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Characterization of GaAs/AlGaAs resonant tunneling diodes with a GaInNAs absorption layer as 1.3 µm photo sensors

F. Hartmann1, F. Langer1, D. Bisping1, A. Musterer2, S. Höfling1, M.Kamp1, A. Forchel1 and L. Worschech1

1Technische Physik, Physikalisches Institut, Universität Würzburg and Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Am Hubland, D-97074 Würzburg,

Germany 2University of British Columbia, Vancouver, BC Canada

ABSTRACT:

Al0.6Ga0.4As/GaAs/Al0.6Ga0.4 double barrier resonant-tunneling diodes (RTDs) were grown by molecular beam epitaxy with a lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer. By means of electron beam lithography and etching techniques RTD mesas with diameters ranging from 1 to 12 µm were realized. Au rings serving as top contacts and apertures were evaporated on top of the RTD mesas which allow efficient optical excitation of charge carriers in the absorption layer. The electrical and optical properties of the RTDs were studied for different samples where the thicknesses ds of a thin GaAs spacer layer incorporated between the AlGaAs barrier and the GaInNAs absorption layer varies. At room temperature current-voltage (I-V) curves were measured and the peak-to-valley ratios (PVRs) were determined. It was found that the PVRs depend sensitively on the thickness ds with PVR values from 1.5 (ds = 1 nm) up to 3.8 for ds = 10 nm associated with different contributions of non-coherent transport. The I-V curves of the RTD mesas were also investigated in the dark and optically excited with a laser emitting at wavelength λ = 1.3 µm. A pronounced photo-effect due to hole accumulation close to the double barrier was found with a sensitivity of about 1 kA/W. A comparison of the experiments with the intrinsic absorption probability shows that the overall internal gain exceeds a factor of 6000. KEYWORDS: GaInNAs, telecommunication wavelength, near infrared, light detection

1. INTRODUCTION Resonant tunneling diodes (RTDs) have been demonstrated as sensitive photo detectors and different layouts and geometries have been proposed to optimize the RTD´s photo detector sensitivity for different optical wavelength ranges. The detection mechanism is based on the conversion of an optical signal into an electrical one by excitation of an electron-hole pair, the subsequent local separation of the charge carriers in the electrically biased RTD and the signal amplification caused by a modulation of the RTD´s transmission properties. By means of a proper band gap engineering photo-generated holes can be efficiently accumulated close to the RTD double barrier (DB) which in turn alter via Coulomb interaction the transmission coefficient of tunneling electrons.1,2

Incorporation of an absorption material in RTDs has been found to enhance the light-induced current changes. Illumination of (In)GaAs/AlAs based RTDs with embedded InAs quantum dots (QDs) has resulted in pronounced shifts of the resonant tunneling peak in the I-V curve at low temperatures. 3,4,5 Besides InAs QDs which form e.g. via strain-induced self assembly on InP 5 or GaAs substrates 3,4 another promising candidate material for low band gap absorption in GaAs-based photo detectors is quaternary Ga1-xInxNyAs1-y. 6,7 GaInNAs can be grown lattice-matched to GaAs/Al(Ga)As by choosing the appropriate composition concentrations x and y. 8 Based on this material system, light detection for the near infrared region has been realized with e.g. GaInNAs/GaAs quantum wells or p-i-n diodes embedded in optical cavities. 9,10,11,12,13

In the present study Al0.6Ga0.4As/GaAs/Al0.6Ga0.4As double-barrier resonant tunneling diodes (RTD) were grown with a lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer by molecular beam epitaxy. Electrical and optical properties of the RTDs were studied for different samples with various thicknesses of a thin GaAs spacer layer imbedded between the AlGaAs barrier and the GaInNAs absorption layer. The current-voltage (I-V) curves of the RTD mesas were investigated in the dark and under illumination with a laser (λ = 1.3 µm). A pronounced photo-effect with sensitivities of around 103 A/W was found. Noise current measurements verify a

Infrared Remote Sensing and Instrumentation XX, edited by Marija Strojnik, Gonzalo Paez, Proc. of SPIE Vol. 8511, 85110G · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.931118

Proc. of SPIE Vol. 8511 85110G-1

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current correlation with a noise access factor of only 30. The noise equivalent power was determined and found to be as low as 34 fW/Hz0.5 for an RTD with diameter d = 2 µm.

