Chongbiao Luan Zhaojun Lin Yuanjie Lv Zhihong Feng Jingtao Zhao Qihao Yang Ming Yang

Transcription

1 Appl. Phys. A DOI /s Enhanced effect of side-ohmic contact processing on the 2DEG electron density and electron mobility of In 0.17 Al 0.83 N/AlN/GaN heterostructure field-effect transistors Chongbiao Luan Zhaojun Lin Yuanjie Lv Zhihong Feng Jingtao Zhao Qihao Yang Ming Yang Received: 19 January 2014 / Accepted: 21 March 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract From the capacitance voltage curves and current voltage characteristics of the In 0.17 Al 0.83 N/AlN/ GaN heterostructure field-effect transistors (HFETs) with side-ohmic contacts and normal-ohmic contacts, twodimensional electron gas (2DEG) electron mobility was calculated. It is found that the polarization Coulomb field scattering (PCF) is closely related to the normal-ohmic contact processing, and the PCF was weakened by side- Ohmic contact processing in In 0.17 Al 0.83 N/AlN/GaN HFETs, similar to that in AlGaN/AlN/GaN HFET devices. Further, due to the stronger spontaneous polarization in the thinner In 0.17 Al 0.83 N barrier layer, the influence of the gate bias on the PCF in In 0.17 Al 0.83 N/AlN/GaN HFETs is greater than that in AlGaN/AlN/GaN HFETs. As a result, the PCF in In 0.17 Al 0.83 N/AlN/GaN HFETs with side-ohmic contacts is stronger than that in AlGaN/ AlN/GaN HFETs with side-ohmic contacts. Moreover, the 2DEG electron density in the In 0.17 Al 0.83 N/AlN/GaN HFETs with side-ohmic contacts is increased by more than twice compared with the 2DEG electron density in the In 0.17 Al 0.83 N/AlN/GaN HFETs with normal-ohmic contacts. C. Luan Z. Lin (&) J. Zhao Q. Yang M. Yang School of Physics, Shandong University, Jinan , China linzj@sdu.edu.cn C. Luan luanchongbiao@163.com Y. Lv Z. Feng National Key Laboratory of Application Specific Integrated Circuit (ASIC), Hebei Semiconductor Research Institute, Shijiazhuang , China 1 Introduction The lattice-matched In x Al 1-x N/AlN/GaN heterostructure field-effect transistors (HFETs), in which In x Al 1-x N is lattice matched to GaN while simultaneously offering a high spontaneous polarization charge (x * 0.18), wide bandgap and large refractive index contrast with respect to GaN, have attracted great attention due to their broad and important applications in high-frequency and highpower electronic devices [1 4]. Yue et al. [5] demonstrated a record current gain cutoff frequency (f T ) of 370 GHz for a 30-nm-gate-length device. The electron mobility in the two-dimensional electron gas (2DEG) channel is also crucial in the ultimate performance of an In x Al 1-x N/AlN/GaN HFET. In the normal-ohmic contact processing, the Ohmic contact metal directly deposited on the In x Al 1-x N barrier layer and the gate bias are the crucial factors which generate the strain variation of the In x Al 1-x N barrier layer [6 9].The polarization Coulomb field scattering (PCF) related to the strain variation of the In x Al 1-x N barrier layer, caused by the normal-ohmic contact processing and the Schottky gate bias, has an important influence on the 2DEG electron mobility in both the rectangular and circular In 0.18 Al 0.82 N/AlN/GaN HFET devices with normal-ohmic contacts [6]. In this work, to remove the influence of the normal-ohmic contact processing on the strain variation of the In Al 0.83 N barrier layer, we used side-ohmic contact processing to form the Ohmic contacts of In 0.17 Al 0.83 N/AlN/ GaN HFET devices [10]. By studying the 2DEG electron mobility in In 0.17 Al 0.83 N/AlN/GaN HFETs with side- Ohmic contacts, the relationship between the PCF and the Ohmic contact processing can be determined, and the comparison between In 0.17 Al 0.83 N/AlN/GaN HFETs and AlGaN/AlN/GaN HFETs made [11]. Moreover, the

