For a long wavelength photodiode of optical communications, p-i-n diode, MSM(Metal-Semiconductor-Metal) diode, photoconductor, APD(Avalanche Photodiode) are used. And photodiodes with high efficiency and broad bandwidth are the key devices for feature wide-band optical communication systems. This device has problems of increasing quantum efficiency and transit time according to the increase of absorption in i-layer. Transit time can be reduced using by thin absorbing layer. The light injects toward the lateral direction in the WGPD(Waveguide p-i-n Photodiode) and widen bandwidth. A conventional surface-illuminated PD requires vertical mounting of the PD chip or 90o bending of input light by optical reflection in the module assembly. These complicated assembly procedures lead to a low yield and high assembly cost. On the other hand, a well-designed WGPD is usually passively aligned to the defined position on a module board, then flip-chip mounted without wire bonding.
In this paper, the WGPD consists of two transparent InGaAsP guiding layers with refractive index of 3.39 at 1.3㎛ and a 1.5㎛ bandgap InGaAs with an index at 1.5㎛ of 3.59.
The epitaxial layers were grown by horizontal LPE(Liquid Phase Epitaxy) apparatus on a n-InP substrate. i-InGaAs epi layer was grown at 630℃ with cooling rate 0.6℃/min by horizontal LPE apparatus. The growth thickness of i-InGaAs layer was about 1.3㎛ and these conditions were not satisfied. Because of source baking time were not enough as 6hr.
And then, we did carry out source baking during a 24hr. It is that the condition of i-InGaAs layer were steadily getting better. For a long source baking time, the source's impurity concentration was reduced. In this case, i-InGaAs epi layer was grown at 630℃ with cooling rate 0.6℃/min. The process of growth was similar previous condition and these thickness was 1.5㎛. It could be found that the characteristics of WGPD fabricated by 24hr backing time were better than by 6hr.
The fabricated device has an external quantum efficiency of 68% as well as a cut-off frequency of 4.5GHz at 1.55㎛ wavelength.
For improving operation characteristics of WGPD, we need both transit time and frequency response. It is considered that the characteristics of the width, length, and thickness of waveguide photodiode's i-InGaAs layer.
Moreover, I will suggest that a new waveguide structure and analysed using BPM(Beam Propagation Method) for increasing coupling efficiency between optical fiber and waveguide photodiode.