Buried heterostructure AlGaAs lasers on semi-insulating substrates

Buried heterostructure (BH) AlGaAs lasers were fabricated on Cr-doped semi-insulating substrates. Low threshold current (8 mA/μm stripe width for cavity length of 300 μm), a high differential quantum efficiency (55%), and stable transverse mode operation were realised

Recent developments in monolithic integration of InGaAsP/InP optoelectronic devices

Monolithically integrated optoelectronic circuits combine optical devices such as light sources (injection lasers and light emitting diodes) and optical detectors with solid-state semiconductor devices such as field effect transistors, bipolar transistors, and others on a single semiconductor crystal. Here we review some of the integrated circuits that have been realized and discuss the laser structures suited for integration with emphasis on the InGaAsP/InP material system. Some results of high frequency modulation and performance of integrated devices are discussed

Very low threshold InGaAsP mesa laser

Very low threshold currents InGaAsP/InP terrace mesa (T-ME) lasers with an unpassivated surface have been fabricated on semi-insulating (SI) InP substrates. Fabrication of the lasers involves a single-step liquid phase epitaxial (LPE) growth and a simple etching process. Lasers operating in the fundamental transverse mode with threshold currents as low as 6.3 mA (for a cavity length of 250 μm) have been obtained. Comparison between the unpassivated lasers and those passivated using the mass transport technique is described

AlGaAs lasers with micro-cleaved mirrors suitable for monolithic integration

A technique has been developed for cleaving the mirrors of AlGaAs lasers without cleaving the substrate. Micro-cleaving involves cleaving a suspended heterostructure cantilever by ultrasonic vibrations. Lasers with microcleaved mirrors have threshold currents and quantum efficiencies identical to those of similar devices with conventionally cleaved mirrors

High-gain AlGaAs/GaAs double heterojunction Darlington phototransistors for optical neural networks

High-gain MOCVD-grown (metal-organic chemical vapor deposition) AlGaAs/GaAs/AlGaAs n-p-n double heterojunction bipolar transistors (DHBTs) and Darlington phototransistor pairs are provided for use in optical neural networks and other optoelectronic integrated circuit applications. The reduced base doping level used results in effective blockage of Zn out-diffusion, enabling a current gain of 500, higher than most previously reported values for Zn-diffused-base DHBTs. Darlington phototransitor pairs of this material can achieve a current gain of over 6000, which satisfies the gain requirement for optical neural network designs, which advantageously may employ neurons comprising the Darlington phototransistor pairs in series with a light source

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

Integrated Semiconductor Ring Lasers

The concept of a semiconductor laser with a circular resonator, its advantages and particular problems are discussed. The pillbox resonator is introduced and its operation on whispering gallery modes is illustrated using a computer model. The experimental evidence of the guiding mechanism is shown, leading to the first demonstration of continous wave operation in a semiconductor ring laser with a threshold current of 24mA. The parameters of the GaAs/AlGaAs material that are relevant for the low threshold current operation are presented and all aspects of the fabrication procedure are covered, emphasizing the processes that led to smooth sidewalls and the low loss circular cavity. A further reduction of the threshold current to 12.5mA is shown, which is owing to a coating of silicon nitride that supresses the non-radiative recombination current and reduces the scattering loss. The excess bending loss is calculated to be 3dB/360 and found to be independent of the radius between 30mum and 145mum. The influence of the Y-junction on the operation characteristic is studied and shown to cause kinks in the L-I curve; it is also held responsible for the relatively low differential quantum efficiency (0.02-0.04) of the devices. Strip-loaded guiding is demonstrated for radii between 300mum and 600mum and proposed as a solution for the problem of degradation that is caused by etching through the active layer. The integration capability of the structure is demonstrated by the succesful operation of a circuit comprising of a ring laser, a low-loss waveguide and a detector, and an optoelectronic integrated circuit featuring a ring laser and a field-effect transistor. The material parameters that are involved in performing these complicated functions are discussed and modeled numerically

High speed modulation and CW operation of AlGaAs/GaAs lasers on Si

Microwave modulation and CW operation of AlGaAs lasers grown by MBE on Si substrates have been obtained for the first time. Ridge waveguide lasers(10µm×380µm) were modulated with a microwave signal up to 2.5GHz which is notable considering the structure used. Near and far field measurements indicated a single transverse mode and a narrow beam angle (4.8°). Finally, polarization measurements appear to show the solely TE nature of the emission