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n-type doping of MCT layers grown by MOVPE (IMP)
Volume
10, number
iMATERIALS
9, IO
February
LETTERS
199
I
n-type doping of MCT layers grown by MOVPE (IMP) J.S. Gough,
M.R. Houlton,
Royal Signals and Radar Establishment.
Received
20 August
S.J.C.
Irvine,
St. Andrew.!
N. Shaw, Road, Malvern,
1990: in final form 13 November
M.L. Young Ubrcestershve,
and A. Royle UK
1990
Mercury cadmium telluride layers were grown at 350°C by metal organic vapour phase epitaxy (MOVPE) using the interdiffused multtlayer process (IMP) and doped with indium or aluminium. Aluminium doping yielded a low electrical activity and displayed large concentration oscillations with the IMP period. This has been attributed to the formation of oxide and carbide inclusions. Indium doping does not display this oscillatory behaviour and the electrical activity was close to 100% after lowtemperature annealing at concentrations up to 3 X 10” cm -’
1. Introduction The ability to control the electrical conductivity of the narrow band gap semiconductor (Hg, Cd)Te by impurity doping is an important step for the fabrication of heterojunction infrared detectors. The growth of such structures is not easy because of the high metal vacancy concentrations [ 1 ] which makes the layers p-type and high diffusion rates of some of the potential dopant elements [3]. Both these difficulties can be overcome by growing the structure at a sufficiently low temperature. Metal organic vapour phase epitaxy (MOVPE) has been developed in the past few years as a method for growing these more complex device structures [ 3 1. The interdiffused multilayer process (IMP) has been used as a means of improving lateral alloy uniformity without any compromise in the control of the heterojunction interface. IMP has been described in detail elsewhere 14Sl. n-type doping of CdTe has been reported by Taskar et al. [6] using indium from triethyl indium (TEIn), reporting electron concentrations from 6~ lOI to 1 x 10” cme3. p-n junctions were fabricated in CdTe using arsine to dope the p-type side of the junction. Indium doping of CdTe grown by molecular beam epitaxy (MBE) has required laser illumination to achieve activation of the dopant. Doped n-isotype heterojunctions in MCT have been reported by Boukerche et al. [7] who used an in0167-577x/91/$
03.50 0 1991 - Elsevier Science
Publishers
B.V.
dium effusion source. However, problems with indium memory in the growth chamber from run to run have also been reported [ 8 1. Doping of MOVPE grown MCT has been reported by Whiteley et al. [9] who used trimethyl indium (TMIn) as a dopant source introduced by effusion and by a carrier gas (bubbler). The MCT layers were grown using the IMP variant, at 410°C from the precursors diethyl telluride (DETe) and dimethyl cadmium (DMCd). Using the TMIn source as a bubbler the MCT layers were doped at donor concentrations in excess of 4x 10” crnp3 and also increased the cadmium content (x) from 0.193 to as high as 0.422. In the effuser mode no change in composition was noted and the range of donor concentrations was extended down to 2~ IOr cm-3. In this Letter both aluminium and indium are investigated as n-type dopants using the bubbler mode in order to achieve greater control of the dopant supply, and at the lower growth temperature of 350°C in order to reduce unwanted dopant diffusion.
2. Experimental The layers grown for this study were produced using the precursors diisopropyl telluride ( DIPTe), dimethyl cadmium (DMCd) and elemental Hg with a growth temperature of 350°C using the interdiffused multilayer process (IMP) described elsewhere (North-Holland
t
393
Volume 10, number 9,lO
MATERIALS LETTERS
type MCT
February 199 1
15 urn were grown using the dopant TEA1 and this alkyl was introduced at different stages of the IMP cycle - CdTe + HgTe, CdTe only, HgTe only, at the rate of 25 cm3/min. Eight layers of between 10 and 20 urn were grown using the dopant TMIn introduced during the CdTe phase of the IMP cycles. Flow rates were investigated in the range of 5-100 cm3/min and for one layer an attempt was made to grow the doped heterostructure shown in fig. 1. The layers were assessed for chemical incorporation using the technique of secondary ion mass spectrometry (SIMS) and electrical activity by variable temperature Hall measurements and at 77 K with variable magnetic field.
x = 0.24 10 pm
SUBSTRATE
Fig. 1. The proposed indium-doped heterostructure.
