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Optical and electrical properties of C+-implanted GaAs
Nuclear Instruments and Methods in Physics Research B39 (1989) 457-460 North-Holland, Amsterdam
OPTICAL
AND ELECTRICAL
Shigeru SHIGETOMI Nobukazu OHNISHI
PROPERTIES
OF C +-IMPLANTED
457
GaAs
I) , Yunosuke MAKITA *), Masahiko MORI 2, Yutaka KOGA ‘I, Paul PHELAN 3), Hajime SHIBATA 2, and Tokue MATSUGORI 4,
2),
‘) Department of Physics, Kurume University, 1635 Mii-maehi, Kurume-shi, Fukuoka-ken 830, Japan ‘) Electrotechnical Laboratory, I-1 -4 Umezono, Tsukuba-shi, Ibaraki-ken 305, Japan ‘) Institute of Fundamental Analysis Inc., Ltd., 3-24-3 Yoyogi, Shinjuku-ku, Tokyo 151, Japan 4’ Department of Electronics, Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa-ken 259-12, Japan
Carbon (C) was implanted into extremely pure GaAs grown by molecular beam epitaxy in a wide range of C dose [C] from to 1 x 10” cmm3. The impurity levels and the process of crystalline recovery were investigated by using Raman scattering, photoluminescence and the Hall effect. The implanted layers were found to form a crystalline layer at an annealing temperature around 200° C for [C] smaller than 1 X10” cmm3, while for [C] over that value this temperature became extremely high with increasing [C] and it reached around 550° for [C] =1 X 10” cmm3. The well-defined below-band gap emission, [g-g], exclusively inherent to acceptor impurities was a dominant one for [C] lower than 3 X 10” cme3 but its intensity was suppressed for [C] higher than this value. From the results of Hall effect, the current peculiar features of [g-g] were found to be attributable to the low substitutional efficiency of C atoms as acceptors. 1 x lOI
1. Introduction
2. Experimental
Carbon (C) is one of the residual impurities in GaAs grown by molecular beam epitaxy (MBE). Thus it is necessary to investigate the behaviour of C atoms in high-quality GaAs used for high speed electronic devices. There are studies available of the optical and electrical properties in implanted layers with a relatively low C dose, [Cl. When [C] is larger than 1 x lOi cmm3, the newly-discovered emission [g-g] due to an acceptor-acceptor pair was found to be created just below the bound exciton emissions by the measurements of low-temperature photoluminescence (PL) [l]. The results of electrical measurements revealed that p-type conduction is produced in the implanted layer when the annealing temperature (T,) is higher than 600 o C [2]. Since the C atom belongs to the column IV group, it is possible for it to work as an amphoteric impurity in GaAs. The primary purpose of this paper is to provide extensive information on the optical and electrical properties of C+-implanted GaAs for an extremely wide range of [C] up to 1 X 10 *’ cm- 3. The degree of crystalline recovery from the damaged state as functions of [C] and T, are evaluated by using Raman scattering (RS). The impurity levels are also presented from the measurements of PL and Hall effect and we discuss the possibility of C atoms as amphoteric impurities in GaAs. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The wafers used for ion implantation were (100) undoped GaAs grown by MBE at 550 o C. Ion implantation of C+ was carried out at three ion energies, 100, 200 and 300 keV. The calculation according to LSS theory predicts that the concentration of implanted C atoms is a flat distribution between 0.01 and 0.70 pm from the front surface [3]. During annealing the samples were sandwiched between two pieces of undoped GaAs in order to prevent the decomposition of GaAs [l]. All RS experiments were performed at room temperature using a quasi-backscattering geometry. The direction of propagation of the incident beam with 514.5 nm line was at an angle of about 40° with respect to the normal against the (100) surface. PL measurements were also made at 2 K by using 514.5 nm line under the condition of extremely low excitation density. The carrier concentration of implanted layers was measured by the van der Pauw method. The measurements of dc Hall voltage were carried out in a magnetic field of 0.5 T and in the temperature range from 35 to 250 K.
3. Results and discussion The annealing behavior for a sample with [C] = 1
x
of unpolarized RS spectra 102’ cm-3 is shown in fig.
V. SEMICONDUCTORS:
GaAs, FIB .
