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A new method of depositing amorphous chalcogenide glass layers by chemical vapor deposition has been developed. It was demonstrated that thicknesses up to 1 μ of compositions in the Si–As–Te system could be deposited on sapphire substrates. The optical constants of these layers were measured in the 2 to 5-μ region and indices of refraction between 3.4 and 3.6, with extinction coefficients less than about 0.02, were observed. One of the major advantages of this technique is that is does not require vacuum. The initial work on the Si–Te–As system resulted in layers which were macroscopically uniform with no apparent crystallinity. The capability of compositional grading is also inherent in the method and its application as a corrosion barrier is demonstrated.

A detailed analysis of the method of thermally stimulated capacitor discharge (TSCD) is presented. Distinction is made between the dielectric relaxation currents and the trap-limited currents. The position, shape, and number of the depolarization peaks caused by the dielectric relaxation currents are shown to depend (i) on the conductivity of the sample as a function of temperature and electric field and (ii) on the geometry of the sample. It is pointed out that the behavior of the dielectric-relaxation-current peaks is quite similar to the trap-limited-current peaks, and an analysis of the peaks may lead to false conclusions, if done without distinguishing them from one another. We assert that no new information about the trap parameters can be obtained from TSCD, unless the structure of the polarization and depolarization curves are different. No such difference was found in the chalcogenide glass studied by us, and the observed TSCD could be explained in terms of dielectric relaxation currents only. We also point out that a low drift mobility, a short screening length, and possibly a large density of localized states near the Fermi level, make the observation of trap-limited TSCD in multicomponent chalcogenides unlikely.

One approach to obtaining electro-optic or magneto-optic modulation in an integrated optic structure is to use a transparent film on an “active” (electro- or magneto-optic) optical substrate. Since the most useful active optical materials have relatively high refractive indices which preclude the use of oxide glasses as the transparent thin film, we have investigated the possibility of using higher index chalcogenide glasses in this configuration. Preliminary waveguiding studies were performed on a model system consisting of an As2S3 film on a LiNbO3 substrate which demonstrated the feasibility of using chalcogenide glasses in this regard. The optical and mechanical properties of a suitable glass system, As-Ge-S, were then characterized. The characterization consisted of developing empirical formulas based on the type of bonding present in the glass which can be used to predict the relevant properties of a glass as a function of its composition. These formulas allow selection of optimum glass compositions to match a variety of electro-optic and magneto-optic substrates.

A number of chalcogenide glasses were investigated for thin film switching applications. Two ranges of threshold voltage were of interest, 15 and 30 V. The switching performance of thin film devices was evaluated and rated on a simple numerical scale. The memory glasses based on the Ge-Te eutectic gave generally satisfactory performance. Selenium-based glasses exhibited high threshold voltage in thin film form, but had limited lifetime. Threshold voltages of about 30 V were obtained from Bi-As-Se glasses; these proved difficult to lock “ON” and possible reasons for this are discussed.

The conducting 'skin' formed on the surface of chalcogenide glasses as a result of chemical interaction with O2 or H2O at elevated temperatures is shown by Raman scattering to be crystalline Te. The DC conductivity and thermoelectric power of the skin are consistent with the interpretation of the Raman data.

An investigation has been carried out into the use of thin film chalcogenide glass memory switches for addressing large-area electroluminescent displays. Material preparation of the bulk glass used as a starting material and the subsequent r.f. sputtering process used to produce thin films are discussed in some detail. Three basic approaches are described for obtaining high-threshold memory switches and their relative merits are compared. The simplest method of producing such a memory switch, by increasing the thickness, is shown to have serious limitations. The threshold fields have also been increased by varying the composition of the thin film but the compositions so far evaluated were difficult to lock-ON and possible reasons for this are discussed. The third approach is to connect electrically a number of identical thin film devices in series; this is shown to be the most successful method.

The current-voltage characteristics and the temperature and electric fieid distributions of the point contacted Te-Se-Ge glass are numerically obtained by solving the thermal model. The maximum temperature rise near the electrodes needed for the switching is about 35°C and the switching field is below 105 V/cm. Both quantities are independent of the ambient temperature and the resistivity. Thermal runaway occurs after the maximum temperature rise reaches 35°C.

