Uncooled operation of the laser diodes up to 85 ℃ is an important requirement, because thermo-electric cooler(TEC) of the laser modules is too expensive.
In this thesis, we have theoretically investigated a 1.3 ㎛ InGaAsP/InP Planar Buried Heterostructure-Laser Diode(PBH-LD) for high temperature operation with low threshold current without temperature control. We adopted the strained MQW structure as an active region and PBH among strongly index-guided structures in order to have low current operation.
Based on the rate equations, which describe rate change between the carrier density and the photon density, we proposed the electrical equivalent circuit of PBH-LD. In the PBH LD, the confinement of the injected current to the active region is achieved through reverse-biased homojunctions. The leakage current paths are identified by a major circuit element in each branch, e.g., the diode(p-n forward biased InP homojunction), the thyristor(p-n-p-n InP) in the electric equivalent circuit model.
For the diode leakage path in the electric circuit model, p-InP blocking layer is equal to a resistor according to the concentration and thickness. For the thyristor leakage path in the model, p-n-p-n current blocking layer is equal to two transistors such as p-n-p and n-p-n type. We calculated the resistor according to the concentration and thickness of p-InP blocking layer and the connection width, which is the minimum distance between active layer and n-InP blocking layer. The transistor gains are calculated according to the concentration of each layer. In order to optimize the concentration and thickness of each layer, the electrical equivalent circuit model was simulated with PSPICE circuit simulator.
Based on the theoretical analysis of electric equivalent circuit model, the PBH-LD is fabricated by using the vertical type Liquid Phase Epitaxy(LPE) system, which has been made by hands. In order to form mesa shape, wet etching and meltback method are executed in turn. And meltback method has the advantage of reduction of damage on a substrate due to wet etching and thermal damage during growth.
After forming mesa shape on the substrate, p-n-p current blocking layers were grown at 611 ℃ with the cooling rate of 1 ℃/min by two phase solution technique. And then, SiNx is removed, p-InP and ohmic contact layers are regrown by 600 ℃ with the same cooling rate.
The light output power versus injection current and the spectrum characteristics of the fabricated PBH-LDs were measured with various temperature. From the measurement, the threshold current was 6 ㎃ and the light output power at 25 ℃ was about 22 ㎽ with 300 ㎛ cavity length at 100 ㎃. The characteristic temperature from 25 ℃ to 45 ℃ is 50 K, and from 45 ℃ to 65 ℃ is 44 K. Also, From the spectrum characteristic measurement, we confirmed the central wavelength was 1310 ㎚ and the temperature dependence of lasing wavelength was 5 Å/℃.
We also measured the far field pattern of fabricated PBH-LD and the full width at half maximum(FWHM) was 35o(∥)×40o(⊥).