摘要:

Here we report a variety of results obtained by using sequences of luminescence spectra excited by 1–20‐keV electron irradiation to carry outin situstudies of depth‐dependent optical activity in luminescent crystals. Data are shown for various samples subjected to localized damage from ion implantation: GaAs implanted with Cu+, ZnS implanted with Ar+and Cu+, and CdS implanted with Ar+and H+. Semiquantitative interpretation of the results shows that the depth‐resolved cathodoluminescence measurements can have unique value in characterizing the effects of ion‐implantation lattice damage. In this case cathodoluminescence can be excited from depths ranging from much shallower to much deeper than typical implant depths. In addition, the use of depth‐resolved measurements on nominally undamaged ZnS crystals reveals the presence of weak near‐surface luminescence bands despite careful surface preparation. This result makes it clear that luminescent center profiling by layer removal methods can lead to erroneous results when recurring spectral features result from inherent near‐surface conditions. More importantly, in several ZnS and CdS samples we find that unexpectedly sharp near‐surface depth resolution (of the order of several hundred Å) can be obtained even though bulk carrier diffusion lengths are of the order of microns. While this effect is presently not understood in detail, the result should permit depth‐resolved measurements on sputtering damage and very shallow implants. The various samples we have studied show that modifications in spectrum from injection‐level effects (particularly GaAs) or from exposure to electron irradiation (particularly CdS) may appear in depth‐resolved or other luminescence measurements. Finally, the depth‐resolved cathodoluminescence measurements are compared with single‐wavelength photoluminescence spectra on the same samples for cases where the photoexcitation light is or is not strongly absorbed. It is pointed out that the cathodoluminescence measurements are more adaptable toin situdepth profiling because the irradiation energy (excitation depth) and current (injection level) can be easily varied over wide ranges, and because the excitation depth is not affected by the optical absorption coefficient (which is often uncertain, particularly in implanted layers). In addition, we find that cathodoluminescence can be more sensitive than photoluminescence for studying optical centers introduced by shallow implants, even when the sample is strongly absorbing to the photoexcitation light.

ISSN:0021-8979    

DOI:10.1063/1.1662736

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Photovoltaic spectra for lead selenide diffusedp‐njunctions have been studied versus temperature and substrate impurity concentration. The shape of the spectra and its variation with these two parameters can be explained by taking into account the contribution to photovoltage of several mechanisms. At low photon energies, absorption near the junction is dominant. On the high‐energy side, the flat response can be assigned to absorption in the substrate material, followed by spontaneous reemission (radiative energy transfer). In the intermediate region, the dip in photoresponse is due to variations in the spatial distribution of photoexcited carriers which are generated in a wide region of chemical impurity gradient. Careful evaluation of those parameters which we measured in ourp‐nstructures fits with the characteristics of PbSe studied by other methods.

ISSN:0021-8979    

DOI:10.1063/1.1662737

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

It is shown that the apparent doping profile obtained from a1/CHF2 vs Vplot (whereCHFis the high‐frequency MOS capacitance andVis the voltage across the capacitor) can be corrected for interface‐state effects to obtain a more accurate doping profile. This more accurate doping density,N(w), at distancewfrom the semiconductor‐insulator interface is related to the uncorrected density,N0(w), obtained by neglecting interface states, byN(w)=N0(w) (1−CLF/Cox) (1−CHF/Cox)−1, whereCLFis the quasistatic MOS capacitance at the voltage corresponding to the depletion widthwandCoxis the oxide capacitance. This scheme is shown to provide the doping profile to within 5% accuracy for distances from three extrinsic Debye lengths out to the maximum depletion width obtainable in thermal equilibrium. Experimental verification of this method is obtained by measuring the doping profile on a single capacitor before and after bias‐temperature stress. The same doping profile is obtained in both cases despite a large change in interface‐state density.

ISSN:0021-8979    

DOI:10.1063/1.1662738

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Wave propagation in optical waveguides deposited on gyrotropic and anisotropic substrates is analyzed in terms of the eigenmodes of the guide. The field composition and the phase velocity of the eigenmodes are determined. The results of the analysis are applied to waveguide modulators and distributed‐feedback lasers for use in integrated optics. Based on the eigenmode analysis, coupled‐wave equations are set up to facilitate the analysis of integrated‐optics problems which involve the use of gyrotropic and anisotropic waveguides.

ISSN:0021-8979    

DOI:10.1063/1.1662739

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Results are presented to show that at threshold the normalized current density increases and the spontaneous electron lifetime decreases, with increasing acceptor concentration in the active region of double heterostructure stripe geometry lasers. It is shown that, regardless of the acceptor concentration in the range from 2×1017to 3×1019cm−3, an injected electron density of about 2×1018cm−3is needed in order to achieve lasing in a 12‐&mgr;m‐wide stripe laser. Our threshold data confirm earlier experimental results of higher optical loss and a theoretical prediction of lower gain at a given current for heavily doped material. Variations in threshold current density and spontaneous lifetime as a function of acceptor concentration indicate that the laser transition proceeds by a band‐to‐band recombination mechanism rather than by a band‐to‐acceptor mechanism.

