GaN HEMTs with various high work function gate metal designs

Microelectronics Reliability 51 (2011) 2163–2167

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Thermal stability investigations of AlGaN/GaN HEMTs with various high work function gate metal designs Hsien-Chin Chiu ⇑, Chao-Wei Lin, Che-Kai Lin, Hsuan-Ling Kao, Jeffrey S. Fu Dept. of Electronics Engineering, Chang Gung University, Taoyuan, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 1 December 2010 Received in revised form 13 May 2011 Accepted 13 May 2011 Available online 12 June 2011

a b s t r a c t The operation of high power RF transistor generates a huge amount of heat and thermal effect is a major consideration for improving the efficiency of power transistors. AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have been studied extensively because of their high thermal conductivity. This study comprehensively investigates the DC, low frequency noise, microwave and RF power performance of Al0.27Ga0.73N/GaN HEMTs on silicon substrates at temperatures from room temperature to 100 °C using high work function metals such as palladium (Pd) and iridium (Ir) gate metals. Although the conventional Ni gate exhibited a good metal work function with AlGaN, which is beneficial for increasing the Schottky barrier height of HEMTs, the diffusion of Ni metal toward the AlGaN and GaN layers influences the DC and RF stability of the device at high temperatures or over long-term operation. Pd and Ir exhibited less diffusion at high temperature than Ni, resulting in less degradation of device characteristics after high temperature operation. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Wide-bandgap AlGaN/GaN high electron mobility transistors (HEMTs) are emerging as excellent candidates for use in RF/microwave power amplifiers (PAs) because of their high frequency and favorable power handling characteristics [1]. However, thermal effects can be an important issue in RF power devices, because a huge amount of heat is generated during their operation [2]. Nibased gate metallization has been widely used to form the Schottky gate contacts in AlGaN/GaN HEMTs because Ni has a high Schottky barrier (0.97 eV). However, the rapid diffusion of Ni into AlGaN/GaN constitutes a reliability issue since the device parameters, such as transconductance, threshold voltage, and gate capacitance are altered at high temperature or in the long-term operation [3,4]. Furthermore, in maintaining a wide gate voltage swing at high power, the thermal stability of the Schottky barrier performance of a GaN HEMT is significant [5,6]. Therefore, a Schottky gate that is stable at high temperatures; exhibits significantly less metal diffusion and has a high gate barrier must be developed. In this study, the temperature-dependent characteristics of AlGaN/ GaN HEMTs with a Pd–GaN and an Ir–GaN Schottky contact are investigated and compared with those of conventional Ni–GaN Schottky contact devices. The inert material work functions of Pd, Ir and Ni are 5.6 eV, 5.67 eV and 5.35 eV, respectively. A metal with a high work function helps to yield a high barrier of the GaN Schottky gate electrode. With respect to their DC, low frequency noise, ⇑ Corresponding author. Tel.: +886 3 2118800; fax: +886 3 2118507. E-mail address: [email protected] (H.-C. Chiu). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.05.012

microwave and RF power performance at temperatures from room temperatures to 100 °C, Pd and Ir exhibit low diffusion in chemical solution and high thermal stability because of their high melting points (1555 °C for Pd and 2466 °C for Ir) [7,8]. The barrier heights obtained from the Schottky current–voltage (I–V) curves of the devices were 0.601 eV, 0.703 eV and 0.639 eV for Ni–GaN, Pd–GaN and Ir–GaN, respectively. Since the conducting carriers are less influenced by temperature and Pd-gate HEMTs and Ir-gate HEMTs have favorable Schottky diode characteristics, the Schottky barrier height and ideality factor of both devices degrade slightly with variation in temperature, improving device reliability. Devices with a Pd-gate and an Ir-gate covered by a thick copper film and then a thin gold film exhibit a lower gate resistance compared to devices without the thick copper film, resulting in better DC and RF characteristics at high temperatures. Therefore, Pd-buried gate and Irburied gate in GaN-based HEMTs are very promising for use in microwave power device applications at high temperature. 2. Structure and fabrication of devices Fig. 1 schematically depicts the cross-section of GaN/AlGaN HEMTs that were grown by metal–organic chemical vapor deposition (AP-MOCVD) on 4-in. silicon substrates. A 0.8-lm-thick undoped GaN layer and a channel layer were grown on top of a 1.8-lm-thick undoped GaN buffer layer, and an 18-nm undoped Al0.27Ga0.73N Schottky layer was inserted between the GaN channel layer and the 1-nm undoped GaN cap layer. The designed structure exhibited a sheet charge density of 1.03  1013 cm 2 and a Hall mobility of 1534 cm2/V s at 300 K. During device fabrication, the

