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CdTe MOCVD grown superlattice epitaxial structures on GaAs by ion beam techniques
Journal of Crystal Growth 117 (1992) 460—464 North-Holland
o,
CRYSTAL GROWTH
Analysis of HgTe/CdTe MOCVD grown superlattice epitaxial structures on GaAs by ion beam techniques L.S. Wielunski, M.J. Kenny CSIRO Division of Applied Physics, Lucas Heights Research Laboratories, PMB-7, Menai, NSW 2234, Australia
and G.N. Pain Telecom Australia Research Laboratories, 770 Blackurn Road, Clayton, Victoria 3168, Australia
Heteroepitaxial MOCVD grown HgTe/CdTe superlattice structures have been examined by Rutherford backscattering spectrometry (RBS) to monitor Hg, Cd and Te concentrations as a function of depth. Individual sublayers thicknesses2C(d, have p)’3C been measured at the same time. Crystal quality has been assessed using ion channeling. In addition the nuclear reaction ‘ was used to detect carbon impurities and proton induced X-ray emision (PIXE) analysis used to detect In and Sb introduced during growth. The results show that the as-grown HgTe/CdTe superlattice has good crystal quality and reasonable lateral uniformity. Mercury concentration is difficult to control during growth and variation between sub-layers is observed. Hg—Cd interdiffusion is observed in heat treated samples. Carbon concentration varies; in a good quality samples 20 ppm is present.
1. Introduction
and results are sensitive to surface conditions and/or contaminations. Polarity determination by
The major issues of concern in I1—IV heteroepitaxial structures grown by MOCVD technique are control of composition (including introduced impurities) and crystal quality. Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) in combination with
ion beam channeling examines the subsurface
channeling are powerful and nondestructive techniques in thin layer material analysis with good mass and depth resolution and the capacity to both detect and depth profile crystal defects such as dislocations and interstitial atoms [1,2]. For homo- and heteroepitaxial structures, the RBS-channeling technique can determine thin layer strain and interface quality [3—5].Face polarity for compound semiconductors can be determined by a number of techniques including: ion beam channeling [6], electron microscopy [7], Xray diffraction [8] and chemical etching. Chemical methods are very sensitive to the first monolayer 0022-0248/92/$05.Ot) © 1992
—
crystal structure and is not sensitive to the surface contaminations or other first monolayer distortion [6,7]. The concentration of Hg is difficult to control in Hg 1~Cd~Tecompounds grown by MOCVD methods and analysis is complicated. Rutherford backscattering analysis can determine the Hg concentration profile as a function of depth with depth resolution of about 40 nm and can assess lateral uniformity [9,10].
2. Material growth HgTe/CdTe superlattices and Hg1~Cd~Te heteroepitaxial structures were grown on GaAs wafers in a MR Semicon Quantax 226 MOCVD reactor. Growth occurs in a flow of hydrogen gas
Elsevier Science Publishers B.V. All rights reserved
L.S. Wielunski et a!.
/ Analysis of HgTe / CdTe MOCVD grown SLs on
at 250—350°C in a horizontal reactor in which pyrolysis of organometallics is induced by the cracking susceptor suspended above the substrate. This process involves the use of organometallic species dimethylcadmium, diethyltelluride and elemental mercury. Details of substrate preparation and growth process have been presented earlier [10]. In some cases the as-grown structures have been annealed to promote Hg—Cd interdiffusion and to generate a uniform Hg1 Cd~Testructure from the HgTe/CdTe superlattice. 3. Material analysis For ion beam analysis, a 1 mm diameter hehum or hydrogen ion beam from an accelerator was directed onto samples mounted on the x/y stage of a three-axis goniometer in the vacuum chamber. Scattered particles and emitted X-rays were detected by solid state detectors and recorded by a multichannel analyser as energy spectra. Random spectra, produced by selecting an orientation away from any possible channeling direction, were used for compositional analysis. Crystal quality and crystal defect numbers were estimated by comparing random and channeling spectra. The count rate in the channeling spectrumrandom should spectrum be about one twentieth for the in the case ofofa that perfect crystal. A higher count rate in the channeling spectrum indicates crystal defects, imperfections or even misorientations [11]. The Hg concentration in uniform Hg 1 ~Cd~Te structure is obtained by comparing the Hg step
height se~emercury with collective profiles in nonuniform of and Te The combination signals depth inthe with a depth resolution random mass spectrum. resolution ofheight RBS (20—50 is Hg, usedCd nm) tosamobin ples including HgTe/CdTe superlattice struc tures. Scattering from Hg has a relatively higher energy and higher RBS cross-section than that from Cd or Te. These two effects energy shift and change in scattering probability generate oscillations in the RBS spectra which correspond to mercury concentration oscillations in the depth —
—
461
GaAs by ion beam technique
profile of the superlattice structure. The period of the RBS spectra oscillations corresponds directly to the thicknesses of individual sublayers involved and the oscillation amplitude corresponds directly to changes in mercury concentration. Rutherford backscattering mass resolution is not high enough to adequately separate Cd and Te signals and is further degraded for low intensity signals from the dopant impurity In or Sb intentionaly introduced in some structures. PIXE was used to separate signals from these elements [131. 12C(d, p)13C was used to The carbon nuclearimpurities reaction [131. 1 MeV deuterium detect ions were employed and high energy protons detected with a solid state detector.
