- Email: [email protected]
Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area
Solid-State Electronics xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area q Wang Chong ⇑, Zhao Meng-Di, He Yun-Long, Zheng Xue-Feng, Wei Xiao-Xiao, Mao Wei, Ma Xiao-Hua, Zhang Jin-Cheng, Hao Yue Key Lab of Wide Band Gap Semiconductor Materials and Devices, Xi’an 710071, China The School of Microelectronics, Xidian University, Xi’an 710071, China
a r t i c l e
i n f o
Article history: Received 21 July 2016 Received in revised form 23 November 2016 Accepted 1 December 2016 Available online xxxx The review of this paper was arranged by Prof. E. Calleja Keywords: AlGaNGaN HEMT Ohmic contact Patterned etching Surface morphology
a b s t r a c t In this paper, the ohmic contact of AlGaNGaN HEMT was optimized by introducing patterned etching in ohmic area, and the conventional structure and whole etching structure were investigated for comparison. The contact resistance decreased from 0.46 X mm for conventional to 0.35 X mm and 0.18 X mm respectively for the whole etching and patterned etching structures. The current-voltage characteristics between the ohmic electrodes presented sharper slope, higher saturation current and lower knee voltage on patterned etching structures. After Cl2 plasma etching on ohmic area surface of AlGaN, the surface oxide layers and the pollutants were removed, therefore, the surface roughness of the ohmic metal reduced obviously, and the surface morphology improved. Meanwhile, the side area induced in patterned etching provided more extra contact area, which increased the tunneling current. The different apertures and the duty factor of patterned etching were investigated, and the results indicated that the quantity of side area produced in patterned etching dominated the reduction effect of ohmic contact resistance. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction GaN material and its heterostructure are very promising candidates in the applications of RF and power devices due to large bandgap, high critical electric field and high saturation velocity [1,2]. Low connect resistance is very important for RF device to enhance the frequency characteristics [3]. In power devices, the channel resistance of on-state is affected by the series resistance induced by ohmic contacts [4]. Therefore, as one of the significant issues which concern GaN devices, the resistance of ohmic contact need to be further reduced. N-doped in source and drain region by ion implantation is a useful method to reduce the contact resistance [5]. A HEMT with regrowed n+ GaN on ohmic area have been made to obtain a very low contact resistance of 0.08 X mm [6]. However, the above process is rather complicated. The surface treatment process was also utilized to reduce contact resistance [7,8]. The ohmic electrode of TiN/TiSi2 on the uneven AlGaN surface formed by dry etching was investigated [9].
q Supported by the National Natural Science Foundation of China under Grant Nos. 61574110, 61574112 and 61474091. ⇑ Corresponding author. E-mail address: [email protected] (C. Wang).
In this paper, a process called ‘‘patterned etching” is presented to reduce the contact resistance on the AlGaN/GaN heterostructure. The surface morphology and the contact resistance characteristics with different etching time and different proportions of etching area under ohmic electrodes were investigated.
2. Experiments The AlGaN/GaN structure used in this paper was grown by metal organic chemical vapour deposition (MOCVD) on a (0 0 0 1) sapphire substrate. The growth process starts with a 30 nm GaN nucleation layer followed by the growth of a 1.8 lm GaN buffer layer, a 20 nm AlGaN barrier layer with an Al content of 27%. The two dimensional electron gas (2DEG) with a sheet carrier concentration of 1.32 1013 and mobility of 1634 cm2/Vs was obtained by Hall measurements. There were three structures of different ohmic pre-process employed in this experiment, as shown in Fig. 1. The first one was conventional structures without extra etching process, the second one was realized by etching the whole area of source and drain area on the AlGaN layers under the contact metal, and the last one was realized by etching selected parts of the regions using specific masks to form the structures. We referred to them as
http://dx.doi.org/10.1016/j.sse.2016.12.001 0038-1101/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
2
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
6
6
'
'
6
'
7L$O 1L$X
7L$O 1L$X
$O*D1
$O*D1
$O*D1
*D1OD\HU
*D1OD\HU
*D1OD\HU
6XEVWUDWH
6XEVWUDWH
6XEVWUDWH
D
E
F
Fig. 1. The fabrication of (a) ‘‘conventional structures”, (b) ‘‘whole etching structures” and (c) ‘‘patterned etching structures”.