The properties of chalocogenide glass semiconductors of Te-Se-Ge and Te-Se-Sb systems are investigated by electrical conduction and differential thermal analysis. When germanium is introduced to a binary Te-Se system, it forms a strong three-dimensional network structure, and increases the glass transition temperature because it is rather easy to form covalent bonds with selenium and tellurium. Introducing antimony into the glass decreases the activation energy remarkably and increases the conductivity, and a further increase of the antimony content promotes the crystallization of the glass; whereas even by increasing the germanium content the crystallization scarcely occurs. The resistivity of the present glasses is expressed by the reiationship, ρ=ρ0exp (ΔE/kT). The pre-exponential factor, mainly determined by the mobility of the carriers, is nearly constant, and independent of the composition.

Fritzsche's measurements (see abstr. A28342 of 1970) of the electrical properties of a chalcogenide glass in the pre-switched regime have been examined and found to agree with a model of hopping conduction within an exponential distribution of localized states.

Measurements on thin chalcogenide-glass threshold switches have shown that the on-state of a formed device consists of several distinct branches which depend on the switching history of the device and the electrode materials. The multiply branched on-state is interpreted in terms of the sequential forming of current channels between conducting islands in a filamentary region between the electrodes.

The thermally stimulated currents (TSC) in a number of amorphous chalcogenide glasses are studied. In order to detect small changes in conductivity, after exposure to light, two samples are used in a bridge configuration. The TSC observed are found to be independent of the heating rate and the decay times are of several hours at 300 K. We find that our observations cannot be explained in terms of thermally stimulated currents. Models are discussed to explain the phenomena observed. We suggest that the observed changes in conductivity are caused by the same photoinduced defects which give rise to the reversible light-induced shifts of the optical-absorption edge in a number of chalcogenide glasses.

A field effect has been measured in a semiconducting chalcogenide glass, of composition Te2AsSi. The sample was in the form of a fine filament, encapsulated within a rod of insulating glass, with the bias field applied radially. The effect is small, and the change in conductivity increases more slowly with field at high fields than at low fields. The results are explained in terms of a model with a uniform density of bulk states near the Fermi energy and a sharp surface level just below the Fermi energy. There is a depletion layer at the surface at zero bias, hence the surface barrier with no external field varied with each sample.

A simple test is developed to establish the existence of an Ohmic thermally activated conductivity in a chalcogenide glass which is carrying uniform current densities sufficient to cause current‐controlled negative differential resistance. The technique involves proper plotting of the turnover voltage data, taken as a function of ambient temperature, according to a theoretical relation obtained for a Joule‐heated thin‐film sandwich structure. This technique is of particular value in the event that insufficient information is available for a detailed analysis of the current‐voltage curve. Experimental data from a thin‐film a‐As2SeTe2 device are plotted according to this relation and evidence is presented that the chosen conductivity function does not obtain for this glass at high current densities, even though this behavior is observed at low current densities.

The optical and electrical properties of the metal photo-doped chalcogenide glasses were studied. The optical absorption of the chalcogenide glasses e.g., the As–S, As–S–Te, and As–S–Se systems) was increased by the photo-doping of such metals as Ag and Cu in the region of their absorption edges. The electrical conductivity of the glasses also increased with the photo-doping of the metals. The effective band gap obtained from the results of the temperature dependency of the electrical conductivity decreased with the increase in the amount of metal which was photo-doped in the glasses. The results obtained in this study suggest that the metals photo-doped in the chalcogenide glasses behaved as structural modifiers of the glasses, and that the state of density in the gap was increased by the metal photo-doping.

Transport measurements have been performed on the conducting state of a memory-type chalcogenide glass, of composition Te81Ge15As4. DC resistivity was measured from 4°K to 300°K and indicates no carrier freeze-out occurs down to 4°K. The resistance ratio, ϱ(300°K)/ϱ(4°K), was found to be 2.5. AC conductivity measurements at room temperature showed only an 18% increase in σ(ω) up to 150 khz. Hall effect experiments at 100 Hz show no dependence of carrier concentration on temperature from 77°K to 300°K, and indicate a room temperature mobility of 85 cm2/V-sec. The sign of the Hall constant implies that the carriers are holes. Resistance was measured as a function of magnetic field from 0 to 140 kOe, at 1.6°K. The magnetoresistance was positive and proportional to H2 at low fields, with saturation beginning to set in above about 80 kOe. The results of these studies, taken in conjunction with the X-ray and NMR data, can be used to suggest a plausible model for memory behavior in chalcogenide glasses.

