A. Yu. Egorov

Electronic states and band alignment in GalnNAs/GaAs quantum-well structures with low nitrogen content

We investigate the electronic states in strained Ga0.62In0.38N0.015As0.985/GaAs multiple- quantum-well structures using photoluminescence and (polarized) photoluminescence excitation measurements at low temperature. From a theoretical fit to the experimental data, a type-I band alignment for the heavy holes with a strained conduction-band offset ratio of about 80% is obtained, while the light holes show an approximately flat band alignment. Additionally, our results suggest an increased effective electron mass in GaInNAs, possibly due to the interaction of the conduction band with nitrogen-related resonant states, an observation prospectively of benefit for GaInNAs-based diode lasers.We investigate the electronic states in strained Ga0.62In0.38N0.015As0.985/GaAs multiple- quantum-well structures using photoluminescence and (polarized) photoluminescence excitation measurements at low temperature. From a theoretical fit to the experimental data, a type-I band alignment for the heavy holes with a strained conduction-band offset ratio of about 80% is obtained, while the light holes show an approximately flat band alignment. Additionally, our results suggest an increased effective electron mass in GaInNAs, possibly due to the interaction of the conduction band with nitrogen-related resonant states, an observation prospectively of benefit for GaInNAs-based diode lasers.

Recombination mechanisms in GaInNAs/GaAs multiple quantum wells

Recombination processes in Ga1−xInxNyAs1−y/GaAs multiple quantum wells (MQWs) were investigated as function of the nitrogen molar fraction. We found a pronounced S-shaped behavior for the temperature-dependent shift of the photoluminescence emission similar to the ternary nitrides InGaN and AlGaN. This is explained by exciton localization at potential fluctuations. Time-resolved measurements at 4 K reveal an increase of the decay time with decreasing emission energy. A model based on lateral transfer processes to lower-energy states is proposed to explain this energy dependence. The formation of tail states in the Ga1−xInxNyAs1−y/GaAs MQWs is attributed to nitrogen fluctuations.

Effect of annealing on the In and N distribution in InGaAsN quantum wells

We analyze the influence of annealing on compositional fluctuations in InGaAsN quantum wells by means of composition-sensitive high-resolution transmission electron microscopy and photoluminescence. In as-grown samples, we find In-concentration fluctuations of ±5% on a length scale of 20 nm in a two-dimensional grown quantum well. No indications for N concentration fluctuations are found within the limits of resolution. Annealing homogenizes the In distribution within the well and causes diffusion of N out of the quantum well. According to our compositional analysis, the blueshift in the photoluminescence can in part be attributed to reduction in N concentration inside the well. The more homogeneous In distribution leads to a reduction in linewidth and Stokes shift.

Gain spectra of (GaIn)(NAs) laser diodes for the 1.3-μm-wavelength regime

Optical gain spectra of (GaIn)(NAs)/GaAs quantum-well lasers operating in the 1.3-μm-emission-wavelength regime are measured and compared to those of a commercial (GaIn)(AsP)/InP structure. Good agreement of the experimental results with computed spectra of a microscopic many-body theory is obtained. Due to the contributions of a second confined subband, a spectrally broad gain region is expected for (GaIn)(NAs)/GaAs at elevated carrier densities.

Growth of high quality InGaAsN heterostructures and their laser application

Focus of this work is the optimization of growth to achieve high quality laser material for emission at 1.3 μm and beyond. GaAs/GaAsN/InGaAsN heterostructures were grown by solid source molecular beam epitaxy. To achieve optimum crystal quality of InGaAsN heterostructures, growth was followed by a high temperature treatment at about 700°C. The high optical quality of our annealed material is attested by large exciton recombination lifetimes (more than 2 ns). Consequently, a decrease of single quantum well transparency current density down to 100 A/cm 2 is found and SWQ lasers with threshold current densities as low as 350 A/cm 2 have been made. This represents clearly the lowest laser thresholds reported so far for emission around 1.3 μm from the InGaAsN material system.

Simultaneous experimental evaluation of In and N concentrations in InGaAsN quantum wells

We propose a method that solves the problem of the independent determination of the indium and nitrogen concentrations in a strained quaternary InGaAsN superlattice. The method is experimentally based on the simultaneous measurement: (i) of the tetragonal lattice distortion of the unit cell from high resolution micrographs and (ii) of the intensity of the chemically sensitive (002) reflection from dark field images. As an example, we evaluate InGaAsN quantum wells with a nominal N concentration of 1.7% and with In concentrations of 10%, 20%, or 35%. We reveal local fluctuations of the In and N concentrations over distances down to 4 nm with a sensitivity of 0.1% for N and 1% for In fluctuations in this distance range.

Optical investigations of GaInNAs/GaAs multi-quantum wells with low nitrogen content

The optical properties of GaInNAs/GaAs multi-quantum wells were investigated by photoluminescence excitation (PLE) spectroscopy, as well as by photoluminescence (PL), under various excitation intensities and at various temperatures. The PLE spectra demonstrated pronounced excitonic features and the corresponding transitions were identified. At low temperatures the PL spectra were sensitive to the excitation intensity. Under fixed excitation intensity, both the peak energy and the linewidth of photoluminescence showed anomalous temperature dependence, specifically an S-shaped temperature dependence of the peak energy and a N-shaped temperature dependence of linewidth in the PL spectra. The observed results are explained consistently in terms of the exciton localization effect due to the local fluctuations of nitrogen concentration.

Infrared absorption study of nitrogen in N-implanted GaAs and epitaxially grown GaAs1−xNx layers

Fourier-transform infrared absorption measurements have been carried out in the two-phonon region of GaAs. Implantation of the nitrogen isotopes 14N and 15N, respectively, into bulk GaAs shows that a local vibrational mode at 471 cm−1 (14N) is due to isolated nitrogen. The band is also found in GaAs1−xNx(0<x<0.03) layers grown by solid-source molecular beam epitaxy. The strength of the band correlates quantitatively with the decrease of the lattice parameter determined by x-ray diffraction for x<0.01 and can be used for the assessment of the nitrogen fraction incorporated substitutionally on anion lattice sites.

InGaAsN/GaAs heterostructures for long-wavelength light-emitting devices

We report on the growth and properties of InGaAsN/GaAs heterostructures and on their applications for lasers emitting at λ≈1.3 µm. Material growth was performed by molecular beam epitaxy using an RF plasma source. Broad area and ridge waveguide (RWG) laser structures based on such quantum wells (QWs) exhibit performances that can compete with those of 1.3 µm InGaAsP lasers. In particular, we have achieved 300 K operation of broad area lasers at 1.3 µm with threshold current densities down to 500 and 650 A cm-2 for 800 µm long, single and triple QW structures. Similar structures with heat-sinking at 10 °C yield a maximum CW output power of up to 8 W. RWG lasers have thresholds down to 11 mA and show CW operation up to 100 °C with a T0 of up to 110 K.

Influence of indium on the electronic states in GaInNAs/GaAs quantum well structures

We use room-temperature photoreflectance spectroscopy to investigate the influence of indium on the electronic structure of Ga1−xInxNyAs1−y/GaAs multiple quantum wells. To fit our experimental data, a semiempirical theoretical model based on the band anticrossing Hamiltonian is successfully applied. Thus, we can extract some information about the Hamiltonian, in particular, the dependence of the coupling parameter CNM on the In concentration in GaInNAs. CNM is shown to decrease with increasing indium mole fraction, confirming theoretical predictions.