摘要:

A three‐layered AlxGa1−xAs&sngbnd;GaAs&sngbnd;AlxGa1−xAs structure has been used to measure the optical absorption and photoluminescence in thin GaAs layers prepared by liquid‐phase epitaxy. The results presented here are for lightly dopedn‐type GaAs with free‐carrier concentrations near 1016cm−3; however, the technique can be used for arbitrarily doped material. The absorption coefficient &agr; was measured between 1.4 and 2.2 eV at 2 and 298 K. The absorption strength at the band gap was found to be (1.15×104)±1000 cm−1at 2 K and (0.99×104)±1000 cm−1at 298 K. At 1.96 eV, the energy of the He&sngbnd;Ne laser commonly used for photoexcitation of GaAs, &agr; at 298 K was measured to be 4.4×104cm−1. A value of 3.8 meV for the room‐temperature exciton binding energy was inferred from the temperature dependence of the interband absorption strength. This value together with previous reflectivity data for high‐purity GaAs gives the energy gap of pure unstrained GaAs at 298 K as 1.424±0.001 eV. The effects of strain due to lattice mismatch in the three‐layered structures were observed in the absorption edge at 2 K. The calculated photoluminescence spectrum obtained through the principle of detailed balance from the absorption data agrees well with the measured photoluminescence at 298 K. A comparison of the photoluminescence from the excited surface and back surface permits assignment of an upper limit of 5×104cm/sec for the room‐temperature GaAs&sngbnd;AlxGa1−xAs interface recombination velocity forx≈ 0.5. This comparison of the front and back photoluminescence also shows that the minority‐carrier diffusion length at 298 K for a sample with an electron concentration of 2×1016cm−3is at least 2.5 &mgr;. The data presented here can be used to calculate the intrinsic carrier concentrationni, the thermal radiative generation rateG, and the radiative constantB(=Rr/np). The values at 298 K (corrected where necessary to a band‐gap energyEg=1.424 eV) are as follows:ni=1.8×106cm−3,G=4500 cm−3/sec, andB=1.4×10−9cm3/sec.

ISSN:0021-8979    

DOI:10.1063/1.1663321

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A new form of glass fiber is investigated. The core of the fiber is eccentrically positioned and contacts only a very small part of the inner surface of the outer tube with a lower index of refraction than that of the core. The total loss measured was 10 dB/km at a 0.80‐&mgr;m wavelength and the pulse broadening measured on a 55‐m section was 0.11 ns, which is much smaller than the value expected for the unclad fiber. Thus, it is shown that the part making contact between the core and tube dominates the transmission characteristics.

ISSN:0021-8979    

DOI:10.1063/1.1663322

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Optical absorption and emission spectra of Cr3+ions in YAlO3have been investigated at 300 and 77°K. In the cubic field approximation, the spectra can be fitted by the parametersDq= 1800 cm−1,B= 590 cm−1,C= 2980 cm−1. An intense vibronic spectrum is associated with2E→4A2fluorescence transitions. Measurements of absorption cross sections and oscillator strengths, thermal broadening and shifts of theR‐line fluorescence, and lifetimes of the2Efluorescence and Cr3+‐Cr3+pair lines are reported. At low temperatures and chromium concentrations, the radiative quantum efficiency of the2Estate is approximately unity. Other relaxation processes become important at temperatures > 400°K. The possibility of observing stimulated emission from YAlO3:Cr3+is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1663323

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

GaAsP red‐light‐emitting diodes containing 40% phosphorus were stressed uniaxially in the [100] direction at 77 K. The spectral emission peak was observed to shift to higher energies at the rate of 2.1 ± 0.2 meV/kbar. From the stress dependence of the recombination current, the band‐gap stress coefficient is 3.6 ± 0.3 meV/kbar. By using the fact that radiative recombination takes place via a band‐acceptor (Zn) process, it is concluded that the acceptor level moves away from the valence band at the rate of 1.5 ± 0.4 meV/kbar. The intensity‐voltage characteristic is consistent with the interpretation that the effect of the uniaxial stress is to deionize the acceptor level, which causes a smaller decrease of intensity with stress than would be expected from band‐gap deformation alone. The effect of uniaxial stress on the indirect‐direct conduction‐band transition is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1663324

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The low‐temperature photoluminescence spectra of AgGaS2are investigated in this work. At 4.2°K, the luminescence spectra are dominated by many sharp lines as well as broad emission bands. A series of emission lines was found to be superimposed on the broad band peaking at ∼ 5000 Å. The series of emission lines are attributed to phonon‐assisted transitions involving an unidentified impurity or defect. Two basic phonons with energies of 5.4 and 7.2 meV are identified from the phonon structure. The 7.2‐meV mode is also seen in the Raman spectrum. They are tentatively identified as the lattice modes.

ISSN:0021-8979    

DOI:10.1063/1.1663325

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A theory for carrier mobilities at weakly inverted Si surfaces is described. The theory treats transport of carriers across barriers in regions of statistically variable surface potentials. The potential variations are related to an earlier model proposed by Nicollian and Goetzberger to explain trapping phenomena. Measurements of surface conductance at inverted Si surfaces are presented which extend to lower free‐carrier populations than previously reported work. For bothpandnsurface channels, the measurements show a region of near constancy in mobility at very low surface densities followed by a rapidly increasing mobility at voltages near the conventional inversion threshold. The general features of these measurements and of their temperature variations correlate well with the theoretical model proposed.

