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Be delta-doped layers in GaAs studied by scanning tunnelling microscopy
MATERIALS SalEIKE & ENGINEERING ELSEVIER
Materials Science and Engineering B35 (1995) 485-488
B
Be delta-doped layers in GaAs studied by scanning tunnelling
microscopy P.M. K o e n r a a d a, M.B. Johnson b, H.W.M. Salemink b, W.C. van der VleutenL J.H. Wolter ~ COBRA lnteruniversity Research Institute, Physics Department, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, Netherlands b IBM Research Division, Zurich Research Laboratory, 8803 Rschlikon, Switzerland
Abstract We have imaged Be del~La-dopedlayers in GaAs with atomic resolution using cross-sectional scanning tunnelling microscopy. In the samples grown at low temperature (480 °C) we observe that the width of doping layers for concentrations up to 1 × 1013 cm 2 is smaller than 1 nm, while for a higher doping concentrations we find that the doping layer thickness increases strongly with doping concentration. This broadening is symmetrical about the intended doping plane. We believe that this broadening of the doping layer at high dc,ping concentrations is due to Coulombic repulsion between individual Be ions. The effect of Coulombic repulsion can also be obse,rved in the spatial distribution of the dopant atoms in the plane of the doping layer.
Keywords: Gallium arsenide; Scanning tunnelling microscopy
I. Introduction As the size of electronic devices decreases, better control over the growth is required. With present day growth techniques (for example, MBE, CBE etc.) atomically sharp interfaces can be obtained. However, in small novel devices not only does the layer structure has to be controlled on the atomic scale, but also the dopant atoms have to be confined to a few atomic layers. The technique of confining dopant atoms to, ideally, a single atomic layer is referred to as delta doping. Moreover, in these devices the local doping concentrations has to be increased too in order to have sufficient free carriers available. Thus, there is a large amount of interest in the physics and applications of delta-doped layers in semiconductors. Delta-doped layers have been studied using many techniques. Mostly these techniques have been used to determine the spreading of the delta layer. Most commonly, CV-profiling [1], SIMS profiling [2] and magneto transport [3] have been used. More recently, T E M Elsevier Science S.A. S S D I 0921-5107(95)01436- 5
[4], LVM spectroscopy [5], and H R X R D [6] have been used. Unfortunately, all these techniques have a limiting depth resolution of a few nanometres. Several authors [7,8] have shown recently that individual ionized dopants can be observed in cross-sectionally cleaved I I I - V semiconductors surfaces with scanning tunnelling microscopy (STM). It is a strong advantage of STM that individual atoms at a surface can be observed so that there is not some kind of averaging in one or two directions as is the case with the other techniques mentioned above. Because the (110) crosssectional surface is perpendicular to the (001) growth surface by means of this technique one can study the spreading of the doping layer as well as the distribution of the dopant atoms within the dopant plane [9].
2. Experimental details The GaAs structures we studied were grown at 480 °C with a growth rate of about 1 p m h i (approxi-
486
P.M. Koenraad et al. / Materials Science and Engineering B35 (1995) 485 488
mately equal to a monolayer per second) on a (001) p + substrate with alignment 0.0 0.2 °. The Be delta-doped layers were obtained by doping the growth surface during a growth interrupt lasting from 10 s up to 360 s, while the surface was under an As flux. In this way, a stack of four doping layers was produced with doping concentrations of 3 × 1012 c m - 2 , 1 x 1013 cm 2, 3 x 1013 cm 2 and 1 x l014 cm -2, respectively. 25 nm of undoped GaAs was grown between the individual doping layers. This stack of four delta-doped layers was repeated three times in the structure where each stack was separated by a doped 2.5 nm Alo.2Gao.sAs marker layer. The intended doping concentration was checked by SIMS measurements and the electric activity of the dopant atoms in the layers by etching CV profiling. The STM measurements were performed in an UHV environment (p ~ 1 x 10 ~1 torr) on freshly in situ cleaved samples.
