- Email: [email protected]
Pb1-xEuxTe heterostructures
Journal of Crystal Growth 127 (1993) 302—307 North-Holland
j~
o~
CRYSTAL
GROWTH
MBE of high mobility PbTe films and PbTe/Pb1 ~Eu~Te heterostructures G. Springholz, 0. Bauer and 0. Ihninger Institut für Haibleiterphysik, Johannes Kepler Universität Linz, A-4040 Linz, Austria
The MBE growth of PbTe and PbEuTe is shown to depend critically on the flux composition (PbTe, Te2, Eu). Using RI-tEED techniques, the flux composition can be determined and adjusted to result in epitaxial layers with high structural 2/Vs at quality, T= 5 K as evidenced have been by achieved. the half-widths Small deviations (30”) of of thethe (222) fluxBragg compositions diffraction. from Electron their optimum mobilities values as high result as in 1.98x a substantial 106 cm decrease of the electron mobilities and an increase of the X-ray diffraction half-widths. In addition, results on the MBE growth of PbTe/Pb 1 _~Eu~Te multi quantum well structures are presented.
1. Introduction Advances in epitaxial growth techniques have recently led to the fabrication of lead salt heterostructure quantum well diode lasers [1,21. In molecular beam epitaxy (MBE) growth of IVVI compounds, the binary compounds (PbTe, PbSe), which vaporize mainly as molecules [31,are usually used as source materials. A chalcogen source is used for the adjustment of the concentration and the type of carriers. For the growth of ternary compounds, an additional metal source (e.g. Eu) is used for the control of composition and a chalcogen source in order to adjust the stoichiometry. In this paper, we present results on the dependence of the MBE growth of PbTe °n the PbTe/Te2 flux composition and on the structural and electronic properties of PbTe epitaxial layers grown with different flux compositions. In addition, structural data on PbTe/Pb1 ~Eu~Te (x <0.04) multi quantum well samples are presented. 2. Experimental procedure Our experiments were carried out in a Riber MBE growth chamber and custom-built preparation and load-lock chambers. For PbTe or PbTe/Pb1 ~Eu~Te heterostructure growth, we 0022-0248/93/$06.00 © 1993
—
used a PbTe, an Eu and two different Te2 effusion cells as beam flux sources. The two different Te cells are used when growing PbTe/Pb1_~ Eu~Teheterostructures in order to adjust both the carrier concentration and the composition independently. In principle, Pb and Te vacancies in the PbTe crystal lattice act as acceptors or donors, respectively, therefore it is impossible to produce undoped intrinsic material. The beam flux rates of the different effusion cells were measured with an ion gauge beam flux monitor. In addition, a quartz crystal thickness monitor was used to determine absolute beam flux rates and to calibrate the ion gauge beam flux monitor ~ Therefore, an exact adjustment of the ratios between the different beam fluxes of PbTe, Eu and Te2 was possible. Films were grown on freshly cleaved (lii) BaF2 substrates with background pressures of 5 x 10 10 mbar during growth. The growth temperatures ranged from 350°Cdown to 160°Cand were calibrated with the melting points of indium (157°C), tin (232°C),bismuth (271°C)and lead (328°C)and with the oxygen desorption temperature of GaAs (582°C). As PbTe vaporizes mainly as PbTe molecules [3], high purity zone refined Bridgman grown PbTe was used as source material. We found that due to fractional dissociation [3] and resublimation of the PbTe within the effusion
