**4. Comparison of properties of the GaP nanocrystals and perfect bulk single crystals**

Jointly with Refs. [1-31] this section is a generalization of the results on long-term observa‐ tion of luminescence, absorption, Raman light scattering, and microhardness in bulk semi‐ conductors in comparison with some properties of the best to the moment GaP nanocrystals. We show that the combination of these characterization techniques elucidates the evolution of these crystals over the course of many years, the ordered state brought about by pro‐ longed room-temperature thermal annealing, and the interesting optical properties that ac‐ company such ordering. We demonstrate that long-term natural stimuli improve the perfection of our crystals, which can lead to novel heterogeneous systems and new semicon‐ ductor devices with high temporal stability.

incide with those obtained from the relevant GaP powders or suspensions. We note that in the GaP/BPVE nanocomposite the position of the luminescent maximum can be changed be‐ tween 2.5 – 3.2 eV and the brightness is 20-30 more than in the PGMA and PGMA-co-POEG‐ MA matrixes. We explain the broadening of the luminescence band and the shift of its maximum to low photon energies in luminescence of the nanocomposite based on the GaP powder in Figure 9, spectrum 1, by the presence of the nanoparticles with the dimensions of 10-100 nm in the powder. Meanwhile, suspensions containing the 10 nm nanoparticles ex‐ hibit pronounced quantum confinement effects since this diameter equals the Bohr diameter

Figures 9 and 10 present a clear image of the quantum confinement effect in the GaP nano‐ particles. In accord with our data [28-30] the shift is about a few tenths of eV and, obviously, it is impossible to explain only through this effect the dramatic 1 eV enhancement to the re‐

In order to explain this interesting phenomenon we postulate that the nanocrystals, much like the ideal long-term ordered bulk GaP single crystals, exhibit this huge increase in blue-shifted luminescence due to: (a) negligibly small influence of defects and non-radiative recombination of electron-hole pairs and very high efficiency of their radiative annihilation, (b) high perfec‐ tion of nanocrystal lattice, and (c) high transparency of nanocrystals due to their small dimen‐ sions for the light emitted from high points of the GaP Brillouin zones, for instance, in the direct

Our first attempts to prepare GaP nanoparticles [26] yielded room temperature lumines‐ cence with maximum shifted only to 2.4 eV in comparison with the new maximum at 3.2 eV. It confirms significant achievements in technology of GaP nanoparticles and GaP/polymers nanocomposites. On the base of these improved technologies for preparation of GaP nano‐ particles and GaP/polymer nanocomposites we can change within the broad limits the main parameters of luminescence and expect to create a framework for novel light emissive de‐

The film device structures demonstrate broadband luminescence in the region from UV un‐ til yellow-red with controlled width and position of maximum with the luminous intensity

**4. Comparison of properties of the GaP nanocrystals and perfect bulk**

Jointly with Refs. [1-31] this section is a generalization of the results on long-term observa‐ tion of luminescence, absorption, Raman light scattering, and microhardness in bulk semi‐ conductors in comparison with some properties of the best to the moment GaP nanocrystals. We show that the combination of these characterization techniques elucidates the evolution of these crystals over the course of many years, the ordered state brought about by pro‐

300°K equal to 2.8 eV [40] and (d) high efficiency of this so called "hot" luminescence.

vice structures using dramatic 1 eV expansion of GaP luminescence to UV region.

up to 1 cd compared with industrial light emitting diodes.

<sup>c</sup> - Γ15v between the conductive and valence bands with the photon energy at

gion of luminescence at 300 K on the high-energy side of the spectrum.

of the bound exciton in GaP.

14 Optoelectronics - Advanced Materials and Devices

transitions Γ<sup>1</sup>

**single crystals**

Our unique collection of long-term ordered perfect GaP single crystals gives opportunities to find deep fundamental analogies in properties of the perfect single crystals and nanopar‐ ticles as well as to predict and to realize in nanoparticles and perfect bulk crystals new inter‐ esting properties and applications.

The long-term ordering of doped GaP and other semiconductors has been observed as an interesting accompanying process, which can only be studied in the situation when one has a unique set of samples and the persistence to observe them over decade time scales.

