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Group V acceptor doping of CdxHg1−xTe layers grown by metal-organic vapour phase epitaxy
June 1988
MATERIALS LETTERS
Volume 6. number 10
GROUP V ACCEPTOR DOPING
OF Cd,Hg,_,Te
GROWN
VAPOUR PHASE EPITAXY
BY METAL-ORGANIC
P. CAPPER,
P.A.C. WHIFFIN,
Philips Research Lahoraiories.
B.C. EASTON,
LAYERS
C.D. MAXEY
Redhill, Surrey. UK
and
I. KENWORTHY .Mullard Southampton.
Millbrook
Industrial Estate, Southampton,
Han&
C’K
Received 7 March 1988: in final form 12 April 1988
Layers of Cd,Hg, _ ,Te have been grown by metal-organic vapour phase epitaxy and doped with arsenic and with phosphorus. This has been achieved by establishing metal-rich gas phase conditions during growth and obviates the need for high temperature “activation” type anneals. Data from Hall measurements and secondary ion mass spectrometry on annealed samples are in close agreement
In order to produce stable p-n junctions for photovoltaic devices made from Cd,Hg,_,Te it is necessary to use intentional doping to fix the carrier concentrations on either side of the junction. For the n-type side In has been used frequently as the donor impurity in LPE (liquid phase epitaxy) and MBE (molecular beam epitaxy ) layers [ l-4 1. The p-type side usually uses a group V element as group I acceptors tend to have higher diffusivities [ l-31. As the group V element needs to occupy a Te site to achieve acceptor activity the stoichiometry at the growth temperature plays a crucial role. In Bridgman growth P, As and Sb were found to be active acceptors [ 5,6 1. A similar picture emerged in Hg-rich LPE material [ 7 1. However for Te-rich LPE layers it has been found necessary to use an “activation” anneal whether the dopant was introduced into the layer via the melt [ 81 or by implantation after growth [ 31. This “activation” anneal consisted of heating samples in Hg vapour at 500°C [ 81 or 400 “C [ 3 ] and was postulated in ref. [ 8 ] to cause transfer of the group V elements from metal sites, where donor activity might be expected, to Te sites, where the dopants will be acceptors. We report here the first MOVPE (metal-organic 0167-577x/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
vapour phase epitaxy) layers doped with group V acceptors. No “activation” anneals are necessary as growth takes place from a “metal-rich” gas phase. Diethyltelluride (DET) and dimethlylcadium (DMC) were used as the Te and Cd sources, respectively, while elemental Hg provided the required Hg vapour pressure. Arsine (0.1% in hydrogen) and phosphine (0.1% in hydrogen) were used for the sources of As and P, respectively. These were introduced, via an injection tube, downstream of the Hg reservoir at ~25 scc/min. Growth took place on a resistively-heated graphite susceptor held at z 4 10” C, with the Hg at z 240°C. Details of the apparatus and IMP (interdiffused multilayer process) growth process were given in ref. [ 91. Both ( 111) and 2” off ( 100) CdTe substrates were used. Following growth layers were cleaved into squares ( z 5 x 5 mm) for Hall measurement and annealing. Samples were etched in dilute Br?/CH,OH and washed in CH30H and H20 prior to contacting with In solder. A field of 3.2 kG was used and all data are quoted at 77 K. Preliminary experiments showed that when the binaries alone were grown As was not present in HgTe (i.e. < 1 x lOI cmp3) but was incorporated into B.V.