3. Results and discussion [ 4,5,10]. The dopant precursors were triethyl aluminium (TEAl) and trimethyl indium (TMIn) and both sources were cooled to 2 OC in temperature controlled baths. A series of 11 layers of between 10 and
3. I. Aluminium doping SIMS depth profiles showed the Al concentration to be non-uniform. A typical result is shown in fig.
10
5 DEPTH
15
Cmlsronm)
Fig. 2. Aluminium concentration variations with IMP cycles in aluminium-doped CMT.
394
Volume 10, number 9,lO
MATERIALS LETTERS
2, where the Al concentration is varying as a series of well-de~ned spikes between levels of 2.0 x 1Oi7 and 1.5~ lOi atoms cmM3. The number of spikes corresponds to the number of IMP cycles used to grow the layer. For this sample, one IMP period gives a thickness of 0.26 urn comprised of 0.21 urn HgTe +0.05 nm CdTe and it can therefore be deduced from this that the Al spikes are coincident with the CdTe parts of the IMP cycles. The Hg profile shows that the IMP layers have interdiffused and resulted in uniform MCT. This makes the behaviour of the Al difficult to understand as it is expected to be substitutional on the group II sublattice (CdTe) and yet has stayed pinned there whilst the CdTe has interdiffused with the HgTe layers. Subsequent Hall effect measurement of this layer gave a p-type carrier concentration of 7 x lOI cme3 which would be expected for an as-grown, undoped MCT layer and hence the Al appears to be inactive. In an attempt to stimulate electrical activity and promote uniform distribution of Al by diffusion, while tilling the Hg vacancies, a piece of the same
February I99 1
sample was subjected to a 48 h anneal in Hg vapour at 250°C. Annealed, undoped CMT would be expected to be n-type due to residual donors with a net concentration between 1 and 5.0~ 10” cmp3. For the doped sample an electron concentration of 5.5 x 1Ot6 cme3 was measured after the anneal suggesting that some of the dopant had become active. A SIMS profile of the annealed sample showed that the Al had not interdiffused at this temperature. A further sample from this layer was annealed in Hg vapour at 500°C for 4 h to hasten the diffusion followed by a low-temperature anneal also in Hg vapour at 230°C for 96 h to reduce the high equilibrium level of Hg vacancies arising from the hightemperature stage. Fig. 3 shows the SIMS profile of this sample where the Al has diffused uniformly throughout the top 2.5 urn of the layer. Given the annealling conditions used we assume this result to be representative of the whole layer. The carrier concentration, although still n-type. had fallen to 3.8 x 10” cmm3 -a value to be expected from nominally undoped material. We suggest that
-‘-I--....-._c,..._“.“.,““_
-“-“^-.‘..~.-.i~.-...“_._~~,__.“_._”.,~..~..~
.._
_.._._“.u-“C..,._
m e,.,.
~~_-._-..,‘_-.‘-^-..-,~,‘...‘~~~’”.”.’,_~..’,.-._._.__,
Cadmium
*
lo-
DEPTH
Fig. 3. Depth profile of aluminium~o~d
(micronas)
CMT after an anneal for 4 h at 500°C and 96 h at 23O’C.
Volume 10, number 9,10
MATERIALS LETTERS
the large number of Hg vacancies ( 10” cmm3) created during the 500°C anneal have complexed with the Al and rendered it inactive in a similar manner to the behaviour of Cd vacancies in Al-doped CdTe
[Ill. Even in samples where the Al showed some activity the levels were very low L mpared to the concentrations of Al found in the L,~=rs. In an attempt to discover how the Al was incorporated the sample was profiled by SIMS for oxygen and carbon. The result of this profile in fig. 4 shows oxygen and carbon spikes which are coincident with the Al spikes and this would suggest that most of the Al is incorporated throughout the layer in the form of aluminium oxide or aluminium carbide inclusions. 3.2. Indium doping SIMS depth profiles of In-doped samples did not show any of the dopant spikes seen previously in the Al-doped samples. Fig. 5 shows the In profile for a MCT layer doped with a TMIn flow of 5 cm3 min- ‘.
