- Email: [email protected]
SiGe superlattices
Journal of Crystal Growth 127 (1993) 435—439 North-Holland
ii RN A Lo,
CRYSTAL GROWTH
Cross-sectional scanning tunneling microscopy of MBE-grown Si p—n junctions and Si/SiGe superlattices E.T. Yu, M.B. Johnson
1,
V.P. Kesan, A.R. Powell, J.-M. Halbout and S.S. Iyer
IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA
We have used cross-sectional scanning tunneling microscopy and spectroscopy to study a Si (001) p—n junction and a Si/Si 0 76Ge024 (001) superlattice grown by molecular-beam epitaxy. The shape of the band-edge energy profile in the p—n junction can be seen with a spatial resolution of better than 100 A, and features in the electronic structure of the Si/Si076Ge024 superlattice have been detected with a spatial resolution of only a few nanometers. Topographic contrast between the Si and Si0 76Ge0 24 layers in the superlattice has also been observed.
1. Introduction The characterization of semiconductor structures with high spatial resolution has become increasingly important as epitaxial crystal growth and semiconductor processing techniques have improved. Structural and electronic studies of semiconductor materials and devices with nanometer resolution will be crucial for continued reductions in device size and for the development of advanced heterostructure devices. Among the more promising characterization tools are scanning tunneling microscopy (STM) and related techniques, which offer both extremely high spatial resolution and sensitivity to electronic properties; these methods have been applied to the study of GaAs/A1GaAs [1—6],InP/InGaAs [71 and Si [8—131device structures. In this paper, we describe cross-sectional scanning tunneling microscopy and spectroscopy of a Si (001) p—n junction and a Si/SiGe (001) superlattice grown by molecular-beam epitaxy (MBE). We have been able to see clear transitions be-
tween p- and n-type material across the p—n junction, and variations in the conduction-bandedge energy have been detected with a resolution of better than 100 A. Studies of the Si/Si0•76Ge024 superlattice have yielded (110) cross-sectional images exhibiting topographic contrast between the Si and Si076Ge024 layers. Threshold voltages for tunneling extracted from current—voltage (I—V) characteristics measured at several points across the heterostructure exhibit the qualitative features expected in the shapes of the conductionand valence-band-edge profiles.
2. Experiment The samples used in these experiments were grown by MBE on 5 inch Si (001) substrates. The growth system and substrate preparation techniques have been described in detail elsewhere [14].The Si p—n junction consisted of an As-doped Si substrate (p 0.005 11 cm), on which were grown a 2000 A Si layer (n 5>< 3) followed by doped a 2000with A Sb B-doped Si 1018 cm layer (p 5 x 10~~ cm3). Dopant levels in this structure were confirmed by secondary ion mass spectrometry (SIMS). The Si/Si 076Ge024 super=
=
=
Present address: Zurich Research Laboratory, IBM Research Division, CH-8803 Rüschlikon, Switzerland.
0022-0248/93/$06.00 © 1993
—
Elsevier Science Publishers B.V. All rights reserved
436
E. T Yu et al.
/ Cross-sectional
STM of MBE-grown Sip—n junctions and Si / SiGe SLs
lattice consisted of 262 A Si alternating with 75 A Si~76Ge024for sixteen periods; the middle 50 A in 3). the ASi1400 layers X l0~ A Si were bufferB-doped layer was (p grown5 on the cm substrate prior to deposition of the superlattice. Because of the low substrate temperature (450°C) employed during the growth, the layers were expected to be coherently strained to the Si substrate [15]. This was confirmed by high-resolution X-ray diffraction measurements. STM measurements were performed using samples cleaved ex situ to expose a (110) crosssectional face. Flat (110) surfaces were obtained by cleaving large samples over a distance typically greater than 5 cm, and the cleaved surfaces were passivated by etching in 1: 100 HF: H
P
20, following the procedure of Johnson and Halbout [101. Flat, electronically unpinned surfaces are essential for spectroscopic measurements of the bulk electronic properties of a semiconductor. STM measurements were carried out in ultra-high vacuum at pressures ranging. from 1 x 10~ to 1 x — . 10 Torr. The sample—tip geometry in our STM and the procedure used to locate the edge of the sample, and subsequently the epitaxial layers, have been described elsewhere [13].