‘‘conventional structures”, ‘‘whole etching structures” and ‘‘patterned etching structures”, respectively. Apart from the different ohmic pre-process technology, the above three kinds of structures had the same device processes. The mesa was formed using Cl2 plasma dry etching in a reactive ion etching (RIE) system. Unlike the conventional structures, patterned etching structures needed a lithograph process in the source and drain ohmic region to form the holes pattern prior to the Ti/Al/ Ni/Au (20 nm/140 nm/55 nm/45 nm) metal stack deposition. The holes were formed using Cl2 plasma dry etching in RIE system, and the etching power was 50 W. The etching times were set to 10 s, 15 s, 20 s, 25 s, 35 s and 45 s, respectively. Then different etching depths were obtained. In addition, different apertures of holes were etched by changing the etching patterns, in which 0.8 lm (small holes), 1.6 lm (medium holes), and 3 lm (large holes) apertures were used, and the duty factor of etching area in every structures were 15%, 30%, and 45%, respectively. The AlGaN surface morphology images of different patterned etching structures are shown in Fig. 2. Then the source/drain ohmic contact electrodes of Ti/Al/Ni/Au were annealed at 830 °C for 30 s. The device parameters were measured using the Keithley 4200 semiconductor parameter analyzer. The contact resistances were determined by current-voltage measurements employing a linear TLM method [4–6].
microscope and by SEM, respectively. The photos revealed that the conventional structures had the roughest surface morphology. Apparently, the whole etching structures showed the smoothest surface. The irregular surface oxide layers and the pollutants were removed during etching process, and the smoother surface was obtained, which benefited the lower contact resistances and the higher breakdown voltages of buffer layer [7–9]. Fig. 3(c1)–(c3) shows the optical micrograph of ohmic contact regions after annealing with different etching apertures and duty factors. Fig. 3(d) shows the SEM photo of patterned etching structures. The 3D atomic force microscope (AFM) was employed to analyze the ohmic contact surface morphology of three kinds of structures, as can be seen in Fig. 4. The root mean square (RMS) value of surface roughness for conventional, whole etching and patterned etching structure were about 39.25 nm, 19.84 nm and 43 nm, respectively. The whole etching process on AlGaN surface can reduce the RMS of ohmic metal surface effectively. However, the RMS of the ohmic metal surface with patterned etching process has no obvious change. The small etching holes area and larger scan scope of AFM may be responsible for the RMS result. Fig. 5 shows the elements spectral on the material surface after etching process by employed SEM. The SEM was used for the detection of elements qualitative. The test result presents that no extra element was founded in AlGaN surface, which was suggested that no impurities were introduced to AlGaN barrier layers surface in the etching process.
3. Results and discussion 3.2. Electrical properties 3.1. Surface morphology As shown in Fig. 3, the squares with the area of 54 lm ⁄ 54 lm was selected in different conditions, the surface morphology of TLM electrode regions after annealing was conducted by optical
Reduction of the AlGaN layer thickness in ohmic electrode region before metal deposition was proposed to reduce contact resistance [10]. In addition, it has been found that there was an optimal thickness of AlGaN layer for good contact resistances char-
Fig. 2. The surface morphologies image of patterned etching structures by SEM.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
3
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
(a)
(b)
(c2)
(c1)
(c3)
(d)
Fig. 3. Surface morphology of TLM electrode regions of (a) conventional structures, (b) whole etching structures, (c1) patterned etching structures with large holes, (c2) with medium holes, (c3) with small holes, (d) the SEM photo of patterned etching structures.
acteristics [11]. The thickness of AlGaN layer in ohmic region can be adjusted by etching time. Therefore, the experiments with different etching times were carried out to find out the best process condition for the optimization of ohmic contact. TLM patterns with spacings (L) of 3, 5, 8, 13, 20 lm were used for the contact resistance measurement, and width (W) is 100 lm. Resistance measurements between each pair of contacts can be used to construct the TLM graph as shown in Fig. 6. The measured total resistance is R = (L/W)Rs + 2Rc. From the graph, the parameter Rc can be determined. In the limit of a zero-length resistor, the total resistance would be just twice the contact resistance. They can be found from Fig. 6 by extrapolating back to L = 0. The contact resistance parameters extracted from the TLM data are shown in Fig. 6. The comparison of the contact resistances between whole etching structures and patterned etching structures on varying etching time is shown Fig. 7. Compared with conventional structures (When etching time is 0 s, it represents the condition of conventional structures), the