Silicon Nitride Thickness Dependent Electrical Properties of InAlN/GaN Heterostructures

Introduction

Lattice-matched InAlN/GaN/Si based high electron mobility transistors (HEMTs) have recently attracted much attention as a candidate for next generation high-power, high-temperature and high-frequency devices, due to their higher charge density than the conventional AlGaN/GaN heterostructure [1]. However, HEMT performance, reliability, integrity and robustness are determined by the contact materials employed and passivation. Various dielectrics, such as SiO₂, Si₃N₄, AlN, MgO, Al₂O₃, HfO₂, Sc₂O₃, etc have been investigated for passivation. Among them, Si₃N₄ is the most widely used. Extensive improvement in electrical properties of the AlGaN/GaN/Si heterostructure achieved by the SiN_x passivation process has been reported [2]. Such improvements by SiN_x passivation were believed to be due to the reduction of the surface states, change in the AlGaN potential barrier height or increase of the 2DEG density at the channel due to tensile stress [3-5]. In spite of various reports on SiN_x pasivation of InAlN/GaN heterostructures, SiN_x thickness dependent studies have not been extensively explored to date. In the present study, we report SiN_x thickness dependent electrical properties and its effect on the device performance of InAlN/GaN heterostructures.

Experiment

Nearly lattice-matched InAlN/GaN/Si epi-wafer, with a sheet resistance of 348.9 Ω/sq, was used in the present study. Prior to ohmic contact formation, InAlN/GaN/Si substrate surface was ultrasonically cleaned with acetone, IPA and DI water. Hf (15)/Al (200)/Ta (20) nm ohmic contacts were deposited by sputtering followed by rapid thermal annealing in vacuum at 600 ^oC for 1 min. No surface treatment was performed prior to passivation. Passivation SiN_x layer of varying thickness ranging from 25 to 200 nm was deposited at 300°C by PECVD using silane and ammonia as precursor gases. SiN_x layer on the ohmic electrodes was removed by etching in buffered hydrofluoric acid. To characterize our samples, room-temperature Hall measurements using van der Pauw geometry and ultraviolet (UV) Photoluminescence (PL) spectra excited using 266 nm laser source were performed.

Results and Discussion

Figure 1 shows the room-temperature Hall measurements of InAlN/GaN/Si as a function of the passivation SiN_x layer thickness. Increase in sheet carrier concentration (n_s), whereas decrease in mobility (μ) is observed with increasing SiN_x passivation layer thickness. A thickness of ~100 nm can be viewed as optimum as it yields a minimum sheet resistance (R_s), as shown in Fig. 1. The corresponding increase in n_s is substantial of almost ~30%, with respect to sample without SiN_x passivation. Although the reason for the significant increase in n_s is not apparent, yet our preliminary studies by means of UV PL reveal a shift in the PL peak position of the GaN channel layer (Fig. 2), which is most likely due to band bending under stress. However, possibility of reduction of surface states of the passivated InAlN and/or an increase in positive charge at the SiN_x/InAlN interface cannot be ruled out. Additionally, the decrease in μ is possibly due to increased electron-electron scattering and/or interface roughness scattering with such a high carrier concentration in 2DEG channel. Interestingly, the trend of increase in 2DEG density is similar to that observed for AlGaN/GaN heterostructure under different SiN_x passivation layer thickness [6]. This observation indicates that the intrinsic property of SiN_x film may be playing a critical role in the enhancement of 2DEG density rather than the interface property. Further studies to establish the mechanisms responsible for the enhancement in 2DEG density as well as to evaluate the influence of passivation layer thickness on the device performance are in progress.

Conclusion

In conclusion, surface passivation of InAlN/GaN heterostructures with PECVD SiN_x of varying thickness has been investigated. A thickness of ~ 100 nm is found to be optimum and has yielded a minimum sheet resistance. The corresponding 2DEG density change is substantially higher (by ~30 %) than sample without SiN_x passivation, and this enhancement is attributed possibly to band bending under stress.

References

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