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Heteroepitaxy between layered semiconductors GaSe and InSe
Applied Surface North-Holland
Science 41/42
HETEROEPITAXY C. TATSUYAMA, Department Received
BETWEEN T. TANBO
LAYERED
1988; accepted
SEMICONDUCTORS
GaSe AND InSe
and N. NAKAYAMA
of Electronics, Faculty of Engineering, 8 November
539
(1989) 539-543
Toyama
for publication
Unruersity, Gofuku, Toyama 930, Japan
5 April 1989
The initial stage of heteroepitaxy of GaSe on InSe, and InSe on GaSe has been studied by means of AES (Auger electron spectroscopy), LEELS (low-energy electron-loss spectroscopy) and XPS (X-ray photoelectron spectroscopy). GaSe(InSe) was deposited onto the substrate by using a W filament. The growth mode was determined by measuring the decrease in the AES signal intensity from the substrate upon deposition, which reveals that GaSe deposited on InSe (or InSe on GaSe) grows layer-by-layer with unit thickness of the primitive layer consisting of four atomic planes in the sequence Se-M-M-Se (M = Ga, In). The crystal quality of the grown layer was characterized by LEELS. The results indicate that the grown layers of about two primitive layers deposited at elevated temperature ( - 300 o C) have the same crystal quality as the bulk crystal, in spite of the large lattice mismatch of about 7% between GaSe and InSe. The valence band discontinuity between GaSe and InSe estimated by XPS was less than 0.1 eV, and the conduction band discontinuity is thus about 0.7 eV due to the band gap difference between the two crystals.
1. Introduction In heteroepitaxial growth of well-known threedimensional crystals such as IV, III-V and II-VI semiconductors, the lattice matching between two crystals forming heterojunctions is one of the severe conditions for obtaining high quality junctions. Growth of strained layer superlattices is one method to relax the limitation of lattice matching. However, if the thickness of the growing layer exceeds some critical thickness, misfit dislocations appear [l]. Recently, it has been reported that the heteroepitaxy between layered materials is possible without generation of misfit dislocations in spite of the large difference between the lattice constants [2]. The layered materials are described by stackings of thin primitive layers consisting of several atomic planes [3]. The bonding between the primitive layers is by a weak van der Waals force. No dangling bonds exist on the cleaved surface. Therefore, the deposited layer on the substrate grows free from strain due to the lattice mismatch. GaSe and InSe belong to III-VI layered semiconductors, in which a primitive layer consists of 0169.4332/89/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
four atomic planes in the sequence Se-M-M-Se (M = Ga. In). The band gaps of GaSe and InSe are 2.0 and 1.3 eV, respectively. The lattice mismatch between GaSe and InSe is about 7% in the cleaved basal plane. These materials have long been of interest from their electrical and optical properties related to the anisotropy of the crystal structure [4,5]. In the present paper, we report for the first time on the growth mode, crystal quality of the grown layers, interfacial reaction and the valence band discontinuity, at the initial stage of the heteroepitaxy of GaSe on InSe, and InSe on GaSe.
2. Experimental All experiments were carried out in an ultrahigh vacuum chamber with base pressure better than 1 X 10e9 Torr. The vacuum chamber was equipped with a double-pass cylindrical mirror type energy analyzer (CMA) for the measurements of AES, LEELS and XPS. GaSe and InSe single crystal were grown by the Bridgman method from the melts. The substrate of about 10 x 10 x 2 mm3 was cut from the crystal, and mounted on a stain-
540
C. Tatsuyama
et al. / Heteroepltuxy
between Iuyered semkondwtor.r
less-steel sample holder via In metal. The substrate was cleaved before insertion into the chamber. The heating of the substrate was performed by a W filament behind the sample holder. The sample temperature was measured by a Chromel-Alumel thermocouple attached to the sample holder. GaSe and InSe were evaporated from a W-solenoidal filament surrounded by liquid N, shrouds. The thickness of the deposited layer was monitored by a quartz oscillator and also estimated by the intensity of AES signals from the substrate. AES and LEELS were measured at a primary electron beam energy of 3 keV and 100 eV, respectively, and a beam current of 0.3 PA. XPS was measured with unmonochromatized Mg Ka,.z Xrays (1253.6 eV) with a power of 400 W. The CMA was operated in the non-retarding mode for AES, and in the retarding mode for XPS and LEELS. All measurements of AES, LEELS and XPS were performed after the sample was cooled down to room temperature.
