- Email: [email protected]
Annealing of ion-implanted GaN
Physica B 273}274 (1999) 96}100
Annealing of ion-implanted GaN A. Burchard!, E.E. Haller", A. StoK tzler!, R. Weissenborn!, M. Deicher!,*, ISOLDE Collaboration# !Fakulta( t fu( r Physik, Universita( t Konstanz, P.O. Box 5560, D-78457 Konstanz, Germany "Department of Materials Science, University of California Berkeley, Berkeley CA 94270, USA #CERN/PPE, CH-1211 Geneva 23, Switzerland
Abstract 111.Cd and 112Cd ions have been implanted into GaN. With photoluminescence spectroscopy and perturbed cc angular correlation spectroscopy (PAC) the reduction of implantation damage and the optical activation of the implants have been observed as a function of annealing temperature using di!erent annealing methods. The use of N or NH 2 3 atmosphere during annealing allows temperatures up to 1323 and 1373 K, respectively, but above 1200 K a strong loss of Cd from the GaN has been observed. Annealing GaN together with elementary Al forms a protective layer on the GaN surface allowing annealing temperatures up to 1570 K for 10 min. ( 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72V; 78.55.CR; 76.80.#Y Keywords: GaN; Ion implantation; Annealing; Photoluminescence; PAC
1. Introduction The advantage of doping semiconductors by ion implantation is the control of concentration, depth and lateral distribution of the dopants. However, ion implantation is always accompanied by structural damage to the crystal requiring thermal annealing to achieve electrical activation of the dopants. The reconstruction of the GaN lattice with a melting point of about 2800 K [1] requires annealing temperatures up to 1900 K and an external N overpressure of several GPa to suppress the 2 decomposition of the GaN due to the loss of N. It has been shown by RBS measurements [2] that a signi"cant amount of damage remains in the material after annealing at 1370 K under N atmosphere. On the other hand, 2 implanted Zn acceptors have been e$ciently optically activated by annealing at 1720 K under a N overpres2 * Corresponding author. Tel.: #49-7531-883865; fax: #497531-883090. E-mail address: [email protected] (M. Deicher)
sure of 1.6 GPa [3]. For the production of GaN-based devices it is necessary to have a practicable annealing technique which e$ciently activates the implanted atoms. Several procedures for thermal processing of GaN have been investigated and were recently reviewed by Pearton et al. [4]. We report on the observation of implantation-induced defects and their annealing using the perturbed cc angular correlation spectroscopy (PAC) [5]. This technique probes the immediate vicinity of a suitable radioactive dopant, 111.Cd in this case. In a previous work, PAC has been used to study the annealing of the implantation-induced damage after implantation of the group-III element 111In [6]. Here, we report on the annealing of Cd-implanted GaN under N , 2 NH , and Al atmosphere. The achieved optical proper3 ties have been checked by photoluminescence spectroscopy (PL).
