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Electrical and electroluminescent properties of GaN light emitting diodes with the contact layer implanted with Mn
Solid-State Electronics 47 (2003) 963–968 www.elsevier.com/locate/sse
Electrical and electroluminescent properties of GaN light emitting diodes with the contact layer implanted with Mn A.Y. Polyakov a,*, N.B. Smirnov a, A.V. Govorkov a, J. Kim b, F. Ren b, M.E. Overberg c, G.T. Thaler c, C.R. Abernathy c, S.J. Pearton c, C.-M. Lee d, J.-I. Chyi d, R.G. Wilson e, J.M. Zavada f a Institute of Rare Metals, Moscow 109107, Russia Department of Chemical Engineering,University of Florida, Gainesville, FL 32611, USA c Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA d Department of Electrical Engineering, National Central University, Chung-Li 32054, Taiwan, ROC e Consultant, Stevenson Ranch, CA 91381, USA f US Army Research Office, Research Triangle Park, NC 27709, USA b
Received 27 May 2002; received in revised form 2 August 2002; accepted 1 November 2002
Abstract Electrical and luminescent properties of GaN/InGaN multiquantum well light emitting diodes (MQW LEDs) with the top p-GaN layer implanted with 3 1016 cm2 Mn ions for potential spin-polarized emission are reported. The forward current in the Mn-implanted diodes was limited by filling of hole traps in the high-resistivity implanted region, the most shallow hole traps having the activation energy of 0.27 eV. Admittance spectroscopy and deep level transient spectroscopy measurements also revealed the presence in the implanted region of traps with apparent activation energies of 0.23, 0.43, 0.5, 0.65 and 0.85 eV. Microcathodoluminescence spectra of the implanted diodes are dominated by two defect bands, the blue band centered near 2.8 eV and the yellow band centered near 2.25 eV, in contrast to the MCL spectra of the virgin diodes dominated by the 2.67 eV band coming from recombination in the GaN/InGaN MQW region. The high resistivity of the implanted region and the high density of deep traps in this region lead to a strong increase in the threshold voltage for the onset of electroluminescence from about 4 V before implantation to about 8 V after implantation. Ó 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Mn implantation into p-GaN has been shown to produce magnetic semiconductor material with the Curie temperature close to room temperature [1,2]. Such a material is of great interest for spintronic applications (see e.g. Ref. [3–14]), i.e. applications in which addi-
* Corresponding author. Address: Institute of Rare Metals, B. Tolmachevsky 5, Moscow 119270, Russia. Tel.: +7-095-2399090; fax: +7-095-230-4753. E-mail address: [email protected] (A.Y. Polyakov).
tional information can be stored and retrieved by using the induced spin polarization of charge carriers in addition to their charge [15–17]. One of the devices actively pursued for such applications is a light emitting diode LED in which the polarization of emitted light can be regulated by the applied magnetic field, a so-called spinLED [18–21]. Preliminary results give some evidence that implantation of Mn to the concentration of about 3 at.% into the top contact p-GaN layer of the multiquantum-well MQW GaN/InGaN LED allows the achievement of some degree of polarization [22]. However, the light emission efficiency of such spin-LEDs was very low and they needed a fairly high driving voltage to
0038-1101/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1101(02)00463-X
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get a reasonable intensity of electroluminescence. It is of interest to understand the nature of the effects involved and to try to optimize the process. To this end we compared the electrical and luminescent characteristics and the spectra of deep centers in GaN/InGaN MQW LEDs with and without Mn implantation into the top p-GaN layer.
2. Experimental The LED structures were grown by metallorganic chemical vapor deposition MOCVD on sapphire substrates. The structures were as follows (counting from the sapphire substrate): (1) 20 nm low-temperature GaN buffer; (2) 4 lm of Si doped n-GaN, (3) 5 QWs GaN/ Inx Ga1x N (In mole fraction about x ¼ 0:3) with the GaN barriers 10 nm-thick and the InGaN wells 30 nmthick, (4) 0.1 mm of p-Alx Ga1x N (Mg) (x 0:1, Mg concentration 1019 cm3 ), (5) 0.3 lm of p-GaN(Mg). These structures were processed into mesa diodes with the area of about 5 105 cm2 by dry etching, the contacts to p-type and n-type regions were prepared by respectively Au/Ni and Al/Ti deposition and annealing at 900 °C. More detailed description of the processing procedure can be found in Ref. [23]. Spin-LEDs were produced by implanting into the top p-GaN contact layer 3 1016 cm2 Mn ions at 250 keV at 350 °C to avoid amorphisation [24] and by subsequent annealing of the implanted samples at 700 °C for 15 s in N2 atmosphere The projected range of the Mn implanted region was about 0.2 lm. On both types of structures we measured current– voltage I–V characteristics in the temperature range 80–400 K, capacitance–voltage C–V characteristics at frequencies from 10 Hz to 10 MHz, capacitance/conductance versus frequency (C=G–f ) and versus temperature (C=G–T ), microcathodoluminescence MCL spectra at 90 and 300 K, electroluminescence spectra at 300 K and deep level transient spectroscopy measurements. The measurement techniques were fairly standard and corresponding set-ups can be found e.g. in Ref. [25–27].