The level of In in this sample was 3 x lOI atoms cmm3 and n-type conduction was not therefore expected. Subsequent Hall measurement gave a p-type result, as predicted, with a level of 5x 1016cme3. In an attempt to increase the In concentration in the MCT layer, subsequent samples were grown with increased In flows. Fig. 6 shows a sample grown with an In flow of 50 cm3 min-’ where 7 urn of the 11 urn thick layer had in excess of 1x 10” atoms cmW3 of In. Hall measurement of this layer gave our first n-type result with a carrier concentration of 2.9 x 1OL6 cme3. This sample was then annealed at 250°C for 50 h in Hg vapour to fill the Hg vacancies and allow the In activity to be assessed. The post-anneal carrier concentration was measured as 3 x 10” cmp3 which showed good agreement with the surface SIMS level in fig. 6. The increasing In concentration with layer thickness is a feature of In doping and indicates considerable adsorption on the TMIn feed pipes and reactor manifold. This effect is being investigated further to produce flatter profiles. To demonstrate the versatility of the MOVPE
1oi9i
Fig. 4. Oxygen, carbon and aluminium concentration variations with depth in an as-grown sample. 396
February 199 1
Volume 10, number 9,lO
MATERIALS LETTERS
February 1991
1Oi .J
10’
cadmium
-n-
t.i.rcut-y
-c
:i0=
~-----~------~~-------I
10
I
_104
10
-IO3
10
:10* \
<-
Indlum
Fig. 5. Depth profile of low-level indium-doped CMT. i0’St
410'
110’
2
4
6 DEPTH
a
i0
12
14
Cmicron8)
Fig. 6. Depth profile of high-levei indium~o~d
CMT. 397
Volume 10, number 9,lO
MATERIALS LETTERS
February 199 I
Fig. 7. SIMS depth profile of the heterostructure with the intended structure overlayed.
technique a doped heterost~cture, of the type that may be of interest for device researchers, was attempted. The intended structure is shown in fig, 1 and fig. 7 is the structure shown in fig. 1 overlayed onto the SIMS profile obtained from the layer. The intention was to grow 7 urn of x=0.30, p-type (undoped) foilowed by 10 urn of x=0.24, p-type (undoped) with a final layer of 5 pm ofx=0.30, n-type (In doped). The Hg profile shows that the intended thicknesses were achieved and a sharp interface exists between the doped and undoped sections of the sample.
is reproducibly grown by this technique and we have now shown that n-type doping is possible. Although Al looked attractive it does not incorporate as an electrically active dopant. In shows none of the problems associated with the Al and at levels > 1 X 10” atoms cm-3 overcompensates for the vacancies and produces n-type material.
Acknowledgement
The authors are grateful for the substrate preparation and growth-run assistance from Miss Deborah Jones.
4. Conclusions Impurity-doped MCT will be required for future generation infrared detectors and MOVPE has the potential to fulfil this requirement. p-type materiai 398
References [ I] H.R. Vydyanath, J. Electrochem. Sot. Solid State Sci. Technol. 128 f 1981) 2619.
Volume IO,number 9,10
MATERIALS LETTERS
[2] M. Brown and A.F.W. Willoughby, J. Crystal Growth 59 (1982) 27. [ 31 S.J.C. Irvine, J. Giess, J.S. Go&, G.W. Blackmore, A. Royle, J.B. Mullin, N.G. Chew and A.G. Cullis, J. Crystal Growth 77 (1986) 437. [4] S.J.C. Irvine, J. Tunnicliffe and J.B. Mullin, Mater. Letters 2 (1984) 305. [5] J. Tunnicliffe, S.J.C. Irvine, O.D. Dosser and J.B. Mullin, J. Crystal Growth 68 (1984) 245. [6] N.R. Taskar, V. Natarajan, I.B. Bhat and SK. Ghandski, J. Crystal Growth 86 (1988) 228.
February I99 1
[ 71 M. Boukerche, S. Yoo, I.K. Sou, M. De Souza and J.P. Faurie, J. Vacuum Sci. Technol. A 6 (1988) 2623. [ 81 M. Boukerche, P.S. Wijewarhasuriya, S. Sivananthan, I.K. Sou, Y.K. Kim, K.K. Mahavadi and J.P. Faurie, J. Vacuum Sci. Technol. A 5 ( 1988) 2830. [9] J.S. Whiteley, P. Koppel, V.L. Conger and K.E. Owens, J. Vacuum Sci. Technol. A 6 (1988) 2804. [lo] J. Thompson, P. Mackett, L.M. Smith, D.J. Cole-Hamilton and D.V. Shenai-Khatkhate, J. Crystal Growth 86 ( 1988) 233. [ 111 N. Shaw, private communication.
