Influence of etching on the surface properties of Cd0.99Mn0.01Te gallium doped

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Influence of etching on the surface properties of Cd0.99Mno.o,Te gallium doped B Bieg, S. KuimiliskP and J. Szatkowski *, Maritime Academy of Szczecin, Waly Chrobrego l/2, 70-500 Szczecin, Poland, *Institute of Physics, Technical University of Wroclaw, Wyb Wyspiariskiego 27, 50-370 Wrocfaw, Poland

The influence of surface treatment on the surface properties of Cd,,,Mn,,,Te gallium doped was studied. Using surface photovoltage spectroscopy (SPS) we have investigated three types of the monocrystalsurfaces : polished with ‘Gamal’, etched in 5% BrJmethanol and etched in KOH/H,O. Changes in the electronic structure of Cd,,Mn,,,Te: Ga surfaces were observed. The temperature dependence of the band gap and surface states have been determined for each sample.

1. Introduction

Among the II-VI type semiconductors the CdTe and CdMnTe mixed crystals are those of which technology of production and doping is the best known. Many papers concerning the optical, electrical and magnetic properties of the intrinsic and doped crystals appeared. Up till now, despite the great possibility of practical applications, there exist only several papers referring to the influence of the surface preparation on their electronooptical propertieslA. In this paper are presented the results of surface photovoltage spectroscopy (SPS) measurements of the CdoPPMnO.,,Te gallium doped crystals. The measurements were carried out for surfaces prepared in different ways.

However, as a result of etching of sample A, a strongly defected layer was removed. The SPS measurements were carried out with a modified Kelvin method in the temperature range between room and liquid nitrogen temperature at a pressure of 10u4 Pa. The sample was illuminated with a constant or modulated illumination using a halogen lamp in the wavelength range between 0.6 and 1.5 Aim. Modulation frequency of 20 Hz was used. Details of the measuring set are described in ref 5. 3. Results and discussion The SPS curves for samples A, B and C obtained at constant and modulated illumination, are presented in Figures I, 2 and 3.

2. Experimental The investigated CdO,,MnO.,,Te gallium doped single crystals were grown from the melt with a Bridgman method by W. Giriat in IVIC, Caracas, Venezuela. The Hall constant and capacityvoltage (C-V) measurements performed for the different samples have shown that the gallium concentration amounts to 8 x lOI cmw3 and in the whole volume of the sample is almost homogeneous. Freezing of the carriers was not observed until liquid nitrogen temperature. The crystals were mechanically cut into slices of 0.5 mm thick along the natural cleavage plane (110). The sample was ground, polished with ‘Gamal’ powder (Gamma alumina A446) and rinsed in methanol. The ohmic contact was made by soldering of indium. The SPS measurements were carried out for the sample prepared in this way (sample A). Then, the same sample was etched in 5% Br, solution in methanol for 5 min. After rinsing the sample in methanol a series of measurements were performed (sample B). Finally the sample was etched in aqueous KOH solution and rinsed in HCl solution and methanol (sample C). According to refs 14 on the surface of samples A and C the composition of Cd, Mn and Te is almost stoichiometric and the surface of sample B is richer in tellurium.

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1.6

1.5

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1.4 c

13

1.2

11

10

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Figure 1. SPS curves for the polished sample A at constant (full line) and modulated (dotted line) illumination at 193 K and the temperature dependence of the electron transitions (inset). 481

B Bieg et al : Surface treatment 1.5

1.5 B

effects on Cd,,,Mn,.,,Te 14

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- etched BrZimerhanol

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1.34 11.255

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Figure 4. Energetic

I ;

scheme of the surface layer of the polished

sample A

at 193 K.

0.8

09

1.0

1Kl

1.2

Xluml Figure 2. SPS curves for sample B etched in BrZin methanol solution at constant illumination at 185 K and the temperature dependence of the band gap (inset).

From the energy position of the dvJdl, the energy values of the electron transition were determined. On the SPS curve for sample A (Figure 1) three electron transitions with energy less than the energy gap value are present. A strong increase in the photovoltage at E4 = 1.225 eV, observed at constant illumination, is connected with the electron transition from the surface states to the conduction band. These transitions do not appear with modulated illumination. This leads to the conclusion that the E4 level corresponds to the slow surface states. The electron transition with energy E3 = 1.34 eV, observed on the SPS and PC curves, probably corresponds to the wide acceptor band lying about 0.2 eV above the top of the valence band’ well known from the literature. For the light energy close to E2 = 1.45 eV quenching of the photovoltage was observed. This level corresponds to the electron transition from the valence band to the third energetic state situated in the energy gap, located at about 0.12 eV below the bottom of the conduction band. A sharp increase in photovoltage at an energy of E 1 = 1.57 eV relates to

