Argon aids p-doping in GaN LEDs
Although GaN LEDs are a great success, the MOCVD process used to make them cannot deliver p-type regions with ideal characteristics. What is needed for really bright emitters, such as those targeting solid-state lighting, are heavily doped p-type regions with minimal electrical resistivity and abrupt doping profiles in the epilayer stack. But today s processes produce high-resistivity material that hampers current spreading, alongside poor ohmic contacts that increase the operating voltage and prevent high drive currents.
This state of affairs stems from the inherent properties of p-type GaN and AlGaN grown by MOCVD using the standard dopant – magnesium. These nitrides have a high electrical resistivity, which is partially addressed by post-growth thermal activation. However, even after this treatment, p-type GaN still has electrical resistivities of 0.1 Ω-cm or more, which limits the efficacy of the brightest LEDs, due to the high operating voltages required.
The high electrical resistivity probably results from passivation by hydrogen – the standard MOCVD carrier gas. Post-growth thermal activation breaks the magnesium–hydrogen complexes and boosts electrical conductivity, although if too much magnesium is added it can degrade material quality.
Magnesium-doped nitrides grown by MOCVD also suffer from a "memory effect". This dopant resides in the growth chamber after shutting the line valve and is then absorbed in subsequent growth layers that should be free of magnesium.
To address these issues we have been developing a growth process that uses no pure hydrogen. This effort, which has been funded by the US Department of Energy s Solid-State Lighting program, uses a HVPE process that delivers faster growth rates and lower dislocation densities. NH3 and HCl are employed as the active gases and argon as the carrier.
MOCVD growth with argon as the carrier gas would not be as effective, because this would demand far higher ratios of NH3 to the group-III source, which would lead to more hydrogen in the grown layer.
Our proprietary multiwafer HVPE reactor was used to deposit p-type GaN layers on (0001) c-plane sapphire at typical rates of 1 µm/min and 1050 °C. The group-III source was high-purity gallium, and magnesium provided the dopant.
We produced GaN layers with thicknesses of 3–15 µm and magnesium atomic concentrations of 2 × 1016 cm–3–2 × 1020 cm–3, according to secondary ion mass spectrometry (SIMS) measurements. All epilayers had a smooth surface, and X-ray diffraction measurements showed that high doping concentrations did not affect crystal quality. Activation was not required to produce p-type conductivity in the samples.
Capacitance-voltage (C-V) measurements with a mercury probe showed p-type conductivity for magnesium atomic concentrations of 1 × 1017 cm–3. C-V tests also revealed net acceptor concentrations (number of acceptors minus number of donors) as high as 3 × 1019 cm–3, a level that is suitable for forming a good ohmic contact.
Electrical measurements on layers with carrier concentrations in the range 4 × 1017–1.5 × 1018 cm–3 revealed resistivities of 0.02–1.00 Ω-cm. Hole mobility was at least as good as those for MOCVD-grown material, which indicates that HVPE-grown LEDs should have excellent current-spreading characteristics. SIMS measurements also show that our material is less susceptible to the memory effect (figure 1).
The high conductivity and net acceptor concentration of our material results from higher levels of magnesium than hydrogen (figure 2). Minimizing hydrogen concentrations is critical for very high doping, according to our measurements on two magnesium-doped samples with atomic concentrations of (1–3) × 1019 cm–3. A sample with a hydrogen concentration of 4 × 1017 cm–3 had a net acceptor concentration of 1.2 × 1019 cm–3. The film with a hydrogen concentration of 1 × 1018 cm–3 had a net concentration of just 1.3 × 1018 cm–3. The variations in hydrogen concentration caused magnesium in one sample to be almost entirely active and in the other to be only 10% active.
We have also demonstrated the versatility of our HVPE growth approach. "Upside-down" LEDs that cannot be made by MOCVD were fabricated on p-GaN-on-sapphire templates. These structures contained InGaN layers with 15–30 mole % of InN and produced emissions in the 450–515 nm range.