S. Shigetomi et al. / C +-implanted
550 “c ANNEAL
TO LO LA mln
IIA i
AS IMPLANTED
Fig. 1. Raman spectra of unimplanted and C+-implanted GaAs with [C] =lXIOzo cmm3 as a function of the annealing temperature.
I. The spectrum of an unimplanted (UI) sample is also indicated for comparison. In the UI sample, a strong peak at 292 cm-’ and a weak one at 269 cm-’ are the longitudinal optical (LO) and transverse optical (TO) phonons at the F point, respectively. According to the selection rules for first order RS in semiconductors having a zinc-blende structure, only the LO phonon is observed from the (100) surface in a backscattering geometry. The appearance of the weak peak attributed thus ascribed to the to the TO phonon should be present quasi-backscattering geometry. For the as-implanted sample the LO phonon intensity decreased in comparison with that of the UI sample, and its peak position shifted slightly towards lower frequency. Furthermore, a broad peak extending from 200 to 280 cm-’ appeared, and another broad but weaker one was formed between 150 and 190 cm-‘. The former peak consists of the LO, TO and longitudinal acoustic (LA) phonons both at the X and L points in the Brillouin zone. The latter peak is the impurity induced acoustic (IIA) phonon near the K point. This observation indicates that the lattice symmetry is destroyed by implantation and the lattice is highly disordered [4]. When T, exceeds 400 o C, the aforementioned two sets of broad peaks from 200 to 280 cm-’ and from 150
GaAs
completely. The to 190 cm-‘, respectively, disappeared LO phonon intensity was, however, weaker than that of the UI samples but that in the implanted sample for r, = 550°C became comparable to that of the UI sample. These observations show conclusively that the implanted layer with this extremely high [C] can recover its crystallinity gradually with increasing T, and attain an almost perfect crystalline state for annealing at a temperature higher than 550 o C. The variation of the LO phonon intensity by annealing is a very important method to evaluate the crystalline recovering procedure. Fig. 2 shows the isochronal annealing behavior of the LO phonon intensity in the samples implanted with [C] from 1 X lOi to 1 x 10” cmp3. The lowest T, at which the LO phonon intensity of the ion-implanted sample acquires around 95% of that of the UI sample is around 2OO’C for [C] smaller than 1 X 10” cmm3. For higher concentrations, this temperature increases rapidly with increasing [C] and reaches around 55O’C for [C] = 1 x 10” cme3. The features presented in fig. 2 indicate that the annealing procedures of the C+-implanted GaAs is substantially dependent upon the amount of damage produced by ion implantation. In order to investigate the properties of radiative processes pertinent to the C atoms in GaAs, we measured the low temperature PL of the C+-implanted samples. High resolution spectra near free (FE) and bound (BE) exciton emissions are shown in fig. 3. For the UI sample, the emission denoted by g, is due to the excited state of an isolated C acceptor [5]. In the implanted samples, the intensities of the sharp emissions associated with FE and BE gradually decreased with increasing [Cl. The intensity of g however, in the sample with [C] = 1 X lOi cme3, was enhanced as is shown in the figure. With further increment of [C] to greater than 1 X 10” cmp3 a new broad emission
I OO
I
I
200 ANNEALING
I
1
I
400 TEMPERATURE
1
600 (“C 1
Fig. 2. Isochronal annealing characteristics of the LO phonon intensity for C+-implanted GaAs with various [Cl.
