Optical and Electrical Studies of the Double Acceptor ... - Springer Link

Journal of ELECTRONIC MATERIALS, Vol. 41, No. 10, 2012

DOI: 10.1007/s11664-012-2104-8 2012 TMS

Optical and Electrical Studies of the Double Acceptor Levels of the Mercury Vacancies in HgCdTe F. GEMAIN,1 I.C. ROBIN,1,3 S. BROCHEN,1 M. DE VITA,1 O. GRAVRAND,1 and A. LUSSON2 1.—CEA, LETI, Minatec, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France. 2.—GEMaC, CNRS, UVSQ, 1 Place Aristide Briand, 92190 Meudon, France. 3.—e-mail: ivan-christophe. [email protected]

Correlations between photoluminescence and temperature-dependent Hall measurements were carried out on unintentionally doped HgCdTe epilayers with cadmium composition of 32.7%. These films were grown by liquid-phase epitaxy and post-annealed under different conditions as follows: a p-type annealing was used to control the mercury vacancy concentration, and an n-type annealing under saturated mercury atmosphere was used to fill the mercury vacancies. Comparison of the results obtained by these two characterization techniques allowed us to identify the two acceptor energy levels of the mercury vacancy. Moreover, the ‘‘U-negativity’’ of the vacancy was evidenced: the ionized state V is stabilized under the neutral state V0 by the dominance of the Jahn–Teller effect over Coulombic repulsion. Finally, three epilayers with different cadmium compositions were also characterized to complete this study. Key words: HgCdTe, photoluminescence, mercury vacancies, U-negativity, Hall effect

INTRODUCTION The success of new-generation HgCdTe (MCT) devices based on p/n junctions requires good control of doping properties, especially extrinsic p-type doping.1,2 This material is naturally p-type doped by native mercury vacancies. Vydyanath and coworkers3,4 estimated the concentration of these intrinsic acceptor impurities in as-grown liquid-phase epitaxy (LPE) samples to be about 1018 cm 3 for growth temperatures T > 300C. The energy levels related to these acceptors are still not well known. Indeed, the Hg vacancy is a double acceptor defect with two levels at different ionization energies. However, to date, only one acceptor activation energy has usually been measured in literature by temperature-dependent Hall measurements.5,6 Also, only one emission peak at about 12 meV to 15 meV lower than the band-to-band emission was reported in literature using photoluminescence (PL) measurements and (Received October 27, 2011; accepted April 5, 2012; published online May 15, 2012)

identified to be potentially related to Hg vacancies.7–10 In this work, several samples from the same epilayer undergoing different annealing to fill or to reinforce the Hg vacancies are used. Systematic comparison of the PL studies performed on the different samples allowed us to observe for the first time the two acceptor levels of the Hg vacancies. The ionization energy of the two acceptor levels could be measured and the correlation with temperature-dependent Hall measurements allowed us to clearly attribute the emission peaks to each acceptor level of the Hg vacancy. Studies of three epilayers with different cadmium compositions (32%, 45%, and 60%) are finally presented.

SAMPLE PREPARATION The studied MCT layer was grown by LPE at 500C. The active layer was 9 lm thick with cadmium composition of 32.7%, corresponding to a cutoff wavelength of 3.75 lm at room temperature. Three samples of this MCT epilayer were compared 2867

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as follows: A first sample was kept as grown. Another sample was p-type annealed under vacuum at 330C for 1 h in order to fix the p-type doping level due to Hg vacancies. A third sample was heated under saturated mercury atmosphere for 5 days at 200C in order to fill the Hg vacancies and to electrically measure the residual doping level. This treatment is called ‘‘n-type annealing.’’ Modulated PL measurements were carried out using a Fourier-transform infrared (FTIR) spectrometer on the as-grown, p-type annealed, and n-type annealed samples. The PL measurements were performed between 2 K and 300 K using a 1064 nm wavelength neodymium-doped yttrium aluminum garnet (YAG) laser for excitation. The signal was detected using a cooled InSb detector. The results are compared with temperature-dependent Hall measurements obtained with a magnetic field of 7 kG.