E
—‘
=
STM H
05
05
) )
n-Si
00
p-Si
00
~-“ -~
—~
—0 5 —1.5 -0.5
0.5
-0 5 -1.5 —0.5 0.5
1.5
1.5
Vb (V) Vb (‘h’) Fig. I. Scanning tunneling spectroscopy across a p—n junction. ~_v characteristics measured at each point in the scan provide information about the electronic structure across the device
model to the experimental data. In this model, the tunneling current 1~ is given by ‘~(V~) cx
j~
E)[f5(E)
—
f1( E
—
eVb)]
3. Si p—n junction results
XT(E,Vh) dE. Here, ~ is the density of states in the sample, f5 and f1 are the Fermi distribution functions in the
Spectroscopic measurements on a p—n junction are illustrated schematically in fig. 1. During each topographic line scan across the device region, the feedback loop is momentarily held open at each point while an I—V characteristic is obtamed. I—V curves for n-type and p-type Si are shown in fig. 1. For an unpinned Si surface, the onset of tunneling current for positive and negative sample voltage is associated with tunneling of electrons into the sample’s conduction band or out of the valence band, respectively. By monitoring a threshold bias voltage for the onset of tunneling, Vth, one should be able to probe the spatial variation of the band-edge energies relative to the Fermi level in the device, The threshold voltage ~‘~h for a given 1—V curve is determined by fitting a simple theoretical
sample and tip, and T is the transmission coefficient for tunneling electrons. The model has been simplified by assuming a constant density of states in the tip, neglecting tip-induced band bending in the sample, and neglecting the dopant-induced components of the tunneling current. While this model does not necessarily yield the precise energy of the band edge for all possible dopant concentrations, it does provide a systematic way to detect variations in the band-edge energies across a device. Figs. 2a and 2b show topography and J”~h(x)in the vicinity of the p—n junction. The solid line in fig. 2b represents an average of the values extracted from I—V curves measured during the four line scans shown in fig. 2a. The dashed line represents the calculated conduction-band-edge profile for the junction, shifted and scaled in
E. T. Yu et a!.
/ Cross-sectional STM of MBE-grown
437
Fig. 4a shows 1950 A x 1950 A constant-current image of thea superlattice, taken at a sample
(a) tot
‘ti
(b)
Sip—njunctions and Si / SiGe SLs
voltage 1.5 V and a tunneling current of the 0.1 nA. Fig.of4b+ shows a single line scan across superlattice. The bright regions in the image, corresponding to the peaks in the line scan, represent the Si076Ge024 layers, while the dark re-
1.25 1,00
gions in the image represent the Si layers. The 0.75
superlattice corrugation amplitude is approximately 10 A; STM studies of GaAs/A1GaAs [4—6] and InP/InGaAs [71heterostructures have revealed similar corrugations with amplitudes rang-
0.50 0.25
‘
0
200
400
600
800
1000
Position (A) Fig. 2. (a) Topographic line scans across the Si p—n junction. (b) V~5(x)extracted from I—V characteristics measured while scanning across the junction (solid line), and the calculated band-edge profile, shifted and scaled in voltage to overlay Vth(x) (dashed line).
ing from 0.3 A to over 100 A. The wide range of corrugation amplitudes observed is probably due to variations in tip and sample conditions in different experiments. While these features were
(001)
(a) voltage to overlay, approximately, V~h(x); the band-edge profile was calculated using only
14OASI
(11c)+—
I
1O6ASi
dopant levels measured by SIMS and known bulk material parameters as input. The shape of the depletion region, extending over approximately 150 A, can be seen quite clearly in the experimental threshold voltage profile, and changes in
~.-
STM tip
75
A
Si0750e024
A
50
I
]
A~
106
B-doped
Si,
16x
S
the band-edge profile occurring over considerably shorter lengths should be resolvable. Potentio metric measurements we have performed on this and other p—n junctions demonstrate that varia tions in band-edge energies can be extracted in a similar manner for electrically biased device structures [13].