2 C. Luan et al. influence of side-ohmic contact processing on the 2DEG electron density of In 0.17 Al 0.83 N/AlN/GaN HFETs can also be determined. 2 Experiments Fig. 1 Schematic illustration of the In 0.17 Al 0.83 N/AlN/GaN HFET with normal-ohmic contacts (a) and with side-ohmic contacts (b) The heterostructure layer was epitaxially grown by metal organic chemical vapor deposition (MOCVD) on a (0001) sapphire substrate (see Fig. 1 for detailed description). Hall measurements indicated a sheet carrier density of around /cm 2 and an electron mobility of 1,900 cm 2 /V s at room temperature. Both rectangular and circular In 0.17 Al 0.83 N/AlN/ GaN HFET devices with normal-ohmic contacts and side-ohmic contacts were fabricated (Fig. 1). The device processing was the same as that in reference [5]. For all the rectangular devices, the source and Fig. 2 The measured C V curves at room temperature for samples 1, 2, 3, 4 (a), samples 5, 6, 7, 8 (b), 1 0,2 0,3 0,4 0 (c), and 5 0,6 0,7 0,8 0 (d)

3 Side-Ohmic contact processing in In 0.17 Al 0.83 N/AlN/GaN transistors drain contacts were both rectangular, 100 lm inwidth and 50 lm in length, and the space between the source and drain was 100 lm. For all the circular devices, the source contact was circular with a radius of 50 lm, and the drain was a ring with an inside radius of 150 lm and an outside radius of 210 lm. These contacts were all annealed in a rapid thermal annealing system, and with transmission line method patterns, the specific resistivity of the devices with normal-ohmic contacts and side-ohmic contacts were measured to be and X cm 2, respectively. For the rectangular devices with side- Ohmic contacts and normal-ohmic contacts, the Ni/ Au rectangular Schottky contacts were both symmetrically placed in the middle between the source and drain, and their sizes were 20/100 lm (length/width), 40/100 lm, 60/100 lm, and 80/100 lm, which were marked as 1, 2, 3, 4 and 1 0, 2 0, 3 0, 4 0, respectively. For the circular devices with side-ohmic contacts and normal-ohmic contacts, the contact sizes of Ni/Au ring Schottky contacts were both 180/220 lm (inside diameter/outside diameter), 160/240 lm, 140/260 lm, and 120/280 lm, which were marked as 5, 6, 7, 8 and 5 0, 6 0, 7 0, 8 0, respectively. Capacitance voltage (C V) curves were measured at room temperature using an Agilent B1520A at 1,000 khz, and current voltage (I V) characteristics were also measured at room temperature using an Agilent B1500A semiconductor parameter analyzer. The scanning electron microscope with energy dispersive spectrometer (SEM EDS) measurements were performed at room temperature using JSM-6700F and INCA x-sight 7421 with an accelerating voltage of 15 kv. 3 Results and discussions Figure 2a, b shows the C V curves of the rectangular and circular samples with side-ohmic contacts, respectively. Figure 2c, d shows the C V curves of Fig. 3 The calculated 2DEG electron density n 2D under different gate biases at room temperature for samples 1, 2, 3, 4 (a), samples 5, 6, 7, 8 (b), 1 0,2 0,3 0,4 0 (c), and 5 0,6 0,7 0,8 0 (d)

