Journal ArticleDOI

# The orientation independence of the CdTe-HgTe valence band offset as determined by x-ray photoelectron spectroscopy

01 Dec 1991-Semiconductor Science and Technology (IOP Publishing)-Vol. 6

Abstract: The authors have investigated the valence band offset ( Delta EV) of the CdTe-HgTe heterojunction for three orientations, (100), (110) and (111)B, using in situ X-ray photoelectron spectroscopy. The difference in energy between the Cd 4d and Hg 5d5/2 core levels, Delta ECL, and consequently Delta EV was found to be independent of surface orientation and the surface structure immediately prior to growth of the uppermost layer. Delta EV was found to be 0.37+or-0.07 eV.
Topics: , Heterojunction (51%)

### 1. Introduction

• Self-consistent tight-binding (SCTB) calculations by Munoz et al [6] have predicted a large dependence on orientation, 180 meV between the (100) and (HO) surfaces (AEv = 0.46 and 0.28 eV, respectively).
• In contrast, Van de Walle et al [7] , using self-consistent local density functional methods, predict no surface dependence, i.e. AEv = 0.27 and 0.28 eV, respectively.
• In fact a more general study by the latter authors suggests that this independence is a characteristic of a number of important interfaces, e.g. CdTe•-HgTe, AIAs-GaAs and Si-Ge.
• Indeed this has been shown to be the case for the GaAs-AIAs heterojunction [8] .
• In order to determine which is correct for the CdTe-HgTe heterojunction the authors have investigated the effect of surface orientation as well as the effect of interface structure on the valence band offset.

### 2. Experimental details

• The resulting heterojunctions were transferred via an ultra-high-vacuum transfer system and investigated by XPS under nearly in situ conditions.

### 3. Results and discussion

• First the HgTe and CdTe spectra were co~bined and fitted to the heterojunction spectrum by means of a least-square procedure.
• In order to more accurately determine the position of the peak.
• The results for the (100), ( 110) and (llJ)B orientations are tabulated in table 1. in the first case HgTe was grown on (100)-(2 x 1) and (111)B-(1 x 1) Te-stabilized CdTe surfaces respectively, whereas in the second case HgTe was grown on a mixed surface with half-order reconstruction in both the [OH] and [010] azimuths for the (lOO) orientation and (2v13 x 2j3)R30° reconstruction for (111)B.
• In each case the two corresponding values are the same within the experimental uncertainities given in table 1 .

### 4. Conclusions

• Furthermore, 6.Ev has been determined to be 0.37 ± 0.07 eV, in good agreement with the literature [3] [4] [5] .

