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Diffusion of nonstoichiometric defects in n-GaP crystals
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 6 (2003) 441–443
Diffusion of nonstoichiometric defects in n-GaP crystals Takenori Tannoa, Ken Sutoa, Yutaka Oyamaa, Jun-ichi Nishizawab,* b
a Department of Materials Science, Tohoku University, Aoba-yama 02, Sendai 980-8579, Japan Semiconductor Research Institute, Semiconductor Research Foundation, Kawauchi Aoba, Sendai 980-0862, Japan
Received 19 March 2003; received in revised form 20 July 2003; accepted 5 August 2003
Abstract Diffusion of nonstoichiometry-related point defects from a LEC-grown GaP substrate to a Te-doped GaP n-type epitaxial layer was investigated by means of photocapacitance. It was revealed that deep donor level at EC 2:1 eV was introduced into GaP substrate with annealing under phosphorus vapor pressure. Thus, the 2.1 eV deep level is thought to be involved with excess P atoms such as interstitial phosphorus atoms. In Te-doped crystal, 2.1 eV level was detected and the density increased as the time of substrate annealing increased. By measuring PHCAP spectra of samples with different thickness of epitaxial layer, diffusion profile of the defects from the substrate interface was obtained. From this, the diffusion coefficient at 850 C is estimated to be B8 10–11 cm2/s. r 2003 Elsevier Ltd. All rights reserved. PACS: 71.55.Eq; 78.66.Fd; 66.30.Lw Keywords: Liquid phase epitaxy; Diffusion; Te-doping; Photocapacitance
1. Introduction It is well known that liquid encapsulated Czochralski (LEC)-grown GaP crystals contain a lot of point defects originating in their nonstoichiometric composition in consequence of high growth temperature. When liquidphase epitaxy is carried out on a LEC-grown wafer, migration of these point defects is expected. In this case difference of defect densities between LEC-grown wafer and LPE layer acts as a driving force, and high growth temperature, even though not so high as LEC growth temperature, enhances the diffusion. Therefore the initial density of defects at the surface region of the substrate should strongly affect the crystallinity of the epitaxial layer. It has been reported that not band-edge green emission but only deep-level emissions at 620 and 700 nm are detected from thin GaP LPE layers in photoluminescence experiments [1]. This result indicates
*Corresponding author. Tel.: +81-22-223-7287; fax: +8122-223-7289. E-mail address: [email protected] (J.-i. Nishizawa).
that larger number of defects exist near the interface to the substrate than other region of the epitaxial layer. It is reasonable to consider that some kind of defects diffuse from the substrate to the epitaxial layer. In this report, we investigate electronic states of diffusing defects and their diffusion property by photocapacitance (PHCAP) method.
2. Experiments PHCAP measurement is a powerful method for detection of deep levels with low concentration. This measurement is based on the photoexcitation of carriers trapped at deep levels in the depletion layer. Details are described elsewhere [2]. In our experiment, the width of depletion layer is in range of 50–500 nm, which is much thinner than the thickness of epitaxial layer. Therefore, profile of defect diffusion from the substrate can be obtained by carrying out PHCAP on Schottky contacts fabricated on various points of an epitaxial layer, where the thickness of epi-layer differs at each position because of inhomogeneity of growth rate within a wafer.
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.08.019
ARTICLE IN PRESS 442
T. Tanno et al. / Materials Science in Semiconductor Processing 6 (2003) 441–443
Te-doped GaP epitaxial layers are grown at 850 C with TDM-CVP (temperature difference method under controlled vapor pressure) [3–5]. Te concentration in grown layer is 6 1016 cm–3. TDM-CVP equipment used in this study has two wells for reserving melt. One of them is filled with Ga melt solving GaP and Te, and another is empty to use for annealing a substrate under phosphorus vapor pressure of 20 kPa. When the substrate is slid under an empty well, annealing under controlled phosphorus vapor pressure begins. Following the annealing procedure, the substrate is moved to under the filled well to successively start growth. After growth, the sample is slid back to beneath the carbon cover and cooled down rapidly by shifting out the furnace. For PHCAP, Schottky and ohmic contacts were fabricated with Au and Au–Ge/Au, respectively. PHCAP measurements were performed at 95 K where shallow levels are thermally ionized. When monochromatic light is irradiated on a Schottky diode, electrons or holes trapped in deep levels are emitted to conduction band or valence band if the incident photon energy is larger than the photoionization energy of the deep level. And then, the space charge changes. In PHCAP with constant-capacitance mode, capacitance of the diode is kept constant with automatic control of bias voltage. From the shift of bias voltage, density of ionized centers can be calculated [2]. In the case of n-type semiconductors, ups and downs in spectra indicate the presence of deep electron traps and deep hole traps, respectively.
