Non-equilibrium and stochastic thermodynamics
Can buried quantum dots make wide-gap nitrides conduct holes?
Open access · cc by · source: Europe PMC
Embedding tiny GaN quantum dots in aluminium-rich AlGaN raised the top of the valence band so magnesium acceptors release holes far more easily, giving good p-type conduction and a better deep-ultraviolet LED.
Study at a glance
- Design
- Other — DFT calculations of Mg acceptor levels near GaN quantum dots in AlN, followed by growth of Mg-doped AlGaN with buried GaN dots, temperature-dependent Hall measurements, optical characterisation and a comparison of two deep-UV LEDs.
- N
- No sample size; grown films with about 50%, 60% and 70% Al content, plus two LED devices (quantum-engineered versus uniformly doped p-layer).
- Population
- Mg-doped AlGaN semiconductor films with embedded GaN quantum dots
- Outcome
- Acceptor activation energy, hole concentration, resistivity, band gap, and LED current-voltage and electroluminescence
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Key findings
Calculations showed the activation energy depends strongly on where the Mg sits: doping inside the dot was poor, while sites in the matrix near the dot interface gave much lower values. In the grown material with 60% aluminium, the activation energy came out at roughly 21-43 meV, far below the 630 meV typical of Mg in AlN, with a room-temperature hole concentration of about 1.25 × 10^18 per cubic centimetre; similar values were found at 50% and 70% aluminium. The LED with the quantum-engineered p-layer turned on at lower voltage, emitted more strongly and showed a single peak at the designed wavelength, while the conventional device showed an extra peak from electron overflow.
Methodology
The authors first used density functional theory to compute how much energy a magnesium acceptor needs to free a hole when it sits at different distances from a GaN quantum dot inside AlN. They then grew Mg-doped AlGaN containing periodically buried GaN dots using an interrupted, non-equilibrium deposition sequence repeated 40 times, and checked the structure with X-ray diffraction, electron microscopy and mass spectrometry. Temperature-dependent Hall and resistivity measurements gave the acceptor activation energy, and two deep-UV LEDs that differed only in their p-type layer were compared.
Limitations
The calculations used a small supercell with very strong quantum confinement, so the computed energies at farther doping sites could not be tested and the authors only argue that larger dots would do better. The two methods of extracting activation energy disagreed by about a factor of two, which the authors attribute to dot size spread and contact problems at low temperature. The LED comparison involves just one device of each type with no reported efficiency figures or lifetime tests, and whether the approach transfers to other semiconductors is only proposed.
How this study connects
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