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Radiation-induced defects in n-type GaN and InN
Physica B 308–310 (2001) 58–61
Radiation-induced defects in n-type GaN and InN V.V. Emtseva,*, V.Yu. Davydova, E.E. Hallerb, A.A. Klochikhina, V.V. Kozlovskiic, G.A. Oganesyana, D.S. Poloskina, N.M. Shmidta, V.A. Vekshina, A.S. Usikova a
Division of Solid State Electronics, Ioffe Physicotechnical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia b Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA 94720, USA c Technical State University, 195251 St. Petersburg, Russia
Abstract The electrical properties of the n-GaN and n-InN, subjected to proton irradiation, are studied. The irradiation of the n-InN results in an increasing concentration of charge carriers, whereas strong compensation effects take place in the proton-irradiated n-GaN. The annealing behavior of the radiation-induced defects in both materials is discussed briefly. r 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 61.80.Lj; 61.82.Fk Keywords: Gallium nitride; Indium nitride; Irradiation
2. Experimental
crystal structure of the layers were well characterized by X-ray diffraction and Raman spectroscopy. The layer thickness ranges from 1.0 to 1.5 mm. The as-grown nInN layers were nominally undoped, with electron concentrations in the low 1020 cm3. In some cases, Mg or Dy impurities were added during growth for reducing the free electron concentration. Auger spectroscopy did not reveal any significant content of oxygen in the n-InN layers. Samples were irradiated with protons of 150 keV. After irradiation, the samples were subjected to isochronal annealing in steps of 501C or 1001C for 20 min in nitrogen. Hall effect and conductivity measurements were carried out using the Van der Pauw technique. The Raman scattering measurements were taken at room temperature.
Layers of hexagonal n-GaN and n-InN on (0 0 0 1) sapphire substrates were grown by the MOCVD and plasma-assisted MBE techniques, respectively. The
3. Results and discussion
*Corresponding author. Tel.: 7-812-247-9952; fax: 7-812-2471017. E-mail address: [email protected]ffe.rssi.ru (V.V. Emtsev).
In Figs. 1 and 2, several typical dependencies of the electron concentration and mobility, nðTÞ and mðTÞ; in one of the n-InN layers are shown. As can be seen from
1. Introduction Interest in experimental studies of the point defects in the GaN produced by irradiation with fast electrons and protons is steadily growing in the hope to obtain an understanding of the complex nature of native and impurity-related defects in this material; see for instance Refs. [1–6]. The situation for the irradiated InN layers is even worse, since the properties of the point defects in the InN are so far unknown. The purpose of the present work is to investigate the production and annealing processes of the proton irradiation-induced defects in the n-GaN and n-InN.
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 5 0 - 0
V.V. Emtsev et al. / Physica B 308–310 (2001) 58–61
Fig. 1. Electron concentration versus reciprocal temperature in the n-InN counterdoped with Mg before the irradiation (open circles), after the proton irradiation (solid circles), and after annealing at T ¼ 3001C (solid triangles). The irradiation dose, F ¼ 1 1016 cm2.
Fig. 2. Electron mobility versus temperature in the n-InN counterdoped with Mg before the irradiation (open circles), after the proton irradiation (solid circles), after annealing to T ¼ 3001C (solid triangles down), T ¼ 4501C (solid triangles up), and T ¼ 5001C (solid diamonds). The irradiation dose, F ¼ 1 1016 cm2.
