Reflectivity of Gallium Phosphide in VUV Spectral Region

REFLECTIVITY OF GALLIUM PHOSPHIDE IN VUV SPECTRAL REGION Grigoryan N.E., Eritsyan G.N., Melkonyan R.A., Martirosyan H.M., Harutunyan V.V., Babayan A.K. Yerevan Physical Institute Markaryan St. 2, 375036 Yerevan, USSR Investigation of semiconductor optical properties provide valuable information about the energy structure of semiconductors. In the vacuum ultraviolet spectral region where the absorption coefficient is of great value and its evaluation presents considerable difficulties the measuring of the reflectivity of a clean surface of a semiconductor acquires great significance for investigation of the crystal band structure energy. In the present paper we report the results of GaP crystal surface reflectiVity measurement at room temperature and with the use of synchrotron radiation. Within 4 to 10 eV spectral region the reflectiVity measurements were carried out with the help of the vacuum monochromator VMR-2 that had been specially altered for the work on the spectroscopic synchrotron radiation channel of Yerevan Physics Institute Accelerator. The specimen surfaces were mechanically polished, residual mechanical stress and oxides were removed chemically. After etching the samples were exposed to air for a minimum period of time reqUired for a sample mounting into a camera. The GaP surface reflectiVity measurements results are given in fig. 1, where the results obtained by other authors are also brought 0, 2.7 (curves', 3). The comparison with them shows that the principle features of reflectivity spectra for all these three curves are maintained with the exception that unlike the works [1, 2, 3] in the present work we have succeeded in investigations of the GaP energy band fine structure details using the advantages of a synchrotron radiation (continuum spectrum, high intensity, etc.). These are transitions at '};"IAI';. 5,9 eV and 11.w= 7,0 eVand also the fine structure of the E; peak (here we follow Cardona's notations [,] ). We assume that the pecularity of the spectrum at -uY = 5,9 eV corresponds to £ a. + A ~ transition at point X in the Brillouin zone in the (100) direction, where Ll~ - value of X,C state orbital splitting, making 0,4 eV

n

294

for GaP [4] . OUr measured value for L1.2.~0,45 eV slightly differs from the value for x~-x~ gap. This is most likely due to overlapping of the x; state spin-orbit-splitting on the x~ state orbiV' c tal one. However we failed to resolve the doublet of the E2(X 5-X 1) peak that is probably attributed to very amall values of the ~~ spin-orbit-split of X state. The experiment optical scheme resolution makes 0,05 eV. Thus the pecularity of the reflectivity -zr c.. spectrum at *n~= 5,9 eV corresponds to overlapping of the X - X 3 5 'V" c.. and x,' - X3 transitions. Such an assumption enables us to evaluate the X5v state splitting as a result of spin-orbital interactions Lj' L.. 0,1 eV• .2.. In the photon energy region at 17 -w = 1, eV the reflec tivi ty spectral dependence reveals fine structure observed in all GaP crystals under investigation. The carrier concentration in samples was less than 3.10 11 sm- 3 thereby impurity defects were negligible. I The reflectivity E1 peak in this spectral region corresponds to interband transitions at point L3 in Brillouin zone. In reflectivity spectra of some A3B5 crystals (GsAs, InSb) the E; maximum reveals ~: splitting due to L3 band spin-orbit-split [1] • The theoretical value of for GaP is 0, 01 - O.08 eV [1, 5] , that is in good agreement with the known experimental of spin-orbit-splitting ~o = 127 eV at point in Brillouin zone as 44'~iL1o In paper [3] the experimental value for ~: is 0,18 eV which is ambiguously ascribed to spin-orbit-splitting at point L. Our experimental results show that the reflectivity spectrum in the region of transitions has a complex structure. It comprises three outlines maxima e 1, e 2 and E{ • We are not declined to consider our obtained results as a consequence of or L~ band spin-orbit-split, because the energy distance bet~een maxima is not correlated with the theoretical values of .:1 1 • Our detected structures e1 and e 2 are probably due to transitions between surface exciton states of the crystal. Similar surface excitons are observed in GaAs and InAs crystals in the region of transitions from the d-electron shell of gallium and indium to c. c, L1 and X1 states. For example, surface exciton binding energy for In4d-X~ transition forms 0,5 eV [6] . For more defined identification of the obtained results the investigations of a transition

5

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L; - L)

r

L;

295

struoture in Ga AsxP1_x

system are required.

REFERENCES M.Cardona: Optioal Absorption above Fundamental Edge, "Semioonduotors and Semimetals", 3, 125, Aoademio Press, (1967). Wooley J.C., Thompson A.G. and Rubenstein M.: Phys. Lett. 12 670, (1965). Sobolev V.V., Sirbue N.N.: Soviet Phys.-Solid State, ~, 2537 (1964). Brailovskii E.U., Demidenko Z.A., Grigoryan N.E., Eritsyan G.N.: Soviet Journal of Contemporary Physics (Izv. AN Arm. SSR, fisika), ~, 19 (1983). Braunstein R., Kane E.O.: Journ. Phys. Chem. Sol., ~, 1423 (1962). Asphes D.E., Cardona M., Saile V., Skibowski M. and Sprussel G.: DESY SR-79/06, March 1979.

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=i 60

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...> 40

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....o

-

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.' r·

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o~-----'----------'-----'-------"----'

4

Fig. 1.

5

6 7 photon energy (eV)

8

Room-temperature reflectivity of GaP. 1 - The results of present report. 2, 3 - The results of Ref. 1 and 2, respectively.