3. Results 3.1. I–V characteristics The room temperature I–V characteristic of the unimplanted structure is presented in Fig. 1. This characteristic is fairly standard for such structures (see e.g. [23]). Measurements at different temperatures showed that for temperatures above 250 K the ideality factor of the forward current was close to 2.1 and the activation energy of the saturation current was close to 0.28 eV while at lower temperatures the ideality factor was
Fig. 1. Room temperature I–V characteristics of the GaN/InGaN MQW LEDs before (solid line) and after (dashed line) implantation of 3 1016 cm2 Mn.
gradually increasing and the activation energy steadily decreasing. These are rather common features of such diodes and they are usually explained by tunneling with thermal activation [23]. Implantation of Mn led to a very strong decrease of the forward current of the diodes. The forward characteristics bear clear evidence of trap-limited current regime with superlinear growth of current with voltage. At high temperatures and high forward voltages one could clearly observe switching from one set of deep traps to the next set. In Fig. 2 we present the log–log plot of forward current versus voltage for measurements at 400 K. One can see the linear growth of current with voltage I V a ða ¼ 1Þ at low voltages, the quadratic growth ða ¼ 2Þ for higher voltages, a sharp rise at about 10 V due to filling of the first set of traps and the beginning of the second quadratic region. These are the classical features of the trap filling current regime [28]. The activation energy at the ohmic and the first trap-filling region was 0.27 eV as measured from the temperature dependence of the current in these regions. Since the carriers in question should be holes this energy corresponds to the first set of hole traps in and near the Mn implanted region. Unfortunately, accurate measurements of the activation energy for the second set of hole traps related to the second quadratic region could not be made. The energy of 0.27 eV is considerably deeper than the Mg level [29] which suggests that the implanted (and possibly, the adjacent) regions are relatively highly-resistive. 3.2. Admittance spectroscopy The dependences of capacitance on frequency at room temperature for the unimplanted LED and the Mn implanted LED are shown in Fig. 3. For the unim-
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Fig. 2. Double logarithmic plot of the I–V curve of the implanted diode taken at 400 K and showing the linear region I Vaða ¼ 1Þ, the quadratic region I Vaða ¼ 2Þ, the jump in current corresponding to the complete filling of the first set of hole traps and the onset of the second quadratic region due to the second set of hole traps. The activation energy shown on the plot was obtained by making I–V measurements at various temperatures.
planted diode the C–f curve shows two steps, the low frequency one and the high-frequency one. Capacitance/ conductance versus frequency C=G–T measurements (so-called admittance spectroscopy) give the activation energies for the high-frequency traps as 0.15 eV which suggests that this region could be due to the series resistance of the Mg doped contact layer. The activation energy for the low-frequency region was quite small,
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about 0.08 eV. The form of the C–f and C=G–T curves observed for our LEDs is very similar to that reported recently by us for p-GaN/InGaN superlattices [29]. In this earlier paper we argued that the low-frequency region is due to the series resistance of the superlattice region which the holes traverse by thermally assistant tunneling. It is very likely that the same is true for the present structures. Mn implantation led to a very considerable decrease of the measured diode capacitance as can be seen from Fig. 3. The C–f curves showed two steps and admittance spectroscopy analysis yielded the activation energy for the low-frequency step as 0.43 eV and for the highfrequency step as 0.23 eV. A detailed explanation of the observed results is lacking at the moment but it seems reasonable to assume that the general decrease of capacitance after implantation is due to the presence of a high resistivity Schottky diode at the surface connected in series with the capacitance of the unimplanted portion of our LED. It would seem that the activation energies observed in C=G–T measurements relate in that case to the hole traps dominant in this high-resistivity space charge region whose thickness is determined by the concentration of deep traps located near the Fermi level and changing with temperature. The 0.23 eV trap is almost certainly the same as observed in I–V characteristics (see previous section), the 0.43 eV hole traps could be the second set of hole traps limiting the current flow at high voltages (see Fig. 2). Hole traps with the activation energy close to the 0.43 eV were observed in Mn implanted single films of GaN. 3.3. DLTS measurements
Fig. 3. C–f characteristics of the virgin GaN/InGaN MQW LED (dashed curve) and the Mn implanted LED (solid curve). The activation energies in the former case relate to the thermally assisted tunneling via the MQW region and to the series resistance of the p-GaN(Mg) contact layer. The activation energies on the implanted curve relate to deep traps in the highresistivity implanted region.
Proper DLTS measurements were not really possible either on the virgin diodes or on the Mn implanted LEDs because of the strong impact of series resistance. However, capacitance modulation with time upon switching the bias on the diode from reverse to forward was observed and allowed some qualitative assessment of the deep levels spectra. In Fig. 4 we present the DLTS spectra taken on unimplanted sample with reverse bias of )3 V and the forward bias pulse of 4 V, with time windows t1 ¼ 100 ms and t2 ¼ 1000 ms. The only feature observed was a very weak and very broad band with apparent activation energy of about 1 eV coming most likely from the QW region of the structure. Mn implanted diodes showed very weak dependence of capacitance on bias. However, if the forward bias was high enough so that efficient hole injection into the implanted high-resistivity region could be achieved the capacitance changes during the pulse were quite appreciable and the resultant spectrum for the reverse bias of )1 V and the bias pulse of 8 V (t1 =t2 ¼ 100/1000 ms) could be easily detected and produced two major peaks with apparent activation energies of 0.65 and 0.85 eV
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Fig. 4. DLTS spectrum taken for the unimplanted diode at reverse bias of )3V, forward bias pulse of 4 V (2 s long), time windows t1 =t2 ¼ 100/1000 ms (dash-dotted curve); the two other spectra are for the Mn implanted diode as measured with reverse bias of )1 V, forward bias pulse of þ8 V (2 s long) (dashed curve) and for the implanted diode as measured with reverse bias of )1 V and with optical injection pulse from the deuterium UV light source (2 s long) (solid curve) (time windows settings in both cases were the same: t1 =t2 ¼ 100/1000 ms).