399
10, number
iMATERIALS
9, IO
February
LETTERS
199
I
n-type doping of MCT layers grown by MOVPE (IMP) J.S. Gough,
M.R. Houlton,
Royal Signals and Radar Establishment.
Received
20 August
S.J.C.
Irvine,
St. Andrew.!
N. Shaw, Road, Malvern,
1990: in final form 13 November
M.L. Young Ubrcestershve,
and A. Royle UK
1990
Mercury cadmium telluride layers were grown at 350°C by metal organic vapour phase epitaxy (MOVPE) using the interdiffused multtlayer process (IMP) and doped with indium or aluminium. Aluminium doping yielded a low electrical activity and displayed large concentration oscillations with the IMP period. This has been attributed to the formation of oxide and carbide inclusions. Indium doping does not display this oscillatory behaviour and the electrical activity was close to 100% after lowtemperature annealing at concentrations up to 3 X 10” cm -’
1. Introduction The ability to control the electrical conductivity of the narrow band gap semiconductor (Hg, Cd)Te by impurity doping is an important step for the fabrication of heterojunction infrared detectors. The growth of such structures is not easy because of the high metal vacancy concentrations [ 1 ] which makes the layers p-type and high diffusion rates of some of the potential dopant elements [3]. Both these difficulties can be overcome by growing the structure at a sufficiently low temperature. Metal organic vapour phase epitaxy (MOVPE) has been developed in the past few years as a method for growing these more complex device structures [ 3 1. The interdiffused multilayer process (IMP) has been used as a means of improving lateral alloy uniformity without any compromise in the control of the heterojunction interface. IMP has been described in detail elsewhere 14Sl. n-type doping of CdTe has been reported by Taskar et al. [6] using indium from triethyl indium (TEIn), reporting electron concentrations from 6~ lOI to 1 x 10” cme3. p-n junctions were fabricated in CdTe using arsine to dope the p-type side of the junction. Indium doping of CdTe grown by molecular beam epitaxy (MBE) has required laser illumination to achieve activation of the dopant. Doped n-isotype heterojunctions in MCT have been reported by Boukerche et al. [7] who used an in0167-577x/91/$
03.50 0 1991 - Elsevier Science
Publishers
B.V.
dium effusion source. However, problems with indium memory in the growth chamber from run to run have also been reported [ 8 1. Doping of MOVPE grown MCT has been reported by Whiteley et al. [9] who used trimethyl indium (TMIn) as a dopant source introduced by effusion and by a carrier gas (bubbler). The MCT layers were grown using the IMP variant, at 410°C from the precursors diethyl telluride (DETe) and dimethyl cadmium (DMCd). Using the TMIn source as a bubbler the MCT layers were doped at donor concentrations in excess of 4x 10” crnp3 and also increased the cadmium content (x) from 0.193 to as high as 0.422. In the effuser mode no change in composition was noted and the range of donor concentrations was extended down to 2~ IOr cm-3. In this Letter both aluminium and indium are investigated as n-type dopants using the bubbler mode in order to achieve greater control of the dopant supply, and at the lower growth temperature of 350°C in order to reduce unwanted dopant diffusion.
2. Experimental The layers grown for this study were produced using the precursors diisopropyl telluride ( DIPTe), dimethyl cadmium (DMCd) and elemental Hg with a growth temperature of 350°C using the interdiffused multilayer process (IMP) described elsewhere (North-Holland
t
393
Volume 10, number 9,lO
MATERIALS LETTERS
type MCT
February 199 1
15 urn were grown using the dopant TEA1 and this alkyl was introduced at different stages of the IMP cycle - CdTe + HgTe, CdTe only, HgTe only, at the rate of 25 cm3/min. Eight layers of between 10 and 20 urn were grown using the dopant TMIn introduced during the CdTe phase of the IMP cycles. Flow rates were investigated in the range of 5-100 cm3/min and for one layer an attempt was made to grow the doped heterostructure shown in fig. 1. The layers were assessed for chemical incorporation using the technique of secondary ion mass spectrometry (SIMS) and electrical activity by variable temperature Hall measurements and at 77 K with variable magnetic field.
x = 0.24 10 pm
SUBSTRATE
Fig. 1. The proposed indium-doped heterostructure.