the low-energy (LE) peak disappear after n-type annealing, while they dominate the PL spectrum after p-type annealing. Thus, those two peaks are attributed to the two acceptor states V0 and V of the Hg vacancy and certainly correspond to eA0 transitions of an electron from the conduction band to the acceptor level. The energy differences between the HE peak and the ME and LE peaks give two ionization energies of 10.7 meV and 27.3 meV, respectively. However, at this point, it is impossible to attribute each ionization energy to the corresponding state of the Hg vacancy V0 or V . This attribution will be done using the temperature-dependent Hall measurements discussed later (Fig. 2). The PL temperature dependence of the peak energies between 2 K and 300 K for the as-grown sample, p-type annealed sample, and the n-type annealed sample is presented in Fig. 2. The HE peak located at 261.8 meV at low temperature presents the same temperature dependence for the as-grown sample and the n-type annealed sample. However, in the case of the p-type annealed sample, all the electrons recombine on acceptor levels for temperature less than 40 K. As a consequence, the HE peak is observed only above 40 K. Between 2 K and 80 K, the PL emission of the HE peak is lower than the theoretical bandgap energy: it comes from the recombination of carriers localized in the Urbach tail caused by alloy fluctuations.11,12 For higher temperatures, it follows the variation of the theoretical bandgap. Thus, this PL emission is attributed to a band-to-band transition. The ME and HE peaks observed in the as-grown sample and the p-type annealed sample disappear above 150 K because of the ionization of the Hg vacancies.

OPTICAL CHARACTERIZATION: PHOTOLUMINESCENCE STUDIES Figure 1a compares the PL spectra of the MCT layer as-grown, after p-type annealing, and after n-type annealing. The measurements were done at 5 K for an excitation density of 20 W/cm2. The spectra were fitted using Gaussian curves to identify the different contributions. The PL spectrum of the as-grown sample is composed of three emission peaks centred at 261.8 meV, 251.1 meV, and 234.5 meV, respectively. The high-energy peak, named ‘‘HE peak,’’ is seen in the as-grown PL spectrum and after n-type annealing. It is attributed to band-to-band recombination, which is confirmed by the temperature-dependent measurements shown in Fig. 1b discussed later. The middle-energy (ME) peak and

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Fig. 1. (a) Comparison of the PL spectra at 5 K of the as-grown sample, after p-type annealing, and after n-type annealing for excitation density of 20 W/cm2. (b) Temperature dependence of the PL peak energies of the as-grown sample, after p-type annealing, and after n-type annealing.

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Optical and Electrical Studies of the Double Acceptor Levels of the Mercury Vacancies in HgCdTe

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Doping level CBE equation Linear fit

Ea = 26,2 meV

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Fig. 3. Inverted ordering of mercury vacancy states in HgCdTe.

-1 Reciprocal temperature (1000/T) (K )

Fig. 2. Temperature-dependent Hall measurements of the as-grown sample.

ELECTRICAL CHARACTERIZATION: TEMPERATURE-DEPENDENT HALL EFFECT Temperature-dependent Hall measurements are presented in Fig. 2. The carrier concentration in the as-grown sample is measured between 10 K and 300 K. Two main regimes can be seen in the graph. At low temperatures, a very small activation energy is measured. This corresponds to a conduction channel possibly due to adsorbed molecules on the surface of the sample.13,14 It dominates the conduction properties up to 20 K. Then, a second regime of ionization is measured. The hole concentration temperature evolution depends on the number of activated acceptors compared with the residual donor doping level. Indeed, if p £ Nres D , p T3/2exp( Ea/kBT), Ea being the activation energy, Nres D the residual doping level, and T the 3/4 exp( Ea/ temperature, whereas if p ‡ Nres D , p T 15 2kBT). Hall measurements on the n-type annealed sample allow the determination of a residual doping level of 1.05 9 1014 electrons cm 3 at 77 K. Moreover, temperature-dependent Hall measurements on the n-type annealed sample show a carrier concentration much lower than the carrier concentration in the as-grown sample at all temperatures. As a consequence, the ionization regime corresponds to ‘‘uncompensated’’ material where p T3/4exp( Ea/ 2kBT). For accuracy reasons, the graph shown in Fig. 2 is fitted by the charge-balance equation repre(CBE) given by NA = p + Nres D , where NA sents the number of ionized acceptors. It is usually assumed that the density n of electrons in the conduction band is negligible and all the donor defects are ionized. It can be noticed that the fitting curve exhibits a change of slope at NA = Nres D , below which the ‘‘compensated’’ case applies. The experimental data are well fitted in the temperature range by assuming only one acceptor state with a single activation energy of 26.2 ± 3.9 meV. This activation

