Semiconductors, Vol. 37, No. 9, 2003, pp. 1042–1046. Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 9, 2003, pp. 1067–1071.Original Russian Text Copyright © 2003 by Ushakov, Klevkov.

ELECTRONIC AND OPTICAL PROPERTIESOF SEMICONDUCTORS

Microphotoluminescence Spectra of Cadmium Telluride Grown under Nonequilibrium Conditions

V. V. Ushakov* and Yu. V. KlevkovLebedev Physical Institute, Russian Academy of Sciences, Leninskiœ pr. 53, Moscow, 119991 Russia

*e-mail: ushakov@mail1.lebedev.ruSubmitted November 10, 2002; accepted for publication December 4, 2002

Abstract—Microphotoluminescence spectroscopy and imaging were used to study impurities and defects inCdTe crystals grown by nonequilibrium techniques. The growth procedure includes low-temperature synthesisand purification of the material via congruent sublimation, with subsequent deposition under the conditions ofgas-dynamic vapor flow and high-rate low-temperature condensation. Although the growth conditions arehighly nonequilibrium, the obtained polycrystalline material with a grain size of 1–2 mm exhibits strong low-temperature exciton luminescence, whose intensity is nearly uniform over the bulk of the ingots. At the sametime, it is found that residual impurities and defects have a tendency to accumulate to form clusters within cer-tain areas which are a hundred micrometers in size; the density of some impurities in these clusters is suffi-ciently high. © 2003 MAIK “Nauka/Interperiodica”.

1. INTRODUCTION

Over the years, cadmium telluride has been an inter-esting object for physical studies and a promising mate-rial for technology. Its application potential has beengrowing steadily and now includes optoelectronics,X-ray and γ-ray spectrometry, and solar-cell manufac-turing [1]. However, despite progress in materials sci-ence, it is not always possible with up-to-date technol-ogy to ensure proper control over the processes respon-sible for the content of impurities and defects and thestructure of the crystal forming during growth. It wasshown [2] that CdTe with superior optical and electricalparameters can be produced by low-temperature syn-thesis and sublimation purification of the compound,with its composition being brought to the lowest-pres-sure point. However, it turned out that the materialobtained still incorporates a number of backgroundimpurities, whose content in some cases exceeds theircontent in the source materials of 99.9999% purity. Toenhance the purification efficiency, in the present studywe used nonequilibrium processes occurring under theconditions of congruent sublimation and gas-dynamicvapor flow in a reactor [3]. Combined with deep purifi-cation of source materials, this technique enabled us, atthe output, to obtain polycrystalline ingots of CdTewith the lowest possible deviation from stoichiometryand a concentration of major background impurities notexceeding 1015 cm–3. The rate of vapor deposition attemperatures around 600°C amounted to ~500 µm/h,which is an order of magnitude larger than the close-to-equilibrium values typical of conventional tech-niques. The properties of the material obtained werestudied by low-temperature microphotoluminescencespectroscopy and imaging; these methods combinehigh sensitivity to the presence of impurities and

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defects in the crystal with the possibility of carrying outlocal analysis.

2. EXPERIMENTAL

The samples under study were [111] textured poly-crystals with a single-crystal grain size of 1–2 mm.They were cut parallel to the 111 planes; the surfacewas treated by grinding, polishing, and etching in a bro-momethanol solution. Subsequently, the grain bound-aries and the structure defects within grains wererevealed by treatment with the selective etchant E-Ag-1.The main structural defects revealed by chemical etch-ing were grain boundaries, twin boundaries, and iso-lated dislocations with a density of less than 103 cm–2

within the grain. According to the Hall data, at roomtemperature, the samples have p-type conductivity anda resistivity of 103–104 Ω cm.

After carrying out measurements of as-grown sam-ples, they were subjected to 70-h annealing in an atmo-sphere of saturated Cd vapor at 700°C.

