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Phase composition of microdefects in heavily doped n-GaAs
ARTICLE IN PRESS Physica B 404 (2009) 4988–4991
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Phase composition of microdefects in heavily doped n-GaAs N.A. Davletkildeev , M.M. Nukenov, N.V. Sologub Omsk Dostoevsky State University, 55a, Mira pr., Omsk 644077, Russia
a r t i c l e in f o
a b s t r a c t
Keywords: Heavily doped n-GaAs Microdefects X-ray diffractometry Phase composition Structural parameters
The phase composition of the microdefects in Te-doped GaAs single crystals grown by the Czochralski technique with a free carrier density n0 = 5 1017–5 1018 cm 3 was investigated. Ga2Te3 phase reflections appeared in the X-ray diffraction patterns. Nonmonotonic dependences of the relative volume fraction and the coherent scattering domain size of Ga2Te3 on Te doping level were obtained. The possible causes of the nonmonotonic variation in the structural parameters of the Ga2Te3 phase with increase of Te concentration in GaAs are discussed. & 2009 Elsevier B.V. All rights reserved.
1. Introduction There are many reports [1–5] on the observation of various microdefects in heavily doped by the VI-group impurities (Te, Se, S) GaAs crystals, however, the question about the nature and formation mechanisms of the observed microdefects still remains open. With the use of transmission electronic microscopy (TEM), it was shown [2–5] that the heavily doped crystals involve the following basic types of microdefects: (1) single and/or multilayer stacking faults 0.1–1.0 mm in size; (2) small (up to dozens of nanometers) prismatic dislocation loops, both full and partial; (3) small size particles of the extrinsic discharges located within the square of a stacking fault or on a dislocation loop. There are various opinions concerning the nature of the observed microdefects and the possible mechanisms of their formation. The majority of authors [2–4] hold the opinion that the interstitial layers and the particles of the extrinsic discharges consist of Ga2(Te, Se, S)3 phase. However, there are no direct data on composition of the microdefects. The aim of this contribution is to investigate the phase composition and the structural parameters of the microdefects in GaAs with various Te doping levels.
2. Experimental The objects of investigations were Te-doped GaAs single crystals grown by the Czochralski technique under flux with a free carrier density n0 =5 1017–5 1018 cm 3. The samples were cut from an ingot in the form of disks with thickness of 2 mm and oriented along the (1 0 0) plane. The Corresponding author. Tel.: + 7 381222 4972; fax: + 7 381 264 77 87.
E-mail address: [email protected] (N.A. Davletkildeev). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.267
deviation of the sample surface from the (1 0 0) plane did not exceed 11. After orientation, the surface of all samples was exposed to standard treatment (mechanical grinding and polishing and also chemical polishing). The samples were analysed by means of DSL etching (diluted Sirtl-like etching used with light), AFM (atomic force microscopy), and XRD (X-ray diffractometry). DSL etching was used for revealing the microdefects on GaAs:Te surfaces. Etching of samples was carried out using a 1:6 aqueous solution of HF–CrO3 (ratio 1:5) at illumination by a halogen lamp [6]. AFM was used to examine the surface morphology and DSLrevealed defect structures. AFM was carried out with a scanning probe microscope Solver PRO (NT-MDT Inc.) in the tapping mode in air using an NSG10 probe (NT-MDT Inc.). XRD was used to characterize the phase composition and the structural parameters of the microdefects. The XRD patterns were recorded on a Shimadzu XRD-6000 diffractometer with CuKa radiation.
3. Results and discussion Fig. 1 shows AFM images of the DSL-etched samples with various free carrier density. It demonstrates the changes of defect structure of GaAs:Te crystals with increase of doping level. The AFM images show that all etch figures are hillocks. Large extended figures represent an elevation (hillock) covered by microhillocks. Small etch figures are single microhillocks. With increase of the Te concentration in GaAs, the size of hillocks decreases (Fig. 1). The characteristic feature of the sample with n0 = 1018 cm 3 is the presence of the area around the hillock that depleted microhillocks over 10–20 mm (Fig. 1a). The area decreases with increase of doping level (Fig. 1b and c).
