Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds

Electron-enhanced reactions in II-VI compounds are shown to be caused by the presence of some mobile defects which diffusion is not enhanced under excitation. At the same time, electron-enhanced diffusion can be imitated in these reactions due to carrier trapping by deep centers that do or even do n...

Повний опис

Збережено в:

Бібліографічні деталі
Дата:	1999
Автори:	Korsunskaya, N. E., Markevich, I. V., Dzhumaev, B. R., Borkovskaya, L. V., Sheinkman, M. K.
Формат:	Стаття
Мова:	English
Опубліковано:	Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України 1999
Назва видання:	Semiconductor Physics Quantum Electronics & Optoelectronics
Онлайн доступ:	http://dspace.nbuv.gov.ua/handle/123456789/117932
Теги:	Додати тег Немає тегів, Будьте першим, хто поставить тег для цього запису!
Назва журналу:	Digital Library of Periodicals of National Academy of Sciences of Ukraine
Цитувати:	Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds / N.E. Korsunskaya, I.V. Markevich, B.R. Dzhumaev, L.V. Borkovskaya, M.K. Sheinkman // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 4246-ХХ. — Бібліогр.: 12 назв. — англ.

Репозитарії

Digital Library of Periodicals of National Academy of Sciences of Ukraine

id	irk-123456789-117932
record_format	dspace
spelling	irk-123456789-1179322017-05-28T03:03:09Z Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds Korsunskaya, N. E. Markevich, I. V. Dzhumaev, B. R. Borkovskaya, L. V. Sheinkman, M. K. Electron-enhanced reactions in II-VI compounds are shown to be caused by the presence of some mobile defects which diffusion is not enhanced under excitation. At the same time, electron-enhanced diffusion can be imitated in these reactions due to carrier trapping by deep centers that do or even do not take part in the reaction. To elucidate the real defect reaction mechanism a detailed study is required in every case. For this purpose, a method of mobile defect detection and their diffusion characteristic direct investigation has been elaborated. 1999 Article Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds / N.E. Korsunskaya, I.V. Markevich, B.R. Dzhumaev, L.V. Borkovskaya, M.K. Sheinkman // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 4246-ХХ. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS 61.72.Ji, 61.72.Yx, 72.40.+w, 72.80.Ey, 78.55.Et http://dspace.nbuv.gov.ua/handle/123456789/117932 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
description	Electron-enhanced reactions in II-VI compounds are shown to be caused by the presence of some mobile defects which diffusion is not enhanced under excitation. At the same time, electron-enhanced diffusion can be imitated in these reactions due to carrier trapping by deep centers that do or even do not take part in the reaction. To elucidate the real defect reaction mechanism a detailed study is required in every case. For this purpose, a method of mobile defect detection and their diffusion characteristic direct investigation has been elaborated.
format	Article
author	Korsunskaya, N. E. Markevich, I. V. Dzhumaev, B. R. Borkovskaya, L. V. Sheinkman, M. K.
spellingShingle	Korsunskaya, N. E. Markevich, I. V. Dzhumaev, B. R. Borkovskaya, L. V. Sheinkman, M. K. Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds Semiconductor Physics Quantum Electronics & Optoelectronics
author_facet	Korsunskaya, N. E. Markevich, I. V. Dzhumaev, B. R. Borkovskaya, L. V. Sheinkman, M. K.
author_sort	Korsunskaya, N. E.
title	Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds
title_short	Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds
title_full	Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds
title_fullStr	Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds
title_full_unstemmed	Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds
title_sort	electron-enhanced reactions responsible for photoluminescence spectrum change in ii-vi compounds
publisher	Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
publishDate	1999
url	http://dspace.nbuv.gov.ua/handle/123456789/117932
citation_txt	Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds / N.E. Korsunskaya, I.V. Markevich, B.R. Dzhumaev, L.V. Borkovskaya, M.K. Sheinkman // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 4246-ХХ. — Бібліогр.: 12 назв. — англ.
series	Semiconductor Physics Quantum Electronics & Optoelectronics
work_keys_str_mv	AT korsunskayane electronenhancedreactionsresponsibleforphotoluminescencespectrumchangeiniivicompounds AT markevichiv electronenhancedreactionsresponsibleforphotoluminescencespectrumchangeiniivicompounds AT dzhumaevbr electronenhancedreactionsresponsibleforphotoluminescencespectrumchangeiniivicompounds AT borkovskayalv electronenhancedreactionsresponsibleforphotoluminescencespectrumchangeiniivicompounds AT sheinkmanmk electronenhancedreactionsresponsibleforphotoluminescencespectrumchangeiniivicompounds
