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Composition and carbon effects on uniformity for implanted active layers fabricated in undoped LEC GaAs
Volume 6, number
56
MATERIALS
LETTERS
March
COMPOSITION AND CARBON EFFECTS ON UNIFORMITY FOR IMPLANTED ACTIVE LAYERS FABRICATED IN UNDOPED K. USUDA
1988
LEC GaAs
and S. YASUAMI
Research and Development Center, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan Received
2 1 September
1987; in final form 18 February
1988
Fluctuations in carrier concentrations in implanted layers were found to depend on the carbon concentration in substrates. Real composition effects, i.e. not via carbon, were clarified, when the carbon concentration was reduced to z 3 x 1Or4 atoms/cm’. Fluctuations in the carrier concentration were least, when a crystal was grown from raw materials with an atomic ratio (Ga/As), around 0.95.
For semi-insulating GaAs crystals grown by the LEC method, a dominant residual impurity is carbon. The carbon atom creates a shallow acceptor level in the crystal and compensates for the native deep donor EL2 to attain the semi-insulating nature [ 11. As far as carbon heaters are employed for melting materials, this carbon incorporation to a certain degree into a crystal is inevitable. Since it kills shallow donors, e.g. Si, doped by ion implantation, threshold voltages of MESFETs fabicated in these GaAs substrates depend linearly on the carbon concentration in the substrates [ 21. On the other hand, the carbon incorporation in a crystal varies with the composition of the raw materials, and hence the resultant threshold voltage appears to depend on the composition, although it merely depends on the carbon concentration [ 31. Consequently, when the carbon concentration is high, the composition effect on the carrier concentration in implanted active layers cannot be discriminated from that of the carbon concentration. In this study, attention was equally paid to carbon and composition, and their effects on the carrier concentration, especially its uniformity in a substrate, were investigated. The substrates were prepared from the head, middle and tail parts of 3 inch diameter semi-insulating GaAs crystals grown by the liquid encapsulated Czochralski (LEC) method. The crystals were pulled in the [ 1001 direction from melts of various com164
positions. Materials with atomic ratios (Ga/As), (0.92, 0.95 and 0.99) were charged in a crucible for respective crystal growth. The concentration of carbon atoms, occupying the As sublattices and acting as shallow acceptors, was determined by local vibrational mode infrared absorption measurements with the Hakone-1986 factor ]41. Si ions were implanted into the substrates at 150 keV with a dose of 2.5 X lOI cmp2. This was followed by annealing for 15 min in an As atmosphere at 850°C. Both Schottky (1 mm0) and ohmic contacts were formed with aluminium. In-depth carrier profiles were measured by the capacitance-voltage (CV) profiling method at 10 points across one substrate. Values where the carrier concentration profiles peak were averaged over these points and an averaged peak carrier concentration N,, and its standard deviation AN, were obtained. Fig. 1 shows the carbon concentration dependence for the percentage standard deviation in the peak carrier concentrations ( o,,,~/N, ) in a substrate. All the substrates were obtained from crystals grown using the 0.95 atomic ratio material. a&/N, increases, as the carbon concentration increases, although its fluctuation increases, as well. Consequently, carbon affects both the carrier concentration [ 31 and its fluctuation in implanted layers. Since the effective segregation coefficient of carbon for GaAs is larger than 1, and the present crys-
0167-517x/88/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
B.V.
MATERIALS LETTERS
Volume 6, number 5,6
the peak carrier concentration is expected to depend chiefly on the carbon concentration in the substrates, provided that the relation shown in fig. 1 holds. The higher the As fraction (the lower the carbon concentration) in the materials used for growing crystals, and/or the nearer to the tail of a crystal a substrate is sampled, the lower a.,, /N, However, the actual G,~~/N, is minimized, not for
.
.
Head A Mlddle n Tall L
h
0
I
I
I
I
I
I
2
3
4
5
Carbon Concentration
(~lO’~crn-~)
Fig. I. Percentage standard deviation in peak carrier concentration as a function of carbon concentration in substrates.
tals were grown from As-rich materials, the carbon concentration and its fluctuation in a crystal should lower as crystal growth proceeds, according to the results obtained in the previous work. However, some of the data do not follow this norm, and hence the data scattering is appreciable. Fig. 2 is a plot of a,.,.,/N, versus the atomic ratio of the materials charged for growing respective crystals. For crystals with a rather high carbon concentration ( 2 1.5 x 10” atoms/cm’), the fluctuation in
s e
00
Head
AA
Middle
q n
Tail i
Concentration
_
a”
0100 Atomic
Ratio
for
Material
March 1988
[Go/As),
Fig. 2. Percentage standard deviation in peak carrier conct Itration plotted against atomic ratio for materials.
