- Email: [email protected]
AlSb quantum wells
262
Materials &'ience and Engineering, t;,21 ( t 9 9 3 26 2-2~7.
Characterization of heterointerfaces and surfaces in InSb on GaAs and in InAs/A1Sb quantum wells J. Wagner, J. Schmitz, A.-L. Alvarez*, P. K o i d l a n d J. D. R a l s t o n Fraunhofer-Institut fiir Angewandte Festkrrperphysik, Tullastrasse 72, D- 79108 Freiburg (Germany) Abstract Resonant Raman scattering by longitudinal optical (LO) phonons and by interface modes was used to study InSb/GaAs and InAs/AISb heterointerfaces for InSb on GaAs and for InAs/AlSb quantum wells respectively, grown b y molecular beam epitaxy. Ramanspectra recorded from samples with InSb layers ranging in thickness from 10 to 300 monolayers indicate that, to achieve two-dimensional growth of InSb on GaAs with good crystalline quality, the layer thickness must exceed around 100 monolayers. From the dependence on excitation power of electric-field-induced LO phonon scattering measured for a series of n- and p-type doped thick InSb layers, it is concluded that the surface Fermi level is pinned at the valence band edge. In InAs/AISb quantum wells, scattering by an InSb-like interface mode is observed which resonates at approximately the InAs Et energy gap.
There is considerable current interest in the narrow band gap III-V semiconductors InSb and InAs, and in related heterostructures such as InAs/A1Sb quantum wells (see for example ref. 1). There is a variety of potential applications for these heterostructures in electronic and IR optoelectronic devices [1]. The fabrication of such devices requires epitaxial growth of the above III-V materials. For technological reasons heteroepitaxy on GaAs substrates is advantageous [2, 3]. The critical layer thickness for the growth of, for example, InSb on GaAs is below two monolayers, which reflects the large lattice mismatch between these two materials. It has been shown that heteroepitaxial growth of InSb on GaAs starts with the nucleation of three-dimensional islands and complete coverage of the GaAs substrate is only obtained for equivalent layer thicknesses exceeding 100 monolayers (ML)[4]. In InAs/AISb heterostructures the lattice mismatch is much smaller than in the InSb/GaAs system. Also, in these heterostructures there is no common anion or cation across the heterointerface which results in two different types of interface, either InSb- or AlAs-like. It has been shown that the in-plane electronic transport properties of the two-dimensional electron gas formed in the InAs channel of an InAs/A1Sb quantum well depend critically on the type of heterointerface [5]. In the present study we used resonant Raman scattering by longitudinal optical (LO) phonons and by
*Permanent address: ETS1 Telecommunicaci6n, Universidad Polit&nica, E-28040 Madrid, Spain. 0921-5107/93/$6.00
interface modes to study the heteroepitaxial growth of InSb on GaAs, including the Fermi level position at the InSb surface, and the interface formation in InAs/A1Sb quantum wells. The samples were grown by solidsource molecular beam epitaxy (MBE) on (100) GaAs substrates. Growth of InSb was performed using an Sb2 molecular beam with an In-to-Sb flux ratio close to unity. The substrate temperatures were in the range 4 0 0 - 4 5 0 °C. Si and Be were used as n- and p-type dopants respectively. Undoped layers showed residual p-type conductivity at low-temperatures, with a hole concentration of approximately 1016 cm -3. The thickness of the layers was typically 1/~m. InAs/AISb single quantum wells with a width of 15 nm were grown on a 1/Jm thick AISb buffer layer followed by a GaSb/A1Sb 2.5 nm/2.5 nm superlattice. The InAs quantum well was sandwiched between a 40 nm wide A1Sb barrier on the substrate side and a 10 nm wide A1Sb layer on the surface side. The shutter sequence for the growth of the InAs/A1Sb heterointerfaces was chosen to promote the formation of InSb-like interfaces [5]. The whole structure was capped with 10 nm GaSb. Growth of the quantum well structure was performed at 500 °C. Figure 1 shows a series of low-temperature Raman spectra of InSb layers grown on GaAs substrates with equivalent layer thicknesses of 10, 40, and 300 ML. Here the equivalent layer thickness was determined from the deposition time, based on growth rate calibration using reflection high-energy electron diffraction (RHEED) oscillations. The spectra were recorded in backscattering mode from the (100) growth surface with the polarization of the incident and scattered light © 1993 - Elsevier Sequoia. All rights reserved
