Effective mass calculation for InSe, InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples using the external electric field shifting

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1 Indian Journal of Pure & Applied Physics Vol. 42, March 2004, pp Effective mass calculation for InSe, InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples using the external electric field shifting Aytunç ATEŞ Department of Physics, Faculty of Art and Sciences, Atatürk University, 25240, Erzurum-Turkey Received 30 June 2003; revised 29 September 2003; accepted 26 December 2003 Undoped InSe and Ho doped InSe single crystals were grown by Bridgman-Stockberger method. The InSe crystals both undoped and doped with different ratios of (InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 ) had no cracks or voids on the surface. No polishing or cleaning treatments were carried out on the cleaved faces of these samples because of the natural mirror-like cleavage faces. The absorption measurements were carried out in the temperature range K and the external electric field effect on the absorption measurements was investigated. The absorption edge shifted towards the longer wavelengths under an electric field as 6 kv/cm. Using the electric field shits, effective mass values are calculated for all the samples. [Keywords: InSe single crystal, absorption, exciton binding energy, Holmium doping] IPC Code: H 01L 27/04 1 Introduction Layer structures that contain highly anisotropic physical properties are formed by atoms strongly bounded by covalent or ionic forces within the layer while individual layers are held together by relatively a much weaker force which is frequently referred to as van der Waals interaction 1. The III-VI compounds such as gallium selenide, gallium sulfide or indium selenide belong to this class of layered structures, and the interest in such compounds originates from their potential usefulness in semiconducting devices such as capacitors, micro batteries and solar energy converter 2,3. On the basis of InSe and GaSe, the metalsemiconductor structure 5,6, the metal-insulatorsemiconductor structure, homo- and hetero-junctions have been constructed 7-9. Undoped indium selenide (InSe) is naturally grown as n-type semiconductor and each covalently bound layer consists of four monoatomic sheets in the order of In-Se-Se-In (Ref. 4). The III-VI layered compounds have attracted attention as new materials for integrated electronics. Good quality single crystals have generally been obtained from non-stoichiometric melts using the Bridgman technique A direct energy gap of 1.3 ev exists at 300 K with an indirect energy gap of about 50 mev below the energy of the direct band gap 11. Mooser and Schlüter 11 reported an exciton line near the fundamental band edge absorption of GaSe 12. Abdullayeva et al. 12 reported the electroabsorption spectra of GaSe x S 1-x in a fundamental absorption edge and referred to the existence of various phases of layered structures 13. The absorption spectra of InSe crystal have been investigated under an electric field as a function of temperature. The absorption edge shifts towards longer wavelengths by applied electric field. The shifts of about 14 nm in the absorption edge corresponding to 21 mev at 10 K and 8.3 nm in the absorption edge corresponding to 11 mev at room temperature are observed for applied electric field of 6 kv/cm. Such shift can be explained on the basis of the Franz-Keldysh effect or thermal heating of the sample under the electric field. On the other hand, applied electric field caused a decrease of intensity in the absorption spectres 14. The direct energy gap (1.3 ev) as well as the other optical and transport properties of this compound are appropriate for photovoltaic solar energy conversion 15,16. The InSe is a promising semiconductor in the preparation of polymer Schottky diodes, capacitors or microbatteries 17. The optical properties near the band edges of pure InSe have been investigated and compared with the calculated band structure 18.