ARTICLE IN PRESS N.A. Davletkildeev et al. / Physica B 404 (2009) 4988–4991
Fig. 1. AFM images of the DSL-etched surface of GaAs:Te crystals with the free carrier density: (a) 1018 cm 3, (b) 1.5 1018 cm 3, and (c) 2.5 1018 cm 3.
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According to Weyher and Van de Van [6], large etch figures form on the places of the grown-in dislocation outcrop. Grown-in dislocations in the material under study have a complex structure that always appeared in tangles surrounded by clouds of large extrinsic faulted Frank loops on {111} planes. Frank dislocation loops limit the extrinsic stacking faults that have one or several interstitial layers [7]. The complex defects containing the grown-in dislocations and dislocation loops have the characteristic sizes 2–6 mm [6]. This is in agreement with the sizes of large etch figures on AFM images (Fig. 1). In addition, the density of large etch figures corresponds to the density of grown-in dislocations in the investigated material [8]. The small etch figures (microhillocks) are produced in the outcrop of single Frank dislocation loops. The width of profile across the microhillock (0.2–1 mm) corresponds to characteristic sizes of dislocation loops [1,5–7,9]. The density of microhillocks corresponds to the density of dislocation loops [7,9]. Thus, in investigated crystals the extrinsic stacking faults surrounded by the Frank dislocation loops are revealed. Prismatic dislocation loops in investigated samples were not observed, because they form at the impurity concentration above 8 1018 cm 3 [3,9]. Also the small size particles of the extrinsic discharges were not revealed by DSL etch, since they are tiny in sizes. The XRD patterns are recorded in the vicinity of the (2 0 0), (4 0 0) GaAs reflections and (2 0 0), (4 0 0) reflections of the expected Ga2Te3 phase. Fig. 2 shows the XRD patterns of GaAs:Te single crystals. The insets show peaks from the (2 0 0) and (4 0 0) planes of Ga2Te3 phase. They confirm the presence of Ga2Te3 phase in GaAs:Te crystals. The relative volume fraction of Ga2Te3 has been estimated from the comparison of the Ga2Te3 and GaAs reflection intensities. The concentration dependences of the volume fraction of Ga2Te3 phase and the microdefects density are presented in Fig. 3. The Ga2Te3 fraction has the same order of value as Te fraction in the investigated samples. The microdefects density was determined from the analysis of AFM images. The Ga2Te3 volume fraction increases monotonically over the range n0 o1.15 1018 cm 3 then slumps at n0 = 1.5 1018 cm 3. In contrast, the microdefects density increases through the whole investigated concentration range (Fig. 3). It should be noted that both the stacking faults and the small particles of the extrinsic discharges, located in the area of stacking faults or on dislocation loop, contribute to Ga2Te3 reflection intensity. According to the TEM data [1,7,9], the density of particles of the extrinsic discharges ten times exceeds the density of stacking faults. With increase of doping level the densities of the particles and the stacking faults increase proportionally. Therefore, reduction of Ga2Te3 volume fraction at n0 41.5 1018 cm 3 is not connected with decrease of the microdefects density. Analysis of the peak broadening factors has shown that the peak broadening from Ga2Te3 is caused by small size of coherent scattering domain (CSD). Applying the Scherrer equation [10], the estimation of the CSD size has been carried out. Fig. 4 displays the concentration dependences of the CSD size and the lattice constant of Ga2Te3. The concentration dependence of the CSD size of Ga2Te3 phase is ˚ This value exceeds nonmonotonic. The CSD size is in average 1000 A. twice the average size of particles of the extrinsic discharges, obtained by TEM [4,7,9]. The increased value of the CSD size is caused by small intensity and narrowness of Ga2Te3 peaks. The average lattice constant of Ga2Te3 is 5.892 A˚ and changes insignificantly with increase of the free carrier density up to n0 = 2 1018 cm 3. For n0 4 2 1018 cm 3 it decreases. The changes in the volume fraction and the structure parameters can be explained as follows. The slump of the Ga2Te3
ARTICLE IN PRESS 4990
N.A. Davletkildeev et al. / Physica B 404 (2009) 4988–4991
Fig. 4. Concentration dependences of the CSD size (full circles) and the lattice constant (open circles) of Ga2Te3 in GaAs:Te single crystals.
