Physics - Theses, Dissertations, and other Required Graduate Degree Essays

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Luminescence studies in cadmium sulphide.

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
1968
Abstract: 

This thesis describes some properties of the luminescence from single crystals and evaporated thin films of CdS under optical and electron beam excitation. A comparison . of optical and electron beam excitation mechan-ismSj as applied to the study of luminescence from solids,, is made. It is concluded that the variable penetration depth of high energy electrons with accelerating voltage can be readily used to distinguish between surface and bulk effects,, and that electron beam excitation can be used to advantage when the efficiency of a radiative process is low and high excitation intensity is required. The main advantage of optical excitation is in the vertical excitation of electrons from lower to higher energy levels which arises from the negligible momentum of the incident photons. Optical excitation spectra of various bound exciton emission lines in single crystals of Cds are , presented . It is shown that -the complex responsible for the so-called I, emission ^ due to the annihilation of an exciton bound to a neutral acceptor,, is created by the direct formation of bound excitons on the impurity site and by the formation of free or intrinsic excitons which are subsequently trapped on the impurity. The complexes responsible for the so-called 1 and I,- emission are also created by the formation of intrinsic excitons but more importantly they are created by direct phonon assisted formation of excitons bound to neutral and ionized impurities. It is concluded that the impurity involved is a donor. Electron beam excited luminescence from evaporated thin films of CdS is obtained. The transitions giving rise to this luminescence are identified as resulting from the recombination of free electrons with bound holes (free-bound emission) and from the recombination of bound electrons with bound holes (bound-bound emission) with the simultaneous emission of n LO phonons (n = 0,, 1, 2, ...). It is shown that if the films are coated with a 2000A thick layer of SiO , causing the CdS energy bands to be bent down at the X SiO - CdS interface,, the peak position of the free-bound emission x is shifted toward higher energies when the incident electrons have a small penetration depth (low energy). This is shown to result from an accumulation layer of electrons at the SiO - CdS inter- X. face .

Document type: 
Thesis
File(s): 
Senior supervisor: 
R.R. Haering
Department: 
Science: Department of Physics
Thesis type: 
(Dissertation) Ph.D.

Laser induced dielectric breakdown and mechanical damage in silicate glasses.

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
1968
Abstract: 

Dielectric breakdown and mechanical damage in silicate glasses under high intensity laser radiation is investigated in detail. A Q-switched ruby laser is used to induce photoconductivity in soda glass., fused quartz and quartz crystal. 27 The number of charge carriers produced per laser pulse of 10 -2 -1 photons cm s is accounted for by multiphoton ionization of nonbridging oxygens in the silicon oxygen network. The relative magnitude of the effect proves that the photoconductivity does not result from ionization of sodium in glass as extensively quoted in literature. The lifetime of the charge carriers produced is estimated to be 10 s. Prom thermoelastic considerations a criterion for the validity of possible damage mechanisms is established. It is shown that stimulated Brillouin scattering cannot give rise to an effective absorption of 50 cm in the focal volume as required by the thermoelastic considerations. It is proposed and established that the mechanical damage is caused by the acceleration of primary electrons produced by multiphoton ionization, leading to a fully developed electronic instability in few nanoseconds. At this electron density the absorption in the focal volume is 10 cm" , and is responsible for the complete absorption of the laser pulse at intensities above the threshold for breakdown. The diffusion and recombination of electrons are found to be negligible, the only rate limiting - iii - process being the loss of electron energy to the lattice. The variation in the threshold intensity for breakdown in different glasses is due to the variations in the elastic scattering cross section. The study of the mechanical damage caused by a laser pulse., leads to the estimation of the surface energy of the (-material, which in the case of soda glass is found to be 10 ergs/cm The enhancement of the photoconductivity signal is obtained when -5 -4 a second laser pulse comes within the time 10 -5 x 10 sec. of the first pulse. The additional number of electrons produced by the second pulse is accounted for by the ionization of the color centers,, caused by trapping the electrons produced by the first pulse. - iv -

Document type: 
Thesis
File(s): 
Senior supervisor: 
K.E. Rieckhoff
Department: 
Science: Department of Physics
Thesis type: 
(Dissertation) Ph.D.

Field effect investigations in thin cadmium sulfide films.

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
1967
Abstract: 

Field effect investigations on thin cadmium sulfide films have yielded a method of controlling the surface potential. The control of the surface potential is achieved by the successive evaporation of two dielectrics, GaF2 and SiO. The SiO produces donor-like surface states at the CdS surface,, while CaF2 produces acceptor-like surface states. By evaporating a thin layer of CaF2 between the CdS and SiO., the effect of the donor-like surface states on the surface conduction of CdS may be reduced. Thus, by proper choice of the CaF3 thickness, any surface potential between the limits of SiO and CaF2 may be obtained. The analysis, construction, and performance of a new evaporated, thin-film transistor capable of withstanding over 300 V is described. This thin-film transistor will switch a current of 100p.a with a gate voltage of less than 50 V, and is particularly suited as a transistor controlled switch. The large increase in operating voltage is achieved, by changes in the device geometry, material characteristics, and through careful control of the CdS surface potential by 'the method, described above. Devices were constructed with an incremental saturation resistance of 200Mi7, and a maximum operating voltage in excess of 350 V. These transistors were used to switch electro- luminescent lamps with gate voltages of 50 V in less than 0.2 msec. The transistors with semiconducting layers of CdS, utilizing CaF3 _, Si03 and Ge03 insulators and Al electrodes^ were fabricated on glass substrates by vacuum evaporation. In related experiment it was found that an evaporated CaP3 layer could be used to vary the surface potential of germanium,, and the effects of controlling the surface potential of a Ge(Li) p-i-n diode were observed. A thickness of 150 to 200 A of CaF3 appeared to restore the surface potential in the intrinsic region to the bulk value,, and thus resulted in a lower surface leakage current. - 11 -

Document type: 
Thesis
File(s): 
Senior supervisor: 
R.R. Haering
Department: 
Science: Department of Physics
Thesis type: 
(Dissertation) Ph.D.