and a shoulder near 200 K. When optical injection pulse from a UV deuterium lamp was used for excitation instead of electrical injection one the magnitude of the signal was much lower than with electrical pulse injection and the dominant feature became the broad band with apparent activation energy of 0.5 eV. The exact location of the observed traps and even their sign are under question because of the series resistance effects but it is felt that the signal due to these traps comes from the high resistivity implanted region.
Fig. 5. 5.90 K MCL spectra taken from the surface of the unimplanted diode (dashed curve) and of the Mn implanted diode (solid curve); the probing electron beam accelerating voltage was 9 kV in both cases.
yellow band centered near 2.25 eV also very prominent. Mind that the position of the blue band became quite measurably different from the MQW signal in the unimplanted region and that the peak of the yellow band also became measurably shifted after implantation. Electroluminescence spectra taken at 300 K are compared for the unimplanted and the implanted sample in Fig. 6. The former spectrum was taken with the forward bias of 4 V, the latter with the forward bias of 10 V. Mind that because of the very high series resistance of the implanted diodes the EL spectrum could not be taken at the same forward current value. From Fig. 6 it can be seen that, although the magnitude of the EL signal was very strongly reduced after implantation the spectral form did not really change and the dominant EL band before and after implantation was the 2.67 eV band due to the recombination in the MQW GaN/InGaN region. Therefore, the strong blue and yellow
3.4. MCL spectra and electroluminescence spectra 90 K MCL spectra of the unimplanted and the Mn implanted LEDs taken from the surface of the mesa diode with the probing electron beam accelerating voltage of 9 kV are shown in Fig. 5. The unimplanted spectrum contains the usual features of the upper pGaN MCL spectrum: the 3.26 eV donor–acceptor pairs peak with phonon replicas and the 2.9 eV blue band. One could also observe a weak 3.46 eV GaN bandedge signal probably coming from the underlying n-GaN. In addition one could see the dominant peak at 2.67 eV coinciding with the dominant peak in the EL spectrum (see Fig. 5) and coming from the GaN/InGaN MQWs region and a weak yellow luminescence centered near 2.2 eV and probably also coming from the MQW region [23]. After the implantation the MCL intensity became very much weaker due to the introduction of multiple radiation defects. The absolutely dominant MCL band became the blue band centered near 2.8 eV with the
Fig. 6. 300 K EL spectra of the unimplanted diode (dashed curve, measurements taken at 4 V forward bias) and of the Mn implanted diode (solid curve, measurements taken at 10 V forward bias).
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MCL bands appearing in the implanted LEDs and measurably shifted towards higher photon energies should be due to production of radiation defects in the upper p-GaN contact layer. This finding is in tune with MCL spectra measurements performed on Mn implanted single layers of GaN showing the emergence of strong blue and yellow bands, the blue band presumably coming from recombination involving deep centers near Ec 0:5 eV and somewhat tentatively associated with Mn acceptors complexes with nitrogen vacancies [30].
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termined previously by implanting protons or Ar into GaN [31]. All this indicates that, although prototype spin-LEDs operating at room temperature can be indeed built using the Mn implanted conventional LED structures the number of defects introduced by such treatment is prohibitively high and it would be very desirable to fabricate GaN spin-LEDs by some growth process not involving Mn implantation.