-_______________
A
1400
S
-_______________
substrote
(001) p-Si
(b) 1.50
4. Si/SiGe superlattice results Fig. 3a shows the superlattice used in these experiments and fig. 3b the calculated conduction- and valence-band-edge profiles for the structure. For the strained Si076Ge024 layers, the band edges shown in fig. 3b represent the lowest (fourfold) conduction-band edge and the highest (heavy-hole-like) valence-band edge; band gaps of 1.12 and 0.91 eV have been assumed for Si and Si076Ge024, respectively, and we have taken the
valence-band offset to be 0.19 eV [16—19].
1.25 1.00
SIGe
-
0.75 -
Si
SIGe
Si
SiGe
C
0.50 0.25 0.00 —0.25 -
O.5o~
100
200
300
400
500
600
700
800
900
1000
(A) Fig. 3. (a) Schematic diagram of the Si/Si076Ge024 superlattice. Scanning tunneling microscopy and spectroscopy are performed on a cleaved (110)profiles face of calculated the sample.for(b)the Conduction- and valence-band-edge superPosition
lattice.
E. T. Yu ci al.
438
/ Cross-sectional STM of MBE-grown Sip—n junctions and Si / SiGe SLs
generally ascribed to variations in tunneling characteristics into different materials rather than actual physical topography, the possibility of structural, as opposed to purely electronic, contributions to the topography we observe has not been eliminated. Fig. 5a shows I—V tunneling spectra for a Si076Ge024 layer (a) and undoped and doped regions of a Si layer (b and c, respectively) in the superlattice. Threshold voltages for the onset of tunneling for positive and negative sample bias have also been shown. As discussed in section 3, the threshold voltages are related although not necessarily equal to the actual band-edge energies; by tracing the variation in threshold voltages across the superlattice, one might hope to probe variations in the band-edge energies across the structure. Positive and negative threshold tunneling voltages for I—V spectra across the superlat-
1.0
a bc
0.8
0.6 0.4
£ ~
~
/0
0.2 00 -02
_____
—0.4 0.6 —1 5—1 0—05 0.0 0,5 1.0 1 5
(a)
Sample Voltage (V)
080 f
0.70
%•4 —
0.60 ~ 050 =
“ ~%fb
.
>0.40 0 30
(b)
a c
b
0.20 -0.20
a’ c’ b .
0.30
—0.40
.~
..:~‘
••~~“.
-060 0.70
‘w’
~. ..~. .•~ ~ ‘
(c)
—0.80
0 100 200 300 400 500 600 700 800 9001000 Position (A) Fig. 5. (a) I—V characteristics for a Si1176Ge024 layer (a), and undoped and doped regions of a Si layer (curves b and c, respectively). t7th’ are Positive markedand on the negative I—V threshold curves. (b)voltages Positivefor andtun(c) neling, threshold voltages across the superlattice structure. negative The labels a, b, and c correspond to the I—V curves shown in (a). The dashed lines represent the conduction- and valenceband-edge profiles calculated for the superlattice, shifted in voltage to overlay the threshold voltages.
tice are shown as solid circles in figs. Sb and Sc. The labels a, b and c correspond to the similarly labeled I—V curves in fig. 5a. Calculated conduction- and valence-band-edge profiles are shown as dashed lines in figs. Sb and 5c, respectively, shifted in voltage to overlay the threshold voltage profiles. In fig. 5b, the lower conduction-band-
___________________________________ 20
I
edge energies in the Si
—10 0
500
1000
Position
1500
2000
(A)
Fig. 4. (a) Constant-current image of the superlattice, near interface between the superlattice and the Si buffer layer; image area is 1950 Ax 1950 A. (b) Line scan across superlattice; the Si075Ge1124 layers appear as peaks in topography.
the the the the
076Ge024 layers and the band bending induced by the doping in the Si layers are evident in the positive threshold voltage profile. Shifts in the negative threshold voltage near the Si/5i075Ge024 interfaces, corresponding to the Si/Si0 76Ge~24valence-band offset, can be seen in fig. Sc; these shifts typically occur over distances of approximately SO A. Peaks in J”~~(x) arising from the modulation doping can also be detected.
