Preparation and characterization of Pbl_xSnxTe pseudobinary alloy semiconductors

Transcription

1 Bull. Mater. Si., Vol. 3, Number 3, November 1981, pp Printed in lndia. Preparation and haraterization of Pbl_xSnxTe pseudobinary alloy semiondutors S C DAS, A K CHAUDHURI and S BHATTACHARYA Department of Physis, Indian Institute of Tehnology, Kharagpur , lndia MS reeived 14 Marh 1981 Abstrat. A p-type pseudo-binary alloy semiondutor, Pb0..~Sn0.TTe, has been prepared from p-type speimens of PbTe and SnTe and lattie onstants determined with an auray of nm. Vauum annealing of Pb0.~Sn0.7 Te reveals two new x-ray powder diffration lines bearing indies (444) and (800), while others beome more sharp, CuKa-doublets get learly resolved and the lattie onstant is inreased by~ nm. Slight deviation from Vegard's law linearity is observed showing that the sample must be onsidered as ternary in nature. Thin films deposited on mia and glass substrates kept at room temperature are found to have a little higher SnTe ontent. The effetive arrier onentration alulated from Hall measurements at room temperature is~ ~ m-l Keywords. Narrow-gap semiondutors; pseudo-binary alloys; x-ray analysis. 1. Introdution The alloy semiondutor Pbl_ x Snx Te is being extensively studied nowadays, as it has small and ontrollable band-gap (Eg), depending on Sn-ontent (X), temperatures (Dimmok et al 1966) and hydrostati pressure (Melngailis et al 1970) et. Thus the emission wavelength of the soures (light-emitting diodes) based on this material is within the IR-region of the spetrum (6-32 Jxm). This makes the material potentially useful as IR-detetors and lasers (Butler et al 1966), speially in the ryogeni range. The variation of the band gap of Pb~_x SnxTe with x has been explained with a band-inversion mehanism (Dimmok et al 1966) whih has also been verified by several experiments viz. measurements of eletrial ondutivity as a funtion of temperature for several alloys near the rossing. (Bis and Dixon 1968). The alloy Pbi-x Snx Te with x > 0.5 has not been so widely studied and so in this work we hoose x = 0.7 beause it is very lose to, and on the SnTe-side of, the band-rossing point at room temperature. 2. Preparation Both PbTe and SnTe have the same rok-salt struture and they exhibit omplete solid solubility over the entire omposition range, Hene a pseudobinary assumption is justifiable for Pb~_x Snx Te for x = 0 to

2 302 S C Das, A K Chaudhuri and S Bhattaharya Here a polyrystatline ingot of p-pb0.3sn0.vte is prepared from p-pbte and p-snte. For p-pbte, the bulk speimen is obtained from Bhabha Atomi Researh Centre, Trombay and p-type SnTe bulk is prepared from 99.99% pure semiondutor grade tin and tellurium. Crushed samples of p-pbte and p-snte are taken in proportions appropriate to their moleular weights for x The relation is [wt/wz = (1-x/x)] where w~ and w~ are the masses of PbTe and SnTe respetively and x is the tool fration of SnTe. Weights are taken in an eletroni mirobalane and then the samples are plaed in a quartz tube, whih is subsequently evauated to,-, 8 10-~Pa and sealed. The sealed ampoule is then heated to,-,975 C in a furnae for 6hr, thorough mixing of the omponents is ensured and finally it is quenhed in old water. 3. Experiments The ingot thus prepared is powdered in an agate mortar, sieved through a 270 ASTM mesh and then a small amount of it is taken in a glass (orning) apillary of internal diameter ~0.25 x 10-~m. The (apillary and the sample) assembly is then kept mounted in a Debye-Sherrer amera (i.d.: m) for 40 hr and an x-ray powder photograph of the ombination is obtained from whih different lines for the sample are learly identified. The same ombination is then annealed at 500 C for 6hr at a vauum of,~ 1.3 x Pa (Hewes et al 1973), after whih another powder photograph of the ombination is taken with the same exposure time. By the same proedure, x-ray powder photographs are also taken for PbTe and SnTe. In all ases, CuK~-radiation (from a NORELCO unit using Ni-foil as /3-filter) is used. Then, with glass and mia as substrates (kept at room temperature), thin films of Pb0.~ Sn0.TTe are deposited by thermal evaporation, using a heating urrent of 70 amps and a vauum of 6.6x 10-~ Pa, the rate of deposition being ~ nm/min. The substrates are kept at room temperature and so the filmsare expeted to be polyrystalline. Diffratometer analysis with these films shows that only few peaks appear and they do so at a slightly higher angle (0) than the orresponding powder diffration lines. This reveals (using Vegard's law) that the films have a slightly inreased SnTe-ontent as ompared to the bulk sample. 4. Measurements For annealed Pb0.~ Sn0.rTe, interplanar spaings (d) of different rystallographi planes and lattie parameter (a) of the rystal are alulated from measurements on its powder photograph. Using the Taylor-Sinlair funtion ~os~0(1/sin 0+ 1[0) (obtained from tables), the most aurate value of a is determined by the extrapolation method (figure 1) with an auray of,,~ nm. Similar measurements are performed with the powder photographs for only unannealed Pb0.sSn0.TTe, PbTe and SnTe. In all ases, lines with 0 ~ 29 only are onsidered for graphial extrapolation.

