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Annealing effects of -implanted CdTe epilayers

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1996 J. Phys. D: Appl. Phys. 29 2162

(http://iopscience.iop.org/0022-3727/29/8/015)

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J. Phys. D: Appl. Phys. 29 (1996) 2162–2164. Printed in the UK

Annealing effects of As +-implantedCdTe epilayers

J M Zhang †§, S C Shen†, S P Guo‡ and S X Yuan ‡† National Laboratory for Infrared Physics, Chinese Academy of Sciences,420 Zhong Shan Bei Yi Road, Shanghai 200083, People’s Republic of China‡ Shanghai Institute of Technical Physics, Chinese Academy of Sciences,420 Zhong Shan Bei Yi Road, Shanghai 200083, People’s Republic of China

Received 12 March 1996

Abstract. The annealing effects of As-ion-implanted (211)B CdTe films grown bymolecular beam epitaxy have been studied by resonant Raman scattering andphotoluminescence spectroscopy. The Raman intensity ratio of 2LO to 1LOphonon modes and the normalized bound-exciton emission intensity indicate thatbetter removal of the implantation-induced damage is obtained with an increase ofthe annealing temperature (TA) beyond 440 ◦C, whereas the lattice perfection dropssharply when TA is higher than 440 ◦C. With an increase of TA in the studied range,more and more As atoms occupy the Te sites as acceptors and the samplesbehave in a p-type manner with a decreasing compensation coefficient and anincreasing hole concentration.

In recent years, considerable attention has been paidto the growth of CdTe films and the control of theirelectrical properties by using different molecular beamepitaxy (MBE) [1, 2] or metal–organic chemical vapourdeposition (MOCVD) [3] techniques and several substrates[4, 5]. This is because of the difficulty of the fabrication ofn- or p-type conducting layers. The interest in CdTe comesnot only from its wide uses in electronic and optoelectronicdevices, but also from its close link to the importantinfrared material HgCdTe and CdTe-based superlatticesand quantum wells such as HgTe–CdTe and CdMnTe–CdTe. Ion implantation is another effective method forintroducing selective dopants. This technique offers goodcontrol of the dopant concentration and depth [6, 7], but itis always accompanied by undesirable structural damageto the lattice. It is lucky that thermal annealing is avery effective way to recover the crystalline perfection.In this paper, we report resonant Raman scattering (RRS)and photoluminescence (PL) studies on the modificationof electrical properties and damage removal by systematicthermal annealing.

Undoped (211)B CdTe epilayers were grown onchemically pre-treated (and later pre-heated to 580◦C)(211)B semi-insulating GaAs substrates. The (211)Borientation was chosen due to the suppression of twindefect formation along this growth direction [4, 8]. Thesamples were grown with a substrate temperature of290◦C using a single CdTe source in a ShenyangScientific Instruments FW-III system. Reflected high-energy electron diffraction (RHEED) was used to determine

§ Present address: Max-Planck-Institut fur Festkorperforschung, Heisen-bergstraße 1, D-70569 Stuttgart, Germany.

the monocrystalline growth of the epitaxial layers. Theimplantations were performed with As ions using an ionenergy of 120 keV and doses in the range 1013–1015 cm−2.The beam current density was lower than 30 nA cm−2.After implantation the samples were annealed under acontinuous N2 gas flow or a Cd atmosphere at temperaturesin the range 300–500◦C for 1 h. The implantation is withina layer of depth less than 0.5 µm, which is much thinnerthan the thickness of the CdTe epilayer (4µm). Thus, weneed not take the effects of interface and lattice mismatchbetween the epilayer and the substrate into account. Ramanexperiments were performed in backscattering geometrywith the sample cooled to 77 K and excited with the 4880Aline of an Ar-ion laser, aiming at theE0+10 gap resonance.Both the Raman and the PL signals were dispersed anddetected by using a SPEX double-grating monochromatorwith a cooled, GaAs photocathode photomultiplier andstandard photon-counting electronics.

It has been shown that, in polar semiconductors,resonant Raman scattering by LO phonons is a powerfuland non-destructive technique by which to analyse thelattice damage [9, 10]. The damage-induced broadeningand energy shift of the corresponding resonance in thescattering efficiency result in a high sensitivity of thescattering intensity ratio of the LO phonons to the crystalperfection. Figure 1 displays a series of Raman spectrarecorded from as-grown CdTe as well as from the samplesimplanted with a dose of 1014 cm−2, and then annealedunder N2 flow at 300, 350, 400 and 450◦C respectively.Due to the lattice damage resulting from the implantation,the intensities of the 1LO and 2LO lines decrease greatlycompared with those off the as-grown samples whereas,after annealing, they become much stronger than those

