Gross R., Sidorenko A., Tagirov L. Nanoscale Devices

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316 A. Casian, Z. Dashevsky, V. Dusciac, R. Dusciac

The results of the numerical calculation of P as a function of

for a p-

type Q1D crystal with the above mentioned parameters and different values

and

D are shown in Fig. 1. It is seen that P has a maximum of

W/cmK 240

near 2.1=

, and

W/cmK 210

=P for 5.1

, a value

very close to one calculated using the approximate expression (9).

Conclusions

The thermoelectric power factor P has been modeled for a series of quasi

one-dimensional organic semiconductors with the goal to find more

efficient materials for the sensitive elements of thermoelectric infrared

detectors. It is found that in some of such crystals, in which the interference

of two main electron-phonon interaction mechanisms is well pronounced,

the value of P can be increased by almost a factor of ten compared to the

respective value in the best PbTe material.

Acknowledgments

The work was supported by INTAS-01-0184 project.

References

1. Dresselhaus MS et al (1999) Proc 18

Intern Conf on Thermoel. Baltimore,

USA (IEEE, Piscataway, NJ) p 92

2. Harman TC, Spears DL, Manfra MJ (1996). J Electron Mater 25:1121

3. Beyer H et al (2002). Appl Phys Lett 80:1216

4. Mironov OA et al (1997). Thin Solid Films 294:182

5. Harman TC, Taylor PJ, Walsh MP, LaForge BE (2002). Science 297:2229

6. Casian A, Sur I, Scherrer H, Dashevsky Z (2000). Phys Rev B 61:15965

7. Casian A, Dashevsky Z, Kantser V, Scherrer H, Sur I, Sandu A (2000). Phys

Low-Dim Struct 5/6:49

8. Sur I, Casian A, Balandin A (2004). Phys Rev B 69:035306

9. Harman TC, Spears DL, Calawa DR, Groves SH, Walsh MP (1997) Proc of

Int Conf on Thermoelectricity. Dresden, Germany p 416

10. Sur I, Casian A, Balandin AA, Dashevsky Z, Kantser V, Scherrer H (2003)

Proc of 21

Intern Conf on Thermoel. Long Beach, USA (IEEE, Piscataway,

NJ):288

Organic Semiconductors – More Efficient Material 317

11. Pope M, Swenberg CE (1999) Electronic Processes in Organic Crystals and

Polymers:2

Ed. Oxford University Press, Oxford

12. Casian A, Dusciac V, Coropceanu Iu (2002). Phys Rev B 66:165404

13. Casian A, Balandin A, Dusciac V, Coropceanu Iu (2002). Phys Low-Dim

Struct 9/10:43

14. Casian A et al (2002) Proc of Intern Conf, CAS. Sinaia, Romania 2:381

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16. Conwell EM (1980). Phys Rev B 22:1761

17. Metzger RM (1981). J Chem Phys 74:3458

Submillimeter Radiation–Induced Persistent

Photoconductivity in Pb

1-x

Te(In)

A. E. Kozhanov

, D. E. Dolzhenko

, I. I. Ivanchik

, D. M. Watson

D. R. Khokhlov

Moscow State University, Moscow 119992, Russia

University of Rochester, Rochester, 14627 NY, USA

Abstract: Persistent photoconductivity in a Pb

0.75

0.25

Te(In) alloy initiated by

monochromatic submillimeter-range radiation at wavelengths of 176

and 241 µm was observed at helium temperatures. This

photoconductivity is shown to be associated with the optical excitation

of metastable impurity states.

Keywords: persistent photoconductivity, impurity states, submillimeter radiation

Introduction

Most of the presently used high-sensitivity non-thermal detectors intended

for use in the far infrared range are based on doped silicon and germanium.

The longest wavelength corresponding to the photoelectric threshold in

such radiation detectors that has been reached in uniaxially strained Ge: Ga

was

= 220 µm [1].

A viable alternative to the silicon- and germanium-based radiation

detectors is offered by lead telluride based narrow-gap semiconductors.