4. Results and discussion Fig. 1 shows typical Hg 1 ~Cd~Te on GaAs RBS spectra in random and channeling modes. The Hg and Cd—Te steps are clear in the high energy part of the random spectrum. The Hg concentration is estimated at 0.87 (x = 0.13) by comparing the relative step simulated height Hg/(Cd—Te) with the family of computer spectra for different mercury concentrations shown in fig. 2. 4
4x10
neled
-______________________________________ 50
100
‘150
200
Channel Fig. 1. 2 MeV He RBS spectra (random and channeled) of 1.6 ~m thicksignal Hg1~Cd~Te structure substrate. surface is in channel 188, on andGaAs the Te and CdThe are Hg in channels 180 and 177 respectively. The GaAs substrate signal is below channel 70. Depth scale is 16 nm/channel.
462
L. S. Wielunski et a!.
350
/ Analysis of HgTe / CdTe MOCVD grown SLs on 4
Energy (MeV) 1.0 1.5
0.5
I
‘
8x10
2.0
GaAs by ion beam technique ‘
6-
a) .~200 ~250a) 300 N ~6 150 ~-s 100
50
~TeH
Pa ad orn
°N~T~
0 ‘6
(0 (__) 0 0
4 0 2
GaAs1 ~
___
50
0 50
___
100 150 Channel
200
Fig. 2. Computer simulated 2 MeV He RBS spectra calculated for different composition Hg, ~Cd~Te layer 5 X lO~~ atoms/cm2 thick on GaAs substrate,
This estimate is obtained from the first 130 nm of the surface. The shape of the random spectrum suggests that the Hg concentration at depth 300— 700 nm is reduced to about 0.73 (x 0.27). Total thickness is about 1.6 p.m. The low channeling level indicates that it is of a reasonably good crystal quality. Rutherford backscattering analysis of a superlattice sample was made at three orthogonal points separated by 5 mm in directions normal and parallel to the gas flow. Typical random and channeling spectra are shown in fig. 3. This superlattice structure contains ten periods and corresponding peaks are seen in the spectra. In the =
100 Channel
150 NurnLaer
200
Fig. 3. 2 MeV He RBS spectra (random and channeled) of a 1.51 ~m thick HgTe/CdTe superlattice structure (10 periods). Depth scale is 16 nm/channel.
the average height of the random spectrum. Concentrations are uniform normal to the gas flow but vary in the gas flow direction. More detailed information about elemental concentration profiles can be obtained using complex RBS computer simulation programs [141. Fig. 4 shows the simulated random spectrum for the ideal crystal corresponding to the sample shown in fig. 3. The Energy (MeV) 0.5
250
200
~
~
150
1.0
1.5
2.0
I
I
I
-
Experiment
‘/
Simulation
U
direction normal to the gas flow, the analysis points indicated that the total thickness of the superlattice layer was 1.51 and 1.48 p.m. However in the direction of the gas flow the points sampled indicated a thickness variation from 1.48 to 1.35 p.m. These results suggest that thickness uniformity in the gas flow direction is not very good. The channeling spectrum indicate reasonable crystal quality; however, there is a significant dechannehing rate observed in this sample (fig. 3) consistent with there being strain and defects on the interfaces between individual sublayers. Average mercury concentrations can be estimated from
.N 100
z
~0
‘~
Cd
-s”
0 50
100
Channel
150
200
Fig. 4. Comparison of experimental HgTe/CdTe superlattice (random) spectrum with computer simulation. The individual Hg, Cd, Te, Ga and As components are shown separately. The details of the composition profile used are shown in table i.