The measurements were carried out using an auto-mated microphotoluminescence scanner, which makesit possible to record luminescence spectra from a givenspot on the sample surface and luminescent images ofthe surface at a given wavelength. The samples weremounted on a cold finger in a cryostat; their tempera-ture could be varied from 100 to 300 K and was moni-tored by a copper–constantan thermocouple. The lumi-nescence was excited by the radiation of a He–Ne laser(632.8 nm) using a focusing optical system. The typicalpower of excitation radiation on the sample surface was2.5–3.0 mW, with the spot diameter being 8–10 µm;optical filters were used to reduce the excitation level.The sample emission in the wavelength range up to

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MICROPHOTOLUMINESCENCE SPECTRA OF CADMIUM TELLURIDE 1043

1 µm was spectrally analyzed by an MDR-12 fast grat-ing monochromator (1200 grooves/mm) with recipro-cal linear dispersion of 2.4 nm/mm and detected by acooled FÉU-83 photomultiplier; the output signal waslock-in detected and processed by a computer. Theposition of the excitation spot on the sample surfacewas monitored visually using a microscope-basedguidance system. The cryostat was mounted on a spe-cial table whose motion was controlled by computercommands. Scanning was performed by translating thetable along two mutually perpendicular axes normal tothe laser-beam direction, which was kept constant. Thestep size was varied according to the requirements ofthe experiment; usually, it amounted to 25 µm. In thecourse of a spatial scan at a given wavelength, the datawere output on the display using color representation ofthe luminescence intensity; at any spot of interest, thescan could be paused to record the emission spectrum.Below, the luminescence spectra obtained in this studyare given with the corrections made to account for thespectral sensitivity of the setup.

3. RESULTS AND DISCUSSION

Excitonic lines were dominant in the low-tempera-ture spatially integrated luminescence spectra of thesamples, while the intensity of the long-wavelengthimpurity and defect bands was low; this is evidence of

A

B

CD

E

F

G

H

I

Fig. 1. The structure of the 1.8 × 1.2-mm single-crystalgrain under study; the points to which the luminescencespectra shown below correspond are indicated.

SEMICONDUCTORS Vol. 37 No. 9 2003

the high quality of the material. The measurementsreported here were carried out for the region of thegrain of 1.8 × 1.2 mm in size, whose photograph isshown in Fig. 1; we indicate in Fig. 1 the spots fromwhich the luminescence spectra shown below wererecorded. The crystal surface was “aged” and coveredwith a stable oxide layer; thus, there was no lumines-cence “degradation,” i.e., no decrease in the emissionintensity with time as a result of the action of a focusedlaser beam [4]. All the data reported below correspondto a sample temperature of 100–110 K; in this case,radiative transitions involving relatively deep-levelimpurity and defect centers become most important.

When comparing the spatially resolved and inte-grated luminescence spectra, one should take intoaccount that the difference in the excitation level forthese two cases may be several orders of magnitude.If the excitation-level dependences of the intensities ofdifferent bands are not the same [5], the shapes of thespectra recorded by the two methods noted may differfrom each other considerably. In particular, a higherintensity of exciton lines in comparison with the impu-rity and defect lines differentiates between cathodolu-minescence and microphotoluminescence spectra; thiscan be easily verified by defocusing the excitationbeam. Taking into account the power of the laser beamincident on the sample, the absorption coefficient forthe laser radiation, the spot size, and the literature dataon the lifetime and diffusion length of nonequilibriumcharge carriers, we estimate the maximum carrier densityin the excitation region to be as high as 1018 cm–3. Thisvalue is 1.5–2 orders of magnitude higher than the exci-tation level typical of conventional, spatially integratedmeasurements.

Figure 2 shows a representative set of microphoto-luminescence spectra recorded at different points of the

1.351.401.451.501.551.601.65Photon energy, eV

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Fig. 2. A representative set of microphotoluminescencespectra recorded at points A–I of the single-crystal grainunder study (see Fig. 1) at 100 K.