Elsevier Science Publishers By. All rights reserved
G. Springholz et a!.
/ MBE of high mobility PbTe films and
cell, the flux composition deviates from the initial composition of the source material becoming increasingly Pb-rich, always resulting in n-type films. The dissociated Pb atoms from the sublimation of PbTe have a lower vapour pressure than PbTe, whereas the dissociated Te2 molecules have a much higher vapour pressure at the effusion cell temperature (520°C).The dissociated Pb atoms consequently evaporated more slowly from the effusion cell which leads to an increase of the overall Pb concentration in the crucible. Consequently, we use a Te-rich PbTe source material with a nominal composition of Pb0 49Te051. A Te effusion cell is used in addition to fine-adjust the beam flux composition and to compensate for the compositional change of the PbTe source material. In situ RHEED experiments were carried out with a 35 keV electron beam directed on the sample at small angles of incidence typically around 0.3°. We have studied the MBE growth of high mobility PbTe films at substrate temperatures between 300 and 350°C and different beam flux compositions adjusted with an additional Te2 beam flux source. First, the freshly cleaved (111) BaF2 substrates were baked at 490°Cfor 15 mm after which sharp streaks are observed in the RHEED diffraction pattern [5]. During the 3D nucleation of PbTe on BaF2 (lattice mismatch of 4.2%), the pattern changes to the spotty transmission RHEED pattern. However, already after 8 mm of growth (= 1400 A) the RHEED pattern returned to the familiar RHEED streak pattern indicating the reformation of a smooth surface. The thickness of the samples was typically about 3—5 ~tm in order to reduce the influence of the highly disordered interface between film and BaF2 substrate. In general, no clear surface reconstruction of the (111) PbTe surface can be seen, except after long term annealing and subsequent cooling to low substrate temperatures, after which weak half order streaks in the sixfold [110] directions appear. 3. Adjustment of the beam flux composition The IV—VI compounds are know to evaporate mainly as PbTe or PbSe molecules [3] with only
PbTe/ Pb
1 - Eu~TeHSs
303
minor fractions of dissociated species. Therefore, a rather stoichiometric flux composition can be expected from the evaporation of PbTe. We found, however, that the flux composition deviates from the initial composition of the PbTe source material, usually having a higher overall Pb content. This we partly account for by using a non-stoichiometric, Te-rich PbTe source material with a nominal composition of Pb049Te~51.This results in a flux composition closer to the needed exact 1: 1 composition. However, the exact flux composition changes after ever reloading of the effusion cells and for the different PbTe source materials which we used. In addition, the composition becomes increasingly Pb-enriched with operation time of the effusion cell. This can in principle be compensated with a small additional Te2 flux, but this is quite delicate to achieve, because of the difficulty to determine the exact flux composition. We have developed a novel in situ method to determine the flux composition using RHEED. This method is based on the special properties of the PbTe (111) surface and is described briefly in the following. Because of the NaC1 type crystal structure, the PbTe (111) surface consists of either only Pb atoms or Te atoms. We have previously shown [4,6] that for elevated substrate temperatures, the equilibrium stable (111) PbTe surface is a Pbstabilized surface due to Te desorption from the surface. For L, lower than roughly 200°C, this surface state can be changed to a Te-stabilized surface with an adequate Te2 flux impinging on the surface, depending on the substrate temperature. This change of surface stoichiometry can be monitored by the RHEED specular spot which has a higher intensity level for the Pb-stabilized than for the Te-stabilized surface [4,6]. If growth is started on a Pb-stabilized surface with a Te-rich flux composition and sufficiently low substrate temperatures around 1, = 160°C,the surface state then gradually changes from the Pb-stabilized to the Te-stabilized surface as growth proceeds. Consequently, the specular spot intensity changes to the lower intensity level characteristic to the Te-stabilized surface. Superimposed on this gradual intensity change, well-developed RHEED intensity oscillations [6] can be observed. On the ..
.
304
G. Springholz et a!.
/ MBE of high mobility PbTe films and
contrary, if a Pb-rich flux is used for growth, the surface stoichiometry does not change, and as growth proceeds the mean specular intensity remains almost constant, apart from the superimposed RHEED intensity oscillations. Under such conditions we have observed up to N = 230 RHEED oscillation periods [7]. This behaviour has been studied in detail in a previous paper [6] for the MBE growth of Pb 1~Eu~Te(x <4%), which shows a similar behaviour. From this intensity change and from the slope of the change, the overall Te to Pb flux ratio can be determined
PbTe /Pb, - Eu~TeHSs
7
1o
PbTe /
(B
/ ~
106
1
(C)