L.S. Wielunski et a!.
/ Analysis of HgTe / CdTe MOCVD grown SLs on
individual elemental contributions to the spectrum are shown separately. Mercury is clearly concentrated in the near surface sublayers. Dctails of concentration profile used in the simulation are shown in table 1. The high diffusion mobility of Hg and Cd in HgTe/CdTe superlattices was observed by RBS analysis before and after annealing. As a result of Hg—Cd interdiffusion, the structure became more uniform and typical superlattice oscillations in the RBS spectrum were reduced in amplitude and finally disappeared. In this case the RBS spectra are similar to shown in fig. 1. The final structure is usually good quality uniform Hg1 ..~Cd~Tewith mercury concentration well controlled by the thickness ratio of the HgTe and CdTe sublayers [9,10]. The detection of low concentrations of In or Sb in CdTe or Hg1.~Cd~Teis not possible by RBS due to signal overlap. The PIXE analysis is capable of very good separation of In, Sb, Hg, Cd, Te, Ga and As signals. Notwithstanding this, PIXE analysis of the sample shown in fig. 3 indicates that In was not present. It should be
GaAs by ion beam technique
463
remembered that PIXE analysis does not provide depth information and also the intensities of observed signals are modified by X-ray absorption. A major concern with the use of organometallie compounds in MOCVD is material contamination with carbon. The nuclear reaction 12C(d, p)’3C was used to detect carbon present in Hg, .. 5Cd5Te and HgTe/CdTe superlattice samples. In good crystal quality structures (with channehing/random ratio <10%) the carbon level was about or below the sensitivity limit of 20 ppm (atomic); however, in some poor quality samples (where channeling was not observed) the carbon signal was as high as 150 ppm. This observation suggests that in good epitaxial structures the carbon level will be insignificant. The techniques reported above are compiementary with other analytical methods. For exampie, X-ray diffraction methods can provide crystal quality information including superlattice anaiysis. However X-ray methods do not provide depth resolution comparable to RBS and cannot be used to generate elemental depth profiles. In a similar way, cross-sectional TEM is a very power-
Table 1 Concentration profile used in computer simulation shown in fig. 4 Layer No.
Thicknes (nm)
Atomic composition (%) Hg Cd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
37.2 76.3 62.5 63.1 75.6 57.1 82.0 57.1 69.4 69.8 69.4 69.8 69.4 69.8 69.4 69.8 69.4 69.8 69.5 114.2 1000
35 20 5 12 3 9 5 5 5
5 4 3
15 50 30 45 38 47 41 50 45 50 45 50 45
50 45 50 46 50 47 50
Te
Ga
As
50
50
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50
464
L. S. Wielunski et a!.
/ Analysis
of HgTe / CdTe MOCVD grown SLs on GaAs by ion beam technique
ful technique with very high resolution [15], but sample preparation is complex, usually affects material structure, and the sample cannot be further analysed.
Research, Telecom Australia to publish this work is acknowledged.