Doping

lead telluride and its solid solutions by group III elements gives

rise to effects such as Fermi level stabilization and persistent

photoconductivity, which are not characteristic of the starting material [2].

particular, the Fermi level in Pb1−xSnxTe(In) alloys with a tin content of

319

R. Gross et al. (eds.), Nanoscale Devices - Fundamentals and Applications, 319–324.

320 A. E. Kozhanov, D. E. Dolzhenko, I. I. Ivanchik, et al.

0.22<x<0.28 stabilizes within the band gap to produce semi-insulating

states of the semiconductor at low temperatures. Because the effect of

Fermi level stabilization brings about homogenization of the

electrophysical parameters of the material and because the characteristic

energy parameters of the alloy, such as the band gap width and the

impurity-state activation energy, are of the order of a few tens of meV, the

possibility of employing these semiconductors as radiation detectors in the

far IR range appears extremely attractive.

This possibility was realized [3], and it was found that the sensitivity

parameters of a Pb

0.75Sn0.25Te(In)-based IR radiometers would substantially

exceed those of their counterparts based on doped silicon and germanium.

However, the key question of the spectral response of the Pb

1–xSnxTe(In)-

based radiation detector, in particular, of the red cutoff of the photoeffect in

this material, has remained unexplored. The phenomenon of persistent

photoconductivity observed in Pb

1–xSnxTe(In) at low temperatures results in

a buildup of nonequilibrium carriers in the allowed band under background

radiation present in each standard spectrometer, thus making standard

spectral measurements impossible. An instrument that has no background

illumination at all but provides the possibility of illuminating a sample with

radiation of a fixed wavelength and calibrated intensity is required.

Fig. 1. Setup for measuring photoconductivity spectra of Pb1–xSnxTe(In) [4]: 1-

blackbody, 2 - entrance window, 3 - “nitrogen” filter, 4 - “helium” filter, 5 -

interference filter, 6 - rotating disk with filters, 7 - sample, and 8 - helium bath.

Submillimeter Radiation–Induced Persistent Photoconductivity 321

Such an instrument was built and described in [4] (see Fig. 1). A sample

was fixed to the bottom of a helium bath in evacuated space. The

background radiation was eliminated by means of screens cooled to liquid-

helium and liquid-nitrogen temperatures. Blackbody radiation with a

temperature of 77 or 300 K impinged on the sample through the entrance

window and a series of cooled filters. Special filters, which were

maintained at the liquid-nitrogen or liquid-helium temperature, transmitted

only the part of the radiation corresponding to the spectral interval under

study. The diaphragm in the “helium” screen made it possible to calibrate

the radiation flux striking the sample. Finally, a narrow radiation line was

isolated by means of an interference filter mounted on a rotating disk within

the helium screen.

It was demonstrated in [4], that the radiation with wavelengths of 90 and

116 µm gives rise to strong persistent photoresponse in Pb

0.75

0.25

Te(In) at

the liquid helium temperature. An important point mentioned in [4] was

that the radiation energy quantum corresponding to the used wavelengths is

smaller than the thermal activation energy of the ground impurity state that

pins the Fermi level. Therefore the persistent photoconductivity was

defined in this case by the excitation of metastable, but not the local ground

states.

However, the interpretation of the results mentioned above has remained

an open issue. Indeed, the thermal activation energy E

a was calculated from

an analysis of the temperature dependence of the resistivity by using the

relation ρ~exp(E

a/2kT) rather than ρ~exp(Ea/kT), as is usually accepted

when dealing with impurity states. The calculations made by using the first

of the above relations were based on a study of the character of the

pressure-induced motion of the impurity level [5]. However, this

substantiation is of an indirect nature. If the activation energy of an

impurity level is calculated using the second relation, the value of E

a will be

one-half of that obtained from the first relation; i.e., it will correspond to a

wavelength of 140 µm. Therefore, the conclusion that the metastable

impurity states provide a major contribution to the photoresponse at

wavelengths of 90 and 116 µm will be invalid. In this paper, we report on

the detection of a photoresponse in a Pb

0.75Sn0.25Te(In) film at wavelengths

of 176 and 241 µm.