4. MBE of high mobility PbTe films This general RHEED behaviour was used for characterization and adjusting the beam flux composition in order to improve the epitaxial conditions and to achieve high mobilities in the MBE grown films. The flux composition was adjusted by varying the additional Te2 flux with a constant PbTe flux of 1 .tm/h and recording the RHEED specular spot traces when growth was started from the Pb-stabilized surface at T = 160 C. For the growth of high mobility films, the overall flux composition was chosen such that the Pb-stabilized surface at 1’~= 160°Cwas just maintamed during growth with only a minimal additional Te2 flux. Then films were grown at the regular substrate temperatures of 350°Cwith same overall flux composition. Typically, the additional Te 2 fluxes needed are of the order of less than 1% of the PbTe beam flux rate. Because of the gradual change of the source material composition in the effusion cell, this optimum flux composition has to be readjusted from time to time in order to maintain film qualities. procedure results inthe the high growth of films with This high mobilities above 106 cm2/V s at 5 K. The temperature dependent Hall mobility of a PbTe film grown under such conditions is shown in fig. 1 (upper trace). This film was grown at = 350°Cusing an additional Te 2 flux of ~Te2 = 2 s, which corresponds 3.1 ax0.07% 1fjil molecules/cm to addition to the PbTe flux of 0.8 mI/s (1 ml = 5.54 x 1014 molecules/cm2). This film exhibits a mobility of 1.98 x 106 cm2/V s at 5 K,
1 0~ _______________________________ 5
10
20
30
50
100 200 300 ( K ) Fig. 1. Hall mobilities of representative PbTe samples grown
Temperature
on (111) BaF2 at T~= 350°C,but with different flux composition: (A) optimized flux composition with an additional Te 2 flux of 0.07% to the PbTe flux of 0.8 ml/s, (B) without additional Te2 flux and (C) after Pb-enrichment of the PbTe source material in the effusion cell and without additional Te2 flux. All films are n-type carrier concentrations between 17 with and —lXlO’7cm. —4X10
which is the highest reported mobility for MBE grown PbTe films, the mobility being almost a factor of 3 larger than has been reported so far. No post-growth anneal of the samples was performed. The carrier 3. concentration of this film is Similar high mobilities nhave = —3.8 x 1017 cm so far only been reported for hot wall epitaxial grown PbTe films, however, using much higher substrate temperatures of 450°C, which are unsuited for heterostructure growth, and an additional long term 20—30 h post-growth annealingDeviations at 300—350°C[8]. from the optimum flux composition can be either to the Te-rich or Pb-rich side. Increasing the overall Te concentration in the flux results in highly compensated films, although
G. Springholz et aL
/ MBE of high mobility PbTefilms and PbTe /Pb1
their structural quality and surface smoothness is similar to the high mobility samples. Increasing the overall Pb concentration of the flux (or decreasing the Te concentration) results in roughening of the samples, strongly reduced low temperature mobilities and less structural perfection (broadening of the X-ray rocking curves). The importance of the adjustment of beam flux cornposition is illustrated in fig. 1, where the Hall mobilities of three different representative samples are shown. For these films, identical growth conditions were used (T~= 350°C, 1 nm/h, 3—4 ~m thickness and low background pressures of 4 x 1’0’° mbar), except for differences in the overall flux composition. Sample A (upper trace) was grown with optimized flux composition as described above. Sample B (middle trace) was grown without additional Te2 flux, and sample C (lowest trace) was grown at a later stage where the PbTe source material was already significantly enriched Pb.with Therespect Te deficiency in this case was about with 1.5% to the overall —
Pb-to-Te ratio. As seen, sample B has already 2/V~~a reduced 5 K mobility of ~.t= 1.24 X 10~cm which is almost a factor of 2 smaller than for sample A, whereas for sample the 5 Kis mobility 2/V s,C, which even a is only ~ = 390 000 cm factor of 5 smaller. In addition, this sample shows an increase of the surface roughness. All films are n-type with about the same carrier concentrations (—3.8 x l0~ cm3 for sample A, 1.0 X 1017 cm3 for sample B and —2.9 x 10~’cm3 for sample C). However, it is interesting to note that in spite of these large differences in the low temperature mobilities, the room temperature —
-
~Eu~Te HSs
305
PbTe (222)
BaF2 (222) 50
1-’
E
>
5)
________________________________ 24.3
24.35
24.4
24.45
24.5
24.55
Omega (deg) Fig. 2. X-ray (222) Bragg rocking curves for two different PbTe samples. high mobility (~z5K>sam10~ 2/V. s) with Lower FWHMtrace: = 30”, upper trace:sample low mobility cm pie (tLIK = 200000 cm2/Vs) with FWHM = 50”. Insert shows BaF 2 substrate (222) Bragg peak.
BaF 2 substrate. This FWHM is not very much larger than the half-width of the BaF2 (222) Bragg reflection which is about of 17”the (see in fig. 2). In contrast, the FWHM lowinsert mobility films is of the order of 50”, indicating the reduced structural perfection of these films. It should be noted, however, that this strict correlation between the mobilities and the X-ray data is not always observed.
5. PbTe / Pb 1 ~Eu~Te heterostructures —
mobilities of the samples are all almost the same. This is due to the limitation of the mobilities by phonon scattering for temperatures down to 77 K, which also partly explains the success of MBE produced IV—VI devices such as lasers and detectors [1,2,9]. In fig. 2, X-ray rocking curves of the PbTe (222) Bragg reflections are shown for a high mobility (lower trace) and a low mobility (upper trace) sample. The typical half-width (FWHM) of the rocking curves of the high quality samples is around 30”, which is quite narrow considering the high 4.2% lattice mismatch of the films to the
Since the energy gap of Pb1 5Eu~Teincreases steeply with Eu content, the system PbTe/ PbEuTe is particularly suited for the fabrication of type I quantum wells consisting of a combination of a narrow gap well and wide gap barrier. Even for an Eu content as low as 0.04, the energy gap of PbEuTe is 430 meV at 4 K, and therefore more than twice the energy gap of PbTe (190 meV). In addition, the PbTe—EuTe system is miscible for almost the entire range of compositions [1,10] and the lattice mismatch is only 0.23% for x = 0.04.