volume fraction at n0 = 1.5 1018 cm 3 can be connected with disorientation of Ga2Te3 crystallites with respect to GaAs matrix. A cause of the disorientation can be a local supersaturation with vacancies of GaAs volume around microdefects. The vacancies can be formed at the nucleation and growth of the interstitial layer [1,9]. The authors [9] use the same arguments to explain a difference between the calculated value and the experimental value for the lattice constants of GaAs and Ga2Te3. The high vacancies concentration leads to reduction of the CSD sizes with increase of doping level. Local supersaturation with vacancies prevents the interstitial layer from further growth. The increase of the Ga2Te3 volume fraction is provided by increase of the microdefects density.
4. Conclusions Fig. 2. Typical XRD patterns from (2 0 0): (a) n (4 0 0), (b) planes of GaAs:Te single crystals. The insets show the Ga2Te3 phase reflections from (2 0 0) and (4 0 0) planes.
To summarize, in the presented work the following basic results were obtained:
The microdefects in GaAs:Te single crystals were revealed by DSL etching.
On the basis of the comparative analysis of the literary TEM
data and the AFM data of the etch figures, it was established that the microdefects of GaAs:Te crystal are Frank dislocation loops. The presence of Ga2Te3 phase in GaAs:Te single crystals was revealed by XRD. On the basis of the XRD patterns analysis the relative volume fraction, the CSD size and the lattice constant of Ga2Te3 phase were obtained. The nonmonotonic dependences of these parameters on the free carrier density were found. The nonmonotonic dependences of the volume fraction and the structural parameters for Ga2Te3 phase on the carrier density are explained by the local supersaturation with vacancies in GaAs volume around microdefects.
References Fig. 3. Dependences of the relative volume fraction of Ga2Te3 phase (full circles) and the microdefects density (open circles) on the carrier density for GaAs:Te single crystals.
[1] M.G. Mil’vidsky, V.B. Osvensky, Structural Defects in Monocrystalline Semiconductors, Metallurgiya, Moscow, 1984, p. 233. [2] T. Kamejima, J. Matsui, Y. Seki, H. Watanabe, J. Appl. Phys. 50 (1979) 3312.
ARTICLE IN PRESS N.A. Davletkildeev et al. / Physica B 404 (2009) 4988–4991
[3] [4] [5] [6] [7]
V.D. Verner, S.K. Maksimov, D.K. Nichugovskii, Phys. Stat. Sol. A 33 (1976) 755. P.W. Hutchinson, P.S. Dobson, Philos. Mag. 30 (1974) 65. D. Laister, G.M. Jenkins, J. Mater. Sci. 3 (1968) 584. J.L. Weyher, J. Van de Ven, J. Cryst. Growth 78 (1986) 191. L.M. Morgulis, V.B. Osvenskii, M.G. Milvidskii, Sov. Phys. Solid State 16 (1974) 223.
