We report a study of the photoconductivity mechanism and transport paths of photoexcited charge carriers in the GeSn/Ge/Si heterostructures. The dark conductivity was studied as a function of temperature, which allowed to identify of the presence of deep levels at EV+(100÷130) meV. We have established that point defects are the source of a band of electronic states and determine the photoconductivity response. The photocurrent dependencies on excitation intensity demonstrate that the main conduction occurs mainly through the Ge layer under low pumping and through the Si substrate under high one, since the GeSn top layer is much thinner has a much higher conductivity. This detailed understanding of the recombination processes is of critical importance for developing GeSn/Ge-based optoelectronic devices.
KEYWORDS: Indium gallium arsenide, Solar cells, Gallium arsenide, Solar energy, Photovoltaics, Molecular beam epitaxy, Quantum wells, Temperature metrology, Video, Current controlled current source
A study of the photovoltaic properties of the GaAs-based solar cells with InGaAs quantum wire had been conducted. The research included the investigation of the photovoltage rise and decay transients, spectral photovoltage dependences at different temperatures. The objects investigated were GaAs-based solar cells with InGaAs quantum wire (QWR) embedded into space-charge-region of p-i-n junction. Samples with different In content and size of InGaAs nanoobjects had been created using molecular beam epitaxy. Unlike the reference cell, the ones containing the InGaAs QWR had shown higher sensitivity in the energy range 1.2 - 1.38 eV. This is caused by the spatial separation of electron-hole (e-h) pairs excited in the QWR due to band-to-band transition. Under selective excitation of the e-h pairs only in the InGaAs quantum wire the photovoltage rise transient is slower compared to the e-h generation in GaAs. This effect is explained by charge carriers release from the InGaAs quantum well into delocalized states of the surrounding GaAs. It was determined that the InGaAs quantum wires increase the recombination rate of the non-equilibrium carriers in the temperature range 80 to 290 K, which means that the quantum wires are the additional recombination centers.
Structures with one-dimensional quantum objects in intermediate band are promising for their application in solar cells and photodetectors. We present analysis of dark current-voltage characteristics, photo-voltage decay and photo-voltage spectra for this structures in comparison with reference GaAs based structures. It has been shown that InGaAs quantum wires make a significant influence on J-V dependences and photo-voltage spectra. InGaAs QWRS are additional recombination centers and transitions between them dominated over by Shockley-Read-Hall recombination at low bias. The InGaAs/GaAs sample shows a significantly higher photo-voltage in the spectral range of 1.25-1.37 eV, as compared to a reference GaAs p-n junction, due to intermediate band transitions in the quantum wires.
The structures consisting of Ge-nanoclusters grown on silicon oxide layer are promising candidates for optoelectronics
as well as for nonvolatile memory circuits . This is due to their infrared photoluminescent and photoconductive
properties. Crystalline germanium nanoclusters (NCs) are grown by a molecular-beam epitaxy technique on chemically
oxidized Si(100) surface at 700°C. It was shown that structures with Ge-nanoclusters, grown on silicon surface
characterized by fluctuations of the electrostatic field, that determined of positive charge trapped by dimensional
quantum states Ge nanoclusters and Ge-nanoclusters/Si interface traps. Field effect on lateral conductivity and
photovoltage spectra in Ge-nanostructures were analized.
Germanium (Ge) nanoclusters are grown by a molecular-beam epitaxy technique on chemically oxidized Si(100) surface at 700ºC. Evidence for long-term photoinduced changes of surface conductivity in structures with Ge nanoclusters (NCs) grown on silicon oxide is presented. Photoexcitation NCs or Si by quanta with different energy allows observing two non-equilibrium steady-states with excess and shortage of conductivity values as compare to equilibrium one. The persistent photoconductivity (PPC) behaviour was observed after interband excitation of electron-hole pairs in Si(001) substrate. This effect may be attributed to spatial carrier separation of photoexcited electron-hole pairs by macroscopic fields in the depletion layer of near-surface Si. Photoquenching of surface conductivity, driven by optical recharging of Ge NC’s and Si/SiO2 interface states, is observed. Conductivity decay is discussed in the terms of hole`s accumulation by Ge-NC states enhancing the local-potential variations and, therefore, decreasing the surface conductivity of p-Si.
Materials with one-dimensional quantum structures are promising for their application in solar cells. The
photo-voltage generation of these structures is caused by spatial separation of electron-hole pairs by a built-in
electric field in the GaAs p-i-n junction. The InGaAs/GaAs sample shows a significantly higher photo-voltage in the
spectral range of 1.25-1.37 eV, as compared to a reference GaAs p-n junction, due to interband transitions in the
quantum wires (QWRs).