Fig. 1. PHCAP spectra of undoped n-type GaP substrates annealed under phosphorus vapor pressure of 20 kPa (solid); and annealed beneath a carbon cover (broken).
3. Results and Discussion First, two annealed undoped n-type LEC-grown substrates without epitaxial growth were examined with PHCAP. Samples were annealed at 850 C for 2 h; one was at phosphorus vapor pressure of 20 kPa, and another was annealed beneath a carbon cover where no additional vapor pressure is applied. PHCAP spectra of these substrates are displayed in Fig. 1. One can find that density of a deep donor level at EC 2:1 eV is obviously higher in the substrate annealed under vapor pressure than another. From this result, the 2.1 eV deep donor is supposed to be P-rich-type nonstoichiometric defects or related complexes. We should note here that annealing under phosphorus vapor pressure did not increase point defects compared to the virgin substrate although defect density was higher than the annealing without additional vapor pressure. It was reported that defect densities in S-doped GaP crystal decreased as a consequence of annealing under controlled phosphorus vapor pressure in a region of 13 Pa to 67 kPa [6]. This fact means that a lot of nonstoichiometric defects are contained in the LECgrown crystal and large part of them is vanished by annealing at 850 C independently of the pressure of
Fig. 2. PHCAP spectra of Te-doped LPE layers grown on substrates pre-annealed under phosphorus vapor pressure for 5 min (triangle) and 60 min (square).
phosphorus vapor. Actually, in the case of unannealed undoped substrates, PHCAP measurement was impossible because of high resistivity of metal–semiconductor contacts. This fact suggests that the densities of nonstoichiometric defects are higher than those of residual-donor impurities. Therefore, densities of deep levels in annealed substrates estimated in this experiment should be lower than in the unannealed substrate.
ARTICLE IN PRESS T. Tanno et al. / Materials Science in Semiconductor Processing 6 (2003) 441–443
443
complex defects diffuse keeping there complicated structures. Such a high diffusion constant implies the interstitial phosphorus atoms.
4. Summary
Fig. 3. Depth profile of defects obtained from PHCAP spectra of Te-doped layers with difference thicknesses.
Next, PHCAP spectra of Te-doped LPE layers were measured (Fig. 2). They were grown on the S-doped ntype substrates that had been previously annealed under phosphorus vapor pressure for 5 or 60 min at 850 C. The thickness of epi-layers are B17 mm. The most significant dissimilarity appears at 2.1 eV. With longer pre-annealing of the substrate, the deep level at EC 2:1 eV increased. From the coincidence of energy, the deep level is thought to exist both in the substrate and the epitaxial layer. Furthermore, other two deep donor at EC 1:5 and 2.25 eV were detected. However, in contrast, they were not affected by the time of preannealing. As mentioned before, by measuring different points of the epi-layer, it is possible to obtain diffusion profile of defects (Fig. 3). The deep level at EC 2:1 eV shows clear dependence on the depth. The concentration of 2.1 eV deep donor decreased as the thickness increased and become lower than detection limit at thickness >16.4 mm. We suppose that 2.1 eV defect diffuses from the substrate into the epitaxial layer. Diffusion coefficient of this defect at 850 C is estimated to be 8 10– 11 cm2/s from the slope of the profile and the growth time of 2 h. At least, it is difficult to suppose that
Two types of undoped GaP substrates annealed under different conditions were examined by means of PHCAP measurement and it was revealed that deep donor level at EC 2:1 eV in the substrate annealed under phosphorus vapor pressure of 20 kPa is higher than in the one annealed beneath the carbon cover. In Te-doped LPE layer grown on pre-annealed S-doped GaP substrate, depth profile of nonstoichiometry-related defects was observed. Among three detected deep levels at EC 1:5; –2.1, and –2.25 eV, only 2.1 eV level showed clear depth dependence. From the slope of the profile, the diffusion coefficient of 2.1 eV defect at 850 C was estimated to be 8 10–11 cm2/s. This defect is considered as a P-rich-type nonstoichiometric point defect such as an interstitial phosphorus atoms.
