Spectroscopic Signatures of Novel Oxygen-Defect Complexes in Stoichiometrically Controlled CdSe

Transcription

1 Physis Physis Researh Publiations Purdue University Year 008 Spetrosopi Signatures of Novel Oxygen-Defet Complexes in Stoihiometrially Controlled CdSe G. Chen J. S. Bhosale I. Miotkowski A. K. Ramdas This paper is posted at Purdue e-pubs. artiles/996

2 PRL 0, 9550 (008) P H Y S I C A L R E V I E W L E T T E R S 7 NOVEMBER 008 Spetrosopi Signatures of Novel Oxygen-Defet Complexes in Stoihiometrially Controlled CdSe G. Chen,* J. S. Bhosale, I. Miotkowski, and A. K. Ramdas Department of Physis, Purdue University, West Lafayette, Indiana 7907, USA (Reeived 0 September 008; published 5 November 008) Growth of single rystals of CdSe with oxygen, introdued by stoihiometri ontrol to suppress the formation of native Se and Cd vaanies, generates oxygen enters replaing Cd (O Cd ) rather than Se (O Se ) as expeted. This antisite substitution is unambiguously singled out by the host isotope fine struture of the nearest neighbor (NN) Se atoms in the loalized vibrational modes (LVMs) of O Cd. When the stoihiometry ontrol favors the formation of Cd vaanies, three infrared signatures, and appear asribable to the LVMs of O Se in assoiation with a Cd vaany in the NN position as (O Se V Cd ) enters. Polarization measurements establish the monolini C s symmetry for these enters. As a funtion of temperature, they display a remarkable two-step symmetry transformation, C s! C v! T d, due to the dynami swithing of the O Se V Cd dangling bond. DOI: 0.0/PhysRevLett PACS numbers: 6.7.J, 6.0.Pw, 78.0.Fs In ompound semiondutors, unlike in the elemental, stoihiometry needs to be stritly ontrolled in order to ahieve rystal growth free from imperfetions suh as ation or anion vaanies, interstitials and antisite substitutions. This is often a prerequisite for the deliberate inorporation of speifi impurities in desired onentrations. Native defets often behave like donors or aeptors; Cd vaanies in CdTe and Se vaanies in CdSe [,] are reported to be aeptors and donors, respetively. Under ertain irumstanes a light substitutional impurity in assoiation with the vaany displays unique infrared signatures harateristi of loalized vibrational modes (LVMs). We reently reported [] suh LVMs of oxygen imputities in CdTe in whih oxygen substitutionaly replaed a tellurium host atom (O Te ), the nearest neighbors (NN) then being the four Cd atoms as O Te, or in assoiation with a Cd vaany in one of the NN site as (O Te V Cd ). The former has T d site symmetry with a single triply degenerate LVM whereas the latter displays two LVMs onsistent with its C v symmetry []. The striking temperature behavior of the doublet in whih the two omponents approahed and oalesed showed that the (O Te V Cd ) dangling bond swithed its ½Š orientation among the four hi diretions and the (O Te V Cd ) omplex thus aquired a temperature averaged T d symmetry above T, the temperature of oalesene. In order to disover additional examples of suh an extraordinary behavior, we have investigated CdSe, with Cd s bonded to Se s and Se s to Cd s in a tetrahedral oordination resulting in a wurtizite rystal with C 6v spae group symmetry. By ontrolling the stoihiometry, our aim was to suppress the native Se vaanies and enourage the formation of O Se enters tetrahedrally bonded to Cd s in NN positions; suh O Se enters would have C v symmetry. However, the inorporation of exess Se in a onentration in exess of that of the native Se vaanies would result in Cd vaanies and (O Se V Cd ) enters. Crystals of CdSe were grown with these strategies. In this Letter we report the surprising nature of the oxygen related LVMs in CdSe disovered in a high resolution study using Fourier transform spetrometer. The absorption spetra were reorded using an ultrahigh resolution BOMEM DA. Fourier transform infrared (FTIR) spetrometer apable of an ultimate unapodized resolution of 0:006 m. A HgCdTe-infrared detetor for the m range and an InSb detetor for m range were employed. A Janis 0DT Supervaritemp ontinuous-flow optial ryostat allowed measurements in the range of :8 00 K and a speially designed high temperature ell in the range of K. The orientation of the uniaxial rystals was determined by the Laue bak-refletion method. A wire grid polarizer on a AgBr substrate was used in the measurements requiring polarized radiation. Single rystal of CdSe was grown with the addition of CdO to provide oxygen and exess Se to suppress the ourrene of Se vaanies [,5]. The infrared spetrum of CdSe prepared in this manner displays two LVMs and entered at 99:77 m and 00: m respetively, when reorded at a relatively low resolution and unpolarized inident light. Beause of the uniaxial nature of the host rystal, a substitutional impurity is expeted to display a loal symmetry of C v hene two infrared ative LVMs, one polarized parallel to ^, the opti axis and the other? ^. Under the high resolution employed in Fig., both and display a remarkable host isotope fine struture strikingly similar to that for Mg Cd in CdSe [6]. As it is dominated by the natural isotope abundane and the atomi masses of the NN atoms, the host isotope fine struture bears unmistakable similarity for different impurities with the same nearest neighbors in the same host [7,8]. Has oxygen replaed Cd as O Cd rather than Se as =08=0(9)=9550() Ó 008 The Amerian Physial Soiety