3. Results
In Fig. 1 we show a large scale As-related image across a full stack of four delta-doped layers. The white hillocks, which are due to individual ionized doping atoms, are clearly visible [6]. Ionized Be closest to the cleaved surface appears brightest while deeper lying dopants appear weaker. We are able to observe ionized dopant atoms up to a depth of about 1.5 nm below the cleaved surface. From expanded images such as Fig. 1 the position of the Be atoms was determined with atomic resolution in both the [001] and the [I10] direction. The height of each white hillock, which relates to the depth position of the dopant atom, was determined from intensity contour plots. Fig. 2 shows the spreading of the dopants in the [001] growth direction and the peak height of the hillock features. The spreading is given in bilayer units where one bilayer consists of a single As layer and a single Ga layer. The zero position in the [001] direction is the intended doping position as determined from the position of the A1GaAs marker layers. A Gaussian fit was used to quantitatively determine the spreading of the dopants.
concentration. The results also show a small, about two bilayers, shift of the dopant plane from the intended doping position towards the growth surface. This shift is probably due to the field induced drift from the surface depletion field which exists at the surface of the semiconductor [10]. However, this surface depletion field cannot be responsible for the broadening of the dopant layers at high doping concentrations because we observe a symmetric broadening of the doping layer centred around the intended dopant plane. We believe that this broadening is due to the mutual Coulomb repulsion between the individual Be ions. This drift/ diffusion process is active in the absence of the surface depletion field as can seen from the ongoing drift/diffusion during growth of the sample. This is confirmed in Fig. 3 where we show that in the different stacks of
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4. Discussion
The unprecedented resolution in Fig. 2 shows the potential of X-STM for the study of delta-doped layers. No other technique is able to get a similar, atomic, resolution as demonstrated here for the case with of X-STM. The histogram shows a near single atomic layer thickness for the doping layers with the lowest doping concentration whereas a considerable broadening is observed for the layers with the higher doping
M
Fig. 1. STM image of Be delta-doped layers. Large scale (300"120 nm 2, compressed) As-related image of one of the stacks consisting each of four delta-doped layers. Tunnelling conditions: sample bias = 1.0 V and tunnel current 20 pA. The grey-scale range is 0.08 nm with a [001] corrugation of approximately 0.03 nm. Electrically active Be dopants appear as white hillocks o f approximately 2.5 nm in diameter and up to 0.05 nm high.
P.M. Koenraad et al. / Materials Science and Engineering B35 (1995) 485-488
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0012 ....... 10113 ........101' 4" t"5'2'0 ..... 2000:': ......4000'.... ' 0 intended 213 Time at growth temp. dopant conc. (cm-2) (s) Fig. 3. Layer width as a function of the intended doping concentration and the time at growth temperature for the three stacks of doping layers. Data from stack 1 (circle), stack 2 (triangle) and stack 3 (square) are shown.
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Fig. 4. Radial distribution function for the delta layers with doping concentrations of (a) 3 x 1012 cm -2, (b) 1 x 1013 cm -2 and (c) 3 x 1013 cm - 2 respectively as determined from the X-STM images (circle). The radial distribution function for a random distribution is shown by the dotted line.
488
P.M. Koenraad et al. / Materials Science and Engineering B35 (1995) 485-488
delta layers the thickness of the dopant layer depends on the time at growth temperature. One of the most exciting possibilities of X-STM is the fact that we are able to study the distribution of the dopants in the plane of the doping layer. The study of the in plane distribution can be used to check if any ordering of the dopants is occurring. A convenient way of showing the presence of structure in a particle distribution is the radial distribution function (frequency plot of the pair distance for all available pairs in the set of particles). In Fig. 4 we show the radial distribution function of the doping layers with doping concentrations of I x 1013 cm 2 and 3 × 1013 cm -2, respectively. In the case of a totally random distribution one would observe a flat radial distribution function, g(r) = 1. Fig. 4 clearly shows that close pairs of dopant atoms are much less common. The distribution of the ionized Be atoms thus has the character of a so called strongly interacting gas. We believe that the absence of the close pairs is due to the mutual repulsion of the two ionized dopants. This is the same Columbic repulsion that spreads the dopants out in the growth direction. Thus, it might be difficult to obtain a long range order in the dopant distribution, because the same effect that orders in the in-plane dimensions pushes the dopants off the plane in the growth direction. To achieve order it may be important to use longer growth interrupts because during this time the drift/diffusion of the dopants is highly anisotropic with motion on the growth surface being far more likely. In the future, it will be very interesting to see what the influence of the structure in the ionized dopant distribution is on the mobility of free carriers that scatter on these ionized impurities. In the ideal case of perfect long range order producing a Wigner crystal
of ionized Be atoms so that the scattering of the free carriers on the impurity distribution is absent. 5. Conclusions
Using STM we have shown that the width of the doping layers for concentrations up to 1 x 1013 c m - 2 is smaller than I nm. In doping layers with a higher doping concentration we find that the doping layer thickness increases strongly due to Coulombic repulsion between individual Be ions. The Coulombic repulsion can also be observed in the spatial distribution of the dopants in the plane of the doping layer.
