Non-polar GaN epilayers and heterostructures for photonic applications

This PhD thesis describes the experimental study of wurtzite III-nitride semiconductors grown on non-polar crystal orientations, namely (1120) a- and (1010) m-planes. Hindered by poor material quality, they were not as extensively investigated as the polar basal (0001) c-plane. Nevertheless, the renewed interest in this topic has been gaining momentum since 2000, thanks to the improvement in material quality. The motivation is to overcome the quantum-confined Stark effect (QCSE) in heterostructures grown on c-plane, which is induced by the intrinsic spontaneous and piezoelectric polarization field. QCSE weakens the performance of c-plane optoelectronic devices such as light emitting diodes (LEDs) and laser diodes and it can be avoided by growing these devices on non-polar planes. In addition, non-polar LEDs are also featured by the spontaneously polarized light emission and thus have a large potential for liquid crystal display backlighting application. This thesis includes two aspects: (1) Since previous study in our laboratory was lacking before this work, the first target was to establish the heteroepitaxial growth procedures for a- and m-plane GaN layers on r- and m-plane sapphire substrates, respectively, by hydride vapor phase epitaxy (HVPE). Then, smooth planar templates have been obtained. Particularly, m-plane GaN has been demonstrated on m-plane sapphire. Further analysis revealed that actually three orientations (two semi-polar and one non-polar) can be grown on m-plane sapphire through the modification of sapphire surface chemistry and/or growth procedure. Nevertheless, the material quality of these non-polar templates is inferior to that of c-plane as they are characterized by the presence of high-density stacking faults and threading dislocations. To reduce the defect density, epitaxial lateral overgrowth (ELO) technique has been applied. As a result, material quality close to that of planar c-plane was achieved in GaN layers overgrown on the mask areas. Overall, the experimental results revealed peculiar characteristics of non-polar orientation, induced by the intrinsic anisotropy of wurtzite structure parallel and perpendicular to the basal plane. It also lays the foundation for further improvement in material quality. (2) Based on these aforementioned results, the next stage was to investigate the heterostructures grown on those HVPE ELO templates. GaN/AlGaN single quantum wells (QWs) and AlN/GaN distributed Bragg reflectors (DBR) have been grown by NH3-source molecular beam epitaxy (MBE). The experiments confirm that QWs are free from QCSE and their optical properties are comparable to those grown on planar c-plane templates. Furthermore, nitride-based microcavities (MCs) have been demonstrated on a-plane ELO templates. Their optical properties are encouraging and open the way for more sophisticated structure aimed at polariton physics. In conclusion, non-polar a- and m-plane GaN planar and ELO templates have been obtained through epitaxy on r- and m-plane sapphire substrates, respectively, by HVPE. Nanostructures such as QWs, DBRs, and MCs have been grown on non-polar HVPE GaN ELO templates by MBE.

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 alloys and heterostructures

Marlene Glauser

The III-nitride semiconductor material system - (InAlGa)N - is of highest interest for optoelectronic applications due to its direct bandgap, tunable from the ultraviolet to the infrared spectral range. The most well-known are white light-emitting diodes, which are presently revolutionizing the general lighting market, and 405 nm laser diodes, the key elements of the Blu-ray Disc[TM] technology. Furthermore, sophisticated devices like electrically-driven verticalcavity surface-emitting lasers, which are microcavity based lasers, have been demonstrated recently, paving the way for electrically-driven low threshold room temperature polariton lasers. In 1996 Imamoglu and coworkers predicted the possibility of achieving such a low threshold coherent light source from a polariton condensate in a suitable microcavity. The eigenmodes of such a microcavity are exciton-polaritons, admixed particles resulting from the strong coupling between a photonic mode and an excitonic resonance. Thanks to their very light effective mass at the center of the Brillouin zone (105 times lighter than that of a free electron) and efficient relaxation mechanism a phase transition toward an exciton-polariton condensate might occur at elevated temperatures (observed up to 340 K in GaN-based microcavities). The goal of the present study is to provide a detailed theoretical analysis of the main emission features of electrically-driven polariton lasers based on planar GaN microcavities with embedded InGaN/GaN multiple quantum wells and to further derive the stringent requirements necessary for their experimental implementation. The complete polariton phase diagram is established for two experimentally relevant pumping geometries. Furthermore, the steady-state, the high-speed current modulation response, the relative intensity noise, and the Schawlow- Townes linewidth of those two geometries are derived. The resulting general expressions can be applied to any inorganic semiconductor polariton laser diodes. Then the building blocks of such microcavities are experimentally analyzed separately, i.e., the bottom III-nitride based distributed Bragg reflector, the active medium, and the top dielectric Bragg reflector. An innovative optical characterization method allows to study the effect of the substrate on lattice-matched InAlN/GaN Bragg reflectors. The best optical quality is obtained for such Bragg reflectors when grown on high quality free-standing GaN substrates. Furthermore, particular attention is paid to the excitonic localization via simulations and optical characterizations. For this purpose, a photoreflectance setup allowing the determination of the Stokes shift in the InGaN alloys grown on free-standing GaN substrates either as thick layers or as heterostructures (quantum wells) was carefully designed to operate from cryogenic to room temperature. The stacking of several In0.1Ga0.9N/GaN quantum wells results in a detrimental increase in the Stokes shift and thus the absorption linewidth, which is incompatible with the requirements of the strong coupling regime. Various possible reasons such as inhomogeneous built-in field distribution among the quantum wells and their partial strain relaxation are then identified, hence explaining why it is presently not possible to achieve the strong coupling regime with such an active region embedded in a semi-hybrid microcavity. Alternative solutions for the microcavity design to achieve the strong coupling regime with the InGaN alloy are discussed, supported by results obtained from transfer matrix simulations. The inhomogeneous absorption linewidth of thick InGaN layers is deduced from transmission measurements and for low In content layers (x < 12%) it is successfully reproduced by a model based on a stochastic distribution of indium atoms at the atomic scale. It is finally proposed that gain dilution in green laser diodes should not be much higher than in blue laser diodes provided that the In content within the QWs is homogeneous and abrupt interfaces are present.