365
Volume 6, number 10
MATERIALS LETTERS
CdTe, agreeing with Ghandhi et al. [ lo] although no direct chemical analysis for As was given in ref. [ 10 ] . The growth rate of CdTe was unaffected but the presence of As increased the HgTe growth rate by a factor of 2. This resulted in a decrease in the expected x value when Cd,Hg, _,Te layers were grown, although this could be easily adjusted by modifying the IMP ratio. Analysis by secondary ion mass spectrometry (SIMS) also showed that As did not diffuse rapidly, during the growth time, when it was introduced halfway through a Cd,Hg,_,Te layer. Dopant flows were maintained constant in both the HgTe and CdTe cycles during the IMP growth of this layer. An As level of x I x 10” crnT3 was found in that portion of the layer grown with the arsine flowing, decreasing to < 3 X lOI cms3 (SIMS .detection limit) for the undoped section of the layer. This decrease occurred over a distance of ~0.5 pm which is at the limit of the spatial resolution of the SIMS analysis in this particular case. In order to produce acceptor activity it was found necessary to reduce the Te alkyl concentration from that used for undoped MOVPE layers by z 30%. This corresponded to a change in the Te: metal ratio of 1.3 : 1 to 0.9: 1. Under these “metal-rich” gas phase conditions three As-doped and one P-doped layers were grown. Fig. 1 shows the results of a standard low temperature ( 200 ’ C ) anneal used to remove Hg vacancies. All the doped samples (x=0.26-0.3) remain virtually unchanged from their as-grown p-type levels whereas undoped controls of similar x values, annealed in the same run, converted to n-type. Both MOVPE and LPE controls were used and the fact that one of the LPE layers produced a very low ( z 2 x 1OL4cmp3) n-type level suggests that the anneal was not introducing significant quantities of other impu~ties. Hall measurements made down to 20 K on these anneaied doped samples showed low activation energies ( fi:6-8 meV), indicative of impurity doping [ 6 1. Anneals carried out at 400°C in Hg, followed by heating at 200°C in Hg, were also carried out on the three As-doped layers. Once again the doped samples remained p-type while the undoped controls converted to n-type. There were indications that the high temperature anneal may have activated more As but this is not proven. Fig. 2 shows typical SIMS profiles for As in both 366
June 1988
o-o
As-doped
o-a
P -
v--o
MOVPE
C.-A
LPE
doped
Metal undoped
undoped
- Rtch control
control
k k s
\
10’5
\
Time
(h)
Fig. 1. Carrier concentrations (77 K) for doped and undoped (control) layers before and after 200°C anneal in Hg.
as-grown and annealed samples from the same layer with an x value of 0.29. Clearly the As does not migrate during a 200°C anneal; the two levels agree within the factor of 2 uncertainty of the SIMS analysis. The carrier concentration decreases slightly after annealing, reflecting the removal of Hg vacancies in this layer with ~~0.26. The Hall carrier concentration agrees well with the As level in the annealed sample. Survey analysis by laser scan mass spectrometry [ 111 of undoped MOVPE layers shows no other potential acceptors at the 10” cmW3level and so we believe the annealed p-type level is due solely to the presence of As. Fig. 3 shows the corresponding plots for the Pdoped layer, x value ~0.26. The SIMS profiles are more uneven as the level is close to the SIMS detection limit ( lOI atoms cm-3) and the interface “spike” in the as-grown sample is omitted for clarity. Once again the annealed carrier concentration agrees with the P level demonstrating that P is also an active acceptor when “metal-rich” conditions are used,
Volume 6. number
IO
MATERIALS
PVC 10’8 __-/--
397
LETTERS
(As-grown)
June 1988
PVC
__..._-c- .---_--.-r__T
397
( Annealed)
‘“‘“F ‘\
Hg
‘B I 1 0
I I
,o161'
0 Depth Fig. 2. SIMS profiles
,D’6 1 LI-_.l.ti_ 0 5 Depth (mcron)
5 (micron)
for As and carrier concentrations
although the lower concentration incroporated suggests As is more efficiently incorporated. In order to confirm that reducing the Te alkyl concentration was the key factor in achieving activation of dopants a layer was grown using the higher “undoped” Te alkyl flow, with arsine. SIMS confirmed the presence of .4s in the layer but on annealing at
PVC 401
(AS -grown
in “metal-rich”
1
layer before and after 200°C anneal in Hg.
