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Effect of non-stoichiometry on near-band-edge absorption and non- radiative recombination in bulk GaAs
Materials Science and Engineering, B14 (1992) 47-52
47
Effect of non-stoichiometry on near-band-edge absorption and nonradiative recombination in bulk GaAs S. T/izemen and M. R. Brozel Department of Electrical Engineering and Electronics and The Centrefor Electronic Materials, University of Manchester Institute of Science and Technology, PO Box 88, Manchester M60 1QD (UK) (Received November 25, 1991 )
Abstract We demonstrate that the total near-band-edge photoluminescence intensity of undoped GaAs grown by the liquid encapsulated Czochralski technique is reduced if the stoichiometry of the starting melt becomes more gallium rich. At the same time, concentrations of the native deep donor level EL2 are reduced, but there is an increase in the concentration of an unknown defect which causes absorption near the band edge in cooled samples. Such absorption is known as reverse contrast (RC). We produce evidence that the defects that give rise to this absorption, RC defects, are also the dominant non-radiative recombination centres in undoped GaAs. It is probable that these defects are gallium rich and may be related to arsenic vacancies.
I. Introduction
There is great technological interest in investigating native point defects in liquid encapsulated Czochralski (LEC) GaAs as it is believed that it is their role in compensating residual chemical acceptors, such as carbon and zinc, that gives rise to the semi-insulating (SI) behaviour of undoped material. Of the several deep native defects in GaAs, the midgap level EL2 is known as being the most important in this compensation mechanism. The majority of EL2 defects in common substrate materials are neutral and it is fortunate for reasons of substrate assessment that they produce a broad absorption signature that extends from the band edge to near 1.5 ,urn. An important property of this defect is its photoquenchability (or bleachability) to a metastable state. Thus, at sample temperatures below 120 K and on illumination by subbandgap light, EL2 centres lose both their electrical and their optical behaviour [1-5]. This property allows us to make accurate determinations of concentrations of neutral EL2 defects in a simple way by observing changes in absorption after bleaching [6]. However, it has been demonstrated that another point defect [7, 8] gives rise to additional low temperature (below 140 K) absorption that is restricted to 0921-5107/92/$5.00
within 65 meV of the band edge. An absorption image produced by these defects is known as reverse contrast (RC). These centres can also be photoquenched [9, 10] but at lower sample temperatures than those used for photoquenching EL2 centres. We have demonstrated previously that these point defects are not related to the EL2 defect in any charge state [6]. The major technological importance of the point defects that cause low temperature near-band-edge absorption is that their IR absorption images exhibit a remarkable resemblance to low temperature cathodoluminescence (CL)[11] and photoluminescence (PL) [12] images. As it is accepted that luminescence intensity in these materials at low temperatures is controlled by non-radiative recombination; it appears that RC absorption mapping [13] can give information regarding the distributions of the dominant non-radiative recombination centre in bulk GaAs in a quantitative and rapid way [13]. In this work, we measure concentrations of EL2 and near-band-edge absorption centres (RC defects) and their corresponding photoquenching as functions of near-band-edge PL intensity and starting melt stoichiomerry in both lightly n-type and SI GaAs. We demonstrate that all these properties are dependent on melt stoichiometry and that trends observed previously © 1992 - Elsevier Sequoia. All rights reserved
4~
,% Tiizemen. M. R. Brozel
/
Near-hand-edge absorption in GaAs
in commercial, "stoichiometric'" material can be extended to these samples.