4 C. Luan et al. the rectangular and circular samples with normal- Ohmic contacts, respectively. The C V measurements were obtained using the source contact and the Ni Schottky contact. From the measured C V curves, the 2DEG electron density n 2D under different gate biases for all the samples can be calculated [12] and the resultsareshowninfig.3a d. From Fig. 3, it can be seen that the 2DEG electron densities of In Al 0.83 N/AlN/GaN HFETs with side-ohmic contacts are increased by more than twice compared with the 2DEG density of the In 0.17 Al 0.83 N/AlN/GaN HFETs with normal-ohmic contacts. The reason is that a huge amount of Ohmic contact metal atoms diffuse into the In 0.17 Al 0.83 N barrier layer, which can induce the crystal structure variation of the In 0.17 Al 0.83 N barrier layer, and the spontaneous polarization of the In 0.17 Al 0.83 N barrier layer will be reduced by the variation of the crystal structure. As a result, the 2DEG electron density in the In 0.17 Al 0.83 N/AlN/GaN HFETs with normal-ohmic contacts is decreased. The I V characteristics for all the prepared samples were measured and are shown in Figs. 4, 5, 6 and 7. In the calculation of the 2DEG electron mobility, the values of current I DS with a source drain voltage of 100 mv at different gate biases were used for all the samples; the calculation method was the same as described in references [12, 13], and the calculated results are shown in Fig. 8a d. The In 0.17 Al 0.83 N/AlN/GaN HFET samples 1, 2, 3, 4, 5, 6, 7, and 8 in Fig. 8a, b correspond to the samples 1 0, 2 0, 3 0, 4 0, 5 0, 6 0, 7 0, and 8 0 in Fig. 8c, d, respectively. With the corresponding samples, the materials and the device sizes are the same, and the difference is that the Ohmic contacts are normal in Fig. 8c, d, and the Ohmic contacts in Fig. 8a, b are formed with side-ohmic contact processing. Fig. 4 The measured I V curves at room temperature for samples 1, 2, 3, and 4

5 Side-Ohmic contact processing in In 0.17 Al 0.83 N/AlN/GaN transistors Fig. 5 The measured I V curves at room temperature for samples 5, 6, 7, and 8 It is generally considered that ionized impurity scattering, LO phonon scattering, interface roughness scattering and PCF are the dominant scattering mechanisms that affect the 2DEG electron mobility in In 0.17 Al 0.83 N/ AlN/GaN HFET samples [6, 14]. Ionized impurity scattering can be ignored because the materials used here are undoped [12, 13]. For the LO phonon scattering and interface roughness scattering, the electron drift mobility decreases with the increasing 2DEG electron density [15 18] and rises with PCF in In 0.17 Al 0.83 N/AlN/GaN HFETs [6]. From Fig. 8, the different electron mobilities according to the gate bias can be explained as follows: Before the deposition of the Ohmic contact metals and the Schottky contact metals, all the polarization charges at the In 0.17 Al 0.83 N/AlN/GaN interfaces are regularly distributed and cannot scatter electrons in the 2DEG electron channel. Figure 9 is the SEM EDS composition map for one of our devices with side- Ohmic contacts (spectrum 1) and one of the devices with normal-ohmic contacts (spectrum 2). It is shown that, from spectrum 1, there are no Ohmic contact metal atoms found in the area near the Ohmic contact metals for the samples with side-ohmic contacts. But, from spectrum 2, it can be seen that there are a large number of Ohmic contact metal atoms existing in the In Al 0.83 N barrier layer, which are comparable with or larger than the indium atoms in quantity. The phenomenon in Fig. 9 (spectrum 1 and 2) can be explained as follows (same as fore AlGaN/AlN/GaN HFETs). In the In 0.17 Al 0.83 N/AlN/GaN HFETs fabricated with normal-ohmic contact processing (spectrum 2), all the Ohmic contact metal atoms were deposited on the In 0.17 Al 0.83 N barrier layer. During rapid thermal annealing, a large number of Ohmic contact metal atoms diffuse into the In 0.17 Al 0.83 N barrier layer, and it is possible that near the source and drain Ohmic contacts, the crystal structure of the In 0.17 Al 0.83 N barrier layer is varied since the Ohmic contact metal atoms are