References 5. Conclusions The present work shows that ion beam methods are very powerful for analysing thin layer epitaxial structures. The results presented show that MOCVD grown Hg1_~Cd~Te structures and HgTe/CdTe superlattices can be good quality heteroepitaxial structures with reasonable lateral uniformity. Because RBS cross-sections and energy losses are well known, the method is quantitative with a precision of about 2%. The typical range of RBS analysis is 1—2 p.m and the depth resolution is of the order of 40 nm. The methods are ideal for HgTe/CdTe superlattice structure analysis and can be used to monitor mercury redistribution during deposition or subsequent thermal treatment. The nondestructive character of the ion beam methods makes them very attrac-
tive. Acknowledgements This research has been partly funded under the Generic Technology component of the industry Research and Development Act 1986, Grant No. 15019. The authors are grateful to Dr. D. Cohen from ANSTO and Dr. S. Sic from CSIRO for PIXE analysis and valuable discussions. The permission of the Executive General Manager,
[1] W.K. Chu, J.W Mayer and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978). [2] L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York, 1982) 131 L.S. Wielunski, S. Hashimoto and W.M. Gibson, NucI. Instr. Methods B 13 (1986) 61. [4] S. Hashimoto, L.S. Wielunski, J.-L. Peng, W.M. Gibson and L.J. Schowalter, NucI. Instr. Methods B 13 (1986) 65. 151 S. Hashimoto, J.-L. Peng. W.M. Gibson, L.J. Schowalter and R.W. Fathauer, AppI. Phys. Letters 47 (1985) 1071. [61L.S. Wielunski, MS. Kwietniak, G.N. Pain and Ci. Rossouw, NucI. Instr. Methods B 45 (1990) 459. [7] S.R. Glanvill, C.J. Rossouw, M.S. Kwietniak, G.N. Pain, T. Warminski and L.S. Wielunski, J. AppI. Phys. 66 (1989) 619. [8] A.W. Stevenson, D. Gao, G.N. Pain and L.S. Wielunski, Acta Cryst. A 47 (1991) 128. [9] L.S. Wielunski, M.J. Kenny and G.N. Pain, Mater. Forum 15 (1991) 150. 110] G.N. Pain, N. Bharatula, T.J. Elms, P. Gwynn, M. Kibel, MS. Kwietniak, P. Leech, N. Petkovic, C. Sandford, J. Thomson, T. Warminski, D. Gao, S.R. Glanvill, C.J. Rossouw, A.W. Stevenson, SW. Wilkins and L.S. Wielunski, J. Vacuum Sci. Technol. A 8 (1990) 1067. [11] L.S. Wielunski, MS. Kwietniak, G.N. Pain and C.J. Rossouw, NucI. Instr. Methods B 45 (1990) 455. [12] G.I. Christiansz, S. Georgiou, MS. Kwietniak, G.N. Pain, B. Usher, T. Warminski, SR. Glanvill, C.J. Rossouw, A.W. Stevenson, SW. Wilkins and L.S. Wielunski, X-Ray Spectrom. 19 (1990) 19. [13] JR. Bird and J.S. Williams, Eds., Ion Beams for Mater,(Academic Press, Sydney,B 1989) pp. 344. 159, 210. [14] als L.R.Analysis Doolittle, Nucl. Instr. Methods 9 (1985) [15] SR. Glanvill, MS. Kwietniak, and G.N. C.J. Phil. Rossouw, T. Warminski, L.S. Wielunski I.J.Pain, Wilson, Mag. Letters 59 (1989) 17.
o,
CRYSTAL GROWTH
Analysis of HgTe/CdTe MOCVD grown superlattice epitaxial structures on GaAs by ion beam techniques L.S. Wielunski, M.J. Kenny CSIRO Division of Applied Physics, Lucas Heights Research Laboratories, PMB-7, Menai, NSW 2234, Australia
and G.N. Pain Telecom Australia Research Laboratories, 770 Blackurn Road, Clayton, Victoria 3168, Australia
Heteroepitaxial MOCVD grown HgTe/CdTe superlattice structures have been examined by Rutherford backscattering spectrometry (RBS) to monitor Hg, Cd and Te concentrations as a function of depth. Individual sublayers thicknesses2C(d, have p)’3C been measured at the same time. Crystal quality has been assessed using ion channeling. In addition the nuclear reaction ‘ was used to detect carbon impurities and proton induced X-ray emision (PIXE) analysis used to detect In and Sb introduced during growth. The results show that the as-grown HgTe/CdTe superlattice has good crystal quality and reasonable lateral uniformity. Mercury concentration is difficult to control during growth and variation between sub-layers is observed. Hg—Cd interdiffusion is observed in heat treated samples. Carbon concentration varies; in a good quality samples 20 ppm is present.
1. Introduction
and results are sensitive to surface conditions and/or contaminations. Polarity determination by
The major issues of concern in I1—IV heteroepitaxial structures grown by MOCVD technique are control of composition (including introduced impurities) and crystal quality. Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) in combination with
ion beam channeling examines the subsurface
channeling are powerful and nondestructive techniques in thin layer material analysis with good mass and depth resolution and the capacity to both detect and depth profile crystal defects such as dislocations and interstitial atoms [1,2]. For homo- and heteroepitaxial structures, the RBS-channeling technique can determine thin layer strain and interface quality [3—5].Face polarity for compound semiconductors can be determined by a number of techniques including: ion beam channeling [6], electron microscopy [7], Xray diffraction [8] and chemical etching. Chemical methods are very sensitive to the first monolayer 0022-0248/92/$05.Ot) © 1992
—
crystal structure and is not sensitive to the surface contaminations or other first monolayer distortion [6,7]. The concentration of Hg is difficult to control in Hg 1~Cd~Tecompounds grown by MOCVD methods and analysis is complicated. Rutherford backscattering analysis can determine the Hg concentration profile as a function of depth with depth resolution of about 40 nm and can assess lateral uniformity [9,10].
