Carrier leakage in 1.3μm SCH quantum well lasers

Abstract

We demonstrate 1300 nm SCH quantum well lasers grown by CBE with low threshold and high efficiency. Spontaneous emission measurements of these lasers indicate strong carrier leakage. This reduces To to values around 55 K. Recently, 1300 nm InGaAsP/InP quantum well lasers have been reported with low threshold currents and high efficiency values (1,2). However, published To values are still around 45 K, which is too low for applications without Peltier coolers, as for example in datacomm systems. We have grown 1300 nm InGaAsP/InP SCH quantum well lasers by CBE. The structure (Fig. 1) consists of three compressively strained quaternary quantum wells (L,=6 nm) with a lattice mismatch of 0.7%. The 8 nm wide barriers and the 0.15 pm wide quaternary confinement layers have a band gap of 1.13 eV. This structure is optimized for optical confinement and yields a confinement factor of 1.2% per quantum well. Broad area lasers with a stripe width of 55 pm have been fabricated from the epitaxial layers. Length-dependent measurements (Fig. 2) yield a transparency current density of 290 A/cm2 and cavity optical losses of about 8.6 cm-'. Despite this low threshold current, To has a value of only 55 K at room temperature. At 60 C this value is reduced to only 30 K (Fig. 3). To clarify the origin of the low To value, the spontaneous emission of the lasers has been measured through the top contact at various temperatures. Figure 4 shows three spontaneous emission spectra. The broad main peak at about 1 eV is the spontaneous emission spectrum of the laser. A second peak emerges at about 1.13 eV, which is merely the band gap of the quaternary confinement layers. The intensity of this second peak relative to the main peak increases strongly with increasing temperature. The ratio between the intensity of the main peak and the second peak increases from 0.085 at 20 C to 0.33 at 60 C. It is thus obvious that with increasing temperatures, a strongly increasing part of the carriers leaks out of the quantum well and recombines in the adjacent confinement layers. This carrier leakage is one reason for the low To values of 1.3 pm quantum well lasers. At the conference, we will present measurements on improved structures with blocking layers that reduce carrier leakage. In conclusion, we have fabricated a 1.3 pm SCH laser with low threshold current and high quantum efficiency. The lasers investigated have a transparency current density of 290 A/cm2, internal losses of 8.6 cm-' and a To of 55 K at room temperature. To our knowledge, these are the best values for 1.3 pm quantum well lasers yet to be published. Spontaneous emission measurements show that part of the low To value is due to carrier leakage into the confinement layers.