An Absolute Radiometer Based on InP Photodiodes

Semiconductor photodetectors based on InP materials are the ones most often used in state of the art long wavelength optical fiber communication system. Mixed compounds such as InGaAs (P) and In(Al)GaAs lattice matched to InP are the materials responsible for detecting long wavelength light, specially the nondispersion wavelength (1.3 μm) and loss minimum wavelength (1.55 μm) of silica optical fibers. The characteristics of these InP-based photodectors are superior to those of conventional photodiodes composed of elemental Ge, which was the only material applicable for wavelengths below 1.55 μm. By using a heteroestructure, which hadn’t been expected 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 can be confined to a limited layer and the InP wide bandgap layer can serve as a transparent layer for specific communication wavelength. Recently InGaAs/InP avalanche photodiodes (APDs) with a SAM (separation of absorption and multiplication) configuration have become commercially available. The SAM configuration is thought to be necessary for high performance APDs utilizing long wavelengths. The photodiodes may be operated under reverse bias, high quality semiconductor layers need to be produced. To obtain photodiodes that operate at a low bias and have a low dark current, it is necessary to produce epitaxial layers that are pure and have few defects (such as dislocations, point defects, and impurity precipitates). To get stable and uniform gain in APDs, in which internal gain is achieved through the carrier avalanche process, the layers in the avalanche region must be uniform and free of dislocations. Furthermore, a planar device structure requires that a guard ring be used to keep the electric field around the photoreceptive area from increasing too much. Fabrication and processing technologies such as impurity diffusion, ion implantation, and passivation will also play important roles in the production of reliable photodetectors. From a radiometric point of view, the photodetectors important characteristics are: Speed of (characterized by the bandwidth of the frequency response or the Full Width Half


1.-INTRODUCTION
These photodetectors are chosen as the first device of interest because of their simple structure, and since their analysis is a natural extension, almost an example, of our discussion of p-n diodes.Whereas the field of photodetectors goes far beyond that of semiconductor photodetectors, we restrict ourselves here to such devices.It will be discussed p-i-n diodes, which are also referred to as photovoltaic detectors, photoconductors or solar cells photodetectors.The distinction between the different devices is somewhat artificial since many similarities exist between these devices but it enables to clearly separate the difference in structure, principle of operation and purpose of the devices [2].
Semiconductor photodetectors based on InP materials are the ones most often used in state of the art long wavelength optical fiber communication system.Mixed compounds such as InGaAs (P) and In(Al)GaAs lattice matched to InP are the materials responsible for detecting long wavelength light, specially the nondispersion wavelength (1.3 m μ ) and loss minimum wavelength (1.55 m μ ) of silica optical fibers.The characteristics of these InP-based photodectors are superior to those of conventional photodiodes composed of elemental Ge, which was the only material applicable for wavelengths below 1.55 m μ .By using a heteroestructure, which hadn't been expected 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 can be confined to a limited layer and the InP wide bandgap layer can serve as a transparent layer for specific communication wavelength.Recently InGaAs/InP avalanche photodiodes (APDs) with a SAM (separation of absorption and multiplication) configuration have become commercially available.The SAM configuration is thought to be necessary for high performance APDs utilizing long wavelengths.
Because photodiodes may be operated under reverse bias, high quality semiconductor layers need to be produced.To obtain photodiodes that operate at a low bias and have a low dark current, it is necessary to produce epitaxial layers that are pure and have few defects (such as dislocations, point defects, and impurity precipitates).To get stable and uniform gain in APDs, in which internal gain is achieved through the carrier avalanche process, the layers in the avalanche region must be uniform and free of dislocations.Furthermore, a planar device structure requires that a guard ring be used to keep the electric field around the photoreceptive area from increasing too much.Fabrication and processing technologies such as impurity diffusion, ion implantation, and passivation will also play important roles in the production of reliable photodetectors [3].
From a radiometric point of view, the photodetectors important characteristics are: Speed of (characterized by the bandwidth of the frequency response or the Full Width Half Maximum (FWHM) of the pulse response), responsivity (determined as the ratio of current out the detector to the incident optical power on the device), sensitivity (defined as the minimal input power that can still be detected which, as a first approximation, is defined as the optical power which generates an electrical signal equal to that due to noise of the diode).One related characteristic is the quantum efficiency of the detector which is the ratio of the number of electron-hole pairs which contribute to current to the number of incident photons.
When the light radiation impinges on a detector, various physical processes occur; part of the incident light is reflected by the sensitive surface, while the rest passes inside the detector, where can be partially, because of losses due to absorption, converted into an electronic signal.The response of each photodetector is conditioned by a quantity of the converted light power, but for evaluating the incident power one has to know the ratios of the reflected, absorbed, and converted portions.
An InP-photodetector is a photodiode based on a p-n or hetero-structure.There is a region, which can be denominated as the depleted or exhausted region,where an electric field sweeps the generated charge carriers and produces an external electrical current.In addition charge generated outside that region also contributes to the photocurrent.Thus, the total photodiode response I can be written as where ( ) λ η is the internal quantum efficiency, which indicates the number of electrons produced by each absorbed photon, q is the electron charge, h is the Planck constant, c is the velocity of light, φ is the radiant flux, λ is the wavelength and ( ) λ ρ is the photodiode's reflectance.From equation (1.26), the responsitivity R can be obtained as: This equation shows that the responsivity depends on the wavelength of the incident light by three ways, directly, via the reflectance of the surface, and through the quantum efficiency.
This equation indicates also, that the responsivity will be known if both the reflectance and the internal quantum efficiency are known at every wavelength and is the quantity usually measured.It is seen from equation 1.26 that the photodiode response depends on a set of parameters inherent in the incident light like the spectral distribution, polarization, modulation frequency, angle of incidence, and radiant power.Furthermore, the response is determined by photodetectors features such as the material refractive index and the structure of diode as well as by some environmental factors,such as temperature, for example [2].