2. Material growth HgTe/CdTe superlattices and Hg1~Cd~Te heteroepitaxial structures were grown on GaAs wafers in a MR Semicon Quantax 226 MOCVD reactor. Growth occurs in a flow of hydrogen gas
Elsevier Science Publishers B.V. All rights reserved
L.S. Wielunski et a!.
/ Analysis of HgTe / CdTe MOCVD grown SLs on
at 250—350°C in a horizontal reactor in which pyrolysis of organometallics is induced by the cracking susceptor suspended above the substrate. This process involves the use of organometallic species dimethylcadmium, diethyltelluride and elemental mercury. Details of substrate preparation and growth process have been presented earlier [10]. In some cases the as-grown structures have been annealed to promote Hg—Cd interdiffusion and to generate a uniform Hg1 Cd~Testructure from the HgTe/CdTe superlattice. 3. Material analysis For ion beam analysis, a 1 mm diameter hehum or hydrogen ion beam from an accelerator was directed onto samples mounted on the x/y stage of a three-axis goniometer in the vacuum chamber. Scattered particles and emitted X-rays were detected by solid state detectors and recorded by a multichannel analyser as energy spectra. Random spectra, produced by selecting an orientation away from any possible channeling direction, were used for compositional analysis. Crystal quality and crystal defect numbers were estimated by comparing random and channeling spectra. The count rate in the channeling spectrumrandom should spectrum be about one twentieth for the in the case ofofa that perfect crystal. A higher count rate in the channeling spectrum indicates crystal defects, imperfections or even misorientations [11]. The Hg concentration in uniform Hg 1 ~Cd~Te structure is obtained by comparing the Hg step
height se~emercury with collective profiles in nonuniform of and Te The combination signals depth inthe with a depth resolution random mass spectrum. resolution ofheight RBS (20—50 is Hg, usedCd nm) tosamobin ples including HgTe/CdTe superlattice struc tures. Scattering from Hg has a relatively higher energy and higher RBS cross-section than that from Cd or Te. These two effects energy shift and change in scattering probability generate oscillations in the RBS spectra which correspond to mercury concentration oscillations in the depth —
—
461
GaAs by ion beam technique
profile of the superlattice structure. The period of the RBS spectra oscillations corresponds directly to the thicknesses of individual sublayers involved and the oscillation amplitude corresponds directly to changes in mercury concentration. Rutherford backscattering mass resolution is not high enough to adequately separate Cd and Te signals and is further degraded for low intensity signals from the dopant impurity In or Sb intentionaly introduced in some structures. PIXE was used to separate signals from these elements [131. 12C(d, p)13C was used to The carbon nuclearimpurities reaction [131. 1 MeV deuterium detect ions were employed and high energy protons detected with a solid state detector.
4. Results and discussion Fig. 1 shows typical Hg 1 ~Cd~Te on GaAs RBS spectra in random and channeling modes. The Hg and Cd—Te steps are clear in the high energy part of the random spectrum. The Hg concentration is estimated at 0.87 (x = 0.13) by comparing the relative step simulated height Hg/(Cd—Te) with the family of computer spectra for different mercury concentrations shown in fig. 2. 4
4x10
neled
-______________________________________ 50
100
‘150
200
Channel Fig. 1. 2 MeV He RBS spectra (random and channeled) of 1.6 ~m thicksignal Hg1~Cd~Te structure substrate. surface is in channel 188, on andGaAs the Te and CdThe are Hg in channels 180 and 177 respectively. The GaAs substrate signal is below channel 70. Depth scale is 16 nm/channel.
