Susumu Kinoshita

Surface emitting semiconductor lasers

A description is given of the research progress in developing a vertical-cavity surface-emitting (SE) injection laser based on GaAlAs/GaAs and GaInAsP/InP systems. Ultimate laser characteristics, device design, state-of-the-art performances, possible device improvement, and future prospects will also be discussed. The authors propose a vertical-cavity surface emitting semiconductor laser. To reduce the threshold current, they improved the laser reflector and introduced a circular buried heterostructure. The microcavity structure, which is 7 mu m long and 6 mu m in diameter, was realized with a threshold of 6 mA. Thus, possibilities of an extremely low threshold current SE laser device and a densely packed two-dimensional array are suggested. >

Room‐temperature continuous wave lasing characteristics of a GaAs vertical cavity surface‐emitting laser

Room‐temperature continuous wave (cw) operation of a GaAs vertical microcavity surface‐emitting laser has been achieved. An ultrashort cavity device with a cavity length of ∼5.5 μm was grown by metalorganic chemical vapor deposition. cw lasing characteristics such as mode properties and temperature characteristics were examined. Single longitudinal mode operation with a side mode suppression ratio of 35 dB was obtained. The temperature range for single mode operation was more than 50 K.

Circular buried heterostructure (CBH) GaAlAs/GaAs surface emitting lasers

For the purpose of introducing a current confining structure to the GaAlAs/GaAs surface emitting (SE) laser, a circular buried heterostructure (CBH) was introduced. The selective meltback/regrowth was employed as a novel burying method of the CBH-SE laser. First, a difference of the meltback-speeds for various AI contents of GaAlAs layers was measured by observing the depth of dissolved groove. For example, the meltback speed for GaAs was 1.6 μm/s and was almost 3 times as fast as that for Ga 0.7 Al 0.3 As. Next, an edge emitting BH stripe laser was fabricated by the selective meltback/regrowth method. The nominal threshold current density of this device was almost the same as that of the DH broad contact laser (≅ 8 kA/cm2/μm). The typical threshold current of CBH-SE lasers was reduced to ∼ 80 mA with a minimum of 68 mA when the active region was \sim 15 \mu m in diameter under pulsed conditions at room temperature. But one extremely low threshold device with an active region of \sim 6 \mu m in diameter was realized. The threshold was 6 mA (300 K, 1 μs pulse) and 4.5 mA (77 K, CW). This means that the diffraction loss is not noticeable when the diameter is > 6 \mu m. This is also the first demonstration of a microcavity GaAlAs/GaAs surface emitting laser with a cavity of 7 μm long and 6 μm in diameter.

GaAlAs/GaAs Surface Emitting Laser with High Reflective TiO2/SiO2 Multilayer Bragg Reflector

First, it has been made clear that the important parameters of the SE laser are its active layer thickness d and the mirror reflectivity R. The required values of three parameters such as threshold gain gth, threshold current density Jth, and differencial quantum efficiency ηd are obtained for d=2~3 µm and R=95%. At this condition gth is estimated as 300 cm-1 (Nth=3×1018 cm-3), Jth is 25~30 kA/cm2, and ηd is 40%. Next, in order to obtain a reflectivity as high as 95%, a TiO2/SiO2 multilayer Bragg reflector was introduced. It was found that a good quality TiO2 film could be evaporated by leaking oxygen through a variable leak valve to hold 1×10-4 Torr. Then, a 7-pair TiO2/SiO2 multilayer Bragg reflector was fabricated; its peak reflectivity was 95% at 8800 A. A room-temperature pulsed operation of a GaAlAs/GaAs SE layer was achieved and the minimum threshold current was reduced to as low as 150 mA when a round mesa was fabricated to 20 µm in diam.

Surface emitting semiconductor laser array: Its advantage and future

In this paper we review the research progress of surface emitting lasers. A two‐dimensional arrayed configuration of surface emitting lasers will open up a new era of the semiconductor laser, since it provides high power capability, parallel optical processing, and vertical optical interconnection of circuit boards. We present experimental results on vertical cavity surface emitting semiconductor lasers. In order to reduce the threshold current, some improvements have been made on the laser reflector and a circular buried heterostructure has been introduced to confine injection current more effectively. The microcavity structure, which is 7 μm long and 6 μm in diameter, yields a threshold of 6 mA. A possibility of realizing an extremely low threshold current surface emitting laser device and a densely packed two‐dimensional array are suggested.

Selective Meltbacked Substrate Inner-Stripe AlGaAs/GaAs Lasers Operated under Room Temperature CW Condition

A novel lateral-mode controlled AlGaAs/GaAs inner-stripe laser is demonstrated which is fabricated by a one-step liquid phase epitaxial process using a newly developed selective meltback technique. The laser consists of V-groove substrate and six AlxGa1-xAs layers with different Al content forming a built-in rib-waveguide structure. In the fabrication, difference of meltback speed between GaAs and Al0.5Ga0.5As crystals was utilized. It was found that GaAs crystal was meltbacked rapidly by a factor of about fifteen compared with Al0.5Ga0.5As layer. The laser fabricated was operated under CW condition at 20°C with threshold current of 65 mA.

0.67 µm Room-Temperature Operation of GaInAsP/AlGaAs Lasers on GaAs Prepared by LPE

Room temperature pulsed operation of 0.67 µm wavelength GaInAsP/AlGaAs lasers on GaAs was achieved, where the lattice constant of GaInAsP active layer neraly matched those of the Al0.6Ga0.4As cladding layers. The threshold current density Jth of the device was 8 kA/cm2.

A Trial Fabrication of Circular Buried Heterostructure (CBH) GaAlAs/GaAs Surface Emitting Laser by Using Selective Meltback Method

For the purpose of introducing a current-confining structure to a GaAlAs/GaAs surface emitting (SE) laser, a circular buried heterostructure (CBH) was proposed. A selective meltback method was employed in order to avoid the oxidization of GaAlAs. The threshold current of an SE laser was 300 mA under a pulsed condition at room temperature. The nominal threshold current density was 20 kA/cm2/µm.

Liquid Phase Epitaxy and Growth Technology

Liquid-Phase Epitaxy (LPE) has many advantages and is capable of producing reliable semiconductor devices. This chapter provides a detailed description of the equipment necessary for LPE and the epitaxial techniques required for double heterostructures. As discussed in Chap. 4, the principle behind LPE is very simple: epitaxial layers can be thermodynamically grown on a substrate with the same orientation when the substrate meets the oversaturated solution. Since semiconductor-laser research began, LPE has been a popular growth technique and it still plays an important role in producing semiconductor lasers, light emitting diodes and photodetectors.

Epitaxy of III–V Compound Semiconductors

This chapter summarizes the fundamental concepts of the epitaxial growth of III–V compound semiconductors. The relevant crystal-growth technologies encompass two categories: bulk crystal preparation for a substrate to be used as a seed, and thin-film epitaxial growth. Later chapters will contain detailed discussion of the epitaxial-growth techniques, and here we overview several characteristics of liquid-phase epitaxy by comparing them with other crystal-growth techniques, such as Metalorganic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), and Chemical Beam Epitaxy (CBE), among others.