10U'NACO, CRYSTALGROWTH

Journal of Crystal Growth 138 (1994) 219-224

Photoluminescence of vapor and solutiongrown ZnTe single crystals

Y. Biao a, M. Azoulay a, M.A. George a, A. Burger *,a, W.E. Collins a, E. Silberman a,

C.-H. Su b, M.E. Volz b, F.R. Szafran b, D.C. Gillies b

a Center for Photonic Materials and Devices, Department of Physics, Fisk University, Nashville, Tennessee 37208, USA

b Space Science Lab, Marshall Space Flight Center, Huntsville, Alabama 35812, USA

Abstract

ZnTe single crystals grown by horizontal physical vapor transport (PVT) and by vertical traveling heater method(THM) from a Te solution were characterized by photoluminescence (PL) at 10.6 K and by atomic force microscopy(AFM). Copper was identified by PL as a major impurity existing in both crystals, forming a substitutional acceptor,CUZn' The THM ZnTe crystals were found to contain more Cu impurity than the PVT ZnTe crystals. The formationof CUZn- VTe complexes and the effects of annealing, oxygen contamination and intentional Cu doping were alsostudied. Finally, the surface morphology analyzed by AFM was correlated to the PL results.

1. Introduction

High purity and good quality single crystals ofZnTe have been grown by various techniquesover the years for their potential application asvisible light emitting devices. Most of them weregrown at temperatures below the melting point(1290°C) of ZnTe, which improved the crystallineperfection and reduced the possibility of contami-nation from or through crucibles. However, rela-tively high levels of unintentional dopants, partic-ularly Cu, have been usually found in the growncrystals [1,2]. Similar to other direct-gap semicon-ductors, the low temperature PL spectrum ofZnTe crystals consists of three regions: (A) shal-low-level free or bound excitons (2.25-2.39 eV);(B) donor-acceptor pair (DAP) recombination

* Corresponding author.

0022-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reservedSSDI 0022-0248(93)E0610-J

(2.10-2.25 eV); and (C) deep-level defects, suchas vacancy complexes (1.60-2.10 eV). The free-exciton recombinations were identified at 2.381eV for (X)n~l' 2.391 eV for (X)Il~2 and 2.392 eVfor (X)n~3' and a direct energy gap (Eg) of 2.3941eV was estimated by photoluminescence at 1.6 K[3]. A center is considered simple when an impu-rity or a vacancy occupies a Zn or the Te latticesite, such as CUZn and VTe, and contributes asingle additional charge carrier; its energy levelsare analogous to that of hydrogen. Consequently,the activation energy of the single carrier boundto the substitutional impurity is close to thatcalculated from the hydrogen model. The typicalemission of A?" a Cu acceptor substituting forZn atoms, has been found at 2.3746 eV in the1.6-4.0 K temperature range, although its emis-sion peak is sometimes observed to broaden andshift towards lower energy as the temperatureand Cu concentration increase [4,5]. In high qual-

220 Y Biao et al. / Journal of Crystal Growth 138 (1994) 219-224

ity ZnTe crystals, the two-hole transition, havingan energy difference A~u_A~u of 125 meV, maybe used as evidence for Cu impurity.Deep level emissions were sometimes observed

in the PL spectra of PVT grown ZnTe crystals.These levels are very broad and have too high abinding energy to be hydrogenic centers, so theyare referred to as complex centers. Besides Znvacancy and Te vacancy complexes, deep levelsassociated with 0, Cu, Mn, Pb, Sn, and someother transition metals could become part of thesecomplexes. As the binding energy of an electronor a hole bound to a single substitutional impu-rity increases, the interaction of the electron withthe lattice vibrations or the electron-phononcoupling gets stronger. In this particular case thelongitudinal optical (LO) phonon coupling is byfar the strongest because of the polarization fieldassociated with it. The phonon energies of LO(T),LO(X), LA, TO(T) and TA were measured to be26.1, 23.6, 13.6, 22.5 and 5.7 meV, respectively[3]. In this work we have performed low tempera-ture PL measurements of ZnTe crystals grown byPVT and THM in order to identify the presenceof impurities and native defects associated withthese growth techniques. The morphology of thecrystal surfaces were analyzed by AFM and corre-lations to the PL spectra were obtained.