E. T Yu eta!. 5.
/
Cross-sectional STM of MBE-grown Si p—n junctions and Si / SiGe SLs
Conclusions
We have used cross-sectional scanning tunneling microscopy and spectroscopy to probe the electronic structure in a Si p—n junction and a Si/Si075Ge024 superlattice. By tracing variations in threshold voltages for tunneling extracted from I—V characteristics, variations in the band-edge energies in a device can be detected. The shape of the depletion region in the p—n junction, extending over approximately 150 A, can clearly be seen, and features in the electronic structure of the Si/Si0 76Ge024 superlattice have been detected with a spatial resolution of only a few nanometers. Topographic contrast between the Si and Si076Ge024 layers in the superlattice has also been observed.
Acknowledgements We would like to acknowledge F. Cardone for performing SIMS measurements on the Si p—n junctions. The STM software used in these experiments was provided by R.M. Feenstra. References [1] P. Muralt, H. Meier, D.W. Pohl and H.W. Salemink, AppI. Phys. Letters 50 (1987) 1352. [2] H.W. Salemink, H.P. Meier, R. Ellialtioglu, J.W. Gerritsen and P.R. Muralt, AppI. Phys. Letters 54 (1989) 1112.
439
[3] D.L. Abraham, A. Veider, C. Schönenberger, H.P. Meier, D.J. Arent and S.F. Alvarado, AppI. Phys. Letters 56 (1990) 1564. [4] J.M. Gómez-RodrIguez, A.M. Baró, J.P. Silveira, M. Váquez, Y. Gonzalez and F. Briones, AppI. Phys. Letters 56 (1990) 36. [5] 0.
Albrektsen, D.J. Arent, H.P. Meier and H.W. Salemink, AppI. Phys. Letters 57 (1990) 31. [61I. Tanaka, T. Kato, S. Ohkouchi and F. Osaka, J. Vacuum Sci. Technol. A 8 (1990) 567. [71F. Osaka, I. Tanaka, T. Kato and Y. Katayama, Japan. J. AppI. Phys. 27 (1988) L1193. [8] S. Hosaka, S. Hosoki, K. Takata, K. Horiuchi and N. Natsuaki, AppI. Phys. Letters 53 (1988) 487. [9] 5. Kordic, E.J. van Loenen, D. Dijkkamp, A.J. Hoeven and H.K. Moraal, J. Vacuum Sci. Technol. A 8 (1990)
[10] MB. Johnson and J.M. Halbout, J. Vacuum Sci. Technol. B 10 (1992) 508. [11] T. Takigami and M. Tanimoto, AppI. Phys. Letters 58 (1991) 2288. [12] C.C. Williams, J. Slinkman, W.P. Hough and H.K. Wickramasinghe, AppI. Phys. Letters 55 (1989) 1662. [13] E.T. Yu, MB. Johnson and J.M. Halbout, AppI. Phys. Letters, to be published. [14] S.S. Iyer, in: Epitaxial Silicon Technology, Ed. B.J. Baliga (Academic Press, Orlando, FL, 1985) p. 97. [15] T.S. Kuan and S.S. Iyer, AppI. Phys. Letters 59 (1991) 2242.
[16] W. Ni, J. Knall and G.V. Hansson, Phys. Rev. B 36 (1987) 7744. [17] G.P. Schwartz, MS. Hybertsen, J. Bevk, R.G. Nuzzo, J.P. Mannaerts and G.J. Gualtieri, Phys. Rev. B 39 (1989) 1235. [18] E.T. Yu, E.T. Croke, T.C. McGill and RH. Miles, Appl. Phys. Letters 56 (1990) 569. [191 C.G. Van de Walle and R.M. Martin, Phys. Rev. B 34 (1986) 5621.
ii RN A Lo,
CRYSTAL GROWTH
Cross-sectional scanning tunneling microscopy of MBE-grown Si p—n junctions and Si/SiGe superlattices E.T. Yu, M.B. Johnson
1,
V.P. Kesan, A.R. Powell, J.-M. Halbout and S.S. Iyer
IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA
We have used cross-sectional scanning tunneling microscopy and spectroscopy to study a Si (001) p—n junction and a Si/Si 0 76Ge024 (001) superlattice grown by molecular-beam epitaxy. The shape of the band-edge energy profile in the p—n junction can be seen with a spatial resolution of better than 100 A, and features in the electronic structure of the Si/Si076Ge024 superlattice have been detected with a spatial resolution of only a few nanometers. Topographic contrast between the Si and Si0 76Ge0 24 layers in the superlattice has also been observed.