Fig. 1, proton irradiation results in a substantial increase in the concentration of the charge carriers. This effect was observed for all the samples, independent of the counterdoping. Evidently, the increase in the electron concentration in the n-InN after the irradiation is due to
59
the production of radiation-induced defects with shallow donor states. The production rate is the same, about 2 104 cm1, over a wide dose range from 1 1015 to 1 1016 cm2. All these experimental facts suggest that these defects are native. We believe that the native defects responsible for the net effect in the irradiated nInN can be attributed to immobile nitrogen vacancies. Up to T ¼ 1001C, there is no change in the nðTÞ and mðTÞ curves. A pronounced decrease in nðTÞ by 30% was observed over a temperature interval of T ¼250–3001C; see Fig. 1. At this annealing stage, the annealed fraction of the electron mobility reached nearly 50%, as can be estimated from Fig. 2. At elevated temperatures, the annealing behavior of defects becomes rather complicated. First, a reverse annealing stage of the electron concentration and mobility takes place in the temperature interval from T ¼ 4001C to 4501C. As a result, the concentration of charge carriers returns practically to the value measured in the irradiated n-InN and the mobility drops by an order-of-magnitude; see Fig. 2. After annealing to T ¼ 5001C, the nðTÞ curves were found to be little affected but the electron mobility became much higher than that in the non-irradiated layers. In accordance with the earlier observations [7], in Raman spectra of as-grown n-InN layers we also revealed the presence of a band at n ¼ 590 cm1; see Fig. 3. This band has been attributed to the L–LO mode of Raman scattering. Our study of the protonirradiated n-InN clearly showed that the intensity of this band is dose dependent; see Fig. 3. Theoretical calculations of the cross-sections for three different models made it possible to conclude that the Raman scattering in the region of interest is associated with the shortrange potential scattering process due to the presence of defects; see Fig. 4. As is seen in Fig. 3, the proposed model gives a satisfactory explanation to the experimental data. Details of calculations will be discussed in a separate paper. Contrary to the n-InN, the proton irradiation of the doped n-GaN : Si leads to a substantial decrease in the concentration of the charge carriers; see Figs. 1 and 5. The electron removal rate estimated from the nðTÞ curves given in Fig. 5 is about 1 104 cm1. Surprisingly, the electron removal rate in the n-GaN : Si with a lower doping level is evidently smaller, at least by a factor of 3; see Fig. 5. It has been reported [1,6] that in the doped n-GaN : Si subjected to fast electron irradiation, the defect production rate is dependent on the doping level, too. There are no significant changes in the nðTÞ dependencies after the annealing of the proton-irradiated nGaN to T ¼ 2001C, whereas the electron mobility decreases noticeably. The first annealing stage of defects takes place over a temperature interval of T ¼300– 4001C. As is seen from Fig. 6, the mobility of the charge
60
V.V. Emtsev et al. / Physica B 308–310 (2001) 58–61
Φ =1*10
LO 16
18
10
cm IE2 /ILO =0.25 E2
3 Φ =5*10
15
initial 400
cm
-2
IE2 /ILO =0.45
500
600
4
6
1000/T, K
650
10
8
12
-1
-1
Fig. 3. The experimental and calculated Raman spectra in the n-InN before the irradiation (curve 1), after the proton irradiation at F ¼ 5 1015 cm2 (curve 2), and F ¼ 1 1016 cm2 (curve 3). Calculated spectra are shown by the broken line. The probability of the short-potential scattering process due to the defects is assumed to vary proportionally with the defect concentration. The ratios of the intensities of the E2 and LO bands are given. The E2 band is used as a reference, since its intensity is not sensitive to the presence of the defects.
1 2 3
Fig. 5. Electron concentration versus reciprocal temperature in the heavily doped n-GaN : Si before the irradiation (open circles), after the proton irradiation (solid circles), and after annealing at T ¼ 4001C (solid triangles up) and T ¼ 6001C (solid triangles down). The irradiation dose, F ¼ 1 1014 cm2. For comparison purposes, two nðTÞ curves for the moderately doped n-GaN : Si before the irradiation (crosses) and after the proton irradiation at the same dose (open triangles up) are also shown.
InN
2
µ, cm /(V*s)
Scattering Cross Sect ion, a.u.
16
10
1
550
Raman shift, cm
17
10
2
IE2 /ILO =1.1
450
n, cm -3
Intensity, a.u.