5. Conclusions 4. Discussion Our results strongly suggest that in Mn implanted MQW GaN/InGaN LEDs the implanted region is rather highly resistive and the current flow through this region is determined by the hole trap filling regime. The first set of hole traps limiting current flow is located near Ev þ 0:27 eV according to I–V –T measurements. These centers seem to be too deep to be associated with Mg acceptors and probably come from some point defects or their complexes with impurities. Admittance spectroscopy suggests that the upper damaged region forms a high-resistivity Schottky diode in series with the unimplanted portion of the LED structure which causes a strong overall decrease of the measured diode capacitance. Both admittance spectroscopy and DLTS indicate the emergence of new centers in the implanted LEDs. These centers are most likely located in the highlydamaged high-resistivity regions and have the apparent activation energies 0.23 eV (probably the same centers as the 0.27 eV traps in I–V measurements), 0.43 eV in admittance, 0.5 eV in DLTS with optical injection (possibly those two are the same traps), 0.65 and 0.85 eV in DLTS with electrical injection. The latter could be the acceptors associated with the yellow luminescence band [31]. Unfortunately, strong interference from the series resistance effects does not allow to quantitatively determine the type, concentration and location of the traps. MCL and EL spectra of the unimplanted LEDs are dominated by the 2.67 eV band due to the recombination in the GaN/InGaN MQW region. After implantation the EL spectrum is still determined by this band although the EL intensity is greatly diminished because of the high-resistivity of the implanted layer and poor double injection efficiency into the MQW region due to strong recombination in the implanted region. The MCL spectra of the implanted LEDs are dominated by the defect blue band centered near 2.8 eV and the defect yellow band centered near 2.25 eV, both originating in the implanted region. The former is very likely due to formation of defect levels near Ec 0:5 eV by the Mn acceptors and the nitrogen vacancies. The latter are due to radiation defects as de-
GaN/InGaN MQW LEDs with Mn implanted top p-GaN contact layer do show quite measurable electroluminescence and, since they also show above-roomtemperature magnetic Curie temperature they could in principle be used as components in magnetic memory devices and other spintronic applications. However, the Mn implantation causes the implanted region to become high-resistivity which results in the forward current through the diode to be limited by filling of a set of hole traps the most shallow of which has the activation energy of 0.27 eV. Admittance measurements suggest that the high-resistivity region forms a Schottky diode in series with the unimplanted portion of the LED. Admittance spectroscopy and DLTS show the presence in this high-resistivity Schottky diode of deep traps with apparent activation energies of 0.23 eV (most likely the same traps as the 0.27 eV trap in I–V characteristics), 0.43, 0.5, 0.65 and 0.85 eV. MCL spectra indicate formation in the implanted regions of high density of defects giving rise to the blue band at 2.8 eV and the yellow band at 2.25 eV so that the MCL spectrum is no longer dominated by the 2.67 eV band due to recombination in the MQW GaN/InGaN region. This high resistivity of the implanted region and high density of deep recombination centers causes the electroluminescence efficiency to become very low, so that the threshold voltage for luminescence increases from about 4 V to about 8 V and the EL intensity is about two orders of magnitude lower compared to the unimplanted diode. It is expected that serious improvements in efficiency should occur if the Mn is introduced not by ion implantation but by during regular growth, although, as will be shown elsewhere, such a process needs optimization to be effective.
Acknowledgements The work at IRM was supported in part by a grant from the Russian Foundation for Basic Research
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(RFBR grant # 01-02-17230). The work at UF is partially supported by NSF(CTS 991173, DMR 0101438) and ARO. The work of RGW is also partially supported by ARO. The work at NCU was supported by the National Science Council of Republic of China under contract no. NSC 90-2215-E-008-038 and the Ministry of Education of Republic of China under the Program for Promoting Academic Excellence of Universities, 91E-FA06-1-4. References [1] Theodoropoulou NA, Hebard AF, Chu SNG, Overberg ME, Abernathy CR, Pearton SJ, et al. Appl Phys Lett 2001;79:3452. [2] Pearton SJ, Overberg ME, Thaler G, Abernathy CR, Theodoropoulou NA, Hebard AF, et al. J Vac Sci Technol A 2002;20:583. [3] Sonoda S, Shimizu S, Sasaki T, Yamamoto Y, Hori H. J Cryst Growth 2002;237–239:1358. [4] Das Sarma S. Am Sci 2001;89:516. [5] Wolf SA, Awschalom DD, Buhrman RA, Daughton JM, von Molnar S, Roukes ML, et al. Science 2001;294:1488. [6] Ohno H. J Vac Sci Technol B 2000;18:2039. [7] Dietl T. J Appl Phys 2001;89:7437. [8] Dietl T, Ohno H, Matsukura F, Cibert J, Ferrand D. Science 2000;287:1019. [9] Reed MK, El-Masry NA, Stadelmaier HH, Ritums MK, Reed MJ, Parker CA, et al. Appl Phys Lett 2001;79:3473. [10] Thaler GT, Overberg ME, Gila B, Frazier R, Abernathy CR, Pearton SJ, et al. Appl Phys Lett 2002;80:3964. [11] Park SE, Lee H-J, Cho YC, Jeong S-Y, Cho CR, Cho S. Appl Phys Lett 2002;80:4187. [12] Theodoropoulou N, Hebard AF, Chu SNG, Overberg ME, Abernathy CR, Pearton SJ, et al. J Appl Phys 2002; 91:7499. [13] Sato K, Katayama-Yoshida H. Jpn J Appl Phys 2001; 40:4485.
