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Magnetophotoluminescence in high purity MOVPE GaAs
Journal of Crystal Growth 77 (1986) 321—325 North-Holland, Amsterdam
321
MAGNETOPHOTOLUMINESCENCE IN HIGH PURITY MOVPE GaAs S.A. ZEMON, P.E. NORRIS and G. LAMBERT GTE Laboratories Inc., 40 Sylvan Roa4 Waltham, Massachusetts 02254, USA
Magnetophotoluminescence (MPL) is shown to be useful for the identification of trace acceptor impurities in MOVPE GaAs. Using MPL a trace concentration of zinc acceptors was detected in a sample where the zinc transitions were obscured in zero magnetic field. Acceptor MPL was observed to be insensitive to V/Ill ratio for values between 17.3 and 54.4. Conduction-band-toacceptor MPL spectral widths as small as 0.3 meV were found. Resolved Landau level transitions and the magnetic splitting of conduction-band-to-acceptor transitions were observed.
1. Introduction The identification of acceptor impurity transitions is very important for semiconductor characterization. Free-to-bound-acceptor [(e, A°)] transitions [1] can often be identified at 4.2 K by observing the photoluminescence (PL) peak energy (Er) at low excitation power densities (Fe) for samples with donor concentrations low enough so that donor-to-acceptor [(D°, A°)] transitions are not significant. For compensated samples, it may be possible to suppress competing (D°, A°) peaks, and thereby identify the specific (e, A°) transitions, by employing high ~e where (D°,A°) luminescence is saturated or taking spectra at temperatures T high enough for the donors to ionize [1]. However, cases arise where these procedures do not lead to definitive, identifiable (e, A°) peaks. For example, one such case occurs in our high purity metallorganic vapor phase epitaxy (MOVPE) GaAs which has acceptor PL spectra dominated by carbon with magnesium and/or beryllium in lower concentrations. If zinc were also present as a trace contaminant, a problem could occur in detecting Zn because luminescence due to the other acceptors can obscure the Zn transitions. In order to explore this situation, we have found it quite helpful to study PL spectra from samples subjected to magnetic fields. It is well known thatstates in a are strong magnetic into fieldLandau the conduction band transformed level
(n) distributions with drastic changes in the density of states [2].At fields high enough to split the n 0 and n 1 (e, A°) transitions, the spectral width is appreciably reduced and E~(ignoring the small Zeeman and diamagnetic effects) shifts unearly with the magnetic flux density (B) as (n + 1/2)hw~/21T, where n 0, 1,..., w~ eB/mec, and me is the electron effective mass [3,4]. Thus, it is possible to identify definitively transitions as (e, A°)due to their characteristic shifts in energy with increasing B [5]. =
=
=
=
2. Experimental The nominally undoped MOVPE GaAs layers studied here were grown at low pressure (50 Torr) on a semi-insulating 50 mm diameter, liquid encapsulated Czochralski (LEC) substrate [(100) 2° off towards (110)]. The film thicknesses ranged from 5 to 13 ~tm. The growth temperatures were between 575 and 6250 C, and the nominal [V/Ill] ratio of the mole fraction concentrations (in 5 SLPM H2 carrier gas) went from 17.3 to 54.4. The growth rate was 15.0 nm/mm. The vertical, RFheated growth system as well as the substrate preparation technique has been described previously [6].Triethylgallium (TEG) and arsine sources were supplied by Stauffer Chemical and Phoenix Research, respectively, and have produced2/V. highs purity GaAs with high mobility (132,000 at 77 K), and very sharp excitonic PL cm structure
0022-0248/86/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
322
S.A. Zemon eg al.
/
Magnetophotoluminescence in high purity MOVFE GaAs
dominated by free-exciton emission [7]. The magnetophotoluminescence (MPL) was measured at 4.2 K with the sample freely suspended in liquid helium. Magnetic flux densities of up to 6.4 T were produced by a superconducting magnet in the Faraday configuration where the magnetic field is perpendicular to the sample surface and the luminescence is observed along the direction of the field. The excitation source, the 676.4 nm line of a krypton laser, was at perpendicular incidence to the sample with a power density of 0.5 mW/cm2 and a beam diameter of 5 mm. The luminescence was analyzed for right circularly polarized (RCP) and left circularly polarized (LCP) components, dispersed with a ~ m double monochromator and detected with a cooled GaAs photomultiplier system applying photon counting techniques. A monochromator resolution of 0.04 nm was used.
