Optimization of NH₃-MBE grown p-doped (Al)GaN layers and their implementation in long wavelength laser diodes and tunnel junctions

Over the last two decades III-nitride optoelectronic devices have experienced an impressive evolution in terms of performance. However, their potential is far from being fully exploited. Although they offer bandgaps from the deep UV to the infrared spectral range, lasing is currently restricted to the 336¿536 nm wavelength region. Several challenges still need to be overcome. For example, the thermal stability of the InGaN active region becomes an issue in long-wavelength (¿ > 500 nm) laser diodes: high indium content (In > 25%) quantum wells (QWs) have been shown to markedly degrade when kept above 900 °C. For this reason, the growth temperature of the subsequent p-type waveguide and cladding layers must be sufficiently low to avoid QW degradation, which is accompanied by a reduction in the internal quantum efficiency. Thanks to the high vacuum environment, molecular beam epitaxy (MBE) allows growing these layers at a much lower temperature compared to metalorganic vapor phase epitaxy (MOVPE), and could be used to reduce the thermal budget on the active region, further extending the wavelength range. Thus the aim of this thesis is the realization of long wavelength laser diodes by combining high efficiency active regions grown by MOVPE with p-doped layers grown by ammonia-MBE. Hence, the p-type doping by ammonia-MBE is first scrupulously studied and optimized on 500 nm thick (Al)GaN layers. In order to comprehensively address the physical processes governing the doping efficiency, the role of the growth temperature, the V-III ratio, and the growth rate is investigated. In addition, the Mg cell configuration resulting in the best doping profile control and reproducibility is studied as well. Electrochemical capacitance-voltage profiling is extensively used to study the compensation mechanisms. The optimization leads to the achievement of layers exhibiting low dopant compensation (below 5%) up to high doping levels and resistivity values as low as 0.4 Ohm cm. These layers are then implemented on top of active regions grown by MOVPE and lasing is obtained for the first time in hybrid structures exhibiting state-of-the-art electrical characteristics. The threshold voltage is as low as 4.3 V in an index guided structure with 800x2 ¿m² ridge dimension designed for violet emission (¿ = 400 nm). These excellent current-voltage characteristics are achieved owing to the doping profile control in the cladding and contact layers which is confirmed by the low contact resistance value of 5E-4 cm². Lasing is also demonstrated beyond 500 nm, keeping state-of-the-art electrical characteristics, despite the reduced doping in the cladding layer. These devices also exhibit values for internal optical losses as low as 6 cm¿1. The growth of tunnel junctions is also attempted. Low specific resistance values are demonstrated in ammonia-MBE grown GaN tunnel homojunctions. The absence of hydrogen passivation and the reduced memory effects of MBE are considered to be crucial for the realization of these devices together with the stability of the doping at reduced growth temperatures. n-p-n structures featuring a tunnel junction in series with a p-n junction are then used to spread the current across light-emitting diodes. A device architecture featuring a buried tunnel junction to be used for current injection inmicro-LEDs or vertical cavity surface emitting lasers is also demonstrated and excellent current confinement properties are obtained.

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

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

Growth and characterization of single quantum dot devices for fiber-based quantum communications

Blandine Alloing

Single photon sources have recently attracted significant interest for their potential role in the future applications of quantum cryptography, and in the longer term, quantum computing. Among the different types of single photon emitters, semiconductor quantum dots (QDs) are good candidates as they combine discrete electronic transitions with the possibility of electrical excitation. Self-assembled InAs QDs on GaAs substrate are by far the most studied system as they can easily be grown into a microcavity structure. Embedding these into a microcavity is important as it increases the extraction efficiency, directionality and speeds up the photon emission. Until now, most studies have concentrated on QDs emitting in the λ

EPFL2006

Lattice-matched AlInN alloys for nitride-based optoelectronic devices

Julien Dorsaz

EPFL2006