Broadband photosensor with a tunable frequency range, built on the basis of nanoscale carbon structure with field localization a
Alexander N. Yakunin ∗a, Garif G. Akchurin a,b, Nikolay P. Aban’shin c, Boris I. Gorfinkel’ c Institute of Precise Mechanics and Control, Russian Academy of Sciences, 24 Rabochaya street, Saratov 410028, Russia b Saratov State University named by N.G. Chernyshevsky, 83 Astrakhanskaya street, Saratov 410012, Russia c Volga-Svet Co.Ltd, Saratov, 102 50-Let Octyabrya avenue, Saratov 410052, Russia ABSTRACT
The work is devoted to the development of a new direction in creating of broadband photo sensors which distinctive feature is the possibility of dynamic adjustment of operating frequency range. The author’s results of study of red threshold control of classic photoelectric effect were the basis for the work implementation. This effect was predicted theoretically and observed experimentally during irradiation of nanoscale carbon structure of planar-edge type by stream of low-energy photons. The variation of the accelerating voltage within a small range allows you to change photoelectric threshold for carbon in a wide range - from UV to IR. This is the consequence of the localization of electrostatic field at tip of the blade planar structure and of changes in the conditions of non-equilibrium electrons tunneling from the boundary surface of the cathode into the vacuum. The generation of nonequilibrium electrons in the carbon film thickness of 20 nm has a high speed which provides high performance of photodetector. The features of the use of nanoscale carbon structure photocurrent registration as in the prethreshold regime, and in the mode of field emission existence are discussed. The results of simulation and experimental examination of photosensor samples are given. It is shown that the observed effect is a single-photon tunneling. This in combination with the possibility of high-speed dynamic tuning determines the good perspectives for creation of new devices working in the mode of select multiple operating spectral bands for the signal recording. The architecture of such devices is expected to be significantly simpler than the conventional ones, based on the use of tunable filters. Keywords: dynamically tunable photodetector, carbon nanoscale structure, single-photon photodetector, broadband photodetector, multiple spectral band photodetector
1. INTRODUCTION Researches [1] electron emission of granular films of gold and silver activated by cesium and oxygen have shown that the photoeffect in such structures is determined by the probability of tunneling nonequilibrium photoelectrons through the potential barrier formed by the active layer. In this case, an exponential decrease of the intensity of the longwavelength part of the spectrum of photoelectron emission with increasing wavelength and the unexpected absence of a clearly defined photoelectric threshold are observed. Increasing the spectral sensitivity in the band width of about 100 nm and non-monotonic character of its dependence on the frequency was related by the authors [2] to the excitation of surface plasmons. This assumption is confirmed by the results of theoretical analysis, obtained in [3]. The phenomena of surface carrier trapping metallic nanoparticles and the search for effective tunneling conditions of undoubted practical interest in connection with the prospects for its use at creation of new high-speed photocathodes with a femtosecond performance [4].
∗
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2. THEORY OF TUNNEL PHOTOEFFECT However, in [5] covered issues which, in our opinion, were beyond the scope of research studies [1-4]. It is a very important regularities of influence on the parameters the tunnel PE level of intense electrostatic fields with high strength, which are located on nanoscale inhomogeneities of the cathode surfaces. Investigation of processes of field emission in planar nanostructures such as that shown schematically in Fig. 1 a [5, 6], prompted the authors to submitting a new task. It was decided to use the structure with field localization to reduce the red tunnel photoeffect threshold [7, 8]. The technology of manufacturing such a structure (see Fig. 1b) is widespread in the manufacture of semiconductor heterostructures, it provides good stability of processes. But the most the remarkable property, as was shown in [9] was to provide the active protection against bombardment of carbon blade by ions of residual atmosphere of technical vacuum molecules. Consider the case of irradiating of emitter surface by an optical beam with a wavelength of λ = a/ν. Then during the onephoton absorption process by equilibrium electron of emitter with boundary Fermi energy EF nonequilibrium electrons are generated. Their energy increases by hν. This spatially local energy distribution of photoelectrons characterized quasi-Fermi level EFn limiting energy photoelectrons EFn = EF + hν. Probability of nonequilibrium photoelectrons tunneling and the corresponding current density photoemission Jph can be determined after taking into account the efficiency of absorption of optical radiation emitter η and modification of Fowler-Nordheim relation [10]:
J ph =
⎛ 8π (2m)1/ 2 (ϕ − hν ) 3 / 2 ΔZϑ ( y ) ⎞ (1 − R )(1 − eαL )Wη ( βU ) 2 e3 ⎟⎟ ⎜⎜ − exp , (1) 8πht 2 ( y ) ΔZ 2 (ϕ − hν ) 3 eh β U h ν Nc ⎠ ⎝
where
ϑ ( y)
and t ( y ) – even slowly decreasing functions [10] with a range of variation of argument 1 > y ≥ 0 ; 2