B 828
830
=
3. MPL residual acceptor identification Fig. 1 shows PL spectra for LCP luminescence at magnetic flux densities of 0, 1.7, and 5.1 T taken on a 5 ~tm thick layer which was grown at 575°Cwith a V/Ill ratio of 54.4. The peaks in the zero field spectrum are identified as (e, A~)[1,8], (e, A~g/Be)[1], and (D°,A~)[8] (and possibly (e, A~~) [1,8]} at 1.4934 eV (830.00 nm), 1.4916 eV (831.00 nm), and 1.4898 eV (832.00 nm), respectively. As the flux density is increased beyond 0.4 T, three sharp peaks appear in the spectra. The spectral width of the (e, A~)line, for example, narrows by a factor of about three from 1.6 to 0.6 meV. The increase in energy of these peaks with B can be clearly observed in the 1.7 and 5.1 T spectra. In fig. 2, the energy peaks of the LCP (/.~m +1) and RCP (~m —1) modes are plotted as a function of B. Straight lines are drawn through the data points and extrapolated =
5.1 T
=
-
-
/
-
832
X (nm) Fig. 1. MPL LCP spectra in the acceptor luminescence region at 4.2 K. Feature I is (e, A~)with n = 0, feature 2 is (e, A~g/~)with n = 0, feature 3 is (e, A~~) with n 0, and feature 4 is (e, A~)with n 1 where n is the Landau level,
1.489 0
I
I
I
2.0 MAGNETIC FLUX 4.0 DENSITY (T)
I 6.0
Fig. 2. Magnetic flux density dependence of the (e, A°)peak energies.
S.A. Zemon et aL
/
Magnetophotoluminescence in high purity MOVPE GaAs
(i.e., averaging out the magnetic sublevel splittings) has a slope consistent with 0.87 meV/T as expected for the movement of the n 0 conduction band back to B Landau 0. Thelevel, center confirming of gravitythat of each all three set peaks are (e, A°)transitions. The symmetric splitting of the (e, A~)components seen at 6.4 T is similar to that observed in ref. [4]. The lowest energy transition in fig. 2 extrapolates back to 1.4892 eV at B 0. Because of departures from linearity in weak fields [3], the zero field values of (e, A~)and A~jg/Be) are larger than the extrapolated values by almost 0.3 extrapolated value to obtain an estimated zero meV. Thus, this quantity is added to the 1.4892 eV field value of 1.4895 eV. The latter is consistent with a free-to-bound transition involving a zinc acceptor [1,8]. Using conventional PL neither the 4.2 K, high Pc spectrum nor the 20 K spectrum exhibit any well-defined (e, A°)peaks other than the one for the carbon acceptor. Thus, MPL data are needed in this case for the definitive identification of the zinc acceptor.
323
=
RCP
=
.
=
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,
4. MPL (e, A°)data versus V/Ill ratio Four samples with V/Ill ratios of 17.3, 27.2, 34.7, and 54.4, were examined for trends in the acceptor MPL. The first three were grown at 625°C and the last one at 600°C. The MPL spectra of all four were quite similar, with the sample grown at 600°C having slightly sharper lines. RCP spectra are shown in fig. 3 at magnetic flux densities of 0, 3.8, and 6.4 T for the 600°C sample. Note that carbon is the dominant acceptor with Mg/Be observable as a trace acceptor only in a magnetic field. From the figure it can be seen that the splitting of the (e, A~)line into two LCP components is easily observable at 3.8 T. In fact, the splitting of the LCP and RCP (e, A~) transitions into the four allowed circularly polarized sublevels could be distiguished at magnetic flux densities as low as 2.5 T, an indication of the sharpness of the lines. The magnetic sublevel resolution for RCP (e, A~)achieved at 6.4 T is better than that obtained for high purity (1.4 x
3.8T
6.4T
I
I
I
I
826
828
830
832
>‘, (‘~) Fig. 3. MPL RCP acceptor spectra at 4.2 K.