200°C the layer, which had an x value of 0.23, converted to n-type at z 1 x lOI cm-‘; demonstrating the inactivity of As in this case. A further As-doped layer was grown, again under “metal-rich” conditions, at an x value of ~0.2. We have previously shown that low x undoped material is easier to convert to n-type, i.e. remove Hg vacan-
)
PVC
401
( Annealed)
-Hall cmm3
,015;’
’
’
Depth
’
’
5
’
’
’
’
’
10
,0'5[-
(micron)
Fig. 3. SIMS profiles for P and carrier concentrations
5 Depth
in “metal-rich”
10
(mIcronI
layer before and after 200°C anneal in Hg.
367
Volume 6, number 10
MATERIALS LETTERS
ties, than is higher x material [ 12 1. The layer had an as-grown p-type level (77 K) of z 3 X lOi crnm3 while SIMS analysis revealed an As content of z 1.5~ 10” cmp3. The difference is due to the Hg vacancy concentration in material of x=02 [ 121. On annealing, the layer produced a p-type carrier concentration of w 1 x 10” cmm3, again agreeing within the SIMS uncertainty factor with the As content. In conclusion, we have demonstrated that “metalrich” gas phase conditions during MOVPE growth of Cd,Hg, _XTe can lead to activation of both P and As in x=0.2-0.3 material. This activation is achieved without recourse to high temperature anneals and opens up possibilities for heterojunction layer growth. The low As diffusivity and lack of any auto-doping from the Hg (due to the use of the injection tube for the dopant) make the growth of such layers feasible. Analyses by SIMS confirm the presence of both P and As in as-grown and annealed samples. Reducing the Te alkyl concentration has been shown to be the critical parameter change that leads to dopant activation. Low activation energies for annealed doped samples were found, from low temperature Hall measurements, which supports the view that the group V elements are active acceptors.
368
June 1988
The authors ,would like to thank Messrs. J.A. Roberts, J.B. Clegg, F. Grainger and I.G. Gale for the SIMS and LSMS analyses. The work was carried out with the support of Procurement Executive, Ministry of Defence, DCVD, sponsored by RSRE. References [ I] L.E. Lapides, R.L. Whitney and CA. Crosson, Mater. Res. Sot. Symp. Proc. 48 (1985) 365.
[ 2 ] T. Tung, M.H. Kahsher. A.P. Stevens and P.E. Herning, Mater. Res. Sot. Symp. Proc. 90 ( 1987) 32 1. [3]L.O. Bubulac, W.E. Tennant, D.S. Lo, D.D. Edwall, J.C. Robinson, J.S. Chen and G. Bostrup, J. Vacuum Sci. Technol. A5 (1987) 3166. [4] M. Boukerche, J. Reno, I.K. Sou, C. Hsu and J.P. Faurie, Appl. Phys. Letters 48 (1986) 1733. [5] P. Capper, J. Crystal Growth 47 ( 1982) 280. [ 61 P. Capper, J.J.G. Gosney, C.L. Jones, I. Kenworthy and J.A. Roberts, J. Crystal Growth 7 1 ( $985) 57. [7 ] M.H. Kalisher, J. Crystal Growth 70 (1984) 365. [8] H.R. Vydyanath, J.A. Ellsworth andC.M. Devaney, J. Electron. Mater. 16 (1987) 13. [9] P.A.C. Whiffin, B.C. Easton, P. Capperand CD. Maxey, J. Crystal Growth 79 (1986) 935. [ lo] SK. Ghandhi, N.R. Taskar and I.B. Bhat, Appl. Phys. Letters 50 ( 1987) 900. [ 11] F. Grainger and J.A. Roberts, Semicond. Sci. Technol., to be published. [ 121 P. Capper, B.C. Easton, P.A.C. Whiftin and C.D. Maxey, J. Crystal Growth 79 (1986) 508.
