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Low temperature growth of (Cd,Hg) Te layers by MOVPE
626
Journal of Crystal Growth 107 (1991) 626—631 North-Holland
Low temperature growth of (Cd,Hg)Te layers by MOVPE F. Desjonquères, A. Tromson-Carli, P. Cheuvart, R. Druilhe, C. Grattepain, A. Katty, Y. Marfaing, R. Triboulet Laboratoire de Physique des Solides de Bellevue, CNRS, 1 Place A. Brian4 F-92195 Meudon Cedex, France
and D. Lorans Société Anonyme de Télécom,nunications, 41 Rue Caniagrel, F-75624 Paris Cedex, France
MCT layers have been grown for the first time at 250°Cusing DATe, DMCd and mercury. First the buffer layer CdTe grown at 365°Cusing DIPTe and DMCd is studied with an emphasis on the influence of the substrate orientation. Indeed the surface morphology and the crystalline quality may change dramatically as a function of the substrate orientation. Then the low temperature MOVPE growth of (Cd,Hg)Te is described: different compositions were achieved and the crystalline and electronic properties are presented.
1. Introduction The presence of semimetals in the Il-VI family allows the preparation of original microstructures (heterostructures) such as type III superlattices which cannot be found in other semiconductor families. On the other hand, new IR detectors based on complex heterostructures have already been proposed with the mercury based materials, This leads to a new challenge for the MOVPE growth of (Cd,Hg)Te: the growth at low temperature in order to reduce the broadening of the interfaces, to lower the diffusion of impurities coming from the substrate and the density of mercury vacancies. Different techniques to this end have been studied such as plasma enhanced chemical deposition [1], precracking of precursors [2], photo-MOCVD using a mercury lamp or a laser [3]. All of them have different drawbacks. A very promising method seems to be the use of alternative precursors with a lower pyrolysis temperature, especially for tellurium. Dihydrotellurophene, dimethylditelluride, ditertiarybutyltelluride and methylallyltelluride have already given 0022-0248/91/$03.50 © 1991
—
some results [4,7] but we chose diallyltelluride because of its acceptable vapor pressure (P = 3 Torr at 45°C) and its low temperature of decomposition [8]. It is well known that not only the surface morphology of epitaxial layers, but also their crystalline and electronic quality depend on the substrate orientation and on the nature of an eventual buffer layer. That is why, before displaying results related to the low temperature growth of (Cd,Hg)Te layers, an experimental study of the surface morphology and structural perfection of CdTe buffer layers grown on CdZnTe and CdTe substrates as a function of the substrate orientation has been undertaken.
2. Experimental Our reactor, a Quantax 226 (MR Semicon), is horizontal, containing a heated Hg cell and an RF heated graphite susceptor. The carrier gas used thoughout the studies was hydrogen. The organometallics DIPTe, DMCd and DMZn were
Elsevier Science Publishers BY. (North-Holland)
F Desjonquères et at.
/ Low temperature growth of (Cd,Hg)Telayers by MOVPE
supplied by SMI. Using partial pressures of 10 ~ atm for a VI/Il ratio of one, buffer layers of 2 ~sm of CdTe and 0.1 ,sm of ZnTe were grown at 365°Conto GaAs (100) 2° (110) substrates. DATe, also supplied by SMI, DMCd and metallic mercury, maintained respectively at 55, 0 and 22°C,were used for the growth of (Cd,Hg)Te at 250°C (substrate temperature). The surface morphology and the quality of the epitaxy were checked by scanning electron microscopy (SEM), electron channeling patterns or pseudo-Kikuchi lines (ECP) and double-crystal X-ray diffraction. SIMS was used to determine homogeneity in depth. The electron microprobe and IR transmission allowed the composition of the MCT layers to be determined. Finally the electrically active carrier concentration, the resistivity and the mobility were measured at room temperature and at 77 K by the Van der Pauw technique. —
—,
627