1014 cm _3) VPE-grown GaAs using an appreciably higher magnetic flux density (10 T) [4]. The full width at half maximum (FWHM) of each (e, A~)line at 6.4 T is estimated to be 0.3 meV as compared to a FWHM in zero field of 0.9 meV. The spectral width of these lines is attributed to a collisional lifetime broadening for the free electrons [9]. We have found that lower mobility (85,000 cm2/V. s at 77 K) samples have noticeably broader (e, A~)lines in a magnetic field. In fact, their MPL acceptor spectra show no splitting into LCP and RCP Zeeman doublets. Thus, the FWHM of the MPL (e, A~)lines appears to be a useful measure of sample quality. The acceptor-region MPL spectra were observed to be substantially independent of the nominal V/Ill ratio. While at first this may seen to contradict previously published V/Ill ratio dependences [10], it must be remembered that TEG is being used as the gallium source, not TMG. Previous work with TEG [11] showed that the acceptor PL did not vary significantly with the nominal V/Ill ratio, in agreement with the results
324
S.A. Zemon et at
/ Magnetophotoluminescence in high purity MOVPE GaAs
well understood. However, it has also been observed in the case of (Al, Ga)As grown from TEG and TEAl [12]. Analysis of the carbon content by SIMS showed presented here.little Thevariation reason for in this C with behavior V/Ill isratio not in marked contrast to the strong variation seen when using TMG and TMA1. Although splitting into RCP (and LCP) doublets was seen in the MPL (e, A~)lines for all four samples, for the MPL a similar (e, A~g/Be) splitting lines didasnot canalways be observed occur from fig. 3. In contrast, 6.4 T spectra from one sample (V/Ill 17.3) did show an (e, A~g/Be) splitting. The observed RCP splitting of 0.36 meV (0.02 nm) is equal to that found for the RCP (e A~)line. The reason for the lack of symmetric behavior often observed for C and Mg/Be (e, A°) transitions is not understood, although the possibility of simultaneous contributions from both Mg and Be in some cases cannot be ruled out.
___~
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OT
Z Ui Ui
0.2T
z w C.)
=
Ui
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/
fl...3
828
o.4T
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5. Landau level transitions
830
831
Fig. 4. Low field MPL (e, A~)spectra at 4.2 K.
We now direct our attention to spectra obtained at low magnetic fields. Fig. 4 shows the LCP (e, A~)line taken at 0, 0.2, and 0.4 T. The dramatic narrowing of the spectral width as the magnetic field increases [3,4] (caused by the peaking in the Landau density of states [3,4]) is quite evident. In addition to the main transition between the neutral carbon acceptors and the n 0 Landau level, lower intensity transitions are seen on the high energy side of the main line, involving n 1, 2, and 3 Landau levels. These data are similar to those for high 4cm3) VPE reported p-type GaAs [3,4].purity ((1—2) x 10’
___________________________________
1.496
-
I
/
The increase of the Landau level energy peaks Landau with data magnetic points level for predictions each density Landau islevel (with shown follow a small in fig. thecontri5.linear The bution due to flux the Zeeman difference between adjacentsplitting). straight The linesenergy at a particular magnetic field gives a magnetic field splitting of hw/2irB 1.71 meV/T which is equivalent to an effective mass of 0.0677 times the mass of an electron. This is within about 2% of the previous values of m~for GaAs [3]. =
1.495
-
LCP
/
~ I /
/ II
=
=
(e,A~)
1,7
/
/I /~
I
>‘
a
(‘
‘I
Ui1.494 Z Ui
‘I,
I
1.493 0.0
I
I
I
2.0 MAGNETIC FLUX DENSITY (T) 1.0
I 3.0
Fig. 5. Magnetic flux density dependence of Landau level peaks at 4.2 K.
