Indirectly Pumped 3.7 THz InGaAs/InAlAs Quantum-Cascade Lasers Grown by Metal-Organic Vapor-Phase Epitaxy

Device-performances of 3.7 THz indirect-pumping quantum- cascade lasers are demonstrated in an InGaAs/InAlAs material system grown by metal-organic vapor-phase epitaxy. The lasers show a low threshold-current-density of ~420 A/cm2 and a peak output power of ~8 mW at 7 K, no sign of parasitic currents with recourse to well-designed coupled-well injectors in the indirect pump scheme, and a maximum operating temperature of Tmax~100 K. The observed roll-over of output intensities in current ranges below maximum currents and limitation of Tmax are discussed with a model for electron-gas heating in injectors. Possible ways toward elevation of Tmax are suggested

Material Growth of InGaAs/InAlAs Superlattices for Quantum Cascade Laser Application

Quantum Cascade Lasers (QCLs) have rapidly advanced to a leading position among infrared light sources, and are being used in a number of chemical sensing and spectroscopic applications. Most of these devices are grown by molecular beam epitaxy (MBE) except for some of those grown by metalorganic chemical vapor deposition (MOCVD). This thesis investigates the material growth of near-infrared (NIR) QCLs of InGaAs/InAlAs lattice-matched system using MOCVD. The effects of various growth parameters were understood by detailed study on the material properties of InGaAs and InAlAs. Also, in-depth study on InGaAs/InAlAs superlattices shows that there exists a growth window to optimize their performance. With proper interruption periods of growth between individual layers, the hetero-interfacial defects can be reduced at high growth temperature without surface degradation of constituent layers. Finally, the preliminary study of NIR QCLs with digital alloys insertion was conducted and some QCLs wafers were grown successfully

Photoluminescence Study of the Interface Fluctuation Effect for InGaAs/InAlAs/InP Single Quantum Well with Different Thickness

Photoluminescence (PL) is investigated as a function of the excitation intensity and temperature for lattice-matched InGaAs/InAlAs quantum well (QW) structures with well thicknesses of 7 and 15 nm, respectively. At low temperature, interface fluctuations result in the 7-nm QW PL exhibiting a blueshift of 15 meV, a narrowing of the linewidth (full width at half maximum, FWHM) from 20.3 to 10 meV, and a clear transition of the spectral profile with the laser excitation intensity increasing four orders in magnitude. The 7-nm QW PL also has a larger blueshift and FWHM variation than the 15-nm QW as the temperature increases from 10 to ~50 K. Finally, simulations of this system which correlate with the experimental observations indicate that a thin QW must be more affected by interface fluctuations and their resulting potential fluctuations than a thick QW. This work provides useful information on guiding the growth to achieve optimized InGaAs/InAlAs QWs for applications with different QW thicknesses

Low-angle misorientation dependence of the optical properties of InGaAs/InAlAs quantum wells

We investigate the dependence of the low-temperature photoluminescence linewidths from InP-lattice-matched InGaAs/InAlAs quantum wells on the low-angle misorientation from the (100) surface of the host InP substrate. Quantum wells were grown on InP substrates misorientated by 0, 0.2, 0.4 and 0.6 degrees; 0.4 degrees was found to consistently result in the narrowest peaks, with the optimal spectral purity of ~4.25 meV found from a 15nm quantum well. The width of the emission from the 15nm quantum well was used to optimize the growth parameters. Thick layers of Si-doped InGaAs were then grown and found to have bulk, low temperature (77 K), electron mobilities up to \mu ~ 3.5 x 10^4 cm2/Vs with an electron concentration of ~1 x 10^16

Indium Phosphide Based Quantum Cascade Lasers Grown on Silicon Substrate

Quantum Cascade Lasers (QCLs) are semiconductor devices that, currently, have been observed to emit radiation from ~ 2.6 µm to 250 µm (1 to 100 terahertz range of frequencies). They have established themselves as the laser of choice for spectroscopic gas sensing in the mid-wavelength infrared (3-8 µm) and long-wavelength infrared (8-15 µm) region. In the 4-12 µm wavelength region, the highest performing QCL devices, in terms of wall-plug efficiency and continuous wave operation, are indium phosphide (InP) based. The ultimate goal is to incorporate this InP-based QCL technology to silicon (Si) substrate since most opto-electronics are Si-based. The main building blocks required for practical QCL-on-Si integrated platforms were demonstrated and will be covered in this presentation. The experimental results of a 40-stage indium phosphide based quantum cascade laser grown on a lattice-mismatched germanium-coated silicon substrate with metamorphic buffer (M-buffer) is discussed. The QCL\u27s strain-balanced active region was composed of Al0.78In0.22As/In0.73Ga0.27As and an 8 µm-thick all-InP waveguide. Since the M-buffer was insulating, the wafer was processed into ridge-waveguide chips with lateral current injection scheme. Lasing was observed from 78K up to 170K for QCL-on-Si devices. Also discussed is the first room temperature operation of QCL grown on a lattice-mismatched gallium arsenide (GaAs) substrate with metamorphic buffer (M-buffer). Similar to QCL-on-Si, a lateral injection scheme was utilized since M-buffer was insulating. Lasing was observed from 78K up to 303 K for QCL-on-GaAs. Material characterization of QCL-on-InP, QCL-on-GaAs, and QCL-on-Si using Transverse Electron Microscopy (TEM) will also be covered in this presentation. A very small section, 10 µm x 10 µm, of the QCL active region was used to give an estimate of the defect density for each of the QCL configuration. Lastly, characterization of the material quality of the remaining 6-inch wafer of QCL-on-Si using photoluminescence spectroscopy (PL) will be discussed. This method helped determined the best portion of the remaining material for subsequent processing into ridge waveguide devices

