**7. Spectral responsivity scale in the near IR range based on single InP/ InGaAs photodiodes**

As in the visible range, semiconductor photodiodes are the best choice for establishing spec‐ tral responsivity scales in the near IR range. The first attempt was to use germanium photo‐ diodes, since its gap allowed to obtain a device responding to wavelengths lower than 1.6 μm, approximately, depending on temperature. However germanium photodiodes have got a rather high dark current and lower shunt resistance than silicon, then they are not so use‐ ful for optical radiation detection. Since optical communications were demanding better de‐ tectors to enlarge their use, other photodiodes were developed in this spectral region of great interest. Since no other single element semiconductor was possible, semiconductor hetero-junctions were developed. A hetero-junction is a junction formed between two semi‐ conductors with different band-gaps. Of course building such devices is not straightforward since the lattice parameters have to be matched, but this is not the subject of this chapter and many good references may be found in literature [17].

The group known as III-V hetero-structures has yield different photodiodes in the near IR range, particularly those based on InP/InGaAs has yield very good devices for the spectral range covered by germanium photodiodes. This hetero-structure has got two junctions in fact. The InGaAs material, having a lower gap, is kept in between two layers on InPwhose gap is bigger and hence transparent to the wavelength region used in optical communica‐ tions: the nondispersion wavelength (1.3μm) and the loss minimum wavelength (1.55μm). The radiometric characteristics of these InP-based photodetectors are superior to those of conventional photodiodes composed of elemental Germanium. Because of that they have re‐ placed germanium in almost every application.

and and germanium photodiode 12) formed an image of the monochromator's exit slit on the sensitive surfaces of photodiodes. The angle of incidence was equal to 7.4 º which was

The measurement method consists in comparing the response from a germanium photo‐ diode to the radiation reflected by the InGaAs/InP photodiode with the response from an

aluminum standard mirror whose reflectance is measured as in [21], so that [20]:

( ) () () ( ) *p*

l

l

*m*

mm in diameter and the other part had a quadratic active area of 8 mm x 8 mm.

*I I*

*m*

<sup>=</sup> (6)

Photodiodes as Optical Radiation Measurement Standards

http://dx.doi.org/10.5772/51462

181

 r l

the InGaAs/InP, *I <sup>m</sup> (λ)*is the response to the light reflected by the mirror, and ρm(λ) is the reflectance of a standard mirror. With this method the reflectance of photodiodes from dif‐ ferent manufacturers hasbeen measured. One part of detectors had a round active area of 5

The polarization degree of light at the output the monochromator was different with vary‐ ing the wavelength.The figures 5and 6 illustrate spectral dependences of the reflectance, which had been obtained from photodetectors belonging to three different manufacturers. Two types (photodiodes 1 and 4 and photodiodes 2 and 5) are 5 mm in diameter sensitive

accepted as the normal incidence in this train of measurements.

**Figure 4.** Experimental set-up for measuring the reflectance InGaAs/InP photodiodes

rl

Here,*I <sup>p</sup> (λ)* is the response to the light reflected by

**9. Analysis of Reflectance of InP Photodiodes**

By using a hetero-structure, which hadn't been used in group IV elemental semiconductors such as Si and Ge, new concepts and new designs for high performance photodetectors have been developed.For example, the absorption region for a specific spectral range can be con‐ fined to a limited inner layer, avoiding typical high recombination rates of charge carriers at the first interfaceof the photodiode and getting a higher internal quantum efficiency.

Recently InGaAs/InP avalanche photodiodes (APDs) with a SAM (separation of absorption and multiplication) configuration have become commercially available. The SAM configura‐ tion is thought to be necessary for high performance APDs utilizing long wavelengths.

InGaAs/InPphotodetectors are used for maintaining the scale of spectral responsitivityup to 1.7 μm in many laboratories [17, 19].In addition they are exploited in instruments for meas‐ uring optical radiation within the near infrared (NIR) range (800 nm -1600 nm). From this point of view, these photodiodes are like other and their response is given by equations (1) and (2). Therefore to know their reflectance and internal quantum efficiency is the key for defining the spectral responsivity scale in this range.

Next experimental values for those properties measured in our laboratory for devices built by different manufacturers will be presented.