0022-3727/96/082162+03$19.50 c© 1996 IOP Publishing Ltd

Annealing of As+-implanted CdTe epilayers

Figure 1. Raman spectra of the as-grown, implanted andvarious annealed CdTe epilayers at 77 K excited by 4880 Aline of an argon laser. A: as-grown; B: implanted; C:300 ◦C; D: 350 ◦C; E: 400 ◦C and F: 450 ◦C annealed undera continuous N2 gas flow.

in the as-grown ones. Beyond the excitonic effectmechanism [11], we suppose that the dipole-forbidden,one-LO phonon resonance enhancement is mainly impurityinduced. With an increase ofTA, the 1LO intensity near170 cm−1 increases monotonically. The 2LO phonon,I2LO ,which is Frohlich electron–phonon interaction induced,to I1LO intensity ratio increases first withTA, until itreaches about 60% of the value for the as-grown case at420◦C. It decreases with further increase of the annealingtemperature, as shown in figure 2(a). Similar results havebeen found in the PL measurements, shown in figure 2(b).The integral PL intensity normalized with respect to thatin the as-grown sample in the bound-exciton range of1.58–1.61 eV is shown in samples with different annealingtemperature. The intensity reaches 60% of the value in thevirgin sample at the annealing temperatureTA = 440◦C.This corresponds to the least number of non-radiativerecombination centres and the best lattice recovery in thesample. IfTA is higher than 440◦C, the lattice perfectiondrops sharply. It looks like this result is because ofan increase of the surface Cd vacancies at too high anannealing temperature. However, the Raman and PL peaksare quite similar in those samples annealed at differenttemperatures under a Cd atmosphere (see figure 2(b)). Itis hard to say that an increase in number of Cd vacanciesdecreases the perfection of the sample enough to change theRaman and PL spectra. A further study, maybe by a ‘face-to-face’ annealing treatment [6], is needed. We supposea more likely explanation to be this: after annealing at ahigh enough temperature, a lot of As atoms are activated,the compensation coefficient decreases and the net carrierconcentration increases. Thus the sample behaves as amoderately or heavily doped material instead of as a closelycompensated one [14]. This causes changes of the structuraland emission properties.

Figure 2. The annealing temperature (TA) dependence of(a) the ratio of Raman intensities of 2LO (I2LO ) over 1LO(I1LO ) (annealed under a continuous N2 gas flow, dots andsquares represent two series of samples) and (b) thebound-exciton emission intensity normalized with respect tothat in the as-grown sample (annealed under Cdatmosphere (triangles) and under a N2 flow (diamonds),respectively). Arrows point to the values for the as-grownand as-implanted samples. Errors are estimated from noiselevels of the spectra and uncertainties of themeasurements.

The 14.5 K PL spectra of the above series of CdTesamples are plotted in figure 3. Three bands can be found;they are the bound-exciton emission at around 1.59 eV, the1.557 eV peak and its LO phonon replica and the 1.51 eVpeak and its LO replicas. Temperature- and incident-laser-power-dependence measurements show that the 1.557 eVpeak is due to a single acceptor level, which has alsobeen observed in CdTe/InSb grown by MBE [12] andin p-type CdTe grown by photo-assisted MBE [1]. Ourmeasurements on samples similarly implanted but annealedin a Cd bath have revealed that the 1.557 eV peak isrelated to the Cd vacancies, since the intensity of this peakdecreases quite a lot (only about 10% of it is left) afterannealing in a Cd bath, compared with annealing in N2.Even though our experiment temperature is only 77 K,three peaks can still be identified in the exciton emissionrange. In the as-grown sample, the exciton emission ismainly from the 1.593 eV donor bound exciton, whereas,after implantation and annealing, the 1.590 and 1.5875 eVacceptor bound-exciton emissions become much stronger.The ratio of the strengths of the lines associated with donorand acceptor bound excitons is not directly proportional tothe ratio of the concentrations of the impurities; however,this datum clearly indicates the change of the conductivity

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J M Zhang et al

Figure 3. PL spectra at 14.5 K of as-grown (A),implanted (B) and annealed samples heated at 300 (C),400 (D), 420 (E), 440 (F), 450 (G) and 500 ◦C (H) for 1 hunder a continuous N2 flow.

in our samples from n-type to p-type. A similar situationhas been confirmed by electrical measurements in In-doped CdTe [5]. We will re-state this phenomenon in thefollowing.