2. Experimental details The GaN samples used were epitaxial layers grown on AlN/c-sapphire by metal organic vapor-phase epitaxy
0921-4526/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 4 1 5 - 9
A. Burchard et al. / Physica B 273}274 (1999) 96}100
(MOVPE) purchased from CREE Research or obtained from Hewlett-Packard Optoelectronics Division. For the PAC measurements the samples were doped by ion implantation (60 keV, 5]1011 ions/cm2) at room temperature with 111.Cd at the ISOLDE mass separator facility at CERN (Geneva). For the PL experiments, the samples have been implanted with stable 112Cd (60 keV, 1]1011 } 1]1014 ions/cm2). The implantation energy of 60 keV corresponds to a calculated depth distribution [7] of the implanted Cd ions centered at 190 As below the surface with a width of 75 As , i.e. the implanted dose range corresponds to mean Cd concentrations between 5]1016 cm~3 and 5]1019 cm~3. To serve as reference, a part of each sample was not implanted. The samples were annealed for 10 min in sealed quartz ampoules either under N or NH atmosphere or in vacuum to2 3 gether with elementary Al up to 1623 K. The PAC technique [5] is sensitive to electric "eld gradients (EFG) present at the site of the probe atom, in our case 111.Cd (t "48 min). An EFG causes a three-fold hyper"ne 1@2 splitting of an excited state of the 111Cd nuclei. This splitting is observed by PAC and is characteristic for the defects in the immediate neighborhood of the probe atom. The EFG is described by the quadrupole coupling constant l "eQ< /h(Q denotes the nuclear quadruQ zz pole moment, < the largest component of the diagonalzz ized EFG tensor) and the asymmetry parameter g. These quantities are unique for each defect and both, the symmetry of a formed probe-atom-defect complex and the fraction of probe atoms involved in this complex, can be determined from the characteristic modulation of the PAC spectrum R(t). A damping of the observed modulation due to the superposition of di!erent EFGs caused by the presence of di!erent defect con"gurations is described by the width *l assuming a Lorentzian distribuQ tion of these EFGs. All spectra have been recorded at room temperature with the c-axis of the GaN layer either oriented in the detector plane with an angle of 453 between two detectors or perpendicular to the detector plane. The photoluminescence (PL) experiments were carried out at 4.2 K using a He-#ow-cryostat. The luminescence was excited by the 325 nm line of a HeCd laser. The luminescence was dispersed using a 0.75 m grating monochromator and detected with a photomultiplier.
3. Results and discussion Fig. 1a shows a PAC spectrum R(t) recorded directly after the implantation of 111.Cd. The relaxation of the PAC signal within a few ns is caused by the superposition of many di!erent EFGs. This is a clear sign for the presence of highly defective regions around all Cd atoms. These defect structures cause di!erent EFGs and lead to the observed strong damping of the spectrum. These
97
Fig. 1. PAC spectra recorded for GaN implanted with 111.Cd ions (5]1011 cm~2, 60 keV): (a) as implanted, after annealing under N atmosphere (b) or NH atmosphere (c) at 873, 1323, 2 3 and 1373 K for 10 min.
defects act as non-radiative recombination centers and reduce strongly the total PL intensity observed from the implanted region of the sample. Increasing the implanted Cd dose to 1014 ions/cm2 reduces the overall PL intensity to about 1% compared to an unimplanted sample. Immediately after the implantation no Cd-related luminescence is observable. Fig. 1b and c show PAC spectra recorded after di!erent annealing steps under N pressure (1 bar at 295 K) and NH #ow, respectively. 2 3 Annealing at 873 K results in a partial recovery of the damage. Now, 60% of the 111.Cd atoms are exposed to an EFG with l "6.0(5) MHz which exhibits a large Q damping (*l "5 MHz). Increasing the annealing temQ perature to 1323 K continuously reduces this damping. For annealing under N atmosphere, raising the anneal2 ing temperature to 1373 K drastically changes the spectrum: The unique EFG with l "6.0 MHz is no longer Q observed and the spectrum looks similar to the one recorded directly after the implantation (Fig. 1a). This is an indication that the surface starts to decompose via the loss of N leading to a high concentration of defects in the GaN lattice reaching the depth where the implanted ions are located. This decomposition is shifted to higher annealing temperatures (above 1373 K) by the presence of NH (Fig. 1c). The EFG characterized by l "6.0 MHz 3 Q is axially symmetric (g"0) and its symmetry axis is oriented along the c-axis of the hexagonal structure of GaN. Both the symmetry and the small value of this EFG favors its association with the intrinsic EFG