6 C. Luan et al. Fig. 6 The measured I V curves at room temperature for samples 1 0,2 0,3 0, and 4 0 comparable with or larger than the indium atoms in quantity. Thus, the spontaneous polarization of the In 0.17 Al 0.83 N barrier layer near the source and drain Ohmic contacts will also be varied, and the polarization charges at the In 0.17 Al 0.83 N/AlN interface which is close to the Ohmic contact metals are distributed irregularly [6]. With the metal atoms concentration gradient, the stress gradient is induced and generated along the strained In 0.17 Al 0.83 N barrier layer. It has been shown that there is two-way coupling between stress and diffusion evolution which includes diffusion-induced stress and stress-enhanced diffusion. Significant contributions to the driving force for diffusion are attributed to the stress gradient [19]. Therefore, with the diffusion driven by the stress gradient, there are a large number of Ohmic contact metal atoms found in the In 0.17 Al 0.83 N barrier layer with normal-ohmic contact processing. But, in the devices fabricated with side-ohmic contact processing (spectrum 1), only a small part of the Ohmic contact metals was deposited on the In 0.17 Al 0.83 N barrier layer (Fig. 1b). Compared with the normal-ohmic contact processing (Fig. 1a), much fewer Ohmic contact metal atoms diffused into the In 0.17 Al 0.83 N barrier layer during rapid thermal annealing; a much smaller concentration gradient for the Ohmic contact metal atoms and the stress gradient along the In 0.17 Al 0.83 N barrier layer were present for the side-ohmic contact processing, which resulted in a much weaker diffusion driving force for the Ohmic contact metal atoms. As a result, few Ohmic contact metal atoms were found in the In 0.17 Al 0.83 N barrier layer with side-ohmic contact processing. In the In 0.17 Al 0.83 N/AlN/GaN HFETs with normal- Ohmic contacts, a large number of Ohmic contact metal

7 Side-Ohmic contact processing in In 0.17 Al 0.83 N/AlN/GaN transistors Fig. 7 The measured I V curves at room temperature for samples 5 0,6 0,7 0, and 8 0 atoms diffused into the In 0.17 Al 0.83 N barrier layer, which could induce the crystal structure variation of the In 0.17 Al 0.83 N barrier layer. As a result, the spontaneous polarization of the In 0.17 Al 0.83 N barrier layer near the source and drain Ohmic contacts was varied [5], and the polarization charges at the In 0.17 Al 0.83 N/AlN interface close to the Ohmic contact metals were distributed irregularly. This irregular polarization charges distribution will result in the PCF to the 2DEG electrons. The crystal structure variation of the In 0.17 Al 0.83 N barrier layer will also decrease the 2DEG electron density in In 0.17 Al 0.83 N/AlN/GaN HFET devices. But in the devices fabricated with side-ohmic contact processing, no Ohmic contact metal atoms were found to diffuse into the In 0.17 Al 0.83 N barrier layer (Fig. 9a); thus there was no variation in the crystal structure of the In 0.17 Al 0.83 N barrier layer. The polarization charges at the In Al 0.83 N/AlN interface close to the Ohmic contact metals were still regularly distributed and could not scatter the 2DEG electrons. Moreover, although there was no strain for the In 0.17 Al 0.83 N barrier layer in the In Al 0.83 N/AlN/GaN heterostructures, the thin AlN interlayer was strained due to the lattice being unmatched to GaN. Moreover, with the converse piezoelectric effect, the gate bias will possibly induce the strain variation of the AlN interlayer in the In 0.18 Al 0.82 N/AlN/ GaN heterostructures [6, 8, 9]. Then, the In 0.17 Al 0.83 N barrier layer will follow the strain variation of the AlN interlayer. But the In 0.17 Al 0.83 N barrier layer is only 4 nm (AlGaN barrier layer is 22.5 nm), the AlN interlayer is 1 nm, and the spontaneous polarization is very strong in our In 0.17 Al 0.83 N/AlN/GaN heterostructures, so the converse piezoelectric effect is stronger in In 0.17 Al 0.83 N/AlN/GaN heterostructures than in AlGaN/ AlN/GaN heterostructures (see Fig. 10); thus, the influence of the gate bias on the variation of the polarization charges density underneath gate metals in the In 0.17 Al 0.83 N barrier layer is much stronger. The