2. Experimental procedure

Single crystals of ZnTe were grown withoutany intentional doping by two different tech-niques: horizontal physical vapor transport (PVTZnTe) and vertical traveling heater method fromtellurium solution (THM ZnTe). In order to con-firm the nature of the impurities in both the PVTand THM grown crystals, Cu was diffused intothe PVT ZnTe crystals by depositing a thin filmof Cu onto its wafers, annealing them for 1 h at250°C, and then removing the remaining Cu filmby etching. As a reference, another as-grown PVTcrystal went through the same annealing proce-dure, but without Cu deposition. Previous chemi-cal analysis data and low temperature IR trans-mission measurements indicated that the PVTZnTe.single crystals had a better overall purity

than the THM ZnTe crystals [2]. Both of themand PVT ZnTe: Cu crystals were further aged atambient for about 6 months, and then annealedunder vacuum at 600°C for 1 h. The samples wereeither cleaved or polished and etched, with a 5%bromine in methanol solution, for PL and AFMmeasurements.Low temperature PL measurements were per-

formed on crystal samples which were cooleddown to 10.6 K by a APD Cryogenic Inc systemequipped with dual HC-4MKI helium compres-sors. The 488.0 nm line, with power of 15mW/mm2, from an ILT 5500A air-cooled argonion laser was selected for excitation. The PLspectra were recorded using a Spex 1877DTriplemate Spectrophotometer and a liquid nitro-gen cooled CCD detector. In the spectrographstage of the spectrometer, a 30 JLm slit and a 300grooves/mm grating were employed.The surface morphology of the single crystals

was studied by atomic force microscopy (AFM),using a Digital Instruments Nanoscope II,equipped with piezoelectric tube scanners allow-ing imaging of cleaved or polished crystals fromatomic resolution up to 130 JLm maximal scans.The cantilevers were commercial nanoprobes,made of gold coated silicon nitride with a forceconstant of 0.06 N/m.

3. Results and discussion

Fig. 1 shows the full range photoluminescencespectra of PVT ZnTe crystals at 10.6 K: annealedat 250°C (Fig. la), aged for about 6 months (Fig.Ib), and annealed under vacuum at 600°C for 1 h(Fig. lc). In the spectrum shown in Fig. Iii, theprincipal bound exciton (PBE) at 2.3746 eV(FWHM = 1.0 meV) has been attributed to theexciton bound to Cu acceptor, A~u. The intervalon the lines above the spectra represent the suc-cessive LO-phonon replicas. The LO-phononreplica peaks and two-hole transition (A~U) at2.2495 eV are relatively sharp and strong to beidentified together with A~u. The energy differ-ence between A~u and A~u, known as Ecu(ls)-

Ecu(2s) for a hydrogenic model, is the same asthat found in the literature [3,5]. The lines la-

Y Biao et al. / Journal of Crystal Growth 138 (1994) 219-224

L6 1.8 2.0 2.1 2.2 2.3 2.4

ENERGY (eV)

Fig. 1. Photoluminescence spectra of a PVT ZnTe sample, at10.6 K; (a) annealed at 2S0°C, (b) aged at ambient for about 6months, and (c) annealed under vacuum at 600°C for 1 h.

beled Cnare also observed in the spectrum and

probably involve excitons bound to Cu complexes[5]. The lines C2, C6 and C7 resulted from anexciton bound to a common Cu-related neutral("isoelectronic") trigonal complex defect [6]. TheC4 and Cs lines appear at positions differentfrom ref. [5]. The donor-acceptor pair (DAP)band and its LO-phonon replica were interpretedas due to the recombination between a donor andCu acceptor defect levels, labeled as (DO,AOc).Additionally, the occurrence of an asymmetricbroad band at 1.72 eV indicates that more thanone kind of vacancy complex may be present,CUZn-VTe and VZn-VTe are two possible complexcenters causing the asymmetry. In Fig. Ib, thepeak broadening due to aging at ambient reducesthe sharpness of the A~u and C

nseries, and the

FWHM of the PBE peak is broadened to 4.5meV, for example. When the temperature of thesample was increased to 30 K, the FB line be-came more intense while the DAP band de-creased in intensity. A slightly more symmetricdeep level band (around 1.75 eV) is interpretedas being caused by the fact that VZn defects aregradually occupied by mobile Cu atoms to formmore CUZn-VTe centers. After annealing at 600°C,the PVT ZnTe crystals became very stable andexhibited a different PL spectrum, as shown in

