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Identification of an ionized-donor-bound-exciton transition in GaN
Solid State Communications, Printed
Vol.
103. No. 9, pp, 533-535. 1997 0 1997 Ekvier Science Ltd in Great Britain. All tights reserved 003%1098/97 $17.ooc.O0
PII: SOO38-1098(!Y7)00231-7
IDENTIFICATION OF AN IONIZED-DONOR-BOUND-EXCITON
TRANSITION IN GaN
D.C. Reynolds,” D.C. Look,” B. Jogai,” V.M. Phanseb and R.P. Vaudob “University Research Center, Wright State University, Dayton, OH 45435, U.S.A. bAdvanced Technology Materials, Inc., 7 Commerce Drive, Danbury, CT 06810, U.S.A. (Received 20 January 1997; accepted 15 May 1997 by A.H. MacDonald)
The transition involving an exciton bound to an ionized donor has been identified in GaN. The linewidth of the transition was not sufficiently narrow to permit identification from Zeeman studies. Instead, an alternative series of measurements including energy ordering of the ionized-donorbound exciton with respect to the neutral-donor-bound exciton, line widths of the above two excitons, screening studies and electron bombardment measurements were used to identify the transition. 0 1997 Elsevier Science Ltd
1. INTRODUCTION Very little has been reported on ionized-donor-boundexciton transitions (D+, X) in GaN. The line widths are broad enough to make Zeeman studies difficult. In general, the identification of the D+, X transition is unmistakable from magnetic field splitting data. A zero field splitting arises from an exchange interaction of an unpaired electron and an unpaired hole in the upper state of the complex, giving a I’s and a F6 component [ 11.The Ps state consists of paired electron and hole spin states while the F6 component consists of parallel electron and hole spin states. The P6 to P 1 (ground state) transition is an unallowed transition in zero magnetic field. As a field is applied, the P6 transition becomes allowed due to magnetic field mixing of states. The zero field splitting is small; therefore, narrow lines are required to observe the magnetic field effect. When the lines are broader, other techniques are required to identify D+, X transitions. It would be expected that both neutral-donor-bound-exciton (Do, X) transitions and D+, X transitions would be observed. One can obtain an energy ordering of these two transitions from the arguments of Hopfield [2]. The binding energy of the exciton to the neutral donor as well as the binding energy of the exciton to the ionized donor depend on the electron and hole effective mass ratio u = rn:/rni for the particular material being investigated. The binding energies are expected to be nearly the same for u values in the range 0.2-0.25. For u values less than this, the binding energy of the exciton to the ionized donor is less
than the binding energy of the exciton to the neutral donor. The reverse is true for u values which are greater. It might also be expected that the D+, X transition would be narrower than the Do, X transition since the P6 component of the former is unallowed in zero magnetic field. In this paper we report Do, X and D+, X transitions in a GaN sample where the lines are reasonably sharp (for GaN) but not sufficiently sharp that magnetic field measurements could be used to identify the transitions. A variety of other techniques were used which we believe positively identify the transitions. 2. EXPERIMENTAL DETAILS The GaN layer used in this study was grown by hydride vapor phase epitaxy (HVPE) to a thickness of 11 pm. The substrate was a 2-inch wafer of (0 0 0 1) A1203 (sapphire) and the PL sample was a 2 mm X 8 mm piece cut from near the middle of this wafer. Hall-effect measurements, performed on an adjacent 7 mm X 7 mm piece, gave a carrier concentration of 3.8 X lOI cme3 at 300 K. However, we note that a simple estimate of the electric-field-induced exciton broadening would predict an exciton line width of 12 meV from this effect [3], whereas the actual line widths are only about 3 meV. Thus, we must assume that the PL emission is coming from the “best” parts of the layer, with much lower carrier concentrations than the average. Strong variations of electrical properties with depth in HVPE material have been recently reported [4].
533
IONIZED-DONOR-BOUND-EXCITON
534 r
I
3.5365
0’ X - 3.475WJ
Vol. 103, No. 9
_.c.-.
1425
3.4895
Energy (eV)
Fig. 1. Low temperature
IN GaN
-
-, 3.5130
TRANSITION
PL spectra for VPE grown GaN.
