Ge/Sb2Te3 nanocomposite multilayer films for high data retention phase-change random access memory application

The amorphous-to-crystalline transition of Ge/Sb₂Te₃ nanocomposite multilayer films with various thickness ratios of Ge to Sb₂Te₃ were investigated by utilizing in situ temperature-dependent film resistance measurements. The crystallization temperature and activation energy for the crystallization of the multilayer films increased with the increase in thickness ratio of Ge to Sb₂Te₃. The difference in sheet resistance between amorphous and crystalline states could reach as high as 10⁴ Ω/□. The crystallization temperature and activation energy for the crystallization of Ge/Sb₂Te₃ nanocomposite multilayer films was proved to be larger than that of conventional Ge₂Sb₂Te₅ film, which ensures a better data retention for phase-change random access memory (PCRAM) use. A data retention temperature for 10 years of the amorphous state [Ge (2 nm)/Sb₂Te₃ (3 nm)]₄₀ film was estimated to be 165 °C. Transmission electron microscopy (TEM) images revealed that Ge/Sb₂Te₃ nanocomposite multilayer films had layered structures with clear interfaces.     

Study of aluminium-modified Ge2Sb2Te5 thin films for the applicability as phase-change storage device material

Electrical and optical studies have been carried out on aluminium-modified Ge₂Sb₂Te₅ thin films to check its applicability as an active material in optical and electrical memory storage devices. Five polycrystalline bulk samples were prepared with compositions: Al_x(Ge₂Sb₂Te₅)_1?x; x = 0, 0.08, 0.14, 0.21, 0.25. Amorphous thin films were deposited from the polycrystalline bulk by thermal evaporation. Temperature-dependent resistance shows the increase in crystallization temperature of Ge–Sb–Te films on aluminium addition. Activation energy for conduction, conductivity, optical band gap, coefficient of refraction and extinction coefficient are studied with respect to Al content in both amorphous and crystalline phases of Ge–Sb–Te alloy films.     

Effect of composition and thermal annealing on the absorption and electrical conduction of Se85−xTe15Sbx glasses

The optical absorption of the as-prepared and thermally annealed Se_85−xTe₁₅Sb_x (0≤x≤9) thin films was measured. The mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap (E₀) decreased from 1.12 to 0.84 eV with increasing Sb content of the as-prepared films from 0 to 9 at.%. The as-prepared Se₇₆Te₁₅Sb₉ films showed an increase in (E₀) with increasing the temperature of annealing in the range above T_g (363 K). The electrical conductivity of the as-prepared and annealed films was found to be of Arrhenius type with temperature in the range 300-360 K. The activation energy for conduction was found to decrease with increasing both the Sb content and temperature of annealing. The results were discussed on the basis of the lone-pair electron effect and of amorphous crystalline transformation.     

Observation of Meyer-Neldel rule in thermally activated Poole-Frenkel photoconduction in a-Se<Subscript>70</Subscript>Te<Subscript>30−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films

In the present paper, we report the compensation effect on photoconductivity of thin films of Se₇₀Te_30−x Cd _x (x = 2, 4, 6) alloys for high field conduction. Steady state photoconductivity measurements have been made in thin films for different electric fields. The photoconduction was found to be ohmic at low fields and Poole-Frenkel type at high fields. Meyer-Neldel rule (MN rule) for the pre-exponential factor (σ _ph)₀ and activation energy (ΔE _ph) of photoconduction in thin films of Se₇₀Te_30−x Cd _x is observed in the present study.     

Effects of Ge doping on the properties of Sb2Te3 phase-change thin films

Ge-doped Sb₂Te₃ films were prepared by magnetron sputtering of Ge and Sb₂Te₃ targets on SiO₂/Si (1 0 0) substrates. The effect of Ge doping on the structure was studied in details by X-ray diffraction, differential scanning calorimetry, and X-ray photoelectron spectroscopy measurements. It is indicated that Ge atoms substitute for Sb/Te in lattice sites and form Ge-Te bonds, moreover, a metastable phase was observed in Ge-doped specimens. Both crystallization temperature and resistivity of amorphous Sb₂Te₃ increase after Ge doping, which are beneficial for improving room temperature stability of the amorphous state and reducing the SET current of chalcogenide random access memory.     

