AlGaN-Free Blue III-Nitride Laser Diodes Grown on c-Plane GaN Substrates

We report on the fabrication of InGaN-based multiple-quantum-well laser diodes (LDs) emitting at 420 nm. Structures with standard claddings (p-and n-AlGaN), asymmetric claddings (p-GaN and n-AlGaN), and AlGaN-free claddings were grown by metal organic vapor phase epitaxy on polar c-plane free-standing GaN substrates. Electrical and optical properties of each LD are presented. Thanks to an optimized design of the InGaN waveguide and active region, cw lasing of a completely AlGaN-free laser diode is demonstrated, with a threshold current density < 5 kA/cm(2) and a differential efficiency per facet of similar to 0.4 W/A without high-reflection coatings. (c) 2010 The Japan Society of Applied Physics

InGaN laser diodes emitting at 500 nm with p-layers grown by molecular beam epitaxy

Antonino Francesco Castiglia, Julien Dorsaz, Eric Feltin, Nicolas Grandjean, Lise Lahourcade, Jean-Michel Jacques Lamy, Marco Malinverni, Denis Martin, Marco Rossetti

We demonstrate hybrid laser diodes by combining n-type layers and an active region grown by metal organic vapor phase epitaxy with p-type layers grown by molecular beam epitaxy. These p-doped layers, grown at 740 degrees C, exhibit state-of-the-art electrical characteristics and prevent the indium-rich quantum wells from thermal degradation. Narrow ridge-waveguide lasers with high-reflectivity coatings show a threshold current density of 9.7 kA.cm(-2), a threshold voltage of 5.4V, and a lasing wavelength of 501 nm. The internal optical loss and material gain of the epitaxial structures are also measured and discussed. (C) 2015 The Japan Society of Applied Physics.

Japan Society of Applied Physics2015

Lattice-matched AlInN alloys for nitride-based optoelectronic devices

Julien Dorsaz

Gallium Nitride (GaN) and its ternary alloys with aluminium and indium have met a growing interest in the last decade. These semiconductors have a large direct bandgap and can be doped with either silicon (Si) for n-type and magnesium (Mg) for p-type layers. Consequently, they are attractive targets for the fabrication of electrically driven light-emitting diodes, laser diodes and photodetectors, operating in the UV and visible range. Nitride semiconductors are also robust ans stable at high temperatures, suggesting their use in high frequency – high power electronic devices. The research on the III-nitride semiconductors is still in its infancy and, although several devices have been demonstrated and commercialized, there are still many issues to be solved. In particular, the III-nitride materials suffer from the lack of an adequate lattice-matched substrate for epitaxial growth, the high density of defects and dislocations, the poor quality of the p-type layers (low hole concentration and mobility), localization effects in indium containing layers, chemical inertness, etc… Also, due to the material quality issues, some electrical and optical parameters are only approximatively known. AlInN alloys have not attracted much attention compared to the InGaN and AlGaN ternary alloys, due to their unstable growth conditions. High-quality AlInN layers were deposited by metalorganic vapor-phase epitaxy (MOVPE). AlInN with an indium content of 18% is lattice-matched to GaN. It has a bandgap energy of about 4.35 eV and a refractive index contrast to GaN of 7% at 450 nm. It is shown that the optical, electrical and structural characteristic of epitaxial AlInN layers start degrading above a critical thickness of 200 nm, due to an ongoing pollution of the MOVPE reactor by products of the reaction. The distributed Bragg reflector (DBR) is an essential building block for advanced optoelectronic devices such as vertical-cavity surface emitting lasers (VCSELs). The introduction of the lattice-matched AlInN quarter-layers allows to grow thick DBR structures, with limited strain issues. It is used with GaN quarter-layers for visible reflectors and AlGaN quarter-layers for UV reflectors Optimizing the growth conditions of AlInN layers leads to the realization of DBRs with reflectivity above 99 % in the blue wavelength range (410-450 nm) as well as in the UV wavelength range (345 nm). The novel AlInN/GaN DBR is introduced in various microcavity LED (MCLED) structures. The basic features of the MCLED devices are presented, as well as optical simulations obtained using the transfer matrix representation (TMR) formalism, coupled with a dipole approximation for source terms. Electrical characterizations on these devices show that an intra-cavity contact (ICC) scheme should be used to achieve a sufficient electron injection in the active region. The external quantum efficiency of MCLED devices with thick AlInN/GaN DBRs (12 pairs) were lower than expected, due to the degradation of the crystalline quality of the active layers caused by the pollution of the reactor. Another requirement for the realization of a nitride VCSEL is the optimization of the electrical injection of carriers in the optical cavity. Assuming that circular metallic contacts are used, the carriers should spread laterally and be injected in the active region under the top DBR. This is generally achieved by the combination of a current spreading cap layer, which helps for the lateral injection of the carriers, and a current confinement layer, which inhibit the injection of the carriers in the active region outside of the vertical cavity. For the III-nitride structures, several methods are available for the lateral injection: n+/p+ tunnel junctions, doped-AlGa(In)N/GaN superlattices, semi-transparent electrodes, etc… A novel technique is developed to oxidize selectively surface or buried AlInN layers and form a stable, semi-insulating compound, with a refractive index of about 1.8. A detailed description of the technique is presented. It can be applied to form current confinement layers in VCSELs, but also layers for high reflectivity distributed Bragg reflectors (DBRs), layers for improved light confinement in planar waveguides or insulating buffer layers for field effect transistors (FETs). Also, oxidized AlInN could be used as a sacrificial layer for the preparation of free-standing GaN substrates. In this work, the technique was applied to form a current confinement aperture in a LED device. A buried AlInN layer was oxidized laterally, leaving a small 5 µm aperture in a 28 µm × 28 µm mesa structure. Optical and electrical confinement are demonstrated. With the availability of a high-reflectivity lattice-matched nitride DBR and a technique to produce small current apertures, both based on the new AlInN material, the realization of an electrically injected VCSEL becomes a realistic goal. An ideal structure is realized and discussed.