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[8] V.A. Bogdanova, N.A. Davletkil’deev, M.M. Nukenov, N.A. Semikolenova, Phys. Solid State 50 (2008) 244. [9] L.M. Morgulis, M.G. Milvidskii, V.B. Osvensky, Izv. Akad. Nauk SSSR, Ser. fiz. 38 (1974) 1447. [10] R. Jenkins, R.L. Snyder, Introduction to X-ray Powder Diffractometry, John Wiley & Sons Inc., New York, 1996, p. 89.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Phase composition of microdefects in heavily doped n-GaAs N.A. Davletkildeev , M.M. Nukenov, N.V. Sologub Omsk Dostoevsky State University, 55a, Mira pr., Omsk 644077, Russia
a r t i c l e in f o
a b s t r a c t
Keywords: Heavily doped n-GaAs Microdefects X-ray diffractometry Phase composition Structural parameters
The phase composition of the microdefects in Te-doped GaAs single crystals grown by the Czochralski technique with a free carrier density n0 = 5 1017–5 1018 cm 3 was investigated. Ga2Te3 phase reflections appeared in the X-ray diffraction patterns. Nonmonotonic dependences of the relative volume fraction and the coherent scattering domain size of Ga2Te3 on Te doping level were obtained. The possible causes of the nonmonotonic variation in the structural parameters of the Ga2Te3 phase with increase of Te concentration in GaAs are discussed. & 2009 Elsevier B.V. All rights reserved.
1. Introduction There are many reports [1–5] on the observation of various microdefects in heavily doped by the VI-group impurities (Te, Se, S) GaAs crystals, however, the question about the nature and formation mechanisms of the observed microdefects still remains open. With the use of transmission electronic microscopy (TEM), it was shown [2–5] that the heavily doped crystals involve the following basic types of microdefects: (1) single and/or multilayer stacking faults 0.1–1.0 mm in size; (2) small (up to dozens of nanometers) prismatic dislocation loops, both full and partial; (3) small size particles of the extrinsic discharges located within the square of a stacking fault or on a dislocation loop. There are various opinions concerning the nature of the observed microdefects and the possible mechanisms of their formation. The majority of authors [2–4] hold the opinion that the interstitial layers and the particles of the extrinsic discharges consist of Ga2(Te, Se, S)3 phase. However, there are no direct data on composition of the microdefects. The aim of this contribution is to investigate the phase composition and the structural parameters of the microdefects in GaAs with various Te doping levels.
2. Experimental The objects of investigations were Te-doped GaAs single crystals grown by the Czochralski technique under flux with a free carrier density n0 =5 1017–5 1018 cm 3. The samples were cut from an ingot in the form of disks with thickness of 2 mm and oriented along the (1 0 0) plane. The Corresponding author. Tel.: + 7 381222 4972; fax: + 7 381 264 77 87.
E-mail address: [email protected] (N.A. Davletkildeev). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.267
deviation of the sample surface from the (1 0 0) plane did not exceed 11. After orientation, the surface of all samples was exposed to standard treatment (mechanical grinding and polishing and also chemical polishing). The samples were analysed by means of DSL etching (diluted Sirtl-like etching used with light), AFM (atomic force microscopy), and XRD (X-ray diffractometry). DSL etching was used for revealing the microdefects on GaAs:Te surfaces. Etching of samples was carried out using a 1:6 aqueous solution of HF–CrO3 (ratio 1:5) at illumination by a halogen lamp [6]. AFM was used to examine the surface morphology and DSLrevealed defect structures. AFM was carried out with a scanning probe microscope Solver PRO (NT-MDT Inc.) in the tapping mode in air using an NSG10 probe (NT-MDT Inc.). XRD was used to characterize the phase composition and the structural parameters of the microdefects. The XRD patterns were recorded on a Shimadzu XRD-6000 diffractometer with CuKa radiation.
3. Results and discussion Fig. 1 shows AFM images of the DSL-etched samples with various free carrier density. It demonstrates the changes of defect structure of GaAs:Te crystals with increase of doping level. The AFM images show that all etch figures are hillocks. Large extended figures represent an elevation (hillock) covered by microhillocks. Small etch figures are single microhillocks. With increase of the Te concentration in GaAs, the size of hillocks decreases (Fig. 1). The characteristic feature of the sample with n0 = 1018 cm 3 is the presence of the area around the hillock that depleted microhillocks over 10–20 mm (Fig. 1a). The area decreases with increase of doping level (Fig. 1b and c).