S.A. Zemon et at
/
Magnetophotoluminescence in high purity MOVPE GaAs
In all four samples a small peak was seen on the low energy side of the (e, A~)line with a zero field maximum at 830.27 nm (1.49291 eV). The MPL behavior of the energy maximum does not exhibit the linear relationships followed by (e, A°) transitions, but rather increases quadratically with field [13] in a fashion similar to (D°, A~) [5]. Thus, the possibility of an (e, A°)identification is eliminated and that of a donor-to-acceptor transilion is suggested. Following Skromme and Stillman [14], we identify the line with a donor-toacceptor transition where the electron is in the n 2 excited state [(D,~2, At)].
325
transitions were resolved in the (e, A~)line, and the Zeeman splittings of the (e, A°) transitions were observed.
Acknowledgements We wish to thank Dr. S.K. Shastry for helpful conversations; Drs. H. Lockwood and P. Haugsjaa for continued support; and M. DeAngelis and D. Zappen for technical assistance.
=
References 5. Summary
[11 D.J. Ashen, P.J. Dean, D.T.J. Hurle, J.B. Mullin and A.M. White, J. Phys. Chem. Solids 36 (1975) 1041.
MPL has been found to be a useful technique for identifying trace acceptor impurities in undoped GaAs. It was possible to deduce the presence of zinc at B < 2 T even though its luminescence peak was obscured in zero field. At B 0.4 T the (e, A~~)line became observable in the center of the (D°,A~)line and moved linearly (as hco~/4~) away from the latter as B increases. The zinc identification was based upon an extrapolation of the various .E~(B) data back to zero field. For samples grown with V/Ill ratios in the range between 17.3 and 54.4 and growth temperatures between 600 and 625°C, the dominant acceptor was found to be carbon. Be/Mg was present as a trace acceptor, but was observable only when using MPL techniques. The MPL spectra for these (e, A°) transitions were found to be insensitive to variations in V/Ill ratio, at least for the range of ratios examined in this study. The utilization of TEG (as compared to TMG) as a source material is thought to be responsible for this insensitivity. MPL (e, A°)spectral widths as narrow as 0.3 meV were found. Finally, for the first time in MOVPE material, the n 0, 1, 2, and 3 Landau level =
[2] L.M. Roth and P.N. Ar~rres, in: Semiconductors and Semimetals, Vol. 1, Ed. R.K. Willardson and AC. Beer (Academic Press, New York, 1972). [3] W. Ruhie and E. Gobel, Phys. Status Solidi (b) 78 (1976) 311. [4] D. Bimberg, Phys. Rev. B18 (1978) 1794. [5] J.A. Rossi, C.M. Wolfe and JO. Dimmock, Phys. Rev. Letters 25 (1970) 1614. [6] P. Norris, J. Black, S. Zemon and G. Lambert, J. Crystal Growth 68 (1984) 437. [7] J. Black, P. Norris, E. Koteles and S. Zemon, in: Proc. 11th Intern. Symp. on GaAs and Related Compounds, Biarritz, 1984, Inst. Phys. Conf. Ser. 74, Ed. B. de Cremoux (Inst. Phys., London—Bristol, 1985) p. 683. [8] M. Ozeki, K. Nakai, K. Dazai and 0. Ryuzan, Japan. J. Appl. Phys. 13 (1974) 1121; 12 (1973) 479. [9] P.J. Dean, H. Venghaus and P.E. Simmonds, Phys. Rev. B18 (1978) 6813. [10] S. Takagishi and H. Mori, Japan. J. Appl. Phys. 23 (1984) [11] L100. K. Kimura, S. Takagishi, S. Horiguchi, K. Kamon, M. Mikara and M. Ishii, in: Extended Abstracts of the 17th Conf. on Solid State Devices and Materials, Tokyo, 1985, pp. 201—204. [12] N. Kobayashi and T. Makimoto, Japan. J. Appl. Phys. 24 (1985) L824. [13] 5. Zemon and G. Lambert, unpublished, 1986. [14] B.J. Skromme and G.E. Stillman, Phys. Rev. B29 (1984) 1982.
