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Very low temperature growth and doping of silicon MBE layers
484
Journal of Crystal Growth 95 (1989) 484--485 North-Holland. Amsterdam
VERY LOW TEMPERATURE GROWTH AND DOPING OF SILICON MBE LAYERS H. JORKE. H. KIBBEL, F. SCHAFFLER, A. CASEL, H.-J. HERZOG and E. KASPER A EG Research Center U/rn, Sedansirasse 10, D- 7900 U/rn. Fed. Rep. of Germany
(100) Silicon molecular beam epitaxy films with etch pit densities below iO~cm 2 and ~ values of 3.3%—3.9% were grown at very low temperatures (250_3500 C). 8-Doping at a monolayer Sb deposition shows a dopant activation of 0.45—0.81 with no detectable broadening at 7~ 200°C growth temperature.
The present short communication deals with a study on layer quality and transport properties of antimony doped (100) silicon films grown in the very low temperature regime (200—350°C). Previous investigations of this dopant at growth temperatures down to 450°C [1] have shown the existence of a transition temperature T * which separates the regimes of strong surface segregation and kinetically limited segregation. At a growth rate of R = I A/s the transition takes place between 520 and 560°C [1]. Surface segregation with samples grown at 500°C was found to be reduced by approximately three orders of magnitude compared to values obtained under standard conditions (7~ 600°C). The films reported here were deposited on thin (200 A) Si buffer layers grown under standard conditions (T,, = 550°C). The growth rate was chosen to be R = I A/s. In a first series 1000 A thick films were homogeneously doped with lOIS 3 by dopant coevaporation. In a Sb atoms/cm second series 1015 Sb atoms/cm2 were deposited on the buffer layer prior to the cooling down to very low process temperatures. Subsequently a 1000 A thick Si cap layer was grown to produce a buried doping spike. Defect etching (Schimmel etch) of homogeneously doped films grown at T,, = 250°C and = 300°C reveals good crystal quality with etch pit densities below i03 cm2. Hall measurements using the Van der Pauw geometry. however, showed a markedly enhanced freeze-out behaviour of these samples compared to routinely grown material. At room temperature only 17% (at T., =
250°C) and 23% (at T~= 300°C) of the impurities appear to be ionized. In both cases, however, the carrier mobility reaches about 80% of the corresponding bulk mobility [2]. To get an increased dopant activation, samples were annealed at successively increased temperatures in an N-, atmosphere. Annealing time was 5 mm at each temperature step. The sheet resistance measured after each cycle shows a first decrease already at 350°C, followed by a second decrease at about 550°C. Such a behaviour may be caused by compensating defects being generated in low-temperature grown Si MBE layers. Another possibility, which cannot be ruled out presently, is the existence of planar Sb impurity complexes [3]. In a second series Si cap layers were grown on Sb terminated (100) Si buffer layers at T, = 200, 250, 300 and 350°C to produce s-doping layers [4.5]. Prior to cap layer growth a well-defined coverage of exactly one monolayer is adjusted by re-evaporation of excess Sb atoms [4]. Consequently, sheet resistance mapping revealed an cxceptional low inhomogeneity of about 2% over a 3 inch wafer. Defect etching (Schimmel etch) also showed good crystallinity with etch pit densities below io~cm2 except for the sample grown at T,, = 200°C. This sample became rough during etching which indicates the cap layer being amorphous. Rutherford backscattering (RBS) spectra using 1.5 MeV 4 He ions with a detector angle of 135° (channeling direction) also revealed the cap layer to be of good crystal quality at 7~ 250°C (x,,
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
0
H. Jorke et 0/.
/
Table 1 ~ (ratio between backscattering yield in aligned and random direction) and dopant activation of 6-doping structures 2 sheet grown on (100) Si with n~= 6.8 x io’~ Sb atoms/cm carrier concentration ~H’ determined by Hall measurements, is taken at 77 K Sample No.
T, (°C)
~
°H(77 K)/n~
485
Very low temperature growth and doping of Si MIlE layers
nH(295 K)/n~
where the cap layer was found to be amorphous.
