Enhancement of AlGaN/GaN High Electron Mobility Transistor Off-State Drain Breakdown Voltage via Backside Proton Irradiation

The demand for radiation-resistant electronic devices spans multiple disciplines such as radar and communication systems, aerospace, nuclear design. For space applications, the most important type of particles is protons with energies up to hundreds of MeV. Early experiments performed that the radiation tolerance of AlGaN/GaN is much higher than for their AlGaAs/GaAs counterparts.  There are many rad-hardness studies have been carried out using protons of various energies on AlGaN/GaN HEMTs. Typically, irradiated HEMTs exhibited degradations on drain saturation current (I_DS) and transconductance (g_m).  Recently, it was reported that the off-state breakdown voltage (V_BR) and the critical voltage during the off-state drain-voltage step-stress improved for the proton irradiated HEMTs. The defects created by the high energy proton irradiation were evenly distributed throughout the entire HEMT structure.  Thus, it would be very difficult to identify the mechanism of drain breakdown voltage and step-stress critical voltage improvement.   In this work, protons were irradiated from the backside of the HEMT structure through via holes fabricated on Si substrate, which was used as the substrate to grow the HEMT structure.  DC performance before and after irradiation was performed and electric field distribution around the gate electrode was simulated with FLOODs and TCAD.

                The AlGaN/GaN HEMT structure was grown by MOCVD on top of Si wafers. The layer structure included an AlN nucleation layer, followed with a series of composition-graded Al_xGa_1-xN transition layers to reduce the strain, an 800 nm GaN buffer layer and capped with a 16 nm unintentionally-doped Al_0.26Ga_0.74N barrier layer. Device fabrication started with Ohmic contacts formation by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å).  The contacts were annealed at 850°C for 45 s under N₂. Device isolation was achieved with multiple-energy and multiple-dose of N⁺implantations. 70 nm silicon nitride deposited by PECVD was used for device passivation. 1-µm gates were defined by lift-off of e-beam deposited Ni/Au metallization.  The wafer front-side process was finished with e-beam evaporated Ti/Pt/Au (300 Å/300 Å /2500 Å) interconnection contacts.

The ranges of proton irradiation energy and dose were 225 to 340 keV and 1×10¹² cm^-2 to 5×10¹² cm^-2, respectively. Prior to the proton implantation, rectangular via holes were formed to remove the Si under GaN HEMT structure by etching through the Si substrate from the back side of the sample with a standard BOSCH process in a STS deep reactive ion etching system using AZ9260 photoresist as the etching mask. Figure 1 shows the SEM picture of the rectangular via holes created on Si substrate.  Those via holes were formed directly beneath HEMT active area.

Stopping and Range of Ions in Matter (SRIM) was used to simulate proton penetration depths and vacancies generated by the implantations. Figure 2 illustrates the vacancy distribution. The tail of the vacancy distribution for the back side implantation is sharper, thus defects, mainly gallium vacancies, can be precisely put in the AlGaN transistor layer, GaN buffer layer or GaN/AlGaN 2 dimension electron gas (2DEG) channel layer.  

The effect of proton irradiation on I_ds, g_m, V_BR is listed in Table I. The V_BR improvement was only found at the condition of proton energy equals to 340keV, in which the defects were created at 2DEG region. Moreover, V_BR improvement irradiation with 340keV and 1×10¹² /cm²was not observed and indicating a certain amounts of defects at 2DEG region needed to form virtual gate to have the improvement.

                FLOODS and TCAD simulation tools were also used to model the electric field distribution around the gate region.  These simulation results will be presented.