In the present publication, we report data gained in experiments on growing PbTe:Ga films on Si substrates and on examining the properties of In/(p-PbTe:Ga) test photosensitive structures based on such films. The various processes for production of PbTe layers on Si have long been under study (see, e.g. [1]). Advantageous features of such heterostructures include the possibility of fabricating large-format IR photodetectors on individual silicon chips and the possibility of fabricating flip-chip photodetectors, for which matching of multiplexer crystal and FPA substrate in terms of thermal expansion coefficients is not required. Also, experiments on doping PbTe films on BaF₂ substrates with gallium were previously reported in the literature [2]. From the standpoint of photodetector applications, the main result obtained in [2] was the revealed possibility of obtaining PbTe films with an extremely low concentration of charge carriers, which, in principle, enables the fabrication of photosensitive structures based on such films.

The process used in the present study for growing CaF₂/BaF₂ buffer layers on (111)-oriented Si substrates was described elsewhere [3]. The Si substrates with CaF₂/BaF₂ layers used for growing PbTe films on such layers were loaded into the MBE facility in the atmosphere of dry pure nitrogen. The thickness of the PbTe films was about one micrometer. The films were doped with gallium in a gas atmosphere as described in [4]. The source of Ga was Gа_0.6Te_0.4 alloy. A typical dependence of current on reciprocal temperature in our PbTe:Ga films is shown in Fig.1.

Fig.1. The electric current through a PbTe:Ga/BaF₂/CaF₂/Si film versus the reciprocal temperature. The sample was shaped as a square, and the bias voltage was U=1.5 V. Curve 1 was measured in the dark, and curve 2, under illumination. The straight line shows an Arrhenius dependence with activation energy ΔE=0.05 eV.

As it is evident from the data in Fig. 1, the sheet resistance of our films was rather high, ρ > 10⁶ Ω at Т=80 K, and the films were quite sensitive to illumination; these findings prove that the film systems obtained offer promise in fabrication of photoconductor-type photodetectors. However, the main focus in the present study was on the possibility of fabricating diode-type photodetectors based on PbTe films.

PbTe:Ga films of p-type conductivity with hole concentration p=(1-50)×10¹² cm^-3 and hole mobility μ_p>10⁴ cm² V^-1 s^-1 at Т=80 K were used. Indium contacts to the samples ranging in area from 75х75 to 1200х1200 μm² were obtained using thermal evaporation of In in vacuum. Anneals were given to the samples in argon ambient. Measurements were made on non-annealed (Schottky barriers) and annealed (shallow n-p junctions) structures. During measurements, the samples were held at temperature T=80 K in a nitrogen cryostat. The spectral sensitivity of measured structures, generally consistent with the behavior of PbTe photosensitivity in the fundamental absorption region of the material, exhibited interference-induced features, which, in principle, enables optimization of photosensitivity at a desired wavelength through film thickness adjustment. Current-voltage characteristics of a structure 200 μm in diameter measured prior to and after the annealing of the sample in argon atmosphere are shown in Fig.2; during measurements, the structure was shielded from background radiation.

Fig. 2. Current-voltage characteristics of a structure 200 μm in diameter measured prior to (1) and after (2) the annealing of the sample in argon on screening the sample from background radiation. The straight lines show approximations of the linear portions of the characteristics.

Approximation of the linear portions of the forward branches of the current-voltage curves has yielded offset voltage values 0.1 V and 0.22 V, which can be identified as the diffusion potential values for Schottky barriers and n-p junctions (before and after annealing of the samples). Photosensitivity and noise measurements performed under low background illumination conditions have yielded for the maximum detectivity a value D*>10¹¹ W×cm×Hz^-0.5. Charge transport mechanisms in the films and available potential for improving characteristics of the photodetectors and for use of obtained film structures in the development of FPA photodetectors on silicon chips are discussed.

            This work was supported by the Ministry of Science and Education of the Russian Federation under Grant No. 2012-1.1-12-000-2003-120.

References

1. C.  Maissen,  J.  Masek,  H.  Zogg, S. Blunier.  Appl. Phys. Lett. 53, 1608 (1988).

2. B.A. Akimov, V.A. Bogoyavlenskii, L.I.Ryabova, et al. Phys. Rev. B 61, 16045 (2000).

3. S.P. Suprun, and D.V. Shcheglov. JETP Letters 88, 365 (2008).

4. L.F. Vasil’eva, A.E. Klimov, N.I. Petikov, and V.N. Shumsky. Inorganic Materials 37, 144 (2001).

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1. Surface morphology and structure of CaF₂/BaF₂–on-Si epitaxial layers and the electronic properties of the interface with the substrate
2. The Improved Procedure of Ga-doped PbTe Films One Stage Synthesis by Modified HWE Technique
3. The Determination of Different Types of Conductivity Areas within Homogeneity Region of Ga Solid Solutions in PbTe films on Si-substrates 
4. The  determination  of solubility limits of gallium impurity  in  PbTe films doped with Ga  on  Si  substrates
5. electrical properties of lead telluride single crystals doped with bromine
6. The function of thiourea coordination compounds composition in synthesis of CdS thin films
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8. The charge states of In impurity atoms in PbTe<In>/Si films prepared by modified HWE technique
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