221

Fig. lc. The free exciton recombination is re-solved from the PBE line, while the LO-phononreplicas of Ayu have almost vanished. Cu has avery high diffusion coefficient in ZnTe and conse-quently has a very high degree of association withother point defects (including other copper atoms)present in the crystal, as indicated by the peaks ofthe C

nseries which are intense enough to overlap

the occurrences of the DAP, the FB and the A~utransitions.Fig. 2 shows the PL spectra of PVT ZnTe :Cu

crystals: (a) after the Cu diffusion at 250°C, (b)after aging, and (c) after annealing under vacuumat 600°C for 1 h. Spectrum 2a shows featureswhich are very different from the spectra foundin the literature and those obtained by us onsamples which were highly contaminated by Cuduring growth. The line at 2.3827 eV and a smallshoulder at 2.3895 eV are attributed to D~ andthe free exciton (x), respectively. A new set of[DAP]j at 2.3215 eV and associated phonon repli-cas may be attributed to interstitial Cu. The oc-currence of ClO line and its phonon coupling aredue to copper diffusion. The spectrum in Fig. 2bis similar to that in Fig. Ib, which indicates thatthe Cu impurity has completely diffused into thecrystal lattice to occupy VZnand form more CUZncenters. The vanishing of the [DAP]j makes DAP

CUZn-VT<

I(a)

xOts':~1.6 1.7 1.8 1.9 2.0 2.1 2.2

ENERGY (eV)

Fig. 2. Photoluminescence spectra of Cu doped PVT grownZnTe sample, at 10.6 K: (a) after the Cu diffusion at 2S0°C,(b) aged at ambient for about 6 months, and (c) annealedunder vacuum at 600°C for 1 h.

2.3 2.4

222 Y Biao et al. / Journal of Crystal Growth 138 (1994) 219-224

and FB well resolved. The Cu diffusion also pro-duced more defects associated with the broadband at 1.78 eV (probably CUZn-VTecenters), thesharp line C9 and its two successive LO-phononreplicas. After annealing the crystal, it can beobserved in the spectrum shown in Fig. 2c thatA~u is broadened and decreased in intensity, andthat the structure of the C

nseries is clearly

resolved. The broad band at 1.78 eV and thetriple lines have almost vanished. New peaks at1.97 eV were produced during the annealing pro-cess. These are thought to be due to in the formof oxygen centers (OTe)' without the fine struc-ture of the Zn-O axial centers.The larger intensity ratio of PBE to C2 in Fig.

lc, as compared to Fig. 2c, indicates that theundoped crystals contain less total Cu impurity,and the PL structure of la is comparable to thatof the crystals having Cu atoms of about 1017cm -3 [5]. Earlier studies on the PVT ZnTe crys-tals with chemical analysis showed that an unin-tentional Cu concentration of 50 ppba (3 X 1016cm-3) from atomic absorption « 3000 ppba fromspark source mass spectrography), while no IRabsorption was observed [2].The full range photoluminescence spectra of a

THM grown ZnTe crystal are shown in Fig. 3: (a)as-grown crystal; (b) aged crystal for about 6

2.32.2

ENERGY (eV)

Fig. 3. Photoluminescence spectra, at 10.6 K, of a ZnTecrystal grown by THM: (a) as-grown crystal, (b) aged crystalfor about 6 months, and (c) annealed at 600°C, in vacuum for1 h.

1.7 1.9 2.1 2.4

months; and (c) re-annealed at 600°C for 1 h. Thebroad band of deep level with the center of 1.9eV has the typical triplet fine structure of a LOphonon replica, and its zero-phonon lines at1.9874, 1.9798 and 1.9724 eV can be explained bythe associated donor-acceptor pair (Zn-O)model, assuming axial symmetry [7,8]. In the THMsample the presence of oxygen impurity causedthe donor-bound-exciton line, D1, at 2.3825 eV tobe well resolved. The oxygen atom might act as adonor and affect the position and the shape ofthe DAP peak and its replica. The intensity ratiosfor PBE/DAP and PBE/Zn-O center are equalto 15 and 5, respectively. The A~u line still ap-pears as a typical PBE, together with the D 1 line.The PL spectrum of the aged crystal, as shown inFig. 3b, shows the occurrence of the successivephonon coupling of A~u. In Fig. 3c the PL spec-trum of the THM ZnTe sample after annealing at600°C for 1 h is shown; when compared to theunannealed sample and the annealed PVT ZnTecrystals different features are evident: (i) thestrongest emission line is C3 instead of A~u, (ii)the triplet structure, together with LO-phonon, inthe deep-level region has vanished, which meansthe decomposition of Zn-O center. The DAPrecombination may contribute some energy ofseveral LO-phonons to the Zn-O center. Thespectrum of Fig. 3c provides evidence that the Cnline series originate from complex centers, distin-guished from A~u, and that these complexes areassociated with Cu and 0 atoms.Fig. 4 shows AFM images of (110) crystallo-

graphic planes cleaved from different crystals.The structure of the cleavage steps appear to beaffected by the crystal growth method or by postgrowth treatments, a phenomenon recently ob-served on the cleaved surfaces of as-grown andannealed CdZnTe crystals [9]. The steps of theTHM ZnTe crystal surface, shown in Fig. 4a, arenot as straight as those of the PVT as presentedin Fig. 4b. The post-growth doping of the PVTgrown crystal with Cu at 250°C caused a modifi-cation of the microstructure, apparently, as aresult of the stress induced during the diffusionprocess. This effect is illustrated in the image ofFig. 4c, where the shape of cleavage steps hasbeen deformed and presumably Cu inclusions are