InN W272
5x10
2
4x10
2
3x10
2
2x10
2
2
10 1 9x10 1 8x10 70 80 90100 400
450
500
550
600
650
-1
Frequency, cm
Fig. 4. Raman cross-section versus frequency in the range of L–LO modes calculated for three different scattering mechanisms in the InN. Perfect crystal, curve 1; defects with screened long-range potential and curve 2; defects with short-range potential, curve 3.
carriers continues to drop strongly. With the temperature increasing to T ¼ 6001C, the electron concentration and mobility in the irradiated n-GaN recover substan-
T,K
200
300
Fig. 6. Electron mobility versus temperature in the n-GaN : Si before the irradiation (open circles), after the proton irradiation (solid circles), after annealing at T ¼ 4001C (solid triangles up) and T ¼ 6001C (solid triangles down). The irradiation dose, F ¼ 1 1014 cm2.
tially. The mobility of the charge carriers becomes even larger than that in the initial n-GaN. After annealing at T ¼ 7001C, the fraction of unannealed defects turned out to be between 20% and 30%.
V.V. Emtsev et al. / Physica B 308–310 (2001) 58–61
61
4. Conclusions
References
The behavior of defects in the n-GaN and n-InN produced by proton irradiation and subsequent annealing has been studied. From the data obtained on the nInN it can be concluded that most likely the nitrogen vacancies with shallow donor states are responsible for the increasing concentration of free electrons in the irradiated n-InN. It has been demonstrated that in the Raman spectra, the intensity of a band at n ¼ 590 cm1 in the proton-irradiated n-InN layers is dose-dependent. Theoretical calculations showed that the model of defects with short-range potential gives a satisfactory explanation for this band in Raman scattering. The first stage of defect annealing in the irradiated n-InN takes place at T ¼ 3001C. The production rate of native defects in the irradiated n-GaN appears to be Fermi-level dependent. Two main recovery stages of the electron concentration over two intervals from T ¼ 3001C to 4001C and from T ¼ 5001C to 6001C have been found.
[1] V.V. Emtsev, V.Yu. Davydov, I.N. Goncharuk, E.V. Kalinina, V.V. Kozlovskii, D.S. Poloskin, A.V. Sakharov, N.M. Shmidt, A.N. Smirnov, A.S. Usikov, in: G. Davies, M.H. Nazare (Eds.), Proceedings of the Materials Science Forum, Vols. 258–263, Trans Tech Publications, Switzerland, 1997, pp. 1143–1148. [2] D.C. Look, D.C. Reynolds, J.W. Hemsky, J.R. Sizelove, R.L. Jones, R.J. Molnar, Phys. Rev. Lett. 79 (1997) 2273. [3] C. Bozdog, H. Przybylinska, G.D. Watkins, V. H.arle, F. Scholz, M. Mayer, M. Kamp, R.J. Molnar, A.E. Wickenden, D.D. Koleske, R.L. Henry, Phys. Rev. B 59 (1999) 12479. [4] F.D. Auret, S.A. Goodman, F.K. Koschnick, J.-M. Spaeth, B. Beaumont, P. Gibart, Appl. Phys. Lett. 74 (1999) 407. [5] K.H. Chow, G.D. Watkins, A. Usui, M. Mizuta, Phys. Rev. Lett. 85 (2000) 2761. [6] V.V. Emtsev, V.Yu. Davydov, V.V. Kozlovskii, V.V. Lundin, D.S. Poloskin, A.N. Smirnov, N.M. Shmidt, A.S. Usikov, J. Aderhold, H. Klausing, D. Mistele, T. Rotter, J. Stemmer, O. Semchinova, J. Graul, Semicond. Sci. Technol. 15 (2000) 73. [7] V.Yu. Davydov, V.V. Emtsev, I.N. Goncharuk, A.N. Smirnov, V.D. Petrikov, V.V. Mamutin, V.A. Vekshin, S.V. Ivanov, M.B. Smirnov, T. Inushima, Appl. Phys. Lett. 75 (1999) 3297.
Acknowledgements The work was supported by CRDF (grant # RP12258) and partly by The Russian Foundation for Basic Research (grant 99-02-18318).