References [1] E.F. Schubert, J.M. Kuo and R.F. Kopf, J. Elect. Mater., 19 (1989) 521. [2] J.J. Harris, J.B. Clegg, R.B. Beall, J. Castagn, K. Woodbridge and C. Roberts, J. Cryst. Growth, I11 (1991) 239. [3] P.M. Koenraad, Electron mobility in deltadoped layers, in E.F. Schubert (ed.), Delta Doping of Semiconductors, Cambrigde University Press, in press. [4] A. Ourmazd, J. Cunningham, W. Jan, J.A. Rentschler and W. Schroeter, Appl. Phys. Lett., 56 (1990) 854 [5] J. Wagner, M. Ramsteiner, W. Stolz, M. Hauser and K.Ploog, Appl. Phys. Lett., 55 (1989) 978. [6] L. Hart, M.R. Fahy, R.C. Newman and P.F. Fewster, Appl. Phys. Lett., 62 (1993) 2218. [7] M.B. Johnson, O. Albrektsen, R.M. Feenstra and H.W.M. Salemink, Appl. Phys. Lett., 63 (1993) 2923. [8] J.F. Zheng, X. Liu, N. Newman, E.R. Weber, D.F. Ogletree and M. Salmeron, Phys. Rev. Lett., 72 (1994) 1490. [9] M.B. Johnson, P.M.Koenraad, H.W.M. Salemink, W.C. van der Vleuten and J.H. W01ter, Phys. Rev. Lett., in press. [10] E.F. Schubert, G.H. Gilmer, R.F. Kopf and H.S. Luftman, Phys.Rev. B, 46 (1993) 15078.
Materials Science and Engineering B35 (1995) 485-488
B
Be delta-doped layers in GaAs studied by scanning tunnelling
microscopy P.M. K o e n r a a d a, M.B. Johnson b, H.W.M. Salemink b, W.C. van der VleutenL J.H. Wolter ~ COBRA lnteruniversity Research Institute, Physics Department, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, Netherlands b IBM Research Division, Zurich Research Laboratory, 8803 Rschlikon, Switzerland
Abstract We have imaged Be del~La-dopedlayers in GaAs with atomic resolution using cross-sectional scanning tunnelling microscopy. In the samples grown at low temperature (480 °C) we observe that the width of doping layers for concentrations up to 1 × 1013 cm 2 is smaller than 1 nm, while for a higher doping concentrations we find that the doping layer thickness increases strongly with doping concentration. This broadening is symmetrical about the intended doping plane. We believe that this broadening of the doping layer at high dc,ping concentrations is due to Coulombic repulsion between individual Be ions. The effect of Coulombic repulsion can also be obse,rved in the spatial distribution of the dopant atoms in the plane of the doping layer.
Keywords: Gallium arsenide; Scanning tunnelling microscopy
I. Introduction As the size of electronic devices decreases, better control over the growth is required. With present day growth techniques (for example, MBE, CBE etc.) atomically sharp interfaces can be obtained. However, in small novel devices not only does the layer structure has to be controlled on the atomic scale, but also the dopant atoms have to be confined to a few atomic layers. The technique of confining dopant atoms to, ideally, a single atomic layer is referred to as delta doping. Moreover, in these devices the local doping concentrations has to be increased too in order to have sufficient free carriers available. Thus, there is a large amount of interest in the physics and applications of delta-doped layers in semiconductors. Delta-doped layers have been studied using many techniques. Mostly these techniques have been used to determine the spreading of the delta layer. Most commonly, CV-profiling [1], SIMS profiling [2] and magneto transport [3] have been used. More recently, T E M Elsevier Science S.A. S S D I 0921-5107(95)01436- 5
[4], LVM spectroscopy [5], and H R X R D [6] have been used. Unfortunately, all these techniques have a limiting depth resolution of a few nanometres. Several authors [7,8] have shown recently that individual ionized dopants can be observed in cross-sectionally cleaved I I I - V semiconductors surfaces with scanning tunnelling microscopy (STM). It is a strong advantage of STM that individual atoms at a surface can be observed so that there is not some kind of averaging in one or two directions as is the case with the other techniques mentioned above. Because the (110) crosssectional surface is perpendicular to the (001) growth surface by means of this technique one can study the spreading of the doping layer as well as the distribution of the dopant atoms within the dopant plane [9].