462
L. S. Wielunski et a!.
350
/ Analysis of HgTe / CdTe MOCVD grown SLs on 4
Energy (MeV) 1.0 1.5
0.5
I
‘
8x10
2.0
GaAs by ion beam technique ‘
6-
a) .~200 ~250a) 300 N ~6 150 ~-s 100
50
~TeH
Pa ad orn
°N~T~
0 ‘6
(0 (__) 0 0
4 0 2
GaAs1 ~
___
50
0 50
___
100 150 Channel
200
Fig. 2. Computer simulated 2 MeV He RBS spectra calculated for different composition Hg, ~Cd~Te layer 5 X lO~~ atoms/cm2 thick on GaAs substrate,
This estimate is obtained from the first 130 nm of the surface. The shape of the random spectrum suggests that the Hg concentration at depth 300— 700 nm is reduced to about 0.73 (x 0.27). Total thickness is about 1.6 p.m. The low channeling level indicates that it is of a reasonably good crystal quality. Rutherford backscattering analysis of a superlattice sample was made at three orthogonal points separated by 5 mm in directions normal and parallel to the gas flow. Typical random and channeling spectra are shown in fig. 3. This superlattice structure contains ten periods and corresponding peaks are seen in the spectra. In the =
100 Channel
150 NurnLaer
200
Fig. 3. 2 MeV He RBS spectra (random and channeled) of a 1.51 ~m thick HgTe/CdTe superlattice structure (10 periods). Depth scale is 16 nm/channel.
the average height of the random spectrum. Concentrations are uniform normal to the gas flow but vary in the gas flow direction. More detailed information about elemental concentration profiles can be obtained using complex RBS computer simulation programs [141. Fig. 4 shows the simulated random spectrum for the ideal crystal corresponding to the sample shown in fig. 3. The Energy (MeV) 0.5
250
200
~
~
150
1.0
1.5
2.0
I
I
I
-
Experiment
‘/
Simulation
U
direction normal to the gas flow, the analysis points indicated that the total thickness of the superlattice layer was 1.51 and 1.48 p.m. However in the direction of the gas flow the points sampled indicated a thickness variation from 1.48 to 1.35 p.m. These results suggest that thickness uniformity in the gas flow direction is not very good. The channeling spectrum indicate reasonable crystal quality; however, there is a significant dechannehing rate observed in this sample (fig. 3) consistent with there being strain and defects on the interfaces between individual sublayers. Average mercury concentrations can be estimated from
.N 100
z
~0
‘~
Cd
-s”
0 50
100
Channel
150
200
Fig. 4. Comparison of experimental HgTe/CdTe superlattice (random) spectrum with computer simulation. The individual Hg, Cd, Te, Ga and As components are shown separately. The details of the composition profile used are shown in table i.
L.S. Wielunski et a!.
/ Analysis of HgTe / CdTe MOCVD grown SLs on
individual elemental contributions to the spectrum are shown separately. Mercury is clearly concentrated in the near surface sublayers. Dctails of concentration profile used in the simulation are shown in table 1. The high diffusion mobility of Hg and Cd in HgTe/CdTe superlattices was observed by RBS analysis before and after annealing. As a result of Hg—Cd interdiffusion, the structure became more uniform and typical superlattice oscillations in the RBS spectrum were reduced in amplitude and finally disappeared. In this case the RBS spectra are similar to shown in fig. 1. The final structure is usually good quality uniform Hg1 ..~Cd~Tewith mercury concentration well controlled by the thickness ratio of the HgTe and CdTe sublayers [9,10]. The detection of low concentrations of In or Sb in CdTe or Hg1.~Cd~Teis not possible by RBS due to signal overlap. The PIXE analysis is capable of very good separation of In, Sb, Hg, Cd, Te, Ga and As signals. Notwithstanding this, PIXE analysis of the sample shown in fig. 3 indicates that In was not present. It should be
GaAs by ion beam technique
463
remembered that PIXE analysis does not provide depth information and also the intensities of observed signals are modified by X-ray absorption. A major concern with the use of organometallie compounds in MOCVD is material contamination with carbon. The nuclear reaction 12C(d, p)’3C was used to detect carbon present in Hg, .. 5Cd5Te and HgTe/CdTe superlattice samples. In good crystal quality structures (with channehing/random ratio <10%) the carbon level was about or below the sensitivity limit of 20 ppm (atomic); however, in some poor quality samples (where channeling was not observed) the carbon signal was as high as 150 ppm. This observation suggests that in good epitaxial structures the carbon level will be insignificant. The techniques reported above are compiementary with other analytical methods. For exampie, X-ray diffraction methods can provide crystal quality information including superlattice anaiysis. However X-ray methods do not provide depth resolution comparable to RBS and cannot be used to generate elemental depth profiles. In a similar way, cross-sectional TEM is a very power-
Table 1 Concentration profile used in computer simulation shown in fig. 4 Layer No.