MATERIALS LETTERS
Volume 6. number 10
GROUP V ACCEPTOR DOPING
OF Cd,Hg,_,Te
GROWN
VAPOUR PHASE EPITAXY
BY METAL-ORGANIC
P. CAPPER,
P.A.C. WHIFFIN,
Philips Research Lahoraiories.
B.C. EASTON,
LAYERS
C.D. MAXEY
Redhill, Surrey. UK
and
I. KENWORTHY .Mullard Southampton.
Millbrook
Industrial Estate, Southampton,
Han&
C’K
Received 7 March 1988: in final form 12 April 1988
Layers of Cd,Hg, _ ,Te have been grown by metal-organic vapour phase epitaxy and doped with arsenic and with phosphorus. This has been achieved by establishing metal-rich gas phase conditions during growth and obviates the need for high temperature “activation” type anneals. Data from Hall measurements and secondary ion mass spectrometry on annealed samples are in close agreement
In order to produce stable p-n junctions for photovoltaic devices made from Cd,Hg,_,Te it is necessary to use intentional doping to fix the carrier concentrations on either side of the junction. For the n-type side In has been used frequently as the donor impurity in LPE (liquid phase epitaxy) and MBE (molecular beam epitaxy ) layers [ l-4 1. The p-type side usually uses a group V element as group I acceptors tend to have higher diffusivities [ l-31. As the group V element needs to occupy a Te site to achieve acceptor activity the stoichiometry at the growth temperature plays a crucial role. In Bridgman growth P, As and Sb were found to be active acceptors [ 5,6 1. A similar picture emerged in Hg-rich LPE material [ 7 1. However for Te-rich LPE layers it has been found necessary to use an “activation” anneal whether the dopant was introduced into the layer via the melt [ 81 or by implantation after growth [ 31. This “activation” anneal consisted of heating samples in Hg vapour at 500°C [ 81 or 400 “C [ 3 ] and was postulated in ref. [ 8 ] to cause transfer of the group V elements from metal sites, where donor activity might be expected, to Te sites, where the dopants will be acceptors. We report here the first MOVPE (metal-organic 0167-577x/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
vapour phase epitaxy) layers doped with group V acceptors. No “activation” anneals are necessary as growth takes place from a “metal-rich” gas phase. Diethyltelluride (DET) and dimethlylcadium (DMC) were used as the Te and Cd sources, respectively, while elemental Hg provided the required Hg vapour pressure. Arsine (0.1% in hydrogen) and phosphine (0.1% in hydrogen) were used for the sources of As and P, respectively. These were introduced, via an injection tube, downstream of the Hg reservoir at ~25 scc/min. Growth took place on a resistively-heated graphite susceptor held at z 4 10” C, with the Hg at z 240°C. Details of the apparatus and IMP (interdiffused multilayer process) growth process were given in ref. [ 91. Both ( 111) and 2” off ( 100) CdTe substrates were used. Following growth layers were cleaved into squares ( z 5 x 5 mm) for Hall measurement and annealing. Samples were etched in dilute Br?/CH,OH and washed in CH30H and H20 prior to contacting with In solder. A field of 3.2 kG was used and all data are quoted at 77 K. Preliminary experiments showed that when the binaries alone were grown As was not present in HgTe (i.e. < 1 x lOI cmp3) but was incorporated into B.V.