contact resistance decreased from 0.46 X mm for conventional to a minimum of 0.35 X mm for the whole etching structure and 0.18 X mm for the patterned etching structure, respectively. On the one hand, the lower contact resistance of the patterned etching structures was attributed to the removal of the irregular surface oxide layers and the pollutants by etching process [12]. On the other hand, patterned etching will produce much sidewall area, which reacts with Ti to form TiN after annealing. It will generate more N vacancies. Meanwhile, fringing effects near the edges resulted from uneven AlGaN layer thickness on the patterned etching structures can provide more 2DEG density [11], thus as shown in Fig. 7, the contact resistances with the patterned etching was lower than that with the whole etching structures. It is also found that patterned etching with 20 s was the optimal condition. In
addition, when the etching time was more than 45 s, the contact resistances increase rapidly, which is far more than the values on the conventional structures (0.46 X mm). This trend can be explained by that very thin AlGaN layers resulted in a decrease in 2DEG concentration induced by the polarization in the AlGaN layer, then the ohmic contact degraded [11]. It is know that N vacancies corresponded to the increase of the donor doping density. Therefore, the tunneling effect can be improved. The result can be illustrated by the following expressions:
@J Rc ¼ @V
1
X cm2
ð1Þ
V¼0
where
J t / exp
Eoo ¼
eh 2
e/Bn E00 sffiffiffiffiffiffiffiffiffiffiffi Nd es mn
ð2Þ
ð3Þ
In which, it can be seen that as the doping concentration Nd increases, the electric field intensity E00 increases, and the tunneling current Jt is increased, thus the contact resistance Rc decreases. N vacancies on the surface of AlGaN can be generated in etching process, and the formation of the TiN in ohmic metal annealing was also benefit to the formation of the N vacancies [13]. The patterned etching process will produce much sidewall area to react with Ti so as to get more N vacancies. Therefore, the N vacancies introduced by the patterned etching process resulted in an increase in tunneling current [14] and a lower contact resistance.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
4
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
(a)Convenonal
RMS=39.25 nm
(b)Whole etching
RMS=19.84 nm
(c) Paerned etching
RMS=43 nm
Fig. 4. Surface scan images of 3D AFM for (a) conventional ohmic contact, (b) whole etching ohmic contact and (c) patterned etching ohmic contact. The images are 3D AFM with a scan area of 10 lm 10 lm.
Intensity / a.u.
10000
Ga
1000 Al
Ga
100 N
10
1
0
2
4 6 Energy / KeV
8
10
Fig. 5. The elements spectral diagram of the AlGaN surface after etching by SEM.
The TLM current-voltage characteristics between 3 lm spacing electrons were examined, which were also set at different etching time, and the voltage swept from 0 to 15 V. The linear regions of curves of I versus V are shown in Fig. 8. The I–V current characteristics between the ohmic electrodes presented sharper slope on 20 s patterned etching structures, which indicated that the ohmic contact of 20 s patterned etching structures is the optimal. As shown in Fig. 9, the test data of optimal patterned etching condition (20 s patterned etching) and the optimal whole etching condition (20 s whole etching) were showed together for comparison. The saturation current was 1080 mA/mm for the patterned etching, 950 mA/mm for the whole etching and 900 mA/mm for the conventional structures, respectively. The higher saturation current and lower knee voltage were obtained on 20 s patterned etching structures. In order to investigate the further optimization result based on patterned etching structures for enhancing the contact resistances,
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
5
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
120
Resistant(Ohm)
100 80 60
y=3.628x+9.213
y=3.189x+7.042
40
I(mA/mm)
conventional: Rc=0.46 ohm mm whole etching: Rc=0.35 ohm·mm patterned etching: Rc=0.18 ohm·mm
TLM Model
20
3 μm
5 μm
8 μm
13 μm
20 μm
y=3.711x+3.555 0
0
2
4
6
Contact resistance (ohm.mm)
whole etching small holes etching medium holes etching large holes etching
0.6 0.5 0.4
Table 1 Parameters of the holes with different etching apertures and duty factors on patterned etching structures. Sample
Length (lm)
Width (lm)
Quantity
Duty factor (%)
Side area produced (lm2)
Contact resistance area Large holes Medium holes Small holes
120 3 1.6 0.8
100 3 1.6 0.8
1 594 1428 2940
100 44.55 30.46 15.68
0 5595 ⁄ h 7174 ⁄ h 7385 ⁄ h
Note: h represents the depth of etching holes which depending on the etching time.
0.3 0.2 0.1 0.0 -5
0
5
10 15 20 25 30 35 40 45 50
Etching time (s) Fig. 7. Dependence of contact resistances on etching time for structures with whole etching and patterned etching.
900 800 700
I(mA/mm)
20s patterned etching
Fig. 9. The optimal TLM I–V characteristics curves of 3 lm spacing electrodes.
Fig. 6. Measurement of contact resistance for samples with patterned etching and whole etching compared to a conventional sample using TLM test structure.