energy corresponds within the experimental error to the energy difference between the LE peak and the HE peak in the PL measurements described before. However, the activation energy corresponding to the 10.7 meV energy difference between the ME peak and the HE peak is not measured by the temperaturedependent Hall measurements; Indeed, if another accepter level with activation energy of 10 meV is taken into account in the CBE, it is not possible to fit the data. The activation energy measured by temperaturedependent Hall measurements certainly corresponds to the V0 state ionization energy. Thus, the LE emission peak in the PL measurements corresponds to an eA0 recombination between an electron in the conduction band and the V0 state of the Hg vacancy. So, the ME peak corresponds to an eA recombination between an electron in the conduction band and the V state of the Hg vacancy. This means that the V state has a smaller ionization energy (10.7 meV) than the V0 state (27.3 meV) and is closer to the valence band. This is the reason why the ionization energy of the V state cannot be measured by thermal activation with temperaturedependent Hall measurements. Indeed, when the V0 state is thermally activated, the V state is not stable and is directly ionized into the V state by thermal activation. On the other hand, the V state is observed in PL measurements because in this case it is photogenerated by an electron transfer on the V0 state. First an electron recombines on the V0 state and ionizes this level, which gives rise to the LE peak, 27.3 meV below the band-to-band emission. Then, an electron can recombine on the previously photogenstate. erated V state, which is ionized into the V This gives rise to the ME peak, 10.7 meV below the band-to-band emission (cf. Fig. 3). These results seem to confirm that the Hg vacancy is a negative-U defect.16,17 It corresponds to a stabilization of the V ionized state compared with the V0 neutral state because of the Jahn–Teller effect, due to lattice distortion overcoming the Coulombic repulsion (termed U). Let us explain these points in more detail. As shown in Fig. 4a, HgCdTe crystallizes in the

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Te atom Te atom

Cd or Hg atom Hg atom Distortion direction

Fig. 4. (a) Crystallographic structure (zincblende) of HgCdTe. (b) Tetrahedral environment of Hg atom.

zincblende structure. An Hg atom is surrounded by four Te atoms in a tetrahedral crystal as described in Fig. 4b. The coupling between the dangling bond orbitals in HgTe produces a fully r symmetric level of low energy and three higher p levels which are degenerate in the undistorted vacancy (cf. Fig. 5). In the neutral Hg vacancy, two electrons occupy the fully symmetric level and the other four lie in the p upper levels. When the mercury atom is removed from the crystal lattice, displacements are made along the Z direction as presented in Fig. 4b. Thus the pX and pY states split from the pZ state: the pX and pY states are stabilized whereas the pZ state rises in energy. Let us denote by EJT the energy splitting between the pX, pY states and the pZ state due to the lattice distortion (Jahn–Teller effect). The energy difference between the pX, pY states and the pZ state depends on the ionization state of the vacancy, and one has to take into account the Coulombic repulsion (U). Let us call Etot = EJT + U the total energy difference between the pX, pY states and the pZ state. Two possibilities exist to fill the energy levels: if the Jahn–Teller effect is small compared with the Coulombic repulsion, in the neutral state, the four electrons occupy energy states as described in Fig. 6a. When filling the vacancy with an electron, two opposite effects affect the energy level of the V-state: The Coulombic repulsion tends to increase the energy level, leading to a positive term DU in the total energy variation. On the other hand the distortion of the lattice is reduced, leading to a reduction of the Jahn–Teller effect and to a negative term DEJT in the total energy variation. In the case of Fig. 6a, where the Coulombic repulsion is dominant, the energy difference between the V and the V0 states DEtot = DU + DEJT is positive because the Coulombic repulsion (DU > 0) is greater than the Jahn–Teller energy reduction (DEJT < 0). In the case described in Fig. 6b, the Jahn–Teller effect is larger than the Coulombic repulsion. Thus, in the neutral

