We have measured the magnitude of the conduction-band discontinuities at heterojunctions for several compositions of InGaAsP grown lattice matched on InP. We find that the conduction-band discontinuity (ΔEc) is related to the difference in band gaps (ΔEg) between the InGaAsP and InP layers via ΔEc =0.39(ΔEg). Thus, 40% of the band-gap difference lies in the conduction band of this material system. The measurements were made on a series of composition of InGaAsP spanning the alloy range from In0.53Ga0.47As (with energy gap Eg =0.75 eV) to InP (Eg =1.35 eV) using capacitance-voltage techniques. Depletion deep into the semiconductor layers was facilitated by the formation of organic-on-inorganic semiconductor contact barriers on the InGaAsP surface.

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We have measured the frequency chirp in gain-guided, weakly index-guided, and strongly index-guided InGaAsP lasers under direct-current modulation. The measured chirp width is largest for gain-guided and smallest for weakly index-guided ridge waveguide lasers. The chirp width for 1.5-μm InGaAsP lasers is about a factor of 2 larger than that for 1.3-μm InGaAsP lasers of the same structure. The frequency chirping results from a modulaton of the carrier density which modulates the effective refractive index of the guided mode. The frequency chirping can introduce a limitation on the performance of single-frequency injection laser sources for high bit-rate digital transmission in fiber communication systems at 1.55 μm.

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