3 Preparation of pseudo-binary alloys 303 d _ % = nm.'e'-t 800) 8 (1) "6 / l, I 0 o sin e Figure l. Lattie onstant (a) determination of annealed Pbo.sSno. Te by graphial extrapolation Room temperature measurements are arried out for eletrial ondutivity (a), thermoeletri power (~)and Hall effet with thin films on glass and mia substrates for a thikness of 200 nm. Film thikness is measured with one film on glass by the Tolansky method (Holland 1966), whih uses an optial interferene mirosope to measure the "Fizeau fringes". The auray attained is ~ 5 nm. For the above measurements, pressure ontats with film surfaes are used along with graphite paint. A magneti field of 0-8 Tesla is used for Hall measurements and readings are taken with diret and reversed fields and their average taken. Different voltages are measured with a d mirovoltmeter. 5. Results The extrapolated values of a for PbTe and SnTe are nm and nm respetively whih agree very well with their standard values. For Pb0.3'Sn0.~Te, the value of a is obtained as nm for the unannealed sample and nm (figure 1) for the annealed one i.e. an inrease of nm in the value of the lattie onstant of the sample is observed due to vauum annealing. Moreover, it is found that two new lines appear in the powder pattern near 57 and 76 bearing indies (444) and (800) respetively when the sample is annealed. Also, in the powder pattern of the annealed sample, the lines are far more sharp and Ko~-doublets are learly resolved. Powder diffration lines bearing only the all-even indies appear in all the ases, whih reveal that Pb0. s Sn0.TTe has the same rok-salt struture as PbTe and SnTe.

4 304 S C Das, A K Chaudhuri and S Bhattaharya ~5 o O. 640 It) 0 J O. 656 t ill? o PbTe M01e fration(x) of SnTe SnTe Figure 2. Vegard's law variation of lattie onstant (a) of Pb~_ x Sn x Te with x. A test of Vegard's law is made using the determineda values of PbTe and SnTe as end-points'(figure 2). The linear relation aording to Vegard's law an be expressed as a 0 (PbSnTe) = a (PbTe) x. (1) A slight variation (.~0.13%) from Vegard's law is noted whihan be viewed as a failure of this alloy to behave exatly as a binary one i.e. it must be onsidered as slightly ternary in nature (Bis and Dixon 1969). PbTe is a diret energy-gap semiondutor whose valene and ondution band-edges have L~+ and La- symmetry respetively while SnTe has an inverted band struture. The energy gaps (Eg) at 300 K for PbTe and SnTe are respetively 0.32 ev and 0.18 ev. Also, it is now established that for Pb~ x Snx Te, Eg varies almost linearly with x (Dimmok et al t966; Tauber and Cadoff 1967). On this basis, for our sample Eg=0.04 ev at 300 K. Its orresponding emission wavelength ts ~-, 30 #m. Obviously, its ondution and valene band-edges will have L6+ and L6- symmetry respetively like SnTe. From Hall voltage (V,,) measurements with the films, we find R,,~-,1.6 x 10-a m~/, and the effetive arrier onentiation (p--l/eb,) omes to be about m -~. Both the bulk and the film are found to be p-type i. e. there is an exess of Te-atoms (or Pb/Sn vaanies or possibly non-metal interstitials) in them. The value of o ~ at 300 K is found to be 540 mho-m -1 and Hall mobility (/x,~r, r) is found to be 8.42 x 10-~ m*-/v-se. The thermoeletri power (~) at 300 K for the thin films of Pb0.3Sn0.~Te is found to be,.o 150 /zv/ K.

5 Preparation of pseudo-binary alloys Conlusion With suh high hole onentrations as in this ase, the arriers would be 'degenerate'. This may possibly explain the observed deviation from Vegard's law; Bis and Dixon (1969) have indeed pointed out that for arrier onentrations exeeding 1 x 102~m -3, relatively large shifts our in Vegardtype relations. This deviation is also onsistent with the observations of Short (1968) and of Wagner and Woolley (1967) that marked deviations from stoihiometry our as in PbSnTe as the SnTe ontent inreases. Aknowledgements The authors thank all those who helped them in this work. Speial mention must be made of Shri B C Mandal for his ooperation in various stages of their work. Referenes Bis R F and Dixon J R 1968 Phys. Rev Bis R F and Dixon J R 1969 J. Appl. Phys Butler J V, Calawa A R and Harman T C 1966 Appl. Phys. Lett Dimmok J O, Melngailis I and Strauss A J 1966 Phys. Rev. Lett Hewes C R, Adler M S and Senturia S D 1973 J. Appl. Phys Holland L 1966 Vauum deposition of thin films (London : Chapman and Hall) 224 Melngailis J, Kafalas J A and Harman T C Conf. on Physis of semimetals and narrow-gap semiondutors (eds D L Carter and R T Bate) (.Texas : Pergamon Press) 407 Short N R 1968 Bri. J Appl. Phys. D1 129 Tauber R N and Cadoff I B 1967 Y. Appl. Phys Wagner J W and Woolley J C 1967 Mater. Res. Bull