8 C. Luan et al. Fig. 8 The relationship between the 2DEG electron mobility and the applied gate bias at room temperature for samples 1, 2, 3, 4 (a) and samples 5, 6, 7, 8 (b) with side-ohmic contacts, and the samples 1 0, 2 0,3 0,4 0 (c) and samples 5 0,6 0,7 0,8 0 (d) with normal-ohmic contacts in the In 0.17 Al 0.83 N/AlN/GaN heterostructures above reasons are the key factors that generate the PCF in In 0.17 Al 0.83 N/AlN/GaN HFETs. AsshowninFig.8a,b,forthesamples3,4,7,and 8 which were fabricated with side-ohmic contact processing, the LO phonon scattering and the interface roughness scattering were stronger than the PCF in these samples; therefore it results in the monotonic decrease for the 2DEG electron mobility with the gate bias. For the samples 2 and 6 which were fabricated with side-ohmic contact processing, the scattering associated with the PCF which is mainly caused by the gate bias is relatively weak, and the PCF is approximately the same as the LO phonon scattering and the interface roughness scattering. Therefore, the mobility of the 2DEG electrons is almost unchanged with different gate biases. For samples 1 and 5, the PCF caused by the gate bias is also stronger than the LO phonon scattering and the interface roughness scattering (but the influence of the gate bias on the PCF in In 0.17 Al 0.83 N/ AlN/GaN HFETs is stronger than that in AlGaN/AlN/ GaN HFETs in reference [1], see Fig. 1, so it results in the monotonic increase of the 2DEG electron mobility with the gate bias. But, for the samples 1 0,2 0,5 0,and6 0 which were fabricated with normal-ohmic contact processing in Fig. 8c, d, the gradient of the polarization charge density variation which is caused by both the normal-ohmic contact processing and the gate bias is large; thus the PCF is stronger compared with LO phonon scattering and interface roughness scattering in these samples and results in the monotonic increase in the mobility of the 2DEG electrons with the gate voltage. From Fig. 8a d, it is shown that the PCF is not

9 Side-Ohmic contact processing in In 0.17 Al 0.83 N/AlN/GaN transistors Fig. 9 The SEM EDS composition map in the area around 1 lm apart from the Ohmic contact for one of our In 0.17 Al 0.83 N/AlN/GaN HFETs with side-ohmic contacts (spectrum 1) and one of the devices with normal-ohmic contacts (spectrum 2) the dominant scattering mechanism in almost all the samples (except samples 1 and 5) with side-ohmic contact processing, while in the samples 1 0, 2 0, 3 0, 5 0, and 6 0 with normal-ohmic contact processing, the PCF is the dominant scattering mechanism. It demonstrates that the PCF is much weaker in the devices with side- Ohmic contacts than that in the devices with normal- Ohmic contacts. Therefore, the conclusion can be made that the PCF caused by the polarization charge density variation at the In 0.17 Al 0.83 N/AlN interface is closely related to the Ohmic contact processing, and side- Ohmic contact processing weakens the PCF in In Al 0.83 N/AlN/GaN HFETs. Moreover, for the thinner In 0.17 Al 0.83 N barrier layer and the stronger spontaneous polarization in the In 0.17 Al 0.83 N barrier layer, the influence of the gate bias on the PCF in In 0.17 Al 0.83 N/ AlN/GaN HFETs is stronger than that in AlGaN/AlN/ GaN HFETs. Thus, as shown in Fig. 1 the PCF in In 0.17 Al 0.83 N/AlN/GaN HFETs with side-ohmic contacts is stronger than that in AlGaN/AlN/GaN HFETs with side-ohmic contacts. In addition, the 2DEG electron density in In 0.17 Al 0.83 N/AlN/GaN HFETs with side- Ohmic contacts is increased by more than twice compared with the 2DEG density in In 0.17 Al 0.83 N/AlN/ GaN HFETs with normal-ohmic contacts. 4 Conclusion In summary, from the measured C V curves and the I V characteristics for the rectangular and circular In Al 0.83 N/AlN/GaN HFETs with side-ohmic contacts and normal-ohmic contacts, the PCF in In 0.17 Al 0.83 N/AlN/ GaN HFETs with side-ohmic contacts has been investigated. It is found that the PCF is closely related to the normal-ohmic contact processing, and side-ohmic contact processing greatly weakens the PCF in the In Al 0.83 N/AlN/GaN HFETs similar to that in AlGaN/AlN/ GaN HFETs. The influence of the gate bias on the PCF in In 0.17 Al 0.83 N/AlN/GaN HFETs is greater than that in AlGaN/AlN/GaN HFETs, and the PCF in In 0.17 Al 0.83 N/ AlN/GaN HFETs with side-ohmic contacts is stronger than that in AlGaN/AlN/GaN HFETs with side-ohmic contacts. Moreover, the 2DEG electron density in In 0.17 Al 0.83 N/AlN/GaN HFETs with side-ohmic contacts is increased by more than twice compared with the

