Journal of Electrical Engineering 2 (2014) 207-212 doi: 10.17265/2328-2223/2014.05.002 D DAVID PUBLISHING Nodar Kekelidze 1, 2, 3, Elza Khutsishvili 1, 2, Bella Kvirkvelia 1, 2, 3, David Kekelidze 2, Vugar Aliyev 4 and George Kekelidze 5 1. Ferdinand Tavadze Institute of Metallurgy and Materials Science, Tbilisi 0160, Georgia 2. Iv.Javakhishvili Tbilisi State University, Tbilisi 0179, Georgia 3. Georgian Technical University, Tbilisi 0171, Georgia 4. Institute of Physics of National Academy of Sciences, Baku 1143, Azerbaijan 5. BoT EUROSOLAR, e.v.bonn 53113, Germany Abstract: Transport phenomena, namely electrical properties of n-type crystals of InAs and InP solid solutions were investigated in the temperature range 4.2-300 K before and after irradiation with fast neutrons and electrons. Maximum integral fluence of fast neutrons was 2 10 18 n cm -2. At the irradiation with 3 MeV electrons, the integrated electron fluence was 5 10 17 e cm -2. We show that point type defects play an important role in the radiation processes. They are effective scattering centers of charge carriers in InAs, InP and InP x As 1-x solid solutions irradiated with 3 MeV energy electrons, especially for InAs-rich solid solutions. The charge carriers scattering mechanisms and accordingly the values of mobility are defined by disordered regions in samples irradiated with fast neutrons. The presence of minimum mobility value in composition dependence of mobility disappears after fast neutrons irradiation, which indicates that the contribution of alloy scattering is negligibly small in crystals irradiated with fast neutrons at both room and low temperatures. Key words: Neutrons and electrons radiation, the point type defects, disordered regions. 1. Introduction The phenomena of mutual compensation of radiation donors and acceptors have been revealed in interesting for optoelectronics and microelectronics InP x As 1-x solid solutions and radiation-resistant materials withstanding high fluencies of fast neutrons have been created [1, 2]. These radiation-resistant materials are important for their application in the Space, accelerators, and nuclear reactors as well as at Chernobyl and Fukushima. Investigation of charge carriers mobility dependence on the composition of InP x As 1-x solid solutions and on the temperature in samples irradiated with fast neutrons and electrons are given in the presented work. It is remarkable that in Ref. [3] there are presented data only for two compositions of irradiated with fast neutrons InP x As 1-x solid solutions. Corresponding author: Nodar Kekelidze, professor, research fields: semiconductor physics, material science and engineering. E-mail: nodar.kekelidze@tsu.ge. 2. Experiment Experimental samples were grown by the horizontal zone melting. The data of carrier mobility are obtained from the measurements of Hall effect and electric conductivity by the compensation circuit at the direct current in the temperature interval 4.2-300 K. The homogeneity of the samples for measurements was determined by X-ray spectroscopy and testing implementation of Vegards Law [4]. Maximum integral fluence of fast neutrons was 2 10 18 n cm -2. At the irradiation with 3 MeV electrons, the integrated electron fluence was 5 10 17 e cm -2. 3. Results and Discussion In the unirradiated crystals, the value of charge carriers mobility is determined by dominating scattering on optical phonons at high temperatures, including 300 K and by scattering on the ions of impurity, prevailing at lower temperatures (T < 100 K).
208 At large concentrations of impurity N 10 18 cm -3 scattering on the impurity ions is already dominating at room temperature. 3.1 Fast Neutrons Irradiated InAs-InP Alloys Investigated samples of InAs-InP alloys system before irradiation had nearly the same charge carriers concentration (n) ~ 10 16 cm -3 initially. The typical experimental results of Hall coefficient and electrical resistivity measurements the carrier mobility (μ) composition dependences for InAs-InP alloys system at 300 K and 100 K before and after irradiation are presented in Figs. 1a and 1b. Composition dependence of InAs-InP alloys carriers mobility before irradiation reveals minimum in the InP-side of alloys system at all investigated temperatures. With lowering of temperature, this minimum is more expressed. The presence of minimum in composition dependence of mobility is defined by alloy scattering [4-8] along with the other scattering mechanisms (on the optical phonons, ionized impurities) existent in InAs and InP. Share of contribution of separate scattering mechanisms into summary scattering is different at various temperatures and composition of InP x As 1-x alloys. The situation gets more complex after irradiation. Fig. 1 demonstrates interesting picture. It is seen that after irradiation characteristic minimum disappears and mobility decreases along the full system of InP x As 1-x alloys. Such character of mobility behavior is connected with the change of contribution of separate components of scattering mechanisms into summary scattering in experimental samples at radiation. One of the most important consequences of the irradiation affects on the electrical properties is the origination of radiation defect centers. Those defects act as scattering centers and this process leads to the reduction of carriers mobility. It is very important that after irradiation the charge carriers concentration changes. Therewith if all investigated samples before irradiation had nearly the same carriers concentration, (a) (b) Fig. 1 Composition dependence of charge carriers mobility (µ) for InP x As 1-x alloys experimental samples with nearly the same initial carriers concentration n ~ 10 16 cm -3 : (a) at 300 K and (b) at 100 K; 1 before irradiation, 2 after fast neutrons irradiation. after irradiation, charge carriers concentration is different in samples. But carriers concentration changes in various samples in different ways. Here it is possible to separate two types of carriers concentration behavior in asymmetrical InAs and InP sides along the InP x As 1-x alloys system. The mobility decreases after irradiation in all types of InP x As 1-x alloys. Absence of minimum on the curve 2 in Figs. 1a and 1b (Fig. 1) means that the contribution of alloy scattering is negligibly small in crystals irradiated with fast neutrons at both room and low temperatures. 3.1.1 InAs side of InP x As 1-x alloys (x < 0.3). Experimental dependences of carriers concentration (n) vs. temperature in InAs side of InP x As 1-x alloys