3. Results
and discussion
3. I. Growth mode The growth mode of GaSe on InSe, and InSe on GaSe was investigated by measuring the decrease in the AES-signal intensity from the substrate upon the deposition. Figs. la and lb show the decrease in intensity of the In(MNN) AES signal of the InSe substrate at 398 eV as a function of the thickness of the GaSe deposited on the substrate at temperatures of 200 and 400 o C, where (a) is a linear plot and (b) a semi-logarithmic plot. The AES intensity I is normalized to that obtained from the cleaved InSe surface, and the thickness of the deposited GaSe was monitored by a quartz oscillator. In fig. la, for both temperatures, the intensity of the In(MNN) AES signal decreases with a constant slope down to I = 0.42 upon GaSe deposition, followed by a different slope for further deposition. These results clearly indicate that the deposited GaSe grows in a layerby-layer fashion.
GO% ’
on
I
,nse
Gase
01 2d GoLie
on
,nse
ln(MNNlk398e”
In(MNNl-398eV
0’ 0
GuSe und InSe
40
60
Thlckness(!,i (0)
80
0
20
40
GaSe
Thickness
60
80
t,!a )
(b)
Fig. 1. Intensity of the In(MNN) AES signal at 39X cV ah ;I function of the thickness of GaSe deposited on InSe at 200 and 400° C. The thickness of GaSe was measured by a quart7 oscillator. The intensity is normalized to that of cleaved In%: (a) linear plot, (b) semi-logarithmic plot
The thickness where the slope of I changes is different for both substrate temperatures. namely about 24 A for 200 OC and about 42 A for 400” C. However, we need to note here that the thickness of GaSe was indirectly monitored by a quartz oscillator placed far from the substrate. Therefore, the thickness measured by a quartz oscillator does not represent the exact thickness of adsorbed GaSe on the substrate. The sticking probability of GaSe should depend on the substrate temperature. This difference may thus be due to the fact that the sticking probability for deposited GaSe is larger for 200°C than for 400°C. For layer-by-layer growth, the AES intensity I from the substrate is expressed by 1 = exp( - 19cl/h, ) ,
(1)
where d is the unit thickness of the grown layer, H is the number of layers deposited on the substrate. X, is the effective escape depth of the AES electrons passing through the deposited layers. In the present experiment, the electrons which escape from the surface at an angle of $I = 42.3” with respect to the surface normal are detected by the CMA. Thus, X, is expressed as h cos + = 0.74h. where X is the escape depth of the electrons. The value of X is estimated by the following relation
[61: x(nm)
= 538N-
‘,l”E -2 + 0.4,Nmml j?El,”
(2)
where N is the atomic density (nm “) and E the kinetic energy of the electron (398 eV). For GaSe.
541
C. Tatsuyama et al. / Heteroepitaxy between layered semiconductors GaSe and InSe
N = 33.27 nmp3, then A is 12.8 A, which gives x, = 0.45 A. In fig. la, the inflection point of the decreasing slope of the AES signal should correspond to the completion of the growth of one (13= 1) unit layer. By substituting X, = 9.450A and 0 = 1 into eq. (1) d is calculated to be 8.2 A. This value of d is very close to the thickness of a primitive layer of GaSe (7.97 A). Thus, the inflection point in fig. la corresponds to the completion of the growth of one primitive layer. In fig. lb, the behavior of the decrease in intensity of AES again follows the layer growth mode, but it has a different slope for the deposition over one primitive layer. This result indicates that the sticking probability of GaSe on InSe changes after 8 = 1 compared with before 8 = 1. After the completion of the growth of one primitive layer, the growth rate of GaSe increases, then the decreasing slope becomes steeper. If we take a primitive layer as a unit layer for the layer-by-layer growth, the escape depth X of the In(MNN) AES electrons passing through the GaSe layer is estimated to be 12.4 A (X, = 9.2 A). After this estimation, the exact thickness of the GaSe deposited on InSe can be obtained by the AES-signal intensity from the substrate. A similar behaviour of the AES intensity has been observed for the deposition of InSe on GaSe. 3.2. Crystal quality of grown films The crystal quality of the grown films was characterized by LEELS. Fig. 2 shows the evolution of the LEELS spectrum for the deposition of GaSe on InSe at 400” C. The thickness of the GaSe deposited on InSe was estimated by the AES-signal intensity from the InSe substrate, as discussed in the preceding subsection. The lowest spectrum in the figure is that of InSe, and the uppermost spectrum is that of GaSe. The assignment of each peak has already been published before [7], and peaks A, B, C and F are loss peaks due to the excitations between the valence and conduction bands, and peaks G, H and I due to transitions between Ga 3d (In 3d) and the conduction band. Peaks D and E are the surface and bulk plasmon excitations, respectively.