Any attempt to accelerate the above noted processes, for instance, through annealing of GaP at increased temperatures cannot be successful because high-temperature processing results in thermal decomposition (due to P desorption) instead of improved crystal quality. There‐ fore successful thermal processing of GaP can only take place at temperatures below its sub‐ limation temperature, requiring a longer annealing time. Evaluated within the framework of the Ising model the characteristic time of the substitution reaction during N diffusion along P sites in GaP:N crystals at room temperature constitutes 10 -15 years [5]. Hence, the obser‐ vations of luminescence of the crystals made in the sixties and the nineties were then com‐ pared with the results obtained in 2009-2012 in closed experimental conditions.

The pure and doped GaP crystals discussed herein were prepared nearly 50 years ago. Throughout the decades they have been used to investigate electro- and photoluminescence (PL), photoconductivity, bound excitons, nonlinear optics, and other phenomena. Accord‐ ingly, it is of interest also to monitor the change in crystal quality over the course of several decades while the crystal is held under ambient conditions.

More specifically, since 2005, we have analyzed the optical and mechanical properties of sin‐ gle crystalline Si, some III–V semiconductors, and their ternary analog CdIn2S4, all of which were grown in the 1960s. Comparison of the properties of the same crystals has been per‐ formed in the 1960s, 1970s, 1980s, 1990s [1-12], and during 2000s [13–25] along with those of newly made GaP nanocrystals [26-28] and freshly prepared bulk single crystals [19-23]. We improved in the preparation of GaP nanocrystals the known methods of hydrothermal and colloidal synthesis taking into account that success of our activity depends on optimal choice of the types of chemical reactions, necessary chemicals and their purity, conditions of the synthesis (control accuracy, temperature, pressure, duration, etc.), methods and quality of purification of the nanocrystals, storage conditions for nanoparticles used in the further operations of fabrication of the GaP/nanocomposites.

Single crystals of semiconductors grown under laboratory conditions naturally contain a varied assortment of defects such as displaced host and impurity atoms, vacancies, disloca‐ tions, and impurity clusters. These defects result from the relatively rapid growth conditions and inevitably lead to the deterioration of the mechanical, electric, and optical properties of the material, and therefore to rapid degradation of the associated devices.

Different defects of high concentration in freshly prepared GaP single crystals completely suppress any luminescence at room temperature due to negligible quantity of free path for non-equilibrium electron-hole pairs between the defects and their non-radiative recombina‐ tion, while the quantum theory predicts their free movement in the field of an ideal crystal lattice. The long-term ordered and therefore close-to-ideal crystals demonstrate bright lumi‐ nescence and stimulated emission repeating behavior of the best nanoparticles with pro‐ nounced quantum confinement effects. These perfect crystals due to their unique mechanical and optical properties are useful for application in top-quality optoelectronic de‐ vices as well as they are a new object for development of fundamentals of solid state phys‐ ics, nanotechnology and crystal growth.

case, the impurity atoms regularly occupy the host lattice sites and affect the band structure of the crystals, which is now a dilute solid solution of GaP-GaN rather than GaP doped by

Advanced Light Emissive Device Structures http://dx.doi.org/10.5772/52416 17

As noted previously, the luminescence of fresh doped and undoped crystals could be ob‐ served only at temperatures below about 80 K. The luminescence band and lines were al‐ ways seen at photon energies less than the value of the forbidden gap (2.3 eV). Now, after 50 years, luminescence of the long-term-ordered bulk crystals similar to the GaP nanocrystals [27-31] is clearly detected in the region from 2.0 eV to 3.0 eV at room temperature [13-25]. We believe, in the long-term-ordered bulk crystals this considerable extension of the region of luminescence at 300°K to the high-energy side of the spectrum is due to: (a) a very small concentration of defects, (b) low contribution of nonradiative electron–hole recombination, (c) considerable improvement of crystal lattice, (d) high transparency of perfect crystals, and

Earlier, in freshly prepared crystals we observed a clear stimulated emission from a GaP:N resonator at 80 K [4] as well as so called superluminescence from the GaP single crystals. Presently, our ordered crystals have a bright luminescence at room temperature that implies their perfection and very lower light losses. Currently we demonstrate [19, 20, 24, 29, 30] that the stimulated emission is developed even at room temperature by direct electron–hole recombination of an electron at the bottom of the conduction band with a hole at the top of

We also have demonstrated the considerable improvement of quality of GaP nanocrystals as the result of elaboration of an optimal for them nanotechnology. Figure 11 compares the lu‐ minescence spectra of our long-term (up to 50 years) ordered GaP single crystals (spectrum 1) to that from high quality GaP nanoparticles [27-31] and their GaP nanoparticles/polymers

The best quality GaP nanoparticles have been prepared by hydrothermal or colloidal syn‐ thesis from white phosphorus at decreased temperature (125°C) and intense ultrasonication.