0.7
conventional 20s whole etching
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 V(V)
8 10 12 14 16 18 20 22 24
Spacing(µm)
0.8
1200 1100 1000 900 800 700 600 500 400 300 200 100 0
600 500
300 200 100 0.5
4. Conclusion In this paper, a new structure called ‘‘patterned etching structures” was introduced and investigated. The lowest contact resistance of 0.18 X mm was realized by patterned etching structures. The I–V current characteristics between the ohmic electrodes presented sharper slope, higher saturation current and lower knee voltage on patterned etching structures. It is explained by formation of sidewall area on surface of AlGaN during the patterned etching process, which will generate more tunneling current and then reduce the ohmic contact resisitance. Furtherly, by setting different apertures of holes on patterned etching structures, it was proved that quantity of side area produced in patterned etching dominated the reduction effect of the ohmic contact resistance.
conventional 15s patterned 20s patterned 25s patterned 35s patterned 45s patterned
400
0 0.0
As illustrated in Fig 7, with patterned etching process, the similar ohmic contact results were obtained by different etching apertures of holes. The side area of different etching apertures of holes is similar, and the quantity of side area produced in patterned etching dominated the reduction of ohmic contact resistance. Therefore, the contact resistance value with different etching apertures is very close.
1.0
1.5
2.0
2.5
V(V) Fig. 8. TLM I–V curves of 3 lm spacing electrodes on patterned structures at different etching time.
different etching apertures of holes were realized by changing the etching patterns, in which 0.8 lm, 1.6 lm, and 3 lm apertures, namely 15%, 30%, and 45% duty factors were used. Hereafter referred to as ‘‘small holes”, ‘‘medium holes” and ‘‘large holes”, respectively. The specific parameters are presented in Table 1.
Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 61574110, 61574112 and 61106106. The measurements were carried out in the Key Lab of Wide Band gap Semiconductor Materials and Devices, Xidian University. References [1] Yue Y, Hu Z, Guo J, Sensale-Rodriguez B, Li G. InAlN/AlN/GaN HEMTs with regrown ohmic contacts and fT of 370 GHz. IEEE Electr Dev Lett 2012;33:988–90.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
6
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
[2] Shinohara K, Regan D, Corrion A, Brown D, Tang Y, Wong J, et al. Self-alignedgate GaN-HEMTs with heavily-doped n+ -GaN ohmic contacts to 2DEG. Int. Electr. Dev. Meeting. 2012:27.2.1–4. [3] Chung JW, Hoke WE, Chumbes EM, Palacios T. AlGaN/GaN HEMT With 300GHz. IEEE Electron Dev Lett 2010;31:195–7. [4] Selvanathan D, Mohammed FM, Bae JO, Adesida I, Bogart KH. Investigation of surface treatment schemes on n-type GaN and Al0.20Ga0.80N. J Vac Sci Technol, B 2005;23:2538–44. [5] Gothard G, German F. CHAPTER 7 - Transmission line modeling method. Time domain electromagnetic; 1999. p. 237–77. [6] Asghari B, Dinavahi V. Novel transmission line modeling method for nonlinear permeance network based simulation of induction machines. IEEE Trans Magn 2011;47:2100–8. [7] Lau W, Tan JB, Singh B. Formation of Ohmic contacts in AlGaN/GaN HEMT structures at 500 °C by Ohmic contact recess etching. Microelectron Reliab 2009;49:558–61. [8] Wang C, Cho SJ, Kim NY. Comparative analysis of nano-scale structural and electrical properties in AlGaN/GaN high electron mobility transistors on SiC and sapphire substrates. J Nanosci Nanotechnol 2013;13:7083–8.
[9] Piazza M, Dua C, Oualli M, Morvan E, Carisetti D, Wyczisk F. Degradation of TiAlNiAu as ohmic contact metal for GaN HEMTs. Microelectron Reliab 2009;49:1222–5. [10] Kim NY, Wang C, Cho SJ. Optimization of ohmic contact metallization process for AlGaN/GaN high electron mobility transistor. Trans Electr Electron Mater 2013;14:32–5. [11] Takei Y, Kamiya M, Tsutsui K, Kakushima K, Saito W, Wakabayashi W, et al. Reduction of contact resistance on AlGaN/GaN HEMT structures introducing uneven AlGaN layers. Phys Status Solidi A 2015;10:1104–9. [12] Buttari D, Chini A, Meneghesso G, Zanoni E, Moran B, Heikman S, et al. Systematic characterization of Cl2 reactive ion etching for improved ohmics in AlGaN/GaN HEMTs. IEEE Electron Dev Lett 2002;23:76–8. [13] Dora Y, Chakraborty A, Heikman S, MacCarthy L, Keller S, Denbaars SP, et al. Effect of ohmic contacts on buffer leakage of GaN transistors. IEEE Electron Dev Lett 2006;27:529–31. [14] Zhou Q, Chen HW, Zhou CH, Feng ZH, Cai SJ, Chen KJ. Schottky source/Drain InAlN/AlN/GaN MISHEMT with enhanced breakdown voltage. IEEE Electron Dev Lett 2012;33:38–40.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area q Wang Chong ⇑, Zhao Meng-Di, He Yun-Long, Zheng Xue-Feng, Wei Xiao-Xiao, Mao Wei, Ma Xiao-Hua, Zhang Jin-Cheng, Hao Yue Key Lab of Wide Band Gap Semiconductor Materials and Devices, Xi’an 710071, China The School of Microelectronics, Xidian University, Xi’an 710071, China