S. Shigetomi
et al. / C + -implanted GaAs
band, denoted by [g-g], was formed on the lower energy side of g. The emission energy of [g-g] shifted moderately towards the lower energy side with [C] increasing from 1 X 10” to 3 x 10” cme3. This behavior has been reported previously for dilutely C+-implanted GaAs [l]. [g-g] is commonly observed in acceptor-incorporated GaAs irrespective of the method of introduction: ion implantation [1,6], incorporation during crystal growth either in the form of an ion beam (Mg+ for MBE) [7] or in the form of elements (Mg for liquid phase epitaxy, LPE) [8]. Since [g-g] appeared only when the acceptor concentration, [A] was relatively high and its emission energy presented a strong energy shift against the increase of [A], [g-g] was confirmed to be related to the pair between excited state of acceptors where the overlapping of excited orbitals of holes occurs making the pairs efficient radiative recombination centers [S]. For the case of implantation of heavier acceptor ion species Zn and Cd, the locking of the energy shift pertinent to [g-g] was usually observed at large [A] of around 1 x 10” cm-3 where [g-g] had already shifted very close to the transition between free electrons and the acceptor level, (e, A) [5]. In the sample with [C] greater than 5 X 1Or7 cmp3, the emission energy of [g-g] seemed to present no further shift towards the lower energy side. Additionally
PHOTON 1.520
EY$RGV
(eV ) 1.500 2x10%ni3
;~~:-lxlo~~
x,o F. E,) n.27
815
.“k’
UNIMPLANTE
820
WAVELENGTH
025 (nm)
Fig. 3. Photoluminescence spectra of unimplanted and C+-implanted GaAs near the free and bound exciton emission regions at 2 K as a function of [Cl. The annealing temperature is 850 o c.
459 TEMPERATURE
( K )
Id88
3’
’ ’
’
’ ’ ’
10
lo;T
’
’
’
20
’
’
’
’
’
30
J
(K-‘1
Fig. 4. The volume hole concentration as a function of the reciprocal temperature for C+-implanted GaAs with various [Cl. The annealing temperature is 850° C. The solid lines indicate the values given by Fermi statistics and two active acceptor levels.
the integrated PL intensity decreased with increasing [Cl. This observation, which was recognized for the first time in the present C+-implanted material, is a new feature of [g-g]. This feature of [g-g] can be explained by the low substitutional efficiency of C atoms as acceptors from the result of Hall effect measurements. Fig. 4 shows the temperature dependence of the volume hole concentration in C+-implanted GaAs for five different values of [Cl. The sheet-hole concentration was calculated from the measured Hall coefficient by using a Hall factor of unity. The volume concentration of free holes p was evaluated by assuming that holes were distributed uniformly over a depth of 0.70 urn, because the distribution of implanted C atoms is homogeneous from the front surface to a depth of 0.70 urn, as was previously described in section 2. For [C] smaller than 1 X 10” cmp3, p decreased sharply with decreasing temperature T, displaying two slopes for p above and below - 8 X 10” cme3. For the samples with [C] = 5 X 10” and 1 x 1O’9 cmm3, p decreased with decreasing T between 60 and 250 K and then increased for T below 50 K. The behavior of p at low T indicates that the conduction mechanism is strongly influenced by impurity conduction. Since the curves of p vs l/T for [C] smaller than 1 x 10” cmp3 have two slopes, we made the analysis V. SEMICONDUCTORS:
GaAs,
FIB
460
S. Shigetomi et al. / C + -implanted GaAs
Table 1 Concentrations of donor (No), shallow acceptor (NA,) and deep acceptor ( NA2) impurities, along with ionization energies of shallow acceptor ( EAI) and deep acceptor ( EA2) levels in
C+-implanted GaAs for various [Cl.
[cl
ND
N
NAZ
(cme3)
(cme3)
A’-3) (cm
(cmm3)
1x10’6
1.8x10’s
1 X10” 1x10’*
1.6~10’~ 1.7~10’~
1.OX1O16 8.6 X 10” 9.8~10’~ 8.0x10’* 1.3~10” 9.2~10”
0.026 0.026 0.025
0.10 0.10 0.10
using Fermi statistics with the assumption of two active acceptor levels [9]. Electron and hole effective mass ratios of 0.07 and 0.5 [lo] and the degeneracy factor for the acceptor level of 4 [9] were used in the calculation. The best fit curves are drawn by solid lines in fig. 4. The values of the parameters for fitting are shown in table 1. The ionization energy of a shallow acceptor obtained agrees with the 0.026 eV level of a C acceptor at As sites (C,,) [ll]. For [C] up to 1 x lOI cmw3, the concentration of C,, working as acceptors is nearly coincident with [Cl. The higher [C] brought about the reduction of the activation efficiency and for a [C] of 1 x 10” crnm3 the activation efficiency reached 13%. Carbon belongs to column IV of the Periodic Table and is supposed to work as an amphoteric impurity in GaAs. However, an increase of donors was not obtained from the results of the present analysis. We suppose that the low activation effeciency for high [C] is principally caused by the interstitial C atoms or the formation of complexes. The position of the deep acceptor energy level is at 0.10 eV above the valence band. The acceptor concentration associated with this level which is larger than [C] increases with increasing [Cl. Brehm and Pearson have reported the deep acceptor level in ray-irradiated GaAs from the results of Hall effect measurements [12]. The value of the deep acceptor level obtained is the same as that of the defect related deep acceptor level, suggesting that the defects from ion implantation or high-temperature annealing remain in the implanted layer.