lens. In the Bragg position the electron beam is diffracted through the crystal and the contrast intensity of backscattering electrons allows the emergence of Kikuchi patterns. These patterns are well known for silicon of diamond structure and are not very different for zinc-blende. An angular scale given by the apparatus allows the determination of angular pole positions and then of the orientation of grains. It is a fast and easy method which has been checked by calculation in the case of twinning bands. A twin is a 60° lattice rotation around a [111] direction. A transformation matrix applied to the coordinates of a grain surface allows an easy calculation of the corresponding coordinates of the twinned grain to be made. Table 1 shows some examples of corresponding orientations of surfaces in the twinning bands. 3.2. Evaluation of the layer morphology as a function of orientation
3. Buffer layer morphology versus substrate orientation 3.1. Determination of the orientation of grains Electron channeling patterns (ECPs), pseudoKikuchi lines, have been used to determine orientation and misorientation of the grams of CdZnTe substrates. A parallel electron beam is deflected over all the angular incidences by a deflecting
In the (111) orientation, pseudo-Kikuchi lines do not depend on Te or Cd faces, while the morphology and crystalline quality of the layer are strongly different on (111)B and (111)A faces and on their corresponding twinned orientations: (511)B and (511)A. Fig. 1 shows Kikuchi patterns of the substrate with two twinned grains of orientation (111) and (151). A common row of [022] type ([022] for (111) surface and [202] for (151)
Table 1 Different quality results as a function of the orientation of the substrate. On the right side the morphology quality is described as a function of the number of polycrystalline defects and the existence of pyramids and facets Calculated twin orientation
Usual orientations
(122) type
(100)
50—100
(110) type (411) type)
(110)
120—160
Many defects
((111)B
250—280 125—180 230—300 130—160
Without defects, but not smooth Few defects, smooth Very perturbed Smooth, without defects
(111) type
(511) type
)(511)B \ (111)A ~(511)A
X-ray rocking curve widths (arc see)
Surface quality
Good
628
F. Desjonquères et al.
/ Low temperature growth of (C4Hg)Te layers by
(111) —
—
~ 22}._~~
r
I
=
—
(151) .
,.
—
[202]
Fig. 1. ECP of the twinned grains (Ill) and (511) types.
surface) indicates the existence of a common plane of (110) type. The tellurium and cadmium faces were identified by etching (A for Cd and B for
MOVPE
Te). This etching method is in agreement with the crystallographic determination described by Fewster and Whiffin [9]. The (111)A surface results in many defects (fig. 2) while the associated (511)A twinning band shows Kikuchi patterns and a morphology of good quality. The opposite (511)B surface results in many hillocks and poor crystalline quality, although (111)B is better (fig. 3). However Kikuchi patterns indicate the existence of (111) twins with depth in six fold symmetry instead of three fold, see the [311] rows for example (fig. 3). These results about (111) faces are in agreement with those obtained by Gibart et al. [10] in the case of 111—V compounds. A (511) surface is built from steps of two atoms in the (100) plane and with a riser of one atom in
nfl (111) IA
(511)
Fig. 2. Upper: ECP of (511)A layer. Lower: layer morphologies of the (511)A and (111)A twinned grains.
..
-4
Fig. 3. Upper: Kikuchi patterns of (Ill )B layer, Lower: layer morphologies on the two twinned grains (111)B and (511)B.
F Desjonquères et a!.
/ Low temperature growth
of (c4Hg,~Telayers by MO VPE
629
Fig. 4. Surface morphologies of the layer grown on two twinned grains (100) on the left side and (122) on the right side,
Fig. 5. (110) observation surfaces of the twinned layer. The equivalent hillocks growth direction makes an angle of 70031~ induced by the twin rotation in a (111) common plane.
the (111) plane. This induces a 15.8° rotation of the (100) surface around the [0111direction. It can be considered as a high misorientation of the (100) surface, which may be an advantage. In a twin the (100) surface is coordinated with a (212) surface. Very different morphologies are obtained on these two surfaces (fig. 4). The (100) type has good crystallinity as shown by Kikuchi
good choice for substrate orientation producing the best quality epitaxial layers.