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secondary-ion mass spectrometry (SIMS) measurements were made. Fig. 2 shows the results. The SIMS measurement conditions were an energy of 25 keV and an operating current of 2 pA. Fig. 2 shows the SIMS depth profiles of AlGaN/GaN with an Ni-gate, Pdgate and Ir-gate. Clearly, the intense peaks were obtained from the Ni, Pd and Ir atoms at the interface between the gate metal and the GaN cap layer. After stable baking, the Ni diffused through the AlGaN Schottky and almost reached the two-dimensional gas (2DEG) of the HEMT. This critical diffusion of Ni might have caused the diffusion of Cu into GaN and generated a strong gate leakage current [10,11]. Because of their high work functions, the Pd and Ir exhibit shallower metal diffusion and higher melting points. Moreover, the electron beam evaporation of Ir made the chamber hot during the evaporation procedure, forming an intermixed material of Ir and AlGaN because of the high melting point of Ir (2466 °C). Therefore, the Pd-gate exhibited the least diffusion of metal into GaN. Fig. 1. The structure cross section of the AlGaN/GaN HEMT.

3. Comparison of DC characteristics of devices To investigate the current–voltage characteristics, the Schottky diode performance of Ni-gate, Pd-gate and Ir-gate HEMTs was examined. Fig. 3a plots the Schottky gate leakage current characteristics. The gate leakage currents of the Pd-gate and Ir-gate HEMTs were lower than that of the Ni-gate HEMTs, because Pd and Ir have a lower diffusivity and a higher work function than Ni [12]. Moreover, the gate turn-on voltages, at which a gate current of 1 mA/mm flowed, were extracted from the Schottky diode I–V curves at room temperature as 1.18 V, 1.3 V and 1.23 V for the Ni-gate, Pd-gate and Ir-gate HEMTs, respectively. A thermally stable Schottky barrier height is important in determining the ability of the gate terminal to control the channel as well as the reliability

Gate-to-Drain Current, I gd (mA/mm)

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Ni-gate Pd-gate Device Ir-gate o -4 -4 -4 Slope eV/ C -6.40x10 -3.33x10 -4.53x10

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Gate-to-Drain Voltage, Vgd (volt)

Ideality Factor, n

active region was protected by a photoresist and the mesa isolation region was removed using BCl3 + Cl2 mixed gas plasma in a reactive ion etching (RIE) chamber. Ohmic contacts of the Ti/Al/Ni/Au (30 nm/125 nm/50 nm/100 nm) metals were formed by electron beam evaporation, and patterned by conventional optical lithography with lift-off. The contacts then underwent rapid thermal annealing (RTA) an 850 °C for 30 s in a nitrogen-rich chamber. After the gate finger lithographic pattern had formed and the surface had been cleaned, the samples were immediately loaded into an electron beam deposition chamber. The 1.0-lm gate-length Pd/ Cu/Au and Ir/Cu/Au (20 nm/200 nm/20 nm) composite metals were deposited to form gate electrodes. For comparison, a traditional Ni/Cu/Au Schottky gate AlGaN/GaN HEMTs was also fabricated. To form the composite gate, Cu was used because of its low resistivity and a top Au layer was deposited to prevent its oxidization. Finally, the interconnection level was a Ti/Cu/Au (30 nm/ 1000 nm/20 nm) metal layer and a 200 nm-SiO2 layer was deposited to passivate the device. The Ti/Cu/Au were sequentially deposited by using electron beam evaporator and a thermally stable photo-resist (LOR) was also adopted for thick metal lift-off process. In addition, the chamber temperature needs to be controlled under 50 °C. The device was then placed in a 300 °C N2-rich oven for 4 h to walk-out the transistors and thereby ensure uniformity [9]. Because of their high work function, the metals had a high resistance, resulting in a higher gate resistance and a smaller device bandwidth. The copper metal that was adopted in this study reduced the gate resistance and significantly reduced the signal loss. To identify the interfacial reactions between the Ni-gate, Pd-gate, Ir-gate contacts and AlGaN/GaN epitaxial layers,

Device o Slope n/ C

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SIMS Depth Profile (nm) Fig. 2. SIMS depth profiles of the Ni-gate, Pd-gate and Ir-gate GaN HEMT.

Fig. 3. Comparison of (a) gate leakage current (b) Schottky barrier height and ideality factor versus temperatures for three devices.