InP PHOTODETECTORS
Photodiodes based on InP materials are becoming the most used for different applications in near IR range [1, 2], since their radiometric characteristics are superior to those of Ge photodiodes.In recent years they also have been studied to develop spectral responsivity scales in the near IR range (800 nm -1600 nm) in different National Laboratories [3][4][5].
In order to do that it is important to know the external quantum efficiency and the reflectance of these photodiodes and try to model them to establish spectral responsivity values at any wavelength, as it has been done by authors of ref. 6 in a limited range.
In this work we will present spectral reflectance and responsivity values of three types of photodiodes made by different manufacturers measured between 800 nm and 1600 nm and will compare the results obtained.

SPECTRAL RESPONSIVITY
A set of two photodiodes from the same batch of every manufacturer (three) has been studied in this work, which makes a total of six photodiodes.Two sets of detectors had a round aperture 5 mm in diameter and the third set had a rectangular aperture 8 mm x 8 mm.

EXTERNAL QUANTUM EFFICIENCY
External quantum efficiency is obtained from the responsivity values according to the equation: Where H, C and e are the usual physical constant and λ is the wavelength.The values obtained are presented in figure 2. It can be more clearly seen that the oldest detector (detector B) presents a lower external quantum efficiency than the other and that detector Chacho presents a higher external quantum efficiency and a "roll-off" at a longer wavelength than detector Xotchil, which starts to decrease its quantum efficiency at a shorter wavelength.

REFLECTANCE MEASUREMENT
Photodiodes' reflectance has been measured with respect to a standard aluminium mirror [7].The reflectance was measured at an incidence angle of 7º with linearly polarized and nonpolarized lights, considering the small change in the standard mirror each time [8]. Figure 3 shows spectral reflectance values for the same photodiodes with linearly polarized light.It can be said that the change in reflectance at this incidence angle is lower than 2 %.Two main features appear in figure 3.In one hand all detectors present an antireflection structure along the long wavelength interval, probably to maximize their performance in the most useful range.In the other hand modern detectors present a lower reflectance than the oldest one.

ACKNOWLEDGEMENTS
Spectral responsivity (R(λ)) has been measured by comparison to an electrically calibrated pyroelectric radiometer (ECPR), obtaining responsivity values with an uncertainty of 1.2 % approximately, roughly the accuracy of the ECPR.The results obtained are shown in figure 1.