Y Biao et al. / Journal of Crystal Growth 138 (1994) 219-224

Fig. 4. Crystal surfaces at ambient, as imaged by atomic forcemicroscopy, with the marker representing 1 ,um: (a) as-cleavedTHM ZnTe, (b) as-cleaved PVT ZnTe, (c) as-cleaved PVTZnTe Cu doped, (d) PVT ZnTe Cu doped, as-cleaved afterthe annealing treatment at 600°C, (e) THM ZnTe sampleafter polishing and etching in Br-methanol solution, and (f)PVT ZnTe sample after polishing and etching in Br-methanolsolution.

present. The induced stress has been relieved bythe annealing treatment at 600°C, and Fig. 4dshows that the step shape has recovered its origi-nal shape of the as-grown crystal presented inFig. 4b. The polished and etched surfaces of theTHM and PVT crystals were also imaged and areshown in Figs. 4e and 4f, respectively. The sizeand the density of the residues, as well as thesurface roughness, vary significantly.The surface morphology of the samples quali-

tatively correlates to the PL spectra. Table 1

223

summarizes the full width at half maximum (F =

FWHM) of A~u and the intensity ratio 1= Afu /C,

where C is the intensity of the deep level broadband of region C (1.60-2.10 eV). It is commonlyaccepted that lower F values indicate a highercrystalline quality, while lower I value is associ-ated with higher defect density [10]. Consideringthe crystal growth techniques, and comparing thePVT grown to the THM grown crystals, it is clearthat the PVT crystal exhibits, both higher crys-talline perfection and lower impurity content.These quantitative results are well correlated tothe morphological images shown in Figs. 4a, 4b,4e and 4f, indicating that both the as-cleaved andthe etched surfaces of the THM grown surfacesare rougher and exhibit higher density of residues,as compared to those of the PVT grown crystal.Further, the intentional doping of the PVT growncrystal increased the F value and decreased the I

value, indicating the deterioration in the crys-talline quality and the increasing content of vari-ous types of impurities and defects. The effects ofthese post growth processes on the morphologythe cleaved surfaces are demonstrated in Figs. 4cand 4d. Considering the PL data together withthe AFM analysis, we may conclude that thedoping process not only produced the diffusion ofCu, but also introduced high density of defects,which are clearly visible in Fig. 4c, where thestructure of cleavage steps has been modified anda region of highly concentrated inclusions (clus-

Table 1Summary of the F and I values as calculated from the PLspectra

Type of sample, treatment FWHM(meV)

1

PVT, polished and etchedPVT, cleavedPVT, agedPVT, annealedTHM, polished and etchedTHM, agedTHM, annealedPVT, Cu doped, etchedPVT, Cu doped, agedPVT, Cu doped, annealed

134.58866846

3200200018003005

6401300.2

660800

224 Y Biao et al. / Journal of Crystal Growth 138 (1994) 219-224

ters) was produced. After annealing, the stresshas been relieved, and the shape of the cleavagesteps was restored to that of the as-cleaved crys-tal. To some extent, the surface preparationmethods were found to affect the PL spectra; thecleaved surface exhibited slightly broader emis-sion peaks, as well as slightly lower I value ascompared to the polished etched surfaces. Inspite of the fact that the cleaved surface issmoother than the etched one, it appears that thecleaving process introduces surface defects whichhave not been observed by AFM, but contributeto a deterioration of the PL spectrum. Appar-ently, the etching process clears away some of thesurface defects which are introduced during themechanical polishing procedure, providing theband sharpening of the PL spectrum.In conclusion, PVT ZnTe crystals have been

confirmed, by low temperature photolumines-cence and atomic force microscopy, to have abetter purity and quality than THM ZnTe crys-tals. The as-grown crystals contain different levelsof Cu impurity and the THM ZnTe crystals alsocontain isoelectronic oxygen substituting for Teatoms. After doping Cu into the PVT ZnTe crys-tals by diffusion, new interstitial Cu acceptorcenters and more vacancies of Te are formed.Additionally, the process of annealing at 600°Ccauses the vanishing of the Zn-O center in THM

ZnTe crystals and reduces the vacancies of PVTZnTe.

4. Acknowledgement

This work was supported by NASA Grantnumber NAGW-2025.

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