J. Wagner et al.
InSb/GaAs
/
Characterization of lnSb on GaAs
hVL= 2.015 eV T=77K
LO(InSb)
x(y.y)R
m f12 ,<
300 ML
__ZXJ
40 ML
_
2LO (GaAs)
10 ML i
I
n
I
=
I
4-00 600 200 RAMAN SHIFT (cm -1)
Fig. 1. Low-temperature (77 K) Raman spectra of InSb grown on (100) GaAs for different equivalent InSb layer thicknesses ranging from 10 to 300 monolayers (ML). The spectra were excited at 2.015 eV in resonance with the InSb E~ band gap energy. Polarization of both incident and scattered light was parallel to the same (100) crystallographic direction.
parallel to the same (100) crystallographic direction Ix(y, y)£]. Optical excitation was at 2.015 eV in resonance with the E~ band gap energy of InSb [6]. For the present scattering configuration intrinsic two-LO phonon scattering is allowed, whereas one-LO phonon scattering by the deformation mechanism is symmetry forbidden. For resonant excitation, however, intrinsic one-LO phonon scattering via the Fr6hlich mechanism as well as defect-induced and electric-field-induced one-LO phonon scattering contribute to the Raman spectrum [7]. The present Raman spectra show that equivalent layer thicknesses of several tens of monolayers are required to produce films which exhibit the one-LO phonon line of crystalline InSb. In addition, scattering by crystalline Sb is observed, with two fines resolved at 116 and 153 cm ~[8], for an equivalent layer thickness of 40 ML. For a film thickness of 10 ML only a broad peak at 150-200 cm- 1 is observed, other than the oneLO and two-LO phonon scattering from the GaAs substrate [9]. This broad peak can be assigned to scattering by highly disordered or amorphous InSb, and possibly by amorphous Sb. For a thickness of 300 ML both the
263
InSb one-LO and two-LO phonon lines are well resolved and there is no further change in the Raman spectrum when the InSb layer thickness is increased up to the micrometre range. The observation of comparatively strong two-LO phonon scattering from InSb for layer thicknesses greater than or equal to 300 ML indicates the good crystalline quality of the fully relaxed InSb film [10] despite the large dislocation density arising from the lattice mismatch to the GaAs substrate. If we combine the present findings with results reported by Zhang et al. [4], which were obtained by transmission electron microscopy and RHEED, the following picture can be constructed. For equivalent InSb layer thicknesses of the order of 10 ML, small islands about 10 nm in height are present with a surface coverage of about 25% [4]. These islands give rise to the broad Raman peak observed in the lowest curve of Fig. 1. With increasing equivalent layer thickness the surface coverage and the island size both increase, forming a connected InSb network [4]. As a consequence, the phonon coherence length also increases leading to the appearance of mainly defectinduced one-LO phonon scattering from crystalline InSb. At this stage also crystalline Sb is present, possibly on the uncovered GaAs surface. Equivalent layer thicknesses exceeding 100 ML are required to achieve complete coverage with InSb [4] and to observe the intrinsic InSb two-LO phonon line, which js indicative of the good crystalline quality of the grown ~material, along with dominantly intrinsic one-LO phonon scattering. In Fig. 2 the intensity of one-LO phonon scattering, normalized to the strength of the purely intrinsic twoLO phonon signal, is plotted vs. the optical power density for a series of approximately 1/~m thick n- and p-type doped InSb layers. The data were again recorded in the x(y,y)2 scattering configuration. Both the nominally undoped, but residually p-type, and the intentionally p-type doped layers show a comparatively low one-LO phonon intensity which shows very tittle variation with power density. The homogeneously n-type doped sample exhibits a much larger one-LO phonon intensity at low power densities along with a marked decrease in intensity with increasing excitation power density. An InSb n-i-p-i structure was also analysed, consisting of 10 nm thick Si and Be doped layers separated by 40 nm wide undoped spacers. Proceeding from the epilayer surface back towards the substrate, the layer sequence starts with a 40 nm thick undoped InSb cap layer followed by the first Be-doped layer. The donor ND and acceptor NA concentrations were in the 1018 cm -3 range with AID> 2NA. Similar to the homogeneously n-type doped layer, the residual n-type n-i-p-i structure also shows a large relative
J. Wagner et al.
264
/
Characterization of lnSb on GaAs
10 2
o ,_1
'\
\.
