CdTe superlattices

~

Pergamon

Solid-State Electronics Vol. 37, Nos 4-6. pp. 112t--I 124. 1994 Copyright ~ 1994 Elsevier Science Ltd 0038-1101(93)E0025-V Printed in Great Britain. All rights reserved 0038-1101/94 $6.00+ 0.00

TIME RESOLVED OPTICAL STUDY OF VERTICAL TRANSPORT IN Cd0.82Mno.lsTe/CdTe SUPERLATTICES PH. ROUSSIGNOL1'2,J. MARTINEZ-PASTOR2'3, A. VINATTIERI2, C. DELALANDEI and B. LUNN4 ILaboratoire de Physique de la Matidre Condens6e de l'Ecole Normale Supdrieure, 24 rue Lhomond, 75005 Paris, France, 2Laboratorio Europeo di Spettroscopie Non Lineari e Dipartimento di Fisica, Universita di Firenze, Largo E. Fermi 2-50125 Firenze, Italy, 3Departament de Fisica Aplicada, Universitat de Valdncia, 46100 Burjassot, Val6ncia, Spain and 4Department of Engineering Design and Manufacture, University of Hull, Hull, England Abstract--We have performed c.w. and time-resolved photoluminescence measurements on a 30/30 Cd0.82Mn0.18Te/CdTesuperlattice containing two enlarged wells with different widths. Excitation above the alloy band gap allows to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed in the 4-100 K temperature range. At low temperatures (< 20 K), mobilities of the order of 4 x 104cm2/Vs and 3 x 102cm2/Vs are estimated for the two processes, respectively.

INTRODUCTION

In the last few years the improvement in molecular beam epitaxy growth has made possible the observation of vertical transport in superlattices[l]. Experimental evidences of this type of conduction first predicted by Esaki and Tsu[2] have been brought in heterostructures based on III-V semiconductors by using various optical or electric techniques[3-5]. Time-resolved photoluminescence in superlattices containing enlarged wells appeared to be one of the most powerful techniques and allowed an extensive study of the carrier transport in the AIGaAs/GaAs system[6]. In this paper, vertical transport is addressed by the same optical technique, in the case of the less investigated II-VI semiconductor heterostructures. The sample was grown by molecular beam epitaxy on an InSb substrate. On this substrate are grown in succession, a 1000,/k thick CdTe layer, a 2000A Cd0s2Mn0.j~Te layer, 40 periods of 30/30 Cd0s, Mn0.jsTe/CdTe superlattice (SL) and a 60A thick enlarged well (EW). The second EW, 200/~ thick, is separated from the first one by 40 periods of SL, and followed by 160 periods of SL. Finally a 2000 A thick Cd0s2Mn0.~sTe cap layer completes the structure. The excitation source for the time-resolved measurements was a N d : Y A G synchronously pumped dye laser, providing 5 ps pulses in the range 560-780nm at the repetition rate of 76MHz. Measurements have been also performed by using, for excitation, the second harmonics of the Nd: YAG laser (80 ps pulse duration at 532 nm). The PL signal was dispersed through a 0.22 m double monochromator (spectral resolution: l meV) and detected by a synchroscan streak camera, with an overall time

resolution of 20 ps. For decay times longer than I ns, the PL signal was analyzed by using a time correlated single photon counting system and a cavity dumper was inserted in the dye laser cavity in order to decrease the repetition rate; in this case the overall time resolution was of the order of 200 ps.

EXPERIMENTAL RESULTS

In order to study the Bloch transport over the SL minibands, the structure is excited at 2.33eV (Nd : YAG second harmonics) or 2.18 eV (dye laser), far above the alloy band gap (!.869 eV at 4 K). In this case all the carriers are photogenerated within the 2000/~ thick CdMnTe cap layer. Typical PL spectra obtained at different temperatures are shown in Fig. 1. At 10 K (lower curve in Fig. I) the main peak at 1.722 eV corresponds to the excitonic recombination in the superlattice. The PL lines observed at 1.595 and 1.645 eV correspond to the excitonic recombination in the 200 and 60/~ EWs, respectively. The PL emission lines on the low energy side of the excitonic peaks are likely related to impurity assisted recombinations (A °X, e - A °. . . . )[7]. As clearly shown in Fig. 1, at low temperatures (4-50 K) the main peaks in the PL spectrum of the heterostructure correspond to the recombination in the SL. Excitonic recombination in the two EWs is also observed evidencing the vertical transport. Nevertheless, considering the rather low intensity of the EWs' PL peaks, transport along the growth axis of the heterostructure does not seem so el~cient; most of the photogenerated carriers get localized and recombine in the superlattice region before reaching the enlarged wells. When the temperature increases the contribution at the PL emission of the EWs increases.