2. GaInNAs PROPERTIES, SAMPLE PREPARATION AND FABRICATION In Figure 1 band gap energies versus lattice constants for different III-V semiconductor materials are

depicted. The dashed lines correspond to ternary compound semiconductors, e.g. InGaAs or AlGaAs. The blue shaded region corresponds to GaInNAs. From the ternary GaNxAs1-x(x) and InNxAs1-x(x) the band gap energy of Ga1-yInyNxAs1-x can be estimated by 8

CyyxyExEyyxE ggg )1()()()1(),( InNAsGaNAsGaInNAs −−+−= , (1) with x the N and y the In composition. C is the bowing parameter of GaNxAs1-x(x) and assumed to be

constant with C = 0.477 eV 14. For a lattice-matched growth of GaInNAs on GaAs the content ratio of indium to nitrogen is about In[%]:N[%] = 3 8. In Figure 1(b) corresponding band gap energies and respective wavelengths for lattice-matched conditions are plotted for different N and In contents. According to this model, to design the band gap energy corresponding to a wavelength of 1.3 µm the material composition should be Ga0.91In0.09N0.03As0.97. However, usually a subsequent annealing procedure is applied, which reduces non-radiative recombination centers and thus enhances significantly the internal quantum efficiency. The annealing procedure however also causes a blue shift of the band gap energy. In order to finally reach a lattice-matched growth of quaternary GaInNAs on GaAs with a proper band gap of the absorption material to serve for a detection of light with a wavelength of 1.3 µm Ga0.89In0.11N0.04As0.96 was found as an ideal composition.

Figure 1: (a) Band gap energies and lattice constants for various III-V materials. The dashed lines correspond to the ternary materials and the shaded region to GaInNAs. (b) Wavelength and band gap energy of lattice matched GaInNAs to GaAs dependent on the In and N composition.

In the left part of Figure 2 a scheme of the layer sequence of the studied structures is shown. On a silicon-doped GaAs substrate, 300 nm GaAs was grown with a decreasing doping concentration from n = 1x1018 cm-3 to 1x1017 cm-3, followed by the double barrier structure (DBS). The undoped DBS consists of a 15 nm thick GaAs layer and two 3 nm thick Al0.6Ga0.4As barriers embedding a 4 nm thick GaAs quantum well. Subsequently, GaAs spacer layers of varying thickness (dS = 1, 5 or 10 nm) were deposited prior to the reduction of the substrate temperature from 595 °C to 370 °C. The reduction of the growth temperature is required for the growth of the GaInNAs absorption material. The spacer layer was introduced to reduce the effect of oxidation and incorporation of impurities at the DBS during a growth interruption of several minutes while lowering the substrate temperature. Samples with a lattice-matched GaInNAs layer with a thickness of 158, 154 and 149 and a 1, 5 and 10 nm thick GaAs spacer layer, respectively, were grown. The Ga0.89In0.11N0.04As0.96 absorption layer consists of a doped layer with a thickness of 144 nm and n = 1x1017 cm-3.



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The initial Ga0.89In0.11N0.04As0.96 deposited directly after the GaAs spacer layer with varying thickness was undoped. Growth was completed with a 556 nm GaAs thick cap layer with n = 1x1018 cm-3 for which the growth temperature was raised again. Dry chemical etching was applied to define RTD mesas with diameters ranging from 12 µm down to 1 µm. On top of the structure, ring shaped Au contacts were deposited (see electron microscopy image on top of the mesa in Figure 2). The aperture allows optical excitation of charge carriers together with an electrical contact. As sketched in the right part of Figure 2, light-excited electron-hole pairs become locally separated by an electric field. The electrons are transferred away from the DBS while the holes accumulate close to the DBS. This alters the intrinsic electric field and thus changes the transmission rate of electrons through the RTDs for a given bias voltage. 1-5

