Photopyroelectric Determination of Thermal Parameters across p→n Transition in Pb Doped Ge-Se Glasses Jacob PHILIP * , Ravindran RAJESH and Preethy C. MENON Department of Instrumentation, Cochin University of Science & Technology, Cochin - 682 022, India The thermal parameters of two series of Pb - modified Ge-Se glasses, Pb 20 Ge x Se 80-x (x = 17 - 24) and Pb y Ge 42-y Se 58 (y = 0 - 20), which exhibit carrier type reversal from p-type to n-type at specific compositions, have been measured simultaneously by the photopyroelectric technique. It is found that thermal diffusivity, thermal conductivity and heat capacity exhibit anomalous variations at the compositions at which carrier type reversals are reported to occur in these two series of glasses. The results are explained in terms of enhancement in electron concentration during the p - to n - type changeover in these materials. (Received June 29, 2000; Accepted October 11, 2000) • To whom correspondence should be addressed. E-mail: jp@cusat.ac.in In general, chalcogenide glasses are p-type semiconductors due to the fact that the number of electrons excited above the conduction band mobility edge is smaller than the number of holes excited below the valence band mobility edge 1 . They also contain positively and negatively charged defect states, known as valence alternation pairs 2 (VAPs), which essentially pin the Fermi level at the middle of the band gap making these materials rather insensitive to doping 3 . It has been found in the recent past that certain charged additives could change the ratio of VAPs to such an extent that the Fermi level gets unpinned 4 . It is seen that appropriate quantities of metallic additives such as Pb or Bi in Ge-Se glasses can enter the network as charged species and alter the concentration of VAPs, changing the conduction to n - type 5 . The phenomenon of p to n transition or carrier type reversal (CTR) has initiated work in the direction of fabricating a new class of p-n junctions based entirely on semiconducting glasses, which has got several manufacturing advantages over conventional p-n junctions. Even though such possibilities still remain uncertain, the physics of the effect has generated great deal of interest among semiconductor scientists. CTR has been reported in Pb or Bi doped Ge-Se glasses 5,6 , Pb doped In-Se glasses 7 , Pb doped Ge-Se-Te 8 glasses etc. A number of papers have been published on electrical conductivity, activation energy, thermoelectric power, thermal analysis, structure and vibrational states of Pb as well as Bi doped Ge-Se glasses across the composition exhibiting CTR 5,6 . Experiments on Pb containing Ge-Se glasses have led to the discovery of CTR in two series of Pb doped Ge-Se glass compositions, viz., Pb 20 Ge x Se 80-x (x = 17 – 24, with CTR occurring at x = 21) and Pb y Ge 42-y Se 58 (y = 0 – 20, with CTR occurring at y = 9) 5 . Since the concentration of lead in the germanium- chalcogenide network in these compositions is rather high, it is more appropriate to use the term 'chemical modification' rather than 'doping'. In this paper we report the anomalous variations in thermal parameters, such as thermal diffusivity, thermal effusivity, thermal conductivity and heat capacity as the lead or germanium concentration is varied through the compositions at which the above two series of Pb modified Ge-Se glasses undergo CTR using the improved photopyroelectric technique. The photopyroelectric technique has distinct advantages over other photothermal methods, such as its extreme simplicity, sensitivity and nondestructive probing ability. Details of the experiment, results obtained and a discussion of the results are given below. Experimental The two series of glass compositions, Pb 20 Ge x Se 80-x with x = 17 - 24 (Series I) and Pb y Ge 42-y Se 58 with y = 0 - 20 (Series II), have been prepared in the bulk form from high purity constituents by the standard melt quenching technique. The glassy nature of the samples have been confirmed by XRD technique. Thin slices of the samples having thickness of the order of 0.5 mm have been prepared by hand lapping and polishing after cutting with a slow speed diamond wheel saw. In the photopyroelectric (PPE) technique to measure thermal diffusivity (α), thermal effusivity (e), thermal conductivity (K) and heat capacity (C p ) simultaneously, a thermally thick pyroelectric film is attached to one side of the sample, which is also thermally thick, and the combination is mounted on a thermally thick backing medium. The other side of the sample is illuminated by an intensity modulated beam of light, which gives rise to periodic temperature variations by optical absorption. The thermal waves so generated propagate through the sample and are detected by the pyroelectric detector. The signal is processed by standard electronic techniques. A 120 mW He-Cd laser modulated at a frequency of 40 Hz has been used as the radiation source and a PVDF film of thickness 52 μm coated with Ni-Cr film as the pyrolectric detector. Measurement of the PPE signal phase and amplitude enables one to determine the thermal diffusivity and effusivity respectively. Once the diffusivity, defined by p C K ρ α= , and effusivity, defined by ρ p KC e = , where ρ being the sample mass density, are measured, the thermal conductivity and heat capacity of the sample can be determined. 2001 © The Japan Society for Analytical Chemistry s99 ANALYTICAL SCIENCES APRIL 2001, VOL.17 Special Issue