H A
o10
m\m\|
,_1
I-f
~n~ l §
~-.--''~'~ |_.~
I
PO
I
Q---"-° ~
o~o
x-,
I
I I I Illl
lOxPo
I
I
I I I IlJ[
]OOxPo
POWER DENSITY
Fig. 2. Normalized LO phonon scattering strength I(LO)/I(2LO) vs. optical power density. P0 corresponds to a power density of approximately 10 Wcm-:. Data are shown for undoped InSb ( x ), p-type InSb:Be (o) with a low-temperature hole concentration of 9.1 x 1017 cm-3, an InSb n-i-p-i doping superlattice (m), and n-type InSb:Si (A) with a low-temperatureelectron concentration of 1.6 x 1017 c m -3. Error bars are indicated for the data points recorded at the lowest power densities. one-LO phonon intensity at low power densities followed by a very pronounced decrease in that intensity for increasing power density. The above findings can be explained by the presence of a surface electric field in the n-type samples which leads to electric-field-induced Raman scattering by the one-LO phonon. The intensity of this scattering is proportional to the square of the strength of the surface electric field averaged over the probing depth in the Raman experiment of about 14 nm. (This value is based on room-temperature ellipsometry data [11] which have been shifted in energy to account for the temperature induced variation in band gap energy.) The surface electric field is progressively screened by photogenerated carriers with increasing optical power density. The slight decrease in relative LO phonon intensity, observed in the homogeneously doped n-type InSb:Si layer at the lowest excitation power density, is within experimental error. The surface electric field, the presence of which has been confirmed by the observation of interference effects between dipole-allowed and electric-field-induced LO phonon scattering [12], is caused by pinning of the surface Fermi level at the
valence band edge [13, 14]. For this situation we expect a surface electric field of the order of 105 V cm f for n-type InSb with a carrier concentration of 1() L~cm [15], but a negligible electric field in p-type material. Thus the present experiment demonstrates how resonant LO phonon Raman scattering can be used to study surface Fermi level pinning in III-V semiconductors such as InSb via the resulting surface electric field, as well as the effects of optical excitation on this surface field. We turn now to the resonant Raman scattering from InAs/AISb heterostrucmres. Figure 3 shows a lowtemperature Raman spectra of a 15 nm wide InAs/ AISb single quantum well. The spectra were excited at 2.71 eV, close to the E~ band gap resonance of bulk InAs [16t. The polarization of the scattered light was either parallel [x(z,z)£] (top curve) or perpendicular [x(y,z)£] (bottom curve) to the polarization of the incident light. The spectra show one-LO phonon scattering by the InAs quantum well and by the A1Sb barriers as well as two-LO phonon scattering from InAs. The presence of an InAs two-LO phonon signal, and the observation that InAs one-LO phonon scattering is stronger for parallel than for crossed polarizations, show clearly the resonant enhancement of LO phonon scattering from the InAs quantum welt [161]. The non-resonantly excited A1Sb one-LO phonon signal, in contrast, is as expected [7, 9] most intense for crossed polarizations. Non-resonant one-LO phonon scattering from the GaSb capping layer overlaps with the InAs one-LO phonon signal [17] and is just resolved, for crossed polarizations, as shoulders on the high- and low-frequency sides of the InAs one-LO phonon line. This observation proves the high sensitivity to scattering from the InAs quantum well achieved in the present experiment by resonant excitation, despite the strong absorption of the incident and the scattered light in the GaSb cap and the A1Sb top barrier layer [17]. In the present sample the GaSb oneLO phonon line is split into a GaSb-like and a GaAslike mode [18] owing to the unintentional incorporation of As [19]. The effect of As incorporation on the AISb LO phonon is to shift this mode to higher frequencies; A1AsxSb I -x displays a single-mode behaviour [20]. In the low-frequency range the Raman spectra of the InAs/AISb quantum well (Fig. 3) show second-order acoustic phonon scattering from the AISb barriers [2TA(A1Sb)] [21] which is strongest for parallel polarizations, and a peak at about 190 cm- 1 [IF(InSb)] which is most intense for crossed polarizations. The latter peak is assigned to a longitudinal InSb interface mode [17, 22]. Scattering by the 190 cm ~ mode was found to resonate at approximately the InAs El gap energy and not at the corresponding gap energy of InSb (see
J. Wagner et al.
LO (InAs)
/
Characterization of lnSb on GaAs
InAs//AISb QW hUL= 2.71 eV
265
for performing the Hall effect measurements. A.-L.A. wishes to thank the Comunidad Aut6noma de Madrid for financial support.
