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HgTe multilayers
Journal of Crystal Growth 68 (1984) 262 -270 North-Holland. Amsterdam
262
RBS AND CHANNELLING CHARACTERISATION OF INTERDIFFUSED MOVPE CdTe/HgTe MULTILAYERS J.A. GRIMSHAW Physics Department, Royal Military College of Science, Shris:enhans, Swindon, Wilts. SNO 8Ll. UK
A variety of growth-related compositional and structural effects have been observed on interdiffused multilayer-grown CMT. The uniformity of composition to a depth of > 1 ~.sm from the surface has been studied using RBS for epitaxial CdTe/l-lgTe multilayers with different individual layer thicknesses grown on CdTe substrates. Owing to partial interdiffusion, the thickness of the outer layers has often been estimated from the measurements. Where there is sufficient uniformity in the near-surface region the parameter .s in Cd~Hg 1.~~Te can be accurately estimated. In one sample a backscatter yield profile due to the in-diffusion of 1-1g. arising from the exposure of the sample to Hg vapour at the end of the growth process, was clearly identified. A trend towards near-surface compositional uniformity for thinner individual layer thicknesses was observed. Channelling measurements on interdiffused multilayers in the near-normal major crystallographic directions have shown qualitatively the degree of lattice order to vary considerably between samples. In one case a change in disorder across interfaces between layers was observed. The influence of changes induced by the analysing beam itself on the channelling was detected and avoided in the measurements. A search for oxygen at substrate---epilayer interfaces, and for hydrogen in the bulk, of both multilayers and photosensitised grown HgTe on CdTe. have shown the oxygen content to be
1. Infroduction The near-surface composition, uniformity and structural order of CMT-related epilayers has been studied by RBS and channelling. The materials investigated consisted mainly of interdiffused multilayers of CdTe and HgTe and related structures grown by MOVPE. The techniques for growth of these thermal-imager associated materials, which were supplied by RSRE Malvern, are described elsewhere in this issue [1,2]. RBS and channelling has been used by other investigators for the analysis of thermal processing of laser deposited CMT films [3] and for ion implanted CMT [4,5]. The results of the present work show up a variety of growth-related, interdiffusion near-surface effects. They serve also to illustrate the power and relevance of this type of ion beam analysis when applied to as-grown material.
2. Experimental methods RBS and channelling measurements employed 2.0 MeV 4He ions from the RMCS Van de Graaff
generator, using a conventional hackscattering setup. 2The beamsample impinged on an of about 0.5 of the surfaces andarea a silicon surface mm barrier detector, placed usually at a backscattering angle of 160°,was used for analysis. All channelling results were obtained using well-collimated beams accurately aligned to the near-normal major crystallographic direction of the samples. Repeat channelling spectra obtained on HgTe/CdTe interdiffused multilayer process (IMP) samples showed that the channelling incasurements were just susceptible to 4He beam damage at the fluences required, corresponding to 1016 4He ions/cm2 at 2 MeV. Consequently samples were adjusted into channelling alignment, prior to measurements, using a proton beam since it introduced much less damage. Random RBS spectra were gathered after completion of channelling measurements, and used continuous sample tilting some 7° around the channelling direction to achieve a good approximation to randomness. The thickness of the multilayer regions of all IMP samples was at least 6 jim. Some of these thicknesses were accurately checked by RBS using
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J.A. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/HgTe multi/ayers
proton beams because of their greater probing depths compared with He ions. For analysis of oxygen content a deuteron beam was used along with measurement of the yield of the 160(d,p)’70 reaction, following the method of Amsel [6]. In searching for oxygen at the interface
3. Results and discussion Fig. 1 shows the high energy portion of a typical random RBS spectrum obtained for 2 MeV 4He ions backscattered off ingot grown CMT. If incident particles of energy E 0 are backscattered at an angle 0 from atoms in the surface of the sample then, as denoted in fig. 1, the associated positions on the spectrum for the constituent atoms at the surface can be calculated, since the backscattered energy E is given by:
between grown layers and substrate, the energy of the deuteron beam was adjusted so that, having penetrated the epilayers, it had slowed down to 860 keV at the interface. Quantitative measurements were then possible for oxygen, using the associated reaction cross-section value of 4.5 mbarn for a detector angle of 150° [6]. The beam energy adjustment was accurately achieved with the aid of calculations on deuteron backseattering spectra obtained on the samples. A search for hydrogen incorporated in IMP grown samples employed a method of elastic recoil analysis based on the work of Doyle and Peercy 4He ions. [7] and used a beam of
E ~
=
k=
(
m 2{ m + M)
~+
3.1. Calculation of x in Cd,~Hg,
2
—sin
~J} ~ 1/2
—
Te
—
-
—
—
—
—
—
—
—
—
—
Charir~t Number _________
2
At energies below any backscatter edge a continous spectrum arises due to scattering of the
—
0 168
2
Te
[d
2000
M m
(1) where m is the mass of scattered particle and M is the mass of the struck nucleus.