patterns and a narrow FWHM of X-ray rocking curve about 50 to 100 arc sec but a significant number of hillocks remain on top of the “pyramids”. The (122) type presents many defects and poor crystalline quality as shown by Kikuchi patterns. The (110) surface is coordinated with another (110) type of surface as inferred from (111)/(511) conjugate observations. The morphology and the crystalline quality are the same on the two twinned grains (fig. 5). In table 1 results of X-ray double diffraction values and morphology qualities are plotted. (511)A orientation seems to be a very
4. MCT layer epitaxial growth MCT layers were grown on various substrates (GaAs, Cd0 96Zn0~Te, CdTe) which were degreased in trichlorethylene, acetone and methanol. The GaAs substrates were etched in 5: 1 : 1 H2S04: H202: H20 and the CdTe substrates in Br—methanol. Then they were preheated at 365°C for 20 mm before the growth at 250°C.The pregrowth bake temperature seems to be sufficient to remove the carbon and oxygen contamination [11]. The growth rate of the alloy was 1 ~tm/h. The results are presented in table 2 and an example of the surface morphology and the ECP is given in fig. 6 and fig. 7. The layers deposited on a buffer layer grown at 365°C produced the best structural perfection.
Table 2 Crystallographic characterizations of different layers HgCdTe/GaAs(100)
Good surface morphology. Kikuchi lines (100); FWHM
650” to 1000”
HgCdTe/CdZnTe(100)2° —. (110)
Many pyramids, very few hillocks, nice Kikuchi lines (100); FWHM = 275”
HgCdTe/CdZnTe(111)
Triangular structures. Kikuchi lines (111); FWHM = 250” to 335”
HgCdTe/CdTe(100)
Large pyramids. Kikuchi lines (100); FWHM = 310” to 345”
HgCdTe/CdTe/ZnTe/GaAs(100)
Buffer layer grown at 365°C, MCT layer grown at 250°C, specular morphology; Kikuchi lines (100); FWHM = 250” to 275”
630
F Desjonquères eta!.
/
Low temperature growth of (Cd,Hg)Te layers by MOVPE Ice.
Fig. 6. Surface morphology of MCT/CdTe/ZnTe/GaAs: x 0.07.
Laye.-
s~i~
ID~ -
=
I 3~
0
Fig. 8. SIMS analysis: HgCdTe/GaAs.
Different alloy compositions were achieved, e.g. x 0.07, 0.1, 0.45, 0.8, 0.98, by varying the DMCd partial pressure around 5 X i0~ atm, which is very small in comparison with the DATe partial pressure (over 5 X i0~ atm). The mercury partial pressure always remained at 102 atm. For the band edge transmittance of the MCT layer, the composition mapping was achieved over 5 cm2 and the radial uniformity was 6%. The SIMS analysis was performed with a CAMECA IMS 4f and an oxygen (Ofl beam. it gave us two types of results: A depth profile where the mercury, the cadmium and the tellurium are very homogeneous. It is =
—
shown in fig. 8 for a (100) MCT layer grown directly on GaAs (100). Another one generally obtained for HgCdTe layers grown on the various substrates. As shown in fig. 9, some heterogeneities for cadmium and mercury, probably due to variations in the very small flow rate of DMCd, are visible. From the electrical properties it has been found that the layers were always N-type with typically n 2 >< 1017 cm3 and p. 6000 cm2/V. s at 300 K, n 1017 cm3 and p. 9000 cm2/V. ~ at 100 —
=
=
=
=
~06
~--
bsi~o~
lo5i~
Fig. 7. ECP of the same layer as in fig. 6.
Fig. 9. SIMS analysis: HgCdTe/CdZnTe.
F Desjonquères et at
/ Low temperature growth of (C4Hg)Te layers by MOVPE
K. These results are usually obtained for HgCdTe/CdTe/ZnTe/GaAs, x 0.35. The lack of purity of DATe, which is a complex molecule, difficult to purify, is probably the cause of these high values of carrier concentration and mobility. Indeed it shows that the purity level of these new precursors with low decomposition temperature is the main obstacle to be overcome in order to get device quality MCT layers. =
5. Conclusion In the case of the MOVPE growth of MCT layers at low temperatures, the structural quality of CdTe buffer layers grown on CdTe substrates has been studied as a function of substrate orientation. In this preliminary study, such surfaces as (111)B or (511)A have been found to give excellent surface morphologies. The growth of MCT layers has been achieved at a temperature as low as 250°C from DMCd, DATe and Hg. The main problem seems to be related to the poor purity of the complex DATe compound. The lack of reproducibility is probably due to a thermal instability of this compound [12]. Other more stable molecules are planned to be used to overcome this drawback, together with an
631
organomercury compound in order to benefit from an all-organometallic configuration.