209 show that at all fixed temperatures n increases significantly after irradiation and practically does not change with increase of temperature. Analysis of the temperature dependence of carrier concentration after irradiation in these samples shows that irradiation introduces radiation donors role of which interstitial atoms of arsenide with shallow energetic levels play. The increase of charge carrier concentration after irradiation weakens at increase of InP composition of InP x As 1-x alloys. The change of carriers mobility (µ) after irradiation in InAs sides correlates with a variation of carriers concentration. After irradiation carriers mobility in InAs sides of InP x As 1-x alloys in the whole investigated temperature range (4.2-300 K) decreases and is equipollent to origination of ~ 10 18-10 19 cm -3 ionized centers. This causes inhibition of carriers scattering on the lattice vibrations. So appreciable increase of mobility at the decrease of temperature is not observed. On the other hand, a substantial increase of the carriers concentration promotes rise of the degree of electrons gas degeneracy. As a result, only an insignificant increase of mobility in the range of relatively high temperatures and fast transition into degeneracy state at lowering of temperature with following permanency of mobility are observed. Detailed quantitative calculation shows that it is not possible to describe carrier mobility behavior in the temperature range 4.2-300 K at the fast neutrons irradiation with great fluxes on the base of the theory on charge carriers scattering on the point defects and lattice vibration: with lowering of temperature electrons mobility sharply falls. After irradiation contribution of alloy scattering is small. In InAs side of InP x As 1-x alloys crystals, the properties of InAs sub lattice retain and the radiation properties of InP sublattice are also perceptible in them. After irradiation there originate not only donor centers but acceptor centers too. 3.1.2 InP side of InP x As 1-x alloys (x > 0.3). Experimental dependences of carrier concentration vs. temperature for n-type InP side of InP x As 1-x alloys show that oppositely to InAs side at all fixed temperatures charge carrier concentration decreases significantly after irradiation. Revealed temperature dependences of carrier concentration indicate that predominately structural defects of catching centers for both types of charge carriers are created at irradiation of alloys by fast neutrons. Furthermore, radiation acceptors interstitial atoms of phosphorus, with deep energy levels in forbidden band are created. Temperature dependences of carriers concentrations before and after irradiation are strongly different. Before irradiation electron concentration practically does not change with temperature in whole investigated temperature range, but after irradiation temperature dependence of carrier concentration intensifies. Irradiation leads to decrease of mobility at all fixed temperatures in the whole investigated temperature range (Figs. 1 and 2). After irradiation much stronger temperature dependence of mobility is observed than it was expected as a result of scattering on the point ionized defects. Such strong temperature dependence of mobility after irradiation with fast neutrons (Fig. 2, curve 2), is defined by the action of large defects-disordered areas introduced into material in the process of irradiation. Comparison with the results received in InAs side shows that after severe irradiation more radiation centers are created in InP side. Apparently InP lattice is destroyed stronger by irradiation because mass of phosphorus is twice lighter than mass of arsenic. So irradiation leads to forming significant number of large disordered regions, which possibly define magnitude of mobility. It is natural to assume that at the same time point type defects are created. For updating mechanisms defining mobility of experimental samples after irradiation quantity studies of μ(t) were implemented. Theory of scattering on the points centers does not describe μ(t). Theoretical calculations of the mobility component μ def provided by radiation defects, introduced at