Properties of LiZn x V2−x O4 and LiMg x V2−x O4 mixed oxides

The phase composition of the mixed oxides, series LiMe _x V_2–x O₄, where Me=Zn, Mg, was studied using X-ray analysis. The minor phases present in the investigated samples together with the basic spinel phase were determined as Li₃VO₄ and Zn₂V₂O₇ or Mg₂V₂O₇ respectively. Their presence is caused by the tendency of tetravalent vanadium ions to the disproportionation. For the compositionsx=0·375 were determined the quantitative phase compositions. The lattice constant of the spinel phase is linearly changing according to Vegard's rule, for the zinc series in the range 0x0·375 and for the magnesium series in the range 0xs0·25. Further substitution in magnesium series caused tetragonal deformation of the cubic spinel lattice.The author is indebted to Dr. A.Bergstein for helpful discussions and to Mrs. E.Hrubá for technical assistance.     

Phases and reactions in the Li x Mn3−x O y system

The dissociation and oxidation processes occurring in Li _x Mn_3–x O _y in air in a temperature range between 0 and 12 000°C were examined by means of thermographic and X-ray methods. Lithium and manganese form oxides with a spinel structure in a broad interval of the oxidation degree of manganese ions. As consequences of the oxidation degree and of the temperature, changes of the lattice parameters, of the cation distribution in the spinel lattice and the occurrence or disappearance of a tetragonal deformation of the lattice are experienced. In the temperature range between 950 and 1100°C probably the Li₂Mn₄O₇ compound and its solid solutions with Mn₃O₄ occur.The authors wish to express their gratitude to A. ápová, J. Mikulíková and E. Kitzinger for their helpful cooperation.     

Optical,photoelectric and electric properties of single-crystalline Sb2Se3

Measurements of absorption coefficient in the region of the absorption edge, of spectral distribution of photoconductivity and dependence of electrical conductivity upon temperature on Sb₂Se₃ single crystals are given. The absorption of light was proved to correspond to indirect forbidden transitions. The value of optical gapE _g ^opt =(1·11±0·02) eV forE a andE c was determined. From photoconductivity and conductivity measurements the values of the gaps areE _g ^opt =1·11 eV andE _g ^el =1·04 eV. The anisotropy of the electrical conductivity parallel and perpendicular to the cleavage plane is 2·2.     

Investigation of Sb-rich Si2Sb2+x Te6 material for phase change random access memory application

Sb-rich Si₂Sb_2+x Te₆ (x=0, 1.4, 10) thin films are proposed to present the feasibility for electronic phase change memory application. The crystallization behavior is improved by adding Sb into the material. The crystallization temperature is about 506, 502, and 450 K for Si₂Sb₂Te₆, Si₂Sb_3.4Te₆, and Si₂Sb₁₂Te₆ films, respectively, and the corresponding activation energy is in the range from 2.70 to 1.69 eV, which is expected for a low-power and high-speed SET operation. In addition, maximum temperature for a 10 year data lifetime is estimated to be 133, 127, and 98°C, respectively. The memory devices are successfully fabricated employing these films, promising that the stability of the low-resistance crystalline state is improved by adding Sb into the stoichiometric Si₂Sb₂Te₆ material, and the reversibility of the device is also realized for the Si₂Sb₁₂Te₆-based cell.     

Galvanomagnetic properties of Hg1−x MnxTe1−y Sey semimagnetic semiconductors

The galvanomagnetic properties of single crystals of the semimagnetic semiconductors Hg_1−x Mn_xTe_1−y Se_y with 0.01<y<0.1 and x=0.05 and 0.14 in the temperature range 4.2–300 K are investigated. The features of the temperature dependence of the Hall coefficient R _H and the complicated behavior of R _H in a magnetic field are attributed quantitatively to the existence of three groups of current carriers, viz., electrons and two types of holes, for which the temperature dependences of the densities and mobilities are obtained. A transition from p-type to n-type conductivity is observed as the Se content is increased, and the negative magnetoresistance simultaneously gives way to positive magnetoresistance. Zh. éksp. Teor. Fiz. 112, 1809–1815 (November 1997)     