y = (e3 βU / ΔZ )1 / 2 /(ϕ − hν ) – relative reduction in the potential barrier height for nonequilibrium photoelectrons; m – mass of the electron; R – reflection coefficient of optical radiation from emitter surface; N – concentration of free electrons in emitter material (cm-3); W – optical power density (W/cm2); α – coefficient of optical radiation absorption of emitter material (cm-1); η – quantum efficiency is a dimensionless quantity equal to the ratio of photoelectrons reaching the emitter surface to the number of absorbed photons in the volume and taking into account the effect of energy relaxation of photoelectrons in the emitter volume; L – effective penetration depth of optical radiation into emitter material (cm), c – light velocity. Classic photoemission in metal samples considering nonzero emitter temperature under weak electrostatic fields (do not change the width and height of the potential barrier) was investigated in Fowler’s work [11]. The theory is allowed to determine the electron work function of the metal for the actual temperatures. These experiments confirmed quadratic dependence of photocurrent on the change of frequency in the vicinity of photoelectric threshold ν0. It should be noted that the presented model allows the existence of photoeffect at low photon energy hν – lower, than hν0. This difference between the vacuum energy level E0 and the Fermi level EF does not exceed of kT. Field emission current density of equilibrium carriers JF is determined by the known formula [10]:
JF =
⎛ 8π (2m)1 / 2 (ϕ )3 / 2 ΔZϑ ( y ) ⎞ e3 ( βU ) 2 ⎜⎜ − ⎟⎟. exp 8πht 2 ( y ) ΔZ 2 (ϕ ) βU ⎝ 3eh ⎠
(2)
The symbols in the formula (2) correspond to the previously introduced in (1). Then the resulting current density JΣ is found by superposition (1) and (2) as
J∑
=
Jph + JF.
(3)
Two different groups of carriers determines a more complex dependence (3) of electrostatic field intensity and the conditions of irradiation by photons.
Light
hν α-C
1
Mo SiO2 Glass 2 (a)
(b) Figure 1. Scheme of planar carbon structure with nanoscale localization field irradiated with light of photon energy hν (a) and photo in cross section structure made by use of electron microscope (b). 1 – carbon film with blade(emitter), 2 – anode. Scale label on the photo corresponds to 200 nm.
3. LOCALIZATION OF OPTICAL AND ELECTROSATIC FIELDS Analysis of the structure of formula (1) shows that, unlike the field emission current density the tunnel photocurrent is determined not only by the level of localization electrostatic field intensity (the field enhancement factor) The sensitivity of the photosensor is affected by a number of factors: photon energy of irradiating flow, reflection coefficient of optical radiation from emitter surface, concentration of free electrons in emitter material, optical power density, coefficient of optical radiation absorption of emitter material, effective penetration depth of optical radiation into emitter material and quantum efficiency. Dimensional effects can significantly alter the character of the interaction of light flow with the material emitter. Finite element simulation of the optical field intensity distribution in the nanoscale structure shown in Fig. 1, was carried out. It was assumed that plane wave is incident. A wide range of wavelengths of incident radiation has been investigated - from 0.6 to 10 microns. Electrophysical parameters of the carbon film and the molybdenum substrate were taken from [12]. In the conjugation of the carbon film with edge of the molybdenum layer significant heterogeneity of electrical component intensity of the optical field is revealed. The distribution of the radiation intensity I/I0with wavelength λ = 0.6 um, calculated in the carbon film, is shown in Fig. 2 depending on the abscissa coordinate (along the film). The degree of optical field localization characterized by a maximum I/I0 = 158. Over the carbon film thickness significant inhomogeneity of the field also observed - from bottom to upper surface of the film I/I0 falls by 6 times. Very unexpected was the fact that on blade point (X = 400 nm) the degree of localization of the optical field is negligible. Quantitative evaluation of changes of I/I0 maximum in wavelength range is given in the table. This dependence is not so critical, which is a favorable factor in achieving proper bandwidth photosensor. Study of parameter I/I0 is important because it determines the concentration of nonequilibrium electrons and directly affects the sensitivity of photosensor.
1000 Lower side Median plane Upper side
I / I0
100
10
1
0.1 0
100
200
300
400
X, nm Figure 2. Dependence of radiation intensity I/I0 in carbon film vs. coordinate X (along the film) for three cross sections in thickness. X=400 nm corresponds to edge of carbon blade.
Table. Dependence of I/I0 maximum in wavelength range
Parameter
Maximum of intensity I/I0
Wavelength (um)
radiation
0.6
2.0
10.0
156
79
64
The maximum intensity of electrostatic field is localized at the tip of the blade of carbon film. The results of modeling the distribution of equipotentials and contours of electrostatic field modulus are shown in Fig. 3 a and 3 b, respectively. Difference of potentials "emitter - anode" in the calculated variant assumed to be equal to 100 V.
⎪E⎥ max=0.6 kV/um
a
b
Figure 3. Distribution of equipotentials U (a) and contours of electrostatic field modulus ⎪E⎥ (b). Step of equipotentials is 5 V, step of contours of electrostatic field modulus is 3 V/um.