first_indexed	2025-07-08T13:02:23Z
last_indexed	2025-07-08T13:02:23Z
_version_	1837083914124918784
fulltext	4 2 © 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 1. P. 42-46. 1. Introduction Processes of two types, namely, dislocation multiplica- tion and electronically enhanced point defect reactions are known to be responsible for II-VI compound light emitting device degradation. The processes connected with dislocation motion and multiplication are well in- vestigated. At the same time, there is a little information about defect reactions in light emitting devices based on II-VI compound crystals and layers. This state of affairs may be due to the difficulty of required information ex- traction under finished device investigation. Such infor- mation can be obtained more easily under investigation of materials used for device preparation. This report deals with mechanisms of point defect re- actions in CdS, CdSe, CdSSe crystals which are used as working elements of pulsed electron-beam-pumped lasers [1]. A number of defect reactions resulting in some elec- tric, photoelectric and optical characteristic changes were found by us in these crystals earlier [2]. These reactions were proved to be processes of rearrangement of pre-ex- PACS 61.72.Ji, 61.72.Yx, 72.40.+w, 72.80.Ey, 78.55.Et Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds N. E. Korsunskaya, I. V. Markevich, B. R. Dzhumaev*, L. V. Borkovskaya, M. K. Sheinkman Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kyiv, 252028, Ukraine, tel: (044) 265-72-34, fax: (044) 265-83-44, e-mail: kors@lumin.semicond.kiev.ua; tel: (044) 265-63-40, Fax: (044) 265-63-40, e-mail: moishe@photel.semicond.kiev.ua. *Turkmen Polytechnic Institute, Ashgabat, 744025, Turkmenistan Abstract. Electron-enhanced reactions in II-VI compounds are shown to be caused by the presence of some mobile defects which diffusion is not enhanced under excitation. At the same time, elec- tron-enhanced diffusion can be imitated in these reactions due to carrier trapping by deep centers that do or even do not take part in the reaction. To elucidate the real defect reaction mechanism a detailed study is required in every case. For this purpose, a method of mobile defect detection and their diffusion characteristic direct investigation has been elaborated. Keywords: electron-enhanced reactions; II-VI compounds; mobile defects. Paper received 25.12.98; revised manuscript received 08.04.99; accepted for publication 19.04.99. isted defects and were shown to be reversible [2]. The activation energies of reaction process, E R , and initial state restoration one, E I therm , were found to be 0.15÷0.35 and 0.4÷1.2 eV ranges, respectively, the relation E R < < E I therm being observed for each reaction [2]. As a rule, point defect rearrangements both in the presence and absence of excitation are supposed to be controlled by defect diffusion. So, the relation E R < < E I therm is usually explained by reduction of defect diffu- sion activation energy under excitation (electron-en- hanced diffusion) [3-5], partially, by phonon-kick mech- anism [5]. In the latter case the equality E eq = E ex + E t takes place, where E ex and E eq are the defect rearrange- ment activation energies under excitation and equilibri- um conditions, respectively, E t is the energy depth of the defect that takes part in the reaction [5]. However, our investigations showed that the same equality can take place when excitation does not affect defect diffusion. 43SQO, 2(1), 1999 N. E. Korsunskaya et al.: Electron-enhanced reactions responsible ... 2. Results and discussion Photoluminescence (PL), photocurrent (PC), thermally stimulated current (TSC) and EPR spectra were mea- sured in CdS, CdSe and CdS x Se 1-x crystals. PL was ex- cited with 0.365 µm mercury lamp line. The results of reaction mechanism investigations for two different de- fect reactions are adduced below. 