ISSN:0021-8979    

DOI:10.1063/1.1663326

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A new experimental technique, uv‐stimulated photocurrent spectroscopy, has been used to study a trapping effect observed in the anodic oxide of sputtered &bgr;‐Ta films. Experimental results indicate the presence of a discrete trap level peaked at 2.1 eV below the oxide conduction band, in addition to the 1.5‐eV trap band as previously reported by Hickmott and later by Thomas. A kinetic model involving a single trap level in the oxide was used to analyze transient data. The results of this analysis indicate that the 2.1‐eV trap is a positive Coulombic center having an electron capture cross section of 3.2×10−14cm2, a photon capture cross section (at 2.1 eV) of 10−17cm2, a density of states of 3×1018/cm3, and a ratio of filled to unfilled states of 0.06 during uv stimulation.

ISSN:0021-8979    

DOI:10.1063/1.1663327

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Thin films of bismuth‐germanium have been fabricated by the method of codeposit vapor quenching onto liquid nitrogen or water‐cooled substrates. Structural, electrical, optical, and photoelectrical characterizations of these films have been accomplished. In this paper, Paper I of a two‐paper series, the structural and electrical characterization results are reported. X‐ray diffraction measurements indicate that a stable amorphous phase exists for the BixGe1−xthin‐film system for 0 ≤x≤ 0.12. Low‐field dc resistivity as a function of temperature from room temperature to liquid‐nitrogen temperature and relative thermopower measurements show that in the amorphous range the BixGe1−xthin films aren‐type semiconductors with a continuously decreasing thermal activation energy asxincreases. The resistivity of amorphous bismuth‐germanium films decreases with an increase in bismuth concentration. Current‐voltage data taken on several films at 300 and 162°K indicate Ohmic behavior with no evidence of Poole‐Frenkel, Schottky‐barrier, or switching effects. A structural model for the amorphous bismuth‐germanium system is proposed with covalent hybrid bonding of monoatomic bismuth to a Polk‐Turnbull amorphous germanium random network. An energy‐band density‐of‐states model is proposed based on the Weaire‐Thorpe interpretation of the Polk‐Turnbull network and considerations from the theory of heavily doped semiconductors. The proposed structural and energy‐band models are utilized to explain the observed electrical and optical properties.

ISSN:0021-8979    

DOI:10.1063/1.1663328

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Thin films of bismuth‐germanium have been fabricated in vacuum by the method of codeposit vapor quenching onto liquid nitrogen or water‐cooled substrates. In this paper, Paper II of a two‐paper series, the optical and photoelectrical characterization of these films are reported. Transmittance, reflectance, and absorptance data are reported in the spectral range 0.2–3.0 &mgr; for BixGe1−xthin films in the composition range 0 ≤x≤ 0.25. Ultraviolet/visible near‐normal reflectance spectra for films in the amorphous phase (0 ≤x≤ 0.2) closely resemble the optical reflectance spectra of amorphous germanium. The reflectance spectra of the amorphous films were static and insensitive to an increase in bismuth concentration. Additionally the spectra were devoid of any optical remnants of the crystalline spectra of either germanium or bismuth. The reflectance spectra of a Bi0.25Ge0.75film which was x‐ray polycrystalline revealed a broad local maximum near 2 eV, indicating crystalline bismuth transitions and the existence of a pure Bi phase. Near‐infrared transmittance, reflectance, and absorptance data indicate that the optical energy gap,Egopt(x)of thea‐BixGe1−xthin‐film system is reduced with an increase in bismuth concentration. An expression forEgopt(x)as a function of bismuth concentration is developed using the formulation of Cardona from calculated absorptance data. ac photoconductivity studies at room temperature and low temperature (T= 162°K) have been accomplished fora‐BixGe1−xthin films in the composition range 0 ≤x≤ 0.03. The binary films in this set were found to be photoconductive with a response of the same order of magnitude and spectral shape as that of amorphous germanium films. The observed photoconductivity was present at room temperature and increased one to two orders of magnitude upon cooling toT= 162°K. The photoconductive spectral edge occurred at a photon energy between 0.6 and 0.8 eV for all films studied. An observed shift in the spectral edge with bismuth concentration is tentatively attributed to the optical band‐gap dependence on the alloy concentration.

ISSN:0021-8979    

DOI:10.1063/1.1663329

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Experimental results are presented on the effect of diluent cooling of a 1721‐Å high‐pressure xenon laser produced with a pulsed beam of high‐energy electrons. Both helium and argon were used as diluents in order to limit the rise in the gas temperature that occurs when the electron beam is stopped in the gas. In the Xe&sngbnd;He mixtures the chemistry was not significantly altered by the presence of He; but in the Xe&sngbnd;Ar mixtures, argon was very important in the absorption and transfer of internal electronic energy. The results demonstrate that both diluents improve the operational characteristics of the laser. Only helium is found to improve the efficiency of conversion of electron‐beam energy into stored laser energy.

ISSN:0021-8979    

DOI:10.1063/1.1663330

出版商:AIP    

年代:1974

数据来源: AIP