4. Conclusion The electrical and optical properties of C+-implanted GaAs as a function of dose and annealing temperature were measured by Raman scattering, photoluminescence and Hall effect. The minimum annealing temperature necessary for obtaining a crystalline layer is strongly dependent upon [C] and is around 200 o C for [C] less than 1 X 10” cme3 while it is higher than 550 o C for [C] = 1 X 10” cme3. [g-g] formed just below the bound exciton emissions is a dominant emission in C+-implanted GaAs. The locking of the energy shift of [g-g] was found to take place at a relatively low [Cl. It is demonstrated from the combined results of photoluminescence and Hall effect that this peculiar behavior of [g-g] is caused by the low efficiency of substitutional C atoms.
References [l] Y. Makita, T. Nomura, M. Yokota, T. Matsumori, T. Izumi, Y. Takeuchi and K. Kudo, Appl. Phys. Lett. 47 (1985) 623. [2] B.K. Shin, Appl. Phys. Lett. 29 (1976) 438. [3] J. Lindhard, M. Scharff and H.E. Schiett, K. Dan. Vidensk Selsk Mat. Fys. Medd. 33 (1963) 1. [4] K.K. Tiong, P.M. Amirtharaj. F.H. Pollak and D.E. Aspnes, Appl. Phys. Lett. 44 (1984) 122. [S] N. Oh&hi, Y. Makita, M. Mori, K. Irie, Y. Takeuchi and S. Shigetomi, J. Appl. Phys. 62 (1987) 1833. [6] Y. Takeuchi, Y. Makita, K. Kudo, T. Nomura, H. Tanaka, K. Irie and N. Ohnishi, Appl. Phys. Lett. 48 (1986) 59. [7] Y. Makita, T. Takeuchi, N. Oh&hi, T. Nomura, K. Kudo, H. Tanaka, H-C. Lee, M. Mori and Y. Mitsuhashi, Appl. Phys. Lett. 49 (1986) 1184. [8] M. Mori, Y. Makita, Y. Okada, N. Ohnishi, Y. Mitsuhashi, H. Tanaka and T. Matsumori, J. Appl. Phys. 62 (1987) 3212. [9] C.S. Fuller, K.B. Wolfstirn and H.W. Allison, J. Appl. Phys. 38 (1967) 2873. [lo] F.D. Rosi, D. Meyerhofer and R.V. Jensen, J. Appl. Phys. 31 (1960) 1105. [ll] Y. Makita, M. Yokota, T. Nomura, H. Tanoue, I. Takayasu, S. Kataoka, T. Izumi and T. Matsumori, Nucl. Instr. and Meth. B7/8 (1985) 433. [12] G.E. Brehm and G.L. Pearson, J. Appl. Phys. 43 (1971) 568.
