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Molecular beam epitaxial growth and characterization of ZnTe and ZnTe-CdTe superlattice
Journal of Crystal Growth 119 ( 992) 322—325 North-Holland
Jou~, o,
CRYSTAL GROWTH
Molecular beam epitaxial growth and characterization of ZnTe and ZnTe—CdTe superlattice Jie Li
~,
Li He
~,
Nanchang Zhu
b,
Pudong Lao
C
and Shixin Yuan
a
Shanghai Institute of Technical Physics, (‘/unese Academy of Sciences, 420 Zhong Shun Bei Yi Street, Shanghai 200083, People ‘s Republic of China Shanghai Institute of Metallurgy. Chinese Academy of Sciences, Shanghai 20050, People Is Republic of China Department of Physics, Fudan Unii’ersitv. Shanghai 200433, People Is Republic of China Received 12 September 1991 manuscript received in final form 17 December 1991
Molecular beam epitaxial (MBE) growth of ZnTe and ZnTc—CdTc superlattice on (iaAs(lOO) substrates was investigated by reflection high energy electron diffraction. Optimal growth parameters were proposed. Characterization of ZnTe film and ZnTe—CdTe superlattices was performed using X-ray diffraction, photoluminescence and Raman scattering. It was found that although growth conditions play an important role, the main condition for achieving a high-quality CdTe—ZnTe superlattice is to avoid the generation of misfit dislocations.
1. Introduction Il—VI alloy semiconductors are of current interest because they offer direct energy gaps coyering the entire infrared and visible bands, but the difficulty in doping the materials to form p—n junctions slowed down their development towards device applications. In this sense, CdTe/ZnTe heterostructures seem promising because ZnTe favors acceptor doping while n- and p-type doped CdTe is available [1,21.Additionally, CdTe/ZnTe is lattice mismatched by 6.4%. The mismatch induced strain is believed to benefit material doping [3]. The strain effect itself is also an interesting research topic which has attracted great attention. Because the growth of CdZnTe ternary alloy is still beyond control in the whole composition range [4], and a strained layer superlattice can be of high-quality [51,a CdTe—ZnTe superlattice may be an ideal substitute of a CdZnTe alloy in terms of effective band gaps and average lattice constants with a variation of the ratio of CdTe to ZnTe thickness. Therefore, a CdTe—ZnTe superlattice is a potential lattice matched buffer layer l)022-0248/92/$l)5.O() © 1992
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for growing HgZnTe on a GaAs substrate, which is a prospective infrared material [6]. As a buffer, CdTe—ZnTe superlattices have further advantages because strained layer superlattices can efficiently bend threading dislocations at the interfaces and thus block the upward propagation of dislocations from the interface of the film/GaAs substrate. Direct transitions of carriers, both in real and in k space, along with strong quantum confinement effects, are expected in a properly designed CdTe—ZnTe superlattice [7], resulting in a high oscillator strength of carrier transitions. Luminescence efficiency was found to be much higher for CdTe—ZnTe superlattices than that for corresponding CdZnTe alloys [81. Recent reports [9,10] on materials of the CdTe—ZnTe system showed excellent properties for applications in optoelectronics. However, the crystalline quality of CdTe—ZnTe superlattices has not, at the present stage, met the needs of device applications [11]. Investigations of both its characterization and crystal growth are still necessary. In this paper, we present our work on MBE growth and characteriza-
Elsevier Science Publishers B.V. Al! rights reserved
Jie Li et al. / MBE and characterization of ZnTe and ZnTe—CdTe superlattice
tion of ZnTe films and CdTe—ZnTe superlattices on GaAs(100) substrates. It was found that although growth conditions play an important role, the main condition for obtaining a high-quality CdTe—ZnTe superlattice is to avoid the generation of misfit dislocations,
2. Experimental procedure All the growth experiments were carried out on GaAs(100) substrates with an FW-III MBE machine made in the People’s Republic of China, A GaAs wafer was purchased, polished and stuck on a Mo block by indium. The surface preparation of the GaAs substrate followed the standard procedure [12]. The temperature of the substrate was measured by a thermocouple installed at the back of the Mo block and was calibrated by the melting point of indium. Three effusion cells were employed containing elemental zinc, tellurium and cadmium separately. We used reflection high energy electron diffraction (RHEED) equipment to monitor the deposition processes in situ and investigate the growth mechanism. Characterization of the prepared samples was performed with several methods. Nomarski interference microscopy was applied to observe the surface morphology of the films. X-ray diffraction scans in the vicinity of (004) Bragg reflection of bulk ZnTe, using a copper target as the radiation source, as well as transmission electron microscopy (TEM), were used for examining the structures of ZnTe films and CdTe—ZnTe multilayers. Photoluminescence spectra were accumulated at 90 K excited by the light of a CW argon laser. Raman scattering measurements were conducted at room temperature under off-resonance conditions.