8000—
~4000
263
ltnerayl
_________________
1000
Fig. 1. High energy region of random 2 MeV 4He RBS spectrum from Hg 1 ~Cd,~Te backscatter edges due to the constituents at the surface are indicated.
with x nominally 0.21. The positions of
264
iA. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/ Fig Ti’ osultilas’er.s
beam from atoms in progressively deeper layers of the material. The Hg step in fig. I shows a nearlyflat region allowing calculation of x for uniform CMT at a depth of 500—700 A. Measurement of the height of the second step, for the same depth, above that of the extrapolated first step, as shown, relies on finding incident particle and energy conditions such that the extrapolation will produce minimal error. Corresponding to an energy increment ~E of any step in the spectrum is an increment of depth ~x in the sample, such that to a very good approximation for small depths, and for incident beam normal to the sample surface, =
~x (k1
+
2/~cos0~),
(2)
A comparative measurement was made using a sample of freshly cleaved single crystal Bi 2Te5. a material with a remarkably similar atomic mass distribution to CMT with x = 0.20. The bismuth content was measured as 39.3%. 3.2. Interdiffused multi/avers grow/i by MO VPE Many of the results obtained on these samples would benefit from analysis by RBS computer simulation. However, pending such analysis it is enlightening to discuss the results qualitatively. Each of the samples investigated possessed a large number of HgTe and CdTe layers grown on a CdTe substrate. The sample which gave rise to the random
where k is given by eq. (1) and ~ and c2 are the average energy loss rates of the scattered particle on its respective inward and outward paths in the sample. There is a scarcity of stopping power data for CMT. However, from tables of semi-empirical data [8—lOl one can predict the energy variation of stopping power for light ions in the material. For
backscatter spectrum of fig. 2 had alternate layers of 700 A thick HgTe and 2000 A thick CdTe grown 2° off (100) orientation, the final layer being CdTe [Ill. Inset in the figure is shown schematically the components of yield near to the backscatter edge from the three constituents for the final three layers. assuming no interdiffusion. The chain curve refers to the yield from the fe. assumed to be a uniform constituent throughout.
our conditions the effect on eq. (2) of the different k values is opposed by the effect of the different . values. So for same ~x the measured ratio of Hg step height to that of the other constituents should be increased by 2%. For results with similar features to those of fig. 1 the required ratio. NHC/NR. where NHg is the density of mercury atoms and NR the density of the remaining constituents, has been calculated from the above-corrected ratio of step heights, Hug/HR using
The height of the solid curve above the chain curve represents the additional yield due to the Cd, and has a dip due to the separation of the two (‘dTe layers by the HgTe layer. The additional yield due to the Hg is represented by the dashed curve. The LH edge of the Hg peak and the LH side of the dip would be due to adjacent layers of Hg and Cd atoms, their different energies being given by eq.
Hug/HR
=
(NHg/NR)(aFIg/aR).
(3)
where aH5/OR is the ratio of the Rutherford scattering cross sections for mercury and remaining constituent nuclei, The results of calculations lead to (1 — x) for the mercury, but poor near-surface uniformity restricted them to only a few epilayer samples. As an example of the capability of the method, for ingot-grown samples with an expected Hg content of 39.5%, the RBS measured value averaged 39.4 ±0.75%, i.e. x = 0.212 for an anticipated x of 0.21,
(1). The measured yield would then be due to the sum of the three yield components. Yield from the deeper layers is of course not shown. Referring to the actual measured yield in fig. 2 we see only the vestiges of a dip and Hg peak. It is clear that interdiffusion had occurred not only for deep layers but also between the HgTe and the CdTe layers near the surface to give a material approaching a uniform composition. Because interdiffusion must be progressive as the layers were grown, at the end of the process we see the remaining Hg-rich region has given rise to a small peak positioned towards the RH edge of the Hg peak shown in the inset. Also the migration of the Hg into the top CdTe layer is detected as a bulge on
iA. Grimshaw
I ~
/
Characterisation of interdiffused MOVPE CdTe/HgTe multi/ayers
265
15000—
—
—
10000-
-
—
5000—
—
~
CHANNEL NUMBER
,
—
IENfRGY)
Fig. 2. Random RBS spectrum obtained using a 2.0 MeV anticipated yield components shown inset.
256
512
4He beam incident on a CdTe/HgTe multilayer sample, with form of
the RH side of the main backscatter edge, where it
The dip in yield in fig. 3 arises because of the
will be seen that the concentration of Hg falls off over a depth corresponding to the final 800 A of CdTe to a low value at the surface.
second interface beneath the surface, as discussed for fig. 2. The width of the peak measured from the bottom of the dip to the average of the back-
We next discuss the effects for a sample of the
scatter edge position for Cd and Hg gives, using
—
same orientation with thicker grown layers.
eq. (2) and stopping power data [8—10],the corn-
In fig. 3 the specified thicknesses of the alternate epilayers of the sample are shown in the inset,
bined thickness of the outer two layers as approximately 6000 A.