Comparing the results for the nanocomposites prepared from GaP powder or suspension (Figure 11, spectra 2 and 3 respectively), it was established that the maximum shift to ultra‐ violet and the best quality in general have the nanocomposites obtained from the nanoparti‐

Nanocrystals stored as dry powder demonstrate rather broad luminescent band with max‐ imum at 2.8 eV (Figure 11, spectrum 2), while the nanocrystals of about 10 nm sizes, thor‐ oughly separated and distributed in a suspension, that prevent their coagulation, mechanical and optical interaction, exhibit bright narrow-band luminescence with maxi‐ mum at 3.2 eV, approximately 1 eV above the position of the absorption edge in GaP at

K (Figure 11, spectrum 3). The thoroughly washed, ultrasonicated and dried nano‐ powders as well as their specially prepared suspensions have been used for fabrication of blue light emissive GaP nanocomposites on the base of some optically and mechanically

(e) low probability of phonon emission at indirect transition.

the valence band and the LO phonon absorption.

cles stored as a suspension in a suitable liquid.

compatible with GaP polymers.

nanocomposites [34-36].

300o

occasionally located N atoms.

Continuing generalization of data on improvement of properties from semiconductor GaP:N crystals prepared nearly 50 years ago and their convergence to the behavior of GaP nanoparticles, here we discuss only the most interesting for fundamentals of solid state physics and application in optoelectronics and photonics data.


Taking into account the above-mentioned results, a model for the crystal lattice and its be‐ havior at a high level of optical excitation for 40-year-old ordered N-doped GaP have been suggested [3]. At relevant concentrations of N, the anion sub-lattice can be represented as a row of anions where N substitutes for P atoms with the period equal to the Bohr diameter of the bound exciton in GaP (approximately 10 nm). At some level of excitation, all the N sites will be filled by excitons, thereby creating an excitonic crystal which is a new phenomenon in solid-state physics and a very interesting object for application in optoelectronics and nonlinear optics [3, 30].

The perfect ordered GaP:N crystals demonstrate uniform luminescence from a broad exci‐ tonic band instead of the narrow zero-phonon line and its phonon replica in disordered and partly ordered (25-year-old) crystals due to the ordered crystals having no discrete impurity level in the forbidden gap. To the best of our knowledge, the transformation of a discrete level within the forbidden gap into an excitonic band is observed for the first time. In this case, the impurity atoms regularly occupy the host lattice sites and affect the band structure of the crystals, which is now a dilute solid solution of GaP-GaN rather than GaP doped by occasionally located N atoms.

Different defects of high concentration in freshly prepared GaP single crystals completely suppress any luminescence at room temperature due to negligible quantity of free path for non-equilibrium electron-hole pairs between the defects and their non-radiative recombina‐ tion, while the quantum theory predicts their free movement in the field of an ideal crystal lattice. The long-term ordered and therefore close-to-ideal crystals demonstrate bright lumi‐ nescence and stimulated emission repeating behavior of the best nanoparticles with pro‐ nounced quantum confinement effects. These perfect crystals due to their unique mechanical and optical properties are useful for application in top-quality optoelectronic de‐ vices as well as they are a new object for development of fundamentals of solid state phys‐

Continuing generalization of data on improvement of properties from semiconductor GaP:N crystals prepared nearly 50 years ago and their convergence to the behavior of GaP nanoparticles, here we discuss only the most interesting for fundamentals of solid state

**1.** Over time, driving forces such as diffusion along concentration gradients, strain relaxa‐ tion associated with clustering, and minimization of the free energy associated with properly directed chemical bonds between host atoms result in ordered redistribution