Figure 2: (Left) Schematic layer sequence of the RTD together with an electron microscopy image of an Au ring contact. The RTDs are based on the GaAs system with highly doped drain and source layers. The DBS consists of 3 nm Al0.6Ga0.4As barriers and a 4 nm GaAs quantum well. The bulk-like GaInNAs absorption layer was grown on top of the DBS with a GaAs spacer layer in between. RTDs mesas with diameters from 12 down to 1 µm have been realized by dry chemical etching and the RTDs are contacted via an Au ring contact to allow illumination of the heterostructure in combination with good contacts. (Right) Schematic valence (VB) and conducting (CB) band profiles of an RTD with a GaInNAs layer biased with an external voltage V. Due to illumination electron-hole pairs are generated. While the electrons are transferred away from DBS, the holes will accumulate close to the DBS.

3. TRANSPORT CHARACTERISTICS OF RTDS WITH AN NEARBY GaInNAs

ABSORPTION LAYER In Figure 3 I(V) characteristics of RTD mesas with a diameter d = 3µm for different spacer layer thicknesses ds = 1, 5 and 10 nm (from (a) to (c)) are shown. All transport measurements were conducted at room temperature. Typical RTD responses were found with a resonance current Ipeak and a valley current Ivalley. The RTDs exhibit also a pronounced bistable switching in the regime of negative differential resistance, which is caused by the load line effect. Remarkable, the resonance currents Ipeak of the three different samples are almost identical with Ipeak ~ 530 µA, while the valley currents differ significantly. The resonance current (or resonant current density Jpeak) is determined mainly by coherent tunneling processes with momentum and energy of electrons conserved during the tunneling process. Contrary, the origin of the valley current is based on hot electrons and non-coherent tunneling, i.e. inelastic scattering processes associated with phonons or defects are involved. 15 As for all three samples the contribution of hot electrons and phonon-electron interactions are be assumed as to be comparable, the variation of Ivalley is interpreted in terms of defect-related scattering mechanisms which occur close to the DBS in the GaInNAs layer. With increasing GaAs spacer layer, the distance between the DBS and GaInNAs increases and the number of defects close to the DBS becomes smaller und thus defects induced scattering is lower for larger ds.



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Figure 3: I(V) characteristics of the GaInNAs integrated RTDs with spacer layer thicknesses from ds = 1 to 5 and 10 nm (a to c). The measurements have been conducted at room temperature with an RTD mesa diameter d = 3µm. The coherent tunneling is constant for a variation of ds, while the non-coherent tunneling contribution is inverse proportional to the spacer layer thickness.

The role of the GaAs spacer layer on the electric properties is now studied in more detail for the three different samples and varying mesa diameters. The peak-to-valley ratio (PVR) was determined by conducting the I-V curves at room temperature in the dark. In Figure 4(a) the resonance currents Ipeak and in Figure 4(b) the valley currents Ivalley are plotted as a function of the RTD area for the different GaAs spacer layer thickness ds. For the three samples the valley and resonance currents (Ipeak,valley) are linear functions of the RTD area. Therefore the current densities Jpeak and Jvalley are almost constant. This indicates that for areas down to about a few µm2 side wall defects and lateral quantization are not important. The PVRs are determined by linear fits of Ipeak and Ivalley with 1.5 (ds = 1 nm), 2.9 (ds = 5 nm) and 3.8 (ds = 10 nm).

Figure 4: Resonance (Ipeak in (a)) and valley (Ivalley in (b)) currents as a function of the RTD area for the samples with ds = 1, 5 and 10 nm. The PVR increases with the thickness of ds caused by a reduction of inelastic scattering via local defects at the DBS. The maximum PVR is 3.8 for ds = 10 nm.

By the introduction of a small spacer layer between the double barrier and the absorption layer the non-coherent transport contribution to the total current can be reduced. A maximum PVR of 3.8 is an excellent value even for resonant tunneling diodes based on GaAs only.

