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On the growth mechanism of Li- and Na-doped Zn chalcogenides on GaAs(001) by means of molecular beam epitaxy
j........ C R Y S T A L GROWTH
ELSEVIER
Journal of Crystal Growth 159 (1996) 376-379
On the growth mechanism of Li- and Na-doped Zn chalcogenides on GaAs(001) by means of molecular beam epitaxy M. Ohishi *, M. Yoneta, S. Ishii, M. Ohura, Y. Hiroe, H. Saito Department of Applied Physics, Okayama Universityof Science, Ridai-cho 1-1, Okayama 700, Japan
Abstract
Sharp and semicircular patterns were observed in RHEED during the MBE growth of Li- or Na-acceptor doped ZnSe and ZnS on GaAs(001). The radius and the separation between the diffraction circles vary with the change of the azimuth of the incident electron beam. Calculated diffraction patterns assuming that Li or Na atoms are arrayed one-dimensionally along the [110] direction of the crystal axis are in good agreement with the experimental results. We conclude that Li or Na atoms are incorporated at the [1 I0] terrace steps, which prevents the further growth from the step edge.
1. Introduction
Conduction control, especially in p-type, is one of the most serious subjects in Zn chalcogenides for developing optoelectronic devices. Only well-known nitrogen acceptors give rise to low resistive ZnSe, but no other species, such as alkali metals or III-group elements, has been introduced successfully to give low resistive p-type conduction even in ZnSe. It is important to find out what kind of mechanisms prevent conduction control in Zn chalcogenides. In these years, we have tried to grow p-type ZnSe and ZnS using Li and Na as acceptor species [1,2]. Experimental results showed that considerable reduction of the growth rate or almost no growth of the epilayer was found under higher doping of Li or Na. Furthermore, unusual curved RHEED patterns were observed under higher doping conditions, which were never observed for undoped surfaces [1,2]. This paper is concerned with detailed observation
* Corresponding author. Fax: + 81 86 255 7700.
of RHEED under higher doping of Li and Na into Z n S e / G a A s and Z n S / G a A s . Observed RHEED patterns are compared with calculated ones. We conclude that Li or Na atoms are arrayed one-dimensionally along the [110] terrace steps. The mechanism of growth reduction under doping is also discussed.
2. Experimental procedure
Z n S e / G a A s and Z n S / G a A s epilayers were grown by using hot molecular beams (PH-MBE) at a growth temperature of 250°C. Zn, Se and S beams were post-heated at 600, 600 and 400°C, respectively. Beam flux ratios used in the present study were merely the ratio of each beam pressure measured by means of an ion gauge at the substrate position. Li and Na are used as the p-type dopants, supplied from elemental Li using a conventional effusion cell or from NaN 3 compound using a valved cracking cell, respectively. The doping levels were changed by changing the effuser temperature for both cases.
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 7 9 4 - 6
M. Ohishi et al./ Journal of Crystal Growth 159 (1996) 376-379
GaAs(001) substrates were chemically etched using the conventional sulfuric acid solution, and were thermally cleaned at a typical temperature of 600°C prior to the growth. The epitaxial growth was monitored by in-situ RHEED observation with an acceleration voltage of 12 kV.
3. Experimental results and calculation 3.1. RHEED pattern for Li-doped ZnSe
RHEED pattems for Li-doped (b) and undoped (a) ZnSe/GaAs(001) surfaces observed from three azimuths are shown in Fig. 1. Growth of Li-doped ZnSe was performed on an undoped ZnSe buffer layer with about 0.5 ftm thickness at the Li effuser temperature of T ( L i ) = 325°C. It seems that the diffraction patterns for [110] and [1~0] in (a) and (b) are very similar to each other. It is noticed, however, that in (b) the diffraction streaks for [110] are rather narrower than those for [110], in contradiction with the case of (a), where the diffraction streaks for [1~0] are broader than those for [110]. In our previous study, we pointed out that the width has a close relationship to the dimension of the surface terraces [3]. In the undoped case, the surface terraces had the dimension of ~ 15 ,~ for [110] and ~ 6 5 A for [1~0]. The present experimental result shows that the surface terraces are elongated toward the [ 110] direction for the Li-doped case (b), indicating that the growth manner has changed by supplying Li and Na, as will be mentioned later.