It is evident that arsenic has been incorporated intoour implanted and annealed CdTe epilayers because AsT e

acceptors are indicated by the donor–acceptor (DAP)behaviour of the 1.51 eV peak. The 1.51 eV DAP bandrelates to a shallow 14 meV donor and the AsT e acceptor(EA = 92 meV) [4, 13]. As shown in figure 3, withan increase ofTA, the intensity of the DAP emissionincreases. This means that more As atoms have occupiedthe tellurium sites as acceptors, in agreement with themonotonically increasing behaviour of the intensity of the1LO phonon peak in our Raman measurements. Fromthe energy shift of the DAP peak with laser power, wecan obtain the compensation coefficientE0 according toI (ω) = I0 exp(Ep/E0), whereI (ω) is the DAP emissionintensity, Ep the peak position andI0 a constant [5, 14].Our fitting shows thatE0 = 12.1, 8.7, 1.23 and 0.85 meVin those samples annealed at 300, 420, 450 and 500◦Crespectively. In [14],E0 = 2.7 meV was obtained forn-type GaAs withND − NA = 3.7 × 1016 cm−3 andND + NA = 2.5 × 1018 cm−3. It is well known thatE0

is smaller in samples which have heavier doping levels andlighter compensation. Thus we can conclude that, with anincrease ofTA, our annealed samples become p-type withhigher hole concentration.

To confirm this result, keeping in mind the fact that thesubstrates are non-conducting, we made a series of Schottkybarriers by direct deposition of gold onto special areas ofthe surfaces of the layers and measured the capacitance–voltage (C–V ) properties [15]. The details of theC–V measurements will be reported elsewhere, but we will

list the main results here. We were not able to obtainthe carrier profiles in the as-grown layer and the sampleannealed at 300◦C, due to high compensations in thesetwo samples. However, it is clearly revealed that the as-grown sample is n-type, whereas after implantation andannealing, the conductivity becomes p-type. This changeof the conductivity matches the PL behaviours of boundexcitons quite well. The highest net hole concentrationalong the direction of the growth of the layer, with depletiondepths about 0.01 µm, increases from 2.9×1016 cm−3 forthe sample withTA = 450◦C to 4.7×1016 cm−3 for thesample withTA = 500◦C. TheseC–V data directly supportour conclusion made above from an analysis of Raman andPL data.

In summary, the annealing of As-ion-implanted CdTeepilayers has been studied by RRS and PL. The Ramanintensity ratio of the 2LO phonon peak over the 1LOphonon peak and the normalized PL intensity indicatebetter removal of the implantation-induced damage withan increase ofTA until 440◦C, after which it deterioratessharply whenTA is higher than 440◦C. More As atomsoccupy the Te sites and the samples behave in a p-typemanner with a smaller compensation coefficient and higherhole concentration with an increase ofTA in the studiedrange. This conclusion is supported byC–V measurements.

Acknowledgment

We thank Dr Zhao Jun for his help with theC–V

measurements.

References

[1] Arias J M, Shin S H, Cooper D E, Zandian M, Pasko J G,Gertner E R, De Wames R E and Singh J 1990J. Vac.Sci. Technol.A 8 1025

[2] Giles N C, Bicknell R N and Schetzina J F 1987J. Vac.Sci. Technol.A 5 3064

[3] Mitsuru Ekawa, Kazuhito Yasuda, Touati Ferid, ManabuSaji and Akikazu Tanaka 1992J. Appl. Phys.71 2669

[4] Gold J S, Myers T H, Giles N C, Harris K A, Mohnkern LM and Yanka R W 1993J. Appl. Phys.74 6866

[5] Bassani F, Tatarenko S, Saminadayar K, Bleuse J, MagneaN and Pautrat J L 1991Appl. Phys. Lett.58 2651

[6] Moriya N, Brener I, Kalish R, Pfeiffer W, Deicher M,Keller R, Magerle R, Recknagel E, Skudlik H, WichertTh, Wolf H and ISOLDE Collaboration 1993J. Appl. Phys.73 4248

[7] Guo S P, Zhang J M, Xie Q X and Yuan S X 1994Proc.SPIE2364505

[8] Lange M D, Sporken R, Mahavadi K K, Faurie J P,Nakamura Y and Otsuka N 1991Appl. Phys. Lett.581988

[9] Lusson A, Wagner J and Ramsteiner M 1989Appl. Phys.Lett. 54 1787

[10] Wagner J, Koidl P, Bachem K H, Uzan-Saguy C, Kalish Rand Bruder M 1993J. Appl. Phys.73 2739

[11] Islam S S, Rath Shyama, Jain K P, Abbi S C, Julien C andBalkanski M 1992Phys. Rev.B 46 4982

[12] Mar H A and Salansky N 1984J. Appl. Phys.56 2369[13] Molva E, Pautrat J L, Saminadayar K, Milchberg G and

Magnea N 1984Phys. Rev.B 30 3344[14] Pankove J I 1971Optical Processes in Semiconductors

(Engelwood Cliffs, NJ: Prentice-Hall)[15] Blight S 1990Solid State Technol.April 175

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