a r t i c l e
i n f o
Article history: Received 21 July 2016 Received in revised form 23 November 2016 Accepted 1 December 2016 Available online xxxx The review of this paper was arranged by Prof. E. Calleja Keywords: AlGaNGaN HEMT Ohmic contact Patterned etching Surface morphology
a b s t r a c t In this paper, the ohmic contact of AlGaNGaN HEMT was optimized by introducing patterned etching in ohmic area, and the conventional structure and whole etching structure were investigated for comparison. The contact resistance decreased from 0.46 X mm for conventional to 0.35 X mm and 0.18 X mm respectively for the whole etching and patterned etching structures. The current-voltage characteristics between the ohmic electrodes presented sharper slope, higher saturation current and lower knee voltage on patterned etching structures. After Cl2 plasma etching on ohmic area surface of AlGaN, the surface oxide layers and the pollutants were removed, therefore, the surface roughness of the ohmic metal reduced obviously, and the surface morphology improved. Meanwhile, the side area induced in patterned etching provided more extra contact area, which increased the tunneling current. The different apertures and the duty factor of patterned etching were investigated, and the results indicated that the quantity of side area produced in patterned etching dominated the reduction effect of ohmic contact resistance. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction GaN material and its heterostructure are very promising candidates in the applications of RF and power devices due to large bandgap, high critical electric field and high saturation velocity [1,2]. Low connect resistance is very important for RF device to enhance the frequency characteristics [3]. In power devices, the channel resistance of on-state is affected by the series resistance induced by ohmic contacts [4]. Therefore, as one of the significant issues which concern GaN devices, the resistance of ohmic contact need to be further reduced. N-doped in source and drain region by ion implantation is a useful method to reduce the contact resistance [5]. A HEMT with regrowed n+ GaN on ohmic area have been made to obtain a very low contact resistance of 0.08 X mm [6]. However, the above process is rather complicated. The surface treatment process was also utilized to reduce contact resistance [7,8]. The ohmic electrode of TiN/TiSi2 on the uneven AlGaN surface formed by dry etching was investigated [9].
q Supported by the National Natural Science Foundation of China under Grant Nos. 61574110, 61574112 and 61474091. ⇑ Corresponding author. E-mail address: [email protected] (C. Wang).
In this paper, a process called ‘‘patterned etching” is presented to reduce the contact resistance on the AlGaN/GaN heterostructure. The surface morphology and the contact resistance characteristics with different etching time and different proportions of etching area under ohmic electrodes were investigated.
2. Experiments The AlGaN/GaN structure used in this paper was grown by metal organic chemical vapour deposition (MOCVD) on a (0 0 0 1) sapphire substrate. The growth process starts with a 30 nm GaN nucleation layer followed by the growth of a 1.8 lm GaN buffer layer, a 20 nm AlGaN barrier layer with an Al content of 27%. The two dimensional electron gas (2DEG) with a sheet carrier concentration of 1.32 1013 and mobility of 1634 cm2/Vs was obtained by Hall measurements. There were three structures of different ohmic pre-process employed in this experiment, as shown in Fig. 1. The first one was conventional structures without extra etching process, the second one was realized by etching the whole area of source and drain area on the AlGaN layers under the contact metal, and the last one was realized by etching selected parts of the regions using specific masks to form the structures. We referred to them as
http://dx.doi.org/10.1016/j.sse.2016.12.001 0038-1101/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
2
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
6
6
'
'
6
'
7L$O 1L$X
7L$O 1L$X
$O*D1
$O*D1
$O*D1
*D1OD\HU
*D1OD\HU
*D1OD\HU
6XEVWUDWH
6XEVWUDWH
6XEVWUDWH
D
E
F
Fig. 1. The fabrication of (a) ‘‘conventional structures”, (b) ‘‘whole etching structures” and (c) ‘‘patterned etching structures”.