321
MAGNETOPHOTOLUMINESCENCE IN HIGH PURITY MOVPE GaAs S.A. ZEMON, P.E. NORRIS and G. LAMBERT GTE Laboratories Inc., 40 Sylvan Roa4 Waltham, Massachusetts 02254, USA
Magnetophotoluminescence (MPL) is shown to be useful for the identification of trace acceptor impurities in MOVPE GaAs. Using MPL a trace concentration of zinc acceptors was detected in a sample where the zinc transitions were obscured in zero magnetic field. Acceptor MPL was observed to be insensitive to V/Ill ratio for values between 17.3 and 54.4. Conduction-band-toacceptor MPL spectral widths as small as 0.3 meV were found. Resolved Landau level transitions and the magnetic splitting of conduction-band-to-acceptor transitions were observed.
1. Introduction The identification of acceptor impurity transitions is very important for semiconductor characterization. Free-to-bound-acceptor [(e, A°)] transitions [1] can often be identified at 4.2 K by observing the photoluminescence (PL) peak energy (Er) at low excitation power densities (Fe) for samples with donor concentrations low enough so that donor-to-acceptor [(D°, A°)] transitions are not significant. For compensated samples, it may be possible to suppress competing (D°, A°) peaks, and thereby identify the specific (e, A°) transitions, by employing high ~e where (D°,A°) luminescence is saturated or taking spectra at temperatures T high enough for the donors to ionize [1]. However, cases arise where these procedures do not lead to definitive, identifiable (e, A°) peaks. For example, one such case occurs in our high purity metallorganic vapor phase epitaxy (MOVPE) GaAs which has acceptor PL spectra dominated by carbon with magnesium and/or beryllium in lower concentrations. If zinc were also present as a trace contaminant, a problem could occur in detecting Zn because luminescence due to the other acceptors can obscure the Zn transitions. In order to explore this situation, we have found it quite helpful to study PL spectra from samples subjected to magnetic fields. It is well known thatstates in a are strong magnetic into fieldLandau the conduction band transformed level
(n) distributions with drastic changes in the density of states [2].At fields high enough to split the n 0 and n 1 (e, A°) transitions, the spectral width is appreciably reduced and E~(ignoring the small Zeeman and diamagnetic effects) shifts unearly with the magnetic flux density (B) as (n + 1/2)hw~/21T, where n 0, 1,..., w~ eB/mec, and me is the electron effective mass [3,4]. Thus, it is possible to identify definitively transitions as (e, A°)due to their characteristic shifts in energy with increasing B [5]. =
=
=
=
2. Experimental The nominally undoped MOVPE GaAs layers studied here were grown at low pressure (50 Torr) on a semi-insulating 50 mm diameter, liquid encapsulated Czochralski (LEC) substrate [(100) 2° off towards (110)]. The film thicknesses ranged from 5 to 13 ~tm. The growth temperatures were between 575 and 6250 C, and the nominal [V/Ill] ratio of the mole fraction concentrations (in 5 SLPM H2 carrier gas) went from 17.3 to 54.4. The growth rate was 15.0 nm/mm. The vertical, RFheated growth system as well as the substrate preparation technique has been described previously [6].Triethylgallium (TEG) and arsine sources were supplied by Stauffer Chemical and Phoenix Research, respectively, and have produced2/V. highs purity GaAs with high mobility (132,000 at 77 K), and very sharp excitonic PL cm structure
0022-0248/86/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
322
S.A. Zemon eg al.
/
Magnetophotoluminescence in high purity MOVFE GaAs
dominated by free-exciton emission [7]. The magnetophotoluminescence (MPL) was measured at 4.2 K with the sample freely suspended in liquid helium. Magnetic flux densities of up to 6.4 T were produced by a superconducting magnet in the Faraday configuration where the magnetic field is perpendicular to the sample surface and the luminescence is observed along the direction of the field. The excitation source, the 676.4 nm line of a krypton laser, was at perpendicular incidence to the sample with a power density of 0.5 mW/cm2 and a beam diameter of 5 mm. The luminescence was analyzed for right circularly polarized (RCP) and left circularly polarized (LCP) components, dispersed with a ~ m double monochromator and detected with a cooled GaAs photomultiplier system applying photon counting techniques. A monochromator resolution of 0.04 nm was used.