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Semicond. Sci. Technol. 6
(1991)
C76-C79. Printed in the UK
The orientation independence of the
CdTe-HgTe
valence band offset as
determined by x-ray
photoelectron
spectroscopy
eR
Becker, Y S Wut, A Waag, M M Kraus and G Landwehr
Physikalisches Insmut
der
Universitat Wurzburg, 0-8700 Wurzburg, Federal
Republic of Germany
Abstract.
We have investigated the valence band offset
(.1.Ev)
of the CdTe-HgTe
heterojunction for three orientations,
(100),
(110)
and (111)8, using
In
situ x-ray
photoelectron spectroscopy. The difference in energy between the
Cd
4d
and
Hg
5d
512
core levels,
.1.E
CL
' and consequently
.1.Ev
was found to
be
independent of
surface orientation and the surface structure
Immediately
prior
to growth
of
the
uppermost layer.
AEv
was found to be 0.37 ±
0.07
eV.
1.
Introduction
The valence band offset (!1Ev) of
an
abrupt
CdTe-HgTe
heterojunction
is
an
important
parameter for the fabrica-
tion of devices as
well
as having a large effect on the
band
structure
of
quantum
wells
and
superlattices (see, for
example, Meyer
et
al
[1
]). !1Ev for the
CdTe-
HgTe inter-
face
has recently been the subject
of
a certain
amount
of
controversy. The
common
anion
rule predicts a small
valence
band
offset which was initially corroborated by a
magneto-optical investigation by
Guldner
et
al
[2],
i.e.
their measurements were consistent with a valence
band
offset
of
40 meV, However, a much larger value
of
350 meV was reported later by two separate x-ray photo-
electron spectroscopy
(xps) investigations
[3,4].
The
valence
band
offset was also found to be independent of
growth sequence
and
the thickness
of
the uppermost
layer from 5 to
35
and
ultraviolet
photoemission spectroscopy
(ups) measurements by
Sporken et
al
[5]
have shown that the valence
band
offset
is
temperature independent
and
therefore the discre-
pancy between these two values can
not
be explained as
the result
of
a temperature dependence. The ensuing
controversy, which has been extensively reviewed by
Meyer
et
al
[1], appears
to
be resolved in favour
of
the
larger value.
Self-consistent tight-binding
(SCTB) calculations by
Munoz
et
al
[6]
have predicted a large dependence on
orientation,
180
meV between the (100)
and
(HO) sur-
faces
(AEv = 0.46
and
0.28 eV, respectively).
In
contrast,
of
of
Science, Beijing. People's Republic
of
China.
0268-1242191/12OC76
+04
03.50 © 1991101' Publishing Lld Van de Walle et al [7], using self-consistent local density functional methods, predict no surface dependence, i.e. AEv = 0.27 and 0.28 eV, respectively. In fact a more general study by the latter authors suggests that this independence is a characteristic of a number of impor- tant interfaces, e.g. CdTe·-HgTe, AIAs-GaAs and Si-Ge. Indeed this has been shown to be the case for the GaAs-AIAs heterojunction [8]. More recent calculations by Muiioz et al [9] resulted in a much smaller depen- dence on orientation, 90 me V between the (100) and (110) surfaces (IlEv = 0.46 and 0.37 eV, respectively). In order to determine which is correct for the CdTe-HgTe heterojunction we have investigated the effect of surface orientation as well as the effect of interface structure on the valence band offset. 2. Experimental details Epitaxial growth was carried out in a four-chamber RIBER 2300, molecular beam epitaxial (MBE) system which has been modified to permit the growth of Hg- based materials. The vacuum in the growth chamber is better than 6 x lO-10 Torr when no Hg has recently been admitted. Three MBE cells were employed, two of which were commercial cells and which contained high- purity CdTe and Te. The third cell, designed by us, is a stainless steel cell for Hg which can be refilled without breaking the vacuum. The flux of the latter cell is stable to within ± 1.5 and ± 3 % over a period of 2 and 30 h, respectively. The growth chamber is connected with the xps chamber (3 x lO-lO Torr) with a transfer system whose vacuum was better than 1 x lO-9 Torr. The CdTe-HgTe heterojunctions were grown on (110) CdTe and on (l00)- and (11l)B-oriented CdTe and CdZnTe substrates which had been degreased, chemo- mechanicaUy polished for several minutes, etched in a weak solution of bromine in methanol and rinsed in methanol. Immediately prior to loading the substrates into the MBE system, they were rinsed in de-ionized water, briefly dipped in hydrochloric acid and then rinsed in de- ionized water so as to remove all of the original oxide and carbon from the substrate surface. We have found that, as a result of this previous step. the newly formed oxide, is much more easily evaporated from the surface. This is accomplished by heating the substrates at temperatures up to about 350 °C while being monitored by reflection high-energy electron diffraction (RHEED) as described elsewhere [10]. Throughout this paper we consistently use the convention of referring to the direction of the incident electrons when referring to reconstruction in a particular azimuth. Approximately 0.1 and 2 1lm of CdTe were grown on CdTe and CdZnTe substrates, respectively. This growth was initiated at 300 and 340 cC, respectively and contin- ued while lowering the temperature to 230°C where the growth was completed. Then a thin layer, 6-40 A, of HgTe was grown at 180°C. For the (100) and (1l1)B orientations. this thin layer of HgTe was on a Te- stabilized surface as well as a surface displaying attri- butes of both Te and Cd stabilization. The former surface structure was established by exposing the CdTe film to a Te flux of 3 x 10 - 7 Torr at 210 0C. The latter surface structure was established by evaporating Te from the surface at about 340°C for several minutes while main- taining a smooth surface as evidenced by the presence of uniform streaks and the absence of spots in the RHEED pattern. In the (100) case this mixture of Te and Cd stabilization of this surface is characterized by half-order reconstruction in the [011] and [010] azimuths. The (1 t t)B surface is more complicated. Here the Te (1 x 1) stabilized surface undergoes a transition to (2j3 x 2j3)R30° reconstruction upon evaporation of Te from HgTe CdTe -----r,--- E~dTe E v _...;v __ -+- __ [CdTe HgTe y, l1E [~dTe ( EH9 Te _ EH9 Te ) Hg5d 5/2 v ~ ( ECdTe_ ECdTe) Cd4d v E~~;~5/2 LL l1ECL v [CdTe Cd4d Figure 1. A schematic diagram of the relevant energy levels 01 an abrupt CdTe-HgTe heterojunction. Valence band offset of CdTe-HgTe by xps the surface. The as-grown surface structure in the (110) orientation could not be changed either with excess Te flux or with an increase in temperature up to 340°C, and therefore HgTe was grown only on the as-grown CdTe surface. The resulting heterojunctions were transferred via an ultra-high-vacuum transfer system and investigated by XPS under nearly in situ conditions. XPS experiments were preformed with a RIBER MAC 2 electron spectrometer using an Mg KO( x-ray source (1253.6 e V) with an acceler- ation voltage of 10 kV, a current of 10 mA and without a monochromator. The energy scans were repeated for at least 12 h in order to achieve an acceptable signal-to- noise ratio. 