[14] Katayama-Yoshida H, Kato R, Yamamoto T. J Cryst Growth 2001;231:438. [15] Fong CY, Gubanov VA, Boekema C. J Electron Mater 2000;29:1067. [16] Akinaga H, Nemeth S, de Boeck J, Nistor L, Bender H, Borghs G, et al. Appl Phys Lett 2000;77:4377. [17] Overberg ME, Gila BP, Thaler GT, Abernathy CR, Pearton SJ, Theodoropoulou NA, et al. J Vac Sci Technol B 2002;20:969. [18] Fiederling R, Kein M, Rerescher G, Ossan W, Schmidt G, Wang A, et al. Nature 1999;402:787. [19] Ohno Y, Young DK, Beschoten B, Matsukura F, Ohno H, Awschalom DD. Nature 1999;402:790. [20] Jonker BT, Park YD, Bennett BR, Cheong HD, Kioseoglou G, Petrou A. Phys Rev B 2000;62:8180. [21] Park YD, Jonker BT, Bennett BR, Itskos G, Furis M, Kioseoglou G, et al. App Phys Lett 2000;77:3989. [22] Pearton SJ, Abernathy CR, Overberg ME, Thaler GT, Onstine AH, Gila BP, et al. Mater Today 2002;(June): 24. [23] Polyakov AY, Smirnov NB, Govorkov AV, Kim J, Luo B, Mehandru R, et al. J Appl Phys 2002;91:5203. [24] Kucheyev SO, Toth M, Phillips MR, Williams JS, Jagadish C, Li G. Appl Phys Lett 2001;78:34. [25] Polyakov AY, Smirnov NB, Govorkov AV, MilÕVidskii MG, Redwing JM, Shin M, et al. Solid-State Electron 1998;42:627. [26] Polyakov AY, Smirnov NB, Usikov AS, Govorkov AV, Pushniy BS. Solid-State Electron 1998;42:1959. [27] Polyakov AY, Smirnov NB, Govorkov AV, Shin M, Skowronski M, Greve DW. J Appl Phys 1998;84:870. [28] Sze SM. Physics of semiconductor devices. NY: Wiley and Sons; 1983. [29] Polyakov AY, Smirnov NB, Govorkov AV, Osinsky AV, Norris PE, Pearton SJ, et al. J Appl Phys 2001;90:4032. [30] Reshchnikov MA, Shahedpour F, Korotkov RY, Wessels BW, Ulmer MP. J Appl Phys 2000;87:3351. [31] Polyakov AY, Usikov AS, Theys B, Smirnov NB, Govorkov AV, Jomard F, et al. Solid-State Electron 2000; 44:1971.
Electrical and electroluminescent properties of GaN light emitting diodes with the contact layer implanted with Mn A.Y. Polyakov a,*, N.B. Smirnov a, A.V. Govorkov a, J. Kim b, F. Ren b, M.E. Overberg c, G.T. Thaler c, C.R. Abernathy c, S.J. Pearton c, C.-M. Lee d, J.-I. Chyi d, R.G. Wilson e, J.M. Zavada f a Institute of Rare Metals, Moscow 109107, Russia Department of Chemical Engineering,University of Florida, Gainesville, FL 32611, USA c Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA d Department of Electrical Engineering, National Central University, Chung-Li 32054, Taiwan, ROC e Consultant, Stevenson Ranch, CA 91381, USA f US Army Research Office, Research Triangle Park, NC 27709, USA b
Received 27 May 2002; received in revised form 2 August 2002; accepted 1 November 2002
Abstract Electrical and luminescent properties of GaN/InGaN multiquantum well light emitting diodes (MQW LEDs) with the top p-GaN layer implanted with 3 1016 cm2 Mn ions for potential spin-polarized emission are reported. The forward current in the Mn-implanted diodes was limited by filling of hole traps in the high-resistivity implanted region, the most shallow hole traps having the activation energy of 0.27 eV. Admittance spectroscopy and deep level transient spectroscopy measurements also revealed the presence in the implanted region of traps with apparent activation energies of 0.23, 0.43, 0.5, 0.65 and 0.85 eV. Microcathodoluminescence spectra of the implanted diodes are dominated by two defect bands, the blue band centered near 2.8 eV and the yellow band centered near 2.25 eV, in contrast to the MCL spectra of the virgin diodes dominated by the 2.67 eV band coming from recombination in the GaN/InGaN MQW region. The high resistivity of the implanted region and the high density of deep traps in this region lead to a strong increase in the threshold voltage for the onset of electroluminescence from about 4 V before implantation to about 8 V after implantation. Ó 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Mn implantation into p-GaN has been shown to produce magnetic semiconductor material with the Curie temperature close to room temperature [1,2]. Such a material is of great interest for spintronic applications (see e.g. Ref. [3–14]), i.e. applications in which addi-
* Corresponding author. Address: Institute of Rare Metals, B. Tolmachevsky 5, Moscow 119270, Russia. Tel.: +7-095-2399090; fax: +7-095-230-4753. E-mail address: [email protected] (A.Y. Polyakov).
tional information can be stored and retrieved by using the induced spin polarization of charge carriers in addition to their charge [15–17]. One of the devices actively pursued for such applications is a light emitting diode LED in which the polarization of emitted light can be regulated by the applied magnetic field, a so-called spinLED [18–21]. Preliminary results give some evidence that implantation of Mn to the concentration of about 3 at.% into the top contact p-GaN layer of the multiquantum-well MQW GaN/InGaN LED allows the achievement of some degree of polarization [22]. However, the light emission efficiency of such spin-LEDs was very low and they needed a fairly high driving voltage to
0038-1101/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1101(02)00463-X
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get a reasonable intensity of electroluminescence. It is of interest to understand the nature of the effects involved and to try to optimize the process. To this end we compared the electrical and luminescent characteristics and the spectra of deep centers in GaN/InGaN MQW LEDs with and without Mn implantation into the top p-GaN layer.
