Investigations of GaN-based vertical field effect transistors for applications in high-power electronics

• Applicants: Professor Dr. Oliver Ambacher; Professor Dr. Ferdinand Scholz
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 339032420

• Final Report Year: 2022

Final Report Abstract

In this project, we have investigated various possibilities how to realize current-blocking layers (CBL) with respective current apertures for ”Current Aperture Vertical Transistors” (CAVETs). Besides the classical approach (dry-etching of the striped openings), UUlm particularly investigated a method where the openings were defined by selective area epitaxy making dry-etching of the semiconductor material obsolete. We obtained a very good definition of such open stripes with still homogeneous properties of the surrounding GaN:Mg CBL. By systematic variations of the MOVPE growth conditions, a fairly flat and feature-less overgrowth of the top HEMT structure on such striped surfaces could be obtained. These epitaxial approaches show slightly worse Mg profiles as compared to the implantation approach studied in Freiburg (see below). Anyway, the Mg profile was sharp enough for VFET applications particularly after the (anyway needed) growth interruption between CBL growth and the HEMT overgrowth. Investigations about the blocking behaviour of the CBL have shown that a fairly low Mg concentration of [Mg] ≈ 5 · 10^18 cm^-3 is sufficient for electrical blocking. The respective break-through measurements, however, were strongly influenced by parasitic break-through processes via surface-related defects. Such effects are not expected to play a role in a final device. With Zn instead of Mg as dopant for the CBL, we observed that an abrupter doping profile can be realized. Also with this dopant, a fair blocking behaviour could be measured for medium Zn concentrations. Hence Zn is a very promissing candidate to be used in a CAVET. Unfortunately, only few CAVET samples with Mg-doped CBLs optimized at project partner UUlm have been processed to full devices. They showed partly puzzling results, but the functionality of the CBL could be clearly confirmed. In Freiburg, two different process variants for CBL realization were analyzed: The CBLs were generated either by selective growth (different approach than studied in Ulm) or Mg implantation and their structural development was discussed. Subsequently, the CAVETs were finalized using AlGaN/GaN overgrowth. The structural and electrical characterization of the CAVET structures gave significantly better results for CAVETs with Mgimplanted CBL, which was explained by the better surface morphology and the lower Mg carryover into the overgrown GaN channel. Further investigations into the two process strategies were carried out by measuring fully processed transistors. Based on the results of the element distribution, the structural and electrical characterization as well as the analysis on the device level, it could be deduced that the fabrication by means of Mg-implantation leads to a significantly improved device performance in terms of on-state, transfer and offstate properties. In particular, it was found that the leakage current through the CBL was comparatively lower, which is owing to the reduced penetration of the electric field into the CBL due to the higher Mg doping concentration. In order to subsequently prove the robustness of the component design in combination with the implantation-based manufacturing process, a suitable way to scale up the components was developed. Largearea CAVET multifinger structures were proposed by combining the optimum intrinsic CAVET design of small transistors with the proven comb structure of the lateral HEMT. Reliable on-state, transfer- and off-state characteristics were demonstrated for the scaled-up components, demonstrating the suitability of the developed layout in combination with the new process technology. The findings of this work were applied to a component design and have led to a CAVET with the highest absolute current of over 20 A, which means an increase by a factor of 103 compared to the prior art. At the same time, it was found that the critical field strength in the largest fabricated devices is close to the small gate width devices shown previously. The demonstrated pulse power stability of almost 1 kW is close to the theoretical limit for lateral HEMTs, showing that the CAVET technology is suitable for use in power electronics.

Publications

• “Investigations of Mg-doped GaN as current-blocking layers in VFET”, 18th European Workshop on Metalorganic Vapour Phase Epitaxy (EWMOVPE XVIII), Vilnius, Lithuania, June 2019
  A. Ipsen, J.-P. Scholz, M. Kateb, P. Döring, M. Mikulla, P. Waltereit, L. Kirste, O. Ambacher, and F. Scholz
  
• “Optimization of Metal-Organic Chemical Vapor Deposition Regrown n-GaN”, Phys. Status Solidi (b) (2019)
  S. Leone, P. Bruckner, L. Kirste, P. Döring, T. Fuchs, S. Müller, M. Prescher, R. Quay, O. Ambacher
  
• “Towards vertical HEMTs – HEMTs overgrown on trenched p-GaN”, 33rd DGKK Workshop Epitaxy of III/V Semiconductors, Dresden, Germany, Dec. 2019
  J.-P. Scholz, C. Chen, P. Döring, L. Kirste, P. Waltereit, and F. Scholz
  
• “Growth and Fabrication of Quasi Vertical Current Aperture Vertical Electron Transistor Structures”, Phys. Status Solidi (a) (2020)
  P. Döring, R. Driad, S. Leone, S. Müller, P. Waltereit, L. Kirste, V. Polyakov, M. Mikulla, and O. Ambacher
  
• “Metal organic chemical vapour deposition regrown large area GaN-on-GaN current aperture vertical electron transistors with high current capability”, Electron. Letters (2021)
  P. Döring, R. Driad, R. Reiner, P. Waltereit, M. Mikulla, and O. Ambacher