ARTICLE IN PRESS N.A. Davletkildeev et al. / Physica B 404 (2009) 4988–4991
Fig. 1. AFM images of the DSL-etched surface of GaAs:Te crystals with the free carrier density: (a) 1018 cm 3, (b) 1.5 1018 cm 3, and (c) 2.5 1018 cm 3.
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According to Weyher and Van de Van [6], large etch figures form on the places of the grown-in dislocation outcrop. Grown-in dislocations in the material under study have a complex structure that always appeared in tangles surrounded by clouds of large extrinsic faulted Frank loops on {111} planes. Frank dislocation loops limit the extrinsic stacking faults that have one or several interstitial layers [7]. The complex defects containing the grown-in dislocations and dislocation loops have the characteristic sizes 2–6 mm [6]. This is in agreement with the sizes of large etch figures on AFM images (Fig. 1). In addition, the density of large etch figures corresponds to the density of grown-in dislocations in the investigated material [8]. The small etch figures (microhillocks) are produced in the outcrop of single Frank dislocation loops. The width of profile across the microhillock (0.2–1 mm) corresponds to characteristic sizes of dislocation loops [1,5–7,9]. The density of microhillocks corresponds to the density of dislocation loops [7,9]. Thus, in investigated crystals the extrinsic stacking faults surrounded by the Frank dislocation loops are revealed. Prismatic dislocation loops in investigated samples were not observed, because they form at the impurity concentration above 8 1018 cm 3 [3,9]. Also the small size particles of the extrinsic discharges were not revealed by DSL etch, since they are tiny in sizes. The XRD patterns are recorded in the vicinity of the (2 0 0), (4 0 0) GaAs reflections and (2 0 0), (4 0 0) reflections of the expected Ga2Te3 phase. Fig. 2 shows the XRD patterns of GaAs:Te single crystals. The insets show peaks from the (2 0 0) and (4 0 0) planes of Ga2Te3 phase. They confirm the presence of Ga2Te3 phase in GaAs:Te crystals. The relative volume fraction of Ga2Te3 has been estimated from the comparison of the Ga2Te3 and GaAs reflection intensities. The concentration dependences of the volume fraction of Ga2Te3 phase and the microdefects density are presented in Fig. 3. The Ga2Te3 fraction has the same order of value as Te fraction in the investigated samples. The microdefects density was determined from the analysis of AFM images. The Ga2Te3 volume fraction increases monotonically over the range n0 o1.15 1018 cm 3 then slumps at n0 = 1.5 1018 cm 3. In contrast, the microdefects density increases through the whole investigated concentration range (Fig. 3). It should be noted that both the stacking faults and the small particles of the extrinsic discharges, located in the area of stacking faults or on dislocation loop, contribute to Ga2Te3 reflection intensity. According to the TEM data [1,7,9], the density of particles of the extrinsic discharges ten times exceeds the density of stacking faults. With increase of doping level the densities of the particles and the stacking faults increase proportionally. Therefore, reduction of Ga2Te3 volume fraction at n0 41.5 1018 cm 3 is not connected with decrease of the microdefects density. Analysis of the peak broadening factors has shown that the peak broadening from Ga2Te3 is caused by small size of coherent scattering domain (CSD). Applying the Scherrer equation [10], the estimation of the CSD size has been carried out. Fig. 4 displays the concentration dependences of the CSD size and the lattice constant of Ga2Te3. The concentration dependence of the CSD size of Ga2Te3 phase is ˚ This value exceeds nonmonotonic. The CSD size is in average 1000 A. twice the average size of particles of the extrinsic discharges, obtained by TEM [4,7,9]. The increased value of the CSD size is caused by small intensity and narrowness of Ga2Te3 peaks. The average lattice constant of Ga2Te3 is 5.892 A˚ and changes insignificantly with increase of the free carrier density up to n0 = 2 1018 cm 3. For n0 4 2 1018 cm 3 it decreases. The changes in the volume fraction and the structure parameters can be explained as follows. The slump of the Ga2Te3
ARTICLE IN PRESS 4990
N.A. Davletkildeev et al. / Physica B 404 (2009) 4988–4991
Fig. 4. Concentration dependences of the CSD size (full circles) and the lattice constant (open circles) of Ga2Te3 in GaAs:Te single crystals.
