E‐band InAs quantum dot laser on InGaAs metamorphic buffer layer with filter layer

InAs quantum dots (QDs) are receiving attention as next-generation Eband light source that offers high-temperature operation and temperature insensitive operation. However, high-density crystal defects occur at the interface between the InGaAs buffer layer and GaAs, resulting in reduced device performance and shortened lifetime. Here, E-band QD lasers are demonstrated on InGaAs buffer layer, which suppressed the spread of dislocation by introducing a high-temperature annealing and a strained layer superlattice filter. In the device, the peak wavelength at room temperature is measured to be 1427 nm and the threshold current density was 440 A/cm2. This result indicates that E-band QD laser structures on low threading dislocation density are promising for the realisation of high-performance E-band lasers.

✉ Email: jkkwoen@iis.u-tokyo.ac.jp InAs quantum dots (QDs) are receiving attention as next-generation Eband light source that offers high-temperature operation and temperature insensitive operation. However, high-density crystal defects occur at the interface between the InGaAs buffer layer and GaAs, resulting in reduced device performance and shortened lifetime. Here, E-band QD lasers are demonstrated on InGaAs buffer layer, which suppressed the spread of dislocation by introducing a high-temperature annealing and a strained layer superlattice filter. In the device, the peak wavelength at room temperature is measured to be 1427 nm and the threshold current density was 440 A/cm 2 . This result indicates that E-band QD laser structures on low threading dislocation density are promising for the realisation of high-performance E-band lasers.

