism (b).
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Figure 8. Specific contact resistivity ρC vs. annealing temperature. All samples were annealed for 1 min, expect for samples with Ti (2.7 nm)/Al/W, which were annealed for 10 min. The inset shows the corresponding channel sheet resistances Rsh. Reprinted with permission from Ref. [33]. Copyright 2018, Wiley.
Figure 8. Specific contact resistivity ρC vs. annealing temperature. All samples were annealed for 1 min, expect for samples with Ti (2.7 nm)/Al/W, which were annealed for 10 min. The inset shows the corresponding channel sheet resistances Rsh. Reprinted with permission from Ref. [33]. Copyright 2018, Wiley.
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Figure 9. Cross sectional high-resolution TEM images of (a) as-deposited, (b) 750 °C-annealed, (c) 850 °C-annealed non-gold ohmic contacts. (d,e): zoomed-in TEM images of Ta/Ni interface (A1) and metal/AlGaN interface (A2) of 850 °C-annealed non-gold contact in (c), respectively, and (f): zoomed-in TEM image of metal/AlGaN interface (A3) of 750 °C -annealed non-gold (Ta/Si/Ti/Al/Ni/Ta) ohmic contact in (b). Reprinted with permission from Ref. [82]. Copyright 2013, IOP science.
Figure 9. Cross sectional high-resolution TEM images of (a) as-deposited, (b) 750 °C-annealed, (c) 850 °C-annealed non-gold ohmic contacts. (d,e): zoomed-in TEM images of Ta/Ni interface (A1) and metal/AlGaN interface (A2) of 850 °C-annealed non-gold contact in (c), respectively, and (f): zoomed-in TEM image of metal/AlGaN interface (A3) of 750 °C -annealed non-gold (Ta/Si/Ti/Al/Ni/Ta) ohmic contact in (b). Reprinted with permission from Ref. [82]. Copyright 2013, IOP science.
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Figure 10. Comparison of RC as a function of annealing temperature of different Au-free ohmic contact to AlGaN/GaN HEMT from various publications.
Figure 10. Comparison of RC as a function of annealing temperature of different Au-free ohmic contact to AlGaN/GaN HEMT from various publications.
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Figure 11. The schematic diagrams of MWA equipment. Reprinted with permission from Ref. [89]. Copyright 2017, IEEE.
Figure 11. The schematic diagrams of MWA equipment. Reprinted with permission from Ref. [89]. Copyright 2017, IEEE.
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Figure 12. (a) TEM image of the post-annealing structure by MWA where the arrows markings indicate dislocations. (b) Magnified image of the interface. (c) TEM image of bulge area of the alloyed ohmic contact treated by RTA. (d) TEM image of bulge area of the alloyed ohmic contact treated by MWA. Reprinted with permission from Ref. [88]. Copyright 2015, IEEE.
Figure 12. (a) TEM image of the post-annealing structure by MWA where the arrows markings indicate dislocations. (b) Magnified image of the interface. (c) TEM image of bulge area of the alloyed ohmic contact treated by RTA. (d) TEM image of bulge area of the alloyed ohmic contact treated by MWA. Reprinted with permission from Ref. [88]. Copyright 2015, IEEE.
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Figure 13. (a) Temperature−dependent characteristics of the RSH and RC of MWA−HEMT. (b) Measured (symbols) and modeled (lines) ρC of MWA−HEMT as a function of temperature. Reprinted with permission from Ref. [89]. Copyright 2017, IEEE.
Figure 13. (a) Temperature−dependent characteristics of the RSH and RC of MWA−HEMT. (b) Measured (symbols) and modeled (lines) ρC of MWA−HEMT as a function of temperature. Reprinted with permission from Ref. [89]. Copyright 2017, IEEE.
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Figure 14. The schematic energy band diagram for the contacts annealed by LA.
Figure 14. The schematic energy band diagram for the contacts annealed by LA.
