Structure for InGaAs photodetectors

Structure for InGaAs photodetectors

The demand of photodetectors fabricated on InGaAs/InP PIN wafer operating at around 1300nm~1550nm has increased significantly. So that is great news for semiconductor wafer foundries, like PAM-XIAMEN, who offer semiconductor substrate and epitaxial wafer for electronic and power devices fabrication. InGaAs wafer for PIN photodetctor is one of important epitaxy wafer from PAM-XIAMEN. Take specific epitaxial structures for InGaAs photodetector below for example:

InGaAs Photodetector wafer

1. Epitaxy Structure of InGaAs/InP PIN Photodetector

No.1 1550nm InGaAs Epi Structure for PIN (PAM211119-1550PIN)

Layer Material Thickness(nm) Dopant Dopant Concentration(cm-3) Type
4 GaxIn1-xAsyP1-y Si N
3 InP Si N
2 GaxIn1-xAs Undoped N
1 InP 0.5-1 Si N
  N+ InP Substrate        


No.2 InGaAs/InP Heterostructure

Material X Thickness (nm) Dopant Doping concentration
InP   1000 N (Sulfur) 3e16
In(x)GaAs 0.53 3000 U/D 5e14
InP   500 N (Sulfur) 3e16
Substrate     SI (Fe)  


2. InGaAs Epitaxial Structure for Photodetector

The No.1 1550nm InGaAs Epi Structure for PIN is discussed in detail. The material structure is InP/InGaAs/InP double heterojunction, and the pin structure is grown by epitaxial technology. The InP layer above the InGaAs absorption layer is a wide band gap material, which is transparent to light with wavelengths of 1.31um and 1.55um, and photo-generated carriers are only in It is produced in InGaAs material, which avoids the generation and diffusion of light-generated minority carriers.

This structure has obvious advantages over that of single heterojunction detectors in terms of quantum efficiency and frequency response. The most important thing in the design of the epitaxial structure is the design of the absorption layer. It must have enough thickness to ensure high responsivity and small junction capacitance, and it must be as thin as possible to shorten the carrier transit time to improve response bandwidth of InGaAs photodetector, so the thickness of the absorbing layer should be selected as a compromise.

The change in the absorption coefficient of semiconductor materials with wavelength is shown in the figure. For InGaAs materials, the absorption coefficient at 1310nm wavelength is 1.15um-1, and the absorption coefficient at 1550nm wavelength is 0.67um-1.

changes in the absorption coefficient of semiconductor materials with wavelength

When growing the epitaxial stack, different InP substrates, such as N-type and semi-insulating, can be used to grow the epitaxial layers according to the contact design. A buffer and contact layer is grown on the InP substrate, and then an InGaAs absorber layer is grown. The doping level and thickness of buffer, contact and absorber layer are optimized for the operating wavelength. According to the characteristics of required InGaAs photodetector and the choice of cut-off wavelength, the epi-layer can be composed of additional stacks: space charge regions, interfaces, and caps. These layers are grown for controlling drift, diffusion and carrier recombination as well as preventing surface current leakage.

3. Relations between Wavelength and Refractive Index of InGaAs

The following diagram shows the longer the wavelength of the InGaAs material, the smaller the refractive index. More about the detailed parameters please email to [email protected].

Relations between Wavelength and Refractive Index of InGaAs

4. InGaAs/InP Material for PIN Photodetector

Among the natural semiconductor materials, only Ge (with a band gap of 0.66eV, corresponding to the absorption edge of 1.88um) can be used to make photodetectors used in the long wavelength (1.3-1.6μm) band of optical fiber communication. However, the band gap of Ge is relatively small, and the manufactured device has higher noise when working at or above room temperature. The photodetector working at the long wavelength of 1.31um-1.55um generally uses InGaAs/InP material. The band gap of InGaAs material is 0.75eV, and its absorption spectrum can cover the optical fiber communication band of 1.00-1.65um, including the 1.3um low dispersion and 1.55um low loss windows of quartz fiber. As a result, InGaAs is an ideal material for photodetectors in the optical communication band.

InGaAs can achieve complete lattice matching with InP material, so a high-quality epitaxial layer can be grown on the InP substrate, and the electron mobility of InGaAs material is very high. These characteristics make the InGaAs PIN photodetector have extremely high response speed and small dark current.

The successful development of the InGaAs PIN photodiode overcomes the shortcomings of the large dark current and poor temperature characteristics of the Ge photodiode, and provides a good-quality key device for the long-relay transmission and multiple-wavelength transmission technology of optical communication.

5. Direction of Light Injection of InGaAs Photodetector

For the light injection direction, the PIN photodetector based on InGaAs/InP epi wafer has two methods: one is the backside light injection, that is, the light enters from the N-type InP substrate; the other is the front light injection type, that is, the light enters from the P-type. For backside light injection devices, the InP substrate (Eg=1.35eV) is a light-transmitting layer for long wavelengths, and the surface reflection coefficient is small. If the material parameters are properly controlled, photo-generated carriers can be directly generated in the depletion layer. This structure has the advantages of high quantum efficiency and small junction capacitance.

However, in this structure, the incident light is far away from the PN junction, and the divergence of the light causes absorption on the sidewall of the ultrafast InGaAs photodetector, which affects the frequency characteristics. Moreover, the quantum efficiency and dark current of the device are greatly restricted by the growth of the material, the repeatability is poor, and the manufacturing process is more complicated. Compared with the back-side light-injection device, the front-side light-injection device has a simpler process, the incident light is very close to the PN junction, and the coupling effect is good. Its shortcomings such as diffusion junction depth control and large junction capacitance can be overcome through reasonable parameter design for InGaAs photodetector wafer array and some special process technologies.

For more information, please contact us email at [email protected] and [email protected].

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