365
Volume 6, number 10
MATERIALS LETTERS
CdTe, agreeing with Ghandhi et al. [ lo] although no direct chemical analysis for As was given in ref. [ 10 ] . The growth rate of CdTe was unaffected but the presence of As increased the HgTe growth rate by a factor of 2. This resulted in a decrease in the expected x value when Cd,Hg, _,Te layers were grown, although this could be easily adjusted by modifying the IMP ratio. Analysis by secondary ion mass spectrometry (SIMS) also showed that As did not diffuse rapidly, during the growth time, when it was introduced halfway through a Cd,Hg,_,Te layer. Dopant flows were maintained constant in both the HgTe and CdTe cycles during the IMP growth of this layer. An As level of x I x 10” crnT3 was found in that portion of the layer grown with the arsine flowing, decreasing to < 3 X lOI cms3 (SIMS .detection limit) for the undoped section of the layer. This decrease occurred over a distance of ~0.5 pm which is at the limit of the spatial resolution of the SIMS analysis in this particular case. In order to produce acceptor activity it was found necessary to reduce the Te alkyl concentration from that used for undoped MOVPE layers by z 30%. This corresponded to a change in the Te: metal ratio of 1.3 : 1 to 0.9: 1. Under these “metal-rich” gas phase conditions three As-doped and one P-doped layers were grown. Fig. 1 shows the results of a standard low temperature ( 200 ’ C ) anneal used to remove Hg vacancies. All the doped samples (x=0.26-0.3) remain virtually unchanged from their as-grown p-type levels whereas undoped controls of similar x values, annealed in the same run, converted to n-type. Both MOVPE and LPE controls were used and the fact that one of the LPE layers produced a very low ( z 2 x 1OL4cmp3) n-type level suggests that the anneal was not introducing significant quantities of other impu~ties. Hall measurements made down to 20 K on these anneaied doped samples showed low activation energies ( fi:6-8 meV), indicative of impurity doping [ 6 1. Anneals carried out at 400°C in Hg, followed by heating at 200°C in Hg, were also carried out on the three As-doped layers. Once again the doped samples remained p-type while the undoped controls converted to n-type. There were indications that the high temperature anneal may have activated more As but this is not proven. Fig. 2 shows typical SIMS profiles for As in both 366
June 1988
o-o
As-doped
o-a
P -
v--o
MOVPE
C.-A
LPE
doped
Metal undoped
undoped
- Rtch control
control
k k s
\
10’5
\
Time
(h)
Fig. 1. Carrier concentrations (77 K) for doped and undoped (control) layers before and after 200°C anneal in Hg.
as-grown and annealed samples from the same layer with an x value of 0.29. Clearly the As does not migrate during a 200°C anneal; the two levels agree within the factor of 2 uncertainty of the SIMS analysis. The carrier concentration decreases slightly after annealing, reflecting the removal of Hg vacancies in this layer with ~~0.26. The Hall carrier concentration agrees well with the As level in the annealed sample. Survey analysis by laser scan mass spectrometry [ 111 of undoped MOVPE layers shows no other potential acceptors at the 10” cmW3level and so we believe the annealed p-type level is due solely to the presence of As. Fig. 3 shows the corresponding plots for the Pdoped layer, x value ~0.26. The SIMS profiles are more uneven as the level is close to the SIMS detection limit ( lOI atoms cm-3) and the interface “spike” in the as-grown sample is omitted for clarity. Once again the annealed carrier concentration agrees with the P level demonstrating that P is also an active acceptor when “metal-rich” conditions are used,
Volume 6. number
IO
MATERIALS
PVC 10’8 __-/--
397
LETTERS
(As-grown)
June 1988
PVC
__..._-c- .---_--.-r__T
397
( Annealed)
‘“‘“F ‘\
Hg
‘B I 1 0
I I
,o161'
0 Depth Fig. 2. SIMS profiles
,D’6 1 LI-_.l.ti_ 0 5 Depth (mcron)
5 (micron)
for As and carrier concentrations
although the lower concentration incroporated suggests As is more efficiently incorporated. In order to confirm that reducing the Te alkyl concentration was the key factor in achieving activation of dopants a layer was grown using the higher “undoped” Te alkyl flow, with arsine. SIMS confirmed the presence of .4s in the layer but on annealing at
PVC 401
(AS -grown
in “metal-rich”
1
layer before and after 200°C anneal in Hg.