T=15K
I--
References
x(z,z)~ <
b.l
--
(InAs)
2LO
)._ 2TA(AISb)
,
I
1
IF(InSb)~ II
,
I
200
J
I
300
,
I
#00
I
,
500
RAMAN SHIFT (cm -1) Fig. 3. Low-temperature (15 K) Raman spectra of an InAs/AISb single quantum well. The spectra, excited in resonance with the InAs E l band gap energy, were recorded with polarization of the scattered light either parallel [x(z,z)£1 (top) or perpendicular [x(y,z)£] (bottom) to that of the incident light. Here x, y, and z denote (100) crystallographic directions.
above), which gives additional support to the assignment of this peak to an interface mode [19]. In conclusion, we have shown how resonant Raman scattering by LO phonons and by interface modes can be used to analyse heterointerfaces and surface Fermi level pinning in InSb/GaAs and InAs/AlSb heterostructures. Tuning the incident photon energy into resonance with an appropriate band gap energy of one of the constituents allows the vibrational properties of that particular layer to be probed with a high degree of selectivity. Acknowledgments We would like to thank G. Bihlmann for valuable technical assistance in MBE growth and P. Hiesinger
1 Proc. 6th Int. Conf. on Narrow Gap Semiconductors, Semicond. Sci. TechnoL, 8(1990) S102-S137, S183-$225. 2 R.A. Stradling, Semicond. Sci. TechnoL, 6 ( 1991) C52. 3 J. R. S6derstr6m, M. M. Cumming, J.-Y. Yao and T. G. Anderson, Semicond. Sci. Technol., 7(1992) 337. 4 X. Zhang, A. E. Staton-Bevan, D. W. Pashley, S. D. Parker, R. Droopad, R. L. Williams and R. C. Newman, J. AppL Phys., 67(1990) 800. 5 G. Tuttle, H. Kroemer and J. H. English, J. AppL Phys., 67 (1990) 3032. 6 W. Dreybrodt, W. Richter and M. Cardona, Solid State Commun., 11 (1972) 1127. 7 J. Men6ndez and M. Cardona, Phys. Rev. B, 31 (1985) 3696. J. Men6ndez, L. Vina, M. Cardona and E. Anastassakis, Phys. Rev. B, 32(1985) 3966. 8 M. Hfinermann, W. Pletschen, U. Resch, U. Rettweiler, W. Richter, J. Geurts and P. Lautenschlager, Surf. Sci., 189-190(1987) 322. 9 R. Trommer and M. Cardona, Phys. Rev. B, 17(1978) 1865. 10 J. Wagner and Ch. Hoffmann, Appl. Phys. Lett., 50 (1987) 682. J. Wagner, AppL Phys. Lett., 52 (1988) 1158. 11 D.E. Aspnes and A. A. Studna, Phys. Rev. B, 27(1983) 985. 12 J. Wagner, A.-L. Alvarez. J. Schmitz, J. D. Ralston and P. Koidl, AppL Phys. Lett., 63 (1993) 349. 13 C.A. Mead and W. G. Spitzer, Phys. Rev., 134 (1964) A713. 14 J. Geurts, W, Pletschen and W. Richter, Surf. Sci., 152-153 (1985) 1123. 15 J. Geurts, PhD Thesis, Technical University of Aachen, 1984. 16 R. Caries, N. Saint-Cricq, J. B. Renucci, A. Zwick and M. A. Renucci, Phys. Rev. B, 22 (1980) 6129. 17 I. Sela, C. R. Bolognesi, L. A. Samoska and H. Kroemer, Appl. Phys. Lett., 60(1992) 3283. 18 D.H. Jaw, D. S. Cao and G. B. Stringfellow, J. Appl. Phys., 69 (1991)2552. 19 J. Wagner, J. Schmitz, M. Maier, J. D. Ralston and P. Koidl, submitted to Appl. Phys. Lett. 20 I. Sela, C. R. Bolognesi and H. Kroemer, Phys. Rev. B, 46 (1992) 16142. 21 Y. S. Raptis, E. Anastassakis and G. Kanellis, Phys. Rev. B, 46(1992) 15801. 22 M. Inoue, M. Yano, H. Furuse, N. Nasu and Y. lwai, Semicond. Sci. Technol., 8(1993)S121.