3. Results and discussion The valence band offset AEv is schematically shown in figure 1 and is given by AEv = (E~:Xd512 - E~gTe) - (E~~Id - E~dTe) + AE CL (1) Therefore, in order to determine AE v , we have to mea- sure these three binding energy differences for HgTe, CdTe and the CdTe-HgTe heterojunction. Where E~mSI2 and E~~Id are binding energies of the Hg 5d sl2 and Cd 4d core levels in HgTe and CdTe, respectively. E~gTc and E~dTe are the energies of the valence band maxima in HgTe and CdTe, respectively, and AEcL is the binding energy difference between the Hg 5d5J2 and Cd 4d core levels in the HgTe-CdTe heterojunction. This procedure results in a value of 0.37 ± 0.07 eV for AEv for the (100) orientation. The large uncertainty is due pri- marily to the difficulty in determining the position of the valence band maximum (see figure 2). As can be seen by comparing the XPS spectra for (100) and (110) CdTe in figure 2, the energy difference between the Cd 4d core level and valence band maximum in CdTe is independent of these two surface orientations. The same is true for 80 60 :l I'd 40 (Jl I- Z :::J 0 20 u 0 12 9 (100l edTe: Full I'n. (t10) CdTe: Dotted l,ne 6 3 BINDING ENERGY (eV) o Figure 2. xps spectra of (100) and (110) CdTe, showing the Cd 4d core level peak and the CdTe valence band maximum. e77 C A Becker at al 60 :J '" 4Q U1 r- Z ::1 0 ....... r.\'J U 0 B 6 ENERGY (eV) Figure 3. An XPS spectrum of a (110) CdTe-HgTe heterojunctlon (full curve) and a least-squares fit of the XPS spectra of a (110) CdTe and (110) HgTe film (dotted curve) in the vicinity of the Cd 4d and Hg 5d core levels. (111)8. This demonstrates that this energy difference is a bulk property and is independent of orientation as is usually assumed. Consequently all orientation or inter- face effects on 6.Ev are contained in 6.E CL which can be determined with greater precision than the position of the valence band. An XPS spectrum for a (110) CdTe-HgTe heterojunc- tion is shown in figure 3 and the corresponding spectra for CdTe and HgTe epitaxial films in the region ofthe Cd 4d and Hg 5d core levels can be ~een in figure 4. 6.E CL can- not be determined directly for the heterojunction due to the overlapping of the Cd 4d, Hg 5d 3 / 2 and Hg 5d core levels. First the HgTe and CdTe spectra were co~bined and fitted to the heterojunction spectrum by means of a least-square procedure. This least-square fit as well as the heterojunction spectrum are plotted in figure 3. In order to more accurately determine the position of the peak 60 ::I III 40 lrl I- Z ::J 0 20 U 0 B 6 ENERGY (eV) Figure 4. xps spectra of (110) CdTe and (110) HgTe films in the vicinity of the Cd 4d and Hg 5d core levels. C78 Table 1. The energy difference between the Cd 4d and Hg 5d s/2 core levels for the (100), (110) and (111) B orientations. Orientation (100) (110) (111 )8 2.96 ± 0.03 3.01 ± 0.03 2.98 ± 0.03 due to the Hg 5d ~/2 core level it was resolved from the HgTe spectrum as depicted by the dotted curve in figure 4. This was accomplished by assuming that this peak is symmetricaL i.e. that the high-energy flank is a mirror image of the low-energy flank. This should be a good assumption because only minimal inelastic scattering. is expected under the low-energy flank. 6.E CL was then determined by measuring the energy difference between the peak centres at half maximum of the Cd 4d and Hg 5d s / 2 core levels. The results for the (100), (110) and (llJ)B orienta- tions are tabulated in table 1. Two different interface structures were investigated for the (100) and (lll)B orientations. in the first case HgTe was grown on (100)-(2 x 1) and (111)B-(1 x 1) Te-stabilized CdTe sur- faces respectively, whereas in the second case HgTe was grown on a mixed surface with half-order reconstruction in both the [OH] and [010] azimuths for the (lOO) orientation and (2v13 x 2j3)R30° reconstruction for (111)B. In each case the two corresponding values are the same within the experimental uncertainities given in table 1. As can be seen in table 1, 6.E CL and therefore tlEv for the CdTe-HgTe heterojunction is, within experimen- tal uncertainty (±0.03 eV), independent of orientation and interface structure. 4. Conclusions In conclusion, by means of in situ XPS experiments we have shown that 6.E CL (and therefore that tlEv) for the CdTe- HgTe heterojunction is independent of the surface orientation and the surface reconstruction immediately prior to the growth of HgTe, whether Te-stabilized or a mixture of Cd and Te stabilization. These results agree with the self-consistent local density predictions of Van de Wane et at [7] but not with the SCTB calculations of Muiioz et 01 [6, 9]. Furthermore, 6.Ev has been deter- mined to be 0.37 ± 0.07 eV, in good agreement with the literature [3-5]. Acknowledgments This project was supported by the Bundesministerium fUr Forschung und Technlogie and the Deutsche F orsch ungsgemeinschaft. References [1] Meyer J R, Hoffman C A and Bartoli F J 1990 Semieond. Sei. Technol. 5 S90 [2] Guldner Y, Bastard G, Vieren J P, Voos M, Faurie J P and Million A 1983 Phys. Rev. Left. 51 907 [3] Kowalczyk S P, Cheung ] T, Kraut E A and Grant R W 1986 Phys. Rev. Left. 56 1605 [4] Faurie J P, Hsu C and Due T M 1987 J. Vae. Sci. Technol. A 5 3074 [5J Sporken R, Sivananthan S, Faurie J P, Ehlers D H. Fraxedas J, Ley L. Pireaux J J and Caudano R 1989 Valence band offset of CdTe-HgTe by XPS J. Vac. Sci. Technol. A 7 427 [6J Muiioz A, Sanchez-Dehesa J and Flores F 1987 Phys. Rev. B 35 6468 [7] Van de Walle C G and Martin R M 1988 Phys Rev. B 374801 [8] Hirakawa K, Hashimoto Y and Ikoma T 1990 Appl. Phys. Lett. 57 2555 (9] Munoz A, Sanchez-Dehesa J and Flores F 1988 Phys. Rev. B 37 4803 [10] Wu Y S, Becker C R. Waag A, Bieknell-Tassius R N and Landwehr G 1991 J. Appl. 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150 citations