It is clear from the random backscatter spectrum
The (100) channelled spectrum corresponding
in the figure that the interdiffusion of the surface layers was far from complete, but that deeper layers had substantially interdiffused. Thus a thick
to fig. 3 is shown in fig. 4 (50% more integrated beam current used). The onset of components of yield for the three consituents at the surface were
Hg-rich region remained from the last HgTe layer,
resolved. But there is a further inflexion in the
giving rise to the prominent wide peak in the spectrum. However, the step at the front edge of the spectrum shows that a large concentration of
yield which is interpreted as relating to the position of the first interface. The depth of the first interface below the surface is then calculated as
Hg had diffused through the outer CdTe layer and
2000 ±300 A which means the width of the Hg-rich
it is calculated (using the method described in section 2) that material in the near-surface region was composed of approximately 27% Hg.
layer was approximately 4000 A, in reasonable
agreement with the specified value. Channelling in this sample was undoubtedly
266
J.A. Grimshaw
-
1
7500
15000
-
/
CharacterisaUon of interdijfused MO VPE CdT
5’ / Fig! s’ snultilavers
N
-__
—
—
\J/~Z~~=~\\
-
2500H
~~OMeV
2,,um [die 0’2 0
-
-
OL+S,ssrn HgTe
I
[die CHANNEL NUMBER
256
(ENERGY)
512
Fig. 3. Random RBS spectrum obtained using a 2.0 MeV 4He beam incident on a CdTe/HgTe individual grown layers, shown inset.
IMP grown sample, with thick.
NI
--
7500
ii 0
~F[~
~g
_I
CHANNEL
NUMBER
256
IENERg]1
512
Fig. 4. Channelled RBS spectrum obtained using a 2.0 MeV 4He beam incident along the near-normal <100> of a ttiick-Ia’ver CdTe/HgTe IMP grown sample.
J.A, Grimshaw
/
Characterisation of interdiffused MO VPE CdTe/HgTe multi/ayers
poor (fig. 4). Furthermore, the channelling angle
267
was narrower than for other samples. This suggests
The spectra of fig. 5 were,obtained for a <100) oriented sample with 700 A thick HgTe layers
the atomic rows and planes were distorted due to localised strain. Also disorder varied with depth:
alternating with 1400 A thick CdTe, with a final layer of CdTe [11]. Both the <100) channelled
near the front of the sample the corresponding Hg yield suggests a high value of Xmin of ~— 0.2, due to locahised strain and possibly also interstitial Hg atoms near the surface; at the first interface, because there is no feature in the random spectrum in fig. 3 corresponding to the inflexion in fig. 4, disorder must have increased at the interface and beyond into the HgTe layer; regarding the second interface, the bottom of the dip in fig. 4 was shifted to the right compared with its position in fig. 3, which implies a different dechannelling rate in the epitaxial CdTe underlayer than in the HgTe
(lower curve) and random RBS spectra are shown,
layer, and hence different disorder on the CdTe
positions due to atoms misplaced from lattice sites
side of the interface. Thus there was a change in disorder with depth across both interfaces, We next discuss effects in samples with thinner grown layers.
in surface oxide. But in the random spectrum there is no sign of a Hg-rich region. There is, however, an abrupt change in slope at position marked A, and it may be inferred that the fall-off in back-
7600~
I
the horizontal axis including only the upper half of the spectra as compared with figs. 2 and 3. The beam integrated current used in obtaining the lower curve was double that for the random curve, having the effect of magnifying the vertical scale by two for the channelled spectrum. The positions for the onset of backscatter yield associated with surface atoms of Hg, Te and Cd are clearly discernable in both spectra, and are marked on the
figure. In fact, in the channelled spectrum all three constituents gave rise to separate peaks at these
I
-
A
CdTe
Hg
5000
—
—
2500
—
—
0 256
CHANNEL NUMBER
400
(ENERGY)
—i”
512
Fig. 5. High energy region of random and <100) channnelled (lower curve) RBS spectra obtained using a 2.0 MeV on a IMP grown sample.
4He beam incident
268
iA. Grimshaw
/
Characterisatton of interdijfused MO VPE C~d7’i’/iig]/inu/tilacers
scatter yield to the right of A is caused by a reducing Hg concentration in the material as the surface is approached (Hg nuclei having a higher scattering cross section than Cd nuclei). The spectrum may then be interpreted as due to a sample with a much thicker CdTe top layer than that for figs. 2 and 3, allowing a more prolonged interdiffusion of the HgTe and CdTe layers. If position A then refers to the point when the growth of the HgTe layer was stopped, the spectrum width from position A to the Hg surface peak gives us the thickness of the outer CdTe layer as approximately 3500 A, using stopping power data. Over the final — 800 A of the CdTe outer layer the Hg yield component can be seen forward of the main backscatter edge in the random spectrum. The concentration gradient of the Hg here is clearly the opposite to that expected if Hg was migrating from a source in the interior. This remarkable result can be understood because at the end of the growth process the sample was maintamed at 410°C in contact with Hg vapour for 10 mm and then cooled down [1]. It is clear that the Hg concentration profile observed near the surface in fig. 5 resulted mainly from the inward migration of Hg from the surface.
If we now look at the channelled yield in fig. 5 for the same near-surface Fig component, we find it to he about 15% of the corresponding random Hg yield peak. Thus the proportion of Hg atoms in interstitial sites exposed to the (100) channelled beam at this point was no more than 15%. Furthermore, the proportion remained low to a depth of — 800 A at least, as can he seen from the variation of the channelled yield, and remembering the beam will have an increasing dechannelled component with depth. These results suggest the Hg atoms which migrated in from the surface were on substitutional sites, although further work is required to verify this. Analysis of other samples in the same manner gave similar numbers, where the near-surface Hg distribution was dominated by migration from the interior. Much better channelling is to be seen in fig. 6 which was obtained on another thin-layer IMP sample with individual CdTe layers grown as 1000 A and the HgTe layers as 900 A on a (Il 1) oriented substrate, the final layer being HgTe [Ifl. Apart from structure in the curves associated with the near-surface region, over most of their length the channelled and random curves run smoothly and nearly parallel, indicating both a material of
12 000
Hg
9000
—
6000
-
3000
-
N
N
-‘-~~.