Materials &'ience and Engineering, t;,21 ( t 9 9 3 26 2-2~7.
Characterization of heterointerfaces and surfaces in InSb on GaAs and in InAs/A1Sb quantum wells J. Wagner, J. Schmitz, A.-L. Alvarez*, P. K o i d l a n d J. D. R a l s t o n Fraunhofer-Institut fiir Angewandte Festkrrperphysik, Tullastrasse 72, D- 79108 Freiburg (Germany) Abstract Resonant Raman scattering by longitudinal optical (LO) phonons and by interface modes was used to study InSb/GaAs and InAs/AISb heterointerfaces for InSb on GaAs and for InAs/AlSb quantum wells respectively, grown b y molecular beam epitaxy. Ramanspectra recorded from samples with InSb layers ranging in thickness from 10 to 300 monolayers indicate that, to achieve two-dimensional growth of InSb on GaAs with good crystalline quality, the layer thickness must exceed around 100 monolayers. From the dependence on excitation power of electric-field-induced LO phonon scattering measured for a series of n- and p-type doped thick InSb layers, it is concluded that the surface Fermi level is pinned at the valence band edge. In InAs/AISb quantum wells, scattering by an InSb-like interface mode is observed which resonates at approximately the InAs Et energy gap.
There is considerable current interest in the narrow band gap III-V semiconductors InSb and InAs, and in related heterostructures such as InAs/A1Sb quantum wells (see for example ref. 1). There is a variety of potential applications for these heterostructures in electronic and IR optoelectronic devices [1]. The fabrication of such devices requires epitaxial growth of the above III-V materials. For technological reasons heteroepitaxy on GaAs substrates is advantageous [2, 3]. The critical layer thickness for the growth of, for example, InSb on GaAs is below two monolayers, which reflects the large lattice mismatch between these two materials. It has been shown that heteroepitaxial growth of InSb on GaAs starts with the nucleation of three-dimensional islands and complete coverage of the GaAs substrate is only obtained for equivalent layer thicknesses exceeding 100 monolayers (ML)[4]. In InAs/AISb heterostructures the lattice mismatch is much smaller than in the InSb/GaAs system. Also, in these heterostructures there is no common anion or cation across the heterointerface which results in two different types of interface, either InSb- or AlAs-like. It has been shown that the in-plane electronic transport properties of the two-dimensional electron gas formed in the InAs channel of an InAs/A1Sb quantum well depend critically on the type of heterointerface [5]. In the present study we used resonant Raman scattering by longitudinal optical (LO) phonons and by
*Permanent address: ETS1 Telecommunicaci6n, Universidad Polit&nica, E-28040 Madrid, Spain. 0921-5107/93/$6.00
interface modes to study the heteroepitaxial growth of InSb on GaAs, including the Fermi level position at the InSb surface, and the interface formation in InAs/A1Sb quantum wells. The samples were grown by solidsource molecular beam epitaxy (MBE) on (100) GaAs substrates. Growth of InSb was performed using an Sb2 molecular beam with an In-to-Sb flux ratio close to unity. The substrate temperatures were in the range 4 0 0 - 4 5 0 °C. Si and Be were used as n- and p-type dopants respectively. Undoped layers showed residual p-type conductivity at low-temperatures, with a hole concentration of approximately 1016 cm -3. The thickness of the layers was typically 1/~m. InAs/AISb single quantum wells with a width of 15 nm were grown on a 1/Jm thick AISb buffer layer followed by a GaSb/A1Sb 2.5 nm/2.5 nm superlattice. The InAs quantum well was sandwiched between a 40 nm wide A1Sb barrier on the substrate side and a 10 nm wide A1Sb layer on the surface side. The shutter sequence for the growth of the InAs/A1Sb heterointerfaces was chosen to promote the formation of InSb-like interfaces [5]. The whole structure was capped with 10 nm GaSb. Growth of the quantum well structure was performed at 500 °C. Figure 1 shows a series of low-temperature Raman spectra of InSb layers grown on GaAs substrates with equivalent layer thicknesses of 10, 40, and 300 ML. Here the equivalent layer thickness was determined from the deposition time, based on growth rate calibration using reflection high-energy electron diffraction (RHEED) oscillations. The spectra were recorded in backscattering mode from the (100) growth surface with the polarization of the incident and scattered light © 1993 - Elsevier Sequoia. All rights reserved