ko//[llO]
(a) undoped
(b) Li-doped
//
377
The pattern for [100] in Li-doped ZnSe is completely different from that of the undoped case, i.e. a long and sharp curved pattern in contrast to a straight and streaky one in (a). The characteristics of the doping experiments are summarized as follows: (1) The curved pattern was observed only under relatively higher doping levels of Li, T(Li) = 250300°C. The grown ZnSe layers using these T(Li) show strong donor-acceptor pair bands and free-toacceptor line in low temperature photoluminescence spectra. Also capacitance measurements show the hole concentration around 1017 cm -3. (2) It took 10 to 30 min after the start of doping for the curved pattern to appear, indicating that some kind of change in the surface structure takes place by supplying Li atoms. (3) A considerable decrease in the growth rate was observed under the higher doping conditions, and almost no growth takes place at T(Li) > 350°C. (4) The curved pattern did not disappear until the sample temperature was raised up to 520-550°C under the supply of neither Zn, Se nor Li. (5) These characteristics are observed under the doping of Na into ZnSe, and also Li and Na into ZnS. 3.2. Experimental and calculated azimuth dependence of diffraction patterns
Fig. 2 shows RHEED patterns from the ZnSe(001) surface doped with Li at T(Li) = 325°C for various azimuths, where 0 is the angle of observation with respect to the [110] direction and k 0 is the wave
ko//[lO0]
ko//[1-fO]
// //
Fig. 1. TypicalRHEED patterns of Li-doped(b) and undoped(a) ZnSe(001)surfaces observedfrom the direction of the [110], [100] and [1~0] azimuths at 180 rain after the start of growth at a growth temperatureof 250°C.
M. Ohishi et al./Journal of Crystal Growth 159 (1996) 376-379
378
vector of the incident electron beam. 0 = 0 °, 45 ° and 90 ° correspond to k011[ll0] (a), [100] (f) and [1~0] (i), respectively. These three patterns are in principle the same as those shown in Fig. 1. When 0 ~ 0 °, the patterns completely changed, and a semicircular ring appeared, as shown in Fig. 2b 0 = 5 ° and Fig. 2c 0 = 10°. With increasing 0, the diameter of the circles becomes larger and the circles get closer to each other. The distance between the circles becomes larger from the left- to right-hand side in each figure, Figs. 2d-2h. In every case, the bright diffraction spots, which are due to the surface lattice, are positioned correctly on the semicircular rings. Finally, at 0 = 90 ° the diffraction pattern consists of long straight lines (i). The experimental results shown in Fig. 2 could be explained only by taking one-dimensional (1D) construction of surface atoms into consideration [4]. We assume that atoms with the electron scattering efficiency different from that of bulk materials are arrayed one-dimensionally, Li or Na in the present case. These atoms occupy their location by replacing Zn atoms, resulting in acceptors. If they are arbitrarily positioned, no diffraction patterns shown in Fig. 2 should be observed. We assume that Li or Na atoms are located linearly along the direction of [ 110] at the same site as Zn atoms of the bulk crystal without any voids. This array constitutes the reciprocal lattice
ll ll ll ll ll ll (a) 0 = 0*
(b) 0 = 5*
(c) 0 =10"
!
(d) 0 =20 °
(e) 0 = 3 0 *
(f) 0 =45 °
(g) O =65*
(h) O =80*
(i) O ---90*
Fig. 2. Azimuth dependence of RHEED patterns of the Li-doped ZnSe(001) surface grown at 250°C.