‘‘conventional structures”, ‘‘whole etching structures” and ‘‘patterned etching structures”, respectively. Apart from the different ohmic pre-process technology, the above three kinds of structures had the same device processes. The mesa was formed using Cl2 plasma dry etching in a reactive ion etching (RIE) system. Unlike the conventional structures, patterned etching structures needed a lithograph process in the source and drain ohmic region to form the holes pattern prior to the Ti/Al/ Ni/Au (20 nm/140 nm/55 nm/45 nm) metal stack deposition. The holes were formed using Cl2 plasma dry etching in RIE system, and the etching power was 50 W. The etching times were set to 10 s, 15 s, 20 s, 25 s, 35 s and 45 s, respectively. Then different etching depths were obtained. In addition, different apertures of holes were etched by changing the etching patterns, in which 0.8 lm (small holes), 1.6 lm (medium holes), and 3 lm (large holes) apertures were used, and the duty factor of etching area in every structures were 15%, 30%, and 45%, respectively. The AlGaN surface morphology images of different patterned etching structures are shown in Fig. 2. Then the source/drain ohmic contact electrodes of Ti/Al/Ni/Au were annealed at 830 °C for 30 s. The device parameters were measured using the Keithley 4200 semiconductor parameter analyzer. The contact resistances were determined by current-voltage measurements employing a linear TLM method [4–6].
microscope and by SEM, respectively. The photos revealed that the conventional structures had the roughest surface morphology. Apparently, the whole etching structures showed the smoothest surface. The irregular surface oxide layers and the pollutants were removed during etching process, and the smoother surface was obtained, which benefited the lower contact resistances and the higher breakdown voltages of buffer layer [7–9]. Fig. 3(c1)–(c3) shows the optical micrograph of ohmic contact regions after annealing with different etching apertures and duty factors. Fig. 3(d) shows the SEM photo of patterned etching structures. The 3D atomic force microscope (AFM) was employed to analyze the ohmic contact surface morphology of three kinds of structures, as can be seen in Fig. 4. The root mean square (RMS) value of surface roughness for conventional, whole etching and patterned etching structure were about 39.25 nm, 19.84 nm and 43 nm, respectively. The whole etching process on AlGaN surface can reduce the RMS of ohmic metal surface effectively. However, the RMS of the ohmic metal surface with patterned etching process has no obvious change. The small etching holes area and larger scan scope of AFM may be responsible for the RMS result. Fig. 5 shows the elements spectral on the material surface after etching process by employed SEM. The SEM was used for the detection of elements qualitative. The test result presents that no extra element was founded in AlGaN surface, which was suggested that no impurities were introduced to AlGaN barrier layers surface in the etching process.
3. Results and discussion 3.2. Electrical properties 3.1. Surface morphology As shown in Fig. 3, the squares with the area of 54 lm ⁄ 54 lm was selected in different conditions, the surface morphology of TLM electrode regions after annealing was conducted by optical
Reduction of the AlGaN layer thickness in ohmic electrode region before metal deposition was proposed to reduce contact resistance [10]. In addition, it has been found that there was an optimal thickness of AlGaN layer for good contact resistances char-
Fig. 2. The surface morphologies image of patterned etching structures by SEM.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
3
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
(a)
(b)
(c2)
(c1)
(c3)
(d)
Fig. 3. Surface morphology of TLM electrode regions of (a) conventional structures, (b) whole etching structures, (c1) patterned etching structures with large holes, (c2) with medium holes, (c3) with small holes, (d) the SEM photo of patterned etching structures.
acteristics [11]. The thickness of AlGaN layer in ohmic region can be adjusted by etching time. Therefore, the experiments with different etching times were carried out to find out the best process condition for the optimization of ohmic contact. TLM patterns with spacings (L) of 3, 5, 8, 13, 20 lm were used for the contact resistance measurement, and width (W) is 100 lm. Resistance measurements between each pair of contacts can be used to construct the TLM graph as shown in Fig. 6. The measured total resistance is R = (L/W)Rs + 2Rc. From the graph, the parameter Rc can be determined. In the limit of a zero-length resistor, the total resistance would be just twice the contact resistance. They can be found from Fig. 6 by extrapolating back to L = 0. The contact resistance parameters extracted from the TLM data are shown in Fig. 6. The comparison of the contact resistances between whole etching structures and patterned etching structures on varying etching time is shown Fig. 7. Compared with conventional structures (When etching time is 0 s, it represents the condition of conventional structures), the