2.1. Reaction resulting in degradation of PL in- tensity and photosensitivity. This reaction occurs in �pure� and Cu-doped CdS, CdSe crystals in 300÷400 K temperature range under illumi- nation with visible light from the regions of photocur- rent maxima. It results in dramatic decrease of crystal photosensitivity and extrinsic PL intensity [6]. After heat- ing up to 450 K and subsequent cooling in the dark the initial state restores. At T < 300 K, when the reaction is �frozen�, the sample characteristics can be investigat- ed both in the initial state and after reaction carrying out. PL spectra at 80 K in these two states for CdS:Cu crystal are shown in Fig. 1. It was found from TSC investigations that the density of shallow electron traps T1 (E 1 = E C - 0.03 eV) decreased and the density of deep electron traps T2 (E 2 = E C - 0.9 eV) increased as a result of reaction (Fig. 2, curves 1,2). It was proved that T2 center appearance resulted from re- arrangement of T1 centers, probably, due to their asso- ciation [6]. As EPR investigations showed, T1 centers were hydrogenlike donors [7]. It was found that T2 de- struction and T1 reappearance under heating in the dark occurred in fact simultaneously with T2 thermal ioniza- tion. The activation energies of both the reaction and initial state restoration were obtained from process rate temperature dependencies by the method described in [8]. These values were found to be equal to in 0.35 and 1.25 eV, respectively. Therefore, the equation E I therm = E R + E 2 takes place. Since the initial state restoration occurs simultaneous- ly with T2 thermal ionization one may suppose that it is the latter which controls restoration process. To verify this supposition we tried to ionize T2 optically. For this purpose the sample after reaction carrying out had been irradiated with IR-light at 80 K and than TSC spectrum was measured. An effective ionization of T2 by λ = = 0.8÷1.1 µm IR light was observed. It was also found that such irradiation enhanced initial state restoration process and decreased its activation energy down to 0.35 eV. Therefore, initial state restoration process can be also the photo-enhanced one, moreover, its activation energy in this case E I opt, is equal to E R . One can see also that E I therm = E I opt + E 2 , i.e. the equal- ity E eq = E ex + E t is valid here. Nominally, such equality allows to assume that the phonon-kick mechanism [5] or the mechanism of diffusion enhancement caused by cen- ter transition into excited state [9] takes place in the pro- cess under consideration. However, proceeding from re- ceived results one can conclude that the investigated re- action as well as initial state restoration after T2 optical ionization is controlled by the T1-center thermal diffu- sion, while E I therm includes E 2 value. Let us show that in this case above mentioned equality really can be valid. The kinetics of T2-center destruction is described by the system of equations: Fig. 1. Luminescence spectra of CdS:Cu crystal in initial state (1) and after reaction (2), T = 80 K. Fig. 2. TSC spectra of CdS:Cu crystal: (1) in the initial state; (2) after degradation reaction; (3,4) after electric field applica- tion at 350 K during 10 min, when the investigated region is the cathode (3) and the anode (4). N. E. Korsunskaya et al.: Electron-enhanced reactions responsible ... 4 4 SQO, 2(1), 1999 dN dt Bp 2 2= − , dp dt A N p Cp Bp2 2 2 2 2( )= − − − − , where N 2 is the T2-center density, p 2 is the hole density on T2-centers, B B e0 E / kTdif= − is the probability of T2- center diffusional dissociation, E dif is the T1 center dif- fusion activation energy, A NC E / kTe T2= −νS is the prob- ability of thermal excitation of electrons from T2-cen- ters to the c-band (v � thermal velocity of electron, S � cross-section of electron capture by T2-center, N C � state density in the c-band), and C = nvS is the probability of electron retrapping (n � free electron density). Since reaching the generation-recombination equi- librium in the system is a much faster process than T2- center destruction, it may be supposed that n = const. The solution of this system of equations may be repre- sented as N C e C e2 1 t / 2 t / 1 2= +− −τ τ , where C 1 , C 2 are integration constants, and 1 t (A B C) (A B C) 4AB 21,2 = + + ± + + − . The activation energy of T2-center destruction is de- termined by the slope of the straight line τ 1,2 (1/T). De- pending on the correlation of A, B and C, E I may take different values from E dif up to E 2 +E dif . If retrapping is available and the probability of empty T2-center destruction is much less than the probability of T2-center thermal ionization (B << A) in the tempera- ture range where p 2 << N 2 and, therefore, p 2 ∼ A, the concentration of dissociated centers ∆N 2 is proportional to B p e e2 E / kT E / kT2 dif⋅ ∼ ⋅− − and E I = E 2 + E dif . Hence, phonon-kick mechanism [5] or excited state mechanism [9] can be imitated when a reaction under ex- citation is controlled by defect thermal diffusion. To elu- cidate the real defect reaction mechanism a method using the mobile defect drift in external electric field was elabo- rated [10]. It was shown that investigation of PC and PL characteristic changes in sample areas near electrodes after electric field (E = 102 ÷ 103 V/cm) application