The PL spectra were excited with a He-Cd laser. The measurements were made at 2 K with the sample immersed in liquid He. The spectra were analyzed with a high-resolution 4 m spectrometer equipped with an RCAC3 1034A photomultiplier tube for detection. 3. EXPERIMENTAL
RESULTS
The photoluminescence (PL) spectra from the GaN sample is shown in Fig. 1. The transition at 3.4818 eV is the free exciton transition associated with the A-band. The transition at 3.4750 eV is the Do, X transition, while the transition at 3.4706 eV is the D+, X transition. Using 0.236mo as the effective-electron mass, calculated by Meyer et al. [5], and 0.40mo as the effective hole mass given by Orton [6], a u value of 0.59 is obtained, which would place the D+, X transition on the low energy side of the Do, X transition (as is observed). The bound exciton energy is the free exciton energy minus the energy with which the exciton is bound to the center. From this relation the exciton is bound to the neutral donor with an energy of 6.8 meV in good agreement with the energy reported by Andrianov et al. [7]. The measured energy with which the exciton is bound to the ionized donor is 11.2 meV. In the case of GaAs the Df, X transition is also on the low energy side of the Do, X transition. The ratio of the binding energy of the exciton to the ionized donor to the binding energy of the exciton to the neutral donor in GaAs is 1.3; in GaN it is 1.6. This is reasonable since the larger u value in GaN will increase the relative energy separation between Do, X and D+, X. The line width, full width at half maximum, for D+, X is 2.8 meV; for Do, X it is 3.4 meV. This is reasonable because the transition associated with the F6 component of D+, X is not allowed in zero magnetic field. The effect of screening on the optical transitions in GaN is to be reported in another paper [8]. The screening
3.5365
3.4895
3.5130
.25
Energy (eV)
Fig. 2. Effect of screening
on sample shown in Fig. 1.
effect is somewhat anomalous in this sample, as shown in Fig. 2. The solid curve in Fig. 2 shows the emission when the sample is excited only with the HeCd laser. In the dashed curve the sample is simultaneously excited with the HeCd laser and an Arf ion laser (5145 A), with a pump intensity of 30 KW cm-*. The Arf ion laser excites additional free electrons in a two-step process involving deep impurities or defects. The additional electrons screen the impurity potentials resulting in reduced ionization energies. If the two observed transitions were due to Do, X transitions resulting from two different chemical donors, one would not expect the one with the greater binding energy to show greater screening. However, if the transition with the greater binding energy is a D+, X transition the increased free electrons will neutralize some of the ionized donors, reducing the number of ionized centers to which excitons can bind. Two effects are observed: (1) there are fewer centers to which the excitons can be localized and thus the free exciton intensity increases, and (2) the number of neutral donor centers is increased which offsets the expected reduction in the intensity of Do, X due to screening. Increasing the Arf ion laser pump intensity to 40 KW cm-*, dot-dashed curve, shows a further increase in free exciton emission intensity and a further decrease in D’, X emission intensity. This behavior supports the identification of the transition at 3.4706 eV as a D’, X transition. An additional experiment was performed in which the sample in Fig. 1 was irradiated with 1 meV electrons to a fluence of 5 X lOI cm-*. The results of this irradiation are shown in Fig. 3. The solid curve shows the emission intensity for the as grown sample. After irradiation the emission is shown as the dashed curve. The relative intensities of the Do, X and D+, X transitions are reversed. This can be understood if the irradiation is producing an excess of acceptor centers which is known to be the case from Hall-effect measurements. The acceptors will ionize some of the neutral
Vol. 103, No. 9
IONIZED-DONOR-BOUND-EXCITON
ASGrown
. . . . . . . _.
,,m&&d
-.-.-.-..
Annealed
1 ! : I ” j! j !
I/
I
3.5365
3.5130
3.4995
535
Acknowledgements-The
authors would like to thank C. Huang and C.W. Litton for technical support. The work of D.C.R., D.C.L. and B.J. was performed at Wright-Laboratory, Avionics Directorate (WL/AADP), Wright Patterson Air Force Base under USAF Contract No. F33615-95-C-1619. This work was partially supported by AFOSR.
I i
\‘\ i ‘X.,
3.4654
IN GaN
would not expect them to behave differently when exposed to electron irradiation. This article reports, to the best of our knowledge, the first identification of the ionized-donor-bound-exciton transition in GaN.
!.