3. Results and discussion 3.1. Growth of ZnTe and ZnTe—CdTe superlattice For MBE growth of a ZnTe(100) film on a GaAs(100) substrate, the most important parameters were the growth temperature and the source
323
flux ratio of Zn to Te2. We defined the flux ratio R PZn/PT~, where P~ and ‘~Te are the corresponding vapor pressures of Zn and Te2 flux, respectively, measured at the growth position. ZnTe films were deposited in a temperature range of 250—330°C. High-quality crystals, in terms of structural perfection and smooth surface, were achieved at temperatures above 300°C.When the growth temperature dropped from 300°C, the brightness of the Kikuchi bands on the RHEED screen degraded gradually, although the diffraction patterns still remained streaky and resolved indicating a smooth surface. At temperatures below 270°C, rough surfaces, observed through RHEED and by Nomarski microscopy, were obtamed and Te precipitates were found by TEM to be simular to early TEM observations [13] on MBE grown ZnTe films. Large lattice parameter mismatch (~7%) between GaAs and ZnTe resulted in the initial growth of ZnTe in a three-dimensional mode. In order to rapidly achieve a two-dimensional growth mode, Te-rich growth conditions (R < 1) were employed in the initial stage of the growth. The processes were monitored by RHEED. Once an atomic flat surface was achieved, R was increased to a value R~to continue the growth, at which the RHEED pattern showed half-order streaks, while the electron beam was in the KOOl ~ direction and showed half-order streaks along [011], but a xl pattern in the [011] direction. This indicated a mixing surface reconstruction of c(2 x 1) and (2 x 1). It was found that the R value adopted affected the crystalline quality and the best crystals were obtained at R R~.This indicated that the surface composition of the constituent elements is correlated with the bulk properties of our MBE films. The mixing surface reconstruction at R R~meant a coexistence of Te-covered and Zn-covered growth fronts; hence, the setup of surface stoichiometry. A minimum number of vacancies of constituent atoms were expected under these conditions. But equal Zn and Te2 fluxes did not lead to surface stoichiometry. The fact [141that the growth from the ZnTe compound source was observed to exhibit the (2 x 1) surface reconstruction gave also evidence that equal Zn and Te2 fluxes cannot lead to =
=
=
Jie Li et al.
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/ MBL
and characterization of ZnTe and Zn Te—CdTe superlattice
surface stoichiometry. R~was around 1.5 for a smooth surface at 300°Cin our experiments, hut it was related to the growth temperature and surface morphology, ZnTe—CdTe superlattices were grown on thick (1—2 ~m) ZnTe buffer layers on GaAs(100) sub-
periods. Clearly defined satellite peaks of the superlattice, marked by their orders, can be ohserved. The zeroth order superlattice peak was identified by a theoretical simulation. It is noted that the shapes of the diffraction peaks are asymmetric. This is due to the contribution of the Kcs7
strates in a temperature range of 270—310°C at various flux ratios. The best result was obtained when the substrate was at 300°Cand the fluxes of Zn, Te, and Cd were adjusted to achieve surface stoichiometry. ZnTe and CdTe were lattice parameter mismatched by 6.4%. The critical thickness beyond which lattice strain will relax is only 5 monolayers for CdTe/ZnTe epitaxy [15]. We prepared various superlattice samples including those having CdTe layers much thinner than the critical thickness because misfit-induced defects might be generated even before approaching the critical thickness [16].
light diffraction. In addition to the satellite peaks, (004) diffractions of the GaAs substrate and the ZnTe buffer layer are also obvious. The angular distance between the peaks of the substrate and the buffer layer is 2.70°, equal to the angular distance between bulk GaAs and ZnTe within the experimental error. This indicates that the ZnTe buffer layer is free-standing on the GaAs substrate. This is correct because the buffer layer thickness deduced from its growth parameter is 1.5 ~m, far exceeding the critical thickness of the epitaxy of ZnTe on GaAs(lOO). The dynamical X-ray diffraction theory [17] was applied to calculate and simulate the diffraction curves. The simulation result is illustrated by the solid curve in fig. Ia. The peak position and intensity distribution are consistent with the experimental curve. From the simulation, we obtained that the mdividual layer thicknesses of the superlattice were CdTe(4.7 A)/ZnTe(33.2 A), and that the interfaces of the ZnTe—CdTe superlattice were coherently grown on the ZnTe buffer layer without
3.2. X-ray diffraction X-ray diffraction measurements for characterization of the structure and strain distribution of the grown ZnTe—CdTe superlattices were carried out. The dotted curve of fig. la is the diffraction scan around (004) Bragg diffraction from sample #9019, which contains 40 repeats of ZnTe/CdTe
b o —
0 o
-
~
0:3
0o -
—1
~
°~
~0
~
°
-
s—..
~e. 0
~
0
co
0
08° -
—2
~‘
c
°
-4
~
A~(DE0REE)
p
0
03
+1
p
Buffer layer—
4
A~(DEGREE)
Fig. I. X-ray diffraction scans (circles) in the vicinity of the (400) Bragg diffraction of ZnTe. The solid curves are the results of dynamical simulations. Samples: (a) #9019; (b) #9033.
Jie Li et al.
/ MBE and
characterization of Zn Te and ZnTe —CdTe superlattice
relaxing mismatch strain through defect generation. The strain is distributed solely within CdTe layers and the ZnTe maintains its bulk material properties. It is known that a superlattice grown on a substrate obeys a critical thickness criterion that is obeyed by an alloy of the same thicknessand volume-averaged composition. According to the theory of People and Bean [18], the entire superlattice thickness of sample #9019 shown in ftg. la is less than its critical value. Thts 15 ifl agreement with the above analyses. Fig. lb shows both the experimental and the theoretical simulation results of X-ray diffraction in the angle vicinity of the (004) peak of bulk ZnTe from sample #9033 which has 10 periods of ZnTe/ CdTe on a 2 ~im ZnTe buffer layer. The peak from the ZnTe buffer layer can be observed while the diffraction from the GaAs substrate is omitted. Eighth diffraction order of the superlattice peaks is obvious in spite of the small number of periods. Theoretical calculations gave that the thicknesses of the constituent layers of this superlattice were CdTe(19.5 A)/ZnTe(104.0 A) and that the mismatch strain within the CdTe had relaxed to a great extent. Therefore, a large number of misfit dislocations are to be expected at the CdTe/ ZnTe interfaces. This is reasonable because the CdTe thickness is beyond the critical value, 3.3. Photoluminescence Photoluminescence measurements were performed at 90 K on the ZnTe films and ZnTe— CdTe superlattices were prepared. The excitation light was higher than the sample band gaps in energy. An exciton peak dominated over all the photoluminescence spectra. The exciton peak from the ZnTe—CdTe superlattices was much stronger in the gross than that from the ZnTe films. This is mainly due to carrier confinement in quantum wells. Fig. 2 displays exciton peaks, denoted by (1), (2) and (3), from three of the prepared superlattice samples. These three spectra were obtained at the same pump intensity, Peaks (1) and (3) are from samples #9019 and #9033, respectively. Peak (2) is from sample #9020 which has a structure analogous to that of
325
-~
(2)
15
16~~ 18 19 4cm~) PHOTON ENERGY (l0 Fig. 2. Exciton peaks of photolumineseenee spectra at 90 K of samples: (1) #9019; (2) #9020; (3) #9033.