306
G. Springholz et a!. i0~
__
0
I I
Buff,,
I 102
>5
10
.E
I
P6EuT, 50 B: 642Al II I QW92A-~~’ I PbE~T, ~
10~
PbTe / Pb, - ,Eu~TeHSs
_______________
io~
U)
~
/ MBE of high mobility PbTe films and
Bragg (222)
0 .i/
I
___
~
2!
B F?
~ ~
in the barriers and the buffer was 2.9%, as determined from the energy gap obtained by infrared The electronic transmission [10]. properties of single quantum well and multi quantum well samples were investigated by photoluminescence, giving information on the confined electron and hole states. Typical luminescence linewidths were ofwere the order of 3 frared transmission experiments performed meV [12]. In addition, low temperature mid in-
100
showing absorption features due to the interband absorption between electronic subbands, yielding
10.1 23.5
24.0
24.5
25 0
Omega (deg) Fig. 3. X-ray (222) Bragg 0—20 curves of a PbTe/Pbi.,EuuTe (x = 0.029) multi quantum well sample with 50 QW periods grows on a 4 ~.omPbEuTe buffer on (Ill) BaF 7. The sample structure is shown in the insert. Dashed line is the corresponding diffractogram calculated using kinematical diffraclion theory and a broadening by a combination of Lorentzian and Gaussian.
similar broadening of the levels. Because of the conducting PbEuTe buffer layer, parallel conduction occurs and Hall effect and conductivity measurements do not give the carrier mobilities in the quantum wells.
6. Conclusion We have grown multi quantum well (MQW) structures60onand PbEuTe with well widths between 160 Abuffers and barrier widths of 680—320 A. The Eu content of the barriers was chosen to vary from 2% to 4%. The structural properties were investigated by in situ RHEED and X-ray diffraction. At the heterointerfaces the growth continues in a 2D growth mode, as the RHEED pattern remains unchanged. So far, no X-ray diffraction data on such structures have been published. As an example, in fig. 3, a (222) Bragg reflection 8—20 curve of of a typical MQW sample is displayed. For this sample, 8 satellite peaks are observed proving the structural quality. Also shown is a calculated diffractogram using kinematical diffraction theory [11]. It was assumed that no strain relaxation occurs. For a homogeneous PbEuTe film with similar Eu content, the FWHM of the (222) Bragg peak was about 55”. Consequently, the calculated diffractogram was convoluted with a broadening function (combination of Lorentzian and Gaussian), where for the latter a FWHM according to the zero order peak was taken. The number of OW periods was 50, the determined superlattice penod was 732.5 A, the well widths were 92 A and the barrier widths were 641.5 A. The Eu content
High films 106 2/V mobility s) can bePbTe grownepitaxial by MBE with(~.t4K> a proper cm choice of source materials and a careful control of the flux composition (PbTe, Te 2). The flux composition deviates from the initial composition of the PbTe source material because of fractional dissociation and resublimation within the effusion cell. The substrate temperatures for PbTe and PbEuTe MBE growth can be as low as = 160°C, which is important for the growth of abrupt IV—VI heterointerfaces.
Acknowledgements We thank G. Griesche and J. Reichow, Humboldt Universität Berlin, Germany, for providing the PbTe source material. This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung, Vienna, Austria (No. 8446 PHY).
References [IJ DL. Partin, in: Semiconductors and Semimetals, Vol. 33, Eds. R.K. Williams and AC. Beer (Academic Press, New York, 1991), and references therein.
G. Springholz eta!.
/ MBE of high mobility PbTefilms and PbTe/Pb1
[2] D.L. Partin, IEEE J. Quantum Electron. QE-24 (1988) 1716. [3] R.F.C. Farrow, in: Molecular Beam Epitaxy and Heterostructures, NATO ASI Series E 87, Eds. L.L. Chang and K. Ploog (Nijhoff, Dordrecht, 1985) p. 227. [4] G. Springholz and G. Bauer, Vacuum 43 (1992) 357. [5] H. Clemens, A. Voiticek, A. Holzinger, G. Bauer and H. Böttner, J. Crystal Growth 102 (1990) 933. [6] G. Springholz and G. Bauer, Appl. Phys. Letters 60 (1992) 1600. [71G. Springholz and G. Bauer, to be published.