: Electron
Energy Loss(eV)
Fig. 2. Evolution of the LEELS spectrum obtained at a primary electron beam energy of 100 eV as a function of the thickness of GaSe deposited on InSe at 400 ’ C.’
The spectrum of InSe gradually changes by the deposition of GaSe, but that of 3.0 A still shows the nature of InSe, while that of 7.5 A becomes almost the same as that of Case, as is especially clear from the changes of peaks C, D, E and F. The thickness of 7.5 A is about the same as one primitive layer of GaSe. The spectrum after deposition of 17.2 A (19 = 2) is completely replaced by that of GaSe. These results indicate that the grown layers are free from the strain usually induced by the large lattice mismatch for three-dimensional crystals, and the crystal quality of the grown layer of about two primitive layers is as good as the bulk crystal. A similar evolution of the LEELS spectrum has also been observed for the deposition of InSe on GaSe at 300 o C. 3.3. Interfacial
reactivity
The interfacial reactivity has been studied by XPS. Figs. 3a and 3b show the evolution of the
C. Tatsuyamu et al. / Heteroepitaq
842
GaSe
between ku_vered semiconductors GuSe und InSe
on InSe
GoSe
hv=1253.6eV
, 000
I
605 Kinetic Energy
Electron
(a
on InSe
hv=l253.6eV
(eV)
Electron
130 Kinetic
135 Energy(eV1
(b)
)
as a function of the thickness of &Se deposited on In% at Fig. 3. Evolution of XPS spectra of: (a) In3ds,> and (b) Ga2p,,,, 300’ C. The peak height of each individual spectrum is multiplied by a factor to give the same height in all cases.
XPS spectra of In 3d,,, and Ga2p,,,, respectively, upon GaSe deposition on InSe at 300” C. Fig. 4 is the FWHM (full width at half maximum) of the XPS spectra shown in fig. 3. In these figures, the thickness of GaSe was measured by a quartz oscillator. The FWHM of Ga2p,,, is slightly wider than that of the bulk crystal shown by the dotted line in fig. 4, thereby suggesting a somewhat poorer crystal quality of the deposited layer than of the bulk. The FWHM of In3d,,, slightly increases with GaSe deposition, which is
,
I
I
I
GoSe on
InSe
Ts = 300°C
;
1.6 -
01 $
_ cl 1.4 ---
Ga *p3/2 -_
0 ____--__ -_Go 2p3,2 (bulk)
_-.--
0 ---
I I5
I 20
z LL_ I.*%‘” 3%/2 1.0
0
I IO
I 5 Case
Fig. 4. FWHM
Thickness
of XPS spectra
(A)
shown in fig. 3.
