A PBH-LD, a kind of strongly index guided laser, has been made by a meltback method by using a vertical LPE system which was made in our laboratory for ourselves.
Formation of a mesa shape by a meltback method has an advantage in the reduction of damages on a substrate due to chemical etching and heating during regrowth.
After the investigation of several characteristics of meltback solutions and meltback temperature, we confirmed that both of chemical etching and meltback method should be used to make a high performance PBH-LD. Therefore, we have formed mesa shapes successively with chemical etching and the meltback method. It is considered that the characteristics of the interface between the substrate and current blocking layers grown after the meltback may be excellent because of high meltback temperature of 610℃. The width of an active layer has been controlled to be 0.8 to 1.2㎛ so that the fabricated LD could operate with single mode in the lateral direction. To reduce the leakage current of current blocking layers, the widths of p-InP and n-InP layer have been grown to be 1.2㎛ and 1.6㎛, respectively.
From the measurement of electric and optical characteristics of the fabricated MQW-PBH-LD, it was confirmed to be operated with low current and high performance. When the length of resonator was 300㎛, its characteristics were as follows: the threshold current of 10mA, the internal quantum efficiency of 82%, the internal loss of 9.2cm-1, and characteristic temperature of 65K. From the measurement of far-field pattern, we confirmed that it was operated with single mode in both directions parallel and normal to the junction interface.
And we observed the variation of threshold current varying the leakage width at a certain cavity length and then applying the same widths to different cavity lengths and as a consequence, we clarified that the threshold current became low in the decrease of the leakage width and in the increase of the ratio of specific resistivity of leakage region to active region. We also made a comparison between the calculated threshold current in the absence of leakage region and the measured threshold current in the opposite case. As a result, the ratio of specific resistivity was about 0.5 in the measured LD, which has the width of a active layer of 1.4㎛ and leakage width of 0.6㎛.