**2.** We observe in the long-term ordered GaP:N single crystals a new type of the crys‐ tal lattice, where host atoms occupy their equilibrium positions, while impurities di‐ vide the lattice in the short chains of equal length in which the host atoms develop

**3.** The nearly half-centennial evolution of the GaP:N luminescence and its other optical and mechanical properties are interpreted as the result of both volumetrically ordered N impurities and the formation of an ordered crystal-like bound exciton system. The highly ordered nature of this new host and excitonic lattices increases the radiative re‐ combination efficiency and makes possible the creation of advanced non-linear optical

Taking into account the above-mentioned results, a model for the crystal lattice and its be‐ havior at a high level of optical excitation for 40-year-old ordered N-doped GaP have been suggested [3]. At relevant concentrations of N, the anion sub-lattice can be represented as a row of anions where N substitutes for P atoms with the period equal to the Bohr diameter of the bound exciton in GaP (approximately 10 nm). At some level of excitation, all the N sites will be filled by excitons, thereby creating an excitonic crystal which is a new phenomenon in solid-state physics and a very interesting object for application in optoelectronics and

The perfect ordered GaP:N crystals demonstrate uniform luminescence from a broad exci‐ tonic band instead of the narrow zero-phonon line and its phonon replica in disordered and partly ordered (25-year-old) crystals due to the ordered crystals having no discrete impurity level in the forbidden gap. To the best of our knowledge, the transformation of a discrete level within the forbidden gap into an excitonic band is observed for the first time. In this

ics, nanotechnology and crystal growth.

16 Optoelectronics - Advanced Materials and Devices

physics and application in optoelectronics and photonics data.

of impurities and host atoms in a crystal.

media for optoelectronic applications.

harmonic vibrations.

nonlinear optics [3, 30].

As noted previously, the luminescence of fresh doped and undoped crystals could be ob‐ served only at temperatures below about 80 K. The luminescence band and lines were al‐ ways seen at photon energies less than the value of the forbidden gap (2.3 eV). Now, after 50 years, luminescence of the long-term-ordered bulk crystals similar to the GaP nanocrystals [27-31] is clearly detected in the region from 2.0 eV to 3.0 eV at room temperature [13-25]. We believe, in the long-term-ordered bulk crystals this considerable extension of the region of luminescence at 300°K to the high-energy side of the spectrum is due to: (a) a very small concentration of defects, (b) low contribution of nonradiative electron–hole recombination, (c) considerable improvement of crystal lattice, (d) high transparency of perfect crystals, and (e) low probability of phonon emission at indirect transition.

Earlier, in freshly prepared crystals we observed a clear stimulated emission from a GaP:N resonator at 80 K [4] as well as so called superluminescence from the GaP single crystals. Presently, our ordered crystals have a bright luminescence at room temperature that implies their perfection and very lower light losses. Currently we demonstrate [19, 20, 24, 29, 30] that the stimulated emission is developed even at room temperature by direct electron–hole recombination of an electron at the bottom of the conduction band with a hole at the top of the valence band and the LO phonon absorption.

We also have demonstrated the considerable improvement of quality of GaP nanocrystals as the result of elaboration of an optimal for them nanotechnology. Figure 11 compares the lu‐ minescence spectra of our long-term (up to 50 years) ordered GaP single crystals (spectrum 1) to that from high quality GaP nanoparticles [27-31] and their GaP nanoparticles/polymers nanocomposites [34-36].

The best quality GaP nanoparticles have been prepared by hydrothermal or colloidal syn‐ thesis from white phosphorus at decreased temperature (125°C) and intense ultrasonication.

Comparing the results for the nanocomposites prepared from GaP powder or suspension (Figure 11, spectra 2 and 3 respectively), it was established that the maximum shift to ultra‐ violet and the best quality in general have the nanocomposites obtained from the nanoparti‐ cles stored as a suspension in a suitable liquid.

Nanocrystals stored as dry powder demonstrate rather broad luminescent band with max‐ imum at 2.8 eV (Figure 11, spectrum 2), while the nanocrystals of about 10 nm sizes, thor‐ oughly separated and distributed in a suspension, that prevent their coagulation, mechanical and optical interaction, exhibit bright narrow-band luminescence with maxi‐ mum at 3.2 eV, approximately 1 eV above the position of the absorption edge in GaP at 300o K (Figure 11, spectrum 3). The thoroughly washed, ultrasonicated and dried nano‐ powders as well as their specially prepared suspensions have been used for fabrication of blue light emissive GaP nanocomposites on the base of some optically and mechanically compatible with GaP polymers.