sample #9019, but the superlattice has sixty CdTe(3.4 A)/ZnTe(54.7 A) periods. For making a comparative investigation, sample #9020 has been purposely grown under conditions away from the optimum. Its growing temperature was 280°C and the source flux ratio was R I for both ZnTe and CdTe growth. It can be seen in fig. 2 that the exciton peak from #9020 is weaker and broader than that from #9019, despite the fact that the X-ray diffraction patterns belong to the unrelaxed case for both samples. We attributed the luminescence degradation of sample #9020 to structural imperfections induced in its growth processes. As described before, R 1 is an offsurface-stoichiometry growth condition. Growth under such conditions might introduce group II element vacancies and worse Te precipitates, especially at lower growth temperatures. These defects provided nonradiative decay paths [19] and thus lowered the total luminescence efficiency, and together with the less smooth superlattice interfaces resulting from the low growth temperature, broadened the excitonic transition. However, it is noticeable in fig. 2 that peak (3) is still much weaker and broader even than peak (2), although sample #9033 was produced with optimal growth parameters. According to the X-ray analysis, sample #9033 was relaxed because its constituent CdTe layers had exceeded the critical thickness. Therefore, rough interfaces and a large number of misfit dislocations are expected in such a heavily mismatched system. This is why its optical properties degrade greatly. From a cornparison between the photoluminescence spectra of the three samples, it is reasonably concluded that although growth conditions play an impor=
=
Jie Li et al.
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/ MBL and
characterization of ZnTe and ZnTe—C~dTesuperlattice
1
-
IL /
>-~
>-
.1 LCi
L~.
120
~
~
~.
170 RAMAN SHIFT
_
220
120
170 PAMAN SHIFT
cm~)
220 [cm
Fig. 3. Spectra of Raman scattering at room temperature under off-resonance excitation conditions. Samples: (a) #9013; (h) #91)16.
tant role in achieving high-quality ZnTe—CdTc superlattices. the main problem is to design the mismatch strain to avoid any relaxations. In this sense, we have reported [20] the growth of multiquantum well materials composed of ultra-thin period ZnTe—CdTe superlattices for improving the structural quality. From this structure, we observed intense and narrow photoluminescence even at room temperature, indicating a great progress relative to the earliest ZnTe—CdTe superlattices [8] from which photoluminescence disappeared at liquid-nitrogen temperature.
Part of the superlattice samples have been examined by Raman scattering. Measurements were carried out at room temperature in the hack scattering geometry. The excitation light was the 5145 A line from an argon laser, which gave rise to an off-resonance scattering. Figs. 3a and 3h show the Raman spectra from sample #9013
nate the lowest confined longitudinal optical (LO) phonon modes of the constituent CdTe and ZnTe layers. These two ZnTe—CdTe superlattices are free-standing on their ZnTe buffer layers according to X-ray diffraction analysis and the theory of People and Bean [18]. It is known that the LO phonon frequencies of ZnTe and CdTe at room temperature are 208 and 170 cm [22], respectively. These frequencies were shifted in the superlattices because of the combinational effects of strain and confinement. That is, iw iw~ + iw~,where iw~ represents the strain-induced frequency shift and iw~designates the shift induced by the confinement effect. Referring to the report by Jusserand et al. [23], we calculated iw, for the samples. It must he mentioned that in our calculations, we used the Grüneisen parameter y of ZnTe [24] representing that of CdTe as art approximation because to the best of our knowledge, the Grhneisen parameter of CdTe has not been reported. Confinement effect makes the LO phonons of CdTe and ZnTe degenerate to LO~1
(CdTe 13 A/ZnTe 34 A) and #9016 (CdTe 11 A/ZnTe 47 A), respectively. LC1 and LZ~desig-
multiple modes [25]. Using the dispersion relation [22]. we calculated iw~ of the first-order con-
3.4. Raman scattering
Table I Experimental and theoretical LO~phonon shifts (cm Sample
I)
ZnTe
CdTe
No.
9013 9016
—5.24 —3.45
=
—0.31) —0.17
jw
jw’
—5.54 —3.62
—6.)) —4.5
Jw~
15.83 17.03
1.97 —2.23 —
isv’
13.86 14.81)
4.5 17.5
Jie Li eta!.
