CC BY
1ФИЗИКА (PHYSICS)
УДК 53
Pashikova T.D.
Magtymguly Turkmen State University (Ashgabat, Turkmenistan)
Hayitkuliyev S.
Magtymguly Turkmen State University (Ashgabat, Turkmenistan)
CREATION OF PHOTODETECTORS OF SOLAR UV RADIATION
FOR ECOLOGICAL INVESTIGATIONS
Abstract: highly efficient photodetectors of the visible and UV radiation were developed on the basis of Au-oxide (Ga2O3)-n-GaAs0.6P0.4 nanostructures. Using filter УФС-2 (ultraviolet glass) filter instead of the sapphire window in the standard case essentially shifts the spectrum maximum of photosensitivity shortwave UV region (hvm—3,65 eV). This detector is sensitive only to the UV radiation of ecological range (hv=3,1-4,43 eV).
Keywords: UV photodetectors, MDS-structure, photosensitivity, filter, environmental
study.
Recently, interest has increased in semiconductor photodetectors in the ultraviolet (UV) range of the spectrum. It is connected with the requirements of modern medicine, biotechnology, ecology, as well as with the problems of "ozone depletion" [1-3]. The most promising devices in this spectral range are Photodetectors based on metal (M) - semiconductor (S) structures with a thin oxide layer (OL) between the semiconductor and the metal [4-6].
This paper presents the results of studies of the photoelectric properties of Au-oxide-n-Ga P0.4 As0.6 nanostructures with different thicknesses of oxide layer (
10-\00Â ) in the visible and UV regions of the spectrum. The creation of highly effective photodetectors of solar UV radiation ( ¿ = 280-400nm, hv = 3-1 -4-43eV) for ecological purposes has been reported.
As the starting material, Ga P0.4 As0.6 /n+ - Ga As (nD 6-lOl6srn\
rt □ 5-1011 sm 3) epitaxial structures were used; the thickness of n-layer Ga P0.4 As0.6 was 40-50 nm. An ohmic contact was created on the n+-Ga As substrate by fusing the 96% In+4% Te alloy. On the surface of the n-Ga P0.4 As0.6 epitaxial layer, an oxide
(dielectric) layer ( Ga2Oз,Eg °k>~5-1eV) was first formed, then a barrier contact (BC) of the semitransparent +Au layer was created. OL an BC were created by a chemical method. The thickness of metal barrier layer was 12-14 nm. The finished structure was installed in a standard "THRESHOLD" photodiode package with a quartz window.
Fig.1. 2D image of the surface of Au- Ga2O3-n-GaP0.4As0.6 nanostructures obtained by AFM. Sample №3
Fig 2. Three - dimensional image of the surface of Au- Ga2O3-n-GaP0.4As0.6 nanostructures obtained by AFM. Sample №3
Current-voltage (I-U), capacitance-voltage (C-U) characteristics and short-
circuit photocurrent spectrum (hv) in the photon energy range I-5-5-5eV at 300 k
were studied. In the studied Gap4As°6 m-s-structures, the dielectric (Ga2°3) layers were of different thicknesses, the largest was 10 nm. The constructive diagram of the photodetector is shown in fig.3,a. and fig.6,a. Surface morphology of Au - oxide- n - Gap0A As0 6 nanostructures was studied using AFM (fig.1, fig.2).
^ 3.
^ 4.
Dependence of direct dark current on voltage (I-U), in the range of current densities 10-7-10-2A/sm2 turned out to be exponential I=I0exp(qU/PkT). From this dependence, the ideality coefficent of P structures was determined. In our experiments, the coefficient P depended on the thickness of intermediate dielectric layer 5. From ellipsometric, I-U and C-U experimental data, an empirical dependence of 5 on P was established. This dependence turned out to be linear ( pic. 3, 6): 5=a(P-1), where the empirical coefficient a=125 Â. This empirical relationship was used to determine 5 in other structures, according to the P values measured for them.
62 0 4э6 400 34o 280 248
111, 33
Fig 5.
When different structures (with different coefficients P) are illuminat ed with ultraviolet light with photon energy h v=3.4 eV, we have established the optimal thickness of the oxide layer (Ga2O3) photodetectors. It was found that the optimal thickness 5-30-60Â. The main results of studies of photosensivity (PS) of Au-oxide-n-GaAs0.6P0.4 structures are illustrated in figures 4 and 5.
Figure 4 shows the photocurrent spectra of Au-oxide-n-GaAs0.6P0.4 structures with different thicknesses of the dielectric layer 5=10 -90 Â (P=1.07-1.70). In structures with very thin dielectric layers (5-10 Â, fig.4, curve-1), there is no obstacle for the transition of photoholes from the semiconductor to the metal, but
there is high probability for the transition of hot photoelectrons from semiconductor to the metal, which reduces short-wavelength PS.
In structures with relatively thick dielectric layers (5—83 A, fig.4, curve-3) the transition of photoholes to the metal is hindered and they accumulate near the semiconductor-dielectric layer interface, which leads to their recombination with photoelectrons and, consequently, to a decrease in the short-wavelength PS. In structures, in which 5—47 A, the dielectric layer is thick enough to noticeably hinder the transition of hot photoelectrons from the semiconductor to the metal, while at the same time it is thin enough not to strongly impede the transition of photoholes from the semiconductor to the metal. Here, the relative short-wavelength PS [If0(4.0 eV) / If0(hvm)] is 1.5-2 times higher than in structures with very thin (5—10 A) and relatively thick (5—83 A) dielectric layers.
Fig 6.
As a result, photodedectors of UV radiation based on Au-oxide-n-GaAs0.6P0.4 nanostructures with an optimal thickness of the oxide layer (5=30-60 A, fig. 4, curve 2) were developed. They can be used in the ecological range of the UV part of the solar spectrum X=280-400 nm (hv=3.1-4.43 eV). PD in the specified range of UV radiation have an almost constant PS (SI-0.15 А/Вт, fig.5, curve-1).
To use the PD as the main element of the intensimeter and dosimeter, the spectrum of its current photosensitivity is corrected with UVC -1 (d=3 мм, fig.5, curve-2) or UVC-2 (d=2 мм, fig.5, curve-3). The use of UVC-2 filter instead of a quartz window in a standard housing significantly shifts the maximum of the PS spectrum to the short-wavelength UV region (hvmax=3.65 eV, fig.5, curve-3). Such a photodetector is sensitive only to UV radiation of the environmental range X=280 -400 nm (fig.6, b). It has been established that under direct solar illumination of a PD with a UVC-2 filter, the dependence of If0 on the radiation flux density P in the range Р=10-3-102 Вт/м2 linear, i.e. If0=aP, where a - is the coefficient of proportionality, for each PD is a constant value. The PD created on the basis of Au-oxide-n-GaAs0.6P0.4 nanostructures was used to measure the intensity of solar UV radiation reaching the Earth's surface and its dose.
Thus, the created nanostructured photodetectors differ favorably in design and technology from the known GaPxAs1-x photodetectors and have a higher PS in the short-wavelength UV region of the spectrum.
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