state, the four electrons fully occupy the pX and pY states. When filling the vacancy with an electron, the reduction of the Jahn–Teller energy (DEJT < 0) is greater than the Coulombic repulsion (DU > 0), leading to a negative DEtot = DU + DEJT. This model explains why the V ionized state has a lower energy than the V0 state and clearly shows the negative-U property of the mercury vacancy in HgCdTe (Fig. 6). At this point we would like to point out that other research groups18–20 introduced donor-like Shockley– Read (SR) centers at about 30 meV under the conduction band to explain the carrier lifetimes measured in their photodiodes and the measured dark currents. Thus, the LE peak could have been attributed to a radiative recombination from this SR center. However, as shown before, our Hall measurements do show the existence of an acceptor level 26.2 meV above the valence band, corresponding to the energy difference between the LE peak and the band-to-band recombination. Thus, the correlation between our PL and Hall measurements makes us confident in our peak attributions. To conclude this part, our PL measurements led to a direct observation of the Hg vacancy double acceptor levels in HgCdTe. The correlation with Hall measurements allowed understanding of the energy ordering of the Hg vacancy states in the HgCdTe bandgap. For Cd composition of 32.7%, the ionization energy of the neutral level V0 is 27.3 meV and the ionization energy of the V ionized state is 10.7 meV. The second part of this paper presents results obtained on samples with different cadmium compositions. STUDIES OF SAMPLES WITH OTHER CADMIUM COMPOSITIONS Three MCT epilayers also grown by LPE with different cadmium compositions were studied. Epilayer A has a 5.9-lm-thick active layer with cadmium composition of 32%, very similar to the one presented before corresponding to a cutoff wavelength of 3.85 lm at room temperature. This layer was studied to check that our observations are reproducible. Epilayer B is 8.3 lm thick with 45% cadmium composition, which corresponds to a cutoff wavelength of 2.5 lm at room temperature. Epilayer C is 8 lm thick with 60% cadmium composition corresponding to a 1.73 lm cutoff wavelength at 300 K. Each epilayer was cut into three samples to perform the same annealing as described before. Here, we focus our studies on the results obtained on the p-annealed samples. The PL measurements were carried out with a 532-nm solid-state laser between 4 K and room temperature. The signal was detected with a cooled MCT or InGaAs detectors depending on the emission wavelength.


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p*antibonding

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Fig. 5. Energy levels and orbital scheme in HgTe.

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Fig. 6. Energy levels in HgCdTe. (a) Mercury vacancy energy levels when Coulombic repulsion is dominant. (b) Mercury vacancy energy levels when Jahn–Teller effect is dominant.

Figure 7a presents the PL spectrum of the p-type annealed sample A. Three Gaussian contributions could be fitted, corresponding to the HE peak, the ME peak, and the LE peak as described before. The energy differences between the HE peak and the ME and LE peaks give two ionization energies of 15.9 meV and 27.3 meV, respectively. The HE peak corresponds to the band-to-band recombination as confirmed by the temperature-dependent PL measurements shown in Fig. 7b. The ME peak disappears above 50 K, whereas the LE peak is followed in temperature until 200 K. This behavior is the same as that found for the mercury vacancy states discussed in the previous part. Moreover, temperature-dependent Hall measurements were fitted by only one activation energy of 24 meV (±3.9 meV), which corresponds to the LE peak ionization energy. Thus, for a sample with a similar composition to the one discussed before, the results obtained by PL and Hall measurements also show an inverted ordering

of the mercury vacancy acceptor levels in the bandgap. The PL spectrum of the p-typed annealed sample B with 45% Cd composition is shown in Fig. 8a. Three peaks also compose this spectrum at low temperatures. The HE, ME, and LE peaks are centred at 486 meV, 468 meV, and 450 meV, respectively. In the temperature-dependent PL measurements presented in Fig. 8b, the HE peak follows the theoretical bandgap corresponding to 44% Cd composition until 300 K. The ME and LE peaks disappear before 100 K, because of the ionization of the acceptor levels. The ionization energies measured are 20 meV and 38 meV. A final epilayer (p-type annealed sample C) with 60% Cd composition was studied as presented in Fig. 9a. The energy difference between the HE peak and the ME and LE peaks gives two ionization energies of 16.5 meV and 31.4 meV, respectively, for the mercury vacancy. Temperature-dependent PL