-
—
NN
~-
RANDOM -
~
—
0 60 HEI ILIM
ION
250 BACKSIATTER
508 FNERNY
CHANNEl SI
Fig. 6. Random and <11)> channelled RBS spectra obtained using a 2.0 MeV
4He beam incident on a thin-layer IMP grown sample.
iA. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/HgTe multi/ayers
269
\\
8000
CdTe
Hg
6000
Random C C —
4000
—
—
c <111>
S
a
0
2000
-
—
100
250 HELIUM
ION
BACKSCATTER
512
,
ENERGY
(CHANNELSI
Fig. 7. Random and <111) channelled RBS spectra obtained using a 2.0 MeV
uniform composition and a well-ordered crystal lattice inwards from a depth of ‘— 0.5 jim to over 2 jim. Fig. 6 shows fairly uniform Hg content for 800 A depth from the surface, but again the structure in the random curve is associated with incomplete interdiffusion between the last three layers put down. The major dip is associated with the interface between the final HgTe layer and CdTe beneath it. In the channelled curve, peaks due to matrix atoms in surface oxide are again observed. The sample relating to fig. 7 possessed an even thinner layer structure, was grown on a <111) substrate and had individual layer thicknesses grown as 400 A for CdTe and 1300 A for HgTe with a final layer of HgTe. The random curve of fig. 7 shows compositional uniformity extended from deep in the material to within ‘- 1500 A of the surface, following which there were regions only slightly rich and then slightly depleted of Hg. But the channelled curve shows the degree of order was not as good overall as that for the sample related to fig. 6.
41-k beam incident on a thin-layer IMP grown sample.
3.3. Oxygen and hydrogen content In using nuclear reaction analysis for detection of oxygen (section 2), although reaction yield peaks were readily observed for oxygen residing on sample surfaces, no oxygen was detected in the vicinity of the epilayer—substrate interfaces both for IMP and HgTe/CdTe photolysis samples. The detection limit in these measurements was 1 monolayer of oxygen in the interface region. A search was made for hydrogen (section 2) in the bulk of both IMP and photolysis-grown samples. It was deduced that the concentration of hydrogen must be below the 2 at% limit of detectabihity over the depth probed of about 1/4 jim.
4. Conclusions As regards IMP grown layers, the results described, and others we have made, suggest strongly that good compositional uniformity near the surface requires small individual layer thicknesses.
270
iA. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/J-igTe niult,laver.s
However, whether this rule could be relaxed for compositional uniformity at depths greater than the final three layers is not completely certain at present. In a poorly interdiffused region, increased disorder across interfaces has been observed. As regards achieving good lattice order at larger depths, no clear trend emerges favouring thin mdividual layers over the range of thickness studies. But there is some evidence of greater disorder in interdiffused layers with a higher proportion of Hg. Channelling measurements on IMP grown material could just detect beam-induced changes 4He ions/cm2 at 2 MeV, by 1016 Oxygen content in the substrate-epilayer interface region must be less than 1 monolayer in total both for IMP grown and photosensitised grown HgTe on CdTe. Hydrogen content was below 2 at% in IMP and photosensitised grown samples.
For
sufficiently
uniform
material, x
in
Cd~Hg 1 ~Te can be measured accurately by RBS in the near-surface region.
epilayer samples and for many helpful and stinlulating discussions. The author is indebted to Professor W.G. Townsend for his support and keen interest in the work. He is also most grateful to D. Diskett for carrying out much ancillary work and to E.E. Barratt, G. Partridge and T. Day for their work concerning accelerator operations.
References Ill
J. Tunnicliffe, S.J.C. Irvine. O.D. Dosser and LB. Mullin. J. Crystal Growth 68 (1984) 245.
[2] S.J.C. Irvine, J.B. Mullin and J. Tunnicliffe. J. Crystal Growth 68 (1984) 188. [3] K L Conway. J F Gibbons and T W Sigmon. J Vacuum Sci. Technol. 21(1982) 212.
141 15]
G. Bahir and R. Kalish. J. AppI. Phys 54 (1983) 3129.
G.L. Destafanis, NucI. Instr. Methods 209/210 (1983 567. [6] 0. Amsel and D. Samuel, Anal. Chem. 39 (1967) 1689. [7] B.L. Doyle and P.S. Peercy. AppI. Phys Letters 34(1979) 811. [8] i.E. Ziegler, Ed.. The Stopping and Ranges of Ions in Matter, Vol. 4. Helium (Pergamon, 1977).
Acknowledgements
Warm thanks are extended to J.B. Mullin. S.J.C. Irvine and J. Tunnicliffe of RSRE for supply of
[9] CF. Williamson, J.-P. Boujot and J. Picard. Report CEAR-3042 (1966). 110] J.W. Mayer and E. Rimini, Eds., Ion Beam Handbook for Material Analysis (Academic Press, 1977). [11] J. Tunnicliffe, personal communication.