the crystal with a minimum concentration of carbon ((Ga/As),=0.92), but for one with (Ga/As),= 0.95. Furthermore, for a crystal with (Ga/As).,= 0.99, a,*, IN,, does not reduce towards the crystal end. Although Q,.~/N, is affected mainly by carbon via composition, some genuine composition effects seem to arise. When the carbon concentration reduces to less than Z~X lOI atoms/cm3, ohrP/NP decreases. However, the general tendency of the relation between gNP/N, and the atomic ratio is mostly retained. Since the carbon concentrations in these crystals are less than 3 x lOI atoms/cm3, fluctuations in the carbon concentration should at least be less than this. Carbon is hence considered merely to cause minor effects on a.,,/Np and not so much as shown in fig. 2. The authors believe that this composition dependence for a,,/N, would be a real composition effect, not involved in carbon concentration variations due to composition variation. The following conclusion is thus reached. For crystals containing a high concentration of carbon, carbon compensation for doped silicon is predominant in implanted active layers. Fluctuation in carrier concentrations is also due mainly to carbon concentration variation. Real composition effects on the implanted active layers hence are mingled with carbon effects via composition. The real composition effects are revealed as the carbon concentration is reduced. A crystal grown from materials with around (Ga/As), = 0.95 realizes least fluctuations in carrier concentrations through a crystal. However, further detailed study is to be carried out for extracting crystallographically meaningful results. We would like to thank Dr. T. Nakanisi for his continual encouragement during this study. We are indebted to Mr. M. Watanabe, Dr. K. Terashima, Mr. J. Nishio and Mr. S. Yashiro for supplying crystals. 165
Volume 6, number 5,6
MATERIALS LETTERS
We also thank Mr. S. Washizuka and Mr. K. Fukuta for their technical assistance. References [ I] D.E. Holmes, R.T. Chen, K.R. Elliot and C.G. Kirkpatrick, Appl. Phys. Letters 40 ( 1982) 46.
166
March 1988
[ 21R.T. Chen, D.E. Holmes and P.M. Asbeck, Appl. Phys. Letters45 (1984) 459. [3] S. Yasuami, K. Fukuta, M. Watanabe and T. Nakanisi, Japan. J. Appl. Phys. 25 (1986) 1905. [4 ] M.R. Brozel, Semi-insulating III-V materials (Ohmsha, Tokyo, 1986) p. 217.
56
MATERIALS
LETTERS
March
COMPOSITION AND CARBON EFFECTS ON UNIFORMITY FOR IMPLANTED ACTIVE LAYERS FABRICATED IN UNDOPED K. USUDA
1988
LEC GaAs
and S. YASUAMI
Research and Development Center, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan Received
2 1 September
1987; in final form 18 February
1988
Fluctuations in carrier concentrations in implanted layers were found to depend on the carbon concentration in substrates. Real composition effects, i.e. not via carbon, were clarified, when the carbon concentration was reduced to z 3 x 1Or4 atoms/cm’. Fluctuations in the carrier concentration were least, when a crystal was grown from raw materials with an atomic ratio (Ga/As), around 0.95.
For semi-insulating GaAs crystals grown by the LEC method, a dominant residual impurity is carbon. The carbon atom creates a shallow acceptor level in the crystal and compensates for the native deep donor EL2 to attain the semi-insulating nature [ 11. As far as carbon heaters are employed for melting materials, this carbon incorporation to a certain degree into a crystal is inevitable. Since it kills shallow donors, e.g. Si, doped by ion implantation, threshold voltages of MESFETs fabicated in these GaAs substrates depend linearly on the carbon concentration in the substrates [ 21. On the other hand, the carbon incorporation in a crystal varies with the composition of the raw materials, and hence the resultant threshold voltage appears to depend on the composition, although it merely depends on the carbon concentration [ 31. Consequently, when the carbon concentration is high, the composition effect on the carrier concentration in implanted active layers cannot be discriminated from that of the carbon concentration. In this study, attention was equally paid to carbon and composition, and their effects on the carrier concentration, especially its uniformity in a substrate, were investigated. The substrates were prepared from the head, middle and tail parts of 3 inch diameter semi-insulating GaAs crystals grown by the liquid encapsulated Czochralski (LEC) method. The crystals were pulled in the [ 1001 direction from melts of various com164
positions. Materials with atomic ratios (Ga/As), (0.92, 0.95 and 0.99) were charged in a crucible for respective crystal growth. The concentration of carbon atoms, occupying the As sublattices and acting as shallow acceptors, was determined by local vibrational mode infrared absorption measurements with the Hakone-1986 factor ]41. Si ions were implanted into the substrates at 150 keV with a dose of 2.5 X lOI cmp2. This was followed by annealing for 15 min in an As atmosphere at 850°C. Both Schottky (1 mm0) and ohmic contacts were formed with aluminium. In-depth carrier profiles were measured by the capacitance-voltage (CV) profiling method at 10 points across one substrate. Values where the carrier concentration profiles peak were averaged over these points and an averaged peak carrier concentration N,, and its standard deviation AN, were obtained. Fig. 1 shows the carbon concentration dependence for the percentage standard deviation in the peak carrier concentrations ( o,,,~/N, ) in a substrate. All the substrates were obtained from crystals grown using the 0.95 atomic ratio material. a&/N, increases, as the carbon concentration increases, although its fluctuation increases, as well. Consequently, carbon affects both the carrier concentration [ 31 and its fluctuation in implanted layers. Since the effective segregation coefficient of carbon for GaAs is larger than 1, and the present crys-
0167-517x/88/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
B.V.