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crystal under study. A line of edge (exciton) emission at1.578 eV is present in all the recorded spectra, the dis-tribution of its intensity over the sample surface beingrelatively uniform (variation by no more than a factorof 2–3). In contrast, the impurity- and defect-relatedemission band at 1.4 eV has a high intensity only insome local areas of ~100 µm in size (see below). In thespectra recorded from these areas at the highest excita-tion level, this band is peaked at 1.48 eV. The bandwidth at half-maximum is 80 meV, and its shape corre-sponds to the envelope of the Poisson distribution ofintensity in a typical “self-activation” band with a

1.351.401.451.501.551.601.65Photon energy, eV

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950

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Fig. 3. Variation in the microphotoluminescence spectrawith the excitation level; the spectra correspond to theregion of intense impurity–defect emission around point Ain Fig. 1.

0 0.2 0.4 0.6 0.8 1.0

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Fig. 4. Excitation-level dependences of (1) the peak spectralposition and (2) the intensity of the impurity–defect band at1.4 eV; curve 3 shows the relevant dependence of the inten-sity for the edge emission band at 1.58 eV. The plot is com-piled from the data in Fig. 3.

Huang–Rhys factor of about 2 [1, 6–10]. When, usingoptical filters, the excitation intensity was reduced(without varying the laser-spot diameter) to 1% of itsinitial level (i.e., to the level characteristic of spatiallyintegrated measurements), the position of the excitonband remained unchanged and the peak of the defect–impurity band shifted steadily from 1.485 to 1.433 eV(Fig. 3). It is noteworthy that this shift was not accom-panied by a change in the band shape and the results ofmicroluminescence mapping of the grain were identi-cal for the highest and the lowest excitation levels.Thus, the shift observed does not result from the factthat several overlapping bands with different depen-dences of intensity on the excitation level were mea-sured. Specifically, this shift represents an intrinsicproperty of the radiative transitions involved. Takinginto account all the spectral characteristics of the bandunder consideration, it is reasonable to attribute it to theemission of donor–acceptor pairs involving shallowdonors and A-center acceptors (VCd–D) [11, 12]. In thiscontext, the position of the band peak at 1.433 eV in thecase of the lowest excitation level (which correspondsto the common case of spatially-integrated lumines-cence) is quite typical. The unusually large shift of theband position with increasing excitation level (50 meVin comparison with 5–10 meV usually observed [6, 13])is due to the very high excitation level attained and tothe local nature of the luminescence measurements car-ried out in the regions of the crystal with a high densityof recombination centers. We can see from Fig. 4 that,with an increase in the excitation level, there appears aclear trend toward flattening-out in the dependences ofthe shift and the intensity of the 1.4-eV band on theexcitation level. Most likely, such behavior is relatedboth to the saturation of the donor–acceptor recombina-tion channel itself and to the rapidly increasing compe-tition from the efficient exciton radiative channel(where the excitation-level dependence of the recombi-nation rate is nearly quadratic).

On the basis of the literature data on the position ofthe zero-phonon lines (ZPLs) in the “self-activation”band for various elements [1, 7, 14, 15], we may try toidentify the local centers responsible for microphotolu-minescence emission. We should use the resultsobtained for the lowest excitation level, make allow-ance for the temperature shift, and take into account thetypical position of ZPLs with respect to the peaks ofself-activation bands in CdTe. The value for the low-temperature position of the ZPL thus obtained is~1.47 eV. This value corresponds to A-centers formedwith the participation of group-VII elements, chlorinebeing the most probable impurity among them.

Luminescence mapping at the 840-nm wavelengthof a 2.5 × 2.5 mm region including a single-crystalgrain shown in Fig. 1 indicates that the intensity of thedefect- and impurity-related band under considerationis high only over certain limited areas of 100–150 µmsize, mainly located relatively close to the grain bound-aries (see Fig. 5). We do not yet understand in detail