Our first attempts to prepare GaP nanoparticles [26] yielded room temperature lumines‐ cence with maximum shifted only to 2.4 eV in comparison with the new maximum at 3.2 eV. It confirms significant achievements in technology of GaP nanoparticles and GaP/polymers nanocomposites. On the base of these improved technologies for preparation of GaP nano‐ particles and GaP/polymer nanocomposites we can change within the broad limits the main parameters of luminescence and expect to create a framework for novel light emissive de‐

Advanced Light Emissive Device Structures http://dx.doi.org/10.5772/52416 19

Semiconductor nanoparticles were introduced into materials science and engineering main‐ ly that to avoid limitations inherent to freshly grown semiconductors with a lot of different defects. The long-term ordered and therefore close to ideal crystals repeat behavior of the best nanoparticles with pronounced quantum confinement effect. These perfect crystals are useful for application in top-quality optoelectronic devices as well as they are a new object

This study of long-term convergence of bulk- and nanocrystal properties brings a novel perspective to improving the quality of semiconductor crystals. The unique collection of pure and doped crystals of semiconductors grown in the 1960s provides an opportunity to observe the long term evolution of properties of these key electronic materials. During this almost half-centennial systematic investigation we have established the main trends of the evolution of their optoelectronic and mechanical properties. It was shown that these stimu‐ li to improve quality of the crystal lattice are the consequence of thermodynamic driving forces and prevail over tendencies that would favor disorder. For the first time, to the best of our knowledge, we have observed a new type of the crystal lattice where the host atoms occupy their proper (equilibrium) positions in the crystal field, while the impuri‐ ties, once periodically inserted into the lattice, divide it in the short chains of equal length, where the host atoms develop harmonic vibrations. This periodic substitution of a host atom by an impurity allows the impurity to participate in the formation of the crystal's en‐ ergy bands. It leads to the change in the value of the forbidden energy gap, to the appear‐ ance of a crystalline excitonic phase, and to the broad energy bands instead of the energy levels of bound excitons. The high perfection of this new lattice leads to the abrupt de‐ crease of non-radiative mechanisms of electron-hole recombination, to both the relevant in‐ crease of efficiency and spectral range of luminescence and to the stimulated emission of light due to its amplification inside the well arranged, defect-free medium of the crystal. The further development of techniques for the growth of thin films and bulk crystals with ordered distribution of impurities and the proper localization of host atoms inside the lat‐

This long-term evolution of the important properties of our unique collection of semicon‐ ductor single crystals promises a novel approach to the development of a new generation of optoelectronic devices. The combined methods of laser assisted and molecular beam ep‐ itaxies [41-43] will be applied to fabrication of device structures with artificial periodicity;

vice structures using dramatic 1 eV expansion of GaP luminescence to UV region.

for development of fundamentals of solid state physics.

**4.1. Conclusions**

tice should be a high priority.

**Figure 11.** Luminescence of perfect bulk GaP single crystals (1) in comparison with the luminescence of GaP nanopar‐ ticles and GaP/polymers nanocomposites (2-3). Nanoparticles prepared from white P by mild aqueous or colloidal syn‐ thesis at decreased temperature, stored as the dry powder (spectrum 2) or suspension in a liquid (spectrum 3).

According to our measurements, the matrix polymers PGMA-co-POEGMA or BPVE used in this work provide no contribution to the spectra of luminescence of the based on these ma‐ trixes, so, the nanocomposite spectra coincide with those obtained from the relevant GaP powders or suspensions.

We note that in the GaP/BPVE nanocomposite the position of the luminescent maximum can be changed between 2.5 – 3.2 eV and the brightness is 20-30 more than in the PGMA and PGMA-co-POEGMA matrixes. We explain the broadening of the luminescence band and the shift of its maximum to low photon energies in luminescence of the nanocomposite based on the GaP powder by presence in the powder of the nanoparticles with the different dimen‐ sions between 10-100 nm. Meanwhile, the nanocomposites on the base of the suspensions containing only approximately 10 nm nanoparticles, exhibit bright luminescence with maxi‐ mum at 3.2 eV due to high transparency of 10 nm nanoparticles for these high energy emit‐ ted photons and pronounced quantum confinement effects since this diameter equals the Bohr diameter of the bound exciton in GaP.