15

15

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12

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the electron band-to-band transitions induced by light with energy close to the band gap. The transitions E 1, E2 and E3 are also present on the SPS curve obtained for the modulated illumination. Therefore, they are related to the fast states, having a good contact with the bulk. In Figure 4 the proposed model of the band structure for Cd,,,,Mn,,,,Te : Ga at 193 K is presented. On the SPS curve for sample B (Figure 2) only one electron transition with energy of El = 1.55 e\! is distinctly seen. As for sample A, this level is related to electron generation from the valence to the conduction band. The intensity of the other transitions, well seen on the SPS curve for sample A, has grown less distinctly than for sample A and determination of their energy was not possible. The shape of the SPS curve obtained at constant illumination for sample C (Figure 3) is almost the same as for sample A. On this curve the electron transitions with energies : El = 1.56 eV, E2 = 1.45 eV and E3 = 1.35 eV can also be distinguished, which means that on the crystal surface an electronic structure characteristic similar to that for sample A can also be distinguished. It is probably related to the restoration of the surface stoichiometry after etching in KOH. A lack of the slow electron state E4 on the surface of the sample C, testifies that this state was related to the presence of a strongly defected layer with a big dislocation density on the surface of sample A. Removal of this - 100 pm thick defected layer also causes deterioration of the electronic contact between the surface and the bulk’. As a result, the surface states E2 and E3, which on the surface of sample A were the fast states, are becoming the slow states and on the SPS curve at the modulated illumination they are not present for sample C. Based on analysis of the SPS curves, measured at different temperatures, the temperature dependence of the band gap El and electron transitions E2, E3 and E4 (Figures 1, 2 and 3) have been determined. The parameters : E(O)+nergy of state at temperature of 0 K, and a-temperature coefficient of the level energy change, calculated from linear regression E(T) = E(O) + aT, for the particular energy states and samples, are presented in Table 1.

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4. Summary

.

0.9 Xhl

1.0

1.1

12

Figure 3. SPS curves for sample C etched in KOH aqueous solution at constant (full line) and modulated (dotted line) illumination at 187 K and the temperature dependence of the electron transitions (inset). 482

For the polished sample three types of the electron levels with energies of E2 = 1.45 eV, E3 = 1.34 eV end E4 = 1.255 eV have been stated on the surface. Additional etching of the sample in the Br,/methanol solution causes an increase of the surface voltage barrier--connected with production of the layer enriched in tel-

B Bieg et al : Surface treatment

effects on Cd,,,Mn,,,Te

Table 1. Energy values E(0) and temperature coefficient c1for the electron transitions El, E2, E3 and E4 in Cd,, ,,Mn, 0, Te : Ga Sample A polished

Sample B etched in Br,/methanol

Sample etched in KOH/H,O

Level

d( (10m4eV K-‘)

E(0) (eV)

a (10m4eV K-‘)

E(0) (eV)

a (10m4eV K-‘)

E(0) (eV)

El E2 E3 E4

-3.5 -3.2 -3.2 -2.1

1.62 1.50 1.40 1.31

-4.0

1.63 -

-3.3 -3.5 -2.8

1.61 1.51 1.40

lurium on the surface of Cd,g,Mn,,,Te : Ga, and causes the disappearance of the energy levels E2 and E3 observed on the polished sample. Etching of this sample in aqueous KOH solution restores the stoichiometric composition of the sample surface and causes the reappearance of the states E2 and E3. At the same time, the electric contact between the surface and bulk becomes worse.

References ‘M H Patterson and R H Williams, J Phys D 11, L83 (1978). ‘P Gaugash and A G Milnes, J Electrochem Sot, 128,924 (1981). ‘A J Ricco, H S White and M S Wrighton, J Vat Sci Technol, A 2,910 (1984). “R D Feldman, R L Opila and P M Bridenbaugh, J Vat Sci Technol, A

Acknowledgements

3, 1988 (1985). ‘S Knzminski and AT Szaynok, Phys Stat Sol (a) 89,623 (1985). “S R Morrison, The Chemical Physics of Surfaces. Plenum Press, New

The authors thank Professor W. Giriat for supplying the semiconducting materials for the investigations.

York and London (1977). ‘M Samimi et al, Phys Stat Sol (a), 100, 251 (1987). ‘1 Hahnert and M Wienecke, Mater Sci Engng B16, 168 (1993).

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