EPFL2014

Light-Matter Interaction in III-Nitride Waveguides: Propagating Polaritons and Optical Gain

Joachim Armand Simonne Ciers

III-nitride waveguides featuring AlInN claddings and GaN/AlGaN quantum wells (QWs) offer promising perspectives for applications in many fields of short-wavelength photonics. Thanks to their nearly lattice-matched nature, these structures exhibit an excellent material quality, leading, e.g., to strong light-matter interaction in such QWs, and several promising phenomena. In the low carrier density regime, the strong coupling between QW excitons and waveguide photons results in propagating hybrid light-matter particles, called (exciton-)polaritons, which combine photon-like propagation and exciton-like interactions. These interactions lead to a strong optical nonlinearity, which could be useful for integrated all-optical devices. Due to their strong exciton binding energy (~40 meV in the present structures), III-nitride devices have the potential to maintain these nonlinearities up to room temperature. In the high carrier density regime, a GaN/AlGaN QW electron-hole plasma can provide gain to an optical field in the UV, which can be useful for realizing near-UV laser diodes and semiconductor optical amplifiers. The performance of current UV devices featuring AlGaN claddings is limited by poor material quality. The improved structural quality of waveguides with lattice-matched AlInN claddings could therefore circumvent these issues. This study aims at an in-depth investigation of the optical properties of III-nitride waveguides with AlInN claddings and GaN/AlGaN QWs grown on freestanding GaN substrates. In a sample with an active region that was optimized for strong exciton-photon coupling, we observe propagating polaritons in the low-density regime. A sample with an active region that was optimized for homogeneous near-resonant excitation with a 355 nm laser shows elevated optical gain in the high-density regime. The nearly lattice-matched nature of the entire structure leads to a high structural and optical quality. We found inhomogeneous broadening values between 8 and 11 meV, and a standard deviation in the QW emission energy well below 1 meV over a 50 × 50 µm

^2

area. We calculated the band structure and transition energies of the QWs using self-consistent Schrödinger-Poisson k ·p calculations and found an excellent agreement with experiments. The waveguide polaritons feature a normal mode splitting as large as 60 meV at low temperature, thanks to the large overlap between the optical mode and the active region, a polariton decay length up to 100 µm for photon-like polaritons and a lifetime of 1-2 ps. These decay lengths and lifetimes are limited by residual absorption occurring in the waveguide. The large normal mode splitting and elevated in-plane homogeneity are important assets for the realization of polaritonic integrated circuits. We also demonstrate optically-pumped waveguides exhibiting narrow bandwidth (3.8 nm) optical gain around 370 nm. Due to the high refractive index contrast between the cladding layers and the active region, the confinement factor is as high as 48% and net modal gain values in excess of 80 cm

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are measured. The results agree well with self-consistent calculations accounting for built-in electric field effects and high carrier density related phenomena. As such, these results open interesting perspectives for the realization of more efficient near-UV laser diodes and semiconductor optical amplifiers.

EPFL2018