1. Introduction The characterization of semiconductor structures with high spatial resolution has become increasingly important as epitaxial crystal growth and semiconductor processing techniques have improved. Structural and electronic studies of semiconductor materials and devices with nanometer resolution will be crucial for continued reductions in device size and for the development of advanced heterostructure devices. Among the more promising characterization tools are scanning tunneling microscopy (STM) and related techniques, which offer both extremely high spatial resolution and sensitivity to electronic properties; these methods have been applied to the study of GaAs/A1GaAs [1—6],InP/InGaAs [71 and Si [8—131device structures. In this paper, we describe cross-sectional scanning tunneling microscopy and spectroscopy of a Si (001) p—n junction and a Si/SiGe (001) superlattice grown by molecular-beam epitaxy (MBE). We have been able to see clear transitions be-
tween p- and n-type material across the p—n junction, and variations in the conduction-bandedge energy have been detected with a resolution of better than 100 A. Studies of the Si/Si0•76Ge024 superlattice have yielded (110) cross-sectional images exhibiting topographic contrast between the Si and Si076Ge024 layers. Threshold voltages for tunneling extracted from current—voltage (I—V) characteristics measured at several points across the heterostructure exhibit the qualitative features expected in the shapes of the conductionand valence-band-edge profiles.
2. Experiment The samples used in these experiments were grown by MBE on 5 inch Si (001) substrates. The growth system and substrate preparation techniques have been described in detail elsewhere [14].The Si p—n junction consisted of an As-doped Si substrate (p 0.005 11 cm), on which were grown a 2000 A Si layer (n 5>< 3) followed by doped a 2000with A Sb B-doped Si 1018 cm layer (p 5 x 10~~ cm3). Dopant levels in this structure were confirmed by secondary ion mass spectrometry (SIMS). The Si/Si 076Ge024 super=
=
=
Present address: Zurich Research Laboratory, IBM Research Division, CH-8803 Rüschlikon, Switzerland.
0022-0248/93/$06.00 © 1993
—
Elsevier Science Publishers B.V. All rights reserved
436
E. T Yu et al.
/ Cross-sectional
STM of MBE-grown Sip—n junctions and Si / SiGe SLs
lattice consisted of 262 A Si alternating with 75 A Si~76Ge024for sixteen periods; the middle 50 A in 3). the ASi1400 layers X l0~ A Si were bufferB-doped layer was (p grown5 on the cm substrate prior to deposition of the superlattice. Because of the low substrate temperature (450°C) employed during the growth, the layers were expected to be coherently strained to the Si substrate [15]. This was confirmed by high-resolution X-ray diffraction measurements. STM measurements were performed using samples cleaved ex situ to expose a (110) crosssectional face. Flat (110) surfaces were obtained by cleaving large samples over a distance typically greater than 5 cm, and the cleaved surfaces were passivated by etching in 1: 100 HF: H
P
20, following the procedure of Johnson and Halbout [101. Flat, electronically unpinned surfaces are essential for spectroscopic measurements of the bulk electronic properties of a semiconductor. STM measurements were carried out in ultra-high vacuum at pressures ranging. from 1 x 10~ to 1 x — . 10 Torr. The sample—tip geometry in our STM and the procedure used to locate the edge of the sample, and subsequently the epitaxial layers, have been described elsewhere [13].
