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Defect specific topography of GaAs wafers by microwave-detected photo induced current transient spectroscopy
Materials Science and Engineering B91– 92 (2002) 371– 375 www.elsevier.com/locate/mseb
Defect specific topography of GaAs wafers by microwave-detected photo induced current transient spectroscopy B. Gru¨ndig-Wendrock a,*, M. Jurisch b, J.R. Niklas a a
b
Technical Uni6ersity Bergakademie, Freiberg, Germany Freiberger Compound Materials GmbH, Freiberg, Germany
Abstract Non-destructive and spatially resolved (35 mm) characterisation methods for high resistivity GaAs and epitaxial substrates are presented. Microwave detected photo induced current transient spectroscopy (PICTS) is a further development of the already existing method Microwave detected photoconductivity (MDP). Using various GaAs samples of different preparations, the possibilities of these new tools are demonstrated along with the first results for technologically relevant extrinsic and intrinsic defects. © 2002 Published by Elsevier Science B.V. Keywords: Microwave absorption; Photoconductivity; PICTS; Contactless; SI-gallium arsenide; Si-GaAs
1. Introduction In semi-insulating (SI) GaAs a dislocation network is formed by dynamical polygonisation near the solid/liquid interface which interacts with native defects, dopants and impurities during the cooling-down procedure after crystal growth. Segregated zones of defects are generated, which are correlated to frozen-in inhomogeneities of the electrical and optical properties scaled by the cell size and sometimes called ‘mesoscopic inhomogeneities’ [1]. These are of great importance for the properties of devices. Therefore, a non-destructive method for the characterisation of these inhomogeneities, particularly of the concentration and distribution of deep defects is required. Defect specific topographic experiments are also important for the investigation of formation- and annihilation processes of intrinsic defects and diffusion- and gettering processes of impurity atoms for the optimisation of defect engineering and materials properties. The well-known deep level transient spectroscopy (DLTS) [2] turned out to be the most appropriate tool to characterise defects by their energetic levels. Unfortunately it is not applicable to higher resistance GaAs material and it provides neither identification of defect * Corresponding author. Fax: + 49-3731-394314. E-mail address: bianca – [email protected] (B. Gru¨ndig-Wendrock).
species nor their spatial distribution. Similar results as with DLTS can be obtained by measuring photoconductivity transients and evaluating them like the capacitance transients in DLTS experiments. These so-called photo induced current transient spectroscopy (PICTS) [3] experiments are feasible for high resistance material, however, an exact information about absolute concentrations of defects is lost.
2. Theoretical considerations A non-equilibrium concentration of electrons Dn and holes Dp is introduced by locally resolved excitation with a small laser spot of a pulse with above bandgap light. A photoconductivity D| = q(vn Dn + vp Dp)
(1)
results, where q is the electron charge and vn,p the electron and hole mobility, respectively. Due to their low mobility the contribution of the holes to the photoconductivity can be neglected in GaAs. The lifetime ~ of excess charge carriers is influenced by recombination and trapping processes through recombination centres and shallow and deep defects. It amounts to about 5 ns in the cell walls and 0.5 ns in the cell interiors [5]. Trapped electrons can be reemitted to the conduction band by thermal excitation. Thus, the time dependence
0921-5107/02/$ - see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 1 0 7 3 - X
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B. Gru¨ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375
of the photoexcitation response is determined by generation, trapping, recombination and reemission processes [4] (Fig. 1a). After the photoexcitation and
recombination and trapping of charge carriers the typical photoconductivity transient based on the thermal excited emission is visible (Fig. 1a): t
D|transient = qvDn =qvnT0~ne − enT t
(2)
nT0 corresponds to the initial density of carriers trapped by a distinct defect, etnT symbolises the thermal excited emission rate of this trap, t is the time after the termination of the photo pulse. Prior to the PICTS-experiments a survey of the cellular structure and possible distribution of defects can be obtained by the method of microwave detected photoconductivity (MDP) [1]. A semiconductor laser modulated at a rate of 100 kHz was used for the photoexcitation. The photoconductivity signal according to Fig. 1a is detected by conventional lock-in technique. Thus the magnitude of the resulting DC-signal also depends on the intensity of thermally excited emission of charge carriers from the traps which determines the time dependence of the photoconductivity transient (laser off in Fig. 1a). There are two cases for the emission