Electronic Raman scattering experiments have been carried out on both molecular beam epitaxy and metal-organic chemical vapor deposition-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (full width at half maximum≅15 cm−1)observed at around 841 cm−1 is attributed to the electronic Raman scattering from neutral Mg impurities in Mg-doped GaN. Our experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about one-half of its binding energy

Chen, Y.

Jiang, H. X.

Koch, C.

Li, J.

Lin, J. Y.

Morkoç, Hadis

Tsen, K. T.

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Virginia Commonwealth UniversityVCU Scholars CompassElectrical and Computer Engineering Publications Dept. of Electrical and Computer Engineering2000Observation of electronic Raman scattering fromMg-doped wurtzite GaNK. T. TsenArizona State University at the Tempe CampusC. KochArizona State University at the Tempe CampusY. ChenArizona State University at the Tempe CampusSee next page for additional authorsFollow this and additional works at: http://scholarscompass.vcu.edu/egre_pubsPart of the Electrical and Computer Engineering CommonsTsen, K.T., Koch, C., Chen, Y., et al. Observation of electronic Raman scattering from Mg-doped wurtzite GaN. AppliedPhysics Letters, 76, 2889 (2000). Copyright © 2000 AIP Publishing LLC.This Article is brought to you for free and open access by the Dept. of Electrical and Computer Engineering at VCU Scholars Compass. It has beenaccepted for inclusion in Electrical and Computer Engineering Publications by an authorized administrator of VCU Scholars Compass. For moreinformation, please contact libcompass@vcu.edu.Downloaded fromhttp://scholarscompass.vcu.edu/egre_pubs/32AuthorsK. T. Tsen, C. Koch, Y. Chen, Hadis Morkoç, J. Li, J. Y. Lin, and H. X. JiangThis article is available at VCU Scholars Compass: http://scholarscompass.vcu.edu/egre_pubs/32Observation of electronic Raman scattering from Mg-doped wurtzite GaNK. T. Tsen,a) C. Koch, and Y. ChenDepartment of Physics and Astronomy, Arizona State University, Tempe, Arizona 85287H. MorkocDepartment of Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia 23284J. Li, J. Y. Lin, and H. X. JiangDepartment of Physics, Kansas State University, Manhattan, Kansas 66506~Received 6 December 1999; accepted for publication 17 March 2000!Electronic Raman scattering experiments have been carried out on both molecular beam epitaxy andmetal-organic chemical vapor deposition-grown Mg-doped wurtzite GaN samples. Aside from theexpected Raman lines, a broad structure ~full width at half maximum>15 cm21) observed at around841 cm21 is attributed to the electronic Raman scattering from neutral Mg impurities in Mg-dopedGaN. Our experimental results demonstrate that the energy between the ground and first excitedstates of Mg impurities in wurtzite GaN is about one-half of its binding energy. © 2000 AmericanInstitute of Physics. @S0003-6951~00!01420-0#The recent surge of activity in wide band gapsemiconductors1 has arisen from the need for electronic de-vices capable of operation at high power levels, high tem-peratures, and caustic environments, and separately, from aneed for optical materials, especially emitters, which are ac-tive in the blue and ultraviolet wavelengths. Electronicsbased upon the existing semiconductor device technologiesof Si and GaAs cannot tolerate greatly elevated temperaturesor chemically hostile environments. The wide band gapsemiconductors with their excellent thermal conductivities,large breakdown fields, and resistance to chemical corrosionwill be the materials of choice for these applications. Amongthe wide band gap semiconductors, the III–V nitrides havelong been viewed as a very promising semiconductor systemfor device applications in the blue and ultraviolet wave-lengths. The wurtzite polytypes of GaN, AlN, and InN forma continuous alloy system whose direct band gaps rangefrom 1.9 eV for InN, to 3.4 eV for GaN, and to 6.2 eV forAlN. Thus III–V nitrides could potentially be fabricated intooptical devices which are active at wavelengths ranging fromthe red all the way into the ultraviolet. Because the lowerohmic contact resistances and larger predicted electron satu-ration velocity2 the research of GaN is of particular interest.P-type doping in nitride-based wide band gap semicon-ductors has been a big challenge to researchers. The recentlysuccessful p-type doping in metal-organic vapor phase epi-taxy GaN using Mg has allowed the realization of blue lightemitting diodes.3,4 Although much work has been devoted tothe study of the properties of Mg impurities in GaN samples,the electronic and optical properties of Mg impurities remaincontroversial and inconclusive. In this letter, we report theobservation of electronic Raman scattering ~ERS! in Mg-doped GaN. Our experimental results provide an importantinformation about the electronic structure of Mg impuritiesin GaN; namely, the energy difference between the groundand first excited states of Mg impurities in GaN is aboutone-half of its binding energy.The Mg-doped wurtzite GaN samples studied in thiswork were grown by molecular beam epitaxy ~MBE! andmetal-organic chemical vapor deposition ~MOCVD! tech-niques on ~0001!