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A. Burchard et al. / Physica B 273}274 (1999) 96}100
detected by probe atoms located on Ga lattice sites with a defect-free nearest surrounding. This has been shown in an earlier work [6] observing the same EFG using the isotope 111In which should occupy Ga lattice sites in GaN and is supported by emission channeling measurements [8] which show that at least 90% of the implanted 111In atoms occupy substitutional lattice sites. In Fig. 2, the results for annealing under N ("lled symbols) and 2 NH (open symbols) are summarized. Annealing above 3 600 K leads to an increase of the fraction of 111.Cd atoms with no defects in the immediate neighborhood (Fig. 2a). At 770 K, about 60% of the probe atoms observe the intrinsic lattice EFG but the rather broad distribution of about *l "5 MHz (Fig. 2b) indicates Q that more distant defects are still present. As shown in Fig. 2b, the width of this distribution becomes more narrow with increasing annealing temperatures indicating a further removal of defects. After annealing between 873 and 1323 K, 60% of the 111.Cd atoms reside on almost unperturbed lattice sites characterized by a minimum of the damping of *l +1 MHz associated with Q l "6 MHz. This residual damping may be caused by Q the remaining implantation damage or dislocations present in the layer. Using NH allows annealing temper3 atures up to 1373 K resulting in a further reduction of the damping *l . But even at these temperatures 40% of the Q implanted ions reside in highly perturbed surroundings. This is consistent with results obtained by RBS measurements and XTM images [2] of Si-implanted GaN which show the presence of implantation-induced defects and an electrical activation of about 50% of the dopants at these annealing temperatures. In Fig. 3, PL spectra of GaN implanted with 112Cd (1]1012 cm~2) recorded after annealing under NH between 1000 and 1373 K are 3 shown. For each annealing step the PL spectrum of an unimplanted part of the sample (`referencea, dotted line) is shown. With increasing annealing temperature the concentration of non-radiative recombination centers decreases which is visible in the increase of the intensities of the transitions due to the donor-bound excitons (DX) and its phonon replicas (DX-LO, DX-2LO) which "nally reach about 50% of the reference spectrum after annealing at 1373 K. At this temperature, the decomposition of the GaN surface already starts. This is expressed by a vanishing of the oscillations on the yellow luminescence (YL) located around 2.2 eV which are caused by Fabry}Perot interferences between the GaN/sapphire interface and the GaN surface. Increasing the annealing temperature to 1423 K destroys the GaN layer. The luminescence band centered at 2.7 eV and the transitions at 3.341, 3.328, and 3.272 eV were attributed to centers involving Cd (Fig. 3). The identi"cation of these transitions is discussed in detail by StoK tzler et al. [9]. Implanting radioactive isotopes makes it easy to follow up losses of the implanted species during annealing via detecting the radioactivity in the sample before and after
Fig. 2. Fraction of 111.Cd on substitutional Ga lattice sites in GaN (a) and the damping of the observed EFG (b) as a function of the annealing temperature for annealing under N (closed 2 symbols) and NH atmosphere (open symbols). 3
Fig. 3. PL spectra recorded for GaN implanted with 112Cd (1]1012 cm~2) after annealing under NH between 1000 and 3 1373 K. The dotted curves show spectra recorded for a nonimplanted part of the sample.
each annealing step. Fig. 4 shows the loss of implanted 111.Cd atoms after annealing under N and NH atmo2 3 sphere. After annealing for 10 min at 1323 K, about 75% of the Cd atoms have left the GaN sample. Increasing the annealing temperature to 1373 K results in a loss of more than 95% of the Cd atoms. This dramatic loss is less pronounced in the PL spectra shown in Fig. 3. It can be seen by comparing the intensities of the transitions related to donor-bound excitons (DX) and Cd, especially between 1323 and 1373 K. The observed PL intensities due to Cd transitions involve only Cd atoms which are optically active. This fraction of optically active Cd
A. Burchard et al. / Physica B 273}274 (1999) 96}100
99
Fig. 4. Loss of Cd atoms out of GaN as a function of the annealing temperature and annealing under N or NH atmo2 3 sphere.