4. EXPERIMENTAL RESULTS AND DISCUSSION Each photodiode comprises a comb_shaped array of emitters (spatially periodic sequence of microscopic carbon blades with an edge length of 200 nm and a thickness of 20 nm) and anodes. The planes of emitters and anodes in various samples of microdiodes are spaced by 1–3 um. The maximum potential difference between the emitter blade and anode does not exceed 100 V. The investigation of microdiodes with carbon emitters included preliminary measurements of the dark current–voltage (I–U) characteristics. The tunneling photoeffect in nanodimensional diode structures was studied while probing their comb-shaped emitter arrays by laser and diode sources, operating at wavelength 380, 405, 473, 683, 849, 950 and 1550 nm. On opposed to the classical photoelectric effect, which occurs only when the photon energy greater than the work function of the material (the tunneling probability is equal to 1), the tunnel photocurrent in a strong (107-108 V/cm) field occurs at a lower photon energy (the possibility of reducing by more than 6 times has been experimentally proved, see Fig. 4). This ensures the control of "red" threshold of tunnel photoeffect in the wavelength range from the UV to the IR. Fig. 4 shows the results describing the possibility of precise control of both the frequency
dependence of the sensitivity of the photoelectric effect, and the level of the photocurrent by small changes in the voltage limits "emitter-anode". The dotted line denotes the "red" threshold of the classical photoelectric effect.
25
Anode potential, V
20
15
10
5
0 100
1000
10000
Wavelength, nm Photoelectric red threshold
Field emission boundary Red threshold of classical photoeffect
Figure 4. The frequency dependence of the "red" threshold of tunnel photoeffect in carbon nanostructures in the range of UV to IR vs. the anode potential
The purpose of the next series of experiments was to study the patterns of influence on photosensor sensitivity of field intensity in the area of tip of emitter it is determined by the accelerating potential difference applied to the gap "emitter anode") and optical power level of irradiation. Blue solid-state laser diode-pumped SHG DPSS (Optronics) with adjustable power level was used for irradiation, the wavelength of 473 nm, the photon energy hν=2.617 eV. Measurement results of the current-voltage characteristics of the photodiode are shown in Fig. 5. By W denotes the dimensionless output power of laser, W=0 corresponds to the absence of radiation. Thus, the bottom curve in Fig. 5 represents the dependence of the dark current from applied potential difference. At a stable operation of device, this current can be subtracted beforehand when taking measurements. Curves in Fig. 6 were obtained in this way. At low power levels of irradiation (W = 1 and W = 2) at them definitely appear areas of saturation when increasing the electric field at the tip of the emitter does not lead to an increase in the tunneling photocurrent. This means that all of the nonequilibrium electrons appearing when irradiated by an external sourceeither relaxed on the way to the tip of the emitter, or have overcome the potential barrier at the emitter surface. Selection of a specific field intensity allows to control the sensitivity of the photosensor. It should be take into account that at the same time "red" threshold of photoeffect changing also. In other words, investigated the structure provides possibility of dynamic control the spectral sensitivity of the photosensor and, as a consequence, the possibility of implementing a multi-frequency tracking mode. The speed of control by emission current is determined by the rate of change of optical power or electrical potential.
1000 100 10 I, nA
W=0
1
W=9 W=2
0.1
W=1
0.01 0.001 0
5
10 15 20 25 30 35 40 45 U, V
Figure 5. The current-voltage characteristics of the photodiode with nanoscale carbon emitter when irradiated with blue solid-state laser diode-pumped SHG DPSS (Optronics), the wavelength of 473 nm, the photon energy hν=2.617 eV, the dimensionless output power of W; W=0 corresponds to the absence of radiation.
250
I, nA
200
W=9 W=2
150
W=1
100 50 0 0
5
10
15
20 25
30
35
40
45
U, V Figure 6. Dependence of tunnel photocurrent vs. the anode potential. The conditions of irradiation and a notations of dimensionless power are the same as in Fig. 5.
5. CONCLUSIONS 1. We have developed the theory of tunnel photo effect, which occurs in carbon nanosized structures with localization of both electrostatic and optic fields. 2. The localization of first one determines the possibility of broad band control of tunnel photoeffect. The experimentally observed shift of the red threshold characteristic for classical photoeffect exceeded six times in frequency of low-energy photons. 3. The localization of second one effects on heterogeneity of concentration of nonequilibrium electrons and, as a result, the quantum yield of photoelectrons into vacuum. 4. Developed the theory of tunnel photo effect allows adequate to describe the phenomenon tunneling saturation photocurrent with increasing electric field intensity backdrop of rising the field emission current. 5. Described dependence of the tunnel photocurrent from electrostatic field intensity makes the perspective of constructing new photosensors with dynamically controlled spectral sensitivity. Acknowledgments Authors are grateful to Yu.A. Avetisyan for a fruitful discussion of the research results. This study was supported by the Russian Foundation for Basic Research, project no. 12-07-12066-ofi_m.
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