J. Wagner et al.
InSb/GaAs
/
Characterization of lnSb on GaAs
hVL= 2.015 eV T=77K
LO(InSb)
x(y.y)R
m f12 ,<
300 ML
__ZXJ
40 ML
_
2LO (GaAs)
10 ML i
I
n
I
=
I
4-00 600 200 RAMAN SHIFT (cm -1)
Fig. 1. Low-temperature (77 K) Raman spectra of InSb grown on (100) GaAs for different equivalent InSb layer thicknesses ranging from 10 to 300 monolayers (ML). The spectra were excited at 2.015 eV in resonance with the InSb E~ band gap energy. Polarization of both incident and scattered light was parallel to the same (100) crystallographic direction.
parallel to the same (100) crystallographic direction Ix(y, y)£]. Optical excitation was at 2.015 eV in resonance with the E~ band gap energy of InSb [6]. For the present scattering configuration intrinsic two-LO phonon scattering is allowed, whereas one-LO phonon scattering by the deformation mechanism is symmetry forbidden. For resonant excitation, however, intrinsic one-LO phonon scattering via the Fr6hlich mechanism as well as defect-induced and electric-field-induced one-LO phonon scattering contribute to the Raman spectrum [7]. The present Raman spectra show that equivalent layer thicknesses of several tens of monolayers are required to produce films which exhibit the one-LO phonon line of crystalline InSb. In addition, scattering by crystalline Sb is observed, with two fines resolved at 116 and 153 cm ~[8], for an equivalent layer thickness of 40 ML. For a film thickness of 10 ML only a broad peak at 150-200 cm- 1 is observed, other than the oneLO and two-LO phonon scattering from the GaAs substrate [9]. This broad peak can be assigned to scattering by highly disordered or amorphous InSb, and possibly by amorphous Sb. For a thickness of 300 ML both the
263
InSb one-LO and two-LO phonon lines are well resolved and there is no further change in the Raman spectrum when the InSb layer thickness is increased up to the micrometre range. The observation of comparatively strong two-LO phonon scattering from InSb for layer thicknesses greater than or equal to 300 ML indicates the good crystalline quality of the fully relaxed InSb film [10] despite the large dislocation density arising from the lattice mismatch to the GaAs substrate. If we combine the present findings with results reported by Zhang et al. [4], which were obtained by transmission electron microscopy and RHEED, the following picture can be constructed. For equivalent InSb layer thicknesses of the order of 10 ML, small islands about 10 nm in height are present with a surface coverage of about 25% [4]. These islands give rise to the broad Raman peak observed in the lowest curve of Fig. 1. With increasing equivalent layer thickness the surface coverage and the island size both increase, forming a connected InSb network [4]. As a consequence, the phonon coherence length also increases leading to the appearance of mainly defectinduced one-LO phonon scattering from crystalline InSb. At this stage also crystalline Sb is present, possibly on the uncovered GaAs surface. Equivalent layer thicknesses exceeding 100 ML are required to achieve complete coverage with InSb [4] and to observe the intrinsic InSb two-LO phonon line, which js indicative of the good crystalline quality of the grown ~material, along with dominantly intrinsic one-LO phonon scattering. In Fig. 2 the intensity of one-LO phonon scattering, normalized to the strength of the purely intrinsic twoLO phonon signal, is plotted vs. the optical power density for a series of approximately 1/~m thick n- and p-type doped InSb layers. The data were again recorded in the x(y,y)2 scattering configuration. Both the nominally undoped, but residually p-type, and the intentionally p-type doped layers show a comparatively low one-LO phonon intensity which shows very tittle variation with power density. The homogeneously n-type doped sample exhibits a much larger one-LO phonon intensity at low power densities along with a marked decrease in intensity with increasing excitation power density. An InSb n-i-p-i structure was also analysed, consisting of 10 nm thick Si and Be doped layers separated by 40 nm wide undoped spacers. Proceeding from the epilayer surface back towards the substrate, the layer sequence starts with a 40 nm thick undoped InSb cap layer followed by the first Be-doped layer. The donor ND and acceptor NA concentrations were in the 1018 cm -3 range with AID> 2NA. Similar to the homogeneously n-type doped layer, the residual n-type n-i-p-i structure also shows a large relative
J. Wagner et al.
264
/
Characterization of lnSb on GaAs
10 2
o ,_1
'\
\.
