Shbrt Notes K181

phys. stat. sol. (a )% K181 (1984) Subject classification: 20.1; 13.1; 22.8

Solid State Physics Laboratory, Delhi2) (b) Composition Dependence of the Optical Energy Gap - - in Pbl ,€&,Te Alloy Thin Films

BY MUKESH JAIN (a), A.V.R. WARFUER (b), and H.K. SEHGAL (a)

Department of physics, Indian Institute of Technology, Delhi 1 ) (a) and

Sb' ion implanted single crystals of Pb Te /1/ have been found use- ful a s photovoltaic infrared detectors upto * 3 . 9 pm. The wavelength response of the detector could be extended to longer wavelengths by decreasing the optical gap of PbTe /2/. This could be achieved by increasing the concentration of HgTe /3/ in the ternary alloy semiconductor. Relaxed conditions of nucleation and growth during flash evaporation of films have been exploited to achieve this aim. Results of optical absorption measurements are reported for mixed films of PbTe and HgTe. The mercury concentration (x) in the films is varied in the limits 0.07 to 0.93.

Mixed films of lead and mercury tellurides, with mercury fraction (x) varying in the range 0.07 5 x 5 0.93, were grown in a vacuum of 1 0-6 T o r r by the ilash evaporation technique on freshly cleaved KBr single crystal sub- strates kept at 25 OC. The starting materials for evaporation were prepared by alloying PbTe with HgTe (mixed in appropriate proportions) in evacuated sealed transparent silica ampoules. Before evaporation, the materials were homogenized by keeping them at slightly below their melting temperatures for 60 h.

Hg 0.95 0.05

Electron diffraction studies on the films in the composition range 0.075~s 0.50 show each of them to be polycrystalline with a single phase f.c.c.

structure. The lattice parameter decreases slightly with increase in the. mercury concentration. Auger depth profiling indicates a slight stoichiometric excess of tellurium in the otherwise homqeneous films. Films in the com- position range 0 .60 5 x 5 0.93 were amorphous and had high relative concen- trations of mercury near the film-substrate interface and lead near the film- air interface.

1 ) New Delhi 110016, India. 2 ) Lucknow Road, New Delhi 110007, India.

13 physica (a)

K182 physica status solidi (a) 82

Fig. 1. Room temperature optical ab- sorption spectra for Pbl -,HgxTe films

grown at 25 OC (x = O 0.07, 0.25, A 0.33, A 0.50, 0 0.75, 0.93;

--- amorphous films)

t zo 73

' 16

polyc rystalline films,

12

The optical behaviour of the 100 to 08

200 nm thick films was investigated

from the transmission and reflection

spectra recorded i n the wavelength

t L c ~ range 2.5 to 25 p m b y a double beam

T, R, and d express the estimated transmittivity, reflectivity, and thickness,

respectively, of the film.

steep decrease in the absorption coefficient is followed by a broad band in the

6 to 1 6 p m region. The steep decrease in oc , indicative of an absorption

edge, is observed to occur at longer wavelengths (lower energies) in films

with larger concentrations of mercury. A linear variation of a: with (hv - - Egf/2 suggests the presence of a direct optical band gap for the film ma-

terials.

The direct optical band gap, computed from (ad) versus energy curves,

The results indicate that in each of the lead rich films (0.07 2 x 5 0.50) the

2

is observed to decrease with an increase in mercury concentration in the films. The variation is shown i n Fig. 2. The observed change i n E with

composition can be described by a parabolic dependence /5/ g

E = a + b x + c x ( l - x ) , g

where a and b a r e constants connected with the band gaps of the starting

compounds, and c is the bowing parameter. The exact equation satisfying

Short Notes K183

Fig. 2. Optical energy gap (eV) as a function of lat- tice parameters in the pbl -xHgxTe (0.07 (= x 5 0.50)

the variation can be expressed as 2

8 010 E (x) = 0.32 - 0.6998 x + 0.6798 x . g The nonlinear variation of optical gap with composition

0 70 20 30 10 50 is probably due to the observed stoichiometric excess of Te in the films. The presence of a uniphase crystal structure and continuous variation of the optical

energy gap with change in the mercury concentration in the films suggest the alloying of PbTe (E = 0.32 eV; a = 6.443 8 , Fm3m - 0;) /6/ with HgTe (semimetal /3/ with inverted band structure similar to that of gray tin /7h

a = 6.46 8; F43m /8/) t o form alloys of the type pbl -xHgxTe. The broad absorption band observed on the low energy side of the funda-

mental absorption in the films is probably due to intraband transitions similar t o !hose observed ih Ge by Kane /9/. The transitions originate because of ex- citation of free carriers in a band resulting from simultaneous scattering of mobile ca r r i e r s by phonons o r by impurities. The presence of the band at about the same spectral position suggests that the structure of the valence band practically remains unchanged in the Pbl xHgxTe (0.07 6 x 2 0.50) films. The behaviour is similar to that reported for G a b /lo/, GaP /lo/, and CdHgTe

The absorption edge in a mercury rich (0.60 5 x 2 0.93) film (shown by

dashed curves in Fig. 1) is observed to lie close to the absorption edge of pure PbTe. In addition, the nature of the absorption in the long-wavelength region (> 8 p m ) is as expected for semimetals. These results are supported by Auger depth prqfiling which indicates the segregation of PbTe and HgTe in the amor- phous films.

0.27 and 0.14 eV in the flash evaporated single phase 0.07 5 x <= 0.50

conLentrotion [ o f % Hg) - zu g

m/.

The study thus shows that it is possible to vary the optical gap between

Hg Te films. & l - X x

The authors are grateful t o Prof. S.C. Jain, Director, Solid State Physics

Laboratory, Delhi, for his encouragement and providing facilities in his lab- oratory.

K184 physica status solidi (a) 82

References / l / D . TRBOJEVIC, E. PEROVIC, P. NIKOLIC, and Y . CVEKIC, Infrared

/2/B.C. GELMONT, T.R. GLOBUS, and A.V. MATREENKO, Solid State

/3/ T.C. HARMAN, J. appl. Phys. 32, 1800 (1961). /4/ I.V. BODNAR, B.V. KORZUN, and A.I. LUKOMSKII, phys. stat. sol.

/5 /A.G. THOMPSON and J.C. WOOLEY, Canad. J. Phys. - 45, 255 (1967). /6/ S.M. SZE (Ed.), Physics of Semiconductor Devices, Wiley Eastern

Phys. - 20, 193 (1980).

Commun. - 38, 931 (1981).

(b) - 105, K143 (1981).

Limited, New Delhi 1979 (p. 15). /7/ S. GROVES and W. PAUL, Phys. Rev. Letters c, 194 (1963). /8/ W. L. ROTH, in: Physics and Chemistry of II-VI Compounds, Ed. M,

AVEN and J.S. PFtENER, North-Holland Publ. Co., Amsterdam 1967

(p. 118). /9/ E.O. KAIW, J . Phys. Chem. Solids - 1 , 83 (1956).

/lo/ J.W. ALLEN and J.W. HOBBY, Proc. Phys. SOC. (Londod82, 315

/11/ J.C. HARMAN, see /8/ (p. 799). (1963).

(Received December 8, 1983)