atoms increases due to annealing whereas the total number of Cd atoms decreases due to out-di!usion. This is plausible because di!usion is enhanced by the presence of defects, especially vacancies. Therefore, optically inactive Cd atoms residing in highly defective regions leave the sample preferentially. Above 1300 K the observed decomposition of the surface may also contribute to the loss of 111.Cd. Another possibility to protect the GaN surface during high-temperature annealing is either the use of a protective layer like sputtered AlN [10,11] or the addition of nitrides like AlN or InN to supply an overpressure of N during the annealing [4]. In the following, we present 2 evidence for an alternative approach by adding elementary Al to the evacuated quartz ampoule containing the GaN sample. Fig. 5 shows PL spectra of GaN implanted with 112Cd (3]1012 cm~2, 60 keV) after using this procedure for annealing at 1473 and 1573 K for 10 min. The PL spectra show that the GaN layer is still intact and strong Cd bands are visible. Obviously, due to the vapor pressure of Al at these temperatures, a protective layer has formed on the GaN surface which e!ectively suppresses both the loss of N and of Cd. The existence of such a layer is visible in PL via a band normally not observed in GaN. Etching the sample with aqueous KOH at 360 K removes this layer. The samples appear optically smooth and the total PL intensity almost approaches the intensity of the non-implanted reference. Annealing above 1600 K destroys the GaN layer as can be seen by the drastic reduction of the observed PL intensity. The chemical nature of the layer formed on the GaN surface is not known up to now, it may consist of a thin AlN layer. XPS studies are on the way to clarify this question. This cheap and fast procedure may open the possibility to enhance the annealing of ion-implanted GaN.
Fig. 5. Photoluminescence spectra recorded after annealing of Cd-implanted GaN (3]1012 cm~2, 60 keV) at 1473 and 1573 K. The dotted curves show spectra recorded for a non-implanted part of the sample.
structural and optical properties. Annealing under N or 2 NH atmosphere leads to a partial structural and optical 3 recovery but is accompanied by a strong loss of Cd at annealing temperatures above 1200 K. First promising results have been presented using elementary Al during annealing of GaN which forms a protective layer and allows annealing temperatures up to 1600 K. The chemical nature of this layer has to be determined in future experiments. Future work will also focus on the annealing behavior of GaN implanted with increasing doses and also the electrical properties will be determined using the same samples for all techniques.
Acknowledgements We acknowledge the Hewlett-Packard Optoelectronics Division for supplying us with GaN samples. We thank H. HofsaK ss for performing some of the 112Cd implantations. This work was "nancially supported by the Bundesminister fuK r Bildung, Wissenschaft, Forschung und Technologie under Grant No. 03-DE5KO1-6.
References 4. Summary and conclusions Using PAC and PL spectroscopy, the annealing of GaN implanted with Cd has been studied via both its
[1] J.A. Van Vechten, Phys. Rev. B 7 (1973) 1479. [2] J.C. Zolper, H.H. Tan, J.S. Williams, J. Zou, D.J.H. Cockayne, S.J. Pearton, M. Hagerott Crawford, R.F. Karlicek, Appl. Phys. Lett. 70 (1997) 2729.
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[3] T. Suski, J. Jun, M. Leszczynski, H. Teisseyre, S. Strite, A. Rockett, A. Pelzmann, M. Kamp, K.J. Ebeling, J. Appl. Phys. 84 (1998) 1155. [4] S.J. Pearton, J.C. Zolper, R.J. Shul, F. Ren, J. Appl. Phys. 86 (1999) 1. [5] Th. Wichert, N. Achtziger, H. Metzner, R. Sielemann, in: G. Langouche (Ed.), Hyper"ne Interactions of Defects in Semiconductors, Elsevier, Amsterdam, 1992, p. 77. [6] A. Burchard, M. Deicher, D. Forkel-Wirth, E.E. Haller, R. Magerle, A. Prospero, A. StoK tzler, Mater. Sci. Forum 258}263 (1997) 1099. [7] J.B. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257.