EPFL2006

Characterization of a-plane grown GaN on sapphire substrates by electron microscopy

Yadira Arroyo Rojas Dasilva

Gallium nitride (GaN) is one of the most interesting materials for devices applications such as blue light emitting diodes, laser diodes and high power and high temperature electronic applications, because of its large band gap (3.39 eV). Several growth techniques including metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hybrid vapor phase epitaxy (HVPE) are employed to synthesize high quality GaN. Sapphire, that is available in several orientations, is the common substrate to grow GaN. The GaN that grown on c-(0001) sapphire, has a wurtzite structure and suffers of spontaneous polarization effects. These effects create internal electric fields that affect the operation and the efficiency of the devices. This work is part of a large effort to grow defects free GaN layers in non-polar geometries undertaken at LASPE-IQEP-EPFL. Among the non-polar geometries, the a-(1120) GaN geometry has been selected. The specimens were produced using the HVPE and Epitaxial lateral Overgrowth (ELO) techniques and were fully characterized mainly by electron microscopy observations. Despite the ELO improvements and smaller lattice mismatch in a-GaN (≈1.1%) than c-GaN (≈16%), a high density of dislocations and stacking faults are still observed. They are results of the lattice mismatch and the difference of the thermal expansion coefficients between the components. The reduction of defect density due to ELO has been found to be approximately two orders of magnitude for dislocations (1 × 1010 cm-2 to 3 × 108 cm-2) and for stacking faults (1 × 106 cm-1 to 4 × 104 cm-1). In the ELO window areas, the threading dislocations have screw and mixed characters with Burgers vector b→ = 1/3 [1120] and b→ = 1/3 [1123] respectively. In the overgrown ELO areas, the dislocations have different characters such as screw character with b→ = 1/3 [1120], edge character with b→ = 1/3 [2110] and partial dislocations. Different types of stacking faults have been observed in the a-GaN film: basal (BSF) I1 and I2, prismatic (PSF) (1120) and (1010). The I2 BSF is bordered by two Shockley partial dislocations of Burgers vector b→ = 1/3 < 1010 > and its energy γ has been calculated: 50 ergs/cm2. Prismatic stacking faults, R→ = 1/2 [1101] are located at the end of the I1- BSFs, where stair rod dislocations are formed at the intersection of the two faults. In order to know the influence of these defects on the optical properties, cathodoluminescence (CL) and microphotoluminescence (µ-PL) experiments were performed. These observations demonstrate that the optical properties are affected by the high density of stacking faults. The CL spectra show 4 different emission peaks where the dominant emission at 3.42 eV is attributed to the I1 BSFs. In (µ-PL spectra, 4 emission peaks are also observed, where the dominant one at 3.44 eV in the window area of the ELO mask is also attributed to the SFs and that at 3.49 eV in the overgrown mask area is the typical NBE transition. This shows that locally the GaN film is of good quality. A single GaN quantum well (SQW) intercalated by two Al1-xGaxN layer were grown on HVPE-ELO templates by MBE. The structural characterization of six specimens have shown a high pits density on the sample surface (1 × 1012 cm-2) associated with the threading dislocations of screw character ending at the bottom of the pits. Moreover, the pits created at the intersection, near the surface, of several dislocations groups have been observed. A high density of BSFs (1 × 105 to 1 × 106 cm-1) was found in the samples, which are formed at Al1-xGaxN/GaN interface and propagated towards the specimen surface. We also observed that these stacking faults affect the optical properties of the SQW-GaN. Our results have shown that the inhomogeneity of the optical properties of a-(1120) GaN layers is associated with specific structural defects, in particular BSFs. These results provide guidelines to design new growth procedures aimed at improving the overall quality of such GaN films.