Radiation-induced defects in n-type GaN and InN V.V. Emtseva,*, V.Yu. Davydova, E.E. Hallerb, A.A. Klochikhina, V.V. Kozlovskiic, G.A. Oganesyana, D.S. Poloskina, N.M. Shmidta, V.A. Vekshina, A.S. Usikova a
Division of Solid State Electronics, Ioffe Physicotechnical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia b Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA 94720, USA c Technical State University, 195251 St. Petersburg, Russia
Abstract The electrical properties of the n-GaN and n-InN, subjected to proton irradiation, are studied. The irradiation of the n-InN results in an increasing concentration of charge carriers, whereas strong compensation effects take place in the proton-irradiated n-GaN. The annealing behavior of the radiation-induced defects in both materials is discussed briefly. r 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 61.80.Lj; 61.82.Fk Keywords: Gallium nitride; Indium nitride; Irradiation
2. Experimental
crystal structure of the layers were well characterized by X-ray diffraction and Raman spectroscopy. The layer thickness ranges from 1.0 to 1.5 mm. The as-grown nInN layers were nominally undoped, with electron concentrations in the low 1020 cm3. In some cases, Mg or Dy impurities were added during growth for reducing the free electron concentration. Auger spectroscopy did not reveal any significant content of oxygen in the n-InN layers. Samples were irradiated with protons of 150 keV. After irradiation, the samples were subjected to isochronal annealing in steps of 501C or 1001C for 20 min in nitrogen. Hall effect and conductivity measurements were carried out using the Van der Pauw technique. The Raman scattering measurements were taken at room temperature.
Layers of hexagonal n-GaN and n-InN on (0 0 0 1) sapphire substrates were grown by the MOCVD and plasma-assisted MBE techniques, respectively. The
3. Results and discussion
*Corresponding author. Tel.: 7-812-247-9952; fax: 7-812-2471017. E-mail address: [email protected]ffe.rssi.ru (V.V. Emtsev).
In Figs. 1 and 2, several typical dependencies of the electron concentration and mobility, nðTÞ and mðTÞ; in one of the n-InN layers are shown. As can be seen from
1. Introduction Interest in experimental studies of the point defects in the GaN produced by irradiation with fast electrons and protons is steadily growing in the hope to obtain an understanding of the complex nature of native and impurity-related defects in this material; see for instance Refs. [1–6]. The situation for the irradiated InN layers is even worse, since the properties of the point defects in the InN are so far unknown. The purpose of the present work is to investigate the production and annealing processes of the proton irradiation-induced defects in the n-GaN and n-InN.
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 5 0 - 0
V.V. Emtsev et al. / Physica B 308–310 (2001) 58–61
Fig. 1. Electron concentration versus reciprocal temperature in the n-InN counterdoped with Mg before the irradiation (open circles), after the proton irradiation (solid circles), and after annealing at T ¼ 3001C (solid triangles). The irradiation dose, F ¼ 1 1016 cm2.
Fig. 2. Electron mobility versus temperature in the n-InN counterdoped with Mg before the irradiation (open circles), after the proton irradiation (solid circles), after annealing to T ¼ 3001C (solid triangles down), T ¼ 4501C (solid triangles up), and T ¼ 5001C (solid diamonds). The irradiation dose, F ¼ 1 1016 cm2.
