Laser Emission in Thin DielectricCoated CdSe LasersVern N. Smiley, Henry F. Taylor, and Adolph L. Lewis Citation: Journal of Applied Physics 42, 5859 (1971); doi: 10.1063/1.1660026 View online: http://dx.doi.org/10.1063/1.1660026 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/42/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Bi-nanoparticle (CdTe and CdSe) mixed polyaniline hybrid thin films prepared using spin coating technique J. Appl. Phys. 105, 034904 (2009); 10.1063/1.3072627 CdSe quantum dot microdisk laser Appl. Phys. Lett. 89, 231104 (2006); 10.1063/1.2402263 Model dielectric function of hexagonal CdSe J. Appl. Phys. 68, 1192 (1990); 10.1063/1.346716 Modification of CdSe resistivity by laser annealing J. Appl. Phys. 50, 5624 (1979); 10.1063/1.326736 Laser annealing of CdSe thin films AIP Conf. Proc. 50, 665 (1979); 10.1063/1.31735

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of the mass of the ions and the mass of the film atoms. It cannot be determined at this time how these Ta ions are formed. Due to the high evapora­tion temperature, the Ta atoms may be thermally ionized. It is also conceivable that Ta atoms are ionized by electron impact. The latter effect should be more probable in Ta than in the more common metals because (i) high currents are needed for evaporation of Ta, and ionization probability in­creases with electron current, (ii) Ta is a heavy atom and, therefore, its velocity is less than that of a light atom; hence, the Ta atom remains in an area of electron impact for a longer time, and (iii) because Ta is a heavy atom, the ionization cross section is large. For these reasons it must be as­sumed that Ta ions are created during evaporation.

It has been demonstrated that during e-beam evapo­ration, high-energy electrons are reflected toward the substrate. These electrons cause floating sub­strates to acquire high negative voltages. The posi­tive Ta ions which are generated during the e-beam evaporation are accelerated toward the high negative

potential, and sputtering results. If the electric field generated by the potential of the substrate is not homogeneous, then the sputtering is not homo­geneous over the substrate and visible nonuniformi­ties are generated.

The authors,are grateful to Mrs. M. Whitaker for her editorial assiatance and careful preparation of the manuscript and figures.

tInformation contained in this paper is the result of re­search supported bv the Advanced Electronic Devices Branch, Air Force Avionics Laboratory, Air Force Sys­tems Command, Wright-Patterson Air Force Base, Ohio, and performed jointly in house under Contract No. F33615-67-C-1312. Basic research project 4150 is the programming authority for this work.

tC. Hayashi, Research/Development 20, 40 (1969). 2S. Tolansky, Multiple Beam Interferometry (Oxford U. P., New York, 1948).

30. S. Heavens, Optical Properties of Thin Solid Films (Dover, New York, 1955), p. 62.

4L. Young and F. G. R. Zobel, J. Electrochem. Soc. 113, 277 (1966).

JOURNAL OF APPLIED PHYSICS VOLUME 42, NUMBER 13 DECEMBER 1971

Laser Emission in Thin Dielectric-Coated CdSe Lasers

Vern N. Smiley, * Henry F. Taylor, and Adolph L. Lewis Naval Electronics Laboratory Center, San Diego, California 92152

(Received 3 May 1971)

We report laser action in optically pumped short­cavity lasers, one to a few J1. thick, in a direction parallel to the cavity length (perpendicular to the crystal film plane) in CdSe platelets with multilayer dielectrics deposited directly on the crystal faces. Excitation was with an Ar laser operating at 5145 and 4880 A as well as other weaker lines.

The use of multilayer coatings facilitates operation in the parellel direction by increaSing the Q in that direction. Similar uncoated crystals, all less than 5 J1. thick, pumped in the same manner in our appa­ratus, would lase only in the plane of the thin crystal.

Other workers have achieved laser action in platelets, some in the short dimenSion, using electron beams, two-photon, or single-photon pumping. 1-5 However, in general, it is difficult to achieve laser action in the short dimension with extremely thin-layer ma­terials because the single-pass gain in the crystal plane is normally much larger than in the perpen­dicular direction. In addition, the unpumped material is transpraent to the laser emission, so that crystal edges can form a cavity. 5 On can "spoil" the Q in the the plane of the film by making sure that no two edges are parallel. However, this does not neces­sarily prohibit oscillation arising from multiple paths in the sample. 5

If coatings are used to enhance the cavity Q in the

short dimension, provision must be made for pump­ing energy to penetrate through to the active film. In the case of optical pumping and metallic films, this requires that one coating be thin, hence its re­flectance is reduced. Our coatings consisted of

0.2

0.1 I LASER I l.-J WAVELENGTH '" i.Y RANGE 0.0 ..... -~-~~--a--"'--...

9000 8000 7000 6000 5000 4000

FIG. 1. Transmittance of fifteen-layer multilayer film stack (ZnS-ThOF J on glass substrate.