2. Experimental The LED structures were grown by metallorganic chemical vapor deposition MOCVD on sapphire substrates. The structures were as follows (counting from the sapphire substrate): (1) 20 nm low-temperature GaN buffer; (2) 4 lm of Si doped n-GaN, (3) 5 QWs GaN/ Inx Ga1x N (In mole fraction about x ¼ 0:3) with the GaN barriers 10 nm-thick and the InGaN wells 30 nmthick, (4) 0.1 mm of p-Alx Ga1x N (Mg) (x 0:1, Mg concentration 1019 cm3 ), (5) 0.3 lm of p-GaN(Mg). These structures were processed into mesa diodes with the area of about 5 105 cm2 by dry etching, the contacts to p-type and n-type regions were prepared by respectively Au/Ni and Al/Ti deposition and annealing at 900 °C. More detailed description of the processing procedure can be found in Ref. [23]. Spin-LEDs were produced by implanting into the top p-GaN contact layer 3 1016 cm2 Mn ions at 250 keV at 350 °C to avoid amorphisation [24] and by subsequent annealing of the implanted samples at 700 °C for 15 s in N2 atmosphere The projected range of the Mn implanted region was about 0.2 lm. On both types of structures we measured current– voltage I–V characteristics in the temperature range 80–400 K, capacitance–voltage C–V characteristics at frequencies from 10 Hz to 10 MHz, capacitance/conductance versus frequency (C=G–f ) and versus temperature (C=G–T ), microcathodoluminescence MCL spectra at 90 and 300 K, electroluminescence spectra at 300 K and deep level transient spectroscopy measurements. The measurement techniques were fairly standard and corresponding set-ups can be found e.g. in Ref. [25–27].
3. Results 3.1. I–V characteristics The room temperature I–V characteristic of the unimplanted structure is presented in Fig. 1. This characteristic is fairly standard for such structures (see e.g. [23]). Measurements at different temperatures showed that for temperatures above 250 K the ideality factor of the forward current was close to 2.1 and the activation energy of the saturation current was close to 0.28 eV while at lower temperatures the ideality factor was
Fig. 1. Room temperature I–V characteristics of the GaN/InGaN MQW LEDs before (solid line) and after (dashed line) implantation of 3 1016 cm2 Mn.
gradually increasing and the activation energy steadily decreasing. These are rather common features of such diodes and they are usually explained by tunneling with thermal activation [23]. Implantation of Mn led to a very strong decrease of the forward current of the diodes. The forward characteristics bear clear evidence of trap-limited current regime with superlinear growth of current with voltage. At high temperatures and high forward voltages one could clearly observe switching from one set of deep traps to the next set. In Fig. 2 we present the log–log plot of forward current versus voltage for measurements at 400 K. One can see the linear growth of current with voltage I V a ða ¼ 1Þ at low voltages, the quadratic growth ða ¼ 2Þ for higher voltages, a sharp rise at about 10 V due to filling of the first set of traps and the beginning of the second quadratic region. These are the classical features of the trap filling current regime [28]. The activation energy at the ohmic and the first trap-filling region was 0.27 eV as measured from the temperature dependence of the current in these regions. Since the carriers in question should be holes this energy corresponds to the first set of hole traps in and near the Mn implanted region. Unfortunately, accurate measurements of the activation energy for the second set of hole traps related to the second quadratic region could not be made. The energy of 0.27 eV is considerably deeper than the Mg level [29] which suggests that the implanted (and possibly, the adjacent) regions are relatively highly-resistive. 3.2. Admittance spectroscopy The dependences of capacitance on frequency at room temperature for the unimplanted LED and the Mn implanted LED are shown in Fig. 3. For the unim-
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Fig. 2. Double logarithmic plot of the I–V curve of the implanted diode taken at 400 K and showing the linear region I Vaða ¼ 1Þ, the quadratic region I Vaða ¼ 2Þ, the jump in current corresponding to the complete filling of the first set of hole traps and the onset of the second quadratic region due to the second set of hole traps. The activation energy shown on the plot was obtained by making I–V measurements at various temperatures.
planted diode the C–f curve shows two steps, the low frequency one and the high-frequency one. Capacitance/ conductance versus frequency C=G–T measurements (so-called admittance spectroscopy) give the activation energies for the high-frequency traps as 0.15 eV which suggests that this region could be due to the series resistance of the Mg doped contact layer. The activation energy for the low-frequency region was quite small,
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about 0.08 eV. The form of the C–f and C=G–T curves observed for our LEDs is very similar to that reported recently by us for p-GaN/InGaN superlattices [29]. In this earlier paper we argued that the low-frequency region is due to the series resistance of the superlattice region which the holes traverse by thermally assistant tunneling. It is very likely that the same is true for the present structures. Mn implantation led to a very considerable decrease of the measured diode capacitance as can be seen from Fig. 3. The C–f curves showed two steps and admittance spectroscopy analysis yielded the activation energy for the low-frequency step as 0.43 eV and for the highfrequency step as 0.23 eV. A detailed explanation of the observed results is lacking at the moment but it seems reasonable to assume that the general decrease of capacitance after implantation is due to the presence of a high resistivity Schottky diode at the surface connected in series with the capacitance of the unimplanted portion of our LED. It would seem that the activation energies observed in C=G–T measurements relate in that case to the hole traps dominant in this high-resistivity space charge region whose thickness is determined by the concentration of deep traps located near the Fermi level and changing with temperature. The 0.23 eV trap is almost certainly the same as observed in I–V characteristics (see previous section), the 0.43 eV hole traps could be the second set of hole traps limiting the current flow at high voltages (see Fig. 2). Hole traps with the activation energy close to the 0.43 eV were observed in Mn implanted single films of GaN. 3.3. DLTS measurements
Fig. 3. C–f characteristics of the virgin GaN/InGaN MQW LED (dashed curve) and the Mn implanted LED (solid curve). The activation energies in the former case relate to the thermally assisted tunneling via the MQW region and to the series resistance of the p-GaN(Mg) contact layer. The activation energies on the implanted curve relate to deep traps in the highresistivity implanted region.