References [1] L.M. Williams, P.Y. Lu, C.H. Wang, J.M. Parsey and S.N.G. Chu, Appl. Phys. Letters 51 (1987) 1738. [21P.Y. Lu, L.M. Williams, S.N.G. Chu and M.N. Ross, Appi. Phys. Letters 54 (1989) 2021. [3] J.B. Mullin, S.J.C. Irvine, J. Giess, J.S. Gough, A. Royle, M.C.L. Ward and G. Grimes, in: Proc. NATO Workshop, Liege, 1988, to be published. [4] L.S. Lichtmann, J.D. Parsons and E.H. Cirlin, J. Crystal Growth 86 (1988) 217. [5] S.K. Ghandhi, I.B. Bhat, H. Ehsani, D. Nucciarone and G. Miller, Appl. Phys. Letters 55 (1989) 137. [6] D.W. Kisker, M.L. Steigerwald, T.Y. Kometani and K.S. Jeffers, Appl. Phys. Letters 50 (1987) 1681. [7] W.E. Hoke and P.J. Lemonias, Appl. Phys. Letters 48 1669. [8] (1986) R. Korenstein, WE. Hoke, P.J. Lemonias, K.T. Higa and D.C. Haths, J. AppI. Phys. 62 (1987) 4929. [9] P.F. Fewster and P.A.C. Whiffin, J. AppI. Phys. 54 (1983) 4668. [10] P. Gibart, A. Tromson-Carli, Y. Monteil and A. Rudra, J. 50 (1989) C5—529. [11] Physique D.S. Buhaenko, S.M. Francis, P.A. Goulding and ME. Pemble, J. Vacuum Sci. Technol. B6 (1988) 1688. [12] J.E. Hails, S.J.C. Irvine and J.B. Mullin, Mater. Res. Soc. Meeting, Boston, 1989, to be published.
Journal of Crystal Growth 107 (1991) 626—631 North-Holland
Low temperature growth of (Cd,Hg)Te layers by MOVPE F. Desjonquères, A. Tromson-Carli, P. Cheuvart, R. Druilhe, C. Grattepain, A. Katty, Y. Marfaing, R. Triboulet Laboratoire de Physique des Solides de Bellevue, CNRS, 1 Place A. Brian4 F-92195 Meudon Cedex, France
and D. Lorans Société Anonyme de Télécom,nunications, 41 Rue Caniagrel, F-75624 Paris Cedex, France
MCT layers have been grown for the first time at 250°Cusing DATe, DMCd and mercury. First the buffer layer CdTe grown at 365°Cusing DIPTe and DMCd is studied with an emphasis on the influence of the substrate orientation. Indeed the surface morphology and the crystalline quality may change dramatically as a function of the substrate orientation. Then the low temperature MOVPE growth of (Cd,Hg)Te is described: different compositions were achieved and the crystalline and electronic properties are presented.
1. Introduction The presence of semimetals in the Il-VI family allows the preparation of original microstructures (heterostructures) such as type III superlattices which cannot be found in other semiconductor families. On the other hand, new IR detectors based on complex heterostructures have already been proposed with the mercury based materials, This leads to a new challenge for the MOVPE growth of (Cd,Hg)Te: the growth at low temperature in order to reduce the broadening of the interfaces, to lower the diffusion of impurities coming from the substrate and the density of mercury vacancies. Different techniques to this end have been studied such as plasma enhanced chemical deposition [1], precracking of precursors [2], photo-MOCVD using a mercury lamp or a laser [3]. All of them have different drawbacks. A very promising method seems to be the use of alternative precursors with a lower pyrolysis temperature, especially for tellurium. Dihydrotellurophene, dimethylditelluride, ditertiarybutyltelluride and methylallyltelluride have already given 0022-0248/91/$03.50 © 1991
—
some results [4,7] but we chose diallyltelluride because of its acceptable vapor pressure (P = 3 Torr at 45°C) and its low temperature of decomposition [8]. It is well known that not only the surface morphology of epitaxial layers, but also their crystalline and electronic quality depend on the substrate orientation and on the nature of an eventual buffer layer. That is why, before displaying results related to the low temperature growth of (Cd,Hg)Te layers, an experimental study of the surface morphology and structural perfection of CdTe buffer layers grown on CdZnTe and CdTe substrates as a function of the substrate orientation has been undertaken.
