Semiconductor materials are extensively used in electronics due to its specific electrical conductivity between conductors and insulators. It is also promising to be used in the optoelectronics such as the photo-detectors, light emitting diodes and semiconductor lasers. The development of semiconductor lasers is nearly as old as the advent of lasers. Compared to other laser sources, the semiconductor lasers have several advantages like good electrical-to-optical efficiency, compact and monolithic in fabrication and widely controlled emission spectral range. Mostly the gain mediums of the semiconductor lasers are direct bandgap semiconductor materials of III-V compounds. The light amplification is achieved by recombination of the electron-hole pairs in conduction band and valence band of active layers in semiconductor gain device induced by the propagating optical fields. The simplest form of semiconductor gain devices is p-n junction diodes in which the population inversion is brought about around junction area by a sufficiently large forward bias.
To reduce pump threshold current and improve laser efficiency, the double heterostructure semiconductor laser diodes have been demonstrated with better carrier and photon confinement. More recently, a novel structure of multiple quantum wells (MQWs) has been demonstrated via advanced semiconductor growth techniques with the precision up to single atomic layer. They exhibit numerous excellent characteristics like lower optical loss, lower threshold current injection, batter carrier confinement, superior room-temperature performance and the flexible tuning parameters in comparison to double heterostructure semiconductor lasers [1]. The
number of wells can be selected to fit the required gain or absorption coefficient and the emission wavelength can be adjusted by varying the bandgap of well layer, barrier height and well thickness.
Different types of semiconductor lasers could be classified as edge-emitting and surface-emitting lasers which will be mentioned in section 1.3. The optical cavity of edge-emitting semiconductor lasers is formed by the cleaved facets perpendicular to plane of active layer structure. Therefore, the emitted photon is propagating in path normal to the epitaxial direction guided by the interfaces between active and confined layers. Schematic description of the edge-emitting lasers or the so-called Fabry-Pérot lasers is shown in Fig. 1.1-1. Because of the long active region length L, the cavity gain of edge-emitting laser is pretty high and the output power is much higher than surface-emitting laser. But the active region height H, which is equivalent to active layer thickness, is typically two orders smaller than the width W and the output beam is elliptically shaped with poor beam quality. Alternatively, the surface-emitting lasers are developed with output light direction parallel to the epitaxial direction. The laser cavity is conventionally formed by two distributed Bragg mirror (DBR) structures which are parallel to the epitaxial layer on top and bottom side of semiconductor gain chip. A simplified schematic illustration of surface-emitting lasers is shown in Fig.
1.1-2. Because there is no restriction of lateral dimensions and the cavity has circular cross-section geometry, the output laser beam could be a single transverse mode non-diffraction limit beam. But the low optical gain due to short active length and the small current injection aperture due to the ultra-short linear-linear cavity limited the output power to just several milli-watts. Therefore, the optical pumping instead of current injection is used in external-cavity surface-emitting lasers which has been developed to obtain high power and good beam quality semiconductor laser source.
Length, L
Height, H Width, W
Dielectric mirror
Cleaved reflecting surface Elliptical laser beam
Active layer
Electrode Stripe electrode
Fig 1.1-1 Schematic illustration of the structure of edge-emitting laser.
Electrode
Electrode
DBRs
DBRs
Substrate
Active layer
Laser emission
Fig 1.1-2 Schematic illustration of the structure of surface-emitting laser.
The detailed evolution of surface emitting lasers will be further discussed in section 1.3.
The material system is important to the emission spectral range of semiconductor lasers which could be tuned from 400 nm in the ultraviolet by GaN-based system, to 2.3 μm by AlGaAsSb-based system. To select suitable substrate and semiconductor alloy epitaxial layers, the use of a lattice constant versus bandgap energy diagram is needed [2]. The chosen semiconductor materials in active layer should have direct bandgap matched to the objective emission photon energy. Besides, the lattice constant should be lattice-matched to the substrate and prevent the generation of large amount of defects. In using the approximation of Vegard’s law, the lattice constant and bandgap shift could be continuously tuned via the mixed ternary of quaternary alloys instead of using binary compounds [3]. The mostly mature and used material system is the compounds composed by AlAs/GaAs/InAs alloys lattice-matched to the GaAs substrate. The wavelength range of AlxGa1-xAs with mole fraction x could be varied from 780 nm to 900 nm with direct bandgap and nearly perfect lattice-matching to GaAs substrate. With the indium-doping in place of aluminum-doping, the compressive strain of the InGaAs on the GaAs substrate will make the bandgap shifted to around 1 μm. Although the more doping fraction of indium could alter the emission wavelength to be longer than 1 μm, the more serious strain effect will introduce more defects which will result in much scattering loss and lower the semiconductor laser efficiency. But this restriction could be eased by low temperature growth or by alternating growth of large and small lattice constant material to cancel the built-in energy of the MQWs which is called “strain compensation” [4]. The high refractive index difference and high thermal conductivity of the AlAs/GaAs distributed Bragg reflectors (DBRs) for GaAs-based system is also beneficial to the
development in surface-emitting lasers.
Semiconductor lasers at 1.3 and 1.55 μm are of great importance in remote sensing, laser ranging and optical communication. The InGaAsP and AlGaInAs quaternary compounds based on InP substrate and InGaNAs quaternary compound based on GaAs substrate are typically used in this spectral region. Although the InGaAsP/InP system is developed earlier than AlGaInAs/InP system, the conduction band offset of the former is smaller than the latter. This means that the carrier confinement and thermal stability under room temperature operation of InGaAsP/InP system is worse than AlGaInAs/InP system. Therefore, in this thesis we choose AlGaInAs/InP system as our MQWs material and a series of characteristics of photoluminescence, stimulated emission and saturable absorption in the surface- emitting scheme have been investigated in the following three chapters. But the DBRs lattice matched to the InP-based systems suffer from low refractive index contrast, low thermal conductivity and high complexity of growth to realize the surface-emitting laser. Alternatively, Fe-doped InP with good transparency at 1-2 μm is chosen as the substrate in our MQW sample instead of the conventionally used S-doped InP. Consequently, the transmitted AlGaInAs MQW chip is achieved and the function of DBRs could be replaced by an external mirror in the surface-emitting laser.
Recently, a new GaAs-based nitride quaternary compound is applied in the semiconductor lasers in the spectral range of 1.1-1.6 μm. By introducing a few fraction of nitride to InGaAs the emission wavelength of this quaternary compound has potential shifted to longer wavelengths and the lattice-mismatching to GaAs can be reduced. However, because the nitrogen ion is smaller than the elements in the InGaAs material, the more doping concentration of nitride will result in the more defect in the alloys. It will give rise to strong scattering loss which is unfavorable to
the semiconductor lasers. So far, several InGaNAs vertical-external-cavity surface-emitting lasers have been demonstrated at wavelength range at 1.2, 1.3 and 1.55 μm. Moreover, the wafer fusion technique which allows combining different semiconductor materials with variant lattice constant is used in the AlGaInAs/GaAs OPSLs and average output power of 2.6 W is obtained at 1.57 μm. Table 1.1-1 shows the performances and material systems of the OPSLs from visible to mid-infrared spectral region to date [5-27]. Because of the absence of efficient and mature commercial pump diode laser module, the semiconductor lasers at the spectral range below 640 nm are mostly realized by frequency-doubling of the infrared semiconductor lasers [28].