-
TRANSITION
3.4425
Energy (eV)
Fig. 3. Electron irradiation on sample in Fig. 1. - as grown, - - - irradiated 5 X lOI electrons, - ’ - . sample annealed 400°C 10 min. donors resulting in an increase in the number of ionized donors. This would result in an increase in intensity of the D+, X transition and a reduced intensity for the Do, X transition. The sample was then annealed at 400°C for 10 min. The results of the anneal are shown as the dot-dashed curve in Fig. 3. Here the relative intensities of the D+, X and Do, X transitions are again reversed. This can be understood if the annealing process preferentially returns the acceptor centers to positions that they occupied prior to irradiation. The interpretation of this experiment also depends on our assignments of the Do, X and D’, X transitions. As was the case for screening, if the two transitions, identified as Do, X and D’, X, were due to two different chemical donors, closely spaced in energy, with excitons bound to their neutral states, one
REFERENCES 1. 2.
3.
Thomas, D.G. and Hopfield, J.J., Phys. Rev., 128, 1962, 2135. Hopfield, J.J., Proc. Seventh Int. Conj Phys. Semicond. Paris, 1964 (Edited by M. Hulin), p. 725. Dunod, Paris, 1964. Look, D.C., Reynolds, D.C., Kim, W., Aktas, O., Botchkarev, A., Salvadore, A. and Morkoc, H., J. Appl. Phys., 80, 1996, 2960.
4.
5.
Goetz, W., Romano, L.T., Johnson, N.M., Walker, J., Bour, D.P. and Molnar, R.J., Mat. Res. Sot. Symp. Proc. (in press). Meyer, B.K., Volm, D., Graber. A., Alt, H.C., Detchprohm, T., Amano, A. and Akasaki, I., Solid State Commun.,
6. 7.
8.
95, 1995, 597.
Orton, J. W., Semicond. Sci. & Technol., 10, 1995, 101. Andrianov, A.V., Lacklison, D.E., Orton, J.W., Dewsnip, D.J., Hooper, S.E. and Foxon, C.T., Semicond. Sci. Technol,, 11, 1996, 366. Reynolds, D.C., Look D.C., Jogai, B. and Molnar, R., Unpublished.
Vol.
103. No. 9, pp, 533-535. 1997 0 1997 Ekvier Science Ltd in Great Britain. All tights reserved 003%1098/97 $17.ooc.O0
PII: SOO38-1098(!Y7)00231-7
IDENTIFICATION OF AN IONIZED-DONOR-BOUND-EXCITON
TRANSITION IN GaN
D.C. Reynolds,” D.C. Look,” B. Jogai,” V.M. Phanseb and R.P. Vaudob “University Research Center, Wright State University, Dayton, OH 45435, U.S.A. bAdvanced Technology Materials, Inc., 7 Commerce Drive, Danbury, CT 06810, U.S.A. (Received 20 January 1997; accepted 15 May 1997 by A.H. MacDonald)
The transition involving an exciton bound to an ionized donor has been identified in GaN. The linewidth of the transition was not sufficiently narrow to permit identification from Zeeman studies. Instead, an alternative series of measurements including energy ordering of the ionized-donorbound exciton with respect to the neutral-donor-bound exciton, line widths of the above two excitons, screening studies and electron bombardment measurements were used to identify the transition. 0 1997 Elsevier Science Ltd
1. INTRODUCTION Very little has been reported on ionized-donor-boundexciton transitions (D+, X) in GaN. The line widths are broad enough to make Zeeman studies difficult. In general, the identification of the D+, X transition is unmistakable from magnetic field splitting data. A zero field splitting arises from an exchange interaction of an unpaired electron and an unpaired hole in the upper state of the complex, giving a I’s and a F6 component [ 11.The Ps state consists of paired electron and hole spin states while the F6 component consists of parallel electron and hole spin states. The P6 to P 1 (ground state) transition is an unallowed transition in zero magnetic field. As a field is applied, the P6 transition becomes allowed due to magnetic field mixing of states. The zero field splitting is small; therefore, narrow lines are required to observe the magnetic field effect. When the lines are broader, other techniques are required to identify D+, X transitions. It would be expected that both neutral-donor-bound-exciton (Do, X) transitions and D+, X transitions would be observed. One can obtain an energy ordering of these two transitions from the arguments of Hopfield [2]. The binding energy of the exciton to the neutral donor as well as the binding energy of the exciton to the ionized donor depend on the electron and hole effective mass ratio u = rn:/rni for the particular material being investigated. The binding energies are expected to be nearly the same for u values in the range 0.2-0.25. For u values less than this, the binding energy of the exciton to the ionized donor is less