al- lows to detect mobile defects and to identify their nature [10,11]. Defect drift activation energy which is equal to defect diffusion activation energy was obtained from the temperature dependence of drift rate [10]. Application of electric field to investigated crystals at T = 350÷400 K was found to result in accumulation of T1 centers near the cathode (Fig. 2, curve 3), which was in accordance with their donor nature [7]. This ef- fect was accompanied by the λ = 4869.5 A o bound exci- ton line intensity increase. This line is known to be caused by radiative annihilation of exciton bound to shallow donor Cd i [11]. Thus, shallow donors that participate in reaction are Cd i atoms. This donor diffusion activation energy, E dif , was found to be independent on nonequi- librium carrier density (intensity of the light) and equal to 0.4 eV. Hence, E dif ≈ E R , i.e., the reaction is really con- trolled by T1 thermal diffusion. Defect rearrangement activation energy can also be influenced by carrier trapping at centers that do not take part in defect reaction at all. This effect takes place, for instance, in donor-acceptor pair (DAP) dissociation reac- tion which is considered below. 2.2. Photosensitizing reaction in CdS x Se 1-x crystals. As it was shown earlier [2], in CdS and CdS x Se 1-x crystals dissociation of DAPs consisting of shallow donors (E d = = E C - 0.03 eV) and deep acceptors (E a = E C - 1.2 eV) occurred under intrinsic band illumination in the 150÷300 K temperature range. The reaction results in the increase of PC and intensity of λ = 1.03 µm PL band which is due to radiative recombination of free electrons at the deep acceptor. Since this acceptor is the center of photosensitivity (r-center [12]), the dissociation reaction leads to the increase of PC both in the intrinsic maxi- mum I ph int and in the extrinsic one I ph ex (Fig. 3, curves 1,2). The latter is due to excitation of electrons from r- centers to the c-band [12]. Heating the sample to 400 K Fig. 3. Photocurrent spectra of type 2 crystal at 80 K after: cooling in the dark from 400 K (1); cooling under intrinsic band illumination from 300 K (2) and followed by heating in the dark (3) to 220 (x), 230 (•), 235 (∇) and 280 K(5); cool- ing under intrinsic band illumination from 300 K and irradia- tion with IR-light at 80 K followed by heating in the dark to 230 K (4). 45SQO, 2(1), 1999 N. E. Korsunskaya et al.: Electron-enhanced reactions responsible ... and subsequent cooling in the dark results in restora- tion of the initial state. In order to obtain the activation energies of the dissociation and association processes both the kinetics of r-center density (N r ) increase under illumination and N r decrease in the dark were investi- gated at different T. N r was estimated from I ph ex value. Two types of CdS x Se 1-x crystals were found. For both types DAP dissociation activation energy was shown to be equal to 0.17 eV. In crystals with 0 < x ≤ 0.5 (type 1) DAP association in the dark was found to occur in the same temperature range (200 ÷ 230 K) as their dissocia- tion under illumination. At the same time, in crystals with 0.5 < x ≤1 (type 2), heating to 200 ÷ 235 K resulted in a very small N r de- crease and only near 280 K did an abrupt photocurrent drop take place (Fig. 3). The curves ∆I ph ex/I ph ex (max) = = f(∆t) (∆t is time interval that the sample was hold for in dark) proved to be exponential for crystals of both types and therefore, the investigated process was a reac- tion of the first order. From the slope of ln1/τ a = f(1/T) plot (τ a was time constant) the activation energies of the association process E eq = 0.17 ± 0.05 eV for type 1 crys- tals and E eq ∼ 0.8 eV for type 2 crystals were obtained (Fig. 4). So, in type 1 crystals the association activation energy coincides with the activation energy of dis- sociation, i.e. E eq = E ex in this case. This means, to all appearances, that both processes are controlled by dif- fusion of the same defect, with a diffusion activation en- ergy that is not affected by excitation. In type 2 crystals the relation E eq > E ex holds. The TSC investigation showed that in type 2 crystals (un- like in type 1 ones) deep electron traps were present, which caused a TSC peak at 300 K. The trap ionization energy E t = 0.8 eV obtained from the TSC curve coin- cides rather well with the DAP association energy (Fig. 4). It may be supposed, therefore, that the DAP Fig. 4. The association reaction kinetics for type 1 crystal (1) and type 2 crystal (2). TSC for type 2 crystal in the 240÷280 K range (3). association kinetics in type 2 crystals is controlled by a trap emptying process. To verify this supposition, the following experiment was carried