In accordance with previous data [27-31, 34-36] the shift due to the quantum confinement effects is about a few tenths of eV and, obviously, it is impossible to explain only through this effect the dramatic 1 eV expansion of the region of luminescence at 300 K to the highenergy side of the spectrum.

In order to explain this interesting phenomenon we postulate that the nanocrystals, much like the ideal long-term ordered bulk GaP single crystals, exhibit this huge increase in blue-shifted luminescence due to: (a) negligibly small influence of defects and non-radiative recombination of electron-hole pairs and very high efficiency of their radiative annihilation, (b) high perfec‐ tion of nanocrystal lattice, and (d) high transparency of nanocrystals due to their small dimen‐ sions for the light emitted from high points of the GaP Brillouin zones, for instance, in the direct transitions Γ<sup>1</sup> <sup>c</sup> - Γ<sup>15</sup> v between the conductive and valence bands with the photon energy at 300°K equal to 2.8 eV [40] and (e) high efficiency of this so called "hot" luminescence.

Our first attempts to prepare GaP nanoparticles [26] yielded room temperature lumines‐ cence with maximum shifted only to 2.4 eV in comparison with the new maximum at 3.2 eV. It confirms significant achievements in technology of GaP nanoparticles and GaP/polymers nanocomposites. On the base of these improved technologies for preparation of GaP nano‐ particles and GaP/polymer nanocomposites we can change within the broad limits the main parameters of luminescence and expect to create a framework for novel light emissive de‐ vice structures using dramatic 1 eV expansion of GaP luminescence to UV region.

Semiconductor nanoparticles were introduced into materials science and engineering main‐ ly that to avoid limitations inherent to freshly grown semiconductors with a lot of different defects. The long-term ordered and therefore close to ideal crystals repeat behavior of the best nanoparticles with pronounced quantum confinement effect. These perfect crystals are useful for application in top-quality optoelectronic devices as well as they are a new object for development of fundamentals of solid state physics.

#### **4.1. Conclusions**

**Figure 11.** Luminescence of perfect bulk GaP single crystals (1) in comparison with the luminescence of GaP nanopar‐ ticles and GaP/polymers nanocomposites (2-3). Nanoparticles prepared from white P by mild aqueous or colloidal syn‐ thesis at decreased temperature, stored as the dry powder (spectrum 2) or suspension in a liquid (spectrum 3).

According to our measurements, the matrix polymers PGMA-co-POEGMA or BPVE used in this work provide no contribution to the spectra of luminescence of the based on these ma‐ trixes, so, the nanocomposite spectra coincide with those obtained from the relevant GaP

We note that in the GaP/BPVE nanocomposite the position of the luminescent maximum can be changed between 2.5 – 3.2 eV and the brightness is 20-30 more than in the PGMA and PGMA-co-POEGMA matrixes. We explain the broadening of the luminescence band and the shift of its maximum to low photon energies in luminescence of the nanocomposite based on the GaP powder by presence in the powder of the nanoparticles with the different dimen‐ sions between 10-100 nm. Meanwhile, the nanocomposites on the base of the suspensions containing only approximately 10 nm nanoparticles, exhibit bright luminescence with maxi‐ mum at 3.2 eV due to high transparency of 10 nm nanoparticles for these high energy emit‐ ted photons and pronounced quantum confinement effects since this diameter equals the

In accordance with previous data [27-31, 34-36] the shift due to the quantum confinement effects is about a few tenths of eV and, obviously, it is impossible to explain only through this effect the dramatic 1 eV expansion of the region of luminescence at 300 K to the high-

In order to explain this interesting phenomenon we postulate that the nanocrystals, much like the ideal long-term ordered bulk GaP single crystals, exhibit this huge increase in blue-shifted luminescence due to: (a) negligibly small influence of defects and non-radiative recombination of electron-hole pairs and very high efficiency of their radiative annihilation, (b) high perfec‐ tion of nanocrystal lattice, and (d) high transparency of nanocrystals due to their small dimen‐ sions for the light emitted from high points of the GaP Brillouin zones, for instance, in the direct

300°K equal to 2.8 eV [40] and (e) high efficiency of this so called "hot" luminescence.


powders or suspensions.