B 828
830
=
3. MPL residual acceptor identification Fig. 1 shows PL spectra for LCP luminescence at magnetic flux densities of 0, 1.7, and 5.1 T taken on a 5 ~tm thick layer which was grown at 575°Cwith a V/Ill ratio of 54.4. The peaks in the zero field spectrum are identified as (e, A~)[1,8], (e, A~g/Be)[1], and (D°,A~)[8] (and possibly (e, A~~) [1,8]} at 1.4934 eV (830.00 nm), 1.4916 eV (831.00 nm), and 1.4898 eV (832.00 nm), respectively. As the flux density is increased beyond 0.4 T, three sharp peaks appear in the spectra. The spectral width of the (e, A~)line, for example, narrows by a factor of about three from 1.6 to 0.6 meV. The increase in energy of these peaks with B can be clearly observed in the 1.7 and 5.1 T spectra. In fig. 2, the energy peaks of the LCP (/.~m +1) and RCP (~m —1) modes are plotted as a function of B. Straight lines are drawn through the data points and extrapolated =
5.1 T
=
-
-
/
-
832
X (nm) Fig. 1. MPL LCP spectra in the acceptor luminescence region at 4.2 K. Feature I is (e, A~)with n = 0, feature 2 is (e, A~g/~)with n = 0, feature 3 is (e, A~~) with n 0, and feature 4 is (e, A~)with n 1 where n is the Landau level,
1.489 0
I
I
I
2.0 MAGNETIC FLUX 4.0 DENSITY (T)
I 6.0
Fig. 2. Magnetic flux density dependence of the (e, A°)peak energies.
S.A. Zemon et aL
/
Magnetophotoluminescence in high purity MOVPE GaAs
(i.e., averaging out the magnetic sublevel splittings) has a slope consistent with 0.87 meV/T as expected for the movement of the n 0 conduction band back to B Landau 0. Thelevel, center confirming of gravitythat of each all three set peaks are (e, A°)transitions. The symmetric splitting of the (e, A~)components seen at 6.4 T is similar to that observed in ref. [4]. The lowest energy transition in fig. 2 extrapolates back to 1.4892 eV at B 0. Because of departures from linearity in weak fields [3], the zero field values of (e, A~)and A~jg/Be) are larger than the extrapolated values by almost 0.3 extrapolated value to obtain an estimated zero meV. Thus, this quantity is added to the 1.4892 eV field value of 1.4895 eV. The latter is consistent with a free-to-bound transition involving a zinc acceptor [1,8]. Using conventional PL neither the 4.2 K, high Pc spectrum nor the 20 K spectrum exhibit any well-defined (e, A°)peaks other than the one for the carbon acceptor. Thus, MPL data are needed in this case for the definitive identification of the zinc acceptor.
323
=
RCP
=
.
=
IZ
I-
z
~ C.) (I’ Ui
,
4. MPL (e, A°)data versus V/Ill ratio Four samples with V/Ill ratios of 17.3, 27.2, 34.7, and 54.4, were examined for trends in the acceptor MPL. The first three were grown at 625°C and the last one at 600°C. The MPL spectra of all four were quite similar, with the sample grown at 600°C having slightly sharper lines. RCP spectra are shown in fig. 3 at magnetic flux densities of 0, 3.8, and 6.4 T for the 600°C sample. Note that carbon is the dominant acceptor with Mg/Be observable as a trace acceptor only in a magnetic field. From the figure it can be seen that the splitting of the (e, A~)line into two LCP components is easily observable at 3.8 T. In fact, the splitting of the LCP and RCP (e, A~) transitions into the four allowed circularly polarized sublevels could be distiguished at magnetic flux densities as low as 2.5 T, an indication of the sharpness of the lines. The magnetic sublevel resolution for RCP (e, A~)achieved at 6.4 T is better than that obtained for high purity (1.4 x
3.8T
6.4T
I
I
I
I
826
828
830
832
>‘, (‘~) Fig. 3. MPL RCP acceptor spectra at 4.2 K.