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MICROPHOTOLUMINESCENCE SPECTRA OF CADMIUM TELLURIDE 1045

how these precipitates form during the growth process.It can only be stated with confidence that neither iso-lated dislocations nor twin boundaries (points C, E, andI in Fig. 1) act as the nucleation centers. A slight non-systematic increase in the impurity-band intensity wasobserved in the neighborhood of the grain boundaries.In the spectra of grain-inside regions, only an intenseedge band is typically present. However, these regionsare also not entirely free of defects. Analyzing the spec-tra of these regions (points D, F, G, and H in Fig. 1)recorded on a considerably expanded sensitivity scale,we observed weak bands peaked at 1.49, 1.47, 1.44,1.41, and 1.36 eV under our experimental conditions).A demonstration set of the corresponding spectra isshown in Fig. 6. These bands are apparently related tothe radiative transitions to different levels of the accep-tors with an energy Ev + 0.15 eV [1, 16], which, thus,

50150300450600750900In

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00 500 1000 1500 2000 2500

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Fig. 5. Microphotoluminescence map of the 2.5 × 2.5-mmarea of the sample including the single-crystal grain shownin Fig. 1; the scan is recorded at a wavelength of 840 nm.

D

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T = 107 K

1.55 1.50 1.45 1.40 1.35 1.30 1.25

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Fig. 6. A demonstration set of the spectra of background impu-rity–defect luminescence of the material under study (points D,F, G, and I within the single-crystal grain, see Fig. 1).

SEMICONDUCTORS Vol. 37 No. 9 2003

form a weak impurity–defect background in the spectraof the crystals under study.

To examine the response of the ensemble of crystalimpurities and defects to thermal treatment, the sam-ples were subjected to annealing at 700°C in an atmo-sphere of saturated Cd vapor. Such a treatment leads toa 100-fold increase in the intensity of all spectral bands.In addition, a new band appears at ~1.41 eV; unfortu-nately, it overlaps strongly with the other bands andthey cannot be analyzed separately. In general, thesechanges were typical of the spectra from any point onthe crystal surface. Thus, on the one hand, the annealingnoted causes a substantial decrease in the density ofnonradiative recombination centers in the material; onthe other hand, it causes the formation of new impu-rity–defect centers. However, the former effect is sostrong that it makes it possible to observe crystal lumi-nescence at room temperature; specifically, the edgeband peaked at 1.502 eV (this band is due to the con-duction-to-valence band transitions and the excitontransition with its phonon replicas [17]).

4. CONCLUSION

Thus, we demonstrated that microphotolumines-cence can be efficiently used to study the impurity anddefect content and the structure of CdTe crystals grownby nonequilibrium techniques, including low-tempera-ture synthesis from deeply purified components withthe subsequent purification of the compound beinggrown under the conditions of congruent sublimation,gas-dynamic vapor flow, and high-rate low-temperaturecondensation. A key feature of the studies performedwas the need for local analysis of the material obtained.Although the material possesses a polycrystalline struc-ture with grains 1–2 mm in size, it exhibits strong low-temperature exciton luminescence with rather uniformintensity distribution over the bulk of the ingots. It isevident, however, that, due to the large deviation fromequilibrium during the growth, the background impuri-ties are distributed inhomogeneously; there is a ten-dency toward accumulation of impurities and defects inregions, about 100-µm-size, with a high density of cer-tain impurities (chlorine). The detailed mechanism ofthis phenomenon is not yet understood. It can be saidwith confidence that neither isolated dislocations nortwin boundaries act as the nucleation centers. A slightnonsystematic increase in the impurity-band intensitywas observed in the spectra recorded from regionsclose to the grain boundaries. Apparently, the accumu-lation of impurities in these regions leads to some puri-fication of the grain bulk from impurity- and defect-related recombination centers. The luminescent proper-ties of the crystals can be enhanced considerably bylong-term annealing in a Cd-vapor atmosphere. Theinfluence of the grain boundaries and other aggrega-tions of extended defects on the ensemble of impuritiesand defects in the crystals under study will be consid-ered in detail in a subsequent publication.

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ACKNOWLEDGMENTS

We are grateful to Prof. A.A. Gippius and Prof.V.S. Bagaev for useful comments and their interest inthis study.

The study was supported by the Russian Foundationfor Basic Research (project nos. 01-02-16500 and00-02-17521) and the Program of Support of the Lead-ing Scientific Schools of the Russian Federation(project no. 00-15-96568).

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Translated by M. Skorikov

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