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FIG. 2. CdSe laser ('" 3. 5 J.l thickness) spectrum obtained with scanning monochromator and oscilloscope. Scan starts at left at 6738 A and wavelength increases to right. Sensitivity-41. 7 A/div. Vertical scale is arbi­trary. Pump intensity was 2x10 5 W/cm2• Output power was approximately 5 mW peak. T= 77 K.

fifteen-layer quarter-wavelength-thick layers of alternate high- and low-index material. The central wavelength (where the layers are a quarter-wave­length thick) is adjusted so that the coated surfaces are highly reflective at the CdSe laser wavelengths but have low reflectance at the pump wavelengths. Figure 1 is a plot of the measured transmittance of a fifteen-layer coated surface. Several pumping lines are indicated: 5145 and 4880 A for Ar and 6471 A for Kr. In practice we used the Ar lines in a pulsed, convertible Ar-Kr laser (Hughes model 3043 H) due to lack of sufficient power in the Kr line.

The Ar lines are far from the CdSe absorption edge, where the absorption is extremely high. The pene­tration depth at 5000 A is only about O. 07 /.J. based on Parson's absorption measurements. 6 However, diffusion of electron-hole pairs is rapid enough to produce a uniform distribution through the sample thickness in a few nsec. 7

A typical laser spectrum obtained from a pulsed Ar laser-pumped CdSe platelet about 3. 5 /.J. thick is shown in Fig. 2. The pump power used here was "" 2-W peak. Threshold power was"" 1-W peak. The large mode widths and total-wavelength range are due to thermal effects causing cavity tuning and mode hopping. 8 The data were taken at 77 K.

Shewchun et al. 9 and Packard et al. 3 have recently pointed out that the appearance of mode structure does not necessarily indicate laser action. We have observed the following characteristics in our thin­cavity lasers supporting the achievement of oscilla­tion threshold: (i) The time history of the total emis­sion follows the shape of the pump pulse until the power is raised to a critical value. At that power, a narrow spike appears at the peak of the output pulse and increases drastically in height and width as the pump power is increased. (This demonstrates

superlinearity. ) (ii) A drastic narrowing of the output spectrum and formation of a single well-defined mode at the same power level. (iii) A sudden de­crease in output-emission angle at this power level. (iv) Visual examination through a microscope re­vealed the sudden appearance of a very bright spot or spots of light smaller than the flourescent spot at the same threshold power. Increasing the pump power further increased the size of the bright areas.

Near- and far-field patterns were observed, the former by photographing a magnified image of the crystal with an image converter camera and the latter by direct photography. Near-field results show the presence of filaments or bright spots much smaller than the pumped region. These bright spots sometimes shift position from pulse to pulse. A far­field pattern is shown in Fig. 3. The film plane was 10 cm from the crystal and the beam diameter ap­proximately 2.5 cm resulting in a beam-spread half-angle of about 7°. This is considerably larger than the diffraction to be expected if the entire pumped region were emitting coherently over a 35-/.J.-diam aperture.

The technique of using coatings providing high re­flectance for the laser wavelength and low reflec­tance for the pump wavelengths can be extended to other thin-film laser systems and used in situations where the pump and laser wavelengths are closer together.

We acknowledge the help of E. R. Schumacher for providing multilayer coatings.

FIG. 3. Far-field image of laser beam. The 2. 5-cm­diam image was formed directly on Polaroid film at a distance of 10 cm from the crystal.

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* Present address: Dese rt Research Institute, University of Nevada, Reno, Nev. 89507

IN. G. Basov, O. V. Bogdankevich, and A. Z. Grasyuk, IEEE J. Quantum Electron. QE-2, 594 (1966).

2G.E. Stillman, M.D. Sirkis, J.A. Rossi, M.R. Johnson, and N. Holonyak, Jr. , Appl. Phys. Letters 9, 268 (1966).

3J. R. Packard, W. C. Tait, and D. A. Campbell, IEEE J. Quantum Electron. QE-5, 44 (1969).

4M.R. Johnson, N. Holonyak, Jr., M.D. Sirkis, and E.D. Boose, Appl. Phys. Letters 10, 281 (1967).

5N. Holonyak, Jr., M.R. Johnson, andD.L. Keune, IEEE J. Quantum Electron. QE-4, 199 (1968). 6R.B. Parsons, W. Wardzynski, and A.D. Yoffe, Proc. Roy. Soc. (London) A262, 120 (1961).

