摘要:

Calculations of the vibrational density of states, Raman and infrared spectra, and inelastic neutron scattering intensity have been performed for model structures representing amorphous Si and Ge. Results for some representations of the random network accord well with available experimental measurements. We are able to explain the principal features of the numerical results in terms of the dominance of central forces and the approximate local tetrahedral order.

ISSN:0094-243X    

DOI:10.1063/1.2945970

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Raman scattering measurements are reported for amorphous 3–5 and 2–6 systems (GaAs,InAs,GaSb,InP,ZnTeandCdTe) prepared by dc sputtering. The spectra, which extend to low frequencies, indicate density of states features in the 3–5 systems related to that of the corresponding crystal as well as distinct wrong bond features in certain materials. The source of these wrong bonds is discussed and suggested to arise from either microscopic phase separation or clustering on an atomic scale. A quantitative estimate fora‐GaSbsuggests that the number of wrong Sb bonds is small but are observed due to their large Raman cross section. The a‐2–6 spectra also suggests wrong bonds, strongly enhanced by the relatively large scattering cross section of a‐Te.

ISSN:0094-243X    

DOI:10.1063/1.2945971

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Recent work has shown that the vibrational properties of solids may be calculated to some approximation by using small clusters such that the surface atoms have an additional potential upon them. This potential, which is complex and frequency dependent, is determined by demanding that the average mean square displacements of the surface and bulk atoms is the same.In this paper, the work is extended from the basic tetrahedral unit of Si/Ge to larger structural units ‐ pentagonal and hexagonal rings. These yield characteristic densities of states which are not very different from those obtained previously for the tetrahedral unit. The implications of this result are discussed.

ISSN:0094-243X    

DOI:10.1063/1.2945972

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

An analytic model structure for amorphous tetrahedrally coordinated substances is presented. A local coordinate system is attached to each atom in a way that the near neighbors of the central atom occupy approximate lattice positions. The deviations of the atomic positions from the local lattice positions grow with the distance from the central atom. The correlation of the relative orientation of the local coordinate systems is decaying with distance, described by an angular correlation length 1/&THgr;o. The nearest neighbor force constants are taken to be the same for all atoms when expressed in the respective local coordinate system. Using a simple bond polarizability the Raman tensor is calculated. The essential features of the experimental curves, including the &ohgr;4dependence for small &ohgr;, can be reproduced and limiting values for &ohgr; and 1/&THgr;oare found.

ISSN:0094-243X    

DOI:10.1063/1.2945973

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Inelastic neutron scattering has been used to study vibrational spectra of amorphous germanium. For large momentum transfers, i.e. several times that of the first maximum in the amorphous x‐ray scattering pattern, neutron energy loss spectra should become insensitive to further changes in momentum transfer and are then simply related to the vibrational density of states. This incoherent limit neutron scattering method in principle avoids the matrix element uncertainties which enter interpretations of infrared absorption and Raman scattering experiments. Preliminary results, obtained with a 5 gram sample of evaporated amorphous germanium, indicate that the lowest energy peak in the density of states is broadened, and its center of gravity is shifted to somewhat lower energy than the corresponding TA phonon peak in the crystalline density of states.

ISSN:0094-243X    

DOI:10.1063/1.2945974

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

It is shown that the first order spectra of tetrahedrally coordinated amorphous semiconductors (TCAS) have a significantly large contribution from the allowed modes of the crystalline case. When these modes are not allowed in the crystalline case, e.g., infrared (IR) spectrum of Ge or Si, the spectrum will be composed entirely of the phonon density of state due to the relaxation of the wave vector (k⇒) conservation rule. A method involving the local changes of inter‐atomic distances or density in the amorphous (&agr;) state is presented which predicts the predominant dynamical disorder broadening of the first order spectra of such amorphous semiconductors.

ISSN:0094-243X    

DOI:10.1063/1.2945976

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

All amorphous materials measured to date show a monotonically increasing absorption (conductivity) proportional to&ngr;&bgr; (&bgr; ≲ 2)in the region above ∼ 0.1 cm−1. Far infrared transmission measurements using a molecular gas laser, and microwave dielectric loss experiments using a cavity perturbation technique establish the existence of such an absorption in evaporated films of &agr;‐Ge. The magnitude of the absorption depends on film preparation. In the microwave region at elevated temperatures a thermally activated conductivity is observed in &agr;‐Ge.

ISSN:0094-243X    

DOI:10.1063/1.2945977

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The annealing properties of both high and low substrate temperature (Ts) a‐Ge samples have been examined. The behavior of high Tssamples is seen to agree with those previously reported. The behavior for low Tssamples is shown to have a wide range of behavior which may or may not be a function of the vacuum during annealing. Depth profiles of oxygen and carbon from Auger spectroscopy have been obtained and results show that high oxygen levels exist in low Tssamples after annealing.In the low temperature regime we show that for our data the variable range tunneling mechanism explains the temperature, annealing and thickness dependence of the conductivity quite well. Some inconsistencies which remain between the data and theory are noted.

ISSN:0094-243X    

DOI:10.1063/1.2945978

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The electrical conductivity of germanium deposited at 80 K has been found to vary with temperature with a small activation energy. On annealing, the characteristics follow more closely the familiar T−1/4law, derived for variable‐range hopping at the Fermi level. The conductivity of all samples increases with electric field above a threshold value. The optical absorption edge sharpens on annealing. Various models to explain the data are considered.

ISSN:0094-243X    

DOI:10.1063/1.2945979

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

We have studied the temperature dependence (in the temperature range of 6–80 K) of electrical conductivity of amorphous Ge films condensed at 8K but subsequently thermally heated to 30K and 80K. The results are described by Mott's type of hopping conduction within the localized states centered at the Fermi level. Below 18K the conduction is due to phonon assisted tunneling between distant hopping sites (ielog &sgr; ∝ 1/T1/4). At higher temperatures the data is consistent with conduction due to phonon assisted hopping between nearest hopping site. Quantitive comparison with the theory leads to very reasonable values of the density of states (∼ 1020eV−1cm−3) and other parameters for these two processes.

ISSN:0094-243X    

DOI:10.1063/1.2945980

出版商:AIP    

年代:1974

数据来源: AIP