Introduction:
The E-band (extended-wavelength band: 1360-1460 nm) is the least common telecommunication band. The main reason is that there is a water (OH) absorption region around the wavelength of 1.38 μm, which is included in the E-band. However, in dry fiber standardised in 2003 (ITU-T G.652.D), the E-band has less attenuation than the O-band and has better wavelength dispersion performance than the S-band [1]. Meanwhile, coarse wavelength division multiplexing (CWDM) technology is expected to use the entire telecommunicationband range from 1270 to 1610 nm. Therefore, to reduce the cost of building a CWDM system, it is necessary to use uncooled lasers. However, it is common to use S-band to L-band in real system implementations. This is because a light source suitable for E-band has not been developed until now due to compatibility with the previous generation optical fibers. Particularly for material systems using GaAs or InP substrates, the limitations of material properties and combinations have limited laser development at around 1400 nm. InAs QD lasers on an InGaAs metamorphic buffer layer are promising candidates for E-band emission devices. Due to QD's unique independent 3D barrier structure, QD lasers are less sensitive to threading dislocation density, even on Si and Ge substrates, and have excellent high-temperature operation and temperature-insensitive operation [2][3][4][5][6]. This point is also suitable as an uncooled laser used in CWDM, which is considered an application of E-band laser.
However, until now, devices only in the wavelength range around 1.46 μm have been reported [7,8]. In addition, high-density crystal defects occur at the interface between the InGaAs buffer layer and GaAs, resulting in reduced device performance and shortened lifetime. In order to suppress the density of point defects and the spread of threading dislocations, thermal annealing [7,9] and dislocation filter layer have been widely introduced to epitaxial structures [10,11]. In particular, strained layer superlattice (SLS) is one of the essential filtering structures for suppressing dislocations.
In this paper, we adopt InGaAs strained layer superlattice filters in InGaAs metamorphic buffer layer on GaAs. By including filter layers, the emission intensity of InAs/InGaAs QDs on metamorphic buffer layer is increased. Finally, we successfully demonstrate E-band InAs/InGaAs QD lasers on an InGaAs metamorphic buffer layer containing InGaAs SLS filter layers.
Experimental method: A typical solid source molecular beam epitaxy (MBE, Riber Compact 21 DZ) was used for this study. Dual filament cells were used to supply Group III elements Ga, In, and Al. Arsenic tetramer was supplied using a valved cracker cell. Be and Si were supplied as doping materials. n-Type 3-inch GaAs substrate with doping concentration of 2.0 × 10 18 cm −3 was used. After the substrate was thermally cleaned at a temperature of 600°C measured by BandiT, all processes were measured with a thermocouple on the back of the substrate. After typical thermal cleaning of the GaAs substrate, a 200 nm GaAs buffer layer was grown. Then, 301.5 nm thick In 0.15 Ga 0.85 As layer including strained layer superlattice buffer was grown. In this layer, 100 nm of In 0.15 Ga 0.85 As was first grown at a temperature of 390°C, and then 5 pairs of 10 nm In 0.15 Ga 0.85 As/ In 0.25 Ga 0.75 As strained layer superlattice (SLS) were grown [12]. In addition, after growing an In 0.15 Ga 0.85 As layer of 100 nm, an AlAs layer of 1.5 nm was grown. The grown structure was annealed at 700°C for 10 minutes [7]. This In 0.15 Ga 0.85 As layer metamorphic buffer layer including strained layer superlattice was repeated 3 times. We grew the laser structure on top of the metamorphic/strained layer superlattice buffer layer. After growing a 300 nm In 0.15 Ga 0.85 As n-type contact layer, a 50 nm sacrificial layer of In 0.15 Al 0.67 Ga 0.18 As was grown. The active layer was sandwiched with a 1500 nm-thick n-/p-type Al 0.35 In 0.15 Ga 0.50 As clad. After the growth of the bottom clad layer, a 40 nm In 0.15 Ga 0.85 As layer was grown. After growing 1 nm of In 0.15 Ga 0.85 As on 2 nm GaAs for planarisation, 0.957 nm of InAs quantum dots were grown. The QD layer was capped with 5 nm of In 0.34 Ga 0.66 As and 2.5 nm of In 0.15 Ga 0.85 As. After that, 2 nm of GaAs was grown and in-flush was performed for 5 minutes. A 40 nm InGaAs layer was grown as a spacer. This layer structure was stacked 8 times in the active layer. After the p-type clad was grown, a 200 nm p-type In 0.15 Ga 0.85 As contact layer was grown.
The Fabry-Perot lasers were fabricated. The mesa structure was formed using citric acid (50% wt.):H 2 O 2 (20:1) solution after patterning using laser lithography. The direction of the mesa structure was formed in the [110] direction with less interference of the cross-hatch and deep grooves were formed by strain [13]. The etch stop layer was etched with HF (20% aq.). For n-type and p-type electrodes, 30 nm AuGeNi/200 nm Au was deposited. These electrodes were deposited on the top-side of the substrate using electron beam deposition and formed using the liftoff method. The cavity width was 80 μm and the length was 1.5 mm.
After the cross-section of the sample was milled to 200 nm by using focused ion beam (FIB), transmission electron microscopy (TEM) images were observed. The QD density was evaluated by an atomic force microscopy (AFM). The optical properties of InAs/GaAs QDs were measured by photoluminescence (PL) measurements. Electroluminescence was observed in a fabricated device under pulse current injection (1 kHz repetition frequency, 1 μs pulse-width, and 0.1% duty-cycle) at room temperature.
Results and discussion: At first, we observed InAs QD structures on In 0.15 Ga 0.85 As layer by AFM. A good QD uniformity is obtained with a density of 1.2 × 10 10 cm −2 . The average height and diameter of QDs were 2.4 nm and 28 nm, respectively. For the PL measurement of the effect of the lower SLS filter on QD emission, one QD layer was sandwiched with a 100 nm top and bottom In 0.15 Al 0.35 Ga 0.50 As clad layer grown on top of two types of metamorphic buffer layers. The first sample was grown on a buffer layer containing SLS filter layer as described in the experimental method. The second sample was formed on a buffer layer with a continuous In 0.15 Ga 0.85 As buffer instead of the SLS structure. The PL emissions from the two samples are shown in Figure 1. Both samples had almost the same emission peak wavelength of 1421 nm; however, the intensity from the sample with SLS filter layer showed 23% brighter than without the SLS filter sample. This is believed to be due to the reduction of the defect density by the SLS filter layers. Although there is an effect of optical absorption by water in the atmosphere and the measuring fiber, the emission linewidths (full-width at half-maximum: FWHM) of two samples were about 26 meV.
In order to observe the propagation of dislocation, a cross-sectional TEM measurement under the acceleration voltage of 200 kV was used after the focused ion beam (FIB) preparation. Figure 2 shows a brightfield TEM image of metamorphic buffer layers underneath the laser structure along the [011] direction. As a result, it was found that most of the misfit dislocations were generated at the interface between GaAs and In 0.15 Ga 0.85 As, and several dislocations propagated to the upper layer, and blocked in the first SLS filter layer. The cross-sectional TEM contains only a limited space because the sample depth is thin (200 nm); however, it is predicted that only threading dislocations density of less than 10 6 /cm 2 order exist, because dislocations have not been observed in the upper layer than the second SLS filter layer. Figure 3 shows light-current (L-I) curve of a QD laser grown on metamorphic buffer layers. The lowest threshold current density (J th ) was 440 A/cm 2 , the slope efficiency was 51 mW/A, and the maximum output power from a single facet was more than 30 mW. Figure 4 shows emission spectra for an E-band QD laser at various injection current. The laser device showed stable ground state lasing with a wavelength of 1427 nm at current injections up to 1000 mA.

Conclusion:
We demonstrate E-band QD lasers on InGaAs buffer layer, which suppressed the spread of dislocation by introducing a hightemperature annealing and a strained layer superlattice filter. In the device, the wavelength at room temperature is measured to be 1427 nm and the threshold current density was 440 A/cm 2 . This result indicates that E-band QD laser structures on low threading dislocation density are promising for the realisation of high-performance E-band lasers. In addition, the strained layer superlattice filter is expected to be used in the growth of InGaAs on Si substrates with higher defect density.