due to the inclusion of the component of In metal as shown by the shaded area of the top figure in fig. 3a. This result indicates that GaSe deposited on InSe at 300 o C reacts with the InSe substrate. For the deposition of GaSe on InSe at RT, and of InSe on GaSe at RT and 300 o C, no interfacial reactions have been seen. 3.4. Band discontinuit_y The valence band discontinuity A E, between GaSe and InSe was measured by XPS. A E,. is given by, AE,=(E;-E,Z,)-(E;-E,‘L,)+AEc,,
(3)
A EcL = E,:_ - E,‘,,
(4)
where EJ and Eb? are the top of the valence band of InSe and GaSe, respectively, and E,‘, and EL\ the energy of In 3d 5,2 and Ga 2p,,,, respectively. The values of (Ei - E,‘,) and (E: - E$) were measured separately for each bulk crystal. and found to be 443.96 and 1116.77 eV, respectively. A EcL is the energy difference between Ga2p,,, on InSe and In 3d 5,2 when GaSe is deposited (InSe on GaSe). The positive value of A E, implies that the valence band of GaSe lies above that of
543
C. Tatsuyama et al. / Heteroepitaxy between layered semiconductors GaSe and InSe I
I
1
GoSe on InSe
-0.5
-1
GaSe- InSe T,= 300°C
i
3
o.50Ldij Overlayer
Thickness
(i
1
Fig. 6. Valence band discontinuity for GaSe on InSe, and InSe on GaSe. Substrate temperature for the deposition was 300 o C.
GaSe Thickness CL
)
Fig. 5. Energy positions of the core levels for the deposition of GaSe on InSe at 300 o C, which were obtained from fig. 3.
References InSe by AE,. The peak positions of Ga 2~~,~ and as a function of the thickness of GaSe In3d,,, deposited on InSe at 300 o C are obtained from fig. 3, and are shown in fig. 5. Substituting these results into eq. (3) the valence band discontinuity of GaSe and InSe for the deposition of GaSe on InSe is obtained as shown in fig. 6, where the result for InSe on GaSe is also shown. For both cases, the band offset of the valence band lies within t-O.1 eV. Thus, the conduction band discontinuity is 0.7 f 0.1 eV due to the band gap difference between the two crystals (2.0 eV for GaSe, and 1.3 eV for InSe).
[l] A. Kobayashi and S. Das Sarma, Phys. Rev. B 37 (1987) 1039, and references therein. [2] A. Koma, K. Sunouchi and T. Miyajima, J. Vacuum Sci. Technol. B 3 (1985) 724. [3] H. Hullinger, in: Physics and Chemistry of Materials with Layered Structures, Vol. 5 of Structural Chemistry of Layer-Type Phases, Ed. F. Levy (Reidel, Dordrecht, 1976). [4] V. Capozzi, Phys. Rev. B 28 (1983) 4620. [5] A. Segura, F. Pomer, A. Cantarero, W. Krause and A. Chevy, Phys. Rev. B 29 (1984) 5708. [6] M.P. Seah and W.A. Dench, NLP Rept. Chem. (82) April 1978. [7] H. Araki, S. Nishikawa, T. Tanbo and C. Tatsuyama, Phys. Rev. B 33 (1986) 8164.
Science 41/42
HETEROEPITAXY C. TATSUYAMA, Department Received
BETWEEN T. TANBO
LAYERED
1988; accepted
SEMICONDUCTORS
GaSe AND InSe
and N. NAKAYAMA
of Electronics, Faculty of Engineering, 8 November
539
(1989) 539-543
Toyama
for publication
Unruersity, Gofuku, Toyama 930, Japan
5 April 1989
The initial stage of heteroepitaxy of GaSe on InSe, and InSe on GaSe has been studied by means of AES (Auger electron spectroscopy), LEELS (low-energy electron-loss spectroscopy) and XPS (X-ray photoelectron spectroscopy). GaSe(InSe) was deposited onto the substrate by using a W filament. The growth mode was determined by measuring the decrease in the AES signal intensity from the substrate upon deposition, which reveals that GaSe deposited on InSe (or InSe on GaSe) grows layer-by-layer with unit thickness of the primitive layer consisting of four atomic planes in the sequence Se-M-M-Se (M = Ga, In). The crystal quality of the grown layer was characterized by LEELS. The results indicate that the grown layers of about two primitive layers deposited at elevated temperature ( - 300 o C) have the same crystal quality as the bulk crystal, in spite of the large lattice mismatch of about 7% between GaSe and InSe. The valence band discontinuity between GaSe and InSe estimated by XPS was less than 0.1 eV, and the conduction band discontinuity is thus about 0.7 eV due to the band gap difference between the two crystals.