Contributions to the growth temperature of amorphous semiconducting films prepared by physical vapor deposition have been identified and explained both theoretically and emprically. Temperature measurements have been made with the aid of thin film thermocouples deposited on the substrate prior to the deposition of a chalcogenide glass (As 2 Se 3 ). These measurements show the importance of substrate bonding to an isothermal reservoir in maintaining low temperature rises and further indicate that, for the particular source used, the heat released by condensation of the evaporant is a significant contributor to the total heat flux incident on the film.

A variety of compositions of glass in the As-Te-I, As-Te-Ge, and As-Te chalcogenide series have been prepared and structurally analyzed. The analytical techniques included x-ray powder diffraction, density determinations, and differential-scanning-calorimetry experiments. The temperature dependence of the Hall mobility, Seebeck coefficient, and dc conductivity was determined for six representative alloy compositions. The Hall mobility was found to be $n$-type, low, and activated with nearly equal activation energies in each case. The conductivity mobility was deduced from the $p$-type Seebeck coefficients and the conductivity measurements to have an activated character with a characteristic energy of 0.19 \ifmmode\pm\else\textpm\fi{} 0.02 eV for all the measured compositions. These results appear to be inconsistent with the familar Cohen-Fritzsche-Ovshinsky (CFO) picture for transport in amorphous semiconductors, and a new model for conduction in these glasses is proposed. This model involves the hopping of holelike small polarons between sites associated with a common As-Te structural entity which is postulated to be present in all the alloys measured here. The sign, magnitude, and temperature dependence of the dc conductivity, Seebeck coefficient, and Hall mobility are all consistent with this model.

The beginning of transmission for a solid occurs at a wavel ength corresponding to the energy of its forbidden gap. The long wavel ength cut off for the solid is usually determined by a lattice or lattice type absorption of the constituent elements. These two wavelength limits are qualitatively predictable from the position of the constituent elements in the Periodic Table; that is, their atomic masses and electronic configurations. Absorption within the transmission range is usually related to impurities or the perfection of the sample and is thus related to the method of preparation.

The general features of the conductivity spectra of chalcogenide glasses in the frequency range 109 to 1013 Hz are briefly reviewed.

The paper reports measurements of the transport and photoconductive properties of non-crystalline Se, As 2 Te 3 , and As 2 Se 3 . The data are briefly discussed in terms of the band structure of these materials.

Current vs. voltage characteristics of thin films made of Te-As-Ge-Si-type chalcogenide glass have been measured at various temperatures. It has been found that two types of field dependence occur. At electric fields above 10 4 V/cm there is Poole-Frenkel emission. At fields above 5 × 10 5 V/cm a stronger field dependence occurs. At low temperatures this can be explained by tunneling of electrons from traps to the conduction band and from the valence band to traps, respectively. A gaussian trap distribution in the mobility gap was used in analyzing the high field currents measured. The measurements of current vs. voltage characteristics have been supplemented by measuring thermally stimulated current.

The paper deals with methods and techniques for the examination of internal defects in opaque dielectric or semiconducting materials: infrared microscopy (normal or polarized radiation; normal or half-dark field), infrared polarimetry and the Schlieren method. Infrared images of various defects found in amorphous and liquid selenium, pure and doped, and in some types of chalcogenide glasses are shown. Measurements of the absorption coefficient of amorphous selenium using defect-free samples are presented. It is shown that defects can seriously affect the measured physical properties of chalcogenide glasses.

Heat capacity measurements have been performed for the phase-separated pseudobinary chalcogenide glass system X PbSe-(1− X )Ge 1.5 As 0.5 Se 3 in the composition range X = 0.0 to 0.6. The heat capacity versus temperature plots for the phase-separated glasses exhibit two glass transition breaks. The composition dependences of the heat capacity changes, ifΔC p , at the two transitions are shown to be in accord with the behavior predictable from the phase diagram of a glass-forming system exhibiting liquid-liquid immiscibility and with the structure of these glasses as determined by electron microscopy. The two composition points on the immiscibility dome at which the temperature is equal to the higher glass transition temperature have been determined from the composition dependence of the ΔC p 's and indicate that the dome spans almost the entire composition range for this system.