2 206 INDIAN J PURE & APPL PHYS, VOL 42, MARCH 2004 Far-infrared absorption of InSe has been studied by several workers Blenkii et al. 20, Gasanly et al. 21, Faradev et al. 22 and Riede et al. 23 reported the lattice absorption spectrum of InSe in the region from 20 to 100 μm. Measurements of absorbance and transmittance in In x Se 1-x thin film in the spectral range 1.1 to 3.9 ev at room temperature were carried out by Abd El-Moiz 24. On the other hand, there has been an increasing interest to study the influence of doping rare-earth (RE) elements to layered crystals. In this paper we calculated the effective mass for InSe and Ho doped InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples. For the optoelectronic applications, it is of great importance that the material should protect its excitonic behaviour up to the room temperature. The Ho doping effect on the effective mass has been investigated using the absorption edge shifting under an external electric field. 2 Experimental Methods The basic elements were to produce InSe single crystals due to purity considerations. These elements were weighed in a stoichiometric ratio accurate to 0.1 mg. The mass of elements was standardised to 50 g. The basic criteria for this choice was to have sufficient yield to justify the cost of one run. This stoichiometric ratio necessary to produce 50 g InSe was calculated using the following relationships: M In =(M Se /A Se )A In with total mass, M(In)+ M(Se)=50 g, (1) (2) where, M and A are total and atomic masses, respectively. The selenium element was weighed as the first element because of the hardness of the material and the difficulty in cutting it to desired accuracy. The InSe single crystals were grown in our crystal laboratories using Bridgman-Stocberger method. In the Bridgman method, the temperature gradient is constant and the furnace is fixed but the quatrz ampoule is moving along the furnace. On the other hand, in the Stockberger method, ampoule and furnace are fixed but the temperature gradient is varied in desired steps. The mass ratios of , , , : The Ho added in order to obtain different Ho concentration. The melting point of the InSe compound, 660±5 C was determined from the phase diagram 6. The samples were cleaved from ingots having thicknesses of about 50 μm parallel to the c-axis. It is found that all samples exhibited n-type conductivity at room temperature using hot probe and Hall effect techniques. The absorption measurements were carried out in the temperature range K in steps of 10 K for E=0 and F=6 kv/cm. The optical measurements were made as a function of temperature. For measurements, a Perkin-Elmer Lambda 2S spectrometer is used with wavelength resolution better than 0.3 nm. 3 Basic Equations The conversation of externally applied voltage in to electric field F inside the crystal has been done by fitting the energy shift δe of the absorption edge 26 and is given by Eq. 3: * 3 3 δ E = ( m )( ehf) (3) 2 where m * is effective mass, e electron charge and h is Planck s constant. 4 Results and Discussion Figure 1 shows the exciton absorption spectra of InSe, InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 at 10 K. On Ho doping, the InSe caused a narrowing in the band gap. In general, we observe that with lower Ho concentrations the intensity of the absorption peak is also reduced. High Ho concentrations cause an increase in the intensity of the Fig. 1 Absorption spectra for InSe, InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples at 10 K.

3 ATES: EFFECTIVE MASS CALCULATION USING EXTERNAL ELECTRIC FIELD SHIFTING 207 Fig. 2 The shifting (δe ) versus wavelength for InSe, InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples at 10 K under an electric field absorption peak. Increasing exciton peak intensity as the Ho concentration increased may be due to the increase in the number of excitons, because certain impurity concentrations are expected to be high in higher concentrations of Ho. These impurities are optically active in the band gap of InSe can be rendered inactive by Ho doping. These results are in agreement with the literature 5,14,27-31.

4 208 INDIAN J PURE & APPL PHYS, VOL 42, MARCH 2004 Fig. 3 The shifting (δe) versus wavelength for InSe, InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples at 300 K under an electric field The absorption spectra of InSe InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples at 10 K have been investigated under an electric field. Figures 2 and 3 show the absorption spectra of InSe, InSe:Ho , InSe:Ho 0.025, InSe:Ho and InSe:Ho 0.05 samples at 10 K and 300 K under an electric field. As seen in Figs 1 and 2, the absorption edge shifts towards longer wavelengths by applied