1014 cm _3) VPE-grown GaAs using an appreciably higher magnetic flux density (10 T) [4]. The full width at half maximum (FWHM) of each (e, A~)line at 6.4 T is estimated to be 0.3 meV as compared to a FWHM in zero field of 0.9 meV. The spectral width of these lines is attributed to a collisional lifetime broadening for the free electrons [9]. We have found that lower mobility (85,000 cm2/V. s at 77 K) samples have noticeably broader (e, A~)lines in a magnetic field. In fact, their MPL acceptor spectra show no splitting into LCP and RCP Zeeman doublets. Thus, the FWHM of the MPL (e, A~)lines appears to be a useful measure of sample quality. The acceptor-region MPL spectra were observed to be substantially independent of the nominal V/Ill ratio. While at first this may seen to contradict previously published V/Ill ratio dependences [10], it must be remembered that TEG is being used as the gallium source, not TMG. Previous work with TEG [11] showed that the acceptor PL did not vary significantly with the nominal V/Ill ratio, in agreement with the results
324
S.A. Zemon et at
/ Magnetophotoluminescence in high purity MOVPE GaAs
well understood. However, it has also been observed in the case of (Al, Ga)As grown from TEG and TEAl [12]. Analysis of the carbon content by SIMS showed presented here.little Thevariation reason for in this C with behavior V/Ill isratio not in marked contrast to the strong variation seen when using TMG and TMA1. Although splitting into RCP (and LCP) doublets was seen in the MPL (e, A~)lines for all four samples, for the MPL a similar (e, A~g/Be) splitting lines didasnot canalways be observed occur from fig. 3. In contrast, 6.4 T spectra from one sample (V/Ill 17.3) did show an (e, A~g/Be) splitting. The observed RCP splitting of 0.36 meV (0.02 nm) is equal to that found for the RCP (e A~)line. The reason for the lack of symmetric behavior often observed for C and Mg/Be (e, A°) transitions is not understood, although the possibility of simultaneous contributions from both Mg and Be in some cases cannot be ruled out.
___~
11f\1LCP~cP
OT
Z Ui Ui
0.2T
z w C.)
=
Ui
-J
/
fl...3
828
o.4T
829
X (nm)
5. Landau level transitions
830
831
Fig. 4. Low field MPL (e, A~)spectra at 4.2 K.
We now direct our attention to spectra obtained at low magnetic fields. Fig. 4 shows the LCP (e, A~)line taken at 0, 0.2, and 0.4 T. The dramatic narrowing of the spectral width as the magnetic field increases [3,4] (caused by the peaking in the Landau density of states [3,4]) is quite evident. In addition to the main transition between the neutral carbon acceptors and the n 0 Landau level, lower intensity transitions are seen on the high energy side of the main line, involving n 1, 2, and 3 Landau levels. These data are similar to those for high 4cm3) VPE reported p-type GaAs [3,4].purity ((1—2) x 10’
___________________________________
1.496
-
I
/
The increase of the Landau level energy peaks Landau with data magnetic points level for predictions each density Landau islevel (with shown follow a small in fig. thecontri5.linear The bution due to flux the Zeeman difference between adjacentsplitting). straight The linesenergy at a particular magnetic field gives a magnetic field splitting of hw/2irB 1.71 meV/T which is equivalent to an effective mass of 0.0677 times the mass of an electron. This is within about 2% of the previous values of m~for GaAs [3]. =
1.495
-
LCP
/
~ I /
/ II
=
=
(e,A~)
1,7
/
/I /~
I
>‘
a
(‘
‘I
Ui1.494 Z Ui
‘I,
I
1.493 0.0
I
I
I
2.0 MAGNETIC FLUX DENSITY (T) 1.0
I 3.0
Fig. 5. Magnetic flux density dependence of Landau level peaks at 4.2 K.