[8] C. Ronning, M. Dalmer, M. Deicher, M. Restle, M.D. Bremser, R.F. Davis, H. HofsaK ss, in: C.R. Abernathy, H. Armano, J.C. Zolper (Eds.), Gallium Nitride and Related Materials II, Material Research Society Symposium Proceedings, Vol. 468, Pittsburgh, 1997, p. 407. [9] A. StoK tzler, R. Weissenborn, M. Deicher, Physica B 273}274 (1999) 144, These Proceedings. [10] J.C. Zolper, D.J. Rieger, A.G. Baca, S.J. Pearton, J.W. Lee, R.A. Stall, Appl. Phys. Lett. 69 (1996) 538. [11] X.A. Cao, S.J. Pearton, R.K. Singh, C.R. Abernathy, J. Han, R.J. Shul, D.J. Rieger, J.C. Zolper, R.G. Wilson, M. Fu, J.A. Sekhar, H.J. Guo, S.J. Pennycook, MRS Int. J. Nitride Semicond. Res. 4S1 (1999) G6.33.
Annealing of ion-implanted GaN A. Burchard!, E.E. Haller", A. StoK tzler!, R. Weissenborn!, M. Deicher!,*, ISOLDE Collaboration# !Fakulta( t fu( r Physik, Universita( t Konstanz, P.O. Box 5560, D-78457 Konstanz, Germany "Department of Materials Science, University of California Berkeley, Berkeley CA 94270, USA #CERN/PPE, CH-1211 Geneva 23, Switzerland
Abstract 111.Cd and 112Cd ions have been implanted into GaN. With photoluminescence spectroscopy and perturbed cc angular correlation spectroscopy (PAC) the reduction of implantation damage and the optical activation of the implants have been observed as a function of annealing temperature using di!erent annealing methods. The use of N or NH 2 3 atmosphere during annealing allows temperatures up to 1323 and 1373 K, respectively, but above 1200 K a strong loss of Cd from the GaN has been observed. Annealing GaN together with elementary Al forms a protective layer on the GaN surface allowing annealing temperatures up to 1570 K for 10 min. ( 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72V; 78.55.CR; 76.80.#Y Keywords: GaN; Ion implantation; Annealing; Photoluminescence; PAC
1. Introduction The advantage of doping semiconductors by ion implantation is the control of concentration, depth and lateral distribution of the dopants. However, ion implantation is always accompanied by structural damage to the crystal requiring thermal annealing to achieve electrical activation of the dopants. The reconstruction of the GaN lattice with a melting point of about 2800 K [1] requires annealing temperatures up to 1900 K and an external N overpressure of several GPa to suppress the 2 decomposition of the GaN due to the loss of N. It has been shown by RBS measurements [2] that a signi"cant amount of damage remains in the material after annealing at 1370 K under N atmosphere. On the other hand, 2 implanted Zn acceptors have been e$ciently optically activated by annealing at 1720 K under a N overpres2 * Corresponding author. Tel.: #49-7531-883865; fax: #497531-883090. E-mail address: [email protected] (M. Deicher)
sure of 1.6 GPa [3]. For the production of GaN-based devices it is necessary to have a practicable annealing technique which e$ciently activates the implanted atoms. Several procedures for thermal processing of GaN have been investigated and were recently reviewed by Pearton et al. [4]. We report on the observation of implantation-induced defects and their annealing using the perturbed cc angular correlation spectroscopy (PAC) [5]. This technique probes the immediate vicinity of a suitable radioactive dopant, 111.Cd in this case. In a previous work, PAC has been used to study the annealing of the implantation-induced damage after implantation of the group-III element 111In [6]. Here, we report on the annealing of Cd-implanted GaN under N , 2 NH , and Al atmosphere. The achieved optical proper3 ties have been checked by photoluminescence spectroscopy (PL).
