Effect of Ultraviolet Radiation on Conductivity of AlN

Effect of Ultraviolet Radiation on Conductivity of AlN

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Aluminum nitride (AlN) has a 6.1eV ultra wide bandgap, which is attractive for manufacturing high-power and high-voltage electronic products, and also has the potential for deep ultraviolet optoelectronics in the wavelength range of about 200nm. However, broadband gap materials pose challenges in achieving high conductivity.

Improving the conductivity in AlN involves reducing the density of threaded dislocations and aluminum vacancy silicon (VAl-nSi) complexes. These defects reduce the effectiveness of silicon dopant by capturing electrons in the donor state as shallow as about 70 meV, thereby lowering the conductivity.

The purpose of ultraviolet light irradiation is to generate excess minority charge carriers, whose presence causes the formation of VAl-nSi complexes to shift upwards, thereby reducing their density. Generating holes requires UV photons with energy higher than 6.1 eV bandgap (i.e. wavelength less than 200 nm). The theoretical design of this technology is defect quasi Fermi level (dQFL) control.

Researchers used AlN substrates with dislocation densities below 103/cm2, and single crystal AlN was processed from pellets grown through physical vapor transport (PVT), followed by the addition of homogeneous epitaxial AlN layers using nitrogen rich metal organic chemical vapor deposition (MOCVD).

N-type doping is achieved by injecting 1014atoms/cm2 of silicon ions at an energy of 100 keV. During the injection process, the AlN substrate is tilted 7° to avoid the channel effect of ions easily passing through aligned gaps in the lattice structure.

Doping activation was achieved by annealing at 1200 °C for 2 hours at a pressure of 100 Torr in nitrogen, at a temperature considered lower than the value required for the system to reach thermodynamic equilibrium.

Irradiate the AlN wafer with a UV lamp from a 1kW mercury xenon lamp. UV irradiation reduced the photoluminescence of the intermediate gap, indicating successful suppression of compensatory VAl-nSi point defects during post implantation annealing process.

Fig. 1 Room temperature photoluminescence spectra of Si injected AlN annealed without (red) and with (blue) ultraviolet irradiation

Fig. 1 Room temperature photoluminescence spectra of Si injected AlN annealed without (red) and with (blue) ultraviolet irradiation

The contacts used for electrical measurements are Vanderberg format electron beam evaporation of vanadium/aluminum/nickel/gold. After depositing in nitrogen at 850 °C for one minute, anneal the contacts.

Measure the conductivity of AlN wafers annealed under various conditions within the temperature range of 300K and 725K. Compared with the AlN wafers annealed at the same temperature but without ultraviolet radiation, the conductivity of the wafers annealed by ultraviolet radiation increased by 30 times over the entire temperature range. As the temperature approaches room temperature, it exhibits poor performance.

Using the temperature dependence of conductivity, researchers estimate that the compensation ratio for AlN irradiated with ultraviolet light is 0.2, while the compensation ratio for AlN wafers annealed without ultraviolet light at 1200 °C is 0.9.

Fig. 2 Temperature dependent conductivity of Si implanted AlN wafers with and without UV annealing at different temperatures

Fig. 2 Temperature dependent conductivity of Si implanted AlN wafers with and without UV annealing at different temperatures

Due to the variation of Gaussian donor concentration and migration rate generated by the implant with depth, it is expected that the Hall measurement results at room temperature will not be very accurate. Therefore, thermal probe communication measurement was conducted.

At temperatures above 400 °C, the estimated free electron concentration is 5×1018/cm3 (assuming the average value of the 200nm layer) and the mobility is 1cm2/V-s. The concentration of the thin-layer carrier is close to the silicon dose, approximately 1×1014/cm2.

Researchers have stated that although ion implantation in AlN has demonstrated high conductivity exceeding 1/Ω-cm at room temperature, despite low compensation rates, the measured carrier mobility is about 100 times lower than that of epitaxial doping.

The AlN annealed at 1200 ° C without UV has a thin wafer carrier concentration of approximately 1×1013/cm2 at similar migration rates. The low migration rate has prompted researchers to further investigate, hoping to improve and achieve higher conductivity.

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