1. Introduction In heteroepitaxial growth of well-known threedimensional crystals such as IV, III-V and II-VI semiconductors, the lattice matching between two crystals forming heterojunctions is one of the severe conditions for obtaining high quality junctions. Growth of strained layer superlattices is one method to relax the limitation of lattice matching. However, if the thickness of the growing layer exceeds some critical thickness, misfit dislocations appear [l]. Recently, it has been reported that the heteroepitaxy between layered materials is possible without generation of misfit dislocations in spite of the large difference between the lattice constants [2]. The layered materials are described by stackings of thin primitive layers consisting of several atomic planes [3]. The bonding between the primitive layers is by a weak van der Waals force. No dangling bonds exist on the cleaved surface. Therefore, the deposited layer on the substrate grows free from strain due to the lattice mismatch. GaSe and InSe belong to III-VI layered semiconductors, in which a primitive layer consists of 0169.4332/89/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
four atomic planes in the sequence Se-M-M-Se (M = Ga. In). The band gaps of GaSe and InSe are 2.0 and 1.3 eV, respectively. The lattice mismatch between GaSe and InSe is about 7% in the cleaved basal plane. These materials have long been of interest from their electrical and optical properties related to the anisotropy of the crystal structure [4,5]. In the present paper, we report for the first time on the growth mode, crystal quality of the grown layers, interfacial reaction and the valence band discontinuity, at the initial stage of the heteroepitaxy of GaSe on InSe, and InSe on GaSe.
2. Experimental All experiments were carried out in an ultrahigh vacuum chamber with base pressure better than 1 X 10e9 Torr. The vacuum chamber was equipped with a double-pass cylindrical mirror type energy analyzer (CMA) for the measurements of AES, LEELS and XPS. GaSe and InSe single crystal were grown by the Bridgman method from the melts. The substrate of about 10 x 10 x 2 mm3 was cut from the crystal, and mounted on a stain-
540
C. Tatsuyama
et al. / Heteroepltuxy
between Iuyered semkondwtor.r
less-steel sample holder via In metal. The substrate was cleaved before insertion into the chamber. The heating of the substrate was performed by a W filament behind the sample holder. The sample temperature was measured by a Chromel-Alumel thermocouple attached to the sample holder. GaSe and InSe were evaporated from a W-solenoidal filament surrounded by liquid N, shrouds. The thickness of the deposited layer was monitored by a quartz oscillator and also estimated by the intensity of AES signals from the substrate. AES and LEELS were measured at a primary electron beam energy of 3 keV and 100 eV, respectively, and a beam current of 0.3 PA. XPS was measured with unmonochromatized Mg Ka,.z Xrays (1253.6 eV) with a power of 400 W. The CMA was operated in the non-retarding mode for AES, and in the retarding mode for XPS and LEELS. All measurements of AES, LEELS and XPS were performed after the sample was cooled down to room temperature.
3. Results
and discussion
3. I. Growth mode The growth mode of GaSe on InSe, and InSe on GaSe was investigated by measuring the decrease in the AES-signal intensity from the substrate upon the deposition. Figs. la and lb show the decrease in intensity of the In(MNN) AES signal of the InSe substrate at 398 eV as a function of the thickness of the GaSe deposited on the substrate at temperatures of 200 and 400 o C, where (a) is a linear plot and (b) a semi-logarithmic plot. The AES intensity I is normalized to that obtained from the cleaved InSe surface, and the thickness of the deposited GaSe was monitored by a quartz oscillator. In fig. la, for both temperatures, the intensity of the In(MNN) AES signal decreases with a constant slope down to I = 0.42 upon GaSe deposition, followed by a different slope for further deposition. These results clearly indicate that the deposited GaSe grows in a layerby-layer fashion.