2. Experimental Our reactor, a Quantax 226 (MR Semicon), is horizontal, containing a heated Hg cell and an RF heated graphite susceptor. The carrier gas used thoughout the studies was hydrogen. The organometallics DIPTe, DMCd and DMZn were
Elsevier Science Publishers BY. (North-Holland)
F Desjonquères et at.
/ Low temperature growth of (Cd,Hg)Telayers by MOVPE
supplied by SMI. Using partial pressures of 10 ~ atm for a VI/Il ratio of one, buffer layers of 2 ~sm of CdTe and 0.1 ,sm of ZnTe were grown at 365°Conto GaAs (100) 2° (110) substrates. DATe, also supplied by SMI, DMCd and metallic mercury, maintained respectively at 55, 0 and 22°C,were used for the growth of (Cd,Hg)Te at 250°C (substrate temperature). The surface morphology and the quality of the epitaxy were checked by scanning electron microscopy (SEM), electron channeling patterns or pseudo-Kikuchi lines (ECP) and double-crystal X-ray diffraction. SIMS was used to determine homogeneity in depth. The electron microprobe and IR transmission allowed the composition of the MCT layers to be determined. Finally the electrically active carrier concentration, the resistivity and the mobility were measured at room temperature and at 77 K by the Van der Pauw technique. —
—,
627
lens. In the Bragg position the electron beam is diffracted through the crystal and the contrast intensity of backscattering electrons allows the emergence of Kikuchi patterns. These patterns are well known for silicon of diamond structure and are not very different for zinc-blende. An angular scale given by the apparatus allows the determination of angular pole positions and then of the orientation of grains. It is a fast and easy method which has been checked by calculation in the case of twinning bands. A twin is a 60° lattice rotation around a [111] direction. A transformation matrix applied to the coordinates of a grain surface allows an easy calculation of the corresponding coordinates of the twinned grain to be made. Table 1 shows some examples of corresponding orientations of surfaces in the twinning bands. 3.2. Evaluation of the layer morphology as a function of orientation
3. Buffer layer morphology versus substrate orientation 3.1. Determination of the orientation of grains Electron channeling patterns (ECPs), pseudoKikuchi lines, have been used to determine orientation and misorientation of the grams of CdZnTe substrates. A parallel electron beam is deflected over all the angular incidences by a deflecting
In the (111) orientation, pseudo-Kikuchi lines do not depend on Te or Cd faces, while the morphology and crystalline quality of the layer are strongly different on (111)B and (111)A faces and on their corresponding twinned orientations: (511)B and (511)A. Fig. 1 shows Kikuchi patterns of the substrate with two twinned grains of orientation (111) and (151). A common row of [022] type ([022] for (111) surface and [202] for (151)
Table 1 Different quality results as a function of the orientation of the substrate. On the right side the morphology quality is described as a function of the number of polycrystalline defects and the existence of pyramids and facets Calculated twin orientation
Usual orientations
(122) type
(100)
50—100
(110) type (411) type)
(110)
120—160
Many defects
((111)B
250—280 125—180 230—300 130—160
Without defects, but not smooth Few defects, smooth Very perturbed Smooth, without defects
(111) type
(511) type
)(511)B \ (111)A ~(511)A
X-ray rocking curve widths (arc see)
Surface quality
Good
628
F. Desjonquères et al.
/ Low temperature growth of (C4Hg)Te layers by
(111) —
—
~ 22}._~~
r
I
=
—
(151) .
,.