Journal ArticleDOI
Yves Guldner1, Gérald Bastard1, J. P. Vieren1, M. Voos1  +2 moreInstitutions (1)
Abstract: Far-infrared magnetoabsorption experiments done in a HgTe-CdTe superlattice are presented. From the results, which are interpreted in terms of interband transitions from valence to conduction subbands, the superlattice band structure has been deduced. These investigations show, in particular, that this superlattice is a quasi zero-energy-gap semiconductor, and yield the first determination of the offset between the HgTe and CdTe valence bands.

140 citations

Journal ArticleDOI
Abstract: We report the results of a detailed investigation on the Te‐stabilized (2×1) and the Cd‐stabilized c(2×2) surfaces of (100) CdTe substrates. The investigation demonstrates for the first time that both laser illumination and, to a greater extent, high‐energy electron irradiation increase the Te desorption and reduce the Cd desorption from (100) CdTe surfaces. Thus it is possible by choosing the proper growth temperature and photon or electron fluxes to change the surface reconstruction from the normally Te‐stabilized to a Cd‐stabilized phase.

34 citations

Journal ArticleDOI
Abstract: We have studied the temperature dependence of the CdTe–HgTe valence‐band discontinuity with x‐ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The samples have been cooled at 110 K for the XPS experiments and at 50 K for UPS. At room temperature, we measure a valence‐band discontinuity of 0.35±0.05 eV, in agreement with previous photoemission results. The valence‐band discontinuity is found to change by only a few millivolts between room temperature and 50 K, with an estimated uncertainty of ∼60 meV.

25 citations

Journal ArticleDOI
15 Apr 1987-Physical Review B

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