200°C the layer, which had an x value of 0.23, converted to n-type at z 1 x lOI cm-‘; demonstrating the inactivity of As in this case. A further As-doped layer was grown, again under “metal-rich” conditions, at an x value of ~0.2. We have previously shown that low x undoped material is easier to convert to n-type, i.e. remove Hg vacan-
)
PVC
401
( Annealed)
-Hall cmm3
,015;’
’
’
Depth
’
’
5
’
’
’
’
’
10
,0'5[-
(micron)
Fig. 3. SIMS profiles for P and carrier concentrations
5 Depth
in “metal-rich”
10
(mIcronI
layer before and after 200°C anneal in Hg.
367
Volume 6, number 10
MATERIALS LETTERS
ties, than is higher x material [ 12 1. The layer had an as-grown p-type level (77 K) of z 3 X lOi crnm3 while SIMS analysis revealed an As content of z 1.5~ 10” cmp3. The difference is due to the Hg vacancy concentration in material of x=02 [ 121. On annealing, the layer produced a p-type carrier concentration of w 1 x 10” cmm3, again agreeing within the SIMS uncertainty factor with the As content. In conclusion, we have demonstrated that “metalrich” gas phase conditions during MOVPE growth of Cd,Hg, _XTe can lead to activation of both P and As in x=0.2-0.3 material. This activation is achieved without recourse to high temperature anneals and opens up possibilities for heterojunction layer growth. The low As diffusivity and lack of any auto-doping from the Hg (due to the use of the injection tube for the dopant) make the growth of such layers feasible. Analyses by SIMS confirm the presence of both P and As in as-grown and annealed samples. Reducing the Te alkyl concentration has been shown to be the critical parameter change that leads to dopant activation. Low activation energies for annealed doped samples were found, from low temperature Hall measurements, which supports the view that the group V elements are active acceptors.
368
June 1988
The authors ,would like to thank Messrs. J.A. Roberts, J.B. Clegg, F. Grainger and I.G. Gale for the SIMS and LSMS analyses. The work was carried out with the support of Procurement Executive, Ministry of Defence, DCVD, sponsored by RSRE. References [ I] L.E. Lapides, R.L. Whitney and CA. Crosson, Mater. Res. Sot. Symp. Proc. 48 (1985) 365.
[ 2 ] T. Tung, M.H. Kahsher. A.P. Stevens and P.E. Herning, Mater. Res. Sot. Symp. Proc. 90 ( 1987) 32 1. [3]L.O. Bubulac, W.E. Tennant, D.S. Lo, D.D. Edwall, J.C. Robinson, J.S. Chen and G. Bostrup, J. Vacuum Sci. Technol. A5 (1987) 3166. [4] M. Boukerche, J. Reno, I.K. Sou, C. Hsu and J.P. Faurie, Appl. Phys. Letters 48 (1986) 1733. [5] P. Capper, J. Crystal Growth 47 ( 1982) 280. [ 61 P. Capper, J.J.G. Gosney, C.L. Jones, I. Kenworthy and J.A. Roberts, J. Crystal Growth 7 1 ( $985) 57. [7 ] M.H. Kalisher, J. Crystal Growth 70 (1984) 365. [8] H.R. Vydyanath, J.A. Ellsworth andC.M. Devaney, J. Electron. Mater. 16 (1987) 13. [9] P.A.C. Whiffin, B.C. Easton, P. Capperand CD. Maxey, J. Crystal Growth 79 (1986) 935. [ lo] SK. Ghandhi, N.R. Taskar and I.B. Bhat, Appl. Phys. Letters 50 ( 1987) 900. [ 11] F. Grainger and J.A. Roberts, Semicond. Sci. Technol., to be published. [ 121 P. Capper, B.C. Easton, P.A.C. Whiftin and C.D. Maxey, J. Crystal Growth 79 (1986) 508.