1121

PH. ROUSSIGNOLet al.

1122

However, above 80-90 K the PL line of the 60/~ EW tends to disappear. As commonly observed[8], this behaviour is certainly due to a worse quality of the thinner well, which made it more sensitive to non radiative processes. Above those temperatures the luminescence from the 200 .~, EW is more important than that of the SL and becomes the dominant feature of the PL spectrum of the structure, indicating that the vertical transport is now more efficient than the localization process. An Arrhenius plot of the PL intensity ratio between the PL from the 200,~ EW and that of the SL shows an activated behaviour above 50 K, with an activation energy of about 28 meV. In the time-resolved measurements excitation powers of 20-200 mW were used corresponding roughly to excitation densities of about 1017-10t8 cm -3. We report in Fig. 2 the PL decay curves of the 200 A EW at 4 and 40 K obtained for two different excitation energies. Under resonant excitation of the E1HI transition the PL decay [dotted lines in Fig. 2(a,b)] is fast with time constants of 20 and 50 ps at 4 and 40 K, respectively. Such a resonant excitation gives the recombination time of the isolated well. The PL decay of the 200 1~ EW is strikingly different when all the carriers are photogenerated in the cap layer [continuous lines in Fig. 2 (a,b)]. The long decay times obtained when exciting the heterostructure above the SL band gap are then characteristic of the vertical transport. In fact, the decay time of the isolated well, being much shorter, influences only the risetime of the measured PL signal as it can be seen in Fig. 2 (the apparent risetime is always related to the faster mechanism involved in the PL dynamics). A similar behaviour is observed for the 60/~, EW. '

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Time (ps) Fig. 2. Semilogarithmicplots of the PL decays of the 200 ,~ enlarged well at 4 K (a) and 40 K (b). The dotted lines correspond to a resonant excitation of the EIHI transition of the EW; the continuous lines are obtained when the excitation energy is higher than the CdMnTe barrier band gap and all the carriers generated in the cap layer of the heterostructure. At low temperatures (4-35 K), the PL decay curves of the EWs under non resonant excitation [transport conditions and continuous line of Fig. 2(a) for the 200/~ EW] can be decomposed as a sum of two exponentials with a risetime corresponding to the decay time of the isolated EW and two decay times characteristic of the transport mechanism which are shown in Fig. 3(a) for the whole temperature range investigated. In fact, the fast component dominates the PL decay curve: the intensity ratio between the slow and the fast component is of the order of 0.10. Moreover, when increasing the excitation power, this ratio tends to decrease. The two characteristic times of the EWs' PL decay seem to be related to two different types of carrier transport, the fastest one being favoured by an increase of the excitation power. Above 40 K a monoexponential decay is observed as shown in Fig. 2(b). The diffusion mobility, /-LD,can be deduced from the transport times according to[9]: eD

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where D is the diffusion coefficient, zR the recombination time along the carrier path and vD the drift velocity, determined by the path length (LsL), and the diffusion time (r D).

Time resolved optical study The diffusion mobilities reported in Fig. 3(b) have been estimated using eqn (I), with LsL = 9600 and 12000/~ (the distance between the cap layer and the EWs 200 and 60 A, respectively) and zR - 350 ps. The PL decay time in the SL exhibits a weak temperature dependence: from 250 to 350ps in the 4-100K temperature range. Therefore, this time, although influenced by the transport, can be mainly attributed to excitonic recombination in the SL. A zR value of 350 ps will give the right order of magnitude for the recombination time of the SL, although somehow underestimated, in the whole range of temperature. In the 4-25 K temperature range, the fastest transport mechanism gives high diffusion mobilities, ranging from 40000 to 2000cm2/Vs. Above 30 K this mobility drops significantly to reach a value of 40cm2/Vs at 70K. The slowest mechanism gives much smaller mobilities, of the order of 300 cm2/Vs. However, when the slow component is observable (4-20 K) its decay time is not accurately determined (intensity ratio ~ 10%) and this mobility should be considered as an order of magnitude.