This may be explained by a discrepancy of about 30
nm found
between
the
thickness
of
the
amorphous layer determined by RBS and the de-
pth of the Sb doping spike (SIMS). This, in turn, suggests that the cap layer deposition starts epitaxially even at 7~ 200°C.The slight increase of =
the sheet carrier density with decreasing temperaB 1619
B 1618 B 1620 B 1621
350
300 250 200
0.033
0.033 0.039 0.88
0.81
0.73
0.78 0.63
ture is yet not well understood (table 1. dis-
0.61 0.50
0.45
crepancy between
n(77 K)/n
5 and ~H (295 K)/n3. A similar behaviour is found with heavily
0.30
doped bulk material where this is attributed to the <0.04, table 1). The layer grown at T5 = 200 °C was amorphous = 0.88). were Beyond that,detectathe Sb doping spikes at (Xmin the interface clearly
ble. At T~ 250 °Cthe random signal corresponds to about 5.9 X iO~cm2 and the channeling signal to about 6 )< 10~~ cm2. This is indicative of a high degree of substitutional dopant incorporation. The RBS peaks have a full-width at halfmaximum of 35 nm except for the sample grown at 1~ 350 °C, where the peak was slightly asymmetrically broadened by segregation. Secondary ion mass spectrometry (SIMS) using Cs~at 15 keV shows the Sb dopants being captured totally ion the very low temperature films. Significant broadening, however, is still visible at T 2 5 350°C. The agrees total of n,, well 8.2 with X 1014 cm detected by SIMS fairly a monolayer coverage at the (100) surface (6.8 x 1014 cm2). The dopant activation of the 8-doping layers
onset of impurity band conduction [6]. The Hall mobility is found to sbeover almost constant at a level 2/V. the whole temperature of about 70 cm There is a slight tendency of jL to range measured. decrease with decreasing process temperature.
Again, carriers in show a mobility of temperature, which being embedded in
the sample grown at 200°C still ~t ~ cm2/V s at room also indicates the doping spike a crystalline matrix. =
=
=
=
grown at very low temperatures is found to be near unity at higher process temperatures (table 1)
and amounts to still about 0.45 at 7
=
200°C
References [11 H. JoTke, Surface Sci. 193 (1988) 569. [21 F.J. Morin and J.P. Maita, Phys. Rev. 96 (1954) 28. [31H. H. Kibbel, F. Schäffler, A. Casel, H,-J. Herzog andJorke, E. Kasper, to be published. 141 A.A. van Gorkum, K. Nakagawa and Y. Shiraki, Japan. J. Appl. Phys. 26 (1987) L1933. [5} H.P. Zeindl, G. Tempel, B. Bullemer and I. Eisele. in: Proc. 2nd Intern. Symp. on Si MBE, Honolulu, Hawaii, 1988, Electrochem. Soc. Proc., Vol. 88-8, Eds. J.C. Bean and .J. Schowalter (Electrochem. Soc., Pennington, NJ, 1988) p. 515. [61 H. Fritsche and M. Cuevas, Phys. Rev. 119 (1960) 1238.
Journal of Crystal Growth 95 (1989) 484--485 North-Holland. Amsterdam
VERY LOW TEMPERATURE GROWTH AND DOPING OF SILICON MBE LAYERS H. JORKE. H. KIBBEL, F. SCHAFFLER, A. CASEL, H.-J. HERZOG and E. KASPER A EG Research Center U/rn, Sedansirasse 10, D- 7900 U/rn. Fed. Rep. of Germany
(100) Silicon molecular beam epitaxy films with etch pit densities below iO~cm 2 and ~ values of 3.3%—3.9% were grown at very low temperatures (250_3500 C). 8-Doping at a monolayer Sb deposition shows a dopant activation of 0.45—0.81 with no detectable broadening at 7~ 200°C growth temperature.
The present short communication deals with a study on layer quality and transport properties of antimony doped (100) silicon films grown in the very low temperature regime (200—350°C). Previous investigations of this dopant at growth temperatures down to 450°C [1] have shown the existence of a transition temperature T * which separates the regimes of strong surface segregation and kinetically limited segregation. At a growth rate of R = I A/s the transition takes place between 520 and 560°C [1]. Surface segregation with samples grown at 500°C was found to be reduced by approximately three orders of magnitude compared to values obtained under standard conditions (7~ 600°C). The films reported here were deposited on thin (200 A) Si buffer layers grown under standard conditions (T,, = 550°C). The growth rate was chosen to be R = I A/s. In a first series 1000 A thick films were homogeneously doped with lOIS 3 by dopant coevaporation. In a Sb atoms/cm second series 1015 Sb atoms/cm2 were deposited on the buffer layer prior to the cooling down to very low process temperatures. Subsequently a 1000 A thick Si cap layer was grown to produce a buried doping spike. Defect etching (Schimmel etch) of homogeneously doped films grown at T,, = 250°C and = 300°C reveals good crystal quality with etch pit densities below i03 cm2. Hall measurements using the Van der Pauw geometry. however, showed a markedly enhanced freeze-out behaviour of these samples compared to routinely grown material. At room temperature only 17% (at T., =
250°C) and 23% (at T~= 300°C) of the impurities appear to be ionized. In both cases, however, the carrier mobility reaches about 80% of the corresponding bulk mobility [2]. To get an increased dopant activation, samples were annealed at successively increased temperatures in an N-, atmosphere. Annealing time was 5 mm at each temperature step. The sheet resistance measured after each cycle shows a first decrease already at 350°C, followed by a second decrease at about 550°C. Such a behaviour may be caused by compensating defects being generated in low-temperature grown Si MBE layers. Another possibility, which cannot be ruled out presently, is the existence of planar Sb impurity complexes [3]. In a second series Si cap layers were grown on Sb terminated (100) Si buffer layers at T, = 200, 250, 300 and 350°C to produce s-doping layers [4.5]. Prior to cap layer growth a well-defined coverage of exactly one monolayer is adjusted by re-evaporation of excess Sb atoms [4]. Consequently, sheet resistance mapping revealed an cxceptional low inhomogeneity of about 2% over a 3 inch wafer. Defect etching (Schimmel etch) also showed good crystallinity with etch pit densities below io~cm2 except for the sample grown at T,, = 200°C. This sample became rough during etching which indicates the cap layer being amorphous. Rutherford backscattering (RBS) spectra using 1.5 MeV 4 He ions with a detector angle of 135° (channeling direction) also revealed the cap layer to be of good crystal quality at 7~ 250°C (x,,