2. Experimental techniques The samples used in this study were either undoped or lightly n-type GaAs grown by the LEC method. The n-type crystals were doped with tin, a fraction of which was radioactive to allow quantitative electrical and compositional assessment. Melt stoichiometries were adjusted to produce melts varying from intentional gallium richness (Ga:As up to 1.1 in the original melt) to intentional arsenic richness (Ga:As down to 0.99 in the original melt). The use of dry boric oxide, B203, as the encapsulant for some growths was intended to retain the original stoichiometry by reducing possible reactions by either the gallium or the arsenic with water in the B203. The tin-doped crystals were n-type for all or most of the volume [14]. A series of commercial SI, LEC GaAs substrates is also investigated. A novel optical absorption bleaching technique [6] was employed to determine neutral EL2 concentrations [EL2 ~] and near-band-edge absorption coefficients as functions of original melt stoichiometry. In this technique, changes in optical transmission at particular wavelengths are measured before and after bleaching to determine quantitative values of [EL2 °] and relative values of RC defect concentrations [RC]. Although this technique was originally devised for the assessment of semi-insulating GaAs, we have shown that it works satisfactorily for all the present samples. EL2 optical absorption measurements were made at a wavelength of 1 /zm at sample temperatures below 120 K. After the entire bleaching of the absorption by EL2 centres, the transmission was re-measured. Nearband-edge low temperature RC absorption bleaching measurements at a wavelength of 0.83 /zm were performed on the same samples, but at lower temperatures (below 45 K). During this and subsequent measurements taken after bleaching, interference from EL2 absorption was absent because all EL2 centres were in their metastable, inert state. Light beams of 0.91 # m and 0.95 /~m wavelength derived from a quartz halogen bulb and interference filters were used to photoquench the EL2 and RC centres respectively. The absorption coefficient A a ( 2 ) ( c m -I) of the photoquenchable defects was found from the change in transmitted intensity at wavelength ). from Aa(2) = (1/d) in[l(t)/l(0)], where 1(0)and I ( t ) a r e the intensities of transmitted light respectively before and after a time t (s) of photoquenching and d (cm) is the sample thickness. Neutral EL2 concentrations [EL2 °] were calculated by using Martin's calibration [1]. At present, it has not been found possible to relate the optical absorption
of the near-band-edge component to defect concentration. The advantages of this optical absorption bleaching technique are as follows. (i) Characterization of the material can be achieved easily using commercial 400 pm thick material. (ii) Because the comparison is made before and after bleaching, there is no need to use a reference sample. This simplifies the experimental system. (iii) Because the experimental technique is based on observing the change in optical absorption before and after photoquenching, the calculated absorption coefficient is only due to these photoquenchable defects and is immune from interference by other absorbing species. It follows that an accurate calculation of the photoquenchable defect concentration is possible if a suitable cross-section is available. Optical absorption images were taken with a simple IR absorption imaging system with a TV monitor connected to an IR-sensitive TV camera [9]. Samples were held at 80 K. Monochromatic light of 0.9 #m and 0.83 /~m wavelengths, again derived from interference filters, was used to image EL2 ° and RC concentrations respectively. The former was used in preference to 1 Arm wavelength (used for quantitative, non-mapping measurements) because the response of our TV camera does not extend to 1 Arm. We have previously shown that this wavelength change does not affect these images. By using an appropriate sample holder in this system, it is possible to characterize two samples at the same time. In previous work, the excellent correlation between low temperature RC absorption images and low temperature PL and CL images was demonstrated [11, 12]. This correlation suggests that bleaching of the RC image may also result in a modification of luminescence efficiency. A novel double-beam PL (DBPL) technique was used to investigate the effects of photoquenching RC centres on luminescence efficiency. In these experiments the samples are held below 20 K by using a closed-cycle, helium cryostat. Two laser beams are used. One, a 30 mW argon ion laser beam, strikes the front of the sample for photoexcitation in the usual way, and the second, a 50 mW beam of 1.06 # m wavelength light from an Nd:YAG laser, is shone from the rear for photoquenching. In order to remove possible breakthrough of the laser light via the monochromator to the detector, cut-off filters are used between the sample and the monochromator. Luminescence produced from the sample at various wavelengths is analysed by a computer-controlled spectrometer. Detection of luminescence is made using a photomultiplier tube operating at a temperature of 100 K. A computer-controlled lock-in amplifier, operating in conjunction with a reference signal obtained from an
S. Tiizemen, M. R. Brozel
/
l~ear-band-edge absorption in GaAs
optical chopper, is used to improve signal-to-noise ratios and to produce difference signals between bleached and unbleached PL [15]. Further details of the experimental apparatus have been given by Tiizemen et al. [15]. As a check on the luminescence efficiency as a function of excitation efficiency over several orders of magnitude selected samples were investigated by cathodoluminescence at a temperature of 10 K. Spatial scans of spectrally resolved subbandedge components were measured on an Oxford Instruments MonoCL system. Spatial maps showed enhanced but broadened band edge luminescence at dislocation cell walls, as has been reported previously on similar samples investigated with a similarly resolvin~ instrument.
3. Experimental results The growth of GaAs from a gallium-rich melt is known to result in material with low [EL2] [16]. An [EL2 °] image of such material exhibits little contrast, especially at low magnification. However, the RC absorption image is very clear. Figure 1 is an RC image of the central region (approximately 3 cm x 2 cm in area) of such a sample. It is clear that polygonized dislocation arrays, both in the form of cell structure near the centre and as lineage, appear bright (less absorbing) in this image. In
49
addition, there are other regions away from the central region which contain bright areas. It was this type of image that led us to investigate the effects of melt stoichiometry on RC images. Figures 2(a) and 2(b) show neutral EL2 ° and RC absorption images obtained from our lightly n-type, arsenic-rich and gallium-rich GaAs respectively. It should be noted that free carrier absorption is not significant at these short wavelengths and that such effects are not an interference with these measurements. In Fig. 2(a), EL2 is seen to be concentrated along the dislocation cell walls in both materials although the mean [EL2] is greater in the arsenic-rich material. In Fig. 2(b), RC is seen to be concentrated inside the cells and the mean [RC] is greater in the gallium-rich material. The apparent spatial anticorrelation between [EL2] and [RC] can also be seen in Fig. 3. Data in this figure were obtained from samples cut from the seed ends of crystals so that the stoichiometry for those particular samples is more easily related to the original melt stoichiometry. Data points towards the right-hand side are from a sample cut from the tail of a gallium-rich crystal where the gallium richness of the melt must have been much larger than near the seed. This uncertainty in crystal stoichiometry is indicated on the graph. In general, we have found that arsenic-rich samples have more EL2 and fewer RC centres, while gallium-rich samples have more RC and fewer EL2 centres. Normalized luminescence intensities averaged over
Fig. 1. Reverse contrast image of the central region of a 3 in diameter, 600 gm thick, GaAs wafer grown from a gallium-rich melt (Ga: As = 1.1 ) viewedwith light of 0.83 #m wavelength.Sample temperature was 100 K.