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measurements presented in Fig. 9b show the ionization of the two acceptor levels (ME and LE peaks) above 110 K. The HE peak is at a lower energy than the theoretical bandgap because of the localization of the carriers in the Urbach tails. To conclude this second part, it seems that the mercury vacancy level ionization energies are little influenced by the Cd composition between 32% and 60% Cd (cf. Fig. 10). The ionization energy of the V0 state that we measured for Cd composition of 32% (27 meV) is close to the one measured by Kenworthy et al.6 for a similar composition [22 meV deduced from formula (8) of Ref. 6]. For Cd composition of 45%, our PL measurements tend to show that the ionization energies of the acceptor levels increase (38 meV for the V0 state). For 60% Cd composition, the ionization energies decrease. This is possibly due to a diminution of the Jahn– Teller effect for higher cadmium compositions. On the other hand, as Cooper and Harrison17 suggested, it can also be assumed that the decrease of

the dielectric constant with increasing Cd content may result in a larger Coulombic repulsion. CONCLUSIONS The double acceptor levels of mercury vacancies in HgCdTe were measured by PL characterization. Two ionization energies were measured by PL between 10 meV and 18 meV for the V state and from 27 meV to 38 meV for the V0 state for Cd composition ranging from 32% to 60%. Temperature-dependent Hall measurements showed only one activation energy corresponding to the V0 state ionization energy for samples with 32% and 32.7% Cd. The correlation between PL measurements and Hall measurements was the key to understanding the energy ordering of the Hg vacancy states in the bandgap. Indeed, the fact that only one activation energy is measured by temperature-dependent Hall measurements shows that the V state is stabilized compared with the V0 state, which corresponds to



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measure samples with smaller Cd compositions to correlate with the measurements of Kenworthy et al.6

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REFERENCES 30 25 20 15 10 5 30

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the negative-U property of the Hg vacancy. The two acceptor states were hence clearly identified. The extended study of samples with different Cd compositions shows a small evolution of the double acceptor ionization energies with Cd composition for compositions between 32% and 60%. This could be due to a diminution of the Jahn–Teller effect for higher cadmium compositions and an increase of the Coulombic repulsion because of a diminution of the dielectric constant.17 Electrical measurements are underway to study the Hg vacancy ordering for samples richer in Cd. We are also planning to

1. O. Gravrand, P. Ballet, J. Baylet, and N. Baier, J. Electron. Mater. 38, 1684 (2009). 2. I.C. Robin, M. Taupin, R. Derone, A. Solignac, P. Ballet, and A. Lusson, Appl. Phys. Lett. 95, 202104 (2009). 3. H.R. Vydyanath and C.H. Hiner, J. Appl. Phys. 65, 3080 (1989). 4. H.R. Vydyanath, J. Cryst. Growth 161, 64 (1996). 5. E. Finkman and Y. Nemirovsky, J. Appl. Phys. 59, 1205 (1986). 6. I. Kenworthy, P. Capper, C.L. Jones, J.J.G. Gosney, and W.G. Coates, Semicond. Sci. Technol. 5, 854 (1990). 7. A.T. Hunter and T.C. McGill, J. Appl. Phys. 52, 5779 (1981). 8. F. Yue, J. Chu, J. Wu, Z. Hu, Y. Li, and P. Yang, Appl. Phys. Lett. 92, 121916 (2008). 9. N. Dai, Y. Chang, X.G. Wang, B. Li, and J.H. Chu, Curr. Appl. Phys. 2, 365 (2002). 10. I.C. Robin, M. Taupin, R. Derone, P. Ballet, and A. Lusson, J. Electron. Mater. 39, 868 (2010). 11. A. Lusson, F. Fuchs, and Y. Marfaing, J. Cryst. Growth 101, 673 (1990). 12. V.I. Ivanov-Omskii, N.L. Bazhenov, and K.D. Mynbaev, Phys. Status Solidi B 246, 1858 (2009). 13. D.C. Look, Semicond. Sci. Technol. 20, S55 (2005). 14. D.C. Look, H.L. Mosbacker, Y.M. Strzhemechny, and L.J. Brillson, Superlattices Microstruct. 38, 406 (2005). 15. P. Blood and J.W. Orton, Rep. Prog. Phys. 41, 157 (1978). 16. G.D. Watkins, Adv. Solid State Phys. XXXIV, 163 (1984). 17. D.E. Cooper and W.A. Harrison, J. Vac. Sci. Technol. A 8, 1112 (1990). 18. R.E. DeWames, private communication. 19. M.A. Kinch, Fundamentals of Infrared Detector Materials (Bellingham, WA: SPIE Press, 2007), chap. 4, p. 39. 20. A. Rogalski, Rep. Prog. Phys. 68, 2267 (2005).