/ MBE and
characterization of ZnTe and ZnTe—CdTe superlattice
fined mode. To sum up the above results, we obtained iw. Calculation results and the experimental frequency shifts iw’ are listed in table 1. It is seen that there is good agreement between the theoretical and the experimental L01 frequency shifts for ZnTe, and that the frequency shift of the ZnTe increases with increasing thickness of the CdTe layers. However, the LC1 peak shift of sample #90 16 does not agree so well with the calculations. This may be due to the y approximation made in the calculations and the weakness of the LC1 peak itself. In addition, we obtained that confinement-induced frequency shifts can be more than 1 cm~, as long as the layer thicknesses of ZnTe—CdTe superlattices are less than 20 A. Confinement effects were neglected in some calculations [26] for strained layer superlattices. Our results suggest that confinement effects should be considered for some extreme cases. The peaks at 130 and 140 cmt in figs. 3a and 3b are ascribed to impurities absorbed on sample surfaces because they appeared in Raman spectra from different samples (Si andt GaAs, peak isfor notexamfully pIe). of the 154 cm clear.The It origin may concern impurities or defects in CdTe layers because it was also observed in Raman spectra of CdTe films. We noted that in fig. 3a there is a weak structure at 142.5 cm_t, very close to the 140 cmt peak. Amirtharaj and Pollak [271 measured Raman scattering from polycrystalline Te on CdTe surfaces and observed a 141 cmt peak originating from phonons with an E symmetry in the trigonal Te crystal. We identified the 142.5 cmt peak as this phonon mode of Te precipitates in sample #9013. The frequency deviation is attributed to the.effect of a tensile stress [27] on Te precipitates in the sample. Sample #9013 was produced in our preliminary growth runs when the growth parameters had not yet been optimised. This sample is poor in quality when comparing it with sample #9016 on the basis of the linewidth of LZ 1 peak [28] and the intensity of Rayleigh scattering of the excitation line [291. Therefore, growth conditions influence the quality of ZnTe—CdTe superlattices and inappropriate growth parameters can even lead to the generation of Te precipitates. This
327
agrees with TEM observations and photoluminescence measurements.
4. Conclusion MBE growth of ZnTe and ZnTe—CdTe superlattices was investigated in detail. Growth ternperature and source flux ratio were found to be the main parameters to influence the crystal qua!ity. Optimal growth conditions require a substrate temperature of 300°C and a flux ratio of the constituents to correspond to the stoichiometry of the growing surface. Although the growth conditions play an important role, the main problem for obtaining a high-quality CdTe—ZnTe superlattice is to avoid the generation of misfit dislocations. A CdTe—ZnTe superlattice can be of highquality with an unrelaxed and properly distributed strain. The lowest confined LO modes of CdTe and ZnTe were first observed by Raman scattering in CdTe—ZnTe superlattices at room temperature under off-resonance conditions. Theoretical calculations havescattering successfully explained the spectra of Raman and suggested that confinement effects should not be neglected for some extreme cases of strained-layer superlattices.
Acknowledgements The authors would like to thank Meifang Yu, Yimin Qiao and Xingyu Cheng for their technical assistance.
References [1] RN. Bicknell, NC. Oiles and J.F. Schetzina. AppI. Phys. Letters 49 (1986) 1735.
121
RN. Bicknell, NC. Giles and J.F. Schetzina, AppI. Phys. Letters 49 (1986) 1095. [31 Shang Yuan Ren, J.D. Dow and S. Klemn, J. AppI. Phys. 66 (1989) 2065. [41RD. Feldman, R.F. Austin, A.H. Dayem and El-I. Westerwick, AppI. Phys. Letters 49 (1986) 797. Osbourn, J. AppI. Phys. 53 (1982) 1586.
[51G.C.
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[6] 5. Sivananthan, X. Chu, M. Boukerehe and J.P. Faurie. AppI. Phys. Letters 47 (1985) 1291. [7] Fl. Mathieu. J. Allegre, A. Chatt. P. l,efehvre and J.P. Faurie. Phys. Rev. B 38 (1988) 7741). [8] RH. Miles. G.Y. Wu, MB. Johnson, T.C. McGill. J.P. Faurie and S. Sivananthan, AppI. Phys. Letters 48 (1986) 1383. [9] A. Partovi, A.M. Glass. DII. Olson, RI). Feldman, RE. Austin, D. Lee. AM. Johnson and DAB. Miller. Appi. Phys. Letters 58 (1991) 334. [I))] D. Lee, J.E. Zucker. AM. Johnson, RD. Feldman and RE. Austin, AppI. Phys. Letters 57 (1990) 1132. Ill] AM. Glass. K. Tai, RB. Bylsma. RD. Feldman, D.H. Olson and R.F. Austin, AppI. Phys. Letters 53 (1989) 834. [12] J.P. Faurie, C. Hsu. S. Sivananthan and X. Chit. Surface Sci. 168 (1986) 473. 1131 J. Petruzzello. Di. Olcgo, X. Chu and J.P Fauric, J. AppI. Phys. 63 (1988) 1783. [14] CL Monfroy, S. Sivananthan, X. C’hu. J.P. Faurie, RI). Knox and J.L. Staudenmann, AppI. Phys. Letters 49 (1986) 152. [15] J. Cihert, Y. Gobil, Le Si Dang, S. Tatarenko. G. Feuillet, P.11. Jouneau and K. Saminadayar, AppI. Phys. Letters 56 (1990) 292. [16] S.V. Ghaisas and A. Madhukar, J. Vacuum Sci. Technol. B 7(1989) 264.
