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We measured the resonant nonlinear optical absorption and refraction due to intersubband transitions in GaN/AlGaN multiple quantum well structures at 1.5 mum. The measured values are much smaller than that predicted by theoretical studies. This is attributed to a larger than expected dephasing of excited electrons. At high excitation intensity, the absorption can be saturated. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We use hydrostatic pressure as an instrument to reveal a strong nonlinearity of the electrical polarization in group-III nitride quantum well structures. From the photoluminescence peak energies of the quantum well emission at different applied pressures we obtain the values of the built-in electric field in the wells and the corresponding well-barrier polarization difference. We found that in both the InGaN/GaN and GaN/AlGaN systems the field and the polarization difference increase with pressure much faster than expected from the conventional (linear) model of polarization. This behavior is explained by the dramatic strain dependence of the piezoelectric coefficients of the group-III nitrides, which constitutes the nonlinear piezoelectric effect.

A review of the recent progress in intersubband transitions in GaN-based superlattice structures grown by plasma-assisted molecular beam epitaxy (MBE) is presented. Careful control of the growth parameters resulted in reduced threading dislocation density as well as optimized interfaces for the superlattices. Intersubband absorption has been observed from a variety of configurations involving GaN single or asymmetric double quantum wells with either thick Al x Ga 1-x N barriers or short-period GaN/Al x Ga 1-x N superlattice barriers. The peak wavelength of absorption can be varied between 1.4-4.2 μm by changing the quantum well thickness. Electron scattering times were measured by the pump-probe technique and were found to be 240-330 fs at 1.55 μm. In addition, intersubband transitions have also been observed for GaN/AlN superlattices grown with a non-polar (1120) orientation.

GaN quantum wells were studied, grown by plasma-assisted molecular beam epitaxy with a sub-band spacing of ∼740 meV (λ=1.67 µm). The GaN quantum wells are clad on both sides with short-period superlattice barriers. Using the time-resolved pump-and-probe technique, with 1.55 µm pump and 1.70 µm probe wavelength, an intersub-band electron scattering time of 370 fs was measured.

We report the results of the growth by molecular beam epitaxy of GaN/Al x Ga 1-x N superlattice (SL) structures with single or double quantum wells and thick Al x Ga 1-x N or short-period SL barriers. The dependence of intersubband absorption on the well width for single well SL structures was systematically investigated. For GaN quantum wells interleaved with SL barriers, the upper state electron confinement was maintained by Bragg reflection from the minigaps formed within the barriers. In addition, evidence of electron transfer from doped SL barriers to undoped GaN quantum wells was obtained. Abrupt interfaces and coherent periodicities of the SL structures were confirmed by transmission electron microscopy and X-ray diffraction measurements. Intersubband absorption at peak wavelengths between 1.4 and 4.2 μm has been observed.

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/AlxGa1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al0.8Ga0.2N quantum wells.

We have grown nonpolar GaN/AlN multiple quantum wells by plasma-assisted molecular-beam epitaxy on R-plane sapphire substrates. X-ray diffraction and selected-area diffraction data show that the III-nitride epilayers are oriented in the [11 20] direction with the [0001] axis lying in the plane of the substrate. The 18-A GaN quantum wells exhibit luminescence at 326 nm, which is in agreement with the transition energy calculated using a flat-band model; that is, without the presence of a built-in electric field.

We measured the resonant nonlinear optical response of the intersubband transitions in GaN/AlGaN multiple quantum well structures. The measured value for the nonlinear susceptibility is found to be much smaller than previous theoretical predictions. This is attributed to a large electron dephasing due to material imperfections. We show that at higher incident intensities, absorption saturation is possible, and measure the saturation intensity for various wavelengths around the transition resonance. We also discuss the prospects of using such structures as building blocks for all-optical nonlinear switches, in light of our experimental findings.

We report on a comparative study of room temperature intersubband electron scattering lifetimes in GaN/AlGaN single and coupled double multiple quantum well (QW) samples with peak absorption wavelengths ranging from 1.4 to 1.7 μm. Using time-resolved pump-probe spectroscopy electron scattering times as short as ∼160 fs have been measured for a coupled QW sample and ≲300 fs for single QW samples. While no significant dependence on the excitation power has been observed, a decrease of the scattering times with increasing probe wavelength has been measured and may be attributed to monolayer fluctuations in the samples.

We present experimental evidence of the nonlinear behavior of the macroscopic polarization in GaN/AlxGa1−xN quantum wells. This behavior is revealed by determining the barrier-well polarization difference as a function of applied hydrostatic pressure. The polarization difference and corresponding built-in electric field in the wells increase with applied pressure at a much higher rate than expected from the linear model of polarization. This result, universally observed in the quantum well structures with different AlN mole fraction in the barriers, is explained by the nonlinear dependence of the piezoelectric polarization in GaN and AlN on the strain generated by pressure.