-oriented sapphire substrates with about1-mm-thick AlN buffer layers. The thickness of GaN layerswas about 2 mm. The z axis of this wurtzite structure GaN isperpendicular to the sapphire substrate plane. The Mg con-centrations were NMg>1019 cm23, 531018 cm23 for MBEand MOCVD-grown samples, respectively, as determined bysecondary ion mass spectroscopy ~SIMS! experiments. Thefree hole concentrations were 331017 cm23, 131017 cm23for MBE and MOCVD-grown samples, respectively, whichwere measured at T5300 K and by Hall measurements.Raman scattering experiments were carried out by usingthe second harmonic of a mode-locked continuous-wave~cw! yttrium–aluminum–garnet ~YAG! laser operating at arepetition rate of about 76 MHz. The laser pulse width wasabout 80 ps and photon energy was about 2.34 eV. Thisexcitation laser, which had an average laser power of about200 mW, was focused into the samples with a spot size ofabout 100 mm. Raman scattering measurements were madein a variety of scattering geometries; however, we shall onlypresent results for Z(X ,X)Z¯ , Y (X ,X)Y¯ scattering geom-etries, where X5~100!, Y5~010!, and Z5~001!. Raman scat-tering signal was detected and analyzed by a standard Ramansystem equipped with a charge-couple device ~CCD! as amultichannel detector.Figures 1 and 2 show two typical Raman scattering spec-tra for a MBE-grown, Mg-doped GaN sample and aMOCVD-grown, Mg-doped GaN sample, respectively, takenat T525 K and in Z(X ,X)Z¯ configuration. These two spectraare very similar and are normalized to their respectiveA1~LO! phonon intensities. The structure centered around757 cm21 comes from scattering of light by the Eg phononmode of sapphire;5,6 the shoulder close to 741 cm21 belongsto the A1~LO! phonon mode of GaN. In addition to thesea!Electronic mail: tsen@asu.eduAPPLIED PHYSICS LETTERS VOLUME 76, NUMBER 20 15 MAY 200028890003-6951/2000/76(20)/2889/3/$17.00 © 2000 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Wed, 22 Apr 2015 18:13:24expected phonon modes, there is a well-defined structureshowing up at about 841 cm21 ~>105 meV!. Because of thefollowing reasons this additional structure was attributed toERS from neutral Mg impurities in GaN:~1! It is well-known that photons can excite holes fromground state to excited states in neutral acceptors insemiconductors—the so-called ‘‘electronic Ramanscattering.’’7~2! This structure has been observed for both MBE andMOCVD-grown, Mg-doped GaN samples but not for ei-ther MBE or MOCVD-grown undoped GaN smaples.~3! The intensity of the structure has been found to decreaseas the lattice temperature increases. This is consistentwith our assignment that the structure comes from elec-tronic Raman scattering from neutral Mg impurities andrules out any possible assignments of either overtone orcombination modes, whose signal is expected to increasewith temperature.~4! The observed signal scales quite well with the neutralMg concentration. As indicated by Figs. 1 and 2, theratio of total neutral Mg acceptor concentration is about2 to 1, and the ratio of ERS signal is also about 2 to 1 forMBE and MOCVD-grown samples.For a shallow acceptor in a simple semiconductor suchas Si, in the effective mass approximation, its electronicstates are made up of Bloch waves chosen from near the topof the valence band, as a result, ERS signal from the acceptoris expected to show the same polarization symmetry as themain phonon line; in other words, there exists a simple se-lection rule for ERS signal for a shallow acceptor in a simplesemiconductor. In our current situation—Mg impurity inwurtzite GaN, Mg impurity is believed to have relativelylarge binding energy (EMg’200 meV!,8,9 and therefore it isa relatively deep acceptor; in addition, GaN is a polar, piezo-electric semiconductor with a relatively complicated valenceband structure. Therefore, it is rather difficult to deduce aselection rule for ERS signal. Experimentally, what we havefound is that ERS signal from neutral Mg impurity is presentin all the scattering geometries that we have tested, but withdifferent scattering strengths. For example, Fig. 3 shows aRaman spectrum for the MBE-grown, Mg-doped GaNsample, taken under the same experimental conditions as inFig. 