S. Lahmann

Composition dependence of polarization fields in GaInN/GaN quantum wells

We report an experimental determination of the internal polarization field in GaInN/GaN quantum wells, due to piezoelectric and spontaneous polarization, utilizing the quantum confined Stark effect, with fields as large as 3.1 MV/cm at 22% In. From its dependence on quantum well composition and strain, we find that the total field in GaInN is a linear combination of polarization charges from GaN and InN. The piezoelectric constants d31 for GaN and InN derived from our data are 1.05±0.05 pm/V and 3.7±0.5 pm/V, in fair agreement with theoretical data.

Influence of low-temperature interlayers on strain and defect density of epitaxial GaN layers

The effect of GaN and InxGa1xN interlayers on the strain state and defect density of GaN buffer layers is studied. The layers are grown by low pressure MOVCD on c-plane sapphire substrates and were investigated by in situ reflectometry, AFM, photoluminescence, high-resolution X-ray diffraction. In addition, etch pit densities are determined by wet chemical etching in hot phosphoric acid. We find that InxGa1xN interlayers slightly decrease the defect density and reduce the strain in the GaN buffer layers. r 2002 Elsevier Science B.V. All rights reserved.

Ultrafast polarization dynamics in optically excited biased quantum wells

We observe ultrafast polarization dynamics in strongly internally biased InGaN/GaN multiple quantum wells during intense femtosecond optical excitation by means of time-resolved detection of THz emission, correlated with time-integrated photoluminescence measurements. We demonstrate that in the case of strong enough excitation the built-in bias field (on the order of MV/cm) can be completely screened by the carriers excited into spatially separated states. This ultrafast screening of the initial bias field across the quantum well leads to dynamical modification of the band structure of the sample, and consequently to dynamical modification of the optical absorption coefficient within the duration of the excitation pulse. We show that such an optically induced dynamical screening of the biased quantum well can be described in terms of discharging of a nano-scale capacitor with a femtosecond laser pulse. The electrostatic energy stored in the capacitor is released via THz emission. A realistic quantum-mechanical model of the temporal evolution of the polarization inside the quantum wells shows that due to its nonlinearity such a process may lead to emission of a THz pulse with bandwidth significantly exceeding that of the excitation pulse.

Terahertz pulse emission from strained GaN/GaInN quantum well structures

Efficient THz pulse generation from optically pumped InGaN/GaN multiple quantum well structures is demonstrated. Such structures incorporate strong electric fields of opposite directions - piezoelectric fields inside the quantum wells and in the barrier. Polarized states inside the quantum well and in the barrier give different contributions to the THz pulse, depending on whether the excitation is real or virtual. If the excitation energy is slightly below the e1-h1 transition in the quantum well, two-photon absorption in the barriers is observed to give the dominant contribution to the THz generation process at high pump fluence. At low pump fluence the virtual excitation of polarized states inside the quantum well dominates.