References [1] Suto K, Adachi S, Yoneyama T, Nishizawa J. Free exciton recombination in GaP and GaP/AlxGa1–xP layers grown by temperature difference method under controlled vapor pressure. J Cryst Growth 1996;160(1–2):13–20. [2] Oyama Y, Nishizawa J, Dezaki K. Deep levels in Te-doped GaAs prepared by annealing under controlled arsenic vapor pressure. J Appl Phys 1991;70(2):833–7. [3] Nishizawa J, Okuno Y. Liquid phase epitaxy of GaP by a temperature difference method under controlled vapor pressure. IEEE Trans Electron Devices 1975;ED22(9):716–21. [4] Nishizawa J, Okuno Y, Tadano H. Nearly perfect crystal growth of III–V compounds by the temperature difference method under controlled vapour pressure. J Cryst Growth 1975;31:215–22. [5] Nishizawa J, Koike M, Miura K, Okuno Y. Deep Levels Studies of N-free and N-doped GaP grown by TDM-CVP. Jpn J Appl Phys 1979;19(1):25–30. [6] Yu TJ, Suto K, Nishizawa J. Investigation of deep levels in GaP liquid phase epitaxial layers on substrates with vapor pressure heat treatment. Mater Res Soc Symp Proc 1999;588:61–6.
Materials Science in Semiconductor Processing 6 (2003) 441–443
Diffusion of nonstoichiometric defects in n-GaP crystals Takenori Tannoa, Ken Sutoa, Yutaka Oyamaa, Jun-ichi Nishizawab,* b
a Department of Materials Science, Tohoku University, Aoba-yama 02, Sendai 980-8579, Japan Semiconductor Research Institute, Semiconductor Research Foundation, Kawauchi Aoba, Sendai 980-0862, Japan
Received 19 March 2003; received in revised form 20 July 2003; accepted 5 August 2003
Abstract Diffusion of nonstoichiometry-related point defects from a LEC-grown GaP substrate to a Te-doped GaP n-type epitaxial layer was investigated by means of photocapacitance. It was revealed that deep donor level at EC 2:1 eV was introduced into GaP substrate with annealing under phosphorus vapor pressure. Thus, the 2.1 eV deep level is thought to be involved with excess P atoms such as interstitial phosphorus atoms. In Te-doped crystal, 2.1 eV level was detected and the density increased as the time of substrate annealing increased. By measuring PHCAP spectra of samples with different thickness of epitaxial layer, diffusion profile of the defects from the substrate interface was obtained. From this, the diffusion coefficient at 850 C is estimated to be B8 10–11 cm2/s. r 2003 Elsevier Ltd. All rights reserved. PACS: 71.55.Eq; 78.66.Fd; 66.30.Lw Keywords: Liquid phase epitaxy; Diffusion; Te-doping; Photocapacitance
1. Introduction It is well known that liquid encapsulated Czochralski (LEC)-grown GaP crystals contain a lot of point defects originating in their nonstoichiometric composition in consequence of high growth temperature. When liquidphase epitaxy is carried out on a LEC-grown wafer, migration of these point defects is expected. In this case difference of defect densities between LEC-grown wafer and LPE layer acts as a driving force, and high growth temperature, even though not so high as LEC growth temperature, enhances the diffusion. Therefore the initial density of defects at the surface region of the substrate should strongly affect the crystallinity of the epitaxial layer. It has been reported that not band-edge green emission but only deep-level emissions at 620 and 700 nm are detected from thin GaP LPE layers in photoluminescence experiments [1]. This result indicates
*Corresponding author. Tel.: +81-22-223-7287; fax: +8122-223-7289. E-mail address: [email protected] (J.-i. Nishizawa).