An Overview on Quantum Cascade Lasers: Origins and Development

This chapter presents an introductory review on quantum cascade lasers (QCLs). An overview is prefaced, including a brief description of their beginnings and operating basics. Materials used, as well as growth methods, are also described. The possibility of developing GaN-based QCLs is also shown. Summarizing, the applications of these structures cover a broad range, including spectroscopy, free-space communication, as well as applications to near-space radar and chemical/biological detection. Furthermore, a number of state-of-the-art applications are described in different fields, and finally a brief assessment of the possibilities of volume production and the overall state of the art in QCLs research are elaborated

Contactless electroreflectance study of the surface potential barrier in n-type and p-type InAlAs van Hoof structures lattice matched to InP

N-type and p-type In0.52Al0.48As van Hoof structures with various thicknesses of undoped In0.52Al0.48As layer (30, 60, 90, and 120 nm) were grown by metal-organic vapor phase epitaxy on InP substrates and studied by contactless electroreflectance (CER) at room temperature. The InAlAs bandgap related CER resonance followed by a strong Franz-Keldysh oscillation (FKO) of various periods was observed clearly for the two structures. This period was decreased with the decrease of thickness of undoped In0.52Al0.48As layer and was slightly narrower for p-type structures. The FKO period analysis indicates that the Fermi level is pinned 0.730.02 eV below the conduction band at In0.52Al0.48As surface. This pinning was attributed to the surface reconstruction combined with the adsorption of oxygen and carbon atoms (consequence of air exposure) which were detected on the In0.52Al0.48As surface by X-ray photoelectron spectroscopy. Also, CER measurements repeated one year after the sample growth shows that the process of InAlAs oxidation in laboratory ambient is negligible and therefore this alloy can be used as a protective cap layer in InP-based heterostructures

High-performance III-V quantum structures and devices grown on Si substrates

III-V material laser monolithically grown on silicon (Si) substrate is urgently required to achieve low-cost and high-yield Si photonics. Due to the material dissimilarity between III-V component and Si, however, several challenges, such as dislocations and antiphase domains, remain to be solved during the epitaxial growth. In this regard, quantum dot (QD) laser diodes have been demonstrated with impressive characteristics of temperature insensitive, low power consumption and defects tolerance, and thus QD material is regards as an ideal material for laser directly grown on Si substrate. In this thesis, both QD laser diodes with 1.3 µm wavelength and quantum dot cascade laser with mid-infrared wavelength have been investigated. To understand the unique advantages of QD material, the comparison of QD and quantum well (QW) materials and devices grown on Si substrate is carried out in chapter 3. Based on identical fabrication and growth conditions, Si-based QW devices are unable to operate at room temperature, while the room-temperature Si-based QD is obtained with threshold current density of 160 A/cm2 and single-facet output power of >100 mW under continuous wave (c.w.) injection current driving. Besides, Si-based QD laser also shows remarkable temperature stability which the c.w. operation temperature reaches 66 ℃. The results point out that QD material has great potential in monolithic growth of III-V on Si for silicon photonics. Then, a novel approach of all-MBE grown QD laser on Si substrate is reported in chapter 4, with the optimization of buffer layer. The all-MBE grown QD laser on on-axis Si substrate with maximum operation temperature of 130 oC is achieved by utilizing thin Germanium (Ge) buffer. The mid-infrared silicon photonics has wide applications and market, but the lack of Si-based mid-infrared laser is a subsistent problem. Because the bandgap of conventional QW and QD materials is impossible to match the wavelength in mid-infrared range (3 µm to 20 µm), the Si-based quantum cascade laser (QCL) devices is regarded as an effective method to meet the requirement. Therefore, the high-performance QCL is firstly explored in chapter 5, and then, several methods in fabrication process are researched to enhance the performance for QCL devices. After the optimization of structure design and development of fabrication process, the InP-based QCL shows impressive properties with 600 mW emission power and over 100℃ operation temperature under c.w. mode. Following the previous work on Si-based QD laser, the quantum dot cascade laser (QDCL) is expected as a suitable solution for Si-based QCL devices. With the continuous improvement in structure design, the QDCL with multilayer QDs shows comparable performance, compared with conventional QCL devices. It is noted that the QDCL generates both TE and TM modes output, which is a breakthrough towards surface emitting QCL because the common QW-based QCL has only-TM emission in principle. Finally, the Si-based QCL is attempted with different structure design based on the pervious results