The paper presents the functional expression of V-A characteristics dependent on hydrostatic pressure up to 5 kbar for chalcogenide glass Ge15Te81S2As2. The measured V-A characteristics and those derived on the assumption of a linear decrease of the energy gap due to pressure have been compared. Temperature changes caused by current passing through the sample in thermal contact with the environment have been taken into consideration similarly as in the case of paper (2).

The paper is concerned with thermal and non-thermal operating conditions of threshold switches made of multicomponent chalcogenide glass alloys [Te40As35Ge7Si18]. The observed current-time and frequency-delay relationships are shown to be inconsistent with thermal and electro-thermal models when applied to threshold conditions, and suggest electronic interpretations.

Studies have been made of the temperature dependence of the switching characteristics of thin and thick chalcogenide switches. The onset of break down in both can be described quantitatively be thermal avalanching, although in the former switching disappears at the glass transition temperature. This observation, together with a large activation energy (1·6 ev) deduced from measurement of the recovery time after a single switching event, have been used to formulate a qualitative model of the switching action, after breakdown. It has been suggested that at the glass transition temperature a hightly-conducting phase is precipitated out of the glass matrix to stabilize the temperature, so preventing dielectric breakdown and enabling repeated switching from a resistive to a conducting state.

The paper is concerned with thermal and non-thermal relaxation processes. The free carrier lifetime, as assessed from the time-decay of the ON-state condition, is shown to be of the order of 0·1 μsec. The short-term memory exhibited by threshold switches is interpreted in terms of this free carrier decay. It is shown that sequential switch operation is, additionally, influenced by the dielectric relaxation time which governs space charge decay. There is also a (very small) thermal contribution to switch recovery. It depends on the initial temperature distribution and thus on the duration of the preceding ON-state.

It is shown that two distinct operating regimes exist, a low voltage regime (close to the threshold point) under which the switching delay is subject to substantial fluctuations and a high voltage regime (high overvoltages) for which the delay is closely determined. The results show that the origin of the scatter lies not primarily in the mechanism whereby the threshold point is approached but in the transient and semi-permanent after-effects of previous switching events. These after-effects can be detected in terms of a diminished pre-threshold conductance.

Glasses based on S, Se, and Te have been under study for a number of years as possible infrared optical materials. Though amorphous in nature, their optical properties are similar to those of crystalline semiconductors. Chalcogenide glasses, as they are called, become transparent on the long wavelength side of an absorption edge while their long wavelength cutoff is determined by a lattice type absorption. In the transparent region, impurity absorption is found. Free carrier absorption is not observed even at elevated temperatures. Other physical properties of chalcogenide glasses related to their use as optical materials, compare favorably with conventional infrared optical materials. Because of their amorphous nature, chalcogenide glasses can be prepared in large, very honogeneous pieces. Optical homogeneity is extremely important when preparing optical components for high resolution optical systems.

Changes in the surface conductance of chalcogenide glass As-Te- Ge by heat treatment in various ambients were studied. Temperature dependence of the surface conductance suggests that the increase of the surface conductance is due to a certain degenerate material chemically formed on the surface with the presence of water.

The reversible formation and growth of filaments between two electrodes placed on the surface of a chalcogenide glass is described. The composition of the filament and the surface temperature during the filament formation were measured.

The dynamic current-voltage characteristics are analyzed for a metal/chalcogenide glass/metal system in which the metal of one or both electrodes interacts with the glass.

An experimental study was made of film diodes based on a chalcogenide glass with symmetric and asymmetric electrodes and with two lower symmetric electrodes. Film diodes with aluminum electrodes can be used as varistors. The electrodes affect the shape of the current-voltage characteristics. Diodes having two lower electrodes have a longer useful lifetime than do those having an upper electrode and a lower one. Generation in the negative-resistance region is described.

The paper gives an account of two series of experiments on threshold switches based on chalcogenide glass films, one concerned with the nature of the on -state, and one concerned with the effect of crystalline semiconductors as contacting materials. The results, including the demonstration of a new “blocked- on -state”, consolidate electronic (non-thermal) interpretations of threshold switching. Asymmetric switches involving one graphite and one Ge contact exhibit asymmetric switching, the direction of the asymmetry being reversed for n-type and p-type Ge. Such switches are also light sensitive.