4. PHOTOCURRENT AND SMALL SIGNAL SENSITIVITY FOR THE TELECOMMUNICATION WAVELENGTH In the left part of Figure 5 I(V) curves conducted for an RTD with spacer layer thickness of ds = 5 nm

in the dark as reference and under illumination with a laser emitting at a wavelength λ = 1.3 µm with a power density of P = 5.6 µW/µm2 are shown. Evidently, one can see pronounced differences between the curves. The resonance threshold Vup as well as the valley threshold Vdown of the RTD shift due to an illumination of the RTD to lower bias voltages. This is associated with a light-induced accumulation of photo-generated holes close to



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the DBS and an enhanced photoconductivity. In the right part of Figure 5 the photocurrent ΔI = I(P)-I(dark) versus the applied bias is plotted. The threshold shifts (Vdown and Vup) in combination with the negative differential resistance leads to negative ΔI values down to about -0.4 mA only between Vdown(P) < V <Vdown(dark) and Vup(P) < V <Vup(dark) respectively. Apart from these regions the photocurrent is positive and reaches even a maximum at a bias voltage range of 3 - 4 V. The positive maximum implies that different loss mechanisms of charge carriers exist. For the hole accumulation several loss mechanisms can occur. E.g., for large bias voltages (> 4 V), the band bending results in a lowering of the hole barrier. The number of accumulated holes at the DBS becomes then reduced as the holes can easily escape via tunneling. On the other hand, for bias voltages lower 3 V, radiative recombination of electron-hole pairs becomes likely as the local separation of photo-excited carriers is not efficient.

Figure 5: I(V) characteristics of RTD mesas conducted in the dark and under illumination with a laser emitting photons with a wavelength λ = 1.3 µm and a power density of P = 5.6 µW/µm2 (ds = 5 nm). Under illumination the I(V) characteristic differ by a threshold voltage shift to lower bias voltages and a maximum output current difference <ΔI> (see right part) is found in the bias voltage range V = 3-4 V.

The photo-induced current changes as a function of the laser power for different samples are shown in Figure 6(a). Here the generated photocurrents ΔI are plotted as a function of the incident laser power for the different samples with an RTD diameter d = 6 µm. The RTDs were biased at V = 3 V. Two trends can be observed. First, the maximum photocurrent <ΔI> dependents on the spacer layer thickness with <ΔI>max ~ 62 µA for ds = 1 nm and 11 µA for ds = 10 nm. Contrary to the electrical response of the RTDs, e.g. enhanced PVR under an increment of ds, the light sensitivity decreases with increasing spacer layer thickness. This observation is associated with a reduction of the Coulomb interaction of the accumulated holes with electrons passing through the DBS due to an increasing spacer layer thickness. Second, the photocurrents are nonlinear functions of the incident light power. Thus the sensitivity is not constant and depends on the input power. In Figure 6(b) the small signal photocurrent <ΔI> is shown for an RTD with diameter d = 11 µm and spacer layer thickness ds = 5 nm. Indeed, the small signal photocurrent is a linear function of the incident light power.



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Figure 6: (a) Photocurrents <ΔI> = I(P) –I(dark) versus incident light power for ds = 1, 5 and 10 nm. The maximum of <ΔI> is decreasing with increasing ds. (b) <ΔI> as a function of the light power for the sample with ds = 5 nm. The corresponding sensitivity is S = 966 A/W.

Thus one can extract the sensitivity S(P) = ∂<ΔI>/∂P from linear fitting with S(P) = (966 ± 15) A/W. With an ideal value of the quantum efficiency ηe = 1 and a spectral response Rλ at 1.3 µm with 1.05 A/W the gain of the RTD light detector can be estimated to be at least 900. However taking into account a quantum efficiency ηe = 14% calculated from the thickness of the absorption layer with an absorption coefficient at the band edge of 104 cm-1, the gain actually exceeds a value of 6000.

5. CURRENT NOISE AND NOISE EQUIVALENT POWER Besides the sensitivity for a detector also the current noise is an important quantity. While the sensitivity mainly reflects the number of charge carriers generated per incident photons, the current noise depends on several physical mechanisms. The system noise limits the resolution of the minimal current change which can be detected for a given integration time. In Figure 7 the I(V) characteristics (top) and the corresponding current noise (bottom) iRTD of an RTD with ds = 5 nm and diameter d = 5µm are plotted. The current noise was recorded in a frequency bandwidth of Δf = 25-125 kHz. From a comparison of the I-V curve with the current noise one can see that iRTD increases similarly to the current. This suggests that the current noise is dominated by shot noise of charge carrier transport through the RTD. For comparison also the theoretical shot noise limit determined by ishot = (2qI)0.5 for the RTD mesa is shown in Figure 7. The calculated shot noise is about a factor of 30 smaller than the experimentally conducted current noise. Thus by assuming that electron transport contributes mainly to the noise, the corresponding excess factor is approximately 30 for a bias voltage range from 3 to 8 V.