-
rEurTe HSs
307
[8] A. Lopez-Otero, Thin Solid Films 49 (1978) 3. [9] H. Holloway, in: Physics of Thin Films, Vol. 11, Eds. G. Hass and M. H. Francombe (Academic Press, New York, 1990) p. 105. [10] R. Suryanarayanan and 5K. Das, J. AppI. Phys. 67 (1990) 1612. [11] H. Krenn, E. Koppensteiner, A. Holzinger, A. Voiticek, G. Bauer and H. Clemens, J. AppI. Phys. 72 (1992) 97. [12] G. Springholz et al., Superlattices Microstruct., to be published.
j~
o~
CRYSTAL
GROWTH
MBE of high mobility PbTe films and PbTe/Pb1 ~Eu~Te heterostructures G. Springholz, 0. Bauer and 0. Ihninger Institut für Haibleiterphysik, Johannes Kepler Universität Linz, A-4040 Linz, Austria
The MBE growth of PbTe and PbEuTe is shown to depend critically on the flux composition (PbTe, Te2, Eu). Using RI-tEED techniques, the flux composition can be determined and adjusted to result in epitaxial layers with high structural 2/Vs at quality, T= 5 K as evidenced have been by achieved. the half-widths Small deviations (30”) of of thethe (222) fluxBragg compositions diffraction. from Electron their optimum mobilities values as high result as in 1.98x a substantial 106 cm decrease of the electron mobilities and an increase of the X-ray diffraction half-widths. In addition, results on the MBE growth of PbTe/Pb 1 _~Eu~Te multi quantum well structures are presented.
1. Introduction Advances in epitaxial growth techniques have recently led to the fabrication of lead salt heterostructure quantum well diode lasers [1,21. In molecular beam epitaxy (MBE) growth of IVVI compounds, the binary compounds (PbTe, PbSe), which vaporize mainly as molecules [31,are usually used as source materials. A chalcogen source is used for the adjustment of the concentration and the type of carriers. For the growth of ternary compounds, an additional metal source (e.g. Eu) is used for the control of composition and a chalcogen source in order to adjust the stoichiometry. In this paper, we present results on the dependence of the MBE growth of PbTe °n the PbTe/Te2 flux composition and on the structural and electronic properties of PbTe epitaxial layers grown with different flux compositions. In addition, structural data on PbTe/Pb1 ~Eu~Te (x <0.04) multi quantum well samples are presented. 2. Experimental procedure Our experiments were carried out in a Riber MBE growth chamber and custom-built preparation and load-lock chambers. For PbTe or PbTe/Pb1 ~Eu~Te heterostructure growth, we 0022-0248/93/$06.00 © 1993
—
used a PbTe, an Eu and two different Te2 effusion cells as beam flux sources. The two different Te cells are used when growing PbTe/Pb1_~ Eu~Teheterostructures in order to adjust both the carrier concentration and the composition independently. In principle, Pb and Te vacancies in the PbTe crystal lattice act as acceptors or donors, respectively, therefore it is impossible to produce undoped intrinsic material. The beam flux rates of the different effusion cells were measured with an ion gauge beam flux monitor. In addition, a quartz crystal thickness monitor was used to determine absolute beam flux rates and to calibrate the ion gauge beam flux monitor ~ Therefore, an exact adjustment of the ratios between the different beam fluxes of PbTe, Eu and Te2 was possible. Films were grown on freshly cleaved (lii) BaF2 substrates with background pressures of 5 x 10 10 mbar during growth. The growth temperatures ranged from 350°Cdown to 160°Cand were calibrated with the melting points of indium (157°C), tin (232°C),bismuth (271°C)and lead (328°C)and with the oxygen desorption temperature of GaAs (582°C). As PbTe vaporizes mainly as PbTe molecules [3], high purity zone refined Bridgman grown PbTe was used as source material. We found that due to fractional dissociation [3] and resublimation of the PbTe within the effusion