3. Results and discussion [ 4,5,10]. The dopant precursors were triethyl aluminium (TEAl) and trimethyl indium (TMIn) and both sources were cooled to 2 OC in temperature controlled baths. A series of 11 layers of between 10 and
3. I. Aluminium doping SIMS depth profiles showed the Al concentration to be non-uniform. A typical result is shown in fig.
10
5 DEPTH
15
Cmlsronm)
Fig. 2. Aluminium concentration variations with IMP cycles in aluminium-doped CMT.
394
Volume 10, number 9,lO
MATERIALS LETTERS
2, where the Al concentration is varying as a series of well-de~ned spikes between levels of 2.0 x 1Oi7 and 1.5~ lOi atoms cmM3. The number of spikes corresponds to the number of IMP cycles used to grow the layer. For this sample, one IMP period gives a thickness of 0.26 urn comprised of 0.21 urn HgTe +0.05 nm CdTe and it can therefore be deduced from this that the Al spikes are coincident with the CdTe parts of the IMP cycles. The Hg profile shows that the IMP layers have interdiffused and resulted in uniform MCT. This makes the behaviour of the Al difficult to understand as it is expected to be substitutional on the group II sublattice (CdTe) and yet has stayed pinned there whilst the CdTe has interdiffused with the HgTe layers. Subsequent Hall effect measurement of this layer gave a p-type carrier concentration of 7 x lOI cme3 which would be expected for an as-grown, undoped MCT layer and hence the Al appears to be inactive. In an attempt to stimulate electrical activity and promote uniform distribution of Al by diffusion, while tilling the Hg vacancies, a piece of the same
February I99 1
sample was subjected to a 48 h anneal in Hg vapour at 250°C. Annealed, undoped CMT would be expected to be n-type due to residual donors with a net concentration between 1 and 5.0~ 10” cmp3. For the doped sample an electron concentration of 5.5 x 1Ot6 cme3 was measured after the anneal suggesting that some of the dopant had become active. A SIMS profile of the annealed sample showed that the Al had not interdiffused at this temperature. A further sample from this layer was annealed in Hg vapour at 500°C for 4 h to hasten the diffusion followed by a low-temperature anneal also in Hg vapour at 230°C for 96 h to reduce the high equilibrium level of Hg vacancies arising from the hightemperature stage. Fig. 3 shows the SIMS profile of this sample where the Al has diffused uniformly throughout the top 2.5 urn of the layer. Given the annealling conditions used we assume this result to be representative of the whole layer. The carrier concentration, although still n-type. had fallen to 3.8 x 10” cmm3 -a value to be expected from nominally undoped material. We suggest that
-‘-I--....-._c,..._“.“.,““_
-“-“^-.‘..~.-.i~.-...“_._~~,__.“_._”.,~..~..~
.._
_.._._“.u-“C..,._
m e,.,.
~~_-._-..,‘_-.‘-^-..-,~,‘...‘~~~’”.”.’,_~..’,.-._._.__,
Cadmium
*
lo-
DEPTH
Fig. 3. Depth profile of aluminium~o~d
(micronas)
CMT after an anneal for 4 h at 500°C and 96 h at 23O’C.
Volume 10, number 9,10
MATERIALS LETTERS
the large number of Hg vacancies ( 10” cmm3) created during the 500°C anneal have complexed with the Al and rendered it inactive in a similar manner to the behaviour of Cd vacancies in Al-doped CdTe
[Ill. Even in samples where the Al showed some activity the levels were very low L mpared to the concentrations of Al found in the L,~=rs. In an attempt to discover how the Al was incorporated the sample was profiled by SIMS for oxygen and carbon. The result of this profile in fig. 4 shows oxygen and carbon spikes which are coincident with the Al spikes and this would suggest that most of the Al is incorporated throughout the layer in the form of aluminium oxide or aluminium carbide inclusions. 3.2. Indium doping SIMS depth profiles of In-doped samples did not show any of the dopant spikes seen previously in the Al-doped samples. Fig. 5 shows the In profile for a MCT layer doped with a TMIn flow of 5 cm3 min- ‘.
