INTI!QOUCTION
large ern c<lIdllium telluride (CdTe) and zinc no-iull telluride (Zn Cd 1 _ X Te) single crystal substrates hllve fllllny potenthl IIpplications iA the fabrication of electroniC and optoelectronic devices II]. Interest in this wide-gap ~teri."l has also developed due to its chemical compatibility and close lattice match to the I.portant infrared detector material HgCdTe . HO'olever , the und erstanding of the native defects and their complex 'oIith residual impurities is important in order to have a better contr ol of both their optical <lind electric."l properties . In the last fe'ol years numerous studies on the photoluminescent (PL) properties of CdTe cryshls have been reported and only few on ZnxCd1_xTe (5) (6) (7) (8) .
INTI!QOUCTION
In the present pape r we r eport on the PL f r om lIS-prepared and Cd vapor ~nnealed Zn)(Cdl_~Te (OSX~. 2S) single cr ystals , For .,,11 the investigated values of x, the prominent emission lines occur In the e~citon region indicating", high cr yshlline qu""l1ty. In pure CdTe cr ystals , the 15K PL spectr "" uhibit t'olO m",in lines In this markallen@example.net. 2 nm (1.S934eV) and 779. 5 nm (1 . 5907eV) . The fo~r line occurs in the energy range where recombinlltion of excitons bound to sh""llow neutral donors (O·,X) has been reported [g) . Our studies on ""nnul.d CdTe S4mples under Cd atlllosphere indicate thllt the 1. 5907 eV line is related with recombination of excitons bound to cad .. iUII vllCllncies [51 . We also obseryed that in the IIlhed cryshls exciton rec_bination """inly occur s yh neutral donor shtes .
EXPERIMENTAL
All samples were grown fro. the llelt by a "'Odified Brid~n .-ethod at G""ltech Se-lconductor M."terlllls COrporllUon . The composition of the ZnxCdl_xTe samples was determined frOll electron .icroprobe analysis <lind frOll! the energy of the uin uciton line observed in the 15K Pl spectra [6). All as-prepared crystals were unintentionally doped and have lin appro-'hI. A ••. Soc. Symp. "roc. Vol. 181 .
EXPERIMENTAL
• , ttO MII.rI.,. A .... fell Society ximate room-temperature resistivity of 5ll0 ' and 8ll0' n cm fo r the mhed and pure crysttlls , respectively. All spectr a in the as~prepared sa mples "'ere taken on (1l0) cleaved surhces at 15K . The hser po",e r , from an argon ion laser , impinging on the sample ",as of 0. 5 mW ",ith the laser beam focused using cylindrical optics . The Pl emission ",as spectrally analyzed ",ith a double monochromator , ",ith slits set to achieve a resolution of less than lA o Annealing "'as ca r ried out in a sealed Quartz ampoule under Cd saturated isothermal conditions at 600·C for t"'o hou r s . Before the Pl ",as measured all annealed sampl es "'er e chemica l ly etched to remove the su r face damage. The chemical treatment ",as as follo",s : ifmlersion in a 1% bromine in ITI!!thanol solution for 30 seconds follo",ed by a 60 seconds ifmlersion in a solution of 3. 5M KOH in 1 liter of H 2 0.
RESULTS AND DISCUSSION
figure shows typical 15K Pl spectra for three as-p r epared ZnxCd I-X Te cleaved surfaces with x .. O, 0. 10 and 0. 25. All three spectr a a r e plotted ",ith the same abscissa . Consider ing the origin of the emission lines . the Pl spectra in the f igure can be divided into three regions : a) the exciton region , for wavelengths near t he band-gap: bl the free to bound (f-A) and bound to bound (O-A) transitions , for intermediate ~' s and, c) emission at lonler :I. ' s associated with crysttll impe r fections and deeper impurity levels 6. 10] . This latt er emission generally appea r s in the Pl spectra as a broad band at approxima t ely 50 to 100 nm above the exciton lines . Notice that none of the three spectra in fig . 1 sho"'s any significant emiSsion in this latter region , which is an indication of high crystalline Quality.
RESULTS AND DISCUSSION
The identification of the emiss i on lines in the intermediat e r egion , as due to electron-accept or and donor-acceptor t r ansitions has been reported in a previous study [11] .
RESULTS AND DISCUSSION
In the exciton region. f ree exciton recombin5 t ion results in the emission of photons ",ith an energy 10 . 5 meV less t he,n th e band gap , ",hich , at 15K is at 772 nm ( 1.606 eV) , 743 . 5 nm (1.668 eV) and 695 . 9 nm ( 1. 78lgeV) for x .. O, 0. 10 and 0. 25 , respectively. This line is denoted by (X)n .. l in the figure . The exist ence of several different donors and acceptors in CdTe has al r eady been pOinted out [12] . free excitons can get trapped by the potential of the impurities becoming bound excHons . The two main lines in the exciton region of our pure CdTe markallen@example.net. 2 nm (1.5934 eV) markallen@example.net nm (1.5907 eV) hftve been identified ftS due to the recombination of excitons bound to shallow neutral donors {DO . X) [9] ftnd to neutral acceptor (AO , X) states . respectively.