Experiment

Pb0.75Sn0.25Te(In) films were grown through molecular-beam epitaxy on a

BaF

2 substrate. The thermal activation energy of the impurity ground state

calculated from the relation ρ ~ exp(E

a/2kT) was 20 meV. The experiment

322 A. E. Kozhanov, D. E. Dolzhenko, I. I. Ivanchik, et al.

was carried out in the setup shown schematically in Fig. 1. The temperature

of the helium screen after filling in the liquid helium reached a stationary

level in ~30 min. The disk with the filters was initially set in the position

where the blackbody radiation impinged on the metal shutter. Since it was

fixed to the bottom of the helium bath, the sample was cooled faster than

the screen and the disk. Therefore, the sample was initially illuminated by

the background radiation of the screen and the disk that had not yet cooled

down. This background illumination gave rise to the generation of long-

lived nonequilibrium carriers in the sample. To transfer the sample to the

unperturbed state, it was warmed with a heater located close to the sample,

after the screen cooling. The increase in the sample temperature to 30 K and

subsequent cooling to liquid-helium temperature transferred the electronic

system of the sample to a close-to-ground state, with practically all the

carriers localized. After this, the disk was rotated such that the radiation

with the wavelength determined by the disk filter was directed onto the

sample. This rotation usually lasted 3 to 4 s.

0 200 400 600 800

0,0

1,0x10

-2

2,0x10

-2

3,0x10

-2

4,0x10

-2

5,0x10

-2

6,0x10

-2

Current, mcA

Time, s

241

176

Fig. 2. Kinetics of the increase and decrease of the photocurrent plotted for a

voltage of 10 mV across the sample and different radiation wavelengths. The

arrows indicate the points in time where the IR illumination is turned on and off.

Submillimeter Radiation–Induced Persistent Photoconductivity 323

We studied the kinetics of the current increase through the sample for

various voltages across the sample and various blackbody temperatures. In

Fig. 2 we have plotted the experimental data obtained for a sample voltage

of 10 mV and a blackbody temperature of 300 K. A noticeable

photoresponse was detected at both wavelengths of the radiation striking

the sample. Because our sensitive measuring equipment made it possible to

record currents of up to 0.25 µA, we could reliably measure the kinetics of

the increase in the photocurrent only at low voltages across the sample,

U<40 mV. At higher voltages, the photocurrent grew so fast that the

amplifier became overloaded in a time comparable to the time required to

rotate the filter disk, i.e., within a few seconds.

Discussion

The following features in the photoconductivity may be of interest here.

First of all, the current rise observed after switching on the illumination

follows a strongly nonlinear kinetics. Switching off the illumination

triggers a fast decay of the photocurrent, with subsequent slow relaxation to

the dark level. If, however, the illumination is switched on again a short

time after its removal, the photocurrent rises very fast (in a time comparable

to the “fast” relaxation time) to the value recorded just before the

illumination removal, after which the previous, relatively slow dynamics of

the increase in the photocurrent sets in again. The fast and the slow

processes are apparently of essentially different natures.

Another feature may also be noteworthy. The energies of photons

corresponding to radiation wavelengths of 176 and 241 µm are substantially

less than the thermal activation energy of the impurity ground state, even if

this energy is calculated using the relation ρ~exp(E

a/kT). Thus, the results

obtained in this study provide direct evidence for the persistent

photoconductivity in Pb

1–xSnxTe(In) originating from photoexcitation of

metastable impurity states. The threshold energy for optical excitation of

these states is very low. The wavelength of the corresponding photon is

longer than at least 241 µm, which, as far as we know, is the largest value

for non-thermal radiation detectors. The photoconductivity cutoff of

the materials studied here apparently lies at substantially longer

wavelengths. One cannot rule out the possibility that the operating range of

1–xSnxTe(In)-based photodetectors extends over the whole submillimeter

range.

324 A. E. Kozhanov, D. E. Dolzhenko, I. I. Ivanchik, et al.

Acknowledgments

This study was supported in part by the Russian Foundation for Basic

Research (project nos. 04-02-16497, 05-02-16657), INTAS (project no.

2001-0184), and a NATO Collaborative Linkage Grant.

References

1. Haller EE, Hueschen MR, Richards PL (1979). Appl Phys Lett 34:495

2. Volkov BA, Ryabova LI, Khokhlov DR (2002). Usp Fiz Nauk 172:875 [(2002)

Phys Usp 45:819]

3. Chesnokov SN, Dolzhenko DE, Ivanchik II, and Khokhlov DR (1994).

Infrared Phys 35:23

4. Khokhlov DR, Ivanchik II, Raines SN, et al. (2000). Appl Phys Lett 76:2835

5. Akimov BA, Zlomanov VP, Ryabova LI, et al. (1979). Fiz Tekh Poluprovodn

(Leningrad) 13:1293 [(1979) Sov Phys Semicond 13:759]

Quasioptical Terahertz Spectrometer Based on a

Josephson Oscillator and a Cold Electron

Nanobolometer

M. Tarasov

, L. Kuzmin

, E. Stepantsov

, A. Kidiyarova-Shevchenko

Institute of Radio Engineering and Electronics RAS, Moscow 125009,

Russia

Chalmers University of Technology, Göteborg SE41296, Sweden

Institute of Crystallography RAS, Moscow 117333, Russia

Abstract: We have developed a low temperature transmission spectrometer

operating in a wide range of frequencies from 100 GHz to 1.7 THz.