50
3: Tiizemen, M. R. Brozel
/
Near-batul-edge absorption in (;aAs
sample areas of about 100 /~m 2 from the M o n o C L system are tabulated with E L 2 and R C concentrations in Table 1, together with normalized intensities of the same luminescence emission as derived by PL. Bearing in mind the very different ways in which these two luminescence data were obtained, there is good agreement, each showing a factor of about 150 difference
between the luminescence intensities from the brightest to the least bright sample. In order to check whether our previously measured correlation between R C absorption images and luminescence efficiency in "stoichiometric" G a A s extends to gross variations caused by melt non-stoichiometry, we have p e r f o r m e d D B P L experiments. We have shown
Fig. 2. (a) EL2 ° absorption from arsenic-rich (left-hand image) and gallium-rich (right-hand image), lightly n-type GaAs samples of 400/am thickness. (b) RC absorption images of the same samples at a wavelength of 0.83/am.
TABLE 1. Concentrations of neutral EL2 centres (as measured by optical absorption photoquenching), absorption coefficients of reverse contrast measured at 830 nm, normalized photoluminescence and cathodoluminescence intensities of the entire, near-bandedge emission and corresponding double-beam photoluminescence response as described in the text Sample number
[EL2] (cm- 3)
1
1.0× 1016
2.9
2 3 4
1.0 × 101~' 1.1 × 1016 0.7 × 1016
6.3 7.0 11.7
AU, arbitrary units. aNot measured.
aRc (cm- 1)
Normalized PI~,t (AU) 90
8.0 8.5 0.6
Normalized Cl~nt (AU)
DBPL response [PL]b/[PL]u
85
1.1 1.3 1.5 2.3
a 10.5 0.7
S. Tiizeme~ M. R. Brozel
,~.o
/
-~0.8 'P-
Original St0ichiometry of melt, (fia/As)
J
Fig. 3. Relative RC concentration (curve 1) and neutral EL2 concentration (curve 2), as functions of original melt stoichiometry.
~2.0
t'-
_~1.0 t-~
e
I
I
I
I
I
I
0.0 2.0 4.0 6.0 8.0 10.0 12.0 t Absorption coefficient at830nm{cr~'}
Fig. 4. The proportionality of PL increase after photoquenching as a function of original near-band-edge absorption.
previously that there is an increase in PL efficiency after photoquenching of the near-band-edge absorption and this increase is almost proportional to the absorption coefficient at 830 nm. We have now determined that samples grown from gallium-rich melts exhibit higher RC absorption and show reduced PL and CL efficiency. The demonstration that this reduction in PL efficiency is due to RC defects is the correlated increase in PL efficiency when the RC absorption is bleached. Such a correlation is shown in Fig. 4 for a series of different samples. These data are also included in Table 1.
