Photoconductivity and bandgap of binary impurity - doped crystalline silicon

The paper presents the results of study of the photoconductivity spectra of sulfur- and -doped samples of silicon (Si, Si) under dark conditions and while samples were subjected to constant light in forward and reverse switching modes. The paper also presents the calculation results of the value of the band gap (Eg ) of  -doped silicon samples (Si). Spatial analogues of generally accepted GaAs structure such as Si2Mn2S and Si2Zn2S are hypothesized to be brought up in the result of above mixed dopings. Tentative investigations of the short-circuit current and open-circuit voltage of p-n-structures, as well as the photoconductivity of such structures, together with the analysis of their lux-ampere characteristic might serve for proposing potential varying band structures for solar cells.


Introduction
In order to make silicon propagate light in the visible and infrared range at room temperature, scientists considered a set of silicon combinations, and here one can note porous silicon, nanoscale silicon clusters in the SiO2 lattice, silicon-silicon dioxide based super lattices, ideal silicon-germanium-type solutions.However, the main problem here remains the question of sustainable electroluminescence at room temperature for a durable period of time.
Naturally, when adding impurities and forming combinations of compounds inside the silicon lattice, taking into account the share of covalent-ionic bonds, taking into account the valence of the impurities itself, its own size and geometric parameters, as well as spatial orientation of the lattices, researchers were to a certain extent able to engineer local distortions inside silicon, such as by changing the geometry and linear lattice parameters of the resulting new structure in such local surroundings and obtaining the similarity of the direct -band semiconductor structure.
The question of studying the possibilities of changing the geometric spatial structure of silicon itself without impurities, was not actively considered by researchers.
In the present paper we consider the spatial analogue of the hypothetical structure of Si2Mn2S (similarly to generally accepted GaAs structure).In for that purpose, we study photoelectric properties of silicon doped with chalcogenides, and in particular sulfur and in parallel with Mn and Zn.The photoconductivity parameters of Si<MnS> and the value of the band gap (Eg) of Si<ZnS> are presented hereunder.

Materials and methods
Recently, studies of optical and photoelectric properties of silicon doped with chalcogenides, and in particular sulfur, which is traditionally known for forming donor centers in silicon, cause great interest among scientists and researchers.This interest is driven partly by the desire to study quantum-optical effects in doped semiconductors.Excited states of sulfur form a permanent spectrum, alike that of hydrogen, while basic state is located much deeper in the band gap of silicon unlike that created by group V impurities [1][2][3][4][5].Sulfur in silicon is quite an interesting impurity because some optical transitions forbidden for shallow donor levels are allowed for the deep center of sulfur [6].
Photoconductivity of sulfur-doped silicon was studied in [7].In the experiment the initial p-type silicon wafers had a resistivity  = 20  • .After doping with sulfur at a temperature of 1200 0 , the initial silicon converted, respectively, into i-type with  = 10 5  •  and thereafter into n-type with  = 10 3 cm.The measurements were carried out using an infrared spectrometer IKS-21 at a temperature of 77K.Dark photoconductivity in samples with  = 10 3  •  started at h = 0.3 eV and had a small peak at h = 0.38 eV.In i-type samples with  = 10 5  • , the dark spectrum was monotonic.In these samples, the dark photoconductivity increased monotonically starting with h  0.5 eV.
The present paper reports the results of a study of the photoconductivity spectrum of silicon samples doped with sulfur under dark conditions and under constant illumination in forward and reverse connect modes and, accordingly, the curves of dependence of the current in the samples on the energy of incident photons are presented.
The authors investigated the photoconductivity spectra of initial p-type silicon samples with  = 1  •  (boron-doped silicon), doped with sulfur in dark and under illumination (source is a conventional 2V incandescent lamp) under forward and reverse switching modes.
Photoconductivity measurements of samples doped with S were carried out on IKS-21 spectrometer equipped with a cryostat, which allowed studying the photoconductivity in a wide temperature range (T =77-350 K).To study only the impurity-induced photoconductivity, a double filter system fabricated of a polished single-crystal silicon plate was used, which was installed in front of the cryostat window after IKS-21 lighting source.
An ohmic contact of InGa (97% -3%) was deposited on the front surface with p-type conductivity, and a contact made of InSn (52% -48%) was applied on the back surface with n-type conductivity.The sample appeared to be sensitive to light.
Photoconductivity in dark conditions and under light were recorded according to the standard diagram.The amperage was recorded with a SH300-type universal nanoammeter.