of Znlc and ZnTe—CdTe ,superlattice [17] CR. Wic. TA. Tombrello and T. Vrecland. J. AppI. Phys. 59(1986)3743. [181 R. People and J.C. Bean, AppI. Phys. Letters 47 (1985) 322. [191 M.D. Camras, J.M. Brown, N. Holonyak, Jr.. Ml). Nixon. R.W. Kaliski. Mi. Ludowise. W.T. Dietze and CR. Lewis. J. Appl. Phys. 54(1983) 6183. [20] Jie Li, Li 1-Ic. Wci Shan. Xingyu Cheng and Shixin Yuan, 3. (‘rystal Growth 111)1991) 736. [2!] H. Mathieu. A. Chait. J. Allegre and J.P. Fauric, Phys. Rev. B 41)199))) 61)82. 122] l).J. Olego, P.M. Raccah and J.P. Faurie, Phys, Rev. B 33 (1986)3819. [23] B. Jusserand, P. Voisin and M. Voos. AppI. Phys. Letters 46 (1985) 678. [24] Shun Le Hong, K. lnoue and K. Murase. Solid Statc (lommun. 62(1987) 621. [25] AK. Sood. J. Menendez and M. (‘ardona. Phys. Rev. Letters 54 (1985) 2111. [26] M.K. Jackson, RH. Miles and T.C. McGill, AppI. Phys. Letters 55 (1989) 786. [27] P.M. Amirtharaj and F.H. Pollak. Appi. Phys. Letters 45 (1984) 789. [28] PD. l,ao, W.C. ‘lang and K.C. Rajknmar. J. AppI. Phys. 67 (1990) 6445. [29] W.C. Tang, PD. Lao and A. Madhukar, AppI. Phys. Letters 52 (198$) 42.
Jou~, o,
CRYSTAL GROWTH
Molecular beam epitaxial growth and characterization of ZnTe and ZnTe—CdTe superlattice Jie Li
~,
Li He
~,
Nanchang Zhu
b,
Pudong Lao
C
and Shixin Yuan
a
Shanghai Institute of Technical Physics, (‘/unese Academy of Sciences, 420 Zhong Shun Bei Yi Street, Shanghai 200083, People ‘s Republic of China Shanghai Institute of Metallurgy. Chinese Academy of Sciences, Shanghai 20050, People Is Republic of China Department of Physics, Fudan Unii’ersitv. Shanghai 200433, People Is Republic of China Received 12 September 1991 manuscript received in final form 17 December 1991
Molecular beam epitaxial (MBE) growth of ZnTe and ZnTc—CdTc superlattice on (iaAs(lOO) substrates was investigated by reflection high energy electron diffraction. Optimal growth parameters were proposed. Characterization of ZnTe film and ZnTe—CdTe superlattices was performed using X-ray diffraction, photoluminescence and Raman scattering. It was found that although growth conditions play an important role, the main condition for achieving a high-quality CdTe—ZnTe superlattice is to avoid the generation of misfit dislocations.
1. Introduction Il—VI alloy semiconductors are of current interest because they offer direct energy gaps coyering the entire infrared and visible bands, but the difficulty in doping the materials to form p—n junctions slowed down their development towards device applications. In this sense, CdTe/ZnTe heterostructures seem promising because ZnTe favors acceptor doping while n- and p-type doped CdTe is available [1,21.Additionally, CdTe/ZnTe is lattice mismatched by 6.4%. The mismatch induced strain is believed to benefit material doping [3]. The strain effect itself is also an interesting research topic which has attracted great attention. Because the growth of CdZnTe ternary alloy is still beyond control in the whole composition range [4], and a strained layer superlattice can be of high-quality [51,a CdTe—ZnTe superlattice may be an ideal substitute of a CdZnTe alloy in terms of effective band gaps and average lattice constants with a variation of the ratio of CdTe to ZnTe thickness. Therefore, a CdTe—ZnTe superlattice is a potential lattice matched buffer layer l)022-0248/92/$l)5.O() © 1992
—
for growing HgZnTe on a GaAs substrate, which is a prospective infrared material [6]. As a buffer, CdTe—ZnTe superlattices have further advantages because strained layer superlattices can efficiently bend threading dislocations at the interfaces and thus block the upward propagation of dislocations from the interface of the film/GaAs substrate. Direct transitions of carriers, both in real and in k space, along with strong quantum confinement effects, are expected in a properly designed CdTe—ZnTe superlattice [7], resulting in a high oscillator strength of carrier transitions. Luminescence efficiency was found to be much higher for CdTe—ZnTe superlattices than that for corresponding CdZnTe alloys [81. Recent reports [9,10] on materials of the CdTe—ZnTe system showed excellent properties for applications in optoelectronics. However, the crystalline quality of CdTe—ZnTe superlattices has not, at the present stage, met the needs of device applications [11]. Investigations of both its characterization and crystal growth are still necessary. In this paper, we present our work on MBE growth and characteriza-
Elsevier Science Publishers B.V. Al! rights reserved
Jie Li et al. / MBE and characterization of ZnTe and ZnTe—CdTe superlattice
tion of ZnTe films and CdTe—ZnTe superlattices on GaAs(100) substrates. It was found that although growth conditions play an important role, the main condition for obtaining a high-quality CdTe—ZnTe superlattice is to avoid the generation of misfit dislocations,
2. Experimental procedure All the growth experiments were carried out on GaAs(100) substrates with an FW-III MBE machine made in the People’s Republic of China, A GaAs wafer was purchased, polished and stuck on a Mo block by indium. The surface preparation of the GaAs substrate followed the standard procedure [12]. The temperature of the substrate was measured by a thermocouple installed at the back of the Mo block and was calibrated by the melting point of indium. Three effusion cells were employed containing elemental zinc, tellurium and cadmium separately. We used reflection high energy electron diffraction (RHEED) equipment to monitor the deposition processes in situ and investigate the growth mechanism. Characterization of the prepared samples was performed with several methods. Nomarski interference microscopy was applied to observe the surface morphology of the films. X-ray diffraction scans in the vicinity of (004) Bragg reflection of bulk ZnTe, using a copper target as the radiation source, as well as transmission electron microscopy (TEM), were used for examining the structures of ZnTe films and CdTe—ZnTe multilayers. Photoluminescence spectra were accumulated at 90 K excited by the light of a CW argon laser. Raman scattering measurements were conducted at room temperature under off-resonance conditions.
