Optical properties of Te doped GaP single crystals

PHY$1CA ELSEVIER

Physica A 233 (1996) 145-152

Optical properties of Te doped GaP single crystals S.B. Y o u s s e f * , M . M . E L - N a h a s s Department of Physics, Faculty qf" Education, Ain-Shams University, Cairo, Eyypt Received 18 January 1996

Abstract

The optical properties of Te doped GaP single crystals were investigated in the spectral range of 0.20-3.0 ~tm. It was found that the spectral distributions of R, n, er and a,. reflect sharp structure due to valence to conduction-band transitions (E,, E0, El and E2) having the energies 2.16,2.75,3.65 and 5.39eV, respectively. It was found that Te doped GaP single crystals exhibit indirect allowed optical transition associated with three phonons of energies 0.047, 0.038 and 0.050 eV. Two scattering mechanisms were detected in GaP:Te crystals, the first one is operating in the wavelength region 0.8-1.3 ~tm and is due to acoustical vibration, while the second one is operating in the wavelength region 1.3 2.85 tam and is due to impurity (Te) ions. Kewvords: Optical properties; Semiconductors; Band gaps

1. Introduction

Gallium phosphide is a very interesting material for scientific and technical investigations. Because of its wide energy gap, higher ionization energy of local impurity levels, the integration of impurity centers with themselves and with charge carriers, GaP differs from other semiconductors [1]. Experimental data concerning the optical constants were determined for pure GaP compound [ 2-4]. However, deep centers in GaP have not attracted much attention, probably because of its good optical properties [5]. Only the isoelectronic traps (N and O) have been studied in detail because of their role in luminescence [6]. The aim of the present work is to determine the optical constants of GaP singlecrystals doped with tellurium viz. the real and imaginary refractive index (n and K) and the absorption coefficient ~. Also the real and imaginary parts of the dielectric constant (~:r, F-i) and those of the optical conductivities (ar, oi) a r e given in the photon energy range 0.40-6.2 eV. * Corresponding author. 0378-4371/96/$15.00 Copyright (~) 1996 Elsevier Science B.V. All rights reserved

PHS0378-4371(96)00195-1

S.B. Youssef M.M. EL-NahasslPhysica A 233 (1996) 145 152

146

2. Experimental work Gallium phosphide single crystals were grown by Czochralski's method [7] and doped with tellurium in the same manner as reported by Mounir et al. [8]. The obtained samples have n-type conductivity with a carrier concentration 1.4 × 1018 cm -3 at room temperature ( ~ 300 K) as calculated from Hall effect measurements. The structural properties of GaP:Te crystals were carried out using X-ray diffractometer, (Philips PW 1410) provided with Co-Ks radiation, operating at 60kV. The transmittance (T) of crystals, at normal incidence, as well as the spectral reflectance (R), at an incident angle of 5 °, were measured at room temperature in the spectral range 0.20-3.0 lam using UV-3101 PC double beam spectrophotometer.

3. Results and discussion 3.1. Structural properties The analysis of X-ray diffraction pattem carried out for GaP:Te; (Fig. that it has zinc-blend structure. The lattice constant (a) as calculated was 5.435 A, which differs slightly from that reported by Morgan [9] for pure (a ----5.415 A). This difference can be attributed to the presence of Te as in the crystal.

1) indicates found to be GaP crystal an impurity

3.2. Valence to conduction-band transitions Fig. 2 illustrates the spectral distribution of both T and R for GaP : Te single crystal (0.0425 mm thick), in the spectral range 0.20-3.0 Ixm. As shown in Fig. 2, the spectral

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26, Fig. 1. The X-ray diffraction pattern of GaP:Te single crystal.

I0

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147

Y o u s s e f M . M . E L - N a h a s s / P h y s i c a A 233 (1996) 145-152

Table 1 Values of E,, E0, El and E2 obtained in the present work as well as theoretical (Th.) and experimenlal (Exp.) results for pure GaP single crystal at 290K

-X~"

Transition [141 Identification [14]

F~-X6C

F sv_ F 6c

l.~_C C

A 4,5-t. A 6c

A 6;"- A 6c

X 6,7 r

Eu

Eo

E0 + A

El

Et + A

E2

Energy values (eV)

2.25(Exp.) II 2.21 (Exp.) 12 2.239(Exp.) 13 2.26(Th.) 14

2.7(Th.) l° 3.59(Th.) l° 2.8(Exp.) 15 3.69(Exp.) 15 2.75(Exp.) 16 2.845(Exp.) 16 3.662(Exp.) 16 2.74(Th.) 14 2.84(Th.) ~4 Y70(Th.) 14

4.92(Th.) t° 5.3(Exp. ) t2 5.27(Exp.) I¢' 5.0(Th. ) 14

2.160

2.755

5.391

Present data (eV)

3.647

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Fig. 2. The spectral distribution of both the transmittance (T) and the reflectance (R) for GaP:Te crystal (0.0425 mm thick) at room temperature in the spectral range 0.2-3.0 lam.