2. Experimental details The GaAs structures we studied were grown at 480 °C with a growth rate of about 1 p m h i (approxi-
486
P.M. Koenraad et al. / Materials Science and Engineering B35 (1995) 485 488
mately equal to a monolayer per second) on a (001) p + substrate with alignment 0.0 0.2 °. The Be delta-doped layers were obtained by doping the growth surface during a growth interrupt lasting from 10 s up to 360 s, while the surface was under an As flux. In this way, a stack of four doping layers was produced with doping concentrations of 3 × 1012 c m - 2 , 1 x 1013 cm 2, 3 x 1013 cm 2 and 1 x l014 cm -2, respectively. 25 nm of undoped GaAs was grown between the individual doping layers. This stack of four delta-doped layers was repeated three times in the structure where each stack was separated by a doped 2.5 nm Alo.2Gao.sAs marker layer. The intended doping concentration was checked by SIMS measurements and the electric activity of the dopant atoms in the layers by etching CV profiling. The STM measurements were performed in an UHV environment (p ~ 1 x 10 ~1 torr) on freshly in situ cleaved samples.
3. Results
In Fig. 1 we show a large scale As-related image across a full stack of four delta-doped layers. The white hillocks, which are due to individual ionized doping atoms, are clearly visible [6]. Ionized Be closest to the cleaved surface appears brightest while deeper lying dopants appear weaker. We are able to observe ionized dopant atoms up to a depth of about 1.5 nm below the cleaved surface. From expanded images such as Fig. 1 the position of the Be atoms was determined with atomic resolution in both the [001] and the [I10] direction. The height of each white hillock, which relates to the depth position of the dopant atom, was determined from intensity contour plots. Fig. 2 shows the spreading of the dopants in the [001] growth direction and the peak height of the hillock features. The spreading is given in bilayer units where one bilayer consists of a single As layer and a single Ga layer. The zero position in the [001] direction is the intended doping position as determined from the position of the A1GaAs marker layers. A Gaussian fit was used to quantitatively determine the spreading of the dopants.
concentration. The results also show a small, about two bilayers, shift of the dopant plane from the intended doping position towards the growth surface. This shift is probably due to the field induced drift from the surface depletion field which exists at the surface of the semiconductor [10]. However, this surface depletion field cannot be responsible for the broadening of the dopant layers at high doping concentrations because we observe a symmetric broadening of the doping layer centred around the intended dopant plane. We believe that this broadening is due to the mutual Coulomb repulsion between the individual Be ions. This drift/ diffusion process is active in the absence of the surface depletion field as can seen from the ongoing drift/diffusion during growth of the sample. This is confirmed in Fig. 3 where we show that in the different stacks of
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4. Discussion
The unprecedented resolution in Fig. 2 shows the potential of X-STM for the study of delta-doped layers. No other technique is able to get a similar, atomic, resolution as demonstrated here for the case with of X-STM. The histogram shows a near single atomic layer thickness for the doping layers with the lowest doping concentration whereas a considerable broadening is observed for the layers with the higher doping
M
Fig. 1. STM image of Be delta-doped layers. Large scale (300"120 nm 2, compressed) As-related image of one of the stacks consisting each of four delta-doped layers. Tunnelling conditions: sample bias = 1.0 V and tunnel current 20 pA. The grey-scale range is 0.08 nm with a [001] corrugation of approximately 0.03 nm. Electrically active Be dopants appear as white hillocks o f approximately 2.5 nm in diameter and up to 0.05 nm high.