3. Results and discussion 3.1. Growth of ZnTe and ZnTe—CdTe superlattice For MBE growth of a ZnTe(100) film on a GaAs(100) substrate, the most important parameters were the growth temperature and the source
323
flux ratio of Zn to Te2. We defined the flux ratio R PZn/PT~, where P~ and ‘~Te are the corresponding vapor pressures of Zn and Te2 flux, respectively, measured at the growth position. ZnTe films were deposited in a temperature range of 250—330°C. High-quality crystals, in terms of structural perfection and smooth surface, were achieved at temperatures above 300°C.When the growth temperature dropped from 300°C, the brightness of the Kikuchi bands on the RHEED screen degraded gradually, although the diffraction patterns still remained streaky and resolved indicating a smooth surface. At temperatures below 270°C, rough surfaces, observed through RHEED and by Nomarski microscopy, were obtamed and Te precipitates were found by TEM to be simular to early TEM observations [13] on MBE grown ZnTe films. Large lattice parameter mismatch (~7%) between GaAs and ZnTe resulted in the initial growth of ZnTe in a three-dimensional mode. In order to rapidly achieve a two-dimensional growth mode, Te-rich growth conditions (R < 1) were employed in the initial stage of the growth. The processes were monitored by RHEED. Once an atomic flat surface was achieved, R was increased to a value R~to continue the growth, at which the RHEED pattern showed half-order streaks, while the electron beam was in the KOOl ~ direction and showed half-order streaks along [011], but a xl pattern in the [011] direction. This indicated a mixing surface reconstruction of c(2 x 1) and (2 x 1). It was found that the R value adopted affected the crystalline quality and the best crystals were obtained at R R~.This indicated that the surface composition of the constituent elements is correlated with the bulk properties of our MBE films. The mixing surface reconstruction at R R~meant a coexistence of Te-covered and Zn-covered growth fronts; hence, the setup of surface stoichiometry. A minimum number of vacancies of constituent atoms were expected under these conditions. But equal Zn and Te2 fluxes did not lead to surface stoichiometry. The fact [141that the growth from the ZnTe compound source was observed to exhibit the (2 x 1) surface reconstruction gave also evidence that equal Zn and Te2 fluxes cannot lead to =
=
=
Jie Li et al.
324
/ MBL
and characterization of ZnTe and Zn Te—CdTe superlattice
surface stoichiometry. R~was around 1.5 for a smooth surface at 300°Cin our experiments, hut it was related to the growth temperature and surface morphology, ZnTe—CdTe superlattices were grown on thick (1—2 ~m) ZnTe buffer layers on GaAs(100) sub-
periods. Clearly defined satellite peaks of the superlattice, marked by their orders, can be ohserved. The zeroth order superlattice peak was identified by a theoretical simulation. It is noted that the shapes of the diffraction peaks are asymmetric. This is due to the contribution of the Kcs7
strates in a temperature range of 270—310°C at various flux ratios. The best result was obtained when the substrate was at 300°Cand the fluxes of Zn, Te, and Cd were adjusted to achieve surface stoichiometry. ZnTe and CdTe were lattice parameter mismatched by 6.4%. The critical thickness beyond which lattice strain will relax is only 5 monolayers for CdTe/ZnTe epitaxy [15]. We prepared various superlattice samples including those having CdTe layers much thinner than the critical thickness because misfit-induced defects might be generated even before approaching the critical thickness [16].
light diffraction. In addition to the satellite peaks, (004) diffractions of the GaAs substrate and the ZnTe buffer layer are also obvious. The angular distance between the peaks of the substrate and the buffer layer is 2.70°, equal to the angular distance between bulk GaAs and ZnTe within the experimental error. This indicates that the ZnTe buffer layer is free-standing on the GaAs substrate. This is correct because the buffer layer thickness deduced from its growth parameter is 1.5 ~m, far exceeding the critical thickness of the epitaxy of ZnTe on GaAs(lOO). The dynamical X-ray diffraction theory [17] was applied to calculate and simulate the diffraction curves. The simulation result is illustrated by the solid curve in fig. Ia. The peak position and intensity distribution are consistent with the experimental curve. From the simulation, we obtained that the mdividual layer thicknesses of the superlattice were CdTe(4.7 A)/ZnTe(33.2 A), and that the interfaces of the ZnTe—CdTe superlattice were coherently grown on the ZnTe buffer layer without
3.2. X-ray diffraction X-ray diffraction measurements for characterization of the structure and strain distribution of the grown ZnTe—CdTe superlattices were carried out. The dotted curve of fig. la is the diffraction scan around (004) Bragg diffraction from sample #9019, which contains 40 repeats of ZnTe/CdTe
b o —
0 o
-
~
0:3
0o -
—1
~
°~
~0
~
°
-
s—..
~e. 0
~
0
co
0
08° -
—2
~‘
c
°
-4
~
A~(DE0REE)
p
0
03
+1
p
Buffer layer—
4
A~(DEGREE)
Fig. I. X-ray diffraction scans (circles) in the vicinity of the (400) Bragg diffraction of ZnTe. The solid curves are the results of dynamical simulations. Samples: (a) #9019; (b) #9033.
Jie Li et al.