262
RBS AND CHANNELLING CHARACTERISATION OF INTERDIFFUSED MOVPE CdTe/HgTe MULTILAYERS J.A. GRIMSHAW Physics Department, Royal Military College of Science, Shris:enhans, Swindon, Wilts. SNO 8Ll. UK
A variety of growth-related compositional and structural effects have been observed on interdiffused multilayer-grown CMT. The uniformity of composition to a depth of > 1 ~.sm from the surface has been studied using RBS for epitaxial CdTe/l-lgTe multilayers with different individual layer thicknesses grown on CdTe substrates. Owing to partial interdiffusion, the thickness of the outer layers has often been estimated from the measurements. Where there is sufficient uniformity in the near-surface region the parameter .s in Cd~Hg 1.~~Te can be accurately estimated. In one sample a backscatter yield profile due to the in-diffusion of 1-1g. arising from the exposure of the sample to Hg vapour at the end of the growth process, was clearly identified. A trend towards near-surface compositional uniformity for thinner individual layer thicknesses was observed. Channelling measurements on interdiffused multilayers in the near-normal major crystallographic directions have shown qualitatively the degree of lattice order to vary considerably between samples. In one case a change in disorder across interfaces between layers was observed. The influence of changes induced by the analysing beam itself on the channelling was detected and avoided in the measurements. A search for oxygen at substrate---epilayer interfaces, and for hydrogen in the bulk, of both multilayers and photosensitised grown HgTe on CdTe. have shown the oxygen content to be
1. Infroduction The near-surface composition, uniformity and structural order of CMT-related epilayers has been studied by RBS and channelling. The materials investigated consisted mainly of interdiffused multilayers of CdTe and HgTe and related structures grown by MOVPE. The techniques for growth of these thermal-imager associated materials, which were supplied by RSRE Malvern, are described elsewhere in this issue [1,2]. RBS and channelling has been used by other investigators for the analysis of thermal processing of laser deposited CMT films [3] and for ion implanted CMT [4,5]. The results of the present work show up a variety of growth-related, interdiffusion near-surface effects. They serve also to illustrate the power and relevance of this type of ion beam analysis when applied to as-grown material.
2. Experimental methods RBS and channelling measurements employed 2.0 MeV 4He ions from the RMCS Van de Graaff
generator, using a conventional hackscattering setup. 2The beamsample impinged on an of about 0.5 of the surfaces andarea a silicon surface mm barrier detector, placed usually at a backscattering angle of 160°,was used for analysis. All channelling results were obtained using well-collimated beams accurately aligned to the near-normal major crystallographic direction of the samples. Repeat channelling spectra obtained on HgTe/CdTe interdiffused multilayer process (IMP) samples showed that the channelling incasurements were just susceptible to 4He beam damage at the fluences required, corresponding to 1016 4He ions/cm2 at 2 MeV. Consequently samples were adjusted into channelling alignment, prior to measurements, using a proton beam since it introduced much less damage. Random RBS spectra were gathered after completion of channelling measurements, and used continuous sample tilting some 7° around the channelling direction to achieve a good approximation to randomness. The thickness of the multilayer regions of all IMP samples was at least 6 jim. Some of these thicknesses were accurately checked by RBS using
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J.A. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/HgTe multi/ayers
proton beams because of their greater probing depths compared with He ions. For analysis of oxygen content a deuteron beam was used along with measurement of the yield of the 160(d,p)’70 reaction, following the method of Amsel [6]. In searching for oxygen at the interface
3. Results and discussion Fig. 1 shows the high energy portion of a typical random RBS spectrum obtained for 2 MeV 4He ions backscattered off ingot grown CMT. If incident particles of energy E 0 are backscattered at an angle 0 from atoms in the surface of the sample then, as denoted in fig. 1, the associated positions on the spectrum for the constituent atoms at the surface can be calculated, since the backscattered energy E is given by:
between grown layers and substrate, the energy of the deuteron beam was adjusted so that, having penetrated the epilayers, it had slowed down to 860 keV at the interface. Quantitative measurements were then possible for oxygen, using the associated reaction cross-section value of 4.5 mbarn for a detector angle of 150° [6]. The beam energy adjustment was accurately achieved with the aid of calculations on deuteron backseattering spectra obtained on the samples. A search for hydrogen incorporated in IMP grown samples employed a method of elastic recoil analysis based on the work of Doyle and Peercy 4He ions. [7] and used a beam of
E ~
=
k=
(
m 2{ m + M)
~+
3.1. Calculation of x in Cd,~Hg,
2
—sin
~J} ~ 1/2
—
Te
—
-
—
—
—
—
—
—
—
—
—
Charir~t Number _________
2
At energies below any backscatter edge a continous spectrum arises due to scattering of the
—
0 168
2
Te
[d
2000
M m
(1) where m is the mass of scattered particle and M is the mass of the struck nucleus.