P.M. Koenraad et al. / Materials Science and Engineering B35 (1995) 485-488
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Fig. 2. (a-d) Histogram showing the position of the dopant atoms in the [001] growth direction for the four doping layers in the second stack. A Gaussian fit (dotted curve) is used to determine the averaged position (solid vertical line) and width. The intended doping position of the delta layer is indicated by the dotted vertical line. (e-h) Histograms of the height of the hillock features. The height of the hillock is determined the depth of the dopant atom below the cleaved surface.
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Fig. 4. Radial distribution function for the delta layers with doping concentrations of (a) 3 x 1012 cm -2, (b) 1 x 1013 cm -2 and (c) 3 x 1013 cm - 2 respectively as determined from the X-STM images (circle). The radial distribution function for a random distribution is shown by the dotted line.
488
P.M. Koenraad et al. / Materials Science and Engineering B35 (1995) 485-488
delta layers the thickness of the dopant layer depends on the time at growth temperature. One of the most exciting possibilities of X-STM is the fact that we are able to study the distribution of the dopants in the plane of the doping layer. The study of the in plane distribution can be used to check if any ordering of the dopants is occurring. A convenient way of showing the presence of structure in a particle distribution is the radial distribution function (frequency plot of the pair distance for all available pairs in the set of particles). In Fig. 4 we show the radial distribution function of the doping layers with doping concentrations of I x 1013 cm 2 and 3 × 1013 cm -2, respectively. In the case of a totally random distribution one would observe a flat radial distribution function, g(r) = 1. Fig. 4 clearly shows that close pairs of dopant atoms are much less common. The distribution of the ionized Be atoms thus has the character of a so called strongly interacting gas. We believe that the absence of the close pairs is due to the mutual repulsion of the two ionized dopants. This is the same Columbic repulsion that spreads the dopants out in the growth direction. Thus, it might be difficult to obtain a long range order in the dopant distribution, because the same effect that orders in the in-plane dimensions pushes the dopants off the plane in the growth direction. To achieve order it may be important to use longer growth interrupts because during this time the drift/diffusion of the dopants is highly anisotropic with motion on the growth surface being far more likely. In the future, it will be very interesting to see what the influence of the structure in the ionized dopant distribution is on the mobility of free carriers that scatter on these ionized impurities. In the ideal case of perfect long range order producing a Wigner crystal
of ionized Be atoms so that the scattering of the free carriers on the impurity distribution is absent. 5. Conclusions
Using STM we have shown that the width of the doping layers for concentrations up to 1 x 1013 c m - 2 is smaller than I nm. In doping layers with a higher doping concentration we find that the doping layer thickness increases strongly due to Coulombic repulsion between individual Be ions. The Coulombic repulsion can also be observed in the spatial distribution of the dopants in the plane of the doping layer.
References [1] E.F. Schubert, J.M. Kuo and R.F. Kopf, J. Elect. Mater., 19 (1989) 521. [2] J.J. Harris, J.B. Clegg, R.B. Beall, J. Castagn, K. Woodbridge and C. Roberts, J. Cryst. Growth, I11 (1991) 239. [3] P.M. Koenraad, Electron mobility in deltadoped layers, in E.F. Schubert (ed.), Delta Doping of Semiconductors, Cambrigde University Press, in press. [4] A. Ourmazd, J. Cunningham, W. Jan, J.A. Rentschler and W. Schroeter, Appl. Phys. Lett., 56 (1990) 854 [5] J. Wagner, M. Ramsteiner, W. Stolz, M. Hauser and K.Ploog, Appl. Phys. Lett., 55 (1989) 978. [6] L. Hart, M.R. Fahy, R.C. Newman and P.F. Fewster, Appl. Phys. Lett., 62 (1993) 2218. [7] M.B. Johnson, O. Albrektsen, R.M. Feenstra and H.W.M. Salemink, Appl. Phys. Lett., 63 (1993) 2923. [8] J.F. Zheng, X. Liu, N. Newman, E.R. Weber, D.F. Ogletree and M. Salmeron, Phys. Rev. Lett., 72 (1994) 1490. [9] M.B. Johnson, P.M.Koenraad, H.W.M. Salemink, W.C. van der Vleuten and J.H. W01ter, Phys. Rev. Lett., in press. [10] E.F. Schubert, G.H. Gilmer, R.F. Kopf and H.S. Luftman, Phys.Rev. B, 46 (1993) 15078.