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600

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slopes of the fitted curves, were 1.073 mA/mm 1/ °C, 0.425 mA/mm 1/ °C and 0.671 mA/mm 1/ °C, respectively. These devices all had a higher DC output resistance at 100 °C, indicating that greater self-heating caused lattice scattering and thereby reduced impact ionization in the channel. These results are corresponding directly to the degradation of Schottky gate performance [2]. Low-frequency noise measurements, which were sensitive to the semiconductor interface, were made to elucidate further the relationship between the low-frequency noise and the interface property of the metal–semiconductor contact. The bias that was applied to make the low-frequency noise measurements was Vds = 8 V, associated with an Ids of 100 mA/mm, for all three devices. Since the series resistance of the device strongly dominates the low-frequency noise, the use of identical Ids bias points is critical to comparing fairly the flicker noise characteristics of the devices. As presented in Fig. 5, the Pd-gate HEMTs and Ir-gate HEMTs had a lower noise spectral density than the conventional Ni-gate HEMTs because Pd and Ir are chemically more inert than Ni and they also have higher melting point such that they diffuse less and also are better diffusion barrier against copper diffusion. 4. Microwave power characteristics of device at high temperatures

400 300

The microwave performance of 1  100 lm2-gate devices was determined from 100 MHz to 30 GHz, using an HP-8364C automatic network analyzer and Cascade Microtech RF on-wafer probes. All devices were biased at a gate voltage with peak gm and Vds of 8 V. The S-parameter matrix for the two-port network

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Ir-gate Ni-gate Pd-gate -2.67x10-2 -1.33x10-2 -1.73x10-2

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of the device. Improving the Schottky barrier height is expected to reduce the gate leakage current, which improvement is particularly important to minimize the power consumption of power transistors and the degradation of their linearity at high power operation [2]. The temperature-dependent barrier heights and ideality factors of the Schottky diodes were determined from the Ig–Vg characteristics and the results are shown in Fig. 3b [13,14]. From room temperature to 100 °C, the barrier height (0.601–0.553 eV) of the Ni-gate HEMTs varies by 7.99% while the ideality factor (1.38–1.53) varies by 10.87%. The corresponding numbers for the Pd-gate devices are 3.56% (0.703–0.678 eV) and 3.91% (1.28– 1.33) and those for the Ir-gate devices are 5.32% (0.639–0.605 eV) and 6.01% (1.33–1.41). Fig. 4a plots the DC characteristic of a device with a gate length of 1.0-lm gate-length. It plots two sets of drain-to-source current (Ids) versus drain-to-source voltage (Vds) curves for the Ni-gate, Pdgate and Ir-gate HEMTs – one set measured at room temperature and the other measured at 100 °C. The Pd-gate exhibits a small variation in drain current at high bias because it has a high Schottky barrier, proving that the Pd-gate characteristics are less affected by temperature. However, a significant drop drain current in the Ni-gate HEMTs at 100 °C is caused mostly by the decrease in channel mobility that is itself caused by lattice scattering. This phenomenon significantly degrades the performance of the Schottky diode [15]. Fig. 4b plots the DC transconductance and maximum drain current of the device. The temperature-variation of the Pd-gate device transconductance (gm) characteristics reveals greater independence from temperature, with a smaller decrease of gm, 6.7%, from room temperature to 100 °C (from 141.5 mS/mm to 132 mS/mm, respectively). Over this temperature range, the corresponding decrease of gm for Ir-gate HEMTs was 12.4% (from 132.25 mS/mm at room temperature to 115.9 mS/mm at 100 °C). For Ni-gate HEMTs, the decrease was almost 23.4% (from 123.35 mS/mm at room temperature to 94.45 mS/mm at 100 °C). The maximum drain currents for Ni-gate, Pd-gate and Ir-gate devices, obtained from the

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Fig. 6. The temperature dependent of fT and fmax from 25 °C to 100 °C for three devices.