rate to be considered: In the first case the time constant for the photoconductivity transient is on the order of the time where the laser is off. This situation is depicted in Fig. 1a, and in more detail in Fig. 1b. The lower trace in Fig. 1b shows the signal behind the synchronous detector of the lock-in amplifier (before the integrator). Averaging this signal yields the final measurement result. If, on the other hand, the emission rate is very high and thus the transient very fast, then the photoconductivity signal approaches the shape shown in the upper trace in Fig. 1c. The same shape is, however, obtained if the emission rate is very small (with a corresponding very long transient time constant). In this case there is almost no signal during the laser off period. The two latter cases (Fig. 1c) yield a higher output signal than the case in Fig. 1b. Summarising, a small signal in the final topograms below (indicated dark) correspond to a medium high emission time constant, whereas high signals (light) correspond to either a very short or a very long time constant. In a PICTS measurement, in contrast, only the photoconductivity transient according to Eq. (2) which now has a minimum length of several 100 ms is analysed using the so-called two-gate technique of DLTS. From etnT the activation energy, EA, of a trap can be calculated using the relation: Fig. 1. (a) Photoconductivity during optical excitation followed by the photoconductivity transient. (b). Analysis of the photoconductivity signal as obtained by conventional lock-in-technique. The lower trace indicates the voltage-signal of the detector diode before the integrator stage of the lock-in amplifier. (c). Analysis as in Fig. 1b, however, for a very fast or a very slow reemission of charge carriers. The resulting output voltage-signal of the detector diode is higher as in Fig. 1b (for details see text.)
etnT = AT 2e − EA/kT
(3)
where A is a material constant. In contrast to DLTS, the magnitude of the peaks in the PICTS-spectra is proportional to the density of trapped carriers and not to the density of traps itself.
B. Gru¨ ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375
Fig. 2. Block diagram of the experimental set-up for MDP (dotted lines) and microwave detected PICTS (dashed lines).
3. Apparatus The experimental set-up for the MDP according to [1] and completed for the purposes of microwave detected PICTS is sketched in Fig. 2. The sample in a flat LN2 cryostat below a quartz window is outside the microwave cavity. A special iris in the wall of the cavity allows the microwave field to ‘leak out’ through the window into the sample. Thus, the complex dielectric constant of the sample influences the resonant frequency and the loss properties of the cavity. For optical excitation a semiconductor LASER with 680 nm wavelength, 3 mW power and a spot size of about 35 mm was used. Microwave absorption by excited charge carriers is detected according to the principle of usual EPR-technique [6]. For MDP the LASER is chopped at a rate of about 100 kHz and the detector signal analysed using conventional lock-in technique. For microwave detected PICTS lengths of optical pulses and intersections between pulses of about 1 ms were used. The signals are recorded by a transient recorder of the PICTS-PC. A stepping mechanism allows the sample to be positioned in the x – y-plane. Further details are described in [7].
373
(VGF) material sample 1 was measured. Sample 2 results from a series of samples with a special temperature-time-regime during annealing [8]. Sample 3 was annealed under the condition of an unusual protecting atmosphere. Samples 2 and 3 are both liquid encapsulated Czochralski (LEC)-GaAs samples. Photoconductivity mappings of samples 1 and 2, measured at 300 K and between 200 and 250 K a second time, are shown in Figs. 3 and 4. Dark areas correspond to a low signal and, therefore, to a medium intense defect emission (compare Fig. 1b). An inversion of the contrast is clearly obvious. Fig. 5 shows the microwave detected photo-conductivity mapping of sample 3 measured at 300 K. Microwave detected PICTS was measured at two points of the topogram, one in the cell wall (thin arrow), the other one within the cell interior (thick arrow). Results are summarised in Fig. 5b. The higher density of the so-called EL5-defect is found in the cell interiors. This defect has its highest thermal emission between 250 and 300 K. Therefore, it is responsible for the dark areas in the mapping in Fig. 5a. This defect is also present in
4. Experimental results and discussion As some first examples for the application of the above described techniques GaAs samples of different preparations were characterised by MDP. The resulting topograms are shown in the following. The first microwave detected PICTS-measurement was used for scaling the contrast of some of the MDP-topograms. All the samples are from SI, single-crystal wafers. As a representative of standard vertical gradient freeze
Fig. 3. MDP-mappings of sample 1 at (a) 300 K; and (b) 200 –500 K. Area 4.8 × 4.8 mm2, step width 80 mm.