OPTICAL
AND ELECTRICAL
Shigeru SHIGETOMI Nobukazu OHNISHI
PROPERTIES
OF C +-IMPLANTED
457
GaAs
I) , Yunosuke MAKITA *), Masahiko MORI 2, Yutaka KOGA ‘I, Paul PHELAN 3), Hajime SHIBATA 2, and Tokue MATSUGORI 4,
2),
‘) Department of Physics, Kurume University, 1635 Mii-maehi, Kurume-shi, Fukuoka-ken 830, Japan ‘) Electrotechnical Laboratory, I-1 -4 Umezono, Tsukuba-shi, Ibaraki-ken 305, Japan ‘) Institute of Fundamental Analysis Inc., Ltd., 3-24-3 Yoyogi, Shinjuku-ku, Tokyo 151, Japan 4’ Department of Electronics, Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa-ken 259-12, Japan
Carbon (C) was implanted into extremely pure GaAs grown by molecular beam epitaxy in a wide range of C dose [C] from to 1 x 10” cmm3. The impurity levels and the process of crystalline recovery were investigated by using Raman scattering, photoluminescence and the Hall effect. The implanted layers were found to form a crystalline layer at an annealing temperature around 200° C for [C] smaller than 1 X10” cmm3, while for [C] over that value this temperature became extremely high with increasing [C] and it reached around 550° for [C] =1 X 10” cmm3. The well-defined below-band gap emission, [g-g], exclusively inherent to acceptor impurities was a dominant one for [C] lower than 3 X 10” cme3 but its intensity was suppressed for [C] higher than this value. From the results of Hall effect, the current peculiar features of [g-g] were found to be attributable to the low substitutional efficiency of C atoms as acceptors. 1 x lOI
1. Introduction
2. Experimental
Carbon (C) is one of the residual impurities in GaAs grown by molecular beam epitaxy (MBE). Thus it is necessary to investigate the behaviour of C atoms in high-quality GaAs used for high speed electronic devices. There are studies available of the optical and electrical properties in implanted layers with a relatively low C dose, [Cl. When [C] is larger than 1 x lOi cmm3, the newly-discovered emission [g-g] due to an acceptor-acceptor pair was found to be created just below the bound exciton emissions by the measurements of low-temperature photoluminescence (PL) [l]. The results of electrical measurements revealed that p-type conduction is produced in the implanted layer when the annealing temperature (T,) is higher than 600 o C [2]. Since the C atom belongs to the column IV group, it is possible for it to work as an amphoteric impurity in GaAs. The primary purpose of this paper is to provide extensive information on the optical and electrical properties of C+-implanted GaAs for an extremely wide range of [C] up to 1 X 10 *’ cm- 3. The degree of crystalline recovery from the damaged state as functions of [C] and T, are evaluated by using Raman scattering (RS). The impurity levels are also presented from the measurements of PL and Hall effect and we discuss the possibility of C atoms as amphoteric impurities in GaAs. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The wafers used for ion implantation were (100) undoped GaAs grown by MBE at 550 o C. Ion implantation of C+ was carried out at three ion energies, 100, 200 and 300 keV. The calculation according to LSS theory predicts that the concentration of implanted C atoms is a flat distribution between 0.01 and 0.70 pm from the front surface [3]. During annealing the samples were sandwiched between two pieces of undoped GaAs in order to prevent the decomposition of GaAs [l]. All RS experiments were performed at room temperature using a quasi-backscattering geometry. The direction of propagation of the incident beam with 514.5 nm line was at an angle of about 40° with respect to the normal against the (100) surface. PL measurements were also made at 2 K by using 514.5 nm line under the condition of extremely low excitation density. The carrier concentration of implanted layers was measured by the van der Pauw method. The measurements of dc Hall voltage were carried out in a magnetic field of 0.5 T and in the temperature range from 35 to 250 K.
3. Results and discussion The annealing behavior for a sample with [C] = 1
x
of unpolarized RS spectra 102’ cm-3 is shown in fig.
V. SEMICONDUCTORS:
GaAs, FIB .
S. Shigetomi et al. / C +-implanted
550 “c ANNEAL
TO LO LA mln
IIA i
AS IMPLANTED
Fig. 1. Raman spectra of unimplanted and C+-implanted GaAs with [C] =lXIOzo cmm3 as a function of the annealing temperature.