H A
o10
m\m\|
,_1
I-f
~n~ l §
~-.--''~'~ |_.~
I
PO
I
Q---"-° ~
o~o
x-,
I
I I I Illl
lOxPo
I
I
I I I IlJ[
]OOxPo
POWER DENSITY
Fig. 2. Normalized LO phonon scattering strength I(LO)/I(2LO) vs. optical power density. P0 corresponds to a power density of approximately 10 Wcm-:. Data are shown for undoped InSb ( x ), p-type InSb:Be (o) with a low-temperature hole concentration of 9.1 x 1017 cm-3, an InSb n-i-p-i doping superlattice (m), and n-type InSb:Si (A) with a low-temperatureelectron concentration of 1.6 x 1017 c m -3. Error bars are indicated for the data points recorded at the lowest power densities. one-LO phonon intensity at low power densities followed by a very pronounced decrease in that intensity for increasing power density. The above findings can be explained by the presence of a surface electric field in the n-type samples which leads to electric-field-induced Raman scattering by the one-LO phonon. The intensity of this scattering is proportional to the square of the strength of the surface electric field averaged over the probing depth in the Raman experiment of about 14 nm. (This value is based on room-temperature ellipsometry data [11] which have been shifted in energy to account for the temperature induced variation in band gap energy.) The surface electric field is progressively screened by photogenerated carriers with increasing optical power density. The slight decrease in relative LO phonon intensity, observed in the homogeneously doped n-type InSb:Si layer at the lowest excitation power density, is within experimental error. The surface electric field, the presence of which has been confirmed by the observation of interference effects between dipole-allowed and electric-field-induced LO phonon scattering [12], is caused by pinning of the surface Fermi level at the
valence band edge [13, 14]. For this situation we expect a surface electric field of the order of 105 V cm f for n-type InSb with a carrier concentration of 1() L~cm [15], but a negligible electric field in p-type material. Thus the present experiment demonstrates how resonant LO phonon Raman scattering can be used to study surface Fermi level pinning in III-V semiconductors such as InSb via the resulting surface electric field, as well as the effects of optical excitation on this surface field. We turn now to the resonant Raman scattering from InAs/AISb heterostrucmres. Figure 3 shows a lowtemperature Raman spectra of a 15 nm wide InAs/ AISb single quantum well. The spectra were excited at 2.71 eV, close to the E~ band gap resonance of bulk InAs [16t. The polarization of the scattered light was either parallel [x(z,z)£] (top curve) or perpendicular [x(y,z)£] (bottom curve) to the polarization of the incident light. The spectra show one-LO phonon scattering by the InAs quantum well and by the A1Sb barriers as well as two-LO phonon scattering from InAs. The presence of an InAs two-LO phonon signal, and the observation that InAs one-LO phonon scattering is stronger for parallel than for crossed polarizations, show clearly the resonant enhancement of LO phonon scattering from the InAs quantum welt [161]. The non-resonantly excited A1Sb one-LO phonon signal, in contrast, is as expected [7, 9] most intense for crossed polarizations. Non-resonant one-LO phonon scattering from the GaSb capping layer overlaps with the InAs one-LO phonon signal [17] and is just resolved, for crossed polarizations, as shoulders on the high- and low-frequency sides of the InAs one-LO phonon line. This observation proves the high sensitivity to scattering from the InAs quantum well achieved in the present experiment by resonant excitation, despite the strong absorption of the incident and the scattered light in the GaSb cap and the A1Sb top barrier layer [17]. In the present sample the GaSb oneLO phonon line is split into a GaSb-like and a GaAslike mode [18] owing to the unintentional incorporation of As [19]. The effect of As incorporation on the AISb LO phonon is to shift this mode to higher frequencies; A1AsxSb I -x displays a single-mode behaviour [20]. In the low-frequency range the Raman spectra of the InAs/AISb quantum well (Fig. 3) show second-order acoustic phonon scattering from the AISb barriers [2TA(A1Sb)] [21] which is strongest for parallel polarizations, and a peak at about 190 cm- 1 [IF(InSb)] which is most intense for crossed polarizations. The latter peak is assigned to a longitudinal InSb interface mode [17, 22]. Scattering by the 190 cm ~ mode was found to resonate at approximately the InAs El gap energy and not at the corresponding gap energy of InSb (see
J. Wagner et al.
LO (InAs)
/
Characterization of lnSb on GaAs
InAs//AISb QW hUL= 2.71 eV
265
for performing the Hall effect measurements. A.-L.A. wishes to thank the Comunidad Aut6noma de Madrid for financial support.