The photoconductivity spectra of the structure nanodimensional Ge/c-Si with Ge quantum wells on a single-crystal substrate surface were measured using infrared spectrophotometer IR-12. The same measurements were also made for the structures Al0.2Ga0.8As/In0.1Ga0.9As/GaAs with further comparison of received results to standard GaAs photodiode. The photoconductivity spectrum of nanodimensional Ge/c-S i structure was received at room temperature. The investigated samples are made by molecular - beam epitaxy method. rectangular frame type (5x5 micron) contact was generated on a surface of Ge layer. The thickness of a contact strip was equaled to 0,5 micron. The second contact was soldered to the back side of the singlecrystal surface. A shifting voltage U =1,5 V was switched in the opposite direction (negative potential to Ge slice) At measurements of photoconductivity of structure. It is necessary to note that photoconductive signal was 3 orders less, than at inverse displacement. It specifies presence heterotransitions between Ge and c-S i layer. The photosensitivity of a standard silicon photodiode was investigated for comparison of such assumption. For example the spectral dependence of photosensitivity of standard silicon photodiode FD-142Κ is represented. The spectral position of a photoconductivity curve was the same to standard silicon photodiode at room temperature. The value of photosensitivity of a researched sample was compared with the standard photodiode. Is established, that both these values are of the same order. It is possible to explain it by presence of a potential barrier between Ge and Si. It is known that longwave border of photoconductivity is defined by width of the forbidden zone of the semiconductor. The increase of photoconductivity is caused by increase of absorption at rising of quantums energy of the exited radiation (at reduction of wavelength). The form of a photoconductivity spectrum of the photodiode FD-142Κ and absence of a hole in the spectrum in short-wave area (1,5-2,1 μm) specifies that the speed of a surface recombination is equal to zero. For the structure nanodimensional Ge/ c-S i, otherwice, significant hole in this area was observed at the room temperature. So, samples had the large speed of surface recombination. To observe the contribution of nonequilibrium charge carriers to the photoconductivity of structure nanodimensional Ge/c-Si it is necessary to cool down to Τ < 100 K. The intersubband transitions can occur in nanodimensional Ge at such temperatures. So, it is necessary to expect observation of a photosensitivity in the infrared, which corresponds energy of these transitions. It is possible to explain photosensitivity of nanostructures by existence of interzoned transitions in nanodimension Ge. The spectral dependence of photosensitivity of structure nanodimension Ge/c-Si in IR- of area is received. Analysis of received results have shown that the spectrum Al0.2Ga0.8As/In0.1Ga0.9As/GaAs differs from standard GaAs photodiode by wider spectral sensitivity range owing to creation of nanodimensional layers Al0.2Ga0.8As/In0.1Ga0.9As on the GaAs substrate. It gives the possibility to detect optical irradiation.
The spectral dependence of photoconductivity of structure nanodimensional Ge/c-Si was measured on infrared spectrophotometer DCS-12 which contains Ge quantum holes on a surface of single-crystal substrate. The photoconductivity spectrum of nanodimensional Ge/c-Si structure was received at room temperature. The investigated samples are made by molecular-beam epitaxy method, rectangular frame type (5x5 micron) contact was generated on a surface of Ge layer. The thickness of a contact strip was equaled to 0,5 micron. The second contact was soldered to the back side of the singlecrystal surface. A shifting voltage of U = 1,5 V was switched in the opposite direction (negative potential to Ge slice) At measurements of photoconductivity of structure. It is necessary to note that photoconductive signal was 3 orders less, than at inverse displacement. It specifies presence heterotransitions between Ge and c-Si layer. The photosensitivity of a standard silicon photodiode was investigated for comparison of such assumption. For example the spectral dependence of photosensitivity of standard silicon photodiode ΦB-142K is represented. The spectral position of a photoconductivity curve was the same to standard silicon photodiode at room temperature. The value of photosensitivity of a researched sample was compared with the standard photodiode. Is established, that both these values are of the same order. It is possible to explain it by presence of a potential barrier between Ge and Si. It is known that longwave border of photoconductivity is defined by width of the forbidden zone of the semiconductor. The increase of photoconductivity is caused by increase of absorption at rising of quantums energy of the exited radiation (at reduction of wavelength). The form of a photoconductivity spectrum of the photodiode ΦB-142K and absence of a hole in the spectrum in short-wave area (1,5-2,1 micron) specifies that the speed of a surface recombination is equal to zero. For the structure nanodimensional Ge/c-Si, otherwise, significant hole in this area was observed at the room temperature. So, samples had the large speed of surface recombination. To observe the contribution of nonequilibrium charge carriers to the photoconductivity of structure nanodimensional Ge/c-Si it is necessary to cool down to T < 100 Κ. The intersubband transitions can occur in nanodimensional Ge at such temperatures. So, it is necessary to expect observation of a photosensitivity in the infrared, which corresponds energy of these transitions. It is possible to explain photosensitivity of nanostructures by existence of interzoned transitions in nanodimension Ge. The spectral dependence of photosensitivity of structure nanodimensional Ge/c-Si in IR- of area is received.
The porous silicon deserves great scientific attention due to intensive photoluminescence can be observed at the room temperature.
With the purpose of modeling photo-electric properties of porous silicon the researches of influence of non-uniform deformation of silicon monocrystal on the form of spectral distribution of photoconductivity were carried out. Measurements of the photoconductivity (PC) and photomagnetic effect (PME) spectra of crystalline silicon were carried out for the sample under the non-uniform bend deformation. This deformation causes a decrease of the photoconductivity spectrum drop in the short-wave region when illuminating the stretched surface. Under constant deformation conditions the PME spectrum form is changed only in the long-wave region. Obtained data are explained by diffusion length decreasing as a consequence of decreasing diffusion coefficient under the influence of a strain gradient. The analysis of the investigated photo-electric properties of porous silicon has resulted in a conclusion about the excitation photoluminescence occurs in amorphous matrix of porous silicon. And the important role plays drift of nonequilibrium carriers of a charge in an internal electrical field of porous silicon at carry of energy the excitation from area of generation to the centre of luminescence.
The work has excellent application for production of porous silicon based photodiodes with improve performances.
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