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B. Gru¨ ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375
samples 1 and 2, but probably with a higher contrast that can be explained by the EL6-defect, which emits between 200 and 250 K. According to conventional PICTS-measurements its density is higher in samples 1 and 2 than in sample 3. It is known from the literature that this defect is mainly found in the cell interiors [9]. A further support of the above arguments is expected from detailed spatially resolved microwave detected PICTS-measurements which are under way.
5. Conclusion The few experimental examples shown above give just a first idea of the potential of MDP and microwave detected PICTS. Many experimental improvements particularly towards higher sensitivity and higher resolution are still possible. So far microwave
Fig. 5. (a) MDP-mapping of sample at a 300 K. Area 2.5 × 2.5 mm2, step width 50 mm. (b) Spatially resolved microwave detected PICTSspectra of the cell wall (thin arrow) and cell interior (thick arrow).
detected PICTS is the only method to deliver information about defects similar to DLTS with the great additional advantage of high spatial resolution. Unlike DLTS it is non-destructive and epitaxial layers can be investigated as well. Due to these advantages these methods may in future also be used for routine experiments in materials production.
References
Fig. 4. MDP-mappings of sample 2 at (a) 300 K and (b) 200 – 250 K. Area 3 × 3 mm2, step width 50 mm.
[1] J.R. Niklas, W. Siegel, M. Jurisch, U. Kretzer, Mater. Sci. Eng. B80 (2001) 206. [2] D.V. Lang, J. Appl. Phys. 45 (1974) 3023. [3] P. Blood, J.W. Orton, The Electrical Characterisation of Semiconductors: Majority Carriers an Electron States, Academic Press, London, UK, 1992. [4] Z.M. Wang, J. Windscheif, D.J. As, W. Jantz, J. Appl. Phys. 73 (3) (1993) 1430. [5] X. Le Cleac’h, Solid State Commun. 85 (9) (1993) 799.
B. Gru¨ ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375 [6] J.-M. Spaeth, J.R. Niklas, R.H. Bartram, Structural Analysis of Point Defects in Solids, Springer, Heidelberg, 1991. [7] B. Gru¨ ndig, Diploma Thesis, Technical University Bergakademie Freiberg, 2001.
375
[8] T. Steinegger, Doctoral Thesis, Technical University Bergakademie Freiberg, 2001. [9] K. Yasutake, H. Kakiuchi, A. Takeuchi, K. Yoshii, H. Kawebe, J. Mat. Sci.: Mater. Electronics 8 (1997) 239.
Defect specific topography of GaAs wafers by microwave-detected photo induced current transient spectroscopy B. Gru¨ndig-Wendrock a,*, M. Jurisch b, J.R. Niklas a a
b
Technical Uni6ersity Bergakademie, Freiberg, Germany Freiberger Compound Materials GmbH, Freiberg, Germany
Abstract Non-destructive and spatially resolved (35 mm) characterisation methods for high resistivity GaAs and epitaxial substrates are presented. Microwave detected photo induced current transient spectroscopy (PICTS) is a further development of the already existing method Microwave detected photoconductivity (MDP). Using various GaAs samples of different preparations, the possibilities of these new tools are demonstrated along with the first results for technologically relevant extrinsic and intrinsic defects. © 2002 Published by Elsevier Science B.V. Keywords: Microwave absorption; Photoconductivity; PICTS; Contactless; SI-gallium arsenide; Si-GaAs
1. Introduction In semi-insulating (SI) GaAs a dislocation network is formed by dynamical polygonisation near the solid/liquid interface which interacts with native defects, dopants and impurities during the cooling-down procedure after crystal growth. Segregated zones of defects are generated, which are correlated to frozen-in inhomogeneities of the electrical and optical properties scaled by the cell size and sometimes called ‘mesoscopic inhomogeneities’ [1]. These are of great importance for the properties of devices. Therefore, a non-destructive method for the characterisation of these inhomogeneities, particularly of the concentration and distribution of deep defects is required. Defect specific topographic experiments are also important for the investigation of formation- and annihilation processes of intrinsic defects and diffusion- and gettering processes of impurity atoms for the optimisation of defect engineering and materials properties. The well-known deep level transient spectroscopy (DLTS) [2] turned out to be the most appropriate tool to characterise defects by their energetic levels. Unfortunately it is not applicable to higher resistance GaAs material and it provides neither identification of defect * Corresponding author. Fax: + 49-3731-394314. E-mail address: bianca – [email protected] (B. Gru¨ndig-Wendrock).
species nor their spatial distribution. Similar results as with DLTS can be obtained by measuring photoconductivity transients and evaluating them like the capacitance transients in DLTS experiments. These so-called photo induced current transient spectroscopy (PICTS) [3] experiments are feasible for high resistance material, however, an exact information about absolute concentrations of defects is lost.