2. Experimental details The GaN samples used were epitaxial layers grown on AlN/c-sapphire by metal organic vapor-phase epitaxy
0921-4526/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 4 1 5 - 9
A. Burchard et al. / Physica B 273}274 (1999) 96}100
(MOVPE) purchased from CREE Research or obtained from Hewlett-Packard Optoelectronics Division. For the PAC measurements the samples were doped by ion implantation (60 keV, 5]1011 ions/cm2) at room temperature with 111.Cd at the ISOLDE mass separator facility at CERN (Geneva). For the PL experiments, the samples have been implanted with stable 112Cd (60 keV, 1]1011 } 1]1014 ions/cm2). The implantation energy of 60 keV corresponds to a calculated depth distribution [7] of the implanted Cd ions centered at 190 As below the surface with a width of 75 As , i.e. the implanted dose range corresponds to mean Cd concentrations between 5]1016 cm~3 and 5]1019 cm~3. To serve as reference, a part of each sample was not implanted. The samples were annealed for 10 min in sealed quartz ampoules either under N or NH atmosphere or in vacuum to2 3 gether with elementary Al up to 1623 K. The PAC technique [5] is sensitive to electric "eld gradients (EFG) present at the site of the probe atom, in our case 111.Cd (t "48 min). An EFG causes a three-fold hyper"ne 1@2 splitting of an excited state of the 111Cd nuclei. This splitting is observed by PAC and is characteristic for the defects in the immediate neighborhood of the probe atom. The EFG is described by the quadrupole coupling constant l "eQ< /h(Q denotes the nuclear quadruQ zz pole moment, < the largest component of the diagonalzz ized EFG tensor) and the asymmetry parameter g. These quantities are unique for each defect and both, the symmetry of a formed probe-atom-defect complex and the fraction of probe atoms involved in this complex, can be determined from the characteristic modulation of the PAC spectrum R(t). A damping of the observed modulation due to the superposition of di!erent EFGs caused by the presence of di!erent defect con"gurations is described by the width *l assuming a Lorentzian distribuQ tion of these EFGs. All spectra have been recorded at room temperature with the c-axis of the GaN layer either oriented in the detector plane with an angle of 453 between two detectors or perpendicular to the detector plane. The photoluminescence (PL) experiments were carried out at 4.2 K using a He-#ow-cryostat. The luminescence was excited by the 325 nm line of a HeCd laser. The luminescence was dispersed using a 0.75 m grating monochromator and detected with a photomultiplier.
3. Results and discussion Fig. 1a shows a PAC spectrum R(t) recorded directly after the implantation of 111.Cd. The relaxation of the PAC signal within a few ns is caused by the superposition of many di!erent EFGs. This is a clear sign for the presence of highly defective regions around all Cd atoms. These defect structures cause di!erent EFGs and lead to the observed strong damping of the spectrum. These
97
Fig. 1. PAC spectra recorded for GaN implanted with 111.Cd ions (5]1011 cm~2, 60 keV): (a) as implanted, after annealing under N atmosphere (b) or NH atmosphere (c) at 873, 1323, 2 3 and 1373 K for 10 min.