4. Discussion and conclusions In this work and previously [10], we have observed that EL2 and RC centres can be photoquenched at different sample temperatures. IR absorption mapping demonstrates the anticorrelation between concentrations of EL2 and RC, as shown in Figs. 2 and 3. It must be asked whether the defect that gives rise to RC
Near-band-edge absorption in GaAs
51
absorption is a different defect from EL2 or whether it is simply the EL2 defect but in a different charge state. EL2 absorption imaging is sensitive only to neutral EL2 ° defects, the absorption spectrum consisting of a broad ionization component where electrons on EL2 ° centres are excited to the conduction band and an intracentre band producing an additive absorption near a wavelength of 1 um [3]. Residual native acceptors convert a fraction of EL2 to EL2 ÷ centres. It is conceivable in SI GaAs containing both EL2 ° and EL2 ÷ that RC absorption is due in some way to the EL2 ÷ component, the reverse contrast reflecting the mapping of non-uniform concentrations of acceptors whilst the total EL2 concentration is relatively constant. However, this cannot be true in low resistivity n-type GaAs where the Fermi level is close to the conduction band edge and all EL2 centres are in their neutral EL2 ° state. It follows that the observation of RC imaging in n-type GaAs proves that RC centres cannot be EL2 centres in any charge state. There is a strong correlation between the increase in PL efficiency by photoquenching and the reduction in near-band-edge RC absorption coefficient by the same treatment, and excellent correlation between RC and luminescence images. We conclude that the low temperature luminescence of bulk GaAs is dominated by the presence of non-radiative recombination centres and that the RC centres are these centres. No electrical activity of these defects has been seen in GaAs in the dark. However, photocurrent data [17] do support the existence of a very deep photoquenchable acceptor level. The close agreement between the spectral forms of this photocurrent and the absorption and bleaching spectra of RC images suggests that RC centres can be identified with these acceptor levels. It is generally accepted that the SI behaviour of most undoped LEC GaAs can be explained by considering the compensation between EL2 centres and residual chemical acceptors or relatively shallow native acceptors only. The presence of very deep acceptors near the conduction band edge does not affect this view as they are unionized at room temperature and do not take part in the compensation process. However, their apparent electrical inactivity in n-type GaAs where the Fermi level is close to the conduction band edge suggests that RC centres may exist at low concentration but exhibit a very high absorption cross-section. Our stoichiometry measurements show that higher concentrations of RC centres are seen in gallium-rich samples and that they behave as the dominant recombination centre in this, as well as in conventional, LEC GaAs. We have previously discussed the possibility of the RC defect being associated with the arsenic vacancy because theoretical studies have shown that an ionization energy of the arsenic vacancy is expected at
52
S Tiizemen, M. R. Brozel
/
approximately E g - 5 0 meV [18, 19J, in close agreement with the optical absorption threshold energy. The present stoichiometry data give further support to such a model although the possibility of more complex defects such as the gallium antisite cannot be ruled out.
Acknowledgments The authors gratefully acknowledge the help of Miss J. Gilmore in the careful preparation of this manuscript. Part of this work has been carried out with the support of Procurement Executive, Ministry of Defence. One of the authors (S.T.) is grateful to Atatiirk University for a grant. References 1 G.M. Martin, Appl. Phys. L ett., 39 (9) ( 1981 ) 747. 2 P. Omling, L. Samuelson and H. G. Grimmeiss, Phys. Rev. B, 29 (8)(1984) 4534. 3 M. Skowronski, J. Lagowski and H. C. Gatos, Phys. Rev. B, 32 (6)(1985) 4264. 4 J.C. Parker and R. Bray, Phys. Rev. B, 37(1988)6368. 5 M. D. Manasreh and D. W. Fischer, Phys. Rev. B, 39 (17) (1989) 13001. 6 S. Tiizemen and M. R. Brozel, E-MRS Fall Meet., Strasbourg, 1990; Appl. Surf. Sci., 50 (1991)395.
Near-band-edge absorption in GaAs 7 M. Skolnick, L. J. Reed and A. D. PitL Appl. Phys. Left., 44 (1984)447. 8 M. R. Brozel and M. S. Skolnick, in H. Kukimoto and S. Miyazawa (eds.), Semi-Insulating II1-V Materials, Proc. 4th Int. Conf., Hakone, 1986, Ohmsha, Tokyo, 1986, p. 109. 9 L. Breivik, M. R. Brozel, A. Mohades-Kassai and J. M. Langer, in A. G. Milnes and C. J. Miner (eds.), Semi-Insulating III-V Materials, Proc. 6th Int. Conf., Toronto, 1990, Hiiger, Bristol, 1990, p. 257. 10 S. Tiizemen, M. R. Brozel and L. Breivik, GaAs and Related Compounds, Jersey, 1990,1in Inst. iPhys. Conf. Ser. 112 (1991) 237. 11 M. R. Brozel, L. Breivik, D. J. Stirland, G. M. Williams and A. G. Cullis, Appl. Surf. Sci., 50(1991) 395. 12 H. C. Alt, M. Miillenborn and G. Packeiser, in A. G. Milnes and C. J. Miner (eds.), Semi-Insulating 111-V Materials, Proc. 6th Int. Conf., Toronto, 1990, Hilger, Bristol, 1990, p. 309. 13 A. Mohades-Kassai and M. R. Brozel, J. Cryst. Growth, 103 (1990) 303. 14 M. R. Brozel, E. J. Foulkes, I. R. Grant and D. T. J. Hurle, J. Cryst. Growth, 80(1987) 323. 15 S. T/izemen, L. Breivik and M. R. Brozel, DRIP4, Wilmslow, 1991; Semicond. Sci. Technol., 7 (1992) A36-A40. 16 E Orito, Y. Yamada, K. Katano, E Yajima and T. Okana, in K. Sumino (ed.), Defect Control in Semiconductors, Vol. 1, Elsevier, Amsterdam, 1990, p. 771. 17 J. Jimenez, E Hernandez, J. A. de Saja and J. Bonnaf~, J. Appl. Phys., 57(12)(1985) 5290. 18 M.J. Puska, O. Jepsen, O. Gunnarsson and R. M. Nieminen, Phys. Rev. B, 34 (4) (1986) 2695. 19 P. J. Lin-Chung and T. L. Reinecke, Phys. Rev. B, 27 (2) (1983)1101.