out. A sample of type 2 crystal was first cooled from 300 to 80 K under intrin- sic illumination. The intrinsic light was then switched off and the sample was illuminated with IR light to ion- ize deep traps and consequently to recharge the r-cen- ters. Emptying traps was controlled by TSC measure- ments. It was found that after IR illumination heating to 220 ÷ 230 K resulted in a considerable decrease of N r (Fig. 3). At the same time IR light did not affect the as- sociation process in type 1 crystals. So, illumination-enhanced diffusion does not take place in the investigated process. The relation E eq > E ex in this case occurs because the acceptor can recharge only after deep electron trap ionization. In principle, donor itself can play the role of trapping center. Let us show that in this case equality E eq = E ex + E dif can take place. A set of equations which describes the association re- action, i.e., the process of the decrease of isolated donor density, is: dN dt BN Nd d + a -= − , dN dt BN N A(N N ) CNd + d + a - d d + d += − + − − , dN dt dN dt d DAP= − , where A C N ed C E / kTd= − , B = 4πDr 0 N a -, C = nC d , N d , N a and N DAP is the donor, acceptor and DAP density, re- spectively, D is the diffusion coefficient of the mobile component, (N N )d d− + the density of donors filled with electrons, C d the coefficient of electron capture by the donor, N C is the density of states in the c-band, n � the free electron density and r 0 is the largest distance between donor and acceptor centers at which association is still possible. Consideration similar to that carried out for degrada- tion reaction shows that when strong retrapping is present and C >> A the equation E eq = E d + E dif will take place. 3. Conclusions Carrier trapping is shown to be able to influence essen- tially on defect reaction in semiconductors. When deep traps are present, the kinetics of defect reaction can be controlled by thermal ionization of these traps. In this case illumination (injection) enhanced diffusion, in par- ticular, by the phonon-kick mechanism may be imitat- ed. This is the case which is proved to take place in CdS and CdSSe crystals. Defect drift in an external electric field is proposed as an effective method for defect reac- tion mechanism study. N. E. Korsunskaya et al.: Electron-enhanced reactions responsible ... 4 6 SQO, 2(1), 1999 References 1. O. V. Bogdankevich, N.N. Kostin, E.M. Krasavina, I.V. Kruk- ova, E.V. Markov, E.V. Matveenko, V.A. Teplitsky, Izv. Akad. Nauk SSSR, Neorg. Mater. 23(10), pp. 1618-1622 (1987). 2. M. K. Sheinkman, N.E. Korsunskaya, I.V. Markevich, T.V. Torchinskaya, The recharge-enhanced transformations of donor-aceptor pairs and clusters in CdS // J. Phys. Chem. Sol. 43(5), pp. 475-479 (1982). 3. D. V. Lang, Recombination-enhanced Reactions in Semi- conductors // Annu.Rev.Matt.Sci., Palo Alto, California 12, pp. 377-400 (1972). 4. M.K. Sheinkman and L.C.Kimerling, The mechanism of elec- tronically enhanced defect reactions in semiconductors, in Defect Control in Semiconductors, eds. K.Sumino, Elsevier Science Publishers B.V., North-Holland (1990). 5. J. D. Weeks, J. C. Tully, L. C. Kimerling, Theory of Recombina- tion-enhanced Defect Reactions in Semiconductors // Phys.Rev. B12(8), pp. 3286-3292 (1975). 6. N. E. Korsunskaya, I. V. Markevich, T. V. Torchinskaya, M. K. Sheinkman, Photosensitivity Degradation Mechanism in CdS:Cu Single Crystals // Phys. stat. sol. (a) 60(1), pp. 565-572 (1980). 7. N. P. Baran, B. R. Dzhymayev, N. E. Korsunskaya, I. V. Mar- kevich, L.Yu. Khomenkova, E.P.Shulga, Investigation of EPR spectra of shallow donors responsible for electronically enhanced defect reactions in CdS // Fiz. Tverd. Tela 38(6), pp. 1735-1741 (1996). 8. N. S. Koparanova, A. Drangazhova, Thermal and Optical Resto- raiton of Photoelectric Sensitivity of CdS Single Crystals after Optical Degradation // Compt. Rendus de l�Academie Bulg. Sci., 30(7), pp. 989-992 (1977). 9. M. K. Sheinkman, New explanation of recombination-enhanced phenomena in semiconductors // Pisma v Jurn. Expe. i Tech. Fiz. 38(6), pp. 278-279 (1983). 10. N. E. Korsunskaya, I. V. Markevich, T. V. Torchinskaya, M. K. Sheinkman, Investigation of defect drift in electric field in CdS:Li crystals // Fiz. Tekn. Poluprov. 13(3), pp. 435-440 (1979). 11. N. E. Korsunskaya, I. V.Markevich, T. V.Torchinskaya, M. K. Sheinkman, Electrodiffusion of shallow donors in CdS crys- tals // J. Phys. p. 13(4), pp. 1275-1278 (1980). 12. V. E. Lashkarev, A. V. Lyubchenko and M. K. Sheinkman, Ner- avnovesnye Procesy v Fotoprovodnikakh, Naukova dumka, Kiev (1981).

Electron-enhanced reactions responsible for photoluminescence spectrum change in II-VI compounds

Репозитарії

Схожі ресурси