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Figure 7: I(V) characteristic (top) and corresponding noise current iRTD (bottom) for the RTD with diameter d = 5µm and spacer layer thickness ds = 5nm. The current noise is correlated with the dark current and exceeds the theoretical shot noise limit (red line) by a factor of around 30.

Having in mind that the internal gain of photo-generated electron hole pairs was estimated to be at least M = 6000, the noise current gain is astonishingly small with a value of 30. This also indicates that mainly the Coulomb interaction of localized holes serving as internal floating gate is responsible for the large photo effect in RTDs as the contribution of shot noise of localized charges is small. It is worth to mention that in standard avalanche photo diodes (APD), the internal gain M is directly correlated with the current noise iAPD with

2 ( )APD sdi q I I f F M= Δ + Δ , (2)

where Isd stands for the dark current, ΔI the photocurrent and F the excess noise factor with frequency bandwidth fΔ . Even for condition that only a dark current flows through the APD, carriers injected into the APD multiplication region become amplified by the same internal gain factor and contribute to the current noise. This is different for the studied RTD light detector, as the gain factors for the photo-induced signal and current noise differ significantly.

The noise equivalent power (NEP) was also determined, which quantifies the minimum detectable light

power. The NEP is defined as the ratio of the current noise iRTD and the sensitivity S with the relation

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In Figure 8 the noise equivalent power calculated from eq. (3) as a function of the RTD light detector area is shown. As reference also the NEP of an InGaAs APD (Ref. (16)) with a NEP of 460 fW/Hz0.5 is presented. The NEP of the studied GaInNAs integrated RTD light detector for an RTD diameter of 2 µm the NEP is around 34 fW/Hz0.5.



NEP = 460 fW /Hi'S

10

ó

1 10

area (pm2)

100

Figure 8: Noise equivalent power versus detector area. Also shown is the minimum NEP from Ref. (16) for an InGaAs avalanche photodiode with a NEP of around 460 fW/Hz0.5.

SUMMARY In summary, Al0.6Ga0.4As/GaAs/Al0.6Ga0.4 double barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby and lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer for light detection at the telecommunication wavelength λ = 1.3 µm at room temperature. Samples with different thicknesses of a thin GaAs spacer layer between the AlGaAs barrier and the GaInNAs absorption layer have been fabricated and the electric-optical properties of the RTDs with diameters from 12 to 1µm have been studied. The peak-to-valley ratios (PVRs) are increasing from 1.5 (ds = 1 nm) up to 3.8 for ds = 10 nm. Herby the coherent tunneling current (Ipeak) is not affected by the spacer layer thickness but the non-coherent current (Ivalley) is significantly reduced with increasing spacer layer thickness. The current-voltage (I-V) curves of the RTD mesas were investigated in the dark and under illumination at λ = 1.3 µm. A pronounced photo-effect is found and contributed to hole accumulation at the double barrier of the RTDs. For small input powers the sensitivity is constant with around 1000 A/W. With an intrinsic absorption probability of 14%, the overall internal gain actually exceeds 6000. The current noise measurements verify that the overall noise is correlated to the shot noise but not correlated with the internal gain. In comparison with a standard single photon detector the noise equivalent powers are significant lower and in the order of tens of fW/Hz0.5. ACKNOWELEMNT:

The authors are grateful for financial support by the BMBF via national project EIPHRIK (FKZ: 13N10710) and the European Union (FPVII (2007-2013) under grant agreement n° 256959 NANOPOWER). Expert technical assistance by M. Emmerling and S. Handel is gratefully acknowledged. REFERNCES: [1] Park, P. W., Chu, H. Y., Han, S.G., Choi, Y.W., Kim, G. and Lee, E.-H., “Optical switching mechanism based on charge accumulation effects in resonant tunneling diodes”, Applied Physics Letters 67, 1241-1243 (1995). [2] Coelho, I. J. S., Martins-Filho, J. F., Figueiredo, J. M. L. and Ironside, C. N., “Modeling of light-sensitive resonant-tunneling-diode devices”, Journal of Applied Physics 95, 8258-8263 (2004). [3] Blakesley, J. C., See, P., Shields, A. J., Kardynal, B. E., Atkinson, P., Farrer, I. and Ritchie, D.A., “Efficient Single Photon Detection by Quantum Dot Resonant Tunneling Diodes”, Physical Review Letters 94, 067401 (2005). [4] Li, H. W., Kardynał, B. E., Ellis, D. J. P., Shields, A. J., Farrer, I. and Ritchie, D. A., “Quantum dot resonant tunneling diode single photon detector with aluminum oxide aperture defined tunneling area”, Applied Physics Letters 93, 153503 (2008). [5] Li, H. W., Kardynał, B. E., See, P., Shields, A. J., Simmonds, P., Beere, H. E. and Ritchie, D. A., “Quantum dot resonant tunneling diode for telecommunication wavelength single photon detection”, Applied Physics Letters 91, 073516 (2007).



[6] Riechert, H., Ramakrishnan, A. and Steinle, G., “Development of InGaAsN-based 1.3 μm VCSELs”, Semiconductor Science and Technology 17, 892 (2002). [7] Kitatani, T., Kondow, M., Nakatsuka, S., Yazawa, Y. and Okai, M., ” Room-temperature lasing operation of GaInNAs-GaAs single-quantum-well laser diodes”, IEEE Journal of Selected Topics in Quantum Electronics 3, 206 (1997). [8] Kudrawiec, R., “Alloying of GaNxAs1−x with InNxAs1−x: A simple formula for the band gap parametrization of Ga1−yInyNxAs1−x alloys”, Journal of Applied Physics 101, 023522 (2007). [9] Han, Q., Yang, X. H., Niu, Z. C., Ni, H. Q., Xu, Y. Q., Zhang, S. Y., Du, Y., Peng, L. H., Zhao, H., Tong, C. Z., Wu, R. H. and Wang, Q. M., “1.55μm GaInNAs resonant-cavity-enhanced photodetector grown on GaAs”, Applied Physics Letters 87, 111105 (2005). [10] Loke, W.K., Yoon, S.F., Wicaksono, S. and Ng, B.K., “Characteristics of non-annealed λ = 1.35 μm closely lattice-matched GaInNAs/GaAs p-i-n photodetector structures grown by solid-source molecular beam epitaxy”, Materials Science and Engineering B 131, 40–44 (2006). [11] Loke, W. K., Yoon, S. F., Tan, K. H., Wicaksono, S. and Fan, W. J., “Improvement of GaInNAs p-i-n photodetector responsivity by antimony incorporation”, Journal of Applied Physics Letters 101, 033122 (2007). [12] David, J. P. R. and Tan, C. H., “Material Considerations for Avalanche Photodiodes”, IEEE Journal of Quantum Electronics 14, NO. 4, 998 - 1009 (2008). [13] Yang, X., Héroux, J. B., Mei, L. F. and Wang, W. I., “InGaAsNSb/GaAs quantum wells for 1.55 μm lasers grown by molecular-beam epitaxy”, Applied Physics Letters 78, 4068 (2001). [14] Veal, T. D., Piper, L. F. J., Jefferson, P. H., Mahboob, I., McConville, C. F., Merrick, M., Hosea, T. J. C., Murdin, B. N. and Hopkinson, M., “Photoluminescence spectroscopy of bandgap reduction in dilute InNAs alloys”, Applied Physics Letters 87, 182114 (2005). [15] Zohta, Y. and Ezawa, H., “Effect of inelastic scattering on resonant tunneling studied by the optical potential and path integrals”, Journal of Applied Physics 72, 3584 (1992). [16] For a review of commercial available APD detectors: www.thorlabs.com.



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