3 PRL 0, 9550 (008) P H Y S I C A L R E V I E W L E T T E R S.5 Res.=0.0m - (a) E 0 Res. = 0.0 m - (b) 7 NOVEMBER 008 E Absorption Coeffiient (m - ).5 µ Absorption Coeffiient (m - ) 8 6 µ FIG. (olor online). The fine struture of the O Cd LVMs in CdSe for (a) E k ^ and for (b) E? ^, where E is the eletri vetor of the inident light and ^, the opti axis. O Se? In the former ase, the fine struture and its similarity with Mg Cd an be explained, sine in both ases, the fine struture is assoiated with NN Se atoms set in vibratory motion by the LVMs of O Cd or Mg Cd. Therefore O Cd as an antisite replaement is the lear but surprising hoie based on these experimental observations. The large frequenies of and are yet another indiation that oxygen has entered CdSe, replaing Cd as an antisite impurity, not Se as an isoeletroni impurity. If oxygen replaes Se, the LVMs of O Se are expeted in the spetral range of m from the knowledge of the LVMs of S Se in CdSe [9]. When measured with a Si bolometer whih overs this spetral range, no evidene for O Se was found. The large frequenies an well be attributed to four extra eletrons surrounding the O Cd enter whih signifiantly inrease the effetive fore onstants responsible for and. The observation of a single pair of and, and its intensity proportional to the CdO added to the starting material during rystal growth, rules out Se Cd instead of O Cd being the ause for and. Experiments with inident radiation polarized parallel to the ^ axis, E k ^, redued dramatially as ompared to, whereas for radiation polarized perpendiular to the ^ axis, E? ^, also dereased to zero. On the basis of these polarization measurements, we assign to the loalized vibration parallel to the ^ axis and to vibration perpendiular to ^ as indiated in Fig.. In the samples of CdSe grown with CdO to provide oxygen and an additional large amount of Se not only to suppress the existing Se vaanies but also to produe Cd vaanies, three absorption peaks are observed at ¼ 09: m, ¼ 07:5 m, ¼ 6: m without and appearing. We propose that in these samples oxygen substitutionally replaes Se (O Se ) in the viinity of a Cd vaany (V Cd ). Beause the host rystal has wurtzite struture, the NN vaany along ^ is different from those at one of the other three sites. In the former onfiguration, the (O Se V Cd ) enter has C v symmetry and will display two infrared ative modes, one along ^ and the other, perpendiular to the ^ axis. A Cd vaany an also oupy any one of the three NN sites in the plane normal to ^ whih are haraterized by a dangling bond not parallel to the ^ axis; suh (O Se V Cd ) enters have a C s loal symmetry and should exhibit three infrared ative modes. Sine three infrared ative modes are observed experimentally, we hoose C s symmetry for the (O Se V Cd ) enters hene the atomi onfiguration shown in Fig.. Consider the symmetry of an (O Se V Cd ) enter shown in Fig.. It has a plane of refletion whih ontains a Cd atom in position, an oxygen impurity (O Se ) and a Cd vaany (in this ase, at position ). In two of the three infrared ative modes O Se will vibrate in the plane of symmetry ( plane) as indiated in Fig., while in the third it will vibrate perpendiular to the plane hene perpendiular to the ^ axis [0]. Thus,, and will be polarized along three diretions, two in the plane and the third?

4 PRL 0, 9550 (008) P H Y S I C A L R E V I E W L E T T E R S 7 NOVEMBER 008 Absorption Coeffiient (m - ) 5 Res. = 0. m - (a) E FIG. (olor online). A three-dimensional model for the (O Se V Cd ) enter with Cd vaany at, two of the nearest neighbor Cd atoms at and and third at on ^. Also shown is the plane of refletion,, on whih the Cd atom at, O Se, and the Cd vaany at are loated. Oxygen vibrations for and are along Rð Þ and Rð Þ lying on and Rð Þ normal to. Rð Þ, Rð Þ, and Rð Þ are mutually perpendiular. Given the (O Se V Cd ) enter with its C s symmetry, the intensities of,, and differ from one to another. The different (O Se V Cd ) enters will all have Cd-O axes parallel to ^ but their -planes defined by Rð Þ at 0 with respet to one another, distributed randomly over the rystal. Distributed into three equal populations of defet orientations defined by the Cd vaany in position,, or, the s will display a orresponding orientational degeneray []. Therefore, averaged over the rystal as a whole,,, and will display isotropy when viewed along ^. This effet is diretly demonstrated by the indistinguishable absorption spetra reorded with radiation inident along ^ and polarized along two mutually orthogonal arbitrary diretions in the plane normal to ^. Consider the infrared ative mode in whih oxygen vibrates perpendiular to the plane hene perpendiular to ^ irrespetive of the position of the Cd vaany. Suh a mode is ompletely polarized for E? ^, but will not ouple to it for E k ^. This predition is diretly onfirmed as shown in Fig.. The mode beomes extremely weak when the inident light is polarized k ^, while it is the most intense when the inident light is polarized? ^ (the very Absorption Coeffiient (m - ) Res. = 0. m small intensities of for E k ^ an be attributed to the finite onvergene of the radiation inident on the rystal). Based on these experimental observations, we assign to the infrared ative mode in whih oxygen vibrates? ^. Depending on the angles they make with respet to the ^ axis, and will differ in their intensities for E k ^ ompared to E? ^ as shown in Fig.. The temperature dependene of the frequenies of,, and for the (O Se V Cd ) enters is remarkable. As the temperature inreases, they do not hange signifiantly from 5 to 00 K, beyond whih, inreases, whereas and derease. As seen in Fig. (d), with inreasing temperature, and approah eah other and oalese into a single, doubly degenerate mode at T 80 K. We propose that a dynami swithing of the O Se V Cd dangling bond among the three equivalent positions,, and ours and transforms the site symmetry of the oxygen-vaany omplex from C s to C v as the O Se V Cd bond swithing ours at an inreasing rate and the (O Se V Cd ) defet enter inreasingly aquires C v sym- (b) FIG. (olor online). The figure shows the polarizations of,, and, with the diretion of propagation of the inident radiation, k, perpendiular to ^, and the eletri vetor, E, parallel to ^ in (a) and perpendiular to ^ in (b). E 9550-