47
Effect of non-stoichiometry on near-band-edge absorption and nonradiative recombination in bulk GaAs S. T/izemen and M. R. Brozel Department of Electrical Engineering and Electronics and The Centrefor Electronic Materials, University of Manchester Institute of Science and Technology, PO Box 88, Manchester M60 1QD (UK) (Received November 25, 1991 )
Abstract We demonstrate that the total near-band-edge photoluminescence intensity of undoped GaAs grown by the liquid encapsulated Czochralski technique is reduced if the stoichiometry of the starting melt becomes more gallium rich. At the same time, concentrations of the native deep donor level EL2 are reduced, but there is an increase in the concentration of an unknown defect which causes absorption near the band edge in cooled samples. Such absorption is known as reverse contrast (RC). We produce evidence that the defects that give rise to this absorption, RC defects, are also the dominant non-radiative recombination centres in undoped GaAs. It is probable that these defects are gallium rich and may be related to arsenic vacancies.
I. Introduction
There is great technological interest in investigating native point defects in liquid encapsulated Czochralski (LEC) GaAs as it is believed that it is their role in compensating residual chemical acceptors, such as carbon and zinc, that gives rise to the semi-insulating (SI) behaviour of undoped material. Of the several deep native defects in GaAs, the midgap level EL2 is known as being the most important in this compensation mechanism. The majority of EL2 defects in common substrate materials are neutral and it is fortunate for reasons of substrate assessment that they produce a broad absorption signature that extends from the band edge to near 1.5 ,urn. An important property of this defect is its photoquenchability (or bleachability) to a metastable state. Thus, at sample temperatures below 120 K and on illumination by subbandgap light, EL2 centres lose both their electrical and their optical behaviour [1-5]. This property allows us to make accurate determinations of concentrations of neutral EL2 defects in a simple way by observing changes in absorption after bleaching [6]. However, it has been demonstrated that another point defect [7, 8] gives rise to additional low temperature (below 140 K) absorption that is restricted to 0921-5107/92/$5.00
within 65 meV of the band edge. An absorption image produced by these defects is known as reverse contrast (RC). These centres can also be photoquenched [9, 10] but at lower sample temperatures than those used for photoquenching EL2 centres. We have demonstrated previously that these point defects are not related to the EL2 defect in any charge state [6]. The major technological importance of the point defects that cause low temperature near-band-edge absorption is that their IR absorption images exhibit a remarkable resemblance to low temperature cathodoluminescence (CL)[11] and photoluminescence (PL) [12] images. As it is accepted that luminescence intensity in these materials at low temperatures is controlled by non-radiative recombination; it appears that RC absorption mapping [13] can give information regarding the distributions of the dominant non-radiative recombination centre in bulk GaAs in a quantitative and rapid way [13]. In this work, we measure concentrations of EL2 and near-band-edge absorption centres (RC defects) and their corresponding photoquenching as functions of near-band-edge PL intensity and starting melt stoichiomerry in both lightly n-type and SI GaAs. We demonstrate that all these properties are dependent on melt stoichiometry and that trends observed previously © 1992 - Elsevier Sequoia. All rights reserved
4~
,% Tiizemen. M. R. Brozel
/
Near-hand-edge absorption in GaAs
in commercial, "stoichiometric'" material can be extended to these samples.
2. Experimental techniques The samples used in this study were either undoped or lightly n-type GaAs grown by the LEC method. The n-type crystals were doped with tin, a fraction of which was radioactive to allow quantitative electrical and compositional assessment. Melt stoichiometries were adjusted to produce melts varying from intentional gallium richness (Ga:As up to 1.1 in the original melt) to intentional arsenic richness (Ga:As down to 0.99 in the original melt). The use of dry boric oxide, B203, as the encapsulant for some growths was intended to retain the original stoichiometry by reducing possible reactions by either the gallium or the arsenic with water in the B203. The tin-doped crystals were n-type for all or most of the volume [14]. A series of commercial SI, LEC GaAs substrates is also investigated. A novel optical absorption bleaching technique [6] was employed to determine neutral EL2 concentrations [EL2 ~] and near-band-edge absorption coefficients as functions of original melt stoichiometry. In this technique, changes in optical transmission at particular wavelengths are measured before and after bleaching to determine quantitative values of [EL2 °] and relative values of RC defect concentrations [RC]. Although this technique was originally devised for the assessment of semi-insulating GaAs, we have shown that it works satisfactorily for all the present samples. EL2 optical absorption measurements were made at a wavelength of 1 /zm at sample temperatures below 120 K. After the entire bleaching of the absorption by EL2 centres, the transmission was re-measured. Nearband-edge low temperature RC absorption bleaching measurements at a wavelength of 0.83 /zm were performed on the same samples, but at lower temperatures (below 45 K). During this and subsequent measurements taken after bleaching, interference from EL2 absorption was absent because all EL2 centres were in their metastable, inert state. Light beams of 0.91 # m and 0.95 /~m wavelength derived from a quartz halogen bulb and interference filters were used to photoquench the EL2 and RC centres respectively. The absorption coefficient A a ( 2 ) ( c m -I) of the photoquenchable defects was found from the change in transmitted intensity at wavelength ). from Aa(2) = (1/d) in[l(t)/l(0)], where 1(0)and I ( t ) a r e the intensities of transmitted light respectively before and after a time t (s) of photoquenching and d (cm) is the sample thickness. Neutral EL2 concentrations [EL2 °] were calculated by using Martin's calibration [1]. At present, it has not been found possible to relate the optical absorption