Elsevier Science Publishers By. All rights reserved
G. Springholz et a!.
/ MBE of high mobility PbTe films and
cell, the flux composition deviates from the initial composition of the source material becoming increasingly Pb-rich, always resulting in n-type films. The dissociated Pb atoms from the sublimation of PbTe have a lower vapour pressure than PbTe, whereas the dissociated Te2 molecules have a much higher vapour pressure at the effusion cell temperature (520°C).The dissociated Pb atoms consequently evaporated more slowly from the effusion cell which leads to an increase of the overall Pb concentration in the crucible. Consequently, we use a Te-rich PbTe source material with a nominal composition of Pb0 49Te051. A Te effusion cell is used in addition to fine-adjust the beam flux composition and to compensate for the compositional change of the PbTe source material. In situ RHEED experiments were carried out with a 35 keV electron beam directed on the sample at small angles of incidence typically around 0.3°. We have studied the MBE growth of high mobility PbTe films at substrate temperatures between 300 and 350°C and different beam flux compositions adjusted with an additional Te2 beam flux source. First, the freshly cleaved (111) BaF2 substrates were baked at 490°Cfor 15 mm after which sharp streaks are observed in the RHEED diffraction pattern [5]. During the 3D nucleation of PbTe on BaF2 (lattice mismatch of 4.2%), the pattern changes to the spotty transmission RHEED pattern. However, already after 8 mm of growth (= 1400 A) the RHEED pattern returned to the familiar RHEED streak pattern indicating the reformation of a smooth surface. The thickness of the samples was typically about 3—5 ~tm in order to reduce the influence of the highly disordered interface between film and BaF2 substrate. In general, no clear surface reconstruction of the (111) PbTe surface can be seen, except after long term annealing and subsequent cooling to low substrate temperatures, after which weak half order streaks in the sixfold [110] directions appear. 3. Adjustment of the beam flux composition The IV—VI compounds are know to evaporate mainly as PbTe or PbSe molecules [3] with only
PbTe/ Pb
1 - Eu~TeHSs
303
minor fractions of dissociated species. Therefore, a rather stoichiometric flux composition can be expected from the evaporation of PbTe. We found, however, that the flux composition deviates from the initial composition of the PbTe source material, usually having a higher overall Pb content. This we partly account for by using a non-stoichiometric, Te-rich PbTe source material with a nominal composition of Pb049Te~51.This results in a flux composition closer to the needed exact 1: 1 composition. However, the exact flux composition changes after ever reloading of the effusion cells and for the different PbTe source materials which we used. In addition, the composition becomes increasingly Pb-enriched with operation time of the effusion cell. This can in principle be compensated with a small additional Te2 flux, but this is quite delicate to achieve, because of the difficulty to determine the exact flux composition. We have developed a novel in situ method to determine the flux composition using RHEED. This method is based on the special properties of the PbTe (111) surface and is described briefly in the following. Because of the NaC1 type crystal structure, the PbTe (111) surface consists of either only Pb atoms or Te atoms. We have previously shown [4,6] that for elevated substrate temperatures, the equilibrium stable (111) PbTe surface is a Pbstabilized surface due to Te desorption from the surface. For L, lower than roughly 200°C, this surface state can be changed to a Te-stabilized surface with an adequate Te2 flux impinging on the surface, depending on the substrate temperature. This change of surface stoichiometry can be monitored by the RHEED specular spot which has a higher intensity level for the Pb-stabilized than for the Te-stabilized surface [4,6]. If growth is started on a Pb-stabilized surface with a Te-rich flux composition and sufficiently low substrate temperatures around 1, = 160°C,the surface state then gradually changes from the Pb-stabilized to the Te-stabilized surface as growth proceeds. Consequently, the specular spot intensity changes to the lower intensity level characteristic to the Te-stabilized surface. Superimposed on this gradual intensity change, well-developed RHEED intensity oscillations [6] can be observed. On the ..
.
304
G. Springholz et a!.
/ MBE of high mobility PbTe films and
contrary, if a Pb-rich flux is used for growth, the surface stoichiometry does not change, and as growth proceeds the mean specular intensity remains almost constant, apart from the superimposed RHEED intensity oscillations. Under such conditions we have observed up to N = 230 RHEED oscillation periods [7]. This behaviour has been studied in detail in a previous paper [6] for the MBE growth of Pb 1~Eu~Te(x <4%), which shows a similar behaviour. From this intensity change and from the slope of the change, the overall Te to Pb flux ratio can be determined
PbTe /Pb, - Eu~TeHSs
7
1o
PbTe /
(B
/ ~
106
1
(C)