distribution of the reflectance R(2) is characterized by sharp structure associated with valence to conduction-band transitions Eo, E0, E1 and E2. The spectral distribution of n { = (1 + X/R)/(1 - x/R)}, shown in Fig. 3, reflects also the same feature. Table 1 summarizes the present data concerning the values of Eo, E0, El and E2. For comparison, the previous theoretical (Th.) and experimental (Exp.) results [10-16] for pure GaP single crystal at 290K are given in the same table. As obvious from Table 1, the present data are in good agreement with the previous theoretical and experimental results. However, the value of E, in the present work is

S.1~ Youssef M.M. EL-Nahass/Physica A 233 (1996) 145-152

148

O

E2

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1.0

1.5

2.0

2.5

3.0

Fig. 3. The spectral dependence of the real refractive index (n) for GaP:Te crystal.

slightly lower than that predicted before for pure GaP crystals [11-14]. This can be attributed to the effect of electron-impurity and electron-electron interactions. Parmenter [17] found that for lower impurity concentration the energy gap decreases in a manner similar to that due to lattice vibration, whereas at higher impurity concentration, a tail of states extends to lower energies. Other workers have extended the calculation to include electron-electron interaction. Wolf [ 18, 19] found that the states near the band edge shift so as to decrease the energy gap with increasing doping, but he did not find a band tailing. Huang et al. [20] concluded that as the free carrier concentration changes, not only the slope of the exponential absorption curve changes, but also the energy-band gap shifts. The red-shift in the absorption was caused by the band-tailing effect and the band-gap shrink effect.

3.3. The absorption coefficient (or) Fig. 4(a) represents the spectral distribution of (cQ calculated using the following formula [21 ]: = 1/t ln{(1 -

R)2/T}.

Here, t is the crystal thickness in cm, T and R are the transmittance and reflectance of the sample at the same wavelength. The trend of the spectral distribution of ct is similar to that of the heavily doped material [21] and is characterized by the existence of a 3 ~tm peak which was observed by Spitzer et al. [22]. Paul [23] suggested that this peak arose from electron transitions between two sets of (l 00) minima Al and A2, split from each other. Measurements of Allen and Hodby [24] on GaPxAst_x alloys support this hypothesis.

S.B. Youssef M.M. EL-Nahass/Physica A 233 (1996) 145 152

149

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I 2.5

(a) I 3.0

Fig. 4. The absorption coefficient (~) as a function o f the w a v e l e n g t h (2) for degenerate n-type G a P : T e (1.4 × 1018 c m - 3 ) crystal: (a) linear scale; and (b) logarithm scale.

The few weak absorption peaks superimposed on the 3 ~m peak can be attributed to the Te absorption at low photon energy. To know the scattering mechanism that may exist in GaP:Te crystal in the spectral range of 0.20-3.0 ~tm, log ~ is plotted as a function of log 2, as shown in Fig. 4(b). This relation yields two distinct linear parts. The first part exists in the wavelength range 0.8 1.3 p.m, where ~ is found to be proportional to 21"6. The second part exists in the wavelength range 1.3 2.85 ~tm, where ~ is found to be proportional to )#.12. The obtained values (1.6 and 4.12) indicate that the scattering occur by acoustical vibrations (1.66) [25] and by impurity ions (4.21) [25] in the wavelength ranges (0.8 1.3~tm) and (1.3-2.85 ~tm), respectively. Fig. 5(a) illustrates (~hy) 1/2 as a function of the photon energy (hy). As observed, there are several straight sections cutting the (hT) axis at (~hT) 1/2 - 0, separated by phonon energies 0.047,0.038, and 0.050eV, indicating that allowed indirect optical transitions, with a tail absorption, associated with phonons are existing. The indirect energy gap of GaP:Te single crystal was found to be 2.165eV. This value is in good agreement with that estimated from the spectral distribution of both R and n. Fig. 5(b) represents logs versus l o g ( h 7 - E~); this plot yields a straight line of slope 2 confirming the existence of indirect allowed optical transitions in GaP:Te crystal.

S.B. Youssef, M.M. EL-Nahass/Physica A 233 (1996) 145-152

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3.4. The dielectric constant o f GaP.'Te crystals Since at any given wavelength the absorption coefficient ~ and the refractive index n have been already determined, thus the absorption index K (-- c(2/4n) and the dielectric constant e(hT) ( = ~r(hT) q- iei(hT)) can also be calculated. Here er ( = n 2 - K 2) is the real part o f the dielectric constant and 8i ( = 2nK) is the imaginary part. The spectral distribution o f K is similar to that o f ~. The spectral distribution o f both e~ and Ei are illustrated in Fig. 6. As shown er(h])) is characterized by the existence of the sharp structure associated with the valence to conduction-band transitions (Eg, E0, El and E2) having the same values previously mentioned (Section 3.2). It is also clear that £i(h7) defines the same value o f E q at 2.165 eV.