18 Optoelectronics - Advanced Materials and Devices

Bohr diameter of the bound exciton in GaP.

energy side of the spectrum.

transitions Γ<sup>1</sup> <sup>c</sup>

This study of long-term convergence of bulk- and nanocrystal properties brings a novel perspective to improving the quality of semiconductor crystals. The unique collection of pure and doped crystals of semiconductors grown in the 1960s provides an opportunity to observe the long term evolution of properties of these key electronic materials. During this almost half-centennial systematic investigation we have established the main trends of the evolution of their optoelectronic and mechanical properties. It was shown that these stimu‐ li to improve quality of the crystal lattice are the consequence of thermodynamic driving forces and prevail over tendencies that would favor disorder. For the first time, to the best of our knowledge, we have observed a new type of the crystal lattice where the host atoms occupy their proper (equilibrium) positions in the crystal field, while the impuri‐ ties, once periodically inserted into the lattice, divide it in the short chains of equal length, where the host atoms develop harmonic vibrations. This periodic substitution of a host atom by an impurity allows the impurity to participate in the formation of the crystal's en‐ ergy bands. It leads to the change in the value of the forbidden energy gap, to the appear‐ ance of a crystalline excitonic phase, and to the broad energy bands instead of the energy levels of bound excitons. The high perfection of this new lattice leads to the abrupt de‐ crease of non-radiative mechanisms of electron-hole recombination, to both the relevant in‐ crease of efficiency and spectral range of luminescence and to the stimulated emission of light due to its amplification inside the well arranged, defect-free medium of the crystal. The further development of techniques for the growth of thin films and bulk crystals with ordered distribution of impurities and the proper localization of host atoms inside the lat‐ tice should be a high priority.

This long-term evolution of the important properties of our unique collection of semicon‐ ductor single crystals promises a novel approach to the development of a new generation of optoelectronic devices. The combined methods of laser assisted and molecular beam ep‐ itaxies [41-43] will be applied to fabrication of device structures with artificial periodicity; together with classic methods of crystal growth, can be employed to realize impurity or‐ dering that would yield new types of nanostructures and enhanced optoelectronic device performance.

**References**

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912-13.

785-89.

phide. *J. Lumin.*, 9-302.

Meeting, Symposium E, 301-192.

*nal of the Physical Sciences*, 1(3), 14-19.

GaP:N Ordered System. *J. Pure Appl. Opt.*, 2-499.

*Physics of Semiconductors*, 2-1189, Moscow.

19-845, Presented by Nobel Prize Laureate Prokhorov AM.

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[2] Ashkinadze, B. M., Pyshkin, S. L., Bobrysheva, A. I., Vitiu, E. V., Kovarsky, V. A., Le‐ lyakov, A. V., Moskalenko, S. A., & Radautsan, S. I. (1968, July 23-29). Some non-line‐ ar optical effects in GaP. *In: Proceedings of the IXth International Conference on the*

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Our long-term ordered and therefore close to ideal crystals repeat behavior of the best nano‐ particles with pronounced quantum confinement effect. These perfect crystals are useful for application in top-quality optoelectronic devices as well as they are a new object for devel‐ opment of fundamentals of solid state physics.

For the first time we also show that well-aged GaP bulk crystals as well as high quality GaP nanoparticles have no essential difference in their luminescence behavior, brightness or spectral position of the emitted light. The long-term ordered and therefore close to ideal crystals repeat behavior of the best nanoparticles with pronounced quantum confinement effect. These perfect crystals are useful for application in top-quality optoelectronic devices as well as they are a new object for development of fundamentals of solid state physics.

Especially important for application in new generation of light emissive devices is the dis‐ covered in framework of the Project [31] dramatic expansion of luminescence region in GaP perfect bulk single crystals as well as in the best prepared GaP nanocrystals and based on them composites with transparent polymers. The broad discussion and dissemination of our results will stimulate development of our further collaboration with reliable partners from the USA, Italy, Romania, France and other countries.