10 C. Luan et al. Fig. 10 The relationship between the 2DEG electron mobility and the applied gate bias at room temperature for rectangular samples 1, 2, 3, 4 (a) and circular samples 5, 6, 7, 8 (b) with side-ohmic contacts in the In 0.17 Al 0.83 N/AlN/GaN HFETs, and the rectangular samples 1, 2, 3, 4 (a) and circular samples 5, 6, 7, 8 (b) with side-ohmic contacts in the AlGaN/AlN/GaN HFETs 2DEG electron density in the In 0.17 Al 0.83 N/AlN/GaN HFETs with normal-ohmic contacts. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No ), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No ) and Graduate Independent Innovation Foundation of Shandong University, GIIFSDU (Grant No. yzc12064). The authors would like to thank Dr. Wu Lu from Ohio State University for useful discussions. References 1. S. Pandey, B. Fraboni, D. Cavalcoli, A. Minj, A. Cavallini, Appl. Phys. Lett. 99, (2011) 2. D.S. Lee, X. Gao, S.P. Guo, D. Kopp, P. Fay, T. Palacios, IEEE Electron Device Lett. 32, 1525 (2011) 3. L. Zhou, J.H. Leach, X.F. Ni, H. Morkoc, D.J. Smith, J. Appl. Phys. 107, (2010) 4. M. Gonschorek, J.-F. Carlin, E. Feltin, M.A. Py, N. Grandjean, V. Darakchieva, B. Monemar, M. Lorenz, G. Ramm, J. Appl. Phys. 103, (2008) 5. Y.Z. Yue, Z.Y. Hu, J. Guo, B. Sensale-Rodriguez, G.W. Li, R.H. Wang, F. Faria, T. Fang, B. Song, X. Gao, S.P. Guo, T. Kosel, G. Snider, P. Fay, D. Jena, H.L. Xing, IEEE Electron Device Lett. 33, 988 (2012) 6. C.B. Luan, Z.J. Lin, Z.H. Feng, L.G. Meng, Y.J. Lv, Z.F. Cao, Y.X. Yu, Z.G. Wang, J. Appl. Phys. 112, (2012) 7. F. González-Posada Flores, C. Rivera, E. Muñoz, Appl. Phys. Lett. 95, (2009) 8. C. Rivera, E. Muñoz, Appl. Phys. Lett. 94, (2009) 9. A.F.M. Anwar, R.T. Webster, K.V. Smith, Appl. Phys. Lett. 88, (2006) 10. Y.J. Ohmaki, M. Tanimoto, S. Akamatsu, T. Mukai, Jpn. J. Appl. Phys. 45, L1168 (2006) 11. C.B. Luan, Z.J. Lin, Y.J. Lv, L.G. Meng, Y.X. Yu, Z.F. Cao, H. Chen, Z.G. Wang, Appl. Phys. Lett. 101, (2012) 12. J.Z. Zhao, Z.J. Lin, T.D. Corrigan, Z. Wang, Z.D. You, Z.G. Wang, Appl. Phys. Lett. 91, (2007) 13. Y.J. Lv, Z.J. Lin, Y. Zhang, L.G. Meng, C.B. Luan, Z.F. Cao, H. Chen, Z.G. Wang, Appl. Phys. Lett. 98, 512 (2011)

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