T
ko//[llO]
(a) 0 = o°
×1/2
(C) 0 = 20*
x1
(e) 0 = 60*
xl
xl
(d) 0=45 °
k0//[100] xl
[llllllllll
(f) 0 = 9 o ° ~ / / [ l i 0 ]
i
xl
Fig. 3. Azimuth dependence of calculated RHEED patterns for a one-dimensional crystal lattice along the [110] direction with respect to the bulk crystal. The small circles on the curves are the diffraction spots due to the surface lattice. The size of (a) is reduced to one half of the other figures.
consisting of equidistant planes or sheets with the spacing of 1.58 ,~,-1 perpendicular to [110]. Fig. 3 illustrates calculated diffraction patterns for several azimuths, where the angle of observation 0 is taken with respect to the [110] direction in the same way as that of the experiments in Fig. 2. We used that the electron beam with the same energy as the experiments is incident with the glancing angle of zero for simplicity. At 0 = 0 °, k 0 is perpendicular to the reciprocal sheets, the cross section of the reciprocal sheets and the Ewald sphere of 57.1 ,~in radius gives rise to a large semicircular ring on the screen. The center is located at the intersection of the primary beam shown by the dotted line in the figure and the screen for observation, as shown in Fig. 3a. The radius on the screen is of 6 cm in the present experimental and calculation conditions, which is observed to be slightly larger on the phosphor screen used in the experiments. With increasing 0, the center of the semicircles shifts to the right-hand side, the diameter of the circles becomes larger and the circles get to closer each other (Figs. 3b-3e). The distance between the circles becomes larger from
M. Ohishi et al./ Journal of Crystal Growth 159 (1996) 376-379
left- to right-hand side in each figure, similar to the experimental results. At 0 = 90 ° the diffraction pattern consists of long straight lines (f). The diffraction spots due to the reciprocal rods of the surface lattice are also calculated and shown by small full circles in each figure. The spots are situated exactly on the semicircles, which justifies the assumption used in the calculation, i.e. the 1D atomic array has the same periodicity as the bulk lattice. Overall features of the calculated results are found to reproduce very well the experimental results shown in Fig. 2. The details of the calculation will be appeared in a separate paper.
4. Discussion
The calculation of the azimuth dependence of diffraction patterns in Section 3.2 has clarified that the 1D array by Li or Na should be oriented along the [110] direction. This fact is also in accordance with the experimental results, i.e. the surface terraces are elongated toward the [110] direction in the case of Li-doped ZnSe, in contrast to the undoped case. It seems natural to assume that Li or Na atoms supplied are adsorbed preferentially by Se atoms at the terrace steps parallel to [110] (B-step), resulting in the easier formation of a 1D array in this direction. Li or Na atoms at the B-steps may not make bonds with further incoming Se atoms, which prevents the terrace elongation toward [1~0]. In contrast, if we assume the 1D array at the A-steps, the terrace steps
379
along [110], Li or Na atoms have to be adsorbed by replacing all Zn atoms at this step edge. This, however, seems to be unlikely. Even if some of Zn atoms at the A-steps are replaced by Li or Na atoms, the elongation of terraces toward [110] may take place at the same rate as in the undoped growth. That is to say, under the doping condition, the terraces grow mainly toward the [110] direction. The rate, however, is much slower than that toward [1~0] in the undoped case, resulting in the decrease of the growth rate of the entire epilayer. The diffraction due to 1D arrays can be observed after the steps along [110] have grown up to enough size. These considerations may explain the experimental results mentioned in Section 3.1, i.e. why the surface terraces are elongated toward the [110] direction for the Li-doped case, why the diffraction due to 1D arrays appeared several decades after the start of doping, and why the growth rate under doping is decreased considerably. Finally, we wish to mention that unfortunately we have no answer how we overcome these difficulties and grow highly doped p-type Zn chalcogenides.
References [1] M. Yoneta, H. Saito and M. Ohishi, J. Crystal Growth 138 (1994) 110. [2] M. Yoneta, H. Saito, M. Ohishi, K. Kitani, H. Kobashi and C. Hatano, J. Crystal Growth 150 (1995) 817. [3] M. Ohishi, H. Saito, H. Torihara, Y. Fujisaki and K. Ohmori, Jpn. J. Appl. Phys. 30 (1991) 1647. [4] B.A. Joyce, J.H. Neave, P.J. Dobson and P.K. Larson, Phys. Rev. B 29 (1984) 814.