contact resistance decreased from 0.46 X mm for conventional to a minimum of 0.35 X mm for the whole etching structure and 0.18 X mm for the patterned etching structure, respectively. On the one hand, the lower contact resistance of the patterned etching structures was attributed to the removal of the irregular surface oxide layers and the pollutants by etching process [12]. On the other hand, patterned etching will produce much sidewall area, which reacts with Ti to form TiN after annealing. It will generate more N vacancies. Meanwhile, fringing effects near the edges resulted from uneven AlGaN layer thickness on the patterned etching structures can provide more 2DEG density [11], thus as shown in Fig. 7, the contact resistances with the patterned etching was lower than that with the whole etching structures. It is also found that patterned etching with 20 s was the optimal condition. In
addition, when the etching time was more than 45 s, the contact resistances increase rapidly, which is far more than the values on the conventional structures (0.46 X mm). This trend can be explained by that very thin AlGaN layers resulted in a decrease in 2DEG concentration induced by the polarization in the AlGaN layer, then the ohmic contact degraded [11]. It is know that N vacancies corresponded to the increase of the donor doping density. Therefore, the tunneling effect can be improved. The result can be illustrated by the following expressions:
@J Rc ¼ @V
1
X cm2
ð1Þ
V¼0
where
J t / exp
Eoo ¼
eh 2
e/Bn E00 sffiffiffiffiffiffiffiffiffiffiffi Nd es mn
ð2Þ
ð3Þ
In which, it can be seen that as the doping concentration Nd increases, the electric field intensity E00 increases, and the tunneling current Jt is increased, thus the contact resistance Rc decreases. N vacancies on the surface of AlGaN can be generated in etching process, and the formation of the TiN in ohmic metal annealing was also benefit to the formation of the N vacancies [13]. The patterned etching process will produce much sidewall area to react with Ti so as to get more N vacancies. Therefore, the N vacancies introduced by the patterned etching process resulted in an increase in tunneling current [14] and a lower contact resistance.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
4
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
(a)Convenonal
RMS=39.25 nm
(b)Whole etching
RMS=19.84 nm
(c) Paerned etching
RMS=43 nm
Fig. 4. Surface scan images of 3D AFM for (a) conventional ohmic contact, (b) whole etching ohmic contact and (c) patterned etching ohmic contact. The images are 3D AFM with a scan area of 10 lm 10 lm.
Intensity / a.u.
10000
Ga
1000 Al
Ga
100 N
10
1
0
2
4 6 Energy / KeV
8
10
Fig. 5. The elements spectral diagram of the AlGaN surface after etching by SEM.
The TLM current-voltage characteristics between 3 lm spacing electrons were examined, which were also set at different etching time, and the voltage swept from 0 to 15 V. The linear regions of curves of I versus V are shown in Fig. 8. The I–V current characteristics between the ohmic electrodes presented sharper slope on 20 s patterned etching structures, which indicated that the ohmic contact of 20 s patterned etching structures is the optimal. As shown in Fig. 9, the test data of optimal patterned etching condition (20 s patterned etching) and the optimal whole etching condition (20 s whole etching) were showed together for comparison. The saturation current was 1080 mA/mm for the patterned etching, 950 mA/mm for the whole etching and 900 mA/mm for the conventional structures, respectively. The higher saturation current and lower knee voltage were obtained on 20 s patterned etching structures. In order to investigate the further optimization result based on patterned etching structures for enhancing the contact resistances,
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
5
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
120
Resistant(Ohm)
100 80 60
y=3.628x+9.213
y=3.189x+7.042
40
I(mA/mm)
conventional: Rc=0.46 ohm mm whole etching: Rc=0.35 ohm·mm patterned etching: Rc=0.18 ohm·mm
TLM Model
20
3 μm
5 μm
8 μm
13 μm
20 μm
y=3.711x+3.555 0
0
2
4
6
Contact resistance (ohm.mm)
whole etching small holes etching medium holes etching large holes etching
0.6 0.5 0.4
Table 1 Parameters of the holes with different etching apertures and duty factors on patterned etching structures. Sample
Length (lm)
Width (lm)
Quantity
Duty factor (%)
Side area produced (lm2)
Contact resistance area Large holes Medium holes Small holes
120 3 1.6 0.8
100 3 1.6 0.8
1 594 1428 2940
100 44.55 30.46 15.68
0 5595 ⁄ h 7174 ⁄ h 7385 ⁄ h
Note: h represents the depth of etching holes which depending on the etching time.
0.3 0.2 0.1 0.0 -5
0
5
10 15 20 25 30 35 40 45 50
Etching time (s) Fig. 7. Dependence of contact resistances on etching time for structures with whole etching and patterned etching.
900 800 700
I(mA/mm)
20s patterned etching
Fig. 9. The optimal TLM I–V characteristics curves of 3 lm spacing electrodes.
Fig. 6. Measurement of contact resistance for samples with patterned etching and whole etching compared to a conventional sample using TLM test structure.