4. MBE of high mobility PbTe films This general RHEED behaviour was used for characterization and adjusting the beam flux composition in order to improve the epitaxial conditions and to achieve high mobilities in the MBE grown films. The flux composition was adjusted by varying the additional Te2 flux with a constant PbTe flux of 1 .tm/h and recording the RHEED specular spot traces when growth was started from the Pb-stabilized surface at T = 160 C. For the growth of high mobility films, the overall flux composition was chosen such that the Pb-stabilized surface at 1’~= 160°Cwas just maintamed during growth with only a minimal additional Te2 flux. Then films were grown at the regular substrate temperatures of 350°Cwith same overall flux composition. Typically, the additional Te 2 fluxes needed are of the order of less than 1% of the PbTe beam flux rate. Because of the gradual change of the source material composition in the effusion cell, this optimum flux composition has to be readjusted from time to time in order to maintain film qualities. procedure results inthe the high growth of films with This high mobilities above 106 cm2/V s at 5 K. The temperature dependent Hall mobility of a PbTe film grown under such conditions is shown in fig. 1 (upper trace). This film was grown at = 350°Cusing an additional Te 2 flux of ~Te2 = 2 s, which corresponds 3.1 ax0.07% 1fjil molecules/cm to addition to the PbTe flux of 0.8 mI/s (1 ml = 5.54 x 1014 molecules/cm2). This film exhibits a mobility of 1.98 x 106 cm2/V s at 5 K,
1 0~ _______________________________ 5
10
20
30
50
100 200 300 ( K ) Fig. 1. Hall mobilities of representative PbTe samples grown
Temperature
on (111) BaF2 at T~= 350°C,but with different flux composition: (A) optimized flux composition with an additional Te 2 flux of 0.07% to the PbTe flux of 0.8 ml/s, (B) without additional Te2 flux and (C) after Pb-enrichment of the PbTe source material in the effusion cell and without additional Te2 flux. All films are n-type carrier concentrations between 17 with and —lXlO’7cm. —4X10
which is the highest reported mobility for MBE grown PbTe films, the mobility being almost a factor of 3 larger than has been reported so far. No post-growth anneal of the samples was performed. The carrier 3. concentration of this film is Similar high mobilities nhave = —3.8 x 1017 cm so far only been reported for hot wall epitaxial grown PbTe films, however, using much higher substrate temperatures of 450°C, which are unsuited for heterostructure growth, and an additional long term 20—30 h post-growth annealingDeviations at 300—350°C[8]. from the optimum flux composition can be either to the Te-rich or Pb-rich side. Increasing the overall Te concentration in the flux results in highly compensated films, although
G. Springholz et aL
/ MBE of high mobility PbTefilms and PbTe /Pb1
their structural quality and surface smoothness is similar to the high mobility samples. Increasing the overall Pb concentration of the flux (or decreasing the Te concentration) results in roughening of the samples, strongly reduced low temperature mobilities and less structural perfection (broadening of the X-ray rocking curves). The importance of the adjustment of beam flux cornposition is illustrated in fig. 1, where the Hall mobilities of three different representative samples are shown. For these films, identical growth conditions were used (T~= 350°C, 1 nm/h, 3—4 ~m thickness and low background pressures of 4 x 1’0’° mbar), except for differences in the overall flux composition. Sample A (upper trace) was grown with optimized flux composition as described above. Sample B (middle trace) was grown without additional Te2 flux, and sample C (lowest trace) was grown at a later stage where the PbTe source material was already significantly enriched Pb.with Therespect Te deficiency in this case was about with 1.5% to the overall —
Pb-to-Te ratio. As seen, sample B has already 2/V~~a reduced 5 K mobility of ~.t= 1.24 X 10~cm which is almost a factor of 2 smaller than for sample A, whereas for sample the 5 Kis mobility 2/V s,C, which even a is only ~ = 390 000 cm factor of 5 smaller. In addition, this sample shows an increase of the surface roughness. All films are n-type with about the same carrier concentrations (—3.8 x l0~ cm3 for sample A, 1.0 X 1017 cm3 for sample B and —2.9 x 10~’cm3 for sample C). However, it is interesting to note that in spite of these large differences in the low temperature mobilities, the room temperature —
-
~Eu~Te HSs
305
PbTe (222)
BaF2 (222) 50
1-’
E
>
5)
________________________________ 24.3
24.35
24.4
24.45
24.5
24.55
Omega (deg) Fig. 2. X-ray (222) Bragg rocking curves for two different PbTe samples. high mobility (~z5K>sam10~ 2/V. s) with Lower FWHMtrace: = 30”, upper trace:sample low mobility cm pie (tLIK = 200000 cm2/Vs) with FWHM = 50”. Insert shows BaF 2 substrate (222) Bragg peak.
BaF 2 substrate. This FWHM is not very much larger than the half-width of the BaF2 (222) Bragg reflection which is about of 17”the (see in fig. 2). In contrast, the FWHM lowinsert mobility films is of the order of 50”, indicating the reduced structural perfection of these films. It should be noted, however, that this strict correlation between the mobilities and the X-ray data is not always observed.
5. PbTe / Pb 1 ~Eu~Te heterostructures —
mobilities of the samples are all almost the same. This is due to the limitation of the mobilities by phonon scattering for temperatures down to 77 K, which also partly explains the success of MBE produced IV—VI devices such as lasers and detectors [1,2,9]. In fig. 2, X-ray rocking curves of the PbTe (222) Bragg reflections are shown for a high mobility (lower trace) and a low mobility (upper trace) sample. The typical half-width (FWHM) of the rocking curves of the high quality samples is around 30”, which is quite narrow considering the high 4.2% lattice mismatch of the films to the
Since the energy gap of Pb1 5Eu~Teincreases steeply with Eu content, the system PbTe/ PbEuTe is particularly suited for the fabrication of type I quantum wells consisting of a combination of a narrow gap well and wide gap barrier. Even for an Eu content as low as 0.04, the energy gap of PbEuTe is 430 meV at 4 K, and therefore more than twice the energy gap of PbTe (190 meV). In addition, the PbTe—EuTe system is miscible for almost the entire range of compositions [1,10] and the lattice mismatch is only 0.23% for x = 0.04.