defects act as non-radiative recombination centers and reduce strongly the total PL intensity observed from the implanted region of the sample. Increasing the implanted Cd dose to 1014 ions/cm2 reduces the overall PL intensity to about 1% compared to an unimplanted sample. Immediately after the implantation no Cd-related luminescence is observable. Fig. 1b and c show PAC spectra recorded after di!erent annealing steps under N pressure (1 bar at 295 K) and NH #ow, respectively. 2 3 Annealing at 873 K results in a partial recovery of the damage. Now, 60% of the 111.Cd atoms are exposed to an EFG with l "6.0(5) MHz which exhibits a large Q damping (*l "5 MHz). Increasing the annealing temQ perature to 1323 K continuously reduces this damping. For annealing under N atmosphere, raising the anneal2 ing temperature to 1373 K drastically changes the spectrum: The unique EFG with l "6.0 MHz is no longer Q observed and the spectrum looks similar to the one recorded directly after the implantation (Fig. 1a). This is an indication that the surface starts to decompose via the loss of N leading to a high concentration of defects in the GaN lattice reaching the depth where the implanted ions are located. This decomposition is shifted to higher annealing temperatures (above 1373 K) by the presence of NH (Fig. 1c). The EFG characterized by l "6.0 MHz 3 Q is axially symmetric (g"0) and its symmetry axis is oriented along the c-axis of the hexagonal structure of GaN. Both the symmetry and the small value of this EFG favors its association with the intrinsic EFG
98
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detected by probe atoms located on Ga lattice sites with a defect-free nearest surrounding. This has been shown in an earlier work [6] observing the same EFG using the isotope 111In which should occupy Ga lattice sites in GaN and is supported by emission channeling measurements [8] which show that at least 90% of the implanted 111In atoms occupy substitutional lattice sites. In Fig. 2, the results for annealing under N ("lled symbols) and 2 NH (open symbols) are summarized. Annealing above 3 600 K leads to an increase of the fraction of 111.Cd atoms with no defects in the immediate neighborhood (Fig. 2a). At 770 K, about 60% of the probe atoms observe the intrinsic lattice EFG but the rather broad distribution of about *l "5 MHz (Fig. 2b) indicates Q that more distant defects are still present. As shown in Fig. 2b, the width of this distribution becomes more narrow with increasing annealing temperatures indicating a further removal of defects. After annealing between 873 and 1323 K, 60% of the 111.Cd atoms reside on almost unperturbed lattice sites characterized by a minimum of the damping of *l +1 MHz associated with Q l "6 MHz. This residual damping may be caused by Q the remaining implantation damage or dislocations present in the layer. Using NH allows annealing temper3 atures up to 1373 K resulting in a further reduction of the damping *l . But even at these temperatures 40% of the Q implanted ions reside in highly perturbed surroundings. This is consistent with results obtained by RBS measurements and XTM images [2] of Si-implanted GaN which show the presence of implantation-induced defects and an electrical activation of about 50% of the dopants at these annealing temperatures. In Fig. 3, PL spectra of GaN implanted with 112Cd (1]1012 cm~2) recorded after annealing under NH between 1000 and 1373 K are 3 shown. For each annealing step the PL spectrum of an unimplanted part of the sample (`referencea, dotted line) is shown. With increasing annealing temperature the concentration of non-radiative recombination centers decreases which is visible in the increase of the intensities of the transitions due to the donor-bound excitons (DX) and its phonon replicas (DX-LO, DX-2LO) which "nally reach about 50% of the reference spectrum after annealing at 1373 K. At this temperature, the decomposition of the GaN surface already starts. This is expressed by a vanishing of the oscillations on the yellow luminescence (YL) located around 2.2 eV which are caused by Fabry}Perot interferences between the GaN/sapphire interface and the GaN surface. Increasing the annealing temperature to 1423 K destroys the GaN layer. The luminescence band centered at 2.7 eV and the transitions at 3.341, 3.328, and 3.272 eV were attributed to centers involving Cd (Fig. 3). The identi"cation of these transitions is discussed in detail by StoK tzler et al. [9]. Implanting radioactive isotopes makes it easy to follow up losses of the implanted species during annealing via detecting the radioactivity in the sample before and after
Fig. 2. Fraction of 111.Cd on substitutional Ga lattice sites in GaN (a) and the damping of the observed EFG (b) as a function of the annealing temperature for annealing under N (closed 2 symbols) and NH atmosphere (open symbols). 3
Fig. 3. PL spectra recorded for GaN implanted with 112Cd (1]1012 cm~2) after annealing under NH between 1000 and 3 1373 K. The dotted curves show spectra recorded for a nonimplanted part of the sample.