Fig. 1, proton irradiation results in a substantial increase in the concentration of the charge carriers. This effect was observed for all the samples, independent of the counterdoping. Evidently, the increase in the electron concentration in the n-InN after the irradiation is due to
59
the production of radiation-induced defects with shallow donor states. The production rate is the same, about 2 104 cm1, over a wide dose range from 1 1015 to 1 1016 cm2. All these experimental facts suggest that these defects are native. We believe that the native defects responsible for the net effect in the irradiated nInN can be attributed to immobile nitrogen vacancies. Up to T ¼ 1001C, there is no change in the nðTÞ and mðTÞ curves. A pronounced decrease in nðTÞ by 30% was observed over a temperature interval of T ¼250–3001C; see Fig. 1. At this annealing stage, the annealed fraction of the electron mobility reached nearly 50%, as can be estimated from Fig. 2. At elevated temperatures, the annealing behavior of defects becomes rather complicated. First, a reverse annealing stage of the electron concentration and mobility takes place in the temperature interval from T ¼ 4001C to 4501C. As a result, the concentration of charge carriers returns practically to the value measured in the irradiated n-InN and the mobility drops by an order-of-magnitude; see Fig. 2. After annealing to T ¼ 5001C, the nðTÞ curves were found to be little affected but the electron mobility became much higher than that in the non-irradiated layers. In accordance with the earlier observations [7], in Raman spectra of as-grown n-InN layers we also revealed the presence of a band at n ¼ 590 cm1; see Fig. 3. This band has been attributed to the L–LO mode of Raman scattering. Our study of the protonirradiated n-InN clearly showed that the intensity of this band is dose dependent; see Fig. 3. Theoretical calculations of the cross-sections for three different models made it possible to conclude that the Raman scattering in the region of interest is associated with the shortrange potential scattering process due to the presence of defects; see Fig. 4. As is seen in Fig. 3, the proposed model gives a satisfactory explanation to the experimental data. Details of calculations will be discussed in a separate paper. Contrary to the n-InN, the proton irradiation of the doped n-GaN : Si leads to a substantial decrease in the concentration of the charge carriers; see Figs. 1 and 5. The electron removal rate estimated from the nðTÞ curves given in Fig. 5 is about 1 104 cm1. Surprisingly, the electron removal rate in the n-GaN : Si with a lower doping level is evidently smaller, at least by a factor of 3; see Fig. 5. It has been reported [1,6] that in the doped n-GaN : Si subjected to fast electron irradiation, the defect production rate is dependent on the doping level, too. There are no significant changes in the nðTÞ dependencies after the annealing of the proton-irradiated nGaN to T ¼ 2001C, whereas the electron mobility decreases noticeably. The first annealing stage of defects takes place over a temperature interval of T ¼300– 4001C. As is seen from Fig. 6, the mobility of the charge
60
V.V. Emtsev et al. / Physica B 308–310 (2001) 58–61
Φ =1*10
LO 16
18
10
cm IE2 /ILO =0.25 E2
3 Φ =5*10
15
initial 400
cm
-2
IE2 /ILO =0.45
500
600
4
6
1000/T, K
650
10
8
12
-1
-1
Fig. 3. The experimental and calculated Raman spectra in the n-InN before the irradiation (curve 1), after the proton irradiation at F ¼ 5 1015 cm2 (curve 2), and F ¼ 1 1016 cm2 (curve 3). Calculated spectra are shown by the broken line. The probability of the short-potential scattering process due to the defects is assumed to vary proportionally with the defect concentration. The ratios of the intensities of the E2 and LO bands are given. The E2 band is used as a reference, since its intensity is not sensitive to the presence of the defects.
1 2 3
Fig. 5. Electron concentration versus reciprocal temperature in the heavily doped n-GaN : Si before the irradiation (open circles), after the proton irradiation (solid circles), and after annealing at T ¼ 4001C (solid triangles up) and T ¼ 6001C (solid triangles down). The irradiation dose, F ¼ 1 1014 cm2. For comparison purposes, two nðTÞ curves for the moderately doped n-GaN : Si before the irradiation (crosses) and after the proton irradiation at the same dose (open triangles up) are also shown.
InN
2
µ, cm /(V*s)
Scattering Cross Sect ion, a.u.
16
10
1
550
Raman shift, cm
17
10
2
IE2 /ILO =1.1
450
n, cm -3
Intensity, a.u.