of the near-band-edge component to defect concentration. The advantages of this optical absorption bleaching technique are as follows. (i) Characterization of the material can be achieved easily using commercial 400 pm thick material. (ii) Because the comparison is made before and after bleaching, there is no need to use a reference sample. This simplifies the experimental system. (iii) Because the experimental technique is based on observing the change in optical absorption before and after photoquenching, the calculated absorption coefficient is only due to these photoquenchable defects and is immune from interference by other absorbing species. It follows that an accurate calculation of the photoquenchable defect concentration is possible if a suitable cross-section is available. Optical absorption images were taken with a simple IR absorption imaging system with a TV monitor connected to an IR-sensitive TV camera [9]. Samples were held at 80 K. Monochromatic light of 0.9 #m and 0.83 /~m wavelengths, again derived from interference filters, was used to image EL2 ° and RC concentrations respectively. The former was used in preference to 1 Arm wavelength (used for quantitative, non-mapping measurements) because the response of our TV camera does not extend to 1 Arm. We have previously shown that this wavelength change does not affect these images. By using an appropriate sample holder in this system, it is possible to characterize two samples at the same time. In previous work, the excellent correlation between low temperature RC absorption images and low temperature PL and CL images was demonstrated [11, 12]. This correlation suggests that bleaching of the RC image may also result in a modification of luminescence efficiency. A novel double-beam PL (DBPL) technique was used to investigate the effects of photoquenching RC centres on luminescence efficiency. In these experiments the samples are held below 20 K by using a closed-cycle, helium cryostat. Two laser beams are used. One, a 30 mW argon ion laser beam, strikes the front of the sample for photoexcitation in the usual way, and the second, a 50 mW beam of 1.06 # m wavelength light from an Nd:YAG laser, is shone from the rear for photoquenching. In order to remove possible breakthrough of the laser light via the monochromator to the detector, cut-off filters are used between the sample and the monochromator. Luminescence produced from the sample at various wavelengths is analysed by a computer-controlled spectrometer. Detection of luminescence is made using a photomultiplier tube operating at a temperature of 100 K. A computer-controlled lock-in amplifier, operating in conjunction with a reference signal obtained from an
S. Tiizemen, M. R. Brozel
/
l~ear-band-edge absorption in GaAs
optical chopper, is used to improve signal-to-noise ratios and to produce difference signals between bleached and unbleached PL [15]. Further details of the experimental apparatus have been given by Tiizemen et al. [15]. As a check on the luminescence efficiency as a function of excitation efficiency over several orders of magnitude selected samples were investigated by cathodoluminescence at a temperature of 10 K. Spatial scans of spectrally resolved subbandedge components were measured on an Oxford Instruments MonoCL system. Spatial maps showed enhanced but broadened band edge luminescence at dislocation cell walls, as has been reported previously on similar samples investigated with a similarly resolvin~ instrument.
3. Experimental results The growth of GaAs from a gallium-rich melt is known to result in material with low [EL2] [16]. An [EL2 °] image of such material exhibits little contrast, especially at low magnification. However, the RC absorption image is very clear. Figure 1 is an RC image of the central region (approximately 3 cm x 2 cm in area) of such a sample. It is clear that polygonized dislocation arrays, both in the form of cell structure near the centre and as lineage, appear bright (less absorbing) in this image. In
49
addition, there are other regions away from the central region which contain bright areas. It was this type of image that led us to investigate the effects of melt stoichiometry on RC images. Figures 2(a) and 2(b) show neutral EL2 ° and RC absorption images obtained from our lightly n-type, arsenic-rich and gallium-rich GaAs respectively. It should be noted that free carrier absorption is not significant at these short wavelengths and that such effects are not an interference with these measurements. In Fig. 2(a), EL2 is seen to be concentrated along the dislocation cell walls in both materials although the mean [EL2] is greater in the arsenic-rich material. In Fig. 2(b), RC is seen to be concentrated inside the cells and the mean [RC] is greater in the gallium-rich material. The apparent spatial anticorrelation between [EL2] and [RC] can also be seen in Fig. 3. Data in this figure were obtained from samples cut from the seed ends of crystals so that the stoichiometry for those particular samples is more easily related to the original melt stoichiometry. Data points towards the right-hand side are from a sample cut from the tail of a gallium-rich crystal where the gallium richness of the melt must have been much larger than near the seed. This uncertainty in crystal stoichiometry is indicated on the graph. In general, we have found that arsenic-rich samples have more EL2 and fewer RC centres, while gallium-rich samples have more RC and fewer EL2 centres. Normalized luminescence intensities averaged over
Fig. 1. Reverse contrast image of the central region of a 3 in diameter, 600 gm thick, GaAs wafer grown from a gallium-rich melt (Ga: As = 1.1 ) viewedwith light of 0.83 #m wavelength.Sample temperature was 100 K.