ELSEVIER
Journal of Crystal Growth 159 (1996) 376-379
On the growth mechanism of Li- and Na-doped Zn chalcogenides on GaAs(001) by means of molecular beam epitaxy M. Ohishi *, M. Yoneta, S. Ishii, M. Ohura, Y. Hiroe, H. Saito Department of Applied Physics, Okayama Universityof Science, Ridai-cho 1-1, Okayama 700, Japan
Abstract
Sharp and semicircular patterns were observed in RHEED during the MBE growth of Li- or Na-acceptor doped ZnSe and ZnS on GaAs(001). The radius and the separation between the diffraction circles vary with the change of the azimuth of the incident electron beam. Calculated diffraction patterns assuming that Li or Na atoms are arrayed one-dimensionally along the [110] direction of the crystal axis are in good agreement with the experimental results. We conclude that Li or Na atoms are incorporated at the [1 I0] terrace steps, which prevents the further growth from the step edge.
1. Introduction
Conduction control, especially in p-type, is one of the most serious subjects in Zn chalcogenides for developing optoelectronic devices. Only well-known nitrogen acceptors give rise to low resistive ZnSe, but no other species, such as alkali metals or III-group elements, has been introduced successfully to give low resistive p-type conduction even in ZnSe. It is important to find out what kind of mechanisms prevent conduction control in Zn chalcogenides. In these years, we have tried to grow p-type ZnSe and ZnS using Li and Na as acceptor species [1,2]. Experimental results showed that considerable reduction of the growth rate or almost no growth of the epilayer was found under higher doping of Li or Na. Furthermore, unusual curved RHEED patterns were observed under higher doping conditions, which were never observed for undoped surfaces [1,2]. This paper is concerned with detailed observation
* Corresponding author. Fax: + 81 86 255 7700.
of RHEED under higher doping of Li and Na into Z n S e / G a A s and Z n S / G a A s . Observed RHEED patterns are compared with calculated ones. We conclude that Li or Na atoms are arrayed one-dimensionally along the [110] terrace steps. The mechanism of growth reduction under doping is also discussed.
2. Experimental procedure
Z n S e / G a A s and Z n S / G a A s epilayers were grown by using hot molecular beams (PH-MBE) at a growth temperature of 250°C. Zn, Se and S beams were post-heated at 600, 600 and 400°C, respectively. Beam flux ratios used in the present study were merely the ratio of each beam pressure measured by means of an ion gauge at the substrate position. Li and Na are used as the p-type dopants, supplied from elemental Li using a conventional effusion cell or from NaN 3 compound using a valved cracking cell, respectively. The doping levels were changed by changing the effuser temperature for both cases.
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 7 9 4 - 6
M. Ohishi et al./ Journal of Crystal Growth 159 (1996) 376-379
GaAs(001) substrates were chemically etched using the conventional sulfuric acid solution, and were thermally cleaned at a typical temperature of 600°C prior to the growth. The epitaxial growth was monitored by in-situ RHEED observation with an acceleration voltage of 12 kV.
3. Experimental results and calculation 3.1. RHEED pattern for Li-doped ZnSe
RHEED pattems for Li-doped (b) and undoped (a) ZnSe/GaAs(001) surfaces observed from three azimuths are shown in Fig. 1. Growth of Li-doped ZnSe was performed on an undoped ZnSe buffer layer with about 0.5 ftm thickness at the Li effuser temperature of T ( L i ) = 325°C. It seems that the diffraction patterns for [110] and [1~0] in (a) and (b) are very similar to each other. It is noticed, however, that in (b) the diffraction streaks for [110] are rather narrower than those for [110], in contradiction with the case of (a), where the diffraction streaks for [1~0] are broader than those for [110]. In our previous study, we pointed out that the width has a close relationship to the dimension of the surface terraces [3]. In the undoped case, the surface terraces had the dimension of ~ 15 ,~ for [110] and ~ 6 5 A for [1~0]. The present experimental result shows that the surface terraces are elongated toward the [ 110] direction for the Li-doped case (b), indicating that the growth manner has changed by supplying Li and Na, as will be mentioned later.