GO% ’
on
I
,nse
Gase
01 2d GoLie
on
,nse
ln(MNNlk398e”
In(MNNl-398eV
0’ 0
GuSe und InSe
40
60
Thlckness(!,i (0)
80
0
20
40
GaSe
Thickness
60
80
t,!a )
(b)
Fig. 1. Intensity of the In(MNN) AES signal at 39X cV ah ;I function of the thickness of GaSe deposited on InSe at 200 and 400° C. The thickness of GaSe was measured by a quart7 oscillator. The intensity is normalized to that of cleaved In%: (a) linear plot, (b) semi-logarithmic plot
The thickness where the slope of I changes is different for both substrate temperatures. namely about 24 A for 200 OC and about 42 A for 400” C. However, we need to note here that the thickness of GaSe was indirectly monitored by a quartz oscillator placed far from the substrate. Therefore, the thickness measured by a quartz oscillator does not represent the exact thickness of adsorbed GaSe on the substrate. The sticking probability of GaSe should depend on the substrate temperature. This difference may thus be due to the fact that the sticking probability for deposited GaSe is larger for 200°C than for 400°C. For layer-by-layer growth, the AES intensity I from the substrate is expressed by 1 = exp( - 19cl/h, ) ,
(1)
where d is the unit thickness of the grown layer, H is the number of layers deposited on the substrate. X, is the effective escape depth of the AES electrons passing through the deposited layers. In the present experiment, the electrons which escape from the surface at an angle of $I = 42.3” with respect to the surface normal are detected by the CMA. Thus, X, is expressed as h cos + = 0.74h. where X is the escape depth of the electrons. The value of X is estimated by the following relation
[61: x(nm)
= 538N-
‘,l”E -2 + 0.4,Nmml j?El,”
(2)
where N is the atomic density (nm “) and E the kinetic energy of the electron (398 eV). For GaSe.
541
C. Tatsuyama et al. / Heteroepitaxy between layered semiconductors GaSe and InSe
N = 33.27 nmp3, then A is 12.8 A, which gives x, = 0.45 A. In fig. la, the inflection point of the decreasing slope of the AES signal should correspond to the completion of the growth of one (13= 1) unit layer. By substituting X, = 9.450A and 0 = 1 into eq. (1) d is calculated to be 8.2 A. This value of d is very close to the thickness of a primitive layer of GaSe (7.97 A). Thus, the inflection point in fig. la corresponds to the completion of the growth of one primitive layer. In fig. lb, the behavior of the decrease in intensity of AES again follows the layer growth mode, but it has a different slope for the deposition over one primitive layer. This result indicates that the sticking probability of GaSe on InSe changes after 8 = 1 compared with before 8 = 1. After the completion of the growth of one primitive layer, the growth rate of GaSe increases, then the decreasing slope becomes steeper. If we take a primitive layer as a unit layer for the layer-by-layer growth, the escape depth X of the In(MNN) AES electrons passing through the GaSe layer is estimated to be 12.4 A (X, = 9.2 A). After this estimation, the exact thickness of the GaSe deposited on InSe can be obtained by the AES-signal intensity from the substrate. A similar behaviour of the AES intensity has been observed for the deposition of InSe on GaSe. 3.2. Crystal quality of grown films The crystal quality of the grown films was characterized by LEELS. Fig. 2 shows the evolution of the LEELS spectrum for the deposition of GaSe on InSe at 400” C. The thickness of the GaSe deposited on InSe was estimated by the AES-signal intensity from the InSe substrate, as discussed in the preceding subsection. The lowest spectrum in the figure is that of InSe, and the uppermost spectrum is that of GaSe. The assignment of each peak has already been published before [7], and peaks A, B, C and F are loss peaks due to the excitations between the valence and conduction bands, and peaks G, H and I due to transitions between Ga 3d (In 3d) and the conduction band. Peaks D and E are the surface and bulk plasmon excitations, respectively.
: Electron
Energy Loss(eV)
Fig. 2. Evolution of the LEELS spectrum obtained at a primary electron beam energy of 100 eV as a function of the thickness of GaSe deposited on InSe at 400 ’ C.’