Proper DLTS measurements were not really possible either on the virgin diodes or on the Mn implanted LEDs because of the strong impact of series resistance. However, capacitance modulation with time upon switching the bias on the diode from reverse to forward was observed and allowed some qualitative assessment of the deep levels spectra. In Fig. 4 we present the DLTS spectra taken on unimplanted sample with reverse bias of )3 V and the forward bias pulse of 4 V, with time windows t1 ¼ 100 ms and t2 ¼ 1000 ms. The only feature observed was a very weak and very broad band with apparent activation energy of about 1 eV coming most likely from the QW region of the structure. Mn implanted diodes showed very weak dependence of capacitance on bias. However, if the forward bias was high enough so that efficient hole injection into the implanted high-resistivity region could be achieved the capacitance changes during the pulse were quite appreciable and the resultant spectrum for the reverse bias of )1 V and the bias pulse of 8 V (t1 =t2 ¼ 100/1000 ms) could be easily detected and produced two major peaks with apparent activation energies of 0.65 and 0.85 eV
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Fig. 4. DLTS spectrum taken for the unimplanted diode at reverse bias of )3V, forward bias pulse of 4 V (2 s long), time windows t1 =t2 ¼ 100/1000 ms (dash-dotted curve); the two other spectra are for the Mn implanted diode as measured with reverse bias of )1 V, forward bias pulse of þ8 V (2 s long) (dashed curve) and for the implanted diode as measured with reverse bias of )1 V and with optical injection pulse from the deuterium UV light source (2 s long) (solid curve) (time windows settings in both cases were the same: t1 =t2 ¼ 100/1000 ms).
and a shoulder near 200 K. When optical injection pulse from a UV deuterium lamp was used for excitation instead of electrical injection one the magnitude of the signal was much lower than with electrical pulse injection and the dominant feature became the broad band with apparent activation energy of 0.5 eV. The exact location of the observed traps and even their sign are under question because of the series resistance effects but it is felt that the signal due to these traps comes from the high resistivity implanted region.
Fig. 5. 5.90 K MCL spectra taken from the surface of the unimplanted diode (dashed curve) and of the Mn implanted diode (solid curve); the probing electron beam accelerating voltage was 9 kV in both cases.
yellow band centered near 2.25 eV also very prominent. Mind that the position of the blue band became quite measurably different from the MQW signal in the unimplanted region and that the peak of the yellow band also became measurably shifted after implantation. Electroluminescence spectra taken at 300 K are compared for the unimplanted and the implanted sample in Fig. 6. The former spectrum was taken with the forward bias of 4 V, the latter with the forward bias of 10 V. Mind that because of the very high series resistance of the implanted diodes the EL spectrum could not be taken at the same forward current value. From Fig. 6 it can be seen that, although the magnitude of the EL signal was very strongly reduced after implantation the spectral form did not really change and the dominant EL band before and after implantation was the 2.67 eV band due to the recombination in the MQW GaN/InGaN region. Therefore, the strong blue and yellow
3.4. MCL spectra and electroluminescence spectra 90 K MCL spectra of the unimplanted and the Mn implanted LEDs taken from the surface of the mesa diode with the probing electron beam accelerating voltage of 9 kV are shown in Fig. 5. The unimplanted spectrum contains the usual features of the upper pGaN MCL spectrum: the 3.26 eV donor–acceptor pairs peak with phonon replicas and the 2.9 eV blue band. One could also observe a weak 3.46 eV GaN bandedge signal probably coming from the underlying n-GaN. In addition one could see the dominant peak at 2.67 eV coinciding with the dominant peak in the EL spectrum (see Fig. 5) and coming from the GaN/InGaN MQWs region and a weak yellow luminescence centered near 2.2 eV and probably also coming from the MQW region [23]. After the implantation the MCL intensity became very much weaker due to the introduction of multiple radiation defects. The absolutely dominant MCL band became the blue band centered near 2.8 eV with the
Fig. 6. 300 K EL spectra of the unimplanted diode (dashed curve, measurements taken at 4 V forward bias) and of the Mn implanted diode (solid curve, measurements taken at 10 V forward bias).
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MCL bands appearing in the implanted LEDs and measurably shifted towards higher photon energies should be due to production of radiation defects in the upper p-GaN contact layer. This finding is in tune with MCL spectra measurements performed on Mn implanted single layers of GaN showing the emergence of strong blue and yellow bands, the blue band presumably coming from recombination involving deep centers near Ec 0:5 eV and somewhat tentatively associated with Mn acceptors complexes with nitrogen vacancies [30].
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termined previously by implanting protons or Ar into GaN [31]. All this indicates that, although prototype spin-LEDs operating at room temperature can be indeed built using the Mn implanted conventional LED structures the number of defects introduced by such treatment is prohibitively high and it would be very desirable to fabricate GaN spin-LEDs by some growth process not involving Mn implantation.