ko//[llO]
(a) undoped
(b) Li-doped
//
377
The pattern for [100] in Li-doped ZnSe is completely different from that of the undoped case, i.e. a long and sharp curved pattern in contrast to a straight and streaky one in (a). The characteristics of the doping experiments are summarized as follows: (1) The curved pattern was observed only under relatively higher doping levels of Li, T(Li) = 250300°C. The grown ZnSe layers using these T(Li) show strong donor-acceptor pair bands and free-toacceptor line in low temperature photoluminescence spectra. Also capacitance measurements show the hole concentration around 1017 cm -3. (2) It took 10 to 30 min after the start of doping for the curved pattern to appear, indicating that some kind of change in the surface structure takes place by supplying Li atoms. (3) A considerable decrease in the growth rate was observed under the higher doping conditions, and almost no growth takes place at T(Li) > 350°C. (4) The curved pattern did not disappear until the sample temperature was raised up to 520-550°C under the supply of neither Zn, Se nor Li. (5) These characteristics are observed under the doping of Na into ZnSe, and also Li and Na into ZnS. 3.2. Experimental and calculated azimuth dependence of diffraction patterns
Fig. 2 shows RHEED patterns from the ZnSe(001) surface doped with Li at T(Li) = 325°C for various azimuths, where 0 is the angle of observation with respect to the [110] direction and k 0 is the wave
ko//[lO0]
ko//[1-fO]
// //
Fig. 1. TypicalRHEED patterns of Li-doped(b) and undoped(a) ZnSe(001)surfaces observedfrom the direction of the [110], [100] and [1~0] azimuths at 180 rain after the start of growth at a growth temperatureof 250°C.
M. Ohishi et al./Journal of Crystal Growth 159 (1996) 376-379
378
vector of the incident electron beam. 0 = 0 °, 45 ° and 90 ° correspond to k011[ll0] (a), [100] (f) and [1~0] (i), respectively. These three patterns are in principle the same as those shown in Fig. 1. When 0 ~ 0 °, the patterns completely changed, and a semicircular ring appeared, as shown in Fig. 2b 0 = 5 ° and Fig. 2c 0 = 10°. With increasing 0, the diameter of the circles becomes larger and the circles get closer to each other. The distance between the circles becomes larger from the left- to right-hand side in each figure, Figs. 2d-2h. In every case, the bright diffraction spots, which are due to the surface lattice, are positioned correctly on the semicircular rings. Finally, at 0 = 90 ° the diffraction pattern consists of long straight lines (i). The experimental results shown in Fig. 2 could be explained only by taking one-dimensional (1D) construction of surface atoms into consideration [4]. We assume that atoms with the electron scattering efficiency different from that of bulk materials are arrayed one-dimensionally, Li or Na in the present case. These atoms occupy their location by replacing Zn atoms, resulting in acceptors. If they are arbitrarily positioned, no diffraction patterns shown in Fig. 2 should be observed. We assume that Li or Na atoms are located linearly along the direction of [ 110] at the same site as Zn atoms of the bulk crystal without any voids. This array constitutes the reciprocal lattice
ll ll ll ll ll ll (a) 0 = 0*
(b) 0 = 5*
(c) 0 =10"
!
(d) 0 =20 °
(e) 0 = 3 0 *
(f) 0 =45 °
(g) O =65*
(h) O =80*
(i) O ---90*
Fig. 2. Azimuth dependence of RHEED patterns of the Li-doped ZnSe(001) surface grown at 250°C.