I. The spectrum of an unimplanted (UI) sample is also indicated for comparison. In the UI sample, a strong peak at 292 cm-’ and a weak one at 269 cm-’ are the longitudinal optical (LO) and transverse optical (TO) phonons at the F point, respectively. According to the selection rules for first order RS in semiconductors having a zinc-blende structure, only the LO phonon is observed from the (100) surface in a backscattering geometry. The appearance of the weak peak attributed thus ascribed to the to the TO phonon should be present quasi-backscattering geometry. For the as-implanted sample the LO phonon intensity decreased in comparison with that of the UI sample, and its peak position shifted slightly towards lower frequency. Furthermore, a broad peak extending from 200 to 280 cm-’ appeared, and another broad but weaker one was formed between 150 and 190 cm-‘. The former peak consists of the LO, TO and longitudinal acoustic (LA) phonons both at the X and L points in the Brillouin zone. The latter peak is the impurity induced acoustic (IIA) phonon near the K point. This observation indicates that the lattice symmetry is destroyed by implantation and the lattice is highly disordered [4]. When T, exceeds 400 o C, the aforementioned two sets of broad peaks from 200 to 280 cm-’ and from 150
GaAs
completely. The to 190 cm-‘, respectively, disappeared LO phonon intensity was, however, weaker than that of the UI samples but that in the implanted sample for r, = 550°C became comparable to that of the UI sample. These observations show conclusively that the implanted layer with this extremely high [C] can recover its crystallinity gradually with increasing T, and attain an almost perfect crystalline state for annealing at a temperature higher than 550 o C. The variation of the LO phonon intensity by annealing is a very important method to evaluate the crystalline recovering procedure. Fig. 2 shows the isochronal annealing behavior of the LO phonon intensity in the samples implanted with [C] from 1 X lOi to 1 x 10” cmp3. The lowest T, at which the LO phonon intensity of the ion-implanted sample acquires around 95% of that of the UI sample is around 2OO’C for [C] smaller than 1 X 10” cmm3. For higher concentrations, this temperature increases rapidly with increasing [C] and reaches around 55O’C for [C] = 1 x 10” cme3. The features presented in fig. 2 indicate that the annealing procedures of the C+-implanted GaAs is substantially dependent upon the amount of damage produced by ion implantation. In order to investigate the properties of radiative processes pertinent to the C atoms in GaAs, we measured the low temperature PL of the C+-implanted samples. High resolution spectra near free (FE) and bound (BE) exciton emissions are shown in fig. 3. For the UI sample, the emission denoted by g, is due to the excited state of an isolated C acceptor [5]. In the implanted samples, the intensities of the sharp emissions associated with FE and BE gradually decreased with increasing [Cl. The intensity of g however, in the sample with [C] = 1 X lOi cme3, was enhanced as is shown in the figure. With further increment of [C] to greater than 1 X 10” cmp3 a new broad emission
I OO
I
I
200 ANNEALING
I
1
I
400 TEMPERATURE
1
600 (“C 1
Fig. 2. Isochronal annealing characteristics of the LO phonon intensity for C+-implanted GaAs with various [Cl.
S. Shigetomi
et al. / C + -implanted GaAs
band, denoted by [g-g], was formed on the lower energy side of g. The emission energy of [g-g] shifted moderately towards the lower energy side with [C] increasing from 1 X 10” to 3 x 10” cme3. This behavior has been reported previously for dilutely C+-implanted GaAs [l]. [g-g] is commonly observed in acceptor-incorporated GaAs irrespective of the method of introduction: ion implantation [1,6], incorporation during crystal growth either in the form of an ion beam (Mg+ for MBE) [7] or in the form of elements (Mg for liquid phase epitaxy, LPE) [8]. Since [g-g] appeared only when the acceptor concentration, [A] was relatively high and its emission energy presented a strong energy shift against the increase of [A], [g-g] was confirmed to be related to the pair between excited state of acceptors where the overlapping of excited orbitals of holes occurs making the pairs efficient radiative recombination centers [S]. For the case of implantation of heavier acceptor ion species Zn and Cd, the locking of the energy shift pertinent to [g-g] was usually observed at large [A] of around 1 x 10” cm-3 where [g-g] had already shifted very close to the transition between free electrons and the acceptor level, (e, A) [5]. In the sample with [C] greater than 5 X 1Or7 cmp3, the emission energy of [g-g] seemed to present no further shift towards the lower energy side. Additionally
PHOTON 1.520
EY$RGV
(eV ) 1.500 2x10%ni3
;~~:-lxlo~~
x,o F. E,) n.27
815
.“k’
UNIMPLANTE
820
WAVELENGTH
025 (nm)
Fig. 3. Photoluminescence spectra of unimplanted and C+-implanted GaAs near the free and bound exciton emission regions at 2 K as a function of [Cl. The annealing temperature is 850 o c.