5 ATES: EFFECTIVE MASS CALCULATION USING EXTERNAL ELECTRIC FIELD SHIFTING 209 Table 1 The shiftings in the absorption edge are (in nanometer (nm) and milli electron volt (mev)) for all Ho doped InSe samples and calculated m * /m 0 ratios. Shifting (10 K) Shifting (300 K) m * /m 0 Sample (nm) (mev) (nm) (mev) 10 K 300 K InSe 11,19 16,1 8,35 11,3 0,022 0,064 InSe:Ho 0,0025 0,62 0,9 0,46 0,5 1,271x10 2 7, InSe:Ho 0,005 17,16 23,5 13,78 16,7 0,0074 0,019 InSe:Ho 0,025 0,31 0,4 0,78 1 6,7 10,01 InSe:Ho 0,05 1,66 2,4 1,71 2,1 1, ,7 electric field. The shifting in the absorption edges is given in Table 1 as wavelength and electron volt. The shift of absorption edge can be explained on the basis of the Franz-Keldysh effect or thermal heating of the sample under the electric field. It is evident from Figs 2 and 3 that, the shifting in InSe:Ho sample is more than the other samples and the shifting in InSe:Ho sample is smaller than that in the other samples. The shifting in InSe:Ho sample is about 27 times larger than in InSe:Ho sample at 10 K. Effective masses are calculated as m //c = m o and m c =0.401m o for n=0 mini band and m //c = m o and m c = m o for n=1 mini band by Gashimzade et al. 32. It is found that Ho is an optically active element and Ho doping caused a shifting towards the long wavelength. The calculated effective mass values are in agreement with the literature. For all of the samples, the shifting values at 10 K are larger than the values at 300 K. References 1 Liang W Y, In Intercalation in Layered Materials, Vol. 148 of NATO Advanced Study Institute, Series B: Physics, edited by M S Dresselhaus (Plenum, New York, (1986) pp Wilson A & Yoffe A D, Adv Phys, 18 (1969) Julien C & Samaras I, Solide State Ionics, 27 (1988) De C, Blasi, Micocci G, Rizzo A & Tepore A, J Cryst Growth, 57 (1982) Giulio M D, Micocci G, Rizzo A, Tepero A, Appl Phys, 54 (1983) Dzhafarov T D, Mekthiev A, Sadigov M S, Phys Status Solidi (A), 69 (1982) Shigetomi S, İkari T, Koga Y, Jpn J Appl Phys, 27 (1988) Katerinchuk V N, Kovalyuk M Z, Fiz Tekh Poluprovod, 25 (1991) Dawar A L, Joshi J C, Mater J Science, 19 (1984) Imai K, Suzuki K, Haga T, Hasegawa Y & Abe Y J, Cryst Growth, 54 (1981) Mooser E & Schlüter M, Nuovo Cimento B, 18 (1973) Abdulleyeva S G, Gadjiyev V A, Kerimova T G & Yu Salaev E, Nuovo Cimento B, 38 (1977) Doboz J Y, Binet F, Rosencher E & Harle V, Mater Sci Eng B, 43 (1997) Ates A, Gurbulak B, Yıldırım M, & Dogan S, Physica E, 16 (2003) Segure A, Guesdon J P, Besson J M & Chevy A, J Appl Phys, 54 (1983) Di Giulio M, Micocci G, Rizzo A, Tepore A, J Appl Phys, 54 (1986) Wittingon M S, Prog Solid State Chem, 12 (1978) Piacentini M, Doni E, Girlanda R, Grasso V & Balzarotti A, Nuovo Cimento B, 54 (1979) Marı B, Segura A & Chevy A, Phys Stat Sol (B), 130 (1985) Blenkii G, Aliev L N, K H Nani R & Salaev E Y, Phys Stat Sol (B), 82 (1977) Gasanly N M, Yavalov B M, Tagirov V I & Vinogradov E A Phys Stat Sol (B), 89 (1978) Faradev J E, Gasanly N M, Marvin Y B M & Melnik N N, Phys Stat Sol (B), 85 (1978) Riede V, Neuman H, Levy F & Sobotta H, Phys Stat Sol (B), 109 (1982) Abd A B El-Moiz, Physica, 191B (1993) Moss T S, Optical prosess in Semiconductors (Butterworts, London), 1959, p Pakove J I, Optical processes in Semiconductors, (Dover, New York), (1975).

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