2. Theoretical considerations A non-equilibrium concentration of electrons Dn and holes Dp is introduced by locally resolved excitation with a small laser spot of a pulse with above bandgap light. A photoconductivity D| = q(vn Dn + vp Dp)
(1)
results, where q is the electron charge and vn,p the electron and hole mobility, respectively. Due to their low mobility the contribution of the holes to the photoconductivity can be neglected in GaAs. The lifetime ~ of excess charge carriers is influenced by recombination and trapping processes through recombination centres and shallow and deep defects. It amounts to about 5 ns in the cell walls and 0.5 ns in the cell interiors [5]. Trapped electrons can be reemitted to the conduction band by thermal excitation. Thus, the time dependence
0921-5107/02/$ - see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 1 0 7 3 - X
372
B. Gru¨ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375
of the photoexcitation response is determined by generation, trapping, recombination and reemission processes [4] (Fig. 1a). After the photoexcitation and
recombination and trapping of charge carriers the typical photoconductivity transient based on the thermal excited emission is visible (Fig. 1a): t
D|transient = qvDn =qvnT0~ne − enT t
(2)
nT0 corresponds to the initial density of carriers trapped by a distinct defect, etnT symbolises the thermal excited emission rate of this trap, t is the time after the termination of the photo pulse. Prior to the PICTS-experiments a survey of the cellular structure and possible distribution of defects can be obtained by the method of microwave detected photoconductivity (MDP) [1]. A semiconductor laser modulated at a rate of 100 kHz was used for the photoexcitation. The photoconductivity signal according to Fig. 1a is detected by conventional lock-in technique. Thus the magnitude of the resulting DC-signal also depends on the intensity of thermally excited emission of charge carriers from the traps which determines the time dependence of the photoconductivity transient (laser off in Fig. 1a). There are two cases for the emission rate to be considered: In the first case the time constant for the photoconductivity transient is on the order of the time where the laser is off. This situation is depicted in Fig. 1a, and in more detail in Fig. 1b. The lower trace in Fig. 1b shows the signal behind the synchronous detector of the lock-in amplifier (before the integrator). Averaging this signal yields the final measurement result. If, on the other hand, the emission rate is very high and thus the transient very fast, then the photoconductivity signal approaches the shape shown in the upper trace in Fig. 1c. The same shape is, however, obtained if the emission rate is very small (with a corresponding very long transient time constant). In this case there is almost no signal during the laser off period. The two latter cases (Fig. 1c) yield a higher output signal than the case in Fig. 1b. Summarising, a small signal in the final topograms below (indicated dark) correspond to a medium high emission time constant, whereas high signals (light) correspond to either a very short or a very long time constant. In a PICTS measurement, in contrast, only the photoconductivity transient according to Eq. (2) which now has a minimum length of several 100 ms is analysed using the so-called two-gate technique of DLTS. From etnT the activation energy, EA, of a trap can be calculated using the relation: Fig. 1. (a) Photoconductivity during optical excitation followed by the photoconductivity transient. (b). Analysis of the photoconductivity signal as obtained by conventional lock-in-technique. The lower trace indicates the voltage-signal of the detector diode before the integrator stage of the lock-in amplifier. (c). Analysis as in Fig. 1b, however, for a very fast or a very slow reemission of charge carriers. The resulting output voltage-signal of the detector diode is higher as in Fig. 1b (for details see text.)
etnT = AT 2e − EA/kT
(3)
where A is a material constant. In contrast to DLTS, the magnitude of the peaks in the PICTS-spectra is proportional to the density of trapped carriers and not to the density of traps itself.
B. Gru¨ ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375
Fig. 2. Block diagram of the experimental set-up for MDP (dotted lines) and microwave detected PICTS (dashed lines).
3. Apparatus The experimental set-up for the MDP according to [1] and completed for the purposes of microwave detected PICTS is sketched in Fig. 2. The sample in a flat LN2 cryostat below a quartz window is outside the microwave cavity. A special iris in the wall of the cavity allows the microwave field to ‘leak out’ through the window into the sample. Thus, the complex dielectric constant of the sample influences the resonant frequency and the loss properties of the cavity. For optical excitation a semiconductor LASER with 680 nm wavelength, 3 mW power and a spot size of about 35 mm was used. Microwave absorption by excited charge carriers is detected according to the principle of usual EPR-technique [6]. For MDP the LASER is chopped at a rate of about 100 kHz and the detector signal analysed using conventional lock-in technique. For microwave detected PICTS lengths of optical pulses and intersections between pulses of about 1 ms were used. The signals are recorded by a transient recorder of the PICTS-PC. A stepping mechanism allows the sample to be positioned in the x – y-plane. Further details are described in [7].
