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RESULTS AND DISCUSSION

1. 
   Photoluminescence of an epitaxial layer of a (Si₂)_1-x(GaN)_x solid solution
   

The photoluminescence (PL) spectrum of the grown epitaxial layer of the solid solution, which is shown in Fig. 2, was studied. PL was excited by laser radiation from the side of the epitaxial layer at liquid nitrogen temperature (77 K). As can be seen from Fig. 2, the PL spectrum of the solid solution has a wide band covering the visible range of the emission spectrum from 400 to 650 nm with an emission maximum at 438 nm, which corresponds to the photon energy E_ph = 2.83 eV. This maximum peak is due to the substitutional solid solution (Si₂)_1-x(GaN)_x, the band gap (E_g) of which is smaller than E_g of gallium nitride – E_g,GaN = 3.43 eV and larger than E_g of silicon – E_g,Si = 1.12 eV. The PL spectrum in the long-wavelength region shows a small emission peak at photon energies E_ph = 1.66 eV. The presence of such a peak against the background of a wide emission spectrum apparently indicates the appearance of a diffuse band of the impurities energy levels (E_i,Si) located in the band gap of the solid solution (Si₂)_1-x(GaN)_x (Fig. 3).
Since the supplied laser radiation with an energy of 3.82 eV is almost completely absorbed in the near-surface region of the epitaxial layer with a thickness of about 2 μm, the luminescent radiation originates from the film sublayer, where the main component is wide-gap GaN, is additionally evidenced by a wide emission band in the short-wavelength region of the PL spectrum with maximum at E_ph = 2.83 eV. The silicon content in the near-surface region of the film is low. Consequently, the smeared band of energy levels at E_ph = 1.66 eV is due to Si-Si bonds, which are surrounded by enriched GaN molecules in the solid solution sublayer (Fig. 1). The ionization energy of the Si-Si bond located in the crystal lattice of a pure silicon matrix at 77 K is 1.16 eV. However, when the Si-Si bond is surrounded by strongly bound Ga-N atoms (Fig. 1), as a result of sp³ hybridization of the electron shells of the molecules of Si₂ and GaN atoms, the Si-Si bond energy increases to ~ 1.66 eV and causes the atoms energy levels appearance of Si₂ molecules in the band gap of the (Si₂)_1-x(GaN)_x solid solution.

Fig. 2. Photoluminescence spectrum of the epitaxial layer of the (Si2)1-x(GaN)x solid solution at a temperature of 77 K.

Thus, the grown (Si₂)_1-x(GaN)_x film is a solid solution of molecular substitution with a wide photosensitivity region and is of great interest for the development of optoelectronic devices which were operating in the visible region of the radiation.

2. 
   Temperature CVC of the p-Si–n-(Si₂)_1-x(GaN)_x structure
   

The temperature dependence of the current-voltage characteristic (CVC) of p-Si–n-(Si₂)_1-x(GaN)_x structures was studied for clarification of the temperature effect on the mechanism of current transfer in p-n-structures fabricated on the basis of grown solid solution. Fig. 4 shows the current-voltage characteristics of the structure under study in the temperature range from 300 to 360 K. This is apparently due to the peculiarities of the (Si₂)_1-x(GaN)_x solid solution formation.
An analysis of the direct branch of the CVC shows that the current (J) dependence on the voltage (V) can be extrapolated in the initial section up to 0.5 V by an exponential dependence, and at voltages V > 0.5 V - by a power dependence of the J=A∙V_m type.
As it is known, in bipolar p-n structures, the equations for the transfer of electrons and holes are usually not considered separately, but an equation is obtained that describes the so-called ambipolar transfer of free carriers in the base of the p-n structure by mathematical transformations [5, 6]. Taking this circumstance into account, as well as the thermal stability of the experimental dependence J = f(V), the analytical expression for the CVC exponential section will be described by the following form [7]:

where A is a parameter characterizing the temperature-independent of the CVC, and q is the elementary charge.

As follows from the theory connected with the ambipolar diffusion of charge carriers, a temperature-independent CVC is observed when there is a band of deep impurity energy levels in the base of the p-Si–n-(Si₂)_1-x(GaN)_x structure.

Fig. 3. Energy band diagram of the (Si2)1-x(GaN)x solid solution with a blurred band of energy levels of atoms of Si2 molecules.

Fig. 4. Current-voltage characteristic of the p-Si–n-(Si2)1-x(GaN)x heterostructure at different temperatures.

Parameter A allows us to find the half-width of the band (∆E) of these energy levels - ∆E=2/A. Based on the experimental results of the exponential section of the CVC, the value of the parameter A was determined, which amounted to A ≈ 7.4 1/eV, and so made it possible to calculate the value E ≈ 0.27 eV. The base of the n-(Si₂)_1-x(GaN)_x structure is a graded-gap substitutional solid solution of a smoothly varying composition with access to the surface on GaN. In the near-surface region of the epitaxial film, the concentration of Si₂ molecules is up to 2-3 mol.%. At such a concentration, apparently, they represent an isovalent impurity, which forms a band of deep electron energy levels with a half-width E ≈ 0.27 eV (Fig. 3).

Thus, the results of the CVC studies show that the current transfer in the p-Si–n-(Si₂)_1-x(GaN)_x structure is stable in the temperature range 300–360 K. and solution (Si₂)_1-x(GaN)_x has the electronic energy band with the half-width E = 0.27 eV.

4.