210 irradiation with maximum dose of fast neutrons of flux Ф = 2 10 18 n cm -2, were fulfilled for InP-side of InAs-InP alloys. The experimental mobility component μ def was defined from the relationship: 1/μ def = 1/μ 2 1/μ 1 (1) where, μ 1 mobility before irradiation, μ 2 mobility after irradiation. The results were compared with the theory of scattering on the points centers and scattering on disorder regions [6, 9]. The calculation has been implemented for all samples. For demonstration we show the result for InP 0.6 As 0.4 solid solution in the middle of InAs-InP system (Figs. 2 and 3). The calculations showed that the theory of scattering on large disordered areas is in a good agreement with experiment. The mobility was calculated according to a theory [6, 9] for the scattering due to large disordered Fig. 2 Temperature dependence of charge carriers mobility for InP 0.6 As 0.4 solid solution (n = 4.7 10 17 cm -3 ). 1 before irradiation, 2 after irradiation with fast neutrons. Fig. 3 Temperature dependence of charge carriers mobility component μ def introduced by fast neutrons irradiation for InP 0.6 As 0.4 solid solution (n = 4.7 10 17 cm -3 ); curve theoretical calculation for the scattering due to large disordered areas, dots experimental data. areas. So at irradiation of InP side of InP x As 1-x alloys with fast neutrons, large disorder areas are main defects, which define the value of mobility. The contribution of point defects into scattering is less by far. The following isochronous annealing results of irradiated crystals InP side of InP x As 1-x alloys show that after annealing at 500-600 C, the temperature dependence of carrier mobility approaches to µ(t) for unirradiated alloys. 3.2 Electrons (3 MeV) Irradiated InAs-InP Alloys Samples of n-type InAs-InP alloys system were selected with nearly the same carriers concentration ~ 10 16 cm -3 initially before irradiation. The carrier mobility (μ) composition dependences for InAs-InP alloys system at 300 K and 200 K before and after irradiation are presented in Fig. 4. Revealed minimum in the composition dependence of InAs-InP alloys carriers mobility before irradiation is preserved after electrons irradiation as opposed to irradiation with fast neutrons (Fig. 1). Analysis of the theoretical and experimental data of mobility shows that electron irradiation changes the situation. Different contribution of carriers scattering on the optical phonons, ionized impurities and alloy scattering at various temperatures and composition of InP x As 1-x solid solutions is complicated by the charge carriers concentration changes after irradiation. 3.2.1 InAs side of InP x As 1-x alloys (x < 0.3) As a result of radiation, in InAs side of system carriers concentration increases with the increase of irradiation dose. It is clear that after irradiation quantity of scattering centers increases that causes the decrease of mobility. But after irradiation by electrons the decrease of the mobility is weaker in comparison with a case of radiation by fast neutrons as it is seen from Figs. 1 and 4. If before radiation there was observed definite dependence mobility on the temperature, after irradiation mobility dependence on the temperature is not observed that is caused by charge carriers gas degeneration. The results show that after 3 MeV energy electrons radiation donor type
211 (a) observed. Therewith carriers concentration decreases with the increase of irradiation dose. Charge carriers mobility decreases after radiation. Analysis of temperature dependence of charge carriers concentration shows that after irradiation structural defects of catching centers are created and introduced with levels located deeply in the forbidden zone. Calculation shows that arisen radiation defects are not simply point ionized centers and the mechanism of carriers scattering changes. Decrease of charge carriers concentration after radiation is attended by decrease of mobility and weakening of mobility temperature dependence. Analysis of temperature dependence of charge carriers mobility shows that after irradiation the effect of large defects are necessary to take into part in InP-rich InP x As 1-x solid solutions irradiated with 3 MeV electrons. 4. Conclusions (b) Fig. 4 Composition dependence of charge carriers mobility (µ) for InP x As 1-x alloys samples in experimental samples with nearly the same initial carriers concentration n ~ 10 16 cm -3 : (a) at 300 K and (b) at 200 K; 1 before irradiation, 2 after electrons irradiation. radiation defects originate. Arisen radiation defects give rise to additional scattering of charge carriers that is attended by disappearance of mobility temperature dependence. Electrons radiation creates defects with shallow levels in forbidden zone like the case at fast neutrons radiation. It is obvious that along with marked radiation defects there are created defects in InP sub lattice which introduce their contribution in the carriers scattering. Comparison of µ with existent theories shows that charge carriers mobility is defined by scattering on the point type defects in InAs-rich alloys irradiated by 3 MeV crystals. 3.2.2 InP side of InP x As 1-x alloys (x > 0.3) In InP side of system after radiation, opposite behavior decrease of carrier concentration is The investigation of transport phenomenon in the irradiated InP x As 1-x solid solutions let conclude that point type defects play an important role in the radiation processes. They are effective scattering centers of charge carriers in InAs, InP and InP x As 1-x solid solutions irradiated with 3 MeV energy electrons, especially for InAs-rich solid solutions. The charge carriers scattering mechanisms and accordingly values of mobility are defined by disordered regions in samples irradiated with fast neutrons. The contribution of alloy scattering is negligibly small in crystals irradiated with fast neutrons at both room and low temperatures. References [1] N. Kekelidze, G. Kekelidze, D. Kekelidze, V. Aliyev, Investigation of InP x As 1-x solid solutions and creation of the radiation-resistant materials on their basis, in: AIP Conference Proceedings, Dec. 2013, p. 101. [2] N. Kekelidze, B. Kvirkvelia, D. Kekelidze, V. Aliyev, E. Khutsishvili, G. Kekelidze, Phenomenon of mutual compensation of radiation donors and acceptors and creation of radiation-resistant materials, Journal of Electrical Engineering 2 (4) (2014) 187-192. [3] E. Khutsishvili, B. Kvirkvelia, D. Kekelidze, V. Aliyev,
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