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
0
H. Jorke et 0/.
/
Table 1 ~ (ratio between backscattering yield in aligned and random direction) and dopant activation of 6-doping structures 2 sheet grown on (100) Si with n~= 6.8 x io’~ Sb atoms/cm carrier concentration ~H’ determined by Hall measurements, is taken at 77 K Sample No.
T, (°C)
~
°H(77 K)/n~
485
Very low temperature growth and doping of Si MIlE layers
nH(295 K)/n~
where the cap layer was found to be amorphous.
This may be explained by a discrepancy of about 30
nm found
between
the
thickness
of
the
amorphous layer determined by RBS and the de-
pth of the Sb doping spike (SIMS). This, in turn, suggests that the cap layer deposition starts epitaxially even at 7~ 200°C.The slight increase of =
the sheet carrier density with decreasing temperaB 1619
B 1618 B 1620 B 1621
350
300 250 200
0.033
0.033 0.039 0.88
0.81
0.73
0.78 0.63
ture is yet not well understood (table 1. dis-
0.61 0.50
0.45
crepancy between
n(77 K)/n
5 and ~H (295 K)/n3. A similar behaviour is found with heavily
0.30
doped bulk material where this is attributed to the <0.04, table 1). The layer grown at T5 = 200 °C was amorphous = 0.88). were Beyond that,detectathe Sb doping spikes at (Xmin the interface clearly
ble. At T~ 250 °Cthe random signal corresponds to about 5.9 X iO~cm2 and the channeling signal to about 6 )< 10~~ cm2. This is indicative of a high degree of substitutional dopant incorporation. The RBS peaks have a full-width at halfmaximum of 35 nm except for the sample grown at 1~ 350 °C, where the peak was slightly asymmetrically broadened by segregation. Secondary ion mass spectrometry (SIMS) using Cs~at 15 keV shows the Sb dopants being captured totally ion the very low temperature films. Significant broadening, however, is still visible at T 2 5 350°C. The agrees total of n,, well 8.2 with X 1014 cm detected by SIMS fairly a monolayer coverage at the (100) surface (6.8 x 1014 cm2). The dopant activation of the 8-doping layers
onset of impurity band conduction [6]. The Hall mobility is found to sbeover almost constant at a level 2/V. the whole temperature of about 70 cm There is a slight tendency of jL to range measured. decrease with decreasing process temperature.
Again, carriers in show a mobility of temperature, which being embedded in
the sample grown at 200°C still ~t ~ cm2/V s at room also indicates the doping spike a crystalline matrix. =
=
=
=
grown at very low temperatures is found to be near unity at higher process temperatures (table 1)
and amounts to still about 0.45 at 7
=
200°C
References [11 H. JoTke, Surface Sci. 193 (1988) 569. [21 F.J. Morin and J.P. Maita, Phys. Rev. 96 (1954) 28. [31H. H. Kibbel, F. Schäffler, A. Casel, H,-J. Herzog andJorke, E. Kasper, to be published. 141 A.A. van Gorkum, K. Nakagawa and Y. Shiraki, Japan. J. Appl. Phys. 26 (1987) L1933. [5} H.P. Zeindl, G. Tempel, B. Bullemer and I. Eisele. in: Proc. 2nd Intern. Symp. on Si MBE, Honolulu, Hawaii, 1988, Electrochem. Soc. Proc., Vol. 88-8, Eds. J.C. Bean and .J. Schowalter (Electrochem. Soc., Pennington, NJ, 1988) p. 515. [61 H. Fritsche and M. Cuevas, Phys. Rev. 119 (1960) 1238.