Thicknes (nm)
Atomic composition (%) Hg Cd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
37.2 76.3 62.5 63.1 75.6 57.1 82.0 57.1 69.4 69.8 69.4 69.8 69.4 69.8 69.4 69.8 69.4 69.8 69.5 114.2 1000
35 20 5 12 3 9 5 5 5
5 4 3
15 50 30 45 38 47 41 50 45 50 45 50 45
50 45 50 46 50 47 50
Te
Ga
As
50
50
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50
464
L. S. Wielunski et a!.
/ Analysis
of HgTe / CdTe MOCVD grown SLs on GaAs by ion beam technique
ful technique with very high resolution [15], but sample preparation is complex, usually affects material structure, and the sample cannot be further analysed.
Research, Telecom Australia to publish this work is acknowledged.
References 5. Conclusions The present work shows that ion beam methods are very powerful for analysing thin layer epitaxial structures. The results presented show that MOCVD grown Hg1_~Cd~Te structures and HgTe/CdTe superlattices can be good quality heteroepitaxial structures with reasonable lateral uniformity. Because RBS cross-sections and energy losses are well known, the method is quantitative with a precision of about 2%. The typical range of RBS analysis is 1—2 p.m and the depth resolution is of the order of 40 nm. The methods are ideal for HgTe/CdTe superlattice structure analysis and can be used to monitor mercury redistribution during deposition or subsequent thermal treatment. The nondestructive character of the ion beam methods makes them very attrac-
tive. Acknowledgements This research has been partly funded under the Generic Technology component of the industry Research and Development Act 1986, Grant No. 15019. The authors are grateful to Dr. D. Cohen from ANSTO and Dr. S. Sic from CSIRO for PIXE analysis and valuable discussions. The permission of the Executive General Manager,
[1] W.K. Chu, J.W Mayer and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978). [2] L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York, 1982) 131 L.S. Wielunski, S. Hashimoto and W.M. Gibson, NucI. Instr. Methods B 13 (1986) 61. [4] S. Hashimoto, L.S. Wielunski, J.-L. Peng, W.M. Gibson and L.J. Schowalter, NucI. Instr. Methods B 13 (1986) 65. 151 S. Hashimoto, J.-L. Peng. W.M. Gibson, L.J. Schowalter and R.W. Fathauer, AppI. Phys. Letters 47 (1985) 1071. [61L.S. Wielunski, MS. Kwietniak, G.N. Pain and Ci. Rossouw, NucI. Instr. Methods B 45 (1990) 459. [7] S.R. Glanvill, C.J. Rossouw, M.S. Kwietniak, G.N. Pain, T. Warminski and L.S. Wielunski, J. AppI. Phys. 66 (1989) 619. [8] A.W. Stevenson, D. Gao, G.N. Pain and L.S. Wielunski, Acta Cryst. A 47 (1991) 128. [9] L.S. Wielunski, M.J. Kenny and G.N. Pain, Mater. Forum 15 (1991) 150. 110] G.N. Pain, N. Bharatula, T.J. Elms, P. Gwynn, M. Kibel, MS. Kwietniak, P. Leech, N. Petkovic, C. Sandford, J. Thomson, T. Warminski, D. Gao, S.R. Glanvill, C.J. Rossouw, A.W. Stevenson, SW. Wilkins and L.S. Wielunski, J. Vacuum Sci. Technol. A 8 (1990) 1067. [11] L.S. Wielunski, MS. Kwietniak, G.N. Pain and C.J. Rossouw, NucI. Instr. Methods B 45 (1990) 455. [12] G.I. Christiansz, S. Georgiou, MS. Kwietniak, G.N. Pain, B. Usher, T. Warminski, SR. Glanvill, C.J. Rossouw, A.W. Stevenson, SW. Wilkins and L.S. Wielunski, X-Ray Spectrom. 19 (1990) 19. [13] JR. Bird and J.S. Williams, Eds., Ion Beams for Mater,(Academic Press, Sydney,B 1989) pp. 344. 159, 210. [14] als L.R.Analysis Doolittle, Nucl. Instr. Methods 9 (1985) [15] SR. Glanvill, MS. Kwietniak, and G.N. C.J. Phil. Rossouw, T. Warminski, L.S. Wielunski I.J.Pain, Wilson, Mag. Letters 59 (1989) 17.