5 PRL 0, 9550 (008) P H Y S I C A L R E V I E W L E T T E R S 7 NOVEMBER 008 N= N= C s O Se V Cd C v (a) (b) () Temperature (K) FIG. (olor online). Shemati energy level diagrams and the fundamental dipole transitions in (a) (O Se V Cd ) enter with C s site symmetry, (b) (O Se V Cd ) enter with dynami swithing of the dangling bond inluded and symmetry of the enter transforms from C s to a C v site symmetry, () (O Se V Cd ) enter with dynami swithing of the dangling bond inluded and the enter aquires a quasi-t d site symmetry. (d) The temperature dependene of,, and for the (O Se V Cd ) enter from 5 to 600 K. metry; this aquired symmetry is refleted in the normal mode also to vibrate normal to the ^ axis resulting in a doubly degenerate mode of C v symmetry as shematially shown in Fig. (b). With a further inrease in temperature, dereases while inreases slightly and merges with, i.e., to beome a single, triply degenerate mode at T T d (d) 560 K, i.e., at higher temperatures, the O Se V Cd dangling bond of the (O Se V Cd ) enter swithes rapidly among all the four Cd positions,,, and with equal probabilities, and the net symmetry of the (O Se V Cd ) enter effetively transforms into a quasi-t d site symmetry as shematially represented in Fig. (). The work reported in this Letter undersores the important role stoihiometry plays in the impurity onfigurations in ompound semiondutors. The antisite replaement of oxygen (O Cd ) unambiguously revealed in the host isotope fine struture expeted for its LVMs, the atomi onfiguration and site symmetry of (O Se V Cd ) enters uniquely revealed by the polarization measurements and a two-step symmetry transformation produed by inreasing temperatures, attributed to the dynami swithing of the dangling bond are the highlights of the present work. The work reported in this Letter was supported by the National Siene Foundation (DMR and DMR ). J. S. B. reeived a grant from the Purdue Researh Foundation. We thank Professor Sergio Rodriguez for many stimulating disussions. *Present address: Department of Physis, University of California San Diego, La Jolla, California 909, USA. [] II-VI Blue/Green Light Emitters: Devie Physis and Epitaxial Growth, edited by R. L. Gunshor and A. V. Nurmikko, Semiondutors and Semimetals Vol. (Aademi Press, New York 997). [] G. Chen, I. Miotkowski, S. Rodriguez, and A. K. Ramdas, Phys. Rev. Lett. 96, (006). [] R. C. Newman, Adv. Phys. 8, 55 (969). [] I. Miotkowski, R. Vogelgesang, H. Alawadhi, M. J. Seong, A. K. Ramdas, S. Miotkowska, and W. Paszkowiz, J. Cryst. Growth 0, 5 (999). [5] F. T. J. Smith, Solid State Commun. 8, 6 (970). [6] M. D. Siaa, A. J. Mayur, H. Kim, I. Miotkowski, A. K. Ramdas, and S. Rodriguez, Phys. Rev. B 5, 878 (996). [7] W. M. Theis, K. K. Bajaj, C. W. Litton, and W. G. Spitzer, Appl. Phys. Lett., 70 (98). [8] G. A. Gledhill, R. C. Newman, and J. Woodhead, J. Phys. C 7, L0 (98). [9] R. Beserman and M. Balkanski, Phys. Rev. B, 608 (970). [0] G. Herzberg, Moleular Spetra and Moleular Struture II. Infrared and Raman Spetra of Polyatomi Moleules (D. Van Nostrand Co., Prineton, NJ, 960), p. 0. [] A. A. Kaplyanskii, Opt. Spetros. 6, 9 (96)