Contents I. Introduction
The development of 1.3-m lasers with excellent temperature characteristics is required for metropolitan and access networks. Conventional lasers on InP substrates are more sensitive to the ambient temperature than those on GaAs substrates. The reported characteristic temperature () of an InGaAs multiple-quantum-well (MQW) laser on a GaAs substrate is 170 K [1], while a 1.3- m InGaAsP MQW laser on an InP substrate exhibits a of 50-70 K [2]. However, it is difficult to realize a 1.3-m emission with an InGaAs MQW laser on a GaAs substrate, because of the critical thickness. There have been several reports describing long-wavelength lasers on GaAs substrates, such as lasers with highly strained InGaAs MQWs [3]–[5], metamorphic buffers [6], InAs quantum dots [7]–[9], and GaInNAs [10]–[12] MQWs as active layer materials. Of the many competing active layer materials for 1.3-m lasers, InGaAs quantum wells on InGaAs ternary substrates are advantageous as regards controlling the lattice constant and the energy band gap, and, thereby, providing a large conduction band offset [13], [14]. Lasers on ternary substrates with high characteristic temperatures ( K: 20-50°C, 99 K: 50-70°C at 1.226 m) [15] and high-temperature operation (210°C/pulsed at 1.226 m) [16] have been reported. These lasers, which are grown on In0.22Ga0.78As substrates, are high-reflectivity (HR)-coated and short-wavelength (1.226-m) devices. A 1.3- m laser has been reported that employs an In0.31Ga0.69As substrate; however, its characteristic temperature is lower than expected ( K) [17]. This was because the crystal quality of the In0.31Ga0.69As substrate was inferior to that of an In0.22Ga0.78As substrate. The full-width at half-maximum (FWHM) of the X-ray rocking curve increased with increasing indium content [18]. In addition, ternary InGaAs (In:0.2-0.3) is disadvantageous in terms of thermal conductivity compared with binary materials such as GaAs and InP since it causes excessive heating around the active region. In this study, to overcome the problems related to thermal conductivity and crystal quality, we have introduced a novel crystal growth method called the traveling liquidus-zone (TLZ) method and fabricated an In0.1Ga0.9As substrate with a low indium content. The TLZ method was invented by Kinoshita of the Japan Aerospace Exploration Agency [19]. It yields better crystal uniformity and can be used to realize large wafers by suppressing convection. This method produces rectangular- and plate-shaped InGaAs wafers. An InGaAs substrate with a low indium content has better thermal conductivity than previously reported lasers [15]–[17]. To achieve 1.3-m lasing on a low indium-content substrate, we have to pay special attention to forming the MQW for the active layer, because of the large difference between the crystal lattice constants of the InGaAs (In:) well and the substrate (In:0.1). We employed low-temperature metal-organic vapor-phase epitaxy (MOVPE) to realize highly strained InGaAs-InGaAs MQWs. These MQWs have a deep potential and efficiently confine the electrons, which improves the temperature characteristics, as found with the 1.2-m-band highly strained InGaAs laser on GaAs [20], [21].