8000—
~4000
263
ltnerayl
_________________
1000
Fig. 1. High energy region of random 2 MeV 4He RBS spectrum from Hg 1 ~Cd,~Te backscatter edges due to the constituents at the surface are indicated.
with x nominally 0.21. The positions of
264
iA. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/ Fig Ti’ osultilas’er.s
beam from atoms in progressively deeper layers of the material. The Hg step in fig. I shows a nearlyflat region allowing calculation of x for uniform CMT at a depth of 500—700 A. Measurement of the height of the second step, for the same depth, above that of the extrapolated first step, as shown, relies on finding incident particle and energy conditions such that the extrapolation will produce minimal error. Corresponding to an energy increment ~E of any step in the spectrum is an increment of depth ~x in the sample, such that to a very good approximation for small depths, and for incident beam normal to the sample surface, =
~x (k1
+
2/~cos0~),
(2)
A comparative measurement was made using a sample of freshly cleaved single crystal Bi 2Te5. a material with a remarkably similar atomic mass distribution to CMT with x = 0.20. The bismuth content was measured as 39.3%. 3.2. Interdiffused multi/avers grow/i by MO VPE Many of the results obtained on these samples would benefit from analysis by RBS computer simulation. However, pending such analysis it is enlightening to discuss the results qualitatively. Each of the samples investigated possessed a large number of HgTe and CdTe layers grown on a CdTe substrate. The sample which gave rise to the random
where k is given by eq. (1) and ~ and c2 are the average energy loss rates of the scattered particle on its respective inward and outward paths in the sample. There is a scarcity of stopping power data for CMT. However, from tables of semi-empirical data [8—lOl one can predict the energy variation of stopping power for light ions in the material. For
backscatter spectrum of fig. 2 had alternate layers of 700 A thick HgTe and 2000 A thick CdTe grown 2° off (100) orientation, the final layer being CdTe [Ill. Inset in the figure is shown schematically the components of yield near to the backscatter edge from the three constituents for the final three layers. assuming no interdiffusion. The chain curve refers to the yield from the fe. assumed to be a uniform constituent throughout.
our conditions the effect on eq. (2) of the different k values is opposed by the effect of the different . values. So for same ~x the measured ratio of Hg step height to that of the other constituents should be increased by 2%. For results with similar features to those of fig. 1 the required ratio. NHC/NR. where NHg is the density of mercury atoms and NR the density of the remaining constituents, has been calculated from the above-corrected ratio of step heights, Hug/HR using
The height of the solid curve above the chain curve represents the additional yield due to the Cd, and has a dip due to the separation of the two (‘dTe layers by the HgTe layer. The additional yield due to the Hg is represented by the dashed curve. The LH edge of the Hg peak and the LH side of the dip would be due to adjacent layers of Hg and Cd atoms, their different energies being given by eq.
Hug/HR
=
(NHg/NR)(aFIg/aR).
(3)
where aH5/OR is the ratio of the Rutherford scattering cross sections for mercury and remaining constituent nuclei, The results of calculations lead to (1 — x) for the mercury, but poor near-surface uniformity restricted them to only a few epilayer samples. As an example of the capability of the method, for ingot-grown samples with an expected Hg content of 39.5%, the RBS measured value averaged 39.4 ±0.75%, i.e. x = 0.212 for an anticipated x of 0.21,
(1). The measured yield would then be due to the sum of the three yield components. Yield from the deeper layers is of course not shown. Referring to the actual measured yield in fig. 2 we see only the vestiges of a dip and Hg peak. It is clear that interdiffusion had occurred not only for deep layers but also between the HgTe and the CdTe layers near the surface to give a material approaching a uniform composition. Because interdiffusion must be progressive as the layers were grown, at the end of the process we see the remaining Hg-rich region has given rise to a small peak positioned towards the RH edge of the Hg peak shown in the inset. Also the migration of the Hg into the top CdTe layer is detected as a bulge on
iA. Grimshaw
I ~
/
Characterisation of interdiffused MOVPE CdTe/HgTe multi/ayers
265
15000—
—
—
10000-
-
—
5000—
—
~
CHANNEL NUMBER
,
—
IENfRGY)
Fig. 2. Random RBS spectrum obtained using a 2.0 MeV anticipated yield components shown inset.
256
512
4He beam incident on a CdTe/HgTe multilayer sample, with form of
the RH side of the main backscatter edge, where it
The dip in yield in fig. 3 arises because of the
will be seen that the concentration of Hg falls off over a depth corresponding to the final 800 A of CdTe to a low value at the surface.
second interface beneath the surface, as discussed for fig. 2. The width of the peak measured from the bottom of the dip to the average of the back-
We next discuss the effects for a sample of the
scatter edge position for Cd and Hg gives, using
—
same orientation with thicker grown layers.
eq. (2) and stopping power data [8—10],the corn-
In fig. 3 the specified thicknesses of the alternate epilayers of the sample are shown in the inset,
bined thickness of the outer two layers as approximately 6000 A.