373
(VGF) material sample 1 was measured. Sample 2 results from a series of samples with a special temperature-time-regime during annealing [8]. Sample 3 was annealed under the condition of an unusual protecting atmosphere. Samples 2 and 3 are both liquid encapsulated Czochralski (LEC)-GaAs samples. Photoconductivity mappings of samples 1 and 2, measured at 300 K and between 200 and 250 K a second time, are shown in Figs. 3 and 4. Dark areas correspond to a low signal and, therefore, to a medium intense defect emission (compare Fig. 1b). An inversion of the contrast is clearly obvious. Fig. 5 shows the microwave detected photo-conductivity mapping of sample 3 measured at 300 K. Microwave detected PICTS was measured at two points of the topogram, one in the cell wall (thin arrow), the other one within the cell interior (thick arrow). Results are summarised in Fig. 5b. The higher density of the so-called EL5-defect is found in the cell interiors. This defect has its highest thermal emission between 250 and 300 K. Therefore, it is responsible for the dark areas in the mapping in Fig. 5a. This defect is also present in
4. Experimental results and discussion As some first examples for the application of the above described techniques GaAs samples of different preparations were characterised by MDP. The resulting topograms are shown in the following. The first microwave detected PICTS-measurement was used for scaling the contrast of some of the MDP-topograms. All the samples are from SI, single-crystal wafers. As a representative of standard vertical gradient freeze
Fig. 3. MDP-mappings of sample 1 at (a) 300 K; and (b) 200 –500 K. Area 4.8 × 4.8 mm2, step width 80 mm.
374
B. Gru¨ ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375
samples 1 and 2, but probably with a higher contrast that can be explained by the EL6-defect, which emits between 200 and 250 K. According to conventional PICTS-measurements its density is higher in samples 1 and 2 than in sample 3. It is known from the literature that this defect is mainly found in the cell interiors [9]. A further support of the above arguments is expected from detailed spatially resolved microwave detected PICTS-measurements which are under way.
5. Conclusion The few experimental examples shown above give just a first idea of the potential of MDP and microwave detected PICTS. Many experimental improvements particularly towards higher sensitivity and higher resolution are still possible. So far microwave
Fig. 5. (a) MDP-mapping of sample at a 300 K. Area 2.5 × 2.5 mm2, step width 50 mm. (b) Spatially resolved microwave detected PICTSspectra of the cell wall (thin arrow) and cell interior (thick arrow).
detected PICTS is the only method to deliver information about defects similar to DLTS with the great additional advantage of high spatial resolution. Unlike DLTS it is non-destructive and epitaxial layers can be investigated as well. Due to these advantages these methods may in future also be used for routine experiments in materials production.
References
Fig. 4. MDP-mappings of sample 2 at (a) 300 K and (b) 200 – 250 K. Area 3 × 3 mm2, step width 50 mm.
[1] J.R. Niklas, W. Siegel, M. Jurisch, U. Kretzer, Mater. Sci. Eng. B80 (2001) 206. [2] D.V. Lang, J. Appl. Phys. 45 (1974) 3023. [3] P. Blood, J.W. Orton, The Electrical Characterisation of Semiconductors: Majority Carriers an Electron States, Academic Press, London, UK, 1992. [4] Z.M. Wang, J. Windscheif, D.J. As, W. Jantz, J. Appl. Phys. 73 (3) (1993) 1430. [5] X. Le Cleac’h, Solid State Commun. 85 (9) (1993) 799.
B. Gru¨ ndig-Wendrock et al. / Materials Science and Engineering B91–92 (2002) 371–375 [6] J.-M. Spaeth, J.R. Niklas, R.H. Bartram, Structural Analysis of Point Defects in Solids, Springer, Heidelberg, 1991. [7] B. Gru¨ ndig, Diploma Thesis, Technical University Bergakademie Freiberg, 2001.
375
[8] T. Steinegger, Doctoral Thesis, Technical University Bergakademie Freiberg, 2001. [9] K. Yasutake, H. Kakiuchi, A. Takeuchi, K. Yoshii, H. Kawebe, J. Mat. Sci.: Mater. Electronics 8 (1997) 239.