P. Wolny

True-Blue Nitride Laser Diodes Grown by Plasma-Assisted Molecular Beam Epitaxy

We demonstrate true-blue (450–460 nm) nitride-based laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy (PAMBE) on c-plane GaN substrates operating in continuous wave mode. The maximum optical power measured for these LDs is 80 mW, while the lifetime at the optical power of 10 mW is longer than 2000 h. We present a new LD design consisting of a separate confinement heterostructure with GaN claddings and thick In0.08Ga0.92N waveguides. We show that the key element responsible for the improvements in performance of the true-blue PAMBE LDs is the growth of InGaN quantum wells using a high nitrogen flux.

Enhancement of optical confinement factor by InGaN waveguide in blue laser diodes grown by plasma-assisted molecular beam epitaxy

The influence of InxGa1−xN waveguide on the properties of blue (λ = 450 nm) laser diodes grown by plasma-assisted molecular beam epitaxy was studied. The threshold current density was reduced by 50% (to 3.6 kA/cm2) when the indium content was increased from x = 0.04 to 0.08. This is explained by a substantial enhancement of the optical confinement factor for a high-In-content waveguide, which increases the differential modal gain. Furthermore, we observed a decrease in optical losses when Mg-doped layers were separated from the active region by a thicker waveguide. This is attributed to the lower overlap between the optical mode and absorptive Mg-doped layers.

Surface properties of c-plane GaN grown by plasma-assisted molecular beam epitaxy

Two series of GaN van Hoof structures with different thicknesses of an undoped GaN cap layer were grown under metal-rich conditions by plasma-assisted molecular beam epitaxy. These were then investigated by contactless electroreflectance (CER) to study the Fermi-level position of the (0001) GaN surface after growth as well as after chemical treatment using Piranha solution. The first and second series of samples were grown on GaN/sapphire templates and high-pressure bulk GaN crystals, respectively. A clear CER resonance followed by Franz–Keldysh oscillations (FKOs) of various periods was clearly observed for both sample series before and after chemical treatment. The Fermi-level position of the GaN surface was determined from the analysis of FKOs related to the built-in electric field in the undoped GaN layer. For the as-grown GaN surface, the Fermi level was found to be located 0.42 and 0.57 eV below the conduction band in samples grown on GaN/sapphire templates and high-pressure bulk GaN crystals, respe...

Strain relaxation in semipolar (202¯1) InGaN grown by plasma assisted molecular beam epitaxy

Strain relaxation in semipolar (202¯1) InGaN layers grown by plasma assisted molecular beam epitaxy (PAMBE) was investigated with high-resolution X-ray diffraction (XRD) reciprocal space mapping, cathodoluminescence (CL), fluorescent light microscopy (FLM), and atomic force microscopy. We find that XRD detects lattice relaxation much later than its actual onset occurs. Other techniques used in this study allowed to detect local footprints of plastic relaxation before it was evidenced by XRD: at the initial stages of strain relaxation, we observed changes in layer morphology, i.e., formation of short trench line segments on the surface along the ⟨ 112¯0⟩ direction as well as dark lines in CL and FLM. The misfit dislocations formation and glide were observed in two slip systems: initially in basal slip system ⟨ 112¯0⟩{0001} and for larger amount of strain in non-basal, prismatic slip system ⟨112¯0⟩{11¯00}. Experimentally determined critical thickness for InGaN layers grown by PAMBE on semipolar (202¯1) bulk...

Dependence of indium content in monolayer-thick InGaN quantum wells on growth temperature in InxGa1-xN/In0.02Ga0.98N superlattices

We investigate the In content in single monolayer (ML)-thick InxGa1-xN quantum wells (QWs) as a function of the growth temperature ranging from 650 °C to 480 °C, stacked in a superlattice (SL). The SLs were grown by plasma-assisted molecular beam epitaxy using high N-flux. For the evaluation of the indium concentrations, scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) studies were combined with local lattice parameter measurements obtained from high-resolution transmission electron microscopy (HRTEM) images. The mean In content in the QWs increases from 11% to 23% when the growth temperature decreases from 650 °C to 610 °C. Further decrease in the growth temperature results in a saturation of the mean In content. Our experiments show that a substantial reduction of the growth temperature is not a practical way to obtain pseudomorphically grown InN MLs on GaN(0001). The InGaN QW thickness is limited to 1 ML and is not affected by a change of growth temperature. For two SL structures grown at constant temperatures of 640 °C and 600 °C, increase in the In content in the QWs causes a shift in the peak emission from 382 to 395 nm, as was measured by cathodoluminescence at 7 K. The application of X-ray diffraction studies to analyze the composition of InGaN ML-thick QWs in SLs is discussed.We investigate the In content in single monolayer (ML)-thick InxGa1-xN quantum wells (QWs) as a function of the growth temperature ranging from 650 °C to 480 °C, stacked in a superlattice (SL). The SLs were grown by plasma-assisted molecular beam epitaxy using high N-flux. For the evaluation of the indium concentrations, scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) studies were combined with local lattice parameter measurements obtained from high-resolution transmission electron microscopy (HRTEM) images. The mean In content in the QWs increases from 11% to 23% when the growth temperature decreases from 650 °C to 610 °C. Further decrease in the growth temperature results in a saturation of the mean In content. Our experiments show that a substantial reduction of the growth temperature is not a practical way to obtain pseudomorphically grown InN MLs on GaN(0001). ...

AlGaN cladding-free 482 nm continuous wave nitride laser diodes grown by plasma-assisted molecular beam epitaxy

The authors demonstrate continuous wave (CW) operating at 482 nm AlGaN-cladding-free laser diodes (LDs) grown by plasma assisted molecular beam epitaxy (PAMBE). The maximum CW output power was 220 mW. The PAMBE LD structures were grown on (0001) GaN substrates obtained by hydride vapor phase epitaxy, with threading dislocation density of 5 × 107 cm−2. The PAMBE process was carried out in metal-rich conditions supplying high nitrogen flux (ΦN) of 2 μm/h during quantum wells (QWs) growth. The authors found that high ΦN improves quality of high In content InGaN QWs. The role of nitrogen in growth of InGaN on wurtzite (0001) GaN surface as well as influence of LD design on threshold current density is discussed.

Broadening of intersubband transitions in InGaN/AlInN multiquantum wells

In this article, the authors report on growth of InGaN/AlInN multiquantum wells (MQWs) by rf-plasma-assisted molecular beam epitaxy on (0001) GaN substrates. Intersubband transitions in InGaN/AlInN MQWs with widths varying from 1.3 to 1.8 nm has been studied experimentally and analyzed theoretically within the electron effective mass approximation. The intersubband absorption between the first and the second electron levels was observed at the wavelength of ∼1.3–1.6 μm. The broadening of intersubband absorption for this system has been found to be much larger than for indium free system (i.e., GaN/AlN MQWs). In addition, it has been found that the broadening increases from ∼280 to ∼390 meV when the nominal quantum well (QW) width decreased from 1.8 to 1.3 nm. On the basis of theoretical calculations combined with the transmission electron microscopy analysis, it has been concluded that the observed broadening of intersubband absorption is due to the QW width fluctuations. It has been estimated that the ma...