S.A. Zemon et at
/
Magnetophotoluminescence in high purity MOVPE GaAs
In all four samples a small peak was seen on the low energy side of the (e, A~)line with a zero field maximum at 830.27 nm (1.49291 eV). The MPL behavior of the energy maximum does not exhibit the linear relationships followed by (e, A°) transitions, but rather increases quadratically with field [13] in a fashion similar to (D°, A~) [5]. Thus, the possibility of an (e, A°)identification is eliminated and that of a donor-to-acceptor transilion is suggested. Following Skromme and Stillman [14], we identify the line with a donor-toacceptor transition where the electron is in the n 2 excited state [(D,~2, At)].
325
transitions were resolved in the (e, A~)line, and the Zeeman splittings of the (e, A°) transitions were observed.
Acknowledgements We wish to thank Dr. S.K. Shastry for helpful conversations; Drs. H. Lockwood and P. Haugsjaa for continued support; and M. DeAngelis and D. Zappen for technical assistance.
=
References 5. Summary
[11 D.J. Ashen, P.J. Dean, D.T.J. Hurle, J.B. Mullin and A.M. White, J. Phys. Chem. Solids 36 (1975) 1041.
MPL has been found to be a useful technique for identifying trace acceptor impurities in undoped GaAs. It was possible to deduce the presence of zinc at B < 2 T even though its luminescence peak was obscured in zero field. At B 0.4 T the (e, A~~)line became observable in the center of the (D°,A~)line and moved linearly (as hco~/4~) away from the latter as B increases. The zinc identification was based upon an extrapolation of the various .E~(B) data back to zero field. For samples grown with V/Ill ratios in the range between 17.3 and 54.4 and growth temperatures between 600 and 625°C, the dominant acceptor was found to be carbon. Be/Mg was present as a trace acceptor, but was observable only when using MPL techniques. The MPL spectra for these (e, A°) transitions were found to be insensitive to variations in V/Ill ratio, at least for the range of ratios examined in this study. The utilization of TEG (as compared to TMG) as a source material is thought to be responsible for this insensitivity. MPL (e, A°)spectral widths as narrow as 0.3 meV were found. Finally, for the first time in MOVPE material, the n 0, 1, 2, and 3 Landau level =
[2] L.M. Roth and P.N. Ar~rres, in: Semiconductors and Semimetals, Vol. 1, Ed. R.K. Willardson and AC. Beer (Academic Press, New York, 1972). [3] W. Ruhie and E. Gobel, Phys. Status Solidi (b) 78 (1976) 311. [4] D. Bimberg, Phys. Rev. B18 (1978) 1794. [5] J.A. Rossi, C.M. Wolfe and JO. Dimmock, Phys. Rev. Letters 25 (1970) 1614. [6] P. Norris, J. Black, S. Zemon and G. Lambert, J. Crystal Growth 68 (1984) 437. [7] J. Black, P. Norris, E. Koteles and S. Zemon, in: Proc. 11th Intern. Symp. on GaAs and Related Compounds, Biarritz, 1984, Inst. Phys. Conf. Ser. 74, Ed. B. de Cremoux (Inst. Phys., London—Bristol, 1985) p. 683. [8] M. Ozeki, K. Nakai, K. Dazai and 0. Ryuzan, Japan. J. Appl. Phys. 13 (1974) 1121; 12 (1973) 479. [9] P.J. Dean, H. Venghaus and P.E. Simmonds, Phys. Rev. B18 (1978) 6813. [10] S. Takagishi and H. Mori, Japan. J. Appl. Phys. 23 (1984) [11] L100. K. Kimura, S. Takagishi, S. Horiguchi, K. Kamon, M. Mikara and M. Ishii, in: Extended Abstracts of the 17th Conf. on Solid State Devices and Materials, Tokyo, 1985, pp. 201—204. [12] N. Kobayashi and T. Makimoto, Japan. J. Appl. Phys. 24 (1985) L824. [13] 5. Zemon and G. Lambert, unpublished, 1986. [14] B.J. Skromme and G.E. Stillman, Phys. Rev. B29 (1984) 1982.