—
[202]
Fig. 1. ECP of the twinned grains (Ill) and (511) types.
surface) indicates the existence of a common plane of (110) type. The tellurium and cadmium faces were identified by etching (A for Cd and B for
MOVPE
Te). This etching method is in agreement with the crystallographic determination described by Fewster and Whiffin [9]. The (111)A surface results in many defects (fig. 2) while the associated (511)A twinning band shows Kikuchi patterns and a morphology of good quality. The opposite (511)B surface results in many hillocks and poor crystalline quality, although (111)B is better (fig. 3). However Kikuchi patterns indicate the existence of (111) twins with depth in six fold symmetry instead of three fold, see the [311] rows for example (fig. 3). These results about (111) faces are in agreement with those obtained by Gibart et al. [10] in the case of 111—V compounds. A (511) surface is built from steps of two atoms in the (100) plane and with a riser of one atom in
nfl (111) IA
(511)
Fig. 2. Upper: ECP of (511)A layer. Lower: layer morphologies of the (511)A and (111)A twinned grains.
..
-4
Fig. 3. Upper: Kikuchi patterns of (Ill )B layer, Lower: layer morphologies on the two twinned grains (111)B and (511)B.
F Desjonquères et a!.
/ Low temperature growth
of (c4Hg,~Telayers by MO VPE
629
Fig. 4. Surface morphologies of the layer grown on two twinned grains (100) on the left side and (122) on the right side,
Fig. 5. (110) observation surfaces of the twinned layer. The equivalent hillocks growth direction makes an angle of 70031~ induced by the twin rotation in a (111) common plane.
the (111) plane. This induces a 15.8° rotation of the (100) surface around the [0111direction. It can be considered as a high misorientation of the (100) surface, which may be an advantage. In a twin the (100) surface is coordinated with a (212) surface. Very different morphologies are obtained on these two surfaces (fig. 4). The (100) type has good crystallinity as shown by Kikuchi
good choice for substrate orientation producing the best quality epitaxial layers.
patterns and a narrow FWHM of X-ray rocking curve about 50 to 100 arc sec but a significant number of hillocks remain on top of the “pyramids”. The (122) type presents many defects and poor crystalline quality as shown by Kikuchi patterns. The (110) surface is coordinated with another (110) type of surface as inferred from (111)/(511) conjugate observations. The morphology and the crystalline quality are the same on the two twinned grains (fig. 5). In table 1 results of X-ray double diffraction values and morphology qualities are plotted. (511)A orientation seems to be a very
4. MCT layer epitaxial growth MCT layers were grown on various substrates (GaAs, Cd0 96Zn0~Te, CdTe) which were degreased in trichlorethylene, acetone and methanol. The GaAs substrates were etched in 5: 1 : 1 H2S04: H202: H20 and the CdTe substrates in Br—methanol. Then they were preheated at 365°C for 20 mm before the growth at 250°C.The pregrowth bake temperature seems to be sufficient to remove the carbon and oxygen contamination [11]. The growth rate of the alloy was 1 ~tm/h. The results are presented in table 2 and an example of the surface morphology and the ECP is given in fig. 6 and fig. 7. The layers deposited on a buffer layer grown at 365°C produced the best structural perfection.
Table 2 Crystallographic characterizations of different layers HgCdTe/GaAs(100)
Good surface morphology. Kikuchi lines (100); FWHM
650” to 1000”
HgCdTe/CdZnTe(100)2° —. (110)
Many pyramids, very few hillocks, nice Kikuchi lines (100); FWHM = 275”
HgCdTe/CdZnTe(111)
Triangular structures. Kikuchi lines (111); FWHM = 250” to 335”
HgCdTe/CdTe(100)
Large pyramids. Kikuchi lines (100); FWHM = 310” to 345”
HgCdTe/CdTe/ZnTe/GaAs(100)
Buffer layer grown at 365°C, MCT layer grown at 250°C, specular morphology; Kikuchi lines (100); FWHM = 250” to 275”
630
F Desjonquères eta!.
/
Low temperature growth of (Cd,Hg)Te layers by MOVPE Ice.
Fig. 6. Surface morphology of MCT/CdTe/ZnTe/GaAs: x 0.07.
Laye.-
s~i~
ID~ -
=
I 3~
0
Fig. 8. SIMS analysis: HgCdTe/GaAs.