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Figure 15. XPS core-level spectra of AlGaN surface as grown, after RTA and after the LA method. Reprinted with permission from Ref. [96]. Copyright 2019, Wiley.
Figure 15. XPS core-level spectra of AlGaN surface as grown, after RTA and after the LA method. Reprinted with permission from Ref. [96]. Copyright 2019, Wiley.
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Figure 16. The schematic diagrams of micron-scale LA.
Figure 16. The schematic diagrams of micron-scale LA.
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Table
Table 1. Wm, ρc, ΦB characteristics of different metals to (Al)GaN.
Table 1. Wm, ρc, ΦB characteristics of different metals to (Al)GaN.
MetalWm (eV)ρc (×10−6 Ω/cm2)ΦB (eV)Refs.
Al4.2880.8[38,45]
Ti4.3330.58[38,39,40,41,43]
Au5.12.350.9[38,43,44]
Ni5.156.991.4[38,41,44]
Mo4.65.30.81[42]
Pd5.129.781.9[38,46]
Pt5.65111.6[38,41]
Cr4.511.80.39[41]
Table
Table 2. Survey of literature data on GaN HEMTs using different Au-free ohmic contact schemes.
Table 2. Survey of literature data on GaN HEMTs using different Au-free ohmic contact schemes.
Au-Free SchemesRC (Ω·mm)Idmax (mA/mm)RON (Ω·mm)Specific On-State Resistivity (mΩ/cm2)Gm (mS/mm)Root-Mean-Square (RMS) Surface
Roughness (nm)		Ref.
Ti/Al/W0.3584229.7/73.2/[33]
Ta/Al/Ta0.226341.3/55/[69]
Ti/Al/Ti/TiN0.21/////[71]
Ta/Si/Ti/Al/Ni/Ta0.24830//250/[72]
Ti/Al/Ni/Pt0.6700/0.771404.6[73]
Ti/Al/TiN0.62/////[74]
Ti/Al/NiV0.13210/2.360/[76]
Ti/Al/Ti/W0.41350///7[78]
TixAly0.063/////[79]
Ti/Al/Ni/Ti/40015.66.39/7.9[80]
Table
Table 3. Comparisons of peak temperature, RC, and RMS characteristics for various annealing conditions. Reprinted with permission from Ref. [90]. Copyright 2016, IEEE.
Table 3. Comparisons of peak temperature, RC, and RMS characteristics for various annealing conditions. Reprinted with permission from Ref. [90]. Copyright 2016, IEEE.
MethodsPeak Temperature (°C)RC (Ω·mm)RMS (nm)
MWA, 3500 W, 20 min579.61.1637.7
MWA, 4200 W, 5 min579.64.5737.5
MWA, 4200 W,10 min5944.0235.8
MWA, 4200 W, 15 min597.61.4449.6
MWA, 4200 W, 20 min597.60.663.5
RTA, 840 °C, 35 s8400.4101
Table
Table 4. EDS Composition (at. %) from the points labeled in Figure 12. Reprinted with permission from Ref. [88]. Copyright 2015, IEEE.
Table 4. EDS Composition (at. %) from the points labeled in Figure 12. Reprinted with permission from Ref. [88]. Copyright 2015, IEEE.
NAlTiNiGaAu
1 26.37 2.76 70.87
2 9.9557.377.103.4222.16
3 89.302.47 8.23
4 8.9683.241.63 6.16
5 97.811.59 0.60
619.5320.2723.75 35.201.26
7 48.77 45.495.75
8 51.35 48.65
Table
Table 5. GaN-based HEMTs devices’ electrical characteristics: MWA vs. RTA [88,89,90,91].
Table 5. GaN-based HEMTs devices’ electrical characteristics: MWA vs. RTA [88,89,90,91].
MaterialMethodsTemperature (°C)RC (Ω·mm)ρc (×10−5Ω/cm2)RMS (nm)Idmax (mA/mm)RON (Ω·mm)Gm (mS/mm)
AlGaN/GaNMWA579.60.6/63.5850//
AlGaN/GaNRTA8400.4/101800//
AlN/GaNMWA572.40.65/29.31400/270
AlN/GaNRTA7500.4/56.51320/267
InAlN/GaNMWA//4.296.79/4.9/
InAlN/GaNRTA875/4.02115/6.1/
Table
Table 6. GaN-based HEMTs devices’ electrical characteristics: LA vs. RTA.
Table 6. GaN-based HEMTs devices’ electrical characteristics: LA vs. RTA.
MethodsRC (Ω·mm)RMS (nm)Idmax (mA/mm)RON (Ω·mm)Gm (mS/mm)Ref.
LA2.1816.532914.4/[94]
RTA0.85/33014.3/[94]
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Zhang, L.-Q.; Wu, X.-L.; Miao, W.-Q.; Wu, Z.-Y.; Xing, Q.; Wang, P.-F. Process of Au-Free Source/Drain Ohmic Contact to AlGaN/GaN HEMT. Crystals 2022, 12, 826. https://doi.org/10.3390/cryst12060826

AMA Style

Zhang L-Q, Wu X-L, Miao W-Q, Wu Z-Y, Xing Q, Wang P-F. Process of Au-Free Source/Drain Ohmic Contact to AlGaN/GaN HEMT. Crystals. 2022; 12(6):826. https://doi.org/10.3390/cryst12060826

Chicago/Turabian Style

Zhang, Lin-Qing, Xiao-Li Wu, Wan-Qing Miao, Zhi-Yan Wu, Qian Xing, and Peng-Fei Wang. 2022. "Process of Au-Free Source/Drain Ohmic Contact to AlGaN/GaN HEMT" Crystals 12, no. 6: 826. https://doi.org/10.3390/cryst12060826

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