5. Conclusions 4. Discussion Our results strongly suggest that in Mn implanted MQW GaN/InGaN LEDs the implanted region is rather highly resistive and the current flow through this region is determined by the hole trap filling regime. The first set of hole traps limiting current flow is located near Ev þ 0:27 eV according to I–V –T measurements. These centers seem to be too deep to be associated with Mg acceptors and probably come from some point defects or their complexes with impurities. Admittance spectroscopy suggests that the upper damaged region forms a high-resistivity Schottky diode in series with the unimplanted portion of the LED structure which causes a strong overall decrease of the measured diode capacitance. Both admittance spectroscopy and DLTS indicate the emergence of new centers in the implanted LEDs. These centers are most likely located in the highlydamaged high-resistivity regions and have the apparent activation energies 0.23 eV (probably the same centers as the 0.27 eV traps in I–V measurements), 0.43 eV in admittance, 0.5 eV in DLTS with optical injection (possibly those two are the same traps), 0.65 and 0.85 eV in DLTS with electrical injection. The latter could be the acceptors associated with the yellow luminescence band [31]. Unfortunately, strong interference from the series resistance effects does not allow to quantitatively determine the type, concentration and location of the traps. MCL and EL spectra of the unimplanted LEDs are dominated by the 2.67 eV band due to the recombination in the GaN/InGaN MQW region. After implantation the EL spectrum is still determined by this band although the EL intensity is greatly diminished because of the high-resistivity of the implanted layer and poor double injection efficiency into the MQW region due to strong recombination in the implanted region. The MCL spectra of the implanted LEDs are dominated by the defect blue band centered near 2.8 eV and the defect yellow band centered near 2.25 eV, both originating in the implanted region. The former is very likely due to formation of defect levels near Ec 0:5 eV by the Mn acceptors and the nitrogen vacancies. The latter are due to radiation defects as de-
GaN/InGaN MQW LEDs with Mn implanted top p-GaN contact layer do show quite measurable electroluminescence and, since they also show above-roomtemperature magnetic Curie temperature they could in principle be used as components in magnetic memory devices and other spintronic applications. However, the Mn implantation causes the implanted region to become high-resistivity which results in the forward current through the diode to be limited by filling of a set of hole traps the most shallow of which has the activation energy of 0.27 eV. Admittance measurements suggest that the high-resistivity region forms a Schottky diode in series with the unimplanted portion of the LED. Admittance spectroscopy and DLTS show the presence in this high-resistivity Schottky diode of deep traps with apparent activation energies of 0.23 eV (most likely the same traps as the 0.27 eV trap in I–V characteristics), 0.43, 0.5, 0.65 and 0.85 eV. MCL spectra indicate formation in the implanted regions of high density of defects giving rise to the blue band at 2.8 eV and the yellow band at 2.25 eV so that the MCL spectrum is no longer dominated by the 2.67 eV band due to recombination in the MQW GaN/InGaN region. This high resistivity of the implanted region and high density of deep recombination centers causes the electroluminescence efficiency to become very low, so that the threshold voltage for luminescence increases from about 4 V to about 8 V and the EL intensity is about two orders of magnitude lower compared to the unimplanted diode. It is expected that serious improvements in efficiency should occur if the Mn is introduced not by ion implantation but by during regular growth, although, as will be shown elsewhere, such a process needs optimization to be effective.
Acknowledgements The work at IRM was supported in part by a grant from the Russian Foundation for Basic Research
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(RFBR grant # 01-02-17230). The work at UF is partially supported by NSF(CTS 991173, DMR 0101438) and ARO. The work of RGW is also partially supported by ARO. The work at NCU was supported by the National Science Council of Republic of China under contract no. NSC 90-2215-E-008-038 and the Ministry of Education of Republic of China under the Program for Promoting Academic Excellence of Universities, 91E-FA06-1-4. References [1] Theodoropoulou NA, Hebard AF, Chu SNG, Overberg ME, Abernathy CR, Pearton SJ, et al. Appl Phys Lett 2001;79:3452. [2] Pearton SJ, Overberg ME, Thaler G, Abernathy CR, Theodoropoulou NA, Hebard AF, et al. J Vac Sci Technol A 2002;20:583. [3] Sonoda S, Shimizu S, Sasaki T, Yamamoto Y, Hori H. J Cryst Growth 2002;237–239:1358. [4] Das Sarma S. Am Sci 2001;89:516. [5] Wolf SA, Awschalom DD, Buhrman RA, Daughton JM, von Molnar S, Roukes ML, et al. Science 2001;294:1488. [6] Ohno H. J Vac Sci Technol B 2000;18:2039. [7] Dietl T. J Appl Phys 2001;89:7437. [8] Dietl T, Ohno H, Matsukura F, Cibert J, Ferrand D. Science 2000;287:1019. [9] Reed MK, El-Masry NA, Stadelmaier HH, Ritums MK, Reed MJ, Parker CA, et al. Appl Phys Lett 2001;79:3473. [10] Thaler GT, Overberg ME, Gila B, Frazier R, Abernathy CR, Pearton SJ, et al. Appl Phys Lett 2002;80:3964. [11] Park SE, Lee H-J, Cho YC, Jeong S-Y, Cho CR, Cho S. Appl Phys Lett 2002;80:4187. [12] Theodoropoulou N, Hebard AF, Chu SNG, Overberg ME, Abernathy CR, Pearton SJ, et al. J Appl Phys 2002; 91:7499. [13] Sato K, Katayama-Yoshida H. Jpn J Appl Phys 2001; 40:4485.
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