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characterization of Zn Te and ZnTe —CdTe superlattice
relaxing mismatch strain through defect generation. The strain is distributed solely within CdTe layers and the ZnTe maintains its bulk material properties. It is known that a superlattice grown on a substrate obeys a critical thickness criterion that is obeyed by an alloy of the same thicknessand volume-averaged composition. According to the theory of People and Bean [18], the entire superlattice thickness of sample #9019 shown in ftg. la is less than its critical value. Thts 15 ifl agreement with the above analyses. Fig. lb shows both the experimental and the theoretical simulation results of X-ray diffraction in the angle vicinity of the (004) peak of bulk ZnTe from sample #9033 which has 10 periods of ZnTe/ CdTe on a 2 ~im ZnTe buffer layer. The peak from the ZnTe buffer layer can be observed while the diffraction from the GaAs substrate is omitted. Eighth diffraction order of the superlattice peaks is obvious in spite of the small number of periods. Theoretical calculations gave that the thicknesses of the constituent layers of this superlattice were CdTe(19.5 A)/ZnTe(104.0 A) and that the mismatch strain within the CdTe had relaxed to a great extent. Therefore, a large number of misfit dislocations are to be expected at the CdTe/ ZnTe interfaces. This is reasonable because the CdTe thickness is beyond the critical value, 3.3. Photoluminescence Photoluminescence measurements were performed at 90 K on the ZnTe films and ZnTe— CdTe superlattices were prepared. The excitation light was higher than the sample band gaps in energy. An exciton peak dominated over all the photoluminescence spectra. The exciton peak from the ZnTe—CdTe superlattices was much stronger in the gross than that from the ZnTe films. This is mainly due to carrier confinement in quantum wells. Fig. 2 displays exciton peaks, denoted by (1), (2) and (3), from three of the prepared superlattice samples. These three spectra were obtained at the same pump intensity, Peaks (1) and (3) are from samples #9019 and #9033, respectively. Peak (2) is from sample #9020 which has a structure analogous to that of
325
-~
(2)
15
16~~ 18 19 4cm~) PHOTON ENERGY (l0 Fig. 2. Exciton peaks of photolumineseenee spectra at 90 K of samples: (1) #9019; (2) #9020; (3) #9033.
sample #9019, but the superlattice has sixty CdTe(3.4 A)/ZnTe(54.7 A) periods. For making a comparative investigation, sample #9020 has been purposely grown under conditions away from the optimum. Its growing temperature was 280°C and the source flux ratio was R I for both ZnTe and CdTe growth. It can be seen in fig. 2 that the exciton peak from #9020 is weaker and broader than that from #9019, despite the fact that the X-ray diffraction patterns belong to the unrelaxed case for both samples. We attributed the luminescence degradation of sample #9020 to structural imperfections induced in its growth processes. As described before, R 1 is an offsurface-stoichiometry growth condition. Growth under such conditions might introduce group II element vacancies and worse Te precipitates, especially at lower growth temperatures. These defects provided nonradiative decay paths [19] and thus lowered the total luminescence efficiency, and together with the less smooth superlattice interfaces resulting from the low growth temperature, broadened the excitonic transition. However, it is noticeable in fig. 2 that peak (3) is still much weaker and broader even than peak (2), although sample #9033 was produced with optimal growth parameters. According to the X-ray analysis, sample #9033 was relaxed because its constituent CdTe layers had exceeded the critical thickness. Therefore, rough interfaces and a large number of misfit dislocations are expected in such a heavily mismatched system. This is why its optical properties degrade greatly. From a cornparison between the photoluminescence spectra of the three samples, it is reasonably concluded that although growth conditions play an impor=
=
Jie Li et al.
326
/ MBL and
characterization of ZnTe and ZnTe—C~dTesuperlattice
1
-
IL /
>-~
>-
.1 LCi
L~.
120
~
~
~.
170 RAMAN SHIFT
_
220
120
170 PAMAN SHIFT
cm~)
220 [cm
Fig. 3. Spectra of Raman scattering at room temperature under off-resonance excitation conditions. Samples: (a) #9013; (h) #91)16.
tant role in achieving high-quality ZnTe—CdTc superlattices. the main problem is to design the mismatch strain to avoid any relaxations. In this sense, we have reported [20] the growth of multiquantum well materials composed of ultra-thin period ZnTe—CdTe superlattices for improving the structural quality. From this structure, we observed intense and narrow photoluminescence even at room temperature, indicating a great progress relative to the earliest ZnTe—CdTe superlattices [8] from which photoluminescence disappeared at liquid-nitrogen temperature.
Part of the superlattice samples have been examined by Raman scattering. Measurements were carried out at room temperature in the hack scattering geometry. The excitation light was the 5145 A line from an argon laser, which gave rise to an off-resonance scattering. Figs. 3a and 3h show the Raman spectra from sample #9013
nate the lowest confined longitudinal optical (LO) phonon modes of the constituent CdTe and ZnTe layers. These two ZnTe—CdTe superlattices are free-standing on their ZnTe buffer layers according to X-ray diffraction analysis and the theory of People and Bean [18]. It is known that the LO phonon frequencies of ZnTe and CdTe at room temperature are 208 and 170 cm [22], respectively. These frequencies were shifted in the superlattices because of the combinational effects of strain and confinement. That is, iw iw~ + iw~,where iw~ represents the strain-induced frequency shift and iw~designates the shift induced by the confinement effect. Referring to the report by Jusserand et al. [23], we calculated iw, for the samples. It must he mentioned that in our calculations, we used the Grüneisen parameter y of ZnTe [24] representing that of CdTe as art approximation because to the best of our knowledge, the Grhneisen parameter of CdTe has not been reported. Confinement effect makes the LO phonons of CdTe and ZnTe degenerate to LO~1
(CdTe 13 A/ZnTe 34 A) and #9016 (CdTe 11 A/ZnTe 47 A), respectively. LC1 and LZ~desig-
multiple modes [25]. Using the dispersion relation [22]. we calculated iw~ of the first-order con-
3.4. Raman scattering
Table I Experimental and theoretical LO~phonon shifts (cm Sample
I)
ZnTe
CdTe
No.
