b
Fig. 1 Device structure diagram.
a Structure diagram of conventional QCL
b Structural diagram of QCL with π-shape electrode. Inset: The size of π-shape electrode.
The conventional double-channal planar buried heterostructure (BH) device structure is shown in Figure 1a, which is processed into a BH configuration with a ridge width of 10 μ m. The fabrication process is the same as that described in Ref. [10]. For packaging, the chips were soldered epi-up on AlN submounts to improve heat removal efficiency. Then the submounts with chips were mounted on the copper heat sinks with indium solder followed by wire bonding. The parasitic capacitance of the entire device can be roughly composed of three parts:Ctotal = Cbypass + CInP:Fe + CAR = ε0εsS/d , as shown in the illustration, where εs is sheet-permittivity of InP:Fe and AR,S is pumped area, d is thickness of the layer. To reduce bypass capacitance, we skillfully removed the redundant Ti/Au contact around the current injection window through standard photolithography and etching as shown in Figure 1b, only leaving the current injection window portion and enough pad area for bonding wires. The size of the pad is as shown in the illustration, which just meets the size of the gold wire. This makes the metal contact electrode look like π-shape. The whole top electrode area is reduced from 6.8 × 105 μ m2to 2.37 × 105 μ m2. The processed wafer was cleaved into 2 mm bars, and a high-reflectivity coating consisting of Al2O3/Ti/Au/Ti/Al2O3(200/10/100/10/120 nm) was applied on the back facet with the front facet left uncoated.
Experiments and discussion: For characteration, the laser was mounted on a holder containing a thermistor combined with a thermoelectric cooler (TEC) to monitor and adjust the sub-mount temperature. The emitted optical power was measured with the calibrated thermopile detector placed in front of the laser facet without any correction. The spectra of devices were tested by a Fourier transform infrared spectrometer with a resolution of 0.25 cm-1.
Figure 2a shows the CW power-current-voltage (P-I-V) characteristics of a QCL with π-shape metal contact electrode at different heat sink temperature and a conventional QCL at 20 °C. The QCL with π-shape metal contact electrode CW output power ranging from 181 mW at 10 °C to 60 mW at 40 °C.