DISCUSSION Let us first consider the low temperature measurements (4-30 K). At the large excitation intensities used in our experiments (10iT-10 Is carriers per cm -3) we can reasonably assume that the main contribution '

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1123

to the transport provided by the fast component is due to ambipolar Bloch transport of carriers[10]. Electron-hole pairs are created in the cap layer of the heterostructure and ambipolar band transport takes place. Nevertheless, for these temperatures, as it can be seen in Fig. 1, localization of carriers occurs. Carriers can be captured by ionized acceptors in order to give neutral acceptors or localized on defects before arriving to the large well. Due to the smaller width of the HH miniband, holes are certainly more sensitive to localization than electrons. The long time constant can then be related to another transport mechanism, hopping through localized states, this mechanism being much slower than Bloch conduction[11]. As a matter of fact, PL spectra of the heterostructure obtained at high c.w. excitation power (150 mW ~ 10iT-10~s carriers/cm 3) are dominated by the excitonic recombinations in the SL and EWs; saturation of the finite number of the sites responsible for the localization of the carriers does occur. In time-resolved measurements a similar carrier density is photocreated, so we mainly see the band transport of free carriers. When the carrier concentration becomes sufficiently low, transport due to localized carriers begins to be observable. Moreover, when the excitation power is increased, due to the finite number of trapping sites, the ratio between free and localized carriers increases and the hopping process is less observable, as experimentally seen. For temperatures higher than 30 K we observe a monoexponential decay for the two EWs. Two main reasons make the observation of the slow component impossible. On one hand, as the temperature increases more and more carriers get detrapped from localized tail states of the SL and are thermally activated upwards the extended states[12]. The activation energy of 28 meV found for our heterostructure is consistent with the calculated localization energy on a two monolayers fluctuation ( ~ 30 meV). On the other hand, carrier scattering by acoustic and optical phonons takes place[l 3] and produces a large drop of the mobility [Fig. 3(b)]. The transport time becomes longer than the hopping characteristic time and makes the observation of the minor component difficult. At low temperatures the mobilities in our 30/30 CdTe/Cd0 s2Mn0.~sTe superlattice depicted in Fig. 3(b) are of the same order of magnitude as those reported for electrons in the GaAs/A1GaAs system[6,14]. In fact, the ambipolar diffusion coefficient is expected to be close to the hole diffusion coefficient (Damb ~ 2 D,)[15]. At 10 K, we estimate a diffusion coefficient of 15 cm2/s which is intermediate between electron and hole diffusion coefficient in a 30/30 GaAs/Al0.15Ga0.ssAs superlattice (30 and 2 cm2/s, respectively)[10]. Hole mobility seems then somewhat higher in the II-VI heterostructure. If we consider the small number of data available in the literature for II-VI systems, the estimated value of 4 × 104 cm"/Vs

PVl. ROUSSIGNOL et al.

1124

seems also to be intermediate between the electron mobility (5 x 104-10 x 104 cm: /Vs[13,16,17]) and the hole mobility (1.5 x 104cm2/Vs in a H g T e / C d T e superlattice[17]). At higher temperatures, the ambipolar mobility ( ~ 20 cm:/Vs at 100 K) is lower than the values reported for holes in these materials (200cm2/Vs in a H g T e / C d T e superlattice[17] and 1200cm:/Vs in bulk CdTe[13] at 170K). However hole mobilities are d e p e n d e n t on the valence b a n d structure and can be hardly c o m p a r e d so directly, especially at high temperatures, where acoustic and optical p h o n o n scattering plays a m a j o r role and is also much sensitive to the valence b a n d dispersion via the Fr6hlich potential. CONCLUSION We have presented experimental results, obtained by means of time-resolved photoluminescence, on carrier t r a n s p o r t in a I I - V I Cd0.a0Mn0~sTe/CdTe superlattice with two CdTe enlarged wells. Excitation above the b a n d gap o f the superlattice allows us to excite the sample near its surface. Band t r a n s p o r t occurs but c.w. PL spectra show that localization of carriers plays an i m p o r t a n t role in this structure. A clear thermally activated b e h a v i o u r is observed. In the time-resolved m e a s u r e m e n t s two t r a n s p o r t mechanisms with different time scales are individuated. The faster one can be assigned to Bloch t r a n s p o r t along the growth axis of the heterostructure, the slower one to hopping between localized states. At low temperature, ambipolar mobilities of 40000 cm2/Vs have been estimated for Bioch transport a n d 300 cm-'/Vs for hopping. The two order of magnitude difference between the two mobilities is consistent with calculations in the G a A s / A I G a A s system[l 1]. These values can be c o m p a r e d and are in reasonable agreement with the values reported in similar systems[l 3,16,17]. Acknowledgements--We would like to thank G. Bastard, E. Deleporte and M. Colocci for helpful discussions. The

Laboratoire de Physique de la Mati6re Condensee is "'Laboratoire associ6 fi l'Universit6 Paris VI et au CNRS". One of the authors (J. M-P.) thanks the Spanish "Ministerio de Educaci6n y Ciencia'" for financial support.

REFERENCES

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