The spectrum of InSe gradually changes by the deposition of GaSe, but that of 3.0 A still shows the nature of InSe, while that of 7.5 A becomes almost the same as that of Case, as is especially clear from the changes of peaks C, D, E and F. The thickness of 7.5 A is about the same as one primitive layer of GaSe. The spectrum after deposition of 17.2 A (19 = 2) is completely replaced by that of GaSe. These results indicate that the grown layers are free from the strain usually induced by the large lattice mismatch for three-dimensional crystals, and the crystal quality of the grown layer of about two primitive layers is as good as the bulk crystal. A similar evolution of the LEELS spectrum has also been observed for the deposition of InSe on GaSe at 300 o C. 3.3. Interfacial
reactivity
The interfacial reactivity has been studied by XPS. Figs. 3a and 3b show the evolution of the
C. Tatsuyamu et al. / Heteroepitaq
842
GaSe
between ku_vered semiconductors GuSe und InSe
on InSe
GoSe
hv=1253.6eV
, 000
I
605 Kinetic Energy
Electron
(a
on InSe
hv=l253.6eV
(eV)
Electron
130 Kinetic
135 Energy(eV1
(b)
)
as a function of the thickness of &Se deposited on In% at Fig. 3. Evolution of XPS spectra of: (a) In3ds,> and (b) Ga2p,,,, 300’ C. The peak height of each individual spectrum is multiplied by a factor to give the same height in all cases.
XPS spectra of In 3d,,, and Ga2p,,,, respectively, upon GaSe deposition on InSe at 300” C. Fig. 4 is the FWHM (full width at half maximum) of the XPS spectra shown in fig. 3. In these figures, the thickness of GaSe was measured by a quartz oscillator. The FWHM of Ga2p,,, is slightly wider than that of the bulk crystal shown by the dotted line in fig. 4, thereby suggesting a somewhat poorer crystal quality of the deposited layer than of the bulk. The FWHM of In3d,,, slightly increases with GaSe deposition, which is
,
I
I
I
GoSe on
InSe
Ts = 300°C
;
1.6 -
01 $
_ cl 1.4 ---
Ga *p3/2 -_
0 ____--__ -_Go 2p3,2 (bulk)
_-.--
0 ---
I I5
I 20
z LL_ I.*%‘” 3%/2 1.0
0
I IO
I 5 Case
Fig. 4. FWHM
Thickness
of XPS spectra
(A)
shown in fig. 3.
due to the inclusion of the component of In metal as shown by the shaded area of the top figure in fig. 3a. This result indicates that GaSe deposited on InSe at 300 o C reacts with the InSe substrate. For the deposition of GaSe on InSe at RT, and of InSe on GaSe at RT and 300 o C, no interfacial reactions have been seen. 3.4. Band discontinuit_y The valence band discontinuity A E, between GaSe and InSe was measured by XPS. A E,. is given by, AE,=(E;-E,Z,)-(E;-E,‘L,)+AEc,,
(3)
A EcL = E,:_ - E,‘,,
(4)
where EJ and Eb? are the top of the valence band of InSe and GaSe, respectively, and E,‘, and EL\ the energy of In 3d 5,2 and Ga 2p,,,, respectively. The values of (Ei - E,‘,) and (E: - E$) were measured separately for each bulk crystal. and found to be 443.96 and 1116.77 eV, respectively. A EcL is the energy difference between Ga2p,,, on InSe and In 3d 5,2 when GaSe is deposited (InSe on GaSe). The positive value of A E, implies that the valence band of GaSe lies above that of
543
C. Tatsuyama et al. / Heteroepitaxy between layered semiconductors GaSe and InSe I
I
1
GoSe on InSe
-0.5
-1
GaSe- InSe T,= 300°C
i
3
o.50Ldij Overlayer
Thickness
(i
1
Fig. 6. Valence band discontinuity for GaSe on InSe, and InSe on GaSe. Substrate temperature for the deposition was 300 o C.
GaSe Thickness CL
)
Fig. 5. Energy positions of the core levels for the deposition of GaSe on InSe at 300 o C, which were obtained from fig. 3.
References InSe by AE,. The peak positions of Ga 2~~,~ and as a function of the thickness of GaSe In3d,,, deposited on InSe at 300 o C are obtained from fig. 3, and are shown in fig. 5. Substituting these results into eq. (3) the valence band discontinuity of GaSe and InSe for the deposition of GaSe on InSe is obtained as shown in fig. 6, where the result for InSe on GaSe is also shown. For both cases, the band offset of the valence band lies within t-O.1 eV. Thus, the conduction band discontinuity is 0.7 f 0.1 eV due to the band gap difference between the two crystals (2.0 eV for GaSe, and 1.3 eV for InSe).
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