volume fraction at n0 = 1.5 1018 cm 3 can be connected with disorientation of Ga2Te3 crystallites with respect to GaAs matrix. A cause of the disorientation can be a local supersaturation with vacancies of GaAs volume around microdefects. The vacancies can be formed at the nucleation and growth of the interstitial layer [1,9]. The authors [9] use the same arguments to explain a difference between the calculated value and the experimental value for the lattice constants of GaAs and Ga2Te3. The high vacancies concentration leads to reduction of the CSD sizes with increase of doping level. Local supersaturation with vacancies prevents the interstitial layer from further growth. The increase of the Ga2Te3 volume fraction is provided by increase of the microdefects density.
4. Conclusions Fig. 2. Typical XRD patterns from (2 0 0): (a) n (4 0 0), (b) planes of GaAs:Te single crystals. The insets show the Ga2Te3 phase reflections from (2 0 0) and (4 0 0) planes.
To summarize, in the presented work the following basic results were obtained:
The microdefects in GaAs:Te single crystals were revealed by DSL etching.
On the basis of the comparative analysis of the literary TEM
data and the AFM data of the etch figures, it was established that the microdefects of GaAs:Te crystal are Frank dislocation loops. The presence of Ga2Te3 phase in GaAs:Te single crystals was revealed by XRD. On the basis of the XRD patterns analysis the relative volume fraction, the CSD size and the lattice constant of Ga2Te3 phase were obtained. The nonmonotonic dependences of these parameters on the free carrier density were found. The nonmonotonic dependences of the volume fraction and the structural parameters for Ga2Te3 phase on the carrier density are explained by the local supersaturation with vacancies in GaAs volume around microdefects.
References Fig. 3. Dependences of the relative volume fraction of Ga2Te3 phase (full circles) and the microdefects density (open circles) on the carrier density for GaAs:Te single crystals.
[1] M.G. Mil’vidsky, V.B. Osvensky, Structural Defects in Monocrystalline Semiconductors, Metallurgiya, Moscow, 1984, p. 233. [2] T. Kamejima, J. Matsui, Y. Seki, H. Watanabe, J. Appl. Phys. 50 (1979) 3312.
ARTICLE IN PRESS N.A. Davletkildeev et al. / Physica B 404 (2009) 4988–4991
[3] [4] [5] [6] [7]
V.D. Verner, S.K. Maksimov, D.K. Nichugovskii, Phys. Stat. Sol. A 33 (1976) 755. P.W. Hutchinson, P.S. Dobson, Philos. Mag. 30 (1974) 65. D. Laister, G.M. Jenkins, J. Mater. Sci. 3 (1968) 584. J.L. Weyher, J. Van de Ven, J. Cryst. Growth 78 (1986) 191. L.M. Morgulis, V.B. Osvenskii, M.G. Milvidskii, Sov. Phys. Solid State 16 (1974) 223.
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[8] V.A. Bogdanova, N.A. Davletkil’deev, M.M. Nukenov, N.A. Semikolenova, Phys. Solid State 50 (2008) 244. [9] L.M. Morgulis, M.G. Milvidskii, V.B. Osvensky, Izv. Akad. Nauk SSSR, Ser. fiz. 38 (1974) 1447. [10] R. Jenkins, R.L. Snyder, Introduction to X-ray Powder Diffractometry, John Wiley & Sons Inc., New York, 1996, p. 89.