than the binding energy of the exciton to the neutral donor. The reverse is true for u values which are greater. It might also be expected that the D+, X transition would be narrower than the Do, X transition since the P6 component of the former is unallowed in zero magnetic field. In this paper we report Do, X and D+, X transitions in a GaN sample where the lines are reasonably sharp (for GaN) but not sufficiently sharp that magnetic field measurements could be used to identify the transitions. A variety of other techniques were used which we believe positively identify the transitions. 2. EXPERIMENTAL DETAILS The GaN layer used in this study was grown by hydride vapor phase epitaxy (HVPE) to a thickness of 11 pm. The substrate was a 2-inch wafer of (0 0 0 1) A1203 (sapphire) and the PL sample was a 2 mm X 8 mm piece cut from near the middle of this wafer. Hall-effect measurements, performed on an adjacent 7 mm X 7 mm piece, gave a carrier concentration of 3.8 X lOI cme3 at 300 K. However, we note that a simple estimate of the electric-field-induced exciton broadening would predict an exciton line width of 12 meV from this effect [3], whereas the actual line widths are only about 3 meV. Thus, we must assume that the PL emission is coming from the “best” parts of the layer, with much lower carrier concentrations than the average. Strong variations of electrical properties with depth in HVPE material have been recently reported [4].
533
IONIZED-DONOR-BOUND-EXCITON
534 r
I
3.5365
0’ X - 3.475WJ
Vol. 103, No. 9
_.c.-.
1425
3.4895
Energy (eV)
Fig. 1. Low temperature
IN GaN
-
-, 3.5130
TRANSITION
PL spectra for VPE grown GaN.
The PL spectra were excited with a He-Cd laser. The measurements were made at 2 K with the sample immersed in liquid He. The spectra were analyzed with a high-resolution 4 m spectrometer equipped with an RCAC3 1034A photomultiplier tube for detection. 3. EXPERIMENTAL
RESULTS
The photoluminescence (PL) spectra from the GaN sample is shown in Fig. 1. The transition at 3.4818 eV is the free exciton transition associated with the A-band. The transition at 3.4750 eV is the Do, X transition, while the transition at 3.4706 eV is the D+, X transition. Using 0.236mo as the effective-electron mass, calculated by Meyer et al. [5], and 0.40mo as the effective hole mass given by Orton [6], a u value of 0.59 is obtained, which would place the D+, X transition on the low energy side of the Do, X transition (as is observed). The bound exciton energy is the free exciton energy minus the energy with which the exciton is bound to the center. From this relation the exciton is bound to the neutral donor with an energy of 6.8 meV in good agreement with the energy reported by Andrianov et al. [7]. The measured energy with which the exciton is bound to the ionized donor is 11.2 meV. In the case of GaAs the Df, X transition is also on the low energy side of the Do, X transition. The ratio of the binding energy of the exciton to the ionized donor to the binding energy of the exciton to the neutral donor in GaAs is 1.3; in GaN it is 1.6. This is reasonable since the larger u value in GaN will increase the relative energy separation between Do, X and D+, X. The line width, full width at half maximum, for D+, X is 2.8 meV; for Do, X it is 3.4 meV. This is reasonable because the transition associated with the F6 component of D+, X is not allowed in zero magnetic field. The effect of screening on the optical transitions in GaN is to be reported in another paper [8]. The screening
3.5365
3.4895
3.5130
.25
Energy (eV)
Fig. 2. Effect of screening
on sample shown in Fig. 1.