E
—‘
=
STM H
05
05
) )
n-Si
00
p-Si
00
~-“ -~
—~
—0 5 —1.5 -0.5
0.5
-0 5 -1.5 —0.5 0.5
1.5
1.5
Vb (V) Vb (‘h’) Fig. I. Scanning tunneling spectroscopy across a p—n junction. ~_v characteristics measured at each point in the scan provide information about the electronic structure across the device
model to the experimental data. In this model, the tunneling current 1~ is given by ‘~(V~) cx
j~
E)[f5(E)
—
f1( E
—
eVb)]
3. Si p—n junction results
XT(E,Vh) dE. Here, ~ is the density of states in the sample, f5 and f1 are the Fermi distribution functions in the
Spectroscopic measurements on a p—n junction are illustrated schematically in fig. 1. During each topographic line scan across the device region, the feedback loop is momentarily held open at each point while an I—V characteristic is obtamed. I—V curves for n-type and p-type Si are shown in fig. 1. For an unpinned Si surface, the onset of tunneling current for positive and negative sample voltage is associated with tunneling of electrons into the sample’s conduction band or out of the valence band, respectively. By monitoring a threshold bias voltage for the onset of tunneling, Vth, one should be able to probe the spatial variation of the band-edge energies relative to the Fermi level in the device, The threshold voltage ~‘~h for a given 1—V curve is determined by fitting a simple theoretical
sample and tip, and T is the transmission coefficient for tunneling electrons. The model has been simplified by assuming a constant density of states in the tip, neglecting tip-induced band bending in the sample, and neglecting the dopant-induced components of the tunneling current. While this model does not necessarily yield the precise energy of the band edge for all possible dopant concentrations, it does provide a systematic way to detect variations in the band-edge energies across a device. Figs. 2a and 2b show topography and J”~h(x)in the vicinity of the p—n junction. The solid line in fig. 2b represents an average of the values extracted from I—V curves measured during the four line scans shown in fig. 2a. The dashed line represents the calculated conduction-band-edge profile for the junction, shifted and scaled in
E. T. Yu et a!.
/ Cross-sectional STM of MBE-grown
437
Fig. 4a shows 1950 A x 1950 A constant-current image of thea superlattice, taken at a sample
(a) tot
‘ti
(b)
Sip—njunctions and Si / SiGe SLs
voltage 1.5 V and a tunneling current of the 0.1 nA. Fig.of4b+ shows a single line scan across superlattice. The bright regions in the image, corresponding to the peaks in the line scan, represent the Si076Ge024 layers, while the dark re-
1.25 1,00
gions in the image represent the Si layers. The 0.75
superlattice corrugation amplitude is approximately 10 A; STM studies of GaAs/A1GaAs [4—6] and InP/InGaAs [71heterostructures have revealed similar corrugations with amplitudes rang-
0.50 0.25
‘
0
200
400
600
800
1000
Position (A) Fig. 2. (a) Topographic line scans across the Si p—n junction. (b) V~5(x)extracted from I—V characteristics measured while scanning across the junction (solid line), and the calculated band-edge profile, shifted and scaled in voltage to overlay Vth(x) (dashed line).
ing from 0.3 A to over 100 A. The wide range of corrugation amplitudes observed is probably due to variations in tip and sample conditions in different experiments. While these features were
(001)
(a) voltage to overlay, approximately, V~h(x); the band-edge profile was calculated using only
14OASI
(11c)+—
I
1O6ASi
dopant levels measured by SIMS and known bulk material parameters as input. The shape of the depletion region, extending over approximately 150 A, can be seen quite clearly in the experimental threshold voltage profile, and changes in
~.-
STM tip
75
A
Si0750e024
A
50
I
]
A~
106
B-doped
Si,
16x
S
the band-edge profile occurring over considerably shorter lengths should be resolvable. Potentio metric measurements we have performed on this and other p—n junctions demonstrate that varia tions in band-edge energies can be extracted in a similar manner for electrically biased device structures [13].
-_______________
A
1400
S
-_______________
substrote
(001) p-Si
(b) 1.50
4. Si/SiGe superlattice results Fig. 3a shows the superlattice used in these experiments and fig. 3b the calculated conduction- and valence-band-edge profiles for the structure. For the strained Si076Ge024 layers, the band edges shown in fig. 3b represent the lowest (fourfold) conduction-band edge and the highest (heavy-hole-like) valence-band edge; band gaps of 1.12 and 0.91 eV have been assumed for Si and Si076Ge024, respectively, and we have taken the
valence-band offset to be 0.19 eV [16—19].
