3. Mid-IR DM laser

## 3.1 Design of InP ridge waveguide FP lasers

A typical InP based Type-I laser structure for an emission wavelength of 1.6–2.1 μm is shown in Figure 8 [14]. For most of the laser structures reported on in Section 3, two to three compressively strained QWs are used as the active region with a width of 10 nm. They are separated by 15–30 nm thick InxGa1-xAs barrier layers which are either lattice matched to the InP substrate or tensile strained in order to reduce the average strain in the active region. The active region is sandwiched between two 200 nm-thick InGaAsP separate confinement guide layers with a bandgap wavelength of λ<sup>g</sup> = 1.3 μm which also acts as an etch stop layer for ridge waveguide definition. A 1800 nm thick p-InP layer is grown on top of the separate confinement heterostructure followed by a 200 nm thick highly p-doped InxGa1-xAs contact layer [3].

For the results shown below, four laser structures with varying In compositions were grown on 75 mm diameter n-type (100)-InP substrates in a metal-organic vapour-phase epitaxy reactor at low pressure. The overlapped photoluminescence spectra for the four wafers are shown in Figure 9 measured at room temperature. The sharp peaks indicate high material quality with low defects.

Figure 8.

Direct bandgap profile of an InP-based type-I laser structure with an emission wavelength of 2.1 μm [14].

#### Figure 9.

The optical waveguiding properties of the ridge waveguide structure was determined by carrying out 2-D numerical simulations and one example for the 2 μm wafer (In = 0.74) is shown in Figure 10. The modal analysis of the structure showed that no higher order lateral modes are supported when the ridge width is 2 μm and that the calculated effective index for the waveguide is 3.2. This effective index is used to calculate the grating pattern spacing required to give single mode emission at the target wavelength which will be described below [3].

### 3.1.1 FP laser fabrication

In order to make good single longitudinal mode lasers, the ability to fabricate uniform, highly reliable FP lasers is essential and complete wafers of FP lasers were processed for material evaluation. For the laser to operate in a single lateral mode, control of the ridge width is critical. Simulations showed that a 2 μm wide ridge results in a stable transverse mode. The waveguide was realised using inductive coupled plasma dry etching, the dry etch chemistry used was Cl/N2 followed by a short wet-etch to remove surface roughness. Electrical contacting was achieved using conventional metals (Ti/Pt/Au) and SiO2 as an insulator for contact definition

Overlapped measured photoluminescence spectra at 25°C for four wafers with varying In composition.

Mid-Infrared InP-Based Discrete Mode Laser Diodes DOI: http://dx.doi.org/10.5772/intechopen.86458

Figure 10. 2D simulation of a ridge waveguide laser diode.

on the heavily doped (<sup>p</sup> <sup>2</sup> <sup>10</sup><sup>19</sup> cm<sup>3</sup> ) p<sup>+</sup> -InGaAs capped layer. A scanning electron microscope (SEM) image of the fabricated ridge waveguide is depicted in Figure 11a. Finally the wafers were thinned to 150 μm by mechanical polishing and the n-metal electrode applied [3]. A completed wafer is shown in Figure 11b. Subsequently bars were cleaved into 900 μm cavity lengths and the front and back facets coated 20 and 95% respectively.