Intersubband absorption in coupled GaN/AlGaN double quantum wells (DQWs) has been measured. The samples were grown by molecular-beam epitaxy on a sapphire substrate and with large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 and 1.52 μm were measured for a symmetric DQW of 12 A wide wells coupled by a 10 A wide barrier, which also showed evidence of excited-state anticrossing. As expected, asymmetric DQWs displayed no such anticrossing, and the ground-state anticrossing energies were found to be much smaller, as a result of the comparatively large effective electron mass, than the energy broadening of individual transitions. Degenerate doping of the DQWs was used to establish a common reference energy at the Fermi level, which allows overcoming uncertainties related to intrinsic internal electric fields. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm.

Intersubband optical absorption in narrow, 15–30 A wide, GaN/AlGaN quantum wells has been measured. The samples were grown by molecular-beam epitaxy on sapphire substrate and the barrier AlN mole fraction was varied from 0.45 to 0.8. Peak absorption wavelengths ranged from 4.2 μm for 30 A wide wells to 1.77 μm for a 15 A wide well. Modeling shows that a large contribution to the considerable spectral width of the absorption of ∼150 meV likely results from monolayer fluctuations. In addition to the composition dependent band offset, the intrinsic electric fields in the wells and barriers are the determining factors for the shortest possible wavelength.

We present a study of the optical properties of different GaN/AlGaN multiple quantum well heterostructure samples. In particular, a comparative study of room temperature intersubband electron scattering lifetimes in a variety of single and coupled quantum well samples is presented for different excitation powers and wavelengths. All samples were grown by molecular beam epitaxy on sapphire substrate. Typical quantum well widths of ~10 to 15 a were used together with AlN bulk and superlattice barriers with an AlN-mole fraction of ~0.65 and ~0.9, respectively. In absorption measurements, the single quantum well samples show intersubband absorption peaks ranging from ~1.4 to ~1.7 μm wavelength. Two absorption peaks at ~1.75 and ~2.45 μm were measured for an asymmetric coupled double quantum well structure. The intersubband electron scattering lifetimes were determined with time resolved pump-probe measurements using pump and probe beams with a pulse width of ~130 fs and wavelengths of ~1.55 and ~1.7 μm, respectively. For the single quantum well samples, intersubband scattering lifetimes of ~260 to ~300 fs have been measured. For the upper excited state of the coupled double quantum well, a lifetime of ~230 fs has been found.

In this paper, we show that different GaN heterostructures exhibit distinct nonlinear optical response. The response is studied using spectrally integrated pump-probe technique. The intersubband (ISBT) relaxation dynamics of excited carriers in GaN quantum well(QW) structures and their nonlinearities is essential for possible ultrafast optoelectronic applications based on these transitions.

Summary from only given. We report on the first comparative study of room temperature intersubband (IS) scattering lifetimes in a variety of single and coupled double GaN/AlGaN MQW samples at different excitation powers and wavelengths. Furthermore, samples with two types of QW barriers are compared: bulk-like barriers (85% AlN mole fraction) and superlattice (SL) barriers (65% AlN mole fraction). All samples were grown by molecular beam epitaxy on sapphire substrates.

We present measurements of intersubband transitions in the communications wavelength range using GaN/AlGaN heterostructures that have multiple single and coupled quantum wells.

Many talks of molecular beam epitaxy as an important technology for the mass production of quantum devices will be presented in this conference. This talk will concentrate on the development and the unique features of quantum cascade lasers such as high-power output, tuning characteristics, and the extension to GaN material. (C) 2001 Elsevier Science B.V. All rights reserved.

Summary form only given. Fueled by the recent success of mid-infrared quantum cascade lasers, optical devices based on intersubband (IS) transitions face a rising interest also in other wavelength ranges. In particular, the intrinsically ultrafast electron dynamics associated with IS-transitions warrants research into the latter for the communications wavelength range (/spl lambda//spl sim/1.55 /spl mu/m). Few material systems, however, provide a sufficiently large band-discontinuity for the quantum wells (QWs) to accommodate the large subband spacing needed for such short wavelengths, Sb-based materials or group-III-nitrides presently being the best candidates. The rapid success of GaN-based interband light emitting diodes and lasers makes this material system particularly attractive. We present the first IS-transitions in the communications wavelength range, with peak absorption wavelengths as short as 1.4 /spl mu/m.

Intersubband optical absorption at /spl lambda/ /spl sim/ 2.1 /spl mu/m wavelength in doped 17.5 /spl Aring/ wide GaN quantum wells (QWs) with 51 /spl Aring/ wide intermediate AlN barriers is reported. A /spl sim/600 nm thick GaN template and 15 QWs have been grown by molecular beam epitaxy on R-plane sapphire substrate and display pure A-plane orientation. QWs with A-plane orientation are essentially free from spontaneous polarisation, which is expected to greatly simplify the design of GaN/Al(Ga)N devices employing intersubband transitions.