that larger number of defects exist near the interface to the substrate than other region of the epitaxial layer. It is reasonable to consider that some kind of defects diffuse from the substrate to the epitaxial layer. In this report, we investigate electronic states of diffusing defects and their diffusion property by photocapacitance (PHCAP) method.
2. Experiments PHCAP measurement is a powerful method for detection of deep levels with low concentration. This measurement is based on the photoexcitation of carriers trapped at deep levels in the depletion layer. Details are described elsewhere [2]. In our experiment, the width of depletion layer is in range of 50–500 nm, which is much thinner than the thickness of epitaxial layer. Therefore, profile of defect diffusion from the substrate can be obtained by carrying out PHCAP on Schottky contacts fabricated on various points of an epitaxial layer, where the thickness of epi-layer differs at each position because of inhomogeneity of growth rate within a wafer.
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.08.019
ARTICLE IN PRESS 442
T. Tanno et al. / Materials Science in Semiconductor Processing 6 (2003) 441–443
Te-doped GaP epitaxial layers are grown at 850 C with TDM-CVP (temperature difference method under controlled vapor pressure) [3–5]. Te concentration in grown layer is 6 1016 cm–3. TDM-CVP equipment used in this study has two wells for reserving melt. One of them is filled with Ga melt solving GaP and Te, and another is empty to use for annealing a substrate under phosphorus vapor pressure of 20 kPa. When the substrate is slid under an empty well, annealing under controlled phosphorus vapor pressure begins. Following the annealing procedure, the substrate is moved to under the filled well to successively start growth. After growth, the sample is slid back to beneath the carbon cover and cooled down rapidly by shifting out the furnace. For PHCAP, Schottky and ohmic contacts were fabricated with Au and Au–Ge/Au, respectively. PHCAP measurements were performed at 95 K where shallow levels are thermally ionized. When monochromatic light is irradiated on a Schottky diode, electrons or holes trapped in deep levels are emitted to conduction band or valence band if the incident photon energy is larger than the photoionization energy of the deep level. And then, the space charge changes. In PHCAP with constant-capacitance mode, capacitance of the diode is kept constant with automatic control of bias voltage. From the shift of bias voltage, density of ionized centers can be calculated [2]. In the case of n-type semiconductors, ups and downs in spectra indicate the presence of deep electron traps and deep hole traps, respectively.
Fig. 1. PHCAP spectra of undoped n-type GaP substrates annealed under phosphorus vapor pressure of 20 kPa (solid); and annealed beneath a carbon cover (broken).
3. Results and Discussion First, two annealed undoped n-type LEC-grown substrates without epitaxial growth were examined with PHCAP. Samples were annealed at 850 C for 2 h; one was at phosphorus vapor pressure of 20 kPa, and another was annealed beneath a carbon cover where no additional vapor pressure is applied. PHCAP spectra of these substrates are displayed in Fig. 1. One can find that density of a deep donor level at EC 2:1 eV is obviously higher in the substrate annealed under vapor pressure than another. From this result, the 2.1 eV deep donor is supposed to be P-rich-type nonstoichiometric defects or related complexes. We should note here that annealing under phosphorus vapor pressure did not increase point defects compared to the virgin substrate although defect density was higher than the annealing without additional vapor pressure. It was reported that defect densities in S-doped GaP crystal decreased as a consequence of annealing under controlled phosphorus vapor pressure in a region of 13 Pa to 67 kPa [6]. This fact means that a lot of nonstoichiometric defects are contained in the LECgrown crystal and large part of them is vanished by annealing at 850 C independently of the pressure of
Fig. 2. PHCAP spectra of Te-doped LPE layers grown on substrates pre-annealed under phosphorus vapor pressure for 5 min (triangle) and 60 min (square).
phosphorus vapor. Actually, in the case of unannealed undoped substrates, PHCAP measurement was impossible because of high resistivity of metal–semiconductor contacts. This fact suggests that the densities of nonstoichiometric defects are higher than those of residual-donor impurities. Therefore, densities of deep levels in annealed substrates estimated in this experiment should be lower than in the unannealed substrate.