InN W272
5x10
2
4x10
2
3x10
2
2x10
2
2
10 1 9x10 1 8x10 70 80 90100 400
450
500
550
600
650
-1
Frequency, cm
Fig. 4. Raman cross-section versus frequency in the range of L–LO modes calculated for three different scattering mechanisms in the InN. Perfect crystal, curve 1; defects with screened long-range potential and curve 2; defects with short-range potential, curve 3.
carriers continues to drop strongly. With the temperature increasing to T ¼ 6001C, the electron concentration and mobility in the irradiated n-GaN recover substan-
T,K
200
300
Fig. 6. Electron mobility versus temperature in the n-GaN : Si before the irradiation (open circles), after the proton irradiation (solid circles), after annealing at T ¼ 4001C (solid triangles up) and T ¼ 6001C (solid triangles down). The irradiation dose, F ¼ 1 1014 cm2.
tially. The mobility of the charge carriers becomes even larger than that in the initial n-GaN. After annealing at T ¼ 7001C, the fraction of unannealed defects turned out to be between 20% and 30%.
V.V. Emtsev et al. / Physica B 308–310 (2001) 58–61
61
4. Conclusions
References
The behavior of defects in the n-GaN and n-InN produced by proton irradiation and subsequent annealing has been studied. From the data obtained on the nInN it can be concluded that most likely the nitrogen vacancies with shallow donor states are responsible for the increasing concentration of free electrons in the irradiated n-InN. It has been demonstrated that in the Raman spectra, the intensity of a band at n ¼ 590 cm1 in the proton-irradiated n-InN layers is dose-dependent. Theoretical calculations showed that the model of defects with short-range potential gives a satisfactory explanation for this band in Raman scattering. The first stage of defect annealing in the irradiated n-InN takes place at T ¼ 3001C. The production rate of native defects in the irradiated n-GaN appears to be Fermi-level dependent. Two main recovery stages of the electron concentration over two intervals from T ¼ 3001C to 4001C and from T ¼ 5001C to 6001C have been found.
[1] V.V. Emtsev, V.Yu. Davydov, I.N. Goncharuk, E.V. Kalinina, V.V. Kozlovskii, D.S. Poloskin, A.V. Sakharov, N.M. Shmidt, A.N. Smirnov, A.S. Usikov, in: G. Davies, M.H. Nazare (Eds.), Proceedings of the Materials Science Forum, Vols. 258–263, Trans Tech Publications, Switzerland, 1997, pp. 1143–1148. [2] D.C. Look, D.C. Reynolds, J.W. Hemsky, J.R. Sizelove, R.L. Jones, R.J. Molnar, Phys. Rev. Lett. 79 (1997) 2273. [3] C. Bozdog, H. Przybylinska, G.D. Watkins, V. H.arle, F. Scholz, M. Mayer, M. Kamp, R.J. Molnar, A.E. Wickenden, D.D. Koleske, R.L. Henry, Phys. Rev. B 59 (1999) 12479. [4] F.D. Auret, S.A. Goodman, F.K. Koschnick, J.-M. Spaeth, B. Beaumont, P. Gibart, Appl. Phys. Lett. 74 (1999) 407. [5] K.H. Chow, G.D. Watkins, A. Usui, M. Mizuta, Phys. Rev. Lett. 85 (2000) 2761. [6] V.V. Emtsev, V.Yu. Davydov, V.V. Kozlovskii, V.V. Lundin, D.S. Poloskin, A.N. Smirnov, N.M. Shmidt, A.S. Usikov, J. Aderhold, H. Klausing, D. Mistele, T. Rotter, J. Stemmer, O. Semchinova, J. Graul, Semicond. Sci. Technol. 15 (2000) 73. [7] V.Yu. Davydov, V.V. Emtsev, I.N. Goncharuk, A.N. Smirnov, V.D. Petrikov, V.V. Mamutin, V.A. Vekshin, S.V. Ivanov, M.B. Smirnov, T. Inushima, Appl. Phys. Lett. 75 (1999) 3297.
Acknowledgements The work was supported by CRDF (grant # RP12258) and partly by The Russian Foundation for Basic Research (grant 99-02-18318).