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A charge profiling of GaInN/GaN hetero-polarization structures typical for high power 525 nm green light emitting diodes is presented. We identify three individual charge maxima of some 1.17 × 1012 cm–2 electrons with a line width as narrow as 17 nm corresponding to three individual but widely separated quantum wells. The sheet charge density indicates negligible screening of the polarization charges. The voltage required to deplete each well is found to be ΔU = –0.4 ± 0.1 V and approximates the voltage drop across the polarization dipole of the quantum well. The modulation frequency dependence reveals a characteristic time constant of 1 μs for the dominant trapping and ionization process. Such analysis of high performance green light emitters proves a suitable tool for further device optimization. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract: We identify, quantify, and correlate the polarization dipole across the well of device-typical piezoelectric GaInN/GaN heterostructures with the luminescence properties of the well. This quantity reflects in the asymmetry of the barrier height on either side of the well. By a detailed comparison of photoreflection, electroreflection, low and high excitation density photoluminescence we find that a very similar splitting occurs in the emission characteristics of the well. We therefore conclude that the electronic band structure within the wells is also to a very large extent controlled by the quantity of the polarization dipole in such polarization heterostructures.

Abstract: We obtained 1 m crack-free AlGaN layers up to an AlN molar fraction of 0.4 by growing directly on low-temperature-deposited buffer layers. The buffer layer is effective for growing AlGaN layers without the stress caused by the lattice mismatch. We also demonstrated nitride-based laser diodes with such a 1 m crack-free n-AlGaN cladding layer/n-AlGaN contact layer/low-temperature-deposited buffer layer/sapphire structure, which showed a clear single spot in a far field pattern. The AlGaN-based structure can suppress optical leakage from the waveguide region to the underlying layer. The threshold current of the laser diode is about 230 mA, which is comparable to or better than that of our laser diodes with the conventional GaN-based structure.

n-AlGaN has been introduced as a new n-type contact layer for a GaN-based laser diode (LD) prepared on sapphire substrate. Cracks, generated by the lattice mismatch between n-AlGaN cladding layer and n-GaN contact layer in a typical LD, have been extremely reduced. Under the pulse lasing conditions at room temperature, the threshold current was about 230 mA. Lasing wavelength was 405 nm. Moreover, there is also great improvement in the optical confinement for this new LD structure. Both near-field and far-field patterns manifested a clear single spot.

We report on the fabrication and the characterization of low-intensity ultraviolet metal–semiconductor–metal photodetectors based on AlGaN (0 ≤ x ≤ 0.44) with low threading dislocation density. Active layers were grown by organometallic vapor phase epitaxy on GaN. For comparison, AlGaN layers grown on sapphire were also prepared. Photodetectors on low threading dislocation layers present a very low dark current level, constant responsivity with optical power, a rejection over two orders of magnitude at the band gap, and an improved response speed. The results were uniform over the whole AlN mole fraction investigated. On the other hand, samples on sapphire present degraded behaviors when the AlN mole fraction becomes higher than 0.11.

The optical properties and their relation to the electronic bandstructure in Si-doped GaInN/GaN multiple quantum well structures typical for near UV laser diode devices are investigated in photoluminescence, photoluminescence excitation and photoreflection spectroscopies. A set of four evenly spaced levels N0 ··· N3 is identified covering the spectral range from the lowest luminescence band (N3) to the GaN barrier energy (N0). The splitting of N3 and N2 appears as a discrete splitting rather than a continuous localization process. The level splitting is well described by a Stark-like ladder defined by the multi interface bandoffset ΔE = FeLz across the piezoelectric GaN/GaInN/GaN structure.

Tetraethylsilane (TeESi) and bis(ethylcyclopentadienyl)Mg (ECp2Mg) were employed as Si and Mg dopant precursors for MOVPE growth of n-type and p-type GaN films, respectively. In Si doping, the electron concentration was observed to increase with the increase of the TeESi flow rate. The temperature dependence of the Hall mobility showed good agreement with n-type GaN films grown using different dopant precursors (SiH4, GeH4, Si2H6). The donor activation energy was estimated to be 27 meV, which is almost the same as the literature values. In Mg doping, we also found that the Mg concentration increases as the ECp2Mg flow rate increases. All of Mg-doped samples in this study showed p-type conduction after annealing. The acceptor activation energy was estimated to be 170 meV, which was close to the reported values.

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Annealing of GaN and GaN based ternaries at high temperatures requires a capping layer, which can withstand applied temperatures and protect the surface of the III-nitride material from decomposition. AlN cap is often used as this capping layer, but it is very resistant to wet etching and requires harsh chemical, largely KOH-based etchants, for removal. In this work NbN was used as a new material for high temperature capable cap that is easily removable using wet processing. It was shown that NbN is capable of avoiding decomposition when used on an InGaN layer that is subjected to annealing temperatures up to 1220°C. In addition, it was demonstrated that the NbN capping layer can be completely removed using a HF/HNO3wet process.

The electron effective mass in hexagonal GaN films grown by metal organic vapor phase epitaxy on sapphire substrates is determined by cyclotron resonance experiments. Its value is mp*= 0.22±0.005 mo. Taking polaron effects into account the band edge mass is mb*= 0.20±0.005 mo. From the resonance linewidth a mobility of 3500 cm2/V·s at 6 K is obtained.