T=15K
I--
References
x(z,z)~ <
b.l
--
(InAs)
2LO
)._ 2TA(AISb)
,
I
1
IF(InSb)~ II
,
I
200
J
I
300
,
I
#00
I
,
500
RAMAN SHIFT (cm -1) Fig. 3. Low-temperature (15 K) Raman spectra of an InAs/AISb single quantum well. The spectra, excited in resonance with the InAs E l band gap energy, were recorded with polarization of the scattered light either parallel [x(z,z)£1 (top) or perpendicular [x(y,z)£] (bottom) to that of the incident light. Here x, y, and z denote (100) crystallographic directions.
above), which gives additional support to the assignment of this peak to an interface mode [19]. In conclusion, we have shown how resonant Raman scattering by LO phonons and by interface modes can be used to analyse heterointerfaces and surface Fermi level pinning in InSb/GaAs and InAs/AlSb heterostructures. Tuning the incident photon energy into resonance with an appropriate band gap energy of one of the constituents allows the vibrational properties of that particular layer to be probed with a high degree of selectivity. Acknowledgments We would like to thank G. Bihlmann for valuable technical assistance in MBE growth and P. Hiesinger
1 Proc. 6th Int. Conf. on Narrow Gap Semiconductors, Semicond. Sci. TechnoL, 8(1990) S102-S137, S183-$225. 2 R.A. Stradling, Semicond. Sci. TechnoL, 6 ( 1991) C52. 3 J. R. S6derstr6m, M. M. Cumming, J.-Y. Yao and T. G. Anderson, Semicond. Sci. Technol., 7(1992) 337. 4 X. Zhang, A. E. Staton-Bevan, D. W. Pashley, S. D. Parker, R. Droopad, R. L. Williams and R. C. Newman, J. AppL Phys., 67(1990) 800. 5 G. Tuttle, H. Kroemer and J. H. English, J. AppL Phys., 67 (1990) 3032. 6 W. Dreybrodt, W. Richter and M. Cardona, Solid State Commun., 11 (1972) 1127. 7 J. Men6ndez and M. Cardona, Phys. Rev. B, 31 (1985) 3696. J. Men6ndez, L. Vina, M. Cardona and E. Anastassakis, Phys. Rev. B, 32(1985) 3966. 8 M. Hfinermann, W. Pletschen, U. Resch, U. Rettweiler, W. Richter, J. Geurts and P. Lautenschlager, Surf. Sci., 189-190(1987) 322. 9 R. Trommer and M. Cardona, Phys. Rev. B, 17(1978) 1865. 10 J. Wagner and Ch. Hoffmann, Appl. Phys. Lett., 50 (1987) 682. J. Wagner, AppL Phys. Lett., 52 (1988) 1158. 11 D.E. Aspnes and A. A. Studna, Phys. Rev. B, 27(1983) 985. 12 J. Wagner, A.-L. Alvarez. J. Schmitz, J. D. Ralston and P. Koidl, AppL Phys. Lett., 63 (1993) 349. 13 C.A. Mead and W. G. Spitzer, Phys. Rev., 134 (1964) A713. 14 J. Geurts, W, Pletschen and W. Richter, Surf. Sci., 152-153 (1985) 1123. 15 J. Geurts, PhD Thesis, Technical University of Aachen, 1984. 16 R. Caries, N. Saint-Cricq, J. B. Renucci, A. Zwick and M. A. Renucci, Phys. Rev. B, 22 (1980) 6129. 17 I. Sela, C. R. Bolognesi, L. A. Samoska and H. Kroemer, Appl. Phys. Lett., 60(1992) 3283. 18 D.H. Jaw, D. S. Cao and G. B. Stringfellow, J. Appl. Phys., 69 (1991)2552. 19 J. Wagner, J. Schmitz, M. Maier, J. D. Ralston and P. Koidl, submitted to Appl. Phys. Lett. 20 I. Sela, C. R. Bolognesi and H. Kroemer, Phys. Rev. B, 46 (1992) 16142. 21 Y. S. Raptis, E. Anastassakis and G. Kanellis, Phys. Rev. B, 46(1992) 15801. 22 M. Inoue, M. Yano, H. Furuse, N. Nasu and Y. lwai, Semicond. Sci. Technol., 8(1993)S121.