It is clear from the random backscatter spectrum
The (100) channelled spectrum corresponding
in the figure that the interdiffusion of the surface layers was far from complete, but that deeper layers had substantially interdiffused. Thus a thick
to fig. 3 is shown in fig. 4 (50% more integrated beam current used). The onset of components of yield for the three consituents at the surface were
Hg-rich region remained from the last HgTe layer,
resolved. But there is a further inflexion in the
giving rise to the prominent wide peak in the spectrum. However, the step at the front edge of the spectrum shows that a large concentration of
yield which is interpreted as relating to the position of the first interface. The depth of the first interface below the surface is then calculated as
Hg had diffused through the outer CdTe layer and
2000 ±300 A which means the width of the Hg-rich
it is calculated (using the method described in section 2) that material in the near-surface region was composed of approximately 27% Hg.
layer was approximately 4000 A, in reasonable
agreement with the specified value. Channelling in this sample was undoubtedly
266
J.A. Grimshaw
-
1
7500
15000
-
/
CharacterisaUon of interdijfused MO VPE CdT
5’ / Fig! s’ snultilavers
N
-__
—
—
\J/~Z~~=~\\
-
2500H
~~OMeV
2,,um [die 0’2 0
-
-
OL+S,ssrn HgTe
I
[die CHANNEL NUMBER
256
(ENERGY)
512
Fig. 3. Random RBS spectrum obtained using a 2.0 MeV 4He beam incident on a CdTe/HgTe individual grown layers, shown inset.
IMP grown sample, with thick.
NI
--
7500
ii 0
~F[~
~g
_I
CHANNEL
NUMBER
256
IENERg]1
512
Fig. 4. Channelled RBS spectrum obtained using a 2.0 MeV 4He beam incident along the near-normal <100> of a ttiick-Ia’ver CdTe/HgTe IMP grown sample.
J.A, Grimshaw
/
Characterisation of interdiffused MO VPE CdTe/HgTe multi/ayers
poor (fig. 4). Furthermore, the channelling angle
267
was narrower than for other samples. This suggests
The spectra of fig. 5 were,obtained for a <100) oriented sample with 700 A thick HgTe layers
the atomic rows and planes were distorted due to localised strain. Also disorder varied with depth:
alternating with 1400 A thick CdTe, with a final layer of CdTe [11]. Both the <100) channelled
near the front of the sample the corresponding Hg yield suggests a high value of Xmin of ~— 0.2, due to locahised strain and possibly also interstitial Hg atoms near the surface; at the first interface, because there is no feature in the random spectrum in fig. 3 corresponding to the inflexion in fig. 4, disorder must have increased at the interface and beyond into the HgTe layer; regarding the second interface, the bottom of the dip in fig. 4 was shifted to the right compared with its position in fig. 3, which implies a different dechannelling rate in the epitaxial CdTe underlayer than in the HgTe
(lower curve) and random RBS spectra are shown,
layer, and hence different disorder on the CdTe
positions due to atoms misplaced from lattice sites
side of the interface. Thus there was a change in disorder with depth across both interfaces, We next discuss effects in samples with thinner grown layers.
in surface oxide. But in the random spectrum there is no sign of a Hg-rich region. There is, however, an abrupt change in slope at position marked A, and it may be inferred that the fall-off in back-
7600~
I
the horizontal axis including only the upper half of the spectra as compared with figs. 2 and 3. The beam integrated current used in obtaining the lower curve was double that for the random curve, having the effect of magnifying the vertical scale by two for the channelled spectrum. The positions for the onset of backscatter yield associated with surface atoms of Hg, Te and Cd are clearly discernable in both spectra, and are marked on the
figure. In fact, in the channelled spectrum all three constituents gave rise to separate peaks at these
I
-
A
CdTe
Hg
5000
—
—
2500
—
—
0 256
CHANNEL NUMBER
400
(ENERGY)
—i”
512
Fig. 5. High energy region of random and <100) channnelled (lower curve) RBS spectra obtained using a 2.0 MeV on a IMP grown sample.
4He beam incident
268
iA. Grimshaw
/
Characterisatton of interdijfused MO VPE C~d7’i’/iig]/inu/tilacers
scatter yield to the right of A is caused by a reducing Hg concentration in the material as the surface is approached (Hg nuclei having a higher scattering cross section than Cd nuclei). The spectrum may then be interpreted as due to a sample with a much thicker CdTe top layer than that for figs. 2 and 3, allowing a more prolonged interdiffusion of the HgTe and CdTe layers. If position A then refers to the point when the growth of the HgTe layer was stopped, the spectrum width from position A to the Hg surface peak gives us the thickness of the outer CdTe layer as approximately 3500 A, using stopping power data. Over the final — 800 A of the CdTe outer layer the Hg yield component can be seen forward of the main backscatter edge in the random spectrum. The concentration gradient of the Hg here is clearly the opposite to that expected if Hg was migrating from a source in the interior. This remarkable result can be understood because at the end of the growth process the sample was maintamed at 410°C in contact with Hg vapour for 10 mm and then cooled down [1]. It is clear that the Hg concentration profile observed near the surface in fig. 5 resulted mainly from the inward migration of Hg from the surface.
If we now look at the channelled yield in fig. 5 for the same near-surface Fig component, we find it to he about 15% of the corresponding random Hg yield peak. Thus the proportion of Hg atoms in interstitial sites exposed to the (100) channelled beam at this point was no more than 15%. Furthermore, the proportion remained low to a depth of — 800 A at least, as can he seen from the variation of the channelled yield, and remembering the beam will have an increasing dechannelled component with depth. These results suggest the Hg atoms which migrated in from the surface were on substitutional sites, although further work is required to verify this. Analysis of other samples in the same manner gave similar numbers, where the near-surface Hg distribution was dominated by migration from the interior. Much better channelling is to be seen in fig. 6 which was obtained on another thin-layer IMP sample with individual CdTe layers grown as 1000 A and the HgTe layers as 900 A on a (Il 1) oriented substrate, the final layer being HgTe [Ifl. Apart from structure in the curves associated with the near-surface region, over most of their length the channelled and random curves run smoothly and nearly parallel, indicating both a material of
12 000
Hg
9000
—
6000
-
3000
-
N
N
-‘-~~.