0.7
conventional 20s whole etching
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 V(V)
8 10 12 14 16 18 20 22 24
Spacing(µm)
0.8
1200 1100 1000 900 800 700 600 500 400 300 200 100 0
600 500
300 200 100 0.5
4. Conclusion In this paper, a new structure called ‘‘patterned etching structures” was introduced and investigated. The lowest contact resistance of 0.18 X mm was realized by patterned etching structures. The I–V current characteristics between the ohmic electrodes presented sharper slope, higher saturation current and lower knee voltage on patterned etching structures. It is explained by formation of sidewall area on surface of AlGaN during the patterned etching process, which will generate more tunneling current and then reduce the ohmic contact resisitance. Furtherly, by setting different apertures of holes on patterned etching structures, it was proved that quantity of side area produced in patterned etching dominated the reduction effect of the ohmic contact resistance.
conventional 15s patterned 20s patterned 25s patterned 35s patterned 45s patterned
400
0 0.0
As illustrated in Fig 7, with patterned etching process, the similar ohmic contact results were obtained by different etching apertures of holes. The side area of different etching apertures of holes is similar, and the quantity of side area produced in patterned etching dominated the reduction of ohmic contact resistance. Therefore, the contact resistance value with different etching apertures is very close.
1.0
1.5
2.0
2.5
V(V) Fig. 8. TLM I–V curves of 3 lm spacing electrodes on patterned structures at different etching time.
different etching apertures of holes were realized by changing the etching patterns, in which 0.8 lm, 1.6 lm, and 3 lm apertures, namely 15%, 30%, and 45% duty factors were used. Hereafter referred to as ‘‘small holes”, ‘‘medium holes” and ‘‘large holes”, respectively. The specific parameters are presented in Table 1.
Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 61574110, 61574112 and 61106106. The measurements were carried out in the Key Lab of Wide Band gap Semiconductor Materials and Devices, Xidian University. References [1] Yue Y, Hu Z, Guo J, Sensale-Rodriguez B, Li G. InAlN/AlN/GaN HEMTs with regrown ohmic contacts and fT of 370 GHz. IEEE Electr Dev Lett 2012;33:988–90.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001
6
C. Wang et al. / Solid-State Electronics xxx (2016) xxx–xxx
[2] Shinohara K, Regan D, Corrion A, Brown D, Tang Y, Wong J, et al. Self-alignedgate GaN-HEMTs with heavily-doped n+ -GaN ohmic contacts to 2DEG. Int. Electr. Dev. Meeting. 2012:27.2.1–4. [3] Chung JW, Hoke WE, Chumbes EM, Palacios T. AlGaN/GaN HEMT With 300GHz. IEEE Electron Dev Lett 2010;31:195–7. [4] Selvanathan D, Mohammed FM, Bae JO, Adesida I, Bogart KH. Investigation of surface treatment schemes on n-type GaN and Al0.20Ga0.80N. J Vac Sci Technol, B 2005;23:2538–44. [5] Gothard G, German F. CHAPTER 7 - Transmission line modeling method. Time domain electromagnetic; 1999. p. 237–77. [6] Asghari B, Dinavahi V. Novel transmission line modeling method for nonlinear permeance network based simulation of induction machines. IEEE Trans Magn 2011;47:2100–8. [7] Lau W, Tan JB, Singh B. Formation of Ohmic contacts in AlGaN/GaN HEMT structures at 500 °C by Ohmic contact recess etching. Microelectron Reliab 2009;49:558–61. [8] Wang C, Cho SJ, Kim NY. Comparative analysis of nano-scale structural and electrical properties in AlGaN/GaN high electron mobility transistors on SiC and sapphire substrates. J Nanosci Nanotechnol 2013;13:7083–8.
[9] Piazza M, Dua C, Oualli M, Morvan E, Carisetti D, Wyczisk F. Degradation of TiAlNiAu as ohmic contact metal for GaN HEMTs. Microelectron Reliab 2009;49:1222–5. [10] Kim NY, Wang C, Cho SJ. Optimization of ohmic contact metallization process for AlGaN/GaN high electron mobility transistor. Trans Electr Electron Mater 2013;14:32–5. [11] Takei Y, Kamiya M, Tsutsui K, Kakushima K, Saito W, Wakabayashi W, et al. Reduction of contact resistance on AlGaN/GaN HEMT structures introducing uneven AlGaN layers. Phys Status Solidi A 2015;10:1104–9. [12] Buttari D, Chini A, Meneghesso G, Zanoni E, Moran B, Heikman S, et al. Systematic characterization of Cl2 reactive ion etching for improved ohmics in AlGaN/GaN HEMTs. IEEE Electron Dev Lett 2002;23:76–8. [13] Dora Y, Chakraborty A, Heikman S, MacCarthy L, Keller S, Denbaars SP, et al. Effect of ohmic contacts on buffer leakage of GaN transistors. IEEE Electron Dev Lett 2006;27:529–31. [14] Zhou Q, Chen HW, Zhou CH, Feng ZH, Cai SJ, Chen KJ. Schottky source/Drain InAlN/AlN/GaN MISHEMT with enhanced breakdown voltage. IEEE Electron Dev Lett 2012;33:38–40.
Please cite this article in press as: Wang C et al. Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area. Solid State Electron (2016), http://dx.doi.org/10.1016/j.sse.2016.12.001