Photocathode Dark Current

Thermal electron emission from a solid (dark current in a photocathode) can be modeled in various degrees of complexity and full models for either classical or NEA emitters [5.139,140] require a fair amount of detail. In simplified form we can consider initially the limit of treating the substance as a perfect electron source (an electron black body ). Here all electrons which are thermally excited to energy greater than the surface barrier (work function or electron affinity) are emitted, and each emitted electron is assumed to be instantly replaced by an electron from the bulk. This is roughly the Richardson model, which applies fairly well to a metal. The dark current computed from this model is an upper limit to emission of a real device. 

For a semiconductor we can alternately assume that any thermally excited and emitted electron is not instantly replaced owing to diffusion delay from the 

Emission from most classical and NBA surfaces is in this same 10 -10 A/cm range, indicating that the surface component dominates the bulk and that it is roughly independent of the substrate material. There is some tendency for higher emission in lower threshold (lower bandgap) emitters, but this is not as pronounced as might be expected from a simple theory for bulk emission over a barrier. The primary exception is the S-1 surface. 

One obvious means of reducing thermal dark emission is to cool the photocathode [5.19, 135, 136, 138, 141-144]. (Another is to reduce either the 

Emission from most classical and NEA surfaces is in this same 10  

---

Additionally, the photocathode dark current will be amplified by a factor G before arriving at the anode. Therefore, the minimum intensity that can be detected at the anode, at room temperature, is given by... 

Photocells The basic construction of a photocell is illustrated in Figure 17. A photocurrent flows when the photocathode is illuminated, this is proportional to the intensity of illumination if the supply potential has been chosen to be higher than the saturation potential. A minimal potential is required between the photocathode and the anode in order to be able to collect the electrons that are emitted. The sensitivity is independent of frequency up to 10 Hz. The temperature sensitivity of evacuated photocells is very small. The dark current (see below) is ca. 10 " A[l]. 

Even in the absence of illumination (darkness) some electrons, excited by thermal energy, are emitted from the photocathode. Since photocathodes are materials with low working functions, the thermal energy can be high enough to induce the emission of electrons. These emitted electrons give rise to what is known as the dark current or, sometimes, the thermo-ionic current. The dark current varies randomly with time, so that it is considered as noise. It has been experimentally determined that the thermo-ionic current, U, due to photoelectrons emitted by a photocathode in the absence of illumination is given by... 

In the particular case of a photocathode, this fluctuation affects both the dark current it) as well as the illumination induced current (/lum)- In the absence of illumination, the only current generated in the photocathode is the dark current, and so the shot noise associated with it is Aif If the light-induced current, /lum. is smaller than the shot noise associated with the dark signal Ai,), then it will be not possible to distinguish any light-induced current. In these conditions, the incident light cannot be detected by the photomultiplier, as it is not possible to separate the noise and the signal. As a consequence, the shot noise associated with the dark current determines the minimum intensity that can be detected by a particular photomultiplier (or by a particular photocathode). This is clearly shown in the next example. 

As we found in Example 3.1, the dark current of this photocathode when operating at room temperature is I, T = 300 K) 2 x 10 a. The minimum current that can be measured is equal to the current dispersion caused by shot noise over the dark current, so that... 

Calculate the dark current intensity at room temperature (T = 300 K) of a metallic photocathode with the following characteristics area = 3 cm, eip = 1.05 eV. If the quantum efficiency of this photocathode is 0.75, calculate the minimum power that can be detected (the wavelength of the incident beam is 808 nm). Discuss the improvements if the photocathode is cooled down to 260 K. 

The discussion in Section II-B indicates that optical emission from 02(1E 7+) or 02(1A9) to the ground state may provide a useful method for the identification and estimation of the excited species. In laboratory studies, the (0, 0) bands, lying at about 7620 A and 1.27 [x, respectively, are likely to be the strongest. The emission at 7620 A is relatively easily detected by suitable photomultipliers, and spectra may even be recorded with photographic emulsions sensitive to the near infrared (such as the Kodak N coating). Trialkali (S20) photocathodes combine a high sensitivity with low dark current, and photomultipliers with an S20 cathode... 

DC amplifiers are the simplest and least expensive of the electronic measurement systems. They are most commonly found in commercially available fluorimeters. Ideally, the amplifier stage of a circuit contributes little or no noise to the system the photomultiplier should produce only the noise associated with perfect performance of the photocathode and the electron multiplication process described above. However, the anode dark current of the photomultiplier adds to the noise in the signal, and the amplifier makes its own contribution to the total noise. It is therefore imperative to select the proper photomultiplier tube with low dark current so as to have a higher signal-to-noise ratio. 

The current-potential curve for the p-InP photocathode under illumination in CO2 (40 atm)-methanol exhibited a relatively large photocurrent (solid line), while the dark current was negligibly small (dotted line, < 1 mA cm 2) at potentials down to -2.0 V vs. Ag-QRE (Fig. 1). The onset photopotential was approximately -0.6 V. When CO2 was replaced with Ar, the onset of the cathodic photocurrent shifted toward the negative direction by 0.4 V (dashed line). This indicates that, in the highly concentrated CO2 solution, CO2 reduction on the p-InP surface occurs in preference to the reaction occurring under Ar atmosphere, which is predominantly hydrogen evolution. The cathodic photocurrent reached 20 mA (approximately 100 mA cm 2) at a potential of -2.4 V vs. Ag-QRE. 

The maximum power point of efficient solar cells is located close to the open circuit voltage (see Figures 2.12 and 2.89). For p-type semiconductors, the open circuit condition is the most anodic potential at which the photocathode is operated and anodic dark currents compensate the cathodic photocurrent at this potential. 

The sensitivity above 500 nm has been increased by the introduction of multialkali and extended red multialkali photocathodes, which provide good sensitivity to 700 or 800 nm (Figure 2.25). Red-sensitive PMTs typically have higher dark current, but with the current generation of multialkali photocathodes the dark current is not a problem. Sensitivity to still longer wavelengths can be obtained... 

---