The spectrometer has utilized the unique properties of high-Tc

superconducting Josephson junctions and the wideband response of

sensitive Cold-Electron Bolometers (CEB). The voltage response of

the CEB integrated with log-periodic and double-dipole antennas, has

been measured using an oscillator consisting of high-Tc Josephson

junction integrated on a separate substrate with a log-periodic

antenna. Superconducting Josephson junctions with high

characteristic voltages (I

larger than 4 mV at 4.2 K) are fabricated

by depositing YBa

-x on miscut sapphire bi-crystal substrates,

where the tilting axis is along the grain boundary. The cold electron

bolometer having a superconductor-insulator-normal metal-insulator-

superconductor (SINIS) structure was 200 nm wide, 10 µm long, and

terminating tunnel junctions were 200x300 nm² area. The response of

the bolometer with a double dipole antenna has resonance shape with

maximum corresponding to the designed central frequency of 300

GHz. A voltage response of the bolometer up to 4·10

V/W

corresponds to a noise equivalent power of the bolometer of 1.2·10

-17

W/Hz

1/2

. Our measurements demonstrate that the Josephson junction

overheated by the transport current up to 3 K at 1 mV bias when it

is placed on a millikelvin stage. A high-Tc Josephson junction

operated at temperatures below 2 K has the advantage of a high I

product that enhances the oscillation frequency to above 2 THz. The

325

R. Gross et al. (eds.), Nanoscale Devices - Fundamentals and Applications, 325–335.

326 M. Tarasov, L. Kuzmin, E. Stepantsov, A. Kidiyarova-Shevchenko

resolution of the spectrometer is determined by the linewidth of

Josephson oscillations and for this temperature is below 1 GHz.

Further developments of such a device are possible by using a carbon

nanotube.

Keywords: proximity effect, superconductivity, hybrid nanostructures.

Cryogenic Detectors

It has become increasingly evident in the last few years that

superconducting devices will play a major role in radiation detection and

the characterization of the electromagnetic spectrum in terahertz frequency

range. The critical difference between detection at terahertz frequencies and

detection at shorter wavelengths lies in the low photon energies (1-10

meV). The ambient background thermal noise almost always dominates

narrow-band signals requiring cryogenic cooling. The use of

superconducting sensors has been further promoted through the recent

development of ultra-low temperature coolers which no longer need liquid

cryogens. Superconducting devices offer the prospect of fundamental

thermodynamic or quantum limited sensitivity, spectral sensitive detection

with ability to measure photons individually, and the ability to manufacture

large arrays with modern thin-film technology. Ultimate NEP of a

bolometer (see [1]) is determined by fundamental thermodynamics and is

essentially the same for all types NEP

=4k

G, where k

is Boltzmann’s

constant, T the electron temperature, and G the thermal conductance. The

main limitation for practical detector NEP is determined by a background

power load P

that gives NEP

=2P

[2]. At present the state-of-the-art

practical bolometers demonstrate NEP below 10

-17

W/Hz

1/2

at 0.1 K. The

main generic types of cold direct detectors are as follows:

The Transition Edge Sensor (TES) is the most acknowledged type of

superconducting bolometer. It consists of a thin superconducting film that

can change its resistance under the incoming radiation power. Depending

on the frequency range, such a bolometer can be integrated with THz band

planar antenna, or attached to an absorber film that interacts with the optical

or X-ray radiation. For X-ray detectors at 100 mK one of the best

NEP=3

-18

W/Hz

1/2

was demonstrated in [3]. The dynamic range of TES

detectors is strictly limited by dc heating power applied before real

operation. Any attempt

to increase the dynamic range lead to additional

Gross R., Sidorenko A., Tagirov L. Nanoscale Devices - Fundamentals and Applications

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