effect is somewhat anomalous in this sample, as shown in Fig. 2. The solid curve in Fig. 2 shows the emission when the sample is excited only with the HeCd laser. In the dashed curve the sample is simultaneously excited with the HeCd laser and an Arf ion laser (5145 A), with a pump intensity of 30 KW cm-*. The Arf ion laser excites additional free electrons in a two-step process involving deep impurities or defects. The additional electrons screen the impurity potentials resulting in reduced ionization energies. If the two observed transitions were due to Do, X transitions resulting from two different chemical donors, one would not expect the one with the greater binding energy to show greater screening. However, if the transition with the greater binding energy is a D+, X transition the increased free electrons will neutralize some of the ionized donors, reducing the number of ionized centers to which excitons can bind. Two effects are observed: (1) there are fewer centers to which the excitons can be localized and thus the free exciton intensity increases, and (2) the number of neutral donor centers is increased which offsets the expected reduction in the intensity of Do, X due to screening. Increasing the Arf ion laser pump intensity to 40 KW cm-*, dot-dashed curve, shows a further increase in free exciton emission intensity and a further decrease in D’, X emission intensity. This behavior supports the identification of the transition at 3.4706 eV as a D’, X transition. An additional experiment was performed in which the sample in Fig. 1 was irradiated with 1 meV electrons to a fluence of 5 X lOI cm-*. The results of this irradiation are shown in Fig. 3. The solid curve shows the emission intensity for the as grown sample. After irradiation the emission is shown as the dashed curve. The relative intensities of the Do, X and D+, X transitions are reversed. This can be understood if the irradiation is producing an excess of acceptor centers which is known to be the case from Hall-effect measurements. The acceptors will ionize some of the neutral
Vol. 103, No. 9
IONIZED-DONOR-BOUND-EXCITON
ASGrown
. . . . . . . _.
,,m&&d
-.-.-.-..
Annealed
1 ! : I ” j! j !
I/
I
3.5365
3.5130
3.4995
535
Acknowledgements-The
authors would like to thank C. Huang and C.W. Litton for technical support. The work of D.C.R., D.C.L. and B.J. was performed at Wright-Laboratory, Avionics Directorate (WL/AADP), Wright Patterson Air Force Base under USAF Contract No. F33615-95-C-1619. This work was partially supported by AFOSR.
I i
\‘\ i ‘X.,
3.4654
IN GaN
would not expect them to behave differently when exposed to electron irradiation. This article reports, to the best of our knowledge, the first identification of the ionized-donor-bound-exciton transition in GaN.
!.
-
TRANSITION
3.4425
Energy (eV)
Fig. 3. Electron irradiation on sample in Fig. 1. - as grown, - - - irradiated 5 X lOI electrons, - ’ - . sample annealed 400°C 10 min. donors resulting in an increase in the number of ionized donors. This would result in an increase in intensity of the D+, X transition and a reduced intensity for the Do, X transition. The sample was then annealed at 400°C for 10 min. The results of the anneal are shown as the dot-dashed curve in Fig. 3. Here the relative intensities of the D+, X and Do, X transitions are again reversed. This can be understood if the annealing process preferentially returns the acceptor centers to positions that they occupied prior to irradiation. The interpretation of this experiment also depends on our assignments of the Do, X and D’, X transitions. As was the case for screening, if the two transitions, identified as Do, X and D’, X, were due to two different chemical donors, closely spaced in energy, with excitons bound to their neutral states, one
REFERENCES 1. 2.
3.
Thomas, D.G. and Hopfield, J.J., Phys. Rev., 128, 1962, 2135. Hopfield, J.J., Proc. Seventh Int. Conj Phys. Semicond. Paris, 1964 (Edited by M. Hulin), p. 725. Dunod, Paris, 1964. Look, D.C., Reynolds, D.C., Kim, W., Aktas, O., Botchkarev, A., Salvadore, A. and Morkoc, H., J. Appl. Phys., 80, 1996, 2960.
4.
5.
Goetz, W., Romano, L.T., Johnson, N.M., Walker, J., Bour, D.P. and Molnar, R.J., Mat. Res. Sot. Symp. Proc. (in press). Meyer, B.K., Volm, D., Graber. A., Alt, H.C., Detchprohm, T., Amano, A. and Akasaki, I., Solid State Commun.,
6. 7.
8.
95, 1995, 597.
Orton, J. W., Semicond. Sci. & Technol., 10, 1995, 101. Andrianov, A.V., Lacklison, D.E., Orton, J.W., Dewsnip, D.J., Hooper, S.E. and Foxon, C.T., Semicond. Sci. Technol,, 11, 1996, 366. Reynolds, D.C., Look D.C., Jogai, B. and Molnar, R., Unpublished.