The level of In in this sample was 3 x lOI atoms cmm3 and n-type conduction was not therefore expected. Subsequent Hall measurement gave a p-type result, as predicted, with a level of 5x 1016cme3. In an attempt to increase the In concentration in the MCT layer, subsequent samples were grown with increased In flows. Fig. 6 shows a sample grown with an In flow of 50 cm3 min-’ where 7 urn of the 11 urn thick layer had in excess of 1x 10” atoms cmW3 of In. Hall measurement of this layer gave our first n-type result with a carrier concentration of 2.9 x 1OL6 cme3. This sample was then annealed at 250°C for 50 h in Hg vapour to fill the Hg vacancies and allow the In activity to be assessed. The post-anneal carrier concentration was measured as 3 x 10” cmp3 which showed good agreement with the surface SIMS level in fig. 6. The increasing In concentration with layer thickness is a feature of In doping and indicates considerable adsorption on the TMIn feed pipes and reactor manifold. This effect is being investigated further to produce flatter profiles. To demonstrate the versatility of the MOVPE
1oi9i
Fig. 4. Oxygen, carbon and aluminium concentration variations with depth in an as-grown sample. 396
February 199 1
Volume 10, number 9,lO
MATERIALS LETTERS
February 1991
1Oi .J
10’
cadmium
-n-
t.i.rcut-y
-c
:i0=
~-----~------~~-------I
10
I
_104
10
-IO3
10
:10* \
<-
Indlum
Fig. 5. Depth profile of low-level indium-doped CMT. i0’St
410'
110’
2
4
6 DEPTH
a
i0
12
14
Cmicron8)
Fig. 6. Depth profile of high-levei indium~o~d
CMT. 397
Volume 10, number 9,lO
MATERIALS LETTERS
February 199 I
Fig. 7. SIMS depth profile of the heterostructure with the intended structure overlayed.
technique a doped heterost~cture, of the type that may be of interest for device researchers, was attempted. The intended structure is shown in fig, 1 and fig. 7 is the structure shown in fig. 1 overlayed onto the SIMS profile obtained from the layer. The intention was to grow 7 urn of x=0.30, p-type (undoped) foilowed by 10 urn of x=0.24, p-type (undoped) with a final layer of 5 pm ofx=0.30, n-type (In doped). The Hg profile shows that the intended thicknesses were achieved and a sharp interface exists between the doped and undoped sections of the sample.
is reproducibly grown by this technique and we have now shown that n-type doping is possible. Although Al looked attractive it does not incorporate as an electrically active dopant. In shows none of the problems associated with the Al and at levels > 1 X 10” atoms cm-3 overcompensates for the vacancies and produces n-type material.
Acknowledgement
The authors are grateful for the substrate preparation and growth-run assistance from Miss Deborah Jones.
4. Conclusions Impurity-doped MCT will be required for future generation infrared detectors and MOVPE has the potential to fulfil this requirement. p-type materiai 398
References [ I] H.R. Vydyanath, J. Electrochem. Sot. Solid State Sci. Technol. 128 f 1981) 2619.
Volume IO,number 9,10
MATERIALS LETTERS
[2] M. Brown and A.F.W. Willoughby, J. Crystal Growth 59 (1982) 27. [ 31 S.J.C. Irvine, J. Giess, J.S. Go&, G.W. Blackmore, A. Royle, J.B. Mullin, N.G. Chew and A.G. Cullis, J. Crystal Growth 77 (1986) 437. [4] S.J.C. Irvine, J. Tunnicliffe and J.B. Mullin, Mater. Letters 2 (1984) 305. [5] J. Tunnicliffe, S.J.C. Irvine, O.D. Dosser and J.B. Mullin, J. Crystal Growth 68 (1984) 245. [6] N.R. Taskar, V. Natarajan, I.B. Bhat and SK. Ghandski, J. Crystal Growth 86 (1988) 228.
February I99 1
[ 71 M. Boukerche, S. Yoo, I.K. Sou, M. De Souza and J.P. Faurie, J. Vacuum Sci. Technol. A 6 (1988) 2623. [ 81 M. Boukerche, P.S. Wijewarhasuriya, S. Sivananthan, I.K. Sou, Y.K. Kim, K.K. Mahavadi and J.P. Faurie, J. Vacuum Sci. Technol. A 5 ( 1988) 2830. [9] J.S. Whiteley, P. Koppel, V.L. Conger and K.E. Owens, J. Vacuum Sci. Technol. A 6 (1988) 2804. [lo] J. Thompson, P. Mackett, L.M. Smith, D.J. Cole-Hamilton and D.V. Shenai-Khatkhate, J. Crystal Growth 86 ( 1988) 233. [ 111 N. Shaw, private communication.
399