306
G. Springholz et a!. i0~
__
0
I I
Buff,,
I 102
>5
10
.E
I
P6EuT, 50 B: 642Al II I QW92A-~~’ I PbE~T, ~
10~
PbTe / Pb, - ,Eu~TeHSs
_______________
io~
U)
~
/ MBE of high mobility PbTe films and
Bragg (222)
0 .i/
I
___
~
2!
B F?
~ ~
in the barriers and the buffer was 2.9%, as determined from the energy gap obtained by infrared The electronic transmission [10]. properties of single quantum well and multi quantum well samples were investigated by photoluminescence, giving information on the confined electron and hole states. Typical luminescence linewidths were ofwere the order of 3 frared transmission experiments performed meV [12]. In addition, low temperature mid in-
100
showing absorption features due to the interband absorption between electronic subbands, yielding
10.1 23.5
24.0
24.5
25 0
Omega (deg) Fig. 3. X-ray (222) Bragg 0—20 curves of a PbTe/Pbi.,EuuTe (x = 0.029) multi quantum well sample with 50 QW periods grows on a 4 ~.omPbEuTe buffer on (Ill) BaF 7. The sample structure is shown in the insert. Dashed line is the corresponding diffractogram calculated using kinematical diffraclion theory and a broadening by a combination of Lorentzian and Gaussian.
similar broadening of the levels. Because of the conducting PbEuTe buffer layer, parallel conduction occurs and Hall effect and conductivity measurements do not give the carrier mobilities in the quantum wells.
6. Conclusion We have grown multi quantum well (MQW) structures60onand PbEuTe with well widths between 160 Abuffers and barrier widths of 680—320 A. The Eu content of the barriers was chosen to vary from 2% to 4%. The structural properties were investigated by in situ RHEED and X-ray diffraction. At the heterointerfaces the growth continues in a 2D growth mode, as the RHEED pattern remains unchanged. So far, no X-ray diffraction data on such structures have been published. As an example, in fig. 3, a (222) Bragg reflection 8—20 curve of of a typical MQW sample is displayed. For this sample, 8 satellite peaks are observed proving the structural quality. Also shown is a calculated diffractogram using kinematical diffraction theory [11]. It was assumed that no strain relaxation occurs. For a homogeneous PbEuTe film with similar Eu content, the FWHM of the (222) Bragg peak was about 55”. Consequently, the calculated diffractogram was convoluted with a broadening function (combination of Lorentzian and Gaussian), where for the latter a FWHM according to the zero order peak was taken. The number of OW periods was 50, the determined superlattice penod was 732.5 A, the well widths were 92 A and the barrier widths were 641.5 A. The Eu content
High films 106 2/V mobility s) can bePbTe grownepitaxial by MBE with(~.t4K> a proper cm choice of source materials and a careful control of the flux composition (PbTe, Te 2). The flux composition deviates from the initial composition of the PbTe source material because of fractional dissociation and resublimation within the effusion cell. The substrate temperatures for PbTe and PbEuTe MBE growth can be as low as = 160°C, which is important for the growth of abrupt IV—VI heterointerfaces.
Acknowledgements We thank G. Griesche and J. Reichow, Humboldt Universität Berlin, Germany, for providing the PbTe source material. This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung, Vienna, Austria (No. 8446 PHY).
References [IJ DL. Partin, in: Semiconductors and Semimetals, Vol. 33, Eds. R.K. Williams and AC. Beer (Academic Press, New York, 1991), and references therein.
G. Springholz eta!.
/ MBE of high mobility PbTefilms and PbTe/Pb1
[2] D.L. Partin, IEEE J. Quantum Electron. QE-24 (1988) 1716. [3] R.F.C. Farrow, in: Molecular Beam Epitaxy and Heterostructures, NATO ASI Series E 87, Eds. L.L. Chang and K. Ploog (Nijhoff, Dordrecht, 1985) p. 227. [4] G. Springholz and G. Bauer, Vacuum 43 (1992) 357. [5] H. Clemens, A. Voiticek, A. Holzinger, G. Bauer and H. Böttner, J. Crystal Growth 102 (1990) 933. [6] G. Springholz and G. Bauer, Appl. Phys. Letters 60 (1992) 1600. [71G. Springholz and G. Bauer, to be published.
-
rEurTe HSs
307
[8] A. Lopez-Otero, Thin Solid Films 49 (1978) 3. [9] H. Holloway, in: Physics of Thin Films, Vol. 11, Eds. G. Hass and M. H. Francombe (Academic Press, New York, 1990) p. 105. [10] R. Suryanarayanan and 5K. Das, J. AppI. Phys. 67 (1990) 1612. [11] H. Krenn, E. Koppensteiner, A. Holzinger, A. Voiticek, G. Bauer and H. Clemens, J. AppI. Phys. 72 (1992) 97. [12] G. Springholz et al., Superlattices Microstruct., to be published.