7D. L. Keune, J. A. Rossi, O. L. Gaddy, H. Merkelo, and N. Holonyak, Jr. , AppL Phys. Letters 14, 99 (1969).

BV.N. Smiley, A.L. Lewis, H.F. Taylor, and D.J. Albares, J. Opt. Soc. Am. 60,1321 (1970).

9J. Shewchun, B.S. Kawasaki, and B.K. Garside, IEEE J. Quantum Electron. QE-6, 133 (1970).

JOURNAL OF APPLIED PHYSICS VOLUME 42, NUMBER 13 DECEMBER 1971

Compensated Silicon-Impurity Conduction Bolometer

M.A. Kinch Texas Insf:n.tments Incorporated, Dallas, Texas 75222

(Received 29 January 1971; in final form 4 June 1971)

The capability and performance of compensated Si (Sb ~ 2 x 10lB cm-3; B ~ 2 X 1017 cm-3) impu­rity conduction bolometers as extremely sensitive detectors of far-infrared radiation (2 mm to 30 ~m) is described. Electrical and far-infrared measurements indicate a NEP ~ 2. 5 X 10-14

W 1Hz I / 2, and a response time ~ 10-2 sec, when ope rated at 1. 5 oK,

The impurity conduction bolometer has been used as a very sensitive detector of far-infrared radiation for a number of years .1,2 Its early history was plagued by a lack of understanding of the basic phe­nomenon responsible for the absorption mechanism of this long wavelength radiation, which resulted in the fabrication of these detectors becoming more an art than a science. The situation was clarified by an analysis by Blinowski and Mycielskp,4 of the absorp­tion of electromagnetic radiation in the "hopping" im­impurity conduction regime in n-type silicon and germanium. Their theoretical treatment has since been verified experimentally by a number of au­thors. 5 This theory served to emphasize the impor­tance of compensating impurities in achieving rea­sonable values of absorption coefficient in the bolo­metric material, and also indicated that for a fixed value of compensation K =NA/ND, where ND and NA represent the concentrations of donor and acceptor impurities in an n-type semiconductor, larger ab­sorption coefficients over a wider range of the far­infrared spectrum could be obtained with silicon than germanium. Essentially, this is because the absorp­tion coefficient is a strong function of the impurity concentration ND (in n-type materia!), but there is an upper limit to this parameter determined by the transition from the "hopping" to the "metallic" im­purity conduction regime. Because the donor wave functions are smaller in Silicon, this transition does not occur until much higher values of ND (typical val­ues for the transition concentrations for shallow im­purities are -3X1016 cm-9 for Ge and -3xl018 cm-3

for si). Silicon has two other potential advantages over germanium: (i) The specific heat should be smaller by a factor of - 8 at helium temperature, which coupled with the larger absorption coefficient should yield bolometer elements of very small ther­mal mass. For a fixed modulation frequency, this means that small thermal conductances can be used

[bolometer response time T-C/G-(w",t1, where C

is the heat capacity of the element, G the thermal conductance to the surroundings, and w'" the modula­tion frequency], which in turn results in larger val­ues of voltage responsivity (0:: l/G), and smaller values of noise equivalent power (NEP) ["'4To(kG)1/2, where To is the temperature of the heat sink, and k is Boltzmann's constant]. 1 (ii) Silicon device fabrica­tion is a very advanced technology and much more is known concerning sources of excess current noise, which has been a major problem with low-frequency operation of germanium bolometers. 6

For the above reasons we have fabricated a number of silicon bolometers for use with two Fourier trans­form interferometers presently in our far-infrared laboratory. These detectors have given excellent service over a period of three years and a descrip­tion of their electrical properties, fabrication, and far-infrared performance now seems appropriate.

The material was grown by the Czochralski technique aiming at a composition of 2 x l018_cm-3 antimony

0.6,--,--,--,--,---r----.---,-,---,----, -0- RESPONSE

0.5 - BIAS CHARACTERISTIC

BOLOMETER Si 2' To~1.5 'K

en I-

z

------------~~ ~ 0.4 a:: ----0- <t "0 ---:--; 0.3

--._------. ~ CD a:: <t

0.2

0.1

LU en z o Q. en LU

0.0 '---,-',::,---L_--'-_.L-----'_--L_--L-_...l--_L----l a:: 10~ 10~

l (amp)

FIG. 1. V-I characteristic and signal response vs bias current for element Si2, size 1 x 1 x 0.2 mm, for signal and background flux wavelengths >40~.

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