Investigation of Au-free AlGaN/GaN HEMT on Silicon with substrate transfer process: From Ohmic contacts to RF performance

Abstract

The technological importance of III-nitride based high electron mobility transistors (HEMTs) for power switching and RF applications is growing rapidly, owing to two advantageous electrical properties. While high sheet carrier concentration and mobility of charge carriers confined in the 2DEG translate to high current levels, high bandgap of III-N based materials leads to high breakdown voltage. However, III-N based devices have not been able to penetrate the semiconductor market as deep as Silicon based technologies. High costs associated with the epitaxial growth of the III-N based HEMT stack on hetero-substrates like Si, SiC or Sapphire as well as high costs associated with III-N based device processing are the two key reasons. While development of III-N growth on Silicon can potentially bring down the net cost associated with growth, CMOS compatible fabrication of III-N based devices can reduce the cost associated with device processing as this will permit the processing of III-N devices in existing CMOS fab lines. The most important difference between conventional and CMOS compatible processing of III-N based devices is the absence of gold (Au) in CMOS process, since Au has high diffusion rates in Si even at moderate temperatures and it also acts as a deep impurity in Silicon leading to formation of electrical trap levels in Silicon bandgap leading to degradation in device performance. Further, sputtering is preferred over electron beam evaporation for metal deposition in CMOS processes from the reliability standpoint. CMOS also uses a lower thermal budget when compared to III-N processing. In this work, we focus on the development of Au-free fabrication process for AlGaN/GaN based RF HEMT devices. The first portion of this work deals with the process development for fabrication of Au-free Ohmic contacts to AlGaN/GaN HEMT. Conventionally, Ti/Al/Ni/Au based metal stack deposition by electron beam evaporation followed by high temperature post metal annealing (PMA) at 850 ℃ in N2 has been used for Ohmic contact formation. In this work, we have developed a Ti/Al/Ti/W based metal stack deposited by sputtering and followed by a PMA step at relatively moderate temperature of 600 ℃ in N2. However, it is preceded by AlGaN barrier recess etch step. In this work, we study the effect of etch chemistry (digital etch using BCl3/O2 and low etch rate BCl3/Cl2 RIE etch) and metallization scheme (using sputtered Ti/Al and Ti/Al/Ti/W stacks) on the recessed Au-free Ohmic contacts. An optimum recess etch of entire AlGaN barrier using digital etching leads to better uniformity in contact resistance (RC). Further, use of Ti/W cap layer on Ti/Al leads to low contact resistance of 0.56 Ω-mm with a smooth contact surface morphology. Possible mechanism for carrier transport through the contacts has been discussed based on temperature dependent electrical characterization and field emission is found to be the dominant mechanism of carrier transport. The second portion of the work deals with electrical performance comparison of Au-based and Au-free AlGaN/GaN High electron mobility transistors (HEMT) on Silicon (Si). The chemical composition of both types of contacts i.e., Ti/Al/Ti/W (Au-free) and Ti/Al/Ni/Au (Au-based) have been studied using transmission electron microscope (TEM). For the former, top W layer is found to be restricted in its interaction with the lower metal layers, leading to a continuous W cap at the contact surface which results in a 20x improvement in the surface roughness for the Au-free contacts. Transistors have been fabricated with Ni/W and Ni/Au based gate metal stacks for Au-free and Au-based processes respectively. From HEMT measurements, devices with Au-free contacts are found to exhibit +0.4 V shift in the threshold voltage and a 10x increase in the gate leakage which is attributed to the strain associated with the sputtered W-capping layer of the Ni/W gate-metal stack and plasma induced damage caused at the barrier surface due to high-power sputter deposition of gate contact. The lowest RC value of 0.4 Ω-mm obtained for the Au-free Ohmic contacts is comparable to the RC value of 0.38 Ω-mm obtained for the Au-based Ohmic contacts used in this work.

Flexible III-N based device technology is rapidly gaining attention for use in RF and optical applications. Efficient III-N based RF devices on flexible substrates can lead to high performing conformal wireless communication and RADAR systems, while efficient III-N based optical devices and associated circuitry on bendable and inexpensive polymer substrates can lead to highly efficient futuristic consumer electronic products like foldable transparent mobiles, billboards etc. A CMOS compatible III-N device process followed by economical and relatively simple wafer-level transfer method onto inexpensive polymer-based flexible substrates can lead to scalable and high-performance III-N device technology on flexible and conformal substrates. In the third portion of this work, we report on the electrical performance of AlGaN/GaN HEMT fabricated using Au-free process, after being transferred onto flexible Kapton. Electrical characteristics of the flexible HEMT indicate5-10% higher on-current when bent with radius of curvature of 2 cm (at low drain bias voltages), while the off-state performance remains unaffected. Initially, 2DEG properties like field-effect mobility and carrier concentration have been extracted. While FATFET measurements indicate negligible change in field-effect mobility, C-V measurements indicate ~10% reduction in 2DEG concentration after transfer. The comparison of electrical characteristics of the Au-free HEMT’s indicates ~50% reduction in on-current of the transferred devices, which is attributed to heating of the transistor channel caused due to low thermal conductivity of the polymer Kapton tape. Electrical characteristics of the flexible HEMT carried out under drain pulsing further support the above observation.

The last portion of this work deals with process development and performance characterization of Au-free AlGaN/GaN RF HEMT. DC, small signal RF and pulsed I-V measurements have been carried out to ascertain the performance of the Au-free RF HEMT. De-embedded fT of > 40 GHz were achieved in 2 x 50 μm devices with 250 nm gate length and source-to-drain spacing of 6 μm. Substrate ramp measurements have been carried out in order to ascertain the significant drain lag current collapse seen in the devices and negative charge stored in the buffer under high drain bias is found to be the reason for the large drain lag seen in the devices. Delay-time analysis and small-signal modelling of the devices were carried out and the small-signal simulated S-parameters match well with the measured S-parameters of the actual devices. In conclusion, we have developed Au-free III-N based HEMT process for fabrication of AlGaN/GaN HEMT and compared their performance with conventionally processed Au-based devices. While these devices have been transferred on low-cost Kapton as a work in the direction of flexible GaN based devices, the same process has been used to fabricate RF devices with good small signal characteristics.