An analysis of the thermoelectric efficiency of n-(Bi,Sb)2(Te,Se,S)3 solid solutions within an isotropic scattering model

A study is reported on the thermoelectric properties of n-type solid solutions Bi₂Te_3?y Se_y (y=0.12, 0.3, 0.36), Bi_2?x Sb_xTe_3?y Se_y (x=0.08, 0.12; y=0.24, 0.36), and Bi₂Te_3?z S_z (z=0.12, 0.21) as functions of carrier concentration within the 80-to 300-K range. It has been established that the highest thermoelectric efficiency Z is observed in the Bi₂Te_3?y Se_y (y=0.3) solid solution containing excess Te at optimum carrier concentrations (0.35×10¹⁹ cm^?3) and at temperatures from 80 to 250 K. The increase in Z in the Bi₂Te_3?y Se_y solid solution compared with Bi_2?x Sb_xTe_3?y Se_y and Bi₂Te_3?z S_z is accounted for by the high mobility μ₀, an increase in the effective mass m/m ₀ with decreasing temperature, the low lattice heat conductivity κ_L, and the weak anisotropy of the constant-energy surface in a model assuming isotropic carrier scattering.     

Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films

Amorphous thin film Ge₁₅Te_85−x Sn _x (1≤x≤5) and Ge₁₇Te_83−x Sn _x (1≤x≤4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bottom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn concentration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigidity percolation in the composition dependence of switching fields of Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with increasing Sn concentration, which is consistent with the composition dependence of switching fields.     

Composition dependence of photoconductivity in amorphous thin films of Se70Te30−xSbx

Summary The present paper reports the composition dependence of steady-state and transient photoconductivity in amorphous thin films of Se₇₀Te_30-xSb_x, wherex is varied from 0 to 10. The results indicate that photoconductivity is highly composition dependent in this system and a discontinuity in various electrical parameters is observed at 4 at. % of Sb. This is explained in terms of increased disorder up to 4 at. % of Sb. However, at higher concentration of Sb, an ordered structure may be established due to the formation of microcrystalline phases as observed in X-ray diffraction patterns.     

The susceptibility of selenides and tellurides of heavy elements

The results of measuring the temperature dependence of the susceptibility (T) of samples of PbSe, Sb₂Se₃, Sb₂Te₃, Bi₂Se₃ and Bi₂Te₃ are given and the measured curves explained. The connection between the molar susceptibility of the characteristic crystal lattice of these semiconductors and the total number of electrons in their molecules is pointed out.     

Optical and photoelectrical properties of glassy GeS x

Absorption edge, index of refraction, density and spectral dependence of the photoconductivity were measured on amorphous layers of stoichiometric GeS and on glasses GeS _x (1<x<2). An attempt is made to estimate the product of the mobility, lifetime and quantum efficiency for=0·9 m and=1·54 Å.Presented in part at the Int. Conf. on High Resistence Semiconductors, Photoelectrets and Electrophotography, Varna, Bulgaria, November 1973.Authors acknowledge valuable discussions with drs S.Koc and B.Velický, as well as the help of dr. J.Drahokoupil with the X-ray experiments. The glassy samples were kindly prepared by ing. A.Hrubý.     

Investigation of the optical properties of In2Se3xTe3(1−x) single crystals

In [1] it was shown that when In₂Te₃ interacts with In₂Se₃ in samples which are mainly indium telluride, substitutional solid solutions having the zinc blende structure are formed in the concentration range 0–40 mol.% In₂Se₃ (0 x 0.4). The solubility of the low temperature hexagonal modification-In₂Se₃ did not exceed 2 mol.%; formation of solid solutions having the medium temperature hexagonal modification of-In₂Se₃ was observed for the interval 0.9 x 0.95. These published data [2, 3] are concerned with electrical properties of the crystals. In this paper the results of an investigation of the optical properties of In₂Se_3xTe_3(1–x) single crystals for temperatures between 300 and 77°K are presented. Crystals suitable for optical measurements were obtained by a modified Bridgman method, which included vibarational mixing. All the samples were n-type. The resistivity varied from 10⁶ to 10⁴ ohm · cm in the course of the transition from-In₂Te₃ to the solid solutions (0 x 0.4).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 65–68, May, 1972.     