1.25 1.00
SIGe
-
0.75 -
Si
SIGe
Si
SiGe
C
0.50 0.25 0.00 —0.25 -
O.5o~
100
200
300
400
500
600
700
800
900
1000
(A) Fig. 3. (a) Schematic diagram of the Si/Si076Ge024 superlattice. Scanning tunneling microscopy and spectroscopy are performed on a cleaved (110)profiles face of calculated the sample.for(b)the Conduction- and valence-band-edge superPosition
lattice.
E. T. Yu ci al.
438
/ Cross-sectional STM of MBE-grown Sip—n junctions and Si / SiGe SLs
generally ascribed to variations in tunneling characteristics into different materials rather than actual physical topography, the possibility of structural, as opposed to purely electronic, contributions to the topography we observe has not been eliminated. Fig. 5a shows I—V tunneling spectra for a Si076Ge024 layer (a) and undoped and doped regions of a Si layer (b and c, respectively) in the superlattice. Threshold voltages for the onset of tunneling for positive and negative sample bias have also been shown. As discussed in section 3, the threshold voltages are related although not necessarily equal to the actual band-edge energies; by tracing the variation in threshold voltages across the superlattice, one might hope to probe variations in the band-edge energies across the structure. Positive and negative threshold tunneling voltages for I—V spectra across the superlat-
1.0
a bc
0.8
0.6 0.4
£ ~
~
/0
0.2 00 -02
_____
—0.4 0.6 —1 5—1 0—05 0.0 0,5 1.0 1 5
(a)
Sample Voltage (V)
080 f
0.70
%•4 —
0.60 ~ 050 =
“ ~%fb
.
>0.40 0 30
(b)
a c
b
0.20 -0.20
a’ c’ b .
0.30
—0.40
.~
..:~‘
••~~“.
-060 0.70
‘w’
~. ..~. .•~ ~ ‘
(c)
—0.80
0 100 200 300 400 500 600 700 800 9001000 Position (A) Fig. 5. (a) I—V characteristics for a Si1176Ge024 layer (a), and undoped and doped regions of a Si layer (curves b and c, respectively). t7th’ are Positive markedand on the negative I—V threshold curves. (b)voltages Positivefor andtun(c) neling, threshold voltages across the superlattice structure. negative The labels a, b, and c correspond to the I—V curves shown in (a). The dashed lines represent the conduction- and valenceband-edge profiles calculated for the superlattice, shifted in voltage to overlay the threshold voltages.
tice are shown as solid circles in figs. Sb and Sc. The labels a, b and c correspond to the similarly labeled I—V curves in fig. 5a. Calculated conduction- and valence-band-edge profiles are shown as dashed lines in figs. Sb and 5c, respectively, shifted in voltage to overlay the threshold voltage profiles. In fig. 5b, the lower conduction-band-
___________________________________ 20
I
edge energies in the Si
—10 0
500
1000
Position
1500
2000
(A)
Fig. 4. (a) Constant-current image of the superlattice, near interface between the superlattice and the Si buffer layer; image area is 1950 Ax 1950 A. (b) Line scan across superlattice; the Si075Ge1124 layers appear as peaks in topography.
the the the the
076Ge024 layers and the band bending induced by the doping in the Si layers are evident in the positive threshold voltage profile. Shifts in the negative threshold voltage near the Si/5i075Ge024 interfaces, corresponding to the Si/Si0 76Ge~24valence-band offset, can be seen in fig. Sc; these shifts typically occur over distances of approximately SO A. Peaks in J”~~(x) arising from the modulation doping can also be detected.
E. T Yu eta!. 5.
/
Cross-sectional STM of MBE-grown Si p—n junctions and Si / SiGe SLs
Conclusions
We have used cross-sectional scanning tunneling microscopy and spectroscopy to probe the electronic structure in a Si p—n junction and a Si/Si075Ge024 superlattice. By tracing variations in threshold voltages for tunneling extracted from I—V characteristics, variations in the band-edge energies in a device can be detected. The shape of the depletion region in the p—n junction, extending over approximately 150 A, can clearly be seen, and features in the electronic structure of the Si/Si0 76Ge024 superlattice have been detected with a spatial resolution of only a few nanometers. Topographic contrast between the Si and Si076Ge024 layers in the superlattice has also been observed.
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