Observations of propagation in shock-crystallization of sputtered amorphous germanium films

Dependence of the propagation of shock-crystallization in amorphous germanium films on the geometrical shape of the specimen has been investigated. Microscopic change in crystallized region has been observed by a scanning electron microscope. Domino-like energy transfer mechanism has been revealed from the observed results.     

Heat-radiation from shock-crystallization of sputtered amorphous germanium films

Temperature dependence of the shock-crystallization, effect of heat treatment, and heat radiation from the specimen during the process of the event have been investigated. Local temperature rise as high as 500°C accompanied with the propagation of crystallization wave has been observed.     

Thermoelectric power and electrical conductivity of sputtered amorphous germanium: Effect of annealing

Detailed measurements of the temperature dependence of dc electrical conductivity(T) and the thermoelectric powerS(T) at different annealing stages of sputtered amorphous (a-Ge) and polycrystalline germanium (c-Ge) layers in the range 80–430 K are reported. The sign ofS of the polycrystalline layers is positive. On the other hand the sign ofS of the a-Ge is always negative. At low temperature theS(T) dependences of a-Ge are constant and weakly depend on annealing. In the high temperature region the ¦S(T)¦ curves reach maximum values and depend relatively strongly on annealing. In the low temperature range the(T) dependences satisfy the T^–1/4 Mott's law. Annealing shifts the(T) towards lower values especially at low temperature. The(T) andS(T) data of a-Ge are discussed on the basis of a proposed model of the hopping conduction within the mobility gap.The author expresses thanks to Dr. S.Koc, Dr. Z.ima, Dr. V.Vorlíek and Dr. I.Gregora for helpful discussions.     

Magnetoresistance in amorphous germanium

Magnetoresistance of sputtered films of a-Ge has been studied. The experiments were performed in the temperature range 78–450 K and in magnetic fields up to 7 kG. The magnetoresistance has been found to be negative up to 450 K and changes its sign at 80 K. The experiments are confronted with a model based on the Zeeman splitting of localized electronic levels in the applied magnetic field. It is shown that such model can lead to relatively good agreement between the theory and the experiment.We are grateful to B.Velický for discussions about the subject matter of this paper and to J.Zemek for preparation of germanium samples.     

Controlled doping of rf sputtered germanium films

Germanium films have been rf sputter deposited on a variety of substrates. A new techniques has been developed to control doping concentrations of the films at predetermined levels for bothp-type as well asn-type films. The hole concentrations of these films could be varied from 10¹⁵ to 2×10¹⁸/cm³ while the electron concentrations could be varied from 10¹⁵ to 5×10¹⁷/cm³ using this technique. Transmission electron microscope studies have been made to study the crystalline quality of the films.     

Optical absorption in sputtered amorphous phosphorus

Optical absorption measurements are reported on r.f. sputtered films of a-P prepared at variable substrate temperatures. With decreasing substrate temperature the absorption spectra shift to lower frequencies and the energy gap decreases. The form of the spectra are, however, similar both in the band edge and Urbach tail regions. The Urbach slope is found to differ substantially from that of a-As, indicating that factors other than coordination number may influence this parameter. The changes in the absorption spectra are suggestive of a possible role of intermediate range order on the electronic states in a-P.     

Negative differential resistance in amorphous germanium

Results of a study of static current-voltage curves of amorphous germanium diodes with gap-type electrode construction are presented. It is shown that with increase in the interelectrode distance there occurs a transition from a switching regime with irreversible memory to a stable negative differential resistance regime. The diode current-voltage curves and data obtained by studying potential distribution over specimen length are explained with a model of space charge-limited currents.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 103–107, March, 1980.     

Two types of magnetoresistance in amorphous germanium

The magnetoresistance ot sputtered films of a-germanium has been measured in the temperature region 78–450 K in magnetic fields up to 25 kG. Two types of dependences of the magnetoresistance on the magnetic field and the temperature have been found. Differences between these two types are ascribed to the influence of deep impurity levels on the electrical conductivity in amorphous Ge.We are grateful to Mr. J.Zemek for preparation of the germanium samples.     

Laser-stimulated conductivity of amorphous germanium films

It is shown that metastable states are created in evaporated -Ge films by laser irradiation. The lifetime of the metastable state is a strong function of the incident laser power density.Supported by IRSIA (Brussels)     

Ion implantation in amorphous germanium and silicon

The effects of Sb and Al implantation on the conductivity of amorphous Ge and Si film are reported. The room temperature conductivity of the vacuum deposited films is found to increase by a factor of 50 to 100 on implantation. The excess conductivity can be removed by annealing at 300°C for 6 hr. Results on a-Si films deposited in a partial pressure of H₂ also indicate that doping effect due to ion implantation is very small. The temperature dependence of conductivity of the films is found to obey a $\text{T}{}^{\mathrm{<mtext>?1</mtext><mtext>4</mtext>}}$ law before and after implantation. The value of T₀ is found to be rather insensitive to both hydrogenation and implantation.     

Impurity effects in amorphous germanium and silicon

A systematic study of the hoping conductivity of amorphous germanium-transition metal (Cr, Co, Fe) films reveals an exponential decrease of the hopping parameter, T_o, as a function of the transition metal concentration. A similar, but often unnoticed, behavior is found in the literature. A linear decrease in the energy gap as a function of concentration is postulated as an explanation.     

The temperature dependence of the electrical conductivity in amorphous germanium

An investigation of the electrical conductivity in amorphous Ge has been made. It has been found that the temperature dependence of the conductivity can differ from the usually acceptedT ^–1/4 law. It has been shown that transition from the hopping-type conductivity can be influenced by the conductivity in extended states.Cukrovarnická 10, Praha 6, Czechoslovakia.We are grateful to dr. B. Velický for discussions about the subject matter of this paper and to Mr. J. Zemek for preparation of germanium samples.     

Investigations of the electron structure of ion-implanted amorphous germanium

Ge amorphised by Sb⁺ implantation has been investigated by means of UV photoemission. Our results differ significantly from earlier data on amorphous Ge. It is suggested that this implantation produces a new kind of amorphous state, characterized by modified local atomic arrangement.     

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.     

Memory switching of germanium tellurium amorphous semiconductor

The dc conductivity and switching properties of amorphous GeTe thin film of thickness 262 nm are investigated in the temperature range 303-373 K. The activation energy ΔE_σ, the room temperature electrical conductivity σ_RT and the pre-exponential factor σ₀ were measured and validated for the tested sample. The conduction activation energy ΔE_σ is calculated. The I-V characteristic curves of the thin film samples showing a memory switching at the turnover point (TOP) from high resistance state (OFF state) to the negative differential resistance state (NDRS) (ON state). It is found that the mean values of the threshold electrical field E_th decreased exponentially with increasing temperatures in the investigated range. The switching activation energy ΔE_th is calculated. Measurements of the dissipated threshold power P_th and the threshold resistance R_th were carried out at TOP point at different temperatures of the samples. The activation energies ΔE_R and ΔE_P caused by resistance and power respectively are deduced. The results obtained support thermal model for initiating switching process in this system.