ARTICLE IN PRESS T. Tanno et al. / Materials Science in Semiconductor Processing 6 (2003) 441–443
443
complex defects diffuse keeping there complicated structures. Such a high diffusion constant implies the interstitial phosphorus atoms.
4. Summary
Fig. 3. Depth profile of defects obtained from PHCAP spectra of Te-doped layers with difference thicknesses.
Next, PHCAP spectra of Te-doped LPE layers were measured (Fig. 2). They were grown on the S-doped ntype substrates that had been previously annealed under phosphorus vapor pressure for 5 or 60 min at 850 C. The thickness of epi-layers are B17 mm. The most significant dissimilarity appears at 2.1 eV. With longer pre-annealing of the substrate, the deep level at EC 2:1 eV increased. From the coincidence of energy, the deep level is thought to exist both in the substrate and the epitaxial layer. Furthermore, other two deep donor at EC 1:5 and 2.25 eV were detected. However, in contrast, they were not affected by the time of preannealing. As mentioned before, by measuring different points of the epi-layer, it is possible to obtain diffusion profile of defects (Fig. 3). The deep level at EC 2:1 eV shows clear dependence on the depth. The concentration of 2.1 eV deep donor decreased as the thickness increased and become lower than detection limit at thickness >16.4 mm. We suppose that 2.1 eV defect diffuses from the substrate into the epitaxial layer. Diffusion coefficient of this defect at 850 C is estimated to be 8 10– 11 cm2/s from the slope of the profile and the growth time of 2 h. At least, it is difficult to suppose that
Two types of undoped GaP substrates annealed under different conditions were examined by means of PHCAP measurement and it was revealed that deep donor level at EC 2:1 eV in the substrate annealed under phosphorus vapor pressure of 20 kPa is higher than in the one annealed beneath the carbon cover. In Te-doped LPE layer grown on pre-annealed S-doped GaP substrate, depth profile of nonstoichiometry-related defects was observed. Among three detected deep levels at EC 1:5; –2.1, and –2.25 eV, only 2.1 eV level showed clear depth dependence. From the slope of the profile, the diffusion coefficient of 2.1 eV defect at 850 C was estimated to be 8 10–11 cm2/s. This defect is considered as a P-rich-type nonstoichiometric point defect such as an interstitial phosphorus atoms.
References [1] Suto K, Adachi S, Yoneyama T, Nishizawa J. Free exciton recombination in GaP and GaP/AlxGa1–xP layers grown by temperature difference method under controlled vapor pressure. J Cryst Growth 1996;160(1–2):13–20. [2] Oyama Y, Nishizawa J, Dezaki K. Deep levels in Te-doped GaAs prepared by annealing under controlled arsenic vapor pressure. J Appl Phys 1991;70(2):833–7. [3] Nishizawa J, Okuno Y. Liquid phase epitaxy of GaP by a temperature difference method under controlled vapor pressure. IEEE Trans Electron Devices 1975;ED22(9):716–21. [4] Nishizawa J, Okuno Y, Tadano H. Nearly perfect crystal growth of III–V compounds by the temperature difference method under controlled vapour pressure. J Cryst Growth 1975;31:215–22. [5] Nishizawa J, Koike M, Miura K, Okuno Y. Deep Levels Studies of N-free and N-doped GaP grown by TDM-CVP. Jpn J Appl Phys 1979;19(1):25–30. [6] Yu TJ, Suto K, Nishizawa J. Investigation of deep levels in GaP liquid phase epitaxial layers on substrates with vapor pressure heat treatment. Mater Res Soc Symp Proc 1999;588:61–6.