Different alloy compositions were achieved, e.g. x 0.07, 0.1, 0.45, 0.8, 0.98, by varying the DMCd partial pressure around 5 X i0~ atm, which is very small in comparison with the DATe partial pressure (over 5 X i0~ atm). The mercury partial pressure always remained at 102 atm. For the band edge transmittance of the MCT layer, the composition mapping was achieved over 5 cm2 and the radial uniformity was 6%. The SIMS analysis was performed with a CAMECA IMS 4f and an oxygen (Ofl beam. it gave us two types of results: A depth profile where the mercury, the cadmium and the tellurium are very homogeneous. It is =
—
shown in fig. 8 for a (100) MCT layer grown directly on GaAs (100). Another one generally obtained for HgCdTe layers grown on the various substrates. As shown in fig. 9, some heterogeneities for cadmium and mercury, probably due to variations in the very small flow rate of DMCd, are visible. From the electrical properties it has been found that the layers were always N-type with typically n 2 >< 1017 cm3 and p. 6000 cm2/V. s at 300 K, n 1017 cm3 and p. 9000 cm2/V. ~ at 100 —
=
=
=
=
~06
~--
bsi~o~
lo5i~
Fig. 7. ECP of the same layer as in fig. 6.
Fig. 9. SIMS analysis: HgCdTe/CdZnTe.
F Desjonquères et at
/ Low temperature growth of (C4Hg)Te layers by MOVPE
K. These results are usually obtained for HgCdTe/CdTe/ZnTe/GaAs, x 0.35. The lack of purity of DATe, which is a complex molecule, difficult to purify, is probably the cause of these high values of carrier concentration and mobility. Indeed it shows that the purity level of these new precursors with low decomposition temperature is the main obstacle to be overcome in order to get device quality MCT layers. =
5. Conclusion In the case of the MOVPE growth of MCT layers at low temperatures, the structural quality of CdTe buffer layers grown on CdTe substrates has been studied as a function of substrate orientation. In this preliminary study, such surfaces as (111)B or (511)A have been found to give excellent surface morphologies. The growth of MCT layers has been achieved at a temperature as low as 250°C from DMCd, DATe and Hg. The main problem seems to be related to the poor purity of the complex DATe compound. The lack of reproducibility is probably due to a thermal instability of this compound [12]. Other more stable molecules are planned to be used to overcome this drawback, together with an
631
organomercury compound in order to benefit from an all-organometallic configuration.
References [1] L.M. Williams, P.Y. Lu, C.H. Wang, J.M. Parsey and S.N.G. Chu, Appl. Phys. Letters 51 (1987) 1738. [21P.Y. Lu, L.M. Williams, S.N.G. Chu and M.N. Ross, Appi. Phys. Letters 54 (1989) 2021. [3] J.B. Mullin, S.J.C. Irvine, J. Giess, J.S. Gough, A. Royle, M.C.L. Ward and G. Grimes, in: Proc. NATO Workshop, Liege, 1988, to be published. [4] L.S. Lichtmann, J.D. Parsons and E.H. Cirlin, J. Crystal Growth 86 (1988) 217. [5] S.K. Ghandhi, I.B. Bhat, H. Ehsani, D. Nucciarone and G. Miller, Appl. Phys. Letters 55 (1989) 137. [6] D.W. Kisker, M.L. Steigerwald, T.Y. Kometani and K.S. Jeffers, Appl. Phys. Letters 50 (1987) 1681. [7] W.E. Hoke and P.J. Lemonias, Appl. Phys. Letters 48 1669. [8] (1986) R. Korenstein, WE. Hoke, P.J. Lemonias, K.T. Higa and D.C. Haths, J. AppI. Phys. 62 (1987) 4929. [9] P.F. Fewster and P.A.C. Whiffin, J. AppI. Phys. 54 (1983) 4668. [10] P. Gibart, A. Tromson-Carli, Y. Monteil and A. Rudra, J. 50 (1989) C5—529. [11] Physique D.S. Buhaenko, S.M. Francis, P.A. Goulding and ME. Pemble, J. Vacuum Sci. Technol. B6 (1988) 1688. [12] J.E. Hails, S.J.C. Irvine and J.B. Mullin, Mater. Res. Soc. Meeting, Boston, 1989, to be published.