H.-C. Chiu et al. / Microelectronics Reliability 51 (2011) 2163–2167

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was the one that is probably the most commonly used and served as the basic building block of higher-order matrices for the transistor intrinsic and extrinsic elements of small-signal networks. The relationship between the reflected power and incident power waves obtained from S parameters, which were used to determine the transistor gain and the bandwidth at microwave frequencies. Calculating the small-signal gain (S21) yields the current gain cutoff frequency (fT) and maximum oscillation frequency (fmax) [2]. Fig. 6 shows the fT and fmax of the three devices versus operating temperature (from room temperature to 100 °C). Both the fT and fmax of the Pd-gate HEMTs and the Ir-gate HEMTs are thermally stable (9.5– 8.5 GHz for fT and 15.7–14.2 GHz for fmax of Pd-gate HEMTs; 7.6– 6.3 GHz for fT and 12–10.2 GHz for fmax of Ir-gate HEMTs); however, for the Ni-gate HEMTs, fT and fmax degraded by almost 32% and 26%, respectively, over this temperature range. Moreover, the slope of linear fit to fT for the Ni-gate was 2.67  10 2 GHz/°C, which was much steeper than those for the Pd-gate and Ir-gate HEMTs, which were 1.33  10 2 GHz/°C and 1.73  10 2 GHz/°C, respectively. Similarly, the slope of the linearly fitted fmax for the Ni-gate was 3.73  10 2 GHz/°C, which was also steeper than those for the Pd-gate and Ir-gate HEMTs, which were 2  10 2 GHz/°C and 2.4  10 2 GHz/°C, respectively. Temperature-dependent microwave power measurements were made using a load-pull system with automatic tuners to measure the optimum load impedance that maximize output power. The microwave load-pull power performance was evaluated at 3.5 GHz, with a drain bias of 8 V, for all three devices, each with 1  100 lm2 gate. The gate bias was chosen for class-AB operation with an output current of 50 mA/mm for each device. The three devices were operated at identical DC power consumptions (Pdc) to compare their power performance fairly. When the temperature was increased to 100 °C, the Ir-gate HEMTs exhibited a lower microwave power degradation owing to the higher melting point of Ir, a lower gate leakage and a higher thermal stability. As shown in Fig. 7, the Ir-gate HEMTs exhibited a Pout shift from 13.83 to 13.61 dBm and a linear Gp from 9.51 to 7.18 dB. These corresponding values for the Pd-gate HEMTs were a Pout shift of 14.58 to 14.15 dBm and a linear Gp of 11.97 to 9.59 dB. However, for the Ni-gate, the microwave power was significantly reduced by raising the temperature to 100 °C. Since FET power performance is heavily determined by the current density, Schottky diode and device linearity, the Pd-gate and Ir-gate HEMTs had better characteristics not only at room temperature but also at high temperatures. Therefore, the microwave power performance of Pd-gate and Irgate HEMTs was more temperature-independent than that of Nigate HEMTs. Based on the materials characterization of the devices discussed above the performance of Pd-gate and Ir-gate HEMTs is thermally

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stable, in contrast to that of Ni-gate HEMTs. Generally, a thermally robust device is also a reliable device. Aging tests were performed on the three devices. Fig. 8 shows the normalized Ids and Schottky barrier height variations for the Ni-gate, Pd-gate and the Ir-gate HEMTs during an aging bias-stress test at room temperature. After 72 h of continuous operation at Vgs = 0 V and Vds = 8 V, the channel current of the Ni-gate HEMTs had dropped by 46% from its original value, whereas those of the Pd-gate and Ir-gate HEMTs dropped by only 25% and 28%, respectively. Moreover, the Schottky barrier heights of the Pd-gate and the Ir-gate HEMTs during after 72 h of operation were reduced from 0.703 eV to 0.543 eV and 0.639 eV to 0.454 eV, respectively, because the high work functions of the metals, low gate leakages and high thermal stabilities. However, the Ni-gate HEMTs exhibited a serious degradation of barrier height from 0.601 eV to 0.337 eV, because of the high gate leakage current. The results in Fig. 8 are consistent with the claim that Pdgate and Ir-gate HEMTs are more thermally stable and electrically reliable. 5. Conclusion The Schottky contacts, DC, RF and microwave power characteristics of devices were examined. Experimental results indicate that the lower diffusivity and gate leakage of Pd-gate and Ir-gate devices than those of Ni-gate devices caused them to exhibit better thermal performance and lower low-frequency noise. Pd-gate and Ir-gate devices also exhibited better thermal stability of DC, RF and the microwave power performance because these metals had high melting points and low diffusivities on AlGaN/GaN Schottky gate contacts. The superior thermal stability, high current driving capacity and favorable device linearity demonstrate that

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Pd-gate HEMTs and Ir-gate HEMTs are very promising for use in microwave power devices.

[6]

Acknowledgements The authors would like to thank the Nano Device Labs (NDL) for providing the low frequency noise measurements. This work was financially supported by the National Science Council, ROC (NSC97-2221-E-182-048-MY3) and facility supports of High Speed Intelligent Communication (HSIC) Research Center of Chang Gung University, Taoyuan, Taiwan.

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