ISSN:0021-8979    

DOI:10.1063/1.1662740

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Threshold chalcogenide switching devices have been maintained in an on‐state using cw functions in conjunction with the original set pulse. The on‐state is distinct from a memory state in that the off‐state can be recovered by lowering the signal amplitude (or frequency) without the need of a high current reset pulse. Once set on, the maintained or preserved on‐state shows no switching discontinuity. Essentially zero current is observed (in either polarity) until a critical voltage level is reached, defined as the barrier voltage, beyond which the current rises almost vertically. TheI‐Vcurve of the preserved on‐state displays its most pronounced temperature dependence at frequencies just above the value at which the device turns off but is relatively temperature independent at significantly higher frequencies (1 MHz and above). The temperature dependence has been investigated near the turn‐off frequency from 500 down to about 20 °K, and consistently shows a maximum in barrier voltage vs ambient temperature at about 35 °K. At room and liquid‐nitrogen ambients, barrier voltage increases superlinearly with decreasing frequency from about 1 MHz to the turn‐off value. With a cw source, the turn‐off which is caused by decreasing frequency appears to be due to allowing a current less than the minimum holding value to persist for a critical time period. Studies of interruptions of the pulse‐switched on‐state show that when the on‐state voltage is forced to zero for a time in excess of about 0.4 &mgr;sec the device will turn off. As the interruption time increases, the reswitching voltage also increases until the original threshold is recovered at interruption times of about 1.5 &mgr;sec. A similar dependence has been found in the reswitching voltage for interruptions which do not force the on‐voltage to zero but simply reduce it.

ISSN:0021-8979    

DOI:10.1063/1.1662741

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The ionization rate of carriers in GaAs has been determined as a function of field from photocurrent in a Schottky barrier. Analysis of the data is based on a model which includes the effects of junction widening and light absorption in the semiconductor. The results agree well with earlier reported values. The technique may be valuable for obtaining avalanche measurements in semiconductors for which fabrication ofp‐njunctions is difficult.

ISSN:0021-8979    

DOI:10.1063/1.1662742

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Coupling of electromagnetic energy between parallel single‐mode dielectric waveguides is investigated theoretically. Expressions giving the decrease in coupling length due to an antisymmetric change in refractive index are derived. The minimum parallel‐guide path length,l, required for switching or modulation is determined from coupling length calculations. It is shown that, for large waveguide separations,&Dgr;nl=3&lgr;/4, where &lgr; is the free‐space wavelength and &Dgr;nis the magnitude of the antisymmetric index change. Calculations for slab and square‐cross‐section waveguide geometries indicate that antisymmetric index changes are much more effective than symmetric changes for switching or modulation.

ISSN:0021-8979    

DOI:10.1063/1.1662743

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The upconversion of a 1.5‐&mgr; pump beam to 0.65‐&mgr; visible fluorescence has been studied in CaF2:Er3+over a concentration ranging from 0.01% to 2.0%. The upconversion process has a cubic dependence upon pump intensity and ion concentration and has been shown to result from a sequence of two cooperative pair interactions.

ISSN:0021-8979    

DOI:10.1063/1.1662744

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The photoluminescence fromn‐ andp‐type In1−xGaxP withx≃0.5, prepared by vapor‐phase epitaxy on GaAs substrates, has been studied between 4.2 and 300 K. This material is of particular practical interest because of its close lattice‐parameter match with GaAs and direct energy band gap of 1.9 eV. At very low temperatures, four major emission bands have been identified, involving intrinsic recombination, donor—to—valence‐band transitions, conduction‐band—to—acceptor transitions, and donor‐acceptor transitions. The intrinsic recombination dominates in all the samples above about 150 K. The spectra are consistent with a shallow donor ionization energy of 7±1 meV, the same value as in InP. The spectral data of Cd‐doped samples (withpvarying from 1.8×1016to 9.3×1017cm−3) suggest a consistent shift of the Cd acceptor ionization energy to lower values with increasing doping. The extrapolated value for very low doping is 59±2 meV at 50 K. The residual donor density is low in all thep‐type samples studied (≲ 1016cm−3). In the case of highlyp‐type Zn‐doped material, a major new emission band is seen 0.19 eV below the band‐gap energy at 77 K and may contribute to the low radiative efficiency of this material. The band is believed to be due to a complex center involving Zn. The photoluminescence data presented for Zn‐doped material indicate that the radiative efficiency falls off steeply with increasing doping at 300 K in In0.5Ga0.5P withp> 1018cm−3, in contrast to the case of GaAs. The relevance of the present data to homojunction laser diode fabrication is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1662745

出版商:AIP    

年代:1973

数据来源: AIP