3.5. Optical conductivity Fig. 6 also shows, the spectral distributions o f both a r and rri as a function o f photon energy (h?), where O"r is the real part o f the optical conductivity ( = (1 - er)co/4n) and ai is the imaginary part ( = ~io)/4~}. Here 09 = 2n7. As observed, o"r reflects the sharp structure associated to valence to conduction-band transitions (E.q, E0, El and E2) with

51

S.B. Youssef M.M. EL-Nahass/Physica A 233 (1996) 145-152

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Fig. 6. The spectral distributions of the real and imaginary parts of the dielectric constant, (c,.,ci) and the optical conductivity (O-r,O'i) in the photon energy range 0.4 6.2eV: (o) (~:,.and el) and (.) (rrr and o-i). the same energy values as determined before and ai confirms the previously obtained value o f E~/. 4. Conclusions (1) Degenerate n-type (1.4 x 10 j8 cm - 3 ) G a P : T e crystals exhibit indirect allowed optical transitions associated with three phonons o f energies 0.047, 0.038 and 0.050 eV.

152

S.B. Youssef M.M. EL-Nahass/Physica A 233 (1996) 145-152

(2) Doping GaP crystals with Te (n = 1.4 x 1018cm -3) decreases the energy gap from 2.24eV (mean value) for pure Gap to 2.165 eV. (3) Two scattering mechanisms are operating, due to acoustical vibrations and impurity (Te) ions in the wavelength region (0.8-1.3 p.m) and (1.3-2.85 gm), respectively. (4) The spectral distribution of R,n, er and ar reflect sharp structure due to valence to conduction-band transitions (E~, E0, E1 and E2), whereas ei and ai confirm the E~ value.

Acknowledgements The authors are grateful to Prof. Dr. A.A. E1-Shazly, Ain-Shams University, for valuable discussions.

References [1] Ya.E. Pokrovskogo, Sovremennye problemy fiziki - sbornik statey (Nauka, Moscow, 1972) ch. 3 (in Russion). [2] B.O. Serphin and H.E. Bennett, Semiconductors and Semimetals, eds. R.K. Willardson and A.C. Bear (Academic, New York, 1967) Vol. 3. [3] D.E. Aspnes and A.A. Studna, Phys. Rev. B 27 (1983) 985. [4] K. Strossner, S. Ves and M. Cardna, Phys. Rev. B 32 (1985) 6614. [5] M.A. Zaidi, M. Zazoui and J.C. Bourgoin, J. Appl. Phys. 74 (1993). [6] P.J. Dean, Deep Centers in Semiconductors, ed. S.T. Pantelides (Gordon and Breach, New York, 1986) ch. 4. [7] V.D. Kuznetsov, Crystals and Crystallization (Nauka, Moscow, 1954) (in Russion). [8] M. Mounir, I.K. Lazaeva and A.E. Yunivich, Egypt. J. Phys. 12 (1981) 57. [9] T.N. Morgan, Phys. Rev. Lett. 21 (1968) 819. [10] F.H. Pollack, C.W. Higginbottam and M. Cardona, J. Phys. Soc. Japan, Suppl. 21 (1966) 20. [l 1] V.K. Subashiev and G.A. Chalikyan, Conf. Semiconductor Phys., Moscow, Vol. 1 (1968). [12] V.K. Subashiev and G.A. Chalikyan, Phys. State. Sol. 13 (1966) K91. [13] P.J. Dean, G.K. Aminsky and R.B. Zetterstrom, J. Appl. Phys. 38 (1967) 3551. [14] Sadao Adachi, J. Appl. Phys. 66 (1989) 6030. [15] M.L. Belle, Zh.1. Alferov, V.S. Grigorev, L.V. Kradinova and B.D. Prochukhan, F.T.T. 8 (1966) 2623. [16] A.G. Thompos, M. Cardona, K.L. Shaklee and J.C. Wool, Phys. Rev. 146 (1966) 601. [17] R.H. Parmenter, Phys. Rev. 100 (1955) 573. [18] P.A. Wolff, Proc. Internat. Conf. Phys. Semicond. Exeter., 1962 (Inst. of Phys. and Phys. Soc. London, 1962). [19] P.A. Wolff, Phys. Rev. 126 (1962) 405. [20] H.C. Huang, S. Yee and M. Soma, J. Appl. Phys. 67 (1990) 1497. [21] Yu.I. Ukhanov, Optical Properties of Semiconductors (Nauka, Moscow, 1977). [22] W.G. Spitzer, M. Gershenzon, C.J. Froschand and D.F. Gibbs, J. Phys. Chem. Solids 11 (1959) 339. [23] W. Paul, J. Appl. Phys. 32 (1961) 2082. [24] J.W. Allen and J.W. Hodby, Proc. Phys. Soc. London 82 (1963) 315. [25] P.S. Kireev, Semiconductor Physics (Mir Publisher, Moscow, 1974) ch. 8.