50
3: Tiizemen, M. R. Brozel
/
Near-batul-edge absorption in (;aAs
sample areas of about 100 /~m 2 from the M o n o C L system are tabulated with E L 2 and R C concentrations in Table 1, together with normalized intensities of the same luminescence emission as derived by PL. Bearing in mind the very different ways in which these two luminescence data were obtained, there is good agreement, each showing a factor of about 150 difference
between the luminescence intensities from the brightest to the least bright sample. In order to check whether our previously measured correlation between R C absorption images and luminescence efficiency in "stoichiometric" G a A s extends to gross variations caused by melt non-stoichiometry, we have p e r f o r m e d D B P L experiments. We have shown
Fig. 2. (a) EL2 ° absorption from arsenic-rich (left-hand image) and gallium-rich (right-hand image), lightly n-type GaAs samples of 400/am thickness. (b) RC absorption images of the same samples at a wavelength of 0.83/am.
TABLE 1. Concentrations of neutral EL2 centres (as measured by optical absorption photoquenching), absorption coefficients of reverse contrast measured at 830 nm, normalized photoluminescence and cathodoluminescence intensities of the entire, near-bandedge emission and corresponding double-beam photoluminescence response as described in the text Sample number
[EL2] (cm- 3)
1
1.0× 1016
2.9
2 3 4
1.0 × 101~' 1.1 × 1016 0.7 × 1016
6.3 7.0 11.7
AU, arbitrary units. aNot measured.
aRc (cm- 1)
Normalized PI~,t (AU) 90
8.0 8.5 0.6
Normalized Cl~nt (AU)
DBPL response [PL]b/[PL]u
85
1.1 1.3 1.5 2.3
a 10.5 0.7
S. Tiizeme~ M. R. Brozel
,~.o
/
-~0.8 'P-
Original St0ichiometry of melt, (fia/As)
J
Fig. 3. Relative RC concentration (curve 1) and neutral EL2 concentration (curve 2), as functions of original melt stoichiometry.
~2.0
t'-
_~1.0 t-~
e
I
I
I
I
I
I
0.0 2.0 4.0 6.0 8.0 10.0 12.0 t Absorption coefficient at830nm{cr~'}
Fig. 4. The proportionality of PL increase after photoquenching as a function of original near-band-edge absorption.
previously that there is an increase in PL efficiency after photoquenching of the near-band-edge absorption and this increase is almost proportional to the absorption coefficient at 830 nm. We have now determined that samples grown from gallium-rich melts exhibit higher RC absorption and show reduced PL and CL efficiency. The demonstration that this reduction in PL efficiency is due to RC defects is the correlated increase in PL efficiency when the RC absorption is bleached. Such a correlation is shown in Fig. 4 for a series of different samples. These data are also included in Table 1.
4. Discussion and conclusions In this work and previously [10], we have observed that EL2 and RC centres can be photoquenched at different sample temperatures. IR absorption mapping demonstrates the anticorrelation between concentrations of EL2 and RC, as shown in Figs. 2 and 3. It must be asked whether the defect that gives rise to RC
Near-band-edge absorption in GaAs
51
absorption is a different defect from EL2 or whether it is simply the EL2 defect but in a different charge state. EL2 absorption imaging is sensitive only to neutral EL2 ° defects, the absorption spectrum consisting of a broad ionization component where electrons on EL2 ° centres are excited to the conduction band and an intracentre band producing an additive absorption near a wavelength of 1 um [3]. Residual native acceptors convert a fraction of EL2 to EL2 ÷ centres. It is conceivable in SI GaAs containing both EL2 ° and EL2 ÷ that RC absorption is due in some way to the EL2 ÷ component, the reverse contrast reflecting the mapping of non-uniform concentrations of acceptors whilst the total EL2 concentration is relatively constant. However, this cannot be true in low resistivity n-type GaAs where the Fermi level is close to the conduction band edge and all EL2 centres are in their neutral EL2 ° state. It follows that the observation of RC imaging in n-type GaAs proves that RC centres cannot be EL2 centres in any charge state. There is a strong correlation between the increase in PL efficiency by photoquenching and the reduction in near-band-edge RC absorption coefficient by the same treatment, and excellent correlation between RC and luminescence images. We conclude that the low temperature luminescence of bulk GaAs is dominated by the presence of non-radiative recombination centres and that the RC centres are these centres. No electrical activity of these defects has been seen in GaAs in the dark. However, photocurrent data [17] do support the existence of a very deep photoquenchable acceptor level. The close agreement between the spectral forms of this photocurrent and the absorption and bleaching spectra of RC images suggests that RC centres can be identified with these acceptor levels. It is generally accepted that the SI behaviour of most undoped LEC GaAs can be explained by considering the compensation between EL2 centres and residual chemical acceptors or relatively shallow native acceptors only. The presence of very deep acceptors near the conduction band edge does not affect this view as they are unionized at room temperature and do not take part in the compensation process. However, their apparent electrical inactivity in n-type GaAs where the Fermi level is close to the conduction band edge suggests that RC centres may exist at low concentration but exhibit a very high absorption cross-section. Our stoichiometry measurements show that higher concentrations of RC centres are seen in gallium-rich samples and that they behave as the dominant recombination centre in this, as well as in conventional, LEC GaAs. We have previously discussed the possibility of the RC defect being associated with the arsenic vacancy because theoretical studies have shown that an ionization energy of the arsenic vacancy is expected at
52
S Tiizemen, M. R. Brozel
/
approximately E g - 5 0 meV [18, 19J, in close agreement with the optical absorption threshold energy. The present stoichiometry data give further support to such a model although the possibility of more complex defects such as the gallium antisite cannot be ruled out.