459 TEMPERATURE
( K )
Id88
3’
’ ’
’
’ ’ ’
10
lo;T
’
’
’
20
’
’
’
’
’
30
J
(K-‘1
Fig. 4. The volume hole concentration as a function of the reciprocal temperature for C+-implanted GaAs with various [Cl. The annealing temperature is 850° C. The solid lines indicate the values given by Fermi statistics and two active acceptor levels.
the integrated PL intensity decreased with increasing [Cl. This observation, which was recognized for the first time in the present C+-implanted material, is a new feature of [g-g]. This feature of [g-g] can be explained by the low substitutional efficiency of C atoms as acceptors from the result of Hall effect measurements. Fig. 4 shows the temperature dependence of the volume hole concentration in C+-implanted GaAs for five different values of [Cl. The sheet-hole concentration was calculated from the measured Hall coefficient by using a Hall factor of unity. The volume concentration of free holes p was evaluated by assuming that holes were distributed uniformly over a depth of 0.70 urn, because the distribution of implanted C atoms is homogeneous from the front surface to a depth of 0.70 urn, as was previously described in section 2. For [C] smaller than 1 X 10” cmp3, p decreased sharply with decreasing temperature T, displaying two slopes for p above and below - 8 X 10” cme3. For the samples with [C] = 5 X 10” and 1 x 1O’9 cmm3, p decreased with decreasing T between 60 and 250 K and then increased for T below 50 K. The behavior of p at low T indicates that the conduction mechanism is strongly influenced by impurity conduction. Since the curves of p vs l/T for [C] smaller than 1 x 10” cmp3 have two slopes, we made the analysis V. SEMICONDUCTORS:
GaAs,
FIB
460
S. Shigetomi et al. / C + -implanted GaAs
Table 1 Concentrations of donor (No), shallow acceptor (NA,) and deep acceptor ( NA2) impurities, along with ionization energies of shallow acceptor ( EAI) and deep acceptor ( EA2) levels in
C+-implanted GaAs for various [Cl.
[cl
ND
N
NAZ
(cme3)
(cme3)
A’-3) (cm
(cmm3)
1x10’6
1.8x10’s
1 X10” 1x10’*
1.6~10’~ 1.7~10’~
1.OX1O16 8.6 X 10” 9.8~10’~ 8.0x10’* 1.3~10” 9.2~10”
0.026 0.026 0.025
0.10 0.10 0.10
using Fermi statistics with the assumption of two active acceptor levels [9]. Electron and hole effective mass ratios of 0.07 and 0.5 [lo] and the degeneracy factor for the acceptor level of 4 [9] were used in the calculation. The best fit curves are drawn by solid lines in fig. 4. The values of the parameters for fitting are shown in table 1. The ionization energy of a shallow acceptor obtained agrees with the 0.026 eV level of a C acceptor at As sites (C,,) [ll]. For [C] up to 1 x lOI cmw3, the concentration of C,, working as acceptors is nearly coincident with [Cl. The higher [C] brought about the reduction of the activation efficiency and for a [C] of 1 x 10” crnm3 the activation efficiency reached 13%. Carbon belongs to column IV of the Periodic Table and is supposed to work as an amphoteric impurity in GaAs. However, an increase of donors was not obtained from the results of the present analysis. We suppose that the low activation effeciency for high [C] is principally caused by the interstitial C atoms or the formation of complexes. The position of the deep acceptor energy level is at 0.10 eV above the valence band. The acceptor concentration associated with this level which is larger than [C] increases with increasing [Cl. Brehm and Pearson have reported the deep acceptor level in ray-irradiated GaAs from the results of Hall effect measurements [12]. The value of the deep acceptor level obtained is the same as that of the defect related deep acceptor level, suggesting that the defects from ion implantation or high-temperature annealing remain in the implanted layer.
4. Conclusion The electrical and optical properties of C+-implanted GaAs as a function of dose and annealing temperature were measured by Raman scattering, photoluminescence and Hall effect. The minimum annealing temperature necessary for obtaining a crystalline layer is strongly dependent upon [C] and is around 200 o C for [C] less than 1 X 10” cme3 while it is higher than 550 o C for [C] = 1 X 10” cme3. [g-g] formed just below the bound exciton emissions is a dominant emission in C+-implanted GaAs. The locking of the energy shift of [g-g] was found to take place at a relatively low [Cl. It is demonstrated from the combined results of photoluminescence and Hall effect that this peculiar behavior of [g-g] is caused by the low efficiency of substitutional C atoms.
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