Results and discussion
Behavior of sulfur in silicon is characterized by the ionization energy Ei that varies in a large diapason as reported by different authors [8].
The authors in [7] explain such an ambiguity in the values of the ionization energy and the ionization energy band in the region h = 0.28-0.38eV by the fact that sulfur in silicon is a relatively shallow single-charged donor, and the formation of deeper impurity levels can be associated with the fact that that sulfur relatively easily forms associations of closely spaced singly charged atoms and the ionization energies of electrons are determined not only by the ionization potential of an isolated sulfur atom, but also by the Coulomb interaction of the ions included in the association.
When studying the curve of the photoconductivity spectrum (where the x-axis is the energy of the incident quanta h in eV, and the y-axis represents the values of photocurrent Iph in A) of the Si <S> sample in reverse mode in dark, the photo response was observed at ℎ = 0.26  and further increase in photoconductivity could be observed at the values ℎ = 0.4  and ℎ = 0.85  (Figure 1). Figure 2 shows the curve of the photoconductivity spectrum at a constant intrinsic illumination from a lamp at 2V in the reverse connection of the diode structure.During experiments, the photoconductivity spectra in dark and under constant intrinsic illumination (Figure 3) in the direct connection mode of the diode structure was also studied.
In [7,8], in silicon samples doped with sulfur, dark photoconductivity in samples with =10 3 cm started at h =0.3 eV, and in i-type Si <S> samples with =10 5 cm under integral illumination, weak quenching was observed at h  0.35 eV, and starting from h  0.5 eV, the quenching depth reached 95%.
The samples investigated under dark conditions in forward connection mode (Figure 3) usually manifest different behavior, and the corresponding photoconductivity spectrum in these samples starts at 0.26 eV and increases further to 0.4 eV.While reaching the point 0.4 eV, the photoconductivity spectrum in dark decreases and its magnitude is less than that of the dark current.Here, we are most likely witnessing what is known as negative photoconductivity while there happens to occur the effect of carrier injection associated with the 0.4 eV level.Under constant illumination, the photoconductivity starts at Ei0.25 eV and decreases within the 0.4 eV range with a sharp downward slope peak at E 0.52 eV, where the phenomenon of infrared quenching of photoconductivity occurs.One can assume that dark current in the forward connection mode, by and large, might be due to the fact that current carriers are possibly being injected from contact points.At voltage "switched-on" mode, electrons are caught at level Ei = 0.4 eV, since their concentration at level 0.25 eV is comparatively low, and at this section the investigated photocurrent is determined by holes [9,10].
Having analyzed and summarizing the above experimental data one can tentatively assume that at direct switching mode of the diode structure, we are witnessing the so-called effect of negative photoconductivity due to the process of injection of current carriers associated with level E = 0.4 eV [8][9][10][11][12].Under constant illumination, photoconductivity starts at Ei0.26 eV and decreases within the 0.4 eV range with a relatively sharp minimum at E  0.52 eV, where the effect of infrared quenching of photoconductivity occurs.This thing clearly manifests itself on Figure 2 under constant illumination when the structure is reversed.
Having compared the photoconductivity spectra in both directions, the authors came to the conclusion that both curves of the photoconductivity spectrum are characterized by the same ionization levels, apart from the fact that we are witnessing a slight difference and it is that in forward direction mode we come across negative photoconductivity associated with the process of injection of current carriers from contacts.The lux-ampere characteristic of such a structure is well shown by authors in the research paper.
The authors also measured the band gap of the p-n junction formed on the basis of n-type conductivity phosphor-doped silicon samples with resistivity of =100 ⋅cm.
At various temperatures, the authors measured the current-voltage characteristics (CVC) of the samples where p-n junction were formed.Their spectral sensitivity was also measured at room temperature by using IKS-12 spectrophotometer in the visible light range.The current-voltage characteristics of the samples were measured using a DC source with a voltage of U=5V and U=12V, a multi-stage potentiometer with a resistance of 10 k, while current measurements were carried out using a Rigol DM3068-type device, voltage measurements were carried out with a Mastech MS8040 device, a thermostat connected to a constant voltage source, digital temperature meter type Espada TPM10 with scale division of Δt=0,1 0 С.In order to prevent significant overheating of p-n structures, the measurements were carried out by applying short-term impulse voltages.The results of the currentvoltage characteristics of samples with a p-n junction are shown in Figure 4.

Conclusion
Tentative investigations of the short-circuit current and open-circuit voltage of p-n-structures, as well as the phase transitions in the above structures, together with the analysis of their lux-ampere characteristic in, showed that these structures could in principle be seen as potential photodiodes and/or solar cells.Further calculation of the fill factor and efficiency ratio of these structures would pave the way for prospective application of these structures as solar cells that would allegedly harness the wider solar spectrum thus increasing their economic efficiency.Also, as the band gap (Eg) is a core parameter of a semiconductor material, so, the authors think that an exact knowledge of the band gap in such materials makes it possible to manipulate key performance characteristics of semiconductor devices developed on the basis of such materials.Adding impurities and forming combinations of compounds inside the silicon lattice, taking into account the share of covalent-ionic bonds, the valence of the impurities itself, its own size and geometric parameters, as well as spatial orientation of the lattices, one might to a certain extent able to engineer structures with increased likelihood of a radiative transition (as is observed in direct-band semiconductors) or with extended spectral sensitivity thus harnessing wider solar spectra.

Figure 1 .
Figure 1.The spectral dependence curve of photoconductivity of Si <S> sample (initial boron-doped silicon with  = 1 cm) under dark conditions at a temperature of 77K in reverse connection mode.

Figure 2 .Figure 3 .
Figure 2. The spectral dependence curve of photoconductivity of Si <S> sample (initial boron-doped silicon with  = 1 cm) under stable illumination at a temperature of 77K in reverse connection mode.