Acknowledgments The authors gratefully acknowledge the help of Miss J. Gilmore in the careful preparation of this manuscript. Part of this work has been carried out with the support of Procurement Executive, Ministry of Defence. One of the authors (S.T.) is grateful to Atatiirk University for a grant. References 1 G.M. Martin, Appl. Phys. L ett., 39 (9) ( 1981 ) 747. 2 P. Omling, L. Samuelson and H. G. Grimmeiss, Phys. Rev. B, 29 (8)(1984) 4534. 3 M. Skowronski, J. Lagowski and H. C. Gatos, Phys. Rev. B, 32 (6)(1985) 4264. 4 J.C. Parker and R. Bray, Phys. Rev. B, 37(1988)6368. 5 M. D. Manasreh and D. W. Fischer, Phys. Rev. B, 39 (17) (1989) 13001. 6 S. Tiizemen and M. R. Brozel, E-MRS Fall Meet., Strasbourg, 1990; Appl. Surf. Sci., 50 (1991)395.
Near-band-edge absorption in GaAs 7 M. Skolnick, L. J. Reed and A. D. PitL Appl. Phys. Left., 44 (1984)447. 8 M. R. Brozel and M. S. Skolnick, in H. Kukimoto and S. Miyazawa (eds.), Semi-Insulating II1-V Materials, Proc. 4th Int. Conf., Hakone, 1986, Ohmsha, Tokyo, 1986, p. 109. 9 L. Breivik, M. R. Brozel, A. Mohades-Kassai and J. M. Langer, in A. G. Milnes and C. J. Miner (eds.), Semi-Insulating III-V Materials, Proc. 6th Int. Conf., Toronto, 1990, Hiiger, Bristol, 1990, p. 257. 10 S. Tiizemen, M. R. Brozel and L. Breivik, GaAs and Related Compounds, Jersey, 1990,1in Inst. iPhys. Conf. Ser. 112 (1991) 237. 11 M. R. Brozel, L. Breivik, D. J. Stirland, G. M. Williams and A. G. Cullis, Appl. Surf. Sci., 50(1991) 395. 12 H. C. Alt, M. Miillenborn and G. Packeiser, in A. G. Milnes and C. J. Miner (eds.), Semi-Insulating 111-V Materials, Proc. 6th Int. Conf., Toronto, 1990, Hilger, Bristol, 1990, p. 309. 13 A. Mohades-Kassai and M. R. Brozel, J. Cryst. Growth, 103 (1990) 303. 14 M. R. Brozel, E. J. Foulkes, I. R. Grant and D. T. J. Hurle, J. Cryst. Growth, 80(1987) 323. 15 S. T/izemen, L. Breivik and M. R. Brozel, DRIP4, Wilmslow, 1991; Semicond. Sci. Technol., 7 (1992) A36-A40. 16 E Orito, Y. Yamada, K. Katano, E Yajima and T. Okana, in K. Sumino (ed.), Defect Control in Semiconductors, Vol. 1, Elsevier, Amsterdam, 1990, p. 771. 17 J. Jimenez, E Hernandez, J. A. de Saja and J. Bonnaf~, J. Appl. Phys., 57(12)(1985) 5290. 18 M.J. Puska, O. Jepsen, O. Gunnarsson and R. M. Nieminen, Phys. Rev. B, 34 (4) (1986) 2695. 19 P. J. Lin-Chung and T. L. Reinecke, Phys. Rev. B, 27 (2) (1983)1101.