Enhancement of AlGaN/GaN High Electron Mobility Transistor Off-State Drain Breakdown Voltage via Backside Proton Irradiation
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 AlxGa1-xN transition layers to reduce the strain, an 800 nm GaN buffer layer and capped with a 16 nm unintentionally-doped Al0.26Ga0.74N 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 Ids, gm, VBR is listed in Table I. The VBR improvement was only found at the condition of proton energy equals to 340keV, in which the defects were created at 2DEG region. Moreover, VBR 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.