MATERIALS LETTERS
Volume 6, number 5,6
the peak carrier concentration is expected to depend chiefly on the carbon concentration in the substrates, provided that the relation shown in fig. 1 holds. The higher the As fraction (the lower the carbon concentration) in the materials used for growing crystals, and/or the nearer to the tail of a crystal a substrate is sampled, the lower a.,, /N, However, the actual G,~~/N, is minimized, not for
.
.
Head A Mlddle n Tall L
h
0
I
I
I
I
I
I
2
3
4
5
Carbon Concentration
(~lO’~crn-~)
Fig. I. Percentage standard deviation in peak carrier concentration as a function of carbon concentration in substrates.
tals were grown from As-rich materials, the carbon concentration and its fluctuation in a crystal should lower as crystal growth proceeds, according to the results obtained in the previous work. However, some of the data do not follow this norm, and hence the data scattering is appreciable. Fig. 2 is a plot of a,.,.,/N, versus the atomic ratio of the materials charged for growing respective crystals. For crystals with a rather high carbon concentration ( 2 1.5 x 10” atoms/cm’), the fluctuation in
s e
00
Head
AA
Middle
q n
Tail i
Concentration
_
a”
0100 Atomic
Ratio
for
Material
March 1988
[Go/As),
Fig. 2. Percentage standard deviation in peak carrier conct Itration plotted against atomic ratio for materials.
the crystal with a minimum concentration of carbon ((Ga/As),=0.92), but for one with (Ga/As),= 0.95. Furthermore, for a crystal with (Ga/As).,= 0.99, a,*, IN,, does not reduce towards the crystal end. Although Q,.~/N, is affected mainly by carbon via composition, some genuine composition effects seem to arise. When the carbon concentration reduces to less than Z~X lOI atoms/cm3, ohrP/NP decreases. However, the general tendency of the relation between gNP/N, and the atomic ratio is mostly retained. Since the carbon concentrations in these crystals are less than 3 x lOI atoms/cm3, fluctuations in the carbon concentration should at least be less than this. Carbon is hence considered merely to cause minor effects on a.,,/Np and not so much as shown in fig. 2. The authors believe that this composition dependence for a,,/N, would be a real composition effect, not involved in carbon concentration variations due to composition variation. The following conclusion is thus reached. For crystals containing a high concentration of carbon, carbon compensation for doped silicon is predominant in implanted active layers. Fluctuation in carrier concentrations is also due mainly to carbon concentration variation. Real composition effects on the implanted active layers hence are mingled with carbon effects via composition. The real composition effects are revealed as the carbon concentration is reduced. A crystal grown from materials with around (Ga/As), = 0.95 realizes least fluctuations in carrier concentrations through a crystal. However, further detailed study is to be carried out for extracting crystallographically meaningful results. We would like to thank Dr. T. Nakanisi for his continual encouragement during this study. We are indebted to Mr. M. Watanabe, Dr. K. Terashima, Mr. J. Nishio and Mr. S. Yashiro for supplying crystals. 165
Volume 6, number 5,6
MATERIALS LETTERS
We also thank Mr. S. Washizuka and Mr. K. Fukuta for their technical assistance. References [ I] D.E. Holmes, R.T. Chen, K.R. Elliot and C.G. Kirkpatrick, Appl. Phys. Letters 40 ( 1982) 46.
166
March 1988
[ 21R.T. Chen, D.E. Holmes and P.M. Asbeck, Appl. Phys. Letters45 (1984) 459. [3] S. Yasuami, K. Fukuta, M. Watanabe and T. Nakanisi, Japan. J. Appl. Phys. 25 (1986) 1905. [4 ] M.R. Brozel, Semi-insulating III-V materials (Ohmsha, Tokyo, 1986) p. 217.