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This investigation is aimed towards using optical spectroscopy for remote identification and quantitative analysis of hazardous substances for safety and security applications. We introduce a new model employing portable photosensor devices that are based on the double-barrier and vertically placed silicon structure, for such applications. The different absorption depths of individual waves allow us to carry out their spectral selection using an algorithm developed for this specific objective. We tested the proposed model on experimental Ag-p-Si-n-Si structures. The algorithm is developed for the spectral analysis without the preliminary calibration. The low dark currents (several dozens of pA) permit us to carry out the spectral analysis of the integral flux of the electromagnetic radiation of low intensity. The quantitative data from light current-voltage characteristics allow us to obtain an intensity distribution spectrum characteristic of the material by using red LED and the green laser. The results of this investigation divulge new possibilities for the creation of a new type of the portable semiconductor spectrophotometer and due to its stand-off detection capability, offer potential pathways to evaluate hazardous substances.

In recent years, increasing demand for the stand-off detection/sensing for hazardous chemical has significantly increased due to their use in safety, security, environmental protection and detect suspicious packages, wire, fragmentation materials, or other signs of improvised explosive devices (IEDs) or crisis situations. Standoff techniques enable detection of threats without contact. The possibility of contamination (for operators and/or equipment) is thus avoided, and the need for subsequent decontamination is eliminated. In addition, standoff detection of explosives allows identification of threats from safe distances, i.e., outside the blast zone. There are now ongoing efforts to extend these methodologies to even biological threats. It is, hence of paramount importance to increase the efficiency of such methods and solution pathways which will respond quickly to such crisis situations. It is particularly important to create user-friendly sensors platform which will likely enable the remote analysis of the different materials in different media.

Remote spectrophotometric sensors are of special interest these days, as they provide us with necessary information on the composition of the mediupm under investigation and hence solve the omnipresent problem of identification from the security stand-point [

It is critical that several sensors are required for monitoring large areas. To optimize cost and to strategically place sensors in regions which are likely to provide a fair assessment of the contamination scenario, it is urgent to develop; a): inexpensive and small-size sensors with high spectral sensitivity, suitable for remote field identification, and, b): appropriate algorithms for accurate registration of data measured by these sensors. In methods that are currently available for spectral analysis, the spectral distribution of the electromagnetic radiation is obtained by light filters, a prism, a diffraction grating and high accuracy mechanical devices [

The effective solution to the above-stated problems is by development of a semiconductor structure in which the electronic processes will provide high accuracy spectral analysis of the electromagnetic radiation. Several researchers have conducted study using multi-colored photoreceivers [^{+}-p-n^{+} structures.

The structure under investigation is a silicon structure, as prepared using standard integrated circuit processing methods. The structure consists of an n^{+}-p-n^{+} structure (_{m} (^{+}-p junction (ohmic contact). The electromagnetic radiation is incident on the left ohmic contact as shown in ^{+}-p junction is dominating and thus it’s depleted layer becomes wider. In the meantime, the depleted layer of the forward biased junction becomes thinner. As a result, the current is mainly governed by the reverse biased p-n^{+} junction. The incident electromagnetic waves have different penetration depths with respect to their wavelengths and intensities.

Since the shift of x_{m} towards the surface (towards “0” in the ^{+} layer towards the left surface with high conductivity can be neglected, the “0” point can be taken as the surface, and the voltage drop at the front contact can be neglected. This allows us to measure of the photocurrent which contains information with respect to their wavelengths and intensities.

The information can be obtained by a relevant algorithm. For the development of such algorithm, it is necessary to find the relationship between x_{m}, applied voltage V, absorption coefficient α and incident intensity F. Considering that the base is covered by the depleted regions of both barriers, we determine the distribution of potential in the spatial charge region of the double-barrier structure, as shown in

In Poisson’s equation, the potential

Since

where

The boundary conditions for the given equation are

(

Taking into account the boundary conditions, we integrate Equation (2) and obtain,

When

It is possible to find the dependences of

where

If both barriers are identical, then

With the help of Equation (5), we can determine the modulation depths of depleted layers, and the position of

For that, we first determine the total photocurrent flowing through the structure. In the investigated photodetectors, the photocurrent has diffusion and drift components. To determine the diffusion photocurrent, it is necessary to find p_{n} density of minority charge carriers-holes in the n-semiconductor. For that, the following one dimensional diffusion equation is solved [

where ^{+}-region, _{0} represents the total flux of incident photons, _{p} is the diffusion coefficient of acceptors in the n^{+}-re- gion, _{n0} represents the equilibrium density of acceptors in the n-region. The solution of Equation (6) under the boundary conditions p_{n} = p_{n}_{0} at х = ∞ and p_{n} = 0 at х = d (p_{n}―is the concentration of minority charge carriers in the rear n^{+}-region) is

Khudaverdyan et al. [

where _{opt}―the radiation power, R―the reflection coefficient, h― Planck constant, n―the frequency of the electromagnetic radiation and q―the electron charge.