1 but in Y (X ,X)Y¯ scattering configuration. No phononmodes associated with either GaN or sapphire are observedin Y (X ,X)Y¯ configuration in this spectral range, as expected.ERS signal associated with Mg impurities is observed withits intensity comparable to that observed in Z(X ,X)Z¯ con-figuration.We have repeated our Raman measurements with cwlasers having photon energies at 1.92 and 2.42 eV, respec-tively. The results are very similar to what we have presentedusing pulsed laser with photon energy at 2.34 eV and repeti-tion rate at 76 MHz. This observation rules out the possibil-ity of the effect of high laser power density used in the pre-sentation.We note that in addition to the expected Raman signalfrom GaN, sapphire and ERS, we have also observed twoRaman signal very close to 260 and 657 cm21 which hasbeen reported by Harima et al.10 as local modes in Mg-dopedFIG. 1. A Stokes Raman spectrum taken for a MBE-grown, Mg-doped GaNsample. The total Mg concentration is 131019 cm23. The structure around841 cm21 is attributed to the electronic Raman scattering from the neutralMg acceptors in Mg-doped GaN.FIG. 2. A Stokes Raman spectrum taken for a MOCVD-grown, Mg-dopedGaN sample. The total Mg concentration is 531018 cm23. The structurearound 841 cm21 is attributed to the electronic Raman scattering from theneutral Mg acceptors in Mg-doped GaN. Figures 1 and 2 are normalized totheir respective A1~LO! phonon mode intensities, so that they are readilycompared. The slight decrease in the Eg phonon mode intensity as comparedto that in Fig. 1 is due to a slight misorientation in the scattering geometry.FIG. 3. A Stokes Raman spectrum for the MBE-grown, Mg-doped GaNsample, taken under the same experimental conditions as in Fig. 1 but atscattering configuration Y (X ,X)Y¯ . The structure around 841 cm21 is due toelectronic Raman scattering from neutral Mg acceptors.2890 Appl. Phys. Lett., Vol. 76, No. 20, 15 May 2000 Tsen et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Wed, 22 Apr 2015 18:13:24GaN. The intensity of these two modes has been found to bevery weak in our experiments—at least one order of magni-tude smaller than that of the optical phonon modes in GaN.We believe that the reason why ERS signal from Mg-dopedGaN was not observed by previous researchers is most likelydue to the quality of Mg-doped GaN samples.The contribution of ERS signal in semiconductors comesprimarily from the transition of hole from the ground ~1s! tofirst excited ~2s! states.7 The reported binding energy of Mgimpurity in wurtzite GaN ranges from 150 to 255 meV.8,9 Ifwe were to use the average value ~>190 meV! for the bind-ing energy, then our experimental results suggest that the1s–2s transition in the Mg impurity is about one-half of itsbinding energy.In conclusion, we have performed electronic Ramanscattering experiments on both MBE and MOCVD-grownMg-doped wurtzite GaN samples. Our experimental resultsdemonstrate that the energy difference between the groundand first excited states of the Mg impurities in wurtzite GaNis about one-half of its binding energy.This work was supported by the National Science Foun-dation under Grant No. DMR-9301100.1 J. I. Pankove and T. D. Moustakas, in Gallium Nitride, in series of Semi-conductors and Semimetals, edited by R. K. Willardson and E. R. Weber~Academic, New York, 1997!, Vol. 50.2 D. K. Ferry, Phys. Rev. B 12, 2361 ~1975!.3 S. Nakamura, T. Mukai, M. Senoh, and N. Iwasa, Jpn. J. Appl. Phys., Part2 31, L139 ~1992!.4 H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, Jpn. J. Appl. Phys.,Part 2 28, L2112 ~1989!.5 T. Azuhata, T. Sota, K. Suzuki, and S. Nakamura, J. Phys.: Condens.Matter 7, L129 ~1995!.6 S. J. Shieh, K. T. Tsen, D. K. Ferry, A. Botchkarev, B. Sverdlov, A.Savador, and H. Morkoc, Appl. Phys. Lett. 67, 1757 ~1995!.7 M. V. Klein, in Light Scattering in Solids I, edited by M. Cardona and G.Guntherodt, Topics in Appl. Phys. ~Springer, New York, 1983!, Vol. 8.8 S. Fisher, C. Wetzel, E. E. Haller, and B. K. Meyer, Appl. Phys. Lett. 67,1298 ~1995!.9 T. Tanaka, A. Watanabe, H. Amano, Y. Kobayashi, I. Akasaki, S.Yamazaki, and M. Koike, Appl. Phys. Lett. 65, 593 ~1994!.10 H. Harima, T. Inoue, S. Nakashima, M. Ishida, and M. Taneya, Appl.Phys. Lett. 75, 1383 ~1999!.2891Appl. Phys. Lett., Vol. 76, No. 20, 15 May 2000 Tsen et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Wed, 22 Apr 2015 18:13:24

Observation of electronic Raman scattering from Mg-doped wurtzite GaN

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Observation of electronic Raman scattering from Mg-doped wurtzite GaN

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