Laser and electrical current induced phase transformation of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> semiconductor thin film on Si(111)

Phase transformation of thin film (∼30 nm)In₂Se₃/Si(111) (amorphous→crystalline) was performed by resistive annealing and the reverse transformation (crystalline→amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In₂Se₃ is of interest for non-volatile phase-change memory. Amorphous In _x Se _y was deposited at room temperature on Si(111) after pre-deposition of a crystalline In₂Se₃ buffer layer (0.64 nm). Upon resistive annealing to 380°C, the film was transformed into a γ-In₂Se₃ single crystal with its {0001} planes parallel to the Si(111) substrate and parallel to Si , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and X-ray diffraction. Laser annealing with 20-ns pulses (0.1 millijoules/pulse, fluence≤50 mJ/cm²) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous In _x Se _y film.     

Structural and optical properties of In35Sb45Se20−xTex phase-change thin films

Physical properties of In₃₅Sb₄₅Se_20−xTe_x thin films with different compositions (x=2.5, 5, 7.5, 10, 12.5 and 15 at %) prepared by electron beam evaporation method are studied. X-ray diffraction results indicate that the as-evaporated films depend on the Te content and the crystallized compounds consist mainly of Sb₂Se₃ with small amount of Sb₂SeTe₂. Transmittance and reflectance of the films are found to be thickness dependent. Optical-absorption data indicate that the absorption mechanism is direct transition. Optical band gap values decrease with increase in Te content as well as with increase in film thickness.     

The fundamental absorption band of amorphous germanium

The index of attenuationk() of evaporated films of amorphous Ge was measured in the spectral range of 1.95 to 5.3 eV by measuring the transmissivity of samples with thicknesses varying from 560 to 1510 Å. It is shown that in comparison with crystalline Ge the absorption is significantly shifted towards lower energies and the peak at 4·5 eV is absent. Using these data and the data onk() determined in other spectral ranges up to 25 eV by various authors, the index of refractionn() was calculated by Kramers-Kronig analysis. The absorption of amorphous Ge is compared with that of crystalline Ge and the differences are discussed.The authors thank Mr. Z. Balej for the preparation of the thin films used in our measurements, Dr. J. Nadrchal for help with the calculations, Dr. T. M. Donovan for the information on his measurements of photoemission before its publication and Prof. R. Grigprovici for helpful discussions.     

Switching phenomenon and optical properties of Se85Te10Bi5 films

Se₈₅Te₁₀Bi₅ films of different thicknesses ranging from 126 to 512 nm have been prepared. Energy-dispersive X-ray (EDX) spectroscopy technique showed that films are nearly stoichiometric. X-ray diffraction (XRD) measurements have showed that the Se₈₅Te₁₀Bi₅ films were amorphous. Electrical conduction activation energy (ΔE_σ) for the obtained films is found to be 0.662 eV independent of thickness in the investigated range. Investigation of the current voltage (I-V) characteristics in amorphous Se₈₅Te₁₀Bi₅ films reveals that it is typical for a memory switch. The switching voltage V_th increases with the increase of the thickness and decreases exponentially with temperature in the range from 298 to 383 K. The switching voltage activation energy (ε) calculated from the temperature dependence of V_th is found to be 0.325 eV. The switching phenomenon in amorphous Se₈₅Te₁₀Bi₅ films is explained according to an electrothermal model for the switching process. The optical constants, the refractive index (n) and the absorption index (k) have been determined from transmittance (T) and reflectance (R) of Se₈₅Te₁₀Bi₅ films. Allowed non-direct transitions with an optical energy gap (E_g^opt) of 1.33 eV have been obtained. ΔE_σ is almost half the obtained value of E_g^opt, which suggested band to band conduction as indicated by Davis and Mott.