Interband optical absorption in semiconductors.

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
1968
Abstract: 

A unified, treatment of the direct. Intel-band optical absorption in semiconductors in the presen'.-;.- f external static electric and magnetic fields., is presented in tne framework of the 'effective mass approximation1. A general expression for tne absorption coefficient Is derived, wnlcn in une appr..?par at el.y which may provide the values of effective masses .in alilerent bands. The experimental conditions,, and the validity criterion for the perturbation theory., are thoroughly discussed. Some attention :.i s paid to the study of magnetic field induced surface states. Whereas suen states have been observed recently in metals, there is no experimental or theoretical evidence as to whether or not these states exist in semiconductors. We have theoretically predicted that such states do in fact exist even in semiconductors. The experimental conditions under which surface states may be detected experimentally in semiconductors are discussed. The arguments are supported with quantitative calculations,, where possible. Throughout the entire presentation we nave restricted ourselves to the study of direct band gap semiconductors. - 11 -

Document type: 
Thesis
File(s): 
Senior supervisor: 
R.R. Haering
Department: 
Science: Department of Physics
Thesis type: 
(Dissertation) Ph.D.

Recombination kinetics in CdS.

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
1967
Abstract: 

In high-purity cadmium sulfide crystals,, at low temperatures and high excitation intensities, emission lines attributed to free and bound exciton recombination are observed in the spectral range 4860 to 5090 A. In addition, the main peaks of two broad emission bands, which are repeated at lower energies with the simultaneous emission of one or more longitudinal optical phonons, are observed at about 51^-0 and 5180 A.. The high energy band, which is d.ominant at liquid, nitrogen temperatures, is d.ue to free electrons recombining with holes bound at cadmium vacancy acceptors. The low energy band, which is dominant at liquid helium temperatures, is due to electrons bound to shallow donors recombining with the bound holes. The photoluminescence efficiency and photoconductivity response of cadmium sulfide crystals were measured and the data interpreted in terms of an energy band model involving the donor and acceptor levels previously established as being involved in the radiative transitions. In addition, an effective recombination center (consisting of deep acceptor-like recombination centers) and non-radiative surface recombination centers are required to account for the non-radiative transitions. The results of the thesis are divided into four topics and are summarized below. The first topic deals with the controversy in the literature regarding the origin of the high energy emission band at about 51^-0 A. Two recent papers, which identify this band, as being due to bound electron-to-bound hole transitions, are analyzed and. it is shown that their conclusions are incorrect. Further ii, analysis and experiments show that their data support the free electron-to-bound electron interpretation of other authors. The second topic was the effect of surface recombination centers on the luminescence efficiency. These states are believed to be mainly chemisorbed oxygen ions. Non-radiative surface recombination is reduced by applying an electric field to counteract the electric field in the charge depletion layer next to the surface, or by phot o-d.es orb ing the oxygen ions. This electric field d.raws minority carriers to the surface where they recombine non-radiatively. The luminescence efficiency is found to be lowest when the electron-hole pairs are generated.,, closest to the surface. This is interpreted as meaning that a greater fraction of the carriers can reach the surface to recombine and that ambipolar diffusion of carriers into the interior of the crystal does not take place. It was also found, that heating CdS briefly in a nitrogen ambient produces free-to-bound and bound-to-bound transitions associated with nitrogen acceptors 130 meV above the valence band. The nitrogen impurities are near the surface since these band.s are removed by a short etch in concentrated hyd.rochloric acid. The third topic was the recombination kinetics of excitons and the bound, electron-to-bound hole luminescence. In all cases, the exciton efficiency increases with increasing excitation intensity as expected since the formation of excitons depends on the prod.uct of the free carrier densities. The bound-to-bound emission efficiency is high and varies slowly with excitation intensity. The efficiency falls slowly both at high and at ill. low excitation intensity. The fall-off in efficiency at high excitation intensity is accompanied by an increase in efficiency of the free-to-bound emission band. The decrease in efficiency at low excitation intensities may be due to non-radiative surface recombination. The last topic was the recombination kinetics of the free electron-to-bound hole luminescence. Using the energy band model mentioned earlier, the data was analyzed to obtain the electron and hole lifetimes, the luminescence efficiency, and the electron and hole capture cross-sections of the cadmium vacancy acceptor and the other d.eep recombination center. The internal luminescence efficiency is near unity as long as the minority carriers (holes) are quickly captured by the radiative recombination centers (cadmium vacancies). At high temperature the luminescence efficiency is low because the cadmium vacancy centers act as traps rather than recombination centers, while at high excitation intensities the efficiency drops because the radiative transitions saturate.

Document type: 
Thesis
File(s): 
Senior supervisor: 
K. Colbow
Department: 
Science: Physics Department
Thesis type: 
(Dissertation) Ph.D.