9013 9016
—5.24 —3.45
=
—0.31) —0.17
jw
jw’
—5.54 —3.62
—6.)) —4.5
Jw~
15.83 17.03
1.97 —2.23 —
isv’
13.86 14.81)
4.5 17.5
Jie Li eta!.
/ MBE and
characterization of ZnTe and ZnTe—CdTe superlattice
fined mode. To sum up the above results, we obtained iw. Calculation results and the experimental frequency shifts iw’ are listed in table 1. It is seen that there is good agreement between the theoretical and the experimental L01 frequency shifts for ZnTe, and that the frequency shift of the ZnTe increases with increasing thickness of the CdTe layers. However, the LC1 peak shift of sample #90 16 does not agree so well with the calculations. This may be due to the y approximation made in the calculations and the weakness of the LC1 peak itself. In addition, we obtained that confinement-induced frequency shifts can be more than 1 cm~, as long as the layer thicknesses of ZnTe—CdTe superlattices are less than 20 A. Confinement effects were neglected in some calculations [26] for strained layer superlattices. Our results suggest that confinement effects should be considered for some extreme cases. The peaks at 130 and 140 cmt in figs. 3a and 3b are ascribed to impurities absorbed on sample surfaces because they appeared in Raman spectra from different samples (Si andt GaAs, peak isfor notexamfully pIe). of the 154 cm clear.The It origin may concern impurities or defects in CdTe layers because it was also observed in Raman spectra of CdTe films. We noted that in fig. 3a there is a weak structure at 142.5 cm_t, very close to the 140 cmt peak. Amirtharaj and Pollak [271 measured Raman scattering from polycrystalline Te on CdTe surfaces and observed a 141 cmt peak originating from phonons with an E symmetry in the trigonal Te crystal. We identified the 142.5 cmt peak as this phonon mode of Te precipitates in sample #9013. The frequency deviation is attributed to the.effect of a tensile stress [27] on Te precipitates in the sample. Sample #9013 was produced in our preliminary growth runs when the growth parameters had not yet been optimised. This sample is poor in quality when comparing it with sample #9016 on the basis of the linewidth of LZ 1 peak [28] and the intensity of Rayleigh scattering of the excitation line [291. Therefore, growth conditions influence the quality of ZnTe—CdTe superlattices and inappropriate growth parameters can even lead to the generation of Te precipitates. This
327
agrees with TEM observations and photoluminescence measurements.
4. Conclusion MBE growth of ZnTe and ZnTe—CdTe superlattices was investigated in detail. Growth ternperature and source flux ratio were found to be the main parameters to influence the crystal qua!ity. Optimal growth conditions require a substrate temperature of 300°C and a flux ratio of the constituents to correspond to the stoichiometry of the growing surface. Although the growth conditions play an important role, the main problem for obtaining a high-quality CdTe—ZnTe superlattice is to avoid the generation of misfit dislocations. A CdTe—ZnTe superlattice can be of highquality with an unrelaxed and properly distributed strain. The lowest confined LO modes of CdTe and ZnTe were first observed by Raman scattering in CdTe—ZnTe superlattices at room temperature under off-resonance conditions. Theoretical calculations havescattering successfully explained the spectra of Raman and suggested that confinement effects should not be neglected for some extreme cases of strained-layer superlattices.
Acknowledgements The authors would like to thank Meifang Yu, Yimin Qiao and Xingyu Cheng for their technical assistance.
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of Znlc and ZnTe—CdTe ,superlattice [17] CR. Wic. TA. Tombrello and T. Vrecland. J. AppI. Phys. 59(1986)3743. [181 R. People and J.C. Bean, AppI. Phys. Letters 47 (1985) 322. [191 M.D. Camras, J.M. Brown, N. Holonyak, Jr.. Ml). Nixon. R.W. Kaliski. Mi. Ludowise. W.T. Dietze and CR. Lewis. J. Appl. Phys. 54(1983) 6183. [20] Jie Li, Li 1-Ic. Wci Shan. Xingyu Cheng and Shixin Yuan, 3. (‘rystal Growth 111)1991) 736. [2!] H. Mathieu. A. Chait. J. Allegre and J.P. Fauric, Phys. Rev. B 41)199))) 61)82. 122] l).J. Olego, P.M. Raccah and J.P. Faurie, Phys, Rev. B 33 (1986)3819. [23] B. Jusserand, P. Voisin and M. Voos. AppI. Phys. Letters 46 (1985) 678. [24] Shun Le Hong, K. lnoue and K. Murase. Solid Statc (lommun. 62(1987) 621. [25] AK. Sood. J. Menendez and M. (‘ardona. Phys. Rev. Letters 54 (1985) 2111. [26] M.K. Jackson, RH. Miles and T.C. McGill, AppI. Phys. Letters 55 (1989) 786. [27] P.M. Amirtharaj and F.H. Pollak. Appi. Phys. Letters 45 (1984) 789. [28] PD. l,ao, W.C. ‘lang and K.C. Rajknmar. J. AppI. Phys. 67 (1990) 6445. [29] W.C. Tang, PD. Lao and A. Madhukar, AppI. Phys. Letters 52 (198$) 42.