RESULTS AND DISCUSSION
The identification of the 1. 5907 eV line as due to a (A" , X) process is based on the changes observed in the Pl spectra induced by the annealing under Cd atmosphe r e . which are sho",n in Fig . 2 . The complete Pl spectra as well as the resol~ed exciton regions a r e sho",n f or as-p r epared (a) and for annealed (b) samp l es . As can be seen . two drastic changes occur upon annealing : one is that the lines in the range of 790-800 nm denoted by (F-A) and (D-A) have been drastically r educed and the other is the disappearance of the (A" . X) line . The broader bands found at 790-800nm are generally accepted to involve deep acceptorll~e le~els associated with a comple~ cented3]. and more recently t hei r origin has been related with cadmiu m vacancies [13 ] . In a re cent publication Seto et al [5] have seen that , under similar annealing conditions , the intensity of a bound e~citon 
,.
Flg, 1. Photoluminescence spectra of Zn~Cd\_~Te single crystals wlth x-o. 0.10 and 0.25. Details on the assigments and identification of various lines can be found in the text .
,.
line at 1. 5896 eV in their 4. 2K PL spectra of high quality p-type CdTe is drastically reduced after the annealing ; their results strongly suggest that this line can be ascribed to the recombination of excitons trapped at Cd vacancies . Our results in annealed CdTe crystals under a Cd atmosphere provide further evidence for the previously proposed assignments for the PL bands in the region of 790-800 nm as well as for the origin of the (A· ,X) line at 1. 5907 eV. The identification of the 1. 5907 eV line is also consistent with its temperature dependence because the PL intensity of acceptor-bound excitons decreases very rapidly with increasing temperature [11] . Figure 3 shows the PL sp'!ctra of an as-prepared (a) and Cd vapor annealed (b) ZnxCd\_x Te with x,,0.09. Similar to the pure CdTe crystals . the Pl spectra of the as-prepared mixed crystals show a dominant nearband-edge emission. In the exciton region the markallen@example.net nm (1.6463eV) has the same exciton binding energy of 2 meV as the line labeled with (O",X) in the PL spectra of pure CdTe . lending evidence that this feature is due to the same transition in both cases. The line at 756 nm (1.6402 eV) has been identified with a (A·,X) transition based on a variety of evidence. In the first place . the exciton binding energy of 7. 5 meV is close to the calculated binding energy of excitons bound to acceptors of 7 meV [14J . In the second place, this line is thermally quenched at temperatures exceeding SDK. This assignment is also in agreement with previous observations in other alloyed compound semiconductors showing that the (Ao.X) lines are much more sensitive to alloying and that the linewidth increases at a rate much greater than that of the (O . X) levels (15) . We have observed that in the exciton region . the Pl spectra of the mixed crystals look very similar regardless of x in the range investigated , tals The result of the annealing under Cd atmosphere for ZnXCd\_X Te crysis quite different than that observed in the pure crystals , In the " CdT.
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• , v," X·Q.09 ,'As -Ii'""","
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.ro Based on the annealing behavior of the Pl spectra of CdTe and Zn it Cd I-it Ie crystals. one can conclude that. in contrast to pure CdTe. the fre~ to bound . the bound to bound and the (Ao . X) bands in the mhed crystals are probably not related with recombination involving cadmium vacancy levels. These results provides further support for the observation that In addition to CdTe imp r oves crystal quality [16. 171 . particularly reducing the density of group II (Cd or Zn) vacancies .
CONCLUSIONS
The photoluminescent properties of ZnilCd -xTe single crystals for 09 'ill. 25 have been studied . To assist in the i~entHlcation of the lines in the spectra . as-prepared and annealed (under Cd atmosphere) ullples were Measured . In all crystals . the d_inant e.isslon occurs at the nearband-edge region. indicating good crystalline quality. In the pure crystal . the two main eilciton lines are identtfied with recombination of e xcitons at neutral shallow donors (778 . 2 nm) and at neutral donors (779. 5 nm). The assignment of the latter line was deduced from its annealing behavior under a saturated Cd atmosphere . We also observed that the doublet in the wavelength range of 790-800 rom disappears after the annealing. which indicates that the levels involved in that transition are aho related to cad.Ium vacancies .
CONCLUSIONS
In the as-prepared mixed crystals, we observed that the Pl spectra is qualitatively sillilar r egardleu of the values of il . By an",logy with the pure crystal . the main eilciton line is identified .. ith '" (D° , X) tranSition and a weak (A" .X ) line is also observed . From the observed changes in the PL spectra of Cd annealed mixed crystals . we concluded that the -.,in lines In the spectra are not related with transitions involving Cd vacancies.

{'paper_id': '67795317', 'title': 'Annealing Behavior of the Photoluminescence Lines in CdTe and Znx Cd1-x Te Single Crystals', 's2_url': 'https://api.semanticscholar.org/CorpusID:67795317', 'source': 'S2ORC'}