-
—
NN
~-
RANDOM -
~
—
0 60 HEI ILIM
ION
250 BACKSIATTER
508 FNERNY
CHANNEl SI
Fig. 6. Random and <11)> channelled RBS spectra obtained using a 2.0 MeV
4He beam incident on a thin-layer IMP grown sample.
iA. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/HgTe multi/ayers
269
\\
8000
CdTe
Hg
6000
Random C C —
4000
—
—
c <111>
S
a
0
2000
-
—
100
250 HELIUM
ION
BACKSCATTER
512
,
ENERGY
(CHANNELSI
Fig. 7. Random and <111) channelled RBS spectra obtained using a 2.0 MeV
uniform composition and a well-ordered crystal lattice inwards from a depth of ‘— 0.5 jim to over 2 jim. Fig. 6 shows fairly uniform Hg content for 800 A depth from the surface, but again the structure in the random curve is associated with incomplete interdiffusion between the last three layers put down. The major dip is associated with the interface between the final HgTe layer and CdTe beneath it. In the channelled curve, peaks due to matrix atoms in surface oxide are again observed. The sample relating to fig. 7 possessed an even thinner layer structure, was grown on a <111) substrate and had individual layer thicknesses grown as 400 A for CdTe and 1300 A for HgTe with a final layer of HgTe. The random curve of fig. 7 shows compositional uniformity extended from deep in the material to within ‘- 1500 A of the surface, following which there were regions only slightly rich and then slightly depleted of Hg. But the channelled curve shows the degree of order was not as good overall as that for the sample related to fig. 6.
41-k beam incident on a thin-layer IMP grown sample.
3.3. Oxygen and hydrogen content In using nuclear reaction analysis for detection of oxygen (section 2), although reaction yield peaks were readily observed for oxygen residing on sample surfaces, no oxygen was detected in the vicinity of the epilayer—substrate interfaces both for IMP and HgTe/CdTe photolysis samples. The detection limit in these measurements was 1 monolayer of oxygen in the interface region. A search was made for hydrogen (section 2) in the bulk of both IMP and photolysis-grown samples. It was deduced that the concentration of hydrogen must be below the 2 at% limit of detectabihity over the depth probed of about 1/4 jim.
4. Conclusions As regards IMP grown layers, the results described, and others we have made, suggest strongly that good compositional uniformity near the surface requires small individual layer thicknesses.
270
iA. Grimshaw
/
Characterisation of interdiffused MOVPE CdTe/J-igTe niult,laver.s
However, whether this rule could be relaxed for compositional uniformity at depths greater than the final three layers is not completely certain at present. In a poorly interdiffused region, increased disorder across interfaces has been observed. As regards achieving good lattice order at larger depths, no clear trend emerges favouring thin mdividual layers over the range of thickness studies. But there is some evidence of greater disorder in interdiffused layers with a higher proportion of Hg. Channelling measurements on IMP grown material could just detect beam-induced changes 4He ions/cm2 at 2 MeV, by 1016 Oxygen content in the substrate-epilayer interface region must be less than 1 monolayer in total both for IMP grown and photosensitised grown HgTe on CdTe. Hydrogen content was below 2 at% in IMP and photosensitised grown samples.
For
sufficiently
uniform
material, x
in
Cd~Hg 1 ~Te can be measured accurately by RBS in the near-surface region.
epilayer samples and for many helpful and stinlulating discussions. The author is indebted to Professor W.G. Townsend for his support and keen interest in the work. He is also most grateful to D. Diskett for carrying out much ancillary work and to E.E. Barratt, G. Partridge and T. Day for their work concerning accelerator operations.
References Ill
J. Tunnicliffe, S.J.C. Irvine. O.D. Dosser and LB. Mullin. J. Crystal Growth 68 (1984) 245.
[2] S.J.C. Irvine, J.B. Mullin and J. Tunnicliffe. J. Crystal Growth 68 (1984) 188. [3] K L Conway. J F Gibbons and T W Sigmon. J Vacuum Sci. Technol. 21(1982) 212.
141 15]
G. Bahir and R. Kalish. J. AppI. Phys 54 (1983) 3129.
G.L. Destafanis, NucI. Instr. Methods 209/210 (1983 567. [6] 0. Amsel and D. Samuel, Anal. Chem. 39 (1967) 1689. [7] B.L. Doyle and P.S. Peercy. AppI. Phys Letters 34(1979) 811. [8] i.E. Ziegler, Ed.. The Stopping and Ranges of Ions in Matter, Vol. 4. Helium (Pergamon, 1977).
Acknowledgements
Warm thanks are extended to J.B. Mullin. S.J.C. Irvine and J. Tunnicliffe of RSRE for supply of
[9] CF. Williamson, J.-P. Boujot and J. Picard. Report CEAR-3042 (1966). 110] J.W. Mayer and E. Rimini, Eds., Ion Beam Handbook for Material Analysis (Academic Press, 1977). [11] J. Tunnicliffe, personal communication.