each annealing step. Fig. 4 shows the loss of implanted 111.Cd atoms after annealing under N and NH atmo2 3 sphere. After annealing for 10 min at 1323 K, about 75% of the Cd atoms have left the GaN sample. Increasing the annealing temperature to 1373 K results in a loss of more than 95% of the Cd atoms. This dramatic loss is less pronounced in the PL spectra shown in Fig. 3. It can be seen by comparing the intensities of the transitions related to donor-bound excitons (DX) and Cd, especially between 1323 and 1373 K. The observed PL intensities due to Cd transitions involve only Cd atoms which are optically active. This fraction of optically active Cd
A. Burchard et al. / Physica B 273}274 (1999) 96}100
99
Fig. 4. Loss of Cd atoms out of GaN as a function of the annealing temperature and annealing under N or NH atmo2 3 sphere.
atoms increases due to annealing whereas the total number of Cd atoms decreases due to out-di!usion. This is plausible because di!usion is enhanced by the presence of defects, especially vacancies. Therefore, optically inactive Cd atoms residing in highly defective regions leave the sample preferentially. Above 1300 K the observed decomposition of the surface may also contribute to the loss of 111.Cd. Another possibility to protect the GaN surface during high-temperature annealing is either the use of a protective layer like sputtered AlN [10,11] or the addition of nitrides like AlN or InN to supply an overpressure of N during the annealing [4]. In the following, we present 2 evidence for an alternative approach by adding elementary Al to the evacuated quartz ampoule containing the GaN sample. Fig. 5 shows PL spectra of GaN implanted with 112Cd (3]1012 cm~2, 60 keV) after using this procedure for annealing at 1473 and 1573 K for 10 min. The PL spectra show that the GaN layer is still intact and strong Cd bands are visible. Obviously, due to the vapor pressure of Al at these temperatures, a protective layer has formed on the GaN surface which e!ectively suppresses both the loss of N and of Cd. The existence of such a layer is visible in PL via a band normally not observed in GaN. Etching the sample with aqueous KOH at 360 K removes this layer. The samples appear optically smooth and the total PL intensity almost approaches the intensity of the non-implanted reference. Annealing above 1600 K destroys the GaN layer as can be seen by the drastic reduction of the observed PL intensity. The chemical nature of the layer formed on the GaN surface is not known up to now, it may consist of a thin AlN layer. XPS studies are on the way to clarify this question. This cheap and fast procedure may open the possibility to enhance the annealing of ion-implanted GaN.
Fig. 5. Photoluminescence spectra recorded after annealing of Cd-implanted GaN (3]1012 cm~2, 60 keV) at 1473 and 1573 K. The dotted curves show spectra recorded for a non-implanted part of the sample.
structural and optical properties. Annealing under N or 2 NH atmosphere leads to a partial structural and optical 3 recovery but is accompanied by a strong loss of Cd at annealing temperatures above 1200 K. First promising results have been presented using elementary Al during annealing of GaN which forms a protective layer and allows annealing temperatures up to 1600 K. The chemical nature of this layer has to be determined in future experiments. Future work will also focus on the annealing behavior of GaN implanted with increasing doses and also the electrical properties will be determined using the same samples for all techniques.
Acknowledgements We acknowledge the Hewlett-Packard Optoelectronics Division for supplying us with GaN samples. We thank H. HofsaK ss for performing some of the 112Cd implantations. This work was "nancially supported by the Bundesminister fuK r Bildung, Wissenschaft, Forschung und Technologie under Grant No. 03-DE5KO1-6.
References 4. Summary and conclusions Using PAC and PL spectroscopy, the annealing of GaN implanted with Cd has been studied via both its
[1] J.A. Van Vechten, Phys. Rev. B 7 (1973) 1479. [2] J.C. Zolper, H.H. Tan, J.S. Williams, J. Zou, D.J.H. Cockayne, S.J. Pearton, M. Hagerott Crawford, R.F. Karlicek, Appl. Phys. Lett. 70 (1997) 2729.
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