Taking into consideration Equations (8), (9) and (7), the expression for the total current flowing through the structure is:

At normal functioning, the term that contains n_{p}_{0} is much smaller than the first term. Thus, it may be neglected, and Equation (10) is:

When irradiated by the integral flux (e.g. of the Sun), Equation (11) for the photocurrent is:

where (i = 1, 2, 3, …) changes in the integral flux with the change of the emission wavelength, and (j = 1, 2, 3, …) changes with the change of bias voltage, F_{0}(l_{i}) is the total flux of incident photons with the wavelength l_{i}.

Since the width of the n?region in Equation (11) was less than L_{p}, the value of L_{p} in Equation (12) was replaced by W (_{i} and its intensity by changing the voltage ∆V, thus obtaining the spectral dependence for wave intensities.

Let us consider some peculiarities of detection of separate waves in the integral flux of radiation using structure containing oppositely directed potential barriers. Such scenarios are used for detection of certain wavelength and its intensity under the conditions of the wave absorption and emission (fluorescence) of the objects under investigation.

In the present investigation, we used the AM0 spectral distribution for Si using radiation of LED (L-813SRC-J4) with a dominant wavelength 660 nm and a laser with the radiation wavelength of 530 nm. Since, with increase of the bias voltage, the registration region of the rear barrier widens towards the surface [

To determine the spectral composition of an integral flux of the electromagnetic radiation, the wave intensities of the radiation are equally divided so that the absorption depth corresponds to the width of the registering environment (~4 - 6 μm). The photocurrent corresponding to the largest possible value of x is normalized by the most deeply penetrated wave. Under these conditions, the process of the selection of separate waves and their intensities from the integral flux of radiation can be described as in the following.

Assume that the leading indicator signal is the photocurrent. We also assume that the largest values of x_{m}_{1} and x_{m}_{2} of x_{m} correspond to the change of bias voltage 1 mV, and the relevant photocurrents I_{1} and I_{2}. They are obtained only from the absorption of the most deeply penetrated wave λ_{i}. With the help of these photocurrents, and using Lambert’s law on radiation absorption in the homogeneous environment, we will have the coefficient of wave absorption,

where

Using

With the help of Equations (5), (13) and (14), we determine the absorption coefficient of the most deeply penetrated wave, the wavelength and the intensity of the wave. Then, with the help of the expression (12), considering the absorption surface, we form the dependence I_{1} = f(V) and subtract it from the experimental dependence I = f(V) of the summed current. As a result, we obtain a new dependence of the summed photocurrent on V, without the dependence I_{1} = f(V).

By means of relevant software, this method helps us determine the lengths and the intensities of all the waves in the radiation. The experiments were carried out on the n^{+}-p-n^{+} structure. The cross-section of the structure with the parameters of the layers is presented in _{к} = 0.89 eV, and the distance of Fermi level from the valence band top of the p-epitaxial layer was E_{F} − E_{V} = 0.29 eV (_{c} = 0.25 eV.

The photocurrents of the two oppositely directed barriers are oppositely directed, and they compensate each other depending on the applied voltage. It is noticeable in the relative spectral characteristics (

The long-wave peak has small height and is governed by the rear barrier, which is located at the depth of 6 µm from the illuminated surface (

(

Thus, at different voltages, different waves penetrate into the registration region of the rear barrier. These waves determined by the corresponding change of the photocurrent. During the experiment, for determining the spectral distribution of the intensity of the absorbed radiation, the L-813SRC-J4 LEDs and laser beam (wavelength of 530 nm) were used. The experimental current-voltage characteristic in the dark (

The design width of the depleted layer of the n-p junction under the zero bias is ~1 µm, and 2.55 µm under the reverse voltage of ~5 V. Thus, we observe the modulation of the width of the registration region of the rear barrier. With the help of Equation (13) and (14), the current-voltage characteristic data in the dark and under illumination, and the developed algorithm [

We have developed a new model of the portable photosensor for the remote identification of optical information. Different absorption depths of electromagnetic waves in the developed silicon photodetector are used to create an algorithm for the determination of the wavelength and its intensity.

Experimental samples have been investigated and possibility of selective spectral sensitivity in them has been proven. That is possible thanks to variation of potential barriers in bases of the samples at the expense of each other when applying external voltage.

Spectral distributions of intensities of red LED and the laser with the radiation wavelength of 530 nm have been received using the developed algorithm. It repeats the reference distribution, with certain approximation.

The results of the testing of the algorithm on the experimental samples open new possibilities for the creation of a new type of the portable semiconductor spectrophotometer. The developed algorithm is used to carry out the spectral analysis without a preliminary calibration.

The authors express their gratitude to the company “RD ALFA Microelectronics” for their assistance in the prototyping of the sample photodetectors. We also acknowledge support of NATO Science for Peace and Security-G4403 for partial support of this investigation.

Khudaverdyan, S., Meliqyan, V., Hovhannisyan, T., Khudaverdyan, D. and Vaseashta, A. (2017) Identification and Analysis of Hazardous Materials Using Optical Spectroscopy. Optics and Photonics Journal, 7, 6-17. http://dx.doi.org/10.4236/opj.2017.71002