T
ko//[llO]
(a) 0 = o°
×1/2
(C) 0 = 20*
x1
(e) 0 = 60*
xl
xl
(d) 0=45 °
k0//[100] xl
[llllllllll
(f) 0 = 9 o ° ~ / / [ l i 0 ]
i
xl
Fig. 3. Azimuth dependence of calculated RHEED patterns for a one-dimensional crystal lattice along the [110] direction with respect to the bulk crystal. The small circles on the curves are the diffraction spots due to the surface lattice. The size of (a) is reduced to one half of the other figures.
consisting of equidistant planes or sheets with the spacing of 1.58 ,~,-1 perpendicular to [110]. Fig. 3 illustrates calculated diffraction patterns for several azimuths, where the angle of observation 0 is taken with respect to the [110] direction in the same way as that of the experiments in Fig. 2. We used that the electron beam with the same energy as the experiments is incident with the glancing angle of zero for simplicity. At 0 = 0 °, k 0 is perpendicular to the reciprocal sheets, the cross section of the reciprocal sheets and the Ewald sphere of 57.1 ,~in radius gives rise to a large semicircular ring on the screen. The center is located at the intersection of the primary beam shown by the dotted line in the figure and the screen for observation, as shown in Fig. 3a. The radius on the screen is of 6 cm in the present experimental and calculation conditions, which is observed to be slightly larger on the phosphor screen used in the experiments. With increasing 0, the center of the semicircles shifts to the right-hand side, the diameter of the circles becomes larger and the circles get to closer each other (Figs. 3b-3e). The distance between the circles becomes larger from
M. Ohishi et al./ Journal of Crystal Growth 159 (1996) 376-379
left- to right-hand side in each figure, similar to the experimental results. At 0 = 90 ° the diffraction pattern consists of long straight lines (f). The diffraction spots due to the reciprocal rods of the surface lattice are also calculated and shown by small full circles in each figure. The spots are situated exactly on the semicircles, which justifies the assumption used in the calculation, i.e. the 1D atomic array has the same periodicity as the bulk lattice. Overall features of the calculated results are found to reproduce very well the experimental results shown in Fig. 2. The details of the calculation will be appeared in a separate paper.
4. Discussion
The calculation of the azimuth dependence of diffraction patterns in Section 3.2 has clarified that the 1D array by Li or Na should be oriented along the [110] direction. This fact is also in accordance with the experimental results, i.e. the surface terraces are elongated toward the [110] direction in the case of Li-doped ZnSe, in contrast to the undoped case. It seems natural to assume that Li or Na atoms supplied are adsorbed preferentially by Se atoms at the terrace steps parallel to [110] (B-step), resulting in the easier formation of a 1D array in this direction. Li or Na atoms at the B-steps may not make bonds with further incoming Se atoms, which prevents the terrace elongation toward [1~0]. In contrast, if we assume the 1D array at the A-steps, the terrace steps
379
along [110], Li or Na atoms have to be adsorbed by replacing all Zn atoms at this step edge. This, however, seems to be unlikely. Even if some of Zn atoms at the A-steps are replaced by Li or Na atoms, the elongation of terraces toward [110] may take place at the same rate as in the undoped growth. That is to say, under the doping condition, the terraces grow mainly toward the [110] direction. The rate, however, is much slower than that toward [1~0] in the undoped case, resulting in the decrease of the growth rate of the entire epilayer. The diffraction due to 1D arrays can be observed after the steps along [110] have grown up to enough size. These considerations may explain the experimental results mentioned in Section 3.1, i.e. why the surface terraces are elongated toward the [110] direction for the Li-doped case, why the diffraction due to 1D arrays appeared several decades after the start of doping, and why the growth rate under doping is decreased considerably. Finally, we wish to mention that unfortunately we have no answer how we overcome these difficulties and grow highly doped p-type Zn chalcogenides.
References [1] M. Yoneta, H. Saito and M. Ohishi, J. Crystal Growth 138 (1994) 110. [2] M. Yoneta, H. Saito, M. Ohishi, K. Kitani, H. Kobashi and C. Hatano, J. Crystal Growth 150 (1995) 817. [3] M. Ohishi, H. Saito, H. Torihara, Y. Fujisaki and K. Ohmori, Jpn. J. Appl. Phys. 30 (1991) 1647. [4] B.A. Joyce, J.H. Neave, P.J. Dobson and P.K. Larson, Phys. Rev. B 29 (1984) 814.